With the development of science and technology, robots have gradually entered people's lives, bringing great convenience to life.
Robots are divided into three categories: series robots, parallel robots and hybrid robots. Among them, the parallel robot consists of some closed-loop kinematic chains, and there are two classical configurations: Delta robot and Stewart robot.
The Delta parallel robot really attracted widespread attention in the 1990s, and has the advantages of high stiffness, high speed, strong flexibility, and light weight. It is the most widely used in light industries such as food, medicine, and electronics, and has unparalleled advantages in material handling, packaging, and sorting. In recent years, with the increasingly widespread application in the market, parallel robots have become a new force for the growth of industrial robot demand.
At present, the base of parallel robots is still small. Compared with the traditional serial robot market, there are not many players participating in it. International companies include ABB, FANUC, Bolix, and Omron. According to data from the Robotics Research Institute (GGII), in 2019, the sales volume of DELTA in the Chinese market was 4,620 units, a year-on-year increase of 30.14%, and its market output value was 594 million yuan, a year-on-year increase of 12.50%. GGII predicts that, as a subdivision of industrial robots, the Delta parallel robot market is enough to support an annual market demand of 1 billion.
Compared with the popularity of the Delta robot, the Stewart robot, as the first parallel robot in the world, has not been fully explored for its application scenarios.
What is a Stewart robot?
The Stewart robot has six degrees of freedom and six branch chains. The two ends of each branch chain are ball pairs, and a moving pair connects two rods in the middle.
Originally invented by Gough in 1947, the Stewart robot was used to detect tire wear under various loading conditions. The connection between Stewart and Gough stems from a paper published by Stewart in 1965, which mentioned Gough's work and the image of such a platform, which had an important influence on the development of parallel robot kinematics. Based on this, parallel robots with this type of configuration are also called Gough-Stewart robots.
Academically, the Stewart robot is also the most classic parallel robot configuration. Parallel robots are introduced in university textbooks, and the first example is mostly Stewart robots; in the textbooks, the kinematics and dynamics calculations of parallel robots are used as examples, and Stewart robots are also used as examples, which shows the status of Stewart robots in parallel robots.
Advantages of Stewart Robots:
1. Heavy load: This is also a common advantage of parallel mechanisms, but the advantage of Stewart robots is particularly obvious. The reason is very simple, it has six "legs", and the weight of the moving platform and the load is distributed on the six branch chains. Studies have shown that with the same load capacity, the weight of the Stewart robot is about 1/20 of the weight of the traditional six-degree-of-freedom serial robot;
2. Full degrees of freedom: The Stewart robot has six complete degrees of freedom, so its moving platform can theoretically complete any pose movement in the workspace.
Due to the characteristics of its configuration and degrees of freedom, the Stewart robot also has its inherent shortcomings, which limit its further development. Its shortcomings are as follows:
1. There is no positive analytical solution: due to its characteristics of six degrees of freedom and configuration, the analytical solution of its kinematics positive solution has not been solved yet, so it is difficult to achieve high-precision motion like the Delta robot (delta robot). The analytical solution of its kinematics positive solution can be solved);
2. The singularity type is complex: Due to the many degrees of freedom, the singularity type of the Stewart robot is very complex. Now the academic community has solved its 8 singularity types, of which the 8th singularity type was solved in 2004. It has been half a century since the paper published by Stewart, that is to say, there may still be singularity types that have not been discovered;
3. Analytical solution without workspace: Since the analytic solution of Stewart robot has not been solved in academia, there is no analytical solution in its workspace. Now the common method is to use Monte Carlo search. The small working space is a major disadvantage of parallel robots, and the Stewart robot has no analytical solution for the working space, which is even more unfavorable for the application of the Stewart robot.
The entry of emerging robot companies
Aware of the unique advantages of Stewart robots, some robot manufacturers have begun to develop Stewart robots, and then it is possible to extend their tentacles to the field of swing tables and simulation tables to compete with traditional "leading" companies that focus on swing tables and simulation tables.
The biggest advantage of robot companies is their control technology. Because the usage scenarios of Stewart robots generally do not require high precision and the movement is relatively simple, the application of traditional robot control algorithms and trajectory planning algorithms to the scenarios of the simulation platform belongs to the competition of dimensionality reduction.
On the other hand, compared with the traditional swing table and simulation table companies, the disadvantages of robot companies are also obvious:
1. Qualification issue: A considerable part of the simulation platform projects are from the military. If companies want to bid, they need to have certain confidentiality qualifications, which are not possessed by a considerable number of robot companies. Without qualifications, companies are not even qualified to bid. However, once the company successfully applies for the confidentiality qualification, the company needs to abide by the regulations, for example, the office computer cannot access the Internet, etc., such conditions are detrimental to the company's office efficiency;
2. Requirements for heavy load: Different from the application scenario of Delta robot, Stewart robot is often used in heavy load conditions, and its installation method is also different from that of Delta. Its fixed platform is on the bottom and the moving platform is on the top. The load on the moving platform may be a warship, an aircraft, or a tank. The load may be several tons, dozens of tons or even hundreds of tons. The sudden change of working conditions will also bring new challenges to the company;
3. Requirements for non-standard design: Most of the companies that do Delta robots now develop different types of robots, and then apply different types of robots to different scenarios. However, each project of the swing table and the simulation table is different, and the working conditions are very different, so it is difficult to rely on several existing models to cope with different projects. It is necessary to carry out separate structural design and simulation of the swing table for different projects. .
The biggest difficulty in non-standard design is that the drawings will only be used once, at most two or three times. In terms of cost and time, non-standard equipment does not allow secondary manufacturing of parts during the production process (somewhat similar to the non-standard design of production lines). This kind of non-standard design requirements of the regular robot body may also make robot companies not very adaptable.