Abstract:
A closed-loop system includes a pump driving a fluid through a series of conduits. A control system is included that minimizes pressure fluctuations in a bootstrap reservoir to maintain a desired minimum pressure at the pump inlet. Moreover, the control system reduces a maximum system pressure by reducing the magnitude of pressure fluctuations encountered by the bootstrap reservoir.

Description:
BACKGROUND 
     This disclosure generally relates to a reservoir for a hydraulic system. More particularly, this disclosure relates to control of fluid pressure to minimize a maximum pump inlet pressure. 
     A closed-loop hydraulic system includes a pump that drives fluid through the system. A reservoir is provided within the system to accommodate changes in the working fluid due to thermal expansion and contraction along with other variables. A bootstrap reservoir includes a piston movable between a high pressure chamber and a low pressure chamber to maintain a desired minimum pump inlet pressure. The minimum fluid pressure at an inlet to the pump is desired to provide efficient operation of the pump. A fluid pressure that is lower than desired can adversely affect pump operation and durability. Accordingly, a minimum pressure provided by the bootstrap reservoir is set well above the minimum desired inlet pressures. Because the low end of the pressure range is fixed by the bootstrap reservoir, the high end of the pressure range may be higher than desired. Higher pressures require that all system components be sufficiently robust to perform at the higher pressures. Accordingly, components in the fluid system are designed to withstand higher pressures that results in increased cost and weight. 
     SUMMARY 
     A disclosed closed loop fluid system includes a pump for pumping fluid at a desired pressure and flow to hydraulically operated devices such as valves or other hydraulic actuators, or devices which exchange heat with the system fluid such as heat exchangers or electronic motor controllers. Volume fluctuations within the system are compensated by a bootstrap reservoir. A control system is included that minimizes pressure fluctuations in the bootstrap reservoir to maintain a desired minimum pressure at the pump inlet. Moreover, the control system reduces a maximum system pressure by reducing the magnitude of pressure fluctuations encountered by the bootstrap reservoir. 
     The control system includes a pressure sensor, controller and valve. The pressure sensor measures pressure indicative of pressure at the inlet of the pump. Measurements from the pressure sensor are utilized to drive and operate the control valve. The control valve modulates pressure within the system to minimize the effects of fluid pressure drops caused by the device on the bootstrap reservoir. The reduction in pressure fluctuations provides for a lower upper pressure limit, and thereby reduces overall system and component requirements. 
     These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a closed loop fluid system including a bootstrap reservoir and a valve for controlling a pressure drop within the system. 
         FIG. 2  is another schematic representation of a closed loop fluid system including a bootstrap reservoir and valve for controlling pressure drop within the system. 
         FIG. 3  is yet another schematic representation of a closed loop fluid system that includes a valve for controlling a pressure within the system. 
         FIG. 4  is another schematic representation of a closed loop fluid system that includes a portion of a bootstrap reservoir in series with a pump. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an example closed loop system  10  includes a high pressure portion  12  and a low pressure portion  14 . Fluid flow  46  proceeds through the closed loop system through a plurality of conduits  44 . Conduits  44  communicate fluid pressure to a device or devices schematically indicated at  16 . The device  16  represents hydraulically operated devices such as valves, other hydraulic actuators or devices which exchange heat with the system fluid such as heat exchangers or electronic motor controllers that require fluid flow at a desired pressure within the closed loop system  10 . 
     A pump  18  drives a fluid flow through the conduits  44  of the closed loop system  10  and includes an inlet  32  and an outlet  34 . Fluid pressure at the inlet  32  is maintained above a minimum desired operating pressure. The desired operating pressure at the pump  18  is set to a minimum level. As appreciated, if the fluid pressure at the inlet  32  drops below a minimum pressure, cavitation can occur within the pump  18  that causes a degraded operating capacity. 
     Accordingly, in the example closed loop system  10 , a bootstrap reservoir  20  is set parallel to the pump  18  to maintain a minimum pressure at the pump inlet  32 . The example bootstrap reservoir  20  includes a high pressure chamber  22  and a low pressure chamber  26 . The high pressure chamber  22  and the low pressure chamber  26  are separated by a piston  30 . A piston  30  moves responsive to system volume change and differential pressures within the high pressure chamber  22  and the low pressure chamber  26 . 
     An area  24  of the high pressure chamber  22  is different than an area  28  of the low pressure chamber  26 . The difference in area provides the balance of the high pressure chamber  22  and the low pressure chamber  26  that sets a minimum pressure level for fluid pressure within the conduits  44  of the closed loop system  10 . In order to maintain a desired minimum operating pressure within the system, the areas  24  and  28  are balanced to provide the desired minimum fluid pressure in view of operation of the device  16 . The bootstrap reservoir  20  further adjusts pressure within the system  10  to accommodate changes in the working fluid encountered during operation. Such changes can include thermal expansion and contraction along with losses due to leakage or other operational functions. 
     The example bootstrap reservoir  20  is designed with a desired ratio by varying the ratio of an area  24  of the piston  30  acted on by fluid in the high pressure chamber  22  with an area  28  acted on by the low pressure chamber  26 . The specific ratio between the high pressure area  24  and the low pressure area  28  sets the minimum desired pressure at the pump inlet  32 . 
