Jenhwa Guo

Dynamic characteristic of a biomimetic underwater vehicle

For predicting the dynamic characteristics of a fishlike AUV testbed, which is under development at National Taiwan University, we developed a practical method to simulate the undulatory locomotion of a flexible slender body. When a flexible slender body, which is divided into a number of segments undulates, the wave passes from the nose to the tail. Reaction forces due to momentum change, friction, as well as cross flow drag acting on each segment are taken into account. Equations of motions described by the body-fixed coordinate are obtained by taking the summation of the longitudinal force, lateral force and yaw moment acting on all the segments. Equations of motions are solved step by step in time axis and the velocity is transferred to space-fixed coordinates for integrating the trajectory. In numerical simulation, a digital filtering technique is applied to avoid drifting in sway and yaw motions and the steady state solutions can be obtained. Based on the results of a series of simulation calculations, the dynamic characteristics of the BAUV testbed are shown and discussed in this paper.

Determining the bodily motion of a biomimetic underwater vehicle under oscillating propulsion

The paper describes a biomimetic autonomous underwater vehicle (BAUV) that mimics the shape and behavior of fish. The swimming motion of the BAUV is achieved using an oscillating body. The body spline is specified by a set of parameters, which are utilized using genetic algorithm (Gas) by evaluating a fitness function of the optimization which is defined as the ratio of the forward velocity to the required driving power of the joint motors. The resulting body spline is found to be better than all other body splines at all tail-beating frequencies. Each body spline has an optimal tail-beating frequency.

A maximum entropy method for multi-AUV grouping

A vector-quantization formulation is used to define a grouping problem for multiple AUVs in a sampled environment. The objective is to minimize a quantization error function. The self-organizing network structure developed by Kohonen is a famous quantization model. Difficulties of applying the Kohonens network is that the convergence property is not guaranteed. In addition, learning gains in the Kohonens network have to be manually adjusted. This paper proposes a control method for the grouping of multiple AUVs under the structure of the Kohonens network. To solve the difficulties encountered in the framework of Kohonens network, we incorporate a Lyapunov function of a thermal statistical model to solve the problem of convergence. The position of each AUV is treated as a probability distribution function under thermal equilibrium. The learning gains are determined using the condition of asymptotically stability of the network. The minimization problem is formulated in a Lagrange optimal form with the constraint of maximum entropy. The intervehicle distance is controlled by the optimal distribution of the entropy. We prove that the global-minimum-error of the cost function can be achieved for the grouping

A practical method for simulating pectoral fin locomotion of a biomimetic autonomous underwater vehicle

In this paper, a practical method to simulate pectoral fin locomotion of a fishlike AUV testbed is presented. Basing on a blade element synthesis scheme, the authors developed a simple mathematical model to evaluate the hydrodynamic forces acting on a pectoral fin in feathering motion and lead-lag motion. The pectoral fin is treated as a number of moving blade element, the lift, cross flow drag and added inertia acting on each blade element are evaluated as a two dimensional oscillating thin foil and they are described in the fin-fixed coordinate system. These forces of blade element are transferred to the fuselage-fixed coordinate system and then integrated to obtain the total forces acting on a pectoral fin. Katos data from model test and calculation on a pectoral fin of a bass are cited to compare with the corresponding calculation results by the present simple mathematical model. The quite satisfactory agreement confirmed the validity of the simple model for evaluating the hydrodynamic forces of a pectoral fin. On this base, the previously developed computer program for simulating body-tail undulatory locomotion is then extended to simulate the pectoral fin locomotion of a biomimetic autonomous underwater vehicle

Motion control and way-point tracking of a biomimetic underwater vehicle

AUVs with rigid hulls and powered by rotary propellers have problems such as low propulsion efficiency, difficulties in positioning, agile turning, and precise hovering. Research and development into biomimetic AUVs (BAUVs) is growing in order to overcome the problems of AUVs. A BAUV testbed is being developed. Our BAUV testbed is composed of several links and joints. We develop a local control law that coordinates body angles by periodically alternating the position of the center-of-mass in the local coordinate. We have found that three parameters are sufficient to coordinate joint angles to perform the carangiform swimming. We then develop a global control law for the way-point tracking problem. The effectiveness of this method is evaluated by computer simulation. Control performance of the BAUV system with model uncertainties to track way-points under the influence of disturbances is also discussed.

