Abstract:
An apparatus and method of transmitting data between two inductively coupled circuits. The method includes providing a transmitting circuit, providing a receiving circuit spaced from the transmitting circuit, and generating a data signal in the transmitting circuit. The data signal includes a first component representing the data, and a second component identifying a portion of the first component. The apparatus includes a transmitting circuit having a transmitting coil. The transmitting circuit generates electrical energy in the transmitting coil. The apparatus further includes a receiving circuit spaced from and inductively coupled to the transmitting circuit. The receiving circuit includes modulating means for generating a carrier signal including a first component encoding a plurality of consecutive data signals, and a second component identifying the beginning of respective ones of the data signals.

Description:
RELATED APPLICATIONS 
     This application is a Division of Ser. No. 08.824,322 filed Mar. 26, 1997, U.S. Pat. No. 5,939,977 and claims the benefit under 35 U.S.C. §119 of co-pending U.S. Provisional Patent Application Serial No. 60/014,763, filed Apr. 3, 1996, which issued as U.S. Pat. No. 5,939,977. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a system for sensing a physical characteristic of a rotating wheel of a vehicle, and in particular, to a method and apparatus for encoding data indicating the pressure and temperature of the gas within the vehicle wheel. 
     It is commonly known to provide vehicles with wheels that include a tire, typically made of rubber or some other elastomeric material. The tire is usually hollow and is usually filled to a given pressure with a gas (commonly air). It is very important that the tire of the vehicle be inflated to the correct pressure for the particular tire. Improper inflation of a vehicle tire adversely effects the wear of the tire and diminishes the efficiency of the vehicle. Therefore, it is important that the vehicle operator check the tires on a regular basis to ensure proper inflation of the tires. However, the process of checking and adding to or subtracting from the air in the tire can be tedious and inconvenient for the typical motor vehicle operator. Moreover, the air pressure in a given tire can vary significantly depending upon the changes in the ambient temperature. 
     Various systems have been developed to make monitoring of tire pressure more convenient and easy. For example, U.S. Pat. No. 4,918,423 describes a tire inspection device for measuring the pressure and temperature of the air in the tire. The device includes a pressure sensor switch  9  connected to a coil  8 , and a hand-held detecting rod  28 . If the tire internal pressure has the proper value, the sensor  9  turns on and a green lamp on the detecting rod  28  lights. If the tire internal pressure has the improper value, the sensor  9  turns off and a red lamp on the detecting rod  28  lights. 
     Another example of a system for detecting tire pressure is shown and described in U.S. Pat. No. 4,768,375 (Eckardt et al.). The system illustrated in Eckardt et al. is passive in that it does not require a person to take any action to detect the tire pressure. The Eckardt et al. system can also detect tire pressure while the vehicle wheel is rotating. 
     SUMMARY OF THE INVENTION 
     The invention provides a system for remote tire pressure sensing that allows remote measurement of the tire pressure and temperature within a vehicle tire. The system utilizes low frequency magnitude field communication, eliminates cross talk between the wheels of the vehicle or the wheels of other vehicles, includes remote power transmission, provides identical transducers for each of the wheels (thereby rendering the wheels totally interchangeable), includes air detection capability, and measures actual pressure and actual temperature of the air in the tire. 
     The remote tire pressure sensing system includes a transponder mounted on the rim of the tire. The transponder includes a temperature sensor and a pressure sensor which measure the temperature and pressure, respectively, of the air in the tire. The remote tire pressure sensing system also includes a transceiver for each pair of tires on the vehicle. The transceiver is mounted on a frame member of the vehicle and includes a twisted wire pair going to each of the vehicle wheels. The twisted wire pairs are connected to a frame mounted transceiver coil. In a preferred embodiment of the invention, the transceiver coil is arcuately shaped so as the match the arc of the inner wall of the wheel rim. This maximizes the communication window between the transponder and the transceiver coil. 
