Controller mismatch

Another common problem, says Motion Science’s Dan McFarland, is mismatching a feedback device, its operating speed, and a controller.

Typical case: To satisfy requirements for high linear speed and accurate resolution, a designer chose a lead screw that required 20 revolutions per inch of linear travel, a motor-mounted encoder with over 4,000 pulses per revolution (ppr), and a specific motion controller. However, during startup, they learned that the controller couldn’t keep up with the feedback signals when the motor was delivering the maximum linear speed. The pulse rate from the encoder (several hundred thousand pulses per second) was above the controller’s capability. He had essentially four choices:

• Install a new controller capable of handling the higher pulse rate.
• Change the lead screw to a coarser thread.
• Use a new encoder with fewer pulses per revolution.
• Combine some or all of the above.

This situation taught several people two lessons — more fully understand all the components and test the machine before shipping it to the customer.

This application also raisies another design consideration — the tradeoffs between position accuracy and repeatability and how these criteria influence the bottom line.

Response or repeatability

With the awesome capabilities of microprocessors and digital signal processors (DSPs), it is often a wise step to forego high resolution and depend on repeatability, comments Mr. McFarland. For example, some applications need the ability to move something from point A to point B by programming the motion controller. Other applications only need to repeat specific move sets that are taught to the motion controller by running the tracer through the steps. Then the machine will follow the instructions it was taught. This is just one way a precision system can compensate for machine variables, such as lead-screw thread variations, yet deliver highly repeatable moves.

Budget

One of the biggest time-wasters is neglecting budgetary considerations early in the design phase. It’s the old story of engineers designing a much better product than the market will buy. As Mr. Mc- Farland notes, a lot of time is often spent by both the equipment builder and the drive supplier before budgetary constraints enter the picture. This oversight frequently sends the project back to the drawing board and generally delays the project.

We gratefully acknowledge the important contributions to this article by engineers from the following companies.

Motion Science, Parvex Servo Systems , Pittman Servo Motors.

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