Pneumatic motion platform myths and ideas.

Hi, I just read many topics from people saying all things from pneumatic platforms, that are not precise, and not encouraging people do to in this way.
Then, I want to publish my position about it, and some knowledge.

When I read these comments or projects, like all projects
are with 5/3 simple valves. To implement a PID control (or simple servo
control ) people try to use this valve in a "PWM" ( talking about the fluid itself
and not the electric control of the valve ). Pneumatic is a fast system,
and good simple valves go to a maximum as 15ms to open and close, so
the maximum switching frequency you get is ~65Hz. This is extremely low,
and even, impractical, as you need 100x or 1000x more than this.

The solution, as the industry know for years, and I dont know why sim
builders don't go this way (about later), is proportional 5/3 valves.
Proportional directional valves control the flow of the fluid, precisely from
0% to 100%, the majority can position the spool in any flow option in
~5ms.

The fact of precision with this ( that of course is dependent on other factors,
like the actuator feedback and PID control ) are this:

Positioning precision of ±0.032 in or ±0.81mm ( source: http://www.hydraulicspneumatics.com/...lse/7223/Issue )

This is an exaggeration to a motion platform, and we can be very happy
with lower values from lower priced valves ( Of course this precision is reflected in velocity control to, resulting in good washout ).

Velocity and force dont need to be explained, but, just for complementing:
A cylinder with 2 inch internal diameter, result in an ~141 Kgf actuating force.
A cylinder with 5 inch internal diameter, result in an ~883 kgf actuation force.
(Calculated considering 100psi pressure and ignoring friction, a think all professional and some hobby compressors today go to values equal to 116psi or greater.)
Only for curiosity, a 5 inch internal diameter cylinder with 116psi pressure result in an ~1024 Kgf actuation force. Sufficient for a real Boeing 747 cockpit ;)

Other solution, a bit new to industry, is hydro-pneumatic control.
Knowing some more advanced products, I draw this idea.

http://img6.imageshack.us/img6/7808/...eumatic.th.gif

This is an tandem cylinder, its like two cylinders with one rod.
The orange part is actuated by pneumatic, equal to a pneumatic
cylinder. The blue part is filled with hydraulic fluid, and have a
flow control valve, that controls the passage of hydraulic fluid from
the cylinder.

So, this is in fact, a pneumatic cylinder with an hydraulic brake.
You can control the pneumatic cylinder with a simple 5/3 valve,
but the brake cylinder is that control the rod, just opening and closing
the flow control valve, that can be a ball valve actuated by a small motor.
You don't need so fast response because the hydraulic fluid is like incompressible, giving more predicated response. Air is the inverse. So,
when the hydro valve is closed, even external force (like anyone
jumping in the cockpit ;) do not move the cilinder. In an only pneumatic
cilinder, the proportinal valve and the PID control need to be fast to
compensate the external force, what is possible with no problem, too ).

Only to be clear, I don't invented the hydro-pneumatic cylinder, just
think in the idea of using the principle, on tandem pneumatic cylinder.

Some links:

Enfield Technologies, very good about pneumatic servo control:
http://www.enfieldtech.com/
http://www.youtube.com/watch?v=Bk6eLsqvBxc
(A good video from Enfield Technologies showing anything like a 3 dof
plataform ;))

Good page with pneumatic myths:
http://www.hydraulicspneumatics.com/...lse/7223/Issue

Thanks,
Diego

Good post. I'm an Automotive Mechanic and for several years I dealt primarily with automatic transmission diag and repair. I often wondered why the hydraulic and pneumatic motions systems don't use duty cycle or variable rate solenoids. It would be simple to convert an analog DC voltage signal to a duty cycle or frequency signal. Then pressurize both sides of the cylinder piston and use pressure release as the control. I drew something up several years ago using a single hydraulic pump, 4 flow equalizers, 4 duty cycle solenoids, and 2 bi-directional slave motors. I never built it so I don't know for sure if it would work but it should.
John

Doon1,

If you have budget to buy and support the maintenance of a hydraulic system,
I don't think you will have much problem with positioning without proportional
valves.

But with pneumatic, if I know what you said, the simple valves I know are to slow in response to do a good control. The way you said is the way people
try to do ( as simple valves are so inexpensive compared to proportional ones ),
but as the response is slow, external forces can disturb the system and the
valve could not respond accurate, causing over-shoot or the inverse.

Compared to hydraulic systems, pneumatic have near zero maintenance, are much less dangerous (for a not well trained people of course), and the best cost/force and speed for the range required for motion platforms.

I think you know this, its just for complementing the reply ;)

Hi,

where go the schematic for the hydro-pneumatic idea:

http://img404.imageshack.us/img404/2...umatic2.th.gif

As its getting a bit difficult to find tandem pneumatic cylinders here, and
using parallel cylinders would require 3 cylinders for each DOF, I'm trying a
pneumatic with variable magnetic brake (replacing the hydraulic part),
or dynamically pneumatic balanced, with linear motor acting.

This would need low force on the linear actuators, so I can use a ratio
based on speed and not on force.

When I have news I will try to post where with some results, but first,
I need to get my Cessna CJ cockpit up ;)

Quote:

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Originally Posted by diegocn

or dynamically pneumatic balanced, with linear motor acting.

---

Hi Diegocn,
Have a look at the HI6SIM MKIV idea.
Douwe

Quote:

---

Hi Diegocn,
Have a look at the HI6SIM MKIV idea.
Douwe

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Hi Douwe.

I knew your site from months or even years. I think I have found it when
searching for home made electric linear actuators. Its really a nice project.

I'm planning my sim for years, and I think that you too, the ideas we have
are a bit close ;)

The difference ( as I can find on your site ), is that you want a manually
balanced, so when in flight, the balance is the same.

I'm thinking in "dynamically" balanced, with pneumatic valves,
so the idea, is getting near zero force on the electric actuators
wherever is possible.
As nut and screw electric linear actuators work as a good break by the
friction, the idea is more like this.

The other idea is like your drawing on the MKIV, but instead of using a
motor with reduction, I think on using a brake with a disk, by friction
or electromagnetic.

Diego

Quote:

---

Originally Posted by diegocn

Hi,
The other idea is like your drawing on the MKIV, but instead of using a
motor with reduction, I think on using a brake with a disk, by friction
or electromagnetic.
Diego

---

A reduction cam problely left out when using direct drive motors ,
(Torque, Hub (e-bike) or pancake).
Douwe

Of course, the speed reduction, is just a impedance matcher, for the
speed/force impedance.

What is the maximum load you are planning for your motion base?

About 150 Kg,
for a glider cockpit and pilot.
Air pressure for the 3 bicycle pumps 50 : 9,5cm2 = +/- 5 ato.
Douwe

You can look up some other ideas at Pneumatic logic