mechanical engineering design projects final status

MECHANICAL ENGINEERING DESIGN PROJECTS

FINAL STATUS REPORT

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SUBMITTED BY

Daniel Chabolla, Sean Gowen, Aimee Kim, Michael Lo, Wenbin Zhao

May 7, 2013


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TABLE OF CONTENTS

PROJECT OVERVIEW...................................................................................................... 3

OVERALL DESIGN........................................................................................................... 4

TESTING/PROTOTYPING RESULTS............................................................................... 8

PROPOSED IMPROVEMENTS/LESSONS LEARNED..................................................... 12

REQUIREMENTS COMPLIANCE...................................................................................... 14 COST.................................................................................................................................. 15


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PROJECT OVERVIEW

An ornithopter is a flying machine which uses flapping wings for propulsion. Due to the close

resemblance to insects and birds in both their physical and flight characteristics, ornithopters have been developed for clandestine surveillance as well as to study the aerodynamics of flapping wings. At the request of Dr.Howard Hu, the team has decided to design and build the smallest ornithopter possible from the ground up. Dr.Hu is interested in the study and manipulation of wing tip vortices which only have significant relevance at small scales. Furthermore, Dr. Hu would like an ornithopter for recreational purposes.

According to the requirements of the team’s customer, the ornithopter should be a maximum of 10 inches in wingspan and be able to operate at normal building altitudes of the University of Pennsylvania at standard room temperature and pressure. Additionally, the ornithopter should be able to produce level flight for at least one minute.

An additional goal which the team set for itself was to have the ornithopter feature at least rudder and throttle control. These control features were not required by the customer.

While there are a few ornithopters on the market at 10 inch wingspan, Dr. Hu wanted a freely customizable model that he could easily change in his lab using the tools available at UPenn.


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OVERALL DESIGN FLAPPING MECHANISM: The team focused on constructing a 2-winged ornithopter. Flapping is achieved through the drivetrain illustrated below:

Two acrylic plates (4), held together by a 0.09” diameter carbon fiber rod, house the DC motor (6). The motor used in the model is a 30:1 High Power Brushed DC Gearmotor purchased from Pololu. The carbon fiber extends beyond the gearbox of the motor where it interfaces with the flapping mechanism bracket (5). The wing actuator component (1) pivots about front plane of the flapping mechanism bracket (5). The wing actuator component (1) interfaces with the connecting rod (2) which in turn interfaces with the motor coupler component (3). The motor coupler component (3) is directly attached to the D-shaft of the DC motor. Two Lithium-Polymer cells, capable of providing a nominal 3.7V each (for a series total of 7.4V), provide the onboard power necessary to operate the DC motor. When a voltage is applied between the two terminals of the DC motor, the shaft of the motor spins. This in turn causes the motor coupler component to rotate. As the motor coupler rotates the end of the connecting rod interfacing with the wing actuator components moves in a vertical fashion. This motion in turn causes the wing actuator to pivot about a point which induces the flapping motion that results in flight.


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MOTOR SELECTION

Typically two types of motors are utilized to operate an ornithopter: DC pager motors and brushless DC motors. Regardless of motor choice, a gearbox is required and the team did not have sufficient equipment for fabricating the gearbox. Pre-fabricated gearboxes available online did not feature the necessary gear ratios or were too heavy. Therefore, the team decided to work with Pololu High Power Gearmotor, which featured integrated, lightweight gearboxes at the appropriate gear ratios. Currently there is no ornithopter in existence that makes use of brushed DC motors.

ELECTRONICS AND CONTROLLER:

A microcontroller onboard a hand-held controller communicates via the nRF24LE1 wireless module to issue commands to the on-board ornithopter microcontroller. The packet structure features a start packet followed by two 8-bit numbers signifying the voltage to be supplied to the motor and electromagnetic actuator. The user is able to control the control number values by manipulating two joysticks which produce a varying voltage that is sampled by the microcontroller on the hand-held. On-board the ornithopter is a wireless receiver which receives signals from the user’s handheld transmitter. The packets received are read by the on-board microcontroller. The microcontroller determines how much voltage to supply to the motor and actuator and regulates these voltages through means of pulse width modulation (PWM) signals that is essentially buffered by a half-bridge. The rate at which the wings flap is dictated by the rate at which the motor shaft rotates, which is proportional to the voltage applied to the motor.


