GB2519956A

GB2519956A - Wheel unit for a wheel unit auto-location system

Info

Publication number: GB2519956A
Application number: GB1319320.6A
Authority: GB
Inventors: Jonathan Barr; Samuel Strahan; Alan Millen
Original assignee: Schrader Electronics Ltd
Current assignee: Schrader Electronics Ltd
Priority date: 2013-11-01
Filing date: 2013-11-01
Publication date: 2015-05-13
Anticipated expiration: 2033-11-01
Also published as: GB2519956B; GB201319320D0

Abstract

A method of sending a transmission from a transmitter unit, particularly a tyre pressure monitoring unit. The method includes initiating the transmission after a variable length delay has elapsed from a reference point in an output signal of a wheel phase angle sensor in the transmitter unit, and selecting the length of the variable delay to compensate for a variable phase delay associated with the output signal. The method provides a reliable technique for processing the output of the wheel phase angle sensor to determine the Angle of Interest and provide correlated transmissions from the wheel mounted unit to a central controller for the purposes of auto-location.

Description

Wheel Unit for a Wheel Unit Auto-Location System

Field of the Invention

The present invention relates to transmitter units, particularly for systems for performing auto-location of wheel monitoring units mounted on the wheels of a vehicle, and more particularly to a wheel monitoring unit for use in same. The invention relates particularly, but not exclusively, to wheel mountable tyre pressure monitoring units and to Phase Auto-Location (PAL) systems.

Background to the Invention

Tire pressure monitoring systems generally include a wheel monitoring unit in the form of a tire pressure monitoring (1PM) unit (commonly referred to as a 1PM sensor) in or at each wheel of a vehicle, and a central controller which receives tire pressure information from each TPM sensor, for reporting to the driver of the vehicle. Auto-location involves identification of each TPM sensor and determination of its position on the vehicle, automatically and without human intervention. Auto-location may be done initially upon installation and subsequently in the event of tire rotation or replacement. Performing auto-location typically involves determining the identity, e.g. by serial number, of a 1PM sensor in each of the wheels in the car. Knowing the identity of the 1PM sensor in each wheel allows a pressure by position display to be implemented and shown to the driver, while in vehicles with different placard tire pressures for front and rear axles, it is desirable to know TPM sensor identities and positions in order to check pressure against a correct threshold for an applicable axle.

International patent application WO 2011/038033 and United States patent application US 2013/0079977 each discloses a TPM system in which the central controller uses wheel phase information transmitted to it by the TPM sensors to perform auto-location of the sensors. These systems are of a type that may be referred to as Phase Auto-Location (PAL) systems.

In a PAL system, the wheel mounted unit includes a wheel phase angle sensor, for example a shock sensor, that produces a signal that is indicative of the angular position of the wheel. As the sensor rotates with the wheel, it tends to produce a sinusoidal output signal. In practice, the output signal produced by such sensors rarely resembles a perfect sinusoid. Imperfect road surfaces tend to result in what appears to be random noise being superimposed on the signal. Additionally, it has been observed that the output signal includes one or more additional frequency components as well as a fundamental frequency component. Unwanted signal components such as noise can be filtered using a suitable filter, for example the tracking filter described in US2012234087. The filtered signal may then be converted to digital signals which can be analysed by a suitably programmed processor.

In a PAL system the wheel monitoring units attempt to reliably detect when a fixed point on the circumference of the wheel reaches a particular angular position, e.g. when the location of the valve is closest to the road surface. The specific location of this point on the wheel circumference or its angular position is not important but should be fixed per journey. In practice, the fixed point may be detected by determining a reference point in the output signal of the wheel phase angle sensor. The fixed point, and more particularly the reference point on the output signal, may be referred to as the Angle Of Interest (Aol). The AOl may be used in determining the timing of transmissions to the central controller and to perform auto-location, for example by correlation with the vehicle's ABS data.

It would be desirable to provide a reliable technique for processing the output of the wheel phase angle sensor to determine the AOl and provide correlated transmissions from the wheel mounted unit to the central controller for the purposes of auto-location.

One consideration when developing a PAL sensor is the relationship between the relative position of a fixed point in the wheel's circumference and a designated reference point in the output signal of the wheel phase angle sensor. The time required for the signal from the wheel phase angle sensor to pass though the measurement system, known as phase delay, must be considered as changes to this time will cause variability in the reference point on the wheel's circumference that the ECU will correlate to. The phase delay can vary due to the behaviour of components of the wheel unit over vehicle speed (and hence frequency) making it difficult to ensure this time is fixed. For example, a shock sensor is a capacitive device which has an associated phase response. This means that as wheel rotation speed (and hence frequency) changes, there may be an associated change in phase delay. Also, amplifiers which are typically used to amplify the small shock sensor signal also have an associated phase response, as does a typical filter, including the above-mentioned tracking filter.

These and other circuit elements introduce variable phase delay into the system that varies the relative position of the reference point in the output signal (AOl) to a fixed point in the circumference of the wheel.

It would be desirable to provide a technique to compensate for variable phase delays in the wheel mounted units.

