Analysis of various types and working principles of acceleration sensors
Published time: 2020-02-19 12:34:56
For most engineering applications, choosing the right test tool will have a big impact on test results. This article will help readers choose the correct acceleration sensor. Let's start with the classification and principle of sensors.
Basic acceleration sensor types
In general, there are two types of acceleration sensors: AC-responsive acceleration sensors and DC-responsive acceleration sensors.
As an AC-responsive acceleration sensor, as its name suggests, its output is AC-coupled. Such sensors cannot be used to test static accelerations, such as gravity and centrifugal acceleration. They are only suitable for measuring dynamic events. The DC-responsive acceleration sensor has a DC-coupled output and can respond to acceleration signals as low as 0 Hz. Therefore, a DC-responsive acceleration sensor is suitable for testing both static and dynamic accelerations. It is not necessary to choose a DC-responsive acceleration sensor only when it is necessary to test static acceleration.
Acceleration, speed, displacement
Many vibration studies require information on acceleration, velocity, and displacement. These are important information that engineers need to design and verify structures. Generally speaking, acceleration provides a good reference, while speed and displacement are the variables needed for calculation. In order to calculate the speed and displacement from the acceleration, the acceleration signal output from the sensor is digitally or analogically integrated once and twice. This can cause problems with AC-coupled sensors. To demonstrate this problem, consider using an AC sensor to measure a wide-pulse half-sine wave signal. Due to the limitation of the inherent AC RC time constant, the output of the sensor does not match the input pulse well. For the same reason, at the end of the pulse, the sensor output will produce a negative zero offset.
This seemingly small difference in magnitude will produce significant errors after integration. DC-responsive acceleration sensors do not have this problem, because their output can accurately follow slowly changing inputs. In practical daily applications, the input signal may not be a simple half-sine pulse, but such problems always exist when testing any slowly changing signal with an AC-coupled sensor.
Now we look at various commonly used acceleration sensor technologies.
AC response acceleration sensor
The most commonly used AC response acceleration sensor uses a piezoelectric element as its sensitive unit. When there is acceleration input, the detection mass in the sensor "moves" to cause the piezoelectric element to generate a charge signal proportional to the input acceleration. From an electrical point of view, the piezoelectric element is like an active capacitor with an internal resistance of 10x9 ohms. The internal resistance and capacitance determine the RC time constant, which also determines the high-frequency pass characteristics of the sensor. For this reason, piezoelectric acceleration sensors cannot be used to measure static events. Piezoelectric elements can come from nature or man-made. They have different signal conversion efficiencies and linearity. There are two types of piezoelectric acceleration sensors on the market-charge output type and voltage output type.
Charge output type acceleration sensor
The main piezoelectric acceleration sensor uses zirconate titanate ceramics, which has a wide operating temperature range, a wide dynamic range, and a wide frequency range (available frequency> 10kHz). The charge output type acceleration sensor encapsulates a piezoelectric ceramic in a gas-tight metal case. Due to its ability to withstand harsh environments, it has very good durability. Due to its high impedance, the sensor needs to be used with a charge amplifier and a low noise shielded cable, preferably a coaxial cable. A low-noise cable is one that has low triboelectric noise2, which is a kind of noise from the cable itself caused by motion. Many sensor manufacturers also provide such low-noise cables. The charge amplifier is connected to a charge output type acceleration sensor, which can eliminate the influence of the cable capacitance and the sensor capacitance in parallel. With an advanced charge amplifier, the charge output acceleration sensor can easily achieve a wide dynamic response ("0dB"). Because piezoelectric ceramics have a wide operating temperature range, some sensors can be used in temperatures from -200 ° C to + 400 ° C, even at wider temperatures. They are particularly suitable for vibration testing at extreme temperatures, such as the monitoring of turbine engines.
Voltage output type acceleration sensor
Another type of piezoelectric acceleration sensor outputs a voltage signal instead of a charge signal. This sensor contains a charge amplifier inside. Voltage mode sensors are available in 3-wire (signal, ground, power) and 2-wire (signal / power, ground). The 2-wire type is also called an integrated circuit piezoelectric sensor (IEPE). IEPE is very popular because it can be easily connected using coaxial wires (2 wires, core wires and shielded wires). In this mode, the AC signal is superimposed on the DC power supply. A coupling capacitor in series at the output can remove the DC bias voltage of the sensor, so that only the sensor signal output is obtained. Many modern instruments provide an IEPE / ICP3 input interface so they can be directly connected to IEPE sensors. If the IEPE power supply interface is not available, a signal amplifier with a constant current source and an IEPE sensor are required for the first phase. A 3-wire sensor requires a separate DC power cord.
