A Method of Robotic Actuation using Control Moment Gyros

AIAA YPSE Conference21 November 2008

A Method of Robotic Actuation using Control Moment Gyros

Presenter: Ian Livingston

AIAA YPSE ‘08

November, 21st 2008


Overview

Outline:-Introduction-What is a CMG-Reactionless Actuation-Joint Torques vs. Body Torques-Planar Robot Design-Simulation Results-Experimental Design-Hardware and GUI-Controller Design-CL Gimbal Control-Feed Forward Control-Video-Future Plans-Acknowledgements-Questions

• Introduction• What is a CMG• Reactionless Actuation• Joint Torques vs. Body Torques• Planar Robot Design• Simulation Results• Experimental Design (Functional Flow)• Hardware and GUI• Controller Design• Closed Loop Gimbal Control• Feed Forward Joint Angle Control• Video• Future Plans• Acknowledgements• Questions


Control Moment Gyros (CMGs)


Components:•Constant speed reaction wheel•Gimbal motor (positioned along g axis)

Applications:•Momentum Storage•Propellant-less Attitude Control

Advantages:•Low Power (100x)1

•Spaceflight heritage

1. Carpenter, Peck 2008

Equation for torque output

Equation for reaction wheel momentumhr = Irw·rw


Reactionless Actuation


CMG Arm from previous project team on the “Vomit Comet”

Reaction Forces from Mechanism

•Actuator Reaction Forces•Caused by direct-drive motors. To rotate an object the motor rotates the base object in the opposite direction.

•Inertial Reaction Forces•Caused by a spinning object not at the center of mass.

Advantages to Reaction-Less Actuation:

•Isolate subsystems

•Reduce dumped momentum to the basestructure.


Joint Torques Versus Body Torques


R21

R2 1

Joint Torques Body Torques

motor cmg

R

R2

•CMGs create body torques

•The motion of the arm remains the same

•Only the torque caused by the off center reaction force needs to be absorbed using body torques.

R2


Planar Robot Arm Design


Purpose:To create a CMG robot arm to demonstrate the advantages of body torques on space applications.

Design:

•Two or more link free floating CMG arms

•Air bearings to provide frictionless surface

•Use Scissored Pair CMGs to remove Gyroscopic effects and off axis torques

Previous work and Motivation:•Past teams built the reaction wheels for a CMG arm test

•Only open-loop results were obtained from zero-g tests

•This experiment was created to perform tests anytime

Photo: A. Soto


Scissored Pair CMGs


Advantages:•Eliminate off-axis torques

•Direction of torque is fixed

•No internal singularities

•Cancelation of unwanted gyroscopic effects

A A/G G /A N G /A A/N

G /A G /AA/N A/N

ˆˆ

ˆ

i igi ri

i

ri

i

i igi ri

ω ω ω

ω

I I ω gτ g

ω gI I ω ω h

Video: M. Peck


Simulation Analysis and Hypothesis1


Joint torque advantage

CMG advantage

From power simulation results:

•CMGs are better for reaching tasks

•Joint torques better for panning tasks

•Simulation to determine power properties of CMG arm•Utilized several set ups for links and properties•Main focus on one link and two link robot arms both in parallel axis and perpendicular joint axes.•Simulation assumes 0 initial and final velocities and accelerations.

D. Brown, 2008


Experimental Design (Functional Flow)

MATLAB GUI

Joint Angle

Start Control

MATLAB code

Wireless Belkin

Transceiver

Data, Graphs

Commands

Data

Computer

Control Voltage

PotentiometerData

Wireless Link

Wireless Belkin USB

Hub

CMG Robot Arm

USB DAQ Board

Reaction Wheels (2x)

Reaction Wheel

Speed Controller

Power Supply

Motor Controller

Gimbal Potentiometer

Joint Potentiometer

Gimbal MotorVoltsVolts

Gimbal Rotation

Volts

Volts

Power Supply

Power Supply

Power Supply Power Supply

Data



Hardware and MATLAB GUI Design



Controller Design


•Gimbal Angle is controlled by a PID closed loop control

•Joint Angle is controlled using a Simulink model with feed forward control


CL Gimbal Angle Control Analysis


Equations of motion for gimbal motor:

Transfer function for gimbal motor:

Root locus, Bode Plots and gains:

Gains:Kp= 3Ki = 1Kd = .1

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1-1.5

-1

-0.5

0

0.5

1

1.5Root Locus

Real Axis

Imag

inar

y A

xis

-150

-100

-50

0

50

100

Mag

nitu

de (

dB)

10-1

100

101

102

103

104

105

106

-180

-135

-90

-45

0

Pha

se (

deg)

Bode DiagramGm = Inf dB (at Inf rad/sec) , Pm = 20.7 deg (at 1.1 rad/sec)

Frequency (rad/sec)


CL Gimbal Angle Control Results


2 4 6 8 10 12 14 16 18 20-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6Linear Simulation Results

Time (sec)

Am

plitu

de


Feed-forward Control Design


•System is non-linear•Small angle approximation was not assumed since can rotate up to 70°

Relationship between Joint angle and gimbal angle:

Current issues with Feed-Forward:•The runtime of the feed forward Simulink block is approximately .11 seconds. •This would limit the control system by 10 data points•At maximum voltage this is equivalent to 30 degree rotation


Video of CMG robot arm



Future task, test, and Goals


Current Issues:•Joint angle control is not fully functional•No support for multi-links•Air canisters are currently leaking

Planned Tests:•Full systems test using feed forward control•Evaluation of power consumption versus joint angle control

Identified Improvements:•Characterize noise in potentiometer and build LQG control•Replace existing reaction wheels with smaller and faster COTS wheels•Reduce the size of the supporting arm and structure•Remove chain and replace with gears


Acknowledgements


•Daniel Brown, PhD. Candidate, Aerospace

•Dr. Mason Peck

•Space Systems Design Studio

•Cornell CMG team

1. DARPA SUMO spacecraft with CMG arms


Questions?

A Method of Robotic Actuation using Control Moment Gyros

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