Panagiotis Chatzakos

Parametric analysis and design guidelines for a quadruped bounding robot

This paper attempts to set the basis for a systematic approach in designing legged robots. A dynamically stable quadruped robot running in the sagittal plane with a bounding gait is used, and a non-dimensional criterion that is based on the robots forward speed and the required power to sustain a passive motion, is introduced. Dimension-less robot parameters ratios and desired motion variables are inputs to an optimization scheme that takes into consideration findings from experimental biology and environment specifications. Basic design guidelines, which derive from simulation results, are provided.

A Service Robot for Subsea Flexible Risers

This article presents the development of a remotely operated vehicle (ROV) for the underwater inspection of subsea flexible riser, proposing effective solutions for the associated robotic manipulation system. Novel, underactuated end-effectors have been conceived and developed to firmly hold the robot on the riser during locomotion or operation, without affecting risers integrity. By using these end-effectors, the robot is able to move along the riser, like an inchworm does, and rotate around the riser at any angle. In principle, the current design could operate up to 2,000 m sea depth with minor modifications. In this study, the service robot carries and deploys digital radiographic equipment for the volumetric inspection of flexible risers. Several preliminary tests, as well as a series of underwater tests, prove the robots encouragingly positive performance.

Autonomous Infrared (IR) Thermography based inspection of glass reinforced plastic (GRP) wind turbine blades (WTBs)

In this paper, the development of an autonomous, novel and lightweight multi-axis scanning system, deploying insitu Infra Red Thermography (IRT) based Non Destructive Testing (NDT), on glass reinforced plastic (GRP) wind turbine blades (WTBs), is presented. The scanner incorporates an intrinsic multi-degree-of-freedom (DOF) end-effector able to comply and achieve successful adjustment on the complex contours of a WTB, automatically aligning at the same time, the IRT equipment in proper position for inspection. The scanning system has been developed to mimic the dexterity and Non-Destructive inspection operations performed by an expert human operator. For this reason the passive adjustment of the end-effector module has been achieved via the utilisation of a half Stewart platform. Simultaneous motion of a mechanically decoupled IRT equipment carrier is realised, that exhibits two degrees of freedom, enabling accurate self positioning, laterally to the target inspection area of the WTB and automatic retraction to home position upon system withdrawal from the inspection site.The overall philosophy inherent in the systems design and development is the maximisation of the blade area coverage in a single run, at a known sensitivity, with the utilisation of the minimum number of system degrees of freedom (DOFs) and the maximum repeatability as well as positional accuracy possible. The entire system, scanning modules and end-effector, are uniquely adapted to operate in remote locations, i.e. the 100m blades utilised in offshore wind farms, as well as in the factory environment when used in a quality assurance capacity.

Self-stabilising quadrupedal running by mechanical design

Dynamic stability allows running animals to maintain preferred speed during locomotion over rough terrain. It appears that rapid disturbance rejection is an emergent property of the mechanical system. In running robots, simple motor control seems to be effective in the negotiation of rough terrain when used in concert with a mechanical system that stabilises passively. Spring-like legs are a means for providing self-stabilising characteristics against external perturbations. In this paper, we show that a quadruped robot could be able to perform self-stable running behaviour in significantly broader ranges of forward speed and pitch rate with a suitable mechanical design, which is not limited to choosing legs spring stiffness only. The results presented here are derived by studying the stability of the passive dynamics of a quadruped robot running in the sagittal plane in a dimensionless context and might explain the success of simple, open loop running controllers on existing experimental quadruped robots. These can be summarised in a the self-stabilised behaviour of a quadruped robot for a particular gait is greatly related to the magnitude of its dimensionless body inertia, b the values of hip separation, normalised to rest leg length, and leg relative stiffness of a quadruped robot affect the stability of its motion and should be in inverse proportion to its dimensionless body inertia, and c the self-stable regime of quadruped running robots is enlarged at relatively high forward speeds. We anticipate the proposed guidelines to assist in the design of new, and modifications of existing, quadruped robots. As an example, specific design changes for the Scout II quadruped robot that might improve its performance are proposed.

On the development of an unmanned underwater robotic crawler for operation on subsea flexible risers

Flexible risers are used for supplying oil from the deep sea to the offshore platforms and FPSOs. This paper presents the design and development of an unmanned robotic crawler for operation on subsea flexible risers. The goal is to provide the oil industry a tool for carrying equipment on flexible risers. The robot has been developed to crawl on the external surface of flexible risers. It moves along and around the riser using an inch-worm type motion. Novel under-actuated end effectors have been conceived and developed to hold the robot on the riser, without permanent influence to the flexible riser surface and geometry. In principle, the current design with modification could operate up to 2000m sea depth. In this paper, customization of the robot operation principles to carry and deploy digital radiographic equipment for the volumetric inspection of flexible risers is described in detail. Currently, there exists no equipment that can perform reliable non-destructive inspection of subsea flexible risers. Robot performance has been experimentally validated and these results are presented here for the first time. The resulting crawling performance has been proved to be independent of its environment.

