Stimulation of the Fingertip by Lateral Skin Stretch

1. THE INFORMATION ON THIS WEBPAGE IS LARGELY OUTDATED. PLEASE REFER TO THE FOLLOWING PAGE FOR AN UP-TO-DATE VERSION.

2. REFERENCES TO THE INFORMATION CONTAINED BELOW SHOULD NOW BE MADE BY CITING THE TECHNICAL REPORT TR-CIM 06.04 AVAILABLE HERE OR THE PUBLICATIONS AVAILABLE HERE.

Why this webpage?

The existence of this webpage holds multiple purposes. First, it is an attempt at putting into perspective all my graduate work at the McGill Haptic lab under Prof. Vincent Hayward's supervision. Since most of our lab research concerning skin stimulation by lateral stretch is reported throughout a few publications available below, the goal is certainly not to give another detailed description of this work. This webpage is meant to elaborate on what has been achieved and to offer an explanation of what we are trying to attain on the long term. This is closely tied to the second objective of this webpage, which is to develop a certain community around the STReSS project. It is clear that both, our Haptic lab and any lab interested in tactile displays would gain tremendously at opening a discussion on the subject, possibly leading to some form of cooperation. In fact, many resarch labs around the world have already shown interest in the STReSS. Finally, this webpage will also display interesting results that cannot be reported in our publications, most of the time because of their akward format (e.g. movies).

Acknowledgment

I would like to stress (no pun intended !) that most of the work that has been accomplished so far is the product of a tight collaboration with one of my lab colleagues, Vincent Levesque. Vincent's research interests are directly related to the STReSS project. In fact, both of our master's work have been combined into a USPTO patent that is currently pending. Vincent is now an essential member of the STReSS development team. For more information or any comment you would like to make, do not hesistate to contact either of us.

I would also like to mention that another lab colleague of ours, Qi Wang, has been involved with the manufacturing of the device since the beginning. His advice and expertise has been invaluable. Finally, and this goes without saying, most of the credit for the key ideas covered in this webpage goes to my supervisor, Prof. Vincent Hayward.

What is the big deal about touch?

Touch is a rich physical faculty. It is the only sense that is bi-directional - you can both create and feel tactile stimuli through skin. Skin is the largest organ in the body in terms of surface area. It allows for a particular exchange of information with the direct surrounding world, contributing greatly to our sense of awareness. We experience everyday shapes and textures through small and big skin deformations, changes in boundary temperatures, and, localized and distributed vibrations at the skin surface. All these interactions stimulate specific skin mechanoreceptors that encode the tactile information before it is being sent to the brain by neural impulses. Skin’s sensitivity is not uniformly distributed across the body; its highest sensitivity occurs at the fingertip because of the high density of mechanoreceptors that are present in this region. While vision and hearing are well understood, touch remains a mystery. Skin's properties, ranging from its biomechanics to its perceptual and cognitive characteristics, continue to be a major topic of debate in research. On the other hand, everyone agrees that touch plays an important role in our everyday experience of the surrounding world and that we would gain from a better understanding of it.

What are Tactile Displays?

Forschungszentrum Karlsruhe

There is a crying need for tools to help us experiment with touch. One of these tools is a comprehensive tactile display capable of providing meaningful sensations to a human subject in a controlled environment. Tactile displays (TD) are devices that try to communicate information through touch, most often at the fingertip. Tactile information includes both, the simulation of natural tactile percepts found in the real-world (e.g. the texture of a fabric) and the development of tactile stimuli used as communication means for human-computer interactions (e.g. the buzzing of a ringing beeper). Human-computer interaction technology has long been making use of touch for information transfer from the human to the computer (through a keyboard for instance). However, it has rarely relied on touch to close the communication loop for the information transfer in the other direction - from the computer to the human. Most of the time, humans rely on their vision (via a computer screen) and audio capabilities (via speakers) to retrieve information from the computer. This emphasizes a great contrast with the real-world where we are continuously relying on touch to explore and feel our environment.

What Are the Applications for TDs?

VisuAide inc

Why are Tactile Displays so Difficult to Build?

