A TRANSDUCER

Abstract: A passive electrically operable monitoring means has an inherent resistance and a characteristic capacitance and inductance only, so that the monitoring means has a predetermined natural frequency of oscillation. Variations in a physical parameter being monitored by the monitoring means will cause the characteristic capacitance and/or inductance to vary thereby varying the natural frequency of oscillation of the monitoring means. A transducer includes an excitation means (32) for exciting the monitoring means and a sensing means for sensing the natural frequency of oscillation of the monitoring means. The excitation means (32) includes a pulse generating means for exciting the monito­ ring means with a string of pulses having a repetition rate suitably lower than the natural frequency of oscillation of the monitoring means so that damped oscillations occur in the monito­ ring means.

Description: THIS INVENTION relates to a transducer for monitoring pressure in a tyre. More particularly it relates to a passive electrically operable monitoring means for such a transducer for providing an electrical signal representative of the pressure in a tyre. The invention also relates to a method of monitoring the pressure in a tyre. The invention then extends to a tyre which carries the transducer. The Applicant is aware of various transducers for monitoring variations in physical parameters. Thus, for example, in the case of DE-A1-3 205 705 an arrangement is disclosed for measuring non-electric physical quantities. This arrangement uses a spring as an inductance in a tuning circuit of an oscillator, the quantity to be measured acting on the spring to deform the spring. Deformation of the spring causes a change in inductance which, in turn, causes a change in the frequency of oscillation of the oscillator. Further, DE-OS-2 262 032 discloses an electro-mechanical pressure or force transducer which utilizes a cavity or line resonator for measuring a non-electric physical quantity. According to one aspect of the invention, there is provided a passive electrically operable monitoring means for a transducer for monitoring pressure in a tyre, the monitoring means having an inherent resistance and a self-capacitance and a self-inductance only, characterized by the monitoring means being arranged such that the pressure being monitored causes variations in the self -capacitance and the self-inductance of the monitoring means thereby to vary the natural frequency of oscillation of the monitoring means in accordance with the variations in the pressure. It will be appreciated that as the monitoring means has only a self-inductance and a self-capacitance, it does not have a discrete capacitance or inductance. In one embodiment of the invention, the monitoring means may comprise a length of conducting wire wound into a coil with spaces being provided between adjacent turns of the coil. Then, the turns may be resiliently arranged relative to one another. Preferably the monitoring means may include a resiliently flexible web arranged between the turns of the coil, with the turns being secured to the web so that, in use, as the web flexes under the effect of the pressure being monitored, the spacing between the turns varies thereby varying the self-inductance and the self-capacitance of the coil, and hence its natural frequency of oscillation. It will be understood that in this form the coils may be arranged next to one another. Those skilled in the art will appreciate that the monitoring means could be defined in any number of ways. Hence, in another embodiment of the invention, the monitoring means may comprise at least two lengths of spaced conductors arranged in a two-wire transmission line or coaxial cable manner. The spacing between the conductors or the effective length thereof may be variable to vary the natural frequency of oscillation of the monitoring means. Similarly, more than two lengths of conductors may be arranged adjacent to and spaced from one another in a resilient manner. The ends of some or all of the conductors may be joined so that the conductors form an endless coil or they may not be joined so that each conductor is a D.C. open circuit. They may even, possibly, be joined by discrete impedance components or the ends of adjacent conductors may be connected by such discrete impedance components. The conductors may be arranged to be resiliently spaced, in any suitable manner. Thus they may be secured to a resiliently flexible material which flexes as the pressure being monitored varies, so that the spacing of the conductors varies. Instead, the spacing between the conductors may be kept constant, and the effective length of the conductors may be varied. Hence, the ends of the conductors may be terminated in spring loaded short circuited terminations which, in use, are responsive to variations in the pressure being applied for varying the effective length of the conductors. As indicated above, with a pressure transducer, one way of varying the self-inductance and/or self-capacitance of the circuit nay be by varying the spacing of the conductors, or the length of the line. However, the same result could be obtained in other ways, for example, by maintaining the spacing of the conductors and the effective length of the line constant, and by varying an electro-magnetic value, such as the permittivity or permeability, of a medium between the turns of the coil or the conductors. Thus, the monitoring means may include a dielectric medium located between the turns of the coil or the length of conductors, as the case may be, the permittivity of the dielectric medium