Matthew Spenko

Frictional adhesion: a new angle on gecko attachment

SUMMARY Directional arrays of branched microscopic setae constitute a dry adhesive on the toes of pad-bearing geckos, natures supreme climbers. Geckos are easily and rapidly able to detach their toes as they climb. There are two known mechanisms of detachment: (1) on the microscale, the seta detaches when the shaft reaches a critical angle with the substrate, and (2) on the macroscale, geckos hyperextend their toes, apparently peeling like tape. This raises the question of how geckos prevent detachment while inverted on the ceiling, where body weight should cause toes to peel and setal angles to increase. Geckos use opposing feet and toes while inverted, possibly to maintain shear forces that prevent detachment of setae or peeling of toes. If detachment occurs by macroscale peeling of toes, the peel angle should monotonically decrease with applied force. In contrast, if adhesive force is limited by microscale detachment of setae at a critical angle, the toe detachment angle should be independent of applied force. We tested the hypothesis that adhesion is increased by shear force in isolated setal arrays and live gecko toes. We also tested the corollary hypotheses that (1) adhesion in toes and arrays is limited as on the microscale by a critical angle, or (2) on the macroscale by adhesive strength as predicted for adhesive tapes. We found that adhesion depended directly on shear force, and was independent of detachment angle. Therefore we reject the hypothesis that gecko toes peel like tape. The linear relation between adhesion and shear force is consistent with a critical angle of release in live gecko toes and isolated setal arrays, and also with our prior observations of single setae. We introduced a new model, frictional adhesion, for gecko pad attachment and compared it to existing models of adhesive contacts. In an analysis of clinging stability of a gecko on an inclined plane each adhesive model predicted a different force control strategy. The frictional adhesion model provides an explanation for the very low detachment forces observed in climbing geckos that does not depend on toe peeling.

Smooth Vertical Surface Climbing With Directional Adhesion

Stickybot is a bioinspired robot that climbs smooth vertical surfaces such as glass, plastic, and ceramic tile at 4 cm/s. The robot employs several design principles adapted from the gecko including a hierarchy of compliant structures, directional adhesion, and control of tangential contact forces to achieve control of adhesion. We describe the design and fabrication methods used to create underactuated, multimaterial structures that conform to surfaces over a range of length scales from centimeters to micrometers. At the finest scale, the undersides of Stickybots toes are covered with arrays of small, angled polymer stalks. Like the directional adhesive structures used by geckos, they readily adhere when pulled tangentially from the tips of the toes toward the ankles; when pulled in the opposite direction, they release. Working in combination with the compliant structures and directional adhesion is a force control strategy that balances forces among the feet and promotes smooth attachment and detachment of the toes.

An Adaptive Shared Control System for an Intelligent Mobility Aid for the Elderly

The control system for a personal aid for mobility and health monitoring (PAMM) for the elderly is presented. PAMM is intended to assist the elderly living independently or in senior assisted living facilities. It provides physical support and guidance, as well as monitoring basic vital signs for users that may have both limited physical and cognitive capabilities. This paper presents the design of a bi-level control system for PAMM. The first level is an admittance-based mobility controller that provides a natural and intuitive human machine interface. The second level is an adaptive shared controller that allocates control between the user and the computer based on metrics of the users performance. Field trials at an eldercare facility show the effectiveness of the design.

Whole body adhesion: hierarchical, directional and distributed control of adhesive forces for a climbing robot

We describe the design and control of a new bio-inspired climbing robot designed to scale smooth vertical surfaces using directional adhesive materials. The robot, called Stickybot, draws its inspiration from geckos and other climbing lizards and employs similar compliance and force control strategies to climb smooth vertical surfaces including glass, tile and plastic panels. Foremost among the design features are multiple levels of compliance, at length scales ranging from centimeters to micrometers, to allow the robot to conform to surfaces and maintain large real areas of contact so that adhesive forces can support it. Structures within the feet ensure even stress distributions over each toe and facilitate engagement and disengagement of the adhesive materials. A force control strategy works in conjunction with the directional adhesive materials to obtain sufficient levels of friction and adhesion for climbing with low attachment and detachment forces.

