Acceleration detecting device including a circuitry configuration having a set of calculations during one rotation of a wheel assembly

An acceleration detecting device includes a power source, an acceleration sensor, an acquiring section, a rotation period calculating section, and an acquisition period setting section. The acceleration sensor is configured to detect a centrifugal acceleration generated by rotation of a wheel assembly. The acquiring section is configured to acquire a detection value from the acceleration sensor with a predetermined acquisition period, thereby acquiring the detection value from the acceleration sensor each time the wheel assembly rotates a constant angle. The rotation period calculating section is configured to calculate a rotation period of the wheel assembly. The acquisition period setting section sets the acquisition period. The acquisition period setting section is configured to increase a number of times the detection value is acquired from the acceleration sensor per rotation of the wheel assembly as the rotation period calculated by the rotation period calculating section becomes longer.

TECHNICAL FIELD

The present invention relates to an acceleration detecting device.

BACKGROUND ART

As described in Patent Document 1, a tire condition monitoring apparatus, which monitors the condition of a tire, includes a transmitter provided in a wheel assembly and a receiver that receives transmission data transmitted from the transmitter. The transmitter includes an acceleration sensor, which detects a centrifugal acceleration generated by rotation of the wheel assembly, and a control section, which acquires a detection value from the acceleration sensor. The control section acquires the detection value of the acceleration sensor several times per rotation of the wheel assembly. The control section is capable of identifying the condition of the wheel assembly from the detection value of the acceleration sensor. The condition of the wheel assembly refers to, for example, whether the rotation angle of the wheel assembly has reached a specific angle or whether the wheel assembly is rotating.

PRIOR ART DOCUMENT

Patent Document

SUMMARY OF THE INVENTION

Problems that the Invention is to Solve

When the number of times the detection value is acquired from the acceleration sensor per rotation of the wheel assembly is small, the condition of the wheel assembly may not be properly acquired. The number of times the detection value is acquired from the acceleration sensor per rotation of the wheel assembly may be increased. This configuration, however, increases power consumption and thus potentially shortens the life of the power source.

Accordingly, it is an objective of the present invention to provide an acceleration detecting device that is capable of reducing power consumption.

Means for Solving the Problems

To achieve the foregoing objective and in accordance with a first aspect of the present invention, an acceleration detecting device is provided that includes a power source, an acceleration sensor, an acquiring section, a rotation period calculating section, and an acquisition period setting section. The acceleration sensor is configured to detect a centrifugal acceleration generated by rotation of a wheel assembly. The acquiring section is configured to acquire a detection value from the acceleration sensor with a predetermined acquisition period, thereby acquiring the detection value from the acceleration sensor each time the wheel assembly rotates a constant angle. The rotation period calculating section is configured to calculate a rotation period of the wheel assembly. The acquisition period setting section sets the acquisition period. The acquisition period setting section is configured to increase a number of times the detection value is acquired from the acceleration sensor per rotation of the wheel assembly as the rotation period calculated by the rotation period calculating section becomes longer.

The acquiring section acquires the detection value from the acceleration sensor a large number of times when the rotation period of the wheel assembly is long. On the other hand, the acquiring section acquires the detection value from the acceleration sensor a small number of times when the rotation period of the wheel assembly is short. Therefore, power consumption can be reduced compared to a case in which the number of times the detection value is acquired from the acceleration sensor is made constant regardless of the rotation period of the wheel assembly. In addition, since the number of times the detection value is acquired from the acceleration sensor increases when the rotation period is long, the acceleration detecting device is capable of properly acquiring the condition of the wheel assembly.

The above-described acceleration detecting device may include a specific angle detecting section that is configured to compare at least two detection values that are acquired consecutively by the acquiring section, and detect that the acceleration detecting device is located at a predetermined specific angle from change in fluctuation of the detection values.

This allows for detection of the rotation angle of the wheel assembly reaching the specific angle.

In the above-described acceleration detecting device, the specific angle detecting section may be configured to determine that the acceleration detecting device has passed a lowest position or a highest position in the wheel assembly when the change in fluctuation of the detection value switches from increasing to decreasing or from decreasing to increasing.

This allows for determination that the acceleration detecting device has passed the highest position or the lowest position.

The above-described may be attached to a back face of a tread portion of a tire. The acceleration detecting device may further include a ground contact determining section that is configured to determine that a section of the tread portion in which the acceleration sensor is located has contacted the ground when the detection value of the acceleration sensor has changed from a value greater than or equal to a predetermined ground contact determination threshold to a value less than the ground contact determination threshold.

This allows for proper determination that the part of the tread portion at which the acceleration sensor is located has contacted the ground.

Effects of the Invention

The present invention is capable of reducing power consumption.

MODES FOR CARRYING OUT THE INVENTION

An acceleration detecting device according to an embodiment will now be described.

As shown inFIG. 1, a tire condition monitoring system30is mounted on a vehicle10.

The vehicle10includes four wheel assemblies11. Each of the wheel assemblies11includes a wheel12and a tire13attached to the wheel12. The wheel assembly11on the right front side will be denoted by FR, the wheel assembly11at the left front side will be denoted by FL, the wheel assembly11at the right rear side will be denoted by RR, and the wheel assembly11at the left rear side will be denoted by RL.

The vehicle10includes an antilock braking system (ABS)20. The ABS20includes an ABS controller25and rotation sensors21to24, which respectively correspond to the four wheel assemblies11. The first rotation sensor unit21corresponds to the left front wheel assembly FL, and the second rotation sensor unit22corresponds to the right front wheel assembly FR. The third rotation sensor unit23corresponds to the left rear wheel assembly RL, and the fourth rotation sensor unit24corresponds to the right rear wheel assembly RR. The ABS controller25is configured by a microcomputer and the like and acquires the rotation angle of each of the wheel assemblies11based on signals from the rotation sensor units21to24.

As shown inFIG. 2, each of the rotation sensor units21to24includes a gear26, which rotates integrally with the wheel assembly11, and a detector27, which is arranged to face the outer circumferential surface of the gear26. The gear26has teeth arranged on the outer circumferential surface at constant angular intervals. The gear26has forty-eight teeth. The detector27detects pulses generated when the gear26is rotated. The ABS controller25is connected to the detector27by wire and obtains the rotation angle of each of the wheel assemblies11based on a pulse count value, which is a detection value of each of the detectors27. Specifically, the ABS controller25counts rising edges and falling edges of the pulses generated in the detector27. The ABS controller25calculates, as the pulse count value, the remainder when the counted number of pulses is divided by the number of pulses per rotation of the gear26(ninety-six). Also, the degree of rotation of the gear26per pulse count is obtained by dividing 360 degrees by the number of pulses generated by the detector27while the wheel assembly11rotates one rotation. In this manner, the rotation angle of the wheel assembly11is obtained from the pulse count value. The pulse count value is from 0 to 95.

The tire condition monitoring system30will now be described.

As shown inFIG. 1, the tire condition monitoring system30includes transmitters31and a receiver50. Each transmitter31is attached to one of the four wheel assemblies11. The receiver50is installed in the vehicle10. The transmitter31is attached to the wheel assembly11so as to be arranged in the inner space of the tire13. The transmitter31is of a type that is fixed to the tire valve, the wheel12, or the tire13. The transmitter31detects the condition of the corresponding tire13and wirelessly transmits transmission data including detected information of the tire13to the receiver50. The tire condition monitoring system30monitors the condition of the tire13by receiving the transmission data transmitted from the transmitter31through the receiver50.

As shown inFIG. 3, each of the transmitters31includes a pressure sensor32, a temperature sensor33, an acceleration sensor34, a transmission control section35, a transmission circuit36, a battery37, and a transmission antenna39. The transmitter31is driven by power supplied from the battery37, and the transmission control section35controls operation of the transmitter31in an integrated manner. The battery37, which is the power source of the transmitter31, may be a primary battery or a power storage device such as a rechargeable battery or a capacitor.

The pressure sensor32detects the air pressure of the corresponding tire13. The temperature sensor33detects the temperature inside the corresponding tire13.

The acceleration sensor34is installed so as to detect the centrifugal acceleration. The acceleration sensor34has a detection axis and detects acceleration in the direction along the detection axis. The acceleration sensor34is attached to the wheel assembly11such that the detection axis is directed in the vertical direction when the transmitter31is located at the lowest position of the wheel assembly11. The acceleration sensor34may be a uniaxial acceleration sensor34or a multiaxial acceleration sensor34as long as it is capable of detecting at least the centrifugal acceleration.

