The Energy Harvesting Tipping Point for Wireless Sensor Applications
Ever since the first watermills and windmills were used to generate electricity, energy harvesting has been an attractive source of energy with great potential. In recent years, energy harvesting technology has become more sophisticated and efficient, and energy storage technologies, such as supercapacitors and thin-film batteries (TFBs), have become more cost-effective.
The wireless sensor node is one of the most important product types being forecast for growth as an energy-harvesting solution. Wireless sensors are ubiquitous and very attractive products to implement using harvested energy. Running mains power to wireless sensors is often neither possible nor convenient, and, since wireless sensor nodes are commonly placed in hard-to-reach locations, changing batteries regularly can be costly and inconvenient.
Low-Power Optimization
Low-power modes on MCUs and wireless transceivers have been optimized in recent years to enable effective power management in wireless sensor applications.
Power consumption can be minimized by optimizing the relative amount of time spent in low-power sleep mode and reducing the active mode time. A fast processing core enables the MCU to execute the control algorithm very quickly, enabling a rapid return to low-power sleep mode and thereby minimizing the power-hungry area under the curve.
Wireless sensor nodes spend most of their time in sleep mode. The only subsystem that stays awake is the real-time clock (RTC). The RTC keeps time and wakes up the wireless sensor node to measure a sensor input. Low-power RTCs typically integrated onto microcontrollers consume only a few hundred nanoamps. It is important to minimize the system's wake-up time because power is consumed during this time.
When a wireless sensor node wakes up, it is usually intended to measure a sensor signal using the analog-to-digital converter (ADC). It is important to note the wake-up time of the ADC as well as the digital wake-up time since there is little point in waking up the CPU very quickly if the ADC takes an order of magnitude longer to wake up.
The radio transmission consumes most of the current in the system. Minimizing the amount of time the radio is on is essential to conserving energy. One way to achieve this is to avoid complicated communications protocols that require the transmission of many bits of data.
Another way to reduce the wireless sensor node's power consumption is to minimize the number of chips used in the system. Fewer chips on the printed circuit board (PCB) result in lower leakage current losses.
Managing Harvested Energy
An important consideration in the development of an energy harvesting sensor node is to ensure that there is always enough energy available to power the system.
This energy harvesting system uses a solar cell array to harvest energy. A solar cell unit, such as a Sanyo AM-1815, delivers approximately 40 μA when a 200 Lx light level is available. It is reasonable to expect this level of light in an office with a window but no direct sunlight on the cell. The 40 μA of current that the array generates is fed into a power management circuit and trickle-charged into a thin-film battery (TFB).
This combination of solar cell, power management and storage technologies provides an adequate level of energy for a wireless sensor node. The next important decision in the design process is the selection of a low-power MCU and wireless transceiver combination that can operate effectively from a limited energy source.
The control problem to be considered is how to operate the wireless sensor node at a duty ratio that does not deplete the TFB capacity that is itself being trickle-charged by the solar cell. Using the low-power design techniques discussed earlier, it is possible to reduce the average current of the wireless sensor node to around 51 μA (including power management leakage) while transmitting sensor data every second for three minutes.
Questions and Answers
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