Image Credits: Keyence and Panasonic

High performance, noncontact, eddy current linear position sensors measure the position and motion of conductive targets.

How Eddy Current Linear Encoders Work

These sensors use a reference coil to produce a magnetic field in a target. As the target gets closer to the sensor, the sensor and target magnetic fields distort each other. The image below shows the fields.

Image Credit: Lion Precision

This video illustrates the operation:

Video Credit: National Instruments

Types of Eddy Current Linear Encoders

Although they sometimes share the “inductive” designation, eddy current sensors should be distinguished from inexpensive inductive sensors. The latter sensors, such as those used in inexpensive proximity switches, are not suited to precision metrology applications. In contrast, eddy current sensors have very linear outputs, high resolution, and are stable with temperature.

Specifications and Features

  • Range of measurement varies (e.g. 0.04 to 0.62 inches).

  • Sensing frequency can vary (e.g. at least 3 to at least 34 kHz).

  • Static resolution (e.g. better than 6.5E-7 to better than 9.6E-5) indicates much change in a quantity is required for sensor detection.

  • Nonlinearity (e.g. better than 5% to better than 0.5%) indicates the deviation from a purely linear response in the measuring region. Lower deviation values denote more accurate reading throughout the measuring range.

  • Output electrical signals can be analog (current, voltage, or frequency) or digital (serial, parallel, or other).

  • Packaging can be a raw sensor element or a housed transducer. Self-contained instruments or meters display output at or near the device. Gauges/indicators have an analog display and no electronic output.

  • Devices can tolerate maximum shocks, maximum vibrations, and certain temperature ranges (e.g. -60 to 300°F).

  • Targets can be magnetic or nonmagnetic, ferrous or nonferrous.

  • Sensors can be shielded (embedded in metal and not side-sensitive), weld field immune (will not false-trigger in the presence of extreme electromagnetic fields and produced by resistance welders), and/or intrinsically safe (incapable of releasing sufficient electrical or thermal energy to cause ignition of hazardous atmospheric mixtures). They also may have temperature compensation (ignore ambient temperatures).

Applications

Eddy current linear encoders have high resolution and are tolerant of dirty environments. They are used to measure position (automation, machine tools, disk drives) and dynamic motion (drive shaft monitoring, vibration measuring).

References

Comparing Capacitive and Eddy-Current Sensors (alternative video presentation)

Designing and Building an Eddy Current Position Sensor

Eddy Current Sensor Overview

Finding the Right Sensor for Linear Displacement

Inductive Eddy Current Sensors for Position/Displacement Measurement and Nano-Positioning

 


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