Abstract: Motion control technology has become a key technology in mechatronics. This paper introduces the classification and various application directions of robots, and analyzes the application and development status of different motion controls in robots. It also introduces the current development status of intelligent mobile robot technology and the development level of intelligent mobile robots in various countries around the world, and then discusses the development trend of motion control in intelligent mobile robots and the prospects for future technologies.
Keywords: Intelligent mobile robot; motion control; development prospects
Foreword
A robot is a programmable and multifunctional manipulator used to transport materials, parts, and tools, or a specialized system with changeable and programmable movements to perform different tasks. An intelligent mobile robot, on the other hand, is a machine system that comprehensively simulates humans in terms of perception, thinking, and action, though its appearance may not necessarily resemble a human. It serves as a comprehensive testing ground for artificial intelligence technology, allowing for a full examination of technologies across various fields of AI and the study of their interrelationships. It can also excel in hazardous environments, performing dangerous work, operating in the air and at sea, and on the battlefield. An intelligent mobile robot should possess three capabilities: the ability to perceive its environment, the ability to influence its environment while performing a specific task, and the ability to connect perception with action. The fundamental difference between intelligent mobile robots and industrial robots lies in the fact that intelligent mobile robots possess perception functions as well as recognition, judgment, and planning capabilities.
As the application fields of intelligent mobile robots expand, people expect them to serve humanity in more areas and replace humans in performing more complex tasks. However, the environments in which intelligent mobile robots operate are often unknown and difficult to predict. The tasks that intelligent mobile robots must perform are also becoming increasingly complex; and manually analyzing and designing the behavior of intelligent mobile robots is becoming increasingly difficult. Currently, research on intelligent mobile robots is deepening both domestically and internationally.
High-speed, high-precision motion control is a key advantage of industrial robots. Fixed at workstations, industrial robots perform tasks such as material handling, welding, painting, polishing, and loading/unloading with high efficiency. However, when changing production lines, robots need to be repositioned and reprogrammed, resulting in significant downtime. Collaborative robots, on the other hand, offer flexible deployment options. They can be mounted on mobile platforms, improving robot mobility and production line flexibility, allowing for flexible deployment. In recent years, large companies have focused heavily on robot mobility, resulting in numerous acquisitions and mergers in the mobile robot sector. The demands for mobility in various service robots are only increasing, with Boston Dynamics' ATLAS robot achieving a remarkably high level of motion capability.
1. Current Status of Intelligent Robot Development
Intelligent mobile robots are the third generation of robots. These robots are equipped with multiple sensors and can fuse information from these sensors to effectively adapt to changing environments. They have strong adaptive, learning, and autonomous capabilities.
Currently, the intelligence level of intelligent mobile robots under development is not high; they can only be considered in their initial stage. The core issues in current research on intelligent mobile robots are twofold: First, improving the autonomy of intelligent mobile robots. This relates to the relationship between intelligent mobile robots and humans, aiming for robots to become more independent of humans and possess a more user-friendly human-machine interface. In the long term, the goal is for operators to simply provide the task, and the machine to automatically generate the steps to complete it. Second, improving the adaptability of intelligent mobile robots, enhancing their ability to adapt to environmental changes. This relates to the relationship between intelligent mobile robots and their environment, aiming to strengthen the interaction between them.
Intelligent mobile robots involve many key technologies that determine their level of intelligence. These key technologies mainly include the following aspects: multi-sensor information coupling technology, which refers to integrating sensor data from multiple sensors to generate more reliable, accurate, or comprehensive information. A fused multi-sensor system can more completely and accurately reflect the characteristics of the detected object, eliminating information uncertainty and improving information reliability; navigation and positioning technology, which requires precise knowledge of the robot's or obstacle's current state and position in autonomous mobile robot navigation, whether for local real-time obstacle avoidance or global planning, to complete tasks such as navigation, obstacle avoidance, and path planning; path planning technology, which involves finding the optimal path from the initial state to the target state in the robot's workspace, avoiding obstacles, based on one or more optimization criteria; robot vision technology, which includes image acquisition, processing and analysis, output and display, with core tasks being feature extraction, image segmentation, and image recognition; intelligent control technology, which improves the robot's speed and accuracy; and human-machine interface technology, which studies how to enable humans to communicate with computers conveniently and naturally.
