WO1988007941A1

WO1988007941A1 - Tyre deflation warning device

Info

Publication number: WO1988007941A1
Application number: PCT/AU1988/000117
Authority: WO
Inventors: Iain Godfrey Saul; Iain Kilburn White
Original assignee: Consolidated Technology Pty. Ltd.
Priority date: 1987-04-16
Filing date: 1988-04-15
Publication date: 1988-10-20

Abstract

Tyre deflation warning device having a body (60) which is screw fittable to a tyre valve and having a chamber (30) into which air from the valve is then admitted to act on a diaphragm (55) which moves a contact (25) to close an electric circuit via a terminal (68) under the condition where tyre pressure on the diaphragm falls below a predetermined level. Admission of air from the valve (3) into the chamber (30) is achieved by means of a spring-loaded actuating element (70) within the body (60) which, as the body is fitted to the valve, is effective to enter the valve and to engage and inwardly displace a valve member of the valve to open the valve. A radio transmitter (80) is included in the body and generates a radio signal when the low pressure condition is detected. A radio receiver (8) is positioned in a vehicle cab to receive the warning signal from the device (6). An alarm unit (10) is detachable from the receiver (8). When attached to the receiver (8), the device (10) is actuated to give a warning on detection by the receiver (8) of an alarm condition. The unit (10) may then be removed and brought adjacent selected ones of the vehicle tyres to determine which tyre has a reduced pressure, by detecting the particular warning device (6) which has been actuated. For this purpose the unit (10) has an in-built R.F. section (480) effective to detect the signal generated by the device (6) independently of the receiver (8).

Description

TYRE DEFLATION WARNING DEVICE

This invention relates to a tyre deflation warning device.

Various proposals have been made in the past for monitoring and warning of the pressure level of a vehicle tyre. These include proposals to provide a pressure sensitive device which is affixed to a vehicle tyre, such as at the tyre valve, and including a radio transmitter effective, on detection of a decrease in tyre pressure, to generate a radio signal which is received by a suitable radio receiver positioned, for example, in the vehicle cabin, to cause generation of a suitable alarm signal such as generation of an audible or visible alarm.

These devices, however, have certain defects. In particular, the frequency of the radio signal generated by the devices has

not been sufficiently stable to enable effective operation to be achieved, particularly to enable the radio transmitters to comply with Government regulations relating to operation of radio transmitters generally. While it is possible, by the use of sophisticated electronic circuitry to ensure sufficient stability in radio transmitters generally, the devices required for the present purposes must be very small so that they do not have a significant effect on the vehicle wheel balance and it has in the past proven difficult to provide a suitably stable and small radio transmitter.

According to one aspect of the present invention there is provided a tyre deflation warning device comprising means fittable to a tyre valve and including a sensor to sense tyre pressure and radio transmitter effective to transmit a radio signal containing information as to tyre pressure, as sensed by said sensor, wherein said radio transmitter in use generates a radio signal at a predetermined radio frequency and wherein the radio transmitter includes a surface acoustic wave device effective to stabilise said predetermined frequency.

Another difficulty with prior tyre alarm devices relates to the arrangements made for detecting low tyre pressure. With multi-axle and multi-wheel trucks, for example, there may be a number of wheels with the result that,if the alarm signal generated at each wheel is the same, the receiver is unable to discriminate as between one wheel and the other so that, although a satisfactory alarm signal is generated, the vehicle driver is not easily able to ascertain which of the wheels has suffered a loss of pressure. If the loss of pressure is catastrophic, this may be readily observed by inspection, but otherwise, particularly with vehicles having adjacent pairs of wheels, it may be very difficult to ascertain, otherwise than by subsequent manual checks of the tyre pressure in each tyre, which tyre has suffered pressure loss. This difficulty might be overcome by providing that each wheel has a radio transmitter arranged to generate a distinctive signal on loss of tyre pressure, and the radio receiver may be arranged to discriminate on the basis of these signals as to which tyre has suffered pressure loss. The difficulty then arises, however, that the receiver must be made relatively more complicated. Furthermore, the transmitters themselves do require to have a certain power output in order to ensure adequate detection by the receiver which is possibly, with trucks for example, positioned some way away from the tyres. There is the difficulty then that vehicles may detect the inadvertent transmission of alarm signals from other vehicles passing thereby. The probability of this arising may be minimised by varying the signal generated by the devices associated with particular trucks but, since there is only a limited number of possible variants to frequency and encoding which may be possible, the adoption of multiple encodings associated with individual vehicles makes it more difficult to ensure that the individual vehicles, as a whole, have different alarm signals relative to other vehicles. In one form of prior device, the receiver is made portable, and instead of being within the vehicle cabin is simply used in an interrogatory fashion, being moved into position adjacent each tyre in turn in order to determine which tyre has suffered pressure loss. While effective, this arrangement is generally thought to be less satisfactory than an arrangement which is capable of continuous monitoring, such as is the case where the receiver is positioned within the vehicle cabin.

