A Guide to Position Sensors

This article provides clarifications on the terminology used when discussing position sensors.

Different terms are used for ‘sensor’ – these include detector, encoder, transmitter, transducer, and even sender. For all intents and purposes, these terms refer to the same thing, albeit some differences do exist. The universal term ‘sensor’ is used in this article.

To add to the confusion, certain sensors - especially proximity sensors – are in fact proximity switches, because they are designed to detect the presence or absence of an object. This means that proximity switches only create a simple on/off or digital output and not an uninterrupted measurement of position.

Instead of focusing on switches, this article concentrates on true sensors – sensors that generate a signal (typically electrical) that is proportional to its position along a measurement path.

In addition, there are other terms that refer to (linear and rotary) position – angular position, displacement, rotary, rotation, linear and angle. In this article, the universal term, ‘position’ covers both angular and linear geometries.

Figure 1. Rotary and linear position sensors

Many position sensors can also be considered as speed or velocity sensors. Since speed or velocity is the rate of change of position any position sensor with a frequently updated position can also be considered a speed sensor. Through modern control systems, speed can be easily determined by differentiating the output of the sensor with respect to time or, by simply counting positional changes relative to time.

Position sensors can be categorized as absolute or incremental. Whenever there is a change in position, the output from an incremental sensor also changes. Whereas, a signal generated from an absolute sensor is proportional to its true position, whether it is moving or stationary. To establish if a sensor is incremental or absolute, it is important to consider what happens at power up – the sensor is an absolute sensor when there is a true position signal without motion.

Position Measurement Basics

Instrumentation theory – resolution, accuracy, repeatability and so on – is normally forgotten. While the terminology used in instrumentation and some cryptic technical concepts can be confusing, the theory is essential when choosing an appropriate position sensor for a specific application.

If the selection is wrong, the position sensor selected could turn out to be very expensive, or the product control system may not have the required critical performance.

Some more definitions are clarified here:

  • The accuracy of a sensor is a measure of output’s veracity
  • The resolution of a sensor is a measure of the smallest positional decrement or increment that can be measured
  • The precision of a sensor is its degree of repeatability
  • The linearity of a sensor is the difference between the actual position being measured and the sensor’s output

The difference between precision and accuracy can be explained with the analogy of an arrow fired at a target. Accuracy denotes how close the arrow is to the bullseye. If many arrows are shot, precision denotes the size of the arrow cluster. If the arrows are close to one another, the cluster is known as highly repeatable, or precise.

Figure 2. An accurate shot (left) and precision shooting (right)

A perfectly linear position sensor is perfectly accurate. For most applications, linearity is believed to be equivalent to accuracy. However, it is not as easy as specifying accurate sensors every time.

The optimal strategy is to specify only what is needed – nothing more, nothing less. For example, in a position sensor meant for an industrial flow meter, linearity is not a major requirement because the flow properties of the fluid will be highly non-linear. Instead, repeatability over fluctuating environmental conditions is likely to be key. The usual key requirements are resolution and repeatability (instead of linearity) in many engineering applications.

For example, the key requirements in a CNC machine tool are precision and accuracy. As a result, a position sensor with a high repeatability, high accuracy (linearity), and high resolution, which are retained in wet and dirty environments over extended periods of time (with no maintenance) are the main requirements.

Common Types of Position Sensor

Position sensors are employed in a wide range of industrial and commercial applications, from high-end military and defense applications to consumer appliances and low-cost automotive applications. Following temperature measurement, position measurement is the most common property to be measured.

Figure 3. Position sensors are used in military and defense applications.

It can be difficult to find the right position sensor as there are so many different types available. The following section highlights the major types of sensor along with their strengths and weaknesses.

Potentiometers

The most common position sensors are potentiometers (‘pots’), although these are slowly losing favor to non-contact sensors.

Pots determine a voltage drop when an electrical contact slides on a resistive track, meaning that the sensor position is in proportion to its voltage output. Pots come in linear, rotary, or curvilinear forms, and are usually compact and lightweight. While a basic potentiometer will not be expensive, a high precision model may cost more than $200 US. Less than 0.01% linearities can be achieved by laser trimming the resistive tracks.

Figure 4. A typical single turn potentiometer

Potentiometers work well in applications with relaxed performance, modest duty cycles and gentle environments. However, they are prone to wear in high-vibration environments and/or with foreign particles such as dust or sand abrading the resistive track. While high-quality devices claim extended service life in terms of the number of cycles, this often ignores the effect of vibration.

Potentiometers usually claim to have ‘infinite resolution’. Although this is possible from a theoretical standpoint, many control systems require digital data and therefore the true resolution is that of the analog-to-digital converter (which also has to be considered in cost calculations).

In certain safety-related applications in the petrochemical, aerospace and medical industries, pots are grouped as ‘simple devices’. This means that while pots are susceptible to different types of failure, they are not subjected to the same rigorous design and selection scrutiny as electronic sensors by the certifying bodies. This can make it complicated to substitute unreliable pots.

Magnetic

A similar measuring principle is used by all magnetic sensors: when a magnet moves with respect to a magnetic detector, the magnetic field changes in proportion to their relative displacement. Hall Effect devices are a common form of a magnetic sensor and are available in chip form. Hall Effect devices are generally used in automotive and electric motor applications that require modest measurement performance.

Figure 6. Hall Effect sensors are the most common type of magnetic sensor.

Magnetic sensors are more tolerant to foreign matter and hence do not have the disadvantages associated with optical devices. These sensors, however, are not often used for high accuracy applications due to magnetic hysteresis as well as the need for precision mechanical engineering between the moving and stationary parts.

