Much current study of legged locomotion has rightly focused on foot traction forces, including on granular media. Future legged millirobots will need to go through terrain, such as brush or other vegetation, where the body contact forces significantly affect locomotion. In this work, a (previously developed) low-cost 6-axis force/torque sensing shell is used to measure the interaction forces between a hexapedal millirobot and a set of compliant beams, which act as a surrogate for a densely cluttered environment. Experiments with a VelociRoACH robotic platform are used to measure lift and drag forces on the tactile shell, where negative lift forces can increase traction, even while drag forces increase. The drag energy and specific resistance required to pass through dense terrains can be measured. Furthermore, some contact between the robot and the compliant beams can lower specific resistance of locomotion. For small, light-weight legged robots in the beam environment, the body motion depends on both legground and body-beam forces. A shell-shape which reduces drag but increases negative lift, such as the half-ellipsoid used, is suggested to be advantageous for robot locomotion in this type of environment.

This paper analyzes and implements two novel turning strategies for underactuated legged robots that leverage contact of an active tail against terrain. The first strategy produces a sustained turn with a tail dragging against the ground during forward locomotion. The second strategy produces a rapid point turn by impacting the tail against the ground. LoadRoACH, a 55 g palm-sized legged robot, is developed to carry the active tail payload used in turning experiments. A steady-state turning model predicts the achievable turn speed of the robot on carpet, and open-loop turning experiments characterize the performance of the two tail contact turning strategies. Tail drag turning provides comparable turning maneuverability to differential drive turning gaits on carpet and gravel surfaces. Tail impact turning can produce rapid point turns on carpet, tarp, and gravel, but has a large variability in turn angle and time to recover from the turn. Finally, tail drag and tail impact turning control methods are implemented in an aggressive closed-loop corner steering maneuver.

Legged robots can explore unstructured environments more effectively than wheeled robots, but high turning rate tracking is still a challenging problem, particularly on varying surfaces. Previous steering methods with small robots have shown high turn rates, but usually only on a limited set of surfaces. This paper proposes a new method for steering a miniaturized legged robot by cooperation between two robots connected by a compliant joint, creating a 73 gram, 12 legged robot. Detailed design issues and an empirical verification are presented for several cooperation strategies, including changing velocities of the 4 sets of leg triples. The robots use their combined traction forces to turn at better than 50 degrees/sec at 1 m/sec on various surfaces. Closed-loop steering using a differential drive strategy is implemented on the connected robots to track a “figure 8” trajectory on a tile surface.

As small legged robots for search and rescue are deployed in collapsed buildings with unstructured terrain and steep drop-offs, they run the risk of flipping over and becoming incapacitated. A single degree of freedom low-mass tail is added to a 77.5 g, 18 cm long VelociRoACH legged robot with protective shell, which gives it the capability to dynamically self-right. Quasi-static analysis of terrestrial self-righting gives design requirements for the tail actuator. Dynamic simulation predicts that terrestrial self-righting is slower than aerial self-righting with a massive tail, but can be achieved without adding significant mass away from the base of leg support. Open-loop experiments on terrain with varying friction and roughness show that VelociRoACH can dynamically self-right using tail contact in as little time as 256 ms. Finally, an autonomous self-righting experiment on an obstacle with multiple step drops demonstrates that the robot can detect inversion and rapidly self-right while walking on challenging terrain.

Cooperation primitives for climbing steps were developed for a system of two 10 cm long VelociRoACH hexapedal legged robots with a removable connection. When performed sequentially, the set of primitives allow the team of two robots to climb a step on the order of their body length. These primitives use a tether between the robots actuated by a winch on one of the robots to form and release connections, run synchronously while connected, and provide a tether assist force while running. For a step with a coefficient of friction of 1 and a height of 6.5 cm, quasi-static analysis correctly predicts that the two connected robots can raise the front robot over the top of the step, while a single robot can only pitch upward against the step. The winch module designed to perform the cooperative climbing experiments meets the system goals of providing controllable forces greater than each robot's body weight while driving a removable connection between the robots. Experiments demonstrate that the robot system can perform each cooperation primitive individually with a reliability of at least 50% using simple strategies of maintaining a constant bounding frequency with the drive motors of each robot and a set tether tension with the winch.

In this paper, we propose a trajectory-adjustable integrated milli-scale jumping-crawling robot with improved ability to overcome obstacles compared to a robot that can only crawl. The robot employs a novel jumping module with enhanced energy storing-capacity and a height-adjustable active trigger. To increase the energy-storing capacity, latex rubber and knee-like joints are employed to utilize large displacement of the elastic material. The active trigger is based on a single DC motor and can release stored energy at any state, enabling the robot to control the take-off speed of jumping. The jumping module is integrated with the lightweight Dash crawler. The integrated jumping-crawling robot weighs 59.4 g and controls its moving trajectory by adjusting both its crawling speed and its jumping take-off speed.

Miniature legged robots have demonstrated fast locomotion at small sizes, but underactuation generally limits their performance in rapid turns and other dynamic maneuvers. In this work, the inertial effect of swinging a dynamic tail was studied as a method to achieve these rapid turns. An analytic model of a robot with a reaction wheel tail was developed to determine the effect of varying system parameters such as tail motor voltage, coefficient of friction, and tail inertia on inertial turning performance. This model was also validated experimentally using a modified off-the-shelf miniature legged robot. The same trends seen in the analytic study were also found in the experiments varying motor voltage, coefficient of friction, and tail inertia.