Devices that are exposed to harsh or protracted vibration are challenging for a number of components, with position and speed sensors suffering the greatest difficulties. This piece lists ten simple guidelines to assist design engineers in choosing position and speed sensors to stand up to shock or vibration.
Figure 1. Airborne equipment is often subject to harsh and prolonged vibration. Image Credit: Celera Motion
Off-road vehicles, airborne avionics and mining equipment are some of the first items which come to mind when considering harsh shock and vibration environments, but less immediately obvious examples, such as pumps, motor drives and refrigeration plants, where vibration may be less extreme, but is prolonged over many years, should also be taken into account.
While characteristics vary between applications, any environment with exposure to vibration or shock can be problematic for position sensors. The ten guidelines offered below will assist engineering in selecting position or speed sensors that will be reliable in the field.
1. Use Non-Contact Sensors
While potentiometers are the most commonly employed type of position sensor, they are typically unsuitable for environments with severe or protracted vibration, as their sliding contacts suffer wear and have a finite lifespan.
A potentiometer with a life-time of one million cycles, for example, would likely be perfectly acceptable for a benign application with around 100 cycles per day, as this would equal a 10,000 day (or 27 year) lifespan.
However, if an identical potentiometer was used in an application with a vibration of 20 Hz, such as an engine or pump, as shown in Figure 1, it would be likely to fail in under 24 hours, as the device’s contacts will register each vibration as a microscopic cycle.
If the potentiometer is typically positioned at a specific point, the impact of the wear is accelerated, and the potentiometer is likely to fail even more rapidly.
Figure 2. Vibration causes potentiometers to wear out quickly. Image Credit: Celera Motion
2. Damp the Sensor Output
By definition, it is probable that the position or speed being measured is varying at the vibrating frequency (or some function of the frequency). A sensor with undamped electronics will output the measured position, and its output will seem to bounce along at the vibration frequency.
However, if the sensor’s output is electrically damped, the output becomes the average of the measured position. Within certain sensors, the period over which the output is averaged can be specified – from a fraction of a second or many seconds- as is appropriate for the application.
3. Measure Directly Rather Than Indirectly
Where position or speed needs to be measured in a vibrating system, various constituents within the system will probably be vibrating at disparate frequencies and amplitudes.
As such, in vibrating environments, it is of increased importance to measure the position of the actual elements whose position or speed is to be measured directly, as opposed to measuring position indirectly, for example, at the end of a gear-train or multi-link mechanism. Without direct measurement, measurement precision will be corrupted.
4. Avoid Delicate Glass Scales in Transmissive Optical Encoders
Transmissive optical encoders employ a glass scale with opaque and transparent lines, which modulate light as the scale turns. These optical sensors offer excellent performance in benign environments, free of contaminants.
However, in applications with heavy shock or vibration, glass scales are vulnerable to fracture. Interferential encoders are more resistant to shock and vibration, as the scale can be mounted on a hub which wholly supports the scale.
5. Use Caution with Magnetic Sensors
Magnetic encoders, both Hall and magnetoresistive, are sturdy, small in size and can be highly economical. However, the magnetic track is somewhat brittle and can be vulnerable to shock.
6. Minimize Sensor Weight
A factor often overlooked when considering the damage to sensors is that it is often a result of the momentum of the sensor’s own components, rather than a direct result of the vibration itself.
Lowering the weight lessens momentum and minimizes the potential for damage. In harsh vibration environments, lightweight sensors are typically less susceptible.
7. Use Heavy-Duty Connectors – Or Preferably No Connectors
Cables and connectors most commonly cause an electrical failure in harsh vibration environments. The insubstantial connectors typically employed on consumer electronics have no place in harsh vibration environments.
Connectors should instead be heavy-duty, such as military standard 38999 (shell types), or at least include jack screws to fix the connector’s male and female elements. Where possible, connectors should be eliminated and electrical interconnections made through direct wiring or flying leads.
8. Potting and Encapsulation
An effective method through which to mitigate problems resulting from vibration is to pot sensors and cables into position. There are a wide variety of two-part epoxies employed for electronic encapsulation, and these offer a highly effective technique for securing position sensors into the host equipment.
Further advantages of this method are that it also offers a barrier against contaminants and improves heat dissipation at increased temperatures.
9. Stress Relieve Connecting Wires
Wires and cables are often forgotten in stress and vibration analyses, but a moving cable is almost guaranteed to cause problems from conductors or electrical joints cracking as a result of fatigue. Potting is a highly effective way to eliminate such problems, but cable wraps, potting or tightly fitting conduits can also be considered as alternatives.
10. Lock Any Fasteners
While this may appear an obvious step, it is still one that is frequently forgotten. Fasteners securing position sensors should be bonded into position with thread lock or, if possible, an anti-rotation fastener such as a tab washer to inhibit hex-headed screws from turning and loosening.
This information has been sourced, reviewed and adapted from materials provided by Celera Motion.
For more information on this source, please visit Celera Motion.