Heading Hold Gyro.

"Peter Dohm" wrote in message
...
"RogerN" wrote in message
m...

"brian whatcott" wrote in message
...
I expect, like me, you thought you knew what heading hold gyros were all
about? If you haven't been around model helicopter enthusiasts lately,
you have no idea! Try googling 'heading hold gyro'.
It turns out, that radio control modelers stick a heading hold gyro on
their model helicopter and hook it to a fast servo that modulates the
tail rotor pitch. The device takes over when a rotate ("pirouette')
command stops, and stops the tail dead on that heading - fast, and holds
it against wind-drift and weather cock effects.
The HH gyro runs $40 to $150 and a fast (digital) servo might add
another $40 on it.

Think of the possibilities for a heading stabilize function in a
homebuilt!
A HH gyro driving a big servo, controlling a servo tab on the rudder.
Something similar could be arranged for pitch hold (a sort of super
cheap altitude hold/augment?)

Brian W

The heading hold gyros are rate gyros and use a microcontroller integrate
the error. They hold heading real well but drift over time. Most of the
time we can trim out the drift well enough to not be a problem but
remember most model helicopter flights last no more than 15 minutes and
if a constant heading was held for an entire 5 minutes it would be a long
and boring time for the pilot.

But gyros in model helicopters do make a world of difference. When I
started trying to learn to fly model helicopters (1981) gyros weren't
very common. I tried for years and was never able to hover out a tank of
fuel before a crash. After purchasing a simple mechanical rate gyro, I
flew some 70 full tanks of fuel before crashing, and that crash was a
result of getting too far away and losing orientation (the heli turned
black in the bright sky!).

On my larger model helicopters I have gyro's that once sold for nearly
$400 and a servo that sells for $130, I bought most of my stuff used and
sometimes crashed. By that time I had so much experience rebuilding
crashes that I would buy heli's needing work for a fraction of the new
cost. I have my own home machine shop and make most of the shafts simply
by cutting drill rod to length and cross drilling the holes for the
bolts, a $20 main shaft costs me about $2 + 10 minutes.

RogerN


As a matter of fact, I really allowed myself to get way off track early on
in this thread--along with nearly everyone else.

Basically IIRC the rate gyro systems are traditionally called wing
levelers, while a real heading gyro is just that--and frequentle slaved to
a flux gate--so that an autopilot on heading hold will really continue to
follow the same magnetic heading. The rate gyro allows the autopilot to
intersept a new course or heading without commanding an acrobatic maneuver
to accomplish it.

Peter

The problem with sensing magnetic heading in model helicopters is that they
want them to hold the heading relative to the model. For example if you
were heading north and did a loop you would be heading south at the top of
the loop, that would be interesting if it tried to auto correct magnetic
heading during a loop or other aerobatics.

But for stabilizing an aircraft that wasn't performing aerobatics perhaps a
rate gyro, magnetic heading, and perhaps an accelerometer might be able to
do the job.

RogerN

"brian whatcott" wrote in message
...
RogerN wrote:
/snip/
Think of the possibilities for a heading stabilize function in a
homebuilt!
A HH gyro driving a big servo, controlling a servo tab on the rudder.
Something similar could be arranged for pitch hold (a sort of super
cheap altitude hold/augment?)

Brian W

The heading hold gyros are rate gyros and use a microcontroller to
integrate the error. They hold heading real well but drift over time.
Most of the time we can trim out the drift well enough to not be a
problem but remember most model helicopter flights last no more than 15
minutes and if a constant heading was held for an entire 5 minutes it
would be a long and boring time for the pilot. /snip/
RogerN


Good thought. Gyros drift.
When I consider my personal heading hold, cross country, I drift way more
than any gyro I could buy, I reckon :-)

Moreover (as I find to my cost) a mini rate gyro won't drive a torquey
servo, unless I beef up the output signal.
It's pretty to watch a little scrap of gyro stretching and shrinking a 1.5
millisecond pulse repeated at 50Hz when I rotate the gyro.
I needed to slap together a pretend receive signal which pulses at 1.5 ms
per 20 ms with a pot to vary the pulse from 1 ms to 2 ms to provide a
command signal. That's one 556 chip and a few Rs and Cs. Next step is a
heading hold - which I should have bought initially, and some mini servos
that can use this mini rate servo.

Brian W

The transmitter frames are about 50Hz but some of the more expensive
gyro/servo combinations are able to update the servo a few times between
transmitter frames. If I'm not mistaken larger servos don't put a higher
drain on the signal lines but they do pull more amps through the power
wires, that are often routed through the gyro. It's been years since I
looked but I think they have wiring harnesses to deliver servo power more
directly through the battery and the signal wire comes through the receiver
or gyro, as opposed to the power being routed through the receiver or gyro.

Instead of

Power switch receiver gyro servo

they have

Power switch servo
and
Power switch receiver gyro signal from gyro servo

Back maybe 15 years ago I flew with one of the gyro manufacturers and at the
time they were using Tokin gyros, and AVR microcontrollers. I think some of
the newer gyro sensors have less drift problem than the Tokin did. The
drift was at least partially caused by change in gyro output due to change
in temperature. When the model heli is setting in the sun, the gyro gets
hot, when they start flying around the air flow helps cool it off and it
seems to be a good bit of drift during the first few minutes of a flight.
They fought this problem using a thermistor and compensating.

I know it would be more expensive but I thought it would be interesting to
use 2 sensors mounted 180 degrees to each other. That way they should both
drift in temperature together, giving nearly 0 differential output but the
actual signal from motion should be doubled.

RogerN

RogerN wrote:

Moreover (as I find to my cost) a mini rate gyro won't drive a torquey
servo, unless I beef up the output signal.
Brian W

/snip/ If I'm not mistaken larger servos don't put a higher
drain on the signal lines but they do pull more amps through the power
wires /snip/
RogerN


I tried driving a Futaba S3010 with a GWS PG-03 rate sensor.
From a 6 volt cad stack.
The servo drives well with my dummy receiver signal - but just sits
there with the PG-03 in line.
But a scope demonstrates the GW-03 is reshaping the pulse with angular
rate....
So I ordered a Futaba 251 HH...

Brian W