Taavi Salumae

Flow-relative control of an underwater robot

This paper describes flow-relative and flow-aided navigation of a biomimetic underwater vehicle using an artificial lateral line for flow sensing. Most of the aquatic animals have flow sensing organs, but there are no man-made analogues to those sensors currently in use on underwater vehicles. Here, we show that artificial lateral line sensing can be used for detecting hydrodynamic regimens and for controlling the robot’s motion with respect to the flow. We implement station holding of an underwater vehicle in a steady stream and in the wake of a bluff object. We show that lateral line sensing can provide a speed estimate of an underwater robot thus functioning as a short-term odometry for robot navigation. We also demonstrate navigation with respect to the flow in periodic turbulence and show that controlling the position of the robot in the reduced flow zone in the wake of an object reduces a vehicle’s energy consumption.

FILOSE for Svenning: A Flow Sensing Bioinspired Robot

The trend of biomimetic underwater robots has emerged as a search for an alternative to traditional propeller-driven underwater vehicles. The drive of this trend, as in any other areas of bioinspired and biomimetic robotics, is the belief that exploiting solutions that evolution has already optimized leads to more advanced technologies and devices. In underwater robotics, bioinspired design is expected to offer more energy-efficient, highly maneuverable, agile, robust, and stable underwater robots. The 30,000 fish species have inspired roboticists to mimic tuna [1], rays [2], boxfish [3], eels [4], and others. The development of the first commercialized fish robot Ghostswimmer by Boston Engineering and the development of fish robots for field trials with specific applications in mind (http://www.roboshoal. com) mark a new degree of maturity of this engineering discipline after decades of laboratory trials.

Against the flow: A Braitenberg controller for a fish robot

Underwater vehicles do not localise or navigate with respect to the flow, an ability needed for many underwater tasks. In this paper we implement rheotaxis behaviour in a fish robot, a behaviour common to many aquatic species. We use two pressure sensors on the head of the robot to identify the pressure differences on the left and right side and control the heading of the fish robot by turning a servo-motor actuated tail. The controller is inspired by the Braitenberg vehicle 2b, a simple biological model of tropotaxis, that has been used in many robotic applications. The experiments, conducted in a flow pipe with a uniform flow, show that the robot is able to orient itself, and keep the orientation, to the incoming current. Our results demonstrate that guidance of a fish robot relative to a flow can be implemented as a simple rheotaxis behaviour using two sensors and a Braitenberg 2b controller.

A Flexible Fin with Bio-Inspired Stiffness Profile and Geometry

Biological evidence suggests that fish use mostly anterior muscles for steady swimming while the caudal part of the body is passive and, acting as a carrier of energy, transfers the momentum to the surrounding water. Inspired by those findings we hypothesize that certain swimming patterns can be achieved without copying the distributed actuation mechanism of fish but rather using a single actuator at the anterior part to create the travelling wave. To test the hypothesis a pitching flexible fin made of silicone rubber and silicone foam was designed by copying the stiffness distribution profile and geometry of a rainbow trout. The kinematics of the fin was compared to that of a steadily swimming trout. Fin’s propulsive wave length and tail-beat amplitude were determined while it was actuated by a single servo motor. Results showed that the propulsive wave length and tail-beat amplitude of a steadily swimming 50 cm rainbow trout was achieved with our biomimetic fin while stimulated using certain actuation parameters (frequency 2.31 Hz and amplitude 6.6 degrees). The study concluded that fish-like swimming can be achieved by mimicking the stiffness and geometry of a rainbow trout and disregarding the details of the actuation mechanism.

Modelling of a biologically inspired robotic fish driven by compliant parts

Inspired by biological swimmers such as fish, a robot composed of a rigid head, a compliant body and a rigid caudal fin was built. It has the geometrical properties of a subcarangiform swimmer of the same size. The head houses a servo-motor which actuates the compliant body and the caudal fin. It achieves this by applying a concentrated moment on a point near the compliant body base. In this paper, the dynamics of the compliant body driving the robotic fish is modelled and experimentally validated. Lighthills elongated body theory is used to define the hydrodynamic forces on the compliant part and Rayleigh proportional damping is used to model damping. Based on the assumed modes method, an energetic approach is used to write the equations of motion of the compliant body and to compute the relationship between the applied moment and the resulting lateral deflections. Experiments on the compliant body were carried out to validate the model predictions. The results showed that a good match was achieved between the measured and predicted deformations. A discussion of the swimming motions between the real fish and the robot is presented.

