Abstract:
A compressor including a pair of opposed pistons disposed in a housing and defining a compression chamber. An electromagnetic actuator reciprocatedly drives the pistons within the housing in cooperation with force accumulator. The force accumulators bank the force during a first reciprocation, decelerating the pistons, and apply the force in a subsequent reciprocation, thereby accelerating the pistons.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates generally to compressors. More particularly, the subject matter disclosed herein relates to electromagnetically driven reciprocating compressors adapted for use in displacing fluids, such as oil or natural gas. 
         [0002]    Reciprocating compressors are widely used in the oil and gas industry to pressurize and displace gas. For example, in gas pipeline transmission systems and distribution networks, reciprocating compressors move natural gas from production sites to end-users by ingesting relatively low pressure gas, and expelling the gas at a higher pressure. Reciprocating compressors also perform this same function used in industrial plants, such as petroleum refineries and chemical plants, where compressors move intermediate and end product gases. 
         [0003]    Reciprocating compressors typically include a piston driven by a rotatory motor, such as an internal combustion engine or motor. In such systems, a crankshaft and connecting rod convert motor shaft rotation to piston translation in a compression chamber. Piston translation within a cylinder bore in turn compresses gas in a compression chamber located at an end of the cylinder bore. Such machines may be single action, where gas compression takes place only when the piston moves in a single direction, or double action, where gas compression takes place when the piston moves in two directions. 
         [0004]    Rotary reciprocating compressors have several disadvantages. 
         [0005]    First, during the majority of each rotation of the motor shaft, the connecting rod applies force to the piston at an angle with respect to the piston translation axis. 
         [0006]    Since the crankshaft is mechanically linked piston, piston travel during each stroke is fixed. Therefore, the volume swept by the piston during a stroke is also fixed. This means that the, in order to change the volume of gas pumped over time, operating speed must be changed. Operating speed change limits the flexibility of the machine insofar as pumping capacity as, in order to change the volume of gas pumped over time, the machine be sped up or slowed down, as would is necessary when gas demand in the distribution network increases or decreases. Changing operating speed is undesirable because it reduces efficiency and changes the vibration frequency imposed on the equipment. 
         [0007]    One solution to these problems is an electromagnetically actuated reciprocating compressor. Such systems use linear motors attached to piston rods to drive opposed pistons in a single compression chamber. When the pistons move in phase with a 0 degree offset, thereby maintaining a fixed distance between the opposed pistons, the compression chamber volume remains constant, and reciprocation effects minimal gas displacement (or gas compression). When the pistons move out of phase with a 180 degree offset, thereby minimizing compression chamber volume when the pistons reach top dead center, and minimizing compression chamber volume when the pistons reach bottom dead center, reciprocation alternately minimizes and maximizes volume to effect maximum gas displacement (or gas compression). Varying the phase angle between these two extremes therefore provides a means for varying displacement (and compression) from a minimum when the pistons move “in phase”, and a maximum when the pistons move “out of phase”. 
         [0008]    Unfortunately, currently available linear motor technology is unsuitable for use in such phased compressors because the associated high inertial rod loads. Existing linear motors can generate a limited amount of force, and the inertia associated with piston rod/compression piston assemblies in machines suitable for use in natural gas systems exceeds that available from existing linear motors. In addition, oppositely arranged pistons in a standard reciprocating compressor would make the machine prohibitively large. And varying the phase between oppositely arranged pistons in a standard reciprocating compressor is not an easy or fast operation. 
         [0009]    Accordingly, there is a need for an electromagnetic actuator for a piston rod where phasing control can be easily obtained by controlling current command on the electromagnetic motor. There is a further need for an electromagnetic actuator that allows for a compact machine. Finally, there is a need for an electromagnetic actuator that can overcome the high inertial forces associated with accelerating and decelerating a piston rod/compression piston assembly. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0010]    Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof. 
         [0011]    In one embodiment, a reciprocating compressor is provided. The reciprocating compressor comprises a housing having an inner surface defining compression chamber, the housing having a first aperture and a second aperture; a first piston having a compression face, the piston being slidably disposed within the compression chamber; a first piston rod having proximate portion and a distal portion, the proximate portion being slidably received within the first aperture and being drivably connected to the first piston; a second piston having a compression face opposed to the first piston compression face, the second piston being slidably disposed within the compression chamber; a second piston rod having proximate portion and a distal portion, the proximate portion being slidably received within the second aperture and drivable connected to the second piston; a first actuator attached to distal portion of the first piston rod; and a second actuator attached to the distal portion of the second piston rod. The piston rods define a translation axis extending through the compression chamber, and the first and second actuators are configured to drivably reciprocate the first and second pistons within the compression chamber along the translation axis. 
         [0012]    In another embodiment of a reciprocating compressor, the compressor comprises a housing having an inner surface defining compression chamber, the housing having an aperture; a first piston having a compression face, the piston being slidably disposed within the compression chamber; a first piston rod having proximate portion and a distal portion, the proximate portion being slidably received within the aperture and being drivably connected to the first piston; a second piston having a compression face opposed to the first piston compression face, the second piston being slidably disposed within the compression chamber; a second piston rod having proximate portion and a distal portion, the proximate portion being slidably received within the first piston rod and drivable connected to the second piston; a first actuator attached to distal portion of the first piston rod; and a second actuator attached to the distal portion of the second piston rod. The first and second piston rods define a translation axis extending through the compression chamber, and the first and second actuators are configured to drivably reciprocate the first and second pistons within the compression chamber along the translation axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other features, aspects, and advantages 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: 
           [0014]      FIG. 1  show a cross-sectional, diagrammatical view of a phased piston reciprocating compressor of an embodiment of the present invention having a double electromagnetic actuator with resonance springs. 
           [0015]      FIG. 2-3  are cross-sectional, diagrammatical views of the compressor of  FIG. 1  illustrating the forces exerted on the reciprocating components during operation of the compressor. 
