Design and development of MH actuator system

Sensors and Actuators A 113 (2004) 118–123

Design and development of MH actuator system Ken Kurosaki∗ , Takehiro Maruyama, Kengo Takahashi, Hiroaki Muta, Masayoshi Uno, Shinsuke Yamanaka Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan Received 5 October 2003; received in revised form 20 February 2004; accepted 6 March 2004 Available online 5 May 2004

Abstract Hydrogen-absorbing alloys have an energy conversion function via a hydriding/dehydriding reaction. In the present study, the simple metal hydride (MH) actuator, consisting of the copper plated hydrogen-absorbing alloy as a power source, Peltier elements as a heat source and a cylinder with metal bellows as a functioning part has been developed. To improve the thermal conductivity of the hydrogen-absorbing alloy, an electro-less copper plating has been carried out. The effects of the electro-less copper plating and the dynamic characteristics of the MH actuator have been studied. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen-absorbing alloy; Metal hydride; Actuator; Electro-less copper plating; Thermal conductivity

1. Introduction It is well known that hydrogen-absorbing alloys can reversibly absorb and desorb a large amount of hydrogen, more than about 1000 times as their own volume. Additionally, the hydrogen-absorbing alloys have an energy conversion function via a hydriding/dehydriding reaction. By heating the hydrogen-absorbing alloys, the hydrogen equilibrium pressure increases and hydrogen is desorbed, whereas by cooling the alloys, the hydrogen equilibrium pressure decreases and hydrogen is absorbed. In this way, it is possible to utilize the mechanical energy of the hydrogen gas pressure change by the manipulating heat. Wakisaka et al. have developed the metal hydride (MH) actuator, which is based on a new concept using the reversible reaction between the heat energy and mechanical energy [1,2]. Because, the MH actuator is characterized by the small size, low weight, noiseless operation and compliance similar to that of the human elbow joint, a lot of people have developed the application to the medical and rehabilitation equipment that assists the aged and disabled [3–6]. Some lifting devices using this type of actuator have already been developed and utilized more than 10 years ago. However, the improvement of performances of those devices is still desired.

∗ Corresponding author. Tel.: +81-6-6879-7905; fax: +81-6-6879-7889. E-mail address: [email protected] (K. Kurosaki).

0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.03.009

The MH actuator does not produce any uncomfortable noise or vibration, so it is suitable for use in the medical and welfare equipment. However, because of the driving mechanism, the response speed depends on the thermal conductivity of the hydrogen-absorbing alloy. In the MH actuator, the alloy is pressed into a pellet, but its thermal conductivity is very poor. In the present study, in order to improve the thermal conductivity, the micro-encapsulation has been carried out by the electro-less plating method, in which the alloy particles made of LaNi5 are coated by a thin porous copper layer. The effect of the copper plating to the hydrogen-absorbing properties and thermal conductivities of the pelletized sample have been evaluated. The driving characteristics of the MH actuator have been studied.

2. Principle of the MH actuator The activate principle of the MH actuator is shown in Fig. 1. The hydrogen gas pressure derived from the hydrogen-absorbing alloy activates the MH actuator. In the present study, a Peltier element has been used as a heat source. By changing the direction of the electric current in the Peltier elements, the alloy can be heated or cooled. The construction of the functional part employs metal bellows that can contain the hydrogen without leakage. When the alloy is heated by the Peltier element, the hydrogen gas is desorbed from the alloy, the internal pressure increases,

K. Kurosaki et al. / Sensors and Actuators A 113 (2004) 118–123

119

Load

Peltier Element Metal Bellows

Hydrogen-Absorbing Alloy

Power Source

Fig. 1. Activate principle of the MH actuator.

and the cylinder is pushed up. On the other hand, when the alloy is cooled, the hydrogen gas is absorbed into the alloy, the internal pressure decreases and the cylinder goes down.

Fig. 3. Appearance of the MH actuator.

and dimension. The thermal conductivity λ of the pellet was evaluated from the thermal diffusivity α, heat capacity CP and density d as the following relationship:

3. Experimental The polycrystalline sample of LaNi5 was prepared by an arc melting method under an argon atmosphere. The chips of lanthanum and nickel were used for the starting materials. The purities of them were above 99.9 mass%. To enhance the homogeneity, the obtained sample was annealed at 1303 K for 7 h under vacuum in quartz ample. Thus, obtained sample was ball milled to the powder with the particle size below 53 ␮m in diameter. In order to enhance the thermal conductivity of the pellet, the electro-less copper plating was performed to the alloy powder. Ishikawa and coworkers has reported the micro-encapsulation of hydrogen-absorbing alloy particles by coating with a thin porous copper layer by the chemical plating method [7–9]. The electro-less copper plating process is described as the following reaction [10]:

λ = αCP d The thermal diffusivity was measured by a laser flash method at room temperature in vacuum. The heat capacity was referred to literature data [11]. The relationship between the copper plating amount and thermal conductivity was studied.Thus, obtained pellet was sandwiched in a couple of the Peltier element and stuck on the inner wall of the tank with silicon grease as shown in Fig. 2. The appearance of the MH actuator developed in the present study is shown in Fig. 3. To increase the heat release ability and decrease its weight, the container of the hydrogen-absorbing alloy is made of aluminum. The driving characteristics of this MH actuator were studied.

