Abstract:
A multi-stroke air cylinder providing a precisely directed and controlled stroke in the face of lateral, torsional and tilting loads on a tooling plate. The multi-stroke cylinder utilizes a plurality of mechanically linked pneumatic or hydraulic pistons having different stroke lengths that can be added together in any combination, allowing the user to select any stroke length up to a predetermined, total combined stroke length, in increments equal to the stroke length of the smallest cylinder. The multi-stroke cylinder includes a head assembly having a fluid inlet for introducing fluid to the cylinder at a first pressure. The cylinder also includes a first positioning system having a plurality of pistons capable of moving a piston rod away from the first positioning system, and a second positioning system located between the head assembly and the first positioning system. The second positioning system comprises a plurality of movable pistons for displacing the piston rod a preselected distance and at least one elongated fluid supply member secured to a respective one of the pistons of the second positioning system for introducing a fluid between adjacent pistons. When a plurality of fluid supply members are used in the second positioning system, they are concentrically arranged and are at least partially coextensive with one another.

Description:
RELATED APPLICATION 
   This application is a continuation of U.S. patent application Ser. No. 09/750,092, filed Dec. 29, 2000, now U.S. Pat. No. 6,651,546, the full disclosure of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to a multi-stroke linear actuator capable of achieving a predetermined number of discrete positions, more particularly, it relates to a linear actuator for accurately moving a tooling member a preselected distance. 
   BACKGROUND OF THE INVENTION 
   Many conventional devices are known for guiding and positioning a tool or an element, such as a parts gripper, with respect to a work piece. These devices range from simple hand-operated mechanical devices to more accurate and automatic, fluid operated devices in which the tool can be located in numerous positions by controlling the pressure and amount of the fluid. Such devices are commonly used in a variety of environments to perform a multitude of work functions such as the pick-up placement of parts in assembly lines, and the positioning of work pieces or tools for operations such as punching, drilling, printing, clamping and so forth. The devices can also be used to position individual parts for automatic assembly, etc. In each of these jobs, repetitive, precise and accurate movement in the face of undesired external loads is essential. 
   Pneumatic and hydraulic operated fluid devices accomplish movement of a tool or work piece by a power mechanism acting on a tooling plate. One conventional power mechanism includes a double action piston located within a cylinder and integrally connected to a piston rod. Pneumatic or hydraulic pressure is applied to either side of the piston so that a pressure differential is created across the piston. The differential pressure in the cylinder controls the location of the piston. It causes the piston to displace within the cylinder until the force on both sides of the piston is equal. The displacement, or stroke, of the piston rod is generally limited to the distance the piston can displace within the cylinder. This type of a system can be disadvantageous if the fluid medium is compressed air and the piston is floating in the cylinder and finally positioned by equal fluid forces being established on opposite sides of the piston. In heavy machine tool work, the forces created between the tools and the work can add to the force on one side of the piston within the cylinder, upsetting the equilibrium and throwing the tool out of alignment. 
   One manner of overcoming this disadvantage has been to utilize a plurality of fluid-actuated cylinders, such as hydraulic cylinders that do not rely on the establishing of equilibrium pressure. These cylinders have piston strokes of varying lengths and are stacked in an end-to-end relationship to provide a more rigid connection between the controlled tool and the positioning device. Such a device is disclosed in U.S. Pat. No. 3,633,465 to Puster. The actuated pistons disclosed in Puster slide the cylinders a distance that is equal to the sum of the stroke lengths of each actuated cylinder. Sizing the cylinders so that each has a different stroke length allows the device to achieve a large number of positions. Conventional multi-stroke, actuated cylinders are not laterally stable and occupy an excessive amount of space during use. In addition, many of these conventional actuators utilize position feedback mechanisms for insuring the accuracy of the positioning of the tooling plate. Typically, these feedback mechanisms include sensitive electrical feedback loops that can cause radio frequency interference with the power and fluid control mechanisms. Also, the use of electrical feedback or position control mechanisms can require shaft encoders that impose a risk of sparks or shorts, thereby creating explosive or otherwise hazardous conditions. 
   It is an object of the present invention to overcome the disadvantages of the prior art. It is also an object of the present invention to provide a multi-stroke cylinder capable of accurately achieving a large variety of positions without the use of a position feedback mechanism. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a multi-stroke air cylinder that provides a precisely directed and controlled stroke in the face of lateral, torsional and tilting loads on a tooling plate. The present invention can use binary techniques or combinations of stroke increments to provide a precise positioner utilizing pneumatic or hydraulic power that provides accurate positioning of a tool without requiring or using position feedback mechanisms. Also, the air cylinder is laterally stable so it can be used in areas such as woodworking, apparel manufacturing, building materials, housing construction and other similar arts. 
   The present invention utilizes a plurality of mechanically linked pneumatic or hydraulic pistons having different stroke lengths that can be added together in any combination, allowing the user to select any stroke length up to a predetermined, total combined stroke length, in increments equal to the stroke length of the shortest stroke piston. For example, if the invention included four pistons having stroke lengths of one inch, two inches, four inches and eight inches, the user can select any stroke length in increments of one inch up to a total combined stroke length of fifteen inches. A three inch stroke would be obtained by extending the one inch stroke piston and the two inch stroke piston. A seven inch stroke would be obtained by extending the one inch stroke piston, the two inch stroke piston and the four inch stroke piston. The activation and extension of all of the pistons would achieve a fifteen inch stroke. The present invention also includes a plurality of pistons that can move the tooling plate by a fraction of an inch. This fractional movement can be added to the movement of the pistons having full inch increments so that positions in increments of the smallest fraction of an inch can be achieved up to the aggregate stroke length of all of the pistons. 
