Joachim Schmidt, DCubed
Leon Christians, DCubed
Joachim Schmidt, Predictive Models for the design of High-Efficiency Architectures for Space Systems, DCubed
1. Motivation/Background
Adaptive space structures enable reversible shape reconfiguration between two or more operational states, allowing spacecraft instruments to dynamically adapt to mission phases and optimize performance.
Conventional deployable structures on the other hand most often do not require multiple stable states, but systems are often limited by weight, size, and complexity. Electric motors, while precise, lack the required power density and compactness for small satellite applications, and they introduce intricate electrical setups. In contrast, elastic springs and Belleville washers offer better power-to-weight ratios but suffer from uncontrolled deployments, risking high, chaotic accelerations that could damage the system.
Antagonistic Shape Memory Alloy (SMA) actuators effectively address these limitations by offering superior power-to-weight ratios compared to traditional springs. They enable smooth and controlled actuation while providing the capability to achieve multiple stable states, which is particularly advantageous for morphing structures. However, antagonistic SMA-based systems typically require complex control algorithms to achieve accurate positioning. To overcome this, a predictive antagonistic model was proposed Schmidt 2024, enabling the determination of highly accurate equilibrium positions of SMA actuators without control loops.
While the recently proposed framework has streamlined the design of SMA spring-based antagonistic actuators, it does not fully exploit the unique strengths of SMA technology. The spring architecture, though effective, fails to achieve the torsional limits of SMA materials when stretched fully. Alternative architectures, such as rotational (torsional) systems or linear tension-based mechanisms, can double or triple the power density. Moreover, SMA can be shaped into virtually any form, enabling designs that directly actuate deployable or morphing systems without the need for linkages or transmissions, further increasing efficiency and compactness.
2. Methodology
To extend the mole-based design framework for antagonistic Shape Memory Alloy (SMA) actuators, a phenomenological, phase-diagram-based modelling approach was adopted. This approach captures the shape memory effect and superelasticity by leveraging the constitutive model introduced by Brinson cite{brinson_one-dimensional_1993}.
The modelling framework is extended to torsional architectures and alternative configurations through the following steps:
Stress-Strain to Force-Displacement/Torque-Rotation: The stress-strain relationship from Brinson’s model was applied to derive the force-displacement and torque-rotation behaviour of the selected architectures.
Geometric Boundary Conditions: Force equilibrium and system length constraints dictate the equilibrium positions. The model assumes equal but opposite displacements of the antagonistic elements to visualize the actuator response in a single diagram.
To address cyclic training effects, the model incorporates a linear decrease in actuator stroke, as observed experimentally. This performance degradation is attributed to defect accumulation, dislocations, and the formation of stabilized martensite.
Finally, one architecture is selected for demonstration. Experimental data from this demonstration provides an initial verification of the extended framework, confirming its capability to predict equilibrium positions and actuation behaviour across alternative designs.
3. Architectures
Several promising Shape Memory Alloy (SMA) actuator architectures have been identified, leveraging the unique properties of antagonistic actuation. These architectures encompass both torsional and tensile configurations, offering high versatility in designing systems with improved power density and compactness.
The identified architectures fall into distinct categories.
Non-continuous rotary actuators produce finite rotation angles, typically below 360°. These include unidirectional actuators, such as simple rotary hinges or folding mechanisms that utilize SMA wires or sheets. Bi-directional actuators, on the other hand, employ antagonistic SMA wires, springs, bias springs, or two-way memory SMA elements to achieve reversible motion.
Torsional architectures utilize helical SMA springs, pre-twisted wires, or SMA-driven flexures to generate torque-based motion. Notable examples include helical spire designs, where pre-twisted SMA wires wrapped around a helical structure enable significant rotational angles, and gimbal drives, which use antagonistic helical springs and ball-screw mechanisms to deliver high torque and compactness.
Linear-to-rotary conversions employ SMA elements to transform linear displacement into rotary motion. This can be achieved through pulleys and cylindrical surfaces, where SMA wires are wrapped around drums or guided through pulley sets to increase output torque and rotational angles. Additionally, torsional bias springs facilitate bi-directional rotation via controlled SMA heating and cooling cycles.
The potential use cases for these SMA actuator architectures in space applications are compelling. Rotary Hold-Down and Release Mechanisms (HDRMs) employing antagonistic SMA springs provide smooth, controlled articulation for deployable structures, such as solar arrays or antenna booms. Torsional actuators enable shape reconfiguration in morphing structures, including adaptive antennas, radiators, or optical systems. Compact rotary release systems, utilizing SMA-based actuators, offer high torque and reliability in critical release mechanisms.
To validate the extended framework, one architecture will be selected for experimental demonstration. The resulting data will serve as an initial verification, confirming the framework’s capability to predict equilibrium positions, output torque, and rotational behaviour for the chosen design