Oregon State’s CASSIE biped: Mechanical design for dynamic ... · Oregon State’s CASSIE biped:...
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Oregon State’s C ASSIE biped: Mechanical design for dynamic locomotion Andy Abate Oregon State University Jonathan W. Hurst Oregon State University, Agility Robotics ABSTRACT CASSIE is a bipedal robot designed for dynamic walking and running over real-world terrain. The mechanical design is focused on creating specific dynamic behaviors in cooperation with software control, rather than motors simply following torque and speed trajectories. Specific aspects of this design include the particular kinematic structure and chosen degrees of freedom, series-elastic behavior, and power quality consi- derations, in addition to a number of implementation details. KINEMATIC STRUCTURE CASSIE has a three-link leg design with a parallel constraint between the thigh and tarsus. Kinematically, the leg is a serial mechanism with motors driving hip and knee flexion/extension relative to the thigh. The heavy motors are kept up near the hip to reduce leg inertia. Series springs connect the knee actuator to the shin and the achilles link to the tarsus. When the springs are undeflected, the tarsus and thigh point in the same direction, but spring deflections break this alignment. In addition to the hip and knee motors, CASSIE has yaw, abduction, and toe motors. The yaw motors allow CASSIE to turn on its own, and the toe motors create an actuated base of support and let the robot stand in place. SERIES- ELASTIC BEHAVIOR We wanted a SLIP-like compliance, so the asymmetric knee and heel springs have tuned stiffnesses to act together as a single leg-length spring [1]. Having a lightweight lower leg connected through series springs reduces impacts and allows CASSIE to store and release gait energy with each stride. POWER QUALITY Even when a gait has fixed energy requirements, the wrong motor placement could require motors to work against each other and waste energy internally. CASSIE’s serial hip and knee configuration does three times less internal work than the parallel ATRIAS leg design for the same gait [2]. For walking and running, motors in this leg configuration are always doing the same sign work rather than contradictory work. I MPLEMENTATION DETAILS Motor/gearhead selection: To determine the optimal pai- ring of motors with gear ratios, we took sample torque/speed trajectories for each joint and exhaustively searched through nearly 100 different motor specifications and gear ratios bet- ween 1:1 and 50:1 for the pair that requires the least energy. Crash-resistant composite covers: Carbon fiber and alu- minum shells protect the robot from falls. After a tumble, the robot can be picked back up and continue operating. CASSIE has already fallen several times on concrete without taking any serious damage (although we like to avoid it). Motor/amplifier modules: Thigh and gimbal electronics and motor stators are treated as modules and can be either independently removed or as a long chain. The leg remains mechanically intact, with the electronics modules, encoders and all, immediately serviceable. Achilles link: The kinematic connection between thigh and tarsus is an out-of-plane link. The heel spring is positioned with a compound out-of-plane angle to maximize perpendicu- larity with this link. ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Graduate Research Fellowship Program Grant No. 1314109, and also under DARPA award number W911NF- 16-1-0002. The CASSIE biped design and all derived work has been licensed to Agility Robotics by Oregon State University. REFERENCES [1] Andy Abate, Ross L Hatton, and Jonathan W Hurst. Passive- dynamic leg design for agile robots. In Robotics and Automation (ICRA), 2015 IEEE International Conference on, pages 4519– 4524. IEEE, 2015. [2] Andy Abate, Jonathan W Hurst, and Ross L Hatton. Mechanical antagonism in legged robots. In Robotics: Science and Systems (RSS), 2016.
Transcript of Oregon State’s CASSIE biped: Mechanical design for dynamic ... · Oregon State’s CASSIE biped:...
Oregon State’s CASSIE biped:Mechanical design for dynamic locomotion
Andy AbateOregon State University
Jonathan W. HurstOregon State University, Agility Robotics
ABSTRACT
CASSIE is a bipedal robot designed for dynamic walkingand running over real-world terrain. The mechanical design isfocused on creating specific dynamic behaviors in cooperationwith software control, rather than motors simply followingtorque and speed trajectories. Specific aspects of this designinclude the particular kinematic structure and chosen degreesof freedom, series-elastic behavior, and power quality consi-derations, in addition to a number of implementation details.
KINEMATIC STRUCTURE
CASSIE has a three-link leg design with a parallel constraintbetween the thigh and tarsus. Kinematically, the leg is a serialmechanism with motors driving hip and knee flexion/extensionrelative to the thigh. The heavy motors are kept up near thehip to reduce leg inertia.
Series springs connect the knee actuator to the shin and theachilles link to the tarsus. When the springs are undeflected,the tarsus and thigh point in the same direction, but springdeflections break this alignment.
In addition to the hip and knee motors, CASSIE has yaw,abduction, and toe motors. The yaw motors allow CASSIE toturn on its own, and the toe motors create an actuated base ofsupport and let the robot stand in place.
SERIES-ELASTIC BEHAVIOR
We wanted a SLIP-like compliance, so the asymmetric kneeand heel springs have tuned stiffnesses to act together as asingle leg-length spring [1]. Having a lightweight lower legconnected through series springs reduces impacts and allowsCASSIE to store and release gait energy with each stride.
POWER QUALITY
Even when a gait has fixed energy requirements, the wrongmotor placement could require motors to work against eachother and waste energy internally. CASSIE’s serial hip andknee configuration does three times less internal work than theparallel ATRIAS leg design for the same gait [2]. For walkingand running, motors in this leg configuration are always doingthe same sign work rather than contradictory work.
IMPLEMENTATION DETAILS
Motor/gearhead selection: To determine the optimal pai-ring of motors with gear ratios, we took sample torque/speedtrajectories for each joint and exhaustively searched throughnearly 100 different motor specifications and gear ratios bet-ween 1:1 and 50:1 for the pair that requires the least energy.
Crash-resistant composite covers: Carbon fiber and alu-minum shells protect the robot from falls. After a tumble, therobot can be picked back up and continue operating. CASSIEhas already fallen several times on concrete without taking anyserious damage (although we like to avoid it).
Motor/amplifier modules: Thigh and gimbal electronicsand motor stators are treated as modules and can be eitherindependently removed or as a long chain. The leg remainsmechanically intact, with the electronics modules, encodersand all, immediately serviceable.
Achilles link: The kinematic connection between thighand tarsus is an out-of-plane link. The heel spring is positionedwith a compound out-of-plane angle to maximize perpendicu-larity with this link.
ACKNOWLEDGMENTS
This material is based upon work supported by the NationalScience Foundation under Graduate Research Fellowship ProgramGrant No. 1314109, and also under DARPA award number W911NF-16-1-0002. The CASSIE biped design and all derived work has beenlicensed to Agility Robotics by Oregon State University.
REFERENCES
[1] Andy Abate, Ross L Hatton, and Jonathan W Hurst. Passive-dynamic leg design for agile robots. In Robotics and Automation(ICRA), 2015 IEEE International Conference on, pages 4519–4524. IEEE, 2015.
[2] Andy Abate, Jonathan W Hurst, and Ross L Hatton. Mechanicalantagonism in legged robots. In Robotics: Science and Systems(RSS), 2016.
