Patent Publication Number: US-9850889-B2

Title: Hydraulic fluid control system for a diaphragm pump

Description:
1. RELATED APPLICATIONS 
     This application claims priority of previously filed U.S. Provisional Patent Application Ser. No. 61/300,786, which was filed on Feb. 2, 2010, the contents of which are herein incorporated by reference in their entirety. 
    
    
     2. TECHNICAL FIELD 
     The present invention relates generally to hydraulic diaphragm pumps. More particularly, the present invention relates to hydraulic fluid control system for a hydraulic diaphragm pump and a method of operating such hydraulic fluid control system to control the volume and/or pressure of hydraulic fluid in the pump. 
     3. BACKGROUND 
     Positive displacement hydraulic diaphragm type pumps are known in the art for delivery of a pumped process fluid by a means of a pumping action between inlet and outlet valves. Hydraulic diaphragm type pumps typically make use of a deformable diaphragm fluidly connected to a hydraulic fluid chamber and located between the inlet and outlet valves between which the process fluid is pumped by constrictive pressure exerted by the diaphragm. The diaphragm is in turn forced to move by a powered hydraulic fluid displacement mechanism that displaces hydraulic fluid into and out of the hydraulic fluid chamber surrounding the hydraulic diaphragm. One particular type of diaphragm is the hose diaphragm. 
     A deformable hose diaphragm is typically a generally cylindrical membrane, or bladder, with 2 openings, one at substantially each end of the hose diaphragm, to separate the process fluid inside of the diaphragm from a hydraulic fluid chamber surrounding the diaphragm. The hose diaphragm is typically constructed from substantially impervious materials permissive of deformation to change the internal volume of the diaphragm, such as pliable and/or elastic materials like polymeric, plastic, metallic foil, rubber materials, in solid or laminated form, for example. Preferably the process fluid flows from one end through to the other end of the hose diaphragm. Due to the substantially straight flow of the process fluid through the hose diaphragm, and the separation between the process fluid and the hydraulic fluid, this type of positive displacement pump is typically suited for pumping highly viscous materials, abrasive, reactive or corrosive materials, slurries and sludges, as well as less viscous fluids at a wide range of pressures. Although hose diaphragm pumps are discussed in particular below, the field of the present invention applies to all forms of hydraulic diaphragm pumps. In the case of hydraulic diaphragm pumps using an alternate diaphragm such as a flat or substantially planar diaphragm, separately or in combination with a hose diaphragm, the description below may be interpreted such that the two working surfaces of the alternate diaphragm correspond to the inside and outside of a hose diaphragm. 
     Hydraulic diaphragm pumps according to the art may typically provide a constrictive pressure around the diaphragm to provide the necessary pumping action of the process fluid inside the diaphragm by displacing the hydraulic fluid in a hydraulic fluid chamber surrounding the diaphragm, to constrict (effectively decreasing the internal volume of the diaphragm and the process fluid within) and expand (effectively increasing the internal volume of the diaphragm and the process fluid within) the diaphragm respectively. During operation of the hydraulic diaphragm pump, changes in the volume of hydraulic fluid in the hydraulic fluid chamber(s) surrounding the hydraulic diaphragm(s) may result due to leaks or losses of hydraulic fluid such as through seals, connections and/or imperfections in the hydraulic fluid system. Such changes in the volume of hydraulic fluid in the hydraulic fluid chamber(s) of the pump may result in undesired changes to the volume and/or range of extension and constriction of the hydraulic diaphragm, such as excessive expansion or stretching of the diaphragm on the suction portion of the pump stroke. Such changes in the extension/constriction operating range of the diaphragm may lead to undesirable reduced pump efficiency, wear, and/or premature failure of the hydraulic diaphragm. 
     Accordingly, there is a need for a hydraulic fluid control system for a hydraulic diaphragm pump that addresses some of the limitations of existing hydraulic diaphragm pump designs, and particularly hose diaphragm pump designs according to the art. 
     4. SUMMARY 
     It is an object of the present invention to provide a hydraulic fluid control system for a hydraulic diaphragm pump that addresses some of the limitations of the prior art. 
     Another object of the present invention is to provide a method for controlling a hydraulic fluid control system for a hydraulic diaphragm pump that addresses some of the limitations of the prior art. 
     According to an embodiment of the present invention, a hydraulic fluid control system for a hydraulic diaphragm pump comprising at least one hydraulic diaphragm containing a process fluid and which is surrounded by at least one hydraulic fluid chamber containing a hydraulic fluid is provided. In such embodiment, the hydraulic fluid control system comprises: 
     a differential pressure sensor operable to detect and measure a pressure difference between the process fluid contained in the at least one hydraulic diaphragm and the hydraulic fluid contained in the at least one hydraulic fluid chamber; 
     a hydraulic fluid reservoir containing hydraulic fluid; and 
     a hydraulic fluid pump fluidly connected to the hydraulic fluid reservoir and the at least one hydraulic fluid chamber, and operable to provide a volume of hydraulic fluid to the at least one hydraulic fluid chamber in response to the pressure difference measured by the differential pressure sensor. 
     According to another embodiment of the invention, a method of operating a hydraulic fluid control system for a hydraulic diaphragm pump comprising at least one hydraulic diaphragm containing a process fluid and which is surrounded by at least one hydraulic fluid chamber containing a hydraulic fluid is provided. In such embodiment, the method of operating the hydraulic fluid control system comprises: 
     detecting a position of the at least one hydraulic diaphragm which corresponds to a desired point of the pump cycle; 
     measuring a pressure differential between the process fluid pressure and the hydraulic fluid pressure; 
     comparing the measured pressure differential with a setpoint pressure differential which corresponds to a desired limit of hydraulic fluid pressure or volume; and 
     providing a volume of hydraulic fluid to the hydraulic fluid chamber with the hydraulic fluid pump if the measured pressure differential is greater than the setpoint pressure differential. 
     Further advantages of the invention will become apparent when considering the drawings in conjunction with the detailed description. 
    
    
     
       5. BRIEF DESCRIPTION OF THE DRAWINGS 
       The hydraulic fluid control system and method of fluid control therefore of the present invention will now be described with reference to the accompanying drawing figures, in which: 
         FIG. 1  illustrates a schematic view of an exemplary hydraulic fluid control system for a hydraulic diaphragm pump according to an embodiment of the present invention. 
         FIG. 2  illustrates a schematic view of two exemplary hydraulic pump diaphragm housings corresponding to a suction stroke, according to an embodiment of the invention. 
         FIG. 3  illustrates a schematic view of two exemplary hydraulic pump diaphragm housings corresponding to a pumping stroke, according to another embodiment of the invention. 
         FIG. 4  illustrates a graphical representation of hydraulic and process fluid pressures of an exemplary hydraulic pump diaphragm, according to an embodiment of the invention. 
         FIG. 5  illustrates a schematic view of an exemplary hydraulic pump diaphragm corresponding to the graph of hydraulic and process fluid pressures shown in  FIG. 4 , according to an embodiment of the invention. 
