Introduction
In a modern world a lot of emphasis is put on gathering and analyzing data from the environment in order to improve the quality of life. Many studies have been done on employing the wireless networks with a large number of sensor nodes to collect information and extract useful data (figure 1).
Figure 1: Example of wireless sensor network used to monitor conditions in a plant [1]
To make the sensor networks cost effective, the wireless sensor node needs to be efficient in every aspect. Currently, the sensor nodes are powered using batteries that are bulky and require multiple replacements to extend the life of the sensor node. This is can be rather difficult and costly job in large sensor networks and the need for more practical power source arises. Making the device that could harvest the energy from the environment and transform it to useful energy for the sensor node would solve this problem by providing an ever lasting energy source for remotely deployed nodes.
Energy Harvesting
One of the most common energy harvesting devices is the solar cell (figure 2a). We find solar cells primary in space applications due to continuous and infinite source of sunlight, while on the surface of the earth the available solar energy changes due to unpredicted weather conditions. Recreantly, the energy scavenging device was developed to collect the energy from the soles of the shoes (figure 2b). This type of energy harvesting is convenient for military purpose to recharge batteries while soldiers are on the move[2]. Another new concept of energy harvesting involves extraction of energy between two points with different temperatures (figure 2c). The thermal energy scavenger is currently being employed by IMEC[3] for powering a small wireless sensors on human body.
Figure 2: Examples of energy scavenging: a)Solar cell, b)Shoe inserts, c)Thermal energy
Vibration Energy
Another source of energy, which has not been exploited extensively, is vibration energy. Vibrations are all around us, the car engine is a perfect example, as well as the building infrastructure and even the house appliances. Figure 2 shows the survey of vibrations found in our close environment quantified in terms of maximum acceleration and the frequency at which maximum acceleration occurs.
Figure 3: Survey of vibrations found in environment [4]
At Koser Lab we decided to pursue vibration energy harvesting not only because we believe that the source of vibration is all around us. In the vibration free environment, the source of vibration itself can represent useful information that end user needs to detect. Vibration energy scavenger in such environment will sit still until vibration occurs, and once it does, the harvester will collect enough energy to report that some disturbance has been detected. This type of sensor can be used for many security applications.
Vibration Energy Harvesting
There are several possible ways to perform vibration-to-electricity conversion each having some advantage over the other. Electrostatic energy conversion [4] is based on variable capacitor with constrained charge or voltage across it (figure 4a). These converters require external power supply, which in a way defeats the purpose of energy harvesting, but they are very easy to integrate with existing CMOS technology which gives them advantage in terms of implementation. Another approach is the electromagnetic conversion (figure 4b) which uses principles of Faraday's law where wire coils move through the permanent magnetic field, inducing a voltage in the coils [5]. The voltages induced in these systems are relatively low and not practical to be used with current electronics.
Figure 4: Vibration-To-Electricity Conversion: a)Electrostatic b)Electromagnetic
The approach that we chose in Koser Lab is based on piezoelectric conversion. Piezoelectric material is a type of ferroelectric materials, the group of materials that exhibit electrical dipole moment within its lattice structure. Once the dipoles get aligned along certain direction the material is able to develop electric field when external stress is applied to it or vice versa (figure 5).
Figure 5: a)Aligned electrical dipoles in piezoelectric material b)Piezoelectric conversion
Smart Sand Project
The Smart Sand concept is a silicon platform based on vibration energy harvester as a building block for variety of highly integrated wireless sensors nodes. It is a sub-centimeter device machined out of silicon which uses piezoelectricity to collect ambient energy. It provides enough silicon area to integrate all the electronics for power management, low power sensor and Radio Frequency (RF) transmitter, resulting in a very compact wireless sensor node (figure 6). Recreantly a temperature sensor has been developed for integration with Smart Sand [6]. The major advantage of Smart Sand Device is its compact size and ability to integrate wireless sensor node it its structure using standard Integrates Circuit (IC) technology and silicon micro-machining processes. To achieve the maximum possible energy conversion efficiency, Smart Sand pushes the limits of piezoelectric material at the frequencies found in the environment. It is important to note that Smart Sand is not a resonating device that is designed to work at specific frequency. This device can operate in a relatively large range of frequencies from 10Hz up to 300Hz.
Figure 6: Schematic of Smart Sand Device with low power energy sensor
Modeling
The mechanical basis for Smart Sand has already been designed in Koser Lab. To simulate the performance of the device we use finite element analysis software packages, as well as MATLAB and Simulink.
Fabrication
Currently the micro fabrication process is under development in Koser Lab for integration of piezoelectric material with mechanical basis of Smart Sand. The prototype structure, cantilever beam, has been used to deploy Lead Zirconium Titanate (PZT) with great success (figure 7a). Thin PZT film is formed on the surface of the silicon with two metal electrodes forming the metal/PZT/metal sandwich (figure 7b).
Figure 7: a)Silicon cantilever beam with piezoelectric material integrated on its surface
b) SEM picture of cantilever beam cross section
By bending the tip of cantilever beam downward or upward, the top surface of silicon gets stretched or compressed applying the strain directly to piezoelectric material. While the strain is being applied the piezoelectric material develops the electric field and forms the potential between two electrodes. Figure 8 shows the measured voltage as a function of induced stress in PZT.
Figure 8: Voltage from piezoelectric material as a function of strain.
References
[1] WiSNet-Wireless Sensor Network; Product of Expert Monitoring Limited;
[2] Leonov V., Fiorini P., Sedky S., Torfs T., Van Hoof C.; ”Thermoelectric MEMS generators as a power supply for a body area network”; 13th International Conference on Solid-State Sensors and Actuators and Microsystems, TRANSDUCERS '05, Seoul, South Korea, 2005, p 291-294
[3] Shenck, N.S.a.P., J A, “Energy scavenging with shoe-mounted piezoelectrics”; IEEE Micro, 2001. 21: p. 30-41
[4] S. Roundy, Energy scavenging for wireless sensor nodes with a focus on vibration to electricity conversion, PHD Thesis, University of California Berkeley, May, 2003
[5] Amirtharajah, R. and A.P. Chandrakasan; “Self-Powered Signal Processing Using Vibration Based Power Generation”; IEEE Journal of Solid-State Circuits, 1998. 33(5): p. 687-695
[6] Kaya, T., Koser, H., Culurciello, E., “A Silicon-on-Sapphire Low-Voltage Temperature Sensor for Energy Scavengers,” Proc. IEEE Inter. Symp. Circuits and Systems (ISCAS), New Orleans, May 2007
This material is based upon work supported by the National Science Foundation under Grant No. EECS-0601630. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and researchers, and do not necessarily reflect the views of the National Science Foundation.
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