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The down scalability of conventional actuators is limited by their physical working principles. This applies especially for micro actuators resulting in a massive disproportion between Workspace and cross section.

These scaling effects don’t apply to Shape Memory Alloys (SMA) resulting in an out-standing potential for micro actuator design. However, like conventional actuators SMA based actuators usually require guidance which negatively affect their cross section, mass and complexity. Hence, avoiding separate guidance would further reduce their mass, cross section and complexity. A possible approach is based on an angular difference between the direction of the SMA wire and working direction. Hereby, the stiffness of an SMA-wire is split into a longitudinal and a transversal component.

This approach possesses to design actuators with different workspace geometries like linear or rotary ones. The design process for the described solid state actuators is characterized by opposing demands. A high blocking force will, for example, lead to a small load-free stroke. This is a complex correlation between the vector of the variables for design and specification. To achieve a straightforward design process it can be formulated as a mathematical optimization problem to calculate the design parameters. This optimization process has been used to develop both a rotary and a linear actuator. For validation the actuators were experimentally investigated.

The rotary actuator was designed for moving ventilation flaps in automotive applications. The actuator consists of frame and end effector which are linked with two groups of SMA wires. Every group is responsible for one direction of movement. For fastening the SMA-wires, screw terminals are used to allow a simple and fast replacement of the SMA-wires. For positioning control the actuator can be equipped with a contactless Hall effect encoder with an angular resolution of approx. 0.1°.

The main performance of the actuator is determined using the motion blocking torque M_block and maximum load free angle ϕmax. The requirements (req.), the computed specifications (c.s.) and the measurement results (m.r.) are listed in Table 1.

Table 1: Motion parameters of the rotary actuator

It is apparent that the calculated values match the measured values with high accuracy. Furthermore the blocking torque exceeds its requirement by approx. 50%. Only the maximum load free angle can’t be achieved, which is caused by the small available space and the minimum size of the end effector.

The design of the linear actuator is based on the demands of a feed axis for small machine tools. Like its rotary counterpart the linear actuator consists of frame, end effector and two groups of SMA wires. The fastening of the wires is realized with aluminum crimps. This increases reliability during the cutting in length process of the SMA wires. Furthermore the actuator has a mechanism for tuning the pre tension of the wires. This enables a simple centering of the actuator. Hence, the risk of tilting the end effector can be minimized. Similar to the rotary actuator the linear one can be characterized with the two motion parameters blocking Force F_Block and maximum load free stroke z_max. The values for requirement, computation and measurement results are listed in the table below.

Table 2: Motion parameters of the linear actuator

Comparison of requirement, calculated and measured values shows very good accordance. The maximum load free stroke has a deviation of approx. 25% between requirement and measurement. This is the result of a difference between assumed and real material parameters of the SMA wire.

Since the selfguiding function is the main feature of the presented approach, special attention is paid to the stiffness of the actuators. For this purpose the end effector of the rotary actuator is loaded in different spatial directions and the position is measured. The rotary actuator has a stiffness of 1,81 N/mm-1 in moving direction. This value differs about 9% from the calculation.

Summary and outlook:

A novel solid state actuator design based on Shape-Memory-Alloy wires was presented. It possesses inherent system stiffness without requiring external guiding elements. Therefore, the force of the SMA-wires was divided into a transversal and a longitudinal component which is achieved by an angular difference between moving direction and wire.

Using a mathematical model the complex design of this type of actuators can be reduced to an optimization process. To validate the design both a linear and a rotary actuator were developed and experimentally investigated.

Further research will focus on the development of control algorithms to consider the strong nonlinearities of the SMA material to obtaining a constant positioning behavior along the complete stroke.