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Bio-inspired, Modular, and Multifunctional Thermal and Impact Protected (TIPed) Embedded Sensing Controls Actuation Power Element (ESCAPE)

Structures

Lawrence S. Gyger, Jr., Brent W. Spranklin, Satyandra K. Gupta, and Hugh A. Bruck,

Department of Mechanical Engineering University of Maryland College Park, MD 20742, USA

Abstract There is a great deal of interest in creating new structural concepts, such as snake robots, from analogous biological systems. These biological systems are typically composed of materials that serve multiple functions. For example, the skin of a snake is a system consisting of a soft, hyperelastic covering, with nerves providing thermal and pressure sensitivity, and a hard, scaly coating to resist wear and tear from hard particles such as sand. Therefore, bio-inspired structures can also be composed of multifunctional materials. There are currently many approaches to fabricating bio-inspired structures with conventional materials. These concepts typically utilize a system-of-systems design approach where the structure will be composed of a structural framework, an actuation system, a power source for the actuation system, controllers, and external sensors. The drawbacks to these structures include the need to assemble all of the components, interfaces between components that can compromise reliability, constraints on scaling down the structure, and very high power consumption and power generation requirements. These problems have been addressed by developing a new scalable approach to creating multifunctional materials for bio-inspired structures where the controllers, actuators, and sensors are integrated into a modular, multifunctional structure. This approach is facilitated by using multi-material multi-stage molding processes with fully embedded electronic systems, and has resulted in new Thermal and Impact Protected (TIPed) Embedded Sensing Controls Actuation Power Element (ESCAPE) Structures for compact and rugged robotic applications. 1. INTRODUCTION Robotic concepts inspired by nature, such as snake-robots, are generating interest in areas such as search and rescue and reconnaissance because of their ability to maneuver into tight spaces in unstructured environments where many other conventional robots cannot travel. They can be made with small cross-sections, and because of their hyper-redundant characteristics can operate even with failed segments. Considerable progress has been made in development of control systems and kinematics for such robots. However, manufacturing reliable and rugged snake-robots remains a major challenge and a significant impediment to their deployment in search and rescue operations in harsh environments. Existing designs of snake-inspired robots consist of many small parts, and thus are costly and time-consuming to assemble. In addition, the fact that they consist of many small parts means that they are more prone to failure. Furthermore, very little attention has been paid to designing the robots to employ the types of materials needed for survival in harsh environments.

Considerable progress has been made in development of control systems and kinematics for such robots [Wolf03,Sait02,Nils98,Ma01,Chos00,Chos99,Dowl99,Hiro03]. However, manufacturing snake-robots remains a major challenge and a significant impediment to their deployment. Currently snake-inspired robots are expensive to produce, as they require manual assembly of many small parts because of the many sections and joints. This is especially important since one of the probable uses for snake robots, dangerous “search and rescue” operations, would place the robots in locations where it is likely that they would be partially or completely

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destroyed. Further, it would be desirable to have a group of snake robots in such a situation so that many can be deployed at the same time, increasing the amount of ground that is covered by the search.

In addition to the issues regarding assembly time and costs, manual assembly also limits the sizes in which snake robots can be made. If snake robots can be made smaller, they can fit into much smaller areas and reduce the amount of energy required to propel the system. Further, if the part count can be decreased, theoretically the failure probability of the robot would be decreased. The reduction in part count, and the use of the robots’ structural material as packaging and fastening for the electronic, power, and actuation systems, could lead to significant weight and size reduction. Finally, the robots will need to have sufficiently rugged construction that can survive minor impacts and be capable of operating for a limited time in harsh thermal environments.

Approaches have been developed for fabricating multifunctional structures with integrated sensing, actuation, and control systems for morphing applications in aerospace systems that are also applicable to the development of snake robots [NRC04]. Most of these concepts employ conventional sensors, such as strain sensors, thermal sensors, and optical sensors. More advanced actuation concepts are typically employed using active materials such as Shape Memory Alloys (SMAs), piezoelectric materials, and electroactive polymers. Each of these concepts has limitations on frequency response, power requirements, and deformation response. What now needs to be developed for snake robot applications is an affordable and scalable approach for integrating power sources, actuators, and sensors into a single platform, as can be found in natural systems such as plants and animals.

