Patent Publication Number: US-2021169402-A1

Title: Wearable Wrist Joint-Action Detectors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of Ser. No. 14/963,136, filed 2015 Dec. 8, and claims priority from provisional application 62/102,494, filed 2015 Jan. 12. 
    
    
     BACKGROUND—PRIOR ART 
     Physical activity is one of the most important factors in reducing the risk of obesity, cardiovascular disease, diabetes, metabolic syndrome, colon cancer, breast cancer, and depression. In addition, a recent study shows that prolonged sitting is linked to higher risk of death from heart disease and other causes, even if a person gets regular exercise. Pedometers are commonly used to monitor physical activity, and are usually worn on the waist, wrist, or foot of a user. A modern pedometer typically incorporates an accelerometer to detect body vibrations for counting steps as a user walks or runs, but cannot detect other types of physical activity. For example, we have found that a pedometer cannot reliably distinguish between sitting and standing, because a user is stationary in both cases. We have also found that a pedometer might mistakenly count steps when a user&#39;s body vibrates on a traveling vehicle, or when a user steers a vehicle while wearing a wrist-worn pedometer. 
     U.S. Pat. No. 7,463,997 (2008) to Pasolini et al. discloses a step-detection algorithm for a pedometer incorporating an accelerometer carried by a user, such as on the user&#39;s belt or shoulder. U.S. patent application Pub. No. 2014/0074431 (2014) by Modi discloses step-detection optimizations for a pedometer incorporating an accelerometer worn on a user&#39;s wrist. The challenges of detecting steps reliably by using the accelerometer data of a pedometer are discussed in these two publications. 
     U.S. Pat. No. 4,651,446 (1987) to Yukawa et al. discloses a pedometer that uses a step sensor mounted on the instep of footwear for detecting the flexing motion of the instep as a user walks or runs. The step sensor can be a strain transducer, a piezoelectric element, or simply a switch which closes during the flexing motion of the instep. Although this pedometer can detect steps reliably without suffering interference from body vibrations not related to walking or running, it still cannot detect whether a user is sitting, standing, or performing any type of physical activity other than walking and running. Furthermore, we have found that this type of step sensor would not be very reliable and durable because of wear and tear of the mechanical parts after repeated use. 
     U.S. Pat. No. 6,005,548 (1999) to Latypov et al. discloses a method for tracking a user&#39;s spatial position and orientation by placing a sensor of relative rotation angles at each of the main joints of the user, such as the hip, knee, shoulder, and elbow joints. This sensor of relative rotation angles of a joint is a strain gauge or an optical-fiber sensor, which is physically attached to the body segments connected by the joint. Although this method can detect many types of physical activity performed by a user, physically attaching a sensor of rotation angles to two body segments connected by a joint is inconvenient and cumbersome. 
     U.S. Pat. No. 8,165,844 (2012) to Luinge et al. discloses a system for capturing motion of a human body by placing a wireless sensor module on each body segment. Each sensor module contains accelerometers and gyroscopes for collecting three-dimensional inertial data, and optionally magnetometers for sensing the earth&#39;s magnetic field. Although this method can detect any physical activity, the relatively complex system requires placing a sensor module on each of the two body segments linked by a joint for determining the degree of flexion of the joint. The system is too complicated and costly for detecting common physical activities such as walking, running, sitting, and standing, so it is not practical for widespread use. 
     The basic element of any physical activity is a joint action, which also includes the degree of the joint action in this description. A joint action is any movement of a joint that is allowed by the anatomy of the joint, such as flexion, extension, abduction, adduction, internal rotation, or external rotation. For this reason, a physical activity performed by a human being or an animal can be determined by detecting the joint actions of the joints (simply called actions of the joints in this description) that are involved in the physical activity. However, the aforementioned prior-art approaches for monitoring physical activities either do not detect joint actions or use complicated devices for detecting joint actions. In short, they suffer from a number of disadvantages: 
     (a) Accelerometer-based pedometers might mistakenly count steps when a user&#39;s body vibrates. 
     (b) A current physical-activity monitor, such as a pedometer, worn on the waist, wrist, or foot of a user, can only detect that the user is walking or running, but not any other type of physical activity, such as sitting or standing. 
     (c) Current joint-action sensors that can detect a user performing a physical activity by sensing the actions of the joints involved in the physical activity are inconvenient, cumbersome, and costly, so they are not practical for widespread use. 
     For improving and simplifying physical-activity monitoring, U.S. Pat. No. 8,050,881 (2011) to Yeung et al. (the same inventors of the present invention) discloses a physical-activity monitoring system that incorporates only three sensors on the body of a user. The sensors are accelerometers worn on the wrist, waist, and thigh (the accelerometer worn on the thigh is for improving detection of physical activities of the lower extremities) of a user. The accelerometers are used for monitoring the orientations and movements of the body segments of a user to detect the type and intensity of physical activity performed. The accelerometers on the wrist are useful in detecting activities involving the upper extremities. The degree of flexion of the hip joint can be determined from the orientations of the waist and thigh with respect to the vertical direction of the earth&#39;s gravitational acceleration, so that stationary activities, such as standing and sitting, can be detected. The waist and thigh accelerometers can also detect more rigorous activities, such as walking, running, or jumping, from simultaneous, fast-changing accelerations. However, we have found that it would be a major improvement in usability for the physical-activity monitoring system if the sensor worn on the thigh were not required for detecting actions of the hip joint. 
     SUMMARY 
     In accordance with a first embodiment of a wearable joint-action sensor, the sensor comprises a capacitive proximity sensor that is worn on the right waist of the torso of a user, above the right thigh of the right upper leg, for detecting flexion and extension of the right hip joint, without requiring the user to attach a sensor to the right thigh. The capacitive proximity sensor detects the amount of separation between the capacitive proximity sensor and the right abdomen of the torso above the right thigh as the right hip joint is flexed or extended. Based on the amount of separation and its rate of change, the wearable joint-action sensor can detect actions of the right hip joint of the user, such as walking, running, sitting, and standing. 
     In accordance with a second embodiment of a wearable joint-action sensor, the sensor comprises a capacitive proximity sensor that is worn on the right shoulder of the torso of a user and attached with a shoulder pad. The capacitive proximity sensor detects the amount of separation between the capacitive proximity sensor and the region comprising both the right shoulder and the top of the right upper arm, as the right shoulder joint is abducted or adducted. Any other action of the right shoulder joint, such as flexion or extension, can be detected by placing an additional sensing plate of the capacitive proximity sensor on the shoulder pad, where the capacitive proximity sensor can detect the amount of separation between the sensing plate of the capacitive proximity sensor and the region comprising both the right shoulder and the top of the right upper arm, as the right shoulder joint performs the action. Based on the amount of separation and its rate of change, the wearable joint-action sensor can detect actions of the right shoulder joint of the user, such as swinging, lifting, jumping rope, and pushing up. 
     In accordance with a third embodiment of a wearable joint-action sensor, the sensor comprises a capacitive proximity sensor that is worn on the wrist region of the right forearm of a user. The capacitive proximity sensor detects the amount of separation between the capacitive proximity sensor and the wrist regions of the right forearm and the right hand as the right wrist joint is flexed or extended. Based on the amount of separation and its rate of change, the wearable joint-action sensor can detect actions of the right wrist joint, such as waving, pushing, and slapping. 
     In accordance with a fourth embodiment of a wearable joint-action sensor, the sensor comprises a capacitive proximity sensor that is mounted on the instep of the right footwear that is worn by a user. The capacitive proximity sensor detects the amount of separation between the capacitive proximity sensor and the instep of the right foot as the right ankle joint is flexed (plantar flexion) or extended (dorsiflexion). Based on the amount of separation and its rate of change, the wearable joint-action sensor can detect actions of the right ankle joint, such as kicking, tip toeing, walking, and running. 
     In accordance with a fifth embodiment of a wearable joint-action sensor, the sensor comprises a capacitive proximity sensor that is worn on the right upper arm of a user above the elbow. The capacitive proximity sensor detects the amount of separation between the capacitive proximity sensor and the right upper arm above the elbow pit (antecubital fossa) as the right elbow joint is flexed or extended. Based on the amount of separation and its rate of change, the wearable joint-action sensor can detect actions of the right elbow joint, such as lifting, stretching, and punching. 
     In accordance with a sixth embodiment of a wearable joint-action sensor, the sensor comprises a capacitive proximity sensor that is worn on the first finger segment of the right index finger of a user. The capacitive proximity sensor detects the amount of separation between the capacitive proximity sensor and a region comprising both the first and second segments of the right index finger near the first finger joint (the proximal interphalangeal joint) as the first finger joint is flexed or extended. Based on the amount of separation and its rate of change, the wearable joint-action sensor can detect actions of the first finger joint of the right index finger, such as typing, flicking, and pointing. 
     In accordance with a seventh embodiment of a wearable joint-action sensor, the sensor comprises a capacitive proximity sensor that is worn on top of two adjacent sections of the upper spinal column of a user. The capacitive proximity sensor detects the amount of separation between the capacitive proximity sensor and a region comprising the two adjacent sections of the upper spinal column as the upper spinal column is flexed or extended. Based on the separation and its rate of change, the wearable joint-action sensor can detect actions of the upper spinal column, such as flexion and extension. The wearable joint-action sensor can inform the user of poor posture because of excessive flexion of the upper spinal column by activating a reminder to produce vibration, sound, voice, display, or light. 
     In accordance with an eighth embodiment of a wearable joint-action sensor, the sensor comprises an active infrared proximity sensor (or a visible- or infrared-light imaging sensor) and a triaxial accelerometer that is worn on the right waist of the torso of a user above the right thigh of the right upper leg. By using four infrared-light sources, the active infrared proximity sensor detects the amount of separation between the active infrared proximity sensor and the right thigh of the right upper leg, and the amount of deviation of the right thigh from the midline of the active infrared proximity sensor. Based on the amount of separation and the amount of deviation of the right thigh from the midline of the active infrared proximity sensor, and its rates of change, the wearable joint-action sensor can detect actions of the right hip joint and many types of physical activity, such as walking, running, sitting, standing, and dancing. Furthermore, by combining the joint actions detected by using the active infrared proximity sensor and the body actions detected by using the triaxial accelerometer, the wearable joint-action sensor can detect additional types of physical activity, such as jumping, somersaulting, sitting up, pushing up, and biking. 
     In addition to being used for detecting many types of physical activity, a wearable joint-action sensor can be used for controlling a computing device or for serving as a reminder as follows. When a wearable joint-action sensor uses a communication device to communicate with a computing device, such as a smartwatch, fitness wristband, smartphone, or computer, a joint action detected by the wearable joint-action sensor can be used for controlling the computing device, such as turning a display on or off, turning an audio recorder on or off, playing back a recorded audio message, or receiving or terminating a phone call. Furthermore, multiple wearable joint-action sensors, each used for detecting actions of a joint (such as a finger joint, wrist joint, toe joint, or ankle joint), can communicate with a computing device, such as an electronic piano (or any electronic music instrument), electronic game console, or computer, to perform complex operations of the computing device. A wearable joint-action sensor can also communicate with a reminder, such as a voice recorder, vibrator, beeper, display, or simply a light-emitting diode (LED), to remind a user or someone else (such as a caregiver of the user) that a predetermined desirable or undesirable joint action is or has been detected. 
    
    
     
       DRAWINGS—FIGURES 
         FIG. 1  is a block diagram showing schematically a wearable joint-action sensor incorporating a capacitive proximity sensor in accordance with the embodiments of the invention. 
         FIG. 2  is a flow diagram illustrating the operation of the wearable joint-action sensor in  FIG. 1  in accordance with the embodiments of the invention. 
         FIG. 3A  is a graphical illustration of a possible location of the wearable joint-action sensor for detecting actions of the right hip joint of a user in accordance with the first embodiment. 
         FIG. 3B  is a front view (the surface that faces the right abdomen of the user) of the wearable joint-action sensor in  FIG. 3A . 
         FIG. 4A  is a graphical illustration of a decrease in the amount of separation between the capacitive proximity sensor (i.e. the sensing plate of the capacitive proximity sensor) and the right abdomen above the right thigh as the right hip joint of the user in  FIG. 3A  is flexed. 
         FIG. 4B  is a graphical illustration of an increase in the amount of separation between the capacitive proximity sensor and the right abdomen above the right thigh as the right hip joint of the user in  FIG. 3A  is extended. 
         FIG. 5  is a graphical illustration of a possible location of the wearable joint-action sensor for detecting actions of the right shoulder joint of a user in accordance with the second embodiment. 
         FIG. 6  is a graphical illustration of a decrease in the amount of separation between the capacitive proximity sensor and the region comprising both the right shoulder and the top of the right upper arm as the right shoulder joint of the user in  FIG. 5  is abducted. 
         FIG. 7  is a graphical illustration of a possible location of the wearable joint-action sensor for detecting actions of the right wrist joint of a user in accordance with the third embodiment. 
         FIG. 8A  is a graphical illustration of a decrease in the amount of separation between the capacitive proximity sensor and the wrist regions of the right hand and the right forearm as the right wrist joint of the user in  FIG. 7  is extended. 
         FIG. 8B  is a graphical illustration of an increase in the amount of separation between the capacitive proximity sensor and the wrist regions of the right hand and the right forearm as the wrist joint of the user in  FIG. 7  is flexed. 
         FIG. 9  is a block diagram showing schematically the communication between the wearable joint-action sensor and a computing device for controlling the operation of the computing device by using an action of a joint. 
         FIG. 10  is a graphical illustration of a possible location of the wearable joint-action sensor for detecting actions of the right ankle joint of a user in accordance with the fourth embodiment. 
         FIG. 11A  is a graphical illustration of a decrease in the amount of separation between the capacitive proximity sensor and the instep of the right foot as the right ankle joint of the user in  FIG. 10  is extended (dorsiflexion). 
         FIG. 11B  is a graphical illustration of an increase in the amount of separation between the capacitive proximity sensor and the instep of the right foot as the right ankle joint of the user in  FIG. 10  is flexed (plantar flexion). 
         FIG. 12  is a graphical illustration of a possible location of the wearable joint-action sensor for detecting actions of the right elbow joint of a user in accordance with the fifth embodiment. 
         FIG. 13  is a graphical illustration of an increase in the amount of separation between the capacitive proximity sensor and the right upper arm above the elbow pit as the right elbow joint of the user in  FIG. 12  is flexed. 
         FIG. 14  is a graphical illustration of a possible location of the wearable joint-action sensor for detecting actions of the first finger joint (the proximal interphalangeal joint) of the right index finger of a user in accordance with the sixth embodiment. 
         FIG. 15  is a graphical illustration of an increase in the amount of separation between the capacitive proximity sensor and the right index finger near the first finger joint as the first finger joint of the right index finger of the user in  FIG. 14  is flexed. 
         FIG. 16  is a graphical illustration of using a glove for attaching the wearable joint-action sensor in the proximity of the wrist joint and in the proximity of each of the first finger joints of the fingers of the right hand of a user. 
         FIG. 17  is a block diagram showing schematically the wearable joint-action sensor communicating with a reminder in accordance with the seventh embodiment. 
         FIG. 18  is a graphical illustration of a possible location of the wearable joint-action sensor for detecting actions of the upper spinal column of a user in accordance with the seventh embodiment. 
         FIG. 19  is a graphical illustration of an increase in the amount of separation between the capacitive proximity sensor and the upper spinal column of the user in  FIG. 18  as the user flexes the upper spinal column. 
         FIG. 20A  is a graphical illustration of a possible location of a wearable joint-action sensor incorporating an active infrared proximity sensor and a triaxial accelerometer for detecting actions of the right hip joint of a user in accordance with the eighth embodiment. 
         FIG. 20B  is a front view (the surface that faces the right abdomen of the user) of the wearable joint-action sensor in  FIG. 20A . 
         FIG. 21A  is a graphical illustration of the separation between the active infrared proximity sensor and the right thigh, and the orientation of the triaxial accelerometer, as the right hip joint of the user in  FIG. 20A  is flexed. 
         FIG. 21B  is a graphical illustration of an increase in the amount of separation between the active infrared proximity sensor and the right thigh, and the orientation of the triaxial accelerometer, as the degree of flexion of the right hip joint of the user in  FIG. 21A  decreases. 
         FIG. 22  is a block diagram showing schematically the wearable joint-action sensor incorporating the active infrared proximity sensor and the triaxial accelerometer in accordance with the eighth embodiment. 
         FIG. 23  is a flow diagram illustrating the operation of the wearable joint-action sensor in  FIG. 22  in accordance with the eighth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A proximity sensor is used in each of the embodiments of the wearable joint-action sensor for detecting actions of a joint, such as flexion and extension, by detecting the amount of separation between the proximity sensor and a target near the joint. We have found that using a proximity sensor to detect actions of a joint for monitoring physical activity of a user has a number of advantages: 
     (a) A single proximity sensor in the wearable joint-action sensor can detect actions of a joint directly without the need for obtaining data from multiple sensors located on the two body segments linked by a joint. As discussed in the first and eighth embodiments below, the wearable joint-action sensor worn on the right waist of a user can detect actions of the right hip joint, such as flexion, extension, abduction, and adduction, so that physical activities, such as walking, running, dancing, standing, and sitting, can be detected without requiring attaching a second sensor to the right thigh, as suggested by the prior arts. 
     (b) A proximity sensor in the wearable joint-action sensor can detect actions of a joint accurately without suffering from being significantly influenced by vibration of a user&#39;s body as an accelerometer-based physical-activity monitor would be, because an accelerometer-based physical-activity monitor detects movements of a single body segment (which moves during vibration) while the proximity sensor in the wearable joint-action sensor detects relative movements of two body segments (which tend to move together during vibration) linked by a joint. 
     (c) The wearable joint-action sensor based on proximity sensing is easy to wear, compact, efficient, and low cost, so it is practical for widespread use for physical-activity monitoring or for controlling a computing device, such as an electronic game console, electronic piano (or any electronic music instrument), smartwatch, fitness wristband, smartphone, or computer. 
     (d) The wearable joint-action sensor based on proximity sensing is very reliable and durable because it does not suffer from wear and tear of mechanical parts after repeated use. 
     Proximity sensors are well known in the art of sensing, and they have a high reliability and long functional life because there is no moving mechanical part. Common proximity sensors include capacitive proximity sensors, active infrared proximity sensors (which use both infrared-light sources and detectors), passive infrared proximity sensors (which use only infrared-light detectors), inductive proximity sensors, magnetic proximity sensors, ultrasonic proximity sensors, single-chip radars (which are widely used in automobiles for safety and convenience), visible- or infrared-light imaging sensors or auto-focus rangefinders (with or without a light source), and time-of-flight proximity sensors (which are advanced active infrared proximity sensors) that measure the time that light (typically infrared light) takes to travel to the target and reflect back to the sensor. 
     Many different types of proximity sensors can be used in the embodiments of the wearable joint-action sensor. An active infrared proximity sensor (including time-of-flight proximity sensors), ultrasonic proximity sensor, or single-chip radar requires emitting a beam of waves to its target for detecting the amount of separation between the sensor and the target, so it consumes additional power for producing the beam of waves. Without emitting a beam of waves, an inductive proximity sensor or a magnetic proximity sensor detects the amount of separation between the proximity sensor and a metal target, but it would be inconvenient for the user to wear the metal target on a body segment. Visible- or infrared-light imaging sensors or autofocus rangefinders can be used as proximity sensors, but they require adequate illumination of their targets. Without adequate illumination, infrared-light imaging sensors or autofocus rangefinders, or passive infrared proximity sensors (often referred to as pyroelectric sensors), can perform proximity sensing based on the infrared radiation of a warm target, such as a body segment of a human being or an animal if the body segment is not covered with thick clothing. 
     A capacitive proximity sensor is simple, low-cost, and does not require emitting a beam of waves or using a metal target. We presently contemplate using a capacitive proximity sensor in the wearable joint-action sensor, although any other type of proximity sensor, such as an active infrared proximity sensor, can replace the capacitive proximity sensor in the embodiments of the wearable joint-action sensor. Using a capacitive proximity sensor for the embodiments should not be construed as limiting the scope of the embodiments, but merely as providing illustrations of some of the presently preferred embodiments. For example, the capacitive proximity sensor in the embodiments of the wearable joint-action sensor can be replaced with an active or passive infrared proximity sensor, visible- or infrared-light imaging sensor or rangefinder (with or without a light source), ultrasonic proximity sensor, single-chip radar, or any other type of proximity sensor, without departing from the spirit and the scope of the embodiments. 
