31A Minimally Invasive Tendril Robot for In-Space Inspection

A Minimally Invasive Tendril Robot for In-SpaceInspectionJoshua S. Mehling and Myron A. Diftler Mars Chu Michael Valvo Automation, Robotics, and Simulation Division TRAC Labs Division Robotics Department NASA / Johnson Space Center Metrica, Inc. Jacobs Sverdrup / ERC Houston, TX 77058 Houston, TX 77058 Houston, TX 77058 jmehling, mdiftler@ mchu@ mvalvo@Abstract– Current work at NASA’s Johnson Space Center is focusing on the identification and design of novel robotic archetypes to fill roles complimentary to current space robots during in-space assembly and maintenance tasks. Tendril, NASA’s latest robot designed for minimally invasive inspection, is one system born of this effort. Inspired by the biology of snakes, tentacles, and climbing plants, the Tendril robot is a long slender manipulator that can extend deep into crevasses and under thermal blankets to inspect areas largely inaccessible by conventional means. The design of the Tendril, with its multiple bending segments and 1 cm diameter, also serves as an initial step in exploring the whole body control known to continuum robots coupled with the small scale and dexterity found in medical and commercial minimally invasive devices. An overview of Tendril’s design is presented along with preliminary results from testing that seeks to improve Tendril’s performance through an iterative design process.Index Terms – tendril, minimally invasive, in-space inspection, continuum robot, space roboticsI.I NTRODUCTIONNASA has a history of augmenting human abilities and activities in space with robotic systems. For example, rovers from NASA’s Jet Propulsion Laboratory (JPL) are currently exploring the surface of Mars well prior to the design of a crew exploration vehicle for human transportation to the planet [1]. Also the shuttle and station remote manipulator systems (RMS’s) have proven invaluable in moving and mating together large modules during the construction of the International Space Station. The Canadian Space Agency (CSA) has also designed their Special Purpose Dexterous Manipulator (SPDM) to service robotically compatible Orbital Replacement Units (ORU’s) [2]. Designs such as these are examples of recognizing a specific need and then designing a system to meet that need. Just as in nature where there are different types of animals suited for specific purposes, robots too can be classified as specialized archetypes designed to perform in certain roles.Perhaps the most immediately useful robotic archetype for space would fill a niche as an assistant to human astronauts. A dexterous and anthropomorphic system such as NASA’s Robonaut, with two human-scale arms and five-fingered hands, is capable of working with an ever growing set of EVA tools and would be a valuable assistant to astronauts on orbit [3]. Robonaut’s fine motion ability matched with its 1:1 strength-to-weight ratio allow it to be a jack-of-all-trades when it comes to augmenting crew in construction, assembly, maintenance, or inspection tasks.While the jack-of-all-trades or all-purpose problem solver archetype filled by human astronauts and Robonaut is of paramount importance to future space activities, current work at NASA’s Johnson Space Center (JSC) is focusing on the identification of other important roles and the design of new robots to compliment existing systems. Having an array of varied archetypes working together, as opposed to a single class of robot working alone, provides more options to mission planners for accomplishing the future tasks NASA will face as a result of its new initiative to explore the Moon and Mars, build large-scale structures in space, and construct and maintain long-term habitats.In identifying and designing new robot archetypes, biology can be an important inspiration. It not only points to the various roles that new robots can play, but it can also inspire the forms that these new robots take. This thinking is apparent in the designs of JSC’s spider-like robot, a large eight-legged system that specializes in walking lightly across web-like truss structures [4], JPL’s LEMUR system that specializes in small-scale manipulation and dexterity [5], and NASA’s latest robot designed for minimally invasive inspection, Tendril.II.B ACKGROUNDThe Tendril robot, seen in Fig. 1, finds its inspiration in the biology of snakes, tentacles, and climbing plants. A long slender manipulator, Tendril has a diameter to length aspect ratio of 1:100 (1cm:1m) and, like its biological analogues, it can fit into tight spaces and through complex geometries. Built to compliment NASA’s Mini-AERCam, a free-flying surface inspector [6], and other similar systems, the Tendril robot will ultimately be used to inspect deep into crevasses, under thermal blankets, or down potential damage holes caused by MicroMeteoroid Orbital Debris (MMOD) or other impacts. Locations such as these are largely inaccessible to both astronauts and existing in-space inspection tools.Small-scale dexterity and operating within a confined workspace have already been extensively researched with minimally invasive surgery in mind. Commercial devices used in laparoscopic procedures, such as Intuitive Surgical’s DaVinci system, have dexterous wrists and a wide range of end effectors that can pass through small ports (~1cm) and work inside