Summary of the Invention

A first aspect of the invention provides a method of sending a transmission from a transmitter unit comprising a sensor, said method including: initiating said transmission after a variable length delay has elapsed from a reference point in an output signal of said sensor; and selecting (which term is intended to embrace calculating or otherwise determining) the length of said variable delay to compensate for a variable phase delay associated with said output signal.

A second aspect of the invention provides a transmitter unit comprising a sensor, said transmitter unit including: means for initiating a transmission after a variable length delay has elapsed from a reference point in an output signal of said sensor; and means for selecting the length of said variable delay to compensate for a variable phase delay associated with said output signal.

In preferred embodiments, the transmitter unit is incorporated into a wheel mountable unit, in particular the wheel unit of a tyre pressure monitoring system.

A third aspect of the invention provides a tyre pressure monitoring system comprising the transmitter unit of the first aspect of the invention.

Typically, the transmitter unit includes a controller for, amongst other things implementing said initiating means and said selecting means.

In some embodiments, selecting the length of said variable delay involves adjusting said reference point to compensate for said variable phase delay, and selecting the length of said variable delay such that said transmission is initiated after a known delay has elapsed from the occurence of said compensated reference point. The variable delay may comprise an applied delay and a processing time and/or one or more other delays. Selecting the length of said variable delay may involve calculating the length of said applied delay.

Determining the length of said variable phase delay may be achieved using one or more mathematical models of electonic circuitry associated with said output signal and/or by determining the length of said variable phase delay from pre-calculated values associated with electronic circuitry associated with said output signal.

Advantageously, compensating for a variable phase delay involves compensating for phase delays caused by one or more components of said wheel unit (or other transmitter unit), for example any one or more of a wheel phase angle sensor, an amplifier, a filter or an analogue-to-digital converter.

The length of said variable delay may be selected to compensate for all or part of the variable phase delay associated with said output signal.

A fourth aspect of the invention provides a method of determining a reference point in a signal having a sinusoidally varying signal level, said method including: determining a first time at which said signal level crosses an first threshold; determining a second time at which said signal level crosses a second threshold; and defining said reference point as the point in said signal occuring at a pre-determined time between said first and second times.

Advantageously, said signal may comprise said sensor output signal, or a signal derived therefrom.

Said pre-determined time may conveniently be halfway between said first and second times.

Said determining a first time may involve determining the time at which said signal level crosses said first threshold in a first direction (increasing or decreasing signal level) and said determining a second time may involve determining the time at which said signal level crosses said second threshold in a second direction (decreasing or increasing signal level) opposite to said first direction.

Advantageously, the frequency period of said signal may be determined as the time between the respective reference point determined in respect of consecutive instances of said first and second times.

In preferred embodiments, the method involves measuring a plurality of consecutive instances of said frequency period, comparing said respective measured frequency periods, and if said respective measured periods are sufficiently similar, determining that said output signal is of a satisfactory quality.

Said consecutive instances of said frequency period may be measured over a measurement period corresponding to a plurality of cycles of said output signal. Preferably, the method includes selecting as said reference point, the reference point defined in respect of a last of said plurality of measured frequency periods.

Other preferred features are recited in the dependent claims.

Other advantageous aspects of the invention will be apparent to those ordinarily skilled in the art upon review of the following description of a preferred embodiment and with reference to the accompanying drawings.

Brief Description of the Drawings

An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic view of a tyre pressure monitoring system incorporated into a vehicle; Figure 2 is a schematic view of a wheel monitoring unit suitable for use with the system of Figure 1; Figure 3 is a graphical representation of an output signal from a wheel phase angle sensor comprising for example an accelerometer or a shock sensor; Figure 4 is a further graphical representation of an output signal from a wheel phase angle sensor being part of the wheel monitoring unit of Figure 2; and Figures 5A and SB are timing diagrams illustrating the transmission and receipt of signals within the system of Figure 1.

Figure 6 is a graphical representation of the output of the phase angle sensor after passing through the sensor interface 207 of Figure 2 at 2Hz and at 10Hz.

Detailed Description of the Drawings

FIG. 1 illustrates a wheel monitoring system in the particular form of a tire pressure monitoring system 100. The system 100 is shown incorporated in a standard vehicle 1 having four wheels, namely a left front wheel (LF), a right front wheel (RF), a left rear wheel (LR) and a right rear wheel (RR), although it may alternatively be used with a vehicle having more or fewer wheels. The system includes wheel units 101, 102, 103 and 104 each being associated with a respective wheel of the vehicle 1.

In typical embodiments, the system 100 includes antilock brake system (ABS) sensors 200, 201, 202 and 203, usually one for each wheel. In this embodiment, ABS sensors 200, 201, 202, 203 are each associated with a respective wheel of the vehicle 1. In other embodiments, ABS sensors may not be associated with all of the vehicle's wheels.