         FIG. 6  illustrates a graphical representation of hydraulic and process fluid pressures of an exemplary hydraulic pump diaphragm, according to a further embodiment of the invention. 
         FIG. 7  illustrates a schematic view of an exemplary hydraulic pump diaphragm corresponding to the graph of hydraulic and process fluid pressures shown in  FIG. 6 , according to a further embodiment of the invention. 
         FIG. 8  illustrates a graphical representation of fluid pressure differential vs. hydraulic fluid loss showing exemplary upper and lower setpoint pressure differential values for an exemplary hydraulic pump diaphragm, according to another embodiment of the invention. 
         FIG. 9  illustrates a schematic view of an exemplary hydraulic fluid control system for a hydraulic diaphragm pump according to yet another embodiment of the present invention. 
     
    
    
     Similar reference numerals refer to corresponding parts throughout the several views of the drawings. 
     6. DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention are described below with reference to the Figures of the drawings. It is intended that the embodiments and Figures disclosed herein are to be considered illustrative rather than restrictive. 
       FIG. 1  illustrates a schematic view of an exemplary hydraulic fluid control system for a hydraulic diaphragm pump according to an embodiment of the present invention. The hydraulic diaphragm pump includes hydraulic diaphragms  122  and  124 , which separate a process fluid  116  within the diaphragm, such as a slurry or other fluid desired to be pumped, from hydraulic working fluid pumping chambers  121  and  123  respectively, within pump compression housings  104  and  106 . The hydraulic working fluid pumping chambers  121  and  123  may preferably be filled with a hydraulic working fluid, such as a hydraulic oil, water or other suitable working fluid operable to exert pressure against the hydraulic diaphragms  122  and  124 . The hydraulic diaphragm  122 ,  124  may typically seal against the shell or ends of the hydraulic fluid compression housings  104 ,  106  to contain the hydraulic fluid in chambers  121 ,  123 , between the housing and the hydraulic diaphragm to facilitate compression and expansion of the hydraulic diaphragms  122 ,  124 . 
     The hydraulic working fluid compression housings  104  and  106  are operable to alternately compress hydraulic diaphragms  122  and  124  during a pumping stroke (effectively decreasing the internal volume of the hydraulic diaphragm and the process fluid within) and expand (effectively increasing the internal volume of the hydraulic diaphragm and the process fluid within) hydraulic diaphragms  122  and  124  during a suction stroke, in response to displacement of the hydraulic working fluid into or out of the hydraulic fluid chambers  121  and  123  in pump compression housings  104 ,  106 , respectively. In one embodiment of the invention, hydraulic working fluid may be displaced into and out of hydraulic fluid chambers  121  and  123 , respectively, in opposite phase to each other, in order to alternatingly displace hydraulic working fluid into one of hydraulic fluid chambers  121  and  123 , while simultaneously displacing hydraulic working fluid out of the other hydraulic fluid chamber. In such opposite phase operation of working fluid chambers  121  and  123 , alternating constricting forces (during a pumping stroke) and expanding forces (during a suction stroke) may be applied to hydraulic diaphragms  122  and  123  in opposite phase (i.e. 180 degree phase difference) to each other, resulting in the alternate pumping of the process fluid  116  through diaphragms  122  and  124 . In one such embodiment, such alternate pumping of process fluid  116  through diaphragms  122  and  124  may desirably result in a substantially constant or steady state flow of pumped process fluid  117  from common process fluid outlet  130 . In other embodiments, two or more hydraulic fluid chambers may operate with different phase differences, such as to provide continuous, discontinuous or other desired process fluid output flow characteristics, for example. 
     Hydraulic fluid compression housings  104  and  106  may typically comprise inlet ends  118  and  120 , and outlet ends  126  and  128 , respectively, which may typically each comprise a unidirectional flow control valve to allow process fluid  116  to enter compression housings  104  and  106  through inlet ends  118  and  120  and to exit through outlet ends  126  and  128 , while substantially preventing or reducing process fluid backflow. Accordingly, inlet ends  118  and  120  and outlet ends  126  and  128  may comprise any suitable type of flow control valve, typically a one-way passively operated valve, such as ball, cone, or poppet check valves, for example. Alternatively, actively operated flow control valves may also be used. Common process fluid flow inlet  114  is fluidly connected to inlet ends  118  and  120  to provide process fluid  116 , and common process fluid flow outlet  130  is fluidly connected to outlet ends  126  and  128  to receive pressurized pumped process fluid  117 . 
     In one embodiment, hydraulic diaphragms  122  and  124  may comprise substantially annular hydraulic hose diaphragms, which may be made from one or more suitable resilient and/or elastic materials such as polymeric, plastic, and rubber materials, within which the process fluid may be pumped. In such an embodiment, hydraulic fluid chambers  121  and  123  may comprise an annular chamber situated between the walls of pump compression housings  104  and  106 , and the outside of hose diaphragms  122  and  124 , for example. In other embodiments, hydraulic diaphragms  122 ,  124  may comprise other types of pump diaphragms, such as planar diaphragms, for example. In yet a further embodiment, the hydraulic diaphragm pump may comprise only one compression chamber  104 , or may alternatively comprise three or more compression chambers connected to a common process fluid inlet  114  and outlet  130 . 
     The hydraulic diaphragm pump of  FIG. 1  further comprises a hydraulic fluid drive source  108  which is fluidly connected to hydraulic fluid chambers  121  and  123  by hydraulic fluid lines  110  and  112 , respectively. Hydraulic fluid drive source  108  is operable to displace hydraulic fluid into and out of chambers  121  and  123  to compress and expand hydraulic diaphragms  122  and  124 , respectively, to produce the pumping action of the pump. Hydraulic fluid drive  108  is powered by a drive motor  102 , to drive the displacement of hydraulic fluid into and out of chambers  121  and  123 . In one embodiment, hydraulic fluid drive source  108  comprises a hydraulic fluid drive cylinder whereby a reciprocating linear motion of a hydraulic fluid piston within hydraulic fluid drive cylinder  108  is effective to alternatingly displace hydraulic fluid in and out of hydraulic fluid chambers  121  and  123 , and thereby to apply alternating constricting forces (during a pumping stroke) and expanding forces (during a suction stroke) on hydraulic diaphragms  122  and  123  in opposite phase to each other, resulting in the alternate pumping of the process fluid  116  through diaphragms  122  and  124 . In an alternative embodiment, more than 2 hydraulic diaphragms may be used collectively to pump a process fluid  116  in response to displacements of hydraulic fluid surrounding the hydraulic diaphragms, such as 3, 4, 6, or 8 hydraulic diaphragms for example. In another alternative embodiment, a single compression housing with one or more hydraulic diaphragms may be used to pump a process fluid  116 , such as in applications not requiring continuous flow of the process fluid, for example. In yet another embodiment, multiple hydraulic diaphragms may be incorporated in each of one or more compression housings  104 , such as a hose diaphragm to contain process fluid  116 , in conjunction with a flat diaphragm separating the hose diaphragm from the hydraulic fluid and hydraulic drive source  108 , for example, as may be desirable for providing redundant protection against hydraulic diaphragm failure in some applications. 
