PLC needed for 0 to -24Vdc analog input

I'm a PLC newbie. I have an eddy current device that measures the "runout" of a shaft. It has an output of
0 to -24Vdc. I will set it at a nominal -8 to -9 Vdc. As I measure the shaft runout, the voltage output from the eddy current device will vary by no more than 200 mV.

I need a PLC (and small HMI panel) to read this single analog input. It will capture the minimum voltage, the maximum voltage, subtract the two to give a Total Incidated Runout (TIR) of the shaft. Want to spend less than $1000. The problem I am having is finding an AI that won't get fried when it sees the -24Vdc during the setup of my shaft runout test.

Thanks,
VE

Greetings and welcome to the forum.

Most likely you are going to have to use a signal conditioner.

This link http://www.dataforth.com/device.view.aspx?deviceid=178 is for a signal conditioner that converts a +/-40Volt input to a +/-10V input.

You are probably going to have some difficulty finding a PLC with analog and an HMI and a signal conditioner for under $1000 so you need to start rethinking the budget unless you buy used off e-bay. You can probably find a solution for under $1500. Nematron makes some single line inexpensive display panels that run a couple of hundred dollars each. An AB Micrologix 1100 is a small inexpensive PLC and the software for the 1100 is free. The ML1100 has a built in analog input but it is only 10 bit resolution, so most likely you will have to use one of the analog expansion modules which will add more expense. Redlion controls has an HMI that can take analog inputs allowing you to do this without a PLC at all.

I doubt you will fry anything... it will just be out of range, I send 24VDC to a MicroLogix 1100 all the time and they have built in analog

I may be able to help with the purchase but a few more details are needed (other power)

http://www.plccable.com/micrologix-1100/

A signal conditioner as TConnolly is also a great option

Best regards
Mark

You probably need a signal conditioner. Bently Nevada and Metrix make devices to convert eddy current prox probe outputs to 4-20 mA. (I believe they call them a proximitter) I suggest contacting one of their application engineers. Once you have a 4-20 mA signal virtualy any PLC or I/O device can accommodate you.

why not just use a resistance bridge to read a bit less than the full signal, it will cost manny time less than a converter...
Use resistances value low enough to avoid to be affected by plc internal input resistance that would also respect the signaling device capacity.
For exemple 2x 500 ohms 1watt connected in serie between - and + of the signal and you connect the plc input between the middle and -. You will get a 0-12v signal out of the 0-24v device. And if the resistance open for any reason, you will get the full signal voltage 0-24v) but it won't burn a normal input card

If you know it won't go above 9v in normal operation, you could connect it directly as stated above, most of the good brand plc analog input will resist to a 24-30vdc surge in case of process issue, you can trigger an alarm when it is above 9 and being in alarm between 9 to 24v...This way, you get better resolution for your 200mv change...

Then just choose a plc with a built in 0-10vdc with a precision low enough to read the device changes...a 10bits input should see +/- 20mV variations steps.
10v / 512 bits ? (I'm not sure if i should include sign or not...?)

vs a 13bits that would read 10 times more precise...?

I'm i right ?

If you are in a budget, AB plc may not be the best cost effective choice but it is one of the reliable brand name and you usually got what you pay for....
For sure, you want something else than AB for HMI because you don't want to deal with AB component temrinals and a normal pv monochrome would raise the bill higher than 1000$ for sure...

You could use a relay logic (Like a LOGO, Zelio etc with a built in display, this wouldn'T cost much and you could display your tags values on it. You won't get any real animation or pictures but if you want a reliable low cost option this would make it.


Your best option is to call your local supplier and tell them what you want to achieve and they will make suggestions of availaible products...

Jeff23spl said:

why not just use a resistance bridge ...

Click to expand...

Depends on the CPU that he would use but a MicroLogix 1100 would be a 250ohm 1/4 watt would drop down the 24 VDC to about 10 VDC

very strange a negative voltage output, but please tell us what sensor you are using?
to get a better resolution a zenerdiode can help.
as display use a simple multimeter, or arduino, a plc is not the best as you will need high resolution.

One of the baby unitronics oplc's may be suitable... The jazz is cheap, has an AI rated to 28v, and 12 bit resolution..

You can contact Entertron. They have done PLC configurations in the past to satisfy analog input requirements of 0-12 VDC and 0-24 VDC.

www.entertron.com

Bom

Touchscreen 4.3"

PLC - Click

24 DC Supply. 2.5 Amp
Analog module. 10 Volt. 13 bit. 100 Volt overload

List so far is under $500.00. Software is free for PLC and HMI.
Conditioner or resistive divider for 24V to 10V conversion, if needed. Enclosure. Program cables.

geniusintraining said:

Depends on the CPU that he would use but a MicroLogix 1100 would be a 250ohm 1/4 watt would drop down the 24 VDC to about 10 VDC

Click to expand...

humm it depend with which other resistance you bridge your 250ohms ... the specs, the ml1100 analog input impedance greater than 199kohms.

If you connect :

0-24v signal
|
RES 500 ohm
|
Connecting point of resistance that connect plc input
|
RES 500 ohm parale with plc impedance
|
0v ground


So plc impedance will bring doen the 500ohms to approx 498 ohm so this won<t make any significant result to the ratio 498/998 = 0.49999% of the input voltage

Jeff23spl said:

it depend with which other resistance you bridge your 250ohms ...

Click to expand...

Very true.... we use it in conjunction with an 1k ohm potentiometer and this give 0-10, depending on a few variables (power supply voltage, res +/-, pot +/-, etc)

Works good and less expensive the other options