design of a drive-mechanism for a flapping wing micro air vehicle satyandra k. gupta mechanical...

Design of a Drive-Mechanism for a Flapping Wing Micro Air Vehicle

Satyandra K. GuptaMechanical Engineering Department and

Institute for Systems ResearchUniversity of Maryland, College Park

Students: Arvind Ananthanarayanan, Wojciech Bejgerowski, and Dominik Mueller

Sponsors: ARO MURI and NSF

Motivation

• Attributes of fixed wing flight─ High forward speeds required for generating lift─ Low maneuverability─ Difficult to operate in confined spaces

• Attributes of rotary wing flight─ Low forward speeds and hovering possible─ High frequency leads to noisy operation

• Attributes of flapping wing flight─ Low frequency flapping leads to quiet flight─ Low forward speeds lead to high maneuverability─ Ability to use in surveillance operations

Design Goals

• Drive mechanism to convert rotary motion to flapping wing motion

• Include symmetry to ensure stability and minimize vibration

• Constraints─ Transmit torque of 0.66 N-mm─ Support wings of total area 260 cm2─ Flap wings at more than 10 Hz─ Achieve flapping range between -12.5°

and +52.5°

• Performance metrics─ Weight ─ Cost─ Power transmission efficiency

Flapping range

Requirement of low weight electronics demands high transmission efficiency

Drive Mechanism

Motor Wing

Flapping range required to generate the right amount of thrust and lift demands highly synchronized drive mechanism

Our exploratory experiments indicated that the drive mechanisms must weigh less than 1.5 g

Design Concept

• Compliant members used in mechanism to minimize power losses

• Molded mechanism frame used to minimize weight• 2-stage gear reduction used to transmit motor torque

CompliantFrame

Rocker

Crank

Wing Supports Wing Supports

Flexural Member

Rocker

Crank

Motor with Pinion

Gears

DESIGN CONCEPT

ACTUAL MECHANISM DESIGN

Problem Formulation

• Primary Objective: Minimize weight• Secondary Objective: Minimize number of mold pieces• Constraints:

─ Structure shape should be such that forces acting do not induce excessive stresses

─ Structure shape should satisfy molding constraints Mold machinability Demoldability of part

─ Weld-lines should be placed in low stress areas of the structure shape

Decomposing the Problem

• Objective function– Minimize weight

• Constraints– Stresses should not

be excessive– Mold machinability

• Decision Variable– Structure shape and

dimensions

• Objective function– Minimize mold pieces

• Constraints– Demoldability

• Decision Variable– Non-critical connector

shapes– Parting lines

• Constraints– Mold filling– Demoldability– Location of weld-lines

• Decision Variable– Number of gates– Sacrificial shape

elements

Mechanism concept

Final molded mechanism

Shape Synthesis:Optimization problem

Mold Piece Design: Optimization problem

Gate Placement: Constraint satisfaction

problem






dimensions

Mechanism concept










elements


problem

Overview of Approach

Elaborate Mechanism

Shape

Parametric Model

3D Model

• Mechanism shape analyzed ─ Forces at different points of the mechanism computed─ Shape altered to allow for low deflection forces on structure

• Forces input into FE model to find stresses─ Mechanism dimensions computed based on allowable stresses─ Moldability constraints need to be met while selecting dimensions

Mechanical Concept

MoldingRules

Design Requirements

Parametric Optimization

Moldability Constraints

Stress Constraints

Kinematic Representation and Modeling

• Force estimated using MSC-ADAMS

krotkrot

ΩFapplied Fapplied

Downstroke Wing Action

krot

Ω

Fapplied

krot

Fapplied

Torsion spring stiffness krot0.7 N-mm/deg

Motion applied Ω 21387 rpm

Wing force resulting from flapping action Fapplied0.19 N

DC

BA

i1

i2

g

b

f

e

d

c

a

E

Upstroke Wing Action

Measurement of Forces Generated by Flapping

• Linear motion using a rigid linear

• MAV is mounted in a clamp fixed to the end of the linear air bearing

• COOPER LFS270 load cell with a 250 g capacity and 0.025 g resolution is used for the measurement

MAV

Clamp

Load Cell

Air Bearing

Vertical Setup

Horizontal Setup

MAV

Clamp

Load Cell

Air Bearing

Shape Elaboration

In-Plane Constraints for the Wing Supports

Two-Point Support for the Gearing Axis

Rounded Fillets around the Sleeve

Crash Impact Protection

• Shape selection: ─ Bi-planar body-frame

Finite Element Analysis (Pro/Mechanica) and Optimization

Maximum induced stresses at one

time instant High Stress Concentration Area

Large Displacement Areas

Undesired Weld-line locations

• FE structural analysis conducted on the body frame using force estimates from ADAMS

• Large displacement and high stress concentration areas identified• Feature sizes based on maximum allowable stresses

Shape Synthesis Result:Optimized 3-D Model

b

t

Width = 16 mm

Length = 41.7 mm

Motor Support Diameter = 7mm

Flexural Members for Compliant Mechanism

x

yz

b = 0.89 mmt = 1.52 mm

• Final dimensions






Mechanism concept







dimensions



elements



problem


Part Model &Parting Lines

PartModel

Parting Line Optimization

Modified Part Model

Perform FEA-based Parametric

Optimization

Part Model & Parting

Lines

Part Model &Parting Lines

Add Sacrificial Shape Elements

Change Connector Shape

No

Yes

NoIs it

possible to change connector

shapes?