           [0016]      FIG. 4  show a cross-sectional, diagrammatical view of a phased piston reciprocating compressor of an embodiment of the present invention having coaxially nested piston rods and a single electromagnetic actuator with resonance springs. 
           [0017]      FIG. 5-6  are cross-sectional, diagrammatical views of the compressor of  FIG. 4  illustrating the forces exerted on the reciprocating components during operation of the compressor. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken as limiting the scope of the invention. 
         [0019]      FIGS. 1-3  show a compressor having phased pistons driven by double electromagnetic actuators with resonance springs according to an embodiment of the present invention. 
         [0020]      FIG. 1  shows a compressor  10  comprising a first drive assembly  20 , a first accumulator assembly  30 , a compression assembly  40 , a second accumulator assembly  50 , and a second drive assembly  60 . A first piston rod  12  connects the first drive assembly  20 , the first accumulator assembly  30 , and the compression assembly  40 . A second piston rod  14  connects the second drive assembly  60 , the second accumulator assembly  50 , and the compression assembly  40 . The first piston rod  12  and the second piston rod  14  are arranged serially and substantially coaxially along an axis  16 , the axis  16  extending through the center of the compression assembly  40 . 
         [0021]    The first drive assembly  20  mechanically communicates with the first accumulator assembly  30  and the compression assembly  40  through the first piston rod  12 . The first accumulator assembly  30  mechanically communicates with the first drive assembly  20  and the compression assembly  40  through the first piston rod  12 . The second drive assembly  60  mechanically communicates with the second accumulator assembly  50  and the compression assembly  40  through the second piston rod  14 . The second accumulator assembly  50  mechanically communicates with the second drive assembly  60  and the compression assembly  40  through the second piston rod  14 . 
         [0022]    As shown in  FIG. 1 , the compression assembly  40  comprises a housing  41 , a first compression piston  42 , and a second compression piston  44 . As more fully described below, a first compression piston  42  and a second compression piston  44  are axially disposed within the housing  41 , and define least one fluidly isolated compression chamber. In one embodiment, the compression pistons ( 42 , 44 ) divide the housing volume into three chambers, each chamber being substantially fluidly isolated with respect to the other chambers. 
         [0023]    The housing  41  further comprises a first aperture and a second aperture, each aperture being substantially aligned with axis  16 , the apertures defining an orifice linking the interior of the housing with environment external to compression assembly  40 . The first aperture slidably and sealably receives the first piston rod  12  along the axis  16 , the first piston rod  12  extending into the housing  41  and connecting to the first compression piston  42 . The second aperture slidably and sealably receives the second piston rod  14  along the axis  16 , the second piston rod  14  extending into the housing  41  and connecting to the second compression piston  44 . 
         [0024]    The first piston  42  comprises a surface. The first piston surface comprises an edge, the edge being configured to slidably and sealably engage an inner surface of the housing. The first piston surface further comprises a proximal face, the proximal face being substantially orthogonal to the axis  16  and facing the second piston  44 . The first piston surface further comprises a distal face opposite its proximal face, the rear face being substantially orthogonal to the axis  16 . In an embodiment, the first piston rod  12  connects to the first compression piston  42  at the rear face of the first compression piston  42 . As used herein, the term “proximal” refers to placement or movement toward the center of the compression assembly  40 . As used herein, the term “distal” refers to placement or movement away from the center of the compression assembly  40 . 
         [0025]    The second piston  44  comprises a surface. The second piston surface comprises an edge, the edge being configured to slidably and sealably engage the housing inner surface. The second piston surface further comprises a proximal face, the proximal face being substantially orthogonal to the axis  16  and facing the proximal face of first piston  42 . The second piston surface further comprises a distal face opposite its proximal face, the rear face being substantially orthogonal to the axis  16 . In an embodiment, the second piston rod  14  connects to the second compression piston  44  at the distal surface of the second compression piston  44 . 
         [0026]    A portion of the housing inner surface, first piston proximal face, and second piston proximal face collectively define a central compression chamber  43 . The central compression chamber  43  in turn is fluidly communicative a fluid source (not shown) and a fluid destination (also not shown) through an inlet/outlet valve  47 . In an embodiment, a portion of the housing inner surface and the first piston distal face further define a first compression chamber  45 . The first compression chamber  45  in turn is also fluidly communicative with the fluid source and the fluid destination through an inlet/outlet valve  48 . In an embodiment, a portion of the housing inner surface and the second piston distal face further define a second compression chamber  46 . The second compression chamber  46  in turn is fluidly communicative with the fluid source and the fluid destination through an inlet/outlet valve  49 . In embodiments, one of the central compression chamber  43 , the first compression chamber  45 , and the second compression chamber  46  are substantially fluidly isolated from one another. As would be recognized by one of skill in the art in view of the disclosure and teachings herein, “fluid” refers materials comprising a liquid, a gas, or a comprising a combination of fluid and gas. 
         [0027]    In embodiments, at least one of the valves ( 47 , 48 , 49 ) comprises a solenoid actuator (not shown). In other embodiments, at least one of the valves ( 47 , 48 , 49 ) comprises a magnetic gearing actuator (not shown). Operatively, the valves ( 47 , 48 , 49 ) cooperate with movement of the pistons ( 42 , 44 ) to allow fluid to enter at least one compression chamber at a first pressure and exit the chamber at a second pressure. As would be understood by one of ordinary skill in the art in view of the disclosure and teachings herein, fluid communication between the chambers ( 43 , 45 , 46 ) and the fluid supply/destination may be accomplished by dedicated individual inlet and outlet valves as shown in  FIG. 1-3 , or through a single valve configured to selectively connect the chamber with a fluid source and fluid destination. 
