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REGULAR PAPER
RAS: a robotic assembly system for steel structure erectionand assembly
Ci-Jyun Liang1 • Shih-Chung Kang1 • Meng-Hsueh Lee1
Received: 4 December 2016 / Accepted: 19 July 2017 / Published online: 13 September 2017
� Springer Nature Singapore Pte Ltd. 2017
Abstract This research focuses on a long-standing, yet
critical problem in the erection of steel structures. In the
current state of practice, steel workers must stand on an
unfinished structure to assist with the assembly of structural
elements manually. They must pull on the wire hanging
under the rigging elements to align the bolting holes of the
moving and fixed elements. This work is often performed
in high places, which can be very risky. Therefore, we have
developed a robotic assembly system (RAS) for steel beam
erection and assembly to prevent workers from having to
work in a high place. The RAS consists of four methods:
rotation, alignment, bolting, and unloading. The rotation
method involves a flywheel installed on top of the rigging
beam, which aims to rotate the beam to the assembly angle.
The alignment method includes both vertical and hori-
zontal alignment. The vertical alignment relies on a camera
and a marker on the column to align the beam altitude. The
horizontal alignment relies on a specially-designed beam,
which allows for it to be smoothly guided into the right
position. The bolting method is used to connect the beam to
a fixed element. We designed an additional guide hole
above each bolt hole. The bolt can be inserted in the guide
hole and slid to the bolt hole. The unloading method is used
to unload the crane cable and the RAS. We use a pin
mechanism for the beam-hook connection so it can easily
be unplugged by a motor. The system is built in a scaled
experimental construction site to validate its feasibility.
The results show that the RAS can operate the assembly
process without humans working at risky heights, and can
complete faster than the traditional method. In conclusion,
we have developed a robotics assembly system that can
help reduce the frequency of accidental falls during the
steel beam assembly process. The RAS adheres to the
process of the current erection method and can be broadly
introduced to existing construction sites.
Keywords Steel beam assembly � Construction robotics �Construction safety � Auto joint � Rotation method
1 Introduction
The steel beam erection and assembly process is always in
the critical construction path and accounts for a high per-
centage of the cost in a large high-rise steel structure
construction project (Chi et al. 2012; Chin et al. 2005;
Pavlovcic et al. 2004); however, it relies largely on manual
labor (Irizarry 2011), which means even a simple human
mistake might result in a serious delay of the entire con-
struction schedule and thus, extra costs to the project
(Peurifoy et al. 2011). Figure 1 illustrates the process of
steel beam erection and assembly. First, ground workers
connect the steel beam to the tower crane hook, then the
tower crane lifts and transports the steel beam to the
assembly position, as shown in Fig. 1a, b. Second, workers
at the construction height align the steel beam to the pre-
cise joint position by hand, by wire, or even by foot, as
shown in Fig. 1c. This step accounts for the highest per-
centage of time spent in the entire process (Chi and Kang
2010). Finally, workers assemble the steel beam with steel
plates and two or three bolts to achieve the temporary
connection, as shown in Fig. 1d. During the process, steel
workers have to stand on a narrow steel bracket or other
& Shih-Chung Kang
1 Department of Civil Engineering, National Taiwan
University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617,
Taiwan
123
Int J Intell Robot Appl (2017) 1:459–476
https://doi.org/10.1007/s41315-017-0030-x
steel beam at a substantial height with only a simple safety
cable. Accidents sometimes happen and can cause serious
injuries or fatalities (Beavers et al. 2009)—falling, being
crushed/struck/hit by an object, and being electrocuted are
the three main categories of fatal events common to the
task of crane erection. Furthermore, the efficiency of the
process is difficult to control due to the impact of manual
labor (Liang and Kang 2014). Therefore, preventing human
workers from having to work at heights is the primary goal
in improving the safety and efficiency of the steel beam
erection and assembly process.
1.1 Crane operation safety and efficiency
Safety and efficiency issues are very important in con-
struction projects (Zhou et al. 2012). Recent erection and
assembly related research has been focused on improving
crane operation (Kang and Miranda 2006) and steel worker
performance (Teizer et al. 2013). Since the crane operator
plays an important role in crane operation, several research
papers focused on crane operator training and blind spot
reduction (Cheng and Teizer 2014; CM Lab 2015; Huang
and Gau 2003). Juang et al. (2013) developed a stereo-
scopic kinesthetic crane training system to train the crane
operator in a realistic approximate approach. Chi et al.
