Magnetically-joined Manipulator to Ensure Safety on Collision

Transcription

1 AIAA Conference <br>and<br>aiaa Unmanned...Unlimited Conference 6-9 April 2009, Seattle, Washington AIAA Magnetically-joined Manipulator to Ensure Safety on Collision Shinichi Kimura *, Yoshihiro Ozawa and Meguru Yamauchi Tokyo University of Science, Noda, Chiba , Japan Robots for intra-vehicular support are being studied for their potential to reduce the work load of astronauts. Intra-vehicular support robots must meet strict safety requirements, and they need to be compact to share limited room with astronauts. A magnetically jointed module manipulator (MagMo) solves these problems in a unique manner. The manipulator is not harmful when it contacts humans unexpectedly because it disassembles by contact force, and it can be easily assembled only when it is needed. In this paper, the basic concept of the MagMo is introduced. F I. Introduction orty years have passed since the first manned space flight. Manned space utilization is becoming a mature endeavor, partly because of the establishment of a manned space infrastructure. The importance of the International Space Station has steadily increased, and it has become an established space base for various types of experiments that may lead to further effective space utilization. The Japanese Experiment Module (JEM) was successfully attached to the International Space Station in It contains various types of equipment and is ready to assume its role as part of the space infrastructure for space utilization. Astronauts are required to perform various tasks, including housekeeping tasks, to maintain their space vehicle as well as to carry out various types of experiments. Reducing and optimizing the workload of the astronauts is of particular interest so that a manned space system can be operated more effectively. Most of these tasks are simple and do not require the astronauts to use their intelligence. 1 It has been proposed that robots may be used to reduce the workload of astronauts. An autonomous robot could perform a simple repetitive task that would not require intelligence. Further, a teleoperated robot may be able to deal with unexpected situations with the support of ground personnel. However, even though some functions can be performed by teleoperation, they still require the support of the astronauts in unforeseen situations. The concepts that underlie the use of robots to reduce the workload of astronauts (i.e., IVA robots) have been studied since the very early stages of the development of the ISS. However, unfortunately, these concepts have not yet been implemented. The reasons why IVA robots have not yet been adopted in manned space systems can be summarized as follows: (1) Optimizing workspace: If a robot is required to support humans in performing a task, it must have sufficient workspace to access the target. However, manned space vehicles such as the ISS are quite small. A robot would be required to share the space with astronauts and other equipment. If a single robot is used, it is required to be sufficiently large to cover the workspace; such a large robot occupies a large amount of the limited vehicle space, and it will be a hindrance to the astronauts. The concept of using a rail-guided mobile robot has been explored to reduce such interference; however, a large number of rail tracks would be required to provide the robot with continuous access to the environment. Using Charlotte, a tethered robot developed by Swaim and co-workers, is a unique approach to optimize the limited workspace. 2 It moves in front of the experimental equipment onboard the Space Shuttle by using tethers hanging from the corners of the equipment. The tethers can be easily removed when the robot is not in use, and no special equipment is required for the robot to move around. However, it still has a disadvantage to occupy the full surface of the experimental equipment. Another distinct approach is to use the Space Humming Bird (SHB) proposed by Tsumaki et al. 3 In this approach, a small lightweight robot is flown inside the vehicle. Since its * Professor, Department of Electrical Engineering, 2641 Yamasaki, Member. Master Course Student, Department of Electrical Engineering, 2641 Yamasaki, Non-member. Master Course Student, Department of Electrical Engineering, 2641 Yamasaki, Non-member. 1 Copyright 2009 by the, Inc. All rights reserved.

