# A Miniature Flapping Mechanism Using an Origami-Based Spherical Six-Bar Pattern
## 1 Introduction  
Flapping-wing micro air vehicles (FWMAVs) have been widely studied due to their agility and maneuverability in the air. These properties can be applied to the areas of reconnaissance for military and exploration missions for disaster areas. For example, they can easily approach tiny cracks at disaster areas or enemy camps without being recognized. To enable successful working of FWMAVs, researchers have employed a variety of actuators such as DC motors [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19], piezoelectric actuators [20, 21, 22, 23], electromagnetic actuators [24, 25, 26, 27, 28], and electrostatic actuators [6, 29]. Among these actuators, in this paper, we suggest a novel transmission mechanism to reduce the assembly process and wear problems for the DC motor-based FWMAVs. In the case of DC motor-based FWMAVs, most of them use linkage structures to transmit the motor's rotating motion to the wing's flapping motion. These transmitting linkages have involve the possibility of mechanical wear originating from the friction between relative moving components and requires an onerous assembly process owing to several tiny components. Types of the transmissions used for the DC motor-based FWMAVs are given in Table 1.  
Previously, a variety of transmission mechanisms have been employed. [PERSON] et al. used a spatial mechanism based on the double Scotch yoke [15]. They proposed to use spherical type of double Scotch yoke to overcome drawbacks of the planar Scotch yoke. [PERSON] et al. and [PERSON] et al. employed four-bar linkages to change the motor's rotating motion to the flapping motion [5, 12]. [PERSON] et al. and [PERSON] et al. also madeflapping mechanisms using crank-rocker four-bar linkages [2; 4]. [PERSON] et al. and [PERSON] et al. made flapping mechanisms using crank-sliders [1; 9]. The sliders enabled the mechanism to have reciprocating motion of the wings. [PERSON] et al. and [PERSON] et al. applied crank-shaft mechanisms [8; 11]. The crank-shaft mechanisms converted motors' rotating movement into an oscillating movement of the wings. [PERSON] et al. proposed a flapping robot, COLIBRI, by combining four-bar linkage and a slider-crank mechanism [16]. To reduce the mass and complexity of the transmission structures, some researchers suggested a string-based flapping mechanism or direct-driven mechanisms [7]. [PERSON] et al. and [PERSON] et al. used string-based flapping mechanisms [3; 13]. KUbeetle [17; 18; 19] mixed four-bar linkages and a string-based pulley mechanism. The string-based transmission enabled the prevention of mechanical wear and fracture of moving parts. The direct-driven mechanisms enabled the extreme reduction in the mechanical structures for transmission [10; 14]. The direct-driven mechanisms achieved a high lift-to-weight ratio by resonating the system. In this paper, we suggest a novel transmission mechanism to reduce the assembly process and wear problems for the DC motor-based FWMAVs. To make this possible, two design approaches have been utilized: origami-based fabrication and geometrically constrained spherical six-bar linkage. The origami-based fabrication method reduces the number of relative moving components by replacing rigid links and pin joints with facets and folding joints, which shortens the assembly process and reduces wear problems [30; 31]. The geometrically constrained spherical six-bar linkage enables the change in the motor's rotating motion to the linear reciprocating motion. Due to the property that every axis passes through a single central point, the constrained spherical six-bar linkage filters the rotating motion of the DC motor and passes only linear motion to the flapping wings. Using the proposed design approaches, a novel flapping mechanism is made as shown in Figure 1.