The transmission control section35is configured by a microcomputer or the like including a CPU35aand a storage section35b, which includes a RAM, a ROM, and the like. The transmission control section35includes a timing function. The timing function is implemented by, for example, a timer or a counter. The transmission control section35may include dedicated hardware (application specific integrated circuit: ASIC) that executes at least part of various processes. That is, the transmission control section35may be circuitry including 1) one or more processors that operate according to a computer program (software), 2) one or more dedicated hardware circuits such as an ASIC, or 3) a combination thereof. The processor includes a CPU and memory such as a RAM and ROM. The memory stores program codes or instructions configured to cause the CPU to execute processes. The memories, or computer readable media, include any type of media that are accessible by general-purpose computers and dedicated computers.

The storage section35bstores an ID code indicating individual identification information of each of the transmitters31. For the illustrative purposes, the ID code of the transmitter31attached to the left front wheel assembly FL is denoted by FLID, the ID code of the transmitter31attached to the right front wheel assembly FR is denoted by FRID, the ID code of the transmitter31attached to the left rear wheel assembly RL is denoted by RLID, and the ID code of the transmitter31attached to the right rear wheel assembly RR is denoted by RRID. The storage section35bstores various programs for controlling the transmitter31.

The transmission control section35generates transmission data and outputs the generated transmission data to the transmission circuit36. The transmission data is digital data and is a data string of binary numbers. The transmission circuit36modulates the transmission data. The modulated transmission data is transmitted from the transmission antenna39as a wireless signal. The wireless signal is a signal including the transmission data. The wireless signal is transmitted as a signal of an RF band, for example, a 315 MHz band or a 434 MHz band.

The transmitter31is capable of executing normal transmission, by which the transmission data is transmitted regardless of the rotation angle of the wheel assembly11, and specific angle transmission, by which the transmission data is transmitted when the rotation angle of the wheel assembly11is a predetermined specific angle.

In the normal transmission, the transmission data is transmitted from the transmitter31at a predetermined interval. The predetermined interval is set to, for example, ten seconds to several tens of seconds.

The specific angle transmission is performed, for example, when the vehicle10has been in a stopped state continuously for a predetermined time or longer. The predetermined time is set to time longer than time required for changing the positions of the wheel assemblies11such as in tire rotations or time required for replacing the wheel assemblies11. The predetermined time is set to, for example, several tens of minutes to several hours.

Whether the vehicle10is traveling can be determined based on the acceleration detected by the acceleration sensor34. The centrifugal acceleration acting on the acceleration sensor34increases as the vehicle speed increases. If the acceleration detected by the acceleration sensor34is greater than or equal to a travel determination threshold, the transmission control section35determines that the vehicle10is traveling. If the acceleration detected by the acceleration sensor34is less than the travel determination threshold, the transmission control section35determines that the vehicle10is in a stopped state. The travel determination threshold is set to a value greater than the acceleration detected by the acceleration sensor34when the vehicle10is in a stopped state while taking factors such as tolerances into consideration.

In the specific angle transmission, the transmitter31transmits the transmission data when the rotation angle of the wheel assembly11is detected to be the predetermined specific angle. Specifically, the transmission control section35transmits the transmission data from the transmitter31when the specific angle is detected and a predetermined time (for example, ten seconds to several tens of seconds) has elapsed since the last transmission of the transmission data.

The process performed by the transmission control section35when performing the specific angle transmission will now be described.

As shown inFIG. 4, the transmission control section35acquires the detection value of the acceleration sensor34in step S1. The transmission control section35functions as an acquiring section. Next, in step S2, the transmission control section35calculates the rotation period [seconds] of the wheel assembly11. Specifically, the transmission control section35calculates the rotation period of the wheel assembly11by using the following expression (1).

where S represents the rotation period [seconds] of the wheel assembly11, G represents the detection value [G] of the acceleration sensor34, and R represents the radius [mm] of the rim or the wheel12. The radius of the rim of the wheel12is stored in the storage section35b. The transmission control section35functions as a rotation period calculating section.

In step S3, the transmission control section35determines whether the rotation period is longer than or equal to a threshold. The threshold is used to determine whether the vehicle speed is low or high. The rotation period correlates with the vehicle speed, and the rotation period becomes shorter as the vehicle speed increases. For example, the threshold is set to a value that corresponds to the vehicle speed of 70 [km/h].

