2011 11th International Conference on Control, Automation and Systems Oct. 26-29, 2011 in KINTEX, Gyeonggi-do, Korea

1. INTRODUCTION

Research on service robots has been paid enormous

attention by many institutions to develop robots that can help human beings in various aspects. The ultimate goal of service robots is to develop human-like robots that can have the mobility and the manipulability just like humans.

ASIMO [1] and HUBO [2] are one of service robots that have the walking capability. The size of ASIMO is designed based on our home environment to perform various functions such as carrying objects and turning light switches. Unfortunately, both robots are not ready to serve human beings yet, but provide the possibility of a future service robot by demonstrating stable walking motions. While ASIMO and HUBO have been focused on stable walking, not manipulation, Mahru has been developed for the manipulation purpose [3].

For service robots to be practical, manipulability is more important than stable walking since the mobility can be achieved by wheels. This leads to research on a mobile manipulator which has two arms based on wheels. Twendy-one is a typical mobile manipulator that demonstrates to serve human beings with foods [4,5]. Two heavy arms to carry objects generate vibration and their movements are slow due to visual processing.

In other aspect to serve human beings, a cloth arrangement robot has been developed. The cloth folding robot arranges T-shirts after laundry [6]. The robot uses a camera to detect the shape of clothes, uses two arms to hold both ends of the top of the shirt, and then folds it in the sequential manner.

There are many different ways to serve human beings. From the examples of aforementioned service robots, it can be realized that serving human beings is quite difficult for the robot to perform like human

beings. However, research has to carry on step by step procedure to improve performances of service robots.

In the framework of wheeled mobility, most mobile manipulators have a four wheeled structure to have a stable posture all the time. Recently, two wheels based mobility demonstrates a better maneuvering performance in the narrow space although it has less stability. Segway is a typical two wheeled mobile robot that has been commercialized [7]. An intelligent method has been applied to robust balancing control of two wheeled mobile robot [8, 9]. Two wheeled mobile robots are controlled under constrained environment [10, 11].

As an extension, the two wheeled structure is adopted to mobile manipulators. Limbo robot having two wheels and one manipulator has demonstrated stable balancing control for the Limbo dance game [12].

As a further extension of research on the balancing robot, an interaction control between ROBOKER and a human operator has been performed [13]. The balancing robot is a kind of a mobile manipulator, but it has only two wheels to maintain balance. When a force induced by a human operator is applied to ROBOKER, the robot maintains balance based on force control.

In this paper, a cooperative task between two balancing robots is performed as shown in Fig. 1. Two balancing robots are constrained to carry a box without dropping it. Two balancing robots form a master and slave structure. The slave robot has the force control capability to follow the master robot by to maintaining a desired. Experimental studies are demonstrated to confirm cooperative tasks between two balancing robots.

Experimental Studies of an Object Handling Task by Force Control between

Two Balancing Robots

Seung Jun Lee and Seul Jung Intelligent Systems and Emotional Engineering Laboratory

Department of Mechatronics Engineering Chungnam National University, Daejeon, Korea

(Tel : +82-42-821-6876; E-mail: jungs@cnu.ac.kr)

Abstract: This paper presents experimental studies of a cooperative control task between two balancing robots, which have two wheels to balance and navigate on the ground. Two balancing robots are constrained to carry a box without dropping it. Two balancing robots form a master and slave structure such that the master robot leads the slave robot and the slave robot follows. The slave robot has a force sensor for the force control capability that tries to maintain a desired force based on impedance formulation. Experimental studies of handling an object are demonstrated to confirm the feasibility of utilizing balancing robots for cooperative tasks. Keywords: balancing robot, force control, cooperative control

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Fig. 1 Concept of cooperation between two balancing robots

2. ROBOKERs Two balancing robots called ROBOKERs are

implemented. Fig. 2 shows ROBOKER 1 and Fig. 3 shows ROBOKER 2. ROBOKER 1 has two arms and each arm has 6 degrees of freedom. ROBOKER 1 has a stretchable waist so that it can reach a higher object.

Fig. 2 ROBOKER 1

ROBOKER 2 is a new version of ROBOKER series

to fit the Korean life style that Korean people live on the floor and still being implemented for its two arms. It has several new design features; a stretchable waist, separable bodies, a sliding waist, and a sliding arm. A stretchable waist allows the robot to reach the higher object. Separable bodies of two parts, the upper manipulation part and the lower mobile robot part allows the mobile robot to perform cleaning tasks. The sliding waist back and forth can help balancing by moving the center of gravity. The sliding arm structure allows the robot to reach objects on the floor easily.

