Encoders are fundamental sensing components used to measure position, speed, and direction of motion in automation, robotics, CNC machinery, material handling, and test systems. They translate mechanical movement into electrical signals that control systems rely on for feedback, synchronisation, and safety.
While encoders are often grouped by type, correct selection depends on resolution, signal output, environmental conditions, and integration with control electronics. This guide is intended to help engineers and procurement teams move from basic identification to confident, application-appropriate specification.
What is an encoder?
An encoder is a sensor that converts rotational or linear motion into electrical signals that can be interpreted by controllers such as PLCs, drives, or microcontrollers.
This article focuses on rotary encoders, which are by far the most common form.
What are rotary encoders?
Rotary encoders are precision electromechanical sensors that measure the angular position, speed, and direction of a rotating shaft. They function by converting rotational mechanical displacements into electrical signals that can be interpreted by controllers such as PLCs, drives, or microcontrollers. These devices are essential for providing the feedback and synchronization required in systems ranging from industrial robotics and CNC machinery to everyday consumer electronics like volume knobs.
What is the primary function of an encoder in a motion control system?
An encoder acts as a bridge between physical movement and digital intelligence. It converts mechanical motion, such as the rotation of a motor shaft or the travel of a linear slide—into electrical signals that represent position, speed, and direction. This data allows a controller to "see" exactly what the hardware is doing in real-time.
Key measurement outputs include:
- Angular position
- Rotational speed
- Direction of rotation
Rotary Encoder types
Incremental Encoders
Incremental encoders generate a series of pulses as the shaft rotates. Position is determined by counting pulses from a known reference point.
Typical features:
- Channels A and B for quadrature output
- Optional index (Z) pulse for reference
- Simple, cost-effective implementation
Incremental encoders are widely used where:
- Relative position is sufficient
- Homing routines are available
- System simplicity is a priority
Optical Encoders
Optical encoders use a light source, code wheel, and photodetector to generate high-resolution signals. As the wheel rotates, light is interrupted in precise patterns, producing clean, repeatable pulses.
Advantages:
- High resolution and accuracy
- Low signal jitter
- Excellent repeatability
Limitations:
- Sensitivity to dust, oil, and condensation
- Typically lower environmental robustness than magnetic alternatives
Absolute vs Incremental (Contextual comparison)
While this category focuses on incremental encoders, it is worth noting:
- Absolute encoders provide a unique position value at all times
- Incremental encoders require a reference on power-up
Understanding this distinction helps prevent mis-specification in safety-critical or power-loss-sensitive systems.
What is the difference between incremental encoders and absolute encoders in motion control?
Incremental encoders track changes in position relative to a starting point and typically require a "homing" sequence after a power loss. Absolute encoders provide a unique digital code for every specific position, allowing the system to retain its exact location data even if the power is cycled or the machine is moved while turned off.
Which encoder types are recommended for robotics applications?
- Absolute encoders - often the primary choice for robotics because they provide immediate position data upon power-up.
- Optical encoders - recommended for high-end robotics, such as pick-and-place systems and robotic surgery, where unmatched resolution and repeatable accuracy are required.
- Incremental encoders - suitable for mobile robotics to monitor wheel speed, gearbox performance, and acceleration where absolute positioning is less critical than real-time velocity feedback.
Resolution, accuracy & signal integrity
Resolution (PPR / CPR)
Resolution is commonly expressed as:
- Pulses per revolution (PPR)
- Counts per revolution (CPR, after quadrature decoding)
Higher resolution improves positional accuracy but increases:
- Signal frequency
- Controller processing load
- Susceptibility to electrical noise
Signal output types
Common incremental encoder outputs include:
- TTL / RS-422
- Open collector
- Line driver differential outputs
Differential outputs are strongly recommended for:
- Long cable runs
- High-noise industrial environments
Mechanical & environmental considerations
Shaft & mounting options
- Solid shaft vs hollow shaft
- Clamping vs set-screw fixation
- Tolerance to misalignment
Incorrect mechanical coupling is a leading cause of bearing wear and premature failure.
Optical encoders should only be specified where environmental conditions are controlled or appropriately sealed.
Encoders are often embedded deep within systems, making replacement costly. Selection should consider:
Higher resolution and optical sensing increase cost but may not deliver value if:
Balanced specification reduces over-engineering and improves system reliability.
Encoders are precision components that directly influence system accuracy, reliability, and control performance. By evaluating encoder type, resolution, signal output, mechanical integration, and environmental suitability, engineers and buyers can confidently specify solutions that meet both technical and operational requirements.
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