     In a further embodiment, drive motor  102  may comprise a linear motor, such as an electromagnetic linear motor which may be electrically controllable. In another embodiment, one or more linear motors may be used to drive hydraulic drive cylinder  108 . In an alternative embodiment, drive motor  102  may comprise a conventional reciprocating drive source such as an electrically driven bellcrank reciprocating drive, for example. 
     The hydraulic fluid control system of  FIG. 1  further comprises a hydraulic fluid reservoir  170  containing hydraulic fluid  172 , which supplies hydraulic fluid through fluid conduits  162  and  164  to controllable hydraulic fluid pumps  158  and  160 . Hydraulic fluid pumps  158  and  160  are controllable to supply hydraulic fluid to hydraulic fluid chambers  121  and  123  through hydraulic fluid lines  150  and  152 , respectively, to allow for adjustment of hydraulic fluid volume in chambers  121  and  123  to compensate for changes in hydraulic fluid volume such as due to leakage or loss of hydraulic fluid from the hydraulic pump system, for example. Accordingly, controllable hydraulic fluid pumps  158  and  160  may supply hydraulic fluid  172  through hydraulic fluid lines  150  and  152  to hydraulic fluid chambers  121  and  123 , via hydraulic fluid chamber pump ends  125  and  127  respectively, which are fluidly connected to hydraulic fluid chambers  121  and  123 . In an optional embodiment, hydraulic fluid reservoir  170  may also comprise individual hydraulic fluid return conduits  166  and  168 , which lead from pumps  158  and  160  to common return conduit  174  into reservoir  170 , such as for the return of hydraulic fluid removed from hydraulic fluid chambers  121 ,  123  by pumps  158  and  160 , for example. 
     The hydraulic fluid control system also comprises differential pressure sensors  138  and  140 , which are in fluid communication with hydraulic fluid lines  150  and  152  (which are in turn fluidly connected to hydraulic fluid chambers  121  and  123 ) through hydraulic fluid sensor conduits  146  and  148 , respectively. Differential pressure sensors  138  and  140  are also in fluid communication with pressurized process fluid  117  in outlet ends  126  and  128  of compression housings  104  and  106 , through process fluid sensor conduits  142  and  144 , respectively. Accordingly, differential pressure sensors  138  and  140  are operable to detect and measure a pressure differential between the pressurized process fluid  117  and the hydraulic fluid in hydraulic fluid chambers  121  and  123 , respectively. In one embodiment of the present invention, pressure differential sensors  138  and  140  may be operable to control hydraulic fluid pumps  158  and  160 , and thereby to control the flow of hydraulic fluid  172  into hydraulic fluid chambers  121  and  123 , respectively. In such an embodiment, differential pressure sensors  138  and  140  may be used to detect and measure a pressure differential between process fluid  117  and hydraulic fluid in chambers  121  and  123  such as due to a loss or leak of hydraulic fluid from chambers  121 ,  123 , hydraulic drive cylinder  108 , or hydraulic lines  110 ,  112 , for example, and to thereby trigger and control the flow of hydraulic fluid  170  to be added to chambers  121 ,  123 , to maintain a substantially constant hydraulic fluid volume in chambers  121 ,  123 , for example. In a particular embodiment, differential pressure sensors  138  and  140  may comprise differential pressure transducers, for example, however, any suitable type of sensor for detecting and measuring pressure differential between process fluid  117  and hydraulic fluid in chambers  121 ,  123  may optionally be implemented. 
     In an automated embodiment of the present invention, the hydraulic fluid control system also comprises a controller  132  which is connected to differential pressure sensors  138  and  140 , and also preferably to controllable hydraulic fluid pumps  158  and  160 , such as by electrical cables, wireless connection or other suitable connection means. In such an embodiment, controller  132  may comprise any suitable electronic control unit, such as a programmable electronic controller, which is operable to control hydraulic pumps  158  and  160  using differential pressure measurements from differential pressure sensors  138  and  140 . In a particular embodiment, controller  132  may comprise a programmable logic controller (or PLC) which executes a control program comprising computer readable instructions to effect control of the hydraulic fluid pumps  158 ,  160  to add hydraulic fluid  170  to hydraulic fluid chambers  121  or  123  in response to pressure differentials measured by sensors  138 ,  140  due to hydraulic fluid loss or leaks. In a further illustrative embodiment, a DMC-A2 controller available from MacroSensors™, may be used as an example of controller  132 . 
     In a further embodiment according to the present invention, the hydraulic fluid control system also comprises position sensors  134  and  136  which are operable to detect the position of hydraulic fluid drive cylinder  108  at the ends of its travel, and therefore, to detect the endpoint of the suction stroke (when the displacement of hydraulic fluid expanding the hydraulic diaphragm ends) and pumping stroke (when the displacement of the hydraulic fluid constricting the hydraulic diaphragm ends) of the hydraulic diaphragms  122  and  124 . In such embodiment, position sensors  134  and  136  may preferably also be connected to controller  132 , and the position sensor information may be used to detect the pressure differential from sensors  138 ,  140  corresponding to the end of the suction stroke of hydraulic diaphragms  122 ,  124 , to control the operation of hydraulic fluid pumps  158  and  160  to add hydraulic fluid  172  to hydraulic chambers  121 ,  123 , for example. In a particular embodiment, position sensors  134  and  136  may comprise Hall Effect sensors operable to detect the position of hydraulic drive cylinder  108  at the end of the suction stroke of a hydraulic diaphragm, however, alternatively, any suitable position sensor operable to detect the end of a suction stroke may be employed. 
     Referring now to  FIG. 2 , a schematic view of two exemplary hydraulic pump diaphragm housings corresponding to a suction stroke are shown, according to an embodiment of the invention. The first hydraulic diaphragm pump housing  204  includes inlet end  218  and outlet end  226  each comprising a unidirectional check valve such as a ball valve, for example, to admit the pumping flow of process fluid  216  in through the inlet end  218 , into housing  204 , to be pressurized and pumped out through the outlet end  226 . Housing  204  also includes a hydraulic diaphragm  222  such as a hose diaphragm, surrounded by hydraulic fluid chamber  221  containing hydraulic fluid, such as hydraulic oil, for example. The hydraulic diaphragm  222  is shown in a compressed or constricted condition, corresponding to the beginning of a suction stroke, during which process fluid  216  may be drawn into the hydraulic diaphragm  222 . In order to apply suction to admit process fluid  216  during the suction stroke, hydraulic fluid is withdrawn from hydraulic fluid chamber  221  by hydraulic fluid line  210 , so as to apply an expansive force to the hydraulic diaphragm  222  causing it to expand and increase in volume inside housing  204 , thereby drawing process fluid  216  into hydraulic diaphragm  222  under suction. 
