CA1266788A - Detection of a low pressure condition of a pneumatic vehicle tyre - Google Patents

Abstract

A B S T R A C T

A tire pressure indicating device includes a coil mounted in a tire forming with a capacitor to form a passive oscillatory circuit.
The circuit is energised by pulses supplied via a coil positioned outside the tire and secured to the vehicle and the frequency in the passive oscillatory circuit is varied with tire pressure due to changes caused to the capacitance value of the capacitor . The frequency in the passive oscillatory circuit is sensed via a coil positioned outside the tire and secured to the vehicle.

Description

BACKGROUND OF THE INVENTION
THE invention relates generally to detecting conditions of rotatable members and has particular application to detecting the pressure of a vehicle tire.

A number of patent specifications have addressed the problem of detecting and indicating tihe pressure of a vehicle tire. These documen-ts disclose methods of using the variations in tire pressure to modify one or more parameters of a resonant circuit which is attached to the vehicle wheel. In many of those cases, resonant primary, secondary or receiving circuits are used, at least two such circuits being electrically closely coupled. For example UK 2 065 898, UK 2 069 209, USA 2 274 557, USA 3 662 335 and European application 0 045 401 disclose systems having closely coupled circuits. Such closely coupled circuits, or mutually I tuned circuits, have the disadvantage that characteristics of the electrical signals generated by the circuits such as their bandwidth and amplitude are dependent ; on this coupling, which is in turn dependent on the .,~ .

distance between the circuits. The result is that these parameters of the electrical signals vary undesirably with variations in the distance between the circuits, which increases the difficulty of o~taining accurate and reliable measurements. French Patent 2 344 007 discloses an arrangement employing an FM transmitter on the wheel.

SUMMARY OF THE I NVENTI ON

The present invention, in one aspect, resides in a tire pressure monitoring means which includes a passive resonant electrical circuit defining means which defines a resonant electrical monitoring circuit and which is securable relative to the tire to be rotatable therewith, at least a part of the circuit defining means being responsive to pressure to vary the lS natural frequency of oscillation of the monitoring circuit and said part being locatable relative to the tire to be influenced by the pressure therein;
an energising means for energising the monitoring circuit to cause oscillations therein, the energising : 20 means including a pulse generating means for generating a series of energising pulses at a repetition rate suitably lower than the resonant frequency of oscillation of the monitoring circuit and a first coil means that is connected to the generating means and is locatable suitably close to the tire to energise the monitoring circuit; and a sensing means for sensing the frequency of oscillation of the monitoring circuit, the sensing means including a second coil means that is also locatable suitably close to the tire to provide a signal representative of the frequency of oscillation of the monitoring circuit.

The invention, in another aspect, resides in a method of monitoring the pressure of a tire which includes ~ ^ . .

-3a-energising a passive resonant electrical monitoring circuit that is located in the tire and which is responsive to pressure in the tire so that the circuit : oscillates at a natural frequency that i5 dependent on the pressure in the tire, by supplying pulses to an energy transmitting device located outside the tire at a reception rate that is suitably lower than the natural frequency of oscillation of the monitoring circuit and transmitting energy to the monitoring circuit in pulse form and in a wireless manner; and sensing the frequency of oscillation of the monitoring circuit.

The present invention further provides a tire pressure monitoriny means which inc].udes a passive resonant electrical circuit defining means which defines a resonant electrical monitori.ng circuit, is secured to a tire to be rotatable therewith and which includes a resonating coil, at least a part of the circuit defining means being responsive to the pressure in the tire, to vary the natural frequency of oscillation of the monitoring circuit;
an energising means Eor energising the monitoring circuit in an inductive manner to cause oscillations therein, the energising means including an energising coil and a pulse generating means for generating an energising pulse with the energising coil being connected to the pulse generating means to be supplied, in use, with the energising pulse; and a sensing means for sensing the frequency of oscillations in the monitoring circuit in an inductive manner and for providing a signal representative of the frequency of oscillations of the monitoring circuit, the sensing means including a sensing coil that is located suitably close to the tire, .

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-3b-the energising coil and the resonating coil being loosely inductively coupled such that the natural frequency of oscillation of the monitoring circuit is substantially independent of the energising means; and the resonating coil and the sensing coil being loosely inductively coupled such that the natural frequency of oscillation of the monitoring circuit is substantially independent of the sensing means.

