What is Z control? How does it differ from PID control, and what is the concept of temperature control that suppresses over?

What is Z-control control?

In temperature control, there is a requirement to "raise the temperature to the set temperature as quickly as possible."

On the other hand, there is also the demand to "not exceed the set temperature."

For example, let's consider equipment that heats up to 200°C.

Even if heating continues because the temperature has not yet reached 200°C, some of the heat previously applied by the heater may remain inside the equipment.

If that heat is transferred to control object with a delay, the temperature can exceed 200°C.

Z control is control that considers how to supply energy to the temperature in order to achieve both a rapid increase in temperature and prevention of overheating near the setpoint.

How is this different from PID control?

In typical PID control, the operation variable is determined by combining three operations—P, I, and D—based on the difference between the setpoint and the measured value.

Z-control is a different control algorithm from PID control.

However, this does not mean that "PID is not used in Z control."

Z control also uses adjustment parameter such as P (proportional band), I (integral time), and D (derivative time).

These parameter are not used directly in the same way as in general PID computation, but rather are utilized internally within Z control.

Developed from temperature regulation of the skunk cabbage plant

origin of the concept for Z control was the "Zazenso" plant.

Symplocarpus renifolius is a plant that blooms in the cold environment of early spring.

A distinctive feature is its ability to generate heat on its own.

Even in environments with large fluctuations in outside temperature, it possesses the ability to regulate temperature, maintaining the spadix (the part of the plant that resembles a spadix) at approximately 20°C.

Skunk cabbage adjusts heat quantity in response to changes in its own temperature, maintaining a constant temperature range.

Z-control is a model that examines the relationship between temperature change and heat generation, and applies it to industrial temperature control.

However, the mechanism of the skunk cabbage was not simply transferred to the machine.

We are developing control methods to achieve the necessary rise time and control accuracy near the setpoint required for industrial temperature control.

What is over?

For example, even if the set temperature is 200°C, the actual temperature may exceed 200°C.

This phenomenon, where the measured value exceeds the set value, is called over.

While process may not pose a problem even if the set temperature is temporarily exceeded, if there is higher limit to the processing temperature, it can affect product quality and process itself.

If it takes time for heat to transfer, it becomes difficult to control.

One of the factors that makes temperature control difficult is "dead time."

For example, if the heater and temperature sensor are far apart, even if power is supplied to the heater, the temperature sensor's reading will not change immediately.

The heater generates heat, that heat is transferred through the equipment, and the temperature at the location to be control changes. Only then does the temperature sensor detect that change.

If you continue to apply a high output during that time, assuming that the temperature is still low, a lot of heat may accumulate inside the equipment.

The delayed arrival of that heat to control object leads to over.

Raise quickly, but don't overdo it.

If the goal is simply to reduce over, one approach is to lower the heating rate and slowly raise the temperature.

However, that would mean it would take a long time to reach the set temperature.

Not only the current temperature, but also the temperature changes

Z-control considers not only the current temperature but also how the temperature is changing.

For example, it's not just the information "the current temperature is 180°C" that's important, but also the change that "the temperature is 180°C and is rising further."

If the temperature is rising towards the set value, it can be assumed that the temperature will continue to rise due to the heat that has been added up to that point.

Therefore, the output is adjusted to prevent supplying more energy than necessary, even before reaching the set value.

Reduce the input of more energy than necessary.

In equipment with significant wasted time, continuing to heat the system because the temperature is still low can result in supplying the equipment with more heat than necessary.

Z control takes into account temperature changes and the energy supplied to the equipment, and suppresses the input of unnecessary energy as it approaches the set value.

Maintains a stable temperature near the set value.

Simply raising the temperature to near the set value is not sufficient for effective temperature control.

Once the target temperature is reached, it needs to be maintained stably.

The Z control combines the concept based on temperature regulation in the Zazen-sou with compensation designed to maintain accuracy near the setpoint, as used in industrial temperature control.

This combines control control stabilize it near the set value.

It's not simply a matter of saying it's superior to PID.

Z-control is not always superior to PID control.

There are many pieces of equipment that can achieve a sufficient temperature response with PID control.

