Dear Friends,
I have my voltage input from monitor terminal from the motor which indicates the speed. When I measure the the voltage between monitor terminal and GND the value displayed on the screen of the multimeter varies from 0.2V to 0.4V. Those measurements are reasonalble. When I use the FPGA input VI to take in data from terminal AI0+ which is connected to the monitor terminal the displayed value on the desk top is always 10, no mater the motor is running or not running. I think this problem might be solved by connecting GND to AI1+ and have the scope displays the difference between AI0 and AI1. But when I did this, the displayed value from AI1+ is always 10 and the difference is kept at zero.
My test shows that the displayed value on the desk top is 10 when the terminal is connected and zero when the terminal is not connected. So I suspect the measurement does not show the real voltage value. Because the SCB-68 board has multifunction which does not allow the value to be transmitted to the computer. So I checked the SCB-68 manual(It does not mention anything about AI- terminals). My setting for SCB-68 is direct feedthrough switch setting, so I should be able to connect terminal directly. I notice that for every analog input there are two terminals, take analog input 1 for example. AI1+ and AI1- both exist. I only connect the signal to AI1+ and leave AI1- floating; for analog output there is only one terminal AO1. The situation is that my analog input is effective because the motor is driven by the analog output. Compare the situation for both AO and AI, I conclude that there should be something wrong about the floating AI-.
So I measure the voltage between the floating AI0- and GND when the motor is not running. The voltage is not zero although the voltage between AI0+ and GND is zero (indicating zero motor speed). Then I measure all the voltage between the AI+ terminals and corresponding AI- terminals, there is some voltage between each pair and the voltages are different.
Since My labmate is running the experiment with SCB-68 without any AI- terminal and I don't have any thing similar to compare. My next step will be trying to connect some wire to the AI- terminals and to see anything that could be changed by that. Do you have anything in mind on my trial on the AI- terminals?