   The multi-stroke cylinder according to the present invention includes a head assembly having a fluid inlet for introducing fluid to the cylinder at a first pressure. The cylinder also includes a first positioning system having a plurality of pistons capable of moving the piston rod away from the first positioning system. A second positioning system is located between the head assembly and the first positioning system. The second positioning system comprises a plurality of movable pistons for moving the piston rod a preselected distance and a plurality of fluid supply members which are each secured to a respective one of the pistons of the second positioning system for introducing a fluid between adjacent pistons. The fluid supply members are concentrically arranged and are at least partially coextensive with one another. The disadvantage previously discussed concerning differential pressure pistons does not occur with the present invention because an equilibrium is not established. Instead, low pressure used to maintain the rest position of the pistons is expelled from the cylinder of the second positioning system as the piston is moved by the higher pressure introduced through the fluid supply members. 
   The first or “fine” positioning system utilizes a plurality of positioning stages having increments of movement in 1/16 of an inch intervals up to a total of 15/16 of an inch. The smallest of the different sized stages is 1/16 of an inch. The second or “coarse” positioning system has increments of movement set in one inch intervals up to a total of fifteen inches. In this system, the pistons would be set to extend at different lengths with the smallest stage length being one inch. By activating the coarse and fine positioning systems, the tooling plate of the present invention can be positively positioned in as many as 256 individual positions. If an additional stage capable of 1/32 of an inch were added, the number of discrete positions that could be achieved would be doubled to 512, thereby increasing the accuracy of the multi-stroke cylinder. Similarly, adding another stage capable of 1/64 of an inch movement could again double the accuracy while quadrupling the original number of discrete positions obtainable to 1024. 
   The present invention accurately positions the head of a piston rod or other similar devices such as a tooling plate in one, two or three planes by activating one or a plurality of pistons within a cylinder. Valves control the flow of the fluid medium within the cylinder and between the pistons. The head of the tooling piston or plate can securely and accurately carry any number or types of tools for performing an application on a work piece. For instance, by attaching a drill, the user could accurately drill a hole anywhere in an X-Y plane to a depth of Z and repeat the same controlled drilling depth at a second location. Alternatively, the hole could be drilled to a different depth at the second location. By attaching a parts gripper, the operator could retrieve a part from a known inventory position and place it accurately in an assembly a predetermined distance away. The present invention allows these applications to occur without the forces generated at the work piece affecting the position of the head of the piston rod. 
   Unlike conventional multi-stroke actuators and their related methods for carrying out the above discussed tasks, the embodiments according to the present invention do not require a feedback mechanism to insure the positioning accuracy of the tooling piston or plate. Selecting the proper combination of valves insures that the piston rod moves positively to the selected position. An additional advantage arises from the exclusive use of fluid power to carry out the positioning, thereby eliminating the necessity of employing electrical counters or shaft encoders which impose the risk of sparks or shorts in explosive or otherwise hazardous conditions. Furthermore, the present invention is completely free of radio-frequency interference since no sensitive electrical feedback loops are required. The multi-stroke cylinders according to the present invention are also compact in size and laterally stable so that they are able to be used in a variety of locations for performing many different operations. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a schematic view of a multi-stroke cylinder according to an embodiment of the present invention; 
       FIG. 2  is a schematic view of the multi-stroke cylinder shown in  FIG. 1  with the stages in an extended state; 
       FIG. 3  illustrates the second positioning system according to the embodiment shown in  FIG. 1  at rest, without the cylinder; 
       FIG. 4  illustrates a cross section of the back plate and pistons of the first positioning system according to the embodiment shown in  FIG. 1 ; 
       FIG. 5  illustrates the back plate and pistons of the first positioning system according to the embodiment shown in  FIG. 1  in an extended state; 
       FIG. 6  is a schematic view of the first positioning system shown in  FIG. 5  at rest; 
       FIG. 7  is a schematic view of a multi-stroke cylinder according to another embodiment of the present invention; 
       FIG. 8  is a schematic view of the multi-stroke cylinder shown in  FIG. 7  with the stages in an extended state; 
       FIG. 9  is an end view of the multi-stroke cylinder according to  FIG. 7 ; 
       FIG. 10  illustrates the connection between the pistons and fluid supply tubes of the embodiment shown in  FIG. 7 ; 
       FIG. 11  is a schematic view of another embodiment of the multi-stroke binary cylinder according to the present invention; 
       FIG. 12  is a schematic view of the multi-stroke cylinder shown in  FIG. 11  with the stages of the first positioning system in an extended state; 
       FIG. 13  is a schematic view of the multi-stroke cylinder of  FIG. 11  with the stages of the first and second positioning systems in an extended stroke; 
       FIG. 14  is a schematic view of the tethered pistons of the first positioning system and second positioning system housing; 
       FIG. 15  shows the pistons of the first positioning system about the second positioning system housing; 
       FIG. 16  shows a surface of the back plate according to the embodiment shown in  FIG. 11 ; 
       FIG. 17  is a schematic view of another embodiment of the multi-stroke cylinder of  FIG. 17  with both positioning stages in their fully retracted states, according to the present invention; 
       FIG. 18  is a schematic view of the multi-stroke cylinder shown in  FIG. 17  with both positioning stages in their fully extended states; 
       FIGS. 19A-C  schematically illustrate a stroke piston as shown in  FIG. 17 ; 
       FIG. 20  illustrates the first stage positioning system with all pistons in their retracted positions as shown in  FIG. 17  but with the cylinder wall removed for better clarity; 
       FIG. 21  illustrates the first stage positioning shown in  FIG. 20  but with all pistons in their extended positions; 
       FIG. 22  is a schematic view of the second stage positioning system with both the 4″ stroke and the 8″ stroke pistons in their retracted positions but with the cylinder wall removed for better clarity; 
       FIG. 23  illustrates the second stage positioning system shown in  FIG. 23  but with the 4″ stroke piston in its extended position; 
       FIG. 24  illustrates the second stage positioning system shown in  FIG. 24  but with both the 4″ stroke and the 8″ stroke pistons in their extended positions; 
       FIG. 25  schematically illustrates the second stage positioning system shown in  FIG. 23  with the enclosing cylinder tube removed; 
       FIG. 26  schematically illustrates the second stage positioning system shown in  FIG. 24  with the enclosing cylinder tube removed and the 4 inch stroke piston extended; 
       FIG. 27  schematically illustrates the second stage positioning system shown in  FIG. 25  with the enclosing cylinder tube removed and with both the 4 inch and 8 inch stroke pistons extended; 
       FIG. 28  illustrates the multi-stroke cylinder as shown in  FIG. 18  but with a color coded Legend which shows the placement of the various seals and bearings; 
       FIGS. 29A and 29B  illustrate the multi-stroke cylinder shown in  FIG. 18  but with the input air manifold assembled to the top of the main housing; 
       FIG. 30  depicts a bottom view of the air input manifold plate showing the grooves which channel compressed air from the plumbing connections to the piston input orifices atop the main housing; and 
       FIG. 31  is an end view of the air input manifold plate of FIG.  30 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A multi-stroke air or hydraulic cylinder according to the present invention is shown in FIG.  1 . This invention utilizes floating, tethered power pistons interconnected in such a manner as to cause an output piston rod  189  to move a distance equal to the sum of all the distances moved by each of the individual pistons.  FIG. 1  schematically illustrates the multi-stroke cylinder  100  in a fully retracted condition.  FIG. 2  illustrates the multi-stroke cylinder  100  with its stages, pistons, in a fully extended condition. The first positioning system  110  includes four pistons having fractional stroke lengths (fractions of an inch) located within an annular cylindrical housing  120 . A second positioning system  150  includes four pistons having longer strokes (multiples of one inch) located within a conventional cylinder  160 . 
