An Active-Passive Variable Stiffness Elastic Actuator for Safety Robot Systems

Transcription

1 The 1 IEEE/RSJ International Conference on Intelligent Robots and Systems October 18-, 1, Taiei, Taiwan An Active-Passive Variable Stiffness Elastic Actuator for Safety Robot Systems Ren-Jeng Wang, Han-Pang Huang, Member, IEEE Abstract For classical robotic alications, robotic systems consist of servo motors, high-ratio reduction and rigid links; mechanical designers refer to designing robotic alications as stiff as ossible to make robots maniulate with remarkable seed and recise osition movements. However, these robotic alications can hardly interact with eole and environments under safety constraints. It oses the very fundamental roblem of ensuring safety to humans and rotecting the robot. This aer resents an active-assive variable stiffness elastic actuator (APVSEA) which is designed for safety robot systems. The APVSEA consists of two DC-motors: one is used to control the osition of the joint and the other is used to adjust the stiffness of the system. The stiffness is generated by two antagonistically nonlinear srings. By changing the reload length of the two antagonistically nonlinear srings, APVSEA has the ability to minimize large imact forces due to shocks, to safely interact with the user and/or become as stiff as ossible to make recise osition movements or trajectory tracking control easier. Eeriment results are resented to show that APVSEA is caable of roviding recise osition movements while offering safe human-robot interaction. T I. INTRODUCTION he develoment of robots that can work alongside humans in our homes and worksaces to assist human daily activities though hysical interaction, informational interaction, etc., has been ongoing for a long time. That is the reason why the research on hysical human-robot interaction that considers the trade-off between safety and erformance has emerged in the field of modern robotics [1]. Traditionally, industrial robots are heavy, fast, strong and owerful. Most of them are without any caacity of interaction with humans under safety constraints. Therefore, under the safe condition, how a robot achieves good erformances in recision and high-seed is becoming one of the most imortant issues. A maniulator s accetable velocity regarded as an imortant erformance inde can be imroved in several ways. For eamle, limiting the velocity commands of the robot, lowering the stiffness of the robot s cover using soft material, reducing the transmission stiffness between the This work was artially suorted by the National Science Council of Taiwan, R.O.C. under the grant number NSC 98-1-E--133-MY3. R. J. Wang is with the Mechanical Engineering Deartment, National Taiwan University, Taiei, Taiwan, 1617, R.O.C. ( d94531@ ntu.edu.tw). H. P. Huang is with the Mechanical Engineering Deartment, National Taiwan University, Taiei, Taiwan, 1617, R.O.C. (hone: ; fa: ; hanang@ ntu.edu.tw). actuator and the outut link via assive comliance [][3]. Recently, in order to efficiently kee robots with intrinsically safe robot actuation, several techniques and aroaches have been devised. SEA, these actuators use low imedance or high comliant and/or viscous elements in series between transmission mechanisms and loads, and moreover, both linear and rotary SEA designs have been resented in [4-6]. The safe joint mechanism(sjm) and the safe link mechanism(slm) which assive mechanisms roosed in are combined with slider structures, linear srings and transmission shafts, The mechanisms erform nonlinear characteristics by using the idea of the transmission angle of the four-bar linkage and the characteristics of the slider mechanism in combination with the srings [7-1]. Variable stiffness actuator uses a variable stiffness transmission mechanism to vary mechanical stiffness of the given system [11]. One actuator with two non-linear mechanical srings working in antagonistic configuration was designed. It allows changing the mechanical imedance of the actuator during motion [1]. In [13-15] most of the roosed imlementations make use of two actuators to control the osition and stiffness of the joint. By changing the link osition or the effective length of the sring results in change in stiffness. In [16]a robot arm with actuator systems using a neumatic artificial muscle is develoed. Most of the roosed imlementations of this kind of actuators make use of two non-linear mechanical neumatic artificial muscles in antagonistic configuration. The neumatic artificial muscle is light weight and fleible. An active-assive variable stiffness elastic actuator (APVSEA), designed for safety robot systems is resented in this aer. It discusses in detail the APVSEA mechanism, the stiffness adjuster