Andrew C. Keefe

Segmented Reinforcement Variable Stiffness Materials for Reconfigurable Surfaces

Reconfigurable and morphing structures may provide a range of new functionalities such as optimization over broad operational conditions and multi-mission capability. This article introduces a new generic approach to achieving large strains in materials with high elastic moduli (5-30 GPa). The work centers on creating variable stiffness composite materials which exhibit a controllable change in elastic modulus (bending or axial) and large reversible strains (5-15%). We have performed a simulation study to better understand the implication of various geometric design parameters on the elastic and deformation behavior. Using this information, a series of prototype materials were prepared using a commercial shape memory polymer, and measurements on these materials indicate a controllable change in stiffness as a function of temperature with large reversible strain accommodation. We have fabricated and tested several design variations of laminar morphing materials which exhibit structural stiffness values of 8-12 GPa, changes in modulus of 15-77x and large reversible axial of 2-10%. Results indicate that significant controllable changes in stiffness are possible. Further, agreement between simulations and prototype material properties indicate that simulations may be used an effective screening tool to specify micromechanical design variations for specific application requirements.

Highly curved image sensors: a practical approach for improved optical performance

The significant optical and size benefits of using a curved focal surface for imaging systems have been well studied yet never brought to market for lack of a high-quality, mass-producible, curved image sensor. In this work we demonstrate that commercial silicon CMOS image sensors can be thinned and formed into accurate, highly curved optical surfaces with undiminished functionality. Our key development is a pneumatic forming process that avoids rigid mechanical constraints and suppresses wrinkling instabilities. A combination of forming-mold design, pressure membrane elastic properties, and controlled friction forces enables us to gradually contact the die at the corners and smoothly press the sensor into a spherical shape. Allowing the die to slide into the concave target shape enables a threefold increase in the spherical curvature over prior approaches having mechanical constraints that resist deformation, and create a high-stress, stretch-dominated state. Our process creates a bridge between the high precision and low-cost but planar CMOS process, and ideal non-planar component shapes such as spherical imagers for improved optical systems. We demonstrate these curved sensors in prototype cameras with custom lenses, measuring exceptional resolution of 3220 line-widths per picture height at an aperture of f/1.2 and nearly 100% relative illumination across the field. Though we use a 1/2.3 format image sensor in this report, we also show this process is generally compatible with many state of the art imaging sensor formats. By example, we report photogrammetry test data for an APS-C sized silicon die formed to a 30° subtended spherical angle. These gains in sharpness and relative illumination enable a new generation of ultra-high performance, manufacturable, digital imaging systems for scientific, industrial, and artistic use.

Dynamically variable negative stiffness structures

A novel active structure supports loads while dynamically and continuously changing stiffness by more than 100× in less than 10 ms. Variable stiffness structures that enable a wide range of efficient load-bearing and dexterous activity are ubiquitous in mammalian musculoskeletal systems but are rare in engineered systems because of their complexity, power, and cost. We present a new negative stiffness–based load-bearing structure with dynamically tunable stiffness. Negative stiffness, traditionally used to achieve novel response from passive structures, is a powerful tool to achieve dynamic stiffness changes when configured with an active component. Using relatively simple hardware and low-power, low-frequency actuation, we show an assembly capable of fast (<10 ms) and useful (>100×) dynamic stiffness control. This approach mitigates limitations of conventional tunable stiffness structures that exhibit either small (<30%) stiffness change, high friction, poor load/torque transmission at low stiffness, or high power active control at the frequencies of interest. We experimentally demonstrate actively tunable vibration isolation and stiffness tuning independent of supported loads, enhancing applications such as humanoid robotic limbs and lightweight adaptive vibration isolators.

