Eli S. Leland

Resonance tuning of piezoelectric vibration energy scavenging generators using compressive axial preload

Vibration energy scavenging, harvesting ambient vibrations in structures for conversion into usable electricity, provides a potential power source for emerging technologies including wireless sensor networks. Most vibration energy scavenging devices developed to date operate effectively at a single specific frequency dictated by the devices design. However, for this technology to be commercially viable, vibration energy scavengers that generate usable power across a range of driving frequencies must be developed. This paper details the design and testing of a tunable-resonance vibration energy scavenger which uses the novel approach of axially compressing a piezoelectric bimorph to lower its resonance frequency. It was determined that an axial preload can adjust the resonance frequency of a simply supported bimorph to 24% below its unloaded resonance frequency. The power output to a resistive load was found to be 65–90% of the nominal value at frequencies 19–24% below the unloaded resonance frequency. Prototypes were developed that produced 300–400 µW of power at driving frequencies between 200 and 250 Hz. Additionally, piezoelectric coupling coefficient values were increased using this method, with keff values rising as much as 25% from 0.37 to 0.46. Device damping increased 67% under preload, from 0.0265 to 0.0445, adversely affecting the power output at lower frequencies. A theoretical model modified to include the effects of preload on damping predicted power output to within 0–30% of values obtained experimentally. Optimal load resistance deviated significantly from theory, and merits further investigation.

Wireless Sensor Networks for Home Health Care

Sophisticated electronics are within reach of average users. Cooperation between wireless sensor networks and existing consumer electronic infrastructures can assist in the areas of health care and patient monitoring. This will improve the quality of life of patients, provide early detection for certain ailments, and improve doctor-patient efficiency. The goal of our work is to focus on health-related applications of wireless sensor networks. In this paper we detail our experiences building several prototypes and discuss the driving force behind home health monitoring and how current (and future) technologies will enable automated home health monitoring.

A MEMS AC current sensor for residential and commercial electricity end-use monitoring

This paper presents a novel prototype MEMS sensor for alternating current designed for monitoring electricity end-use in residential and commercial environments. This new current sensor design is comprised of a piezoelectric MEMS cantilever with a permanent magnet mounted on the cantilevers free end. When placed near a wire carrying AC current, the magnet is driven sinusoidally, producing a voltage in the cantilever proportional to the current being measured. Analytical models were developed to predict the applicable magnetic forces and piezoelectric voltage output in order to guide the design of a sensor prototype. This paper also details the fabrication process for this sensor design. Released piezoelectric MEMS cantilevers have been fabricated using a four-mask process and aluminum nitride as the active piezoelectric material. Dispenser-printed microscale composite permanent magnets have been integrated, resulting in the first MEMS-scale prototypes of this current sensor design.

A new MEMS sensor for AC electric current

This paper presents new results in the testing and characterization of a MEMS sensor for AC electric current. The sensor is comprised of a piezoelectric MEMS cantilever with a microscale permanent magnet mounted to its free end. When placed near a wire carrying AC current the magnet couples to the oscillating magnetic field around the wire, deflecting the cantilever and generating a sinusoidal voltage proportional to the current. Unlike inductive sensors, this sensor does not need to encircle the conductor and it can measure current in a two-wire “zip-cord”. It is also self-powered, and is thus more suitable for wireless sensor node applications than a powered sensor device. The theoretical basis of this new sensors operation is presented, as well as the fabrication of a MEMS sensor device, and the first test results of this new sensor measuring current in single-wire and two-wire conductors. Sensor response is linear (R2 > 0.99) with sensitivity in the range of 0.1–1.1 mV/A. An integrated self-powered sensor device is also presented, which employs a piezoelectric energy harvester to power the sensors signal conditioning circuitry at a 2.6% duty cycle.

Self-powered MEMS sensor module for measuring electrical quantities in residential, commercial, distribution and transmission power systems

Ongoing initiatives for energy conservation present the need for ubiquitous sensing of electrical power use in residential and commercial settings. Inexpensive and massively distributed electrical sensors installed in power distribution and transmission systems will enable collection of highly granular information regarding the operation of the power grid. Incorporated into the upcoming Smart Grid infrastructure, we envision this data-collecting capability to enhance the overall stability of the grid, as well as improve its diagnostic capabilities. In this paper, we present our ongoing work towards developing self-powered MEMS sensor modules that can be installed in both residential and commercial settings, as well as in power distribution and transmission systems. The sensor modules will measure electrical quantities such as voltage, current and instantaneous power using a suite of MEMS sensors, and will scavenge the energy needed for their operation from the current flowing in the energized conductor onto which they are attached.

Design and Fabrication of a MEMS AC Electric Current Sensor

The need for energy efficiency combined with advances in compact sensor network technologies present an opportunity for a new type of sensor to monitor electricity usage in residential and commercial environments. A novel design for a self-powered, proximity based AC electric current sensor has been developed. This sensor device is constructed of a piezoelectric cantilever with a permanent magnet mounted to the cantilevers free end. When the sensor is placed in proximity to a wire carrying AC electric current, the permanent magnet couples to the wires alternating magnetic field, deflecting the piezoelectric cantilever and thus producing a sinusoidal voltage proportional to the current being measured. Analytical models were developed to predict the magnetic forces and piezoelectric voltage output pertaining to this design. MEMS-scale cantilevers are currently under development using a three-mask process and aluminum nitride as the active piezoelectric material. Very small (300 μm) permanent magnets have been dispenser-printed using magnetic powders in a polymer matrix. Previously presented meso-scale (2-3 cm3) prototype devices exhibited sensitivities of 74 mV/A, while simulations suggest MEMS device sensitivity of 2-4 mV/A.

Proximity-Based Passive Current Sensors for Real-Time Monitoring of Power Usage in Low and Mid-Voltage Applications

Traditional monitoring of current flow in power lines involves breaking the circuit for through-the-sensor type monitoring or separating individual wires for around-the-conductor “clamp” metering. Such techniques for measurement are invasive and disruptive for many applications. Using 0.9 cm diameter permanent magnets attached to piezoelectric bimorph elements in conjunction with the fluctuating magnetic fields generated by alternating currents, sensors have been developed that enable in-situ, self-powered measurement of current flow without the need to encircle or disconnect the wire being measured. Whereas previous development [1, 2] of these devices has focused on two-wire applications for low (<600V) voltages, single-wire configurations for higher-voltage (up to 15kV) applications are also considered here. Outputs have been found to be linear through a wide ranges of current (0–200A on 0.6–120V AC), with resolution limited primarily by sensor distance from the target wire (3cm or less from wire edge). Sensitivity studies have been performed on conductors of varying dimensions (18 gauge to 4 gauge) and geometries (single conductor and two-wire) for known sensor placements and current flows. Initial integration has been performed with a wireless radio device to produce a self-contained sensor package 8cm by 10cm by 3cm that can sample data at 2 kHz and transmit results to a base station for further analysis. Initial numerical modeling of the magnetic field gradients around current-carrying wires has been performed to optimize sensor placement. Potential applications include customer (home, business, industry) and distribution grid-level monitoring.Copyright