The system 100 includes a central controller in the form of an Electronic Control Unit (ECU) 300, and an associated wireless receiver 400 (which may comprise a transceiver) for receiving transmissions from the wheel units 101, 102, 103, 104. The ECU 300 is coupled to the ABS sensors 200,201,202, 203 via a communication bus such as a Controller Area Network (CAN) bus and receives ABS data from the ABS sensors 200 201, 202, 203. The ECU 300 includes a processor 302 and data storage 304. The ECU 300 may be implemented by any suitable means, for example a microprocessor, microcontroller, an Application Specific Integrated Circuit (ASIC), or other suitable data processing device programmed to perform the functions described herein.

In use, the ECU 300 also receives data from the wheel units 101,102, 103 and 104 via the receiver 400. For example, the wheel units 101, 102, 103 and 104 may be configured to transmit radio frequency or other wireless communication signals conveying data and other information to the ECU 300. The respective wheel units include a suitable radio (or other wireless) transmission circuit and the ECU 300 includes a suitable radio (or other wireless) reception circuit for wireless communication. The system may be equipped to provide two-way communication between the ECU and the wheel units in which case each may include a suitable transceiver rather than just a receiver or transmitter.

The system 100 is configured to perform auto-location of the wheel units 101, 102, 103, 104 using data transmitted from the units 101, 102, 103, 104 tothe ECU 300. In preferred embodiments, the transmitted data includes wheel phase angle information and so the system 100 may be referred to as a Phase Auto-Location (PAL) system, for example of the type disclosed in WO 2011/038033 or US 2013/0079977. As such, the ECU 300 is configured to correlate the data received from the wheel units 101, 102, 103 and 104 with ABS data received from the ABS sensors in order to perform auto-location, for example in the manner described in WO 2011/038033 or US 2013/0079977. In alternative embodiments, the system may include one or more sensors, devices, sub-systems, or mechanisms that provide wheel phase and/or speed data for use in addition to, or instead of data supplied by an antilock brake system when performing auto-location. The system 100 is typically operable in more than one mode, including for example a first mode in which it performs a task such as tire pressure monitoring, and a second mode in which it performs auto-location.

Referring to FIG. 2, the structure of a typical wheel unit 101 is described in more detail. The wheel units 102-104 may incorporate the same structure as that of the wheel unit 101. The illustrated wheel unit 101 includes a controller, conveniently a microcontroller 202, a battery 204 (or other electrical power source), a transponder coil 206, a sensor interface 207, a pressure sensor 208, a wheel phase angle sensor 212, a transmitter 214 and an antenna 216. In other embodiments, the wheel unit 101 may have a different structure from the structure illustrated in FIG. 2, and some of the illustrated components may be omitted depending on the application. For example, in the present embodiment it is assumed that the system 100 is operable as a tyre pressure monitoring system and so the wheel unit 101 includes a pressure sensor 208 for monitoring the pneumatic pressure in the wheel's tyre. In other embodiments, the system may be configured to monitor one or more other characteristics of the wheel, for example temperature, in which case it may include one or more other sensors, for example a temperature sensor, instead of or as well as the pressure sensor 208.

The controller 202 may be implemented by any suitable means, for example a microprocessor, microcontroller, an Application Specific Integrated Circuit (ASIC), or other suitable data processing device programmed to perform the functions described herein, and may include or have access to suitable data storage as required.

The microcontroller 202 is coupled to the sensor interface 207. The sensor interface 207 is coupled to the wheel phase angle sensor 212. The wheel phase angle sensor 212 produces an output signal that is indicative of the angular position of the wheel as the wheel rotates, the output signal being provided to the sensor interface 207. It may be said that the wheel phase angle sensor 212 provides wheel phase angle measurements, i.e. data indicating the angular position of the wheel as the wheel rotates, to the sensor interface 207. Alternatively, or additionally, the wheel phase angle sensor 212 provides other value(s) or information indicative of wheel phase angle. The sensor interface 207 receives the output of the wheel phase angle sensor 212 in the form of an electrical signal. The sensor interface 207 receives the electrical signal and processes it, the processing typically involving amplification and filtering. As such, the interface 207 includes one or more suitable filters (not shown) and one or more suitable amplifiers (not shown). The filter may for example be a tracking filter as disclosed in US2012234087. In the case where a tracking filter is used, the filter adjusts its filter parameters depending on the speed of the vehicle to remove noise.

The sensor interface 207 may send the processed signal to an analog to digital converter (not shown) in order to convert the signal into a digital signal. Alternatively, the signal may be converted to digital before it is processed, and then processed digitially by a suitable digital signal processor.

The microcontroller 202 receives the digital form of the signal from the wheel phase angle sensor 212 for processing. Alternatively still, the sensor interface and/or microcontroller may perfrom a threshold analysis of the analogue signal without digitizing the signal. In any event, measurements of wheel phase angle, i.e. the angular position of the wheel, can be taken directly or indirectly from the output (digitized or not) of the sensor 212 by the microcontroller 202.

In the illustrated embodiment, the pressure sensor 208 detects the pneumatic air pressure of the tire with which the wheel unit 101 is associated. In alternative embodiments, the pressure sensor 208 may be supplemented with or replaced by a temperature sensor or other devices for detecting tire data. An indication of the tire pressure data is sent to the microcontroller 202 via an analog-to-digital converter (not shown).

