How does a cascade control work?

A cascade control consists of at least two control loops connected in series. The main controller (master) controls the primary process variable, while the secondary controller (slave) manages an intermediate variable that directly affects the primary variable.

A typical example is temperature control in an industrial oven. The primary controller monitors the product temperature and sends the setpoint value to the secondary controller, which controls the heating elements. This ensures that temperature fluctuations are compensated quickly, without the main controller having to intervene directly.

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What is a cascade controller?

A cascade controller is an instrument or software module that enables this hierarchical control. Through clever interaction between the main and secondary controller, higher control precision is achieved, especially in processes where disturbance can occur quickly.

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What is a cascade system?

A cascade system comprises the complete set-up of sensors, actuators and control technology that together form a cascade control. This system is used in various industrial processes where fast and stable controls are required.

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Set-up cascade control

The structure divides the system to be controlled into at least 2 parts. The setpoint of the system is specified on the co-called master controller, which regulates the actual value to the desired value:

In relation to the output level of the master controller, the slave controller regulates with an additional setpoint. The output level setting determines how large the additional setpoint should be at an output level of 0% or 100% of the master controller.

This structure can generally be used if an actual auxiliary value (xAux) can be measured in a control process and needs to be controlled relative to the output level of the master controller.

See diagram below:

Cascade versus PID control

The difference between cascade and PID control lies mainly in their structure and the way they address process disturbances.

A standard PID (Proportional, Integrating, Differentiating) control uses one control loop to control a process variable (e.g. temperature, pressure or level). The PID controller continuously adjusts the output (e.g. controlling a valve, pump or heating element) to keep the process value as close as possible to the desired  setpoint.

A cascade control uses two or more PID controllers connected in series. The main controller (master) thus monitors the primary process variable and sends its output as a setpoint to the secondary controller (slave), which manages an intermediate process variable. This allows the secondary controller to react faster to disturbances, keeping the master controller more stable.

Characteristics

PID control

Cascade control

Numer of controllers

1

Min. 2

Reaction speed to disruptions

Slower in complex processes

Faster thanks to secondary controller

Complexity

Simple

Advanced, requires more settings

Applications

Simplified processes

Complex systems with intermediate process steps

Example

Regulating final temperature in oven

Control of both product temperature and control
of heating elements in an oven

At JUMO we apply cascade controls in various industrial processes, using f.e. JUMO DICON touch, JUMO meroTRON and JUMO variTRON

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Cascade— user cases for the industry

Temperature control in heating and cooling processes

In chemical reactors and food processing, cascade control is used to minimise temperature variations. The main controller controls the product temperature, while the secondary controller controls the temperature of the heating of cooling medium. This prevents overshoot and optimises heat transfer.

Pressure control in steam boilers and compressed air systems

For a stable steam supply, the main controller monitors the boiler pressure and sends a set point to the secondary controller, which controls the burner. This ensures constant pressure and prevents inefficient fuel consumption.

Level control in tanks and reservoirs

In the water and wastewater industry, cascade control is used to stabilise the liquid level. The main controller measures the level and controls the secondary controller, which regulates the pump speed. This prevents sudden level fluctuations and improves process efficiency.

Speed control of pumps and fans

In HVAC systems, cascade control is used to efficiently control pumps and fans. For example, the main controller controls temperature or pressure, while the secondary controller controls the frequency converter of the fan of pump. This leads to energy savings and longer equipment life.

Speed control in conveyors and extruders

In the plastics and food industries, the speed of extruders or conveyors needs to be accurately adjusted. The main controller monitors product quality, while the secondary controller controls motor speed. This prevents product deviations and improves final quality.

Conclusion

Cascade is a powerful technique for complex industrial processes where accurate and fast control is required. By combining main and secondary controllers, disturbances can be handled better, leading to more stability, higher efficiency and better product quality.

JUMO is happy to help you implement optimal cascade control for your process. Contact us without any obligation!

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About the writer

My name is Saskia van der Laan and I have worked at JUMO for 35 years, combining my professional skills with my passion for writing. My passion is to use written text to translate technology into practice with the aim of informing and inspiring the reader. In my spare time, I like to do sports and enjoy a long walk with the dog.