Ajallooeian, Mostafa

D'Haene, Michiel

Degrave, Jonas

Gay, Sebastien

Ijspeert, Auke Jan

Möckel, Rico

Schrauwen, Benjamin

Sproewitz, Alexander

Tuleu, Alexandre

Vespignani, Massimo

English

Ghent University Academic Bibliography

Towards Dynamically Running Quadruped Robots:Performance, Scaling, and ComparisonAlexander Sproewitz*, Alexandre Tuleu*, Michiel D’Haene**,Rico Mo¨ckel*, Jonas Degrave**, Massimo Vespignani*,Sebastien Gay*, Mostafa Ajallooeian*, Benjamin Schrauwen**, Auke Jan Ijspeert**Biorobotics Laboratory, EPFL, Lausanne, Switzerland, **Reservoir Lab, UGhent, Belgiumemail:alexander.sproewitz@epfl.ch1 Hardware platforms and project ideasThe field of robotic research applying legged machinesand robots is growing quickly, but it is still facing a majordiscrepancy of performance, quality, and complexity, com-pared to their biological counterparts.A lack of appropriate off-the-shelf hardware platforms, alack of open-access to dynamically running legged robotplatforms, and missing comparative studies between exist-ing quadruped platforms (mis)leads researchers to re-inventand re-design own quadruped robots, from the scratch. Asresearch of legged locomotion should be the goal, and notresearch about its tools (the robots per se), this can be afrustrating and time consuming process. Research in dy-namic legged locomotion would benefit from free and easyaccess to blueprints, and studies where different robot de-sign choices can be tested and compared rigorously againsteach other. A future, better-performing platform could beassembled from the best-performing components (actuators,sensors, driver code, controllers) and principles (type of ac-tuation, type of control, sensing, leg design, trunk design,compliant design) of existing systems.We shortly present two quadruped robot platforms.CheetahCub-robot was developed at BioRob [1], of an olderversion with lower speed and capabilities [2]. Oncilla-robot was developed at BioRob/EPFL in cooperation withResLab/UGhent. Both robot platforms use a four-segment,pantograph-based leg design [3], and are roughly similarin dimensions. Of the two platforms, Oncilla-robot is thesuccessor of CheetahCub-robot, with more sensor capabil-ities (Table 1), improved actuation, but also higher com-plexity and cost. CheetahCub-robot and Oncilla-robot weredesigned with multiple purposes; one of them includes re-search on the pantograph-like leg structure, and its implica-tions for dynamic locomotion.Oncilla-robot is being developed as the quadruped robotplatform of the European AMARSi project, “. . . aim[ing] ata qualitative jump in robotic motor skills towards biologi-cal richness.”[4], and is also meant to help comparing andanalysing data from Biology and Robotics [5]. The en-tire Oncilla-robot platform will become open-source soon.This will include blueprints of all mechanical and electricalcomponents and assemblies, firmware, communication, anddriver programming code.Due to its similarity in structure, both systems are easy tocompare: CheetahCub-robot reached dynamic trot-gaits (upto 1.4ms−1, 6.9 body lengths per second, Froude number[6]FR=1.3, cost of transport COT=6JN−1m−1) [7]. Chee-tahCub shows self-stabilizing properties, where open-looplocomotion patters lead to decreasing pitch and roll anglesat increasing robot speed. The robot also performed well atstep-down obstacles, again without using sensory feedback.Oncilla-robot is currently undergoing rigorous tests, show-ing already good trot gait speed (v=0.55ms−1), at lower costof transport, around COT=3JN−1m−1. Hence, the effect ofusing stronger, more heavy motors and smaller gear box ra-tios is reflected by the lower cost of transport of Oncilla-robot.Oncilla-robot was designed with having certain draw-backs of CheetahCub-robot in mind. The RC servo motorsof CheetahCub robot are modular, very compact and light-weight, easy to control, relatively cheap, and require not ex-ternal control PCB. However, their small