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Robot machining: Recent development and future research issues

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Early studies on robot machining were reported in the 1990s. Even though there are continuous worldwide researches on robot machining ever since, the potential of robot applications in machining has yet to be realized. In this paper, the authors will first look into recent development of robot machining. Such development can be roughly categorized into researches on robot machining system development, robot machining path planning, vibration/chatter analysis including path tracking and compensation, dynamic, or stiffness modeling. These researches will obviously improve the accuracy and efficiency of robot machining and provide useful references for developing robot machining systems for tasks once thought to only be capable by CNC machines. In order to advance the technology of robot machining to the next level so that more practical and competitive systems could be developed, the authors suggest that future researches on robot machining should also focus on robot machining efficiency analysis, stiffness map-based path planning, robotic arm link optimization, planning, and scheduling for a line of machining robots.
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ORIGINAL ARTICLE
Robot machining: recent development and future
research issues
Yonghua Chen &Fenghua Dong
Received: 5 March 2012 /Accepted: 29 July 2012 /Published online: 12 August 2012
#The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract Early studies on robot machining were reported
in the 1990s. Even though there are continuous worldwide
researches on robot machining ever since, the potential of
robot applications in machining has yet to be realized. In
this paper, the authors will first look into recent develop-
ment of robot machining. Such development can be roughly
categorized into researches on robot machining system de-
velopment, robot machining path planning, vibration/chatter
analysis including path tracking and compensation, dynam-
ic, or stiffness modeling. These researches will obviously
improve the accuracy and efficiency of robot machining and
provide useful references for developing robot machining
systems for tasks once thought to only be capable by CNC
machines. In order to advance the technology of robot
machining to the next level so that more practical and
competitive systems could be developed, the authors sug-
gest that future researches on robot machining should also
focus on robot machining efficiency analysis, stiffness map-
based path planning, robotic arm link optimization, plan-
ning, and scheduling for a line of machining robots.
Keywords Robot machining .NC path planning .
Machining efficiency .Industrial robots .Joint stiffness
1 Introduction
Modern industries are heavily dependent on robots that have
a wide range of applications such as material transfer, pre-
cision assembly, welding, and machining [14]. Statistical
data from International Federation of Robotics has shown
that there is a steady increase in annual robotic sales except
2009 when the financial tsunami had seriously hit the world
economy [1]. In 2011, the annual industrial robot sale was
estimated to reach 139,300 units, making the worldwide
population of operational industrial robots to reach
1,035,000 units. However, this data is dwarfed by the recent
reports about ambitious plans in Asian industrial companies
to rapidly increase their industrial robots population. For
example, there were many media reports in 2011 about an
Asian company Foxconn [2] who is going to install one
million industrial robots in the next 3 years. Foxconn Tech-
nology [2] is a subcontractor for worlds leading electronics
product companies such as Apple, Microsoft, etc. It employs
over one million production workers in China. Due to
dozens of suicidal death in a single year in 2010 inside its
factories (most were bored due to routine assembly line
work), the company announced an ambitious plan to devel-
op and install over one million robotic arms in the next
3 years. Even though most of the robots will be used in
operations traditionally performed by robots, some
machining-related operations are also expected. This will
definitely boost the applications of robots in machining.
This instance reflects that the future growth of industrial
robots will be even more dramatic when emerging countries
start to automate their factories.
According to a white paper published by The Robotic
Industries Association in 2009 [3], robotic machining prod-
ucts and services constitute less than 5 % of existing robotic
sales, but was seen as a growth application segment over the
next 35 years. Applications involve the pre-machining of
parts made from harder materials, with robots performing
various processes at lower tolerances. It was believed that
robotic machining could not replace computer numerical
control (CNC) machining for three to four axis applications,
but is currently viewed as an immediate viable alternative
Y. Chen (*):F. Dong
Department of Mechanical Engineering,
The University of Hong Kong,
Pokfulam, Hong Kong, China
e-mail: yhchen@hku.hk
Int J Adv Manuf Technol (2013) 66:14891497
DOI 10.1007/s00170-012-4433-4
tool for non-metallic materials and for metals depending on
the degree of hardness, required surface finish, and part
complexity.
The white paper also revealed that the barrier to more
widespread adoption of robot machining was a general lack
of knowledge by the end-user community regarding the
capabilities and advantages of robots in machining applica-
tions. A significant effort is therefore required to educate
end-users on the capabilities of robot machining before
significant increases in robotic machining applications are
realized. Realizing the potential of robotic machining, the
worlds leading industrial robot manufacturers are starting to
provide machining robots together with relevant software
packages [58]. Even traditional computer-aided design
(CAD)/computer-aided manufacturing (CAM) software
developers such as Delcam [9] have incorporated offline
CAD-based robot machining capabilities into their tradition-
al CAD/CAM software packages.
Before being applied to direct machining, robot arms
have been used to machining-related jobs. Some studies
have shown that robots can perform well in polishing
[1012], grinding [1316], and deburring [17,18]. The
major purpose of polishing is to generate a glossy or smooth
surface, not to modify a parts dimensions. Polishing tools
are often soft or flexible, thus positional accuracy require-
ment is not very high. This has created an excellent oppor-
tunity for robots to excel in polishing operations because an
articulated robot arm can easily position the polishing tool to
any positions that are needed. It was argued that robot
grinding and polishing could produce surface quality better
than that from three axis CNC milling machines [18,19].
The better surface roughness from robot grinding/polishing
(0.52 μm against 1.30 μm in three-axis milling) is mainly
due to the ability of the robot to easily change the orientation
of the tool, therefore, it can always keep the tool normal to
the polished surface.
As for milling operations, many studies have reported
mixed results showing that many improvements must be
made before robot milling could be readily applied to mill-
ing operations. It is interesting to notice that articulated
robots have some problems such as low repeatability, yet
the robots were first successfully applied to the finishing
operations of machining (polishing and grinding) where part
surface quality requirements are high. This may be partly
explained by the material removal rate. At the rough cutting
(or milling) and finish cutting stages, a large amount of
material must be removed. This will subject the robot arm
to a large load yet the rigidity of current robot design is not
big enough to withstand such a large load in machining.
Thus large error in machining may occur.
