A number of tyre pressure sensors exist. One is described in published International Patent Application No WO-A1-9728975 (University of British Columbia). The sensor indicates the degree of inflation of a tyre. The sensor includes an annular contact which rotates with the tyre.

Another sensor is described in granted European Patent EP-B 1-610737 (Michelin & Cie). The sensor obtains values directly from tyres, compares these with those previously obtained and alerts a driver in the event of a deviation of tyre pressure from a pre-set value. A processor performs comparisons sequentially with signals received from each one of a plurality of sensors.

Published International Patent Application No WO-A-9606747 (Otter Controls Limited) describes a tyre condition monitoring system for a vehicle. A transmitter sends a signal to a receiver which is indicative of the condition of the tyre and wheel. The transmitter is mounted on the wheel, inside the tyre; the receiver is mounted behind the vehicle's dashboard. The system is complex and requires calibration. A disadvantage of the system described is that the tyre must be manufactured with a transducer, battery and transmitter encased within it. This adds to cost of tyre manufacture and entails that a tyre must be discarded if a unit fails, because units are difficult to replace.

There is now described an alternative and improved tyre pressure sensor and related alarm system.

According to a first aspect of the present invention there is provided a tyre pressure sensor which in use is in fluid communication with a pneumatic tyre, the sensor comprising: a body, within which is located a diaphragm which displaces in accordance with a pressure variation within said tyre; a transducer is arranged to provide a signal indicative of displacement of said diaphragm; and means for transmitting said signal characterised in that the sensor is adapted to be connected directly to the valve of a tyre.

A depressor is preferably supported by the body of the sensor. The depressor is circularly symmetric and presents a protuberance which urges against the tyre valve as the sensor is connected thereto. Minimum air leaks from the tyre in this preferred embodiment. Preferably connection is by way of a threaded connector.

Means may be provided to prevent overtightening of the sensor onto a valve stem.

Preferably a shearable collar is provided. Such a shearable collar also ensures that if there is an accidental impact against the sensor, the sensor body shears away at a predetermined position and does not deflate the tyre; but rather leaves only a threaded collar remaining attached to the tyre. If this occurs an alarm sounds and a driver is alerted to the fact that the sensor has been inadvertently dislodged.

According to a second aspect of the invention there is provided a tyre pressure sensor, which in use is connected to a valve of the tyre by way of a threaded collar, the sensor comprising a body and a depressor to open the valve, there being a shearable collar between the body and threaded collar, said shearable collar being dimensioned and arranged to shear at a predetermined shear stress so that the collar is detached from the sensor and depressor.

The sensor according to the second aspect of the invention preferably includes all the features of the first aspect; namely: a diaphragm, a transducer and a transmitter.

Calibration is not required by the sensor, because it is effectively self calibrating.

Tyre pressure is sensed when the sensor is first connected and this initial value is used as a'datum'. Any discrepancy below a specified percentage of this datum indicates a leak and triggers an alarm.

Preferably the means for transmitting the signal includes a radio frequency (rf) transmitter. The rf transmitter preferably includes a Surface Acoustic Wave (SAW) device. However, other means for transmitting a signal may be used. For example, the transmitter may be an optical transmitter; a microwave transmitter or an acoustic (ultra sound) transmitter.

The diaphragm which displaces in accordance with a pressure variation, is preferably circularly symmetric. It is ideally formed from a material of very low mass.

Most preferably the diaphragm forms part of a solid state device, for example it is integrated with a semi-conductor device. Because the mass of the displaceable diaphragm is so small (typically less than 10 x 10-Skg and more preferably less than 10 x <BR> <BR> <BR> 10-6 kg) the effect of centripetal force (F = m w 2/r) is negligible. This is important, because at very high rotational velocities, the effect of centripetal force tends to provide an erroneous signal. The erroneous signal has generally indicated a higher internal tyre pressure than was actually present. Thus if"m"is very small this effect is negligible.

