Patent Publication Number: US-7717025-B2

Title: Fluid actuator with limit sensors and fluid limit valves

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is derived in part from the provisional patent Application No. 60/743,796 filed Mar. 27, 2006 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   This invention relates the construction of a valve for hydraulic fluid leak detection and correction in hydraulic fluid linkages, insuring that the hydraulic fluid linkages are accurate at all times. The reliable accurate hydraulic fluid linkages with adjustable limit stops can be used to replace complex mechanical linkages. 
   2. Description of Prior Art 
   Hydraulic circuits have not been able to fully replace mechanical linkages in precision applications, such as vehicle steering and other systems requiring accurate reliable correlation which linkages provided. Mechanical linkages reliably correlate the movement of mechanical components. Hydraulic circuits used to replace mechanical linkages use two or more linear actuators, rotary actuators or fluid motors to control the movement of mechanical components. In a hydraulic circuit these hydraulic actuators or motors are connected by a hydraulic fluid conduit with possible intermediary fluid control valves and fluid pumps. The hydraulic circuits used to replace mechanical linkages are hydraulic linkages. Replacing mechanical linkages with hydraulic linkages have significant advantages over mechanical linkages. Hydraulic conduits required to construct hydraulic linkages can be easily routed. Hydraulic circuits can easily switch operating modes. In each operation mode the hydraulic circuit can form a hydraulic linkage between a different set of mechanical components or the mechanical components can be controlled independently in a completely uncorrelated manner. To replace mechanical linkages, hydraulic circuits need to be able to detect and correct fluid loss in hydraulic linkages and require limit stops to prevent damage caused by extending or retracing too far and/or too hard. Through the use of limit sensors and fluid limit valves, the hydraulic linkage can include leakage compensation and leakage location detection and allow for accurate control over the extension and retraction of a piston in the fluid actuator. Mechanical stops prevent over extension and over retraction and are strong enough to resist the full force of the hydraulic actuator or the full force of the mechanical load. Conventional actuators include mechanical stops. However, the mechanical stops included in conventional actuators are not adjustable. Mechanical components in different orientations may require mechanical limit stops to be repositioned. Without adjustable mechanical actuator stops, actuator movement often cannot be stopped before damaging over extension or over retraction occurs. 
   Piston bypass valves have been constructed to activate the piston when it is in proximity to the end cap. For example, see U.S. Pat. Nos. 5,425,305, 6,170,383 (Mauritz). The described art does not provide location adjustable piston bypass valves. 
   Re-phasing hydraulic circuits have been constructed of re-phasing cylinders utilizing fluid bypass ports. For example, see U.S. Pat. No. 4,463,563 (Krehbiel). The described art does not provide location adjustable piston bypass valves. Also, neither the location nor the pressure at which these pistons are re-phased, by means of a bypass port, is adjustable. 
   Hydraulic steering linkages have been constructed from a pair of hydraulically linked hydraulic cylinders. For example see U.S. Pat. Nos. 6,179,315 (Boriack) 3,212,793 (Pietrotroia). There is no means of detecting or correcting for hydraulic fluid leakage from the hydraulic linkage described in this prior art. Hydraulic leakage occurs in virtually all hydraulic circuits. Leakage can occur as hydraulic fluid lost from the hydraulic circuit, or as hydraulic fluid leaking across actuator seals. 
   Cushioning devices are constructed for decelerating and stopping pistons by restricting fluid flow. For example, see U.S. Pat. Nos. 4,397,218 (Spring), 6,557,456 (Norton). The presented cushioning devices do not include a means of adjusting the extension and/or retraction limits at which the cushioning limit valves are activated or proportionally activating the cushioning limit valves. 
   A limit switch with a sensing element is actuated when the operating actuator reaches an end position. The switching element stops the supply of hydraulic fluid to the operating actuator in response to the actuated sensing element. For example see U.S. Pat. Nos. 3,920,217 (Danfoss) and 3,941,033 (Danfoss). Proximity switches are used to detect the proximity of components before they come into contact. When proximity switches detect the limit position, valves are controlled to interrupt hydraulic supply to actuators preventing them from over extending or retracting. For example U.S. Pat. No. 4,165,674 (Weight). Rather than stopping or interrupting the hydraulic supply to actuators, the hydraulic flow can also be reduced, slowing the actuators&#39; movement as it approaches the limit. A limit valve which controls the driving hydraulic flow by reducing the hydraulic pump stroke when the limit valve is moved to a predetermined limit is able to reduce the hydraulic flow as desired. For example see U.S. Pat. No. 5,117,935 (Hall). However, the limit sensors and valves designed to stop or reduce hydraulic supply to actuators cannot detect or correct hydraulic leakage in circuits. Separate mechanical stops are required to prevent mechanical loads from over extending or retracting the actuators. 
   Leakage in hydraulic linkages can be compensated by continuously monitoring the position by a sensor on the control element and by a sensor on the driven element. The hydraulic flow to the actuators is continuously adjusted according to the monitored positions of the control and driven element. For example U.S. Pat. No. 7,028,469 (Porskrog). Here the elements are linked by an electronic control system. This electronic monitoring system requires an electrical power supply at all times for the position sensors and the control valve solenoids. The electronic monitoring systems is only able to compensate for slow hydraulic leaks. The present invention is able to compensate for slow hydraulic leaks without requiring continuous monitoring of either the control or driven elements. The prior art method of compensating for hydraulic leaks by continuously monitoring the control and driven elements does not include adjustable mechanical stops required to prevent mechanical loads from over extending or retracting the actuators. Whereas the present invention does include adjustable mechanical stops. 
   OBJECTS AND ADVANTAGES 
   The present invention enables mechanical linkages to be replaced by hydraulic linkages in precision applications such as vehicle steering and other systems requiring accurate reliable correlation between mechanical components. The present invention integrates adjustable mechanical stops or cushions into hydraulic actuators. This allows hydraulic linkages to be used in operating situations where over limit movement must be prevented. The present invention enables detection and correction of hydraulic leakage at actuator&#39;s extension and retraction limits. Slow hydraulic leaks do not require continuous monitoring. Intermittent detection and correction of hydraulic leakage is sufficient. The present invention does not require a hydraulic source to be constantly available to correct for hydraulic leakage. More time is available to recharge the hydraulic pressure source between its usage for hydraulic leakage correction. The present invention provides hydraulic leakage detection and correction without the requirement of continuous monitoring and an electronic control system. As a result hydraulic linkages constructed are simpler and more reliable and can be safely used as a replacement for mechanical linkages. 
   SUMMARY 
   In accordance with the present invention, a fluid linkage circuit with limit sensors is integrated into a fluid actuator, such that when the fluid actuator extends to its extension limit or retracts to its retraction limit, the limit sensors will activate fluid valves to redirect fluid to bypass the fluid actuator&#39;s piston, thereby preventing over extension or over retraction and correcting for fluid leakage within a fluid linkage circuit. The location of the limit sensors are adjusted by adjusting the mechanical limit stops or cushions of the fluid actuator. 

   
     DRAWINGS 
     Figures 
       FIG. 1  Isometric view of a Prior Art Fluid Actuator. 
       FIG. 2  Cross Section view of the Prior Art Fluid Actuator shown in  FIG. 1  taken along Cutting Plane A-A. 
       FIG. 3  Isometric view of the Base Portion of the Prior Art Fluid Actuator shown in  FIG. 1   
       FIG. 4  Cross Section view of the Base Portion of the Prior Art Fluid Actuator shown in  FIG. 2  taken along Cutting Plane B-B. 
       FIG. 5  Isometric view of the Base Portion of the Fluid Actuator with an Integrated Fluid Limit Valve that has No Moving Parts. 
       FIG. 6  Cross Section view of the Base Portion of the Fluid Actuator with an Integrated Fluid Limit Valve that has No Moving Parts shown in  FIG. 5  taken along Cutting Plane C-C. 
       FIG. 7  Isometric view of the Base Portion of the Fluid Actuator with a Fluid Limit Valve integrated into the Piston. 
       FIG. 8  Cross Section view of the Base Portion of the Fluid Actuator with a Fluid Limit Valve integrated into the Piston shown in  FIG. 7  taken along Cutting Plane D-D. 
       FIG. 9  Isometric view of the Base Portion of the Fluid Actuator with a Fluid Limit Valve integrated in the End Cap. 
       FIG. 10  Cross Section view of the Base Portion of the Fluid Actuator with a Fluid Limit Valve integrated in the End Cap shown in  FIG. 9  taken along Cutting Plane E-E. 
       FIG. 11  Isometric view of the Hydro Pneumatic Cylinder with Adjustable Mechanical Limits. 
       FIG. 12   a  Cross Section of Hydraulic Cylinder with Adjustable Mechanical Limits shown in  FIG. 11  taken along Cutting Plane F-F 
       FIG. 12   b  Cross Section of Hydro Pneumatic Cylinder with Adjustable Mechanical Limits shown in  FIG. 11  taken along Cutting Plane F-F 
       FIG. 13   a  Detailed Side Cross Section of Fluid Limit Valve shown in  FIG. 12   a ,  FIG. 12   b  which is taken along Cutting Plane F-F 
       FIG. 13   b  Detailed Side Cross Section of Fluid Limit Valve with External Fluid Leakage Correction Supply shown in  FIG. 12   a ,  FIG. 12   b  which is taken along Cutting Plane F-F 
       FIG. 14  Basic fluid linkage utilizing the Fluid Limit Valve With Moving Parts 
       FIG. 15  Basic fluid linkage utilizing the Fluid Limit Valve Without Moving Parts 
       FIG. 16  Fluid Actuators with External Mechanical Limit Stops 
   

   REFERENCE NUMERALS 
   
       
         101  piston rod 
         102  main piston 
         103  end cap for cylinder base 
         104  cylinder tube 
         105  base end cap piston stop 
         106  base fluid limit valve outlet holes 
         115  end cap for cylinder head 
         116  base poppet plunger of bidirectional fluid limit valve 
         117  base fluid limit valve cover 
         144  poppet return spring of fluid limit valve 
         205  line to cylinder base connection 
         207  fluid limit valve outlet 
         208  line to cylinder head connection 
         223  base end cap ports for poppet fluid limit valve 
         225  poppet fluid limit valve bypass port 
         229  return spring of check ball 
         310  high-pressure fluid pump 
         320  fluid actuator 
         322  fluid actuator 
         330  fluid check valve 
         331  fluid check valve 
         340  mechanical limit sensor that can apply force to fluid limit valve  500   
         341  mechanical limit sensor that can apply force to fluid limit valve  510   
         345  mechanical limit sensor that can apply force to fluid limit valve  550   
         346  mechanical limit sensor that can apply force to fluid limit valve  560   
         410  fluid control valve 
         411  fluid control valve crossover line 
         412  fluid control valve straight-through line 
         500  fluid limit valve with moving parts 
         501  fluid limit valve  500  in disconnect state 
         502  fluid limit valve  500  in connect state 
         510  fluid limit valve with moving parts 
         511  fluid limit valve  510  in disconnect state 
         512  fluid limit valve  510  in connect state 
         540  fluid limit valve without moving parts 
         541  fluid limit valve  540  in self-connect state 
         542  fluid limit valve  540  in through-connect state 
         550  head limit sensor and fluid limit valve 
         551  base limit sensor and fluid limit valve 
         555  counter balance valve 
         560  fluid limit valve without moving parts 
         561  fluid limit valve  560  in self-connect state 
         562  fluid limit valve  560  in through-connect state 
         620  internal piston hydraulic pump 
         621  head gas or hydraulic head chamber inlet 
         622  piston gas inlet 
         623  base gas or hydraulic base chamber inlet 
         630  head hydraulic inlet 
         631  base hydraulic inlet 
         636  hydraulic lines manufactured into cylinder body  689   
         650  outer head gas chamber 
         651  head chamber 
         652  head chamber 
         653  base chamber 
         654  base chamber 
         655  hydraulic adjustable head chamber 
         656  hydraulic adjustable base chamber 
         665  gas damping valve or fluid limit valve 
         671  fluid limit valve outlet 
         672  fluid limit valve inlet 
         673  fluid limit valve return spring 
         674  fluid limit valve poppet plunger 
         676  fluid limit valve body 
         677  fluid leak correction supply inlet 
         678  check valve plunger or ball 
         679  fluid limit valve cavity 
         680  cylinder head shell 
         681  base cap with base stops 
         682  head cap with head stops 
         683  piston shaft 
         684  head cap with head stops 
         685  base piston stub 
         686  hydraulic floating head piston 
         688  hydraulic floating base piston 
         689  cylinder body 
         700  hydraulic cylinder mounting joint 
         701  separated pivot joint 
         702  adjustable mechanical limits 
         705  separation between floating base piston  688  and mechanical limit piston  720   
         706  side frame 
         707  pivot connecting frame 
         710  prior art hydraulic cylinder shown in  FIG. 2   
         720  mechanical limit piston 
         721  head chamber 
         722  vent 
         901  fluid pump  310  intake line from fluid reservoir 
         903  low-pressure return line from fluid control valve to fluid reservoir 
         910  high-pressure line from fluid control valve to head connection of fluid actuator  320  and to the fluid limit valve 
         911  high-pressure line from fluid control valve to head connection of fluid actuator  322  and to fluid limit valve 
         915  high-pressure line connecting base connection of fluid actuators  320  and  322  to fluid limit valves and fluid check valves 
         930  high-pressure line from fluid pump  310  to fluid control valve 
     
  
   DETAILED DESCRIPTIONS OF PRIOR ART EMBODIMENTS AND THEIR OPERATIONS 
   FIGS.  1  and  2   
   Description of Complete Prior Art Fluid Actuator 
   Fluid actuators are used to extend and/or retract in order to displace a load.  FIG. 1  is an isometric view of a complete prior art fluid actuator.  FIG. 2  is a sectional view taken along the cutting plane A-A of  FIG. 1 . 
