Patent ID: 7355321

Claim:
A mechanical quantity sensor comprising: two piezoelectric vibrators arranged to receive stresses caused by a mechanical quantity in opposite directions; a voltage signal applying circuit arranged to apply a voltage signal to the two piezoelectric vibrators; a current-to-voltage converter circuit arranged to convert electric current signals flowing through the piezoelectric vibrators into voltage signals; a phase difference signal processing circuit arranged to detect a phase difference between the voltage signals output from the current-to-voltage converter circuit and output a mechanical quantity detection signal; and resistors arranged in electric current paths of the two piezoelectric vibrators; wherein the voltage signal applying circuit includes: a voltage amplifier and amplitude limiter circuit arranged to amplify the voltage of an added signal corresponding to an added value of currents flowing through the two piezoelectric vibrators and to limit the amplitude of the voltage signal output from the voltage amplifier circuit to a predetermined value; a phase control circuit arranged to detect the phase difference between a feedback voltage signal applied to both of the two piezoelectric vibrators and the added signal and control the phase of the feedback voltage signal so that the phase difference equals a predetermined value; and a filter circuit arranged to minimize unwanted frequency components of the feedback voltage signal; and wherein the mechanical quantity sensor is oscillated by the piezoelectric vibrators, the voltage amplifier circuit, the amplitude limiter circuit, the phase control circuit, and the filter circuit.