The battery 204 is a power source of the wheel unit 101. The transponder coil 206 detects external activation of the transponder by a signal applied by a remote exciter and may modulate a signal to communicate data to a remote detector from the wheel unit 101. The wheel unit 101 provides data including tire pressure from the pressure sensor 208 and the wheel phase angle information from the wheel phase angle sensor 212 through the transmitter 214 and the antenna 216 to the ECU 300.

Upon rotation of a wheel, the wheel phase angle sensor 212 operates to measure the wheel phase angle, i.e. provides a signal indicating the angular position of the wheel at any given time during rotation. Wheel phase angle measurements need not necessarily be made with respect to an absolute reference. A reference may be arbitrarily selected based on accuracy capability and ease of implementation. In other words, the phase measurements do not have to be measured from a top of wheel, or road striking point or other specific point. In some embodiments, the key piece of information required by the system 100 may be a phase difference, or a phase delta of the wheel, and therefore, the requirement may be that two different phase angles are measured relative to the same reference angle. Alternatively, or additionally, in other embodiments, the key piece of information may comprise a single angle measurement during a rotation of a wheel.

The wheel phase angle sensor 212 (and more generally the wheel mounted units) may be mounted in any suitable location on or in the wheel, for example on a rim of the wheel, e.g. coupled to the tyre's valve, or on the tyre itself, typically on an internal surface of the tyre. In one embodiment, the wheel phase angle sensor 212 comprises a rotation sensor. For example, the rotation sensor may be a piezoelectric rotation sensor which measures a wheel phase angle based on gravitational force experienced by the sensor. Specifically, as the wheel rotates, the gravitational force causes a sensing element of the rotation sensor to experience different forces which results in a different output signal representing a wheel phase angle (or wheel angular position). In that way, the rotation sensor produces an output signal indicating a wheel phase angle over time as the wheel rotates. The output signal of the rotation sensor may have different amplitude and/or different polarity depending on the wheel phase angle. For instance, the rotation sensor may produce an output signal having amplitude Mat 0 degrees and having the amplitude -M at 180 degrees. Alternatively, or additionally, any conventional rotation sensor may be used as the wheel phase angle sensor 212.

PJternatively, the wheel phase angle sensor 212 comprises a shock sensor of the type that produces an electrical signal in response to changes in acceleration. The electrical signal is indicative of, typically proportional to, the experienced change in acceleration. Alternatively, the wheel phase angle sensor 212 may comprise an accelerometer or a micro-electromechanical systems (MEMS) sensor. In any case, in preferred embodiments, the wheel phase angle sensor 212 produces a sinusoidal output signal in response to rotation of the wheel on which it is mounted, or more particularly an output that includes a fundamental sinusoidal component. Alternatively, the wheel phase angle sensor may produce an output signal that may be represented sinusoidally, e.g. after suitable processing. For example, the wheel phase angle sensor may alternatively comprise a Hall Effect sensor or a road strike sensor.

By way of example, in embodiments where shock sensors or accelerometers are used as the wheel phase angle sensor 212, FIG. 3 is a graph illustrating a wheel phase angle or a wheel angular position as a function of the gravitational force or acceleration experienced by the sensor as the wheel rotates. In the illustrated embodiment, the wheel rotates counter clockwise, and acceleration along the z axis 304 leads acceleration along the x axis 302 by approximately 90 degrees. The output signal is a sinusoid with a period equal to one revolution of the wheel. The magnitude of the output signal is a voltage proportional to the change in acceleration or acceleration experienced by the wheel phase angle sensor 212 as it rotates. Typically one cycle of the fundamental sinusoidal component of the sensor 212 output corresponds to one revolution of the wheel. The microcontroller 202 may be programmed to recognize a repeating pattern in the signal produced by the sensor 212 and so to identify respective rotations of the wheel. Preferred embodiments of the invention may utilize the output signal corresponding to either one of the axes.

Hence, via the sensor interface 207, the microcontroller 202 receives signals representing wheel phase angle from the wheel phase angle sensor 212. The microcontroller 202 determines wheel phase angle data from the received signal to enable auto-location to be perfromed by the ECU 300.

The microcontroller 202 typically stores the wheel phase angle data for transmission to the ECU 300 at an appropriate time. The microcontroller 202 encodes and transmits data via the transmitter 214 and the antenna 216. The data preferably includes wheel phase angle data, typically amongst other things such as tire pressure information and an identifier of the wheel unit 101.

Wheel phase angle data may be determined in respect of a measurement period which may correspond to one or more revolutions of the wheel. The microcontroller 202 may not transmit every time the output signal has been received, or in respect of every wheel revolution. The nature of the wheel phase data gathered by the microcontroller 202 depends on the auto-location scheme being implemented by the system and may for example be as described in WO 2011/038033 or US 2013/0079977. For example, the microcontroller 202 may determine a difference between two wheel phase angles measured at respective time instants in a wheel's rotation. The microcontroller 202 may determine a second time based on a predetermined known time delay from a first time. For instance, the microcontroller 202 may consider the first time as a single measurement point of a wheel phase angle during the rotation of a wheel and the second time as a data transmission point for a communication to the ECU. The microcontroller 202 may include a clock or time base, or other circuit or module for measuring time increments and operating at specified times or during specified time durations. The wheel phase angle data may include actual wheel phase angles measured at different times. In another embodiment, the wheel phase angle information may include a wheel phase angle measured at a transmission time, such as the second time, and a difference in wheel phase angle measured at two different times. Alternatively, the wheel phase angle data may include only the difference in wheel phase angles. In another embodiment, the wheel phase angle information may include no actual wheel phase angle. Instead, the wheel phase angle data includes a wheel phase angle indication.