   FIGS.  1  and  2   
   Operation of Complete Prior Art Fluid Actuator 
   Fluid flowing into cylinder base connection  205  forces piston  102  to move and piston rod  101  to extend. When the piston  102  moves and the piston rod  101  extends, fluid is forced out of the cylinder head connection  208 . Fluid flowing into the cylinder head connection  208  forces piston  102  to move and piston rod  101  to retract. When the piston  102  moves and the piston rod  101  retracts, fluid is forced out of the cylinder base connection  205 . The prior art fluid actuator has a fluid connection  205  in the base and another fluid connection  208  in the head. The piston  102  does not pass over either the base  205  or the head  208  connection. The base connection  205  is always on the base side of the piston  102 . And the head connection  208  is always on the head side of the piston  102 . When the prior art fluid actuator is operating as designed, fluid does not flow from the head side of the piston  102  to the base side of the piston  102 . 
   FIGS.  3  and  4   
   Description of Bottom Portion of Prior Art Fluid Actuator 
     FIG. 3  is an isometric view of the bottom portion of the complete prior art fluid actuator shown in  FIG. 1  and  FIG. 2 .  FIG. 4  is a sectional view taken along the cutting plane B-B of  FIG. 3 . Since the operation of a fluid actuator is symmetric, it is only necessary to examine either the top portion or bottom portion for purpose of understanding the fluid actuator&#39;s operation. 
   FIGS.  3  and  4   
   Operation of Bottom Portion of Prior Art Fluid Actuator 
   Fluid flowing into cylinder base connection  205  forces piston  102  to move and piston rod  101  to extend. When the piston  102  moves and piston rod  101  extends, fluid is forced out of the cylinder head connection  208 . Fluid flowing into the cylinder head connection forces piston  102  to move and piston rod  101  to retract. When the piston  102  moves and piston  101  retracts, fluid is forced out of the cylinder base connection  205 . The prior art fluid actuator has a fluid connection  205  in the base and another fluid inlet/outlet in the head. The piston  102  does not pass over either the base connection  205  or the head connection  208 . The base connection  205  is always on the base side of the piston  102 . And the head connection  208  is always on the head side of the piston  102 . When the prior art fluid actuator is operating as designed, fluid does not flow from the head side of the piston  102  to the base side of the piston  102 . 
   DETAILED DESCRIPTIONS OF EMBODIMENTS AND THEIR OPERATIONS 
   Except where specified, the fluid used in these circuits is incompressible with insignificant foaming characteristics, a vapour point well above expected operating temperatures, and a freezing point well below expected operating temperatures. Also, the viscosity cannot be prohibitively high; if gelling occurs, it is well below expected operating temperatures. 
   The previous figures describe embodiments of prior art, whereas the following figures describe embodiments of the new invention being claimed. 
   FIGS.  5  and  6   
   Description of Fluid Actuator with a Fluid Limit Valve Containing No Moving Parts 
     FIG. 5  is an isometric view of the bottom portion of a fluid actuator with a fluid limit valve containing no moving parts.  FIG. 6  is a sectional view taken along the cutting plane C-C of  FIG. 5 . 
   FIGS.  5  and  6   
   Operation of Fluid Actuator with a Fluid Limit Valve Containing No Moving Parts 
   The piston  102  can be either on the head side or base side of the base fluid limit valve outlet holes  106 . The base fluid limit valve cover  117  covers the valve outlet holes  106  and provides a base fluid limit valve outlet  207 . A check valve will be attached to the fluid limit valve outlet  207  as later shown in fluid circuits. The check valve prevents fluid flowing into the fluid outlet  207 . 
   Consider the situation where the piston  102  is on the base side of the base fluid limit valve outlet holes  106  and the piston  102  is extending. Fluid is forced into the base connection  205  and a check valve prevents fluid flowing into the base fluid limit valve outlet  207 . The fluid forced into the base connection  205  forces the piston  102  to extend which in turn forces fluid out of the head connection  208 . The combined fluid actuator with fluid limit valve is functioning as a conventional prior art fluid actuator. Until the piston  102  extends past the base fluid limit valve outlet holes  106 , it continues to function as a conventional prior art fluid actuator. 
   Consider the situation where the piston  102  is on the head side of the valve outlet holes  106  and the piston  102  is retracting. Fluid forced into the head connections  208  causes the piston  102  to retract. As the piston  102  retracts, fluid is forced out the base connection  205  and out the base fluid limit valve outlet  207 . The combined fluid actuator with fluid limit valve is functioning as a conventional prior art fluid actuator until the piston  102  retracts past the base fluid limit valve outlet holes  106 . Once the piston  102  has retracted past the valve outlet holes  106 , fluid forced in the head connection  208  can freely flow through the valve outlet holes  106  and out the base fluid limit valve outlet  207 . As a result, the piston  102  applies negligible force against the base end cap  103 . Later circuits describe how fluid limit valves used in this manner can detect and correct for fluid loss. 
   FIGS.  7  and  8   
   Description of Fluid Actuator with an Open Fluid Limit Valve Containing Moving Parts 
     FIG. 7  is an isometric view of the bottom portion of a fluid actuator with a closed fluid limit valve with moving parts.  FIG. 8  is a sectional view taken along cutting plane D-D of  FIG. 7 . 
   FIGS.  7  and  8   
   Operation of Fluid Actuator with an Open Fluid Limit Valve Containing Moving Parts 
   While the piston  102  shown in  FIG. 8  has not retracted or extended sufficiently for the fluid limit poppet valve  674  to come in contact with either the head or base poppet plungers, the combined fluid actuator with fluid limit valve is functioning as a conventional prior art fluid actuator. 
   Consider the situation where the piston  102  is retracting. Fluid forced into the head connections  208  causes the piston  102  to retract. As the piston  102  retracts, fluid is forced out of the base connection  205 . The combined fluid actuator with fluid limit valve functions as a conventional prior art fluid actuator until the piston  102  retracts sufficiently for the fluid limit poppet valve  674  to come into contact with the base poppet plunger  116 . Force of the base poppet plunger  116  against the fluid limit poppet valve  674  compresses the poppet fluid limit valve return spring  673  and opens fluid limit poppet valve  674 . Piston  102  can retract until it comes in contact with the base poppet plunger  116 . When fluid pressure on the head side of piston  102  is greater than the base side, this fluid pressure displaces the fluid bypass head check ball  678  allowing fluid to enter the head fluid limit valve hydraulic inlet  672  of the fluid limit poppet valve  674 . Fluid forced into the head fluid limit valve hydraulic inlet  672  can freely flow through the poppet fluid limit valve bypass port  225 , the open fluid limit poppet valve  674 , the base end cap ports  223  and finally out of the base connection  205 . When piston  102  is forced to retract as a result of an external force, base cap piston stop  105  is required to restrain the piston  102  and protect against over retraction. Later circuits describe how fluid limit valves used in this manner can detect and correct for fluid loss. 
   Similarly consider the situation where the piston  102  is extending and assume the fluid limit poppet valve  674  is initially in contact with the base poppet plunger  116  with sufficient force to open the fluid limit poppet valve  674 . When fluid pressure on the base side of piston  102  is equal or greater than the head side, return spring  229  of fluid bypass head check ball  678  holds the bypass head check ball  678  closed. As a result fluid cannot flow from the base side to the head side of the piston  102 . Fluid forced into the base connection  205  causes the piston  102  to extend. As the piston  102  extends, fluid is forced out of the head connection  208 . The combined fluid actuator with fluid limit valve functions as a conventional prior art fluid actuator until the piston  101  extends sufficiently for the fluid limit poppet valve  674  to come into contact with the head poppet plunger. When the force exerted by the head poppet plunger against the fluid limit poppet valve  674  is sufficient, the poppet fluid limit valve return spring  673  is compressed and fluid limit poppet valve  674  opens. The fluid limit poppet valve  674  operates in this piston  102  extension fluid limit valve as described previously for the case of piston  102  retraction fluid limit valve. Later circuits describe how fluid limit valves used in this manner can detect and correct for fluid loss. 
   FIGS.  9  and  10   
   Description of an Alternate Embodiment of Fluid Actuator with an Open Fluid Limit Valve Containing Moving Parts 
     FIG. 9  is an isometric view of the bottom portion of a fluid actuator with a closed fluid limit valve with moving parts.  FIG. 10  is a sectional view taken along cutting plane E-E of  FIG. 9 . The fluid limit valve outlet  207  is connected to the line to the cylinder head connection  208 . 
   FIGS.  9  and  10   
   Operation of an Alternate Embodiment of Fluid Actuator with an Open Fluid Limit Valve Containing Moving Parts 
   When piston  102  has not retracted sufficiently to come in contact with the base poppet plunger  674 , and has not extended sufficiently to come into contact with the head poppet plunger, the combined fluid actuator with fluid limit valve is functioning as a conventional prior art fluid actuator. 
   Consider the situation where the piston  102  is retracting. Fluid forced into the head connections  208  causes the piston  102  to retract. As the piston  102  retracts, fluid is forced out of the base connection  205 . The combined fluid actuator with fluid limit valve functions as a conventional prior art fluid actuator until the piston  102  retracts sufficiently to come into contact with the base poppet valve  674 . The force against the base poppet valve  674  compresses the poppet fluid limit valve return spring  144  and opens fluid limit poppet valve  674 . Piston  102  can retract until it comes in contact with base cap piston stop  105 . Fluid forced into the head connection  208  is also forced into the fluid limit valve outlet  207 . Fluid forced into the fluid limit valve outlet  207  can freely flow through the base poppet valve  674  and through the fluid limit valve out  671  and finally out of the base connection  205 . When piston  102  is forced to retract as a result of an external force, base cap piston stop  105  is required to restrain the piston  102  and protect against over retraction. Later circuits describe how fluid limit valves used in this manner can detect and correct for fluid loss. 
