Transporting an Object by a Passive Mobile Robot with Servo Brakes in Cooperation with a Human

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<ul><li><p>This article was downloaded by: [Temple University Libraries]On: 08 December 2014, At: 05:33Publisher: Taylor &amp; FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK</p><p>Advanced RoboticsPublication details, including instructions for authors andsubscription information:</p><p>Transporting an Object by aPassive Mobile Robot with ServoBrakes in Cooperation with aHumanYasuhisa Hirata a , Zhidong Wang b , Kenta Fukaya c &amp;Kazuhiro Kosuge da Department of Bioengineering and Robotics, TohokuUniversity, 6-6-01, Aoba, Aramaki, Aoba-ku, Sendai980-8579, Japan;, Email: Department of Advanced Robotics, Chiba Institute ofTechnology, Narashino, Chiba 275-0016, Japanc Department of Bioengineering and Robotics, TohokuUniversity, 6-6-01, Aoba, Aramaki, Aoba-ku, Sendai980-8579, Japand Department of Bioengineering and Robotics, TohokuUniversity, 6-6-01, Aoba, Aramaki, Aoba-ku, Sendai980-8579, JapanPublished online: 02 Apr 2012.</p><p>To cite this article: Yasuhisa Hirata , Zhidong Wang , Kenta Fukaya &amp; Kazuhiro Kosuge(2009) Transporting an Object by a Passive Mobile Robot with Servo Brakes in Cooperationwith a Human, Advanced Robotics, 23:4, 387-404, DOI: 10.1163/156855309X408745</p><p>To link to this article:</p><p>PLEASE SCROLL DOWN FOR ARTICLE</p><p>Taylor &amp; Francis makes every effort to ensure the accuracy of all the information(the Content) contained in the publications on our platform. However, Taylor&amp; Francis, our agents, and our licensors make no representations or warrantieswhatsoever as to the accuracy, completeness, or suitability for any purposeof the Content. Any opinions and views expressed in this publication are theopinions and views of the authors, and are not the views of or endorsed byTaylor &amp; Francis. The accuracy of the Content should not be relied upon and</p><p></p></li><li><p>should be independently verified with primary sources of information. Taylor andFrancis shall not be liable for any losses, actions, claims, proceedings, demands,costs, expenses, damages, and other liabilities whatsoever or howsoever causedarising directly or indirectly in connection with, in relation to or arising out of theuse of the Content.</p><p>This article may be used for research, teaching, and private study purposes.Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expresslyforbidden. Terms &amp; Conditions of access and use can be found at</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Tem</p><p>ple </p><p>Uni</p><p>vers</p><p>ity L</p><p>ibra</p><p>ries</p><p>] at</p><p> 05:</p><p>33 0</p><p>8 D</p><p>ecem</p><p>ber </p><p>2014</p><p></p></li><li><p>Advanced Robotics 23 (2009)</p><p>Full paper</p><p>Transporting an Object by a Passive Mobile Robot withServo Brakes in Cooperation with a Human</p><p>Yasuhisa Hirata a,, Zhidong Wang b, Kenta Fukaya a and Kazuhiro Kosuge a</p><p>a Department of Bioengineering and Robotics, Tohoku University, 6-6-01, Aoba, Aramaki,Aoba-ku, Sendai 980-8579, Japan</p><p>b Department of Advanced Robotics, Chiba Institute of Technology, Narashino,Chiba 275-0016, Japan</p><p>Received 2 May 2008; accepted 7 August 2008</p><p>AbstractIn this paper, we introduce a passive mobile robot called PRP (Passive Robot Porter) to realize transportationof an object in cooperation with human, which is developed based on a concept of passive robotics. PRPconsists of three omni-directional wheels with servo brakes and a controller. It can manipulate an objectby controlling an external force/moment applied by a human based on the control of the servo brakes. Weconsider the characteristics of the servo brakes and control the brake torque of each wheel based on thebrake force/moment constraint so that several motion functions of PRP are realized based on the appliedforce. This allows PRP to track a path which includes motion perpendicular to the pushing direction of thehuman without using servo motors. The impedance-based motion control is also realized with respect to theperpendicular to the pushing direction. These functions are implemented on PRP experimentally, and theexperimental results illustrate the validity of PRP and its control method. Koninklijke Brill NV, Leiden and The Robotics Society of Japan, 2009</p><p>KeywordsPassive mobile robot, passive robotics, object handling, humanrobot interaction, brake control</p><p>1. Introduction</p><p>Most robots have been used as industrial robots in factories to replace humans do-ing tasks that humans do not want to do or could not do, and have been isolatedfrom humans. Recently, however, we expect to utilize robot systems not only in theindustrial fields, but also fields such as the home, office and hospital in cooperationwith humans. For realizing physical support for a human being using robot systems,we have to consider two main points: achieving high performance and user safety.