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QUESTIONS
- Identify the parts of a closed-loop feedback
system.
- Explain the terms proportional, integral, and derivative.
- Discuss four statements that can be made about a P-only controller.
- Explain the term bias (offset) and how it's used to
enhance the P-only controller.
- Explain how you would determine the proper value of gain, reset, and rate for a new closed-loop system.
- Explain what would happen to your process alarms if you changed
the setpoint. Explain the function of deviation alarms.
- 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
- When a system is in open loop the feedback signal is sent to the
summing junction.
- Process alarms are fixed alarms used to protect the system.
- When a system is placed in manual mode, the output signal is changed
by the controller.
- Deviation alarms are fixed alarms used to protect the system.
- A bump in a process control system is any change in setpoint or
any change in the process variable.
MULTIPLE CHOICE
- Bumpless transfer causes the_____.
- setpoint to be set equal to the output when the controller
is changed from manual to automatic mode.
- setpoint to be equal to the process variable when the controller
is changed from manual to automatic mode.
- process variable to be set equal to the setpoint when the
controller is changed from manual mode to automatic
mode.
- The proportional band is the ______ .
- amount of change in error that will cause the output to change
from 0% to 100%.
- amount of error that can occur before the deviation alarm
is activated.
- amount of error that can occur before the process alarm is
activated.
- A heating system that uses ratio control to control a heating
contactor _____ .
- turns the contactor on for a percentage of time equal to the
percentage of the out put signal.
- provides a voltage signal to the coil of the heating contactor that's between 0-100% of the applied voltage.
- provides an analog voltage signal directly to the heating element that's equal to the percentage of time the output signal
is on.
- The term tuning a loop refers to ______ .
- adjusting the offset for the feedback signal.
- adjusting the gain, reset, or rate for a process control system.
- adjusting the setpoint for a process control system.
- If the integral time of a process control system is decreased
while the gain is held constant, the system will ______ .
- respond more rapidly.
- respond more slowly.
- not change its response since the gain did not change.
PROBLEMS
- 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.
- 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 Ku is 15, and the value for Pu. is 5 minutes.
- 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).
- Determine the proportional band if the gain is set to 1.2 for the
system.
- 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.
![](images/9-28_table.jpg)
ill. 1
![](images/9-31_formulas.jpg)
ill. 2
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