The diaphragm, which preferably comprises piezoelectric (PZT) material, is advantageously arranged in the sensor so that it is located within a generally cylindrical pressure chamber which in use is defined by the body of the sensor and the valve to which it is connected. The pressure chamber is in fluid communication with the pneumatic tyre, when the sensor is screwed onto the tyre valve. This is because the depressor is presented to the valve; and as the sensor is screwed onto the valve's thread, the valve is depressed and opened. Minimum air leaks from the tyre because valve and sensor have relatively long threaded portions.

The pressure chamber of the sensor, is advantageously isolated from the interior of the sensor and its other components by an 0-ring seal.

A transducer preferably contacts the diaphragm and is connected to an analogue- to-digital converter (ADC). The ADC provides a signal to a micro-processor also housed within the sensor. The transducer preferably also incorporates a differential amplifier. However, the differential amplifier may be a separate component. The transducer and differential amplifier are disposed on one half of a printed circuit board.

Preferably the analogue to digital converter (ADC) forms part of the micro- processor. The micro-processor provides a digital signal, indicative of the status of the tyre to a further amplifier. Control of all functions of the sensor are performed by the micro-processor. Amplified signals indicative of the status of the tyre are sent to the rf transmitter.

The rf transmitter is arranged to transmit at 433.92 or 418 MHz (or any approved frequency greater than 300 MHz) and has a range of about 100 m. Preferably at regular intervals a tyre status and tyre identification signal is transmitted. However, this feature can be disabled and the sensor arranged to transmit an alarm signal only when there is a pressure drop in the tyre.

Current to drive all components, including the rf transmitter, is supplied to the sensor by"coin type"batteries. All components are preferably potted in a resin which provides mechanical strength and stability, so that the sensor is capable of withstanding high frequency vibrational and sudden impact forces.

Advantageously all electrical connections are made using a gold alloy connection material. The advantage of such a material is that it withstands the harsh environment to which the sensor is subjected. Other, more brittle electrical connections may shatter or perish. Tests have proven that the gold alloy connection material is extremely durable.

According to another aspect of the invention there is provided a tyre pressure sensor alarm system comprising: a sensor having a transducer arranged to supply a signal indicative of tyre status to a micro-processor, said signal comprising two portions; the first portion being indicative of the sensor's origin (the identification portion); the second portion being indicative of the tyre's status (the status portion), and means for transmitting said signals to a receiver, characterised in that means is provided in the receiver so that it can be reconfigured to receive a signal from a different sensor or group of sensors.

This aspect of the invention is considered particularly advantageous as it means the system is sufficiently versatile to enable the receiver to be removed and used with different trailers. Only the receiver unit needs to be reconfigured with each sensor of a new trailer. The receiver unit overcomes the problem of having to remove a large number of sensors associated with one system and reconnect them to a new trailer. Instead a user simply has to remove the receiver unit and interrogate each sensor of a new trailer.

Preferably an alarm is arranged to trigger under certain conditions. Some of these conditions are described below.

A status portion of the signal is preferably derived from two sequential measurements obtained from the transducer and computed according to an algorithm stored in the micro-processor. If any two measurements differ, one from another by more than a predetermined amount, typically 15%, during a predetermined interval, an alert status is accorded to the status portion of the signal. The signal is then transmitted to a remote receiver. When the signal is received an alarm is triggered. If however, the two sequential measurements are substantially identical, or differ by an amount less than the aforementioned predetermined amount, then the status accorded to the signal is"ok". In this event, no alarm i s triggered. However, if the receiver detects no signal from a sensor within a pre-set time period, then the system may be modified to trigger a different alarm.

This alarm is called a fault alarm. This might occur if a sensor has been stolen, is damaged or if its battery supply is low.

A relay receiver optionally receives signals from a plurality of a first group of sensors and stores information about them, then may be used to store information as to status and"id"of a second group of sensors. The receiver is preferably configured to be used on trailers which are disconnected from haulage equipment for example tractor units of heavy goods (articulated) lorries.

Remote Excitation mode is used in some prior art systems. Such a function is not required by the present system as if a sensor unit fails, it is removed and discarded then replaced with a new one.

