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A rate gyro is an electro-mechanical device that can sense
rotational motion. A few years ago, all rate gyros on the market
utilized an electric motor to spin a flywheel. The spinning
flywheel has an angular momentum and it will tilt when the motor-flywheel
assembly is turned due to an outside force. This gyroscopic
precession phenomenon has often been demonstrated in physic class
with a spinning bicycle wheel mounted on two handles. The teacher
usually tells the student to sit in a barber chair and hold the
spinning bicycle wheel in front of him. As the teacher turns
the barber chair, the student will feel the spinning bicycle
wheel wanting to tilt.
As the name implies, a rate gyro only senses a rate input.
The faster the teacher turns the barber chair, the more the
bicycle wheel tilts. The gyro motor/flywheel assembly will tilt
more when there is a large rate perturbation. The electric motor
and brass flywheel assembly can be seen in my picture of the
Airtronics SG-X gyro. Notice that there are two springs used
to restrain the motor/flywheel assembly. If we remove the springs,
then the gyro will measure the "turn angle" instead
of "turn rate." Then it becomes a heading gyro that
measures heading changes instead of rate changes.
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| Author's home made heading
hold gyro circuit and gyro from 1985. |
The first electro-mechanical rate gyro for R/C use was made
by Kavan corporation in Germany in 1978. It sold for about $50.
I purchased one at that time and it worked quit well. And it
made RC helicopter flying a lot easier. Kavan was also one of
the first companies in the world to manufacture RC helicopters.
The Kavan 60-size Jet Ranger was introduced at around 1974 and
it was the first RC helicopter that started looping and rolling.
In 1978, Ernie Huber of New Hampshire became the first one to
fly an RC helicopter inverted. He used a Kavan Jet Ranger and
Ernie wired up his own invert switch in a 5-channel RC airplane
transmitter.
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Old style electro-mechanical
rate gyro. |
The next mass produced RC gyro was manufactured by KO in Japan
at around 1980 and it was sold in the US by Kraft. It also worked
very well by the standards of the early 80's. Then came the
JR-100 and the Digi gyros made by Digi Corp in 1981. The JR-100
was the first rate gyro to allow the pilot to change the gyro
sensitivity from the transmitter. The Digi gyro was the first
to have a two-position sensitivity control box.
Piezo gyros for RC use got its start at around 1988 when a
few independent modelers from California designed and built their
own gyros using piezo ceramic crystals. Futaba became one of
the first large manufacturers to offer piezo gyros successfully.
Their G-501 gyro dominated the contest circuit and was used
by all the top F3C contestants at the 1995 World Championships.
Cliff Hiatt became the first to win a F3C world Championship
with a piezo gyro in 1995.
A piezo gyro has higher performance than electro-mechanical
gyros because the piezo gyros have no moving parts, hence they
last longer, can stand greater shocks, and are more sensitive
to even the smallest perturbation. In technical term, it is
called having a larger bandwidth. A large bandwidth means the
sensor can sense from low frequency slow perturbations up very
high frequency small and fast perturbations.
All helicopters have three axes of motion. Yawing is defined
as a rotation about the helicopter main rotor shaft. The roll
axis is a an imaginary axis that runs from the nose of the helicopter
to the tail. The pitch axis runs from the left side of the
aircraft to the right side of the aircraft. For a model helicopter
we do not need to install rate gyros for the roll or the pitch
axes because we have the flybar which acts like a psuedo mechanical
gyro to help stabilize the model's pitching and rolling motions.
Full-size helicopters do not have a flybar, but they are bigger
helicopters and they have slower reaction time as compared to
model helicopters, so they do not require a flybar to tame them
down. Helicopters are usually least stable in the yaw axis, hence,
a gyroscope is employed to sense the helicopter yawing motion,
then a feedback signal is fed to the tail control servo to help
tame down heading changes. Most commercial, small helicopters
do not have rate gyros, but most military helicopters and very
large commercial helicopters have a rate gyro for the tail.
The future of rate gyros will be the laser gyros. Laser gyros
are already used in commercial airplanes and helicopters for
over a decade. At $10,000 per unit they are still too expensive
and too bulky to fit in model helicopters. A ring laser gyro
utilizes two laser beams traveling in opposite directions in
a triangle container to sense yaw rate changes. The two beams
go round and round inside the triangular shaped container. When
there is a yaw rate, one light beam will then travel a shorter
distance then the other. This reduction in travel time provides
the rate information.
I predict classical electro-mechanical gyros will soon be
extinct from the RC helicopter market because it is cheaper to
manufacture piezo ceramic crystal rate gyros and the performance
of piezo gyros is much better. The only companies that still
manufacture mechanical gyros are JR, Futaba and Airtronics.
Futaba has the G-153, G-154 and G-155, and JR has the JR 120
and 130 and Airtronics has the SGX. These gyros are hard to
find on the market now and they have been priced at around $100
each for the last ten years. Nowadays, we can buy an inexpensive
piezo gyros for as little as $50. But the cheap piezo gyros
have drift problems.
A piezo crystal is a special crystal that will generate a
tiny voltage change when the crystal stress and strain are build
up in the crystal. You introduce stress to the piezo crystal
by bending it slightly. When there is a yaw rate perturbation,
the inertia of the motion will deflect the crystal very slightly
and it is enough to generate a tiny voltage that can be amplified
and used as a feedback signal. Piezo crystals are also used
in cheap microphones. When you talk into a microphone, your
sound wave vibrates the piezo crystal inside and that generates
a tiny voltage. On the other hand, if voltages are applied to
a piezo crystal, the crystal will deflect.
