WO1989002837A1

WO1989002837A1 - Tyre pressure monitor

Info

Publication number: WO1989002837A1
Application number: PCT/AU1988/000390
Authority: WO
Inventors: Harley Maxwell; Michael Robert Osborne
Original assignee: Copeland Industries Pty. Ltd.
Priority date: 1987-10-05
Filing date: 1988-10-05
Publication date: 1989-04-06

Abstract

A tyre pressure monitoring apparatus includes a tyre pressure sensor (9) with a sensor chamber (11) for communication with the tyre whose pressure is to be monitored, biasing means (15) biasing a movable member (14) in one direction, the air pressure in the chamber (11) acting on the movable member (14) in the opposite direction. A switch means (17, 18) is responsive to the movable member to generate a sensor signal related to the sensed tyre pressure. A transmitter (19) includes a coil (20) and a drive circuit (21) operative in response to the sensor signal to supply a time varying current to the coil (20) whereby a magnetic field is generated. A receiver member (30, 31) is arranged so as to be magnetically coupled to the transmitter coil (20) so as to generate a receiver signal indicative of the sensed tyre pressure. At least one further tyre pressure sensor (9) and associated transmitter (19) can be operatively mounted to a further tyre of the vehicle to enable the sensing of tyre pressures of at least two vehicle tyres.

Description

TYRE PRESSURE MONITOR This invention relates to apparatus for monitoring the pressure in a vehicle tyre so that an abnormal condition can be detected and signalled. 5 In the past it has been proposed to monitor the pressure in a vehicle tyre so that an abnormal pressure, particularly a low pressure, can be signalled to the vehicle driver allowing corrective action to be taken. Such apparatus is particularly advantageous for trucks such as articulated vehicles having a trailer with a

10 relatively large number of wheels. In such vehicles if a tyre is under-inflated or becomes punctured the faulty condition can be unnoticed _ by the driver for a considerable period of time and during this time a tyre and wheel can be irreparably damaged. Not only are such tyres and wheels expensive but failure to detect and rectify the

15 faulty condition can produce unpredictable road holding of the vehicle leading to road accidents.

In the past, one proposal to monitor vehicle tyre pressures has included the provision of a mechanical indicator associated with each vehicle tyre so that if the pressure in one tyre drops below a

20. predetermined -level, a visual indication is given at the tyre. A difficulty with this kind of system is that manual inspection is required of all wheels in order to detect a low pressure. Obviously this means that if a tyre is punctured or a fault develops as the vehicle is travelling, the .driver can be unaware of this, possibly

25 until irreparable damage has occurred.

Another apparatus proposed for overcoming the problem uses a tyre pressure sensor which activates a radio transmitter when an abnormal tyre pressure is sensed. The transmitter is mounted to the vehicle wheel. A radio receiver tuned to the transmission frequency of the

30 transmitter is mounted to the vehicle or trailer chassis or at the driver's cabin so that a signal can be given to the driver if the. radio transmitter signals an abnormal tyre pressure. This kind of system using radio transmission must transmit at a closely controlled radio frequency in order not to interfere with other radio frequency

35 bands. Also the system itself can be susceptible to interference from other radio sources leading to spurious signals being given. It is an object of the present invention to provide a tyre pressure monitoring apparatus which is effective to automatically detect and provide an indication of an abnormal tyre pressure and which can be conveyed to a vehicle driver. 5 It is a preferred object to provide a tyre pressure monitoring apparatus which is not susceptible to external interference or causing interference to external equipment.

According to the present invention there is provided a tyre pressure monitoring apparatus including a tyre pressure sensor for 0 sensing the pressure in an associated tyre and for generating the sensor signal related to the sensed tyre pressure; a transmitter including a coil and a drive circuit operative in response to the sensor signal to supply a time varying current to the coil whereby a magnetic field is generated by the time varying current flowing 5 through the coil, the magnetic field being related to the sensed tyre pressure; a receiver which includes a receiver member for being arranged so as to be magnetically coupled to the transmitter coil so as to generate a receiver signal upon generation of a magnetic field by the transmitter, the receiver signal thereby being indicative of 0 the sensed tyre pressure, the receiver further including a receiver circuit responsive to the receiver member and operative in response to the receiver signal to generate an indicator signal related to the « sensed tyre pressure.

