LINEAR COMPRESSOR AND METHODS OF SETPOINT CONTROL

Abstract

A linear compressor and methods of operation, for example, to control a setpoint and vary cooling capacity of the linear compressor, are provided herein. An appliance may include a linear compressor having a reciprocating piston movable in a negative axial direction toward a chamber and positive axial direction away from the chamber. The appliance may further include a motor operatively coupled to the reciprocating piston, the motor having a resting setpoint, an inverter configured to supply a variable frequency waveform to the motor, and a controller configured to control the variable frequency waveform. The controller may be configured to direct a positive DC voltage to the motor to shift the resting setpoint to increase a cooling capacity of the linear compressor.

Claims

1. An appliance, comprising: a linear compressor, the linear compressor having a reciprocating piston movable in a negative axial direction toward a chamber and positive axial direction away from the chamber a motor operatively coupled to the reciprocating piston, the motor having a resting setpoint; an inverter configured to supply a variable frequency waveform to the motor; and a controller configured to control the variable frequency waveform, the controller being further configured to direct a positive DC voltage to the motor to shift the resting setpoint to increase a cooling capacity of the linear compressor.

2. The appliance of claim 1, wherein the positive DC voltage is a constant voltage applied across a plurality of sinusoidal cycles of the variable frequency waveform.

3. The appliance of claim 1, wherein the controller is further configured to apply an amplitude skew increasing half-cycle amplitude in the positive axial direction for a plurality of sinusoidal cycles of the linear compressor.

4. The appliance of claim 1, wherein the controller is further configured to apply a phase skew increasing half-cycle wavelength in the positive axial direction for a plurality of sinusoidal cycles of the linear compressor.

5. The appliance of claim 1, wherein the controller is further configured to: determine that additional cooling capacity is required at the refrigerator; and increase the positive DC voltage in response to determining that additional cooling capacity is required.

6. The appliance of claim 5, wherein increasing the positive DC voltage comprises increasing the positive DC voltage by a predetermined voltage value.

7. The appliance of claim 6, wherein the predetermined voltage value is chosen from a predetermined range of voltage values associated with increased cooling capacities.

8. The appliance of claim 1, further comprising a temperature sensor configured to signal that a pull-down event is required at the refrigerator.

9. The appliance of claim 1, further comprising a temperature selection control configured to select additional cooling capacity from the linear compressor.

10. The appliance of claim 1, wherein the controller is further configured to decrease the positive DC voltage to zero volts to reset the resting setpoint to return to a nominal cooling capacity of the linear compressor

11. A method for operating a linear compressor of a refrigerator, the linear compressor comprising a motor and a reciprocating piston movable in a negative axial direction toward a chamber and positive axial direction away from the chamber, the method comprising: supplying a variable frequency waveform to the motor of the linear compressor to produce a reciprocal motion in the piston at a first cooling capacity; determining that an increase in cooling capacity is required; and directing a positive direct current (DC) voltage to the motor to induce an extension force at the motor in the positive axial direction during at least a portion of the supplying step in response to the determining step.

12. The method of claim 11, wherein the positive DC voltage is a constant voltage applied across a plurality of sinusoidal cycles of the linear compressor.

13. The method of claim 11, further comprising applying an amplitude skew increasing half-cycle amplitude in the positive axial direction for a plurality of sinusoidal cycles of the linear compressor.

14. The method of claim 11, further comprising applying a phase skew increasing half-cycle wavelength in the positive axial direction for a plurality of sinusoidal cycles of the linear compressor.

15. The method of claim 11, wherein the determining that the increase in cooling capacity is required comprises determining that a pull-down event is occurring in the refrigerator.

16. The method of claim 11, wherein the determining that the increase in cooling capacity is required comprises receiving a selection to pull-down a temperature of a storage area of the refrigerator.

17. The method of claim 11, further comprising: determining whether additional cooling capacity is required after applying the extension force; and increasing the positive DC voltage in response to determining that additional cooling capacity is required.

18. The method of claim 17, wherein increasing the positive DC voltage comprises increasing the positive DC voltage by a predetermined voltage value.

19. The method of claim 18, wherein the predetermined voltage value is chosen from a predetermined range of voltage values associated with increased cooling capacities.

