Vibration sensors are sensors for measuring, displaying and analyzing linear velocity, displacement and proximity, or else acceleration. They can be used on a stand-alone basis, or in conjunction with a data acquisition system. Vibration sensors 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.
Vibration sensors 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 vibration sensors are piezoelectric, capacitance, null-balance, strain gage, resonance beam, piezoresistive and magnetic induction. An alternative to traditional vibration sensors is one manufactured using MEMS technology, a micro-machining technology that allows for a much smaller device and thus package design.
Five main features must be considered when selecting vibration sensors: measuring range, frequency range, accuracy, transverse sensitivity and ambient conditions. Measuring range can be in G’s for acceleration, in/sec for linear velocity (or other distance over time), and inches or other distance for displacement and proximity. Frequency is measured in Hz and accuracy is typically represented as a percentage of allowable error over the full measurement range of the device. Transverse sensitivity refers to the effect a force orthogonal to the one being measured can have on the reading. Again, this is represented as percentage of full scale of allowable interference. For the ambient conditions, such things as temperature should be considered, as well as the maximum shock and vibration the vibration sensors 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 vibration sensors can measure.
Electrical output options depend on the system being used with the vibration sensors. Common analog options are voltage, current or frequency. Digital output choices are the standard parallel and serial signals. Another option is to use vibration sensors with an output of a change in state of switches or alarms. In addition, these sensors can have acceleration, velocity, or displacement as output by either integrating or differentiating their primary output.
When mounting vibration sensors, many choices must be weighed based on application and ability. Probably the most secure method is stud mounting. Many vibration sensors 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 vibration sensors to be mounted on an electrically isolated surface to provide ground isolation between the mounting surface and signals from the vibration sensors. Triaxial mounting cubes can also be purchased to mount three vibration sensors 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.
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