Principle of Configuration Synthesis for Low-DOF Industrial Robots

Abstract : This paper proposes a theoretical method for configuration synthesis of industrial robots with few degrees of freedom. First, based on given operational requirements, the desired degree of freedom type of the mechanism is analyzed. Then, by analyzing the constraint screw system and geometric conditions, the relationship between the kinematic pair screws and the constraint screw system in the mechanism is obtained. Finally, combining the geometric conditions of the configuration constraints and considering the actual situation of mechanism control, the kinematic pair screws are linearly combined, and the instantaneous nature of the mechanism is verified, thus obtaining the mechanism configuration with the desired degree of freedom. An industrial robot with a degree of freedom type of 2R3T is used as an example for illustration.

Keywords : industrial robot; configuration synthesis; helical system; instantaneous performance

1. Introduction
Industrial robots with fewer than six degrees of freedom are called low-degree-of-freedom (DOF) industrial robots. While low-DOF robots cannot perform complete spatial positioning and attitude adjustment, they are sufficient for certain specific tasks such as arc welding, spot welding, spraying, handling, and gluing. Furthermore, compared to robots with six or even redundant degrees of freedom, low-DOF industrial robots offer advantages in configuration design and control. However, in practical applications, the configuration synthesis of industrial robots still relies heavily on experience-based design, followed by continuous experimentation and improvement. Although this has led to some feasible solutions, a universally applicable configuration synthesis method for low-DOF industrial robots has not yet been fully established.
Currently, few studies have focused on the configuration design synthesis of industrial robots with few degrees of freedom. Besides empirical methods, Yang Tingli et al. synthesized the topological structure of serial mechanisms based on the motion output characteristic equations and the output characteristics of serial mechanisms of different scales. Furthermore, a large number of scholars, when designing parallel robots, synthesized the structural types of serial branches according to the constraint types (pure force, couple, and force helix) provided by the serial branches to the moving platform, a method known as the helix theory branch method.
With the increasing demand for industrial robots in structured working environments, the use of robots with fewer degrees of freedom (DOFs) to complete tasks has become a trend. Current development of serial robots mainly focuses on the introduction of new robot types and the enrichment and development of robot application technologies. However, the theoretical and systematic design of DDF serial robot configurations remains an easily overlooked but urgently needed solution.
This paper proposes a configuration synthesis principle for industrial robots with few degrees of freedom. First, based on given operational requirements, the desired degree of freedom type of the mechanism is analyzed. Then, the degree of freedom is represented in helical form, and the corresponding constraint helical system is identified. Helical theory is used to obtain the relationship between the kinematic pair axes and the constraint helical system. Finally, combining the geometric conditions of the configuration constraints and considering the actual situation of mechanism control and configuration, the kinematic pair helices are linearly combined, and the instantaneous nature of the mechanism is verified, thus obtaining the mechanism configuration requiring the desired degree of freedom. This principle is illustrated using an industrial robot with 2R3T degrees of freedom as an example.

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