# PID Temperature Controllers

## What is a PID Temperature Controller?

PID temperature control is a loop control feature found on most process controllers to improve the accuracy of the process. PID temperature controllers work using a formula to calculate the difference between the desired temperature setpoint and current process temperature, then predicts how much power to use in subsequent process cycles to ensure the process temperature remains as close to the setpoint as possible by eliminating the impact of process environment changes.

PID temperature controllers differ from On/Off temperature controllers where 100% power is applied until the setpoint is reached, at which point the power is cut to 0% until the process temperature again falls below the setpoint. This leads to regular overshoots and lag which can affect the overall quality of the product.

Temperature controllers with PID are more effective at dealing with process disturbances, which can be something as seemingly innocuous as opening an oven door, but the change in temperature can then have an impact on the quality of the final product. If the PID temperature controller is tuned properly it will compensate for the disturbance and bring the process temperature back to the setpoint, but reduce power as temperature approaches the setpoint so that it doesn’t overshoot and risk damaging the product with too much heat.

## The P, I & D

PID control belongs to the “optimal” category of control theory which specifies that a certain process variable is optimally achieved. For temperature controller PID, the optimal variable is maintaining the process temperature at the setpoint for the desired period of time, avoiding any severe changes from lag, overshoot or disturbances.

The three elements of the PID algorithm are the Proportional, the Integral, and the Derivative. These elements each relate to the variance in the process temperature versus the setpoint in a period of time.

• Proportional - the variance between the setpoint and the current process temperature
• Integral - the previous variance from the setpoint
• Derivative - the predicted future variance based on previous and current variance

These variances over time are then calculated using a PID formula, either manually by an engineer or automatically by the temperature controller, and the result is how much power needs to be applied to the process to maintain the temperature at the setpoint.

## The History of PID Temperature Controllers

Mechanical feedback devices have been in use since the late 18th century in the form of governors. These were limited to only one or two elements from Proportional, Integral or Derivative and were originally intended to maintain a consistent operating speed in steam engines that were being used to drive factory machinery.

The first full PID controller was developed in 1911 by Elmer Sperry for the US Navy to automate ship steering. Sperry designed his system to emulate the behaviour of the helmsmen, who were capable of compensating for a persistent variance, as well as anticipating how the variance will change in future.

Subsequently in 1922 the engineer Nicolas Minorsky published the first theoretical analysis of PID control, similarly based on observations of a helmsman’s ability to adapt to changing conditions. Minorsky rendered the helmsman’s ability to adapt to changing conditions as a mathematical formula, which formed the basis for modern PID control.

## The Different Tuning Methods for PID Controllers

There are two primary ways of tuning a temperature controller with the PID values.

1. An engineer manually works out the P, I & D variables and the level of power required in the process to maintain the setpoint.
2. By entering target values and using the self-tune function the temperature controller automatically calculates the PID to directly control the process.

In either case the PID formula provides a level of power to apply in the process to maintain the setpoint, which is either inputted by the engineer or set by the PID controller itself.

## Which PID Temperature Controller?

PID loop tuning is used in a variety of temperature controllers and for varying numbers of loops. The most basic setup is for one temperature controller to calculate the PID and manage a single process.

Medical cleaning equipment often uses a single loop PID temperature controller to ensure that the process runs at the right temperature for long enough to properly sterilise implements. A temperature sensor would measure the temperature inside the sterilising tank, which the PID temperature controller would then interpret and use to increase or decrease power to the heating element.

A more complicated temperature controller PID setup is multiloop, in which a single temperature controller manages several processes simultaneously. However each process is discrete and therefore operates on individual loops, so a disturbance on one process will have no impact on another. For example a bakery might have several ovens operating with the same setpoint, but not affecting each other, which would be run by a multiloop PID temperature controller.

### PID Controllers with Cascade Control Loops

Some PID temperature controllers have enhanced capabilities which allow them to operate multiple loops that relate to each other, rather than each loop operating discreetly under central control.

Cascade control is where two control loops operate in relation to each other in the form of a primary and secondary loop. The primary loop controls the main element of the process being heated, however it does not have a direct heating element working on it. Instead there is a secondary element that is often a jacket around the first and is controlled by a heating element.

The PID controller measures both the primary and secondary loops and adjusts the power level affecting the heat of the secondary element so that it in turn heats the primary element to the setpoint.

A familiar example for this is melting chocolate, where if chocolate is directly exposed to heat it is likely to burn, but it can be melted in a bowl over hot water. The chocolate is the primary loop, the delicate substance which ultimately needs to be heated, and the bowl of water is the secondary loop, the intermediary between heat application and the primary loop. Cascade loops work on the same principle, but at a much larger scale and with precise temperature control.

The PID tuning in cascade loops is essential as otherwise there can be excessive overshoot waiting for the primary element to reach the setpoint. The PID controller reduces power as the temperature approaches the setpoint to meet and then maintain the setpoint.