What is cascade control? It's a mechanism that stabilizes temperature using two control loops.

First, let's consider general temperature control.

A temperature sensor measures the current temperature, and the temperature controller monitors the difference between that temperature and the set temperature to determine how much operation the heaters and other equipment.

This mechanism, which uses measured results to determine the next operation, is called feedback control.

PID control is also one of the widely used methods in feedback control.

In cascade control, another control loop is added to this one.

To understand cascade control, it's helpful to consider "why it's necessary to have two control loops."

The heater and temperature sensor are far apart.

For example, in large furnaces, the heater may be located near the furnace wall, far from the furnace interior or the product where temperature stabilization is truly required.

Even if you supply power to a heater, it takes time for that heat to reach the area you want to control.

The system determined that the temperature was still too low and continued heating.

If the temperature controller only looks at the final temperature sensor reading, it will conclude that "the temperature is still below the set temperature" because the measured value does not rise immediately even when the heater is heated strongly.

Therefore, we will continue heating it.

However, heat is still being stored in the heater during that time.

Delayed heat delivery can lead to over

Eventually, when the stored heat is transferred to the area you want to control, the temperature may exceed the set value.

Even if you reduce the output because it has exceeded the set value, the temperature may not drop immediately.

In equipment where there is a long time between operation and the appearance of results, over and hunting are more likely to occur.

Therefore, in cascade control, not only the final temperature to be control is measured, but also the state of the heater and other components preceding it.

For example, consider a system where the furnace temperature needs to be kept constant, and the heater temperature also needs to be measured.

On the primary side, while monitoring the furnace temperature, the system determines "what temperature the heater side should be set to at this moment."

The secondary side operation the heater to achieve the required temperature.

In cascade control, the two control loops are called the "primary loop" and the "secondary loop".

A linear loop handles the "ultimate value you want to control".

In a first-order loop, the value that you ultimately want to stabilize is measured and control.

Temperature control can include furnace temperature, container temperature, and product temperature.

A primary loop is sometimes called the "main loop."

A quadratic loop handles values that are "closer to operation side".

In the secondary loop, values close to those of the actual equipment being operation, such as heater temperature, are measured and control.

A quadratic loop is sometimes called a "secondary loop."

In cascade control, the primary and secondary sides do not operate independently of each other.

The primary side determines "what value to set the secondary side to bring the final temperature closer to the target."

That value will be used as the setting value for the secondary side.

How would this be expressed using SV, PV, and MV?

In control, the setpoint is called SV, the measured value is called PV, and the operation variable is called MV.

In cascade control, configuration is common to use the MV obtained on the primary side as the SV on the secondary side.

In other words, the "amount to be operation" on the primary side becomes the "target value" on the secondary side.

A major advantage of cascade control is that it allows for the creation of configuration where changes can be detected on the secondary side before they affect the final temperature.

In the case of only one loop

Even if a change occurs on the heater side, the controller cannot detect that change until the effect is transmitted to the furnace temperature and detected by the primary temperature sensor.

If there is a quadratic loop

If a secondary sensor, such as one placed on the heater side, configuration detect the change earlier, then correction can be performed more quickly on the secondary side.

As a result, it may be possible to address the issue before it has a significant impact on the temperature you ultimately want to control.

Meeting these conditions helps to reduce the effects of response delays and disturbance, thereby suppressing over and hunting.

Cascade control is considered for equipment where it makes sense to divide control loop into two.

Equipment where the heater and the area to be control are far apart.

In large furnaces, it can take time for heat to control from the heater to the desired location.

In such cases, configuration in which the heater temperature control on the secondary side can be considered.

Equipment subject to disturbance such as self-heating.

Some equipment generates heat through chemical reactions or other processes, even without external heating.

If such changes can be configuration more quickly on the secondary side, cascade control may be used to minimize their impact on the final temperature.

Equipment that changes temperature by operation other values such as flow rate.

Temperature control isn't limited to directly operation the heater.

For example, there is equipment that adjusts temperature by changing the flow rate of cooling water.

In this case, configuration can be considered in which the temperature is control on the primary side and the flow rate control on the secondary side.