     Many techniques have been developed for measuring capacitance. A simple technique is to charge an unknown capacitor with a constant current source to produce a voltage change across the unknown capacitor, so that the value of capacitance of the unknown capacitor is simply equal to the charge (i.e. the constant current multiplied by the charging time) divided by the voltage change. A capacitive proximity sensor can measure an increase in capacitance when a target moves closer to the sensing plate of the capacitive proximity sensor, and vice versa, so the capacitance measured with the sensing plate of the capacitive proximity sensor can be used for representing the separation between the capacitive proximity sensor and the target, if the target remains the same. 
     In general, there are two types of capacitive proximity sensing systems: mutual-capacitive proximity sensing and self-capacitive proximity sensing. In a mutual-capacitive proximity sensing system, a sensing target (such as a finger) alters the mutual capacitive coupling between two sensing pads (i.e. two electrodes) on a sensing plate. The capacitive change is sensed by a capacitance sensor for detecting the separation between the sensing pads (i.e. the sensing plate, because the thickness of a sensing pad is usually negligible) and the target. In a self-capacitive proximity sensing system, a target (such as a finger) capacitance to ground increases the parasitic capacitance of a sensing pad (on a sensing plate) to ground. This is sensed by a capacitance sensor for detecting the separation between the sensing pad (i.e. the sensing plate) and the target. Either mutual-capacitive proximity sensing or self-capacitive proximity sensing can be used for the wearable joint-action sensor, although we presently contemplate using self-capacitive proximity sensing because of the availability of low-cost self-capacitive proximity sensors. 
     Capacitive proximity sensors have been used in mobile phones for detecting close proximity of a user&#39;s cheek as the user holds the phone next to the ear during a call, so that the touchscreen of the phone can be disabled to prevent accidental touchscreen taps. An advanced capacitive proximity sensor available from Semtech Corporation of California (Product Number: SX9300) can be used in a mobile phone for accurately discriminating between an inanimate object and human body proximity, so that the mobile phone can reduce its radio-frequency emission power in the presence of a human body to control the radio-frequency power absorbed by the human body to be within a safe level. The advanced capacitive proximity sensor consists of two capacitive proximity sensors and two sensing pads (one sensing pad for each capacitive proximity sensor) of equal surface areas, with one sensing pad (the inner sensing pad) completely surrounded by the other sensing pad (the outer sensing pad). By using this approach for capacitive proximity sensing, the advanced capacitive proximity sensor is able to discriminate between proximity generated by a low-permittivity object (such as a table) and a high-permittivity object (such as a human body). 
     A capacitive proximity sensor adjusted to a very short range can be used as a touch key, such as a key on the key pad of a mobile phone. For the embodiments of the wearable joint-action sensor, touch sensing can also be used, because it is a form of proximity sensing. Any proximity sensor, such as a capacitive proximity sensor, active infrared proximity sensor (including time-of-flight proximity sensors), passive infrared proximity sensor, inductive proximity sensor, magnetic proximity sensor, ultrasonic proximity sensor, single-chip radar, and visible- or infrared-light imaging sensor or auto-focus rangefinder (with or without a light source) can also be used for touch sensing to detect joint actions in the embodiments of the wearable joint-action sensor. 
     For the embodiments of the wearable joint-action sensor, we contemplate using an 8-channel capacitive touch sensor with proximity detection and signal guard that is available from Microchip Technology Inc. of Arizona (Product Number: CAP1298), although a capacitive proximity sensor manufactured by another company, such as Semtech Corporation of California (Product Number: SX9300, as discussed above), can also be used. Alternatively, an active infrared proximity sensor, such as an infrared proximity sensor available from Maxim Integrated Products Inc. of California (Product Number: MAX44000) operating with one or more external infrared light-emitting diodes (LEDs, the active light sources of the active infrared proximity sensor), or any other type of proximity sensor, can be used for the embodiments of the wearable joint-action sensor. We presently contemplate using one or more channels of the capacitive proximity sensor for self-capacitive proximity sensing, and the remaining channels for capacitive touch sensing of conventional touch keys, so that a user can interact with the wearable joint-action sensor by using the tip of a finger to touch the touch keys. 
     FIG.  1 —Wearable Joint-Action Sensor 
       FIG. 1  shows a block diagram of a wearable joint-action sensor  100  incorporating a capacitive proximity sensor  102 , which has multiple capacitive proximity-sensing channels.  FIG. 1  also shows a side view of a sensing pad  108  (with optional signal guards  310   a  and  310   b  shown in  FIG. 3B  removed to expose sensing pad  108 ) on a sensing plate  118  of wearable joint-action sensor  100 , as well as a user&#39;s body target  114 , which is one or both (i.e. the region comprising both of the two body segments) of the two body segments linked by a joint, of which actions are sensed by wearable joint-action sensor  100 . User&#39;s body target  114  serves as the target for capacitive proximity sensor  102  (i.e. for sensing pad  108  on sensing plate  118  of capacitive proximity sensor  102 ). A body segment, such as the torso (including the abdomen, waist, chest, shoulder, etc. of the torso), a section of the spinal-column, the head, the neck, an upper arm, a forearm (including the wrist region, etc. of the forearm), a hand (including the wrist region, back, palm, fingers, etc. of the hand), a finger segment, an upper leg (including the thigh, buttock, etc. of the upper leg), a lower leg (including the ankle region, etc. of the lower leg), a foot, or a toe segment, is one of the constituent parts into which the body of a human or an animal is divided or marked off by natural boundaries for the discussion of the embodiments. 
     Sensing pad  108  on sensing plate  118  is connected to a channel  104  of capacitive proximity sensor  102  for capacitive proximity sensing, so that the amount of a separation  130  between sensing pad  108  on sensing plate  118  and user&#39;s body target  114  can be detected for detecting actions of the joint (the joint is not shown in  FIG. 1 ), such as flexion and extension. To simplify discussion in this description, the separation between sensing plate  118  and user&#39;s body target  114  is also separation  130 , because the thickness of sensing pad  108  is usually negligible. Furthermore, separation  130  is also used as the separation between capacitive proximity sensor  102  and user&#39;s body target  114 , because capacitive proximity sensor  102  uses sensing pad  108  on sensing plate  118  for detecting separation  130 . 
     In the embodiments of wearable joint-action sensor  100  for sensing joint actions, wearable joint-action sensor  100  is strategically worn on a first body segment of a joint, which links the first body segment to a second body segment. User&#39;s body target  114  is one of the two body segments linked by the joint or both body segments in a region near the joint (i.e. a region comprising both body segments). Sensing pad  108  on sensing plate  118  is in an appropriate proximity of user&#39;s body target  114  for joint-action detection by detecting separation  130  with capacitance, which represents separation  130 , obtained from capacitive proximity sensor  102 . When the surface of user&#39;s body target  114  is not parallel to the surface of sensing pad  108  on sensing plate  118 , the amount of separation  130  is represented by an average capacitance sensed by sensing pad  108 . Furthermore, when user&#39;s body target  114  for detecting actions of a joint is a combination of the first body segment and the second body segment near the joint (i.e. user&#39;s body target  114  is a region comprising both the first body segment and the second body segment near the joint), the amount of separation  130  is also represented by an average capacitance detected by using sensing pad  108  on sensing plate  118 , where the body segment that is larger and closer in front of sensing pad  108  contributes more to the average capacitance of the two body segments in the region near the joint. 
     Any additional clothing material (not shown in  FIG. 1 ) that covers user&#39;s body target  114  or any additional protective material (not shown in  FIG. 1 ) that covers sensing pad  108  can change the capacitance between user&#39;s body target  114  and sensing pad  108 , even though separation  130  is unchanged. When the air gap between the surfaces of the covering materials of user&#39;s body target  114  and sensing pad  108  is large, the capacitance between user&#39;s body target  114  and sensing pad  108  largely results from the air gap. In this case, the capacitance resulting from the air gap is connected in series with the capacitances resulting from the covering materials (the covering materials are usually relatively thin), which are usually much larger than the capacitances resulting from the air gap, so that the capacitance resulting from the air gap dominates the total capacitance of the series combination of capacitances. 
     When the air gap between the surfaces of the covering materials approaches the total thickness of the covering materials, the capacitances resulting from the covering materials, which remain constant as the capacitance resulting from the air gap changes, contribute significantly to the capacitance between user&#39;s body target  114  and sensing pad  108 . In this case, the capacitance between user&#39;s body target  114  and sensing pad  108  approaches the constant series combination of the capacitances of the covering materials as the air gap decreases. This constant series combination of capacitances is in series with a capacitance  116  (a relatively constant capacitance after the user wears wearable joint-action sensor  100  on the body) of user&#39;s body target  114  to ground, and if possible, the total series capacitance can be determined by having the user perform a joint action that closes the air gap and then obtaining the corresponding capacitance data from capacitive proximity sensor  102 . To compute the capacitance resulting from the air gap, this capacitance data can be stored and used along with the capacitance data obtained from capacitive proximity sensor  102  when the air gap is not closed, because the capacitance data obtained when the air gap is not closed is a serial combination of the stored capacitance data (obtained when the air gap is closed) and the capacitance resulting from the air gap. 
     The capacitance obtained from capacitive proximity sensor  102  is typically a binary number (such as the count of a binary counter) that is proportional to the standard measurement of capacitance in Farads. Converting the capacitance obtained from capacitive proximity sensor  102  to the capacitance measurement in Farads is usually unnecessary. Using the capacitance obtained from capacitive proximity sensor  102 , the computed separation of the air gap (i.e. the size of the air gap), which is obtained from the computed capacitance resulting from the air gap, can also be a binary number that is related to the separation in a standard measurement unit (such as in millimeter). Separation  130  (i.e. the capacitance that represents separation  130 , as discussed below) is equal to the separation of the air gap plus the total thickness of the covering materials (not shown in  FIG. 1 ), which can either be determined though the method described above (for the protective material on sensing pad  108 ) or is relatively thin and therefore negligible (for the clothing on user&#39;s body target  114 ). After the user wears wearable joint-action sensor  100  on the body, capacitance  116  of user&#39;s body target  114  to ground is relative constant, so the capacitance obtained from capacitive proximity sensor  102  can be directly used for representing separation  130 , without computing the separation of the air gap, because the capacitance increases as separation  130  decreases, and vice versa, and each value of the capacitance corresponds to a specific value of separation  130  in this case. 
     Optional signal guards  110   a ,  110   b ,  110   c ,  310   a , and  310   b  (in  FIGS. 1 and 3B ) around sensing pad  108  are driven by a signal-guard output  106  of capacitive proximity sensor  102  with low output impedance to match the voltage of sensing pad  108 , so that there is almost no electrical field between sensing pad  108  and signal guards  110   a ,  110   b ,  110   c ,  310   a , and  310   b . By having signal guards  110   a ,  110   b ,  110   c ,  310   a , and  310   b  around sensing pad  108 , the base capacitance of sensing pad  108  is almost zero, so the capacitance detected by using sensing pad  108  of capacitive proximity sensor  102  is largely affected by capacitance  116  of user&#39;s body target  114  to ground and separation  130  between sensing pad  108  and user&#39;s body target  114 . When wearable joint-action sensor  100  is worn on the user&#39;s body (this part of the user&#39;s body is not shown in  FIG. 1 ) with sensing pad  108  detecting separation  130  between sensing pad  108  on sensing plate  118  and user&#39;s body target  114 , the capacitance detected by using sensing pad  108  on sensing plate  118  increases as separation  130  decreases, and vice versa. 
     A ground plane  112  on the back of sensing plate  118  shields sensing pad  108  and signal guards  110   a ,  110   b ,  110   c ,  310   a , and  310   b  from unwanted electric fields in the environment around sensing plate  118 . Ground plane  112 , sensing pad  108 , and signal guards  110   a ,  110   b ,  110   c ,  310   a , and  310   b  are made of any electrically conductive material, such as the copper layers of a multi-layer printed circuit board, and signal guards  110   a ,  310   b ,  110   b  and  310   a  join together to become the four sides of a single electrically conductive trace around sensing pad  108 , as illustrated in  FIG. 3B . If optional signal guards  110   a ,  110   b ,  110   c ,  310   a , and  310   b  are not used, the base capacitance of sensing pad  108  is mostly the capacitance between sensing pad  108  and ground plane  112 . When necessary, this base capacitance can be reduced to improve sensitivity of capacitive proximity sensor  102  to separation  130  by increasing the separation between sensing pad  108  and ground plane  112 , or by cross-hatching ground plane  112 . 
     The circuit ground  120  of capacitive proximity sensor  102  is connected to ground plane  112 , so that capacitive proximity sensor  102  senses capacitances of channel  104 , channel  124 , and channel  126  to ground plane  112 . Capacitive proximity sensing of channel  124  and channel  126  is adjusted to a very short range for sensing optional touch keys  134  and  136 , respectively, which is used for user interface. Although three capacitive proximity sensing channels  104 ,  124 , and  126  of capacitive proximity sensor  102  are illustrated in  FIG. 1 , more or fewer capacitive proximity sensing channels can be employed as needed for an embodiment. Similarly, although one signal-guard output  106  is shown in  FIG. 1 , capacitive proximity sensor  102  can have more signal-guard outputs or no signal-guard output at all. 
     Capacitive proximity sensor  102  uses a bus  122 , such as an inter-integrated circuit bus or a serial peripheral interface bus, to communicate with an embedded controller  132 , which usually comprises a microprocessor with memory for data storage. The capacitance sensed by using sensing pad  108  on sensing plate  118 , as well as the capacitive touch statuses of touch keys  134  and  136 , are sent to embedded controller  132  through bus  122 . On the other hand, embedded controller  132  uses bus  122  to configure capacitive proximity sensor  102  for appropriate proximity and touch-key sensing. Embedded controller  132  uses an output bus  142  to drive an optional display  140 , such as an LCD module, for user interface. Embedded controller  132  also uses a bus  128 , such as an inter-integrated circuit bus or a serial peripheral interface bus, to communicate with an optional communication device  138 . Communication device  138 , such as a USB (Universal Serial Bus), Bluetooth®, Wi-Fi®, or NFC (near-field communication) device, or any radio-frequency, optical, acoustic, or wired communication device, provides wired or wireless communication for embedded controller  132  to communicate with a computing device (not shown in  FIG. 1 ), such as a smartwatch, fitness wristband, smartphone, or computer. Embedded controller  132  can use the display of the computing device for displaying any data or result, so that display  140  is not needed. Communication device  138  can also connect wearable joint-action sensor  100  to the internet to make wearable joint-action sensor  100  a part of the Internet of Things (IoT). 
     Embedded controller  132  processes capacitance data obtained from capacitive proximity sensor  102 , which represents data of separation  130 , to determine joint actions, such as flexion and extension, and the result can be shown on display  140 , or the display of a computing device by using communication device  138 . Alternatively, by using communication device  138 , the capacitance data obtained from capacitive proximity sensor  102  can be downloaded to a computing device (not shown in  FIG. 1 ) or to any device that is connected to the internet (such as the cloud or a smartphone) for data processing and display, so that embedded controller  132  and display  140  are not needed in wearable joint-action sensor  100 . Similarly, the computing device or any device that is connected to the internet can send configuration data and instructions to capacitive proximity sensor  102  directly to control its operation, so that touch keys  134  and  136  are not needed. 
     Joint-Action Detection Operation—FIG.  2   
       FIG. 2  is a flow diagram illustrating the joint-action detection operation of wearable joint-action sensor  100  in  FIG. 1 . In  FIG. 2 , after wearable joint-action sensor  100  has been turned on, embedded controller  132  sends configuration data, such as gain and sensitivity, through bus  122  to set up capacitive proximity sensor  102  at step  200 , for proximity sensing using sensing pad  108  and for touch sensing using conventional touch keys  134  and  136 . 
     At step  202 , embedded controller  132  uses bus  122  to initiate capacitive proximity sensor  102  to calibrate sensing pad  108  for determining the base capacitance of sensing pad  108  when user&#39;s body target  114  is furthest away from sensing pad  108 , and if possible, for determining the total capacitance resulting from the covering materials in series with capacitance  116  when the air gap between the covering materials is closed. The calibration process can be performed by instructing the user to perform a predetermined joint action (such as a full flexion or extension of a joint), or during the manufacturing process or the operation of wearable joint-action sensor  100  (such as during the operation of the first embodiment, as discussed below), so that the user does not need to perform the calibration process at step  202 . Capacitive proximity sensor  102  uses the base capacitance of sensing pad  108  as the reference to obtain capacitance data when user&#39;s body target  114 , with its capacitance  116  to ground, approaches sensing pad  108  on sensing plate  118  with separation  130 . 
     At step  204 , embedded controller  132  uses bus  122  to initiate capacitive proximity sensor  102  to calibrate touch keys  134  and  136  at a predetermined time interval, which may be set up at step  200 . For touch keys  134  and  136 , the calibration process is performed periodically at the predetermined time interval for determining the base capacitances of touch keys  134  and  136 , so that a touch is detected when the capacitance of touch key  134  or  136  exceeds a threshold capacitance (which may also be set up at step  200 ) above the base capacitance of touch key  134  or  136 . After a user wears wearable joint-action sensor  100  and moves around, the periodic calibration reduces the environment&#39;s influence on changes in capacitances during the touch-detection process for touch keys  134  and  136 . On the other hand, a user is required to perform a predetermined joint action, such as a full flexion or extension of a joint, for calibrating sensing pad  108  on sensing plate  118 , and the calibration process is performed much less frequently (or not at all) after the user wears joint-action sensor  100 , because the capacitance sensed by using sensing pad  108  is mostly a series combination of the capacitance resulting from separation  130  (between sensing pad  108  on sensing plate  118  and user&#39;s body target  114 ) and capacitance  116  of user&#39;s body target to ground, which is a relatively constant capacitance after the user wears wearable joint-action sensor  100  on the body. 
     By using bus  122  at step  206 , embedded controller  132  receives capacitance data that represents data of separation  130  between sensing pad  108  on sensing plate  118  and user&#39;s body target  114  from capacitive proximity sensor  102 . At step  208 , a software or hardware joint-action detector in embedded controller  132  uses the capacitance data and the rate of change of the data (if necessary) to detect a joint action, such as flexion or extension, of the joint sensed by wearable joint-action sensor  100 . The joint-action data obtained from step  208 , as well as the raw capacitance data obtained from step  206 , are stored in the memory of embedded controller  132  at step  210 . By using bus  122  at step  212 , embedded controller  132  receives the touch statuses of touch keys  134  and  136  from capacitive proximity sensor  102  to find out at steps  214  and  216  if the user wants to control the operation of embedded controller  132 , such as to stop capacitance-data acquisition for joint-action detection, to display data, or to communicate with a computing device, such as a smartwatch, fitness wristband, smartphone, or computer. Under computer program control, embedded controller  132  can also stop capacitance-data acquisition for joint-action detection, display data, or communicate with a computing device, without requiring the user to use touch key  134  or  136 . 
     If capacitance-data acquisition is not stopped at step  214  under computer program control, embedded controller  132  waits for a predetermined time interval at step  220  before it returns to step  206  to receive more capacitance data from capacitive proximity sensor  102 . On the other hand, if capacitance-data acquisition is stopped at step  214  under computer program control (such as when a predetermined joint action or a specific pattern of the touch statuses of touch keys  134  and  136  has been detected), embedded controller  132  can end all the processes shown in  FIG. 2  at step  216 . It can also go to step  218  to display any data stored in the memory of embedded controller  132  by using display  140  or communicate with a computing device by using communication device  138 . If embedded controller  132  does not end all the processes shown in  FIG. 2  at step  216 , embedded controller  132  returns to step  206  to receive more capacitance data from capacitive proximity sensor  102  after displaying data or communicating with a computing device at step  218 . 
     Although a user of wearable joint-action sensor  100  is typically a human being, a user can be a four-limbed animal, such as a monkey, dog, cat, goose, or elephant, for which joint actions are being detected by wearing wearable joint-action sensor  100  using similar proximity sensing techniques as disclosed in the embodiments. Furthermore, the following description of the embodiments is not intended to limit the invention to these embodiments, but rather to enable any person skilled in the art to make and use the invention. 
