Weighing Scale application using NAU7802 and 8051

[RohanDey]

Hello,

I am trying to develop a weighing scale application using 8051 and NAU7802 ADC. So far I am able to get the counts from the ADC.

I am using a 10Kg, 2mV/V Loadcell, with 5V excitation voltage.
For the ADC I am using a reference voltage of 4.5V. The ADC is a 24-bit one.

My question is, from the ADC counts, how do I convert them to weight values in Kg?

The counts which I retrieved from the ADC, at PGA gain = 1 are:
1. At no load - 16,000
2. At 1Kg load - 19,080
3. At 2Kg load - 22,170

Please help..

Thanks

 

[milan.rajik]

I assume your sensor gives linear output. I hope you understand this.

No load = 16000

@ 1 Kg = 19080

@ 2 Kg = 22170

19080-16000 = 3080

22170-16000 = 6170 (6170 / 2 = 3085, approx 3080)


Post datasheet of sensor.


Vref = 4.5V

Resolution is 24 bit = 16777216 counts

0 to 16777215


If your sensor gives linear output then

16777215 - 16000 = 16761215

4.5V = 16761215 raw ADC value

x = 3080 = 8.269090277763277e-4 V = 1 Kg

3080 = 1 Kg

16761215 = y Kgs

y = 5441.952922077922 Kgs

With 4.5V Vref for ADC, 5441.952922077922 Kgs can be measured.


16761215 * z = 5441.952922077922

z = 3.246753246753247e-4

1/z = 3080 = (19080 - 16000)


WeightInKgs = (RAW_ADC_VALUE - 16000) / 3080;

If RAW_ADC_VALUE = 22170 then

(22170 - 16000) / 3080 = 6170 / 3080 = 2.003246753246753 Kgs.

 

[RohanDey]

That's a brilliant explanation. Thank you very much.

What happens if I change the Loadcell?

Here we are calculation the weight from the equation:
WeightInKgs = (RAW_ADC_VALUE - 16000) / 3080, where the value 3080 comes from testing different loads using this particular Loadcell. If I change the Loadcell to a 50Kg, 2mV/V one will this value (3080) be same?

 

[milan.rajik]

What is the 50 Kg max Loadcell's o/p without load and with 1 Kg load?

Yes, it will be same. 5000+ Kgs I mentioned is with 2mV/V sensor and 24 bit ADC. The max value the ADC can show is 5000+ kgs but if your Loadcell range is 0 - 50 Kgs and 2mV/V then for 50 Kgs the adc value will be 3080 * 50 = 184800.

No load = 16000 and 50 Kgs = 184800

Actual value for 50 Kgs will be 184800 - 16000 = 168800.

(168800 - 16000) / 3080 = 49.61038961038961 approx 50 Kgs.