     The second hydraulic diaphragm pump housing  206  shown in  FIG. 2  similarly includes an inlet end  220  and outlet end  228  comprising unidirectional check valves, to admit process fluid  216  into and out of housing  206 . Housing  206  also includes hydraulic diaphragm  224  such as a hose diaphragm, surrounded by hydraulic fluid chamber  223  which is filled with hydraulic fluid. The hydraulic diaphragm  224  is shown in an expanded condition, corresponding to the end of a suction stroke, when the withdrawal of hydraulic fluid from hydraulic fluid chamber  223  through hydraulic fluid line  212  has caused the hydraulic diaphragm  224  to be filled by process fluid  216  by suction, and to expand the hydraulic diaphragm  224  within housing  206 . Accordingly, it can be seen that the hydraulic diaphragm elements  222  and  224  may typically act as a flexible membrane between the hydraulic fluid filled chamber  221 ,  223  and the process fluid  216  inside the flexible hydraulic diaphragm  222 ,  224 . The hydraulic diaphragm element  222 ,  224  may typically respond to displacement of hydraulic fluid from chambers  221 ,  223  by deforming so as to maintain a substantially constant pressure on either side of the diaphragm, such that there is typically little strain of the hydraulic diaphragm material  222 ,  224  over the range of expansion and constriction during a suction stroke of a hydraulic diaphragm pump using one or more such hydraulic diaphragms  222 ,  224  housed in hydraulic diaphragm pump housings such as those shown in  FIG. 2 . 
     Referring now to  FIG. 3 , a schematic view of two exemplary hydraulic pump diaphragm housings corresponding to a pumping stroke are shown, according to an embodiment of the invention. Similar to as described above in reference to  FIG. 2 , the first hydraulic diaphragm pump housing  304  includes inlet and outlet ends  318  and  326  each comprising a unidirectional check valve to admit the pumping flow of process fluid  317  to be pressurized and pumped out through the outlet end  326 . Housing  304  also includes a hydraulic diaphragm  322  such as a hose diaphragm, surrounded by hydraulic fluid chamber  321  containing hydraulic fluid. However, the hydraulic diaphragm  322  is shown in an expanded condition, corresponding to the beginning of a pumping stroke, during which process fluid  317  may be pumped out of the hydraulic diaphragm  322  and out through outlet end  326 . In order to pump process fluid  317 , hydraulic fluid is forced into hydraulic fluid chamber  321  by hydraulic fluid line  310 , so as to apply a constrictive force to the hydraulic diaphragm  322  causing it to constrict and decrease in volume inside housing  304 , thereby pumping process fluid  317  out of the hydraulic diaphragm  322  under pressure. 
     The second hydraulic diaphragm pump housing  306  shown in  FIG. 3  similarly includes an inlet end  320  and outlet end  328  comprising unidirectional check valves, to admit process fluid  317  into and out of housing  306 . Housing  306  also includes hydraulic diaphragm  32 , surrounded by hydraulic fluid chamber  323  which is filled with hydraulic fluid. The hydraulic diaphragm  324  is shown in constricted condition, corresponding to the end of a pumping stroke, when the flow of hydraulic fluid into hydraulic fluid chamber  323  through hydraulic fluid line  312  has caused the hydraulic diaphragm  324  to be constricted or squeezed under pressure, which contracts the hydraulic diaphragm  324  within housing  306  and forces process fluid  317  through outlet end  328  under pressure. Similar to as described above in reference to  FIG. 2 , the hydraulic diaphragm elements  322  and  324  typically act as a flexible membrane and may typically respond to displacement of hydraulic fluid into chambers  321 ,  323  during the pumping stroke by deforming so as to maintain a substantially constant pressure on either side of the diaphragm, such that there is typically little strain of the hydraulic diaphragm material  322 ,  324  over the range of expansion and constriction during a pumping stroke of a hydraulic diaphragm pump using one or more such hydraulic diaphragms. 
     In one embodiment of the invention, in the case where the hydraulic fluid system as illustrated in  FIGS. 2 and 3  is a closed system with a constant volume of hydraulic fluid, the repeated cycling of the hydraulic diaphragm pump between the suction and pumping stroke positions shown in  FIGS. 2 and 3  may result in a substantially constant degree of expansion and constriction of the hydraulic diaphragm member within the hydraulic pump housings, which may result in substantially little strain or stretching of the hydraulic diaphragm. However, in another embodiment where leaks or other losses or changes of volume of hydraulic fluid in the hydraulic fluid system may occur during continued operation of a hydraulic diaphragm pump, the relative position of the hydraulic diaphragm and thereby the degree of constriction of the diaphragm during the pumping stroke, and particularly the degree of expansion of the diaphragm during the suction stroke may increase due to the change in hydraulic fluid volume. Such an increase in one or more of the degree of constriction and/or expansion of the hydraulic membrane during continued pump operation may then result in the strain or “stretching” of the hydraulic diaphragm at the extremes of the pumping and suction strokes, for example. 
     In a further embodiment of the present invention, it may be preferred to limit the degree of strain or “stretching” of the hydraulic diaphragm experienced over the range of suction and pumping strokes of a hydraulic diaphragm pump, due to the increased wear such strain or stretching may induce to the material of hydraulic diaphragm. In particular, common hydraulic pump diaphragms such as flat diaphragms and/or hose diaphragms, for example may typically be constructed from elastomeric materials, which may commonly be sensitive to repeated strain or stretching under cyclic loading conditions. In particular, increased cyclic strain of such elastomeric diaphragm materials may result in premature diaphragm failure, such as may be due to the exacerbation of minor structural manufacturing defects, which may grow under cyclic strain loading until the diaphragm material fractures or ruptures, for example. 
     In reference to  FIG. 4 , a graphical representation of hydraulic and process fluid pressures of an exemplary hydraulic pump diaphragm is shown, according to an embodiment of the invention. In  FIG. 5 , a schematic view of an exemplary hydraulic pump diaphragm corresponding to the graph of hydraulic and process fluid pressures shown in  FIG. 4  is illustrated, according to an embodiment of the invention. Similar to as described above in reference to  FIG. 2 , the hydraulic pump diaphragm shown in  FIG. 5  comprises a hydraulic diaphragm pump housing  504  including inlet end  518  and outlet end  526  which each comprise a unidirectional check valve to admit the pumping flow of process fluid  516  in through the inlet end  518 , into housing  504 , to be pressurized and pumped out through the outlet end  526 . Housing  504  also includes a hydraulic diaphragm  522  surrounded by hydraulic fluid chamber  521  containing hydraulic fluid. The exemplary hydraulic diaphragm  522  of  FIG. 5  is shown in a relaxed condition, corresponding to the end of a suction stroke. In such a relaxed condition, the hydraulic diaphragm  522  has been filled with process fluid  516  under suction, due to the withdrawal of hydraulic fluid from hydraulic fluid chamber  521  by hydraulic fluid line  510 , however, the hydraulic diaphragm  522  is only expanded and filled with process fluid  516  until it reaches a resting shape which corresponds to an unstressed condition where the diaphragm material is under substantially no strain or stress. 
