anolog signal scanning

[awautomation]

Does anyone knows how can i scan many analog signals with only one analog input?

 

[jdbrandt]

see http://www.sensorpulse.com/Products/Analog_IO/MSP_Features.htm
if you want an analog input with no analog input card

-or-
http://www.omega.com/pptst/OME-DB-889D.html

 

[rsdoran]

Not sure that would be a good idea to try, the only way I can think of that may work would be to use a selector switch to change between the different signals. If the plc uses the analog signals to control outputs this could create problems.

While typing the above I had an idea. Depending how often you need the analog signals to update you MIGHT be able to use digital outputs to alternate (time based) between the incoming analog signals.

I have no idea if either could work, not sure what issues there may be when running the signals through a relay, switch, or output. Someone here has probably done it though. What PLC will you be using, more may be provided if full details are offered.

 

[Tom Jenkins]

Ron's concept would certainly work, but if these are loop powered (2-wire) devices you would have to allow time for the transmtiter to re-stabilize after powering up for each cycle. You could minimize this by using a SPDT relay to swap in a 250 Ohm resistor instead of the card, assuming there is a fair amount of ride through on the transmitters. If your analog input is voltage instead of 4-20 mA the switching is simpler (one leg each) and you don't have the power cycle problem. You can make a 4-20 mA signal 2-10 VDC by using a resistor.

In my opinion these schemes are not cost effective. AutomationDirect analog I/O is aroung $30 US per channel, and most manufacturers have analog I/O for much less than $100 per channel. By the time you get done with engineering, programming, relays, wiring, testing, and generaly screwing around you aren't really going to save much.

 

[aldarin]

Hello,
A couple years ago I have scanned 10 analog sources with one analog input module. We have designed a card with MUX chips and adressed the chennel with digital IO points. For example with 3 DO you could address 8 channels. An with this I could scan 10 channels with a single AI channel. But however I think it was totally loss of time and money since the scan time is slow and it is somehow unreliable due to EMC distortion. Since industrial analog cards are immune to EMC interference they are much more useful. I agree with Tom because time and reliability is more important constraint on engineering because you will probably loose time and money as you waste your time on unreliable solution. As I said before you could use MUX chips and voltage or current switch based cards and then you can scan these adresses with DO.
Best Regards.

 

[Jiri Toman]

You can use commercially available analog multiplexer such as:

http://www.advantech.com/products/model_detail.asp?model_id=1-1TWZ65

I have no idea if either could work, not sure what issues there may be when running the signals through a relay, switch, or output. Someone here has probably done it though.

I have had some experience with switching between analog signals on a molding machine. I have originally used small DIP based relays to do the switching, however I was having lots of problems and after lots of testing I have discovered that the contact bounce was wreaking havoc onto analog signals. Subsequently I switched to MOSFET switches and all the problems went away.
The MOSFET switches I used were made by Clare (they still make them).

 

[RoTaTech]

I have used (at customer's request) a high quality relay to switch between analog signals. Worked fine for us. I think we read at a frequency of 2 or 4 times/minute.

I've also a similar setup to drive two printers and 2 barcode readers, from a ML1200 with 2 serial ports.

 

[Lancie1]

Yes, I did something similar once for a Automation Direct DL-205 PLC-controlled motor tester. The analog input signals were various voltage levels. I used one PLC analog voltage input, that was switched to 7 different 0-10 VDC input signal voltages by the Relay Outputs on the PLC! It worked well, but then the analog levels were constant voltages, and the switching time was slow at every 0.6 seconds.

At the time I thought it was a novel concept to use the PLC outputs to control which multiplexed input I was reading.