In the development of intelligent mobile robots worldwide, the United States has consistently led the world in this field. Its technology is comprehensive, advanced, highly adaptable, reliable, feature-rich, and highly accurate. Its artificial intelligence technologies, such as vision and tactile sensing, are widely used in the aerospace and automotive industries. Japan has experienced rapid development in various types of robots, including intelligent mobile robots, thanks to a series of supportive policies. European countries are recognized as world leaders in the research and application of intelligent mobile robots. China started later but has since entered a period of vigorous development, aiming to use robots as a medium to drive the transformation of the entire manufacturing industry and promote the growth of the entire high-tech industry.
2. Development and Application Prospects of Motion Control Technology
With the development of mechatronics technology, countries around the world are paying increasing attention to motion control technology. Various new motion control technologies and products are emerging rapidly. Below are some representative motion control technologies.
2.1 Full Closed-Loop AC Servo Drive Technology
Digital AC servo systems are widely used in mechatronic products that require high positioning accuracy or dynamic response. The driver in this type of servo system samples the position of the photoelectric encoder at the rear end of the motor shaft, forming a closed-loop control system for position and speed between the driver and the motor. This system offers high position control resolution and good reliability.
Typically, servo systems with position loops rely on the encoder of the servo motor for feedback. This means they cannot compensate for backlash and errors in the transmission chain, resulting in a semi-closed-loop position control system. Panasonic recently introduced a more sophisticated, fully digital, fully closed-loop servo system that achieves higher precision. This system overcomes the shortcomings of the semi-closed-loop system. The position loop sampling can be directly obtained from position feedback elements (such as linear encoders, magnetic encoders, and rotary encoders) mounted on the final stage of the machine. The encoder on the motor then only serves as feedback for the speed loop. This eliminates some mechanical backlash and compensates for errors in the mechanical transmission, achieving true full closed-loop functionality and high-precision position control. This system is widely used in high-precision CNC equipment such as CNC machine tools.
2.2 Linear Motor Driving Technology
In recent years, the application of linear motors in machine tool feed servo systems has gained attention in the global machine tool industry. The biggest difference between direct linear motor drive and traditional rotary motor transmission in machine tool feed systems is that the former eliminates all mechanical transmission links between the motor and the worktable, shortening the machine tool feed transmission chain to zero. This transmission method is called zero-drive! It is precisely this zero-drive method that brings superior performance and characteristics that the traditional rotary motor drive method cannot achieve: First, fast response. Because some mechanical transmission components with large response time constants, such as lead screws, are eliminated in the system, the dynamic response performance of the entire closed-loop control system is greatly improved. Second, high precision. The linear drive system eliminates transmission errors caused by mechanical mechanisms such as lead screws, reducing tracking errors caused by transmission system lag during interpolation, thus greatly improving the positioning accuracy of the machine tool. Third, high transmission stiffness. Due to direct drive, the lag caused by elastic deformation, friction wear, and backlash in intermediate transmission links during starting, speed change, and reversal is avoided, greatly improving its transmission stiffness. Fourth, high speed and short acceleration/deceleration processes. Linear motors were first used in maglev trains (speeds up to 8300 m/min). Therefore, using linear motors in machine tool feed drives can fully meet the maximum feed speed (60-100 m/min) of ultra-high-speed cutting. Their high-speed response also significantly shortens acceleration and deceleration processes. Fifth, the stroke length is unlimited. By connecting linear motors in series on the guide rail, the stroke length can be extended indefinitely. Sixth, the movement is quiet. Because mechanical friction from components such as lead screws is eliminated, and the guide rail can be either a rolling guide or a magnetic levitation guide, noise during movement is greatly reduced. Seventh, high efficiency. The absence of intermediate transmission links eliminates energy loss due to mechanical friction, improving transmission efficiency. Therefore, this system is widely used in maglev trains, high-precision machine tools, and other equipment.
Motion control mainly includes position control, speed control, acceleration control, and torque or force control. Based on the power implementation method, motion control can be divided into electric motor drive, pneumatic, and hydraulic systems. Industrial robots primarily use AC servo motor control systems, and the trends in AC servo motion control are essentially the same as those in robot motion control, such as performance improvements, intelligence, modularization, and networking. Service robots, on the other hand, employ more diverse motion control methods and utilize a wider range of technologies.