In another aspect this invention provides a receiver device for receiving a radio signal from any one of a number of tyre deflation warning devices fitted to respective tyres of a vehicle and which signal is conditioned to a particular state when the tyre pressure as sensed by the respective warning device falls below a predetermined level, said receiver device including a first receiver the output of which is caused to change state when said signal is conditioned to said particular state, the receiver device further including an alarm device responsive to the condition of the output of the first receiver to cause an alarm condition to be indicated pursuant to said change of state, the alarm device being demountably coupled relative to the receiver means whereby to permit the alarm device, once a said alarm condition is indicated, to be succesively positioned adjacent ones of said alarm devices and including a second receiver for receiving said radio signal and responsive, when the alarm device is brought adjacent an alarm device the radio signal from which is conditioned to said particular state, to indicate a further alarm condition. Further difficulty arises with devices of the type in question in the means whereby these are affixed to the vehicle tyre. Generally, the devices are provided with a screw threaded recess which is threaded on to the externally threaded tyre valve stem. At the same time, as this screw threaded engagement takes place, a spigot or other actuating element associated with the device is brought into engagement with a valve member of the valve so as to depress the valve member to allow airflow out of the tyre into the device. This outflow is permitted into, for example, a closed chamber within the body of the device, to act against a suitable pressure sensor which pressure sensor detects loss of tyre pressure by virtue of resultant loss of pressure within the chamber. However, there are a number of different types of construction for the valves in question. In particular, the amount of travel required for the valve member axially within the valve stem, in order to effect opening, varies as between different valves. Thus, if the actuating element is formed as a fixed abutment, as is usual, on the body of the device, it may, with certain valves fail to adequately depress the valve member to allow outflow of air into the device chamber, as the device is screwed onto the valve, or the valve element may press the valve member inwardly to a point where it seats against a fixed obstruction within the valve so that, as the device is screwed onto the valve, the user may not detect that a position has been reached at which the valve member has been depressed to its maximum extent. In the latter case, excessive screwing may occur to the point where the valve member, or the actuating element, is subjected to excessive compression forces causing damage to the valve or to the device itself. In another aspect, the invention provides a tyre deflation warning device comprising a body fittable_to a vehicle tyre valve, a pressure sensor and a radio _ transmitter effective to transmit a radio signal containing information as to pressure sensed by said sensor, the device further including an actuating element which, as the body is fitted to the valve, is effective to enter the valve and to engage and inwardly displace a valve member of the valve to open the valve and cause, without substantial loss of air from the tyre, air pressure within the tyre to act upon the sensor, for sensing of such pressure by the sensor, the sensor being effective to control said radio transmitter whereby the output thereof is conditioned to a particular state when the pressure of the tyre, as sensed by the sensor, falls to a predetermined level, wherein said actuating element is interconnected with said body via a resiliently compressible element adapted to permit the actuating element to be moved, against resilient bias provided by the resiliently compressible element, inwardly relative to the body under the condition where a predetermined force is applied to the actuating element from the valve member, during fitment of the body to the valve.

The invention is further described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram showing the components of a tyre deflation warning system constructed in accordance with the invention;

Figure 2 is a schematic side view of a vehicle, illustrating the position of components of the warning system of figure 1;

Figure 3 is an enlarged partly diagramatic axial cross-section of a tyre valve of a kind with which the warning device of the invention is useful;

Figure 4 is an enlarged partly diagramatic axial cross-section of a warning device constructed in accordance with the invention;

Figure 5 is a plan view of a spring element incorporated into the warning device of- figure 4;

Figure 6 is an axial cross section of the spring element of figure 5;

Figure 7 is a plan view of a further spring element included in the device of figure 4;

Figure 8 is an axial cross-section of the spring element of figure 7;

Figures 9 and 10 are respectively electric circuit diagrams of two different forms of radio transmitter incorporatable into the device of figure 4;

Figure 11 is a circuit diagram of a radio receiver adapted to receive signals from a transmitter incorporated into the device of figure 4;

Figure 12 is a block diagram of a modified form of the receiver and alarm unit constructed in accordance with the invention; Figure 13 is a block diagram of a portion of the receiver of figure 12;

Figure 14 is a block diagram of a further portion of the receiver of figure 12; and

Figures 15 and 16 are diagrams illustrating the mechanical arrangement of separable portions of the receiver of figure 12; and

Figures 1 and 2 show the locations of principal components of an exemplary tyre deflation warning system including alarm devices constructed in accordance with the invention. In figure 1, vehicle tyres 4 are shown assembled onto wheels 2 having valves 3 for inflating the tyres. The warning devices 6 are mounted on the respective valves in place of the dust caps usually provided on such valves, so that the levels of the air pressures within the tyres can be monitored. The sensing devices each include a small telemetry radio transmitter for producing warning radio signals 14 and directing these to a vehicle cab 5 when the tyre pressure drops below a preset level. A visual and audible alarm unit 10 is positioned in the front of the vehicle cab and a receiver 8, for receiving warning signals and actuating unit 10, is also positioned at a convenient location within the cab. An antenna 12 is positioned on the vehicle for receiving the warning signals from the sensing devices.

