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Originally Posted by Andrew Blair
Darn, you're right. I didn't consider this. Perhaps if a flow control was used you could slow the air loss on the open side long enough to correct, then close it again. I'm considering an arm load where most of the time the load force is enough to allow control by opening only the lifting side for a moment. However, the arm itself wouldn't be heavy enough to lower the arm if it's not carrying something, so you would have to force it down using the other isde of the cylinder. Any ideas?
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The best thing to do would be to use something like a gas spring to pull the thing down and use a special type of 3-way valve called a 3 position closed center double soleniod valve.
this valve would allow you to alternate one side of the cylinder between pressurized, sealed off and vented while only using a single valve. When neither soleniod is energized, the output is closed off. When one of the coils in energized, the output gets pressure. When the other is pressurized, the cylinder is vented. I did some tests with this sort of setup and it worked pretty well. The biasing spring also leads to increased stability when lifting light loads. You could also use a 4-way version of one of these if you want to control both sides of the cylinder.