Hydraulic Cylinders - Multiple Stop Positions

Sure! Watch an excavator or bulldozer sometime. These machines are operated routinely by 'stopping' the hydraulic cylinders in multiple positions.One at a time of course :). Skies.

It can be done with a proportional directional control valve - see the diagrams on page 5 of this datasheet.

There are linear feedback positioners that can be mounted on cylinders. That will give you linear positioning.

http://www.isecontrols.com/cylinder-positioning.php

I have applied air cylinders on several circuits with 4 posirive, repeatable positions with unequal stroke cylinders mounted Cap to Cap. you can have Home, Pos. 1, Pos. 2 and Pos. 3 that will acyrately repeat until metal wears, usually from fast cycle peening.

Works on hydraulcs cylinders also but usually not neessary if position accuracy is not close tolerance. lose tolernce and infinite positioning requires Proportional or Servo valves and their added expense.

A hydraulic cylinder may be stopped and held in any position by use of a 4 way, 3 position directional control valve with the center position configured to block both cylinder ports and allow flow between the pressure port to the exhaust port. Because the hydraulic fluid is incompressable, the piston in the cylinder cannot move in either direction.

If a precise mid position is required, a tandom cylinder arangement may be used. These are available from many cylinder manufacturers. This type of cylinder is controled by two independant valves. A primary cylinder, the section closest to the rod end, provides the full desired stroke. A secondary cylinder mounted coaxially behind the primary cylinder when actuated provides a positive stop at he desired mid position. This position would be the stroke of the promary cylinder - the stroke of the secondary cylinder.

It is done every minute of every day. Look at a small hydraulic jack, such as would be used to lift one corner of a car. There is nothing requiring that it go to full extension in use, or to full compression for storage. It can be left at any intermediate position simply by closing the main valve; a check valve will hold whatever amount of fluid you pump into the cylinder, even from a partial stroke of the lever, and the jack will remain in that position indefinitely.

This does NOT mean that it is safe to crawl underneath; jacks and their seals can fail, and may do so very suddenly.