Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B60—VEHICLES IN GENERAL
  • B60C—VEHICLE TYRES; TYRE INFLATION; TYRE CHANGING; CONNECTING VALVES TO INFLATABLE ELASTIC BODIES IN GENERAL; DEVICES OR ARRANGEMENTS RELATED TO TYRES
  • B60C23/00—Devices for measuring, signalling, controlling, or distributing tyre pressure or temperature, specially adapted for mounting on vehicles; Arrangement of tyre inflating devices on vehicles, e.g. of pumps or of tanks; Tyre cooling arrangements
  • B60C23/02—Signalling devices actuated by tyre pressure
  • B60C23/04—Signalling devices actuated by tyre pressure mounted on the wheel or tyre
  • B60C23/0408—Signalling devices actuated by tyre pressure mounted on the wheel or tyre transmitting the signals by non-mechanical means from the wheel or tyre to a vehicle body mounted receiver
  • B60C23/0474—Measurement control, e.g. setting measurement rate or calibrating of sensors; Further processing of measured values, e.g. filtering, compensating or slope monitoring
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B60—VEHICLES IN GENERAL
  • B60C—VEHICLE TYRES; TYRE INFLATION; TYRE CHANGING; CONNECTING VALVES TO INFLATABLE ELASTIC BODIES IN GENERAL; DEVICES OR ARRANGEMENTS RELATED TO TYRES
  • B60C23/00—Devices for measuring, signalling, controlling, or distributing tyre pressure or temperature, specially adapted for mounting on vehicles; Arrangement of tyre inflating devices on vehicles, e.g. of pumps or of tanks; Tyre cooling arrangements
  • B60C23/02—Signalling devices actuated by tyre pressure
  • B60C23/04—Signalling devices actuated by tyre pressure mounted on the wheel or tyre
  • B60C23/0408—Signalling devices actuated by tyre pressure mounted on the wheel or tyre transmitting the signals by non-mechanical means from the wheel or tyre to a vehicle body mounted receiver
  • B60C23/041—Means for supplying power to the signal- transmitting means on the wheel
  • B60C23/0413—Wireless charging of active radio frequency circuits
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B60—VEHICLES IN GENERAL
  • B60C—VEHICLE TYRES; TYRE INFLATION; TYRE CHANGING; CONNECTING VALVES TO INFLATABLE ELASTIC BODIES IN GENERAL; DEVICES OR ARRANGEMENTS RELATED TO TYRES
  • B60C23/00—Devices for measuring, signalling, controlling, or distributing tyre pressure or temperature, specially adapted for mounting on vehicles; Arrangement of tyre inflating devices on vehicles, e.g. of pumps or of tanks; Tyre cooling arrangements
  • B60C23/02—Signalling devices actuated by tyre pressure
  • B60C23/04—Signalling devices actuated by tyre pressure mounted on the wheel or tyre
  • B60C23/0408—Signalling devices actuated by tyre pressure mounted on the wheel or tyre transmitting the signals by non-mechanical means from the wheel or tyre to a vehicle body mounted receiver
  • B60C23/0422—Signalling devices actuated by tyre pressure mounted on the wheel or tyre transmitting the signals by non-mechanical means from the wheel or tyre to a vehicle body mounted receiver characterised by the type of signal transmission means
  • B60C23/0433—Radio signals
  • B60C23/0447—Wheel or tyre mounted circuits
  • B60C23/0455—Transmission control of wireless signals
  • B60C23/0462—Structure of transmission protocol
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B60—VEHICLES IN GENERAL
  • B60C—VEHICLE TYRES; TYRE INFLATION; TYRE CHANGING; CONNECTING VALVES TO INFLATABLE ELASTIC BODIES IN GENERAL; DEVICES OR ARRANGEMENTS RELATED TO TYRES
  • B60C23/00—Devices for measuring, signalling, controlling, or distributing tyre pressure or temperature, specially adapted for mounting on vehicles; Arrangement of tyre inflating devices on vehicles, e.g. of pumps or of tanks; Tyre cooling arrangements
  • B60C23/02—Signalling devices actuated by tyre pressure
  • B60C23/04—Signalling devices actuated by tyre pressure mounted on the wheel or tyre
  • B60C23/0408—Signalling devices actuated by tyre pressure mounted on the wheel or tyre transmitting the signals by non-mechanical means from the wheel or tyre to a vehicle body mounted receiver
  • B60C23/0471—System initialisation, e.g. upload or calibration of operating parameters

Definitions

• the present invention relates to monitoring pressures in pneumatic tires on motor vehicles and, more particularly in conjunction with transponders associated with the tires for transmitting pressure and temperature measurements to an external (e.g., on-board) receiver (reader, or reader/interrogator).
• an external e.g., on-board receiver (reader, or reader/interrogator).
• run-flat driven deflated
• run-flat tires may incorporate reinforced sidewalls, mechanisms for securing the tire bead to the rim, and a non-pneumatic tire (donut) within the pneumatic tire to enable a driver to maintain control over the vehicle after a catastrophic pressure loss, and are evolving to the point where it is becoming less and less noticeable to the driver that the tire has become deflated.
• the broad purpose behind using run-flat tires is to enable a driver of a vehicle to continue driving on a deflated pneumatic tire for a limited distance (e.g., 50 miles, or 80 kilometers) prior to getting the tire repaired, rather than stopping on the side of the road to repair the deflated tire.
• a limited distance e.g. 50 miles, or 80 kilometers
• USP 4,578,992 discloses a tire pressure indicating device including a coil and a pressure-sensitive capacitor forming a passive oscillatory circuit having a natural resonant frequency which varies with tire pressure due to changes caused to the capacitance value of the capacitor.
• the circuit is energized by pulses supplied by a coil positioned outside the tire and secured to the vehicle, and the natural frequency of the passive oscillatory circuit is detected.
• the natural frequency of the coil/capacitor circuit is indicative of the pressure on the pressure- sensitive capacitor.
• a radio frequency (RF) signal indicative of the tire pressure may be transmitted to a remotely-located receiver.
• a "transmitting device” may have its own power supply and may be activated only when the pressure drops below a predetermined threshold.
• the transmitting device may be activated ("turned ON") by an RF signal from the remotely-located receiver, in which case the receiver is considered to be an "interrogator”.
• the transmitting device may be powered by an RF signal from the interrogator.
• the electronic device which monitors the tire pressure may have the capability of receiving information from the interrogator, in which case the electronic device is referred to as a "transponder".
• a "transponder” is an electronic device capable of receiving and transmitting radio frequency signals, and impressing variable information (data) in a suitable format upon the transmitted signal indicative of a measured condition (e.g., tire pressure) or conditions (e.g., tire pressure, temperature, revolutions), as well as optionally impressing fixed information (e.g., tire ID) on the transmitted signal, as well as optionally responding to information which may be present on the received signal.
• a measured condition e.g., tire pressure
• conditions e.g., tire pressure, temperature, revolutions
• fixed information e.g., tire ID
• Passive transponders are transponders powered by the energy of a signal received from the interrogator.
• “Active” transponders are transponders having their own power supply (e.g., a battery), and include active transponders which remain in a “sleep” mode, using minimal power, until “woken up” by a signal from an interrogator, or by an internal periodic timer, or by an attached device.
• the term “tag” refers either to a transponder having transmitting and receiving capability, or to a device that has only transmitting capability.
• tags 5 which are transponders are preferred in the system of the present invention.
• the term “tire-pressure monitoring system” indicates an overall system comprising tags within the tires and a receiver which may be an interrogator disposed within the vehicle.
• pneumatic tire monitoring system (20) comprising a transponder (22) and a receiving unit (24).
• the described transponder systems include interrogation units, pressure sensors and/or temperature sensors associated with the transponder, and various techniques for establishing the identity of the tire/transponder in multiple transponder systems. In most cases, such transponders require battery power.
• a transponder may be implemented as an integrated circuit (IC)
• the IC chip and other components are mounted and/or connected to a substrate such as a printed circuit board (PCB).
• PCB printed circuit board
• the sensors e.g., pressure, temperature
• the pressure sensor preferably will have an operating temperature range of up to 125°C, and should be able to withstand a manufacturing temperature of approximately 177°C.
• the pressure sensor must have an operating pressure range of from about 50 psi to about 120 psi (from about 345 kPa to about 827 kPa), and should be able to withstand pressure during manufacture of the tire of up to about 400 psi (about 2759 kPa).
• the accuracy including the sum of all contributors to its inaccuracy, should be on the order of plus or minus 3 % of full scale. Repeatability and stability of the pressure signal should fall within a specified accuracy range.
• a tire transponder (tag) must therefore be able to operate reliably despite a wide range of pressures and temperatures. Additionally, a tire transponder must be able to withstand significant mechanical shocks such as may be encountered when a vehicle drives over a speed bump or a pothole.