being variable in response to variations in the pressure applied thereto, for varying the natural frequency of oscillation of the monitoring means. According to another aspect of the invention, there is provided a transducer for monitoring a physical parameter, the transducer including a monitoring means as described above; an energising means for energising the monitoring means; and a sensing means for sensing the natural frequency of oscillation of the monitoring means. The energising means may include an energising coil which may be coupled electrically and/or magnetically to the monitoring means in a wireless manner for energising the monitoring means to cause the monitoring means to oscillate at its natural frequency of oscillation, it being appreciated that the energising coil is not mechanically coupled to the monitoring means. The energising coil (or some other energy transmitting device that transmits energy in a wireless manner) may be located, or locatable, sufficiently close to the monitoring means to energise it and to cause oscillations therein at its natural frequency of oscillation. Preferably, the monitoring means minimally influences the energising means and vice versa. Thus, the energising coil may be loosely coupled to the monitoring means. The transducer may include a pulse generating means for generating a string of pulses at a repetition rate sufficiently lower than the natural frequency of oscillation of the monitoring means so that damped oscillations occur in the monitoring means, in use. The sensing means may include a sensing coil which is electrically and/or magnetically coupled to the monitoring means in a wireless manner. The sensing means is preferably also not mechanically connected to the monitoring means. The monitoring means is also, preferably, not influenced by the sensing means and vice verse, and hence the sensing coil may be loosely coupled to the monitoring means. The invention extends still further to a tyre which includes a casing; and a monitoring means as described above mounted within and rotatably fast with the casing. If the monitoring means comprises a coil, the invention will accordingly extend to a tyre having the coil fast therewith. The monitoring means will have at least a part thereof, and preferably all, in communication with the tyre which, in use, gets pressurised. Thus, the monitoring means may be secured to an inside surface of the tyre, conveniently of a side wall portion. It will also be understood that the monitoring means could just as well be secured to a tube for the tyre, or to a rim with which the tyre is to be engaged. The energising means and the sensing means of the transducer may be mountable adjacent an exterior surface of a side wall of the casing. It will be appreciated that a transducer as described above has a simple yet elegant construction, which is also extremely robust and reliable. The transducer is also extremely rugged and balanced so that it does not perturb a tyre or anything else with which it is used. According to yet a further aspect of the invention, there is provided a method of monitoring pressure in a tyre which includes, energising a passive electrically operable monitoring means having an inherent resistance and a self-capacitance and a self-inductance only so that the monitoring means oscillates at a natural frequency of oscillation; varying the natural frequency of oscillation of the monitoring means in response to variations in the pressure; and sensing the variation in the natural frequency of oscillation of the monitoring means to provide an indication of the variation in the pressure being monitored, characterized in that the natural frequency of oscillation of the monitoring means is varied by varying the self-capacitance and the self-inductance of the monitoring means in response to variations in the pressure being monitored. As described above, the monitoring means may comprise a length of conducting wire wound into a coil to provide a plurality of spaced turns, and the method may then include varying the spacing between the coils to vary the self-capacitance and self-inductance of the coil, and hence its natural frequency of oscillation, in response to variations in the pressure being applied to the coil. Further, as previously described, when the monitoring means comprises at least two length of conductors which are arranged in a transmission line or co-axial cable manner, the method may include varying the spacing between the conductors. When the monitoring means comprises a length of conducting wire wound into a coil to provide a plurality of spaced turns, the method may include causing the permittivity or permeability of a medium between the turns to vary, thereby to vary the natural frequency of oscillation of the coil. The method may then include locating a suitable material between the turns, the material being allowed to expand or to be compressed as the pressure being monitored. and which is exerted on the material, varies so that the permittivity of the material varies. Instead, the method may include introducing a suitable material between the elements in proportion to the pressure being monitored, so that the permittivity of the medium between the turns varies. The method may include energising the monitoring means with a string of pulses which has a repetition rate suitably lower than the natural frequency of oscillation of the monitoring means so that damped oscillations occur in the monitoring means. The invention is now described, by way of examples, with reference to the accompanying drawings in which:- Figure 1 shows a sectioned view of part of a wheel