• UMD255 — Up/down motor
down and PWM value
So, a command like LMF000
will turn the left motor off; LMF128
will run the left motor forward at
half power; and LMR255 will run
the left motor in reverse at full
power. The forward/reverse and
left/right values read from the
joystick are run through a simple
wheelchair algorithm which
smoothes out the power changes
as the joystick is moved.
These six-character commands
are sent over two Arduino digital
pins using the SoftwareSerial
library. Assigned transmit and
receive pins on the Arduino are
hardwired through the tether to
their opposites on the second
Arduino. The ROV Arduino receives
the six-character command and
enables the specified motor and sets the PWM, which is
then fed to the motor driver. The motor driver acts as a
slave and powers the appropriate motor.
The circuitry in the ROV can also send data back up the
tether to be viewed. Given that there is a large battery in
the ROV running the motors, it is useful to be able to check
on its voltage and how much charge is available. A simple
two-resistor voltage divider can be read by an analog pin on
the Arduino and — with a little math — converted into the
battery voltage. Additional possible sensors are a depth
gauge, compass, accelerometer, and gyroscope. Each piece
of data can be coded in a fashion similar to the commands.
For example, BAT followed by
battery voltage as a float, CMP
followed by 359, compass bearing
going clockwise, and so on.
Construction of the
Frame and Main Tube
The frame is made from 1/2
inch schedule 40 PVC pipe which is
available in any home
improvement store. The fittings
used are 12 elbows and 20 tees.
The uprights that the motors are
mounted on were extended
outward to give more room for the
propellers, but this may have not
been necessary. Additionally, the
total height could have been
reduced as extra room for ballast
was not needed.
Using ceramic tile for ballast
worked very well, and the height
was perhaps not needed. In general, a cube shape of
appropriate size is what works. Two brass petcocks were
threaded into the 1/2 inch PVC tubes: one at the top and
bottom. Opening these in the water allows you to very
carefully trim the neutral buoyancy by adding a small
amount of water into the frame.
The main tube has a six inch PVC cap on one end
that’s attached with PVC cement. A 3-1/2 inch hole was
drilled in the end of the cap, and a clear piece of 1/2 inch
thick Plexiglas was cemented with marine epoxy over the
opening to provide a window for the video camera. The
opposite end requires an end cap that can be removed to
40 SERVO 03.2016
Figure 5. The
Hall-effect
switch and
and attached
magnet.
Figure 3. Back of the PVC frame with rubber end cap removed.
Figure 4. Front of the PVC frame showing headlight and
camera window.
