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62 SMT007 MAGAZINE I MARCH 2021 is error signal drives the proportional and integral elements. e resulting signals are added together and are used to drive the pro- cess response. e alternate placement for the proportional element can be seen with the dotted lines. is can be a better loca- tion for the proportional element, depending on how one wants the system to respond to commands. To see the effect of these control actions, Figure 2 shows the four controller response algorithms and their responses to a change in load or setpoint. On-off control is the sim- plest, slowest, and never returns to exactly the same conditions. PID is the fastest and the most complex because it responds to the Feature by Martin Tarr ELECTRONICS MANUFACTURING EXPERT—RETIRED An excerpt om Automation and Advanced Procedures in PCB Fabrication by Happy Holden with Martin Tarr. Control Dynamics Control dynamics determine the signals to the solution replenishment equipment. Fig- ure 1 displays the traditional control loop and proportional-integral-derivative (PID) con- troller. In this case, the derivative element is being driven only from process feedback. e process feedback is subtracted from the com- mand signal (setpoint) to generate an error. A Short Overview of Control Theory Figure 1: Basic PID controller. (Source: Tim Wescott [1] )

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