The disturbance rejection tests demonstrate the controller's ability to compensate for external disturbances, which impact the motor position. In this test a load torque is applied to the system, held for short period of time, and then removed from the system. Figure 6-10 is an example of a disturbance rejection test. The response of the controller is measured using the maximum position error and settling time. The maximum position error shows the deviation from the goal position, and is an indication of how aggressively the controller is tuned. Aggressive tuning produces a low maximum error. In Figure 6-10, the PI controller presents a greater maximum error than the SpinTAC controller, indicating that the SpinTAC controller is more responsive in compensating for system error.

 

Settling time refers to the amount of time from the point when the disturbance occurs until the position feedback returns to a fixed band around the goal position reference. This is also an indication of how aggressively the control loop is tuned. If the controller is tuned too aggressively it will have a long settling time, and will oscillate around the goal position before settling. Figure 6-10 shows that the PI controller has a longer settling time than the SpinTAC controller. Note that there is little oscillation from either controller as they return to the goal position.

When doing disturbance rejection testing it is important to test at multiple speed and load combinations. In order to properly evaluate the effectiveness of your position controller, tests should be done across the entire speed range since the speed at which the motor is running will impact the ability to control the position. The test results will indicate whether the controller will meet the application specifications or whether the controller needs to be tuned multiple times for different operating points. The disturbance rejection tests were conducted at nine different speed and load combinations (Table 6-7 and Table 6-8) in order to test a wide range of operation.

It is also important to be able to create repeatable disturbances. This can be accomplished using a dynamometer or a disturbance motor. Creating repeatable disturbance is an important factor when evaluating multiple controllers. If test conditions cannot be replicated, it is difficult to adequately compare the responses of multiple controllers.

For the following test results, a disturbance load profile was created that applied 25%, 50%, and 100% of rated torque to the motor. The test results compare the performance of SpinTAC Position Control to a standard PI position control system. The following parameters were measured:

• Average Recovery Time (from the point of disturbance until within 2% of the reference position) - the average recovery time was measured when the load was applied, and when the load was removed from the system.
• Absolute Average Position Error - positive or negative deviation from the reference position (in mechanical degrees) when a system disturbance is introduced.
• Maximum Position Error - maximum deviation (in mechanical degrees) from the goal position when a disturbance is introduced.