: This invention still further provides a method of monitoring the pressure of a tire which includes energising a passive resonant electrical monitoring circuit that is located in the tire and which is responsive to pressure in the tire so that the circuit oscillates at a natural frequency that is dependent on the pressure ln the tire, by supplying pulses :Erom a pulse generating means to an energy transmitting coil located outside the tire, the transmitting coil being loosely inductively coupled with the monitoring circuit such that the natural frequency of oscillation of the monitoring circuit is substantially independent of the pulse generating means and the transmitting coil; and sensing the frequency of oscillation of the monitoring circuit by means of a sensing coil that is located outside the tire and which is also loosely inductively coupled with the monitoring circuit.

The tire pressure is sensed by a device which is arranged to adjust a capacitive component of the passive oscillatory circui~.

The oscillatory circuit can have separate capacitive devices arranged to be selectively switched into the 7~

oscillatory circuit and the pressure sensing device is arranged to control the selective switching.

The output signal may be arranged to produce a warning signal whenever its value differs from a predetermined value or range of values. Such a signal may be a visual signal and/or an audible signal.

ln indicating devices of the present invention the passive oscillatory circuit is excited withou-t causing undesirable interference between the exciting energy and the oscillations of the passive oscillatory circuit.
This is accomplished on the one hand by using coupling circuits having srnall coefficients of coupling, that is the coupling circuits are not closely coupled.
Even so the frequency detector circuit is able to pick up and respond to flux changes caused by the passive oscillatory circuit to detect the natural frequency of the passive oscillatory circuit as required. On the other hand the exciting energy is provided in the form of single pulses transmitted at intervals many times greater than the oscillations of the passive oscillatory circuit. This means that errors in the detection of the natural frequency can be eliminated and direct interference between the excitation energy source and the detecting ciruit is not produced and/
or need not be catered for.
i As mentioned above, in the prior art, primary windings, secondary windings and receiving circuit windings are provided, at least two of which are closely coupled, and the secondary winding being fixed to rotate with the wheel.
Mutual inductance in the close coupled windings means that in cases where the frequency at any ,ime $2~i~7~

in the secondary winding relates to the parameter to be measured, the close coupling interferes with the measurements to be ma~e. Because in mutually tune~
circuits the degree of coupling is directly proportional to the separation between windings, if this separation varies in use undesirably an unpredictable v~riation in ampli~ude and bandwidth occur~ In applications such as tire pressure indicating and warning devices this is likely to happen in practice and represents a serious disadvantage. In embodiments of this invention, the coefficient of coupling is made deliberately small and therefore becomes unimportant. In fact, the passive oscillatory circuit, rotating with the wheel, can be treated as an isolated circuit. The natural frequency at any time of the oscillatory circuit, which is dependent on tire pressure, can be detected in a manner which is independent of amplitude, coefficient of coupling or feedback and any other variations normally associated with tne physical and electrical requirements of maintaining a Stable or predictable relationship between mutually coupled windin~s.

As me~tioned earlier, in embodiments of tAe invention pulsed excitation is also used for driving the passive oscillatory circuit. The pulses are at a 'frequency' much less than the natural frequency of the passive oscillatory circuit. For example in one preferred embodiment the 'frequency' of the pulses is one sixteenth the natural frequency-of the passive oscillatory circuit.
In fact, the pulses may be provided at greater intervals and need no~ be produced 'in pAase' with the oscillations of the passive oscillatory circuit.

~66 ,~8 In mutually coupled or clos~-iy coupled circuits the frequency of the energising signals are often the same as the natural frequt~;e~ of the passive oscillatory circuit and are essentiall~ in phase with the oscillations o~ the oscillatory circuit.

The detecting circuit is normally positioned so that coupling between the energising circuit and the detecting circuit is a minimum distance to the passive circuit in the tire but of course the tire must be free at all times to rotate without fouling the stationary coils.
Maturally the separation between the coils will vary to some extent as the vehicle is driven along the road but as stated above variations in separation does not affect the satisfactory operation of the device. It will be noted also that if the detecting circuit is energised directly and inadverten~ly by the energising pulses, the relatively infrequent occurrence can easily be distinguished electrically from the natural frequency oscillation signals picked up for determination of the frequency thereof in the passive oscillatory circuit.

