Piston stroke control circuit for an oscillating compressor

Abstract: A linear hermetic compressor is provided which may be employed in refrigerators or air conditioners. The compressor includes a linear motor (3) and a piston oscillation controller (18), controlling the control oscillations of the piston so as to bring an actual top dead center position of the piston into agreement with a reference top dead center position and altering the reference top dead center position based on a given parameter such as the ambient temperature or a required thermal load of, for example, a refrigerator.

Description: The present invention relates generally to an improved structure of a linear compressor which may be used in refrigerators or air conditioners. Japanese Patent First Publication No. 8-247025 discloses a linear compressor for use in a refrigerator which is designed to keep a top clearance between a cylinder head and the head of a piston when lying at the top dead center constant for avoiding an overstroke of the piston. The linear compressor, however, has the drawback in that a decrease in ambient temperature or thermal load of the refrigerator results in excess of a refrigerating capacity because of the constant top clearance. EP-A-0,774,580 discloses a compressor which is controlled so that the difference between the actual top dead centre position and a reference top dead centre position are reduced. The conventional linear compressor also encounters another drawback in that the efficiency of a linear motor which moves the piston linearly is poor because of an increased copper loss of a coil. It is therefore an aim of the present invention to avoid the disadvantages of the prior art. It is another aim of the present invention to provide an improved structure of a compressor which has a high efficiency. According to the present invention, there is provided a linear compressor comprising: a hermetic casing; a block having formed therein a cylinder within which is provided a piston which can oscillates to change the volume of a compression chamber; a linear motor for moving the piston within the cylinder in a first direction; biassing means connected at a first portion to said block and at a second portion to the piston to urge the piston in a second direction opposite the first direction in response to the movement of the piston through said linear motor so that the piston oscillates in the cylinder; a piston position sensor for detecting a position of said piston to provide a position signal indicative thereof; a top dead center position determining circuit for determining an actual top dead center position of said piston based on the position signal provided by said piston position sensor; an oscillation control circuit for controlling oscillation of the piston so that a difference between the actual top dead center position and a reference top dead center position of the piston is decreased; and a reference top dead center position changing circuit for changing the reference top dead center position. In the preferred mode of the invention, an ambient temperature sensor is further provided which measures an ambient temperature. The reference top dead center position changing circuit changes the reference top dead center position based on the ambient temperature measured by the ambient temperature sensor. An operating condition determining circuit is further provided which determines a compressor output request. The reference top dead center position changing circuit changes the reference top dead center position based on the compressor output request determined by the operating condition determining circuit. Optionally, according to the invention, the linear motor may comprise (a) a rotor; and (b) a stator including a coil and an annular yoke retaining therein the coil, the yoke being made of a plurality of arc-shaped blocks each of which has formed therein a groove in which a peripheral portion of the coil is fitted. Also optionally according to the invention, the linear motor may comprise (a) a rotor; and (b) a stator including an annular yoke, the yoke being formed with a plurality of blocks each constituting a circumferential portion of the yoke, each of the blocks including a central section and first and second outer sections disposed on both sides of the central section, the central section being made consisting of laminations made from a material having a higher permeability, the first and second outer sections each being made from a material having a permeability lower than that of the central section and an electric resistance greater than that of the central section. In the preferred mode of the invention, the laminations making up the central section of each of the blocks of the yoke are laid to overlap each other in a circumferential direction of the yoke. The central section of each of the blocks of the yoke is quadrangular prism-shaped, and each of the first and second outer sections is triangular prism-shaped. The central section and the first and second outer sections are joined to each other using resin. The first outer section of each of the blocks of the yoke has a protrusion formed on a surface facing in a circumferential direction of the yoke. The second outer section has formed in a surface facing the circumferential direction of the yoke a groove in which the protrusion is fitted for joining the blocks to each other