Rotary encoders monitor the position and movement of electric motor shafts – but how do they work?
And what kind do you need?
What is an Encoder?
An encoder is a device used to provide feedback to a control system by sensing a change in position. Rotary encoders work with electrical motors to sense rotational motion related to the motor shaft. Depending on the type of encoder used, they can provide information about change in position, absolute position, and even the speed of the shaft.
Encoders are invaluable to the small servo motors that function with great precision and accuracy in a 3D printer. They are just as critical to the massive motors used with cranes to lift heavy, cumbersome objects. In fact, encoders are found in just about every industry, from petrochemical to paper and pulp manufacturing.
How Does an Encoder Work?
We’ve already talked about the fact that a rotary encoder tracks the position and speed of a motor shaft. Based on that data, it generates a closed-loop feedback signal. That signal is then used by a control system to make decisions about motor operation parameters.
There are several different technologies that an encoder can use to generate the feedback signals. These include resistive, magnetic, mechanical, and optical. Of these, optical is the easiest to understand: the encoder provides speed and position feedback based on the interruption of light.
To understand how encoders work, we can look at a basic optical encoder. An optical encoder system consists of the following components:
- Code disk, which has a pattern that represents different positions of the shaft
- Light source, which illuminates the code disk
- Photodetector assembly, which generates a signal based on the pattern detected from the code disk
- Electronics board, which is interprets the signal generated by the photodetector assembly and forwards it to a motion control system
- Motion control system, which adjusts motor parameters based on the feedback received from the rotary encoder
As another more advanced example, consider a magnetic encoder. In a permanent magnet motor encoder, this feedback signal ensures that the motor stator and rotor positions are synchronized to the drive-supplied current. Current is applied to the windings with the rotor magnets fall within a proper position range so that motor torque is maximized.
Encoder Mounting Options
There are three basic mounting options for a rotary encoder: hub/hollow shaft, shafted, and bearingless. The hollow shaft encoders are directly mounted to the shaft using a simple spring-loaded tether. This makes them easy to install but it is important to ensure that the encoder remains electrically isolated from the motor itself.
In a shafted encoder mount, a coupling connects the motor shaft and the encoder shaft. One of the benefits of this approach is that it allows the encoder to be both electrically and mechanically isolated from the motor. One the other hand, the coupling and longer shaft will increase the cost.
Encoder Signal Types and Their Applications
There are two types of signals that a motor encoder can produce: incremental and absolute. Incremental signals indicate that the position has changed, but do not provide any information about specific position. Incremental encoders are often used in industrial applications with an AC induction motor. They are chosen when budget limitations make cost an issue or when only relative position information is needed.
Absolute signals, however, indicate that both that the position has changed and what the absolute position currently is. This is accomplished by generating a different binary output (sometimes called a “word”) for each position. Absolute encoders are used when a high degree of accuracy and precision is needed, but they aren’t going to be as robust as their incremental counterparts. Also, absolute encoders will often be seen in servo applications with permanent-magnet brushless motors and are used when precision and accuracy are critical.
Encoders are a powerful tool for electric motors, but with so many options regarding the sensing technology, absolute or relative position, and mounting options it can quickly become a challenge to find an appropriate encoder for your applications. Here at HECO, we can leverage our knowledge and experience to help you find the right encoder solutions for your motors. Contact us today!
Author:Hunter Shields, [email protected];312-415-2096