MEGA 2560: DIFFERENTIAL ADC WITH PGA LIBRARY AND 4-WIRE LOAD CELL

Good day,

I am trying to verify theoretical estimates of load distribution of a statically indeterminate frame.

We have steel 'bedplates' that machinery is mounted on and supported by (4), (6), or (9) rubber pads. We need to report the load at each pad and have a good correlation between the calculated reaction forces and FEA results.

We are using these (2000) Kg load cells: http://www.micelect.es/en/micelect-load-weighing-sensors/ccp-elevator-bed-frame-sensor which have shielding and:

RED +Vdc BLACK -Vdc GREEN + SIGNAL YELLOW - SIGNAL

The distributor does not speak english so we are limited in our support as far as additional details on the load cell for now but I have a MEGA2560 and am trying to understand and use the analogReadGain() or analogReadDiff() commands to get usable readings that i can calibrate with known weights.

I have read the reference on analogread() and am using these two resources to guide me:

https://www.davidpilling.com/wiki/index.php/DiffADC https://lawas.nahdhi.com/arduino-differential-adc-and-gain-adc.html

I would appreciate any suggestions in general but in particular with the following:

  • Would it be correct for me to connect the load cell RED and BLACK to the 2560 5V and GND respectively? What about the shielding?
  • Would it be correct to connect the load cell YELLOW and GREEN to the GND and A0 pin on the 2560? Is there a better way of using the load cell -SIGNAL in this setup?
  • How would i best safely set-up (9) load cells to be read individually?
  • Once i have the load cells connected, which libraries do i need to include and how best would i apply them? I understand using the internal reference voltage will give me better resolution

I am a mechanical engineer and firefighter so i apologize if i am not presenting information adequately for those more inclined and appreciate all the assistance i have received so far.

Would it be correct for me to connect the load cell RED and BLACK to the 2560 5V and GND respectively? What about the shielding? Would it be correct to connect the load cell YELLOW and GREEN to the GND and A0 pin on the 2560? Is there a better way of using the load cell -SIGNAL in this setup?

No, although the thin documentation doesn't tell details I assume it's a standard load cell which needs an amplifier as p.e. the HX711.

How would i best safely set-up (9) load cells to be read individually?

Setup a different pair of pins for every cell. The Mega2560 has more than enough pins.

Thank you pylon,

I understand the need for an amp like the HX711 but does the Arduino ADC library not have the ability to read from the load cell directly and amplify the signal in the same way an extrnal amp like the HX711 does albeit in 10bit instead of 24bit resolution that the HX711 offers?

Thank you again for your help.

I understand the need for an amp like the HX711 but does the Arduino ADC library not have the ability to read from the load cell directly and amplify the signal in the same way an extrnal amp like the HX711 does albeit in 10bit instead of 24bit resolution that the HX711 offers?

No. The ADC of the Arduino doesn't include an amplifier. The signal is not strong enough to be fed directly to the ADC.

pylon: The ADC of the Arduino doesn't include an amplifier.

Actually, the 2560 does have a PGA (gains of 10x or 200x) on four differential inputs. See page 268 of the datasheet.

The 1284 has similar capabilities. See page 243 of its datasheet.

Likewise the Leonardo's 32u4 has a PGA (gains of 10x, 40x, and 200x). See page 297...

For all, resolution at the higher gains is degraded to only 7 or 8 bits....which may not be enough to demonstrate "good correlation between the calculated reaction forces and FEA results"...but only the OP can make that call.

Thank you all for your responses.

If we can get +/-5% accuracy, that would be enough.

With regards to using the internal ADC and higher gain, would the pins be connected as described in the first post? I'm still unclear on how to use the code properly but i think getting the wiring setup first might give me some insight on how to proceed.

I appreciate all of the responses and time/effort from you kind folks thus far.

It would be considerably simpler to use HX711s.

If you really want to try the 2560, study Pilling's webpage, the library, and the datasheet. If you go that way, it appears that you are headed into relatively unexplored territory. Good luck with your project.