Selecting position sensors in high-vibration environments

Zettlex UK Ltdvisit website


Machines that are subject to harsh or prolonged vibration present challenges for many components – none more so than position and speed sensors. Here, Mark Howard of Zettlex lists 10 simple rules for design engineers when selecting position and speed sensors that must cope with shock or vibration.


The following 10 simple rules should help design engineers select position or speed sensors that will not fail once installed in the field:

1. 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. This is because a potentiometer’s sliding contacts wear and 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 because this equates to 10,000 days (or 27 years). However, place the same potentiometer in an application which is vibrating at 20Hz such as an engine or a pump, and the same potentiometer is likely to fail in less than a day. This is because the 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.

2. Damp the sensor electrical output

By definition, the position or speed being measured is likely to be changing at the vibrating frequency (or some function of the frequency). A sensor with undamped electronics will output the measured position and so its output will appear to bounce along at the vibration frequency. 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.

If a switch or solenoid is to be activated at some point in the measurement cycle (e.g. a pump switch when a tank is full), 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. 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.

3. Measure position or speed directly not indirectly


4. Avoid glass scales for optical sensors

Optical position sensors often use a glass scale through which they shine and measure their light path. In benign environments most optical sensors will perform satisfactorily, provided that there is no foreign matter to interfere with the optical path. However, any glass scale is susceptible to fracture in environments with heavy shock or vibration. Of course, this results in catastrophic failure of the optical sensor, with little or no warning.

5. Minimise the weight of the sensors

An often overlooked phenomenon is that damage imparted to sensors is usually not directly due to the vibration itself but rather as a result of the momentum of the sensor’s own components. Minimising weight will minimise momentum and hence minimise the potential for damage. Lightweight sensors are generally less susceptible in harsh vibration environments.

6. Use heavy duty connectors – or preferably no connectors

The single largest cause of electrical failure in hash vibration environments is cables and connectors. Harsh vibration environments are no place for the flimsy connectors normally used on consumer electronics. Instead, connectors should be heavy duty – such as military standard 38999 (shell types) – or at least include jackscrews to bind the connector’s male and female elements. If possible, connectors should be eradicated and electrical interconnections made by direct wiring or flying leads.

7. Potting and encapsulant

An excellent way to mitigate problems due to vibration is to pot sensors and cables into position. There are a wide variety of two-part epoxies used for electronic encapsulation and these are an excellent method of securing position sensors into the host equipment. This has added advantages of providing a barrier against contaminants and improving heat dissipation at elevated temperatures.

8. Stress relieve connecting wires

Wires and cables tend to be forgotten in stress and vibration analyses but a moving cable is a sure way to generate problems from conductors or electrical joints cracking due to fatigue. Potting is the preferred method of eradicating such problems, but alternatives include cable wraps, potting or tightly fitting conduit.

9. Lock any fasteners


10. Use caution with magnetic sensors


For more information on new generation inductive position sensors, visit the website at or email .

© Copyright 2006-14 Damte Ltd.