Patent Application
20150275890
Typically, an integrated pump and energy recovery system is used to pump an input stream of fluid for purification through a membrane or filter, such as a reverse osmosis membrane, at a higher pressure. A stream of brine or other concentrated unpurified material is then discharged under pressure from such a membrane or filter. Typically energy is recovered from the discharge stream still under pressure, and then such recovered energy is used for a useful purpose, for example, to reduce the amount of energy that the pump would otherwise have to expend in order to pump the input stream of fluid into the system, thereby making operation of the purification system more efficient.
When a semi-permeable membrane divides two fluids of different salinities, osmosis occurs. To achieve equilibrium of the chemical potential across the membrane, liquid flows through the membrane into the more concentrated solution. This flow continues until concentrations on either side of the membrane are equal and the osmotic pressure is reached. Reverse osmosis (RO) is a membrane-technology filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of the selective membrane. The result is that the solute is retained on a pressurized side of the membrane and the pure solvent is allowed to pass to the other side of the membrane.
Efficiency of the reverse osmosis process may be improved by recovering energy from the high pressure waste brine. Known systems for pumping and energy recovery include, for example, some combination of: plunger pumps with belt drives and pulsation dampeners, centrifugal pumps, sumps and sump pumps, reverse flow pump and Pelton wheel energy recovery turbines, hydraulic turbo chargers, flow work exchangers, and variable frequency drives.
In an integrated pump and energy recovery system using a plurality of piston-cylinder arrangements and valves for pumping the fluid medium, there are issues associated with generation of pressure ripples in the high pressure feed-water fed to a reverse osmosis (RO) membrane stack and variation in total flow of the feed-water per cycle. Such issues affect the RO membrane stack.
In accordance with one exemplary embodiment, a system for controlling an integrated pump and energy recovery system is disclosed. The system includes a processor-based device communicatively coupled to a position sensor, a pressure sensing unit, and a hydraulic pump of a hydraulic power unit. The processor-based device is configured to receive a position signal representative of a measured position of a piston assembly movable between a top dead center and a bottom dead center of a hydraulic cylinder and a pumping cylinder, from the position sensor. The hydraulic pump is coupled to the hydraulic cylinder. The processor-based device is also configured to receive a pressure signal representative of a measured pressure of at least one of a fluid medium fed from the pumping cylinder to a filtering device, a hydraulic fluid fed from the hydraulic pump to the hydraulic cylinder, from the pressure sensing unit. The processor-based device is further configured to generate a control signal based on the position signal and the pressure signal, and transmit the control signal to the hydraulic power unit to control the measured position of the piston assembly and a pressure of the fluid medium.
In accordance with another exemplary embodiment, a method implemented via a processor-based device for controlling an integrated pump and energy recovery system is disclosed. The method involves receiving a position signal representative of a measured position of a piston assembly moving between a top dead center and a bottom dead center of a hydraulic cylinder and a pumping cylinder, from a position sensor. The hydraulic cylinder is coupled to a hydraulic pump of a hydraulic power unit. The method also involves receiving a pressure signal representative of a measured pressure of at least one of a fluid medium fed from the pumping cylinder to a filtering device, a hydraulic fluid fed from the hydraulic pump to the hydraulic cylinder, from a pressure sensing unit. The method further involves generating a control signal based on the position signal and the pressure signal. The method also involves transmitting the control signal to the hydraulic power unit to control the measured position of the piston assembly and a pressure of the fluid medium.
In accordance with yet another exemplary embodiment, a non-transitory computer readable medium having instructions to enable a processor-based device to receive a position signal representative of a measured position of a piston assembly moving between a top dead center and a bottom dead center of a hydraulic cylinder and a pumping cylinder, from a position sensor. The hydraulic cylinder is coupled to a hydraulic pump of a hydraulic power unit. The non-transitory computer readable medium further includes instructions to enable a processor-based device to receive a pressure signal representative of a measured pressure of at least one of a fluid medium fed from the pumping cylinder to a filtering device, a hydraulic fluid fed from the hydraulic pump to the hydraulic cylinder, from a pressure sensing unit. The non-transitory computer readable medium further includes instructions to enable a processor-based device to generate a control signal based on the position signal and the pressure signal. The non-transitory computer readable medium further includes instructions to enable a processor-based device to transmit the control signal to the hydraulic power unit to control the measured position of the piston assembly and a pressure of the fluid medium.
