Patent Publication Number: US-2012026029-A1

Title: Motion detection system and method with null points

Description:
The technical field of this disclosure is motion detection systems and methods, particularly, motion detection systems and methods with null points. 
     Wireless communication and control networks are becoming increasingly popular for home automation, building automation, healthcare infrastructure, low power cable-less links, asset control, and other applications. One benefit of such networks is the ability to locate a network device or tag. For example, lighting commissioning personnel can quickly identify a specific wireless device, so installation costs can be reduced. Expensive equipment may be tagged, and tracked in and around a building, allowing staff to easily locate the tagged equipment when needed for use, for calibration, or in an emergency. Tagged equipment can also generate an alarm when moved beyond specified boundaries. 
     Although a number of methods are available to determine locations of mobile devices, such as asset tags, or fixed devices, such as lights or control units, all require that one device transmit a message and another device receive the message. Unfortunately, transmitting and receiving messages requires power. In battery powered devices, battery life is directly affected by the amount of time spent transmitting or receiving messages. This is particularly true for applications requiring real time location information, such as small form factor/high volume asset tags, for which battery capacity is limited. Precise location must be sacrificed for available battery capacity. 
     One approach has been to equip each asset tag with a mercury switch or an accelerometer, which is used to determine whether the asset tag is moving. The rate of transmitting messages and the time spent receiving messages is reduced when the accelerometer indicates that the asset tag is not moving. Unfortunately, equipping each asset tag with a mercury switch or accelerometer increases the number of parts, increasing the cost, assembly time, and complexity of the asset tag. 
     One problem encountered in range estimation for wireless communication and control networks is the presence of null points in the signal field. Original signals and reflected signals cancel each other at the null points. Because range estimation often depends on the orderly, regular decay of signal strength to determine distance, the null points are anomalies in the signal field and create errors in range estimation. The presence of null points is undesirable in range estimation and requires corrective measures for accuracy. 
     It would be desirable to have a motion detection system and method with null points that would overcome the above disadvantages. 
     One aspect of the present invention relates to a motion detection method including transmitting a signal; detecting the signal at a first device; determining whether signal strength of the detected signal is less than an expected signal strength; transmitting at least one additional signal; detecting the at least one additional signal at the first device; determining whether signal strength of the detected at least one additional signal is less than the expected signal strength; and determining that the first device is in a null point when the signal strength of the detected signals is less than the expected signal strength for a predetermined number of the detected signals. 
     Another aspect of the present invention relates to a motion detection system including a first device operable to transmit a signal; a second device operable to detect the signal; and a processor operable to determine whether signal strength of detected signals at the second device is less than an expected signal strength, and operable to determine that the second device is in a null point when the signal strength of the detected signals is less than the expected signal strength for a predetermined number of the detected signals. 
     Yet another aspect of the present invention relates to a motion detection method including transmitting a first signal; detecting the first signal at a plurality of first devices; determining a greatest signal strength of the first signal detected by the plurality of first devices; determining that one of the plurality of first devices is in a null point when signal strength of the detected first signal at the one of the plurality of first devices is less than the greatest signal strength less a predetermined signal strength offset; transmitting a second signal; detecting the second signal at the plurality of first devices; determining that the one of the plurality of first devices is in the null point when signal strength of the detected second signal at the one of the plurality of first devices is less than the greatest signal strength less the predetermined signal strength offset; and determining that the one of the plurality of first devices is stationary when the one of the plurality of first devices is in the null point for the first signal and the second signal. 
    
    
     
       The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention, rather than limiting the scope of the invention being defined by the appended claims and equivalents thereof. 
         FIG. 1  is a schematic diagram of a motion detection system in accordance with the present invention; 
         FIG. 2  is a block diagram of a radio frequency (RF) unit for use with a motion detection system and method in accordance with the present invention; 
         FIG. 3  is a block diagram of a motion detection system in accordance with the present invention; and 
         FIG. 4  is a flowchart of a motion detection method in accordance with the present invention. 
