ABCM Symposium Series in Mechatronics - Vol. 2 - pp.409-416 Copyright © 2006 by ABCM
Proceedings of COBEM 2005 Copyright © 2005 by ABCM
18th International Congress of Mechanical Engineering November 6-11, 2005, Ouro Preto, MG
ELECTRICAL ACTIVATION UNDER CONSTANT LOAD OF Ti-Ni AND Cu-Zn-Al SMA WIRE ACTUATORS Carlos José de Araújo Department of Mechanical Engineering Universidade Federal de Campina Grande Caixa Postal: 10069 Cep: 58109-970 Campina Grande – PB
[email protected]
Igor Silva Teixeira de Lima Department of Mechanical Engineering Universidade Federal de Campina Grande Caixa Postal: 10069 Cep: 58109-970 Campina Grande – PB
[email protected]
Abstract: Electrical actuation cycles in shape memory alloy (SMA) wires working against a dead weight (isotonic actuation) have been performed on different wire actuators. Ti-Ni, Ti-Ni-Cu and Cu-Zn-Al SMA wires were studied by means of an experimental set-up specially designed to perform these electromechanical cycles between 10 and 200 MPa. The obtained electrical current versus strain loops as a function of the applied stresses are used to determine important parameters for these actuators, like transformation currents, current hysteresis, electrical current slopes and shape memory effect (SME) under load. Results concerning the performance of the actuators are discussed on the basis of the particularities of each studied SMA. Keywords: SMA actuators; Smart materials; Electrical activation; Shape memory alloys.
1.
Introduction
At present, the materials most attractive for electromechanical force transduction are piezoceramics, piezopolymers, magnetostrictive materials , and shape memory alloys (Fletcher, 1996). Shape memory alloys (SMA) are one of the active materials more promising for use as artificial muscles in robotics and smart structures (Srinivasan and McFarland, 2001). SMA wires are linear actuators that can contract and elongate as a function of temperature due to a reversible solid state phase transformation. The displacement originated by this thermoelastic martensitic transformation is non-linear and present a temperature hysteresis (Otsuka and Wayman, 1998). If SMA are activated by resistive electrical heating, these materials are naturally considered as electrical actuators. In general, actuator motion is controlled as a function of temperature. However, for thin SMA wires temperature is difficult to measure and must be estimated from the electrical current using the static thermal equilibrium equation for the steady-state (Nascimento et al, 2004). For these actuators, experimental determination of displacement versus temperature loops is fundamental for the design of smart structures incorporating them. On the other hand, it can be sometimes important to verify the displacement versus current behavior instead of displacement – temperature. In this work, Ti-Ni and Cu-Zn-Al SMA wire actuators were studied by means of an experimental set-up specially designed to perform electromechanical cycles between 10 and 200 MPa. The obtained electrical current versus displacement loops as a function of the applied stresses are used to determine important parameters for these actuators, like transformation currents, current hysteresis, electrical current slopes and shape memory effect (SME) under load.
2.
Experimental procedure
Table 1 show the several types of SMA wire actuators used in this study. As-received SMA wires can present or not the shape memory effect (SME) phenomenon. For wires supplied in the cold-drawing state, SME is not initially verified and a heat treatment is necessary to release the martensitic transformation, as indicated in Tab. 1.