2 mobility is based on free flight, it requires very little space. However, its functionality such as its ability to generate force and carry objects is rather limited. (2) Guaranteeing safety: IVA robots are required to share their workspace with astronauts and expensive equipment, without causing any harm to them. While collision avoidance is undoubtedly an important issue, unfortunately, we still cannot guarantee efficient operation in unpredictable situations. (3) Monitoring robot movements: Although there have been great advances in robot control systems, we still cannot guarantee absolute safety. Therefore, robot behavior must be carefully monitored when they are operated in risky situations. In case of unexpected activity, the observer should be able to stop the robot Figure 1. Magnetically-joined modular by using an emergency button. Therefore, at least one observer robot (MagMo) must be dedicated to monitoring the robot and, if necessary, activating the emergency button. However, this would negate some of the advantages of using this robot. In this paper, a magnetically-joined modular robot has been proposed to solve these three problems in a unique manner. If we cannot prevent unpredictable collisions, the best way to guarantee safety is to ensure that such collisions do not cause any harm. The robot consists of modules that are joined together by magnets, and it falls apart on collision to avoid injury to the astronauts or damage to the equipment. In case of an unexpected activity, it would simply need to be given a shock to make it break apart. Such a robot could be disassembled and stored efficiently, and the parts could be easily reassembled into a robot with an optimum size for carrying out specific tasks. In previous studies, we have studied a modular manipulator system based on a decentralized control architecture. 4-7 Depending on the task and constraints, a modular manipulator can be flexibly constructed using modules such as actuators and decentralized controllers. Such a manipulator can flexibly adapt to changes in its structure and to partial faults, owing to its decentralized autonomous control architecture. We have extended the concept of this manipulator to the magnetically-joined modular robot(magmo) as a new approach to realize an IVA robot (Fig. 1). II. Basic Concept of MagMo If we utilize robots for human support in manned space vehicles, robots must share limited space with astronauts. It is impossible to separate the robot work area from human activities, as with terrestrial factory robots. One of the most serious concerns in such human robot interactive situations is safety in the case of collisions between humans and robots. In addition, equipment surrounding the robots may be irreplaceable and expensive. Some of them are life-threatening if damaged. Robots must not pose a hazard to astronauts or equipment. To avoid collisions with robots, collision avoidance control technologies have been studied in great detail. Undoubtedly, these technologies are important for using robots in the real world, but they are based on sensing or modeling the environment. Unfortunately, these approaches cannot guarantee perfect safety because they cannot be free from sensor and model faults. A logical, intelligent approach is not suited to this problem, and a passive, mechanical approach, low-tech in nature, seems the best way to realize guarantee safety. Here, let us consider another approach for safety. Even if robots may come into contact with humans, safety will be achieved if the robot is not hazardous to humans. If robots consist of soft shielded modules that break apart from each other upon unexpected contact, the robot may collide with a human or other object but not be hazardous. The MagMo is based on this concept. Soft shielded module robots, including an actuator and decentralized controller, are linked to each other by a joint mechanism using magnetic force. The joint mechanism maintains the connection despite low-level tension and slow deformations in normal 2 Figure 2. MagMo s components detach easily in the case of collision

