Basics of Encoders for Motion Systems

From Advanced Micro Controls Inc. (AMCI)

Editor’s Note:  This article is excerpted from the white paper, “Encoders and Resolver Based Encoder Technology,” from Advanced Micro Controls Inc. (AMCI). Download the full paper to learn about common types of encoders, choosing the correct electrical interface, mechanical considerations, resolver-based encoders, communicating to the PLC, application examples, case studies and how to select an encoder.

A modern control system is fundamentally a feedback control system, such as a push button that executes an operation that eventually starts a motor. That push produces an output that rotates a shaft. A feedback loop feeds information back to the controller to verify the operation and make the necessary corrections.

That feedback can be, but is not limited to, limit switches, temperature sensors, or in some cases, position feedback. Position feedback can be used to monitor or adjust the motor.

Multiple ways exist to get position feedback, but the most common types are rotary sensors, encoders, resolvers or even potentiometers. For linear applications, magneto restrictive sensors or linear variable transformers (LVTs) are commonly used. And for noncontact applications lasers are often used to provide position feedback for linear measurement.

Encoders are critical elements in a motion system because they provide position and/or velocity feedback to the controller. Simply put, an encoder is a sensor that converts mechanical movement into an electrical signal and relays that data to other devices.

The most common method is optical position decoding. This consists of a coded disc or linear slide, an LED, and a light-sensing circuit. As the coded disc rotates, the light is shuttered from the LED and is received and transmitted by the sensing circuit as a square wave.

Let’s start by looking at the basics of encoders. You can download the white paper to get a deep dive into types to use for various applications.

Common Types of Encoders

The four common types of encoders are rotary, linear, incremental and absolute.

1. Rotary. A rotary encoder converts angular position in an analog or digital signal. It works best for applications where angle of the shaft, number of turns for a rotating axis, or velocity data of an axis are needed.
2. Linear. Linear coders convert linear distance movement to an electrical signal. It’s selected to measure distance traveled or linear position information. Linear encoders are available in a wide range of options, but here they are reduced to three basic types.
3. Incremental vs. 4. Absolute. Incremental encoders produce electrical pulses, or counts, with a change in position. The signals are typically square wave but can be sinusoidal.

Incremental encoders do not retain position after a power failure or cycle. They can be thought of as sensors that monitor a relative change in position and not a sensor that reports the actual position. These encoders are typically used to monitor speed or applications where absolute position is not required.

Absolute encoders, on the other hand, provide a unique position value for every shaft or linear position. Absolute encoders retain their positions during a power cycle even if there’s movement and the position changes. It always knows where it is.

Signals are available in ever-growing varieties, but the most common types are Synchronous Serial Interface (SSI), parallel, networked or fieldbus interfaces (EtherNet/IP™, EtherCAT^®, PROFINET^®, DeviceNet™, CANopen^®, etc.). Absolute encoders are used in applications where current position information is required. 

With global headquarters in Terryville, Connecticut, Advanced Micro Controls Inc. (AMCI) is a Rockwell Automation Technology Partner. The company designs and manufactures specialty I/O modules, position sensing solutions and motion control technology. Position sensing technology includes Resolver, SSI, LDT and high-speed programmable limit switch controllers. Motion control technology includes stepper/servo controllers, drives and motors.

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