Table 1 – General data of the SMA wires. Specimens
Alloy
TN1 TN2
Ti-Ni Ti-Ni
Diameter (mm) 0.20 0.37
TN3
Ti-Ni
0.29
TNC
Ti-Ni-5%Cu
0.29
CZA
Cu-Zn-A l
0.50
Supplier Dynalloy (USA) AMT Inc. (Switzerland) Memo ry-Metalle (Germany) AMT Inc. (Switzerland) Tréfimetaux (France)
Code or market name FlexinolTM ------
Shape memory effect yes yes
Heat treatment -----------
Alloy H
no
350o C/20min
7326 HT
yes
------
------
no
650o C/15min water quench
The SMA wire actuators were tested using a platform specially designed for this task, as illustrated in Fig. 1. In this platform, wire sample is loaded in a uniaxial constant tensile stress mode while electrical heating – cooling cycles is performed using an Agilent model E3633A DC power supply. Contraction and elongation of the SMA wire actuator is continuously measured by a Solartron model DC miniature DF5 LVDT displacement sensor. All samples had lengths between 95 and 100mm were tested under constant load (dead weight) corresponding to the stress range 10 – 200MPa. To stabilize shape memory properties, each sample of SMA wires shown in Tab. 1 was trained under 150MPa during 100 cycles. Figure 2 show the contraction displacement by shape memory effect (SME) of the TN2 specimen. For the first cycle, SMA wire is in the low temp erature position (LTP) corresponding to zero electrical current (i = 0) and the application of a current pulse (i > 0) leads sample to the high temperature position (HTP) by SME contraction, as indicated in Fig. 2. The electrical current is determined experimentally depending of the type of SMA and its diameter.
(a) Fixed grip Frame SMA wire LVDT sensor Mobile grip
Dead weight
Figure 1 – Experimental test bench. (a) Design. (b) Laboratory construction.
(b)
Proceedings of COBEM 2005 Copyright © 2005 by ABCM
18th International Congress of Mechanical Engineering November 6-11, 2005, Ouro Preto, MG
4,0
σ=150MPa
3,5
HTP
Displacement (mm)
3,0 2,5 2,0
i=0
i>0
1,5
SME Displacement
HTP 1,0
SME
LTP 0,5 0,0
LTP
-0,5 0
10
20
30
40
50
60
70
80
90
100
Number of cycles
Figure 2 – Training of the SMA wire actuators. TN2 specimen.
3.
Results and discussions
Figure 3 show the electromechanical behavior of some SMA wire actuators for three different applied loads. These curves reveal some changes occurring in the current - displacement loops as a function of the applied stress as well as the influence of the phase transformation on the current – tension (electrical resistance) behavior during electrical heating and cooling. Improvement of the SME displacement, increasing of electrical currents and reduction of current hysteresis as a function of applied stresses can be observed in Fig. 3(a) for the TN1 SMA wire actuator. The current – tension relationship verified in Fig. 3(a) show three regions corresponding to martensite phase (region I), phase transformation (region II) and austenite phase (region III). Figures 3(b) and 3(c) confirms similar behaviors for TNC and TN2 SMA wire actuators. However, Fig. 3(d) show that the cooper based SMA wire actuator (CZA) has a poor displacement by SME, which is associated with a limited phase transformation as can be observed from its current – tension behavior. For this specimen, the three characteristic regions shown in Fig. 3(a) and detected in all Ti-Ni SMA wires are not present. Some important shape memory parameters like transformation currents, current hysteresis, electrical current slopes and shape memory effect under load can be obtained from Fig. 3.