Demoldability or Excessive Flash

Problems?

Yes

Changing Connector Shape to Reduce Mold Pieces

• Consider different polygonal and circular shapes for non-critical connector shapes

• For each shape, determine the total number of mold pieces (used MoldGuru a software developed by my students)

─ identify candidate parting directions─ compute the mold piece regions for each direction

• Select the connector shape that minimizes the mold pieces

Triangular shape element

Mold Piece Optimization Result:Optimized Mold Pieces

Side Mold Cores

Top Mold Piece

3 Piece Middle Layer

Assembly

Bottom Mold Piece

Step 3b: Removal of middle layer piece

Step 3a: Removal of middle layer pieces

Step 1: Removal of top and bottom layer of mold pieces post injection

Step 2: Removal of cores

Injection molded body frame• Mold Piece Design:

─ five pieces─ five side-cores




elements

Mechanism concept



problem





dimensions








• Identified allowable gate locations

─ Low stress areas from FE analysis

─ Permissible location for flash

• Filling simulations conducted in Moldflow Plastics Insight for different number of gates and sacrificial shape elements

PartModel

Insert Gate

Move gates

Add Sacrificial Shape Elements

Insert additional Gate

Simulate Flow

Yes

No

Yes

No

Yes

No

No

Yes Gate addition

necessary?

Gate move possible?

Cavity fills?Weld-lines

at acceptable locations?

Final Mold Design

Filling Analysis

• Single gated mold leads to asymmetric filling─ Causes warpage in molded body frame

Gate locationGate locations

(a) Single gate mold (b) Two gate mold

Appearance of Weld-lines

• Weld-lines appear in critical areas due to use of two gated mold• Third gate introduced to move weld line to non-critical area

Gatelocations Gate

locationsUndesirable weld-line location Weld-line moved to

desired location

(a) Two gate mold (b) Three gate mold

• Weld-lines still present in other critical areas

Weld-lines

Introduction of Sacrificial Shape Elements

• Sacrificial shape elements added to:

─ Absorb Weld-lines from the critical areas

─ Absorb flash from the critical areas

─ Provide for better material flow within the cavity

─ Ensure that the part is sticking to only one mold piece during demolding

• Features sheared off and removed after molding completed

Sacrificial element 1 absorbs weld lines

Sacrificial element 2 ensures part sticks to one mold piece

Gate Placement Results:Gate Locations

Sacrificial shape element 1

Sacrificial shape element 2

Location of Weld-lines Location

of the Gates

Gate 1

Gate 2

Gate 3

• Resulting gate placement:• Sacrificial shape elements:

•Sacrificial shape element 1:─ completely eliminated the weld-line on the top of the compliant members─ provided a better melt flow between the cavities─ ensured safe demolding

•Sacrificial shape element 2:─ eliminated the weld-line around the hole

Molded Mechanism Frame

Top View Side View

In-Plane Constraints for the Wing Supports

Two-Point Support for the Gearing Axis

Rounded Fillets around the Sleeve

Crash Impact Protection

• In-mold fabricated Body-frame:• Bi-planar Design:

Assembly of MAV

• Mechanism Integration into MAV

“Small Bird”

Overall Weight 12.9 g

Payload Capability 2.5 g

Flapping Frequency 12.1 Hz

Wing Area 260.0 cm2

Wing Span 34.3 cm

Flight Duration 5 min

Flight Velocity 4.4 m/s

“Big Bird”

Overall Weight 35.0 g

Payload Capability 12.0 g

Flapping Frequency 4.5 Hz

Wing Area 691.7 cm2

Wing Span 57.2 cm

Flight Duration 7 min

Flight Velocity 3.75 m/s

“Big Bird with Vision”

• We have build a version of big bird that flies with a miniature video camera─ Camera, transmitter, and battery weigh 10.0g─ Total weight is 45.0g

“Big Bird” with Folding Wing

Weight: 36.9 g

Wing Span: 57.2 cm

Flapping Frequency: 4.5 Hz

Pay Load Capability: 10.0 g

Summary

• Concurrent optimization of shape satisfying functionality and moldability constraints using multi-piece multi-gate mold design

─ Weight

─ Cost

─ High transmission efficiency drive mechanism developed to convert rotary motion to flapping wing motion

• Tools used─ ADAMS

─ Pro/Mechanica

─ MoldGuru

─ MoldFlow

─ Pro/Manufacturing

• We had to rely on physical tests to estimate aerodynamic forces

• Designed and developed successfully flying flapping wing MAV

design of a drive-mechanism for a flapping wing micro air vehicle satyandra k. gupta mechanical...

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