Enhancing the Vertical Mobility of a Robot Hexapod Using Microspines Matt Martone∗1, Catherine Pavlov∗2, Adam Zeloof2, Vivaan Bahl3, and Aaron M. Johnson2 Abstract— Modern climbing robots have risen to great heights, but mechanisms meant to scale cliffs often locomote slowly and over-cautiously on level ground. Here we introduce T-RHex, an iteration on the classic cockroach-inspired hexapod that has been augmented with microspine feet for climbing. T- RHex is a mechanically intelligent platform capable of efficient locomotion on ground with added climbing abilities. The legs integrate the compliance required for the microspines with the compliance required for locomotion in order to simplify the design and reduce mass. The microspine fabrication is simplified by embedding the spines during an additive manufac- turing process. We present results that show that the addition of microspines to the T-RHex platform greatly increases the maximum slope that the robot is able to statically hang on (up to a 45◦ overhang) and ascend (up to 55◦) without sacrificing ground mobility. I. INTRODUCTION In nature, animals have adapted a wide variety of ap- Fig. 1. The new T-RHex robot platform statically clinging to rough proaches to climbing, but even with finely-tuned sensorimo- aggregate concrete. Each leg includes 9 individually compliant microspines. tor cortices most can’t measure up to the reliability of insects. promises in order to climb. Some like Spinybot [4] are Cockroaches are able to scale nearly any natural material designed to only work in scansorial locomotion. LEMUR3 using microscopic hairs along their legs and feet for surface is a quadruped which uses active microspine grippers on its adhesion [1–3]. These hairs catch on surface irregularities, feet to achieve mobility on vertical and overhung natural known as asperities, and can support the insect’s body rock surfaces, but requires carefully planned foot placement through load sharing across many hairs. The cockroach’s for each step that limits travel speed [8,9]. DROP [10] and scant cognitive intelligence severely limits its ability to plan TBot [11] are small, lightweight wheeled platforms capable actions or move dexterously; rather it relies on mechanical of both vertical and level ground mobility, but microspine intelligence for locomotion. This approach is promising for adhesion quickly degrades as the tips dull on flat ground. field robots that need to reliably climb on an array of surface RiSE [6] is a hexapedal robot with microspine feet capable materials and geometries. of 0.25 m/s locomotion on flat ground, but uses different feet Microspine technology can effectively mimic the adhesion for walking and climbing. of a cockroach’s foot on a larger scale [4], though heavily directional without the aid of Van der Waals forces (as used RHex is a cockroach-inspired ground robot that uses six in gecko-like adhesives [5]). Microspine grippers can attach legs to traverse uneven terrain [12–14]. Its curved leg shape to rough surfaces by distributing the load across hundreds of and compliance are key to simplifying control architecture millimeter scale hooks sprung with hierarchical compliance. through mechanical intelligence [15,16], and it can move arXiv:1906.04811v2 [cs.RO] 19 Sep 2019 Legged robots such as RiSE [6], Spinybot [7], and LEMUR effectively on a wide array of structured and unstructured [8] have demonstrated the effectiveness of microspines for terrains [17–20]. RHex can carry a relatively large payload robotic rock climbing. However, while these systems can due to the structure of the legs [21], enabling it to carry scale walls efficiently, their microspine feet and slow joints many sensors for search and rescue, reconnaissance, or en- limit mobility on flat ground. vironmental monitoring missions [22]. While quite mobile on Microspines have been utilized to achieve climbing with horizontal terrain, the platform’s utility and viability would numerous morphologies, though all must make design com- be improved greatly by adding the ability to scale slopes and walls. This is especially important in areas with incomplete ∗M. M. is supported by a DOE Traineeship in Robotics. C. P. is supported infrastructure such as disaster zones or construction sites. by a NASA Space Technology Research Fellowship. Both contributed equally to the work presented here. In this paper we present a new leg design (in Sec. II- 1Robotics Institute, Carnegie Mellon University, Pittsburgh, PA, USA A) that uses microspines to enable a RHex-like robot to [email protected] hang onto vertical or even overhanging surfaces, such as the 2 Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA, vertical wall in Fig. 1, as well as climb inclined surfaces USA fcpavlov,azz,[email protected] ◦ 3Electrical & Computer Engineering, Carnegie Mellon University, Pitts- up to 55 . The