Precision Pointing in Space Using Arrays

of Shape Memory Based Linear Actuators

Nikhil SonawaneDr. Jekanthan Thangavelautham

Dr. Huei-Ping HuangDr. Kiran Solanki

1

Motivation

Galileo probe, Io and Jupiter MarCO,first interplanetary cubesat mission [1]

Interplanetary space exploration at low cost[1] Source: NASA Jet Propulsion Laboratory

Challenges Space structures increase in surface

area for deep space applications. Ex. Communication antennas Solar Panels : Space telescopes

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22max /4 cAfG Antenna gain :

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Objectives

Developing highly efficient (weight bearing, low power) mechanical systems at low cost

Mechanism need to be free of mechanical complexity and simple in operation with low mass

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Pointing Mechanisms: Current Technologies

Antenna pointing mechanism2 axis gimbal based KARMA 4 with twist capsule and

Slip ring

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Configuration of Antenna Pointing Mechanisms –

Principle of APM using Gimbals

Antenna Pointing Plate

Satellite Structure Interface

Interactive G35 Gimbal Model• Large angle deflection• Widely used for large satellites• Heavy in weight• Expensive

Current Technologies

1. Gimbal Based

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Antenna mounting plate

Screw jack mechanism and stepping motor assembly

Base plate ( spacecraft main structure )

Flexural pivot

• Very light weight• Used for very precise fine pointing• Mechanically complex• Low weight bearing Fine pointing assembly

Current Technologies2. Pivot Based

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Current and Proposed Technology

 

APM technolo

gy Power Mass Cost

Specific work ratio

Operating temperature

Gimbal based 2

axisStepper motor 3.9 W 2.7 kg high low -40 to 60 °C

Gimbal based 1

axisStepper motor 27.4 W 1.8 kg high low -50 to 105 °C

KARMA5

stepper motor 25 W  10 kg high low -75 to 170 °C

SADMstepper motor 5 W 5 kg high low -45 to 75 °C

Proposed APM

SMA based 4 W 0.5 kg low high -100 to 300 °C

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Research Tasks• Demonstrate laboratory bench-top scale

linear actuation using the contraction of SMA structures.

• Demonstrate SMA based pointing mechanism on lab scale.

• Demonstrate a mechanism for efficient latching and position retention to a pre-determined level of accuracy.

• Develop a system level concept design for implementation on small satellites.

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Shape Memory Alloy Concept

Shape Memory Effect

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Shape Memory Alloy Concept

Superelastic Effect

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SMA based Linear actuators

Actuation of SMA wire

Applications -

. MigaOneTM Linear actuator Solar Paddle Actuator

Latching Mechanism

The SMA wire in deactivated position. (Extended position)The SMA wire in activated position. (Contracted position)

Applications -

• Frangibolt

• Deployment of Solar panels,antennas

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Methodology

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Material Selection

• For linear actuation purposes, the best possible configurations used widely are springs and straight wires for their ability to support large and heavy objects.

• The decision of down-selection to the SMA wire and spring was based on ease of commercial availability

wire /spring

wire diameter

pulling capacity

0.38 mm 2250 g0.51 mm 3560 g0.51 mm 243g

wire diamete

rpulling capacity

0.050 mm 36 g0.076 mm 80 g0.010 mm 143 g

Properties for linear actuation mechanismProperties for latching wire

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Technical characteristics for SMA wires

Maximum pulling force, Bias force vs Wire Diameter

Technical characteristics for SMA wires

Current vs wire diameter

Technical characteristics for SMA wires

Cooling time vs wire diameter

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Characterization of SMA spring behavior

Deflection study of SMA springs under varying load to know about its actuation abilities as it is heated.

SMA spring deactivated SMA spring activated

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Characterization of SMA spring behavior

Experimental Setup

Measuring Scale

DC Power source

SMA spring

Weights

Camera

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Characterization of SMA spring behavior

Experimental Setup

• Camera arrangement to avoid parallax

Reference for readings

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Characterization of SMA wire behavior

Deflection study of SMA wires under varying load to know about its actuation abilities as it is heated.

