case study: term project...case study: final design project enae 788x - planetary surface robotics u...

Case Study: Final Design Project ENAE 788X - Planetary Surface Robotics

U N I V E R S I T Y O FMARYLAND

Case Study: Term Project• Expectations for the term project• An example of a previous term project• Quickie assignment – one person from each

team go on Zoom chat right now and list the names of the members of your team, and also your team name if you’ve selected it

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© 2020 David L. Akin - All rights reserved http://spacecraft.ssl.umd.edu

http://spacecraft.ssl.umd.edu



2020 Design Project Statement• Perform a detailed design of a BioBot rover,

emphasizing mobility systems– Chassis systems (e.g., wheels, steering, suspension…)– Support systems (e.g., energy storage)– Navigation and guidance system (e.g., sensors,

algorithms...)

• Design for Moon, then assess feasibility of systems for Mars, and conversion to Earth analogue rover

• This is not a hardware project for this class - but it will be built during the Spring term!

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Course Overview ENAE 788X - Planetary Surface Robotics


Level 1 Requirements (Performance)1. Rover shall have a maximum operating speed of

at least 4 m/sec on level, flat terrain.2. Rover shall be designed to accommodate a 0.3

meter obstacle at minimal velocity.3. Rover shall be designed to accommodate a 0.1 m

obstacle at a velocity of 2.5 m/sec.4. Rover shall be designed to safely accommodate a

20° slope in any direction at a speed of at least 1 m/sec and including the ability to start and stop.

5. The rover shall have a nominal sortie range of 54 km at an average speed of 2.5 m/sec.

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Level 1 Requirements (Payload)6. Rover shall be capable of carrying one 170 kg EVA

crew and 80 kg of assorted payload in nominal conditions.

7. Payload may be modeled as a 0.25 m box8. Rover shall be capable of also carrying a second

170 kg EVA crew in a contingency situation. Payload may be jettisoned if design permits.

9. Rover design shall incorporate roll-over protection for the crew and all required ingress/egress aids and crew restraints.

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Level 1 Requirements (Operations)10.A nominal sortie shall be at least eight hours long.11.Two rovers must be launched on a single CLPS

lander.12.A single rover shall mass ≤250 kg.13.Rovers shall be developed in time to be used on

the first Artemis landing mission.14.Rover shall be capable of operating indefinitely

without crew present.

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Level 1 Requirements (GN&C)15.Rover shall be be capable of being controlled

directly, remotely, or automated.16.Rover shall be capable of following an astronaut,

following an astronaut’s path, or autonomous path planning between waypoints.

17.Rover shall be capable of operating during any portion of the lunar day/night cycle and at any latitude.

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Final Project Expectations• Final design of rover

– Solid models of design – Design evolution throughout as the analysis progressed– Details of mass, power, etc.

• Trade studies (NOT an exhaustive list!)– Number, size, configuration of wheels– Diameter and width of wheels– Size and number of grousers– Suspension design– Steering design– Alternate design approaches (e.g., tracks, legs, hybrid)

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Final Design Expectations (2)• Vehicle stability

– Slope (up, down, cross)– Acceleration/deceleration– Turning– Combinations of above

• Terrain ability (“trafficability”)– Weight transfer over obstacles– Climbing/descending vertical or inclined planes– Hang-up limit (e.g., high-centering, wheel capture)

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Final Design Expectations (3)• Suspension dynamics• Development of drive actuator requirements• Detailed wheel-motor design• Development of steering actuator requirements• Detailed steering mechanism design• Mass budget (with margin)• Power budget (with margin)• Other design aspects as included

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Final Project Presentations• Monday, Dec. 7, and Wednesday, Dec. 9• Each final project will have 25 minutes for a class

presentation• All teams should submit their slides prior to class

on Dec. 7 (submit as both powerpoint and pdf)• Slide deck should be comprehensive and reflect all

of your work - slides which can’t be presented in the time allotted should be included as backup

• Would also like to collect model files created during the course – details TBD

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Interim Project Progress Report • Due Wednesday, Oct. 28 • Format: presentation slides showing work to date• Submitted electronically and presented in class (10

minutes/group)• Would like to see solid progress and use of

concepts from lectures to date• Certainly expect to see basic rover concept, trade

studies leading to configuration choices (number and sizes of wheels, general configuration)

• Opportunity to see other designs and interact

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Terrestrial Lunar Rover (TLR)

ENAE788X Planetary Surface Robotics

Design Project

Team Members Cagatay Aymergen • Jignasha Patel

Syed Hasan • John Tritschler

ENAE-788X • Cagatay Aymergen • Jignasha Patel • Syed Hasan • John Tritschler 13

Overview• Project Requirements and Objectives • Concepts Explored • TLR Design Overview • Terramechanics and Energetics • Stability and Breaking • Steering • Suspension system • Chassis • Motors and Gearing • Track Wheel Hybrid Mobility Unit Details • TLR Design Details • Operations • Sensors • Mapping • Command and Control • Mass Budget • Reliability and Fault Tolerance • Earth Analog Considerations • Possible Improvements to TLR

12/11/2008


• Project Description • Perform a detailed design of the mobility systems for a small pressurized rover

– Chassis systems (e.g., wheels, steering, suspension...) – Navigation and guidance system (e.g., sensors, algorithms...)