Referring again back to FIG. 1, the ECU 300 receives the wireless communication from the wheel unit 201. The ECU 300 extracts wheel phase angle data from the communication along with any other relevant information e.g. the tire pressure and/or the wheel unit identifier. In typical embodiments, the ECU 300 correlates the wheel phase angle data with the ABS data from the ABS sensors 200, 201, 202, 203 for example as described in WO 201 1/038033 or US 2013/0079977.

Referring now to Figure 4 of the drawings, there is shown a graphical representation of the output signal from the wheel phase angle sensor 212. The graph illustrates how the amplitude (y-axis) of the output signal varies with time and with the phase angle (angular position) of the wheel. Hence, the x-axis, using appropriate respective scales, may be indicative of time and phase. Figure 4 shows a sinusoidal signal and it is assumed therefore that any necessary filtering has been performed.

In typical embodiments, for the performance of auto-location, the wheel unit 101, 102, 103, 104 periodically measures the filtered sensor output signal for a configurable duration that corresponds to at least one and preferably a plurality of revolutions of the wheel. This measurement is conveniently performed by the microcontroller 202. The measured signal may be stored for analysis at a convenient time, or may be analysed in real time. The wheel unit 101, 102, 103, 104, conveniently by means of the microcontroller 202, determines the period of the sensor output signal. This may be achieved by any convenient means, e.g. detecting peaks or troughs in the sinusoidal signal, or locations where the signal crosses zero signal level, or locations where the signal crosses a preselected amplitude threshold(s). In preferred embodiments, this involves detecting when the amplitude of the sensor output signal crosses first and second distinct amplitude thresholds AT1, AT2, one (the upper threshold) being higher than the other (the lower threshold). Preferably, thresholds AT1, AT2 have opposite polarity, one (AT1 in this example) defining a positive threshold and the other (AT2 in this example) defining a negative threshold. The detection preferably involves determining when the sensor output rises above the upper threshold and when it falls below the lower threshold, although in alternative embodiments the detection may involve determining when the sensor output falls below the upper threshold and when it rises above the lower threshold.

Detection of when the thresholds are crossed may be achived by causing a respective interrupt signal to be generated each time a threshold is crossed (in the relevant sense i.e. by a rising or falling signal). In this example, successive interrupt signals are generated in response to the crossing of the upper and lower thresholds alternately, and in particular by falling below one threshold and rising above the other alternately. In this example, a rising edge interrupt signal is generated when the sensor output signal rises above the positive (or upper) threshold, and a falling edge interrupt signal is generated when the sensor output falls below the negative (or lower) threshold.

Alternatively, a rising edge interrupt signal may be generated when the sensor output signal rises above the negative (or lower) threshold, and a falling edge interrupt signal is generated when the sensor output falls below the positive (or upper) threshold.

In any case, a reference point RP in the sensor output signal is defined as the point ocurring at a specified location between the occurance of successive crossings of the upper and lower thresholds, i.e. between successive interrupt signals. In the illustrated embodiment, the location of the reference point is calculated with respect to a rising interrupt and the next falling interrupt, although it may alternatively be calculated with respect to a falling interrupt and the next rising interrupt. Similary, in the illustrated embodiment, the location of the reference point is calculated with respect to a relevant crossing of the upper threshold and the next relevent crossing of the lower threshold, although it may alternatively be calculated with respect to a relevant crossing of the lower threshold and the next relevant crossing of the upper threshold. In the illustrated embodiment, the location of the reference point is conveniently calculated as the midpoint between the relevant successive threshold crossings / interrupts. However, the location of the reference point may alternatively be defined at any other location between the relevant successive threshold crossings / interrupts, or the interrupts themselves so long as a consistent approach is taken during analysis of the sensor output. The reference point RP corresponds with a specific wheel phase angle (or angular position) of the wheel during each wheel revolution. The reference point may be used as the Angle of Interest (AOl) from which the timing of transmissions from the wheel unit 101, 102, 103, 104 can be determined and/or a reference point with respect to which one or more wheel phase angle measurements may be taken for the purpose of auto-location.

The period of the sensor output signal is calculated as the time between successive reference points (RP5) being detected in the sensor output signal. To this end the calculation of the reference point as described above is performed over a period corresponding to at least one, and preferably a plurality of, revolutions of the wheel based on all relevent threshold crossings during that period.

For example, with reference to Figure 4, the signal period may be calculated as: Period = (tl)12 ± t2 + (13)12 [1] Where ti is the time between first and second consecutive threshold crossings between which a first reference point is defined, t2 is the time between the second threshold crossing and the first threshold crossing of the next cycle of the signal, and t3 is the time between said next first threshold crossing and its corresponding second threshold crossing between which the next PP is defined.

