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Reading motor voltage

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canoo

New Member
I have a brushed DC motor which operates from 0-40VDC. It gets a supply signal from a motor controller. There is also a tachometer output from the motor (0-40VDC as well) which gets fed back into the motor controller.

I need to have a reading of the the motor supply voltage and tach voltage fed into a computer DAQ system. The DAQ system I'm using has a 0-10VDC analog input.

My question is: How do I signal condition The motor supply and the tach output to be properly read by the DAQ system?

Thanks in advance! :)
 

MikeMl

Well-Known Member
Most Helpful Member
Is the ground side of the motor drive common to your ADC's analog ground?

Can it be, or do you need optoisolation?
 

canoo

New Member
Is the ground side of the motor drive common to your ADC's analog ground?

Can it be, or do you need optoisolation?
The motor controller ground is not tied to the DAQ card's ground. I believe that's what you were asking. Correct me if I'm wrong.
 

MikeMl

Well-Known Member
Most Helpful Member
That is what I was asking.

Is the motor driven in some kind of H-Bridge?

Seems like you have much greater issues than simply reducing the motor voltage to the input range of your ADC card which can be done simply by using resistors and capacitors to make a precision voltage divider (like a 10x scope probe).

You might need two ADC channels to read the voltage (referenced to the ground on the motor driver) on each end of the motor, and then subtract those two readings in software to determine the voltage across the motor.

Another possibility is that you cannot tie the analog ground of your ADC to the drive circuit (because of safety or noise), in which case you will need opto-isolation, which gets quite complex.
 

canoo

New Member
You might need two ADC channels to read the voltage (referenced to the ground on the motor driver) on each end of the motor, and then subtract those two readings in software to determine the voltage across the motor.
I don't quite understand what you mean here. Could you explain a little bit more?
 

MikeMl

Well-Known Member
Most Helpful Member
I think my explanation is pretty clear. Ask a specific question. Post a diagram of your motor drive.
 
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canoo

New Member
Post a diagram of your motor drive.
Here's a diagram of the whole system. Maybe it will help. :)

The broken lines is what I'm looking to add.



Also to clarify something I said earlier. The ADC ground will actually need to be tied to the motor driver ground. Sorry :eek:, wasn't thinking.
 
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Mr RB

Well-Known Member
I've seen industrial systems where the motor and tacho voltages are converted to freq, linearly, then optocouplers used to transmit the freq. Good for noisy environments, long transmission distances, fully isolated etc. Google should bring up some "voltage to frequency" converter chips.
 

canoo

New Member
I've seen industrial systems where the motor and tacho voltages are converted to freq, linearly, then optocouplers used to transmit the freq. Good for noisy environments, long transmission distances, fully isolated etc. Google should bring up some "voltage to frequency" converter chips.
Hmm yeah I never thought of that. It looks like the best input range Analog Devices has for their voltage to frequency converters is +/-14V. I'll keep looking though. Thanks.
 

Ubergeek63

Well-Known Member
I've seen industrial systems where the motor and tacho voltages are converted to freq, linearly, then optocouplers used to transmit the freq. Good for noisy environments, long transmission distances, fully isolated etc. Google should bring up some "voltage to frequency" converter chips.
an RC filter works fine. especially if you sacrifice a little resolution and make the source a higher voltage than you need.
 
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