• a device which can be used to indicate if a transponder or the tire has been exposed to excessive, potentially damaging temperatures is the "MTMS" device or Maximum Temperature Memory Switch developed by. Prof. Mehran Mehregany of Case Western Reserve University. It is a micro-machined silicon device that switches to a closed state at a certain high-temperature point. The sensor switches from an "open" high resistance state of, for example, over 1 mega-ohm to a "closed" low resistance state of, for example, less than 100 ohm.
• Suitable pressure transducers for use with a tire-mounted transponder include: (a) piezoelectric transducers;
• USP 4,893,110 (Hebert; 1990), incorporated in its entirety by reference herein, discloses a tire monitoring device using pressure and temperature measurements to detect anomalies.
• a ratio of temperature and pressure provides a first approximation of a number of moles of gas in the tire, which should remain constant barring a leak of inflation fluid from the tire, (column 1, lines 18-26).
• the computer processes the measured values for pressure and temperature for each tire, and estimates for the pressure/temperature ratio (P/T estimate) are calculated for each wheel.
• P/T estimate estimates for the pressure/temperature ratio
• the ratio for one of the tires is compared with the ratio for at least another one of the tires, and an alarm is output when a result (N) of the comparison deviates from a predetermined range of values.
• FSK frequency-shift key
• USP 4,703,650 discloses a circuit for coding the value of two variables measured in a tire, and a device for monitoring tires employing such a circuit.
• the coding circuit comprises an astable multivibrator which transforms the measurement of the variables, for instance pressure and temperature, into a time measurement.
• the astable multivibrator delivers a pulse signal whose pulse width is a function of the temperature and the cyclic ratio of which is a function of the pressure.
• RF radio frequency
• a radio-frequency (RF) transponder comprises circuitry capable of transmitting information unique to an object with which the transponder is associated to an external reader/interrogator. Additionally, one or more transponder sensors (transducers) provide real-time parameter measurement at the transducer location. These measurements are transmitted to the external reader/interrogator, in the form of data, in a data stream on a signal which is output by the transponder, such as by impressing (modulating) the data stream onto an RF signal transmitted by the transponder to the external reader/interrogator.
• transponder sensors transmitters
• At least two real-time parameters are measured: pressure and temperature.
• Pressure is preferably measured by a separate (“off- chip”) pressure sensor, which is of a type that varies its capacitance value in a known way, such as a polynomial, or preferably as a substantially linear function of ambient pressure.
• the temperature sensor is embedded (“on-chip”) in the IC chip of the transponder and disposed so as to be subject to substantially the same ambient temperature as the pressure sensor so that a true, temperature-compensated pressure can readily be calculated.
• the transponder temperature reading (N ⁇ ) is a number which is directly proportional to the temperature, but the transponder pressure reading (N P ) is a function of both temperature and pressure.
• the ratio of readings (N ⁇ /N p ) is a number which is directly proportional to the capacitance of the pressure sensor, which is preferably directly proportional to the pressure. The ratio is insensitive to variations in transponder power level or reader-to-transponder coupling over a wide range of operating conditions.
• the individual transponders are calibrated, and the calibration information is stored in the transponder to be transmitted along with the measurement and identification data to a reader/interrogator.
• the reader/interrogator can most accurately calculate pressure and temperature readings from the raw measurement data transmitted to it.
• the calibration method employs a calibration chamber with highly accurate reference pressure and temperature sensors deployed therein.
• the transducer to be calibrated is placed in the chamber and exposed to a number of predetermined temperatures (e.g., 25°C) and pressures (e.g., 700 kPa) (as measured by the reference sensors) at a number of calibration points.
• a temperature reading (N ⁇ ) and a pressure reading (N P ) sensed by the transponder are recorded along with the reference temperature and pressure. From these readings and reference measurements the calibration constants (N- ⁇ s, m ⁇ , and ni p ) are calculated.
• the readings are range-checked to be sure that the calibration constants will be within acceptable ranges. Out-of-range readings cause rejection of the transponder.
• the calibration procedure is preferably designed to produce calibration constants which can be used in linear equations by the reader/interrogator, and the pressure equation will use the count ratio as its independent variable.
• a suitable form of linear equation is the "point slope" form, which describes a line by a "defining point” on the line and a slope at that defining point.
• the calibration procedure uses a minimum of three, and preferably at least five, calibration points.
• the calibration points are selected to bracket the range of expected temperatures and pressures in the monitored object.
• the temperature defining point is selected to be at a standardized temperature for the monitored object, and the pressure defining point is selected to be at a nominal pressure expected at the standardized temperature in the monitored object.
• cross-sectional views, if any, presented herein may be in the form of "slices", or “near-sighted" cross-sectional views, omitting certain background lines which would otherwise be visible in a true cross-sectional view, for illustrative clarity.
• Figure 1 A is a generalized diagram of an RF transponder system comprising an external reader/interrogator and an RF transponder within a pneumatic tire, according to the prior art;
• FIG. IB is a schematic diagram of a typical tire pressure monitoring system (TPMS), according to the prior art
• FIG. 2 is a block diagram of major components of an RF transponder, according to the present invention.
• Figure 3 is a schematic diagram of major portions of the RF transponder of Figure 2, according to the present invention
• Figure 3 A is a schematic diagram of a portion of the RF transponder of Figure 2, according to the invention
• Figure 3B is a schematic diagram of a portion of the RF transponder of Figure 2, according to the invention
• Figure 3C is a diagram of a memory space within the RF transponder of Figure 2, illustrating how data may be arranged and transmitted, according to the invention
• Figure 4 is a schematic block diagram of major portions of a reader/interrogator, according to the invention.
• Figure 5A is a graph of the temperature response of a transponder, according to the invention.
• Figure 5B is a graph of the pressure response of a transponder, according to the invention.
• Figure 6A is a block diagram of a transponder calibration system, according to the invention.
• Figure 6B is a flowchart of a transponder calibration procedure, according to the invention.
• Figure 6C is a bit map for a concatenated string of transponder calibration constants, according to the invention.
• Figure 7 is a graph of transponder readings as received by a reader/interrogator versus power supplied to a transponder, according to the invention.
• Figure 1A illustrates an RF transponder system 100 of the prior art, comprising an
• RF (radio frequency) transponder 102 disposed within (e.g., mounted to an inner surface of) a pneumatic tire 104.
• An antenna, not shown, is mounted within the tire 104 and is connected to the transponder 102.
• the transponder 102 is an electronic device, capable of transmitting an RF signal comprising unique identification (ID) information (e.g., its own serial number, or an identifying number of the object with which it is associated -in this example, the tire 104) as well as data indicative of a parameter measurement such as ambient pressure sensed by a sensor (not shown) associated with the transponder 102 to an external reader/interrogator 106.
• ID unique identification
• the external reader/interrogator 106 provides an RF signal for interrogating the transponder 102, and includes a wand 108 having an antenna 110, a display panel 112 for displaying information transmitted by /from the transponder 102, and controls (switches, buttons, knobs, etc.) 114 for a user to manipulate the functions of the reader/interrogator 106.
• the present invention is directed primarily to implementing the RF transponder. Nevertheless, certain functionality for a reader/interrogator to be compatible with the transponder of the present invention is discussed hereinbelow with respect to Figures 4 to 7.
• the ID and/or parameter measurement information may be encoded (impressed) in a variety of ways on the signal transmitted by the transponder 102 to the reader/interrogator 106, and subsequently "de-coded” (retrieved) in the reader/interrogator 106 for display to the user.
• the RF transponder 102 may be "passive", in that it is powered by an RF signal generated by the external reader/interrogator 106 and emitted by the antenna 108.
• the RF transponder 102 may be "active", in that it is battery- powered. Transponder systems such as the transponder system 100 described herein are well known.
• AN EXEMPLARY VEHICLE SYSTEM Figure IB illustrates a typical tire pressure monitoring system (TPMS) 150 of the prior art installed on a vehicle 152 (shown in dashed lines), such as a typical passenger vehicle having four pneumatic tires 104a, 104b, 104c and 104d (104) installed on four respective wheels (not shown).
• TPMS tire pressure monitoring system
• Each of the four tires 104a..104d (104) is equipped with an electronic module ("tag") 102a..102d (102), respectively, and associated sensor (not shown, well known) capable of monitoring one or more conditions such as air pressure and/or air temperature within the tire, and transmitting a radio frequency (RF) signal indicative of (e.g., modulated as a function of) the monitored condition(s) within the respective vehicle tire.
• tags 102 are suitably transponders, but may alternatively simply comprise one or more condition sensors and a radio frequency transmitter, as described hereinabove. Suitable tags 102 for use with the present invention are described in greater detail hereinbelow with respect to Figures 2 to 3C.
