Manipulators – Part 7 of 8 Articles
The Robot Continuous Path, Singularities and the Tool Center Point
Robot paths, not endpoints, are key for thermal spray operations. In some point-to-point robotic operations (such as in palletizing), the robot may pause at each end-point in the path. This is unacceptable for many thermal spray operations; therefore, the continuous path is used. The path is continuously and smoothly controlled by the coordinated motion of the robot joints.
| The Robot Continuous Path, Singularities and the Tool Center Point by Plasma Powder and Systems, Inc. Robot paths, not endpoints, are key for thermal spray operations. In some point-to-point robotic operations (such as in palletizing), the robot may pause at each end-point in the path. This is unacceptable for many thermal spray operations; therefore, the continuous path is used. The path is continuously and smoothly controlled by the coordinated motion of the robot joints. In addition to specifying each segment of the path, the programmer needs to select how the robot transitions in moving from one path segment to the next. For a continuous positioning path, the robot approaches a path segment end-point, does not stop at the point and moves along to the next segment. How closely the robot approaches each segment end-point may be defined by specifying a value for each motion command, usually from 0 to 100. When 100 is specified, the robot moves along to the next segment without decelerating, only performing a change in direction at the segment end-point. When 0 is specified, the robot decelerates in order to approach close to the end-point of the segment before moving on to the next path segment. A “Fine” positioning option may also be included for the case where the robot is to move precisely to a segment end-point before proceeding along the next segment. Thermal spray operations normally do not require “Fine” positioning. The singularity must also be considered when teaching linear or circular motion path segments. A singularity occurs when two of the robot axes are parallel. If a moving statement is taught near or through a singularity, the robot may move differently than when taught. When a six-axis robot travels through or near a wrist singularity (axis 4 and 6 inline), motion performance becomes undesirable because 1) axis 4 and axis 6 may each rotate through a large angle, 2) the overall speed of the robot will slow down as a result of limiting the wrist axes joint speed and 3) the path might deviate from the programmed path if motor speed limit is exceeded. The programmer needs to be aware of singularities when developing the motion program and position the robot to avoid singularities. In addition to the end-points of each segment, a path velocity is defined. However, for an articulated robot, each part of the robot may be moving at a different speed. Robot speed is in reference to what part of the robot? This is where the Tool Center Point (TCP) comes into play. The TCP is usually referenced to the final axis of the robot and is defined by the operator. For thermal spray operations, the point of interest is the center of the spray on the part. When the gun is mounted to the robot, the TCP is a point in front of the gun at the stand-off distance required for the coating. It needs to be taught for each gun and for each unique stand-off distance for that gun. Three approaches are generally available for teaching the TCP: the Three-Point Method, the Six-Point Method, and Direct Entry. Direct Entry requires measurements on the mounted gun and can be prone to error. The Three-Point Method is fine for operations where the tool orientation is of minor importance. However, for thermal spray operations, the gun orientation (which way the gun is pointing) is very important. Therefore, the Six-Point Method is recommended. For the Six-Point Method, a “straw” is usually inserted in the gun nozzle and extends in front of the nozzle the required stand-off distance. A point target is located in the spray area and is “touched” by the end of the straw with three different orientations of the gun. The next three points are used to teach the robot the axis of the gun and the up direction for the gun. Once taught, the gun is easily moved along the part during programming. For example, when jogging the robot in the tool coordinate system, only one key is needed to move the gun closer or further from the piece being coated. The next and final article of eight in our Manipulators series addresses “ |