Alicat MC Series Mass Flow Controller - Manual (Page 32)

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To adjust the Proportional and Differential (P&D) terms via RS-232 / RS-485:
Type *@=A followed by “Enter” to stop the streaming mode of information.
To adjust the “P” or proportional term of the PID controller, type *R21 followed
by “Enter”.
The computer will respond by reading the current value for register 21 between
0-65535. It is good practice to write this value down so you can return to the
factory settings if necessary. Enter the value you wish to try by writing the new
value to register 21. For example, if you wished to try a “P” term of 220, you
would type *W21=220 followed by “Enter” where the bold number denotes the
new value.
The computer will respond to the new value by confirming that 21=220. To
see the effect of the change you may now poll the unit by typing A followed
by “Enter”. This does an instantaneous poll and returns the values once. You
may type A “Enter” as many times as you like. Alternately you could resume
streaming mode by typing *@=@ followed by “Enter”. Repeat step 3 to remove
the unit from the streaming mode.
To adjust the “D” or proportional term of the PID controller, type *R22 followed
by “Enter”.
The computer will respond by reading the current value for register 22 between
0-65535. It is good practice to write this value down so you can return to the
factory settings if necessary. Enter the value you wish to try by writing the new
value to register 22. For example, if you wished to try a “D” term of 25, you
would type *W22=25 followed by “Enter” where the bold number denotes the
new value.
The computer will respond to the new value by confirming that 22=25. To see the
effect of the change you may now poll the unit by typing A followed by “Enter”.
This does an instantaneous poll and returns the values once. You may type A
“Enter” as many times as you like. Alternately you could resume streaming mode
by typing *@=@ followed by “Enter”. Repeat.
You may test your settings for a step change by changing the set-point. To do this
type A32000 (A is the default single unit address, if you have multiple addressed
units on your RS-232 / RS-485 line the letter preceding the value would change
accordingly.) followed by “Enter” to give the unit a ½ full scale set-point. Monitor
the unit’s response to the step change to ensure it is satisfactory for your needs.
Recall that the “P” term controls how quickly the unit goes from one set-point to
the next, and the “D” term controls how quickly the signal begins to “decelerate”
as it approaches the new set-point (controls the overshoot).