20. The method of claim 11, wherein the extension force is sufficient to shift a resting setpoint of the linear compressor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures.

[0011] FIG. 1 is a front elevation view of a refrigerator appliance according to an example embodiment of the present disclosure.

[0012] FIG. 2 is schematic view of certain components of the refrigerator appliance of FIG. 1.

[0013] FIG. 3 provides a perspective view of a linear compressor according to an example embodiment of the present disclosure.

[0014] FIG. 4 provides a side section view of the linear compressor of FIG. 3.

[0015] FIG. 5 provides an exploded view of the linear compressor of FIG. 4.

[0016] FIG. 6 provides a plot of cooling capacity and associated efficiency for a conventional linear compressor compared to the linear compressor of FIG. 3.

[0017] FIG. 7 provides a method for operating a linear compressor according to an example embodiment of the present disclosure.

[0018] FIG. 8 provides a flow chart illustrating a method for operating a linear compressor according to an example embodiment of the present disclosure.

[0019] FIG. 9 provides a movement plot of a linear compressor model.

[0020] FIG. 10 provides a plot of a variable frequency waveform with associated DC voltage for setpoint control, according to an example embodiment of the present disclosure.

[0021] FIG. 11 provides a plot of a variable frequency waveform with an applied phase or amplitude skew for setpoint control, according to an example embodiment of the present disclosure.

DETAILED DESCRIPTION

[0022] Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0023] FIG. 1 depicts a refrigerator appliance 10 that incorporates a sealed refrigeration system 60 (FIG. 2). It should be appreciated that the term refrigerator appliance is used in a generic sense herein to encompass any manner of refrigeration appliance, such as a freezer, refrigerator, refrigerator/freezer combination, and any style or model of conventional refrigerator. In addition, it should be understood that the present subject matter is not limited to use in appliances. Thus, the present subject matter may be used for any other suitable purpose, such as vapor compression within air conditioning units or air compression within air compressors. As illustrated, the refrigerator appliance 10 includes one or more compartments 14 and 18 for chilling food or other items by manner of refrigeration as described herein.

[0024] FIG. 2 is a schematic view of certain components of refrigerator appliance 10, including a sealed refrigeration system 60 of refrigerator appliance 10. A machinery compartment 62 contains components for executing a known vapor compression cycle for cooling air. The components include a compressor 64, a condenser 66, an expansion device 68, and an evaporator 70 connected in series and charged with a refrigerant. As will be understood by those skilled in the art, refrigeration system 60 may include additional components, e.g., at least one additional evaporator, compressor, expansion device, and/or condenser. As an example, refrigeration system 60 may include two evaporators.

[0025] Within refrigeration system 60, refrigerant flows into compressor 64, which operates to increase the pressure of the refrigerant. This compression of the refrigerant raises its temperature, which is lowered by passing the refrigerant through condenser 66. Within condenser 66, heat exchange with ambient air takes place so as to cool the refrigerant. A fan 72 is used to pull air across condenser 66, as illustrated by arrows A.sub.C, so as to provide forced convection for a more rapid and efficient heat exchange between the refrigerant within condenser 66 and the ambient air. Thus, as will be understood by those skilled in the art, increasing air flow across condenser 66 can, e.g., increase the efficiency of condenser 66 by improving cooling of the refrigerant contained therein.

[0026] An expansion device (e.g., a valve, capillary tube, or other restriction device) 68 receives refrigerant from condenser 66. From expansion device 68, the refrigerant enters evaporator 70. Upon exiting expansion device 68 and entering evaporator 70, the refrigerant drops in pressure. Due to the pressure drop and/or phase change of the refrigerant, evaporator 70 is cool relative to compartments 14 and 18 of refrigerator appliance 10. As such, cooled air is produced and refrigerates compartments 14 and 18 of refrigerator appliance 10. Thus, evaporator 70 is a type of heat exchanger which transfers heat from air passing over evaporator 70 to refrigerant flowing through evaporator 70.

[0027] Collectively, the vapor compression cycle components in a refrigeration circuit, associated fans, and associated compartments are sometimes referred to as a sealed refrigeration system operable to force cold air through compartments 14, 18 (FIG. 1). The refrigeration system 60 depicted in FIG. 2 is provided by way of example only. Thus, it is within the scope of the present subject matter for other configurations of the refrigeration system to be used as well.

