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Open Access Dissertation
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Year Degree Awarded
Computer Engineering | Computer Sciences | Electrical and Computer Engineering
The key limitation in mobile computing systems is energy - without a stable power supply, these systems cannot process, store, or communicate data. This problem is of particular interest since the storage density of battery technologies do not follow scaling trends similar to Moore's law. This means that depending on application performance requirements and lifetime objectives, a battery may dominate the overall system weight and form factor; this could result in an overall size that is either inconvenient or unacceptable for a particular application. As device features have scaled down in size, entire embedded systems have been implemented on a single die or chip, resulting in the battery becoming the form factor bottleneck.
One way to diminish the impact that batteries have on mobile embedded system design is to decrease reliance on buffered energy by providing the ability to harvest power from the environment or infrastructure. There are a spectrum of design choices available that utilize harvested power, but of particular interest are those that use small energy buffers and depend almost entirely on harvested power; by minimizing buffer size, we decrease form factor and mitigate reliance on batteries. Since harvested power is not continuously available in embedded computing systems, this brings forth a unique set of design challenges.
First, we address the design challenges that emerge from mobile computing systems that use minimal energy buffers. Specifically, we explore the design space of a computational radio frequency identification (RFID) platform that uses a small solar harvesting unit to replenish a capacitor-based energy storage unit. We show that such a system's performance can be enhanced while in a reader's field of interrogation and also allows for device operation while completely decoupled from reader infrastructure. We also provide a toolset that simulates system performance using a set of experimentally obtained light intensity traces gathered from a mobile subject.
Next, we show how energy buffered from such a harvesting-based system can be used to implement an efficient burst protocol that allows a computational RFID to quickly offload buffered data while in contact with a reader. The burst mechanism is implemented by re-purposing existing RFID protocol primitives, which allows for compatibility with existing reader infrastructure. We show that bursts provide significant improvements to individual tag throughput, while co-existing with tags that do not use the burst protocol.
Next, we show that energy harvesting can be used to enable a novel security mechanism for embedded devices equipped with Near Field Communications (NFC). NFC is growing in pervasiveness, especially on mobile phones, but many open security questions remain. We enable NFC security by harvesting energy via magnetic induction, use the harvested energy to power an integrated reader chip, and selectively block malicious messages via passive load modulation after sniffing message contents. We show that such a platform is feasible based on energy harvested opportunistically from mobile phones, successfully blocking a class of messages while allowing others through. Finally, we demonstrate that energy harvested from mobile phones can be used to implement wirelessly powered ubiquitous displays. One drawback of illuminated displays is that they need a continuous source of power to maintain their state - this is an undesirable property, especially since the display is typically the highest power consumption system component of embedded devices. Electronic paper technologies eliminate this drawback by providing a display that requires no energy to maintain state. By combining NFC energy harvesting and communication, and electronic paper technologies, we implement a companion display for mobile phones that obtains all the energy required for a display update while communicating with a user application running on a mobile phone. The companion display assists the phone in displaying static information while the power hungry display remains unpowered.
Gummeson, Jeremy Joel, "Exploiting Energy Harvesting for Passive Embedded Computing Systems" (2014). Doctoral Dissertations. 8.