to complete the voke. Also optionally according to the invention, the linear motor may comprise (a) a rotor; (b) a stator including an annular yoke, the yoke being formed with a plurality of blocks each constituting a circumferential portion of the yoke, each of the blocks being formed with laminations made from a material having a higher permeability; and (c) an annular retainer retaining the blocks on an inner surface thereof at regular intervals in a circumferential direction of the retainer, the retainer including a strip member and holders, the strip member being made from a material having a permeability lower than that of the yoke and an electric resistance greater than that of the yoke, each of the holders holding therein one of the blocks and being glued on the strip member. In the preferred mode of the invention, each of the holders is made of a C-shaped member having disposed therein one of the blocks. The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Fig. 1 is an illustration which shows a linear compressor according to the first embodiment of the invention; Fig. 2 is a -graph which shows the efficiency of a linear compressor and a reference top dead center position in terms of the refrigerating capacity; Fig. 3 is a partially cross sectional view which shows a linear motor may be used in linear compressors of the first embodiments; Fig. 4 is a top view which shows an outer yoke of the linear motor of Fig. 3; Fig. 5 is a longitudinal cross sectional view taken along the line VII-VII in Fig. 4; Fig. 6 is a perspective view which shows fabrication of a coil fitted in the outer yoke of Figs. 4 and 5; Fig. 7 is a perspective view which shows one of separate blocks making up the outer yoke of Figs. 4 and 5; Fig. 8 is a top view which shows a modification of the outer yoke in Fig. 3; Fig. 9 is a side view of Fig. 8; Fig 10 is a perspective view which shows one of the separate blocks of Figs. 8 and 9; Fig. 11 is a top view which shows a second modification of the outer yoke in Fig. 3; Fig. 12 is a perspective view which shows one of separate blocks in Fig. 11; and Fig. 13 is a perspective view which shows a block retainer in Fig. 11; Referring now to the drawings, particularly to Fig. 1, there is shown a linear compressor (also called a vibrating compressor) according to the present invention which. The following discussion will refer to the linear compressor as being used in a refrigerating system such as a refrigerator, but the present invention is not limited to the same. The linear compressor includes generally a hermetic casing 1 and a compressor mechanism 2. The compressor mechanism 2 includes a linear motor 3, a cylinder 4, a piston 5, a hollow block 6, a cylinder head 7, and a spring assembly 8 and is supported within the casing 1 through suspension springs (not shown). A lubricating oil 11 is settled on the bottom of the casing 1. The block 6 has formed therein the cylinder 4 within which the piston 5 reciprocates to suck, for example, a refrigerant from the inlet 7a into a compression chamber 9 and to discharge it from the outlet 7b to the refrigerating system. The motor 3 consists of an annular stator 3a and an armature 3b mounted on the piston 5. The stator 3a is mounted in a peripheral wall of the block 6. Instead of the motor 3, any other known moving mechanisms may be used which are capable of reciprocating the piston 5. The spring assembly 8 which consists of four spring discs 8a laid to overlap each other and outer and inner annular spacers 8b and 8c disposed between adjacent two of the spring discs 8a. Each of the spring discs 8a has formed therein, for example, arc-shaped slits to define spring arms and is connected at the center to the periphery of the piston 5 and at an outer edge to an inner wall of the hollow block 6. The linear compressor also includes a piston position sensor 14, an A/D converter 15, a top dead center determining circuit 16, a reference top dead center position determining circuit 23, an oscillation controller 18, and an ac power supply 17. The oscillation controller 18 includes a reciprocating motion controller 21 and a base driver 22. The reciprocating motion controller 21 includes a reference top dead center position memory 19, an amplifier 20, a comparator 25. The piston position sensor 14 is implemented by a differential transformer and consists of a cylindrical coil 14a and a core 14b made of a permanent magnet. The cylindrical coil 14a is connected to an inner end wall of the hollow block 6. The core 14b is connected to the bottom of the piston 5. The movement of the core 14b (i.e., the piston 5) causes the coil 14a to output an electric signal indicative of the position of the piston 5 in an analog form to the A/D converter 15. The A/D converter 15 converts the input signal into a digital signal and outputs it to the top dead center determining circuit 16. The top dead center determining circuit 16 monitors the position of the piston 5 based on the signal from the A/D converter 15 to output a top dead center signal to the reciprocating motion controller 21 when the piston 5 has reached the top dead center. The reciprocating motion controller 21 compares through the comparator 25 an actual top dead center position of the piston 5 indicated by the top dead center signal with a reference top dead center position stored in the reference top dead center position memory 19 to change