     More particularly, the invention provides an apparatus including a transmitting circuit adapted to be mounted on the frame member. The transmitting circuit includes a transmitting coil and generates electrical energy in said transmitting coil. The apparatus also includes a receiving circuit adapted to be mounted on the vehicle wheel. The receiving circuit includes a sensor for measuring a physical characteristic of the vehicle wheel and modulating means connected to the sensor for generating a carrier signal including a first component encoding a plurality of consecutive data signals corresponding to the physical characteristic of the vehicle wheel, and a second component identifying the beginning of respective ones of the data signals. 
     It is an advantage of the invention to provide a system that provides remote measurement of the pressure and temperature of the air within a vehicle tire. 
     It is another advantage of the invention to provide a system for encoding and transmitting data between a transponder or transmitting circuit and a transceiver or receiving circuit that is in motion relative to the transponder. 
     It is another advantage of the invention to provide a method of encoding the data that allows the transceiver to synchronize to a given data packet. 
     Other features and advantages of the invention are set forth in the following detailed description and claims. 
    
    
     BRIEF SUMMARY OF THE DRAWINGS 
     FIG. 1 is a partial schematic diagram of a vehicle including a system for remotely sensing the pressure and temperature of air in a vehicle tire. 
     FIG. 2 is an enlarged view taken along line  2 — 2  in FIG.  1 . 
     FIG. 3 is a view taken along line  3 — 3  in FIG.  2 . 
     FIG. 4 is an electrical schematic diagram of the transponder of the system. 
     FIG. 5 is an electrical schematic diagram of the transceiver of the system. 
     FIG. 6 is an electrical schematic diagram of a transponder that is another embodiment of the invention. 
     FIG. 7 is an electrical schematic diagram of a transponder that is a third embodiment of the invention. 
     FIG. 8 is a waveform diagram illustrating an example of quadrature phase shift keyed (“QPSK”) encoding. 
    
    
     Before one embodiment of the invention is 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 Ccomponents set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Shown in FIG. 1 of the drawings is a portion of a vehicle  10  embodying the invention. The vehicle  10  includes a frame  14  (only a portion of which is shown in FIG. 1) and a pair of frame members  18  and  22  each supporting a wheel end housing  26  and  30 , respectively. As is commonly known in the art, the wheel end housings  26  and  30  are mounted for rotation relative to the frame members  18  and  22  about an axis  32 . The wheel end housings  26  and  30  include a wheel mounting surface  34  and a plurality of threaded rods  38  extending axially from the mounting surface  34 . The vehicle  10  also includes wheels  42  and  46  mounted on the wheel end housings  26  and  30 , respectively. While only two wheels are shown mounted on the vehicle  10 , as is commonly known in the art, the vehicle  10  can have any number of wheels in any of a variety of configurations. The number of wheels on the vehicle  10 , as well as the particular construction of the frame members  18  and  22  and wheel end housings  26  and  30  may also vary depending upon the type of vehicle, e.g., automobile, truck or trailer, and may vary depending upon the type of drive used by the vehicle, e.g., rear wheel drive, front wheel drive, four wheel drive, etc. 
     As shown in the drawings, the wheels  42  and  46  are identical, and accordingly, only the wheel  46  will be described in detail. The wheel  46  includes a rim  50  having a cylindrical or drum-like portion  54 . The cylindrical portion  54  has axially opposite end portions  58  and  62 . The rim  50  also includes an end wall  66  adjacent the end portion  62 . The end wall  66  has therein a series of apertures (not shown) for receiving threaded rods  38 . The threaded rods  38  extend through the apertures on the rim  50  and lug nuts  74  are threaded onto the rods  38  to secure the rim  50  to the wheel end housing  30 . 
     As best shown in FIG. 2, the cylindrical portion  54  of the rim  50  includes an inner surface  78 , an outer surface  82  and an aperture  86  extending between the inner and the outer surfaces  78  and  82 . Referring again to FIG. 1, the wheel  46  also includes a tire  90  mounted on the rim  50  adjacent the outer surface  82  of the cylindrical portion  54 . As is commonly known in the art, the tire  90  is circular and extends around the rim  50 . The tire  90  includes an inner surface  94  which, with the outer surface  82  of the rim  50 , forms a cavity  98  that is typically inflated to a specified pressure with a gas or gas mixture such as air. The tire  90 , along with the other tires of the vehicle  10 , supports the vehicle  10  on the travelling surface  99 . 
