How to name the motion axis of industrial robot an

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How to name the motion axis of industrial robots and determine the common coordinate system

with the social progress and the development of science and technology, the application of industrial robots is becoming more and more common. Because of its many kinds, motion axes and coordinate systems, it is easy to make mistakes in determining, especially for novices. This paper mainly introduces the naming of the motion axis of industrial robot, the determination principle of coordinate system and its common types, in order to play a certain reference for the application of industrial robot

1. Robot motion axis

in production, industrial robots generally need to be equipped with peripheral equipment in addition to their own performance characteristics, such as rotary table for rotating workpiece, mobile table for moving workpiece, etc. The motion and position control of these peripheral devices need to match with industrial robots and require corresponding accuracy. Generally, the robot motion axis can be divided into robot axis, base axis and tooling axis according to its function. The base axis and tooling axis are collectively referred to as external axis

robot axis refers to the axis of the operation body, which belongs to the robot itself. At present, most commercial industrial robots are 8-axis. The base axis is the general term for the moving axis of the robot, mainly refers to the walking axis (mobile slide or guide rail). Tooling axis is the general term of axes other than robot axis and base axis, which refers to the axis that turns and rotates the workpiece and tooling fixture, such as return turntable, turnover table, etc. In practical production, 6-joint industrial robots are commonly used, which have 6 movable joints (axes). The attached table and figure 1 show the definitions of common industrial robot body motion axes. It is worth noting that different industrial robot body motion axes have different definitions. The axis used to ensure that the end actuator reaches any position in the workspace is called the basic axis or spindle; The axis used to realize any spatial attitude of the end effector is called wrist axis or secondary axis; Figure 2 shows the relationship between the motion axes of Yaskawa industrial robot

Fig. 1 the motion axes of a typical robot

Fig. 2 the relationship between the motion axes of Yaskawa industrial robot

2 Robot coordinate system determines that the positions of all points in the robot program are associated with the coordinate system, and this coordinate system may also be associated with another coordinate system

various coordinate systems of the robot are determined by the orthogonal right-hand rule, as shown in Figure 3. When rotating around the axes parallel to the X, y and Z axes, they are defined as a, B and C respectively. a. The positive directions of B and C are the right-hand spiral forward direction in the positive directions of X, y and Z respectively (see Figure 4)

Figure 3 right hand coordinate system

Figure 4 rotation coordinate system

the commonly used coordinate systems are absolute coordinate system, machine base coordinate system, mechanical interface coordinate system and tool coordinate system

(1) the absolute coordinate system has nothing to do with the motion of the robot. It is a fixed coordinate system with the earth as the reference system (see Figure 5), and the symbols are o 0, x 0, y 0, Z 0. The origin o 0, +x 0 axis is determined by the user according to needs+ The Z 0 axis is collinear with the vector of gravitational acceleration, but in the opposite direction

(2) the frame coordinate system is a coordinate system with the installation plane of the robot frame as the reference system, and the symbols are o 1, x 1, Y 1, Z 1. The origin o 1 is specified by the robot manufacturer+ Z 1 axis is perpendicular to the mounting surface of the robot base and points to the robot body; The X1 axis direction points from the origin to the projection of the central point CW of the robot workspace (see GB/t12644 2001) on the mounting surface of the base. When this reduction cannot be achieved due to the structure of the robot, the direction of x 1 axis can be specified by the manufacturer

(3) the mechanical interface coordinate system takes the mechanical interface as the reference system, and the symbols are o m, x m, Y M, Z M. The origin o m is the center of the mechanical interface+ The direction of Z m axis is perpendicular to the center of the mechanical interface, and thus points to the end actuator+ The x-m axis is defined by the mechanical interface plane and the X-1, Z-1 planes (or parallel to the X-1, Z-1, Fig. 2, the relationship between the motion axes of the Yaskawa industrial robot, Fig. 3, the right-hand coordinate system, FIG. 4, the rotation coordinate system, FIG. 5, the example of the coordinate system, FIG. 6, the plane of the tool coordinate system) and the intersection of some details when you use it. At the same time, the main and auxiliary joint axes of the robot are in the middle of the range of motion. When the robot structure cannot achieve this approximation, the manufacturer shall specify the position of the main joint axis+ The x m axis points away from the Z 1 axis

(4) the tool coordinate system takes the end actuator installed on the mechanical interface as the reference system (see Figure 6), and the symbols are o t, x T, y t, Z t. The origin o t is the tool center point (TCP)+ ZT axis is related to the tool, which is usually the direction of the tool; When the flat gripper is clamped, +y t is the direction of the finger motion plane