A bio-mimetic design and control of a fish-like robot using compliant structures

This paper presents a bio-mimetic approach to the design and control of a fish-like robot with compliant parts. One of the key contributions of this work is the use of continuous structures instead of discrete assemblies. In this framework, the motion of the robot is accomplished by copying the kinematics of a biological fish swimming in a sub-carangiform mode. The flexible part referred to as the tail is modeled as a cantilever beam with non-uniform cross-section actuated by a time varying moment. The geometrical and inertial properties of the tail are known. The method of assumed mode is used to derive the equations of motion of the tail; a relationship between the applied torque and the lateral line deflections is calculated. The expression of the torque mimicking the midline kinematics of a biological fish is then computed. A prototype implementing the proposed approach is built. Experiments are performed on a given tail in air and in water. The calculated and experimental midline deflections are then compared.

A bio-inspired compliant robotic fish: Design and experiments

This paper studies the modelling, design and fabrication of a bio-inspired fish-like robot propelled by a compliant body. The key to the design is the use of a single motor to actuate the compliant body and to generate thrust. The robot has the same geometrical properties of a subcarangiform swimmer with the same length. The design is based on rigid head and fin linked together with a compliant body. The flexible part is modelled as a non-uniform cantilever beam actuated by a concentrated moment. The dynamics of the compliant body are studied and a relationship between the applied moment and the resulting motion is derived. A prototype that implements the proposed approach is built. Experiments on the prototype are done to identify the model parameters and to validate the theoretical modelling.

Biomimetic mechanical design for soft-bodied underwater vehicles

This paper describes a biomimetic underwater fish robot prototype and its design methodology. The key question directing our design is the transfer of functionality from fish to a fish robot with respect to efficient mobility. We want to minimize mechanical complexity and achieve a low-cost fabrication. We argue for the case of morphological computation, i.e. achieving high mobility and efficiency by duplicating fish physical body structure. In this way, a possibly large part of the fish motion ability is outsourced to the embodiment, i.e. achieved by the interaction of the fish body parts and the water flow. This approach makes us focus on the material properties of a compliant tail propulsion mechanism. The tail is actuated by a single motor and we want to make it efficient by exploiting the energy propagation from the body to the surrounding fluid. We explain our design constraints, material choices and describe the design process. We draw conclusions about the relevance of our design parameters and design choices.

Design principle of a biomimetic underwater robot U-CAT

This paper presents a design rationale and describes the design of a biomimetic underwater robot U-CAT. U-CAT is an autonomous, small-size and low cost vehicle, currently under development, for shipwreck penetration. The robot will assist archaeologists during possibly dangerous and expensive shipwreck exploration missions. It reduces the need of using divers, while being able to go to places that remain unreachable for ROVs. U-CAT uses a novel 4-fin actuation that gives the vehicle high maneuverability for operating in complex environments with walls, ropes, nets and other obstacles. The use of fins also allows more quiet motion with respect to the traditional propellers. Fins beat up less sediment from the bottom and the walls, and thus maintain higher visibility for recording a video. The U-CAT will be also equipped with sensors specifically customized for shipwreck exploration tasks.

Flow-aided path following of an underwater robot

This paper describes an underwater robot navigation strategy in flow. Our aim is to demonstrate that knowing the relative flow speed is advantageous because it permits using more energy efficient and stable control for trajectory following. We use a biomimetic robot that moves in uniform flow using a side-slipping maneuver. Side-slipping permits the robot to move laterally with respect to the incoming flow by exploiting its passive dynamics. The side-slipping maneuver is controlled by adjusting the heading of the robot with respect to the flow. We implement simple PID controllers for controlling the motion of the side-slipping robot laterally and transversely. Also, we compare the performance of the robot in the case where the robot does not know the flow speed. In this latter case the robots heading towards the waypoint is controlled and the flow effect is considered as a disturbance compensated by the control algorithm. Comparative experiments demonstrate that it is advantageous for a robot to know not just its speed and orientation with respect to the worlds frame of reference but also its local flow-relative speed. It permits the robot to follow trajectories more stable and using less energy. In the discussion section we propose possible future directions for implementing the on board flow-relative control.