Development of a Shape Memory Alloy Heat Engine Through Experiment and Modeling

Few technologies can produce meaningful power from low temperature waste heat sources below 250°C, particularly on a per-mass basis. Since the 1970’s energy crisis, NiTi shape memory alloy (SMA) and associated thermal engines have been considered a viable heat-to-power transducer but were not adopted due to previously poor material quality, low supply, design complexity, and cost. Decades of subsequent material development, research, and commercialization have resulted in the availability of consistently high-quality, well-characterized, low cost alloys and a renewed interest in SMA as a waste heat energy recovery technology. The Lightweight Thermal Energy Recovery System (LighTERS) is an ongoing ARPA-E funded collaboration between General Motors Company, HRL Laboratories, Dynalloy, Inc., and the University of Michigan. In this paper we will present initial results from investigations of a closed loop SMA thermal engine (a refinement of the Dr. Johnson design) using a helical coil element and forced-air heat exchange. This engine generates mechanical power by continuously pulling itself through separate hot and cold air streams using the shape memory phase transformation to alternately expand and contract at frequencies between 0.25 and 2 Hz. This work cycle occurs continuously along the length of the coil loop and produces steady state power against an external moment. We present engine features and the thermal envelope that resulted in devices achieving between 0.1 and 0.5 W/g of shape memory alloy material using only forced air heat exchangers and room temperature cooling.Copyright

Theoretical and experimental investigation of architected core materials incorporating negative stiffness elements

Structural assemblies incorporating negative stiffness elements have been shown to provide both tunable damping properties and simultaneous high stiffness and damping over prescribed displacement regions. In this paper we explore the design space for negative stiffness based assemblies using analytical modeling combined with finite element analysis. A simplified spring model demonstrates the effects of element stiffness, geometry, and preloads on the damping and stiffness performance. Simplified analytical models were validated for realistic structural implementations through finite element analysis. A series of complementary experiments was conducted to compare with modeling and determine the effects of each element on the system response. The measured damping performance follows the theoretical predictions obtained by analytical modeling. We applied these concepts to a novel sandwich core structure that exhibited combined stiffness and damping properties 8 times greater than existing foam core technologies.

Lightweight thermal energy recovery system based on shape memory alloys: a DOE ARPA-E initiative

Over 60% of energy that is generated is lost as waste heat with close to 90% of this waste heat being classified as low grade being at temperatures less than 200°C. Many technologies such as thermoelectrics have been proposed as means for harvesting this lost thermal energy. Among them, that of SMA (shape memory alloy) heat engines appears to be a strong candidate for converting this low grade thermal output to useful mechanical work. Unfortunately, though proposed initially in the late 60s and the subject of significant development work in the 70s, significant technical roadblocks have existed preventing this technology from moving from a scientific curiosity to a practical reality. This paper/presentation provides an overview of the work performed on SMA heat engines under the US DOE (Department of Energy) ARPA-E (Advanced Research Projects Agency - Energy) initiative. It begins with a review of the previous art, covers the identified technical roadblocks to past advancement, presents the solution path taken to remove these roadblocks, and describes significant breakthroughs during the project. The presentation concludes with details of the functioning prototypes developed, which, being able to operate in air as well as fluids, dramatically expand the operational envelop and make significant strides towards the ultimate goal of commercial viability.

Active materials for automotive adaptive forward lighting Part 1: system requirements vs. material properties

Adaptive Frontlighting Systems (AFS in GM usage) improve visibility by automatically optimizing the beam pattern to accommodate road, driving and environmental conditions. By moving, modifying, and/or adding light during nighttime, inclement weather, or in sharp turns, the driver is presented with dynamic illumination not possible with static lighting systems The objective of this GM-HRL collaborative research project was to assess the potential of active materials to decrease the cost, mass, and packaging volume of current electric stepper-motor AFS designs. Solid-state active material actuators, if proved suitable for this application, could be less expensive than electric motors and have lower part count, reduced size and weight, and lower acoustic and EMF noise1. This paper documents Part 1 of the collaborative study, assessing technically mature, commercially available active materials for use as actuators. Candidate materials should reduce cost and improve AFS capabilities, such as increased angular velocity on swivel. Additional benefits to AFS resulting from active materials actuators were to be identified as well such as lower part count. In addition, several notional approaches to AFS were documented to illustrate the potential function, which is developed more fully in Part 2. Part 1 was successful in verifying the feasibility of using two active materials for AFS: shape memory alloys, and piezoelectrics. In particular, this demonstration showed that all application requirements including those on actuation speed, force, and cyclic stability to effect manipulation of the filament assembly and/or the reflector could be met by piezoelectrics (as ultrasonic motors) and SMA wire actuators.