School of Information Science and Technology, ShanghaiTech University
No. 8 Building, 319 Yueyang Road, Shanghai, China, 200031
ABSTRACT
The synchronized switch interface circuits, e.g., synchronized switch harvesting on inductor (SSHI), can significantly
enhance the harvesting capability of piezoelectric energy harvesting (PEH) systems. In these power conditioning circuits,
the piezoelectric voltage is flipped with respect to a bias voltage at the instants when the piezoelectric element is at
maximum deforming positions. Voltage peak detection and in time switching action are required for implementing these
functions. The state-of-the-art solutions are mostly realized by electronic methods, i.e., both functions are carried out by
electronic comparators and electronic switches. However, the peak detectors usually introduce switching phase lag;
while the electronic switches function only when the vibration magnitude is above a threshold level. When the vibration
is lower than such threshold, the SSHI interface shows no improvement. In this paper, we propose a mechanical solution
for constructing the self-powered SSHI interface for PEH systems. This technique is realized by installing a low cost
vibration sensor switch (VSS) at the free end of a piezoelectric cantilever. It senses the maximum deflecting places of the
cantilever and automatically carries out synchronized switching actions. Compared to the existing electronic solutions,
this mechanical solution is compact and has relative low switching threshold. Therefore, with this self-powered solution,
the advantage of SSHI interface circuit can be sufficiently released, in particular, at low level vibration. Experiment
shows the feasibility of this mechanical solution. The advantages and limitations are also discussed in this paper.
Keywords: piezoelectric energy harvesting, synchronized switch interface circuits, self-powered SSHI, vibration sensor
switch
1. INTRODUCTION
Piezoelectric energy harvesting (PEH) technology provides eco-friendly alternative power supplies for low-power
wireless sensors [1-4]. Based on the capacitive nature of PEH elements, synchronized switch harvesting on inductor
(SSHI)[5] and other switch interface circuits [6] were proposed. It was reported that the SSHI can enhance the harvesting
capability by several hundred percent [5]. In SSHI, the synchronized switches should detect the instants when a
piezoelectric element is at maximum deformation and simultaneously carry out voltage flipping action in time. The
extreme voltage detection and switch action were delivered by a controller, which is powered by external power supply
[3, 5], until the inventions of some self-powered schemes [7-8]. Compared the externally powered solutions in the early
stage, the self-powered solutions can diminish the additional power demand and thus make SSHI more applicable in
practice.
A self-powered SSHI circuit is composed of three functional blocks, i.e., voltage peak detector, comparator, and
electronics switch. The non-ideal peak detector lowers the open circuit voltage of the piezoelectric cantilever; the non-
ideal comparator introduces a switching delay; the non-ideal switch can properly work only when the open circuit
voltage is above a threshold voltage. The voltage drop of diodes and transistors [9] is an important issue in self-powered
circuits, it might introduce considerable energy losses [8, 10] and therefore undermine the improvement by using SSHI.
Aiming to reduce the threshold voltage and diminish the energy losses in diodes and transistors, Liu et al. proposed a
mechanical solution, in which the synchronized switches are realized by the mechanical contacts of a piezoelectric
cantilever and two mechanical stoppers at each side of the cantilever [9, 11-12]. Yet, in their design, the vibration
magnitude can be neither too small (cantilever cannot touch the stopper in small magnitude vibration) nor too large
Active and Passive Smart Structures and Integrated Systems 2015, edited by Wei-Hsin Liao,
Proc. of SPIE Vol. 9431, 94311G · © 2015 SPIE · CCC code:
Proc. of SPIE Vol. 9431 94311G-1