Waalbot: An Agile Small-Scale Wall Climbing Robot Utilizing Pressure Sensitive Adhesives Michael P. Murphy, William Tso, Michael Tanzini, Metin Sitti Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Abstract— This paper proposes a small-scale agile wall climb- ing robot able to navigate on smooth surfaces of any orientation, including vertical and inverted surfaces, which uses adhesive elastomer materials for attachment. Using two actuated legs with rotary motion and two passive revolute joints at each foot the robot can climb and steer in any orientation. Due to its compact design, a high degree of miniaturization is possible. It has onboard power, sensing, computing, and wireless communication which allow for semi-autonomous operation. Various aspects of a functioning prototype design and performance are discussed in detail, including leg and feet design and gait control. The current prototype can climb 90 ◦ slopes at a speed of 6 cm/s and steer to any angle. This robot is intended for inspection and surveillance applications and, ultimately, space missions. I. I NTRODUCTION Mobile robots with the ability to climb and navigate on surfaces of any orientation without leaving residue or damag- ing the surface have many potential applications. One of the most notable situations where such a robot could be useful, and perhaps life-saving, is for spacecraft hull inspection and repair. Terrestrial uses include surveillance or inspection in hazardous or difficult to reach areas. Researchers have proposed a great variety of climbing robots for various applications. Many of the first wall scaling robots were intended for cleaning in hazardous environments such as nuclear reactors [1]. In general, climbing robots use one of three types of attachment mechanisms; vacuum suc- tion [1]–[7], magnetic attraction [8], or gripping with claws or grasping mechanism [9], [10]. Each of these mechanisms has advantages and drawbacks. For instance, magnetic attraction can be very strong and can have good power failure mitigation, but is only applicable for ferromagnetic surfaces. Suction adhesion relies on a good seal with the surface, so cracked or non-smooth surfaces are problematic and power efficiency limits their untethered climbing duration. Recently, robots using micro-claws have shown good performance on surfaces such as brick and stone [9], but clawed and grasping robots cannot climb smooth surfaces like glass or painted structures. To avoid the drawbacks associated with the aforementioned climbing mechanisms, the robot presented in this paper is designed to ultimately utilize a different type of adhesion mechanism found in biology called dry adhesion. This paper proposes a semi-autonomous small-scale robot theoretically capable of navigating on smooth flat surfaces of any orientation using adhesive elastomer for attachment. The robot is actuated by two motors, each controlling a set of three-footed wheels. Unlike many previous implementations Fig. 1. Photograph of prototype Tri-Foot Waalbot climbing a 90 ◦ (vertical) surface. of wheel-leg designs such as the ground walking RHex [11], Whegs [12] and miniWhegs [13] robots, Waalbot’s legs are specialized for climbing. A Mini-Whegs climbing robot has been developed which is able to climb and transition reliably using adhesive tape, but can only steer very gradually [14]. In contrast, Waalbot climbs with high speed and is able to make sharp turns. II. ATTACHMENT MECHANISM The Gecko lizard’s ability to climb surfaces, whether wet or dry, smooth or rough, has attracted scientists’ attention for centuries. By means of compliant micro/nano-scale high aspect ratio beta-keratin structures at their feet, geckos and spiders manage to adhere to almost any surface with a controlled contact area [15]. It has been shown that adhesion is mainly due to molecular forces such as van der Waals forces [16] (from which Waalbot draws its name). Tiny fibers on the animals feet form weak attractive bonds with the surface, the combination of billions of contacting fibers creates a large adhesion (up to 100 kPa). Since dry adhesion does not rely heavily on the surface material or atmospheric pressure, it allows climbing on a wide variety of surfaces and is uniquely suitable for use in the vacuum of space. Synthetic fibrillar dry adhesive technology is not currently mature enough to be used for climbing robots, however Waal- bot is designed with the intention of eventually utilizing the