4、 Key Technologies of Industrial Robots 1. Basic System Composition of Robots Industrial robots consist of three major parts and six subsystems.
The three major parts are the mechanical part, sensing part, and control part.
The six subsystems can be divided into mechanical structure system, driving system, perception system, robot environment interaction system, human-machine interaction system, and control system.
The mechanical structure system of industrial robots consists of three main parts: the base, arms, and end effectors. Each large component has a mechanical system with several degrees of freedom.
If the base has a walking mechanism, it constitutes a walking robot; If the base does not have a walking and bending mechanism, it constitutes a single robot arm.
The arm is generally composed of the upper arm, lower arm, and wrist.
The end effector is an important component directly mounted on the wrist, which can be a second-hand or multi finger gripper, as well as a spray gun, welding tool, and other work tools.
2) The driving system, in order to make the robot operate, requires the placement of transmission devices on each joint, i.e. each degree of freedom of motion, which is the driving system.
The driving system can be hydraulic transmission, pneumatic transmission, electric transmission, or a combination of them to apply a comprehensive system. It can be directly driven or indirectly transmitted through mechanical transmission mechanisms such as synchronous belts, chains, gear trains, harmonic gears, etc.
3) The perception system consists of internal sensor modules and external sensor modules, which are used to obtain meaningful information in the internal and external environmental states.
The use of intelligent sensors has improved the mobility, adaptability, and intelligence level of robots.
The human sensory system is extremely agile in perceiving information from the external world. However, for some special information, sensors are more effective than the human sensory system.
4) The robot environment exchange system is a system for modern industrial robots to exchange and coordinate with equipment in the external environment.
Industrial robots are integrated with external devices into a functional unit, such as processing units, welding units, assembly units, etc.
Of course, it can also be a functional unit that integrates multiple robots, machine tools or equipment, and multiple parts storage devices to perform complex tasks.
5) The human-machine exchange system is a device that operators control and communicate with robots, such as the standard terminal of a computer, command console, information display board, danger signal alarm, etc.
The system can be classified into two categories: instruction giving devices and information display devices.
6) The robot control system is the brain of the robot, which is the main factor determining the functionality and performance of the robot.
The task of the control system is to control the execution mechanism of the robot to complete the specified movements and functions based on the robot’s operation instructions and signals feedback from sensors.
If the industrial robot does not have information feedback characteristics, it is an open-loop control system; If it has information feedback characteristics, it is a closed-loop control system.
According to control principles, control systems can be divided into program control systems, adaptive control systems, and artificial intelligence control systems.
According to the form of control operation, the control system can be divided into point control and trajectory control.
The point type only controls the accurate positioning of the actuator from one point to another, and is suitable for machine tool loading and unloading, spot welding, and general handling, loading and unloading operations; Continuous trajectory type can control the actuator to move according to the given trajectory, suitable for continuous welding and painting operations.
The task of the control system is to control the execution mechanism of the robot to complete the specified movements and functions based on the robot’s operation instructions and signals feedback from sensors.
If the industrial robot does not have information feedback characteristics, it is an open-loop control system; If it has information feedback characteristics, it is a closed-loop control system.
According to control principles, control systems can be divided into program control systems, adaptive control systems, and artificial intelligence control systems.
According to the form of control operation, the control system can be divided into point control and trajectory control.
A complete set of industrial robots includes the robot body, system software, control cabinet, peripheral mechanical equipment, CCD vision, fixtures/grippers, peripheral equipment PLC control cabinet, and teaching pendant/teaching box.
The following focuses on introducing the driving system and perception system of industrial robot equipment.
2. The driving system of industrial robots is divided into three categories based on power sources: hydraulic, pneumatic, and electric.
According to needs, these three basic types can also be combined to form a composite driving system.
These three types of basic driving systems each have their own characteristics.
Hydraulic drive system: As hydraulic technology is a relatively mature technology.
It has the characteristics of high power, large force (or torque) to inertia ratio, fast response, and easy implementation of direct drive.
Suitable for application in robots with high load-bearing capacity, large inertia, and working in welding resistant environments.
However, hydraulic systems require energy conversion (converting electrical energy into hydraulic energy), and in most cases, speed control uses throttling speed regulation, which is less efficient than electric drive systems.
The liquid leakage of hydraulic systems can cause environmental pollution and high working noise.
Due to these weaknesses, in recent years, electric systems have often replaced robots with loads below 100kg.
The fully hydraulic heavy-duty robot developed by Qingdao East China Construction Machinery Co., Ltd. is shown in the figure.
