Comparing multiple values

I have a large moving block that I'm monitoring the four corners of, and would like to make sure that any single corner doesn't move too far or too little compared to the others.

I've done this with long, narrow, things before by just subtracting the two values and making sure the difference between the two values doesn't exceed a set amount. If I expanded this to four corners I'd end up with 6 different combinations of numbers, and I think that while it would be easy to understand, there has to be a better way to do it. How would you go about making sure four values don't get too far apart?

Just interested in how other people would do it. Maybe I only really need to monitor the difference between the two sets of opposite corner to ensure that the block is moving straight?

I'm using a Micrologix 1400 and RSLogix 500

What about using four LIM instructions ( one for each corner) with the same High/Low Lim values?
1 AND 2 AND 3 AND 4 = OKAY

dmargineau said:

What about using four LIM instructions ( one for each corner) with the same High/Low Lim values?

Click to expand...

Oh, it's a moving block. So the values are all changing constantly. I want to make sure they are all changing by the same amount (within a limit).

So the limit would be constantly changing as well, and then computing the limit would be the issue.

I guess I could choose one of the four as the "master" and then use the limit instructions to make sure the other three are within the tolerance of that one master.

Start:
Min = (maximum possible reading)
Max = 0

For corners 1 to 4
if reading > Max then Max = reading
if reading < Min then Min = reading
Next corner

Diff = Max - Min
If Diff > tolerence then take action

Repeat while moving

So, compare only the two extremes, ignore the other two. That would do it.