Manipulators – Part 6 of 8 Articles
The Thermal Spray Robot
A key to the success of the articulated robot is the advanced controller software. Thermal spray operators are generally most comfortable with X-Y-Z manipulators as it is difficult to visualize a position based on the angular position of six rotary joints. The advanced robot software addresses this transformation and allows the programmer/ operator to calculate in Cartesian coordinates.
| The Thermal Spray Robot by Plasma Powder and Systems, Inc. Robots used for thermal spray operations are typical of the articulated style with either five or six axes, six being preferable for flexibility. The articulated robot employs rotary joints for motion and functions much like a human arm. In fact, human anatomy terms, such as wrist and arm, are often used to describe robotic operations. A key to the success of the articulated robot is the advanced controller software. Thermal spray operators are generally most comfortable with X-Y-Z manipulators as it is difficult to visualize a position based on the angular position of six rotary joints. The advanced robot software addresses this transformation and allows the programmer/ operator to calculate in Cartesian coordinates. Three different options are provided for the operator to manually move or jog the robot using a joy-stick or series of motion buttons: 1.) Joint coordinate system. The joint coordinate system is used to control individual robot joints. The position and attitude of the robot are defined by the angular position of each joint. The joints are numbered (one through six for a six-axis robot) starting with the base or main body of the robot. The programmer often selects this coordinate system for the initial positioning of the robot. 2.) X-Y-Z or world coordinate system. This coordinate system is fixed in the workspace and is generally set with the origin on axis 1 and at the intersection of the orthogonal plane of axis 2. This coordinate system uses the same nomenclature that was developed for airplanes: X towards the front (forward direction of travel), Y for motion to the left when facing the same direction of the robot, Z for up (altitude) and W (yaw), P (pitch) and R (roll) for the three rotational components. Programmers often select this system for manually moving the robot to selected positions along the part to be coated. 3.) A tool coordinate system is a Cartesian coordinate system that defines the position and the attitude of the thermal spray gun. For this system, the origin (Tool Center Point or TCP) is usually the point of application on the part to be coated. This coordinate system is especially useful when adjusting the position of the thermal spray gun with reference to the part being coated. Note that these coordinate systems are for manually jogging the robot. Once the points and paths are defined for a coating program, the robot will follow the prescribed motion program regardless of the coordinate system used for manually jogging the robot during programming. The coating program will consist of a number of housekeeping tasks (i.e. resetting counters that keep track of the number of passes), discrete commands to sequence the various units (dust collectors, etc.) and a series of motion commands. Motion commands are instructions to move the gun between two points in the coating motion program. Typically, there are three motion command types available: 1.) Joint. All joints rotate at a specified speed and will arrive at the final point at the same time. The path of motion is usually non–linear and the attitude of the gun is not controlled. Therefore, this motion is usually not used during a coating pass. 2.) Linear. The gun path is controlled from one point to the next and along a linear path. The attitude of the gun is also controlled. This is the motion most used in coating operations. 3.) Circular. The circular motion mode controls the path of the gun from a start point to an endpoint through a central or passing point. The next article and seventh in the series references “The Robot Continuous Path, Singularities and the Tool Center Point”. |