If the rotation period is greater than or equal to the threshold, the transmission control section35executes the process of step S4. If the rotation period is less than the threshold, the transmission control section35executes the process of step S5. In step S4, the transmission control section35sets an acquisition count to twenty times. In step S5, the transmission control section35sets the acquisition count to ten times. The acquisition count is the number of times the detection value is acquired from the acceleration sensor34per rotation of the wheel assembly11. Thus, it can be said that the transmission control section35increases the number of times the detection value is acquired from the acceleration sensor34per rotation of the wheel assembly11as the rotation period calculated with the expression (1) becomes longer.

The acquisition count, which is set in correspondence with the rotation period, is preferably changed by being multiplied by integers. In the present embodiment, two values of the acquisition count are set in correspondence with the rotation period. Specifically, the acquisition count is set to ten times or twenty times, which is obtained by multiplying ten times by two. If three values of the acquisition count are set in correspondence with the rotation period, the acquisition count is preferably set to ten times, twenty times, or thirty times.

Next, in step S6, the transmission control section35calculates an acquisition period, which is a period in which a detection value is acquired from the acceleration sensor34, from the set acquisition count. The acquisition period is set such that the detection value is acquired the number of times corresponding to the set acquisition count per rotation of the wheel assembly11. The acquisition period is calculated by dividing the rotation period by the acquisition count. The transmission control section35functions as an acquisition period setting section.

Next, in step S7, the transmission control section35detects that the rotation angle of the wheel assembly11matches the specific angle. First, the transmission control section35acquires a detection value from the acceleration sensor34with the calculated acquisition period.

As shown inFIG. 5, the gravitational acceleration always acts in the vertical direction. The detection axis of the acceleration sensor34changes its orientation as the wheel assembly11rotates. Thus, the gravitational acceleration detected by the acceleration sensor34fluctuates depending on the rotation angle of the wheel assembly11. More specifically, the detection value of the acceleration sensor34changes along a sine wave with the centrifugal acceleration being the center. In the present embodiment, the detection value is the centrifugal acceleration+1[G] when the acceleration sensor34is located at the lowest position in the wheel assembly11, and the detection value is the centrifugal acceleration−1[G] when the acceleration sensor34is located at the highest position in the wheel assembly11. For the illustrative purposes, the origin (0°) of the rotation angle of the wheel assembly11is set to the angle at which the transmitter31is located at the most forward position in the wheel assembly11. The rotation angle of the wheel assembly11when the transmitter31is located at the lowest position in the wheel assembly11is defined as 90°. The rotation angle of the wheel assembly11when the transmitter31is located at the rearmost position in the wheel assembly11is defined as 180°. The rotation angle of the wheel assembly11when the transmitter31is located at the highest position in the wheel assembly11is defined as 270°. The highest position is the highest position in the vertical direction of the wheel assembly11, and the lowest position is the lowest position in the vertical direction in the wheel assembly11.

When the detection value is acquired from the acceleration sensor34with the acquisition period, the detection value is acquired each time the wheel assembly11rotates by 36° if the acquisition count is ten times. The detection value is acquired each time the wheel assembly11rotates by 18° if the acquisition count is twenty times. In correspondence with the acquisition count, the detection value is acquired at constant angular intervals. Thus, for each value of the acquisition count, the angle at which detection value is acquired is a constant angle. For example, when the acquisition count is ten times, the detection value is acquired at constant angular interval of 36°, or at 0°, 36°, 72°, . . . . When the acquisition count is twenty times, the detection value is acquired at constant angular interval of 18°, or at 0°, 18°, 36°, . . . .

FIG. 5shows acquisition points P1to P10, which represent points in time at which the detection value is acquired when the acquisition count is ten times. Unless the vehicle10is abruptly accelerated or stopped, the vehicle speed scarcely changed significantly during rotation of the wheel assembly11, and changes in the detection value per rotation of the wheel assembly11is assumed to be caused by changes in the position of the acceleration sensor34. In other words, the rotation angle of the wheel assembly11is acquired by monitoring changes in the detection value of the acceleration sensor34.