These and other features and aspects of embodiments of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
In accordance with certain embodiments of the present invention, a system includes a processor-based device communicatively coupled to a position sensor, a pressure sensing unit, and a hydraulic pump of a hydraulic power unit. The processor-based device is configured to receive a position signal representative of a measured position of a piston assembly movable between a top dead center and a bottom dead center of a hydraulic cylinder and a pumping cylinder, from the position sensor. The hydraulic pump is coupled to the hydraulic cylinder. The processor-based device is further configured to receive a pressure signal representative of a measured pressure of at least one of a fluid medium fed from the pumping cylinder to a filtering device, a hydraulic fluid fed from the hydraulic pump to the hydraulic cylinder, from the pressure sensing unit. The processor-based device is further configured to generate a control signal based on the position signal, the pressure signal, and transmit the control signal to the hydraulic pump to control the measured position of the piston assembly and a pressure of the fluid medium. The piston assembly is movable between the top dead center and the bottom dead center of the pumping cylinder to intake the fluid medium from a source into the pumping cylinder, feed the fluid medium from the pumping cylinder to the filtering device; intake a brine into the pumping cylinder from the filtering device, and eject the brine from the pumping cylinder to a drain. In accordance with some other embodiments, an associated method implemented via a processor-based device is disclosed. In accordance with another exemplary embodiment, a non-transitory computer readable medium having instructions to enable a processor-based device to implement the associated method is disclosed. The exemplary control strategy facilitates to minimize pressure ripples in the high pressure fluid medium fed to a filtering device. The control strategy specifically involves switching between controlling a position of a piston assembly and the pressure of high pressure feed-water depending upon a measured position of the piston assembly and pressure of the fluid medium fed to the filtering device or pressure of a hydraulic fluid fed to the hydraulic cylinder. Such a controlled switching operation ensures that the piston assembly moves through a complete stroke of each cycle. Hence, a total flow of the liquid medium per cycle is maintained constant, while simultaneously minimizing the generation of pressure ripples in the liquid medium fed to the filtering device.
Referring to
The system 10 mainly includes a hydraulic power unit 11 having a hydraulic pump 12 and a hydraulic cylinder 14, a fluid pumping cylinder 16, and a processor-based control device 18. The hydraulic pump 12 is coupled to the hydraulic cylinder 14. The hydraulic cylinder 14 is coupled to the fluid pumping cylinder 16. The hydraulic cylinder 14 includes a piston 20 movable between a top dead center (TDC) and a bottom dead center (BDC). The hydraulic pump 12 is also coupled to a reservoir 22. In other embodiments, two or more hydraulic cylinders, fluid pumping cylinders, hydraulic pumps, and reservoirs may be used depending on the application.
In the illustrated embodiment, the piston 20 is a double acting piston. The hydraulic pump 12 feeds a hydraulic fluid 24 from the reservoir 22 alternately to either sides of the piston 20 to move the piston 20 between the TDC and BDC in the hydraulic cylinder 14.
In the illustrated embodiment, the pumping cylinder 16 has a piston assembly 26 movable between a top dead center (TDC) and a bottom dead center (BDC). It should be noted herein that the terms “piston” and “piston assembly” may be used interchangeably. The piston assembly 26 is coupled via a piston rod 28 to the piston 20. The hydraulic pump 12 drives the hydraulic cylinder 14 which in turn drives the fluid pumping cylinder 16. Specifically, when the piston 20 moves to the TDC of the hydraulic cylinder 14, the piston assembly 26 is also moved to the TDC of the pumping cylinder 16, and vice versa. In the illustrated embodiment, the piston assembly 26 is also a double acting piston.
A fluid medium 29, for example seawater, brackish water, groundwater, boiler feed water or wastewater, is fed from a feed water source 30 to the fluid pumping cylinder 16 via a low pressure feed pipe 32. The fluid medium 29 is pressurized within the fluid pumping cylinder 16 and directed to a filtering device (for example, a RO membrane) 34 via a high pressure feed pipe 36.
The filtering device 34 is used to separate the fluid medium 29 into a low pressure stream of low-solute permeate 38 and a high pressure stream of high-solute brine 40. The brine 40 is directed back to the pumping cylinder 16 via a high pressure brine pipe 42.
