Optimal packet scheduling for a piezoelectric energy harvesting node

We consider a wireless sensor node equipped with a piezoelectric vibration energy harvester, which is excited by a series of periodic base acceleration impulses. The magnitude and arrival times of the impulses are known a priori. The node also contains a finite energy storage system, a data queue, and a data transmitter. While most piezoelectric energy harvesters trickle-charge the storage system, in this study, we model a PWM-controlled H-bridge circuit at the transducer and energy storage system interface, which can realize any desired transducer current including two-way power flow. Our goal is to maximize the bits of data transmitted from the node over a fixed period of time through the control of the transmission power and transducer current. However, there are parasitic losses associated with the circuitry used to extract power, and therefore, the optimal scheduling policy must balance energy resources between the two energy-consuming tasks: harvesting energy and transmitting data. Using barrier methods, we develop the optimal off-line control policy to maximize data transmission, and compare how this optimal control policy behaves with a finite versus an infinite energy storage system. The resulting off-line control policy provides an upper bound on the performance of a real-time controller, and the analysis can be used for design purposes.