1 Introduction

The steel beam erection and assembly process is always in the critical construction path and accounts for a high percentage 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 construction 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 precise joint position by hand, by wire, or even by foot, as shown in Fig. 1c. This step accounts for the highest percentage 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 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.

Fig. 1
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Steel beam assembly process: a lifting, b transporting, c aligning and d bolting

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1.1 Crane operation safety and efficiency

Safety and efficiency issues are very important in construction 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 stereoscopic 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-exploring 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 collisions. 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) analyzed the steel erection process and presented the factors 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 suspension 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 construction system and used on a reinforced concrete building site (Wakisaka et al. 2000). Alternately, the motor controllable 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).

Fig. 2
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Three steps for typical steel beam erection and assembly

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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 addition, 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 Automated 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 positioning 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 construction 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 construction machinery. ABCS (Obayashi Corporation), Akatuki 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 steel 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 structures fail, which are the design implications of the proposed 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 introduced to existing construction sites. The system is validated 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 rigging 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 temporary 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.

Fig. 3
figure 3

The procedure of beam erection and assembly with: a current process, and b with the RAS

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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 perspective 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.

Fig. 4
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The side perspective view of the rotation box

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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 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

$$ \omega_{b} = \frac{{I_{w} }}{{I_{b} }}\omega_{w} $$
(1)
Fig. 5
figure 5

The mathematical model of the rotation method (side view)

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where \( I_{w} \), \( I_{b} \) represent the moment of inertia of the flywheel and the rigged beam, and \( \omega_{w} \), \( \omega_{b} \) 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 minute of which is \( \omega_{m} \). The angular velocity of the flywheel 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 \( \omega_{w} = \alpha t_{a} \), where \( t_{a} \) is the accelerating time to reach the maximum revolution per minute \( \omega_{m} \). 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.

Fig. 6
figure 6

Schematic diagram of angular velocity of the rigging beam as a function of rotating time

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In order to select a proper motor for the rotating system, we have to calculate the maximum power \( P_{max} \) of the motor. The angular acceleration \( \alpha \) is given from the motor

$$ \tau = I_{w} \alpha $$
(2)

where \( \tau \) is the torque of the motor. We can then calculate the power \( P \) by (1) and (2)

$$ P = \tau \omega_{m} = \left( {I_{w} \alpha } \right)\omega_{m} = I_{w} \frac{{\omega_{w} }}{{t_{a} }}\omega_{m} $$
(3)

Knowing that when \( \omega_{w} = \omega_{m} \), \( P \) is the maximum value

$$ P = P_{max} = I_{w} \frac{{\omega_{m}^{2} }}{{t_{a} }} $$
(4)

From (4) and Fig. 6, we find that the maximum revolution 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 \( \delta \) is the vertical 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 \( d + \delta \) 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.

Fig. 7
figure 7

The vertical alignment method (partial side view)

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Fig. 8
figure 8

The bolting steel plate (front view)

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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 \( \Delta \) is

$$ \Delta = 2L\tan \theta $$
(5)
Fig. 9
figure 9

The camera captures the image and searches for the marker

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Fig. 10
figure 10

The mathematical model of the influence on marker size of the erection swag: a left side view and b back view

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where \( L \) represents the distance between the camera and the column and \( \theta \) represents the pendulum angle. The pendulum equation, according to Kuo and Kang (2014), is

$$ \frac{{d^{2} \theta }}{{dt^{2} }} = \frac{a}{l}\cos \theta - \frac{g}{l}\sin \theta $$
(6)

where \( a \) represents the crane operation acceleration, \( l \) represents the crane cable length and \( g \) represents the gravity. The marker width \( B \) is

$$ B = 2l\sin \theta $$
(7)

Therefore, we can determine the marker length \( \Delta \) and width \( B \) with (5), (6) and (7). The marker size and location needs to be set on a correct column location in the manufacturing 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 alignment, 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 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.

Fig. 11
figure 11

The horizontal alignment method: a top view and b side view

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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 finishing the horizontal alignment, the bolts have been positioned 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 attachment 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.

Fig. 12
figure 12

The concept of the bolting method: a, c Before releasing and b, d after releasing

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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. 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 Earthquake Engineering (Lin et al. 2013), which contains one beam and two columns, and was scaled with length ratio \( \alpha = 0.4 \). 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).

Fig. 13
figure 13

The scaled physical experiment: a the scaled tower crane, b the scaled steel structure, and c the scaled rotation box

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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 Bluetooth 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 contains 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 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.

Table 1 The detail specification of the scaled physical experiment
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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 horizontal 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.

Fig. 14
figure 14

The procedure: a preparing for the beam erection and assembly, b checking the horizontal status, c transporting, and d rough alignment

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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. 15
figure 15

The vertical alignment: a adjusting the altitude of the beam and b achieving the vertical alignment

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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.

Fig. 16
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Rotating the beam

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Fig. 17
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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

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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.

Table 2 The comparison of the rotation time
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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.

Fig. 18
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Assembling the bolts

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Fig. 19
figure 19

Unloading and repositioning

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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 steel reinforced concrete construction site located in Taiwan. 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),

$$ \alpha = \gamma^{2} $$
(8)
Table 3 Comparison of the traditional method and the RAS operation time
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where \( \alpha \) represents length ratio and \( \gamma \) represents time ratio. Thus, the time ratio \( \gamma = 0.63 \).

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 operation 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 \( \gamma \). The alignment and bolting operation shows a significant improvement with the 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 proposed system needs less operation space than the traditional method.

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 manually, 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 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 feedback 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 further 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 specification. 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 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 feasibility 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 construction. This could greatly reduce accidental falls as well as improve the efficiency of the steel structure assembly process.