• the system 150 comprises a single reader/interrogator 166 (compare 106) and an associated display unit 162 (compare 112).
• One or more antennas 160a..160d may be disposed on the vehicle chassis to receive RF transmissions from the tags 102 and, optionally, to interrogate and/or power the tags 102.
• four antennas 160 are illustrated, each antenna being disposed at a fixed position on the vehicle adjacent a respective one of the tires 104, within the near field of the respective tag 102.
• the use of near field transmission has many distinct advantages over transmitting an inherently greater distance from each tire 104 to a centrally located antenna on the vehicle 152.
• antenna 160 adjacent each wheel is entirely optional and is well known, for example, as disclosed in USP 3,553,060; USP 3,810,090; USP 4,220,907; USP 5,541,574; and USP 5,774,047, all of which are incorporated in their entirety by reference herein.
• monitored condition information carried by the RF signals from the respective tags 102 can be decoded (e.g., demodulated) for subsequent display (162) to the operator of the vehicle 152. It is within the scope of the invention that suitable discernable visual and/or audible warnings can be used at the option of the vehicle manufacturer.
• the aforementioned TPMS 150 is presented merely as an example of an overall system installed on a vehicle, and should not be construed as limiting the present invention to a particular implementation, such as having an antenna at each of the wheel wells. Alternatively, for example, the TPMS 150 may employ a single antenna disposed in a suitable location to receive the transmissions of all of the tags 102.
• FIG. 2 is a block diagram of the RF transponder 200 (compare 102) of the present invention, illustrating the major functional components thereof.
• This exemplary system is described as an embodiment which preferably measures pressure and temperature, but it is within the scope of the invention to include measurement of other parameters which employ suitable sensors.
• the transponder 200 is preferably implemented on a single integrated circuit (IC) chip shown within the dashed line 202, to which are connected a number of external components. Other dashed lines in the figure indicate major functional "blocks" of the transponder 200, and include a transponder "core" 204 and a sensor interface 206.
• the components external to the IC chip 202 include an antenna system 210 comprising an antenna 212 and typically a capacitor 214 connected across the antenna 212 to form an L-C resonant tank circuit, an external precision resistor (Rext) 216, an external pressure-sensing capacitor (C P ) 218, and an optional external maximum temperature measurement switch (MTMS) 220.
• the antenna 212 may be in the form of a coil antenna, a loop antenna, a dipole antenna, and the like.
• the signal output by the transponder may be provided on a transmission line.
• the capacitor 214 may be omitted since it would not be of benefit in tuning such an antenna system.
• a transponder having a coil antenna is described.
• the pressure-sensing capacitor C p is preferably a rugged, low temperature coefficient, sensor with a capacitance versus pressure response having good sensitivity and linearity in the pressure range of interest.
• An example is an all-silicon "touch mode" capacitive pressure sensor such as are known in the art, and mentioned hereinabove.
• the transponder core 204 includes interface circuitry 222 for processing an RF signal, such as a 125 kHz (kiloHertz) un-modulated carrier signal received by the antenna 212, for rectifying the received RF signal, and for providing voltages for powering other circuits on the IC chip 202.
• the interface circuitry provides a regulated supply voltage (Vdd) of 2.5 volts, and a temperature-independent bandgap voltage (Vbg) of 1.32 volts.
• Vdd regulated supply voltage
• Vbg temperature-independent bandgap voltage
• the interface circuitry 222 also provides the received RF signal, preferably at the input frequency (Fi) it is received, to a clock generator circuit 224 which generates clock signals in a known manner for controlling the timing of other circuits on the IC chip 202, as well as the output frequency (Fc) of a signal which is transmitted by the transponder 200 to the external reader/interrogator (e.g., 106, 166).
• a clock generator circuit 224 which generates clock signals in a known manner for controlling the timing of other circuits on the IC chip 202, as well as the output frequency (Fc) of a signal which is transmitted by the transponder 200 to the external reader/interrogator (e.g., 106, 166).
• a timing generator/sequencer circuit 226 receives the clock pulses from the clock generator circuit 224 and processes (e.g., divides) the clock pulses to generate timing windows (W ⁇ and W P , described hereinbelow) for predetermined periods of time (t*,. and t P , respectively) during which parameter (e.g., temperature and pressure) measurements are made.
• the timing windows W ⁇ and W P may either be of substantially equal duration or of unequal duration.
• the timing generator/sequencer circuit 226 also controls the timing and sequence of various functions (e.g., pressure measurement and capture, temperature measurement and capture, described in greater detail hereinbelow) performed in the sensor interface 206, and is preferably implemented as an algorithmic state machine (ASM).
• ASM algorithmic state machine
• the transponder core 204 further includes a register/counter circuit 230 which includes a temperature register 232 (e.g., 12-bit) and a pressure register 234 (e.g., 12-bit) for capturing and storing temperature and pressure measurements (counts), respectively, and a block 236 of addressable memory (e.g., 120-bit), which includes an EEPROM array.
• the registers 232 and 234 and EEPROM array 236 are shown in a dashed line 238 representing a block of addressable memory on the IC chip 202.
• the register/counter circuit 230 also includes a multiplexer and column decoder 240, as well as a row decoder 242 for controlling the sequence in which signals (i.e., data) are output on a line 244 to a modulation circuit 246 which, via the interface circuitry 222, communicates selected measured tire operating characteristics in a data stream via the antenna system 210 to an external reader/interrogator (e.g., 106, 166).
• signals i.e., data
• a modulation circuit 246 which, via the interface circuitry 222, communicates selected measured tire operating characteristics in a data stream via the antenna system 210 to an external reader/interrogator (e.g., 106, 166).
• the transponder core 204 also includes a baud rate generator 248 which controls the rate at which modulating information (e.g., the temperature or pressure measurement) is applied to the modulation circuit 246.
• the baud rate generator 248 also provides a data carrier clock controlling the output frequency Fc of the transponder and a data rate clock controlling a rate at which the data stream including measurements, calibration information, identification, etc. is modulated onto the transponder 200 output carrier signal.
• the sensor interface 206 includes a circuit 250 for generating an output current I(T)/N on a line 251 which is related to a predictable characteristic voltage of a temperature-sensitive component (e.g., Vbe of a transistor Ql, described hereinbelow) which is superimposed on the external resistor (Rext) 216.
• the output current I(T)/N on the line 251 is provided to a relaxation oscillator 252.
• the relaxation oscillator 252 oscillates at a frequency controlled by a rate of voltage change (dV/dT) which is a function of the output current I(T)/N on line 251 and of internal capacitances C ⁇ , C ⁇ Q associated with the relaxation oscillator 252 as well as an external capacitance (C P ) 218 that can be switched into the oscillator circuit.
• An output signal Fosc' from the relaxation oscillator 252 is provided on a line 253 which, as will be explained in greater detail hereinbelow, is indicative of both ambient temperature and ambient pressure.
• the term "ambient" refers to the parameter being measured in the vicinity of the transponder 200, or more particularly in the vicinity of the respective sensors associated with the transponder 200.
• a pneumatic tire e.g., 104
• ambient pressure and “ambient temperature” refer to the pressure and temperature of the inflation medium (e.g., air) within the tire 104.
• an RF signal from an external source i.e., reader/interrogator, not shown, compare 106, 166
• This RF signal is rectified and used to power the RF transponder 200.
• Modulating information applied to the modulation circuit 246 is used to alter characteristics of the antenna system 210 (e.g., impedance, resonant frequency, etc.). These alterations are sensed by the external reader/interrogator 106, 166 and are decoded, providing communication of temperature and pressure information back from the RF transponder 200 to the external reader/interrogator 106, 166.
• the timing generator/sequencer circuit 226 controls when the external pressure- sensing capacitance (C P ) 218 is included in the generation of a signal at frequency Fosc' which is output by the relaxation oscillator 252, and also controls the capturing of the pressure and temperature counts via the data capture circuit 254. For example, to measure temperature, the temperature-sensitive current I(T) passes through the internal oscillator capacitors (Cpx-, and C ⁇ ), but the pressure-sensing capacitor (C P ) 218 is disconnected from (not included in) those capacitances. This means that the frequency Fosc' of the oscillator output signal seen on line 253 is a function of temperature alone.
• the output frequency Fosc' of the oscillator 252 on the line 253 will, as explained in greater detail hereinbelow, be a function of both pressure and temperature.
• an algorithm is employed in the reader/interrogator 106, 166 to extract a "pressure-only" reading from the pressure-temperature measurement.
• references made herein to "pressure readings”, “pressure counts”, “pressure response”, “pressure register” and the like generally refer to “pressure” as measured by this transponder technique which actually produces a hybrid pressure- temperature reading.
• this hybrid reading has been processed to remove its temperature component, the reading will be referred to as a “pressure-only” reading.