and comarative erformances between an actuator with maimum and minimum stiffness. This aer is structured as follows: section II resents the main ideas of recise osition movement actuation and active-assive variable stiffness actuator (safe actuation). In section III, the design of an active-assive variable stiffness elastic actuator (APVSEA) is elained. The APVSEA consists of two DC-motors, one ball screw, one screw and four variable stiffness srings. The rotation of the ais is measured by an encoder which is fied on the outut link. Section IV resents eeriment results to show that APVSEA is caable of roviding recise osition movements and safer human-robot interaction. Finally, the conclusions are made in Section V /1/$5. 1 IEEE 3664

2 II. PRECISE POSITION MOVEMENT ACTUATION / SAFE ACTUATION The recise osition movement actuation and safe actuation are resented in this section. The main concet of the recise osition movement actuation is the use of a ball-screw and time belt for greater efficiency and making accurate reservations. For the safe actuation, active and assive variable stiffness serial configurations (adatable comliance) are discussed. A. Motor- drive system To kee the ability of the actuator to make recise osition movements and trajectory tracking control easier, as in classic robotic systems, a motor-ball screw drive system was designed, as shown in Fig. 1. Note that a ball-screw is driven and rotated by a DC-motor1; because the ball screw is rotated, the moving lant which is fied on the ball screw will advance or draw back in its own aial direction. There is a time belt fied on the moving lant and connected with the outut link; when the DC-motor1 drives the ball screw, the moving lant is moved. Finally, the outut link is rotated and moves because of the time belt which is connected to the moving lant. B. Passive Variable Stiffness Serial Configuration The design of a assive variable stiffness serial configuration can be eressed by the series combination. In order to elain the roerties of the assive variable stiffness serial configuration, a simle sring system with a linear sring is used, as shown in Fig. is the osition of the moving lant (block), and is the osition where no forces are generated. The forces on the block are given by: F = k ( ), if (1) Differentiating (1) with resect to the osition, the stiffness is eressed as: K = = k () d This result shows that the stiffness of the comliance is uncontrollable. When the srings with a quadratic characteristic are used, the force is eressed as: F = k ( = k ) k + k The stiffness of the nonlinear sring comliance system is obtained (3) K = = k k (4) d Where the stiffness varies as a linear function of the block osition. Motor 1 Time belt Moving lant Outut link Fig. 1. Motor- Drive System. Motor P Fig.. Passive variable stiffness serial configuration. C. Active Variable Stiffness Serial Configuration From (4), the stiffness of the nonlinear sring comliance system is deendent on the osition of the block. In order to obtain the ability to control the stiffness of the sring comliance system so as not to deend on the osition of the block, a new structure model, active variable stiffness serial configuration, was built. To elain the roerties of the active variable stiffness serial configuration, a simle linear antagonistic setu with two linear srings having the same sring constant is used, as shown in Fig 3. is the osition of the moving lant (block), is the osition where no forces are generated, and and are the controllable osition (reload distance of the sring). The forces on the block equal the sum of the forces of both srings: F = k( + ( = k + k( )) k( ) + k + ( )) Differentiating (5) with resect to the osition, the stiffness becomes k = = k (6) d This result shows that the stiffness of the comliance is uncontrollable. When the srings with a quadratic characteristic are used, the force is eressed as: (5) 3665

3 F = k( = k( + ( )) k( + ) + k( + ( )) ) + k( + ) (7) Motor Connecter The stiffness of the nonlinear sring comliance system, active variable stiffness serial configuration, is obtained: K = = k( + ) (8) d Fig. 4 shows the relation between the block movement and the force on the block ( k = 1, = 6 ), where the control arameters and are ( = 1,, 3 and = 1,, 3 ). Note that the stiffness of the active variable stiffness serial configuration is controllable. D. Active-Passive Variable Stiffness Elastic Actuator (APVSEA) A motor-ball screw drive system has the roerty of making recise osition movements and the active-assive variable stiffness serial configurations has the ability to minimize large imact forces due to shocks, thereby safely interacting with the user. In this aer, the main idea of the active-assive variable stiffness elastic actuator designed for safety robot systems is to combine these two imortant roerties. As shown in Fig. 5, there is a connecter between the motor ball screw recise drive system and the active-assive variable stiffness serial configuration; the connecter connects these two configurations, and as a result, an active-assive variable stiffness elastic actuator was build. The actuator has the ability to minimize large imact forces due to shocks, thereby safely interacting with the user and/or becoming as stiff as ossible to make recise osition movements or trajectory tracking control easier. The force on the block Motor P P Fig. 3. Active Variable Stiffness Serial Configuration. 