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"Bart" - Argonaut Jr's
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We are using RC (radio controlled) hobby type motors with props added on for Bart's thrusters. These are widely available and relatively inexpensive, small, but powerful motors than can be run even in salt water with the motor completely exposed to the water. Read more here: DC Motors and Props.
We tested a 1000 Kv Outrunner Brushless RC Motor and 30 amp and 30 amp BEC ESC (Electronic Speed Controller) $8.50 for both, plus another $8 for shipping from China. The results were mixed. One of the three motors caused the ESC to shutdown after a short time of running underwater. The cause is likely due to the way the ESC senses an overload condition in the motor, so it shut off power to the motor even when there was not really an overload.
However the performance at just a 5 amp draw. We also learned that we need and ESC for RC Cars that have the ability to run the motor in reverse.
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Mounting a prop to this motor was fairly easy. The motor's dimensions are 51.6mm x 23.9mm x 8.5mm. The hub of a fan from a PC power supply is about 14mm, so we turned a piece of 1" Sch 40 PCV pipe in a lathe to fit over the motor and into the fans hub. It would take more time, but you could do this with a drill, small drum sander attachment to fit inside the PVC pipe so it spins the pipe, and sand paper to cut down the outside dimension of the PCV.
We purchased 5 - 45 amp ESCs for cars because they can run the brushless motors in forward and reverse. Add we went with 5, even more powerful motor that runs at 800Kv or slower which is better for turning bigger props and getting more power. Finally we got a couple of 1/4 scale servos for tilting the camera and rotating the sonar transducer.
More on Brushless Motors and ESCs
The following is a good summary of the construction and considerations for using RC hobby motors on an ROV, courtesy of John on the Yahoo RobotROV forum.
Doug
Your thread on cheap brushless motors & ESC got me
looking into brushless motors because it seemed to be a good way to
go (either in oil or open to the water) especially at that price.
The problem was that I really did not know much about them apart
from the fact that they are popular with RC airplanes.
Turns out they are a lot more complicated than I expected. Here is
what I have found for what its worth.
Inrunner / Outrunner – These are the two major brushless motor
designs. An inrunner brushless motor has stationary coils, and a
rotating permanent magnet inside the coils on the motor shaft.
An outrunner brushless motor is the opposite, it has a rotating
permanent magnet, placed outside the stationary coils on the motor
shaft . Outrunner motors have lower KV ratings, so they run at a
lower speed with more torque.
Outrunner can allow RC planes to direct drive larger props without a
gearbox. RC cars and boats tend to require inrunner brushless
motors, rather than outrunners. {Likely outrunner RPM is still to
high and torque too low to direct drive a car or boat prop}
All RC brushless motors are 3 phase rotating fields. Most inrunners
have either 2 or 4 poles on the rotor. Poles are always pairs (north
& south) so always multiples of two. The stator holds the stationary
field coils and for 3 phase systems the number of coils will always
be a multiple of 3. The positions on the stator where coils can be
placed are called teeth (or slots). The simplest motor is 2 poles
(N-S) and 3 coils. This motor is designated as 3s2p which means 3
stator teeth (or slots) 2 poles. The number of poles and field coils
are never equal to provide a static mismatch that allows the motor
to self start.
The number of turns of wire per coil is a major design choice. More
turns gives more torque and less rpm, less turns gives less torque
and more rpm. Torque and current are related and fewer turns also
means high current (and thicker wires).
In general, the Outrunner design has more torque than an inrunner,
and a 12 stator motor is more efficient than a 9 stator design (the
more stator positions, the finer the control over the rotating field
position to reduce torque ripple and increase power output).
With an outrunner there is a lot more space to install many more
poles and the magnetic fields are operating much further out from
the axis of rotation so they can apply more rotational torque.
12s10p and 12s14p are popular designs for outrunners. The more poles
there are the slower the motor can rotate compared to the rotational
speed of the stator fields. For the 12s14p motor the stator field
rotates through 360 degrees 7x for each rotation of the rotor shaft.