One of the most scalable processes known to the human kind is the molding process. Once a mold has been created, parts can be manufactured using the molds quickly in high volume. As the volume of production goes up, the cost of making individual parts goes down due to amortization of the mold cost over a larger number of parts. Molds are being used to make parts as large as several meters in size and as small as few microns in size. The traditional molds are single-stage molds. These molds are used in industry today because of their simple design and availability of mold design tools. However, such molds cannot be used for making geometrically complex heterogeneous structures. Recently, we have been developing a multi-material multi-stage molding process for making geometrically complex heterogeneous structures [Bruck04, Gouk06, Gupt04, Kuma02, Li03, Li05, Priy04]. This process allows us to fabricate heterogeneous structures with material interfaces that are geometrically complex that can exhibit superior fracture and fatigue behavior as well as improved functional performance. Specifically, multi-material multi-stage molds have the following benefits:

• They can be used to create multi-material objects. A partially assembled mold is used to pour one material.

After completing one molding stage, the mold assembly is modified by removing and adding mold components and a different material is poured to produce a different portion of the object. By using multiple stages, a geometrically complex multi-material object can be manufactured. The ability to create multi-material objects allows designers to select different materials for different portions of the object, thus helping to improve material-functional compatibility for the overall object. Figure 1 shows an example of a multi-material compliant mechanism fabricated in our lab using a two-stage mold. Figure 2 shows a multi-material vibration-damping panel that has been fabricated in our lab using a three-stage mold.

• They can be used to create objects in which prefabricated sensors/actuators can be embedded in the object

during the molding process. Figure 3 shows a housing for an inertial sensor that has been fabricated in our lab using a three-stage mold. The ability to embed prefabricated sensors/actuators during the molding process helps to eliminate the need for post-molding assembly and significantly improving reliability.

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As illustrated by the above examples, multi-material multi-stage molds can be used to create geometrically complex heterogeneous ceramic and polymer objects that are impossible to create using traditional molding techniques. The ultimate goal is to design a modular, multifunctional snake robot structure for extreme environments that could be inexpensively produced, using multi-material molding and embedded components. The module that will come out of a mold completely assembled, with integrated actuation, power, and control systems, and a mechanism that facilitates connection to other modules. This design would be inexpensive, mass-producible, and scalable. The device should be inexpensive enough that it can be considered to be disposable. The modularity of the robot would also allow users to determine how long of a snake that they want, and what features are needed so it can be quickly assembled and immediately put to use. A model of such a snake robot assembled from individual modules that we have designed is shown in Figure 4.

Figure 2: Example of multi-material vibration control

Hard Soft

Figure 1: Example of multi-material compliant mechanism

Hard Material

Soft

Figure 3: Examples of Embedded Inertial Sensor

sensor

housing

connector

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Figure 4. An assembled snake robot.

In this paper, we discuss the use of multi-stage molding with conventional electronics and polymeric materials to develop new approach to creating multifunctional materials. This approach is facilitated by using multi-material multi-stage molding processes with fully embedded electronic systems, and has resulted in new Thermal and Impact Protected (TIPed) Embedded Sensing Controls Actuation Power Element (ESCAPE) Structures for compact and rugged robotic applications. A prototype TIPed ESCAPE structure that consists of a molded universal joint and embedded electronics is also presented.