   Similarly, consider the situation where the piston  102  is extending. Fluid forced into the base connections  205  causes the piston  102  to extend. As the piston  102  extends, fluid is forced out of the head connection  208 . The combined fluid actuator with fluid limit valve functions as a conventional prior art fluid actuator until the piston  102  extends sufficiently to come into contact with the head poppet plunger. Force of the head poppet plunger against the head fluid limit poppet valve compresses the head poppet fluid limit valve return spring and opens the head poppet fluid limit valve. Piston  102  can extend until it comes in contact with head cap piston stop. Fluid forced into the base connection  205  is also forced into the fluid limit valve head fluid connection. Fluid forced into the fluid limit valve head fluid connection can freely flow through the head poppet fluid limit valve and through the base connection and finally out of the head connection  208 . When piston  102  is forced to extend as a result of an external force, head cap piston stop is required to restrain the piston  102  and protect against over extension. Later circuits describe how fluid limit valves used in this manner can detect and correct for fluid loss. 
   FIGS.  11 ,  12   a,    12   b,    13   a  and  13   b    
   General Description of Hydraulic Cylinder with Adjustable Mechanical Limits 
   Hydro pneumatic and hydraulic cylinders with adjustable mechanical limits are used in steering, load leaving, roll control and many other fluid circuits requiring fluid actuators with adjustable mechanical limits or fluid leakage detection and correction within hydraulic linkages. The hydro pneumatic, hydraulic cylinders and the associated fluid limit valves with limit sensors are shown in  FIG. 11 ,  12   a,    12   b,    13   a,    13   b .  FIG. 11  is an isometric view of the hydro pneumatic cylinders used in the load balance and roll control circuits. For simplicity the hydro pneumatic cylinder shown in  FIG. 11  does not include head or base mountings. A cross section of the hydraulic cylinder with adjustable mechanical limit sensors and fluid limit valves  550 ,  551  is taken along the cutting plane F-F and shown in  FIG. 12   a . A cross section of the hydro pneumatic cylinder is taken along the cutting plane F-F and shown in  FIG. 12   b . In  FIGS. 12   a  and  12   b , the location of the hydraulic mechanical limit sensors and fluid limit valves  550 ,  551  is shown. The detail cross section of the hydraulic fluid limit valve without external fluid leak correction supply is shown in  FIG. 13   a . The detailed cross section of the hydraulic fluid limit valve with external fluid leak correction supply is shown in  FIG. 13   b.    
   FIGS.  11 ,  12   a,    12   b,    13   a  and  13   b    
   Detail Description and Operation of Hydraulic Cylinder with Adjustable Mechanical Limits 
   The preferred embodiments, implementing the adjustable extension limit and associated head fluid limit valve  550  and the adjustable retraction limit and associated base fluid limit valve  551  are presented. The preferred methods of adjusting the extension or retraction limits is adjusting the measured value of extension and retraction limits when the piston position is measured electrically, or piston extension when the piston activates the mechanical limit sensor at extension and retraction limits. Both electrically activated limit sensors and mechanically activated limit sensors have advantages and disadvantages. 
   In the first embodiment the piston position is electrically measured. In this embodiment the limit sensors are activated when the piston reaches a predetermined measured position. The measured position corresponding to the extension limit of the head limit sensor can be reprogrammed. The extension limit of the head limit sensor and the retraction limit of the base limit sensor are adjusted by reprogramming the measured positions. Limit sensors activated a programmed measured piston locations do not need to be integrated into the cylinder construction. These electrically activated limit sensors can easily be housed in a separate control box attached to the cylinder or near the cylinder. This allows conventional cylinders with integrated electrical position measurement to be used without modifications. However, the electrical limit sensors require electrical power source for the cylinder position measurements and to drive solenoids opening and closing the fluid limit valves. When using the electrical limit sensors, electric solenoids&#39; shutoff valves are required in addition to the fluid limit valves to close the actuators fluid inlet and outlet to prevent over extension and over retraction. The additional head shutoff valve prevents fluid from leaving the cylinder head, and the cylinder&#39;s piston  102  from over extending. And the additional base shutoff valve prevents fluid from leaving the cylinder base, and the cylinder&#39;s piston  102  from over retracting. The additional cylinder head shutoff valve is closed when head fluid limit valve  550  is opened. The additional cylinder base shutoff valve is closed when the base fluid limit valve  551  is opened. Time required to close the solenoid shutoff valves prevents exact enforcement of the fluid actuator&#39;s extension and retraction limits. As a result the adjustable mechanical limits are a more favourable embodiment. 
   In the second embodiment the mechanical limit sensors are activated by the piston mechanically forcing the fluid limit valves to open. The mechanical limit sensors can be activated directly by the piston as show in  FIG. 12   b  or indirectly by means of fluid linkage. A fluid linkage can connect an external fluid limit valve to a hydraulic limit sensor integrated into the actuator. The limit sensor is activated when the main piston  102  is a distance away from the cylinder end by a floating piston. Rather than adjusting the position of the limit sensor, the distance at which the main piston  102  is away from the cylinder end is adjusted. As shown in  FIG. 12   a , it is easy to adjust the distance the limit sensor and head fluid limit valve  550  is from the main piston  102 . The mechanical limit sensor is activated by the hydraulic floating head piston  686  mechanically forcing the head fluid limit valve  550  to open. It is easy to control the distance of the hydraulic floating head piston  686  from the main piston  102  by adjusting the amount of fluid between them. Similarly it is easy to adjust the distance of the limit sensor and the base fluid limit valve  551  from the main piston  102 , by adjusting the amount of hydraulic fluid between the floating base piston  688  and the main piston  102 . Mechanical limit sensors and fluid valves operate independently of an external power source. The hydraulically adjustable head chamber  655  between the main piston  102  and the floating head piston  686  mechanically limits the extension of the main piston  102 . The extension of the floating head piston  686  is limited by the cylinder head stops. And the minimum separation between the main piston  102  and the floating head piston  686  is controlled by the amount of incompressible fluid in the head chamber  655 . As a result the minimum separation of the main piston  102  from the cylinder head stops is adjusted by the amount of fluid in the head chamber  655 . When the floating head piston  686  reaches the cylinder head stops, it also activates the mechanical limit sensor which opens the head fluid limit valve  550 . The open head fluid limit valve  550  allows additional fluid destined for the hydraulic base chamber  654  to bypass the fluid actuator. Additional fluid forced into the hydraulic base chamber  654  would force the floating base piston  688  to extend and consequently the main piston  102  would extend. The head fluid limit valve  550  prevents fluid forced into the base of the fluid actuator from forcing the main piston  102  to over extend. Similarly the hydraulically adjustable chamber  656  between the main piston  102  and the floating base piston  688  mechanically limits the retraction of the main piston  102 . And the base fluid limit valve  551  prevents fluid forced into the hydraulic head chamber  651  of the fluid actuator from forcing the main piston  102  to over retract. Similarly the hydraulically adjustable chamber  656  of the base limit switch valve  551  prevents the hydraulic cylinder from being over retracted. The main piston  102  is mechanically prevented by the floating pistons  686  and  688  from over shooting the hydraulically adjusted extension and retraction limits. 
   The hydro pneumatic or hydraulic cylinder shown in  FIG. 11  has an outer cylinder head shell  680  which slides over the cylinder body  689 . The cylinder head shell  680  protects the integrated mechanical limit sensors and the head fluid limit valve  550 . The mechanical limit sensors operate as force sensors in hydro pneumatic cylinders. The cylinder head shell can also structurally support the piston shaft  683 , reducing the bending load on the piston shaft  683 . The cylinder head shell  680  also protects the piston shaft  683  oil seals from dirt. The cylinder head shell  680  shown in  FIG. 11  is optional, but is included because of the benefits it provides. The head gas inlet  622 , the head gas or hydraulic head limit adjustment chamber inlet  621  and the base gas or hydraulic base limit adjustment chamber inlet  623  are located in the cylinder head shell  680 . In  FIG. 12   a  the inlets  621 ,  623  are head and base limit adjustment chamber inlets. In  FIG. 12   b  the inlets  621 ,  623  are head and base gas inlets. The head hydraulic inlet  630  and the base hydraulic inlet  631  are located in the cylinder body  689 . The hydro pneumatic cylinder in  FIG. 12   b  does not include hydraulic limit adjustment chambers  655 ,  656 . In the hydro pneumatic cylinder in  FIG. 12   b , gas head  652  and base  653  chambers are used in place of the hydraulic limit adjustment chambers  655 ,  656 . The gas damping valve  665  can open between the gas pressure chambers  652  and  653 . The gas damping valve  665  and gas chambers  652  and  653  can operate to damp the main piston&#39;s  102  movement. The gas pressure in the piston gas base chamber  652  of the hydro pneumatic cylinder is able to damp piston extension. And similarly, gas pressure in the piston gas base chamber  653  of the hydro pneumatic cylinder is able to damp piston retraction. As the piston  102  approaches its extension or retraction limits, the gas in the head  652  and base  653  chambers is compressed. The increasing gas pressure is able to slow down the piston  102  as it approaches the extension or retraction limits. And gas pressure of the head  652  and base  653  chambers opposes the hydraulic pressure moving the piston  102 . Hydro pneumatic cylinders are not often used in applications requiring precise position control. Hydraulic fluid pressurized by means of accumulators is often used instead of directly using a compressible gas in hydro pneumatic cylinders. 
   Consider the hydro pneumatic fluid actuator shown in  FIG. 12   b  is controlled as hydraulic cylinder. As the floating head piston  686  approaches the cylinder head stops, it applies increasing force on the head mechanical limit sensor which increasingly opens the head fluid limit valve  550 . As the head fluid limit valve  550  increasingly opens, it allows a greater amount of fluid to bypass the fluid actuator. As more fluid bypasses the fluid actuator, less fluid goes to extending the main piston  102 . As a result the main piston slows down as the floating head piston  686  approaches the cylinder head stops. Also, the gas pressure in the head chamber  652  counters the hydraulic fluid pressure in the head chamber  651 . As a result the extension force exerted on the main piston  102  reduces as the floating head piston  686  approaches the cylinder head stops. The gas pressure within the head chamber  652  is adjustable. Increasing the gas pressure within the head chamber  652  results in the floating head piston  686  approaching the cylinder head stops with reduced force. The extension of the floating head piston  686  slows further away from the cylinder head stops. The extension of the floating head piston  686  effectively stops further away from the cylinder head stops. Similarly increasing the gas pressure within the gas base chamber  653  results in the floating base piston  688  approaching the cylinder base stops with reduced force. And the retraction of the floating base piston  688  slows and effectively stops further away from the cylinder base stops. 
   This arrangement is useful for load sensitive steering hydraulic circuits and similar circuits requiring reduced fluid actuator travel length at low load settings. During high speed vehicle operation, the steering load decreases and the maximum safe steering angle correspondingly decreases. A hydraulic steering circuit with this operating characteristic can be constructed utilizing the hydro pneumatic fluid actuator with adjustable gas head  652  and base  653  pressure chambers. At lower steering loads, the hydraulic pressures in head  651  and base  654  chambers can be reduced. The reduced hydraulic pressures in head  651  and base  654  chambers results in the pistons approaching the cylinder stops with reduced force and slowing and stopping further away from the cylinder stops. During low speed vehicle operation, full fluid actuator steering power needs to be available. Also, maximum manoeuvrability is required during lows speed operation and the extension and retraction limits of the fluid actuator should not be reduced. During low vehicle speed operation, the hydraulic pressure in head  651  and base  654  chambers is greater than the gas pressures in the fully compressed gas head  652  and base  653  pressure chambers. In this situation, the head  652  and base  653  gas chambers will remain fully compressed and there will be no gap between the floating head  686  and base  688  pistons and the main piston  102 . The main piston  102  will approach the cylinder end stops at full power and do not slow or stop before reaching the cylinder end stops. Upon the main piston  102  reaching the extension and retraction limits, the mechanical limit sensors will be activated and the fluid limit valves will allow fluid to bypass the fluid actuator. The fluid limit valves  550 ,  551  by allowing fluid to bypass the fluid actuator, prevent excessive force against the cylinder end stops. Also, when head  652  and base  653  gas chambers remain fully compressed, the limit sensors are only activated at the main piston  102  extension and retraction limits within the hydro pneumatic cylinder. and limit sensor activation is not gradual. When the limit sensor activation is not gradual, the fluid limit valves can detect and correct hydraulic fluid loss. At high hydraulic pressures where head  652  and base  653  gas chambers remain fully compressed, hydraulic fluid loss within an hydraulic linkage is detectable and correctable. The ability to detect and correct hydraulic fluid loss in a hydraulic steering circuit greatly reduces the reliability of the steering system. As a result the described hydro pneumatic cylinder with adjustable gas head  652  and base  653  chambers is well suited for load sensitive steering hydraulic circuits. 