     FIGS.  3 A and  3 B—First Embodiment 
     A first embodiment of the wearable joint-action sensor is illustrated in  FIGS. 3A and 3B .  FIG. 3A  is a side view of wearable joint-action sensor  100  inside a housing  301  worn on the right waist of the torso (the torso is the first body segment in this embodiment) of a user (a human being in this embodiment) with a waist belt  300  for detecting joint actions, such as flexion and extension, of the right hip joint of the user. Wearable joint-action sensor  100  inside housing  301  incorporates sensing plate  118  (details in  FIGS. 1 and 3B ), which is illustrated by a dotted outline. The right hip joint links the right abdomen of the torso to the right thigh of the right upper leg (the right upper leg is the second body segment in this embodiment), and sensing pad  108  on sensing plate  118  of wearable joint-action sensor  100  is in the proximity of the first body segment (the right abdomen of the torso above the right thigh) for detecting separation  130  (in  FIG. 1 ) between capacitive proximity sensor  102  (i.e. sensing plate  118  of capacitive proximity sensor  102 ) and the first body segment, which serves as user&#39;s body target  114  for sensing plate  118 . 
       FIG. 3B  is a front view (the surface that faces the right abdomen of the user) of wearable joint-action sensor  100  inside housing  301  in  FIG. 3A . Sensing pad  108  is at the center of sensing plate  118 , with optional signal guards  110   a ,  110   b ,  310   a , and  310   b  around sensing pad  108 . Signal guards  110   a ,  310   b ,  110   b  and  310   a  join together to become the four sides of a single electrically conductive trace around sensing pad  108 . Optional signal guard  110   c  (in  FIG. 1 ) is underneath sensing pad  108 , as well as underneath signal guards  110   a ,  110   b ,  310   a , and  310   b , so it is not shown in  FIG. 3B . In  FIG. 3B , although sensing pad  108  on sensing plate  118  shown in the front view of housing  301  is exposed with no covering material, sensing pad  108  on sensing plate  118  can be covered with a protective material or be placed inside housing  301  without being exposed. Furthermore, sensing pad  108  and sensing plate  118  can be of any shape, although they are shown to be rectangular in  FIG. 3B . 
     Wearable joint-action sensor  100  inside housing  301  can be attached to waist belt  300  by using a mounting clip, which is commonly used for wearing a mobile phone or pager on the waist of a user. Wearable joint-action sensor  100  inside housing  301  can be attached to waist belt  300  by any other mechanism, such as by sewing, or by using any type of magnetic, adhesive, or adhering device. Wearable joint-action sensor  100  inside housing  301  can also be attached directly, such as by adhesive or adhesive tape, to the right waist of a user without using waist belt  300 , or be clipped to a pants pocket that is close to the right waist. Furthermore, wearable joint-action sensor  100  can be incorporated inside the housing of a smartphone that is worn on the right waist of a user, so that housing  301  is not needed. As illustrated in  FIG. 3A , there is an appropriate separation between sensing plate  118  and user&#39;s body target  114  when the user is standing. We presently contemplate using a square copper pad (approximately 2.5 cm×2.5 cm) on a printed circuit board as sensing pad  108  (in  FIGS. 1 and 3B ) with ground plane  112  but without any signal guard on sensing plate  118 . In this case, separation  130  between sensing pad  108  on sensing plate  118  and user&#39;s body target  114  (i.e. between sensing plate  118  and user&#39;s body target  114 , because the thickness of sensing pad  108  is negligible) is less than 1.5 cm, although sensing pad  108  of any other size or shape, with or without signal guard or ground plane, and a different amount of separation  130  between sensing pad  108  on sensing plate  118  and user&#39;s body target  114  can also be used. User&#39;s body target  114  at the right abdomen of the torso is usually covered with clothing, but we have found that sensing plate  118  can still be used for sensing the amount of separation  130  and the rate of change between sensing plate  118  and user&#39;s body target  114 , because typical clothing materials do not alter the electric field between sensing pad  108  on sensing plate  118  and user&#39;s body target  114  severely. 
     Although only one sensing plate  118  is illustrated in  FIGS. 3A and 3B , multiple smaller sensing plates, which can be arranged in a one- or two-dimensional array, or in any spatial pattern, can be incorporated in wearable joint-action sensor  100  to obtain more separation data for the right abdomen of the torso as a user flexes or extends the right hip joint. Wearable joint-action sensor  100  with multiple sensing plates can be used for detecting distortion of the right abdomen of the torso of a user when the user abducts or adducts the right hip joint, or when the user performs a combination of actions (for example, flexion combined with abduction) of the right hip joint, such as when the user is dancing. Wearable joint-action sensor  100  with multiple sensing plates can also facilitate obtaining appropriate separation data for the right abdomen of the torso even when a user does not wear wearable joint-action sensor  100  exactly as directed, because in this case some of the sensing plates could still be located at an appropriate proximity of the right abdomen of the torso for detecting actions of the right hip joint of the user. 
     Joint-Action Detection Operation—FIGS.  4 A and  4 B 
       FIG. 4A  illustrates a decrease in the amount of separation  130  (in  FIG. 1 ) between sensing plate  118  of capacitive proximity sensor  102  (in  FIG. 1 ) and user&#39;s body target  114  (i.e. between capacitive proximity sensor  102  and user&#39;s body target  114 ) as the right abdomen of the torso bulges out when a user flexes the right hip joint. The bulging out of the right abdomen of the torso is initiated by contractions of the right psoas major and iliacus muscles, which flex the right hip joint and cause bulging out of the soft tissues of the right abdomen of the torso. The decrease in the amount of separation  130  results in an increase in capacitance of sensing pad  108  (in  FIG. 1 ) on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . In  FIG. 4A , although there is still a small amount of separation  130  between sensing plate  118  and user&#39;s body target  114  when the right hip joint of the user is flexed, sensing plate  118  can touch (or almost touch) user&#39;s body target  114  for detecting flexion of the right hip joint, such as when the user is sitting. 
     On the other hand,  FIG. 4B  illustrates an increase in the amount of separation  130  between sensing plate  118  and user&#39;s body target  114  as the right abdomen of the torso stretches out when the user extends the right hip joint. The stretching out of the right abdomen of the torso is caused by relaxation of the right psoas major and iliacus muscles and by pulling of the soft tissue of the right abdomen of the torso when the right hip joint extends. The increase in the amount of separation  130  results in a decrease in capacitance of sensing pad  108  on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . 
     When a user walks or runs, the right hip joint flexes as the user takes a step forward with the right leg, and the right hip joint extends as the user takes a step forward with the left leg (in  FIG. 4B ). Wearable joint-action sensor  100  inside housing  301  in the first embodiment can be used as a physical-activity monitor by using sensing pad  108  on sensing plate  118  to sense the periodic variation of the capacitance to ground, which represents the periodic variation of separation  130 , and measuring the rate of change of the capacitance to ground when the user walks or runs. The number of steps that the user takes can be detected by analyzing the waveform of the capacitance to ground with various techniques, such as by detecting the total number of peaks and valleys of the waveform, which corresponds to the total number of maximum flexions and maximum extensions of the right hip joint, respectively. Alternatively, multiplying the number of significant flexions of the right hip joint by a factor of two can produce the number of steps that the user takes, where significant flexion is any degree of flexion that is larger than the minimum degree of flexion of the right hip joint required for the user to take an ambulating step forward with the right leg. A significant flexion can be detected from the waveform of the capacitance to ground by using a capacitance threshold (i.e. a threshold for the degree of flexion), which can be determined based on the amplitude of the waveform, such as using a threshold at a predetermined percentage of the amplitude of the waveform above or below the midline of the waveform, depending on the specific design of wearable joint-action sensor  100 . Unlike a pedometer that uses an accelerometer to detect steps, wearable joint-action sensor  100  does not mistakenly count steps when a user&#39;s body vibrates on a traveling vehicle, because the right hip joint does not flex and extend alternately in this case. 
     One advantage of using wearable joint-action sensor  100  as a physical-activity monitor is that it can detect whether a user is sitting or standing when the user is not walking or running. As illustrated in the block diagram (in  FIG. 17 ) for the seventh embodiment, wearable joint-action sensor  100  in the first embodiment can also be used as a sitting detector for reminding a user (such as by vibration, sound, voice, light, or a message on display  140  or on the display of a computing device) that the user has been sitting for too long. The timer for reminding the user can be implemented by a hardware or software timer in embedded controller  132  or in reminder  1700  (in  FIG. 17 ). Furthermore, reminder  1700  can be incorporated inside housing  301  of wearable joint-action sensor  100 . 
     Wearable joint-action sensor  100  can detect sitting and standing accurately after capacitive proximity sensor  102  is calibrated for the degree of flexion of the right hip joint of a user. With less elaborate calibration, wearable joint-action sensor  100  can still detect sitting and standing quite accurately by using a timer to determine the duration of significant flexion or no significant flexion of a user&#39;s right hip joint, respectively. This is based on the fact that the right hip joint is in significant flexion for a long duration when the user is sitting, and the right hip joint is not in significant flexion for a long duration when the user is standing. When a user is doing squats for exercise instead of either sitting or standing, the right hip joint is alternating between significant flexion and no significant flexion, which can be detected by wearable joint-action sensor  100 , so long as the duration of significant flexion or no significant flexion (or both) is longer than a predetermined duration for walking or running, indicating that the user is not walking or running. 
     The duration of significant flexion or no significant flexion, or the duration of any detected action of a joint, especially of a joint that is directly involved in ambulation, can be determined by a hardware or software timer implemented in embedded controller  132  (at least one hardware timer is usually incorporated in a microprocessor, and a software timer can be implemented by counting the number of times a computer program loop with a predetermined execution time has been executed). Sitting or standing (i.e. the user is not ambulating) can be detected by wearable joint-action sensor  100  when the duration of any detected action of a joint exceeds a predetermined duration. This can also distinguish between sitting and standing if the joint is a hip joint, as discussed above. If wearable joint-action sensor  100  is only used for detecting sitting, capacitive proximity sensor  102  can simply be a capacitive touch sensor, which is commonly incorporated in many microprocessors for sensing a capacitive touch key, and sensing pad  108  on sensing plate  118  of capacitive proximity sensor  102  can be designed to touch or be in close proximity to the right abdomen (likely though clothing) of a user when the user is sitting. 
     For example, instead of sensing actions of a hip joint for detecting sitting or standing, sitting or standing can be detected with less accuracy by using a hardware or software timer implemented in embedded controller  132  to determine the duration of the degree of flexion of a knee joint, which tends to be flexed significantly (i.e. the degree of flexion exceeds a predetermined threshold) when sitting, and not significantly flexed (i.e. the degree of flexion below the predetermined threshold) when standing. By using a similar detection principle, sitting or standing can be detected with even less accuracy by sensing actions of an elbow joint (instead of a hip or knee joint), which tends to be flexed (in  FIG. 13 ) for a long duration when sitting, and extended (in  FIG. 12 ) for a long duration when standing (detecting flexion and extension of the right elbow joint is discussed in the fifth embodiment). Furthermore, when a user is either sitting or standing instead of ambulating, the actions (also including the degree of the actions) of the user&#39;s joints, especially of the joints that are involved in ambulation directly, tend to decrease significantly. When the duration of a decrease in action of a joint (i.e. a decrease in the movement of a joint) determined by a timer exceeds a predetermined duration, sitting or standing (i.e. the user is not ambulating) is detected, although a decrease in action of most of the user&#39;s joints cannot be used for distinguishing between sitting and standing, except for some of the joints discussed above. 
     Wearable joint-action sensor  100  can use its communication device  138  and a signal link  1702  to communicate with reminder  1700  (in  FIG. 17 ), which can be a voice recorder, vibrator, beeper, display, or simply a light-emitting diode (LED), for reminding a user that a predetermined action of the right hip joint, such as sitting or standing, is or has been detected. Reminder  1700  can also be far away from the user for reminding someone else, such as a caregiver of the user. Furthermore, wearable joint-action sensor  100  in the first embodiment can be used for detecting how much time a user spends in each of sitting, standing, and ambulating (i.e. either walking or running, with or without counting steps). The time distribution among these three types of physical activity that the user performs could be more relevant to the user for maintaining a good health than the number of steps counted with a prior-art pedometer. 
     Although  FIGS. 4A and 4B  illustrate that wearable joint-action sensor  100  is worn on the right waist of the torso of a user using waist belt  300  for detecting actions of the right hip joint, wearable joint-action sensor  100  can be worn on the left waist of the torso of a user with waist belt  300  for detecting actions of the left hip joint by using the same detection principle. Furthermore, wearable joint-action sensor  100  can be worn on the lower back of the torso above the buttock (i.e. the gluteal region of the upper leg) of the left or right upper leg using waist belt  300  to detect actions of the left or right hip joint, respectively, by using the same detection principle, because the buttock of the upper leg bulges out as the hip joint of the upper leg extends, and the buttock of the upper leg stretches out as the hip joint flexes. In this case, the change in separation  130  between sensing plate  118  and the top of the buttock of the upper leg, which serves as user&#39;s body target  114 , is used for detecting actions of the hip joint. Wearable joint-action sensor  100  in the first embodiment can be used for detecting any type of physical activity that involves a hip joint, without the inconvenience of attaching a sensor to the thigh of the upper leg of a user, as necessary in the prior arts for sensing actions of a hip joint. 
     FIG.  5 —Second Embodiment 
     A second embodiment of the wearable joint-action sensor is illustrated in  FIG. 5 .  FIG. 5  is a side view of wearable joint-action sensor  100  inside a housing  501  worn on the right shoulder of the torso (the torso is the first body segment in this embodiment) of a user (a human being in this embodiment), on a shoulder pad  500  with a shoulder strap  502 , for detecting actions, such as abduction and adduction, of the right shoulder joint of the user. Wearable joint-action sensor  100  inside housing  501  incorporates sensing plate  118 , which is illustrated by a dotted outline. Sensing plate  118  of wearable joint-action sensor  100  is in the proximity of the user&#39;s right shoulder of the torso and the top of the right upper arm (the right upper arm is the second body segment in this embodiment), and both the right shoulder of the torso and the top of the right upper arm (i.e. the region comprising both the first and second body segments) serve as user&#39;s body target  114  for sensing plate  118 . The right shoulder joint is adducted in  FIG. 5 , with soft tissues of the right shoulder and the top of the right upper arm stretched out. 
     Wearable joint-action sensor  100  inside housing  501  can be attached to shoulder pad  500  by using a mounting clip, or it can be attached to shoulder pad  500  by any other mechanism, such as by sewing, or by using any type of magnetic, adhesive, or adhering device. Furthermore, wearable joint-action sensor  100  can also be attached directly, such as by adhesive or adhesive tape, to the right shoulder of a user without using shoulder pad  500  and shoulder strap  502 . As illustrated in  FIG. 5 , there is an appropriate separation between sensing plate  118  and user&#39;s body target  114  when the user&#39;s right shoulder joint is adducted. 
     Although only one sensing plate  118  is illustrated in  FIG. 5 , multiple sensing plates can be arranged around shoulder pad  500  to detect other actions of the right shoulder joint, such as flexion and extension. Wearable joint-action sensor  100  with multiple sensing plates can also facilitate obtaining appropriate separation data for the region comprising both the right shoulder and the top of the right upper arm even when a user does not wear wearable joint-action sensor  100  exactly as directed, because in this case some of the sensing plates could still be located at an appropriate proximity of the right shoulder and the top of the right upper arm for detecting actions of the right shoulder joint of the user. 
     Joint-Action Detection Operation—FIG.  6   
       FIG. 6  illustrates a decrease in the amount of separation  130  (in  FIG. 1 ) between sensing plate  118  of capacitive proximity sensor  102  (in  FIG. 1 ) and user&#39;s body target  114  (i.e. between capacitive proximity sensor  102  and user&#39;s body target  114 ) as both the right shoulder of the torso and the top of the right upper arm become closer to sensing plate  118  when the user abducts the right shoulder joint, with soft tissues of the right shoulder and the top of the right upper arm bulging out. The decrease in the amount of separation  130  results in an increase in capacitance of sensing pad  108  (in  FIG. 1 ) on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . In  FIG. 6 , although there is a small amount of separation  130  between sensing plate  118  and user&#39;s body target  114  when the right shoulder joint of the user is abducted, sensing plate  118  can touch (or almost touch) user&#39;s body target  114  for detecting abduction of the right shoulder joint. 
     When the user performs a physical activity that involves the right shoulder joint, such as swinging, lifting, or jumping rope, one or more sensing plates can be attached to shoulder pad  500  to detect the activity, so long as capacitive proximity sensor  102  of wearable joint-action sensor  100  can detect the amount of separation  130  and the rate of change between sensing plate  118  and user&#39;s body target  114  on the right shoulder and the top of the right upper arm (i.e. the region comprising both the right shoulder and the top of the right upper arm) as the right shoulder joint performs the action. 
     Although  FIG. 6  illustrates that wearable joint-action sensor  100  inside housing  501  is worn on the right shoulder of the user with shoulder pad  500  and shoulder strap  502  for detecting actions of the right shoulder joint, wearable joint-action sensor  100  can be worn on the left shoulder of the user for detecting actions of the left shoulder joint by using the same detection principle. 
     FIG.  7 —Third Embodiment 
     A third embodiment of the wearable joint-action sensor is illustrated in  FIG. 7 .  FIG. 7  is a side view of wearable joint-action sensor  100  inside a housing  701  worn on the wrist region of the right forearm (the right forearm is the first body segment in this embodiment) of a user (a human being in this embodiment) with a wristband  700  for detecting joint actions, such as flexion and extension, of the right wrist joint of the user. Wearable joint-action sensor  100  inside housing  701  incorporates sensing plate  118 , which is illustrated by a dotted outline. Sensing plate  118  of wearable joint-action sensor  100  is in the proximity of the wrist region of the right hand (the right hand is the second body segment in this embodiment) and the wrist region of the right forearm. The right wrist joint links the wrist region of the right forearm to the wrist region of the right hand, and both the wrist region of the right forearm and the wrist region of the right hand (i.e. the region comprising both the first and second body segments) serve as user&#39;s body target  114  for sensing plate  118 . 
     Wearable joint-action sensor  100  inside housing  701  can be attached to wristband  700  by using any mechanism for attaching a wristwatch to a wristband, or by using any type of magnetic, adhesive, or adhering device. Wearable joint-action sensor  100  can also be attached directly, such as by adhesive or adhesive tape, to the wrist region of the right forearm of a user without using wristband  700 . Furthermore, wearable joint-action sensor  100  can be incorporated inside the housing of a smartwatch, which is worn on the wrist region of the right forearm of a user, so that housing  701  is not needed. As illustrated in  FIG. 7 , there is an appropriate separation  130  (in  FIG. 1 ) between sensing plate  118  and user&#39;s body target  114  when the right wrist joint is not flexed or extended. 
     Although only one sensing plate  118  is illustrated in  FIG. 7 , multiple sensing plates can be incorporated in wearable joint-action sensor  100  to obtain more separation data for the region comprising both the wrist region of the right hand and the wrist region of the right forearm as the user flexes or extends the right wrist joint. Wearable joint-action sensor  100  with multiple sensing plates can facilitate obtaining appropriate separation data for the region comprising both the wrist region of the right hand and the wrist region of the right forearm even when a user does not wear wearable joint-action sensor  100  exactly as directed, because in this case some of the sensing plates could still be located at an appropriate location in the proximity of the wrist region of the right hand and the wrist region of the right forearm for detecting actions of the right wrist joint of the user. 
     Joint-Action Detection Operation—FIGS.  8 A and  8 B 
       FIG. 8A  illustrates a decrease in the amount of separation  130  (in  FIG. 1 ) between sensing plate  118  of capacitive proximity sensor  102  (in  FIG. 1 ) and user&#39;s body target  114  (i.e. between capacitive proximity sensor  102  and user&#39;s body target  114 ) as the wrist region of the right forearm and the wrist region of the right hand become closer to sensing plate  118 , with soft tissues at the top of the wrist regions bulging out, when the user extends the right wrist joint. The decrease in the amount of separation  130  results in an increase in capacitance of sensing pad  108  (in  FIG. 1 ) on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . In  FIG. 8A , although there is a small amount of separation  130  between sensing plate  118  and user&#39;s body target  114  when the right wrist joint of the user is extended, sensing plate  118  can touch (or almost touch) user&#39;s body target  114  to detect extension of the right wrist joint. 
