Basics of a Motion Control System

Backlash Instability

A technique called dual-loop control is sometimes used in a motion control system to compensate for instabilities caused by backlash. Backlash arises in motion control systems where a motor is coupled to a load through gears or lead screws. When a position encoder is installed on such loads, a sufficient level of backlash can make the feedback oscillate. This phenomenon can be a limiting factor for precise movements.

A motion control system featuring dual-loop control uses two feedback encoders, one on the motor, the other attached to the load. The motor encoder closes one loop that provides motor control. The load encoder closes a second supervisory loop that monitors load-position errors and then commands the motor-control loop to move.

A motion control system with a dual-loop algorithm would first command the motor to move a certain interval at a specified speed. Once this move is complete, the motion control system will then run a program designed to check the position of the load encoder to determine if any position error remained, then command the motor to move until the error hit zero.

Another use for this algorithm is where a clutch couples the motor to a load. A Dual-loop motion control system can provide smooth motion through motor velocity and load-position profiles. For example, say that the motor used in a motion control system must accelerate to some constant velocity, hold this velocity for a time, then decelerate to a stop. With no slippage, the load position is simply the integral of the motor velocity profile. One approach to the algorithm is to give the motor a command to reduce velocity when the load position is a certain number of counts from its final destination. When the load is a few counts closer to the end, the motor is commanded to stop.

The solution is independent of slip because it makes the motor run longer to make up for the slip in rotation. Of course, the motion control system could feature a profile, which contains a number of set points and velocity changes for more complex profiles.

Motion Control System : Introduction to Motion Control System & Design Engineering

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