DESIGN AND DEVELOPMENT OF SHAPE

MEMORY ALLOY WIRE LINEAR ACTUATOR

Faculty Guide Presented by

Dr.P. Radhakrishnan Sivasankar M

12MI36

OBJECTIVE

• In my project the focus is to design a shape memory alloy wire

for the linear actuator and to develop the prototype of the

actuator model.

INTRODUCTION

• Smart materials for the engineering applications.

• This advances in technology reduces the size of the component

in compact reliable.

• Shape memory alloy is more versatile with other conventional

actuation.

• The most common shape memory is an alloy of nickel and

titanium called Nitinol.

• A shape memory element can be actuated thermally or

electrically.

SHAPE MEMORY ALLOY

• Shape memory alloys (SMAs) are smart materials that

respond with a change in shape, in other words recovery

of strain to change in external thermomechanical

conditions. The material exhibits two unique properties.

• Shape memory effect – the ability of SMAs to be

severely deformed and returned to their shape simply by

heating them.

• Superelasticity or pseudoelasticity.

SHAPE MEMORY EFFECT

• If a mechanical load is applied to the material in the twinned

martensite phase, it is possible to detwin the martensite by

reorienting a certain number of variants.

• The detwinning process results in a macroscopic shape chage,

where the deformed configuration is retained when the load is

released.

• A subsequent heating of the SMA to temperature above Af will

result in a reverse phase transformation and will lead to shape

recovery.

SHAPE MEMORY EFFECT OF SMAs UNDER

THE APPLICATION OF LOAD

DESIGN OF SHAPE MEMORY ALLOY WIRE ACTUATOR

• In our configuration the SMA wire works against a constant force.

• If a dead weight is suspended by a shape memory wire, at low

temperature the wire will be deformed to a length L1.

• When the wire is heated above the Af temperature it recovers to

length L2.

COMPUTATION ALGORITHM

• The most important relations involved for the SMA wire actuator design are:

Wire Diameter:

where, G = weight, in [N]

= maximum design stress at high temperature, in [MPa]

High temperature strain:

Where Eh is the value of the young’s modulus for the material at high

temperature, in [MPa];

• Required (free) length:

Where is the low temperature strain.

• The length increment needed at Af temperature to

produce the force G:

Length increment = Lf [mm]

• High temperature length:

Lh = Lf + Length increment [mm]

• Low temperature length:

Ll = Lh + stroke [mm]

[mm]

• Low temperature stress:

Where E1 is the value of the young’s modulus for the

material at low temperature, in [MPa]

• Reset force:

R = [N]

[MPa]

• For design calculations, wire diameter and length are found by

constraining the maximum tensile stress (at high temperature) and

strain ( at low temperature) .

SMA Wire Design:

• Wire Diameter [mm] = 0.5

• Low temperature tensile stress [MPa] = 138

• Required length [mm] = 60

Biasing Spring Design:

Bias rate:

Kb = (Fh – Fl)/ Stroke [N/mm]

Fh = Force exerted by the biasing spring at hight

temperature

Fl = Force exerted by the biasing spring at low temperature

Bias Wire Diameter:

db = [mm]

Db = Average bias spring diameter

T= Maximum bias shear stress

• Number of bias active turns:

n =

G = steel spring shear modulus.

• Using standard steel spring design procedure,

assume that the maximum shear stress for the wire is T = 675MPa. The bias spring shear modulus is G = 79300 MPa.

Parameter Value

Number of Active Coils, n 4

Diameter of Wire, d 0.5mm

Mean Diameter of the Spring, D 9mm

Optimized Parameter of the Biased Spring

DESIGN APPROACH

• The design and develop the prototype of the SMA wire linear

actuator.

• This prototype mechanism utilizes SMA wire to actuate a

high force.

• It is self-resetable using a bias return spring.

• The current heats the wire directly via resistance in the wire.

• Through a strain recovery process in the material, the wire

contracts when heated and returns when cooled using the

compression bias spring.

ASSEMBLED MODEL

FUNCTIONAL CHARACTERIZATION OF THE

SMA ACTUATOR

• Force – Deflection characteristics of the SMA wire.

