Conference Paper

Millimeter-scale nearly perpetual sensor system with stacked battery and solar cells

Univ. of Michigan, Ann Arbor, MI, USA
DOI: 10.1109/ISSCC.2010.5433921 Conference: IEEE International Solid-State Circuits Conference, ISSCC 2010, Digest of Technical Papers, San Francisco, CA, USA, 7-11 February, 2010
Source: DBLP


Sensors with long lifetimes create new applications in medical, infrastructure and environmental monitoring. Due to volume constraints, sensor systems are often capable of storing only small amounts of energy. Several systems have increased lifetime through VDD scaling [1][2][3]. This necessitates voltage conversion from higher-voltage storage elements, such as batteries and fuel cells. Power is reduced by introducing ultra-low-power sleep modes during idle periods. Sensor lifetime can be further extended by harvesting from solar, vibrational and thermal energy. Since the availability of harvested energy is sporadic, it must be detected and stored. Harvesting sources often do not provide suitable voltage levels, so DC-DC up-conversion is required.

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    • "Recently emerging micro-watt applications, such as micro-sensor networks, handset electronics, implantable biomedical devices, etc place a primary criterion on minimum energy consumption or high energy efficiency to prolong battery life time [1]-[3]. To improve the energy efficiency, operating voltage (V DD ) in these applications is positioned near or below threshold voltage (V th ), known as the near-or sub-threshold region. "
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    • "However, storage capacity may be limited by cost, size, or weight. For example, a 9-cubic millimeter solar-powered sensing system recently developed uses a 12µAh battery [34]. Many existing approaches to solar-powered sensing choose a fixed sampling rate that minimizes the chance of fully depleting the energy supply, based on long-term predictions of solar energy levels . "
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    ABSTRACT: Daylight harvesting is the use of natural sunlight to reduce the need for artificial lighting in buildings. The key challenge of daylight harvesting is to provide stable indoor lighting levels even though natural sunlight is not a stable light source. In this paper, we present a new technique called SunCast that improves lighting stability by predicting changes in future sunlight levels. The system has two parts: 1) it learns predictable sunlight patterns due to trees, nearby buildings, or other environmental factors, and 2) it controls the window transparency based on a quadratic optimization over predicted sunlight levels. To evaluate the system, we record daylight levels at 39 different windows for up to 12 weeks at a time, and apply our control algorithm on the data traces. Our results indicate that SunCast can reduce glare by 59% over a baseline approach with only a marginal increase in artificial lighting energy.
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    • ". Our low-power switched capacitor network up-converts the 0.5V voltage of the solar cells to charge up the 3.6V Li-film battery[7]. Figure 4. On-demand clocking improves the power management unit efficiency by 20% at low load levels [7] "
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    ABSTRACT: Sensors with long lifetimes are ideal for infrastructure monitoring. Miniaturized sensor systems are only capable of storing small amounts of energy. Prior work has increased sensor lifetime through the reduction of supply voltage , necessitating voltage conversion from storage elements such as batteries. Sensor lifetime can be further extended by harvesting from solar, vibrational, or thermal energy. Since harvested energy is sporadic, it must be detected and stored. Harvesting sources do not provide voltage levels suitable for secondary power sources, necessitating DC-DC upconversion. We demonstrate a 8.75mm3 sensor system with a near-threshold ARM microcontroller, custom 3.3fW/bit SRAM, two 1mm2 solar cells, a thin-film Li-ion battery, and integrated power management unit. The 7.7μW system enters a 550pW data-retentive sleep state between measurements and harvests solar energy to enable energy autonomy. Our receiver and transmitter architectures benefit from a design strategy that employs mixed signal and digital circuit schemes that perform well in advanced CMOS integrated circuit technologies. A prototype transmitter implemented in 0.13μm CMOS satisfies the requirements for Zigbee, but consumes far less power consumption than state-of-the-art commercial devices.
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