     On the other hand,  FIG. 8B  illustrates an increase in the amount of separation  130  between sensing plate  118  and user&#39;s body target  114  as the wrist region of the right forearm and the wrist region of the right hand become farther away from sensing plate  118 , with soft tissues at the wrist regions stretching out, when the user flexes the right wrist joint. The increase in the amount of separation  130  causes a decrease in capacitance of sensing pad  108  on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . 
     When a user performs a physical activity that involves the right wrist joint, such as waving, pushing, slapping, weightlifting, one or more sensing plates can be incorporated in wearable joint-action sensor  100  to detect the activity, so long as capacitive proximity sensor  102  of wearable joint-action sensor  100  can detect the amount of separation  130  and the rate of change between sensing plate  118  and user&#39;s body target  114  as the right wrist joint performs the action. 
     Although  FIGS. 7, 8A, and 8B  illustrate that wearable joint-action sensor  100  inside housing  701  is worn on the wrist region of the right forearm of a user for detecting actions of the right wrist joint, wearable joint-action sensor  100  can be worn on the wrist region of the left forearm of the user for detecting actions of the left wrist joint by using the same detection principle. The same detection principle can also be used for sensing plate  118  of wearable joint-action sensor  100  located in the proximity of the user&#39;s palm of the right hand (the second body segment) and the wrist region of the right forearm (the first body segment, where a user wears wearable joint-action sensor  100 ), with both the first and second body segments (i.e. the region comprising both the first and second body segments) serving as user&#39;s body target  114  for sensing plate  118 . Furthermore, the same detection principle can be used for detecting abduction and adduction of a wrist joint when sensing plate  118  of wearable joint-action sensor  100  is placed in the proximity of one side (or one sensing plate  118  on each side) of the wrist region of the hand and the wrist region of the forearm of a user. 
     Many common daily physical activities involve complex actions of a wrist joint, and wearable joint-action sensor  100  in the third embodiment can be used for detecting these physical activities involving a wrist joint by incorporating multiple sensing plates around the wrist joint (i.e. around the wrist regions of the wrist joint) of a user. For example, wearable joint-action sensor  100  with multiple sensing plates around a wrist joint can be used for detecting handwashing, so that the number, duration, and time of handwashing performed by the user can be recorded for monitoring the user&#39;s compliance in maintaining excellent personal hygiene. This is important if the user is a child, and is particularly important if the user is a physician, nurse, or healthcare provider. Wearable joint-action sensor  100  with multiple sensing plates around a wrist joint can detect the actions of the wrist joint during handwashing with water, soap, or any hand sanitizer. If running water is used for handwashing, another sensor, such as a sound transducer, water-droplet detector, or humidity sensor, can be added to further confirm detection of handwashing by sensing the sound (audible or inaudible) when water impacts the hand (or the wash sink) or when water exits the faucet, by detecting water droplets in the proximity of or in contact with the water-droplet detector, or by sensing an increase in humidity around a steam or pool of water, respectively. 
     Control of Computing Devices with Joint-Action Detection— FIG. 9   
       FIG. 9  shows a block diagram of a computing device  900  using wearable joint-action sensor  100  for controlling the operation of computing device  900 . Computing device  900  can be a smartwatch, fitness wristband, smartphone, computer, or any device that has a CPU (central processing unit) or logic circuits (for performing logical operations), with an appropriate wired or wireless communication device, which can simply be an electrical cable, for communicating with wearable joint-action sensor  100 . As illustrated in  FIGS. 1, 2, and 9 , wearable joint-action sensor  100  can detect actions of a joint and use its wired or wireless communication device  138  (in  FIG. 1 ) and a signal link  902  for controlling computing device  900  by informing computing device  900  that a predetermined joint action is or has been detected. Signal link  902  can simply be a wired connection, such as electrical conductors or an electrical cable (for example, a Universal-Serial-Bus cable), or any wireless connection, such as a radio-frequency (for example, a Bluetooth®, Wi-Fi®, or near-field-communication connection), optical, or acoustic connection. Computing device  900  uses the received information of the joint action detected by wearable joint-action sensor  100  to determine its operation. On the other hand, computing device  900  can use its communication device (not shown in  FIG. 9 ) and signal link  902  to download a computer program to embedded controller  132  (in  FIG. 1 ), so that embedded controller  132  can execute the downloaded computer program for controlling the operation of wearable joint-action sensor  100 . 
     For example, if computing device  900  is a smartphone, it can download an appropriate computer program to wearable joint-action sensor  100 , so that wearable joint-action sensor  100  is programmed to inform computing device  900  when wearable joint-action sensor  100  detects a user waving the right hand by flexing and extending the right wrist joint alternately. Based on the received information of the joint action or actions detected by wearable joint-action sensor  100 , computing device  900  can perform a predetermined operation, such as turning on or off its display, playing back a voice recorded message, or receiving or terminating a phone call. 
     When wearable joint-action sensor  100  is used for controlling computing device  900 , wearable joint-action sensor  100  can be incorporated in the housing of computing device  900  and communicate with computing device  900  by a bus (for example, an inter-integrated circuit bus or a serial peripheral interface bus) if computing device  900  is also a wearable device, such as a smartwatch or fitness wristband. In this case, sensing plate  118  of wearable joint-action sensor  100  can be incorporated on the bottom of the housing of computing device  900  in the proximity of the wrist region of the hand (or the palm of the hand, if computing device  900  is worn at the bottom of the wrist) and the wrist region of the right forearm. Based on the received information of the joint action or actions detected by wearable joint-action sensor  100 , computing device  900  can perform a predetermined operation, such as turning on or off its display of time, reporting time audibly, or recording an audio message. It should be noted that sensing plate  118  can be incorporated on the side, instead of the bottom, of the housing of computing device  900  that faces the hand if wearable joint-action sensor  100  only detects extension of the wrist joint (or flexion of the wrist joint, if computing device  900  is worn at the bottom of the wrist). Alternatively, wearable joint-action sensor  100  can be physically separated from computing device  900  and use its wired or wireless communication device  138  and signal link  902  to communicate with computing device  900 . For example, wearable joint-action sensor  100  can use USB (Universal Serial Bus), Bluetooth®, Wi-Fi®, NFC (near-field communication), or any radio-frequency, optical, or acoustic communication to control computing device  900 , such as a smartphone or computer, which can be nearby or far away from wearable joint-action sensor  100 . 
     In  FIG. 9 , computing device  900  can be controlled by any of the embodiments, not just the third embodiment for the wrist joint. For example, when wearable joint-action sensor  100  is used for detecting abduction and adduction of the right shoulder joint, as discussed in the second embodiment, computing device  900  can increase the volume of recorded music being played back after receiving information from wearable joint-action sensor  100 , indicating that the user has raised the right arm by abducting the right shoulder joint. Furthermore, when wearable joint-action sensor  100  is used for detecting flexion and extension of a finger joint, as discussed in the sixth embodiment, wearable joint-action sensor  100 , each worn on a finger of one or both hands of a user (in  FIG. 16 ), can be used by the user to perform complex control of computing device  900 , such as an electronic piano (or any electronic music instrument), electronic game console, or computer, when it is located far away from the user, or when there is no physical keyboard or control panel for the electronic piano (or any electronic music instrument), electronic game console, or computer. 
     FIG.  10 —Fourth Embodiment 
     A fourth embodiment of the wearable joint-action sensor is illustrated in  FIG. 10 .  FIG. 10  is a side view of wearable joint-action sensor  100  inside a housing  1001  mounted on the instep of a right footwear  1000  that is worn on the right foot (the right foot is the first body segment in this embodiment) by a user (a human being in this embodiment) for detecting joint actions, such as flexion (plantar flexion) and extension (dorsiflexion), of the right ankle joint, which links the right foot to the ankle region of the right lower leg (the right lower leg is the second body segment in this embodiment) of the user. Wearable joint-action sensor  100  inside housing  1001  incorporates sensing plate  118 , which is illustrated by a dotted outline. Sensing plate  118  of wearable joint-action sensor  100  is in the proximity of the instep of the right foot, and the instep of the right foot (the first body segment) serves as user&#39;s body target  114  for sensing plate  118 . When compared with prior-art instep sensors that are constructed with strain transducers, piezoelectric elements, or switches, the fourth embodiment of the wearable joint-action sensor is more reliable and durable because there is no wear and tear of the mechanical parts after repeated use. 
     Wearable joint-action sensor  100  inside housing  1001  can be attached to the instep of right footwear  1000  by using a mounting clip. Wearable joint-action sensor  100  can also be attached to the instep of right footwear  1000  by any other mechanism, such as by sewing, or by using any type of magnetic, adhesive, or adhering device. Furthermore, wearable joint-action sensor  100  inside housing  1001  can be attached directly, such as by adhesive or adhesive tape, to the right foot of a user without using right footwear  1000 . As illustrated in  FIG. 10 , there is an appropriate separation  130  (in  FIG. 1 ) between sensing plate  118  and user&#39;s body target  114  when the right ankle joint is not flexed or extended. 
     Although only one sensing plate  118  is illustrated in  FIG. 10 , multiple sensing plates can be incorporated in wearable joint-action sensor  100  to obtain more separation data for the instep of the right foot as the user flexes or extends the right ankle joint. Wearable joint-action sensor  100  with multiple sensing plates can facilitate obtaining appropriate separation data for the instep of the right foot even when a user does not wear wearable joint-action sensor  100  exactly as directed, because in this case some of the sensing plates could still be located at an appropriate proximity of the instep of the right foot for detecting actions of the right ankle joint of the user. 
     Joint-Action Detection Operation—FIGS.  11 A and  11 B 
       FIG. 11A  illustrates a decrease in the amount of separation  130  (in  FIG. 1 ) between sensing plate  118  of capacitive proximity sensor  102  (in  FIG. 1 ) and user&#39;s body target  114  (i.e. between capacitive proximity sensor  102  and user&#39;s body target  114 ) as the instep of the right foot becomes closer to sensing plate  118 , with soft tissue at the instep of the right foot bulging out, when the user extends the right ankle joint. The decrease in the amount of separation  130  results in an increase in capacitance of sensing pad  108  on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . In  FIG. 11A , although there is a small amount of separation  130  between sensing plate  118  and user&#39;s body target  114  when the right ankle joint of the user is extended, sensing plate  118  can touch (or almost touch) user&#39;s body target  114  for detecting extension of the right ankle joint. 
     On the other hand,  FIG. 11B  illustrates an increase in the amount of separation  130  between sensing plate  118  and user&#39;s body target  114  as the instep of the right foot becomes farther away from sensing plate  118 , with soft tissue at the instep of the right foot stretching out when the user flexes the right ankle joint. The increase in the amount of separation  130  causes a decrease in capacitance of sensing pad  108  on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . 
     When a user performs a physical activity that involves the right ankle joint, such as kicking, tip toeing, walking, or running, one or more sensing plates can be incorporated in wearable joint-action sensor  100  to detect the activity, so long as capacitive proximity sensor  102  of wearable joint-action sensor  100  can detect the amount of separation  130  and the rate of change between sensing plate  118  and user&#39;s body target  114  at the instep of the right foot as the right ankle joint performs the action. 
     Although  FIGS. 10, 11A, and 11B  illustrate that wearable joint-action sensor  100  inside housing  1001  is worn on the instep of right footwear  1000  of the user for detecting actions of the right ankle joint, wearable joint-action sensor  100  can be worn on the instep of the left footwear of the user for detecting actions of the left ankle joint by using the same detection principle. The same detection principle can also be used for detecting internal rotation and external rotation of an ankle joint when sensing plate  118  of wearable joint-action sensor  100  is located on one side (or one sensing plate  118  on each side) of the footwear in the proximity of a user&#39;s foot and appropriately separated from the user&#39;s foot. 
     FIG.  12 —Fifth Embodiment 
     A fifth embodiment of the wearable joint-action sensor is illustrated in  FIG. 12 .  FIG. 12  is a side view of wearable joint-action sensor  100  inside a housing  1201  worn on the right upper arm (the right upper arm is the first body segment in this embodiment) of a user (a human being in this embodiment) with an armband  1200  for detecting joint actions, such as flexion and extension, of the right elbow joint of the user. Wearable joint-action sensor  100  inside housing  1201  incorporates sensing plate  118 , which is illustrated by a dotted outline. Armband  1200  is preferably made of an elastic material, although other types of materials can also be used, so long as armband  1200  can hold wearable joint-action sensor  100  in an appropriate location on the right upper arm when the muscles of the right upper arm change their shapes during flexion and extension of the right elbow joint, which links the right upper arm to the right forearm (the right forearm is the second body segment in this embodiment). Sensing plate  118  of wearable joint-action sensor  100  is in the proximity of the user&#39;s right upper arm (the first body segment) above the elbow pit (antecubital fossa), which serves as user&#39;s body target  114  for sensing plate  118 . 
     Wearable joint-action sensor  100  inside housing  1201  can be attached to armband  1200  by using a mounting clip, or it can be attached to armband  1200  by any other mechanism, such as by sewing, or by using any type of magnetic, adhesive, or adhering device. Furthermore, wearable joint-action sensor  100  inside housing  1201  can also be attached directly, such as by adhesive or adhesive tape, to the upper arm of a user without using armband  1200 . Although  FIG. 12  shows a small amount of separation  130  between sensing plate  118  and user&#39;s body target  114  when the user&#39;s right elbow joint is fully extended, sensing plate  118  can touch (or almost touch) user&#39;s body target  114  to detect full extension, or any degree of extension, of the right elbow joint. 
     Although only one sensing plate  118  is illustrated in  FIG. 12 , multiple sensing plates can be incorporated in wearable joint-action sensor  100  to obtain more separation data for the right upper arm above the elbow pit as the user flexes or extends the right elbow joint. Wearable joint-action sensor  100  with multiple sensing plates can also facilitate obtaining appropriate separation data for the right upper arm above the elbow pit even when a user does not wear wearable joint-action sensor  100  exactly as directed, because in this case some of the sensing plates could still be located at an appropriate proximity of the right upper arm above the elbow pit for detecting actions of the right elbow joint of the user. 
     Joint-Action Detection Operation—FIG.  13   
       FIG. 13  illustrates an increase in the amount of separation  130  (in  FIG. 1 ) between sensing plate  118  of capacitive proximity sensor  102  (in  FIG. 1 ) and user&#39;s body target  114  (i.e. between capacitive proximity sensor  102  and user&#39;s body target  114 ) as the biceps muscle of the right upper arm bulges out when the user flexes the right elbow joint. The increase in the amount of separation  130  results in a decrease in capacitance of sensing pad  108  (in  FIG. 1 ) on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . 
     When a user performs a physical activity that involves the right elbow joint, such as lifting, stretching, or punching, one or more sensing plates can be incorporated in wearable joint-action sensor  100  to detect the activity, so long as capacitive proximity sensor  102  of wearable joint-action sensor  100  can detect the amount of separation  130  and the rate of change between sensing plate  118  and user&#39;s body target  114  on the right upper arm above the elbow pit as the right elbow joint performs the action. 
     Although  FIGS. 12 and 13  illustrate that wearable joint-action sensor  100  inside housing  1201  is worn on the right upper arm of the user for detecting actions of the right elbow joint, wearable joint-action sensor  100  can be worn on the left upper arm of the user for detecting actions of the left elbow joint by using the same detection principle. Furthermore, the user&#39;s right forearm (the second body segment) instead of the right upper arm (the first body segment) can serve as user&#39;s body target  114  for detecting actions of the right elbow joint. In this case, wearable joint-action sensor  100  can be worn on the right upper arm just above the elbow pit with sensing pad  108  on sensing plate  118  in the proximity of the right forearm (instead of the right upper arm) to detect the amount of separation  130  between sensing plate  118  and the right forearm as the user flexes the right elbow joint. Alternatively, wearable joint-action sensor  100  can use an active infrared proximity sensor, which typically has a longer sensing range than a capacitive proximity sensor, for sensing the separation between the active infrared proximity sensor (which is worn on the right upper arm) and the right forearm. The operation of the active infrared proximity sensor for detecting the degree of flexion of the right elbow joint is similar to the operation of active infrared proximity sensor  2004  (in  FIGS. 20A, 21A, and 21B ) for detecting the degree of flexion of the right hip joint in the eighth embodiment. 
     FIG.  14 —Sixth Embodiment 
     A sixth embodiment of the wearable joint-action sensor is illustrated in  FIG. 14 .  FIG. 14  is a side view of wearable joint-action sensor  100  inside a housing  1401  worn on the first segment (the proximal phalanx, which is the first body segment in this embodiment) of the right index finger of a user (a human being in this embodiment), with a ring  1400  for detecting joint actions, such as flexion and extension, of the first finger joint (the proximal interphalangeal joint) of the right index finger of the user. Wearable joint-action sensor  100  inside housing  1401  incorporates sensing plate  118 , which is illustrated by a dotted outline. Sensing plate  118  of wearable joint-action sensor  100  is in the proximity of the first segment and the second segment (the middle phalanx, which is the second body segment in this embodiment) of the right index finger near the first finger joint, which links the first segment to the second segment of the right index finger. Both the first segment and the second segment of the right index finger (i.e. the region comprising both the first and second body segments) serve as user&#39;s body target  114  for sensing plate  118 . 
     Wearable joint-action sensor  100  inside housing  1401  can be attached to ring  1400  by using a mounting clip, or it can be attached to ring  1400  by any other mechanism, such as by sewing (if ring  1400  is made of a fabric material), or by using any type of magnetic, adhesive, or adhering device. Furthermore, wearable joint-action sensor  100  inside housing  1401  can also be attached directly, such as by adhesive or adhesive tape, to the first segment of the right index finger of a user without using ring  1400 . In  FIG. 14 , although there is a small amount of separation  130  between sensing plate  118  and user&#39;s body target  114  when the first finger joint of the user&#39;s right index finger is fully extended, sensing plate  118  can touch (or almost touch) user&#39;s body target  114  for detecting extension of the first finger joint. 
     Although only one sensing plate  118  is illustrated in  FIG. 14 , multiple sensing plates can be incorporated in wearable joint-action sensor  100  to obtain more separation data for the region comprising both the first and second segments of the right index finger near the first finger joint of the right index finger as the user flexes or extends the first finger joint. Wearable joint-action sensor  100  with multiple sensing plates can also facilitate obtaining appropriate separation data for the region comprising both the first and second segments of the right index finger near the first finger joint even when a user does not wear wearable joint-action sensor  100  exactly as directed, because in this case some of the sensing plates could still be located at an appropriate proximity of the region comprising both the first and second segments of the right index finger near the first finger joint for detecting actions of the first finger joint of the right index finger. 
     Joint-Action Detection Operation—FIG.  15   
       FIG. 15  illustrates an increase in the amount of separation  130  (in  FIG. 1 ) between sensing plate  118  of capacitive proximity sensor  102  (in  FIG. 1 ) and user&#39;s body target  114  (i.e. between capacitive proximity sensor  102  and user&#39;s body target  114 ) as the user flexes the first finger joint of the right index finger. During this movement, soft tissues at the top of the first and second segments of the right index finger near the first finger joint stretch out. The increase in the amount of separation  130  results in a decrease in capacitance of sensing pad  108  (in  FIG. 1 ) on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . 
     When the user performs a physical activity that involves the first finger joint of the right index finger, such as typing, flicking, or pointing, one or more sensing plates can be incorporated in wearable joint-action sensor  100  to detect the activity, so long as capacitive proximity sensor  102  of wearable joint-action sensor  100  can detect the amount of separation  130  and the rate of change between sensing plate  118  and user&#39;s body target  114  near the first finger joint of the right index finger as the first finger joint performs the action. 
