The first generation of control systems emerged between 1930 and 1940, primarily represented by mechanical control technology, exemplified by stationary instruments. The second generation, emerging in 1950, mainly consisted of relay control technology (primarily electrical control) and analog control technology, represented by regulators. The control systems currently referred to are the third generation, which arose in the 1970s. Their main technological representatives are distributed control systems ( DCS ) for process industries and programmable logic controllers ( PLCs ) for discrete industries.
Currently, automation control systems are evolving towards fourth-generation control systems. These fourth-generation systems exhibit two trends: system openness and the integration of process industry control and discrete control, reflected in the fusion of DCS and PLC functions. Currently, PC-based control systems, embedded control systems, and fieldbus control systems all aspire to become the mainstream systems of fourth-generation control systems. However, due to their respective shortcomings, it is generally believed that fourth-generation control systems will ultimately be comprised of flexible control systems—TCS—that combine the characteristics of current third-generation control systems with openness and integration.
To correctly understand the meaning of control systems, there are some control-related terms that must be understood, which will be introduced here.
I/O points:
When discussing control systems, I/O points are one of the most frequently heard terms. It refers to input/output points; "I" stands for INPUT, and "O" stands for OUTPUT. Input and output are both relative to the control system. Input refers to the measured parameters entering the control system from instruments, while output refers to the parameters output from the control system to actuators. Each parameter is called a point. The size of a control system is sometimes determined by the maximum number of I/O points it can control.
Analog and digital signals:
In control systems, another common term is analog quantity and digital quantity. Whether input or output, a parameter is either analog or digital. Analog quantities refer to continuous values that vary within a certain range, such as temperature (0-100 degrees Celsius), pressure (0-10 MPa), liquid level (1-5 meters), and the opening degree of an electric valve (0-100%). Digital quantities, on the other hand, refer to physical quantities with only two states, such as the on/off state of a switch, the closed/open state of a relay, and the on/off state of a solenoid valve. For control systems, since the CPU is binary, each bit of data has two states: "0" and "1". Therefore, digital quantities can be represented using only one bit within the CPU; for example, "0" represents on and "1" represents off. Analog quantities, depending on their precision, typically require 8 to 16 bits to represent a single value. The most common analog quantity is 12-bit, with a precision of 2-12, and the highest precision is approximately 0.025%. Of course, in actual control systems, the accuracy of analog quantities is also limited by the accuracy of analog-to-digital converters and instruments, and it is usually impossible to achieve such a high level.
Control loop:
Typically, in analog signal control, a controller determines an output based on an input variable according to certain rules and algorithms. This input and output form a control loop. Control loops can be open-loop or closed-loop. In an open-loop control loop, the output is determined by a reference variable, and the input and output are not directly related. A closed-loop loop, on the other hand, feeds back the control loop's output as its input, comparing it with the setpoint or expected output value. Closed-loop control, also known as feedback control, is the most common control method in control systems. Several common feedback control modes are introduced below.
Two-position control:
This is the simplest form of feedback control, sometimes called on/off control. This type of control sends a switching signal when the measured value reaches its highest or lowest value. Although the measured quantity may be analog, the control output is an on/off signal, hence the name two-position control. Many temperature controllers and level switches in industrial settings use this method.
Proportional control:
The controller's output value is proportional to the deviation of the measured value and setpoint of the controlled parameter or a reference point. Proportional control is smoother than two-position control, eliminating the oscillations in the controlled variable that occur in two-position control. For example, regarding the liquid level in a reaction tank, if the set liquid level is 2700 mm, when the liquid level decreases, the valve on the feed pipe should increase its opening, and when the liquid level is high, the opening should decrease. The ratio of increase and decrease is proportional to the magnitude of the deviation between the liquid level and the setpoint.
