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68 SMT007 MAGAZINE I MARCH 2021 Differential In general, if you cannot stabilize a process with proportional control, you cannot stabilize it with PI control. Proportional control deals with the present behavior of the process, and integral control deals with the past behavior of the process. If there was an element that pre- dicted the process behavior, then this might be used to stabilize the process. A differentiator will do the trick. e example shows the differential term of a PID controller. I prefer to use the actual pro- cess position rather than the error because this makes for smoother transitions when the command value changes. e differential term itself is the last value of the position minus the current value of the position. is provides a rough estimate of the velocity (delta position/ sample time), which predicts where the posi- tion will be in a while. With differential control, the precision actu- ator system can be stabilized. Figure 8 shows the heating system with PID control. Using the full PID control with this process can result in performance improvements. SMT007 Reference 1. Wescott, T., copyright 2018, reprinted with per- mission. Download your copy of Automation and Advanced Procedures in PCB Fabrication by Happy Holden at I-007eBooks. Figure 8: Response of the precision actuator system with PID control. This system settles in less than half a second compared to multiple seconds for other systems [1] .