In the above system, a number of sensing devices 6 are thus used, each for monitoring the air pressure of a respective vehicle tyre. One warning device 6 is illustrated in figure 4. This is intended for use with the particular type of tyre valve 3 shown in figure 3. This tyre valve is of the common "Schrader" type, having an elongate tubular housing 3a which extends radially inwardly from the tyre 4 through an opening in the rim of the wheel 2. The housing 3a has therewithin a valve member 50 which is axially movable within the housing 3a. When the tyre is inflated, the valve member 50 is biased, by air pressure in the tyre acting thereon, to a closed condition at which outflow from the tyre through the housing 3a is prevented. In this condition, an inner head portion 51 of the member 50 then bears against an internal annular valve seat 53 formed in the housing 3a. To permit outflow of air from the tyre 4, the valve member 50 may be depressed inwardly relative to the valve housing by pressing on an axial stem 52 formed on the valve member 50. Then, the head portion 51 is released from valve seat 53 and air can flow from the tyre through the housing 3a to exit at an open outer end of the housing 3a. To facilitate this inward pressing, the outer end of the valve stem is positioned adjacent the outer, open, end of the housing 3a. The housing 3a is shown as having a screw thread 54 at its outer end, to receive a removable screw-threaded dust cap 55 provided to prevent ingress of contaminant.,

The device 6 shown in figure 4 includes a hollow body 60 including a generally cylindrical, hollow, housing member 18. At an outer end, the housing member is closed by an end cap 11 of somewhat cylindrical form, having a threaded outer periphery which is threadedly received in an internally threaded portion of the member 18. An O-ring 13 is positioned between these components so as

SUBSTITUTE SHEET to seal the annular space between the end cap 11 and the housing member 18 and to render the end of the body 60 airtight..

The end of the member 18 opposite end cap 11 is closed by a further end cap 65. The end cap 65 is of generally cup-shaped form and is inserted into the adjacent end of the housing member 18, being affixed in position by any suitable means such as glueing, screw-threading or welding.

The end cap 65 is provided with an axially outwardly extending hollow cylindrical spigot 15 which spigot has an internal axial bore 15a which bore is internally threaded. Bore 15a is open at its inner end to the interior of the device 6 via an axial passage 37 extending through end cap 65.

Aside from the opening to the interior of the body 8 formed by the passage 17, the interior of the body 60 is substantially closed by the end caps 2 and 11 to define an interior cavity. This cavity is divided transversally of the housing member 18 by a flexible diaphragm 55.

The diaphragm 55 is of somewhat disc-like form, having a raised peripheral rim 57. The diaphragm 55 is held in position between a peripheral internal step 24 formed on the interior of housing member 18 and a stepped wall portion 65b of the end cap 65, being sandwiched between compressed O-rings 28, 29 around the periphery thereof and to either side of the rim 57.

SUBSTITUTESHEET The diaphragm 55 divides the ^interior of the body 18 into two cavities 30 and 36. Cavity 30 is defined between the diaphragm and the end cap 65, whilst cavity 36 is defined between the diaphragm 55 the end cap 11. The housing member 18 has an internal annular flange 40, which flange has an axial aperture therethrough. The diaphragm 55 is positioned adjacent this flange, and a resilient spring element 19 is positioned between the flange 40 and the diaphragm 55, being arranged to provide a resilient bias which tends to press the centre of the diaphragm 55 in a direction from chamber 36 and towards the end cap 65. The spring element 19 is shown in more detail in figures 7 and 8 as comprising a disc-like element having a plurality of equiangularly disposed fingers 19a radiating from a central portion 19b. the fingers 19a are positioned, at outer ends thereof, on foot portions 19c which foot portions rest against the flange 40, adjacent the periphery thereof, in the fashion shown in figure 4. The spring element 19 is dished, as shown in figure 8 and is arranged so that the convexly curved side of the element is positioned adjacent the diaphragm 55, with the diaphragm resting thereagainst, substantially over the whole of the surface of the element 19. For purposes of locating the element 19 and diaphragm 55, the diaphragm 55 has a central cylindrical nub 55a which is received within a recess portion 26 of the element, positioned centrally within the central portion 19b.

The concave side of the element 19 faces away from the diaphragm 55 within the chamber 36. There is also positioned, within chamber .36, a strip-like contact 25 which has a portion 25a positioned immediately, below

SUBSTITUTESHEET the recess portion 26 of the element 19. The contact 25 is fixed at a fixed end 25c to a circuit board 17 carried by a carrier piece 21 itself fixed within the chamber 36. The contact 25 extends from the fixed end 25c in cantilevered fashion to the portion 25a thereof and thence to a free end 25b. Batteries 16 for powering the device 6 are contained within the end cap 11, adjacent carrier piece 21. The circuit board 17 carries a contact 42 which, when the end cap 11 and batteries 16 are in position, engages the batteries to provide a connection to the batteries. A spring contact 44 is also carried by the circuit board 17 and has a cantilevered free end 44a which also engages the batteries 16, when these are in position. The contacts 42 and 44 provide electrical connection to components carried by the circuit board 17, for operating an electric circuit described later.

A screw terminal 68 is threadedly received within a threaded opening in contact 44, and extends lengthwise of the housing member 18, generally parallel to the axis thereof. This has an enlarged head portion 69 positioned adjacent free end portion 25b of the contact 25.

Under conditions where air pressure within the chamber 30 substantially exceeds that within chamber 36, the excess pressure force of in chamber 30 presses against diaphragm 55 which in turn resiliently compresses spring element 19. The spring element 19 assumes the condition shown in figure 4, and the portion 26 thereof is moved downwardly to press against the portion 25a of contact 25, thus to move the contact 25 against natural resilient bias thereof so that the

SUBSTITUTESHEET portion 25b of the contact is moved to the position shown in Figure 4 where if is spaced away from the underside of portion 69 of the terminal 68. Under conditions when there is less than a substantial excess air pressure in the chamber 30,there is less pressure force on the diaphragm 19 and resiliant bias of the element 19 causes the diaphragm and the portion 26 of the element 19 to be moved upwardly as shown in figure 4, whereby the free end portion 25a of the contact 25 is moved in a direction which is upward as shown in Figure 4, under natural resilient bias of the contact 25 to cause the end portion 25b to engage the underside of portion 69 of terminal 68. In the position where the pressure in chamber 30 does not substantially exceed that in chamber 36, there is thus an electrical connection provided between the terminal 68 and the contact 25 but, in the condition where there is a substantially higher pressure in the chamber 30„ that electrical connection is broken by virtue of the described disconnection of coupling between the end portion 25b of element 25 and the head portion 69 of terminal 68.