Although ROBOKER 2 shown in Fig. 3 is still required to be developed further, all of design features are included except robot arms.

Fig. 3 ROBOKER 2

3. CONTROL SCHEMES 3.1 Balancing control The balancing mobile robot requires both control of

position and balance with two wheels. Although the robot is a nonlinear non-holonomic system whose dynamics is quite complicated, linear control methods are used for control performances.

Two wheel torques,LR ττ , , are required to control the

position, p , the orientation φ , and the balancing angle θ . Here linear control methods are used for the balancing and navigation control. Each variable is controlled by PD or PID control methods. Position and orientation are sensed by encoders attached to motors. The balancing angle is detected by a gyro sensor.

)()(..

θθθθ θθθ −+−= dddp kku

)()()( vvkdtppkppku ddpdipdppp −+−+−= ∫ (1)

)()()( ωωφφφφ φφφφ −+−+−= ∫ dddidp kdtkku

where iidipi kkk ,, are controller gains for each variable.

Then each wheel torque is designed as a sum of each control input in (1).

φθ

φθ

τ

τ

uuu

uuu

pL

pR

−+=

++= (2)

where Rτ is the right and

Lτ is the left wheel torque.

Fig. 4 shows the balancing control block diagram. After completing balancing control performance of two ROBOKERs, force control is applied to the slave robot.

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Fig. 4 Position control block diagram

3.2 Impedance Force Control The concept of an impedance control method is to

regulate the contact force by the relationship between position/velocity and force. Adjusting impedance parameters controls the force indirectly. An applied force can be regulated by selecting the impedance relationship with position [14].

The original impedance function is given as

ccce kxxbxmf ++=...

(3)

where kbm ,, are impedance parameters selectively decided by a user, ef is the contact force and cx is the displacement.

To implement the position based impedance control, the Laplace transform of (3) is given as a second order filter of the measured force, ef to generate the

positional correction term, cx [15].

)(1

)(2

sFkbsms

sX ec ++= (4)

where s is the Laplace operator.

From (4), the sensed force )(sFe is filtered by the impedance function to generate the position correction term )(sX c . Therefore, cx can be either the

positional correction term cp or the orientation

correction term cφ . Those correction terms are added to the position and the orientation to form the position based impedance force control algorithm.

PID control methods for the position and the orientation are given.

)()()( ''' vvkdtppkppku ddpdipdppp −+−+−= ∫ (5)

)()()( '.. ωωφφφφ φφφφ −+−+−= ∫ dddidp kdtkku

where the modified position is

cdd ppp +=' , (6)

and the modified orientation is

cdd φφφ +=' . (7)

Fig. 5 shows the impedance force control block diagram with adjustments of position and orientation. Each wheel torque for the right and left wheel can be represented same as (2). The difference from the explicit force control method is that no switching part is necessary in the impedance formulation in Fig. 5.

Fig. 5 Impedance Control block diagram

4. EXPERIMENTS 4.1. Experimental Setup

Two balancing robots are tested for balancing control a priori before applying force control methods [16].

PID controller gains for two ROBOKERS are listed in Tables 1 and 2.

Table 1 PID gains for ROBOKER 1

Roboker1 PID-Gains Angle P-gain 150

I-gain 0 D-gain 8

Position P-gain 12 I-gain 0.3 D-gain 14

Heading P-gain 300 I-gain 1 D-gain 10

Table 2 PID gains for ROBOKER 2

Roboker2 PID-Gains Angle P-gain 104

I-gain 0 D-gain 5

Position P-gain 5 I-gain 0.3 D-gain 10

Heading P-gain 100 I-gain 1 D-gain 5

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Those controller gains are found through experimental studies.

ROBOKER 1 is a master and ROBOKER 2 is a slave such that the master leads the slave robot. The slave robot has a force sensor to capture the force from the master, and control the applied force to maintain a box without falling. Since the arms of ROBOKER 2 are not ready, a single bar is used as an arm to hold an object. Fig. 6 shows the physical setup of ROBOKERs for handling and control a box between two robots. The control frequency is 100Hz.

Fig. 6 Experimental setup of cooperation control of two balancing robot systems

4.2 Cooperative Task between Two ROBOKERs

Next experiment is the object handling control between robots, which form the master and the slave. Impedance parameters are selected based on experimental studies and listed in Tables 3 and 4. Table 3 shows the impedance parameters for position and Table 4 for orientation.