2. INTEGRATION OF ACTUATORS USING MULTI-MATERIAL MULTI-STAGE MOLDING PROCESS

Multi-stage molds can be used to create multifunctional structures with integrated actuators that can replace existing joined, multi-piece assemblies, such as the rotor shown in Figure 5. A multifunctional structure will be produced using a sequence of molding stages. Each stage is defined by: (1) the mold assembly being used in the current stage, (2) the already fabricated portion of the target object, (3) the component to be added to the already fabricated portion of the target object in the current stage. In each molding stage two types of transformations are achieved. First, the new mold assembly will be created from the previous mold assembly. Second, the new component will be added to the already fabricated structure by pouring the required material. By using more then two mold pieces in each molding stages, we can alter mold assemblies significantly and therefore can create geometrically complex structures and interfaces. The bending actuation element itself can be fabricated using a graded distribution of one-way SMA wires, which has been used to create bio-inspired morphing wing structures (Figure 6) [Bruc02a,Bruc02b]. A process is currently under development in which both sacrificial as well permanent mold pieces can be used at multiple length scales. This is accomplished by using new micro-injection molding system to create scaled down micromolded parts (Figure 7).

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Figure 5. Illustration of multi-material multi-stage molding process for fabricating a morphing rotor structure

Figure 6. Bio-inspired morphing wing structure using graded one-way SMA wire distribution as actuators for use in the multi-material multi-stage molding process in Figure 5.

Figure 7. A micro molded geometrically complex part

Fabricated Prototype

Bending Actuation Element in Stage 1

Mold Pieces in Stage 2

CAD Model of Joined Rotor Assembly Redesigned as

Heterogeneous Structure with Bending Actuation

Elements

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3. INTEGRATION OF SENSORS A variety of sensors can be integrated into the bio-inspired structure, including thermocouples for determining temperature, strain gages for determining strain, and inertial sensors for determining motion. The research issues for sensor integration are similar to those encountered for actuator integration. However, many of the sensors will typically be located closer to the surface of the structure, and will interact less with the morphing structure than the actuators will. It is important to have good interfacial adhesion between the sensor and structure in order to maintain reliability. Furthermore, circuits have to be designed to draw signal out of the sensors for use in controlling the actuators. We are currently in the process of molding structures with embedded sensors. Our main focus is to study how the sensor performance is degraded as a result of molding process.

4. INTEGRATION OF CONTROLS AND POWER ELEMENTS

A bio-inspired, modular, and multifunctional structure was developed that contained a multi-material molded universal joint, allowing for 2-DOF, and contained integrated controls and power elements. A solid model of the module is shown in Figure 8a, and the actual module is shown in Figure 8b. The mechanical structure of the module was assembled entirely in-mold, and consists of a universal joint, along with snapping docking connectors. The male and female connectors are located at opposite ends of the structure, and are geometrically correct, although a more flexible material must be used in order for them to be functional (something that can easily be done with an additional shot of a different grade of polyurethane or a thermoplastic material). The module was assembled via a two-stage transfer molding process where a second shot of material was used to make the universal joint. The entire module consisted of only three parts, the two on either side of the joint, and the cross-shaped piece making the joint, needing no additional fasteners or shafts. The concave interior feature of the female side of the docking mechanism was created using a split-core technique.

Two Solarbotics G10 pager motors were used to actuate the 2-DOF, and a simple circuit was used to initiate their firing. The motors were glued to the structure after molding, and were connected to the opposite side of the universal joint through the use of small tie rods. Alternatively, the bending actuators shown in Figure 6 could be employed, but it was simpler to employ the pager motors for the prototype module. The control circuit used to intermittently actuate the 2-DOF consists of a small PIC12F629 microcontroller chip manufactured by Microchip, three capacitors, and a voltage regulator to maintain 5 volts supplied to the microcontroller and pager motors. The entire circuit was embedded in the cast polyurethane and has been shown to work. The complete embedded circuit has been shown to operate for a few minutes at a time drawing approximately 30 mA, in other tests, an embedded microchip was shown to operate for >30 minutes at a time without failure. A microchip was also embedded into hot injection-molded polyethylene without failure. Since the circuit was embedded in the polymer, no fastening was required; the complete circuit was simply placed into the proper location in the mold.

Figure 8a. Solid Model of Multifunctional Structural Module.

