Unmanned planetary rovers have traversed kilometers of Lunar and Martian terrain while performing valuable science. However, they still face mobility challenges including steep slopes and unstable soil that can entrap vehicles, as demonstrated by NASA's Spirit rover. Vehicles on Earth can depend on a human operator or rescue vehicle to tow them out of an entrapment, but remote rovers cannot, limiting their route to highly conservative path selections. To increase rover mobility on slopes and unstable soils, we present a resettable anchor launcher for independent self-rescue. The device launches a tethered land anchor away from the rover and then uses a winch to tow the rover up a hill or out of an entrapment. This paper presents the design of the launcher and its integration into a half-meter-long rover mobility platform with field testing at the NASA Glenn Research Center SLOPE Lab. We demonstrate repeatable launching and winching to help the rover climb a 17° slope of loose GRC-1 Lunar regolith simulant that it otherwise could not climb. Our work presents an alternative method to increase rover mobility, especially up slopes, and enables independent rover rescue, which could eventually increase mission duration and reduce risk of entrapment during extraterrestrial exploration.

Series elastic actuators primarily use linear springs in their drivetrains, which introduces a design tradeoff: soft springs provide higher torque resolution at the cost of system bandwidth, whereas stiff springs provide a fast response but lower torque resolution. Nonlinear springs (NLSs) potentially incorporate the benefits of both soft and stiff springs, but such springs are often large. An NLS design was recently proposed that combines a variable radius cam with a rubber elastic element, enabling a compact spring design. However, the rubber introduces hysteresis, which can lead to poor torque tracking if not accounted for in the controller. To overcome this limitation, we here propose a state observer that captures hysteretic effects exhibited by the rubber to provide an accurate estimate of actuator torque. We perform torque-control experiments with this observer on an actuator testbed and compare the performance of the NLS to both soft and stiff linear metal springs. Experiments show that the NLS exhibits improved output impedance compared to both linear springs, and comparable bandwidth to the stiff linear spring up to 1.5 Hz. However, the hysteresis in the urethane rubber introduces instability in higher-frequency conditions, suggesting that future NLS designs can be improved by use of a different rubber as the elastic element.

Animals and robots balance dynamically by placing their feet into proper ground targets. While foot placement controls exist for both fully robotic systems and powered prostheses, none enable the dynamism and reactiveness of able-bodied humans. A control approach was recently developed for an ideal double pendulum dynamical system that places feet into ground targets for a wide range of initial conditions and in the presence of significant locomotion disturbances. While its performance in simulation make it an attractive candidate to control legged robotic systems, it is unclear if the approach can be used on real-world systems. In this paper we transfer and evaluate the approach on robotic hardware. Our results show that the controller can be transferred to robotic systems and allow them to achieve comparable foot placement accuracies to an ideal double pendulum simulation, both when motion is undisturbed, as well as when obstacles are encountered in early, mid, and late swing. These results suggest that the proposed approach is a potential alternative control method for legged robots and powered prostheses, enabling recovery from sudden swing-leg disturbances such as trips and unexpected obstacle encounters. The results also point out the need for additional considerations when tuning the controller in order to generate human-like swing trajectories and durations.

Series elastic actuators often use linear metal springs in their drivetrains, which requires design compromises between torque resolution and actuation bandwidth. Nonlinear springs (NLSs), with variable stiffness, overcome this limitation, enabling both high torque resolution and high bandwidth. Current NLS designs combine variable cam structures with off-the-shelf linear springs, which increases the overall size of these torque transmitting elements. NLS size could be reduced by using other materials as an elastic element. We present an optimization-based synthesis method for NLSs that are compact and encode a user-defined torque-deflection profile using elastic elements with an arbitrary stiffness profile. We experimentally validate the proposed method by creating a NLS prototype and testing it on an actuator testbed. The prototype uses rubber as the elastic element, resulting in a compact design that generates the desired torque profile, although hysteresis of the rubber material partially compromises performance. The results suggest that the proposed method successfully generates compact NLS designs, but that rubber elements need to be carefully chosen to mitigate hysteresis.

We investigate the existence of computationally inexpensive first and second order statistics that uniquely describe grass for application in an autonomous lawnmower. We then segment images based on these statistics to determine locations of driveable terrain in an image. Tight statistical clustering of illuminated grass versus artificial texture suggests that this method is sufficient for identifying driveable terrain for an autonomous lawnmower.