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Ch56-I044963.fm  Page 279  Thursday, July 27, 2006  8:22 AM
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                    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
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