3 work, but the modules separate from each other upon encountering a large force or fast deformation caused by unexpected collision. Isaac Asimov noted the Three Laws of Robotics in his science fiction classic I, Robot. The first law is A tool must be safe to use. The MagMo strictly obeys the first law by breaking itself upon contact. Another problem with using robots in manned space vehicles is the space that robots occupy in the vehicle. For a robot to support a task, it must have sufficient workspace to access the Figure 3. MagMo s structure target. The more tasks the robot supports, the wider its working area must be. If we expect a single robot to cover the entire area of the vehicle, we need quite a large robot that may be a hindrance for human activities. Several strategies have been considered to acquire a large work area in limited space in manned space vehicles. Gantry-type mobile robots, such as the Mobile Servicing System (MSS) of the International Space Station, are one possible solution. 1 In this idea, the robot moves on a gantry rail installed in front of the equipment that the robot may need to access to perform tasks. The robot and gantry rails are occupied even when the robots are not used in this type of a system. Another unique idea is a tethered robot, e.g., Charlotte. 2 It moves in front of the experimental equipment on the Space Shuttle by tension from a tether that hangs from the corners of the equipment. The tether can be removed easily when the robot is not required, and no special structure is required for the robot to move. For extra-vehicular activities (EVA), Oda et al. consider a similar structure to achieve quite a large work area with a small robot. 8 However, the robot still has the disadvantage of occupying the full surface of the experimental equipment. A small free-flying robot called Space Hummingbird (SHB) is another unique concept. 3 Since its mobility is based on free flight, it requires very little space. However, its functionality, such as its ability to generate force and carry objects, is rather limited. MagMo addresses this problem as well. MagMo s parts can be attached and detached very simply by moving them near each other or separating them (Fig. 2). Just before it is required, the MagMo components will be attached to each other, and the manipulator will be constructed according to the task request. When it is not required, they easily detach from each other and can be stored compactly. Unlike Charlotte, MagMo works only as required, and does not occupy the entire surface of experimental equipment. Tightly coupled with safety requirements, the need to observe a robot s situation and motion is also a barrier for the utilization of robots in manned space vehicles. While there have been great advances in robot control, we still cannot guarantee absolute safety. Therefore, robot behavior must be observed carefully when they are operated in delicate situations. In the worst case in a manned space vehicle, unexpected motion may be a fatal problem. In the case of unexpected movement, the observer should be able to stop the robot by means of an emergency button. Therefore, at least one observer s time must be dedicated to observing the robot and, if necessary, activating the emergency button. However, this would negate some of the advantages of robot use. If the MagMo performs unexpected motion, it is fairly easy to stop. Simple touching or chopping makes the MagMo components detach from each other. Mental resistance to using robots in manned situations will be reduced because astronauts can control the situation at any time with easy and instinctive action. In summary, MagMo solves the problems that impede robotic assistance in manned space vehicle in a unique manner. In the next section, the structure of MagMo is introduced. III. MagMo Structure Figure 3 shows the basic structure of MagMo. Since reconfiguration depending on needs and tasks is one of the most important features of modular robots, MagMo should possess all the capabilities required to construct a decentralized autonomous system, e.g., autonomous control, communications, and actuators to generate manipulator function. In addition to such basic structures, MagMo should also realize an important feature, the ability to separate its parts from each other upon unexpected collision. To realize this feature, MagMo has a specially designed joint mechanism and gear system, as introduced below. 3

4 A. Joint Mechanism The MagMo achieves safe operation by breaking apart when an unexpected collision occurs. On the other hand, the MagMo maintains its connections against low and static forces caused by normal tasks. M-tran, which is well known for joint mechanisms using magnetic force, is a reconfigurable modular robot developed by Miurata et al.. 9 The joint mechanism of M-tran focuses mainly on maintaining contact at any time, and is separated only when a control signal is applied. Therefore, the objectives of this joint mechanism are nearly contrary to those of the MagMo. In order to separate upon unexpected collision without any sensing or control, the joint mechanism must be perfectly passive in attachment and separation. A combination of magnetic force and an elastic structure fulfill these antithetical requirements in the MagMo. Magnetic force steeply decays according to the distance between the magnets. The force is very high when they contact each