Industrial robots can only cope with various work processes if they are programmed, such as spraying, welding, and palletizing. All of these cannot be achieved without programming. Robots without programming are like uncut knives.
When it comes to programming, you need to know the coordinate system of industrial robots. Industrial robots rely on coordinates to locate the desired position. For example, we need the end axis of the robot, and when walking from A to B, we need to give it a coordinate.
Coordinates are quite complex, mainly because there are too many types.
The commonly used coordinate systems for industrial robots include the base coordinate system, world coordinate system, tool coordinate system, workpiece coordinate system, user coordinate system, joint coordinate system, etc.
The base coordinate system and world coordinate system are fixed coordinate systems, the tool coordinate system and workpiece coordinate system are moving coordinate systems, and the joint coordinate system and flange coordinate system are used to describe joint motion and tool posture.
The choice of different coordinate systems depends on specific task requirements; for example, welding requires a tool coordinate system, assembly requires a workpiece coordinate system, and multi-robot collaboration relies on the geodetic coordinate system.
The coordinate system of a robot itself is very complex, and this article mainly discusses the most commonly used coordinates of the Braun robot: world coordinates and joint coordinates.
The teaching pendant of this Braun robot, where the W/J button can quickly help switch between world coordinates and joint coordinates.
World coordinate system
The world coordinate system, also known as the geodetic coordinate system, is a coordinate system that is mostly consistent with the base coordinates. It is a standard Cartesian coordinate system fixed in space, usually with the fixed position of the robot unit or workstation as the origin. Its zero point is located at a fixed position on the robot's foot or workstation, used to describe the absolute position of the robot in three-dimensional space.
The base coordinate system is a coordinate system fixed on the robot base and serves as the reference origin for the robot's motion. When the robot is inverted, the world coordinate system and the base coordinate system become inconsistent.
All other coordinate systems (such as base coordinate system, workpiece coordinate system, tool coordinate system, etc.) are directly or indirectly related to the world coordinate system.
The world coordinate system is usually represented by the X, Y, and Z axes, and the values are obtained by adding the linkage parameters (geometric parameters of the mechanical structure) of each joint of the robot, which is used to indicate which point in space the robot is located at.
Of course, theories are quite complex. In practical operation, we only need to enable the world coordinate system mode in the teaching pendant, start recording from point a of the robot, operate the robot to move to the desired point b and mark it, and the robot can move in the desired direction.
Joint coordinate system
The joint coordinate system is a coordinate system set in the joints of a robot, where each joint corresponds to an independent coordinate system and its motion is described by rotation axes (X, Y, Z). The degree of joint rotation is based on the origin of the joint coordinate system. The origin of the joint coordinate system is related to the numerical value of the motor encoder. The system will record the encoder value of a state as the origin, and in this state, the joint coordinate values are all 0.
Will the origin be lost after a power outage? The answer is no. For example, the Braun robot uses an absolute value encoder motor, which is powered by a battery when the power is off. After restarting, the system will read the absolute encoder value of the motor from memory to ensure that the origin is not lost.
The advantage of using a joint coordinate system is that when it is necessary to manually adjust the robot's posture (such as bypassing obstacles or fine-tuning the end angle), we can directly control the angles of each joint (such as J1 rotating 30 °, J2 raising 45 °), instead of specifying the XYZ position of the target point. This method is more flexible and is commonly used for teaching, complex posture adjustment, or fault recovery.
The coordinate system of a robot may seem complex, but in essence, it is constantly developed and tested to better adapt to the task. Each coordinate system has its advantages and disadvantages.
For example, if a robot requires complex posture adjustment (such as bypassing obstacles), a joint coordinate system would be more convenient as it directly controls the robot's joint angles (J1-J6), making it suitable for adjusting posture, obstacle avoidance, teaching, and other scenarios. If it is only linear motion, then the world coordinate system is more efficient as it can directly control the end position (XYZ), suitable for precise point motion.