     The vehicle  10  also includes a system  100  for remotely sensing the pressure and temperature of the air in the cavity  98 . In general terms, the system  100  includes a transponder  104  mounted in the aperture  86  of the rim  50  for rotation with the rim  50  about the axis  32 , and a transceiver  108  mounted on the frame  14  so that the transceiver  108  is in a fixed reference relative to the wheel  46 . As is commonly known in the art, power can be provided to the transceiver using virtually any electrical power supply. In the preferred embodiment, power is provided to the transceiver  108  by a +/−5.0 volt, 500 kHz switching power supply (not shown). While the system  100  is described in the context of a system for remotely sensing the pressure and temperature of air in a vehicle wheel, it should be understood that the system  100  can be used in any environment where it is desired to transfer data of any kind between inductively coupled circuits or through other wireless short range communications such as, for example, radio frequency identification tags. 
     As shown in FIG. 2 of the drawings, the transponder  104  includes a molded plastic bobbin  112 . The bobbin  112  is circular in shape and has an outer surface  116  defining a recess  120 . A wire conductor is wound around the bobbin  112  to form an inductive coil  124  in the recess  120 . The bobbin  112  also includes an inner surface  128  defining a circular aperture. A circular ferrite pole piece  136  is mounted in the aperture. The bobbin  112  also includes a top surface  140 . An electronic circuit or circuit assembly  144  is mounted on the top surface  140 . 
     The transponder  104  also includes electrically conductive leads  148  mounted in the bobbin  112  and extending between the circuit assembly  144  and the inductive coil  124  to electrically connect the inductive coil  124  to the circuit assembly  144 . A cap  152  is mounted on the top surface  140  of the bobbin  112  to enclose the circuit assembly  144 . The cap  152  includes a disk-like top-portion  156  having an outer edge  160  and a cylindrical sidewall  164  extending downwardly from the outer edge  160 . The cylindrical sidewall  164  engages the top surface  140  of the bobbin  112  so that the cap  152  covers the circuit assembly  144 . The top-portion  156  of the cap  152  includes an aperture or port  168  to allow the pressure and temperature of air in the cavity to reach the circuit assembly  144 . A filter  172  is mounted in the port to prevent dirt and debris from coming into contact with the circuit assembly  144 . The cap  152  and bobbin assembly are encased within a molded rubber housing  176 . The molded rubber housing  176  is resilient so that it can be forced into the aperture  86  of the rim  50  to form a seal therewith. 
     FIG. 4 is an electrical schematic of the transponder  104 . As shown in FIG. 4 the inductive coil  124  forms an antenna for the circuit  144  and a capacitor  180  is connected in parallel with the inductive coil  124  so that one end of the capacitor and inductive coil combination forms a virtual ground  184 . The capacitor  180  and inductive coil  124  form a resonant circuit. The opposite end of the parallel combination is connected to a node  188 . An inverting Schmitt Trigger  192  is connected to node  188  and includes an output  196  connected to the input  200  of a frequency divider  204 . The frequency divider  204  has two outputs  208  and  212 . The output  208  is a “divide by two” output. The output  212  is a “divide by sixteen” output. 
     The transponder  104  includes a microprocessor  216  connected to the output  212 . In the preferred embodiment, the microprocessor  216  is a PIC16C74 microprocessor and, as is commonly known in the art, the microprocessor  216  includes an analog-to-digital converter (“ADC”) and a serial communications interface (“SCI”). The SCI is set up for synchronous slave mode operation and  9  bit data communication and utilizes the 11.25 kHz signal to shift data out of the SCI registers (not shown). 
