20th International Conference on Adaptive Structures and Technologies

October 20-22, 2009, Hong Kong

Automotive Research Advances in Smart Materials and Devices at the GM/UM SMS Collaborative Research Laboratory

Jonathan Luntz1, Diann Brei1*, Poorna Mane1, John Redmond1, Brian Barnes1, John Shaw2 ,

Nancy L. Johnson3, Alan L. Browne3, Paul W. Alexander3, Nilesh Mankame3 1 Mechanical Engineering Dept., University of Michigan, Ann Arbor, MI 48109-2125 USA

2 Aerospace Engineering Dept., University of Michigan, Ann Arbor, MI 48109-2125 USA

3 General Motors R&D, Warren, MI 48090-9055 USA

ABSTRACT

The field of smart materials and structures has evolved in the last 10 years to the point of viability

for commercial mass-production industries. A good example of this is the automotive industry where

there has been a 600% increase in patent applications. While the fundamental science is present for

many smart materials such as shape memory alloys or piezoelectrics, there have been only a few

instances of successful commercialization due to lack of technology and industrial infrastructure. To

make a clear path to hasten the transition from all the progress made during the past decades of

research into launched competitive products, General Motors (GM) has teamed with the College of

Engineering at the University of Michigan (UM) to establish a $2.9 Million Collaborative Research

Laboratory (CRL) in the area of Smart Materials and Structures. The mission of the SMS CRL is to

exploit the emerging capabilities of smart materials and structures to create and support innovative

advanced device technologies for application within the automotive industry. The SMS CRL is

structured into three thrust areas: Smart Device Technology Innovation, Smart Materials Maturity,

and Mechamatronic System Design Methodology. This paper presents a brief overview of the

mission, global infrastructure and collaborative philosophy of the GM/UM SMS CRL and its recent

advancements including: devices for pedestrian protection, Shape Memory Alloy (SMA)

technologies such as ultrafast latches and linear ratchet drives, SMA material modeling,

characterization, and shakedown studies, and model-based design tools for SMA wires. The most

important accomplishment has been the new research and development paradigm that has at the core

a truly collaborative, cross–talk relationship between all the multi-level thrust areas and the SMS

CRL team composed of researchers, students, and engineers across the world resulting in synergism

between projects to accelerate the time between theory and concept to fruitful applicable results.

Keywords: smart materials, active materials, shape memory alloys, mechamatronics

* Corresponding author: University of Michigan, Mechanical Engineering, 2350 Hayward St., 2250 G.G. Brown, Ann

Arbor, MI 48109-2125,Tel: (734) 763 6617; Fax: (734) 647 3170; E-mail: dibrei@umich.edu

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1 INTRODUCTION

For several decades, the military, industrial, and medical sectors have invested in smart

materials-based actuation to enable elegant, energy dense actuation solutions for a variety of devices

[1]. This effort has provided an enhanced understanding of a suite of active materials, and provided a

rudimentary foundation for the analysis of smart materials devices. Yet until recently, only a handful

of niche smart materials-based products have successfully transitioned to market. Examples include

piezoelectric actuators in compact audio speakers, accelerometers and micro-positioning systems

[2,3], magnetorheological shock absorbers or struts for use in vehicle suspension systems [4], shape

memory polymer repair material for auto repair applications, and SMA arterial stents, valves, and

couplings [5]. Smart material actuators often exceed the energy and power densities of conventional

actuators by orders of magnitude, are simplistic in architecture relative to typically complex

assemblies found in conventional actuation, and create opportunities for compact, economical

actuation. These can lead to true benefits for industry as evidenced by over 600% growth in smart

material patents in the last decade alone [6].

One good example of this growth is the automotive industry with important applications such as

adaptive aerodynamics for increased fuel economy, pedestrian protection and enhanced energy

dissipation for crash safety, active suspensions, and controllable closures [7-10]. Despite the

potential of smart materials to transform the automotive industry, it has been difficult to transition the

technology to market due to material complexities and a lack of design tools to support engineers.

For example, shape memory alloy (SMA) is highly desirable for automotive applications due to its

robustness, high energy density, and ability to manufacture high yields at low cost. For many years,

several technical barriers have stood in the way of its widespread use such as low frequency,

difficulty to accurately predict performance under varying conditions, performance degradation over

time, and the need to package long or many wires within compact form factors. The complex

behavior of the material requires a fairly detailed level of material knowledge to design devices.