An embodiment of the sensor, associated receiver and alarm system, will now be described, by way of example only, and with reference to the Figures in which: Figure 1 shows a block diagram of a tyre pressure alarm system; Figures 2a and 2b show plan and sectional views respectively of a sensor, Figures 3a and 3b shown plan and sectional views of a cap, which hermetically seals the sensor; Figure 4 is a general assembly showing key components and their location in the sensor; Figures 5a and 5b show front and rear views of a sensor board, identifying key components thereon; Figures 6a and 6b show front and rear views of a circuit board for a transmitter portion of the sensor; Figure 7 shows a block diagram of a circuit board for use in the sensor; Figure 8a is an overall view of transducer on a circuit board; Figures 8b, c and d show plan, underplan and side views (respectively) of the transducer; Figure 9a is an electrical connection diagram showing mounting pads of the transducer; Figure 9b is an equivalent circuit diagram to the connection diagram of Figure 9a; and Figures 10a, b, c and d show printed circuit board layouts.

Manufacture of the sensor is made with reference to the associated parts list headed"A"and"B". Manufacture of the receiver is made with reference to the parts list headed"C". Lists A, B and C are appended to this description and form part of the detailed description of the invention.

Referring now to Figures 1 to 4, a sensor 100 is of a generally cylindrical form and includes a Schrader (Trade Mark) screw thread 102. Thread 102 permits the sensor to be connected to a stem of a valve (not shown) of a pneumatic tyre. A depressor 104 is located in a generally central region within a pressure chamber 106 of the sensor 100. Depressor 104 depresses the tyre valve when the sensor 100 is screwed onto the threaded portion of the valve. An initialisation stage then occurs. This stage monitors the internal pressure of the tyre and transmits a signal to receiver 108.

Sensors 100 are designed to determine the pressure of the tyres on which they are installed, store this initial pressure value in a memory and, subsequently, trigger a fault condition if/when the pressure change within the tyre is, for example, more than 15% from the pressure ori<Jinally measured. There is therefore no requirement for triggering of a fault condition, at predetermined absolute pressures. Thus there is no need to calibrate the sensors to an absolute pressure reading. The pressure variations are relative to the initial pressure obtained.

Sensors 100 a, b, c and d are located on each tyre of a vehicle 101. The sensors 100 screw onto a tyre valve, effectively replacing its dust cap. On a four-wheel vehicle the system is as shown in Figure 1, with a sensor attached to a valve of each wheel. The sensor 100 can however be used on two wheel vehicles, or vehicles or trailers with more than four tyres.

The pressure transducers used in the sensors operate under micro-processor control. Each sensor 100 is configured to transmit to a receiver 108 (or transceiver relay) an identification code when it is installed on a tyre. Sensors 100 also measure and record a value corresponding to the pressure each one encounters when installed on a tyre. This value is stored in an electronic memory and is used for comparison with the tyre pressure during use, in order to determine whether the tyre pressure has changed by more than a predetermined amount. Preferably this change is monitored as a percentage of the first encountered pressure. Transducers have an operating range from 0.5 to 10 bar and this enables the same sensors to be used on a wide range of vehicles.

A data processing unit 111 is provided in the receiver 108 in the cab of the vehicle, in a position where it can be viewed by a driver. The data processing unit 111 is coupled to a display 110 which is used to display tyre deflation/inflation warning information to the driver. The display 110 is preferably an alphanumeric display of, for example, the liquid crystal type. Thus it can provide other instructions and general information to a the driver. Alternatively a hand-held diagnostic device (not shown) may be used. Optionally the display 110 also includes an audible alarm.

If the pressure in a tyre changes from a first encountered pressure by more than the predetermined amount, a micro-processor controller 10 sends a warning signal to (SAW) device 7 for transmission via transmitter 9. The warning signal comprises an identification portion identifying the sensor and a tyre status portion, indicating the fault.

In one particular embodiment of sensor an identification signal may be a 16-bit coded signal which enables over 65,000 different sensor identification codes to be used. Alternatively, 4 bits may be used to indicate the tyre's status and 12 bits for an identification ("id") code. Thus the chance of receiving a false signal from a different sensor with a corresponding"id"is very remote.