The rotation angle of the wheel assembly11when the detection value is acquired at the acquisition points P1to P10varies in a range between a value obtained by adding, to the rotation angle, half the value obtained by dividing 360° by the acquisition count and a value obtained by subtracting the same value from the rotation angle. Thus, if the acquisition count is ten times, the rotation angle of the wheel assembly11when the detection value is acquired at the acquisition points P1to P10varies in a range of ±18°. For example, when the rotation angle at which the detection value is acquired at the acquisition points P2is 36°, the acquisition point P2varies in a range of 36°±18°. The above-described constant angle is employed to accept this error.

Comparison between the detection value obtained at each of the acquisition points P1to P10and the detection value obtained at the last one of the acquisition points P1to P10show that the acquisition points P1to P10include acquisition points at which the detection value switches from increasing to decreasing and acquisition points at which the detection value switches from decreasing to increasing. InFIG. 5, sign “+” indicates an increase, and sign “−” indicates a decrease. In the present embodiment, the position at which the gravitational acceleration is detected to be the greatest corresponds to the acceleration sensor34being located at the lowest position in the wheel assembly11. Thus, when the acceleration sensor34passes the lowest position in the wheel assembly11, the detection value switches from increasing to decreasing. In contrast, when the acceleration sensor34passes the highest position in the wheel assembly11, the detection value switches from decreasing to increasing. Thus, the position of the acceleration sensor34is acquired from change in fluctuation of the detection value.

As shown inFIG. 5, the detection value is acquired at the acquisition points P1to P20when the acquisition count is twenty times. For the acquisition points P1to P10, which are half the acquisition points P1to P20, the detection value is acquired at the same rotation angles of the acquisition points P1to P10in a case in which the acquisition count is ten times. When the acquisition count is twenty times, the rotation angle of the wheel assembly11when the detection value is acquired at the acquisition points P1to P20varies in a range of ±9°. For example, the rotation angle of the wheel assembly11when the detection value is acquired at the acquisition point P2is within a range of 36°±9°. Each of the acquisition points P11to P20is an intermediate angle between an adjacent pair of the acquisition points P1to P10.

The transmission control section35detects that the transmitter31is located at the specific angle when the change in fluctuation of at least two detection values that are acquired consecutively has switched from increasing to decreasing. When the acquisition count is ten times, the transmission control section35determines that the transmitter31is located at the specific angle when the detection value decreases after the change in fluctuation of the detection value switched from increasing to decreasing. That is, the transmission control section35determines that the transmitter31is located at the specific angle when the change in fluctuation of the detection value is in order of increase, decrease, and decrease.

When the acquisition count is twenty times, the transmission control section35determines that the transmitter31is located at the specific angle when the change in fluctuation of the detection value is in order of increase, decrease, decrease, and decrease. In the present embodiment, the specific angle is 144°. The specific angle varies in accordance with the acquisition count. Specifically, when the acquisition count is ten times, the specific angle is 144°±18°. When the acquisition count is twenty times, the specific angle is 144°±9°. The transmission control section35determines that the transmitter31has passed the lowest position in the wheel assembly11by detecting the specific angle based on the fact that the change in fluctuation of the detection value switches from increasing to decreasing. The transmission control section35functions as a specific angle detecting section.

As shown inFIG. 4, the transmission control section35transmits transmission data in step S8Accordingly, the transmission data is transmitted at the specific angle.

The transmission control section35may calculate the rotation period by using an expression obtained by modifying the expression (1) by replacing the numerator of the expression (1) with a value obtained from a table. The storage section of the transmission control section35stores a table in which the numerator of the expression (1) is associated with the radius R of the rim of the wheel12. The transmission control section35determines that a calculated rotation period has an error when the set acquisition count is different from the number of times the detection value is actually acquired per rotation of the wheel assembly11. In such case, the transmission control section35changes the value of the numerator of the expression (1). The transmission control section35determines that a calculated rotation period is correct when the set acquisition count is equal to the number of times the detection value is actually acquired, and uses the value of the numerator. That is, the rotation period can be calculated even if the radius of the rim of the wheel12is not stored in the storage section35b.

As described above, the transmission control section35functions as the acquiring section, the rotation period calculating section, the acquisition period setting section, and the specific angle detecting section. Thus, the battery37, which is a power source, the acceleration sensor34, and the transmission control section35constitute an acceleration detecting device40. The transmitter31includes the acceleration detecting device40, which is located at the same position as the transmitter31.

The receiver50will now be described.

