[Daily Question 20090210] What are the complex control systems?

What are complex control systems? answer: 1. Cascade control system 2. Ratio control system 3. Feedforward control system 4. Uniform control system 5. Split-range control system 6. Selective control system 7. Double-impulse, three-impulse control system This post was last edited by Mobei Yihai on 2009-2-10 11:27 ]

1. Cascade control system 2. Ratio control system 3. Feedforward control system 4. Uniform control system 5. Split-range control system 6. Selective control system

Complex control systems have: 1. Ratio control system 2. Feedforward control system 3. Cascade control system 4. Uniform control system 5. Split-range control system 6. Selective control system 7. Three-impulse control system 8. Fuzzy control and other new control systems

The ones I have used include: cascade control loop, fixed ratio control loop, variable ratio control, three-impulse control, feedforward control

In general, single-loop control systems can meet most production control requirements. However, when the capacity lag of the controlled object is relatively large, the load changes violently and frequently, or the process has high product quality requirements, it is difficult for the single-loop control system to meet the control requirements, which requires the design of a complex control system to control the production process. There are many types of complex control systems. According to the structure of the system and the tasks it undertakes, common complex control systems are divided into two categories.: ①Control systems that improve response curve performance indicators, such as cascade, feedforward, pure lag compensation, etc. ②Systems developed according to certain specific requirements, such as ratio, uniformity, division, selection, inference, etc.

The ones we have used generally include: ratio control system ； Three impulse control system ； Feedforward control system ； uniform control system ； Cascade control system ； Split range control system ； selective control system ；

There are many types of complex control systems. According to the structure of the system and the tasks it undertakes, common complex control systems are divided into two categories.: ①Control systems that improve response curve performance indicators, such as cascade, feedforward, pure lag compensation, etc. ②Systems developed according to certain specific requirements, such as ratio, uniformity, division, selection, inference, etc. Cascade control system Ratio control system Feedforward control Uniform control Split-range control Selective control I have a detailed introduction to complex control systems, please contact me if necessary. This post was last edited by jfhou126 on 2009-2-10 13:47 ]

Complex control systems include cascade control systems, feedforward control systems, ratio control systems, uniform control systems, and selective control systems.

Cascade control system, ratio control system, uniform control system, split-range control system, selection control system, three-impulse control system, fuzzy control system, override control system, stick-and-stick control system, program control system, simulation control system, there are many types of combination control, such as: Programmed and feed-forward plus predictive control of polymerization temperature ；