   High pressure fluid is introduced between the pistons through a fluid inlet  114 . This introduced fluid causes the pistons to separate to the extent permitted by respective tethering mechanisms in order to move piston rod  189  a predetermined distance. A low pressure fluid, at approximately ¼ to ½ the pressure of the high pressure fluid, is introduced at the end of the second positioning system  150  closest to piston rod  189  to return the pistons of both positioning systems and piston rod  189  to their rest positions. In a preferred embodiment, air or line air is provided at a high pressure of substantially between 80 PSI and 250 PSI with the low pressure being substantially between 20 PSI and 125 PSI. The cross-hatching shown in  FIG. 1  between piston  156  and head assembly  190  illustrates the presence of low pressure air. The lack of cross-hatching and the extended condition of the device as shown in  FIG. 2  illustrates when high pressure air has been introduced between the pistons. 
   As shown in  FIG. 1 , the first positioning system  110  includes the annular cylindrical housing  120  having an opening  111  through its center section  121  for the passage of tubes  161 - 164  which supply compressed air to the second positioning system  150 . A first stroke piston  115  is positioned against a back plate  112  of housing  120  when it is at rest. The piston  115  is moved a predetermined distance when the introduction of compressed air via a port  113  extending through the rear plate  112  overcomes the low pressure holding the pistons at rest. The remaining pistons  116 - 118  are supplied with high pressure fluid through input ports  114  which enter the annular cylinder wall  125  at right angles to the direction in which pistons  115 - 118  move. Input ports  114  can be positioned at other angles relative to the direction that pistons  115 - 118  move. 
   In order to facilitate the entry of the compressed air into and out of the spaces between each of the moveable pistons  115 - 118 , a shallow slot  131  is formed in each piston wall  132  on one or both sides of the piston seal slot  133 . Slots  131  extend parallel to the direction of travel of the pistons and are aligned with input port orifices  114 , as shown in  FIGS. 1 ,  5  and  6 . In  FIG. 5 , shallow grooves  135 , cut into the perimeter of each piston, connect each of the slots  131  to three grooves  136  cut radially into the piston faces. Grooves  136  are cut into the pistons 120° apart from each other. Once compressed air is delivered between all or some of the pistons  115 - 118 , the selected pistons are spaced apart a predetermined distance for causing a predetermined amount of movement of positioning rod  189 . The result is a calibrated movement of the piston rod  189  outward as high pressure air fills the precise voids between the pistons and overcomes the force of the low pressure air tending to push them toward the back of the housing  120 . Any number of grooves  136  such as two to six, can be formed on the piston faces so that fluid will flow between adjacent pistons. 
   For the sake of clarity,  FIG. 4  shows a cross section of the first positioning system at full extension but without the confining cylindrical housing  120  or center tube  121 . Sets of locked tethering screws  142  extend between adjacent pistons for limiting their relative and total movement. While tethering screws are discussed with this embodiment, other known tethering members such as those discussed below could also be used. Each set of tethering screws  142  includes at least three screws that limit the travel of their respective piston to a predetermined distance relative to the rear plate  112  or to the piston at its left (as shown in the figures). The tethering screws  142  are secured within the adjacent pistons so that they are slidable relative thereto. Three rigid inter-stage pusher rods  148  extend from positioning system  110  and transmit the cumulative movement of all four pistons  115 - 118  to a fractional stroke piston  152  in the second positioning system  150 . O-rings  141  seal the tethering screw cavities  140  containing tethering screws  142 . A seal  143  such as an O-ring is positioned in each slot  133  for preventing fluid from passing between each piston and the inner surface of the cylinder  120 . Seal  143  is also used between the inner surface of the pistons  115 - 118  and the outer surface of center tube  121 .  FIG. 5  shows an outside view of FIG.  4  and illustrates the slots  131  machined axially along the outer, circumferential edge of the annular pistons which connect with the grooves  136  formed across the faces of the pistons in a direction perpendicular to the path of travel of the pistons for the purpose of allowing quick flow of high pressure air from its introduction at ports  114  along the perimeter of the pistons to the working faces thereof. The grooves  136  and slots  131  can be formed by any well known process such as machining, abrading, etc. Additionally tubes or other fluid conduits could be used to present the line air introduced through port  114  to the facial grooves  136 .  FIG. 6  shows the annular pistons in the fully retracted condition and illustrates the axial slots  131  and the facial grooves  136 . 
   An intermediate plate  122 , shown in  FIG. 2 , connects the first positioning system  110  to the second positioning system  150  and contains three linear bearings  123  for guidance of the inter-stage pusher rods  148 . Plate  122  provides support for both the inside tube  121  and the cylinder tube  160  which is held in place by four tensioned tie rods (not shown) between the intermediate plate  122  and the head assembly  190 . 