     The transponder  104  also includes a pressure sensor  220  and a temperature sensor  224  mounted appropriately so that the pressure sensor  220  can measure the pressure of the air in the cavity and so that the temperature sensor  224  can measure the temperature of the air in the cavity. The pressure sensor  220  is preferably a silicon surface micro-machined integrated circuit pressure sensor as shown and described in U.S. Pat. No. 5,507,171, which is incorporated herein by reference. The temperature sensor  224  is preferably an integrated circuit temperature sensor as shown and described in U.S. patent application Ser. No. 08/286,784, titled “Method and Apparatus for Measuring Temperature”, which is incorporated herein by reference. The pressure sensor  220  and temperature sensor  224  each have an output,  228  and  232 , respectively, connected to the microprocessor  216 . The pressure sensor  220  and the temperature sensor  224  are also connected to the virtual ground  184  through switches  236  and  240 , respectively. The operation of the switches  236  and  240  is controlled by the microprocessor  216  to enable sampling of the pressure and temperature by the pressure sensor  220  and temperature sensor  224 , respectively. 
     The transponder  104  also includes an exclusive “LOR” (“XOR”) gate  244  having two inputs  248  and  252 . The first input  248  is connected to the output (i.e., the “divide by two” output) of the frequency divider  204 . The second input  252  is connected to the SCI output  256  of the microprocessor  216 . XOR gate  244  also includes an output  260  upon which is generated a modulation signal for carrying data encoded by the microprocessor  216 . While any appropriate modulation scheme can be used, the modulation signal output by the XOR gate  244  shown in the drawings is bipolar phase shift keyed (“IBPSK”) at 90 kHz. The output  260  is connected to a switch select controller  264  through input  266 . The switch select controller  264  also has an input  268  connected to the microprocessor  216  and includes outputs  272  and  276 . The switch select controller  264  uses the BPSK signal to generate a quadrature phase shift keyed (“QPSK”) signal at 90 kHz by selecting either switch  288  or  332  at the appropriate time. 
     The transponder  104  also includes a rectifying diode  280  connected to node  188 . The diode  280  includes a cathode  284  connected to a switch  288 . The switch  288  includes a control terminal  290  connected to the output  272  of switch select controller  264  and an opposite terminal  292  connected to the virtual ground  184  through capacitor  296  and zener diode  300 . The capacitor  296  and zener diode  300  form a voltage supply for the transponder  104 . 
     The transponder  104  also includes a diode  308  having a cathode  312  connected to the node  188  and having an anode  316 . The transponder  104  also includes zener diode  320  having an anode  324  connected to the anode  316  of diode  308  and a cathode  328  connected to the virtual ground  184  through switch  332 . The output  276  of switch select controller  264  is connected to the control input  336  of the switch  332 . Together, diode  280 , switch  288 , diode  308 , zener diode  320  and switch  332  form a modulation block  304  for the transponder. 
     As shown in FIG. 1 of the drawings, the system  100  also includes a transceiver  108 . For each wheel of the vehicle  10 , the transceiver  108  includes a transceiver circuit  340  (shown in FIG. 5) and a stationary coil package  344  connected to the transceiver circuit  340  via a twisted pair of electrical conductors  348 . As shown in FIGS. 2 and 3 the stationary coil package  344  is mounted on the frame member  22  via bracket  352  and using appropriate hardware as shown in FIGS. 2 and 3. The stationary coil package  344  includes a bobbin  356  having an arcuate, or generally non-planar surface  360 . The arcuate contour of the surface  360  is approximately the same as the arcuate contour of the rotational pathway of the transponder  104  as the transponder  104  rotates about the axis  32  so that, as the transponder  104  rotates past the stationary coil package  344 , a virtually constant radial air gap is maintained between the transponder  104  and the stationary coil package  344 . In the preferred embodiment, the radius of curvature of the surface  360  is approximately 174 mm (to accommodate different rim sizes) and the arc length of the surface  360  is approximately 77 mm. 