With this apparatus the transmission and reception of information 5 relating to the sensed tyre pressure occurs by magnetic coupling and hence there can be insignificant radio interference caused by the apparatus and conversely insignificant interference with the operation of the monitoring apparatus by external radio frequency signals. ° Preferably the tyre pressure sensor has an on state and an off state, the tyre pressure sensor changing from one state to the other upon the sensed tyre pressure reaching a predetermined threshold. In this embodiment, the tyre pressure sensor preferably includes a sensor chamber for communication with the tyre whose pressure is to 5 be monitored, a movable member having associated therewith biasing means biasing the movable member in one direction, the pressure in the chamber acting on the movable member in the opposite direction to the biasing means, the sensor including a switch means associated with the movable member, the switch means adopting a closed or .open state corresponding respectively to the on and off states of the sensor, or vice versa, the switch means being operable to change from its closed to its open condition and vice versa in response to movement of the movable member. Preferably the tyre pressure sensor exhibits hysteresis in changing from one state to the other upon the tyre pressure reaching the threshold so that the tyre pressure sensor positively adopts one state without a significant tendency for continuously changing state when the tyre pressure is close to the predetermined threshold. In this embodiment, preferably the tyre pressure sensor includes a snap over mechanism having -two stable conditions corresponding to the on and off states of the tyre pressure sensor, the snap over mechanism initially resisting movement away from either stable condition under the effect of a change in tyre pressure and subsequently assisting change of its condition upon continued change in the tyre pressure.

The transmitter coil may of any suitable construction and configuration. For example, it has been found that a ferrite core may be used for the coil windings. A 6mm core may for example result in an effective range of in the order of 1 to 3 metres so that the recei-ver device can be conveniently located at one of a large number - of points at the vehicle chassis or frame. A larger coil will of course give a greater effective distance for achieving mutual inductance.

A coil creating a magnetic field by a current flowing through the coil is moderately directional so that the magnetic field receiver device which is magnetically coupled to the transmitter coil needs to be located with some consideration being given to this directionality.

When installed in a vehicle, the tyre pressure sensor and transmitter are preferably mounted to a wheel of the vehicle, the transmitter coil being located with its axis directed generally axially relative to the rotational axis of the wheel, the receiver member being located on the vehicle inwardly of the wheel so that magnetic coupling between the transmitter coil and the receiver member is achieved for substantially all rotational positions of the wheel. The metallic components of a conventional vehicle or trailer chassis or frame would not significantly affect the magnetic coupling achievable provided that the transmitter coil and receiver device are properly located. The receiver device may be a receiver coil or Hall effect device or other magnetic field sensing device. In the case of a receiver coil being provided, the coil may be air cored or may be for example a ferrite loop aerial. 5 The drive circuit may include an oscillator for generating a carrier frequency determining the frequency of the current supplied to the transmitter coil, the drive circuit further including a modulator operative to modulate the carrier frequency in response to the tyre pressure sensor. The receiver circuit which is coupled to 0^' the receiver member may be tuned to the carrier frequency of the oscillator so as to minimise interference to the operation of the receiver circuit..

The receiver circuit may include a timing circuit operative to generate a timing signal, the receiver circuit further including a

15 counter which is operative to count the oscillations of the receiver signal at t^ne carrier frequency for a period of time determined by the timing signal whereby if a pre-deter ined number of oscillations is reached within the time period, the indicator signal is generated to provide information regarding the sensed tyre pressure. The time

20 interval during which counting by the counter of the oscillations of the receiver 'signals occurs is preferably in the order of several seconds, say 4 seconds.

The tyre pressure monitoring apparatus when installed in a vehicle, may include at least one further tyre pressure sensor and

25.associated transmitter operatively mounted to a further tyre o-f the vebicle so as to monitor the pressure of that further tyre, the apparatus including at least one further receiver member operatively associated with a respective one of the transmitters and operative to cause- generation of an indicator signal distinguishable from the

30 first indication signal, both the first and second indication signals being applied to a common output line whereby the indicator signals on the common output line indicate the sensed tyre pressures of at least two vehicle tyres.

In this way. an indication may be given not only of the existence

35 of a sensed abnormal tyre pressure but also either the identity of the tyre, or the general location of the tyre having the abnormal pressure as one of group of tyres. For example, a left side group of wheels of a trailer may each be provided with a tyre pressure sensor and^" transmitter, there being a receiver device located to generate a receiver signal upon any one of the left wheel tyres having an abnormal pressure, and similarly for a right side group of tyres. A single receiver circuit may be coupled to the left side and right side receiver devices and may be operative to switch between the two 5 devices and generate a respective indicator signal in response to the tv/o receiver devices.

