Sensors and placement key to vibration monitoring
16 Jun 2010
Monitran operations director, Andy Anthony, guides on vibration sensor selection and placement:
High Wycombe, UK – Vibration monitoring and analysis are now playing integral roles within many a plant’s condition-based predictive maintenance routines. However, the accuracy of the data around which the routines are based can only ever be as good as the suitability of the sensors used and their correct placement.
Even before considering the electrical characteristics of which sensor to use you must consider the environment in which it is to be used. The nature of the installation, permanent or temporary, will guide you towards sensors with connectors or integral cables. And if the installation is to be temporary, perhaps employing magnetic mounts, consider using paired ID tags/labels on the sensors and at the exact locations on the machines/structures where vibration levels are to be observed.
Next, the presence (or risk) of water, high humidity levels or hazardous atmospheres will steer you towards certain sensor sealing characteristics (IP67, 68…) or intrinsically safe (ATEX, IECEx…) devices. Temperature needs to be considered too, as output drift (with temperature) may be an issue - but only if you intend to operate the sensor close to its (g) limits.
As for the type of sensor to use, there are essentially two main ones to consider; those with AC outputs and those with DC. The former are best suited to analysis, as the AC output can be put through an FFT analyser to yield information on the amplitude and nature of the vibration. The latter are best suited to ‘machine protection’ as they produce an output proportional to velocity or acceleration, in the range 4-20mA, and are thus ideal for interfacing with monitoring and shut-down circuitry.
Next, you must strive to make the best of the sensor’s output, and here it’s the old argument of “range versus sensitivity.” For example, many sensors operate across the range ±80g. Hence, if the sensor’s sensitivity is 100mV/g the output will swing between -8 to 8V across its full operating range.
However, if the sensor is likely to be exposed to no more than ±8g, then it would be wiser to select a sensor with a sensitivity of 1V/g, as the output voltage swing will provide better resolution (10x in fact) of the vibration levels.
Another electrical consideration is the frequency response. For example, most standard sensors’ responses start at about 0.8Hz but if you need to measure movement in a large structure, such as a bridge, then you may need to use a dedicated low frequency sensor, one that can measures down to DC (0Hz).
Hopefully, the above advice will help you narrow down on your choice of sensor; and even for seemingly novel applications, many ‘standard’ products will fit the bill; if the right ones are chosen.