(2012) developed a tele-operated crane interface for a
worker-free construction site. The interface demonstrated
the crane erection status and planning path and informed
the operator when a collision was about to happen. Lee
et al. utilized location tracking sensors and building
information modeling (BIM) (Volk et al. 2014) to set up a
crane navigation system to assist with blind lifting (Lee
et al. 2012). Ray and Teizer (2012) presented a mobile
crane operator head motion estimator to build a map of
dynamic blind spots with a range camera. In addition, path
planning algorithms and visualization techniques have
been utilized to help crane erection operation without
guidance from ground workers on construction site (Chang
et al. 2012; Kang and Miranda 2006; Kang and Miranda
2009; Wang et al. 2011). The genetic algorithm (Yoo et al.
2012) and configuration space (C-Space) (Kang et al. 2009)
methods are normally used in erection path planning.
Zhang and Hammad (2012) proposed the Rapidly-explor-
ing Random Trees Connect-Connect Modified (RRT-Con-
Con-Mod) and Dynamic RRT-Con-Con Modified (DRRT-
Con-Con-Mod) methods to improve the erection path
planning and re-planning. Lei et al. (2013) utilized the
Configuration Space Obstacle (C-Obstacle) method to
check the mobile crane lift path in order to prevent colli-
sions. Hung et al. (2016) proposed HBCD strategies
(Hoisting, Boundary, Capacity, and Direction) to accelerate
the computing time of the mobile crane path planning
algorithm.
Worker safety is also an important issue in erection and
assembly related research (Irizarry and Abraham 2006;
Kim and Kim 2012; Vijay et al. 2006). Irizarry (2011)
Fig. 1 Steel beam assembly
process: a lifting,
b transporting, c aligning and
d bolting
460 Ci-Jyun Liang et al.
123
analyzed the steel erection process and presented the fac-
tors that affect worker performance. Teizer et al. (2013)
utilized Ultra-Wideband sensors to track the ironworkers’
location in the training environment, and virtual reality
techniques to demonstrate the training process for
improving the ironworkers’ education and training method.
Park and Brilakis (2012) proposed a construction worker
detection algorithm to identify the construction worker in
video frames. Lee et al. (2012) utilized an RFID sensor to
monitor the workers’ location at the construction site.
1.2 Steel beam erection and assembly
Based on observations of the steel structure erection and
assembly at a real construction site, we separate the process
into three steps: rotation control, alignment, and bolting, as
shown in Fig. 2. First, the workers rotate the rigging beam
to the correct orientation. Second, the beam is aligned to
the correct position relative to the column. Third, the beam
is assembled to the column with two or three bolts to
complete the temporary connection. The rotation control of
the rigging beam is intended to rotate the beam to the
assigned position and maintain its orientation. A suspen-
sion unit controlled by gyroscopic moment (GYAPTS)
(Wakisaka et al. 2000) and a motor controllable hook block
(Lee and Lee 2014) are two different ways to achieve this
rotation control. The GYAPTS is a gyroscopic device that
attaches to the lifted beam and contains a flywheel. It can
stabilize the lifted beam (passive control) or rotate it to a
precise angle using the moment provided by the flywheel
(active control) (Gajamohan et al. 2012; Gams et al. 2007).
The GYAPTS was implemented in an automatic con-
struction system and used on a reinforced concrete building
site (Wakisaka et al. 2000). Alternately, the motor con-
trollable hook block provides a power resource from the
crane hook. It can simply rotate the rigging beam and
maintain the orientation with a motor connected to the
hook (Lee and Lee 2014; Lee et al. 2012).
In order to automate construction, a suitable manipulator
must be implemented (Gambao et al. 2000; Kahane and
Rosenfeld 2004; Yu et al. 2009). The main purpose of the
manipulator is to align the construction component to the
assigned position and connect it to fixed components. Feng
et al. (2015) developed a marker detecting algorithm for a
mobile robotic manipulator to identify, grasp, and assemble
the construction components. Garg and Kamat (2014)
designed a robotic fabrication mechanism for rebar cages
in concrete construction. Viscomi et al. (1994) utilized a
six degree-of-freedom Stewart platform crane—a three-
dimensional fully controllable manipulator—and an
ATLSS connection to attach the rigging beam. The ATLSS
connection is a joint for fast and easy assembly. In addi-
tion, Quicon� (The Steel Construction Institute 2004), plug
and play joints (Bijlaard et al. 2009), and ConX� (2015)
are all joint innovations well-known for fast connections.
Lee et al. (2012) presented a non-powered multi-beam
lifting system for improving efficiency of the steel beam
erection process. On the other hand, several construction
robot are also used in industry, such as Auto-Claw, Auto-
Clamp, Robotic End-Effector for Big Canopy, and Auto-
mated Building Construction System (ABCS) developed
by Obayashi Corporation (Bock and Linner 2016a). The
ABCS contains an alignment and accuracy measurement
system to check the alignment by vision and laser sensors.