As discussed herein, the low-solute permeate (desalted fluid medium) 38 is ejected from the filtering device 34 and the brine 40 is fed from the filtering device 34 to the pumping cylinder 16 for recovering energy from the brine 40. Advantageously, such energy recovery action reduces the overall energy that would otherwise have to be provided by the hydraulic cylinder 14 in order to reciprocate the piston assembly 26 during a pumping cycle and raise the pressure of the input fluid medium 29 upto a pressure needed by the filtering device 34. In a preferred double-action embodiment, a first working volume on one side of the piston assembly 26 diminishes during a forward stroke of the piston assembly 26, and a second working volume on the other side of the piston assembly 26 diminishes during a backward stroke of the piston assembly 26. Similar principles are applicable to the hydraulic cylinder 14.
In the illustrated embodiment, the working of the system 10 involves feeding the fluid medium 29 to be desalted via the pumping cylinder 16 to the filtering device 34 at a high pressure “P1”. The source 30 of the fluid medium 29 is typically provided at a low pressure “P0”. The pumping cylinder 16 thus increases the pressure of the input fluid medium 29 from a low pressure P0 to a high pressure P1. The filtering device 34 allows some of the fluid medium 29, e.g., 25-55% of the fluid medium, to pass there through and exit as permeate 38. The rest of the fluid medium 29 exits the filtering device 34 as a brine stream 40. The brine stream 40 is at a high pressure P2, where pressure P2 is less than P1, but is still relatively high. The brine stream 40 is fed through the pumping cylinder 16 to recover the energy from the brine stream 40.
The processor-based control device 18 is communicatively coupled to a position sensor 46 and the hydraulic pump 12. In a specific embodiment, the control device 18 controls flow of the hydraulic fluid 24 to the hydraulic cylinder 14 based on an output from the position sensor 46 used to detect position of the piston assembly 26 within the pumping cylinder 16. The control device 16 controls flow of the hydraulic fluid 24 to the hydraulic cylinder 14 to control the actuation/position of the piston assembly 26 within the pumping cylinder 16. The velocity and time duration of the movement of the piston assembly 26 within the pumping cylinder 16 is dependent on the flow rate of the hydraulic fluid 24 to the hydraulic cylinder 14. In one embodiment, the control device 18 controls actuation of the hydraulic pump 12, for example, swash plates of the pump 12 to control flow of the hydraulic fluid 24 to the hydraulic cylinder 14. In another specific embodiment, the control device 18 controls flow of the hydraulic fluid 24 to the hydraulic cylinder 14 based on an output from the sensor 46 used to detect position of the piston assembly 26 within the pumping cylinder 16.
The processor-based control device 18 may be a general purpose processor, a controller, or a server. The processor-based control device 18 may use software instructions from a disk or from a memory. The software may be encoded in any language, including, but not limited to, assembly language, Hardware Description Language, high level languages, and any combination or derivative of at least one of the foregoing languages. The processor-based control device 18 may also read instructions from a non-transitory encoded computer medium having instructions to control the hydraulic pump 12 and the hydraulic cylinder 14 in accordance with the exemplary embodiments of the present invention. In one embodiment, the processor-based control device 18 is a proportional-integral-derivative controller.
The processor-based control device 18 is further configured to generate an error position signal (xerror) by comparing the measured position of the piston assembly with a desired position and is represented by:
x
error
=x
des
−x (1)
In accordance with the illustrated embodiment, the equation pertaining to a control signal (y) generated from the control device 18 is represented by:
y=k
p(xdes−x)+(kff×vdes) (2)
where y is the control signal transmitted to the hydraulic pump 12, xdes is the desired position of the piston assembly, x is the actual position of the piston assembly, vdes is the desired velocity of the piston assembly, kff is the feed forward constant, kp is the proportional gain. In other words, the control signal (y) is representative of a sum of a scaled velocity feed-forward term and a scaled error position signal of the piston assembly. A swash-plate of the hydraulic pump 12 is controlled based on the control signal, thereby increasing (or decreasing) the flow from the hydraulic pump 12 to the hydraulic cylinder 14 which in turn accelerates (or decelerates) the movement of the piston assembly 26.
