Harsha Prahlad

Electroadhesive robots—wall climbing robots enabled by a novel, robust, and electrically controllable adhesion technology

This paper describes a novel clamping technology called compliant electroadhesion, as well as the first application of this technology to wall climbing robots. As the name implies, electroadhesion is an electrically controllable adhesion technology. It involves inducing electrostatic charges on a wall substrate using a power supply connected to compliant pads situated on the moving robot. High clamping forces (0.2-1.4 Newton supported by 1 square centimeter of clamp area, depending on substrate) have been demonstrated on a wide variety of common building substrates, both rough and smooth as well as both electrically conductive and insulating. Unlike conventional adhesives or dry adhesives, the electroadhesion can be modulated or turned off for mobility or cleaning. The technology uses a very small amount of power (on the order of 20 microwatts/Newton weight held) and shows the ability to repeatably clamp to wall substrates that are heavily covered in dust or other debris. Using this technology, SRI International has demonstrated a variety of wall climbing robots including tracked and legged robots.

Comparative Evaluation of Shape Memory Alloy Constitutive Models with Experimental Data

This paper presents research aimed at reviewing and comparing constitutive models for Shape Memory Alloy (SMA) behavior under uniaxial loading with experimental data over the entire thermomechanical range of test conditions. Three commonly used representative constitutive models predicting quasistatic SMA behavior – Tanaka, Liang and Rogers, and Brinson are examined, and are compared with experiments under quasistatic loading conditions. Important distinctions regarding the definition of material constants in these models are pointed out, and the method of obtaining material parameters from experiments is presented. Observations regarding substantial differences between resistive and environmental heating methods of the wire are also presented. It is shown that most of the models are in good overall agreement with isothermal and constrained recovery test data. Several observations regarding the comparison of model predictions with experimental test data and the implementation of the models in a formal code are discussed. Experimental characteristics observed under nonquasistatic loading are used to point out some deficiencies in these models, and the necessity for modeling refinements incorporating these effects is discussed.

Shape control of large lightweight mirrors with dielectric elastomer actuation

Space-based astronomy and remote sensing systems would benefit from extremely large aperture mirrors that can permit greater-resolution images. To be cost effective and practical, such optical systems must be lightweight and capable of deployment from highly compacted stowed configurations. Such gossamer mirror structures are likely to be very flexible and therefore present challenges in achieving and maintaining the required optically precise shape. Active control based on dielectric elastomers was evaluated in order to address these challenges. Dielectric elastomers offer potential advantages over other candidate actuation technologies including high elastic strain, low power dissipation, tolerance of the space environment, and ease of commercial fabrication into large sheets. The basic functional element of dielectric elastomer actuation is a thin polymer film coated on both sides by a compliant electrode material. When voltage is applied between electrodes, a compressive force squeezes the film, causing it to expand in area. We have explored both material survivability issues and candidate designs of adaptive structures that incorporate dielectric elastomer actuation. Experimental testing has shown the operation of silicone-based actuator layers over a temperature range of -100 °C to 260 °C, suitable for most earth orbits. Analytical (finite element) and experimental methods suggested that dielectric elastomers can produce the necessary shape change when laminated to the back of a flexible mirror or incorporated into an inflatable mirror. Interferometric measurements verified the ability to effect controllable shape changes less than the wavelength of light. In an alternative design, discrete polymer actuators were shown to be able to control the position of a rigid mirror segment with a sensitivity of 1800 nm/V, suggesting that sub-wavelength position control is feasible. While initial results are promising, numerous technical challenges remain to be addressed, including the development of shape control algorithms, the fabrication of optically smooth reflective coatings, consideration of dynamic effects such as vibration, methods of addressing large-numbers of active areas, and stowability and deployment schemes.

