Power plants – Motor operated by expansion and/or contraction of a unit of... – Unit of mass is a gas which is heated or cooled in one of a...
Reexamination Certificate
2002-05-30
2004-02-24
Nguyen, Hoang (Department: 3748)
Power plants
Motor operated by expansion and/or contraction of a unit of...
Unit of mass is a gas which is heated or cooled in one of a...
C060S524000
Reexamination Certificate
active
06694730
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention relates generally to cryocoolers, and more particularly to Stirling cycle cryocoolers.
BACKGROUND OF THE INVENTION
Recently, substantial attention has been directed to the field of superconductors and to systems and methods for using such products. Substantial attention also has been directed to systems and methods for providing a cold environment (e.g., 77 K or lower) within which superconductor products such as superconducting filter systems may function.
One device that has been widely used to produce a cold environment within which superconductor devices may function is the Stirling cycle refrigeration unit or Stirling cycle cryocooler. Such devices typically comprise a displacer unit and a compressor unit, wherein the two units are in fluid communication and are driven by one or more linear or rotary motors. Conventional displacer units generally have a “cold” end and a “hot” end, the warm end being in fluid communication with the compressor unit. Displacer units generally include a displacer having a regenerator mounted therein for displacing a fluid, such as helium, from one end, i.e., the cold end of the displacer unit, to the other end, i.e., the warm end, of the displacer unit. A piston assembly of the motor functions to apply additional pressure to the fluid when the fluid is located substantially within the warm end of the displacer unit, and to relieve pressure from the fluid when the fluid is located substantially within the cold end of the displacer unit. In typical cryocoolers, the piston and displacer units oscillate at 60 Hz. In this fashion, the cold end of the displacer unit may be maintained, for example, at 77 K, while the warm end of the displacer unit is maintained, for example, at 15 degrees above ambient temperature. Devices such as superconducting filters are then typically placed in thermal contact with the cold end of the displacer unit via a heat acceptor. Heat is transferred from the device to the heat acceptor. The heat transferred to the heat acceptor then passes to the helium gas contained in the displacer unit.
A typical motor used in a cryocooler comprises a piston assembly on which there is mounted a magnet ring assembly that transforms an oscillating magnetic energy field generated by motor coils to reciprocating mechanical energy that is applied to the piston assembly. For example,
FIGS. 1 and 2
illustrate a prior art piston/magnet assembly
10
, which includes a piston assembly
12
and a magnet ring assembly
14
mounted thereon. Referring specifically to
FIGS. 3-6
, the magnet ring assembly
14
includes eight magnets
16
that are cylindrically arranged to provide a radial magnetic field. To affix the magnets
16
in place, the magnet ring assembly
14
comprises an upper magnet holder
18
, which includes an annular recess
20
that captures the tops
22
of the magnets
16
, and a lower magnet holder
24
, which includes an annular recess
26
that captures the bottoms
28
of the magnets
16
. Preferably, the walls that straddle the annular recesses
20
and
26
are as thin as possible (e.g., 0.0050 inch), so that the thickness of the magnets
16
can be maximized. For purposes of structural integrity, the magnets
16
are held in place by bonding the tops
22
and bottoms
30
of the magnets
16
within the respective annular recesses
20
and
26
. The magnet ring assembly
14
further comprises eight ring rods
32
, which are located between the respective eight magnets
16
and TIG welded through corresponding holes
34
within the upper and lower magnet holders
18
and
24
to maintain the structural integrity of the magnet ring assembly
14
.
Referring back to
FIGS. 1 and 2
, the piston assembly
12
comprises a cylinder
36
having a bore
38
, a cylindrical piston
40
that axially moves within the bore
38
of the cylinder
36
, a piston end cap
42
disposed mounted in the end of the piston
40
, and a piston bracket
44
disposed on the opposite end of the piston
40
. As best shown in
FIGS. 1 and 4
, the upper magnet holder
18
of the magnet ring assembly
14
comprises eight radially circumferentially disposed mounting apertures
46
, and the piston bracket
44
comprises eight corresponding circumferentially disposed mounting apertures
48
, which are used to firmly bolt the magnet ring assembly
14
to the piston assembly
12
, as illustrated in FIG.
1
. So that the top surface of the upper magnet holder
18
is flush with the mounting surface of the piston bracket
44
, the piston bracket
44
further includes eight radially disposed apertures
50
between the mounting apertures
48
to accommodate the ends of the ring rods
32
(shown best in
FIG. 3
) protruding through the upper magnet holder
18
.
Referring still to
FIG. 2
, the piston assembly
12
further comprises gas bearings
52
that receive gas, e.g., helium, from a sealed cavity
54
within the piston
40
. It should be noted that any suitable of gas bearings
52
can be used. In the illustrated embodiment, four circumferentially disposed pairs of gas bearings
52
(only two pairs shown) are used. A check valve
56
(best shown in
FIG. 1
) provides a unidirectional flow of gas from the front of the piston
40
, through the sealed cavity
54
and out through the gas bearings
52
. Preferably, the gas bearings
52
comprise orifices that are on the order of a one or two mils (e.g., 1.5 mils), so that only a small amount of gas escapes from the sealed cavity
54
though the gas bearings
52
, thereby preserving the pressure that has built up in the sealed cavity
54
until the next stroke of the piston
40
. Typically, only 2-5 percent of gas that is displaced by the piston
40
enters the sealed cavity
54
through the check valve
56
.
Because the smallest drill bit currently is around 2.9 mils with a maximum length of about 30 mils, the orifices of the gas bearings
52
cannot be drilled. Instead, each of the gas bearings
52
includes an aperture
58
in which there is disposed a gas bearing restrictor in the form of a screw
60
that can be turned to adjust the rate of gas that flows through the gas bearing
52
. That is, the length of the passage created by the threaded helix between the screw
60
and the aperture
58
can be decreased or increased by carefully rotating the screw
60
in and out of the aperture
58
until the correct flow rates are attained in all gas bearings
52
. Alternatively, sapphire/ruby or glass orifices (not shown) with very small diameters can be used as the gas bearing restrictor to provide a consistent gas flow at the designed rate without requiring adjustment. These orifices, however, can only be made so long, and as will be described in more detail below, have reliability problems. The piston assembly
12
further comprises centering ports
62
(shown in FIG.
1
), which provide a return gas circuit from region adjacent the back of the piston
40
to the region adjacent the front of the piston
40
.
Due to the tight tolerances (typically, about 5 mils) between the magnet ring assembly
14
and adjacent laminations (only internal lamination
28
shown) that are disposed on both the inside and outside surface of the magnet ring assembly
14
, the circularity of the magnet ring assembly
14
must be perfect or near-perfect, so that it does not rub against the adjacent laminations. For the same reason, the concentricity between the piston
40
and the magnet ring assembly
14
must be perfect or near-perfect. In addition, the magnets
16
must be in a perfect or near-perfect cylindrical equidistant arrangement, so that the generated magnetic field is radially uniform. In this manner, a uniform load will be provided to the gas bearings
52
, thereby maximizing the efficiency of the piston assembly
12
. Thus, it can be appreciated that great care must be taken when assembling the magnet ring assembly
14
, resulting in often tedious and time consuming process that is magnified by the relatively large number of parts (eighteen—eight magnets, eight ring rods, two ma
Kunimoto Wallace Y.
O'Baid Amr H.
Nguyen Hoang
O'Melveny & Myers LLP
Superconductor Technologies Inc.
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