1. Field of the Invention
The present invention relates to a electrostatically-driven/capacitance-detection type gyro sensor
2. Description of Related Art
As one type of the gyro sensor used for sensing angular velocity, there is known a electrostatically-driven/capacitance-detection type gyro sensor. As shown in FIG. 17, the gyro sensor of this type is constituted by a sensing element 101 including a movable part displaceable in a certain plane (referred to as “motion plane” hereinafter), and a detector unit 102 driving the movable part for detecting the angular velocity of rotative motion imparted to the sensing element around an axis orthogonal to the motion plane (refer to Published Japanese Translation No. 2001-515201 of a PCT Application, for example).
The sensing element 101 includes a drive electrode generating an electrostatic force with the movable part to vibrate the movable part along a predetermined direction (refereed to as “driving direction” hereinafter) in the motion plane, a monitor electrode forming a monitoring variable capacitor with the movable part whose capacitance varies depending on the displacement of the movable part along the driving direction, a sense electrode forming a sensing variable capacitor with the movable part whose capacitance varies depending on the displacement of the movable part in the motion plane along a direction orthogonal to the driving direction, and a movable electrode through which the movable part is applied with a bias voltage.
By properly controlling the voltage applied between the movable electrode and the drive electrode, it is possible that the movable part continues to vibrate along the driving direction. If the sensing element 101 is imparted with rotative motion around an axis orthogonal to the motion plane when the movable part is vibrating along the driving direction, the movable part develops vibration in the direction orthogonal to the driving direction by the action of the Coriolis force which depends on the angular velocity of the sensing element 101.
The detector unit 102 includes a reference voltage generating section 108 generating a reference voltage Vref, a high voltage generating section 106 amplifying the reference voltage Vref to generate the bias voltage VK (=kRK·Vref, kRK being the gain of the high voltage generating section 106) to be applied to the movable electrode, a drive buffer 105 generating a sinusoidal drive signal VD(t) having an offset voltage proportional to the reference voltage Vref, a CV (Capacitance to Voltage) converting section 103 generating a monitor signal VM(t) whose voltage level varies depending on the variation of the capacitance of the monitoring variable capacitor (that is, the vibration state of the movable part vibrating under the action of the drive signal), and generating a sense signal VS(t) whose voltage level varies depending on the variation of the capacitance of the sensing variable capacitor (that is, the vibration state of the movable part vibrating under the action of the Coriolis force), a drive signal controlling section 104 controlling the amplitude and phase of the drive signal VD(t) generated by the drive buffer 105 in accordance with the monitor signal generated by the CV converting section 103 such that the vibration of the movable part along the driving direction continues, and a sensor output signal generating section 107 outputting a sensor output signal VYAW that has an offset voltage proportional to the reference voltage Vref and has a voltage level varying depending on the Coriolis force applied to the movable part (that is, the angular velocity of the movable part).
In the gyro sensor having the configuration described above, the drive force driving the movable part depends on the voltage difference between the movable electrode and the drive electrode, and the conversion gain of the CV converting section 103 generating the monitor signal VM(t) and sense signal VS(t) depends on the voltage differences between the movable electrode and the monitor electrode or sense electrode. On the other hand, the gain for converting the rotative motion imparted to the sensing element 101 into the Coriolis force (referred to as “element sensitivity” hereinafter) depends on the vibrating state of the movable part vibrating under the action of the drive force.
Accordingly, if the reference voltage Vref varies depending on the variation of the power supply voltage VCC, the gain for converting the angular velocity of the sensing element 101 into the sensor output signal VYAW (referred to as “sensor sensitivity” hereinafter) changes, since the element sensitivity and the circuit characteristics of the detector unit 102 including the CV converting section 103 change depending on the variation of the power supply voltage VCC.
Accordingly, in order that the sensor output signal VYAW is not affected by the variation of the power supply voltage VCC, the reference voltage generating section 108 is configured to generate the reference voltage Vref by use of a constant voltage not affected by the variation of the power supply voltage VCC, such as a band-gap voltage.
In a microcomputer-based system using such a gyro sensor, it is common that the sensor output signal VYAW outputted from the gyro sensor is converted to a digital signal by use of an A/D converter, and then supplied to a microcomputer.
Generally, in such a system, a reference voltage which the A/D converter uses for performing the A/D conversion is generated by dividing down the power supply voltage VCC. Since the sensor output signal VYAW varies between both polarities with respect to a predetermined offset voltage, the microcomputer processes the digital signal outputted from the A/D converter by using the center value of the output range of the A/D converter as a zero point. Generally, in order to maximize the dynamic range of the sensor output signal VYAW, the offset voltage corresponding to the zero point is set at half the power supply voltage VCC.
Accordingly, when the power supply voltage VCC is 5V, the offset voltage thereof is set at 2.5V. As explained above, although the zero point of the sensor output signal VYAW outputted from the gyro sensor is set in accordance with the reference voltage Vref, and is therefore maintained unchanged even if the power supply voltage VCC varies, the zero point recognized by the microcomputer varies in proportion to the power supply voltage VCC. Accordingly, if the power supply voltage VCC deviates from its rated value for some reason, there arises a difference between the zero point recognized by the microcomputer and the zero point of the sensor output signal VYAW outputted from the gyro sensor. This lowers the accuracy of angular velocity detection.
Furthermore, such a system has another problem in that, if the power supply voltage VCC deviates from its rated value, although there occurs no change in the sensor output signal VYAW, there occurs a change in the dynamic range of the A/D converter. This causes a change in the incremental value of the sensor output signal VYAW per bit of the digital signal which the A/D converter generates. This changes the conversion ratio of the A/D converter, which also lowers the accuracy of angular velocity detection.
It may occur that the accuracy of the angular velocity detection can be avoided from being lowered if the microcomputer operates to detect the deviation of the power supply voltage VCC or the deviation of the reference voltage Vref due to the deviation of the power supply voltage VCC, and to compensate for the deviation of the zero point or the sensitivity in accordance with the result of the detection. However, this considerably increases the burden of the microcomputer.