Introduction

As an intro here is a video of the system running

and another using the LCD display

A little theory

To find the Center of Gravity of a plane (or anything else) you just need one scale and a ruler !

You will need to know the total weight of the plane and measure a moment from the leading edge of the wing:

if W is the total Weight, Wb the weight read on the back scale and L the length of the lever between front and back pivots, then

CoG * W = L * Wb

or

CoG = L * Wb / W

This was the easiest solution when you have a single scale. Building such a device was extremely easy and is explained here : a "manual" CoG finder on thingiverse

The main "drawback" of this system is that it requires to weight the plane and then compute the formula anytime you change something into the mass of your model.

So I decided to add a second digital scale and a micro controller to have something more automatic !

When you have "front scale" and "back scale" weights (Wf and Wb)  then the formula to find the CoG position relative to the leading edge of the wing is:

CoG = L * Wb/ (Wf + Wb)

That's frankly easy and the electronics to build such a system should be simple as well.

Electronics schematics

• the heart of the system is a Lolin32 lite ESP32 MCU.
• and 2 load cells + HX711 amplifiers are needed as "digital scales"
• an Oled display is optionnaly added (if you don't have an Android phone !)
• a rotary encoder can help to tune values (if you don't have an Android phone)

And that's it... not complex at all !

Bill of Material can be found into the file sectin of this project

PCB

I have designed a nice and compact PCB allowing to fit the ESP32 and to connect all the sensors

The PCB was kindly sponsored by PCBWay and is as usual of excellent quality.

You can order it herediretly on this project page !

I must admit that this PCB is probably one of the simplest I have ever designed, but It's so cheap, delivered so fast and so professional looking that it worthed the time spent to design it !

And if you are new to PCBWay please use this affiliated link : https://pcbway.com/g/o35z4O

Power considerations

To power this board I will simply use an USB chord plugged into the ESP32 connector on one side and on any 5V power bank on the other one !

The lolin32 is equiped with a 3.3V linear regulator strong enough to deliver 1A current. Fairly enough for two loadcells and an 1" LCD !

LoadCells

I have choosen two 2kg loadcells. This is a good balance between sensitivity an max load. Most of my planes are gliders in the range 200g to 3kg. As the total weight of the plane is almost shared on the two scales, 3kg will put max 1.5kg on each scale... 2kg load cells add some safety margin !

These devices need an amplifier HX711 to adapt their output.

Connection between load cell and amplifier follows this schematics:

Plane stand and scales

The plane will be sitted on a stand, its wing leading edge touching the vertical side of the stand.

The stand itself will be put right above the two loadcells. Perpendicularity of feet must be insured so that the horizontal bars of the stand are trully horizontal. This is mandatory for accuracy of the "moment" computation

Both the stand and the scales must be adjustable in width and lenght to accomodate most of the RC planes.

building the stand

As you can see the length "L" is measured between the two vertical sides of the stand.

Here is the stand with a small foamy flying wing:

The stand can be 3d printed or laser cut. Both options are available.

If you choose the laser cut option (what I did) then cut the “CoGfinder_flat.dxf” file into 5mm plywood. Then glue each foot with 3 parts (the slot one in the middle). Let the glue dry and drill the square slot at 7mm (start at 5 then 6 then 7)… Adjust the hole so that your carbon arrow slides with a some friction into the holes.

If you choose the 3d printed option, then it's even easier : print “front_3d.stl” and “back_3d.stl” files twice, and adjust the holes!

building the scales

The scales are 3d printed. 

The loadcells can be screwed on the scales with 5mm screws. While the top of the loadcell has 4mm holes… (don't ask me why) so that the “ruler” will be screwed with 4mm screws.

You will have to insert the HX711 amplifiers into the slot and solder the wires, then glue the cover.  

The 2 scales are strictly identical and can slide one into the other with the help of carbon arrow pipes inserted into the holes. One side is fixed, while the other (through hole) must slide around the arrow. Adjust the holes to get this behavior.

While finished the scales should look like this !

Once the stand and the scales are finished, you can put one above the oher and use your CoG finder device !

This was with a very small plane... But will it work with bigger ones ?

Tune width anf length of the CoGfinder and... The answer is yes ! It was easier to install the plane rather than shooting a picture with these 2.7m long wings !

Software

Software will be composed of

• firmware embeded into the ESP32
• Android Application (optionnal) communicating with the ESP32 via UDP over Wifi (either in "station mode" or in"Access Point" mode)

If you don't have an Android phone, you will need to use the "standalone" option where all the settings and displays will be done with a rotary encoder and a small 1" Oled display.

