Sensors get a new role as energy scavengers

Sensors get a new role as energy scavengers

Sensor makers are warming up to the idea that their products can double as generators of electricity. Now they are starting to get creative.

A car zipping over a bridge, an oven with sides warm to the touch, and a radio-controlled model car — who would have ever guessed these every-day facets of modern life could be sources of energy? That is increasingly the case, thanks to improvements in sensors that double as energy harvesting devices. Plop a vibration-harvesting sensor next to a busy roadbed and it may be able to produce enough energy to power a small circuit for keeping tabs on structural conditions. Inputs for energy scavenging sources can be found in the human body, may be emitted by machinery such motors, or be found in water and soil.

Moreover, the integrated circuits that work with such sensors are becoming smaller, are more sensitive to low-level signals, and need less power for their own operation. These qualities now make energy scavenging systems candidates for a wide range of applications in medical, automotive, environmental, military, commercial, consumer and infrastructure areas.

There are several methods for converting mechanical or thermal energy into electricity. The technologies employed to do so include piezoelectric, capacitive, RF, inductive coupling, solar, and temperature difference principles. These sensing technologies are getting more efficient, converting more physical energy to electricity.

Several IC manufacturers are trying to encourage the development of energy harvesters, particularly for powering wireless communications. Because the design of energy harvesting equipment involves working with low voltages and currents, it can be tricky to put together working circuits. To simplify the process, makers of energy harvesting ICs are joining up with sensor manufacturers to offer development kits that include demo boards, software, and firmware necessary to get up and running.

The piezoelectric effect

One of the most common forms of energy harvesting makes use of the piezoelectric effect, wherein some materials such as certain ceramics and crystals, can generate electricity in response to an applied mechanical strain. This capability is also reversible.

However, piezoelectric sensors aren’t necessarily traditional crystals. Smart Materials Corp. takes advantage of the piezoelectric effect with its macro fiber composites. It developed them at the National Aeronautics and Space Administration (NASA) in the late 1990s. These MFC sensors/actuators are aimed at picking up mechanical vibrations and body sounds and at ultrasound applications. They serve a wide range of flow-metering and structural-health monitoring uses.

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The MFC sensors are essentially encapsulated piezo ceramics to which metal is added as an interstitial passive material. They’re available in elongator (d33) and contractor (d31) modes which act as powerful actuator/sensitive sensor and low-impedance sensor/ energy generator devices, respectively. As an actuator, an MFC device can operate over a range of 1 Hz to 10 kHz. As a sensor, it has a range of 0.5 Hz to 500 kHz. They’re flexible and robust, available in ready-to-use packages that overcome the disadvantages of solid PZT plates or patches based on solid wafers. And they’re reliable, being rated at more than 109 cycles of actuation and more than 1011 cycles for energy harvesting. Smart Materials says that it is possible to integrate them with electronic components.

The piezoelectric principle is also used by Arveni s.a.s. in France. Its ARxx series of energy harvesters produce medium (AR02) and high (AR01) outputs of 767 μJ and 2.1 mJ, respectively, in response to respective forces of 2.6 and 3.4 N. The respective output voltages are 38.8 and 38.6 V.

Late last year, Arveni demonstrated a proof-of-concept battery- less TV remote control driven by piezoelectric generators for French IP TV provider SFR. Arveni is targeting membrane switch, home automation metering, industrial remote-control, and wireless sensing applications. It claims levels of energy efficiency ranging from 10% to 20%.

For use in load cell applications, SensorTech uses a patentpending thin-film conductive polymer of polyphenylene sulfide (PPS). The polymer sits between the two electrodes in the load path, one at the top and the other at the bottom of the polymer composite. The polymer has a non-linear response (its resistivity falls with a rising load) which is linearized by signal conditioning.

The fact that the resistance drops sharply at low mechanical loads means the load cell is inherently more accurate at lower loads, the opposite response of stain-gauge-based sensors. SensorTech has reduced the normally high hysteresis levels, a bugaboo of this conductive polymer, to as little as 1% and is constantly improving them. Ditto for the repeatability hysteresis levels which are poised to replace straingauge technology, SensorTech claims.

Harnessing temperature gradients

Temperature gradients are one of the oldest sources of energy available for harvesting. Thermocouples, in use for generations, are the most common example of this technique. Also in wide use is the thermoelectric electric effect -- often called the Seebeck effect, the Peltier effect and the Thomson effect. This involves the direct conversion of temperature differences to an electric voltage, a process that’s also reversible (converting a voltage difference to a temperature). Micropelt GmbH in Germany is one of the leading thermoelectric-effect energy harvesters. The company is funded by Infineon Technologies and receives R&D support from Fraunhofer IPM.

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Micropelt makes use of an aluminum-oxide substrate on which multiple thermocouples are laid, thermally in parallel and electrically in series. Up to 128 thermocouples can be laid on a 1,600-mm2 substrate to produce roughly 50 mV/°K, with the generated voltage being proportional to the number of thermocouples laid down on the substrate. A Micropelt TEG MPG-D751 can convert 1 W of thermal energy into about 2 mW at 2.5 mA, energy comparable to that from a button-sized battery.

Another company that harvests energy from temperature differences is Germany’s EnOcean GmbH, which has pioneered patented self-powered wireless technology. It’s STM 300 and STM 300C energy scavenging transceiver modules feature extremely low-power dissipation with various power-down and sleep modes that consume just 0.2 μA. Power comes from a small solar cell or a thermal harvester. The unit operates at 868/315 MHz using an external antenna and is widely applied in energy-saving building and industrial automation applications.

Vibratory motion is another rich source of energy harvesting. Energy scavenged from motion has been used for a number of years in the biomedical community to power prosthetics, pace makers, artificial organs, and implants. However, while this work continues, challenges lie ahead for coming up with devices that meet size, power, and biocompatibilit standards.