[0028] FIG. 3 provides a perspective view of a linear compressor 100 according to an example embodiment of the present disclosure. FIG. 4 provides a side section view of linear compressor 100. FIG. 5 provides an exploded side section view of linear compressor 100. As discussed in greater detail below, linear compressor 100 is operable to increase a pressure of fluid within a chamber 112 of linear compressor 100. Linear compressor 100 may be used to compress any suitable fluid, such as refrigerant, a working fluid, or air. In particular, linear compressor 100 may be used in a refrigerator appliance, such as refrigerator appliance 10 (FIG. 1) in which linear compressor 100 may be used as compressor 64 (FIG. 2). As illustrated, linear compressor 100 defines an axial direction A, a radial direction R, and a circumferential direction C. Linear compressor 100 may be enclosed within a hermetic or air-tight shell (not shown). The hermetic shell can, e.g., hinder or prevent refrigerant from leaking or escaping from refrigeration system 60.

[0029] Turning now to FIG. 4, linear compressor 100 includes a casing 110 that extends between a first end portion 102 and a second end portion 104, e.g., along the axial direction A. Casing 110 includes various static or non-moving structural components of linear compressor 100. In particular, casing 110 includes a cylinder assembly 111 that defines a chamber 112. Cylinder assembly 111 is positioned at or adjacent second end portion 104 of casing 110. Chamber 112 extends longitudinally along the axial direction A. Casing 110 also includes a motor mount mid-section 113 and an end cap 115 positioned opposite each other about a motor. A stator, e.g., including an outer back iron 150 and a driving coil 152, of the motor is mounted or secured to casing 110, e.g., such that the stator is sandwiched between motor mount mid-section 113 and end cap 115 of casing 110. Linear compressor 100 also includes valves (such as a discharge valve assembly 117 at an end of chamber 112) that permit refrigerant to enter and exit chamber 112 during operation of linear compressor 100.

[0030] A piston assembly 114 with a piston head 116 is slidably received within chamber 112 of cylinder assembly 111. In particular, piston assembly 114 is slidable along a first axis A1 within chamber 112. The first axis A1 may include a negative axial direction A() and a positive axial direction A(+), and may be substantially parallel to the axial direction A. Thus, piston assembly 114 may alternately slide or oscillate, e.g., the piston head 116, in the negative axial direction A() and the positive axial direction A(+). During sliding of piston head 116 within chamber 112, piston head 116 compresses refrigerant within chamber 112. As an example, from a top dead center position (i.e., top dead center point), piston head 116 can slide within chamber 112 towards a bottom dead center position (i.e., bottom dead center point) along the positive axial direction A(+), i.e., an expansion stroke of piston head 116. When piston head 116 reaches the bottom dead center position, piston head 116 changes directions and slides in chamber 112 along the negative axial direction A() back towards the top dead center position, i.e., a compression stroke of piston head 116. It should be understood that linear compressor 100 may include an additional piston head and/or additional chamber at an opposite end of linear compressor 100. Thus, linear compressor 100 may have multiple piston heads in alternative example embodiments.

[0031] Linear compressor 100 also includes an inner back iron assembly 130. Inner back iron assembly 130 is positioned in the stator of the motor. In particular, outer back iron 150 and/or driving coil 152 may extend about inner back iron assembly 130, e.g., along the circumferential direction C. Inner back iron assembly 130 extends between a first end portion 132 and a second end portion 134, e.g., along the axial direction A.

[0032] Inner back iron assembly 130 also has an outer surface 137. At least one driving magnet 140 is mounted to inner back iron assembly 130, e.g., at outer surface 137 of inner back iron assembly 130. Driving magnet 140 may face and/or be exposed to driving coil 152. In particular, driving magnet 140 may be spaced apart from driving coil 152, e.g., along the radial direction R by an air gap AG. Thus, the air gap AG may be defined between opposing surfaces of driving magnet 140 and driving coil 152. Driving magnet 140 may also be mounted or fixed to inner back iron assembly 130 such that an outer surface 142 of driving magnet 140 is substantially flush with outer surface 137 of inner back iron assembly 130. Thus, driving magnet 140 may be inset within inner back iron assembly 130. In such a manner, the magnetic field from driving coil 152 may have to pass through only a single air gap (e.g., air gap AG) between outer back iron 150 and inner back iron assembly 130 during operation of linear compressor 100.