Shimizu Corporation and Samsung Corporation developed
a robot crane end-effector Mighty Jack, Auto-Shackle, and
Mighty Shackle Ace for assisting with steel beam posi-
tioning and installing (Bock and Linner 2016b). Saidi et al.
(2006) proposed a RoboCrane system end-effector to
manipulate rigging beam precisely.
Jung et al. (2013a) developed a robot-based construction
automation system for high-rise buildings. The system
included a construction factory (Kim et al. 2009), a scissor
jack-type manipulator (Jung et al. 2008), and the robotic
beam assembly system (Jung et al. 2013b). The construc-
tion factory is a large and safe workspace, also named Sky
Factory, which is assembled outside the unconstructed
building and can move vertically during the construction
process, like a tower crane, carrying workers and con-
struction machinery. ABCS (Obayashi Corporation), Aka-
tuki 21 (Fujita Corporation), FACES (Goyo), MCCS
(Maeda Corporation), SMART (Shimizu Corporation), and
T-Up (Taisei Corporation) are well-known construction
systems in industry featuring construction factory (Bock
and Linner 2016b). The scissor jack-type manipulator can
lift the steel beam to the assembly location and the robotic
beam assembly system will assemble the steel beam to the
existing column. The robotic beam assembly system
includes a teleoperation system (Jung et al. 2013b), a
transport mechanism (Jung et al. 2013a), a robotic bolting
device (Chu et al. 2013), and a specially-designed steelFig. 2 Three steps for typical steel beam erection and assembly
RAS: a robotic assembly system for steel structure erection and assembly 461
123
beam with an automatic guide rope (Kim et al. 2016). Nam
et al. (1946) introduced a boom-mounted, combined
robotic system and wire-suspended positioning system for
automatic steel beam assembly.
A key aspect of the manipulator is the bolting robot. The
main purpose of the bolting robot is to attach the steel
beam to the column with bolts. The bolting robot utilizes a
camera and a computer vision method to detect bolt holes
(Mo et al. 2014), once detected, the robotic bolting device
will install the steel bolts (Chu et al. 2013).
1.3 Research goal
In this study, we develop a robotic assembly system (RAS)
for steel beam erection and assembly, which aims to
improve the safety and the efficiency of the steel beam
assembly process. The RAS can rotate, align, and bolt the
steel beam without help from steel workers at the height of
construction. Removing steel workers from heights on the
construction site during the steel beam erection and
assembly process prevents falls and injuries when struc-
tures fail, which are the design implications of the pro-
posed robotic assembling system. In addition, the
efficiency of the operation can easily be controlled since
the manual factor has been excluded from the process. In
comparison with previous research, this system is easily-
removable and light-weight, which meets the requirements
of the current erection method and can be broadly intro-
duced to existing construction sites. The system is vali-
dated by a scaled physical experiment in our laboratory.
We compare the RAS with the traditional method on a
basis of operation space and operation time. In Sect. 2, we
describe the system architecture of the RAS. Details of the
assembly method are illustrated in Sect. 3, Sect. 4, and
Sect. 5. In Sect. 6, we introduce the scaled physical
experiment for validation. The experimental results are
shown and discussed in Sect. 7. Finally, we discuss the
limitation and conclusion of the study in Sect. 8.
2 Robotic assembly system architecture
The system was designed by observing and reproducing the
current steel structure erection and assembly process, as
shown in Fig. 2. We utilize a rotation method to rotate the
rigging beam to the right angle. A vertical and horizontal
alignment method is developed to align the beam; a bolting
method is developed to attach the beam; and an unloading
method is developed to unload the crane cable. We will
discuss all the methods in the following section.
Two workers are required for the RAS, one is the ground
operator and the other is the tower crane operator. The
detailed procedure of steel beam erection and assembly
using the RAS is illustrated in Fig. 3, with a comparison
of current process. The RAS consists of four key methods:
the rotation method, the vertical and horizontal alignment
method, the bolting method, and the unloading method.
First, the ground operator attaches the steel beam to the
tower crane hook and prepares for the erection. The rig-
ging beam and the RAS must be adjusted such that they
are fully horizontal. Second, the tower crane operator
transports the beam to the assembly position, and aligns
roughly. Third (the vertical alignment method), the RAS
helps the operator to align the height of the beam to a
proper level, such that the beam can later be successfully
connected to the column. The ground operator has to
double check whether the beam is aligned correctly
through the camera. Fourth (the rotation method), the RAS
rotates the beam to the assembly orientation. Fifth (the
horizontal alignment method), the crane operator adjusts
the horizontal position of the beam accurately. The ground
operator has to check that all the bolts are in the correct
bolt holes through the camera. Notice that if the beam
fails to get to the right position, the RAS has to go back to
the rotation step and repeat the process. Sixth (the bolting
method), the beam is attached with bolts, and the tem-
porary connection is completed. The ground operator has
to check whether all bolts have been installed correctly.
