Abstract:
A rotation sensor configured to be mounted on a rim of a wheel. The rotation sensor includes a band sized and shaped to fit around the rim of the wheel, a first element mounted on the band that generates a first time-varying electrical signal in response to a rotational movement, a second element mounted on the band that generates a second time-varying electrical signal in response to the rotational movement, a processor mounted on the band that receives the first and second time-varying electrical signals and processes the first and second time-varying electrical signals to determine a rotational speed, and a rechargeable power source that receives the first and second time-varying electrical signals, consumes at least a portion of the first and second time-varying electrical signals to recharge the rechargeable power source, and generates a power signal. The processor is connected to the rechargeable power source to receive the power signal.

Description:
BACKGROUND 
     The present invention relates to rotation sensors. More particularly, the invention relates to rotation sensors that detect wheel rotation, from which the speed of a vehicle can be determined. 
     Some rotation sensors include components (often targets) in the wheel or wheel rim and other components (that process information from the target) that are located on the chassis. The rotation sensors determine the time it takes for targets to pass the sensor. In some technologies, the angular separation of the targets and the elapsed time is used to determine the speed of the wheel. 
     SUMMARY 
     A number of challenges are created by the location of the targets, which are in or on a rotating member (e.g., the wheel), and the sensing elements, which are in or on a non-rotating member (e.g., the chassis). First, since there is relative motion between the components, a simple wired connection between the two can not be used. Either a slip ring (or similar connector) or a wireless connection must be used. Second, in many instances, the placement of the sensor components exposes them to the environment (e.g., water, snow, cold, dirt, dust, stones, rocks, and the like.). The sensor components may also be exposed to heat from the vehicle brakes. A third challenge relates to providing power to the sensor components. Components located on a vehicle chassis can, in many instances, be connected to a vehicle power system. However, providing power to sensor elements located on rotating components is difficult, because, as was noted, a simple wire connection can not be used between a rotating component (e.g., a sensor target) and a stationary component (e.g., the vehicle power system). Currently, many sensor components are battery-powered (by a battery that is separate from the vehicle battery) to avoid having to transmit power from the vehicle power system to the rotating sensor element. To meet the goals of vehicle manufacturers, such elements must operate for 100,000 miles or 10 years. However, many batteries are not capable of meeting this requirement. 
     To overcome at least some of these disadvantages, the inventors have developed a technology where the rotation sensor is located entirely in the wheel. The rotation sensor includes a rechargeable power source (e.g., conversion equipment and a battery) and an onboard power generator that recharges the storage device. The storage device provides power to a microprocessor and wireless transmitter. The rotation sensor includes no sensing elements that require power. Rather, the rotation sensor includes power-generating elements that generate a voltage or signal when subjected to mechanical deformation, such as bending. The microprocessor receives the signals produced by the power-generating elements. The power generating elements have a dual function and also act as sensing elements. The microprocessor processes the signals from the power-generating elements to determine rotation information. Ultimately, this information is used to determine vehicle speed. The signals from the power-generating devices are also provided to the rechargeable power source. 
     In one embodiment, the invention provides a rotation sensor configured to be mounted on a rim of a wheel. The rotation sensor includes a band, sized and shaped to fit around the rim of the wheel. A first element, a second element, and a processor are mounted on the band. The first element generates a first time-varying electrical signal in response to a rotational movement. The second element generates a second time-varying electrical signal in response to the rotational movement. The processor receives the first and second time-varying electrical signals and processes them to determine a rotational speed. The rotation sensor also includes a rechargeable power source that receives the first and second time-varying electrical signals. The rechargeable power source consumes at least a portion of the first and second time-varying electrical signals to recharge the rechargeable power source. The power source also outputs a power signal to the processor. 
     In another embodiment, the invention provides a rotation sensing system for determining a rotational speed of a wheel of a vehicle. The rotation sensing system includes a wheel that rotates with respect to the vehicle, and the wheel includes a rim. The rim is substantially cylindrically shaped with an inner surface and an outer surface, has a substantially circular cross-sectional area, and is operable to rotate about an axis that passes substantially through a center of the substantially circular cross-sectional area. A tire surrounds the rim, and the tire and the rim form an airtight space therebetween. The rotation sensing system also includes a rotation sensor coupled to the outer surface of the rim and positioned in the airtight space. The rotation sensor includes two sensing elements (a first element and a second element). Each element is positioned on the outer surface of the rim and generates a time-varying electrical signal in response to rotation of the wheel. A processor receives the time-varying signals from the elements and processes the time-varying signals to determine the rotational speed. The rotation sensing system also includes a rechargeable power source that provides power to the processor. The rechargeable power source takes the form of or includes a power storage device and receives the first and second time-varying electrical signals to recharge the power storage device. 