A parametric study on the rolling motion of dynamically running quadrupeds during pronking

This paper examines the passive dynamics of straight-ahead level ground quadrupedal running and explores its use in formulating design guidelines that would: a) reduce steady-state roll and b) self-stabilize the rolling motion, thus making the control of the robot more straightforward. To study the effect of mechanical design in the rolling motion, a simple bounding-in-place (BIP) template is introduced as a candidate frontal plane model that captures the targeted steady-state behavior of a straight-ahead level ground running quadruped robot. This model is parametrically analyzed and local stability analysis shows that the dynamics of the open loop passive system alone can confer stability of the motion! These results might explain the success of simple, open loop running controllers on existing experimental robots and can be further used in developing control methodologies for legged robots that take advantage of the mechanical system.

Dynamically running quadrupeds self-stable region expansion by mechanical design

Dynamic stability allows running animals to maintain preferred speed during locomotion over rough terrain. It appears that rapid disturbance rejection is an emergent property of the mechanical system. In running robots, simple motor control seems to be effective in the negotiation of rough terrain when used in concert with a mechanical system that stabilizes passively. In this paper, we show that a quadruped robot could be able to perform selfstable running behavior in significantly broader ranges of forward speed and pitch rate with suitable mechanical design. The results presented here are derived by studying the stability of passive dynamics of a quadruped robot running in the sagittal plane in a dimensionless context and can be summarized as: (a) the self-stabilized behavior of a quadruped robot for a particular gait is related to the magnitude of its dimensionless inertia, (b) the values of hip separation, normalized to rest leg length, and the leg relative stiffness of a quadruped robot affect the stability and should be in inverse proportion to its dimensionless inertia, and (c) the self-stable regime of quadruped running robots is enlarged at relatively high forward speeds.

Maintaining static stability and continuous motion in rough terrain hexapod locomotion without terrain mapping

Locomotion on rough unknown terrain has been a major challenge for legged robotic systems. Hexapods offer the advantage of static stability due to their capability of maintaining their center of gravity within their support polygon. Various approaches have been proposed for moving on rough terrain that use mapping of the ground or control schemes that result to discontinuous or oscillating motion of the hexapod body. In these approaches, stability is not taken into account, and increased tip-over risk occurs. This work presents a novel approach for continuous and smooth locomotion of a hexapod on rough terrain while maintaining static stability at predefined values regardless of the terrain profile and the existence of obstacles and slopes. The locomotion of the body is adjusted through a correction algorithm that facilitates smooth body motion following the variation of the terrain while static stability is maintained. The effect of the body correction algorithm gains on the body motion behavior with respect to terrain variation is thoroughly analyzed and the approach is evaluated using the force-angle stability measure. Results using multibody dynamics simulations show the effectiveness of the developed approach.

IMPLEMENTATION OF A QUADRUPED ROBOT PRONKING / BOUNDING GAIT USING A MULTIPART CONTROLLER

This paper presents a multipart pronking/bounding controller for a quadruped robot, as well as the corresponding experimental results. The controller achieves given apex height and forward velocity in a quadruped robot with only one actuator per leg. A quadruped is designed and built and described in some detail. Experimental results obtained using internal sensors and high-speed camera captions show that the implemented quadruped robot performs pronking gaits and achieves bounding gaits with the desired characteristics.Copyright

Optimized energy harvesting from mechanical vibrations through piezoelectric actuators, based on a synchronized switching technique

Increasing demand in mobile, autonomous devices has made energy harvesting a particular point of interest. Systems that can be powered up by a few hundreds of microwatts could feature their own energy extraction module. Energy can be harvested from the environment close to the device. Particularly, the ambient mechanical vibrations conversion via piezoelectric transducers is one of the most investigated fields for energy harvesting. A technique for optimized energy harvesting using piezoelectric actuators called “Synchronized Switching Harvesting” is explored. Comparing to a typical full bridge rectifier, the proposed harvesting technique can highly improve harvesting efficiency, even in a significantly extended frequency window around the piezoelectric actuator’s resonance. In this paper, the concept of design, theoretical analysis, modeling, implementation and experimental results using CEDRATs APA 400M-MD piezoelectric actuator are presented in detail. Moreover, we suggest design guidelines for optimum selection of the storage unit in direct relation to the characteristics of the random vibrations. From a practical aspect, the harvesting unit is based on dedicated electronics that continuously sense the charge level of the actuator’s piezoelectric element. When the charge is sensed, to come to a maximum, it is directed to speedily flow into a storage unit. Special care is taken so that electronics operate at low voltages consuming a very small amount of the energy stored. The final prototype developed includes the harvesting circuit implemented with miniaturized, low cost and low consumption electronics and a storage unit consisting of a super capacitors array, forming a truly self-powered system drawing energy from ambient random vibrations of a wide range of characteristics.