As shown inFIG. 1, the receiver50includes a reception control section51, a reception circuit52, and a reception antenna56. The reception control section51is connected to a display57mounted on the vehicle10. The reception control section51is configured by a microcomputer or the like including a reception CPU54and a reception storage section55(a ROM, a RAM and the like). The reception control section51includes a timing function. The timing function is implemented by, for example, a timer or a counter. The reception control section51may include dedicated hardware (application specific integrated circuit: ASIC) that executes at least part of various processes. That is, the reception control section51may be circuitry including 1) one or more processors that operate according to a computer program (software), 2) one or more dedicated hardware circuits such as an ASIC, or 3) a combination thereof. The processor includes a CPU and memory such as a RAM and ROM. The memory stores program codes or instructions configured to cause the CPU to execute processes. The memories, or computer readable media, include any type of media that are accessible by general-purpose computers and dedicated computers.

The reception circuit52demodulates the wireless signal received from each of the transmitters31via the reception antenna56and outputs the transmission data from the transmitters31to the reception control section51.

The reception control section51acquires the pressure in the tire13and the temperature of the tire13, which represent the condition of the tire13, based on the transmission data from the reception circuit52. When there is an anomaly in the tire13, the reception control section51displays warning on the display57.

The reception storage section55stores the ID codes of the transmitters31mounted on the four wheel assemblies11. With this, the transmitters31are associated with the receiver50.

In some cases, it is desired to determine which one of the four wheel assemblies11includes the tire13to which the received transmission data corresponds. For example, in some cases, it is desired to display on the display57the wheel assembly11in which pressure anomaly has occurred in the tire13or it is desired to display on the display57the pressures of the tires13of the respective wheel assemblies11. In such cases, it is required to determine the received transmission data corresponds to which the wheel assembly11. In other words, the reception control section51needs to associate the ID codes of the respective transmitters31with the positions of the wheel assemblies11.

A wheel assembly position identifying process for identifying one of the four wheel assemblies11to which each transmitter31is attached will now be described. The wheel assembly position identifying process is performed when the vehicle10is activated by a start switch, which switches the state of the vehicle10between the activated state and the deactivated state. The activated state of the vehicle10refers to a state in the vehicle10can travel through operation of the accelerator pedal. The deactivated state of the vehicle10refers to a state in which the vehicle10will not travel even if the accelerator pedal is operated.

As shown inFIG. 6, the reception control section51transmits transmission data in step S11. Next, the reception control section51acquires the vehicle speed from the ABS controller25. Next, in step S13, the reception control section51obtains the pulse count value of each of the rotation sensor units21to24upon reception of the transmission data. The processes of step S12and step S13are executed upon acquisition of the transmission data.

Next, in step S14, the reception control section51performs position identification to identify one of the four wheel assemblies11to which each of the transmitters31is attached. The position identification is performed by collecting pulse count values each time the transmission data is received. The rotation speeds of the wheel assemblies11differ, for example, due to the influence of the differential gear. Thus, the relative positions of the transmitters31attached to the wheel assemblies11change in accordance with travelling of the vehicle10. Thus, in a case in which the transmitters31transmit the transmission data at the specific angle, the rotation angles of the four wheel assemblies11is synchronized with the rotation angle at which the transmission data from one of the four transmitters31is transmitted. Thus, in a case in which the transmitters31transmit the transmission data at the specific angles, when the pulse count value is obtained upon reception of the transmission data, the rotation sensor units21to24include a rotational sensor unit that has a small variation of the pulse count value in correspondence with each transmitter31. The reception control section51identifies one of the four wheel assemblies11to which each of the transmitters31is attached based on the variation of the pulse count value collected each time the transmission data is acquired.

When the collected pulse count values fall within a predetermine range, the reception control section51associates the rotation sensor unit that detected the pulse count values with the transmitter31. The predetermined range is set by taking into consideration variations of the pulse count value and is used to determine which one of the rotation sensor units21to24has small variation in the pulse count values. The reception control section51changes the predetermined range in accordance with the vehicle speed. The reception control section51sets the predetermined range to be wider when the vehicle speed is greater or equal to a speed threshold than when the vehicle speed is less than the speed threshold. The speed threshold corresponds to the threshold set for the rotation period. In the present embodiment, the threshold set for the rotation period corresponds to the vehicle speed of 70 [km/h], so that the speed threshold is 70 [km/h]. The difference between the predetermined range in a case in which the vehicle speed is less than the speed threshold and the predetermined range in a case in which the vehicle speed is greater than or equal to the speed threshold is, for example, obtained by adding a margin to the pulse count value corresponding to 18°. That is, the predetermined range is set by taking into consideration variation of the specific angle corresponding to the acquisition count.