• the data capture circuit 254 directs the relaxation oscillator output signal Fosc' either to the temperature register 232 via line 255 or to the pressure register 234 via line 257, depending upon whether temperature or pressure is being measured.
• Counters convert the oscillator frequency Fosc' into counts which are stored in the registers 232, 234.
• the timing "window" provided by the timing generator/sequencer circuit 226 has a known, controlled duration.
• the count remaining in (captured by) the respective temperature or pressure register (232, 234 respectively) when the timing window "closes" is a function of (proportional to) the oscillation frequency Fosc' of the relaxation oscillator 252, and therefore a function of temperature or pressure, whichever is being measured during that timing window.
• the EEPROM array 236 is used to hold calibration constants that the reader/interrogator (e.g., 106, 166) uses to convert temperature and pressure counts (N ⁇ and N P , respectively, described in greater detail hereinbelow) into temperature and pressure readings which can be displayed (e.g., via display 112, 162) to a user.
• the EEPROM array 236 can also store the ID of the transponder, calibration data for the transponder, and other data particular to the given transponder.
• Figure 3 is a more-detailed schematic diagram 300 of several of the components of the transponder 200 of Figure 2, primarily those components described hereinabove with respect to the sensor interface section 206 of Figure 2.
• CMOS transistors are of the FET (field effect transistor) type, each having three “nodes” or “terminals” - namely, a "source” (S), a “drain” (D), and a “gate” (G) controlling the flow of current between the source and the drain.
• FET field effect transistor
• CMOS transistors are connected in a "current-mirroring" configuration.
• the concept of current- mirroring is well known, and in its simplest form comprises two similar polarity transistors (e.g., two PMOS transistors) having their gates connected with one another, and one of the pair of transistors being diode-connected.
• Current-mirroring generally involves causing a current to flow through the diode-connected transistor, which results in a gate voltage on the diode-connected transistor required to produce that current.
• the gate voltage of the diode-connected transistor is forced to become whatever voltage is necessary to produce the mirrored current through that transistor.
• the diode-connected transistor since the diode-connected transistor, by definition, has no gate current, by applying the gate voltage of the diode-connected transistor to any other identically-connected transistor, a mirrored-current will flow through the identically-connected transistor.
• the current-mirroring transistors all have the same physical area, in which case the mirrored current will be essentially the same as the current which is being mirrored. It is also known to produce a mirrored current which is either greater than or less than the current being mirrored by making one of the transistors physically larger or smaller (in area) than the other.
• their scaled (larger or smaller) areas will produce correspondingly scaled (larger or smaller) currents.
• the antenna system 210 comprises a coil antenna 212 and an optional capacitor 214 (connected across the antenna 212 to form an L-C resonant tank circuit) providing an alternating current (AC) output to a full- wave rectifier circuit 302.
• the full-wave rectifier circuit 302 (compare 222) comprises two PMOS transistors and two diodes, connected in a conventional manner, as shown, and outputs a full wave rectified direct current (DC) voltage on a line 303.
• a capacitor 304 is connected between the line 303 and ground to "smooth out” (filter) variations ("ripple") in the full wave rectified DC voltage on the line 303.
• the voltage on the line 303 thus becomes a usable voltage for the remaining components of the transponder - in this case, a positive supply voltage Vcc on the line 303.
• a temperature-sensing circuit 306 corresponding approximately to the base-emitter voltage-to-current converter 250 of Figure 2, is connected between the line 303 (Vcc) and ground, and includes four CMOS transistors labeled PI, P2, Nl and N2 and a lateral bipolar transistor labeled Ql, and is connected to the external resistor 216 (Rext).
• the transistors P2 and Nl are diode-connected, as illustrated.
• the two transistors PI and P2 are connected in a current-mirroring configuration, and the two transistors Nl and N2 are also connected in what can generally be considered to be a current-mirroring configuration.
• the source (S) of the transistor Nl is connected via the transistor Ql to ground, and the source of the transistor N2 is connected via the external resistor (Rext) 216 to ground.
• the resistor (Rext) 216 is an external, precision, reference resistor, whose value is substantially independent of temperature (as contrasted with the temperature dependency of the transistor Ql).
• a suitable value for the resistor (Rext) 216 is, for example, 20.5 kilohms or 455 kilohms.
• the transistor N2 is connected between the transistor P2 and the external resistor 216 (Rext) in a "source-follower” mode.
• a voltage is impressed on the gate (G) of the transistor N2
• its source voltage will “follow” its gate voltage (minus an inherent voltage drop (Vgs) between its gate and its source).
• Vgs inherent voltage drop
• As current flows through the transistor Nl, its gate voltage will be offset by its gate- source voltage drop (Vgs) above the emitter voltage at the transistor Ql. Since the transistors Nl and N2 are essentially identical, with the same current flowing through each of the two transistors Nl and N2, they will have identical gate-source voltage drops (Vgs).
• the voltage at the source of the transistor N2 across the external resistor 216 (Rext) will be essentially identical to the voltage at the emitter of the transistor Ql.
• the current through the external resistor 216 (Rext) will equal the emitter voltage of the transistor Ql divided by the resistance of the external resistor 216 (Rext).
• the circuit 306 ensures that the current I(T) flowing through the external resistor (Rext) is predictable, and is a function of the absolute temperature (T) of the transistor Ql.
• this temperature-dependent current I(T) flowing through the external resistor (Rext) 216 is mirrored to a relaxation oscillator (312, described hereinbelow) to provide a signal indicative of the temperature of the transistor Ql to the external reader (106, Figure 1).
• the output frequency Fosc' of the relaxation oscillator 312 will be a function of the absolute temperature (T) of the transistor Ql.
• the transponder circuit advantageously employs an inherent characteristic of such a transistor implemented in CMOS technology that the base-emitter voltage of the transistor Ql will vary by a predictable amount of -2.2 mv/°C (millivolts per degree Celsius).
• the transponder of the present invention is described in terms of a "passive" device, relying on RF energy being supplied to it by an external source (106, Figure 1) to power up its circuitry.
• the transponder contains its own power supply, such as in the form of a battery.
• first powering up circuitry such as described with respect to the temperature- sensing circuit 306, it is important to ensure that they "ramp up” to their normal operating state from their quiescent state in a reliable and predictable (controlled) manner.
• two lines 305 and 307 are illustrated which are connected between the temperature- sensing circuit 306 and a "startup" circuit 308.
• the startup circuit 308 (also part of the base-emitter voltage-to-current converter 250 of Figure 2) is connected between the supply voltage (Vcc) on the line 303 and ground, and serves two main purposes: (i) to get current flowing in the temperature-sensing circuit 306 when the transponder (200) first starts up from a powered down state; and (ii) to mirror and convert the current flowing through the transistor P2 from a supply-referenced current to a ground-referenced current.
• the transistor P3 is fabricated to have high channel resistance so as to function in a "weak pull-up” mode. With its gate connected to ground, it will always be “on”, and will behave essentially like a resistor having a fairly high resistance (e.g., > 1 megohm).
• the transistor P3 Since, at startup, no current flows elsewhere in the circuit, the transistor P3 operates to pull the gate of the transistor N3 towards the supply voltage (Vcc), thereby turning the transistor N3 "on", which effectively connects the grounded source of transistor N3 to its drain (D) which, in turn, grounds the gates of transistors PI, P2, and P4, and also grounds the drain of diode-connected transistor P2.
• This causes current to flow through the transistor P2 of the temperature-sensing circuit 306 into the drain of the transistor N3. Since the transistors PI, P2 and P4 are current-mirror connected (via the "Pbias" line 305), the current now flowing through transistor P2 will be mirrored in the transistors PI and P4.
• the transistor N5 is connected in a current-mirroring configuration with the transistor N4 (and, as described hereinbelow, with the transistor N6). Therefore, essentially, with a current equivalent to the current through the external resistor (Rext) 216 flowing through the transistor N5, the same current flows through the transistor N4, thereby establishing a reference voltage (Nbias) on the line 309.
• the reference voltage (Nbias) on the line 309, as well as a supply voltage (Vdd) on a line 309', are provided to a current scaling circuit 310.
• the supply voltage (Vdd) on the line 309' is provided in any suitable manner, such as a multiple of a bandgap voltage (Vbg) generated in a conventional manner elsewhere on the chip, and its magnitude (e.g., 1.32 volts) should be independent of temperature, such as inherent to the silicon process which is employed in making the chip.
• a stable (e.g., bandgap) voltage (e.g., Vbg) and the supply voltage (e.g., Vdd) derived therefrom is well within the purview of one having ordinary skill in the art to which the present invention most nearly pertains, and is described in greater detail hereinbelow with respect to Figure 3B.
• the current scaling circuit 310 (also part of the base-emitter voltage-to-current converter 250 of Figure 2) is constructed in the following exemplary manner.
• the sources of the transistors P5 and P6 are connected to supply voltage Vdd.