It should be noted that such work looks promising. One indication: Electric Potential Sensors (EPSs) announced in July by scientists at England’s University of Sussex. The sensors can detect human heartbeats up to 1 m away without any physical contact with the body. The exact method of sensing has yet to be made clear. However, the researchers say that the sensitivity of these sensors means that they can detect muscle and eye movements, and eventually will be able to pick up brain and nerve-fiber signals.

A major developer and manufacturer of energy scavenging motion sensors is Perpetuum Ltd. in the U.K. Its free-standing harvester (FSH) combines electromagnetic vibration energy harvesting with a selectable set of energy charging, storage and management options. The FSHs are optimized for industrial machinecondition monitoring. Machine vibrations are converted into electrical energy to power wireless sensor networks.

The FSH is designed to charge an external device with up to 4 mA at 5 V while reporting power levels via a standard 3-pin IEC interface. The FSHs are also used in the monitoring of helicopter rotor blades for military applications.

Midé Technology Corp is also pursuing the development of vibration energy harvesting, as well as, the capturing of solar energy. It is employing piezoelectric transducers to do this. As a first step, Midé stresses the importance of understanding vibration qualities, and as such has developed the VR001 vibration recorder for accurately characterizing vibrations. The three-axis 3.2-kHz unit measures a vibration’s frequency content, amplitude, and duty cycle, data that is accessible to a computer via a USB interface. Midé says that it has a target cost for the device of less than $10 each, in large volumes.

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The race to cash in on novel energy harvesting ideas has companies outside the traditional transducer market bringing out products. For example, Japan’s Brother Industries Ltd., better known for producing printers, has developed vibration energy harvesters in form factors that let them replace AA and AAA batteries. The compact size enable the devices to generate energy with a simple shake of the controller.

Inside the battery-shaped case are an electromagnetic induction generator and a 500-mF capacitor for energy storage. The output of the AA-size generator is 10 to 180 mW. Proof-of-concept versions have reportedly produced about 1.6 and 3.2 V for the AA-size generator and 2 V for the AAA-size generator. The units are only suitable for low-duty-cycle operation as in TV remote controls which typically consume between 40 to 100 mW.

RF energy

Someday the electromagnetic transmissions beamed from your garage door opener could be a source of electrical power. That is the idea behind devices from Powercast Corp. which take advantage of widely available RF energy in the 902 - 928 MHz ISM band. This part of the RF spectrum is occupied by low-power, non-licensed devices such as simple toys, wireless security systems, wireless telemetry, and wireless automatic meter-reading systems. The Powercast devices typically use the energy they harvest to power MiWi point-to-point wireless sensor networks.

The Powercast transducers have a peak efficiency of about 70% for input power in the 20 mW to 40 mW range. They will also harvest power from signals outside the ISM band, though with diminishing sensitivity. The Powercast P2110 battery-free RF energy harvester receiver converts RF energy to a dc voltage. It mounts on a pc board that includes a Microchip Technology PIC 24 XLP microcontroller and an MRF24 radio module. On the board, which requires 3.3 V for operation, is a 50-mF capacitor for energy storage (It can be as low as 3,300 μF.), as well as discrete temperature, humidity and light sensors.

Some makers of microcontrollers, such as Microchip Technology, have developed low-power controllers with energy harvesting applications in mind. In fact, semiconductor manufacturers like Jennic in the U.K. target wireless energy harvesting by developing specific supporting parts like the firm’s JN5148 microcontroller. The unit includes a proprietary 32-bit RISC processor, a 2.4-GHz radio and 128 kbytes of RAM and peripherals.

The part harvests energy from vibration, heat, light, and RF energy. The firm teamed up with Micropelt for thermo generation; Cymbet for solar power conversion and storage; AdaptivEnergy for piezoelectric harvesting of mechanical impulse, shock and vibration; and Powercast for RF energy harvesting.

Cymbet also makes the EnerChip EP CBC 915 energy processor, which it claims is the world’s first universal energy harvesting power management unit across all energy harvesting transducer technologies, including photovoltaic, thermoelectric, piezoelectric and electromagnetic. It uses patent-pending Maximum Peak Power Tracking technology that constantly matches any energy harvester transducer input impedance. It is designed for optimal power management for Cymbet’s CBC050 EnerChip rechargeable solid-state thin-film battery that has no liquid or gel constituents.

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Other firms, like STMicroelectronics with its EnFilm unit, also make solid-state batteries for energy harvesting. OptiXtal claims that its SuperXcap ultra-thin capacitor- in-a-pouch provides economical energy storage. It puts out a 5-A current at maximum and store up to 3,020 W/L. It’s said to be more efficient at energy conversion from the environment than systems based on batteries which recharge slowly and lose their charging capabilities after a few hundred cycles.

SuperXcaps recharge in seconds and offer a 50,000 charge/discharge cycle life. The wafer-light and ultra-light units consist of outer-membrane metallic shells. The flexible and bendable packets can be configured in a variety of shapes and sizes.

Industry chipping in

One of the more interesting products for complementing energy harvesting transducers is the LTC 3108/3109 highly integrated dc-dc boost converter and power manager from Linear Technology. It is designed to start and run from extremely low-input voltage sources such as thermoelectric generators, thermopiles and small solar cells. Its self-resonant topology steps up input voltages as low as 20 mV. It harvests thermal energy with differences as little as 1ºC.

Linear Technology’s offering shows the semiconductor industry’s eagerness toward energy harvesting and scavenging applications. Many chip makers are joining with transducer manufacturers to offer development kits that expedite sensing system designs. In this camp are chip manufacturers that include Texas Instruments, Microchip Technology, NXP, Atmel, and STMicroelectronics to name a few.

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