Quadruped robot capable of climbing smooth surfaces, such as glass, acrylic and whiteboard using directional adhesive.

As mentioned in adhesion model, surface conformation is essential in adhesion since Van der Waals force is very weak unless there is intimate contact between two surfaces. Van der Waals force is known to be very weak, although its usage is ubiquitous in our life. Most common example is conventional tape that uses very soft material for compliance. Vertical climbing on various surfaces requires more sophisticated system than single thin layer of soft polymer used in tape. In natural and artificial environment, roughness in many length scale is presented. Thus, in order to maximize the number of molecules in intimate contact between feet and wall surfaces, corresponding length scale compliance is needed. Gekco species also present hierarchical compliance in their body. Flexible body and leg can conform at the centimeter scale. Toes and soft skin are responsible for 1~2 millimeter scale. Within a millimeter scale, specialized hair structures are composed of lamellae, setae and nano scale spatulae covering up to nanometer scale. Stickybot inherits similar characteristic comprising 12 active actuator, 8 DOF serial and 4 passive compliant DOFs in leg and 16 segmented toes controlled via two stage differential cable driven system.

Directional adhesion
The most extraordinary feature of Stickybot is Directional adhesive. Climbing robot are not It has Anisotropic structure featured controllable adhesion with directionality in adhesion force. The movie clip above demonstrates directional adhesion compared to conventional double sided tape. Unlike conventional tape, it sticks on smooth surface with very small preload and is also able to detach with by reducing load. If it is loaded in desired direction, it creates maximum contact minimizing stress concentration along the contact area. if it is loaded in wrong direction, the adhesion force is very low.

What makes Stickybot unique is its extraordinary design of foot. Its foot has four segmented toes controlled by single push-pull cable via two stage differential system. Three different polymers are used and stiffened by fine fabric to minimize shear stress concentration along adhesive pads. It took me about three month to finalize design iterating more than three times. Utilizing high surface energy of poly-urethane, we can test various adhesive pads on Stickybot feet.

Its interesting curvature of cable path is designed carefully in order to achieve well distributed normal force on the contact area. Since each toes has seven segments, it can conform non flat surfaces. Despite its flexible structure, reliable adhesion requires elaborated force transmission system from cable tension to normal pressure on contact surface. Its desired profile is calculated in such a way shown on the diagram on right.

Basic idea is to create uniform normal pressure that caused by cable angle differentiation. Each segment should have same amount of force assuming constant cable compression force.

Differential cable system is employed in order of minimizing number of actuators and force balance among four toes. One equivalent mechanical system is double rocker bogie system.Upper stage differential actuation is rocker and one cable connecting two toes enables lower stage differential system.
 

Sprawl robots are designed by inspiration based on study of cockroaches.

Whereas other sprawl robots utilize pneumatic system, iSprawl is equipped with battery and electric motor and power transmission system that convert rotary motion to reciprocal leg thrust. What most makes iSprawl unique is push-pull cable transmission system. Since Sprawl family turn its orientation by rotation of their leg with respect to hip joint, power transmission path is not fixed. Which is frequent problem in legged robot with centered engine.

Usage of steel cable as pulling transmission is very common as you can find very easily in bicycle brake and car throttle. iSprawl's cables, however, thrust and pull the legs in high speed (up to 17Hz) without significant energy loss. This enables iSprawl's light and fast movement of legs leading to fast locomotion up to 15 body-lengths/sec (2.3m/s).

Predecessor of iSprawl, the first sprawl robot that operates with autonomous power. It uses hydraulic power transmission system. The idea was to explore hydaulic power transmission to distribute power to the legs. The tubes contain water and the system works like hydraulic brakes on a car, with a single master piston and slave pistons at each leg. Aqua Sprawl is assembled in a bathtub to prevent air bubbles.

After many design iterations, including remachining the original Sprawlita pistons to put in double seals to handle negative pressure on the return stroke, a version of Aqua Sprawl was produced that ran fairly well.

Although Aqua Sprawl ran reasonably well, there is a fundamental limitation to the achievable speed because while retracting the pistons, only atmospheric pressure is available. On the other hand, this could be a good design for underwater use!