Equation [1] holds for the case where the RP is defined as being midway between the relevant crossing points. In alternative embodiments, the factor of 0.5 applied to tI and t2 would take a value corresponding to relative position of the PP between the relevant crossing points.

Advantageously, in order to verify the stability and quality of the sensor output signal, the wheel unit 101, 102, 103, 104, conveniently by means of the microcontroller 202, measures the sensor output signal during a measurement period that corresponds to a plurality of periods of the wheel revolution, preferably consecutive periods such as Periods 1, 2 and 3 illustrated in Figure 4, and determines if the measured periods are within a specified tolerance of each other. If the measured periods are deemed to be sufficiently similar, the sensor output signal is considered to be satisfactory. In preferred embodiments, the last reference point RP detected in a measurement period is deemed to contain a valid sensor signal and is designtated as the Aol. Subsequent transmssions from the wheel unit may be synchronised to the AOl, and/or wheel phase angle measurements may be based on it.

The wheel unit 101, 102, 103, 104 may then send a wireless transmission to the ECU 300 that contains data to identify it as a correlated (or valid) transmission, and preferably also data to allow the ECU 300 to perform one or more tasks relating to auto-location, for example calculating the Look Back Time (LBT). In some embodiments, the LBT may be used by the ECU to correlate the AOl detected by the wheel unit 101, 102, 103, 104 to an ABS tooth on each of the vehicle wheels using a rolling history or buffer. The information required to calculate the LBT could consist of a frame number, an inter-frame spacing sequence number and a fixed delay, or could contain a time since the AOl was detected. Alternatively or in addition the data included in the transmission may include any of the above mentioned wheel phase angle data. If the sensor output signal cannot be validated within the specified measurement period the wheel unit may include data in the transmission indicating that correlation was not possible.

Referring now in particular to Figures 5A and 5B, a preferred method by which the AOl may be used as a reference point from which to time the transmissions from the wheel unit 101, 102, 103, 104 is described, including a method for compensating for variation in phase delays within the unit 101, 102, 103, 104.

Various components of the wheel unit 101, 102, 103, 104, including the wheel phase angle sensor 212, amplifier(s) and filter circuitry, introduce phase delays to signals that they process, the phase delay varying with wheel frequency and thus vehicle speed. Without appropriate compensation for such phase delays, the location of the AOl in relation to a fixed point on the wheel's circumference (or ABS tooth) would shift with vehicle speed, potentially causing the system 101 to fail to perform auto-location.

Figure 6 shows a representation of the phase angle sensor output after passing through the sensor interface 207 in Figure 2 at 2Hz 601 and at 10Hz 602 in order to illusTrate the effect of the variable phase delay. The scale of the x axis has been adjusted between 601 and 602 to show two periods of the output signal. At time T0 the sensor is at a specific location in the wheel circumference e.g. top dead centre (TDC). If the positive peak is selected as the AOl then the time between the sensor reaching the top dead centre position and the AOl is 125ms or in terms of phase at 2Hz 90 degrees.

At 10Hz 602 the time from the sensor reaching the TDC position and the AOl is lOms or in terms of phase 36 degrees. In accordance with one aspect of the invention therefore, the wheel unit 101, 102, 103, 104 accounts for phase delays, and in particular phase delays that vary over the operating frequency of the wheel unit, when determining when to initiate a transmission to the ECU 300. As is described in more detail hereinafter, this is achieved by adjusting the time between detecting the AOl and transmission to the ECU by a variable amount, the variable amount depending on the difference between a predetermined transmission delay known to the ECU and one or more delays including a phase delay for the relevant components of its circuitry.

Figure 5A shows a portion 500 of the output signal from the wheel phase angle sensor 212 in respect of which the Angle of Interest AOl has been determined. The AOl has been calculated with respect to a crossing by the sensor output signal of a threshold value, in this example a lower threshold LT, at time Ti. Calculation of the AOl is preferably performed in the manner described above with reference to Figure 4, although other methods may be used. Txs denotes the time at which a transmission from the wheel unit 101, 102, 103, 104 to the ECU 300 is initiated. In this example Txs more particularly denotes the time at which a transmission routine begins, data subsequently being transmitted to the ECU in frames, beginning in the illustrated example with transmission frame 1 at time TXf1, then in frame 2 at TXf2 and so on. The wheel unit 101, 102, 103, 104 takes a fixed amount of time to perform the processing required to initiate a transmission (e.g. in detecting the threshold crossing, calculating the AOl and initiating a transmission routine), which is represented in Figure 5A as processing time PT. The wheel unit 101, 102, 103, 104 applies a variable delay AD before the transmission begins, the purpose of the applied delay AD being to implement phase compensation as described in more detail below. In Figure 5A the applied delay AD is shown as a delay introduced by the wheel unit after the AOl is determined and in addition to the processing time PT.