Its large-span load-bearing capacity can reach 2000kg, and the robot’s activity radius can reach nearly 6m, which is applied in the casting and forging industry.
The pneumatic drive of fully hydraulic heavy-duty robots has advantages such as fast speed, simple system structure, convenient maintenance, and low price.
However, due to the low working pressure of the pneumatic device and its difficulty in precise positioning, it is generally only used for driving the end effector of industrial robots.
Pneumatic grippers, rotating cylinders, and pneumatic suction cups can be used as end effectors for gripping and assembling workpieces with medium and small loads.
The pneumatic suction cup and pneumatic robot gripper are shown in the figure.
Pneumatic suction cups and pneumatic robot gripper motors are a mainstream driving method for modern industrial robots, divided into four categories of motors: DC servo motors, AC servo motors, stepper motors, and linear motors.
DC servo motors and AC servo motors adopt closed-loop control and are generally used for high-precision and high-speed robot drive; Stepper motors are used in situations where precision and speed requirements are not high, and open-loop control is adopted; Linear motors and their drive control systems have become increasingly mature in technology and have superior performance that traditional transmission devices cannot match, such as adapting to very high and very low speed applications, high acceleration, high precision, no idle return, small wear, simple structure, and no need for reducers and gear screw couplings.
Given the significant demand for linear drive in parallel robots, linear motors have been widely used in the field of parallel robots.
3. The robot perception system transforms various internal state and environmental information of the robot from signals to data and information that can be understood and applied by the robot itself or between robots. In addition to sensing mechanical quantities related to its own working state, such as displacement, velocity, acceleration, force, and torque, visual perception technology is an important aspect of industrial robot perception.
The visual servo system uses visual information as feedback signals to control and adjust the position and posture of the robot.
This application is mainly reflected in the semiconductor and electronics industries.
Machine vision systems have also been widely used in various aspects such as quality inspection, workpiece recognition, food sorting, and packaging.
Usually, robot visual servoing control is based on position based visual servoing or image-based visual servoing, also known as 3D visual servoing and 2D visual servoing, respectively. These two methods have their own advantages and applicability, but also have some shortcomings. Therefore, some people have proposed the 2.5D visual servoing method.
A position based visual servo system utilizes camera parameters to establish a mapping relationship between image information and the position/pose information of the robot’s end effector, achieving closed-loop control of the robot’s end effector position.
The position and attitude errors of the end effector are estimated by extracting the position information of the end effector from real-time captured images and the geometric model of the positioning target. Then, based on the position and attitude errors, new pose parameters for each joint are obtained.
Position based visual servoing requires the end effector to always be observable in the visual scene and calculate its three-dimensional position and posture information.
Eliminating interference and noise in images is the key to ensuring accurate calculation of position and attitude errors.
A two-dimensional visual servo compares the features of an image captured by a camera with a given image (not three-dimensional geometric information) to obtain an error signal.
Then, by adjusting the joint controller, visual controller, and the current operation status of the robot, the robot completes servo control.
Compared to 3D visual servoing, 2D visual servoing has strong robustness to camera and robot calibration errors. However, in the design of visual servoing controllers, it is inevitable to encounter problems such as singularity of image Jacobian matrix and local minima.
Regarding the limitations of 3D and 2D visual servo methods, F Chaumet et al. proposed a 2.5-dimensional visual servo method.
It decouples the closed-loop control of camera translational displacement and rotation, and based on image feature points, reconstructs the orientation and imaging depth ratio of objects in three-dimensional space. The translational part is represented by the coordinates of feature points on the image plane.
This method can successfully combine image signals with pose signals extracted from images, and integrate the error signals generated by them for feedback, greatly solving problems such as robustness, singularity, and local minima.
However, there are still some issues that need to be addressed in this method, such as how to ensure that the reference object is always within the camera’s field of view during the servo process, and the existence of non unique solutions when decomposing the homography matrix.
When establishing a visual controller model, it is necessary to find a suitable model to describe the mapping relationship between the robot’s end effector and camera.
The method of image Jacobian matrix is widely used in the field of robot visual servo research.
The Jacobian matrix of an image is time-varying, so it needs to be calculated or estimated online.
4. The key basic component of the robot is the robot, which consists of four major components. The body cost accounts for 22%, the servo system accounts for 24%, the reducer accounts for 36%, and the controller accounts for 12%.
The key basic components of a robot refer to the components that make up the robot’s transmission system, control system, and human-machine interaction system, which play a crucial role in the performance of the robot and have universality and modularity.