In the example ofFIG. 7, the variation of the pulse count value detected by the first rotation sensor unit21corresponding to the left front wheel assembly FL is the smallest. Therefore, the transmitter31of the FLID can be determined to be attached to the left front wheel assembly FL. In the example shown inFIG. 7, the pulse count values that are detected by the first rotation sensor unit21when the acquisition count is ten times are expressed by white circles, and the pulse count values that are detected by the first rotation sensor unit21when the acquisition count is twenty times are expressed by black circles. The graph shows that the variation of the pulse count values is small since the variation of the specific angle at which the transmission data is transmitted is small. The reception control section51identifies the wheel assemblies11to which the transmitters31of the FFID, RLID, and RRID are attached. The reception control section51associates the four ID codes with the positions of the respective wheel assemblies11and stores the relationship in the reception storage section55. The processes of the steps S11to S14are repeated each time the transmission data is received until the correlation between all the transmitters31and the wheel assemblies11are identified. When the four ID codes are associated with the positions of the wheel assemblies through the process of step S14the reception control section51finishes the wheel assembly position identifying process.

The transmission control section35detects the specific angle from the detection value of the acceleration sensor34. The transmission control section35intermittently acquires the detection value of the acceleration sensor34in order to reduce the power consumption of the battery37. Accordingly, there are period in which the transmission control section35cannot acquire detection values.

In the present embodiment, the number of times the detection value is acquired per rotation of the wheel assembly11is increased when the specific angle transmission is performed and the rotation period is longer than or equal to the threshold. By increasing the acquisition count of the detection value only when specific conditions are met, the detection accuracy of the specific angle is increased. In the present embodiment, the acquisition count is increased from ten times to twenty times, so that the variation of the specific angle is reduced from ±18° to ±9°. As a result, the reception control section51reduces the predetermined range when performing the position identification. This reduces the pulse count values contained in the predetermined range, reducing the time required to identify the wheel assembly11to which each transmitter31is attached.

The acquisition count may always be increased regardless of the rotation period. However, in this case, the specific angle transmission may be impossible during high speed traveling. Increasing the acquisition count shortens the acquisition period. Further, since the acquisition period is shortened in proportion to the vehicle speed, an increased acquisition count during high sped traveling excessively shorts the acquisition period. When acquiring the detection value from the acceleration sensor34, the transmission control section35requires process time for executing various processes such as comparison between the acquired value and the detection value in the last cycle. If the acquisition period becomes shorter than the process time, the detection value would be acquired before the process is completed, and the process would not be executed properly. In this case, the specific angle transmission would not be performed. That is, the acquisition period needs to be longer than the time obtained by adding a margin to the process time, and the acquisition count cannot be increased regardless of the rotation period. Also, increasing the acquisition count regardless of the rotation period increases the power consumption and thus may reduce the life of the battery37.

In the present embodiment, the acquisition count is increased when the rotation period is longer than or equal to the threshold. This reduces the time required to identify the wheel assembly11to which each transmitter31is attached when the rotation period is longer than or equal to the threshold. Particularly, if the threshold for the rotation period is set by taking into consideration the normal use ranges of various countries, remarkable advantages will be achieved.

For example, in Japan, vehicles are predominantly driven at speeds up to 70 [km/h] except for expressways. Thus, the threshold for the rotation period being set to 70 [km/h] allows the advantages of the acceleration detecting device40of the present embodiment to be enjoyed in various time periods of the day in which the vehicle10is used.

The present embodiment has the following advantages.

(1) The transmission control section35increases the number of times the detection value is acquired from the acceleration sensor34when the rotation period of the wheel assembly11is long. On the other hand, the transmission control section35decreases the number of times the detection value is acquired from the acceleration sensor34when the rotation period of the wheel assembly11is short. Therefore, power consumption can be reduced compared to a case in which the number of times the detection value is acquired from the acceleration sensor34is made constant regardless of the rotation period of the wheel assembly11. In addition, since the number of times the detection value is acquired from the acceleration sensor34increases when the rotation period is long, the transmission control section35is capable of properly acquiring the condition of the wheel assembly11.

