Energy Is Increasingly Drawn From Surroundings
Harvesting energy from the environment is not new: Wind and water have served as energy sources for hundreds of years. yet a wireless device capable of operating from ambient energy sources has only recently been achievable. such a device is ideal for use in the sensor and control market. Principal energy sources are kinetic energy, light, and heat. Harvesting these sources is critical for achieving adequate energy to maintain a practical communications range for a wireless device.
Of course, power consumption may not be constant, and some devices—like kinetic harvesting switches—do not operate over long periods. fast, reliable transmission protocols are therefore vital. Although energy may be freely available, extracting it from the environment is not trivial. Incredibly efficient micro-energy converters are needed to supply power to wireless sensors, switches, and controls. they are required to take advantage of energy harvested in the following ways:
Motion
Linear motion is the most obvious solution for any switching device. but finding suitable and sustainable methods to extract energy is not simple. Piezoelectric and inductive generators are the most common approaches. Piezoelectric devices are small, but also can be inefficient and mechanically unstable.
Inductive solutions are larger, but lower in cost and more efficient. With the latter, there is little mechanical movement and only a few Newtons in the switching action. as a result, sophisticated techniques are required to generate enough energy to power a wireless switch. It is possible to design a product to transmit three data packets or telegrams per button push. such a product will allow on/off and dimming commands to be transmitted.
Light
Indoor solar cells are an ideal energy source for most areas. A small, eight-cell design can deliver 11 to 14 µA at 3 to 4 V. Light sources can be variable in most buildings and absent at night. as a result, suitable energy management and storage schemes need to be employed to avoid shutting down devices in the absence of light sources.
Rechargeable nickel-cadmium or lithium batteries are low in cost. yet their lifespan can be short, depending on energy usage. they also require complex charging circuits. if a maintenance-free system is required for decades, a better approach is to use a polyacenic-semiconductor (PAS) capacitor. A PAS is smaller than a double-layer capacitor. It has a high capacity per square millimeter with low self-discharge rates. the PAS also is environmentally friendly, as it contains no cadmium, mercury, or lead. A PAS coupled with an effective energy-management scheme makes battery-less PIRs, thermostats, and CO2 sensors simple to implement at minimum expense.
Temperature Differences
Differences in temperature can be used to power a number of remote applications. A thermoelectric device creates a voltage when there is a temperature variant on two junctions of two metals—a property discovered by Seebeck in 1821. Conversely, when a voltage is applied, it creates a temperature difference (known as the Peltier effect).
A number of low-cost Peltier elements are available that can be used “in reverse” as generators for small wireless monitors. the voltage generated from these elements is very low. It requires innovative direct-current-to-direct-current (DC-to-DC) conversion to transform the voltage to a level suitable for use by a typical wireless controller. for example, EnOcean’s ECT310 low-voltage DC-to-DC converter uses a blocking oscillator design.
With a 2°K temperature difference and a standard, low-cost Peltier element, the DC-to-DC converter starts operating at around 20 mV. Its output depends on the actual temperature difference of the Peltier element. An input voltage range of 20 to 50 mV corresponds to an output voltage range of 3 to 4 V. A typical thermo-driven sensor consists of a sensor element, a small Peltier element, a DC-to-DC converter, and a radio module.
System Implementation
Implementing a wireless system using harvested energy constrains a microcontroller design. A balanced approach is required to make sure that energy is available for sensing control. the start-up time of a microcontroller plays a strategic role. such timing is usually influenced by oscillator delay. for example, crystals and ceramic resonators can take several milliseconds to stabilize.
RC oscillators, by contrast, provide fast startup. but they generally suffer from poor accuracy over temperature and supply voltage. to save time, it is advisable to use a microcontroller that can start with an RC oscillator and subsequently switch to a crystal oscillator.
In addition to saving energy, rapid switching of a sensor is particularly effective when measuring parameters that change slowly. Given suitable power strategies, it is possible to achieve an average current consumption just above the total current consumption of the continuously running processor blocks.
While several circuit blocks can be switched off, others must be operated permanently. for example, threshold switches activate the electronics and timers that trigger periodic activities, such as sensor readings. these circuit blocks rapidly dominate the entire energy requirements. as a result, they must be aggressively optimized. Because the timers of typical harvesting modules should only require approximately 20 nA, they are typically analog. these timers switch off all components during sleep periods. this approach can enable a power reserve via the PAS capacitor of up to one week with solar devices—even in complete darkness.
for any highly dynamic processes that need to be analyzed, it also is worth doing the following: pre-processing the data in the sensor, reducing the data to be transmitted, and only transmitting minimal measurement data as needed. Simple, well-architected software reduces execution time—saving more energy in the process. Another element to consider when waking a central processing unit (CPU) is oscillator startup and power-down times (see figure). Finally, minimizing writing to memory will save power.
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