As the most mature robot product, SCARA robots are gradually becoming a highly competitive "red ocean" due to the increasing number of entrants and increasingly lower profit margins. Currently, companies are continuously improving the performance of SCARA robots based on their specific positioning, and new technologies are constantly emerging.
However, recently, from a structural design perspective, GGII Robotics Network discovered a problem: some companies use two harmonic reducers for their SCARA robots; while some companies use three reducers for their SCARA robots, including cases where all three are harmonic reducers and cases where two harmonic reducers and one planetary reducer are used.
What are the differences between a SCARA with two gearboxes and a SCARA with three gearboxes? How do companies make their choices? We will reveal the answers below.
■Where is the third speed reducer used?
The general shape of a SCARA robot is shown in the image below. The first and second axes have the same structure, both using servo motors and harmonic reducers for transmission. However, in the internal structure of the head (also known as the Z-axis), as shown in the red box below, we find that different companies have different designs. Currently, Epson uses a gear and belt (synchronous belt) method for speed reduction, and some domestic companies such as Zowell and MASTU also use this method.
However, some companies use speed reducers directly in this area. For example, Turing Robot uses harmonic speed reducers, while Dongguan Zhiying, Kaibao Robot, and Huasheng Control use planetary speed reducers.
■Why did different structural design methods emerge?
Liu Yangsheng, general manager of Dongguan Zhiying, said that although Zhiying's SCARA imitated Epson's product design in some structural aspects, Zhiying made some changes to the internal structure of the head. One of these changes was replacing Epson's gear and belt (synchronous belt) speed reduction device with a planetary gear reducer.
“Epson’s approach can reduce costs. Using a planetary gear reducer costs about 300 yuan more, but it’s simpler to assemble and saves on labor,” Liu Yangsheng said.
Some industry insiders have stated that the cost of using planetary gear reducers is not significantly different from that of synchronous belts. However, when it comes to cost, Turing Robot's design using three harmonic gear reducers is undoubtedly the most expensive. What considerations led Turing Robot to choose this design?
Xu Jianping, founder and general manager of Turing Robot, said that the reason for choosing to use three reducers is that some customers make very heavy grippers, and using harmonic reducers can still ensure the stability of the robot under such circumstances.
“We used to use two speed reducers, but the customers weren’t satisfied and kept blaming our robots. So we switched to three speed reducers. Although the cost increased by about 1,000 yuan, the after-sales costs were reduced,” Xu Jianping said. In fact, this was also an improvement made to cooperate with some domestic system integrators.
Regarding the impact of using a reducer or timing belt on robot accuracy in the Z-axis, Zhang Zhiqiang, Marketing Director of MASTU, stated that the Z-axis accuracy requirements for SCARA are not that high in general applications, so the impact is not significant.
From the structure of SCARA, the Z-axis is a moving joint used to complete the movement of the end part in a plane perpendicular to the plane. Therefore, the lead screw has a greater impact on its accuracy, while the reducer or timing belt mainly plays the role of speed reduction.
However, an industry insider also pointed out that using a synchronous belt structure allows the robot to move faster because synchronous belts are more suitable for high-speed, low-torque motion scenarios.
“Belts are characterized by a low reduction ratio, so they rotate quickly, but their rigidity is not as good as that of gearboxes,” said Yang Jipeng, technical director of Zowell Technology. “For loads with high inertia, some companies may choose to use gearboxes in order to increase the inertia of the end shaft.”
Regarding the fact that some domestic companies do not use the gear and belt method like Epson, Zhang Zhiqiang has another opinion besides the difficulty of assembly. He pointed out that some companies may not use the belt and gear method because of materials, such as the inability to buy high-performance belts.
■ Are there any advantages or disadvantages to different structural design methods?
"The structure is naturally simplified after adopting a planetary reducer, but the planetary reducer will greatly increase the weight and inertia of the two joints, and the backlash of the planetary reducer is generally larger. Planetary reducers with a backlash of less than 1 arc minute are very expensive." A relevant person pointed out, "The synchronous belt reduction design is larger in size, so some companies choose planetary reducers."
Yang Jipeng believes that the choice of which method a company ultimately chooses depends mainly on its inertia requirements. In fact, each model has its own terminal inertia design value, and the different methods chosen by different companies are actually a manifestation of product differentiation.
The aforementioned industry insider stated that each of these three methods has its advantages and disadvantages, and none of them has a particularly prominent advantage. Therefore, it ultimately depends on the company's own considerations. When speed is the primary concern for small loads, while power and stability are prioritized for large loads, using a synchronous belt for small loads and a speed reducer for large loads might be a good choice. Yang Jipeng also pointed out that using a speed reducer at the head of a heavy-duty SCARA robot does indeed improve its stability.
