Design and Construction
RTV Rubber Tail & Polycarbonate Head
tail of the fish was made out of Room Temperature Vulcanizing (RTV) rubber,
which is waterproof and flexible. The head of the fish was made by using a
custom made vacuum formed shell of polycarbonate. The tail and head are each
bonded to ABS flanges, and the flanges are screwed together with a gasket
between them creating a waterproof fish body to protect the electronics.
The tail was first sculpted with soft-clay. A wooden box was constructed to
house the clay tail as it was cast in REPRO, a solidifying molding mixture.
One half of the clay tail was submerged in the REPRO at time. The two halves
together created a negative impression of the tail. The RTV would be poured
inside this void, creating a flexible tail. However, a void inside the
tail was required, so a REPRO "plug" was made. REPRO was poured into the
tail mold and allowed to harden. Once removed, it was sanded down until it
was the size of the required void. Now, an RTV mold could now be made from
the three REPRO pieces. The original tail looked great but functioned less
than adequately, so a second was made. The second tail had no top or bottom
fins, and pieces were even shaved off to create a thinner skin in order to
allow the servos to move easier.
head was sculpted from a material called RENShape. It was sanded down until
it reached a desired size and shape. Measurements were taken of the board
and batteries to produce a perfectly sized head. A sheet of polycarbonate
plastic was heated and vacuum formed on the RENShape head, creating a
beautiful plastic head. This rigid head needed to connect to the
not-so-rigid tailpiece. Two interfacing plastic flanges were machined to fit
the head and the tail together. This connection created a waterproof seal
with the help from a VCR drive belt which functioned as an o-ring.
Tail Servo and Gear Assembly
One of the highest
priorities of the team was to have the fish swim as life-like as possible.
The movement of the fish tail was controlled by two servos in series. In
order to move the tail with the necessary amplitude to propel the fish
forward at an acceptable speed it was necessary to increase the torque
output of the servos. This was accomplished by adding an additional gear
assembly for each servo. In doing this modification, which was necessary for
the basic movement, the maximum amplitude of the tail's movement was limited
|The BOTBoard with sonar electronics
A 68hc811e2 micro-controller was used to control all operations of the
fish. A prefabricated board, called a
was purchased, and the 68hc11 and sonar circuitry was mounted onto it. This
board-chip combination was chosen because it has an internal timer, multiple
input and output ports, and the capability to control up to four servos.
language used to create the main program. SBASIC is a language designed
specifically for the 68HC11 and HC12 chips. The SBASIC interpreter converts
SBASIC code to assembly code, and then an assembler generates the
downloadable S19 file. Finally, a special downloading program talks with the
68HC11 in bootstrap mode via an RS232/TTL cable connected to BOTBoard's
serial port, and it loads the EEPROM with the correct object code.
The program perform three main functions: 1) move the tail servos in the
correct pattern to produce forward and turning movements, 2) to sense
objects in the water using sonar, and 3) avoid any objects in the water.
View the final
The transducer has a natural frequency of 200 kHz; therefore a 200 kHz input
signal will cause it to generate an ultrasonic chirp in the water. Using an
analog swtich IC, the same transducer can then act as a microphone listening
for the resulting echo. The transducer converts this echo back into a
sinusoidal voltage. The echo signal is passed though an analog band-pass
filter/amplifier, which amplifies signals around 200 kHz, and rejects all
other frequencies. This amplified echo signal is compared against a
reference voltage, and the comparator's output is sent to the MCU. The
circuit shown accomplishes this task.
Sonar Filter/Amplifier and Signal Routing Circuit
The software controlled the sonar as follows: for every timer overflow
(signaled by the timer overflow interrupt) the program chirps the sonar,
records time of chirp, waits briefly for ring to go away, and then starts
listening for an echo. If an echo is heard (echo signal is connected to
input capture pin and generates an interrupt) before the next TOF, the
distance is calculated. If no echo is heard, the distance is null (or zero).
Once an echo is heard, the board stops listening for echoes until the next
chirp has occurred.
For the purposes of this application, the actual distance to a
sound-reflective obstacle is not needed. Knowing that an object is in the
way is all that is necessary.
NOTE: Transducer needs to be in the water to work correctly.
NOTE: Speed of sound in 20 deg C water = 0.0583 in/usec
Both the aesthetics and performance of the mechatronic fish turned out
quite well. The fish is about 2 feet long and weighs approximately 5 lbs. It
travels at about .2 ft/s (.14 mph) in calm water. The fish project was an
absolute success, and it may well be the world's first autonomous robotic
There were, however, a few things the team would change if a second
prototype were to be constructed. The amount of control that the team
expected to have with the tail was not as good as initially expected. The
servos initially produced little torque, which corresponded to little
movement. The team modified the servos to produce more torque creating a
more forceful movement of the tail. Once the servos were modified to produce
a powerful tail movement for swimming, the desired range of motion was
limited from the gearing down of the design. It was determined that the
servos also needed to be able to produce a larger range of motion when the
fish is turning. The team concluded that the servos used were underpowered
and unable to meet the performance expectations. It would be recommended to
use servos with a higher torque output to allow for quicker and more
forceful, full-range propulsion from the tail. This improvement will provide
faster swim speeds and better turning abilities.
The RTV rubber used for the tail skin was of acceptable flexibility when the
thickness was under 0.1 inches thick. As the walls increased in thickness
the force required to bend the RTV rubber became too high for acceptable
flexing for the low-torque servos used. It would be recommended to pay
closer attention to the details in the skin thickness and geometry when
constructing another tail skin. A slightly larger body would also be
recommended to allow for more freedom in the design of the tail mechanics.
The sonar system used was very simple and allowed for distance sensing only.
It would be recommended to add a second sonar transducer to give the fish a
sense of relative closeness. In other words, it would know if an obstacle
was on the left or right. This would lead to intelligent on course
adjustments. Right now, the fish is simply programmed to always turn right.