Resistance to analog.

I have 2 devices that I need to read the resistance of with a SLC based AB PLC.

The range I need to be able to read is from 256 Kohms to 13 Megohms. An AB RTD card has a maximum range of 3K and creating a parallel circuit reduces the resolution too much.

I need 1 of 2 types of devices.
1. A module that would send a digital input if it sees a resistance from x to y. Kind of like a relay that will energize when it sees a resistance value within range.
2. A module that would take a resistance value and convert it to a 0-10V or 4-20ma Output. A DMM with this output option would be great.

Has anyone here used a device like this in the past or seen one that they could recommend.

Thanks in advance,
Tim

Try these guys "http://www.apcs.net.au/" it's an Aussie company but they may have a presence in the states or it may just give you some idea's on what to search for.

If an RTD card or a DMM would be OK, then you must be able to put a voltage across the resistor or a current through it. Is that the case?


If so, then with a 1 microamp excitation current, you will see 2.56 to 13 volts across the resistor.

That's pretty wide range you have to measure too. Do you mind telling us what the application is?

I don't know if either of these will work for your application but I have used both.

http://www.abb.us/cawp/usabb046/fbb041b4834d6924c125719b00576090.aspx?

http://www.actionio.com/products/slimpak/g438.html


edit: The link to the US site of APCS (see Consys's post)is http://www.apcs.com/index.htm but they don't list the part number for the resistance transducer on the Aussie site. You could call at the listed number and inquire however.

few years ago i was facing same problem. normally i would use RTD transducer but they don't cover such range. i was only looking for accuracy of some 1-2% so i made divider and wired it to 1746-NI4. (known voltage source plus voltage divider of one known and one unknown resistance). it worked very well but i didn't like the separate power supply and with load changes on existing 24V, reading was affected (although very little). since i still had three spare analog inputs, i added voltage divider to replace dc regulator and simply monitored that voltage by second analog input. the result was just three resistors wired to two analog inputs (it was one precission resistor plus two standard ones as divider to derive some 9V from 24V). unknown resistance was connected accross one of the analog inputs. i also had to take into account internal resistance of the analog input but that was not big deal. it was advertised as 750k or more. in my case it was someting like 762k. in only two hours i got really nice autoranging ohmmeter with just few rungs of code and three resistors. covered range was 0-20MOhm. Worked for me...

These guys have lots of signal conditioners and alarm units:
http://www.api-usa.com/

Use a Wheatstone Bridge.

http://www.geocities.com/CapeCanaveral/8341/bridge.htm

The above link is a good illustration. Sub R1,R2, and R3 for a fixed 256k ohms and let your "256k - 13 Meg" be R4. If you excite the bridge with 10 volts, you will get a reading range across the bridge. If R4 = 256K then Measured voltage (G) = 0 volts (5 volts on each side; if R4 = 13 meg ohm, then (G) = 4.806 Volts. The left side will be 5 volts, the right will be 9.806. You should be able to scale that ok. This gives a total range of 4.806 volts for 256K to 13 megohm.

(G) is called "Detector voltage" in the illustration.

Thank you all for the responses.

I work for an automotive OEM and we assemble the overhead systems for multiple US manufacturers. You would not belive all the widgets and gadgets that the automakers like to stuff into the roof of your car. We do not produce any of these components, but we are responsible for their connections to the wire harness and in some ways functionality.
These specific devices are intrusion sensors that tie into the cars vehicle theft system. From outward appearance they look to be an ultrasonic transmitter and receiver set. How they work, I do not know. I am just supposed to make sure the wiring to them is correct.

All of the websites had some great modules and I will keep them for reference.
1 in perticular met my complete range of resistance and I will look into it further.
I will also be crunching the numbers for a voltage divider or wheatstone circuit. This was my initial thought but we try to stay away from custom circuitry do to the level engineering required any time there is a product change.

Thanks again,
Tim