Unfortunately, general methodologies or design tools to aid inexperienced engineers are rare. In

addition, while SMA allows many options in terms of device architecture to provide tailored

performance and packaging of devices, there currently does not exist a “critical mass” of device

architectures with corresponding performance models to support the model-based design of SMA

devices; thus, designers are typically forced to start from scratch. This is compounded by the fact that

the workforce is unfamiliar with SMA and how to engineer with it. Thus, in addition to inventing

new technologies, the foundational science, design tools, and education regarding smart material

actuation (and all its nuances) are crucial in enabling the incoming workforce to be savvy smart

materials engineers. All of these issues are common across smart materials, and are not unique to

SMA.

To speed the transition from the past decades of research into launched competitive products,

General Motors (GM) has teamed with the College of Engineering at the University of Michigan

(UM) to establish a $2.9 million Collaborative Research Laboratory (CRL) in the area of Smart

Materials and Structures. The mission of the SMS CRL is to develop the knowledge base to exploit

the emerging capabilities of smart materials and structures to create and support innovative advanced

device technologies for application within the automotive industry. This paper presents a brief

overview of the mission, global infrastructure of the GM/UM SMS Collaborative Research

Laboratory along with a set of examples highlighting the synergistic nature of the work within the

SMS CRL with cross-talk between all areas. The example projects center around a particular device:

the SMART (SMA ReseTable) hood lift for pedestrian protection, from which a wide variety of

projects have spawned. Two main SMA technologies are presented, SMA driven ratcheting and

ultrafast latches, along with fundamental research in SMA material modeling, characterization, and

shakedown behavior that enabled the model-based design of these core technologies. General

packaging methods are also described such as spooled SMA wire actuators and SMA cables without

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which many of the technological developments would not be practical. A general approach to

creating design tools is also discussed which enable general engineers, without experience in smart

materials, to develop smart materials based devices and to take advantage of advancements in smart

materials. Stressed in the paper is the collaborative, synergistic research paradigm adopted by the

SMS CRL which empowers this team composed of researchers, students, and engineers across the

world to advance the field of smart materials and efficiently and effectively bring theoretical

developments to marketable application inside and outside the automotive industry.

2 GM/UM SMS CRL STRUCTURE

The overriding goal of the GM/UM SMS CRL is to exploit the emerging capabilities of smart

materials and structures to create and support innovative advanced device technologies for

automotive applications of value and interest to GM and extension to dual technologies for strategic

partners. To this effect the SMS CRL has a constantly evolving mission as new technologies emerge

and the knowledge base grows. To achieve this mission the SMS CRL has identified key thrust areas

which identify research issues and develop tools and design methodologies to integrate smart

materials into mature automotive applications.

2.1 Mission

The SMA CRL was started in 2006 with the overall goal of exploiting smart materials based

research to propel transformative device technologies into the marketplace towards the reinvention of

the automobile. There are many facets to this mission including:

Invention: Utilize emerging smart materials to invent embryonic leap-frog device

technologies with potential to transform the marketplace.

Science: Conduct theoretical and experimental research to expand the knowledge base of

smart materials to facilitate the development of new technologies for automotive

applications.

Tools: Develop design methodologies and tools to support smart material technologies,

quicken the development cycle and successfully progress them to marketable automotive

products.

Application: Foster a collaborative and synergistic environment between researchers and

engineers at GM, UM and their strategic partners, bringing together a breadth and depth of

expertise to improve the progression from basic materials research to innovative

technology development and ultimately its inclusion in successful, marketable products.

Education: Provide research opportunities for graduate students and enhance GM‟s

ability to attract exceptional engineering talent with training in areas of interest to GM.

2.2 Research Thrust Areas

To achieve this mission the SMS CRL is focused initially on three thrust areas: 1) Smart Device

Technology Innovation, 2) Smart Material Maturity, and 3) Mechamatronic System Design

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Methodology. It should be noted that these thrust areas, their projects, and this organizational

structure as a whole are expected to evolve over time in response to changing needs and the SMS

CRL structure allows this adaptability.

The primary focus of the Smart Device Technology Innovation thrust area is to invent new smart

material based device technologies that push the frontier of capabilities. While it is foreseen that most

of these technologies will be initiated by specific automotive applications, the SMS CRL seeks to

develop a robust technology base rather than a “gadget”. It is expected that by utilizing smart

materials, substantial advances can be made leading to disruptive, leap–frog type technologies versus

incremental steps beyond conventional technologies within the market place. By developing new

technologies versus specific artifacts/devices, these technologies will have broad application across

many vehicle components/systems and relevance to many other industries such as appliance, oil,

military, aerospace and medical.

