RE: [Autopilot] Hello. (Glider system)

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Hello,
  From what you have said, it sounds like you have a good plan.  I would
have to say that thermal flying would be the easier in terms of a flight
controls and navigation point of view.  I too fly sailplanes and have flown
both thermal and slope, but mostly thermal.  A simple GPS receiver, even in
differential mode, will not have the precision required for slope soaring.
It would be good enough for thermalling.
  Barometric altimetry can be very good, but it is all in the analog
circuitry.  A pitot tube will be fine for flight control.  It is important
to use a very low-pass filter on this input to reject wind noise.  Also,
play with different tube fronts.  A prop would be too much drag.  If the
sailplane is very stable, polyhedral for example, then attitude stablization
is not required.  A yaw-damper is a good thing to have though.  
   Most of all, my only suggestion is to start simple and work up from
there.  

take care,
Aaron Kahn

-----Original Message-----
From: Nick Shaw
To: aut...@li...
Sent: 10/28/2002 9:26 AM
Subject: [Autopilot] Hello. (Glider system)

Dear all,

I have spent the weekend gleaning information from the web about this
project and thinking about my own implementation.
These are a few of my thoughts, interspersed with some details about
myself.
The first important detail is that I will be installing my system in a
glider rather than a helicopter.
Whether or not this makes the implementation easier, remains to be seen,
but I hope that by using the same components as for the
helicopter control system, that reasonable comparisons can be made.

From a first principles point of view the inputs to the control system
are all important, so I wrote down the following list
(comments are welcome)...

1.Barometric altitude (above MSL) using a gauge pressure sensor.
2.Air speed using a Pitot tube and a differential pressure sensor.
3.Electronic magnetic compass (to give instantaneous bearing when
horizontal). Could be automatically set/calibrated using input
from DGPS (differential GPS) when in motion? Used to augment the
inertial navigation system.
4.Thermometer - very short time constant (maybe fast enough to measure
warm air once centred in a thermal). In any case, could be
used to calibrate sensors.
5.DGPS (WAAS is not available here in the UK) - most GPS I have seen
only update their info. every 1 second. Therefore I would use
GPS to give absolute position (x,y). Altitude information from GPS is
very poor, but could be used to recalibrate barometric
altitude for very long flights. Also get the absolute ground velocity
from the DGPS. Finally, they seem to output the error of the
measurement - which is IMPORTANT. Used to augment the inertial
navigation system.
6. Voltmeter - measuring for low batteries.
7. Ammeter - for interest and checking that there is not too much
current drain if servos are stalled.
8. Precalculated polar curves for the glider (stored in memory).
9. Sonar range finder (height above ground). This could be used to
create its own digital elevation map. (Maximum distance is rather
short though).
10.Most tricky - the attitude system. I would have thought  the best is
using 3 gyros and 3 accelerometers. Is this the plan for the
next version?

I fly my radio controlled gliders both on the slope and thermal soaring
from flat fields. These two activities require different
inputs to the control system. On the slope, you have a quasi-static lift
field - which needs to be evaluated and then stored in
memory, along with a digital elevation map - so that you don't keep on
hitting the slope.
For thermalling off the flat, the need for a digital elevation map
disappears, replaced by the need to circle for long times in a
coordinated turn (bad news for the inertial navigation system).

I imagine having several different flight regimes:
Normal - i.e. cruising flight either at minimum sink speed or best glide
speed.
Aerobatic - simple acrobatics should be feasible with a good inertial
navigation system. I'm thinking of loops and stall turns for
the moment (same controls as a helicopter)
Searching for lift - either slope lift or thermal lift - using
information from all of the inputs.
Landing - needs to know the ground level, wind velocity and lie of the
land.

For a glider, there are also many factors that can be calculated from
the inputs and used to gain height better. These include -
Variometer (including total energy compensation)
Glide efficiency and optimal glide speeds, McCready values.
Final glide speed computation for competitions.

So those are my thoughts at the moment. Can I now ask a couple of
questions...

1.Does the above sound like dreaming or is it a realistic aim?
2.Am I right thinking that there is no consensus at the moment about the
gyros and the accelerometers to be used in version 3?
3.Are there going to be 3 gyros and 3 accelerometers in version 3?
4.Does anyone know if a Pitot tube with a differential pressure sensor
is better (accuracy/stability) than an air speed propeller?

For the record, my background is in Physics, with a mathematical
leaning. I have been programming in C for the past 4 years and am
currently writing up my PhD thesis on genetic algorithms and MRI design
of the main B field. I also hold the world duration record
for model flying (36 hours) which I broke last September (see
http://www.rc-soar.com/wr_duration/index.html).

Cheers,
Nick.

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