     In  FIG. 14 , sensing plate  118  of wearable joint-action sensor  100  is in the proximity of the first and second segments of the right index finger near the first finger joint, and both the first and second segments of the right index finger (i.e. the region comprising both the first and second body segments) serve as user&#39;s body target  114  for sensing plate  118 . If sensing plate  118  is placed on top of the second segment of the right index finger (by using a different design of housing  1401 ) and is distant from the first segment of the right index finger, then only the second segment of the right index finger (i.e. only the second body segment) serves as user&#39;s body target  114 , because there is insignificant change in the separation between sensing plate  118  and the first segment of the right index finger (i.e. the first body segment) when the extended first finger joint is flexed. In this arrangement, the capacitance contribution of the first body segment for detecting the separation is insignificant, because the relatively constant capacitance to ground of the first body segment contributes to the total base capacitance instead. This illustrates the fact that when both the first and second body segments (i.e. when a region comprising both the first and second body segments) serve as user&#39;s body target  114  in any embodiment, one of the two body segments can dominate or even completely control the detection of separation  130  (i.e. the capacitance that represents separation  130 ) for detecting actions of the joint that links the first body segment to the second body segment. 
     Although  FIG. 15  illustrates that wearable joint-action sensor  100  inside housing  1401  is worn on the first segment of the right index finger of a user for detecting actions of the first finger joint of the right index finger, wearable joint-action sensor  100  can be worn on the first segment of the left index finger of a user for detecting actions of the first finger joint of the left index finger by using the same detection principle. In fact, wearable joint-action sensor  100  can be worn on the first segment (the proximal phalanx), the second segment (the middle phalanx), or the third segment (the distal phalanx) of any finger of a user, including the thumb (which has only two segments; the proximal and distal phalanxes), to detect any action of any finger joint, such as the joint at the base of a finger (the metacarpophalangeal joint), the first finger joint (the proximal interphalangeal joint), or the second finger joint (the distal interphalangeal joint). For example, wearable joint-action sensor  100  can be worn on the first segment of a thumb for detecting a thumbs-up action of the first finger joint of the thumb. 
     Furthermore, by using a similar detection principle, wearable joint-action sensor  100  can be worn on the thigh above a knee cap or on the lower leg below a knee cap with an athletic knee brace for detecting actions of the knee joint, which links the thigh of the upper leg to the lower leg, because the actions of a knee joint and those of the first finger joint of a finger are very similar. When wearable joint-action sensor  100  is worn on the thigh of the upper leg above the knee cap, the thigh of the upper leg is the first body segment and the lower leg is the second body segment, and the thigh of the upper leg (the first body segment) is user&#39;s body target  114  for sensing plate  118 . Separation  130  (in  FIG. 1 ) between sensing plate  118  and the thigh decreases when the knee joint is extended, because the soft tissue of the thigh above the knee cap (such as the quadriceps tendon) bulges out. Likewise, separation  130  between sensing plate  118  and the thigh increases when the knee joint is flexed, because the soft tissue of the thigh above the knee cap stretches out. On the other hand, if wearable joint-action sensor  100  is worn on the lower leg below the knee cap, the lower leg becomes the first body segment and the thigh of the upper leg becomes the second body segment, and the lower leg (the first body segment) becomes user&#39;s body target  114  for sensing plate  118 . Separation  130  between sensing plate  118  and the lower leg decreases when the knee joint is extended, because the soft tissue of the lower leg below the knee cap (such as the patellar ligament) bulges out. Likewise, separation  130  between sensing plate  118  and the lower leg increases when the knee joint is flexed, because the soft tissue of the lower leg below the knee cap stretches out. 
     The detection principle for detecting actions of the knee joint can also be used for detecting actions of an elbow joint (i.e. an alternative for the fifth embodiment for the right elbow joint) that links an upper arm to a forearm. In this case, wearable joint-action sensor  100  can be worn on the upper arm above the olecranon (the bony prominence of the elbow) or on the forearm below the olecranon with an athletic elbow brace, and the upper arm or the forearm serves as user&#39;s body target  114 , respectively. The soft tissues around the olecranon bulge out when the elbow joint is extended, and the soft tissues around the olecranon stretch out when the elbow joint is flexed, so the decrease and increase in separation  130  between sensing plate  118  and the soft tissues can be used for detecting extension and flexion of the elbow joint, respectively. 
     As discussed in the third embodiment and illustrated in  FIG. 9 , wearable joint-action sensor  100  of the sixth embodiment or of any embodiment can also be used for controlling a computing device, such as a smartwatch, fitness wristband, smartphone, or computer. In the prior arts, a capacitive touch sensor (i.e. a capacitive proximity sensor adjusted to a very short range) is used for controlling a computing device by using the tip of a finger to touch a capacitive sensing pad. Instead of using the prior-art approach for controlling a computing device, wearable joint-action sensor  100  is worn on the first body segment for detecting an action of a joint that links the first body segment to the second body segment with a proximity sensor. The proximity sensor detects a separation between the proximity sensor and the first body segment, the second body segment, or both the first and second body segments (i.e. the region comprising both the first and second body segments), for detecting the joint action, which is used for controlling a computing device. This approach for controlling a computing device does not require a user to use the tip of a finger to touch a capacitive sensing pad, as required by the prior-art approach. For example, as illustrated in  FIGS. 14 and 15 , flexion of the first finger joint of the right index finger, with wearable joint-action sensor  100  worn on the first segment of the right index finger, can be used for controlling a computing device, without requiring a user to touch sensing pad  108  on sensing plate  118  (in  FIG. 1 ) with the tip of any finger. 
     Using wearable joint-action sensor  100  to control a computing device is a compact and efficient control mechanism, because an action of just a single joint, with wearable joint-action sensor  100  worn on one of the two body segments linked by the joint, is used to control a computing device. For example, wearable joint-action sensor  100  in the sixth embodiment can be used for unlocking a secured computing device, such as a smartphone or a computer, or for sending a secured passcode to a computer program running on a computing device by using communication device  138  to transmit a joint-action signature of the first finger joint of the right index finger to the computing device. The joint-action signature is created by a sequence of actions (i.e. flexion and extension) of the first finger joint of the right index finger, and each action has a predetermined degree of action and duration that a user typically uses, resembling a signature by the user. The computing device can use a timer (a microprocessor usually incorporates at least one timer) and a software or hardware comparator to decode and verify the joint-action signature when the sequence of joint actions, which also includes the degree of each joint action, is received from wearable joint-action sensor  100 . Furthermore, as discussed below, when wearable joint-action sensor  100  is worn on each finger of both hands of a user, an action of a joint of each finger can be used for controlling a computing device, without requiring the user to touch sensing pad  108  on sensing plate  118  with the tip of any finger. 
     Complex Control of a Computing Device with Multiple Joint-Action Detections— FIG. 16   
       FIG. 16  illustrates wearable joint-action sensor  100  inside a housing (housings are not illustrated with reference numerals in  FIG. 16  for simplicity) worn in the proximity of the first finger joint of each finger of the right hand of a user for detecting flexion and extension of the first finger joint. A glove  1600  that encloses most of the first segment (the proximal phalanx) of each finger is used for attaching each wearable joint-action sensor  100  in the proximity of a first finger joint, with sensing plate  118  underneath wearable joint-action sensor  100  (the side view is similar to that shown in  FIG. 14 ). In  FIG. 16 , sensing plate  118  and body target  114  are not illustrated because they are not visible from the top of wearable joint-action sensor  100 .  FIG. 16  also illustrates wearable joint-action sensor  100  worn on the wrist region of the right hand of the user with glove  1600  for detecting flexion and extension of the right wrist joint. The operation of wearable joint-action sensor  100  attached to glove  1600  at the wrist region of the hand for detecting flexion and extension of the right wrist joint is similar to that of wearable joint-action sensor  100  in the third embodiment, except that wearable joint-action sensor  100  is attached to glove  1600  at the wrist region of the right hand (the right hand is the first body segment in this case) with sensing plate  118  in the proximity of the wrist regions of the right hand and the right forearm (the right forearm is the second body segment in this case) as shown in  FIG. 16 , instead of attached to the wrist region of the right forearm with wristband  700  as shown in  FIG. 7 . 
     Wearable joint-action sensor  100  can be attached to glove  1600  by using a mounting clip, or it can be attached to glove  1600  by any other mechanism, such as by sewing, or by using any type of magnetic, adhesive, or adhering device. Furthermore, wearable joint-action sensor  100  can also be attached directly, such as by adhesive or adhesive tape, to the right hand of a user without using glove  1600 . 
       FIG. 16  shows that wearable joint-action sensor  100  in the proximity of the first finger joint of each finger communicates with a sensing hub  1602  with an electrical cable  1604   a ,  1604   b ,  1604   c ,  1604   d , or  1604   e , and wearable joint-action sensor  100  in the proximity of the right wrist joint communicates with sensing hub  1602  with an electrical cable  1610 . Sensing hub  1602  can be an embedded controller that serves each wearable joint-action sensor  100 . Sensing hub  1602  uses an electrical connection  1608  to connect to a communication device  1606  for communicating with a computing device (not shown in  FIG. 16 ), such as an electronic piano (or any electronic music instrument), electronic game console, smartwatch, fitness wristband, smartphone, or computer. Sensing hub  1602  and communication device  1606  can be incorporated in the same housing instead of being separated as shown in  FIG. 16 . Communication device  1606 , such as a USB (Universal Serial Bus), Bluetooth®, Wi-Fi®, or NFC (near-field communication) device, an electrical cable, or any radio-frequency, optical, acoustic, or wired communication device, provides wired or wireless communication for sensing hub  1602  to communicate with the computing device. Wearable joint-action sensor  100  can also use communication device  138  (in  FIG. 1 ) to communicate with sensing hub  1602  wirelessly, instead of using electrical cable  1604   a ,  1604   b ,  1604   c ,  1604   d ,  1604   e , or  1610 . 
     Sensing hub  1602  can perform some or all of the function of embedded controller  132  (in  FIG. 1 ) of wearable joint-action sensor  100 , so that the hardware in wearable joint-action sensor  100  can be reduced. For example, capacitive proximity sensor  102  (in  FIG. 1 ) in wearable joint-action sensor  100  can communicate directly with sensing hub  1602 , which can be an embedded controller, so that embedded controller  132  is not needed in wearable joint-action sensor  100 . In addition, electrical power can be delivered to wearable joint-action sensor  100  from a battery (not shown in  FIG. 16 ) in sensing hub  1602  through electrical cable  1604   a ,  1604   b ,  1604   c ,  1604   d ,  1604   e , or  1610 , so that a battery is not needed in wearable joint-action sensor  100 . Hardware in wearable joint-action sensor  100  can be further reduced by moving its capacitive proximity sensor  102  to sensing hub  1602 , which can have capacitive proximity sensor  102  with multiple channels for proximity sensing, so that only sensing plate  118  is needed in wearable joint-action sensor  100 . 
     Although  FIG. 16  illustrates wearable joint-action sensor  100  worn on the right hand of a user with glove  1600 , wearable joint-action sensor  100  can be worn on the left hand of a user with a glove that is similar to (such as a mirror image of) glove  1600 . Furthermore, glove  1600  can be designed for attaching wearable joint-action sensor  100  near any joint of a finger, including the second finger joint (the distal interphalangeal joint) or the finger joint at the base of a finger (the metacarpophalangeal joint), to detect actions of the joint of the finger. 
     Complex control of a computing device, such as an electronic piano (or any electronic music instrument), electronic game console, or computer, can be performed by a combination of the degrees of flexions or extensions of the first finger joints of all the fingers of a user, and optionally the degree of flexion or extension of the wrist joint of the user. For example, if the left hand of a user also wears a glove that is similar to (such as a mirror image of) glove  1600 , the user can play an electronic piano (or any electronic music instrument) or electronic game console, provide typing input to a computer that is far away from the user, or provide input when there is no physical keyboard or control panel for the electronic piano (or any electronic music instrument), electronic game console, or computer. 
     For detecting actions of a joint of a toe, a sock that has all or part of the toe exposed can be used for attaching wearable joint-action sensor  100  in the proximity of the joint of the toe. For example, a sock with an entire big toe exposed can be used for attaching wearable joint-action sensor  100  in the proximity of the big-toe joint at the base of the big toe (the metatarsophalangeal joint) for detecting flexion and extension of the big-toe joint. In fact, wearable joint-action sensor  100  can be attached to a sock or glove (such as an elastic surgical glove) that does not expose a toe or finger, respectively, so long as the sock or glove is made of a flexible material or has extra space near a joint that can facilitate an increase in separation between sensing plate  118  and user&#39;s body target  114  near the joint, so that flexion and extension of the joint can be detected. An elastic surgical glove with wearable joint-action sensor  100  attached in the proximity of one or more finger joints can allow a surgeon who wears the surgical glove to control a surgical assistance robot during a surgery. 
     FIGS.  17  and  18 —Seventh Embodiment 
     A seventh embodiment of the wearable joint-action sensor is illustrated in  FIGS. 17 and 18 .  FIG. 17  shows wearable joint-action sensor  100  using its communication device  138  (in  FIG. 1 ) and a signal link  1702  to communicate with a reminder  1700 , which can be a voice recorder, vibrator, beeper, display, or simply a light-emitting diode (LED), for reminding or notifying a user or someone else (such as a caregiver of the user) that a predetermined joint action is or has been detected. Depending on the type of communication device  138 , signal link  1702  can be simply electrical conductors or an electrical cable, or any wired or wireless connection, such as USB (Universal Serial Bus), Bluetooth®, Wi-Fi®, or NFC (near-field communication) connection, or any radio-frequency, optical, or acoustic connection. 
       FIG. 18  is a side view of wearable joint-action sensor  100  inside a housing  1801  and reminder  1700  worn on a shirt  1800  on top of the upper spinal column of a user (a human being in this embodiment) for detecting flexion and extension of the upper spinal column of the user. Wearable joint-action sensor  100  inside housing  1801  incorporates sensing plate  118 , which is illustrated by a dotted outline. Although  FIG. 18  shows that reminder  1700  (housing of reminder  1700  is not illustrated with a reference numeral in  FIGS. 18 and 19  for simplicity) is separated from housing  1801  of wearable joint-action sensor  100 , reminder  1700  can be incorporated in housing  1801  of wearable joint-action sensor  100 , and signal link  1702  (in  FIG. 17 ) can be a bus, such as an inter-integrated circuit bus or a serial peripheral interface bus, or simply electrical conductors. 
     A spinal column has many facet joints that link the vertebrae of the spinal column together and give them the flexibility to move against each other as the spinal column flexes (bends forward) or extends (bends backward). In this embodiment, each vertebra or section of the spinal column (i.e. a section of vertebrae), including the soft tissue around the section of the spinal column, is treated as a body segment, with the body segments linked by the facet joints. For this reason, flexion or extension of a section of the spinal column involves actions of many facet joints, not just an action of one facet joint. During flexion or extension of a section of the spinal column, the type of joint action (i.e. the type of facet-joint movement) of all the facet joints in the section of the spinal column is the same, although the degrees of the joint action (i.e. the amount of facet-joint movement) of the facet joints in the section of the spinal column can be different, with the degrees of the joint action of the facet joints increasing as the degree of flexion or extension of the section of the spinal column increases, and vice versa. 
     Wearable joint-action sensor  100  is worn by the user on top of a first section of the upper spinal column (the first section of the upper spinal column is the first body segment in this embodiment) and adjacent to a second section of the upper spinal column (the second section of the upper spinal column is the second body segment in this embodiment), and both the first and second sections of the upper spinal column (i.e. the region comprising both the first and second body segments) serve as user&#39;s body target  114  for sensing plate  118  of capacitive proximity sensor  102  (in  FIG. 1 ) in wearable joint-action sensor  100 . Although user&#39;s body target  114  is covered with shirt  1800 , sensing plate  118  can still be used for detecting the amount of separation  130  (in  FIG. 1 ) and the rate of change between sensing plate  118  and user&#39;s body target  114 , because typical clothing materials do not alter the electric field between sensing plate  118  and user&#39;s body target  114  severely. 
     Wearable joint-action sensor  100  inside housing  1801  and reminder  1700  can be attached to shirt  1800  by using a mounting clip or pin, or it can be attached to shirt  1800  by any other mechanism, such as by sewing, or by using any type of magnetic, adhesive, or adhering device. Furthermore, wearable joint-action sensor  100  can also be attached directly, such as by adhesive or adhesive tape, to the upper spinal column of a user without using shirt  1800 . In  FIG. 18 , although there is separation  130  between sensing plate  118  and user&#39;s body target  114  when the user&#39;s posture is balanced and upright, sensing plate  118  can touch (or almost touch) user&#39;s body target  114  to detect a balanced and upright posture. 
     Although only one sensing plate  118  is illustrated in  FIG. 18 , multiple sensing plates can be incorporated in wearable joint-action sensor  100  to obtain more separation data for the upper spinal column as the user flexes or extends the upper spinal column. Wearable joint-action sensor  100  with multiple sensing plates can also facilitate obtaining appropriate separation data for the upper spinal column even when a user does not wear wearable joint-action sensor  100  exactly as directed, because in this case some of the sensing plates could still be located at an appropriate proximity of the upper spinal column for detecting joint actions of the upper spinal column. 
     Joint-Action Detection Operation—FIG.  19   
       FIG. 19  illustrates an increase in the amount of separation  130  (in  FIG. 1 ) between sensing plate  118  of capacitive proximity sensor  102  (in  FIG. 1 ) and user&#39;s body target  114  (i.e. between capacitive proximity sensor  102  and user&#39;s body target  114 ) as the user flexes the upper spinal column. The increase in the amount of separation  130  results in a decrease in capacitance of sensing pad  108  (in  FIG. 1 ) on sensing plate  118  to ground, which is detected by capacitive proximity sensor  102 . When the amount of separation  130  exceeds a predetermined threshold, wearable joint-action sensor  100  uses its communication device  138  (in  FIG. 1 ) and signal link  1702  (in  FIG. 17 ) to communicate with reminder  1700 , which can speak, vibrate, beep, display, or generate any noticeable signal for reminding the user that a poor posture is or has been detected, so that the user can maintain a good upright posture of the upper spinal column. On the other hand, although not illustrated in a figure here, the amount of separation  130  between sensing plate  118  and user&#39;s body target  114  decreases as the user extends the upper spinal column, and such a decrease in separation  130  can also be detected by capacitive proximity sensor  102  for reminding the user (by using signal link  1702  and reminder  1700 ), so that the user can maintain an appropriate amount of extension of the upper spinal column, such as when the user is doing certain exercise. Reminder  1700  can produce different levels of intensity of vibration, sound, voice, light, or produce different verbal or visual messages (for encouragement or discouragement to the user), corresponding to the degree of flexion or extension of the upper spinal column of the user. 
     When the user&#39;s spinal column is involved in a physical activity, such as sitting, standing, walking, running, or dancing, one or more sensing plates can be incorporated in wearable joint-action sensor  100  worn on top of the spinal column to detect actions of the spinal column, and to remind the user with reminder  1700  that a desired or undesired action of the spinal column is or has been detected, so long as capacitive proximity sensor  102  of wearable joint-action sensor  100  can detect the amount of separation  130  and the rate of change between sensing plate  118  and user&#39;s body target  114  on the sections of the spinal column that are involved in the physical activity. 
     Although  FIG. 19  illustrates that wearable joint-action sensor  100  inside housing  1801  and reminder  1700  are worn on the upper spinal column of a user to remind the user that a poor posture involving the upper spinal column is or has been detected, wearable joint-action sensor  100  inside housing  1801  and reminder  1700  can be worn anywhere along the spinal column to remind the user that a desired or undesired action of certain sections of the spinal column is detected by using the same detection principle, so long as all the facet joints in the sections of the spinal column have the same joint action during flexion or extension of the sections of spinal column. Furthermore, reminder  1700  can be located far away from wearable joint-action sensor  100  to remind someone else (such as a caregiver of the user) that a poor posture is or has been detected. Reminder  1700  can also be implemented in a computing device, such as a smartwatch, fitness wristband, smartphone, or computer, and use the display, speaker, light-emitting diode (LED), or vibrator of the computing device to remind a user or a caregiver of the user that a desired or undesired action of certain sections of the spinal column is or has been detected. 
     Reminder  1700  can be used in any embodiment to remind a user, or someone else, such as a caregiver of the user, that a predetermined joint action is or has been detected. For example, reminder  1700 , which can include a timer and be incorporated in housing  301  (in  FIGS. 3A and 3B ) of wearable joint-action sensor  100  in the first embodiment, can remind a user that the user has been sitting for too long. Furthermore, in any embodiment, reminder  1700  can include a timer and be incorporated in wearable joint-action sensor  100  to remind a user or someone else, such as a caregiver of the user (for example, the mother of a child), that a predetermined joint action is detected or not detected for a predetermined period of time. 