     In another embodiment, the invention provides a method of sensing an angular speed of a wheel of a vehicle. The method includes generating a first time-varying signal with a first element in response to a rotation of the wheel, generating a second time-varying signal with a second element in response to the rotation of the wheel, providing at least one of the first time-varying signal and the second time-varying signal to a rechargeable power source to charge the rechargeable power source, providing the first time-varying signal and the second time-varying signal to a processor, providing a power signal to the processor, and comparing the first time-varying signal and the second time-varying signal to determine a difference between the first time-varying signal and the second time-varying signal, the difference at least partially indicative of a rotational speed of the wheel. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of a wheel illustrating one mechanism for mounting a rotation sensor on a rim. 
         FIG. 2  is a sectional view of the wheel of  FIG. 1  and illustrates a rotation sensor. 
         FIG. 3  is a schematic illustrating one embodiment of a circuit for the rotation sensor of  FIG. 2 . 
         FIG. 4  is a schematic illustrating a second embodiment of a circuit that may be used with the rotation sensor of  FIG. 2 . 
         FIG. 5  is a schematic illustrating four possible positions of sensing elements on the rim. 
         FIG. 6  is a flowchart of one embodiment of logic that a processor may use to process signals received from the sensing elements. 
         FIG. 7  is a continuation of the flowchart of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 1  is an exploded view of a wheel  10  having a rim  14 . The rim  14  is designed to have a tire  18  mounted thereon. When the tire  18  is mounted on the rim  14  a defined, toroidally-shaped space is created between the tire  18  and the rim  14 . A rotation sensor  19  is mounted on the rim  14  within the space between the rim  14  and the tire  18 . As best seen by reference to  FIG. 3 , the rotation sensor  19  includes a plurality of sensing elements  26 ,  30 ,  34 , and  38 , a microprocessor  42 , a rechargeable power source  46 , and a wireless transmitter  50 . 
     Referring back to  FIG. 1 , the components of the rotation sensor  19  are mounted on a band  22  that is fitted to the rim  14  and secured thereto to substantially prevent movement of the rotation sensor  19  with respect to the rim  14 . The band  22  may be formed of an elastic material, metal, plastic, etc. The band  22  may be secured in place by for example, an adhesive, tightening of the band  22  around the rim  14 , a combination of both, or other suitable means. In other embodiments, the rotation sensor  19  may not include a band  22  and the components may be mounted directly to the rim  14  via, for example, soldering, adhesive, pins, or other suitable means. 
     Preferably, the rotation sensor  19  is configured for easy attachment and removal from the rim  14  such as by a band  22  that may be tightened around and loosened from the rim. A suitable band  22  may fasten with, for example, hook-and-loop fasteners or other fasteners that can be tightened or loosed with hand tools. Thus, when the rim  14  and tire  18  are replaced during the lifetime of the vehicle, the rotation sensor  19  may be removed from the rim  14  by loosening the band  22 . The band  22  and rotation sensor  19  may then be placed around a new rim and tightened to secure the rotation sensor in position before a new tire is mounted on the rim. Due to the variety of aftermarket rims and tires, which are available in different designs and sizes, a removably mounted rotation sensor  19  is desirable so it can be removed from an original rim  14  and mounted on a desired aftermarket rim. 
     As noted, the rotation sensor  19  includes a plurality of sensing elements and, in the embodiment illustrated in  FIG. 3 , the sensor includes four elements:  26 ,  30 ,  34 , and  38 . The sensing elements  26 ,  30 ,  34 , and  38  are piezoelectric elements that produce a voltage when deformed. The magnitude of the voltage varies with the amount of deformation of the piezoelectric element  26 ,  30 ,  34 , and  38 . The polarity of the voltage produced varies with the direction of the deformation. The voltage produced may vary in time if the piezoelectric element  26 ,  30 ,  34 , and  38  is subjected to deformations that vary in time. Thus, the voltages produced in time may be referred to as an information signal because the time-varying changes may provide information about the rim  14  (such as forces that are acting upon it, as is discussed in greater detail below). In other embodiments, the rotation sensor  19  may include as little as two sensing elements or may include more than four sensing elements. It is preferable that the rotation sensor  19  include an even number of sensing elements positioned opposite each other along the circumference of the rim  14  such that pairs of elements may be identified. For example, as illustrated in  FIG. 5 , the first sensing element  26  and the third sensing element  34  form one sensing pair and the second sensing element  30  and the fourth sensing element  38  form a second sensing pair. 
