An object is bound to experience vibration or shock when it experiences external force or mechanical excitations. These shock and vibration forces may completely damage the object if they exceed the threshold limit that can be tolerated by the object. Vibrations and shocks are common occurrences in some industrial phenomena such as the motion of a tuning fork, operations of industrial pumps or machinery and many more. In laboratory environment, researchers may also purposely recreate shocks to understand how the objects behave in these conditions. Therefore, there is a need for a device that can accurately measure the vibration level or shocks experienced by an object.
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. The shocks or forces experienced by the body can be static or dynamic. In general, accelerometer comprises two major system: a mechanical sensing element and a mechanism to convert mechanical motion into electrical output.
How accelerometer works?
Accelerometers sense the pressing of mass on the surface/axis due to an external force applied to the object. Accelerometers mainly comprise a Sensing element (Proof Mass), a spring connected to the casing and a Dashpot. A Dashpot is a device that resists motion by inducing Viscous Friction. In this case, the Dashpot is included so as to facilitate the Damping Effect. In the circuital arrangement, the Dashpot is attached in parallel or in series with the mass and casing.
The accelerometer is mounted on the object whose acceleration we need to determine. 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.
Types of accelerometers available
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.
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 typically utilizes MEMS (Micro-Electro-Mechanical System) technology. It is available as both damped and un-damped models with measuring range of up to 200,000 g-level. Piezoresistive accelerometers generally are DC-responding accelerometers which means they are able to measure steady state acceleration close 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 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 principle of piezoelectricity to convert mechanical energy into electrical output. 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 such as in predictive maintenance, aerospace, automotive, medical and process control.
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 which offers superior performance comparable to the 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 acceleration from 0 Hz to 1 kHz. It also has a high sensitivity for detecting even a tiny vibration while provides 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 average of 10mV/g in piezoelectric accelerometer.
Accelerometers applications in the industry
Accelerometers have a very extensive range of applications, depending on the industry they are being used in. From the automobile industry to product manufacturing, Accelerometers have a very important role to ensure the safety of the device, accuracy in measurements, functioning of laboratory equipments and much more. Let’s look at some of the typical applications of accelerometers in the industry.
- Used as a protective shield in laptops to launch protection system to the hard drive in the event of free-fall.
- Ensuring automobiles safety by launching airbag system in the event of crash.
- Missiles and ballistic testing
- Aircraft flight test
- Measuring vibration on gas turbine
- Automotive crash test in laboratory
- Seismic monitoring
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