• Design for moon, then assess feasibility of systems for Earth analogue rover

• The following are the level one requirements provided to impact our design: • L1-1: Rover shall have a maximum operating speed of at least 15 km/hour on level,

flat terrain • L1-2:Rover shall be designed to accommodate a 0.5 meter obstacle at minimal

velocity • L1-3: Rover shall be designed to accommodate a 0.1 meter obstacle at a velocity of

7.5 km/hour • L1-4: Rover shall be designed to accommodate a 20° slope in any direction at a speed

of at least 5 km/hour with positive static and dynamic margins

• The following are the specifications provided to impact our design: • L1-5: Rover shall be capable of supporting a mass (exclusive of chassis and mobility

system) of at least 1000 kg • L1-6: Rover shall be capable of accommodating a cylindrical pressurized cabin that is

1.80 meters in diameter and 1.83 meters long • L1-7: Target overall vehicle mass shall be less than 1800 kg with positive margin

Project Requirements & Specifications

12/11/2008


Project Requirements & Specifications• The following are the Level 2 requirements derived to impact our design: • L2-1: The vehicle shall be designed to be operational on the surface of the moon with

the environmental constraints given in Table 1. • L2-2: An analog test vehicle shall be designed to be operational on the surface of the

earth with the environmental constraints given in Table 1.

• The following are the design goals derived to impact our design: • G-1: Safety factors - at least 1.5 to 2.0 (this might be driven by the earth analog

requirements) • G-2: Fault tolerance - Every subsystem should be single fault tolerant • G-3: Mobility - 360 degrees on the spot turns and movement • G-4: Adaptability - Don't be limited to only this size payload (mass, weight…etc)

Table 1

Earth Moon

Gravitational Acceleration 9.8 m/s2 (1g) 1.545 m/s2 (0.16g)

Atmospheric Density 101.350 pa (14.7 psi) -

Atmospheric Constituents 78% N2 – 21% O2 -

Temperature Range 120 F – -100 F 250 F – -250 F

Length of Day 24 Hr 28 Days

12/11/2008


Concepts Explored

12/11/2008


TLR Design Overview

Supported Payload Accommodates All

Sensors and Avionics

4 Track-Wheel Hybrid Mobility Unit

Large Wheel Driving Wheel

Houses the MotorsSmall Wheel Free Running

Aluminum Chassis

Wheel Connector BarTracks

Suspension System

Wheel to Chassis Connection

• Each mobility unit is capable of rotating about the center of the large wheel • Each large wheel houses two motors that are cross strapped to operate the wheel and the actuator to rotate the wheel connector bar

12/11/2008


Terramechanics and Energetics• Trades

– Draw Bar Pull vs. Wheel Diameter vs. Wheel Width – Grousers vs. No-Grousers – Power vs. Wheel Diameter vs. Wheel Width – Number of wheels vs. Wheel Diameter vs. Wheel Width – Wheels vs. Tracks

• Wheels – Wheel diameter varying from 0.3 to 1.0 m – Wheel width varying from 0.1 to 0.6 m

• Tracks – Large wheel diameter varying from 0.3 to 1.0 m – Small wheel diameter 2/3 of the large wheel

• Study Cases (for each trade above) – Flat terrain with 15km/hr velocity – 20o slope with 5km/hr velocity – 10 cm obstacle with 7.5km/hr (assuming all wheels encounter the obstacle at the

same time) – 50 cm obstacle at minimum velocity (assuming all wheels encounter the obstacle

at the same time)

12/11/2008


Wheeled System – Draw Bar Pull – No GrousersFlat Terrain

4 w

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Wheel Diameter 0.30 m Wheel Diameter 0.40 mWheel Diameter 0.50 m Wheel Diameter 0.60 mWheel Diameter 0.70 m Wheel Diameter 0.80 mWheel Diameter 0.90 m Wheel Diameter 1.0 m

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Wheel Diameter 0.30 mWheel Diameter 0.40 mWheel Diameter 0.50 mWheel Diameter 0.60 mWheel Diameter 0.70 mWheel Diameter 0.80 mWheel Diameter 0.90 mWheel Diameter 1.0 m

20o Slope

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Turtle Performance is Highlighted

12/11/2008


Wheeled System – Draw Bar Pull – With GrousersFlat Terrain

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20o SlopeTurtle Performance is Highlighted

12/11/2008


Wheeled System – Obstacles – Draw Bar Pull – With Grousers

10 cm Obstacle

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12/11/2008


Wheeled System – Obstacles – Draw Bar Pull – With Grousers

50 cm Obstacle On All Wheels

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12/11/2008


Wheeled System – Power – With GrousersFlat Terrain

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1000.001500.002000.002500.003000.003500.004000.004500.005000.005500.006000.006500.007000.007500.008000.008500.009000.009500.00

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20o Slope

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Wheel Diameter 0.30mWheel Diameter 0.40mWheel Diameter 0.50mWheel Diameter 0.60mWheel Diameter 0.70mWheel Diameter 0.80mWheel Diameter 0.90mWheel Diameter 0.10m

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Wheel Diameter 0.30mWheel Diameter 0.40mWheel Diameter 0.50mWheel Diameter 0.60mWheel Diameter 0.70mWheel Diameter 0.80mWheel Diameter 0.90mWheel Diameter 0.10m


