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.

Incremental

The output signals of incremental rotary encoders are evaluated by an electronic counter in which the measured value is determined by counting "increments." These encoders form the majority of all rotary encoders being used today.

When dealing with incremental rotary encoders for length measurement, you are usually dealing with them on slides of machines and fixtures which are generally driven by leadscrews. Encoders are normally positioned and coupled to the rear of the servomotor or to the screw and opposite to the drive via a backlash-free precision coupling.

Standard rotary encoders for general length measurement applications—and in particular the measurement of slide movements with a recirculating ballscrew as the scale—is represented by a shaft encoder with incorporated digitizing electronics. The number of square-wave cycles of the output signals per shaft rotation is identical to the number of lines on the graduation disc.

Incremental rotary encoders with integral couplings used for length measurement are also on the market. This design features some favorable characteristics, especially the version in which the coupling is not mounted on the rotor side, that is, between the driving spindle or motor shaft and the encoder shaft, but is permanently fixed to the stator. In these cases, the spindle or motor shaft is directly connected to the shaft of the rotary encoder. The scanning unit is connected to the encoder shaft by ball bearings, however, without a rigid connection to the housing. Instead, a coupling is located between these components and compensates for alignment errors between both shafts.

All of these rotary encoder versions are, in principle, angular measuring systems and are, providing accuracy requirements are fulfilled, used in many cases for this purpose. The resolution of such encoders can be increased by means of electronic interpolation. There are, of course, the precision rotary encoders specifically designed for angle measurement.

If finer resolution is required, standard rotary encoders often utilize electronic signal interpolation. Rotary encoders for applications in dividing heads and rotary tables, with very small measuring steps (down to 0.36 arc second) have in principle the same basic design features as standard rotary encoders, but incorporate some overall varying construction.

Absolute

Absolute rotary encoders provide an angular position value which is derived from the pattern of the coded disc. The code signal is processed within a computer or in a numerical control. After system switch-on, such as following a power interruption, the position value is immediately available. Since these encoder types require more sophisticated optics and electronics than incremental versions, a higher price is normally to be expected.

The most commonly used coder is Gray coder, a unit-distance coder where only one single bit changes with the transition from one measuring step to the next. Natural binary coder is also frequently used for very high resolutions. With this code, more than one signal may change when going from one measuring step to the next. Precautions must therefore be taken to avoid ambiguities.

Apart from these two codes, a range of other codes have been employed, though they are losing their significance since modern computer programs usually are based on the binary system for reasons of high speed.

There are many versions of absolute encoders available today, such as single-turn or multi-stage versions to name only two, and each must be evaluated based on its intended application.

Linear Encoders

As the present trend of machine tools evolves toward increasingly higher accuracy and resolution, increased reliability and speeds, and more efficient working ranges, so too must feedback systems. Currently, linear feedback systems are available that will achieve resolutions in the submicron range.

Submicron resolutions, for example, are required in the semiconductor industry and in ultra-precision machining. Achieving these resolutions is possible with the use of linear scales which transmit displacement information directly to a digital readout, NC controller, or other peripheral device for display or evaluation.

As in rotary, linear scales operate on the same photoelectric scanning principle, but the linear scales are comprised in an overall straight construction, and their output signals are interpolated or digitized differently in a direct manner. One of these signals is always used by the accompanying digital readout or numerical control to determine and establish home position on the linear machine axis in case of a power interruption or for workpiece referencing.

Overall, there are two physical versions of a linear scale: exposed or enclosed. The versions themselves dictate, fairly accurately, the type of application.

With an enclosed or "sealed" scale, the scanning unit is mounted on a small carriage guided by ball bearings along the glass scale; the carriage is connected to the machine slide by a backlash-free coupling that compensates for alignment errors between the scale and the machine tool guideways. A set of sealing lips protects the scale from contamination. The typical applications for the enclosed linear encoders are primarily machine tools and cutting type machines, or they may be any type of machine located in harsh environments.

Exposed linear encoders also consist of a glass scale and scanning unit, but the two components are physically separated. The typical advantages of the non-contact system are easier mounting and higher traversing speeds since no contact or friction between the scanning unit and scale exists. Exposed linear scales can be found in coordinate measuring machines, translation stages, and material handling equipment.

Another version of the scale and scanning unit arrangement is one that uses a metal base rather than glass for the scale. With a metal scale, the line grating is a deposit of highly reflective material such as gold that reflects light back to the scanning unit onto the photovoltaic cells. The advantage of this type of scale is that it can be manufactured in extremely great lengths, up to 30 meters, for larger machines. Glass scales are limited in length, typically three meters.

There are several mechanical considerations that need to be understood when discussing linear encoders. It is not a simple matter to select an encoder based just on length or dimensional profile and install the encoder onto a machine. These characteristic considerations include permissible traversing speeds, accuracy and resolution requirements, thermal behavior and mounting guidelines.

The Interferential Scanning Principle

The newest linear encoders operate on a principle of a unique diffraction of light waves to obtain very fine measuring steps, and in turn offer machinists an even rarer combination of high accuracy, extremely fine resolution and excellent repeatability of the measuring values not found in any other optical system before.

The semiconductor light source together with a condenser produces a plane light wave, which is represented by a beam normal to the wave front. While striking the scanning grating A, the plane wave is essentially diffracted into three directions. At the phase grating of the scale M, the light is reflected and diffracted once again. After another diffraction series, they come to interference. Three interfering unidirectional light beams are collected by the lens and projected onto three solar cells which convert the light intensities into electrical signals. Since the interferential scanning signals are inherently free of harmonic distortions, the resulting signal periods can undergo a high subdivision.

Measuring systems which operate on the interferential scanning principle combine the advantages of a very fine grating (high resolution and accuracy) with a wide tolerance for the gap between scanner and scale. Due to its high accuracy, extremely fine resolution and—above all—the excellent repeatability of the measuring values, this device can be considered an alternative to the laser interferometer for shorter distances (less than or equal to one meter). Moreover, a measuring system having a steel scale such as that of an interferential scanning setup is better matched to the thermal expansion of a steel workpiece. Therefore, in many cases, it proved superior to a laser interferometer.

Meeting Every Need

All types of linear and rotary encoders are increasingly being employed (as sensors) for acquisition of angular and linear position as well as for acquisition of travel in measurement and control, machine tools, industrial robots and many other sectors of technology. A large selection of sizes and performance characteristics allows machine builders to meet the various demands presented by their customers.

About the author: Rick Korte is president and managing director of the Heidenhain Corporation, Schaumburg, Illinois.