The key basic components of robots are mainly divided into the following three parts: high-precision robot reducer, high-performance AC/DC servo motor and driver, high-performance robot controller, etc.
1) The reducer is a key component of robots, and currently two types of reducers are mainly used: harmonic gear reducers and RV reducers.
The harmonic transmission method was developed by American inventor C Walt Musser was invented in the mid-1950s.
The harmonic gear reducer mainly consists of three basic components: a wave generator, a flexible gear, and a rigid gear. By relying on the wave generator, the flexible gear generates controllable elastic deformation and meshes with the rigid gear to transmit motion and power. The single-stage transmission speed ratio can reach 70-1000, and with the help of the flexible gear deformation, reverse backlash free meshing can be achieved.
Compared with a general reducer, when the output torque is the same, the volume of a harmonic gear reducer can be reduced by 2/3, and the weight can be reduced by 1/2.
The flexible wheel bears a large alternating load, therefore its material has high requirements for fatigue strength, processing and heat treatment, and its manufacturing process is complex. The performance of the flexible wheel is the key to high-quality harmonic gear reducers.
The transmission principle of harmonic gear reducer was proposed by German Lorenz Baraen in 1926, and the RV reducer was first developed by Japanese company TEIJINSEIKI Co., Ltd. in the 1980s.
The RV reducer consists of a front stage of a planetary gear reducer and a rear stage of a cycloidal pinwheel reducer.
Compared to harmonic gear reducers, RV reducers have better rotational accuracy and accuracy retention.
Chen Shixian invented the live tooth transmission technology for the gearbox.
The fourth generation of active tooth transmission – full rolling active tooth transmission (ORT) has been successfully applied to various industrial products.
The Compound Oscillary Roller Transmission (CORT) proposed on the basis of ORT not only has the advantages similar to RV transmission, but also overcomes the disadvantages of RV transmission crankshaft bearing capacity and low lifespan, further improving the service life and bearing capacity; The structure of CORT results in smaller backlash, higher motion accuracy and stiffness under the same precision index, alleviating the defect of RV transmission requiring high manufacturing accuracy, which can relatively reduce machining requirements and manufacturing costs.
CORT is independently developed in China and has independent intellectual property rights.
Anshan Wear resistant Alloy Research Institute and Zhejiang Hengfengtai Reducer Manufacturing Co., Ltd. have both successfully developed CORT reducers for robots.
ORT reducer CORT reducer currently has a 75% market share in high-precision robot reducers, with two Japanese gear companies monopolizing the market, namely Nabtesco from Japan, which provides RV cycloidal pinwheel reducers, and Harmonic Drive from Japan, which provides high-performance harmonic reducers.
The reducers of international mainstream robot manufacturers, including ABB, FANUC, KUKA, and MOTOMAN, are all provided by these two companies. Unlike the general models chosen by domestic robot companies, international mainstream robot manufacturers have signed strategic cooperation relationships with these two companies, and most of the products provided are specialized models that have been improved based on the special requirements of each manufacturer on the basis of the general models.
The research on high-precision cycloidal pinwheel reducers started relatively late in China, and only in some universities, all relevant studies have been conducted.
At present, there are no mature products applied to industrial robots.
In recent years, some domestic manufacturers and universities have begun to focus on the localization and industrialization research of high-precision cycloidal pinwheel reducers, such as Zhejiang Hengfengtai, State Key Laboratory of Mechanical Transmission at Chongqing University, Tianjin Reducer Factory, Qinchuan Machine Tool Factory, Dalian Railway University, etc.
In terms of harmonic reducers, there are already substitute products in China, such as Beijing Zhongji Kemei and Beijing Harmonic Transmission Institute. However, there is still a considerable gap between the corresponding products and Japanese products in terms of input speed, torsion height, transmission accuracy, and efficiency. The mature application in industrial robots is still in its early stages.
The performance comparison of mainstream high-precision harmonic reducers for industrial robots at home and abroad is shown in the table below.
Table 1 Performance Comparison of Mainstream High Precision Harmonic Reducers Note: The comparison data in the above table comes from similar models: HD: CSF-17-100, Zhongji Kemei: XB1-40-100. Transmission efficiency test conditions: input speed 1000r/min, temperature 40 °. Torsion stiffness test conditions: 20% rated torque. The performance comparison of mainstream high-precision cycloidal pinwheel reducers for domestic and foreign industrial robots is shown in the table below.
