Page 295 - Mechatronics for Safety, Security and Dependability in a New Era
P. 295
Ch56-I044963.fm Page 279 Thursday, July 27, 2006 8:22 AM
Page 279
8:22 AM
Ch56-I044963.fm
Thursday, July 27, 2006
279
279
the material to be milled is relatively soft for the robot. However, the amount of points have to be
reduced due to limited performance of the robot controller. This can be seen as an actual velocity of
robot arm.
3.2 Off-line programming
3.2.1 Calibrations of the robot work cell
The fundamental problem of robot systems is a requirement for use of one program in several
robots. Usually it is not possible even if they are manufactured by the same manufacturer and they
should be the same-like, i.e. same model. The reason comes mainly from the manufacturing
deviations between the different manipulators. To overcome these inaccuracies the kinematic model
and coordinate transformations can be corrected and in that way improve the accuracy of the robot
systems. When using robots attached with sensors to different kind of measuring and manipulating
task and when they are off-line programmed, following three calibrations should be done:
calibration of the sensor internal parameters, hand-eye calibration and calibration of the robot.
3.2.2 Managing with various size of "workpieces
Machining of large work pieces is carried out by splitting the paths and CAD models of the work
pieces. This is carried out by geometrical information of the robot workcell, i.e. the reachability of
the manipulator arm with tools used in the cell. By using this set-up, the full performance of the
robot workcell can be used. Using the classical approach, a very large robot would have been
chosen. However, the disadvantage of this way is lack of accuracy because the larger robot you
choose, the more inaccurate it is. By using medium-size robot, it is a compromise of accuracy but
we can still manufacture large work pieces up to several meters. After splitting the paths and CAD
information, paths are optimized for proper robot respectively. In the optimization phase, path is
converted into form that movements of the robot are minimized between the points in the path. The
optimization includes reach and collision check to prevent the unexpected situations.
3.3 Actual machining
3.3.1 Localization of work piece in robot workcell
To be able to manage the split pieces, each piece has to localize very carefully. In the localization,
we use methods presented in previous paper Sallinen & Heikkila (2000). The idea is to fit the
measured points to the reference model of the work piece. Method is fast and flexible to use. The
method is open for different surface forms including plane surface, cylinder surface and spherical
surface. The localization is here carried out using touch sensor which is rather reliable to use in
foundry environment.
3.3.2 Robotic Milling
The postprocessor outputs the programs ready-to-run in the robot including commands for running
of spindles, tools and tool changers. Depending on the equipments of the robot, the machining
parameters such as velocity, rotation speed of the tool and length of the path has to be find out.
These depend also from the sand material in a terms of size of the grains and hardening material.
To solve these parameters, a certain test-run has to be go trough to find out the best surface quality
which is one of the main objectives of the system. The inaccuracies coming from the flexibility of