12/11/2008


Wheeled System – Obstacles – Power10 cm Obstacle

4 w

heel

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6 w

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500.001000.001500.002000.002500.003000.003500.004000.004500.005000.005500.006000.006500.007000.007500.008000.008500.009000.009500.00

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Wheel Diameter 0.30mWheel Diameter 0.40mWheel Diameter 0.50mWheel Diameter 0.60mWheel Diameter 0.70mWheel Diameter 0.80mWheel Diameter 0.90mWheel Diameter 1.0 m

0.00500.00

1000.001500.002000.002500.003000.003500.004000.004500.005000.005500.006000.006500.007000.007500.008000.008500.009000.009500.00

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Wheel Diameter 0.30mWheel Diameter 0.40mWheel Diameter 0.50mWheel Diameter 0.60mWheel Diameter 0.70mWheel Diameter 0.80mWheel Diameter 0.90mWheel Diameter 1.0 m


12/11/2008


Wheeled Terramechanics and Energetics Conclusions

• There is substantial amount of gain from using grousers. • There is not a substantial difference between different grouser heights • It is possible to achieve a positive draw bar pull for all wheel sizes and diameters on

flat terrain, on a slope, and going over 10cm obstacle with all wheels. • A large amount of power is required to overcome the resistance from these cases • It is not possible to achieve enough drawbar pull to go over a 50 cm obstacle,

assuming all wheels will encounter the obstacle at the same time, for reasonable size wheels.

• A wheeled system is not a good option FOR THIS APPLICATION unless: – A Lunar Monster Truck is created or – A system with more than 4 wheels and the same number of actuators (increased

mass and complexity) is produced or – An inefficiency in mobility is accepted or – An inefficiency in power consumption, hence operation time is accepted

• Therefore; need to look at: – Tracked vehicles to achieve larger drawbar pull and lower resistance (less power

use) – Clever concepts that would help overcome 50cm obstacles instead of large

wheels

12/11/2008


Track-Wheel Hybrid System Terramechanics and Energetics

• Four two-wheel track system • Large wheel is attached to chassis and drives the system • Small wheel is free running and is ran by tracks. It is connected to the large wheel by

two beams (one on each Side) • The small wheel can be rotated about the center of the large wheel. • Grouser height used = 0.01m for all calculations • 10% of the total resistance has been added to all calculations as internal resistance to

accommodate for possible unknowns

Wheel 1Wheel 20.2 m

Rotate 360o

12/11/2008


Track-Wheel Hybrid System – Draw Bar Pull – With Grousers

Flat Terrain 20o Slope

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Wheel 1 Diameter 0.03 mWheel 1 Diameter 0.04 mWheel 1 Diameter 0.05 mWheel 1 Diameter 0.06 mWheel 1 Diameter 0.07 mWheel 1 Diameter 0.08 mWheel 1 Diameter 0.09 mWheel 1 Diameter 1.0 m

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TLR Performance is Highlighted

12/11/2008


Track-Wheel Hybrid – Draw Bar Pull – With Grousers

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Only the Small Wheel Acting on the Obstacle

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Both Small and the Large Wheel Acting on the Obstacle

50 cm ObstacleThrust Capacity

Tc1

Resistance R1

R2

Tc2

Thrust Capacity

Resistance


12/11/2008


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Track-Wheel Hybrid System – Power – With Grousers

Flat Terrain 20o Slope

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12/11/2008


Wheel-Track Hybrid Terramechanics and Energetics Conclusions

• Wheel-Track hybrid is superior in all cases to a wheeled system • Wheel-Track hybrid system provides positive drawbar pull for all four cases. • Wheel-Track hybrid system requires significantly less power. • Wheel-Track hybrid system power requirements meet the Turtle average and

maximum power draw requirements for all three cases • The 50 cm obstacle is overcome by the design choice and

implementation: – Rotating the small wheel at an optimum angle to place on the 50cm obstacle and

driving over it – Leveraging the vehicle on front wheel to go over the obstacle or – Riding on the small wheel and rolling over the obstacle with the large ones

12/11/2008


Wheel-Track Hybrid – Power Use• Requirements:

– Average power for Turtle driving system is 0.821 kW – Defined as operations over 3 days

– Maximum power draw for Turtle driving system is 6.19 kW – Allocated power for the driving system is 0.86 kW – Allocated power for the avionics is 0.59 kW in use, 0.2 kW in standby mode

• Based on the power calculations for a 1m diameter, 0.30m width wheel: – Turtle could support only ~6 hours of drive time a day on average (driving

half the time over 10cm obstacles half the time on flat terrain). • Tack Wheel Hybrid System:

– Nominal power usage: for flat terrain ~0.9 kW – Maximum power usage: for 10 cm obstacle is ~1.6 kW – Power usage for 20o slope is ~1.7 kW

• Based on the power calculations: – Track-Wheel hybrid system can support ~16 hours of drive time a day on

average (driving half the time over 10cm obstacles half the time on flat terrain or half time on slope) and almost continuously on flat terrain.

– This would allow for more autonomous applications and a larger range of operations from a base.