     In one embodiment, the fluid pressure traces shown in  FIG. 4  correspond to the expansion and constriction of a hydraulic diaphragm pump which is operated such that the condition of the hydraulic diaphragm at the end of the suction stroke is relaxed or unstressed, as shown in  FIG. 5 . In such a hydraulic diaphragm pump, the fluid pressures of the hydraulic fluid in hydraulic fluid chamber  522  and of the process fluid pumped out of outlet end  526  may be substantially equal, both during the pumping and suction strokes of the pump. As can be seen in  FIG. 4 , during first and second suction strokes, the hydraulic fluid pressure traces  402  and  406  are substantially identical to the process fluid pressure traces  412  and  416 , and therefore, the pressure difference between the hydraulic fluid and the process fluid is substantially zero as shown in pressure difference traces  422  and  426 . Similarly, during the pumping stroke, the hydraulic fluid pressure trace  404  is substantially identical to the process fluid pressure trace  414 , and the pressure difference between the hydraulic fluid and the process fluid is therefore also substantially zero as shown in pressure difference trace  424 . 
     In reference to  FIG. 6 , a graphical representation of hydraulic and process fluid pressures of another exemplary hydraulic pump diaphragm is shown, according to an embodiment of the invention in which the hydraulic diaphragm is not substantially relaxed or unstressed at the end of a suction stroke, for example. In  FIG. 7 , a schematic view of an exemplary hydraulic pump diaphragm corresponding to the graph of hydraulic and process fluid pressures shown in  FIG. 6  is illustrated, according to an embodiment of the invention. Similar to as described above in reference to  FIG. 5 , the hydraulic pump diaphragm shown in  FIG. 7  comprises hydraulic diaphragm pump housing  704  including inlet and outlet ends  718  and  726  which each comprise a unidirectional check valve to admit the pumping flow of process fluid  716  in through the inlet end  718 , into housing  704 , to be pressurized and pumped out through the outlet end  726 . Housing  704  also includes a hydraulic diaphragm  722  surrounded by hydraulic fluid chamber  721  containing hydraulic fluid. The exemplary hydraulic diaphragm  722  of  FIG. 7  is shown in a slightly strained or stretched condition, corresponding to the end of a suction stroke in which the expansion of the hydraulic diaphragm is continued past the point of the diaphragm being in a relaxed or substantially unstressed condition. In such slightly strained or stretched condition of the hydraulic diaphragm  722 , the diaphragm  722  has been filled with process fluid  716  under suction, due to the withdrawal of hydraulic fluid from hydraulic fluid chamber  721  by hydraulic fluid line  710 , however, the hydraulic diaphragm  722  has been expanded and filled with process fluid  716  until it reaches a stretched shape which corresponds to an strained condition where the diaphragm material is under at least a slight degree of strain or stress (somewhat analogous to the slight inflation of a balloon, where the balloon “diaphragm” is slightly stressed or expanded). 
     In such an embodiment, the hydraulic diaphragm  722  does not act as a completely flexible membrane between the process fluid  716  and hydraulic fluid in chamber  721 , since the expansion or stretching of the diaphragm  722  at the end of the suction stroke requires a stretching force to overcome the modulus of the diaphragm material. Accordingly, at the end of the suction stroke when the hydraulic diaphragm is stretched beyond its relaxed condition, the pressure of the process fluid  716  inside the hydraulic diaphragm  722  is at least slightly greater than the hydraulic fluid pressure in the hydraulic fluid chamber  721  which surrounds the diaphragm, providing a pressure difference or differential pressure sufficient to expand or stretch the diaphragm. 
     In one embodiment, the fluid pressure traces shown in  FIG. 6  correspond to the expansion and constriction of a hydraulic diaphragm pump which is operated such that the condition of the hydraulic diaphragm at the end of the suction stroke is at least slightly strained or stretched beyond its relaxed shape, as shown in  FIG. 7 . In such a hydraulic diaphragm pump, the fluid pressures of the hydraulic fluid in hydraulic fluid chamber  722  and of the process fluid  716  in the hydraulic diaphragm  722  may be substantially equal during the pumping stroke, but the process fluid pressure may be at least slightly greater than the hydraulic fluid pressure at the end of the suction stroke, as the hydraulic diaphragm  722  begins to be stretched. As can be seen in  FIG. 6 , during first and second suction strokes, the hydraulic fluid pressure traces  702  and  706  are substantially similar to the process fluid pressure traces  712  and  716  until the end of the suction stroke, when the process fluid pressure is greater, and therefore, there is a discernable increase in the pressure difference between the hydraulic fluid and the process fluid towards the end of the suction stroke as shown in pressure difference traces  722  and  726 . However, since the diaphragm is not substantially stretched while it is constricted or squeezed during the pumping stroke, the hydraulic fluid pressure trace  704  is substantially identical to the process fluid pressure trace  714 , and the pressure difference between the hydraulic fluid and the process fluid is substantially zero during the pumping stroke, as shown in pressure difference trace  724 . 
     In an embodiment where a hydraulic hose diaphragm is used in the pump, the stretching or expansion of the diaphragm beyond its relaxed state may typically represent a radial tension or positive hoop stress in the hose diaphragm. 
     In one embodiment, the operation of a hydraulic diaphragm pump such that the hydraulic diaphragm is at least slightly stretched at the end of the suction stroke may occur due to the leakage or loss of hydraulic fluid from the hydraulic fluid system of the pump. Such leakage may occur through common sources such as leakage of hydraulic seals, lines or other components through damage, wear or just typical operating conditions, for example. In such an embodiment, as the volume of hydraulic fluid in the closed hydraulic fluid system decreases, the position of the hydraulic diaphragm at the end of the suction stroke may become more stretched or expanded, such as represented by a greater radial tension in a hose type diaphragm for example. Such increased radial tension in the hose diaphragm may then typically result in a greater discernable pressure differential between the process fluid pressure and hydraulic fluid pressure towards the end of the suction stroke. Conversely, if hydraulic fluid is added to the hydraulic fluid system and the volume of hydraulic fluid in the closed system increases, the position of the hydraulic diaphragm at the end of the suction stroke may become relatively less stretched, or under less radial tension, which may typically result in a smaller discernable pressure differential between the process fluid pressure and hydraulic fluid pressure towards the end of the suction stroke. 
