Torsion Coupling
6.2 FSMA-Based Actuator
by a 4 × 106 A/m magnetic fi eld, however, the electromagnet will be too huge to use for practical applications. A conventional compact solenoid cannot produce such a high magnetic fi eld.
Mechanism (b) is not achievable for polycrystal FSMA because only a very small strain is available. For example, only the order of 10−4 strain can be obtained for polycrystal FePd in a fi eld up to 8 × 105 A/m [61]. Even though a large strain can be produced by a small magnetic fi eld in single crystals of NiMnGa using mecha- nism (b), the output force is still as small as several MPa.
Th erefore, for the current available FSMA such as the FePd alloy, mechanism (c) (hybrid mechanism) shows the best performance among these mechanisms.
portable electromagnet (H = 8 × 104 A/m). Both beams have the same geometry and Figure 6.9a shows the experimental setup. It is clearly demonstrated in Figure 6.9c that the FePd beam has a much larger defl ection than the Fe beam. Th e defl ection is due to the hybrid mechanism. Since the magnetization of FePd in aus- tenite is similar to that of Fe and the ratio of the Young’s modulus of Fe (210 GPa) to FePd (80 GPa in the austenite phase, 30 GPa in the martensite phase) is about in the 3–7 range, the ratios of the bending defl ection of FePd to Fe could be also 3–7. However, the demonstration results shows that the tip of the FePd beam exhibits a 7 mm displacement during the application of the magnetic fi eld, while the Fe beam shows almost no defl ection. Th is implies that the FePd beam is not only very ductile but also undergoes austen- ite → martensite phase transformation (SIM). As the gap between the specimen and the electromagnet becomes smaller during the deformation, the larger magnetic force leads to more phase change and deformation. Th is comparison shows that the FePd FSMA as an actuator material is superior to the conventional material. Both the cantilever beams spring back to the original position when the electromagnet is turned off .
Because FePd is very ductile, it is also possible to make FePd into various shapes. Figure 6.10 shows the various shapes of polycrystalline FePd specimens, such as rod, wire, and helical spring. Th erefore, this material can be used without limitations of manufacturing. Th e helical FePd spring can be produced by winding and shape memorizing the FePd wire, which can be extruded from a solid cylinder. A series of pictures were taken to exhibit the FePd spring actuated under a compact electromagnet and shown in Figure 6.11 [64]. Th e spring actuation is driven by a modifi ed electromagnet [65], which can produce a much higher magnetic fi eld gradient than a conventional solenoid. By gradu- ally increasing the electrical current, the FePd spring partially shrinks due to magnetic attraction. Th e partial shrinkage of the spring becomes part of the yoke and attracts the rest of the turns of the spring as the electrical current increases. Figure 6.11b shows the complete shrinkage of the FePd spring. Upon removing the electrical power, the spring completely returns to its initial length.
Although FePd appears as a promising actuator material based on the hybrid mechanism, it is very expensive because of FePd
Fe
(b) (c) 5 mm
H field
Side View Solenoid
magnet Fe FePd
(a)
FIGURE 6.9 Bending of polycrystal FePd and Fe cantilever beams under applied magnetic fi eld: (a) the schematic of the experimental setup (the side view from the tip of the beams) (b) before and (c) aft er applying the fi eld.
FIGURE 6.10 Diff erent shapes of polycrystalline FePd specimens.
Fe-Pd helical spring D = 22 mm, L= 55 mm Fe-Pd wire
f 1.6 mm
Fe-Pd wire f 1 mm
Fe-Pd rod f 5 mm
Pd. An alternative way to reduce the cost of the material is to use the FSMA composite, which is composed of the ferromagnetic material and superelastic SMA, where the ferromagnetic mate- rial is provides a large force due to the magnetic fi eld gradient, resulting in a large deformation on superelastic SMA (i.e., NiTi) due to the SIM transformation. Several cases of FSMA compos- ites have been studied and the laminated FSMA composite is easily made without losing its performance and is the most cost- eff ective [66], for example, the laminated plate (Figure 6.12a) and wire of concentric cylinders (Figure 6.12b). As shown in Figure 6.12a, the outer layer of superelastic NiTi SMA can sustain large stress and the inner core ferromagnetic material is subjected to modest stress, thus, leveraging the extra stress bearing capacity of superelastic NiTi while protecting an otherwise brittle soft ferromagnetic material.