A furnace that takes time for heat to transfer.

Cascade control may be considered even in equipment such as semiconductor manufacturing equipment, where it takes time for heat to transfer from the heating element to the actual object being processed.

Learn more about temperature measurement in diffusion furnaces and CVD furnaces.

Cascade control is not a system where simply increasing the number of control loops to two automatically improves control performance.

What's particularly important is "what to measure on the secondary side."

Can we quickly detect changes on the secondary side?

It is important that the value measured on the secondary side changes before the final control value, and that this value can be control independently.

If the two measurements are nearly the same, the effect is small.

If the primary and secondary sides change at almost the same time, the benefit of splitting control loop into two becomes small.

If a single PID is sufficient, there's no need to make it more complex.

If the desired response can be obtained with a single PID control, there is no need to complicate the system by adding sensors or control loops.

While the actual terminal connections and device settings will vary depending on the equipment used, the basic flow for considering cascaded control can be summarized.

1. Decide what you ultimately want to stabilize.

First, decide which value you actually want to control.

For example, furnace temperature, product temperature, container temperature, etc. These are the primary side measurements.

2. Decide what to measure on the secondary side.

Next, we consider values that can capture changes before they affect the final temperature.

Heater temperature, jacket temperature, and flow rate are some possible candidates.

3. Create a state where control is possible using only the secondary side.

First, we use the secondary sensor and operation device to create a state where the secondary value can be control to a set value.

4. Transferring the set value from the primary side to the secondary side.

The operation variable obtained on the primary side can be used as a setting value on the secondary side.

5. Adjust the secondary side.

Adjust the inner secondary loop to ensure stable operation.

6. Adjust the primary side.

After establishing a stable state on the secondary side, the primary side, which handles the final control value, is adjusted.

In cascade control, operation on the primary side are transmitted through the secondary side to the final control object.

Therefore, if you adjust the primary side while the secondary side is unstable, it becomes difficult to determine which PID setting is causing the problem.

Even when using auto tuning, be sure to check specifications and procedure of the controller you are using, and proceed from the secondary side to the primary side.

What is PID tuning? The basics of tuning from the perspective of temperature control waveforms.

What is auto tuning? The mechanism and precautions for automatically adjusting PID constant.

Simply attaching two sensors is not the solution.

The important thing is what each sensor measures.

On the primary side, we measure the value we ultimately want to control, while on the secondary side, we measure a value closer to operation side, allowing us to capture changes more quickly.

The secondary side settings are not necessarily fixed.

In typical temperature control, the system may operate with a fixed set temperature, such as 200°C.

In cascade control, the primary side changes the setpoint on the secondary side.

In other words, the secondary side does not continuously pursue a fixed target value, but rather follows the target value given by the primary side.

It is important that the secondary side can respond faster than the primary side.

If the response of the secondary side is as slow as, or slower than, that of the primary side, it becomes difficult to reap the benefits of cascade control.

We will verify that the secondary side is configuration to detect and control changes more quickly.

configuration is needed that allows values to be passed from the primary side to the secondary side.

When using two controllers, configuration is required that allows the value obtained by the primary side to be received as the set value on the secondary side.

The specific signal models and setting method will vary depending on the controller used.

In explanations of cascade control, configuration using two devices, a primary controller and a secondary controller, is often used.

However, the essence of cascade control is not simply using two controllers.

Therefore, in addition to combining two controllers, there are also methods of configuration two loops within a controller or instrumentation system that has multiple control functions.

It's easier to understand the "two controllers" setup if you think of it as one way to achieve cascade control.

Cascade control and PID control are not mutually exclusive; you cannot choose one over the other.

PID control is control method that determines operation variable based on set values and measured values.

Cascade control is a control configuration concept that focuses on how to connect two control loops.

Therefore, in cascade control, there are configuration that use PID control in both the primary and secondary loops.

What is PID control? The mechanism of PID and the basics of temperature control

What I've explained so far is the basic concept of cascade control.

Our company also publishes technical information on specific construction methods using two controllers, as well as cascade constant that can be used with particular controllers.