Nonlinear servocontrol

June 16, 2005
Optimum servomotor control may not always follow the straightest path. Nonlinear-control algorithms help keep servocontrol processes in line.

Nonlinear control technology (NCT) for Yet's motor control is said to provide tight command tracking, near-zero settling times, and quick servotuning. Here the motor undergoes a load change, forcing the command speed to increase power to maintain 2,400 rpm. The regular servomotor's feed-forward branch creates a large tracking error between actual and target motor speed, before settling back to its optimal rpm. Nonlinear control produces just a small tracking error at the application of the load, and practically no settling time for return to normal speed.

A new servocontrol process not only optimizes servo performance, but allows seamless connectivity to virtually any rotary or linear servomotor. Developed by Yet US Inc., (Manchester, N.H.) a joint venture between Yaskawa Electric Corp., Japan, and RoboGroup T.E.K. Ltd., Israel, the new process uses nonlinear control algorithms in the feed-forward branch. The company claims the new process provides extremely tight command tracking for minimum following error, near-zero settling time at final position, and quick servotuning and adaptation to changing machine conditions.

Feed-forward in most servodrives is temperamental during tuning. And even though motor speed is stable in its initial tuning, varying load conditions can cause instability and cripple the machine. The nonlinear control technology Yet developed continually adapts, maintaining optimization even under large inertia mismatches. The oscillationcanceling algorithm tunes the system as needed, creating a variable notch filter.

Position and velocity control are integrated into a single servoloop that includes four independent feed-forward branches. Each branch includes a variable function gain dependent on machine conditions. This results in gains greater than 10 that available from conventional servos. These higher gains come without machine vibrations or servo instability. And it autotunes in a matter of minutes rather than hours.

An integration process uses the sum of the four feed-forward branches to avoid overshoot during final approach to position.


Yet US Inc.
(866) 938-8080,

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