Abstract:
The wheel locations for wireless tire pressure monitoring sensors are automatically learned by comparing the timing of sensor message transmissions to directly measured wheel rotation, such as from an anti-lock brake system. In order to synchronize the data to be detected and compared across a distributed electronic system, first timestamps are applied to RF sensor messages upon reception in a receiver. Second timestamps are applied to decoded sensor data in the receiver when transmission of the data and the first timestamps to a control module begins. The control module applies third timestamps upon reception by the control module. The control module calculates sensor message times by subtracting a difference of the first and second timestamps from the third timestamp.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to tire pressure monitoring systems in automobiles, and, more specifically, to automatically learning to associate transmissions from identified wheel-mounted sensors with the wheel locations where the respective wheels are installed. 
     Monitoring of tire pressure provides a useful safety feature as the result of being able to automatically inform a driver when a low pressure appears in any tire. A typical tire pressure monitoring system (TPMS) sensor unit includes a battery-powered device that is remotely mounted on each respective wheel. Pressure data from a transducer is wirelessly transmitted (e.g., via RF) to a vehicle-mounted receiver for analyzing the transmitted messages and to associate the measurements with respective wheel locations. Each sensor unit includes a unique identifier or serial number which is included in each message that the receiver can learn to associate with respective wheel positions so that the location of a tire experiencing low pressure can be reported to the driver. Because of the possibility of tire rotation (i.e., swapping of wheel locations for evening out the tire wear) or the replacement of a tire with a spare or new tire on a different wheel, these associations must be continually re-learned during vehicle operation. 
     One technique for learning the wheel location for data obtained from wheel-mounted TPMS sensors involves time correlation between data received from the TPMS sensors with data received from an anti-lock brake system (ABS) that directly monitors wheel positions. More specifically, the sensor unit may include an orientation sensor such as an accelerometer in order to time the broadcasting of messages according to a particular rotational position of the wheel, such as at the top of a wheel rotation. By triggering the transmission of messages once per wheel rotation, the timing of a string of tire pressure messages can be compared with direct measurements from each of the wheel locations in the ABS data. Since tire slippage, vehicle trajectory, and other factors result in differential overall rotation between the individual wheels, timing information from each respective TP MS sensor unit eventually matches only one of the sets of corresponding ABS data measurements. This general technique for learning the wheel associations is shown in U.S. Pat. No. 7,336,161 to Walraet, U.S. Pat. No. 8,528,393 to Craig et al., and U.S. patent application publication 2014/0019035 to Fink et al., for example. 
     In order to reliably associate the TPMS sensor ID&#39;s with respective wheel locations within a reasonable period of time, a high accuracy of the measured time of occurrence for each TPMS sensor message is needed. In an electrical architecture wherein the TPMS data and ABS data is received and processed by the same microprocessor or microcontroller based on a single timing reference (e.g., clock), synchronization between the data sets and the overall accuracy of the timing data itself is fairly straightforward to obtain. In a typical electrical architecture of an automotive vehicle, however, a distributed architecture is employed wherein the RF receiving and decoding circuits are located in one module and the processing (e.g., comparison) of the TPMS data with the ABS data in order to find the wheel associations is performed by circuits located in a different module (e.g., a body control module). The detection and decoding of the RF messages from the TPMS sensors in the RF module may take an amount of time that varies from one message to another. When the decoded messages are repackaged and sent to the other module doing the comparison, the time at which the message arrives at the comparing module is not sufficiently accurate for purposes of the comparison because of the variable delay between the time that the sensor unit was at the reference position and the time that the comparing module receives the corresponding message. Walraet &#39;161 discloses a shared clock signal generated in one module and coupled directly to other modules for use in detecting the times for the TPMS data and ABS data. However, the dedicated provisioning of wiring for sharing a clock signal is undesirable. 
     Synchronization of separate clock references with different modules over existing communication lines (such as a multiplex bus) has provided limited accuracy due to bus delays for the associated messages and due to clock drift that continues to occur between synchronization messages. Therefore, improved timing measurements are needed in the context of a distributed processing system. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a tire pressure monitoring system (TPMS) is provided for a vehicle with a plurality of wheel locations. A plurality of wheel-mounted TPMS sensor units transmit RF sensor messages that are triggered when the respective sensor units detect being in respective rotational positions. The system includes a TPMS receiver unit and a control module, wherein the TPMS receiver unit and the control module have independent time references. A multiplex bus is coupled between the TPMS receiver unit and the control module. An anti-lock brake (ABS) unit is coupled to the control module providing ABS wheel rotation data. The TPMS receiver unit has an RF section, a decoding section, and a bus section, wherein the receiver unit generates TPMS wheel rotation data sent to the control module as multiplex bus messages. Each bus message is comprised of an identifier for a sensor unit that transmitted a respective RF sensor message, a first timestamp triggered when the RF section received the respective RF sensor message, and a second timestamp triggered when the bus section begins to transmit a respective multiplex bus message having contents decoded by the decoding section. The control module assigns a third timestamp to each of the bus messages when received. The control module calculates an RF sensor message time for each bus message in response to subtracting a difference between the first and second timestamps from the third timestamp. The control module compares the TPMS wheel rotation data and the ABS wheel rotation data to associate each TPMS sensor unit with a respective wheel location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an automotive vehicle and a distributed electrical architecture for implementing a tire pressure monitoring system. 
