Copyright (c) 2013 University of Pennsylvania All rights reserved. http://repository.upenn.edu/meam_papers Recent documents in Departmental Papers (MEAM) en-us Thu, 09 May 2013 10:25:55 PDT 3600 http://repository.upenn.edu/meam_papers/297 http://repository.upenn.edu/meam_papers/297 Wed, 27 Feb 2013 14:33:34 PST A general model is presented for electron emission yield from planar photocathodes that accounts for arbitrary cathode thickness and finite recombination velocities at both front and back surfaces. This treatment is applicable to negative electron affinity emitters as well as positive electron affinity cathodes, which have been predicted to be useful for energy conversion. The emission model is based on a simple one-dimensional steady-state diffusion treatment. The resulting relation for electron yield is used to model emission from thin-film cathodes with material parameters similar to GaAs. Cathode thickness and recombination at the emissive surface are found to strongly affect emission yield from cathodes, yet the magnitude of the effect greatly depends upon the emission mechanism. A predictable optimal film thickness is found from a balance between optical absorption, surface recombination, and emission rate.

]]> Kunal Sahasrabuddhe et al. http://repository.upenn.edu/meam_papers/296 http://repository.upenn.edu/meam_papers/296 Wed, 27 Feb 2013 14:33:32 PST The paper presents the development and demonstrates the capabilities of a new laboratory-based environmental X-ray photoelectron spectroscopy system incorporating an electrostatic lens and able to acquire spectra up to 0.4 Torr. The incorporation of a two-dimensional detector provides imaging capabilities and allows the acquisition of angle-resolved data in parallel mode over an angular range of 14° without tilting the sample. The sensitivity and energy resolution of the spectrometer have been investigated by analyzing a standard Ag foil both under high vacuum (10⁻⁸ Torr) conditions and at elevated pressures of N₂ (0.4 Torr). The possibility of acquiring angle-resolved data at different pressures has been demonstrated by analyzing a silicon/silicon dioxide (Si/SiO₂) sample. The collected angle-resolved spectra could be effectively used for the determination of the thickness of the native silicon oxide layer.

]]> F. Mangolini et al. http://repository.upenn.edu/meam_papers/295 http://repository.upenn.edu/meam_papers/295 Wed, 27 Feb 2013 14:33:31 PST The effect of target cross-sectional geometry on drop splashing is investigated using surfaces with length scales comparable to the drop diameter. The target cross-sectional geometries are regular polygon shapes that vary from a triangle (n = 3) to a decagon (n = 10), where n is the number vertices. The impacting cross-sectional surface area of all targets is constrained to equal the cross-sectional area of the impacting drop which is 6.38 mm².

]]> Gabriel Juarez et al. http://repository.upenn.edu/meam_papers/294 http://repository.upenn.edu/meam_papers/294 Wed, 27 Feb 2013 14:33:30 PST Conventionally, a microscopic particle that performs a reciprocal stroke cannot move through its environment. This is because at small scales, the response of simple Newtonian fluids is purely viscous and flows are time-reversible. We show that by contrast, fluid elasticity enables propulsion by reciprocal forcing that is otherwise impossible. We present experiments on rigid objects actuated reciprocally in viscous fluids, demonstrating for the first time a purely elastic propulsion set by the object’s shape and boundary conditions. We describe two different artificial “swimmers” that experimentally realize this principle.

]]> Nathan C. Keim et al. http://repository.upenn.edu/meam_papers/293 http://repository.upenn.edu/meam_papers/293 Wed, 27 Feb 2013 14:33:29 PST The authors demonstrate “smart-cut”-type layer transfer of single-crystal silicon carbide (SiC) by using spin-on-glass (SoG) as an adhesion layer. Using SoG as an adhesion layer is desirable because it can planarize the surface, facilitate an initial low temperature bond, and withstand the thermal stresses at high temperature where layer splitting occurs (800–900 °C). With SoG, the bonding of wafers with a relatively large surface roughness of 7.5–12.5 Å rms can be achieved. This compares favorably to direct (fusion) wafer bonding, which usually requires extremely low roughness (<2 Å rms), typically achieved using chemical mechanical polishing (CMP) after implantation. The higher roughness tolerance of the SoG layer transfer removes the need for the CMP step, making the process more reliable and affordable for expensive materials like SiC. To demonstrate the reliability of the smart-cut layer transfer using SoG, we successfully fabricated a number of suspended MEMS structures using this technology.

