18/22/2011

Automated Refueling for Hovering Robots

Nigel Cochran, Janine Pizzimenti, Raymond Short

WPI Major Qualifying Project with MIT Lincoln Laboratory Group 76

Project Presentation Day

19 April 2012

28/22/2011

Problem Statement

• Currently, there is an insufficient mission duration for small hovering robots compared to the down-time required to charge their batteries

• An autonomous apparatus for exchanging and charging batteries quickly is needed

38/22/2011

Project Goals

UAV to Base Communication

Reliable UAV Positioning

Exchange and Store Batteries

Charge and Balance Lithium-

Polymer Batteries

Maximize In-Flight Duty

CycleEasily Adapted for other UAVs

48/22/2011

Requirements and Assumptions

• Navigating: UAV navigates itself to landing zone in negligible time• Landing: X-Y disp. of ± 6”, Yaw of ± 30°, Pitch/Roll of ± 15°• Base Size: < 3’x3’x2’ and < 30 lbs (excluding batteries)• UAV Modifications: < 100 g added to UAV (excluding battery)• Battery: 5000 mAh, Discharge time 18 mins• Battery Exchange: < 3 mins, Hot swap capable • Electrical Constraints: Balance charging batteries, < 200 W,

Conversion will be provided• Software Requirements: ROS communication via Ethernet/WiFi,

Initiate request for landing give battery voltage• Environment: Indoors (no weather)

58/22/2011

• Off-the-shelf charge/balancing with custom interface• Enough batteries and short service to allow missions of any

length with high duty cycle• ~ 2’x2’ open landing area with active alignment• Custom battery mount and UAV skids

– Can be made universal for many UAVs

• Rotating battery track• ROS communication

– UAV reports battery level– Base signals when UAV can land

Solution Features

68/22/2011

• Venom Easy Balance AC LiPO Charger– Small: 4.01”x2.44”x1.39”– Charge Rate: 0.1-4.5 A (Mechanical Dial)– Balances: 1 Battery, 2-4 Cells– Design Advantage: COTS

• Reverse Engineering– Dial glued at 4.5A (Maximum)– Pololu output port to activate start button– Monitor LED Voltage to know charger’s state

Lithium Polymer Battery Charging

78/22/2011

Model of Batteries Required

1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Number of Batteries

% o

f Cyc

le T

ime

in F

light

In Flight Duty Cycle

Swap and Recharge SystemCharge Only System

Flight Time = 18 minsCharge Time = 80 minsService Time = 4 mins

1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Number of Batteries

% o

f Cyc

le T

ime

in F

light

In Flight Duty Cycle

Swap and Recharge SystemCharge Only System

Flight Time = 18 minsCharge Time = 80 minsService Time = 2 mins

Ideal System Prototype

88/22/2011

• Aligns UAV in center of base orientated in increments of 90°• Two Servo-actuated four-bar linkages

– One servo per four-bar– Required torque of 200 oz-in, using 582 oz-in servos– L-shaped arms interlock for consistent positioning

UAV Alignment Device

98/22/2011

• Based on Pelican’s skid design• Includes extended feet to widen base for easy battery

exchange

• Weight added to UAV: 109g

Universal UAV Skids

108/22/2011

• Raise/Lower Batteries– Scissor lift– Same 582 oz-in servo

• Move between dock and UAV– Rack/pinion & linear bearing– 582 oz-in continuous servo

Enough for 2 cars Limit switches

• Rotate between charging docks– Turntable– 219.5 oz-in Stepper motor

Battery Transfer System

118/22/2011

• Connects correctly to base and UAV independent of rotation

• 0.2” of compliance in mechanical alignment– Pyramid guided touch latch

2.6lb of holding force• 3D printed ABS Plastic Casing

Battery Mating and Alignment

128/22/2011

• Battery (8x)– Start Charge Relay: Digital-Out– Monitor LED State: Analog-In– Presence Limit Switch: Digital-In

• Battery Cart– Endpoint Limit Switch (2x): Digital-In– Middle Limit Switch: Digital-In– Positioning Motor: PWM– Scissor Lift Servo : PWM– Scissor Lift Current Sense: Analog-In