[0033] As may be seen in FIG. 4, driving coil 152 extends about inner back iron assembly 130, e.g., along the circumferential direction C. Driving coil 152 is operable to move the inner back iron assembly 130 along a second axis A2 during operation of driving coil 152. The second axis A2 may be substantially parallel to the axial direction A and/or the first axis A1. As an example, driving coil 152 may receive a current from a current source (not shown) in order to generate a magnetic field that engages driving magnet 140 and urges piston assembly 114 to move along the axial direction A in order to compress refrigerant within chamber 112 as described above and will be understood by those skilled in the art. In particular, the magnetic field of driving coil 152 may engage driving magnet 140 in order to move inner back iron assembly 130 along the second axis A2 and piston head 116 along the first axis A1 during operation of driving coil 152. Thus, driving coil 152 may slide piston assembly 114 between the top dead center position and the bottom dead center position, e.g., by moving inner back iron assembly 130 along the second axis A2, during operation of driving coil 152.

[0034] A piston flex mount 160 is mounted to and extends through inner back iron assembly 130. A coupling 170 extends between piston flex mount 160 and piston assembly 114, e.g., along the axial direction A. Thus, coupling 170 connects inner back iron assembly 130 and piston assembly 114 such that motion of inner back iron assembly 130, e.g., along the axial direction A or the second axis A2, is transferred to piston assembly 114. Piston flex mount 160 defines an input passage 162 that permits refrigerant to flow therethrough.

[0035] Linear compressor 100 may include various components for permitting and/or regulating operation of linear compressor 100. In particular, linear compressor 100 includes a controller (not shown) that is configured for regulating operation of linear compressor 100. The controller is in, e.g., operative, communication with the motor, e.g., driving coil 152 of the motor. Thus, the controller may selectively activate driving coil 152, e.g., by supplying current to driving coil 152, in order to compress refrigerant with piston assembly 114 as described above.

[0036] The controller includes memory and one or more processing devices such as microprocessors, CPUs or the like, such as general or special purpose microprocessors operable to execute programming instructions or micro-control code associated with operation of linear compressor 100. The memory can represent random access memory such as DRAM, or read only memory such as ROM or FLASH. The processor executes programming instructions stored in the memory. The memory can be a separate component from the processor or can be included onboard within the processor. Alternatively, the controller may be constructed without using a microprocessor, e.g., using a combination of discrete analog and/or digital logic circuitry (such as switches, amplifiers, integrators, comparators, flip-flops, AND gates, field programmable gate arrays (FPGA), and the like) to perform control functionality instead of relying upon software.

[0037] Linear compressor 100 also includes a spring assembly 120. Spring assembly 120 is positioned in inner back iron assembly 130. In particular, inner back iron assembly 130 may extend about spring assembly 120, e.g., along the circumferential direction C. Spring assembly 120 also extends between first and second end portions 102 and 104 of casing 110, e.g., along the axial direction A. Spring assembly 120 assists with coupling inner back iron assembly 130 to casing 110, e.g., cylinder assembly 111 of casing 110. In particular, inner back iron assembly 130 is fixed to spring assembly 120 at a middle portion 119 of spring assembly 120.

[0038] During operation of driving coil 152, spring assembly 120 supports inner back iron assembly 130. In particular, inner back iron assembly 130 is suspended by spring assembly 120 within the stator or the motor of linear compressor 100 such that motion of inner back iron assembly 130 along the radial direction R is hindered or limited while motion along the second axis A2 is relatively unimpeded. Thus, spring assembly 120 may be substantially stiffer along the radial direction R than along the axial direction A. In such a manner, spring assembly 120 can assist with maintaining a uniformity of the air gap AG between driving magnet 140 and driving coil 152, e.g., along the radial direction R, during operation of the motor and movement of inner back iron assembly 130 on the second axis A2. Spring assembly 120 can also assist with hindering side pull forces of the motor from transmitting to piston assembly 114 and being reacted in cylinder assembly 111 as a friction loss.

