Load cells and NI USB 9215

[Gustavo Eiji Kaneto]

HI!

I'm looking for a cheap solution to connect 4 load cells to a computer. I need high acquisition rates (such as 20 kS/s per channel).

I found the USB 9215, advertised as "USB DAQ with Signal Conditioning". Support by phone here in Brazil indicated this product telling that I can connect load cells directly to it, without further amplification or excitation source. But the manual don't specify nothing... Somebody knows the true? I tried again support here in Brazil but they are saying that the product is too new, and they don't have lot information about it...

Furthermore, there's a page "lost" in Knowledge Base explaining that the maximum sample rate is not 20 kS/s/channel as I can understand by "20 kS/s simultaneous-sampling". What now?

Why NI writes information that can lead to misinterpretations? And why they don't even explain the features of their products to their people in Brazil?

Thanks,
Gustavo.

[LucaP.]

Hi Gustavo,

Load cell measurements usually require some signal conditioning (bridge excitation,signal amplification, calibration,...) The USB-9215 doesn't provide this kind of functionality directly. You need some external HW which is doing that. If you want to use the 9215, then you must do all the signal conditioning yourself. NI doesn't provide load cell signal conditioning for the USB-9215. But there are other NI HW which can do that. Check this for more informations: Load cell measurements

The sampling rate of the USB-9215 is determined by the highest channel number read, not by the number of channels read.
Highest Channel Number Max. Sample rate
0 20 kS/s
1 10 kS/s
2 6.67 kS/s
3 5 kS/s

For example, if you scan only channel 3, the fastest you can acquire without receiving an error is 5 kS/s.

I agree with you that the documentation is not clearly enough and I hope that my answers can help you.

[KristiH]

Gustavo,
I have just a couple of things to add to Luca's response. The USB-9215 are simultaneously sampled using track-and-hold technology (as the data sheet explains). Therefore, the signals on all channels are sampled simultaneously, but the sampling rate will decrease as more channels are sampled. In addition, the USB-9215 does provide signal conditioning - it provides isolation from channel-to-earth ground. However, it does not provide the bridge conditioning necessary to for load cell measurements.

I hope that helps clarify things a little more.