PID roasting temperature control

The Israeli company Coffee-tech has been producing practical automatic roasters for several years without the need for human intervention in the roasting process. How does it actually work? Will you turn on the roaster, pour green coffee and tell you to empty the coffee just before burning? Or do you have to stand at the roastery and control the roasting cycle even though it is controlled automatically, so you actually roast and all the benefits of this machine will be lost?


Of course, the problem is a bit more complicated, so let's get a little closer to the issue of PID control not only of the roasting process, but basically anything we expect the process to change over time.

Imagine that you are a regulator and your task is to fill the bathtub just to the edge. So you come to the bathroom and see that the tub is empty. So you release the water. And in full, let it be as soon as possible. The water in the tub is growing and you can see that it will be full. You slow down the tap a little because you are afraid that the water will overflow. But you forgot to put a plug in the tub, so the level starts to drop. So you add again. (You don't put that plug in there because you're a regulator who can't). And the bathtub will be full. You only take a little. The water begins to overflow. So you take more. The level drops slowly, so you only add a little. The result is that the tub is still full, even if it is not plugged, and water still flows into it. However, the water flow is set so that the level does not rise or fall. So you work very well as a regulator.

The second example, already a bit technically, let's understand what PID is. The task of the controller is to regulate the temperature in the roasting drum. The controller will supply power (in the form of electricity to the resistance coils in the roaster) and will measure the temperature to know how much power it is supposed to supply. One more important thing the controller must know is the temperature to which the roaster is to melt. So this will be the desired temperature. All users "catch" this temperature themselves according to local conditions such as ambient temperature, air pressure, rate of heat removal from the roaster (depending on the height and diameter of the chimney), but of course also the humidity and grain density. All these parameters are absolutely essential for the roasting process and their change results in a large change in the roasting result, but the automatic adjustment of the PID control ensures that the input (same) roasting quality remains the same when the input parameters remain unchanged.

Proportional component of the controller

This is the first letter in the name of the PID controller. The controller subtracts the measured temperature from the required one and multiplies the difference - we will call it the deviation - by a constant. The result is the power at which the roaster will heat (perhaps in percentages). So if the measured temperature is much lower than required, the power will be great. The closer the measured temperature is to the required one, the lower the output, if the measured temperature is the same as the required one, the output will be zero. The proportional component is therefore explained. Now let's play with the setting of the proportional component of the controller. It was mentioned that the controller multiplies the deviation by a constant. If the proportional component constant is zero, that component will not work. The power will be zero, no matter how large the deviation. Set the constant to 1. When the temperature difference is 10 degrees, the power will be 10 percent. Set the constant to 100. Now we have a thermostat. When the measured temperature is one degree lower, it will heat to 100 percent, when the deviation is zero, it will not heat.

Integration component of the controller

This is the second letter in the PID controller name. The integration controller takes the deviation, multiplies it by a constant and adds it to its component. This means that if the measured value is lower than required, the integration component will increase. If the measured temperature is higher than required, the integration component will decrease. The higher the deviation, the faster the integration component will change. If the controller is only integrated, it will heat a little at first, the output will increase and after reaching the required temperature and exceeding it, the output will decrease. After the temperature has stabilized at the required value, the integration component will be set to the power required to maintain a stable temperature (we supply the same power as the roaster is cooled).
Setting the constant for the integration folder. If the constant is zero, the integration component will not be reflected in the controller at all. If it is too high, the power will be high when the desired temperature is reached and the temperature will exceed the setpoint too much. If set optimally, it will exceed the temperature, but there will be only one overshoot.

Derivative component of the controller

This is the third letter in the PID controller name. The derivative controller takes the rate of change of the deviation and multiplies it by a constant. Thus, as the temperature decreases, the derivative component increases performance. The faster the temperature drops, the higher the output of the shunt regulator. If the temperature rises, the shunt regulator will reduce the power. This will show very well the moment we start to heat up the heated roastery. The temperature suddenly starts to decrease and the derivative component can react to it immediately by increasing the power. On the other hand, when the temperature starts to rise too fast, the performance will decrease. If the constant for the derivative component is too large, the temperature will reach the desired value quite slowly, but the reaction to the change will have a very sharp effect on performance. If the constant for the derivative component is low, the controller will respond more slowly to temperature changes.

This is a comprehensive overview of how the Coffee-tech roaster works to achieve stable roasting results. PID has been operating since the 1970s in automation technology from thermal control to process control. I consider its use in roasting technologies to be a great benefit for the comfort of service in bars and cafes, but its contribution remains emotional for discussion - does a flawless chef or a strong personality full of mistakes, contradictions and moods cook food better? I will leave this assessment to the taste buds of the guests and their current mood.