26-03-2011, 09:33 AM
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ABSTRACT
Sensitive skin is a large-area, flexible array of sensors with data processing capabilities, which can be used to cover the entire surface of a machine or even a part of a human body. Depending on the skin electronics, it endows its carrier with an ability to sense its surroundings via the skin’s proximity, touch, pressure, temperature, chemical/biological, or other sensors. Sensitive skin devices will make possible the use of unsupervised machines operating in unstructured, unpredictable surroundings among people, among many obstacles, outdoors on a crowded street, undersea, or on faraway planets. Sensitive skin will make machines “cautious” and thus friendly to their environment. This will allow us to build machine helpers for the disabled and elderly, bring sensing to human prosthetics, and widen the scale of machines’ use in service industry. With their ability to produce and process massive data flow, sensitive skin devices will make yet another advance in the information revolution. This paper surveys the state of the art and research issues that need to be resolved in order to make sensitive skin a reality.
1. INTRODUCTION
This seminar focuses on the principles, methodology, and prototypes of sensitive skin-like devices, and the related system intelligence and software that are necessary to make those devices work. Sensitive skin represents a new paradigm in sensing and control. These devices will open doors to a whole class of novel enabling technologies, with a potentially very wide impact. Far-reaching applications not feasible today will be realized, ranging from medicine and biology to the machine industry and defense. They will allow us to fulfill our dream for machines sensitive to their surroundings and operating in unstructured environment.
Some applications that sensitive skin devices will make possible are yet hard to foresee. Flexible semiconductor films and flexible metal interconnects that will result from this work will allow us to develop new inexpensive consumer electronics products, new types of displays, printers, new ways to store and share information (like electronic paper and “upgradeable” books and maps). New device concepts suitable for large area flexible semiconductor films will lead to new sensors that will find applications in space exploration and defense, specifically in mine detection and active camouflage.
An ability of parallel processing of massive amounts of data from millions of sensors will find applications in environmental control and power industry. These areas will be further developed because of the highly interdisciplinary nature of the work on sensitive skin, which lies at the intersection of information technology, mechanical engineering, material science, biotechnology, and micro- and nano electronics. Availability of sensitive skin hardware is likely to spur theoretical and experimental work in many other disciplines that are far removed from robotics.
1.1. MACHINES IN UNSTRUCTURED ENVIRONMENTS
Today’s machine automation is almost exclusively limited to the structured environment of the factory floor. The rest of the world, with perhaps 99% of all tasks that involve motion and could in principle be automated, goes unautomated. Think of the unstructured environments in agriculture, construction sites, offices, hospitals, etc. The majority of tasks that are of interest to us take place in unstructured environments, to which today’s automation simply cannot be applied.
Automated moving machines can be divided into unattended those that can operate without continuous supervision by a human operator, and semi-attended, which are controlled by the operator in a remote (teleoperated) fashion. Today the use of both types of machines is limited exclusively to highly structured environments - a factory floor, a nuclear reactor, a space telescope. Such machines can operate successfully with relatively little and fairly localized sensing. Many existing machines could, in principle, be useful in an unstructured environment, if not for the fact that they would endanger people, surrounding objects, and themselves.
The same is true for remotely controlled machines. Unless the work cell is “sanitized” into a structured environment, no serious remote operation could be undertaken. Otherwise, at some instant the operator will overlook a small or occluded object, and an unfortunate collision will occur. And so the designers take precautions, either by “sanitizing” the environment, or by enforcing maddeningly slow operation with endless stops and checks. Much of the associated extra expense would not be necessary if the machines had enough sensing to cope with unpredictable objects around them.
The Way Out is All-Encompassing Sensing:
To operate in an unstructured environment, every point on the surface of a moving machine must be protected by this point’s “own” local sensing.
1.2. SOCIETAL NEEDS AND CONCERNS OF SENSITIVE SKIN
1.2.1. HEALTH INDUSTRY
Sensitive skin will supplant sensing ability of the human skin in limb prosthetics and as a replacement of damaged human skin. It will augment human sensing in wearable clothing, by monitoring, processing, and wireless transfer of information about the well-being of the person wearing sensitive skin. This will advance the post-traumatic health care, care for disabled and elderly persons, and monitoring of military personnel on the battlefield.
