Luther R. Palmer

System Design of a Quadrupedal Galloping Machine

In this paper we present the system design of a machine that we have constructed to study a quadrupedal gallop gait. The gallop gait is the preferred high-speed gait of most cursorial quadrupeds. To gallop, an animal must generate ballistic trajectories with characteristic strong impacts, coordinate leg movements with asymmetric footfall phasing, and effectively use compliant members, all the while maintaining dynamic stability. In this paper we seek to further understand the primary biological features necessary for galloping by building and testing a robotic quadruped similar in size to a large goat or antelope. These features include high-speed actuation, energy storage, on-line learning control, and high-performance attitude sensing. Because body dynamics are primarily influenced by the impulses delivered by the legs, the successful design and control of single leg energetics is a major focus of this work. The leg stores energy during flight by adding tension to a spring acting across an articulated knee. During stance, the spring energy is quickly released using a novel capstan design. As a precursor to quadruped control, two intelligent strategies have been developed for verification on a one-legged system. The Levenberg-Marquardt on-line learning method is applied to a simple heuristic controller and provides good control over height and forward velocity. Direct adaptive fuzzy control, which requires no system modeling but is more computationally expensive, exhibits better response. Using these techniques we have been successful in operating one leg at speeds necessary for a dynamic gallop of a machine of this scale. Another necessary component of quadruped locomotion is high-resolution and high-bandwidth attitude sensing. The large ground impact accelerations, which cause problems for any single traditional sensor, are overcome through the use of an inertial sensing approach using updates from optical sensors and vehicle kinematics.

Intelligent control of an experimental articulated leg for a galloping machine

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.

Screenbot: Walking inverted using distributed inward gripping

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.

A body joint improves vertical to horizontal transitions of a wall-climbing robot

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-WhegsTM robot 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-WhegsTM robots. 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.

Quadrupedal running at high speed over uneven terrain

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.

3D control of a high‐speed quadruped trot

Purpose – Legged vehicles offer several advantages over wheeled vehicles, particularly on broken terrain, but are presently too slow to be considered for many high‐speed tasks. This paper presents an effective 3D controller for a high‐speed quadruped trot.Design/methodology/approach – To successfully regulate forward velocity and heading, secondary motions such as body pitch and roll must be stabilised. The complicated coupling between pitch and roll motion causes the control effort on one axis to disturb the motion and control effort of the other. Unlike the modular methods in previous research, the algorithm presented here employs a cooperative approach where pitch stability effort is directly accounted for by the roll controller.Findings – When the secondary motions such as pitch and roll are well stabilized, forward velocity and heading can be regulated up to 3 m/s and 20°/s, respectively.Research limitations/implications – For many quadrupeds, trotting is usually employed as the precursor to gallopin...

Force Redistribution in a Quadruped Running Trot

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 bodys 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.

Design of a wall-climbing hexapod for advanced maneuvers

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.

Attitude Control of a Quadruped Trot While Turning

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

Toward Gravity-Independent Climbing Using a Biologically Inspired Distributed Inward Gripping Strategy

The biologically inspired strategy of distributed inward gripping (DIG) is presented in this study as a method for foot attachment and adhesion during gravity-independent climbing. As observed in nature, this strategy enables climbing animals to maneuver rapidly on surfaces in any orientation with respect to gravity, and does not require significant energy expenditure for attachment or detachment. DIG is an advanced implementation of directional attachment mechanisms that directs contralateral legs to engage their cockroach-inspired prehensile spines by pulling inward toward the body, rather than downward opposing gravity. By using opposing foot forces to engage the spines, the dependency on gravity is removed and the experimental system designed to test the attachment strategy, DIGbot, is able to climb and make turns on both vertical and inverted mesh screen. This behavior has not been achieved previously by a legged system, and requires novel design and algorithmic features that will be discussed. The spacing in the mesh screen requires each foot to perform a local search for an adequate foothold, which mimics what has been observed in climbing insects. The inward gripping principle is also suited for use with microspine arrays and gecko-inspired dry adhesive pads that require pulling tangential to the surface for attachment, and ultimately will allow for rapid and complex maneuvers on irregular terrain.