To overcome the drawbacks of articulated robots in ma-
chining, in 2009, European Commission had funded a proj-
ect called COMET (plug-and-produce components and
methods for adaptive control of industrial robots enabling
cost-effective, high-precision manufacturing in factory of
the future) [19]. This project was aimed at overcoming
challenges facing European manufacturing industries by
developing innovative machining systems that are flexible,
reliable, and predictable with an average cost efficiency
savings in comparison to machine tools. Industrial robot
technology was chosen as the backbone of the project. The
project investigators are aware of the inherent weakness of
industrial robots, that is, low positioning accuracy, vibration
due to process force, and lack of reliable programming tool.
It is widely anticipated that this project will greatly progress
the technology in robot machining.
For articulated robots, the repeatability is inherently de-
pendant on its reach distance. The larger the reach distance
is, the lower the repeatability will be. This inherent charac-
teristic can be easily explained as when the robot is fully
stretched, it is a cantilever beam. The compliance of a
cantilever is heavily dependent on the cantilever length. In
fact, this characteristic has been manifested by the data
provided by commercial robot suppliers (for examples,
ABB, Motoman, Fanuc, Kuka). Table 1shows some data
for three selected ABB robot models [7]. It can be seen that
as the reach distance increases, the repeatability error is
increased too. This table also shows that the repeatability
of todays industrial articulated robot can be as high as
±0.01 mm which is sufficient for many low- to medium-
accuracy part machining jobs. In fact, most machining jobs
do not need a large reach distance. If the reach distance of
robot design is further reduced, it will be possible to im-
prove the repeatability even further.
The following will classify recent research on robotic
machining into three areas, namely rapid prototyping, vibra-
tion/chattering analysis, path planning, and automatic robot
programming.
2 Robot machining for rapid prototyping
Articulated industrial robots are flexible, cost-effective, and
normally have a large working envelope. However, they
have low positional accuracy and rigidity which confine
early robotic machining researches be aimed at making large
prototypes that are difficult to be made by both CNC
machines and layer-based rapid prototyping machines.
Table 1 A robots reach distance and repeatability
Robot model Reach distance (mm) Repeatability (mm)
ABB IRB 120 580 ±0.01
ABB IRB 140 810 ±0.03
ABB IRB 1410 1,440 ±0.05
1490 Int J Adv Manuf Technol (2013) 66:14891497
2.1 Rapid prototyping based on machining
Early robot machining research was aimed at making parts
with complicated geometry and limited accuracy (say
1.00 mm). Vergeest and Tangelder had first reported a
robotic machining system that was consisted of an articulat-
ed industrial robot, a rotatable horizontal platform for stock
material fixation and a milling device mounted on the end
effector of the robot [20]. The system was capable of mak-
ing parts within an 80-cm cube. Offline robot programming
capability was mentioned yet no detail was reported. Almost
at the same time, Chen and Tse also reported a robotic
system for rapid prototyping purpose [20]. Apart from the
major components as reported in Vergeest and Tangelders
system, the robot arm in Chen and Tsessystemwas
mounted on a 3-m-long linear track which could significant-
ly extend the robot systems machining capability. A de-
tailed robot machining path planning method was reported
as well. Further refinement of the robotic machining system
was documented in their subsequent publications [2123].
Figure 1a shows the hardware system setup. Robot machin-
ing path simulation and verification are shown in Fig.1b.
In order to further increase the efficiency of robot ma-
chining, Huang and Lin reported a dual robot machining
system [24]. In their system, the stock is installed on a fixed
working table, and two robotic arms are used to collabora-
tively machine a 3D part. A similar robot machining system
with two arms could be seen from the research conducted by
Owen et al. [25,26]. They used two robotic arms with one
serving as the stock fixtures and the other as a machining
tool. This system has more degrees of freedom allowing
parts with more complicated geometry be made. However,
the system is more compliant thus needs more careful mon-
itoring of the machining force. Forces acting on the end
effectors were monitored to identify the onset of a distur-
bance so that the system could be slowed down before
saturation actually occurred. In response to disturbances, a
time-scaling method could reduce the tool speed, thereby
reducing the demand on the joint torques and allowing the
pre-computed path to be followed more accurately.
The merits of robot machining were further extended by
Lee and Tsai et al. [27] who had tested Internet-based robot
machining scheduling and collaboration. This system is
good for resource sharing, autonomous repairing, and re-
placement of damaged parts in hazardous environment or
space. To machine a part with better surface quality, Zielin-
ski et al. developed a robotic system that was capable of
both milling and polishing [28]. It was believed that large
parts machined by a robot machining system could have
good surface finish.
Even though articulated robot arms have very good ac-
cessibility compared to traditional CNC machines, yet some
intricate geometric features such as cavities or internal holes
could not be made by articulated robots. Figure 2a shows a
sectioned view of a toilet flush model. The internal channels
of the model could not be machined by any existing CNC
machines. Chen et al. [29,30] had developed a layer-based
method using robot machining that could build parts with
complicated internal features such as the ones shown in
Fig. 2b and c. The layers do not have uniform thickness.
Instead, layer thickness is adaptive to curvature change and
accessibility analysis along the build orientation.
2.2 Robot calibration
In order to develop a robot machining system with better
accuracy, various calibration methods have been reported.
CCD cameras were frequently used to identify the kinematic
parameters of the actual machining setup so that positional
accuracy of the robot could be improved [23,24]. Figure 3
shows a vision-based robot calibration method where a
gauge cylinder was placed in pre-defined locations on the
rotary table [23]. Positional errors Δx,Δy, and Δzwere
measured using the two cameras installed in orthogonal
positions.
Morris et al. reported a robot calibration method using
coordinate measuring machine (CMM). Experimental meas-
urements of some robot poses are taken using a CMM.
Based on the measurements, a kinematic model is developed
(a)
The system set-up
(b)
Machining path simulation and verification
Fig. 1 A robotic machining
system. aThe system setup; b
machining path simulation and
verification
Int J Adv Manuf Technol (2013) 66:14891497 1491
for the robot arm. Its relationship to the world coordinate
frame and the tool is also established [31].
Andres et al. has reported a novel method for the cali-
bration of a complex robotic workcell with eight joints
devoted to milling tasks [32]. A planar calibration method
is developed to estimate the external joint configuration
parameters by means of a laser displacement sensor, thus
avoiding direct contact with the calibration pattern. A re-
dundancy resolution scheme on the joint rate level is inte-
grated with a CAM system for the complete control of the
robotic workcell during the path tracking of a milling task.
