Applied optimal control for dynamically stable legged locomotion

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2004.Includes bibliographical references (p. 79-84).Online learning and controller adaptation will be an essential component for legged robots in the next few years as they begin to leave the laboratory setting and join our world. I present the first example of a learning system which is able to quickly and reliably acquire a robust feedback control policy for 3D dynamic bipedal walking from a blank slate using only trials implemented on the physical robot. The robot begins walking within a minute and learning converges in approximately 20 minutes. The learning works quickly enough that the robot is able to continually adapt to the terrain as it walks. This success can be attributed in part to the mechanics of our robot, which is capable of stable walking down a small ramp even when the computer is turned off. In this thesis, I analyze the dynamics of passive dynamic walking, starting with reduced planar models and working up to experiments on our real robot. I describe, in detail, the actor-critic reinforcement learning algorithm that is implemented on the return map dynamics of the biped. Finally, I address issues of scaling and controller augmentation using tools from optimal control theory and a simulation of a planar one-leg hopping robot. These learning results provide a starting point for the production of robust and energy efficient walking and running robots that work well initially, and continue to improve with experience.by Russell L. Tedrake.Ph.D

Design of Passive Dynamic Walking Robots for Additive Manufacture

Ongoing research in the direction of printable, non-assembly mechatronic systems give rise to the need for multi-material printing, including electronics. However, there are robotic systems that do not use electronic components and still exhibit complex dynamic behavior. Such passive dynamic systems have the potential to save energy and component cost in the field of robotics compared to actuated systems. Ongoing research in computational design synthesis of passive dynamic systems aims at automatically generating robotic configurations based on a given task. However, an automated design-to-fabrication process also requires a flexible fabrication method. Towards the goal of printing functional, non-assembly passive dynamic robots using Fused Deposition Modeling (FDM), this paper explores designing and fabricating passive walking robots and all necessary components using single material FDM. Two configurations of passive dynamic walkers are re-designed and fabricated in this paper. For one of them all components are printed in one job and only little assembly after printing is needed. However, the gait cycle of the second configuration is much more sensitive to small parametric changes and therefore more flexible prototyping is needed in order to allow adjusting of the robot after printing. Moreover, FDM printed robotic joints with sufficient smoothness and axial stiffness are required and a variety of different joint assemblies are designed and tested for the robot prototypes. Even though the most stable gait for the second robot is achieved using a metal bearing instead of the FDM printed ones, this is not necessary for the first robot example. The approach to prototyping with FDM presented in this paper allows achieving functionality through design iteration without incurring significant cost. To arrive at feasible solutions, a modular design approach allows to combine different joints, legs, feet and balancing weights and the connection points of the different elements are adjustable after printing, which makes it possible to shift the center of gravity and other variables of the robot.Mechanical Engineerin

Design and implementation of series elastic actuation in a biomorphic robot leg

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.Includes bibliographical references (leaf 21).Fluid, efficient, robust bipedal locomotion is hard by some approaches. Today's most advanced bipedal robots require flat and level floors, but are still prone to trips and falls. They have trouble interacting with objects in their surroundings, and adapting to them. We think that new approaches may make bipedal control easier. The following work details the design of the BOB (Bag of Bones) biomorphic robot leg that is a continuation of an effort to achieve a better understanding of the sensorimotor neurocontrol of locomotion, particularly in humans. Such an understanding will not only lead to robots that move as well or better than people while being easier to control, but will also enable powerful therapies for ataxia patients. One of the main design requirements for BOB was to incorporate series elastic actuation, but with hobby servo motors as the power source. The use of hobby servos was intended to keep costs low, as was the extensive use of off the shelf parts whenever possible. With the recent advances in hobby servo motors, it was expected that reasonable if not high performance would be possible. The specific contribution of this work includes the entire series elastic actuation system powered by servo motors.(Cont.) The elements of the actuation system include circular servo horns, wire rope used in loops, turnbuckles, and series elastic elements that use compression springs in extension. It was found that the knee joint can flex from 0 to 90 degrees and back in about 0.7 seconds. Similarly, the ankle cycled from approximately 20 degrees of extension to approximately 35 degrees of flexion in about 0.7 seconds. These performance figures indicate that the gearing ratios at the knee and ankle are appropriate and that the current design is sufficiently powered for walking.by Nathaniel K. Chan.S.B

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

Design and control of a clutch for a minimally-actuated biped based on the passive-dynamic simple walker

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (leaf 41).Passive-dynamic walking robots are remarkable mechanical devices capable of maintaining dynamically stable walking gaits with no actuation or control. These systems, however, depend on ideal environmental conditions for stability. Robustness and control capabilities are increased with actuation, but so is the power consumption. Such actuated robots are designed to minimize the actuation requirement by exploiting the system natural dynamics system, but still need actuation to compensate for energy dissipated by friction and collision events, as well as for more control capabilities. A simple clutch mechanism is developed for such systems to allow intermittent control of otherwise passive joints, allowing controllers to exploit the passive or actuated control when desired. The clutch is tested on a hip actuated simple 3D walker to evaluate the performance capabilities of clutched control. Preliminary tests of several control strategies suggest the clutched actuation may provide good performance at a higher efficiency compared to fully actuated systems. This paper describes the development of the clutch device and the hip-actuated biped on with which the clutch is tested, and evaluates the performance of intermittent clutch-control for several control strategies.by Arlis Reynolds.S.B

Interactive simulation of stylized human locomotion

Animating natural human motion in dynamic environments is difficult because of complex geometric and physical interactions. Simulation provides an automatic solution to parts of this problem, but it needs control systems to produce lifelike motions. This paper describes the systematic computation of controllers that can reproduce a range of locomotion styles in interactive simulations. Given a reference motion that describes the desired style, a derived control system can reproduce that style in simulation and in new environments. Because it produces high-quality motions that are both geometrically and physically consistent with simulated surroundings, interactive animation systems could begin to use this approach along with more established kinematic methods.Singapore-MIT GAMBIT Game LabNational Science Foundation (U.S.) (Fellowship 2007043041)Pixar (Firm