     
    
    
       FIG. 1  is a schematic diagram of a motion detection system in accordance with the present invention. In this example, a transmitter transmits a signal detected by a receiver, which determines when the receiver is in a null point and stationary with respect to the transmitter. Referring to  FIG. 1 , in one embodiment, the motion detection system  20  includes a transmitter  30  and a receiver  40 . The transmitter  30  transmits a source signal  32  including source troughs  34  at which the source signal  32  is a minimum. The receiver  40  is operable to detect signals at the carrier frequency of the source signal  32 . In some embodiments, the transmitter  30  can transmit signals over a range of carrier frequencies and the receiver  40  detects signals over a range of carrier frequencies, so the motion detection system  20  can shift carrier frequencies during operation. The source signal  32  reflects from an interfering object  50  as a reflected signal  52  including reflected peaks  54  at which the reflected signal  52  is a maximum. Superposition of the source signal  32  and the reflected signal  52  results in variations in signal strength about the transmitter  30  and receiver  40 . Null points  36  occur when a source trough  34  intersects with a reflected peak  54 . The signal strength at the null points  36  is minimal because the source signal  32  and reflected signal  52  cancel each other. 
     Interference between the source signal  32  and the reflected signal  52  creates the null points  36 . The null points  36  tend to be small in size (typically a few centimeters or less for a 2.4 GHz signal), which makes the position of the null point sensitive to even a very small movement of the transmitter  30 , the receiver  40 , and/or the interfering object  50 . When the receiver  40  is located in a null point, a very small movement of the receiver  40  moves the receiver  40  out of the null point. In addition, an object moving into the area around the transmitter  30 , interfering object  50 , or the receiver  40  can interfere with the source signal  32  and/or the reflected signal  52 , causing the null point to move or disappear. Once a receiver is identified as being in a null point, the receiver can be determined to be in a null point and stationary with respect to the transmitter when the signal strength of the detected signal is less than the expected signal strength for a predetermined number of detected signals. 
     The transmitter  30  and/or the receiver  40  can be fixed or moveable as desired for a particular application. In one embodiment, the motion detection system  20  includes a number of transmitters and/or receivers. The transmitters and/or receivers are located within an area, i.e., the transmitters and/or receivers are located to communicate with each other and establish a field including null points. The transmitter  30  and the receiver  40  can be combined in a single radio frequency (RF) unit when there are a number of transmitters and/or receivers. The transmitter  30  and the receiver  40  can communicate using any desired protocol, such as a ZigBee protocol operating on top of the IEEE 802.15.4 wireless standard, WiFi protocol under IEEE standard 802.11 (such as 802.11b/g/n), Bluetooth protocol, Bluetooth Low Energy protocol, or the like. In one embodiment, the transmitters and/or receivers can be arranged in a predetermined pattern, such as approximate collocation of at least three transmitters and/or receivers to assure that the area of interest is covered by the source and reflected signals. 
     Approximate collocation as defined herein as arrangement of at least three transmitters and/or receivers so that at least two of the transmitters and/or receivers are unobstructed at any time, even when one of the transmitters and/or receivers is obstructed. Approximate collocation assures that at least two of the transmitters and/or receivers are available to process the signal even when an interfering object, such as a metal plate, wall, person, or other object, is near one of the transmitters or receivers and obstructs the signal to another transmitter or receiver. This assures that the motion detection system has sufficient information to estimate an expected signal strength when the expected signal strength is based on current or prior signals. In one embodiment, the approximately collocated transmitters and/or receivers are arranged along a line. In another embodiment, the approximately collocated transmitters and/or receivers are enclosed within a single enclosure. 