     As shown in FIG. 3, the bobbin  356  has a substantially rectangular shape and includes parallel side portions  364  and arcuate end portions  368 , which, together with the side portions  364 , form an endless side surface  372 . The side surface  372  defines an aperture  376  (FIG.  2 ). The stationary coil package  344  includes a wire conductor wound around the bobbin  356  in the aperture  376  to form an inductive antenna coil  380  that, as viewed from the perspective shown in FIG. 2, follows the arcuate shape of the surface  360 . The bobbin is over-molded with a thermoplastic or other appropriate material to form a housing  382  which protects the stationary coil package  344 . The arcuate shape of the surface  360  and the inductive antenna coil  380  maximizes the communication window between the inductive antenna coil  380  and the inductive coil  124  of the transponder  104 . That is, the amount of time during which the inductive antenna coil  380  and the inductive coil  124  of the transponder  104  are inductively coupled is increased. The stationary coil package  344  also includes a capacitor  384  connected in parallel to the inductive antenna coil  380  to form a resonant circuit. As shown in FIG. 5, the twisted pair of electrical conductors  348  are connected to the parallel combination of the inductive antenna coil  380  and capacitor  384 . 
     Referring to FIG. 5, the transceiver circuit  340  includes an H-bridge filter  388  connected to the twisted pair of conductors  348 . The H-bridge filter  388  is a two-pole band-pass filter that is used to simultaneously transmit power to the transponder  104  and receive data signals from the transponder  104 . The H-bridge filter  388  includes a first leg having a serially-connected inductor  392 , resistor  396  and capacitor  400  and second leg including a serially connected inductor  404 , resistor  408  and capacitor  412 . The legs include nodes  416  and  420  between the respective capacitors,  400  and  412 , and resistors,  396  and  408 . As shown in FIG. 4, the capacitors  400  and  412  are connected to the outputs  424  of an H-bridge driver  428 . The H-bridge filter  388  maximizes the current through the inductive antenna coil  380  and is split into a differential configuration to increase the noise rejection of the system  100 . Any external noise picked up by the H-bridge filter  388  will cancel itself out as a result of the differential configuration of the H-bridge filter  388 . Moreover, the filter  388  reduces the harmonic content from the H-bridge driver  428  and also prevents damage to the H-Bridge filter  388  in the event of a short on the twisted pair of conductors  348 . 
     The driver  428  includes an input  432  connected to a system clock or frequency generator  436  that outputs to the driver  428  an approximately 180 kHz (actually 179.7 kHz) square wave drive signal. As a result, the H-bridge driver  428  outputs to the H-bridge filter  388  a large sinusoidal voltage resulting in an approximately 25 volt peak (17.7 volt RMS) power supply across the antenna coil  380 . 
     The regulator  436  also is connected to a frequency divider  440  that divides the 180 kHz signal to an approximately 90 kHz (actually 89.85 kHz) transceiver reference signal. The transceiver reference signal consists of two approximately 90 kHz. signals with a 90 degree phase shift between them obtained by dividing the 180 kHz (actually 179.7 kHz) drive frequency. The frequency divider  440  includes an output  444  connected to a digital signal processor  448 . The digital signal processor  448  includes a QPSK demodulator  452  for receiving the reference signal. The QPSK demodulator  452  operates by comparing the phase of the reference signal with the phase of the signal received from the transponder  104 . The digital signal processor  448  also includes a decoder  456  connected to the demodulator  452 . The decoder  456  includes a pair of outputs  460  and  464  connected to digital-to-analog converters,  468  and  472 , respectively. The digital-to-analog converters  468  and  472  have analog outputs,  476  and  480  respectively. The analog output  476  generates an analog signal related to the temperature of the air in the wheel  46  and the analog output  480  generates an analog signal related to the pressure of the air in the wheel  46 . 
     The transceiver circuit  340  also includes a down converter  484 . The down converter  484  has two inputs  488  and  492 . The input  488  is connected to node  416  and the input  492  is connected to node  420 . The down converter  484  changes the signals from a modulated 90 kHz carrier into two base band signals that represent the in-phase and quadrature components of the transferred signal. The down converter  484  includes outputs  496  and  500 . The outputs  496  and  500  are connected to A/D converters  504  and  508 , respectively. The A/D converters  504  and  508  include respective outputs  512  and  516 , each of which are connected to the QPSK demodulator  452  of the digital signal processor  448 . 