Sensors 100 are arranged to transmit, typically every 24 hours, an"OK"signal to the receiver if a"no fault"signal is detected. The purpose of the"no fault"signal is to ensure that the receiver has information indicating that all sensors on a vehicle are still present and are operating correctly. Thus, if a fault develops on a unit or its battery is low, no"OK"signal is transmitted to the receiver 108 and this may indicate to a driver (via display 110) that there is a problem with a sensor on a particular tyre. Furthermore, the sensor is capable of detecting faults with the sensor itself and to transmit to the receiver 108 a sensor fault signal.

When a fault condition is detected the transmitter 9 transmits a fault signal repeatedly over a period of 30 seconds to receiver/transmitter 108. The transmission is then terminated to save the battery life in the sensor. Timings and duration of signal transmission can of course be varied as desired.

Referring specifically to Figures 2 and 3, which show plan and sectional views respectively, of a preferred embodiment of the sensor 100, a depressor 104 urges a valve open as sensor 100 is screwed onto the threaded portion of a valve, by way of threaded portion 106. Region 120 is an undercut between the body of sensor 100 and the collar region 103 which sur-rounds the threaded portion of the valve. This undercut portion acts as a shareable collar. It has a predetermined shear strength and is dimensioned so as to shear on impact with a kerb, stone or other object. When this happens the sensor 100 and depressor 104 break away form the collar 103. Instantly an alarm signal is transmitted by transmitter 109. However, most importantly the tyre valve closes and nothing remains to depress the valve. This is an important safety feature of the invention, as otherwise if a sensor were dislodged, by for example impact against a stone, the depressor may urge the valve to a part opened position hence air could leak and the tyre deflate.

Figures 3a and 3b show plan and sectional views of the cap 112 which holds all components in the sensor 100 and hermetically seals sensor 100 from egression of dirt and moisture. Projections or protuberances are shown which engage with recesses in the sensor body 100 to prevent tampering. Removal of the cap 112, for example in order to replace battery 5, is by way of a special tool (not shown).

Figure 4 and 5 shows a general assembly of the sensor 100. Transducer 12 (shown in greater detail in figures 8 and 9) is in the form of a generally circular symmetric disc and is supported in the sensor 100 by an O-ring seal 13. Current is returned, via Surface Acoustic Wave (SAW) transmitter. Figures 5 and 6 show the location of electronic components as listed on parts lists A and B.

Figure 7 shows a block diagram of a sensor including a micro-processor 119.

Micro-processor 119 supplies power to transducer and amplifier 120 and amplifier 122 to the transmitter 124 when required.

Cap 112 hermetically seals the sensor 100. Spring 113 touches a metal contact in the cap 112 and completes a circuit. The cap is therefore positive and current, from batteries 5, is conducted to printed circuit board 8. Output of amplifier 122 is measured by an ADC within the micro-processor 119. The micro-processor 119 generates a modulated signal with transmitter 124 when power is applied. The values of the resistor/capacitor circuit in the oscillator 126 are an important part of the sensor and ensure that phase of clock pulses drift. This further enhances the security of the transmitter. The ADC reference voltage is taken from the or each battery.

Figures 8 and 9 show views of the transducer and electrical contacts of how this is connected with the printed circuit board (PCB). Contact pads 200 to 210 connect the transducer to the PCB. A bridge arrangement (shown in Figure 9a) shows the equivalent electrical circuit.

Figures 10 a to d illustrate circuit board layouts used within the parts lists.

The present invention has been described by way of example only and variation may be made to the embodiments described without departing from the scope of the invention. Similarly, although reference has been made to the sensor and system for specific use with vehicles; the sensor and system may be incorporated into aircraft or any other moving vehicle or craft having pneumatic tyres; or indeed in any other situation where pressures need to be sensed remotely.

