Very early attempts at this continuous control of systems under load were seen as far back as the 1600s. One of the earliest examples was invented in the 17th-century when a centrifugal governor mechanism (based on rotating weights) was designed to compensate for increasing differential in rotation speeds between larger and smaller millstone wheels under higher loads.

In 1922, Nicolas Minorsky made a real breakthrough towards what we now recognise as true PID control. Minorsky's observations and mathematical treatment would eventually evolve to give us the electronic industrial control mechanisms widely used as PID systems today.

PID controllers are now used widely in many industrial processes and they have become some of the most common items of automation and control gear used across multiple sectors and industries. This is largely due to the fact that PID controllers are ideally suited to delivering dependable, robust performance across a broad spectrum of environments and applications. They are also inherently user-friendly and simple in terms of both design and operation.

In order to learn more about PID temperature controllers, it’s helpful to understand how other types of temperature control devices function in order to highlight the key differences.

In the simplest terms, all temperature control devices effectively work in a similar way. Their core role is to monitor and calculate, via an ongoing series of measurements from a sensor such as a thermocouple, the precise difference between the operator’s desired process temperature (the setpoint) and the current process temperature at any given time.

To be able to perform automatic temperature adjustments, all control devices will require a form of controller module which allows them to calculate an appropriate response function based on the readout from the sensor. The resulting instruction is then output to the control element. This could be a heater, fans, a closed-loop liquid cooling system, or a combination of all three.

The key fundamental differences between various types of temperature control devices lie within the precise method in which they use and react to this sampled information and the steps that they then take in order to exercise their respective versions of temperature control. As a result, there are three core temperature control device types, as seen below:

PID controllers are widely used in all manner of industries, sectors and applications.

In addition to being relatively easy to set and adjust, the accuracy of their control loop feedback mechanisms makes PID instruments highly effective at digitally tracking - and automatically regulating - various processes and environmental factors across a wide range of variables.

In fact, while this guide is specifically concerned with PID temperature controllers, the basic function of a PID device can be applied to a much broader spectrum of automation and control processes including flow rate, pressure, speed, frequency and more.

Temperature Control

As previously mentioned, the PID temperature controller shares certain characteristics with other types of temperature control device. In particular, it still takes its readings from a sensor (often a thermocouple or similar), and typically outputs instructions via a digital control component to a mechanical actuator device such as a heater or fan array.

The highly universal nature of this basic monitoring and adjustment model means that PID controllers have a broad spectrum of potential temperature control applications in many industries, including:

• Furnace and batch temperature control
• Curing and conditioning of various sheet materials
• Temperature-critical drying and evaporation processes
• Heat treatment and tempering of industrial parts and equipment
• Medical sciences and pharmaceutical development
• Food production and preparation

PID devices are typically shipped by their manufacturers with default settings programmed for the proportional, integral and derivative. Operators installing and using any type of PID instrument must first calibrate the device, making sure it is properly programmed and adjusted to suit the specific needs of the industrial process in question. This also includes ensuring that the environmental parameters that it is required to operate between are appropriate for the potential variables occurring within that process. Until this process has been completed, the PID controller cannot be left to automatically handle its assigned workload.

This is equally true for PID temperature controllers, and there are a number of different ways in which this tuning can be achieved. In practice, all the different methods aim for the same result, which is to adjust, or tune, each of the proportional, integral and derivative terms individually in order for the module as a whole to deliver the desired performance.

Simple trial and error is often viewed as the most practical method of tuning PID controllers for many systems and scenarios. It is based on installing the device in a working system and effectively zeroing out all the settings. The operator then begins with proportional value, adjusting the gain upward until it reaches a point where it is oscillating around the setpoint.

Once this state is reached - which would be unsatisfactory in an accuracy-critical system and exposes one of the key limitations of proportional-only temperature control devices - the values for integral and derivative can be adjusted respectively. The former, if done correctly, should start to limit the oscillation rate down to almost zero, whereas the latter will increase response speed as optimum settings for the system and device type installed are neared.

An alternative to trial and error is the Ziegler-Nichols method. This approach involves observation of both continuous cycling and damped oscillation under a closed-loop system. However, this method has certain limitations and many operators still prefer to use trial and error to achieve their desired results.