Answer / carl_ellis

The first PI proportional controllers were analog electronic
controllers. On those analog controllers, the integral
adjustment was a front panel rotary knob (or pot) which
adjusted the time constant of the error integrator circuit.

Integration is the mathematical operation on the error
signal in a PI or PID controller. Explaining how it works
as a calculus math function is meaningless to technicians
who haven't studied calculus.

Back then, the integral adjustment was called 'reset'
because the adjusting the knob was said to 'reset' the
process variable's droop back towards the setpoint.
Controllers were frequently recorder/controllers so the
trended PV action could be seen over time - the droop from
setpoint and the resulting reset towards the setpoint when
integral action was applied.

Hence, 'reset' became the term used for integral action
rather than 'integral'.

Carl Ellis
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