Shape Memory Alloys

(Grant & Hayward, 1999)

Unfortunately, building a useful and cost-effective TD is not a trivial task and none of the proposed designs seems to yield satisfying results.Tactile displays are complex by nature because they must be composed of high-density arrays of high-performance actuators. In addition, the sense of touch is still widely unknown. For instance, designing an optimal tactile display requires a precise knowledge of the skin and its neuroanatomy, which in turn, is one of the aspects we are trying to study with the device. Hence, we are caught in a vicious circle. An optimal device can only be achieved through iterative designs.

Pneumatic

(Chiang & Fearing, 2000)

Most of the attempts at implementing practical tactile displays have exploited normal skin indentation with pins going up and down against the fingertip in order to create a discrete representation of a texture or a small shape. Other techniques make use of actuators that vibrate, that heat up, that blow pressured air, that change shape when submitted to an electric or magnetic field, that create small currents through the skin, and the list goes on. Among the most used actuator technologies we can find shape memory alloys (SMA), piezoelectric ceramics, motors, pneumatic valves, Peltier elements, rheological fluids, pistons, electrodes, and more. Each possible skin interaction mode combined with an appropriate technology (e.g. skin indentation created with SMA actuators) yields its characteristic advantages and drawbacks (for a table on the state-of-the art, click here).

Rheological

(Lederer et al., 2000)

To our knowledge, most of the tactile displays built to this day fail to convey meaningful tactile information and to be practical at the same time. The devices are too bulky or too big, they do not yield enough force or are constrained to a low bandwidth, they are limited to a small number of actuators with low-spatial density, they require constant maintenance or are simply too expensive and too complex. These failures are reflected by the very low number of tactile displays that have made it to the commercial world. Except for a few devices targeted for the visually-impaired (Braille Players, Optacon, etc.) and even fewer consumer electronic gadgets with limited useful functionalities (e.g. a vibrating cellular phone), there does not exist any commercial tactile display that can be bought for a reasonable price (say, a fraction of the cost of a PC).  

A New Type of Tactile Display

STReSS

VBD

What is the STReSS?

STReSS display

The Stimulator of Tactile Receptors by Skin Stretch (STReSS) is a computer-peripheral device that stimulates the fingertip  by lateral skin stretch. It is composed of a miniature array of 100 (prototype #1) or 50  (prototype #2) bending actuators that induce time-varying programmable strain fields at the fingertip to convey tactile information such as texture or small-scale shape. Tactile signals are generated on a personal computer and then fed to the display via a universal serial bus (USB). My master's thesis (Pasquero, 2003) describes the construction method as well as the drive electronics of the first STReSS prototype. It also reports informally on some tactile sensations that the display is capable of producing. Hopefully the device's unique capabilities will help shed some light on the touch's most intriguing properties.

STReSS system

An entire webpage will soon be dedicated to the STReSS. We intend to post open-source code and open-hardware plans on everything that we have implemented so far concerning the STReSS. Our hope is to develop a small community around the STReSS to get an insight on how to improve on the device and on how to use it in a meaningful way.

In the meantime, interesting movies showing the piezoelectric actuators moving can be found here.

What is the VBD?                  

Current Braille displays consist of a linear array of Braille cells, each capable of reproducing a single Braille character by controlling the positions - up or down - of eight pins. The display is read the same way as paper Braille, by scanning the surface with the fingers. Unfortunately, Braille displays are currently limited to a single line because of cost considerations. We believe it might be feasible to simulate an entire Braille page at a very low cost by using just a few movable cells using lateral skin stretch . This claim is supported by the fact that touch is a local sense. When creating a virtual Braille page, there is no need to display it entirely since only what is currently under the reading fingers is felt.

Computer Braille cell

As it turns out, the VBD is not limited to displaying Braille dots. We are currently investigating a strong illusion that was discovered in the process of building the VBD. As for the STReSS, we will shortly have an entire webpage dedicated to the VBD. In the meantime, an interesting movie showing how the device can be used to read Braille dots can be found here.

VBD Principle