(1) Cascade control system 1. Purpose of cascade control Among complex control systems, cascade control systems are the most widely used. Taking the control of the distillation tower as an example, as shown in Figure 2-69, the bottom temperature of the distillation tower is an important basis for ensuring the separation purity of the bottom product. It generally needs to be constant, so high control quality is required. To this end, we use the tower kettle temperature as the controlled variable and the heating steam that has the greatest influence on the tower kettle temperature as the manipulated variable to form a "temperature control system", as shown in Figure 2-69 (a). However, if the steam flow fluctuates frequently, it will cause temperature changes in the column kettle. Although the simple temperature control system in Figure (a) can overcome this disturbance. However, this overcoming is carried out after the disturbance has already had an effect on the temperature and caused the temperature to change. This is bound to have a great impact on product quality. So this solution is not very ideal. Therefore, making the steam flow stable becomes a problem that must be solved. It is a very common method to use the controlled variable as the one you want to be stable. The control scheme in Figure (b) is a control scheme to keep the steam flow stable. This is a plan to prevent disturbance, and it should be said to be very good in terms of overcoming the influence of steam flow. However, for the distillation tower, it is not just the steam flow that affects the temperature of the column still. For example, the interference of feed flow, temperature, and composition will also cause the temperature of the column still to change, which is beyond the control of plan (b). So, the best way is to combine the two. That is, the most important and strongest interference is pre-processed (coarse adjustment) in the way of flow control in Figure (b), and the influence of other interferences is finally completely resolved (fine adjustment) in the way of temperature control in Figure (a). However, if Figures (a) and (b) are mechanically combined, two control valves will appear on one pipeline, and there will be a phenomenon of mutual influence and neglect of one (i.e. correlation). Therefore, the two are processed into Figure (c), that is, the output of the temperature controller is connected in series to the external setting of the flow controller. Since the signals are connected in series, the system is called "stripping section temperature cascade control system." What needs to be explained here is that the ultimate purpose of the combination of the two is to introduce a "complex control system" composed of a secondary variable (flow rate) in order to stabilize the main variable (temperature). 2. Composition of the cascade control system Interference 2 Interference 1 Figure 2-70 Distillation tower bottom temperature-flow cascade control system From the previous analysis, it is obvious that there are two measurement transmitters, two controllers, two objects and one control valve in the cascade control system. The system composition block diagram is shown in Figure 2-70. In order to distinguish them, we describe them as primary and secondary, so there are the following common terms. Main variable - the controlled variable that is ultimately required to be controlled by the process, such as the temperature of the distillation tower still in the above example. Auxiliary variable - an auxiliary variable introduced to stabilize the main variable, such as steam flow in the above example. Main object - the production equipment that represents the main variable, such as the process equipment between the distillation tower bottom including the reboiler and the temperature detection point in the above example. Sub-object - represents the production equipment that represents the sub-variable, such as the steam pipe in the above example. Main controller - works based on the deviation between the main variable and the process set value. Its output serves as the external set value of the secondary controller and plays a leading role in the system, such as TC in the above example. Secondary controller - works according to the deviation of the secondary variable and the external set value from the main controller, and its output directly controls the control valve, such as FC in the above example. Main measuring transmitter - a transmitter that measures the main variable and converts the signal, such as TT in the above example. Secondary measurement transmitter - a transmitter that measures secondary variables and converts signals, such as the FT in the above example. Main loop - refers to the outer loop composed of the main measuring transmitter, main and auxiliary controllers, control valves and main and auxiliary objects, also called the main loop or outer loop. Secondary loop - refers to the internal loop composed of secondary measurement transmitter, secondary controller, control valve and secondary objects, also called secondary loop or inner loop. As can be seen from the figure, the output of the main controller is used as the external setting of the secondary controller, which is a feature of the cascade control system. 3. Characteristics of cascade control (1) The main loop is a fixed value control system, while the auxiliary loop is a follow-up control system. (2) Structurally, the main and auxiliary controllers are connected in series. The output of the main controller is used as the external setting of the auxiliary controller, forming two main and auxiliary loops. The system controls the actuator through the auxiliary controller. (3). It has strong anti-interference ability, stronger suppression of disturbance entering the secondary loop, high control accuracy and small control lag. Therefore, it is particularly suitable for systems with large hysteresis such as temperature. (4) Selection of loops and variables (1) The secondary loop should include as many disturbances as possible, especially the main disturbances. (2) The time constant of the secondary loop should be small and the response should be fast. Generally, the secondary loop is required to be at least three times faster than the main loop. (3) The selected secondary variable must be a direct factor affecting the main variable. (2) Uniform control system 1. Purpose of uniform control The production equipment of industrial production equipment are closely connected before and after. The discharge of the former equipment is often the feed of the latter equipment. As shown in Figure 2-71, the feed to the depropanizer tower (tower B for short) comes from the still of the first deethanizer tower (tower A for short). For Tower A, it is necessary to ensure the stability of the liquid level in the tower kettle, so there is a liquid level fixed value control system as shown in Figure 2-71. For Tower B, it is hoped that the feed amount will be relatively stable, so there is a flow rate control system as shown in Figure 2-71. Assuming that the liquid level in tower A rises due to disturbance, the liquid level control system will open valve 1 wider so that the liquid level in tower A reaches the requirement. However, as a result of this action, the feed volume of tower B increases higher than the set value, and the flow fixed value control system will close valve 2 to keep the flow stable. In this way, there is a conflict between the supply and demand of the two towers. On the same pipeline, the two valves are "opened" and "closed", making the continuous flowing fluid at a loss. In order to solve the contradiction between supply and demand in the front and rear processes and enable the two variables to take into account and coordinate operations with each other, a uniform control system is adopted. In fact, uniform control is named according to the function to be completed by the system. 2. Characteristics of uniform control. Most uniform control systems are required to take into account the two variables of liquid level and flow, and some also take into account both pressure and flow. Their characteristics are:: Not only should the controlled variable remain unchanged (not fixed value control), but also the two interconnected variables should change slowly within the allowable range. 