In general, a calibration method should serve two purposes.
First, it should establish a relationship between the robot
coordinate system and the workpiece coordinate system.
Second, it should take some measurements so that the kine-
matic parameters of the robot can be modified to accurately
describe the actual position and orientation of the robotic
links.
3 Vibration or chattering analysis
Articulated robot arms are very agile and flexible with good
accessibility. When used for machining, there is always a
tradeoff between low dynamic accuracy and good accessi-
bility. This is why early researches on robot machining were
targeted at making prototypes with large size and compli-
cated geometry [2023,33]. Accuracy of part making was
not a major concern. When used in machining hard or metal
materials, the low stiffness of robot machining systems
presents a bigger problem. To make things worse, a robot
arms stiffness varies significantly in different directions.
For example, the static stiffness of a robot machining system
was reported to be 83.65 μm/N in Xdirection, 20.35 μm/N
in Ydirection, and 68.76 μm/N in Zdirection [34,35]. Due
to the difference of stiffness in different directions, cutting
accuracy was also found different in different cutting direc-
tions. In order to improve robot cutting accuracy, correlation
between vibration/chatter and machining parameters must
be established through experiments. For example, Zaghbani
et al. had collected vibrations and cutting force signals with
analysis in order to find a reliable dynamic stability machin-
ing domain with respect to spindle speed [36]. Feedrate is
also an important machining parameter. It has a large impact
on the machining accuracy as well [35]. Therefore it is
desirable to plan an optimal feedrate with a compromise
between machining efficiency and machining quality. It
was found that a constant feedrate was always preferred if
possible throughout the machining process [37].
Given the fact that the low stiffness of a robot arm may
cause machining errors, researchers have developed some
methods to compensate the errors. Zhang and Pan had
reported a method to control the machining error based on
deflection compensation and adaptive material removal rate
(MRR) [3840]. The deflection compensation was based on
a stiffness matrix in Cartesian space that the researchers had
developed based on experiments. The MRR was adaptive to
the cutting forces which were measured real-time in the
machining process. Based on both deflection compensation
and the controlled MRR, it was reported that the machining
accuracy of foundry parts could be improved from 0.9 to
0.3 mm.
For automatic offline robot machining programming,
Abele et al. developed an offline error compensation model
for the machining path [41]. The model can be used for the
prediction of the cutting force so that the anticipated cutting
deviation could be compensated based on the robots com-
pliance matrix during actually cutting. Since the cutting tool
path could be controlled, the accuracy of an industrial robot
for machining could be increased [42].
Because an articulated robot has heterogeneous stiffness
within its working envelope, it will be best for a robot to
perform a machining job within its possible range of best
stiffness. Vosniakos and Matsas had presented a method that
the robot milling operation could be performed in regions of
the robots workspace where manipulability, both kinematic
and dynamic was the highest, thereby exhausting the robots
potential to cope with the process. By selecting the most
suitable initial pose of the robot with respect to the work-
piece, a reduction in the range of necessary joint torques
(a) CAD model (b) A toilet flush model (c)A castle model
Fig. 2 Large parts with internal
features made by layer-based
robot machining. aCAD mod-
el; ba toilet flush model; ca
castle model
1492 Int J Adv Manuf Technol (2013) 66:14891497
could be reached, to the extent of alleviating the heavy
requirements on the robot. Genetic algorithm was used to
minimize the joint torque loads given the milling forces.
[43]. Similar to Vosniakos and Matsass work, Lopes and
Pires proposed an approach to optimize the workpiece loca-
tion based on machining trajectory and machining forces.
Again, a genetic algorithm was used for the optimization of
initial workpiece location [44].
Realizing that the accuracy of robot machining is affected
by many factors, Andrisano et al. proposed an integrated
approach for robot machining accuracy enhancement based
on robotic process simulation, tailored design of mechanical
apparatus for the machining system, and software modules
for robot control and programming [45]. They also high-
lighted the importance of machining strategy validation,
automatic robot path generation, workcell calibration, and
robot code commissioning.
In order to accurately define the dynamic behavior of a
machining robot, both experimental method and analytical
method have been reported. Bisu et al. have used a frequen-
cy method to measure the dynamic response when milling at
designated points [46]. Since only very few points could be
measured, this method is not directly useful for robot tra-
jectory planning in machining. A more useful method for
robot joint stiffness identification is reported by Dumas et al.
[47]. They evaluate joint stiffness values with consideration
of both translational and rotational displacement of the robot
end effector for a given applied wrench (force plus torque).
Based on the joint stiffness values, they have also developed
the robots Cartesian stiffness matrix which is more useful
for robot machining path planning.
4 Robot machining path planning
There are many literature reports about automatic CNC
machining path planning based on CAD models [48]. In
the past 30 years, the authors estimate that at least tens of
thousands articles on CNC machining path planning have
been published by international journals and conferences.
Even by now, articles in this field are still constantly
emerging [49]. Compared to the wealth of path planning
for CNC machining, very little publications on automatic
robot machining path planning could be found. There might
be a misconception that robot machining path planning is
the same or similar to NC path planning. This misconcep-
tion may have hindered the development of robot machin-
ing. It is true that there are some similarity between NC path
planning and robot machining path planning. Yet the differ-
ence is substantial. For example, the impact of stiffness on
robot machining path planning is significant yet a CNC
machines stiffness has much smaller impact on NC path
planning.
Apart from academic research on automatic path plan-
ning for robot machining, some commercial companies
are also actively engaged in developing software pack-
ages capable of generating robot trajectory automatically
from a CAD model, or from a tool path. Robotmaster ®
has reported a software solution for CAD/CAM-based
programming for robot milling and trimming [50].
Robotmaster can create accurate six-axis robot trajecto-
ries from tool path data. Singularity, collision, out of
reach, and joint extension errors are checked when the
robot trajectory is generated. The functionality of Robot-
master has more or less reflected achievement from early
research on automatic robot machining path planning
[2123,51]. However, the dynamic features of a robot
are not considered in Robotmaster.
