What is a Rotary Encoder?

Reviewed by Stephen Bettles (Technical Support Engineer, January 2025)

Rotary encoders are electromechanical devices that are widely used in industry to monitor and control equipment. They work by converting the angular position or rotation of a shaft into digital signals or analogue signals (voltage proportional to the shaft’s speed or position). In doing so, they accurately convey information about shaft speed, acceleration, and position.

What does a rotary encoder do? As well as controlling automation, rotary encoders are used for servo motor feedback applications (that is, controlling motor speed/position) for example in packaging, textiles, and materials handling applications.

Other applications for rotary encoders include automated vehicle control and other robotics jobs, radar systems, and rheometers (devices that measure viscosity and viscoelasticity).

What does a rotary encoder do? Rotary encoders enable calculation of shaft position and speed by converting mechanical motions into signals. The analogue or digital signal the encoder generates can be used for feedback on performance or to control equipment. One of the reasons rotary encoders are widely used is because they are simple, cost effective and reliable.

There are a number of rotary encoder technologies available, and they are typically classified as either absolute or incremental types, though both can be optical or magnetic. Read on to learn more.

There are a number of ways that rotary encoders read the disc or wheel attached to the shaft to generate data. One of the most common ways is to use optical sensors. This type is known as an optical rotary encoder design.

What is an optical rotary encoder? In an optical rotary encoder, the disc features apertures: slits or slots through which light can travel. As the shaft spins, the sensor measures light pulses and their interruption by the slits. By tracking these pulses, it’s possible to generate signals that show the exact position and speed of the axle or shaft. The optical rotary encoder uses an LED and a photosensor to operate. What is the output from a rotary encoder that uses a photosensor? Output types include push-pull, TTL and IO link.

Absolute rotary encoders transmit a unique signal that shows not only shaft speed and direction but also absolute position. Absolute rotary encoders retain this position information even when power is stopped, which means that that position data is immediately available when power is reapplied. An absolute rotary encoder is often used in applications that need extremely precise positioning, especially after power failure. It uses discs that feature a distinct set of marks for each shaft position.

One big plus point of absolute rotary encoders is that they do not need to be calibrated on start-up; rather, the relationship between encoder value and machinery is set at the time of assembly, meaning the encoder doesn’t have to be re-calibrated. This is particularly advantageous when it comes to machinery start-up performance. Absolute rotary encoders are also useful in situations where accurate motion detection on multiple axes is needed, they tend to have higher overall resolution than incremental encoders, and they’re better when recovery from power failure is required.

In contrast to absolute rotary encoders, incremental rotary encoders are relatively simple and inexpensive and thus widely used. They are good at providing speed and distance information in servo motor applications and for automation and control. In an incremental optical encoder, a typical assembly will feature a printed circuit board containing a sensor array that generates signals concerning position and speed. As the shaft rotates, information captured by the sensors is translated into pulses. An incremental encoder gives a specified number of pulses for each rotation, plus, commonly a single pulse at a set zero position. Incremental encoders are versatile, cost effective and can be easily customised for a wide range of requirements.

What is the working principle of an encoder if it’s magnetic, on the other hand? Magnetic rotary encoders are often used where high reliability and robustness or small size and light weight are requirements. They work by using magnets and wheels to create outputs that show position and speed. The magnetic rotary encoder detects rotation as changes in a magnetic field and converts that information into electrical signals.

A simple magnetic rotary encoder features a permanent magnet attached to the shaft (a typical example might be the shaft of a motor in a servo motor feedback application) and a magnetic sensor on a printed circuit board (PCB). As the shaft rotates, so does the permanent magnet. The PCB-mounted magnetic sensor detects changes in the direction of the magnetic field, conveying information about rotational speed and position.

What should you be considering when it comes to how to choose a rotary encoder for the job? The number of pulses per revolution must be considered depending on the resolution required, bearing in mind the shaft rpm and the maximum pulses per second the external equipment you are connecting to can accept. The output signal level must also be compatible. Physical properties such as shaft size, IP rating etc. must also be considered. 

Absolute rotary encoders, on the other hand, understand positioning information themselves without the need for extra electronics. They are suited to precision work in applications such as determining orientation of CNC machine tools, where positioning accuracy is crucial.

In general, rotary encoders are widely used in applications where precise control of moving components is needed. As noted, one of the most common is motor control. For example, when the load on a motor increases, slowing it down, the rotary encoder will provide feedback ensuring it speeds up to maintain a constant speed. This is critical in applications such as robots that rely on large numbers of servo motors to operate.

So whether it’s controlling a conveyor belt, setting the position of a machine tool cutting head, or transporting goods across the floor of a warehouse, these versatile devices have a crucial role to play in modern industry.

And whatever the application, if you need rotary encoders, RS can help.

Did you know we also carry rotary encoder brackets and motion control sensors? Explore our range today.