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What is cascade control, and how is it implemented in a DCS?
Cascade control: For example, if a storage tank needs to be heated to 80°C using steam, then the thermometer on the tank and the control valve in the steam pipeline together form a cascade control system
A cascade control system is a system in which two regulators operate in series, with the output of one regulator serving as the setpoint for the other regulator. This system is mainly applied in situations where the lag and time constant of the object are large, interference is strong and frequent, load variations are significant, and high control quality is required. The main characteristics of a cascade control system are: (1) In terms of its structural design, it is a double-loop control system composed of two controllers that operate in series ； (2) The purpose of the system is to improve the control quality of the primary variable by introducing secondary variables ； (3) Due to the presence of the secondary loop, there is a leading control effect on the disturbances that enter this loop, thereby reducing the impact of those disturbances on the primary variable ； (4) The system has a certain degree of adaptability to changes in load.
In my understanding of the dual PID loop, the output of the inner loop serves as the input for the outer loop
Cascade control is a system in which two regulators operate in series, with the output of one regulator serving as the setpoint for the other regulator. In a cascade control system on a DCS, the secondary loop is usually activated first, followed by the primary loop.
A cascade control system is a system in which two regulators operate in series, with the output of one regulator serving as the setpoint for the other regulator. A cascade control system uses two sets of sensing transmitters and two regulators; the output of the first regulator serves as the setpoint for the second regulator, while the output of the second regulator is sent to the control valve. The previous regulator is called the main regulator, and the variable it detects and controls is referred to as the main variable (the primary parameter under control), that is, the process control indicator ； The latter regulator is called a secondary regulator, and the variable it detects and controls is referred to as a secondary variable (secondary controlled parameter); it is an auxiliary variable introduced to stabilize the primary variable.