     In a particular embodiment, it is desired to be able to maintain a correct or optimum range of hydraulic fluid volume in the hydraulic fluid system, in order to prevent the hydraulic diaphragm from being under-expanded if the hydraulic fluid volume is too high (which may prevent proper filling of the diaphragm with process fluid and may decrease pump efficiency), and to prevent over-expansion of the diaphragm if the hydraulic fluid volume is too low (which may result in undesirable stress or positive tension in the hydraulic diaphragm and may lead to premature diaphragm failure or rupture). Accordingly in one embodiment of the present invention, the pressure differential between the process fluid pressure in the hydraulic diaphragm and the hydraulic fluid pressure may be measured at a particular point of the hydraulic pump cycle, such as the end of the suction stroke of the pump, and the measured pressure differential may then be used to control the addition (or removal) of hydraulic fluid from the hydraulic fluid system in order to maintain a desired volume of hydraulic fluid and corresponding desired degree of strain or stretch (or positive tension in the case of a hose-type diaphragm) in the hydraulic diaphragm. 
     Referring now to  FIG. 8 , a graphical representation of fluid pressure differential vs. hydraulic fluid loss showing exemplary upper and lower setpoint pressure differential values for an exemplary hydraulic pump diaphragm is shown, according to another embodiment of the invention. In an embodiment of the invention directed to a hydraulic fluid control system, a relation between the pressure differential  800  measured between the process fluid pressure and hydraulic fluid pressure in a hydraulic diaphragm pump, and a corresponding percentage of hydraulic fluid volume loss  802  may be established to define a substantially linear relationship  804 , for example. After such relationship  804  is determined, such as by empirical testing of the measured pressure differential  800  against particular changes in hydraulic fluid volume  802 , upper and lower setpoints  808  and  806  may be selected to define a desired range of hydraulic fluid volumes for correct or optimum operation of the hydraulic diaphragm pump. In order to provide a control mechanism for maintaining the hydraulic fluid volume of the pump system within the desired range, the corresponding upper and lower setpoint pressure differential values  808  and  806  may be used to control the addition and/or removal of hydraulic fluid from the hydraulic fluid system. For example, in one embodiment, a hydraulic fluid volume controlling pump may be controlled to add hydraulic fluid from the pump system if a measured pressure differential value exceeds the upper setpoint value  808 , or optionally to remove hydraulic fluid from the pump system if a measured pressure differential value is less than the lower setpoint value  806 . 
     In another embodiment directed to a hydraulic diaphragm pump system similar to as shown in  FIG. 1 , for example, controllable hydraulic fluid pumps  158 ,  160  may be controlled to add hydraulic fluid  172  from hydraulic fluid reservoir  170  to hydraulic pump chambers  121 ,  123 , if the pressure differential measured by differential pressure sensors  138 ,  140  exceeds a selected upper setpoint value, for example. In an automated embodiment, a control module  132  may execute a control program comprising instructions to:
         a) detect a position of the hydraulic drive cylinder  108 , such as may be detected by position sensors  134 ,  136 , which corresponds to a desired point of the pump cycle, such as the end of the suction stroke, for example;   b) measure a pressure differential value between a process fluid pressure and a hydraulic fluid pressure, such as by measuring a signal from a differential pressure sensor  138 ,  140 ;   c) comparing the measured pressure differential value with a setpoint pressure differential value which corresponds to a desired limit of hydraulic fluid pressure or volume; and   d) if the measured pressure differential value is greater than the setpoint value, control a hydraulic fluid pump to add hydraulic fluid to the hydraulic fluid system;
 
Optionally, the control module may also determine if the measured pressure differential value is less than a setpoint value, and if so, may control a hydraulic fluid pump to remove hydraulic fluid from the hydraulic fluid system. In one such optional embodiment, a single bidirectional controllable hydraulic fluid pump may be used for each hydraulic diaphragm, which may be controlled to either add or remove hydraulic fluid from the hydraulic fluid system for its hydraulic diaphragm, as controlled by the control module in response to measured differential pressure values between the process fluid pressure and hydraulic fluid pressure for the hydraulic diaphragm in question, for example.
       

     Referring now to  FIG. 9 , a schematic view of an exemplary hydraulic fluid control system for a hydraulic diaphragm pump according to yet another embodiment of the present invention is shown. Similar to as described above in reference to  FIG. 1 , the hydraulic fluid control system for a hydraulic diaphragm pump of the embodiment of  FIG. 9  includes hydraulic diaphragm pump compression housings  104  and  106  which house hydraulic diaphragms  122  and  124 , which separate a process fluid  116  within the diaphragm from hydraulic working fluid pumping chambers  121  and  123  respectively. The hydraulic working fluid pumping chambers  121  and  123  may preferably be filled with a hydraulic working fluid and hydraulic diaphragms  122 ,  124  may typically seal against the shell or ends of the hydraulic fluid compression housings  104 ,  106  to contain the hydraulic fluid in chambers  121 ,  123 , between the housing and the hydraulic diaphragm. Accordingly, the hydraulic fluid is operable to facilitate compression and expansion of the hydraulic diaphragms  122 ,  124  such as to alternately compress hydraulic diaphragms  122  and  124  during a pumping stroke (effectively decreasing the internal volume of the hydraulic diaphragm and the process fluid within) and expand (effectively increasing the internal volume of the hydraulic diaphragm and the process fluid within) hydraulic diaphragms  122  and  124  during a suction stroke, in response to displacement of the hydraulic working fluid into or out of the hydraulic fluid chambers  121  and  123 . 
     In one embodiment of the invention, hydraulic working fluid may be displaced into and out of hydraulic fluid chambers  121  and  123 , respectively, in opposite phase to each other, in order to alternatingly displace hydraulic working fluid into one of hydraulic fluid chambers  121  and  123 , while simultaneously displacing hydraulic working fluid out of the other hydraulic fluid chamber. In such opposite phase operation of working fluid chambers  121  and  123 , alternating constricting forces (during a pumping stroke) and expanding forces (during a suction stroke) may be applied to hydraulic diaphragms  122  and  123  in opposite phase (i.e. 180 degree phase difference) to each other, resulting in the alternate pumping of the process fluid  116  through diaphragms  122  and  124 . In one such embodiment, such alternate pumping of process fluid  116  through diaphragms  122  and  124  may desirably result in a substantially constant or steady state flow of pumped process fluid  117  from common process fluid outlet  130 . In other embodiments, two or more hydraulic fluid chambers may operate with different phase differences, such as to provide continuous, discontinuous or other desired process fluid output flow characteristics, for example. 
     Hydraulic fluid compression housings  104  and  106  may typically comprise inlet ends  118  and  120 , and outlet ends  126  and  128 , respectively, which may typically each comprise a unidirectional flow control valve to allow process fluid  116  to enter compression housings  104  and  106  through inlet ends  118  and  120  and to exit through outlet ends  126  and  128 , while substantially preventing or reducing process fluid backflow. Similar to the system described in reference to  FIG. 1 , inlet ends  118  and  120  and outlet ends  126  and  128  may comprise any suitable type of flow control valve, typically a one-way passively operated valve, such as ball, cone, or poppet check valves, for example. Common process fluid flow inlet  114  is fluidly connected to inlet ends  118  and  120  to provide process fluid  116 , and common process fluid flow outlet  130  is fluidly connected to outlet ends  126  and  128  to receive pressurized pumped process fluid  117 . 
