yaw rates to coincide. Clearly, they usually won't exactly coincide because of all sorts of
disturbances (wind, pitch pumps, etc.), and there will be a difference between desired and actual
yaw rate.
The gain setting determines how aggressively the gyro reacts to a difference between
desired and actual yaw rate
. In the most extreme case of very large gain, the gyro would, when it
detects a difference, pull the servo to the maximum left or right end point in order to bring the
tail as fast back to the desired yaw rate as it's possible. However, this is a bit too aggressive, and
the tail would permanently swing back and forth.
If the gain is too large then the tail will
permanently wag
. On the other hand, in the case of small gain the gyro would very cautiously
change the servo position just by tiny bits, and it therefore takes for long until the gyro manages
to get desired and actual yaw rate to coincide.
If the gain is too low then the tail will respond
very slowly upon disturbances
. Obviously, choosing a gain in between such that the tail responds
quickly yet doesn't wag is optimal.
You can compare the situation to driving a car, with you (the driver) being the gyro. If you detect
a deviation from the desired direction, you pull the steering wheel. If you would do that very
aggressively, i.e., whenever you detect a minor deviation would always pull the wheel as much
as it's possible to the left or right, then the car would go zig-zag. On the other hand, if whenever
you detect a deviation from the desired path you would only pull the wheel very gently then you
most likely would crash into the roadside. So, in between is optimal. You, the driver, are smarter
than a gyro and have learned to adapt your "gain" properly. A heli gyro needs to be told.
6.2. PID I
In heading hold mode we do not want the gyro to only stop any undesired yaw, but want it to in
fact hold the tail in a fixed position. In order to be able to do so, however, the gyro needs a sort
of memory, such that it can recall what the desired position was before a disturbance (wind gust,
pitch pump, etc) occurred. Such a memory is available in heading hold mode, and enabled by the
parameter PID I. As before for gain, the gyro could react aggressively to a difference between
the actual tail position and its memory of it, or could do so slowly.
The value of PID I determines
how aggressively the gyro takes into account its knowledge about previous tail positions.
The
consequences of too large or too small value of PID I are similar to that for gain.
If the value of
PID I is too large then the tail will permanently wag, and if it is too small then the previous
position of the tail will be resumed only slowly after a disturbance
.
In the driving-a-car picture, you may consider driving in a huge desert providing nothing to
orient at the horizon. For orientation you may resort to your sense of balance (= rate mode).
After a disturbance you may get the car back driving on a straight line, but you may not go into
the same direction as before. Alternatively you may watch your compass (= heading hold mode).
However, in either case if you turn the steering wheel too wildly upon a deviation from your
desired path then you will go zig-zag (= gain and/or PID I too large). If you decide to first follow
your sense of balance and only after a while to take into account the compass reading (= gain OK
but PID I too small), then you get back on track but your path would be offset by a bit. Ideally,
you react to both your sense of balance and compass in a balanced way (= gain and PID I OK).
As you see, the consequences of both too large gain and PID I are rather similar, i.e., tail wag,
and it is therefore that adjusting these two parameters is not "simple" in as much as a pure trial-
and-error approach may not lead to optimal behavior.
6.3. Debounce CCW/CW
With gain and PID I adjusted properly, the gyro will do great in rejecting disturbances such as
wind gusts or pitch pumps, i.e., the tail will hold well. However the tail will not behave well
upon sudden piro stops, but show a more or less pronounced back bouncing. Why? Well, if you
quickly move the rudder stick to center you tell the gyro that you want to have the tail stop at
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