Chapter 17 - Position-Control Loops

Abstract

The majority of motion-control applications require precise velocity and position control. Most use position-control loops, which are control laws designed for the unique requirements of motion systems. Several loop structures are commonly used in the motion-control industry. One technique is to place a velocity loop inside a position loop. The position loop is tuned after the velocity loop. One needs to start by configuring the system as a position loop. Acceleration feed-forward is a natural extension of velocity feed-forward. A current proportional to the acceleration command is added to the velocity loop output. Feed-forward allows the command response of an outer loop to approach that of an inner loop. Here, velocity feed-forward allows the position controller bandwidth to approach the velocity loop bandwidth. Acceleration and velocity feed-forward together allow the position-controller bandwidth to approach the current-loop bandwidth. In most applications, the motor feedback device is used to calculate the load position. This calculation requires scaling to account for the transmission components such as gear trains, belt/pulley sets, and lead screws.