4 - = = = = 1 = = Block movement Fig. 4. The relationshi between the block movement and the force on the block. Motor 1 Fig. 5. A concet of an Active-Passive Variable Stiffness Elastic Actuator. Actuator. III. DESIGN OF AN ACTIVE-PASSIVE VARIABLE STIFFNESS ELASTIC ACTUATOR (APVSEA) The two main mechanisms, a motor-ball screw system and an active-assive variable stiffness serial configuration, are designed to obtain the two desired characteristics of APVSEA, namely: accurate movement and safety in human-robot interaction. In the motor-ball screw system, a ball screw is driven and rotated by the DC-motor1and a moving lant is laced on the ball screw. When the ball screw is rotated by the DC-motor1, the moving lant will advance or draw back in its own aial direction. There is a time belt fied onto the moving lant and connected with the outut link; when the moving lant moves, the outut link will move and then rotate. According to the abovementioned moving structure, the APVSEA has the ability to move accurately; the 3-D model of APVSEA is shown in Fig. 6. In the active-assive variable stiffness serial configuration, a screw is rotated by a DC-motor, and two moving lants move in oosite directions on the screw. There are two variable stiffness srings between the moving lant and connecter; when two moving lants are moved by the screw, the reload length of the variable stiffness srings will be changed; the 3-D model of APVSEA is shown in Fig. 7. The APVSEA has the ability to minimize large imact forces due to shocks, to safely interact with the user and/or to become as stiff as ossible to make recise osition movements or trajectory tracking control easier. The key oint of the APVSEA mechanical structure is the relation between inut shaft and ball screw. In the APVSEA actuator design, an inut shaft asses through the center of the ball screw. When the inut shaft is driven and rotated by the DC-motor, the ball screw will be driven and rotated. When the ball screw is rotated, the moving lant, which is fied on the ball screw, will advance or draw back in its own aial direction. There is a time belt fied on the moving lant and connected to the outut link; when the DC-motor drives the inut shaft, the ball screw is rotated and the moving lant is moved. Finally, the outut link is rotated because the time belt, which is connected to the moving lant with the outut link, is moved. According to the 3666

4 abovementioned moving theories, the APVSEA has the ability to make recise osition movements or trajectory tracking control easier, as shown in Fig. 8. Moving lant Time belt 1 Motor 1 Moving lant Encoder Inut shaft Fig. 8. 3D model of Main structure of APVSEA. Time belt Outut link Moving lant Fig. 6. 3D model of Active-Passive Variable Stiffness Elastic Actuator (APVSEA). Eternal forces Time belt Variable stiffness sring Connecter Inut shaft Fig. 9. 3D model of Main structure of APVSEA when APVSEA with eternal forces. Screw Time belt Motor Moving lant Fig. 7. Front view of Active-Passive Variable Stiffness Elastic Actuator (APVSEA). In addition, when eternal forces, imacts or shocks are eerted on the outut link, the eternal forces will ush/ull the moving lant by the time belt; then all of the structures, including connecter, ball screw and moving lant will be moved. Because the inut shaft goes through the center of the ball screw, the ball screw will slide in the same aial direction as the inut shaft, as shown in Fig. 9. In all cases, the APVSEA has the ability to minimize large imact forces due to shocks, to safely interact with the user. To sum u, the APVSEA can minimize large imact forces and make recise osition movements; the key oint is the relation between the inut shaft and the ball screw. The ball screw can be rotated by the motor and makes the moving lant move. Besides, the ball screw can also be ushed/ulled by the time belt and slides on the inut shaft when eternal forces are eerted on the outut link. IV. SYSTEM EXPERIMENT EVALUATION In this section, eeriments were conducted to evaluate the roerties