of a vehicle illustrating one embodiment of the invention; Figures 2, 3, 4 and 5 show similar views of further embodiments in accordance with the invention; Figure 6 shows a partly section side view of a pressure transducer in accordance with a further aspect of the invention; Figure 7, 8 and 9, show sectional views of various embodiments of the transducer of Figure 6; Figure 10 shows a partly sectioned three dimensional view of a tyre including the transducer of Figure 6; and Figure 11 shows in block diagram form circuitry for exciting the transducers shown in Figures 1 to 10 for sensing the resulting oscillations in the transducers. Referring to Figure 1, a first embodiment of a transducer in accordance with the invention is designated generally by reference numeral 10. The transducer 10 is associated with a pneumatic tyre 12 which is mounted on a rim 14 to form a wheel for a vehicle (not shown). The transducer 10 comprises a passive monitoring means which is essentially formed from a length of copper wire which is coiled to form a coil 11 having a plurality of turns 16. The diameter of the turns increases in a direction away from a side wall 24 of the tyre 12 so that the coil is substantially frusto-conical. The turns 16 are secured to a flexible membrane 18 which is of a suitable resiliently flexible rubber or plastics material. The smaller end 20 of the membrane 18 is secured to the inner surface of the side wall 24 of the tyre 12 and the other, larger end 22 is secured to the inner surface of a tread portion 26 of the tyre 12, so that the membrane stretches between the side wall portion 24 and the tread portion 26 inside the tyre. The membrane 18 accordingly defines a chamber 28 together with that portion of the inside surface of the tyre 12 which it spans. The ends of the first turn and the last turn are open-circuited. It will be appreciated that the ends of the first turn and the last turn could also be terminated with a discrete impedance component. It will be appreciated that the coil 11 formed by the turns 16 will have a self-inductance and a self-capacitance only and it will oscillate at a particular natural frequency. It will further be appreciated that the selfinductance and the self-capacitance will vary in accordance with the spacing of the turns. Thus, if the membrane 18 flexes, thereby varying the spacing of the turns 16, the natural frequency of oscillation will vary. Thus, as the pressure with in a general chamber 30 defined by the tyre 12 and the rim 14 varies in relation to the pressure in the chamber 28, so the membrane 18 will flex and the resulting natural frequency of oscillation will be representative of the pressure within the wheel. The coil 11 formed by the turns 16 may be excited by means of an excitation coil 32 positioned adjacent the wheel in register with the turns 16. Similarly, once the coil 11 has been excited, the frequency of oscillation may be sensed by a similar coil 34 (see Figure 11) which is also positioned adjacent the wheel, but in a diametrically opposed position to the coil 32. Thus, referring to Figure 11, the coil 32 is supplied with pulses from an amplifier 36 that is connected in turn to a pulse generator 38. The amplifier 36 supplies the excitation coil 32 with pulses at a repetition rate that is substantially lower than the natural frequency of oscillation of the coil 11 in the tyre, so that damped oscillations occur in the coil 11. Further, the excitation coil 32 and the coil 11 are loosely electrically and magnetically coupled so that the coil 11 is sufficiently excited without perturbing the signal in the coil 32 to any significant extent. Similarly, the sensing coil 34 is connected to a series circuit comprising an amplifier 40, a signal-conditioning circuit 42, a frequency-to-D.C. voltage convertor 44 and a comparator 46. Thus, the coil 34 senses the oscillations in the coil 11 and provides a signal representative thereof. The coil 34 and the coil 11 are also loosely magnetically and electrically coupled so that the sensing coil 34 does not affect the natural frequency of oscillation of the coil 11. The signal provided by the sensing coil 34 is amplified by the amplifier 40, conditioned by the circuit 42, and is converted to a D.C. voltage signal which is then compared by the comparator 46 with a predetermined value and if the signal provided by the convertor 44 varies relative to the predetermined value to indicate that the pressure in the tyre has dropped below a predetermined value, then the comparator 46 provides an alarm signal. It will be appreciated that the components of the circuitry illustrated in Figure 11 are standard items which can be readily realised by a person skilled in the art. Accordingly, these components need not be described any further. Referring to Figure 2, a further embodiment of a transducer 50 is shown therein. This transucer 50 is similar to the embodiment 10 shown in Figure 1 and is similarly referenced. However, with this embodiment the turns 16 of the coil 11 are all in the same plane in a spiral manner and the membrane 18 is annular. One end of the membrane 18 is secured to the side wall portion 24 close to the rim 14 and the other end 22 is secured to the side wall portion close to the tread portion 26 to define a chamber 52. The operation of this system is similar to that of Figure 1 and is not discussed further. Referring to Figure 3 a further embodiment of a transducer 60 is shown. This transducer 60 has four layers, with a first layer of turns secured to the side wall portion 24 and successive turns 16 being secured to three membranes 18. A series of chambers 54 are defined and