Linear feedback devices control motion precisely

Scan heads for open linear encoders can be made very small to fit tight machine spaces. Component-style construction and a separate interface board go along with miniaturization, as in this RGH34 read head from Renishaw Inc. (Schaumburg, Ill.).

L inear 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.

Frank J. Bartos, executive editor fbartos@cahners.com

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