     The microprocessor  42  receives and processes information signals from the sensing elements  26 ,  30 ,  34 , and  38 . The microprocessor  42  may process the information signals according to a predefined logic, as illustrated in  FIGS. 6-8 , to determine information about the wheel  10  that may be used to determine wheel speed. In the present embodiment, the microprocessor  42  determines a wheel speed for every quarter turn (90 degrees of rotation) of the wheel  10 . 
     As best seen by reference to  FIG. 4 , the rechargeable power source  46  (which may take the form of a storage device such as a rechargeable battery) is connected to the microprocessor  42  and provides power to the microprocessor  42 . Each sensing element  26 ,  30 ,  34 , and  38  is connected in parallel across the battery  46  and the microprocessor  42 . The battery  46  is configured to receive the signals produced by the sensing elements  26 ,  30 ,  34 , and  38  and use those signals to recharge the battery  46 . In some embodiments, the signals generated by the sensing elements  26 ,  30 ,  34 , and  38  are processed in a conditioning circuit (e.g., rectifying diodes  66 ,  70 ,  72 , and  76 ), as illustrated in  FIG. 4 , and then provided to the battery  46  as a power signal. Thus, no additional power source is required for the rotation sensor  19  to operate. The battery  46  may be recharged during normal use of the sensor  10 . 
     As illustrated in  FIG. 3 , the wireless transmitter  50  communicates with a vehicle electronic control unit (ECU)  62  of the vehicle. The microprocessor  42  processes the information signals received from the sensing elements  26 ,  30 ,  34 , and  38  and determines a wheel speed. The transmitter  50  wirelessly communicates with a receiver  58  to send the wheel speed to the vehicle ECU  62 . The ECU  62  may use the wheel speed information in other systems, such as speedometers, vehicle stability control systems, and traction control systems. The transmitter  50  may also transmit the output signal to other devices that require rotation information. 
       FIG. 5  schematically illustrates placement of the sensing elements  26 ,  30 ,  34 , and  38  on the rim  14 . For convenience, the positions of the sensing elements  26 ,  30 ,  34 , and  38  will be referred to as 0 degrees, 90 degrees, 180 degrees, and 270 degrees. For example, in position A the first sensing element  26  is at 0 degrees, the second sensing element  30  is at 90 degrees, the third sensing element  34  is at 180 degrees, and the fourth sensing element  38  is at 270 degrees. In position B, the wheel (i.e., tire  18  and rim  14 ) has rotated 90 degrees clockwise with respect to position A. In position C, the wheel has rotated 180 degrees clockwise with respect to position A. In position D, the wheel has rotated 270 degrees clockwise with respect to position A. Thus, each sensing element  26 ,  30 ,  34 , and  38  rotates 90 degrees clockwise from position A to position B, from position B to position C, from position C to position D, and from position D to position A. 
     Signals produced by each sensing element  26 ,  30 ,  34 , and  38  vary with the position of the sensing element. Gravity acts on the tire  18 , rim  14 , and sensing elements  26 ,  30 ,  34 , and  38  in the direction of the arrow G shown  FIG. 5 . When the wheel  10  is stationary and in position A, the first sensing element  26  is oriented horizontally, whereby gravity bends the first sensing element  26  toward the center of the rim  14 , which causes the sensing element  26  to output a positive voltage. The third sensing element  34 , also oriented horizontally, bends away from the center of the rim  14  due to the force of gravity and outputs a negative voltage. The second and fourth sensing elements  30  and  38  output substantially zero voltage because the second and fourth sensing elements  30 ,  38  are oriented vertically and the force of gravity does not cause either the second or fourth sensing elements  30  and  38  to bend. 
     As the wheel  10  turns, the sensing elements  26 ,  30 ,  34 , and  38  are positioned as shown in position B of  FIG. 5 . In this position, the fourth sensing element  38  outputs a positive voltage, the second sensing element  30  outputs a negative voltage, and the first and third sensing elements  26 ,  34  output substantially zero voltage. Similarly, when the wheel  10  turns another 90 degrees, the sensing elements  26 ,  30 ,  34 , and  38  are positioned as shown in position C, and after another 90 degrees, the sensing elements  26 ,  30 ,  34 , and  38  are positioned as shown in position D. In general (and when considering gravity alone), the sensing element in the 0 degree position outputs a positive voltage, the sensing element in the 180 degree position outputs a negative voltage, and the sensing elements in the 90 degree and 270 degree positions output a substantially zero voltage. Thus, as the wheel  10  rotates, the outputs of the sensing elements  26 ,  30 ,  34 , and  38  change. As discussed in greater detail below, forces other than gravity may act on the sensing elements  26 ,  30 ,  34 , and  38 . 
     During rotation of the wheel  10 , centrifugal forces are exerted on the sensing elements  26 ,  30 ,  34 , and  38 , changing the outputs of the sensing elements  26 ,  30 ,  34 , and  38 . During driving, other events may cause other forces to be exerted on the sensing elements  26 ,  30 ,  34 , and  38 . The forces may be generated as a result of traveling over bumps, braking, acceleration, collisions, etc. As will be discussed below, these events affect the information signals received by the microprocessor  42  and are accounted for during the processing of the signals. 
       FIGS. 6 and 7  illustrate one example of logic used by the microprocessor  42  to determine wheel speed in rotations per minute (rpm) from the signals received from the sensing elements  26 ,  30 ,  34 , and  38 . In general, the microprocessor  42  detects 90 degree increments of rim  14  rotation and an associated elapsed time in seconds (sec). From that information, the microprocessor  42  calculates wheel speed using the following equation: 
     