Possible and preferred features of the present invention will now be described with particular reference to the accompanying drawings. However it is to be understood that the features illustrated in and 0 described with reference to the drawings are not to be construed as limiting on the scope of the invention. In the drawings:

Fig. 1 is a cross sectional view of one possible tyre pressure sensor for use in a monitoring apparatus according to the present invention, 5 Fig. 2 is a circuit diagram of one possible transmitter circuit to be associated with a tyre pressure sensor,

Fig. 3 is a schematic block diagram of a receiver circuit for use in a tyre pressure monitoring apparatus according to the present invention, and 0 Figs. 4 and 5 are cross sectional views of an alternative possible tyre pressure sensor, Fig 4 showing the sensor in its normal condition when the tyre pressure is above the threshold and Fig. 5 showing the sensor after switching in response to a tyre pressure drop. 5 Referring to Fig. 1 there is shov/n a tyre pressure sensor 9 which includes a lower body portion 10 which is screw threaded for connection to the valve stem of a vehicle tyre. The lower body 10 has a chamber 11 which in use is in communication with the air within the tyre. An upper body portion 12 is threaded to the lower body -0 portion 10 and a diaphragm 13 closes the upper end of the chamber 11. Within the upper body 12 is a pin 14 which is biased downwardly against the diaphragm 13 by spring 15, the pin 14 having a head 16 bearing against the diaphragm 13. The pressure with which the pin 14 bears against the diaphragm 13 depends on the spring force. 5 Associated with the upper body 12 is an electrical contact 17 shown as an annular ring. Associated with the pin 14 is a further electrical contact shown as a contact ^• ring IS. When the tyre pressure falls to a predetermined extent, and hence the air pressure in the chamber 11 falls, the spring 15 operates to urge the pin 14 downwardly to a sufficient extent to bring the contacts 17, 18 together to close an electrical circuit. The air pressure at which the contacts 17, 18 close is determined principally by the force of the spring 15 (as well as by the resistance of the diaphragm 13 to deformation). Changing springs in assembly enables use with a variety of predetermined air pressures at which the electrical contacts will close.

Figs. 4 and 5 show an alternative sensor 9 which exhibits hysteresis in changing from one state to another. In particular the sensor 9 includes a snapover mechanism (60) having two stable conditions (Figs. 4 and 5) corresponding to the off state and the on state respectively of the tyre pressure sensor 9.

In particular the sensor 9 in Figs. 4 and 5 includes a body 10 for connection to the value stem of a vehicle tyre. The body 10 has a chamber 11 which communicates with the tyre. A diaphragm 13 defines a closed end of the chamber 11.

Within the body 10 is a movable member 14 which has a central bulbous nose 61 in contact with the diaphragm 13. The movable member 14 is biased against the diaphragm 13 by biasing spring 15 with a pressure which is adjustable by means of a grub screw 62 so as to adjust the force with which the movable member acts in the opposite direction to the air pressure in the chamber 11, i.e. the tyre pressure.

A switching means in the form of microswitch 63 is operated by movement of the movable member 14. When the tyre pressure is normal the movable member 14 is in the position as shown in Fig. 4. When the tyre pressure, and hence the pressure in chamber 11 drops below a threshold, the movable member 14 moves under the influence of the biasing spring 15 against the air pressure in chamber 11 and adopts the position shov/n in Fig. 5. The movement of the movable member 14 causes opening and closing of the microswitch 63 so as to supply

The snapover mechanism 60 which provides hysteresis in the operation of the Figs. 4 and 5 embodiment comprises a shallow conical washer 64 which has two stable conditions as shown in Figs. 4 and 5 corresponding to the closed and open states of the microswitch 63. Initially the washer 64 resists movement of the movable member 14 away from its normal Fig. 4 position towards the Fig. 5 condition. However once the washer reaches substantially the mid-way position, it rapidly changes to its condition to that shown in Fig. 5 during which phase of its movement, the microswitch 63 changes state. This arrangement ensures that the sensor positively adopts one state without significant tendency for continuously changing state when the tyre pressure is close to the predetermined threshold.

In Fig. 2 the transmitter circuit includes a coil 20 and a drive circuit 21 to supply a time varying current to the coil 20. When installed in use, the drive circuit 21 receives power at the input terminal 22, e.g. when the contacts 17, 18 in Fig. 1 close or microswitch 63 in Figs. 4 and 5 closes. The drive circuit 21 includes a modulator 23 which in the illustrated circuit is a square wave modulator operating at say 1.2 KHz. The modulator 23 operates to switch the transistor Q1 on and off at the modulator frequency. The drive circuit also includes a carrier oscillator circuit 24 which is a Hartley oscillator or emitter tapped oscillator operating at say 455 KHz. The diode D1 prevents saturation in the transistor Q3.

