Assist System for Carrying a Long Object with a Human - Analysis of a Human Cooperative Behavior in the Vertical Direction -

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1 Assist System for Carrying a Long with a Human - Analysis of a Human Cooperative Behavior in the Vertical Direction - Yasuo HAYASHIBARA Department of Control and System Engineering Toin University of Yokohama 64 Kurogane-cho, Midori-ku, Yokohama, Kanagawa, , JAPAN Tomohito TAKUBO Institute of Engineering Mechanics University of Tsukuba - Tennoudai, Tsukuba, Ibaraki, , JAPAN Yukinobu SONODA System Electronics Tokyo Engineering University 44- Katakura-cho, Hachiouji, Tokoyo, , JAPAN Hirohiko ARAI, Kazuo TANIE Robotics Department Mechanical Engineering Laboratory/MITI -2 Namiki, Tsukuba, Ibaraki, , JAPAN Abstract: This paper deals with an assist system for carrying a long object with a human operator When we carry such an object, we often grasp both ends and move it cooperatively Our purpose is to establish how to design the assist system which can achieve such a task It is difficult to apply conventional control laws On the other hand, human can achieve such a task Therefore, we measure the human cooperative behaviors and analyze them to find the cooperative rules Based on the rules, we propose a control law of the assist system Furthermore, we construct a prototype system and verify the validity of the control law Introduction We have studied about an assist system for carrying a long object with a human operator When we carry a long object, we often grasp both ends and move it cooperatively By grasping both ends, the operators should not generate large torque at their arms tips Furthermore, this style of conveyance is easy to move a long object precisely as compared with other styles Our purpose is to establish how to design the assist system which can achieve such a task with a human operator Figure The assist system under carrying a long object with a human naturally The assist system contains a robot arm, a force sensor, a controller and so on In the system, since it is difficult to measure the force applied by the operator, we can not apply the conventional power assist method [,2] On the other hand, the human can achieve such a task based on only the force applied to his/her hands Therefore, we measure the human cooperative behaviors under carrying a long object and analyze them to find the cooperative rules After that, based on the rules, we design a control law for the assist system Such a research on the human cooperative behavior has been already studying [3] However, the proposed cooperative rules are not enough to construct the practical assist system In the other research, the cooperative conveyance of a long object has been realized without the analysis of the human behavior [4] However, they didn t consider such a conveyance as a human operator and a manipulator grasp each end of the object It may be difficult to apply to this style of conveyance Therefore, we have studied about the human cooperative behavior for applying to the practical assist system We have already proposed a control law in the horizontal direction [5] In this paper, we discuss the control law in the vertical direction At first, we introduce an experimental setup for measuring the human cooperative behaviors Next, we explain how to measure and analyze them Then, we design a model of the human cooperative behaviors in the vertical direction and propose a control law Finally, we introduce a prototype of the assist system based on our proposed control law and verify its effectiveness by a simple experiment 2 Experimental Setup The figure 2 shows the experimental setup for measuring the human cooperative behaviors It is the same as the setup for measuring the cooperative behaviors

2 NTSC 28inch display RGB NTSC converter z l y RGB Strage (every 5s) (Master) Computer Forces CCD camera Tracking NTSC Position Figure 2 The experimental setup (Slave) Video tracker in the horizontal direction It measures tracking motions and forces of the subjects under carrying a long object cooperatively Two subjects grasp the handles and carry an object face to face One subject is given the target area on a computer display He/She tries to carry the object to a target area as fast as possible We define him/her as a master Another subject is blindfolded and not given the target area He/She tries to carry the object based on only the force applied to his/her hands We define him/her as a slave The specifications of the object are as follows The distance between the handles is chosen between 5 and (m) The whole weight including force sensors is 7(kg) The six-axes force/torque sensors (NITTA JR3) are attached to both ends Each sensor measures the force which is applied to the arm s tip of each subject respectively However, in this experiment, we do not use the measured forces for the analysis The video tracker (OKK G22) with a CCD camera (HITACHI VM-HL) tracks two black markers, which are placed on the tops of the handles The measured data is sent to a computer (NEC PC-982As) and transformed to the values of the position/posture of the objects These transformed data are stored in the computer each 5(s) On the computer display, two symbols are indicated only to the master A symbol corresponds to the target area and a symbol corresponds to the center of the object Each symbol moves according to each position respectively The size of the symbol implies the size of the target area (5(m) 5(m)) Each line drawn from the center of each symbol indicates each posture respectively The subjects also try to rotate the object according to the target posture In this experiment, the target posture is always horizontal Its margin is ±π/6 Figure 3 The coordinate system The subjects carry the object to the target area After keeping the object inside the target area for 2(s), the target symbol moves discretely according to the position/posture of the next target When this task is repeated 6 times or the period of the experiment is over 2(s), the experiment is finished 3 Experimental Method Using the experimental setup, we measured the cooperative behaviors in the vertical direction The origin of the measurement was placed near the center of the object when the experiment was started as shown in the figure 3 At that time, the subjects stood with straightening their back and grasped the handles with closing their sides The positions of the target area were determined randomly on a vertical line 4(m) long previously Other conditions of the experiments are listed in the table In the experiment (a), the subjects carry a shorter object than one in the experiment (b) These conditions are prepared for investigating the influence of the length of the object In the experiment (c), the handle at the slave side can rotate freely This condition is prepared for confirming the importance of the information of the object s posture Before these experiments, we explained subjects about the experiments and asked the followings: ) move the object to the target area as quick as possible, 2) keep the object s posture horizontal, 3) fix their bodies and move only their arms Furthermore, we also asked the subjects to practice the operation until they thought themselves to become skilled (at least 5(min)) In this practice, we used a set of target areas which was different from one for the experiments The experiments were performed by 4 males in their 2 s Table The conditions of the experiments Distance between the Handle handles l (m) (a) 5 Fixed (b) Fixed (c) 5 Rotative