2.2.1 Applications of motors: mobile robot platforms and small humanoid robot platforms
Motor control is widely used in robots, especially in humanoid robots and mobile platforms, where at least dozens of motors are used, and sometimes hundreds.
2.2.2 Hybrid application of hydraulics and electric motors: Humanoid robots
The robotics competition held by DARPA (Defense Advanced Research Projects Agency) in the United States showcases the most advanced capabilities of humanoid robots. The robots in the competition mainly follow two different technological approaches: purely electric drive and a hybrid hydraulic-electric drive. However, in reality, except for ATLAS, which is hydraulic, all others are electrically driven. The first-place winner in the 2015 competition was a robot from KAIST of South Korea, which used an all-electric solution to drive an 80-kilogram robot, requiring not only a large number of motors but also high power. The second-place winner was Boston Dynamics' ATLAS, which uses a hybrid hydraulic-electric drive, with the main power coming from hydraulic drive, and only some joints using motors due to space constraints.
3. Development Prospects of Motion Control
The motion control market largely depends on the prosperity of downstream machinery manufacturing industries, with the machine tool industry being the largest downstream market for motion control products. According to IHS estimates, in 2015, machine tools (metal cutting machine tools and metal forming machine tools) accounted for over 40% of motion control sales, particularly CNC products, which are highly concentrated in the machine tool industry, with over 80% of CNC product sales originating from this sector. IHS quarterly machinery industry tracking data shows that after two consecutive years of growth, the machine tool industry experienced a decline in 2015, particularly a significant drop in the Asia-Pacific and US markets. China is the world's largest producer and consumer of machine tools. With the slowdown in economic growth and industry investment, both metal cutting and forming machine tools experienced double-digit declines in 2015, severely impacting demand in the motion control market. The US machine tool industry also faced a downturn; industry data shows that machine tool orders in 2015 decreased by 17.4% compared to 2014. Other traditional automation industries, such as rubber, plastics, textiles, and papermaking, also experienced weak demand. Demand remained stable in consumer goods-related downstream industries such as food and beverage and packaging machinery. In addition, the electronics assembly industry related to robots and smartphones performed exceptionally well, strongly supporting the demand for general motion control products.
The global macroeconomic situation and the performance of the machinery and equipment industry are the two main factors that profoundly influence the trend of the motion control market. Data from IHS, which has been tracking the global motion control market for many years, shows that the industry has experienced sharp fluctuations in recent years due to changes in the global economic environment and changes in demand from downstream industries.
The widespread adoption and gradual implementation of concepts such as Industry 4.0, smart manufacturing, and the Industrial Internet of Things (IIoT) have greatly promoted the development of motion control products in areas such as industrial Ethernet, modular and distributed servo drives, and advanced software development. Currently, fieldbus remains the mainstream communication technology for servo drives and controllers, with 53.5% and 79.4% of drives and motion controllers connected to networks via fieldbus in 2015, respectively. With the promotion of Industry 4.0 and the IIoT, there is a growing demand for real-time and efficient communication between industrial equipment. IHS predicts that from 2015 to 2020, the application of industrial Ethernet technology in the motion control market will maintain a high annual growth rate of nearly 20%, gradually replacing fieldbus technology. Among the many industrial Ethernet protocols, PROFINET, led by Siemens, and EtherNet/IP, promoted by Rockwell, will benefit from the two companies' dominant positions in the PLC and motion control markets and maintain strong growth. In addition, EtherCAT technology, led by Beckhoff, has gained increasing recognition from motion control manufacturers in recent years due to its open-source nature, and its growth rate is higher than other communication protocols used in motion control products. IHS predicts that by 2020, over 60% of drives and nearly 30% of motion controllers will use industrial Ethernet technology.
4. Conclusion
The motion control market relies heavily on downstream machinery industries such as machine tools, packaging machinery, robotics, semiconductors, and electronics, and fluctuates significantly with these downstream market conditions. However, with the continuous expansion of industry applications and the deepening of industrial intelligence and networking, the motion control market will move towards a more mature and stable development trajectory.