the body with reduced trauma to patients [7]. Additionally, actuation of tendril-like, high aspect ratio surgical tools has been investigated with various robotic endoscopes [8, 9]. Of particular interest is a 4mm diameter prototype for laryngeal surgery developed at Johns Hopkins [10]. Minimally invasive inspection using commercial articulating borescopes such as those designed by Olympus Corporation is also common in the aircraft and chemical industries [11].In addition to the small-scale tip articulation found in medical and commercial systems, the design of Tendril also finds inspiration from the current research being done on continuum robots of which Robinson and Davies provide a thorough survey [12]. “Invertebrate” robots such as Walker’s elephant’s trunk robot and tentacle robot provide excellent examples of the design and control of systems that have a continuous bending ability along their entire body lengths [13]. Currently, the majority of hyperredundant and continuum robots are larger in diameter and shorter in length than the Tendril robot. To begin to explore these ideas at the small scale of minimally invasive devices, the first generation Tendril has two active bending sections near its tip. The lessons learned from experimentation with this system will lead to the addition of longer and many more bending segments in future design iterations. Thus, Tendril can be seen as an initial step towards a new small-scale, hyperredundant, continuum robot. The following examines Tendril’s mechanical and electrical design in greater detail accompanied by an explanation of control techniques, system testing, and potential upgradesthat will be explored as part of an iterative design process.III.TENDRILThe first generation Tendril (Fig. 1) is comprised of three primary subsystems: the Tendril limb and its associated actuation mechanism, the body housing structure, and the avionics system. Each is designed for straightforward integration and to facilitate the accomplishment of the first generation system’s design requirements. These requirements include: the aforementioned 1:100 aspect ratio with video ability at the tip, two active bending sections along the body to begin to explore coupling and hyperredundancy at a small diameter scale, and active extension, retraction, and rotation of the Tendril limb. A.The Tendril LimbThe Tendril limb is constructed from a series of extension and compression springs that are joined together by threaded links (Fig. 2). The extension spring sections provide passive bending over the majority of the bodylength while the compression springs are used for actively controlled bending. This active bending is actuated by setsof antagonistic tendons that are attached at specific links, run the entire length of the limb, and are terminated at pulleys inside the body housing structure. Each of the Tendril’s two bending sections (Fig. 3) use two pairs of tendons routed perpendicular to each other. Thus, when a specific pulley rotates, one tendon in a pair is retracted and another is allowed to extend. This creates a compressive force on one side of a section’s spring causing bending tooccur in one of the controlled directions (positive or negative pitch and yaw) without causing excessive slack to form in any of the tendons.Figure 1: The Tendril robotFigure 3: Tendril’s two bending sections and tip mounted camera Figure 2: Tendril limb springs joined by threaded links (Inset shows threaded links)The tendons that control the distal bending section are offset 45 degrees from those that control proximal bending. This difference is easily compensated for in the kinematics control algorithm but it is valuable in preventing any variation in the diameter or force required to actuate different bending sections. The antagonistic way in which the tendons are used also has the added benefit of minimizing the number of actuators required to obtain two degree-of-freedom (DoF) motion across each bending section.In addition to all eight of the actuation tendons (four for each bending section) passing through the threaded links, each of the links also has a center bore for power and video wires to pass from a small CCD camera mounted at the Tendril’s tip (seen in Fig. 3) back into the housing structure and out to an external video monitor.Using external springs as opposed to an internal backbone is found to be effective at this size scale. The modular fashion in which individual spring sections attach to one another is also of benefit because it allows for easy adjustment of the length and location of the passive and active bending sections along the Tendril’s limb. This is valuable for testing various configurations in real-time as well as ensuring optimal performance in 1g of a system that is designed to primarily function in a 0g environment.The Tendril actuation assembly (shown as a 3D model in Fig. 4) consists of the four pulleys and brushless DC motors required to drive the eight internal tendons as well as a routing structure to direct the tendons from the actuators into the Tendril limb. Inside the conduits of this routing structure as well as in the limb itself, the tendons pass through Teflon tubing to ease friction down the system’s meter length. Both the material used for the tendons as well as the configuration of the tubing is a subject of current testing. The 2.5 