Introduction to Control Systems

|Learning Objective | |Understand the basic concept in control systems. |

|Basic control theory |

Open-loop control system

An open-loop control system is one in which the control signal of the process is independent of the process output. The control accuracy is determined by the calibration of the plant. Fig. 1-1 contains the functional block diagram of the open-loop system.

| | |[pic] | |Fig 1-1 Block diagram of an open-loop control system |

Advantages of open-loop control system

․ simple and inexpensive

․ no stability problem

Disadvantage of open-loop control system

․ cannot compensate for any disturbances that add to the controller’s driving signal

Closed-loop control system

A closed-loop control system depends on the output of the process to adjust the signal controlling the closed loop. A feedback device measures the process output. The process output is compared to the user command and an output from the plant. Fig. 1-2 illustrates the functional block diagram of a closed-loop control system. The plant controls the process and determines any error between what was commanded and what is actually happening. The process may be any controllable quantity such as voltage, current, speed, torque, temperature, or position.

| | |[pic] | |Fig 1-2 Block diagram of a closed-loop control system |

Advantages of closed-loop control system

․ less sensitive to noise, disturbances and changes in the environment ․ transient response and steady-state error can be controlled more conveniently and with greater flexibility

Disadvantages of closed-loop control system

․ relatively expansive

․ may be unstable if not properly designed

Architecture of a closed-loop control system

• Controlled Variable (CV).

• Set point.

• Error = set point – current value of CV.

• Manipulated Variable.

• Feedback Loop.

•

|[pic] | |Fig 1-3 Architecture of a closed-loop control system |

Feedback control real-time scheduling

• Choices for control variables, manipulated variables, set points. • Choice of appropriate control functions. Is PID (the P stands for proportional control, I for integral control and D for derivative control) enough? • Stability problem of feedback control in the context of real-time scheduling? • How to tune control parameters?

• How significant is the overhead and how to minimize it? • How to integrate a runtime analysis of time constraints with scheduling algorithms?

Using negative feedback control system

• they typically become more stable

• they become less sensitive to variation in component values • it makes systems more immune to noise

Transfer function of a control system

The transfer function of a control system may be defined as the ratio of the output to the input.

On this basis, it is possible to predict how the system will perform if the transfer function is known. It is sometimes difficult to obtain...