• The avionics power use is well below the 0.59 kW • There is 10% margin on all calculations for drawbar pull & power to account for

internal resistance or other unknownns

12/11/2008


Track-Wheel Hybrid Mobility Unit - Details• Wheel:

– The wheel well is made out of titanium – Houses the in-hub motor – Interior is protected by a flexible cover to avoid dust collection on critical components

• Tire: – Modified Lunar Rover wheel construction:

• Thicker woven flexible steel mesh tires with titanium track engagement threads.

• Track: – Same construction as the tires.

• Thicker woven flexible steel mash with titanium grousers on the outer surface and titanium wheel engagement threads on the inner surface

* No CTE mismatch between tracks, tires, wheel wells, and the wheel connector bar * Tire can operate without the track in place in emergencies * Easily maintained - installed/removed, replaced - tracks

Titanium wheels

Steel Woven Mash Tires

Titanium Grousers

Titanium Track Engagement Threads

Steel Woven Mesh Track

Flexible Cover on both wheels

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Small supporting rollers to distribute pressure evenly on the tracks between the wheels (not shown)

12/11/2008


• CG – Nominal CG (x, y, z): (1.2, 1.3, 0.73) meters – Fluctuation (x, y, z): (±0.2, ±0.1, ±0) meters – Critical slope: 48◦

• TRADES – Cg height versus length of vehicle (flat terrain and 20 ۫ slope) – Vehicle width versus cg height, turning radius, and velocity (flat terrain

and 20◦ slope)

Stability

cg

x

cgz

yy

x

z

z

12/11/2008


Stability – Flat Terrain – CG Location vs. Vehicle Length

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

Vehicle Length Needed for Stability for flat terrain

CG

Hei

ght o

ff o

f the

Gro

und

(m)

Vehicle Length

1.33

TLR Limit

12/11/2008


Stability – Slope – CG Location vs. Vehicle Length

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Vehicle Length Needed for Stability for a 20 Degree Slope (m)

CG

Hei

ght o

ff o

f the

Gro

und

(m)

Vehicle Length

2.11

TLR Limit

12/11/2008


Stability – Flat Terrain – Vehicle Width vs. Turning Radius and CG Height

00.10.20.30.40.50.60.70.80.9

11.11.21.31.41.51.61.71.81.9

22.12.22.32.42.5

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.0

0

11.0

0

12.0

0

13.0

0

14.0

0

15.0

0

Vehicle Width Needed for Stability, Velocity = 4.167 m/s

CG

Hei

ght o

ff of

the

Gro

und

(m)

Turning Radius: 2 mTurning Radius: 4 mTurning Radius: 6 mTurning Radius: 8 m2.77

7m Vehicle Width = 2.37

12/11/2008


Stability – Slope – Vehicle Width vs. Turning Radius and CG Height

00.10.20.30.40.50.60.70.80.9

11.11.21.31.41.51.61.71.81.9

22.12.22.32.42.50.

00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

2.20

2.40

2.60

2.80

3.00

3.20

Vehicle Width Needed for Stability, Velocity = 1.388 m/s

CG

Hei

ght o

ff o

f the

Gro

und

(m)

Turning Radius: 2 mTurning Radius: 4 mTurning Radius: 6 mTurning Radius: 8 m

12/11/2008


Stability• Braking

– Main brakes: Disk brakes within each wheel. – Back up:

• Slow or stop the motor to come to a gradual stop. • Stop the motor and lock the tracks to come to a halt.

• Max Deceleration rate – Flat Terrain: 2.66 m/s2 – 20 ۫ slope: 1.94 m/s2

• Stopping distance (flat terrain and 20◦ slope) – Flat Terrain: 3.3 m – 20 ۫ slope: 0.50 m

• Stopping time (flat terrain and 20◦ slope) – Flat Terrain: 1.57 s – 20 ۫ slope: 0.72 s

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12/11/2008


Stability – Going Over Obstacles

Low CG and wide base contribute to stability in handling obstacles.

Rover Overturn Due to Collision With Immovable Obstacle

* Solid lines assume 5% energy lost at impact* Dashed lines assume 25% energy lost at impact

5

6

7

8

9

10

11

12

13

14

15

0.05 0.1 0.15 0.2 0.25 0.3

Obstacle Height [m]

Rov

er S

peed

[km

/hr]

Level Terrain5 deg slope10 deg slope15 deg slope20 deg slopeLevel Terrain5 deg slope10 deg slope15 deg slope20 deg slope

* Solid line denotes 5% energy dissipated at impact; dashed line denotes 25%

12/11/2008


SteeringFlat Terrain 20o Slope

Skid Steering • The larger the track width the better the performance • Extra mass and complexity for actuators to steer is avoided • Zero turning radius at rest Steerability Criteria: Fo ≤ c b l +(w tan(Φ))/2 Steerability = (c b l +(w tan(Φ))/2) - Fo

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

1000.00

1100.00

1200.00

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55Wheel Width - b - (m)

Stee

rabi

lity


-300.00

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

400.00

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55


Stee

rabi

lity


V1 V2V

12/11/2008


Suspension – Human Factors

Frequency (Hz) Effect0.05 – 2 Motion sickness, peak incidence occurs at ~0.17 Hz

1 – 3 Side-to-side and fore-and-aft bending resonances of the unsupported spine

2.5 – 5 Strong Vertical resonance in the vertebra of the neck and lower lumbar spine

4 – 6 Resonances in the trunk20 – 30 Resonances between head and shoulders

Up to 80 Hz Localised resonances of tissues and smaller bones

12/11/2008


Suspension – TradeType Description Examples Advantages Disadvantages

Dependent • Movement of wheel on one side of the vehicle affects the movement of wheel on the other side of the axle. •Commonly used on commercial and off road vehicles.