     In one embodiment, hydraulic diaphragms  122  and  124  may comprise substantially annular hydraulic hose diaphragms similar to as described above. In other embodiments, hydraulic diaphragms  122 ,  124  may comprise other types of pump diaphragms, such as planar diaphragms, for example. In yet a further embodiment, the hydraulic diaphragm pump may comprise only one compression chamber  104 , or may alternatively comprise three or more compression chambers connected to a common process fluid inlet  114  and outlet  130 . 
     Similar to as described above, the hydraulic diaphragm pump of  FIG. 9  further comprises a hydraulic fluid drive source  108  which is fluidly connected to hydraulic fluid chambers  121  and  123  by hydraulic fluid lines  110  and  112 , respectively. Hydraulic fluid drive source  108  is operable to displace hydraulic fluid into and out of chambers  121  and  123  to compress and expand hydraulic diaphragms  122  and  124 , respectively, to produce the pumping action of the pump. Hydraulic fluid drive  108  is powered by a drive motor  102 , to drive the displacement of hydraulic fluid. In one embodiment, hydraulic fluid drive source  108  comprises a hydraulic fluid drive cylinder whereby a reciprocating linear motion of a hydraulic fluid piston within hydraulic fluid drive cylinder  108  is used to displace hydraulic fluid in and out of hydraulic fluid chambers  121  and  123 , and thereby to apply alternating constricting forces (during a pumping stroke) and expanding forces (during a suction stroke) on hydraulic diaphragms  122  and  123  in opposite phase to each other, resulting in the alternate pumping of the process fluid  116  through diaphragms  122  and  124 . In an alternative embodiment, more than 2 hydraulic diaphragms may be used collectively to pump a process fluid  116  in response to displacements of hydraulic fluid surrounding the hydraulic diaphragms, such as 3, 4, 6, or 8 hydraulic diaphragms for example. In another alternative embodiment, a single compression housing with one or more hydraulic diaphragms may be used to pump a process fluid  116 , such as in applications not requiring continuous flow of the process fluid, for example. In yet another embodiment, multiple hydraulic diaphragms may be incorporated in each of one or more compression housings  104 , such as a hose diaphragm to contain process fluid  116 , in conjunction with a flat diaphragm separating the hose diaphragm from the hydraulic fluid and hydraulic drive source  108 , for example, as may be desirable for providing redundant protection against hydraulic diaphragm failure in some applications. 
     In a further embodiment, drive motor  102  may comprise a linear motor, such as an electromagnetic linear motor which may be electrically controllable. In another embodiment, one or more linear motors may be used to drive hydraulic drive cylinder  108 . In an alternative embodiment, drive motor  102  may comprise a conventional reciprocating drive source such as an electrically driven bellcrank reciprocating drive, for example. 
     Similar to as described above in reference to  FIG. 1 , the hydraulic fluid control system of the embodiment shown in  FIG. 9  further comprises a hydraulic fluid reservoir  170  containing hydraulic fluid  172 , which supplies hydraulic fluid through hydraulic fluid conduits  162  and  164  to bidirectional controllable hydraulic fluid pumps  958  and  960 . Hydraulic fluid pumps  958  and  960  are bidirectional, and in one direction of operation are controllable to supply hydraulic fluid to hydraulic fluid chambers  121  and  123  through hydraulic fluid supply lines  950  and  952 , respectively, to allow for addition of hydraulic fluid volume in chambers  121  and  123  to compensate for changes in hydraulic fluid volume such as due to leakage or loss of hydraulic fluid from the hydraulic pump system, for example. Bidirectional controllable hydraulic fluid pumps  958  and  960  are further operable in a second direction of operation to controllably withdraw hydraulic fluid from hydraulic fluid chambers  121  and  123  through hydraulic fluid withdrawal lines  150  and  152 , respectively, to allow for the removal of hydraulic fluid volume from chambers  121  and  123 , such as to compensate for over-filling of the hydraulic system, or changes in desired pump stroke volume or pressure within the hydraulic diaphragms  122  and  124 , for example. Hydraulic fluid reservoir  170  may also comprise individual hydraulic fluid return conduits  166  and  168 , which lead from pumps  958  and  960  to common return conduit  174  into reservoir  170 , for the return of hydraulic fluid removed from hydraulic fluid chambers  121 ,  123  by pumps  958  and  960 , for example. 
     Accordingly, in one direction of operation, bidirectional controllable hydraulic fluid pumps  958  and  960  may supply hydraulic fluid  172  through hydraulic fluid lines  950  and  952  to hydraulic fluid chambers  121  and  123 , via hydraulic fluid chamber inlet ends  918  and  920  respectively, which are fluidly connected to the inlet ends of hydraulic fluid chambers  121  and  123 . In a second direction of operation, bidirectional hydraulic fluid pumps  958  and  960  may withdraw hydraulic fluid  172  through hydraulic fluid withdrawal lines  150  and  152 , via hydraulic fluid chamber outlet pump ends  125  and  127  respectively, which are fluidly connected to the outlet end of hydraulic fluid chambers  121  and  123 . In an alternative embodiment, hydraulic fluid may be added and/or withdrawn from either the inlet or outlet end of hydraulic fluid chambers  121  and  123 , or both, however in a preferred embodiment hydraulic fluid may be added to the inlet end of chambers  121  and  123  in order to desirably reduce any air or other gas bubbles in the hydraulic fluid. In a further optional embodiment, an optional hydraulic fluid filter may also be installed on hydraulic fluid withdrawal lines  150  and  152 , or between pumps  958  and  960  and the hydraulic oil reservoir  170 , to filter hydraulic fluid returning to the reservoir  170 . 
     In a further embodiment, the hydraulic fluid control system may further comprise check valves on hydraulic fluid addition or withdrawal lines, such as to control or prevent backflow and/or pressure surges in hydraulic fluid lines. For example, hydraulic fluid addition lines  950  and  952  may include check valves  954  and  956 , and hydraulic fluid withdrawal lines  150  and  152  may include check valves  154  and  156 . Similarly, hydraulic fluid return lines  166  and  168  may also comprise check valves, such as valves  966  and  968 , for example. In another optional embodiment, bidirectional hydraulic fluid pumps  958  and  960  may include check valves integrated within the pump body, to avoid the need for independent check valves such as valves  954 ,  956 ,  154  and  156 , for example. In a further optional embodiment, hydraulic fluid return lines  166  and  168  may further comprise at least one flow throttling or flow control device  966 ,  968 , such as a needle valve or pressure relief valve for example, located between hydraulic fluid pumps  958  and  960  and hydraulic fluid reservoir  170 . 