BRIEF DESCRIPTION OF THE DRA~INGS
Tire pressure measuring devices according to the invention will now be described by way of example with re~erence to the acompanying drawings in which :
I Figure 1 is a cross-section of part of a tire showing scAematicaly part of the device;
Figure 2 is a block circuit diagram of the device;
Figures 3 and 4 are circuit diagrams of parts of the device;
Figure 5 is a representation of the signals at various points in the device during operation thereof, at one operating frequency;

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Figure 6 is a representation of -the signals at various points in the device during operation thereof, at a lower operat.ing frequency;
Figure 7 shows a 0-10 volt voltmeter calibrated to indicate tire pressure;
Figures 8 and 9 show circuit diagrams of part of the circuit of the device; and Figures 10 and 11 are further cross-sectional views of the tire.

DESCRIPTION OF A PREFERRED EMBODII~ENT
Referring to the drawings,in Figure 1 a vehicle tire 10 is mounted on a rim 12 to which is fitted an air pressure sensing device 14 at-tached to the tire in the form o~ -two capacitor plates 14A and 14B separated by a layer of elastic dielectric material 14C. Tne device 14 is electrically connected to an inductance coil 16 extending around the inside wall of the tire 10.
The coil 16 is positioned generally midway between the rim 12 and the tread of tne tire 10 at the widest part of the tire 10. A transmitting or energising inductance coil 18 and receiving inductance coil 20 are attached to a fixed (non-rotatable) structural member (not shown) of the vehicle. The coils 16, 18 and 20 are positioned so as to be loosely coupled.
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In Figure 2, a passive oscillatory circuit which comprises the capacitor device 14 and the coil 16 are shown on the rotating part of the drawing. The inductance coil 18 is driven by a pulse generator 26 via an amplifier 28, the repetition rate of the pulses being much lower than the natural frequency of oscillation of ~6~7~3 the passive oscillatory circuit. The inductance coil 20 is connected to a series circuit comprising an amplifier 30, a divider circuit 32, a frequency-to-direct curren~vol~age-convertor 3 A~ an~ ~ c~mparatory indicator circuit 36.

In use, whether the vehicle -tire is stationary or rotating, the passive oscillatory circuit is inductively energised by pulses generated by the pulse generator 26 and transmitted by tne coil 18. The passive oscillatory circuit oscillates at its natural frequency, the value of which depends on capacitL~e value of the device 14 in series wi~n the coil 16, the frequency being given by the expression f = ~
and the wavef'orm produced in tne passive oscilla~ory circuit oy the exciting pulses takes the general form f(t)--le d sin~ t, where ~is tne attenuation constant of the circuit.

Figures 5 and 6 show at A and B the exciting pulses and the naturally decaying waveform of the passive oscillatory circuit, respectively. The frequency of oscillation is sensed by inductive coupling between the coils 16 and 20 and the frequencyof oscillation is detected by the circuits 30, 32, 34 and 36 to provide an output corresponding to that frequency. According to the chosen range of values of the capacitor device 14, the indicated pressure can be made to correspond directly to the pressure of the tire. At C is shown the received signal at the coil 20, while D shows the amplified and conditioned signal after it has passed through the receiving amplifier 30. It can be seen that the frequency information is retained. At E is shown the signal after processing by " ~2i~7~

the frequency divider circuit 32. The original frequency of, for example 1,6 MHz must be divided to a frequency in the range below 100 l~Hz, which is the practical upper frequency limit for commercially or generally available frequency to voltage convertors. At F in Figure 5 is shown the DC voltage corresponding to the received frequency. T~is voltage is supplied to a meter such as that shown in Figure 7, and tne meter can be calibrated to read pressure directly.

If the tire pressure falls to an unsatisfactory low level, the detected frequency that corresponds to a low tire pressure causes an ou-tput signal at the circuit 36 which can be used to ~enera-te a visual or audible warning to the driver o~ the ve~icle. As an alternative in Figure 9 the devi.ce can be arranqed so that the.re is a fixed value capacitor, which is short-circuited by a pressure sensing switching device when the tire pressure drops below or exceeds a predetermined level, thereby in turn causing actuatlon of an alarm.

