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DESIGN, SIMULATION AND TESTING OF SHRIMP ROVER USING RECURDYN

Shivesh Kumar, Raghavendra S, Mihir R Bhagat, Gangadharan K V,Department of Mechanical Engineering

12th Symposium on Advance Space Technologies in Robotics and Automation,ESA / ESTEC, Noordwijk, The Nethelands

NATIONAL INSTITUTE OFTECHNOLOGY KARNATAKA

Surathkal, INDIA

CONTENT

• Introduction- Shrimp

- RecurDyn

• Work in RecurDyn- Modeling of Shrimp

- Dynamic simulation on various terrain profiles

• Shrimp: Manufacturing• Experimental Validation• Results & Discussions• Conclusions

ASTRA 2013: ESA / ESTEC, Noordwijk, The Netherlands

INTRODUCTION

• Rovers: Most suitable for planetary exploration

• Over 300 design concepts developed already

• Shifting trends: Long-term, reusable rovers

• Fewer solo-missions. Regular inclusion of a greater bigger objective.

Rover deliverables increase:

• Long range• Greater mobility• Low power consumption• High modularity• Ease of control


SHRIMP ROVER

• Innovative 6 wheeled rover

• Design at the Swiss Federal Institute of Technology (EPFL), Lausanne

• Design objective: Long-range mission for Martian exploration

• Fully functional prototype was demonstrated at ESTEC (2000)

Image: EPFL


SHRIMP ROVER: KEY FEATURES

• 6 motorized wheels

• 2 parallel bogies, front and rear forks

• Passive control

• Excellent mobility:

- Obstacle climbing - 2 x wheel dia.

- Topple resistance - Upto 40 degrees

- Pure steering possible

Image: EPFL

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RECURDYN

• Multi-body dynamics software

• Advantages over other softwares:

- No more excessive simplification

- High solving efficiency

- Good solving stability

• Equations of motion theory in recursive formation

• High precision, fast solving.

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MODELING OF SHRIMP

• Initial design based on SHRIMP III by BlueBotics (www.bluebotics.com)

• Design made in CATIA V5

• Imported to RecurDyn and joints defined

Type of JointType of Joint No. of Joints

Type of Actuation

Revolute joints

Between body and wheels

6 Active

Revolute joints

In front fork 4+1 4 Passive, 1 Active

Revolute joints In rear fork 1 Active

Revolute joints

In right parallel bogie

6 Passive

Revolute joints

In left parallel bogie

6 Passive

Other parameters defined as well:

• Wheel speed: 30 rpm

• Friction: μstatic = 0.5 μdynamic = 0.3

• Tire and road surface properties:

• Spring coeff. = 1000 N.mm

• Damping coeff. = 1

• Damping friction coefficient = 0.9


http://www.bluebotics.com

http://www.bluebotics.com

DYNAMIC SIMULATION

• Kinematic and Dynamic simulation done using the multi-body dynamics solver of RecurDyn

• Simulation time and time steps chosen suitably

• The rover capabilities are tested on 3 types of terrains:

- Ability to climb steps

- Ability to climb a inclined surface

- Ability to adapt passively with concave/convex terrains


CLIMBING STEPS

• Obstacle: Step

• Step size: 200 mm

• Under review: Torque requirements of the wheels


CLIMBING STEPSFRONT WHEEL

REAR WHEEL


CLIMBING INCLINATION

• Obstacle: Slope

• Slope inclination: 40 degree

• Under review: Torque requirements of the wheels


CLIMBING INCLINATIONFRONT WHEEL

REAR WHEEL


CURVED SURFACE ADAPTABILITY

• Shrimp makes use of a passive suspension system

• Its is tested to check its adaptability to curved surfaces.

• Ex.: Concave


TORQUE ANALYSIS

Wheel-Bogie Joint Position Max Torque (Kg-cm)

Front Fork 32.1

Right Front 7.5

Right Rear 7.4

Left Front 7.0

Left Rear 6.8

Rear Fork 16.3

• Maximum torque required is close to 32.1 kg-cm

• Factor of Safety: 1.25

• Hence, Torque requirement becomes 40.1 kg-cm

• Available option: 45 kg-cm motors.


