[simpits-tech] Using real instruments (long)

[Jim S]

Just some observations on using RC servos for the altimeter.

I tried this last year. I took a triple-needle Kollsman altimeter
and stripped out the bellows and gear quadrant part and replaced
them with a servo system. I took the electronics out of a standard
RC servo, added my own motor with a 96:1 gearbox to drive the
100ft needle directly. I then used a precision 10-turn potentiometer
geared by 5:1 to the 100ft drive. This allowed the servo electronics
to drive the 100ft pointer over a 50 turn mechanical range. I left the
gearing for the 1,000ft and 10,000ft needles in the framework. I then
added a couple of trim potentiometers to the main 10-turn so that
I could limit the electronic range given by the standard drive pulses
to only 40 turns. I offset the pointer by -500 feet because I live in
Holland and needed below-sea-level capability. Using a 12-bit digital
source for the drive pulses (4096 steps), I ended up with an altimeter
that went from -500ft to +40450ft in 10ft increments. This actually
worked over the range quite well with a couple of drawbacks, which is
why this mailing is so long. I wanted to set the scenario first so that
you can better understand the problems.

Problem 1; these servos actually run at two speeds. When they are within
a certain range of the target position they go into a "creep" mode and slow
down so that the target position is not overshot. This gets amplified by the
mechanical gearing and ends up as a 500ft slow-down either side of the
target altitude. This is really very slow and not normally seen in the standard
non-modified servo.

Problem 2; the servos have a "dead zone" where they will not move. This
is to prevent the servo from twitching about the target position. This dead
zone also gets multiplied by the mechanical ratio and ends up at about
50ft either side of the target. This means that you have to change altitude
by at least 60ft or more before the servo electronics recognise a change.

The electronics in most standard servos (esp the Hitec) are based on a
custom IC from the manufacturer. This gives little or no adjustment that
the user can play with. The net result is that to resolve both problems,
you need to specially design your own servo control electronics. Something
which most of us would be pushed to do.

The best that I can say is that servos are OK for instruments that require
less that 360 degrees of rotation on the instrument, but become more and
more of a problem for multiple rotations.

However, I think that I have found a solution. I was lucky enough to have a
look at a prototype altimeter from a new company. This uses stepper-motors
for the pointers. The movement was beautiful, smooth and accurate. I found
out that the resolution was 1 foot with a range up to 99,999 feet. The 
altimeter
could handle changes up to about 30,000 feet/min. I decided there and then
that I wanted one. I found their website at www.sim-instruments.com . It might
be worth looking at as I think that they are going to market a range of 
different
instruments covering both commercial and military aircraft. I asked about
pricing and was informed that the instruments would be "within the hobbyist
range". I don't know exactly what that means, but it will be interesting to 
find out.

Sorry for the long mail, but there has been a lot of mails about converting
real instruments and the associated problems and complex solutions that
I thought that I would share some of my experiences.

Jimbo.