DEVELOPMENT OF LIGHTWEIGHT DUAL ARM ROBOT BY

USING HOLLOW SHAFT SERVO ASSEMBLY

Min-Kyu Park

Division of Mechanical Engineering, Yeungnam College of Science & Technology

274 Hyeungchungro, Deagu, South Korea

Seok-Jo Go

Division of Mechanical Engineering, Dongeui Institute of Technology, 152 Yangjiro, Pusan, South Korea

Young-Jin Lee

Autopower Co. LTD., 1048-9 Obang-dong, Kimhae, Kyungnam, South Korea

Keywords: AUTOMAN, Dual Arm Robot, Hollow Shaft Servo Assembly, Lightweight Robot Arm.

Abstract: Studies on dual arm robots have been continually carried out because the robots provide human-like

flexibility of movement. In previous study, the dual arm robot with solid shafts had been designed.

However, the robot has some defects in its heavy weight and big shape. In this study, a hollow shaft servo

assembly for lightweight robot arm has been developed. And then, the dual arm robot (AUTOMAN) has

been designed using a hollow shaft servo assembly.

1 INTRODUCTION

Single arm industrial robots are applied in the

process where the simple repetitive operation is

much like the automotive industry and the

semiconductor industry. There are many tasks that

are difficult or impossible to do with the single arm

robots. The dual arm robot is able to solve these

problems (University of Texas at Austin, DLR,

Yaskawa Co.). In the previous study, the dual arm

robot with solid shafts had been developed as shown

in Figure 1 (Park, 2008). The robot with solid shafts

is easily implemented. However, as the degree of

freedom of a robot rises, when implementing the

complicated motion, some problems are appeared

such as kink and abrasion of cables. As to the dual

arm robot with hollow shafts, the inside of an arm

and a motor shaft are empty. It is free from a

constraint by the electrical wire in the

implementation of complicated motion in

comparison with the robot with solid shafts (Han,

2008). Also, the load to weight ratio of a robot is

improved. Accordingly, basic researches on the

hollow shaft servo assembly had been carried out in

the previous studies (Han, 2008; Lee, 2008; Go.

2009).

The goal of this study is to develop a dual arm

robot with the hollow shaft servo assembly. Firstly,

the hollow shaft servo assembly for applying to the

dual arm robot is developed. Secondly, the

performance of the completed servo assembly is

evaluated. Finally, the industrial hollow shaft

articulated dual arm robot (called as AUTOMAN)

tries to be developed.

Figure 1: Dual arm robot developed in previous study.

409

Park M., Go S. and Lee Y. (2010).

DEVELOPMENT OF LIGHTWEIGHT DUAL ARM ROBOT BY USING HOLLOW SHAFT SERVO ASSEMBLY.

In Proceedings of the 7th International Conference on Informatics in Control, Automation and Robotics, pages 409-414

DOI: 10.5220/0002944304090414

 SciTePress

2 HOLLOW SHAFT SERVO

ASSEMBLY

In order to apply hollow shaft to industrial dual arm,

the specifications of each axis have to be

determined. The payload of the robot is set as 20kg.

The specifications of the each hollow shaft servo

assembly are determined by analyzing the maximum

torque and the steady state torque under tracking

trajectories at full reach within given time like

Figure 2. The capacity, torque and maximum

velocity of each axis are finally obtained by

applying a safety factor to analyzing value as shown

in Table 1 (Lee, 2008).

The assembly drawing of the hollow shaft servo

assembly for the dual arm robot is shown in Figure 3.

The key components comprising the servo assembly

are the hollow shaft servo motor, the hollow shaft

multi-turn absolute encoder, the hollow shaft brake,

and, the hollow shaft harmonic drive. Table 2 shows

the axis specification of the developed hollow shaft

servo assembly.

Figure 2: Dynamic critical conditions of dual arm robot.

Table 1: Specifications of servo motors.

Axis Capacity Torque Velocity

1, 2 2.0 kW 6.8 Nm 3,000 RPM

3, 4 0.9 kW 2.9 Nm 3,000 RPM

5 0.6 kW 1.9 Nm 3,000 RPM

6, 7 0.4 kW 1.3 Nm 3,000 RPM

Table 2: Specifications of hollow shaft servo assembly.

Axis

Capacity

[kW]

Torque

[Nm]

Velocity

[rpm]

Reduction

gear ratio

Weight

[kg]

Length

[mm]

2.0 6.8

3,000

120 23.5 270

2 120 23.9 270

0.9 2.9

120 11.6 236

4 120 12.5 236

5 0.6 1.9 80 5.1 237

0.4 1.3

80 4.9 213

7 80 4.3 213

harmonic drive

rotor

stator

electric brake

absolute encoder

hollow shaft motor

frame

harmonic drive

rotor

stator

electric brake

absolute encoder

hollow shaft motor

frame

Figure 3: Hollow shaft servo assembly.

