NI 9221 Multiple motors, polarity change

[Jani777]

Hi kind people, 

I need your help. I am quite new to LabVIEW and the whole elektrotechnical things. 
I have a cDAQ NI 9189 device and NI 9221 card. With this card I want to measure voltage over multiple electrical motors. 
So far so good, but I need some to run, than stop and than go in in the reverse direction. And hier comes the problem. 

9221 has 8 AI (+), but only one -, so if I change the polarity on one motor, i would destroy my 9221 card. (At least I think so...) 

My question: Do you know any solutions how to make it work correctly please? I'm sending you a circuit of one motor and the problem, which I tried to solve with diods, but I can't figure out how... (See picture) 
I would be very thankful if you can give me the solution or any hint. 
Thank you very much! 
Jani 

[GerdW]

Hi Jani,

---

@Jani777 wrote:

And hier comes the problem. 
9221 has 8 AI (+), but only one -, so if I change the polarity on one motor, i would destroy my 9221 card. (At least I think so...) 

---

You are not very good at reading manuals, aren't you?

Do you spot that "±" character all over the specs?

[Jani777]

Hi Gerd, 

thank you very much for your answer. 
Of course I read the manual, but probably I am not very good in it. 
I do spot all the +/-. But I can't see what you see. 
Could you please explain it to me? 
Thank you very much! 
Jani

[GerdW]

Hi Jani,

even though the input pins are labelled "AI+" and "AI-" the allowed input range is ±60V.

So you can also measure negative voltages easily (without fearing to destroy the NI9221)…

[Jani777]

Dear Gert, 

oh, yes, i understand that...
But is it the same when i have more motors than one on this card? 
My thought is: if the motor goes in the another direction, the poles switch and i am going to measure voltage with a negative value. That wouldn't be problem at all, but i thought it's not right thing to do, because all the channels have one commen - port. 
Of course i would be happy if i am wrong. 🙂 

Thank you! 
Jani

[GerdW]

Hi Jani,

I guess you drive the motors using the same supply: connect NI9221 "AI-" to GND of supply.

To switch the motor direction you need to switch the 2 cables from the motor, but not the NI9221 inputs…

[Jani777]

Hello Gerd, 

thank you for the answer. 

The motors will be driven by the same DC supply, yes. It's a control unit and there are only two cabels which go out of it - blue and brown one. No ground.

Now i understand the switching more, thank you. But i am not really sure if i understand how this work... am i getting closer with the drawing, please? 

The switches would be synchronised.
Thank you, 
Jani

[Jani777]

🙂