"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