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A feature of robots, compared to special machines, is the ability to program the robots.
The programming of robots can be a relatively simple or a highly complex operation. This depends upon the type of robot, the nature of the work it has to perform and its interaction with the environment and with peripheral devices. The task is compounded when more than one robot is working within the manufacturing area. A typical example would here be the automotive industry where robots are used for welding and spray painting car bodies
Two basic methods are used to program robots: on-line and off-line programming.
On-line programming is defined as the process of an operator directly interacting with the robot and the robot controller to teach the robot the new program. In on-line programming, the robot cannot be performing any other tasks while the operator is teaching the new program.
Off-line programming the robot program is developed on a computer which is not attached to the robot, so that robot can continue to perform an application. The following sections provide more detail concerning the processes of on-line and off-line programming.
Before proceeding, two terms must be defined: position and step.
A position is a set of numbers within a robot program which define the angles of each of the robot axes when the robot manipulator is at a point in space. The position is used by the robot controller to move each of the robot axes to the correct point during execution of the robot program.
A step is a command which is written in the robot programming language. The step causes the robot system to perform an action other than moving the manipulator. Examples of steps are: ``close the gripper'' and ``wait for the operator to push the start button''. (These example steps are written in English for clarity-the actual steps would be written in the robot programming language.)
On line programming involves teaching the robot both the steps to perform and the positions to move to. In some robots, the positions are separate from the steps, in others programs contain both the steps and the positions. Typically the steps are entered into the robot controller through some type of text editor. The positions can be also entered through the text editor, or can be ``taught'' by moving the robot physically to the desired location.
Two methods are used to teach positions: the lead-through, and the power-teach methods.
In the lead-through method, the robot controller stores all of the positions of the robot as the operator moves the robot through the desired path. This method is used to store complex paths such as the paths used for spray painting. To move the robot, the operator moves a joy-stick, and the robot tracks the motion of the joy-stick.
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In the power-teach method, the operator uses a teach-pendant to move the robot to each position. Buttons on the teach pendant are pushed by the operator to cause single joints to move. Once the robot is at a position, the operator uses the teach pendant to store that position in the memory of the controller by pressing another button on the teach pendant. This method is used for pick-and-place operations, along with all other point-to-point moves of a robot. The teach pendant used to program a robot from ABB is shown in the picture beside. |
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Three levels of off-line programming are in popular use:
In the basic and intermediate off-line programming techniques, a text editor is used to define the steps and positions required. If the system which is used to develop the robot program includes the capability to check the syntax of the program, or determine if the robot can reach all of the positions, then this system can be classified as an intermediate system. Such intermediate systems are available from the robot vendors themselves at an additional cost. After the program is developed using either the basic or intermediate system, the program must be loaded onto the robot and tested before using it for production.
The graphic simulation systems technique graphically portrays the programmed motion of the robot on a computer screen and can be quite helpful. Today when product life cycles are getting shorter and shorter it is very important to minimize the time from design to manufacturing. A case study shows that an Off-Line Programming system would help to shorten this time considerably.
Other motivations for using an OLP system are amongst others:
1) Frees the robot for productive use.
2) Makes it possible for an engineer to evaluate equipment in a work cell and the layout of it. That is, it will assist engineers in making critical resource requisitions.
3) When a proposed work cell has been designed, the entire application may be simulated in order to assess potential risk.
4) Verifying the program, making preliminary confirmation of the cycle times.
5) Such a system would work as a single reference point for multiple level decision-makers within a company. These can then collectively participate in the total work cell design.
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Picture
on the right shown a work cell done in
CimStation.
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| An Irb2000 is used to weld the exhaust system of a car. |
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