]]> Jae-Hyung Lee et al. http://repository.upenn.edu/meam_papers/292 http://repository.upenn.edu/meam_papers/292 Tue, 21 Aug 2012 10:39:20 PDT The Intuitive da Vinci system enables surgeons to see and manipulate structures deep within the body via tiny incisions. Though the robotic tools mimic one's hand motions, surgeons cannot feel what the tools are touching, a striking contrast to non-robotic techniques. We have developed a new method for partially restoring this lost sense of touch. Our VerroTouch system measures the vibrations caused by tool contact and immediately recreates them on the master handles for the surgeon to feel. This augmentation enables the surgeon to feel the texture of rough surfaces, the start and end of contact with manipulated objects, and other important tactile events. While it does not provide low frequency forces, we believe vibrotactile feedback will be highly useful for surgical task execution, a hypothesis we we will test in future work.

]]> Katherine J. Kuchenbecker et al. http://repository.upenn.edu/meam_papers/291 http://repository.upenn.edu/meam_papers/291 Tue, 21 Aug 2012 10:39:19 PDT New robots for teleoperation and autonomous manipulation are increasingly being equipped with high-bandwidth accelerometers for measuring the transient vibrational cues that occur during con- tact with objects. Unfortunately, the robot's own internal mechanisms often generate significant high-frequency accelerations, which we term ego-vibrations. This paper presents an approach to characterizing and removing these signals from acceleration measurements. We adapt the audio processing technique of spectral subtraction over short time windows to remove the noise that is estimated to occur at the robot's present joint velocities. Implementation for the wrist roll and gripper joints on a Willow Garage PR2 robot demonstrates that spectral subtraction significantly increases signal-to-noise ratio, which should improve vibrotactile event detection in both teleoperation and autonomous robotics.

]]> Katherine J. Kuchenbecker et al. http://repository.upenn.edu/meam_papers/290 http://repository.upenn.edu/meam_papers/290 Tue, 21 Aug 2012 10:39:17 PDT White canes give the visually impaired the freedom to travel independently in unknown environments, but they cannot warn the user of overhead hazards such as tree branches. This paper presents the development and evaluation of a device that provides haptic cues to warn a visually impaired user of low-hanging obstacles during white cane navigation. The Haptic Alerts for Low-hanging Obstacles (HALO) system is a portable and affordable attachment to traditional white canes. By pairing distance data acquired from an ultrasonic range sensor with vibration feedback delivered by an eccentric mass motor, the device aims to alert users of low-hanging obstacles without interfering with the standard functionality of a white cane. We conducted a preliminary validation study wherein twelve blindfolded subjects navigated a custom obstacle course with and without vibration alerts from HALO. The results showed that this new device is intuitive and highly effective at enabling the user to safely navigate around low-hanging obstacles.

]]> Katherine J. Kuchenbecker et al. http://repository.upenn.edu/meam_papers/289 http://repository.upenn.edu/meam_papers/289 Tue, 21 Aug 2012 10:39:16 PDT It is well known that the human chest exhibits a strong force displacement hysteresis during CPR, a stark contrast to the non hysteretic behavior of standard spring manikins. We hypothesize that individuals with experience performing CPR on humans would perceive a manikin with damping as more realistic and better for training. By analyzing data collected from chest compressions on real patients, we created a dynamic model that accounts for this hysteresis with a linear spring and a one-way variable damper, and we built a new high-fidelity manikin to enact the desired force displacement relationship. A linkage attached to the chest plate converts vertical compression motions to the horizontal displacement of a set of pneumatic dashpot pistons, sending a volume of air into and out of the manikin through a programmable valve. Position and pressure sensors allow a microcontroller to adjust the valve orifice so that the provided damping force closely follows the desired damping force throughout the compression cycle. Eight experienced CPR practitioners tested both the new manikin and an identical looking standard manikin; the manikin with damping received significantly higher ratings for haptic realism and perceived utility as a training tool.

]]> Katherine J. Kuchenbecker et al. http://repository.upenn.edu/meam_papers/288 http://repository.upenn.edu/meam_papers/288 Tue, 21 Aug 2012 10:39:15 PDT We present four wearable tactile actuators capable of recreating physical sensations commonly experienced in human interactions, including tapping on, dragging across, squeezing, and twisting an individual’s wrist. In seeking to create tactile signals that feel natural and are easy to understand, we developed movement control interfaces to play back each of these forms of actual human physical contact. Through iterative design, prototyping, programming, and testing, each of these servo-motor-based mechanisms produces a signal that is gradable in magnitude, can be played in a variety of temporal patterns, is localizable to a small area of skin, and, for three of the four actuators, has an associated direction. Additionally, we have tried to design toward many of the characteristics that have made high frequency vibration the most common form of wearable tactile feedback, including low cost, light weight, comfort, and small size. Bolstered by largely positive comments from naive users during an informal testing session, we plan to continue improving these devices for future use in tactile motion guidance.

