Arduino based weight measuring design using load cells

Project Info

Case

Meee-services received a request from a client who was a medical device design specialist for making a special design. He asked us to make a weight measuring design for a device that could fit into his hardware.

Requirements

  1. The circuit was to output the measurement of each sensor in Kg (the measurement were to be made every 0.1 sec or on a demand basis based). The total weight measured by all the sensors were up to 150 kg with the accuracy of 100 grams+.
  2. All the load cells were to measure the weight separately and show the data on a serial monitor
  3. The data were to be saved in the JASON file format and also on an SD card.

Challenges

The main challenge in this project was that the existing load cells in the market were all 3-wire type and it was not possible to connect them to the 4-wire modules.

Solution

Due to the capabilities of Arduino in integrating different modules on the same board, an Arduino Nano board was used for the initial design. In making the design the following issues were considered.

  1. Regarding the problem of wiring the 3-wire sensors to 4-wire modules, after a research on the viable options, the following design was used. In this solution, a small circuit using some resistors was designed and used to connect the sensors to the modules.
  2. Based on the requirement for a 0.1 sec time interval between successive measurements (about 30 bytes in 0.1 sec), the baud rate was selected at 38,400 for sending the data through the serial monitor software.
  3. Also, in order to adjust the sampling rate of HX711 balance module to be interfaced with the load cells, the sampling rate was set at 10. This was done by grounding the pin 15 of the IC. Also, in order to achieve higher rates like 100 ms requirement in this project, this pin was connected to Vcc.
Description

Results and Impacts

The final project met all the requirements of the client as the following:

1.This circuit could simultaneously send data from the board to a computer and a phone via USB and Bluetooth.
2.Per request of the client, the transferred data to the computer was in CSV format which could be easily read.
3.The data was saved on the SD card in Jason format.
4.In live transfers, such data as the start date and time, the number of samples, the interval and total time and the end ending
time was displayed. The same data was saved on the SD card.

Having received the design, the client was quite satisfied.

Client:
Raymond R
Date:
February 21, 2018
Location:
US
Category:
Electrical and Electronic Engineering