     In the example of  FIG. 1 , the transmitter  30  and the receiver  40  are located in the middle of an open space, so the line-of-sight signal strength of a message received from the receiver  40  at the transmitter  30  as the source signal  32  along a first signal path is a certain value X. When a metal plate, wall, person, or other reflective object is positioned near the transmitter  30  and receiver  40  as an interfering object  50 , a second signal path is created from the transmitter  30  to the receiver  40 , i.e., the signal path from the transmitter  30  to the interfering object  50  and from the interfering object  50  to the receiver  40 . The path length of the first and second signal paths are different. At some points, the source signal  32  and the reflected signal  52  combine positively, producing a signal larger than the certain value X (perhaps even twice X). At other points, the source signal  32  and the reflected signal  52  are out of phase, producing a signal smaller than the certain value X (perhaps even a null signal). The receiver  40  is in a null position with respect to the transmitter  30  when the signal at the receiver  40  is at or near a null. Those skilled in the art will appreciate that  FIG. 1  is a simplification of the situation typically present for a motion detection system. Typically, a number of reflecting objects, such as several walls, are present at any location, so the null points occur in a varied and irregular pattern. The null points are very small, e.g., a few centimeters or less for a 2.4 GHz signal, making them useful for detecting small motions and/or lack of motion. 
       FIG. 2  is a block diagram of a radio frequency (RF) unit for use with a motion detection system and method in accordance with the present invention. In this example, the RF unit can be a transmitter, a receiver, or a transmitter and receiver, and can be moveable or fixed. The motion detection system includes a first device, such as a transmitter, operable to transmit a signal; a second device, such as a receiver, operable to detect the signal; and a processor operable to determine whether signal strength of detected signals at the second device is less than an expected signal strength, and operable to determine that the second device is in a null point when the signal strength of the detected signals is less than the expected signal strength for a predetermined number of the detected signals. In one embodiment, the second device is one of a number of second devices, the expected signal strength is the greatest signal strength detected by the number of second devices, and the second device is determined to be in the null point when the signal strength of the detected signal at the one of the number of second devices is less than the expected signal strength less a predetermined signal strength offset for the predetermined number of detected signals. 
     The RF unit  70  includes memory storage  72 , a processor  74 , a transmitter portion  76 , and a receiver portion  78 . The memory storage  72  can be any memory storage suitable for storing data and/or instructions. The memory storage  72  exchanges information with the processor  74 , which controls operation of the RF unit  70 . The transmitter portion  76  and receiver portion  78  communicate wirelessly with other RF units and/or central control centers, and can include antennas. The transmitter portion  76  can receive data and instructions from the processor  74 , and transmit a signal from the RF unit  70 . In one embodiment, the transmitter portion  76  is responsive to a command signal from the processor  74  to reduce transmission frequency when the processor  74  determines the receiver is in a null point and stationary with respect to the transmitter. Transmission frequency is defined herein as how often the transmitter transmits and is independent of the carrier frequency. The receiver portion  78  can receive a signal from outside the RF unit  70 , and provide data and instructions to the processor  74 . In one embodiment, the receiver portion  78  is responsive to a command signal from the processor  74  to reduce reception frequency when the processor  74  determines the receiver is in a null point and stationary with respect to the transmitter. Reception frequency is defined herein as how often the receiver receives and is independent of the carrier frequency. Reducing the transmission and/or reception frequency conserves power and extends battery life. The receiver needs to receive less often when the transmitter sends less often, so the receiver can be turned off when no signal is expected. 
     The RF unit  70  can operate as a transmitter, a receiver, or a transmitter and receiver. In one embodiment, the transmitter portion  76  can be omitted and the RF unit  70  operated as a receiver. In another embodiment, the receiver portion  78  can be omitted and the RF unit  70  operated as a transmitter. In one embodiment, the RF unit  70  operates under the ZigBee communications protocol operating on top of the IEEE 802.15.4 wireless standard. Those skilled in the art will appreciate that the RF unit  70  can operate under any wireless protocol desired for a particular application. In other embodiments, the RF unit  70  operates under the WiFi protocol under IEEE standard 802.11 (such as 802.11b/g/n), Bluetooth protocol, Bluetooth Low Energy protocol, or the like. When the RF unit  70  is both a transmitter and receiver, the receiver portion  78  can be turned off when the receiver portion  78  does not expect and/or need to receive a signal. The RF unit can be associated with another object, such as a lighting fixture, lighting control unit, asset to be tracked, a medical patient, or any other object. The RF unit can also control and/or monitor the associated object. 
