# Development of an Insect-like Flapping-Wing Micro Air Vehicle with Parallel Control Mechanism
## 1 Introduction  
Insects are one of the most remarkable flying species in nature. Despite their small size, insects are highly skilled fliers. Mimicking insect flight has been a hot topic of research for many years. The mechanics of insect flight are a fundamental issue for FMAV design. In the past years, a lot of research has revealed aerodynamics at the insect scale. Some researchers have developed computable quasi-steady or unsteady aerodynamic models for insect flight. For example, [PERSON] et al. applied the blade element theory (BET) in insect flapping [1] and [PERSON] et al. proposed a quasi-steady aerodynamic model that could evaluate the steady translational force coefficients of flapping wings [2]. Some researchers have used computational fluid dynamics (CFD) to study the non-constant aerodynamic model. CFD is a widely used tool in aerodynamics and its utility was shown in [3, 4]. For example, [PERSON] et al. proposed a novel CFD-informed quasi-steady model for insect flight [5] and [PERSON] et al. assessed the effect on the aerodynamic hovering performance of wing shapes by the CFD tool [6]. Based on the advances in the mechanics of insect flight, micro-manufacturing, and avionics technologies, a number of successful flapping-wing micro air vehicles (FMAV) have been developed. The developed insect-like FMAV can currently be divided into two categories based on weight. One category weighs on the mg scale. These FMAVs have a similar weight and size to the microscopic insects in nature. However, the load capacity is limited due to the drive and power. They usually require external energy and control signals and have poor sensory capabilities of their own such as RobeBee, which was developed by the Micro robotics Laboratory at Harvard University [7]; the flapping-wing robot developed by Purdue University [8]; and the flapping-wing robot developed by Shanghai Jiao Tong University [9; 10]. The other category weighs on the g scale. These FMAVs can carry a variety of sensors, control modules, and onboard energy due to their increased size and weight and keep themselves in the air without external assistance such as the nano hummingbird designed by AeroVironment Company [11]; Delfly aircrafts designed by Delft University of Technology [12; 13; 14]; the robotic hummingbird designed by Texas A&M University [15]; the KUBeetle by Konkuk University [16; 17; 18; 19]; hummingbird robot designed by Purdue University [20]; NUS-hummingbird designed by the National University of Singapore [21]; and the COLIBRI robot designed by Universitr' Libre de Bruxelles [22]. Although these FMAVs vary in size and construction, their control methods can be broadly classified as flapping amplitude modulation [7; 15; 20]; flapping rate modulation [7; 14; 20; 21]; wing twist modulation [15; 17; 18; 19; 22]; and stroke plane modulation [14; 16; 17; 21]. The starting point of all of these control methods is to change the flapping pattern of the wings, which in turn produces different control toques. Although there are many ways to generate the control torque, the combination of a flapping wing mechanism with a control mechanism that simultaneously satisfies the requirements for low weight, compactness, and vibration resistance is an important challenge. The coupling of the flapping-wing mechanism and the control mechanism poses strength and reliability problems. The complex structure of the control mechanism and the high number of drives significantly increases the weight and size, and the machining and assembly of these components can become difficult in small dimensions. The traditional serial control mechanism uses a single drive to generate a single torque. The advantage of this approach is that the individual torques are naturally decoupled from each other and the control system is relatively simple. However, parallel mechanisms are used in a wide range of applications in recent years such as the agile eyes [23] and milliDelta [24]. These designs all enable multiple degrees of freedom control in a small space.