     FIG. 3  illustrates a sub-assembly of the pistons of the second positioning system without cylindrical housing  120 , the pistons of the first positioning system and cylinder  160 .  FIG. 3  shows four power pistons  153 ,  154 ,  155  and  156  at rest in their fully retracted positions against the fractional piston  152  and four concentric, co-axial conduits or tubes  161 - 164 . The retraction force produced by the low pressure line air works against the reduced effective area of the retract piston  156  which is the result of using an oversized piston rod  189  having one-half or less the surface area of the advancement pistons  152 - 155 . Tubes  161 - 164  tether each of the pistons  153 - 156  to a respective one of the stroke limiting collars  165 - 168  and limit their distances to those discussed herein. Tubes  161 - 164  are formed of rigid material such as aluminum, brass, steel or any high strength plastic such as delrin, nylon, etc. The rigidity of the tubes contributes to the ability of cylinder  100  to resist lateral and torsional forces applied during its operation. 
   Each concentric tube  161 - 164  is sized so that its outside diameter is sufficiently smaller than the inside diameter of the tube in which it moves to provide an annular cross-sectional area large enough to convey the high pressure fluids, such as air, rapidly to the next succeeding cavity. The wall thickness of each tube is carefully sized to ensure that its strength is sufficient to withstand the tensile and compressive forces it will encounter during the operation of the multi-stroke cylinder  100 . These wall thicknesses can vary depending on the intended use of the cylinder  100 , the materials of the tube and/or the magnitude of the forces that will be applied to the tube. In a preferred embodiment, the wall thickness of each tube  161 - 164  can be substantially 1/32 inch or ⅛ inch. Alternatively, the thickness can be between 1/32 inch and ⅛ inch. The advantages of using coaxial tubes  161 - 164  include less friction, fewer sealing problems, simpler inter-stroke stop mechanisms, reduction in off-center piston loads and increased stability. 
   High pressure compressed air is introduced through collars  165 - 168  and channeled between pistons  152 - 156  by tubes  161 - 164 . The outside and shortest tube  161  rigidly connects the fractional stroke piston  152  to the collar  165 . Collar  165  channels high pressure air between tubes  161  and  162 . This air travels through the fractional stroke piston  152  to move the piston  153 . Similarly, the tube  162  connects the piston  153  to the collar  166  which channels compressed air between tubes  162  and  163 , which in turn introduce the compressed air between pistons  153  and  154 . The air between pistons  153  and  154  moves piston  154  away from piston  153 . Tube  162  is dimensioned in length to limit movement between the fractional piston  152  and the piston  153  to a precise, predetermined length such as one inch. In this same manner, the stroke limiting collar  167  supplies compressed air between tubes  163  and  164  for contacting and moving piston  155  away from piston  154 . Compressed air is supplied to piston  156  through stroke limiting collar  168  which is tapped, as is piston  155 , to receive the much heavier walled center tube  164  which provides structural support to the entire tethering, co-axial tube sub-assembly. The piston  156  is tethered to the piston  155  through a plurality of the steel shafts  157  which allow precisely eight inches of movement between the two pistons  155 ,  156 . 
   As shown in  FIG. 3 , the pistons  152 ,  153  and  154  and stroke limiting collars  165 ,  166  and  167  which contain tubes  161 - 163 , respectively, each include an assembly  180  having two pieces  181 ,  182  formed to complement, capture and retain the flared ends  183  of their respective tubes. Two O-ring static seals  184  within each assembly  180  prevent fluid leakage and each two-part, stroke limiting collar  165 - 167  contains a dynamic seal  185  to prevent leakage between it and the outside wall of the tube on which it slides. 
   Conventional NPT entry ports  186  located in each of the two-part collars  165 - 167  channel the line air into a connecting radial cavity  187  which distributes it through several holes  188  in its associated fluid supply tube to allow flow into the space between adjacent tubes. 
   The piston rod  189  is secured to piston  156  and is capable of being rotated within piston  156  so that outside torque forces are not be transmitted to the internal mechanisms which link pistons  155 - 156  to each other. 
   An alternative form of tethering the pistons is illustrated in FIG.  7 . The same reference numerals are used to indicate common elements between the embodiment shown in FIG.  1  and that shown in FIG.  7 . In  FIG. 7 , the inlet tubes  210  are not concentric with one another. Instead, each extends through one of four linear bearings  211  mounted in a square array within rear plate  112 . A stroke limiting collar  212  is rigidly attached to tube  221  about one inch outside rear plate  112  when the pistons are in their retracted position. The spacing between this collar  212  and plate  112 , as well as the length of pusher rods  148 , allows a fractional stroke piston  252 , attached to tube  221 , to move a full 15/16 of an inch. Tube  221  extends into fractional stroke piston  252  but does not pass through it. Instead, tube  221  stops at a face of piston  252  closest to piston  253 . 
   The three remaining tubes  222 ,  223 ,  224 , all similar to tube  221 , pass through seals  230  and bearings  231  mounted in a square array within fractional stroke piston  252 . The square array of fractional stroke piston  252  is substantially identical to that of plate  112  so that the tubes remain straight as they extend along the length of the multi-stroke cylinder. Tube  222  is attached to the 1″ stroke piston  253  and the other two tubes  223 ,  224  pass through a bearing in piston  253  and are attached to the 2″ stroke piston  254  and the 4″ stroke piston  255 , respectively. Like tube  221 , tubes  222 - 224  have collars  212  rigidly attached at precise positions along their lengths so the collars on adjacent shafts contact one another, as shown in  FIG. 8 , and limit the relative movement between the adjoining shafts and adjacent pistons. In this manner, collar  212  is positioned on tube  222  so the movement of the 1″ stroke piston  253  relative to the fractional stroke  252  piston is limited to one inch. Collar  212  is positioned on tube  223  so the movement of the 2″ stroke piston  254  relative to the 1″ stroke piston  253  is limited to two inches. Collar  212  is positioned on tube  224  so stroke piston  255  only moves four inches relative to 2″ stroke piston  254 . 
