I. General
A proportional–integral–derivative is a control loop feedback mechanism commonly used in industrial control systems. PID control continuously calculates an error value as the difference between a measured process variable and a desired set point.
P accounts for present values of the error (e.g. if the error is large and positive, the control variable will be large and negative).
I accounts for past values of the error (e.g. if the output is not sufficient to reduce the size of the error, the control variable will accumulate over time, causing the controller to apply a stronger action).
D accounts for possible future values of the error, based on its current rate of change.
In this project, it contains 2 control modes and 2 output modes.
II. Command
S1: target value
S2: current value
S3: operation parameter
D: destination device
