{"id":3874,"date":"2020-05-08T08:45:53","date_gmt":"2020-05-08T08:45:53","guid":{"rendered":"https:\/\/novanta.com\/robotics-automation\/technical-paper\/position-sensors\/"},"modified":"2026-04-23T17:41:17","modified_gmt":"2026-04-23T17:41:17","slug":"position-sensors","status":"publish","type":"novanta_tech_paper","link":"https:\/\/novanta.com\/robotics-automation\/technical-paper\/position-sensors\/","title":{"rendered":"Types of Position Sensors: A Practical Selection Guide"},"content":{"rendered":"\n
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Position sensors are used in a wide range of automation and measurement applications. A key step in selecting a suitable position sensor is understanding the requirements of sensor size, resolution, repeatability, accuracy, mounting constraints and environmental ruggedness. This paper discusses the available position sensing technologies and concludes with a key feature comparison.<\/strong><\/p>\n\n\n\n

This paper explores various position sensing technologies and concludes with a key feature comparison, making it an ideal resource for engineers, technicians, and students. It is especially useful for those looking to quickly grasp the fundamentals of position sensing and position sensors.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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Position Sensors \u2013 Key Terminology & Definitions<\/strong><\/h2>\n\n\n\n

Engineers love jargon \u2013 it helps us differentiate engineers from mere mortals. Unfortunately, jargon also makes it difficult for a competent engineer from one area of engineering to get to grips with another. Position sensing is no exception, so let\u2019s start with some clarifications on terminology.<\/p>\n\n\n\n

All position sensors can be classed as either absolute <\/b>or incremental<\/b><\/a>.<\/p>\n\n\n\n

Incremental Sensor<\/strong><\/h3>\n\n\n\n

An incremental sensor provides position change information only. This means that the actual position of the object is unknown at startup. A once-per-revolution index\/marker signal is used to define the zero position or null of the device, detected during a homing routine. In brushless motor commutation, three magnetic Hall Sensors provide coarse absolute position information for preliminary alignment of the magnetic fields. Incremental sensors are typically small, accurate and cost-effective.<\/p>\n\n\n\n

Absolute Sensor<\/strong><\/h3>\n\n\n\n

An absolute sensor provides the actual physical position of the object within one revolution or within the range of linear travel. The motor does not require Halls and homing is only necessary for rotary applications if the range of movement exceeds one revolution. The sensors are usually bigger and more expensive than incremental devices.<\/p>\n\n\n\n

How to tell whether a sensor is absolute or incremental:<\/strong><\/h4>\n\n\n\n

A good test to determine whether a sensor is absolute or incremental is to consider what happens at power up. If there\u2019s a true position signal without any motion, then it\u2019s an absolute sensor<\/p>\n\n\n\n

Multi-Turn<\/strong><\/h3>\n\n\n\n

Rotary devices, the sensor provides actual position over multiple revolutions. Homing can be completely eliminated. Multi-turn devices incorporate internal gearing and are the most bulky, expensive solution.<\/p>\n\n\n\n

Resolution<\/strong><\/h3>\n\n\n\n

Defines the smallest position increment that can be moved or measured and is typically expressed in “counts”. High resolution is required for high performance servo systems. A positioning system “dithers” between two counts so the higher the resolution the smaller the dither. Resolution also has a significant impact on velocity ripple at low speed. Since velocity is derived from position feedback, if the resolution is low there may be insufficient data in a sample to accurately derive velocity. At high speeds, high resolution devices can generate data rates beyond the tracking capability of the controller or servo drive.<\/p>\n\n\n\n

Interpolation<\/strong><\/h3>\n\n\n\n

As will be seen, many sensors generate sine and cosine signals. The period of these signals is defined by the inherent “pitch” of the device. With sin\/cos information it is theoretically possible to have infinite resolution by computing the ratio of the signals. This technique is known as interpolation. In practice, the fidelity of the sin\/cos signals and signal to noise ratio limit the realizable resolution.<\/p>\n\n\n\n

Accuracy<\/strong><\/h3>\n\n\n\n

Defines how close each measured position is to the actual physical position. Accuracy is very much a system issue and can be dominated by mechanical errors such as eccentricity, straightness and flatness. Sensor errors include non-accumulating random variations in pitch (linearity), accumulating pitch errors (slope) and variations in fidelity of internal sin\/cos signals. Precision machine builders typically calibrate out errors via a lookup table of offsets. More detail can be found in our Technical Paper \u2018Understanding Resolution, Accuracy, and Repeatability\u2019.<\/p>\n\n\n\n

Repeatability<\/strong><\/h3>\n\n\n\n

Defines the range of measured positions when the system is returned to the same physical position multiple times. Repeatability can be more important than absolute accuracy. For system inaccuracies to be effectively calibrated it is important for each position reading to be consistent. Sensor hysteresis (different readings depending on direction of approach to measure position) is an important factor in repeatability.<\/p>\n\n\n\n

Modular<\/strong><\/h3>\n\n\n\n

The most common form of rotary feedback device is packaged in an enclosure with internal bearings and a shaft for connection to a motor via a flexible coupling. The enclosures are available with a range of sealing ratings and are bulky. Modular devices have no enclosure or bearings and must be built into the mechanical system. They are significantly more compact but may require a more benign environment depending on the technology.<\/p>\n\n\n\n

On\/Off Axis<\/strong><\/h3>\n\n\n\n

For rotary applications the sensor is typically positioned off-axis on the circumference of a scale which wraps around the axis of rotation. Some implementations position the sensor on-axis minimizing size when radial space is constrained.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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Common Types of Position Sensor<\/h2>\n\n\n\n

Position sensors are used in a wide variety of industrial and commercial applications, from high end military and defence applications through to low cost automotive and consumer appliances. In fact, after temperature measurement, position measurement is the second most common property that we need to measure in our lives.<\/p>\n\n\n\n

There is such a wide array of position sensors to choose from nowadays, so how do you know which is right for you? This section outlines the main types of sensor and their respective strengths and weaknesses.<\/p>\n\n\n\n

Potentiometers<\/h3>\n\n\n\n

Although there is a trend towards non-contact sensors, potentiometers (\u2018pots\u2019) are still widely used in lower-end applications. Pots measure a voltage drop as a contact(s) slides along a resistive track. They are available in rotary, linear or curvilinear forms and are generally compact and lightweight. A simple device will cost pennies, whereas a higher precision version may cost upwards of $200. Linearity of less than 0.01 per cent is possible by laser trimming the resistive tracks.<\/p>\n\n\n\n

Potentiometers are best applied in lower performance applications with low duty cycles in benign environments. They are susceptible to wear and foreign particles such as dust or sand. Pots theoretically have infinite resolution but in practice resolution is limited to the analog to digital converter (ADC) interface and the overall noise environment.<\/p>\n\n\n\n

Potentiometers Strengths & Weaknesses<\/h4>\n\n\n\n
Strengths<\/th>Low cost; simple; compact; lightweight. Can be made accurate<\/th><\/tr><\/thead>
Weaknesses<\/td>Wear; vibration; contamination<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Further Reading on Potentiometers<\/strong><\/h4>\n\n\n\n