It is fully expected that during the course of the technology development, that many basic

research issues will be faced. Thus, the objective of the second thrust area, Smart Material Maturity,

is to carry out theoretical and experimental research to expand the knowledge of smart materials with

regards to fundamental issues affecting implementation such as material uniformity and reliability

and the effect of environmental factors and use history on performance and use life. Such issues are

currently viewed as unknown frontiers impacting the commercial viability of smart materials. The

intent is not to develop new materials but instead usher existing materials to a level of maturity which

allows for successful integration into products.

The third thrust area is Mechamatronic System Design Methodology where mecha- stands for

mechanical, ma- stands for smart materials and –tronic stands for electronics [11]. The objective of

this thrust area is to build a better understanding of the design process suited for these systems

enabling the creation of synthesis and analysis methods/tools to quicken the design cycle and enable

engineers both familiar and unfamiliar with smart materials to quickly and with confidence develop

robust systems based upon the new technology foundations established in the SMS CRL.

2.3 Infrastructure

A key feature of the SMS CRL is its cooperative and collaborative infrastructure. First, the

leadership roles within the SMS CRL are cooperative. Each thrust area and the projects within each

thrust area has leaders from both GM and UM and the entire SMS CRL is jointly directed by a

director from UM and a director from GM. The project co-leaders are responsible for the execution of

the project and any identified deliverables. The Laboratory co-directors, under the recommendations

from the Thrust Area Leaders, have the responsibility for coordinating research and selecting

personnel to work in these areas of technical focus. The co-directors also review and approve all new

project requests and business plans consistent with the technical thrusts of the Lab with the assistance

of an external Laboratory Review Committee.

Second, cross-functional and cross-organizational communication is expected. All the leaders are

expected to be proactive members throughout the process with expanded roles and strong

communication between all – at least on a weekly basis and in many cases on a daily basis. Nothing is

“thrown over the wall”. In addition, the entire SMS CRL meets informally bi-monthly for seminars,

tutorials, logistic meetings, etc. Finally, there are quarterly formal reviews with the SMS CRL team

along with the external General Motors Institute Review Committee to assess the progress and

guide/redirect efforts where needed. This aids in the identification of the core needs and real basic

issues that are at the heart of both institutional missions and which can be addressed through a

coupled approach.

Third, research projects, and hence students‟ theses/dissertations, span multiple thrust areas. A

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typical research project is expected to lead to a new (perhaps patentable) technological development

from the Smart Device Technology Innovation Thrust that leverages basic research findings from the

Smart Material Maturity Thrust, and the student follows through to establish the design knowledge

base and methodology for the Mechamatronic System Design Methodology Thrust so that engineers

in industry can utilize the student‟s contribution for real applications. Students are co-advised by

different faculty within the three thrust areas during different phases of their research and work

closely with various researchers/engineers at GM as the project advances. In addition to aiding in

cross-communication between thrust areas, this method is effective in training a broader,

well-rounded researcher in basic, fundamental research methods who is also, through this experience,

aware of the pragmatic issues faced in industry so that she/he can have a true impact on society. This

is critical to help form the necessary industrial workforce to support the field, and several UM

engineering graduates have already moved on to careers at GM. Thus, programs like these help to

train future leaders in industry.

Since its inception in May 2006, the SMS CRL has expanded to include many other strategic

partners beyond the initial researchers at University of Michigan (from the Mechanical and

Aerospace Engineering Departments) and General Motors Research and Development (GM R&D) in

Warren Michigan. It includes participation from other GM divisions and affiliated organizations with

which GM R&D already had pre-existing long term collaborations in the area of smart materials,

these including GM Engineering, Opel, India Science Lab, and HRL Laboratories. It is a true global

effort. While the primary focus is to develop technology for the automotive industry, many of the

technologies along with the supporting basic research and design methods are applicable across

multiple industries.

3 SYNERGISTIC RESEARCH ACTIVITY

The success of the SMS CRL is due, in large part, to its synergistic paradigm in which each

project draws upon and provides motivation for other projects, blending the lines between projects

and between thrust areas. Each project spans the spectrum from basic science to applied research to

technology development, producing more complete, more relevant results. To illustrate this, this

paper provides a tour of recent advancements within the SMS CRL via a unifying project: the

SMART Lift Device for Pedestrian protection. Pedestrian protection is a major focus of automotive

crashworthiness with new regulations taking effect worldwide [9,10]. While there are many

approaches to reducing the head-injury-criteria (HIC), a leading approach is to actively lift the hood

to increase the crush distance to rigid underhood components [12-15]. One of the key technological

challenges is the very tight timing window (between 25 and 30 ms) given the large displacements

(over 120 mm) and forces (over 1400 N) - too fast a lift may damage the hood under its own inertia

and too slow a lift will not create sufficient clearance in time to reduce HIC. Most current lift devices

are single-use, requiring the hood to be manually returned to a drivable position, and could possibly

damage the hood during lift due, as indicated, to too rapid lift rates which damage would necessitate

hood replacement. The SMART (SMA ReseTable) Lift Device (Figure 1), developed within the

