[PSUBS-MAILIST] Pneumatic Manipulator

Sat Mar 17 08:05:45 EDT 2018

 Alan,I did some experimenting with air cylinders and started testing joints with mechanical resistance to reduce or stop the bounce.  I made up disks that went between the arm joints that could be tensioned to act like brake disks.  Kinda like a motorcycle clutch.  This does work to an extent but counterproductive from an energy conservation point of view.With your knowledge of brushless motors and controlling them, it would seem your direction should be to modify a Lenco actuator by removing the motor and replacing it with a brushless motor to oil fill it.   There is one small challenge thought, and that is to relieve the oil trapped behind the threaded rod.  The threaded rod spins inside a fixed nut with very tight tolerance.  the tight tolerance may not let the oil flow when a void is created by the rod extending.  I started to drill holes in the ss rod to create an oil passage way but stopped for fear of creating a bigger problem thus holding up the project.  If you want to collaborate on making an oil filled actuator by converting a Lenco actuator, I would be in.If your concern with these actuators is the short stroke, don't be.  The arm can be designed with stroke multipliers, that is already in my plan for the next generation arm.Hank

    On Saturday, March 17, 2018, 3:54:24 AM MDT, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Thanks Steve,I thought there might be a solenoid valve that gave a good variable control overthe flow.I did experiment quite a bit with linear actuators, but thought pneumatics mightbe a lot simpler. The commercial deep sea linear actuators are oil filled & use brushless motors. As I found out, the small brushed motors don't run so well inoil. They have external oil reservoirs to compensate for the piston movement &an overpressure. Can always re-look at them.Cheers Alan

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On 17/03/2018, at 3:58 PM, Stephen Fordyce via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

Hi Alan,I've done a little with pneumatic cylinders and I think it will be very difficult to achieve any kind of accurate positioning. Pneumatic is fine if you only need a cylinder to be fully extended or retracted (as is used for industrial automation) but anything mid way is likely to need a complicated feedback/control system.
Even considering as a perfect system, if you have varying force/load, to avoid movement you must have an equally varying pressure in the cylinder. Increasing pressure in the cylinder doesn't help because you then need to increase pressure in the opposite side and then the tendency to resist a disturbing force is not much different anyway. (And if using a single rod cylinder, the pressures in each end need to be different to achieve the same force due to area of the piston) Throw in real world considerations of static friction and dynamic friction and it's harder again to reach and hold a desired position.
I'm a bit surprised you haven't already ordered a cheap pneumatic cylinder for some concept testing :).  That should give you a feel pretty quickly if it's doable or not.
Cheers,Steve Fordyce
On 17 Mar 2018 7:27 am, "Alan via Personal_Submersibles" <personal_submersibles at psubs.org> wrote:

Have thought a lot about pneumatic manipulators.
Hank has commented several times that they are too "spongy".
If you had pressure relief valves or similar in the system set at something
like 30 psi, so there was always a minimum of 30 psi either side of the piston;
would this make it a lot less "spongey".
You would now need to put an extra 30psi in to get the same force.
It would mean more air, but it is not likely you would be using the manipulator
every dive, & it would be a small quantity compared with blowing a ballast tank.
Has this been done before?
Cheers Alan

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