Cu2+ +2HCHO+4OH− → Cu + H2 + 2H2 O + 2HCOO− The amount of the copper was determined by an ICP–AES analysis. The copper plated LaNi5 powder was pressed into the pellet with 24 mm in diameter and 5 mm in thickness. The density of the pellet was evaluated by the measured weight

4. Results and discussion The X-ray diffraction patterns of copper plated LaNi5 (target plating rates are 0, 10, 20, 30 and 40 mass%) are

Fig. 2. MH (LaNi5 ) pellet sandwiched in a couple of the Peltier element (left) and container (right).

K. Kurosaki et al. / Sensors and Actuators A 113 (2004) 118–123 -1

Thermal conductivity at room temperature,λ (Wm K )

120

-1

0 mass%-Cu

10 mass%-Cu

Intensity (arb. unit)

20 mass%-Cu

30 mass%-Cu

40 mass%-Cu

25

25

20

15

Fitting line;λ= 0.58x-0.43 10

5

0 0

10

20

30

40

50

Copper plating rate, x(mass%-Cu)

LaNi5

Fig. 6. Relationship between the copper plating rate and thermal conductivity of the compressed pellets.

Cu

Table 1 Packing density of the compressed pellets

30

35

40

45

50

55

2θ (degree) Fig. 4. XRD patterns of the copper plated LaNi5 ; target plating rates are 0, 10, 20, 30 and 40 mass%.

shown in Fig. 4. As the copper plating rate increases, the intensity of the X-ray diffraction peaks of copper increases and that of LaNi5 decreases. The copper plating does not change the crystal structure of LaNi5 and no peaks of other compounds are observed. The SEM images of the copper plated LaNi5 (target plating rates are 0 and 20 mass%) are shown in Fig. 5. The initial size of the LaNi5 particles is below 53 ␮m in diameter. After the copper plating, a copper layer is observed on the surface of the LaNi5 particles and the particle size is almost same as the initial one. The thickness of the copper layer is about 1 ␮m in the case of the sample with 20 mass% copper

Copper plating rate (mass% Cu)

Packing density (g/cm3 )

0 10 20 30 40

5.34 5.36 5.44 5.50 5.53

plating. At the surface of the copper layer, a porous structure is observed. The generated hydrogen gas generated in the copper plating reaction fabricates this porous structure. The estimated pore size is in nano-orders. Because of this structure, the hydrogen gas can easily pass through the copper layer. Fig. 6 shows the thermal conductivities of the compressed LaNi5 pellets with the copper plating (target plating rates are 0, 10, 20, 30 and 40 mass%) at room temperature in vacuum. The packing densities of the compressed pellets are 5.3–5.5 g/cm3 (see Table 1). As the copper plating rate increases, the thermal conductivity of the pellets increases and that of the copper 40 mass% plated pellet is as 80 times

Fig. 5. SEM images of the copper plated LaNi5 ; target plating rates are 0 (left) and 20 mass% (right).

K. Kurosaki et al. / Sensors and Actuators A 113 (2004) 118–123

121

Displacement (mm)

LaNi5 10 mass%-Cu plated LaNi5 20 mass%-Cu plated LaNi5 30 mass%-Cu plated LaNi5

0.01 0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

Hydrogen content (mass%-H)

Fig. 7. PCT curves of the copper plated LaNi5 ; target plating rates are 0, 10, 20 and 30 mass%.

as large than the initial pure LaNi5 pellet. This result clearly shows the great effectiveness of the copper plating to the thermal conductivity. Fig. 7 shows the PCT curves of the copper plated LaNi5 (target plating rates are 0, 10, 20 and 30 mass%) at 303 K. As the copper plating rate increases, the plateau region in the PCT curves becomes narrower, indicating that the hydrogen capacity decreases. However, the equilibrium pressure is not changed. Fig. 8 shows the relationship between the copper plating rate and hydrogen-absorbing speed of the copper plated LaNi5 (target plating rates are 0, 10, 20 and 30 mass%). As the copper plating rate increases, the hydrogen-absorbing speed increases. The hydrogen-absorbing speed of the initial pure LaNi5 is enhanced up to two times higher by 30 mass% of the copper plating. Comparing to the relationship between the copper plating rate and thermal conduc-

Hydrogen-absorbing speed (mass%/sec)

0.07

0.06

0.05

0.04

0.03

Temperature: 303 K Equilibrium pressure:1.0 MPa 0.02 0

10

20

30

40

Copper plating rate, x(mass%-Cu)

Fig. 8. Relationship between the copper plating rate and the hydrogenabsorbing speed.

