A PID temperature controller does precisely as it sounds—it controls temperatures. Without having manual operator interventions to reach the desired outcomes, it’s an instrument that controls the temperature based on a setpoint or desired setting. Using a thermocouple or RTD as an input compares the setpoint to the actual temperature, then relays an output.
Let’s take a closer look at how they function and how to use them!
What Is a PID Temperature Controller?
A PID temperature controller works by using a specific formula that calculates the difference between current process temperatures and the desired temperatures. This system allows the controller to predict how much power is necessary for future cycles. The goal is to ensure the process temperatures maintain consistent levels and deter any impacts from environmental changes.
PID temperature controllers can handle any process disturbances more effectively. A disruption to the process can be minor or major. Still, something as little as opening an oven door can change the process by changing the temperature and significantly impacting the product’s quality.
Compared to on/off temperature controllers, which apply 100 percent of the power, the power cuts to 0 percent upon reaching the setpoint value. This drastic drop can result in a temperature drop and lead to regular lagging and overshooting, which further impacts the product quality.
The Breakdown of PID
The PID control is part of the optimal category of the control theory, which specifies the achievement of optimal process variables. The optimal variable in PID temperature controllers is maintaining the process temperature for the desired length of time at the specified setpoint, ultimately avoiding any changes from disturbances, lagging, or overshooting.
PID has three significant elements in the algorithm: the Proportional, the Integral, and the Derivative. Together, these elements interact with the variances between the process temperatures and a setpoint from a time period.
- Proportional – the difference between the current process temperature and the setpoint
- Integral – the previous variation from the setpoint
- Derivative – the variance prediction based on prior and current variances
The only two ways to calculate the variances over time with the PID formula are an engineer doing math manually or a temperature controller. Whatever the results are will equate to how much power is necessary to maintain temperatures at the specified setpoint.
A PID controller is one of the most accurate ways to receive the information. However, it’s vital that you tune the three elements individually to a particular system.
As the temperatures approach the setpoint, the proportional control will decrease the amount of power to the heater. This will essentially slow down the heater to avoid overshooting the setpoint while maintaining a stable temperature. By turning the output on and off for short intervals, you can achieve the proper proportioning. Time proportioning uses the time on and the time off to control the temperature.
A proportional band helps the proportional control stay within the setpoint temperature. The controller will only function as an on/off unit outside of this band. Inside the band, the ratios between the setpoint measurement differences turn on and off.
Should the temperatures be further from the setpoint, the on and off times will vary depending on the difference in temperatures. To summarize, if the temperature is too high, the output is off for a longer span of time; if the temperature is too low, the output is on longer.
Tuning Methods for PID Controllers
Tuning a temperature controller can happen one of two ways:
- The engineer will manually work out the P, I, and D elements with the required level of power to sustain a setpoint.
- Utilizing the self-tune function, the temperature controller can automatically calculate the PID with the target values.
Either method will provide a level of power that sustains the setpoint—manual or the controller itself. However, it’s best and often easier to use a controller to ensure accuracy.
A key feature of most temperature controllers is their interchangeability. Typically, a temperature controller gets mounted on the inside of an instrument panel. Therefore, properly cutting the panel to the size of the controller is essential. Most controllers use DIN sizes as the standard.
- 1/4 DIN – 92 x 92 mm
- 1/8 DIN – 45 x 92 mm
- 1/16 DIN – 45 x 45 mm
- 1/32 DIN – 22.504 x 45 mm
The History of PID
In the late 18th century, mechanical feedback devices in the form of governors received the most use. While today’s has all three elements of PID coexisting, they initially only had one or two elements out of the Proportional, the Integral, and the Derivative. The original intention was to maintain consistency in operating speeds in steam engines used to drive machinery in factories. Compare this to the modern use of cannabis distillation operations.
Elmer Sperry developed the first complete PID controller in 1911, and the US Navy used it to automate ship steering. Sperry’s design imitated the helmsmen’s behaviors, who compensated for a constant variance and made predictions on how the variance would change in the future.
Several years later, in 1922, the first theoretical analysis of PID controls was published by Nicolas Minorsky. Some of the same observations came to light with the helmsman’s ability to adapt to a change in conditions. Minorsky declared this ability by the helmsman a mathematical formula, and that is how the modern PID control was born.
A PID temperature controller is only a small piece of the entire puzzle. Analyzing the system you have will help clarify if this is the correct controller for you. When you begin to think about a temperature controller, take into consideration a few things:
- Algorithm controls needed – PID, proportion, on/off
- Number of outputs and type – alarms, limits, heating, and cooling
- Output type required – analog, SSR, electromechanical relays
- Input sensor and temperature required – RTD, thermocouple
Beaker & Wrench is happy to help you choose the best PID temp controller for your setup. It’s imperative that your controller houses the correct number of channels. Get in touch with us today!