the amplitude of a voltage signal outputted through the amplifier 20 to the base driver 22 according to the difference between the actual top dead center position and the reference top dead center position. The base driver 22 controls the supply of power (i.e., an output voltage) from the power supply 17 to the motor 3 based on the inputted voltage signal to adjust the movement of the piston 5 so that the difference between the actual top dead center position and the reference top dead center position is decreased to zero. The reference top dead center position determining circuit 23 is connected to a temperature sensor 70 and an operating condition sensor 75. The temperature sensor 70 measures an ambient temperature. The operating condition sensor 75 monitors a given operating condition such as a required thermal load (i.e., a refrigeration demand) of the refrigerating system as a function of the temperature within a refrigerating chamber of the refrigerating system (e.g., a refrigerator). The reference top dead center position determining circuit 23 alters the reference top dead center position stored in the memory 19 according to the ambient temperature measured by the temperature sensor 70 and the operating condition of the refrigerating system monitored by the operating condition sensor 75. In operation, when the power supply 17 is turned on, the rotor 3b of the motor 3 is excited and attracted in a horizontal direction, as viewed in the drawing, along with the piston 5, pressing the spring assembly 8. The spring assembly 8 produces a reactive force to move the piston 5 in the opposite direction, thereby causing the piston 5 to oscillate, increasing and decreasing the volume of the compression chamber 9 alternately. The oscillation controller 18 adjusts, as described above, the output voltage from the power supply 17 to the motor 3 so that the difference between the actual top dead center position of the piston 5 monitored by the piston position sensor 14 and the reference top dead center position stored in the memory 19 may be decreased to zero. The top clearance of the piston 5 (i.e., the gap between the head of the piston 5 at the top dead center and an inner wall of the cylinder head 7) is, thus, kept constant. When the ambient temperature and/or the required thermal load monitored by the temperature sensor 70 and the operating condition sensor 75 are decreased, it will cause a required refrigerating capacity of the linear compressor to be lowered. The reference top dead center position determining circuit 23 selects one of reference top dead center positions by look-up using a map based on the monitored ambient temperature and the thermal load of the refrigerating system and alters the reference top dead center position now stored in the memory 19 of the reciprocating motion controller 21 so as to increase the top clearance of the piston 5. An increase in top clearance of the piston 5 causes the refrigerating capacity of the linear compressor to be lowered, thereby avoiding the excess of the compression capacity of the linear compressor (i.e., the capacity of the refrigerating system). The linear compressor may also include a quick refrigeration manual switch (not shown) which is installed in an operation panel of the refrigerator. When the quick refrigeration manual switch is turned on by an operator, the reference top dead center position determining circuit 23 alters the reference top dead center position stored in the memory 19 so as to decrease the top clearance of the piston 5 for enhancing the refrigerating capacity regardless of the outputs from the temperature sensor 70 and the operating condition sensor 75. Fig. 2 shows results of tests performed by the inventors of this application and shows that a change in reference top dead center position to increase the top clearance of the piston 5 causes the refrigerating capacity of the linear compressor to be lowered, but the efficiency of the linear compressor hardly changes until a ratio of the volume of the top clearance of the piston 5 to the volume of the cylinder 4 reaches 10%. Specifically, within a range of the ratio of the volume of the top clearance of the piston 5 to the volume of the cylinder 4 of 10%, the refrigerating capacity may be lowered to approximately half according to the environmental and refrigerating conditions without degrading the efficiency of the linear compressor and the entire refrigerating system. Fig. 3 shows a linear motor 3 which may be employed in the above embodiment. The linear motor 3 consists essentially of a stator 3a and a armature 3b. The stator 3a includes an annular inner yoke 101, an annular outer yoke 104, and a coil 106. The inner yoke 101 and the outer yoke 104 are secured on the hollow block 6, as shown in Fig. 1. The outer yoke 104 has two magnetic poles 102 and 103. The rotor 3b includes a cylindrical permanent magnet 107 and has the piston 5 pass through the bottom thereof. The outer yoke 104 includes, as shown in Fig. 4, a plurality of separate blocks 104a, 104b, 104c, and 104d whose end surfaces 114a, 114b, 114c, and 114d are glued or welded to each other. Each of the blocks 104a to 104d has, as shown in Figs. 5 and 7, formed in an inner wall thereof a groove 113 in which the coil 106 is fitted. The outer yoke 104 may be formed with at least two separate blocks. In fabrication of the coil 106 and the outer yoke 104, the coil 