The control algorithm can have a far-reaching effect on system stability, accuracy and cost. Four common temperature control algorithms are on-off, proportional, proportional with integral and derivative (PID), and heuristic.
On-off control turns cooling fully on (or off) exactly at the setpoint temperature. This is the simplest and often the most accurate type of control algorithm. As long as thermal lag in a system is modest and cooling capacity is approximately matched to the load, it is hard to beat on-off control.
Proportional control modulates the amount of cooling in some way so that cooling is applied gradually as the temperature approaches setpoint, and it does not reach its full capacity until the temperature exceeds the setpoint by a predetermined amount. The range of temperature from where cooling is first applied to where it is fully on is called the proportional bandwidth. This range is usually centered on the setpoint.
Proportional control is the first step toward a more sophisticated control algorithm and often is used when a system is difficult to control due to a longer thermal lag time or mismatched cooling capacity. It can add stability by preventing large oscillations in temperature, but the increased stability comes at the cost of accuracy, as the controlled temperature can settle anywhere within the bandwidth, depending on the load conditions.
PID control starts with a proportional control algorithm and adds corrections based on the integrated difference between the actual and setpoint temperature (the “I” or integral term) as well as the rate of rise of load temperature (the “D” or derivative term). The added terms correct for the tendency of plain proportional control to settle anywhere within the bandwidth and exhibit small oscillations. However, some effort is needed to “tune” the parameters, and a longer overall time is required to reach setpoint.
Heuristic control throws away any “cookbook” approach to control type and employs whatever method appears necessary to achieve adequate control of a particular system. It can be thought of as mechanizing what a smart, quick human might do if they were reading the process temperature and had their hand on a cooling amount knob. Heuristic control often uses a combination of on-off and proportional load switching mixed together in a complex timing sequence.
Before choosing an algorithm, you should discuss the options with your equipment vendor. For some processes, a simple control type, such as on-off, might provide the best performance, while a more sophisticated (and more expensive) control, such as PID, might spoil a well-designed thermal system by mistuning. Heeding a vendor’s advice on this selection can result in substantial cost and performance advantages.
October 1, 2008