Tom Wyatt
Heidenhain Corp.
Schaumburg, Ill.

Angle encoders from Heidenhain come in a wide variety of packages including hollow-through-shaft as well as solid-shaft models.

Typically, an angle encoder is probably best if an application needs positioning accuracy of 5 angular sec (0.001°) or better and a minimum line count of 10,000 before subdivision. Typical angle-encoder jobs include indexing, rotary tables, swivel heads on machine tools, radio and optical telescopes and antennas, and high-resolution digital printing machines and robotics.Encoder accuracy determines the position integrity of the rotary axis. Several factors besides resolution and pulse count contribute to encoder accuracy. First is the quality of the graduation regardless of the scanning technique. Other influences are the eccentricity of the graduation to the bearing and the torsional rigidity of the encoder shaft, driveshaft, and stator coupling. The electronics are a factor as well, including signal processing.

Angle encoders come in several mechanical options with varying degrees of accuracy. There are three basic categories depending on whether or not the encoder has an integral bearing.

Angle encoders with integral bearings, a hollow shaft, and integral stator coupling lead the pack in dynamic performance. They eliminate the need for a separate shaft coupling, which would increase the chance of introducing more error. The low-profile design also simplifies mounting.

The interferential scanning principle takes advantage of light diffraction and the interference of light on an extremely fine graduation. The method generates two signal periods from the relative motion of just one grating period. In other words, an angle encoder with a grating of 90,000 steps on its disk will produce 180,000 signal periods before interpolation. The signals are for the most part harmonic free which leads to high interpolation, high resolution, and better accuracy.

One such encoder uses the imaging scanning principle with line counts from 9,000 to 36,000 counts/rev. Accuracies range from ±5 arc sec to ±0.4 arc sec without compensation. Another encoder type employs the interferential scanning principle. These encoders deliver 180,000 signal periods/rev.

Traceable calibration charts come with some encoders and even include the error of the coupling. Error gets calculated over five forward and five reverse revolutions to determine the accuracy. Such charts show not only the long-range error but also the position error within one signal period. The only critical data not included in the calibration chart is the reversal error which can range from 0.6 to 0.8 arc sec and depends on the encoder model.

In the second category are angle encoders with an integral bearing, solid shaft, and separate shaft coupling. The shaft coupling compensates if the drive-shaft bearing experiences axial and radial run-out to the encoder shaft. But the two mating shafts must be optimally aligned to ensure accurate measurements. Accuracies are from ±5 to ±1 arc sec, depending on the model. They use the imaging scanning principle and provide 18,000 to 36,000 line counts.