1 Feedback Control

1.1 Closed Loop FeedbackAll Activities

The figure below shows a plant (motor) with and without feedback control. The controller is that part of the system that receives the measurement of the plant output, then generates the plant input, hence closing the loop.Control system design is the task of designing this controller such that the closed-loop system has satisfactory performance. Broadly speaking, some goals of most closed-loop control systems are listed below. Experiment with the figure to see how the controller affect the system performance in these areas.

  • Command Tracking ...cause the output to track the reference input closely. Observe how closely the output is to the desired value under different conditions
  • Disturbance Rejection ...isolate the output from unwanted disturbance inputs. Add some disturbance to the system (a load on the motor in this example) and observe the difference between in uncontrolled and controls motor.
  • Parameter Sensitivity ...reduce the effect on the output of variations in plant parameters. Change the plant and observe the difference between in uncontrolled and controls motor. In the example changing parameters might equate to changes in the motor performance characteristics as it heats up with use.
These are goals of both continuous and digital control systems.

Open loop vs closed loop comparison
Figure 1: Advantages of Feedback Control

Change the input type, disturbance level and motor parameter using the controls below. Observe the difference, in Figure 1, between the top system without feedback and the bottom system with feedback. Compare the desired speed with the measured speed.

  

  1. How does disturbance affect the system without a controller?
  2. How does disturbance affect the system with a controller?
  3. Change the Motor %Change value. This corresponds to using a motor with characteristics other than those originally specified. Notice how the two responses change. Why might you have a system where some components are not to the original specification?