[0039] FIG. 6 provides a plot of cooling capacity and associated efficiency for a conventional linear compressor compared to the linear compressor 100 of FIG. 3. As illustrated, a conventional linear compressor may operate along curve 604, in a substantially linear manner. The curve 604 depicts decreased cooling capacity with decreased stroke length or current amplitude. It follows that as current and stroke length are increased, cooling capacity also increases linearly.

[0040] In contrast, the linear compressor 100 may operate along curve 606. As shown, a basic linear curve portion 608 exists such that there is a conventionally linear relationship between increasing current and cooling capacity until approximately capacity 610. Upon reaching capacity 610, a direct current (DC) voltage can be injected which offsets a resting setpoint (e.g., L.sub.0, described more fully below) of the motor, and therefore offers increased cooling capacity with a decrease in efficiency. However, as illustrated, the overall efficiency of the linear compressor 100 is greater than that of a conventional linear compressor. For example, because the resting setpoint L.sub.0 of the linear compressor 100 is decreased as compared to a conventional compressor at rest, there is reduced friction when compressing gas, which results in less heating of gases. However, when the resting setpoint L.sub.0 of the linear compressor 100 is shifted due to injection of positive DC voltage, cooling capacity is increased while still retaining overall efficiency much higher than the conventional compressor cycle curve 604.

[0041] Turning now to FIG. 7, a method 700 is illustrated for operating a linear compressor according to an example embodiment of the present disclosure. Method 700 may be used to operate any suitable linear compressor, such as linear compressor 100 (FIG. 3). Moreover, it is understood that the entirety (or a portion) of the method 700 may be utilized as part of, or as an alternative to, any of the described methods herein. In particular, the method 700 may be utilized for selectively supplying or directing a DC voltage as a time varying voltage is supplied to the motor of linear compressor 100. As described above, the DC voltage may induce a positive extension force in the motor. Furthermore, the DC voltage may effectively shift a resting setpoint L.sub.0 of the motor (at least only during application of the DC voltage).

[0042] As an example, the mechanical dynamic model for linear compressor 100 may be

F.sub.m=i=M{umlaut over (x)}+C{dot over (x)}+K(xL.sub.0)F.sub.gas

[0043] where [0044] M is a moving mass of linear compressor 100; [0045] is a motor force constant; [0046] {umlaut over (x)} is an acceleration of the motor of linear compressor 100; [0047] C is a damping coefficient of linear compressor 100; [0048] {dot over (x)} is a velocity of the motor of linear compressor 100; [0049] K is a spring stiffness of linear compressor 100; [0050] x is a position of the moving mass of linear compressor 100; [0051] L.sub.0 is resting setpoint of linear compressor 100; and [0052] F.sub.gas is a gas force.

[0053] Accordingly, a different L.sub.0 can be obtained, at least temporarily, by changing DC voltage. Positive DC Voltage will increase the stroke length and further increase cooling capacity at a low clearance. Generally, the control objective of method 700 is to add V.sub.dc to increase L.sub.0 and stroke length when higher cooling capacity is needed, required, or selected. For example, a control signal, temperature sensor, temperature selection apparatus, or other suitable control signal may be used to signal that higher cooling capacity is needed.

[0054] With respect to FIG. 7, the DC voltage is indicated as a variable value at V.sub.dc. The time varying voltage is indicated at V.sub.ac. A resulting applied voltage function for the combined DC voltage (V.sub.dc) and time varying voltage (V.sub.ac) is indicated at V(t), which controls a duty cycle generator to the motor. An index value for the DC voltage is indicated at V.sub.dc. An index limit for the combined DC voltage (V.sub.dc) may be provided in some embodiments. For instance, a lower index limit, such as 0 (e.g., as shown at FIG. 7) may be provided. Additionally or alternatively, although not shown in FIG. 7, an upper index limit (e.g., between 2 Volts and 5 Volts) may be provided. An index rate (e.g., between 0.25 second and 1.5 seconds) is indicated at T.sub.EC, such that a delay in the combined DC voltage (V.sub.dc) is indicated at Z.sup.TEC.