     Similar to as described above in reference to  FIG. 1 , the hydraulic fluid control system embodiment shown in  FIG. 9  also comprises differential pressure sensors  138  and  140 , which are in fluid communication with hydraulic fluid lines  150  and  152  (which are in turn fluidly connected to hydraulic fluid chambers  121  and  123 ) through hydraulic fluid sensor conduits  146  and  148 , respectively. Differential pressure sensors  138  and  140  are also in fluid communication with pressurized process fluid  117  in outlet ends  126  and  128  of compression housings  104  and  106 , through process fluid sensor conduits  142  and  144 , respectively. Accordingly, differential pressure sensors  138  and  140  are operable to detect and measure a pressure differential between the pressurized process fluid  117  and the hydraulic fluid in hydraulic fluid chambers  121  and  123 , respectively. In one embodiment, process fluid sensor conduits  142  and  144  and hydraulic fluid sensor conduits  146  and  148  may each comprise check valves such as valves  942  and  944 , and  946  and  948 , for example. Incorporation of check valves on the pressure sensor conduits may desirably provide for controlling backflow and/or pressure surges in conduits  142  and  144  and  146  and  148 , or to allow for isolation of differential pressure sensors  138  and  140 , such as for sensor protection or maintenance, for example. 
     In one embodiment of the present invention, pressure differential sensors  138  and  140  may be operable to control bidirectional hydraulic fluid pumps  958  and  960 , and thereby to control the addition and withdrawal of hydraulic fluid  172  into or out of hydraulic fluid chambers  121  and  123 , respectively. In such an embodiment, differential pressure sensors  138  and  140  may be used to detect and measure a pressure differential between process fluid  117  and hydraulic fluid in chambers  121  and  123  such as an increase in pressure differential at the end of a suction stroke which may be due to a loss or leak of hydraulic fluid from chambers  121 ,  123 , hydraulic drive cylinder  108 , or hydraulic lines  110 ,  112 , for example, and to thereby trigger and control the flow of hydraulic fluid  170  to be added to chambers  121 ,  123 , to maintain a substantially constant hydraulic fluid volume in chambers  121 ,  123 , for example. In another embodiment, differential pressure sensors  138  and  140  may also be use to detect and measure a decrease in pressure differential at the end of a suction stroke, such as may be due to an overfilling of hydraulic fluid in chambers  121  and  123 , for example. 
     Similar to as described above, in a particular embodiment, differential pressure sensors  138  and  140  may comprise differential pressure transducers, for example, however, any suitable type of sensor for detecting and measuring pressure differential between process fluid  117  and hydraulic fluid in chambers  121 ,  123  may optionally be implemented. 
     In an automated embodiment of the present invention, the hydraulic fluid control system also comprises a controller  132  which is connected to differential pressure sensors  138  and  140 , and also preferably to bidirectional controllable hydraulic fluid pumps  958  and  960 , such as by electrical cables, wireless connection or other suitable connection means. In such an embodiment, controller  132  may comprise any suitable electronic control unit, such as a programmable logic controller (or PLC), which is operable to control bidirectional hydraulic pumps  958  and  960  using differential pressure measurements from differential pressure sensors  138  and  140 . In a particular embodiment, controller  132  may comprise a programmable logic controller such as a DMC-A2 controller available from MacroSensors™, which executes a control program comprising computer readable instructions to effect control of the bidirectional hydraulic fluid pumps  958 ,  960  to either add hydraulic fluid  172  to hydraulic fluid chambers  121  or  123  in response to pressure differentials measured by sensors  138 ,  140  such as due to hydraulic fluid loss or leaks, or to withdraw hydraulic fluid  172  from chambers  121  or  123  in response to pressure differentials measured by sensors  138 ,  140 , such as due to overfilling of hydraulic fluid, for example. 
     Similar to as described above, in a further embodiment according to the present invention, the hydraulic fluid control system also comprises position sensors  134  and  136  which are operable to detect the position of hydraulic fluid drive cylinder  108  at the ends of its travel, and therefore, to detect the endpoint of the suction stroke (when the displacement of hydraulic fluid expanding the hydraulic diaphragm ends) and pumping stroke (when the displacement of the hydraulic fluid constricting the hydraulic diaphragm ends) of the hydraulic diaphragms  122  and  124 . In such embodiment, position sensors  134  and  136  may preferably also be connected to controller  132 , and the position sensor information may be used to detect the pressure differential from sensors  138 ,  140  corresponding to the end of the suction stroke of hydraulic diaphragms  122 ,  124 , to control the operation of bidirectional hydraulic fluid pumps  958  and  960  to add or withdraw hydraulic fluid  172  to or from hydraulic chambers  121 ,  123 , for example. In a particular embodiment, position sensors  134  and  136  may comprise Hall Effect sensors operable to detect the position of hydraulic drive cylinder  108  at the end of the suction stroke of a hydraulic diaphragm, however, alternatively, any suitable position sensor operable to detect the end of a suction stroke may be employed. 
     In yet a further embodiment of the present invention, the controller  132  of hydraulic fluid control includes a control program which may be stored on a computer readable medium such as a logic chip, RAM (randomly accessible memory) or ROM (read only memory) chip, magnetic, optical or magneto-optical computer readable medium, for example. Such control program may comprise computer readable instructions to effect control of the bidirectional hydraulic fluid motors  958 ,  960 , such as to control the addition and/or withdrawal of hydraulic fluid to and/or from chambers  121 ,  123 , in a desired manner, as is described above in various embodiments. In a particular embodiment, the controller  132  may include a control program comprising computer readable instructions to:
         a) detect a position of the hydraulic drive cylinder  108 , such as may be detected by position sensors  134 ,  136 , which corresponds to a desired point of the pump cycle, such as the end of the suction stroke, for example;   b) measure a pressure differential value between a process fluid pressure and a hydraulic fluid pressure, such as by measuring a signal from a differential pressure sensor  138 ,  140 ;   c) comparing the measured pressure differential value with a setpoint pressure differential value which corresponds to a desired limit of hydraulic fluid pressure or volume; and   d) if the measured pressure differential value is greater than the setpoint value, control a hydraulic fluid pump  958 ,  960  to add hydraulic fluid to the hydraulic fluid system and/or if the measured pressure differential value is less than a setpoint value, control a hydraulic fluid pump  958 ,  960  to remove hydraulic fluid from the hydraulic fluid system.       

     In one embodiment, a control program of the controller  132  may further include instructions to control the bidirectional hydraulic fluid pump  958 ,  960  to continue to add and/or withdraw hydraulic fluid to and/or from the hydraulic chamber  121 ,  123 , for at least one of: a predetermined time, a predetermined number of pump strokes, and/or a predetermined volume of hydraulic fluid, such as may be based on the magnitude of the pressure differential measured by the sensor  138 ,  140 , for example. In another embodiment, such predetermined time, predetermined number of pump strokes and/or predetermined volume of hydraulic fluid to be added and/or withdrawn may be user adjustable, and/or set by according to the control program of the controller  132 , for example. 
     The exemplary embodiments herein described are not intended to be exhaustive or to limit the scope of the invention to the precise forms disclosed. They are chosen and described to explain the principles of the invention and its application and practical use to allow others skilled in the art to comprehend its teachings. 
     As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.