In addition, to the control circuit, a battery was also embedded directly into the plastic as a power element and has been shown to function properly (Figure 9). The battery is connected to the control circuit through a 3-way switch and thus, in addition to switching power to the circuit on and off, the battery can be charged in the module by means of two exposed leads. The battery has been successfully recharged inside the module with no negative

Motor

Batter

IC

Switch

Figure 8b. Molded Multifunctional Structural Module.

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effects. Like the circuit, no additional assembly needs to be performed with the battery; it was merely placed into the appropriate position in the mold. Since all of the electronics were fully embedded in the polymer, there was no need for additional packaging like a circuit board, and thus more efficient use of the three dimensional space of the module was used. The circuit was pre-assembled into a three dimensional shape and then placed in the middle of the module.

Figure 9. Embedding of power element in the multifunctional structure within the molding process

The embedded power elements are also being augmented with a new solar cell technology has recently been developed that is capable of harvesting enough energy to power morphing structures [Bruc05]. The solar cells take advantage of a new gradient architecture design that substantially improves efficiency and power output (Figure 10). These sensors are capable of producing 10 mW outputs at 500 mV for a 1 cm2 area, and are easily printed on flexible, conductive polymer substrates. Standard sunlight irradiates at 100 mW/cm2, resulting in a 10% efficiency. We are working on optimizing the architectures to enhance efficiency and increase power output, but it is currently possible to generate 1 W for every 10 cm x 10 cm area. This is enough power and voltage for nine 5-mil FlexinolTM SMA wire 1 inch in length to generate nearly 5 lbs. of actuation force at approximately 1 Hz. It is also possible to decrease the current and increase the voltage by configuring multiple cells in series, so 100 cells that are 1 cm2 in area could be used to produce 1 W at 50 V. This power can then be used for the integrated sensors, or to regenerate integrated flexible batteries if greater power requirements are needed. Variations in power output can also be used to determine gradients in light intensity, thus allowing the flexible solar cell to also act as a photovoltaic sensor that can be used to determine the optimal positioning of the morphing structure for maximizing power output.

Figure 10. Solid state solar cell with an interfacial gradient through the thickness. The interface consists of a pure dye-sensitized TiO2 layer, followed by a 66%/33% TiO2/CuSCN layer, a 33%/66% TiO2/CuSCN layer, and finally a

pure CuSCN layer 5. DESIGN ANALYSIS FOR MULTIFUNCTIONAL STRUCTURES With the ability to embed sensors, controls, actuators, and power elements into a multifunctional structure, there is a need for analytical and computational tools to optimize the performance of the bio-inspired concepts that the integration will facilitate. At the core of the development of these multifunctional structures are simplified analytical

Battery embedded in mold

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models for guiding the integration of the components, and more detailed multi-physics numerical analyses for refining the analysis. These tools can be summarized as follows:

Actuation: Coupled thermomechanical constitutive models and continuum-based structural models

Mechanical: Dynamic and static continuum-based structural models with internal stress and complex geometries and material distributions

Electrical: Internal resistance heating and active cooling models

Thermal: Conductive and convective heat transfer models

Power Generation: Model of surface area exposed to sunlight and cell efficiency.

The coupling of these models, the associated performance prediction, and the role of sensors in the design of the structures can be summarized as follows:

Critical to this effort is the ability to understand how the design of each component ultimately affects the performance of the bio-inspired actuation concept. For example, the flexibility of the solar cells will directly affect the deformation response, while their power output will dictate the electrical capabilities of the system. The electrical and thermal requirements will be dictated by the desired actuation performance. In this effort, sensing and control aspects of the system will not influence actuation performance.