The rough alignment step must be repeated if the RAS
failed to install any of the bolts. Seventh (the unloading
method), the RAS unloads the beam-hook connecting
cable. Finally, the tower crane operator removes the RAS
and repositions for the next beam. We will provide
detailed descriptions of the four methods listed above in
the following sections.
3 Rotation method
We employ the principle of conservation of angular
momentum to realize the rotation method. A rotation box
with a flywheel is installed on top of the rigging beam to
generate angular momentum and the beam generates an
inverse angular momentum. Figure 4 shows a side per-
spective of the rotation box. The flywheel is rotated by a
motor through an axle and gears. A motor controller and a
wireless router are used to control the flywheel by the
ground operator. After the beam has arrived at the proper
position, the operator turns on the flywheel until the beam
rotates to the correct angle. The rotation box is connected
by two pairs of connecter bracket that clip to the beam
during the process. The camera on the rotation box is used
to realize the alignment method.
Figure 5 shows the mathematical model of the rotation
method. Since the friction of the bearing between the crane
hook and the block can be minimized, we simply assume
462 Ci-Jyun Liang et al.
123
the friction is zero. We also neglect the effect of the wind
given the massive weight of the rigging beam. The angular
velocity of the beam given by the conservation of angular
momentum equation is
xb ¼Iw
Ibxw ð1Þ
where Iw, Ib represent the moment of inertia of the flywheel
and the rigged beam, and xw, xb represent the angular
velocity of the flywheel and the rigged beam.
The angular velocity of the flywheel is provided by a
motor inside the rotation box, the maximum revolution per
Fig. 3 The procedure of beam
erection and assembly with:
a current process, and b with the
RAS
Fig. 4 The side perspective view of the rotation box
Fig. 5 The mathematical model of the rotation method (side view)
RAS: a robotic assembly system for steel structure erection and assembly 463
123
minute of which is xm. The angular velocity of the fly-
wheel can be split into two periods. First is the accelerating
period and second is the constant velocity period. In the
accelerating period, the angular velocity is xw ¼ ata,where ta is the accelerating time to reach the maximum
revolution per minute xm. In the constant velocity period,
we assume the angular velocity of the motor always
reaches the maximum revolution per unit time. Therefore,
the angular velocity of the rigged beam can be derived
from (1) and the angular velocity of the flywheel, as shown
in Fig. 6.
In order to select a proper motor for the rotating system,
we have to calculate the maximum power Pmax of the
motor. The angular acceleration a is given from the motor
s ¼ Iwa ð2Þ
where s is the torque of the motor. We can then calculate
the power P by (1) and (2)
P ¼ sxm ¼ Iwað Þxm ¼ Iwxw
taxm ð3Þ
Knowing that when xw ¼ xm, P is the maximum value
P ¼ Pmax ¼ Iwx2
m
tað4Þ
From (4) and Fig. 6, we find that the maximum revo-
lution per minute of the motor and the accelerating time
influence the rotation time of the rigging beam and the type
of the motor we must select.
4 Alignment method
The alignment method consists of the vertical alignment
and the horizontal alignment. The objective of the vertical
alignment is to check whether the rigging beam reaches the
right height. We use a camera to detect the marker on the
column and inform the crane operator whether the beam
reaches the right height by a transmission signal to the
control cabin. Figure 7 illustrated the vertical alignment
method. If the marker lies at the center of the camera
frame, the vertical alignment is completed. In order to
attach the beam, the vertical alignment position of the
rigging beam must be slightly higher than the bracket.
Length d represents the distance from the center of the
camera lens to the beam top surface. Length d is the ver-
tical distance between the beam and the bracket, which is
also the distance from the center of the guide hole to the
bolt hole, as shown in Fig. 8. Therefore, the centroid of the
marker is d þ d higher than the bracket. Length L is the
distance from central of the camera lens to the column,
which we will use to determine the marker size.
