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Fabrication and optimisation of an electrical motorisation for mini-UAV in hovering

Nicolas Achotte, Jérôme Meunier-Carus,

G. Poulin, J. Delamare, O. Cugat

Laboratoire d’Electrotechnique de Grenoble - France

L.E.GL.E.G

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Electric propulsion chain for hoveringGlobal/elementary optimisation

• Traction

• Output power

• Rotation speed

• Figure of Merit

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• Hovering

• Dimensions

• Mass

• Autonomy

• Noise

• Payload

Specification sheet

Optimisation tool necessary !

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• Voltage

• Current

• Efficiency

• Capacity (A.h)

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Electrical motorisation for mini-UAV

• Experimental study– Hovering power evaluation– Test bench– Experimental results and characterisations– Realisation of global traction chain

• Design of an planar miniature magnetic motor– Modelling– Structure choice & dimensioning

• Optimisation of the entire chain

• Perspectives - Conclusion

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Power needed to hover a given mass

Theory : Momentum theory (Rankine, 1865; Froude, 1885; Betz, 1920)

Figure of merit : ~ hovering ‘’ efficiency ‘’

Mechanical power for hovering :

P

TvM

air2

mg

R.M

mgP

R

m 23

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Test bench

Propeller

Motor

Speed sensor

Thrust sensor

Torque sensor

Speed controller

Fully automated

laptop

Batteries

Ball bearings

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Tests on propellers

Dimensions (50 cm) and non compressible fluid condition

Low Reynolds Number <100000

• Experimental study necessary.• Modeling of Performances• Implementation into Pro@Design Optimisation framework

High speed propeller necessary to build and optimise the electrical chain

Mass of the motor and converter 1/rotation speed

For a given power, Ibatteries 1/rotation speed

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Tests on propellers

0

100

200

300

400

500

600

700

800

0 1000 2000 3000 4000 5000 6000 7000

Speed propeller (rpm)

Th

rust

(g

)

0.11x0.2

0.12x0.25

0.18x0.28

0.2x0.38

0.28x0.51

0.257x0.512

Results in Hovering

0

100

200

300

400

500

600

700

800

0 10 20 30 40 50

Mechanical Power (W)

Th

rus

t (g

)

0.11x0.2

0.12x0.25

0.18x0.28

0.2x0.38

0.28x0.51

0.257x0.512

Best working point

• Diametre = 50 cm

• Thrust = 500 g

• Pmechanical= 26 W

• Rotation speed = 1630 rpm

• Figure of merit = 0.6

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Tests on converters and motors

Brushless Motors : Inner rotor (high speed, low torque): gearbox necessary.

Outer rotor (low speed, high torque).

Test on Model motor AXI 221226 + speed controller Jeti advance 18-3P (Direct drive)

Working point: speed > 3500 rpm for efficiency > 60 % Gearbox (still !) needed…

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Tests on batteries

Our application : high power and energy density required

Lithium Chemistry ( 3,6 V; Idischarge >2 C; Energy density = 140 Wh/kg)

Kind of battery

Nb of elements

Mass (g)

Av Voltage1 Element

(V)(at 1 C)

Capacity(Ah)

Max Continuous discharge current (A)

P max continous

(W)

Energy density(Wh/kg)

Li-IonPanasonic

CGR-18650A

3 141 3,6 1,832,18 C= 4 A

35 (32 min)

135

Li-PolyKokamSLPB

526495

2 137 3,6 31,66 C= 5 A

30,5 (35 min)

132

Tests results for 2 suitable batteries:

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Global traction chain test in hovering

50 cm

Results:

• Autonomy = 33 min • Payload = 141 g• Pelectrical= 35 W• Efficiency = 65 %

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50

100

150

200

250

300

350

400

450

500

550

0 5 10 15 20 25 30 35 40

Time (min)

Th

rust

(g

)

Mass of components

Payload = 141 g

0

50

100

150

200

250

300

350

400

450

500

550

0 5 10 15 20 25 30 35 40

Time (min)

Th

rust

(g

)

Mass of components

Payload = 141 g

Off-the-shelf components can fullfill the specification sheet but…

Important improvements are possible:

• On the propeller mass (85 g at present).• On the propeller speed (for motor and converter optimisation).• On the motor (better torque for direct drive and high efficiency).

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Design of a new dedicated planar motor

Objective : build a brushless motor adjusted to the propeller

Specification sheet : mechanical power and low rotation speed

Model : based on the electromotive force created by a conductor under a magnetic flux variation

Software : Pro@Design

Optimisation goal : minimise the mass of the motor and maximise its efficiency

Constraints : width and thickness of the windings, diametre of the stator and rotor, etc.

Results : Pareto curves (point = minimised mass for a given efficiency)

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Structure

Disk rotor

Planar stator

Rotor sandwich

Stator sandwich Single gap structure

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0

5

10

15

20

25

30

35

20 40 60 80 100efficiency (%)

mas

s (g

)

structure 1

structure 2

structure 3

Structure choice

30 g

20 g

-10 %

-7 %

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Optimisation of the entire chain

m

23

k

mP

Propeller model Motor modelPj = R.i2, (Joule losses)

e = B.L.v, (e.m.f)

Pin = e.i + Pj,

efficiency = Pout/Pin

mass = .V

Batteries model

Batteries data baseVoltage, Current,

Energy, Power, MassPropellers data base, k, Diameter, Mass

Overall mass Autonomy

The best solution

Objectives : maximise the autonomy and the payload for a given overall mass

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Conclusion

• Carefully selected off-the-shelf components can presently comply with the specification sheet

• if smartly associated

• but multi-constraint optimisation is necessary

• dedicated planar motors can enhance the performances

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• Macro Fibre Composite actuators as an alternative to drive morphing structures

• Applications : flapping wings, flaps,…

Threshold of 770 V, deflection of 20 mm

To be optimised in terms of number, optimal speed, MFC dimensions control to be applied to UAV

Perspectives

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Thank you for your attention !

Any questions ?Any answers ?