Motor controllers
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PID (Proportional Integral Derivative) controllers
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is a control loop feedback mechanism (controller) commonly used in industrial control systems. A PID controller  
continuously calculates an error value as the difference between a desired setpoint (SP) and a measured process  
variable (PV) and applies a correction based on proportional, integral, and derivative terms  
(denoted P, I, and D respectively), hence the name.