Power conditioning usually contains a storage element, such as a capacitor or a battery to support the peak demand usually radio transmission of the applications. The reasons for this include: — The harvester output voltage is rarely at or near common Vdd levels. Contributing to this is the nearly universal requirement to operate oscillating vibrational harvesters in resonance. Voltage and power drop off dramatically when operating away from resonance. For some harvesting technologies, output voltage will be low, sometimes less than 1 V. Although supply voltages for low-power electronics have been reducing over time, unless a custom integrated solution is available it may prove difficult to locate off-the-shelf components that can manage the low voltages generated by energy harvesting technologies.
Every application operates in a unique environment, and each environment draws on different sources of energy. Consequently, in most cases, both the energy harvester and its power-conditioning circuit must be uniquely optimized. Certain classes of circuits, or blocks, may be shared across applications. For instance, almost all harvesting systems contain storage of some kind, typically a rechargeable battery or a capacitor.
Most will have voltage conversion or regulation blocks that, if sufficiently parameterized, can be used in various applications. Figure 3 decomposes several simple prototype energy harvesting systems implemented in various research projects by the authors Seeman, Sanders, and Rabaey ; Chee et al. All these examples were built using commercial off-the-shelf COTS components.
Energy Harvesting and Systems
The designs share some similar blocks, but note that no two are identical. Although the examples are sufficient for simple demonstration circuits or prototypes, power management is moving from PCB-based circuits employing simple regulator ICs and discrete passive devices to on-chip blocks custom designed specifically for the electronics on a particular chip with some external passive elements such as large capacitances. However, the general decomposition should not change a great deal. This research device was implemented as a stand-alone IC, but the intent was that it be used as a building block core in future integrated designs.
Using a COTS system, a new device from Linear Technologies, the LTC, was used to integrate a rectifier, comparator, switch, and regulator for a piezoelectric harvester.
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This section illustrates the proposed methodology. A test case is presented in which the vibrations of an electromagnetic harvester are used to power a wireless sensor node for the condition based monitoring of expensive manufacturing equipment.
The goal is to design a condition based monitoring system for an expensive water pump, integral to the semi-industrial infrastructure of a large micro-fabrication laboratory. The sensor node, including everything necessary for operation, must be small in mass with respect to the water pump so that the inherent vibration spectrum from the pump remains unaltered. A single sensor reading is desired about once per second.
The device must not require maintenance such as battery replacement. It will be transmit-only, so a receiver is not necessary. Given this general background, the design goal is to answer: How much power is desirable to support the monitoring system? The sources of energy in the manufacturing space included light, vibration, waste heat, fluid flow, and energized conductors.
An informal audit quickly showed that the space has no windows, so photonic energy was only present when the room lights were on, which was seldom since the room was usually unoccupied. For this reason, light was not a good candidate.
Fluid flow was a possibility since water flows though the pumps and associated pipes. However, a drawback to water flow would have been the need to penetrate the pipe or pump housing to insert the harvesting device.
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The nearby HVAC ductwork was another potential source, yet the HVAC airflow was not co-located near the pump, so issues of energy transfer between the source and sink would have arisen. Each pump has AC power supplied to the motor, but the wiring was encased in galvanized steel conduit.
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Thus, the magnetic fields of the conductors were shielded and unavailable to harvesters. Appropriately, designed harvesters could be installed in the junction box at the pump, but installation would have required a skilled electrician and taking the pump out of service for safety. Waste heat Strasser et al.
With appropriate power conditioning such levels might reach satisfactory power levels and ongoing research is in progress: but, in summary, vibration emerged as the most attractive option for powering the system. In addition, the vibrational energy was available whenever the pump was operating and hence whenever measurements were desired. Figure 4 characterizes the vibration associated with the pump motor. There are several spectral peaks that are candidates for harvesting. Input acceleration magnitudes are very small and generally below 0. Acceleration y axis versus frequency x axis spectra for a 15 kW process cooling-water pump motor three axes of a vibration sensor.
From step S1, it is apparent that the general case of vibrational energy harvesting is a desirable candidate for powering the senor node.http://indfinity.net/marx-y-lennon-historias-desde-la-jaula.php
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This could involve piezoelectric cantilevers, capacitance designs, or electromagnetic designs. A particular vibration energy harvester was selected that utilized electromagnetic transduction and had interchangeable springs for manual adaptation to a target frequency.
