Instruction

Trying to figure out which instruction to use: I am taking a value from a laser and comparing that against the thickness that our sander heads have been set to and if the value from the laser is greater than our thickness setpoint it would through an alarm.

So for instance my laser reading may be 1.245 and the sander is set to sand at 1.25 and the tolerance is set to 10 thousandths... so I need a way to compare the difference between the laser reading and sander setting and compare that to the tolerance.. hope this makes sense...

I would calculate the +/- tolerance then use a LIM (limit) instruction to check that your withing tolerance.

(If you’re using AB PLC’s)
• Start with a “greater than” comparison with the compared value being the maximum thickness allowed
o Example, if A (laser value) is greater than B (thickness set point) then set bit B3:0/0 high
• Next, compare the difference between the laser value and the set point to the tolerance
o Example:
 if B3:0/0 is high, subtract B from A putting the value into N7:0 (or a tag).
 Compare N7:0 to C (the tolerance value)
• If N7:0 is greater than C set an alarm bit
• If N7:0 is less than C do nothing.
I’ve not put that into a PLC program so I can’t say for sure it will work but the concept should be sound. Basically you need to find out what the difference between the set point and the laser value and if it’s greater than the set point you need to see if it is more than the tolerance.

Is it +- 10 or is it +10 -0?
Either way, use an add instruction to calculate your maximum acceptable and a sub instruction to calculate the minimum acceptable.
Then use a less than or equal instruction to compare your measured value with your max, a greater than or equal to compare it to your minimum.

I believe I have figured it out.. Thanks to all you guys for all the help. I really really appreciate you all!!!