1.2.2. ENVIRONMENT – FRIENDLY TECHONOLOGY
For the first time in history, machines will be endowed with a capacity to be careful. By its very nature, sensitive skin will contribute in a dramatic way to the reversal of the well-known negative impact of machines on our environment, across a wide spectrum of natural and man-made settings.
We often hear about the role of computer revolution and office automation in the growth of economy and improved efficiency, which in turn affects the quality of life. Note the difference: while unstructured machine automation will have a similar effect on the economy, its use in service industry will have a direct impact on the quality of human life. Biology and medical science thrive to prolong human life; the unstructured machine automation will constitute a systematic effort by engineers to improve the quality of life.
1.2.3. DIFFICULTIES OF ACCEPTANCE
As with any fundamentally new and powerful technology, sensitive skin technology may evoke adverse psychological reactions, with a potential of diminishing its impact. Today we are psychologically unprepared for automatic moving machines operating in our midst. We are not sure we need them. We are uneasy about the idea of living side by side with a powerful unattended moving machine. It is difficult to imagine that one could stand next to a powerful moving machine and trust it enough to turn one’s back to it, or expect it to step aside when passing. Do we not have more than enough invasion of machinery in our lives? To need a very new product, one must first experience it.
2. SYSTEM CONCEPT
Figure-1 Sketch of interconnects between sensors, intelligence, and actuators
The system consists of a number of distributed sensor, actuator, and intelligence units, which are connected by some network of interconnects. The interconnects are necessary for providing power to the system as well as for communication. The sensors/actuators themselves may have intelligence associated with them, but there are other higher levels of intelligence to which they are connected.
The interconnects shown in the system might be electrical (conventional wires) or optical (fibers). The communication via the individual units might in some cases be “wireless” (implying also fiber-less) for some structures.
For delivering power, it was thought that the system probably would require physical interconnects (i.e. power delivered through fibers or wires), and that “harnessing” energy from the environment, such as via solar or RF pick-ups, would not be practical for most applications (especially for wireless systems). Therefore in all cases there would have to be a physical interconnect between the individual sensor / actuator / intelligence blocks, and so a major part of this report addresses issues associated with this physical level of interconnection.
3. SKIN METERIALS
- Sensitive Skin material will hold embedded sensors and related signal processing hardware. It needs to be flexible enough for attaching it to the outer surfaces of machines with moving parts and flexible joints.
- The skin must stretch, shrink, and wrinkle the way human skin does, or to have other compensating features. Otherwise, some machine parts may become "exposed" due to the machine's moving parts, and have no associated sensing.
- Wiring must keep its integrity when Sensitive Skin is stretched or wrinkled. This requirement calls for novel wire materials, e.g. conductive elastomers or vessels carrying conductive liquid, or novel ways of wire design with traditional materials, such as helical, stretchable wires.
3.1. AERAS OF DISCUSSION
Three areas of potential discussion were considered:
1. What materials might be used for sensors, actuators, and intelligence (transistors) in such a system?
2. How can we make an interconnection network that can flex and bend?
3. How can we physically combine sensors/intelligence/actuators with the interconnect substrate?
3.2. SUBSTRATE / INTERCONNECT ISSUES
3.2.1. STRECHING AND BENDING
A central issue for sensitive skin is that the skin be able to conform to surfaces of arbitrary shape, and be able to flex, bend, and stretch. Flexing, bending, and stretching are important not only for applications (e.g. covering moving arms and joints), but also for initial installation (like putting on clothes).
When a thin planar foil is deformed into “developable” surface such as a cylinder or a cone, the average strain in the foil is zero, and there exists a “neutral plane” within its bulk where the strain locally is zero. The strain on the surfaces scales as the thickness over the radius of curvature.
Therefore by making the substrate thin and /or placing interconnects at the neutral plane, bending to thin radii of curvature appears possible. However, deforming into arbitrary shapes (e.g. spheres), bending in multiple dimensions, and stretching require a finite strain, and hence may cause failure of the interconnects (e.g. if the strain is larger than 1%).
Three different models for the substrate/interconnect system evolved. Adding sensors/actuators/intelligence to the substrate will be discussed in the next major section.