Linear feedback devices offer variety

Feedback devices for linear motion and position sensing continue to evolve. More product varieties and enhanced features can be expected.


F eedback devices for linear motion and position sensing continue to evolve. More product varieties and enhanced features can be expected.

Open encoders (see main article ) are manufactured in a special two-coordinate version, as well. A device of this type incorporates a planar phase grating on the glass surface to allow optical position sensing in two directions-as on an X-Y plane. A reference signal is incorporated for each axis. In a capacitive-type (non-optical) device the conductive grating patterns are likewise applied in both directions. Two-coordinate encoders are suited to such applications as electronics/semiconductor manufacturing and high-performance X-Y positioning equipment.

In linear encoders, type of material used for the optical scale affects device accuracy (resolution). Glass scales can be made with generally higher resolution, with grating line spacing down to 65 mm (0.0002 in.). On metal scales-thin steel strips-the grating lines can have pitch as fine as 20 mm.

Scales for open linear encoders can also be supplied as adhesive-backed steel 'tapes' that permit simple mounting to a stationary surface of the machine or system. However, flatness of the mounting surface must be assured to obtain the expected encoder accuracy. For encoders with very long scanning lengths, continuous rolls of steel tape are available that can be cut to the length needed.

For example, Renishaw Inc . (Schaumburg, Ill.) supplies the scale in 'cut-and-tape' format, and its RGH Series scan heads include a built-in light emitting diode (LED) set-up indicator. The two-color LED helps users simplify and speed up an installation by indicating the best possible alignment of the scale and scan head.

Linear feedback with a `twist'

The principle of magnetostriction is applied with a 'twist' to linear position sensing by MTS Systems Corp., Sensors Division (Cary, N.C.). Its non-contact Temposonics linear position sensing device relies on the momentary interaction of two magnetic fields to induce a 'torsional strain pulse' (twist) in the device's structure called a magnetostrictive waveguide.

A magnet that rides along the outside of the unit's sensing tube supplies one magnetic field. (The magnet would be connected to the moving part of an actuator whose position is the target measurement.) The second field comes from a current pulse (interrogation pulse) generated along a wire inside the waveguide. Momentary interaction between the two magnetic fields creates the strain pulse in the wire. Then, the pulse travels at the speed of sound along the waveguide until detected at the sensor head. Position of the moving magnet (and actuator) can determined accurately by measuring the elapsed time between the start of the interrogation pulse and arrival of the strain pulse at the sensor head. There is no wear involved on the sensor parts.

Interface electronics provides a variety of outputs. Hydraulic and pneumatic cylinders are among the many applications of magnetrostrictive linear feedback devices.

Despite the popularity of optical linear feedback, several alternative methods are available. For example, Danaher Controls (Gurnee, Ill.) offers inductive sensing in its Dynapar LR/LS Series Linear Encoder. It combines favorable features of optical and magnetic encoders, without the fragility of a glass scale or use of a magnetized metal scale. Typical measuring step capability is 0.0002 in. for incremental quadrature outputs. Finer resolutions are available through electronic interpolation or via sine/cosine outputs.

At the National Manufacturing Week show (March 2001) in Chicago, Power Components of Midwest Inc . (Mishawaka, Ind.) introduced a non-contact linear displacement sensor based on Hall-effect technology. LD-50 sensor offers a 0.5-30 in. travel range, 12-bit resolution, and programmable analog or pulse-width modulated (PWM) outputs. Independent linearity is9257.

Frank J. Bartos, executive editor

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