     FIGS.  20 A,  20 B, and  22 —Eighth Embodiment 
     An eighth embodiment of the wearable joint-action sensor is illustrated in  FIGS. 20A, 20B, and 22 .  FIG. 20A  is a side view of a wearable joint-action sensor  2000  inside a housing  2001  worn on the right waist of the torso (the torso is the first body segment in this embodiment) of a user (a human being in this embodiment) above the right thigh of the right upper leg (the right upper leg is the second body segment in this embodiment) with a waist belt  2008  to detect joint actions, such as flexion, extension, abduction, and adduction, of the right hip joint, which links the right thigh to the torso. Wearable joint-action sensor  2000  inside housing  2001  incorporates an active infrared proximity sensor  2004 , which is illustrated by a dotted outline. An overlay  2006 , which is usually made of glass or plastic, is in front of active infrared proximity sensor  2004  and allows infrared light to pass through. In this embodiment, a user&#39;s body target  2100  for active infrared proximity sensor  2004  is the right thigh of the right upper leg (the second body segment), which is illustrated in  FIGS. 21A and 21B . 
     An active infrared proximity sensor comprises an infrared proximity sensor and an infrared-light source (the active element for infrared proximity sensing), such as an infrared light-emitting diode (LED). The reflected infrared-light intensity from a target in the proximity of the active infrared proximity sensor is sensed by the infrared proximity sensor to detect the separation between the active infrared proximity sensor and the target. A time-of-flight proximity sensor is also an active infrared proximity sensor (it is an advanced active infrared proximity sensor), but it uses an infrared laser diode instead of an infrared light-emitting diode (LED) as the infrared-light source. It measures the time light takes to travel from the time-of-flight proximity sensor to the target and reflect back to the time-of-flight proximity sensor to detect the separation, which is the range measured by the time-of-flight proximity sensor. The range obtained from a time-of-flight proximity sensor can be in a standard measurement unit, such as millimeter, or it can be in any unit (such as picosecond of time of flight or the count of a time counter) that is proportional to a standard measurement unit. The advantage of a time-of-flight proximity sensor is that the detected separation (i.e. the range based on the time of flight of light) does not depend on the infrared-light reflectance of the target as the detected separation of active infrared proximity sensor  2004  does. For this reason, a time-of-flight proximity sensor can replace active infrared proximity sensor  2004  in wearable joint-action sensor  2000  for achieving an improved performance, in cases where the likely higher cost and power consumption of a time-of-flight proximity sensor are acceptable. 
     In  FIG. 20A , the user is standing, and the right thigh is not in the vertically downward line of sight of active infrared proximity sensor  2004 , so no user&#39;s body target for active infrared proximity sensor  2004  is illustrated. In this case, the target of active infrared proximity sensor  2004  is the floor or ground (not shown in  FIG. 20A ) on which the user is standing. Although the floor or ground is not a user&#39;s body target per se, because it is not a part of the user&#39;s body, it can still be a useful target for detecting actions of the right hip joint when it is in the proximity of the right hip joint. For example, wearable joint-action sensor  2000  can detect abduction of the right hip joint as the user opens out the right leg to the right side, with both the right and left foot remaining on the floor or ground, bringing active infrared proximity sensor  2004  on the right waist closer to the floor or ground. When the floor or ground falls into the sensing range of active infrared proximity sensor  2004 , the detected separation between active infrared proximity sensor  2004  and the floor or ground decreases as the degree of abduction increases. 
     Wearable joint-action sensor  2000  inside housing  2001  also incorporates a triaxial (three-axis) accelerometer  2002 , which is illustrated by a dotted outline, to sense acceleration in three orthogonal axes X, Y, and Z for detecting the user&#39;s body actions. In this case, the torso is the body segment sensed by triaxial accelerometer  2002 , and a body action comprises movement and orientation of the body segment. To simplify discussion of the orientation of triaxial accelerometer  2002  with respect to the vertically downward direction of the earth&#39;s gravitational acceleration G, the directions of the X, Y, and Z axes are chosen such that the static earth&#39;s gravitational acceleration sensed along each axis is set to G when the axis points vertically downward, even if the acceleration data obtained from triaxial accelerometer  2002  for the axis is −G in this orientation. Only the Y and Z axes of triaxial accelerometer  2002  are illustrated in  FIG. 20A , because the X axis points out of the figure. 
     The eighth embodiment combines the detected joint actions and body actions of a user to detect the types of physical activity performed by the user. Although triaxial accelerometer  2002  is used in the eighth embodiment for detecting body actions of the user, a one-axis (also called single-axis or uniaxial) or a two-axis (also called dual-axis or biaxial) accelerometer, or a combination of the same or different types of accelerometers, can also be used instead, depending on the types of body actions of a body segment needed to be detected for determining the types of physical activity performed by a user. Furthermore, if necessary, a gyroscope can be used to detect rotation of the body segment, which is a type of movement of the body segment. 
       FIG. 20B  is a front view (the surface that faces the right abdomen of the user) of wearable joint-action sensor  2000  inside housing  2001  in  FIG. 20A . Active infrared proximity sensor  2004  (illustrated by a dotted outline) and overlay  2006  are placed along the lower portion of housing  2001  to facilitate detection of abduction (opening out to the side of the user&#39;s body) and adduction (opposite to abduction) of the right hip joint, so long as the right hip joint is at least slightly flexed such that the right thigh is in the line of sight of active infrared proximity sensor  2004 . Otherwise the floor or ground could become the target as discussed above.  FIG. 20B  also shows triaxial accelerometer  2002  (also illustrated by a dotted outline) inside housing  2001 , and only the X and Z axes of triaxial accelerometer  2002  are illustrated, because the Y axis points into the figure. 
     Wearable joint-action sensor  2000  inside housing  2001  can be attached to waist belt  2008  by using a mounting clip, which is commonly used for wearing a mobile phone or pager on the waist of a user. Wearable joint-action sensor  2000  inside housing  2001  can be attached to waist belt  2008  by any other mechanism, such as by sewing, or by using any type of magnetic, adhesive, or adhering device. Wearable joint-action sensor  2000  inside housing  2001  can also be attached directly, such as by adhesive or adhesive tape, to the waist of a user without using waist belt  2008 , or be clipped to a pants pocket that is close to the waist. Furthermore, wearable joint-action sensor  2000  can be incorporated inside the housing of a smartphone, which is worn on the waist of a user, so that housing  2001  is not needed. As illustrated in  FIG. 20A , the right thigh is not in the vertically downward line of sight of active infrared proximity sensor  2004  when the user is standing. 
     We presently contemplate using an infrared proximity sensor that is available from Maxim Integrated Products Inc. of California (Product Number: MAX44000) as the infrared proximity sensor in active infrared proximity sensor  2004 , although an infrared proximity sensor manufactured by another company, or another type of proximity sensor, such as an ultrasonic proximity sensor or a single-chip radar, can also be used. One or more infrared LEDs in active infrared proximity sensor  2004  serve as the infrared-light sources for active infrared proximity sensing. As discussed above, a time-of-flight proximity sensor, which is an advanced active infrared proximity sensor, instead of active infrared proximity sensor  2004 , can also be used in the eighth embodiment. A time-of-flight proximity sensor module, which includes an infrared laser diode, is available from STMicroelectronics of Switzerland (Product Number: VL6180X). We presently contemplate using a triaxial accelerometer that is available from M-CUBE Inc. of California (Product Number: MC3610) as triaxial accelerometer  2002 , although an accelerometer manufactured by another company can also be used. 
       FIG. 22  shows a block diagram of wearable joint-action sensor  2000  incorporating active infrared proximity sensor  2004  and triaxial accelerometer  2002 .  FIG. 22  also shows a side view of active infrared proximity sensor  2004  illustrating its infrared proximity sensor  2200 , and infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d , which serve as the infrared-light sources. As illustrated in  FIG. 22 , when an infrared-light beam  2206  emitted from infrared LED  2202   d  is reflected by user&#39;s body target  2100 , which is the user&#39;s right thigh in this embodiment, a reflected infrared-light beam  2208  reaches infrared proximity sensor  2200 . Using a small rectangular user&#39;s body target  2100  to illustrate the user&#39;s right thigh and treating the surface of user&#39;s body target  2100  as a specular reflector instead of a diffuse reflector simplifies discussion of the operation of active infrared proximity sensor  2004 . Furthermore, user&#39;s body target  2100  can be covered with clothing that has certain reflection and transmission characteristics for infrared light. The effect of variations in infrared-light reflection characteristic of user&#39;s body target  2100  (with or without clothing) can be reduced by performing a calibration process after wearable joint-action sensor  2000  is worn on the waist of a user. When a user walks or runs, active infrared proximity sensor  2004  can use the waveform of the reflected infrared-light-intensity to determine a reflected infrared-light-intensity threshold for detecting significant flexion of a user&#39;s right hip joint, where significant flexion is any degree of flexion that is larger than the minimum degree of flexion required for the user to take an ambulating step forward with the right leg by flexing the right hip joint, as discussed below. 
     When an infrared-light beam  2210  emitted from infrared LED  2202   c  is reflected by the edge of user&#39;s body target  2100 , a less intense reflected infrared-light beam  2212  reaches infrared proximity sensor  2200 , because the reflecting surface is substantially reduced near the edge. On the left side of active infrared proximity senor  2004 , any infrared-light beam emitted from infrared LED  2202   a  or  2202   b  cannot produce a reflected infrared-light beam from user&#39;s body target  2100  that can reach infrared proximity sensor  2200 , unless user&#39;s body target  2100  moves to the left side of active infrared proximity sensor  2004 . 
     In  FIG. 22 , a separation  2201  between active infrared proximity sensor  2004  and user&#39;s body target  2100  is not to scale. Optional optical components, such as lens and reflectors, can be used for infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d , as well as for infrared proximity sensor  2200 , to change the directions and shapes of the emitted infrared-light beams from the infrared LEDs and the reflected infrared-light beams to infrared proximity sensor  2200 . The direction of the emitted infrared-light beam from an infrared LED can also be changed by tilting the infrared LED toward the desired direction of the infrared-light beam when mounting the infrared LED on active infrared proximity sensor  2004 . 
     Under the control of an embedded controller  2214  through buses  2216  and  2218 , active infrared proximity sensor  2004  sequentially emits an infrared-light beam from one of infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d  (the particular infrared LED is selected by embedded controller  2214  using bus  2218 ), and the resulting reflected infrared-light intensity for each infrared LED is sensed by infrared proximity sensor  2200 . Bus  2216  is a standard communication bus, such as an inter-integrated circuit bus or a serial peripheral interface bus, while bus  2218  is a custom bus whose purpose is to select one of the four infrared LEDs at a time to be driven by active infrared proximity sensor  2004 . The reflected infrared-light intensity for each of the four infrared LEDs is sent to embedded controller  2214  sequentially through bus  2216 , and the reflected infrared-light intensities for all four infrared LEDs are used together to detect the amount of separation  2201  (i.e. the height of an isosceles triangle formed by infrared-light beams  2210  and  2212 , and the distance between infrared proximity sensor  2200  and infrared LED  2202   c ) between active infrared proximity sensor  2004  and user&#39;s body target  2100 . This detection process also detects the deviation of user&#39;s body target  2100  from the midline of active infrared proximity sensor  2004  at infrared proximity sensor  2200 . For example, user&#39;s body target  2100  in  FIG. 22  is deviated to the right side of the midline (i.e. the right side of infrared proximity sensor  2200 ) of active infrared proximity sensor  2004 . Embedded controller  2214  also uses bus  2216  to configure active infrared proximity sensor  2004  for appropriate proximity sensing, such as to set the drive current for the infrared LEDs and the sampling rate for the reflected infrared-light intensities. The amount of separation  2201  between active infrared proximity sensor  2004  and user&#39;s body target  2100 , as well as the amount of deviation of user&#39;s body target  2100  from the midline of active infrared proximity sensor  2004 , is used for detecting actions of the right hip joint (the right hip joint is not shown in  FIG. 22 ), such as flexion, abduction, and adduction. 
     Infrared light can pass through overlay  2006  of active infrared proximity sensor  2004 , which can be transparent or be coated with a film that absorbs visible light. Active infrared proximity sensor  2004  drives each of infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d  sequentially with very short pulses to reduce power consumption and to facilitate cancellation of ambient infrared radiation. Infrared proximity sensor  2200  of active infrared proximity sensor  2004  typically has a mechanism for cancelling (such as by subtraction) the ambient infrared-light intensity, which is the baseline infrared-light intensity sensed by infrared proximity sensor  2200  before the reflected infrared-light beam of an infrared LED is sensed. This cancellation mechanism allows active infrared proximity sensor  2004  to operate under a large amount of ambient infrared radiation, such as under the sun. When there is a large 100 or 120-Hertz fluctuation of ambient infrared-light intensity, such as when incandescent light bulbs are powered by the mains electricity, the effect of this fluctuation of ambient infrared-light intensity can be reduced by obtaining data from active infrared proximity sensor  2004  at a 100 or 120-Hertz sampling rate, depending on the frequency of the mains electricity. Light barriers  2204   a  and  2204   b , one on each side of infrared proximity sensor  2200 , form a complete optical block. Light barriers  2204   a  and  2204   b  can be made of any material, such as natural rubber or copper, that does not allow infrared light to pass through, so that an infrared-light beam emitted from any infrared LED  2202   a ,  2202   b ,  2202   c , or  2202   d  is not able to reach infrared proximity sensor  2200  by reflection under overlay  2006 . The need for light barriers  2204   a  and  2204   b  can be reduced by placing infrared proximity sensor  2200  and infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d  as close to overlay  2006  as possible. 
     When wearable joint-action sensor  2000  inside housing  2001  is mounted on waist belt  2008  (in  FIGS. 20A, 21A, and 21B ), it is important to keep a minimum separation between active infrared proximity sensor  2004  and user&#39;s body target  2100 , so that at least one of infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d  can produce a strong reflected infrared-light beam that reaches infrared proximity sensor  2200  when user&#39;s body target  2100  is at the minimum separation. Such minimum separation can be guaranteed by various designs, such as by designing housing  2001  to be mounted high enough on waist belt  2008 , or by placing overlay  2006  (or active infrared proximity sensor  2004  without overlay  2006 ) in a recess at the bottom of housing  2001 . Another possible design is to place overlay  2006  away from light barriers  2204   a  and  2204   b , forming a partial optical block (i.e. a certain acceptable amount of infrared-light reflection under overlay  2006  can reach infrared proximity sensor  2200 ) instead of the complete optical block illustrated in  FIG. 22 , so that at least one of infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d  can produce a strong reflected infrared light beam (from user&#39;s body target  2100 ) that reaches infrared proximity sensor  2200  when user&#39;s body target  2100  presses on overlay  2006 . 
     The reflected infrared-light intensity obtained from infrared proximity sensor  2200  is typically a binary number that is proportional to a standard measurement of the reflected infrared-light intensity in watts per square meter, and converting the reflected infrared-light-intensity binary number to a standard measurement of reflected infrared-light intensity is usually unnecessary. The reflected infrared-light intensity obtained from infrared proximity sensor  2200  can be used directly in embedded controller  2214  after a calibration process that relates the reflected infrared-light-intensity to the amount of separation  2201  between active infrared proximity sensor  2004  and user&#39;s body target  2100 , as well as the amount of deviation of user&#39;s body target  2100  from the midline of active infrared proximity sensor  2004 . This calibration process can be performed during the manufacturing of wearable joint-action sensor  2000  with objects similar in shape and size to human thighs, it can be performed by instructing a user to perform certain actions of the right hip joint while wearing wearable joint-action sensor  2000  on the right waist of the user, or it can be performed while a user is walking or running, producing a periodic reflected infrared-light-intensity waveform that can be analyzed with various techniques, as discussed below. 
     Although four infrared LEDs of active infrared proximity sensor  2004  are illustrated in  FIG. 22 , more or fewer infrared LEDs can be used in active infrared proximity sensor  2004 . In fact, only one infrared LED is needed for detecting separation  2201  between active infrared proximity sensor  2004  and user&#39;s body target  2100 , if user&#39;s body target  2100  does not significantly deviate from the midline of active infrared proximity sensor  2004  (i.e. the right hip joint is not significantly abducted or adducted). However, at least two infrared LEDs, with one infrared LED on each side of infrared proximity sensor  2200 , are required for sensing the amount of deviation of user&#39;s body target  2100  from the midline of active infrared proximity sensor  2004  based on the relative intensities of the reflected infrared-light beams obtained from the two infrared LEDs. Instead of arranging the infrared LEDs in a one-dimensional array, as illustrated in  FIG. 22 , the infrared LEDs can be arranged in a two-dimensional array, or in any spatial pattern to facilitate determination of the amount of separation  2201  between active infrared proximity sensor  2004  and user&#39;s body target  2100 , as well as the amount of deviation of user&#39;s body target  2100  from the midline of active infrared proximity sensor  2004 . Furthermore, active infrared proximity sensor  2004 , including its infrared LEDs, can be mounted on a flexible printed circuit or other surface, so that it can be incorporated inside waist belt  2008 , or along the surface of waist belt  2008 . In fact, the entire wearable joint-action sensor  2000 , including active infrared proximity sensor  2004  and its infrared LEDs, can be incorporated inside waist belt  2008  or along the surface of waist belt  2008  by using similar mounting techniques. 
     Embedded controller  2214  uses a bus  2220 , such as an inter-integrated circuit bus or a serial peripheral interface bus, to configure triaxial accelerometer  2002 , such as to set the sensitivity and sampling rate, and to receive data from triaxial accelerometer  2002  with regard to accelerations along its X, Y, and Z axes. For a user interface, embedded controller  2214  reads signals  2224  and  2226  from push buttons  2222   a  and  2222   b , respectively, and uses an output bus  2230  to drive an optional display  2228 , such as an LCD module. Embedded controller  2214  also uses a bus  2232 , such as an inter-integrated circuit bus or a serial peripheral interface bus, to communicate with an optional communication device  2234 . Communication device  2234 , such as a USB (Universal Serial Bus), Bluetooth®, Wi-Fi®, or NFC (near-field communication) device, or any radio-frequency, optical, acoustic, or wired communication device, provides wired or wireless communication for embedded controller  2214  to communicate with a computing device (not shown in  FIG. 22 ), such as a smartwatch, fitness wristband, smartphone, or computer. Embedded controller  2214  can use the display of the computing device for displaying any data or detection result, so that display  2228  is not needed. Communication device  2234  can also connect wearable joint-action sensor  2000  to the internet to make wearable joint-action sensor  2000  a part of the Internet of Things (IoT). 
     Embedded controller  2214  processes the reflected infrared-light-intensity data obtained from active infrared proximity sensor  2004  and the acceleration data obtained from triaxial accelerometer  2002  to detect a joint action with a software or hardware joint-action detector and a body action with a software or hardware body-action detector. Embedded controller  2214  also combines the detected joint-action and body-action to detect the type of physical activity performed by a user with a software or hardware physical-activity detector. Alternatively, by using communication device  2234 , embedded controller  2214  can send the reflected infrared-light-intensity data and the acceleration data to a computing device (not shown in  FIG. 22 ), such as a smartwatch, fitness wristband, smartphone, or computer, or to any device that is connected to the internet (such as the cloud) for detecting joint actions, body actions, and physical activities with the corresponding software or hardware detectors, and for displaying the data or detection result (so that display  2228  is not needed in wearable joint-action sensor  2000 ). The computing device or any device that is connected to the internet can also send configuration data and instructions to embedded controller  2214  to control its operation, so that push buttons  2222   a  and  2222   b  are not needed. 
     Joint-Action and Physical-Activity Detection Operations—FIGS.  21 A,  21 B, and  23   
     In  FIG. 21A , wearable joint-action sensor  2000  incorporating active infrared proximity sensor  2004  inside housing  2001  is worn on the right waist of the user above the right thigh with waist belt  2008 .  FIG. 21A  shows separation  2201  (in  FIG. 22 ) between active infrared proximity sensor  2004  (illustrated by a dotted outline) and user&#39;s body target  2100  (the right thigh of the user) when the user flexes the right hip joint, such as when the user is sitting. User&#39;s body target  2100  reflects the infrared-light beam emitted (through overlay  2006 ) from an infrared LED of active infrared proximity sensor  2004 , and the reflected infrared-light intensity is sensed (also through overlay  2006 ) by infrared proximity sensor  2200  to detect separation  2201  between active infrared proximity sensor  2004  and user&#39;s body target  2100  for detecting actions of the right hip joint. Wearable joint-action sensor  2000  also incorporates triaxial accelerometer  2002 , which is also illustrated by a dotted outline, for detecting body actions of the torso of the user. Only the Y and Z axes of triaxial accelerometer  2002  are illustrated in  FIG. 21A , because the X axis points out of the figure. 