     The RF unit  70  can send and receive signals at a single carrier frequency or at a number of carrier frequencies. Wavelength changes with carrier frequency, so the locations of the null points are different at different carrier frequencies. In one embodiment, the processor  74  can switch operation of the RF unit  70  between different carrier frequencies, so that the transmitter portion  76  is operable to transmit the signal at different carrier frequencies. Different null points can be found at different locations for different carrier frequencies by switching carrier frequencies for the RF units in the motion detection system. The processor  74  can be operable to determine that a receiver is in a null point when the signal strength of the detected signal is less than the expected signal strength for a predetermined number of detected signals at least one of the different carrier frequencies. 
     The processor  74  can be operable to allow the motion detection system to take a predetermined action when the receiver is determined to be in a null point and stationary with respect to the transmitter. In one embodiment, the processor  74  is operable to measure the time the receiver is determined to be in a null point and stationary with respect to the transmitter. The processor  74  can also be operable to initiate an alarm when the time the receiver is determined to be in a null point and stationary with respect to the transmitter is greater than a predetermined time. In another embodiment, the processor  74  is operable to detect an increase of the signal strength of the detected signal when the receiver is determined to be in a null point and stationary with respect to the transmitter. Such an increase can indicate the presence of a body near the transmitter and/or receiver which changes the location of the null point. 
       FIG. 3  is a block diagram of a motion detection system in accordance with the present invention. In this example, the motion detection system  80  includes a number of RF units  82  in communication with each other as indicated by the dashed lines. In one embodiment, at least some of the RF units  82  communicate with each other wirelessly. In another embodiment, at least some of the RF units  82  are hard wired to communicate with each other. At least one of the RF units  82  can also be in communication with an optional control unit  84 . In another embodiment, the optional control unit  84  can be included in one of the RF units  82 . The relative position of the RF units  82  and reflecting objects in their vicinity results in null points around the motion detection system  80 . The RF units  82  can be fixed or moveable as desired for a particular application. In one embodiment, at least some of the RF units  82  are contained in a single housing. 
       FIG. 4  is a flowchart of a motion detection method in accordance with the present invention. The method  100  includes transmitting a signal  102 , such as transmitting a signal from a transmitter; detecting the signal at a first device  104 , such as a receiver; determining whether signal strength of the detected signal is less than an expected signal strength  106 ; transmitting at least one additional signal  108 , such as transmitting at least one additional signal from the transmitter; detecting the at least one additional signal at the first device  110 ; determining whether signal strength of the detected at least one additional signal is less than the expected signal strength  112 ; and determining that the first device is in a null point  114  when the signal strength of the detected signals is less than the expected signal strength for a predetermined number of the detected signals. The method  100  can be carried out with a motion detection system as described in  FIGS. 1-3  above. 
     Referring to  FIG. 4 , the first device, such as a receiver, can be one of a number of first devices, the expected signal strength can be the greatest signal strength detected by the first devices, so that one of the first devices is determined to be in the null point and stationary with respect to the transmitter when the signal strength of the detected signal at the one of the first devices is less than the expected signal strength less a predetermined signal strength offset for the predetermined number of detected signals. In one example, the predetermined signal strength offset is 15 dB. In another embodiment, the transmitting a signal comprises transmitting a signal from at least one of a number of second devices, such as a number of transmitters; the first device, such as a receiver, is one of a number of first devices; and each of the first devices is associated with one of the second devices as a radio frequency (RF) unit. Those skilled in the art will appreciate that there are different ways to determine the expected signal strength. In one embodiment, the expected signal strength is based on previous values of the detected signal strength, such as the previous value, an average of a number of the previous values, or a time weighted average of the previous values. In one embodiment, the expected signal strength is calculated by modeling the motion detection system and its surroundings. In one embodiment, the predetermined number of detected signals can be a predetermined number of consecutive detected signals. 
