In recent years, with the development of various engineering technologies such as aviation and aerospace, more and more occasions require the acquisition of accurate dynamic data, among which the measurement of parameters such as vibration, impact, pressure, and sound intensity occupies an important position. Various sensors , signal adapters, analysis, recording, and processing instruments have been developed to meet this requirement. Among them, (1) sensors are the first step in obtaining accurate and true data; (2) the purpose of using sensors is to convert the physical quantity to be measured into a physical quantity that is easy to measure.
In the development of sensing technology, the manufacturing technology of sensitive elements plays a decisive role. It is precisely because of the successful development of various ferroelectric materials, the successful development of high-quality silicon strain sensitive elements and capacitive sensitive elements, and the success of silicon wafer microfabrication technology that it is possible to manufacture various high-quality new sensors.
Today, in vibration and shock measurement, various types of piezoelectric, integrated circuit piezoelectric, piezoresistive, and variable capacitance accelerometers are widely used.
piezoelectric accelerometer
A piezoelectric accelerometer is a spontaneous sensor whose output charge is proportional to the sensed acceleration. It features high accuracy, wide frequency response, large dynamic range, small size, light weight, long lifespan, easy installation, and good stability. While natural quartz can be used, with appropriate cutting to form the sensing element, it has low sensitivity and high cost. Currently, ferroelectric materials are commonly used. These are artificial ceramics that have undergone artificial polarization to acquire piezoelectric properties. With proper formulation and sintering processes, high piezoelectric sensitivity and operating temperature can be achieved. After aging treatment, long-term temperature stability can be guaranteed. It is easy to fabricate sensing elements of various shapes, and various structural forms of accelerometers have been developed from it.
Single-end compression type features high sensitivity and high resonant frequency, making it suitable for general measurements. Base-isolated compression type minimizes the influence of base coupling, making it more suitable for low-vibration level measurements and also for locations with strain or temperature instability on the mounting surface.
The ring shear type is characterized by its small size and light weight, making it suitable for measuring the vibration of impact or small, lightweight structural components. Because the sensing element is well isolated from the base, the effects of base bending and noise are effectively avoided. Since the sensing element is only subjected to shear force, the pyroelectric effect is reduced. The center-hole mounting ring shear type allows for arbitrary selection of wiring direction. The isolated shear type employs multiple crystals and passive compensation plates, improving sensitivity, widening the temperature range, ensuring stability, and achieving the highest signal-to-noise ratio.
Integrated circuit piezoelectric accelerometer
The fabrication of integrated circuit piezoelectric accelerometers is a result of advancements in microelectronics technology. These sensors incorporate microelectronic signal adaptation circuitry within their housings, achieving low impedance output, a large output signal, and low sensitivity to interference from cables and connectors. They are also insensitive to various environmental factors. A single two-wire cable or two twisted plastic-insulated wires can simultaneously provide power and transmit signals. Using longer cables does not reduce sensitivity or increase noise. The sensors are simple in structure, low in cost, and offer improved performance, making them particularly suitable for various engineering sites and locations requiring long-distance measurements. High-sensitivity sensors can even be directly connected to recording instruments, offering ease of use. Endevco calls this type of sensor ISOTRON; the equivalent circuit diagram is shown below.
Piezoresistive accelerometer
The novel piezoresistive accelerometer is constructed from a single piece of silicon. It uses a silicon piezoresistive strain gauge as its sensing element, which is essentially a fixed silicon resistor whose resistance change is proportional to the mechanical stress it experiences. This can be microfabricated from a very small monolithic silicon wafer, hence the name "monolithic sensor," thus avoiding the inaccurate mechanical connections of older structures that involved bonding silicon strain gauges to a cantilever beam. The piezoresistive accelerometer is configured as a Wheatstone bridge in the circuit, generating an electrical signal proportional to the vibration acceleration. Because it is used in pairs, the output stability is ensured under different temperature conditions.
This type of sensor is characterized by its ability to measure signals down to DC without phase distortion. It features low output impedance, high output level, low internal noise, and low sensitivity to electromagnetic and electrostatic interference, making it easy to adapt to signals. Some piezoresistive accelerometers are sensitive enough to directly drive recorders. They are insensitive to base strain and thermal transients, exhibit no zero-point drift under large impact accelerations, and can be calibrated using the flip-flop method. Therefore, they are widely used in measuring low-frequency vibrations and long-duration impacts, such as vibration and impact measurements during transportation, packaging testing, shock wave research, automotive crash testing, modal analysis, flutter research, and the study of biomedical phenomena.
Variable capacitance accelerometer
Variable capacitance accelerometers are silicon accelerometers made using a different physical principle. Compared to piezoresistive accelerometers, they offer higher sensitivity, better resistance to environmental vibration and shock, are insensitive to temperature, have good stability, and high linearity. The parameters describing the dynamic characteristics of the various accelerometers mentioned above include: sensitivity, amplitude-frequency response, phase-frequency response, mounting resonant frequency, lateral sensitivity, and amplitude nonlinearity.