and abilities of the active-assive variable stiffness elastic actuator (APVSEA). Fig. 1 is the icture of the APVSEA which consists of two DC-motors, one ball screw, one screw and four variable stiffness srings. The rotation of the ais is measured by an encoder which is fied on the outut link. Dimensions of the APVSEA, design arameters and some detailed secification are listed in the Table I. A. Adative Comliant Proerty An eeriment was designed to interret the adative comliant roerty of the APVSEA. The eeriment comrises four stages. First, by using a simle PID controller, the outut link of the APVSEA was rotated and ket in a vertical direction. Second, the outut link of APVSEA was manually deflected in a counterclockwise direction away from the (equilibrium oint) with the situation whereby the motor was still working. Third, the outut link was deflected in a clockwise direction. Fourth, the link was released. As shown in Fig. 11, the result is a lot of the angle with time and the hotograh shows the beginning and finishing osition of the APVESA. The eeriment shows that an adative comliant configuration was used to make the interaction between robots and humans safer and more natural and that APVSEA has the ability to interact with eole or unknown environments under safety constrains by an adative comliance configuration. 3667

5 Outut link Encoder Fig. 1. Active-Passive Variable Stiffness Elastic Actuator (APVSEA). Link osition ( deg ) ( Angle ) ( second ) Time (s) Fig. 11. Adative comliant roerty for Active-Passive Variable Stiffness Elastic Actuator min stiffness 5 ma stiffness Time( (Second) (s) S ) Fig. 1. Resonse to osition command with various stiffness. Link osition (Angle) ( ( deg ) ) TABLE I THE SPECIFICATIONS OF APVSEA PARAMETERS Value Mass (include two motors). kg Length*Width*Height *1*116 mm DC-motor Ma. Outut Torque 9 Nm Ma. Outut Seed 4 rm Ma. Outut Link Deflection o ± 9 * The inut motors used in this rototye design are Faulhaber DC-micromotor 64W4CR with 3/1 gear head of which reduction ration is 1:14. The ste resonse of the designed APVSEA system with different reload distance of the nonlinear sring (minimum and maimum) is given in Fig. 1. The osition of the APVSEA changes from to 3 by using a simle PID controller; the result is a lot of the angle with time. As shown in Fig. 1, when the reload distance of the APVSEA is minimum (stiffness is minimum), vibration due to the osition command over 1.4 sec and the actuator is at a 7 angle offset because of the gravity, the recise osition movement ability is worse. If the reload distance of the APVSEA is maimum (stiffness is maimum), vibration does not occur, and the APVSEA has good recise osition movements. The eeriment shows that the APVSEA has the ability to obtain different stiffness by changing the reload distance of the nonlinear sring, and the APVSEA with maimum stiffness shows better resonse than the APVSEA with maimum B. Active Variable Stiffness Proerty By changing the reload distance of nonlinear srings, the APVSEA is able to vary the stiffness. In this eeriment, a force sensor is used to measure the eternal force at the end-oint of the outut link and a otential meter is used to measure the dislacement of the nonlinear sring. As shown in Fig. 13, X is the reload distance of nonlinear srings. The o stiffness of the APVSEA is changed with the different reload distance of the nonlinear sring. Alied Torque (Nm) 8 6 Xo= Xo=6 4 Xo=1 Xo=14 Xo=18 Xo= Xo= Angular Deflection (deg) Fig. 13. Measure stiffness of the APVSEA. C. Safe Robot System As shown in Fig. 14, a table-tennis ball and an aluminum cube are hit by the APVSEA outut link which is rotated with maimum and minimum stiffness at.38rad / s (the reload distance of variable stiffness srings are maimum and minimum); the outut link of APVSEA is 5mm long and the objects (table-tennis ball and aluminum cube) are hit at the height of 5mm. The flight distances of the objects are measured. The mean value of the flight distance data is taken after 1 measurements, as shown in Table II. The result shows that for the table-tennis ball and aluminum cube cases, the objects fly farther if the APVSEA has maimum stiffness. The stiffness of APVSEA and flight distances have been shown to be negative correlated to one another. Since the objects are static before being hit by the outut link and the mass of the objects does not change, the arameter that affects the objects flight distance is the acceleration of the objects after collision. Less stiffness leads to less acceleration of the object. Because the safety of the object deends on the objects acceleration after collision, the eeriment imlies that it is safer to imact with less stiffness than with higher stiffness. 3668