Development of a Strain-Rate Dependent Model for Uniaxial Loading of SMA Wires

This paper describes a modeling approach to incorporate the effects of nonquasistatic loading on the extensional behavior of an SMA wire. Experimental results under a variety of loading conditions indicate that the instantaneous temperature of the material is closely related to the applied strainrate in the material. In order to predict this behavior, a coupled thermo-mechanical analysis with the rate form of SMA constitutive models is formulated. The temperatures and temperature rates are not prescribed, but are derived from energy conservation of the material. The stress rate is simultaneously derived using the rate form of SMA constitutive models. Important parameters governing heat transfer such as specific heat, heat transfer and latent heat are modeled and validated with experimental data. For the material tested (0.38 mm diameter SMA wire), the quasistatic strain rate below which no significant deviation in material characteristics are observed was empirically determined to be about 0.0005/s. At a strain rate of 0.01/s, the transformation stresses in the material are increased by approximately 0.5 × 10 8 Pa, accompanied by an increase in temperature of about 2°C over the quasistatic values. The predictions for the stress-strain curves are compared with experimental data at different strain rates over a range of environmental temperatures, and are found to be in good qualitative agreement. The model is also shown to be in good qualitative agreement with the experimental behavior under more complex loading conditions involving two different strain rates of loading. Parametric variation of the model coefficients is discussed. Although the qualitative aspects of the model are in good agreement with experimental data, it is argued that more comprehensive estimation of the model parameters is required to assess the quantitative aspects of the model. The model described in the paper uses the rate forms of the Brinson model. However, it is equally applicable with a modified rate-dependant form of any other quasistatic model describing SMA behavior.

Programmable surface deformation: thickness-mode electroactive polymer actuators and their applications

Many different actuator configurations based on SRI International’s dielectric elastomer (DE) type of electroactive polymer (EAP) have been developed for a variety of applications. These actuators have shown excellent actuation properties including maximum actuation strains of up to 380% and energy densities of up to 3.4 J/g, using the planar mode of actuation. Recently, SRI has investigated different configurations of DE actuators that allow complex changes in surface shape and thus the creation of active surface texture. In this configuration, the “active” polymer film is bonded or coated with a thicker passive layer, such that changes in the polymer thickness during actuation of the DE device are at least partially transferred to (and often amplified by) the passive layer. Although the device gives out-of-plane motion, it can nonetheless be fabricated using two-dimensional patterning. The result is a rugged, flexible, and conformal skin that can be spatially actuated by subjecting patterned electrodes on a polymer substrate to an electric field. Using thickness-mode DE, we have demonstrated thickness changes of the order of 0.5 - 2 mm by laminating a passive elastomeric layer to a DE polymer that is only 60 μm in thickness. Such thickness changes would otherwise require a very large number of stacked layers of the DE film to produce comparable surface deformations. Preliminary pressures of 4.2 kPa (0.6 psi) in a direction normal to the plane of the DE film have been measured. However, theoretical calculations indicate that pressures of the order of 100 kPa are feasible using a single layer of DE film. Stacking multiple layers of DE film can lead to a further increase in achievable actuation pressures. Even with current levels of thickness change and actuation pressures, potential applications of such surface texture change are numerous. A thin, compliant pad made from these actuators can have a massaging or sensory augmentation function, and can be incorporated into garments if desired. The bumps and troughs could act as valves or pumping elements in a fluidic or microfluidic system. Such a device could also be the basis of a smart skin that controls boundary-layer flow properties in a boat or airplane so as to reduce overall drag. The DE elements of the pad can also be used as sensors to make a touch-sensitive skin for recording human interaction with the environment. By driving a thin, compliant vibrating layer at resonant frequencies, one can also configure these devices as solid or fluidic conveyors that transport material on a macroscopic or microscopic scale.

Force requirements for artificial muscle to create an eyelid blink with eyelid sling.

OBJECTIVE To determine the force requirements, optimal vector, and appropriate materials of a novel eyelid sling device that will be used to rehabilitate eyelid closure (blink) in congenital or acquired permanent facial paralysis with an artificial muscle. METHODS The force required to close the eyelids in human cadavers (n = 6) were measured using a load cell system. The eyelid sling using either expanded polytetrafluoroethylene (ePTFE) or temporalis muscle fascia was implanted. The ideal vector of force and placement within the eyelid for a natural eyelid closure were compared. RESULTS The eyelid sling concept was successful at creating eyelid closure in a cadaver model using an upper eyelid sling attached to the distal tarsal plate. Less force was necessary to create eyelid closure using a temporalis muscle fascia sling (627 +/- 128 mN) than for the ePTFE eyelid sling (1347 +/- 318 mN). CONCLUSIONS The force and stroke required to close an eyelid with the eyelid sling are well within the attainable range of the electroactive polymer artificial muscle (EPAM). This may allow the creation of a realistic and functional eyelid blink that is symmetric and synchronous with the contralateral, normally functioning blink. Future aims include consideration of different sling materials and development of both the EPAM device and an articulation between the EPAM and sling. The biocompatibility and durability studies of EPAM in a gerbil model are under way. The successful application of artificial muscle technology to create an eyelid blink would be the first of many potential applications.