The amount of the applied delay is dependent on a determined phase delay for the wheel unit 101, 102, 103, 104. The determined phase delay is the phase delay introduced by the wheel unit 101, 102, 103, 104 during the capture and processing of signals for use in auto-location. The total phase delay may be made up of individual phase delays caused by respective parts of the wheel unit, in particular the wheel phase angle sensor 212, amplifier(s) and filter circuitry, and any other relevant circuitry that may be present, e.g. an analogue-to-digital converter. The relevant circuitry typically comprises analogue circuitry, in particular analogue circuitry used by the wheel unit before the relevant digitised signal is supplied to the controller 202 (e.g. the wheel phase angle sensor 212 and the sensor interface 207 in the example of Figure 2).

The phase delay may be determined in any convenient manner. For example, it may be determined empirically or it may be determined using a mathematical model(s) of the relevant circuitry. To determine the phase delay empirically, the phase delay caused by the wheel unit 101, 102, 103, 104 may be measured or simulated over a range of operating conditions of the wheel unit, for example comprising a range of operating frequencies with which the wheel unit is operable, and preferably also over a respective range of parameter values applicable to the wheel unit, for example filter parameter values. Alternatively a mathematical model of the relevant circuitry may be used to determine the phase delay through the wheel unit for all relevant operating conditions (e.g. including operating frequencies and parameter values as applicable).

It is not necessary to compensate for the absolute phase delay in the wheel unit's measurement system. For example, the amount of compensation required may be calculated with respect to one or more reference values, the reference value(s) representing a reference phase delay, preferably a maximum or minimum phase delay. One option in this regard is to devise a mathematical model for calculating the time that is required to compensate for the phase delay through the circuit for all operating conditions so that it equals the phase delay of a set of reference input parameters. It is preferable that these reference parameters have values that represent the largest and/or smallest possible phase delay through the wheel unit. For example, the maximum operating frequency and filter parameters for the highest possible cut-off frequency would represent the lowest phase delay through the circuit. In this case the angular position of the AOl that would be calculated by the ECU is adjusted to match the angular position of the AOl that would be calculated by the ECU at the maximum operating speed and filter position.

Once the relevant phase delay is determined, it is used to determine a corresponding amount by which the time between the AOl detection and start of the transmission routine is adjusted to compensate for the phase delay. Preferably, the phase delay is determined for each measurement period of the wheel unit 101, 102, 103, 104, and as indicated above is dependent on the relevant operating conditions during the respective measurement period. In the present example, the determined phase delay is used to determine the applied delay AD, as is described in more detail below.

In preferred embodiments, the calculated AOl is adjusted by an amount corresponding to the determined phase delay for the respective measurement period to produce a phase compensated AOl (Figure 5A). The phase delay may be calculated in real time as required using the relevant mathematical models, or may be retrieved when required from a look-up table or other storage facility (especially where the phase delays are pre-calculated by simulation or testing). Two methods that can be used to apply the phase compensation delay are described below by way of example.

In the first method the determined phase delay can be subtracted from one wheel period as measured by the sensor and the result used to determine the required time from the calculated AOl to the transmission routine. In the second method the determined phase delay is subtracted from a pre-determined delay known to the ECU to calculate the required time from the calculated AOl to the start of the transmission routine.

In the first method the phase compensated AOl occurs after the detected AOl and outside the region of measured data and is therefore susceptible to error if the wheel period is changing i.e. if the vehicle is accelerating or braking. Additionally, if the determined phase delay is less than half of the measured period the time between the detected AOl and the start of the transmission routine will be greater than for the second method adding additional error due to microprocessor timing tolerances.

To implement the second method the wheel unit 101, 102, 103, 104 is programmed with a pre-determined transmission delay FD, representing a desired delay between the Angle of Interest AOl and the beginning of a transmission to the ECU. Mechanisms may be incorporated to vary this value but the ECU must always be capable of determining this value.

As can best be seen from Figure 5A, the applied delay AD can be determined as the delay known to the ECU FD less the processing time PT and the time between the detected threshold crossing at TI and the phase compensated AOl. The time between the detected threshold crossing at Ti and the phase compensated AOl is in turn determined by the determined phase delay PD (or more particularly the time corresponding thereto) and the lime corresponding to the offset between Ti and the occurence of the non-compensated AOl. In the illustrated embodiment, the time corresponding to the offset between Ti and the occurence of the non-compensated AOl is 1/4 of the last measured period of the sensor output signal (or (t3)/2 from equation [1]) but may be different depending on how the reference point RP is defined with respect to the threshold crossings.

Once the applied delay AD is known, the wheel unit knows when to initiate a transmission to the receiver ECU 300. In this example, the start time Txs of the transmission routine can be calculated by adding the processing time PT and the applied delay to Ti. Hence, Txs occurs after the compensated AOl by the amount of the known delay FD, which is what is expected by the ECU 300 when receiving transmissions from the wheel unit.

Figure 5B shows a timing diagram corresponding to the reception of the transmissions from the wheel unit 101, 102, 103, 104 at the ECU 300. The ECU 300 is either pre-programmed with or can determine from the encoded RE information the delay between the calculated AOl and the start of the transmission routine in the transmitter. On receipt of the transmission from the wheel unit the ECU uses the value of this delay in addition to the known delays between transmitted frames to calculate the look back time (LBT) required for any received frame to correlate the phase compensated AOl to the ABS information.

