An Overview of MEMS Variable Capacitance Accelerometers

09-08-2019

MEMS Accelerometers are manufactured using microelectronics fabrication techniques. MEMS-based accelerometers measure physical parameters such as vibration or acceleration through the combination of microscopic mechanical sensing structure, typically silicone, coupled with microelectronics circuit. Unlike IEPE accelerometer, MEMS accelerometers are generally used for measuring low frequency vibration down to 0 HZ (DC acceleration) , motion and constant acceleration. Therefore, MEMS accelerometers can also be termed as DC accelerometers.

There are two types of DC Accelerometers that are available in the market: Variable Capacitance Accelerometers and Piezoresistive Accelerometers. These two accelerometers operate based on MEMS technology and differ in terms of sensitivity, measuring ranges and types of applications. In this article, we will discuss Variable capacitance Accelerometers in more details.

What are Variable Capacitance Accelerometer?

Variable capacitance accelerometer provides DC response. Therefore, they are ideal for measuring low frequency vibration down to 0 Hz, motion and steady state acceleration. The Variable Capacitance Accelerometers are known for their high sensitivities, narrow bandwidths and an outstanding temperature stability. They offer accurate and precise measurement results and are ideal in measurement highly sensitive environments such as in seismic or underground applications. Due to their DC responding feature, variable capacitance accelerometers are greatly useful in measuring centrifugal forces or measuring the accelerations or decelerations experienced by devices such as elevators.

How Variable Capacitance Accelerometer work?

The sensor element operates based on the principle of Differential Capacitance, which is a voltage-dependent capacitance. As soon as the sensor experiences acceleration or mechanical force, the silicone structure displaces which results in a change in capacitance. This change in capacitance is detected by the CMOS circuit and it is then transformed into an output voltage proportional to acceleration. To attain the maximum benefit from the DC response generated by this accelerometer, the readout device must also be in a DC coupled state. A VC MEMS accelerometer Sensor consists of a voltage regulator inbuilt in it, to ensure that the sensor is powered by any unregulated DC voltage source.

Characteristics of Variable Capacitance Accelerometer

The Variable Capacitance Accelerometers have some unique characteristics which makes them capable of providing a highly stable and accurate measurement.

  • High sensitivity reading which allows for the measurement of low frequency vibration with a high output level.
  • They can operate over a broad temperature range and can provide a high degree of stability.
  • DC-responding. Let’s consider an Accelerometer placed in position at which the sensitive axis is parallel to the earth’s gravity. In such conditions without external any vibration interference, an output equal to 1g will be produced by the accelerometer. This phenomenon is known as DC responding as it measures acceleration when the object is not moving. This feature makes the accelerometer ideal to be used for measuring centrifugal forces or steady state acceleration such as in movement of an elevator.
  • The thermal shift and sensitivity shifts can be as low as 1.5% in a variable capacitance accelerometer over a temperature range of 180°C.
  • These accelerometers have a frequency response of 0 Hz to 1 kHz, depending on the sensitivity required.

Applications of MEMS DC Accelerometer

Due to their ability to operate under constant acceleration and broad temperature range, there are many industries and applications where variable MEMS Accelerometers are used. Let’s look at these different applications:

  • In the areas of vibration testing, MEMS variable capacitance accelerometers are widely used in aircraft flutter testing.
    Automotive testing and transport applications, such as testing of suspension systems as well as automotive noise, harness and vibration (automotive NVH) testing.
  • Because of their low-frequency trait, they are used for ride quality measurements in automobiles, trucks, and railroad equipment.
  • The DC response given by these accelerometers makes it possible to measure acceleration level experienced by person in a car during a high-speed cornering maneuvre.
  • Cases where the measurement of a whole body’s motion, after subject to any shock or vibrations, is required, a VC Accelerometer comes into play. Such cases are seen in low level accelerations motorsports and high-speed rail applications.
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