Controlling Air driven linear actuators

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jgmdesign
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Posted by jgmdesign: Fri. Jul 21, 2017 - 02:09 AM
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Greets freaks!

 

I have been wanting to do some experimentation with some linear actuators and I am looking for some advice on how to control the actuator position.  I am looking at using an actuator with two air inlets and a lift/pull capacity of 150 pounds.

 

I have been looking at Automation Direct for components, which is rather the easy part.  Finding the device to control the actuator the way I want seems to be the hard part.

 

https://www.automationdirect.com...

 

What I want the actuator to do is move the shaft proportional to the air in the cylinder.  Think of a hobby servo whose position is based on the pulse width of the control signal applied.  That is what I am looking to do.

 

the goal is to have it controlled by an AVR, but baby steps forst.

 

Has anyone done this sort of thing?

 

JIm

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

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valusoft
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Posted by valusoft: Fri. Jul 21, 2017 - 02:32 AM
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Aside: I wonder if I could drive that from my alarm clock to get me out of bed .... hmmmm.

Ross McKenzie ValuSoft Melbourne Australia

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Kartman
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Posted by Kartman: Fri. Jul 21, 2017 - 02:54 AM
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You normally wouldn't use pneumatic stuff for servo applications - which is probably why you're having trouble finding suitable stuff. That's not to say it can't be done, but the usual application is simple push/pull and the position sensors are simple switches.

Hydraulics are normally used for servo applications simply because hydraulic fluid is not compressible vs air that is. Heat the air and it expands significantly - proportionality of the air is not a good method. You can get linear pots to measure the shaft position, but depending on the speed of travel, you could use bang-bang control for low speed (basically pulsing the valve until you get to where you want) or a proportional valve for faster response. I've not seen a proportional servo valve for pneumatics - not that I've actually gone looking.

For moderate loads, electric is probably the cheapest solution, for size constrained or applications requiring significant force, then you look at hydraulics and you start talking big $$$ especially for the servo valves.

 

[edit] Just did a Google and our friends at Festo have a range of servopneumatics. I'd be sitting down when you ask for a price methinks.

https://www.festo.com/net/Suppor...

 

On another note - once you go to pnuematics, then you have the issue of compressing the air - just compressing ambient air has the side effect of condensing the humidity, so you need filters etc to get dry air suitable for control. Otherwise you get corrosion and then headaches. I once repaired a compressor for a Range Rover that has air adjustable suspension. The dessicant only lasts a finite time and if not replaced, then the pump corrodes. The solution was to replace the dessicant and machine up some brass to replace the corroded bits.

Last Edited: Fri. Jul 21, 2017 - 03:02 AM
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Someguy22
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Posted by Someguy22: Fri. Jul 21, 2017 - 04:27 AM
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You can't proportionally control the position of an air cylinder. You can control the speed it gets actuated with proportional valves. Air is compressable, even if you shut down both valves in the middle of the stroke somehow you have just created a shock absorber. 

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Paulvdh
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Posted by Paulvdh: Fri. Jul 21, 2017 - 01:57 PM
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Because of the high compressability of air this is probably not the way to go.

I've seen pnumatic actuators with pretty good positon regulation but that was done by putting 2 cylinders in parallel.

One air cylinder and the 2nd cylinder was oil filled.

The air cylinder pushes the oil around and there were valves to controll the oil flow.

Complicated & messy.

 

Have you thought about "linear actuators" such as:

https://www.aliexpress.com/item/...

These things consist of a DC motor, a gearbox and a spindle.

As far as I know all have built in end switches but they come in a lot of different models Voltages, Speeds, Force, and lengths.

some even have built in encoders.

 

The next step "up" is with a stepper motor and ball screw such as in:

https://www.aliexpress.com/item/...

 

The important factors to design for are:

- Speed.

- Length.

- Force (during hold & movement).

- Accuracy (  Better than 100uM with the ballscrew )

- Reliability / service life ( Those DC motors and the spindels have a limited life span).

    Great for windows (1 or 2 movements a day) but not so great for continuously moving up / down 24/7 ).

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Fri. Jul 21, 2017 - 01:58 PM
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jgmdesign
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Posted by jgmdesign: Fri. Jul 21, 2017 - 02:06 PM
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Thanks for the feedback.

 

I knew air was probably not a good choice, but I has seen it used in the figures in Disney and it appears they were controlling the distance traveled.  I am still going to order an air driven actuator from somewhere just to play with.

 

I knew about Festo, and I have actually been to their offices her by me, but I cannot afford those prices. surprise

 

The stepper motor idea is a great one and I might do something with that as well in the future.