3 4 Experimental Results The figure 4 shows experimental results for a subject Other results are similar It indicates two kinds of graphs in each experiment The upper one illustrates vertical trajectories of the object and the target The lower one illustrates the angle of the object with the horizon The target angles are always (rad) In the experiment (c), we can find that the subjects can not complete the task because they can not keep the object s posture horizontal Position (m) Position (m) Position (m) Experiment (a) (l = 5(m), fixed handle) Experiment (b) (l = (m), fixed handle) Experiment (c) (l = 5(m), rotative handle) Figure 4 Results of the tracking motions (subject SK) On the other hand, in the experiment (a) and (b), they can complete the task They can carry the object into the target area smoothly Considering about that, for completing this task, the slave needs the information of the object s posture Therefore, we design a control law whose input is the object s posture Based on this idea, we compare the slave s behaviors with the object s angles The figure 5 shows the velocity of the arm s tip in the experiment (a) It is derived by calculating the difference of the position of the arm s tip and applying it a low pass filter with the time constant t = 5(s) Comparing the figure 5 with the figure 4(a), we can find that the peaks of the angle θ and the velocity y& s are almost proportional as the following equation: Velocity (m/s) Velocity (m/s) Velocity (m/s) y& s = aθ (a < ) () 2 Figure 5 The velocity of the arm s tip of the slave in the experiment (a) Measured Estimated 2 Experiment (a) (a = -23(m/s/rad)) Tvs + Tvs a Figure 6 The model of the human cooperative behavior in the vertical direction (T v : time constant) Measured Estimated 2 Experiment (b) (a = -24(m/s/rad)) Figure 7 Comparison of the measured velocity and the estimated velocity (T = (s)) ys ys

4 where a is a proportional coefficient This model implies that the slave always try to keep the object s posture horizontal It is a reasonable model By such a behavior of the slave, the conveyance in the vertical direction can be achieved However, when the subjects stay the object, these values are not proportional Although the velocity is nearly, the angle changes slowly and it is not always nearly Considering about that, these values are not proportional in a low frequency band Therefore, we also apply a high pass filter for eliminating the value in the low frequency band Based on this idea, we design a model of the cooperative behavior in the vertical direction The figure 6 shows the model Using this model, we calculate estimated velocities The figure 7 shows the results It indicates the comparison of the estimated velocity and the measured velocity The time constant T v is determined heuristically Its value is (s) The proportional coefficients a are determined by the least square method However, between these signals, there is a little time lag Then, before calculating, we shift the signal of the angle to 5(s) later for fitting the peaks of the signals In the figure 7, we find that the model is almost appropriate However, when we apply this model to the control law in the assist system, the system can not keep the object s posture horizontal by the effect of the high pass filter When we apply the high pass filter, the assist arm does not follow the slow motion of the operator Therefore, in this paper, we do not apply the high pass filter to the prototype system The proportional coefficients a for all subjects are indicated in the table 2 Comparing these values, the values in the experiment (b) are always smaller However, since the differences are not so large, we apply an average of them to the prototype system X F Coordinate Transformation y z fy fz fy ϕ Ths - Ths a m Ix b bx s s fz fy zd yd d yd zd d Xd ϕ J( ϕ) Angular Velocity Controller F = [ f y, f z, τ ] T : force vector applied to the assist arm j : joint angle vector X = [y, z, θ ] T : position vector of the assist arm s tip T h : time constant for the high pass filter in the horizontal direction m, b : desired inertia and viscous friction coefficients in the horizontal direction I x, b x : desired moment of inertia and viscous friction coefficients on the axis X & [,, ] T d = y d z d θ d : desired velocity vector J(j ): Jacobian matrix j& d : desired joint angular velocity vector t m : torque vector which the actuators should generate Figure 8 The concept of the controller in the system d τm Table 2 Proportional coefficients experiment (a) (m/s/rad) experiment (b) (m/s/rad) subject SK subject SH subject NS subject SF average Prototype of the Assist System Based on these results, we constructed a prototype of the assist system It is used for verifying the effectiveness of the control law The figure 8 shows the concept of the controller in the system The controller in the horizontal direction is designed according to our proposed control law [4] In the previous experiment, we confirmed the Figure 9 The photograph of the prototype

