Do’s and don’ts of selecting high-vibe position sensors 09 May 2012

For plant engineers covering machines subject to harsh or prolonged vibration, Mark Howard, of Zettlex, has issued a list of 10 simple rules for selecting position and speed sensors.

First, use non-contact sensors: "Potentiometers are by far the most common form of position sensor, but are generally not suitable for environments with either extreme or prolonged vibration," comments Howard.

Why? Quite simply because a potentiometer's sliding contacts wear, so they have a finite lifetime.

"If we consider a potentiometer with a lifetime of 1 million cycles, for example, this is likely to be fine for a benign application which cycles perhaps 100 times per day," advises Howard.

"However, place the same potentiometer in an application vibrating at 20Hz – such as an engine or pump – and the same potentiometer is likely to fail in less than a day."

Howard explains thatthe potentiometer's contacts will see each vibration as a cycle on a microscopic scale. "If the potentiometer is normally positioned at a particular point, the wear effect is accelerated and the potentiometer is likely to fail even more quickly."

For his second point, Howard moves on to damping – and suggests its use on the sensor electrical output, because the position or speed being measured is likely to be changing at the vibrating frequency, or some function of it.

"A sensor with undamped electronics will output the measured position, so its output will appear to bounce along at the vibration frequency," he says.

"However, if the sensor is damped, the sensor's output becomes the average of its measured position. In some sensors the length of time over which the output is averaged can be varied – from a fraction of a second or many seconds – to suit the application."

Howard also points out that, if a switch or solenoid is to be activated at some point in the measurement cycle then it may be that the switching point is made repeatedly over a short period, due to the cyclic nature of the vibration.

"This will cause the switch or solenoid to open or close rapidly, in turn causing rapid start-stop of the host system," warns Howard. "This can be solved by either damping the sensor or by introducing some hysteresis, which will only allow it to switch after a set time."

For Howard, the number three requirement is to measure position or speed directly, not indirectly. Why? "If position or speed is to be measured in a vibrating system, it is likely that different components within the system will be vibrating at various frequencies and amplitudes. [So] it is more important to measure the position of the elements whose position or speed is to be measured directly."

Forth, he says, avoid glass scales for optical sensors. "Any glass scale [for transmitting and measuring light path] is susceptible to fracture in environments with heavy shock or vibration... This results in catastrophic failure of the optical sensor, with little or no warning."

Fifth, minimise the weight of the sensors, advises Howard. "An often overlooked phenomenon is that damage imparted to sensors is usually not due to the vibration itself but rather as a result of the momentum of the sensor's own components," he explains.

"Minimising weight will minimise momentum and hence minimises the potential for damage. Lightweight sensors are generally less susceptible in harsh vibration environments."

And so he continues. Howard's remaining points are: use heavy-duty connectors, or preferably no connectors at all; mitigate problems by potting sensors and cables into position; stress relieve the connecting wires; lock any fasteners; and use caution with magnetic sensors.

Why the latter? "If a magnetic sensor is to be used, then extreme care should be exercised in its selection," explains Howard.

"First, modern rare earth magnets are extremely brittle and subject to catastrophic failure if subject to shock. Magnetic reed switches are prone to fatigue over prolonged periods... Magnetostrictive sensors rely on delicate and precise location of amorphous crystal ribbons in wave guides. The fixture of the ribbon in the wave guide is susceptible under conditions of either shock or vibration."

Brian Tinham

Related Companies
Zettlex (UK) Ltd

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