• Hotchkiss (leaf springs) • Trailing arms • Leaf spring • 4-bar

• Simple to design • Low cost • Low mass

• Negatively affects ride and handling compared to independent systems

Semi-dependent

• Beam that can bend and flex

• Trailing twist axle • Simple to design • Design flexibility

Independent • Widely used today in the commercial vehicle industry

• Macpherson Strut • Double Wishbone • A-arm • Multi-link

• Better drive and handling over independent passive suspensions. • Design flexibility • Better reliability than active/semi-active. • Better cost and mass over active/semi-active

Semi-Active • Suspension dynamics change continuously but is not electronically monitored

• Hydropneumatic • Hydrolastic • Hydragas

• Continuous improvements to road handling and ride

• Cost and design maturity

Active • Electronic monitoring of vehicle conditions, coupled with the means to impact vehicle suspension.

• Bose Suspension • Active body control

• Continuous monitoring of vehicle motion for improved bounce, roll, pitch and wrap modes.

• Increase in cost and mass, negative affects to reliability, and design maturity

12/11/2008


Suspension AnalysisNatural Frequency of the Wheel versus Spring Diameter

1.81.85

1.91.95

22.05

2.12.15

2.22.25

2.32.35

2.42.45

2.52.55

2.62.65

2.72.75

2.82.85

2.9

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Spring Diameter (m)

Nat

ural

Fre

quen

cy (H

z)

Coil Diameter = 0.06mCoil Diameter = 0.08mCoil Diameter = 0.10mCoil Diameter = 0.12mCoil Diameter = 0.14m

Critical Distance of the Wheel versus Spring Diameter

11.05

1.11.15

1.21.25

1.31.35

1.41.45

1.51.55

1.61.65

1.71.75

1.81.85

1.91.95

22.05

2.12.15

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Spring Diameter (m)

Cri

tical

Dis

tanc

e (m

)


Natural Frequency of the Suspension versus Spring Diameter

0.0000.0500.1000.1500.2000.2500.3000.3500.4000.4500.5000.5500.6000.6500.7000.7500.8000.8500.9000.9501.0001.0501.1001.150

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Spring Diameter (m)

Nat

ural

Fre

quen

cy (H

z)


Critical Distance of the Suspension versus Spring Diameter

050

100150200250300350400450500550600650700750800850900950

10001050

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.011

Spring Diameter (m)

Cri

tical

Dis

tanc

e (m

)


Mass of Body

Mass of Wheel

MODEL

12/11/2008


Suspension – Macpherson Strut

• Material: 2014-T6 • Density = 2800 kg/m3 • Modulus of Elasticity = 72.4 GPa • Poisson's Ratio = 0.33 • Bulk Modulus = 27.2 GPa

• Number of Coils: 7 • Coil diameter = 0.003 m • Spring diameter = 0.1 m • Length = 0.24 m • Ks = 40 N/m

12/11/2008


Chassis AnalysisMaterial: AL 6061-T6

Density: 2700 kg/m3

Yield Strength: 310 Mpa

Ultimate Strength: 27 Mpa

Youngs Modulus (E): 69 Gpa

Poisson’s Ratio: 0.33

Axial Launch Load 6 g

Area Moment of Inertia (m): 8.33E-7

Critical Axial Load (N/m2): 1.52E+5

Safety Factor: 2.88

Margin: 180401.05%

Static Loads: 1 g

Area Moment of Inertia: 8.33E-7

Maximum Deflection (m): 0.005

Stress in Beam (N/m2): 2.05E+7

Max Sheer Stress (N/m2): 1.42E+3

Safety Factor: 13.42

Margin: 1242.01%

Lateral Launch Load: 2 g

Area Moment of Inertia: 8.33E-7

Maximum Deflection (m): 0.055

Stress in Beam (N/m2): 2.46E+8

Max Sheer Stress (N/m2): 1.70E+4

Safety Factor: 1.12

Margin: 11.61%

12/11/2008


Chassis Dimensions

1.9 m

1.93 m 0.08 m

0.08 m

0.08 m

0.02 m

Mass: 90 kg

12/11/2008


Track-Wheel Hybrid Mobility Unit – Wheel Connector Beam

Length of beam (m)

Maximum Deflection - Y

(m)Mass of

Beam (kg)

Maximum Stress in

Beam (N/m2)Safety

Factor (SF)

Desirable Angle to the 50 cm

Obstacle

Optimum Angle to the 10 cm

Obstacle

0.70 0.026 ~ 0.5 4.11E+08 ~ 2 34.85o 0.00o

• Wheel 1 Diameter: 0.6 m • Wheel 2 Diameter: 0.4 m • Material: Titanium (6% Al, 4% V) • Yield Strength: 1.05x1011 • Beam Thickness: 0.004 m • Beam Width: 0.06 m • Load Applied: ~ 734 N