SMA wire deactivated SMA wire activated

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Characterization of SMA wire behavior

Experimental Setup

Measuring Scale

DC Power source

SMA straight wire

Weights

Camera

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Characterization of SMA wire behavior

Experimental Setup

• Camera arrangement to avoid parallax

Reference for readings

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Mechanism Selection• SMA wires consume lesser power for the same amount of pulling force when compared to SMA springs. • The amount of pulling force, limits the applicability of SMA springs.

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Mechanism Selection

• Simple lever type was chosen

• Feasibility of manufacturing and available stroke is high

• Provides mechanical advantage to gain higher deflection.

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Design Parameters Setup

Size of the SMA

Resistance per unit length

The bias force when cool

The maximum breaking force

The cooling rate (small wires cool faster than the large ones)

Minimum bending radius

The pulling force when heated

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Design Parameters Setup

• Temperature

• Initial Shape setting

• Stroke

• Environmental conditions/ cooling conditions

• Stress/Strain

• Cycle rate

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Design Parameters Setup• Bias force

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Strength of multiple SMA wire bundles

• If tradeoff between size vs number of wires is done, we can achieve higher pulling force with minimum power

Wire size (μm)

Quantity

Total lift (kg)

Total power (Watt)

50 10 0.35 150 50 1.75 650 100 3.5 1350 250 8.75 32

Wire size (μm)

Quantity

Total lift (kg)

Total power (Watt)

100 10 1.5 5100 50 7.5 24100 100 15 49100 250 37.5 122

Wire size (μm)

Quantity

Total lift (kg)

Total power (Watt)

150 10 3.3 8150 50 16.5 40150 100 33 80150 250 82.5 200

Wire size (μm)

Quantity

Total lift (kg)

Total power (Watt)

250 10 9.3 20250 50 46.5 100250 100 93 200250 250 232.5 500

2

Parabolic dish antenna

Ball and socket joints

Vertical ratcheted armLatching mechanism

Mechanical lever

Slotted cylinder

Constant force spring

SMA wire

SMA wire stand

Inner cylinder

Prototype Design

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• SMA wire contracts

• Mechanical lever forces itself to perform circular motion

• Circular motion translated to linear motion through inner cylinder in turn connected to ratcheted vertical arms.

Prototype Design

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Prototype Design• Antenna movement modulated by 3 control points

• Elevation controlled by upward/ downward movement of vertical ratcheted arms

• Downward motion stopped by latching mechanism at desired angle

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Mechanism Design for Linear Actuation

• Mechanical lever

Calculating for 45° tilt angle –

From characterization curve, for 20 cm wireAt 900 g , deflection observed = 0.07 cmAssuming a = 1, the new angle is 44.43

Therefore using 20 cm wire for prototype.

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Placement of vertical ratcheted arms -

Schematic for movement of antenna by vertical ratcheted arms

• The term Y is the perpendicular distance between vertical arms as seen from top. From trigonometry value of Y = 7 cm

• From trigonometry, The value of Z = 8.08 cm

Mechanism Design for Linear Actuation

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Mechanism Design for Latching

• The 7cm vertical distance is divided by 45 degrees to have ratchets at a distance 0.155 cm which is equivalent to one degree.

• The SMA wire length was chosen based on the construction of the assembly for the linear actuator

• Right angle pull mechanism for 3-4 % contraction.

0.4 cm

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Results and Discussions

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Experiments were conducted to determine range of downward speed of the vertical arm.

Determination of wire size for latching mechanism

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Determination of wire size for latching mechanism

• Based on the results, calculating the time of passing 1 teeth of ratcheted actuator

• The SMA wire with cooling time less than 0.3 secs has to be chosen.

• SMA wire with 0.050 mm with cycle time 0.7 secs is selected for the application,

• The decision also takes into consideration the availability of bias spring available commercially.

