Undetected structural weaknesses in buildings, bridges, and aircraft are potentially disastrous. These complex structures vary widely in design and construction materials, and are subject to multiple stresses across various fatigue points. Whether ancient monuments visited by tourists, 100+ year-old bridges enduring traffic loads never envisaged for the structure, or daringly modern high rises and ‘sea bridges’, infrastructure monitoring has become the norm. Advances in sensor and wireless technologies have highlighted structural health monitoring as a growth market for wireless sensor networks.
However, despite the significant advantages of cable-free, ultra-low-power operation, limitations, including extremely hostile environments and battery lifetime, have proven to be a barrier in some sectors.
Figure 1: Old bridges like this one over Hurricane Gulch, Alaska, or new, such as the Great Belt Bridge, Denmark, require constant monitoring to ensure structural integrity. Vibration energy harvesting provides an ideal means of powering remotely-located wireless sensors.
New research developments in MEMS-based piezoelectric energy-harvesting technology are addressing these challenges to provide extremely-robust, high-reliability, battery-free power for sensors in structural monitoring applications. This article will review recent developments, from the UK and the US, to illustrate the potential for energy harvesting in this market.
Such devices, as they become commercially available, will require associated electronic circuitry, which must be both ultra-low-power itself and provide power management at low levels with power conversion, rectification, and voltage boosting.
Emerging technology, such as sensors, fiber optics, computer vision, wireless sensor networks, and energy harvesting are providing innovative solutions to ongoing applications. MEMS technology for vibration and piezoelectric-based, self-powered sensors, with application in structural health monitoring, is the subject of advanced research around the world.
Cutting-edge university-led research
Established in 2011, the Centre for Smart Infrastructure Construction (CSIC) at the UK’s University of Cambridge¹ has developed a MEMS-based energy harvester, comprising a micro-cantilever structure and a transducer. CSIC’s key breakthrough was in the use of parametric resonance techniques that allow the device to overcome the mismatch between the narrow operational-frequency bandwidth of conventional designs and the wideband vibrations found in structures such as bridges, tunnels, and roads. In operation, force is applied to the cantilever, perpendicular to the length instead of transversely. This results in parametric resonance, which effectively generates more energy from the same amount of vibration.
Further, it can be manufactured using standard large-scale, high-volume semiconductor processes, facilitating integration with sensors and electronic circuitry. Prototype devices have demonstrated the efficacy of the design, with improved power output and a wider operational bandwidth compared to traditional parts. The technology is in the process of being commercialized.
The key advantage of this technology, according to the researchers, is that it removes the cost and effort involved in changing batteries in wireless sensor nodes used for remote structural-monitoring applications, particularly in hard-to-access places, such as under bridges or in tunnels. Modern civil engineering techniques have resulted in much longer road and rail bridges and tunnels, which require sensor nodes at regular intervals. Even with battery lifetimes extending to 10 years, the sheer number of batteries that would require replacement and subsequent disposal, makes battery-operated wireless sensor nodes impractical.
Taking a different approach, Cranfield University² has demonstrated a system based on a piezoelectric energy harvester based on macro-fiber composites. Designed initially to harvest energy from wing vibration of aircrafts while flying, the energy harvester provides the power for sensors and wireless communication in a structural health-monitoring device. Again, the elimination of batteries is cited as a prime advantage, as well as reducing the time and costs of installation and maintenance.
The demonstrator showed the device integrated into a working system comprised of piezoelectric energy harvesters, power management, energy storage, and a wireless sensor network. In operation, the device has been shown to produce 1.8 to 12 mW of power at a low vibration frequency up to 10 Hz. This was sufficient to power the wireless communication circuitry at intervals of 1.4 to 15 seconds, with data transfers at a rate of 500 kbps. In sleep mode, current consumption was just 1.6 µA. Sensors include a 3-axis accelerometer, light detector, and temperature sensor.
The power conversion/conditioning circuitry utilizes a technique to boost the output of the generator by accumulating the harvested energy in a storage reservoir consisting of two supercapacitors. Ongoing work will optimize the charge transfer between the piezoelectric generator and supercapacitors under a range of vibration, amplitude, and frequency conditions.
According to the researchers, the system has potential throughout the avionics, automotive, and transport industries, as well as for monitoring wind turbines, water, oil and gas pipelines, and bridge and tunnel infrastructures.
Mass-market technology
US-based MicroGen Systems, a spin-out from the University of Vermont, is very much on the energy-harvesting radar today with a suite of products based on its proprietary piezoelectric vibrational energy-harvesting technology. Developed initially at Cornell University, the MEMS-based device features a single end-mass-loaded micro cantilever containing a piezoelectric thin film. External vibration force bends the cantilever up and down, producing AC electricity. At resonance, the AC power output is maximized. Typical figures are: 100 µW at 120 Hz and 0.1 g, and 900 µW at 600 Hz and 0.5 g.
MicroGen’s recently-announced BOLT family of micro-power generators exploits this MEMS piezoelectric technology, which can be fabricated in volume on semiconductor production lines. A 300 µF storage capacitor is incorporated, which enables an output of 25 to 500 µW at 3.3 VDC, depending on configuration and frequency.
