Linear Encoder vs. Rotary Encoder: Key Differences

This article explores the differences between linear and rotary encoders, both of which are crucial components in position encoding. So, what exactly is a position encoder? Think of it as a highly precise ruler or protractor that translates physical position into a coded electrical signal. These devices come in various sizes and specifications and are used across numerous industries, including electronics, aerospace, medicine, motion control, semiconductor manufacturing (particularly for wafer handling robotics), robotics, flat panel display production, Coordinate Measuring Machines (CMMs), renewable energy, solar PV, printing, metrology, and more. They are essential in processes like wire bonding and pick-and-place technology.

Encoders are broadly categorized based on the type of motion they measure: linear and rotary. They can also be classified by their measurement method: incremental or absolute. Incremental encoders track relative motion, while absolute encoders pinpoint the actual position.

Linear Encoder

Linear Encoder

• A linear encoder, as the name suggests, measures linear motion. The image above showcases a linear encoder from Renishaw plc.
• It directly measures the position along the axis slide, functioning in what’s known as a closed-loop system.
• Because the position is measured directly at the axis slide, changes in spindle temperature have little to no effect on accuracy.
• Direct position measurement with linear encoders enables precise machining.

Rotary Encoder

Rotary Encoder

• A rotary encoder measures rotary motion. The image above shows a rotary encoder also from Renishaw plc.
• It measures the rotational movement of a spindle.
• The control system calculates the axis position based on the rotary motion and screw pitch. This setup is considered a semi-closed loop system.
• Thermal expansion can cause slight deviations in position, as the spindle may expand slightly with heat, leading to axis drift.

Benefits of Using Encoders for Controlling Moving Axes

Using encoders to manage the movement of machine axes provides several key advantages:

• It reduces both manufacturing costs and time.
• It lowers the risk of the axis losing control or crashing.
• It offers instant position feedback upon startup.

In summary, both linear and rotary encoders play crucial roles in various applications, each tailored to different types of motion and measurement needs. Understanding their differences is key to selecting the right encoder for a given task.