]]> Katherine J. Kuchenbecker et al. http://repository.upenn.edu/meam_papers/287 http://repository.upenn.edu/meam_papers/287 Tue, 21 Aug 2012 10:39:13 PDT Haptics research has seen several recent efforts at understanding and recreating real vibrations to improve the quality of haptic feedback in both virtual environments and teleoperation. To simplify the modeling process and enable the use of single-axis actuators, these previous efforts have used just one axis of a three-dimensional vibration signal, even though the main vibration mechanoreceptors in the hand are know to detect vibrations in all directions. Furthermore, the fact that these mechanoreceptors are largely insensitive to the direction of high-frequency vibrations points to the existence of a transformation that can reduce three-dimensional high-frequency vibration signals to a one-dimensional signal without appreciable perceptual degradation. After formalizing the requirements for this transformation, this paper describes and compares several candidate methods of varying degrees of sophistication, culminating in a novel frequency-domain solution that performs very well on our chosen metrics.

]]> Katherine J. Kuchenbecker et al. http://repository.upenn.edu/meam_papers/286 http://repository.upenn.edu/meam_papers/286 Tue, 21 Aug 2012 10:39:12 PDT Sliding a probe over a textured surface generates a rich collection of vibrations that one can easily use to create a mental model of the surface. Haptic virtual environments attempt to mimic these real interactions, but common haptic rendering techniques typically fail to reproduce the sensations that are encountered during texture exploration. Past approaches have focused on building a representation of textures using a priori ideas about surface properties. Instead, this paper describes a process of synthesizing probe-surface interactions from data recorded from real interactions. We explain how to apply the mathematical principles of Linear Predictive Coding (LPC) to develop a discrete transfer function that represents the acceleration response under specific probe-surface interaction conditions. We then use this predictive transfer function to generate unique acceleration signals of arbitrary length. In order to move between transfer functions from different probe-surface interaction conditions, we develop a method for interpolating the variables involved in the texture synthesis process. Finally, we compare the results of this process with real recorded acceleration signals, and we show that the two correlate strongly in the frequency domain.

]]> Katherine J. Kuchenbecker et al. http://repository.upenn.edu/meam_papers/285 http://repository.upenn.edu/meam_papers/285 Tue, 21 Aug 2012 10:39:11 PDT Individuals learning a new sport often repeat a motion hundreds or thousands of times to try to perfect their form. The quintessential example of this process may be a beginning golfer struggling to learn to putt, where strokes must be precise and consistent in order to place the ball in the hole. This paper presents a four-cable haptic device designed to help golfers learn to improve their putting accuracy. This planar three-DOF system provides feedback that consists of two Cartesian forces and one angular moment. We present the system’s design and kinematics, along with a closed-loop controller that helps the user keep the putter head at the correct angle in the plane. We evaluated our design through a study in which five subjects used the system to repeatedly putt at a target both with and without assistance. While assistance did not change the mean of the putting distribution, it did significantly affect the variance for some subjects

]]> Katherine J. Kuchenbecker et al. http://repository.upenn.edu/meam_papers/284 http://repository.upenn.edu/meam_papers/284 Wed, 20 Jun 2012 11:48:24 PDT Dragging a tool across a textured object creates rich high-frequency vibrations that distinctly convey the physical interaction between the tool tip and the object surface. Varying one’s scanning speed and normal force alters these vibrations, but it does not change the perceived identity of the tool or the surface. Previous research developed a promising data-driven approach to embedding this natural complexity in a haptic virtual environment: the approach centers on recording and modeling the tool contact accelerations that occur during real texture interactions at a limited set of force-speed combinations. This paper aims to optimize these prior methods of texture modeling and rendering to improve system performance and enable potentially higher levels of haptic realism. The key elements of our approach are drawn from time series analysis, speech processing, and discrete-time control. We represent each recorded texture vibration with a low-order auto-regressive moving-average (ARMA) model, and we optimize this set of models for a specific tool-surface pairing (plastic stylus and textured ABS plastic) using metrics that depend on spectral match, final prediction error, and model order. For rendering, we stably resample the texture models at the desired output rate, and we derive a new texture model at each time step using bilinear interpolation on the line spectral frequencies of the resampled models adjacent to the user’s current force and speed. These refined processes enable our TexturePad system to generate a stable and spectrally accurate vibration waveform in real time, moving us closer to the goal of virtual textures that are indistinguishable from their real counterparts.

]]> Heather Culbertson et al. http://repository.upenn.edu/meam_papers/283 http://repository.upenn.edu/meam_papers/283 Wed, 20 Jun 2012 11:14:57 PDT We present and evaluate an arm-motion guidance system that uses magnetic tracking sensors and low cost vibrotactile actuators. The system measures the movement of the user’s arm and provides vibration feedback at the wrist and elbow when they stray from the desired motion. An initial study was conducted to investigate whether adding tactile feedback to visual feedback reduces motion errors when a user is learning a new arm trajectory. Although subjects preferred it, we found that the addition of tactile feedback did not affect motion tracking performance. We also found no strong preference or performance differences between attractive and repulsive tactile feedback. Some factors that may have influenced these results include the speed and the complexity of the tested motions, the type of tactile actuators and drive signals used, and inconsistencies in joint angle estimation due to Euler angle gimbal lock. We discuss insights from this analysis and provide suggestions for future systems and studies in tactile motion guidance.

