Parallel Mechanism, referred to as PM, can be defined as a closed-loop mechanism in which the moving platform and the fixed platform are connected by at least two independent motion chains, and the mechanism has two or more degrees of freedom and is driven in parallel.
Research into high-speed parallel robots can be traced back to the Delta manipulator, invented by Dr. Clavel in 1985. The robot's driving arm is driven by an outwardly rotating pair, the driven arm is a parallelogram structure, and the end effector can achieve 3D high-speed translational movement in the workspace.
Research Theory: Spiral Theory
Spiral theory is a very important mathematical tool in the study of space mechanisms, especially in the past 20 years, it has played a crucial role in the synthesis of the mechanism configuration of parallel robots with few degrees of freedom.
Professor Huang Zhen of Yanshan University is the earliest scholar in China to study and apply spiral theory, and in his monograph, he introduced the spiral theory and its application in mechanism in detail and systematically, thus greatly promoting the development of spiral theory in mechanism, especially in parallel robot mechanism. Professor Fang Yuefa of Beijing Jiaotong University creatively used the spiral theory to solve the problem of comprehensive analytical analysis of the configuration of parallel robots with few degrees of freedom, which accelerated the popularization of the application of spiral theory.
In the early two decades of the 21st century, with the upsurge of parallel robot mechanism research, the spiral theory has been favored by many roboticists. Almost one-third of the articles on parallel robot mechanics in the world have adopted the method of spiral theory, and it can be said that spiral theory has become an important tool for the study of robot mechanics.
Parallel robot features
The characteristics of parallel robots are that there is no cumulative error and high accuracy; The drive device can be placed on or close to the fixed platform, so that the moving part is lightweight, high speed and good dynamic response. The details are as follows:
(1) No cumulative error, high accuracy;
(2) the driving device can be placed on the fixed platform or close to the position of the fixed platform, so that the moving part is light in weight, high in speed, and has good dynamic response;
(3) Compact structure, high stiffness and large bearing capacity;
(4) The perfectly symmetrical parallel mechanism has good isotropy;
(5) Smaller working space;
Based on these characteristics, parallel robots are widely used in fields that require high stiffness, high precision, or large loads without large working space.
Classification of parallel robots
From the perspective of motion form, parallel mechanism can be divided into plane mechanism and space mechanism; The subdivision can be divided into plane moving mechanism, plane moving rotating mechanism, space pure moving mechanism, space pure rotating mechanism and space mixed motion mechanism.
According to the number of degrees of freedom of the parallel mechanism, it can be divided into 2 degrees of freedom parallel mechanism, 3 degrees of freedom parallel mechanism, 4 degrees of freedom parallel mechanism, 5 degrees of freedom parallel mechanism, and 6 degrees of freedom parallel mechanism.
Among them, the 6-degree-of-freedom parallel mechanism is a major category of parallel robot mechanism, which is the most studied parallel mechanism by scholars at home and abroad, and is widely used in the fields of flight simulator, 6-dimensional force and torque sensor and parallel machine tool. However, there are many key technologies of this type of mechanism that have not been fully solved or have not been fully solved, such as the correct solution of its kinematics, the establishment of dynamic models, and the accuracy calibration of parallel machine tools. From the point of view of complete parallelism, such a mechanism must have 6 chains of motion. But in the existing parallel mechanism, there are also 6 degrees of freedom parallel mechanism with 3 motion chains, such as 3-PRPS and 3-URS and other mechanisms, and there is also a 6 degree of freedom parallel mechanism that attaches 1 5-bar mechanism to each branch of the 3 branches as the 6 degrees of freedom parallel mechanism of the driving mechanism.
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