Robotic Personal Aids for Mobility and Monitoring for the Elderly

Two rehabilitation devices, or personal aids for mobility and monitoring (PAMM), for use by the elderly are presented. The devices are intended to delay the transition from eldercare (assisted living) facilities to nursing homes. The robotic PAMMs provide support, guidance, and health monitoring. Two experimental systems are described: a cane and a walker. Issues of mobility, sensing, and control, as well as experimental data from trials in an assisted living facility using both systems are presented

Directional adhesion for climbing: theoretical and practical considerations

Using the gecko as inspiration, important principles are revealed for reliable maneuvering on vertical surfaces. Foremost among these is the directional behavior of the gecko adhesive system, which permits control of adhesion via control of the tangential forces at the feet. Multiple hierarchical levels of compliance are also important for conforming intimately to surfaces with varying degrees of roughness and different length scales. In light of these requirements, most previously developed synthetic adhesives are not well suited for application on a climbing robot. We describe a synthetic fibrillar adhesive, termed Directional Polymer Stalks, made from relatively soft polyurethane (modulus of elasticity ≈ 300 kPa). The fibrils are angled 20° with respect to vertical and are approximately 1 mm long and 380 μm in diameter. Rather than having a flat top, they have angled faces at 45°. The directional nature of these angled stalks is shown, achieving a maximum adhesion of approximately 1 N for a 3.9 cm2 patch when pulled in the direction in which the stalks are angled. When pulled in the non-adhesive direction, the adhesion forces are negligible. The application to a climbing robot is presented and limitations of the current design are discussed along with ongoing efforts to address them.

Omni-Directional Mobility Using Active Split Offset Castors

An omni-directional mobility platform design concept using two active split offset castors (ASOC) and one or more conventional castors is presented. An ASOC module consists of two coaxial conventional wheels driven independently and connected to the platform via an offset link. The kinematics and implementation of the omni-directional platform is described and analyzed. Particular attention is paid to the system performance on uneven floors. The fundamental mechanics of the ASOC wheel scrubbing, which is critical to system wear and energy use, is analyzed and compared to conventional active castor designs. The effectiveness of the design is shown experimentally using an intelligent mobility aid for the elderly. @DOI: 10.1115/1.1767181#

Directional Adhesive Structures for Controlled Climbing on Smooth Vertical Surfaces

Recent biological research suggests that reliable, agile climbing on smooth vertical surfaces requires controllable adhesion. In nature, geckos control adhesion by properly loading the compliant adhesive structures on their toes. These strongly anisotropic dry adhesive structures produce large frictional and adhesive forces when subjected to certain force/motion trajectories. Smooth detachment is obtained by simply reversing these trajectories. Each toes hierarchical structure facilitates intimate conformation to the climbing surface resulting in a balanced stress distribution across the entire adhesive area. By controlling the internal forces among feet, the gecko can achieve the loading conditions necessary to generate the desired amount of adhesion. The same principles have been applied to the design and manufacture of feet for a climbing robot. The manufacturing process of these directional polymer stalks is detailed along with test results comparing them to conventional adhesives

Hazard Avoidance for High-Speed Mobile Robots in Rough Terrain

Unmanned ground vehicles have important applications in high speed rough terrain scenarios. In these scenarios, unexpected and dangerous situations can occur that require rapid hazard avoidance man ...

Perching and takeoff of a robotic insect on overhangs using switchable electrostatic adhesion

Making small robots stick Aerial views offer the chance to observe a wide range of terrain at once, but they come at the cost of needing to stay aloft. Graule et al. found that electrostatic forces could keep their insect-sized flying robot stuck to the underside of a range of surfaces (see the Perspective by Kovac). They mounted an electrostatically charged pad to the top of their robot, which could then reversibly stick to existing elevated perches—including a leaf—using less power than would be needed for sustained flight. Science, this issue p. 978; see also p. 895 Electrostatic adhesion enables a robotic insect to efficiently perch on and take off from natural and artificial structures. For aerial robots, maintaining a high vantage point for an extended time is crucial in many applications. However, available on-board power and mechanical fatigue constrain their flight time, especially for smaller, battery-powered aircraft. Perching on elevated structures is a biologically inspired approach to overcome these limitations. Previous perching robots have required specific material properties for the landing sites, such as surface asperities for spines, or ferromagnetism. We describe a switchable electroadhesive that enables controlled perching and detachment on nearly any material while requiring approximately three orders of magnitude less power than required to sustain flight. These electroadhesives are designed, characterized, and used to demonstrate a flying robotic insect able to robustly perch on a wide range of materials, including glass, wood, and a natural leaf.