Topic: Development of a Biomimetic Robotic Fish using Ionic Polymer-Metal Compositesas Fin Actuators

Name: Xiao Wu and William Kraig

Advisor: Erik Cheever

Keywords: Biomimetry, Ionic-Polymer-Metal Composites, Fish, Biorobotic, Hydrodynamics, marine exploration

Abstract

More and more as our technology progresses, humans look towards nature to inspire our designs and to help us create novel, more advanced systems. This is especially true in the realm of aquatic technology, where the performance of traditional man-made systems falls far short of biological systems, such as fish. Natural selection has provided the fish with a very efficient propulsion system. For this project, we propose to mimic the highly efficient swimming motion of a fish using the relatively new material, ionic polymer metal composite (IPMC), which has biomimetic properties, as an actuator. Hopefully, using this new type of material, we will be able to better mimic the dexterity and efficiency of the propulsion systems used by fish. The final aim of this project is to create a six degree of freedom biomimetic fish that will perform preprogrammed actions.

Introduction

The oceans and seas are the most ecologically diverse places on our planet, and yet we know very little about them. Man made systems developed for ocean exploration, up to this point, have been very limited in their capabilities. For our engineering solutions to become better suited to marine exploration, we must look to nature for inspiration. While man made marine systems, such as submarines and ships, have been optimized over the years to perform long range cruising, they lack the agility and maneuverability of biological systems. A good measure of the maneuverability for water borne systems is turn radius. The minimum turning radius for a fish is 0.00-0.47 body lengths, while for sea-lions it is about 0.09-0.16 body lengths. In contrast, rigid-body submarines have a turning radius of 2-3 body lengths. In general the performance of man-made systems, constructed from rigid members with propulsion from propellers, fall far short of biological systems.

This project in biomimetics is intended as both a culminating academic exercise as well as a step towards the development of a superior marine exploration system.

Technical Discussion

In designing and building a biomimetic fish, there are several major problems that must be solved. These include: actually determining how a real fish moves and how to mimic this movement; characterizing IPMC and designing the actuators with them; waterproofing the body of the fish to protect the electronics; maintaining a suitable level of buoyancy; and determining the power source to use. All of the possible solutions must be evaluated in terms of weight, cost, and size. The weight of the finished biomimetic platform will affect our determination of neutral buoyancy. The cost of each solution must fit within our budgetary limits, and the size of each solution must fit within the confines of the biomimetic platform.
Each of the above mentioned problems are outlined in more detail below.

Power Source:

Several considerations must be taken into account when choosing and designing a power source for the biomimetic fish. The remote, autonomous nature of this project requires the power source to be some sort of battery. Possible power sources must be evaluated in terms of weight, power output, size, cost, and lifetime. Requirements for the power output of the power supply will be determined once the circuit and mechanisms are designed, and rely on the specifications of the electronic and actuator components we choose for our design. Another requirement of our power supply is that it must hold its charge for a reasonable period of time, to minimize downtime spent changing batteries or charging them.

Design of Propelling Mechanism:

The main design problem involved in the biomimetic fish project, is modeling the propulsion mechanisms of the fish. This part of the project involves choosing an appropriate swimming mode from the choices nature has to offer, and then modeling that motion using our actuator of choice: Ionic Polymer-Metal Composites.

Forces acting on the fish platform:

In designing a biomimetic fish, it is necessary to investigate the different forces and moments that affect the motion of a fish through water. One type of force that affects fish motion is drag. One type of drag is friction drag, or the friction between the fish and the water, which is dependent on both the surface area of the fish and the velocity of the fish. A second type of drag is form drag, which is caused by the pressures formed as the fish displaces water to make room for it to move. This can be minimized by making the body of the fish more streamlined. A final type of drag is known as induced, or vortex drag. As the fish propels itself through the water with its fins, the propelling fins (pectoral or caudal fins) form vortices that result in energy losses. These losses are known as induced drag.

fish diagram
Figure 1: Diagram of fish with fins labeled

Assymetries in the flow of water around the fish can cause differences in pressure on different sides of the fish. These differences, like those on an airplane wing, cause lift forces perpendicular to the flow of the water.

A third type of force is that of acceleration reaction. When the velocity of the fish changes relative to that of the water, the water reacts by resisting the acceleration. Other forces that affect a fish are buoyancy and weight.