In the illustrated embodiment, the processor-based control device 18 is also communicatively coupled to pressure sensors 66, 67 of a pressure sensing unit 69. The processor-based control device 18 is further configured to receive a pressure signal representative of a measured pressure of the fluid medium 29 fed from the pumping cylinder 16 to the filtering device 34, from the pressure sensor 66 and a measured pressure of the hydraulic fluid 24 fed to the hydraulic cylinder 24, from the pressure sensor 67. The control device 18 is configured control the pressure of the fluid medium 29 fed from the pumping cylinder 16 to the filtering device 34 based on the measured pressure from the sensors 66 and 67. Specifically, the control device 18 controls flow of the hydraulic fluid 24 to the hydraulic cylinder 14 based on an output from the pressure sensors 66, 67 to control the pressure of the fluid medium 29 fed from the pumping cylinder 16 to the filtering device 34. In one embodiment, the control device 18 controls the hydraulic pump 12 to control flow of the hydraulic fluid 24 to the hydraulic cylinder 14. In another embodiment, the control device 18 controls a flow control device such as a four-way valve to control flow of the hydraulic fluid 24 from the hydraulic pump 12 to the hydraulic cylinder 14.
In another specific embodiment, only the pressure sensor 66 may be used. In yet another specific embodiment, only the pressure sensor 67 may be used.
As discussed previously, the processor-based control device 18 is further configured to generate an error position signal by comparing the measured position of the piston assembly with a desired position. Further, in the illustrated embodiment, the processor-based control device 18 is further configured to generate an error pressure signal by comparing the measured pressure with a desired pressure and is represented by:
p
error
=p
des
−p (3)
In accordance with the illustrated embodiment, the equation pertaining to a control signal (y) generated from the control device 18 is represented by:
y=α(kp
where α is the weighting factor and is a measure of contribution of position error and pressure error, xdes is the desired position of the piston assembly, x is the actual position of the piston assembly, vdes is the desired velocity of the piston assembly, kff is the feed forward constant, pdes is the desired pressure, p is the actual pressure, kp-poss is the proportional gain for the position error, and kp-pres is the proportional gain for the pressure error. In other words, the control signal (y) is representative of a weighted sum of a scaled velocity feed-forward term and a scaled error position signal of the piston assembly and a scaled error pressure signal. The weighting factor α is determined based on the measured position of the piston assembly.
In accordance with the exemplary embodiments discussed herein, the control technique based on the measured position of the piston assembly and the measured pressure of the fluid medium and/or of the hydraulic fluid, facilitates to reduce generation of pressure ripples in the high pressure fluid medium fed to the filtering device and maintains a total flow of the fluid medium per cycle constant.
Referring to
The pistons 52 are mechanically coupled to each other by the connecting rod 28. The connecting rod 28 extends through a dividing wall 62 between the concentrate working chambers 56, 60 and out of the pumping cylinder 16 through bearing and seal assemblies (not shown) provided to minimize or prevent pressure or fluid leaks. The connecting rod 28 and the pistons 52 are collectively referred to as the piston assembly 26. It should be noted herein that for the top piston 52, the TDC is at the top of the pumping cylinder 16 and BDC is at the middle of the pumping cylinder 16. Similarly for the bottom piston 52, TDC is at the middle of the pumping cylinder 16 and BDC is at the bottom of the pumping cylinder 16.
The pumping cylinder 16 includes a plurality of valves 64 that control the flow of the fluid into and out of the pumping cylinders 16. Opening and closing of the valves 64 may be controlled by a controller in association with the movement of the piston assembly 26. When the piston assembly 26 moves upwards to the TDC, the valves 64 are configured such that the fluid medium in the working chamber 54 flows out to the high pressure feed pipe 36; the brine flows into the concentrate working chamber 56 from the pipe 42; brine flows out of the concentrate working chamber 60 to the pipe 44; and the fluid medium flows into the working chamber 58 from the pipe 32. When the piston assembly 26 moves downwards to the BDC, the valves 64 are configured such that the fluid medium flows into the working chamber 54 from the pipe 32; the brine flows out of the concentrate working chamber 56 to the pipe 44; the brine flows into the concentrate working chamber 60 from the pipe 42; and the fluid medium flows out of the working chamber 58 to the pipe 36. In such a way, energy is recovered from the pressurized brine to help provide pressurized fluid medium to the filtering device.
