Principle and application of piezoelectric acceleration sensor

Photocoupler

Each accelerometer technology has its advantages and disadvantages. It is important to clarify their differences and test needs before making a choice. First and foremost, for applications that need to measure static acceleration or low frequency acceleration (<1 Hz), or applications that require acceleration to calculate speed and displacement, an acceleration sensor with DC response is required. For most engineering applications, choosing the right test tool will have a big impact on the test results. This article will help readers choose the correct acceleration sensor. Let's start with the classification and principle of the sensor.

Basic accelerometer type

In general, there are two types of accelerometers: an AC-responsive accelerometer, a DC-responsive accelerometer, as an AC-responsive accelerometer, and as its name, its output is AC-coupled. Such sensors cannot be used to test static accelerations such as gravitational acceleration and centrifugal acceleration. They are only suitable for measuring dynamic events. The DC-responsive accelerometer has a DC-coupled output that can respond to acceleration signals as low as 0 Hz. The DC-responsive accelerometer is therefore suitable for testing both static and dynamic accelerations. It is not only the acceleration sensor that selects the DC response when it is necessary to test the static acceleration.

Acceleration, speed, displacement

Many studies of vibration require information on acceleration, velocity, and displacement, which are important information that engineers need to design and validate structures. In general, acceleration provides a good reference, while speed and displacement are the variables required for calculation. In order to calculate the velocity and displacement from the acceleration, the acceleration signal output from the sensor is divided into one and two times by digital or analog form. This can cause problems with AC-coupled sensors. To demonstrate this problem, imagine using an AC sensor to measure a wide pulsed half sine wave signal. Due to the inherent AC RC time constant, the sensor output does not match well with the input pulse. For the same reason, at the end of the pulse, the sensor output will produce a negative zero offset. The figure below shows the relationship between the sensor output (red curve) and the wide pulse half sinusoidal acceleration input (blue curve). This seemingly small difference in magnitude will produce a significant error after the integration. A DC-responsive accelerometer has no such problem because its output can accurately follow a slowly changing input. In practical everyday applications, the input signal may not be a simple half-sine pulse, but the problem of testing any slowly changing signal with an AC-coupled sensor will always exist.

Now let's take a look at the various commonly used acceleration sensor technologies.

The most commonly used AC response accelerometer for AC response accelerometers uses piezoelectric elements as their sensitive unit. When there is an acceleration input, the proof mass in the sensor "moves" causes the piezoelectric element to produce a charge signal that is proportional to the input acceleration. From an electrical angle, the piezoelectric element acts as 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 linearities.

There are two types of piezoelectric acceleration sensors on the market - charge output type, voltage output type.

Charge-Output Accelerometer The main piezoelectric accelerometer uses zirconium titanate ceramics with a wide operating temperature range, wide dynamic range, and wide frequency range (available frequencies >10 kHz). The charge output type acceleration sensor encapsulates the piezoelectric ceramic in a hermetically sealed metal case. It has very good durability due to its ability to withstand harsh environments. 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 means that it has low triboelectric noise, which is a motion-generated noise from the cable itself. Many sensor manufacturers offer this low noise cable at the same time. The charge amplifier is connected to the charge output type accelerometer to eliminate the effects of cable capacitance and sensor capacitance in parallel. With advanced charge amplifiers, charge output type accelerometers are easy to achieve wide dynamic response (>120dB). Due to the wide operating temperature range of piezoceramics, some sensors can be used in environments from -200 ° C to +400 ° C or even wider temperatures. They are especially suitable for vibration testing at extreme temperatures, such as turbine engine monitoring.

Voltage Output Type Accelerometer Another type of piezoelectric acceleration sensor outputs a voltage signal instead of a charge signal. The inside of this sensor contains a charge amplifier. The voltage mode sensors are 3-wire (signal, ground, power) and 2-wire (signal/power, ground). The 2-wire type is also known as an integrated circuit piezoelectric sensor (IEPE). IEPE is very popular because it can be easily connected by coaxial lines (2-wire, core and shielded). In this mode, the AC signal is superimposed on the DC power supply. A coupling capacitor in series with the output removes the DC bias voltage of the sensor and only obtains the sensor signal output. Many modern instruments provide an IEPE/ICP input interface for direct connection 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. The 3-wire sensor requires a separate DC power cord for power. Unlike the charge output type accelerometer, the voltage output type accelerometer includes a microcircuit, and the operating temperature range of the circuit limits the overall operating temperature range of the sensor, usually not exceeding 125 °C. There are also some designs that have increased to 175 ° C, but it will be reduced in other performance. Available dynamic range - Due to the extremely wide dynamic range of piezoceramic components, charge-discharge-type accelerometers are flexible in terms of range definition because their full-scale can be freely adjusted by the user via a remote charge amplifier. The voltage output type accelerometer has a predetermined full scale, which is determined by the internal charge amplifier. Once produced by the factory, it will no longer be able to change.