   Each of the hollow tubes  221 - 224  are attached to a high pressure fluid source for introducing air between adjacent pistons. Tube  221 , attached to the fractional stroke piston  252  supplies air between stroke pistons  252  and  253  to move stroke piston  253  one inch; tube  222 , attached to the 1″ stroke piston  253 , supplies air between stroke pistons  253  and  254  to move the 2″ stroke piston  254  two inches; and tube  223 , attached to the 2″ stroke piston  254 , supplies air between stroke pistons  254  and  255  to move stroke piston  255  four inches. The 8″ stroke piston  256  is moved by the fluid supplied between stroke pistons  255  and  256  through tube  224  attached to the 4″ stroke piston  255 . As with tube  221 , tubes  222 - 224  terminate at the face of the piston to which they are attached. The relative movement of piston  256  with respect to piston  255  is limited by a pair of stroke limiting shafts  257  which are rigidly attached to the 4″ stroke piston  255  but pass through the 8″ stroke piston  256  via bearings  258  and seals  259 . The piston rod  189  is capable of being rotated within stroke piston  256  so that outside torque forces cannot be transmitted to the internal mechanisms which link the floating pistons to each other.  FIG. 10  depicts the stroke limiting action of the collars  212  between the fractional stroke piston  252  and the 1″ stroke piston  253  as they would appear if removed from the confining cylinder. Linear bearings  231  and dynamic tube seals  230  provide low friction, leak proof, relative movement between the air supply tubes and the monolithic pistons. O-rings  265  provide hermetic seals where the tubes are attached to the pistons as shown in FIG.  10 . 
   When high pressure air is vented from the space between any two of the pistons, the retraction force of the low pressure air (shown by hatching in  FIG. 7 ) in cylinder  160  between head assembly  190  and piston  156  causes piston  156  to move toward the rear plate  112 . The force of the low pressure air expels the residual air between the two adjacent pistons and moves the pistons and the piston rod  189  inward from their extended positions as shown in FIG.  8 . The pistons and piston rod  189  move an amount equal to the length of the distance between them. The air is vented to the atmosphere through the exhaust port in the three-way valve which supplies high pressure air to the various pistons. Low pressure air returns between piston  256  and head assembly  190  through fluid port  191 . A self compensating type of pressure reducer is used to return the lower pressure fluid between piston  256  and the head assembly  190 . 
   A co-axial multi-stroke cylinder  100 ′ according to another embodiment of the present invention is illustrated in  FIGS. 11-16 . This embodiment utilizes coaxial cylinders for housing its piston rod positioning systems. Elements of this embodiment that are similar to those previously described will be identified using the same numerals. The embodiment shown in  FIG. 11  eliminates the need for low pressure air to retract a piston rod  189 ′. Instead, this embodiment takes advantage of line air for cylindrical and piston rod retraction. 
   With all of the embodiments discussed herein, the use of line air operating against smaller piston areas has the advantage of not requiring a self-relieving pressure reducing valve which increases system costs and plumbing complexity. Also, the prior art systems which use air must vent their air to the atmosphere when any of the pistons advance. Line air is not vented from the system but is pumped back into the supply line by the advancing pistons, thus saving the costs of producing compressed air—a fairly expensive commodity in an industrial plant. By including a three-way valve to handle the line air used for retraction, one could remotely vent this air and thereby effectively double the push power of the cylinder should the occasion arise. 
   As illustrated in  FIG. 11 , cylinder  100 ′ includes first positioning system  110 ′ and second positioning system  150 ′. As with the multi-stroke cylinders discussed above, common elements have the same reference numerals as used with the description of the previous embodiments. The total stroke length of cylinder  100 ′ is 15 and 15/16 inches. However, the individual stroke lengths of each positioning system  110 ′ and  150 ′ are different from those discussed above. Contrary to the multi-stroke cylinders discussed above, first positioning system  110 ′ is capable of moving piston rod  189 ′ a total of 1 and 15/16 inches. Second positioning system  150 ′ is only capable of moving piston rod  189 ′ a total of 14 inches. Nevertheless, the combined total possible stroke length of cylinder  100 ′ is 15 and 15/16 inches when the cylinder has been fully extended as shown in FIG.  13 . 
   First positioning system  110 ′ operates in a similar manner to that discussed above with respect to positioning system  110 . First positioning system  110 ′ includes annular cylindrical housing  120  surrounding a plurality of pistons  115 - 119 . Housing  120  includes an outer surface  124  and an inner surface  126 . Input port orifices  114  extend between surfaces  124  and  126  for introducing compressed air from a conventional source into housing  120  and between pistons  115 - 119 . As discussed above, conventional three-way solenoid or pilot operated valves can be used with the embodiments of the present invention. Such valves which are able to be used with each embodiment described herein are produced by companies such as MAC valves, ASCO, Humphrey and Parker Hannifin. As shown in  FIGS. 14 and 15 , pistons  115 - 119  each include a seal  143 , positioned in slot  133 , that engages with inner surface  126  to prevent the introduced air from passing between each piston  115 - 119  and inner surface  126 . Pistons  115 - 119  also include an inner seal  143  for engaging the outer surface of a housing  151 ′ of second positioning system  150 ′. Tethering members  142  are used to limit the travel of pistons  115 - 119  relative to each other and back plate  112 , as discussed above. Like piston  153  of second positioning system  150 , piston  119  has a total stroke length of one inch. This one inch, when added to the combined 15/16 of an inch stroke of pistons  115 - 118 , provides positioning system  110 ′ with its total stroke length of 1 and 15/16 inches. 
   Second positioning system  150 ′ operates in a similar manner to that discussed above with respect to positioning system  150 . Second positioning system  150 ′ includes housing  151 ′, a rear plate  152 ′ and a plurality of power, stroke pistons  154 - 156  for imparting movement to piston rod  189 ′. As seen in  FIGS. 11-13 , housing  151 ′ has an elongated, generally tubular shape that extends within and through housing  120  such that they are coaxially aligned and mutually supported. This overlapping, coaxial positioning of housings  120  and  151 ′ forms a more stable multi-stroke cylinder when compared to those of the prior art. The overlapping, coaxial positioning of the housings also creates a compact, multi-stroke cylinder  100 ′ that does not occupy as much space, when activated and when at rest, as prior art multi-stroke cylinders. The multi-stroke cylinder  100 ′ is more compact and better able to resist the forces created when piston rod  189 ′ moves. The present invention eliminates the conventional back to back piston relationship used in the prior art. The coaxial positioning also makes the cylinder easier and less costly to manufacture when compared to conventional multi-stroke cylinders. 
