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Brushless DC Motor Tacho Output

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Hi Folks,

 

I’m trying to monitor the speed of brushless DC motor (Thomas 1410 series 12v). using a cDAQ -9174 chassis.

The motor runs as expected but when I attach an oscilloscope to the Tacho output I’m only seeing signals between 0.3v and 0 (but at the correct frequency).  

Is this enough of a signal to bring directly into the chassis (what module would people recommend)?

If not, what should I do to the signal to achieve the 0-5v TTL described in the spec sheet?

(again, what module would be best suited?)

 

Please excuse my inexperience as I’ve never played with frequency measurement before.

 

Regards

 

 

Spec sheet and Brochure attached 

 

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Moved to counter/timer hardware 

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Solution
Accepted by topic author Simon-lee

The spec sheet shows "FG", which a quick Google tells me might be "floating ground".

How are you connecting this to your DAQ device?

 

I'd also further note that "black" is the ground for Vsupply and Speed Control, but Tacho Out isn't mentioned.

 

So you might need to pull up the voltage and measure the difference from the black wire - although this wasn't how I understood your datasheet it is a common option for encoders (and it matches your description).


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Hi Cbutcher,

 

Thank you for your reply, a floating ground would make sense. I assume a couple of k-ohms will suffice for a pull up.

I’ve not bought a module yet, so I was looking at the NI9401 or NI9361, looking at the motor specs I’m only after a 5khz read, any insight would be welcome.

The boss wants to try one of his home-brew instrument amplifiers, so my chance to play might be delayed ,which gives me a few weeks to get a suitable module in.

 

Thanks

Simon

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Well, the downside to the 9361 is that it only has counters, so you can't easily use it for other tasks.

 

The upside is it has counters! Which for this is exactly what you want. Further, it has toggleable pull ups to 5V, which avoid the need for external power supply and resistors in your case (although of course you'll still need the power supply for the motor at 12(?)V).

 

I don't want to push particularly one way or the other on others' purchasing decisions, but I really like the 9361.

 

Regarding the 5kHz reading, at 18 pulses per revolution you'll need quite a high rotation speed to read that fast and have good accuracy. Is that important? (I currently have a related question I'm trying to work our with the 9361 - I'm measuring an encoder with 4P/rev at around 70rpm (giving ~4.6 pulses per second) and yet seemingly I get reasonable results sampling at 50Hz. Not sure what kind of magic is enabling this measurement... But we have a 3600 P/rev encoder ordered)


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Hi Cbutcher,

 

Thanks for the insight, the 9361 looked like the better option, I will get push back from the boss because of the extra expense, so fingers crossed.

The pump itself should top out around 900hz but from what little I’ve read about frequency and Nyquist between 6 and 10 times would be better.

Accuracy isn’t critical but I will need confidence in the figures.

All the best with your project

Regards

Simon

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Regarding Nyquist and sampling, I'd suggest just that you read the manual on the velocity measurements (effectively frequency/period) and NI has some more detailed guides somewhere too.

Edit:

 

If you're using a digital or analog input to read the values then yes - sample fast. But effectively the sample speed relevant to the Nyquist condition for the counter is the counter clock speed (I think 100MHz for the 9361 - so that covers the "sample fast" bit).

The sampling rate that you're setting is the rate at which you want to know the velocity/frequency/period of your input signal.

Making this faster will decrease accuracy typically, but give more results in a fixed time. So it depends on your need for time resolution of velocity measurements.


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