cm diameter pulleys are sized such that their full range of motion allows for greater than ±90 degree travel in both pitch and yaw at each bending section. The Tendril limb actuators’ 0.4 Nm maximum torque, coupled with the pulley diameters, also provides sufficient strength to operate the system when hanging vertically, when gravity is most detrimental to Tendril’s bending motion.The entire length of the Tendril limb can be covered in a custom Teflon fabric sheath. This sheath, currently being integrated into the system, will aid in reducing friction between the limb and the housing structure during extension and retraction. More importantly, this novel covering will protect both the Tendril and any surface it might be operating on from damage due to materials rubbing or scratching against one another. B.Tendril Body HousingTendril’s body housing provides both infrastructure and operational support for the system. Its main purpose is to house the Tendril actuation assembly as well as the system’s avionics and to actuate the system’s extension/retraction and auxiliary rotation degrees-of-freedom. The housing’s overall dimensions are 23 cm in outside diameter and 13 cm in width and the volume of the housing is balanced such that each dimension is minimized while maintaining sufficient space to mount the other subsystems. The housing (shown in Fig 5 and Fig. 6) consists of three sections: the Tendril reel, a body rotation actuator, and an exterior structure.The Tendril reel is a drum of short axial length with the Tendril actuation assembly and avionics mounted inside its cavity. The reel rotates relative to the fixed reference of the exterior structure and either pushes or pulls the Tendril limb for extension or retraction. A spiral groove is formed into the circumference of the reel to guide the Tendril limb during this motion. Furthermore, the groove confines the limb to a single layer during multiple revolutions so that a complex reeling mechanism is not required to prevent binding. The Tendril reel is driven by a centrally located brushless DC actuator. This actuator can produce 1.7 Nm of continuous torque at 120 RPM which is more than sufficient to achieve the desired Tendril limb feed rate of 5 cm/s.The body rotation actuator provides an additional degree-of-freedom for the Tendril body housing. This DoF creates a roll axis for Tendril when the tip is straight and gives Tendril an auxiliary method of panning a full360 degrees when the Tendril’s limb is bending. TheFigure 4: 3-D model of the Tendril actuation assembly Figure 5: External view of the Tendril body housingactuator uses a brushless DC motor and can produce 2.5 Nm of continuous torque at 91 RPM. In the vertical orientation, the actuator only requires a minimum torque output for rotating the body housing. However, when the Tendril is placed in a non-vertical orientation, a much higher output torque is needed. The actuator can produce a holding torque of 24.5 Nm in these cases. The proximal (non-rotating) end of the body rotation actuator can be attached to a fixed base while the distal (rotating) end is attached to the exterior structure through a quick-disconnect locking ring.The exterior structure provides an enclosure for the mechanical and electrical components of the Tendril as well as the Tendril reel, and consists of a central spine with two flanking covers. The central spine supports the reel along coincident axes using a set of large bore bearings for relative rotation and stability. This component is generally cylindrical in shape with a tangential salient (see Fig. 6). On one side of the salient, an outlet port allows Tendril to extend its limb out from and retract it into the body housing. The other side of the salient attaches to the body rotation actuator for support to a fixed base and for an additional DoF as mentioned above.Several features are incorporated into the housing to both confine and guide the Tendril limb during extension and retraction. For example, the outlet port of the exterior structure is aligned tangentially to the reel. This allows the limb to extend and retract with minimal bend angles. Additionally, Tendril’s limb is guided through an internal conical channel formed into the central spine before exiting the housing. The distance between the outer diameter of the reel and the inner diameter of the central spine is also minimized such that the Tendril limb will not cross over the groove walls and create binding. Lastly, Teflon liners are installed along the inner diameter of the central spine to reduce friction between the limb and the adjacent surfaces to facilitate smoother operations.Absolute position sensing is an important component in the multi-turn Tendril extension and retraction system to ascertain current limb position during system initialization or in the case of a power loss. Conventional sensors, however, are not suitable due to size constraints within theTendril body housing. Therefore, an absolute position sensor was custom designed using an inductive sensor to measure the linear distance to a transducer disk. The transducer disk serves two purposes: relating rotational motion of the reel to the linear motion of the Tendril limb and providing a hard stop to prevent the limb from over-extending or over-retracting. The disk engages internal threads that correspond to the