Electronic centre of gravity scale

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For my small fleet of very different model aircraft, I was looking for an electronic centre of gravity scale. Of course I asked myself whether this expenditure is justified for 2-3 missions a year, but the comfort and the fun of it led to a clear decision. Unfortunately, the DIY construction proposals I was familiar with could only incompletely fulfil my requirements. They did not fit the high range of hull widths, wing depths and weights of my fleet. I had tried out a DIY construction proposal and found significant problems with the reproducibility of the measurement results. According to my analysis, this was mainly due to the design of the suspension of the support points, which could not adapt to the wings of the aircraft with little friction. In addition, none of the construction proposals was able to determine the centre of gravity of the planes in flight. This may not be a problem with extremely slim fuselages with directly pinned wings as in F3J etc., but with many true-to-the-original planes this leads to notable errors. And since I had to construct new mounts for my planes anyway, I decided to redesign the complete superstructure and implement the improvements discussed above in the process.

Key features:

• Parametric design with FreeCad (adjustments via a spreadsheet)
• Adaptable for surface depths from 120mm to >300mm
• Open width between supports 30mm to >150mm
• Open height of supports 30mm to >100mm
• Load bearing points mounted on ball heads
• Rear supports optionally individually height-adjustable for centre of gravity in flight position and tolerance compensation
• Power supply via USB
• Display on PC / laptop
• load cells can be calibrated individually
• High repeat accuracy of centre of gravity measurement (0.2mm)
• maximum load >5kg
• Total cost < €30

Mechanical construction

I have carried out the design with FreeCAD. And since the essential parameters are constructed parametrically, most adjustments to one's own aircraft arsenal can also be carried out by a CAD beginner. Only the parameters in a table (spreadsheet) have to be changed in the design. The 3D construction then changes automatically. Afterwards the .stl file is generated and with it you then go into your slicer and printer - done. With this you can adjust the distance of the support points and the open width and height for the fuselage to your requirements. If you want, you can also influence the stability of the superstructure by changing the thickness and width of the arms.

Windows Software

The Arduino Nano sends the 24-bit raw data with a frequency of approx. 10Hz via USB to the PC. Everything else is done by the Windows software. The search for the correct COM port happens automatically. The two load cells can be calibrated individually. It is best to calibrate with the same weight near the maximum load. Before that, you should set the zero point. For the scale to work really precisely, it has to acclimatise for a few hours until the zero point no longer runs away. With a resolution of 100mg you can judge this well.

Electronics

The electronics are extremely simple due to the absence of a display and controls. The HX711 modules are supplied with the load cells. All you need is an Arduino Nano V3.

You can build this on a breadboard with 5mm spacers as feet. With an Arduine on headers you have exactly the height of the cut-out for the USB socket in the housing.

The power is supplied via the USB connection.

Parts list

Download

And here you can find everything you need to build a replica: