"Peter Gottlieb" wrote in message et...
I would think that the "softness" of an air system would limit its
usefulness. I had heard that there were problems with fluid compressibility
and component expansion on prototype high-pressure hydraulics (over 5000
psi) and air is much worse than that.

The problems with the use of compressed air which relate mainly to its
'compressibility' and the 'stiction' of the piston actuators you speak
of have been overcome and highly precise and dynamic pneumatic
positioning systems are now available commerially. These are 'non
linerarites' and modern self tuning and adaptive control systems deal
with them with ease.

A simple PID (Proportinal Integral Differential) controller can
position a pneumatic jack to within +/-1cm. This is clearly not
enough for a flight control surface or a simulator.

This is why pneumatic positioning systems work.
1/ They are closed loop and use a positioning sensor.
2/ The position of the actuator can be used to estimate the
compressibility of the volume in the cylinder.
3/ Use of pressure sensors on the pistons to determin the actual
pressure is obviously expensive so a mathematical model estimates the
pressure.
4/ To get rid of 'stiction' of the piston a 'dither' or samll AC
signal just above resonant frequency is added in to break stiction.
Precision hydraulic use dither as well.
5/ The controller estimates the load and adjusts itself accordingly.

The result is a positioning accuracy of 0.02mm or better.

It isn't even expensive.

Some links for you:
High Steady-State Accuracy Pneumatic Servo Positioning System with
PVA/PV Control and Friction Compensation Abstract
http://robotics.mcmaster.ca/Publications/icra2002_1.pdf

ORIGA-SERVOTEC
Servo-Pneumatic Positioningsystem Series OSP-P
http://www.hoerbiger-origa.com/catal...7_Servotec.pdf

Servopneumatics Can Meet Accuracy Requirements of up to 80% of All
Positioning Applications
http://www.motionshop.com/fluid/pr/festo1.shtml

"Smart" Servopneumatics Positioning Systems
Axis Controller SPC-200
http://www.festo.com/INetDomino/us/e...39005e64b0.htm





"Clark" stillnospam@me wrote in message
...
"The Enlightenment" wrote in news:gF1wb.23785
:

[snip]

I predict incidentally that compressed air servo systems will replace
hydraulics in many aircraft. Safety, economy and ease of service will
promote that.


Ever had one of those air control lines freeze? I have and its not fun...