Recent research on robot machining has focused more
on the influence of robotic dynamics on machining accu-
racy and efficiency [46,47,52,53]. Olabi et al. have pro-
posed to optimize the tool-tip feedrate in Cartesian space
for a given tool path using a smooth jerk limited pattern
with consideration of the joints kinematics constraints
[52]. That is, the dynamic characteristics of the robot are
considered in determining one of the key machining
parameters feedrate.
Xiao et al. propose a robot trajectory planning method
based on cutter location (CL) data generated by convention-
al CAD/CAM [53]. When doing inverse kinematics, a re-
dundant mechanism is analyzed to avoid the singular
configurations and joint limits. A gap bridging strategy is
applied to reduce the jerk motion caused by tool retraction
and cutting paths connection.
Apart from the above-mentioned articles for path/trajec-
tory planning in robot machining, not much else could be
found. Almost all reported robot cutting trajectory planning
methods are based on CL data generated by either using an
existing method, or by an existing CAD/CAM software
package. Based on the authorsexperiences, when generat-
ing CL data, robot dynamics should be considered. A gen-
eral principle about CL data generation should be to
minimize joint motion when the robot moves from one
cutter location to the next cutter location.
Camera 1
Gauge
cylinder
Calibration
tool
XO
Z
Camera 2
Xi
Zi
Rotary platform
Workpiece
coordinate
X'
O'
Z'
Base coordinate
Fig. 3 Calibration of a robot arm
Int J Adv Manuf Technol (2013) 66:14891497 1493
5 Future research issues
Progress in robot machining research is relatively slow in
recent years. This may have been the results of a variety of
factors. In order to advance the science and technology of
robot machining, the following issues are identified and
must be studied in the future.
1. Robot machining efficiency has never been investigat-
ed: In fact, this is one of the major issues in robot
machining that must be addressed in order to extend
robot machining to more applications. Normally, robot
machining has much bigger advantages when machin-
ing large components compared to CNC machining. Yet
when machining a large component, in general, more
material must be removed. However, due to limited
robot rigidity and payload, feed rate, depth of cut, and
cutter diameter must be kept to small values. This will
limit the material removal rate or machining efficiency.
It is desirable to develop some machining strategies
such as special cutting path patterns so that machining
efficiency could be increased. Figure 4a shows a 3D
model of a crane bird. If it is to be made by robot
machining based on a rectangular block raw material,
a lot of material must be removed. If the excess material
is removed bit by bit in a traditional zigzag pattern, it
will take a lot of time. Figure 4b shows the rough
machining of the part to near shape in the projection
plane XY. It may be followed by make the part to near
shape in YZplane and XZplane. After these rough
cutting, the excess material is significantly reduced.
This will greatly increase the efficiency of robot ma-
chining.
It is also possible to use dual robots machining with
one robot for rough machining and the other for finish
machining. Robot machining can afford the luxury of
multi-robots due to its low cost. This demarcation of
machining job may greatly improve robot machining
accuracy as well as efficiency because some robot arms
are best designed for higher payload and others are
designed for greater precision.
2. Develop a rigidity map within a robots working enve-
lope: For a given point within the working envelope, it
can be reached with many possible robot joint config-
urations, joint configurations many have quite different
rigidity which will affect the machining quality. If the
rigidity map is known and easily available, optimal joint
configurations could be identified for a given machining
path. This will help improving the machining quality. In
previous robot machining research, almost all reported
systems used an existing industrial articulated robot arm
(a) the crane bird model (b) machine to near shape
Fig. 4 Machining of a crane bird. aThe crane bird model; bmachine
to near shape
wrist wrist
w
w
nn
wrist
w
n
spindle
spindle
spindle
L
E
E
(a)parallel attachement (b) vertical attachment (c) design into wrist
Fig. 5 Spindle attachments to a
robots end link (wrist)
1494 Int J Adv Manuf Technol (2013) 66:14891497
with a retrofitted tool spindle. The two popular spindle
attachments are shown in Fig. 5a and b. Both attach-
ment methods will weaken the already weak stiffness of
a robot arm and complicate the system calculation. A
suggestion here is to design the tool spindle into the
robots end link (or wrist) as shown in Fig. 5c so that
eccentric force could be avoided and computation
simplified.
3. Optimized robot machining system configuration: Ro-
bot machining were currently researched based on exist-
ing industrial robots that are best suited to material
transfer and welding applications. To get the best ma-
chining results, research on robot machining should not
be restricted to current industrial robot configurations.
Investigation on the proportion and design of the links
L1, L2, and L3 as shown in Fig. 6for optimal machin-
ing accessibility and rigidity should be conducted.
We all know that serial robots have accuracy prob-
lems mainly due to the error magnifying effect of the
arm design and the low arm stiffness. One approach is
to scale down the robot arm since this can reduce the
effect of error magnification and increase the robot
arms stiffness. The reduced reach range may be com-
pensated by introducing a linear stage for the position-
ing of the workpiece or mounting of a robot arm to a
precision XYZ stage as shown in Fig. 6. It is not neces-
sary to control the XYZ stage during machining. Instead,
positions of the robot arm on the XYZ stage can be pre-
computed so that optimum machining operations in
terms of accessibility and rigidity could be performed
for a given part. Since XYZ stage design and manufac-
turing is a mature technology which could provide
stages with good rigidity and sub-micron accuracy.
The addition of the XYZ stage will not have visible
impairment of the robot systems accuracy and stiffness.
4. Robotic machining lines: The advantages of robotics are
best illustrated when a line of robots are used to perform
jobs automatically as those frequently seen in a factorys
automatic assembly lines. There are researches on iso-
lated robot applications to machining, deburring, grind-
ing, or polishing. Yet no effort has been reported about
the development of an automatic machining line that has
all of the above functionalities. Figure 7shows a pro-
posed such robot machining line. The authors do not
agree with the concept of concurrent machining with
multiple robots since this may create a lot of extra
problems such as vibration and torsion. Instead, a ded-
icated robot for a dedicated operation will make parts
with best quality. For example, the robot machining line
shown in Fig. 7has a dedicated rough machining robot
and a finish machining robot. The robot for rough
machining may be designed for high stiffness, yet the
finishing robot might be designed for greater accuracy
as the material removal rate in finish machining is
normally very small thus the load capacity requirement
is low. Other finish machining operations such as grind-
ing or polishing may be added when necessary. If need-
ed, a painting robot may also be added so that a product
can be completely made in a single line. Of course, a lot
of research work must be done in order to make the
automatic robot machining line a reality.