         [0028]    As further shown in  FIG. 1 , the first drive assembly drive  20  comprises a stator  22  and a core  24 . The core  24  is attached to a distal end of the piston rod  12 , and the stator  22  is fixed with respect to the core  24 . Operatively, the stator  22  is configured to exert an electromagnetic force the core  24 , thereby reciprocatedly driving the core  24  in the distal and proximal directions along the axis  16 . 
         [0029]    As also shown in  FIG. 1 , the second drive assembly drive  60  comprises a stator  62  and a core  64 . The core  64  is attached to a distal end of the piston rod  14 , and the stator  62  is fixed with respect to the core  64 . Operatively, the stator  62  is configured to exert an electromagnetic force the core  64 , thereby reciprocatedly driving the core  64  in the distal and proximal directions along the axis  16 . 
         [0030]    In an embodiment, the electromagnetic drive  20  is a linear motor wherein the stator  22  comprises a succession of adjacent coils selectively connectable to a power supply through a controller. When a selected coil is connected to the power supply, the coils exert an electromotive force on the coil, thereby driving the piston rod/compression piston axially along axis  16 . When a group of adjacent coils is connected to the power supply, the electromagnetic force increases. When an adjacent coil in the direction of piston rod/compression assembly translation is added to the set of coils connected to the power supply, and an adjacent coil opposite to the direction of translation is removed from the set of coils connected to the power supply, the stator  22  maintains a constant level electromagnetic force on the core  24 . As such, the controller is configured to dynamically select the group of coils connected to the power supply at any given time, and by energizing and de-energizing coils, configured to controllably displace the coil along the axis  16 . In an embodiment of the invention, the electromagnetic drive comprises a commercially available linear motor. 
         [0031]    As additionally shown in  FIG. 1 , the first accumulator  30  comprises a first flange  32 , a first resilient member  34 , a first post  38 , a second resilient member  37 , and a second flange  39 . In an embodiment, one or both the flanges ( 32 , 39 ) may be defined by the piston rod  12 . In other embodiments, one or both of the flanges may be constructed by attaching assemblies to the piston rod  12 . The first post  38  comprises an aperture  36  that slidably receives the piston rod  12 , and is fixed with respect to the piston rod  12 . Each resilient member ( 34 , 37 ) comprises a first end and a second end. The first resilient member  34  attaches to the first flange  32  at the first end, and the first resilient member  34  attaches to the first post  38  at the second end. The second resilient member  37  attaches to the second flange  39  at the first end, and the second resilient member  34  attaches to the first post  38  at the second end. 
         [0032]    As further shown in  FIG. 1 , the second accumulator  50  comprises a third flange  52 , a third resilient member  54 , a second post  56 , a fourth resilient member  57 , and a fourth flange  59 . In an embodiment, one or both of the flanges ( 54 , 59 ) may be defined by the piston rod  14 . In other embodiments, one or both of the flanges may be constructed by attaching assemblies to the piston rod  14 . The second  56  comprises an aperture  58  that slidably receives the piston rod  14 , and is fixed with respect to the piston rod  14 . Each resilient member ( 54 , 57 ) comprises a first end and a second end. The third resilient member  54  attaches to the third flange  52  at the first end, and the third resilient member  54  attaches to the second post  56  at the second end. The fourth resilient member  57  attaches to the fourth flange  59  at the first end, and the fourth resilient member  57  attaches to the post  56  at the second end. 
         [0033]      FIG. 2  and  FIG. 3  show the forces exerted on piston rod/compression piston assemblies ( 12 , 42 ;  14 , 44 ) by the drive assemblies ( 20 , 60 ). As used herein, the phrase “top dead center” refers to a positional arrangement wherein a piston ( 42 , 44 ) disposed in the compression assembly  40  is substantially at its most distal point of translation along axis  16 . As used herein, the phrase “bottom dead center” refers to a positional arrangement wherein a piston ( 42 , 44 ) disposed in the compression assembly  40  is substantially at its most proximal point of translation along axis  16 . 
         [0034]      FIG. 2  shows the forces exerted to drive first piston rod/compression piston assembly ( 12 , 42 ) in the proximal direction along axis  16 . At the start of the stroke the assembly is substantially motionless, the piston  42  being substantially positioned at top dead center. Four forces are exerted on the assembly during proximal translation. First, the first drive assembly  20  accelerates the assembly by exerting the above-discussed electromotive force F 1  on the assembly, thereby driving the assembly in the proximal direction along axis  16 . Second, at the start of the stroke and for a portion of the stroke, the deformed (elongated) first resilient member  34  returns to its normal shape, thereby exerting a proximally-oriented accelerating force F 2  on the assembly. Third, as the volume within central compression chamber  43  decreases, gas resident in the chamber exerts a distally-oriented force F 3  on the proximal face of the compression piston  42 . Finally, at a point prior to the end of the stroke and continuing until the piston  42  reaches bottom dead center, the second resilient member  37  deforms (elongates), thereby exerting a distally-oriented decelerating force F 4  on the assembly. 
         [0035]      FIG. 2  also shows the forces exerted to drive second piston rod/compression piston assembly ( 14 , 44 ) in the proximal direction along axis  16 . At the start of the stroke the assembly is substantially motionless, the piston  44  being substantially positioned at top dead center. As described above, four forces are exerted on the assembly during proximal translation. First, the second drive assembly  60  accelerates the assembly by exerting the above-discussed electromotive force F 5  on the assembly, thereby driving the assembly in the proximal direction along axis  16 . Second, at the start of the stroke and for a portion of the stroke, the deformed (elongated) third resilient member  57  returns to its normal shape, thereby exerting a proximally-oriented accelerating force F 6  on the assembly. Third, as the volume within the central compression chamber  43  decreases, gas resident in the chamber exerts a distally-oriented force F 7  on the proximal face of the compression piston  44 . Finally, at a point prior to the end of the stroke and continuing until the piston  44  reaches bottom dead center, the fourth resilient member  54  deforms (elongates), thereby exerting a distally-oriented decelerating force F 8  on the assembly. 