Smart Device Technology Innovation thrust area, solves these problems by storing energy in a spring

and very quickly releasing it (within 4 ms) to lift the hood upon detection of an impact with a

pedestrian [9,10]. The device provides enhanced functionality over other alternatives in that the hood

can be automatically lowered to a drivable position immediately following deployment, and that the

deployment energy can be restored in the device for repeated deployment without need for user

intervention. Automatic lower and reset are critical in the case of false deployments that might result

in the case of conservative sensing strategies, an example being collisions with non-pedestrian

objects such as trash cans. For this device to be successful, a broad range of synergistic research

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activity was necessary ranging from technological developments such as high force and stroke SMA

ratchets and ultrafast latches, to basic materials research to model and characterize the SMA material

and understand its shakedown behavior over cycles, to design methodology to explore tradeoffs

between packaging and performance in terms of force and stroke, and develop methodologies and

tools for streamlining the design process. While these synergistic activities are described in this paper

in the context of the SMART Lift as an example, all the activity within the SMS CRL has this highly

collaborative and synergistic nature with a three-way supporting relationship between each of the

thrust areas, and a great deal of cross-talk between all projects. Nurturing this synergy is a strong

priority within the SMS CRL and a key to the depth and breadth developed throughout all the projects

and thrust areas.

3.1 TOoThed Linear RAtchet Drive: TOTLRAD (Smart Device Technology Innovation

Thrust)

Unique to the SMART Lift approach for pedestrian protection was the reset actuation. This

required a non-backdriveable actuator capable of safely compressing the spring used to lift the hood,

producing very large forces (over 1400 N) and large strokes (over 120 mm) while compactly

packaging inside the spring itself. Since the timing of the reset was not critical in that it could be done

over a longer period of time to arm the lift device after the hood was lowered, a time leveraging

approach was employed where an SMA wire driven ratchet compresses the spring [9,10]. Ratcheting

with SMA is particularly challenging because the intermittent, non-monotonic nature of the ratchet

mechanism and the nonlinear hysteretic behavior of the SMA makes it difficult to predict the

performance without a highly detailed model and characterization of both the ratchet mechanism and

Figure 1. SMART Hood Lift Device operational states. A single lift device mounted under each rear corner of the

hood is armed with energy stored in the compressed spring, lifts when an ultrafast latch releases the spring, raising

the hood, lowers after collision to a drivable position, and resets automatically storing energy in the spring via a

toothed linear ratchet drive.

Armed Lifted Lowered Reset

Latch Opens

Device

Lifts

Crescent

Latches

Open

Device

Gravity

Drop

Spring

Compressed

SMA Ratchet

Operates

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the SMA performance. In addition, since SMA shakes down in performance over cycles [16,17], and

this shakedown is highly load dependent, designing a high performance SMA ratchet requires a

deeper understanding of this shakedown process. The highly compact design requirements of the

SMART lift device necessitated the use of a spooled packaging technique while the large forces

required bands of multiple SMA wires, the design of which could be further improved through the use

of SMA cables. All of this has culminated in the development of predictive performance models to

enable design, analysis, and control of SMA driven ratchets. A closed form kinematic model of the

ratcheting system was developed and implemented in a condition based state machine to account for

the various intermittent contacts experienced in a ratchet (Figure 2). The tooth and pawl kinematics

and geometry were coupled to the SMA model developed in the Smart Material Maturity thrust area

enabling real time simulation of a ratchet drive via Simulink. This technology model is applicable not

only to the SMART Lift device, but to any SMA driven ratchet useful within the automobile in

devices such as car seats, automated jacks and sunscreens, and suspension pre-loaders, and outside

the automobile in applications such as medical implant devices [18,19] and structural panel

deployment [9,10]. The many design issues inherent to an SMA driven ratchet couple closely to a

variety of SMS CRL projects including SMA characterization, shakedown, cabling, spooled

packaging, and design tool frameworks.

3.2 Modeling and Characterization (Smart Material Maturity Thrust)

The design and analysis of SMA actuators such as those within the SMART lift is complicated

due to several factors such as the material nonlinearity, hysteresis, and time and path dependent

behavior. While many different constitutive models have been developed over the past two decades

[20-29], they can be difficult to utilize in practical engineering applications due to their complexity,

the dearth of experimental validation and the need for reliable material property databases. Through

the SMS CRL, rigorous thermodynamically-based models for the coupled thermo-mechanical

behavior have been developed, along with lumped reduced-order models to aid in the swift design of

Figure 2. TOoThed Linear RAtchet Drive (TOTRAD) prototype device and analytical and finite element model

developed by the SMS CR combining efforts in device innovation, shakedown, and material maturity.