Integral control:
In integral control, there is a pre-defined relationship between the change in the value of the controlled variable and the time it takes for the control system output to take effect. The actuator output gradually reaches the set value. This control method arises because it often takes a certain amount of time for the actual control element and actuator to send an output signal and for the controlled variable to reach the set value. The most common example is temperature control. For instance, suppose we know that when the gas valve is opened to 60%, the water temperature in the water heater can reach a comfortable 45 degrees Celsius for showering. However, if you turn the valve all the way to 60%, the water will still be cold; you have to wait for the water temperature to rise to around 45 degrees Celsius and then stabilize. If the control system uses proportional control instead of integral control, then when the valve output is 60%, the input temperature might still only be 20 degrees Celsius. Under proportional control, since the deviation still exists, the valve opening will continue to increase. Thus, when the water temperature rises to 45 degrees Celsius, the valve opening might reach 90% or even higher. At this point, although the control system will instruct the valve to remain still, the water temperature will continue to rise, possibly reaching 50 or even 60 degrees Celsius. The valve opening will then decrease, but the water temperature will continue to rise until it reaches 60%. When the valve opening decreases to 60%, the water temperature might still be 70 degrees Celsius. Only when the valve opening becomes 20% will the water temperature reach 45 degrees Celsius. At this point, the valve will stop moving, but the water temperature will continue to drop until it becomes cold water. If this is winter, the situation might be even worse. This is what happens with a temperature controller without integral control. If you have children, this is a very similar situation when they operate the water heater valve for the first time.
Differential control:
Derivative control is often used in conjunction with proportional and integral control. Because integral control has a lag, derivative control allows the control to react to deviations earlier, preventing the control system from becoming too sluggish. Using derivative control in conjunction with proportional and integral control allows the controlled state to reach a steady state more quickly without the oscillation phenomenon mentioned above.
PID control:
In practical control systems, depending on the actual variables, sometimes only one of the three control methods is used, sometimes two, and sometimes all three are employed simultaneously. Proportional control is represented by P, integral control by I, and derivative control by D. Depending on the method used, they are respectively called P control, PI control, and PID control. Among these, PID control is the most common control mode in control systems.
Delay control:
Typically used in applications involving on/off control, when a switch changes state (e.g., from "on" to "off"), the controller's output action is delayed for a period of time. For example, in proximity switches commonly used on production lines, when a workpiece is in place, the proximity switch sends a signal, and because the next roller is installed at a distance from the proximity switch, it usually starts rolling after a few seconds.
Chain control:
This is also commonly used in switch control applications. For example, if there are three switches, A, B, and C, switch C can only open when A and B are both open; or when A is open, C must open. This relationship is called interlocking control. In industrial settings, especially in situations involving safety control, interlocking control is very common. For example, in a reactor's vent valve, when the pressure reaches a certain value, the pressure switch signal changes, and the vent valve must open immediately.
Electric control:
The output of a control system is achieved through electrical quantities or electronic signals, and the controlled object is an electrically driven element, such as a relay, stepper switch, solenoid valve, servo drive , and frequency converter. Most automatic controls involve electrically driven control elements to some extent.
Hydraulic control:
In the operation of machines and equipment, many controls are performed using hydraulic control mechanisms. In applications requiring continuous speed control, hydraulic control is generally more convenient and inexpensive. When energy conversion efficiency is high, hydraulic control is often used in conjunction with servo control in electric control systems. This results in electro-hydraulic actuators with high efficiency and precision.
Pneumatic control:
There are three situations in which pneumatic actuators are used.
First, the movement path uses a standard combination of one-way pneumatic valves to complete the control logic function;
Second, components without moving parts are used in gas pipelines; these components operate by relying on the characteristics of the flowing gas.
Third, the motion logic control system employs modular built-in diaphragm, wound, or sleeve types. All three types of pneumatic components use compressed air as the power source for signal transmission or execution mechanisms. In factories, compressed air is readily available, clean, pollution-free, and safe, and its control functions and designs are very simple; therefore, pneumatic tools are now used on many production lines.