The camera captures the image and searches for the
marker, as shown in Fig. 9. If the beam reaches the right
level, the marker can be found on the image and the
operator will be informed. The marker size is influenced by
the erection swag. Figure 10 shows the mathematical
model of the influence on the marker size due to the
erection swag. The marker length D is
D ¼ 2L tan h ð5Þ
where L represents the distance between the camera and the
column and h represents the pendulum angle. The pendu-
lum equation, according to Kuo and Kang (2014), is
d2hdt2
¼ a
lcos h� g
lsin h ð6Þ
where a represents the crane operation acceleration, l
represents the crane cable length and g represents the
gravity. The marker width B is
B ¼ 2l sin h ð7Þ
Therefore, we can determine the marker length D and
width B with (5), (6) and (7). The marker size and location
needs to be set on a correct column location in the manu-
facturing factory before delivering to construction site. A
minor adjustment based on the environmental condition is
also required on-site before starting the erection and
assembly process.
The camera has to stay at the right orientation during the
rough positioning and vertical alignment, in other words,
facing the column; we use a gyro sensor and a motor to
control the orientation of the camera. Before the vertical
alignment step starts, the motor will rotate the camera in
the direction of the column.
The objective of the horizontal alignment is to adjust the
rigging beam to the assigned position. Since the beam has
been aligned at the correct height during vertical align-
ment, the horizontal alignment will only consider planar
positioning. We change the shape of the flange plates to
parallelograms so that the beam will not get stuck during
Fig. 6 Schematic diagram of angular velocity of the rigging beam as
a function of rotating time
464 Ci-Jyun Liang et al.
123
the rotation process. In addition, this shape allows the beam
to be easily controlled by the tower crane in case the beam
is not at the right position. Figure 11 shows the horizontal
alignment method. The bolting steel plates are used to
validate the accuracy of the alignment. The operator has to
check whether all bolts are positioned in the guide holes.
5 Bolting and unloading method
For providing a faster bolting process, we use a ‘‘plug and
play’’ method instead of a traditional ‘‘tightening bolts’’
method. Figure 8 shows the front view of the bolting steel
plate. We add two additional guide holes through the bolt
holes because only two bolts are needed for temporary
connection. The bolting steel plates are attached to the
bracket and the rigging beam before erection. After fin-
ishing the horizontal alignment, the bolts have been posi-
tioned in the guide holes. The crane operator then releases
the rigged beam and the bolts slide into the bolt holes, as
shown in Fig. 12, completing the temporary bolt attach-
ment step. We have designed a new nut for this method.
The nut has two parts: the sliding part and the attachment
part. The sliding part is used to connect the bolting steel
plate to the beam and slide down through the guide hole to
the bolt hole. These newly designed nuts will be assembled
and welded before the erection process in order to prevent
detachment at the assembly elevation. Then the beam will
be assembled by the attachment part to fully achieve the
temporary connection.
The unloading method is used to remove the RAS and
unload the crane cable. The rotation box is connected to the
crane hook by the cable before the erection process. We
also utilized a simple gripping mechanism to mount the
rotation box on the rigged beam. Therefore, the RAS can
simply be removed by the tower crane during the unloading
step. The cable connecting the rigging beam and the beam
hook also needs to be unloaded during this step. We use a
pin mechanism, cable, and motor to realize the unloading
operation. The cable attaches to the pin bar and the motor.
Fig. 7 The vertical alignment
method (partial side view)
Fig. 8 The bolting steel plate
(front view)
Fig. 9 The camera captures the image and searches for the marker
RAS: a robotic assembly system for steel structure erection and assembly 465
123
After the temporary bolting assembly step is completed, the
motors start to roll the cable and extract the pin bar from
the pin hole. Then, the cable will release and unload the
rigged beam. Finally, the tower crane will remove the RAS
from the attached beam and reposition for the next target.
6 Scaled physical experiment
For validating the RAS, we implemented a scaled physical
experiment, which includes a tower crane and a steel
structure. We used KUKA KR 16 CR (KUKA 2005) to
simulate the tower crane, as shown in Fig. 13a. The KUKA
is a six degree of freedom industrial robot arm which
connects with the cable and the hook on the end effector.
We used block board and steel bracket to build the steel
structure. The steel structure model is an experimental
structure from the National Center for Research on Earth-
quake Engineering (Lin et al. 2013), which contains one
beam and two columns, and was scaled with length ratio
a ¼ 0:4. Figure 13b shows the scaled steel structure. The
bolting steel plates with guide and bolt holes were
manufactured by OMAX 2652 JetMachining� Center
(OMAX 2016).
