Machine Design

Getting the right angle

Angle encoders differ from standard duty rotary encoders in terms of accuracy as well as mechanical complexity and the number of counts per revolution.

Angle encoders from Heidenhain come in a wide variety of packages including hollow-through-shaft as well as solid-shaft models.
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.
The imaging scanning principle is a more conventional method using either reflected or transmitted light depending on the measuring substrate. The scanning assembly consists of the light source, a focusing lens, a scanning reticle with equally spaced lines, and photocells. The scale or glass disk carries the lines matching the scanning reticle. Relative motion of the components creates pulses of light. When the lines of both the reticle and the glass disk exactly align, no light passes through. Once they begin to move, light gets through until it reaches maximum intensity and the reverse starts to happen, diminishing intensity. This alternating sequence of light passes through to the photocells creating sinusoidal electrical signals.

Tom Wyatt
Heidenhain Corp.
Schaumburg, Ill.

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.

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.

The last angle-encoder category is modular encoders or encoders without an integral bearing. These encoders are for integration into a system. Their scanning head must be adjusted to the measuring standard. They're also well suited for applications requiring no additional starting torque, like an air bearing table, or for systems with large internal diameters to 10 m. Because these encoders have no integral bearing, certain models can reach mechanical speeds of 40,000 rpm. A tape scale lets them measure a segment of 360° with as little as 50° of traverse.

Consider several factors when evaluating modular encoders. For starters, look at the system accuracy which includes the positional integrity within one revolution and the position deviation within one signal period. Also, consider the error caused by eccentricity of the graduation to the bearing as well as error from radial deviation of the bearing. If the encoder is to deliver optimal performance it must adhere to the mounting tolerances for that particular model. Ignoring installation tolerances can lead to additional error. The amount of error will differ depending on the scanning technique or type of encoder.

When making the final choice of an angle encoder, there are several considerations other than the mechanical package. For example, the proper interface and output of the encoder are important. There are also mechanical and environmental issues such as contamination and washdown. Protective covers are a good idea for modular encoders. Devices with an integral bearing can accommodate an air purge system for extra protection. Mechanical issues to consider include temperature, acceleration, shock, and vibration.

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