   Alternately consider the hydro pneumatic fluid actuator shown in  FIG. 12   b  is controlled as pneumatic cylinder or shock absorber. The fluid in the hydraulic base  654  and head  651  chambers is adjusted and the position of the main piston  102  is controlled by the gas chamber  652  and  653  pressures. The hydraulic head chamber  651  between the cylinder head and the floating head piston  686  mechanically limits the maximum extension of the main piston  102 . The main piston  102  is operating as a pneumatic cylinder or shock absorber with its maximum extension reduced by the amount of fluid in the hydraulic head chamber  651 . If there is enough hydraulic fluid in the head chamber  651  such that the floating head piston  686  does not activate the mechanical limit sensor, then the head fluid limit valve  550  plays no significant role in this operation mode. Similarly the piston  102  is operating as a pneumatic cylinder or shock absorber with its maximum retraction reduced by the amount of fluid in the hydraulic base chamber  654 . Again if there is enough hydraulic fluid in the base chamber  654  such that the floating base piston  688  does not activate the mechanical limit sensor, then the base fluid limit valve  551  plays no significant role in this operation mode. 
   Head and base limit sensors can also be located in the main piston  102  or the head limit sensor in the floating head piston  686  and the base limit sensor in the floating base piston  688 . The extension of the main piston  102  is limited by the floating head piston&#39;s  686  distance from the cylinder head. The distance the floating head piston  686  is from the cylinder head is adjusted by the amount of fluid in the hydraulic head chamber  651 . When the main piston  102  is at the extension limit determined by the location of the floating head piston  686 , the head limit sensor located in the piston is activated which opens the head fluid limit valve. The open head fluid limit valve allows additional compressible fluid destined for the base gas chamber  653  to bypass the hydro pneumatic fluid actuator. Additional compressible fluid forced into the base gas chamber  653  would force the main piston  102  to extend. The head fluid limit valve prevents compressible fluid forced into the base of the hydro pneumatic fluid actuator from forcing the main piston  102  to over extend. Similarly the retraction limit of the main piston  102  is controlled by the amount of fluid in the hydraulic base chamber  654 . The retraction of the main piston  102  is mechanically limited by the floating base piston  688 . And opening the base fluid limit valve at the main piston  102  retraction limit prevents the compressible fluid from excessively forcing the main piston  102  against its retraction limit. 
   The construction of the hydro pneumatic cylinder with gas chambers  652 ,  653  shown in  FIG. 12   a  and the hydraulic cylinder with adjustable mechanical retraction and extension limits shown in  FIG. 12   b  are very similar. In  FIGS. 12   a  and  12   b , a base cap with base stops  681  is attached to the cylinder body  689 . The base limit switch valve  551  is mounted on the base cap  681 . The base limit switch poppet plunger  674  extends past the base stops. When the hydraulic floating base piston  688  or the floating head piston  686  retracts to the base stops, it activates the base limit switch valve  551 . The hollow base piston stub  685  is attached to the centre of the base cap  681 . The cylinder head shell  680  slides over the cylinder body  689  as shown in  FIG. 12   b . A head cap with head stops  682  is attached to the cylinder head shell  680 . The hollow piston shaft  683  is attached to the centre of the head cap  682 . The hollow piston shaft  683  slides over the hollow base piston stub  685 . The hydraulic head cap  684  is attached to top of the cylinder body  689 . The head limit switch valve  550  is mounted on the hydraulic head cap  684 . The main piston  102  corresponds to the hydraulic piston of the common prior hydraulic cylinders. The main piston  102  is attached to bottom of the hollow piston shaft  683 . The hydraulic cylinder with adjustable mechanical limits is constructed with the hydraulic floating head piston  686  freely moving between the main piston  102  and the hydraulic head cap  684 . The hydraulic cylinder shown in  FIG. 12   a  with adjustable mechanical limits is constructed with the hydraulic floating base piston  688  freely moving between the main piston  102  and the base cap  681 . The hydro pneumatic cylinder shown in  FIG. 12   b  is constructed with the floating head piston  686  freely moving between the main piston  102  and the head cap  684 . The hydro pneumatic cylinder is constructed with the floating base piston  688  freely moving between the main piston  102  and the base cap  681 . 
   In  FIG. 12   b , the head gas expansion chamber  650  is filled through the piston gas inlet  622  in the head cap  682 . The base hydraulic chamber  654  is filled through the hydraulic base input  631  in the base cap  681 . The head hydraulic chamber  651  is filled through the hydraulic base input  630  in the base cap  681 . The hydraulic base chamber  656  of the hydraulic cylinder shown in  FIG. 12   a  is filled through the hydraulic base chamber inlet  623  in the piston shaft  683 . The hydraulic head chamber  655  of the hydraulic cylinder shown in  FIG. 12   a  is filled through the hydraulic head chamber inlet  621  in the piston shaft  683 . The base gas extension chamber  653  of the hydro pneumatic cylinder shown in  FIG. 12   b  is filled through the gas base inlet  623  in the head cap  682 . Inlet  623  of the head cap  682  is connected to inlet  623  of the piston shaft  683 . The head gas expansion chamber  652  of the hydro pneumatic cylinder shown in  FIG. 12   b  is filled through the gas head inlet  621  in the head cap  682 . Inlet  621  of the head cap  682  is connected to inlet  621  of the piston shaft  683 . 
   In  FIG. 12   b , an optional piston hydraulic pump  620  is located inside a hollow piston shaft  683 . The internal hydraulic piston pump  620  can be used to pump hydraulic fluid into a hydraulic accumulator. This hydraulic accumulator may be used as a hydraulic pressure source or as a fluid leak correction supply. The fluid limit valve shown in  FIG. 13   b  has an inlet from the fluid leak correction supply  677 . The fluid leak correction supply can compensate for fluid loss occurring in hydraulic linkage circuits without requiring hydraulic supply pump in the hydraulic linkage circuit. The internal hydraulic piston pump  620  supplying the fluid leak correction supply can eliminate any need for external pressure supply in hydraulic linkage circuits. The system is self sustaining, the internal hydraulic piston pump  620  pumps hydraulic fluid into a hydraulic accumulator used as the fluid leak correction supply. Eventual fluid loss from the hydraulic linkage circuit is provided from the fluid leak correction supply inlet of the fluid limit valves  550  and  551 . The internal hydraulic piston pump  620  also damps the extension and retraction movement of the piston  102 . If the optional internal piston hydraulic pump is not required, the hollow base piston stub  685  can be eliminated and a solid piston stub will be used in its place. 
   More details of the fluid limit valves used in  FIG. 12   a,    12   b  are shown in  FIG. 13   a,    13   b . The base mechanical limit sensor and fluid limit valve  551  can be mounted externally on the base cap  681  as shown in  FIG. 12   b . Or the base mechanical limit sensor and fluid limit valve  551  can be mounted within the base cap  681  as shown in  FIG. 12   a . In either case the outlet  671  of the base fluid limit valve  551  is directly connected to the base hydraulic chamber  654 . Hydraulic lines are directly connected to the hydraulic fluid limit valve inlets of the externally mounted base fluid limit valve  551 . The internally mounted base fluid limit valve  551  requires inlets manufactured into the base cap  681 , which are connected to the base fluid limit valve  551  inlets. As with the externally mounted base fluid limit valve  551 , hydraulic lines are connected to the base cap  681  inlets. 
   The head mechanical limit sensor and fluid limit valve  550  can be mounted externally on the head cap  684  as shown in  FIG. 12   b . Or the head mechanical limit sensor and fluid limit valve  550  can be mounted within the head cap  684  as shown in  FIG. 12   a . In either case the outlet  671  of the head fluid limit valve  550  is directly connected to the head hydraulic chamber  651 . In  FIG. 12   b , when an outer cylinder head shell  680  is not used and the head cap  684  is exposed, hydraulic lines are directly connected to the hydraulic fluid limit valve inlets of the externally mounted head fluid limit valve  550 . The internally mounted head fluid limit valve  550  or the head cap  684  concealed by the outer cylinder head shell, requires inlets manufactured into the head cap  684 . The required inlets manufactured into the head cap  684  are connected to the head fluid limit valve  550  inlets. As shown in  FIG. 12   b , when the outer cylinder head shell  680  is used, the head cap  684  inlets are connected to hydraulic lines  636  which are manufactured into the cylinder body  689 . As with the exposed head fluid limit valve  550 , hydraulic lines are connected to the head cap  684  inlets or cylinder body hydraulic lines  636 . 
   In  FIG. 12   a , the main hydraulic piston  102  of the hydraulic cylinder with adjustment hydraulic chambers  655 ,  656  is retracted by hydraulic fluid flowing through the head hydraulic inlet  630  into the hydraulic head chamber  651 . The main piston  102  of the hydraulic cylinder with adjustment hydraulic chambers  655 ,  656  is extended by hydraulic fluid flowing through the base hydraulic inlet  631  into the hydraulic base chamber  654 . 
   To set the retraction limit of the hydraulic cylinder with a hydraulic adjustable base chamber  656 , the main piston  102  is first retracted or extended to the desired location of the retraction limit. The main piston  102  is extended by forcing fluid into the hydraulic base chamber  654 . The main piston  102  is retracted by forcing fluid into the hydraulic head chamber  651 . Once the main piston  102  is set at the desired location of the retraction limit, the fluid in the hydraulic adjustable head chamber  655  and hydraulic head chamber  651  is fixed as required to prevent the main piston  102  from moving. 
   The required amount of hydraulic fluid in the hydraulic base chamber  656  of the hydraulic or hydro pneumatic cylinder can now be set. Hydraulic fluid is forced through the hydraulic base chamber inlet  623  into the hydraulic base chamber  656 , retracting the hydraulic floating base piston  688  until it is prevented from further retracting by the cylinder base stops. The amount of hydraulic fluid forced into the hydraulic base chamber  656  is the required amount of hydraulic fluid between the main piston  102  and the floating base piston  688  for the desired retraction limit. The amount of hydraulic fluid in hydraulic base chamber  656  of the hydraulic or hydro pneumatic cylinder has been set according to the desired retraction limit. The correct amount of hydraulic fluid in the hydraulic base chamber  656  is indicated by the floating base piston  688  activating the base mechanical limit sensor. At this point the operator observes that hydraulic fluid flowing into the hydraulic base chamber  656  has ceased. Based on this observation, the operator closes off the hydraulic base chamber inlet  623 . Closing off the hydraulic base chamber inlet  623  fixes the amount of fluid in the hydraulic base chamber  656  and fixes the retraction limit as desired. Alternately the retraction limit can be automatically set by utilizing the base mechanical limit sensor with an optional electric switch. Automatically setting the retraction limit does not require an operator to observe the hydraulic flow into the hydraulic base chamber  656  and close off the hydraulic base chamber inlet  623 . When the floating base piston  688  is at the cylinder base stops, the base fluid limit valve poppet plunger  674  is compressed. The action of compressing the base fluid limit valve poppet plunger activates the base mechanical limit sensor. The activated base mechanical limit sensor changes the state of the optional base electrical switch from normal closed to open or from normal open to closed. During the procedure of setting the retraction limit, the base electrical switch changing from its normal state and signals a shutoff valve to close off the hydraulic base chamber inlet  623 . When not setting the retraction limit, changing the state of the optional electrical switch has no effect on the hydraulic base chamber inlet  623  shutoff valve. The optional base electrical switch will indicate to the operator when the floating base piston is fully retracted. When setting the retraction limit, the operator can overcome a failure of the shutoff valve to close off the hydraulic base chamber inlet  623 . This is done by manually closing off the hydraulic base chamber inlet  623  when indicated by the base electrical switch. When not setting the retraction limit, the base electrical switch indicator also informs the operator that the hydraulic cylinder is at its retraction limit. This is useful as it indicates when hydraulic fluid leakage has occurred in a hydraulic linkage circuit connecting hydraulic cylinders together. Hydraulic fluid leakage in a hydraulic linkage circuit is indicated by the connected hydraulic cylinders not reaching their corresponding retraction and extension limits simultaneously. It also indicates to the operator it is pointless to attempt to further retract the hydraulic cylinder currently at its retraction limit. The extension limit of the hydraulic or hydro pneumatic cylinder is set by means of a similar procedure. In the event of a slow hydraulic leakage effecting the amount of fluid in the hydraulic adjustment chambers  655 ,  656 , the extension and retraction limits can be reset by repeating the procedures. 