3 2
50MPa 100MPa 200MPa
heating
cooling
5
Displacement (mm)
Displacement (mm)
4
(b)
(a)
5
1
50MPa 100MPa 200MPa
4 3 2
cooling heating
1 0
0
-1 0,0
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70 0,75
0,1
0,2
0,3
0,4
2,00
0,7
0,8
0,9
1,0
1,1
0,7
0,8
0,9
1,0
1,1
1,75
region I
1,50 1,25
region II
region III
1,50
Tension (V)
Tension (V)
0,6
2,00
2,25 1,75
0,5
Electrical current (A)
Electrical current (A)
heating
1,00
cooling
0,75
1,25
heating
1,00 0,75
0,50 0,25
0,50
0,00
0,00
cooling
0,25 0,0
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70 0,75
0,1
0,2
0,3
Electrical current (A)
0,4
0,5
0,6
Electrical current (A)
(c) 4 3 2
(d) 5
50MPa 100MPa 200MPa
cooling
1
heating
0
Displacement (mm)
Displacement (mm)
5
20MPa 60MPa 120MPa
4 3 2
cooling
1 0
-1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6
heating 0,0
0,5
1,0
1,5
1,50
1,50
1,25
1,25
1,00 0,75 0,50
heating cooling
2,5
3,0
3,5
4,0
4,5
5,0
3,5
4,0
4,5
5,0
Electrical current (A)
Tension (V)
Tension (V)
Electrical current (A)
2,0
1,00
heating
0,75 0,50
cooling
0,25
0,25
0,00
0,00
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6
Electrical current (A)
0,0
0,5
1,0
1,5
2,0
2,5
3,0
Electrical current (A)
Figure 3 – Electromechanical behavior of some studied SMA wire actuators. (a) TN1. (b) TNC. (c) TN2. (d) CZA. Figure 4 show the definition of some of these parameters by means of a current – displacement loop associated with a current – tension curve for the TN1 SMA wire under 200MPa. Critical electrical currents, named iMs , iMf , iAs and iAf , corresponds to the start and finish of austenite to martensite transformation during cooling and to the start and finish of martensite to austenite transformation during heating, as defined in Fig. 4.
Proceedings of COBEM 2005 Copyright © 2005 by ABCM
18th International Congress of Mechanical Engineering November 6-11, 2005, Ouro Preto, MG
5
2,5
iMs 2,0
iAf 3
1,5
Hi
cooling
SME
2
1,0
heating 1
Tension (V)
Displacement (mm)
4
0,5
iMf
iAs
0 0,0
0,1
0,2
0,3
0,0 0,4
0,5
0,6
0,7
0,8
Electrical current (A)
Figure 4 – Main eletromechanical shape memory parameters. TN1 specimen under 200MPa. Figure 5 show the behavior of these currents as a function of applied stresses for TN1, TNC, TN2 and CZA SMA actuator wires. (a)
(b)
0,5
0,40 0,35
Electrical current (A)
Electrical current (A)
0,4 0,30 0,25 0,20 0,15
iAs iAf iMs iMf
0,10 0,05
0,3
0,2
iAs iAf iMs iMf
0,1
0,00
0,0 0
20
40
60
80
100
120
140
160
180
200
0
220
20
40
60
Applied stress (MPa)
80
100
120
140
160
180
200
(c)
(d)
0,90
4,5
IA s IA f IM s IM f
4,0
0,75
Electrical current (A)
3,5
Electrical current (A)
220
Applied Stress (MPa)
0,60
0,45
IA IA IM IM
0,30
S
F
0,15
3,0 2,5 2,0 1,5 1,0
S
F
0,5 0,0
0,00 0
20
40
60
80
100
120
140
Applied stress (MPa)
160
180
200
220
0
20
40
60
80
100
120
140
Applied Stress (MPa)
Figure 5 – Linear relationships between transformation currents and applied stresses. (a) TN1. (b) TNC. (c) TN2. (d) CZA.
As expected, a linear relationship between electrical currents and applied stresses is verified, similarly to the results noticed in literature for transformation temperatures (Otsuka and Wayman, 1998). Recently, Wang et al (2004) studying electrothermal driving characteristics of Ti-Ni SMA springs have obtained an optimum driving current density between 7.6 and 10.2 A/mm2 for a good recovery rate by two-way shape memory effect. Activation electrical currents calculated for a current density of 10.2 A/mm2 leads to the values of 0.32 A, 0.67 A and 1.1 A for TN1, TNC and TN2 specimens, respectively. These currents are sufficient to originate a maximum contraction of the TNC and TN2 SMA wire, but not for the TN1, as can be verified in Figs 3 and 5. The characteristic slopes for these currents, here named CiMs , CiMf , CiAs and CiAf , are determined as defined in Fig. 5 and summarized in Tab. 2. These slopes reveal the linear increase of transformation currents with applied loads. Table 2 – Characteristic slopes of transformation currents. CiMs (mA/ MPa) 0.65 0.53 0.79 0.17 12.5
Specimen TN1 TNC TN2 TN3 CZA
CiMf (mA/ MPa) 0.75 0.62 1.66 0.26 10.4
CiAs (mA/ MPa) 0.25 0.52 0.75 0.23 16.4
CiAf (mA/ MPa) 0.29 0.59 0.43 0.31 14.7
The displacement by shape memory effect under load as well as the current hysteresis defined in Fig. 4 is plotted as a function of applied stress in Fig. 6.
