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Analog Lagging Techniques

Analog Lagging Techniques

Analog Lagging Techniques

(OP)
I am programming a system that will have a 4-20mA linear position sensor that will go from 4-20mA then 20-4mA in one full cycle. This cycle will continuously repeat itself at a set speed. Next, I have an identical hardware setup that I need to lag behind or follow the first cycle at roughly 30% of the cycle. It will be controlled by an analog output signal. I have had several ideas of how to do this that would work as the leading value is increasing from 4-20mA but I am running into a roadblock of how to get the second value to continue to increase to 20mA and then follow the leading value back to 4mA. I am programming in RSLogix 5000. Any help or ideas would be greatly appreciated!

RE: Analog Lagging Techniques

You say "set speed". That's the key. Both ramp/cycles should execute temporally accurately and identically. That means the second one should NOT need to track the leading one at all. What you need to have is a single sync point. The first cycle should provide a trigger pulse at the moment it starts that the second cycle accepts then delays initiation of its cycle start until the appropriate time period has passed providing the 30% lag you desire. This method also allows easy tuning by simply altering that delay period. Both cycles do their own up and down so it's not an issue with the second cycle having to carry thru and return to the start. Both cycles should be written identically with only the second using the initial delay.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Analog Lagging Techniques

You can just do a compare in the plc.

There are scaling on inputs and other features on the analog cards that are kind of nice. Like rail limits, scaling inputs, and all that other nice features.

20mA = 100
4ma = 1

Once the first one gets to 30 then latch a bit till the 2nd one gets to desired output then reset bit and disable function. There are a hundred ways to program, but if you are with a company just try to follow whatever standards they have established.

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