What is an Encoder?

If you search the definition of an encoder, you will get a vast and confusing array of responses. For our purposes, encoders are used in machinery for motion control. Encoders are found in machinery in all industries. You will find encoders used in cut-to-length applications, plotters, robotics, packaging, conveying, automation, sorting, filling, imaging, and many, many more.

What is an Encoder?
You may have never noticed them, but they are there. In this blog post, we will give you a very basic introduction into what an encoder is and what it does.

Simply put, an encoder is a sensing device that provides feedback. Encoders convert motion to an electrical signal that can be read by some type of control device in a motion control system, such as a counter or PLC. The encoder sends a feedback signal that can be used to determine position, count, s peed, or direction. A control device can use this information to send a command for a particular function.

For example:

• In a cut-to-length application, an encoder with a measuring wheel tells the control device how much material has been fed, so the control device knows when to cut.
• In an observatory, the encoders tell actuators what position a moveable mirror is in by providing positioning feedback.
• On railroad-car lifting jacks, precision-motion feedback is provided by encoders, so the jacks lift in unison.
• In a precision servo label application system, the encoder signal is used by the PLC to control the timing and speed of bottle rotation.
• In a printing application, feedback from the encoder activates a print head to create a mark at a specific location.
• With a large crane, encoders mounted to a motor shaft provide positioning feedback so the crane knows when to pick up or release its load.
• In an application where bottles or jars are being filled, feedback tells the filling machines the position of the containers.
• In an elevator, encoders tell the controller when the car has reached the correct floor, in the correct position. That is, encoder motion feedback to the elevator's controller ensures that elevator doors open level with the floor. Without encoders, you might find yourself climbing in or out of an elevator, rather than simply walking out onto a level floor.
• On automated assembly lines, encoders give motion feedback to robots. On an automotive assembly line, this might mean ensuring the robotic welding arms have the correct information to weld in the correct locations.

In any application, the process is the same: a count is generated by the encoder and sent to the controller, which then sends a signal to the machine to perform a function.

How Does an Encoder Work?

Encoders use different types of technologies to create a signal, including: mechanical, magnetic, resistive and optical – optical being the most common. In optical sensing, the encoder provides feedback based on the interruption of light.

The graphic at right outlines the basic construction of an incremental rotary encoder using optical technology. A beam of light emitted from an LED passes through the Code Disk, which is patterned with opaque lines (much like the spokes on a bike wheel). As the encoder shaft rotates, the light beam from the LED is interrupted by the opaque lines on the Code Disk before being picked up by the Photodetector Assembly.

This produces a pulse signal: light = on; no light = off. The signal is sent to the counter or controller, which will then send the signal to produce the desired function.

What is the Difference Between Absolute & Incremental Encoders?

Encoders may produce either incremental or absolute signals. Incremental signals do not indicate specific position, only that the position has changed. Absolute encoders, on the other hand, use a different "word" for each position, meaning that an absolute encoder provides both the indication that the position has changed and an indication of the absolute position of the encoder.