Active Exploration of Surfaces for Legged Locomotion of Robots

This paper presents some results of an ongoing research project in the GRASP Lab in the area of active exploration and perception for the legged locomotion of robots. We propose an active perceptual scheme that is based on the ability of the robot to extract material properties from a surface during locomotion. This ability is provided to the robotic system through a compliant sensing device which is used to monitor the response of the surface when exploratory procedures are executed during the stepping and walking motions of the leg. Such a system will actively perceive changes in the surfaces properties and prevent the robot from slipping, falling, or sinking during locomotion. The paper describes the proposed perceptual scheme, the system set-up, and the implementation of the exploratory procedures

Push recovery with stepping strategy based on time-projection control

In this paper, we present a simple control framework for on-line push recovery with dynamic stepping properties. Due to relatively heavy legs in our robot, we need to take swing dynamics into account and thus use a linear model called 3LP which is composed of three pendulums to simulate swing and torso dynamics. Based on 3LP equations, we formulate discrete LQR controllers and use a particular time-projection method to adjust the next footstep location on-line during the motion continuously. This adjustment, which is found based on both pelvis and swing foot tracking errors, naturally takes the swing dynamics into account. Suggested adjustments are added to the Cartesian 3LP gaits and converted to joint-space trajectories through inverse kinematics. Fixed and adaptive foot lift strategies also ensure enough ground clearance in perturbed walking conditions. The proposed structure is robust, yet uses very simple state estimation and basic position tracking. We rely on the physical series elastic actuators to absorb impacts while introducing simple laws to compensate their tracking bias. Extensive experiments demonstrate the functionality of different control blocks and prove the effectiveness of time-projection in extreme push recovery scenarios. We also show self-produced and emergent walking gaits when the robot is subject to continuous dragging forces. These gaits feature dynamic walking robustness due to relatively soft springs in the ankles and avoiding any Zero Moment Point (ZMP) control in our proposed architecture.Comment: 20 pages journal pape

Sabertooth: A High Mobility Quadrupedal Robot Platform

Team Sabertooth aimed to design and realize an innovative high mobility, quadrupedal robot capable of delivering a payload over terrain impassable by wheeled vehicles at a speed of 5fps. The robot is designed to ascend and descend stairs. The robot uses a spring system in each of its legs for energy efficient locomotion. The 4\u27x3\u27x3\u27 freestanding four legged robot weighs approximately 300lbs with an additional payload capacity of 30lbs. The passive two degree of freedom body joint allows flexibility in terms of robot motion for going around tight corners and ascending stairs. The system integrates sensors for staircase recognition, obstacle avoidance, and distance calculation. A distributed control and software architecture is used for world mapping, path planning and motion control

Sabertooth: A High Mobility Quadrupedal Robot Platform

Team Sabertooth aimed to design and realize an innovative high mobility, quadrupedal robot platform capable of delivering a payload over terrain otherwise impassable by wheeled vehicles at a speed of 5 feet per second. The robot uses a spring system in each of its legs for energy efficient locomotion. The 4ft x 3ft x 3ft freestanding four legged robot weighs approximately 300 pounds with an additional payload capacity of 30 pounds. An important feature of the robot is the passive, two degree of freedom body joint which allows flexibility in terms of robot motions for going around tight corners and ascending stairs. A distributed control and software architecture is used for world mapping, path planning and motion control

Robotic Exploration of Surfaces With a Compliant Wrist Sensor

This paper presents some results of an ongoing research project to investigate the components and modules that are necessary to equip a robot with exploratory capabilities. Of particular interest is the recovery of certain material properties from a surface, given minimal a priori information, with the intent to use this information to enable a robot to stand and walk stably on a surface that is unknown and unconstrained. To this end, exploratory procedures (ep\u27s) have been designed and implemented to recover penetrability, material hardness and surface roughness by exploring the surface using a compliant wrist sensor. A six degree-of-freedom compliant wrist sensor, which combines passive compliance and active sensing, has been developed to provide the necessary flexibility for force and contact control, as well as to provide accurate position control. This paper describes the compliant wrist and sensing mechanism design along with a hybrid control algorithm that utilizes the sensed information from the wrist to adjust the apparent stiffness of the end-effector as desired

Analysis of walking and balancing models actuated and controlled by ankles

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 181-182).Experimental data show that ankle torque is the most important actuator in normal human locomotion. I investigate the dynamics of simple models actuated by ankles alone. To assess the contribution of ankle actuation to locomotion, I first analyze the dynamics of some passive walkers without any joint torque. These passive walkers include a rimless wheel model and springy-legged models with and without a double stance phase. I analyze the stability of the period-one gait of each passive walker to compare it with the stability of the period-one gait of an ankle actuated model. Subsequently, I investigate whether balancing of a double inverted pendulum model whose shape and mass distribution are similar to a human can be achieved by control of ankle torque in a frontal plane. I study the dynamics of the model and design a controller that makes the model balance with biologically realistic ankle torque and a reasonable foot-floor friction coefficient. I conclude that an ankle-actuated model can make a stable period-one gait in a sagittal plane. Also, I deduce that the ankle torque control in a frontal plane can stabilize a double inverted pendulum model whose shape and mechanical properties are similar to those of humans.by Jooeun Ahn.S.M

Knee design for a bipedal walking robot based on a passive-dynamic walker

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (leaf 30).Passive-dynamic walkers are a class of robots that can walk down a ramp stably without actuators or control due to the mechanical dynamics of the robot. Using a passive-dynamic design as the basis for a powered robot helps to simplify the control problem and maximize energy efficiency compared to the traditional joint-angle control strategy. This thesis outlines the design of a knee for the robot known as Toddler, a passive-dynamic based powered walker built at the Massachusetts Institute of Technology. An actuator at the knee allows the robot to bend and straighten the leg, but a clutch mechanism allows the actuator to completely disengage so that the leg can swing freely. The clutch operates by using a motor to rotate a lead screw which engages or disengages a set of spur gears. Control of the knee is accomplished by utilizing the robot's sensors to determine whether or not the knee should be engaged. The engagement signal is then fed through a simple motor control circuit which controls the motor that turns the lead screw. The knee design was successfully implemented on Toddler but more work is required in order to optimize his walking. In order to study the dynamics of walking with knees, we also built a copy of McGeer's original passive walker with knees.by Andrew Griffin Baines.S.B