     In operation, the system clock  436  generates a 180 kHz square wave drive signal that is simultaneously supplied to the H-bridge driver  428  and to the “divide by two” frequency divider  440 . The frequency divider  440  outputs the 90 kHz reference signal for the QPSK demodulator  452 . The 180 kHz drive signal is supplied via the H-bridge filter  388  to the antenna coil  380 . As the vehicle  10  moves along the traveling surface  99 , the wheel  46  rotates and the rim mounted transponder  104  rotates with the wheel  46 . When the transponder  104  rotates to a position where it is adjacent the stationary coil package  344  of the transceiver  108 , the inductive coil  124  and the antenna coil  380  are inductively coupled and the 180 kHz drive signal at approximately 25 volts RMS is transmitted to the inductive coil  124 . The drive signal is half-wave rectified by diode  280  and the resulting d.c. voltage is stored in capacitor  296  whenever switch  288  is closed to provide power to the transponder circuit  144 . Simultaneously, the drive signal is input to inverting Schmitt Trigger  192  which outputs a 180 kHz square wave that is transmitted to the frequency divider  204 . The frequency divider  204  outputs a 90 kHz signal to XOR gate  244  and outputs a 11.25 kHz signal to microprocessor  216 . 
     The microprocessor  216  receives the 11.25 kHz signal as well as pressure and temperature data from the pressure and temperature sensors  220  and  224 , respectively. The microprocessor  216  features a sleep mode to reduce the power requirements for the microprocessor  216 . While in the sleep mode, only the peripheral hardware devices, e.g., the SCI and the ADC, of the microprocessor  216  are enabled. This can be done because the ADC functions and the SCI functions require most of the operating time of the microprocessor  216 . In a typical processing cycle, the microprocessor  216  determines which ADC measurement should be taken, i.e., pressure or temperature, and initiates the ADC measurement. The processing circuitry of the microprocessor  216  enters the sleep mode, and the ADC measurement continues without any assistance from the processing circuitry of the microprocessor  216 . When the ADC measurement is complete, an interrupt is generated which awakens the processing circuitry of the microprocessor  216 . The processing circuitry then processes the data, prepares the data for transmission, initiates the transmission and again enters the sleep mode. After the transmission is complete, another interrupt is generated which awakens the processing circuitry to begin the cycle again. 
     The microprocessor  216  acquires data and generates data output signals using a transponder interrupt service routine software algorithm (“ISR”). The transponder ISR performs the bulk of the work in the transponder  104  and is called whenever the transmit register is empty. The purpose of the transponder ISR is to acquire a new sensor reading and to transmit the sensor reading via the SCI to the transceiver  108 . 
     Although the SCI is set for 9 bot data transmission, the SCI can simulate transmission of an 18 bit data word or packet by transmitting two consecutive 9 bit data words. The microprocessor  216  keeps track of which half of the 18 bit data word has been sent via a high/low packet flag (not shown). The eighteen bit data word includes, in order, a first reference bit, eight synchronization bits, eight data bits and a final reference bit. The first reference bit is a synchronization bit which indicates the beginning of the eighteen bit data word. The synchronization bits provide information that allows the transceiver  108  to synchronize to and decode the data word. The data bits represent the digital value of the physical characteristic, i.e., pressure, temperature, etc., of the vehicle wheel, and the final reference bit indicates the end of the data word. Seventeen bits of the eighteen bit data word are transmitted in the I-channel. 
     The first reference bit is transmitted in the quadrature or Q-channel. The Q-channel first reference bit indicates the beginning of the data packet and allows error checking to be conducted by the transceiver  108 . It should be understood that reference bits, synchronization bits and the data bits indicating the physical characteristic of the vehicle wheel can be encoded in either the I-channel or the Q-channel and can be arranged in any format that allows the data word to be decoded by the transceiver  108 . Moreover, the synchronization bits in the eighteen bit data word can be eliminated if it is determined that the system is operating reliably without both the first reference bit and the synchronization bits. If the synchronization bits in the eighteen bit data word are eliminated, they can be replaced with additional data bits representing a physical characteristic of the vehicle wheel. 
