The function of Pulse Width Modulation is contained in the logic not the card type so you do the exact same thing you would do with a standard DO card.
Andre Dicaire
In reply to Andre Dicaire:
ok, I work in the sugar industry and control of a continuous pan feeding involves feeding molasses to the pan, the input is a density measurement to pid and the output is an on-off valve to control the density within the pan. the duty cycle must be possible adjust via a value in the screen of Delta V. all valves are in a Asi-bus network. had never worked with this type of control before, which is why I'm very concerned
My mistake. I forgot that you had to set the duty cycle of the module. Your ramp idea is probably the best solution.
In reply to Irving_Aguilar:
As I mentioned, the key issue is the process time constant for your Density. Since you are manipulating the DO channel from the module, the resolution of your duty cycle is a function of the module exec time and period for the valve cycle. It will be hard control the loop tightly if the time constant is short. If you shorten the period, you will loose resolution on the duty cycle as you cannot speed up the module exec time past 100 ms. That's why this type of control is typically done with a continuous pulse IO Channel.
You might want to run the DO channel from a 100 ms module and run your PID loop at a more moderate rate. That way you don't waste CPU on the PID and user interface portion of the logic. Since this has to be a slow process for this to work, your PID loop could be run at 2 seconds. Move the Ramp and compare logic to the DO Module. Embed this module in the PID module and wire the PID Out to the Duty Cycle % value.
Do you know what the period of the valve cycle is? (Duty cycle is the percent on time over the period). If this is <10 seconds, your resolution will be greater than 1% i.e. at 5 seconds, the module exec time of 100 ms represents 2%, and so on.
DeltaV AS-I card does not support analog values. This is a 3.0 AS-I spec.
Certainly a control challenge when you don't have the right control signals driving the control element.