## 1 Introduction
[PERSON] [14] first proposed a virtual coefficient index for motion/force transmission based on screw theory, defining the virtual coefficient between the transmission wrench screw (TWS) and the output motion screw (OTS) as the transmission coefficient for evaluating the transmission performance of spatial mechanisms. [PERSON] and [PERSON] [15] proposed a transmission coefficient for the normalized transmission index based on the geometric characteristics of the mechanism, but it was difficult to use for evaluating the transmission performance of the mechanism with moving joint output. [PERSON] et al. [16] solved the transmission index of a 6-RSS parallel mechanism by fixing all inputs except one, based on the minimum cosine of the pressure angle at the connection point between the chain and the moving platform of a single-degree-of-freedom parallel mechanism. [PERSON] et al. [17] proposed a generalized transmission index (GTI) for single-loop spatial mechanisms based on the virtual coefficient between the transmission wrench screw and the output motion screw and proved that the transmission index proposed by [PERSON] was a special case of this index. They also calculated the GTI of the RSCR single-loop spatial mechanism, proving that it has good transmission performance within the workspace. [PERSON] et al. [18; 19; 20] proposed a generalized transmission index for evaluating the motion/force transmission efficiency of fully driven parallel mechanisms independent of the coordinate system, based on virtual coefficients. They provided a calculation method for the output motion screw of each chain of parallel mechanisms and established the evaluation system for the motion/force transmission performance index of the parallel mechanisms. On this basis, [PERSON] et al. [21] skillfully used the folding motion of the folding part of the single-loop mechanism as the output motion screw, provided an analysis method for the motion/force transmission performance index of the sub-chain of the single-loop overconstrained mechanism, and applied the input index of the deployable 7R overconstrained single-loop mechanism to aerospace platforms. It is worth studying how to promote the method and index proposed by [PERSON] et al. to the solution for the motion/force transmission performance index of input-to-output components of single-loop mechanisms and apply it to the design of single-degree-of-freedom single-loop vehicle side-door latches. The overall motion/force transmission performance index for single-loop mechanisms was constructed, and the corresponding calculation procedure is defined here. The main content is as follows: for the multifunctional needs of high-end electric vehicle side-door latches within limited spaces, a single-motor-driven electric releasing and cinching spatial compliant mechanism was designed with timing controlled by incomplete-gear intermittent transmission. The forward and inverse kinematics of the spatial RRURR mechanism are analyzed, and the relationship between each joint angle and the input angle is obtained, laying the foundation for chain-driving transference. Matching methods for the moving/fixed platform inversion and chain-driving transference are proposed. The index for the overall motion/force transmission performance of a single-loop mechanism and its calculation procedure is constructed. ADAMS VIEW 2020 is used to simulate the electric releasing and cinching mechanism. The motion/force transmission performance of electric releasing mechanisms with high motion/force transmission performance requirements is studied; the performance atlas of the mechanism is plotted. The actuating torque required for electric releasing and cinching of the vehicle side-door latch driven by a single motor is calculated. Synthesis and Application of a Single-Motor-Driven Electric Releasing and Cinching Vehicle Side-Door Latch Mechanism  
The operational requirements and current technological landscape of single-motor-driven electric releasing and cinching of vehicle side-door latches are analyzed here.