   Furthermore, the fluid actuator retraction limit can be dynamically controlled by continuously adjusting the amount of hydraulic fluid in the hydraulic base chamber  656 . The fluid actuator may be hydraulic and hydro pneumatic cylinder or hydraulic and hydro pneumatic rotary actuator. Control of the fluid actuators&#39; retraction and extension limits is very useful for imposing limits on the 3D movement of a mechanical component. When a fluid actuator&#39;s retraction and extension limits depends on the extension length and/or rotation angle of other fluid actuators. dynamically controlled mechanical extension and retraction limits are required. 
   The hydraulic base  656  and head  655  chambers can be dynamically controlled by hydraulically linking them to the extension or rotation of other fluid actuators. The complexity of the hydraulic circuits required to hydraulically link the hydraulic base  656  and head  655  chambers increases exponentially with the number of other fluid actuators on which the fluid actuator&#39;s extension and retraction limits depend. Controlling the hydraulic fluid in the base  656  or head  655  chambers by means of hydraulic linkages is preferred, when the fluid actuator&#39;s extension and retraction limits each only depend on a very small number of other fluid actuators. When the extension length and/or rotation angle of several fluid actuators affect the fluid actuator&#39;s required extension and retraction limits, it is preferred the position of the floating pistons  688 ,  686  is measured. A feed back based controller using the measure position of the floating pistons  688 ,  686  opens and closes valves accordingly to insure the correct measured displacement of the floating pistons  688 ,  686 . 
   The main piston  102  of the hydro pneumatic cylinder shown in  FIG. 12   b  is retracted by either hydraulic fluid flowing through the head inlet  630  into the head chamber  651  or pressurized fluid/gas forced through the head inlet  621  into the head chamber  652 . The main piston  102  of the hydro pneumatic cylinder is extended by either hydraulic fluid flowing through the base inlet  631  into the base chamber  654  or pressurized fluid/gas forced through the base inlet  623  into the base chamber  653 . 
   In  FIG. 12   b , a gas damping valve or fluid limit valve  665  may be built into the main piston  102 . The gas damping valve  665  between the gas head chamber  652  and the gas base chamber  653  may be opened and closed to control the damping frequency and damping stiffness. The pressure in the gas head chamber  652  required to open the gas damping valve  665  allowing flow from the gas head chamber  652  to the gas base chamber  653 , may be controlled by a reference pressure. The reference pressure regulating the flow from the gas head chamber  652  to the gas base chamber  653  is supplied via an inlet in the head cap  682 . The pressure in the gas base chamber  653  required to open the gas damping valve  665  allowing flow from the gas base chamber  653  to the gas head chamber  652 , may be controlled by a reference pressure. The reference pressure regulating the flow from the gas base chamber  653  to the gas head chamber  652  is supplied via an inlet in the head cap  682 . The damping stiffness of the hydro pneumatic cylinder is determined by the reference pressures controlling the flow between the gas head chamber  652  and the gas base chamber  653 . The damping frequency is indirectly controlled by the volume of pressurized fluid flowing through the gas damping valve  665 . 
   The hydro pneumatic cylinder controlled in this manner is operated as previously described. The hydraulic head  651  and base  654  chambers can be used to adjust the main piston  102  extension and retraction limits. The head  550  and base  551  mechanical sensors and fluid limit valves are not usable when attached to the head  684  and base  681  end caps as shown in  FIG. 12   b . Head and base mechanical sensors and fluid limit valves can be located in the piston as shown in  FIG. 8 . The retraction limit of the main piston  102  is controlled by the amount of fluid in the hydraulic base chamber  654 . The retraction of the main piston  102  is mechanically limited by the floating base piston  688 . And opening the base fluid limit valve at the main piston  102  retraction limit prevents the compressible fluid from excessively forcing the main piston  102  against its retraction limit. Similarly the extension limit of the main piston  102  is controlled and is prevented from exerting excessive force against its extension limit. Also, the described hydro pneumatic cylinder with adjustable gas head  652  and base  653  chambers is well suited for load sensitive hydraulic circuits. 
   Alternately main piston  102  of the hydro pneumatic cylinder can be retracted by either pressurized fluid/gas forced through the head inlet  630  into the head chamber  651  or hydraulic fluid flowing through the head inlet  621  into the head chamber  652 . The main piston  102  of the hydro pneumatic cylinder can be alternately extended by either pressurized fluid/gas forced through the base inlet  631  into the base chamber  654  or hydraulic fluid flowing through the base inlet  623  into the base chamber  653 . An external gas damping valve can be located between external fluid lines connecting the head chamber  651  and base chamber  654 . The external gas damping valve between the gas head chamber  651  and the gas base chamber  654  may be opened and closed to control the damping frequency and damping stiffness. The pressure in the gas head chamber  651  required to open the external gas damping valve allowing flow from the gas head chamber  651  to the gas base chamber  654 , may be controlled by a reference pressure. The reference pressure regulating the flow from the gas head chamber  651  to the gas base chamber  654  is connected to the external gas damping valve. The pressure in the gas base chamber  654  required to open the external gas damping valve allowing flow from the gas base chamber  654  to the gas head chamber  651 , may be controlled by a reference pressure. The reference pressure regulating the flow from the gas base chamber  654  to the gas head chamber  651  is connected to the external gas damping valve. The damping stiffness of the hydro pneumatic cylinder is determined by the reference pressures controlling the flow between the gas head chamber  651  and the gas base chamber  654 . The damping frequency is indirectly controlled by the volume of pressurized fluid flowing through the external gas damping valve. 
   Consider the situation where the main piston  102  of the hydro pneumatic cylinder is extended and retracted by pressurized fluid/gas forced into the head  651  and base  654  chambers. When operating as described, the hydro pneumatic fluid actuator acts as pneumatic cylinder or shock absorber with adjustable extension and retraction limits. The base  551  and head  550  mechanical limit sensors and fluid limit valves are integrated into the hydro pneumatic cylinder as shown in  FIG. 12   b . The hydraulic fluid in the base  653  and head  652  chambers is adjusted and the position of the main piston  102  is controlled by the gas base  654  and head  651  chamber pressures. The hydraulic fluid in the head chamber  652  between the main piston  102  and the floating head piston  686 , mechanically limits the maximum extension of the main piston  102 . The main piston  102  is operating as a pneumatic cylinder or shock absorber with its maximum extension reduced by the amount of hydraulic fluid in the head chamber  652 . The extension of the main piston  102  is limited by the floating head piston&#39;s  686  distance from the main piston  102 . The distance the floating head piston  686  is from the main piston  102  is adjusted by the amount of hydraulic fluid in the head chamber  652 . When the main piston  102  is at the extension limit. The floating head piston  686  activates the head limit sensor located in the cylinder head cap  684  which opens the head fluid limit valve  550 . The open head fluid limit valve  550  allows additional compressible fluid destined for base gas chamber  654  to bypass the hydro pneumatic fluid actuator. Additional compressible fluid forced into the base gas chamber  654  would force the main piston  102  to extend. The head fluid limit valve  550  prevents compressible fluid forced into the base of the hydro pneumatic fluid actuator from forcing the main piston  102  to over extend. Similarly the retraction limit of the main piston  102  is controlled by the amount of hydraulic fluid in the base chamber  653 . The retraction of the main piston  102  is mechanically limited by the floating base piston  688 , and the base fluid limit valve  551  prevents the compressible fluid from excessively forcing the main piston  102  to over retract. 
   FIGS.  13   a  and  13   b    
   Detail Description and Operation of Fluid Limit Valves 
   The detail cross-section of the fluid limit valves are shown in  FIG. 13   a  and  FIG. 13   b . The fluid limit valve body  676  of the fluid limit valve shown in  FIG. 13   a  has one hydraulic inlet  672  and one hydraulic outlet  671 . The fluid limit valve body  676  of the fluid limit valve with external fluid leak correction supply shown in  FIG. 13   b  has two hydraulic inlets  672 ,  677  and one hydraulic outlet  671 . The poppet plunger  674  extends from the limit switch body  676  out of the outlet  671 . A return spring  673  is forcing the poppet plunger  674  to close until the piston mechanically forces the poppet plunger  674  into the fluid limit valve body  676 . The hydraulic pressure at the hydraulic outlet  671  has relatively little effect on the poppet plunger  674  because of the small poppet plunger  674  area. When poppet plunger  674  is closed, it is seated against the fluid limit valve body  676  and fluid cannot flow from the fluid limit valve fluid cavity  679  out of the hydraulic outlet  671 . 
   The check valve plunger  678  is located inside the fluid limit valve body  676  at the inlet  672 . A return spring  673  is forcing the check valve plunger  678  to close until there is sufficient hydraulic pressure to compress the return spring  673  and force the check valve plunger  678  deeper into the fluid limit valve body  676  away from the inlet  672 . The check valve plunger  678  is only opened when the hydraulic pressure at inlet  672  is sufficiently greater than the fluid limit valve cavity  679  pressure to overcome the return spring  673  force. The hydraulic pressure at the hydraulic inlet  672  has a large effect on the check valve plunger  678  because of the large check valve plunger  678  area. When the check valve plunger  678  is closed, it is seated against the limit switch body  676  and fluid cannot flow from the fluid limit valve fluid cavity  679  out of the hydraulic inlet  672 . 
   The fluid limit valve with external fluid leak correction supply shown in  FIG. 13   b  has an additional hydraulic inlet  677 . The check valve plunger  678  is located inside the fluid limit valve body  676  at the inlet  677 . A return spring  673  is forcing the check valve plunger  678  to close until there is sufficient hydraulic pressure to compress the return spring  673  and force the check valve plunger  678  deeper into the fluid limit valve body  676  away from the inlet  677 . The check valve plunger  678  is only opened when the hydraulic pressure at inlet  677  is sufficiently greater than the fluid limit valve cavity  679  pressure to overcome the return spring  673  force. The hydraulic pressure at the hydraulic inlet  677  has a large effect on the check valve plunger  678  because of the large check valve plunger  678  area. When the check valve plunger  678  is closed, it is seated against the fluid limit valve body  676  and fluid cannot flow from the fluid limit valve cavity  679  out of the hydraulic inlet  677 . 
   FIG.  14   
   Description of Basic Cross Connect and Leak Compensation Illustrating The Fluid Limit Valve with Moving Parts 
   Fluid limit valves are used to compensate and correct for fluid loss in the fluid circuit. There are coordinated piston displacements of equal magnitude but opposite direction in each cylinder because of the cross connect. Fluid check valves establish unidirectional fluid flow. In addition, fluid limit valves serve several purposes. 
   FIG.  14   
   Operation of Basic Cross Connect and Leak Compensation illustrating the Fluid Limit valve with moving parts 
   The operation and functions of the fluid limit valves with moving parts are as follows: 
   First, fluid limit valves  500  and  510  can be in either a connect state or disconnect state. In connect state, fluid flows through the valves. In disconnect state, fluid flowing through the valves is prevented. 
