This concerns a feasibility study for a small helium-filled R/C blimp.
It will have to navigate in a relatively constant-intensity magnetic
field of 1-2 Teslas that will follow a toroidal shape. Of course there
will be induced voltages and eddy currents, even if the speed is max 1
m/s.

An existing blimp working under "normal" conditions is propelled by
4mm, 0.5W, 300mA, 5grams Portescap DC motors, 9/60 reductor gears and
hand-made propellers delivering 5 to 6 grams of thrust. Speed of the
propeller is not really known. The new motor should be in the same
working area.

- Could it be possible to use these standard DC motors despite the
strong magnetic field, or would they require a shielding? The blimp
will have a payload of only 400g, so no heavy solution is possible.

- While the idea of using the ambient magnetic field to replace the
permanent magnet stator seems attractive, in practice it would be too
complicated to make the motor follow the magnetic lines and still keep
the correct orientation.

- Piezoelectic motors: I know they are high-torqued at low speed (like
the Canon lens USM ultrasonic DC motors), but what about their
efficiency and max rotation speed? There are linear actuators and
"inchworms", but they are not useful for this applications.

Furthermore, the use of magnetic materials might later be stricly
prohibited (no iron, nickel, cobalt, ferrites etc.), so the
piezoelectric might be the only solution. However, could it be
possible to build a rather crude motor with a simple coil that would
turn rather "square" (badly), being influenced by ambient EM field?
Thank you for your inputs. In another post, I discuss an alternative
propulsion system: by gas.

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Clarification of Question by knightmare34th-ga on 05 May 2004 02:08 PDT

As requested by neilzero-ga, here are some clues.
For the magnetic field of 1 to 2 teslas, it can be compared to the
Earth's magnetic field, which is estimated at about 5x10-5 tesla [1 T
= 1 kg/Cs = 1 N/Am], or is 0.5 gauss. A standard electromagnet
produces 2T, while 25T can be reached by water cooled coils, 35T by
supraconductive coils, and even 100T by big capacitor discharges. In
the "Palais de la Découverte" in Paris there is a big electromagnet of
9 tons delivering 1T: it is sufficient to hold straight horizontally a
massive steel chain and to ruin a mechanical watch by magnetization of
the needles...
Here is a link about magnetic saturation and B-H curves:
http://www.oz.net/~coilgun/theory/saturation.htm
magnetic materials:
http://www.oz.net/~coilgun/theory/materials.htm
(useful: 1 Tesla = 10000 Gauss).
The shape of my magnetic field will be in the order of 40 meters (I
don't know exactly).

The R/C acronym means "radio-controlled". Sorry I thought it was common notation.
Portescap is a brand name well known for small DC motors (as are Maxon
Motor and Faulhaber among others). The blimp will be around 2*0.5*0.5
meters (not defined yet), mass around 300g. The propellers of the
current blimp (1.1x0.63.x0.50m, 180g) are ~15cm of diamater but very
lightweight (handmade from balsa). They generate a thrust of 5-6
gram-force (~50 milliNewtons), they run at ~1600 rpm thanks to a 9/60
reductor. The motors are weighing 3.5g and are 8mm diameter (not 4mm
as stated) and deliver 10700 rpm without load and max 0.6 mNm of
torque.

The Laplace force F created by a current i flowing into a wire a of
length l in a magnetic field B is: F = i*l x B (where F, l and B are
vectors, and x means vector product.
The electromotive force emf (voltage) created at the tips of a mobile
of length l moving at velocity v perpendicularly to a magnetic field B
is: emf = -B*l*v. And if the mobile is conductive of resistance R, a
current i = emf/R will be induced.

As to the proposition of using 3 perpendicular motors, I've thought
about it but it triples the weight, and still they would be saturated
by the ambient magnetic field. I have the confirmation that I cannot
use magnetic materials (ferro- or ferrimagnetic, i.e. everything
attracted by a magnet).

Concerning the piezoelectric rotary motor, I've found something from
Physik Instrumente (PI Ceramic):
http://www.physikinstrumente.de/pdf/P915K094PZT_RotMot_crop.pdf
It is 3mm in diameter, 6mm long, 1000 rpm, 0.4 mNm, totally
insensitive to magnetic fields. That is close to what I need.

I'll be interested in more details of several technologies you
mentioned. You should not assume the successful reaearcher will be
highly skilled in physics, science and engineering, so teach us some
of the technology and basic fundamentals. Most of the researchers are
quick learners and can find details if you get them started in the
correct direction. One of them may be willing to invest hundreds of
hours in your project as they love learning. I'm guessing a two tesla
field is much stronger than the Earth's magnetic field, but weaker
than the field produced by licquid helium cooled super conducting
magnets used in MRI machines = magnetic resonence imaging. Perhaps the
motors (if any) used in MRI machines can give you some clues.
 My guess is you can use 3 motors, of any design, mounted mutually
perpendicular connected to one propeller and use a simple computer to
disconnect electrically and mechanically the motor with the lowest
efficiency due to the ambient magnetic field. Two perpendictular
motors will be sufficient if one dimention is predictable. Is portecap
a brand name? Do the letters R/L stand for remote control, radio
control or both?  Some approximate dimentions on the blimp and
propeller may be helpful to the researchers which I am not. Is the
toroidal magnetic field measured in meters or kilometers?  Neil

Hello,

This may be a good starting point for a shielded solution :

http://www.lessemf.com/mag-shld.html (See the High Saturation Magnetic
Alloy multi layered solution at the bottom of the page.)

One solution may be to load test the motor using a magnetically stable
area as your standard. You could monitor the the output of the motor
versus the oersted reading at points near the motor housing. Then run
a repeat test under your required conditions using a solution similar
to the above - and compare the results for efficiency. I'm not sure
how practical this would be in your situation, but this would give you
the detailed information you would require for a successful project.

- SgtCory

In response to SgtCory, I found the same page about shielding, but if
you look at the material (MuMetal), it is made at 80% of nickel and
15% of iron. So the shielding would somewhat protect the DC motor, but
it would be attracted by the magnetic field like a nail under a car
scrapping facility electromagnet... It seems that shielding is only
possible with materials with a high permeability (mu), because they
must deviate the magnetic lines, it's impossible to cut them. And a
high magnetic permeability means being "magnetic". But I'm not 100%
sure of this, maybe there are new materials that go around this law?