LIST<BR> PIc /Sensor Board<BR> Sensor Side<BR> Comp. Value Description Supplier Manufacture @ R1 33K Surface Mounted 0603 Nickel Barrier 5% Witch Electronice N/A R2 33K Surface Mounted 0603 Nickel Barrier 5% Witch Electronice N/A R3 4K7 Surface Mounted 0603 Nickel Barrier 5% Witch Electronice N/A R4 33K Surface Mounted 0603 Nickel Barrier 5% Witch Electronice N/A R5 10K Surface Mounted 0603 Nickel Barrier 5% Witch Electronice N/A R6 33K Surface Mounted 0603 Nickel Barrier 5% Witch Electronice N/A R7 33K Surface Mounted 0603 Nickel Barrier 5% Witch Electronice N/A R8 18K Surface Mounted 0603 Nickel Barrier 5% Witch Electronice N/A R9 33K Surface Mounted 0603 Nickel Barrier 5% Witch Electronice N/A IC1 Operation Amplifier Witch Electronics MAXIM MA@ IC2 Operation Amplifier Witch Electronics MAXIM MA@ IC3 SENSOR Bourns BOURNS PIC Side R10 4K7 Surface mounted 0603 Nickel Barrier 5% Witch Electronics R11 4K7 Surface Mounted 0603 Nickel Barrier 5% Witch Electronics R12 10K Surface Mounted 0603 Nickel Barrier 5% Witch Electronics C1 100nF Surface Mounted 0806 Multilayer Cerarnic 5% Witch Electronics C2 22pF Surface Mounted 0603 Multilayer Ceramic 5% Witch Electronics Q1 BAS16 Surface Mounted SOT23 Witch Electronics Q2 BC858B Surface Mounted SOT23 Witch Electronics IC3 Micro Processor Witch Electronics Arizona Micro Chip PI@ PCB Printed Circuit Board PC Protypes LIST B<BR> Other Sensor Parts<BR> Comp. Value Descriptions Suppiler Manufacture @ Spring Inner Battery Spring Ashfield Springs Ltd Ashfield Springs Ltd @ Spring Outer Battery Spring Ashfield Springs Ltd Ashfield Springs Ltd Resin Aladite 2020 Clba Clba 'O' Rings Schrader Seal 'O' Rings Ltd 'O' Rings Ltd 'O' Rings Cap Seal 'O' Rings Ltd 'O' Rings Ltd N 'O' Rings Sensor Seal 'O' Rings Ltd 'O' Rings Ltd N PCB Sattery Board PCP PCP Glue Supper Glue Local Shop N/A Body Sentinel Main Body Kingsnorth Engineering Kingsnorth Engineering Cap Sentinel Cap Kingsnorth Engineering Kingsnorth Engineering Sleevesentinel battery Sleeve Kingsnorth Engineering Kingsnorth Engineering LIST C<BR> Transmitter<BR> Components Side<BR> Comp. Value Descriptions Supplier Manufacture @ R1 220R Surface Mounted 0805 Nickel Barrier 5% Witch Electronics N/A R2 68R Surface Mounted 0805 Nickel Barrier 5% Witch Electronics N/A R3 10R Surface Mounted 0805 Nickel Barrier 5% Witch Electronics N/A R4 6K8 Surface Mounted 0805 Nickel Barrier 5% Witch Electronics N/A R5 10K Surface Mounted 0805 Nickel Barrier 5% Witch Electronics N/A R6 4K7 Surface Mounted 0805 Nickel Barrier 5% Witch Electronics N/A C1 100pF Surface Mounted 0603 Multilayer Ceramic 5% Witch Electronics N/A C2 3.3pF Surface Mounted 0603 Multilayer Ceramic 5% Witch Electronics N/A C3 5.6pF Surface Mounted 0603 Multilayer Ceramic 5% Witch Electronics N/A C4 4.7pF Surface Mounted 0603 Multilayer Ceramic 5% Witch Electronics N/A C5 1nF Surface Mounted 0603 Multilayer Ceramic 5% Witch Electronics N/A C6 1.5pF Surface Mounted 0603 Multilayer Ceramic 5% Witch Electronics N/A C7 0.1nF Surface Mounted 0603 Multilayer Ceramic 5% Witch Electronics N/A D1 BAS21 Surface Mounted SOT23 Witch Electronics Motorola Q1 BFS17P Surface Mounted SOT23 Witch Electronics N/A Saw Side SAW1 RFM SAW R02023 RFM RO@ PC8 Printed Circuit Board PC Protypes