Position & Speed Sensors for High Vibration Environments

2 April 2012 Machines that are subject to harsh or prolonged vibration present challenges for many components - none more so than position and speed sensors. In this article, Mark Howard of Zettlex, lists 10 simple rules for design engineers selecting position and speed sensors that must cope with shock or vibration. There are many examples of harsh shock and vibration environments: off-road vehicles, airborne avionics and mining equipment. There are also some less obvious examples - such as pumps and refrigeration plant – where the vibration is less extreme but persists over many years. Of course, characteristics will vary from application to application but generally all environments with vibration or shock can present significant problems for electrical control systems and position / speed sensors in particular. The following 10 simple rules should help design engineers select position or speed sensors that won’t fail once installed in the field. Zettlex sensors for high vibration applications High vibration applications are many and varied 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 say a life-time of 1 million cycles, 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 as shown in Figure 1 (like an engine or pump, for example), 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. Zettlex as alternative to potentiometers Figure 1 – Potentiometers are generally not suitable for harsh vibration applications 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 closed rapidly, in turn causing rapid start-stop of host the system. This can be solved by either damping the sensor or introducing some hysteresis which will only allow it to switch after a set time. 3. Measure position or speed directly not indirectly 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. Accordingly, it is more important in vibrating environments to measure the position of the actual elements whose position or speed is to be measured directly. This is opposed to measuring position indirectly – say at the end of a gear-train or multi-link mechanism. Without direct measurement, measurement accuracy will be degraded. 4. Avoid glass scales for optical sensors Optical positions sensors often use a glass scale through which they shine and measure their light path. In benign environments most optical sensors will