No legged walking robot yet approaches the high reliability and the low power usage of a walking person, even on flat ground. Here we describe a simple robot which makes small progress towards that goal. Ranger is a knee-less four-legged 'bipedal' robot which is energetically and computationally autonomous, except for radio controlled steering. Ranger walked 65.2 km in 186,076 steps in about 31 h without being touched by a human with a total cost of transport [TCOT ≡ P/mgv] of 0.28, similar to human's TCOT of ≈ 0.3. The high reliability and low energy use were achieved by: (a) development of an accurate bench-test-based simulation; (b) development of an intuitively tuned nominal trajectory based on simple locomotion models; and (c) offline design of a simple reflex-based (that is, event-driven discrete feed-forward)stabilizing controller. Further, once we replaced the intuitively tuned nominal trajectory with a trajectory found from numerical optimization, but still using event-based control, we could further reduce the TCOT to 0.19. At TCOT = 0.19, the robot's total power of 11.5 W is used by sensors, processors and communications (45%), motor dissipation (≈34%) and positive mechanical work (≈21%). Ranger's reliability and low energy use suggests that simplified implementation of offline trajectory optimization, stabilized by a low-bandwidth reflex-based controller, might lead to the energy-effective reliable walking of more complex robots.
We present the design and control of an energy-efficient, knee-less, essentially planar, four-legged bipedal robot called Ranger. In separate trials, Ranger: 1) walked a 40.5 mile ultra-marathon on a single charge and without human touch, setting a robot distance record; and 2) walked stably at Total Cost Of Transport (TCOT= total energy used per unit weight per unit distance travelled) of 0.19, apparently less than that of any other legged robot to date. Key design features are: a light weight and high strength box body, low-inertia leg design for fast and efficient swing, foot actuation that combines toe-off and ground clearance, a steering mechanism that enables turning of this essentially planar robot, and a low-power modular networked electronics hardware system. The model-based control approach uses a simplified offline trajectory optimization with a reflexbased feedback controller for stabilization. Ranger's reasonable success suggests that these design and control ideas could be extended to the development of an energy-efficient higher degree of freedom, 3-D bipedal robot.
Although 3D printing has the potential to provide greater customization and to reduce the costs of creating actuators for industrial applications, the 3D printing of actuators is still a relatively new concept. We have developed a pneumatic actuator with 3D-printed parts and placed sensors for position and force control. So far, 3D printing has been used to create pneumatic actuators of the bellows type, thus having a limited travel distance, utilizing low pressures for actuation and being capable of only limited force production and response rates. In contrast, our actuator is linear with a large travel distance and operating at a relatively higher pressure, thus providing great forces and response rates, and this the main novelty of the work. We demonstrate solutions to key challenges that arise during the design and fabrication of 3D-printed linear actuators. These include: (1) the strategic use of metallic parts in high stress areas (i.e., the piston rod); (2) post-processing of the inner surface of the cylinder for smooth finish; (3) piston head design and seal placement for strong and leak-proof action; and (4) sensor choice and placement for position and force control. A permanent magnet placed in the piston head is detected using Hall effect sensors placed along the length of the cylinder to measure the position, and pressure sensors placed at the supply ports were used for force measurement. We demonstrate the actuator performing position, force and impedance control. Our work has the potential to open new avenues for creating less expensive, customizable and capable actuators for industrial and other applications.
In this paper, we present a theoretical study on the control of a compass gait walker using energy regulation between steps. We use a return map to relate the mid-stance robot kinetic energy between steps with two control inputs, namely, foot placement and ankle push-off. We show that by regulating robot kinetic energy between steps using the two control inputs, we are able to (1) generate a wide range of walking speeds and stride lengths, including average human walking; (2) cancel the effect of external disturbance fully in a single step (dead-beat control); and (3) switch from one periodic gait to another in a single step. We hope that insights from this control methodology can help develop robust controllers for practical bipedal robots. IntroductionCurrent research on bipedal robots has focussed on planning and control of a singe steady state (periodic) walking gait. We characterize a walking gait by a given combination of step length and step velocity. However, for bipedal robots, to be able to walk in man-made environments, they need to be able to walk at a wide range of steady state periodic gaits and be able to switch from one periodic gait to another quickly.In this paper, we show theoretical calculations on how to use energy regulation between steps to plan and control walking of a compass gait model. We first review past work on compass gait models and common control approaches, followed by a discussion on our model.
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