[0055] A determination may be made whether an increase in cooling capacity is required by signaling a change in DC voltage. If increased cooling capacity is required, the DC voltage (V.sub.dc) is indexed higher (e.g., from a starting value of 0). In particular, the DC voltage (V.sub.dc) is increased by the index value (V.sub.dc). Moreover, the DC voltage (V.sub.dc) is combined as a positive value with the time varying voltage (V.sub.ac) to form the voltage function [V(t)]. Additionally, the DC voltage (V.sub.dc) may be repeatedly increased by the index value (V.sub.dc). Moreover, the repeated increases may occur at the index rate (T.sub.EC) until the DC voltage (V.sub.dc) exceeds the index limit (e.g., upper index limit) or until increased cooling capacity is no longer required. If increased cooling is no longer required, the DC voltage (V.sub.dc) is decreased by the index value (V.sub.dc) immediately, or as an indexed value, to zero volts DC.

[0056] FIG. 8 provides a flow chart illustrating a method 800 for operating a linear compressor 100 according to an example embodiment of the present disclosure. Generally, the method 800 is substantially similar to the method 700. For example, the method 800 includes supplying a variable frequency waveform to the motor of the linear compressor to produce a reciprocal motion in the piston at a first cooling capacity, at block 802. The waveform may be the voltage function [V(t)] of FIG. 7.

[0057] Generally, the first cooling capacity may be a base capacity related to a resting setpoint L.sub.0 of the linear compressor 100. Other first cooling capacities may be chosen, including those already having a small offset of the resting setpoint L.sub.0 due to DC voltage injection or other scenarios.

[0058] Any suitable time varying voltage waveform may be supplied to the motor of linear compressor 100 at step 802. For example, the time varying voltage may have at least two frequencies components at step 802. Thus, the time varying voltage may be

v.sub.(t)=v.sub.0[sin (2f.sub.1t)+sin(2f.sub.2t)]

[0059] where [0060] v.sub.a is a voltage across the motor of linear compressor 100; [0061] f.sub.1 is a first frequency; and [0062] f.sub.2 is a second frequency.

[0063] The first and second frequencies f.sub.1, f.sub.2 may be about the resonant frequency of linear compressor 100. In particular, the first and second frequencies f.sub.1, f.sub.2 may be just greater than and just less than the resonant frequency of linear compressor 100, respectively. For example, the first frequency f.sub.1 may be within five percent greater than the resonant frequency of linear compressor 100, and the second frequency f.sub.2 may be within five percent less than the resonant frequency of linear compressor 100. In alternative example embodiments, the time varying voltage may have a single frequency at step 802.

[0064] The method 800 further includes determining that an increase in cooling capacity is required, at block 804. The determining may include receiving a discreet input indicating that a user has requested an increase in cooling capacity. The determining may also include determining that a pull-down event has occurred (such as by leaving a refrigerator door open, initiating an ice maker, or other scenarios). The determining may also include receiving indication from a temperature sensor, a temperature control interface, of other temperature control apparatuses.

[0065] The method 800 further includes directing a positive direct current (DC) voltage to the motor to induce an extension force at the motor in the positive axial direction during at least a portion of the supplying step in response to the determining step, at block 806. This extension force positively shifts the resting setpoint L.sub.0, as described below.

[0066] An example of shifting the resting setpoint L.sub.0 is illustrated generally at FIG. 9. In particular, FIG. 9 illustrates an example movement plot of a linear compressor model, e.g., taken during step 806. As may be seen in FIG. 9, the movement or oscillation of piston assembly 114 may be plotted as a sinusoidal wave wherein x corresponds to piston assembly 114 position (i.e., relative to the chamber 112). Thus, the position at which x=0 is understood to correspond to the base portion of chamber 112 (e.g., a cylinder head). As shown, the sinusoidal wave is defined across one or more strokes of the piston assembly 114. Thus, the sinusoidal wave may be formed from one or more sinusoidal cycles defined by movement (e.g., of piston head 116) from a midpoint to a top dead center point, to a bottom dead center point, and back to the midpoint. The position 909 is the actual midpoint of the sinusoidal wave. In other words, 909 is the midpoint of stroke length (i.e., x.sub.SL) between bottom dead center (i.e., x.sub.BDC) and top dead center (i.e., x.sub.TDC). In a free-floating or ideal system, piston assembly 114 would naturally oscillate about its equilibrium point L.sub.0 (i.e., x.sub.mid=L.sub.0). However, positive DC voltage effectively moves L.sub.0 upward in the positive axial direction A(+). In other words, extension of the piston assembly 114 in the positive axial direction A(+) is greater than extension in the negative axial direction A(). Accordingly, while the physical setpoint of L.sub.0 remains unchanged, the midpoint of oscillation (909, effectively L.sub.0) is shifted through application of a positive DC voltage.