Propulsion Modes:

There are two basic methods used by fish for propulsion. One method uses mainly the fish body and the caudal fin (BCF), while the other method uses mainly the paired fins (pectoral and pelvic) (MPF). Most fish use a combination of the two methods to propel themselves. BCF propulsion is most efficient for swimming at a constant velocity over distance, while MPF propulsion is better for maneuvering and turning the fish. Most fish use mainly BCF propulsion, but rely on MPF for maneuvering and turning.

Most methods of BCF propulsion can be best described as undulatory propulsion. Within the category of BCF propulsion there are several different undulatory propulsion modes. In each of these modes, a propulsive wave is passed along the body of the fish, generating a force that adds momentum to both the water and the fish. The force on the fish is composed of both a force of forward thrust, and a lateral force that results in losses.

The first of the BCF modes is called anguilliform motion, exemplified by the motion of the eel. In this mode, the entire body undulates. Because an entire wavelength of the propulsive wave can be found within the body of the fish, the lateral forces become canceled out.

Subcarangiform and carangiform motion are similar to anguilliform motion. While the motion is similar, the undulations only occur in the rear portions of the fish. This allows for greater overall speed, but limits the fish’s ability to accelerate or turn.

The final undulatory BCF mode is known as thunniform motion. Thunniform mode is considered to be the most efficient mode for swimming at higher velocities. It is however, quite inefficient for maneuvering, turning and swimming at lower velocities.

Fish that mainly use BCF propulsion also utilize MPF propulsion methods for maneuvering, turning, and low speed swimming. MPF propulsion can be either undulatory or oscillatory. Oscillatory MPF propulsion methods can be divided into drag-based and lift based modes. Drag-based propulsion can be thought of as a rowing motion, while lift-based propulsion can be thought of as the motion of a bird’s wings. Drag-based propulsion is more efficient for slower swimming speeds, while lift-based motion is more efficient for faster speeds [2]. Most fish utilize a combination of the two methods.

In the interest of creating a flexible biomimetric model, it would be best to design our biomimetric fish to utilize either the subcarangiform or carangiform methods of propulsion. We will also probably use some sort of combination of the drag-based and lift-based oscillatory MPF propulsion methods to give our biomimetric model a greater degree of maneuverability and acceleration.

Use of IPMC in designing the actuating mechanisms

Waterproofing:

Another major issue that must be solved is waterproofing. Certain parts of the biomimetic fish, namely the electronic components and the power supply, must be kept from the watery environment that the fish is expected to work in. Possible methods of waterproofing must be evaluated in terms of weight, cost, size, reliability, ease of construction, and the ability to make changes to the components inside. Our final solution to the waterproofing problem must be able to ensure that the components inside will be protected from water, and that we can easily make changes to the components inside. Failing that, we must be able to ensure that our circuits and power supplies are working properly before we encapsulate them.

One possible solution would be to waterproof the entire hull. This option would give us the greatest flexibility in making changes to the circuits inside. However, it would also probably be one of the harder solutions to implement, as its design must be quite foolproof as the hull would then be the only line of defense for the components inside.

Another, more practical solution would be to waterproof the individual components inside. This could be accomplished by coating each individual component in silicon. This solution would be the cheapest option, would be relatively simple to implement, and would be very reliable. A major downside to this option would be that making changes to the components would be nearly impossible, as they would be encased in silicon. It would thus be necessary to be sure that the circuits and components were working properly before they are coated.

Maintaining Neutral Buoyancy:

In order for the fish to operate within a watery environment, it must be able to maintain the same density as the surrounding water. There are two possible ways to achieve a state of neutral buoyancy: To design a ballast system similar to those on submarines, where water and compressed air can be mixed in ballast tanks until neutral buoyancy is achieved, or to design the biomimetic platform to have neutral buoyancy in a single density of water. Any potential buoyancy solution must again be evaluated in terms of weight, cost, size, and its efficiency at solving the buoyancy problem.

Project Plan

Research Phase - Questions to be Answered

What waterproofing methods are employed by current small scale UAVs? What new methods of waterproofing could be employed?

  • Research currently employed methods of waterproofing UAVs –evaluate in terms of applicability, cost, size, reliability, weight, ability to modify enclosed components. Evaluate the limitations and advantages of these waterproofing methods.
  • Research possible new methods of waterproofing UAVs
  • Determine and outline the necessary qualities and requirements of a waterproofing method to be used in our biomimetic fish. This outline will include such qualities as applicability, cost, size, reliability, weight and the ability to modify enclosed components
  • Evaluate these waterproofing methods in terms of our outlined qualities and requirements.
  • Evaluate the limitations and advantages of these waterproofing methods.