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practical systems [22,23]. Because both the full and reduced order models are based on the physical

understanding of the material, fewer parameters are required to calibrate the model against a

particular alloy relative to an empirical model. To support these models and activities, in-depth

material characterization studies of off-the-shelf materials, including Nitinol wire of two

compositions and pre-conditioned Flexinol wire (Dynalloy), have been conducted to accurately

measure transformation temperatures, specific heats, and latent heats by differential scanning

calorimetry and thermo-mechanical responses using custom-designed experimental setups [30,31]. A

series of isothermal experiments were conducted to show the dramatic range of tensile responses in a

temperature window spanning the respective stress-free transformation temperatures (Figure 3).

Measurements on two Nitinol alloys with slightly different compositions showed, for example, that

while the specific heat was always near 0.45 J/(g-K), the latent heats of transformation were

quantitatively different, 19.7 versus 15 J/g (M→R→A) for (room temperature) shape memory wire

and superelastic wire, respectively. Characteristic transformation stresses and strains were mapped to

quantify the thermomechanical sensitivities in the material. The different alloy compositions not only

shifted the transformation temperatures significantly, the different latent heats resulted in different

Clausius-Clapeyron slopes in plateau stresses, 9.2 versus 6.7 MPa/K, respectively. In these studies

techniques were developed for improved temperature and strain control. Infrared imaging was used to

measure localization of temperature fields to quantify the sensitivites to loading rate and the thermal

environment. Such characterization studies add to the existing SMA database and provide valuable

insight towards model development and more efficient application of the technology. In the SMS

CRL these modeling and characterization results feed into the design of devices in the Device

Innovation thrust area and provide crucial understanding for the development of design

methodologies.

3.3 Shakedown (Smart Material Maturity Thrust)

One of the key nuances of SMA is the degradation in performance as actuation cycles accumulate,

particularly at larger loads, motivating manufacturers to recommend very conservative operating

limits. Constant-tension thermal cycles were applied to SMA wires at several load levels, recording

strain, temperature, and electrical resistivity simultaneously using a thermoelectric heating/cooling

technique which enables relatively rapid temperature rate (1 °C/s) while preserving temperature

Figure 3. Example experimental thermomechanical results on superelastic Nitinol

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uniformity within 1.5 °C. The least shakedown occurred around 191 MPa (near the supplier‟s

specification), with a small amount of reverse shakedown at lower loads and progressively larger

shakedown at higher loads (Figure 4). Measurements strongly suggested that shakedown occurs

primarily at high temperatures during transformation to the austenite phase. Identifying when

damage occurs will be helpful for constructing a shakedown model useful across arbitrary

thermomechanical loading paths. To expand the operating regimes, the SMS CRL has studied

techniques to cycle or shake down SMA wires under controlled conditions prior to installation

enabling the designer to design to the stable post-shakedown specification of the wire to produce

actuators with repeatable, larger forcing capabilities [16,17]. This is particularly critical for high

performance multi-cycle devices such as the reset ratchet in the SMART lift. Experimental studies

were conducted to explore the functional dependence of shakedown performance on loading and

strain history. SMA wires were thermally cycled under electrical heating under a range of applied

loads from 29 to 78 N (where the manufacturer‟s specification on the 15 mil diameter wire is 20 N)

and allowed strains from 4 to 7%. The steady-state and shakedown-rate performance was

characterized with a double-exponential empirical model fit to explore the functional dependence of

performance on loading conditions which was found to vary greatly (Figure 5). For example, the

absolute strain lost through shakedown was found to increase linearly both with maximum allowed

strain and with load applied, while relative motion loss varied only with applied load. Understanding

these relationships, allows the designer to incorporate shakedown in design decisions. For example,

tradeoffs can be made between motion produced and fatigue life by adjusting the allowed strain, or

between efficiency of material use and overall package length by varying the number of wires (or

wire diameter) sharing a load.

To develop a better understanding of the fundamental mechanism behind shakedown, further

experimental studies were conducted with the aim of supporting the development of a complete

shakedown model. Such a model would be useful, for example, to predict the shakedown response of

the complex loading conditions experienced in a toothed ratcheting device. These experimental

studies expose important shakedown parameters affecting SMA actuator performance and provide a

first step towards creating detailed SMA wire shakedown protocols tailored to the application that

will enable the design of higher performance stable SMA actuators.

