Category Archives: instrumentation project

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Arduino Tutorial: Digital scale

In the previous post has been written about the calibration of load cell sensor for measuring the thrust ofpropeller and the motor/engine. In a posting has been found a constant magnitude of the sensor output voltage per gram about 0.267012014. So every 1 gram increase in weight of load will raise the output voltage of the sensor about 0.267012014 mV.

In my project, this output voltage must be converted into digital numbers using the ADC on the Arduino Nano. Henceforth, the data is sent serially to the PC to be displayed in the monitor. PC programs used in visual basic 6. As a development, in terms of measuring the thrust, required the measurement results are presented in graphical form.

What about the working principle of the Arduino program ?

  1. At just the Arduino turn on, will assume the load is 0.
  2. Arduino will sent periodically value of ADC to the PC every 200mS.
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A simple calibration of load cell amplifier using loadcell 15Kg

As written in previous posting, I will design and make a test equipment to measure the thrust of propeller and the motor/engine in aeromodelling parts. This measure will help to design rc model, such as fixed wing and multicopter. Before designing a total airframe, need to know first the characteristics of the motor/engine and matching propeller size. I used the load cell with specifications was written on this posting. My load cell is 15Kg of full scale with output voltage about 2mV/V. I used the amplifier capable to gained up to 1000x with source 5V for load cell reference voltage. This mean will give output about 10mV in full scale of 15Kg load.

How to a simple calibrate  ?

Since I do not have the standard weight of high-priced, this way is simple to use a digital scale which has an accuracy of about 0.1gram and can measure up to 1Kg. Next, weigh any load with digital scale, record the result. Then move to the load cell and measuring with a digital voltmeter on the load cell amplifier output voltage.

Photo of digital scale Photo of Load cell
362.2 gram 105.1 Volt
 528.5 gram  151.8 Volt
 63.5 gram  19.8 volt
 275.9 gram  80.7 volt
 34.1 gram  11.7 volt
14.7 gram 6.1 volt
30.2 gram 10.6 volt
130.2 gram 39.1 volt
40.4 gram 13.2 volt
101.2 gram 30.7 volt

Plot data using SCILAB

The commands to plot the datas using SCILAB as follow:

->weight = [362.2,528.5,	63.5,275.9,34.1,	14.7,30.2,130.2,	40.4,101.2]
 weight  =
 
    362.2    528.5    63.5    275.9    34.1    14.7    30.2    130.2    40.4    101.2  
 
->volt=[105.1,151.8,19.8,80.7,11.7,6.1,10.6,39.1,13.2,30.7]
 volt  =
 
    105.1    151.8    19.8    80.7    11.7    6.1    10.6    39.1    13.2    30.7  
 
->size(weight)
 ans  =
 
    1.    10.  
 
->size(volt)
 ans  =
 
    1.    10.  
 
->plot(weight,volt,'ro-.')
 
->xgrid
->title("Loadcell amplifier calibration")
 
->xlabel("Load (gram)")
 
->ylabel("Loadcell amplifier output (mV)")

Will result a graph as follow:

How much’s the output voltage of amplifier at zero load ?

On zero load output voltage at zero load is 2.8mV. My amplifier can not adjust to 0.0 volt, because there null offset voltage. On arduino using 10bit adc with 5V ref, will convert into (2.8/5000)*1024 =  0.57344 less than 1, or will known as 0 decimal.

How to calculate voltage per Kg or mV/Kg ?

From the above data, I would find the average value using Microsoft Excel, will be obtained as follows:

 

So output voltage of amplifier will be: 0.267012014 mV/gram or 267.012014 mV/Kg.

The output amplifier according to specifications from 0 up to 4.3 volts. So that the maximum load that can be read are: 4300/267.012014 = 16.104Kg, near specification of loadcell about 15Kg.

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