Moreover, the data sheet of a magnetic sensor should be carefully studied with respect to the temperature coefficient, operating temperature and installation tolerances.

Another consideration is proximity of magnetic materials or electrical cables. One major source of failure is the accumulation of particulates or swarf over time caused by the magnetic attraction of certain foreign particles. As modern NdFeB magnets are notoriously fragile, magnetic sensors cannot be used for applications involving impact or shock conditions.

Strengths: Most liquids have no effect; fairly robust.

Weaknesses: Hysteresis; temperature; nearby steel/DC sources and poor impact/shock performance; precision mechanical engineering.

Magnetorestrictive

A unique phenomenon called ‘magnetostriction’ (present only in some materials) is used by magnetorestrictive sensors. Magnets cause energy passing along a material to reflect when it comes to the end of the material. A thin strip or wire is often employed as a magnetostrictive strip, and the position can be determined from the time it takes a pulse of energy to move along and back in the thin strip.

Figure 7. Magnetostrictive sensors are nearly always linear.

Since the fragile magnetostrictive strip needs to be cautiously held in a housing such as an aluminum extrusion, nearly all magnetostrictive sensors are linear. As a result of the housing, magnetostrictive devices do not experience wear or lifetime issues, and they are suited for high pressure applications including hydraulic rams. The precision housing and manufacturer’s calibration make magnetostrictive sensors quite expensive.

The method is also sensitive to any other effects on the time of flight – especially temperature. The data sheets for magnetostrictive sensors usually quote accuracy at a constant temperature, meaning design engineers need to use the quoted temperature coefficient to perform their own calculations.

Compact magnetostrictive sensors are fragile and the mounts at the two ends of its length are critical. Therefore, magnetostrictive sensors are not suited for harsh shock or vibration environments.

Strengths: Well suited to high pressures; robust; percentage accuracy increases with length.

Weaknesses: Fairly expensive; temp. effects; shock; inaccurate over short distances (<100 mm).

Capacitive

A capacitor is an electrical device where charge accumulates; they usually consist of an insulator that separates two conductive plates. The amount of charge stored by the capacitor varies based on the size of the plates, their percentage overlap, their separation and the material’s permeability between the plates.

In its simplest form, a capacitive position sensor measures the separation of plates. Usual displacements are less than one millimeter for load, strain and pressure measurement.

Another type capacitive sensor can be used for linear or rotary position sensing. Here, an array of plates is cut or etched along the measurement axis. The circuits’ capacitance along the axis changes when another plate moves across them, indicating the relative position of the two parts.

Capacitive position sensors are not common and are rarely used in safety-related applications. In addition to overlap of the plates, capacitance varies with temperature, humidity, the surrounding materials and foreign matter, which makes it more difficult to design a high accuracy, stable position sensor.

Among experienced engineers, capacitive sensors are known to have a poor reputation and are not likely to be selected for safety-related applications. To add confusion, some manufacturers do not refer to them as ‘capacitive’ and instead use other terms such as electric effect, charge coupling or charge storage.

Several different things can go wrong with capacitive sensors, so it is best to avoid them unless there is a need for accurate measurements in highly stable and clinical applications.

Strengths: Low power; compact.

Weaknesses: Sensitive to foreign matter; significant temperature and humidity coefficients; tight installation tolerances.

Traditional Inductive

Used for more than 100 years, conventional inductive position sensors are based on inductive or transformer principles. They are relatively safe and reliably operate in harsh conditions and are virtually an automatic choice in a number of safety-related applications.

Figure 8. Traditional inductive sensors have an excellent reputation for safe and reliable operation.

Linear inductive position sensors are also called variable reluctance or linearly variable differential transformers (LVDTs). Rotary forms are referred to as RVDTs, resolvers or synchros. LVDTs use a transformer construction with a minimum of three wire spools – a primary and two secondaries.

As the rod moves, the electromagnetic coupling between the primary and secondary spools also changes. The ratio of the induced signals indicates the rod’s position with regards to the spools. This ratiometric technique enables the high stability and measurement performance of the LVDT.

Inductive sensors are capable of displacing the electronics away from the sensing area, unlike magnetic and optical sensors, which need electronic circuitry adjacent to the sensing point. This allows inductive sensors to be located in adverse environments with their electronics in benign locations. However, due to their wound transformer design, inductive sensors are large, bulky and expensive.

Strengths: High accuracy; robust; reliable; widely available; extreme environments.

Weaknesses: Heavy; expensive; bulky.

New Generation Inductive or Incoders

A new generation of inductive sensors, commonly known as incoders, use the same principles as conventional inductive sensors, and hence are able to provide good, non-contact measurement performance in rugged environments. These sensors use printed circuits on flexible or rigid substrates, instead of bulky spools of wire.

Other advantages of the printed windings design are as follows:

  • Greater flexibility in form factor
  • Huge reduction in production cost, size and weight
  • Elimination of sources of inaccuracy by the winding process
  • Complex measurement geometries including curvilinear, 2D and 3D position sensing
  • Numerous sensors can be placed in the same space using multi-layer circuit boards (for example, redundant sensors in safety-related applications)

Figure 9. Example of new generation Incoders

Usually, EMC performance is equal to that of LVDTs or resolvers. This is demonstrated by the increasing choice of next-generation inductive devices for aerospace and military applications.Strengths: High accuracy; robust; reliable; multiple geometries; lightweight; compact.

Weaknesses: More expensive than potentiometers

Celera Motion

This information has been sourced, reviewed and adapted from materials provided by Celera Motion.

For more information on this source, please visit Celera Motion.

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