The box cover comes with a lightweight gasket; this
was replaced with one cut to shape from 1/8 inch thick
rubber. When the cover is screwed tightly down, the plastic
cover bends slightly at the corners which does not give a
good seal. This was remedied by placing a 6-3/4 inch
square piece of 1/4 inch aluminum on top of the plastic
cover with matching holes. The holes for the sheet metal
type screws attaching the cover were drilled out and
tapped for larger 10-24 machine screws.
The entire Quad_ROV is quite buoyant and needs
additional weight for ballast. For testing, I duct taped
together a group of 3-1/4 inch ceramic tiles. A square of
four of these matches the box cover size nicely. Several
layers of these squares duct taped together were
temporarily attached to the box with two bungee cords. For
a more permanent solution, a 6-3/4 inch square of thick
aluminum would work well.
With four matching screw holes aligned with the top
cover, this would serve double-duty by also flattening the
plastic box cover for a good seal. I found that using just
enough weight to make the Quad_ROV negatively buoyant
was not enough and led to instability. The Quad_ROV
needs a little extra weight to help with stability, and this
weight needs to be centered at the bottom of the box
(against the cover) to keep a low center of gravity.
I found five layers of the 3-1/4 inch ceramic tiles
provided the right weight. Iron or steel should not be used
for ballast, as this would affect the magnetic sensor in the
Adafruit 9-DOF sensor. In addition, non-magnetic stainless
steel screws and nuts (as well as nylon) were used
throughout this project.
Driving the ESC controllers with a Teensy 3.1 takes a
special work-around. Normally, a sophisticated flight control
board drives the ESCs on a quadcopter.
There are two things of absolute necessity to keep in
mind when using ESCs. First, the ESCs must be calibrated,
and second, the ESCs must only be powered on after servo
signals set to the minimum value are sent to the ESCs. This
minimum servo signal is detected immediately by the ESCs.
The throttle to the ESCs is controlled by a traditional
servo signal. This signal typically varies from a 1,000
microsecond (one millisecond) pulse every 20 milliseconds
to a 2,000 microsecond pulse every 20 milliseconds. This
varying one to two millisecond pulse drives the output of
the ESCs from minimum to maximum throttle.
As with most servos, the exact values for minimum and
maximum output vary slightly and need to be tested. I
found my ESCs did not shut down at exactly one
millisecond. These values can also change depending on the
setting of the calibration values.
If you power the Teensy 3.1 and the ESCs at the same
time, it takes a second or two for the Teensy 3.1 to begin
sending servo signals to the ESC. This results in the ESC
being powered up in error mode and non-functioning.
On the previous design for the ROV_Manatee, I used a
12 volt automotive relay turned on with a Hall effect switch
to power the system after the enclosure for the electronics
was water sealed. Relays produce a magnetic field that will
interfere with the operation of the flight sensors, so this is
not an option for delaying power to the ESCs. Instead, a
quad two-input multiplexer (74LS157) was used. One set of
four inputs of the multiplexer are tied together and
connected to a simple 555 timer circuit carefully tuned to
produce the minimum servo signal.
When the Teensy 3.1, ESCs, multiplexer, and 555 timer
are all powered up at the same time, the ESCs will
immediately receive the minimum servo signal and power-up correctly. After the Teensy 3.1 has had a few seconds to
power up, it switches the multiplexer inputs to receive the
varying servo signals from the Adafruit servo driver board
for normal control.
The 555 timer circuit shown in Figure 4 uses two
additional diodes to produce a 50 Hz pulse with duty cycle
less than 50%. The diodes assure that the charge current
for the 4. 7 µF capacitor only goes through the 2K variable
resistor, while the discharge current goes through the 20K
30 SERVO 12.2017