106 is, as shown in Fig. 6, first made by tensioning and winding wire around a bobbin 150 outward in a radial direction at high density. Next, the bobbin 150 is drawn from the coil 106 and fitted into the grooves 113 of the blocks 104a to 104d of the outer yoke 104. Finally, the blocks 104a to 104d are glued or welded to each other to make up the outer yoke 104. The coil 106 may be impregnated with a resin liquid for avoiding deformation and protection thereof. In conventional linear motors, a stator is usually made by winding wire within a groove formed in an inner wall of a one-piece annular outer yoke inward in a radial direction. It is, thus, difficult to wind the wire under high tension. In contrast to this, the stator 3a of this embodiment is, as described above, made by fitting the coil 106, which is formed by winding the wire outward in the radial direction under high tension, in the grooves 113 of the blocks 104a to 104d and joining the blocks 104a to 104d to complete the outer yoke 104. The coil 106 of this embodiment, therefore, has a higher space factor and a greater number of turns per volume as compared with the conventional linear motors. This reduces a copper loss that is proportional to the square of current flowing through the coil, thereby resulting in greatly improved efficiency of the linear motor 3. Figs. 8, 9 and 10 show the first modification of the outer yoke 104 used in the linear motor 3. The outer yoke 104 of this embodiment includes twelve fan-shaped separate blocks 104a identical in size and shape with each other. Each of the separate blocks 104a consists of a central portion 80 and first and second outer portions 90 and 101. The central portion 80 is formed with quadrangular prism-shaped laminations 70 made from a material having a higher permeability. The first and second outer portions 90 and 101 are each formed with a triangular prism-shaped member made from, for example, resin having the permeability lower than that of the central portion 80 and the electric resistance greater than that of the central portion 80. Each of the blocks 104 is fabricated by sandwiching the central portion 80 between the first and second portions 90 and 101 and impregnating them with resin. Each of the first outer portions 90 has, as shown in Figs. 8 and 10, formed therein a circular recess or groove 210. Each of the second outer portions 101 has, as clearly shown in Fig. 10, formed therein a cylindrical protrusion 220. The assembly of the outer yoke 104 is accomplished by inserting the cylindrical protrusion 220 of each of the second outer portions 101 into the circular groove 210 of one of the first outer portion 90 from an axial direction thereof to join all the blocks 104a. Each of the blocks 104 may be surface-finished before the outer yoke 104 is assembled. Alternatively, the outer yoke 104 may be surface-finished after being assembled. The eddy current loss of a linear motor is usually proportional to the square of the thickness of a yoke, but decreased greatly in this embodiment by the formation of the central portion 80 of the outer yoke 104 with the laminations 70 having the higher permeability, which results in greatly improved efficiency of the linear motor 3. Each of the blocks 104, as described above, has disposed on both sides of the central portion 70 the first and second outer portions 90 and 101 each having the lower permeability and higher resistance, thereby minimizing the leakage of magnetic flux from the central portion 70 of each of the blocks 104 to adjacent one, thus resulting in a decrease in copper loss. The outer yoke 104 of this embodiment does not have the coil 106, but may have the same as needed. The inner yoke 101 shown in Fig. 3 may be fabricated in the same manner as described above. Figs. 11, 12 and 13 show the first modification of the outer yoke 104 used in the linear motor 3. The outer yoke 104 of this embodiment includes twelve separate blocks 215 and an annular block retainer 216. Each of the blocks 215 is, as clearly shown in Fig. 12, formed with quadrangular prism-shaped laminations 217 made from a material having a higher permeability. The block retainer 216, as clearly shown in Fig. 13 consists of a strip member 219 and twelve C-shaped holders 218. The strip member 219 is flexible and made of, for example, resin which has the permeability lower than that of the blocks 215 and the resistance greater than that of the blocks 215. This avoids the flow of magnetic flux through the strip member 219 between adjacent two of the blocks 215, thereby minimizing the copper loss. The C-shaped holders 218 are glued on an inner surface of the strip member 219 at regular intervals and hold therein the blocks 215, as shown in Fig. 11, in a circumferential direction with regular gaps 220. The formation of the gaps 220 between adjacent two of the blocks 215 increases a surface area of the outer yoke 104, thereby facilitating the cooling of the linear motor 3. The outer yoke 104 of this embodiment does not have the coil 106, but may have the same as needed. The inner yoke 101 shown in Fig. 3 may be fabricated in the same manner as described above. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate a better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modification to the shown embodiments which can be embodied without departing from the scope of the invention as set forth in the appended claims.

