Matthew A. Estrada

Dynamic surface grasping with directional adhesion

Dynamic surface grasping is applicable to landing of micro air vehicles (MAVs) and to grappling objects in space. In both applications, the grasper must absorb the kinetic energy of a moving object and provide secure attachment to a surface using, for example, gecko-inspired directional adhesives. Functional principles of dynamic surface grasping are presented, and two prototype grasper designs are discussed. Computer simulation and physical testing confirms the expected relationships concerning (i) the alignment of the grasper at initial contact, (ii) the absorption of energy during collision and rebound, and (iii) the force limits of synthetic directional adhesives.

Modeling the dynamics of perching with opposed-grip mechanisms.

Perching allows Micro Aerial Vehicles (MAVs) avoid the power costs and electrical and acoustic noise of sustained flight, for long-term surveillance and reconnaissance applications. This paper presents a dynamic model that clarifies the requirements for repeatable perching on walls and ceilings using an opposed-grip mechanism and dry adhesive technology. The model predicts success for perching over a range of initial conditions. The model also predicts the conditions under which other directional attachment technologies, such as microspines, will succeed. Experiments conducted using a launching mechanism for a range of different landing conditions confirm the predictions of the model and provide insight into future design improvements that are possible by modifying a few key damping and stiffness parameters.

Aggressive Flight With Quadrotors for Perching on Inclined Surfaces

Micro aerial vehicles face limited flight times, which adversely impacts their efficacy for scenarios such as first response and disaster recovery, where it might be useful to deploy persistent radio relays and quadrotors for monitoring or sampling. Thus, it is important to enable micro aerial vehicles to land and perch on different surfaces to save energy by cutting power to motors. We are motivated to use a downwards-facing gripper for perching, as opposed to a side-mounted gripper, since it could also be used to carry payloads. In this paper, we predict and verify the performance of a custom gripper designed for perching on smooth surfaces. We also present control and planning algorithms, enabling an underactuated quadrotor with a downwardsfacing gripper to perch on inclined surfaces while satisfying constraints on actuation and sensing. Experimental results demonstrate the proposed techniques through successful perching on a glass surface at various inclinations, including vertical.

Perching and Vertical Climbing: Design of a Multimodal Robot

We present a robot capable of both (1) dynamically perching onto smooth, flat surfaces from a ballistic trajectory and (2) successfully transitioning to a climbing gait. Merging these two modes of movement is achieved via a mechanism utilizing an opposed grip with directional adhesives. Critical design considerations include (a) climbing mechanism weight constraints, (b) suitable body geometry for climbing and (c) effects of impact dynamics. The robot uses a symmetric linkage and cam mechanism to load and detach the feet while climbing. The lengths of key parameters, including the distances between each the feet and the tail, are chosen based on the ratio of required preload force and detachment force for the adhesive mechanism.

A robotic device using gecko-inspired adhesives can grasp and manipulate large objects in microgravity

A load-sharing robotic device can grasp, manipulate, and release objects in microgravity using space-qualified dry adhesives. Grasping and manipulating uncooperative objects in space is an emerging challenge for robotic systems. Many traditional robotic grasping techniques used on Earth are infeasible in space. Vacuum grippers require an atmosphere, sticky attachments fail in the harsh environment of space, and handlike opposed grippers are not suited for large, smooth space debris. We present a robotic gripper that can gently grasp, manipulate, and release both flat and curved uncooperative objects as large as a meter in diameter while in microgravity. This is enabled by (i) space-qualified gecko-inspired dry adhesives that are selectively turned on and off by the application of shear forces, (ii) a load-sharing system that scales small patches of these adhesives to large areas, and (iii) a nonlinear passive wrist that is stiff during manipulation yet compliant when overloaded. We also introduce and experimentally verify a model for determining the force and moment limits of such an adhesive system. Tests in microgravity show that robotic grippers based on dry adhesion are a viable option for eliminating space debris in low Earth orbit and for enhancing missions in space.

