Accelerometers are electromechanical devices used to measure acceleration forces acting on a body which can be static or dynamic. An accelerometer comprises two major systems: a mechanical sensing element and a mechanism to convert mechanical motion into electrical output. The measurement of acceleration can be categorized into the measurement of shock, vibration, motion and displacement.
Why use accelerometers?
Any object is bound to experience vibration or shock when it is disturbed by external forces or mechanical excitations. These forces may damage the object if they exceed the threshold limit that can be tolerated by the object.
Industrial equipment such as tuning forks, industrial pumps, conveyor belts, vibrating meshes, are examples of equipment which potential failures can be diagnosed from the change in vibration. This equipment can exhibit an excessive level of oscillation which may indicate loose fittings, damaged parts. If left alone, it will progressively lead to total failure which can be unsafe and force the company into operational shutdown for maintenance. Accelerometers can be used to monitor their vibration and provide the maintenance crew with an indication to schedule timely maintenance works if anomalies occur.
Accelerometers are not only used for industrial condition monitoring and structural health monitoring applications, but they are also extensively used in research and testing applications. For example, these vibration measuring sensors have been used for the development of vibration testing systems for earthquake testing, shaker tables, automotive crash test and product integrity testing. For some of these applications, accelerometers are also used to accurately measure excitation levels as researchers may also purposely recreate shocks to understand how the objects behave under these conditions.
How does an accelerometer work?
Accelerometers sense the pressing of mass on the surface/axis due to an external force applied to the object by mounting them on the tested objects. Users should fully understand their applications before choosing the accelerometers and integrating them as the mounting configuration will impact how the vibration is measured. In the case of linear acceleration, a force acts on the proof mass, causing it to deflect. This deflection is then further converted into an electrical signal by the accelerometer. The movement of the Proof Mass is opposed by the Damper and Spring, making the system reach an equilibrium state.
Different Types of Accelerometers
Accelerometers are available in different sizes, shapes and technology. They can measure from as low as 2g-level to 200,000g-level. In general, there are three types of accelerometers used in industrial testing applications, shown below.
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 a change in the electrical resistance of the piezo materials. The output voltage varies with the amount of stress applied to the accelerometer. Piezoresistive accelerometers utilize MEMS (Micro-Electro-Mechanical System) technology making it suitable for measuring DC-response, steady-state acceleration with minimum zero shift. This allows the bridge element to returns to its ideal state immediately after the shock event which minimizes errors in long-duration shock measurement applications.
Working Principle of MEMS Accelerometer
Piezoelectric Accelerometer
The sensing elements in piezoelectric accelerometers are made of quartz crystals, piezoceramics (Lead Zirconate titanate) or tourmaline crystals or lithium niobate which allow them to operate in high-temperature environments. Acceleration is measured by measuring the changes in electrical charge as the result of mechanical forces. Piezoelectric accelerometers are ideal for general condition monitoring applications or the measurement of high-frequency vibration.
IEPE (Integrated Electronics Piezoelectric) accelerometers are piezoelectric accelerometers that are designed with built-in integrated electronics to amplify the signal before transmitting it to the data acquisition system. The electronics converts the high impedance signal of the piezoelectric material into a voltage signal with low impedance of typically 100 Ω. This configuration allows for signal transmission across long cable lengths without significant loss of quality.
Working Principle of Piezoelectric Accelerometer
Variable Capacitance Accelerometer
Variable Capacitance accelerometers utilize MEMS capacitive technology which offers superior performance compared to the popular piezoelectric accelerometer. They are manufactured using the silicon microfabrication technique which introduces economic feasibility for high volume manufacturing applications. Its popularity amongst OEM manufacturers is also due to its low power consumption, excellent measurement linearity and independence of temperature.
The variable capacitance accelerometer is ideal for measuring a very low, DC-response acceleration such as motion or seismic measurement. It also has a high sensitivity for detecting even a tiny vibration while providing excellent linearity and temperature independence. For example, a capacitive accelerometer with a frequency response from 0Hz to 15Hz will provide a sensitivity of 1V/g compared to an average of 10mV/g in a piezoelectric accelerometer.
Working Principle of MEMS Accelerometer
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