An alternative switching technique for piezoelectric energy harvesting is presented. The energy harvester based on piezoelectric elements is a promising method to scavenge ambient energy. Several non-linear techniques such as SSHI have been implemented to improve the global harvested energy. However, these techniques are sensitive to load and should be tuned to obtain optimal power output. This technique, called Series Synchronized Switch Harvesting (S3H), has both the advantage of easy implementation and independence of the harvested power with the load impedance. The harvesting circuit simply consists of a switch in series with the piezoelement and the load. The switch is nearly always open and is triggered-on each time the piezoelectric voltage reaches an extremum. It is opened back after an arbitrary on-time t0. The energy scavenging process happens when switch is closed. Based on linear motion assumption, the harvester structure is modeled as a “Mass-Spring-Damper” system. The analysis of S3H technique is considered with harmonic excitation. An analytical model of S3H is presented and discussed. The main advantage of this approach compared with the usual standard technique is that the extracted power is independent of the load within a wide range of load impedance, and that the useful impedance range is simply related to the defined switch on-time. For constant displacement excitation condition, the optimal power output is more than twice the power extracted by the standard technique as long as the on-time interval is small comparatively with the vibration period. For constant force excitation, an optimal on-time can be defined resulting in an optimally wide load bandwidth. Keywords: piezoelectric; energy harvesting; non-linear harvesting techniques; switching techniques.
Any piezoelectric generator structure can be modeled close to its resonance by an equivalent circuit derived from the well known Mason equivalent circuit. This equivalent circuit can therefore be used in order to optimize the harvested power using usual electrical impedance matching. The objective of this paper is to illustrate the full process leading to the definition of the proper passive load allowing the optimization of the harvested energy of any harvesting device. First, the electric equivalent circuit of the generator is derived from the Mason equivalent circuit of a seismic harvester. Theoretical ideal impedance matching and optimal load analyze is then given emphasizing the fact that for a given acceleration a constant optimal output power is achievable for any frequency as long as the optimal load is feasible. Identification of the equivalent circuit of an experimental seismic harvester is then derived and matched impedance is defined both theoretically and experimentally. Results demonstrate that an optimal load can always be obtained and that the corresponding output power is constant. However, it is very sensitive to this impedance, and that even if impedance matching is a longtime well known technique, it is not really experimentally and practically achievable.
This paper focuses on the influence of the topology of a network of piezoelectric harvesters using the SSHI (Synchronized Switch Harvesting on Inductor) technology. Generally, an energy harvester is used as a localized and standalone system. In the case of large structure and for large harvested energies, it is usually not easy to increase the size of the piezoelectric patches. In order to harvest energy in the regions of maximum strain of the structure, a networked piezoelectric harvester including many separated piezoelectric patches must be set up with only one output. The main concern is how to connect the piezoelectric elements together and how to implement accurately the SSHI strategy for maximizing the total output power. This paper presents 5 different circuit topologies with or without SSHI enhancement. This work is based upon simulations of a structure with embedded piezoelectric harvesters, made in the Matlab/Simulink environment and using the Simscape library for defining and simulating the electric network. The simulations are done exclusively in pulse mode. For each circuit topology, the total output energy is computed and the optimal harvesting capacitance is defined. The results show the feasibility of grouping various harvesters within a network connected onto a common harvesting capacitance without affecting the extracted energy. The interest of SSHI for networked configuration is confirmed as well as the need for multiple switching units. The effect of the parasitic capacitances due to the bonding of the piezoelectric patches on a metallic structure is also investigated. This capacitance corresponds to the isolation layer between the structure and the bottom electrode of the piezoelectric patches. Results show that an optimal bonding layer thickness can be found that does not affect significantly the coupling coefficient of the piezoelectric patches and which induces parasitic capacitances that do not affect the network functionality.