         [0036]      FIG. 3  shows the forces exerted to drive first piston rod/compression piston assembly ( 12 , 42 ) in the distal direction along axis  16 . At the start of the stroke the assembly is substantially motionless, the piston  42  being substantially positioned at bottom dead center. Four forces are exerted on the assembly during distal translation. First, the first drive assembly  20  accelerates the assembly by exerting the above-discussed electromotive force as a force F 9  on the assembly, thereby driving the assembly in the distal direction along axis  16 . Second, at the start of the stroke and for a portion of the stroke, the deformed (elongated) second resilient member  37  returns to its normal shape, thereby exerting a distally-oriented accelerating force F 10  on the assembly. Third, as the volume within first compression chamber  45  decreases, gas resident in the chamber exerts a proximally-oriented force F 11  on the distal face of the first compression piston  42 . Finally, at a point prior to the end of the stroke and continuing until the piston  42  reaches top dead center, the first resilient member  34  deforms (elongates), thereby exerting a proximally-oriented decelerating force F 12  on the assembly. 
         [0037]      FIG. 3  also shows the forces exerted to drive second piston rod/compression piston assembly ( 14 , 44 ) in the distal direction along axis  16 . At the start of the stroke the assembly is substantially motionless, the piston  44  being substantially positioned at bottom dead center. As described above, four forces are exerted on the assembly during distal translation. First, the second drive assembly  60  accelerates the assembly by exerting the above-discussed electromotive force F 13  on the assembly, thereby driving the assembly in the distal direction along axis  16 . Second, at the start of the stroke and for a portion of the stroke, the deformed (elongated) third resilient member  54  returns to its normal shape, thereby exerting a distally-oriented accelerating force F 14  on the assembly. Third, as the volume within the second compression chamber  46  decreases, gas resident in the chamber exerts a proximally-oriented force F 15  on the distal face of the compression piston  44 . Finally, at a point prior to the end of the stroke and continuing until the piston  44  reaches top dead center, the fourth resilient member  57  deforms (elongates), thereby exerting a distally-oriented decelerating force F 16  on the assembly. 
         [0038]    During the stroke, the sum of the forces dictates the rate at which the assembly accelerates and decelerates during its translation along axis  16 . When the assembly is accelerating, the inertia of the assembly increases. When the assembly is decelerating, the inertia of the assembly decreases. When the assembly travels at a fixed velocity, the inertia of the assembly is constant. Hence, at the beginning of the stroke, relaxation of the first resilient member accelerates the assembly, thereby increasing the inertia resident in the assembly. During a point of travel the second resilient member begins to deform, decelerating the assembly, thereby decreasing the inertia resident in the assembly. Collectively, the resilient members have the technical effect of storing the inertial energy resident in the assembly during a first stroke, and imparting that stored energy to the assembly during a subsequent stroke, thereby conserving the energy present in the piston rod/compression piston assemblies ( 12 , 42 ; 14 , 44 ) during reciprocation. 
         [0039]    As would be readily apparent to one of ordinary skill in the art in view of the disclosure and teachings herein, the configuration of the resilient member pairs described above may be altered to change the timing at which the associated forces are applied. For example, it is within the scope of the present invention for the illustrated pairs of resilient members ( 34 , 37 ; 54 , 57 ) to have different spring constants. Alternatively, the distance over which the resilient member applies force may be different within a pair of resilient members ( 34 , 37 ; 54 , 57 ). Finally, it is within the scope of the present invention for the a single resilient member to perform the above-discussed functions, for example to start the stroke elongated in a distal direction at the start of the stroke, relax during the course of the stroke, and deform in the proximal direction during a terminal portion of the stroke. 
         [0040]    In an embodiment, the resilient member comprises a resonant spring having a spring constant, a resonant frequency, and harmonics of the spring resonant frequency. In the illustrated embodiment, the resonant spring  34  is configured to be deformed as the piston approaches top dead center by the distal translation of first flange  32  with respect to first post  38 , thereby stretching the resonant spring, causing the spring to absorb energy, the spring further decelerating the piston rod/compression piston assembly ( 12 , 42 ) as it approaches top dead center. In the embodiment, the stretched resonant spring  34  returns to its normal shape during the subsequent stroke, thereby accelerating the piston rod/compression piston assembly ( 12 , 42 ) proximally, thereby accumulating inertial energy resident in the assembly during a first distal stroke along axis  16 , and returning energy to the assembly during a second proximal stroke along axis  16  by accelerating the assembly proximally along axis  16 . 
         [0041]    In certain embodiments, the spring is a resonant spring configured to absorb more energy when the frequency of its oscillations (reciprocations) matches the natural frequency of the resonant spring, or a harmonic thereof. For example, when the reciprocating rate of the piston rod  12 /compression piston  42  substantially matches the natural frequency of resonant spring  34 , the above-described cyclic spring deformations maximize energy accumulated and applied by the spring in successive reciprocations. In such embodiments, operating the compressor  10  such that the piston rod/compression piston assembly reciprocates at a rate substantially matching the spring resonant frequency or a harmonic thereof minimizes the drive force requirement. 
         [0042]    Embodiments of the compressor may run in a partially loaded state. In one mode, the load on the distal face of piston  42  may be modulated by controlling the timing of fluid communication between the chamber  45  and the fluid source/destination through selective operation of valve  48 . For example, piston  42  may be partially unloaded by operating valve  48  such that the pressure difference between fluid entering and leaving chamber  45  is reduced, or substantially minimized, during a portion of piston movement. Similarly, the load on the distal face of piston  44  may be modulated by controlling the timing of fluid communication between the chamber  46  and the fluid source/destination through selective operation of valve  49 . For example, piston  44  may be partially unloaded by operating valve  49  such that the pressure difference between fluid entering and leaving chamber  46  is reduced, or substantially minimized, during a portion of piston movement. In another mode, the load on proximal faces of the pistons ( 42 , 44 ) may be modulated by controlling the timing of fluid communication between chamber  43  and the fluid source/destination through operation of valve  47 . For example, the pistons ( 42 , 44 ) may be partially unloaded by operating valve  47  such that the pressure difference between fluid entering and leaving chamber  43  is reduced, or substantially minimized, during a portion of piston movement. Such modes of operation allow for flexible operation, such as periods where fluid demand changes, such as when natural gas demand changes in a natural gas distribution network. 