What is known about the properties of Ionic Polymer-Metal Composites. What are their strengths and limitations?

  • Research Ionic Polymer-Metal Composites and determine what is known about them.
  • Determine the strengths and weaknesses of IPMCs and use this knowledge to decide how to use these IPMCs in our biomimetic design.
  • What are the power requirements for IPMCs?

What types of power supplies are available and feasible to use with a biomimetic UAV?

  • Research current methods of powering UAVs and biomimetic devices.
  • Determine and outline the necessary qualities and requirements of a power supply to be used in a biomimetic UAV. This outline will include such qualities as applicability, cost, size, weight, power output, lifetime, etc.
  • Evaluate these power supplies in terms of our outlined qualities and requirements.
  • Evaluate the limitations and advantages of each power supply

What applicable and inexpensive components are readily available for use in our circuits?

  • Gather information and data sheets on available components
  • Contact vendors and manufacturers of these components
  • Compare the available components in terms of applicability, cost, size, efficiency of operation, weight, etc.
  • Evaluate the limitations and advantages of each component.

What methods of maintaining neutral buoyancy are used in current biomimetic and non-biomimetic UAVs?

  • Research the concept of maintaining neutral buoyancy in a watery environment.
  • Research methods of maintaining neutral buoyancy that are currently used with UAVs
  • Research the requirements of a system for maintaining neutral buoyancy in a biomimetic UAV.
  • Determine and outline the necessary qualities and requirements of a system for maintaining neutral buoyancy in a biomimetic UAV.

How exactly does a fish move? What types of propulsion mechanisms do these animals employ?

  • Research fish propulsion mechanisms. Determine the strengths and weaknesses of each type of propulsion.
  • Determine and outline the strengths and weaknesses of the different types of propulsion in relation to our project.
  • Evaluate these different propulsion types in terms of efficiency, cost to mimic, and difficulty to mimic.

Design Phase

Determine the preferred method of waterproofing for a biomimetic UAV.

  • Using our research on and evaluation of waterproofing methods for a biomimetic fish, determine which waterproofing method is best suited for use with our UAV.

Determine the preferred power source for a biomimetic UAV.

  • Using our research on and evaluation of power supplies for biomimetic UAVs, determine which method of supplying power is best suited for use with our fish.

Determine the preferred method of maintaining neutral buoyancy for a biomimetic UAV.

  • Using our research on and evaluation of the methods of maintaining neutral buoyancy for UAVs, determine which method is best suited for use with our device.
  • Design the system to be used to maintain neutral buoyancy: ballast tanks, compressed air, valves, etc.

Determine the optimal method of fish propulsion to be modeled by our biomimetic fish.

  • Using our research on fish propulsion methods, determine which method is the optimal method for use with a biomimetic UAV.
  • Design, using our research on IPMCs and our research on fish propulsion methods, the propulsion elements and actuators to be used in our biomimetic UAV.

Design of the biomimetic platform

  • Using our research on available components and our design decisions on achieving neutral buoyancy, waterproofing our UAV, supplying power to the UAV and fish propulsion methods to design the biomimetic platform. This platform will be designed according to specs... MORE

Prototyping Phase

  • Purchase components from vendors, order PCBs
  • Fabricate the platform
  • Fabricate the actuating mechanisms
  • Fabricate the control mechanisms/components
  • Fabricate the ballast system
  • Waterproof the components and/or platform

Testing and Correction Phase

  • Purchase/Build a testing medium (tank)
  • Write a testing program for the biomimetic fish
  • Test the biomimetic fish

Project Qualifications

This project will be executed jointly by B.Sc. candidates Wynn Xiao Wu and William Kraig under the guidance of Professor Erik Cheever of Swarthmore College. Both of the student project members have taken three years of engineering courses at Swarthmore and are both intensely interested in the field of aquatic bio-mimetics. Mr. Wu has visited the Ocean engineering department and Sea Grant lab at MIT where extensive work has been done on autonomous underwater vehicles, and has had personal contact with students there who are also pursuing similar research.

In addition through networking, we will try to enlist the help of people knowledgeable in the area of biomimetics.