Referring to
In the illustrated embodiment, a curve 84 is representative of the variation of the weighting factor α. The desired movement of the piston assembly corresponds to a flow rate of the fluid medium corresponding to one cycle per minute where the weighting factor α is a function of the position of the piston assembly. The control system is designed to ensure that the piston assembly moves the distance equal to a full stroke. Hence, weighting α is chosen to be unity when the piston assembly is proximate to the BDC and TDC (i.e. corresponding to portions 74, 76, 78 of the curve 72). For the remaining part of the piston stroke (i.e. corresponding to portions 80, 82 of the curve 72), weighting factor α is chosen to be zero. In other words, the weighting factor α is equal to one for the position control mode and equal to zero for the pressure control mode.
Referring to
In the illustrated embodiment, a curve 102 is representative of the variation of the weighting factor α. The illustrated weighting factor α is a linear weighting factor. It should be noted herein that compared to the embodiment of
Referring to
In the illustrated embodiment, a curve 118 is representative of the variation of the weighting factor α. The illustrated weighting factor α is a quadratic weighting factor. It should be noted herein that compared to the embodiment of
Referring to
Referring to
Referring to
In the illustrated embodiment, the pistons 132, 134, 136 are double acting pistons. The hydraulic pumps 138, 140, 142 feed a hydraulic fluid 146 from the reservoir 144 alternately to either sides of the pistons 132, 134, 136 to move the pistons 132, 134, 136 between the TDC and BDC in the respective hydraulic cylinders 126, 128, 130. In some other embodiments, one hydraulic pump is coupled via three flow control devices, for example, four-way valves to the corresponding hydraulic cylinders 126, 128, 130. In such embodiments, the flow control devices control flow of the hydraulic fluid between the hydraulic pump and the three hydraulic cylinders 126, 128, 130. It should be noted herein that the number of hydraulic cylinders, valves, hydraulic pumps, and reservoirs may vary depending on the application.
In the illustrated embodiment, three pumping cylinders 148, 150, 152 have piston assemblies 154, 156, 158 respectively. The piston assembly 154 is coupled via a piston rod 160 to the piston 132. Similarly, the piston assembly 156 is coupled via a piston rod 162 to the piston 134, and the piston assembly 158 is coupled via a piston rod 164 to the piston 136. Specifically, when the pistons 132, 134, 136 move to the TDC of the hydraulic cylinders 126, 128, 130, the piston assemblies 154, 156, 158 are also moved to the TDC of the pumping cylinders 148, 150, 152, and vice versa. In the illustrated embodiment, the piston assemblies 48, 50, 52 are also double acting pistons.
A fluid medium 166, for example seawater, brackish water, groundwater, boiler feed water or wastewater, flows from a feed water source 168 to the pumping cylinders 148, 150, 152 via a low pressure feed pipe 170. The fluid medium 166 is pressurized within the pumping cylinders 148, 150, 152 and directed to a filtering device 172 via a high pressure feed pipe 174.
The filtering device 172 separates the fluid medium into a low pressure stream of low-solute permeate 176 and a high pressure stream of high-solute brine 178. The brine 178 is directed back to the pumping cylinders 148, 150, 152, via a high pressure brine pipe 180. Low-pressure brine, after being used to generate fluid medium pressure, is directed outwards from the pumping cylinders 148, 150, 152 via low pressure pipes 182, 184, 186.
In a preferred embodiment, the three pumping cylinders 148, 150, 152 are disposed in parallel, such that the piston stroke of each pumping cylinder are phased appropriately compared to the piston stroke of the other pumping cylinders so as to minimize flow or pressure surges. For example, when three such pumping cylinders 148, 150, 152 are used, then the piston strokes are phased to be 120 degrees apart.
A processor-based control device 188 is communicatively coupled to position sensors 190, 192, 194 and the hydraulic pump 12. In a specific embodiment, the control device 188 controls flow of the hydraulic fluid 146 to the hydraulic cylinders 126, 128, 130 based on an output from the position sensors 190, 192, 194 used to detect position of the piston assemblies 154, 156, 158 within the pumping cylinders 148, 150, 152. The control device 188 controls flow of the hydraulic fluid 146 to the hydraulic cylinders 126, 128, 130 to control the actuation/position of the piston assemblies 154, 156, 158 within the pumping cylinder 148, 150, 152.
