PID Digital Temperature Controller

Temperature controllers are instruments used to control heating equipment. They are used in industrial and domestic environments to maintain a set temperature throughout time with minimal fluctuations. In this article, we will be talking about the PID Temperature Controllers, how they work and how to use them.

What is a PID Temperature controller and how does it work?

Let’s talk about temperature controllers in general before discussing PID controllers. Temperature controllers are devices that can control heating elements/coils to provide necessary temperature for a process. These can be electronic or electromechanical devices like thermostats. Their basic functionality is to turn on the heating element when temperature is below a minimum, and turn them off when the desired temperature is reached.

There are three main types of temperature controllers; on-off, proportional and PID loop type controllers. PID type is the most advanced type of temperature controller of them. It is the most accurate and fastest responding controller.

The acronym ‘PID’ stands for ‘Proportional-Integral-Derivative’ control, which is a highly popular and effective closed loop control method used in rapidly changing environments. It belongs to the ‘optimal’ category of control theory which describes the attempt to optimally achieve a certain process variable.

In the case of PID temperature controllers, the optimal variable is the process temperature. The device should work towards reaching the set temperature as fast as possible in the most accurate way without overshoot, lag or disturbance. To monitor the process value/current temperature, the PID temperature controllers use one or more thermocouple/RTD or some other form of temperature measurement. Using this value as an input against the set point, the controller then adjust the power supplied to the actuator (heater) to increase the temperature. If the current temperature is higher than the setpoint, it cuts off the power to the heater. The difference between the process value and the set point is called error. The controller tries to maintain the error close to zero all the time.

However, the idea behind the implementation of PID is to never exceed the temperature set point while reaching the set temperature as fast as possible. For this, the PID temperature controllers take three different, yet connected approaches:

    • Proportional: when the process value (current temperature reading) is lower than the setpoint, the output is increased proportionally to the error. Larger error means higher power is supplied to the heater to heat up fast. Smaller error causes the controller to reduce power.
  • Integral: Integral portion of the controller tries to increase the output power to the heater to reduce the time taken to reach the set point. If the power is not enough to reduce the error, integral controller tries to increase power to the heater.
  • Derivative: The derivative control is influenced by the time that has passed. As the time passes and the temperature error reduces, the output power is also decreased to prevent overshoot.

These three controllers ultimately control the power to the heater to obtain a response as shown in the figure below. The setting point marked on the x-axis is the desired temperature.

PID temperature controller circuit

PID temperature controllers are available in many configurations. Usually, the controller only reads the process temperature through a sensor and controls an external power control device like an SSR to control the power applied to the heater. The image below shows such a kit that has the PID temperature controller, an SSR (Solid state Relay), the heatsink and the temperature sensor.

To wire this system, the following diagram can be followed. The thermocouple wires should not be swapped as it interferes with the controller’s ability to read the process temperature. Invalid temperature reading can cause the PID temperature controller to malfunction.

Some controllers have error detection features such as open thermocouple detection for added safety. Such controllers can stop functioning and remove power to the heater if they detect the thermocouple is disconnected.

According to the internal calculations done by the controller, it controls the solid state relay (SSR) to control the applied average power to the heating element. This is done by momentarily turning on and off the power control device. When properly tuned, the system can reach the desired temperature and maintain the conditions even under external disturbances.

What are the Different Types of Temperature Control Devices?

As we discussed above, PID temperature controllers are the most accurate and fastest responding industrial temperature controllers. There are two more types of temperature control devices that are less accurate but useful in certain applications.

On/Off Temperature Controllers

This is the most simple form of temperature controllers. The on-off temperature controller has two parameters, the setpoint and the differential. The setpoint is the desired temperature the system must have. The differential (also known as histeresis) is the two extremes that define the boundaries when the temperature controller should turn on and off. The minimum defined at which temperature the heater should turn on and vice versa.

On/off temperature controllers are often the easiest to wire. They need three external connections to function:

  • Power supply – Supplies power to the temperature controller.
  • Sensor – A temperature sensor such as an RTD or a thermocouple for obtaining the current temperature from the system.
  • Actuator – This can be a relay or a SSR that controls a. high power heater, or a direct heater connection if the device has a built-in relay
  • User Input – Modern temperature controllers have digital displays with button inputs to configure the parameters. Some devices have rotatable potentiometers to manually set the limits. 

On/off type temperature controllers are used in systems where the temperature changes are very slow and precise control is not needed.

Proportional Temperature Controllers

Proportional temperature controllers are a simplified version of PID temperature controllers. Unlike the on/off type controllers that activate when the temperature drops below or rises above the thresholds, proportional controllers drive the output almost always to maintain the temperature.

These types of controllers regulate the temperature by varying the power supplied to the heater. This involves solid state control like SSR to adjust the power output. The temperature range the device works in is called ‘proportional band’. Similar to the on/off type, these also have upper and lower limits.

Upon startup, proportional temperature controllers behave similar to on/off type. To bring the system temperature into the proportional band, the controller operated the heater at 100% power. Once the temperature exceeds the minimum threshold of the proportional band, the power is reduced to maintain the temperature within the required region. 