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CONTROL SURFACES:

It is important to note that the only control surface on the ornithopter is a rudder. The rudder was fabricated from 1/16’’ balsa wood and actuation was achieved using an electromagnetic coil. The position and area of the control surface was determined through test flights. Elevation control was achieved by altering the flap rate of the ornithopter, which it turn altered thrust produced and moment generated. A faster flapping rate would result in climb while a smaller flapping rate would result in the descending of the ornithopter. An image of rudder design with implemented magnetic actuator follows:

WINGS:

Fixed to the wing actuator components were carbon fiber rods of 0.025’’ diameter. The rods functioned as the main structural member for the wing. Tissue paper wings were fabricated using a wing template to achieve consistency in fabrication, and were attached to the carbon fiber rods using adhesive. Tissue paper material for wing fabrication was attractive because of its low weight and its deformation properties, which resulted in larger thrust generation. To improve lift performance carbon fiber struts were added to wing to stiffen the material.


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TAIL:

Unlike the rudder, the tail was set in a fixed position. It was fabricated from three carbon fiber rods, laminate, and balsa wood. As with the wings, the carbon fiber rods provided structural members on which the laminate rested. The three carbon fiber rods interfaced with a balsa assembly that is presented in the image below. The balsa assembly interfaced with the 0.09’’ carbon fiber rod (main body rod). Trimming of the ornithopter was achieved by varying the angle of the middle balsa brackets of the tail angle adjuster. This angle was changed before each flight test to find an optimal angle.


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TESTING/PROTOTYPING RESULTS

BEFORE MIDREPORT: Previous Tests:

The team completed multiple flight tests in the lobby of the Levine building. Precautions were taken

so that when the ornithopter landed, no parts were broken. This was done by having team members catch the ornithopter. Many ornithopter components were laser cut multiple times in order to ensure that quick fixes could be made during the testing process in the case of any breakages. The team’s initial test of was of the original acrylic model with a single laminate wing without a vertical stabilizer. In the video (Test 1) the ornithopter flew a couple feet then drastically veered to the right and fell. Around 5-7 attempts were made and all had the same conclusion, a vertical stabilizer was needed so the ornithopter flew straight and the weight needed to be reduced.

Test 1: http://www.youtube.com/watch?v=Hkihjml_GW4

The second flight test (Test 2) consisted of the same model as the previous test but included a vertical stabilizer fabricated from laminate and 2 thin carbon fiber rods as support and parts were made from 1/16” acrylic. The results of this test were significantly better. The ornithopter flew straighter and further than in the first field test. In order to get this model to fly further and more level the team worked toward reducing the weight as well as increasing the force produced by the wings.

Test 2: http://www.youtube.com/watch?v=1KRnKT1msB4

The third flight test (Test 3) addressed the ornithopter weight problem by attempting to use a double layer of tissue. This change was made on the test 2 model. Although the weight was slightly reduced, the tissue paper did not produce enough lift. This prototype failed to provide stability which caused the ornithopter to drift off in different directions.

Test 3: http://www.youtube.com/watch?v=2slzwaJ55AA

The fourth test (Test 4) addressed the lack of force issue by attempting to use a double layer of laminate in hopes of creating a stronger thrust and greater stability. Multiple tests were run and it was concluded that double layered laminate wings was the least effective material of all the materials the team tested. The videos of the flights showed definitely lack of force and stability. Even with the vertical stabilizer the ornithopter did not fly straight. The material was too stiff and did not allow enough motion to create thrust upward and downward.

Test 4: http://www.youtube.com/watch?v=YXiYeBI1loI

http://www.youtube.com/watch?v=Hkihjml_GW4

http://www.youtube.com/watch?v=1KRnKT1msB4

http://www.youtube.com/watch?v=2slzwaJ55AA

http://www.youtube.com/watch?v=YXiYeBI1loI


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AFTER MIDREPORT:

The test stand, shown in the photo below, provided the means to compare the performance of different wing materials and designs in thrust generation. The test stand was composed of an L-shaped beam pivoting about an aluminum rod inserted through the trapezoidal stand.

The initial design was to mount the ornithopter directly on top of the L-beam with the bottom of the L-beam resting on a scale. The idea was to determine exact thrust by measuring the reaction force on the scale caused by the thrust generated—because the L-beam would be static the thrust-moment and scale-moment would be equal. However, a consistent reading on the scale could not be achieved due to the vibration caused by the flapping wings.

To resolve this problem, the team decoupled the measuring system from the flapping system. As shown in the figure, the ornithopter was attached to a fixed stand next to the L-beam. A plate of laminate sheet was attached to the top of the L-beam to catch the wind produced from the flapping. As the thrust hit the laminate cover, it created a moment on the L-beam, pushing the bottom of the L-beam onto the scale. Multiple trials were conducted in order to ensure consistent readings. Although this method could only measure relative thrust, the results were repeatable.