     The method  100  can further include taking a predetermined action when the first device, such as a receiver, is determined to be in a null point and stationary with respect to the second device, such as a transmitter. In one embodiment, the predetermined action is reducing transmission frequency for the second device when the first device is determined to be in a null point. Reducing transmission frequency conserves power at the transmitter. In another embodiment, the predetermined action is reducing reception frequency for the first device when the first device is determined to be in a null point. Reducing reception frequency conserves power at the receiver. In another embodiment, the predetermined action is measuring a time the first device is determined to be in the null point, and optionally initiating an alarm when the time measured is greater than a predetermined time. Measuring the time permits analysis of the time a tracked movable component attached to either the transmitter or receiver spends at a fixed location. This can be used to study how long a part is in an assembly station or how long a medical patient is resting quietly in bed. Initiating an alarm provides notice of a condition of concern when the movable component has not moved for a predetermined time, such as when the part has not moved from the assembly station or the medical patient has not been active. 
     The method  100  can further include detecting an increase of the signal strength of the detected signal when the first device is determined to be in the null point. When the receiver is determined to be in the null point and stationary with respect to the transmitter, an increase in signal strength can indicate the presence of a body near the transmitter and/or receiver which changes the location of the null point. The motion detection system can be used as an occupancy detector when the receiver is in a fixed position with respect to the transmitter. 
     The transmitting at least one additional signal  108  can further include transmitting signals of different carrier frequencies. The null points are at different locations at different carrier frequencies, so a receiver can be in a null point with respect to the transmitter at one carrier frequency and not in a null point with respect to the transmitter at a different carrier frequency. Shifting signals over a number of carrier frequencies can find different null points at different carrier frequencies, which can then be used to determine when the receiver is in a null point and stationary with respect to the transmitter. In one embodiment, the transmitting is performed a number of times at a carrier frequency, then the transmitting is performed a number of times at another carrier frequency different from the original carrier frequency. 
     In another embodiment, the carrier frequency is changed after each signal transmission, so that the signal is transmitted at a first carrier frequency, then a second carrier frequency, then a third carrier frequency, et cetera. The transmitting can be performed for a predetermined number of carrier frequencies to determine the expected signal strength. For example, the expected signal strength can be the highest signal strength detected for the different carrier frequencies. In another example, the expected signal strength can be a statistical product of the signal strengths detected over the predetermined number of carrier frequencies, such as the average of the signal strengths detected over the predetermined number of carrier frequencies. When the detected signal strength at one of the carrier frequencies is less than the expected signal strength less a predetermined signal strength offset, that carrier frequency can be identified as being associated with a null point. For an example using five as the predetermined number of carrier frequencies, the sequential signal strengths detected for different carrier frequencies could be −10, −11, −40, −5, and −10. The expected signal strength can be the highest signal strength detected, i.e., −5. The carrier frequency with a detected signal strength of −40 indicates a carrier frequency associated with a null point, because the detected signal strength of −40 is less than the expected signal strength of −5 less a predetermined signal strength offset, such as −15. The detected signal strength at the carrier frequency associated with a null point can be checked for a predetermined number of detected signals to determine whether the receiver is in a null point and stationary with respect to the transmitter. Those skilled in the art will appreciate that null points can occur for one receiver and transmitter pair at multiple carrier frequencies. 
     One implementation of the method uses two signals as the predetermined number of detected signals for which it is determined that the receiver is stationary with respect to the transmitter. The method includes transmitting a first signal, such as transmitting a first signal from a transmitter; detecting the first signal at a number of first devices, such as a number of receivers; determining a greatest signal strength of the first signal detected by the number of first devices; and determining that one of the number of first devices is in a null point when signal strength of the detected first signal at the one of the number of first devices is less than the greatest signal strength less a predetermined signal strength offset. The method further includes transmitting a second signal, such as transmitting a second signal from the transmitter; detecting the second signal at the number of first devices, such as the number of receivers; and determining that the one of the number of first devices is in the null point when signal strength of the detected second signal at the one of the number of first devices is less than the greatest signal strength less the predetermined signal strength offset. The one of the number of first devices can be determined to be stationary when the one of the number of first devices is in the null point for the first signal and the second signal. Those skilled in the art will appreciate that the predetermined number of detected signals can be selected to any number as desired for a particular application considering such factors as interference, environment, the selected predetermined signal strength offset, the number of approximately collocated receivers available, the degree of control over carrier frequency (e.g., the number of frequency channels used), the relative impacts of a false positive or false negative reading, and the like. 
     While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.