One example of an angular velocity sensor driving circuit is disclosed in pages 30-31 of the Journal of Nippondenso Engineering Society (Vol.38, No. 3, 1994).
The angular velocity sensor driving circuit of the prior art includes a driving part for giving vibration to a tuning fork vibrator, vibration level detecting means for detecting a vibration level of the vibrator, coriolis force detecting means for detecting a coriolis force generated according to an angular velocity, a first amplifier for amplifying an output signal of the vibration level detecting means, a rectifying circuit for rectifying an output signal of the first amplifier to get a DC voltage, a comparator for comparing the output voltage of the rectifying circuit and a reference voltage and a variable gain amplifier, which amplifies a 90 degrees phase shifted output voltage of the first amplifier, and returns the output signal to the driving part to control the amplitude of the tuning fork vibrator to be constant.
At the prior art mentioned above, just after the power source is turned on, the gain of the variable gain amplifier becomes maximum among some gains satisfying necessary condition. And it works to suddenly increase the amplitude of the tuning-fork type vibrator and it has a little effect to shorten the starting time of the sensor.
However, because the variable range of the gain of the variable gain amplifier is limited and can not make large enough, a certain long time has been necessary until the amplitude level of the tuning-fork type vibrator reaches to a predetermined value.
If the maximum value of the gain of the variable gain amplifier is set larger than the predetermined value, the starting time of the sensor can be shortened. But it is difficult to make the maximum value of the gain of the variable gain amplifier larger than the predetermined value from total view points such as saturation problem of the voltage waveform of the variable gain amplifier, keeping the noise reduced, stability when the amplitude of the tuning-fork type vibrator is controlled to be constant, etc.
Therefore, it is difficult to shorten the time (start-up time) to make the vibration amplitude of the tuning fork type vibrator constant without making the maximum value of the gain of the variable gain amplifier larger than a predetermined value.
In the case such as a quartz tuning fork type vibrator having a large value of sharpness Q (vibration energy per input energy), it is difficult to shorten the time constant t=Q/2 f (f: driving frequency) without outstanding increase of frequency f. Accordingly, it becomes big obstacle not to be able to make the maximum value of the gain of the variable gain amplifier larger than a predetermined value to reduce the start-up time.
The present invention solves the above-mentioned problem in the prior art and aims to provide an angular velocity sensor driving circuit which make it possible to shorten the start-up time of the angular velocity sensor.