       
         
           
             
               
                 
                   
                     Wheel 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Speed 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       rpm 
                       ) 
                     
                   
                   = 
                   
                     60 
                     
                       4 
                       ⁢ 
                       
                         ( 
                         
                           Elapsed 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           Time 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ( 
                             sec 
                             ) 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     For simplicity, the output voltage due to gravity that is produced by a sensing element oriented in the 0 degree position is assigned an arbitrary value of +1 g, and the output voltage due to gravity that is produced by a sensing element oriented in the 180 degree position is assigned an arbitrary value of −1 g. Sensing elements positioned in the 90 and 270 degree positions output voltages due to gravity that are assigned values of 0 g. Centrifugal forces due to the turning of the wheel are applied substantially evenly, in a radially outward direction, on the sensing elements as the wheel  10  rotates and cause the sensing elements to each output a more negative voltage. 
       FIGS. 6 and 7  illustrate the logic used by the microprocessor  42 . Of course, in other embodiments, the microprocessor  42  can use a different logic. The logic illustrated in  FIGS. 6 and 7  is used for exemplary purposes and is not intended to limit the present invention. 
     At block or step  100  of  FIG. 6 , the microprocessor  42  receives the information signals corresponding to the sensing elements  26 ,  30 ,  34 , and  38  and records a current time t n  when the information is received. The current time t n  corresponds to an internal clock of the microprocessor  42 . 
     At block or step  104 , the microprocessor  42  converts the information signals corresponding to the sensing elements  26 ,  30 ,  34 , and  38  into values S 1 , S 2 , S 3 , and S 4 , respectively, wherein each value has a unit of g. Thus, S 1  represents the information signal corresponding to sensing element  26  in units of g, S 2  represents the information signal corresponding to sensing element  30  in units of g, S 3  represents the information signal corresponding to sensing element  34  in units of g, and S 4  represents the information signal corresponding to sensing element  38  in units of g. 
     There are six possible pairs that can be formed from the values S 1 , S 2 , S 3 , and S 4 . The absolute values of the differences between the values of each pair are calculated at block or step  108  and include |S 1 -S 3 |, |S 2 -S 4 |, |S 1 -S 2 |, |S 1 -S 4 |, |S 3 -S 2 |, and |S 3 -S 4 |. The absolute values can be subdivided into a first and second group. The first group includes the values that correspond to sensors positioned opposite each other, or 180 degrees apart. Thus, the first group includes the absolute values |S 1 -S 3 | and |S 2 -S 4 |. The second group includes the other pairs of values, namely, the absolute values |S 1 -S 2 |, |S 1 -S 4 |, |S 3 -S 2 |, and |S 3 -S 4 |. 
     It was empirically determined that at a point in time, the absolute values in the second group are all equal to each other when there is no horizontal acceleration of the wheel  10 . However, when the wheel  10  experiences horizontal acceleration, the absolute values in the second group are not all equal to each other. More specifically, the values |S 1 -S 2 | and |S 3 -S 4 | are equal to each other, and the values |S 1 -S 4 | and |S 2 -S 3 | are equal to each other but different from the values of |S 1 -S 2 | and |S 3 -S 4 |. Thus, the absolute values of the second group |S 1 -S 2 |, |S 1 -S 4 |, |S 3 -S 2 |, and |S 3 -S 4 | are compared to each other at block or step  112  to determine if horizontal acceleration is present (block or step  116 ) or absent (block or step  120 ). When the wheel  10  does not experience horizontal acceleration, the microprocessor  42  defines a threshold TH as 2|S 1 -S 2 |g. 