other, but suddenly decays when they are distant from each other. The MagMo parts interconnect by magnetic force. However, this type of connection alone cannot distinguish fast and slow forces. Therefore, an elastic structure is installed to connect modules and magnets. As shown in Figure 4, the magnet is attached on the elastic structure at the end of the module. This magnet can follow slower motion but it cannot follow faster motion. Therefore, the joint separates when fast and/or large forces are applied, as in a collision. Since the joint mechanism cannot maintain the geometrical position between modules by the interconnection of the elastic structure alone, a guide structure is installed at the end of the modules. As shown in figure 5, we developed several functional models using several kinds of elastic materials. A 50-mm diameter elastic material is used for the elastic structure of each functional model. A 3300-gauss and 12-mm-diameter neodymium magnet is installed at the center of the elastic material. Force profile of each functional model is recorded during fast and slow forces detach process. Figure 6 shows the experiments on the detachment when slow force is applied. Recording applied force, the force sensor is slowly pressed on the one side of joint mechanism. Figure 7 shows typical example when the joint mechanism is detached each other by slow force. Whereas the force rises suddenly and detaches shortly when force is applied on the joint mechanism using rigid material, the force gradually rises until the joint mechanism detaches each other in the case of cell sponge. The joint mechanism using cell sponge resists slow force. Seven kinds of material are examined on the resistance against slow and fast force. Table 1 shows the maximum force when slow or fast force is applied on the joint mechanisms, that correspond to resistance against the force. It is suitable for joint mechanism that the material perform high resistance for slow force and low resistance for fast force, because the joint mechanism with such material will be detached easily by collision and keep connection against the slow force caused by regular work. So we focused on the ratio of the resistance against fast force to the resistance against slow force. As shown in table, thin rubber structure gets maximum ratio, and the ration is about 3 times as much as solid case. These results suggest that the joint mechanism with elastic structure is 4 Figure 4. Schematic view of MagMo s joint mechanism (a) Solid (b) Thin Rubber (c) Silicon Figure 5. Examples of Joint Mechanism Functional Models Figure 6. Experiment Equipments on the Slow Force Detachment

5 effective for the separation between fast force caused by collision and slow force of normal work, and thin rubber structure is the most suitable among seven materials examined here. In addition to the mechanical connection between modules, the joint mechanism supports functions such as power transmission and communication capabilities. To avoid unexpected interconnection and acquire enough admittance, a magnet and a guide structure are used for power transmission. A wireless communication system is adopted, since wireless communication technologies are mature and easy to use. B. Gear System If MagMo consists of uniform modules, any module can be exchanged with any other, and their previous positions are not recognized once they are separated. Therefore, we can take advantage of the MagMo s modularity as much as possible. To achieve three-dimensional motion using uniform modules, each module should have at least two degrees of freedom on nonparallel axes. From the viewpoint of inverse kinematics, the case where these two axes cross at link axis a right angle is much more convenient than other configurations. On the other hand, the MagMo s actuator may lose its torque at any time, since the joints between modules may separate on unexpected collision for safety, and the power supply may suddenly cut off upon separation. In general, a gear system may passively change its joint angle when the motor torque is cut off. This passive motion is called back drive. If the MagMo s joint angles might be changed in a sudden power loss situation, the MagMo s controller would lose the actual joint angles upon module separation and need to calibrate the angle after reassembly. Such calibration is not only a nuisance but also poses difficulties, since the working area of the MagMo is quite limited. Therefore, the gear system of the MagMo must maintain its joint angle in power-off situations, and remain free of back drive. We developed a new gear system called a Cross-Gear Universal Joint to solve these problems. In the Cross-Gear Universal Joint, two circular gears that intersect at right angles are