     FIG. 8 illustrates an example of QPSK data encoding. As shown in FIG. 8, for most of the data packet transmission, only the in-phase component of the carrier is modulated. Thus, the phase of the carrier is shifted between zero and 180 degrees depending upon the data. However, once per data packet, the in-phase component is set to zero and the quadrature (“QUAD”) component is set to either positive or negative one, depending upon the data. In FIG. 8, a data value of zero occurs during the synchronization pulse. This results in a phase shift of the carrier to 270 degrees. If the data had been one during the synchronization pulse, the phase would have moved to 90 degrees. 
     A down counter (not shown) in the microprocessor  216  maintains a variable to keep track of the number of complete 18-bit data words that have been transmitted to the transceiver  108 . When down counter indicates that ten pressure data words have been transmitted to the transceiver  108 , the variable changes value and the transponder  104  stops measuring from the pressure sensor and begins measuring from the temperature sensor. After ten temperature data words have been transmitted, the transponder switches back to measuring the pressure sensor. 
     The down counter described above is set to a number of different values depending upon the state of the program. Upon reset, the down counter is set to zero so that the first time the ISR is entered after reset, the microprocessor checks to see if the voltage is high enough to perform an ADC conversion (the sensors  220  and  224  need 5.0 volts d.c. or greater for operation). This would allow the transponder  104  to immediately send out valid data provided that the power supply is adequate to perform an ADC conversion. 
     The microprocessor also employs a transponder On-Acquire-Sensor software algorithm (“OAS”). OAS selects which sensor (pressure or temperature) to sample, and then completes the sampling of that sensor. The OAS differentially encodes the resulting sensor value. During the ensuing ADC conversion, the microprocessor  216  is put into sleep mode to conserve power. 
     The sensors  220  and  224  are also disabled when not being sampled. In order to sample a sensor, the sensor  220  or  224  must be enabled and a period of time must elapse before it is actually sampled. This time delay allows the sensor  220  or  224  to settle to a steady state value. 
     Since currently both the pressure and temperature synchronization patterns are fixed values and can be differentially encoded before run-time, the only data that need be differentially encoded at run-time are the sensor ADC values. This differential encoding is done by table look-up. The look-up table contains  128  entries that represent the differentially encoded value of the input data as described above. Whenever a data “1” is detected, the phase of the carrier signal is shifted 180 degrees to indicate to the transceiver that the data bit is a “1”. 
     The data output from the microprocessor  216  at 11.25 kHz and the 90 kHz carrier signal output from the frequency divider  204  are multiplied by XOR gate  244  to generate a modulation signal. The modulation signal is a BPSK signal at 90 kHz and is used to control switches  288  and  332  to convert the BPSK signal to a QPSK signal at 90 kHz. The modulation signal causes the switch  288  to open and close on alternating peaks of the 180 kHz drive signal. Data consisting of pressure and temperature values and synchronization information is “sent” to the antenna coil  124  by closing switch  288  on either the earlier or later set of alternating peaks. The quadrature pulse is “sent” to antenna coil  124  by closing switch  332 . Whenever switch  332  is closed, energy stored in the resonant circuit formed by coil  124  and capacitor  180  lost to the virtual ground  184 . 
     As long as the transceiver  108  is inductively coupled to the transponder  104 , energy is constantly being transferred from the transceiver coil  380  to the transponder antenna coil  124 . The 180 kHz input signal is rectified by diode  280 . Whenever the switch  288  is closed, the resulting rectified voltage is stored in capacitor to provide power to the transponder components. When the switch  288  is open, the transferred energy is stored in the resonant circuit formed by antenna coil  124  and capacitor 180 until the next time that switch  288  is closed. 
