Understanding Accelerometer Sensors and Selecting the appropriate Data Acquisition Hardware
Accelerometers can be used for a variety of test applications, for example product validation, or part of industrial automation monitoring system. Machine condition monitoring (such as factory automation), noise-vibration-harshness testing, and structural testing are some examples of where a vibration sensor can be used. While these test applications span a vast area, the basic principles remain the same.
As a side note, many everyday consumer electronics contain accelerometers. Most smartphones make use of accelerometers so that the phone “knows” its’ orientation. These sensors are typically referred to as MEMS accelerometers (Micro-Electro-Mechanical-Systems) and can range from a single axis accelerometer to three axis accelerometers (triaxial). Below is an example of a MEMS Accelerometer that might be found in your smartphone.
Piezoelectric accelerometers (or sometimes referred to as piezoelectric transducers) generate an electrical signal that is proportional to acceleration. The production of electricity by applying a mechanical stress to certain crystals is generally referred to as the piezoelectric effect. Electrodes in the sensor collect the total accumulated charge generated by a mass stressing a crystal. This all occurs within the accelerometer housing. Wires from the electrodes transmit the signal to a signal conditioner first, then to data acquisition hardware (which might contain additional signal conditioning). Voltage mode sensors contain the signal conditioner built into the sensor, and are classified as Integrated Electronics Piezoelectric (IEPE). Charge mode sensors require external signal conditioning. Below is an image of a typical accelerometers.
The construction of the accelerometers can be divided up into several categories, the most common being the Shear Mode, followed by Flexural Mode. Trying to keep the physics of these sensor constructions to a minimum, just note that:
A general purpose IEPE accelerometer will have the following specifications:
The data acquisition hardware for accelerometers can provide some of the signal conditioning, therefore providing a unified hardware solution, rather than separate modules. For example, in order to properly read the acceleration of an IEPE type of accelerometer, a current must be applied to the sensor (typically 2ma). Additionally, the acquisition hardware can provide low pass filters before digitizing the accelerometer signal to avoid any high frequency noise interference.
For example, suppose our data acquisition hardware specification for dynamic range is 102 dB, and a maximum input of 5V. Then the smallest possible voltage that the device can detect is 40 µV. Having high dynamic range allows for the data acquisition hardware to detect signals ranging from 40 µV to 5 V. This is especially apparent in the frequency domain. A high dynamic range will allow the system to detect the large and small amplitudes of frequencies and provide a much better picture of harmonics.
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