Once the team was confident in the effectiveness of the test stand, it was used to determine the most desirable set of parameters for thrust generation, specifically the flapping range, wing material and wing design. Wings of different materials were flapped at the same voltage and wings made of tissue paper


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produced the most thrust of the materials tested, as shown in the figure below. This result differed from the previous conclusion that laminate was the best material for wings. Slow motion video showed that the tissue paper deformed much more fluidly than the laminate which may have contributed to its efficiency in thrust generation.

Flights tests showed, however, that a wing composed of plain tissue paper was not rigid enough to generate sufficient lift force at the applicable velocities since it featured too much “give”. It was clear that reinforcement was needed to provide the rigidity for the tissue paper. Thin carbon fiber rods were an obvious choice since they were relatively strong and light. The team experimented with various arrangements of carbon fiber rods on the wing. The wing design as shown previously produced the most thrust while still capable of sustaining level flight. Test stand results showed that carbon fiber stiffeners did not significantly vary the thrust characteristics of the flapping wing.

The team also experimented with various lengths of the motor coupler to determine the best flapping range. As indicated in the figure below, the thrust output is nearly linearly proportional to the length of the motor coupler. However, a motor coupler which was too long created instability in the flight of the ornithopter by making the average wing position anhedral. The team chose the longest motor coupler which maintained a dihedral configuration.


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The optimal components found from these tests were incorporated into the final design. As evident in flight Test 5, the ornithopter exhibited high lift potential. Afterwards, the team introduced the on-board electronics for controlled flight. The added 12 grams from the electronics proved too heavy for this prototype. To resolve this issue, the team increased the wingspan to 18”. At this size, thrust control was achieved. However, yaw control required a more powerful actuator than the one the team possessed. Level flight with on-board electronics can be viewed in the Test 6 video linked below.

Test 5: http://www.youtube.com/watch?v=n16VXJRbaMM&feature=youtu.be

Test 6: http://www.youtube.com/watch?v=JPMnAx2E6xg&feature=youtu.be

http://www.youtube.com/watch?v=n16VXJRbaMM&feature=youtu.be

http://www.youtube.com/watch?v=JPMnAx2E6xg&feature=youtu.be


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PROPOSED IMPROVEMENTS/LESSONS LEARNED

Problems - Wingspan and Lack of Literature

Currently, there is no definitive literature on thrust generation from flapping wings and none of the faculty has had any experience studying ornithopters. This resulted in a lot of guess and check work as well as time spent in iterative development based on empirical results. Simple aerodynamic wing and propellor theory was used to guide progress but never matched up adequately to test results.

A task in which lack of theory proved especially problematic was motor selection. During most of the project, the group was in the dark as to which motor would be most appropriate for the ornithopter and designing flapping mechanism around different motors was time consuming. Only through extensive testing did the team finally arrive upon the Pololu Micro Gearmotor, which is not typically used to power aircraft, as the final selection.

Once the team designed a prototype that produced adequate thrust, the 10 inch wingspan requirement was next to cause problems for the team. At such a scale, careful machining and manufacturing is required as even small discrepancies in each wing can cause huge differences in flight characteristics and stability.

Lessons Learned - Parallel Path Approach, Setting Feasible Goals, and Lead-Time Management The team had most success approaching the project on parallel paths--with one team focusing on

robust design and the other on weight reduction. It seemed that when one team encountered difficulty, the other would find success. The most difficult part of this process, however, is knowing when to reconvene and work together. The process thrives, to some degree, on competition to drive individual paths, yet excessive competition and stubbornness can cause a delay or failure in the reintegration process.

The team originally set out to make a hovering ornithopter with the knowledge that it took Aerovironment 4.5 years with full time engineers and incredible amounts of funding to accomplish such a feat. The group questioned all the current methods of building ornithopters and attempted to innovate on every front. Many of these ideas had to be dropped due to lack of time or simply because they were not feasible and, in the end, the group had to rush to design a functional ornithopter. Also, it should be noted that a previous senior design group that had tried to make an uncontrolled ornithopter and failed to make it fly at all, which should have been an indication to the current team exactly how difficult it is to fly with flapping wings at all.

The group also learned the importance of factoring in lead-time for purchasing parts. This lesson has significant importance moving forward because we will most likely deal with purchasing procedures in our future careers. These procedures, such as submitting purchase orders for approval, will add lead time to part delivery. This will affect the scheduling for deliverables on future projects.

Future Improvements/Changes Given greater funds, the team would have been able to purchase micro-machined gearboxes

allowing the use of light, high-efficiency coreless 6-7mm motors or even high-efficiency, expensive


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Maxxon motors. Alternatively, had there been more time, the team could have produced a high efficiency, light gearbox out of thin acrylic, bushings, and low module gears. Using these alternate gearboxes would have allowed for greater thrust generation and more robust flight.