         [0043]    In an embodiment the compressor is a variable capacity compressor. For example, the controller may be configured to vary piston phase, and thereby compressor capacity, by being programmed with set of instructions recorded on a non-transitory, machine-readable media that cause the controller to (i) receive a compressor phase setting, the phase setting comprising a piston offset between 0 degrees and 180 degrees; (ii) select the group of coils from the plurality of coils necessary to connect to the power supply during a stroke of the piston rod/compression piston to define respective stroke lengths; (iii) define the time that each selected coil must be connected to the power supply, define period of time in which the coil be connected to the power supply during the respective stroke, and define the time at which point the coil be disconnected from the power supply during the respective stroke; and (iv) selectively connect the identified coils to the power supply at the defined time, allow the selected coils to remain connected to the power supply for the defined period of time, and selectively disconnect the identified coils at the defined time, to drive the piston rod/compression piston assemblies. In an embodiment, the controller may also be configured to receive a stroke length setting for use in selecting the coils and defining the connection time, connection duration, and disconnect time. 
         [0044]      FIGS. 4-6  show a compressor having phased pistons driven by a single electromagnetic actuator with resonance springs according to an embodiment of the present invention. 
         [0045]      FIG. 4  shows a compressor  200  comprising a drive assembly  220 , a first accumulator assembly  230 , a compression assembly  240 , and a second accumulator assembly  250 . A first piston rod  212  connects the drive assembly  220 , the first accumulator assembly  230 , and the compression assembly  240 . A second piston rod  214  connects the drive assembly  220 , the second accumulator assembly  250 , and the compression assembly  240 . 
         [0046]    The second piston rod  214  is hollow, comprising a corridor (not shown) having a distal opening  228  one its distal end and having a proximal opening  215  on its proximal end. The second piston rod is adapted to slidably and sealably receive a portion of the first piston rod  212  along its axial length, the first and second piston rods being coaxially aligned along ax axis  216 . As shown in  FIG. 4 , dashed lines  218  indicate a portion of the first piston rod  212  received within the second piston rod  214 . Operatively, the piston rods are configured such that the piston rods ( 212 , 214 ) may translate independently with respect to the other along the axis  216 . 
         [0047]    The drive assembly  220  mechanically communicates with the first accumulator assembly  230  and the compression assembly  240  through the first piston rod  212 . The first accumulator assembly  230  mechanically communicates with the drive assembly  220  and the compression assembly  240  through the first piston rod  212 . The drive assembly  220  also mechanically communicates with the second accumulator assembly  250  and the compression assembly  240  through the second piston rod  214 . The second accumulator assembly  250  mechanically communicates with the drive assembly  220  and the compression assembly  240  through the second piston rod  214 . 
         [0048]    As shown in  FIG. 4 , the compression assembly  240  comprises a housing  241 , a first compression piston  242 , and a second compression piston  244 . A first compression piston  244  and a second compression piston  242  are axially disposed within the housing  241 , and define least one fluidly isolated compression chamber. In the embodiment shown in  FIG. 4 , the compression pistons ( 242 , 244 ) divide the housing volume into three chambers, each chamber being substantially fluidly isolated with respect to the other chambers. 
         [0049]    The housing  241  further comprises an aperture substantially aligned with axis  216 , the aperture defining an orifice linking the interior of the housing with environment external to compression assembly  240 . The first aperture slidably and sealably receives the second piston rod  214  along the axis  216 , the second piston rod  214  extending into the housing  241  and connecting to the second compression piston  242 . 
         [0050]    The second compression piston  242  comprises a surface. The second compression piston surface comprises an edge, the edge being configured to slidably and sealably engage an inner surface of the housing  241 . The first piston surface further comprises a proximal face, the proximal face being substantially orthogonal to the axis  216 . The first piston proximal face further comprises the aperture  215 , the first piston rod  212  extending through the aperture  215  and attaching to the first compression piston  244 . The first compression piston surface further comprises a distal face opposite the proximal face, the rear face being substantially orthogonal to the axis  216 . In an embodiment, the second piston rod  214  connects to the second compression piston  242  at the rear face of the second compression piston  242 . 
         [0051]    The first compression piston  244  comprises a surface. The first compression piston surface comprises an edge, the edge being configured to slidably and sealably engage the housing inner surface. The first compression piston surface further comprises a proximal face, the proximal face being substantially orthogonal to the axis  216  and facing the proximal face of second compression piston  242 . The first piston surface further comprises a distal face opposite its proximal face, the rear face being substantially orthogonal to the axis  216 . In the embodiment shown in  FIG. 4 , the first piston rod  212  connects to the first compression piston  244  at its proximal surface. 
         [0052]    A portion of the housing inner surface, first piston proximal face, and second piston proximal face collectively define a central compression chamber  243 . The central compression chamber  243  in turn is fluidly communicative a fluid source (not shown) and a fluid destination (also not shown) through an inlet/outlet valve  247 . In an embodiment, a portion of the housing inner surface and the first piston distal face further define a first compression chamber  245 . The first compression chamber  245  in turn is also fluidly communicative with the fluid source and the fluid destination through an inlet/outlet valve  248 . In an embodiment, a portion of the housing inner surface and the second piston distal face further define a second compression chamber  246 . The second compression chamber  246  in turn is fluidly communicative with the fluid source and the fluid destination through an inlet/outlet valve  249 . In embodiments, one of the central compression chamber  243 , the first compression chamber  245 , and the second compression chamber  246  are substantially fluidly isolated from one another. 
