Motion Control Requirements for Medical Instruments

Tuning Motion Control Loops

The accuracy of delivery can also be improved by tuning of the control loop away from “optimal” from the control engineer’s view. The closed-loop control system can be tuned to emulate a low-pass filter to help reduce the high-frequency content in the jerk (third derivative of position) in the motion. Several changes are involved. Feed-forward is an open-loop method often used to pre-compensate the effective motion command to the servo system to help overcome (hide) some of the filtering properties of the servo system. For this reason, feed-forward should be significantly reduced in liquid delivery systems, allowing the filtering aspects of the control system to be effective. The viscous inertial damping may be increased, as well as the estimated velocity feedback terms, to help over-damp the control system so as to match the motions to the following capabilities of the liquids.

High-torque motors allow either direct-drive or single-stage reduction to be used in many applications. This, in turn, generally allows these motors to be back-driven with no damage to either the motor or the mechanical drive components. Brushless motor designs using high-grade ball bearing construction should be used in systems requiring long-life capabilities.

This article was written by Donald P. Labriola P.E., President of QuickSilver Controls, Covina, CA. For more information, visit http://info.hotims.com/34453-320.

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