2.1 Hollow Shaft Servo Motor

As to the development of the hollow shaft servo

motor, because all cables enter into the hollow shaft,

the inner diameter of hollow shaft is as big as

possible and it should not have a problem in an

assembly between the hollow shaft encoder and the

hollow shaft brake. Moreover, because the hollow

shaft rotates, the protecting duct in which it protects

the cable has to be installed and the axis of motor

has to be designed by considering the connection

with the axis of a reduction gear.

Accordingly, the hollow shaft servo motor was

developed in consideration of the hollow size and

the axis capacity of the motor. Figure 4 shows the

developed hollow shaft servo motor. The cores of

stator and rotor used the Si-steel S18 and S30

respectively. And the magnets of these used Nd-Fe-

B and N-35SH.

Since being the servo system, sensors such as a

resolver or a hollow shaft absolute encoder is

essential. Accordingly, the hollow shaft absolute

encoder is selected as shown in Table 3 (Tamagawa

Seiki Co.).

Figure 4: Hollow shaft servo motor.

Table 3: Specifications of applied encoder.

Axis

Resolution

[bit]

Revolution

[RPM]

Weight

[kg]

1,2

1,500 2.1

3~7 3,000 1.2

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2.2 Hollow Shaft Brake

The development of the hub shape type brake is

needed, because the hollow shaft of the servo motor

can be penetrated. The excitation functional type

brake stops a rotator when a current is electrified in

a coil. Therefore, this brake is suitable to the

machine with horizontal motion. On the other hand,

as to the electro-magnetic operable brake, when a

current is electrified in a coil, the braking power

loses and a rotator is free. This type brake is usually

applied by the machines with vertical motion. In this

research, since the vertical motion had to be possible

in the small and narrow place, the hollow shaft brake

of the non-excited operation type is developed by

considering the required torque and dimension of

each axis. Figure 5 shows the developed hollow

shaft brake and the specifications are shown in Table

Figure 5: Hollow shaft brake.

Table 4: Specifications of developed hollow brake.

Axis

Frictional

Tor

[

]

Weight

[

Outer Dia. / Inner

Dia. / Thickness

1,2 6.86 2.77 147 / 51 / 38.3

3,4 2.94 1.18 104 / 41 / 36/2

5,6,7 2.45 0.54 74.8 / 26 / 29/8

2.3 Hollow Shaft Harmonic Drive Unit

The reduction gear applied to the dual arm robot has

to be fitted the hole size of the hollow. And not only

the high-torque and but also the high precision are

needed. The hollow shaft harmonic drive made by

Harmonic Driver Systems in Japan is applied

(Samick HDS). The motor shaft and the axis of

reduction gear are connected as shown in Figure 6.

The test results of eccentricity and torsion stress are

satisfied the required specifications as shown in

Table 5. The specifications of developed hollow

shaft harmonic drive are presented by Table 6.

2.4 Performance Test of Hollow Shaft

Servo Assembly

After making one side of the dual arm robot as

Figure 7, the test of repeatability carried out. Table 7

shows the specifications of the developed hollow

shaft robot arm. In the performance evaluation about

the robot arm, the repetition precision about the

location tracking was verified according to the KS

7082 standard (KS B 7082,) After from P1 to P5

designated the coordinate in which a robot has to

follow according to the three-dimensional space-

phase like Figure 8, the repetition tracking was

performed by 30 time series. The laser tracker was

used to measure the positions. Results of

repeatability test are as shown in Figure 9. The

average repeatability about each site is ± 0.05 mm

within and it is satisfactory.

Figure 6: Hollow shaft harmonic drive.

Table 5: Performance test results of the unit.

Spec. Test 1 Test 2

Concentricity1

0.02 and

less

0.0103 0.0122

Concentricity2 0.0047 0.0185

Diameter[mm] 50 + 0.015 50.0144 49.9985

Length[mm]

300±0.3

300.0625 300.1394

Torque[Nm]

More than

4.7

274.5 246.37

Table 6: Specifications of the hollow harmonic drive.

Axis Ratio

Torque

[Nm]

Speed

[RPM]

Weight

[kg]

Outer/Inner

[mm]

1,2 120 523 3,000 6.9 190 / 52

3,4 120 178 3,000 3.1 142 / 36

5,6,7 80 44 3,000 0.89 90 / 21

Table 7: Specifications of developed hollow Shaft robot

arm.

Weight Length Payload

Specification 94kg 1,574mm 20kg

DEVELOPMENT OF LIGHTWEIGHT DUAL ARM ROBOT BY USING HOLLOW SHAFT SERVO ASSEMBLY

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Figure 7: Developed hollow-type robot arm.

Figure 8: Reference trajectory.

Figure 9: Results of repeatability test. [unit : mm].

DESIGN OF DUAL ARM ROBOT

In the previous research, the solid type dual arm

robot like Figure 1 had been developed (Park, 2008).

