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VibrationEngineer

PLC Selection for 0 to -24Vdc analog input

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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 Edited by VibrationEngineer

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You can use a voltage divider, a couple of resistors in series, and measure the voltage across the one resistor whose one side is at the reference or ground (otherwise the analog input sees the full -24V "common mode"). The fraction of the full voltage dropped by the resistor can be lower than the 5V or 10V span of a typical PLC analog input. There are lots of voltage divider tutorials on the web, including some youtube vids that will explain how a divider loads the source (the eddy current device) and how it also affects the accuracy of the measurement by being a parallel resistance with the analog input.

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Omron CP1L-E serie have built-in analog input (0-10V), with max rated input of 15V. It's 20 I/O model is less than 300$. But this built-in input resolution is only 1000. If you need more, you can add an analog input option board, for maybe 125$, wich has a resolution of 4000. All you need for a voltage divider is 2 resistances, I suggest 2x 500 ohm resistances so you read half voltage.

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it is not quite so simple, there are several things to be considered. and it is always good idea to use a bit of math (for sanity check at least). using voltage divider one can increase input range but: - using analog input with resolution of 1000 counts, and input span of 24V, one count is 24mV. that is too coarse for signal that may only change 200mV (12%). - if the resolution is 4000 counts, then one count is 6mV (3%) etc. in other words, the higher the resolution, the smaller change can be detected. not sure what your application needs are but in general I would rather see 1% or better. also your signal is negative, so one needs something like: - analog input that can measure negative voltages or - analog input plus something to reverse measured polarity (and/or scaling) such as signal conditioner or stable voltage reference (used as a common for your voltage divider or for input signal offset). then one needs to consider loading of the sensor (what is the required minimum input impedance of sensing channel?). pair of 500ohm resistors in series is only 1k total. that is ok for instrument measuring small current but it is a very poor figure for a voltmeter. even analog voltmeters used to have >20kOhm/volt or good half a MOhm on 25V range. in other words input would simply draw way too much current from test device (up to 24mA in this case), which exceeds range even for analog inputs measuring current (0-20mA or 4-20mA). standard multimeters have 10MOhm on voltage range (and with good reason). good analog card will be at least few hundred kOhm. devices with voltage output are designed to provide 'voltage output' but usually can only handle very low current (<1mA). and there is self heating of resistors. at such low value, current will be too large and temperature changes will affect their value and stability (and therefore measurement). we are already looking at itsy-bitsy value change in a much larger signal, so it is better to avoid things that could spoil results. then there is sampling rate etc. - how fast your signal changes during measurement?

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