Detection of a low pressure condition of a vehicle tire

Claim 2

A tire pressure indicating device which comprises mounted on a tire a passive electrical oscillatory circuit, tire pressure responsive means positioned in the tire arranged to adjust the natural frequency of oscillation of the circuit in response to changes in the tire pressure; coil means secured to the vehicle and positioned adjacent the tire for exciting the passive oscillatory circuit including a pulse generator arranged to produce electrical pulses in the coil means at a repetition rate much lower than the natural frequency of the passive oscillatory circuit; and a frequency detector circuit secured to the vehicle including coil means positioned adjacent the tire to sense the frequency of oscillation of the passive oscillatory circuit and to provide an output signal corresponding to the tire pressure.

Claim 3

A device according to claim 1 in which the tire pressure responsive means is arranged to adjust a capacitive component of the passive oscillatory circuit.

Claim 4

A device according to claim 2, in which the capacitive component comprises at least two separate capacitors electrically connected to a coil of the passive oscillatory circuit and the tire pressure responsive device comprises a switch which is operated when the tire pressure varies beyond a predetermined value to alter the connections of the capacitors to the coil.

Claim 5

A device according to claim 3, in which the switch includes means for adjusting its operating characteristics, and a sealed aperture in the wall of the tire to provide access to the adjusting means.

Claim 6

A device according to claim 1 in which the output signal is arranged to produce a warning signal whenever its value falls outside a range of predetermined values.

Claim 7

A device according to claim 1 in which the passive oscillatory circuit comprises a coil which extends around the tire and is fixed to an inside wall of the tire.

Claim 8

A device according to claim 6, in which the coil is secured mid-way between the rim of the tire and the tread of the tire.

Claim 9

A device according to claim 1 in which the frequency detector circuit includes two coil means connected in parallel which are radially displaced around the circumference of the tire.

Claim 10

A tire pressure indicating device which comprises a passive electrical oscillatory circuit mounted inside the tire including a coil secured to an inside wall of the tire and extending around the tire, a pressure responsive means mounted in the tire arranged to adjust the natural frequency of the passive oscillatory circuit in response to changes in tire prssure; an electrical energising coil mounted on the vehicle and positioned adjacent the outside of the tire opposite the coil of the passive oscillatory circuit for exciting that circuit, a drive circuit for the energising coil including a pulse generator which provides pulses at a repetition rate much lower than the natural frequency of the passive oscillatory circuit; and a frequency detector circuit mounted on the vehicle having a receiver coil positioned adjacent the outside of the tire opposite the coil of the passive oscillatory circuit, the frequency detector circuit including a frequency to analogue convertor to produce an analogue signal corresponding to the tire pressure.

Claim 11

A tire pressure indicating device which comprises a passive electrical oscillatory circuit mounted inside the tire including a coil secured to an inside wall of the tire and extending around the tire, a pressure responsive means mounted in the tire arranged to adjust the natural frequency of the passive oscillatory circuit in response to changes in tire prssure; an electrical energising coil mounted on the vehicle and positioned adjacent the outside of the tire opposite the coil of the passive oscillatory circuit for exciting that circuit, a drive circuit for the energising coil including a pulse generator which provides pulses at a repetition rate much lower than the natural frequency of the passive oscillatory circuit; and a frequency detector circuit mounted on the vehicle having a receiver coil positioned adjacent the outside of the tire opposite the coil of the passive oscillatory circuit, the frequency detector circuit including a frequency to analogue convertor to produce an analogue signal corresponding to the tire pressure.