fixed reference of the exterior structure and pins that are attached to the reel. As the reel rotates the disk slides along the pins. Using calibrated data, the linear position of the disk can be converted into the current position of Tendril limb.C. AvionicsTendril’s power and avionics systems find their heritage in NASA’s Robonaut system [14]. Many of the same components and custom board designs were chosen for Tendril due to the success and continued functionality they have shown in other NASA robots. The CompactPCI computer, which is recognized by NASA as viable flight hardware, is used as the main processor for Tendril. The computer, along with a power system consisting of a rugged chassis with several modules to provide different power levels, a wireless emergency stop receiver, and a series of solid state relays for each motor are mounted inside a power box that is part of the Tendril’s mobile test stand.The six motors in the Tendril system are controlled using the same custom, embedded, three-axis motor amplifiers used in Robonaut. Also, two custom Field Programmable Gate Array (FPGA) boards handle motor commutation and encoder feedback. These avionics stacks are mounted inside the Tendril reel alongside the Tendril actuation assembly (Fig. 6).IV.C ONTROL S OFTWAREAs with the avionics, much of the control software used for Tendril has heritage in the existing Robonaut system. This makes software development and troubleshooting faster and more reliable. The RTI ControlShell software tool is used to provide a real-time control architecture [15]. ControlShell has the ability to run many C++ program blocks on one or more clocks, group these blocks together by functionality, and create different modes for specific applications. The development of the Robonaut system led to an extensive catalogue of software repositories, libraries, and drivers some of which have now been utilized in Tendril.Existing software blocks were modified to create both joint and Cartesian control software for Tendril. Unlike Robonaut, however, which uses a proportional-derivative (PD) controller, Tendril’s position control is based upon a proportional-integral (PI) control loop (Fig. 7).Another new addition in the Tendril control system is a washout filter (Fig. 7). While the actuators have sufficient power to bend the Tendril limb, this required power level, if left across the motors for longer durations can cause them to near their temperature limits. Thus, theFigure 6: Internal view of the Tendril body housing (starboard cover removed)Command Command PWM Rate PWMcontrol system takes advantage of the hysteretic friction inherent in a tendon drive design and uses what is in essence a high pass filter to maintain limb position and minimize motor power. When a high frequency command is entered, the system responds in an effort to reach the desired state, but as the system nears its goal and approaches steady state the now low frequency, steady, motor command is washed out to 20% of its original value. The Tendril is then relying on a small motor command aided by its inherent friction to maintain a desired limb position. In this configuration, it is, of course, important to have a cutoff frequency for the filter very close to zero to prevent a washout from occurring before steady state is reached.Tendril can be controlled from a remote computer using RTI’s network data delivery service (NDDS). Using this protocol, information can be sent and received via Ethernet on computers within a specified network. A graphical user interface (GUI) has been created to receive position data via NDDS and from the GUI a user can issue commands to control Tendril. Joystick control has also been implemented along with the GUI to give the user a more comfortable and intuitive way of operating Tendril.V.C URRENT T ESTINGAs is the case with any iterative design process, the design and performance of Tendril is constantly being tested and improvements are continually being made. While it is hard to analyze each individual modification in a development atmosphere such as NASA where getting the job done is of primary importance, recent testing has sought to quantify performance across two generations of Tendril limb after a series of modifications.The original Tendril limb (Unit 1) has two, 4 cm bending sections actuated by 0.66 mm diameter Teflon coated, braided steel cables. The tendons in this version run inside Teflon tubes that extend partially along the limb’s length. Additionally, links along the passive bending sections are spaced 45 cm apart.The latest version (Unit 2) has Teflon tubing that runs the entire length of the Tendril limb. This limb also has two, 8 cm bending sections that are actuated by 0.25 mm diameter Spectra gel spun polymer tendons. Links on this version’s passive sections are spaced only 4 cm apart.Using the test setup shown in Fig. 8, the applied force to bending angle relationship was compared between the two generations of Tendril limb. Each limb was mounted vertically and known masses varying from 100g to 500g were attached to the positive yaw tendon of each limb’s distal bending section. The angle of bend away from the neutral axis was then recorded. This test is of interest, as it is desirable to increase the efficiency of Tendril limb bending. Minimizing the force required to bend the limb and