The planar and energetically conservative Spring-Loaded Inverted Pendulum (SLIP) model with a linear spring has been modified in recent years to include an articulated knee, friction and contact losses, and energy thrusts during stance to stabilize forward velocity and hopping height. The work presented here advances the SLIP template toward a biological anchor by adding a third parallel articulating joint, which provides redundancy in the inverse kinematics solution space. Instead of one actuator into which an energy thrust can be added, the total thrust can be distributed between two actuators operating on the lower two leg joints (the hip joint remains passive during stance as in the original SLIP model). It is shown that a careless distribution of the thrust can lead to significant energy loss, so an offline search protocol is presented as a general approach for resolving the optimal thrust distribution ratio between these joints. The results of the offline search are then approximated for rapid computation of this ratio during hopping tests in a high-fidelity simulation engine.

A Force Threshold-based Position (FTP) controller is presented for cyclic legged locomotion over irregular terrain. Force feedback in the form of threshold comparison is used to control each leg independently. The FTP controller decouples the control of individual legs and uses only localized feedback to generate joint torques. No feedback of body state is needed and no multi-leg Jacobian is computed. Preliminary results suggest that the FTP controller is robust to terrain elevations without extracting any information about the ground or inertial sensing of the body. The algorithm is applied to a simulated hexapod walking blindly on irregular terrain, and an experimental system has been built. The controller has also been applied and tested on different gaits without modification. The FTP controller also has potential for expansion to bipeds, quadrupeds and other biologically-inspired forms.

A legged system that can negotiate moderately uneven terrain without human intervention or a vision-based control system can allow those resources, if available, to be concentrated on high-level tasks. This paper presents results of preliminary research on the application of a Force Threshold-based Position (FTP) Controller on an upright quadruped system blindly walking on irregular terrain. The algorithm requires no interleg communication, such as needed for a multi-leg Jacobian, or inertial sensing of the body. Instead, each leg relies upon local feedback and simple time triggers for gait phasing. Results are shown for sagittal plane implementation in a dynamics-based simulator, while an experimental system is being designed for further testing. Concurrent research applies the FTP controller to a sprawled-leg hexapod system in hopes of extracting general leg control principles for innate stability over moderate terrain.

This paper is an update on the investigation of Distributed Inward Gripping (DIG) as a rapid and robust attachment mechanism for vertical and inverted climbing. DIG is implemented on an 18-DOF hexapod, DIGbot, with onboard power and control system. Passive compliance in the foot, which is inspired by the flexible tarsus of the cockroach, increases the robustness of the adhesion strategy and enables DIGbot to execute large steps and stationary turns while walking vertically on mesh screen. Results of vertical climbing are shown.

A hexapod designed for wall climbing with a body joint and six 3-DOF legs can perform complex maneuvers such as sharp turns, making both interior and exterior transitions between vertical and horizontal surfaces, and traversing obstacles on both surfaces. This paper presents work toward the design and construction of the hexapod DIGbot, named for its utilization of distributed inward gripping (DIG) to generate adhesive forces. The biologically-inspired DIG approach allows robots to climb on surfaces of any orientation with respect gravity, including ceilings, or in zero gravity environments.

Several recently-designed robots are able to scale steep surfaces using animal-inspired strategies for foot attachment and leg kinematics. These designs could be valuable for reaching high vantage points or for overcoming large obstacles. However, most of these robots cannot transition between intersecting surfaces. For example, our previous Climbing Mini-WhegsTMrobot cannot make a 90deg transition from a vertical wall up onto a flat horizontal surface. It is known that cockroaches bend their body to accomplish such transitions. This concept has been simplified to a single-axis body joint which allows ground-walking robots to cross uneven terrain. In this work, we examine the effect of a body joint on wall-climbing vehicles using both a kinematic simulation and two prototype Climbing Mini-WhegsTMrobots. The simulation accurately predicts that the better design has the body joint axle closer to the center of the robot than to the front wheel- legs for orthogonal exterior transitions for a wide range of initial conditions. In the future, the methods and principles demonstrated here could be used to improve the design of climbing robots for other environments.

Insects and geckos use claws and adhesive pads to negotiate both rough and smooth surfaces. Climbing robots have been designed to mimic various aspects of these and other biological systems to operate in specific vertical environments. Robots that adhere to the surface through suction cups, magnetic end-effectors, or adhesive pads can climb featureless, flat, or smoothly curved surfaces. Vortex-generating climbers do not require smooth surfaces. Robots have been designed with end-effectors that match specific features of the environment, such as peg-holes, handrails, climbing-wall footholds, and poles. Robots have also been fitted with insect-inspired spines to scale rough vertical surfaces.

