System for detecting abnormality in internal pressure of tire

A system for detecting abnormality in the internal pressure of tires having a first device disposed at each of a plurality of wheels including tires and a second device disposed at a non-rotating part, said system comprising:

• a. each of said first devices, which includes:
• a-1. a pressure responsive element which is provided with a crystal resonator, said element being arranged to operate to cause a stress applied to said crystal resonator to be varied by variations of the internal pressure of said tire and thus to vary the natural frequency of the crystal resonator accordingly, and
• a-2. a first antenna connected to said pressure responsive element;
• and
• b. said second device, which includes:
• b-1. a transmitter arranged to excite said crystal resonator by sending out one after another therefrom a plurality of different exciting energies having different frequency values within a range over which the natural frequency of said crystal resonator varies when the internal pressure of the tire is within a normal range thereof,
• b-2. a receiver arranged to receive an oscillatory energy which is discharged from the crystal resonator immediately after said excitation of the crystal resonator by the exciting energy from said transmitter comes to a stop with the oscillatory energy accumulated by the exciting energy at the crystal resonator and discharged therefrom immediately after the ceasing of said exciting action on the crystal resonator,
• b-3. second antennas provided for said plurality of wheels, each of said second antennas being connected to said transmitter and receiver, each second antenna being arranged to send said exciting energy to said first antenna and to receive said discharged oscillatory energy from said first antenna of a corresponding first device disposed at one of said plurality of wheels,
• b-4. A switch-over circuit arranged to stop the exciting energy sending out action of said transmitter after the exciting energy is sent out from the transmitter and to allow said receiver to begin the receiving action thereof,
• b-5. an antenna switch-over circuit arranged to switch over the connection of at least the transmitter with the second antennas from one of said second antennas to another at the end of every cycle of the switching action of said switch-over circuit, and
• b-6. an alarm circuit connected to said receiver, said alarm circuit being arranged to announce abnormality of each individual wheel when the abnormality thereof is detected by the receiver, the abnormality being determined on the basis of that the oscillation frequency of the discharged oscillatory energy comes outside of a preset range of oscillation frequency.