]]> Katherine J. Kuchenbecker et al. http://repository.upenn.edu/meam_papers/282 http://repository.upenn.edu/meam_papers/282 Tue, 19 Jun 2012 11:51:31 PDT We develop a general procedure for estimating the effective constitutive behavior of nonlinear dielectrics. The procedure is based on a variational principle expressing the effective energy function of a given nonlinear composite in terms of the effective energy functions of the class of linear comparison composites. This provides an automatic procedure for converting well-known information for linear composites, in the form of estimates and bounds for their effective dielectric constants, into corresponding estimates and bounds for the effective behavior of nonlinear composites. Further, the procedure is easily implemented, and leads in some cases to exact results. This, exact estimates are given herein for isotropic weakly nonlinear composites with general nonlinearity, and bounds of the Hashin-Shtrikman type are given for the class of two-phase, isotropic dielectric composites with strongly and perfectly non-linear constitutive behavior. The optimality of the bounds is addressed briefly.

]]> Pedro Ponte-Castañeda et al. http://repository.upenn.edu/meam_papers/281 http://repository.upenn.edu/meam_papers/281 Thu, 31 May 2012 14:32:36 PDT Highly sp³-bonded, nearly hydrogen-free carbon-based materials can exhibit extremely low friction and wear in the absence of any liquid lubricant, but this physical behavior is limited by the vapor environment. The effect of water vapor on friction and wear is examined as a function of applied normal force for two such materials in thin film form: one that is fully amorphous in structure (tetrahedral amorphous carbon, or ta-C) and one that is polycrystalline with sp³ to disordered sp² bonding is observed, no crystalline graphite formation is observed for either film. Rather, the primary solid-lubrication mechanism is the passivation of dangling bonds by OH and H from the dissociation of vapor-phase H₂O. This vapor-phase lubrication mechanism is highly effective, producing friction coefficients as low as 0.078 for ta-C and 0.008 for UNCD, and wear rates requiring thousands of sliding passes to produce a few nanometers of wear.

]]> Andrew Konicek et al. http://repository.upenn.edu/meam_papers/280 http://repository.upenn.edu/meam_papers/280 Wed, 28 Mar 2012 10:45:21 PDT We study theoretically magnetohydrodynamic (MHD) motion of a binary electrolyte in a concentric annulus subjected to a uniform, axial magnetic field. The annulus’ cylindrical surfaces serve as electrodes. When a potential difference is imposed across the cylindrical electrodes, radial electric current flows in the solution and interacts with the axial magnetic field to induce a Lorentz body force that drives azimuthal fluid flow. When the annulus is infinitely long, a purely azimuthal flow (analogous to the classical Dean flow) is possible. We determine the velocity profile, ion concentration fields, and current density as functions of the electrodes’ potential difference and study the linear stability of the azimuthal flow. Of particular interest is the effect of the ions’ concentration fields on the centrifugal Dean instability. When the current is directed outwardly, electrochemical effects destabilize the flow, and the MHD flow loses stability at a Dean number much lower than its analogous, pressure driven flow. The supercritical flow consists of convective cells in the transverse plane. In contrast, when the current is directed inwardly, electrochemical effects stabilize the flow and the azimuthal flow is linearly stable for all Dean numbers. When the annulus is capped, purely azimuthal flow is no longer possible, and the flow in the annulus is always three-dimensional. In this case, the secondary flow is mostly driven by pressure gradients induced by the no-slip floor and ceiling. The intensity of the transverse convection depends then only weakly on the current's direction.

]]> M. Qin et al. http://repository.upenn.edu/meam_papers/279 http://repository.upenn.edu/meam_papers/279 Thu, 02 Feb 2012 10:49:19 PST The shape dynamics of soft, elastic particles in an unbounded simple shear flow is investigated theoretically under Stokes flow conditions. Three types of motion—- steady-state, trembling, and tumbling—- are predicted, depending on the shear rate, elastic shear modulus, and initial particle shape. The steady-state motion is found to be always stable. In addition, the existence of a trembling regime is documented for the first time in nonvesicle systems, and a complete phase diagram is developed. The rheological properties of dilute suspensions of such soft particles generally exhibit shear-thinning behavior and can even display negative intrinsic viscosity for sufficiently soft particles.

]]> Tong Gao et al. http://repository.upenn.edu/meam_papers/278 http://repository.upenn.edu/meam_papers/278 Thu, 13 Oct 2011 11:45:16 PDT Han-Sheng Chuang et al.