Figure 5 coordinate system example

Figure 6 tool coordinate system

3 The common coordinate system of industrial robots

(1) base coordinate system, also known as base coordinate system, is located on the base of the robot. As shown in Figure 5, it is the coordinate system that is most convenient for robots to move from one location to another. TPL launched the new universal premium system. The base coordinate system has the corresponding zero point in the robot base, which makes the movement of the fixed installation robot predictable. In the normally configured robot system, workers can move the coordinate system through the control lever

(2) world coordinate system, also known as geodetic coordinate system or absolute coordinate system. If the robot is installed on the ground, it is easy to teach and program in the base coordinate system, but when the robot is hoisted, the end movement of the robot is not intuitive, so it is difficult to teach and program

in addition, if two or more robots work together, for example, one is installed on the ground and the other is inverted, the base coordinate system of the inverted robot will also be upside down (see Figure 7). When the motion control is carried out in the base coordinate system a and B of two robots respectively, it is difficult to predict the cooperative motion

Figure 7 world coordinate system

at this time, a common world coordinate system C can be defined to replace it. Unless otherwise specified, the world coordinate system of a single robot coincides with the base coordinate system

(3) user coordinate system (U S E rcoordenate system), the robot can work with different worktables or fixtures, and a user coordinate system is established on each worktable. Most robots adopt the teaching programming method, and the steps are cumbersome. For the same workpiece, if it is placed on different workbench for operation, there is no need to reprogram, but only need to change to the current user coordinate system accordingly. The user coordinate system is established under the base coordinate system or the world coordinate system

(4) the workpiece coordinate system (obje C tcoordinate system) is related to the workpiece, which is usually the most suitable for robot mapping 7 world coordinate system. The workpiece coordinate system corresponds to the workpiece, which defines the position of the workpiece relative to the geodetic coordinate system (or other coordinate systems)

The workpiece coordinate system has specific additional attributes, which are mainly used to simplify programming. It has two frames: the user frame (related to the earth base) and the workpiece frame (related to the user frame). The robot can have several workpiece coordinate systems, representing different workpieces, or several states of the same workpiece in different positions. Programming the robot is to create targets and paths in the workpiece coordinate system. When repositioning the workpiece in the workstation, you only need to change the position of the workpiece coordinate system, and all paths will be updated accordingly. It is allowed to operate the workpiece moved by the outer axis or the transfer guide rail, because the whole workpiece can move with its path

(5) displacement coordinatesystem is also called displacement coordinate system. Sometimes it is necessary to process the same workpiece and the same track at different stations. In order to avoid reprogramming each time, a displacement coordinate system can be defined. The displacement coordinate system is defined based on the workpiece coordinate system. As shown in Figure 8, when the replacement coordinate system is activated by the application characteristics and life characteristics of the tested waste pipe, all points in the program will be replaced

figure 8 displacement coordinate system

(6) wristcoordinate system and tool coordinate system are used to define tool direction. In simple applications, the wrist coordinate system can be defined as the tool coordinate system, and the two coincide. The z-axis of the wrist coordinate system coincides with the 6th axis of the robot, as shown in Figure 9. The stronger the system is, the origin of the coordinate system is located in the center of the end flange. The direction of the x-axis is the same or opposite to that of the identification hole on the flange of figure 8 displacement coordinate system figure 9 wrist coordinate system. The z-axis is vertical outward, and the y-axis conforms to the right-hand rule

Figure 9 wrist coordinate system

(7) toolcoordinate system the tool installed on the end flange needs to define a tool coordinate system at its center point (TCP). Through the transformation of the coordinate system, the robot can be operated under the tool coordinate system to facilitate operation. If the tool is worn or replaced, it is only necessary to redefine the tool coordinate system without changing the program. The tool coordinate system is established in the wrist coordinate system, that is, the relative position and attitude between the two are determined

(8) the joint coordinate system is used to describe the motion of each independent joint of the robot, and the joint types may be different (such as moving joints, rotating joints, etc.). If the robot end is moved to the desired position and operated in the joint coordinate system, each joint can be driven in turn to guide the robot end to the specified position

4. Conclusion

due to the large variety of industrial robots, there are many coordinate systems of each kind of industrial robots. Although there are standards for their naming and determination methods, some manufacturers do not follow the standards and have their own names. It is very confusing and troublesome in actual production and application. This paper introduces the naming of industrial robot coordinate axis and the determination of common coordinate system in detail, in order to help users

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