With the Fig. 2 circuit, when power is applied to the circuit, e.g. 3 volts from a small nickel cadmium battery (65 in Figs. 4 and 5) or the like, when the tyre pressure sensor switch is closed, a current oscillating at 455 KHz is supplied to the transmitter coil 20 so as to generate a magnetic field oscillating _. at that same frequency.

In Fig. 3 there is shov/n a receiver having tv/o receiver members or devices 30, 31 e.g. coils. The receiver device 30 may be inductively coupled to transmitters say at the rear left side of the vehicle so as to be responsive to the transmitters associated with the rear left tyres and the receiver device 31 may be mounted to be inductively coupled to the transmitters at the rear right side of the vehicle. Of course further receiver devices can be provided for other tyres or tyre groups. The receiver devices 30 and 31 are alternately switched by the multiplex switch 32. The switch 32 operates in response to the timing circuit 33 which includes a crystal oscillator 34 operating at say 32 KHz. This signal is applied to a divider 35 to produce a signal at say 0.125 Hz. The output of divider 35 is applied to switch 32 directly and also through an inverter 36 so that the switch 32 can pass signals from either device 30 or device 31 for alternate four second intervals. Signals from the switch 32 are passed to a receiver circuit section 40 (e.g. an AM sensitive IC chip such as an I.C. circuit type TEA5550) and through a filter 41 tuned to the carrier frequency. The filter 41 passes its output to a dual comparator 42 which provides an 5 output on one of its output lines depe^'nding on whether the input signal is above or below a predetermined level. The comparator outputs are passed to the square wave shaper circuit 43 which in turn is coupled to counter 44.

The counter 44 is reset under the control of reset circuit 48 0 consisting of flip flop 48a and OR gates 49 at the same time as the switch 32 so that at the end of the four second^' time interval the pulse count in the counter 44 represents the number of magnetic field oscillations received by the respective receiver device 30, 31 and hence provides a measure of the proportion of the time interval of 5 four seconds that the transmitter coil was being supplied with current at the carrier frequency. This counter 44 is connected so that it provides an output when a predetermined count is reached corresponding to the transmitter co.il being transmitting for substantially the entire four second time interval. This relatively 0 long time interval ensures that transient transmissions from the transmitter, e.g. as a result of transient switchings due to shocks or vibrations of the vehicle tyre or wheel causing intermittent closing of the sensor contacts, will not result in the generation of an output at the counter 44. 5 The output of counter 44 is applied to a flip flop 45 so that the flip flop 45 can provide an output signal indicative of the predetermined count having been reached. The output of flip flop 45 is applied to flip flops 46 and 47 which are gated by the same signals which operate the switch 32 so that the flip flops 46, 47 0 correspond to respective receiver devices 30, 31. The outputs of flip flops 46, 47 are applied through respective deffering value resistors R10, R11 to current source 50 which applies a signal through amplifier 51 to the output line 52. With this arrangement the current on line 52 has a level uniquely representing reception of 5 a magnetic field at receiver device 30 or 31. This single output line 52 therefore can carry a signal to the vehicle cabin, e.g. through one of the spare plugs of the standard electrical connecting plug between a prime mover and a trailer, this signal being conveyed

— to the vehicle cabin where an appropriate visible and/or audible signal can be generated to alert the vehicle driver to the detected abnormal tyre pressure.

It will be seen that the preferred embodiment of the tyre pressure monitoring apparatus according to the present invention as herein described and illustrated can be used to sense and signal an abnormal tyre pressure. The apparatus can be relatively immune to outside interference, particularly radio interference since substantially pure magnetic coupling is used between the transmitter coil and the receiver device. This also means that great precision and care does not need to be exercised in selecting and generating the transmitted signal since no substantial external radio interference need be caused.

Furthermore it will be seen that the apparatus can be used to monitor specific individual tyres or groups of tyres and appropriate signals can be generated to not only signal to the vehicle driver the detection of an abnormal tyre pressure but can also indicate which tyre or which group of tyres has the abnormal tyre pressure.

The particular preferred embodiment of the tyre pressure sensor described and illustrated in relation to Fig.. 1 enables ready testing of the sensor and of the monitoring apparatus as a v/hole simply by loosening of the sensor from the vehicle valve stem. Of course this results in a drop of pressure in the chamber and closure of the sensor switch.