On the other hand, for temperature control that are difficult to adjust with PID control, such as over and long dead time, Z control becomes an option.

The important thing is not to ask "which is better, PID or Z control?", but to consider control method based on what kind of temperature response you want to achieve.

Equipment that wants to suppress over

For equipment where exceeding the set temperature affects quality or process, the extent to which over can be suppressed is crucial.

Z control aims to suppress over by limiting the input of more energy than necessary as the system approaches the setpoint.

Equipment that takes time for heat to transfer

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

In equipment with such long periods of wasted time, control while only monitoring the measured temperature means it takes a long time to determine the results of the heating process.

Z control can also be used in temperature systems like this.

Equipment that requires consideration of temperature stability and energy consumption.

By preventing excessive heating, we can reduce wasted energy input.

However, this does not mean that a certain amount of energy savings can always be obtained by using Z control.

The effects will vary depending on the equipment structure, operating conditions, and conventional control settings.

Metal heat treatment

In metal heat treatment, there is process where the material is heated to a set temperature, and then that temperature is maintained for a certain period of time.

In process like this, not only the time it takes to reach the set temperature, but also the time it takes to over and stabilize near the set value are important.

Z control is also applied to temperature control in these heat treatment processes.

Plasma nitride reactor

Plasma nitriding is a surface treatment method used to harden metal surfaces.

Temperature management during processing is necessary, and there are examples of temperature control using Z control.

Observe the temperature control of a plasma nitriding furnace.

High-frequency induction heating

In high-frequency induction heating, a magnetic field generated by a coil is used to heat the metal itself.

This is a heating method used for processes such as quenching, annealing, brazing, and melting.

Even with this type of heating equipment, there are temperature control systems that allow you to select Z control.

Observing the hardening of engine valves by high-frequency induction heating.

Equipment with long periods of wasted time, such as large vacuum furnaces.

In large vacuum furnaces, there are walls or insulation materials between the heater and the furnace interior, which can cause a long delay before the heat from the heater reaches the control location.

In such equipment, there are also control methods that combine the concepts of Z control and cascade control.

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

Z control is one option for temperature control.

Simply using Z control does not solve all temperature control problems.

The results of temperature control are influenced by factors such as the temperature sensor, sensor placement, heater capacity, equipment heat capacity, heat transfer, dead time, disturbance, and control parameter.

If the current PID control is providing the required temperature response, there is no need to necessarily switch to Z control.

First, let's identify what the problem is with the current temperature control.

• over is large.
• It takes time for the value to stabilize near the set value.
• There is a long period of wasted time, making PID tuning difficult.
• The temperature changes significantly due to disturbance.
• Excessive heating is occurring.

After adjusting the PID constant, checking the sensor position and heater capacity, if it becomes necessary to review control method itself, Z control becomes an option.

PID can also be set in Z control.

In Z control, P (proportional band), I (integral time), and D (derivative time) are used as tuning parameter.

However, because the internal algorithm differs from that of typical PID control, simply setting the same PID values used in PID control may not necessarily result in the same response.

The adjustment is made while checking characteristic of control object and the actual temperature response.

Some models offer auto tuning capabilities.

Some temperature controllers that support Z control have models that can determine the P.I.D. through auto tuning.

However, the values obtained through auto tuning may not necessarily represent the optimal response for all equipment.

It is important to check the actual waveform depending on the desired temperature response.

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

Settings and features vary depending on the model.

Even with controllers that support Z control, the available settings and auxiliary functions vary depending on the model.

Before actually using the product, please check the product specifications and instruction manual for the applicable model.

This article will not go into detail about how to configure individual products.

origin of the Z control concept lies in the skunk cabbage plant, but it is not a direct reproduction of the skunk cabbage's mechanism.

Simply modeling the temperature regulation of the skunk cabbage plant presents challenges in terms of the speed at which it reaches the setpoint and control accuracy near the setpoint, which are required for industrial control.

Therefore, we have developed control that takes into account both the rise to the set value and stability around the set value, so that it can be used for industrial temperature control.

For more detailed information on control characteristic of Z control and its application to equipment with long dead times, please refer to the following academic papers and presentation materials.