What Is Cascade Control?

Cascade control system is a system where two regulators work in series, and the output of one regulator is used as the given value of the other regulator. The system is mainly applied to the occasions where the lag and time constant of the object are large, the interference is strong and frequent, the load changes greatly, and the control quality is required.

Example: Cascade Control System of Furnace Outlet Temperature and Furnace Temperature

The cascade control system uses two sets of detection transmitters and two regulators. The output of the former regulator is used as the setting of the latter regulator, and the output of the latter regulator is sent to the regulating valve.

The former regulator is called the main regulator, and the variable that it detects and controls is called the main variable (the main controlled parameter), which is the process control index; the latter regulator is called the sub regulator, and the variable that it detects and controls is The secondary variable (secondary controlled parameter) is an auxiliary variable introduced to stabilize the primary variable.

The entire system includes two control loops, a primary loop and a secondary loop. The secondary circuit is composed of the secondary variable detection and transmission, the secondary regulator, the control valve, and the secondary process; the primary circuit is composed of the primary variable detection and transmission, the main regulator, the secondary regulator, the control valve, the secondary process, and the main process.

One perturbation: Perturbation that acts on the main controlled process, but does not include the secondary loop. Secondary disturbance: The disturbance acting on the secondary controlled process, that is, the disturbance included in the secondary loop range.

When disturbance occurs, the steady state is destroyed and the regulator works. According to the position of the disturbance application point, it is analyzed in different cases:

* 1) The disturbance action is used for the secondary circuit

* 2) disturbance action is used in the main process

* 3) Disturbance acts on both the secondary circuit and the main process

The analysis shows that in the cascade control system, the introduction of a secondary loop can not only overcome the disturbance entering the secondary loop early, but also improve the process characteristics. The secondary regulator has the function of "coarse adjustment", and the main regulator has the function of "fine adjustment", thereby further improving its control quality.

* Improved the dynamic characteristics of the process and the quality of system control. ^[1]

* Applied to processes with large capacity lag

* Applied to the process with large pure delay

* Applies to processes with drastic changes and large amplitudes

* Applied to the process of parameter correlation

* Applied to non-linear processes

system design

* Selection of main parameters and design of main circuit

* Selection of secondary parameters and design of secondary circuit

* Selection of control system control parameters

* Selection of control law for main and auxiliary regulators of cascade control system

* Determination of the positive and negative modes of the main and auxiliary regulators of cascade control system

Design of main circuit

The main loop of the cascade control system is fixed value control. The design of the single loop control system is similar. The design process can be carried out according to the simple control system design principles. Here mainly solves the problem of the coordination of the two loops in the cascade control system. It mainly includes how to select the secondary controlled parameters, determine the principles of the main and secondary loops, and so on.

2. Design of the secondary circuit

Since the secondary circuit is a follow-up system, it has strong suppression and adaptive ability to the secondary disturbances contained in it. The secondary disturbance has little effect on the main controlled quantity through the adjustment of the primary and secondary circuits. In the secondary circuit, as far as possible, the main disturbances that change drastically, frequently and with large amplitude during the controlled process should be included in the secondary circuit. In addition, as many disturbances as possible should be included.

They are summarized as follows.

(1) The primary disturbance shall be included in the secondary circuit in the design.

(2) Include more disturbances in the secondary circuit.

(3) The hysteresis of the secondary controlled process should not be too large to maintain the fast corresponding characteristics of the secondary loop.

(4) A part of the controlled object with obvious nonlinear or time-varying characteristics should be attributed to the sub-object.

(5) When it is necessary to accurately track the flow, the optional flow is the secondary controlled quantity.

It should be noted here that (2) and (3) have obvious contradictions. Including more disturbances in the secondary circuit may cause the secondary circuit to have too much hysteresis, which will affect the rapid control of the secondary circuit. Therefore, the integration of (2) and (3) must be considered in the design of the actual system.

For example, the cascade control system shown in Figure 1 with the material outlet temperature as the main controlled parameter, the furnace temperature as the sub controlled parameter, and the fuel flow as the control parameters, assuming that changes in fuel flow and gas-heat value are the main disturbances, The design of the disturbance is reasonable in the secondary circuit.

3. Matching of primary and secondary circuits

1) Matching of the number of disturbances and time constants contained in the main and auxiliary circuits

The design considers that the secondary loop should include as many disturbances as possible. At the same time, attention should also be paid to the matching of the number of disturbances in the primary and secondary loops. If the secondary loop includes more disturbances, the longer the channel is, the larger the time constant is, the secondary loop control effect is not obvious, and the effect of its fast control will be reduced. If all disturbances were included in the secondary circuit, the primary regulator would lose control. In principle, it is necessary to ensure that the ratio of the number of disturbances and the time constant of the primary and secondary circuits is between 3 and 10 in the design. The ratio is too high, that is, the time constant of the secondary circuit is much smaller than the time constant of the main circuit. The secondary circuit is sensitive and controls quickly, but the number of disturbances contained in the secondary circuit is too small, which is not good for improving the control performance of the system. Too low, the time constant of the secondary circuit is close to the time constant of the main circuit, or even greater than the time constant of the main circuit. Although the secondary circuit is beneficial to improve the dynamic characteristics of the controlled process, the control effect of the secondary circuit lacks fastness and cannot be timely and effectively Overcome the influence of disturbance on the controlled quantity. In severe cases, there will be a "resonance" phenomenon between the main and auxiliary circuits, and the system will not work properly.