The micro-power generators are designed specifically to enable autonomous wireless-sensor operation in applications where it is too difficult or costly to replace batteries. However, the devices can also be used in conjunction with the company’s in-house developed, plug-in energy-storage modules or Power Cells, providing a rechargeable battery function. In supercapacitor, coin cell, or battery format, the Power Cells incorporate circuitry for rectification, impedance matching, and voltage regulation. They are specifically designed for the low-level operation and can be targeted for fixed-frequency industrial, structural monitoring, and commercial applications. For example, Power Cells can operate at harmonics of 50/60 Hz in the 100 to 1500 Hz range.
Power partner
Interestingly, MicroGen has partnered with Linear Technology, incorporating the LTC3588-1 piezoelectric energy-harvesting AC to DC converter. The LTC3588-1 is well established and widely used in this type of application with other energy harvesters. It is optimized for high-impedance sources such as piezoelectric transducers. It contains a low-loss, full-wave bridge rectifier and a high-efficiency synchronous buck converter. The design enables a transfer of energy from an input storage device (such as a supercapacitor) to an output at a regulated voltage, capable of supporting loads up to 100 mA.
Figure 2: The LTC3588-1 is designed specifically for operation with piezoelectric energy-harvesting transducers, converting the energy for directly powering a sensor or to an energy-storage device.
The device requires the output voltage of the transducer to be above the undervoltage-lockout rising threshold limit for the specific output voltage set at the D0 and D1 input pins (see Figure 2 above). For maximum energy transfer, the energy transducer must have an open circuit voltage of twice the input operating voltage, and a short-circuit current of twice the input current required. These conditions must be met at the minimum excitation level of the source to achieve continuous output power.
Low power network
Application examples of MicroGen’s BOLT Power Cells have also been demonstrated using ultra-low-power wireless sensor motes, both from Linear Technology (LTC5800-IPM SmartMesh) and from Texas Instruments. Based on TI’s ultra-low-power MSP430 microcontroller and CC2500 2.4 GHz low-power wireless transceiver, the eZ430-RF2500 is a complete development tool providing the hardware and software to create and evaluate a wireless sensor-based monitoring network.
Figure 3: A detailed application note on using the eZ430-2500³ evaluation kit (above) for structural monitoring applications is available.
An Application Note³ explains the expected current consumption of typical applications, which will depend on the frequency of sensor measurements and the frequency and length of data transmissions. For applications where battery backup is viable, the designer is able to predict battery life. However, it warns that battery leakage and battery voltage dissipation will have an impact.
For very long-term and/or large sensor-count structural-monitoring applications, the benefits of being able to draw power from an autonomous source, such as vibration, are apparent. Not only can the lifetime of the system be better predicted and extended, but it also makes smart monitoring much more viable for many structural-monitoring applications.
Buck converter
Texas Instruments has just revised the specification of its popular TPS6273X family of programmable output voltage, ultra-low-power buck converters. These devices are particularly well suited to energy-harvesting applications. An externally-programmable regulated supply preserves the overall efficiency of the power management stage, compared to a linear step-down converter, the company claims. The device is intended to step-down the voltage from an energy-storage element, such as a supercapacitor, in order to supply the rail for low-voltage electronics.
In the case of the TPS62736, the regulated output has been optimized to provide high efficiency, better than 90% at 15 µA, and features ultra-low active current, better than 350 nA. Input voltage range is 2 to 5.5 V. Output voltage is set to 1.8 V, but is adjustable from 1.3 to 5.3 V with external resistors.
Boost buck converter
A more highly-integrated device from TI is the bq25504 highly-efficient boost charger IC. It is specifically aimed at the stringent demands of ultra-low-power applications, such as structural monitoring including movement within buildings, bridges, and infrastructure. The device manages microwatts to milliwatts of power generated from energy-harvesting sources, including vibration. Once started, the boost converter/charger can effectively extract power from low voltage-output harvesters. The boost converter can be started from an input voltage as low as 330 mV, and can continue to harvest at levels down to 80 mV. It features low quiescent current (330 nA typical).
Figure 4: TI’s bq25504 boost converter and power-management solution, as set out for harvesting energy from a solar cell. It can be applied to piezoelectric vibration energy harvesting in the same way.
The device optimizes the transfer of incoming power through a programmable, maximum power-point tracking sampling network. Sampling the Vin-DC open-circuit voltage is programmed using external resistors, and held with an external capacitor (CREF), see Figure 4 above. The bq25504 supports a variety of energy-storage devices, including rechargeable batteries, supercapacitors, or conventional capacitors. The device monitors both maximum and minimum voltages against user-programmed over- and under-voltage levels.
Summary
Energy harvesting from piezoelectric vibration is proving a viable means of implementing long-term, battery-free structural monitoring of large-scale infrastructure applications such as tunnels, bridges, and tall buildings. Piezoelectric energy harvesters are benefitting from the application of MEMS technology to make them more efficient and reliable.
This article has highlighted a couple of research projects nearing completion, and reviewed a number of power management ICs that are ideal for use with piezoelectric energy harvesters in this type of application.
References:
- University of Cambridge Centre for Smart Infrastructure & Construction
- Cranfield University School of Applied Sciences
- Texas Instruments: Application Note using eZ430 for monitoring
