Digital technology comes to motion control

In 1989, Korte became general manager/president of Heidenhain, the North American subsidiary of Dr. Johannes Heidenhain GmbH, Traunreut, Germany. In this capacity, he authors many technical articles on linear- and rotary-encoder technology on behalf of Heidenhain. Korte began his career at the company as a salesman, and became national sales manager in 1986. Before joining Heidenhain, he worked for MXM Corp. Korte served as a sales technician at Trionics Corp. from 1978 to 1981. His experience with electronics began in the Air Force in 1971.

Solid and mature, motion-control has been around as long as there has been motion to control. But Rick Korte explains that the industry has also benefited greatly from digital techniques, leveraging computer technology to improve performance.

Design News: What are some current trends in rotary encoder designs and applications?

Korte: One of the most prevalent new trends is the move to digital speed control. If you review a typical numerically controlled machine, you will find a microprocessor is controlling the position loop. However, the velocity loop is probably using an analog circuit, which uses feedback from a tachometer and employs Hall sensors for commutation of the brushless motor.

At Heidenhain we see a move in the newer control systems to a fully digital solution to both position feedback and velocity control. With the demand to reduce the number of components and the number of lines running between the motor and the control, it is desirable to let the encoder provide position, speed, and commutation data.

Q: How is Heidenhain improving resolution and accuracy?

A: Our continued work in the area of optical gratings and special scanning methods has allowed us to produce finer gratings with higher accuracy--and the ability to have large interpolation values producing higher resolutions. Our use of optical filtering allows increased accuracy in the signal; net quality by using a special scanning method that eliminates various signal elements caused by diffracted light.

Q: What should engineers look for when choosing encoders?

A: Rather than describing specific technical details, the most important decision when choosing an encoder is the vendor itself. Engineers today must be able to have a strong partnership with their supplier. It is important that the encoder manufacturer be able to provide a full range of solutions. The manufacturer must be reliable and dependable, able to provide the best possible technical solution for numerous applications, and forward-thinking. The manufacturer's R&D group must be one step ahead of the requests of the engineers. When a vendor/customer partnership is established on mutual trust and confidence, the outcome can be very dynamic and rewarding.

Q: What is being done to toughen encoders and provide better mechanical performance?

A: One area we have recently addressed is the coupling between the encoder and the motor shaft. The conventional way of connecting an encoder is with a coupling located on the rotor side of the encoder. This configuration is more often becoming unsuitable. The problem is that even the best coupling acts as a torsional spring which, in conjunction with the moment of inertia of the encoder rotor, makes up a spring-mass system. Upon reaching its natural frequency, this design limits drive stability.

Heidenhain now incorporates a coupling on the stator side of the encoder between the scanning unit and the encoder housing. This coupling is not strained during acceleration. The encoder rotor can then rigidly mount to the motor shaft. The stator side coupling sees only the low torque of the bearings. An internal spring-parallelogram coupling compensates axial motion and radial misalignment between the motor shaft and the encoder.

Q: Which areas for encoders will show the greatest growth during the next five years?

A: The area of digital speed control shows very promising potential. And we expect continued growth in the use of higher accuracy encoders for angular position applications.

Q: What changes in encoder design should engineers expect in the future?

A: Watch for the introduction of encoders (both linear and rotary) that can handle higher vibration environments. The continued push toward higher output frequency will also be a dominant trend. Encoders with built-in commutation signals and new ab solute data transmission formats, such as ENDAT (encoder-data-interface), will be available. Rotary encoders will now have internal, programmable memory to store data on the parameters of the system, along with details of the machine application itself.

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