1、 - 1 - Introduction to Control System The design of people to control natures forces successfully has been the catalyst for progress throughout history. Our goal has been to control these forces in order to help perform physical tasks, which were beyond our own capabilities. During the dynamic and h
2、ighly motivated 20th century, the control-system engineering has transformed many of our hopes and dreams into reality. Control-system engineers have made very important contributions to our advancements. As we look back, control-system engineers have made contributions to robotics； space-vehicle sy
3、stem, including the successful accomplishment of the lunar soft landing； aircraft autopilots and control； control-system for ships and submarines, and automatic control systems for hydrofoil, surface-effect ships, high-speed rail system； and most recently, control system for the magnetic-levitation
4、rail systems. Yet all these types of control, if they are to be successful, have certain features in common. A control system is the collection of components connected in such a way as to effect control over certain aspects of the domain in which the system operate. In addition, Control systems exis
5、t that require no human interaction, such as aircraft automatic plots and automobile cruise control system. In dealing with control system, particularly engineering control system, we will deal with a variety of components, indicating that the subject is an interdisciplinary one. The control enginee
6、r needs a working knowledge of mechanics, electronics, electrical machines, fluid mechanics, thermodynamics, structures, material properties, and so on. Computers are used widely to implement control schemes, and an increasing knowledge of information technology and software engineering is therefore
7、 being demanded of control engineers. Obviously not every control system contains elements form each of the above domains but most useful control system contain elements form more than one discipline. In general, control systems can be categorized as being either open-loop or closed-loop. The distin
8、guishing feature between these two types of control system is the use of feedback comparison for closed-loop operation. - 2 - Open-loop Control System Open-loop control systems represent the simplest form of controlling devices. Figure 2.1 illustrates a simple tank-level control system. We wish to h
9、old the tank level h within reasonable acceptable limits even though the outlet flow through value v1 is varied. This can be achieved by irregular manual adjustment of the inlet flow rate by value V2. The system is not a precision system, as it does not have the capability of accurately measuring th
10、e output flow rate through value V1, the input flow rate through value V2, or the tank level. Fig.2.2 shows the simple relationship that exists in this system between the input (the desired tank level) and the output (the actual tank level). This signal-flow representation of the physical system is
11、called a block diagram. Arrows are used to show the input entering and the output leaving the control system. Fig.2.2 Tank-level control-system black diagram Fig.2.1 Tank-level control system - 3 - This control system does not have any feedback comparison, and the term open loop is used to describe
12、this absence. Fig2.3 illustrate a field-controlled dc motor turning a cutting wheel at a constant speed. When a piece of wood is applied to the surface of the cutting wheel, it acts as a disturbing torque of the motor any results in a reduction of the speed of the cutting wheel, assuming that contro
13、l signal remains constant. This situation can be represented as shown in Fig.2.4. The symbol appearing between the motor and the load represents a substractor. The effect of disturbance torques, or other secondary inputs, is detrimental to the accurate functioning of an open loop control system, it
14、has no way of automatically correcting its output, because there is no feedback comparison. Closed-loop Control System Closed-loop control systems derive their valuable accurate reproduction of input from feedback comparison. An error detector derives a signal proportional to the differences between the input and output. The closed-loop control system drives the output until it equals the input and the error is zero. Any differences between the Fig.2.3 Field-controlled dc motor Fig.2.4 Field-controlled dc motor having a disturbance torque
