5. Common pitfalls
The following is a list of the most common mistakes that engineers make with regards to position sensors:-
Not calculating the cost of sensor failure. All engineers want to select a low cost solution. This is not the same as simply selecting the lowest cost sensor. As a general rule, the cost of sensor failure in the field is going to be more trouble and many times more expensive than the cost of a position sensor. In other words, it is usually the best and least overall cost solution to select a sensor that will not fail in the field. Further, there’s also the nature of the failure to be considered. A sensor that malfunctions and stops working is usually far less problematic and costly than a sensor that fails and produces a credible but wrong reading. The consequences of a wrong sensor reading in terms of cost and safety can be even higher than a sensor that simply stops working or produces an error warning.
Not understanding the difference between repeatability, resolution and accuracy. Have a look back at section 3 and make sure you understand these basics. You should avoid the mistake (often propagated by the position sensors industry) of confusing resolution and accuracy. Just because an optical encoder produces a million counts per rev, does not means that it is accurate to one millionth of a rev – far from it. Conversely, repeatability is often the key requirement in many engineering applications and high accuracy (and hence high cost) sensors need not be specified.
Mismatching sensor type and environment. Man has devised ways of harnessing most of the basic physical phenomena to measure position by using optical, magnetic, capacitive, resistive and inductive techniques. Each technique has its own strengths and weaknesses. As a general rule don’t select
- Resistive (potentiometric), optical or capacitive sensors for dirty or wet environments. Condensation and surface ice in outdoor equipment is a common cause of failure.
- Optical, magnetic or capacitive sensors with applications with extreme operating temperatures (most will not operate above 125C)
- Magnetic sensors where high measurement performance is required, unless it’s also possible to eradicate magnetic fields and arrange precision mechanical sensor mounting
- Potentiometers in applications with harsh or prolonged vibration. This because their sliding electrical contacts are subject to wear and failure from lots of vibration induced microscopic movements.
Inferring a measurement rather than measuring directly. A good design rule for position sensors is to measure the position of the object that you’re interested in. In other words, measure its position directly. Try not to infer or calculate a component’s position by measuring the position of another component such as a gear at the end of transmission line or the position of a drive motor. There is likely to be backlash, clearances, part-to-part variability, mechanical failure, differential thermal expansion/contraction etc. that will inevitably degrade measurement performance and reliability.
Forgetting cables & connectors. Cables and connectors are a primary cause of sensor failure. Ensure that they are accounted for in any design and in particular the cables are strain relieved in any applications that experience motion, shock or vibration.
Not reading the small-print of a sensor’s datasheet. The position sensor industry is a competitive one. Unfortunately, this has led to some manufacturers being a bit too commercially sharp with specification data. Often they get away with it because the industry also knows that many engineers won’t have read a paper like this. The consequence is that sensors will be publicised with, for example, a resolution 10,000 counts per rev – but no mention of accuracy. Another example, is sensors with impressively high resolution but much less repeatability – in other words lots of resolution but also plenty of noise on the sensor’s output. The trick is not to be misled by head-line figures of a datasheet – read the small print.
6. How to specify a Position Sensor
The first and most important step in choosing a position sensor for your project is to be absolutely clear about what is needed, particularly with respect to sensor resolution, repeatability and linearity. Over-specifying any of these attributes will cause unnecessary expense. The trick is to find a sensor that is fit-for-purpose at minimum overall cost – remembering to include an allowance for field failure in your analysis.
You can use the following as a check list to ensure you’ve considered all the important stuff in your specification. Providing this to a position sensor supplier together with a mechanical drawing of the envelope will also provide a solid basis for your discussions:
- Geometry – for example, linear or rotary or curvi-linear or 2D or 3D
- Space envelope– mechanical fixing points, cable routings and space envelope
- Measurement type– incremental or absolute
- Full-scale– for example, 360 degrees or 600mm
- Resolution – in other words, the smallest change that must be measured – for example 0,1 degrees or 0,2mm
- Repeatability – in other words, the stability of the measurement in terms of going back to the same point – for example repeatability = +/-0,025mm
- Linearity – the maximum allowable deviation from a perfectly accurate reading. You might want to think carefully about this since we often find that what is most important for many applications is actually repeatability.
- Operating and store temperature range– -40 + 85Celsius is most typical
- Electrical supply – for example, 5V, 12V or 24V
- Electrical output– for example Serial Data, A/B pulses, 0-10V, 4-20mA
- Unusual stuff – such as – “we want to keep power consumption as low as possible” or “it’s for submersion in hot sulphuric acid” or “we’re using a capacitive device and we have reliability problems”