3. Uniform control scheme (1) Simple uniform control system The simple uniform control system is shown in Figure 2-72. Its structure is exactly the same as the general single-loop fixed value control system. There is only a difference in the parameter settings of the controller. (2) Cascade uniform control system is simple. The uniform control system has a very simple structure and is easy to operate. However, there is often a problem of control lag for complex process objects. The best way to reduce lag is to add a secondary loop to form a cascade control system, which forms a cascade uniform control system, as shown in Figure 2-73. The structure of the cascade uniform control system is exactly the same as that of the general cascade control system, but the purpose is different. The difference mainly lies in the parameter settings of the controller. The entire system requires a "slow" word, which is opposite to the "fast" requirement of the cascade system. The difference between the main variable and the auxiliary variable is only in name. The main variable does not necessarily play a leading role. The status of the main and auxiliary variables is determined by the value of the controller. The values ​​of the two controller parameters are processed according to the requirements of uniform control. Generally, the proportional action is enough for the auxiliary controller. Sometimes a little integral action is added. The purpose is not entirely to eliminate the residual difference, but just to make up for the weaker proportional control action for smooth control. The main controller uses proportional control. In order to prevent the control range from being exceeded, a little integral action can be added appropriately. The larger the proportion of the main controller, the higher the stability of the auxiliary variable. In actual work, the proportion of the main controller can be large enough to prevent loss of control. When setting the parameters of the controller, it is advisable to first debug the controller first and then the main controller, and use the empirical trial and error method to gradually debug the proportion from small to large based on the specific situation, and find a slow attenuation non-periodic process. (3) Ratio control system 1. Purpose of ratio control In industrial production, we often encounter the problem of mixing two or more materials in a certain proportion (ratio) or performing a chemical reaction. For example, in the ammonia synthesis reaction, the hydrogen-nitrogen ratio needs to be strictly controlled at 3:1, otherwise the ammonia production will decrease. ； The fuel amount of the heating furnace and the oxygen intake amount of the blower are also required to meet a certain ratio, otherwise, the combustion effect will be affected. The purpose of ratio control is to achieve the proportional relationship between two or more materials. 2. Ratio coefficient Among the two materials that need to maintain a ratio relationship, one material must be in a dominant position, called the main material (main flow), and the variable characterizing this material is called the active quantity F1 ； The other material is proportioned according to the main material. During the control process, it changes with the change of the main material and is called the slave material (auxiliary flow). The variable characterizing its characteristics is called the slave momentum F2. And the ratio of F1 and F2 is called the ratio coefficient, represented by K. K= F1/ F2 3. Ratio control scheme (1) Open-loop ratio control system Figure 2-74 shows the open-loop ratio control system. F1 is the uncontrollable active quantity and F2 is the driven quantity. When F1 changes, F2 is required to track the change of F1 to maintain F1/F2=K. Since the adjustment of F2 will not affect F1, it is an open-loop system. The open-loop control scheme is simple in structure, uses few instruments, and only requires a pure proportional controller or a multiplier. In essence, the open-loop ratio control system can only maintain a certain proportional relationship between the valve opening and F1. When F2 fluctuates due to changes in pressure difference before and after the valve, the system has no control function, and it is essentially difficult to guarantee the ratio between F1 and F2. This solution has no anti-interference ability for F2 and is only suitable for situations where F2 is very stable, so it is rarely used in actual production. (2) Single closed-loop ratio control system In order to solve the problem that open-loop ratio control has no anti-interference ability for the secondary flow, we added a secondary flow closed-loop control system, which constitutes a single closed-loop ratio control system, as shown in Figure 2-75. It is very similar to the cascade control system in structure, but because the active variable F1 of the single closed-loop ratio control system is still in an open-loop state, while the main and auxiliary variables of the cascade control system form two closed loops, there are still differences between the two. In this scheme, the closed-loop control system of the secondary variable has the ability to overcome various disturbances that affect the secondary flow and stabilize the secondary flow. The output of the main momentum controller F1C serves as the external set value of the auxiliary momentum controller F2C. When F1 changes, the output of F1C changes, causing the set value of F2C to change accordingly, causing the auxiliary flow rate to also change proportionally. Ultimately, it is guaranteed that F1/F2=K. The single closed-loop ratio control system has a simple structure, uses fewer instruments, and is easier to implement. In particular, the ratio is more accurate, so it is widely used. It is especially suitable for occasions where the main material is not allowed to be controlled technologically. However, since the active quantity is uncontrollable, the total flow rate cannot be fixed. In addition to the ratio control system introduced above, there are also double closed-loop ratio control systems and variable ratio control systems, which will not be introduced here. (4) Split-range control system 1. Composition of the split-range control system The split-range control system is driven by the output of one controller to drive two or more control valves with different working ranges. Most control valves are pneumatic diaphragm control valves, which are divided into two types: air opening and air closing. Its working signal is a gas signal of 20~100kPa. For a gas-opening valve, the greater the gas pressure signal sent by the controller, the greater the opening of the valve. That is, when the signal is 20KPa, the valve is fully closed (the opening is 0%), when the signal is 100KPa, the valve is fully open (the opening is 100%), and the opposite is true for the gas-closing valve. The signal sent by the controller to the control valve is generally a DC current of 4~20mADC, and an electric/gas converter or electric/gas valve positioner is required to achieve the conversion from 4~20mADC to 20~100kPa. The split-range control system uses the function of the valve positioner to divide the controller's output into several segments, and uses each segment to control a valve respectively. For example, by adjusting the valve positioner, valve A can complete the journey within the signal range of 20~60kPa, and valve B can complete the journey within the signal range of 60~100kPa. The selection of the action direction of the control valve in the split-range control system (air opening or air closing) should be determined according to the actual needs of the production process. 