         FIG. 2  is a block diagram of a TPMS sensor. 
         FIG. 3  illustrates the contents of an RF sensor message for transmitting wirelessly from the TPMS sensor. 
         FIG. 4  is a block diagram showing an RF receiver module and a body control module in greater detail. 
         FIG. 5  is a block diagram showing message processing in the RF receiver in greater detail. 
         FIG. 6  is a block diagram showing message process in the body control module in greater detail. 
         FIG. 7  illustrates the comparison of TPMS wheel rotation data with ABS wheel rotation data for associating sensor ID numbers with their corresponding wheel locations. 
         FIG. 8  is a flowchart showing one preferred method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a vehicle  10  has a left front wheel  11 , a right front wheel  12 , a left rear wheel  13 , and a right rear wheel  14 . Each wheel location includes a tire mounted on a respective wheel containing a respective TPMS sensor unit  15 - 18 . TPMS sensor units  15 - 18  transmit respective RF sensor messages to an RF module  20  mounted in vehicle  10 . RF module  20  is coupled with a body control module (BCM)  21  via a multiplex bus  22 . BCM  21  includes a TPMS algorithm  23  for the purpose of associating identifying serial numbers provided by sensor units  15 - 18  with respective vehicle locations. 
     Based on the pressure measurements contained in the respective sensor messages from TPMS sensor units  15 - 18 , a tire having a pressure below a threshold pressure may generate a warning via a message center  24  which is coupled to bus  22 . Based on the wheel location associations derived, the warning via message center  24  can inform the driver which wheel location contains the underinflated tire. 
     An anti-lock brake system (ABS) controller  25  is connected to ABS wheel units  26 - 29  mounted at wheels  11 - 14 , respectively, for obtaining direct position/timing measurements for the wheels (e.g., from the respective positions of toothed wheels in position sensors mounted on each wheel). 
       FIG. 2  shows a TPMS sensor unit  15  in greater detail. The unit is battery powered from a battery  30 . A pressure transducer  31  provides pressure measurements to a logic block  32 . Logic block  32  may be comprised of a microprocessor with an associated memory  33 . An orientation sensor  34  is coupled to logic block  32  so that a respective rotational position (such as when TPMS sensor unit  15  reaches its highest point above the ground) triggers a transmitter  35  to send an RF sensor message over an antenna  36 . The sensor message includes a sensor ID serial number and other data. 
       FIG. 3  shows a format for a sensor message  37  which may be assembled in the sensor unit has a multi-byte message. A sensor ID  38  is followed by pressure data  39 . Other sensor data may be included such as temperature data  40  and status information  41  (which may include a battery charge state, for example). A checksum  42  may also be included for error detection and correction. 
     As shown in  FIG. 4 , RF receiver module  20  includes an RF detector chip  44  and an antenna  45  for receiving the RF sensor messages. The multiple bytes of each message are converted to readable data by RF chip  44  which outputs the data to a microprocessor  46 . When the data becomes available, an interrupt signal is coupled to microprocessor  46  which identifies an instant in time close to the original sending time of the sensor message. Upon reception of the interrupt, microprocessor  46  uses a timing circuit  48  to obtain a timestamp which is stored in a memory  47 . This critical timing information may be shared with control module  21  via a bus interface  49  which transfers a bus message from microprocessor  46  over bus  22  to a bus interface  50  in control module  21 . A microprocessor  51  and memory  52  in control module  21  act upon the received bus messages and apply timing information from a timing circuit  53  to the data in each bus message as described in greater detail below. 
     The present invention employs a series of timestamps generated in connection with each RF sensor message and the subsequent sending of a corresponding bus message in order to provide accurate timing information for each transmission from the TPMS sensor units.  FIG. 5  illustrates the time stamping process in RF receiver module  20  in greater detail. An RF data in block  58  receives the RF data from the RF receiving chip as soon as a message has been detected by the RF chip. Simultaneously, an interrupt is triggered by interrupt circuit  54  and coupled to logic block  55 . A memory location or buffer  56  is used to assemble a bus message corresponding to each RF sensor message received. Upon receipt of an interrupt, logic block  55  obtains a time value from timing circuit  48  to be applied as a first timestamp T 1  in a multi-byte portion  57  of buffer  56 . RF input data from block  58  is decoded and verified in a decoder block  60  as known in the art. The decoded data (including a sensor serial number and tire pressure data) is stored in a multi-byte portion  61  of buffer  56 . 