The rotation box was also implemented in the scaled
physical experiment. Figure 13c shows the scaled rotation
box. We used plywood to fabricate the outer covering. The
flywheel, the motor, the controller and the connector were
demonstrated by the TETRIX� (2014) and LEGO�
Mindstorms NXT (2014). The TETRIX� is a robotic
toolkit which contains metal members, motors, controllers
and batteries. We used the metal member to fabricate the
connector and the flywheel, which was connected to the
motor, as well as the motor controller and the battery. The
LEGO Mindstorms NXT was utilized as a process unit. We
connected to the LEGO Mindstorms NXT through Blue-
tooth to control the motor revolution velocity. LabVIEW
(2014) was used to program the controller software. For the
vertical alignment, we used the GHI. Net Gadgeteer kit
(2014) and green paint as marker. The Gadgeteer kit con-
tains a mainboard, a camera module, and a multicolor LED
module. We used the camera module to capture the column
image. When the camera detects the green paint, the
multicolor LED module will start to flash and inform the
Fig. 10 The mathematical model of the influence on marker size of the erection swag: a left side view and b back view
Fig. 11 The horizontal
alignment method: a top view
and b side view
466 Ci-Jyun Liang et al.
123
operator that the beam has reached the right height. We
utilized green paint since the camera is most sensitive to
green color (Brown 2004). The detailed specifications of
the scaled experimental scenario are listed in Table 1.
7 Scaled experiment result
The results of the scaled physical experiment are illustrated
in the following section. We discuss the comparison
between the traditional method and the proposed method
based on two factors: the operation space and the operation
time.
7.1 Result
The procedure of the experiment follows the process of
beam erection and assembly with the RAS, as shown in
Fig. 3. First, the ground operator prepares for the beam
erection and assembly process, as shown in Fig. 14a. The
rotation box must be set up and installed on the top of the
rigging beam. The ground operator also checks the hori-
zontal status of the beam before transporting it, to ensure
that all bolts can be positioned in the guide holes later, as
shown in Fig. 14b. Second, the crane operator transports
the beam to the assembly position, as shown in Fig. 14c.
Third, the beam is roughly aligned above the assembly
position, as shown in Fig. 14d.
Fourth, the crane operator adjusts the altitude of the
beam using the vertical alignment method. The camera on
the rotation box is rotated to the column orientation and
captures the image, as shown in Fig. 15a. We use image
processing to detect the color at the center of the image. If
the camera detects the green color, the LED light will start
to flash and inform the crane operator, as shown in
Fig. 15b.
Fig. 12 The concept of the
bolting method: a, c Before
releasing and b, d after
releasing
RAS: a robotic assembly system for steel structure erection and assembly 467
123
Fifth, the ground operator starts the rotation box motor
and rotates the beam to the assembly angle, as shown in
Fig. 16. The rotating beam is stopped when the steel
bolting plates reaches the proper assembly position. Sixth,
the crane operator adjusts the horizontal position of the
beam with the horizontal alignment method. The ground
operator has to check through the camera that the beam is
at the right position and that all bolts are in the guide holes,
as shown in Fig. 17.
The rotation time of the proposed system is illustrated in
Table 2. The Beam I is the real size of the experiment
structure (Lin et al. 2013) and the Beam II is a steel beam
from a real steel structure. We use a motor with 1500 rpm
and a 1500 kg m2 flywheel for the real rotation box. The
accelerating time is 10 s. The angle of the rotation is set at
90 degrees. The resulting rotation time and motor power,
calculated by (4) and Fig. 6, are listed in Table 2.
Seventh, the crane operator releases the beam and lets
the bolts slide into the bolt holes, as shown in Fig. 18. The
ground operator must check that all the bolts are in the bolt
holes and completely fastened. Eighth, the RAS unloads
the pin mechanism of the beam-cable connection and the
rotation box. Then the rotation box is repositioned by the
tower crane and prepares for the next beam attachment
process, as shown in Fig. 19.
To validate the RAS, we compared the RAS with the
traditional method. Table 3 shows the operation time
results. The traditional method is tested on a thirty-floor
Fig. 13 The scaled physical experiment: a the scaled tower crane, b the scaled steel structure, and c the scaled rotation box
Table 1 The detail
specification of the scaled
physical experiment
Items Components Specification
Construction equipment Tower crane KUKA KR 16 CR robot arm
Steel structure Steel beam
Steel column
H 290 9 160 9 15 9 15 9 1720 (mm)
BOX 280 9 280 9 15 9 1000 (mm)
Rotation box Outer covering 400 9 160 9 220 (mm)
Flywheel Cuboid 290 9 225 9 30 (mm)
Motor TETRIX� DC Gear Motor
12-Volt, 152 rpm and 300 oz-in
Motor controller TETRIX� DC Motor Controller
Process unit LEGO Mindstorms NXT
Vertical alignment GHI.Net Gadgeteer
72 MHz. 32-bit ARM Mainboard
320 9 240 20FPS Camera module
Multicolor LED Module
Battery TETRIX� 12-Volt NiMH Battery
Software Control program NI LabVIEW 2010
468 Ci-Jyun Liang et al.
123
steel reinforced concrete construction site located in Tai-
wan. The steel beam size is illustrated in Table 2 Beam II.