2. Application occasions of split-range control (1) To implement several different control methods. Sometimes the process requires the use of two or more media or means to control a controlled variable. Figure 2-76 shows a temperature range control system for a reactor. After the reactor is equipped with materials, steam is used to heat the reactor to start the reaction process. Since the synthesis reaction is an exothermic reaction, after the chemical reaction starts, the heat of reaction needs to be removed with cold water in time to ensure product quality. Here it is necessary to use split-range control methods to realize two different control projects. In the figure, valve A is an air-closing valve, and valve B is an air-opening valve. At the beginning, the temperature in the reactor did not reach the set value, that is, the measured value was very small, so the output of the "forward-acting controller" (controllers are divided into forward-acting and reverse-acting. When acting, the controller output increases as the measured value increases, and when acting reversely, the controller output decreases as the measured value increases) is very small, and the pneumatic signal converted by the valve positioner is also very small, close to 20KPa. As a result, valve A is fully open, valve B is fully closed, and steam enters the jacket, causing the temperature in the reactor to rise. As the temperature increases, the controller output value increases. It can be seen from the split-range relationship diagram that the A valve opening decreases and the steam volume decreases. When the reaction starts, the exothermic reaction causes the temperature in the reactor to rise higher. At this time, the output value of the controller will become larger and larger. The signal converted by the valve positioner is greater than 60KPa, so valve A is fully closed and the steam is stopped. At the same time, valve B is gradually opened, and cold water enters the jacket to cool down the reactor. Thus, the task of controlling a controlled variable using two means is completed. (2) Used to expand the adjustable range of the control valve and improve the control quality. In the production process, sometimes the control valve is required to have a large adjustable range to meet production needs. For example, in the pH value control of chemical "neutralization process", sometimes the flow rate changes greatly, and sometimes there is only a small range of fluctuations. Fine adjustments cannot be made with large-diameter valves, and small-diameter valves cannot adapt to large changes in flow rate. At this time, two control valves with different diameters, large and small, can be connected in parallel as shown in Figure 2-77. (5) Selective control system 1. Purpose of selective control General process control systems can only work when the production process is in a normal state. If a special situation occurs, there are usually two processing methods. One is to use the interlocking protection system to automatically alarm and then stop the vehicle. ； The second is to switch to manual operations to gradually return production to normal. However, both methods have shortcomings, because although emergency parking is safe, it causes great economic losses and takes a long time. Although manual emergency treatment is economical, manual operations are stressful, error-prone, and operation reliability is poor. The selective control system can overcome the shortcomings of both. The so-called selective control system means that there are two sets of control systems to choose from. Under normal working conditions, choose one set, and when production is in an abnormal state in the short term, choose another set. This achieves the purpose of automatic protection without stopping the vehicle. Therefore, selective control is also called replacement control or override control. If automatic interlocking is hard protection, then selective control is soft protection. 2. Types of selective control systems When to use selective control and which control system should be selected by the selector. Depending on the position of the selector and the content of the selection, the selective control system can be divided into the following types: (1) The selector is between the transmitter and the controller to select the controlled variable. (2) The selector is between the controller and the control valve to select different controllers or manipulated variables. 3. Application examples Figure 2-78 shows the ammonia cooler selective control system. The ammonia cooler uses liquid ammonia to evaporate and absorb heat to cool the material. This solution is to ensure that the outlet temperature of the cooled material is the value required by the process. When the material outlet temperature is high, the liquid ammonia feed amount should be increased so that more liquid ammonia evaporates and the material outlet temperature decreases. However, if the liquid level in the ammonia cooler is too high, the evaporation space will be reduced, affecting the evaporation of liquid ammonia, and the temperature will not drop. It may even cause the gas ammonia to carry liquid, causing a "liquid hit" phenomenon when entering the ammonia compressor, causing a safety accident in the compressor. Therefore, it is required that the liquid level in the ammonia cooler cannot exceed a certain limit. To this end, a liquid level controller LC must be added, and a low-value selector LS is used to select between the two controllers according to working conditions to form a selective control system "selecting different controllers." Under normal operating conditions, the liquid level is lower than the set value, and the output of the reaction liquid level controller is very large. The low value selector selects the temperature controller with a low output signal to control the air valve opening. When the temperature of the outlet material is very high, the amount of liquid ammonia fed increases. If the liquid level approaches or exceeds the set value, the output of the liquid level controller (reaction) decreases. When it drops below the output value of the temperature controller, the low value selector cuts off the output of the temperature controller, and selects the liquid level controller to control the control valve (gas opening valve) to lower the liquid level, increase the evaporation space, and reduce the material outlet temperature. The output of the temperature controller also decreases, and when it is less than the output of the liquid level controller, it is selected again. This is how a selective control system works. The principles for other types of choices are the same and will not be discussed here. (6) Feedforward control system 1. Purpose of feedforward control Most control systems are closed-loop control systems with feedback. For this kind of system, no matter what interference causes changes in the controlled variable, it can be eliminated. This is the advantage of feedback (closed-loop) control systems. For example, the feedback control of the heat exchanger outlet temperature shown in Figure 2-79, whether it is changes in steam pressure, flow rate, or feed flow rate, temperature, as long as it ultimately affects the outlet temperature, this system has the ability to overcome it. However, this kind of control is carried out after the disturbance has caused an impact and the controlled variable deviates from the set value, so the control effect lags behind. Especially when disturbances are frequent and the object has a large lag, the impact on the control quality will be greater. So if we predict that a certain disturbance (such as feed flow) is the main disturbance, it is best to suppress it before it affects the outlet temperature. As shown in Figure 2-80, as soon as the feed volume increases, FC immediately opens the steam valve and uses the increased steam to deal with the excess cold material. If the design is good, it can basically ensure that the outlet temperature is not affected. This is the feedforward control system. The so-called feedforward control system refers to a system that controls according to the size of the disturbance change. Its purpose is to overcome the lag and overcome the disturbance before it affects the controlled variable. 2. Characteristics of feedforward control (1) Feedforward control works based on the principle of invariance and is more timely and effective than feedback control. If the influence of the control system on the controlled variable and the influence of the disturbance on the controlled variable can be made equal in magnitude and opposite in direction, the influence of the disturbance on the controlled variable can be completely overcome. (2) Feedforward control is an open-loop control system. The control results of feedback control can be tested through feedback, but it is unknown whether the control results of feedforward control meet the requirements. Therefore, in order to completely overcome the disturbance, it is necessary to conduct in-depth research and thorough understanding of the characteristics of the controlled object. (3) There is no universal controller for feedforward control, but a "special" controller depending on the object. (4) One kind of feedforward can only overcome one kind of interference. (3) Feedforward-feedback control As mentioned earlier, feedback control can ensure that the controlled variable is stable at the required set value, but the control effect lags behind. Although feedforward control functions in advance, it cannot know and guarantee the control effect. Therefore, the ideal approach is to combine the advantages of both to form a feedforward-feedback control system, as shown in Figure 2-81. Feedforward is used to overcome the main disturbance, and feedback is used to overcome other disturbances to stabilize the controlled variable at the required set value.

