Motion control speeds ahead

The demand for higher speeds and accuracies is being met by today's motion control technology, but design engineers face a challenge in pushing the mechanical limits of their designs.

Design News: With all the alternatives in motion-control technology available today, how can a design engineer sort through the choices and make the best selection for his or her application?

Meshkat: One way that a design engineer can get a better handle on the large number of choices in motion-control technology is to understand what kind of performance speed and accuracy he or she needs for a particular application. That right away is going to greatly limit the number of choices.

With many controllers basically at parity today, a design engineer then has to examine the capabilities of the individual vendors and their relative experience in the marketplace. At minimum, a user should be able to expect a full set of development tools and support from a supplier.

Q: Given the move toward Windows NT, some industry observers are saying that eventually the need for standalone motion-control cards is going to go away entirely. What is your thinking?

A: There has been a lot of speculation over whether or not software will ultimately close the loop inside the computer itself, eliminating the need for a standalone control card. While that may be true in some cases, those applications that are time-critical will still be dependent on a standalone card. That's because there is simply not enough computational power in today's microprocessor to handle all of the application demands placed upon it, particularly given the fact that operating systems are getting more complicated. Any application that has stringent performance requirements is going to need to rely on a dedicated control card.

Q: Everything seems to be speeding up today, from faster machine tools, to higher throughputs in semiconductor processing. What impact is this trend having on motion-control technology?

A: While, certainly, as controllers become more integrated, they are getting faster and design engineers are tending to do more with them. In our own card, we now perform vector control of an ac induction motor, which means that design engineers can get servo-like performance out of an $80 motor instead of an $850 motor. Features such as cubic spline and NURBS interpolation, "S-curve" acceleration on circle, and electronic camming are now standard. And, of course, higher speeds means higher sample rates needed from the motion controller.

Industrial motion controllers that meet the requirements of high speed must, at a minimum, be able to: generate command trajectories consistent with the machine's operating bandwidth and resonances; offer a control algorithm that can deal with variable load, parasitic, and other resonances; and perform commutation and digital current loop to improve servo stiffness and torque/force regulation.

One not-so-surprising trend, is that many engineers are deciding that more speed is always better. To some extent the same thing is true with computers--a consumer is going to get a Pentium III, whether they need it or not, because eventually down the road they are going to need it. The really interesting thing happening, however, is that engineers are pushing these systems to their limits mechanically.

Q: What challenges does this pose to the equipment designer?

A: Certainly for some time to come, motion control cards will be able to provide sufficient processing speed. In fact, today a design engineer can get 5X improvement in performance just by using a better control technology. What engineers are up against, however, is figuring out how to push the capabilities of their machines. In many cases, there are machines that are using parts that were designed for different, less stringent limits. And engineers are going to have to figure out how to deal with the mechanical resonances and other boundary conditions that they are pushing up against. Some of these challenges can be solved with today's motion control technology, but not all.

Q: What major trend do you see happening in the area of motion control in the near future?

A: We are going to see much greater use of linear motors, because of their inherently high accuracies and speeds and ability to move a load with no backlash or slippage. For the right application, a linear motor and DSP-based motion controller is a pretty unbeatable combination.

Certainly for some time to come, motion control cards will be able to provide sufficient processing speed Dr. Meshkat is founder and president of DSP Control Group, Inc. Formerly, he was the director of advanced servo technologies at Electrocraft Corporation, consulted with DSP division of Texas Instruments, and taught at the department of electrical engineering at the University of Minnesota. He has published many technical articles and three books on motion-control topics. Among his works is the popular "DSPs in Motion Control". Dr. Meshkat holds patents in various areas of parallel DSP design and auto-tuned motion-control systems.