Table 2 Performance Comparison of Mainstream High Precision RV Cycloidal Pinwheel Reducers Note: The comparison data in the above table comes from similar models: RV: 100CCYCLO: F2CF-C35. Transmission efficiency test conditions: output speed of 15r/min, rated torque. 2) In terms of servo motor and drive, currently the drive part of European robots is mainly provided by companies such as Rentz, Lust, Bosch Rexroth, etc. These European motors and drive components have overload capacity and good dynamic response, The driver has strong openness and a bus interface, but it is expensive.
The key components of Japanese industrial robots are mainly provided by companies such as Yaskawa, Panasonic, and Mitsubishi. Although their prices are relatively low, their dynamic response ability is poor, their openness is poor, and most of them only have analog and pulse control methods.
In recent years, China has also carried out basic research and industrialization of high-power AC permanent magnet synchronous motors and drive parts, such as Harbin Institute of Technology, Beijing Helishi, Guangzhou CNC and other units, and has some production capacity. However, its dynamic performance, openness, and reliability still need to be verified by more practical robot project applications.
3) In terms of robot controllers, currently mainstream foreign robot manufacturers have independently developed controllers based on a universal multi axis motion controller platform.
At present, the commonly used multi axis controller platforms are mainly divided into motion control cards with embedded processors (DSP, POWER PC) as the core and PLC systems with industrial control computers and real-time systems as the core, represented by Delta Tau’s PMAC card and Beckhoff’s TwinCAT system.
In terms of motion control cards in China, Gugao Company has developed corresponding mature products, but their application in robots is still relatively limited.
5. A universal robot operating system (ROS) is a standardized construction platform designed for robots, allowing every robot designer to use the same operating system for robot software development.
ROS will promote the development of the robotics industry towards hardware and software independence.
The independent development model of hardware and software has greatly promoted the development and rapid progress of PC, laptop, and smartphone technology.
The development difficulty of ROS is greater than that of computer operating systems. Computers only need to handle some well-defined mathematical operations, while robots need to face more complex practical motion operations.
ROS provides standard operating system services, including hardware abstraction, underlying device control, common function implementation, inter process messaging, and packet management.
ROS is divided into two layers, with the lower layer being the operating system layer and the higher layer being various software packages contributed by the user group to achieve different functions of robots.
The existing robot operating system architecture mainly includes an open-source operating system based on Linux and Ubuntu.
In addition, institutions such as Stanford University, MIT, and the University of Munich in Germany have developed various ROS systems.
The Microsoft robotics development team also launched a Windows robot version in 2007.
6. In order to improve work efficiency and enable robots to complete specific tasks in the shortest possible time, there must be a reasonable motion planning for robots.
Offline motion planning can be divided into path planning and trajectory planning.
The goal of path planning is to make the distance between the path and obstacles as far as possible and the length of the path as short as possible; The main purpose of trajectory planning is to minimize the running time or energy of the robot during joint space movement.
On the basis of path planning, trajectory planning incorporates time series information to plan the speed and acceleration of the robot during task execution, in order to meet the requirements of smoothness and speed controllability.
Teaching reproduction is one of the methods to achieve path planning. Teaching is carried out through the operating space and the teaching results are recorded, which are reproduced during the working process. On site teaching directly corresponds to the actions that the robot needs to complete, making the path intuitive and clear.
The disadvantage is that it requires experienced operators and consumes a lot of time, and the path may not be optimal.
To solve the above problems, a virtual model of the robot can be established, and path planning for job tasks can be completed through virtual visualization operations.
Path planning can be carried out in joint space.
Gasparetto uses a fifth degree B-spline as the interpolation function for joint trajectories, and optimizes the integration of the square of the acceleration relative to the motion time as the objective function to ensure smooth joint motion.
Liu Songguo interpolated the joint trajectory of the robot using a fifth order B-spline, and the velocity and acceleration endpoint values of each joint of the robot can be configured arbitrarily according to smoothness requirements.
In addition, trajectory planning in joint space can avoid singularity issues in the operating space.
Huo et al. designed a joint trajectory optimization algorithm to avoid singularity in joint space, utilizing the redundancy of a joint function in a 6-degree-of-freedom arc welding robot during the task process. The robot singularity and joint constraints were used as constraints, and the TWA method was used for optimization calculation.
The comparison between joint space path planning and operation space path planning has the following advantages: ① avoiding the singularity problem of robots in the operation space; ② Due to the fact that the motion of robots is controlled by joint motors, a large amount of forward and inverse kinematics calculations are avoided in joint space The trajectory of each joint in the joint space is optimized for easy control.
5、 Industrial robots can be classified into the following types according to different methods: Industrial robot classification 1. From the perspective of mechanical structure, it is divided into series robots and parallel robots.
