Instrumentation Monthly Test | Measurement | Control
Temperature control is critical for many industrial processes, and although in many instances a simple controller can be sufficient, a growing number of applications require high precision temperature control. Ian Collins, Product Manager at West Control Solutions, outlines the role of cascade control in temperature control and how it can be used to improve process quality.
What is cascade control?
Directly heating a product can cause damage in manufacturing processes, so a secondary medium, usually liquid or gas, is often used to transfer heat to the product. The downside is that this often introduces a high level of themal lag to the system, leading to overshoot and instable control. It is difficult for a single instrument to control the temperature precisely when these conditions exist.
Cascade control is a temperature control method that links two control loops in sequence to eliminate this problem. The first controller is known as the master and its role is to monitor the temperature of the end product. The output of the master controller is then used to drive the setpoint of the second control loop, known as the slave. The role of the slave is to control the main heater load and the setpoint driven by demand from the master. As a result, the control system monitors changes at both the end product and at the heater, controlling the system accordingly. This method of control is particularly effective in applications where process variables have a high risk of disturbance and may not remain constant
Cascade control in application
Cascade control can be utilised in many applications across industry. For example, in dairy pasteurisation where water is used to heat milk. In this application water reaches temperature quickly while the milk heats more slowly, which can often lead to overheating. It’s crucial that the milk in this process must not overheat for reasons of reduced product quality. With single loop control the water temperature during the heating process would continue to rise until the milk reached its setpoint – a satisfactory outcome at this stage. Although single loop control generally achieves setpoint efficiently and at speed, problems occur in the moments that follow because the lag created by the process of heating the water to in turn heat the milk results in temperature overshoot of the milk, which seriously compromises the product quality. The answer is to utilise cascade control using two loops, with one loops measuring the temperature of milk and a second loop measuring the water temperature. In this example the cascade control system relies on the measurement recorded by the rising temperature of the milk to slowly decrease the heat of the water, allowing the milk to continue heating at a pace that will not result in overshoot.
Although cascade control is popular in the food and beverage industry, cascade temperature control can be used in many more applications than only food and drink manufacture. Such as in heat exchnagers for industrial building heaters, which can benefit from using cascade control to overcome temperature overshoot. A typical example in this application would see a product setpoint temperature programmed on a master controller that is then compared to the product temperature, in which case a maximum input value restricts the jacket temperature. At start-up the master loop compares the product temperature to its setpoint and gives maximum output. This sets the maximum setpoint on the slave, which is compared to the jacket temperature that is giving maximum heater output. As the jacket temperature rises, the slave’s heater output falls and the product temperature also rises at a rate dependant on the transfer lag between the jacket and product. This causes the master’s output to decrease, reducing the ‘jacket’ setpoint on the slave, effectively reducing the output to the heater. This continues until the system becomes balanced.
Setting up and tuning
As the aforementioned application examples outline, there are a wide range of possible uses for this temperature control method. Once cascade control has been correctly specified for an application, it is then essential that it is correctly configured to meet the requirement of that specific application. To ensure correct configuration the first step is to ensure the secondary loop is tuned before the primary loop, because if the primary loop is tuned first then the secondary loop setpoint will constantly change and the autotune function will be constantly recalculating and unable to tune. Therefore, a fixed setpoint should be entered on the secondary loop so that other parameters can be tuned. Once complete, the primary loop can be tuned and the secondary loop setpoint can then become dependent upon the primary loop.
Cost-effective control
In applications where multiple substances cannot be adequately controlled by a single controller, the installation of highly effective cascade temperature control equipment enables the reduction of temperature overshoot and fluctuation in control. By consulting with a temperature control specialist, manufacturers across from industry can successfully preserve product quality in a cost-effective manner and optimise application efficiency.