Armfield Instruction Manual
82
magnitude and duration, then select the
icon to finish data logging. If the fluid
level falls continuously, wait until the vessel is half empty then stop logging.
Close both valves SOL 2 and SOL 3 and allow the fluid to return to the set point.
Pressure Switch Set Point
Investigate the effect of changing the Set Point for the pressure sensor.
Create a new results sheet by selecting the
icon.
In the software, alter the pressure switch set point to 100mm.
Select the
icon to begin data logging.
Continue logging until the oscillations reach constant magnitude and duration, then
select the
icon to finish data logging.
Repeat the procedure, altering the set point and recording the change in fluid level
until the oscillations reach stable values. Suggested values for the pressure switch
set point value are 150, 250 and 300mm. Remember to create a new results sheet
for each set of data.
PID Settings and optimisation
With proportional control action, the controller produces a signal that is proportional
to the error (the difference between the monitored variable and the set point value).
This creates an offset between setpoint value and actual value (the controller only
supplies an output when there is an error, so there is no controller output when the
value is not at the set point). It also generates an overshoot (the system will oscillate
above and below the setpoint value at the start of the control period until stability is
attained).
Without logging the data:
Alter the setting for Proportional Band (P) in the software to 100%, and continue to
observe the variations in fluid level.
Alter the P setting to 1%. Observe the changes in fluid level.
Experiment with other values, making notes of the effect on fluid level oscillation and
level control.
Return the P setting to 10%.
With integral control action, the controller gives an output that is proportional to the
time integral of the error. Integral control action can potentially be used alone to
control a process, but is normally used in conjunction with proportional action. It can
be used with proportional action to eliminate offset. It can also cause higher
maximum deviation and a longer response time than with proportional action alone.
Without logging the data:
Alter the setting for Integral Time Constant (I) to 5, and observe the variations in fluid
level.
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