Position sensing – 5 top tips to avoid getting fired

February 09, 2018

 

The chief engineer says “We’ll need a sensor to feedback position” and you say “Sure thing, boss”. What you’re actually thinking is “I’ve no idea what he’s talking about. I’ll get on Google and find out what a position sensor is before the next meeting.”

Sound familiar? OK – this article’s for you.

This article explains what a position sensor is and how you can select the right sensor for your design project. We’ll talk to you like you’re a competent design engineer but one that’s not had that much to do with position sensors. By the time you’ve reached the end, you’ll probably know as much as your boss.

 

Position Sensing Tip #1 – Use the right terminology.

Position sensors get called a whole range of things (….especially if they go wrong) and this can cause confusion.  They get referred to as encoders, transducers, transmitters, transformers, resolvers, synchros and even senders. They can also include a description of their geometry such as rotary transducer, angle encoder, linear transducer etc. There’s some subtle differences but to all intents and purposes they all refer to the same thing:- a device whose electrical output is proportional to position or change in position. ‘Position sensor’ is a useful catch all phrase and you won’t sound daft when you’re using it.

 

Position Sensing Tip #2 – Understand the difference between incremental & absolute sensors.

There are 2 basic types of position sensor – incremental and absolute. The output from an incremental sensor is proportional to change in position. The output from an absolute sensor is proportional to position. The two types can categorised by what happens at power up. An incremental sensor will only provide an output when position changes whereas an absolute sensor will output a position signal without any need to move.

There are more incremental position sensors than absolute sensors. This is because traditionally absolute position sensors were more expensive but the price difference is diminishing and absolute position sensors are usually preferred.

 

Position Sensing Tip #3 – Understand what’s important for good measurement performance.

Sounds dumb right? But this is one area that more design engineers get wrong than right. Measurement performance for a position sensor is usually specified by:-

  • Accuracy – the maximum error between the sensor’s output and actual position
  • Resolution – the smallest change in position that the sensor can measure
  • Repeatability or Precision – a measure of the sensor’s reproducibility
  • Linearity – a measurement of the deviation between the sensor’s output to a straight line formed by the output of a perfect sensor and actual position. (Assuming no off-sets, a perfectly linear measuring device is also perfectly accurate).

 

Most engineers get confused about the differences between precision and accuracy.  We can explain the difference between accuracy and precision using the analogy of an arrow fired at a target.  Accuracy describes the closeness of an arrow to the bullseye.

If many arrows are shot, precision equates to the size of the arrow cluster.  If all arrows are grouped together, the cluster is considered precise – in other words, highly repeatable.

Since you’re aiming not to get fired (pun intended), make sure you know the difference between the various measurement terms and what your project requires. The most important parameter for most applications is precision rather than accuracy.

Think about what’s most important in your overall design. Many engineers either over-specify and pay over the odds for the sensor; or they’ll under-specify and not achieve the required performance.

 

Position sensing Tip #4 – Match the kind of sensor to the application.

Position sensors are used in many applications, from ultra-high-spec spacecraft through to low cost automotive and consumer appliances. There’s a myriad of different position sensor manufacturers, sizes, shapes and technologies. Of course, you’ll need to match the cost of the sensor with the allowable budget but it’s simply not true to think that the more expensive the sensor the better it will be for your project.

Key to success is matching the right kind of sensor to your project.

Each of the different techniques used to measure position has its own strengths and weaknesses. You should aim to match the characteristics of your application with strengths of the position sensing technology.

An important point to know is the difference between contacting and non-contacting sensors. Although there is a trend towards non-contact position sensors, potentiometers (‘pots’) remain the most common position sensor. They measure a voltage drop as a contact(s) slides along a resistive track.  Potentiometers operate well in applications with modest duty cycles, benign environments and relaxed performance. Unfortunately, pots are susceptible to wear and foreign particles such as dust or sand. They are typically only suited to low-end, low-cost applications and they have an (undeservedly) poor reputation.

The following is a table showing the various kinds or technologies used for position sensors versus their relative merits.

 

Position sensing types

 

The table should be taken as a general guide and undoubtedly there will be exceptions. It’s meant to help you seek out the kind of position sensor that might be right for your project. Get this selection wrong and you’ll pay the price later – see Tip #5.

 

Position sensing Tip #5 – Don’t skimp! Understand the full cost of a sensor.

Your job as a design engineer will be to produce a design which will meet all aspects of the brief at minimum possible cost.

Watch out! The true cost of sensor is not just the purchase cost. To pick the minimum cost solution be sure to consider cost of failure.

In a best case scenario, sensor failure is spotted at the design stage when an alternate sensor can be found. Impact is usually not that great.

In a slightly worse case scenario, failure is found at the test or qualification stage. Swopping out one type of sensor may have some knock on consequences – but it’s only rarely a disaster.

When the trouble really starts is when position sensors fail in the field. In such scenarios, the few cents or dollars between different sensors pale in to insignificance. Often, the wrong kind of sensor was chosen for the environment. The potential scenarios are:-

– Higher than anticipated repair/replacement costs
– Product recall for fitment of alternate sensor
– Widespread field failure causing loss of reputation in the market
– Field failure causing catastrophic damage.

The best thing any design engineer can do in such an instance is to avoid it ever happening. This is best done by choosing the right position sensor for the job – not necessarily the sensor with the lowest purchase price.

One point worthy of note is the term ‘field failure’. Often, when a position sensor fails it stops working and the host equipment reverts to a safe state. When a position sensor fails and carries on working but outputs a credible but wrong signal is when things can go really wrong. Think aileron position or landing gear position sensing in an aeroplane. This is why inductive position sensors are most usually selected for the most difficult or dangerous applications – since they are very robust and tolerant to extreme environments.

 

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