Project Overview:
The goal of the ShiverPaD project is to be able to display buttons, switches, and other surface features on touchscreens and touchpads. Using the ShiverPaD's finger-forcing capability, we are able to create compliance, viscosity or any other force field confined to the plane of the glass. These forces can create the haptic illusion of surface features such as bumps and edges. By displaying the edges of a button on a touch screen/pad, a user should be able to locate a button by using the same intuitive tactile exploration used to find real buttons on a real keypad/keyboard.
While this concept has already demonstrated its potential to be extended to 2 forcing dimensions, the ShiverPaD prototype discussed here is a 1DOF device capable of rendering 1D virtual environments. As an example of what the 1DOF ShiverPaD is capable of, this video shows a finger flipping a virtual toggle switch.
Publications:
- ShiverPaD: A Glass Haptic Surface That Produces Shear Force on a Bare Finger. Erik C. Chubb, J. Edward Colgate and Michael A. Pehskin. Transactions on Haptics, IN REVIEW.
- Master's Thesis, Erik C. Chubb, Northwestern University, Chicago, IL, USA. September 2009.
- ShiverPad: A Device Capable of Controlling Shear Force on a Bare Finger Erik C. Chubb, J. Edward Colgate and Michael A. Pehskin. World Haptics Conference 2009, March 18-20, Salt Lake City, Utah, pp. 18-23.
Videos:
- Video showing the method the ShiverPaD uses to generate forces on a finger.
- Video showing a finger flipping a virtual toggle switch displayed by the ShiverPaD.
- Video showing the ShiverPaD applying forces to objects: an earplug and a brass weight. In this case, the ShiverPaD frequency is 220Hz, and the forcing direction is arbitrarily chosen to alternate between left and right 3 times per second.
Presentations:
How it works
At the heart of the ShiverPaD is the TPaD variable friction device. It modulates the friction of a glass surface by using 39kHz out-of-plane vibrations to form a cushion of air (or "squeeze film") between the finger and the glass. To generate shear forces, the TPaD is oscillated in-plane while alternating between low and high friction within each cycle (between 40 and 854Hz). During each low friction phase the TPaD slips beneath the finger without applying significant force. During each high friction phase the finger is pushed by the high-friction glass. The time average of these impulses creates a non-zero net force. In Figure 1, the process for creating a rightward net force is illustrated. Figure 2 is a video of the process.
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| (2) A video illustrating how the ShiverPaD produces force. |
The five components of our ShiverPaD prototype are labeled in Figure 3, and explained here,
- The voice coil provides the actuation for the lateral, or "shiver", motion.
- The linear slide ensures that the motion is constrained to the x-direction.
- The "shiver spring" together with the mass of the TPaD create a simple harmonic oscillator. The amplification at resonance is leveraged to achieve greater lateral oscillation amplitudes.
- The TPaD is a variable friction display capable of altering friction on the millisecond time scale or better.
- The laser-based Finger Positioning Sensor with a workspace of 49x49mm and a 0.1mm resolution. (not shown in the figure)
The remaining components in the figure are used to measure the forces on the fingertip.
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| (3a) A conceptual schematic of ShiverPaD. The components outside of the gray dashed box are used for sensing forces on the finger. |
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| (3b) The ShiverPaD device and the force measuring setup. |
Examples of Virtual Environments
Toggle Switch:
Here we create a force field that is a collection of two line sinks and one line source (see Figure 4a). Like a real toggle switch, this field has two low-potential positions with an unstable high-potential position between them.
When a finger is on the surface, it tends to be pushed away from the high-potential line source, and naturally gets pulled into a stable position in one of the line sinks. Therefore, when trying to move the finger back and forth smoothly, it naturally "flips" between the two low-potential areas. It approximates the feel of flipping a toggle switch (Figure 4b is a video of the experience).
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| (4a) The force field used to display the virtual toggle switch |
(4b) A video of a finger flipping the virtual toggle switch |
Edge Tracing:
The edges of a button give us important information about the shape and location of the button. During the exploration of a real edge, the guiding forces keeping the finger constrained to the edge are roughly orthogonal to the direction of travel. Since the ShiverPaD is capable of creating forces orthogonal to finger motion, it has the potential to display edge-like features.
In Figure 5 we show how two different contours (or "edges") can be displayed using the ShiverPaD. The line-sink force field can also be viewed as a vertical line (Figure 5a). Since the entire dashed line has the same low potential, it is easy to run a finger along its length. With the notch contour (Figure 5b) the low potential region of the force field is shifted left and the finger tends to follow its shape.
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| (5a) The line contour |
(5b) The notch contour |
At any instant in time, the ShiverPaD has a constant force field across its surface (finger position feedback is used to create the illusion of spatially varying force fields). An interesting consequence to this method of force generation is that features much smaller than the finger pad are experienced across the whole fingerpad. A real-life analogy to this experience is a small probe glued to the fingertip (a cartoon of this idea is in Figure 6a). Or probably more true to the haptic experience of the ShiverPaD, a small cylindrical magnet glued to the end of the fingertip and interacting with a contour of ferromagnetic material embedded below the surface (Figure 6b).
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| (6a) Probe on the fingertip falls into a grove cut in the glass |
(6b) Magnet on the fingertip interacts with a strip of ferromagnetic material embedded below the surface of the glass |
To demonstrate that the ShiverPaD improves edge-following capability, an experiment was conducted to show that subjects are faster at finding, and more accurate at tracing contours with the ShiverPaD when compared to the TPaD variable friction device. In a 5-subject experiment, it was found with high statistical significance that when using the ShiverPaD, there was a large decrease in the time to find the contour and a large improvement in the subjects ability to accurately trace the contour.
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