The first generation of control systems came into being between 1930 and 1940, mainly representing the mechanical control technology represented by ground-based instruments. The second generation was produced in 1950, mainly electrical control technology based relay control technology and regulator represented by analog control technology. At present, the control system is the third generation of control system, was born in the 1970s, the main technology is used for the process industry distributed control system (DCS) and programmable controller for discrete industry (PLC).
1, I/O points
I/O point is the term most often heard when discussing control systems. It refers to the INPUT/OUTPUT point, with I for INPUT and O for OUTPUT. Input/output are for the control system, input refers to the measurement parameters from the instrument into the control system, output refers to the parameters from the control system output to the actuator, a 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.
2. Analog quantity and switching quantity
In control systems, another common term is analog and switching. Regardless of input or output, a parameter is either analog or switching. Analog quantity refers to the size of the control system quantity is a continuous value that changes in a certain range, such as temperature, from 0-100 degrees, pressure from 0-10mpa, liquid level from 1-5 meters, electric valve opening from 0-100%, etc., these quantities are analog quantities. And the switching quantity refers to the physical quantity of only two states, such as the state of the switch on and off, the closure and opening of the relay, the solenoid valve on and off and so on.
For the control system, because the CPU is binary, each of the data has "0" and "1" two states, therefore, the switch quantity as long as the CPU internal one can be expressed, for example, with "0" to represent open, with "1" to represent off. Analog quantities, on the other hand, usually require between 8 and 16 bits to represent an analog quantity, depending on accuracy. The most common analog quantity is 12 bits, that is, an accuracy of 2-12, with a maximum accuracy of about 2.5 parts per million. Of course, in actual control systems, the accuracy of analog quantities is limited by the accuracy of analog/digital converters and meters, which is usually not possible to achieve such a high.
3. Control loop
Usually for analog control, a controller according to an input, in accordance with certain rules and algorithms to determine an output, so that the input and output form a control loop. There are differences between open and closed control loops. Open loop control loop, in which the output is based on a reference quantity, the input and output are not directly related. The closed-loop loop feeds back the output of the control loop as the input of the loop and compares it with the set value or the supposed output value of the quantity. Closed-loop control, also called feedback control, is the most common control mode in the control system. Several general feedback control modes are described below.
Two control
This is the simplest feedback control, sometimes called switch control. This control is to give a switch signal when the highest or lowest value is measured. Although the measured quantity may be analog, the control output is switched, so it is called two-digit control. In the industrial field, many thermostats and level switches are controlled in this way.
Proportional control
The output value of the controller is proportional to the deviation of the measured and set values of the controlled parameters or a reference point. Proportional control is smoother than two position control, eliminating the two position control when the oscillations of controlled quantity up and down. For example, for a reaction tank level, if the level is set at 2700 mm, the valve opening in the feed line will be increased when the level is lower, and decreased when the level is higher. The proportion of increase and decrease is proportional to the level and the deviation of set value.
Integral control
In integral control, the change of the value of the controlled variable has a predetermined relationship with the time from the output control of the control system to the actual effect. The output of the actuator gradually reaches the set value. The production of this control mode is due to the actual control elements and actuators from the output signal to the controlled variable to reach the set value often need a period of time.
Is the most common example of temperature control, for example, assume that we know to the gas valve opening to 60% of the time, can achieve the water temperature of the water heater is suitable for take a shower of 45 degrees, but, when you are in the position of the valve immediately turned to 60%, the water is still cold, you have to wait a moment, the water temperature rise to about 45 degrees, will be stable.
If the control system does not use integral control, but only proportional control, then when the valve output is 60%, then the input temperature value may still be only 20 degrees, then according to proportional control, since the deviation still exists, the valve opening will continue to increase. So, when the water temperature rise to 45 degrees, the valve opening could reach 90% or even higher, at this time, while keeping will notify the valve control system, but the water temperature will continue to rise, may in the 50 or 60 degrees, at this time, the valve opening will decrease, but before reduced to 60%, the water temperature will continue to rise, when the valve opening down to 60%, The water temperature may still be 70 degrees, until the valve opening becomes 20% and the water temperature becomes 45 degrees, at which point the valve will stop, but the water temperature will continue to drop until it becomes cold water. If this is winter, you may be in even worse condition. That's what happens with a temperature controller without integral control. If you have young children, this is what happens when the child first operates the valve on the water heater.
Differential control
Differential control is usually used in conjunction with proportional and integral control, and since integral control has a lag, differential control allows the control to respond to deviations earlier than the control system is too slow to respond. Differential control can be used together with proportional and integral control to make the controlled state reach a stable state more quickly, without the oscillation phenomenon mentioned above.
PID control
In the actual control system, according to the actual variables, the above three control methods sometimes only one, sometimes two, sometimes three at the same time. Proportional control is represented by P, integral control is represented by I, and differential control is represented by D. According to the way adopted, they are called P control, PI control, PID control. PID control is the most common control mode of the control system.
Time delay control
Usually used in the case of switching quantity control, when a switch state changes (such as from on to off), the output action of the controller will be delayed for a period of time. For example, the proximity switch commonly used in the production line signals when the workpiece is in position, and the next drum usually starts rolling several seconds later because it is some distance from the position where the proximity switch is installed.
Chain control
Is also commonly used for switch control occasions, such as three switches, A, B and C, C switch must be opened at the same time A and B, can be opened; Or when A is on, C must be on; This relationship is called linkage control. In the industrial field, especially where safety control is involved, interlocking control is very common. For example, the release valve in the reaction kettle, when the pressure reaches a certain value, the signal of the pressure switch changes, then the release valve must be opened immediately.
Electric control
Refers to the output of the control system is through the electrical volume or electronic signal to carry out, the control object is the electric actuator, such as relay, step switch, solenoid valve, servo drive and frequency converter and so on, most of the automatic control more or less will have electric control components.
The hydraulic control
In the operation of machines and equipment, much control is carried out by hydraulic control mechanisms. In the case of continuous speed control, hydraulic control is usually more convenient and cheap, when the energy conversion efficiency is high, hydraulic control and electric control servo control is often used at the same time. At this time, the formation of high efficiency and precision electro-hydraulic actuator.
Pneumatic control
Pneumatic actuators are used in three situations:
First, there is a standard one-way pneumatic valve combination on the movement line to complete the control logic function;
Two, in the gas pipeline using some components without moving parts, these components rely on the characteristics of the flow of gas and switch action;
Three, the motion logic control system, using modular built-in diaphragm, winding or sleeve type. All three pneumatic components are powered by compressed air as a transmission signal or execution mechanism. In the factory, because compressed air is easy to obtain, clean, pollution-free, and safe, the control function and design are very simple, therefore, many production lines now use pneumatic tools.