Different from the charge output type acceleration sensor, in addition to the piezoelectric ceramic element, the voltage output type acceleration sensor contains a miniature circuit. The operating temperature range of the circuit limits the overall operating temperature range of the sensor, which usually does not exceed 5 ° C. There are also designs that have increased to 175 ° C, but their performance has decreased in other aspects.
Available dynamic range-Due to the extremely wide dynamic range of the piezoelectric ceramic element, the charge output type acceleration sensor appears very flexible in the definition of the range, because its full range can be freely adjusted by the user through a remote charge amplifier. The voltage output type acceleration sensor has a predetermined full range, which is determined by the internal charge amplifier. Once produced by the factory, it can no longer be changed.
Piezoelectric acceleration sensors can be made into small packages, so they are suitable for dynamic testing of light structures.
DC response acceleration sensor
Two technologies are often used to make DC-responsive acceleration sensors: capacitive piezoresistive
Capacitive sensors (variation in acceleration due to changes in capacitance caused by the detection mass) are the most commonly used acceleration sensors today. There are no alternatives in some areas, such as airbags, mobile devices, etc. The high output makes these sensors inexpensive. However, this low-cost sensor is subject to a lower signal-to-noise ratio and limited dynamic range. All capacitive acceleration sensors have an internal clock. This clock (~ 500kHz) is an indispensable part of the detection circuit. Leaks often interfere with the output signal. The frequency of this noise is much higher than the frequency of the measurement signal, and generally does not affect the measurement result, but it always superimposes with the test signal. Due to the built-in amplifier chip, it generally has a 3-wire (or 4-wire differential output) interface. It works as long as it has DC power.
The working bandwidth of a capacitive acceleration sensor is generally limited to a few hundred Hz, in part because of its large internal structure and heavy air damping. Capacitive acceleration sensors are suitable for measuring low-range acceleration, and the upper limit is generally within 100g. In addition to these limitations, modern capacitive acceleration sensors, especially instrument-grade devices, have good linearity and high stability.
Capacitive acceleration sensors are usually suitable for on-board testing, and the low cost is one reason. For low frequency motion testing, acceleration is generally low, and they are an ideal choice. For example, vibration testing in civil engineering.
Piezoresistive acceleration sensor is another widely used DC response acceleration sensor. Unlike capacitive acceleration sensors that measure acceleration through changes in capacitance, piezoresistive acceleration sensors output acceleration signals through changes in the value of strain resistance, which is part of the sensor's inertial sensing system. Many engineers are familiar with strain gauges and know how to measure their output. Most piezoresistive sensors are sensitive to temperature changes, so their output signals need to be temperature compensated inside or outside the sensor. Modern piezoresistive acceleration sensors include an application specific integrated circuit for on-board signal processing, as well as temperature compensation.
The working frequency of the piezoresistive acceleration sensor can reach 5000Hz. Many piezoresistive acceleration sensors use either air damping (MEMS type) or liquid damping (sticky strain gauge type). Damping characteristics are an important factor in selecting a sensor. In some applications, the input mechanical vibration contains high-frequency components (or excited high-frequency response), and a damped sensor can prevent itself from ringing (resonance), thereby preserving or increasing the available dynamic range. Because the output of the piezoresistive acceleration sensor is differential pure resistance information, the signal-to-noise ratio is usually very good; its dynamic range is limited only by the quality of the DC amplifier connected afterwards. For high acceleration impact tests, some piezoresistive acceleration sensors can measure accelerations exceeding 10,000g.
Due to its wide frequency response capability. Piezoresistive acceleration sensors are suitable for pulse and collision tests, where the frequency and acceleration are usually very high. As a sensor with DC response capability, through its acceleration output, users can get speed and displacement information without integral errors. Piezoresistive acceleration sensors are commonly used in automotive safety testing, weapon testing, and seismic testing.
Each acceleration sensor technology has its advantages and disadvantages. Before making a choice, it is important to understand their differences and test requirements. First and foremost, for applications that need to measure static acceleration or low-frequency acceleration (<1hz), or applications that use acceleration to calculate speed and displacement, you need to choose an acceleration sensor with a DC response. Both DC and AC-responsive acceleration sensors can measure dynamic signals. When only dynamic signals need to be measured, users can choose their own. Some users do not like to deal with the zero offset of the DC response acceleration sensor, but prefer the AC-coupled, single-ended output piezoelectric acceleration sensor. Other users don't care about handling the zero offset and are accustomed to the 3-wire or 4-wire interface. They like the load resistance self-test (shunt) and gravity acceleration self-test (2g rollover). They will choose a DC response acceleration sensor.Tag: acceleration sensors