Several FSMA composite actuators have been made. One of the examples is the torque actuator based on the FSMA compos- ite [67] and its design concept is illustrated in Figure 6.13 where (a) and (b) denote the cases of switch on and off of the actuator
system, respectively. Th e torque actuator consists of an inner rod, which will rotate counterclockwise upon switching on the electromagnet system, thus, attracting the FSMA plate spring to its inner wall. Th e rotating motion of the inner rod will provide the torque work for a dead load that is hanging on the rod by a pulley or belt. Th e FSMA composite is composed of a ferromag- netic material and a superelastic grade SMA, and it is subjected to a bending moment, which is not uniform over the length of the FSMA composite due to its varying curvature. In the fi rst prototype torque actuator, the FSMA composite consists of NiTi superelastic wires and several cylindrical soft iron rods as shown in Figure 6.14a. Th e requirement for designing the FMSA com- posite based torque actuators is to induce a large stress so that the SMA plate of the FSMA composite can reach the onset of SIM transformation, while the stress in the ferromagnetic rods remains below its plastic yield stress. Th e FSMA composite will be attracted to the inner wall of the actuator due to strong mag- netic fl ux gradient upon switching on the electromagnetic system. Th e fi rst prototype torque actuator has produced 0.736 N Fe-Pd spring
(b) 5 cm Electromagnet
(a)
FIGURE 6.11 Actuation of the FePd spring actuator: (a) power off and (b) power on.
(b) Wire torsion (helical spring)
Superelastic TiNi tube Ferromagnetic core M
M
q l
D Df
Stress-induced transformation
tfm t
tTiNi tSIM
(a) Plate bending
Stress distribution
Stress induced s
sfm
sTiNi
sSIM Transformation Superelastic TiNi Ferromagnetic layer
FIGURE 6.12 Two types of FSMA composites composed of soft ferromagnetic core and superelastic NiTi, (a) laminated composite plate and (b) concentric cylinder composite.
m with a 40° maximum angle. Further improvement has been done by using the FSMA composite plate spring, which is made of a superelastic Ni–Ti sheet and square soft iron bars (Figure 6.14b). Because of the higher stiff ness of the structure and the stronger magnetic attracting force, it can provide a 4.8 N m torque with a 102° rotation angle.
A synthetic jet actuator based on the FSMA composite mem- brane has also been constructed [68]. Th e composite membrane
was driven by the electromagnetic system and oscillated to create a synthetic jet fl ow through the exit hole. Th e FSMA composite membrane was composed of a superelastic NiTi thin sheet and a ferromagnetic soft iron pad. Figure 6.15a and b shows the parts of the membrane actuator, respectively. It has two chambers in the center divided by the composite membrane. Th e thin lami- nated yoke is used in the electromagnet unit in order to eliminate the eddy current for the high-frequency operation. When the 20 mm
20 mm (a) (b)
FSMA composite
FIGURE 6.13 Photos of the torque actuator made of FSMA composite (a) switch on and (b) switch off .
Ferromagnetic square bars
NiTi plate
(b) NiTi wires Ferromagnetic cylindrical bars
(a)
FIGURE 6.14 Two types of FSMA composites for the torque actuator (a) made of NiTi superelastic wires and cylindrical soft iron bars and (b) made of NiTi superelastic plate and square soft iron bars.
Electromagnet FSMA composite (b)
) a (
Chamber#1 Chamber#2 Exit hole
FIGURE 6.15 (a) Photo of the synthetic jet membrane actuator and (b) the schematic of actuator parts. (From Liang, Y., Kuga, Y., and Taya, M., Sens. Actuators A, 125, 512, 2005. With permission.)
FSMA composite membrane oscillates close to its resonance frequency, the system effi ciency is optimized and the mem- brane exhibits a large stroke to produce a strong jet fl ow. Since the NiTi sheet of the composite membrane is superelastic, it can sustain large stresses without plastic deformation. Th e syn- thetic jet fl ow velocity from the membrane actuator was mea- sured by the hotwire probe at the exit hole. Figure 6.16a shows the results of jet fl ow velocities as a function of frequency by using diff erent thicknesses of NiTi thin sheets of the composite membrane. Th e maximum jet velocity, 190 m/s at 220 Hz, was obtained by using the 0.3 mm thick NiTi sheet of the composite membrane. Its displacement–frequency response is shown in Figure 6.16b where the peak to peak stroke of the membrane is 3.72 mm at 195 Hz. Th e energy density of the FSMA composite membrane was reported as 30 kJ/m3 and its power density was 6000 kW/m3 [68]. Both the energy density and the power den- sity can be further improved if the composite membrane exhib- its a much larger stroke. Also, the higher frequency response of the composite membrane can further increase the power density.
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