Position sensors are a common component in many engineering projects. Measuring position or speed accurately and reliably is a tricky business. Selecting the wrong sensor or designing the wrong installation often results in overly complex design, poor performance, high cost or unreliability.
1.Misunderstanding the measurement performance
There are 3 key parameters for position sensors – resolution, repeatability and linearity. The greater the resolution, repeatability or linearity – the higher the sensor price. Sensor price is heavily related to linearity. For many engineering systems, it’s actually repeatability which is the key measurement parameter, not linearity.
2. Underestimating the cost of failure
Whilst it’s tempting to install the cheapest possible sensor, any savings will quickly disappear if it causes failure in the field. This can lead to equipment downtime; lost production; disruption; cost of equipment dis-assembly; cost of technician travel and time; loss to reputation. As a rough ball-park, the replacement of a failed sensor in the field will cost at least 10 times and maybe as much as 1000 times the original cost of the sensor.
3. Misunderstanding the safety requirements
As with cost of field failure, skimping on safety is never a good idea. That’s not to say that you should overspend on an over-specified system; the trick is to specify the right system – by matching the position sensor and control system with the probability and impact of failure.
4. Selecting a potentiometer that wears out rapidly
Despite the trend towards non-contact sensors, potentiometers remain the most common position sensor. They use electrical contacts sliding along a resistive track and they can work well in benign environments. They are subject to wear and short life if the contacts get dirty or there is protracted vibration at a fixed position.
5. Optical encoders in dirty environments
Optical encoders are a common form of position sensor and can provide accurate and reliable results. Their level of precision can be staggering due to the tiny features that can be etched on to the optical gratings. These features can also become susceptible to failure of the optical path from dust, sand or other foreign matter. In the best case, an error code or ‘no-read’ will be generated.
6. Underestimating installation tolerances
Make sure you read the datasheet’s small print – especially those for optical ring encoders where a read-head may require installation to tolerances of <10microns to achieve the head-line stated accuracy. Don’t ignore the effect of tight installation tolerances on material or assembly costs.
7. Calibrating each individual sensor
The calibration produces a look-up table so the output from the low-cost sensor is corrected by the host control system. In some situations (e.g. high precision electro-optics or weapon pointing systems) this works well. These are sophisticated manufacturing and service environments with products that will only undergo highly skilled service or maintenance.
8.Inaccuracy from indirect measurement
A surprisingly large number of parameters come in to play whenever indirect measurement is used such as gear-backlash, misalignments, differential thermal expansion and so on. Whenever possible arrange the design to measure directly rather than indirectly.
9.Capacitive encoders in wet environments
As with optical encoders, capacitive encoders work well in clinical conditions but dirt, dust, grease, condensation and static can produce incorrect sensor signals. Capacitive encoders are seldom a good choice for wet environments (condensation is a well-known problem), whereas magnetic or inductive encoders (incoders) are unaffected by most liquids.
10.Forgetting about cables & connectors
To minimise failure, cables should be tightly secured, gently radiused and non-flexing – especially in harsh shock and vibration environments – so that connectors remain unstrained and conductors unbroken. In wet or dirty environments, the number of connectors should be minimised through the use of integral cables. Cables should also be rooted away from sources of electromagnetic noise; extreme temperatures or the harshest environments.
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