To use the device 6, the device is screwed on to the valve 3. In particular, the internally threaded bore 15a is screwed onto the threaded housing 3a of the valve 3. It is necessary, however, for operation of the device 6, that the air pressure within the tyre should act upon the diaphragm 55. To enable this, the device 6 is provided with an actuating element 70 which projects axially into the bore 15a and engages the outer end of the member 50 of valve 3 to depress this and open the valve as previously described, whereupon air within the tyre may pass through the valve 3 and into the chamber 30, via the passage 37, to act against the diaphragm 55.

The element 70 has a spigot portion 70a which is slideably retained within the passage 17 to allow air to freely pass from the valve through the passage 37 and around the spigot portion 70a into the chamber 30. The element 70 also has a peripheral flange 70b located around the spigot portion 70a, at a position intermediate the ends thereof. The element 70 is normally biased to a position at which the flange 70b rests against a portion of the interior surface of the end cap 65 surrounding the passage 37, by action of a leaf spring 41. Leaf spring 41 is shown in figures 5 and 6 as being of somewhat dished configuration having end portions 41a, 41b and a central circular opening 41c. A downwardly depending portion of the spigot portion 70a is neatly engaged within the opening 41c to locate the element 70 thereon.

As shown in figure 4, the ends 41a, 41b of the element 41 are retained at opposite sides of the device 6, being held in position between the inner surface of the end cap 65, at its internal periphery, and an inwardly stepped peripheral portion 55d of the diaphragm 55. The convex side of- the spring element 41 is directed towards, and is adjacent to, the inner surface of the end cap 65.

During the aforementioned screwing of the device 6 onto the valve 3, the spigot portion 70a, of element 70 engages the stem 52 valve member 50 to effect the operating of the valve. This screwing action may be continued until such time as an annular sealing ring 31 at the end of the bore 15a engages the free end of the valve to seal against the valve and prevent escape of air from the tyre outwardly through the bore 15a.

In different commercial forms of the valves 3, the valve stem 52 may be of differing lengths and may extend to different locations adjacent the outer end of the valve. Thus, were the element 70 to be fixed in position relative to the body 60, there is the possibility that, with some valves 3, the spigot portion 7.0a may not be of sufficient axial length to engage the stem 52 sufficiently to cause the valve 3 to open. On the other hand, in other constructions of the valve 3, the spigot portion 70a may extend too far into the valve, causing the valve member 52 to be pressed inwardly to too great an extent, such as to cause the valve member to bind against internal portions of the valve 3 (not shown in figure 3) or to otherwise place too great a compresive strength against the valve stem 52 or against the spigot 70a, possibly causing damage thereto. However, because of the resilient loading provided by the element 41, there is a considerable degree of flexibility insofar as the operation of the element 70 is concerned, with different types of valve 3. More particularly,^' it ■ is possible to select a length for the spigot portion 70a which is sufficient to ensure that most commercially available forms of the valve 3 may be satisfactorily operated. In those instances where the element might otherwise project too far into the bore 15a with possible risk of damage as above described, the valve member 50, upon reaching its maximum extent of inward movement relative to the remainder of valve 3 will so bear against the element 70 as to cause the element 70 to be moved axially inwardly relative to the housing 18 against the resilient bias provided by the spring element 41. Thus, although the spring element 41 is made of sufficient strength to provide adequate force for inwardly depressing the valve member 50 for the various commercial forms of the valve 3 likely to be encountered in practice, the element 41 may be readily designed so as to still permit the required inward movement of the spigot portion 70a to occur, relative to body 60, under the described conditions, without risking substantial damage to the valve or to the device 6.

The circuit board 17 carries thereon a radio transmitter 80 which is normally unactuated but which is actuated under conditions where the pressure within the tyre with which the device 6 is to be used falls below a predetermined level. More particularly, when the device 6 is positioned on a valve 3, the tyre air pressure acts as described against the valve diaphragm 55 to press the contact 25 to a location at which it does not engage the portion 69 of terminal 67. There is then an equilibrium position at which the pressure exerted on the diaphragm 55 by the air within the chamber 30 and the tyre balances the forces at the other side of the diaphragm, the latter forces being principally due to the action of the spring element 19, although further forces may be present due to fixed air pressure within the chamber 36 and some resilient force applied via contact 25. In any event, the equilibirum position so reached will be varied under the condition where the tyre pressure falls from a normal operating level, whereupon the pressure in the chamber 30 correspondingly falls and the combined forces acting at the other side of the diaphram 55, principally again via the spring element 19, move the diaphragm in the direction towards the end cap 65 so that a new rest position is assumed (due to the then existing lesser forces to the opposite side such as lesser spring force applied by the element 19). By suitable selection of the location of the portion 69 of the terminal relative to the contact 25, the portion 25b of the contact 25 which, under normal operating pressure in the tyre, is moved away from the portion 69 is caused to move, when a predetermined pressure drop has occurred in the tyre, to bring the portion 25b of element 25 into engagement with the portion 69, thus bringing the contact pair represented by the contact 25 and terminal 68 from an open position prevailing under conditions of normal tyre pressure, to an open position, prevailing under conditions where a predetermined fall in tyre pressure has occurred. Under these conditions, the radio transmitter formed on the circuit board 17 is brought into operation to generate an alarm signal, as next described. The operating pressure at which this actuation occurs can be varied by rotating terminal 68 to axially move portion 69 thereof relative to contact 25.