   Housing  151 ′ includes a raised, first positioning system engaging portion  148 ′ that transfers the cumulative stroke of pistons  115 - 119  from first positioning system  110 ′ to second positioning system  150 ′ and to piston rod  189 ′. As shown in  FIG. 14 , piston  119  is secured to the engaging portion  148 ′ by a plurality of fastening screws  149 ′. The engaging portion  148 ′ passes through a guide bushing and kinetic seal  123 ′ in plate  122 ′ and reduces the effective area of the return side of piston  119  to provide the force differential needed to extend and retract housing  151 ′ relative to housing  120 . The engaging portion  148 ′ can be varied in diameter from model to model to provide modest variations in the ratio between the forces needed to extend and retract the cylinder. Piston  154  is moved by introducing a high pressure fluid through input port  161 ′ and between back plate  152 ′ and piston  154 . Pistons  155  and  156  are moved by the introduction of fluid via tubes  163  and  164 , as discussed above. Tube  164  passes through a guide bushing/seal arrangement in stroke limiting collar  167 . As with those discussed above, this seal arrangement, shown in  FIG. 13 , prevents the escape of fluid within tube  163  from between collar  167  and the outer wall of tube  164 . 
   After the pressurized fluid exits tube  164  through openings  169 ′, it forces hollow piston rod  189 ′ and rod cap  200 ′ a distance of eight inches away from piston  155 . Piston rod  189 ′ is secured to piston  156  so that no relative movement exists therebetween. As shown in  FIG. 13 , an eight inch tethering rod  157 ′ extends through a guide bushing and a kinetic seal contained within an insert  166 ′ at the end of hollow piston rod  189 ′ where it is secured to piston  156 . Tethering rod  157 ′ includes a tethering head  158 ′ for contacting the insert  166 ′ in order to limit the movement of the piston rod  189 ′. Piston rod  189 ′ includes a hollow center for receiving tethering rod  157 ′ when piston  156  is in contact with piston  155 , such as when the cylinder  100 ′ is at rest, as shown in FIG.  11 . Cylinder  100 ′ is compact and space efficient, in part, due to the piston rod  189 ′ receiving tethering rod  157  while the cylinder  100 ′ is at rest. Low pressure air is introduced into ports  165 ′ and  191  for returning the advanced pistons to their rest positions. 
     FIG. 15  shows an external view of the same pistons in the extended mode. These pistons are slightly reduced in diameter on one or both sides of the full diameter section  144  which contains the seal slots  133  and kinetic seals  143 . This arrangement allows full flow of air in and out of the cavities between the pistons  115 - 119  to the various ports  114  as the pistons  115 - 119  move relative to these ports  114  within the cylinder walls. The reduced diameter sections  135  provide the same function as the parallel slots  131  shown in  FIGS. 5 and 6  but allow the input ports  114  to be placed at any convenient position around the circumference of the piston. As discussed above, shallow lateral slots  131  machined at multiple places across the face of each piston allow quicker movement of compressed air between adjoining pistons as they separate or come together. 
     FIG. 16  shows an end view of the top of the cylinder with the 1/16 inch stroke port  113  at top. Also shown are the 2 inch stroke stop  168 , the 4 inch stroke stop  167  and the 2 inch stroke port  161 ′. Four screws  158 ′ attach the rear end plate  112  to the housing  110 . Up to eight tapped input ports  201  conduct compressed air axially through the solid portions of the housing to connect with radial ports  114  located between adjacent pistons or to other ports machined into the forward plate  122 . This approach simplifies the complicated plumbing of conventional cylinders and is made possible by the reduced diameters  135  on the outside of the annular pistons as described heretofore. 
     FIG. 17  illustrates another embodiment of a multi-stroke cylinder  100 ″ that is similar and operates in essentially the same manner as the multi-stroke cylinder  100 ′ shown in FIG.  11 . As a result, a discussion of its components that are also included in cylinder  100 ′ and its operation will not be repeated. Contrary to the embodiment of  FIG. 11 , the two inch stroke piston  154 ′, according to this embodiment, is housed in the first positioning system  110 ″. As a result, the second positioning system  150 ″ only includes two pistons  155 ,  156  and one fluid introduction tube  164 . First positioning system  110 ″ has a total stroke length of 3 and 15/16 inches. Second positioning system  150 ″ has a total stroke length of only twelve inches.  FIG. 17  schematically illustrates the multi-stroke cylinder  100 ″ in a fully retracted condition. This embodiment is easier, more compact, more stable and more economical to manufacture when compared to conventional cylinders. Also, as with the embodiment shown in  FIGS. 1 and 11 , this embodiment is more accurate and better able to resist the forces created during its operation. 
