Introduction
This was my solo project for my third year major project module. The aim of this project was to design a biologically inspired aquatic robotic system that is capable of autonomous motion in an aquatic environment. The design was based on the anatomy of a fish. The project investigated fish locomotion, and the use of ionic polymer-metal composite (IPMC) as artificial muscle. Control was provided using a MBED microcontroller and infra-red Sensor modules, which were connected to a designed PCB that was attached to the robot chassis.
Ionic Polymer-Metal Composite is a uniquely composed material made from an ionic polymer such as Nafion or Flemion, which show artificial muscle like properties when a voltage is applied across its thickness. IPMC is a suitable material for use as artificial muscle due to its low required input voltage, and ability to work well in water. The development of an aquatic robotic system requires choosing a model based on a specific swimming mode and shape, as different types of fish use different swimming modes for motion. The robotic design will need to be streamlined, as shape can greatly affect the experience of drag in water and therefore the speed of the robot. Powering the robot with off-board circuitry provides a slowing force to the robot, making it less efficient. However the use of on-board electronics will make the robot heavier, and can make its shape bulkier which will require a greater force for propulsion. A balance needs to be made with the added weight of the onboard electronics and the force generated by the IPMC actuators. A single IPMC actuator may not provide enough force to make the robot move, so multiple strips of IPMC are proposed. Polystyrene foam acts as a useful tool for buoyancy control. Undulating Rajiform robots have a slow propulsion speed in comparison to the amount of IPMC actuators that are used. Therefore a fish robot using a tail like a Carangiform or Thunniform fish for faster, more efficient motion is proposed.
Testing
A Program in LabView was created in order to test the IPMC strips that were obtained. The program allowed the voltage and frequency across the electrodes, holding the strip of IPMC, to be varied to the desired values for testing. The resulting data was plotted to show the relationship between voltage, frequency and displacement. The results show that the voltage increases the displacement of the IPMC strip, and as the frequency increases, the displacement decreases. Lower frequencies give the IPMC strip more time to move from one side to the other, which results in higher displacement. However, the lower frequencies slow the movement down, resulting in a lower force output. During testing, it could be seen that the IPMC strip’s displacement was uneven from its origin to positive and from its origin to negative. This suggests an inconsistency with the IPMC material. Uneven + to – displacements will cause problems in directional control, as the robot will find it difficult to swim straight. This inconsistency makes the material very unreliable. Out of the many different sizes of available IPMC strips, only a select few were still functioning. It appears that the IPMC strips can stop functioning without any cause, suggesting that IPMC is an unreliable material for use as artificial muscle.
Design
The cost of IPMC is approximately 50$ per square centimetre, which makes it very expensive to purchase new strips. The high cost is due to each IPMC strip being custom made, making it difficult to obtain multiple strips with identical dimensions and properties. Due to the unreliability and expensive cost of IPMC, only two 55 x 10 x 0.3 mm strips were available for use and no more could be bought in. This limited the propulsion of robot. So with only two IPMC strips available, the design options were limited. The body of the new design was to be made from Perspex, where the strips of IPMC were placed at the rear end of the model. The connection between the body and tail were to be made by a thin Perspex assembly. The tail is made out of a thin sheet of heated laminate and attached to the tail IPMC strips by two small strips of Salotape. The IPMC strips are held between the two sets of brass contacts by a 2.4 mm diameter screw and nut. The main circuit board is screwed onto the centre of the body, and the infra-red modules are screwed onto the head of the body. The chassis body was cut using an automatic milling machine by using a SolidWorks model and programmed using TechSoft Visual Toolpath for the milling machine.
System electronics and control
The robot will be controlled by taking inputs from infra-red sensors when close to a wall and turning accordingly. Three infra-red emitters and receivers are needed to detect walls and obstacles left, right and to the front of it. This requires 3 inputs and 3 outputs from a microcontroller. To control the IPMC strips, an analogue output is needed to create an alternating voltage in the shape of a sin wave. The IPMC strips can be connected in series, so only one analogue output is needed. The MBED microchip was programmed using its online compiler to give it autonomous control. The circuitry was first designed on a breadboard, and then CADSOFT Eagle to produce the schematic and .brd file which was used to create the pcb.
Conclusion
After characterizing the IPMC strips, designing and building the robot chassis and circuitry, and programming the C++ code for control, a fully functioning robotic fish had been developed using IPMC actuators as artificial muscle. However due to the very limited force and displacement output of the IPMC strips, it has been concluded that the robot will not move in water. The principal of using the IPMC actuators as the tail fins still remains the same, and can be shown in a normal environment without the need of waterproofing. IPMC actuators do have potential as artificial muscle, however they are inconsistent and unreliable, and due to the limited quantities and the high cost of IPMC actuators, they’re full potential was unable to be explored.
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This is only a brief summary of my project, if you are interested to find out a bit more, feel free to contact me. I would have made the blog post a lot tidier, but the blog designer only lets me view the first picture, so i have to keep re-editing with lots of paragraphs etc to make it fit on my screen. i'm not too good with html, however i may start looking into it if this blog editor doesn't get improved.
On another note, I am doing a research project with Farnell on the electronics industry and how a student can build his or her career there. I have compiled a short questionnaire which only take 3 minutes to complete. I would be very grateful if you could fill it in and submit it. Click on the link for more info:
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Thanks!