The only ones I have used are cascade control and ratio control. The one upstairs is very detailed. I’ll learn from it.* Why can't I see the picture?

Cascade control system Ratio control system Uniform control system Split-range control system Selective control system Three-impulse control system Fuzzy control system Override control system Robust control system There are so many!

Three-impulse control system, feedforward control system, cascade control system, ratio control system, selective control system. Split-range control system.

According to the structure of the system and the tasks it undertakes, common complex control systems include: cascade, uniform, ratio, split-range, feedforward, substitution, three-impulse and other control systems

Cascade control system, ratio control system, feedforward control system, uniform control system, split-range control system

Complex control systems include cascade control systems, ratio control systems, feedforward control systems, uniform control systems, split-range control systems, selective control systems, double-impulse, three-impulse control systems, etc.

I don’t know whether I should say this, but I feel that follow-up posts on this post are of little value. Can be closed. I recommend everyone to take a look: "Modern Control Theory" and other books.

There are many types of complex control systems. According to the structure of the system and the tasks it undertakes, common complex control systems are divided into two categories.: ①Control systems that improve response curve performance indicators, such as cascade, feedforward, pure lag compensation, etc. ②Systems developed according to certain specific requirements, such as ratio, uniformity, division, selection, inference, etc.

Complex control is compared with traditional single-loop control. The more commonly used ones include cascade control system, ratio control system, uniform control system, split-range control system, selective control system, etc. . . Robust control and fuzzy control are methods of handling control loops and should have nothing to do with complex loops or simple loops.~~~