1. Classification of Automatic Control Systems
Generally, process control often involves the flow of matter and energy. By controlling the magnitude of these flows, the process control objectives can be achieved. The actuator in motion control is an electric motor, while the actuator in process control is a regulating valve. Both motion control and process control are governed by the same automatic control theory.
However, due to the different characteristics of the controlled objects, the two types of control systems also exhibit different response characteristics. For example, motion control systems have short settling times and fast response speeds; while process control systems often contain large inertial elements and large time delay elements, resulting in long settling times and slow response speeds. These differences will exist when applying control theory to solve specific control problems. Within these two main categories of control systems, further classifications can be made based on the specific requirements of the controlled object, the characteristics of the input signal, whether the signal transmission process is continuous, whether the parameters are time-varying, and whether the system contains nonlinear elements.
2. Composition of an automatic control system
To describe the composition of an automatic control system, the parts that perform different functions in the system are often described using the term "links". A typical control system includes the links shown in Figure 4-14.
1) Given stage
A given input quantity is applied to the system through a given element. For example, some control systems use a given potentiometer to apply a voltage signal to the control system. In order to reduce the large impact on the control system caused by the given potentiometer suddenly applying a constant voltage signal to the control system at the moment the switch is closed, the given element sometimes uses an integrator to gradually increase the applied signal.
2) Comparison phase
The comparison stage compares the given quantity with the feedback quantity. For a subtraction of the feedback quantity from the given quantity, the feedback quantity and the given quantity must be connected with opposite polarities, so that the effect of the feedback quantity weakens the given quantity; this is called negative feedback comparison. Conversely, for an addition of the feedback quantity to the given quantity, the polarity of the feedback quantity must be the same as the polarity of the given quantity; that is, the effect of the feedback quantity increases the given quantity; this is positive feedback comparison. In multi-loop control systems, to obtain good response performance, sometimes one inner loop is connected with positive feedback, while all outer loops are connected with negative feedback.
3) Amplification stage
A closed-loop control system controls the output by using the difference signal between the setpoint and the feedback signal. Since the difference signal is very small (zero in a zero-difference system), it is insufficient to make the system work if applied directly to the control equipment; therefore, it must be amplified by an amplification stage.
4) Execution phase
The execution element, also known as the actuator, is the final link in a control system, controlling the controlled variable through its actions. In motion control systems, the actuator is often an electric motor, whose rotational or linear motion drives the load. In process control systems, the actuator is typically a regulating valve; automatically adjusting the valve's opening controls the flow rate or pressure of fluid in a pipeline, thus achieving control over the controlled object.
5) Control links
The control element, sometimes called the calibration element, controller, or regulator, is a manually configured element. Its purpose is to achieve good control performance. Good control performance means that the output follows the input changes faster, more stably, and more accurately.
6) Controlled object
The controlled object, also known as the controlled variable, refers to the physical quantity controlled by the system. The controlled object is often chosen as the output quantity. For example, the controlled object in a speed control system should be the running speed of the motor, and the controlled object in a temperature control system should be the temperature being controlled.
7) Feedback process
The feedback loop transmits the detected controlled variable back to the input, where it is compared with the setpoint to achieve closed-loop control. Some systems directly connect the measured quantity to the comparison loop; this is called unity feedback.
8) Disturbance Link
During operation, automatic control systems are inevitably affected and disturbed by various factors such as the environment and the equipment itself.