0.6 m0.4 m0.2 m

Rotate 360o

0o point

12/11/2008


Motors and Gearing – Design Space

Planetary Gear Systems

Harmonic Drives

Multi-Staged/ Combinations

12/11/2008


Motors and Gearing – Motors Trade Space

Type Advantages Disadvantages Typical Application Typical Drive

Brushless DC Electric Motor

• Long lifespan • Low maintenance • High efficiency

• High initial cost • Requires a controller

• Hard drives • CD/DVD players • Electric vehicles

• Multiphase DC

Brushed DC Electric Motor

• Low initial cost • Simple speed control (Dynamo)

•High maintenance (brushes) • Low lifespan

• Treadmill • Exercisers • Automotive starters

• Direct (PWM)

AC Induction (Shaded Pole)

• Least expensive • Long life • High power

• Rotation slips from frequency • Low starting torque

• Fans • Uni/Poly-phase AC

AC Induction (Split-Phase Capacitor)

• High power • High starting torque

• Rotation slips from frequency • Appliances

• Uni/Poly-phase AC

AC Synchronous

• Rotation in-sync with freq • Long-life (alternator)

• More expensive• Clocks • Audio turntables • Tape drives

• Uni/Poly-phase AC

Stepper DC • Precision positioning • High holding torque• Slow speed • Requires a controller

• Positioning in printers and floppy drives

• Multiphase DC

Motor Comparison, Circuit Cellar Magazine, July 2008, Issue 216, Bachiochi, p.78

12/11/2008


Motors and Gearing – Legacy and Future Rovers

Mars Exploration Rover (180 kg)

Apollo Lunar Roving Vehicle (210 kg)

Mars Science Laboratory (900 kg)

Motors• Independently driven wheels; 28 VDC brushed motors • Identical motors used for steering front and rear wheels.

• Independently driven wheels; 36 VDC brushed motors

• Selected brushless DC motor; low temperature/low-mass gearbox. •• A failure in testing of the proposed dry lubrication to support motor actuator operations at very cold temperatures is contributing to MSL project delays. Gearing

• Two-stage planetary gearbox powers a harmonic drive. (1500:1)

• Harmonic drive (80:1)

Motors/Gearing for TLR will likely require significant R&D. Legacy and Future rovers provide a starting point for design/analysis.

12/11/2008


Motors and Gearing – TLR Motors• The design for the drive system consists of tracks independently driven by brushless DC motors. • BluWav Systems has a line of DC brushless motors that show promise, though further R&D would be necessary.

The brushless DC motors were chosen for: • Low maintenance • High efficiency (>95%) • High reliability • High controller TRL (SAE J1939; RS-232/485)

These areas would need further R&D: • Gearing options (planetary vs. harmonic) • Lower power requirements • Minimum operating temperature range*

BluWav In-Hub Motorhttp://www.bluwavsystems.com/whitepapers/46kWHubMotor.pdf

* Note: a low-temperature failure in testing of the brushless DC motors is contributing to MSL project delays

12/11/2008


Motors – Lifting the Vehicle About Small Wheels

• Use the in-hub motor to raise the small wheel while driving and to pivot about the small wheel to lift the vehicle

Gearing ratio and Torque Required: • Assuming even distribution of the weight over the four tracks…

– Each motor has to lift ~734 kg of mass • Moment arm about the small wheel = 0.7m • Torque required to lift wheel about the small wheel = ~514 Nm • Main motor torque = ~85 Nm • Gear ratio used = 8:1 • Torque generated = 680 Nm to lift the vehicle

Rotate about small wheel to lift vehicle

W 4 W

4

W 4

W 4

Rotate small wheel about large wheel to

change angle of approach

12/11/2008


Design Details – Dimensions

2.6 m

2.1 m

0.3 m

1.9 m1.93 m3.1 m

1.87 m

0.30 m

0.60 m

0.40 m

y

x

z

12/11/2008


Design Details – Dimensions

3.67 m

2.7 m

2.47 m

0.9 m

0.07 m

12/11/2008


Design Details – Mobility ConfigurationsNominal Driving Configuration

• All four tracks flat on ground • Front and rear tracks at same configuration: Large rear and small front wheel

• Drive on Flat Terrain • Drive on slope

• Easily avoid nosing in

Other possible Configurations• Rear wheels can be rotated 180 from nominal condition to increase foot print

• Front wheels can be rotated 180 from nominal condition to decrease foot print • This would be the launch configuration

• Jamming is easily avoided in every configuration

12/11/2008


Design Details – Mobility Configuration

• Each track can be adjusted to take on a different size obstacle at optimum angle of attack • Can adjust wheels to provide a level chassis in all directions up to 18.7o slope

• Used mainly for obstacles. • Main configuration to overcome the 50cm obstacle.

• All tracks can be configured to drive on the small wheel only.

• This method can be used to approach 50cm obstacle. After the approach the vehicle can roll over it while rotating the small wheels in the –X direction.

