CONTROLLER LABORATORY
Background Information
The term bumpless transfer describes the manner in which the output of the controller responds to a change in setpoint value brought on by switching from manual control. In a cascade control system, the act of switching from a local setpoint to an external setpoint can cause a bump as well. Most modern controllers have a setpoint tracking circuit (or subroutine in computer systems) that allows switching between these modes without a bump in output. If a controller does not have an automatic tracking circuit, then a more involved procedure must be performed. Generally, switching from manual to automatic requires adjusting the setpoint of the controller to the actual controlled point and then switching the mode, i.e., aligning the setpoint indicator lo the measurement indicator. On the other hand, switching from automatic to manual may only require a simple repositioning of the mode selector. It should be noted that the automatic-to-manual and manual-to-automatic switching procedure varies according to the manufacturer’s equipment specifications.
When switching between manual and automatic control, a technician must first match the process variable to the setpoint and then switch to automatic or the controller will react aggressively lo match the setpoint. If the PV and setpoint are significantly different, the controller will produce an output change to the final control element that will cause a spike in the process. In some cases, this is not a problem but in others it could be problematic.
Materials Needed
a control loop or process simulator
instruction manual for the automatic controller
Safety Requirements
Safety glasses should always be worn while in the lab.
Procedure
1. Slat up the process plant in the lab and identify a specific control loop to manipulate.
2. Switch the controller from automatic to manual.
Line out the process so that the process variable is in a steady-state condition.
With the controller in Auto, the PV indicator pointer should match the setpoint indicator pointer.
Note the output pressure to the control valve.
Switch auto lo manual.
3. Operate the controller in Manual.
Under dynamic conditions, the controlled variable may have changed because the control valve is now frozen in position. Therefore, adjust the manual output knob lo achieve the same desired value that you had while in automatic control.
Periodically check the process PV and tweak the output knob as needed if the PV is not at desired value.
Adjust the output as often as needed to maintain the desired process value.
4. Switch between manual and automatic.
Adjust the setpoint knob lo place the setpoint indicator pointer lo match the process PV. Switch from manual to automatic.
Monitor the pressure to see if the valve changes or if the PV drifts.
Once in auto, adjust setpoint if it is not at the desired value.
5. Make a remote setpoint adjustment.
With the cascade controller selected lo remote setpoint (control), make a setpoint adjustment.
Change the setpoint by moving it above the measurement indicator and record what happens to the output.
Change the setpoint by moving it below the measurement indicator and record what happens to the output.
Additional Information
All controllers have instruction manuals containing specific operating instructions. If you have a cascade control system, transferring control from local (secondary controller’s set- point) to remote (primary controller’s setpoint) requires the same logical approach as transferring from manual to automatic in a single automatic controller. With the primary controller in manual and the secondary controller in local automatic, line out the primary controller’s PV. Make sure the output from the primary controller (its output) is equal to the secondary controller’s setpoint. Do this by adjusting the primary controller’s output manually. Then switch the secondary controller from local setpoint to remote setpoint. Now the primary controller should be directing the entire cascade loop in manual. Finally, switch the primary controller from manual to automatic in the same manner mentioned in the lab procedure.
Findings
Compile your observations and readings into a report that includes your recorded data, an explanation of the results of the experiment, and a conclusion. Include a paragraph explaining the differences between a local setpoint and a remote setpoint. Be sure to discuss the importance of bumpless transfer when switching an analog controller from automatic to manual control and from manual to automatic. Follow your instructor’s guidelines on how to submit these findings.
We need at least 10 more requests to produce the solution.
0 / 10 have requested this problem solution
The more requests, the faster the answer.