What is the difference between cascade control and split range control?
Achieving accurate and effective control is essential for process automation in order to accomplish operational objectives and preserve system stability. To fulfil particular process needs, two popular control strategies Split Range Control and Cascade Control are used.
In order to manage complex systems where disturbances must be addressed closer to their source, cascade control employs a hierarchical method with two controllers cooperating. Split range control, on the other hand, uses a single controller whose output is split to run several actuators, allowing for the smooth operation of processes that call for distinct actions, like heating and cooling.
Choosing the best approach for a particular application requires an understanding of how different techniques differ from one another. A thorough comparison of split range control and cascade control is given in the table below.
| Aspect | Cascade Control | Split Range Control |
|---|---|---|
| Definition | A control strategy using two controllers in a hierarchy: a primary (master) and a secondary (slave). | A single controller whose output is divided into two or more ranges to control multiple actuators or control devices. |
| Controllers Used | Requires two controllers: one for the primary process variable and another for the secondary process variable. | Only one controller is used, but its output is split to operate multiple actuators or valves. |
| Signal Handling | The output of the primary controller becomes the setpoint for the secondary controller. | The output signal is split into multiple ranges, each controlling a specific device (e.g., heating and cooling valves). |
| Purpose | To improve control performance by isolating and addressing disturbances closer to the source. | To allow a single control variable to control multiple devices for handling different process requirements. |
| Control Objective | Focuses on improving system stability and response by addressing fast-acting disturbances. | Focuses on distributing control actions to different actuators for achieving specific system requirements. |
| Key Components | Primary controller (master), Secondary controller (slave), Feedback from both controllers. | A single controller, Two or more actuators/valves, each with a specific operating range of the control signal. |
| Operation | The master controller controls the main process variable, while the slave handles disturbances affecting the secondary loop. | A single controller outputs a signal divided into ranges, each activating specific actuators based on process needs. |
| Control Output | Output of the master controller is used as the setpoint for the slave controller. | Output is split (e.g., 4-12 mA for cooling, 12-20 mA for heating) to operate actuators for distinct operations. |
| Disturbance Handling | Better disturbance rejection as the secondary controller quickly corrects local disturbances. | Not designed for fast disturbance rejection; used to manage multiple control objectives with one controller. |
| Typical Applications | Temperature control with flow control as a secondary variable, Boiler drum level control. | Heating and cooling control using separate valves, Pressure control with venting and compression actuators. |
| Example | Reactor temperature control where the primary controller regulates the temperature, and the secondary controls the flow rate. | Reactor temperature control using a single controller to manage both hot and cold water valves through split signals. |
| Advantages | Provides precise control by isolating primary and secondary variables, Reduces interaction between control loops. | Simplifies control for systems needing multiple actuators, Reduces hardware requirements (single controller). |
| Disadvantages | Requires additional hardware and configuration for secondary control loop. | Can lead to overlap or interaction if not configured properly, May not handle disturbances effectively. |