For bending actuators fabricated by embedding SMA wires, the design is developed from the following experimentally-refined continuum-based structural model [Bruc02a]:

[ ])()]()[(

)(22 o

at TTd

dk −+−−−+

−−= αε

βγγβαβγβγβγγβγαβ

(11)

[ ])()]()[( 2

2

oat TT

de −+

−−−+= αε

βγγβαβγβγβγ

(1)

Where k2 is the bending curvature, e the elongation of the actuator, d the location of the SMA wire, T-To is the temperature change in the SMA wire, εt is the transformation strain determined from the thermomechanical behavior of the wire, αa is the coefficient of thermal expansion, γ is the fraction of the recovery strain transferred to the matrix, and:

)EA/(AEI/A

aa2

≡β

≡α (2)

where A is the cross-sectional area of the beam, Aa is the cross-sectional area of the wire, E is the Young's Modulus for the beam, Ea is the Young's Modulus of the beam, and I2 is the moment of inertia of the beam cross section.

The mechanical behavior from events such as impact is modeled using standard finite element analysis with commercial codes, such as SolidWorks. A model three layer spherical module fabricated with an outer layer of Nylon, a middle layer of ABS, and an inner layer Polyethylene, all of which are compatible with injection molding machines, is shown in Figure 11 along with the results from a dynamic analysis conducted for a 20 ft. drop normal

Power Generation

Electrical Thermal Actuation Frequency and Deformation response

Sensors

Mechanical

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to Plane 2. Thermal analysis can also be performed using SolidWorks, while characterization of thermal conductivity, especially across interfaces, is conducted using a Hilton Heat Transfer Service Unit 111. The specimen holder, test assembly, and results from some conventional polymers used for the multifunctional structures can be seen in Figure 12 as a function of the test time. Note that the thermal conductivity tends to be closer to the lower thermal conductivity material for the bilayer specimens, which indicates that the thermal conductivity across the interface may be lower than the base material due to a lack of interfacial adhesion from polymer crosslinking or plastic welding. Thus, the thermal conductivity characterization can be used to insight into these adhesion mechanisms, which is difficult via more direct means. Thermal performance can also be validated experimentally using a simple electrical resistance heating model, where the control circuit is considered as a lumped resistor of experimentally-defined finite volume. Power generation is being determined by the efficiency of the flexible solar cells and the exposure of surface area to sunlight, with standard sunlight being 100 mW/cm2 and efficiencies of approximately 10%. Integrated strain and temperature sensors are being used to validate performance, along with more detailed thermomechanical full-field analysis using a thermal imaging system and dynamic Digital Image Correlation previously demonstrated for welding applications [Bruc98].

(a) (b) (c)

Figure 11. Model 3-layer spherical module with dynamic analysis conducted for a 20 foot drop normal to Plane 2.

(a) (b) (c)

Figure 12. Thermal characterization of multifunctional structures: (a) specimen holder, (b) test assembly, (c) results from some conventional polymers used for the multifunctional structures

6. PROTOTYPE SNAKE ROBOT STRUCTURES To complete this research effort, a prototype snake robot structure has been fabricated and characterized. The robot is assembled from a module similar to the one shown in Figure 8b. This design is inexpensive, mass-producible, and scalable. The device is inexpensive enough that it can be considered to be disposable, with a current cost of less than $40 per module for parts. The modularity of the robot allows users to determine how long of a snake that they want from the number of modules they put together, and what specific sensing features are needed in each module so it can be easily assembled and immediately put to use. For a snake robot application, the microcontrollers for all of the modules are connected in series to a data bus which is ultimately controlled by a single, more powerful microcontroller located at the front of the snake. This main microcontroller sends packets of data down the bus addressed to a particular module. All of the modules receive this packet, but it is ignored by all except the one with that specific address. The module’s microcontroller then reads the data and generates the appropriate signal for the intended servo. In addition to the data bus, a clock bus is used to keep all of the modules in sync and a ground line runs along the length of the snake. Using

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this control system, we have successfully developed and demonstrated a locomotion gait. In order to test this gait we developed and used a prototype. All snakes are capable of lateral undulation, in which the body is flexed side-to-side, and the flexed areas propagate from the posterior, giving the overall shape of a sine wave propagating from the posterior. This is a fairly common gait in this field; however, most hobby snake robots rely on wheels embedded in the bottom of each link of the snake robot to accomplish forward progress using the gait. We have duplicated this motion in the vertical plane, allowing the same forward progression, however instead of the snakes body flexing in-plane, it flexes out-of-plane. We have built a serpentine robot and tested the locomotion gait on it. Also, the robot has no reliance on wheeled sections and hence can easily navigate rougher terrain. The prototype snake robot structure fabricated by connecting individual modules with the out-of-plane locomotion gait is shown in Figure 13.

