###### Abstract  
Most of the driving mechanisms used in micro flapping wing vehicles are gear and linkage transmission mechanisms, which convert the unidirectional rotation of the motor into the reciprocating flapping of the driving mechanism. However, gear and linkage transmission mechanisms occupy a larger space and weight with certain energy losses. This article introduces a drive mechanism that is different from gear and linkage mechanisms, namely the string-based mechanism. In this study, the working principle and various parameters in string-based mechanisms are analyzed, and the flapping trajectory and amplitude of the mechanism is simulated. Following that, this article proposes an improved method of the cam mechanism, designs the trajectory of a cam mechanism, and a physical design is carried out with a driving mechanism width of 22 mm. Subsequently, the experiments are conducted to compare the flapping trajectory of the actual mechanism with the simulation trajectory, thereby verifying the simulation accuracy. The results prove that, in a string-based mechanism, the ratio of the radius of the pulley to the installation radius of the double-layer bearings has a direct impact on the flapping amplitude, and optimizing the design of cam mechanisms can effectively reduce the tensile and relaxation phenomena in string-based mechanisms. The above conclusion was verified in physical experiments, where the modified cam mechanism effectively reduced the stuck phenomenon in the string-based mechanism. F 2023 13 13209 1320  
drive, both direct-drive rotorcraft or non-direct-drive rotorcraft all convert the one-way rotation of the motor into the one-way rotation of the blades, which is simpler and more efficient. But the reciprocating motion features in the FMAV require a conversion in the motor unidirectional rotation to wing flapping, which is complex and inefficient [4; 5; 6]. To acquire the ability of flapping flight, in nature, birds use contractile pectoral muscles [7; 8], and insects use the resonant thorax and hydraulic wing veins [9; 10; 11] for better flapping flight (which is not practical for design in FMAVs). Therefore, the drive mechanism has higher requirements in a mechanism design that is based on the purely mechanical method of achieving flapping to achieve the flying of FMAVs. The mechanisms employed by existing flapping MAVs to achieve wing flapping can be broadly categorized as follows: gear-bar linkage-based mechanisms [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22], spring-bar linkage-based mechanisms [23]; string-based mechanisms [24; 25; 26; 27], \"Smart Composite Microstructures' (SCM) and voltage-driven piezoelectric bimorphs mechanisms [28; 29; 30; 31], and motor direct drive mechanisms [32; 33]. Among these, the voltage-driven piezoelectric bimorph mechanism boasts a high flapping speed but suffers from a low driving capacity. At present, it is solely utilized in millimeter-scale insect-like flapping wing aircraft. On the other hand, motor direct-drive mechanisms necessitate controlling the motor to reverse, thus affording flexible control. However, their low efficiency hinders practical applications, which is typically used by Purdue Hummingbirds [33]. The gear-bar linkage-based mechanism, spring-bar linkage-based mechanism, and string-based mechanism are commonly employed in decimeter-level to meter-level flapping wing aircraft, with the string transmission mechanism exhibiting unique advantages over the other two. The string, with an appropriate Young's modulus, is not as rigid as the gear-bar linkage-based mechanism, and it exhibits slight ductility in the face of vibration and resonance, thus safeguarding the mechanism and ensuring a smoother flapping trajectory. Furthermore, compared to the spring-bar-based mechanism, the string-based mechanism boasts greater flexibility without affecting the preset flapping trajectory, thus rendering it effective and reliable for flapping flight control.