In preferred embodiments, the transmission routine implemented by the wheel units 101, 102, 103, 104 has an inter-frame space that has a duration selected from a pseudo random sequence that is included between entry of the (RE) transmission routine and the transmission of the first frame. This randomises the angular position that the RF transmission is sent by the PAL sensor to reduce the probability of the first frame of the RF transmission being lost due to a null spot in the RF polar pattern or collision with a frame from another sensor.

The invention is not limited to the embodiment(s) described herein but can be amended or modified without departing from the scope of the present invention.

Claims

CLAIMS: 1. A method of sending a transmission from a transmitter unit comprising a sensor, said method including: initiating said transmission after a variable length delay has elapsed from a reference point in an output signal of said sensor; and selecting the length of said variable delay to compensate for a variable phase delay associated with said output signal.
2. A method as claimed in claim 1, wherein selecting the length of said variable delay involves adjusting said reference point to compensate for said variable phase delay, and selecting the length of said variable delay such that said transmission is initiated after a known delay has elapsed from the occurence of said compensated reference point.
3. A method as claimed in claim 1 or 2, wherein said variable delay comprises an applied delay and a processing time and/or one or more other delays, and wherein selecting the length of said variable delay involves calculating the length of said applied delay.
4. A method as claimed in any one of claims ito 3, further including determining the length of said variable phase delay using one or more mathematical models of electonic circuitry associated with said output signal.
5. A method as claimed in any one of claims ito 4, further including determining the length of said variable phase delay from pre-calculated values associated with electronic circuitry associated with said output signal.
6. A method as claimed in any preceeding claim wherein said transmitter unit is included in a wheel unit of a wheel monitoring system, for example a tyre pressure monitoring system.
7. A method as claimed in claim 6, wherein said sensor comprises a wheel phase angle sensor, for example a shock sensor.
8. A method as claimed in any preceding claim, wherein said transmission comprises a wireless transmission.
9. A method as claimed in any one of claims 6 to 8, wherein compensating for a variable phase delay involves compensating for phase delays caused by one or more components of said wheel unit, for example any one or more of a wheel phase angle sensor, an amplifier, a filter or an analogue-to-digital converter.
10. A method as claimed in any preceding claim, wherein the length of said variable delay is selected to compensate for all variable phase delay associated with said output signal.
11. A method as claimed in any one of claims ito 9, wherein the length of said variable delay is selected to compensate for part of the variable phase delay associated with said output signal.
12. A method as claimed in claim ii, wherein the length of said variable delay is calculated with respect to a reference phase delay.
13. A method as claimed in claim 12, wherein said reference phase delay corresponds to a maximum or minimum phase delay associated with said output signal.
14. A method as claimed in any preceding claim, wherein the output signal, or a signal derived therefrom, has a sinusoidally varying signal level, the method comprising: determining a reference point in said signal by: determining a first time at which said signal level crosses an first threshold; determining a second time at which said signal level crosses a second threshold; and defining said reference point as the point in said signal occuring at a pre-determined time between said first and second times.
15. A method as claimed in claim 14, wherein said pre-determined time is halfway between said first and second times.
16. A method as claimed in claim 14 or 15, wherein said determining a first time involves determining the time at which said signal level crosses said first threshold in a first direction (increasing or decreasing signal level) and wherein said determining a second time involves determining the time at which said signal level crosses said second threshold in a second direction (decreasing or increasing signal level) opposite to said first direction.
17. A method as claimed in claim 16, wherein said first direction is a direction of increasing signal level and said second direction is a direction of decreasing signal level.
18. A method as claimed in any one of claims 14 to 17, wherein said first threshold is higher than said second threshold.
19. A method as claimed in any one of claims 14 to 18, including determining a frequency period of said signal as the time between the respective reference point determined in respect of consecutive instances of said first and second times.
20. A method as claimed in claim 19, including measuring a plurality of consecutive instances of said frequency period, comparing said respective measured frequency periods, and if said respective measured periods are sufficiently similar, determining that said output signal is of a satisfactory quality.
21. A method as claimed in claim 20, wherein said consecutive instances of said frequency period are measured over a measurement period corresponding to a plurality of cycles of said output signal.
22. A method as claimed in claim 20 or 21, further including selecting as said reference point, the reference point defined in respect of a last of said plurality of measured frequency periods.
23. A transmitter unit comprising a sensor, said transmitter unit including: means for initiating a transmission after a variable length delay has elapsed from a reference point in an output signal of said sensor; and means for selecting the length of said variable delay to compensate for a variable phase delay associated with said output signal.
24. A transmitter unit as claimed in claim 23, wherein said initiating means and said selecting means are implemented by a controller.
25. A transmitter unit as claimed in claim 23 or 24 incorporated into a wheel mountable unit.
26. A transmitter as claimed in claim 25, wherein said wheel mountable unit is a wheel unit of a tyre pressure monitoring system.