Optical encoders are electronic devices which are used to determine the angular or linear position of an object. They work on the principle of optics and provide high resolution digital outputs. In this article, we will look at the working, types, applications and key features of optical encoders.
At its core, an optical encoder consists of a light source, a scanning element and light sensors. The scanning element is usually a disk or strip with contrast patterns. As the scanning element rotates or moves linearly, it modulates the light from the source which falls on the sensors. This modulation is then decoded by the optical encoder electronics to give a digital output representing the position.
In a rotary Optical Encoders the scanning disk has alternating opaque and transparent tracks. As it spins, the tracks periodically block and pass light to a set of photosensors. By counting the pulses, the rotation angle can be accurately measured. Linear optical encoders work on a similar principle except that the scanning strip moves back and forth rather than rotating. This modulation of light allows optical encoders to precisely sense angular or linear displacements.
Types of Optical Encoders
Based on their scanning element and application, optical encoders are broadly classified into the following types:
Linear Encoders: A linear scanning strip provides positional feedback for linear motion guidelines, stages etc.
Multi-turn Encoders: Used where the angular range exceeds a single rotation. Have additional tracks to count total revolutions.
Absolute Encoders: Encode absolute position within full rotational or linear range using gray/binary code patterns.
Non-contact Encoders: Have an air gap between scanning disk and sensors for applications requiring isolation from environmental factors.
Applications
With their ability to accurately sense position with high resolution, optical encoders find application across many industries:
Robotics: Stepper and servo motor feedback for robot joint position control.
Manufacturing: CNC machines, 3D printers, assembly lines use encoder feedback for precision motion control.
Automotive: ECU programming, steering/transmission position sensing, autonomous driving.
Avionics: Flight control surface position, landing gear, throttle valve angle sensing.
Medical Equipment: Imaging tables, surgical robots, rehab equipment use encoders.
3D Metrology: CMM, 3D scanners employ encoders for precise probe positioning.
Key Features and Benefits
Some key features that make optical encoders a preferred choice over other technologies include:
High Resolution: Up to 12,000 counts per revolution for rotary and nanometer scale for linear encoders.
No Wear & Tear: Being contactless, optical encoders have an unlimited operational life.
Reliability: Robust solid-state design with no moving electrical contacts ensures long term reliability.
Non-Contact Sensing: Requires no physical contact between sensing and measured components.
Speed: Capable of sensing at speeds exceeding 10,000 RPM for rotary and meters/second for linear.
Durability: Sealed construction withstands shock, vibration, moisture, dust in harsh industrial environments.
Precision: Sub-arc-minute accuracy achievable for rotary and better than micrometer for linear encoders.
Feedback Loops: Provide closed loop position control for motion systems requiring high stability and repeatability.
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