       FIG. 21B  illustrates an increase in the amount of separation  2201  between active infrared proximity sensor  2004  and user&#39;s body target  2100  when the user decreases the degree of flexion of the right hip joint, such as when the user gets up from the sitting position illustrated in  FIG. 21A . The increase in the amount of separation  2201  results in a decrease in the reflected infrared-light intensity sensed (through overlay  2006 ) by infrared proximity sensor  2200  of active infrared proximity sensor  2004 , so wearable joint-action sensor  2000  inside housing  2001  can detect a decrease in the degree of flexion of the right hip joint. Wearable joint-action sensor  2000  can also detect the upright posture of the torso of the user from the acceleration data received from triaxial accelerometer  2002 , because the acceleration sensed along its Z axis is about −G, where G is the earth&#39;s gravitational acceleration that is pointing vertically downward (not shown in  FIG. 21B ). The accelerations sensed along the X and Y axis are about zero (the X axis is not illustrated in  FIG. 21B  because it is pointing out of the figure), because the X and Y axis are almost orthogonal to the direction of the earth&#39;s gravitational acceleration G. Wearable joint-action sensor  2000  combines the detected joint action (a decrease in the degree of flexion of the right hip joint) and the detected body action (the torso maintains an approximately upright posture) to detect that the physical activity performed by the user is the action of getting up from a sitting position. On the other hand, if the detected joint action is still a decrease in the degree of flexion of the right hip joint, but the acceleration sensed along the Y axis is about G, and the accelerations sensed along the X and Z axis are about zero, then wearable joint-action sensor  2000  detects that the physical activity performed by the user is the action of crawling on the floor instead, because the detected body action of the torso is that the torso is about orthogonal to the earth&#39;s gravitational acceleration G, with the abdomen of the torso facing the floor or ground. 
     Triaxial accelerometer  2002  is inside housing  2001 , which is attached to the waist of a user with waist belt  2008 . Techniques of using acceleration data from a waist-worn triaxial accelerometer for counting steps have been developed for prior-art waist-worn pedometers, and these techniques can be combined with those for detecting actions of the right hip joint in wearable joint-action sensor  2000  to improve accuracy in detection of steps when the user walks or runs, and to detect other types of physical activity performed by the user. As discussed above, the orientation of the torso with respect to the direction of the earth&#39;s gravitational acceleration G is detected by triaxial accelerometer  2002 , and the orientation of the torso is important for detecting the type of physical activity performed by the user. 
     The orientation of the torso (i.e. the body action of the torso) sensed by triaxial accelerometer  2002  is also important for detecting extension, abduction, or adduction of the right hip joint of the user indirectly. When the right hip joint is extended, the right thigh is not in the line of sight of active infrared proximity sensor  2004 , so the reflected infrared-light intensity is very low (the floor or ground, which is significantly farther away, could be out of the sensing range of active infrared proximity sensor  2004 ), but a forward inclination of the torso of the user (a body action for balancing) can be detected by triaxial accelerometer  2002  and be used by wearable joint-action sensor  2000  for detecting extension of the right hip joint indirectly. Similarly, by detecting a sideways inclination of the torso of the user (a body action for balancing) in combination with a very low reflected infrared-light intensity sensed by infrared proximity sensor  2200  of active infrared proximity sensor  2004  when the right hip joint is not flexed significantly, wearable joint-action sensor  2000  can detect abduction or adduction of the right hip joint indirectly. When the user is standing, the right thigh is not in the vertically downward line of sight of active infrared proximity sensor  2004  (in  FIG. 20A ), so the reflected infrared-light intensity is very low, but the acceleration along the Z axis of triaxial accelerometer  2002  is about −G, so wearable joint-action sensor  2000  detects standing by combining the joint-action data (which shows that the right hip joint is not flexed) obtained from active infrared proximity sensor  2004  and the body-action data obtained from triaxial accelerometer  2002 . 
     When the user walks or runs, the right hip joint flexes as the user takes a step forward with the right leg, and the right hip joint extends as the user takes a step forward with the left leg. Wearable joint-action sensor  2000  can be used as a physical-activity monitor by using active infrared proximity sensor  2004  to sense the periodic variation of the reflected infrared-light intensity, which represents the periodic variation of separation  2201 , and measuring the rate of change of the reflected infrared-light intensity when the user walks or runs. The number of steps that the user takes can be detected by analyzing the waveform of the reflected infrared-light intensity with various techniques, such as by detecting the total number of peaks of the waveform, which corresponds to the total number of maximum flexions of the right hip joint. Multiplying the number of maximum flexions or the number significant flexions of the right hip joint by a factor of two can produce the number of steps that the user takes, where significant flexion is any degree of flexion that is larger than the minimum degree of flexion of the right hip joint required for the user to take an ambulating step forward with the right leg. A significant flexion can be detected from the waveform of the reflected infrared-light intensity by using an infrared-light intensity threshold (i.e. a threshold for the degree of flexion), which can be determined based on the amplitude of the waveform, such as using a threshold at a predetermined percentage of the amplitude of the waveform above or below the midline of the waveform, depending on the specific design of wearable joint-action sensor  2000 . Unlike a pedometer that use just an accelerometer to detect steps, wearable joint-action sensor  2000 , which also detects actions of the right hip joint, does not mistakenly count steps when a user&#39;s body vibrates on a traveling vehicle, because the right hip joint does not flex and extend alternately in this case. 
     The repeated flexions of the right hip joint when the user walks or runs can be detected by using just one infrared LED in active infrared proximity sensor  2004  for detecting separation  2201  between active infrared proximity sensor  2004  and the right thigh. In this case, if processing data obtained from triaxial accelerometer  2002  with prior-art techniques for step counting indicates that the user has taken a number of steps, but processing data obtained from active infrared proximity sensor  2004  indicates that the user has taken a significantly fewer number of steps, this indicates that the user has not been walking or running in a typical manner. For simplicity, this type of atypical walking or running activity is called dancing activity in this description. 
     Wearable joint-action sensor  2000  detects dancing activity when a user actually dances or when the user just taking small irregular steps when doing chores, without ambulating (i.e. without walking or running) from one place to another place a distance away. When the user actually dances, the right hip joint flexes and extends with the right thigh deviate frequently away from the line of sight of active infrared proximity sensor  2004  with just one infrared LED, so wearable joint-action sensor  2000  might not be able to detect some of the dancing steps from the reflected infrared-light-intensity data obtained from active infrared proximity sensor  2004 . On the other hand, based on the acceleration data obtained from triaxial accelerometer  2002 , wearable joint-action sensor  2000  likely detects more steps than the user actually takes for dancing, because the user tends to wiggle the waist while dancing, resulting in counting steps when the user just wiggles triaxial accelerometer  2002  in housing  2001  worn on the waist without actually taking an ambulating step. Similarly, when the user takes small steps irregularly when doing chores, such as when the user is sweeping the floor, the degrees of flexion of the right hip joint could be too small (i.e. the right hip joint is not significantly flexed) that wearable joint-action sensor  2000  might not count some of these small steps as ambulating steps based on the reflected infrared-light-intensity data obtained from active infrared proximity sensor  2004 . On the other hand, wearable joint-action sensor  2000  likely counts these small steps as ambulating steps based on the data obtained from triaxial accelerometer  2002 , because the waist of the torso of the user still vibrates when the user takes small steps. 
     Wearable joint-action sensor  2000  can differentiate between actual dancing and taking small irregular steps by using one or more infrared LEDs on each side of infrared proximity sensor  2200  (in  FIG. 22 , there are two infrared LEDs on each side of infrared proximity sensor  2200 ) of active infrared proximity sensor  2004  to detect frequent deviations of the right thigh to the left and right sides of the midline of active infrared proximity sensor  2004  when a user actually dances. When a user takes small steps, the right thigh does not deviate to the left and right sides of the midline of active infrared proximity sensor  2004  as frequently as when the user dances. Dancing activity, which includes actual dancing and taking small irregular steps in this description, consumes a different number of calories in a period of time when compared to walking or running, so it should be treated as a different type of physical activity for estimating energy expenditure. While prior-art pedometers often mistakenly treat dancing activity as ambulation, wearable joint-action sensor  2000  in the eighth embodiment can detect dancing activity. 
     Another advantage of using wearable joint-action sensor  2000  as a physical-activity monitor is that it can also detect whether a user is sitting or standing when the user is not walking or running. As illustrated in the block diagram for the seventh embodiment (in  FIG. 17 ), wearable joint-action sensor  2000  in the eighth embodiment can also communicate with reminder  1700  that includes a timer to be used as a sitting detector for reminding a user (such as by vibration, sound, voice, light, or a message on a display) that the user has been sitting for too long. The timer function can also be implemented by a hardware or software timer in embedded controller  2214 , without requiring that reminder  1700  includes a timer. 
     Wearable joint-action sensor  2000  can detect sitting or standing accurately after the reflected infrared-light intensity sensed by infrared proximity sensor  2200  of active infrared proximity sensor  2004  is calibrated for the degree of flexion of the right hip joint when a user is sitting or standing, respectively. With less elaborate calibration, wearable joint-action sensor  2000  can still detect sitting and standing quite accurately by sensing the duration of significant flexion and no significant flexion of a user&#39;s right hip joint, respectively, where significant flexion is any degree of flexion that is larger than the minimum degree of flexion required for the user to take an ambulating step forward with the right leg by flexing the right hip joint. This is based on the fact that the right hip joint is in significant flexion for a long duration when the user is sitting, and the right hip joint is not in significant flexion for a long duration when the user is standing. The duration of significant flexion or no significant flexion, or the duration of any detected joint action, can be determined by a hardware or software timer implemented in embedded controller  2214 . When the duration of a detected significant flexion of the right hip joint exceeds a predetermined duration, sitting is detected. On the other hand, when the duration of a detected no significant flexion of the right hip joint exceeds a predetermined duration, standing is detected. When a user is doing squats for exercise instead of either sitting or standing, the right hip joint is in significant flexion and no significant flexion alternately, which can be detected by wearable joint-action sensor  2000 , so long as the duration of significant flexion or no significant flexion (or both) is longer than a predetermined duration for walking or running, indicating that the user is not walking or running. 
     As discussed above, wearable joint-action sensor  2000  can also use its communication device  2234  and signal link  1702  to communicate with reminder  1700  (in  FIG. 17 ) for reminding a user. Reminder  1700  can be incorporated inside housing  2001  and use simple electrical connections for communicating with wearable joint-action sensor  2000 , or it can be far away from the user for reminding someone else, such as a caregiver of the user. Communication device  2234  can be a USB (Universal Serial Bus), Bluetooth®, Wi-Fi®, or NFC (near-field communication) device, or any radio-frequency, optical, acoustic, or wired communication device that provides wired or wireless communication. Wearable joint-action sensor  2000  can use communication device  2234  to communicate with a computing device, such as a smartwatch, fitness wristband, smartphone, or computer, to control the computing device (as illustrated in  FIG. 9 ) to deliver the reminding or notifying messages with its display or speaker, instead of using reminder  1700  to deliver the reminding or notifying messages. Wearable joint-action sensor  2000  can also detect how much time (with a hardware or software timer implemented in embedded controller  2214 ) a user spends in each of sitting, standing, dancing, and ambulating (i.e. either walking or running, with or without counting steps). The time distribution among these types of physical activity performed by the user could be more relevant for improving the user&#39;s health than the number of steps counted with a prior-art pedometer, which cannot reliably distinguish between sitting, standing, and dancing. 
     Wearable joint-action sensor  2000  can be used for detecting any type of physical activity that involves the right hip joint, without the inconvenience of attaching a sensor to the right thigh of a user, as suggested by the prior arts for sensing actions of the right hip joint. Although  FIGS. 20A, 21A, and 21B  illustrate that wearable joint-action sensor  2000  is worn on the right waist of a user above the right thigh using waist belt  2008  for detecting actions of the right hip joint, wearable joint-action sensor  2000  can be worn on the left waist of a user using waist belt  2008  for detecting actions of the left hip joint by using the same detection principle. 
       FIG. 23  is a flow diagram illustrating the joint-action and body-action detection operations of wearable joint-action sensor  2000  in  FIG. 22 . In  FIG. 23 , after wearable joint-action sensor  2000  has been turned on, embedded controller  2214  sends configuration data through bus  2216  to set up active infrared proximity sensor  2004  at step  2300 , and sends configuration data through bus  2220  to set up triaxial accelerometer  2002  at step  2302  for detecting a joint action of the right hip joint and a body action of the torso, respectively. 
     At step  2304 , embedded controller  2214  sequentially selects one of infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d  as the infrared-light source, and then uses bus  2216  to receive the corresponding reflected infrared-light-intensity data from active infrared proximity sensor  2004 . Embedded controller  2214  processes the reflected infrared-light-intensity data for infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d  together at step  2304  for detecting an action of the right hip joint with a software or hardware joint-action detector implemented in embedded controller  2214 . For example, if significant reflected infrared-light intensity is detected only when infrared LEDs  2202   c  and  2202   d  on the right side of the midline of active infrared proximity sensor  2004  are used as the infrared-light sources, the user&#39;s right thigh (i.e. user&#39;s body target  2100 ) must be deviated to the right side of the midline of active infrared proximity sensor  2004  (infrared proximity sensor  2200  is at the midline of active infrared proximity sensor  2004 ) when the right hip joint is abducted, as illustrated in  FIG. 22 . On the other hand, if significant reflected infrared-light intensity is detected only when infrared LEDs  2202   a  and  2202   b  on the left side of the midline of active infrared proximity sensor  2004  are used as the infrared-light sources, the user&#39;s right thigh must be deviated to the left side of the midline of active infrared proximity sensor  2004  when the right hip joint is adducted (not illustrated in  FIG. 22 ). 
     In order for infrared proximity sensor  2200  of active infrared proximity sensor  2004  to sense any reflected infrared light from the right thigh, the right hip joint has to be at least slightly flexed to bring the right thigh into the line of sight of active infrared proximity sensor  2004 . Also, when wearable joint-action sensor  2000  is worn on the right waist above the right thigh, the line of sight of active infrared proximity sensor  2004  (i.e. the midline of active infrared proximity sensor  2004  at infrared proximity sensor  2200 ) should approximately aim at the midline of the right thigh, where the reflected infrared-light intensity detected by infrared proximity sensor  2200  for infrared light beams emitted from infrared LED  2202   a  and  2202   d  (or from infrared LED  2202   b  and  2202   c ) are about the same if the right hip joint is at least slightly flexed but not abducted or adducted. 
     At step  2306 , embedded controller  2214  receives acceleration data along the X, Y, and Z axes from triaxial accelerometer  2002 , and then embedded controller  2214  processes the data to detect a body action, such as bending of the torso of a user forward, backward, sideways, or in any other direction, with a software or hardware body-action detector implemented in embedded controller  2214 . As discussed above, embedded controller  2214  can use data from triaxial accelerometer  2002  for joint-action detection indirectly when the user is standing or when the right hip joint is extended, abducted, or adducted. Embedded controller  2214  combines the reflected infrared-light-intensity data for one or more infrared LEDs that produce significant reflected infrared-light intensities (such as infrared LEDs  2202   c  and  2202   d  in  FIG. 22 ) to estimate separation  2201  between active infrared proximity sensor  2004  and user&#39;s body target  2100  (i.e. the right thigh in this embodiment), and the very low reflected infrared-light intensities of the other infrared LEDs (i.e. infrared LEDs  2202   a  and  2202   b  in  FIG. 22 ) are not used for the estimation. As discussed above, embedded controller  2214  uses the reflected infrared-light-intensity data for all four LEDs to estimate the amount of right or left deviation of the right thigh from the midline of active infrared proximity sensor  2004 . 
     At step  2308 , embedded controller  2214  uses the joint-action data obtained from step  2304  and the body-action data obtained from step  2306  to detect a type of physical activity, such as jumping, somersaulting, dancing, sitting up, pushing up, or biking, with a software or hardware physical-activity detector implemented in embedded controller  2214 . For example, when a user is doing push-ups, the user&#39;s abdomen is facing the floor or ground, and embedded controller  2214  can detect push-up activity by combining acceleration data received from triaxial accelerometer  2002  along the Y axis (which varies around G) and along the X and Z axes (which varies around very small fractions of G, because X and Z axes are almost orthogonal to the earth&#39;s gravitational acceleration G), and the very low reflected infrared-light intensities for infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d , because the degree of flexion of the right hip joint is small during push-ups. The number of push-ups performed by the user is equal to the number of peaks or valleys of the acceleration along the Y axis. An additional active infrared proximity sensor  2004  for proximity sensing along the Y axis can be incorporated in wearable joint-action sensor  2000  for sensing the periodic variation of the separation between housing  2001  and the floor or ground when the user is doing push-ups (not illustrated in any figure here) to further confirm the push-up activity. This additional active infrared proximity sensor  2004  is particularly important for detecting push-up activity if triaxial accelerometer  2002  is not incorporated in wearable joint-action sensor  2000 . As discussed below, an altimeter (i.e. barometer) with a high resolution can also be used for sensing the ups-and-downs of the waist during push-ups to confirm the push-up activity. Embedded controller  2214  can also detect sit-up activity based on the variation of the degree of flexion of the right hip joint and the large variation of the Z-axis acceleration between approximately zero (when the torso is horizontal) and −G (when the torso is upright). Similarly, embedded controller  2214  can detect biking from the forward-inclined or upright torso of a user and the cyclic variation of the degree of flexion of the right hip joint of the user. 
     At step  2310 , the joint-action data obtained from step  2304 , as well as the physical-activity data obtained from step  2308 , is stored in the memory of embedded controller  2214 . The body-action data obtained from step  2306  can also be stored in the memory of embedded controller  2214  at step  2310  if necessary. At step  2312 , embedded controller  2214  reads signals  2224  and  2226  from push buttons  2222   a  and  2222   b , respectively, to find out at steps  2314  and  2316  if the user wants to control the operation of embedded controller  2214 , such as to stop data acquisition for joint-action and physical-activity detections, to display data, or to communicate with a computing device, such as a smartwatch, fitness wristband, smartphone, or computer. Under computer program control, embedded controller  2214  can also stop the data acquisition for joint-action and physical-activity detections, display data, or communicate with a computing device, without requiring the user to use push button  2222   a  or  2222   b.    
     If the data acquisition is not stopped at step  2314  under computer program control, embedded controller  2214  waits for a predetermined time interval at step  2320  before it returns to step  2304  to receive more reflected infrared-light-intensity data from active infrared proximity sensor  2004 . On the other hand, if the data acquisition is stopped at step  2314  under computer program control (such as when a predetermined joint action, a predetermined physical activity, or a specific pattern of signals  2224  and  2226  from push buttons  2222   a  and  2222   b , respectively, is detected), embedded controller  2214  can end all the processes shown in  FIG. 23  at step  2316 . It can also go to step  2318  to display any data or detection result stored in the memory of embedded controller  2214  by using display  2228  or communicate with a computing device by using communication device  2234 . If embedded controller  2214  does not end all the processes shown in  FIG. 23  at step  2316 , embedded controller  2214  returns to step  2304  to receive more reflected infrared-light-intensity data from active infrared proximity sensor  2004  after displaying data or communicating with a computing device at step  2318 . 
