High-Power Shape Memory Alloy Catapult Actuator for High-Speed and High-Force Applications

Nickel-Titanium (NiTi) based shape memory alloy (SMA) wires are already often used in industrial actuator applications. Their high energy density allows the building of light-weight actuator systems with high forces using small installation spaces. Combined with the biocompatibility of NiTi, a huge field of applications can be covered by SMA actuated systems. In systems like emergency brakes or switch disconnectors, which require high forces as well as high actuation speed, the high-power capability of NiTi actuators is exploited. The presented work details the development and characterization of a giant power catapult demonstrator, that combines the high-speed and high-force capability of SMA wires. To illustrate the vast force, speed, and power potential of SMA wires, a bowling ball is launched from its resting position vertically into the air using SMA wires. For demonstration purposes, a target altitude for the bowling ball of 500 mm is chosen. With the height and the overall accelerated mass given, an actuation force <inline-formula> <tex-math notation="LaTeX">$F \cong ~920$ </tex-math></inline-formula> N is needed. The instantaneous energy release from the designed power source results in the targeted flight height and an overall peak power of <inline-formula> <tex-math notation="LaTeX">$P \cong ~0.5$ </tex-math></inline-formula> MW.

The notion Nitinol, as a common name for the alloy, 23 is an acronym for the two given components and the 24 place of its discovery. The discovery of the shape memory 25 effect of a binary nickel and titanium alloy is attributed to 26 William J. Buehler and Frederick Wang in 1959 over the 27 course of their research at the Naval Ordnance Laboratory. 28 The shape memory effect is based on the property of the 29 alloy to perform a reversible phase transformation. One dis- 30 tinguishes between the monoclinic low-temperature phase 31 The associate editor coordinating the review of this manuscript and approving it for publication was F. R. Islam . martensite and the cubic space centered high-temperature 32 phase austenite [13], [14]. In contrast to plastic deformation 33 in conventional metals, which leads to a displacement of 34 atoms within the crystal lattice, shape memory alloys show 35 a phase shift in the martensite structure without a displace-36 ment of atoms. This so called pseudoplastic deformation 37 is reversible and can be revoked [15], [16]. Heating the 38 structure leads to a phase transformation towards austenite, 39 which comes with a change in macroscopic shape. In the 40 case of SMA wires, the temperature induced transformation 41 to the austenite phase results in a contraction of the SMA 42 wire, which is used to perform mechanical work in actuator 43 systems [17]. 44 Current research in the field of SMA actuators has a strong 45 focus on identifying new areas of application like the field of 46 bio-inspired motion [18], [19], soft robotics [20], [21] as well 47 as modeling and control of the oftentimes complex nonlinear 48 material behavior [22], [23]. 49 Based on their performance capabilities, SMA actuator 50 applications can be classified into fast responding systems 51 VOLUME 10, 2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ [24], [25], [26], [27], [28] and those that move very high 52 loads [29], [30]. 53 The content of this paper deals with combining both topics 54 in a descriptive way, resulting in a mechatronic system for 55 technology demonstration purposes. Dana,Vollach and Shilo 56 give an overview of these high-rate SMA applications [27], 57 which can be used for the development of quick release mech-58 anisms in safety applications [26]  height.

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After defining the design and in consideration of the nec-74 essary mechanical and electrical parameters, a demonstrator 75 is designed, built, and put into operation.

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The remainder of this paper is composed as follows.

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In section II, an overview of the overall system concept  To gain the maximum effect of the demonstrator, a concept 115 was elaborated combining both parts in an efficacious fully 116 integrated mechatronic system. 117 FIGURE 1. Mechanical stress-strain behavior of SMA wires in low temperature (martensitic) and high temperature (austenitic) states [27].

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As concept base, all needed kinematic parameters have to 119 be evaluated and set. The activation scheme in this demon-120 strator aims at full SMA wire contraction with maximum 121 velocity and thus generating an acceleration of a mass. Even 122 though the mechanical stress-strain behavior of SMA wires 123 is highly non-linear and hysteretic ( Figure 1, [27]), in this 124 specific case the only constraint is to reach the fully austenitic 125 branch (2) from resting position (1) as fast as possible. 126 This means, no further control of the actuator is needed 127 or intended, the only goal is maximum power release upon 128 activation.

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Based on energy conservation, with gravity g and the 130 required height of h 1 = 500 mm, the necessary velocity v 1 131 of the launched bowling ball can be calculated as: Furthermore, with the chosen SMA wire length of 134 l martensite = 400 mm, the presumed strain of 4% and the 135 resulting wire-length-based acceleration travel of 136 x = 16 mm, the remaining translatory motion parameters, 137 acceleration time t 1 and the corresponding acceleration a 1 , 138 can be evaluated: The correlative combined force F can be calculated, con- For the fabrication and assembly of the used SMA bundles, 172 a specific preconditioning routine including a defined biasing 173 mechanism is used, which ensures homogeneous elongation 174 and stress level of σ 1 = 250 MPa of each wire in a bundle. 175 This guarantees that all SMA wires have the same precon-176 ditions before their installation and minimizes individual 177 residual strains.