• the gate of a transistor N6 receives the reference voltage (Nbias) on the line 309.
• the transistor N6 is connected in a current-mirroring configuration with the transistor N5 (as well as with the aforementioned transistor N4) and will therefore mirror the flow of current I(T) through the transistors N4 and N5. Consequently, the flow of current through the diode-connected transistor P5 will mirror the flow of current through the transistors N4, N5 and N6.
• the transistors P5 and P6 are connected in a current-mirroring configuration, but are fabricated (using conventional CMOS fabrication techniques) such that current flowing through the transistor P6 is scaled up or down by a ratio (N) of the physical area of the transistor P5 to the physical area of the transistor P6. For example, if the transistor P6 is smaller in size than the transistor P5 (i.e., the transistor P5 is "N" times larger in area than the transistor P6), then the current flowing through the transistor P6 will be commensurately (N times) smaller than the current flowing through the transistor P5.
• the "scaled" current flowing through the transistor P6 is labeled “I(T)/N” in the figure, and is provided on a line 311 (compare 251) to a relaxation oscillator circuit 312 (compare 252). It is well known that the ratio of the currents between the transistors P5 and P6 can readily be established by conventional circuit processing techniques, such as by simply making one of the transistors larger than the other, or by implementing a one of the two transistors as the aggregate of two or more same-size transistors so that their aggregate area is larger than the area of the other of the two transistors.
• the relaxation oscillator circuit 312 (compare 252) is of fairly conventional design, and includes two pair of transistors at the "front end" of each of its two phase paths - a pair of complementary transistors P7 and N7 at the front end of a one phase path ( 1) 314a, and another pair of complementary transistors P8 and N8 at the front end of another phase path ( 2) 314b.
• each phase path 314a and 314b has a duty cycle (i.e., its "on" time), which may be the same as or may be different than the duty cycle of the other phase path 314b and 314a, respectively.
• each pair of transistors e.g., P7 and N7
• Each phase path 314a and 314b of the relaxation oscillator 312 has a comparator
• the capacitors C FXl and C ⁇ Q have exemplary capacitance values of 2-5 pf (picofarads) and 2-5 pf, respectively, and are preferably implemented as equal-valued "on- chip” devices, such as poly-to-poly capacitors exhibiting a low temperature coefficient (e.g., less than 20ppm).
• the positive (+) inputs (terminals) of the comparators 316a and 316b are tied together and are set to a reference threshold voltage Vbg, such as 1.32 volts, which is independent of temperature.
• a "NOR" logic gate 318a and 318b is connected at the output of each phase path 314a and 314b, respectively, and the two NOR gates 318a and 318b are cross-connected to form a latching circuit having an output on a line 319a and 319b.
• the cross-connected NOR gates 318a and 318b are thus capable of functioning as a flip flop, or an RS (re-set/set) latch.
• the relaxation oscillator 312 will oscillate at a frequency Fosc determined by the rise time of the capacitors C ⁇ and C ⁇ and, importantly, by the scaled current I(T)/N being supplied to the capacitors C ⁇ and C ⁇ . With greater current I(T)/N being supplied, the voltages of the capacitors C FX1 and will rise faster, crossing the threshold voltage faster, and causing the relaxation oscillator 312 to oscillate faster, thereby increasing the frequency Fosc of the signal on the line 319a.
• the signal on the line 319a is inverted by an inverter 320, as shown, to provide a signal of frequency Fosc 1 on the line 321.
• the oscillator 312 is controlled to run in two mutually-exclusive modes, a temperature-sensing mode (between times tO and tl) and a pressure-sensing mode (between times tl and t2), as controlled by the timing generator/sequencer 226.
• the frequency of the oscillator output signal Fosc (and Fosc') will be different in each of these two modes.
• a typical passenger vehicle tire may be properly inflated at about 32 psi (about 221 kPa). Since tire inflation pressures are normally specified as “cold” pressures (pressure measured when the tire is not heated by operation), and since a monitoring device will be reporting pressures measured in tires which are most likely in use and therefore "hot”, it is secondarily desirable to determine the temperature of the inflation medium (e.g., air) within the pneumatic tire.
• the inflation medium e.g., air
• a monitoring system e.g., TPMS 150
• This "cold” pressure could be considered a "temperature-independent” pressure, which is also an indication of the mass of air contained by the tire.
• the hybrid "pressure” measurement it produces must be converted (by different calculations detailed hereinbelow) to a true pressure-only measurement before it can be used in such gas-law calculations.
• the Goodyear EMT (extended mobility technology) series of tires is an example of a "run-flat" tire, an overall purpose of which is to allow a driver to travel up to 50 miles (approximately 120 kilometers) on a deflated (“flat”) tire, at "reasonable” operating speeds (e.g., 60 miles per hour, or 144 kilometers per hour), while maintaining normal control over the vehicle.
• Such run-flat tires are generally well known, and do not form a portion of the present invention, per se.
• the relaxation oscillator 312 By allowing the relaxation oscillator 312 to run, the frequency of its output signal Fosc (and Fosc') will be a function of the absolute temperature of (sensed by) the transistor Ql. This is true in both the temperature-sensing mode and the pressure-sensing mode of operation. In the temperature-sensing mode, and in the case that the capacitance values for C ra ⁇ and Cpj ⁇ are equal, which is preferred, the relaxation oscillator 312 will have a symmetrical (balanced, 50%) duty cycle.
• the pressure-sensing capacitor (C P ) 218 is switched by a semiconductor switch 350 across C ⁇ , which changes the duty cycle and output frequency Fosc (and Fosc') of the relaxation oscillator 312.
• Fosc and Fosc'
• the temperature-sensing mode only the fixed capacitors are being alternately charged (and discharged) resulting in a 50% duty cycle with a period proportional to ambient temperature.
• the pressure-sensing capacitor (C P ) 218 is switched into phase path 314b of the oscillator 312.
• phase path 314a behaves in the same manner as in the temperature-sensing mode
• phase path 314b behaves in a manner that is proportional to the capacitance value of the fixed capacitor C ⁇ plus the capacitance value of the pressure-sensing capacitor (C P ) 218.
• This slows down the oscillator and changes its duty cycle.
• the change in the duty cycle is indicative of the ratio of C P to
• the pressure-measurement window (W P ) can be increased in size (changed in duration) so as to allow for the capture of an appropriate number of pressure counts in the pressure register 234. This can readily be accomplished simply by establishing a larger (than otherwise) value for the time t2 which establishes the end of the pressure-measurement window (W P ) in the pressure-sensing mode (between times tl and t2), as controlled by the timing generator/sequencer 226.
• the temperature-measurement window W ⁇ (between times tO and tl) can be on the order of several ones (e.g., eight) of milliseconds, and the pressure-measurement window W P can be on the order of tens or dozens (e.g., eighty) of milliseconds.
• the scaled current (I(T)/N) flowing out of the current scaling circuit 310 to the relaxation oscillator 312 could be increased during the pressure-measurement window (W P ) to increase the fundamental frequency of the relaxation oscillator 312, thereby increasing the overall resolution of the pressure count.
• transistor P6 being smaller in size (area) than the transistor P5
• transistor P6' in lieu of the transistor P6, the transistor P6 1 having a larger area than the transistor P6 so that the ratio of the areas of the transistors P5 and P6 is closer to unity (i.e., less scaled down) and the current to the relaxation oscillator 312, hence its counting rate, is increased.
• Such switching in of another transistor P6' is readily effected with a switch (not shown) comparable to the aforementioned switch 350 which switches in the pressure-sensing capacitor (C P ) 218.
• certain parameters of the transponder circuit may be established to maximize its pressure-responsiveness and therefore improve the accuracy of the pressure reading displayed by the external reader/interrogator (e.g., 106, 166).
• the transponder responds to the changing capacitance of the pressure sensor (C P ) 218 by changing the value of a binary 12-bit word that is transmitted to the external reader/interrogator 106, 166.
• This binary word is the count of an oscillator frequency during a timing window W P (between tl and t2) established by the timing generator/sequencer 226.
• the pressure response can therefore be described as the change in counts per unit change in capacitance of the pressure-sensing capacitor (C P ) 218.
• Pressure-responsiveness (and resolution) of the transponder has been found to be dependent on a number of factors, each of which can be analyzed. For example, it has been determined that: (a) Increasing the pressure-measurement window W p to make it larger than the temperature-measurement window W t will increase the pressure count N p (and not the temperature count N ⁇ ) for a given value of the pressure-sensing capacitor (C P ) 218, to make up for the relatively lower oscillator frequency which occurs during pressure measurement compared to temperature measurement (as detailed hereinabove).
• Figure 3A illustrates the components involved in the final step of capturing temperature and pressure measurements in the transponder.