Pressure (MPa)

0.1

0.8 0.7 0.6 0.5 0.4 0.3

Temperature (K)

1

375 350 325 300 275 250

Electric Power (W)

Equilibrium pressure (MPa)

Load: 10kg 50 25 0 -25 -50 -75

150 75 0 -75 -150 0

25

50

75

100

125

150

Time (sec)

Fig. 9. Dynamic characteristics of the MH actuator; weight load is 20 kg.

tivity, this result means that the thermal conductivity is one of the most important factors of the hydrogen-absorbing speed. Figs. 9 and 10 show the driving behavior of the MH actuator. The MH actuator, containing only about 10 g of MH alloy, can easily lift the weight load of 20 kg as shown in Fig. 10. Around 313 K, the temperature is saturated in seconds because the heat of formation of the hydride is too large for the Peltier element to cool the alloy. From the time–displacement pattern of the cylinder, the velocity during the rise time is approximately 2.3 mm/s. In this driving procedure, the efficiency of the MH actuator is about 0.6% during the rise time. The periodic driving characteristics of the MH actuator with the weight load of 10 kg are shown in Fig. 11. The displacement of the cylinder is approximately controlled in the range from −5 to 25 mm by heating and cooling in 10 and 25 s, respectively. From the time–displacement pattern, it is found that the velocity during the rise time and fall time are approximately 2.8 and 1.2 mm/s, respectively. In this driving procedure, the efficiency of the MH actuator is only about 0.3% during the rise time. The temperature of the Peltier elements and the pressure of the MH actuator are varied with the electric power.

122

K. Kurosaki et al. / Sensors and Actuators A 113 (2004) 118–123

Fig. 10. Driving behavior of the MH actuator; weight load is 20 kg.

30 20 10 0 -10 0.45 0.40 0.35

Temperature (K)

0.30 340

Electric Power (W)

Pressure (MPa)

Displacement (mm)

Load: 10kg

150

been carried out. The thermal conductivity of the copper 40 mass% plated LaNi5 pellet is approximately 80 times larger than that of the initial state. The hydrogen-absorbing speed also increases as copper plating rate. On the other hand, the hydrogen capacity decreases with increasing the copper plating rate. The MH actuator, which contains only 10 g of the hydrogen-absorbing alloy, can easily lift the weight load of 20 kg with the displacement of 80 mm. The velocities of the cylinder during the rise time and fall time were 2.3 and 1.6 mm, respectively. The displacement of the cylinder can be controlled in the periodic driving. Although, the efficiency of the MH actuator is below 1%, no uncomfortable noise is generated during its driving procedure.

320

References

300 280 260

75 0 -75 -150 0

50

100

150

200

Time (sec)

Fig. 11. Periodic driving characteristics of the MH actuator; weight load is 10 kg.

5. Conclusion By using the hydrogen-absorbing alloy (LaNi5 ) as a power source, the MH actuator has been developed in the present study. In order to improve the thermal conductivity of the hydrogen-absorbing alloy, the electro-less copper plating has

[1] Y. Wakisaka, M. Muro, T. Kabutomori, H. Takeda, S. Shimizu, S. Ino, T. Ifukube, Application of hydrogen-absorbing alloys to medical and rehabilitation equipment, IEEE Trans. Rehabilitat. Eng. 5 (2) (1997) 148–157. [2] Y. Wakisaka, H. Takeda, M. Muro, K. Onishi, Development of MH (metal hydride) actuator, Nihon Seikousyo Gihou 46 (1992) 71– 76. [3] T. Sasaki, T. Kawashima, H. Aoyama, T. Ogawa, T. Ifukube, Development of an actuator using a metal hydride and its application to a lifter for the disabled, Adv. Rob. 2 (1987) 277–286. [4] S. Shimizu, S. Ino, M. Sato, T. Odagawa, T. Izumi, M. Takahashi, T. Ifukube, A basic study of a force display using metal hydride actuator, IEEE Int. Workshop Rob. Human Commun. (1993) 211–215. [5] S. Shimizu, S. Ino, M. Sato, T. Izumi, T. Ifukube, M. Muro, H. Takeda, Y. Wakisaka, A new method of variable compliance for a force display system using a metal hydride actuator, IEEE Int. Workshop Rob. Human Commun. (1994) 265–270. [6] M. Shahinpoor, K.J. Kim, A mega-power metal hydride anthroform biorobotic actuator, Proc. SPIE—Int. Soc. Opt. Eng. 4327 (2001) 110–117. [7] H. Ishikawa, K. Oguro, Method of the preparation of hydrogenabsorbing materials, Jpn. Kokai Tokyo Koho (1985) S60–S190570.

K. Kurosaki et al. / Sensors and Actuators A 113 (2004) 118–123 [8] H. Ishikawa, K. Oguro, A. Kato, H. Suzuki, E. Ishii, Preparation and properties of hydrogen storage alloy-copper microcapsules, J. Less-Common Met. 107 (1) (1985) 105. [9] H. Ishikawa, K. Oguro, A. Kato, H. Suzuki, E. Ishii, Preparation and properties of hydrogen storage alloys microencapsulated by copper, J. Less-Common Met. 120 (1986) 123–133.

123

[10] H. Kimura, Electro-less copper plating solution, Jpn. Kokai Tokyo Koho (1973) S46–S52802, S46–S52803, S46–S52804. [11] The SGTE Pure Substance and Solution Databases, GTT-DATA SERVICES (1996).