The electric circuit for transmitter 80 is shown in figure 9. The batteries 16 are shown coupled via the contact 25 and terminal 68 to provide electric supply to the various components of the circuit.

There is a transistor 155 arranged with its emitter coupled to positive supply via the aforementioned contact 25 and terminal 68, and with its base connected to the junction between two series connected resistors 132, 134. These resistors are connected from the emitter of the transistor 155 to the collector of another transistor 153. The collector of transistor 153 is also coupled via a load resistor 136 to terminal 68. The emitter of transistor 153 is coupled to ground and the base and the collector of transistor 153 are interconnected via a phase shift circuit 156 including of three series connected capacitors 167, 169 and 171. The junctions between capacators 167 and 169 and between capacitors 169 and 171 are connected to ground via respective resistors 173, 177 also forming part of circuit 156. The base of the transistor 153 is connected to the collector thereof via a resistor 175. The transistor 153, and associated components, form a phase shift oscillator 130 operating at frequency of about, say, 400 hertz, when supply from the battery 16 is coupled thereto by a closing of the contact 25 and to terminal 68.

The transistor 155 is switched, in accordance with the operation of the phase shift oscillator 130, to modulate electric supply on a line 181 from the emitter of the transistor 155 to a radio frequency oscillator 190► Oscillator 190 includes a transistor 151 having its collector connected to line 181 via an antenna 146 and having its emitter connected to ground via a resistor 193 and parallel connected capacator 195. The emitter and collector are also coupled together via a capacitor 154. A surface acoustic wave device 152 is coupled between ground and line 181 via a resistor 201 and is also coupled to the base of the transistor 151. The surface acoustic wave device 152 presents a high impedence at one particular frequency to bias the

SUBSTITUTESHEET transistor 151 in a fashion which produces oscillations at that frequency, that is to say to cause the transistor 151 to switch at that frequency, thus generating an electric current through the antenna 146 at such frequency. Because the supply to the oscillator 190 varies in sympathy with the switching of the transistor 155, the high frequency oscillation produced by the oscillator 190 is amplitude modulated at the frequency of oscillation of the oscillator 130.

A capacitor 188 is provided coupled to line 181 for purposes of voltage stabilisation.

Figure 10 shows an alternative form of transmitter. This transmitter 100 is somewhat similar to the transmitter of figure 9 and like reference numerals denote like components in the two Figures. The following description is confined substantially to differences between the circuits, it being noted that the oscillator 130 in figure 9 is constructed in similar form to the oscillator 130 shown in figure 10, save that the collector of the transistor 153 is coupled via resistor 136 to the supply line 181 for operation of the oscillator when contacts 25 and terminal 28 are closed. Output from the oscillator 130 is in this instance taken via a resistor 166 which is connected to one gate of a dual gate field effect transistor 160. The antenna 162 is connected to the source of transistor 166 and whilst the drain and the other gate of transistor 166 are connected to terminals of the surface acoustic wave device 152. The drain of the transistor 160 is also connected to ground via the series connected resistor 183 and capacitor 185 shown, whilst the second gate of the- transistor 160 is

SUBSTITUTE SHEET also connected to ground via resistor 187. The oscillator 190 oscillates at the required radio frequency for generating radio signals and the output is again amplitude modulated in accordance with output from the oscillator 130.

It has been found that radio frequencies of the order of several megahertz may be satisfactorily employed. The modulating frequency may be in the range 100 hertz to 10 kilohertz.

The radio receiver 8 included in the vehicle may be of simple form such as shown in figure 11. Here, the receiver is shown as comprising a radio frequency amplifier 212 which is coupled via a coupling capacitor 214 to the antenna 12. The output from amplifier 212 is coupled via a coupling capacitor 216 to two detector diodes 218, 220. Diode 218 is connected from the output side of capacitor 216 to a positive supply rail 222 of the receiver. Diode 220 is connected from the output side of capacitor 216, via a resistor 224 to the base of the transistor 226. The junction between diode 220 and resistor 224 is connected to supply rail 222 whilst the collector is connected to ground via resistor 230. The collector is also connected via a resistor 232 to the base of a second transistor 234. Transistor 234 has its collector connected to positive supply and its emitter connected to ground via a resistor 238. A piezo element buzzer device 240 has terminals connected to the base and emitter of the transmitter 234 and a further terminal connected to ground. Signal picked up by the antenna 12 is passed to the amplifier 212 via capacitor 214. The signal is rectified and smoothed by the diodes 218, 220 and capacitor 228 and the resultant uni-directional voltage applied to the transistor 226 which acts as a switch. If signal magnitude applied to the base of the transistor 226 will be sufficient to turn the transistor 226 from an off to an on condition which will then cause the transistor 234 to generate an oscillatory output signal applied to the buzzer element 240 whereby an audible alarm will be generated by the buzzer element. The buzzer element in this case forms the alarm unit 10 of figure 2.