mounted in the center of a universal joint. These gears are called the Cross-Gear (Fig. 8). Two gears, called driver gears, are fixed on the universal joint, and mesh with the Cross-Gear on each side. Once the driver gears are driven by motors, the driver gears travel on the Cross- Gear, and two degree of freedom can be controlled by each motor. Thus, the gear system realizes two degrees of freedom at a right angle at the one point. However, the Cross-Gear as described is not free of back drive. Therefore, a worm gear system is applied on the motor side of the driver gears. The worm gear system consists of a worm gear and a worm wheel; the worm gear can drive the worm wheel, but the worm wheel never drives the worm gear. Using these combinations of gears, the Cross-Gear Universal Joint achieves two degree of freedom at a right angle and is back drive free (Fig. 9). Figure 8 shows a functional model of the Cross-Gear Universal Joint. To increase the driving torque, six gears are installed between the worm wheel and the Cross Gear. The overall gear ratio of this system is 1:319. A highperformance motor called EC-max22 from Maxon Corporation is used for the torque driver; it can generate a maximum torque of 11.4 mnm. Using this torque driver, the joint moves about 90 deg/s at maximum speed in each 5 Figure 7. Force Profile During Detachment Process Resistance against Slow Force[Nm] Resistance against Fast Force[Nm] Slow / Fast Ratio Solid Urethane Rubber Cell Sponge (T=10mm) Cell Sponge (T=20mm) Thin Rubber Silicon Uretane

6 Fig 8. Cross Gear Fig 9. Cross Gear Universal Joint degree of freedom, and these specifications are fit for the MagMo. To verify resistance for back drive, the functional model is driven in constant load and power supply is suddenly stopped. Typical result is shown in figure 10. Power supply was stopped at arrow timing, but the functional model keeps its joint angle constantly. This result suggest that the Cross-Gear Universal Joint achieves back drive free. Since the torque driver of the Cross-Gear Universal Joint is mounted in an offset position from the joint axis, the mechanism is quite compact and suitable for use in very small applications. IV. Conclusion In this paper, a unique modular robot concept is proposed to solve three major problems, such as safety requirements, workspace limitations, and the need for observation, in robotic support in manned space vehicles. The concept is called a magnetically jointed module manipulator (MagMo) and realizes safety by breaking apart in the case of a collision. In addition, two important elements, the joint mechanism and gear system, are also introduced. We believe that this unique concept will reduce the barriers to the use of robotic assistance in the human environment and expand the possible uses of robots. References 1 Freund, E., and Rossmann, J., Control of Multirohot-Systems for Autonomous Space Laboratory Servicing, Proc. Int. Symp. Artificial Intelligence Robotics and Automation in Space, 1989, pp Swaim, P. L., Thompson, C. J., and Cambell, P. D., The Charlotte Intra-Vehicular Robot, Proceedings of The Third International Symposium on Artificial Intelligence, Robotics, and Automation for Space, 1994, pp Tsumaki, Y., Yokohama, M., and Nenchev, D. N., Intra-Vehicular Free-Flyer System, Proceedings of Intl. Conference on Intelligent Robotics and Systems, 2003, pp Kimura, S., and Okuyama, T., Processor Performance Required for Decentralized Kinematic Control Algorithm of Module- Type Hyper-Redundant Manipulator, Journal of Robotics and Mechatronics, Vol. 8, No. 5, 1996, pp. 442, Kimura, S., Takahashi, M., Okuyama, T., Tsuchiya, S., and Suzuki, Y., "A Fault-Tolerant Control Algorithm Having a Decentralized Autonomous Architecture for Space Hyper Redundant Manipulators, IEEE Transactions on Systems Man and Cybernetics Part A, Vol. 28, No. 4, 1998, pp. 521, Kimura, S., and Okuyama, T., A Fault-torelant Control Algorithm of Redundant Space Manipulator for Remote Inspection, Journal of Space Technology and Science, Vol. 12, No. 1, 1999, pp. 27, Kimura, S., Tsuchiya, S., Takegai, T., and Nishida, S., Fault Adaptive Kinematic Control Using Multiprocessor System and its Verification Using a Hyper-Redundant Manipulator, Journal of Robotics and Mechatronics, Vol. 13, No. 5, 2001, pp. 540, Oda, M., "REXJ. Demonstration of an Astronaut Support Robot (Astrobot) on the International Space Station Japanese Experiment Module (ISS/JEM:KIBO)", in Proceedings of International Astronautical Congress, Glasgow, U.K., 2008, IAC- 08-B Murata, S., Yoshida, E., Kamimura, A., Kurokawa, H., Tomita, K., Kokaji, S., M-TRAN: Self-Reconfigurable Modular Robotic System, IEEE/ASME Transactions of Mechatronics, Vol.7, No.4, 2002, pp Figure 10. Back Drive Free Profile 6