2) Matching and selection of the control laws of the main and auxiliary regulators

In a cascade control system, the roles of the primary and secondary regulators are different. The main regulator is a fixed value control, and the secondary regulator is a follow-up control. The system has different requirements for the two circuits. The main loop is generally required to have no difference. The control law of the main regulator should be selected from the PI or PID control law; the secondary loop requires fast control and can have a margin. Generally, the P control law is selected without introducing I or D control. If I control is introduced, the control process will be prolonged and the fast control effect of the secondary circuit will be weakened. It is not necessary to introduce D control because the P control of the secondary circuit has already played a fast control role. The introduction of D control will make the control valve act too much Not conducive to the control of the entire system.

3) Determination of the positive and negative action modes of the main and auxiliary regulators

One

1. Used to overcome the large capacity lag of the controlled process

In the process control system, the capacity lag of the controlled process is large, especially when some controlled quantities are parameters such as temperature, the control requirements are high. If a single-loop control system is used, it often cannot meet the requirements of the production process. The use of a cascade control system to improve the dynamic characteristics of the process, increase the system operating frequency, reasonably structure the secondary loop, reduce the impact of capacity lag on the process, and speed up the response. When constructing the secondary circuit, a secondary circuit with a small lag should be selected to ensure a fast-acting secondary circuit.

2. Pure lag used to overcome the controlled process

The existence of pure hysteresis in the controlled process will seriously affect the dynamic characteristics of the control system, making the control system unable to meet the requirements of the production process. The cascade control system is used to form a secondary loop at a position closer to the regulating valve and with less pure hysteresis. The main disturbance is included in the secondary loop to improve the control ability of the secondary loop to the system, which can reduce pure hysteresis to the main controlled The impact of the amount. Improve the control quality of the control system.

3. For suppressing large-scale disturbances

The secondary loop of the cascade control system has a strong ability to suppress disturbances in the loop. As long as the large-scale disturbance is included in the secondary circuit during design, its influence on the main controlled quantity can be greatly weakened.

4. Used to overcome the nonlinearity of the controlled process

In process control, there is a certain degree of non-linearity in the general controlled process. This will cause the characteristics of the entire system to change when the load changes, affecting the dynamic characteristics of the control system. Single-loop systems often cannot meet the requirements of the production process. Since the secondary loop of the cascade control system is a follow-up control system, it has a certain degree of self-adaptability, which can compensate the effect of nonlinearity on the dynamic characteristics of the system to a certain extent.

Cascade control system applied to objects with large capacity lag

When the capacity lag of the object is large, if single-loop control is adopted, the control time of the system is long and the overshoot is large, and the control quality often cannot meet the production requirements. If cascade control is used, the analysis of its characteristics shows that a secondary parameter with a small lag can be selected to form a secondary loop, so that the time constant of the equivalent object can be reduced to improve the system's working efficiency and speed up the reaction. , Can get better control quality. Therefore, for many objects whose temperature or quality parameters are adjusted parameters, the capacity lag is often large, and the quality of control of these parameters in production is relatively high. At this time, a cascade control system should be used. ^[1]

Cascade control system applied to objects with purely large lag

When the pure lag of the object is large and the single-loop feedback control system cannot meet the process requirements, sometimes a cascade control system can be used to improve the control quality of the system. Because after the cascade control system is used, it is possible to choose an auxiliary parameter as a secondary parameter near the regulating valve and the pure hysteresis to form a secondary circuit with a small pure hysteresis. When the disturbance is applied to the secondary loop, the primary loop is used to control the main disturbance before it affects the main parameters by the purely lagging main object, so as to overcome the influence of the pure lag. The secondary circuit has a small pure lag and timely control, which can greatly reduce the influence of the disturbance on the main parameters.

The cascade control system is applied to the objects with large and varied disturbances.

The secondary loop of the cascade control system has a strong ability to correct the disturbance entering it. Therefore, when the system is designed, as long as the drastically changing and large-scale disturbance is included in the secondary circuit, the influence of the drastically changing and large-scale disturbance on the main parameters can be greatly reduced. ^[1]

Cascade control system applied to objects whose parameters are related

In some production processes, sometimes two interrelated parameters need to be controlled using the same medium. In this case, if a single-loop feedback control system is used, two units need to be installed. As mentioned earlier, installing two regulating valves on the same pipeline is not only uneconomical, but also inoperable. For such objects a cascade control system can be used. The primary and secondary of the interrelated parameters are distinguished to form a cascade control to meet the technological requirements. ^[1]