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Robust energy harvesting system solution for wireless sensors
Wireless sensors are gaining significant traction in the market due to their unique advantages. They are particularly suitable for applications where human access is difficult or where a large number of sensors are required, making it impractical to connect them all to a wired data network. In many such scenarios, using a primary battery is not feasible. For instance, a temperature sensor used during meat transportation must be securely installed and inaccessible for maintenance. Similarly, HVAC sensors spread across various air sources may rely on batteries because of their widespread deployment. Energy harvesting technology offers a viable solution by providing power without the need for traditional batteries.
However, energy harvesting alone typically does not generate enough power to continuously operate a sensor-transmitter system. While energy harvesters can produce between 1mW and 10mW, most active sensor-transmitter systems require 100mW to 250mW. Therefore, it's essential to store the harvested energy for use when needed. The system must manage its duty cycle carefully to ensure it doesn’t exceed the energy storage capacity. Additionally, the sensor-transmitter may need to function even when no energy is being collected.
When the stored energy is nearly depleted and the system is about to shut down, it should perform necessary housekeeping tasks, such as sending a shutdown message or saving critical data in non-volatile memory. This highlights the importance of continuously monitoring the available energy levels.
A complete energy harvesting system is illustrated in Figure 1, which uses an LTC3588-1 energy harvester and buck regulator IC, two LTC4071 parallel battery chargers, two GM BATTERY GMB301009 8mAh batteries, and a simulated sensor-transmitter (modeled as a 12.4 mA load with a 1% duty cycle). The LTC3588-1 includes a low-leakage bridge rectifier with inputs at PZ1 and PZ2 and outputs at VIN and GND. VIN also powers a buck regulator with very low quiescent current, and the output voltage is set to 3.3V via D1 and D0.
The system is driven by a PFCB-W14 piezoelectric sensor from Advanced Cerametrics, capable of generating up to 12mW, though our setup only utilizes around 2mW. The LTC4071 manages battery charging with programmable floating voltage and temperature compensation, ensuring safe operation. It can detect battery temperature through an NTC signal and adjust accordingly to prolong battery life.
The LTC4071 provides 50mA of internal parallel current but draws only about 600nA when the battery is below the floating voltage. The GM BATTERY GMB301009 has an 8mAh capacity and an internal resistance of approximately 10Ω. The sensor-transmitter was modeled using a PIC18LF14K22 microcontroller and an MRF24J40MA RF transceiver, simulating a 12.4 mA load with a 0.98% duty cycle.
The system operates in two modes: charge-send and discharge-send. In charge-send mode, the battery is charged while the sensor-transmitter runs at a 0.5% duty cycle. During discharge-send mode, the sensor-transmitter is active, but no energy is being collected from the PFCB-W14.
In the charge-send mode, the PFCB-W14 delivers about 1.7mW on average. This power must both charge the battery and supply the buck regulator that drives the sensor-transmitter. The sensor-transmitter consumes about 0.41mW on average, leaving some current for battery charging. With an 85% efficiency and a quiescent current of 8μA, the system balances energy consumption effectively.