How Does A MEMS Accelerometer Work?

Working principle
The MEMS accelerometer is composed of three parts: an upper capacitor, a middle capacitor board (movable) and a lower capacitor board.When acceleration, the middle capacitor plate will move, and the distance from the upper and lower capacitor plates will change, and the capacitance value of the upper and lower capacitors will change. The change of capacitance value is proportional to the acceleration, and after the output voltage is digitally processed, the digital signal is output.

There are three types of mature MEMS accelerometers: piezoelectric, capacitive, and thermal. Piezoelectric MEMS accelerometers use the piezoelectric effect. Inside, there is a mass supported by a rigid body. When there is movement, the mass will generate pressure, and the rigid body will generate strain, which will convert acceleration into electrical signal output.

There is also a mass inside the capacitive MEMS accelerometer. The piezoelectric accelerometer consists of a mass block, a piezoelectric material (usually piezoelectric ceramics or quartz crystals), and a shell: according to the piezoelectric effect, that is, certain types of crystals produce a voltage when subjected to pressure. The acceleration of the measured object is transmitted to the mass block and the corresponding force is generated on the piezoelectric crystal. This force causes the crystal to produce a voltage that is proportional to the applied force and therefore to the acceleration. A practical piezoelectric accelerometer requires a built-in amplifier circuit to amplify the generated voltage signal and minimize sensitivity to external noise and crosstalk. Finally, the acceleration value can be measured by the voltage signal at the output end. This kind of piezoelectric accelerometer with integrated amplifier circuit is called "ICP accelerometer". From a single unit point of view, it is a standard flat capacitor. The change in acceleration drives the moving mass to change the distance between the two poles of the plate capacitor and the area directly facing it. The acceleration is calculated by measuring the change in capacitance.

There is no mass inside the thermal MEMS accelerometer. There is a heating body in the center, a temperature sensor in the periphery, and a closed air cavity. When working, the gas forms a hot air mass in the interior. The specific gravity of the mass is different from the surrounding cold air. The change in the thermal field formed by the movement of the inertial hot air mass makes the sensor sense the acceleration value.

Due to the rigid body support inside the piezoelectric MEMS accelerometer, usually, the piezoelectric MEMS accelerometer can only sense "dynamic" acceleration, but not "static" acceleration, which is what we call gravity acceleration. . Capacitive and thermal sensing can sense both "dynamic" acceleration and "static" acceleration.

Application

Communication/mobile devices

A large number of sensors are used in mobile devices based on smart phones to increase their intelligence and improve user experience.

Wearable/implantable areas

Wearable/implantable MEMS is an important part of the Internet of Things (IoT), and its main function is to provide information directly to users in a more portable, fast and friendly way. Wearable/should be said to be the most concerned by users, the most interesting topic. Most users have no sense of MEMS in cars and printers, and there are several layers of intermediary between these devices and users. But wearable/direct contact with users, enhance consumer sense of technology, more popular with young users.

Products in the consumer sector include fitness bands mentioned earlier, as well as smart watches. The health field mainly includes diagnosis, treatment, monitoring and care. Such as hearing aid, indicator detection (such as blood pressure, blood sugar levels), posture monitoring. MEMS can realize almost all sensory functions of the human body, including vision, hearing, taste, smell, touch, etc. Various health indicators can be monitored by combining MEMS with biochemistry. The sampling accuracy, speed and applicability of MEMS can reach a high level, and because of its size advantage, it can be directly implanted into the human body, which is a key component of medical auxiliary equipment.

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