     Once data portion  61  and first timestamp portion  57  have been written, steps are begun in order to transmit a corresponding bus message by initiating conversion to a bus format suitable for the multiplex bus using a universal asynchronous receiver/transmitter (UART)  62 . When the conversion begins, an interrupt signal is generated by an interrupt block  63 . The interrupt is handled by logic block  55  by initiating a second timestamp T 2  which is obtained from timing circuit  48  and is stored in a multi-byte portion  64  of buffer  56 . Timestamp T 2  is written quickly enough that it is available by the time when the last bytes of buffer  56  are being converted by UART  62 . 
     UART  62  may also process initiation messages and terminate messages from the bus sent by the main control module (e.g., the BCM) in order to begin or stop the auto-learning process. In order to avoid excessive bus loading, the auto learning process is preferably discontinued as soon as the wheel associations are obtained during each particular driving cycle. 
       FIG. 6  shows the elements of control module  21  that are relevant to receiving and processing the bus messages from the RF receiver module. Thus, a UART  66  receives bus messages from the bus and places the sensor ID number and data payload (e.g., timestamps T 1  and T 2 ) into a memory buffer  68 . A logic block  67  (e.g., a microprocessor) determines that a new bus message with tire pressure sensor data and timing data has been received, and then applies a third timestamp T 3  to a multi-byte buffer section  69  using timing information obtained from a timing circuit  53 . Using timestamps T 1 , T 2 , and T 3 , logic block  67  calculates an RF sensor message time by subtracting the difference between timestamps T 2  and T 1  from third timestamp T 3  (i.e., T 3 −(T 2 −T 1 )). The difference between timestamps T 2  and T 1  represents the processing delays resulting from decoding and verifying the sensor message. The time required to complete transmission of a bus message from the receiver module and store it in the control module is relatively insignificant in comparison to the processing delays that occur within the RF receiver module for decoding the message. Therefore, RF message timing based on corresponding timestamp T 3  minus the difference of T 2  minus T 1  may be sufficiently accurate to associate the wheel locations. If additional accuracy is desired, the relatively smaller delays from bus propagation and message recognition and timestamping in the control module may be accounted for by subtracting an additional predetermined fixed offset (i.e, constant) from third timestamp T 3 . The resulting sensor message time is associated with (e.g., stored together with) the data from the corresponding bus message, and all the message data is organized according to the identification numbers of each transmitting TPMS sensor unit. 
     As shown in  FIG. 7 , a comparison block  70  receives the TPMS wheel rotation data organized according to the sensor identifier numbers. Similarly, ABS-based wheel rotation data for each of the wheel locations is provided to comparison block  70  which produces the learned associations as known in the art. 
       FIG. 8  shows a preferred method of the invention which typically begins at the start of each drive cycle (e.g., upon the starting of the vehicle engine and during subsequent movement of vehicle). The body control module (BCM) sends an initiate message to the RF receiver module in step  71 . Once the learning process is initiated, the RF receiver module processes RF sensor messages from the TPMS sensor units to provide the sensor ID and message timing information needed by the BCM for learning the wheel associations. In step  72 , the TPMS sensors detect predefined target positions. The transmission of sensor messages are triggered at the moment the target positions are reached. The RF receiver module detects a new incoming sensor message and then applies a first timestamp T 1  in step  73 . In step  74 , the receiver module decodes the sensor message and forms a bus message including timestamp T 1 . In step  75 , the RF receiver module starts conversion of the bus message into a bus format (e.g., converting from parallel bits to a serial bit string), and then applies second timestamp T 2  to the end of the bus message. The BCM receives the bus message including timestamps T 1  and T 2  in step  76 , and then applies the third timestamp T 3  based on the timing reference (clock circuit) within the BCM. A return is made to step  73  for processing additional sensor messages while the auto-learning process continues. 
     In step  77 , the BCM organizes the TPMS wheel rotation data according to the sensor IDs and the sensor message times calculated using the difference between timestamps T 2  and T 1 . In step  78 , the BCM organizes the ABS wheel rotation data for comparison with the TPMS wheel rotation data. In step  79 , a comparison is utilized in order to associate the sensor ID numbers with respective wheel locations. Successive approximations may be conducted as the data accumulates until a final determination is made after enough data has been processed to converge to a solution. In step  80 , the BCM sends a termination message to the receiver module in order to terminate further processing of the timing information, thereby reducing the traffic on the multiplex bus. Thereafter, the forwarding of pressure data from the continuing RF sensor messages continues so that any undesirable pressure levels can be detected.