the operation of the system is again similar to the transducers 10 and 50 and will be understood by those skilled in the art without further discussion thereof. Referring to Figure 4, a further embodiment of a transducer 70 is shown. With this embodiment different turns 16 of the coil are separated by members 72 which are of a suitable dielectric material having a predetermined permittivity. The members 72 are connected to a cylindrical base portion 78, the ends of which are flexibly connected to the inner surface of the tread portion 26 by membranes 76 to define a chamber. The members 72 are mounted in any suitable resilient manner, for example by spring elements 74 so that the degree to which the members 72 extend between the turns 16 varies in accordance with the pressure within the tyre 12. A single turn coil 79 is provided adjacent the side wall portion 24 of the tyre 12 to act as a pick-up. The operation of this embodiment 70 is similar to the previous embodiments, it being appreciated that the self-inductance and self-capacitance of the coil formed by the turns 16 will vary in accordance with the extent to which the members 72 extend between the turns 16. Referring to Figure 5, a further embodiment of a transducer 80 is shown. With this embodiment the turns 16 of the coil 11 are secured to raised formations 82 which project inwardly from the inner surface of the side wall portion 24. With this embodiment, as the pressure in the tyre 12 varies, so the degree of flexing of the side wall portion 24 will vary, thereby varying the separation of the turns 16 which accordingly affects the self-inductance and self-capacitance to vary the natural frequency of oscillation thereof. Referring now to Figure 6, yet a further embodiment of a transducer is shown, and with reference to the previous figures, is similarly referenced. In this embodiment of the invention, the transducer 10 comprises a coil 90, the coil 90 being formed from a spiral circular winding 92 of conductive wire. As illustrated in Figure 7, the windings 92 are arranged in layers, with adjacent layers, and adjacent turns of the coil 90 within each layer being separated by corresponding turns of a resilient hollow tube 94 of a non-conductive material such as a synthetic plastics material. Each section of tube 94 separates adjacent turns of the conductors 92. The tube 94 and the conductors 12 are encapsulated in a resilient potting compound 96 such as silicone rubber. Figures 8 and 9 illustrate various embodiments of the tranducer 10. In the embodiment of the invention illustrated in Figure 8, the conductors 92 are wound in three layers, with the turns of adjacent layers being separated by corresponding turns of the tube 94. Adjacent turns in the same layer are separated only by the resilient potting compound 96. In Figure 9, the conductors 12 are arranged about a central resilient tube 98 which is of a non-conducting material, such as a synthetic plastics material. The conductors 92 are held in place about the tube by the potting compound 96. If the resilient tubes 94 or 98 are sealed, changes in the pressure of air surrounding the tranducer 10 will tend to compress them or to allow them to expand. The degree of compression or expansion which occurs is increased by the relative ease with which the tubes 94 or 98 can be compressed. The pressure which is applied to the transducer 10 will cause the tubes 94 or 98 to be compressed thereby varying the self-capacitance or self-inductance of the coil 90, and in so doing the frequency of oscillation of the coil 90 will vary. The variation of the frequency of oscillation of the coil 90 will be directly proportional to the pressure applied to the coil 90 by the air within the tyre 12. As an example, a coil having a natural frequency of oscillation of about 1600 kHz may be excited with pulses having a repetition rate of about 160 k-pulses per second in order that suitable damped oscillations are generated in the coil.

Claims: A passive electrically operable monitoring means (10, 50, 60, 70, 80,) for a transducer for monitoring pressure in a tyre, the monitoring means having an inherent resistance and a self-capacitance and a self-inductance only, characterized by the monitoring means being arranged such that the pressure being monitored causes variations in the self-capacitance and self-inductance of the monitoring means thereby to vary the natural frequency of oscillation of the monitoring means in accordance with the variations in the pressure. The monitoring means (10, 50, 60, 80) as claimed in Claim 1, characterized in that said monitoring means consists of a length of conducting wire wound into a coil (11, 90) with spaces being provided between adjacent turns (16, 92) of the coil (11, 90). The monitoring means (10, 50, 60, 80) as claimed in Claim 2 characterized in that the turns (16, 92) are resiliently arranged relative to one another. The monitoring means (10, 50, 60, 80) as claimed in Claim 3 characterized in that a resiliently flexible web (18) is arranged between the turns (16) of the coil (11), with the turns (16) being secured to the web (18) so that, in use, as the web (18) flexes under the effect of the pressure being monitored, the spacing between the turns (16) varies thereby varying the self-inductance and self-capacitance of the coil (11), and hence its natural frequency of oscillation. The monitoring means as claimed in Claim 1 characterized in that said monitoring means consists of at least two lengths of spaced conductors arranged in a two-wire transmission line or coaxial cable manner. The monitoring means as claimed in Claim 5 characterized in that the