6 TABLE II FLIGHT DISTANCE OF OBJECTS HIT BY OUTPUT LINK Object Stiffness Flight distance Table-tennis ball Ma stiffness 51mm Table-tennis ball Min stiffness 498mm Metal block (3g) Ma stiffness 61mm Metal block (3g) Min stiffness 19mm * The mean value of the data is taken after 1 measurements. APVSEA Outut link The ball are hit by the link at the height 5mm Fig. 14. Eeriment setu for hitting-object eeriment. V. CONCLUSION In this aer, an active-assive variable stiffness elastic actuator for safety robot system was resented. By changing the reload length of the two antagonistically nonlinear srings, the APVSEA has the ability to minimize large imact forces due to shocks, to safely interact with the user and/or to become as stiff as ossible to make recise osition movements or trajectory tracking control easier. The reliminary results obtained from these eeriments show that the APVSEA has the caability to interact with eole or unknown environments under safety constrains by assive comliance, and also has the ability to obtain different stiffness by changing the reload distance of the nonlinear sring; the APVSEA with maimum stiffness shows better resonse than the APVSEA with minimum stiffness. When objects were hit by the outut link of APVSEA with maimum stiffness, the flight distance of the object was further than in the case of APVSEA with minimum stiffness. Since, the safety of the object deends on the acceleration after the collision, the eeriment showed that a joint with less stiffness was safer in case of uneected imact with humans. REFERENCES [1] A. L. Edsinger, Robot Maniulation in Human Environments, Doctoral Dissertation, Deartment of Electrical Engineering and Comuter Scienc, Massachusetts Institute of Technology, 7 [] J. Versace, A Review of the Severity Inde, in Proceeding of the 15th Sta Car Crash Conference, , [3] A. Bicchi and G. Tonietti, Fast and Soft-arm Tactics, IEEE Robotics and Automation Magazine, vol. 11,. -33, 4. [4] J. W. Sensinger and R. F. Weir, Design and Analysis of a Non-backdrivable Series Elastic Actuator, Chicago, IL, United States, vol. 5, , 5.. [5] E. a. B. Torres-Jara, J., A Simle and Scalable Force Actuator, in Proceeding of the 35th International Symosioum on Robotics, 4. [6] G. Wyeth, "Demonstrating the safety and erformance of a velocity sourced series elastic actuator, in Proceedings of the IEEE International Conference On Robotics and Automation, Pasadena, CA, USA, May 8, [7] J. J. Park, Y. J. Lee, J. B. Song, and H. S. Kim, Safe joint mechanism based on nonlinear stiffness for safe human-robot collision, in Proceedings of the IEEE International Conference On Robotics and Automation, Pasadena, CA, USA, May 8, [8] K. S. Kim, J. J. Park and J. B. Song, Safe joint mechanism using double slider mechanism and sring for humanoid robot arm, in Proceedings of the 8th IEEE-RAS International Conference On Human Robots, Daejeon, Korea, December 1~3, 8, [9] J. J. Park, B. S. Kim, J. B. Song and H. S. Kim, Safe Link Mechanism based on Passive Comliance for Safe Human Robot Collision, in Proceedings of the IEEE International Conference On Robotics and Automation, Roma, Italy, Aril 7, [1] J. J. Park, H. S. Kim, J. B. Song and H. S. Kim, Safe robot arm with safe joint mechanism using nonlinear sring system for collision safety, in Proceedings of the IEEE International Conference On Robotics and Automation Kobe Internal Conference Center, Kobe, Jaan, May 9, [11] N. G. Tsagarakis, M. Laffranchi, B. Vanderborght and D. G. Caldwell, A Comact Soft Actuator Unit for Small Scale Human Friendly robot, in Proceedings of the IEEE International Conference On Robotics and Automation Kobe Internal Conference Center, Kobe, Jaan, May 9, [1] S. A. Migliore, E. A. Brown and S. P. D. Weerth, Biologically Insired jint stiffness control, in Proceedings of the IEEE International Conference On Robotics and Automation, Barcelona, Sain, Aril 5, [13] S. Wolf and G. Hirzinger, A new variable stiffness design Matching requirements of the net robot generation, in Proceedings of the IEEE International Conference On Robotics and Automation, Pasadena, CA, USA, May 8, [14] J. Choi, S. Hong, W. Lee and S. Kang, A variable stiffness joint using leaf srings for robot maniulators, in Proceedings of the IEEE International Conference On Robotics and Automation Kobe Internal Conference Center, Kobe, Jaan, May 9, [15] R. Ghorbani and Q. Wu, Closed loo control of an intentionally adjustable comliant actuator, in Proceedings of American Control Conference, Minneaolis, Minnesota, USA, June 6, [16] N. Saga, T. Saikawa and H. Okano, Fleor mechanism of robot arm using neumatic muscle actuators, in Proceedings of the IEEE International Conference On Mechatronics & Automation, Niagara Falls, Canada, July 5,