Cascade control

Cascade control has two controllers, with the output of the main controller serving as the input for the secondary controller. Step control features a controller that operates two or more valves

Cascade control involves two parameters and one control valve, while split-range control involves one parameter controlled by two control valves.

Cascade control involves two parameters and one control valve, while split-range control involves one parameter that is controlled by two control valves. This is a very precise way of putting it

Cascade Control Systems I – Concept of Cascade Control Systems (1) Example 1: Temperature control in a continuous tank reactor. Controlled parameter: Reaction temperature T1. Means of regulation: Changing the flow rate of cooling water. Object of regulation: The reactor. Interfering factors: Material flow rate, inlet temperature, chemical composition of the material, temperature at the cooling water outlet, pressure before the control valve. 1. Using a simple control system: (1) Working process: Suppose there is a disturbance in the cooling water flow; for example, if the temperature of the cooling water rises, then T1 also rises. This triggers a signal from the regulator, which causes the valve to open, thereby reducing T1. (2) Problem: From the moment the valve opens until T1 drops, the process must pass through the jacket and tank walls, resulting in a long regulation time and large dynamic deviations. 2. Adoption of a cascade control system: (1) Design concept: After an interference occurs, first control the jacket temperature, and then control the outlet temperature. (2) Block diagram: (3) Operating process: (i) If there is interference with the cooling water, it first affects the temperature T2 of the jacket water; T2C is used to control the cooling water in order to overcome this interference before it affects temperature T1. (ii) If there is interference in the feed: T1 must change, and the T1C output signal is sent to T2C, thereby shortening the control path and increasing the control speed as well. (II) Concept of cascade control systems 1. Cascade control system: It utilizes two or more regulators, which are connected in series; the output of one regulator serves as the input for another regulator. 2. Block diagram of a general cascade control system: Several terms: as shown in the figure. II. Characteristics of cascade control systems (1) It has a strong ability to suppress disturbances entering the secondary loop. Compared to a simple loop, a cascade control system has an additional secondary loop, GC1 and GC2, added to its structure: the transfer functions of the primary and secondary regulators. 1. When the perturbation is f2, the block diagram becomes: Figure 3-7 Simple: Cascade: By comparing the two equations above, it can be seen that due to the control action of the secondary loop, its negative feedback reduces the impact of the perturbation f2 on C2. III. Cascade System Design The key to cascade design lies in the secondary loop ； The key to designing the secondary loop lies in selecting the secondary parameters, while also taking into account the connection between the primary and secondary loops as well as process feasibility. (1) Design of the main and auxiliary circuits 1. Selection of main parameters: A measurable process parameter that can reflect the control requirements in the most direct, accurate, and rapid manner should be chosen. 2. Design of the secondary circuit: (1) The selection of secondary parameters should result in a small time constant for the secondary circuit, a short control path, and rapid response. (2) The secondary parameters should be selected so that multiple disturbances enter the secondary loop, particularly the main disturbances with the most severe, frequent changes and the largest amplitudes. There is a contradiction: (3) the primary and secondary time constants must match. In principle: the matching of time constants should ensure that the ratio of the operating frequencies of the secondary and primary circuits is greater than 3, while the ratio of the time constants of the primary and secondary systems should be between 3 and 10. (4) The rationality and feasibility of taking the *art exam. (5) Pay attention to production economics. In the design of the secondary circuit, when there are several alternative options available, economic principles should be combined with quality control requirements in order to minimize equipment investment. (II) Selection of control laws for the main and auxiliary controllers: Main controller: The same as in a simple loop. PI, D: To ensure product quality, the secondary controller is usually chosen as P; when a flow rate secondary loop is used, integration can also be appropriately incorporated due to its small time constant. Selection of the controller’s positive and negative actions: Principle for determination: To ensure that the entire circuit forms a negative feedback system. Specific steps: (1) Select the opening and closing mode of the control valve based on the safety principles of the production equipment. (2) Secondary regulator: Ensures that the product of Kc2KvKp2Km2 is positive ； (3) Main regulator: Ensures negative feedback in the main circuit. Example: Reactor control system. Solution: (1) From the perspective of production safety, the cooling water control valve should be of the air-actuated shut-off type. Kv takes the value of “-” (2) When the cooling water flow increases, T2 decreases. ∴Since Kp2 is “-”, Kc2 is “+”. It is a feedback controller. (3) T2↓ T1↓ ∴ Kp1 is “+” ∴ Kc1 is “+”; that is, it is a feedback controller. Verification: As the reaction temperature T1 increases, the output of the main controller decreases; in other words, the set value of the secondary controller decreases (with the given value remaining unchanged and the measured value increasing). The output of the secondary controller decreases, the opening degree of the control valve increases, the flow rate of cooling water rises, and the reaction temperature drops. II. Design of the stage-controlled system: 1. Application scenarios: Expanding the range of control valves, meeting the requirements of special processes, and ensuring safety. 2. Selection of the positive and negative actions of the regulator: to ensure negative feedback ； 3. Actuator’s position range: Determine the position range ； 4. Combination methods for actuator position control: III. Several issues in implementation: 1. The leakage rate of the control valve should be low ； 2. Step control can be achieved using valve positioners or other instruments. 3. Reasonably select the flow characteristics of the valves: A smooth transition is required at the intersection point of the flow characteristics of the two control valves; however, since the gains Kv of the two control valves differ in magnitude, a sudden change may occur at their intersection point. To mitigate the gain discontinuity at the split points, a signal overlap method is employed, such as: 0.02 ~ 0.065 Mpa ； 0.055 ~ 0.1Mpa.

As the name implies, cascade control involves two regulators working in series to control one control valve, while split-range control involves one regulator controlling two control valves.