(2) The transmission control section35is capable of detecting that the transmitter31is located at the specific angle from the change in fluctuation of the detection value. Being able to cause the transmission data to be transmitted at the specific angle, the transmission control section35is capable of causing the receiver to identify the wheel assembly11to which each transmitter31is attached.

(3) The transmission control section35determines that the transmitter31is located at the specific angle when the change in fluctuation of the detection value switches from increasing to decreasing. This allows for determination that the transmitter31has passed the lowest position.

The present embodiment may be modified as follows. The present embodiment and the following modifications can be combined as long as the combined modifications remain technically consistent with each other.

The transmission control section35may detect whether the part of the tire13at which the acceleration sensor34is located has contacted the ground from the detection value of the acceleration sensor34. That is, instead of detecting the specific angle as in the above-illustrated embodiment, the acceleration detecting device40may be used to perform ground contact determination. In this case, as shown inFIG. 8, the transmitter31is provided on a back face15of a tread portion14of the tire13, that is, on the surface opposite to the surface contacting the ground. A part of the tread portion14of the tire13in which the acceleration sensor34is provided is referred to as a sensor mounting section.

During traveling of the vehicle10, when the sensor mounting section of the tire13contacts the ground, the part of the tread portion14that is contacting the road surface is crushed, so that the acceleration sensor34receives a force in a direction opposite to the direction of the centrifugal acceleration. Thus, the detection value of the acceleration sensor34is lowered when the sensor mounting section contacts the road surface. Therefore, the transmission control section35is capable of determining that the sensor mounting section of the tread portion14of the tire13has contacted the ground when the detection value of the acceleration sensor34has changed from a value greater than or equal to a predetermined ground contact determination threshold to a value less than the ground contact determination threshold. The ground contact determination threshold is a value greater than the detection value of the acceleration sensor34when the vehicle10is in a stopped state. The transmission control section35functions as a ground contact determining section. Determination that the sensor mounting section has contacted the ground may be made using a change amount of the detection value in addition to the determination using the ground contact determination threshold. When the sensor mounting section contacts ground, the centrifugal acceleration is cancelled, so that the detection value drops significantly. Therefore, it may be determined that the sensor mounting section is contacting the ground when the amount of change of the detection value becomes greater than or equal to a predetermined value.

The above described acceleration detecting device40is used in the transmitter31, which, for example, detects the condition of the road surface. In this type of transmitter31, the condition of the road surface is estimated from a detection result of a sensor acquired when the sensor mounting section of the tire13contacts the ground. Therefore, detection of the sensor mounting section contacting the ground surface is important. The time during which the sensor mounting section contacts the ground is short. Thus, a smaller number of the acquisition count may prevent ground contacting of the sensor mounting section from being detected. However, the acceleration detecting device40increases the acquisition count in correspondence with the rotation period of the wheel assembly11. Therefore, it is possible to determine that the sensor mounting section has contacted the ground, in other words, that the rotation angle of the wheel assembly11has reached the angle at which the sensor mounting section contacts the ground.

The transmission control section35may determine that the transmitter31has reached the specific angle based on the change in fluctuation of the detection value being switched from decreasing to increasing. In this case, the transmission control section35is able to determine that the transmitter31has passed the highest position in the wheel assembly11.

The specific angle at which the transmission control section35transmits the transmission data may be changed. In this case, the specific angle simply needs to be changed by changing the change in fluctuation of the detection value that triggers the transmission.

Multiple specific angles may be set.

The acquisition count may be changed as necessary.

The acquisition count may be linearly increased as the rotation period is extended, without using the threshold.

The acceleration sensor34may be arranged in any manner as long as it is capable of detecting the centrifugal acceleration.

Various types of power generating elements may be used as the power source of the transmitter31. Even if a member that is capable of being charged or generating power is used as the power source, there is a limit to power that can be used. Therefore, it is preferable to reduce the power consumption by transmission of the transmission data. Thus, by transmitting the transmission data without data indicating the angle information, the limited power is used effectively.

In each embodiment, the vehicle10only has to include multiple wheel assemblies11, and for example, the vehicle10may be a motorcycle.

The receiver50may be a portable terminal carried by an occupant of the vehicle10.

DESCRIPTION OF THE REFERENCE NUMERALS