Exercise D
69
Setting the On/Off Controller
In the PCT40 software select ‘Control’ below the Tank Level sensor display, to open
the PID controller window.
Set the Proportional Band (P) to 0%, the Integral Time (I) to 0s and the Derivative
Time (D) to 0s. Click on ‘Apply’. The controller is now set as a simple On/Off
controller with fixed hysteresis of 0.5% of full scale.
Type in the Set Point value at which the on/off controller will operate. A
recommended starting value is 200mm for level.
Apply the settings and close the controller window.
From the Sample menu at the top of the screen, select ‘Configure…’ and set data
logging to automatic at intervals of five seconds. Check that the reading L1 is varying
with the rising water levels.
Partially close the drain valve at the base of the large process vessel, limiting flow out
of the vessel.
Select the
icon to begin data logging.
Note the behaviour of SOL1 as the fluid level approaches the pressure set point.
Continue logging until the oscillations in fluid level reach stable values for magnitude
(height) and duration. Select the
icon to finish data logging.
Disturbances
Within the software it is possible to introduce disturbances in the outflow from the
vessel by opening the Normally Closed solenoid valves SOL 2 and SOL 3.
Create a new results sheet by selecting the appropriate icon in the tool bar of the
software.
Select the
icon to begin data logging.
In the software, select the switch to open valve SOL 2.
Allow the fluid level oscillations to settle.
Open valve SOL 3. Both SOL2 and SOL3 should now be open
Note whether the fluid level oscillates around the set point or whether the fluid
continues to drain from the process vessel.
Close both valves SOL 2 and SOL 3 and allow the fluid to return to the set point.
Select the
icon to finish data logging.
Set Point Value
Investigate the effect of changing the level at which the switch operates.
In the PID controller window, alter the Set Point to 100mm.
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