We recorded the steel beam erection and assembly process,
counted the operation time for ten times, and calculated the
average operation time. We performed two different tasks.
The first was the low-level operation, which took place at
the level of the 3rd floor, and the second was the high-level
operation, which took place at the level of the 20th floor. In
the RAS low-level task, the ground operator can directly
monitor the whole operation and inform the crane operator
when the alignment and bolting are completed. Conversely,
for the high-level task, the ground operator can only
monitor the whole operation through a camera. In addition,
the rotation time is affected by the scale ratio. According to
Kuo and Kang (2014),
a ¼ c2 ð8Þ
where a represents length ratio and c represents time ratio.
Thus, the time ratio c ¼ 0:63.
The results show that the traditional method took 501 s
to position and attach one steel beam at the low-level, while
the RAS took 55 s, which amounts to a reduction in oper-
ation time by about 89%. The rotation method of the RAS
took 19 s, which are similar to the calculation result from
Table 2 applied with time ratio c. The alignment and bolt-
ing operation shows a significant improvement with the
Fig. 14 The procedure: a preparing for the beam erection and assembly, b checking the horizontal status, c transporting, and d rough alignment
RAS: a robotic assembly system for steel structure erection and assembly 469
123
assistance of camera alignment and since the RAS only
needed to release the beam for bolting. The unloading
method is also reduced to a simple unplugging process in
comparison with traditional loosing bolt process. At the
high-level, the traditional method took 514 s to position and
attach one steel beam and the RAS took 69 s, reducing the
operation time by about 86%. The alignment method took
almost double the time to achieve since the ground operator
can only utilize the camera to check the accomplishment.
7.2 Discussion
We discuss the comparison between the traditional method
and the proposed system considering two main factors: the
operation space and the operation time. The operation
space is the space for operating the process; the operation
time is the time for operating the process.
7.2.1 Operation space
The operation space is the space size for operating the
process, including the rigging path, the rotation and
alignment area and the steel workers working area. For the
rigging path, the traditional method and the proposed
system are almost the same. They simply have to transport
the beam to the assembly position. For the rotation and
alignment operation, the proposed method is significantly
shorter than the traditional method. The alignment method
in the proposed system reduces the unnecessarily manual
alignment process. For the steel workers working area, the
traditional method needs a working area on the bracket and
the beam for steel workers. The proposed system does not
need the steel workers working area. Therefore, the pro-
posed system needs less operation space than the tradi-
tional method.
Fig. 15 The vertical alignment: a adjusting the altitude of the beam and b achieving the vertical alignment
Fig. 16 Rotating the beam
470 Ci-Jyun Liang et al.
123
7.2.2 Operation time
The operation time is the time for operating the process.
The operation time of the proposed system is listed in
Table 2 and Table 3. In the rotation step, the traditional
method uses manual drag to rotate the beam, which is
time-consuming and requires more human workers. In the
alignment step, the traditional method relies on human
workers to align. In the bolting step, the traditional
method needs much more time than the proposed system
since in this system we utilize the plug and play method
instead of the tightening of bolts method. In the unloading
step, the pin mechanism can accelerate the unloading
process.
7.3 Limitation
The limitations of the RAS are listed in this section. First,
we have assumed that the rigging beam can remain fully
horizontal at all times; however, the beam might be not
horizontal during the process and this would cause the RAS
fail. We will design a horizontal mechanism to address this
issue in future work. Second, the RAS only operates the
process until the temporary connection is completed. The
full connection of the beam still requires human workers to
finish. Third, when the rotation method is operated manu-
ally, the overshoot issue will happen and cause structural
damage; thus, a suitable rotation controlling method needs
to be implemented in the future. In addition, utilizing a
Fig. 17 The horizontal alignment: a checking that the beam is at the right position and b checking that all the bolts are in the guide holes
Table 2 The comparison of the
rotation timeBeam I Beam II Scaled Beam
Beam section (mm) H 800 9 400 9 22 9 32 H 800 9 300 9 14 9 26 H 290 9 160 9 15 9 15
L (mm) 5000 10,000 1720
Weight (kg) 1630 2099 5
Ib (kg m2) 3396 17,492 1.989
xm (rpm) 1500 1500 152
Iw (kg m2) 2 2 8.982 9 10-3
xb (�/s) 5.298 1.032 7.717
ta (s) 10 10 7. 9 10-2
Rotation time (s) 21.99 92.21 11.74
Power (W) 4934.80 4934.80 33.72
RAS: a robotic assembly system for steel structure erection and assembly 471
123
dual flywheel system instead of the single flywheel system
could also maintain the rigging beam and reduce the
swaying effect. Fourth, the marker detection for the vertical
alignment can be influenced by outdoor light conditions
causing the alignment to be unstable. In our method, we
used color detection for tracking the marker, instead a
suitable marker tracking algorithm could be applied in the
RAS, such as AprilTag (Olson 2011) or KEG tracker (Feng
and Kamat 2013). These trackers can work well under
outdoor light conditions. Fifth, the vertical alignment is
operated by the crane operator, who relies on signal feed-
back from the rotation box. The delay of the signal might
cause the vertical alignment to be insufficient or overshoot.
