The principle of variable capacitance acceleration sensor

By changing the vibration capacitance of the sensor, the parameters of common capacitance or vibration capacitance can be obtained. Capacitive vibration sensors can be divided into variable gap type and variable common area type. The former can be used to measure linear vibration displacement, and the latter can be used to measure torsional vibration angular displacement of .

CapacitiveAccelerometerThe principle of operation is similar to piezoresistive accelerometers in that they measure changes in a bridge circuit. However, instead of resistance, they measure changes in capacitance. The sensing element consists of 2 parallel plate capacitors, operating in differential mode .They rely on a carrier demodulation circuit or its equivalent to produce an electrical output proportional to acceleration. There are several types of capacitive elements. One is composed of a metal sensing diaphragm and an alumina capacitor plate. 2 fixed plates will The diaphragms are sandwiched to form 2 capacitors. Each has a fixed plate and 2 normal diaphragms act as movable plates.

When exposed to acceleration, a capacitive accelerometer experiences a shift in capacitance caused by a change in the distance between the electrodes. This change is due to the movement of the bend, which is also used as an electrode.

When the sensor is placed in the earth's gravitational field or accelerated by vibrations on the test structure, the force on the spring mass is proportional to the mass of the spring mass and is determined by F=ma. Therefore, the spring mass is linear according to the spring equation deflection:

This deflection causes a change in the distance between the electrode and the spring mass. These changes have a direct effect on each relative capacitor gap according to the following factors:

Capacitive accelerometers require a built-in circuit that has 2 functions: allowing capacitance changes to help measure static and dynamic events, and converting the changes into a voltage signal compatible with the readout instrument.

Silicon micromachining can be used to fabricate capacitive, piezoelectric, and piezoresistive accelerometers. The single-crystal nature of silicon, the elimination of mechanical joints, and the chemical treatment of mechanical stops yield sensors with high range capability. Using Gas instead of silicone oil as the damping medium can extend the damping characteristics over a wide temperature range. The central mass of a series of grooves and holes squeezes out the gas, and through the mass displacement structure, the thermal viscosity of the gas changes little relative to that of the silicone oil .Capacitive MEMS accelerometers can measure accelerations from <2 g="" to="" hundreds="" of="" g,="" at="" frequencies="" up="" to="" 1="" khz,="" and="" can="" withstand="" shock="" levels="" of="" 5000="" g="" or="" more.="" most="" of="" them="" use="" electronics="" that="" inject="" signals="" into="" components="" to="" bridge="" and="" condition="" the="" signal.="" the="" negatives="" of="" these="" accelerometers="" are="" limited="" high-frequency="" range,="" relatively="" large="" phase="" shift,="" and="" higher="" background="" noise="" compared="" to="" comparable="" piezoelectric="">

A capacitive acceleration vibration sensor usually includes two capacitors, a modulation circuit, a gain and a filter.

Servo (Force Balance) Acceleration Vibration Sensor

The acceleration-vibration sensors described so far can be classified as "open-loop" devices. The deflection of the seismic mass, which is proportional to the acceleration, is measured directly using piezoelectric, piezoresistive, or variable capacitance techniques. Due to the non-linearity of the deflection, the This mass displacement is associated with some small but finite error. Servo accelerometers are a "closed loop" device. They keep the internal deviation of the proof mass within very small limits. The mass is kept in "balanced" mode, which is almost Errors due to non-linearity are eliminated. The bending system can be linear or drooping. An electromagnetic force proportional to the feedback current keeps the mass at zero. When an object tries to move, a capacitive sensor detects its movement.

Servo accelerometers are expensive open-loop. Their range is typically less than 50 grams and their accuracy is high enough for guidance and navigation systems. For navigation, a 3-axis servo accelerometer is often combined with a 3-axis rate gyroscope in a thermally stabilized mechanical In the isolation component, as an inertial measurement unit (IMU). The IMU can determine the 6 degrees of freedom needed for space navigation. The frequency they measure is DC (0 Hz), which is usually not popular because of its high frequency response. But it is usually used in the low frequency field.

favorable conditions

1) Good temperature stability

The capacitance value of the capacitive sensor is generally independent of the electrode material, which is conducive to the selection of materials with a low temperature coefficient. Because of its small calorific value, it has little effect on stability. Resistive sensors have copper loss, and are prone to zero point drift due to heating.

2) Simple structure

The capacitive sensor has a simple structure, is easy to manufacture, and has high precision. It can be very small to achieve some special measurements; it can work in harsh environments such as high temperature, strong radiation, and strong magnetic field, and can withstand large temperature changes, high pressure, high pressure, etc. Shock, overload, etc.; it can measure ultra-high temperature and low pressure difference, and can also measure magnetic work.

3) Non-contact measurement and high sensitivity

It can measure the vibration or eccentricity of the rotating shaft and the radial clearance of the small ball bearing without contact. When using non-contact measurement, the capacitive sensor has an average effect, which can reduce the influence of the surface roughness of the workpiece on the measurement.

In addition to the above advantages, the capacitive sensor can also measure very low pressure, force and small acceleration and displacement, because it has a small electrostatic attraction between the charged electrode plates, and the input force and energy are also small. It can Very sensitive, high resolution, and can sense 0.01μM or less. Due to the small loss of medium such as air, the zero residue generated when the differential structure is connected to the bridge is very small, thus allowing the circuit to be amplified at a high magnification, so the instrument has High sensitivity.