Not all roasters will choose to use PID controllers for the roasting process, and that is their choice, as it should be. However, in order to make a valid choice, a roaster must understand existing technologies; what they can and cannot do for their businesses. A choice made without evaluating all available information is a gamble, and why gamble with good coffee? This article attempts to clarify some of the mystery that surrounds PID controllers and to look at what one roastery was able to do with one roaster in one installation. PID Basics So what does PID mean? What is a PID controller? What is the difference between a PID controller and PID profiling controller? PID logic control is used in many of the better off-the-shelf digital controllers (Watlow, Omron, Honeywell, Siemens, etc.) and most, if not all, proprietary coffee roasting control
systems produced by roaster manufacturers. PID controllers make mathematical calculations to help keep the actual temperature as close as possible to a desired set point temperature. In the case of coffee roasting, the set points aregenerated along a positive sloping curve. If the PID settings in a PID array are incorrect, then the system will either be constantly running to catch up to the desired curve, or constantly overshooting and undershooting
as the controller attempts to bring the actual temperature to the set point. A fully functional PID controller will generate set points regardless of whether the PID settings are correct (See Graphs 1 & 2, pages 58–59). For the roaster, the trick is to find the correct PID settings for their roaster in its installation. The proper use of PID controllers is the next logical step up from manually profiling coffee through manipulation of the existing time and temperature curve. A roaster’s existing time and temperature curve is the curve that naturally occurs when a single piece of roasting equipment in a set environment is
roasting a particular coffee, and no changes are undertaken by the operator until the end of the roast. So the question becomes: how do you find the correct PID settings for your roaster and its control system? For most roasters,using a PID controller with a ramping (ramp and soak) or profiling function, the PID settings will be different than those used by most proprietary roasting programs.