In other e~bodiments, two or more fixed value capacitors are employed in place of a 3ingle fixed or variable capacitor. Whereas capacitance of the variable capacitor varies with the pressure of the a'ir in the tire, with a resulting change in the natural resonant ~requency of the circuit 16, a tire pressure responsive switching arrangement is used to switch the fixed capacitors into and out of circuit with the coil 16 as required. The circuits of such switched capacitors are shown in Figure 8.

The circuit details of the pulse generator 26 and the amplifier 28 are shown in Figure 3. The pulse transmitter circuit 26 includes a pulse generator IC.l arranged in such a fashion as to produce a 1 micro-second pulse every ~l2~

10 micro-seconds. The pulse width and period between pulses is so selected as to produce negligible errors in the frequency to be measured from the rotating member.
The amplifier 28 consis-ts of a single stage transistor driver amplifier TR.l arranged to drive transmitting coil 18. The pulse generator and amplifier circuits 28 are mounted in the same housing as the receiver circuit in the cab of the vehicle, but could be mounted in separate housing and at any convenient space in the vehicle.
Transmitting coil 18 is connected to TR.l by means of a coaxial cable (not shown). Coil 18 is mounted on the frame of the vehicle adjacent to the road wheel diagonal to the coil 16 inside the tire. Coils 18 and 16 are loosely coupled to ensure a small coefflcient of coupling.

The circuit details of the amplifier 30, the divider circuit 32, the frequency to direct current voltage convertor 34 and the comparator/indicator circuit 36 are shown in Figure 4. The circuit diagram is shown in two parts for convenience, the circuit is in practice provided~on one circuit board and is connected together.
The amplifier 30 consists of a high grain wide band transistor amplifier incorporating TR.l, TR.2, TR.3, TR.4 and TR.5. The divider 32 is a 4 bit binary counter IC. 1 and the frequency to direct current voltage convertor 34 comprises a precision frequency to voltage convertor IC.2.
The circuit 36 includes a voltage comparitor circuit with alarm threshold adjustment IC.3, status indicating lamps IL.l, IL.2 and IL.3 and audio alarm latching and reset circuit IC.4. The coil 20 is mounted on a member of the vehicle adjacent to the inside wall of the tire and is loosely coupled with coil 16 mounted inside the tire.
Coil 20 is connected to the receiver circuit mounted in the cab of the vehicle by means of a coaxial cable.

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The signal present in the passive oscillating circuit is detected by coil 20 and is applied to the input of the amplifier 30 where it is amplified and limited to contain frequency information only. The received signal is between 400 kHz and 1,6 MHz depending on the pressure inside the tire. The frequency information, which is inversely proportional to the pressure inside the tire, is divided by 16 by IC.l, the 4 bit binary counter, to produce a signal in the 25 kHz to 100 kHz band. The frequency information is translated to a dc voltage level using IC.2. The dc output from IC.2 is applied to a 0 to 10 volt meter to indicate the tire pressure in analogue fashion as shown in Figure 7.

The output from IC.2 is also applied to IC.3, the voltage comparator circuit. R25 is used to set the high voltage (low pressure) alarm threshold and R26 is used to set the low voltage (high pressure) alarm threshold. When the tire pressure is within limits, IL.l a green L E D
is energised. When pressure is outside the preset limits red L.E.D IL.2 for high pressure or IL.3 for low pressure are energised and the alarm circuit IC.4 is enabled causing an audio alarm which can be reset using the pushbutton indicated.

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The output of the comparator/in~icator so provided may be used in various other ways to produce audible or - visual alarms if the pressure of the tire decreases to below or exceeds predetermined pressure levels, as desired.

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In Figures lO and 11 two forms of pressure sensitive switch are shown. A membrane 50 stretches over a micro-switch 52 which is normally closed when the air pressure in the tire is sufficiently high. When the pressure drops below a predetermined safe or satisfactory pressure, the microswitch 52 is opened by a biassing spring (not shown) to short-out a capacitor or to alter the connection to the coil 18 of capacitors such as shown in Figures 8 or 9 as the case may be. In Figure : lO 11, the microswitch 52 is provided with a screw adjustment so that by using an ~ll~n i~2y inserted through a sealed aperture in the wall of the tire, the pressure at which the switch 52 operates can be adjusted.

l~any variations to the device are envisaged. For example, the passive oscillatory circuit may be arranged with - switched and separate inductance coils 16, or include a variable inductance coil having an inductance which is varied with tire pressure. Further, the passive oscillatory circuit can be arranged to respond to another condition such as temperature. Pressure and temperature ' devices may be provided in combination and the frequency oscillations in the coil 16, or other coils, may be separately measured to determine either the pressure or temperature in turn.