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System Diagram

Antenna PointingMechanism

8 Channel RelayModule

DC powersource

Arduino UnoMicrocontrollerboard

Commandinterface

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• Complete system prototype for experimental setup

Experimental Setup

Pictorial representation for 1 cycle -

• Unlatching the ratcheted actuator

Latched Unlatched

Experimental Setup

Experimental Setup

• The vertical ratcheted actuator is activated the moment it gets unlatched and moves up due to the contraction of the SMA wire

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• The normal angle made by the antenna drops as the wire cools down

• Bias force mechanism pulls the actuator assembly towards its direction.

Bias force springs pulling the actuator

Experimental Setup

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• As per the command the latching mechanism gets activated • Locks the movement of vertical actuator.

Unlatched Latched

Video Demonstration

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System and Subsystem Repeatability

Experiments were performed to analyze the repeatability of the latching mechanism for varyinglatch time delays, current being provided to the wire for 1 second.

Sources of error –

• Design and Manufacturing errors

• Heating/Cooling of SMA wire

• Positioning/working of the constant force spring

System repeatability

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System and Subsystem Repeatability

Latching mechanism

• Error free operation

• Simple system in terms construction

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System and Subsystem Repeatability

Linear Actuation mechanism

• Is the error determining subsystem.

• Sources of error being same as system level

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Performance Analysis

Cumulative degrees achieved vs time for arm 1

• The linear movement of the 3 actuators was studied under the payload of a dummy antenna• Angles achieved were compared to determine the performance of each. • The amount of time for which current was provided was varied in each case.

• For Actuator 1

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Performance Analysis• For Actuator 2

Cumulative degrees achieved vs time for arm 2

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Performance Analysis• For Actuator 3

Cumulative degrees achieved vs time for arm 3

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Performance AnalysisSuperimposed graph for all arms

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Performance AnalysisAnalyzing derivative curve for one of the arms -

• SMA’s work on the joule’s effect.

Heat generated = I²Rt

• I is current through SMA, R represents wire resistance, t is the time the current is passed

• Sudden rise due to SMA reaching its Transition temperature range

Mission Concept

Antenna Pointing Mechanism

3-axis Attitude control system

Power SystemComputer Board

S Band Radio

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2

Concept of Operation

Contributions1. SMA  based antenna pointing mechanism has been developed.

2. Angular tip/tilt of 45 degrees accurate to 1 degree has been achieved.

3. Latching mechanism using SMA’s has been developed.

Conclusions The  developed mechanism demonstrates potential of linear

actuation of SMA’s to be used for pointing mechanism of antennas in space with steps of 1 degree and error of ±2 degrees.

The angle of tilt achievable from the experiments and calculations are verified to be same. i.e. 20 degrees.

Laboratory bench-top demonstration of latching using SMA wires achieved without error in operation.

The error propagation in the linear actuation system due to manufacturing and design errors was observed leading to uncertainty in achieving precise control of APM.

Future Work Development and demonstration of active feedback

control system. Thermal modelling of the system and study of

temperature profile for SMA’s. Studies and methods to achieve tip/tilt angle higher

than 45°. Modifications in design and use of precise

manufacturing techniques for accuracy up to 0.1 degrees.

Design studies towards optimizing high strain SMA stowage for maximum packing efficiency.

Development of space worthy system.

Thank you !

39

Determination of wire size for latching mechanism

Based on the results, calculating the time of passing 1 teeth of ratcheted actuator is given by –

Distance between each teeth – 0.155 cm

Maximum speed of the downward motion – 0.4 cm/sec

Time for passing 1 teeth (i.e. 1 degree equivalent distance) = 0.3875 secs

Thus, the SMA wire with cooling time less than 0.3 secs has to be chosen.

SMA wire with 0.002 inches with cycle time 0.7 secs is selected for the application, the decision also takes into consideration the availability of bias spring available commercially.