Wednesday, August 21, 2019

Importance of Accelerometer Selection

Importance of Accelerometer Selection Accelerometer: Vibrations are everywhere in our lives. They may be generated and transmitted by different things, such as motors, turbines, machines and plants, structures and buildings, or even by human body. Some vibrations are desirable, and some of them can be disturbing or destructive. So understanding the causes of vibrations, measuring and reducing them is important. Accelerometer is a device, designed to measure vibrations in different situations and environments. Function: The heart of an accelerometer is a piezoelectric material (like quartz or ceramic crystal). Figure shows this material as a disc for piezoelectric effect explanation. Imagine that piezoelectric material (the disk) is located between two electrodes, like a capacitor. When a force is applied perpendicular to the disk, a voltage is produced at the electrodes. This phenomenon is called piezoelectric effect. Main elements of an accelerometer are piezoelectric material and seismic mass (Figure ). A seismic mass is attached to one side of the disk, and another side is attached to a sensor. When the accelerometer is subjected to vibration, a generated force acts on the piezoelectric element. According to Newtons Law, this force is equal to the product of the seismic mass and the acceleration (m.a). Based on the piezoelectric effect, an output charge is generated, proportional to the applied force. Since the seismic mass is constant, the output charge signal is proportional to the acceleration of the mass. In wide frequency ranges, sensor and seismic mass have the same acceleration amount, as a result, the sensor measures the acceleration of the test sample (Ref). Different types: In sum, four main types of accelerometers are available; Premium grade accelerometers are made from premium crystals and use low noise circuitry to produce a premium. They have steel cover and this cover can protect them in harsh industrial environments. Industrial grade accelerometers are prevalent in different industries and used on different machines, equipment and plants. High vibration accelerometers are used to monitor high vibration levels, and have a lower output (10 mV/g) and lower mass, in compared with industrial accelerometers. Triaxial accelerometers measure vibration in the X, Y, and Z axes. They have three crystals and they react to vibration in different axis. As a result, the output has three signals, for X, Y, and Z axes (Ref) Accelerometer selection: For getting the best result of using accelerometer in a test, sort of issues must be considered. Based on these issues we can determined the most suitable accelerometer, based on its features, for our test. The first and foremost issue that we must consider is the amplitude of the vibration that we are going to monitor. Frequency range that we are going to study is important to choose an appropriate accelerometer (Ref). Another issues are effective in accelerometer selection, such as the temperature range of the installation area, size of the sample in the test, existence of magnetic field in the test environment, and existence of electrical noise in the test area. Other environmental issues are considerable, for example amount of humidity of test environment, or if accelerometer installation can alert the vibration characteristics in the test or not (Ref). Accelerometer installation: As it was discussed before, accelerometer selection has an important role to get more accurate results. But to achieve this goal, installation of accelerometer is important too. Some points and issues that must be considered during the installation of accelerometer: the surface of installation must be clean and grounded (Ref). The selected location for installation must provide a short and rigid path to the vibration source (Ref). For example, for getting better result in a vibration transmission from road surface test,the closest point to the wheels on the unsprung mass and the chassis near the suspension are recommended. Selected Accelerometer: Selected accelerometer for this test, based on needs and test conditions, is PBC model: 356A32. This model is a Triaxial type, with high sensitivity ( ±10%) 100 mV/g (10.2 mV/(m/s ²)), measurement range of  ±50 g pk ( ±491 m/s ² pk), broadband resolution: 0.0003 g rms (0.003 m/s ² rms), and frequency range of ( ±5%) 1.0 to 4000 Hz (Ref). Figure shows this accelerometer. Selected accelerometer for this test, based on needs and test conditions, is PBC model: 356A24. This model is a Triaxial type, with low sensitivity ( ±10%) 10 mV/g (1.02 mV/(m/s ²)), measurement range of  ±500 g pk ( ±4905 m/s ² pk), broadband resolution: 0.004 g rms (0.04 m/s ² rms), and frequency range of ( ±5%) 1.0 to 9000 Hz (Ref). Figure shows this accelerometer.

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