Insights from biology have helped reduce the weight and increase the climbing ability of mobile robots. This paper presents Screenbot, see Fig. 1, a new 126 gram biologically-inspired robot that scales wire mesh substrates using spines. Like insects, it walks with an alternating tripod gait and maintains tension in opposing legs to keep the feet attached to the substrate. A single motor drives all six legs. Mechanisms were designed and tested to move the spines into and out of contact with the screen. After the spine engages the substrate, springs along the leg are compressed. The opposing lateral spring forces constitute a distributed inward grip that is similar to forces measured on climbing insects and geckos. The distributed inward gripping (DIG) holds the robot on the screen, allowing it to climb vertically, walk inverted on a screen ceiling and cling passively in these orientations.

In this paper, an attitude control strategy is developed for a high-speed quadruped trot. The forces in the trot are redistributed among the legs to stabilize the pitch and roll of the system. An important aspect of the strategy is that the controller works to preserve the passive dynamics of quadruped trotting that are accurately predicted by the spring-loaded inverted pendulum (SLIP) model. A hybrid control strategy is presented which allows the quadruped to reach a speed of 4.75 m/s and turn at a rate of 20 deg/s in simulation under operator control. The discrete part of the controller runs once per trot step and outputs a stance thrust energy and hip angles for touchdown. The stance thrust energy accounts for losses during the step, especially at touchdown. Both the stance thrust energy and hip angles dictate the natural dynamics during stance. The force redistribution algorithm continuously operates during stance to stabilize the body's tilt axes, roll and pitch, with minimal effect on the prescribed natural dynamics. The 1.0 m/s increase in speed over previously presented work is largely due to the more dynamically-consistent force redistribution algorithm presented in this paper. The controller also tracks desired changes in heading, for which the biomimetic method of banking into a high-speed turn is also realized.

We previously presented a remote-controlled robot, Climbing Mini-Whegstrade, which scales vertical glass walls using passive compliant feet on simple rotating wheel-legs [1]. With a single drive motor, the legs of the robot press the feet against the glass, then gradually peel them away, mimicking the foot motions observed in insects. With feet made of scotch tape, the robot was able to walk on glass walls and ceilings and transition between surfaces intersecting at interior angles. This paper demonstrates that using passively compliant feet on simple rotating wheel-legs a robot can apply adhesive feet, spines, and Velcro in order to climb various steep substrates.

High-speed legged locomotion is complicated by the challenge of uneven terrain because the system must respond to the fast-changing terrain elevation under each foot, and quickly secure a solid foothold after touchdown. This paper presents a leg stretch reflex and anti-slip retraction algorithm that are added to a previously presented controller to stabilize a high-speed trot over uneven terrain. Together with fuzzy control and a force redistribution algorithm, these control mechanisms stabilize a quadruped trot at 5.25 m/s. The quadruped can turn at 30 deg/s when running at 3.0 m/s, and can maneuver over uneven terrain with standard deviation of height variation of 3 cm at 4.0 m/s. This appears to be the first reported control of high-speed quadrupedal running over uneven terrain.

During a complete running stride, which involves significant periods of flight during which no legs are contacting the ground, a quadruped cannot employ static stability techniques. Instead, the corrective forces necessary to maintain dynamic stability must be applied during the short stance intervals inherent to high-speed running. Because of this complexity and the large coupled forces required to run, much of the research on the control of quadruped running has focused on planar systems which are not required to simultaneously control attitude in all three dimensions. The 3D trot controller presented here overcomes these and other complexities to control a trot up to 3.75 m/s, approximately 3 body lengths per second, and turning rates up to 20 deg/s. The biomimetic method of banking into a high-speed turn is also investigated here. Along with the details of the attitude control algorithm, a set of control principles for high-speed legged motion is presented. These principles, such as the need to counteract the disturbance of swing leg return and the usefulness of force redistribution during stance, are not dependent on a particular scale or actuation scheme and can be applied to a wider range of legged systems

Intelligent controllers are being used with increasing effectiveness on complex systems. This work verifies the effectiveness of fuzzy control, an intelligent method, on a single, articulated-leg that was designed to be used on a high-speed galloping quadruped. Intelligent methods are compared to other control methods in simulation and on the OSU DASH (Dynamic Articulated Structure for High-performance) leg. It is shown that the intelligent controllers outperform non-learning methods. Using fuzzy control, the OSU DASH leg performs stable hopping on a treadmill moving at 2.0 m/s.