! 25 Instead of a single coil 20, two similar coils may be arranged radially displaced around the tire at each side of the coil 18, electrically connected in parallel. Thus, in Figure 4 there are two coils 20 connected in parallel. In practice this has the effect that any movement of the tire and relative to the coils 20 tends to be compensated in the signals sensed by each of the two coils 20.
It is, of course, possible to have three or more coils 20 connected in parallel if desired.

Claims (37)

1. A tire pressure monitoring means which includes a passive resonant electrical circuit defining means which defines a resonant electrical monitoring circuit and which is securable relative to the tire to be rotatable therewith, at least a part of the circuit defining means being responsive to pressure to vary the natural frequency of oscillation of the monitoring circuit and said part being locatable relative to the tire to be influenced by the pressure therein;
an energising means for energising the monitoring circuit to cause oscillations therein, the energising means including a pulse generating means for generating a series of energising pulses at a repetition rate suitably lower than the resonant frequency of oscillation of the monitoring circuit and a first coil means that is connected to the generating means and is locatable suitably close to the tire to energise the monitoring circuit; and a sensing means for sensing the frequency of oscillation of the monitoring circuit, the sensing means including a second coil means that is also locatable suitably close to the tire to pro-vide a signal representative of the frequency of oscillation of the monitoring circuit.

2. A monitoring means as claimed in Claim 1, in which the first coil means is locatable adjacent and outside the tire.

3. A monitoring means as claimed in Claim 1 in which the second coil means is locatable adjacent and outside the tire.

4. A monitoring means as claimed in Claim 1 in which the entire circuit defining means is locatable within the tire.

5. A monitoring means as claimed in Claim 1 in which said part of the circuit defining means responsive to pressure is a capacitive component that is pressure variable to vary its capa-citance.

6. A monitoring means as claimed in Claim 1 which includes a comparison means for comparing the signal provided by the sensing means with a predetermined value and for providing an alarm signal when the signal provided by the sensing means equals the predetermined value.

7. A monitoring means as claimed in Claim 1 in which the circuit defining means includes a coil.

8. A monitoring means as claimed in Claim 1 in which the energising means is operable to supply pulses at the repitition rate independently of the circuit defining means.

9. A monitoring means as claimed in Claim 1 in which the energising means is not electrically connected to the circuit defining means and energises the monitoring circuit, in use, in a wireless manner.

10. A monitoring means as claimed in Claim 1 in which the circuit defining means is operable independently of the sensing means.

11. A monitoring means as claimed in Claim 1 in which said part of the circuit defining means that is responsive to pressure includes two capacitive components and a pressure switch, the capacitive components being switchable into and out of the cir-cuit in accordance with the pressure in the tire.

12. A monitoring means as claimed in Claim 11 in which the switch includes a mounting means for mounting it in the tire and an adjusting means for externally adjusting it to vary the value of pressure at which it operates.

13. A monitoring means as claimed in Claim 1 in which the natural frequency of oscillation of the monitoring circuit is independent of the energising means and the sensing means.

14. A pressurised pneumatic tire of a vehicle which includes a tire pressure monitoring system as claimed in Claim 1.

15. A tire as claimed in Claim 14 which has the circuit defining means rotatably fast therewith with its pressure respon-sive part suitably located to be responsive to the pressure in the tire.

16. A tire as claimed in Claim 15 in which the first coil means is secured to a part of the vehicle adjacent to and outside the tire.

17. A tire as claimed in Claim 15 in which the second coil means is secured to a part of the vehicle adjacent to and outside the tire.

18. A tire as claimed in Claim 15 in which the circuit defining means is located within a pressure chamber defined by the tire and a rim on which it is mounted.

19. A tire as claimed in Claim 18 in which the circuit defining means includes a coil that is secured to an inner sur-face of the tire.

