Open- and Closed-Loop Feedback Systems:
QUIZ

QUESTIONS

1. Identify the parts of a closed-loop feedback system.
2. Explain the terms proportional, integral, and derivative.
3. Discuss four statements that can be made about a P-only controller.
4. Explain the term bias (offset) and how it's used to enhance the P-only controller.
5. Explain how you would determine the proper value of gain, reset, and rate for a new closed-loop system.
6. Explain what would happen to your process alarms if you changed the setpoint. Explain the function of deviation alarms.
7. Explain what would happen to your deviation alarms if you increased the setpoint of the system from 150°F to 170°F.

TRUE OR FALSE

1. When a system is in open loop the feedback signal is sent to the summing junction.
2. Process alarms are fixed alarms used to protect the system.
3. When a system is placed in manual mode, the output signal is changed by the controller.
4. Deviation alarms are fixed alarms used to protect the system.
5. A bump in a process control system is any change in setpoint or any change in the process variable.

MULTIPLE CHOICE

1. Bumpless transfer causes the_____.
   1. setpoint to be set equal to the output when the controller is changed from manual to automatic mode.
   2. setpoint to be equal to the process variable when the controller is changed from manual to automatic mode.
   3. process variable to be set equal to the setpoint when the controller is changed from manual mode to automatic mode.
2. The proportional band is the ______ .
   1. amount of change in error that will cause the output to change from 0% to 100%.
   2. amount of error that can occur before the deviation alarm is activated.
   3. amount of error that can occur before the process alarm is activated.
3. A heating system that uses ratio control to control a heating contactor _____ .
   1. turns the contactor on for a percentage of time equal to the percentage of the out­ put signal.
   2. provides a voltage signal to the coil of the heating contactor that's between 0-100% of the applied voltage.
   3. provides an analog voltage signal directly to the heating element that's equal to the percentage of time the output signal is on.
4. The term tuning a loop refers to ______ .
   1. adjusting the offset for the feedback signal.
   2. adjusting the gain, reset, or rate for a process control system.
   3. adjusting the setpoint for a process control system.
5. If the integral time of a process control system is decreased while the gain is held constant, the system will ______ .
   1. respond more rapidly.
   2. respond more slowly.
   3. not change its response since the gain did not change.

PROBLEMS

1. Use the table in ill. 1 (below) to determine the value for gain, reset, and rate for a P-only loop, a PI loop, and a PID loop if the value for TC is 12 minutes, the value for DT is 2 minutes, and the process gain is 5.
2. Use the table in ill. 2 (below) to determine the values for gain, reset, and rate for a P-only loop, a PI loop, and a PID loop if the value for K_u is 15, and the value for P_u. is 5 minutes.
3. Draw the block diagram for a typical servo system and identify the setpoint, process variable, error, summing junction, controller amp, and output (final control element).
4. Determine the proportional band if the gain is set to 1.2 for the system.
5. If the + deviation alarm is set to 15°F, and the - deviation alarm is set to -12°F, calculate the temperatures the PV must go above or below to actuate these alarms when the SP is set to 190°F.

ill. 1

ill. 2

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