     The pressure at the high pressure chamber  22  of the bootstrap reservoir  20  fluctuates in direct proportion to the system pressure. Therefore pressure changes encountered due to operation of the device  16  are translated to changes in pressure in the high pressure chamber  22  of the bootstrap reservoir  20 . Changes in the high pressure chamber  22  result in a wide range of corresponding pressures in the lower pressure chamber  26  of the bootstrap reservoir  20 . In turn, pressure within the low pressure region  14  and at the pump inlet  32  falls within a wide range. For this reason, the bootstrap reservoir  20  is designed to satisfy the minimum operating pressures for the inlet  32  under all operating conditions, including the lowest pressure drops. This results in an overall higher system operating pressure during normal conditions to compensate for the lowest pressure drops. 
     In the example closed loop system  10  shown in  FIG. 1 , a control system  52  is provided that includes a pressure sensor  38 , controller  40  and valve  36 . The pressure sensor  38  is disposed within the conduits  44  to obtain a pressure measurement indicative of pressure at the inlet  32  of the pump  18 . Measurements of the pressure within the conduits  44  are utilized to drive and operate the control valve  36 . The control valve  36  modulates pressure within the system  10  and specifically within the high pressure chamber  22  to minimize the effects of fluid pressure drops caused by the device  16 . Because the valve  36  controls pressure drops within the system  10 , a range of pressure fluctuations in the high pressure chamber  22  is reduced. The reduction in pressure fluctuations further provides for a lower upper pressure limit, and thereby reduces overall system and component requirements. In other words, system components can be designed lighter in view of the lower upper pressure limits. 
     In the example closed loop system  10 , the valve  36  is modulated by a controller  40  in response to a pressure measured by the pressure sensor  38 . Modulation of the valve  36  varies the pressure drop between node X and node Y which provides for control of pressure within the low pressure chamber  26  and therefore control of pressure at pump inlet  32  in response to varying pressure drops produced by actuation and operation of the device  16 . 
     The example valve  36  can comprise a variable orifice valve that changes the flow area in order to control pressure drops within the system and at the low pressure chamber  26  of the reservoir  20 . The valve  36  may also be an on/off valve that is modulated between open positions and closed positions to limit the range of pressures encountered at the bootstrap reservoir  20 . 
     In operation, the pump  18  outputs fluid at desired flow and pressure through the outlet  34 . Fluid pressure and flow is communicated from the pump  18  to both the high pressure chamber  22  of the bootstrap reservoir  20  and the device  16 . The valve  36  is disposed between the device  16  and the pump  18 . In the event of a pressure drop caused by actuation of the device  16 , the pressure sensor  38  will communicate the change in pressure to a controller  40 . The controller  40  commands the valve  36  to move to a more closed position to minimize the effects of pressure drops within the high pressure portion  12  of the system  10 . Closing of the valve  36  reduces the impact the pressure drop experienced behind the valve caused by actuation and operation of the device  16  such that the high pressure chamber  22  and low pressure chamber  26  do not experience a large drop in pressure. The valve  36  reduces the effect of varying pressure drops of the device  16  on the system and particularly on the pump inlet  32 . 
     The reduced range of pressure drops between node X and node Y provides a corresponding reduction in a range of pressures encountered in the low pressure region  14  and thereby at the pump inlet  32 . In response to an increase in pressure drop caused, for example by the closing of valves of the device  16 , the controller  40  will command the valve  36  to move to a more open condition to reduce pressure within the high pressure chamber  22 . 
     Referring to  FIG. 2 , another example closed loop system  10  includes the bootstrap reservoir  20  disposed in parallel with the pump  18 . In this example the pressure sensor  38  is disposed close to or directly at the inlet  32  of the pump  18 . This position provides an accurate representation of pressure at the inlet of the pump  18 . Pressure measurements from the pressure sensor  38  are communicated to the controller  40  that then drives the valve  36  to the desired position to maintain minimum pressure at the pump inlet  32  resulting in minimum pressure within the system  10 . As appreciated, providing the pressure sensor  38  at the pump inlet  32  provides a direct indication of the desired minimum pressure without any interpretation or extrapolation. 
     Referring to  FIG. 3 , another example closed loop system  10  includes the bootstrap reservoir  20  disposed in parallel with the pump  18  and a differential pressure sensor  42  that measures pressure at at least two different locations  48 , 50  within the system  10 . In this example the pressure sensor  42  is measuring pressure at a first point  50  in the low pressure portion  14  and at a second point  48  in the high pressure portion  12  of the closed loop system  10 . The controller  40  obtains the differential pressure provided by the pressure sensor  42  controls the valve  36  to provide the desired opening required to maintain a desired pressure at the pump inlet  32 . The differential pressure reading can be used to provide additional data indicative of pressure differentials within the system  10 . The differential pressure measurements are communicated to the controller  40  that thereby commands the valve  36  to provide a desired pressure drop within the system  10  that maintains a minimum level of pressure at the pump inlet  32 . 
     Referring to  FIG. 4 , an example closed loop system  54  includes a bootstrap reservoir  56  with a high pressure chamber  22  and a low pressure chamber  58 . In this example, the low pressure chamber  58  is disposed in series with the pump  18 . The in series configuration provides a flow through low pressure chamber  58 . It is also within the contemplation of this disclosure that the high pressure chamber  22  may also be arranged in series with the pump  18 . Moreover, both or just one of the high pressure chamber  22  and the low pressure chamber  58  may be disposed in series with the pump  18  as may be desired to meet application specific requirements. 
     Accordingly, the example system provides for the minimizing of pressure variations within a closed loop system and thereby provides for a reduction in overall system maximum design operating pressure. 
     Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this invention.