On Thu, 24 Sep 1998, Oyvind Kaurstad wrote:

> >Sensor output ~= 0.3..0.8 mV / (degree * second)
>
> Oh.
> In the datasheet I've seen it says 1.11 mV/deg/sec.
> That means if the rotational speed changes from zero to 1 degree per second
> the sensor will change its output with 1.11 mV, right?

I measured each sensor I used in the final circuit. I have a limited trust
in translated datasheets ;( Also, my sensors were Tokin.

> If I use the MAX 187, I will get 1 mV resolution over the range 0 to 4.096 V.
> That should suffice....

a) Do you plan to accomodate turn rates of ~1845 deg/sec ? (about 300 rpm)
If not, amplify the signal somewhat and concentrate on accurate
directional stability imho (30-50 mV/deg at the AD input is more like
it. This will saturate the system at 40 degrees/sec or so. How fast do
you plan to turn ? ;).
b) Why not use a PIC A/D converter ? A 16C671 will do that, no ?

You have servo pulses coming into the PIC from the Rx. The PIC computes
the PID or fuzzy algorythm for the gyro and then adds or substracts a
value to the incoming pulse width, after which it generates a new pulse to
the servo with the new width.

> The amount of error correction will depend on both gyro gain and how fast
> the heli is turned by the wind.

Yes. But the wind is not the main factor imho, unless you fly between
buildings etc. with a lot of turbulence (which is illegal and dangerous
BTW).

> Does it still sound like I'm totally lost here?

No, but you are looking forward to an interesting few weeks/months.
Closed loop servo systems subjected to un-calculable perturbations from
the outside are fun to tune. All heli tail gyros have controls to be tuned
by the pilot as was said before, and this is not by accident. There is
also in-flight gain control sometimes.

I think that you should invest in one of those articulated strut heli
training stands when you check out the system on the bird.

> Yes, but I suppose I will have to experiment a little with this...

Better start with the theory. It took about 80 years for control theory to
reach the point where planes don't fall out of the sky, motors do not
explode by over-revving etc.

> I have no idea what it's meant to do, but since they use an RC highpass filter
> immediately on the sensor output, the DC component is eliminated.
> The filter has a cutoff frequency of approx. 0.3 Hz.

You want dc coupling and low pass filtering for your application.

> >> >Yes. You can even make the chopper turn slowly if the radio signal is
> >> >lost... which is a very good idea in view of the price of the bird imho.
> >>
> >> But how would you determine for how long it should turn?
>
> >Until the ground stops it or radio liaison is re-established, whichever
> >happens first... ;) This assumes that the heli knows how to auto-rotate
> >w/o help (ok, skip the flare, use a shovel instead, but it's still
> >better than a *sieve* and *magnifiying glass*).
>
> Have you seen a heli that's auto-rotating on its own?

Most sane heli radios I've seen (not many, this is not my hobby) are set
up to select dead center stick, idle engine and very slightly down (?)
collective if radio signal is lost. Am I missing something ? This is not
auto-rotation in the proper sense of the word but it is better than flying
clothes iron style landings. If the coning angle is reasonable then the
bird should fall on the struts at less than 6 m/sec. Thus you can use the
shovel to recover the more expensive parts, as opposed to the magnifying
glass & sieve if it hits real hard...

> And in an auto rotation most model helis will have the tail rotor disengaged
> anyway, so the gyro has no control over things.

I forgot about that. Does the Heim mechanism disconnect the tail with the
centrifugal clutch ?

> Aha. That's not a bad idea, although it will have to have quite a long time
> constant, since pirouetting is a very common thing to play with...

Hmm. If you pirouette so much, why do you need a tail rotor <g> ?

Andy Kunz said that the integration can be used for directional control.
I'd say that it can be used, but with great caution. This type of sensor
drifts a lot and it cannot really be relied on for direction control. A
compass module from Jameco will do this much better imho. It would also be
illegal to use it for this in some countries.

Peter