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ACC-28E Potentiometer Input


Brian
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I am trying to read a 10k string pot that is connected to an analog input on an ACC-28E. The problem is that the input is not linear. The resistance change of the pot is linear, but apparantly due to the input impedance, the voltage input is not linear. How do I get around this? This must be a common application.
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I am using the -5VDC and +5VDC from the ACC28E. Analog- is connected to -5VDC and one end of the pot. +5VDC is connected to the other end of the pot. Analog+ is connected to the wiper. I originally tried this using +15VDC through a resistor to give 10VDC at the pot, but I had the same results. The channel jumpers are set up for unipolar, and the resistor packs are in the default positions.
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Brian,

 

I did a test with a rotary pot and ACC-28E and it matches the voltages read on DMM. Is it possible that the voltage output of your potentiometer is not linear? Can you check the voltage of the wiper versus ACC-28E reading? Also I had set my ACC-28E as Bipolar which gives double the resolution on -5 to 5 volt.

 

Regards,

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Sina-

 

Please confirm that your voltages are linear. My DMM readings match the ACC-28E values, but they are not linear. The string pot by itself has linear resistance, but when connected to the ACC-28E, the voltage output is non-linear. That tells me that the ACC-28E input impedance is rather low, and affecting the readings, but I couldn't find the specs for it. Your DMM is probably about 10G Ohms. I don't remember if I checked voltages without the ACC-28 connected.

 

I am not using bipolar, but unipolar 0-10VDC. I have it wired for differential input, so it is really looking at a 0-10VDC range. Also, the readings occasionally jump to a large value, but still within the normal range of the register. I doubt that it is noise. Is it possible that something else is writing to the that address? All my cards are addressed differently.

 

Regards,

 

Brian

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Brian,

 

Here is a plot of the ACC-28E reading versus the encoder reading the position of the potentiometer. The setup is the same as explained in previous post.

 

764779407_Acc-28ETest.png.7218f658d1d4bdd9768bbc1903a10637.png

 

But I can't guarantee that my potentiometer resistance changes linearly though its course.

 

Also, those addresses are only written to by the card whose address is set by its DIP switch. If you're getting spikes, check your grounding and shielding of the signal.

 

Regards,

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Sina-

 

It appears that unless you have an oddball potentiometer, then you are seeing the same issues I am seeing. I've only ever seen 2 types of rotary pots. Ones with linear tapers, and ones with audio tapers. What your graph shows is neither, so I am inclined to believe that the internal circuitry of the ACC-28E is affecting the reading.

 

Do you know of any fixes for this, other than a signal conditioner?

 

Regards,

 

Brian

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