• the signal Fosc' output by the relaxation oscillator 312 is provided on line 321 (compare 253) to an input of each of two AND gates 360 and 362 in the data capture circuit 254.
• a signal (“Capture Temp") is provided by the timing generator/sequencer 226 to the other input of the AND gate 360 during the temperature-sensing window (W ⁇ ) so as to load the temperature register 232 via line 255 with the count ("data,” or “reading”) N ⁇ indicative of measured temperature.
• Another data signal (“Capture Press”) is provided by the timing generator/sequencer 226 to the other input of the AND gate 362 during the pressure-sensing window (W P ) so as to load the pressure register 234 with the count (“data,” or “reading”) N P indicative of measured pressure.
• Each of the registers 232, 234 has a counter (not shown) associated with it to convert the incoming oscillating signal Fosc' to a stored count. The two counts N ⁇ , N P are then shifted out of the registers 232 and 234, via the MUX 240, to the modulation circuit 246 described hereinabove.
• each of the temperature and pressure parameters can be transmitted back to the reader/interrogator 106, 166 as 12-bit data words in selected (known) portions of a larger (e.g., 144-bit) data stream.
• One bit in the overall data stream may be dedicated to the state (e.g., "closed” or "open") of the MTMS switch 220.
• Temperature is suitably measured by counting the number of cycles output from the oscillator 312 during a fixed time period (window W ⁇ of time from tO to tl) having a time period t,-.
• a down-counter (not shown, but associated with the temperature register 232) may be clocked by the oscillator, such that at the end of the window W ⁇ time period t,-, a temperature count N ⁇ is generated.
• the relationship between temperature count N ⁇ and temperature is substantially linear for the circuitry 300 of this embodiment.
• pressure can be measured by counting the number of cycles output from the oscillator 312 during a fixed time period (window W P of time from tl to t2) having a time period t P .
• a down-counter (not shown, but associated with the pressure register 234) may be clocked by the oscillator, such that at the end of the window W P time period t P , a temperature count N P is generated.
• the relationship between pressure count N P and pressure is a predictable function of both actual pressure and temperature for the circuitry 300 of this embodiment. As explained hereinbelow, by manipulating the temperature and "pressure" counts (N ⁇ and N P ) this hybrid pressure-temperature value can be used to determine a pressure-only value.
• the fundamental frequency of the oscillator 312 is set by parameters in the IC chip
• N P is a function (hybrid) of both temperature and pressure, and the relationship of N P to C P is nonlinear. Therefore, using a linear equation for calculating the pressure response would inevitably lead to significant errors over a range of pressures being measured. However, for limited ranges of pressures being measured, for example over a 20 psi (138 kPa) range of pressures, using a linear equation may be acceptable. A better approximation might be obtained using a polynomial equation, but this would complicate the reader/interrogator logic, making for slower response, and would require additional calibration constants.
• N ⁇ (t T *I(t)/n T )/(Nbg*(C FX1 +C TO )) [EQ. A]
• n ⁇ and rip are values for the scaling factor N in the scaled current I(T)/N which could be different during the temperature and pressure measurement windows, respectively.
• N T /N P (t T /t P )*(np/n T ) * (C FX . +C ⁇ +C P V ⁇ PX ⁇ C PX ,) or
• N T /N P (t T /t P )*(np. n T ) * (1 + (C ⁇ C ⁇ +C, ⁇ )) [EQ. B] Since everything to the right of the equals sign is a defined constant except for the pressure- sensing capacitance C P , it can be seen that there is a linear relationship between N T /N P and C P (and thus pressure). This means that N T /N P is only a function of pressure, and is insensitive to temperature or capacitor-charging current variations.
• the transponder could transmit only the counts N ⁇ and N P to a reader/interrogator, and the reader/interrogator would have to use assumed average values for slope and intercept in order to determine temperature and pressure.
• the preferred embodiment as described herein stores calibration constants in the transponder memory (e.g., 236) and transmits these calibration constants with the measurement counts N ⁇ and N P so that the reader/interrogator (e.g., 106, 166) can accurately calculate temperature and pressure using equations customized/optimized for the individual transponder generating the measurements.
• the calibration process will be described hereinbelow.
• the positive (+) inputs (terminals) of the comparators 316a and 316b are tied together and are set to a reference "bandgap" voltage Vbg, such as 1.32 volts, which is independent of temperature.
• Vbg the reference bandgap voltage
• the supply voltage (Vdd) on the line 309' may be provided as a multiple of the reference bandgap voltage (Vbg) so as to be a stable operating voltage for the current scaling circuit 310 and the relaxation oscillator 312.
• Figure 3B illustrates a circuit 370 suitable for generating the supply voltage Vdd.
• a temperature-independent calculable bandgap voltage Vbg is readily derived, based on the processing techniques employed in fabricating the IC chip, as being inherent to the selected process (e.g., CMOS).
• This bandgap voltage Vbg is provided to the positive (+) input of an operational amplifier 372, connected as shown, in a feedback loop having gain, to provide supply voltage Vdd as an integral multiple of the bandgap voltage Vbg.
• AN EXEMPLARY DATA STREAM As mentioned hereinabove, information (data) from the transponder is transmitted to the external reader/interrogator in the form of a data stream, a portion of which is the temperature count N ⁇ , another portion of which is the pressure count N P , and another portion of which represents the state (e.g., "closed” or "open") of the MTMS switch (220). Remaining portions of the data stream may contain information which is personalized to a given transponder unit such as its ID information (e.g., serial number), calibration constants, and the like.
• ID information e.g., serial number
• Figure 3C illustrates an exemplary architecture for information which is stored in memory (e.g., 238) within the transponder 200, as well as a data stream which is transmitted by the transponder 200 to the external reader/interrogator 106, 166.
• the memory 238 of the transponder core 204 has, for example, a 144-bit address space which includes 119 (one hundred nineteen) bits of programmable memory and one address location dedicated to the state of the MTMS switch 220 - these 120 (one hundred twenty) bits of programmable memory constituting the EEPROM 136 - plus two 12-bit temperature and pressure registers 232 and 234, respectively.
• Each of the 119 programmable memory bits can separately be written to with any combination of data, including synchronization (sync) pattern information, general data, error checking codes, and temperature and pressure calibration data.
• the EEPROM is 'block writeable', meaning that in the 'write' mode, the entire 120 bits of EEPROM are programmed to a logical (binary) value of "1". Individual bits can be 'erased' (set to a logical value of "0" simply by clocking the chip to the bit's physical address and placing the chip into the 'erase' mode). The address location is preserved.
• the first twelve data locations (000..011 in ROW 1) are reserved for sync.
• the next seventy one data locations (012..082 in ROWs 2 through 7) are for general information and a value for a data validation algorithm such as CRC (Cyclic Redundancy Check).
• the next data location (083) contains the logic level (state) of the MTMS switch 220.
• a logical value of "1" indicates that the MTMS switch is open and a logical value of "0" indicates that the MTMS switch is closed.
• each transponder unit is suitably calibrated prior to its installation in a tire.
• the next twelve data locations (096..107 in ROW 9) hold pressure calibration (e.g., defining point) data (“PRESS COMP”).
• N ⁇ and N P for temperature and pressure are generated, as described hereinabove, they are stored in ROWs 11 and 12 of the overall memory space, which correspond to the temperature and pressure registers 232 and 234, respectively.
• Various predetermined values can be stored to indicate error conditions such as overflow and short- circuit.
• the transponder of the present invention is not limited to any particular operating frequency.
• the choice of operating frequency will depend largely upon factors such as where the transponder is mounted in relationship to the object it is monitoring, the location of the reader/interrogator antenna (108), and relevant government regulations permitting (conversely, restricting) data transmissions of the type set forth herein in selected portions of the overall RF frequency spectrum.
• An example of suitable operating frequencies for operating the transponder in the United States is 60 KHz to 490 KHz.
• the transponder can be polled (and powered) by the reader/interrogator 106, 166 at a first "interrogation" frequency (Fi), and the data stream can be transmitted back to the reader/interrogator at a second "data carrier” frequency (Fc) which is, conveniently, a whole number multiple or fraction of the interrogating frequency.
• Fc Fi/2.
• Fc Fi/4.
• the frequency (Fc) at which the data stream is transmitted back to the reader/interrogator is independent of the data rate, which is established by the clock generator 224 and the baud rate generator 248.
• the range of available baud rates will typically be significantly less than the interrogation frequency (Fi).
• the baud rate is preferably derived from the interrogation frequency (Fi) of the reader/interrogator, such as a whole number fraction thereof.
• the interrogation frequency (Fi) may be 125 KHz
• the data carrier (Fc) may be set to 62.5 KHz, or half of the interrogation frequency.
• an interrogation frequency (Fi) of 13.56 MHz has been found to be suitable.