• Turntable– Stepper Motor Direction: Digital-Out– Stepper Motor Number of Steps: Digital-Out– Photo Interrupter (8x): Digital-In

• UAV Centering– Arm Actuation Servo (2x): PWM– Arm Actuation Current Sense (2x): Analog-In

• Total I/O Required: 44

I/O Devices

2x 24-pin Pololu Maestro USB Controllers

138/22/2011

High-Level Program Flow

148/22/2011

• BaseStation– Manages system state progression using ROS– Contents:

Callback - handle messages from UAVs Init - initialize starting variables and ROS parameters State - state functions for actions and state transitions Thread - run state and listener threads

• MaestroController– Manages control of peripherals (sensors, servos, etc.)– Contents:

Actions - abstracted functions called by the BaseStation Batteries - all battery-related functions Cart - all cart and scissor-lift related functions Inputs - handling for gathering information from the controllers Low_Level - abstraction for basic USB communications Servos - general servo functions Stepper - all stepper motor-related functions

Program Structure

158/22/2011

• Total Time: Between 4 and 5 minutes– Scissor Lift took approximately 3 of the 4 minutes

• Video:– mqp complete.wmv will be shown at this time

• Future Recommendations:– New battery mating systems– Lighter aluminum or plastic frame– Hot-swapability– Faster motor on scissor lift– Second cart– GUI with system states and battery life– Sensor feedback on alignment system, like limit switches– Shorter stepper motor

Results

168/22/2011

• Lincoln Lab Staff:– Brian Julian (Group 77)– Mike Boulet (Group 76)– Byron Stanley (Group 76)– Mike Stern (Group 77)– Mike Crocker (Group 72)– Group 76 Technicians– Emily Anesta and Seth Hunter

• WPI Faculty:– Prof. Ken Stafford (ME/RBE), Advisor– Prof. Bill Michalson (ECE/RBE/CS), Advisor– Prof. Ted Clancy (ECE), MITLL Site Director– Joe St. Germain (RBE), Robotics Lab Manager

Acknowledgements

178/22/2011

• Problem Statement & Goals• Requirements & Assumptions• Solution Features• LiPoly Battery Charging• UAV Alignment Device• Universal UAV Skids• Battery Transfer System• Battery Mating & Alignment• Controls & Communications• Program Structure

Summary

Questions?

188/22/2011

• Pole LED voltage 3 times over 1 second to determine states

• Possible States:

Charger LED Color Code

LED Colors StateAll orange (2.4V) Charging

All green (4.7V) Charged

Orange and no light (0V) Waiting: Battery connected

Green and red (0.6V) Waiting: No battery connected

Red and no lightErrorAll red

No light

198/22/2011

• Approximate Number of Batteries Required (Worst Case)

(Tf + Ts)n = Tc + Tf + Ts

– Tf = Flight time = 15 minutes (min from specification)

– Ts = Service time = ~1.5 minutes

– Tc = Charge time = 5 Ah battery / 4.5 A charge = ~ 75 minutes

– N = Number of Batteries = (Tc+Tf+Ts)/(Tf+Ts) = (91.5)/(16.5) = ~ 5 batteries

• With a safety factor of about 1.5, we choose 8 batteries

System Battery Requirements

Tf Ts Tf Ts Tf Ts Tf Ts

TfTs

2Tc

Ts

2

208/22/2011

Estimated Analysis of Base Power – 8 Batteries

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

Pow

er U

sed

(Wat

ts)

Time (minutes)

218/22/2011

UAV Communications Simulation in ROS

228/22/2011

Prior Art

UMichigan (2010):Swap/Recharge

KAIST (2011):Swap/Recharge

MIT/Boeing (2011):Swap/Manual

Recharge

MIT (2007):Recharge Only

• 2 min Service• No Hot-swap• Sloped Landing Area• Servo-actuated Magnet Mating• Offset Battery Ring

• 21.8 sec Service• Hot-swap• 2 Arm Catching• Rail Battery Contacts • 2 Vertical Battery Drums

• 47.5 sec Service• No Hot-swap• Mechanical Arm Catching• Electromagnets• Battery Ring with Pusher

• 30-70 min Service• Charge Via Contacts on Feet• Sloped Landing Area w/ Recess