motor size (around12W) and the extremely high gear ratio (around 300/1)leads to a very high power consumption at fast and high fre-quency gaits, up to 10JN−1m−1. This showed to be a limitfor long-lasting experiments, RC-servo motors defected atrobot speeds higher than 1ms−1, without taking long cool-down breaks. Oncilla-robot was designed with high-power(90W) brushless motors, which can deal better with heatinduced by fast oscillating motors, and rapidly switching,high torque profiles required for legged locomotion. Dueto the high motor output torque, a lower gear ratio leads tobetter cost of transport performance. Equally, stronger mo-tors allow for adequate load capabilities of Oncilla-robot.Hence, more sensors, and power on-board could be imple-mented. On-board sensing, improved actuation, and higherweight however also require a more expensive (10 times),more complex, and more load-bearing robot setup. From theexisting results with CheetahCub we expect robot speeds upto 1ms−1 with Oncilla robot, but with significant lower costof transport, and typically at slightly higher gait frequencies,above 3s−1.Table 1: Rough comparison of CheetahCub-robot and Oncilla-robot. We use the Hildebrand version of the Froude number: FR= v2g−1L−1,with v the average robot speed, and the standing hip height as the leg length (L). Cost of transport is defined as COT=Pm−1g−1v−1,with P the full electrical power consumption for locomotion, and m the mass of the robot.CheetahCub-robot Oncilla-robotWeight 1.1kg 3.7kgLeg length, width foot-foot, hip-shoulder 0.15m, 0.11m, 0.21m 0.17m, 0.14m, 0.225mComputation, communication, power onboardRB110, wireless and PWM50Hz, tetherRB110, wireless, RS485 high speed bus, bat-tery on boardSensors RC-internal absolute encoderfor hip and kneeIMU 9axis, 3x absolute joint encoders per leg,horizontal and vertical force sensing per leg,hip and knee relative encoders per leg, robotpower consumption, RC servo encoderActive DOF per leg hip, knee hip, knee, ablationLeg design pantograph, 3-spring system,4 segmentspantograph, 3-spring system, 4 segmentsActuation per leg 2x RC servo motor 2x 90W brushless 4-pole motors, 1x RC servomotorGear ratio hip, knee, ablation 297/1, 297/1, none 84/1, 56/1, 297/1Speed max trot, walk 1.4ms−1, 0.75ms−1 0.55ms−1, . . .FR max trot, walk 1.33, 0.38 0.33, . . .COT best, max FR 5.9JN−1m−1, 10JN−1m−1 3JN−1m−1, . . .(a) Oncilla-robot, CAD(b) CheetahCub-robot, CADFigure 1: Links to movies of both robots can be found in sec-tion 3. Both robots are roughly house cat-sized, Chee-tahCub being less heavy (1.1kg) than Oncilla-robot(3.7kg).2 Open questionsResearch questions for both robot platformsCheetahCub-robot and Oncilla-robot are related to thedesign of the mechanism, the controller, and the resultinggait characteristics. Above robot platforms are very similarin mechanics, this includes the leg design, but also the centerof mass. With the upcoming development of bio-inspiredquadruped and legged robots, it becomes interesting toidentify required “blueprints”, and see their dependence onthe expected gaits.1. Leg design: Raibert’s quadruped robot design [8],featuring prismatic legs, and a three-parted controllerproved that engineering a quadruped robot can workextremely well. Succeeding robots (BigDog [9], Al-phaDog) seemingly feature similar base-controllers,but have switched to different leg design. The exactinfluence of leg segmentation and leg design willrequire more work, and presents a challenging task.2. Until recently, gait and locomotion controllers forquadruped robots, at higher comparable speed, re-lied on feedback loops. Reflex-based controllers(e.g. CPG [10]) were implemented, or model basedcontrol (e.g. Scout 2 [11]). With CheetahCub-robotwe can show that with the appropriate leg configura-tion, and an adapted controller, higher-speed, open-loop, and self-stabilizing locomotion is possible. Wepropose that for level-running, a robot should be capa-ble of open-loop