• the data stream such as the exemplary data stream described with respect to Figure 3C is impressed by the modulator circuit 246 onto the antenna 212, and transmitted to the reader/interrogator 106, 166.
• any suitable modulation scheme be employed, including amplitude modulation (AM), frequency modulation (FM), frequency shift keying (FSK), and phase shift keying (PSK).
• AM modulation is not particularly well-suited to digital transmission.
• Frequency modulation schemes such as FM or FSK may be somewhat problematic with regard to propagating the data-modulated transponder output signal through the medium of a pneumatic tire (e.g. , 104).
• the relaxation oscillator 312 outputs a lower than normal frequency signal Fosc' and thus reduces the temperature and pressure counts N ⁇ and N P below what they should be for a given temperature or pressure.
• the effect is illustrated by the downward curve on the plot 710 of temperature count N ⁇ and on the plot 720 of pressure count N P as the plots extend below the minimum power PWR ⁇ Fortuitously, the low-power effect is proportionally the same for both counts, so that the ratio N T /N P (plot 730) becomes relatively steady for all power levels down to a minimum power PWR Q needed to operate the transponder 200.
• FIG. 4 illustrates a relevant portion 400 of a reader portion of a reader/interrogator (e.g., 106, 166). It should be clearly understood that the transponder 200 of the present invention is suitable for use with virtually any suitably configured reader/interrogator. The description that follows is limited to broad architectural functions that would be performed in the reader/interrogator. One having ordinary skill in the art to which the present invention most nearly pertains would be able, from the description set forth herein, to implement these functions in an otherwise “generic" reader/interrogator.
• the data-modulated transponder output signal is received by the antenna 410
• the received signal is de-modulated and de-coded in a de-modulator/decoder circuit 420 (DE- MOD/DECODE) so that the different portions of the data stream can be properly segregated from one another.
• the data relating to temperature and pressure calibration (TEMP COMP, PRESS COMP, TEMP/PRESS SLOPES), the temperature count (N ⁇ ) and the pressure count (N P ) are provided to an arithmetic logic unit 422 (ALU) capable of generating a calibrated pressure-only signal ("PRESSURE") on a line 423 to the display 412 (compare 112, 162) as well as a calibrated temperature signal (“TEMPERATURE”) on the line 423.
• ALU arithmetic logic unit 422
• the reader/interrogator 400 may be designed to use the temperature and pressure signals to calculate (in the ALU 422) a "cold" tire pressure, or a similar reading indicative of the actual amount of air present in the pneumatic tire 104.
• the reader/interrogator 400 may also include a barometric pressure sensor so that a gauge pressure can be calculated from the typically absolute pressure reading of the transponder pressure sensing capacitor C P 218. This information can be displayed to the user either selectively or simultaneously with other relevant information such as the state of the MTMS switch 220, as well as data (DATA) relating to tire identification and the like. NORMALIZING DATA COMING FROM THE TRANSPONDER
• an exemplary RF transponder 200 capable of measuring temperature and pressure within a pneumatic tire 104, and transmitting digital information indicative thereof to a remote reader/interrogator 400.
• an oscillator 312 is operated during two timing windows (W ⁇ and W P ).
• the frequency Fosc' of the oscillator 312 is a function of a temperature-dependent scaled current (I(T)/N) that is dependent on the temperature of a diode (Ql) junction.
• a temperature count (or "reading") N ⁇ is captured and accumulated in the temperature register 232 during this first timing window W ⁇ .
• the oscillator frequency Fosc' is changed by switching in the pressure sensor capacitance C P .
• this approach causes the oscillator frequency Fosc' during the second timing window W P to be a function (hybrid) of both pressure and temperature.
• a "pressure” count (or “reading”) N P is captured and accumulated in the pressure register 234 during this second timing window W p .
• the so-called pressure count N P is understood to be, in actuality, the hybrid of pressure and temperature, as detailed hereinabove.
• the timing windows may either be of substantially equal duration or of unequal duration.
• the timing windows may suitably be of equal duration to one another, such as 8.192 ms.
• a transponder transmission may consist of 144 bits (compare Figure 3C), arranged as follows: 12 sync bits;
• the transponder transmission is received by an external reader/interrogator 400, whereupon the temperature and pressure counts (N ⁇ and N P ) may be stripped from the received data stream and operated upon (e.g., by ALU 422) using the calibration information in the 36 calibration bits, to generate a calibrated temperature signal and a calibrated pressure (only) signal.
• the oscillator frequency during the second timing window W P hence the pressure count “N P " is a function of both pressure and temperature - hence, the pressure count “N P " is not a linear function of pressure.
• a ratio N T /N P is a linear function of pressure, and pressure only.
• the capacitive pressure sensor 218 responds with a capacitance C P which is related in a reasonably linear way to pressure.
• Using the ratio N T /N P in a linear equation makes the determination of pressure from transponder counts simpler and more accurate than using the "pressure" count N P .
• To calculate pressure from the pressure count N P requires a nonlinear equation, such as a polynomial, and also requires temperature-related correction factors. A linear approximation of this nonlinear equation would be subject to significant inaccuracies for pressures only slightly more or less than the center point of the linear approximation curve.
• Figure 5 A is a graph 500 illustrating the temperature response of the transponder.
• the horizontal axis 502 represents temperature (T)
• the vertical axis 504 represents temperature count (N ⁇ )
• a line 506 is characteristic of the temperature response of the transponder.
• the temperature count (N ⁇ ) varies substantially as a linear function of the temperature (T).
• the line 506 has a negative slope: i.e., the temperature count N ⁇ decreases as the temperature T increases.
• a defining point 508, the significance of which is discussed below, is shown at the coordinate (T.,N T1 ) on the line 506.
• Figure 5B is a graph 510 illustrating the pressure response of the transponder.
• the horizontal axis 512 represents pressure (P)
• the vertical axis 514 represents a ratio (N T /N P ) of transponder's temperature count (N ⁇ ) divided by its pressure count (N P ).
• N T /N P a ratio of transponder's temperature count (N ⁇ ) divided by its pressure count (N P ).
• N T /N P varies substantially as a linear function of the pressure (P) and has a positive slope (N T /N P increases as the pressure P increases).
• a defining point 518 is shown at the coordinate
• (x ! ,y,) is the defining point; and m is the slope.
• the purpose of calibrating the transponder is to generate values for "calibration constants" N TO , m ⁇ , and m- to be used in linear equations (set forth hereinabove) for the lines 506 (temperature response) and 516 (pressure-only response) which will calculate temperature and pressure from the transponder temperature and pressure counts N ⁇ and N P . Due to variances in the manufacturing of the ICs and the pressure sensors, the values of the calibration constants cannot be assumed to be the same for all transponders without introducing large errors in the calculations of temperature and pressure.
• the transponder e.g., 200
• the transponder memory 236 for subsequent transmission to a reader/interrogator (e.g., 400).
• the reader/interrogator 400 utilizes these calibration constants, along with real-time temperature and pressure data (counts N ⁇ and N P ) to calculate the temperature at and pressure acting upon the sensor associated with the transponder 200.
• the air within a tire with which the transponder 200 is associated would then be pressured to be substantially identical to the transponder temperature.
• the tire during its rotation under load generates the heat that raises the internal air temperature about ambient levels.
• Figure 6A illustrates a calibration system 600 comprising a calibration chamber 602 capable of receiving one or more transponders 604 (compare 200) so as to subject the transponder(s) 604 to a number of predetermined temperatures and pressures.
• Figure 6B is a flowchart illustrating the calibration procedure 650.
• the transponder 604 is exposed to a number of temperatures and pressures, at a number of distinct calibration (measurement) points (A- E). For example:
• a first measurement at a first temperature and a first pressure e.g. , B
• a second measurement at a second temperature and second pressure which may be equal to the first pressure (e.g., D)
• Additional readings can be taken at additional calibration points, for example: F) 0° C, 600 kPa
• the transponder 604 is exercised (e.g., in the case of a passive transponder polled/powered by an external reader/interrogator 610 or, in the case of an active transponder, simply powered up), and two values are generated by the transponder: a temperature count N ⁇ and a pressure count N P . It is within the scope of the invention that, rather than transmitting these counts in an RF signal, the temperature and pressure counts may be directly read by probing exposed terminals on the transponder 604, which may be only partially packaged. As illustrated, a reader 610 (compare 400) receives the transponder transmission and provides the temperature and pressure counts to a microprocessor-based controller 620.
• the controller 620 controls one or more heating elements 622 disposed within the calibration chamber 602 and a pressurized air supply 624 in fluid communication with the calibration chamber 602 in response to a stored calibration protocol or to temperature and pressure setpoint inputs 626 and 628, respectively.
• highly accurate temperature and pressure sensors 612 and 614 are disposed suitably in the calibration chamber 602, preferably close to the transponder 604 being calibrated.
