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Employing energy harvesters for wireless sensors

Posted: 13 May 2016 ?? ?Print Version ?Bookmark and Share

Keywords:microcontrollers? sensor? energy harvesting? piezoelectric transducers? buck converter?

Recent developments in ultra-low power microcontrollers have produced devices that offer unprecedented levels of integration for the amount of power they require to operate.

These are systems on a chip with aggressive power saving schemes, such as shutting down power to idle functions. In fact, so little power is needed to run these devices that many sensors are going wireless, since they can readily run from batteries. Unfortunately, batteries must be regularly replaced, which is a costly and cumbersome maintenance project. A more effective wireless power solution may be to harvest ambient mechanical, thermal, or electro-magnetic energy in the sensor's local environment.

The LTC3588-1 shown in figure 1 is a complete energy harvesting solution optimised for high impedance sources such as piezoelectric transducers. It contains a low loss full wave bridge rectifier and a synchronous buck converter, which transfer energy from an input storage device to an output at a regulated voltage capable of supporting loads up to 100mA. The LTC3588-1 is available in 10-lead MSE and 3x3mm DFN packages.

Figure 1: Complete energy harvesting solution optimised for high impedance sources such as piezoelectric transducers.

Ambient energy sources
Ambient energy sources include light, heat differentials, vibrating beams, transmitted RF signals or any other source that can produce an electrical charge through a transducer. Small solar panels have been powering handheld electronic devices for years and can produce 100s of mW/cm2 in direct sunlight and 100s of?W/cm2 in indirect light.

Seebeck devices convert heat energy into electrical energy where a temperature gradient is present. Sources of heat energy vary from body heat, which can produce 10s of?W/cm2 to a furnace exhaust stack where surface temperatures can produce 10s of mW/cm2. Piezoelectric devices produce energy by either compression or deflection of the device. Piezoelectric elements can produce 100s of?W/cm2 depending on their size and construction. RF energy harvesting is collected by an antenna and can produce 100s of pW/cm2.

Successfully designing a completely self-contained wireless sensor system requires power-saving microcontrollers and transducers that consume minimal electrical energy from low energy environments. Now that both are readily available, the missing link is the high efficiency power conversion product capable of converting the transducer output to a usable voltage.

Figure 2 shows an energy harvesting power system that includes the energy source/transducer, an energy storage element and a means to convert this stored energy into a useful regulated voltage. There may also be a need for a voltage rectifier network between the energy transducer and the energy storage element to prevent energy from back-feeding into the transducer or to rectify an AC signal in the case of a piezoelectric device.

Figure 2: Energy harvesting system components.

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