</p><p>* To whom correspondence should be addressed. E-mail:</p><p> Koninklijke Brill NV, Leiden and The Robotics Society of Japan, 2009 DOI:10.1163/156855309X408745</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Tem</p><p>ple </p><p>Uni</p><p>vers</p><p>ity L</p><p>ibra</p><p>ries</p><p>] at</p><p> 05:</p><p>33 0</p><p>8 D</p><p>ecem</p><p>ber </p><p>2014</p></li><li><p>388 Y. Hirata et al. / Advanced Robotics 23 (2009) 387404</p><p>To achieve high performance, humanrobot cooperation issues have been stud-ied by many researchers [15] for augmenting human performance. Most of theseconventional intelligent robot systems have servo motors that are controlled basedon the sensory information from sensors such as force/torque sensors, ultrasonicsensors and laser range finders. The high performance for intelligent systems is re-alized in the form of functions such as power assist, collision avoidance, navigation,variable motion characteristics, etc.</p><p>However, if we cannot appropriately control the servo motors of the intelligentsystems, they can move unintentionally and might be dangerous for human beings.In particular, in Japan, the legislation has to be formulated for using them in a liv-ing environment practically. In addition, these systems with servo motors tend tobe heavy and complex, because of the servo motors, reduction gears, sensors, con-troller, batteries, etc. The battery problem is also very severe for long time working,as the servo motors need much electricity to work.</p><p>On the other hand, Goswami et al. have proposed concept of passive robotics [6],in which the system moves passively based on the external force/moment withoutusing the actuators, and have dealt with the passive wrist, whose components aresprings, hydraulic cylinders, dampers, etc. The passive wrist responds to an appliedforce by computing a particular motion and changing the physical parameters ofthe components to realize the desired motion. Peshkin et al. have also developeda handling system of an object called Cobot [7] based on passive robotics, whichconsists of the caster and the servo motor for steering its caster.</p><p>The concept of the passive robot has been extended to many fields. Wasson etal. [8] and Rentschler et al. [9] proposed a passive intelligent walkers. In most ofthese walkers, a servo motor is attached to the steering wheel, similar to the Cobotsystem [7], and the steering angle is controlled based on environmental informationfor navigating the user. We have also proposed a passive intelligent walker calledRT Walker (Robot Technology Walker) [10]. It differs from other passive intelligentwalkers in that it controls servo brakes appropriately to realize several functionswithout using any servo motors.</p><p>In addition, PADyC (Passive Arm with Dynamic Constraints) has been proposedas an assistant tool for surgeons [11]. Other applications for surgical robots are seenin Ref. [12]. Applications to rehabilitation have also been considered by many re-searchers. One example is shown in Ref. [13]. Applications of the concept to hapticdisplays have been proposed in Refs [14, 15]. These passive systems are intrinsi-cally safe because they cannot move unintentionally with a driving force. Thus,passive robotics will prove useful in many types of intelligent systems throughphysical interaction between the systems and humans.</p><p>In this research, we develop a new passive-type mobile robot system for han-dling a single object in cooperation with a human. The passive object handlingrobot called PRP (Passive Robot Porter) consists of three omni-directional wheelswith servo brakes, which can realize omni-directional motion, and its motion iscontrolled based on the servo brakes similar to RT Walker proposed by us [10].</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Tem</p><p>ple </p><p>Uni</p><p>vers</p><p>ity L</p><p>ibra</p><p>ries</p><p>] at</p><p> 05:</p><p>33 0</p><p>8 D</p><p>ecem</p><p>ber </p><p>2014</p></li><li><p>Y. Hirata et al. / Advanced Robotics 23 (2009) 387404 389</p><p>Different from the object handling robot called Cobot [7], PRP does not utilize anyservo motors for realizing the passive robotics concept; it utilizes the servo brakes,which have some unique characteristics. The constraints based on the characteris-tics of the servo brake make the control of a passive robot difficult compared torobot with servo motors. In this paper, we especially address the characteristicsof the servo brake and derive the servo brake condition for controlling the omni-directional passive mobile robot. Finally, experimental results on the path trackingmotion control and the impedance-based motion control for transporting an objectin cooperation with a human illustrate the validity of the proposed system.</p><p>2. PRP</p><p>We have developed a passive object handling robot called PRP as shown in Fig. 1based on the concept of passive robotics [6]. PRP consists of three omni-directionalwheels with servo brakes and a controller. The omni-directional wheel consists ofseveral small free rollers so that the wheel can move in all directions. Each omni-directional wheel is directly connected to a servo brake and the three wheels areequally spaced with axes sets spaced apart by 2/3 radi. Three encoders are alsoinstalled on three wheels for odometry. The brake systems of the whole wheels arepowered by batteries.