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DIY CoG Meter

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Peter Balcombe
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DIY CoG Meter

Post by Peter Balcombe »

Following on from interest expressed in the Albion build thread, I've started this thread to provide more details of my DIY Centre of Gravity measurement device.
This device is similar to commercial items available from Hyperflight etc. but these commercial units seem to be aimed at the smaller & 'flying broomstick' type models as Pat Teakle would call them. (A Hyperflight unit currently costs approx. £115 & can handle narrowish fuselage models weighing up to about 4Kg).
T9HobbySport also do 2 very similar looking units for £103, capable of 4Kg & 8Kg. They are apparently designed for F3x models & are limited to a maximum fuselage size of 52mm.
I wanted to make a device which could handle 1/4 scale sailplanes weighing up to 5Kg or more.

So .....
The device uses 2 beam type load cells to measure the model weight as sensed at 2 positions across the wing chord, then calculates the overall model weight and the point between the two measurement points that this weight acts. By positioning the weighing points at a known separation & distance from a wing LE reference, the CoG position can then easily be displayed with respect to the LE.

There are various articles & uTube videos on the web relating to weight measurement with Load Cells using the HX711 amplifier and Arduino processor units, plus an article I found on Soaring Planet describing how to calculate the CoG from 2 weights.

The pictures below show my bare device, plus a schematic of the electronics used to measure the weights display the overall model weight plus CoG position behind the LE.

The 2 weight sensors are simple beam load cells which each contain 2 strain gauges mounted across the thinned part of the beam. The strain gauge resistors are internally connected as a Wheatstone bridge arrangement, which when connected to a HX711 amplifier module, produce digital data which can easily be processed by an Arduino processor.
I use the Arduino Mini Pro, but the Arduino Nano would probably be slightly easier to use in hindsight as it has a built-in USB computer interface.
The serial connection (I2C) 16x2 LCD display module gives a readout of overall model weight & current CoG location after a short initialisation process.

The load cells are available with different measurement ranges (e.g. 3Kg, 5Kg, 10Kg etc.), so it is possible to select units which best cover the range needed for your purposes. The load cells need to be used initially with a calibration program running the Arduino & known weights placed on each sensor separately in order to get a calibration value to provide accurate measurement values.

Use of the Arduino processor modules is fairly easy, but means using the Arduino IDE application on a PC/Mac to be able to upload the firmware code to the device.
The overall unit is fairly cheap to produce as the load cells are available for £7.50 each, the HX711 modules for £2.50 each (from China), the Arduino for £4, and an I2C display for <£5 (from China).
Attachments
Schematic diagram
Schematic diagram
Overall view
Overall view
Electronics &amp; load cells
Electronics & load cells
CoG meter display
CoG meter display
Last edited by Peter Balcombe on 11 Mar 2021, 15:38, edited 2 times in total.
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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: DIY CoG Meter

Post by Peter Balcombe »

On the software front,
I have attached a number of zipped files which contain the firmware program ('Sketch' in Arduino IDE terminology), plus a calibration program and the 2 libraries (HX711 module & LCD module) needed to compile the program.
I also include an I2C scanner sketch which enables you to identify if the display is properly connected and what its communication address is, as this may be different from the one applicable to my display module.

If anyone interested in building the device is not 'au fait' with the Arduino IDE, then I can provide more detailed instructions.

Note that the HX711 library examples includes a calibration sketch & examples suitable for single load cell measurement.
Attachments
CoG_Measurement_I2C.zip
(1.86 KiB) Downloaded 61 times
I2C_Scanner.zip
(824 Bytes) Downloaded 51 times
LiquidCrystal_I2C.zip
(7.94 KiB) Downloaded 58 times
Load_Cell_Calibration.zip
(1.25 KiB) Downloaded 52 times
HX711_ADC-master.zip
(250.83 KiB) Downloaded 50 times
Last edited by Peter Balcombe on 01 Apr 2021, 20:19, edited 1 time in total.
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mjcp
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Re: DIY CoG Meter

Post by mjcp »

Do you have to shuffle the airframe fwd and back? Or do you place it at a known distance from the leading edge and allow maths to work it out based on the pair of load cells?

Assume it is only valid for straight / traditional wings?

