Heat Manager Pro User Manual V1.5 Page 68
B.2.3 Integral action
I stands for integral action. It takes a summation of the deviations between set value and
process value from the past into account. This action is necessary for removing the small offset
error that remains between set value and process value when only a P action would be used.
The integral control action is also referred to as reset. The degree of integral action is expressed
as an integral time in seconds. The purpose of the integral action is to automatically
compensate for any steady state offset that is inherent with a controller that only works with a
proportional action.
The integral action moves or resets the proportional band up or down depending on the offset.
The integral time of the controller is adjustable and determines how fast the proportional band is
moved.
A too high setting of the integral action will result in a very slowly reacting process with big
over- and undershoots.
A too low setting of the integral action will result in a control system that can never reach the set
value exactly. There will remain a small offset error and the desired set value is not reached.
The trick here is also to reach the setting that more or less corresponds to the actual inertia of
the system under control.
Now we go back to the speed control of our car with the gas pedal. When you push in the gas
pedal fully it takes some time to rev up the engine and speed up. Also when you let go of the
pedal it takes some time to let the engine rev down. This is mainly because of mechanical
inertia inside the engine. The weight of the car itself is also very important: a big heavy truck
takes much more time to speed up then a small compact car.
In a laboratory, on rollers, we determine the speed of the car as a function of the gas pedal
position: this can now be considered as the P-action.
This situation does not correspond to a real road situation where you have wind and different
kinds of tarmac.
To compensate for this the gas pedal needs to be pushed in a bit more (or less) depending on
the situation. This last action can be considered as the integral action: it fine tunes the actual
gas pedal setting, but in a very slow way, otherwise we overshoot the desired speed.
