Acceleration is a physical quantity that describes how quickly an object's velocity changes. By measuring the acceleration caused by gravity, you can calculate the tilt angle of the equipment relative to the horizontal plane. By analyzing dynamic acceleration, you can analyze how the equipment moves.
Accelerometers were developed to measure and calculate these physical quantities.
Accelerometer
An accelerometer is an electronic device that measures acceleration force and converts acceleration into an electrical signal. Acceleration force is the force acting on an object during acceleration, much like Earth's gravity. Acceleration force can be a constant, such as g, or a variable. There are two types of accelerometers: angular accelerometers, which are improvements on gyroscopes (angular velocity sensors), and linear accelerometers.
Accelerometers can be applied in industrial control and instrumentation; vibration and shaking of handles, toys, and mice; automotive braking and starting detection and alarm systems; structural and environmental monitoring; engineering vibration measurement, geological exploration, and earthquake detection; vibration testing and analysis of railways, bridges, and dams; and dynamic characteristics and security vibration detection of high-rise building structures.
Classification and Principles of Accelerometers
According to Newton's second law: A (acceleration) = F (force) / M (mass)
The acceleration of an object of known mass can be obtained simply by measuring the force F. By balancing this force using electromagnetic force, the relationship between the force and the current (voltage) can be obtained. An accelerometer can be designed based on this simple principle.
Therefore, the essence of an accelerometer is to cause deformation of the sensitive components inside the sensor by applying force, and to obtain the corresponding acceleration signal by measuring the deformation and converting it into a voltage output using relevant circuits.
Accelerometers are classified into four types according to their working principle:
1. Piezoelectric accelerometer
Piezoelectric accelerometers operate based on the piezoelectric effect of piezoelectric crystals.
When certain crystals are deformed by force in a certain direction, polarization occurs inside them, and opposite charges are generated on their two surfaces. When the external force is removed, they return to an uncharged state. This phenomenon is called the "piezoelectric effect".
Crystals exhibiting the "piezoelectric effect" are called piezoelectric crystals. Commonly used piezoelectric crystals include quartz and piezoelectric ceramics.
When the accelerometer is subjected to vibration, the force exerted by the mass block on the piezoelectric element also changes accordingly. When the frequency of the measured vibration is much lower than the natural frequency of the accelerometer, the change in force is proportional to the measured acceleration.
Structure of a piezoelectric accelerometer
S is the spring, M is the mass block, B is the base, P is the piezoelectric element, and R is the clamping ring.
Figure a shows a centrally mounted compression type, where the piezoelectric element-mass-spring system is mounted on a circular central support, which is connected to the base. This structure has a high resonant frequency. However, when the base B is connected to the test object, deformation of the base B will directly affect the output of the pickup. In addition, changes in the temperature of the test object and the environment will affect the piezoelectric element and cause changes in the preload, which can easily lead to temperature drift.
Figure b shows a ring-shaped shear type, where the piezoelectric element is clamped to the central pillar of a triangle by a clamping ring. When the accelerometer senses axial vibration, the piezoelectric element is subjected to shear stress. This structure provides excellent isolation from base deformation and temperature changes, and exhibits a high resonant frequency and good linearity.
Figure c shows a triangular shear shape, a simple structure that can be used to create a very small, high-resonance-frequency accelerometer. The annular mass block is attached to an annular piezoelectric element mounted on a central support. Because the adhesive softens with increasing temperature, the maximum operating temperature is limited.
Piezoelectric accelerometers are characterized by their large dynamic range, wide frequency range, robustness, low susceptibility to external interference, and the fact that piezoelectric materials generate their own charge signals under stress without requiring any external power source. They are the most widely used vibration measurement sensors.
Although piezoelectric accelerometers have a simple structure and a long history of commercial use, their performance is closely related to material properties, design, and manufacturing processes. Therefore, the actual performance parameters, stability, and consistency of similar sensors sold on the market vary greatly. Compared to piezoresistive and capacitive accelerometers, their biggest drawback is that piezoelectric accelerometers cannot measure zero-frequency signals.
2. Piezoresistive accelerometer
Piezoresistive accelerometers were among the earliest developed silicon micro-accelerometers (based on MEMS silicon micromachining technology). The elastic element of a piezoresistive accelerometer typically consists of a silicon beam with an external mass, supported by a cantilever beam. Resistors are fabricated on the cantilever beam and connected to form a measuring bridge. Under inertial force, the mass moves up and down, causing the resistance on the cantilever beam to change with the stress, resulting in a change in the output voltage of the measuring bridge, thus enabling the measurement of acceleration.
Schematic diagram of piezoresistive accelerometer
There are many typical structural forms for piezoresistive silicon micro-accelerometers, including cantilever beam, double-beam, four-beam, and double-island-five-beam structures. The structure and dimensions of the elastic element determine the sensor's sensitivity, frequency response, and range. A mass block can increase the stress on the cantilever beam under relatively small accelerations, thereby improving the sensor's output sensitivity.
Under high acceleration, the effect of the mass block may cause the stress on the cantilever beam to exceed the yield stress, resulting in excessive deformation and ultimately, the cantilever beam fracture. Therefore, for high gn-value accelerations, a single-arm beam and a double-arm beam structure with equal mass block and beam thickness are proposed, as shown in the figure.
