In Situ/Laboratory Instruments
Instrument Class: Magnetic/Motion Sensors
Instrument Type: Accelerometers
Instrument Subtype: ACCELEROMETERS
UND CITATION II
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Accelerometers are sensors and instruments for measuring, displaying and analyzing acceleration and vibration. They can be used on a stand-alone basis, or in conjunction with a data acquisition system. Accelerometers are available in many forms. They can be raw sensing elements, packaged transducers, or as a sensor system or instrument, incorporating features such as totalizing, local or remote display and data recording. Accelerometers can have from one axis to three axes of measurement, the multiple axes typically being orthogonal to each other. These devices work on many operating principles. The most common types of accelerometers are piezoelectric, capacitance, null-balance, strain gage, resonance, piezoresistive and magnetic induction.
Three main features must be considered when selecting accelerometers: amplitude range, frequency range, and ambient conditions. Acceleration amplitude range is measured in Gs, whereas frequency is measured in Hz. For the ambient conditions, such things as temperature should be considered, as well as the maximum shock and vibration the accelerometers will be able to handle. This is the rating of how much abuse the device can stand before it stops performing, much different from how much vibration or acceleration accelerometers can measure.
Electrical output options depend on the system being used with the accelerometers. Common analog options are voltage, current or frequency. Digital output choices are the standard parallel and serial signals. Another option is to use accelerometers with an output of a change in state of switches or alarms.
When mounting accelerometers, many choices must be weighed based on application and ability. Probably the most secure method is stud mounting. Many accelerometers have the option of a threaded section that can be fastened to the machinery or object being monitored. For applications where this is not possible or desirable, many other options are available: wax, magnets and adhesive. Some applications require accelerometers to be mounted on an electrically isolated surface to provide ground isolation between the mounting surface and signals from the accelerometers. Triaxial mounting cubes can also be purchased to mount three accelerometers together in an orthogonal configuration to each other. This way, only one mounting surface on the monitored device has to be used for all three.
To minimize frequency response errors, care must be taken to relieve cable strain by securing the cables attached to accelerometers. To do this, affix the cables to the same device the accelerometers are attached to. This will prevent flexing of the cables between the anchor point and the vibrating surface, thus keeping the accuracy of the readings as high as possible.
[Summary provided by 1999-2003 GlobalSpec Inc.]