   The multi-stroke, hydraulic cylinder  100 ″ is shown in  FIG. 18  with all of its stages extended. This invention utilizes floating, tethered pistons, interconnected in such a manner as to cause an output piston rod  189  to move a distance equal to the sum of all the distances moved by each of the individual, activated pistons. The first positioning system  110 ″ includes six annular pistons  115 ,  116 ,  117 ,  118 ,  119  and  154 ′ having respective stroke lengths of 1/16″, 1/18″, ¼″, ½″, 1″ and 2″ which operate within annular cylindrical housing  120 . The first positioning system is thus capable of stroking 3 15/16″ in increments of 1/16″. The second positioning system  150 ″, extending within the first positioning system, includes two conventional pistons  155  and  156  having respective stroke lengths of 4″ and 8″ and is thus capable of stroking  12 ″ in increments of 4″. The 2″ stroke piston  154 ′ is rigidly attached to the second stage cylinder tube  151 ′ and to the steel extension tube  148 ″ which acts to guide it through the head plate  122 ′ of the first positioning system as its pistons  115 - 119 ,  154 ′ advance and retract. The piston  154 ′ can be integrally formed with the extension tube  148 ″ as a single unit. The outside diameter of the extension tube  148 ″ is sized so that the area left between it and the inside diameter of the annular cylinder  121  approximately one-half the face area of the other annular pistons  115 - 119 ,  154 ′. As a result of this size relationship, compressed air at line pressure acting against this area creates a retraction force against the extended 2″ stroke piston  154 ′ which forces all the first stage pistons  115 - 119 ,  154 ′ to the rear of plate  112  of the annular cylinder  121 . The piston tube  189  of the second stage is sized in a similar manner with respect to piston  156  so that line pressure acting on the retraction face of the 8″ stroke piston  156  forces it against the 4″ piston  155  and pushes both to the rear of the second stage cylinder tube  151 ′. Air orifices  191  placed near the left end of the extension tube  148 ″ and the right end of the second stage cylinder tube  151 ′ allow compressed air to flow in and out of the retraction sides of both cylinders, thus maintaining constant retraction forces regardless of the positions of the pistons within the two cylinders. 
   The introduction of line air through a port  113  or a port  114  between any two pistons will create extension forces that are approximately twice those of the retraction forces needed to return the extended pistons to rest as discussed above. The extension forces cause the affected piston to move toward the head of its respective cylinder (rightward as shown in  FIG. 18 ) the precise distance allowed by the inter-piston tethering mechanisms. 
     FIGS. 19A-C  illustrate the construction details of the 1″ stroke piston  119  which is typical of the fractional movement annular pistons  115 - 119 ,  154 ′. The piston body  132  would typically be fashioned of an easily machined metal, such as aluminum, or a plastic, such as delrin. The piston  119  includes three or more slotted wells  136 ′ machined into each piston face at regular intervals and of sufficient depth to accommodate approximately one half the length of I-shaped metal tethers  142 ′ which link it to the pistons on either side  118 ,  154 ′. Flat steel rings  134 , fastened to both faces of the piston body by multiple through-bolts  180 ′ as shown in  FIGS. 19A-C , contain three or more matching rectangular slots  131 ′ which are aligned with the piston body wells and capture the T-shaped ends of the metal tethers  142 ′, which precisely limit the movements of the various pistons relative to one another and ensure that the piston faces are maintained parallel to each other in the tethered positions. These flat steel rings  134  also prevent the end faces of their respective pistons from being damaged (scratched, broken, nicked, etc.) by an adjacent piston. They also prevent the forces applied by the tethers  142 ′ from damaging the end faces of their respective pistons. The tethers  142 ′ are formed from relatively thin, heavy, high strength, heat treated sheet metal stampings with a slight curvature about their long axes for extra rigidity. The thin cross section of these tethers  142 ′ allow a thinner walled, annular piston and, therefore, greater compactness in overall design. Additionally, the tethers are contained in wells  136 ′ when the pistons are in a retracted position for additional compactness of the air or hydraulic cylinder  100 ″. A plurality of bolt holes  280  extends through each piston and its rings  134  for securing the portions of the piston together. O-rings  141  are installed beneath a bolt head  281  to prevent the passage of air through the bolt holes  280  and preserve the pneumatic integrity of each piston. The outer cylindrical surface  135 ′ of each piston body, on one or both sides  137  of the outer sealing slot lands carrying dynamic seal  133 ′, is stepped down in diameter in order to provide a passageway  135  for compressed air to move into and out of the piston actuating area regardless of the respective piston&#39;s movement or position. As discussed above, dynamic seals  133 ′ on both the inner and outer diameters of each piston  115 - 119 ,  154 ′ prevent passage of compressed air past the piston as it moves back and forth within the containing cylinder  121 . 
     FIG. 20  depicts the first stage positioning system  110 ″ without the enclosing cylinder tube  121  and with all pistons fully retracted against the rear housing plate  112 . The tip ends of the 1/16 stroke piston tethers  142 ′ appear to the left of the 1/16″ piston  115 . Compressed air entry ports  113  and  114  for actuation of the six annular pistons  115 - 119 ,  154 ′ are represented by arrows and are positioned just to the rear (left as shown in  FIG. 20 ) of the dynamic seal lands  137  for each piston. 
     FIG. 21  illustrates the first stage positioning system shown in  FIG. 20  with all six pistons extended to the limits allowed by their tethers  142 ′. The overall piston length is designed to provide adequate depth for containing the associated tethers  142 ′ within their slotted wells  136 ′. The width and placement of the lands  137  and seal grooves  133 ′ are designed to provide adequate lengths for the reduced diameter sections  135  so that compressed air can flow unimpeded through the side input orifices  113 ,  114  and  165  to and from the piston cavities  138  regardless of the position of the pistons within the confining cylinder. 
     FIGS. 22 and 25  schematically illustrate the second stage positioning system  150 ″ without the confining cylinder tube  151 ′ and with both the 4″ stroke piston  155  and the 8″ stroke piston  156  forced into their fully retracted positions by line air pressure  124 ″ working against the right hand face (as seen in  FIG. 22 ) of the 8″ stroke piston.  FIG. 22  illustrates the second stage positioning system  150 ″ in cross section and the direction of the effective air pressure. 
     FIGS. 23 and 26  depict the second stage positioning system shown in  FIG. 22  as it would appear with line air pressure  124 ″ entering through orifice  161 ′ and working against the left hand face of the 4″ stroke piston  155  thus forcing both 4″ stroke piston  155  and 8″ stroke piston  156  outward (rightward as seen in  FIG. 23 ) the precise 4″ allowed by the adjustable tethering stop nuts  168 .  FIG. 23  illustrates the second stage positioning system  150 ″ in cross section and the direction of the effective air pressures. 
