Mobility and manipulation are often considered separately, with independent degrees of freedom (DOF) for each. However, here we show that using the legs for both walking and grasping increases the versatility of both tasks. Our robot has four DOFs: drive and lift for left and right pairs of legs. The legs use a reduced actuation Klann mechanism. The lift DOF rotates the entire trajectory of the legs, which enables gait modulation, climbing, and grasping. This demonstrates the feasibility of a novel operational concept: a robot that can approach, climb onto and securely grasp an object that can then be lifted via a load-bearing tether. Specifically, we show the kinematics to enable small robots to climb onto rectangular objects up to 67% robot height and grasp objects between 43% to 72% of the robot length. With these kinematics, a robot can be scaled for specific terrains and object sizes, with potential application in construction, search and rescue, and object retrieval.

Natural terrain is uneven and walking over these substrates may benefit from grasping into the depressions or "valleys" between obstacles. To examine how leg geometry influences walking across obstacles with valleys, we (1) modeled the performance of a two-linkage leg with parallel axis "hip" and "knee" joints to determine how relative segment lengths influence stepping across rocks of varying diameter and (2) measured the walking limbs in two species of intertidal crabs, \textit{Hemigrapsus nudus} and \textit{Pachygrapsus crassipes}, which live on rocky shores and granular terrains. We idealized uneven terrains as adjacent rigid hemispherical "rocks" with valleys between them and calculated kinematic factors such as workspace, limb angles with respect to the ground, and body configurations needed to step rocks. We first find that the simulated foot tip radius relative to the rock radius is limited by friction and material failure. To enable force closure for grasping and assuming that friction coefficients above 0.5 are unrealistic, the foot tip radius must be at least 10 times smaller than that of the rocks. However, ratios above 15 are at risk of fracture. Second, we find the theoretical optimal leg geometry for robots is with the distal segment 0.63 of the total length, which enables traversal of rocks with a diameter 37\% of the total leg length. Surprisingly, the intertidal crabs' walking limbs cluster around the same limb ratio of 0.63, showing deviations for limbs less specialized for walking. Our results can be applied broadly when designing segment lengths and foot shapes for legged robots on uneven terrain, as demonstrated here using a hexapod crab-inspired robot. Furthermore, these findings can inform our understanding of the evolutionary patterns in leg anatomy associated with adapting to rocky terrain.

Legged robots can have a unique role in manipulating objects in dynamic, human-centric, or otherwise inaccessible environments. Although most legged robotics research to date typically focuses on traversing these challenging environments, many legged platform demonstrations have also included “moving an object” as a way of doing tangible work. Legged robots can be designed to manipulate a particular type of object (e.g., a cardboard box, a soccer ball, or a larger piece of furniture), by themselves or collaboratively. The objective of this review is to collect and learn from these examples, to both organize the work done so far in the community and highlight interesting open avenues for future work. This review categorizes existing works into four main manipulation methods: object interactions without grasping, manipulation with walking legs, dedicated non-locomotive arms, and legged teams. Each method has different design and autonomy features, which are illustrated by available examples in the literature. Based on a few simplifying assumptions, we further provide quantitative comparisons for the range of possible relative sizes of the manipulated object with respect to the robot. Taken together, these examples suggest new directions for research in legged robot manipulation, such as multifunctional limbs, terrain modeling, or learning-based control, to support a number of new deployments in challenging indoor/outdoor scenarios in warehouses/construction sites, preserved natural areas, and especially for home robotics.