A Fingertip Shear Tactile Display for Communicating Direction Cues Scott K. Horschel, Brian T. Gleeson, William R. Provancher

Haptics and Embedded Mechatronics Lab, University of Utah

ABSTRACT Fingertip skin stretch has been found by our lab to be a reliable means for communicating direction. This demonstration will consist of two different parts to illustrate this means of communication. The first part of the demonstration will give participants some perspective on the importance of properly selecting stimulus parameters when rendering skin stretch. It will utilize a precision bench-top shear display. This device is a precision Parker X-Y stage that is used to impose fingertip skin stretch shear stimuli with displacements of 0.05-1.00 mm at rates of 0.5-4 mm/s. The second part of the demonstration will utilize a portable fingertip shear display to provide navigation cues to guide participants through the conference center.

open-bottomed thimble, as described in [4]. Thimbles of different sizes will be used to accommodate a range of finger sizes. A hinge mechanism will prevent the thimble from moving in the proximal/distal and lateral directions but allow the thimble to move up and down (Figure 3). The user will thus be able to regulate the force applied to the shear tactor but is constrained from moving in the plane of the stimuli. The device makes contact with the user’s fingerpad through a sandpaper-like IBM ThinkPad TrackPoint tactor, measuring approximately 7 mm in diameter.

KEYWORDS: Tactile devices and display, Perception and psychophysics INDEX TERMS: H.1.2 [Models and Principles]: User/Machine Systems--Human information processing; H.5.2 [Information Interfaces and Presentation]: User Interfaces—Haptic I/O 1

INTRODUCTION

We have developed two different devices, which have the capability to provide shear feedback in any direction. Despite the high-resolution capabilities of each device, we have confined our present studies to communicating direction in the distal, proximal, ulnar, and radial directions of a person’s fingertip. These directions will be referred to as North (N), South (S), East (E), and West (W), respectively (Figure 1). The ability to communicate direction tactilely may have advantages over direction cues that are provided through visual (maps) or auditory (spoken instructions) pathways, as suggested by multiple resource theory [1]. This would be ideal for drivers, first responders in navigation of buildings, or soldiers in an urban setting. Skin stretch was chosen over other modes of tactile communication for reasons of mechanical design and perceptual acuity. Studies, such as [2], have found direction detection thresholds to be lower for skin stretch than for other forms of tactile stimulation. Additionally, the fingertip is more sensitive to tangential forces than normal forces [3]. 2

Figure 1: Concept for communicating direction via skin stretch.

BENCH-TOP DESIGN

The bench-top device is a Parker Two Axis Linear Stage driven by geared Maxon RE36 DC motors with a gear ratio of 4.8:1 (Figure 2). Position is measured by US Digital E2 encoders with 1250 ticks/revolution, providing position resolution of approximately 0.4 µm. The user’s finger is constrained with an Department of Mechanical Engineering 50 S. Central Campus Dr., Salt Lake City, Utah, 84112-9208 {brian.gleeson, shorschel}@gmail.com, [email protected]

Figure 2: Test setup. The user sits with his/her right index finger in a thimble with the tactor contacting the fingerpad. The device cover is shown pulled back for documentation purposes only. A graphical user interface used for prompting and recording user responses is shown on the left.

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MINIATURE SERVO DESIGN

We have also developed a miniature device capable of providing tactile shear feedback to a user’s fingertip. A secure attachment is made to the fingertip by again using an open-bottom thimble. The thimble’s open bottom permits the device’s shear tactor access to the user’s fingerpad and to reliably render skin stretch. This device embodiment uses two radio controlled (RC) hobby servos and a flexure stage to create planar motion (see Figure 4). Pins protrude upward from each RC servo output wiper into orthogonal slots in the flexure. The flexure is designed such that rotation of an RC servo induces motion on a single axis and so that the motions of the two servos are completely decoupled. The flexure stage was designed to have a travel of ±1 mm. The RC servos used are Cirus CS-101 4g micro servos. The servos have a rotational range of 180 degrees. In the current design, we are using about 50% of the range, equating to about 45 degrees of

rotary motion to achieve 1 mm of linear motion. The device also uses a ThinkPad TrackPoint tactor as the interface from the device to the user’s fingerpad.

Figure 3: The shear tactor is shown in contact with the finger. The thimble and thimble mount are shown translucent so that the finger and tactor can be seen. The thimble is free to move up and down so that participants can regulate their contact force, but is constrained in the plane of tactor motion.

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DEMONSTRATION

We will present a demonstration consisting of two parts. The first part of our demo will use the bench-top design to give the participant direction cues (Figure 1). The demonstration will consist of stimuli that convey direction as effectively as possible, but each stimulus will have a different perceptual difficulty. The range of stimuli presented to conference participants will represent a condensed version of the experiments described in [6]. The direction stimuli will consist of shearing motions with various levels of displacement and speed. We will only communicate four directions: N, S, E, and W. The stimuli will have three distinct portions: an outbound move, a pause (300ms), and a return move. During the outbound and return move, the tactor will move in a straight line in the given direction, at a constant speed, for the given displacement. The return path speed is set at 66% of the outbound speed. The second part of our demonstration will consist of navigating blindfolded conference participants around designated portions of the conference center using only direction cues from our portable shear display (Figure 4). This miniature device will be placed on a blindfolded participant’s finger and will render shear skin stretch to the user’s fingerpad. There will be different types of stimuli given to the participants, including both DC and periodic signals. These various types of direction stimuli have been evaluated in preliminary pilot testing and each has been found to have its own advantages and disadvantages. 5

ACKNOWLEDGEMENTS

This work was supported, in part, by the National Science Foundation under awards IIS-0746914 and DGE-0654414. We would also like to acknowledge our collaboration with Massimiliano Solazzi and Dr. Antonio Frisoli of Scuola Superiore Sant'Anna towards miniaturizing the shear display device. Our thanks to Dr. Hong Tan of Purdue University for her advice. REFERENCES

Figure 4: 3-D solid model of the compact RC servo shear display.

The flexure stage is comprised of two different types of polyurethanes from Innovative Polymers, Inc.: IE-90A (soft) and IE-80D (hard). The two different urethanes were used to create a flexure that is both compact and functional. The harder urethane was used for the main structure of the device and flexible joints were made from the softer urethane (see Figure 5). The flexure was fabricated using shape deposition manufacturing (SDM) [5].

Figure 5: SDM flexure showing the main support frame and the flexible connections.

[1] C. Wickens, Engineering Psychology and Human Performance, 2th ed.: Merrill, 1992. [2] U. Norrsell and H. Olausson, "Human, tactile, directional sensibility and it peripheral origins.," Acta physiologica scandinavica, vol. 144, no. 2, pp. 155-161, 1992. [3] J. Biggs and M. A. Srinivasan, "Tangential versus normal displacements of skin: relative effectiveness for producing tactile sensations.," in In Proc. HAPTICS '02, Orlando, FL, 2002. [4] W. R. Provancher, M. R. Cutkosky, K. J. Kuchenbecker, and G. Niemeyer, "Contact Location Display for Haptic Perception of Curvature and Object Motion," International Journal of Robotics Research, vol. 29, no. 4, pp. 691-702, 2005. [5] L. E. Weiss et al., "Shape Deposition Manufacutring of Heterogeneous Structures," Journal of Manufacturing Systems, vol. 16, pp. 239-248, 1997. [6] B. Gleeson, S. Horschel, and W. Provancher, "Communication of Direction through Lateral Skin Stretch at the Fingertip," Submitted to World Haptics, 2009.