Design of a variable twist tilt-rotor blade using shape memory alloy (SMA) actuators

This paper presents research aimed at actively altering the twist distribution of a tiltrotor blade between hover and forward flight. Three different concepts-extension-twist coupled composites, bimoment actuation and discrete SMA torque tube actuation - are considered, and the torque tube appears the most feasible. Parametric design of the torque tube and attachment technique is presented with actuation torque, heat transfer and bandwidth issues being considered to arrive at the configuration of the tube. The effect of heat treatment of the SMA in tuning the actuation characteristics is discussed. A dramatic improvement in the actuation cooling time is demonstrated through the use of active cooling using thermodelectric modules. An extension of the one-dimensional formulation of Brinsons model to the torsional case is presented. The model is shown to have good correlation with room temperature characteristics. The criterion for impedance matching between the actuator and the host structure is derived. The torsional actuator is tested both under no load and acting against a restoring spring and shows repeatable actuation characteristics.

Modeling and Experimental Characterization of SMA Torsional Actuators

This article presents the material modeling and experimental characterization of SMA rod and tube actuators undergoing pure torsional deformations. The investigation of the torsional characteristics of SMAs is carried out in order to gain a fundamental understanding of the behavior of SMA torsional actuators. The proposed application is to alter the twist distribution of a tiltrotor blade from hover to forward flight modes, providing improved flight efficiency in both modes of flight. To describe the behavior of the actuator, a torsional model involving the extension of the one-dimensional (uniaxial) formulation of SMA phenomenology is presented. As an example, Brinsons model is chosen as the representative uniaxial model; however, the approach used here to extend the uniaxial model into the torsional domain is applicable to nearly any uniaxial SMA model. The parameters for the uniaxial model are derived from extensional testing of an SMA rod or tube, and are then used to predict the torsional characteristics of the same material. A major advantage of this method compared to prior art in torsional modeling is that the simplicity of deriving and implementing the parameters from uniaxial testing is carried over into the torsional domain. This simplicity derives from the fact that the measured properties (forces and displacements) in tensile loading are more readily suited to determining the model parameters than the measured properties in torsional loading which have been used by previous researchers (angles and torques which are, in fact, integrated quantities from strains and stresses). This article also presents a comprehensive experimental investigation and model correlation with both tension and torsional behavior of SMA rods and tubes. The model is compared with experimental data including torsional actuation tests against torsional springs. It is shown that the theoretical model demonstrates good agreement with the experimental data over a wide range of thermomechanical conditions. In addition, experimental phenomena associated with the torsional behavior of the SMA actuator such as the effects of heat treatment, twist rate, and loading pattern are examined.

Polymer-Based Flexible Visuo-Haptic Display

We report a flexible visuo-haptic display that allows for interactive haptic feedback on the visual display. The visuo-haptic display is fabricated by integrating a dielectric elastomer (DE) based thin film actuator array into a flexible display and pressure sensors. The DE actuator array consists of nine active cells, which generate thickness-mode deformation in response to voltage signal. The flexible display presents images of the aligned three alphabet characters at each section in 3 × 3 matrix during light propagation via optical multiwaveguide. The pressure sensors are placed on the bottom of the DE actuator array for haptic feedback. The performance of the DE actuators is proved to be capable of realizing sufficient vibro-tactile sensation in the perceivable range of human touch sense. The integrated system enables the visual display to provide interactive haptic feedback such as key pressing, contact vibration sensations, etc., in accordance with user input.

CHARACTERIZATION OF SMA TORSIONAL ACTUATORS FOR ACTIVE TWIST OF TILT ROTOR BLADES

This paper presents the material modeling and experimental characterization of SMA rod and tube actuators undergoing torsional deformations. The investigation of the torsional characteristics of SMAs was carried out in order to gain fundamental understanding of the behavior of torsional actuators for actively altering the twist distribution of a tiltrotor blade between hover and forward flight. A torsional model involving the extension of the one-dimensional formulation of Brinson’s model for prediction of SMA behavior is presented. The model is shown to have good correlation with experiment over a wide range of thermomechanical conditions including constant temperature torque-angle and actuation tests against torsional springs. Experimental phenomena associated with the torsional behavior of the SMA actuator such as the effects of heat treatment, twist rate and loading pattern are examined, and limitations of the current model pointed out. It is shown that the theoretical model demonstrates good agreement with the experimental data over a wide thermo-mechanical range of conditions.