Figure 4. Evolution of cyclic strain-temperature behavior of Flexinol wire at constant moderate (left) and high

(right) stress levels

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3.4 Shape Memory Alloy ReseTable (SMART) Latch (Smart Device Technology Innovation

Thrust)

Another key component to the SMART Lift device is the ultrafast slot latch which was

demonstrated to be capable of releasing the lifter in less than 4 ms to achieve precise lift times [32].

Latching is core to the SMS CRL because of its universal usage in the car for applications such as

closures [33] and reconfigurable crashworthiness [10], and is widely useful outside the automotive

industry, for example as electronic assembly latches or spacecraft door latches [34-36]. The SMS

CRL has developed both linear latches (i.e. SMART Lift), and rotary latches (i.e. panel lockdown)

[32]. A good example that demonstrates the challenges and basic research issues is the rotary T-latch

[23,32] (Figure 6) which operates by passively engaging via a pair of ramps, retaining a strong

structural connection in multiple degrees of freedom with zero power consumption, and actively

releasing very quickly (demonstrated in less than 20 ms) when the spooled SMA actuator is

electrically heated, and automatically resetting, ready for engagement when the SMA cools. While

the T-latch was designed specifically for the application of hood latchdown for tailorable

Figure 5. Design-oriented shakedown study results depict the evolution of motion over hundreds of cycles at

different loads (left) and the % motion lost as a function of load (right)

Figure 6. Diagram of the Shape Memory Alloy ReseTable (SMART) Latch. The T-latch (left) is shown with its

main components. The latch connects two bodies, one body is connected rigidly to the gate and the other is

connected rigidly to the upper plate. The torsional reset spring is mounted between the T shaft and the upper plate

such that it opposes the torque applied by the SMA wire. The latch is demonstrated (right) to release very quickly

(less than 30 ms) in this example.

Latch structure

T

Gate

Spooled SMA wire

Insulated sheath

Torsion reset spring

T shaft

T shoulders

T terminal end

Engagement ramp

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crashworthiness, it represents a general technology which is captured by analytical operational

behavioral models based on a simplified constitutive law for SMA and first-order mechanics

regarding the loads applied to the T-latch throughout its operation. These predictive analytical

models were used to define design constraints ensuring feasible operation throughout the entire

latching cycle (engagement, retention, release, and reset). A full-scale T-latch prototype was

designed and built to meet standard automotive specifications for hood lockdown, and demonstrated

successfully, releasing extremely quickly (less than 20 ms) while requiring only minor amounts of

energy (about 20 J – a fraction of a percent of that stored in a typical AA battery). This work

motivated a study within the Design Methodology thrust area to better understand the design

tradeoffs between packaging, losses, and range of motion within spooled actuators. The ultrafast

latch development within the SMS CRL represents advancement over traditional latch technologies

with extremely fast release times, compact form factors, light weight, and small release energies with

the promise to improve upon and enable a wide range of devices both inside and outside the

automobile.

3.5 Packaging (Mechamatronic System Design Methodology Thrust)

The high energy density of SMA enables lightweight, low cost actuation but is limited by

packaging constraints. To provide large strokes, devices require long lengths of wire, while to

provide large forces, devices require either larger diameter wires or for shorter full cycles several

wires acting in parallel. Packaging for enhanced stroke and force while keeping within design

constraints is a challenge faced throughout the SMS CRL, particularly in the SMART lift which

requires large forces and strokes and in the T-latch which must provide large rotary motions in a very

compact form. Two of the primary packaging techniques studied in the SMS CRL are: spooling to

provide large stroke in a small package, and cabling to provide large forces in a simple form.

3.5.1 Spooling (stroke)

The spooled packaging technique wraps SMA wires around pulleys and mandrels for increased

compactness and tailorability while providing large linear or rotary motions. Spooling allows wires

to be routed to accommodate form factor constraints that do not otherwise allow long wires such as

was done in the SMART Lift device, and the actuator to be compacted down to as little as a single

mandrel to reduce the overall footprint such as was done in the T-latch. Spooling creates a design

tradeoff since form factor improvements come at the expense of friction losses between the SMA

wire and the mandrel surface and bending losses and potentially accelerated fatigue due to additional

bending strains. An analytical model was developed to relate the motion of a generalized spooled

actuator to its geometry, material properties, and the external load properties [37] (Figure 7). The

model accounts for distributed friction mechanics as well as the non-linear, phase and stress

dependent properties of the active material, and is flexible, accommodating a variety of

configurations including linear and rotational motion outputs and topologies involving multiple

spools. Experimental studies demonstrated the model‟s accuracy with respect to applied load, wrap

angle, and spool position and correlated very well with theory in both form and magnitude. Design

tradeoffs were exposed, for example, that increasing wrap angle reduces overall package size with a

tradeoff of reduced performance and an upper limit on the amount of packaging due to binding

[37,38]. Based on the results of this study, spooled packaging techniques and the accompanying

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predictive model provides a useful foundation for analytical actuator design for synthesizing high

performance actuators with compact packaging and minimized losses.

