# A Dual Stage Low Power Converter Driving for Piezoelectric Actuator Applied in Micro Mobile Robot
## 1 Introduction  
Microrobots represent a new type of bionic robots inspired by insects to achieve ultracompact size, high mobility, and remote controllability; these robots have many applications, including those in bioengineering, disaster relief, microsurgery, and surveillance [1; 2; 3]. Among microrobots, micro mobile robots (MMRs) are widely used in environmental monitoring, rescue operations, and agricultural production, due to their high energy efficiency and high mobility. If the energy conversion efficiency of the frame, including the actuator, power circuit design, and mechanical transmission for stroke amplification can be improved, then the application prospects of such robots will be extended. As a displacement device, actuators are widely used in many types of micromechanical devices like walking, swimming, jumping robots and so on [4; 5; 6]. Among them, especially, the actuator plays a critical role in obtaining sufficient lift force. Prior works on actuators in MMR applications include studies on an electromagnetic actuator, a shape-memory alloy actuator, an electrostatic actuator, and a dielectric elastomer actuator [7; 8; 9; 10]. However, the disadvantages of low response rate, small mechanical displacement, and low conversion efficiency limit their applications to MMRs. The breakthrough in piezoelectric bimorph actuators with high bandwidth and high power density achieved by [PERSON] et al. in a multi-segmented centipede robot. Meanwhile, [PERSON] et al. considered a complete fabrication solution for actuators, links, flexures, integrated wiring, and structural elements using high-performance materials; they then developed a highly efficient transmission link for a specific device [11]. However, a power electronic unit connected to a piezoelectric actuator facestwo major challenges [12]. Firstly, high voltages within the range of 100-120 V are required to generate sufficient force and displacement. Most energy sources compatible with the weight budget of MMR applications have output voltages of 3-5 V. Consequently, power interface circuits require high step-up ratio. Secondly, a piezoelectric actuator provides only a fraction of the input energy required for effective displacement. In addition, prior work in dynamic performance analysis of piezoelectric actuator include [PERSON] et al., which demonstrated comprehensive analytical model of both unimorph and bimorph structure [13; 14]. Different drive frequencies and signal amplitudes can affect the electromechanical coupling factor and conversion efficiency of the piezoelectric actuator. Hence, the drive circuit must support drive signals with specific frequency and efficient bidirectional energy flow to fully utilize the unused energy stored in actuators, and consequently, maximize system efficiency. Several researchers have discussed the miniaturization of voltage conversion circuits for microrobotic applications. A boost converter cascaded with a switched capacitor circuit was adopted by [PERSON] et al. to obtain high voltage [15]. However, this structure requires a large number of pump capacitors, which increases the size of the circuit and reduces its power density, particularly at a high output power. [PERSON] proposed a piezoelectric transformer-based power amplifier with high voltage gain and power density [16]. Although the circuit topology can boost low voltage, the size of the structure faces difficulty in meeting the requirements of current manufacturing levels. Meanwhile, a piezoelectric transformer should operate close to the mechanical resonance frequency to obtain high voltage gain and efficiency. Other topologies with more components and a high step-up ratio are also suitable for driving high-voltage reactive loads; however, most efforts have focused on large-scale and high-power applications [17; 18; 19]. In another part of the circuit, a simple push-pull driver described in Reference [15] can generate a unipolar square wave voltage across the load without energy recovery.