The test stand is built in a way so that the motor holder can be exchanged and replaced with a different one for other motor types and sizes. The whole test stand is semi-automatic, it measures not only the thrust but also the voltage and current applied to the motor under test. The only manual operation is to change the applied voltage.
The Motor Test Stand Project was created to test the small coreless motors used in small RC drones, specifically the 8520 (8.5mm x 20mm) one, with different propeller sizes. This for both the Drone project and also for future small and simple plains or other flying RC projects. All the analog signal conditioning and conversion is done by the STM32G4 DAQ . This simplifies and speeds up the development of both the hardware and software significantly. The electronics is very simple due to this, no additional components besides the sensing elements are needed. The software uses the already developed C# library for the STM32G4 DAQ, only the GUI part had to be implemented.
To transfer the force/thrust of the motor + propeller a hinged arm is used. The arm has an equal length from the pivot point to the motor center, the thrust force center, and to the strain gauge mounting holes. This guarantees a 1:1 force transfer from the motor to the strain gauge, when disregarding losses due to friction in the pivot point and losses to to ductility of the printed plastic parts. The 3D render of all pieces can be seen in the figure bellow:
The arm is composed of two parts, the Main Arm and a Motor Holder piece, which is fixed with a M3 bolt to the main arm piece. This way the motor holder can easily be exchanged with holders for other motors. It has a large opening in the front for reduced air “drag”. The pivot point of the arm is fixed in place with the Arm Holder and a 3mm brass rod. The main arm piece has two ball bearings (683ZZ) so that it can rotate on the brass rod, the pivot point.
The arm is fixed to the strain gauge with two M3 bolts and the strain gauge in turn is mounted to the base with the Strain Gauge Holder, also with two M3 bolts. Both the arm holder and the strain gauge holder are fixed down to the baseplate, a slap of wood (9cm x 25cm) here, with M3 bolts and M3 nuts that are pressed into the plastic pieces.
All four plastic parts were 3D printed with PETG. The 3D files, in STL format, can be downloaded from bellow:
3D Files: Motor_Test_Stand_STL.zip
By using the STM32G4 DAQ, the circuitry is very simple as all the analog signal conditioning and conversion is done on the DAQ board. The strain gauge used is a 100g one from SparkFun and it is powered with 5V supplied by DAQ board. The differential output of the strain gauge, each arm of the wheatstone bridge, are connected to the differential analog input 1 (AI1+ & AI2-) of the DAQ. The strain gauge wheatstone bridge is well balanced, the output is very close to 0V differential when no force is applied, in the rest position. This avoids the need for offset compensation in hardware. The differential signal is amplified by the DAQ (gain of 128).
To measure both the voltage and the current supplied/consumed by the motor, a very simple PCB was soldered. The voltage is directly measured at the Motor Out point, again with a differential input of the DAQ (AI7+ & AI8-). The current is measured with a current sense resistor, with a value of 0.01 Ohm, with the differential voltage on its terminal measured by another differential input of the DAQ (AI5+ & AI6-). This signal is amplified by the DAQ (gain of 32).
Bellow is a Figure showing all the connections and the simple Power Measurement PCB:
To get the conversion factor, uV to grams, the strain gauge readout has to be calibrated. This is done by using different known masses and reading the output voltage of the strain gauge to trace a conversion curve. The calibration masses used here was a cup with water, which works pretty well and is easy to use. Water was added to the cup with a syringe so that precise amounts could be added. The standard 1 ml is 1 gram ratio was used and the calibration test was performed with two different set-ups:
Standing: Here the cup is placed on top of the strain gauge, as can be seen in the figure bellow:
Hanging: Here the cup is hanged from the motor holder, simulating the thrust force of the motor + propeller, as shown in the figure bellow:
The obtained calibration/conversion curve can be seen in the figure bellow. In blue is the curve for the cup standing on top of the strain gauge and in red for the hanging cup set-up.
It is visible that the characteristics of the strain gauge is very linear, as expected, which simplifies the uV to gram conversion as only the slope of the linear trend line has to be used. The initial value/offset is not relevant as it will be calibrated for every experiment, like the TARA on a weight scale. The slopes for both set-ups are slightly different, which is expected due to losses and other not ideal force transfer, the value used for thrust experiments is from the hanging cup set-up (18.784 uV/gram). Also, the sign of both the slope and initial value/offset depend on the strain gauge wiring to the DAQ, if the two readout wires are crossed the sign inverts.
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