View Full Version : Pneumatic Servo/Postion Control

04-29-2010, 05:09 PM
I have been searching the net for stuff to this kind of thing and can't find much, or anything I can understand.

What I want to do is be able to control a pneumatics cylinder's position using the signal that a normal servo would use. Yes it is possible, Disney does it all the time in their animatronics.

I just want to know where can I get stuff like this, or even how can I do it.

The Mad Hatter
04-30-2010, 12:38 PM
Well to give you a heads up Disney uses hydraulic Cylinders to control their Robots, with hydraulics you can control the piston position by the amount of hydraulic fluid that is pumped into it. Controling these devices and be very very difficult and EXPENSIVE!

Now their are things called Actuators that look like pneumatic cylinders that extend when a coil unravels in the inside. The problem with most actuators is that you would need to buy one that extends fast for robotics unless you want it to move like a snail. This can be a price issue specially when you go to buy big Actuators. You would also need to buy a drive that can range anywhere from a few hundred to a few thousand.

Stay with a pneumatic cylinder all the way up or down...... You can never control a pneumatic cylinder how you say you want to, because there is an Air compression issue!! You cannot defy the laws of physics!

Hope this helps,

Frank Balzer

Allen H
04-30-2010, 12:58 PM
Why not skip the cylinder and use a liner actuator that does the same thing and can be treated like a servo? Would this work? I hope this helps.
Allen H

05-03-2010, 01:21 PM
For now I will probably stay with basic pneumatics, I was just wondering what it would cost. I did find one place in my travels that uses servo valves with a position sensor and a micro controller to control pneumatics but it was close to $1500 per unit not including the cylinder.

Those linear Servos are real cool, I may try one out to see how it performs, especially that 2inch per second one.

One the other hand to set the record straight, Disney does use both pneumatics and hydraulics in their A-100 animatronics. However they are only used in situations where fluid could leak onto a guest in the case of a malfunction, such as the Enchanted Tiki Room, Under New Management.

Thanks anyways guys.

Allen H
05-03-2010, 01:30 PM
I didnt mean to offend you at all, sorry if I did. Im actually just learning servos and did not know if you were aware of the linear servos. I have a bit of a bias to pneumatics and try to avoid them when
I can.
Allen H

05-03-2010, 03:41 PM
Didn't mean to give you that impression. I didn't know about linear servos, so thanks. I was just pointing that Disney does indeed pneumatics not just hydraulics, and position control is possible to the mad hatter.

Thanks for that idea about the servos. I just wasn't sure about using a servo because of the low power output.

In Fact, if only this stuff was cheaper: http://machinedesign.com/video/proportional-control-of-pneumatic-cylinders-0311

Allen H
05-03-2010, 05:45 PM
I think the key words there was "pneumatic cylinder potentiometer" When he said that in the video it perked my ears. I googled that and found this
Could that help you?
I also really seem to remember seeing a display exactly like that one in the video from guilderfluke at Iaapa a few years back. You might show that video to someone from there and see if they can help you out. I wish I knew more and could help you further.
Allen H

The Mad Hatter
05-06-2010, 10:41 PM
Holy Crap, I take that Back. This is Really Cool! Thank you for showing this... Someone from my shop told me you can have precision when controlling pneumatics! I will show them now. I know now what system I will put on my 89 Movement robot now! thank you

Frank Balzer

05-07-2010, 08:46 PM
Before you get carried away with this stuff mad hatter. Just remember that for each cylinder you plan to have, it will cost around 500 bucks per cylinder setup to control it. Also, they do not yet make multi cylinder cards so the space required for control electronics it much greater than regular valves. Have fun though.

The Mad Hatter
05-10-2010, 01:30 PM
I will have lots of fun with this system, thanks again for enlightenment on this subject!

06-09-2010, 02:06 PM
The machinedesign.com video, showing proportional control of position for pneumatics, is very similar to what's going on inside a hobby servo. The idea is to feed back an error signal, which is the difference between the reference / set point (where you want it) and the output (actual position). The error signal is multiplied with the reference to shift the output, and this reduces the error until the position is correct.
Hobby servos have all the electronics and actuators built in, but in pneumatics, you've got to link up the valve controller (reference) with the cylinders (output)- as these are separate systems. Additionally, you have to have a sensor on the cylinder to determine the actual position for calculating error.
Ok, so say you have all of that. Now how do you generate a reference? Say you want the cylinder to extend 25%, if you try to do pulse width modulation (http://en.wikipedia.org/wiki/Pulse_width_modulation) (on for 25% of the time, off for 75% of the time), you're going to run into the devastating compressibility of air that the Mad Hatter was talking about. I found this out the hard way.
One way around this is to use a voltage to pressure transducer, like the Bellofram T-1000 (http://www.bcgroupintl.com/Process/Bellofram/IP/T1000.htm). Similar to a typical pneumatic valve, but it's regulator that can provide a set pressure, rather than just on & off. Now your reference is a pressure value, and your electronics need only increase the voltage (or current - different model) until the cylinder position is where you want it. I haven't seen anyone build this particular setup, but I think it would work.
Worth noting that this setup would only work for mechanisms that require more force for more extension, but that's typically the case, anyway. I've bought the T-1000s on ebay for about $80 each.

06-09-2010, 08:28 PM
The problem with trying to use pressure to regulate position, is that normally in animatronics it is not a controllable ratio. Most of the time when there is enough pressure for they cylinder to begin moving, it will continue to do so. This could potentially work if the load was directly centered above the force of the cylinder.

06-10-2010, 08:13 PM
You're right. It would depend on the mechanism it was controlling. For this setup, a single-acting cylinder (spring return) would probably cooperate better, too.