Programming robots may seem straightforward, like a high-tech version of Connect the Dots, where the goal is simply to move from one point to another. However, different motion commands—linear, joint, and circular—affect robot movement in distinct ways, with each designed for specific applications.
Robots have become a vital part of the factory floor, thanks to their flexibility. With the ability to attach various custom grippers and the advent of collaborative robots that don't require safety guarding, robots are increasingly used across a wide range of industries. To program a robot for smooth, quick, and safe movements, it's essential to understand how to direct its motion from one point to another effectively.
The Cartesian Coordinate System ("Frames")
Before diving into motion commands, it's crucial to understand how a robot determines direction. Robots use a Cartesian coordinate system, similar to the graphing systems you encountered in algebra. The Cartesian space is defined by two perpendicular horizontal axes—the x-axis and y-axis—along with the z-axis, which occupies the vertical space, forming a 3D grid, much like a square cardboard box. Each robot operates with a base Cartesian coordinate system, and additional coordinate systems (referred to as "frames" by some manufacturers) are used for the tool, the surrounding work environment, and the part being handled.
It's essential to know which frame is in use when programming the robot's movements. Failing to do so could lead to dangerous outcomes. For instance, if we program a position at x=100, y=0 (100mm directly in front of the robot) and then switch to a coordinate system located 100mm along the y-axis, the robot will move to x=100, y=100 (relative to the base frame). This new position may fall outside the robot's guarding area, potentially causing damage or injury. At the very least, the robot will miss the intended target.
To avoid these issues, always record positions in the correct frame and ensure that the correct frame is active when the robot makes the move.
Types of Robot Movement
Robots primarily use three types of movement to navigate the physical world: linear, joint, and circular. While the objective of each movement is to move from point A to point B, the path taken by the robot differs significantly depending on the type of movement used.
Robot Linear Motion
As the name suggests, a linear move involves the robot traveling in a straight line from its current position to the destination. During this motion, all joints rotate as needed to ensure that the tool center point (TCP) follows the path while maintaining a consistent orientation. Linear motion is commonly used when the robot needs to move from an offset or pounce position to a part, or when entering confined spaces. It is also critical for applications like welding, where precise and controlled linear movement is required.
The main downside of linear motion is that it can be slower than joint motion. To maintain the straight-line path, some joints must rotate more than others, which can lead to slower movement overall. Since each joint has a maximum speed, if a joint must rotate faster than its maximum speed to stay on the path and maintain the desired TCP speed, the overall TCP speed will be reduced to prevent errors or damage. Linear movements are measured in millimeters per second, unlike joint movements, which are expressed as a percentage of the maximum speed, as explained in the next section.
While linear moves offer precision and predictability, relying solely on this type of motion could significantly reduce cycle speed and efficiency.
Robot Joint Motion
A joint move is similar to a linear move with the exception that the robot does not move along a straight path. The robot will move to the destination along a non-linear path - even for very short travel distances. In a joint move, the robot's path can vary, swinging outward in front of itself or inward toward its base. Unlike linear moves, joint moves are less predictable, which is why they are typically used in situations where the robot is in an open space and not confined by safety guards. Since different joints have varying maximum rotational speeds, it's possible for the robot to shift its path when transitioning from a slow test speed to a higher operating speed, relying on the faster base joints to reach higher velocities.
Joint moves are ideal when the robot is moving through open spaces, such as traveling from the home position to a pounce position. As long as the positions and speeds remain constant, the robot will follow the same path every time. If a joint move is determined to be safe, there is no concern that the robot will change its movement unexpectedly. Joint moves are typically much faster than linear moves, which can help reduce cycle times. When a linear move can be safely converted into a joint move, it can significantly improve efficiency and speed.
Robot Arc (Circular) Motion
Arc or circular motion is used to move the robot's TCP along a constant radius, creating a smooth curve. This type of motion is commonly used in applications like welding, where maintaining a consistent TCP orientation while moving around the circle is essential for a smooth path. To complete a full circle, four points are typically required: the start point, first midpoint, first endpoint, and second midpoint, returning to the start point. In some programming languages, this may be represented as two separate arcs, requiring careful measurement of points along the radius. Other languages may allow the use of just three points along the radius, similar to how a 3-point circle is drawn in CAD software.
While a complete circle is not always necessary, arc motion can be used to create a curved path with a defined endpoint. The placement of the points is crucial for the accuracy of the robot's movement. The midpoint should be approximately halfway between the start and end of the arc. If the midpoint is placed too close to one of the endpoints, the robot may end up outside its allowable work envelope when the full arc is calculated.
The Tool Center Point (TCP) or Tool Frame
The Tool Center Point (TCP), also referred to as the tool frame, is a Cartesian coordinate system centered on the robot's end effector. The TCP represents the specific point at which the robot pivots during movement. If this position is not accurately defined, offset commands and calculated positions may not produce the desired results. Additionally, the TCP is critical for determining the speed of the robot's movements. When a speed is specified in the move command, the robot will aim to maintain that speed at the TCP throughout the motion.
Motion Programming in Robotics
Linear, joint, and circular motion commands offer the programmer flexibility in choosing the most appropriate path to accomplish a task. The goal is not merely to move from point A to point B as quickly as possible but to do so in the most effective way for the specific job at hand.