         [0053]    In embodiments, at least one of the valves ( 247 , 248 , 249 ) comprises a solenoid actuator (not shown). In other embodiments, at least one of the valves ( 247 , 248 , 249 ) comprises a magnetic gearing actuator (not shown). Operatively, the valves ( 247 , 248 , 249 ) cooperate with movement of the pistons ( 242 , 244 ) to allow fluid to enter at least one compression chamber at a first pressure and exit the chamber at a second pressure. As would be understood by one of ordinary skill in the art in view of the disclosure and teachings herein, fluid communication between the chambers ( 243 , 245 , 246 ) and the fluid supply/destination may be accomplished by dedicated individual inlet and outlet valves as shown in  FIG. 4-6 , or through a single valve configured to selectively connect the chamber with a fluid source and fluid destination. 
         [0054]    As further shown in  FIG. 4 , the drive assembly drive  220  comprises a stator  222 , first core  226 , and a second core  228 . The first core  226  is attached to the first piston rod  212 , the second core  228  is attached to the distal portion of the second piston rod  214 , and the stator  222  is fixed with respect to the cores ( 226 , 228 ). Operatively, the stator  222  is configured to exert an electromagnetic force the cores ( 226 , 228 ), thereby reciprocatedly driving the cores ( 226 , 228 ) in the distal and proximal directions along the axis  216 . In embodiment of the invention, the stator is configured to independently drive the cores ( 226 , 228 ) with respect to one another. 
         [0055]    In an embodiment, the drive assembly  220  comprises a linear motor wherein the stator  222  comprises a plurality of coils  225  selectively connectable to a power supply (not shown) through a controller (not shown). When an individual coil from the plurality of coils  2251  is connected to the power supply, the coils exert an electromotive force on the cores ( 226 , 228 ), thereby driving the piston rod/compression piston attached to the respective core axially along axis w 16. When a coil is added to the set of coils connected to the power supply, the electromagnetic force increases. When a coil is removed to the set of coils connected to the power supply, the electromagnetic force decreases. When an adjacent coil in the direction of piston rod/compression assembly translation is added to the set of coils connected to the power supply, and an adjacent coil opposite to the direction of translation is removed from the set of coils connected to the power supply, the stator  222  maintains a constant electromagnetic force on the respective core  24 —the electromagnetic force in effect following the core as is translates along the axis. In one embodiment of the invention, the electromagnetic drive comprises a commercially available linear motor. 
         [0056]    As additionally shown in  FIG. 4 , the first accumulator  230  comprises a first flange  232 , a first resilient member  234 , a first post  238 , a second resilient member  237 , and a second flange  239 . In an embodiment, one or both the flanges ( 232 , 239 ) may be defined by the first piston rod  212 . In other embodiments, one or both of the flanges may be constructed by attaching assemblies to the first piston rod  212 . The first post  238  comprises an aperture  236  that slidably receives the first piston rod  212 , and the post  238  being fixed with respect to the piston rod  212 . Each resilient member ( 234 , 237 ) comprises a first end and a second end. The first resilient member  234  attaches to the first flange  232  at its first end, and the first resilient member  234  attaches to the first post  238  at its second end. The second resilient member  237  attaches to the second flange  239  at its first end, and the second resilient member  234  attaches to the first post  238  at its second end. 
         [0057]    As shown in  FIG. 4 , the second accumulator  250  comprises a third flange  252 , a third resilient member  254 , a second post  256 , a fourth resilient member  257 , and a fourth flange  259 . In an embodiment, one or both the flanges ( 254 , 259 ) may be defined by the second piston rod  214 . In other embodiments, one or both of the flanges may be constructed by attaching assemblies to the second piston rod  214 . The second post  256  comprises an aperture  258  that slidably receives the second piston rod  214 , and is fixed with respect to the second piston rod  214 . Each resilient member ( 254 , 257 ) comprises a first end and a second end. The third resilient member  254  attaches to the third flange  252  at its first end, and the third resilient member  254  attaches to the second post  256  at its second end. The fourth resilient member  257  attaches to the fourth flange  259  at its first end, and the fourth resilient member  257  attaches to the post  256  at its second end. 
         [0058]      FIG. 5  and  FIG. 6  show the forces exerted on piston rod/compression piston assemblies ( 212 , 242 ;  214 , 244 ) by the drive assembly ( 220 ) in compressor  200 . 
         [0059]      FIG. 5  shows the forces exerted to drive first piston rod/compression piston assembly ( 212 , 244 ) to drive the piston  244  in the proximal direction along axis  216 , as would be applied during a first reciprocation of compressor  200 . At the start of the stroke the assembly is substantially motionless, the piston  242  being substantially positioned at top dead center. Four forces are exerted on the assembly during proximal translation. First, the drive assembly  220  accelerates the assembly by exerting the above-discussed, distally-oriented, electromotive force F 101  on the assembly, thereby driving the assembly in the distal direction along axis  216 . Second, at the start of the stroke and for a portion of the stroke, the deformed (elongated) first resilient member  237  returns to its normal shape, thereby exerting a distally-oriented accelerating force F 102  on the assembly. Third, as the volume within central compression chamber  243  decreases, gas resident in the chamber exerts an opposing force F 103  on the proximal face of the compression piston  244 . Finally, at a point prior to the end of the stroke and continuing until the piston  244  reaches bottom dead center, the second resilient member  3234  deforms (elongates), thereby exerting an opposing force F 104  on the assembly, thereby decelerating the assembly as it approaches its bottom dead center position. 
