﻿1.2 Open-Loop

# Open-Loop System

The system below consists of a small electric DC motor that drives a dial. You can control the position of the dial by changing the input voltage to the motor using the slider or the "d", "f" and "s" keys. You can also change the characteristics of the system by changing the inertia of the dial and the system friction. As you change the input voltage to the motor you should observe that the speed of the motor is proportional to the input voltage. The mathematical model is derived in Example 2.1

The system below does not include a controller. You need to act as the controller. Experiment with controlling the position of the dial. To do this you will need to turn the motor on and off (or even reverse its direction) until the dial stops at the desired location.

### Questions on Terminology

• What is the plant? (answer)
The plant is the system you are trying to control. In this example it is the motor and dial combined.
• What is the plant output? (answer)
The plant output is something you can measure to determine the status or behavior of the plant. Usually the output is what you are trying to control. In this example it is the position of the dial.
• What is the plant input? (anwer)
The plant input is what you use to change the plant output. In this example it is the voltage to the motor.
• What happens if you approach the desired position with too much speed? (answer)
You may "overshoot" the desired position if you approach it with too much speed. In that case you need to reverse the voltage to drive the motor backwards. You can reverse the motor voltage even before you reach your desired position in order to "brake" the dial

### Experiment

1. Use the "d", "f" and "s" keys to change and zero the input voltage
2. Set the motor speed (voltage) to zero (you can press reset).
3. Practice using the keys until you can move the motor 90° with your eyes closed. This is a matter of memorizing the timing and key strokes. For example, press "f", wait 2 seconds, press "s". This is open-loop control.
4. Change the system friction, and move the motor using the same timing you used previously with your eyes closed.

### Question

• Based on this experiment, what is one limitation of open-loop control? (answer)
Open-loop control assumes the plant dynamics are constant. Every time the system changes, even a small amount, you will need to adjust the control algorithm.
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