1) The document discusses experimental setup for robotic machining of hard materials using Comau robots and a C4G controller with integrated compensation controls. 2) CAD models are converted to G-Code using CAM software and further processed in EasyRob to generate robot programs in PDL2 language. 3) On-line algorithms were implemented to adjust machining speed based on robot configurations to improve workpiece quality by governing velocities.

1. Advanced methods for robotic
machining of hard materials
MasterCAM NX G-CODE
EasyRob PDL2 language
C4G controller with PDL2 real-time condition-
handler and override adjustment

2. Introduction
• The removal of materials is one of those least-utilized
techniques within robotic applications.
• Almost all robotic producers offer today robots for
manufacturing applications, however, they are
mainly for robot finishing.
• CAM packages do not consider specific performance
and limitations in robotic system kinematics,
dynamics and control.

3. Experimental set-up
• Comau robots are chosen because
of innovative C4G Open Control
interfaces (access controller (servo
and programming) via UDP/IP in
real time with a rate of 1 ms)
• Algorithms for force, impedance
control and path governor are
running real-time on RT patches
(RTAI, RTNet) on external Linux
(Ubuntu) PC. New commands are
integrated in the standard robot
language (PDL2) to manage and
monitor external control algorithms
at the programming layer

4. Machining off-line planning
• Geometries are designed in a CAD
program or scanned by a 3D scanner,
converted in surface model before
processed by CAM software
• Any standard CAM software can be
used (NX and MasterCAM programs
were applied to generate standard
NC-machine G-Code in this project)
• G-Code is then processed and
simulated by the robot simulation
software (EasyRob) and converted
into PDL2 language after testing
• PDL2 is then executed by the C4G
controller

5. Machining off-line planning
• Before the final generation of robot
program, several specific robot
milling problems (detection and
avoidance of collisions, singular
configuration, restricted work space,
bad manipulability, etc. must be
addressed)
• Algorithms are implemented in C++
and MATLAB and integrated in the
EasyRob environment (by means of
open APIs) as advanced robot
machining Toolbox. The Toolbox
involves kinematic, dynamic and
control robot models (implemented
in MATLAB). Specific robot signature
is taken into account

6. On-line compensation controls
• Despite careful planning, initial
experiments with standardized
parts demonstrate higher
position errors at specific robot
configurations. During direction
changes, one axes remains
jammed while other axes achieve
maximum speed to keep required
Cartesian velocity causing
unaccepted quality of work piece.
• To cope with this problem on-line
algorithm referred to a real-time
velocity path governor is
implemented.

7. On-line compensation controls
• The control algorithm has been
implemented in the C4GOpen
robot control system on the
external PC in C++ and using
language control interface. It is
running at 10 ms and is adjusting
programmed real-time override
within range 5-100% of the
nominal speed using C4GOpen
functions (PDL2 language real-
time condition-handler and real-
time override adjustment
functionalities)
• Blue - without path-governor,
• Red - with path-governor

8. Conclusion
• Very hard material milling (e.g. Inconel, granite) is beyond the scope of
this project. Chattering effects cause additional and dominant errors due
to oscillations caused by elastic effects in robot structure. Path-governor
algorithm becomes insufficient to cope with such kind of errors.
• Robot grinding and polishing operations requires even higher precision
(hundredth of mm) than milling to achieve good quality. Which is beyond
robot positioning accuracy.
• Advanced approaches address an inclusive set of robotic machining
operations providing a systematic methodology to meet requirements of
end-users, imposing “all or nothing” mind-set and requiring complete
solution for hard materials machining by robots, rather than only pre-
machining.