Shape memory alloys (SMA) are often used to drive actuators, whereby SMA have a very specific fatigue behavior, which influences the lifetime of these actuators. The optimization of the actuator’s lifetime is an important topic to make these innovative actuators more interesting and economic for the sales market. Usually, an actuator is developed first, and a lifetime investigation of the complete actuator is carried out afterwards. The reason is the missing handling guidelines for the SMA wires which provide information on influences limiting the service life of SMA-based actuators before the actuator is fully developed and manufactured. While investigating the influences on the SMA wire during production, storage, and use of an actuator, one frequently appearing case stands out and will therefore be examined in more detail in the further course. In many actuator applications the wire is deflected by a pulley system to generate a greater actuator stroke while keeping the assembly space small. Although a large stroke is required, a wire is used instead of a spring to additionally take advantage of the higher forces generated by the wire compared to the spring. Since the wire experiences inconsistent mechanical stress at the point of deflection and a modified heat conduction occurs due to surface contact, it is questionable if this damages the wire, leading to an earlier structural or functional fatigue. The experiments will be carried out in two different ways. First, the SMA wire is tested regarding lifetime as a reference value. Afterwards, a pulley-system is designed and used deflecting the SMA wire by 90°. Subsequently, the respective lifetimes of the wires from both experiments can be compared to each other and conclusions can be drawn to what extent a deflection of the wire influences the wires lifetime. Furthermore, the damage caused by the deflection is put in relation to an influence of the wire by overheating — in case of too long electrical activation — and to the influence of the mechanical pretension of the wire. Through investigating these impairments of the SMA wire by deflecting it, handling guidelines for these specific cases can be formulated to be considered in future before the actual development of an actuator takes place. Thus, a more effective development with fewer iteration steps can be achieved and the lifetime of the actuator can be determined more precisely in advance. Furthermore, the design can be optimized easier regarding lifetime, by having knowledge apropos of the insight of these influences.

Shape Memory Alloys (SMA) exist in several forms, such as wires, tubes, springs and several more. Most commonly available SMA forms are wires, contracting when heated. There are several ways to activate the effect in wires, but mostly they are electrically heated (Joule Heating) using direct current (DC). There are examples of using alternating current (AC) for the activation of SMA wires, but rather for a highspeed activation that can be considered loss-free, since the wire is heated rapidly. This work investigates the long term behavior of SMA wires activated using AC. An experimental setup according to Design of Experiment (DoE) was proposed to investigate the different influence of the long term behavior between an activation using AC and DC. 15 different levels of the activation parameters voltage, activation time and stress — each for the AC and the DC — were chosen. The value of the DC voltage was the same as the effective voltage for the AC. For each set of parameters, according to the experimental design, two specimens were investigated. The different activation techniques differ in one important aspect: the source generating a DC is controlled, whereby the output voltage is independently kept constant from the change in electrical resistance induced by the phase transformation of the SMA wires when heated. The AC power does not offer such a function. Therefor the applied value of the effective Voltage is only an initial value at the beginning of the activation. During the activation the values of the voltage and the current change due to the phase transformation. For further comparison, the activation energy for both activation methods was calculated. During activation the current and the voltage were measured at 600 Hz for the AC and at 50 Hz for the DC. The energy per activation is calculateto increase comparability between AC and DC.

The integration of shape memory (SM) wires in additive manufacturing (AM) processes and components, either as actuators or sensors, holds enormous potential for future developments. For example, a sensor or an actuator function could be integrated into a component already during the manufacturing process. This integration can eliminate downstream assembly steps and improve the flexibility of the component design. In addition to (complex) components, the AM process can also be used to attach connecting elements to the wire, ensuring an improved connection of the wire to other components like an actuator housing. The integration of SM wires into an AM process will be further investigated in the following and feasibility validated by initial trials. Powder bed fusion – laser beam/metal (PBF-LB/M) is used as an AM process; thereby blocks are welded to the wire in the context of this work as proof of concept. Long-term and tensile tests are carried out to evaluate the functional and mechanical properties of the manufactured compounds. Moreover, images of a scanning electron microscope (SEM) in combination with energy dispersive X-ray spectroscopy (EDX) provide further information about the generated compound.

Today, therapy of neurologically induced functional losses of the upper limbs is mainly carried out manually. Recently, with the progress in automation technology, device-assisted therapy has established itself internationally as an additional treatment option. Thereby, proven therapy methods are applied automatically, which can relieve therapists, increase treatment frequency and lead to a better outcome. However, many of these solutions are still costly, heavy, bulky, or unsuitable for non-specialists because of the device’s and external actuator unit’s complexity. These limitations deny regular and otherwise immensely beneficial self-training by demanding the continuous presence of a therapist for setup and oversight. In this context, Shape-Memory-Alloy-based actuators’ usage may permit new design approaches with enhanced physical properties and usability. Thereby, inhibition thresholds are overcome by reducing the size and weight of available devices and their peripherals, which use standard actuators. For SMA, the necessary strokes and forces are a challenge. To meet the given demands of automated grasp therapy, a suitable actuator build is designed using VDI 2248. The build bases on an antagonistic SMA approach consisting of Nitinol spring combinations to match given boundary conditions, like necessary stroke and gripping forces for a physiologically correct hand movement. Furthermore, optimization for properties like a minimal size to stroke ratio is conducted. This paper delivers an early proof of concept based on a prototype for an SMA-actuated grasp therapy device for neurological rehabilitation.

Actuators based on the shape memory effect have recently become more and more economically important due to the many advantages of shape memory alloys (SMAs), such as their high energy density. SMAs are usually used to control the end/maximum positions, thus the actuators always move between two positions. The repeatable control of intermediate positions has so far proven difficult, because in most cases, external sensors are necessary to determine the length of the SMA element. Additionally control strategies for SMA actuators are rather complex due to the non-linear behavior of this material. The SMA actuator presented here is able to control intermediate positions with repeatable accuracy without the need of a separate control technology. The integrated control unit is based on a mechanical principle using a shaft with a circumference groove. This groove has a height profile that turns the shafts rotation, generated by the SMA, into a translational movement. Therefore, the SMA wire generates a partial stroke at each complete activation, turning the shaft partially. With several activation cycles in a row, the stroke adds up until reaching the maximum. A further activation cycle of the wire resets the actuators stroke to its initial position. Each part of the stroke can, thereby, be controlled precisely and repeatedly within the scope of each complete cycle of the actuator. Based on an integrated ratchet, each stroke of the actuator can hold energy free.