13-11-2012, 01:30 PM
HUMAN POWERED WEARABLE COMPUTING
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ABSTRACT
Batteries add size, weight, and inconvenienceto present-day mobile computers. This paper explores the possibility of harnessing the energy expended during the user's everyday actions to generate power for his or her computer, thus eliminating the impediment of batteries. An analysis of power generation through leg motion is presented in depth, and a survey of other methods such as generation by breath or blood pressure, body heat is also presented.
Possibilities for the “parasitic” collection of power from human walking motion are explored. The focus is on the walking action as a source of power because of the extensive range of motion and large dynamic forces associated with the heel strike and the bendingof the sole. Two piezoelectric systems: a multilayer laminate made from PVDF foil and aunimorph piezoceramic composite (PZT); as well as a mechanical system utilizing a shoe mounted electric generator are examined.
INTRODUCTION
In today’s world, computers, as well as electronic devices, are becoming more and more integrated into everyday life. These seamless integrations focus on mobility, but at the same time strive to be self-effacing to the end user. With the introduction of personal data assistants (PDA’s) and advanced cellular phones capable of searching the web, true mobile computing is closer than ever.
Unfortunately, battery technology which powers most of these mobile connectivity solutions, has not kept up the same pace of improvement. Single use batteries continue to be bulky, expensive, and unreliable, in addition to costing millions to dispose of each year. And, while impressive gains have been made in rechargeable battery technology over the past decade, these types of reusable energy sources are still struggling to reach single-use levels one alternative to carrying relatively large-capacity batteries is to harvest the energy of everyday human motion and use it to power mobile devices.
Wearable computing is an effort to make computers truly part of our everyday lives by embedding them into our clothing (e.g., shoes) or by creating form factors that can be used like clothing (e.g., sunglasses).1 This level of access to computation will revolutionize how computers are used. Although the computational hardware has been reduced in size to accommodate this vision, power systems are still bulky and inconvenient. Even today’s laptops and PDAs (personal digital assistants) are often limited by battery capacity, output current, and the necessity of having an electrical outlet within easy access for recharging. However, if energy can be generated by the user’s actions, these problems will be alleviated.
WALKING
Using the legs is one of the most energy-consuming activities the human body performs. In fact, a 68 kg man walking at 3.5 mph, or 2 steps per second, uses 280 kcal/hr or 324 W of power.3 Comparing this to a standing or strolling rate implies that up to half this power is being used for moving the legs. While walking, the traveller puts up to 30 percent more force on the balls of his feet than that provided by body weight (Figure 1).
STORAGE CONSIDERATIONS
Every power generation system proposed, with the possible exception of heat conversion, would require some power storage device for periods between power generation cycles. Thus, some attention is necessary regarding the efficiency of storage. Electrical storage may be preferable because of its prevalence and miniaturization. First, however, the power must be converted to a usable form. For the piezoelectric method, a step-down transformer and regulator would be needed. Current strategies for converting the high voltages generated by piezoelectric materials to computer voltage levels can attain over 90 percent efficiency.Care is needed to match the high impedance of the piezo generator properly, and, due to the low currents involved, the actual efficiency may be lower. For the other generation methods, power regulators would be needed as well, and aggressive strategies can attain 93 percent efficiency. The most direct solution to the problem of electrical storage is to charge capacitors that can be drained for power during periods of no power generation. However ,simply charging the capacitor results in the loss of half the available power.23 Unfortunately, a purely capacitive solution to the problem is also restricted by size.
CONCLUSION
Although computing, display, communications, and storage technology may become efficient enough to require unobtrusive power supplies, the desire for the fastest CPU speeds and highest bandwidth possible will offset the trend. In addition, dependence on power cells requires the user to “plug in” occasionally. This is impossible in some military and professional contexts. If body motion is used, it may be significantly more convenient to shift weight from one foot to another, for example, than to search for an electrical outlet.
Each of the generation methods has its own strengths and weaknesses, depending on the application. However, power generation through walking seems best suited for general-purpose computing. The user can easily generate power when needed, and, in many cases, the user's everyday walking may be sufficient. A surprising amount of power (5–8 W) may be recovered while walking at a brisk pace, possibly without stressing the user. If less power is needed, piezoelectric inserts may be used, reducing the mechanical complexity of the generation system. However, issues of energy storage and human factors still have to be
resolved. Thus, the natural next step is to prototype a generator.