The last time I really got my hands dirty playing with a piezoelectric transducer was for a small control system I designed back in college. That was a generic device that was more or less plug-and-go, so I can't recall much about it. However, given the recent uptick in interest in energy-harvesting technology as a possible solution to powering remote and embedded devices, I figured it was time to take a closer look at piezoelectric (PE) transducers and their related electronics to see how far they'd come. So, I got hold of the Joule-Thief demonstration kit from AdaptivEnergy, performed an exam, and was blown away.
Before going any further, let's look at the forces driving energy harvesting, aka energy scavenging. While it would be convenient to say the technology's rise is tied directly to the "green" movement, it really results from a confluence of factors: Device output voltage is increasing, power-management circuits have lower losses and higher efficiency, and ICs that actually do the intelligent work and data transmission are operating at ever-decreasing voltage and power levels.
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All these factors combine to make the function-per-microwatt of scavenged energy from vibration, body heat or temperature differentials commercially attractive for the development of wireless embedded sensors in many forms. These have applications that include structural integrity monitoring and analysis for bridges and aircraft, health monitoring, motor-bearing monitoring and the replacement of weighty and troublesome harnesses in automobiles. The ability to harness energy from the environment and avoid battery replacement, which can run into hundreds of dollars, is not only critical to the success of these applications, but in many cases the applications couldn't even exist without the "place and forget" capability energy harvesting can provide.
Energy harvesting also has application in the area of active shock and vibration mounts. These mounts are generally passive--those that are powered have a cabling overhead. It was while developing active mounts for aircraft carriers and submarines for Northrop Grumman that AdaptivEnergy's chief technology officer, Troy Tanner, first started exploring energy harvesting. "Now the whole 'green' thing has greatly increased interest," he said.
AdaptivEnergy is one of many companies benefiting from that interest, thanks to the Joule-Thief, a PE energy-harvesting module the company announced on June 16th. The datasheet says it's tuned to 60-Hz or 120-Hz vibrations, provides a 3.6-V DC output, uses capacitive or Lithium-ion battery storage, weighs 45 g and measures 56.5 mm x 35 mm x 16.5 mm. The module is magnetically mounted and the energy collected is a function of both frequency and amplitude of the vibration source.
However, the datasheet also mentions two technologies at the heart of the device's power density and associated efficiency: the RLP Smart Energy Beam and Energy Key collection electronics. According to Tanner, the RLP prestressed-ceramic design provides 10 times the voltage output of non-stressed designs, while the electronics provides an efficiency of 62 percent. "I've seen academic papers [with power efficiencies] in the sixties, but they were powered devices [i.e., from a line source]," said Tanner.
To find out more about these technologies I used the Joule-Thief kit, comprising the module, an air-pump-based 60-Hz vibration source, a multifunction sensor board (light, air pressure and temperature sensors), a Texas Instruments eZ430 MSP430-based RF interface and associated drive software with a Joule-Thief custom user interface.
I installed the software, attached the Joule-Thief to a metal plate on the pump, placed the sensor board on top of the module using the supplied screws, inserted the tube from the pump into the pressure sensor, attached the DC voltage output connector from the Joule-Thief module to power the sensor board with its included RF interface and turned on the pump. Within seconds the capacitors were charged to the nominal energy level of 3.9 millijoules (mJ), and the device started transmitting automatically to the eZ430 USB radio on my laptop at intervals of 1.4 seconds, the time required to recharge the capacitors.
Typically, the pump supplied with the kit gives a vibration of 1 g. So, in sum, it took 1.4 seconds to scavenge 3.9 mJ from a vibration of 1 g with a frequency of 60 Hz. With the eZ430 radio, that's sufficient for data transmission within a 50-m radius. "It can operate down to 8 mg, but then the transmit intervals become longer," said Tanner. The lower the vibration amplitude, the longer it takes to generate enough energy to transmit. The transmit power can be adjusted for mesh designs, he added.