     At the same time, and still assuming that the transponder  104  and transceiver  108  are inductively coupled, data is continuously transferred from the transponder  104  to the transceiver  108 . As stated above, the transponder  104  generates a large modulated analog QPSK signal at one half the drive frequency. The QPSK signal is received by the stationary coil  380 , passes back through the H-bridge filter  388 , and is transferred to the down converter  484 . The down converter  484  multiplies the received 90 kHz carrier by the 90 kHz transceiver reference signal The output of the down converter  484  consists of two channels that contain the modulation information that was present on the received 90 kHz carrier. The two channels are commonly referred to as the In-phase (“I”) and Quadrature (“QN”) channels. 
     The I and Q channels are digitized using Analog to Digital converter (A/D). The digital signals are then sent to the digital signal processor  448 . The digital signal processor  448  performs all the processing to demodulate the signal, convert it to data, check the data, and output the digital value that was sent by the transponder  104 . 
     The digital signal processor  448  checks the data to ensure: that the data has a minimum signal threshold that is greater than 75% of the average amplitude of all data received during the last one second period; that the synchronization bit in the Q-channel is at least twice the signal level of the I-channel; that the data transmitted does not include part of a pressure data packet mixed with part of a temperature data packet; that all of the synchronization bits are correct; that the data is less than five seconds old; and that the standard deviation of the data is less than approximately one pound per square inch. The system  100  could also be programmed to perform many other tests or manipulations of the data. 
     The output of the digital signal processor  448  is the data received from the transponder  104 . The digital signal processor  448  checks the data for errors. If a data packet contains errors, the data packet is discarded and the next data packet is used. As discussed above, each data packet contains a temperature or pressure value, plus error detection and synchronization bits. The loss of one or even several data packets does not significantly affect performance of the system  100 . This is because the system  100  can transfer many data packets during one wheel revolution at low vehicle speeds. As vehicle speed increases, fewer packets can be transferred per revolution, but the coils  124  and  380  pass each other, i.e., are inductively coupled, more often. Thus, the average data packet transfer rate is 15 pressure and 15 temperature packets per second. The packet rate is only slightly dependent on wheel RPM. 
     If there are no errors for a given data packet, then the data is formatted, and, if necessary, is sent to the digital to analog (“D/A”) converter. The D/A converter is only used for interfacing to systems that can not accept a digital value. In either the analog or digital form, the data can be transmitted to a system for displaying the correct tire pressure for each tire on a readable display mounted inside the passenger compartment of the vehicle  10 . 
     FIG. 6 illustrates a portion of a transponder  600  that is another embodiment of the invention. The transponder  600  is identical to the transponder  104  except for the modulation block  604  shown in FIG.  6 . 
     Like parts or components are identified using like reference numerals. Like the modulation block  304 , the modulation block  604  shown in FIG. 6 is also a quadrature phase shift keyed (“QPSK”) modulation block. The modulation block  604  includes switch  608  connected to the output  276  of switch select controller  264 . As shown in FIG. 6, the node  612  of the parallel combination of antenna  124  and capacitor 180 is connected to ground  184  through diode  616 . Likewise, the node  620  of the parallel combination of antenna  124  and capacitor 180 is connected to ground through diode  624 . Node  620  is connected to node  628  through diode  630  and switch  608 . 
     In operation, data is encoded the same as in the transponder  104 . However, energy stored in the resonant circuit formed by coil  124  and capacitor  180  is transferred to the capacitor  296  whenever switch  608  is closed. This additional energy transfer makes the transponder  600  more efficient than transponder  104 . 
     FIG. 7 illustrates a portion of a transponder  700  that is another embodiment of the invention. The transponder  700  is identical to the transponder  104  except for the modulation block  704  shown in FIG.  7 . The modulation block  704  is a bi-polar phase shift keyed (“BPSK”) modulation block. As is commonly known in the art, BPSK modulation is a “sub-set” of QPSK modulation. In particular, BPSK modulation uses only the I-channel to encode and transmit data. Therefore, there is no Q-channel transmitted and any synchronization or reference bits must be encoded in the I-channel. 
     Other features of the advantages and invention are set forth in the following claims.