An accelerometer is an electromechanical device that is used to measure acceleration forces on a body. They are commonly used for measurement of shock, vibration, motion and sometimes displacement in industrial application.
Accelerometers are available with different size, shape and technology. They can measure from as low as 2g-level to 200,000g-level. With so many accelerometers to choose from, sometimes it can be challenging for the end-users to choose the suitable accelerometer for their applications. There are three types of generally used accelerometers, each possesses distinct characteristics that are specifically designed, as described below.
Different types of accelerometers
Piezoresistive Accelerometer
Piezoresistive accelerometer utilizes strain gauges with a full Wheatstone bridge configuration to convert mechanical stress to a DC output voltage. The mechanical stress caused by acceleration, shock, vibration or external forces causes change in electrical resistance of the piezo materials. The output voltage varies with the amount of stress applied to the accelerometer.
Piezoresistive accelerometers are typically MEMS (Micro-Electro-Mechanical System) technology available in both damped and undamped models with measuring capability up to 200,000g. They are also DC-responding accelerometer; able to measure steady state acceleration down to 0 Hz with minimum zero shift. This means that the bridge element returns to its ideal state immediately post shock event which minimizes errors in long duration shock measurement applications.
Piezoelectric Accelerometer
Piezoelectric accelerometers are made commonly made of quartz crystals, piezoceramics (Lead Zirconate titanate) or tourmaline crystals or lithium niobate for high temperature applications. The accelerometer utilizes the “piezo electric effect” applying a transient load to the device gives an output voltage. Therefore, they are ideal for measuring general vibration or high frequency acceleration in the industry. The piezoelectric accelerometers are widely used for measuring vibration in many scientific and industrial applications in predictive maintenance, aerospace, automotive, medical and process control.
Some variant of piezoelectric accelerometer may also have built-in electronics to amplify the signal before transmitting it to the data acquisition system. This type of accelerometer is defined as IEPE accelerometer. The electronics converts the high impedance signal of the piezoelectric material into a voltage signal with low impedance of typically 100 Ω. Low impedance is beneficial because it can be transmitted across long cable lengths without significant loss of signal quality.
Variable Capacitance Accelerometer
Variable Capacitance accelerometer utilizes MEMS capacitive technology with superior performance that is comparable to the highly popular piezoelectric accelerometer. A variable capacitance accelerometer is manufactured using silicon microfabrication which introduces economic feasibility for high volume applications. It is popular for OEM manufacturers due to its low power consumption, excellent linearity and independent of temperature.
The variable capacitance accelerometer is ideal for measuring a very low, DC-responding frequency from 0 Hz to 1kHz. It also gives a very high sensitivity for detecting even a tiny vibration. For example, a capacitive accelerometer with a frequency response from 0Hz to 15Hz will provide a sensitivity of 1V/g.
General Applications of Accelerometers
Generally speaking, accelerometers are used for measuring accelerations, velocity and displacement of a body. They are critical variables engineers seek for when designing products or validating structural integrity of objects. Typically, acceleration measurement can be divided into four broad categories:
Vibration
Piezoelectric accelerometers are ideal choice for most general vibration measurement due to their wide frequency response, good resolutions and sensitivity. They are also relatively easy to install, which make them popular for monitoring vibration of rotational machinery in the factory.
Shock
Shock measurement require an accelerometer that can measure at high g-level with minimum zero shift to minimize error in the measurement. Piezoresistive accelerometers are ideal for this type of measurement due to their capability to maintain minimal zero shift post-shock event. Some examples of shock measurement in the industry are:
- Automotive crash testing
- Mobile phone drop test
- Missile / Ballistic testing
Motion
Motion is a slow-moving movement with low frequency vibration. Variable capacitance accelerometers excel in this type of measurement due to their high sensitivity when measuring at low frequency response. Motion measurements are typically encountered in measuring movement of robotic arm, Automotive NVH (Noise, Vibration & Harness) testing and flight test.
Seismic
Seismic measurement requires an accelerometer with low noise level and high-resolution reading. A variable capacitance accelerometer fits well for this type of application. It should be noted that seismic measurement is special and not all variable capacitance accelerometers are suitable for this type of measurement. A specifically-designed IEPE accelerometer with ultra-low noise, DC responding and capable of measuring at ultra-low g-level is also suitable for seismic measurement.
For engineers and technicians, selecting a suitable accelerometer for their test and measurement tasks may be daunting due to the wide range of accelerometers available in the market. Knowing what type of measurement you are after and types of accelerometers may assist you in identifying the suitable sensors for your applications.
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