While the simple receiver above described has been found satisfactory, it is preferred to provide an alternative arrangement next described with particular reference to figures 12 to 16.

More particularly, in this instance, the receiver 8 is arranged, as shown in figure 12, for operation from the vehicle battery 245, and the alarm unit 10 is as shown in figures 15 and 16 removably coupled to a carrier device 240 which carrier device is coupled to the receiver 8 by electrical conductors 242, 244 and 246. These conductors connect to fixed contacts 248, 250 and 252 of the carrier device shown diagramatically only in figure 12. The alarm unit 10 has similar contacts 248a, 250a and 252a (Figure 14), which, when the unit 10 is positioned on the carrier device, make interconnections to the contacts carried by the carrier device, thus to couple the unit 10 via the conductors 242, 244 and 246 to the receiver 8. However, on removal of the unit 10 from the carrier device 240, the unit 10 may operate independently, as described later.

Figure 13 shows the circuitry of the receiver 8. More particularly, the receiver 8 includes a power supply 264, of conventional form, having a bridge rectifier 266 coupled via a fuse 270 to the vehicle battery. Output from this is coupled to ground and to a supply rail 290 via a regulator, such as commercially available regulator type LM317T, this having associated capacitors 294, 296 and resistors 298, 300, and rectifier 302. An R.F. section 304 is provided, being for example, of similar form to that described in figure 11, in this instance being shown as comprising a radio frequency amplifier 158 connected to the antenna 12, a mixer 160 coupled to receive output from the radio frequency amplifier and from a local oscillator 164, with output from the mixer being passed through an intermedicate amplifier 162 to a discriminator 166, the output from which is taken on an output line 306 from the R.F. section via an end amplifier 168. The R.F. section 304 includes logic circuitry (not shown) of conventional type effective to provide an output on the output line 306 only under the condition where the radio frequency signal from device 6 is received, and where the particular modulating frequency (400Hz in the example given above) from the device 6 is also detected.

The receiver 8 includes*a divide by ten counter 350 which receives clock pulses from a clock 352 comprised of a unijunction transistor 354 having a base thereof connected to ground via a capacitor 356 and having the gate and second base connected to positive supply via resistors 358 and 360.

SUBSTITUTE SHEET In the absence of detection by the R.F. section of signal indicating loss of tyre pressure, the signal applied via the line 306 to the counter 350 is such as to continually reset the counter 350 so that no output therefrom arises. However, in the presence of detection of a signal for a period exceeding ten clock cycles (whereupon the counter 350 is not reset for such period), the counter will provide an output, via a resistor 370. Typically, the clock may have a frequency of about 1 hertz so that it is required that an alarm condition be present, as detected by the R.F. section 304, for ten clock cycles, that is to say for 10 seconds, before the output of the counter 350 is conditioned in accordance with such detection.

Output via resistor 370 is provided to the base of a transistor 372 having its emitter connected to ground and its connector connected to positive supply via the coil 380 of a relay 378. The relay coil is connected in parallel with a diode 382. When the alarm . condition, as registered via the output from the counter 350 and applied to the base of the transistor 372 occurs, the relay coil is energised by turning on of the transistor 372, whereby contacts 380a associated with the relay 378 are moved from the condition shown in figure 13, at which no electric current passes through the contacts, to a condition at which connection is made to the previously described electrical conductor 244.

The three conductors 242, 244 and 246 interconnecting the receiver 8 and the carrier device 240 are shown in figure 13, together with the associated contacts 240, 250 and 252 of the carrier device. Of these, the contact 244 is connected to negative battery supply (ground) via conductor 244, and terminal 252 is connected to positive supply from the power supp.ly 246 via conductor 246. The contact 248, which provides connection by conductor 242 to the relay contacts 380a is, as described later, normally at near ground condition when the alarm unit 10 is positioned on the carrier device 240. Accordingly, when the relay contacts 380a are operated, as described, the contacts close and provide connection to ground via contact 248. • The relay 378 is then placed substantially between positive supply and ground and the relay is thus latched in the "on" condition regardless of any subsequent change of state of transistor 372. A capacitor 381 is shown in Figure 13, connected from ground to conductor 246, for noise filtering.

Referring now to figure 14, there are shown the three contacts, 250a, 248a and 252a which are disposed on the unit 10 and which, when the unit is in position on the carrier device 240 make connection to respective ones of the contacts 248a, 250a, 252a. There is provided, coupled across contacts 250a, 248a, a series circuit comprising a reset switch 390, such as a push button switch, and a resistor 392. The switch 390 is normally closed and since resistor 342 is of low value, is thus effective to provide he aforedescribed near ground signal condition on the terminal 248.