1) The characteristic of a series robot is that the motion of one axis will change the coordinate origin of another axis. In terms of position solving, the forward solution of the series robot is easy, but the reverse solution is very difficult; 2) Parallel robots use parallel mechanisms, and the motion of one axis does not change the coordinate origin of the other axis.
Parallel robots have the advantages of high stiffness, stable structure, high load-bearing capacity, high micro motion accuracy, and low motion load.
Its positive solution is difficult, but its reverse solution is very easy.
Series robots and parallel robots are shown in the figure.
Series robot parallel robot 2. Industrial robots are divided into the following categories according to the coordinate form of the manipulator: (The coordinate form refers to the form of the reference coordinate system taken by the manipulator’s arm during motion.).
）1) The motion part of a Cartesian industrial robot consists of three mutually perpendicular linear movements (i.e. PPP), and its workspace shape is rectangular.
Its movement distance in various axes can be directly read on various coordinate axes, with strong intuitiveness, easy programming calculation of position and posture, high positioning accuracy, no coupling control, simple structure, but the body occupies a large space volume, has a small action range, poor flexibility, and is difficult to coordinate with other industrial robots.
2) The motion form of cylindrical coordinate industrial robots is achieved through a motion system composed of one rotation and two movements. The workspace shape is cylindrical. Compared with Cartesian coordinate industrial robots, under the same workspace conditions, the body occupies a small volume and has a large range of motion. Its position accuracy is second only to Cartesian coordinate robots, making it difficult to coordinate with other industrial robots.
3) Spherical coordinate industrial robot, also known as polar coordinate industrial robot, consists of two rotations and one linear movement (RRP, one rotation, one pitch, and one telescopic motion) for its arm movement. Its workspace is a sphere, which can perform up and down pitch actions and grasp coordinated workpieces on the ground or at low positions. Its position accuracy is high, and the position error is proportional to the arm length.
4) Multi joint industrial robots, also known as rotary coordinate industrial robots, have arms similar to the human upper limb. The first three joints are the rotary joint (RRR), which is generally composed of a column and a large and small arm. The column and the large arm form a shoulder joint, and the large arm and small arm form an elbow joint, which can make the large arm perform rotary motion and pitch swing, and the small arm perform pitch swing.
Its structure is the most compact, flexible, occupies the smallest area, and can coordinate with other industrial robots, but its position accuracy is low, there are balance problems, and control coupling. This type of industrial robot is becoming increasingly widely used.
5) A planar joint type industrial robot adopts one moving joint and two rotating joints (i.e. PRR), which achieve up and down motion, while the two rotating joints control forward, backward, left and right motion.
This form of industrial robot is also known as SCARA (Selective Compliance Assembly Robot Arm) assembly robot.
In the horizontal direction, it has flexibility, while in the vertical direction, it has greater rigidity.
It has a simple structure and flexible movements, and is commonly used in assembly operations. It is particularly suitable for the insertion and assembly of small-sized parts, such as in the electronic industry.
3. Industrial robots can be divided into two types based on program input methods: programming input type and teaching input type: 1) programming input type is the process of transmitting pre programmed job program files on a computer to the robot control cabinet through RS232 serial port or Ethernet communication methods.
2) There are two types of teaching methods for teaching input: teaching through a teaching box and teaching by the operator directly leading the execution mechanism.
The teaching box is taught by the operator using a manual controller (teaching box) to transmit command signals to the driving system, allowing the executing mechanism to perform the required action sequence and motion trajectory once.
Industrial robots that use teaching boxes for teaching are commonly used. Generally, industrial machines are equipped with teaching box teaching functions. However, for situations with complex work trajectories, teaching box teaching cannot achieve the desired effect, such as painting robots used for painting complex surfaces.
The robot teaching box is taught by the operator directly leading the execution mechanism, which is performed once according to the required action sequence and motion trajectory.
During the teaching process, the information of the working program is automatically stored in the program memory. When the robot operates automatically, the control system detects the corresponding information from the program memory and transmits the instruction signal to the driving mechanism, allowing the executing mechanism to reproduce various actions of the teaching.
6、 The performance evaluation indicators of industrial robots represent the basic parameters and performance indicators of robot characteristics, mainly including workspace, degrees of freedom, payload, motion accuracy, motion characteristics, dynamic characteristics, etc.
Core research& Nbsp; Core research; Robot system integration for the development and application of special construction robots& Nbsp; Robot system integration • End effector and fixture design • On site industrial control software and information exchange • Offline programming simulation and production line virtual design • On site sensing/measurement and control • Advanced manufacturing process integration application
