Encoders are motion-sensing devices used in a closed-loop control system for feedback. There are multiple types of encoders currently being used in different types of motions. The selection of an appropriate encoder for your system could be a challenging task. Encoders can be classified in different ways based on the type of motion, technology, number of parameters, etc. The article differentiates between multiple types of encoders.
Selection Consideration
The encoder is selected based on the requirements of the application. The desired output and environmental conditions are also considered. The consideration includes.
- What kind of motion is being measured i.e., linear, rotational?
- Which parameters are required to measure i.e., position, speed, angle?
- What environmental conditions like dust, vibration, chemicals, shocks, and temperature will the encoder face?
- Is the re-homing of the encoder required in case power outage or not?
- What should be the required resolution of the encoders?
- Which packaging is best suited for your application?
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Types of Encoders by Motion
The most common way to characterize the encoders is by the type of motion the encoder is monitoring. As we all know there are three basic types of motions i.e., linear, rotary, and oscillatory motion.
Linear Encoders
Linear encoders deal with motion along a line and find out the distance between the two points. The linear encoder uses cables or rods as transducers to generate analog or digital output accordingly.
Rotary Encoder
The rotary encoder helps monitor the rotational motion of the position. The most common design is a thru-bore encoder that can be directly mounted over the shaft.
Angle Encoder
Angle encoders are like rotary encoders that can accurately measure the angle of the rotating object. They are used in an oscillatory object or rotating object for measuring angular movement.
Types of Encoders by Position
All the linear and rotary encoders can be classified as an absolute or incremental encoder. Absolute encoders support rehoming in contrast to incremental encoders. Let’s dive in.
Absolute Encoder
Absolute encoders generate specific digital signatures on every position. In case of a power outage, the position of the encoder remains intact. Such encoders are used where a pause in working is common practice or the process is slow. The absolute encoder can identify the current position of the object by reading the current digital signature.
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Rotary absolute encoders are available in single-turn and multi-turn designs. Single-turn design can be used only if the object’s maximum rotation is up to one turn. In contrast to single-turn design, multi-turn encoder design can support any number of object rotations.
Incremental Encoder
In contrast to absolute encoders, incremental encoders do not generate specific digital signatures. Rathe the incremental encoder only relies on courting the pulses. In case of a power outage, the incremental encoders are unable to monitor the object’s position. The encoder should be referenced back to the home position for correction.
Types of Encoders by Sensing Technology
Encoders can be classified based on how the encoder senses the motion and speed. Some of the most common sensing technologies are listed below.
Optical Encoder
As the name suggests, optical encoders use light to monitor the motions of the object. Optical encoders consist of opaque and translucent blocks on the linear scale or rotary disks for linear and rotary encoders respectively. The translucent block passes the light and the opaque reflects the light to the light-sensing device called a photosensor. Based on the reflected light pulses are generated that help the encoder monitor the motion.
Magnetic Encoders
The magnetic encoder uses a magnetic disk with poles on its surface for the rotary encoder. As the disk rotates the changing magnetic flux generates a sinusoidal voltage in the nearby sensor that is based on the Hall effect. Afterward, the waveform is converted into a square wave for counting purposes.
Capacitive Encoders
The capacitive encoder uses changing capacitance with a high-frequency reference signal to detect motion. The capacitive encoder consists of a stationary transmitter and receiver for high-frequency signals. Where a sinusoidal pattern is stamped over the rotor. When the HF signal passes through the rotor, it modulates the AC signal by changing the capacitive reactance.
Types of Encoders by Channel
Encoders can be classified based on the number of channels. Adding another channel can help in detecting another dimension of information.
Single Channel Encoder
A single-channel encoder can only monitor motion and movement. But it cannot detect the direction of motion. For example, rotary encoders can generate the same signal clockwise and counterclockwise.
Quadrature Encoder
Quadrature encoders add another channel for the detection of the direction of the motion. Both channel signal is completely out of phase. For example, in the rotary quadrature encoder, channel A will lead the channel B signal in the case of a clockwise direction and vice versa.