Mechanical Design and Control of the New 7-DOF Cybermotion Simulator
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2011 IEEE International Conference on Robotics and Automation Shanghai International Conference Center May 9-13, 2011, Shanghai, China Mechanical Design and Control of the new 7-DOF CyberMotion Simulator Carlo Masone, Paolo Robuffo Giordano, Heinrich H. Bulthoff¨ Abstract— This paper describes the mechanical and control [9], [10]. This is particularly the case of the CyberMotion design of the new 7-DOF CyberMotion Simulator, a redun- Simulator, a 6-DOF anthropomorphic robot arm based on the dant industrial manipulator arm consisting of a standard 6- commercial KUKA Robocoaster [11] for which a complete DOF anthropomorphic manipulator plus an actuated cabin attached to the end-effector. Contrarily to Stewart platforms, control framework for simulating general vehicle dynamics an industrial manipulator offers several advantages when used was recently presented and experimentally validated in [12], as motion simulator: larger motion envelope, higher dexterity, [13]. Compared to more advanced designs that have recently and possibility to realize any end-effector posture within the appeared in the motion simulation community [14], [15], workspace. In addition to this, the new actuated cabin acts as an the CyberMotion Simulator presents a good tradeoff between additional joint and provides the needed kinematic redundancy to cope with the robot actuator and joint range constraints, development/maintenance costs and motion capability. which in general can significantly deteriorate the desired motion Exploiting a serial manipulator as motion platform poses, cues the robot is reproducing. In particular, we will show that, of course, several challenges: first, suitable inverse kinemat- by suitably exploiting the redundancy better results can be ics schemes must be conceived to deal with an unpredictable obtained in reproducing sustained acceleration cues, a relevant and arbitrary desired cabin motion, generated online as a problem when implementing vehicle simulators. function of the (unpredictable) user inputs to the simulated I. INTRODUCTION vehicle. At the same time, one has to cope with the typical manipulator constraints such as joint limits and actuator The realization of realistic immersions in virtual environ- saturations. Second, the design of washout filters, and in ments is an active research field in robotics [1]. Usually, general of the whole motion cueing block [16], [17], [18], in robotics-related fields, immersion may refer to providing [19], must be tailored to the specific motion envelope of a visual/haptic sensory cues to a human operator to enhance serial manipulator. In [12], [13], a general solution to these his situational awareness of remote locations, to allow him problems is discussed but only validated for the particular interacting with remote environments, or to train him in case of the CyberMotion Simulator, i.e., a 6-DOF anthro- specific tasks [2], [3]. In this respect, when dealing with pomorphic manipulator with no redundancy w.r.t. a general training procedures for pilots, an additional relevant cue Cartesian task. It is clear that a major limitation of such is represented by the sense of self-motion that usually a system is represented by the limited range of the joints: completes the haptic/visual information to improve the im- whenever a joint reaches its limit during the operation, the mersiveness. In this context, simulators of vehicle motion reproduction of a desired motion can be heavily distorted represent one major application of interaction with a virtual because of the partial loss of mobility of the robot, thus environment for training purposes [4], but also for educating inducing false cues on the pilot onboard the cabin. and entertaining as reported in [5]. In this respect, the goal of this paper is to present a novel Usually, motion simulators are based on fully actuated extension of the CyberMotion Simulator that features an hexapods (Stewart platforms). Although with motion ca- actuated cabin acting as an additional seventh joint for the pabilities in degrees of freedom (DOF), these platforms 6 whole system, thus granting a consistent increase of mobility suffer from their limited workspace and the impossibility thanks to the introduced redundancy. This redundancy can to achieve large linear and angular displacements and rates be exploited online to optimize the robot configuration and because of the the closed chain nature of their actuation avoid singularities/joint limits, with the latter being particu- system. As a possible improvement to this design, a novel larly critical for the reproduction of the desired motion cues motion platform based on a serial manipulator arm carrying as explained before. a cabin on the end-effector is also gaining ground in the The paper is organized as follows: Sect. II describes the community because of its higher