How to Lock in a level value for subtracting

[jtspeed99]

Rslogix 500
Im trying to subtract 10 from the level of a tank we have and then use a less than or equal compare to start a pump. How can I subtract 10 from the value at a certain time and use that number in the compare? My first guess is use a one shot to subtract 10 from the level value and that should give me a static number for the compare....maybe

 

[gscure]

That's how I would do it. It's simple and clean and very easy to understand what you are doing if anyone needs to troubleshoot. I like it.

 

[daba]

RSLogix500 is the programming software.....

What PLC are you using ?

 

[jtspeed99]

Slc 5/03

 

[daba]

Suppose Tank Level is in N7:85

Choose an existing unused integer location (eg. N7:121)

SUB, N7:85, 10, N7:121
LEQ, N7:121, {target value}, OTE, {start pump}

 

[jtspeed99]

as the tank level N7:85 drains wont it continue to subtract 10 from the level and never meet the conditions of the LEQ?

 

[daba]

If the tank level continues to drop after starting the pump, the LEQ will continue to call for the pump to run.

Is the pump emptying the tank, or filling it, or is the pump used elsewhere ?

 

[jtspeed99]

Sorry should have explained better. I want to gravity drain 10% off the current tank level and then start the pump to drain the tank after the 10% gravity drain is complete

 

[gscure]

If you use the one shot to trigger the start also then it would only happen at when you have conditioned it to do so. This could be time based as you had mentioned or other conditions as you need.

 

[Lancie1]

I want to gravity drain 10% off the current tank level and then start the pump to drain the tank after the 10% gravity drain is complete

Use whatever your indicator is for a "10% gravity-drain level" in front of a one-shot that starts the pump. Use some other indication to STOP the pump when the tank is empty.

 

[Lancie1]

One way is shown below.

 

[jtspeed99]

I just ran this with emulator and it seemed to work well. Use the OSR to subtract 10% from the current tank level and move that number to my LEQ, when that condition is met it triggers N15:15 which will enable my pumps