• the controller 620 can use the sensors 612 and 614 for feedback control of the calibration chamber 602 temperature and pressure, or can display their readings to an operator manually controlling the procedure.
• the temperature in the chamber 602 is first allowed to "settle” before a temperature measurement is taken (i.e., before the temperature count N ⁇ is read by the reader 610).
• the pressure in the chamber 602 is first allowed to "settle” before a pressure measurement is taken (i.e., before the pressure count N P is read by the reader 610).
• the controller 620 Since the equations used by a reader/interrogator according to this invention assume that the defining point counts (N T1 , N P1 ) are exactly at the selected point of interest (25°C, 700 kPa for this example), the controller 620 must either obtain counts (N-r ⁇ , N ⁇ ,, ⁇ ) when the chamber 602 is at the proper reference values (25 °C, 700 kPa), or must use multiple reference values and their corresponding recorded counts to interpolate the value for the count at the defining point.
• the exemplary transponder values (five temperature counts and five pressure counts) recorded during a calibration run and the associated reference values are summarized in the following table.
• the temperamre and pressure counts (N ⁇ and N P ) received from the transponder are range checked to ensure that they are within certain predetermined ranges (step 656 of Figure 6B). Transponders that have temperamre and pressure counts that fall outside of these ranges are considered defective and may be rejected prior to being calibrated.
• the range check procedure 656 is as follows:
• Nra temperature defining point
• the count at the defining point of the temperamre response line 506 Must fall within a predetermined range of counts.
• N T50 and N w The difference between these two counts represents the slope of the temperamre response line 506. The absolute value of the difference must fall within a predetermined range of counts.
• N P70Q25 pressure defining point i.e., the count at the defining point of the pressure-only response line 516): Must fall within a predetermined range of counts.
• N P60025 and N Pg0O25 The difference between these two counts represents the slope of the pressure-only response line 516. The absolute value of the difference must fall within a predetermined range of counts.
• the calibration constants are then calculated (step 658, Figure 6B) for the transponders that pass the range check, as follows:
• the N-r ⁇ value is the temperamre count N ⁇ generated by the transponder 604 when it is known (via reference temperamre sensor 612) to be at a temperamre of 25°C. This value is converted to binary, and is assigned a suitable number of binary bits (e.g. , 10 bits). The conversion optionally includes subtracting a minimum value (e.g., 1900) from the raw count value in order to zero-base the range of values for the constant.
• a minimum value e.g., 1900
• the temperatures T 2 and T can be any two reference temperatures, and N ⁇ and N T1 are the corresponding temperamre counts ("calibration temperamre readings").
• One of the temperatures (T,) is conveniently the temperamre used as the temperamre defining point with reading N-r ⁇
• the other temperamre (T 2 ) is conveniently plus or minus 25 °C from the temperamre defining point.
• the two temperatures T 2 and T are as far apart from one another as possible, straddling the temperamre defining point, and within the operating range of the transponder (and more preferably within the anticipated temperamre range of the tire), since a greater span between the two temperamre points will inherently provide greater resolution and accuracy for the slope of the temperamre line.
• two temperatures such as 50°C and 0°C which are in addition to the temperamre defining point (25°) provide a large span for calculating the slope of the line 506.
• Using values at two points straddling the temperature defining point also allows two more slope calculations (between the endpoints and the defining point) as a check for non-linear response and for calibration procedure errors.
• m ⁇ (N T50 -N TO )/(T 50 -T 0 ) [EQ. 5]
• the value of m ⁇ is a negative number, and must fall within a range of a predetermined number of counts per °C (as determined by the range check on N T50 -N TO ). Any transponder having a temperamre response slope falling outside of this range can be rejected.
• a binary conversion is performed, and the result is assigned a suitable number of binary bits (e.g., 7 bits).
• the conversion optionally includes multiplying the calculated slope m ⁇ by a number (e.g., -20) to turn the value into a positive whole number, and then subtracting a minimum value (e.g., 60) in order to zero-base the range of values for the constant.
• a number e.g., -20
• a minimum value e.g. 60
• a value for the pressure defining point T /N PTOO ⁇ is calculated by dividing the temperamre counts N ⁇ generated by the transponder 604 when it is known (via reference temperamre sensor 612) to be at a temperamre of 25 °C, by the pressure counts N P generated by the transponder 604 when it is known (via reference sensors 612, 614) to be at a pressure of 700 kPa and a temperamre of 25°C.
• a binary conversion is performed, and the result is assigned a suitable number of binary bits (e.g., 12 bits).
• the conversion optionally includes multiplying the calculated ratio by a number (e.g., 1000) to turn the value into a positive whole number, and then subtracting a minimum value (e.g., 2400) in order to zero-base the range of values for the constant.
• a number e.g., 1000
• a minimum value e.g., 2400
• Pressure slope (m__) (Calibration Constant D): The slope m- of the pressure response line 516 is (from above): As was the case with calculating the slope of the temperamre line 506, preferably two pressure values (P,, P 2 ) are used (providing they are at the same temperamre): preferably representative of the operating range of pressures, and preferably on either side of the defining pressure value. The resulting pressure counts ("calibration pressure readings") are N P1 and N n , respectively.
• Two calibration pressures such as 600 kPa and 800 kPa are employed for example, both at the same temperature (e.g., 25°C) and spanning the pressure defining point (700 kPa) so as to provide a large span for greater resolution and accuracy. Also, using values at two points straddling the pressure defining point allows two more slope calculations (between the endpoints and the defining point) as a check for non-linear response and for calibration procedure errors. Substituting these values for N ⁇ /Np-, and N T /N P1 in EQ. 4 yields the following equation for the slope m_.
• ni p (N ⁇ /N Pg00 ⁇ 25 - N T /N P60025 )/(800-600) [EQ. 6]
• the value of ni p is a positive number, and must fall within a predetermined, acceptable range (as determined by the range check on N Pg0025 - N P60025 ). Any transponder having a pressure response slope falling outside of this range can be rejected.
• a binary conversion is performed, and the result is assigned a suitable number of binary bits (e.g., 7 bits).
• the conversion optionally includes multiplying the calculated slope ni p by a number (e.g., 10000) to turn the value into a positive whole number, and then subtracting a minimum value (e.g., 40) in order to zero-base the range of values for the constant.
• a number e.g. 10000
• a minimum value e.g. 40
• the binary values of the four calibration constants N ra , m ⁇ , N T /N ⁇ and ni p are concatenated (step 660, Figure 6B) into a single string of calibration bits (e.g., 36 bits). These bits are stored (step 662, Figure 6B) in the transponder 604 in the bit cells assigned for calibration bits (e.g., rows 8, 9 and 10 of Figure 3C).
• the concatenated string 680 of calibration constants (N-r ⁇ , m ⁇ , and ni p , respectively) is illustrated in the exemplary bit map of Figure 6C.
• the reader (e.g., reader/interrogator 106, 166, 400) utilizes these calibration constants (682, 684, 686 and 688) as received from the transponder, along with real-time temperamre and pressure counts N ⁇ and N P from the transponder to calculate the temperamre and pressure sensed by the transponder, hence of the object (e.g. , air within the tire) with which the transponder is associated.
• these calibration constants (682, 684, 686 and 688) as received from the transponder, along with real-time temperamre and pressure counts N ⁇ and N P from the transponder to calculate the temperamre and pressure sensed by the transponder, hence of the object (e.g. , air within the tire) with which the transponder is associated.
• the reader 400 converts the counts to engineering units using the calibration constants 682..688 as terms of the equations.
• the equations stored in the reader 400 are the same as those used to generate the calibration constants, except now the counts are the independent variable and the temperamre and pressure are the dependent variables.
• N T ((N T -N TO )/ m ⁇ ) + 25°C [EQ. 7] where: N ⁇ is the count received from reading the transponder;
• N-n j is Calibration Constant A; and m ⁇ is Calibration Constant B.
• N T /N P is the temperamre count N ⁇ divided by the pressure count N P , both as received from the transponder; is Calibration Constant C; and m p is Calibration Constant D.
• an important advantage of the invention is that for the transponder 200 of this invention, the relationship between N T /N P and the pressure sensor capacitance is linear, and independent of the temperamre measurement of the transponder 200, thereby greatly simplifying the design of the reader/interrogator 400.
• the data transmitted by the transponder 200 is also normalized with respect to (relatively insensitive to) power drops, coupling strength variations, leakage currents and the like.

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• 1999
  • 1999-12-14 JP JP2001545104A patent/JP2003517150A/ja active Pending
  • 1999-12-14 WO PCT/US1999/029606 patent/WO2001043997A1/en not_active Application Discontinuation
  • 1999-12-14 EP EP99967303A patent/EP1240035A1/en not_active Withdrawn
  • 1999-12-14 AU AU23608/00A patent/AU2360800A/en not_active Abandoned
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