What is cascade control?
Cascaded control is a method of temperature control using a main controller (installed at the control terminal) and secondary controllers (installed on the control element). In cascaded control applications, in addition to the main controller, secondary variables are also measured and controlled to achieve better control effects. For example, monitoring and controlling the system's steam flow rate can achieve stable temperature control for heating objects.
Why use cascaded control?
For precision applications, it is difficult to control a single device without affecting product quality. Disturbances can cause excessive temperature overshoot, and thermal inertia can pose problems during slow system recovery. A solution to these problems is to connect two or more controllers (each with a separate input) in series to form a single regulating device.
Temperature control application example -- pasteurization of dairy products
In dairy pasteurization, the water temperature rises rapidly when heating milk, but the milk temperature rises very slowly. Crucially, the milk cannot be overheated. Traditional single-loop control systems use only one sensor to directly control the milk temperature: the water temperature continues to rise before the milk reaches the set temperature. While single-loop control technology can quickly and efficiently bring the milk to the set temperature, thermal inertia can cause the water temperature to exceed the milk temperature, continuing to heat the milk and causing temperature overshoot, ultimately severely impacting milk quality.
Solving this problem requires the use of dual-loop cascaded control technology. One control loop measures the milk temperature, while the other measures the water temperature. The cascaded control system slowly lowers the water temperature by recording the continuously rising milk temperature measurements, thus maintaining stable heating of the milk without temperature overshoot.
Process stability
In cascaded control, the disturbances generated by the rapid changes in the secondary controller will not affect the main controller, making cascaded control the ideal solution for precision applications.
Application Example - Heat Exchangers for Industrial Building Heating Systems
Another typical application of cascaded control to avoid temperature overshoot is in heat exchangers in industrial building heating systems. In this type of application, operators typically set a setpoint temperature on the main controller, and the control system then compares this temperature with the actual temperature to achieve control.
The maximum input value limits the sheath temperature. At startup, the master control loop compares the actual temperature with the setpoint temperature and outputs the maximum value as the setpoint for the slave controller. The slave controller then compares this setpoint with the maximum sheath temperature output by the heater. As the sheath temperature rises, the heater output controlled by the slave controller decreases. The actual temperature rises at a certain rate—depending on the thermal inertia between the sheath and the product. This causes the master controller output to decrease, reducing the "sheath" setpoint on the slave controller, effectively reducing the output to the heater. This process continues until the system reaches equilibrium.
A single multi-loop controller can achieve cascaded control, with the main loop power directly input to the slave loop; alternatively, two separate controllers can be connected together, with the main controller power being input as a remote setpoint to the slave controller and scaled proportionally to meet application temperature requirements—ideally, the slave controller's response time should be at least 5 times faster than the main controller.
How to configure the controller and adjust the cascade control?
First, the relationship between the variables and measured values in the main and secondary loops should be determined. The secondary control loop must be able to affect the main control loop; any disturbance to the system should act on the main control loop.
When configuring cascaded control in an application, the key is to adjust the secondary control loop first. If the primary control loop is adjusted first, the setpoint of the secondary control loop will constantly change, causing the automatic adjustment function to repeatedly calculate and fail to complete the adjustment. The correct approach is to set a fixed setpoint for the secondary control loop, then adjust all other parameters, and finally adjust the primary control loop. In this case, the setpoint of the secondary control loop will depend on the primary control loop.
Conclusion
When two or more functions cannot be fully controlled by a single device, cascading two or more control devices provides a simple yet highly effective solution for process temperature control. In many applications, cascaded control can significantly improve product quality and energy efficiency.
About WEST
As a global expert in process and temperature control, WEST Control Solution products comprise four main brands: WEST, PMA, Partlow, and Cal. For over 90 years, WEST has provided high-performance, high-quality products to industries such as plastics, heat treatment, packaging, food and beverage, and life sciences.
WEST is committed to developing close partnerships with its customers. The company regularly engages in discussions and research with clients, fully considering their requirements in new product development to ensure their needs are met. WEST provides reliable, easy-to-use products with dedicated support staff offering unparalleled service. While offering standard products, we are also committed to continuous innovation, providing customized products to our customers, earning the support and trust of customers worldwide.
