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How to select a position sensor

How to select a position sensor that maximizes product performance.

Selecting a position sensor is a common task in product design because position is one of the most commonly measured attributes in electromechanical design. Every day, millions of machines use position measurements for a host of common applications, such as motor control, valve actuation, speed measurement, and presence detection.

 

In response, position sensing solutions are constantly evolving to offer performance that can add significant benefits to a wide range of machines. If you are still considering the same solutions you used 5 or 10 years ago, we’d like to persuade you to pause and consider your selection criteria carefully; finding the right position sensor can be critical to unlocking significant improvements in performance. 

This post is a guide to specification that looks at both established and new position sensing techniques and walks you through some of the decisions and trade offs that need to be made.

2      Background

Terminology or jargon – what this guide covers

Like a lot of things in engineering, position sensing has built up a set of specific vocabulary (or, to be blunt – jargon) that doesn’t always provide clarity. So some background is helpful when reading this guide.

The first important definition is that we are talking here about position sensors (also known as encoders, transducers, detectors, transmitters or senders) that provide a continuous measurement of position across a full range, either on a linear or rotary axis. Proximity switches, which measure the presence or absence of a component, are not covered in this white paper.

It’s also helpful to think about the difference between contact or non contact position sensors, because the two types have different specifications and cost profiles. A popular rule of thumb is that the cheapest contact position sensors tend to have a lower upfront cost, while non contact sensors will tend to be easier to integrate and can offer longer lifetimes with lower wear. It is, however, a bit more complicated than that – as we will look at in the next section.

Another distinction that is very important in some applications is between absolute and incremental sensors. Incremental sensors will only report movement as a difference in value from their starting point; when powered off they may return to a ‘home’ position to ensure that they continue to report from the same point each time. Absolute sensors, in contrast, report a specific position with no reference to another point. When powered off and on they will continue to report the unique value for that position, and do not return to a home point. 

What do we mean by better performance?

On a tight schedule, the quickest approach to sensor selection is either to go for what has always worked, or compare performance numbers from a few data sheets, get quotes and go for the lowest cost option to fit the spec.

So in that situation ‘better performance’ is what you’d expect: more accuracy, resolution, sample rate etc. The sensor with the best numbers that fits the design and cost envelope wins.

However, if there is time, most engineers would prefer to consider options for bigger improvements in the design arising from better sensing. New sensor solutions provide opportunities to make significant changes in a whole range of product types, by offering more durability, smaller mechanical footprint, faster update rates, higher resolution and better linearity across wider ranges, as just a few options. We look at some of the implications in more detail in section 4.

To take advantage of these opportunities you need to consider your choice of position sensor technology carefully. There are a wide range of position sensor types and picking the wrong one can mean an uphill fight to get the performance you want.

3      Your guide to position sensor types – pros and cons

A quick glance at Wikipedia reveals a very long list of devices that can be used to measure position, but the most widely used solutions in the marketplace fall into 5 main classes

Potentiometers

Pot 2

Widely used, Potentiometers (pots) are a contacting position sensor that use a wiper contact, attached to the moving part, to move along a resistive track. The output will vary depending upon the wiper’s position on the track, so resistance is proportional to position. By making the track design curved, pots can be used for rotary as well as linear measurements.

This simple measuring principal means pots have been used for many years in low cost applications with undemanding performance criteria and at their simplest are some of the cheapest position sensors available. They are easy to use and available in a great range of sizes and geometries.

However, they are not recommended for anything other than benign environments; the resistive track is easily eroded by dirt or vibration and accuracy and repeatability are low. If high performance and durability are a concern then pots are not a good fit, although as with most things if enough money is spent on seals, mounting and protection their performance can be improved.

Magnetostrictive position sensors

 

Magnetostrictive 2


Some ferromagnetic materials (cobalt, nickel, iron) change size or shape when placed in a magnetic field. Magnetostrictive sensors use this effect. Applying a magnetic field to these materials creates stress, and this can be used to estimate the position of the magnetic field.

A basic magnetostrictive position sensor will attach a magnet to the target object, alongside a waveguide wire attached to the stationary part of the machine. The magnet is used to produce a pulse of magnetic field which will stress the waveguide wire at the point where the target is. This stress creates a strain pulse which travels at a known rate along the waveguide to the pick up coils, where the timing between pulse and pick up is measured.

This is a technique that works well for linear position rather than rotary position. The waveguide strip and delicate pickup units must be well protected to avoid external stress sources, meaning these sensors are challenging to manufacture and require individual calibration. Consequently, they tend to be higher priced (typically >$100) and used in high value applications.

Even with the housing, these are not sensors well suited to high vibration or shock applications. The time of flight calculation means there is also a minimum length (roughly <100mm) below which measurement is not possible.

Less obvious is the effect of temperature – extremes of temperature, by causing expansion, can affect the accuracy of magnetostrictive sensors, and data sheets should be read with this in mind.

Encoder – magnetic

 

Magnetic encoder 2

Encoders generally use a scale to measure the position of the target object against a fixed part of the machine. In magnetic encoders, that scale is marked out with a number of magnetic poles – by attaching a sensor to the fixed point and measuring the magnetic poles as they pass the sensor, it’s possible to determine the position of the target.

There are a number of sensor options to measure the magnetic poles, but most popular are Hall effect sensors, which vary their voltage in proportion to magnetic field.