         [0060]      FIG. 5  also shows the forces exerted to drive second piston rod/compression piston assembly ( 214 , 242 ) to drive the piston  242  in the proximal direction along axis  216 , as would be applied during a first reciprocation of compressor  200 . At the start of the stroke the assembly is substantially motionless, the piston  242  being substantially positioned at top dead center. Four forces are exerted on the assembly during second piston proximal translation. First, the drive assembly  220  accelerates the assembly by exerting the above-discussed, proximally oriented, electromotive force F 105  on the assembly, thereby driving the assembly in the proximal direction along axis  216 . Second, at the start of the stroke and for a portion of the stroke, the deformed (elongated) third resilient member  254  returns to its normal shape, thereby exerting a proximally-oriented accelerating force F 106  on the assembly. Third, as the volume within the central compression chamber  243  decreases, gas resident in the chamber exerts an opposing force F 107  on the proximal face of second compression piston  242 . Finally, at a point prior to the end of the stroke and continuing until the piston  242  reaches bottom dead center, the fourth resilient member  257  deforms (elongates), thereby exerting an additional opposing force F 108  on the assembly thereby decelerating the assembly as it approaches its bottom dead center position. 
         [0061]    Operatively, the forces at play sum together, and the resultant force causes piston movement. In an embodiment, the forces applied by the resilient members are applied for only a portion of the stroke and complement the drive assembly force. For example, one resilient member of an accumulator begins the stroke in an elongated state, thereby having the technical effect of reducing the force otherwise required of the drive assembly by applying additional force at the start of the stroke. Similarly, the complementary resilient member of the accumulator begins the stroke in a normal state, and becomes elongated towards the end of the stroke, thereby having the technical effect of decelerating the assembly and storing inertial energy for the subsequent reciprocation of the assembly. 
         [0062]      FIG. 6  shows the forces exerted to drive first piston rod/compression piston assembly ( 212 , 244 ) in the distal direction along axis  216 , as would be applied during a second reciprocation of compressor  200 . At the start of the stroke the assembly is substantially motionless, the first compression piston  244  being substantially positioned at bottom dead center. Four forces are exerted on the assembly during distal piston translation. First, the drive assembly  220  accelerates the assembly by exerting a proximally-oriented electromotive force F 109  on the assembly, thereby driving the piston in the distal direction along axis  216 . Second, at the start of the stroke and for a portion of the stroke, the deformed (elongated) first resilient member  234  returns to its normal shape, thereby exerting a proximally-oriented accelerating force F 110  on the assembly. Third, as the volume within first compression chamber  245  decreases, gas resident in the chamber exerts an opposing force F 111  on the distal face of the first compression piston  244 . Finally, at a point prior to the end of the stroke and continuing until the piston  42  substantially reaches the top dead center position, the second resilient member  237  deforms (elongates), thereby exerting an opposing force F 112  on the assembly, thereby decelerating the assembly as it approaches its top dead center position. 
         [0063]      FIG. 6  also shows the forces exerted to drive second piston rod/compression piston assembly ( 214 , 242 ) in the distal direction along axis  216 , as would be applied during a second reciprocation of compressor  200 . At the start of the stroke the assembly is substantially motionless, the second compression piston  242  being substantially positioned at bottom dead center. Four forces are exerted on the assembly during distal translation of the second compression piston  242 . First, the drive assembly  220  accelerates the assembly by exerting the above-discussed electromotive force F 113  on the assembly, thereby driving the piston  242  in the distal direction along axis  216 . Second, at the start of the stroke and for a portion of the stroke, the deformed (elongated) fourth resilient member  257  returns to its normal shape, thereby exerting a distally-oriented accelerating force F 114  on the assembly. Third, as the volume within the second compression chamber  246  decreases, gas resident in the chamber exerts an opposing force F 115  on the distal face of the compression piston  242 . Finally, at a point prior to the end of the stroke and continuing until the second compression piston  242  reaches its top dead center position, the third resilient member  254  deforms (elongates), thereby exerting an opposing force F 116  on the assembly, thereby decelerating the assembly as it approaches its top dead center position. 
         [0064]    During the stroke, the sum of the forces dictates the rate at which the assembly accelerates and decelerates during its translation along axis  216 . When the assembly is accelerating, the inertia of the assembly increases. When the assembly is decelerating, the inertia of the assembly decreases. When the assembly travels at a fixed velocity, the inertia of the assembly is constant. Hence, at the beginning of the stroke, relaxation of the first resilient member accelerates the assembly, thereby increasing the inertia resident in the assembly. During a point of travel the second resilient member begins to deform, decelerating the assembly, thereby decreasing the inertia resident in the assembly. Collectively, the resilient members have the technical effect of storing the inertial energy resident in the assembly during a first stroke, and imparting that stored energy to the assembly during a subsequent stroke, thereby conserving the energy present in the piston rod/compression piston assemblies ( 212 , 244 ; 214 , 242 ) during reciprocation. 
         [0065]    In an embodiment, the resilient member comprises a resonant spring having a spring constant, a resonant frequency, and harmonics of the spring resonant frequency. In the illustrated embodiment, the resonant spring  34  is configured to be deformed as the piston approaches top dead center by the distal translation of first flange  32  with respect to first post  38 , thereby stretching the resonant spring, causing the spring to absorb energy, the spring further decelerating the piston rod/compression piston assembly ( 12 , 42 ) as it approaches top dead center. In the embodiment, the stretched resonant spring  34  returns to its normal shape during the subsequent stroke, thereby accelerating the piston rod/compression piston assembly ( 12 , 42 ) proximally, thereby accumulating inertial energy resident in the assembly during a first distal stroke along axis  16 , and returning energy to the assembly during a second proximal stroke along axis  16  by accelerating the assembly proximally along axis  16 . 
         [0066]    In certain embodiments, the spring is a resonant spring configured to absorb more energy when the frequency of its oscillations (reciprocations) matches the natural frequency of the resonant spring, or a harmonic thereof. For example, when the reciprocating rate of the piston rod  12 /compression piston  42  substantially matches the natural frequency of resonant spring  34 , the above-described cyclic spring deformations maximize energy accumulated and applied by the spring in successive reciprocations. In such embodiments, operating the compressor  10  such that the piston rod/compression piston assembly reciprocates at a rate substantially matching the spring resonant frequency or a harmonic thereof minimizes the drive force requirement. 