5
1,1
TN1 TNC TN2 TN3 CZA
1,0 0,9
4
0,8 0,7
SME (%)
3
Hi (A)
0,6 0,5
2 0,4 0,3 1
0,2 0,1
0
0,0 0
40
80
120
160
200
Applied stress (MPa)
240
0
40
80
120
160
200
240
Applied stress (MPa)
Figure 6 – Shape memory effect and current hysteresis versus applied stress. Figure 6 allows concludes about the best SMA wire in terms of actuator performance. Despite of cost aspects, maximum SME associated with reduced and nearly constant hysteresis can be considered an optimum operation condition (Huang, 2002). Then, from Figs. 3 and 6, the studied SMA actuators are classified in the following sequence: TNC – TN3 – TN1 – TN2 – CZA. In the studied stress range, TNC specimen demonstrates highest SME associated with a reduced current hysteresis between 10 and 200 MPa while CZA present an opposite behavior. 4.
Conclusions
Several SMA wire actuators were studied by electrical activation under constant load aiming to determine some important parameters like transformation currents, current hysteresis, electrical current slopes and stress assisted shape memory effect. These parameters are fundamental for the design of smart systems incorporating these artificial muscles.
Proceedings of COBEM 2005 Copyright © 2005 by ABCM
18th International Congress of Mechanical Engineering November 6-11, 2005, Ouro Preto, MG
As expected, all SMA wire specimens have presented a linear increase of transformation currents as a function of applied loads. Among all tested Ti-Ni wires, Ti-Ni-Cu (TNC) presents a maximum SME associated with reduced and nearly constant current hysteresis between 10 and 200MPa that can be considered optimum operation conditions for these actuators. An opposite behavior was observed in the unique studied Cu-Zn-Al SMA (CZA) that presents a poor SME and an increasing current hysteresis as a function of applied stress.
5.
Acknowledgments
This work was supported by the Brazilian agencies: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/CT-Energ Grant 400724/2003-0 and Universal Grant 472506/2004-8) and Fundação de Amparo à Pesquisa do Estado da Paraíba (Fapesq-PB, PPP Grant 035/03).
6.
References
Fletcher, R., 1996. “Force transduction materials for human technology interfaces”, IBM Systems Journal, Vol. 35, No 3-4, pp. 630-638. Huang, W., 2002. “On the selection of shape memory alloys for actuators”, Materials and Design, Vol. 23, pp. 11-19. Nascimento, M. M., Rocha Neto, J. S., Lima, A. M. N., Almeida, L. A. L. and de Araújo, C. J., 2004. “A model for strain-temperature loops in shape memory alloy actuators”, ABCM Symposium Series in Mechatronics, Vol. 1, pp. 264-271. Otsuka, K. and Wayman, C. M., 1998. “Shape Memory Materials”, Cambridge University Press, Cambridge, UK, 284p. Safak, K. K. and Adams , G. G., 2002. “Modeling and simulation of an artificial muscle and its application to biomimetic robot posture control”, Robotics and Autonomous Systems , Vol. 41, pp. 225-243. Srinivasan, A.V. and McFarland, D.M., 2001. “Smart Structures – Analysis and Design”, Cambridge University Press, Cambridge, UK, 228p. Wang, Z. G., Zu, X. T., You, L. P., Feng, X. D. and Zhang, C. F., 2004. “Investigation on the two-way shape memory effect and alternating current electrothermal driving characteristics of TiNiCu shape memory alloy”, Journal of Materials Science, Vol. 39, pp. 3391-3395.