Double-arm beam structure
Based on world-leading MEMS silicon micromachining technology, piezoresistive accelerometers are characterized by small size and low power consumption, and are easy to integrate into various analog and digital circuits. They are widely used in fields such as automotive crash tests, testing instruments, and equipment vibration monitoring.
The sensitive core of the strain gauge piezoresistive accelerometer is a resistance measuring bridge made of semiconductor material, and its structural dynamic model is still a spring-mass system.
The development of modern microfabrication technology has enabled the design of piezoresistive sensing elements to be highly flexible in order to suit a wide variety of measurement requirements. In terms of sensitivity and range, piezoresistive accelerometers are available for everything from low-sensitivity, high-range impact measurements to high-sensitivity, low-frequency DC measurements.
Meanwhile, piezoresistive accelerometers can measure frequencies ranging from DC signals to high-frequency measurements with high stiffness and frequencies up to tens of kilohertz. Miniaturization is also a highlight of piezoresistive sensors. It should be noted that although the design and application of piezoresistive sensing elements offer great flexibility, the application range of a particular piezoresistive element design is generally smaller than that of a piezoelectric sensor.
Another drawback of piezoresistive accelerometers is their significant susceptibility to temperature fluctuations; practical sensors generally require temperature compensation. In terms of price, piezoresistive sensors used in large quantities are highly competitive in terms of cost, but the manufacturing cost of sensitive cores for specialized applications will be far higher than that of piezoelectric accelerometers.
Piezoresistive accelerometer
3. Capacitive accelerometer
A capacitive accelerometer is a capacitance sensor based on the principle of capacitance and characterized by varying electrode spacing. One electrode is fixed, while the other, a variable electrode, is an elastic diaphragm. The diaphragm displaces under external force (air pressure, hydraulic pressure, etc.), causing a change in capacitance. This type of sensor can measure the vibration velocity (or acceleration) of airflow (or liquid flow) and can further measure pressure.
Schematic diagram of capacitive accelerometer
Capacitive accelerometers have advantages such as simple circuit structure, wide frequency range of about 0 to 450 Hz, linearity of less than 1%, high sensitivity, stable output, small temperature drift, small measurement error, steady-state response, low output impedance, and simple and convenient formula for the relationship between output charge and vibration acceleration, making them highly valuable for practical applications.
However, its shortcomings include nonlinear input and output signals, limited measurement range, susceptibility to cable capacitance, and the fact that the capacitive sensor itself is a high-impedance signal source. Therefore, the output signal of a capacitive sensor often needs to be improved through subsequent circuitry. In practical applications, capacitive accelerometers are mostly used for low-frequency measurements. Their versatility is not as good as that of piezoelectric accelerometers, and their cost is also much higher.
Capacitive accelerometer
Capacitive accelerometers are a common type of acceleration sensor, irreplaceable in certain fields such as airbags and mobile devices. They utilize microelectromechanical systems (MEMS) technology, making mass production economical and ensuring lower costs.
4. Servo-type accelerometer
When the vibrating object being measured passes through the accelerometer housing and receives acceleration input, the mass block deviates from its static equilibrium position. The displacement sensor detects the displacement signal, which is amplified by the servo amplifier and outputs a current. This current flows through the electromagnetic coil, thereby generating an electromagnetic restoring force in the magnetic field of the permanent magnet, forcing the mass block back to its original static equilibrium position. In other words, the accelerometer is working in a closed-loop state, and the sensor output is an analog signal that is proportional to the accelerometer value.
Servo Accelerometer Schematic Diagram
A servo accelerometer is a closed-loop testing system characterized by good dynamic performance, a large dynamic range, and good linearity. Its working principle involves a vibration system consisting of an "m-k" system, similar to a general accelerometer, but with an electromagnetic coil connected to the mass m. When there is an acceleration input to the base, the mass deviates from its 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, generating an electromagnetic restoring force in the magnetic field of a permanent magnet, attempting to keep the mass in its original equilibrium position within the instrument housing. Therefore, the servo accelerometer operates in a closed-loop state.
Because of its feedback function, servo acceleration measurement technology enhances anti-interference capabilities, improves measurement accuracy, and expands the measurement range. It is widely used in inertial navigation and inertial guidance systems, and is also applied in high-precision vibration measurement and calibration.
5. Triaxial accelerometer
It also works based on the fundamental principle of acceleration. Acceleration is a spatial vector. On the one hand, to accurately understand the motion state of an object, it is necessary to measure its components on the three coordinate axes. On the other hand, when the direction of motion of the object is unknown in advance, only a triaxial accelerometer can be used to detect the acceleration signal.
Since triaxial accelerometers are also based on the principle of gravity, they can achieve tilt angles of ±90 degrees or 0-360 degrees on both axes. After calibration, their accuracy is higher than that of dual-axis accelerometers when measuring angles greater than 60 degrees.
Most current triaxial accelerometers/accelerometers use piezoresistive, piezoelectric, and capacitive working principles. The acceleration generated is proportional to the changes in resistance, voltage, and capacitance, and is collected through corresponding amplification and filtering circuits.
Triaxial accelerometers are characterized by their small size and light weight (gm). They can measure spatial acceleration and comprehensively and accurately reflect the motion properties of objects, and are widely used in aerospace, robotics, automotive and medical fields.
Comparison of several accelerometer sensors