6 Discussions and conclusions
This paper has provided a review of recent research and
development related to robot machining. It is found that
there is still a long way to go before robot machining
systems are widely used in practical applications. In current
researches, most researchers have chosen to use existing
industrial robots that are not designed or optimized for
machining operations. The inherent problems of low dy-
namic accuracy, vibration, and chattering could never be
resolved based on current research effort. This has hindered
the development of robot machining in recent years.
In this paper, the authors have suggested ways of im-
proving robot machining accuracy and efficiency so that
robot machining systems could be widely used in the future.
Four future research issues on robot machining are outlined.
L1
y
x
z
L2
L3
Fig. 6 A proposed robot machining cell
Fixture platform
loading
Rough
machining
Grinding/de-burring
Off loading
polishing
Finish
machining
Fig. 7 A proposed robot machining line
Int J Adv Manuf Technol (2013) 66:14891497 1495
It is hoped that research on these issues may eventual
advance the technology of robot machining.
Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution License which permits any use, distribution,
and reproduction in any medium, provided the original author(s) and
the source are credited.
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... und auch in der Forschung wird die Eignung von IR für solche Prozesse kritisch hinterfragt [6,7,8,9]. Faktisch sind IR in dieser Hinsicht nicht mit einer Werkzeugmaschine (WZM) konkurrenzfähig. Bisher konnten sich IR lediglich in Anwendungsfeldern durchsetzen, in denen WZM weniger gut geeignet sind, wie beispielsweise das Bearbeiten großräumiger Bauteile im Flugzeugbau [10] oder in der Holzbranche [11,12]. ...
... Zusammenfassend lässt sich ableiten, dass es einen potenziellen Markt gibt, der aufgrund einer zu geringen dynamischen Bahngenauigkeit von IR nicht erschlossen werden kann. Zur dynamischen Bahngenauigkeit trägt nicht nur das vergleichsweise schlechte Bahnfolgeverhalten [13] bei, sondern auch eine hohe Anfälligkeit gegenüber Schwingungen [8] und Störeinflüssen sowie bei der Fräsbearbeitung das Problem der Ratterneigung [6,14,15]. Diese Herausforderungen sind in der Literatur vielfach 1 1 Einleitung und Problemstellung beschrieben und zahlreiche Lösungsansätze wurden bereits vorgeschlagen [4,5,8,16]. ...
... Zur dynamischen Bahngenauigkeit trägt nicht nur das vergleichsweise schlechte Bahnfolgeverhalten [13] bei, sondern auch eine hohe Anfälligkeit gegenüber Schwingungen [8] und Störeinflüssen sowie bei der Fräsbearbeitung das Problem der Ratterneigung [6,14,15]. Diese Herausforderungen sind in der Literatur vielfach 1 1 Einleitung und Problemstellung beschrieben und zahlreiche Lösungsansätze wurden bereits vorgeschlagen [4,5,8,16]. Trotz dieser Bemühungen bleibt der Wunsch nach einer weiteren Verbesserung der dynamischen Bahngenauigkeit bei IR bestehen. ...
Book
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Die Genauigkeit von Industrierobotern ist seit Jahrzehnten Gegenstand kontinuierlicher Entwicklungs- und Forschungsaktivitäten. Einen wesentlichen Einfluss auf die dynamische Bahngenauigkeit haben die elastischen Antriebsstränge, deren Verhalten sich aus der gekoppelten Dynamik von Antriebsregelung und Getriebemechanik zusammensetzt. Eine negative Folge der Elastizität sind genauigkeitsreduzierende Schleppfehler, die selbst dann auftreten, wenn der Lageregelkreis über das Positionssignal der Getriebeabtriebswelle geschlossen wird. Ein vielversprechender Ansatz zur Schleppfehlerreduktion ist die Methode der semiaktiven Dämpfung, die im wissenschaftlichen Umfeld bereits erfolgreich bei Werkzeugmaschinen umgesetzt wurde. Das Funktionsprinzip basiert darauf, die Getriebemechanik durch bedarfsgerechte Bremseingriffe eines Zusatzaktuators zu dämpfen. Aus Sicht der Antriebsregelung resultiert daraus ein robusteres Regelstreckenverhalten, wodurch performantere Regelparameterwerte einstellt werden können. Die Parameteranpassung verbessert auf Achsebene sowohl das Führungs- als auch das Störverhalten und wirkt sich somit positiv auf die dynamische Bahngenauigkeit des Gesamtsystems aus. Das Ziel dieser Arbeit ist es, die semiaktive Dämpfung mit einem kostengünstigen Zusatzaktuator auf kaskadiert geregelte Knickarmroboter zu übertragen, um eine Verbesserung der dynamischen Bahngenauigkeit zu erreichen. Ausgehend von einer experimentellen Systemanalyse an einem Industrieroboter werden Anforderungen an die Lösung abgeleitet und Achse 1 als Integrationsziel für den Dämpfungsaktuator festgelegt. Zur Aktuatoransteuerung wird ein neues, semiaktives Regelgesetz vorgeschlagen, das modellfrei auf Basis vorhandener Zustandsgrößen arbeitet. Umfangreiche Simulationen zeigen dessen Wirkungsweise und belegen die höhere Dämpfungswirkung im Vergleich zu bekannten Regelgesetzen aus dem Stand der Forschung. Für die Konstruktion des Dämpfungsaktuators werden Auslegungsregeln und eine Möglichkeit zur Integration vorgeschlagen, die sich leicht auf andere Robotersysteme übertragen lassen. Der entwickelte Dämpfungsaktuator weist eine große Bandbreite und hohe Spitzenkräfte auf, sodass eine effektive Bedämpfung der Regelstrecke möglich ist. Zur anforderungsspezifischen Parametrierung der Antriebs- und Aktuatorregelung wird eine H∞-Entwurfsmethodik eingeführt und inbetriebnahmegerechte Einstellregeln daraus abgeleitet. Eine umfangreiche experimentelle Validierung belegt die Wirksamkeit der Gesamtlösung auf Achsebene sowie anhand von Trajektorienfolge- und Fräsversuchen. Basierend auf den Ergebnissen werden abschließend Potenziale und Grenzen der semiaktiven Dämpfung diskutiert, mit dem Fazit einer breiten Einsetzbarkeit in unterschiedlichen Anwendungen.
