tatobari
commented
8 years ago Hey u-l-m-i! I'm sorry that the code is not that tidy yet. Down to business.
Code Updated I changed the "one_kilo" vars and methods to "reference_unit". Makes more sense.
How I "fixed" the code To figure out what was wrong, I decided to print the 8bit arrays the code extracts from the HX711 when reading. I found out that they were being interpreted as LSB Bytes with MSB Bits while they where MSB Bytes with MSB Bits.
Remember? MSB means "Most Significant Bit First" and LSB means "Less Signifficant Bit First".
It's was little odd (it's wierd how you can't use the word "bit" to say "little" on this kind of topics), because most ICs use the LSB rule. Once I changed that part of the code to switch the bytes before building the integer and build up an integer from the three bytes in the correct order, without any load cell wired to the HX711, I began to get readings of 8385000 +/- 100. I new I was on the right track. It wasn't jumping from -4 million to +16 millon. It was making much more sense.
Getting a reference unit I took a leap of faith and wired the load cell and run the code again. The value changed to 8160500 +/- 100. It was still pretty stable. After that, I added 2kg, 2L plastic bottle filled with water (water density is 1). The reading changed to 8348600 +/100 and was still very stable. So I assumed that 8160500 - 8348600 = 188100 should be about 2kg, hence, 94050 should be 1kg.
If you think about it, 0,94kg are 940g which are 94000cg (centigrams). It depends on the loadcell obviously, but this was a great clue that I was on the right track.
In the code I've set the reference unit to 92. I'm doing all this process again while I write this and I'm using a different loadcell now.
Tare
hx.tare() gets an average of 5 readings, sets the "OFFSET" variable and
will substract that variable from every raw reading. The "scale" and the
"reference_unit" values are all used after using the "OFFSET" variable.
How I meassure
hx.set_scale(1000) will divide every raw value by the in specified in the
argument.
set_reference_unit(92) will also divide the value by the integer
specified in the argument keeping three decimals to the result of the
division.
How you should meassure Use the get_value() method. It will give you a raw value so you can check what is the relation between the returned values, your load cell and the weight you are using. The example code would be:
############# EXAMPLE
hx = HX711(5, 6)
hx.power_down()
hx.power_up()
hx.tare()
while True:
try:
val = hx.get_value()
print val
hx.power_down()
hx.power_up()
time.sleep(0.5)
except (KeyboardInterrupt, SystemExit):
cleanAndExit()
Hope it helps.
PS: I don't know why isn't mark down working.
u-l-m-i
arthurmoises
faurepa
SooranSaeed
ello Tato,
thanks for providing your code and explanation, but I’m still struggling with a few things.
First: Can you please explain how to evaluate the tare value and the value for the defined (1kg)
Second: Maybe depended on above
With changing the Vcc to 3,3 V the results get better, but still a very inconstant. My scale consists out of 4 Load Cells which are linked as Wheatstone Bridge. (tare & 1kg as in our example)
e.g. Empty scale: pi@raspberrypi:~/scale/hx711 $ python hx.py 0.000 1.076 -0.043 4.500 -0.033 0.000 -0.033 0.000 0.022 -0.022 -0.033 0.022 -0.011 -0.011 -0.022 ^CCleaning... Bye! pi@raspberrypi:~/scale/hx711 $ python hx.py ## with 1kg 0.000 0.043 0.043 0.033 18.261 0.011 0.196 0.022 0.087 0.087 0.000 0.011 ^CCleaning... Bye! pi@raspberrypi:~/scale/hx711 $ python hx.py ##wtih 5kg
0.000 0.022 0.033 0.033 0.076 0.043 0.043 0.065 -0.011 0.022 0.033 ^CCleaning... Bye! pi@raspberrypi:~/scale/hx711 $
Any idea or advice?
Thanks u-l-m-i