Figure 13. Assembled snake robot with out-of-plane locomotion gait

7. CONCLUSIONS A new approach has been developed to creating bio-inspired, modular, and multifunctional structures where the actuators and sensors are integrated into the structure. This approach is facilitated by using multi-material multi-stage molding processes with fully embedded electronic systems, and has resulted in new Thermal and Impact Protected (TIPed) Embedded Sensing Controls Actuation Power Element (ESCAPE) Structures for compact and rugged robotic applications. The robot is actuated with pager motors, but bending actuation elements utilizing one-way SMA wires can also be employed along with sensing elements. Control of sensing and actuation is accomplished through a compact microcontroller, powered by a miniature battery, with the possibility of adding additional power generation through flexible solar cells. A design approach has been proposed to consider the actuation, mechanical, thermal, electrical, and power generation aspects of the structure, and is currently under further development. A prototype module has been developed with a current cost of less than $40 for parts, and multiple modules connected to create a snake robot with a locomotion gait designed to operate out-of-plane.

The ability to manufacture bio-inspired, modular, and multifunctional structures using scalable concepts with fully embedded electronic systems allows for the development of new robots with the following benefits:

• Affordable: The modules utilize mass production processes and hence enjoy economy of scales. By utilizing assembly-free manufacturing techniques such as multi-material molding, robots can be easily assembled in-mold, requiring less time and thus less labor costs to create. Because robots would be modular, and would often contain many modules, it would be beneficial to mass produce these modules in a low cost means such as molding. This is especially true, given that a group of robots may be used in just one single rescue mission.

• Scalable: Molding processes are inherently scalable; therefore the bio-inspired, modular, multifunctional structures will be more scalable than conventional approaches. This will enable the structures to be employed in a wider range of applications, and to be more easily redesigned for those applications.

• Easy to Transport: Individual modules can be stacked and easily transported to the incident site. Using these modules, robots can be built within few minutes on site. Assembly of the entire robot will be easy because of components that snap together, and because of the identical modules. Additionally, the reduction of parts and material will make the robots smaller and more lightweight, and thus easier to transport.

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• Rugged Construction: Embedding of electronics inside the housing will provide impact resistance and enable high temperature operation for at least limited periods of time. The multi-material molding process would also allow for modules that have increased material-function compatibility, and thus are more adept to withstand the rigors of the mission. For example, the modules will be designed to have graded distribution of material layers with varying degrees of compliance (packaging) and rigidness (support) to both absorb impact energy and keep the components in place. In addition, a reduction in part count should lead to an improvement in reliability.

• Long Operation Range: On board batteries will make the robots untethered, and hence expand its range. Further, because the weight of the robots can be significantly decreased by the dual use of the structural material as packaging and fastening for the embedded electronics. This eliminates the need for much of the traditional fastening and packaging materials that usually accompanies electronic and power components in mechanical and robotic systems, and thus because weight is reduced, power consumption can also be reduced. The weight and thus power consumption can further be reduced by utilizing advanced composite materials optimally designed for this purpose. Finally, weight can also be reduced by creating multifunctional structures where the power storage components also serve as structural material.

8. ACKNOWLEDGEMENTS This worked was supported by NSF grants EEC0315425 and DMI0457058, and by the National Institute of Aerospace, Hampton, VA. Opinions expressed in this paper are those of the authors and do not necessarily reflect opinions of the sponsors. 9. REFERENCES [Bruc02a] H.A. Bruck, C.L. Moore, and T. Valentine, “Characterization and Modeling of Bending Actuation in

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