 

Going to have to think about this some more.  Thanks for some great ideas all.

 

JIm

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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keith v
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Posted by keith v: Fri. Jul 21, 2017 - 02:17 PM
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It CAN be done, but it's hard. expensive air cylinders with low friction help, but like stated above the compressibility of air and the stickyness of the cylinders makes life difficult.

A common work around is to use mechanical stops or multiple cylinders.

The mechanical stop can be a ratchet type that indexes on each step to the next position.

 

Pneumatics has it's place for sure and I really like it, but linear motion isn't one of them!

 

a stepper and a leadscrew is way easier to control.

And the lead screw can just be a piece of all thread and a nut, not as good, but really cheap

Keith Vasilakes

Firmware engineer

Minnesota

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jgmdesign
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Posted by jgmdesign: Fri. Jul 21, 2017 - 02:33 PM
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Keep in mind I want to move 150 pounds

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Paulvdh
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Posted by Paulvdh: Fri. Jul 21, 2017 - 02:33 PM
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Do you have a good reason for wanting to use air?

Air cylinders are usually controlled with a 5/3 way valve such as in:

https://www.aliexpress.com/item/...

 

"5" = 5 connections ( 1 air inlet, 2 to both ends of the cylinder, 2 air outlets)

"3" = 3 positions of the valve, cylinder goes in, Cylinder goes out, cylinder stops.

 

The speed of the cylinder can be (somewhat) controlled by a device like this:

https://www.aliexpress.com/item/...

The screw has a conical end to restrict the amount of air flowing through the valve.

These are connected in such a way to restrict the air going out of the cylinder.

That way both sides of the air cylinder have (at least some) pressure in them.

But the max control you can expect will be very springy.  Difference between no load and full load is probably half the length of you air cylinder.

Unless you get an oversized cylinder but even then you will probably have multiple cm (inches) difference between no load & full load.

 

For position feedback you can use a "lineair displacement sensor"

https://www.aliexpress.com/item/...

They look like an air cylinder because of their ruggedness, but have electric wires instead of air hoses connected to them.

The electrical part is just a potentiometer which can be read out by a regular ADC.

 

You might get some acceptable position control if you use such a sensor with a 5/3 valve and active PID control.

But response time will not be to great because of all the air which has to be moved around.

Changes in air pressure makes the whole thing non-liniair wich complicates PID control

 

Another Idea is to use an electro magnet or a clamp actuated by a 2nd air cylinder to hold a certain position.

Possible, but mechanical complicated.

 

My final advice:

Air pressure is unlikely to be a good solution.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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Paulvdh
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Posted by Paulvdh: Fri. Jul 21, 2017 - 02:41 PM
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jgmdesign wrote:
Keep in mind I want to move 150 pounds

150 pounds is not so much. This one is rated for 500kg (about 1000 pound).

https://www.aliexpress.com/item/...

 

Maybe you can add a counter weight?

 

Or even an electric hoist?

https://www.aliexpress.com/item/...

 

What kind of accuracy / precision / speed are you trying to achieve?

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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digitalDan
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Posted by digitalDan: Fri. Jul 21, 2017 - 04:49 PM
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An air-motor driven screw might give decent linear positioning provided that the velocity and force requirements can be met.

Letting the smoke out since 1978

 

 

 

 

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sbennett
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Posted by sbennett: Fri. Jul 21, 2017 - 06:38 PM
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Industrial pneumatic actuators, commonly diaphragm types for valves, have an attached positioner receiving actual position feedback for control. This would be quite difficult to add to an actuator not designed to include it.

 

Reconsider the compressed air part? A satellite dish actuator may work for your application. I bought a SuperJack® HARL-3618+ from All Electronics for about $50 many years ago, still in the box. Operates off 36VDC, a cam with microswitches and diodes limits travel, a single reed switch with rotating disk magnet monitors gear rotations/travel (not quadrature), slow speed like 5 mm/sec. Four leads, two for motor and two for reed switch.

 

Stan

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Paulvdh
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Posted by Paulvdh: Fri. Jul 21, 2017 - 09:56 PM
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sbennett wrote:
Industrial pneumatic actuators, commonly diaphragm types for valves, have an attached positioner receiving actual position feedback for control. This would be quite difficult to add to an actuator not designed to include it.
Not quite true, the "lineair displacement sensor" I mentioned in #9 is relatively easy to add parallel to an air cylinder.

The problem is more in the inherent fricton of cylinders. Those diaphragm types have inherent low friction making PID control easyer / more lineair.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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