</p><p>The control performance of PRP depends on the characteristics of the servobrakes. In the first prototype of PRP, we used the MR Brake (Magneto-Rheologicalfluid Brake: Load Corp., MRB-2107-3, maximum on-state torque: 5.6 N m) as thepassive actuator. The braking torque of the MR Brake is generated by chain mech-anisms of iron powder from the free-flow state which are reacting to the appliedmagnetic field. This provides high response and good linearity for controlling thebraking torque of wheels. In addition, it consumes a relatively small amount ofpower compared to servo motors and its weight is similar to a motorgear compo-nent with the same output torque.</p><p>Figure 1. PRP.</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Tem</p><p>ple </p><p>Uni</p><p>vers</p><p>ity L</p><p>ibra</p><p>ries</p><p>] at</p><p> 05:</p><p>33 0</p><p>8 D</p><p>ecem</p><p>ber </p><p>2014</p></li><li><p>390 Y. Hirata et al. / Advanced Robotics 23 (2009) 387404</p><p>3. Characteristics of the Servo Brake</p><p>In this section, we first explain the characteristics of the servo brake and de-rive the servo brake condition for controlling PRP. The servo brakes have someunique characteristics. PRP with servo brakes can only move based on the exter-nal force/moment applied to it, because it does not have any active actuators suchas servo motors. This is a very important feature in realizing safety actions. Also,some constraints make the control of the passive robot difficult compared to therobot with servo motors. Let us consider how the output torque of an actuator isapplied to a mobile robot in the case of active and passive actuators such as a servomotor and a servo brake, respectively.</p><p> Output torque of servo motor. It is well known that the torque applied to thewheel w will be equal to the output torque of the servo motor as:</p><p>w = kmIm, (1)where Im denotes the input current of a motor and is expressed as Im_max Im Im_max. km denotes the torque constant of the motor. Without losing gen-erality, the gear ratio is assumed as 1.</p><p> Output torque of servo brake. To control the motion of PRP based on the ex-ternal force few applied to the wheel with the servo brake, we can derive thefollowing relationships with the respect to the angular velocity of the wheelwith servo brakes w:(i) For w = 0</p><p>w =kbIb sgn(w). (2)(ii) For w = 0:</p><p>w ={fewRw |few|Rw kbIbkbIb sgn(few) |few|Rw &gt; kbIb, (3)</p><p>where sgn() is the function to have sign of a parameter. w is the brake torqueapplied to the wheel which is generated by the servo brake of PRP. Ib is theinput current for the servo brakes and is expressed as 0 Ib Ib_max. kb is thepositive coefficient expressed as the relationship between the brake torque andthe input current, and Rw is the radius of the wheel.</p><p>Different from the control of the robot with servo motors, we can only controlthe motion of PRP under the relationships of the servo brake. It is obvious thatthe characteristics of the brake system of wheels are complicated compared to amotorwheel system. The characteristics of the brake system depend on the wheelrotational direction. The sign of the output torque of the wheel is decided by thedirection of the wheel rotation and the magnitude of the torque is proportional to theinput current of the brake. From (2), we can have the following condition betweenthe angular velocity of the wheel and the braking torque of a brakewheel system:</p><p>ww 0. (4)</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Tem</p><p>ple </p><p>Uni</p><p>vers</p><p>ity L</p><p>ibra</p><p>ries</p><p>] at</p><p> 05:</p><p>33 0</p><p>8 D</p><p>ecem</p><p>ber </p><p>2014</p></li><li><p>Y. Hirata et al. / Advanced Robotics 23 (2009) 387404 391</p><p>This condition is the servo brake control constraint of the system and indicates thatthe system cannot have arbitrary torque from a servo brake. Therefore, we need toconsider the feasible brake torque w during motion control of a robot.</p><p>4. Control Method of a Passive Mobile Robot with a Servo Brake</p><p>4.1. Kinematics and Motion Type of PRP</p><p>As mentioned in the previous section, PRP consists of three omni-directionalwheels and the axes of the three wheels intersect on a single point. The kinematicsrelation between the motion vector of PRP rq = [rx ry r ]T and angular velocityvector of wheels = [w1 w2 w3]T can be expressed as the following equationwith Jacobian J:</p><p>rq = J, (5)where:</p><p>J =</p><p>0 Rw3</p><p>Rw3</p><p>2Rw3</p><p>Rw</p><p>3</p><p>Rw</p><p>3</p><p>Rw3L</p><p>Rw3L</p><p>Rw3L</p><p>. (6)</p><p>Here, Rw denotes the radius of the wheel and L denotes the distance between thecenter of the wheel and intersection point of three axes of wheels in the horizontalplane. The robot coordinate system r is set as shown in Fig. 2.</p><p>Since the brake torque of each wheel depends on the direction of the wheel rota-tion, we classify the motion of PRP into eight different cases based on the signs ofthe angular velocities of the three wheels (sgn(wi ), i 1,2,3) as shown in Table 1.</p><p>Figure 2. Configuration and robot coordinates o...</p></li></ul>