[0067] In certain example embodiments, the DC voltage of step 806 may be directed continuously or constantly after the determination is made at step 804. Thus, the positive DC voltage may be a constant voltage that is applied during both the positive axial movement and negative axial movement of the piston assembly 114. Moreover, the positive DC voltage may be applied across a plurality of sinusoidal cycles (i.e., strokes) of the piston assembly 114 as it travels between bottom dead center (x.sub.BDC) and top dead center (x.sub.mc). Notably, directing a constant DC voltage may preserve the existing harmonics for the sinusoidal motion within linear compressor 100.

[0068] FIG. 10 provides a plot of a variable frequency waveform with associated DC voltage for setpoint control, according to an example embodiment of the present disclosure. As shown, the waveform 1000 has a midpoint L0 which has been shifted upwards due to injection of positive DC voltage constantly over multiple cycles of the waveform 1000. It is apparent then, that the area under the sinusoidal curve represented at 1002 is greater than 1004, which allows for increased cooling capacity on-the-fly, without physically altering the structure of the linear compressor 100.

[0069] In additional or alternative example embodiments, the DC voltage of step 806 may be directed intermittently after the determination is made at step 804.

[0070] The intermittent DC voltage may be applied according to a set amplitude skew or phase skew. FIG. 11 provides a plot of a variable frequency waveform with an applied phase or amplitude skew for setpoint control, according to an example embodiment of the present disclosure.

[0071] In particular, the amplitude skew may increase the amplitude of sinusoidal motion for the linear compressor 100 in the positive axial direction A(+). The amplitude skew is applied across a plurality of sinusoidal cycles (i.e., strokes) of the piston assembly 114 as it travels between bottom dead center (x.sub.BDC) and top dead center (x.sub.TDC). Thus, the amplitude skew may increase half-cycle amplitude in the positive axial direction A(+), e.g., such that half-cycle amplitude in the positive axial direction A(+) 1102 (e.g., amplitude of movement above L.sub.0) is greater than half-cycle amplitude in the negative axial direction A() 1104 (e.g., amplitude of movement below L.sub.0).

[0072] As another example, the intermittent DC voltage may be applied according to a set phase skew. In particular, the phase skew may increase the wavelength of sinusoidal motion for the linear compressor 100 in the positive axial direction A(+). The phase skew is applied across a plurality of sinusoidal cycles (i.e., strokes) of the piston assembly 114 as it travels between bottom dead center (x.sub.BDC) and top dead center (x.sub.TDC). Thus, the phase skew may increase half-cycle wavelength in the positive axial direction A(+), e.g., such that half-cycle wavelength in the positive axial direction A(+) (e.g., wavelength or time of movement above L.sub.0) is greater than half-cycle wavelength in the negative axial direction A() (e.g., wavelength or time of movement below L.sub.0).

[0073] Thus, as described above, methods for controlling a resting setpoint and cooling capacity of an appliance have been provided. Technical effects and benefits of the above examples may include higher operational efficiency at low cooling capacity. This may result in better energy savings over the life of an appliance because high cooling capacity is generally used during only a small percentage of the operational life of an appliance. Additionally, due to a lower resting physical setpoint L.sub.0, there is lower friction at low cooling capacity (e.g., friction is reduced by about 30% because the stroke length is reduced). This may further increase the longevity of an appliance due to decreased failures related to mechanical wear at the linear compressor 100. Finally, lower recompression losses (e.g., gas at low capacity is not recompressed and therefore the overall cycle is cooler) and higher peak efficiency (e.g., cooler cylinder and suction gas vs high capacity) may also be realized.

[0074] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.