O ver the past decade, embedded digital electronics have proliferated in both number and variety. Applications such as cellular phones, portable multimedia devices, and sensor networks have kept pace with dramatic increases in computing power and functionality. Battery technology, however, has not. Batteries limit the operating lifetime of portable devices and add undesirable weight and volume. They can't store sufficient energy to support longlifetime embedded applications such as monitoring civil infrastructure or studying the environment. Their replacement cost poses a major barrier to scaling wireless sensor networks to hundreds or thousands of nodes.Energy harvesting from human or environmental sources is a promising alternative to address these limitations and open new frontiers for integrating digital computation with sensing and actuation. Several alternative energy harvesting paradigms are possible, as either a substitute or a complement for batteries. The commercial sector has adopted mechanical energy harvesting as a redundant power source. Products already on the market include radios, flash-lights, and cell-phone chargers powered by handcranked electrical generators or shake-to-recharge electronics. But these mechanisms aren't suitable for all applications: first, they have low energy and power densities; second, they require active user involvement.Researchers have explored various passive energy-harvesting power sources for portable or wearable devices. These include gravity-driven and vibration-driven electromagnetic generators, piezoelectric shoe inserts, and thermocouples for harvesting energy from human body thermal gradients. Passive power sources for sensor networks, another target area for energy harvesting, 1 include ambient mechanical vibration. One project, which some of us worked on, developed a MEMS variable-capacitor transducer and accompanying chips that could harvest machine vibrations for sensor signal processing. 2 Despite a promising start, energy harvesting is still in its infancy. For current sensor network nodes, vibration-based energy harvesting allows an RF transmit duty cycle of less than 3 percent, excluding any computation that occurs at the transmitter. 1 Communication typically dominates power consumption, so many applications must maximize the computation done at a particular node. 3 Required off-chip power electronics can increase system cost and volume, and AC/DC conversion losses can limit energy-harvesting operation. Energy harvesting from human or environmental sources shows promise as an alternative to battery power for embedded digital electronics.Digital signal processors that harvest power from ambient mechanical vibration are particularly promising for sensor networks.

Passive energy harvesting from mechanical vibration has wide application in wearable and embedded sensors to complement or replace batteries. Energy harvesting efficiency can be increased by eliminating AC/DC conversion. A test chip demonstrating self-timing, power-on-reset circuitry, and memory for energy harvesting AC voltages has been designed in 180 nm CMOS and tested. Circuit operation is confirmed for supply frequencies between 60 Hz and 1 kHz with power consumption below 130 µW.

The recent explosion in capability of embedded and portable electronics has not been matched by battery technology. The slow growth of battery energy density has limited device lifetime and added weight and volume. Passive energy harvesting from vibration has potentially wide application in wearable and embedded sensors to complement or replace batteries. We propose increasing energy harvesting efficiency by eliminating AC/DC conversion electronics. We investigated self-timed circuits, power-on-reset circuitry and memory for energy harvesting AC power supplies. Our power-on-reset circuit achieves a substantial improvement over conventional approaches with 4.1 nW of simulated power dissipation and frequency-independent turn-on voltage. A chip is being fabricated to test the circuits presented here.

Passive energy harvesting from mechanical vibration has wide application in wearable devices and wireless sensors to complement or replace batteries. Energy harvesting efficiency can be increased by eliminating AC/DC conversion. A test chip demonstrating self-timing, power-on reset circuitry, and dynamic memory for energy harvesting AC voltages has been designed in 180 nm CMOS and tested. An energy scalable DSP architecture implements FIR filters that consume as little as 170 pJ per output sample. The on-chip DRAM retains data for up to 28 ms while register data is retained down to a supply voltage of 153 mV. Circuit operation is confirmed for supply frequencies between 60 Hz and 1 kHz with power consumption below 130 μW. Reaching the limits of miniaturization will require approaching the limits of power dissipation. We extrapolate from this DSP architecture to find the minimum volume required for mechanical vibration energy harvesting sensors.

The recent explosion in capability of embedded and portable electronics has not been matched by battery technology. The slow growth of battery energy density has limited device lifetime and added weight and volume. Passive energy harvesting from solar radiation, thermal sources, or mechanical vibration has potentially wide application in wearable and embedded sensors to complement batteries. The amount of energy from harvesting is typically small and highly variable, requiring circuits and architectures which are low power and can scale their power consumption with user requirements and available energy. We describe several circuit techniques for achieving these goals in signal processing applications for wireless sensor network nodes such as using Distributed Arithmetic to implement energy scalable signal processing algorithms. In addition, we propose increasing vibration energy harvesting efficiency by eliminating AC/DC conversion electronics, and have investigated self-timed circuits, poweron-reset circuitry, and memory for energy harvesting AC power supplies. These techniques can also be applied to energy harvesting from other sources. A chip will be fabricated to test the proposed circuits.