Claims: A linear compressor comprising: a hermetic casing (1) a block (2) having formed therein a cylinder (4) within which is provided a piston (5) which can oscillate to change the volume of a compression chamber; a linear motor (3) for moving the piston within the cylinder in a first direction; biassing means (8) connected at a first portion to said block and at a second portion to the piston to urge the piston in a second direction opposite the first direction in response to the movement of the piston through said linear motor so that the piston oscillates in the cylinder; a piston position sensor (14) for detecting a position of said piston to provide a position signal indicative thereof; a top dead center position determining circuit (16) for determining an actual top dead center position of said piston based on the position signal provided by said piston position sensor; an oscillation control circuit (18) for controlling oscillation of the piston so that a difference between the actual top dead center position and a reference top dead center position of the piston is decreased; and a reference top dead center position changing circuit (23) for changing the reference top dead center position. A linear compressor according to claim 1, further comprising an ambient temperature sensor (70) for measuring an ambient temperature, and wherein said reference top dead center position changing circuit (23) is adapted to change the reference top dead center position based on the ambient temperature measured by said ambient temperature sensor. A linear compressor according to claim 1 or 2, further comprising an operating condition determining circuit (75) for determining a compressor output request, and wherein said reference top dead center position changing circuit (23) is adapted to change the reference top dead center position based on the compressor output request determined by said operating condition determining circuit. A compressor according to claim 1, 2 or 3 wherein said linear motor comprises: an armature (3b); and a stator (3a) including a coil (106) and an annular yoke (101, 104) retaining therein the coil, the yoke being made of a plurality of arc-shaped blocks (104 c-d) each of which has formed therein a groove (113) in which a peripheral portion of the coil is fitted. A compressor according to claim 1, 2 or 3 wherein said linear motor comprises: an armature (3b); and a stator (3a) including an annular yoke (101, 104) the yoke being formed with a plurality of blocks (104a) each constituting a circumferential portion of the yoke, each of the blocks including a central section (80) and first and second outer sections (90, 101) disposed on both sides of the central section, the central section being made consisting of laminations (70) made from a material having a higher permeability, the first and second outer sections (90,101) each being made from a material having a permeability lower than that of the central section and an electric resistance greater than that of the central section. A compressor according to claim 5, wherein the laminations (70) making up the central section of each of the blocks of the yoke are laid to overlap each other in a circumferential direction of the yoke. A compressor according to claim 6, wherein the central sections (80) of each of the blocks of the yoke is quadrangular prism-shaped and each of the first and second outer sections (90,101) is triangular prism-shaped. A compressor according to claim 5, 6 or 7 wherein the central section (80) and the first and second outer sections (90, 101) are joined to each other using resin. A compressor according to claim 5, 6, 7 or 8, wherein the first outer section (101) of each of the blocks of the yoke has a protrusion (220) formed on a surface facing in a circumferential direction of the yoke, and the second outer section (90) has formed in a surface facing the circumferential direction of the yoke a groove (210) in which the protrusion of an adjacent block is fitted for the joining the blocks to each other to complete the yoke. A compressor according to claim 1, 2 or 3 wherein said linear motor comprises: an armature (3b); a stator (3a) including an annular yoke (104), the yoke being formed with a plurality of blocks (215) each constituting a circumferential portion of the yoke, each of the blocks being formed with laminations (217) made from a material having a higher permeability; and an annular retainer (216) retaining the blocks (215) on an inner surface thereof at regular intervals in a circumferential direction of the retainer, said retainer including a strip member (219) and holders (218), the strip member (219) being made from a material having a permeability lower than that of the yoke and an electric resistance greater than that of the yoke, each of the holders (218) holding therein one of the blocks and being glued on the strip member. A compressor according to claim 10, wherein each of the holders (218) is made of a C-shaped member having disposed therein one of the blocks.

Inventors: INAGAKI KOH, KATAYAMA MAKOTO, KOBAYASHI MASANORI, MORITA ICHIRO, INAGAKI, KOH, KATAYAMA, MAKOTO, KOBAYASHI, MASANORI, MORITA, ICHIRO
Applicants: MATSUSHITA REFRIGERATION, MATSUSHITA REFRIGERATION COMPANY
Application year: 1998
Main classification: F04B35
IPC codes: F04B49, H02K33, H02K7, F04B39, H02K1, F04B35, F04B17