microspine toes, Sec. II-B, are built using burgh, PA, USA [email protected] additive manufacturing to simplify the fabrication process. We leverage the existing leg compliance to avoid the need for multiple smaller flexures to enable successful adhesion (Sec. II-C). By orienting the spines backwards, we ensure that the existing RHex forward gaits are unimpeded while taking advantage of the microspines for climbing with the typically unused backward walking gait space (Sec. III). We test, in Sec. IV, the new leg design’s ability to climb and hang on a variety of inclined surfaces while maintaining good level-ground mobility. Finally, we conclude the paper in Sec. V by considering a pathway towards full horizontal and vertical mobility with the platform. II. MECHANICAL DESIGN The robot described in this paper is an implementation of the standard RHex configuration named “T-RHex,” shown in Fig. 1. Like RHex, T-RHex is a hexapedal robot with six single degree-of-freedom compliant semicircular legs. Each leg is actuated by a Dynamixel MX-64 servo motor capable of continuous 360◦ rotation. Unlike the standard RHex platform, the legs are comprised of multiple independently sprung microspine flexures, whose design is discussed in the rest of this section. The fully assembled T-RHex platform weighs 2.5kg and measures 254mm between front and back legs. In addition to the new legs, T-RHex is equipped with a “climbing tail”. The tail is mounted to what is the front of the robot during regular walking, which becomes the rear of the robot during climbing. Past tails on RHex have been primarily oriented upwards and towards the rear (while walking) [23], and work by using inertial reorientation. The tail on T-RHex is lightweight and intended to contact the surface in order to increase the adhesion forces a the front toes, as with the tail in other climbing robots [11,24,25]. The tail is actuated with a Dynamixel MX-106 servo motor and Fig. 2. A fully assembled T-RHex forked wheel leg (top) and a closeup of ◦ the microspines on the toes (bottom). Both images show a variety of spine can rotate 180 . angles, denoted by the color on the end of each leg slice. A. Leg Design B. Microspine Design RHex’s legs were redesigned to enable T-RHex to climb on high-angle terrain. While climbing, T-RHex faces backwards, Most prior microspine assemblies were fabricated through such that the tips of its appendages are the only points in a multistep additive and subtractive process of polymer cast- contact with the surface (an orientation used by some other ing, milling, and spine embedding called shape deposition legged robots with semicircular leg designs [26]). A T-RHex manufacturing (SDM) [27–29]. Here, we take a different leg consists of multiple stacked, thin slices, as seen in Fig. 2. approach that uses 3D printing to simplify the fabrication Each individual leg slice, Fig. 3, has a single microspine, and process. Each microspine is attached to the leg slice with a the stacked slices are able to deflect in plane independently 3D printed PLA piece. These slice “toes” were 3D printed in of each other, with their shared attachment point concentric large batches, and paused mid-print to insert the microspines, with the driving servo horn. The thickness of the slices was which were then printed over. Three different spine angles (f determined by microspine size, as each slice had to fully in Fig. 3) were fabricated: 60◦, 90◦, and 120◦. Each spine enclose its microspine leaving only the tip exposed. angle allows for adhesion in a different range of the leg’s Because the microspines protrude only from the tip of each rotation, and as such each leg has a mix of spine angles. leg, they do not interfere with the ground during normal Unlike prior legged microspine climbers, which keep the forward walking. This means that ground mobility is not angle of attack of the microspines very constant while in impacted, as evaluated in Sec. IV-D, and the robot can travel stance, T-RHex must maintain adhesion over a large angular at the same speed regardless of whether the legs contain range, making this mix of toe angles key to the success of spines or not. This is also useful to prevent surface harm the climbing gait. while the robot walks, and to protect the spines from dulling The microspines on T-RHex consist of size 12 plain shank when not in use. fish hooks cut to length. At approximately 0.6mm thick, these Fig. 3. Drawing of a single microspine flexure. On T-RHex, D is 100mm, W is 10 mm, and f is 60◦, 90◦, or 120◦. A full leg contains nine stacked Fig. 4. The “inchworm” climbing gait in action on a 55◦ slope showing the microspine flexures. three pairs of legs moving symmetrically. In this frame the front legs (right) are holding position until the back legs (left) complete a clockwise rotation, hooks are small enough to allow for 1.5mm thick leg slices.