The unit 10 has an internal rechargable battery, such as a nickel cadmium battery 400, having one terminal connencted to ground and the other terminal connected to receive supply from conductor 246 via contacts 252, 252a, a resistor 402 and a diode 404, for charging of the battery when the unit 10 is in position on the carrier device 240. There is an external switch 406 on the unit 10 which is operable to permit operation of certain of the components of the unit 10 from the battery 400. However, when the unit is in position on the carrier device 240, the unit 10 is powered from the receiver 8, particularly from the supply rail 290 via the conductor 246 and the engaged contacts 252, 252a. The supply is taken from contact 252a on a supply rail 408. Under the condition where the relay 378 is operated and the contact 248a placed in near ground condition, pursuant to detection of an alarm condition, a transistor 410 is turned on. This transistor has its base connected to contact 248a via a resistor 412 and its emitter connected to ground. The collector is connected to rail 408 via a lamp 414. Contact 248a is connected to ground via a resistor 416. Turning on of transistor 410 provides a circuit from the rail 408 to ground, via the lamp 414, to cause the lamp to be illuminated to indicate an alarm condition. The lamp 414 is positioned on the exterior of the unit 10 where it may be readily observed.

Unit 10 includes a time out timer 420 having an input connected to rail 408 via resistor 424 and also connected to the collector of transistor 410 via a capacitor 422. In the condition where no alarm has been detected, and transistor 410 is off, output from the timer 420, on a line 440 therefrom, is conditioned such that a transistor 442 coupled to the timer 242 is an off condition. When, however, transistor 410 is turned on, a negative going pulse is produced at the junction between capacitor 422 and the resistor 424, the pulse being.of a duration determined by the values of capacitor 422 and resistor 424. More particularly, on switch on transistor 410 occurring, the input to the timer 420 remains at a low condition for a certain time until sufficient charging of the capacitor 422 via resistor 424 occurs as to revert the intput of the timer 420 to its previous state to initiate time out of timer 420. The output on the line 440 is connected to ground via resistor 446. The transistor 442 is turned on for a period dependent on the time out period of the timer 420, following operation of transistor 410.

A buzzer 448 is connected to positive supply via a diode 450 and to ground via transistor 442. Thus, when transistor 442 is turned on, the supply voltage is applied to the buzzer 448 to cause it to emit an audible alarm. This alarm will last for a period of time corresponding to the time for which the output of the timer 420 changes state pursuant to operation of transistor 410.

SUBSTITUTE SHEET Thus, when an alarm condition is detected by the receiver 8, the lamp 414 is illuminated, indefinitely, whilst the buzzer 448 is operated for a period of time. That period of time may be arranged to be, for example, one or two minutes, as required.

The lamp 414 will remain illuminated while the unit 10 is in position on the carrier device 240, even under the condition where an alarm signal is no longer detected and transistor 272 is not turned on, pursuant to the described self-latching action of the relay 378. If, however, the transistor 272 is turned off pursuant-to an alarm condition no longer prevailing, the lamp 414 can be extinguished by operating switch 390 on unit 10 to break the circuit connection between the terminals 248a, 250a via switch 390 so that terminal 248a is no longer at near ground potential. In this case, the relay coil 380 is de-energised and the contacts 380a are reverted to their inactivated condition.

As described, it is possible to remove the unit 10 from the carrier 240. If this is effected under a condition where transistor 272 has been turned off, relay coil 380 is de-energised. Otherwise the relay coil remains energised. In any event, on removal of the unit ten from the carrier device 240, the terminal 248a is no longer at ground potential and transistor 410 is turned off. In the absence of the operation of switch 406 from battery 400, the buzzer 448 will also be turned off this time. It is possible once the unit 10 is removed from the carrier device 200 to effect operation thereof from the battery 400 by operating switch 406 to supply power to a supply rail 470. The supply rail 470 is coupled to rail 408 via a diode 472 and the aforementioned diode 450. The buzzer 448 is connected to the junction between the diodes 450 and 472 and is thus capable of operation, under certain circumstances described next, from the battery 400 via the diode 472. It will be noted, however, that the diode 450, now being of relatively reversed polarity, will not permit conduction to supply rail 408 so that the time out timer 420 which is coupled, to the rail at this location, will not under this circumstance be capable of operation. Supply rail 470 is coupled to an R.F. section 480 of the unit, this being of generally of similar form to the receiver described with reference to figure 11. Components of the receiver of figure 11 and of the R.F. section 480 which are like are denoted by like reference numerals and the following description is confined to differences therein. Here, the output of the R.F. section is taken directly from the transistor 226 without being passed to field effect transistor 240 and transistor 234 and associated components as described in figure 11. The antenna for the R.F. section is shown as comprising an antenna 482 being an antenna separate from the previously described antenna 12, and being positioned within the unit 10. The R.F. section is energised when the switch 406 is operated and will be sensitive to detect signals from the transmitter of any particular device 6. One such detection, the transistor 226 is turned on and the output therefrom, taken via a line 488, is passed to the base of a transistor 490 having its collector connected to the buzzer 448 and its emitter connected to ground. Under this condition, the transistor 490 is turned on to cause continuous buzzing tone indicative that an alarm condition has been detected f om a device 6, by operating buzzer 448.

SUBSTITUTE SHEET The supply rail 470 also supplies a pulse clock 492 which is turned on and rendered operative, when the switch 406 is operated, to apply positive supply to the rail 470. The output of this is connected to ground via a resistor 494 and thence, via a resistor 496, to the base of the transistor 498. Transistor 498 has its emitter connected to ground and its collector connected to buzzer 448. The pulse timer generates a pulse output which is applied to repetitively turn on the transistor 498 which thus effects operation of the buzzer 448, in the absence of turn on of transistor 490, to cause the buzzer to emit repetitive tone bursts.