... It is known that the robot's stiffness and stability may decrease when it operates with its arm fully extended or near its limits. Therefore, it is preferable for the robot to perform figuring tasks within its optimal stiffness range [34]. Consequently, additional machining positions were introduced to allow the robot to machine the reflector surface. ...
... ates with its arm fully extended or near its limits. Therefore, it is preferable for the robot to perform figuring tasks within its optimal stiffness range [34]. Consequently, additional machining positions were introduced to allow the robot to machine the reflector surface. ...
... This is due to the robot's stiffness not being stable throughout the entire figuring path. Therefore, path planning is crucial, as the robot's stiffness varies along the path, significantly affecting machining accuracy and efficiency [20,34]. ...
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The demand for large-scale components continues to grow with the development of frontier technologies. Traditionally, these components are machined using machine tools, which are costly and have functional limitations. High-flexibility robots provide a cost-effective solution for machining large-scale components. This research proposes a dual-robot fabrication system for producing a 2.4 m × 4.58 m carbon fiber reinforced polymer (CFRP) antenna reflector. First, the kinematic model of the in-house developed robot was established to compute its theoretical workspace, which was subsequently used to partition the machining regions. Based on laser tracker measurements and theoretical calculations, a method and procedure for calibrating the Tool Center Point and Tool Control Frame of the robot were proposed. Subsequently, the dual-robot fabrication system was configured based on the determined machining regions for each robot. To further improve the figuring accuracy of the system, the support structure and figuring path were investigated and determined. Finally, processing experiments were conducted, and the material removal function for the flexible processing tool was computed to shape the reflector surface. The final results achieved the required surface figure accuracies for areas ≤ φ1750 mm, ≤φ2400 mm, and the whole surface were improved to 13.5 μm RMS, 23.4 μm RMS, and 45.8 μm RMS, respectively. This validates the processing capability and demonstrates the potential application of the dual-robot fabrication system in producing large-scale components with high accuracy.
... Compared to machine tools, IRs have poor dynamic accuracy [1] and are susceptible to vibrations [2]. These issues are widely described in the literature, with numerous proposed solutions [3][4][5]. ...
... This is reflected in the mechanical frequency response by a dominant resonance frequency that corresponds approximately to the characteristic frequency 0,m1 , whose magnitude depends on . ⏟⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞ ⏟⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞⏞ ⏟ elasticity (2) from motor torque M to motor angular velocity ̇ describes this using two fractional polynomial factors [8]. The first represents the rigid body dynamics of the drive, while the second describes the elastic behavior. ...
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The dynamic accuracy of industrial robots is significantly influenced by the elastic drive trains of the axes. Their behavior is composed of the coupled dynamics of drive control and gear mechanics. As an undesirable consequence, increasing elasticity leads to growing tracking errors. One approach to reduce tracking errors is semi-active damping. The functional principle is based on damping the gear mechanics by selective braking of an additional actuator. From a drive control perspective, this results in a more favorable system behavior, which, in turn, allows the selection of more performant control parameter values. This leads to better tracking and disturbance behavior. The aim of this paper is to transfer the semi-active damping with a low-cost additional actuator to cascade-controlled industrial robots. For this purpose, a novel semi-active control law is proposed for actuator control. A damping actuator for the first robot axis is designed, design rules are derived, and an integration concept is proposed. Finally, a H ∞ synthesis methodology for simultaneous parameterization of the drive and actuator control is introduced. An experimental validation proves the effectiveness of the solution at axis level resulting in an average 17.3 % reduction in tracking errors, and in a milling experiment, reducing the average Euclidean tracking error by 42.7 %.
... However, these processes (grinding, polishing, machining, etc.) are often time-consuming and expensive, as they require specialized equipment, skilled operators, and extended machining time. This can limit their practicality for large-scale or costsensitive production environments, highlighting the need to balance surface quality with economic and operational efficiency [207][208][209]. ...
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Full-text available
The rapid progress in additive manufacturing (AM) has unlocked significant possibilities for producing 316/316L stainless steel components, particularly in industries requiring high precision, enhanced mechanical properties, and intricate geometries. However , the widespread adoption of AM-specifically Directed energy deposition (DED), selective laser melting (SLM), and electron beam melting (EBM) remains challenged by inherent process-related defects such as residual stresses, porosity, anisotropy, and surface roughness. This review critically examines these AM techniques, focusing on optimizing key manufacturing parameters, mitigating defects, and implementing effective post-processing treatments. This review highlights how process parameters including laser power, energy density, scanning strategy, layer thickness, build orientation, and pre-heating conditions directly affect microstructural evolution, mechanical properties, and defect formation in AM-fabricated 316/316L stainless steel. Comparative analysis reveals that SLM excels in achieving refined microstructures and high precision, although it is prone to residual stress accumulation and porosity. DED, on the other hand, offers flexibility for large-scale manufacturing but struggles with surface finish and mechanical property consistency. EBM effectively reduces thermal-induced residual stresses due to its sustained high preheating temperatures (typically maintained between 700 °C and 850 °C throughout the build process) and vacuum environment, but it faces limitations related to resolution, cost-effectiveness, and material applicability. Additionally, this review aligns AM techniques with specific defect reduction strategies, emphasizing the importance of post-processing methods such as heat treatment and hot isostatic pressing (HIP). These approaches enhance structural integrity by refining microstructure, reducing residual stresses, and minimizing porosity. By providing a comprehensive framework that connects AM techniques optimization strategies, this review serves as a valuable resource for academic and industry professionals. It underscores the necessity of process standardization and real-time monitoring to improve the reliability and consistency of AM-produced 316/316L stainless steel components. A targeted approach to these challenges will be crucial in advancing AM technologies to meet the stringent performance requirements of various high-value industrial applications. Citation: Aziz, U.; McAfee, M.; Manolakis, I.; Timmons, N.; Tormey, D. A Review of Optimization of Additively Manufactured 316/316L Stainless Steel Process Parameters, Post-Processing Strategies, and Defect Mitigation. Materials 2025, 18, 2870. h ps://doi.