     FIGS. 24 and 27  depict the second stage positioning system shown in  FIGS. 22 and 23  with line air pressure flowing through the air supply tube  164  and orifices  169  into the cavity between the 4″ stroke piston  155  and the 8″ stroke piston  156 . This cavity or space is eventually vacated by the 8″ stroke piston  156  as the pistons  155 ,  156  separate. The tethering stop nuts  158  provide a lockable adjustment for precisely setting the 8″ tethered travel between the 4″ stroke piston  155  and the 8″ stroke piston  156 . Other well known adjustable locking members could also be used.  FIG. 24  illustrates the second stage positioning system  150 ″ in cross section and the direction of the effective air pressures. 
     FIGS. 28  illustrate the multi-stroke cylinder of  FIG. 18  but with a color-coded Legend which shows position of the various static O-ring seals, linear motion bearings, U-cup type dynamic seals and Quad Ring type dynamic seals. 
     FIG. 29A  illustrates the multi-stroke cylinder of  FIG. 17  with the air distribution manifold assembly  170  mounted in position atop the annular cylinder  121  housing.  FIG. 29B  depicts an end view of the cylinder in  FIG. 29A  with the nine air input connections  172  which channel compressed air between the eight individual pistons and the back plate  112 , and to the return air chambers in the front of the two cylinders (right side as shown in FIG.  29 A). 
     FIG. 30  depicts a bottom view of air input manifold plate  171  showing the grooves  173  which channel compressed air from the plumbing connections  172  to the orifices  113 ,  114  atop the annular cylinder housing  121 . These air flow grooves can be formed by any well known procedure such as machining. 
   The following description applies to the operation of the above discussed embodiments. By limiting the stroke of the first piston  115  to 1/16 of an inch and allowing each succeeding power piston to move a distance precisely double that of the preceding piston, a total stroke length of 15 15/16 can be achieved in discrete intervals of 1/16 inch. The eight individual power pistons  115 - 118  and  153 - 156  or  115 - 119  and  153 - 156  (depending on the described embodiment) thus have stroke lengths of 1/16, ⅛, ¼, ½, 1, 2, 4, and 8 inches, as discussed above. 
   For example, in the embodiment shown in  FIG. 1 , if the required stroke were 11 11/16 inches, valves (not shown) would be opened and high pressure air would be introduced for powering the ½″ stroke piston  118 , the ⅛″ stroke piston  116  and the 1/16″ stroke piston  115 . The introduction of air between these pistons causes the inter-stage pusher rods  148  to advance and move the fractional stroke piston  152  a total of 11/16 of an inch. Simultaneously, valves would also open to power the 8″ stroke piston  156 , the 2″ stroke piston  154  and the 1″ stroke piston  153 , thus moving the piston rod  189  the required total of 11 11/16 inches. 
   While the operation is similar in the embodiment shown in  FIG. 11 , the opening of the valves and introduction of pressurized fluid between the pistons results in the first system engaging portion  148 ′ advancing housing  151 ′ a distance of 1 and 11/16inches. As a result, only the 2″ stroke piston  154  and 8″ stroke piston  156  are moved in system  150 ′. Moreover, by balancing the number of pistons used in the first and second positioning systems against the combined strokes of the various systems, a maximum output stroke can be achieved by a device having a relatively small retracted length. Moreover, in the embodiment shown in  FIG. 17 , the movement of piston rod  189  is effected by the first positioning system  110 ″ moving the extension tube  148 ″ a distance of 3 and 11/16 inches. Air introduced between plate  112  and stroke piston  115 , between stroke pistons  115  and  116 , between stroke pistons  117  and  118 , between stroke pistons  118  and  119 , and between stroke pistons  119  and  154  cause engaging portion  148 ′ to move the predetermined distance. Air introduced between stroke pistons  155  and  156  cause piston rod  189  to move the remaining 8 inches to achieve the total 11 and 11/16 inches. Moving all the valves to an exhaust position would cause the piston rod  189  to retract to its original position. Exhausting through only the 1/16″ stroke valve and the 2″ stroke valve would cause the piston rod to retract to the 9 and ⅝ inches stroke position, etc. Opening or exhausting any other combination of valves would move the piston rod  189  to whatever other position was desired among the 256 discrete positions it would be capable of assuming. The movements would be quick and positive and there would be no doubt about the extended position of the piston rod in the properly sized and powered system. 
   Although the present invention includes a 256 position mechanism, the addition of another fractional piston having a 1/32″ stroke could easily double the obtainable positions to 512. Similarly, further adding a 1/64″ stroke piston could increase the useful strokes to 1024. 
   In practice, a user of the invention would either manually or automatically, possibly using a programmable logic controller, select the stroke length desired in inches and fractions of an inch. One such programmable logic controller is a MITSUBISHI F1-ZONER. However, other well known controllers such as those produced by G.E. or ALLEN BRADLEY may also be used. 
   Any suitable 3-way valve can be used with the embodiments of the present invention. Well known valves which may be used are produced by ASCO, MAC valves, Parker Hannifin or Humphrey. 
   The kinetic seals used in the embodiments of this application are formed elastomeric rings which fit into grooves machined into pistons for the purposes of preventing air or liquid flow past the piston as it moves back and forth within a cylinder. The shapes of these rings are designed to exploit the differential fluid pressures existing on either side of the rings so that the surfaces of the seals are pressed against the groove walls and the moving surfaces of the cylinder in such a manner that no fluid can escape past the seal. Additionally, these seals provide little friction force against the movement of their piston. These seals take on many shapes and forms and are produced and sold by companies such as Parker Hannifin and Minnesota Rubber. 
   Numerous characteristics, advantages and embodiments of the invention have been described in detail in the foregoing description with reference to the accompanying drawings. However, the disclosure is illustrative only and the invention is not limited to the illustrated embodiments. Various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention. For example, although the movement of the stroke pistons is described with respect to 1/16 inch increments, the stroke of each piston can be any increment including 1/10 of an inch. Also, the total stroke length is not limited to 15 and 15/16 inches. The cylinder according to the present invention could have a total stroke length that is greater or less than 15 and 15/16 inches. The embodiments including a shorter stroke length will be more compact and easier to manufacture than the 15 and 15/16 inch version. As is common, the symbol ″ has been used in this application as an abbreviation for the term “inch”.