Therefore, the motion controller combined with the marker
tracking algorithm for vertical alignment needs to be fur-
ther developed to address this issue. Sixth, the scaled
experiment result is based on the indoor environment. We
neglected some outdoor environmental factors such as
wind and weather issues.
With the aspect from two experts in academic structural
engineering and one in construction industry, the special
cutting shape needs further verification for welding speci-
fication. In addition, it is not cost-effectiveness for material
using. Therefore, in the future work, we need to develop a
robust beam orientation controlling system for assembling
a regular shape beam.
8 Conclusion
We developed a robotic assembly system (RAS) for steel
structures. The rotation method, the alignment method, the
bolting method, and the unloading method are the four
main operations performed by the RAS. The rotation
method utilizes a flywheel and the conservation of angular
momentum to rotate the rigging element. The alignment
method utilizes a camera and a marker on the column to
ensure the altitude of the beam is correct. By using a
parallelogram flange plate, the beam can be easily aligned.
The bolting method uses a plug and play method. We add
an additional guide hole above the bolt hole on the steel
bolting plate; therefore, the bolt can plug into the guide
Fig. 18 Assembling the bolts
472 Ci-Jyun Liang et al.
123
Fig. 19 Unloading and repositioning
Table 3 Comparison of the traditional method and the RAS operation time
Operation time (s) Task 1 Task 2
Low level (3rd floor) High level (20th floor)
Traditional (T) RAS (R) Reduction rate (%) Traditional (T) RAS (R) Reduction rate
Rotation 76 19 75.00 82 20 75.61
Alignment 201 27 86.57 197 41 79.19
Bolting 191 1 99.48 195 1 99.49
Unloading 33 8 75.76 40 7 82.50
Total 501 55 89.02 514 69 86.58
RAS: a robotic assembly system for steel structure erection and assembly 473
123
hole and slide to the bolt hole. The unloading method is a
pin mechanism and can be easily unloaded. A scaled
physical experiment was implemented to verify the feasi-
bility of the system. We found that the RAS can operate the
steel beam placement and connection process without steel
workers having to be in high places. In addition, the RAS
needs less operation space and can finish the process faster.
To sum up this research, the system described is intended
to replace the human workers in high-rise building con-
struction. This could greatly reduce accidental falls as well
as improve the efficiency of the steel structure assembly
process.
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Ci-Jyun Liang is currently a
graduate student in Robotics
Institute at University of
Michigan, Ann Arbor. He is
working with Prof. Vineet
Kamat in the Laboratory for
Interactive Visualization in
Engineering (LIVE). His
research interests include
autonomous construction site
robot, computer vision, machine
learning, and virtual/augmented
reality. He also received a M.S.
and B.S. in Civil Engineering
from National Taiwan
University.
Prof. Shih-Chung Kang,Ph.D. of Stanford University, is
currently a professor in depart-
ment of civil engineering in
National Taiwan University. His
research focuses on the
advanced visual and robotics
tools for engineering purposes.
He developed multiple methods
on the automation and simula-
tion for crane operations. He is
now the Editor-in-Chief of
Visualization in Engineering.
He edited multiple special
issues on the topics related to
the visualization and robotics applications. He is also an active
researcher on innovative engineering education. He offers courses on
engineering graphics, game development, data visualization and
robotics and was awarded NTU excellent teacher. His course on
engineering graphic was ranked top 2% MOOCs among Chinese
learners.
RAS: a robotic assembly system for steel structure erection and assembly 475
123
Dr. Meng-Hsueh Lee is a Post
Doc researcher in Center for
Weather Climate and Disaster
Research at National Taiwan
University. He received his
Ph.D. and M.S. from the
Department of Civil Engineer-
ing at National Taiwan Univer-
sity in 2009 and 2004, and his
B.S. in Department of Civil and
Construction Engineering at
National Taiwan University of
Science and Technology in
2002. His research interests
include construction knowledge
management map, construction enterprise resource planning, seman-
tic analysis, and data mining.
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