US 4,110,732 A · Tire pressure monitor

Abstract

Low pressure in a vehicle tire is detected by an electronic system which compares the difference in phase between two cyclic signals and produces an output signal having a negative going pulse and a positive going pulse for each revolution of the tire. Each negative going pulse represents a decrease in phase between the two signals and is generated in response to a shorted coil passing through an electromagnetic field produced by an induction coil in a parallel resonant circuit which produces the second of the two signals. The first signal is produced by an oscillator and is used to excite the resonant circuit and as a reference signal. The coil is shorted by a closed switch in the tire when the pressure is normal. Abnormal tire pressure opens the switch and renders the coil ineffective. Each positive going pulse represents an increase in phase between the two signals and is generated in response to a ferrous metal plate passing through the field. A logic circuit includes a capacitor charged at a fixed rate and operative to activate a warning light when the capacitor charge exceeds a predetermined level. When the tire speed is above 5 mph, the negative going pulses cause the capacitor to discharge faster than it is charged. When the vehicle speed is below 5 mph, a low speed inhibit circuit responsive to the positive going pulses discharges the capacitor faster than it is charged.

Claims8

Dependent claims

An electronic system for detecting rotation and a change in condition of a pneumatic tire mounted for rotation about an axis, said system comprising:

means for producing a first cyclic signal;
a resonant circuit including reactive means and having an effective impedance, said resonant circuit excited by said first cyclic signal and operative to produce a second cyclic signal shiftable in phase relative to said first signal in response to changes in the effective impedance of said resonant circuit;
first and second means disposed to pass in close proximity of said reactive means in response to rotation of said tire, said first and second means respectively operative when passing said reactive means to momentarily change the effective impedance of said resonant circuit in opposite directions for effecting a momentary phase shift of said second signal in one direction and then in the other direction;
means for rendering said first means ineffective to change said effective impedance, and thereby terminate the phase shifts in said one direction, in response to said change in condition of said tire;
and
control means operative to activate a warning device indicating said change in condition in response to termination of said phase shifts in said one direction and continuation of said phase shifts in said other direction.

3 more elements

An electronic system for detecting rotation and a condition change of a pneumatic tire mounted for rotation about an axis, said system comprising:

means for producing a first cyclic signal;
a parallel resonant circuit excited by said first cyclic signal and including an inductor producing an electromagnetic field, said resonant circuit excited by said first cyclic signal and operative to produce a second cyclic signal shiftable in phase relative to said first signal in response to changes in the effective impedance of said resonant circuit;
a coil disposed to pass through said field in response to rotation of said tire, said coil operative to change said effective impedance and shift the phase of said second signal in one direction when said condition of said tire is normal;
means for rendering said coil ineffective to change said effective impedance and shift said phase in said one direction in response to said condition changing to abnormal;
metallic means disposed to pass through said field in response to said rotation and operative to change said effect impedance and shift said phase in said other direction;
and
control means operative to activate a warning device indicating said abnormal condition in response to termination of said phase shifts in said one direction and continuation of said phase shifts in said other directions.

4 more elements

Dependencies are inferred from the claim text.

Tire pressure monitor

Life of this patent

Tire pressure monitor

Life of this patent

Abstract

Claims8