• Easily avoid bottoming out on obstacles less than 0.9m tall

Other Possible Configurations

θ

12/11/2008


Operations – Logic DiagramInitialize

OperationsUse Sensors and

Imaging to Generate Map

Calculate Path

Nominal Driving Condition Tracks are flat to ground

Detect Obstacles Detect Slopes

Every 15 seconds Compare to previous Categorize obstacle height Categorize slope angle

No Obstacle in Path No Slopes in Path

Increase Speed to 15 km/hr

Obstacle in Path Change Angle of ApproachObstacle ≤ 10cm

1 2 3 4 5 6

Lower Speed to 7.5 km/hr

7

1

3

4 Obstacle in Path 10 cm ≤ Obstacle < 30cm Change Angle of ApproachLower Speed to

5 km/hr

Obstacle in Path Obstacle > 50cm2 Re-plan path to Avoid Obstacle

Operate on Flat Terrain

12/11/2008


Operations – Logic DiagramOperating on Flat Terrain

Use Sensors and Imaging to

Generate MapCalculate Path

Nominal Driving Condition Tracks are flat to ground 15 km/hr velocity


Every 15 seconds Compare to previous

1 2 3 4 5 6 7

5 Obstacle in Path 30 cm ≤ Obstacle ≤ 50cm

Change Angle of Approach

Come to a Stop

A Climb and Drive Over the Obstacle

Lift Vehicle onto Small WheelsC

Approach Obstacle

Roll Large Wheels Onto the Obstacle

Rotate Small Wheels Back

Change Angle of ApproachB

Place Small Wheels Onto the Obstacle

Lift Vehicle, Level off, and Drive Forward

Drive Over the Obstacle with Large Wheels

in Front Small Wheels in Back

A B C

Categorize obstacle height Categorize slope angle

12/11/2008


Operations – Logic DiagramOperating on Flat Terrain

Use Sensors and Imaging to

Generate MapCalculate Path

Nominal Driving Condition Tracks are flat to ground 15 km/hr velocity


Every 15 seconds Compare to previous

1 2 3 4 5 6 7

6 Slope in Path Slope > 20o

Categorize obstacle height Categorize slope angle

Re-plan path to Avoid Slope

7 Slope in Path Slope ≤ 20o

Keep Nominal Driving Condition B

Approach Slope and Climb

Keep Nominal Driving Condition A

Approach Slope And Start Climb

Lift Vehicle Onto Small Wheels Partially to Keep Vehicle Level

A B

θ

θ

Lower Speed to 5 km/hr

12/11/2008


Sensors – Obstacle Detection and Avoidance

• The scanning LIDAR (Light Detection And Ranging) will be the rover’s obstacle detection system.

• It is a rotating unit which utilized multiple LIDAR sensors. • All of the sensors measure the distance to surrounding objects and altitude of terrain while rotating. • This scan will be done once every 15 seconds so that the rover will stay updated on passable paths.

• TLR will also employ cameras for remote control applications

Some benefits of the scanning LIDAR are:

• 360 degree field of view (compared to RADAR and Stereo vision which have only 10 and 90 degrees field of view)

• Maps output to navigation computers which generate drive and steering commands to go around obstacles (necessary for rover requirements)

• Capable of operating at night and permanent shadowed regions (many on lunar surface) http://www.cowi.com/menu/services/society/mappingandgeodata/laserscanning/Pages/laserscanning.aspx

12/11/2008


Sensors – Odometry SystemDead Reckoning • Deduce position after moving for a known time at a known direction with a known velocity

Forward Motion: ∆d(p) = fd(∆d1(p), …, ∆dn(p), ∆β1(p), …, ∆βn(p))

Angular Motion: ∆β(p) = fβ(∆d1(p), …, ∆dn(p), ∆β1(p), …, ∆βn(p))

where n = number of wheels

∆β(p)

∆d(p)

P P+1

We want to obtain position P+1 from the position at P

The difference ∆x(p) = x(p+1) – x(p) may be deduced from ∆d(p), ∆β(p)

12/11/2008


Sensors – Angular Positioning Sensors

r

Forward motion may be measured by a sensor by multiplying wheel radius r by

angular motion

The transversal angle of angular motion may be measured with a sensor

(for wheels and robotic arm)

Sensor options for angular positioning are:Sensor Advantage Disadvantage

Potentiometer Low cost and simple interface Easily dirty and sensible to noise

Synchros/Resolvers Easily mounted, can withstand extreme environments Require AC signal source, heavy

Optical encoders Higher resolution, digital High cost, not very robust* Incremental optical encoders will be used for TLR’s angular positioning sensors

∆d(p)

∆β(p)

12/11/2008


Sensors – Guidance Sensors• Odometry is not very reliable • TLR also is equipped with sensors:

• To detect heading • Orientation • Inclination.

• TLR will employ rate sensors, gyroscopes and accelerometers integrated into an Inertial Measurement Unit (IMU) will cover this.

IMU provides attitude and acceleration information during surface operations and convert to outputs used by vehicle control systems for guidance

Yaw

RollPitch

12/11/2008


Mapping• Local map will be created using fixed decomposition with LIDAR

system. • Position and ranging will be updated with 75 meter range accuracy.

• Continuous representation method not preferred for lunar exploration due to 3D surface obstacle and slope concerns. (only good for 2D representation)

• Occupancy grid will be updated using Bayesian method.

• Since Lidar scan will occur every 15 seconds it is safe and effective to update map using this technique.