These are high volume, low cost sensors and good for applications which don’t need linearity below 1%, in other words, applications where accuracy is not a prime concern. Other design considerations to think about:

  • They work to tight mechanical tolerances which can add some complexity and cost to manufacturing. Read data sheets carefully for this.
  • There are sizeable temperature effects. If accuracy is important, temperature will need to be carefully controlled. If this is not mentioned in data sheets proceed with caution
  • Magnetic encoders can be sensitive to a range of external factors including magnetic hysteresis, external DC/AC fields and the distorting effects of magnetically permeable materials (e.g.steel)
  • Over time, magnetic encoders can attract metal particles (swarf) which can also affect their accuracy

So, in summary, these are high volume sensors suitable for cost sensitive designs where linearity/accuracy is not the first concern and where other factors such as temperature and mechanical positioning, can be well controlled.

Encoders – optical

Optical encoder 2

A popular alternative to magnetic encoders, optical encoders, as the name suggests, use light to identify the position of the target. By shining a light onto a light detector and interrupting that light source with a grid or grating that encodes the position of the target, the light detector will produce a digital output that can be used to find the target position.

As with magnetic encoders, this simple principle covers a lot of different variants: the light can be shone through gratings or reflected back, visible or infra-red light can be used, the encoder disc can be glass or other transparent material.

Optical encoders can be used to determine both linear and rotary position and more expensive products can offer high accuracy and resolution, if used correctly. As a result they are popular in many industrial automation applications.

In undemanding environments their performance is good, but great care must be taken to mount them accurately because any misalignment has a significant impact on linearity.  For this reason the encoder disc, sensor and processing electronics are often packaged inside a housing including bearings.  Seals are usually added for environments where dust, shock or moisture feature. These problems with moisture mean that optical encoders are also a poor fit for products that operate in cold or high humidity environments; condensation can affect reliability.

LVDT/RVDT

LVDT 2

Inductive sensors have a good reputation for robustness and accuracy and they are first choice for a range of applications requiring high reliability in challenging environments.

Several types of sensors are based on inductive principles, including simple proximity switches, variable inductance sensors and synchros. These offer a range of performance in respect of cost, resolution and linearity.

Typical examples are LVDTs for linear measurement and RVDTs for rotary.  In these sensors, primary coils excite secondary coils in the housing of the sensor. The EMF in the secondary coils will then vary in proportion to their coupling to a moving ferromagnetic core. By using multiple receive coils and calculating the position based on the ratio of signal in each coil confounding factors like temperature can be removed.

Inductive sensors are unaffected by dirt and moisture and as the sensing components can be located well away from sensitive electronics, are well suited to very harsh environments.

They are not without drawbacks; some types of sensor need shielding from stray magnetic fields, for LVDTs/RVDTs the precision windings make them bulky and relatively expensive, limiting their use to higher value applications in areas like aerospace and process industries.

Resonant inductive position sensing

DualAxisSensing 1100

A modern evolution of inductive sensors that is gaining traction in many applications, resonant inductive position sensing reduces the drawbacks of bulk and cost by embedding the coils in PCB. This offers greater precision at a much reduced cost and footprint.

By using an electrical resonator in the target, the system can use a pulse echo approach – the resonator is inductively powered by an excitation circuit, then the power is removed and the system detects the response of the position sensing coils to the target. This removes cross coupling errors from the system and produces much better signal and higher tolerance for misalignment, free from any calibration.

Overall, resonant inductive position sensing is very well suited for any application which requires high performance – it can be engineered to very high levels of linearity and offers high sample rates. It is even more robust than traditional inductive position sensing approaches, requiring little protection from dirt, moisture, vibration or shock.

For high value, high volume applications such as automotive, pan and tilt control, motor control or intelligent valves, this new approach can unlock higher levels of performance.

4      What better position sensing can offer your product

It’s most important when specifying a sensor to be completely clear about what the sensor is required to do, as well as the additional cost to the design of ensuring that the sensor performs as specified. Protection, connections and processing all add additional costs.

On the other side of the equation, as well as identifying additional costs it’s also important to be consider additional performance in the system as a whole that can be unlocked by the right sensor, and add these into your requirements.

Better understanding of position can allow for a range of system improvements, such as:

  • Better position measurement (more accurate, more timely, more direct) allows control systems to match inputs with more precision, producing efficiency wins.
  • Important in motor control for applications like low speed tracking systems for surveillance cameras
  • Reliability/durability
  • Offering product analytics and services through the collection of better data

Position sensor specification checklist

Having narrowed down your choices, a thorough review of the products available would consider:

    1. Geometry
    2. Footprint
    3. Full scale
    4. Type of measurement – absolute/incremental
    5. Resolution (the smallest difference that can be measured)
    6. Precision (aka repeatability) how closely a sensors measurements repeat each other
    7. Linearity (how closely a sensor’s measurement reflects the true value of what is being measured).
    8. Operating temperature range and temperature stability
    9. Electrical supply and output (eg 4-20mA)
    10. Connections (eg SSI)
    11. Sampling rate and speed of response

5. Find out more

We hope the guidelines above have helped you get a clear perspective on the requirements for your position sensor and the class of solution that might offer the best fit.

If you’d like to specify a CambridgeIC solution we’d be delighted to help you with any questions about the performance of our solutions in a wide range of applications.

We offer off the shelf sensor designs and position sensing ICs for a wide range of designs, and can support you with designs for your application if our off the shelf solutions don’t suit.

To find out more, download a data sheet, order an evaluation kit or speak to an engineer on our website. Or get in touch!