       FIG. 23  can also be the flow diagram for illustrating the joint-action and physical-activity detection operations if a time-of-flight proximity sensor, which is an advanced active infrared proximity sensor, is used in the eighth embodiment instead of active infrared proximity sensor  2004 . In this case, infrared LEDs  2202   a ,  2202   b ,  2202   c , and  2202   d  in  FIG. 22  are replaced with infrared laser diodes, and infrared proximity sensor  2200  is replaced with a time-of-flight detector that measures the time light takes to travel from each laser diode to user&#39;s body target  2100  and reflect back to the time-of-flight detector to determine the range, such as the distance travelled by infrared-light beams  2210  or  2212 . Based on the geometry of the isosceles triangle formed by the equal distances travelled by the emitted and reflected infrared-light beams (such as the equal distances travelled by infrared-light beams  2210  and  2212 ), and the known distance between the infrared laser diode (that emits the infrared-light beam) and the time-of-flight detector, the height of the isosceles triangle, which is separation  2201  between the time-of-flight proximity sensor and user&#39;s body target  2100 , can be detected. If only one infrared laser diode is used in active infrared proximity sensor  2004  for sensing just separation  2201  between the time-of-flight proximity sensor and user&#39;s body target  2100 , the infrared laser diode can be very close to the time-of-flight detector (that detects the range based on the infrared-light travelling time), so that the range obtained from the time-of-flight proximity sensor is approximately equal to separation  2201  between the time-of-flight proximity sensor and user&#39;s body target  2100 . 
     Since a time-of-flight proximity sensor measures the infrared-light travelling time, but not the reflected infrared-light intensity, it can measure separation  2201  between the time-of-flight proximity sensor and user&#39;s body target  2100  accurately even when an emitted infrared-light beam, such as infrared-light beam  2210 , reaches an edge of user&#39;s body target  2100 , so long as the reflected infrared-light intensity is adequate for the time-of-flight detector to function properly. When the emitted infrared-light beam misses user&#39;s body target  2100  entirely, the next closest target, such as the floor or ground, reflects the emitted infrared-light beam instead, and the time-of-flight detector could detect that the next closest target is out of its maximum detection range if the reflected infrared-light intensity is too low for it to function properly. For this reason, a time-of-flight proximity sensor can locate an edge of user&#39;s body target  2100  more accurately than a conventional active infrared proximity sensor, which would sense a gradual change in reflected infrared-light intensity that represents a gradual change in the separation between the conventional active infrared proximity sensor and user&#39;s body target  2100  when an emitted infrared-light beam reaches an edge of user&#39;s body target  2100 . 
     Instead of using a single time-of-flight detector and multiple infrared laser diodes to implement an advanced active infrared proximity sensor similar to the one illustrated in  FIG. 22 , one or more time-of-flight proximity sensor modules, each including a time-of-flight detector and an infrared laser diode, such as the VL6180X module available from STMicroelectronics of Switzerland, can be used to measure separation  2201  between each time-of-flight proximity sensor module and user&#39;s body target  2100 . Multiple time-of-flight proximity sensor modules, with one module activated at a time, can be arranged in an array of one or two dimensions or in any special pattern for detecting the location of user&#39;s body target  2100  with respect to the time-of-flight proximity sensor modules. 
     Although a user of wearable joint-action sensor  2000  is typically a human being, a user can be a four-limbed animal, such as a monkey, dog, cat, goose, or elephant, for which joint actions and physical activities are being detected by using the same joint-action and physical-activity detection principles as disclosed in the eighth embodiment. Furthermore, the description of the eighth embodiment above is not intended to limit the invention to the eighth embodiment, but rather to enable any person skilled in the art to make and use the invention. For example, capacitive proximity sensor  102  (in  FIG. 1 ) can replace active infrared proximity sensor  2004  in the eighth embodiment, and active infrared proximity sensor  2004  can replace capacitive proximity sensor  102  in the other embodiments, without departing from the spirit and the scope of the embodiments. Furthermore, when the ambient infrared-light intensity is adequate, such as when wearable joint-action sensor  2000  is worn by a user under the sun, active infrared proximity sensor  2004  can use sunlight instead of any of the infrared LEDs as the infrared light source, and use infrared proximity sensor  2200  to detect any change in the separation between infrared proximity sensor  2200  and the right thigh (i.e. to detect the relative motion between the right thigh and infrared proximity sensor  2200 ). In this case, a one- or two-dimensional visible- or infrared-light imaging sensor instead of active infrared proximity sensor  2004  can be incorporated in wearable joint-action sensor  2000  for sensing motion of the right thigh to detect actions of the right hip joint. 
     Alternate Sensor Combinations in the First and Eighth Embodiments—FIGS.  1 ,  3 A,  3 B,  4 A,  4 B,  20 A,  20 B,  21 A,  21 B, and  22   
     Both the first and eighth embodiments are used for detecting actions of the right hip joint (or the left hip joint by using the same detection principle) and physical activities of a user. In the first or eighth embodiment, wearable joint-action sensor  100  or  2000 , respectively, is worn on the waist of the torso of the user (the torso is the first body segment in the first or eighth embodiment) above the right thigh of the right upper leg (the right upper leg is the second body segment in the first or eighth embodiment). In the first embodiment (in  FIGS. 1, 3A, 3B, 4A, and 4B ), capacitive proximity sensor  102  is used for detecting separation  130  between sensing pad  108  on sensing plate  118  and the right abdomen of the torso (the first body segment), while in the eighth embodiment ( FIGS. 20A, 20B, 21A, 21B, and 22 ), active infrared proximity sensor  2004  is used for detecting separation  2201  between active infrared proximity  2004  and the right thigh of the right upper leg (the second body segment) instead. 
     Although triaxial accelerometer  2002  is incorporated in wearable joint-action sensor  2000 , triaxial accelerometer  2002  can also be incorporated in wearable joint-action sensor  100  for detecting body actions of the user by using the same detection principle. The three sensors in the first and eighth embodiments, capacitive proximity sensor  102 , active infrared proximity sensor  2004 , and triaxial accelerometer  2002 , can be combined in any form (i.e. by using any one, two, or all three of the sensors) in any embodiment, not just in the first or eighth embodiment, to function synergistically (when more than one of the three sensors are used) to create a new embodiment. Furthermore, another sensor, such as an ultrasonic proximity sensor, single-chip radar, visible- or infrared-light imaging sensor (a two-dimensional imaging sensor or one-dimensional line-scan imaging sensor), altimeter (i.e. barometer), or gyroscope, can also be used to cooperate with any one, two, or all of the three sensors for joint-action and physical-activity detections in the first or eighth embodiment, or in any embodiment. 
     For example, when active infrared proximity sensor  2004  of wearable joint-action sensor  2000  is replaced by capacitive proximity sensor  102 , push buttons  2222   a  and  2222   b  can also be replaced by capacitive touch keys  134  and  136  (in  FIG. 1 ). One or more proximity sensing plates can be placed at the bottom of housing  2001  of wearable joint-action sensor  2000  to replace active infrared proximity sensor  2004  and its overlay  2006  (in  FIGS. 20A and 20B ). One or more proximity sensing plates can also be placed at the front of housing  2001  that faces the right abdomen of a user, similar to the arrangement in the first embodiment, so that embedded controller  2214  can detect flexion, extension, abduction, and adduction of the right hip joint directly. 
     When capacitive proximity sensor  102  or active infrared proximity sensor  2004  is used as a simple sitting and standing sensor (in  FIGS. 3A, 4A, 20A, and 21A ), wearable joint-action sensor  2000  can use data received from triaxial accelerometer  2002  for counting steps only when a user is not sitting or standing, so that wearable joint-action sensor  2000  does not count steps mistakenly when the waist of a user just vibrates, such as when the user is sitting or standing in a travelling vehicle. Furthermore, a high-resolution altimeter (low-resolution barometers have been incorporated in prior-art activity trackers for sensing climbing a flight of stairs, etc.), such as a digital output barometer that is available from STMicroelectronics of Switzerland (Product Number: LPS22HB), can be added to confirm detection of sitting or standing by wearable joint-action sensor  2000  as follows. A large increase in the degree of flexion of the right hip joint (sensed by active infrared proximity sensor  2004 ) in combination with a rapid decrease in altitude of at least 30 cm (sensed by an altimeter), for example, is strong evidence that a user changes from standing to sitting. On the other hand, a large decrease in the degree of flexion of the right hip joint in combination with a rapid increase in altitude of at least a 30 cm, for example, is strong evidence that a user changes from sitting to standing. If a user changes from standing to sitting or from sitting to standing rapidly, the acceleration along the Z-axis sensed by triaxial accelerometer  2002  can also be used for confirming detection of sitting or standing. This example illustrates some of the benefits of using multiple sensors that work synergistically in wearable joint-action sensor  2000  to improve accuracy of joint-action and physical-activity detections. 
     Wearable joint-action sensor  2000  can incorporate both capacitive proximity sensor  102  and active infrared proximity sensor  2004  for detecting actions of the right hip joint, because capacitive proximity sensor  102  can sense extension of the right hip joint (in  FIG. 4B ) directly even through clothing, without relying on detecting the orientation of the torso of a user with triaxial accelerometer  2002 , as discussed above. On the other hand, active infrared proximity sensor  2004 , especially with a number of infrared LEDs, can directly sense actions (except extension) of the right hip joint, such as flexion, abduction, and adduction, more accurately, because active infrared proximity sensor  2004  can have a longer sensing range than that of capacitive proximity sensor  102 , unless the size of sensing plate  118  is large. 
     When both capacitive proximity sensor  102  and active infrared proximity sensor  2004  are incorporated in wearable joint-action sensor  2000 , a software or hardware physical-activity detector in embedded controller  2214  can use a first joint-action detector with data obtained from capacitive proximity sensor  102  to detect flexion and extension of the right hip joint, and use a second joint-action detector with data obtained from active infrared proximity sensor  2004  to detect abduction and adduction, which cannot be reliably detected by the first joint-action detector with data obtained from capacitive proximity sensor  102  using one sensing plate  118 . The physical-activity detector in embedded controller  2214  can use the second joint-action detector with data obtained from active infrared proximity sensor  2004  and a body-action detector with data obtained from triaxial accelerometer  2002  to detect standing, as discussed above. When the physical-activity detector detects standing (i.e. when the right hip joint is not flexed or extended) by using the second joint-action detector and the body-action detector, the first joint-action detector can be calibrated to distinguish between flexion and extension of the right hip joint by storing the corresponding capacitance data obtained from capacitive proximity sensor  102  as the reference. After the calibration process, the physical-activity detector can use the first joint-action detector with data obtained from capacitive proximity sensor  102  to detect walking, running, standing, and sitting, because capacitive proximity sensor  102  tends to consume less power than active infrared proximity sensor  2004 , as discussed above. At any time when the physical-activity detector detects standing by using the second joint-action detector and the body-action detector, the calibration process can be repeated. Furthermore, the physical-activity detector can use the second joint-action detector with data obtained from active infrared proximity sensor  2004  (instead of the first joint-action detector with data obtained from capacitive proximity sensor  102 ) to detect a physical activity, such as dancing activity (discussed above), that cannot be detected as reliably by using the first joint-action detector with data obtained from capacitive proximity sensor  102 . 
     Although body actions can also be detected by using proximity sensors instead of triaxial accelerometer  2002 , such as by placing sensing plate  118  of capacitive proximity sensor  102  in the proximity of the upper spinal column of a user for detecting flexion and extension of the upper spinal column, as discussed in the seventh embodiment, using triaxial accelerometer  2002  on the waist of the torso of a user for detecting body actions of the torso is more convenient in this case, because active infrared proximity sensor  2004  (or capacitive proximity sensor  102 ) for detecting actions of the right hip joint and triaxial accelerometer  2002  can both be incorporated in housing  2001  worn on the right waist. When necessary, triaxial accelerometer  2002  can be worn on a different body segment of the user, such as on the head, for detecting body actions of the body segment, and acceleration data from triaxial accelerometer  2002  can be sent to wearable joint-action sensor  2000  worn on the right waist by wired or wireless communication. Depending on the types of body action to be detected, a one-axis, two-axis, or three-axis (triaxial) accelerometer can be used in any embodiment to detect body actions of a body segment of the user, such as the wrist region of the right forearm (the third embodiment) or a finger segment (the sixth embodiment). Furthermore, if necessary, a gyroscope can be used to detect rotation of the body segment, which is a type of movement of the body segment. Types of physical activity can then be detected by combining joint-action data with body-action data. 
     There is a significant benefit for wearable joint-action sensor  2000  to incorporate any two or all three of active infrared proximity sensor  2004 , capacitive proximity sensor  102 , and triaxial accelerometer  2002  for detecting actions of the right hip joint. As discussed above, each of the three sensors has its own advantages and disadvantages in detecting actions of the right hip joint, so they can calibrate against each other when they are incorporated in wearable joint-action sensor  2000 . For example, when a user is ambulating, a software or hardware physical-activity detector in embedded controller  2214  of wearable joint-action sensor  2000  can use triaxial accelerometer  2002  to detect ambulation, and then use the capacitance waveform obtained from capacitive proximity sensor  102  to determine a capacitance threshold for detecting significant flexion of the right hip joint, where significant flexion is any degree of flexion that is larger than the minimum degree of flexion of the right hip joint required for the user to take an ambulating step forward with the right leg, as discussed above. If the amplitude of the capacitance waveform obtained from capacitive proximity sensor  102  is so small that no capacitance threshold can be reliably determined to differentiate between significant flexion and no significant flexion of the right hip joint, but data from triaxial accelerometer  2002  indicates that the user is ambulating, the user&#39;s body must be vibrating instead of ambulating. In this case, the capacitance sensed by capacitive proximity sensor  102  is high if the user is sitting with significant flexion of the right hip joint, and the capacitance is low if the user is standing with no significant flexion of the right hip joint. Similarly, when ambulation is detected by the physical-activity detector using data obtained from triaxial accelerometer  2002 , the reflected infrared-light-intensity waveform obtained from active infrared proximity sensor  2004  can be used to determine a reflected infrared-light-intensity threshold for detecting significant flexion of the right hip joint, as discussed above. 
     Wearable joint-action sensor  2000  can use the capacitance data from capacitive proximity sensor  102  and a capacitance threshold to count steps, as well as to detect sitting (i.e. the right hip joint is in significant flexion for a long duration) and standing (i.e. no significant flexion of the right hip joint for a long duration), especially when combining the body-action data from triaxial accelerometer  2002 . For example, if the capacitance sensed by capacitive proximity sensor  102  is constantly high for a long duration (i.e. the duration determined by a timer in embedded controller  2214  exceeds a predetermined duration), indicating that a user is sitting, but the body-action data from triaxial accelerometer  2002  indicates that the user is actually lying on the back, wearable joint-action sensor  2000  with housing  2001  worn on the right waist must be pulled down toward the right abdomen of the torso by the earth&#39;s gravitational force when the user is lying on the back, instead of the right abdomen of the torso bulging out toward sensing plate  118  of capacitive proximity sensor  102  (at the front of housing  2001  that faces the right abdomen) as when the user is sitting. Similarly, data from active infrared proximity sensor  2004  and data from triaxial accelerometer  2002  can also be calibrated against each other to optimize detection of actions of the right hip joint. 
     Active infrared proximity sensor  2004  of wearable joint-action sensor  2000  can also be replaced by a different type of proximity sensor, such as by one or more single-chip radars or by an ultrasonic proximity sensor with one or more ultrasonic transducers. Multiple ultrasonic transducers of an ultrasonic proximity sensor can form a phased array that is similar to that of an ultrasonic imaging sensor for imaging the surface of the right thigh to detect actions of the right hip joint. Similarly, multiple infrared proximity sensors (i.e. more than a single infrared proximity sensor  2200  in  FIG. 22 ) with one or more infrared-light sources can be used for tracking the position of the right thigh. A visible- or infrared-light imaging sensor (or multiple visible- or infrared-light photodiodes arranged in any spatial pattern, such as a line-scan imaging sensor), with or without a light source, can also replace active infrared proximity sensor  2004  for detecting actions of the right hip joint by imaging the surface of the right thigh. For example, deviation of the right thigh from the midline of an imaging sensor can be detected by image processing, and the right thigh appears bigger in an image when it is closer (corresponds to a larger degree of flexion of the right hip joint) to the imaging sensor, and vice versa. Furthermore, an autofocus imaging sensor can detect the separation between the imaging sensor and the right thigh when the right thigh is brought into focus of the imaging sensor. Other imaging processing techniques, such as two-dimensional autocorrelation, can be used for detecting actions of the right hip joint from the infrared- or visible-light video images of the right thigh. When necessary, instead of using embedded controller  2214  to process the video-image data received from a video imaging sensor, embedded controller  2214  can use bus  2232  and communication device  2234  to send all the data, including the data from triaxial accelerometer  2002 , to a more powerful computing device for joint-action and physical-activity detection operations. In fact, this technique for reducing the processing burden of the embedded controller in the wearable joint-action sensor can be used in any embodiment when necessary. 
     By combining data obtained from a wrist-worn device (such as the acceleration data obtained from a smartwatch or a fitness wristband, or the wrist joint-action data obtained from wearable joint-action sensor  100  in the third embodiment) of a user, wearable joint-action sensor  2000  can be used in a physical-activity monitoring system for detecting many types of physical activity, without requiring the user to attach an accelerometer to the thigh. The wrist-worn device is useful in detecting activities involving the upper extremities, while wearable joint-action sensor  2000  worn on the waist can detect activities involving the torso and the thigh of the user. For example, when a user is weightlifting, a wrist-worn device incorporating an accelerometer and wearable joint-action sensor  100  in the third embodiment can detect actions of the wrist joint and the orientation of the wrist region of the forearm with respect to the vertical direction of the earth&#39;s gravitational acceleration G, respectively, while wearable joint-action sensor  2000  can detect the orientation of the torso and the degree of flexion of the right hip joint of the user. An altimeter with high resolution can also be incorporated in the wrist-worn device for sensing the ups-and-downs of the wrist during weightlifting, providing additional evidence that the user is weightlifting. U.S. Pat. No. 8,050,881 (2011) to Yeung et al. (the same inventors of the present invention) discloses techniques for synchronization of data collected by a sensor (such as an accelerometer) worn on the wrist with data collected by wearable joint-action sensor  2000  worn on the waist to facilitate detection of the types of physical activity performed by a user. 
     For example, in a physical-activity monitoring system that employs wearable joint-action sensor  2000  and a wrist-worn device with an accelerometer (such as an accelerometer in a smartwatch or fitness wristband) to detect the types of physical activity performed by a user, display  2228  of wearable joint-action sensor  2000  can give the user a rough estimate of the level of physical activity by showing the step count, total sitting time, total standing time, total dancing time, and total ambulating time. Other types of physical activity can be detected after the data from active infrared proximity sensor  2004 , triaxial accelerometer  2002 , and the accelerometer in the wrist-worn device, are downloaded (by wired or wireless communication) to a computing device, such as a smartphone or computer, for joint-action detection, body-action detection, and physical-activity detection and display. The step count obtained from wearable joint-action sensor  2000  would be more reliable than the step count that can be obtained from a prior-art pedometer, because wearable joint-action sensor  2000  uses both joint-action data of the right hip joint and body-action data to detect ambulating steps. The computing device can provide the user a detailed breakdown of different types of physical activity performed by the user, so that the user can make appropriate modification to the types and distributions of physical activity for personalized health improvement. Furthermore, the calibrations of active infrared proximity sensor  2004  (or capacitive proximity sensor  102 ) and triaxial accelerometer  2002  of wearable joint-action sensor  2000  (or wearable joint-action sensor  100 ) for joint-action and physical-activity detections can be personalized for a user by using machine learning if wearable joint-action sensor  2000  (or wearable joint-action sensor  100 ) is worn by the same user regularly, so that fast and accurate joint-action and physical-activity detections can be achieved for the user. 
     Although the description above contains many specificities, these should not be construed as limiting the scope of the embodiments but as merely providing illustrations of some of the embodiments. For example, the above-described embodiments can be modified by one skilled in the art, especially in the combination of various described features, without departing from the spirit and the scope of the embodiments. 
     Furthermore, the figures in the description above illustrate the architecture, functionality, and operation of possible implementations of the embodiments, and each function specified in the flow diagrams may comprise one or more executable computer program instructions. In some alternative implementations, the functions noted can occur out of the order noted in the flow diagrams, and more or fewer functions in each block, as well as more and fewer blocks, can be used. For example, two blocks shown in succession may be executed substantially concurrently or in the reverse order, depending on the functionality involved. It should also be noted that each block and combination of blocks in the block diagrams and flow diagrams can be implemented by hardware, software (i.e. computer program), or a combination of hardware and software. 
     Thus the scope of the embodiments should be determined by the appended claims and their legal equivalents, rather than by the examples given.