Акселерометр — это датчик, который преобразует информацию об ускорении в электрические сигналы. Обычно он состоит из материального блока, демпфирующего механизма, упругого корпуса, чувствительного элемента и отладочных компонентов.
Принцип
При ускорении датчика и измеряемого объекта измеряется сила инерции, действующая на массовый блок, и значение ускорения рассчитывается по формуле a=FMa = \frac{F}{M}a=MF (секунда Ньютона закон).
Types (Based on Sensing Elements)
- Piezoelectric Accelerometer
- Capacitive Accelerometer
- Strain Gauge Accelerometer
- Piezoresistive Accelerometer
- Inductive Accelerometer
- Servo Accelerometer
Piezoelectric Accelerometer
Piezoelectric Accelerometer (Piezoelectric Accelerometer):
Principle: Utilizes the piezoelectric effect of piezoelectric ceramics or quartz crystals. When the accelerometer moves, the force applied to the piezoelectric element by the mass block changes, causing the piezoelectric ceramic or quartz crystal to deform and generate an electrical signal. The electrical signal is proportional to the acceleration, indicating changes in acceleration. Note: The vibration frequency of the measured object should be much lower than the resonant frequency of the accelerometer.
Advantages: High sensitivity, high signal-to-noise ratio, large dynamic range, wide frequency range, simple structure, easy installation, long lifespan.
Disadvantages: High resonant frequency, susceptible to sound interference; high output impedance, weak output signal, requiring amplification circuits for detection.
Piezoelectric Shear-Type IEPE Accelerometer Profile
MEMS Capacitive Accelerometer
Capacitive Accelerometer (Variable Capacitive Accelerometer):
Principle: Based on the capacitance principle, where the distance between electrodes changes. One electrode is fixed, while the other is a flexible diaphragm. Under external forces (e.g., air pressure, hydraulic pressure), the diaphragm moves, causing a change in capacitance. This type of sensor can measure vibration velocity (acceleration) in air or liquid flow and can also measure pressure.
MEMS Variable Capacitive Accelerometer:
Principle: The sensitive element is composed of three monocrystalline silicon wafers bonded together. The upper and lower wafers form two fixed electrodes, while the middle wafer, chemically etched to form a flexible membrane supporting a rigid central mass, acts as the sensitive element. The thickness of the membrane determines the sensor’s range. Small holes are etched in the membrane. As the membrane moves with the mass, air flows through the holes, providing damping force. The change in capacitance generates a current variation that indicates the acceleration.
Advantages: Good low-frequency characteristics, high sensitivity, excellent environmental adaptability, minimal temperature effect. Suitable for measuring both dynamic and steady-state accelerations, low-frequency low-G measurements, and can tolerate high-G shocks.
Disadvantages: Non-linear input-output relationship, high output impedance, poor load capacity, significantly affected by cable capacitance.
Applications: Acceleration and deceleration testing in elevators, flutter testing on aircraft, launch and flight tests of spacecraft, irreplaceable in fields like airbags and mobile devices.
Strain Gauge Accelerometer
Strain Gauge Accelerometer:
Principle: The mass block is fixed at one end of a cantilever beam, with the other end fixed to the sensor base. Both sides of the cantilever beam are attached with strain gauges, forming a Wheatstone bridge. The surrounding of the mass block and cantilever is filled with damping liquid (e.g., silicone oil) to generate the necessary damping force. The motion of the object being measured causes the sensor to move, and the base transmits the motion to the mass block via the cantilever beam. The inertial force deforms the cantilever, causing a change in resistance of the strain gauges. Under constant excitation, the Wheatstone bridge generates a voltage output signal that is proportional to the acceleration, indicating the acceleration value.
Advantages: High precision, wide measurement range, simple structure, good frequency response, easy miniaturization and integration.
Disadvantages: Large non-linearity for high strains, weak output signal requiring compensation; higher measurement accuracy leads to increased fragility.
Piezoresistive Accelerometer
MEMS Piezoresistive Accelerometer:
Principle: Based on the piezoresistive effect of semiconductor materials (monocrystalline silicon), the core components (mass block, cantilever beam, and bracket) are etched from a single crystal silicon wafer, and resistors are diffused at the base of the cantilever beam to form a Wheatstone bridge.
Advantages: Low output impedance, high output signal level, low intrinsic noise, low sensitivity to electromagnetic and electrostatic interference, easy signal conditioning; minimal zero drift under high shock acceleration; wide frequency band.
Disadvantages: Low sensitivity, significant temperature effects.
Applications: Integrated into various analog and digital circuits, widely used in vibration and shock measurement, flutter studies, etc., such as automotive crash tests, test equipment, and vibration monitoring.
Inductive Accelerometer
Inductive Accelerometer Measurement:
Principle: Based on electromagnetic induction, the sensor’s mass block moves within a coil, changing the self-inductance or mutual inductance of the coil, which is then converted into a voltage or current change by the measurement circuit, indicating changes in acceleration.
Advantages: Simple structure, reliable operation, high measurement accuracy, stable zero point, relatively high output power.
Disadvantages: Sensitivity, linearity, and measurement range are interdependent; the sensor’s resolution is related to the measurement range. A large measurement range results in lower resolution, and vice versa; requires high stability of excitation frequency and amplitude; the sensor’s own frequency response is low, making it unsuitable for high-speed dynamic measurements.
Servo Accelerometer
Servo Accelerometer:
Principle: The sensor’s vibration system consists of an “m-k” system, similar to a standard accelerometer, but with an electromagnetic coil attached to the mass block. When acceleration input is applied to the base, the mass block deviates from the equilibrium position. This displacement is detected by a displacement sensor, amplified by a servo amplifier, and converted into a current output. This current flows through the electromagnetic coil in a permanent magnetic field, generating a restoring force that attempts to bring the mass block back to its original equilibrium position, operating in a closed-loop state.
Advantages: A closed-loop testing system with excellent dynamic performance, large dynamic range, and good linearity. Feedback action enhances resistance to interference, improves measurement accuracy, and expands the measurement range. Servo accelerometer technology is widely used in inertial navigation and guidance systems, as well as high-precision vibration measurement and calibration.
Disadvantages: High cost.
Technical Indicators
The primary operating indicators of sensors are divided into Effective Response and Spurious Response.
Effective Response: The sensor’s response in the direction of the sensitive axis due to the mechanical vibration or shock input. This response is desired for reliable data measurement.
Spurious Response: The sensor’s response caused by other physical factors present while measuring mechanical vibrations or shocks. This response interferes with correct measurement and is undesired.
Effective Response Main Technical Indicators: Sensitivity, amplitude-frequency response, and phase-frequency response; non-linearity.
Spurious Response Main Technical Indicators: Temperature response, transient temperature sensitivity, transverse sensitivity, rotational motion sensitivity, base strain sensitivity, magnetic sensitivity, installation torque sensitivity, and response to special environments.
Sensor Selection
The primary focus is on the following indicators:
- Sensor type
- Range
- Sensitivity
- Frequency response bandwidth
- Weight