... El abanico de posibilidades y expectativas de las aplicaciones robóticas en tareas de mecanizado y premecanizado se refleja en la gran colección de revisiones y artículos de la literatura especializada [1], [2], [3], [4], [5]. ...
Conference Paper
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The aim is to improve the productivity of the machining cells through a redistribution of tasks between machining centers and robot. The real working capacity of the robot is established considering the forces of the process and the precision of the product to be manufactured by modeling its behavior. The reallocation is performed by determining the behavior of the robot in the work area and where the best results are produced in the machining direction. The possible reallocation of operations, the positioning point and the orientation of the part are obtained. It is necessary to have an estimation model of the robot requests, being, this input information modified, from the robot feedback. The results of the modeling of the complete process and the tests performed show that the process characteristics and the point of the work area are significantly influenced by the cutting forces.
... El rango de posibilidades y expectativas para aplicaciones con robots en tareas de mecanizados y pre-mecanizados, se refleja en la amplia colección de reseñas y trabajos que existen en la literatura especializada [1][2][3] [4]. Los grandes retos a los cuales se enfrenta el mecanizado con robots frente al mecanizado con máquinas herramientas siguen siendo hoy [5] la caracterización de la rigidez y la configuración del robot [6], la planificación de la trayectoria y su dinámica, la vibración durante el mecanizado [7] y la deformación del robot y su compensación [8]. ...
... While such deviations from the theoretical path are acceptable in many industrial applications that primarily require accurate positioning of the programmed points, they can become problematic in processes where strict trajectory control is essential. This limitation has become increasingly relevant as more industrial processes demand precise automation through robotic systems [5]. ...
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Ensuring consistent contact between the tool and the workpiece is a key challenge in robotic finishing operations, especially when dealing with positioning errors or morphological variations in the workpiece. Tool compliance has become a widely adopted strategy to tackle this issue, offering greater adaptability in these scenarios. At the same time, feed rate plays a fundamental role in process performance; however, achieving a uniform feed rate is not always possible due to robot limitations in executing the programmed trajectory. This study introduces an innovative approach that leverages tool compliance systems not only to maintain contact but also to evaluate the performance of contouring trajectories with varying discretization levels, all without the need for additional measurement equipment. Thus, by continuously monitoring radial compliance, the proposed methodology quantifies both the contact consistency with the workpiece and the time required to complete the motion. The insights gained from this process enable an objective assessment of contour following, balancing speed, and positioning accuracy and provide a practical tool for optimising trajectory design. This approach provides a practical solution for optimising trajectory design and promotes smoother and more fluid transitions between control points, establishing a new standard for precision and efficiency in robotic finishing operations.
... Research on robotic arm face milling has been crucial for improving precision and compensating deviations, ensuring surface flatness. Key advancements include the development of predictive algorithms to address flatness deviations caused by tool wear and other variables, enabling timely adjustments and proactive tool replacements [33][34][35][36]. Offline path compensation and real-time error correction methods, enhanced by advanced sensors and measurement systems such as laser trackers, have been instrumental in achieving these goals. ...
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The current trend in machining with robotic arms involves leveraging Industry 4.0 technologies to propose solutions that reduce path deviation errors. This approach presents significant challenges alongside promising advancements, as well as a substantial increase in the cost of future industrial robotic cells, which is not always amortizable. As an alternative or complementary approach to this trend, methods encouraging the occasional use of Industry 4.0 devices for characterizing the behavior of the actual physical cell, calibration, or adjustment are proposed. One such method, called FlePFaM, predicts flatness errors in face milling operations using robotic arms. This is achieved by estimating tool path deviation errors through the integration of a simple model of the robot arm’s mechanics with the cutting forces vector of the process, thereby optimizing machining conditions. These conditions are determined through prior empirical estimations of mass, stiffness, and damping. The conducted tests enabled the selection of the most favorable combination of variables, such as the robot wrist configuration, the position and orientation of the workpiece, and the predominant milling orientation. This led to the identification of the configuration with the lowest absolute flatness error according to the model’s predictions. The results demonstrated a high degree of similarity—between 97% for the closest case and 57% for the farthest case—between simulated and experimental flatness error values. FlePFaM represents a significant step forward in adopting innovative robotic arm solutions for reliable and efficient production. FlePFaM includes dimensional flatness indicators that provide practical support for decision making.
... The moment verified in each joint, in the X and Z directions, has proportionality associated with the force in the Y direction, however, the angle between the arms and joints, and the consequent decomposition of forces, results in different values in each of them. According to Chen and Dong [28], the repeatability error is proportional to the distance reached by the arm. Lower repeatability is correlated to the variation in the process due to variations, such as different forces, moments, etc. ...
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Competitiveness inherent to global markets shows the need to develop and manufacture high-quality products at a low cost. The automobile industry shows this scenario well as it is significantly relevant in the global economy. Applying the most recent technologies in manufacturing processes is a productive field that favors the development of studies and research in the field of Engineering. Most automobile manufacturers integrate powertrain machining of rotational and prismatic components into their manufacturing process where tolerance and surface finish requirements are very low. In recent years, these requirements have been achieved using computer numerical control (CNC) machine tools and the development of material technology. As the drilling process in aluminum alloys is a relevant step in machining the mentioned components, this article evaluates the application of anthropomorphic robots in drilling in Al–Mg-Si 6351 T6 aluminum alloy. Robotic drilling applications can bring advantages such as flexibility, maneuverability and cost competitiveness, although problems related to dynamics and rigidity make their application challenging. This work evaluated the dynamic, dimensional, geometrical conditions, and tolerances found in the robotic drilling process in Al–Mg-Si 6351 T6 alloy. CAD, CAT, and CAE (multibody) modeling were the basis for manufacturing a robotic cell and contributed to planning with assertiveness (90%) when compared to the physical one. The average diameters of the holes, position error in the coordinates, cylindricity, and circularity presented values in the order of hundredths of millimeters, while the perpendicularity errors obtained were in the order of thousandths of millimeters. The surface finish (Ra) of the holes presented an acceptable average compared to conventional machining processes (3.09 μm), and it was observed that the results can be attenuated by 20% through robot posture optimization.
