How to keep that valve open for certain period of time and shut off automatically

Question

Hi everyone,
The objective of the VI is to measure pH and then open acid or base valve based on pH valve. Below is my UNFINISHED VI
My question is:  if, for example, the  pH is 10, then, the program will open acidic valve for 5 seconds and then shut off automatically. After that, wait for 5 second. this process will go on and on as I use while loop, but I just dont know how to control the valve on for 5sec and off and then wait for 5sec.
Anyone can help me out?    Thank you very much
In the picture, please enter those values before running
P.S The VI is based on previously posted vi by someone who made it 1 yrs ago.
Attachments:
project_trial_1_26.vi ‏27 KB
Capture.JPG ‏79 KB

Answer 1

1. The reason the valves close immediately is that the Elapsed Time VI does not cause any wait, it only measures the time. So the valves closed within a small fraction of a second of the time they opened.
2. I modified your VI to wait until the Elapsed Time has ended.  See the "wait" state.  I probably modified some of your other functionality in the process, but this shows one way you could do the timing. 
3. You do not need any Value property nodes.  They should never be used if the value can be wired directly.
4. You do not need any sequence structures. Dataflow will determine the order of things occurring.
5. If you use an enum for the state machine (and it is a good way to name and select states), you should make it a type def. Then when you need to change it as I did to add several states, you only need to change the Type Def in one place and the changes propagate through to every place you used the control or constants derived from it.
6. I added a Halt state which will close both valves before stopping the program.  When you are controlling a real world process, it is important to consider the start up and shutdown requirements. For example the digital outputs of the USB-6008 default to inputs when the device is first powered up. And, the inputs are pulled high by a 4700 ohm resistor (at least I think I recall that value). Will this open your valves before the program starts running and sets the I/O lines to outputs and forces them low?
The USB-6008 has rather limited drive on the digital lines. You will probably need a buffer between it and the valve coils.
PID and PWM are more complicated than I want to get into here.  First, are your valves proportional or on/off? Second, (assuming that they are on/off) how fast can you open and close them without destroying them in the first week? What is their expected lifetime in terms of the number of operations? How fast do you need to be able to change them to keep your pH where you want it?  Are these values compatible?
Lynn
Attachments:
pH State.ctl ‏12 KB
project_trial_1_28.2.vi ‏66 KB