See the following web site for an explanation and demo.
http://www.aerodesign.de/peter/2001/LRK350/Warum_dreht_er_so_eng.html
The 3 phases can be wired to the motor coils in one of two patterns:
a Delta or a Wye (star). The Wye is a 3 spoke star with one end of
each coil in the center and the other end of each coil attached to
each phase of the controller. The Delta has each coil attached to
the next to form the 3 sides of a triangle and the controller
applies the 3 phases to the vertices. The two patterns have
different performance: the Wye has more torque, the Delta has more
RPM. The conversion factor between the two is 1.732; the Wye has 1.7
times the torque of the Delta, the Delta has 1.7 times the RPM. Also
for the same power output (and Kv) the Wye connected motor requires
1.732 more turns to be wound on each coil.
ESC specs often specify minimum number of turns for the motor coils
and a voltage range for the battery, in addition to their peak and
continuous current ratings.
Many do not support reverse as a command, you must physically swap 2
of the 3 motor wires (sensorless design does not care which motor
wires connect to which ESC wires). No-reverse ESC are common for RC
airplanes, helicopters and some cars (racers). ESC for RC cars may
only have forward and braking; particularly the high current ESC
(motor turns < 6) for car racing.
Some ESC support command reverse but only after a time delay.
Some ESC support command reverse but not at full power.
Many ESC except those for RC rock crawlers may not have good control
over very low speeds or startup under load. Best low speed control
comes from sensored motors and ESCs which can take the sensor input.
Sensored motors have Hall Effect sensors which detect the passage of
the magnetic poles of the rotor to precisely located the rotor
position. Sensorless ESC depend on Back EMF from the motor to detect
zero crossing for commutation and Back EMF is unreliable or unusable
at very low speeds. Also at low speeds motors with fewer poles may
have significant cogging (jerky rotation).
ROV Notes:
=========
Many RC Brushless motors and ESC are designed for very high power
output for short periods, for RC airplanes (200-600W for 5-15min)
and currents can be very high (80-150A). LiPo battery packs can
provide these current levels but they are generally depleted in a
5-15min period. By comparison a bilge pump running at 5A and 12v is
using only 60W, but it is also putting out much less mechanical
power. Many of the ESC for RC crawlers, which support command
reverse and accurate sensored low speed performance, are also
designed for 6-9v, which means 17-100A per motor for the 100-600W
each. An ROV with 4 thrusters could easily require 70-400A which is
a lot of current to send down a wire (or it's a large battery pack;
like for a golf cart).
For direct drive to the ROV prop (no reduction gears) RC inrunners
will not likely work; they usually require gears to drive an
airplane propeller and even then the RPM is high. Outrunners can run
much slower (eg 7x slower) but the RPM is still very high for direct
drive to a prop. The most promising of the RC hobby motors is the
high torque LRK 12s14p outrunner. However, for direct drive of ROV
prop perhaps we should be looking at the low RPM high torque
brushless motors made for something like electric bicycles.
Remember, the larger the number of poles the slower the motor can
operate.
The ESC can run very hot and many have fans. For an ROV the ESC heat
sink will need to be thermally coupled to the water to dissipate the
heat (circulate water to ESC heat sink or thermally couple the ESC
heat sink to the wall of an aluminum pressure hull). LiPo
battery packs have very high energy density (power per pound or per
cubic inch) and can support very high continuous current output
which is why they are used for RC airplanes; but they are very
expensive. A 3 AHr LiPo pack can cost more than a 30-50 AHr gel cell
battery which makes LiPo batteries 10-20x more expensive for the
same capacity. The small gel cell and AGM lead acid batteries have
the capacity but they do not support the high currents for very
long. For sustained high currents it would take large batteries like
those for golf carts. --John
Hydrostatic Test Chamber
Steel Pipe Ball Park PSI calculation
P=2*S*t*F/D
P-pressure
S-SMYS (specified minimum yield strength of your pipe, e.g. 52,000
psi)
t-Pipe wall thickness
F--Design Factor (between 0 and 1, generally between .5 and .72 for
DOT reguated work)
D-Pipe outer diameter
52,000 psi is a middle of the road standard pipe commercially
availble steel pipe strenght. You can get pipe in the following
grades, Grade B (35,000 psi), X42, X52, X60, X65, X70, and a limited
number of mills make X80. (where X52=52,000 psi SMYS).
Example 24 in Pipe with a 3/4" thick wall: (2 * 42 000 *
.71 * .75) / 24 = 1,863.75
Resources
www.hobbyking.com Good
selection of RC Motors and ESC.
http://www.ertyu.org/steven_nikkel/ethernetcables.html How to
wire Ethernet Cables