Sample - Hold Data Aquisition in a PLC...

Hi all,

Got another question. We have an application for a sample-hold circuit, and I'm wondering if you guys think it might be possible to do in a PLC?? I think it might be do-able, but will a PLC be fast enough?? I know complared to a SH analog integrated circuit, it won't be, but just as a brainstrorming idea, do you think it would be possible?? We are trying to eliminate having to hand make a interface board, with an analog IC, we dont like to do that anymore, we like off the shelf parts.. Does anyone one know of a SH module on the market, that all you have to do is wire up your input, output and trigger?? I have looked on the net for SH modules, but came up with nothing, only SH IC's, ie : from Analog devices, or SH aquisition boards that are propietory(spelling??) to a certain manufacter's aqusition system. So, if anyone has any idea, I'd love to hear from them...

Thanks!

Andrew Evenson

How fast is fast enough? A PLC can san in a few miliseconds, depending on the brand and model. For some units you may have to add a few more miliseconds to accommodate the A to D conversion on the analog card. You can easily program the PLC to read and store the analog data from either a switch or other external trigger or from an internal logic trigger based on a threshold value.

Yeah really, how fast do you need your sampling? The analog input on most PLC's is measuring faster than the PLC can scan it. You probably get close to hundred A-D samples on the analog input card and depending how it's setup the PLC will report the average of this for each scan it takes, usually every 5 to 15 milliseconds. Programming the PLC to hold the data, stop the process, or whatever you want it to do is not very difficult. I'm wondering what your application is?

High speed PLCs are provided by all brands, but usually industrial analog I/Os are scanned separately and an amount of time is added for A/D - D/A conversions and exchanges with CPUs, depending on your accuracy.

For basic sample & hold features (up to 12 bits AD + DA converters) : compare Mitsubishi FX and Aromat FP0 low cost solutions. Most compact size with FP0 + A21 expansion (total 50x60x90mm) for 2AD+1DA+I/Os. See what's offered by Keyence too.

Refresh rates, including conversion and "CPU I/O refresh" times, should be from 500hz to 1khz max, when scanning. However, if your trigger input is an interrupt one, the sample delay could be greatly reduced.

Hope this helps

Thanks!!

hi guys,

Thanks for the response! Yeah..I should have asked myself that same question - How fast is fast enough. Sorry, dumb question .

I guess I was really asking if would be possible in a PLC, and after posting the question, I came to the conclusion I would be, at least for the application I am looking at.

Sorry to waste your time with a stupid qustion.

Again, thanks for the responses!!!!

Till next time...

Andrew