SHRIMP: MANUFACTURING

• Major Parts

- Main body

- Parallel bogies

- Front & Rear fork

- Electronics sub-system

• Built in-house at NITK

• Undergraduate project: Resources and facilities greatly constrained.


MAIN BODY

• Twin Alumnium pentagons form the base

• Supporting aluminium blocks:

- To join the two plates

- Support the load

- Mounting point for revolute joints

• Bearings used for revolute joints

• Similar setup for the front fork


PARALLEL BOGIE

• Aluminium bogies mounted on either side of the rover

• Frames of C-section links that form a couple

• Mounted on freely rotating central pivot

• NOTE: Left and right bogie should be greatly identical to avoid mismatch in travel


FRONT & REAR FORKS

• Wheel mounted at the forks

• Front fork is similar to a 4 bar mechanism; wheel made to travel upward when it encounters obstacle

• Steering is achieved using servos whose axis is perpendicular to the ground and the axis of wheels


ELECTRONICSKey Features

• 2 parts: Base station & On-board control system

• Completely wireless control and data transmission using Xbee module

• Wireless video feed

• 2 ATMEGA16 processors in master slave configuration

• Light and portable LiPo battery along with battery protection circuit

• Interactive GUI using Matlab

User

MATLAB based GUI

Intel Core 2 DuoProcessor (2.4 GHz)

based PC

Sensor DataLive Video feed

Rover Control Commands

Wireless Xbee Module (Transmitter)

USB TV

Tuner

RFReceiver

A/V feed FTDI based USB - UART bridge

Wireless Xbee Module (Receiver)

Atmega 16 based developer board (AVRiboard2.0)

RFTransmitter

AVR-iboard

2.0

Master Controller Slave Controller

3 units of:8-28 V, 5A dual DC motor

drivers with current sensing

Right Bogie

Left Bogie

Front & Rear Fork

Front & Rear Fork’sServo for Steering

Servo Pod2 servos

SHARP IR Range Sensor

Wireless Video Camera

BASE STATION

ON-BOARD CONTROL SYSTEM



EXPERIMENTAL VALIDATION

Process

• Make the rover traverse obstacle paths to prove capabilities

• Measure armature current drawn by the motors and thus, estimate the driving torque requirements

• Compare Driving Torque v/s Time graphs with those of RecurDyn

Obstacles

• Step Test:8cm height

• Slope Test:30 degree inclination


MEASURING THE TORQUEHow to calculate torque?

τ = Kt × I



THE EXPERIMENTS

VALIDATION RESULTS

• Step Test

• Front Wheel

• Mean Absolute Percentage Error (MAPE) = 35.62%

• Slope Test

• Rear Wheel

• Mean Absolute Percentage Error (MAPE) = 42.87%


JUSTIFICATION FOR ERRORS

MAPE typically varied between 35 - 45 % for all cases.

• Effects of electrical DC drive model not included in our simulation. Non-linearities like BEMF voltage, friction between bearings, etc affect the system

• Incorrect data / estimation used in modeling: friction in revolute joints, contact friction, etc

• Mismatch between modeled and actual mass-inertia properties

• Simplified model: Nuts, bolts, clamps, etc not modeled


CONCLUSIONS

• Modeling & dynamic simulation of Shrimp through RecurDyn

• Virtual testing of rover on different terrains

• RecurDyn helped in selecting actuators for given payload requirements

• Experimental validation gave satisfactory results

• RecurDyn can be further exploited by better handling of problem at hand


ACKNOWLEDGEMENTS

Mr. B Sridhar, Director of Function Dynamics India Pvt. Ltd.

for issuing provisional licenses of RecurDyn for our project.

NITK Alumni Associationfor providing a soft loan to attend this conference.


THANK YOU!


Please contact:[email protected]