3.5.2 Cables (force)

While spooled packaging is aimed at providing increased stroke in a smaller package, SMA

cables are aimed at providing large forces in a compact, robust, easy to manage form factor. Applied

to either shape memory or superelastic alloys, this form has the potential to provide increased bending

flexibility for packaging, better fatigue performance, energy absorption and damping, reduced

thermal lag, redundancy, and significant design flexibility as compared to monolithic SMA rods of

comparable cross-sections. SMA cables have benefits over monolithic SMA elements as force or

displacement amplifiers and leverage the excellent properties of SMA wire in built-up large-scale

Figure 7. Spooled packaging model (left) predictions match experimental results at different wrap angles over a

range of loads (right).

Figure 8. Thermomechanical Response of the 7x7x0.239 mm SMA cable

Wire stress σ(M)

dSMAInput tail

length = ℓt,in

Fext – externally

applied load

State 0 wire, ε=0

State 1 wire, martensite (ξ(M)=1)

State 2 wire, austenite (ξ(M)=0)

ℓ - actuator

range of motion

R

Wrapped portion

length = ℓw

angle = θw

curvature = ρSMA

=(R + ½dSMA)-1

Output tail

0

,t out

,

A

t out

,

M

t out

Linearly sliding

motion output

Spool

Position

dsp = ℓt,in

A

A

0 5 10

0

5

10

15

20

0 50 100

Applied Load (N)

Str

ok

e (

mm

)

Applied stress (MPa)

strokedata fittheory

0 5 10

0

5

10

15

20

0 50 100

Applied Load (N)

Str

ok

e (

mm

)Applied stress (MPa)

strokedata fitμ=0.1μ=0.125

Page 13

tension structures. They also inherit many of the desirable attributes of conventional stranded rope in

terms of load carrying redundancy and increased bending compliance for packaging and spooling.

Exploratory thermomechanical experiments were conducted for a conventional cable design and its

sub-components [39] (Figure 8). The 7x7 design behaved similar to wires loaded in parallel, making

this design a natural choice for designers who wish to leverage the superior properties of SMA wires,

but need higher forces in a compact, easy to handle, and flexible package. Some important differences

do exist between the 7x7x0.279 mm cable and a single 0.279 mm straight wire: (1) an extra 1.3% of

transformation strain at the cost of about 40 MPa of loading plateau stress, and (2) a reaction torque

under zero-rotation constraint due to the chiral nature of the cable. Surprisingly, the reaction torque

has a maximum at the onset and a negative slope during the A → M+ transformation. Also,

self-heating/cooling effects and localized transformation are similar to that of straight Nitinol wire in

a range of loading rates. Experimental results also show nearly indistinguishable mechanical response

of a lubricated cable from a dry cable, a demonstration of the shape memory effect in an SMA cable,

and a relatively linear relationship between the axial plateau stresses and the specimen temperature.

In addition to demonstrating large forces in a convenient package the tests have shown that cables

with high helix angles (~50°) leverage displacement with recoverable „global‟ strains of ~12% as

compared to the low helix angle cables (~8°) which have strains similar to single SMA wires. These

initial experiments provide a preview of the advantages and design flexibility that SMA cables

provide.

3.6 SMA Wire Actuator Modular Design Framework (Mechamatronic System Design

Methodology Thrust)

As new design options become available such as spooled packaging and SMA ratchet drives, and

new fundamental knowledge is gained regarding material models, characterization, and shakedown

behavior, design tools are required which are modular and extensible in nature such that designers

with a variety of background and experience can incorporate these new developments into their

designs. The SMS CRL has developed the means of developing such tools centered on a modular

framework (Figure 9) upon which such tools can be based [40]. The inherently layered structure of

the SMA actuator design framework is broken up by function: modeling, evaluation, optimization,

and design guidance. Many of the components are represented as sets of “cards” which can be

switched to make local changes to the system. Optimization fits naturally under the framework since

the structure containing the identity and location of the design variables is clearly defined, as does

guidance through the design process based on various heuristics, expert experience, and discrete

optimization and automated design of experiment. An example design tool was implemented in

MATLAB based on the framework, and functions as an aid in the design of actuators employing

multiple straight SMA wires working in parallel against linearly varying (spring-like) and constant

quasi-static loads. This tool is powerful enough to allow a large variety of designs, and provide

detailed evaluation of design performance including automated parametric studies, but is structured

to guide the user through the actuator design process without requiring a deep knowledge into SMA

behavior. The developed tool is just an example of the variety of specialized and general purpose

design tools enabled by the framework which promise to enable designers with a variety of

backgrounds to overcome the difficulties involved in SMA wire actuator design, and to streamline the

wide use of SMA wire actuation in commercial and industrial applications.

