Some differences between capacitive and thermal MEMS sensors

Some differences between capacitive and thermal MEMS sensors
2 min read

Design engineers often need to choose among different kinds of accelerometers when developing heavy equipment. For example, in heavy machinery such as cranes, tractors, woodcutters, and construction projects, designers need to measure the pitch and roll angles of the equipment by using accelerometers.

In most applications, they usually choose based on the main characteristics of the sensor, such as structure, resonant frequency, reliability, stability, bandwidth, power consumption, and cost.

For capacitive sensors and thermal MEMS sensors, the biggest difference between the two technologies is their different sensing techniques.

Typical dual-axis thermal MEMS accelerometers are based on single-chip integration technology. The chips for sensor and control circuits are integrated into a hermetic package. The sensor consists of a cavity created by silicon etching and a set of heaters and temperature measuring cells placed in the cavity.

Unlike capacitive devices, thermal sensors measure acceleration by monitoring the movement of the heated mass within the package cavity. In the absence of acceleration, the hot air mass will be symmetrically distributed over the heater. Under the action of acceleration, the hot air mass will move in the direction of the acceleration. Since the device does not contain structures that can be bent or displaced, very high device reliability can be provided.

Some disadvantages of capacitive MEMS accelerometers

Capacitive MEMS accelerometers use a displaceable cantilever structure. For low-acceleration devices used for tilt measurement, the inherent bandwidth of the cantilever structure is usually greater than 5 kHz, and the resonant frequency is around 2 kHz. When the vibration energy is too large or the vibration frequency is close to the resonance frequency of its cantilever structure, the output signal of the capacitive acceleration sensor may be distorted or resonated.

In most cases, the distorted or resonant signal will cause a huge zero drift (especially the Z-axis), making the sensor unable to correctly restore the real signal in a high-intensity vibration environment. Zero-point drift in high-vibration environments is an inherent disadvantage of capacitive accelerometers, often requiring additional techniques to isolate or mitigate the effects of vibration.

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