Using cFP-AI-110 to measure rpm:capacitance in the card?

[mpatt]

I am attempting to measure the rpm of a number of different augers. The augers have a magnet on them that closes a magnetic switch when the magnet passes the switch. I have built a circuit with a very small amount of power traveling through a resistor and measure the voltage across the resistor. The magnetic switches are in series with voltmeters. I have attached a picture.

The idea was that by measuring the time between closed circuits, I could calculate the frequency of the augers. When I use multimeters, the simple circuit I built works perfectly; a closed switch will cause a meter to read (say) 9v, an open circuit reads 0. However, when I swap in the analog input card for the multimeters I encounter a problem. When the circuit is closed MAX instantly shows the proper voltage, but when it becomes open the voltage decays slowly. In other words I get a triangle shaped waveform, and since some of the augers spin rather quickly the peak to trough distance is almost non-existent.

Does anyone know why the voltage is measured properly as soon as I close the circuit, but why it decays slowly once it is opened? This problem only occurs if I use the NI card.

Thanks for your time

[Scott_A]

Hi there,

How quickly are the augers spinning?  Is it fast enough that you would be pushing the 5 sample/sec limitation of the cFP AI-110?  

How slowly does the voltage decay?  Is it over milliseconds, seconds, or longer?

[mpatt]

The fastest auger spins around 60 rpm so the sampling rate shouldnt be a problem. The rotational speed is only fast relative to the voltage decay. The voltage decays very slowly. While the decay is noticeable immediately after the switch is open, it takes around a minute to drop from say 9 to 6 volts.

Thanks for your reply

[AnalogKid2DigitalMan]

Try adding a pull down resistor from the open (floating) side of the switch to ground, say 1000 ohms.

-AK2DM

[mpatt]

Thanks, I'll give that a try. Did you mean:

[Henrik_Volkers]

If the drawing is right, a pull up seems more reasonable to me 😉

You don't want floating inputs at your cFP.

 If the contact opens, the - input floats and need to be pulled to to + input to measure 0V