P(A| not B) =P(not B|A)P(A)

P(not B|A)P(A)+P(not B| not A)P(not A)

12/11/2008


Command and Control3 RAD750 radiation hardened single board computers will be used

to: • Format and process navigation data for output • Process path commands from the autonomous driving computer • Command the rover through passable paths • Build and output range maps to the autonomous driving computer.

http://www.corelis.com/images/BAE-RAD750-board.jpg

http://www.maxwell.com/images/me/_sbc/scs750d_press.jpg6

BAE Systems RAD750

Maxwell Technologies SCS750

* A maximum of 5 watts of power are required for each 133 mHz RAD750 computer

1 SCS750 high space-qualified super computers will be used to:

• Rover’s autonomous driving computer • Used to compute passable paths for rover to follow

* A maximum of 20 watts of power are required for each 800 mHz SCS750 computer

* Maximum of 35 watts processing for entire rover computer system

12/11/2008


Command and Control

SCS750 RAD750Path Commands

Range Maps

COMPUTING

IMU

Attitude and Acceleration

Optical Encoders

Angular Position

Motor Commands based off possible paths

Motor Controllers

• IMU, optical encoders, and Lidar sensors will provide computers with position information. • Computing will be programmed based off rover surface requirements. • Motor controllers will be updated based off computer processing.

LIDAR

Obstacle Ranging

12/11/2008


Mass Budget

Total Mass: ~723 kg (11% margin)

Mass [kg] Number Total [kg]

Wheel-Track System 152.6 1 152.6Large wheels 13.93 4 55.72

Small wheels 9.29 4 37.16

Arm 0.5 8 4Track 13.93 4 55.72

Suspension & Breaking Systems 50 1 50

Motors & Gears 360 1 360Motors & Gearing - drive 45 4 180

Motors & Gearing - arm control 45 4 180

Structure 90 1 90

Sensors 29 2 58

Cameras 3 2 6

Data management hardware 3 2 6

12/11/2008


ReliabilityReliability for Loss of Mission: 0.9930

Reliability for Loss of Crew: 0.9977Reliability Number Total [kg]

Wheel-Track System 0.9988 1 0.9988

Large wheels 0.9999 4 0.9996

Small wheels 0.9999 4 0.9996

Arm 0.9999 4 0.9996

Suspension & Breaking Systems 0.999 1 0.999

Structure 0.9999 1 0.9999

Reliability Number Total [kg]

Wheel-Track System 0.9960 1 0.9960

Track 0.999 4 0.9960

Motors & Gears* 0.9920 1 1.0000

Motors & Gearing - drive 0.999 4 0.9960

Motors & Gearing - arm control 0.999 4 0.9960

Sensors 0.999 2 0.999

Cameras 0.999 2 0.999

Data management hardware 0.999 2 0.999

Note that high reliability for extended periods requires performance of preventive maintenance and inspections between sorties

12/11/2008


Fault Tolerance• Drive motors and arm control motors provide redundancy

– They are cross-strapped. If one fails the other can operate both.

• Contingency operation possible after track malfunction using wheels

• Significant safety margin (minimum of 12%) in structural calculations

• Manual controls available in the event of a failure of the autonomous control system

12/11/2008


Earth Analog Considerations• Braking characteristics for 1-G trainer should be “tune”-able to emulate

braking conditions on moon… stopping distance on the moon is six times the stopping distance on Earth

• Turning radius of the 1-G trainer should be modified to emulate the turning radius of the TLR (you need a turn radius six times larger one the Moon than on Earth to maintain the same amount of lateral stability)

• Natural frequency for the suspension decreases… dcrit on the moon is ~5.5m as opposed to ~2m on Earth

• Rollover due to obstacle impact at velocity is lessened in 1-G… the 1-G trainer will have sensors to indicate if a driver’s technique would have resulted in rollover on the moon

• The 1-G trainer should be “equipped with removable seat pads which allow comfortable operation in a ‘shirt sleeve’ training session”

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Possible Improvements to TLR – Future Expansion Possibilities for the Mobility Unit –

• Each two track segment can be designed to operate as a single system – Need redundancy on power, mobility controls, and sensor systems. – Critical systems mentioned above needs to be supported between the two wheels

and not the capsule – Easy to attach/detach docking to the capsules is needed – No need for stabilization for flat terrain and certain slopes

• Possible Utilization: – Each two track system can mobilize independently to support different tasks – Two systems can pick up and drop capsules autonomously to support a lunar

base (no need for multiple capsules with dedicated rover capabilities) – The system can be used independently by astronauts in case of an emergency

* If certain units can be separated from the capsule, with a clever design such a vehicle can be created with little mass, power, and budget impact to what has already been designed.

Tracks as designed in this system

Critical systems separated from the capsule and packaged on the wheels. (power, mobility controls, sensors…)Simple platform to support

manned transport

Suspension as designed in this system

View From Top

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References• [Apollo] Lunar Roving Vehicle Operations Handbook. April 19,1971. • Traction Drive System Design Considerations for a Lunar Roving

Vehicles. November 25, 1969. • Digging and Pushing Lunar Regolith: Classical Soil Mechanics and

the Forces Needed for Excavation and Traction. Wilkinson and DeGennaro. September 7, 2006. • High Speed Craft Human Factors Engineering Design Guide. Human Sciences & Engineering Ltd. January 31, 2008. • Human Spaceflight: Mission Analysis and Design. Larson and

Pranke.

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