Encoder Vs. Linear Scale: Differences And Applications

In industrial automation, encoders and linear scales are the two most widely used position detection devices, serving as the "eyes" of closed-loop control systems. Both enable displacement measurement but differ significantly in working principle, performance characteristics, and application scenarios. Proper selection and application are critical to improving equipment accuracy and stability.

1. Core Principles and Structural Differences

Both devices rely on photoelectric conversion, but their measurement methods and structures are distinct.

Encoder

An encoder is a rotary detection device that measures angle or speed by reading patterns on a rotary code disc.

Mounted directly on the motor shaft, converting mechanical rotation into electrical pulse signals.

Calculates rotation angle and speed by counting pulses.

Compact, easy to install, often integrated inside servo motors.

Core component: rotary code disc.

Linear Scale

A linear scale is a linear detection device composed of a light source, moving scale, fixed scale, and photoelectric sensors.

The moving scale travels with the measured object; the fixed scale remains stationary.

Measures linear position/displacement by detecting relative grating movement.

Outputs 90° phase-shifted square or sine waves for high-precision measurement.

Complex structure with high installation accuracy requirements.

Core component: long linear grating; higher manufacturing precision and cost.

2. Performance Comparison

Parameter Encoder Linear Scale

Resolution Moderate; typically hundreds to thousands of pulses per revolution. Superior; can reach micron or even nanometer level (e.g., 0.1 μm).

Linearity Affected by motor rotation and transmission errors; lower linearity. Excellent; unaffected by mechanical transmission errors; linearity up to 0.01 mm/m.

Environmental Resistance Strong; sealed, resistant to dust, vibration, and harsh conditions. Weak; sensitive to temperature, humidity, and vibration; requires clean, stable environments.

Cost Lower; simple structure and low manufacturing cost. Higher; 2–5 times the cost of an encoder with similar precision.

3. Typical Applications

Encoder Applications

Rotary motion measurement and control.

Servo motors, CNC machine tool spindles, robot joints.

Semi-closed loop control: Mounted on the motor tail to indirectly measure displacement.

Types:

Incremental: Measures relative displacement; used for speed and direction detection.

Absolute: Outputs absolute position; strong anti-interference, no homing required after power loss.

Linear Scale Applications

High-precision linear motion measurement.

High-end CNC machine tools, precision measuring instruments, electronic manufacturing equipment.

Full closed-loop control: Directly detects actual table displacement to compensate for transmission errors.

Essential for high-precision grinders, coordinate measuring machines (CMMs), etc.

Combined Use

Many complex machines (e.g., gantry machining centers) use both:

Encoder on the motor for fast angle feedback.

Linear scale on the worktable for high-precision position feedback.

Achieves high-performance full closed-loop control.

4. Selection and Installation Guidelines

Selection

Motion type: rotary → encoder; linear → linear scale.

Accuracy demand: high precision → linear scale; general precision → encoder.

Environment: harsh conditions → encoder; clean environment → linear scale.

Budget: prioritize cost-effective encoders when performance is sufficient.

Installation

Encoder: Ensure coaxiality between motor shaft and encoder shaft to avoid eccentricity errors.

Linear scale: Maintain parallelism with the guide rail to prevent tilt errors.

Maintenance: Regularly clean the linear scale surface; protect encoders from water and dust.

Summary

Encoders and linear scales are both key position detection components, but they serve different purposes.

Encoders excel in rotary measurement, environmental resistance, and cost efficiency.

Linear scales dominate high-precision linear measurement.

Understanding their differences helps select the right device to maximize equipment accuracy and stability. With the development of intelligent manufacturing, both will continue to play vital roles in industrial automation.