Currently these options are not yet developped (stay tuned !)

If you have an Android phone then you are lucky as you will get much more user fiendly interface !

Android App

This app will offer two modes of operation:

• CoG finder
• CoG finder + "lead" helper
• 
  

CoG mode

here is the screen when selecting "CoG" mode of operation

You'll just have to enter the L value, it will be sent to the ESP32 board, then scales will output the "front scale" and "back scale" weights and the CoG position will be computed !

Move your battery into the plane and the CoG will move in real time !

Now if you can't get the right CoG position then you will have to add lead either to the nose or to the tail of your plane. But how much ? This will be easier when using the "CoG-lead" mode of operation

CoG-lead mode

Here is the screen of this mode

Centering the plane could be done either with lead into the nose at L1 distance from the leading edge of the wing or with lead into the tail at L2 distance from the leading edge.

After entering the target CoG position, L1 and L2 values, the app will automatically tell you where to add lead and compute the mass to be added.

(in the above example, CoG is at 76.7mm and should be at 80mm, the plane is "nose heavy" you have to add 4.2g of lead into the tail at 800mm from the leading edge.)

Note that target CoG, L, L1 and L2 will be sent to the ESP32 board and permanently stored into its flash. So you can center your plane during several days without loosing your settings !

Your plane database

The android App allows now to keep track of all planes into ta database.

The data base is accessible here on top of the screen

to add a new plane click on the "new" item

You can modify the values and when ready save again the plane with the menu

Similarly you can delete the current plane.

There is also an option to export the whole database into a .csv file that you can open with any spreadsheet viewer

Using Bluetooth Low Energy

If your Android phone is modern enough (BLE appeared in 2010) then you are lucky as you can use the Android App and the CoGfinder almost "magically"

• turn on Bluetooth on your phone
• launch the App
• power the ESP32

and that's it, no password, no pairing... It just works !

configuring the wifi

If you have an old phone, you can still use Wifi.

First uncomment the line "HAS_WIFI" to allow this option

//#define HAS_WIFI //leave this line commented if you only want Bluetooth Low Energy ; uncomment for both Wifi and BLE

Then compile the firmware and follow this procedure . 

There are two ways to configure the wifi:

• The ESP32 connects to your router (Station mode)
• The ESP32 acts as an Access Point (Access Point mode)

Station mode 

To configure the wifi simply follow this procedure:

I have used the ESP32 WifiManager library capabilities to enter wifi credentials on the ESP32 board.

So if you boot the board (or reset it with the reset button) while shorting the "CONF" pin header on the lower right corner of the board, then the ESP32 exposes a Wifi hotspot (named "JP RC_CoG_finder" on which you can connect any smartphone (or PC)

Connecting to it will automatically open a configuration window

You then have to choose an exisitng wifi network for which you have the credentials, and enter them into the form.

Your ESP32 board is now configured to run with wifi and you would be able to monitor your scales with the android App. (don't forget toremove the shortonthe CONF pin header !).

Your phone will still be connected to internet. So this is the best solution to use if you have wifi close to your current position !

Access Point mode

If you are away from wifi then you can still use your phone but with the following limitations:

• your phone must be disconnected from internet (5G and wifi)
• your phone can connect to the Access Point that will exppose the ESP32

To activate this mode you will have to reset the ESP32 while pressing the rotary encoder button

Then the access point (named "JP RC_CoG_finder") will be into the list of Wifi routers in your phone. Connect to it and launch the App !

loadcells calibration

As any digital scales yours need to be calibrated.

tare

The first (obvious) calibration is the "tare". Tare button (or menu) will give you 0 g on both scales. Tare is automatically done during boot but must manually be done when you place the plane stand on the scales.

scales sensitivity calibration

This calibration should be done only once (with new cells) or when you detect a drift in the measured weights.

• remove the plane stand
• push the tare button
• place 500g of calibrated weight on the front scale

• click on the three dots menu (top right corner) and select "calib front scale" menu
• do the same for the back scale

If you want to calibrate for another weight then you'll have to enter this weight with the "set Calib weight" menu

Oled display and menus

if you don't have an android phone you can still use the device in stand alone mode.

The rotary encoder and the Oled display can be used to control the CoGfinder.

Any modification you do into the parameters will be synchronized between android App and ESP32 board. Easy !

Firmware compilation

Some of the libraries require quite old configuration for the Espressif board manager.

So select "board manager", type "ESP32" into the search box, then select version 2.0.17 and install