dexterity, larger motion whole system (manipulator arm + cabin) and summarizes envelope, the possibility to realize any end-effector posture the main mechanical properties. Sect. III then focuses on within the workspace, and the ability to displace heavy the cabin mechanical design and kinetic analysis. Indeed, loads with large accelerations and velocities [6], [7], [8], the cabin presents a non-trivial mechanical design because C. Masone, P. Robuffo Giordano, and H. H. Bulthoff¨ are with the of its actuation system: it can switch from being a lin- Max Planck Institute for Biological Cybernetics, Spemannstraße 38, 72076 ear/rotation/hybrid joint during the motion. In Sect. III, Tubingen,¨ Germany. we will show how to handle all the possible cases both H. H. Bulthoff¨ is also with the Department of Brain and Cognitive Engineering, Korea University, Anamdong, Seongbuk-gu, Seoul, 136-713 for the forward and inverse kinematics analysis. After this, Korea. Sect. IV focuses on the control design for the whole system: 978-1-61284-385-8/11/$26.00 ©2011 IEEE 4935 here, an extension of the framework introduced in [12], q1 q2 q3 q4 q5 q6 q7 = qC [13] to deal with the redundant case is discussed. Finally, qmin -130 -128 -45 -180 -58 -180 0 Sect. V reports simulation results showing the improvements qmax 130 -48 92 180 58 180 1.7317 of the novel manipulator/cabin design in reproducing sus- q_max 69 57 69 76 76 120 0.34 tained acceleration cues, as the ones typical of race cars. q¨ 98 70 128 33 95 77 0.6 Reproducing high sustained accelerations was not easily max achievable with the original CyberMotion Simulator setup TABLE I: Joint range, velocity and acceleration limits of the because of the aforementioned joint limit constraints. We 7-DOF manipulator with actuated cabin. Entries q1 to q6 are will show that, by exploiting the redundancy offered by the expressed in degrees, q7 in meters. new cabin joint, better results can be obtained for this kinds of situations. Section VI concludes the paper and discusses future directions. Therefore, we model the robot as a single integrator II. PRELIMINARIES q_ = u; (1) The CyberMotion Simulator is based on the standard six- where u 2 R7 is the commanded joint velocity. All the joint anthropomorphic robot arm KUKA Robocoaster (a higher-lever control schemes will be built on top of (1). modified KR-500 industrial robot with a 500 [Kg] payload), The joint limitations of the robot (limited range, maximum which was originally designed for use in amusement parks. velocity and maximum acceleration) are reported in Table I. This setup, together with a seat rigidly attached to the robot The kinematic model of the actuated cabin will be described end-effector, has been used to simulate the motion of a race in the following section. car [12], [13], and of a helicopter at the ILA Berlin air show III. CABIN KINEMATICS 2010 [20]. ~ ~ ~ In order to improve the quality of the simulated expe- Let FF : fOF ; XF ; YF ; ZF g be a frame fixed to the center ~ rience, we designed a novel actuated cabin (Fig. 1) that of the flange (see Fig. 1 and Fig. 3), with ZF perpendicular ~ ~ will replace the original fixed seat. The cabin consists of to the flange itself, and XF and YF perpendicular and two main parts: a 1:6 × 1:8 × 1:9 [m] closed shell gondola parallel to the rotation axes of the two engines, respectively. ~ ~ ~ that encapsulates the seat, and a rigid flange (the dashed red Let also FC : fOC ; XC ; YC ; ZC g be a frame attached to ~ ~ ~ ~ box in Fig. 1) attached to the end-effector of the robot. The the gondola, with YC k YF and XC , ZC aligned with two parts are connected to each other by two identical and the gondola forward/upwards direction, respectively. Due parallel rails, fixed to the back of the gondola, and bound to its mechanical design, the gondola cannot move along ~ to move along a metal guide on the flange. The motion is YC (it is perpendicular to the rails) and the displacement provided by two servomotors capable of 328 [Nm] output between OC and OF along this direction is set to zero. We torque. With respect to the previous setup, the new cabin introduce the simplified situation depicted in Fig. 2, obtained extends the motion capability of the robot and also eliminates by projecting the kinematics on the plane Σ spanned by ~ ~ the unwanted external cues from the environment thanks to fXF ; ZF g. The projection of the rails on Σ consists of two the closed shell design. Moreover, the gondola is equipped segments, AB;d CDd 1, having length L and connected by a with two projectors for stereo visualization and can mount quarter of a circle BCd having radius λ. In order to simplify different input devices, in particular, a full force-feedback the following analysis, we place OC at the center of BCd helicopter control device composed by a cyclic stick, a (see Fig. 2). We also define the points P1 and P2 as the collective stick and actuated pedals, and a force-feedback 1 steering wheel and pedal module.