Proportional Controller

<quote>so until there is error it should take action to eliminate error, but it sets on other value. </quote>

This is incorrect. There will be NO (corrective output) action taken until there is an error. Closed loop proportional systems are always playing catch up. (some are quite fast)

Offset to me, simply indicates the error detecting sensor input, and the output feedback sensor are not scaled the same. Hence the need for an offset factor. Bare in mind the offset may need to be inverted.

That's because you are trying to see "something" in a feedback proportional controller and there is nothing there.

Lets play with our brain. imagine a simple transfer function in an open loop with a "step" temporary function.

!!!There is an Offset¡¡¡

Every system fight against change!!! you can see it in the expresions.

The proportional closed loop simple loses strenght when the error is becoming zero.

So this Offset will remain.

Sorry if i wrote something badly. english is not my first lenguage.

Proportional control calculates an output, based on how the error relates to the 'size' of the proportional band. In response to the output change, the process changes reflected in a process variable change.

At some point, the capacity or ability of the output to change the process can run to its limit. At that point, some error remains because the output of the controller is not capable of changing the process.

Here's an arbitrary example with the red line showing droop at 388 degrees even though setpoint is 400 degrees.

Temperature scale 0-600 Deg. Span = 600 deg.
Setpoint (SP) = 400 deg
Process variable (PV) is the measured temperature, the input to the controller)
Output is scaled 0-100%

Proportional band (PB) = 20%
20% of 600 deg = 120 deg proportional band
The proportional band (PB) is split in half around the setpoint, so the PB range is 60 deg below setpoint to 60 deg above setpoint.

The upper limit of the proportional band is 460 degrees.
- When the PV is above 460 it is outside the PB and the output is 0%
The lower limit of the proportional band is 340 degrees.
- When the PV is below 340 deg, it is outside the PB and the output is 100%

When the PV is between 340 deg and 460 deg, it is inside the PB and the output modulates between 100% and 0%. Heating is reverse action, so the PB is 100% at the lower temperature, decreasing as the PV (temperature increases).

When the SP = 400 deg and the PV is 388 deg, the error is -12 deg.

A -12 deg error calls for a 60% output, because 388 deg is 40% from 340 deg or 60% from 460 deg (reverse action for heating).

If the 60% output capacity is not capable of increasing the load temperature, then the temperature will hold at that value. The error due to the inability to achieve setpoint due to proportional only control is called 'droop'. The droop is due to an output calculated for the given error and the fact that the calculated output does not have the capacity to achieve the setpoint, and there is no other factor or term, like reset action, to correct for the error over time.

In the example, the error remains at -12 degrees, with an output at 60%. Droop is stable control, but not control at setpoint.

Early proportional controllers had a knob on front. Rotating the knob would adjust the entire proportional band 'up' or 'down' to adjust the output to one that could achieve setpoint. It came to be called manual reset, because the operator was 'resetting' the output.