Linear feedback devices control motion precisely

Linear motors and motion systems depend on position feedback for best performance. Various feedback devices convert linear motion information into electronic signals needed for accurate control of position and motion. Linear optical encoders are probably best known, but devices based on magnetic, inductive, or capacitive methods can also be an option (see below).


Linear motors and motion systems depend on position feedback for best performance. Various feedback devices convert linear motion information into electronic signals needed for accurate control of position and motion.

Linear optical encoders are probably best known, but devices based on magnetic, inductive, or capacitive methods can also be an option (see below).

In basic terms, a linear optical encoder consists of a scan head that moves with the motor, and a glass or steel scale mounted to the stationary part of the system. The scan head contains a light source, reticle, photo cells, and processing electronics. When the scan head moves, light shining through the scale is modulated by fine grating marks on the scale's surface, producing sinusoidal outputs from the photocells. This "through-beam" sensing applies to glass scales. Reflective sensing is used with metal scales. Photocell outputs are phase shifted to obtain two sinusoidal signals 90° (electric) apart. This results in the popular quadrature encoder. Electronic circuits further process the signals to square-wave and digital form.

Like their rotary cousins used with standard motors, linear optical encoders have two basic versions. An incremental type provides relative position feedback, while an absolute type provides a unique position. (See more detail in CE , July 2000, p. 156.)

Sealed or open

Encoder structure varies also. In a sealed unit, a metal housing protects the encoder's internals from harsh industrial environments. A flexible lip seal encloses the scan head that rides on a guide way with bearings, but adds some amount of friction. The housing also offers EMI protection.

An open encoder is frictionless since the scan head and linear scale have no physical contact. However, the unit is exposed to contaminants and thus limited to "cleaner applications." Also, EMI protection has to be separately provided.

Typical capabilities of sealed linear encoders include measuring steps (resolution) down to 0.1 mm (4 min.) and scan lengths up to 30 m (98 ft) for incremental units; absolute units offer up to 3 m length. Some open encoders have still finer resolution, down to 0.001-mm. Sophisticated electronic interpolation circuits and multipliers are needed to obtain these high resolutions.

Accuracy of sealed models ranges down to 62 mm (0.00008 in.); open linear models can do 60.5 mm—even better in custom units. Scan speed is a further consideration; it can run as high as 15 m/sec (49 ft/sec) in some open incremental linear units.

For longer scanning lengths, the linear scale is made of thin steel strip (0.3-mm typical thickness). The flexible strip comes in precut lengths or in continuous rolls. Gold plating is used to improve reflectivity and corrosion resistance. Other surface coatings serve to make the scale more durable.

A linear variable differential transformer (LVDT) is a rugged noncontact electromechanical device with a movable central magnetic core, surrounded by cylindrical coils. It produces an ac output proportional to the core's movement and is linear over a specified range. An LVDT is limited to measuring relatively short displacements due to its inherent construction. However, device resolution is virtually unlimited, as set by capability of the external electronics.

Capacitive sensing methods provide still another choice. One recent innovative encoder design uses circuit board production methods to print conductive patterns on the scale and scan head. The scale (transmitter) and the scan head (receiver) interact electrically to produce "pure" sine and cosine dc signals—in a combination coarse/fine output mode that determines absolute position. Submicron resolution is derived from the signals.

An incremental version of the device has only fine-mode output. The reportedly low-cost approach offers typical accuracy of 10 mm or better. Benefits include low power consumption and magnetic/EMI immunity.

Miniaturization is also visibly at work for linear feedback devices (see photo).


Go to for more on linear feedback devices

No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
Control Engineering Leaders Under 40 identifies and gives recognition to young engineers who...
Learn more about methods used to ensure that the integration between the safety system and the process control...
Adding industrial toughness and reliability to Ethernet eGuide
Technological advances like multiple-in-multiple-out (MIMO) transmitting and receiving
Virtualization advice: 4 ways splitting servers can help manufacturing; Efficient motion controls; Fill the brain drain; Learn from the HART Plant of the Year
Two sides to process safety: Combining human and technical factors in your program; Preparing HMI graphics for migrations; Mechatronics and safety; Engineers' Choice Awards
Detecting security breaches: Forensic invenstigations depend on knowing your networks inside and out; Wireless workers; Opening robotic control; Product exclusive: Robust encoders
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
News and comments from Control Engineering process industries editor, Peter Welander.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
Anthony Baker is a fictitious aggregation of experts from Callisto Integration, providing manufacturing consulting and systems integration.
Integrator Guide

Integrator Guide

Search the online Automation Integrator Guide

Create New Listing

Visit the System Integrators page to view past winners of Control Engineering's System Integrator of the Year Award and learn how to enter the competition. You will also find more information on system integrators and Control System Integrators Association.

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.