running, i.e. feedback, either modelor reflex-based, should only be necessary when facingobstacles, or perturbations.3. Somewhat like airplanes, legged robots such asquadruped robots require a low mass to size ratio, tosucceed with high-speed gaits. Calculated, controlledpassive deflections in leg or trunk structure can be ad-vantageous. Compliant elements can reduce the robotweight, by replacing actuators, and by redirecting, in-tegrating, and accumulating/freeing forces and ener-gies. However, simple, linear, and passive compliantsystems typically restrict a robot system to one pre-ferred frequency. How can one force or stimulate asimple passive compliant system into multi-mode lo-comotion with a large range of dynamics?4. Together with intrinsic stability characteristics ofthe robot, the number and types of sensors definethe robot’s capabilities. Certain controllers requirestate estimations, e.g. a standard SLIP-like controlwould require the information of the instantaneous hipheight, speed, and the leg angle in world frame co-ordinates. While off-the-shelve sensors become eas-ier accessible nowadays, good quality ones can stillbe very expensive (IMU, load cells). Which sensors,and which controller present a robust and easy toimplement trade-off for a good legged robotic de-sign?3 MaterialMovie links to CheetahCub-robot and Oncilla-robot areavailable here:http://tinyurl.com/d3okppf (CheetahCub-robot)http://tinyurl.com/crypjy8 (Oncilla-robot)AcknowledgementsThe research leading to these results has received fund-ing from the European Community’s Seventh FrameworkProgramme FP7/2007 − 2013−Challenge 2-CognitiveSystems, Interaction, Robotics-under grant agreementNo. 248311 (AMARSi).References[1] A. Sproewitz, A. Tuleu, M. Vespignani, M. Ajal-looeian, E. Badri, and A. Ijspeert, “Towards dynamic trotgait locomotion—design, control, and experiments withcheetahcub, a compliant quadruped robot,” submitted toIJRR, 2012. under review.[2] S. Rutishauser, A. Sproewitz, L. Righetti, and A. J.Ijspeert, “Passive compliant quadruped robot using centralpattern generators for locomotion control,” in 2008 ProcBIOROB, 2008.[3] H. Witte, R. Hackert, W. Ilg, J. Biltzinger,N. Schillinger, F. Biedermann, M. Jergas, H. Preuschoft, andM. Fischer, “Quadrupedal mammals as paragons for walk-ing machines,” in Proc AMAM, pp. TuA–II–2.1 – TuA–II–2.4, 2003.[4] “Webpage AMARSi, EU-FP7,” 2012.http://www.amarsi-project.eu/.[5] F. L. Moro, A. Sprowitz, A. Tuleu, M. Vespignani,N. G. Tsagarakis, A. J. Ijspeert, and D. G. Caldwell, “Horse-like walking, trotting and galloping derived from kinematicmotion primitives (kmps) and their application to walk trottransitions in a compliant quadruped robot,” Biological Cy-bernetics, 2013. in press.[6] R. M. Alexander, “Walking and running,” The Math-ematical Gazette, vol. 80, no. 488, pp. 262—266, 1996.[7] A. Kuo, “Choosing your steps carefully,” Robotics& Automation Magazine, IEEE, vol. 14, no. 2, pp. 18–29,2007.[8] M. Raibert, M. Chepponis, and H. Brown, “Runningon four legs as though they were one,” Robotics and Au-tomation, IEEE Journal of, vol. 2, no. 2, pp. 70–82, 1986.[9] M. Buehler, R. Playter, and M. Raibert, “Robots stepoutside,” AMAM2005, 2005.[10] H. Kimura, Y. Fukuoka, and A. H. Cohen, “Adaptivedynamic walking of a quadruped robot on natural groundbased on biological concepts,” The International Journal ofRobotics Research, vol. 26, pp. 475–490, May 2007.[11] D. Papadopoulos and M. Buehler, “Stable running ina quadruped robot with compliant legs,” in IEEE Interna-tional Conference on Robotics and Automation, 2000. Pro-ceedings. ICRA ’00, vol. 1, pp. 444–449 vol.1, IEEE, 2000.

Towards dynamically running quadruped robots: performance, scaling, and comparison

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Towards dynamically running quadruped robots: performance, scaling, and comparison

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Ghent University Academic Bibliography