For this first encounter with the eMotor, we show only priliminary result by comparing open-loop and close-loop responses. The PID gains are chosen at some “decent” values, with no fine tuning attempted yet. Watch the video at bottom of this article to see how the experiment was set up. The command speed is set to 3 rev/sec. Actual speed data of 100 points is sent to PC via serial port. (I use a USB-to-serial cable from FTDI and basic terminal software to capture the data.)
Around middle of the data transfer, I pressed the “torque disturbance” push-button on the eMotor. This resembles putting more load to a real motor, say, trying to stop rotation by squeezing the shaft with your fingers, an action my students didn’t like much.) In the open-loop case, we could visualize the reduction of rotation speed. In contrary, the PID algorithm compensates such action by commanding more (e)torque to the eMotor. Figure 3 and 4 shows the time responses of the two cases. We see that the open-loop response in Figure 3 already has significant steady-state error since the beginning, and the response gets worse after torque disturbance is applied. The closed-loop response (Figure 4) drops at the instant the button is pressed but recovers the commanded speed after some time.
Feedback always helps when we use it wisely. The eMotor is no exception.