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The bowling ball is set on a brim of aluminum, operating 179 as a catapult platform. One side of the SMA bundles is 180 attached to this brim in a circular arrangement of 120 • spac-181 ing. Additionally, the platform is mechanically connected to 182 slides guided on linear rails. These rails are connected to 183 a transparent acrylic tube, which has the task of keeping 184 the linear rails in position and guiding the bowling ball 185 on its flight trajectory. To limit the maximum travel of the 186 catapult platform to 16 mm, which is the maximum SMA 187 wire contraction and thus the highest possible acceleration 188 path length, limit stops are attached to the linear rails. This 189 prevents the platform from overshooting and ensures safe 190 separation of the bowling ball from the platform after the 191 acceleration travel, to allow the bowling ball to lift off. The 192 selected position of the end stops was determined, and no 193 variation was made, as the highest possible performance 194 (flight altitude) was aimed for. Therefore, the longest possible 195 acceleration path, i.e., the highest possible active stroke of the 196 SMA bundles used, determines the position of the end stops. 197 A reduction or increase would have no or a negative effect 198 on the performance. For overall sturdiness, the mentioned 199 arrangement is put in a frame of extruded aluminum profiles. 200 The framework is divided into two chambers, the upper one 201 housing the kinematics and the lower the control and power 202 electronics. The other side of the SMA bundles is attached to 203 the upper area of the aluminum frame (Figure 3c). To provide the calculated capacity, the circuit board is 237 designed to house up to twelve parallel connected 460 µF 238 capacitors (Figure 5a), which allows an additional buffer of 239 20% to the needed ten capacitors.

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As main switching component, two parallel connected 241 insulated-gate bipolar transistors (IGBT) are used, which 242 allow a maximum current flow of 960 A (Figure 5b). 243 To enable a high-current flow, extra copper plates and rods 244 were made and applied to the circuit board (Figure 5b). 245 Additionally, the circuit board is equipped with all neces-246 sary state-of-the-art safety and regulation components like a 247 blocking diode and discharge power resistor, to ensure the 248 proper behavior.  Figure 6b. Additionally, a portion of 284 the overall accelerated kinematic mass is put on a linear 285 slide with a launch pad, attached to one side of the SMA 286 bundle, in relation to the used count of SMA wires and 287 capacitors. As the capacitors are discharged, the proportional 288 weight is accelerated and launched equally to the bowling 289 ball into midair ( Figure 7). As the height of the launched 290 weight reached is also measured, the charging voltage of 291 the capacitors can be compared to the theoretical value of 292 U 1 = 230 V and later adjusted to reach the aimed flight 293 height. Regarding the measuring results of the validation 294 process it is shown, that with the designed electronics an 295 almost instantaneous discharge via the SMA wires is possible 296 (Figure 8, center diagram) leading to the targeted 16 mm 297 travel (Figure 8, bottom diagram). Furthermore, with an 298 adjusted charging voltage of U 2 = 280 V (Figure 8, center 299 diagram) all relative combinations of SMA wire and capacitor 300 count and their corresponding weight the resulting perfor-301 mance can be measured. A maximum current flow of up 302 to I 2 = 560 A (Figure 8, top diagram) can be monitored, 303 leading to a discharge time of t 2 ≤ 11 ms (Figure 8, 304 bottom diagram). The travel exceeding the 16 mm mark, 305 can be explained by the not completely stiff carrier, leading 306 to the overshooting as shown in Figure 8. These experi-307 ments are repeated with every power circuit board, until 308 all three needed circuit boards are validated and equipped 309 with ten 460 µF capacitors, providing the needed capacity 310 of 4.6 mF each. 311 VOLUME 10, 2022 Subsequently, all electronics and mechanical components 312 are assembled, combined, and put into operation, which is 313 described in the following section III.

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To examine the combined potential of the three power circuit 316 boards and to compare the reached height of the bowling ball 317 with the calculated height, all components are assembled.

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As mentioned above, the upper chamber of the aluminum 319 framework serves as sturdy and reliable structure for the 320 mechanical components (Figure 9). Apart from that, the 321 bottom chamber contains all the electronics, such as the 322 power circuit boards, the control circuit board, the microcon-323 troller and necessary secondary components. The chambers 324 are locked up with acrylic and aluminum composite panels 325 (Figure 9) to ensure safe operation and handling. As the 326 aimed current flow and voltage represent a high risk for 327 human health, all the electronic connections are precisely 328 built and thoroughly checked. Consequently, the whole setup 329 is put into operation by applying stepwise rising charg-330 ing voltages to the capacitors. The following discharge of 331 the capacitors via the SMAs and the resulting movement 332 is monitored and then compared to the expected behavior. 333 Since the observed behavior matched the predictions, the 334 charge voltage is incrementally increased until the target 335 voltage of U 2 = 280 V.

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The bowling ball is then placed on top of its launching 337 platform and a square patterned sheet is applied at the back 338 side of the demonstrator. Considering the square edge length 339 of 5 cm, the travelled height of the bowling ball can be easily 340 determined.

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The first set of attempts is performed with a charging volt-342 age of U 2 = 280 V, leading to a travel height of h 2 = 41 cm 343 (Figure 10a, dotted red lines), a measured overall system 344 activation energy W 2 = 435 J and a combined overall system 345 power P 2 = 0.43 MW (Figure 10b). The sequence is repeated 346 five times with always comparable results. Although the 347 reached height presented itself very impressive, it minimally 348

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In this article, a vivid SMA catapult demonstrator that dis-370 plays the enormous power potential of SMA technology has 371 been presented. In addition to the development and design 372 of the mechanical structure and actuator components, which 373 successfully launch a bowling ball to the height of about 374 450 mm, a high-power circuit board is developed, that can 375 deliver a current flow of up to 700 A per board. With a series 376 of fundamental experiments, the reliable functionality and 377 repeatability of the circuit boards is validated. Furthermore, 378 it is shown that the launching sequence can be repeated 379 without any loss of efficiency being displayed in the con-