Parallel robots, or PMs, can be defined as closed-loop mechanisms in which a moving platform and a fixed platform are connected by at least two independent kinematic chains, the mechanism has two or more degrees of freedom, and is driven in parallel.
Research on high-speed parallel robots can be traced back to the Delta manipulator invented by Dr. Clavel in 1985. The robot's active arm is driven by an external revolute joint, the driven arm has a parallelogram structure, and the end effector can achieve high-speed 3D translation within the workspace.
Research Theory: Spiral Theory
Spiral theory is a very important mathematical tool in the study of space mechanism, especially in the last 20 years, where it has played a crucial role in the configuration synthesis of parallel robot mechanisms with few degrees of freedom.
Professor Huang Zhen of Yanshan University was one of the earliest scholars in my country to research and apply spiral theory. His monographs provided a detailed and systematic introduction to spiral theory and its applications in mechanism science, greatly promoting its development, particularly in the field of parallel robot mechanisms. Meanwhile, Professor Fang Yuefa of Beijing Jiaotong University creatively utilized spiral theory to solve the problem of comprehensive analytical analysis of the configuration of parallel robots with few degrees of freedom, accelerating the widespread application of spiral theory.
In the first two decades of the 21st century, with the surge of research into parallel robot mechanisms, spiral theory gained favor among many roboticists. Nearly one-third of international research on parallel robot mechanisms employed spiral theory, making it a crucial tool in robotics research.
Characteristics of parallel robots
Parallel robots are characterized by zero cumulative error and high precision; the drive unit can be placed on or near a fixed platform, resulting in lightweight, high-speed, and dynamically responsive moving parts. Details are as follows:
(1) No cumulative error, high accuracy;
(2) The drive unit can be placed on or near a fixed platform, so that the moving parts are lightweight, have high speed, and good dynamic response;
(3) It has a compact structure, high rigidity, and large load-bearing capacity;
(4) Completely symmetrical parallel mechanisms have good isotropy;
(5) Small workspace;
Based on these characteristics, parallel robots have been widely used in fields that require high rigidity, high precision, or large loads but do not require a large workspace.
Parallel Robot Classification
From the perspective of motion, parallel mechanisms can be divided into planar mechanisms and spatial mechanisms; further subdivided into planar traversal mechanisms, planar traversal and rotational mechanisms, spatial pure traversal mechanisms, spatial pure rotational mechanisms, and spatial hybrid motion mechanisms.
Parallel mechanisms can be classified according to the number of degrees of freedom: 2-degree-of-freedom parallel mechanisms, 3-degree-of-freedom parallel mechanisms, 4-degree-of-freedom parallel mechanisms, 5-degree-of-freedom parallel mechanisms, and 6-degree-of-freedom parallel mechanisms.
Six-DOF parallel mechanisms are a major category of parallel robot mechanisms and are the most studied parallel mechanisms by scholars both domestically and internationally. They are widely used in flight simulators, six-dimensional force and torque sensors, and parallel machine tools. However, many key technologies for these mechanisms have not been fully resolved, such as their forward kinematics, the establishment of dynamic models, and the accuracy calibration of parallel machine tools. From a fully parallel perspective, these mechanisms must have six kinematic chains. However, some existing parallel mechanisms also include six-DOF parallel mechanisms with three kinematic chains, such as 3-PRPS and 3-URS, as well as six-DOF parallel mechanisms with a five-bar linkage added to each of the three branches as the driving mechanism.
