Motors & Drives Information

Machines that move need a means of measuring movement. Since the machine tools, inspection machines, material handling equipment and the like have themselves evolved from basic rudimentary manual machines to highly sophisticated automated pieces, so have the internal measuring mechanisms. The most common type of measurement component today is the encoder.

Encoders can be generally categorized into optical (photoelectric), magnetic encoders, and mechanical contact types. Photoelectric encoders in particular—due to their high accuracy, high reliability and relatively low cost, play a significant role in machine tool technology.

There are two basic types of encoders: rotary and linear. While the technical principles behind them are similar, their specific applications most often are not. And while the basic principle of operation developed many years ago is still the basis of today's encoders, a revision of even that technology, highlighted for especially high accuracy needs, is now available.

The Basic Principle

Most of today's linear and rotary encoders operate on the principle of the photo-electrical scanning of very fine gratings.

The so-called scanning unit in an encoder consists of a light source, a condenser lens for collimating the light beam, the scanning reticle with the index gratings, and silicon photovoltaic cells. When the scale is moved relative to the scanning unit, the lines of the scale coincide alternately with the lines or spaces in the index grating. The periodic fluctuation of light intensity is converted by photovoltaic cells into electrical signals. These signals result form the averaging of a large number of lines. The output signals are two sinusoidal signals that are then interpolated or digitized as necessary.

Rotary Encoders

In various sectors of machine technology, angular positions and angular motions need to be transduced into electrical signals, either for display, automation or numerical control. Rotary encoders are used for this purpose of measurement of rotational movement drives. They are also often used in measuring linear movements, for example when used with spindles and especially with recirculating ballscrews.

The most significant characteristics of rotary encoders are summarized in Table I below.