Analog data

I would like to use analog data from a few scans prior to the drop off of the analog signal. How may I accomplish this. I am working with a AB micrologix 1400.

To operate on an event in the past, you have to save it when it happens for use at a future time (because we have not yet discovered time shifting or time travel). You could set up 2 or 3 (or however many past analog signal readings you want to consider) N7 memory locations (for example N7:0, N7:1) and on each scan or when a timer expires, first move N7:0 to N7:1, then save New Analog Signal reading to N7:0. Now at any time, you have 3 values for Analog Signal: Current or New, Past Event N7:0, Past Event N7:1, and so on.

You could then use those past analog signal stored values as desired.

Lancie1, Sorry, should have outlined my application. Read a part length. I have used a SCP instruction and addressed the output N7:0 and using that as my test in a LIM. If within my limits it sets a bit as a go. I have a slight (20ms) delay on the leading edge as the part is seen by a sensor which is my analog signal input but having problems with the trailing edge. The part is measuring fine with a mic but my program is giving a no go.

Sprintsrule,

I think then you have a sensor that detects the part, and when a part is detected, you read the length with a measurement device. If the length is within certain limits, you produce a go-no go signal. Is that correct?

It seems that you are working close to the limits of your PLC. Using the SCP instruciton can produce rounding errors because SCP does an internal division calculation. You could try using Floating Point numbers in the SCP to maybe increase the accuracy, or you could eliminate the SCP and use the raw analog data directly, and set your LIM limits in raw analog counts instead of in engineering units.

If using raw counts doesn't work, then probably your analog input does not have enough "resolution" (number of bits used to represent the signal) to accurately measure your part. The fewer bits in the analog signal count, the larger distance each bit has to represent, meaning that small distance differentials can get rounded up or down.

Lancie1,

Yes, that is correct. I'm using the embedded analog input/output of the micrologix 1400, sounds as if I would need to go with a analog module add on to obtain the higher resolution if your suggestion does not help.

Sounds like you have a very time sensitive application also. Mosrt likely a little late in the game for this advice but a micrologix is really not the best processor choice for this application. Contrologix /compact logix with these actions in a periodic task with high priority would have been a better choice IMHO.

The CompactLogix might be a better choice but on Logix5000 systems the IO update is asynchronous to the PLC scan and this can cause you problems in a high speed application if you aren't careful.

The ML1400 is a speedy little brick. The problem is its on board analog IO isn't very good and is very low resolution. I suggest you try an analog expansion card with higher resolution. You need to pay close attention to the A/D conversion time of the analog input. Even though your scan is fast, the A/D conversion time determines how fast you can get new data. The A/D conversion time is usually much longer than several scans.