         [0067]    Embodiments of the compressor may run in a partially loaded state. In one mode, the load on the distal face of piston  242  may be modulated by controlling the timing of fluid communication between the chamber  245  and the fluid source/destination through selective operation of valve  248 . For example, piston  242  may be partially unloaded by operating valve  248  such that the pressure difference between fluid entering and leaving chamber  245  is reduced, or substantially minimized, during a portion of piston movement. Similarly, the load on the distal face of piston  244  may be modulated by controlling the timing of fluid communication between the chamber  246  and the fluid source/destination through selective operation of valve  249 . For example, piston  244  may be partially unloaded by operating valve  249  such that the pressure difference between fluid entering and leaving chamber  246  is reduced, or substantially minimized, during a portion of piston movement. In another mode, the load on proximal faces of the pistons ( 242 , 244 ) may be modulated by controlling the timing of fluid communication between chamber  243  and the fluid source/destination through operation of valve  247 . For example, the pistons ( 242 , 244 ) may be partially unloaded by operating valve  247  such that the pressure difference between fluid entering and leaving chamber  243  is reduced, or substantially minimized, during a portion of piston movement. Such modes of operation allow for flexible operation, such as periods where fluid demand changes, such as when natural gas demand changes in a natural gas distribution network. 
         [0068]    In an embodiment the compressor is a variable capacity compressor. For example, the controller may be configured to vary piston phase, and thereby compressor capacity, by being programmed with set of instructions recorded on a non-transitory, machine-readable media that cause the controller to (i) receive a compressor phase setting, the phase setting comprising a piston offset between 0 degrees and 180 degrees; (ii) select the group of coils from the plurality of coils necessary to connect to the power supply during a stroke of the piston rod/compression piston to define respective stroke lengths; (iii) define the time that each selected coil must be connected to the power supply, define period of time in which the coil be connected to the power supply during the respective stroke, and define the time at which point the coil be disconnected from the power supply during the respective stroke; and (iv) selectively connect the identified coils to the power supply at the defined time, allow the selected coils to remain connected to the power supply for the defined period of time, and selectively disconnect the identified coils at the defined time, to drive the piston rod/compression piston assemblies. In an embodiment, the controller may also be configured to receive a stroke length setting for use in selecting the coils and defining the connection time, connection duration, and disconnect time. 
         [0069]    In an embodiment, the nested piston rods ( 212 , 214 ) of compressor  200  result in a smaller, more compact compressor and allow for the compressor to be constructed from a single drive assembly. As a result, the overall dimensions of the machine are smaller, reducing the size of the facility required to house the compressor. 
         [0070]    As would be readily apparent to one of ordinary skill in the art in view of the disclosure and teachings herein, the configuration of the resilient member pairs described above may be altered to change the timing at which the associated forces are applied. For example, it is within the scope of the present invention for the illustrated complementary resilient members ( 234 , 237 ; 254 , 257 ) to have different spring constants. Alternatively, the distance over which the resilient member applies force may be different between complementary resilient members ( 234 , 237 ; 254 , 257 ). Finally, it is within the scope of the present invention for the a single resilient member to perform the above-discussed functions, for example to start the stroke elongated in a distal direction at the start of the stroke, relax during the course of the stroke, and deform in the proximal direction during a terminal portion of the stroke. 
         [0071]    According to an embodiment, a capacitor having a first conductor fixed and a second conductor attached to either the first or the second piston rod are separated by a dielectric (e.g. air); in this way, the capacitor has moving plates (to be precise one plate moves with respect to the other plate) and thus has a variable capacitance. According to a variant of this embodiment, the dielectric-occupied distance between the two conductive plates varies with translation of the piston rods. The first and second conductors may be charged once-for-all and left isolated during operation of the compressor, or may be charged differently and left isolated during distinct operating periods of the compressors, or may be permanently connected to a constant voltage generator during operation of the compressor, or may be permanently connected to a variable voltage generator during operation of the compressor (typically the voltage of the generator is varied slowly with respect to the oscillation period of the translatable assembly). Such an accumulator stores a changeable electric charge corresponding to movement of the piston rods, the capacitor thereby banking the inertial energy of the piston rods and being configured to supply the charge to power a subsequent translation of the piston rods. The use of one or more capacitor may be combined with the use of one ore more springs that may have a constant or a variable spring constant. 
         [0072]    It is worth noting that the springs of the embodiments of the present invention may have a spring constant that is constant with respect to time and space which corresponds to the most common case for helical springs; alternatively, the spring constant may vary in time and/or in position, in particular along its length (i.e. it depends on the degree of compression of the spring). 
         [0073]    According to an embodiment, a variable accumulator is provided being configured to vary compressor capacity by increasing stroke and maintaining actuation time, thereby allowing for magnet position to be optimized. In an illustrative manner, the accumulator comprises a resilient member having a plurality of selectable parallel springs. The number of springs used in a stroke can change, thereby altering the spring constant, thereby varying the stroke length and optimizing the magnet position. 
         [0074]    More in general, such accumulator may comprise a spring assembly having a first end coupled to either the first or the second piston rod and a second end fixed with respect to either the first or the second piston rod. A spring assembly may comprise a plurality of springs and the spring constant of this spring assembly may be adjustable; the springs may have different spring constant and be arranged in parallel so to be selectively effective. Alternatively, a spring assembly may comprise a plurality of springs having different lengths and be arranged in parallel so to have different effective strokes (i.e. in a first displacement range of the translatable assembly a first set of springs are active on the translatable assembly, in a second displacement range a second set of springs are active, in a third displacement range a third set of springs are active, . . . .) The expression “arranged in parallel” is to be interpreted from the functional point of view; in fact, the axes of the springs may be parallel to each other (even coincident as a limit case) or inclined to each other. 
         [0075]    While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.