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Robotic grinding of large walls faces challenges such as difficulty in trajectory planning to meet overall flatness and unstable contact forces. This paper presents a surface geometric mapping-based optimal trajectory planning method to generate grinding paths and optimize wall surface flatness. In this method, wall features are measured using distance sensors to establish a 2D mapping model between the surface and a reference plane. Trajectory points are searched based on the relationship among the feature points, material removal model, and radial height. Then, the surface is segmented into several micro-surface cells, and the posture relationship among them is explored to generate the end-effector’s pose data. We also propose a multimodal force control algorithm that switches force control strategy based on radial height states, dynamically adjusting the end-effector’s pose through force fusion to enhance contact force stability. Comparative experimental results with two representative methods show our method’s advantages, featuring a mean overall wall flatness of 1.997 mm and grinding efficiency improvements by 18.51% and 12.7%, respectively, indicating its potential for grinding large surfaces in real applications.
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The robotic machining is one of the most versatile manufacturing technologies. Its emerging helped to reduce the machining cost of complex parts. However, its application is sometimes limited due to the low rigidity of the robot. This low stiffness leads to high level of vibrations that limit the quality and the precision of the machined parts. In the present study, the vibration response of a robotic machining system was investigated. To do so, a new method based on the variation of spindle speed was introduced for machining operation and a new process stability criterion (CS) based on acceleration energy distribution and force signal was proposed for analysis. With the proposed method the vibrations and the cutting force signals were collected and analyzed to find a reliable dynamic stability machining domain. The proposed criterion and method were validated using data obtained during high speed robotic machining of 7075-T6 blocks. It was found that the ratio of the periodic energy on the total energy (either vibrations or cutting forces) is a good indicator for defining the degree of stability of the machining process. Besides, it was observed that the spindle speed with the highest ratio stability criterion is the one that has the highest probability to generate the best surface finish. The proposed method is rapid and permits to avoid trial-error tests during robot programming.
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This paper presents a practical approach to adapt the trajectory planning stage for industrial robots to realize continuous machining operations. Firstly, L1 interpolation is introduced to generate efficiently the tool-paths in the form of shape-preserving quintic splines. Then, the tool-tip feedrate planning in Cartesian space is done using a smooth jerk limited pattern and taking into account the joints kinematics constraints. Experimental validations conducted on a 6-axis industrial robot demonstrate the effectiveness of the proposed methodology of trajectory planning in the context of machining.
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Although robotics based flexible automation is considered as an ideal solution for foundry pre-machining operation, very few successful installations have been seen due to many major challenges involved in robotic machining processes using conventional articulated robot, such as limited material removal rate, low surface quality, and chatter/vibration. This paper explains the reasons for low surface quality in robotic machining processes and analyzes the stiffness properties of robot structure. Then a real-time compensation algorithm based on a robot stiffness model and force control scheme is introduced. The experimental results show that much better surface quality can be achieved without extending the process cycle time.
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Although robots tend to be as competitive as CNC machines for some operations, they are not yet widely used for machining operations. This may be due to the lack of certain technical information that is required for satisfactory machining operation. For instance, it is very difficult to get information about the stiffness of industrial robots from robot manufacturers. As a consequence, this paper introduces a robust and fast procedure that can be used to identify the joint stiffness values of any six-revolute serial robot. This procedure aims to evaluate joint stiffness values considering both translational and rotational displacements of the robot end-effector for a given applied wrench (force and torque). In this paper, the links of the robot are assumed to be much stiffer than its actuated joints. The robustness of the identification method and the sensitivity of the results to measurement errors and the number of experimental tests are also analyzed. Finally, the actual Cartesian stiffness matrix of the robot is obtained from the joint stiffness values and can be used for motion planning and to optimize machining operations.
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Force/torque sensing is very important for several automatic and industrial robotic applications. Basically, if precise control of the forces that arise from contact between tools and parts is required to successfully complete the automatic task, then a force/torque sensor is needed along with some force/torque control technique. In this paper we focus on force/torque sensing aspects applied to industrial robotic tasks. Concentrating on a particular type of force/torque sensor, we demonstrate how to use them and how to integrate them into force/torque control applications using robots. Finally, an industrial application is presented where force control was fundamental for the success of the task.
Chapter
This paper describes the development of an automatic polishing system for a three dimensional sculptured surface with an industrial robot. The robot is installed with a magnetic force sensor and a magnetically pressed polishing tool, both of which have been newly developed.
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This paper presents the development of a robot machining centre for rapid prototyping (RP) applications. Given a three-dimensional model of an object, surface features of the model were extracted and analysed. Cutter paths for both rough and finish cutting were then generated on the basis of the extracted surface features. In order to detect collision between the robot arm and its environment, an accurate model was built using a computer aided design (CAD) system and the Denavit-Hartenberg (D-H) notation was used to represent the robot arm transformation matrix. When collision was detected for a given contact point, both cutter location and robot arm positions were modified. A simple ship model was machined to demonstrate the effectiveness of the robot machining centre. Finally machining errors were quantified with the use of a coordinate measuring machine (CMM).
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This paper presents the development of an international collaborative virtual multi-axis cutting verification system and the framework of a remote dual-robot machining system between the Centre for Advanced Manufacturing Research (CAMR), University of South Australia (UniSA), in Australia, and the Metal Forming Laboratory, (MFL) of the National Cheng Kung University (NCKU), in Taiwan, to establish computer-supported cooperative work (CSCW) through the Internet. Multi-media tools such as text, audio, images and a videoconference system are used to facilitate the communication between geographically distributed engineers. A remote control software is adopted in NCKU to remote control the dual-robot machining system in UniSA; it is also used to facilitate collaborative discussion on multi-axis cutting verification between Taiwan and Australia. A preliminary network analysis and evaluation is also achieved to serve as a reference prior to the execution of remote control. Several case studies for various connection types between campuses, universities and continents were tested through the Internet.
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The paper describes a prototype robot which due to its serial-parallel structure exhibits, high stiffness and has a large work envelope. These features make this robot suitable for relatively high precision machining operations on large workpieces. The conroller for this robot was based on MRROC++, which is a robot programming framework. Thus the controller could be tailored to the tasks at hand, including the capability of in-program switching of kinematic model parameters. To obtain those parameters for different locations in the work-space a calibration procedure using linear measurement guides has been devised.