An experiment was conducted to plot the force-deflection

characteristics of SMA Wire to determine the maximum force

and deflection that can be obtained from the wire. The

experimental setup is illustrated.

FORCE – DEFLECTION CHARACTERISTICS OF THE

0.5MM DIA SMA WIRE

Current (Amps) Temp(°C) Stroke (mm) Load (N)

3 65 18 5

3 65 10 10

3 65 3 20

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25

De

fle

ctio

n(m

m)

Force(N)

Force-Deflection

Force-Deflection

TEMPERATURE -DEFLECTION

CHARACTERISTICS

• The deflection of the SMA Wire at various temperatures is an

important factor in the design of the actuator since it depicts the

actuation of the actuator. The SMA spring actuator is actuated

by passing electric voltage through the wire. The deflection at

various temperatures were noted.

• The deflection of the SMA wire at various temperatures were

experimentally determined with the setup as shown.

TEMPERATURE – DEFLECTION

CHARACTERISTICS OF 0.5MM DIA SMA WIRE

Current(Amps) Temperature(°C) Deflection(mm)

1 40 5

2 50 10

3 60 13

4 70 16

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 60 70 80

De

fle

ctio

n(m

m)

Temperature(°C)

Temperature-Deflection

Temperature-Deflection

HEATING AND COOLING TIME OF THE SMA

WIRE

• The thermodynamics of the heating and cooling cycles is a critical point in the

design, both for the power consumption and for the frequency of the activation

cycles. Hence, the heat transfer is considered to be determined by the heat transfer

coefficient. The heat transfer coefficient of the SMA wire is calculated using the

equation given below:

Where,

h – Heat transfer coefficient (W/mm2 0C)

I – Input current (A)

ρ – Resistivity of the SMA wire (ohm-mm)

r – Radius of the SMA wire (mm)

T – Target temperature (0C)

Ta – Ambient temperature (0C)

)(322

2

aTTr

Ih

Cont.

Condition Current

(A)

HTC

(W/mm2 0C)

Power

(Watts) Heating Time(s)

Cooling

Time(s)

1 4 3.80E-04 2.38 7 8.2

DIFFERENTIAL SCANNING CALORIMETER

TEST

• The DSC test was carried out to determine the austenite finish

temperature of the SMA material that was used to fabricate the

SMA actuator.

PROTOTYPE OF THE ACTUATOR

• The prototype of the SMA wire linear actuator has been

fabricated as per the design concept.

CONCLUSION

• The current work dealt with the design and development of a

SMA wire linear actuator.

• The maximum force-deflection required by the SMA wire to be

actuated is found.

• The Temperature-Deflection characteristics of the SMA wire is

determined.

• The Heating and cooling time of the SMA wire used in the

actuator is found.

• The final Prototype of the actuator model has been fabricated.

FUTURE WORK

• The primary research can be in the reduction of the actuation

time of the actuator and increase the frequency of operation of

the actuator.

• Proper heating and cooling methods can be introduced in the

actuator to control the actuation time of the actuator.

• In order to achieve this, electronic control of the valve can be

introduced to prevent the overheating of the actuator.

• These parameters improvement may leads to the prototype of

the actuator into the final product.

REFERENCES

[1] Sonia DEGERATU, Horia O. MANOLEA, Nicu G. BIZDOACĂ, Gheorghe

MANOLEA, Anca PETRIŞOR, 2009, “Characterization And Design Of A

Shape Memory Alloy Wire Actuator”, 7th International Conference On

Electromechanical And Power Systems.

[2] Sonia Degeratu, Nicu G. Bizdoaca, Gheorghe Manolea, Ilie Diaconu, Anca

Petrisor, Vasile Degeratu,2008, “On the Design of a Shape Memory Alloy

Spring Actuator Using Thermal Analysis” , WSEAS Transactions on Systems,

Vol. 7, pp. 1006-1015.

[3] Adelaide Nespoli, Stefano Besseghini, Simone Pittaccio, Elena Villa, Stefano

Viscuso, 2010, “The high potential of shape memory alloys in developing

miniature mechanical devices: A review on shape memory alloy mini-

actuators”, Sensors and Actuators A, Vol. 158, pp. 149-160.