310202b On-Off Controllers Flashcards
What does a direct acting controller calculate the error as in a control loop.
A direct acting controller calculates the air as process variable (PV) minus set point (SP)
PV - SP
What does the reverse acting controller calculate the error as in a control loop?
A reverse acting controller calculates the error as set point (SP) minus process variable (PV)
Given a direct acting on off controller that has a set point (SP) of of 50%, calculate the controller output (in percentage) for process variable (PV) of 25%.
Direct acting controller = PV - SP
error = PV - SP = 25% - 50% = -25%
The error is negative, therefore, the output of the controller is 0%.
Given a direct acting on-off controller that has a set point (SP) of of 50%, calculate the controller output (in percentage) for process variable (PV) of 75%.
Direct acting controller = PV - SP
error = PV - SP = 75% - 25% = 25%
The error is positive, therefore, the output of the controller is 100%
Why is hysteresis built into an on off controller?
Hysteresis is built into an on off controller to prevent noise from causing the controller’s output to cycle back-and-forth rapidly when the process variable is close to this set point
Hysteresis provides a band in which the controller output cannot change.
Hysteresis is the difference between the two measurement values (the band) where the switching occurs.
Where is the band set in relation to the set point in reverse Hysteresis?
In reverse Hysteresis the band is set below the set point
Where is the band set in relation to the set point in forward Hysteresis?
In forward hysteresis, the band is set above the set point.
Where is the band set in relation to the set point in differential gap hysteresis?
In differential gap hysteresis, the hysteresis band is centered around the set point
Define differential gap control.
Differential gap control is a process control strategy in which the controller output remains at a maximum or minimum value until the controlled variable crosses a band or gap, causing the output to reverse
If a differential gap controller has a set point (SP) of 50% and a differential gap of 50%. What is the upper and lower trip points?
UTP = 50% + 1/2(50%) = 75%
LTP = 50% - 1/2(50%) = 25%
What is the upper and lower trip point for a differential gap controller used to control the level in a vessel if the set point (SP) is 2 m and the gap is 1 m.
UTP = 2.0m + 1/2(1.0m) = 2.50m
LTP = 2.0m - 1/2(1.0m) = 1.50m
An application requires the upper trip point to be at 80%, and the lower trip point to be if 30% to set up the differential gap controller, calculate the gap and the set point
When the controlled variable rises above the upper trip point in a direct acting differential gap controller, the controller output goes to ___________
When the controlled variable drops below the lower trip point, the controller output goes to ___________
When the controlled variable rises above the upper trip point in a direct acting differential gap controller, the controller output goes to 100% (ON)
When the controlled variable drops below the lower trip point, the controller output goes to 0% (OFF)
When the controlled variable rises above the upper trip point in a reverse acting differential gap controller, the controller output goes to __________
When the controlled variable drops below the lower trip point, the controller output goes to __________
When the controlled variable rises above the upper trip point in a reverse acting differential gap controller, the controller output goes to 0% (OFF)
When the controlled variable drops below the lower trip point, the controller output goes to 100% (ON)
How do you adjust the gap on a pneumatic differential gap controller?
The differential gap is set by adjusting the pivot point.
Moving the pivot point towards the set point (SP) and process variable (PV) bellows makes the controller more sensitive to changes in the pressure of these bellows, which lowers the differential gap required to switch the controller.