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Futaba GY-501 heading hold
gyro. |
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The piezo crystals used inside all piezo gyros are sensitive
to temperature. As the temperature changes, the crystal will
introduce a voltage change. These false voltage changes will
introduce an unwanted command to move the servo. The difference
between high quality and expensive piezo gyros and the inexpensive
ones is the high quality ones have more superior circuitry design
to automatically compensate for temperature drift. I usually
would not recommend getting the cheapest gyro that one could
find. A gyro can reduce the learning time tremendously. I recommend
every beginner purchase as good a rate as he can afford. (Editor's
note - James is VERY happy with his new Telebee, watch for full
report in August).
Out of all the non-heading hold type piezo gyros, I like the
Futaba G-501 (discontinued), JR-3000 (will be discontinued),
and the Thunder Tiger TG-8000. I would recommend a high quality
gyro like these three over the super cheap gyro. I use the TG-8000
on my 3-D helicopters, too. The benefit of a non-heading hold
gyro is they are simple to setup and you do not have to worry
about error drift from the heading hold feature. The error drift
is a different problem than the temperature drift, it is noticed
in heading hold gyros only.
A heading hold gyro is a gyro that basically tries to act
like a heading gyro or a positioning gyro. It basically takes
the yaw rate signal from a standard piezo rate gyro and then
mathematically integrates it to the angular position information.
Special circuitry must be introduced to minimize or filter out
the error from time integration. In principle, it is quite easy
to modify your own mechanical rate gyro or regular piezo gyros
into becoming a heading hold gyro by adding an Op-Amp integrator.
I have tried this in 1984 with a Digi gyro and it worked quite
well. I offered the idea to one of the big RC radio manufacturers
at that time but they were not interested. So for a while I
had the only heading hold gyro in the world. But if you look
at the picture, my homemade heading hold gyro from 1985 had a
very big circuit board using old analog technology. Colin S
Mill, an astrophysics researcher from England then became the
first one to commercially offer a heading hold gyro. He quit
his university job and named his company CSM and the CSM 360
gyro was born at around 1997.
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CSM 540 heading hold gyro |
In 1997, mechanical gyros became extinct in F3C contests,
everyone used a non-heading hold piezo gyro. (Heading hold was
only ruled legal one month before the 1997 Championship. Most
contestants did not have enough time to acclimate to heading
hold, therefore, only one used the CSM 360 heading hold gyro.)
In 1999, more than half the contestants used the CSM, Futaba
GY-501, or other heading hold gyros. Interestingly, many F3C
contestants do not use the heading hold mode for all maneuvers.
For example, Mr. Manabu Hashimoto, the 1997 and 1999 F3C World
Champion says that he only uses the heading hold mode on his
GY-501 for the backward recover maneuver. At the 1999 World
Championships, many still used JR's top of the line non-heading
hold JR-3000. (If you would like to learn more about what the
world top contestants used, order a copy of the May/June 2000
Rotory Modeler magazine from Heliproz or from Rotory Modeler;
in there I have a table listing of the equipment used by the
1997 and 1999 World Championship contestants.)
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JR 5000 heading hold gyro
and JR 8700 Super servo and JR 8417 high speed digital servo. |
In 2000, JR introduced their first heading hold gyro, the
JR-5000. This is an excellent gyro.
It is probably one of the best heading hold gyro, but it
is not cheap. There is a less expensive version called the JR-550.
I have tried many gyros on the market, I think the JR-5000 and
the GY-501 and the CSM 540 are probably the best heading hold
gyros. They cost $200+ for the gyro alone, but work extremely
well. The GY-501s almost never drift due to integrator error.
The GY-501 has a very nice feature that automatically zero out
the drift error when you quickly toggle between the Normal gyro
mode and the heading hold mode. In Japan, Futuba has just revealed
four new heading hold gyros: the GY-502 and GY-601 will replace
the GY-501. Two less expensive GY-240 and GY-401 will also be
introduced. (The 401, 240 and 502 are due here at HeliProz in
late August).
On the other hand, the new Telebee heading hold gyro is an
affordable heading hold gyro at less than $100 each. It does
have drift, but for beginners and intermediates this gyro will
be sufficient to hold the tail for learning backward flight and
3-D. I will tell you more about the very affordable Telebee
gyro in the next article and describe how to set it up to get
the most out of it.
In Germany, there is a new gyro that features an innovative
way to eliminate temperature drift. The piezo element is heated
up to a constant 70 degree F temperature regardless of the outside
ambient temperature. This gyro was helped designed by Rudiger
Feil, the Germany F3C and European F3C Champion. This new gyro
will be distributed by Miniature Aircraft in the US starting
in fall, 2000.
Finally, it is important to get a good high speed servo for
the rate gyro. Ideally, get one of those servos that can travel
60 degrees in less than .10 second. The JR 8700G and 8417 and
Futaba 9205 all fall into this category. But Futaba, JR, Airtronics,
and Hitec have now introduced many other high speed digital and
non-digital servos with alloy gears which also give splendid
performance. Ask Heliproz for recommendations and price. Again,
if you have a tight budget, then get the Hitec HS-525BB or the
Airtronics 94743. They are traditional ferrite motor servos,
but they have a .16 second transit time for 60 degrees travel
and are less than $50 each. Even with these two less expensive
servos, your tail will lock on fairly well. Next time we will
discuss the Telebee heading hold gyro.
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