   Second, fluid limit valves  500  and  510  are used to compensate and correct for fluid loss in the fluid circuit. Fluid loss occurs when there is a leak in the fluid circuit. Normally, as the piston of fluid actuator  320  extends, the piston of fluid actuator  322  correspondingly retracts by the same displacement volume. Also, as the piston of fluid actuator  320  retracts, the piston of fluid actuator  322  correspondingly extends by the same displacement volume. However, over time as there is fluid leakage in the fluid circuit, the piston displacement volumes will not be the same without leak compensation. 
   Third, a fluid limit valve at the cylinder head connection prevents the piston from overextending and pushing too hard against the cylinder ends. 
   Fourth, a fluid limit valve at the cylinder base connection prevents the piston from retracting too hard against the cylinder ends. This extension/retraction limiting reduces wear and tear, thus reducing the need for maintenance and increasing the lifetime of the fluid actuator. The operation of fluid limit valves is described below. 
   Fluid is drawn from the fluid reservoir by high-pressure main fluid pump  310  through line  901 . Then the fluid is pumped through fluid control valve  410  by way of line  930 . There are two possible states for fluid control valve  410 : crossover state  411  and straight-through state  412 . 
   Crossover state  411  causes the piston of fluid actuator  320  to extend and the piston of fluid actuator  322  to retract. Straight-through state  412  causes the piston of fluid actuator  320  to retract and the piston of fluid actuator  322  to extend. The process by which this occurs is described below. 
   In crossover state  411 , fluid from line  930  goes to line  911  through fluid control valve  410  and then to the cylinder head connection of fluid actuator  322  and to fluid limit valves  510 . The fluid entering the cylinder head connection of fluid actuator  322  forces its piston to retract. There are two possible cases here resulting in two different states for fluid limit valve  510 . 
   In the first case, the piston of fluid actuator  322  does not retract sufficiently to apply force to mechanical activator  341  and hence does not activate fluid limit valve  510 . Therefore, fluid limit valve  510  is in disconnect state  511  and fluid cannot flow between line  911  and line  915 . The retraction of the piston into the cylinder of fluid actuator  322  displaces fluid from the cylinder base connection of fluid actuator  322  into line  915 . Fluid flows from line  915  into fluid actuator  320 . 
   In the second case, the piston  322  retracts sufficiently to apply force to mechanical activator  341  and hence activates fluid limit valve  510 . Therefore, fluid limit valve  510  is in connect state  512 . Fluid from line  911  flows through the fluid limit valve  510  and through fluid check valve  331  into line  915 . Fluid check valve  331  prevents fluid from flowing from line  915  to line  911 ; it only allows fluid to flow from line  911  to line  915 . fluid limit valve  510  is in connect state  512  so fluid flows through it into line  915  and the cylinder base connections of fluid actuators  320  and  322 . Fluid flowing into the cylinder base connection of fluid actuator  322  counteracts the piston retraction, thus preventing the piston from retracting too hard against the cylinder ends. If piston of fluid actuator  322  is under significant external extension force, the reduced retraction force applied by the fluid bypassing the piston may allow the piston to extend until it does not activate fluid limit  510 . After reverting to the first case, the piston of fluid actuator  322  will retract until it again activates the fluid limit valve  510 . This covers the two states for fluid limit valve  510 . 
   In both cases fluid flows from line  915  into the cylinder base connection of fluid actuator  320  where it forces the piston to extend. The piston extension forces fluid out of the cylinder head connection of fluid actuator  320  into line  910 . Fluid flows from line  910  to line  903  through fluid control valve  410  in crossover state  411 . Line  903  returns the fluid to the fluid reservoir. 
   In crossover state  411 , fluid loss can be seen to have occurred when the piston of fluid actuator  322  is fully retracted and the piston of fluid actuator  320  is not fully extended. In this situation the piston of fluid actuator  322  is fully retracted, and no more fluid can be forced out of its cylinder base connection. However, the piston of fluid actuator  320  has not fully extended, therefore fluid loss has occurred. The amount of required fluid flowing through the fluid limit valve  510 , bypassing the fluid actuator  322  and extending fluid actuator  320 , is equal to the fluid loss that has occurred. Hence the circuit in the crossover state  411  with fluid limit valve  510  can both compensate and measure fluid loss. 
   In straight-through state  412 , fluid from line  930  goes to line  910  through fluid control valve  410  and then to the cylinder head connection of fluid actuator  320  and to fluid limit valves  500 . The fluid entering the cylinder head connection of fluid actuator  320  forces its piston to retract. There are two possible cases here resulting in two different states for fluid limit valve  500 . 
   In the first case, the piston of fluid actuator  320  does not retract sufficiently to apply force to mechanical limit sensor  340  and hence does not activate fluid limit valve  500 . Therefore, fluid limit valve  500  is in disconnect state  501  and fluid cannot flow between line  910  and line  915 . The retraction of the piston into the cylinder of fluid actuator  320  displaces fluid from the cylinder base connection of fluid actuator  320  into line  915 . Fluid flows from line  915  into fluid actuator  322 . 
   In the second case, the piston of the fluid actuator  320  retracts sufficiently to apply force to mechanical limit sensor  340  and hence activates fluid limit valve  500 . Therefore, fluid limit valve  500  is in connect state  502 . Fluid from line  910  flows through the fluid limit valve  500  and through fluid check valve  330  into line  915 . Fluid check valve  330  prevents fluid from flowing from line  915  to line  910 ; it only allows fluid to flow from line  910  to line  915 . fluid limit valve  500  is in connect state  502 , so fluid flows through it into line  915  and the cylinder base connections of fluid actuators  320  and  322 . Fluid flow into the cylinder base connection of fluid actuator  320  counteracts the piston retraction, thus preventing the piston from retracting too hard against the cylinder ends. If piston of fluid actuator  320  is under significant external extension force, the reduced retraction force applied by the fluid bypassing the piston may allow the piston to extend until it does not activate fluid limit  500 . After reverting to the first case, the piston of fluid actuator  320  will retract until it again activates the fluid limit valve  500 . This covers the two states for fluid limit valve  500 . 
   In both cases, fluid flows from line  915  into the cylinder base connection of fluid actuator  322  where it forces the piston to extend. The piston extension forces fluid out of the cylinder head connection of fluid actuator  322  into line  911 . Fluid flows from line  911  to line  903  through fluid control valve  410  in straight-through state  412 . Line  903  returns the fluid to the fluid reservoir. 
   In straight-through state  412 , fluid loss can be seen to have occurred when the piston of fluid actuator  320  is fully retracted and the piston of fluid actuator  322  is not fully extended. In this situation, the piston of fluid actuator  320  is fully retracted and no more fluid can be forced out of its cylinder base connection. However, the piston of fluid actuator  322  has not fully extended, therefore fluid loss has occurred. The amount of required fluid flowing through the fluid limit valve  500 , bypassing fluid actuator  320  and extending fluid actuator  322 , is equal to the fluid loss that has occurred. Hence, the circuit in the straight-through state  412  with fluid limit valve  500  can both compensate and measure fluid loss. 
   FIG.  15   
   Description of Basic Cross Connect and Leak Compensation Illustrating The Fluid Limit Valve with No Moving Parts 
   This diagram is similar to  FIG. 14 , but the fluid limit valves have no moving parts. This fluid limit valve has no disconnect state. Instead, the outlet of the fluid limit valve is connected to either the fluid on the base side of the piston or the fluid on the head side of the piston. If the fluid limit valve is integrated into the base of the cylinder actuator, the situation when the fluid limit valve outlet is connected to the base we will call self-connect, and the situation when the fluid limit valve outlet is connected to the head we will call through-connect. Similarly, if the fluid limit valve is integrated into the head of the cylinder actuator, the situation when the fluid limit valve outlet is connected to the head we will call self-connect, and the situation when the fluid limit valve outlet is connected to the base we will call through-connect. There are coordinated piston displacements of equal magnitude but opposite direction in each cylinder because of the cross connect. Fluid check valves establish unidirectional fluid flow. In addition, fluid limit valves serve several purposes. 
   FIG.  15   
   Operation of Basic Cross Connect and Leak Compensation Illustrating The Fluid Limit Valve with No Moving Parts 
   The operation and function of the fluid limit valves with no moving parts are as follows: 
   First, fluid limit valves can be in either self-connect state or through-connect state. When the fluid limit valve is integrated into the base of the cylinder actuator, the outlet of the fluid limit valve is normally connected by a fluid line to the base outlet of the cylinder actuator. Similarly when the fluid limit valve is integrated into the head of the cylinder actuator, the outlet of the fluid limit valve is normally connected by a fluid line to the head outlet of the cylinder actuator. In this configuration, during the self-connect state the fluid limit valve does not allow fluid to flow between the head and the base of the cylinder actuator. In the through-connect state the fluid limit valve does allow fluid to freely flow between the head and the base of the cylinder actuator. 
   Second, fluid limit valves are used to compensate and correct for fluid loss in the fluid circuit. Fluid loss occurs when there is a leak in the fluid circuit. Normally, as the piston of fluid actuator  320  extends, the piston of fluid actuator  322  correspondingly retracts by the same displacement volume. Also, as the piston of fluid actuator  320  retracts, the piston of fluid actuator  322  correspondingly extends by the same displacement volume. However, over time as there is fluid leakage in the fluid circuit, the piston displacement volumes will not be the same without leak compensation. 
   Third, a fluid limit valve at the cylinder head connection prevents the piston from over-extending and pushing too hard against the cylinder end. 
   Fourth, a fluid limit valve at the cylinder base connection prevents the piston from retracting too hard against the cylinder ends. This extension/retraction limiting reduces wear and tear, thus reducing the need for maintenance and increasing the lifetime of the fluid actuator. The operation of fluid limit valves is described below. 
   Fluid is drawn from the fluid reservoir by high-pressure main fluid pump  310  through line  901 . Then the fluid is pumped through fluid control valve  410  by way of line  930 . There are two possible states for fluid control valve  410 : crossover state  411  and straight-through state  412 . 
   Crossover state  411  causes the piston of fluid actuator  320  to extend and the piston of fluid actuator  322  to retract. Straight-through state  412  causes the piston of fluid actuator  320  to retract and the piston of fluid actuator  322  to extend. The process by which this occurs is described below. 
   In crossover state  411 , fluid from line  930  goes to line  911  through fluid control valve  410  and then to the cylinder head connection of fluid actuator  322  and to fluid limit valves  560 . The fluid entering the cylinder head connection of fluid actuator  322  forces its piston to retract. There are two possible cases here resulting in two different states for fluid limit valve  560 . 
   In the first case, the piston of fluid actuator  322  does not retract sufficiently to activate fluid limit valve  560 . Therefore, fluid limit valve  560  is in self-connect state  561  and fluid cannot flow between line  911  and line  915 . The retraction of the piston into the cylinder of fluid actuator  322  displaces fluid from the cylinder base connection of fluid actuator  322  into line  915 . 
   In the second case, the piston retracts sufficiently to activate fluid limit valve  560 . Therefore, fluid limit valve  560  is in through-connect state  562 . Fluid from line  911  flows through the fluid limit valve  560  and through fluid check valve  331  into line  915 . Fluid check valve  331  prevents fluid from flowing from line  915  to line  911 ; it only allows fluid to flow from line  911  to line  915 . Fluid limit valve  560  is in through-connect state  562  so fluid flows through it into line  915  and the cylinder base connections of fluid actuators  320  and  322 . Fluid flow into the cylinder base connection of fluid actuator  322  counteracts the piston retraction, thus preventing the piston from retracting too hard against the cylinder ends. If piston of fluid actuator  322  is under significant external extension force, the reduced retraction force applied by the fluid bypassing the piston may allow the piston to extend until it does not activate fluid limit valve  560 . After reverting to the first case, the piston of fluid actuator  322  will retract until it again activates the fluid limit valve  560 . This covers the two states for fluid limit valve  560 . 
