## 1 Introduction  
With the intelligent development of vehicles, the multifunctional requirements of high-end vehicle side-door latches are often achieved by adding chains [1]. There are two main approaches for mechanical design in the limited space of vehicle side-door latches. First is designing multifunctional chains with fewer drives based on the spatial mechanism to reduce the number of motors and make effective use of the internal space [2]. Second is improving the structure of the chains to enhance the transmission efficiency of the mechanism, thereby avoiding the use of high-power motors and reducing the size of the door latch. Extant vehicle door latches use multiple motors to control multiple chains to achieve multiple functions, which has increased the size of the door latches. In order to adapt to the limited space inside the door latch, [PERSON] et al. [3; 4] added an electric releasing chain through the proposed compliant joint to a vehicle side-door latch equipped with an electric insurance chain without adding an additional motor. They proposed a single-drive electric releasing and insurance vehicle side-door latch and designed a double-layer compliant intermittent mechanism for the insurance's engaging and disengaging. The patent [5] proposed a child-protection chain that shared a motor with the central control latch. A motor controls the connecting bar to rotate clockwise or counterclockwise, locking or unlocking the central control latch and child-protection latch. The drive structure was mounted directly on the shell of the central control latch, with no additional parts required. [PERSON] et al. [6; 7; 8] introduced spatial mechanisms into the designs of latches, further utilizing the limited space inside the latches. During the process of electric releasing, the vehicle side-door latch should overcome a sealing-reaction force of 300 N. However, the motor torque of the electric insurance chain was relatively small. They proposed and optimized a new type of electric releasing space-compliant mechanism with force-compliance characteristics, which improved the transmission efficiency of the mechanism and reduced energy consumption during the electric releasing process. In addition, [PERSON], [PERSON] et al. achieved lightweight design using composite materials and dynamic structural optimization, which had proven effective in enhancing both transmission performance and spatial compactness under constrained geometric conditions [9; 10]. In response to the problem of long motion chains in vehicle side-door latches, which had resulted in low transmission efficiency and frequent jamming, [PERSON] et al. [11] used multi-component coaxial rotation and cylindrical cam intermittent mechanisms to develop a compact, electric ciching door latch with fewer parts. [PERSON] used Pro/E 5.0 software to analyze the motion of the mechanism and optimized the cam curvature at the jerk point of the acceleration curve. [PERSON] [12] innovatively proposed a vehicle side-door latch that achieved single-drive electric ciching and electric releasing by controlling a single motor to drive the ratchet and pawl. This design met the requirements of single-drive multifunctionality but did not take into account the limited space inside the door latch for choosing the large-size motor. How to achieve single-drive electric releasing and cinching without increasing motor size while improving the mechanism's motion/force transmission efficiency is a key research issue. Motion/force transmission performance is an important index for evaluating the transmission efficiency of mechanisms. [PERSON] [13] proposed the local condition index (LCI) and global condition index (GCI) based on the condition number of the Jacobian matrix and constructed an optimization evaluation index for parallel mechanisms by using the vector matrix mapping of the end-effector speed and the drive-joint rate. However, when the mechanism was in a singular position or there were mixed degrees of freedom, it was difficult to solve the condition index.