AUV obstacle avoidance and navigation using image sequences of a sector scanning sonar

We use the continuous image sequences generated by an electronic scanning sonar to achieve the aim of obstacle avoidance and visual navigation for an autonomous underwater vehicle (AUV). Using sonar systems for sensing of unknown underwater environments is the best selection in practice. However, the critical demand for real-time signal processing and the uncertainties of AUVs dynamics make online detection of obstacles a challenging task. We first use the track-before-detect algorithm to extract information contained in image sequences to estimate the dynamics of the AUV, then we apply the dynamic programming algorithm to solve for the problem of detection. This method reduces the computational cost to meet the real-time demand on obstacle avoidance and navigation of the AUV system.

Design of an Underwater Glider with Fore and Aft Buoyancy Engines

In this paper, we discuss design issues in applying buoyancy engines as the device to vary net buoyancy and to alternate the position of the center of gravity of a glider. The buoyancy engines arrangement contains two tanks located at the fore and end aft part of the hull. Buoyancy engines considered here are those of piston-type. Forces equations which model buoyancy, gravity, and hydrodynamic forces in gliding are derived. Performances of different sizes of buoyancy engines are compared. Operational constrains considering the power consumption of buoyancy engines are also specified. Gliders with rectangular wings of various shape and wing location are then examined in terms of the energy cost for gliding controlled by buoyancy engines.

Modular modeling of maneuvering motions of an underwater glider

In the present paper, based on modular modeling concept, a mathematical model for the coupled nonlinear 6-DOF motions of underwater gliders is developed, and applied to investigate the characteristics of vertical and lateral motions of an autonomous underwater glider under development at National Taiwan University. In the proposed modular model, the component forces of main hull, wings, stern vertical fin and interaction among them are included. The advantage of the present model is that the hydrodynamic coefficients of these components may be estimated using existing database or empirical formula. The longitudinal motions of the autonomous underwater glider designed at NTU are controlled by separate two buoyancy engines located fore and aft, while the lateral motions are controlled by rolling an eccentric weight. Three different configurations of main wings and stern vertical fin are investigated, and the effects of main wings location to the longitudinal motion characteristics as well as the effects of stern vertical fin location to the lateral stability are clarified through a series of simulation. As the results of the present study, a configuration of main wings and stern vertical fin for the underwater glider with two separate buoyancy engines that has most preferred performance is identified, and it has also been confirmed that the developed tool is able to be used in the preliminary design stage for developing an autonomous underwater glider. In addition, the proposed modular modeling can be easily extended to another kind of underwater vehicles, such as regular AUV.

POWER REDUCTION BY CONTROLLING JOINT COMPLIANCE FOR THE PROPULSION OF A BIOMIMETIC UNDERWATER VEHICLE

Abstract The work describes a compliance control scheme for the caudal joint motion of a biomimetic autonomous underwater vehicle (BAUV). The purpose of the control method is to use the motor power more effectively for propulsion. A symmetric foil executing large-amplitude sway and yaw motions in a flow imitates a flapping tail fin which is used as the propulsive device of BAUV. Motions of the oscillating foil are then actuated by motors through springs. A control method was derived for the determination of the spring compliance for better use of the motors’ driving power. It is verified that the compliance control method can reduce the amount of energy for the foil propulsion and is beneficial for the energy saving design for future BAUVs.

Artificial lateral line design for robotic fish

Fishes can avoid colliding with obstacles and track baits depending on the lateral distributed sense nodes which can sense the pressure variances of the surrounding flow field. Moreover, the interacted acoustic pressure field between the tail and the wall can provide useful information of the fish location by extracting the physical meaning of the sense nodes measurement. Therefore, this study provides a high efficiency but low cost piezoelectric material, PVDF, and discusses how the external disturbances affect it and result the output form in voltage which could be used as a biomimetic sensor that attached on the sides of a robotic fish to measure the underwater acoustic pressure. By using the image source method, the most efficient sensing position of the PVDF sensor could be decided. Lastly, we discuss the optimal choosing strategy of the PVDFs length as pressure sensor to make sure to obtain the best performance.