5 dynamics of the human cooperative behaviors in the horizontal direction could be described as a sum of a constant inertia m and a viscous friction b Therefore, in the horizontal direction, we apply an impedance controller We also apply a high pass filter according to the experimental results In this conveyance, the torque generated by the slave at his/her arm s tip is little Therefore, we suppose the arm s tip is like a free joint We apply an impedance controller with a small moment of inertia I x and a viscous friction b x The figure 9 is a photograph of the prototype system A robot arm is controlled with an angular velocity controller (MITSUBISHI PA-) It has 7 joints but we use 3 joints now Other joints are fixed At the tip of the robot arm, a 6 axes force/torque sensor (NITTA JR3) is implemented It measures the force applied by a human operator and the object The computer (IBM PC Compatible) acquires the joint angle vector j and the force vector F Then, it calculates the desired joint angular velocity vector j& d The calculation is repeated every 2(ms) The values of the parameters are determined considering the experimental results The impedance parameters m, b, I x and b x are 54(kg), 26(kg/s), 5(kgm 2 ) and 5(kgm 2 /s) respectively The time constant T h is (s) The proportional coefficient a is 25(m/s/rad) The parameters m, b and a are the averages of the values which are derived by the least square method 6 Verification of the control law For verifying the effectiveness of the control law, we performed a simple experiment using the assist system The experimental method is similar to the above experiment The figure shows the experimental setup The system assists the conveyance in the vertical plane instead of a human operator In this experiment, the video tracker is not used because the position/posture of the object are calculated from the joint angles of the assist arm Positions of the target area are selected in the vertical plane randomly (y:-~(m), z:-~(m)) The distance between the arms tips is about 7(m) The experiment was performed by a male in their 2 s Vertical Horizontal Position (m) Position (m) (a) The position/posture of the object and the target NTSC 28inch display z (m) 5 RGB RGB NTSC converter Assist Arm Area Assist System Side Side 5 5 y (m) Forces Strage (every 5s) Computer Joint Angles 5 Angular Velocity Controller Figure Experimental Setup with the Assist System (b) A part of the trajectory of the object Figure The experimental results (Subject KO)

6 The figure illustrates the experimental result The upper one illustrates the positions/postures of the object and the target The lower one illustrates a part of the trajectory of the object From these figures, we find the assist system works appropriately and carry the object to the target area smoothly 7 Conclusion In this paper, we have discussed about how to design the controller in the assist system, which carry a long object with a human operator Since human can achieve such a task, we try to obtain the cooperative rule from a human behavior In this paper, we measure and analyze the behavior in the vertical direction In consequence, we propose a control law based on an idea as keeping the object s posture horizontal Then, the controller generates the velocity in proportion to the object s angle with the horizon It has a particular advantage that the assist arm should not generate large torque at the arm s tip Based on this control law, we construct a prototype of the assist system We also verify it to be able to assist the conveyance task in a vertical plane In this paper, we do not apply the high pass filter to the controller in the vertical direction We will consider the role of the filter in the future To establish the control law in the assist system, the following will be also studied in the future: measuring the human cooperative behavior in all directions, 2 confirming the effectiveness of the control law in various tasks 8 Acknowledgement The Ministry of International Trade and Industry provided founding for this work We also thank members of the Biorobotics laboratory in the MEL/MITI for their generous assistance References: [] Hayashibara, Y, Tanie, K, Arai, H, Power Assist System - A Proposed method with Consideration of Actuator Saturation -, Proc IFToMM Ninth World Congress on the Theory of Machine and Mechanisms, Italy, 995, pp a robot for cooperation with a human, IEEE International Conference o Robotics and Automation, Japan, 995, pp [4] Kosuge, K, Kazamura, N, Sato, M, Arao, M, Egi, M, Kawano, J, Shimamura, J, Human-Robot Cooperation System with Human Friendly Robot Marvel (Japanese), The 6th Annual Conference of the Robotics Society of Japan, Japan, 998, pp 5-6 [5] Hayashibara, Y, Tanie, K, Arai, H, Sonoda, Y, Power assist system for carrying a long object with a human (Analysis of human cooperative behavior),proc IFToMM Tenth World Congress on the Theory of Machine and Mechanisms, Finland, 999, pp [6] Kosuge, K, Fujisawa, Y, Fukuda, T, Mechanical System Control with Man-Machine-Environment Interactions, IEEE Connference on Robotics and Automation, 993, pp [7] Kosuge, K, Hashimoto, S, Takeo, K, Coordinated Motion Control of Multiple Robots Manipulating a Large, IEEE/RSJ International Conference on Intelligent Robots and Systems, France, 997, pp [8] Kazerooni, H, Mahoney, SL, Dynamics and Control of Robotic Systems Worn by Humans, ASME Jornal of Dynamic Systems, Measurement and Control, Vol33, No3, 99, pp [2] Hayashibara, Y, Tanie, K, Arai, H, Development of Power Assist System with Individual Compensation Ratios for Gravity and Dynamic Load, IEEE/RSJ International Conference on Intelligent Robots and Systems, France, 997, pp [3] Ikeura, R, Inooka, H, Variable impedance control of