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DROs: a closer look.

As far back as the early '50s, digital readout systems (DROs) were used by a few progressive machine shops that realized the moneymaking potential of these new distance-measuring gadgets. But back then DROs were expensive, bulky devices that tended to raise the eyebrows of most realibility-conscious managers. A lot has changed since then.

Innovations in solid-state electronics have made low-cost, high-realibility units available. And integrated circuitry has added memory and programability. The latest DROs can slice manual machine-slide positioning time in half and reduce calculations, miking and operator fatigue, while simultaneously ensuring dead-on accuracy. So attractive are the benefits that we project 50 percent of the manual machine tools will be DRO equipped by 1990--a fourfold increase over today's figures.

The most prominent component of a DRO system is the display where LEDs numerically indicate position. Yet, the least visible, but most critical, component is the measuring device, i.e., some type of transducer or scale. It's this devicehs resolution and accuracy that's the heart of the system.

Although resolution and accuracy often are used interchangeably--they shouldn't be. Resolution in a DRO only refers to the units displayed on the counter. They can read in fractions of an inch or millimeter.

Accuracy, on the other hand, is the measuring device's ability to make correct linear measurement with respect to a recognized standard. For example, the more lines, precisely placed, per millimeter or inch on a glass scale ensures that measurement will be determined with a minimum of interpolation.

There are a number of measuring devices available for DROs. Here is a brief overview of the most common ones. Glass scales

Glass is a relatively inert substance that efficiently transmits light. This fundamental property permits high accuracy in scale manufacturing, and an uncomplicated mechanical-to-electronic energy conversion. In most glass-scale systems you will find little electronic interpolation, the belief being that more interpolation means more opportunity for error.

Most precision glass-scale systems employ a reader head that converts photoelectric signals into square waves. This signal form is less prone to electrical interference that may cause faulty readings or rolling digits in a controller.

These scales should be cloned from a precisely prepared master. The glass scales with which I am most familiar are photoetched from a master scale produced by a laser-interferometer-controlled ruling engine lathe that is anchored to bedrock 150 ft below Bausch & Lomb's David Richardson Grating Laboratory in Rochester. Once completed, the master is checked for accuracy by laser interferometry to ensure millionths of an inch precision.

There have been many improvements in glass-scale technology over the past 5 years. Today, for instance, these scales can resolve to millionths of an inch, fit in tight or awkward spots, are coated to improve their optical properties, and are better protected to minimize maintenance even in the dirtiest of environments.

In the OEM market, glass scales have proven invaluable as linear position feedback systems for CNCs. Scales marked with FTO (fiducial trigger output) points are capable of monitoring a machine slide's true position at all times. This enables a CNC to accurately return a slide to its home position after completing a program.

Even though glass scales are versatile, they aren't appropriate in all cases. Glass and other linear scales aren't applicable to robotics because of the required rotary (nonlinear) measurements, for example. Also, current manufacturing technology limits glass-scale lengths to approximately 120". If longer measurements are required, a combination of rack and pinion and glass can be used. Magnetic scales

A main advantage of magnetic scales are their ruggedness. They are well protected against contamination, temperature, and shock, and for the most part, are sound mechanical assemblies. Moreover, they are backlash-proof and fairly easy to install in any length.

Electronically, however, the're complicated. The demand of electronic interpolation often can result in spurious counts and periodic errors, possibly requiring field recalibration. If improperly shielded, these transducers can pick up magnetized dust as well. Finally, DROs using magnetic scales are more expensive than those using glass. Mechanical rotary encoders

Many digital measuring systems incorporate mechanical rotary encoders, which may use rack-and-pinion transducers, friction roller transducers, or tape or wire transducers. All have the advantage that installation length is practically unlimited. There are, however, other considerations.

Rack and pinion. The beauty of this transducer is simplicity of the mechanism--meshing gears. It can be calibrated to any machine or travel requirement, and is among the most rugged of all transducers.

Rack-and-pinion units usually are sold as segments, which are butted end to end to achieve the necessary travel length. But, field experience shows that these transducers arent' economical for short travels, and it's no easy task to hold accuracy to within 0.0001" because of inevitable tooth wear.

Friction roller. This easy-to-install transducer is the only type that pulls travel measurement data right off the machine. No wires, gear teeth, or scale; just a mounting surface and a flat machine surface to roll against. Low backlash and price are other pluses, but there must be friction and plenty of it. Unfortunately, sooner or later wear sets in and slippage begins, thereby degrading a DRO's accuracy.

Tape or wire. For travel lengths more than 10 ft, this type of transducer is the least expensive; however, repeatability is questionable. At less than 10 ft, these systems generally don't show a cost advantage, and in many cases are more expensive than some of the alternatives. Inductive transducers and laser interferometers

Of all the different measuring devices used in DROs, the inductive types are probably the least commonly used. The primary reason is high cost. Nevertheless, phase-analog technology is very accurate, and inductive hardware is impervious to the harsh environment of a metalworking shop.

Regarding laser interferometers, it's doubtful that more than a few percent of the metalworking shops in this country are able to justify their cost, or will ever need that sophistication for measuring travel. You might think of a laser as a very special scale that uses light waves as extremely small graduations.

To measure travel distance using these wavelengths requires employing an interferometer system to optically manipulate the light and electronically calculate signal deviations. Add to this an elaborate error-correction system to compensate for varying ambient conditions, and you've provided overkill for the needs of most shops. Where this technology is useful is for measuring travel lengths more than 100 ft or accuracies less than 1 micron. An abbey of errors

Regardless of which measuring device is used in a DRO, beware that no machine system is immune to inaccuracy. Contributing causes range from poor fit up between mating surfaces to temperature fluctuations during the course of a shift. An especially troublesome form of inaccuracy in machine tools is called Abbe error.

Abbe error (also called transfer error) is a transverse inaccuracy in a machine table or other moving member caused by insufficiently straight motion of machine slides. This error is present to some degree in all machine tools, occurring on any axis of motion. The culprit is gravity, which induces deflection on overhanging tables (such as on a milling machine), or when workpieces are too heavy or too large for a table.

Ernest Abbe, a 19th century physicist, was first to define this error and its function in travel measurement. He observed that the maximum accuracy for any measuring system can be obtained only when the measuring standard is in line with the workpiece being measured. A machine member whose motion is affected by gravitational deflection (or worn ways and slides for that matter) will always interfere with in-line accuracy or travel measurements, whether made by a glass scale, magnetic scale, or rack-and-pinion transducer. Some DROs can electronically compensate for this error.

In summary, while the metalworking industry remains the principal user of DRO's, they also can be found in such diverse environments as the microelectronics industry (e.g., aligning PC boards for drilling), on microscope stages of comparators, and in hospitals for positioning lasers for delicate eye surgery. In fact, virtually any call for accurate measurement along X-Y-Z axes can be handled by a DRO. We believe there's no limit to applying digital measuring technology.

For more information about DROs, circle E45.
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Author:Tarana, Victor
Publication:Tooling & Production
Date:Jun 1, 1984
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