   In both cases, fluid flows from line  915  into the cylinder base connection of fluid actuator  320  where it forces the piston to extend. The piston extension forces fluid out of the cylinder head connection of fluid actuator  320  into line  910 . Fluid flows from line  910  to line  903  through fluid control valve  410  in crossover state  411 . Line  903  returns the fluid to the fluid reservoir. 
   In crossover state  411 , fluid loss can be seen to have occurred when the piston of fluid actuator  322  is fully retracted and the piston of fluid actuator  320  is not fully extended. In this situation, because the piston of fluid actuator  322  is fully retracted, no more fluid can be forced out of its cylinder base connection. However, the piston of fluid actuator  320  has not fully extended, therefore fluid loss has occurred. The amount of required fluid flowing through the fluid limit valve  560 , bypassing fluid actuator  322  and extending fluid actuator  320 , is equal to the fluid loss that has occurred. Hence, the circuit in the crossover state  411  with fluid limit valve  560  can both compensate and measure fluid loss. 
   In straight-through state  412 , fluid from line  930  goes to line  910  through fluid control valve  410  and then to the cylinder head connection of fluid actuator  320  and to fluid limit valves  540 . The fluid entering the cylinder head connection of fluid actuator  320  forces its piston to retract. There are two possible cases here resulting in two different states for fluid limit valve  540 . 
   In the first case, the piston of fluid actuator  320  does not retract sufficiently to activate fluid limit valve  540 . Therefore, fluid limit valve  540  is in self-connect state  541  and fluid cannot flow between line  910  and line  915 . The retraction of the piston into the cylinder of fluid actuator  320  displaces fluid from the cylinder base connection of fluid actuator  320  into line  915 . 
   In the second case, the piston retracts sufficiently to apply force to activate fluid limit valve  540 . Therefore, fluid limit valve  540  is in through-connect state  542 . Fluid from line  910  flows through the fluid limit valve  540  and through fluid check valve  330  into line  915 . Fluid check valve  330  prevents fluid from flowing from line  915  to line  910 ; it only allows fluid to flow from line  910  to line  915 . Fluid limit valve  540  is in through-connect state  542  so fluid flows through it into line  915  and the cylinder base connections of fluid actuators  320  and  322 . Fluid flow into the cylinder base connection of fluid actuator  320  counteracts the piston retraction, thus preventing the piston from retracting too hard against the cylinder ends. If piston of fluid actuator  320  is under significant external extension force, the reduced retraction force applied by the fluid bypassing the piston may allow the piston to extend until it does not activate fluid limit  540 . After reverting to the first case, the piston of fluid actuator  320  will retract until it again activates the fluid limit valve  540 . This covers the two states for fluid limit valve  540 . 
   In both cases, fluid flows from line  915  into the cylinder base connection of fluid actuator  322  where it forces the piston to extend. The piston extension forces fluid out of the cylinder head connection of fluid actuator  322  into line  911 . Fluid flows from line  911  to line  903  through fluid control valve  410  in straight-through state  412 . Line  903  returns the fluid to the fluid reservoir. 
   In straight-through state  412 , fluid loss can be seen to have occurred when the piston of fluid actuator  320  is fully retracted and the piston of fluid actuator  322  is not fully extended. In this situation, the piston of fluid actuator  320  is fully retracted and no more fluid can be forced out of its cylinder base connection. However, the piston of fluid actuator  322  has not fully extended, therefore fluid loss has occurred. The amount of required fluid flowing through the fluid limit valve  540 , bypassing fluid actuator  320  and extending fluid actuator  322 , is equal to the fluid loss that has occurred. Hence, the circuit in the straight-through state  412  with fluid limit valve  540  can both compensate and measure fluid loss. 
   FIG.  16   
   General Description of Fluid Actuator with External Mechanical Limit Stops 
   Adjustable mechanical limit stops can be integrated into hydro pneumatic and hydraulic cylinders as shown in  FIG. 11 ,  12   a,    12   b,    13   a,    13   b  and previously described. However, adjustable mechanical limit stops need not be integrated into hydro pneumatic and hydraulic cylinders.  FIG. 16  shows prior art fluid actuators utilizing external mechanical limit stops with associated limit sensors and fluid limit valves  551 . A prior art fluid actuator could be a hydraulic cylinder, hydraulic rotary actuator, hydro pneumatic cylinder, hydro pneumatic rotary actuator, pneumatic cylinder or pneumatic rotary actuator. For illustration, a prior art hydraulic cylinder  710  was chosen as an example prior art fluid actuator. In apparatus shown in  FIG. 16 , the articulation of two separation pivots  701  is coordinated. Many applications in industry require coordination of separated pivots, such as steering, self levelling to name a few. Two prior art hydraulic cylinders  710  are used to articulate each separated pivot  701 . The two prior art hydraulic cylinders are linked together in a hydraulic circuit. The components shown in  FIG. 16  are hydraulically linked by a basic fluid linkage utilizing the fluid limit valve with moving parts as shown in  FIG. 14 . The detail cross section of the prior art hydraulic cylinders  710  is shown in  FIG. 2 . The external adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551  are same as the integrated adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551  used by the hydraulic and hydro pneumatic cylinders as shown in  12   a ,  12   b . The external adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551  lack a main piston  102  required to convert fluid pressure into an applied mechanical force. Externally the adjustable mechanical limit stop with associated limit sensors and fluid limit valves  551  appears as a smaller version of the hydraulic and hydro pneumatic cylinders as shown  FIG. 11 . In  FIG. 16 , the limit sensors and fluid limit valve  551  is shown as block diagram within the cross-section of the adjustable mechanical limit stop. A detail cross section of the fluid limit valve  551  is shown in  FIG. 13   a.    
   FIG.  16   
   Detail Description and Operation of Fluid Actuator with External Mechanical Limit Stops 
   The side frames  706  are connected to the pivot connecting frame  707  by separated pivot joints  701 . Two external adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551  are mounted on both ends of one side frame  706 . The adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551  are mounted with the mechanical limit pistons facing the other side frame  706 . Hydraulic cylinders  710  are mounted between the pivot connecting frame  707  and each side frame  706  as shown in  FIG. 16 . Each hydraulic cylinder  710  is mounted to the pivot connecting frame  707  and side frame  706  by a hydraulic cylinder mounting joint  700 . As illustrated by  FIG. 16 , the hydraulic cylinder mounting joint  700  and pivot joints  701  allows the hydraulic cylinders  710  side frame  706  and pivot connecting frame  707  to move within a plane. The side frame  706  with mounted mechanical limit stops with associated limit sensors and fluid limit valves  551  is fixed. The other side frame  706  is movable under the control of the hydraulic cylinders  710 . One hydraulic cylinder  710  is mounted between the upper half of the fixed side frame  706  and the pivot connecting frame  707 . The other hydraulic cylinder  710  is symmetrically mounted between the lower half of the movable side frame  706  and the pivot connecting frame  707 . The hydraulic circuit shown in  FIG. 14  is used to connect the hydraulic cylinders and external adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551  together. The hydraulic cylinders  710  shown in  FIG. 16  are the fluid actuators  320  and  322  labelled in the hydraulic circuit shown in  FIG. 14 . Also, the fluid limit valves  551  shown in  FIG. 16  are the fluid limit valves  500  and  510  activated at the retraction limits of the fluid actuators  320  and  322  as labelled in hydraulic circuit shown in  FIG. 14 . The mechanical limit sensor of a fluid limit valve  551  in  FIG. 16  is activated by the floating base piston retracting and compressing the poppet plunger  674  of the fluid limit valve. This is mechanical limit sensor in  FIG. 16  is the same mechanical limit sensors  340  and  341  shown in the hydraulic circuit of  FIG. 14 . In  FIG. 16 , the upper external adjustable mechanical limit stop associated with the upper hydraulic cylinder  710  is in the same manner by which the fluid limit valve  500  is associated with the fluid actuator  320  in  FIG. 14 . The lower external adjustable mechanical limit stop associated with lower hydraulic cylinder  710  is in the same manner by which the fluid limit valve  510  is associated with the fluid actuator  322  in  FIG. 14 . 
   The external adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551  are constructed as follows. The fluid limit valve  551  is located in the base portion of the adjustable mechanical limit stop. The fluid inlet  672  and outlet  672  of the fluid limit valve  551  connect to a corresponding fluid inlet and outlet of the adjustable mechanical limit stop. The fluid limit valve  551  located within the adjustable limit stop is activated by the floating base piston  688  retracting and compressing poppet plunger  674 . The floating base piston  688  is prevented from overextending and damaging the fluid limit valve  551  by base stops built into the adjustable mechanical limit stop body  702 . The mechanical limit piston  720  extends out of the head of the adjustable mechanical limit stop body  702 . The separation between floating piston  688  and the mechanical limit piston  720  is determined by the amount of hydraulic fluid in the base chamber  656 . The mechanical limit stop is adjusted by adjusting the separation between the floating piston  688  and the mechanical limit piston  720 . The separation  705  between the floating piston and the limit piston is adjusted and the amount of hydraulic fluid in the base chamber  656  is set following mechanical limit stop adjustment procedure. The same procedure described for setting the adjustable mechanical limit stop integrated into a hydro pneumatic or hydraulic cylinder is not repeated here. The floating base piston  688  is contained between the base stops integrated into the mechanical limit stop body and extension stops. The extension stops prevents the floating base piston  688  from crossing over to the wrong side of the base chamber  656  fluid inlet. The mechanical limit piston  720  is constructed with shoulder to ensure that it does not block off the base chamber  656  fluid inlet. The head chamber  721  between the mechanical limit piston  720  and the mechanical limit head is vented by means of vent  722 . The mechanical limit stop is similar to a single acting hydraulic cylinder with fluid limit valve  551  attached. Alternately the fluid limit valve  551  could be mounted externally on a prior art single acting hydraulic cylinder. The hydraulic chamber of the single acting hydraulic cylinder is equivalent to the base chamber  656  of the external adjustable mechanical limit stop. The amount of hydraulic fluid in the chamber of the single acting hydraulic cylinder is again set according to the same procedure used with the hydro pneumatic and hydraulic cylinders with adjustable mechanical limit stops. Compressing the single acting hydraulic cylinder will also compress the externally mounted fluid limit valve. Compressing the single acting hydraulic cylinder with sufficient force will activate the mechanical limit sensor by compressing the poppet valve plunger of the fluid limit valve  551 . 
   The hydraulic cylinders  710  in  FIG. 16  are connected as the fluid actuators in  FIG. 14 . As one hydraulic cylinder  710  retracts, the other hydraulic cylinder  710  correspondingly extends. As a hydraulic cylinder  710  retracts, it draws the movable side frame  706  to-wards its external adjustable mechanical limit stop. The other hydraulic cylinder  710  correspondingly extends and pushes the movable side frame  706  to-wards the external adjustable mechanical limit stop of the retracting hydraulic cylinder  710 . As the hydraulic cylinder  710  continues to retract, the side frame  706  will come in contact with its adjustable mechanical limit stop. The adjustable mechanical limit stop is prevented form sliding along the movable side frame  706  by either a notch on the upper end of the movable side frame  706  or by the hydraulic cylinder  710  mounting on the lower end of the movable side frame  706 . As the retracting hydraulic cylinder  710  retracts further, the movable side frame  706  will compress the adjustable mechanical limit stop. Compressing the adjustable mechanical limit stop will activate the associated limit sensor and open the fluid limit valve  551 . The open fluid limit valve  551  of the associated retracted hydraulic cylinder allows fluid to flow into the base of the extending hydraulic cylinder  710 . The hydraulic components used in the apparatus shown in  FIG. 16  are external adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551  and hydraulic cylinders  710 . The hydraulic fluid flow between the hydraulic components shown in  FIG. 16  is described in the operation of the hydraulic circuit shown in  FIG. 14 . Once the retracting hydraulic cylinder  710  has compressed its associated adjustable mechanical limit stop, it is prevented from further retracting further by the external mechanical limit stop. The external adjustable mechanical limit stops with associated limit sensor and fluid limit valve  551  prevents over retraction that could damage the side frames  706 . Retraction limit of the external adjustable mechanical limit stops is adjustable by the operator as required to prevent damaging the side frames  706 . Even though the two hydraulic cylinders are linked, relative piston displacements can be assumed. The length of the retracting hydraulic cylinder which causes the movable side frame  706  to reach its minimum safe distance from the fixed side frame  706  is unknown. In this apparatus shown in  FIG. 16 , the extending hydraulic cylinder  706  is not prevented by the fluid limit valve from attempting to extend further after the retraction hydraulic cylinder  706  has reached its retraction limit. Activating the fluid limit valve  551  to prevent further retraction of the retracting hydraulic cylinder  706  is not sufficient to prevent the two side frames  706  from becoming too close. The external adjustable mechanical limit stop mechanically prevents the two side frames  706  from becoming too close. This apparatus serves as an example of the operation and usage of external adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551 . In this apparatus shown in  FIG. 16 , hydraulic cylinders with external adjustable mechanical limit stops are advantageous over hydraulic cylinders with integrated adjustable mechanical limit stops. The maximum wheel steering angle of many vehicles is a function of the vehicle ride height and tire size. External adjustable mechanical limit stops with associated limit sensors and fluid limit valves  551  can be incorporated into the hydraulic steering circuit of such vehicles. By means of the external adjustable mechanical limit stop, the steering limits determined by the vehicle ride height and tire size can be statically or dynamically adjusted to prevent damaging scrubbing between tire and vehicle body. 