20. A tire as claimed in Claim 19 in which the coil is co-axial with the tire.

21. A tire as claimed in Claim 19 in which the coil is secured to a side wall portion of the tire.

22. A tire as claimed in Claim 15 in which said part of the circuit defining means responsive to pressure is a capacitive component that is pressure variable to vary its capacitance.

23. A pneumatic tire which has secured to an inside surface a circuit defining means for a monitoring means as claimed in any one of Claims 1 to 3.

24. A pneumatic tire which has secured to an inside surface a circuit defining means for a monitoring means as claimed in any one of claims 4, 5 or 6.

25. A pneumatic tire which has secured to an inside surface a circuit defining means for a monitoring means as claimed in any one of claims 7, 8 or 9.

26. A pneumatic tire which has secured to an inside surface a circuit defining means for a monitoring means as claimed in any one of claims 10, 11 or 12.

27. A pneumatic tire which has secured to an inside surface a circuit defining means for a monitoring means as claimed in claim 13.

28. A method of monitoring the pressure of a tire which includes energising a passive resonant electrical monitoring circuit that is located in the tire and which is responsive to pressure in the tire so that the circuit oscillates at a natural frequency that is dependent on the pressure in the tire, by supplying pulses to an energy transmitting device located outside the tire at a repetition rate that is suitably lower than the natural frequency of oscillation of the monitoring circuit and transmitting energy to the monitoring circuit in pulse form and in a wireless manner; and sensing the frequency of oscillation of the monitoring cir-cuit.

29. A tire pressure monitoring means which includes a passive resonant electrical circuit defining means which defines a resonant electrical monitoring circuit, is secured to a tire to be rotatable therewith and which includes a resonating coil, at least a part of the circuit defining means being respon-sive to the pressure in the tire, to vary the natural frequency of oscillation of the monitoring circuit;
a energising means for energising the monitoring circuit in an inductive manner to cause oscillations therein, the energising means including an energising coil and a pulse generating means for generating an energising pulse with the energising coil being connected to the pulse generating means to be supplied, in use, with the energising pulse; and a sensing means for sensing the frequency of oscillations in the monitoring circuit in an inductive manner and for providing a signal representative of the frequency of oscillations of the monitoring circuit, the sensing means including a sensing coil that is located suitably close to the tire, the energising coil and the resonating coil being loosely inductively coupled such that the natural frequency of oscilla-tion of the monitoring circuit is substantially independent of the energising means; and the resonating coil and the sensing coil being loosely inductively coupled such that the natural frequency of oscilla-tion of the monitoring circuit is substantially independent of the sensing means.

30. A monitoring means as claimed in Claim 29 in which the energising coil is located adjacent and outside the tire.

31. A monitoring means as claimed in Claim 29 in which the sensing coil is located adjacent and outside the tire.

32. A monitoring means as claimed in Claim 29 in which the entire circuit defining means is located within the tire.

33. A monitoring means as claimed in Claim 29 in which the part of the circuit defining means responsive to pressure is a capacitive component that is pressure variable to vary its capa-citance.

34. A monitoring means as claimed in Claim 29 which includes a comparison means for comparing the signal provided by the sensing means with a predetermined value and for providing an alarm signal when the signal provided by the sensing means equals the predetermined value.

35. A monitoring means as claimed in Claim 29 in which the part of the circuit defining means that is responsive to pressure includes two capacitive components and a pressure switch, the capacitive components being switchable into and out of the cir-cuit in accordance with the pressure in the tire.

36. A monitoring means as claimed in Claim 35 in which the switch includes a mounting means for mounting it in the tire and an adjusting means for externally adjusting it to vary the value of pressure at which it operates.

37. A method of monitoring the pressure of a tire which includes energising a passive resonant electrical monitoring cir-cuit that is located in the tire and which is responsive to pressure in the tire so that the circuit oscillates at a natural frequency that is dependent on the pressure in the tire, by supplying pulses from a pulse generating means to an energy transmitting coil located outside the tire, the transmitting coil being loosely inductively coupled with the monitoring circuit such that the natural frequency of oscillation of the monitoring circuit is substantially independent of the pulse generating means and the transmitting coil; and sensing the frequency of oscillation of the monitoring cir-cuit by means of a sensing coil that is located outside the tire and which is also loosely inductively coupled with the monitoring circuit.