     When the wheel  10  experiences horizontal acceleration, without experiencing any vertical acceleration, then there is no effect on the values corresponding to the sensors positioned in the 0 and 180 degree positions. Thus, the absolute difference between the values corresponding to the sensors positioned in the 0 and 180 degree positions is equal to 2 g. For example, when the rotation sensor  19  is orientated as shown in position A of  FIG. 3  and assuming each sensing element  26 ,  30 ,  34 , and  38  experiences a centrifugal force in units of g (Cg), the microprocessor  42  converts the information signals into (1−C)g for the first sensing element  26 , and (−1−C)g for the third sensing element  34  at block or step  104 . Then, the absolute value of the difference between the first sensing element  26  and the third sensing element  34  is equal to |S 1 -S 3 |=|(1−C)g−(−1−C)g|=2 g. Regardless of the amount of centrifugal force Cg experienced by each of the sensing elements  26 ,  30 ,  34 , and  38 , the absolute difference between the sensing elements positioned in the 0 and 180 degree positions is substantially equal to 2 g. 
     However, when the wheel  10  experiences both horizontal and vertical acceleration, all of the values S 1 , S 2 , S 3 , and S 4  are affected and the values corresponding to the sensors positioned in the 0 and 180 degree positions are not equal to 2 g. Thus, at block or step  128 , the microprocessor  42  compares the values in the first group to determine the presence or absence of vertical motion. Specifically, the microprocessor  42  determines if |S 1 -S 3 |=2 g or if |S 2 -S 4 |=2 g (block or step  128 ). When no vertical acceleration is detected (block or step  132 ), the microprocessor  42  defines the threshold TH to be equal to 2 g. When the microprocessor  42  determines that vertical acceleration is present (block or step  140 ), the microprocessor  42  calculates and compares a predefined set of values to determine an appropriate threshold value. 
     The predefined set of values was determined empirically and includes the following eight values: 2|S 1 -S 2 |±|S 2 -S 4 |, 2|S 1 -S 4 |±|S 2 -S 4 |, 2|S 1 -S 2 |±|S 1 -S 3 |, 2|S 1 -S 4 |±|S 1 -S 3 |. The microprocessor  42  identifies equal value pairs from the results, at block or step  144 . Of the equal value pairs identified, the microprocessor  42  determines the greatest absolute value (block or step  152 ) and assigns it to the threshold TH (block or step  154 ). 
     At block or step  158 , the microprocessor  42  determines if |S 1 -S 3 |=TH. If yes, the microprocessor  42  assumes that the sensing element corresponding to S 1  and the sensing element corresponding to S 3  are positioned in the 0 and 180 degree positions. If no, the microprocessor  42  determines if |S 2 -S 4 |=TH (block or step  166 ). If no, then neither the sensing elements corresponding to the values S 1  and S 3  nor the sensing elements corresponding to the values S 2  and S 4  are in the 0 and 180 degree positions. Thus, the microprocessor  42  determines an error and disregards the information signals received. If |S 2 -S 4 |=TH, then the microprocessor  42  knows that the sensing elements corresponding to the values S 2  and S 4  are in the 0 and 180 degree positions. 
     After the microprocessor  42  determines whether the sensing elements corresponding to the values S 1  and S 3  are positioned in the 0 and 180 degree positions (block or step  162 ) or the sensing elements corresponding to the values S 2  and S 4  are positioned in the 0 and 180 degree positions (block or step  174 ), the microprocessor  42  determines whether the wheel rotated 90 degrees, as shown at block or step  178 . The microprocessor  42  compares the current state to the previous state to determine whether the wheel  10  rotated 90 degrees. If the wheel  10  did not rotate 90 degrees, the data is discarded. If the wheel  10  did rotate 90 degrees, the microprocessor  42  calculates the elapsed time t n -t n−1 , where t n−1  is the time at which one sensing element pair is in the 0 and 180 degree positions and t n  is the time at which the other sensing element pair is in the 0 and 180 degree positions (block or step  186 ). The microprocessor  42  uses the elapsed time and Equation 1 to calculate the wheel speed, as shown at block or step  190 , for the current time period. The wheel speed is wirelessly transmitted to the vehicle ECU  62  or other vehicle systems, as described above, for further processing. 
     Thus, the invention provides, among other things, a rotation sensor that determines rotational information about a wheel  10  mounted on a vehicle. Various features and advantages of the invention are set forth in the following claims.