   CONCLUSION, RAMIFICATIONS, AND SCOPE 
   Accordingly, the reader will see that prior art hydraulic circuits have not been able to fully replace mechanical linkages in precision applications. Precision applications where hydraulic circuits have not been able to fully replace mechanical linkages include vehicle steering and other systems requiring accurate reliable correlation which linkages provide. Mechanical linkages reliably correlate the movement of mechanical components. Hydraulic circuits used to replace mechanical linkages use two or more linear actuators, rotary actuators or fluid motors to control the movement of mechanical components. In a hydraulic circuit, these hydraulic actuators or motors are connected by a hydraulic fluid conduit with possible intermediary fluid control valves and fluid pumps. The hydraulic circuits used to replace mechanical linkages are hydraulic linkages. Replacing mechanical linkages with hydraulic linkages have significant advantages over mechanical linkages. Hydraulic conduits required to construct hydraulic linkages can be easily routed. Hydraulic circuits can easily switch operating modes. In each operation mode the hydraulic circuit can form a hydraulic linkage between a different set of mechanical components or the mechanical components can be controlled independently in a completely uncorrelated manner. To replace mechanical linkages, hydraulic circuits need to be able to detect and correct fluid loss in hydraulic linkages and require limit stops to prevent damaging over extension or over retraction. Through the use of limit sensors and fluid limit valves, the hydraulic linkage can include leakage compensation and leakage location detection and allow for accurate control over the extension and retraction of a piston in the fluid actuator. Mechanical stops prevent over extension and over retraction and are strong enough to resist the full force of the hydraulic actuator or the full force of the mechanical load. Conventional actuators include mechanical stops. However, the mechanical stops included in conventional actuators are not adjustable. Mechanical components in different orientations may require mechanical limit stops to be repositioned. Without adjustable mechanical actuator stops, actuator movement often cannot be stopped before damaging over extension or over retraction occurs. 
   To fully understand the advantages of a hydraulic linkage, some existing systems that could benefit from fluid linkages should be considered. Using a hydraulic circuit to construct a hydraulic linkage in a steering system has numerous advantages in that
         It permits a simplified vehicle design. With the hydraulic linkage, there is no need for a mechanical linkage to connect the operator&#39;s steering wheel with the vehicle&#39;s turning wheels and there is no need for a mechanical linkage to connect the left and right turning wheels together. Thus, the engineer has more flexibility on how turning wheels are attached to a vehicle.   It permits a vehicle to be designed without the need to penetrate the body with a mechanical linkage because left and right turning wheels can be connected without a mechanical linkage. Thus, the body will be stronger and can easily be made airtight and waterproof.   It permits a vehicle to be designed without the need to protect an external mechanical steering linkage from road hazards.   It permits a vehicle to be designed without the need to accommodate the mechanical steering linkage.   It permits a vehicle to be designed without a collapsible steering linkage because no mechanical linkage is required between the operator&#39;s steering wheel and the vehicle&#39;s turning wheels.   It permits a trailer to follow in the tracks of the towing vehicle because trailer wheels can easily be steered in coordination with the vehicle. Thus, there is a reduced turning radius and much improved handling with no need to take wide turns around corners.   It permits coordination of the turning wheels of the trailer with the turning wheels of the vehicle. Also, it is easy to disable the coordination by disconnecting couplings or stopping fluid flow through valves.   It permits coordinated turning of the vehicle and turning of the trailer, so the trailer tracks the same wheel path as the vehicle. This allows for different modes of operation to be selected depending on the speed of the vehicle or the desired handling characteristics of the operator, whereas a mechanical linkage system can only be efficiently designed for one mode of operation.   It permits the steering system to be designed such that on soft surfaces, the trailer wheels can be designed to track the vehicle wheels. Substantially less pulling power is required when the trailer follows in the path already cut by the pulling vehicle.   It permits the steering system to be designed such that when passing a vehicle, the trailer wheels will steer with the vehicle wheels to a lesser degree to reduce vehicle spinning, fishtailing, and jackknifing induced by lane changes.   It permits the steering system to be designed such that when parking a vehicle, the trailer wheels can be steered in the same direction as the vehicle wheels or in the opposite direction of the vehicle wheels. Also, the trailer wheels can be left stationary. This versatility allows much greater mobility of the vehicle and trailer in parking.   Similarly, it permits the vehicle to have front and rear attachments like a snowplow, snowblower, or lawn mower that can also be steered.   It permits two or more vehicles to be hooked together and the steering of all of these can be coordinated.   It permits complete redundancy in the steering system through identical but independent fluid linkage circuits.       

   The advantages of using a hydraulic linkage for self-levelling are as follows:
         It permits a simpler and more cost effective design with no mechanical linkage required.   It permits a bucket tip hydraulic cylinder at the end of a telescopic loader to be connected to hydraulic lift cylinders through a fluid linkage.   It permits design of a self-levelling system with a multiple piece lift arm. Several hydraulic lift cylinders will be used to control the multiple piece lift arm. The fluid displaced by these multiple hydraulic lift cylinders from the multiple piece lift arm can be combined to control the self-levelling bucket tip hydraulic cylinder.   It permits self-correction for fluid leakage, unlike conventional hydraulic flow divider valves that require adjustment and tuning.   It permits the operator to feel a feed load on the control actuator proportional to servomotor actuator load.   It permits a vehicle operator to detect a reduction of wheel grip on the road through the ability to feel the load on the vehicle turning wheels. Thus, the driver has better vehicle control and can prevent skidding more effectively.   It permits an operator to control and prevent stall through the ability to feel the load on aerodynamic control surfaces.   It permits a crane or excavator operator to perform very delicate work safely through the ability to feel load.       

   Although the above description contains many specificities, these should not be construed as limiting on the scope of the invention, but as merely providing illustrations of some of the presently preferred embodiments of this invention. Many other variations are possible. For example, all embodiments using linear fluid actuators with pistons moving linearly within a cylinder can equivalently use rotary fluid actuators with vanes rotating within a cylinder. Also, a fluid actuator with adjustable mechanical limits having one or more additional piston ( 690 ,  691 ), which have an adjustable separation from the main piston ( 102 ), can be equivalent constructed from multiple standard fluid actuators and motors. A standard fluid actuator or motor without limit sensors or fluid limit valves provides the function of the main piston ( 102 ). When the limit sensors are activated, fluid limit valves ( 550 ,  551 ) open and allow fluid to bypass this fluid actuator or motor in the same manner as the main piston ( 102 ) was bypassed. If adjustable mechanical limit stops are not required, no additional fluid actuators are required. Standard fluid actuator or motor along with limit sensors and fluid limit valves is sufficient. Also, where hydraulic fluid is used in the embodiments any other incompressible fluid could alternately be used in place of the hydraulic fluid. 
   If adjustable mechanical limit stops are required, additional fluid actuators can be used to replace the adjustable mechanical limit stops. The additional fluid actuator to be used as an adjustable mechanical limit stop is connected in the appropriate location as required to stop movement of the mechanical components. The piston inside the cylinder of the fluid actuator operating as an adjustable mechanical limit stop will extend and retract freely until it is prevented from extending further by the fluid between the piston and the cylinder head, or it is prevented from retracting further by the fluid between the piston and the cylinder base. By adjusting the amount of fluid between the piston and the cylinder head and between the piston and the cylinder base, the limits of extension and retraction of the fluid actuator operating as an adjustable mechanical limit stop are adjusted. The fluid actuator operating as an adjustable mechanical limit stop is mounted with limit sensor. When the piston of this fluid actuator is prevented from extending further by the fluid between the piston and the cylinder head, or it is prevented from retracting further by the fluid between the piston and the cylinder base, it applies force to the limit sensor. When force is applied to the limit sensor used with a fluid actuator containing incompressible fluid operating as an adjustable mechanical limit stop, the limit sensor activates. Fluid limit valves open when the limit sensor is activated as described in the embodiment of the hydraulic cylinder with additional pistons. When force is applied to the limit sensor used with a fluid actuator containing compressible fluid operating as an adjustable mechanical limit damper, the limit sensor activate in proportion to the applied force. Fluid limit valves open in proportion to the degree the limit sensors are activated, when the limit sensor is activated as described in the embodiment of the hydro-pneumatic cylinder with additional pistons. The additional piston provided by the fluid actuator operation as an adjustable mechanical limit stop is equivalent to the additional piston ( 690 ,  691 ) of the preferred embodiment of fluid actuator with adjustable mechanical limits. Described component embodiments may be assembled to form a variety of embodiments equivalent to the presented preferred embodiment of the fluid actuator with adjustable mechanical limits. 
   If a servomechanism with adjustable limits is required, the fluid actuators  320  and  322  are connected to form a fluid linkage servo feedback control. In such a servomechanism, the operator controls the position of one fluid actuator  320 , and the other fluid actuator  322  is attached by a mechanical or magnetic connection to the drive actuator. Fluid conduits  911 ,  915  are further connected to servo drive valve actuators. As a typical servomechanism, when the feedback actuator  322  does not track the movement of the control actuator  320 , fluid displaces the servo drive valve actuators. The servo drive valve actuators act on the drive actuator&#39;s control valve which in turn causes the drive actuator and connected feedback actuator  322  to move such that the feedback actuator  322  tracks the position of the control actuator  320 . The adjustable limits of the control actuator  320  limit the maximum extension and retraction of the drive actuator by means of the servomechanism. 
   The fluid limit valves  550 ,  551  are either normally closed or normally open. When normally closed, they open when activated by limit sensors  344 ,  345 . When normally open, they close when activated by limit sensors  344 ,  345 . Normally open and normally closed fluid limit valves  550 ,  551  can be used in combination or separately. A hydraulic linkage actuator incorporating adjustable soft limit stops will include either a floating piston, with a gas damping valve, subdividing the outer head gas chamber  650  or an additional pneumatic actuator connected by a fluid linkage to the outer gas chamber  650  containing a floating piston. The floating piston activates a normally open fluid limit valve. As the main piston  102  approaches its retraction limit, the gas in the subdivided head gas chamber  650  compresses in proportion to the applied force. As the gas in the head gas chamber  650  compresses, the floating piston proportionally retracts and activates the normally open fluid limit valve. Activation of the normally open fluid limit valve results in restricted fluid flow through it. As the main piston  102  approaches its retraction limit, the normally open fluid limit valve is progressively activated. This restricts fluid flow, thereby resulting in a reduced retraction speed and reduced applied impact force between the base cap  682  and pistons. 
   Thus the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.