The dual arm robot developed in the previous

research has the both arms of 6 degree of freedoms

with the payload 20 kg, and the body (torso) of 2

degree of freedom. However, as to the developed

dual arm robot, some problems have occurred such

as the overweight compared with payload, the

increase of the servo motor capacity due to a volume

and weight enlargement, a kick and an abrasion of

cables. In order to solve these problems, the hollow

shaft servo assembly is developed. In the

development of the robot, it tried to lighten through

the stress analysis and optimal design. The servo

control systems like servo drivers and controller are

used by commercial controller.

3.1 Design and Fabrication of Robot

The dual arm robot makes to put an emphasis on the

improvement of the weight and movement. Housing,

frame, cover and the key element parts of robot are

developed by considering the design specifications

as shown in Figure 10. Especially, in the case of the

housing, in order to satisfy the maximum allowable

deflection 3 mm at full reach of the horizontal

direction, the target rigidity is obtained by the CAE

analysis (Lee, 2008). Figure 11 shows the dual arm

robot (AUTOMAN) in which it makes in this

research. It can show the reduction of weight and the

improvement of design. AUTOMAN is composed of

two arms and one twist joint. Each arm has 7 DOF.

Therefore, AUTOMAN totally has 15 DOF. The

robot can be designed so that cables can pass

through the central part of a robot. The length of the

arm is about 1,500 mm. Table 8 shows the

specification comparison with the solid type dual

arm robot.

Figure 10: Design of motor frame and housing.

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Figure 11: AUTOMAN.

Table 8: Specifications of developed dual arm robot.

Item

Solid type dual

arm robot (Fig. 1)

AUTOMAN

Controlled axes

14(Right 6, left 6,

rotate 2)

15(Right 7, left 7,

rotate 1)

Payload 20 kg 20 kg

Mass 500 kg 300 kg

Repeatability ±0.7 mm ±0.1 mm

Maximum Speed 100 °/s 100 °/s

3.2 Development of Control System

The robot control system is designed for considering

convenience of developing environment, powerful

computing, real-time operation ability. Figure 12

shows the control system of the robot. The MEI

ZMP controller (The Motion Engineering) and

PMAC controller (DELTA TAU) were examined in

order to select the controller in which it is suitable

for the dual arm robot (MEL, DELTA TAU). It is

altogether able to use with these controllers in case

the moving track of a robot is predetermined.

Figure 12: Control system of AUTOMAN.

However PMAC controller has evaluated that it

could not be guaranteed a real-time ability in case

the moving track of a robot is determined in real

time such as a visual servoing control and a force

control. Accordingly, in order that the target

platform of the control system was determined, the

ZMP board (DELTA TAU), that is the commercial

controller in which a performance is verified, was

selected as the controller of the dual arm robot.

4 CONCLUSIONS

This research tried to be comprised the light weight

of the dual arm robot by the hollow shaft servo

assembly, and a miniaturization and highly-rigid in

order to solve a problems reflected in the previous

developed dual arm robot. The hollow shaft servo

assembly is developed by designing and fabricating

the hollow shaft servo motor, the hollow shaft brake,

and so on. And the dual arm robot (AUTOMAN) for

the flexible package operation is designed and made.

The control system is made in consideration of a

convenience and operation processing speed of the

development environment. The performance of the

arm is confirmed that the average repeatability is

measured by ± 0.05 mm within and it is satisfactory.

Moreover, by installing in the cable, kink and

friction problems were solved. It is finally remarked

that the comparison between the solid type robot and

the developed hollow type robot will be undertaken

as a second phase of this work.

ACKNOWLEDGEMENTS

This work was supported by the Ministry of

Commerce, Industry and Energy under Grant No.

10025003.

REFERENCES

University of Texas at Austin, http://www.robotics.

utexas.edu/rrg/.

DLR, http://www.dlr.de/en/.

Yaskawa Inc., http://www.motoman.com/.

Park, C. H., Park K. T., and Kim, D. H., “Design of dual

arm robot manipulator for precision assembly of

mechanical parts,” 2008 International Conference on

Smart Manufacturing Application, pp. 424-427, April

2008.

Han, H. G., Go, H. Y., Lee, Y. C., Jung, C. G., Shin, Y. S.,

Go, S. J., and Lee, Y. J., “Development of Hallow

DEVELOPMENT OF LIGHTWEIGHT DUAL ARM ROBOT BY USING HOLLOW SHAFT SERVO ASSEMBLY

413

Shaft Servo Assembly for Light Weight Robot,”

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Shin, Y. S., and Lee, Y. J., “Development of Hallow

Shaft Servo Assembly for Dual Arm Robot and Its

Performance Evaluation,” KACC 2009, pp. 58-61,

September 2009.

Samick HDS, http://www.samickhds.co.kr.

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MEI, Inc., Product Manual.

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