The operation of the arrangement described is thus such that, when one of a number of devices 6 associated with a vehicle effects detection of an alarm condition, with resultant generation of the modulated radio frequency signal described, the signal is detected by the receiver 8 and, provided the unit 10 is in position on the carrier 240, will cause the lamp 414 to be operated whereby to register that an alarm condition has developed. The buzzer 448 is operated for a predetermined time to provide a similar indication. At this stage, however, the particular one of the devices 6 which has been operated is not known and it is not possible for the driver to ascertain which tyre is suffering from a low inflation rate.^" Accordingly, the vehicle driver may then remove the unit 10 from the carrier device 240, operating the switch 406 to provide for operation of the unit via the battery 400, and may then bring the unit 10, which may be made of quite small hand held size, in turn adjacent each- of the vehicle tyres. After removing unit 10 from carrier 240, and on turning on of switch 406, the buzzer 448 is operated intermittently as described to indicate that the unit is in .operation. When the unit is brought close to a tyre deflation warning device 6 which is generating a radio signal indicative of low tyre pressure, the signal is detected by the R.F. section 480 which then causes the buzzer 448 to be operated continuously. The R.F. section 480 may be made of relatively low sensitivity so that it will provide an output on line 488 only when it is in the close vicinity of a device 6 which is generating an alarm signal. Thus, it is possible to effect an interegatory operation by use of the unit 10, to readily determine which of the tyres have suffered pressure loss.

Upon detection of the unit 6 which has generated the alarm signal, the unit 10 may be turned off by opening switch 406 and the unit 10 returned to the device 240. In that case, the lamp 414 will remain actuated until reset by opening a switch 390, the latter however only being possible to effect under the condition where the output from the counter 350 is no longer applied to turn on transistor 372. That is to say, resetting will not be possible until such time as no one of the devices 6 associated with the vehicle in question is generating an alarm signal.

The described arrangement has been advanced merely by way of explanation, and many modifications and variations may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

CLAIMS :

1. A tyre deflation warning device comprising a body fittable to a vehicle tyre valve, a pressure sensor and a radio transmitter effective to transmit a radio signal containing information as to pressure sensed by said sensor, the device further including an actuating element which, as the body is fitted to the valve, is effective to enter the valve and to engage and inwardly displace a valve member of the valve to open the valve and cause, without substantial loss of air from the tyre, air pressure within the tyre to act upon the sensor, for sensing of such pressure by the sensor, the sensor being effective to control said radio transmitter whereby the output thereof is conditioned to a particular state when the pressure of the tyre, as sensed by the sensor, falls to a predetermined level, wherein said actuating element is interconnected with said body via a resiliently^' compressible element adapted to permit the actuating element to be moved, against resilient bias provided by the resiliently compressible element, inwardly relative to the body under the condition where a predetermined force is applied to the actuating element from the valve member, during fittment of the body to the valve.

2. A warning device as claimed in claim 1, wherein the sensor includes a deformable sensing element open to a chamber within said body, which chamber is in use of the device subjected to said tyre pressure, said sensing element being moved, against resilient bias applied thereto to in use assume a position influenced by the pressure of said tyre, and contact means associated with said deformable or moveable sensing element, the contacts being switched between closed and open conditions or between open and closed positions under a condition of movement of the sensing element to a condition corresponding to reduction in said tyre pressure below a predetermined value, said transmitter being interconnected with said contact means to cause the transmitter to be conditioned to provide said output in said particular state condition under such movement.

3. A warning device as claimed in claim 2, wherein said sensing element is a diaphragm forming one wall of said chamber, said chamber being provided with a bore which is internally threaded to enable the body to be screwed onto a threaded portion of said valve, within an axial passageway of which threaded portion said valve member is axially moveable, said actuating element having a portion extending axially relating to said bore which portion is, at the' free outer end thereof, brought into engagement with the valve actuating member when the body is screw threadedly engaged on the threaded portion of said valve, said resiliently compressible element comprising a compression spring positioned in said chamber and resiliently biasing said actuating element axially relatively to the bore and outwardly relative to the body.

4. A warning device as claimed in claim 3, wherein said compression spring is in the form of a leaf spring engaged at opposite outer ends thereof at opposite sides of the chamber with said body and engaged, at a central portion thereof, with the said actuating element.

5. A receiver device for receiving a radio signal from any one of a number of tyre deflation warning - devices fitted to respective tyres of a vehicle and which signal is conditioned to a particular state when the tyre pressure as sensed by the respective warning device falls below a predetermined level, said receiver device including a first receiver the output of which is caused to change state when said signal is conditioned to said particular state, the receiver device further including an alarm device responsive to the condition of the output of the first receiver to cause an alarm condition to be indicated pursuant to said change of state, the alarm device being demountably coupled relative to the receiver means whereby to permit the alarm device, once a said alarm condition is indicated, to be succesively positioned adjacent ones of said alarm devices and including a second receiver for receiving said radio signal and responsive, when the alarm device is brought adjacent an alarm device the radio signal from which is conditioned to said particular state to indicate a further alarm condition.

6. A tyre deflation warning device comprising means fittable to a tyre valve and including a sensor to sense tyre pressure and radio transmitter means effectice to transmit a radio signal containing information as to tyre pressure as sensed by said sensor, wherein said radio transmitter generates a radio signal at a predetermined radio frequency and wherein the radio transmitter includes a surface acoustic wave device effective to stabilise said predetermined frequency.