Free-flyer acquisition of spinning objects with gecko-inspired adhesives

We explore the use of grippers with gecko-inspired adhesives for spacecraft docking and acquisition of tumbling objects in microgravity. Towards the goal of autonomous object manipulation in space, adhesive grippers mounted on planar free-floating platforms are shown to be tolerant of a broad range of incoming linear and angular velocities. Through modeling, simulations, and experiments, we characterize the dynamic “grasping envelope” for successful acquisition and derive insights to inform future gripper designs and grasping strategies for motion planning.

Planning and Control of Aggressive Maneuvers for Perching on Inclined and Vertical Surfaces

It is important to enable micro aerial vehicles to land and perch on different surfaces to save energy by cutting power to motors and to perform tasks such as persistent surveillance. In many cases, the best available surfaces may be vertical windows, walls, or inclined roof tops. In this paper, we present approaches and algorithms for aggressive maneuvering to enable perching of underactuated quadrotors on surfaces that are not horizontal. We show the design of a custom foot/gripper for perching on smooth surfaces. Then, we present control and planning algorithms for maneuvering to land on specified surfaces while satisfying constraints on actuation and sensing. Experimental results that include successful perching on vertical, glass surfaces validate the proposed techniques.Copyright

A Multimodal Robot for Perching and Climbing on Vertical Outdoor Surfaces

Perching can extend the useful mission life of a micro air vehicle. Once perched, climbing allows it to reposition precisely, with low power draw and without regard for weather conditions. We present the Stanford Climbing and Aerial Maneuvering Platform, which is to our knowledge the first robot capable of flying, perching with passive technology on outdoor surfaces, climbing, and taking off again. We present the mechanical design and the new perching, climbing, and takeoff strategies that allow us to perform these tasks on surfaces such as concrete and stucco, without the aid of a motion capture system or off-board computation. We further discuss two new capabilities uniquely available to a hybrid aerial–scansorial robot: the ability to recover gracefully from climbing failures and the ability to increase usable foothold density through the application of aerodynamic forces. We also measure real power consumption for climbing, flying, and monitoring and discuss how future platforms could be improved for longer mission life.

Perching failure detection and recovery with onboard sensing

Perching on a vertical surface carries the risk of severe damage to the vehicle if the maneuver fails, especially if failure goes undetected. We present a detection method using an onboard 3-axis accelerometer to discriminate between perching success and failure. An analytical model was developed to calculate acceleration differences for success and failure and set decision times. Two distinct decision times were shown to be effective, corresponding to properly engaging the gripper and overloading the grippers capabilities. According to a machine learning feature selection algorithm, the maximum Z axis acceleration of the quadrotor and the presence of near-zero readings are the most relevant features within these two time frames. Using these features, the detection algorithm discriminated between success and failure with a 91% accuracy at 40 ms, and 94% at 80 ms. Real-time detection and failure recovery experiments with a 20 g quadrotor verify the detection method. An improved approach that combines various decision times correctly identified success/failure for all 20 trials with an average total falling distance of 0.8m during recovery. We discuss the feasibility of extending our method to other quadrotor platforms.

Composite force sensing foot utilizing volumetric displacement of a hyperelastic polymer

This paper illustrates the fabrication and characterization of a footpad based on an original principle of volumetric displacement sensing. It is intended for use in detecting ground contact forces in a running quadrupedal robot. The footpad is manufactured as a monolithic, composite structure composed of multi-graded polymers which are reinforced by glass fiber to increase durability and traction. The volumetric displacement sensing principle utilizes a hyperelastic gel-like pad with embedded magnets that are tracked with Hall-effect sensors. Normal and shear forces can be detected as contact with the ground which causes the gel-like pad to deform into rigid wells. This is all done without the need to expose the sensor. A one-time training process using an artificial neural network was used to relate the normal and shear forces with the volumetric displacement sensor output. The sensor was shown to predict normal forces in the Z-axis up to 80N with a root mean squared error of 6.04% as well as the onset of shear in the X and Y-axis. This demonstrates a proof-of-concept for a more robust footpad sensor suitable for use in all outdoor conditions.