sbRIO-9632 Load Cell

[Nathan_B.]

I have the Robotics Starter Kit (DaNI) and I am looking at incorporating a load cell into our robot.  The NI sbRIO-9632 says it has 16-bit resolution and can do a range of ±0.2V on the analog input pins.  One of the load cells that I'm looking at is a LCM201. It has an output of 2mV/V and an excitation voltage of 10V.

So, this gives me:

(10V * 2mV/V) / 500N = 0.04 mV/N

If I use the ±0.2V range on the ADC that gives me 0.006mV/bit resolution.  Combining these, I should be able to get 0.1526 N/bit = 15.5 g/bit.  I'm not sure if I'm off by a factor of two because of the fact the sensor is made for tension and compression (±500N).

Does this all seem correct?  Based on my calculations I shouldn't have any problems using this load cell with my sbRIO (getting 15.5 g/bit).

Does anyone see any issues here because I see modules specific to strain guage bridges that have 24 bit resolution with ±25mV?

[REDS]

Hello Nathan_B,

I don't see a problem with the calculations, you should be able to get 0.1526 N/bit. You could use a higher resolution-bit module, like the 9237 (24-bit and ±25 mV/V) if you want to have higher resolution bits. However, you will have a maximum sampling rate of 50 KS/s as opposed to the 9632's 250 KS/s in page 39.

Now, if you are using 10 V of excitation, notice that your voltage range will be ± 250 mV on the 9237, not ± 25 mV. This is because the 9237 doesn’t do the conversion of input mV / excitation volts. It takes the ratio and does the conversion internally. If you have questions, here is a couple of links that will help you:

How Do I Configure the Input Range of the NI 9237?

Incorrect Voltage Readings on the NI 9237

[Nathan_B.]

I don't currently plan on adding any modules to the system and would prefer to use the sbRIO alone.  It may be worth mentioning that I don't really need a very high resolution of the load, anything less than 400grams should be sufficient (I think) since we are looking at 100+ kg.  Having said that, I will need to get a load cell with a greater capacity which will reduce my mass resolution.

Now for a question,  since I am going to be measuring a differential voltage, do I need to be concerned about where my 10V comes from since the sbRIO doesn't have that high of voltage output?  I will have a 12V regulator and the sensor says it can handle it and I was hoping to power the load cell with that and have the sbRIO read the differential voltage output.

[REDS]

As a matter of fact, the 9632’s specs say that you can have 10V from the analog outputs, page 43. However, you are limited to 3mA or current, which might or might not be a big deal, as the LCM201 datasheet you linked doesn’t specify the current load.

If you want to use an external power supply, then you would have to read it in differential mode, as you said. The AI- would have to be connected to the regulator’s ground and AI+ to the actual positive voltage you will be reading from. Table 1 on this white paper shows how the connections should be made. Take a look at the picture on “Grounded Signal Source” and “Differential (DIFF)”.