The processor-based control device 188 is further configured to receive a pressure signal representative of a measured pressure of the fluid medium 166 fed from the pumping cylinders 148, 150, 152 to the filtering device 172, from the pressure sensor 191 and a measured pressure of the hydraulic fluid 146 fed to the hydraulic cylinders 126, 128, 130 from the pressure sensors 113, 115, 117. The control device 188 is configured control the pressure of the fluid medium 166 fed from the pumping cylinder 146 to the filtering device 172 based on the measured pressure from the sensors 113, 115, 117, and 191. Specifically, the control device 188 controls flow of the hydraulic fluid 146 to the hydraulic cylinders 126, 128, 130 based on an output from the pressure sensors 113, 115, 117, and 191 to control the pressure of the fluid medium 166 fed from the pumping cylinders 148, 150, 152 to the filtering device 172. In one embodiment, the control device 188 controls the hydraulic pumps 138, 140, 142 to control flow of the hydraulic fluid 146 to the hydraulic cylinders 126, 128, 130. In another embodiment, the control device 188 controls a flow control device such as a four-way valve to control flow of the hydraulic fluid 146 from the hydraulic pumps 138, 140, 142 to the hydraulic cylinders 126, 128, 130.
In another specific embodiment, only the pressure sensor 191 may be used. In yet another specific embodiment, only the pressure sensors 113, 115, 117 may be used.
Referring to
Referring to
In the illustrated embodiment, with reference to the threshold position with overlap 210, the first piston assembly transitions to a pressure control mode at approximately 38 seconds which is before the third piston assembly transitions to a position control mode at 39.5 seconds, thereby creating an interval of around 1.5 seconds when more than one piston assemblies are operated in pressure control mode. With reference to the threshold position without overlap 212, the reverse phenomena occurs where the third piston assembly transitions to the position control mode at 37.5 seconds before the first piston assembly transitions to the pressure control mode at 40 seconds. Such a phenomena leads to an interval of around 2.5 seconds where none of the piston assemblies are in pressure control mode. The time intervals discussed herein may be controlled by choosing the thresholds appropriately.
Referring to
In a conventional system, when the piston assembly reverses direction from one end towards another end, the pressure in the fluid medium working chamber of the pumping cylinder is equal to a supply pressure which is substantially lower than the pressure in the high pressure feed pipe. The valve remains closed till pressure in the fluid medium working chamber is above the pressure in the high pressure feed pipe. Hence, flow rate of the fluid medium is reduced causing the pressure in the high pressure feed pipe to drop. Another cause of reduction in pressure in the high pressure feed pipe is due to a sudden in-flow of the brine to the concentrate working chamber in the pumping cylinder. Another cause for the pressure variations is due to inherent imperfections, for example, leaks in the fluid medium working chambers.
Now again referring to
In accordance with an exemplary embodiment, the processor-based control device is configured to generate a first error velocity signal by comparing an estimated velocity of the piston assembly with a target velocity. In such an embodiment, the control device generates a control signal (y) representative of a scaled first error velocity signal and is represented by:
y=k
c(vtarget−vactual) (5)
where vtarget is the target velocity of the piston assembly and vactual is the actual velocity of the piston assembly, kc is the proportional gain for the velocity error. The actual velocity of the piston assembly may be determined based on the measured position of the piston assembly.
The target velocity is representative of a sum of a design velocity of the piston assembly and the scaled error pressure signal and is represented by:
V
target
=V
design
+k
v(ptarget−pactual) (6)
where Vdesign is the design velocity, Kv is the proportional gain for controlling the designed velocity, ptarget is the target pressure in the high pressure feed pipe, pactual is the actual pressure in the high pressure feed pipe.
The processor-based control device is further configured to generate a second error velocity signal by comparing a design velocity with the estimated velocity of the piston assembly, and adjust a desired pressure of the fluid medium representative of a scaled second error velocity signal. The target pressure is adjusted based on the following equation:
where kp is a proportional gain for controlling desired pressure.
In accordance with the exemplary embodiments discussed herein, the PID-based control system actuates piston assembly based on the measured position of the piston assembly and pressure of the fluid medium and/or hydraulic fluid. The control strategy involves switching between position and pressure control mode so as to ensure that the piston assembly does not hit against end walls of the respective cylinders. Further, a constant pressure of fluid medium is maintained and a set flow is delivered to the filtering device
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