maximizing its range of motion will in turn increase the usefulness of the system in future applications. Results of this test are summarized in Table 1.Figure 7: The Tendril control systemIt is apparent from the data that the modifications made in the second version of the Tendril limb dramatically improve performance. The maximum deflection created by a 4.9 N applied force increased from 52 degrees to 138 degrees. The general trend of bend angle also appears somewhat more linear with respect to applied force in the second Tendril version. These results are, of course, preliminary but they suggest that the design modifications to this point are on the right track. Testing will continue in hopes that further improvements can be achieved.TABLE 1APPLIED FORCE VS. BEND ANGLE DATABend Angle of Tendril’s Tip Away fromNeutral Axis Applied ForceTest Unit 1Test Unit 2 0.98 N (100g mass) 25.5° 27.0° 1.96 N (200 g mass)31.5°55.0°2.94 N (300g mass) 40.0° 80.0°3.92 N (400g mass) 51.5° 110.0°4.91 N (500g mass) 52.0°138.0°VI.C ONCLUSION Tendril shows great promise for use in space as a minimally invasive inspector. Its slender aspect ratio and imaging system allow it to reach through small openings and survey areas that are inaccessible to most other systems. The combination of Tendril’s aspect ratio, novel housing structure, and multiple bending sections along itslimb represent a true advancement in state-of-the-artrobotics technology. With this combination of features,Tendril can be used as a baseline comparison and testbedas work continues to develop new techniques and technology to further reduce diameters and increase the degrees-of-freedom of slender manipulation systems. In fact, future generations of the Tendril robot will strive for limb diameters approaching 5mm, limb lengths as long as5m, and many more active bending segments.In the near term, Tendril will be involved incollaborative testing with other new NASA robots.Determining the breadth of tasks Tendril is capable ofperforming as well as developing cooperative teaming scenarios with other new robotic archetypes will aid in refining the eventual role Tendril will take during in-space assembly and maintenance tasks. A CKNOWLEDGMENTThis work was partially supported by NASA’sExploration Systems and Missions Directorate through the “In Space Robotic Assembly and Maintenance” project.R EFERENCES[1]Hubbard, G. S. “The Exploration of Mars; Historical Contextand Current Results.” 42nd Annual AIAA Aerospace Sciences Meeting and Exhibit. Reno, NV, January 2004.[2]Hunter, D. G. “The Space Station Freedom Special PurposeDexterous Manipulator (SPDM).” in proceedings of National Telesystems Conference (NTC 1991). Atlanta, GA, March 1991. pp. 371-376.[3]Bluethmann, W., et al. “Robonaut: A Robot Designed toWork with Humans in Space.” Autonomous Robots. Kluwer Academic Publishers, Hingham, MA. vol. 14, no. 2/3. 2003. pp. 179-197.[4]Rehnmark, F., et al. “Innovative Robot Archetypes for In-Space Construction and Maintenance.” in proceedings of Space Technology and Applications International F orum (STAIF 2005).February 2005. pp. 1070-1077.[5]Kennedy, B., et al. “LEMUR: Legged Excursion MechanicalUtility Rover.” Autonomous Robots. 11(3), pp. 201-205 (2001).[6]Fredrickson, S., Duran, S., and Mitchell, J. “Mini-AERCam Inspection Robot for Human Space Missions.” AIAA Space 2004 Conference and Exhibit. San Diego, CA, September 2004.[7]Guthart, G. S. and Salisbury, J. K., Jr. “The Intuitive™Telesurgery System: Overview and Application.” in proceedings of IEEE International Conference on Robotics and Automation (ICRA 2000). San Francisco, CA, April 2000. pp. 618-621.[8]Maeda, S., et al. “Active Endoscope with SMA (Shape Memory Alloy) Coil Springs.” in proceedings of IEEE International Workshop on Micro Electro MechanicalSystems (MEMS 1996). San Diego, California, 1996, pp. 290-295.Figure 8: Applied force vs. bend angle test setup. Clockwise from left:overall view, close-up of Tendril limb mounting, and close-up of theTendril tip. [9]Ikuta, K., Tsukamoto, M., and Hirose, S. “Shape MemoryAlloy Servo Actuator System with Electric ResistanceFeedback and Application for Active Endoscope.” in proceedings of IEEE International Conference on Robotics and Automation (ICRA 1988). Philadelphia, Pennsylvania, 1988, pp. 427-430.[10]Simaan, N., Taylor, R., and Flint, P. “A Dexterous Systemfor Laryngeal Surgery – Multi-Backbone Bending Snake-Like Slaves for Teleoperated Dexterous Surgical Tool Manipulation.” in proceedings of IEEE International Conference on Robotics and Automation (ICRA 2004). NewOrleans, Louisiana, 2004. pp. 351-357.[11]Mehan, A. “Remote Visual Inspection Equipment for Quality Control.” WESCON/94. ‘Idea/Microelectronics’. Conference Record. Anaheim, CA, 2004. pp. 214-216.[12]Robinson, G. and Davies, J.B.C. “Continuum Robots – AState of the Art.” in proceedings of IEEE International Conference on Robotics and Automation. Detroit, MI, May 1999. pp. 2849-2854.[13]Walker, I. D., “Some Issues in Creating ‘Invertebrate’ Robots,” in proceedings of International Symposium onAdaptive Motion of Animals and Machine s. Montreal,Canada, 2000.[14]Ambrose, R. O. et al. “Robonaut: NASA’s Space Humanoid.”IEEE Intelligent Systems and Their Applications .15(4), pp. 57-63 (2000).[15]ControlShell User’s Manual, Version 7.0. Sunnyvale, CA:Real-Time Innovations, Inc. January 2001.。