Marc
m̶j̶c̶p̶ Marc

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Some (now) pristine models that are un-flown for a year.
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Peter Balcombe
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Re: DIY CoG Meter

Post by Peter Balcombe »

Marc,
On the pic of the overall unit, you will see two vertical pins forward of the front rest pads.
The model is sat on the unit with its LE up against those pins, thus putting the measurement points a known distance behind the LE at this point.
(The distance between measurement points and the reference point are all adjustable in the firmware sketch prior to compiling the code.)
Thereafter, the model stays in the same place as you adjust ballast to get the CoG where you want it behind the LE at the pin spacing.

For a swept LE, unless it’s a very sharply swept wing then it probably won’t make too much difference, but you could either make the spanwise pin width adjustable (which may lead to location errors), or just adjust the required CoG distance to account for the spanwise difference in root LE & measurement LE point for a particular model.
(I’m not sure if commercial units are adjustable anyway).

Peter
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mjcp
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Re: DIY CoG Meter

Post by mjcp »

Ah, so this helps to ballast --> to a correct CoG, without the need to add/recheck / adjust by hand?

Think that makes sense.

Marc
m̶j̶c̶p̶ Marc

Hanger -
Some (now) pristine models that are un-flown for a year.
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Peter Balcombe
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Re: DIY CoG Meter

Post by Peter Balcombe »

Yes Marc, once it’s on the stand you can leave it there & just adjust ballast until the CoG position is where you want it.
Gives you overall model weight as a bonus :)
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Peter Balcombe
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Re: DIY CoG Meter

Post by Peter Balcombe »

I’m also looking at a 4 load cell version for heavier/larger models.
This version replaces the 2 ‘U’ frames/3D printed units in commercial versions (which constrain the fuselage width & depth capacity) with separate left & right assemblies which can either be fixed at a suitable separation, or slid apart on 3 rods.
The front & rear pairs of load cells generate the equivalent 2 weights as for the 2 load cell version.

The idea is that with two sets of 4 different 3D printed parts and a few 6mm carbon/metal rods, the support/weighing setup can easily be adjusted to cope with different model sizes & weights. The electronics just needs 2 extra HX711 amplifier modules & a different firmware program loaded in the Arduino.
The load cells can easily be uprated to higher load versions.

I’ve just received 2 extra sets of 10Kg load cell + HX711 amplifier. These only cost me £7.54 in total & arrived within 10 days :)
Robcrad
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Re: DIY CoG Meter

Post by Robcrad »

Hi Peter,
I am going to have a go building this system, had enough of trying to balance a glider on my finger tips. i am a complete beginner with Arduino and you mentioned you had some more detailed instruction, would it be possible to get a copy of these please?

Many thanks
Rob
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Peter Balcombe
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Re: DIY CoG Meter

Post by Peter Balcombe »

Hi Rob,
The first thing is to download & install the Arduino IDE onto your PC (There are also Mac & Linux versions but I have only used the PC version, so any instructions will only be for that).
Latest version of the IDE appears 1.8.13 & is available from https://www.arduino.cc/en/software.

Once you have the IDE installed & have an Arduino Nano (onboard USB interface makes it much easier to program & debug), plus the load cells, HX711 amplifiers & an I2C 16x2 LCD display connected up as per my schematic diagram then we can set about getting the firmware loaded.

I’m assuming you are ok with following the schematic & soldering things up, plus making up the two model rest arrangements to support the model via the two load cells.
The details of the mechanics are up to you, depending on the type & size of models you want to cater for.
It may be worth initially just making a simple bed to fix the load cells down a known distance apart to check everything out - and then make up your mechanics later.

Rather than clog up the forum thread, it may be better if you send me a PM with your email address so that I can get you going.
The first steps will be to making sure you can get the IDE to recognise the Arduino unit when connected via a suitable USB cable. Once you can successfully do that, we can compile & upload a test program.
Peter
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Peter Balcombe
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Re: DIY CoG Meter

Post by Peter Balcombe »

For anyone that has downloaded the HX711_ADC library from an earlier post, please note that I have just replaced this library with an improved version.
If you have already installed the previous HX711 library into your IDE libraries folder, please delete the previous library folder & replace with the new one. Note that when you extract the library folder, change the name to just "HX711_ADC" - i.e delete the "-Master" bit.

The huge advantage of the new library is that the calibration sketch contained within the examples folder is much easier to use, so use this instead of the previous one. You just place a known weight on the load cell & enter the weight via the serial terminal (& press the Return key). The sketch automatically calculates the precise load cell calibration factor rather than having to manually adjust. :)

I am currently working on an improved version of the 2 load cell CoG meter which includes the auto calibration feature.
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