MicroLogix 1400 (math based?) programming question

[Skidood]

Hi Group,

I'm the new owner of an AB MicroLogix 1400 processor and have been trying to decide the best way to de-pressurize a compressed air tank using a solenoid valve which releases some pressure.
I want to de-pressurize the tank at a rate of 0.8 psi per minute by energizing the valve as needed in short bursts.

Your wisdom would be greatly appreciated !

 

[harryting]

Interesting project, the big picture is to compartmentalize your logic:

A more suitable valve for the application would be an actuated control-valve, but you know that already.

If all you can do is open/close then I would try:

- Make logic to pulse the solenoid based on Rate.
- Use PID or roll your own logic to control the Rate

 

[Skidood]

I should have said the de-pressurization rate cannot be faster than 0.8 psi per minute or slower than 0.4 psi per minute.
And the tank has an inherent bleed-down rate of its own outside of external control of approx. 0.2 psi per minute.

 

[Peter Nachtwey]

This is not a linear process. The air flow is proportional to the square root of the pressure drop.

 

[Skidood]

Yes I'm aware of that. The valve would have to open more frequently as the pressure drops.

 

[Skidood]

Interesting project, the big picture is to compartmentalize your logic:

A more suitable valve for the application would be an actuated control-valve, but you know that already.

If all you can do is open/close then I would try:

- Make logic to pulse the solenoid based on Rate.
- Use PID or roll your own logic to control the Rate

let me ask this: how can I configure ladder logic to pulse an output, say for example, once every 20 seconds for a duration of 1.5 seconds.
If I can figure this out, I can set up 5 or 6 rungs where each one starts with a LIM instruction (using the pressure reading) and each rung is responsible for a certain portion of the pressure decay (say the portion between 7 psi and 4 psi.

 

[Epy]

let me ask this: how can I configure ladder logic to pulse an output, say for example, once every 20 seconds for a duration of 1.5 seconds.
If I can figure this out, I can set up 5 or 6 rungs where each one starts with a LIM instruction (using the pressure reading) and each rung is responsible for a certain portion of the pressure decay (say the portion between 7 psi and 4 psi.

Two timers?

 

[Steve Bailey]

let me ask this: how can I configure ladder logic to pulse an output, say for example, once every 20 seconds for a duration of 1.5 seconds.

Set up a timer with a twenty second preset. Have the timer's "Done" bit reset it. Use a "Less Than" instruction to compare the timer's accumulated value to 1.5 seconds and turn on the output when the "Less Than" instruction is true.

 

[Skidood]

Thanks Steve, your a star.......that's the kinda trick I was looking for. I am not too experienced with programming a PLC but I am definitely getting hooked.