Page 14

3.7 From Science to Technology Transition: The SMART Lift

The synergy among the work across all three thrust areas comes together to enable a fast

progression - blending basic research with application to produce technologies ready to be

transitioned to component suppliers for the automotive industry. The SMART hood lift device for

pedestrian protection is a prime example of such synergism, integrating advancements throughout the

SMS CRL. The architecture of the SMART hood lift device was built upon SMS CRL efforts from

the Technology Innovation thrust area on ultrafast latches for the release and lower functions, toothed

ratchet drives for reset, and spooled packaging to maintain a compact form within the interior of the

spring. The SMA wire actuators were designed with predictable performance with the aid of the

newly established tools and methodologies in the Design Methodology thrust area, where these tools

were based on the basic material understanding learned in the Smart Materials Maturity thrust area.

The device was designed based on analytical and simulation models of lift performance and full-scale

prototypes were built and installed in a hood lift vehicle testbed (Figure 10). The full functionality of

release, lift, automatic lower and reset were all repeatedly demonstrated, with ultrafast latch release

times of less than 4 ms and application-appropriate hood lift times just over 30ms. This enables the

SMART hood lift device to meet the demanding time window without damaging the hood and

provides the useful added adaptive functionality of automatic lower and reset upon a false deploy and

to de-energize when under-hood maintenance is required. This complicated technological success

was only possible in the quick time frame due to the synergistic interaction and indepth understanding

gained across all aspects of the CRL and the close communication.

Figure 9. Modular framework structure. The basic four layer design framework structure separates the

modeling, evaluation, optimization, and guidance functions into sub-function blocks within each layer

User Interface, Educate user, Identify design task, Aid Selection, Determine optimization

objectives

Material

Design

Evaluation

Design Guider

Optimizer

Architecture

Solution Process

Design Decisions

Data

Mapping Functions

σ, ε, T

Properties

Genetic Algorithm,

Simulated Annealing,

Fsolve, etc

Intersection

Solver

Actuated

System

Load

Profiles

Internal

Architecture,

External

Architecture,

Interfaces

Metrics for

Architecture,

Actuator,

Material

Gu

ide

r

La

ye

r

Mo

de

ling

La

ye

r

Eva

lua

tion

La

ye

r

Op

timiz

atio

n

La

ye

r

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4 CONCLUSIONS

This paper describes General Motors new collaborative research laboratory with the University of

Michigan in the area of smart materials and structures. The intent of the SMS CRL is to significantly

aid in leading the smart materials and structures field to a competitive status within the automotive

industry and impact the marketplace with revolutionary ideas and products that have the ability to

disrupt and redefine the industry. The mission of the SMS CRL is to establish the scientific

knowledge base and necessary design tools/methodologies while training the workforce leadership

required to exploit the emerging capabilities of smart materials and structures to create innovative

advanced device technologies for application within the automotive industry and extension to dual

technologies for strategic partners. This SMS CRL is unique in its breadth of approach, ranging from

basic research in the Smart Material Maturity Thrust, scientific methods in the Mechamatronic

System Design Methodology Thrust, to applied research in the Smart Device Technology Innovation

Thrust. Crucial to the success is the synergy gained through the close partnership between researchers

at General Motors, University of Michigan and other strategic partners based upon strong

cross-functional, cross-organization communication ties. The culture in the SMS CRL fosters a

strong synergy between projects and between thrust areas with a great deal of cross-pollination

ensuring that the work done has a broad and complete focus and that each project incorporates aspects

of basic science and technology application to maintain relevance. With the success of the variety of

research projects underway, many more significant advances are expected in the future years that will

have impact beyond General Motors and the automotive industry.

5 ACKNOWLEDGEMENTS

The authors would like to thank Alan Taub and Jan Aase from General Motors Research &

Development for their support and encouragement and for helping establish the SMS CRL as well as

VC Tom Stephens from General Motors and Dean David Munson from the University of Michigan

College of Engineering for their leadership of the General Motors / University of Michigan Institute

for Automotive Research and Education.

Figure 10. SMART Hood Lift Device developed by the SMS CRL. A single lift device (left) is mounted under each

rear corner of the hood in the vehicle testbed (middle) and can lift the rear of the hood more than 120 mm in just

over 30 ms (right).

Page 16

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