# Design of a Parallel Quadruped Robot Based on a Novel Intelligent Control System
## 1 Introduction  
Quadruped robots have received extensive research because of their excellent stability and carrying capacity [1; 2], which are less complicated than the hexapod and eight-legged robots [3; 4]. These advantages lead to the acceptance of quadruped robots in various working environments, both on rugged and flat ground [5]. The legs are the key component determining the application performance of the quadruped robots. Therefore, many studies have focused on the design of mechanical structure, gait planning, and motion control algorithm to optimize the kinematic performance of the legs. The mechanical structures of the legs of quadruped robots are mainly divided into series and parallel forms. With respect to the study on the series structure of the robot's legs, [PERSON] et al. [6] developed telescopic column legs using springs to reduce weight and inertia, but this led to a reduction in load capacity. [PERSON] et al. [7] constructed articulated legs with offset-arranged joints to obtain a larger range of motion during the movement of a robot. [PERSON] et al. [8] designed articulated legs using a two-link mechanism composed of a hip flexion joint and a knee joint to simplify the mechanical structure. As mentioned above, the robot's legs of the series structure are simple and have a large working space, but also have the disadvantages of low load capacity, complicated control systems, and high cost. These shortcomings are usually addressed using parallel structural legs. [PERSON] et al. [9] developed Stanford Doggo with the legs of a parallel four-link mechanism to achieve eight-degree-freedom movement with high speed and load capacity performances. Unfortunately, the two motors located in one joint resulted in reduced stability during motion from friction. Simultaneously, a great deal of effort has been made to improve the stability and rhythm of the gait. Methods of static stability margin (SSM) [10] and central mode processor (CPG) [11; 12; 13; 14; 15] were adopted to plan the gait of quadruped robots. In particular, gait trajectory planning based on bionics methodology has recently become a hot topic [16]. Moreover, a control strategy based on structure design also plays an important role in completing the prescribed gait. A proportional-integral-differential (PID) controller was used to control the robot motion with a predetermined, trajectory to achieve small errors [17; 18; 19; 20]. To ensure that the controller has acceptable performance, [PERSON] et al. used fuzzy systems, and these gains were adaptively tuned for the best performance of the robot against uncertainties in system parameters, external disturbances, and measurement noises [21; 22]. However, the control system is complicated. Some more effective and novel swarm intelligence optimization algorithms, such as the Particle Swarm Optimization algorithm [23], Genetic algorithm [24], and Fruit Fly algorithm [25], have emerged for the parameter optimization of a control strategy. Among them, the sparrow search algorithm (SSA) [26] had fewer parameters and stronger robustness compared with other algorithms; thus, it was used to optimize the controller in this study. This paper aims to solve the problem of quadruped robot walking forward with low accuracy and stability; thus, a quadruped robot with a new mechanical structure, optimized foot trajectory and a novel intelligent control system was designed. The quadruped robot was designed with a five-link leg structure to mimic a dog's diagonal trot. Each leg was a parallel five-link structure with two degrees of freedom, which leads to more even force distribution on the mechanical structure and more carrying capacity. In addition, two joint motors were used to drive two independent kinematic chains simultaneously to reduce kinematic errors. To determine the relationship between the foot position and the angle of the driving joint, the D-H method was used to analyze the kinematics model of the robot's leg.