Switch to AC Induction Motor Cuts Costs, Improves Stability

A flexible motor controller allowed a quick switch from a DC brushless motor to an AC induction motor to drive the gantry on this nuclear imaging system. All that needed to be done was modify a few pr programmable software parameters.

A motor-control module designed for use with both DC brushless and AC induction motors enabled GE Medical Systems to make an easy design change on a dual-head nuclear imaging system while still in the prototype stage, reducing problems with oscillations while cutting costs.

The imaging system consists of a rotating gantry structure and patient positioning table. Two heads, each holding a nuclear detector enclosed in an 800-lb lead box, are mounted to the gantry and rotate about its curved beams. They can be positioned up to 180° from each other. To control the gantry's DC brushless servo motor, GE engineers selected the SB1381 control module, which includes an integrated digital drive. The motor also drives the two heads via a 1,000:1 gear ratio.

Unfortunately, the first prototype suffered from oscillations due to the high load-to-motor inertia ratio. It was also impossible to tune the system for stability at all head positions.

To solve the problem, engineers replaced the low-inertia DC brushless motor with a high-inertia ac induction motor. This change eliminated the oscillation problem, since the change in the reflected inertia of the load caused by repositioning of the heads is relatively small compared to the inertia of the rotor.

What made this simple change possible is the fact that the control module is capable of controlling induction motors using vector control technique by quickly modifying a few programmable software parameters. While theoretically the position and velocity frequency response and acceleration is lower due to higher inertia of the motor, in this case the large load was the limiting factor. So the solution solved the stability problem with no sacrifice in performance, while providing a significant side benefit: cost savings. The AC induction motor was less than $300 vs. more than $1,000 for the DC brushless motor.

(Please Note: This application story was taken from the 6 June 2000 Design News article "Motion control without breaking the bank". )