Claims

1. A tyre pressure monitoring apparatus including a tyre pressure sensor (9) for sensing the pressure in an associated tyre and for generating the sensor signal related to the sensed tyre pressure; a transmitter (19) including a coil (20) and a drive circuit (21) operative in response to the sensor signal to supply a time varying current to the coil (20) whereby a magnetic field is generated by the time varying current flowing through the coil (20), the magnetic field being related to the sensed tyre pressure; a receiver (29) which includes a receiver member (30,31) for being arranged so as. to be magnetically coupled to the transmitter coil (20) so as to generate a receiver signal upon generation of a magnetic field by the transmitter (19), the receiver signal thereby being indicative of the sensed tyre pressure, the receiver (29) further including a receiver circuit (39) responsive to the receiver member (30,31) and operative in response to the receiver (29) signal to generate an indicator signal related to the sensed tyre pressure.

2. A tyre pressure monitoring apparatus as claimed in Claim 1 wherein the tyre pressure sensor (9) has an on state and an off state, the tyre pressure sensor (9) changing from one state to the other upon the sensed tyre pressure reaching a predetermined threshold.

3. A tyre pressure monitoring apparatus as claimed in Claim 2 wherein the tyre pressure sensor (9) includes a sensor chamber (11) for communication with the tyre whose pressure is to be monitored, a movable member (14) having associated therewith biasing means (15) biasing the movable member (14) in one direction, the pressure in the chamber (11) acting on the movable member (14) in the opposite direction to the biasing means (15), the sensor (9) including a switch means (17,18) associated with the movable member, the switch means (17,18) adopting a closed or open state corresponding respectively to the on and off states of the sensor (9), or vice versa, the switch means (17,18) being operable to change from its closed to its open condition and vice versa in response to movement of the movable member (14).

4. A tyre pressure monitoring apparatus as claimed in Claim 2 wherein the tyre pressure sensor (9) exhibits hysteresis in changing from one state to the other upon the tyre pressure reaching the threshold so that the tyre pressure sensor (9) positively adopts one^" state without a significant tendency for continuously changing state when the tyre pressure is close to the predetermined threshold.

5. A tyre pressure monitoring apparatus as claimed in Claim 4 wherein the tyre pressure sensor (9) includes a snap over mechanism (60) having two stable conditions corresponding to the on and off states of the tyre pressure sensor (9), the snap over mechanism (60) initially resisting movement away from either stable condition under the effect of a change in tyre pressure and subsequently assisting change of its condition upon continued change in the tyre pressure.

6. A tyre pressure monitoring apparatus as claimed in Claim 1 and installed in a vehicle, the tyre pressure sensor (9) and transmitter

(19) being mounted to a wheel of the vehicle, the transmitter coil

(20) being located with its axis directed generally axially relative to the rotational axis of the wheel, the receiver member (30,31) being located on the vehicle inwardly of the wheel so that magnetic coupling between the transmitter coil (20) and the receiver member (30,31) is achieved for substantially all rotational positions of the wheel.

7. A tyre pressure monitoring apparatus as claimed in Claim 1 wherein the drive circuit (21) includes an oscillator (24) for generating a carrier frequency determining the frequency of the current supplied to the transmitter coil (20), the drive circuit (21) further including a modulator (23) operative to modulate the carrier frequency in response to the tyre pressure sensor (9).

8. A tyre pressure monitoring apparatus as claimed in Claim 7 wherein the receiver circuit (39) which is coupled to the receiver member (30,31) is tuned to the carrier frequency of the oscillator (24) so as to minimise interference to the operation of the receiver circuit (39).

9. A tyre pressure monitoring apparatus as claimed in Claim 8 wherein the receiver circuit (39) includes a timing circuit (33) operative to generate a timing signal, the receiver circuit (39) further including a counter (44) which is operative to count the oscillations of the receiver (29) signal at the carrier frequency for a period of time determined by the timing signal whereby if a pre-determined number of oscillations is reached within the time period, the indicator signal is generated to provide information regarding the sensed tyre pressure.

10. A tyre pressure monitoring apparatus as claimed in Claim 9 wherein the time interval during which counting by the counter (44) of the oscillations of the receiver signals occurs is in the order of several seconds.

11. A tyre pressure monitoring apparatus as claimed in Claim 1 and installed in a vehicle, the apparatus including at least one further tyre pressure sensor (9) and associated transmitter (19) operatively mounted to a further tyre of the vehicle so as to monitor the pressure of that further tyre, the apparatus including at least one further receiver member (30,31) operatively associated with a respective one of the . transmitters (19) and operative to cause generation of an indicator signal distinguishable from the first indication signal, both the first and second indication signals being applied to a common output line (52) v/hereby the indicator signals on the common output line (52) indicate the sensed tyre pressures of at least two vehicle tyres.