With more time the team would have been able to professionally make the finalized PCB’s, which would allow for more robust mechanical interface between the electronics and main frame--a step necessary to stabilize the ornithopter.

The team would have liked to design a protective structure to encapsulate the flapping mechanism, preventing various components from breaking when the ornithopter impacts the ground. This would save tremendous amount of time spent laser cutting parts, disassembling the broken ornithopter and reassembling. This could drastically reduce up to an hour in downtime between test flights. Using ABS instead of acrylic may also have made the flapping mechanism less brittle and prone to breakage.

The team would have had professionals manufacture the wings as the team had clearly reached the fullest potential of hand manufacturing. Professional manufacturing would most likely stabilize the ornithopter as the lift on either wing would be nearly identical and the dihedral configuration would be able to compensate for the minute amount of roll instability remaining.

Finally, the team would have begun sourcing carbon fiber rods from BP Hobbies in New Jersey rather than DragonPlate in California, which would allow for more rapid prototyping because of a two-week reduction in lead-time.

Overall, the team believes that, with a couple more weeks, it could have fixed the instability problem simply by using more careful and time-consuming manufacturing techniques and methods.


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REQUIREMENT COMPLIANCE

Customer Requirements: Three functional requirements were set forth by Dr. Howard Hu at the opening of the project: 1) Maximum 10” wingspan 2) Minimum 1 minute flight time 3) The ornithopter should operate at normal Penn building altitudes

The team designed a model with a 10 inch wingspan that achieved level flight for about 50 feet (as seen in our testing/prototyping video). The lithium-polymer batteries that were purchased did allow for more than a minute of continuous flapping. This is known because the 160mAh batteries would be able to provide 6 minutes of power at the 1.6A stall (max) current draw of the system, and the motor drew about half that amount during flight.

Team Goals: 1) Yaw Control 2) Thrust Control

The team set the two personal goals stated above. Yaw control was achieved by using an electromagnetic actuator attached to the rudder. Although the team could not demonstrate the rudder control in the final test due to broken leads, the group tested the functionality of the electromagnetic actuator prior to the breakage. The team was able to manipulate the rudder using the wireless controller.

Thrust control was achieved by PWM signals generated by a microcontroller. During the final presentation and in test videos, the team was able to demonstrate how the thrust could be varied using the joystick on the wireless controller. Unfortunately the ornithopter faced issues with stability mainly due to limits of manufacturing

accuracy. Should the project have continued, the group would have worked on improving manufacturing

techniques or, more likely, had wings made by professional manufacturing companies.


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COSTS

Company Description Amount

Radical RC 34756 Pinion Gears $12.00

Dragonplate 15915 Carbon Fiber $45.50

Ebay - ZHANGZIXING2008 Universal Joints $43.96

McMaster Carr PO 291727 M2 screws, aluminum rods $14.84

Reliance Precision Bearing Spacers $92.50

Technobots Worm Gears $28.15

Dragon Plate Carbon Fiber Carbon Fiber $21.15

VXB Bearings Bearings $55.93

Robotshop C8209726 1:30 Pager Motor $26.90

Robotshop 1676-7268 1:30 Pager Motor $24.67

McMaster Carr PO 2938115 Aluminum Rods, $61.50

BP Hobbies.com 467070-13 Carbon Fiber $32.77

Dragon Plate 16985 Carbon Fiber $24.81

Digikey Force Sensors $124.06

Amazon E-flite Motors $194.53

DragonPlate Carbon Fiber $23.45

Amazon.com Magnetic Actuator $19.78

Amazon.com E-Flite Batteries $24.20

Tower Hobbies Balsa Sheets $46.78

Solarbotics 1:30 Pager Motor $61.53


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Mouser Molex Headers $22.39

Amazon Fog Liquid $9.53

Amazon Fog Machine $31.79

Amazon Magnetic Actuator $38.56

Pololu Polulu Motors $44.85

BP Hobbies Pager Motors $34.58

BP Hobbies Pager Motors, Carbon Fiber $42.93

BP Hobbies Pager Motors, Carbon Fiber $44.98

Digi-Key Atmega32U4 $34.75

Digi-Key Resistors, Capacitors $23.46

Digi-Key Voltage Regulator, Motor Drivers $43.64

Digi-Key Atmega32U4 $37.01

Digi-Key PCB Material $9.60

Digi-Key Joysticks, Wireless Chips $39.43

Total

$1,436.51

Max budget: $1500.

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