In the diagram shown below, the brown plot is a pure proportional controller. We can observe how the temperature is constantly varying within a narrow region between 10 to 18 Celcius..

Advantages and disadvantages of PID Temperature Controller

PID temperature controllers are very useful in dynamic systems. They are widely used in applications where the temperature fluctuates often. PID temperature controllers can maintain the preset temperatures regardless of the changing system conditions.

As any other industrial controller, there are advantages and disadvantages associated with PID temperature controllers.

PID Temperature Controller Advantages

Here are some of the advantages of using a PID temperature controller:

  • Easy install and implementation
    • PID temperature controllers are integrated devices that only need few external components to function
  • Increased stability of the system
    • PID controllers can rapidly compensate for external disturbances to the system. This is very important in temperature sensitive applications.
  • Reduces steady-state error
    • Normal on-off type controllers often have a large overshoot. This means that the system temperature can almost always be higher than the desired value, even if it is for a fraction of time. Properly tuned PID controller can eliminate this issue by reaching the set temperature without overshooting.
  • Faster response
    • PID controllers reach the setpoint faster than any other controller in most cases. This is very useful in highly dynamic systems to reach and maintain the required temperature.

PID Temperature Controller Disadvantages

PID controllers also have some inherent disadvantages that it may be problematic to use in some situations. For example, 

  • Difficulties in initial tuning
    • Most PID controllers require manual tuning of the proportional, derivative and integral constants of the control loop. This can be cumbersome at the beginning since it may take a lot of time since the parameters are not known. To tune a PID controller, you can follow the steps shown in this video.
  • PID temperature controllers 
    • PID controllers are, in general, linear. This means that they work best in linear (predictable) systems. If the system is non-linear, the performance can vary.

PID control is a feedback control system that relies on the error between the set point and the process value. When an external disturbance occurs that increases the error, the PID controller intervenes and tries to bring the error to zero. This works well for disturbances in higher magnitudes. However, for small changes in a system, the PID loop can take longer to compensate and this can be less desirable in some cases.

How do you set PID for temperature control?

There are two types of PID tuning, auto and manual tuning. Automatic tuning follows an algorithm to automatically determine the proportional, integral and derivative constants for the controller. The manual process needs trial-and-error to properly tune the controller. Automatic controllers can ease this process by narrowing down the values for the particular constants.

To find out how to configure the P, I and D constants of a particular controller, please refer to its user manual first. Also, making any modifications, ensure that the adjustments will not cause any serious effects to the system. Ideally, you will need a controlled environment to carry out the tuning process.

PID tuning generally starts with determining the proportional gain while leaving the other two values constant. Set the proportional constant to a value that the system starts oscillating around the setpoint. You can increase the current P value by a factor of two, and if it causes too much oscillation, reduce it by 50% of the increased amount. 

After reaching a fairly stable oscillation, the integral term can be tuned the same way. When tuning the integral constant, adjust it so that the system reaches the set point in the least amount of time. When tuning the integral constant, there can be overshoots and oscillations.

Finally, adjust the derivative constant to minimize the oscillations under external disturbances.

If you are using a more advanced PID controller such as the Omega Platinum Series PID Controllers, the manufacturer may offer specialized software to tune the system in a more accurate way. There can also be additional features such as latching outputs, alarms and intelligent auto tuning algorithms.

Applications of PID temperature Controllers

PID temperature controllers are generally used in applications where faster response time and higher accuracy is needed.

One such application is the tyre manufacturing industry. When preparing the raw material and mixing the compounds, the temperature of the rubber mixture has to be maintained at a very fine margin to ensure the material is treated properly.

In food and beverage processing industries such as milk pasteurization, the temperatures need to be very precise to prevent bacteria from growing and loss of important nutrients. PID temperature controllers are used to maintain temperature in the milk during the pasteurization process.

Another application of PID controllers is the healthcare sector. Machines such as test equipment, medical refrigerators, incubators and growing chambers must maintain temperature under very tight margins. PID temperature controllers almost always find applications in these systems.

How to Choose a PID Temperature Controller?

When buying a PID temperature controller, look for the following key specifications:

  • Input type
    • The temperature controller needs a suitable sensor to obtain the process value. This is done through a temperature sensor. It can be a thermocouple (K, J, T-type or any other), an RTD or even a digital temperature sensor in some cases. Select one that is specialized to be used in the particular application.
  • Temperature range
    • It is important to know the range of temperature the system will be working under. Account for any extremities that the system may undergo during operation.
  • Output type
    • Output type can be electromechanical (relay). SSR or a digital output.
  • Control action
    • This can be simple on/off, relative or PID control.
  • Additional features
    • Check if the device supports advanced tuning and any additional features such as alarm outputs, programmable profiles and support for integration with SCADA systems if the application requires it.

Conclusion

PID temperature controllers are used in many automation systems to accurately control and maintain temperatures. There are alternatives to PID controllers that can be used in temperature control applications where such accuracy and speed is not required.

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