The relatively small size, adaptable resonant frequency, and relatively high power output of the selected prototype made it well suited to the application environment where harvestable vibration peaks vary in frequency and magnitude for every piece of equipment. The prototype, shown in Figure 5 , employed a magnetic circuit similar to a coreless axial flux motor containing a Halbach array. Six block magnets created a flux loop entirely through permanent magnetic material to develop a large gap flux density of 0.
The high gap flux density and large gap length resulted in a great deal of induced voltage for the small input accelerations associated with ambient industrial sources. For an input acceleration of 0. Power versus frequency and load graphs are shown in Figure 6. The shape of the frequency curve illustrates the high sensitivity to resonance typical of vibrational energy harvesters.
Basic performance parameters for the device included an RMS output voltage, expected average power at resonance 0. Frequency and load sweeps for selected harvester, from the harvester characterization part of its attribute set. The application specified one sample every second, operating wirelessly from harvested energy with an appropriate energy buffer. A common platform for this class of functions consists of a sensor or sensors , a microcontroller, and a radio. Figure 7 shows a block diagram of a typical load. The microcontroller transferred data samples from the sensor to the radio for transmission.
The sensor issued interrupts to the microcontroller at specified intervals. Even if the controller were continuously powered, it would spend most of the time in a low-power sleep mode. In the event that the sleep mode power demand exceeded the harvester capability, power could be completely disconnected by means of a solid-state switch until enough charge accumulated in the storage element.
The radio s considered ranged from a simple transmit front-end upconversion, modulation, and amplification to a highly configurable mixed-signal transceiver with conformance to standards such as Both have been commonly used in sensors designs, have attractive power profiles, and are relatively inexpensive. This platform was available as a development kit eZRF and was used as a first-order estimate for a load specification.
Microcontroller maximum active power of about 8. The estimate indicated that the energy source needed to cope with a relatively large burst of current once per second. Also the average power provided should meet the quiescent power needs as well as the surplus power for the sensing and transmitting period.
Energy should be presented to the load at a voltage between the supply voltage limits. Finally, there needed to be two power steps: one when the circuit became active and one during transmission.
Bench tests using a profile similar to that specified yielded the numbers shown in Table 2. However, the harvester power cannot be applied directly to the sensor-node electronics, because individual components have specific requirements for voltage, noise, and regulation. Following full-wave rectification, a buffering capacitor was therefore added for storage Figure 9. The circuit was first evaluated using a vibration table to excite the harvester. The acceleration was increased until the device was operating as intended: 0. Figure 10 shows voltage and power profiles for the two primary modes of operation: charging and transmitting.
Blue trace is a marker where transitions indicate phases of operation, e. Since the duty cycle was very small, the harvester was almost always charging the capacitor between 2. Implementing a passive rectification design, capacitive energy buffer, and dynamic load results in harvester performance that deviates significantly from the conditions of a characterization using a purely resistive load. The non-ideal diodes of a bridge rectifier result in voltage drop. Matching the ideal impedance of the harvester becomes challenging. Additionally, operating components such as the charge-pump voltage doubler at lower than the 1.
In static bench testing, as much as 1 mW can be drawn when operating the doubler below the rated input voltage. Figure 11 shows a photograph of the complete system magnetically mounted on the water pump. The system harvested the 51 Hz y -axis vibration peak shown in Figure 4. Figure 12 shows that seven packets could be sent from one discharge of the capacitor before recharging was needed.
Base line vibration characteristics of a new pump were compared with data over time in order to make sure that the pump was operating well without need for maintenance. Energy harvesting scavenging is a satisfactory power generation option for electronic systems in many scenarios, including inaccessible locations, high-density deployments, and long-term applications with no battery replacements.
We presented an overview of these considerations, with a proposed methodology for system design Figure 1.
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The role of the overall environment was emphasized in any design for energy harvesting. The need for advanced circuit design for power conditioning was also emphasized. A test case was presented in which an electromagnetic harvester powered a wireless sensor node for monitoring the condition of a water pump in a micro-fabrication facility.
Bansal, A. Howey, and A.
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