Table 3 Impedance parameters for position

Impedance Filter Parameter (Fx) – Position Mfx 6 Bfx 0.01 Kfx 0.1

Desired Xforce 8.5N LPF taufx 0.9

Table 4 Impedance parameters for orientation

Impedance Filter Parameter (Fy) – Heading Mfy 5 Bfy 0 Kfy 0.1

Desired Yforce 0N (threshold :±1N) LPF taufy 0.8

As the master robot pushes and pulls against the

object, the slave robot reacts to it. Fig. 7 shows the actual demonstration of handling a box between two robots. The plot shows that the master robot pushes against the slave robot to the right while maintaining a box.

Fig. 7 Actual demonstration of the cooperative task between ROBOKER 1 and ROBOKER 2

When the master robot pushes the box, the slave

robot moves back to react to the applied force from the master robot. The corresponding results are shown in Fig. 8, 9 and 10. The desired force is set to 8.5N as shown in Fig. 8. The actual force tracking plot of the slave robot is shown in Fig. 8. Although the force plot shows the overshoot, robots maintain the box successfully. Fig.9 shows the induced position from the impedance filter and the actual position. Fig. 10 shows the position tracking plot of the master robot and the slave robot. We clearly see that the slave robot follows the master robot.

Fig. 8 Force control

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Fig. 9 Position control

Fig. 10 Position control of the master and slave robots

5. CONCLUSIONS

This paper demonstrates experimental results of a cooperative control ability of two balancing robots forming a master and slave structure. Although maintaining balance of each robot is difficult, a cooperation task of carrying an object between two robots is executed. The object handling task between two balancing robots has been performed in limited conditions. Robots performed back and forth movements without arms.

A current service robot focuses on performing one task, however, a future service robot has to take care of every possible tasks to serve human beings. For service robots to be more practical, there are many constraints and problems to be solved. For the future ROBOKERs, robots can handle objects with arms and move on the plane.

ACKNOWLEDGEMENT

This work was supported by the Korea Research Foundation under basic research program (R01-2008-000-10992-0) and AIM under Human Resources Development Program for Convergence Robot Specialists (Ministry of Knowledge Economy), Korea.

REFERENCES

[1] ASIMO, “http://www.asimo.com” [2] HUBO, http://www.hubolab.co.kr [3] Marhu, “http://www.kist.re.kr/en [4] Twendy-one, “http://www.twendy.com” [5] H. Iwata, and S. Sugano, "Design of Human

Symbiotic Robot TWENDY-ONE", IEEE International Conference on Robotics and Automation, pp. 580-586, 2009.

[6] J. Maitin-Shepard, M. Cusumano-Towner, J. Lei and P. Abbeel, “Cloth Grasp Point Detection based on Multiple-View Geometric Cues with Application to Robotic Towel Folding”, the International Conference on Robotics and Automation (ICRA), 2010

[7] Segway, www.segway.com [8] S. S. Kim and S. Jung, “Control experiment of a

wheel-driven mobile inverted pendulum using neural network”, IEEE Trans. on Control Systems Technology, vol. 16, no. 2, pp. 297-303, 2008.

[9] H. J. Lee and S. Jung, "Gyro sensor drift compensation by Kalman filter to control a mobile inverted pendulum robot system”, IEEE International Conference on Industrial Technology, pp. 1026-1031, 2009.

[10] K. Sasaki and T. Murakami, “Pushing operation by two-wheel inverted mobile manipulator”, IEEE Workshop on Advanced Motion Control, pp. 33-37, 2008

[11] S. J. Lee, and S. Jung, “Stabilization of a two wheeled mobile robot system under external force”, URAI, pp.225-228, 2010

[12] K. Teeyapan, J. Wang, T. Kunz, and M. Stilman,“Robot limbo: Optimized planning and control for dynamically stable robots under vertical obstacles”, ICRA 2010

[13] J. K. Ahn, S. J. Lee, and S. Jung, “Force Control Application to a Mobile Manipulator with Balancing Mechanism”, IEEE IECON, pp. 1522-1527, 2010

[14] H. J. Lee, S. H. Yoon, Y. H. Kang, and S. Jung, "A wheeled-mobile robot with human interaction based on force control”, IEEE International Conference on Automation and Logistics, pp. 1072-1076, 2009.

[15] Z.D. Wang, Y. Hirata, and K. Kosuge, "Impedance-based Motion Control of Passive-type Robot Porter for Handling an Object", Proceedings of IEEE International Conference on Robotics and Biomimetics, pp.709-714 ,2006

[16] S. J. Lee, and S. Jung, “Balancing Control of a Home Service Robot, ROBOKER”, ICROS Daejeon-Chungchung Regional Conference, 2010

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