spacing between the conductors or the effective length thereof is variable to vary the natural frequency of oscillation of the monitoring means. The monitoring means (70) as claimed in Claim 2 characterized in that a dielectric medium (72, 78) is located between the turns (16) of the coil (11), the permittivity of the dielectric medium (72, 78) being variable in response to variations in the pressure applied thereto, for varying the natural frequency of oscillation. The monitoring means as claimed in Claim 5 characterized in that a dielectric medium is located between the lengths of conductors, the permittivity of the dielectric medium being variable in response to variations in the pressure applied thereto, for varying the natural frequency of oscillation. A transducer for monitoring pressure in a tyre, characterized in that it includes a monitoring means (10, 50, 60, 70, 80,) as claimed in any preceding Claim; an energising means for energising the monitoring means (10, 50, 60, 70, 80,); and a sensing means for sensing the natural frequency of oscillation of the monitoring means (10, 50, 60, 70, 80,). The transducer as claimed in Claim 9 characterized in that the energising means includes an energising coil(32)which is coupled electrically and/or magnetically to the monitoring means (10, 50, 60, 70, 80,) in a wireless manner for energising the monitoring means (10, 50, 60, 70, 80,) to cause the monitoring means (10, 50, 60, 70, 80,) to oscillate at its natural frequency of oscillation. The transducer as claimed in Claim 10 characterized in that a pulse generating means (38) is connected to the energising coil (32) for generating a string of pulses at a repetition rate sufficiently lower than the natural frequency of oscillation of the monitoring means (10, 50, 60, 70, 80,) so that damped oscillations occur in the monitoring means (10, 50, 60, 70, 80,), in use. The transducer as claimed in Claim 11, characterized in that the sensing means includes a sensing coil (34) which is electrically and/or magnetically coupled to the monitoring means (10, 50, 60, 70, 80,) in a wireless manner. A tyre(12)having a casing (24, 26), characterized in that a monitoring means as claimed in any of Claims 1 to 8 is mounted within and rotatably fast with the casing (24, 26). A method of monitoring pressure in a tyre which includes energising a passive electrically operable monitoring means (10, 50, 60, 70, 80,) having an inherent resistance and a self-capacitance and a self-inductance only so that the monitoring means (10, 50, 60, 70, 80,) oscillates at a natural frequency of oscillation; varying the natural frequency of oscillation of the monitoring means (10, 50, 60, 70, 80,) in response to variations in the pressure; and sensing the variations in the natural frequency of oscillation of the monitoring means (10, 50, 60, 70, 80,) to provide an indication of the variation in the pressure being monitored, characterized in that the natural frequency of oscillation of the monitoring means (10, 50, 60, 70, 80,) is varied by varying the self-capacitance and the self inductance of the monitoring means (10, 50, 60, 70, 80,) in response to variations in the pressure being monitored. The method as claimed in Claim 14, characterized in that the monitoring means (10, 50, 60, 70, 80) comprises a length of conducting wire wound into a coil (11, 90) to provide a plurality of spaced turns (16, 92), and characterized further in that the method includes varying the spacing between the turns (16, 92) to vary the self-capacitance and self-inductance of the coil (11, 90), and hence its natural frequency of oscillation, in response to variations in the pressure being applied to the coil (11, 90). The method as claimed in Claim 14 characterized in that the monitoring means comprises at least two lengths of conductors which are arranged in a two-wire transmission line or co-axial cable manner, and characterized further in that the method includes varying the spacing between the conductors. The method as claimed in Claim 14 characterized in that the monitoring means (70) comprises a length of conducting wire wound into a coil (11) to provide a plurality of spaced turns (16), and characterized further in that the method includes causing the permittivity or permeability of a medium (72, 78) between the turns (16) to vary, thereby to vary the natural frequency of oscillation of the coil (11). The method as claimed in Claim 17 characterized in that it includes locating a suitable material between the turns (16), the material being allowed to expand or to be compressed as the pressure being monitored, and which is exerted on the material, varies so that the permittivity of the material varies. The method as claimed in Claim 18 characterized in that it includes introducing a suitable material (72) between the elements in proportion to the pressure being monitored, so that the permittivity of the medium (72, 78) between the turns varies. The method as claimed in Claim 14 characterized in that it includes energising the monitoring means (10, 50, 60, 70, 80,) with a string of pulses. The method as claimed in Claim 20 characterized in that it includes energising the monitoring means (10, 50, 60, 70, 80,) with a string of pulses which has a repetition rate suitably lower than the natural frequency of oscillation of the monitoring means (10, 50, 60, 70, 80, 100) so that damped oscillations occur in the monitoring means (10, 50, 60, 70, 80,).

Inventors: GALASKO, PHILIP ELLIOT, KRUGER, FRANS JOHANNES
Applicants: GALASKO, PHILIP ELLIOT
Application year: 1987
Main classification: G01D5
IPC codes: G01D5, G01L9, G01L17, G01N27, G01K7