Electrical generator or motor structure – Dynamoelectric – Linear
Reexamination Certificate
1999-11-22
2001-08-21
Enad, Elvin (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Linear
C414S935000
Reexamination Certificate
active
06278203
ABSTRACT:
FIELD OF THE INVENTION
This disclosure is directed to the cooling of linear motors. More particularly the disclosure relates to a structure for cooling the coil assembly of a linear motor and preventing heating of the surrounding environment.
BACKGROUND
Excessive heating of the coils of a linear motor causes an increase in the resistance of the coils, exacerbating the heat problem and reducing the performance of the motor. In addition, this heat is carried away to the outside air and often to the rest of the machine in which the motor is utilized. Heat changes the index of refraction of air and consequently reduces the accuracy of laser interferometers and other optical systems. In addition, the heat causes thermal expansion of machine components, resulting in inaccuracy of precision mechanical systems.
Most commercially available linear motors are not actively cooled. Typically the coils are potted in a moderately conductive epoxy and the motor is cooled through convection into the surrounding air. Trilogy Systems provides an option to their motor where cooling fluid is circulated through a metal mounting bracket of the coil assembly. Because this bracket is mounted only to the top of the motor, not all of the heat is carried away from the motor and a significant portion of it is still convected into the surrounding environment.
U.S. Pat. No. 4,749,921 issued to Anwar Chitayat describes a linear motor. Included is a concept for cooling the linear motor coils. FIG. 8 in this patent shows a system of hollow tubes that are potted with the coil assembly. Coolant can flow through these tubes providing cooling. In U.S. Pat. No. 4,625,132 also issued to Anwar Chitayat, a controlled flow of cooling gas is directed between the motor stator and the moving element with flexible seals on each arm of a U-shaped channel mount a wound stator. In another Chitayat U.S. Pat. No. 4,839,545 an armature of a linear motor is cooled by coolant flowing through a lower serpentine channel in thermal contact with laminations of the motor armature.
U.S. Pat. No. 4,906,878 discloses a linear motor with cross-flow passageways or tubes connecting between inlet and outlet manifolds to remove heat from the motor coils. U.S. Pat. No. 4,916,340 utilizes heat insulating materials with a cooling medium (water) flowing through passageways on coil supporting members. U.S. Pat. No. 5,073,734 discloses a coolant for cooling between linear motor spacers and a screen support having cooling fins.
Yaskawa Japan Laid Open Application Heisei 8-168229 provides a linear motor that is enclosed in a stainless steel can (housing). This can has a small gap along the outside of the coils, which enables (not disclosed) coolant to be forced along the gap between the can and the coils to provide cooling of the motor. Yaskawa Utility Model Application Heisei 5-45102 includes a coil bobbin with a cooling path inside the bobbin.
Typical linear motors that are not cooled have inefficient motor operation due to increased coil resistance with temperature, heating of surrounding air, and heating of surrounding machine elements as discussed above. Motors that are only cooled through the mounting bracket do not provide direct cooling of the coils and suffer from the same disadvantages. The cooled motor of U.S. Pat. No. 4,749,921 and others of the above patents require cooling passages to be created within the coil assembly. This is difficult and can typically only be done by wrapping the coils with tubing and encapsulating the assembly in an epoxy. It also does not completely isolate the motor from the outside air because the cooling tubes do not completely enclose the coils. The Yaskawa disclosures include cooling arrangements which cannot be completely adapted to all motor configurations. In addition, both rely on an exterior thermal insulation or an exterior can (13 and 29, respectively) that may be difficult to fabricate. In both Yaskawa disclosures the cooling is on the inside of the bobbin or inside the can; coil heat may be transferred directly from the coil outer surfaces to the outside environment resulting in detriment to the machine in which the motor is being utilized.
SUMMARY
This disclosure is directed to novel cooling structures for linear motors. In accordance with some embodiments, no extra cooling tubes or components are needed within the coil assembly itself and cooling is accomplished by flowing coolant in a passage or a space between the surfaces of the coils and the coil enclosures. This prevents heat from the coils from reaching, for example, nearby interferometer or other optical systems, where the heat can change the index of refraction of air and reduce the accuracy of such systems or cause thermal expansion of machine components with resultant inaccuracies of the precision mechanical systems.
Typically in a lithographic (e.g. stepper) machine used in the processing of semiconductors wafers and the like, as many as eight linear motors are used to drive positioning elements (such as the reticle stage and wafer stage) of the stepper. This multiplicity of motors obviously compounds the problem of detrimental heat from the individual linear motors. In most applications, motors are cooled to prevent the motor from overheating and the coolant transfers the motor heat to the environment.
In the case of lithographic machines, the problem as recognized by the present inventors is not motor overheating but preventing the motor heat from reaching the environment and thereby adversely affecting the machine's interferometry systems. Hence here the motor heat is confined to the coil and coolant so that it is not transferred to the motor coil housing. Thus direct thermal contact between the motor coils and their housing is minimized.
One embodiment solves these problems in a band coil arrangement by providing integrally cast recesses forming cooling channels in a cast encapsulant block partially surrounding the coil assembly, along with closure members affixed over the cooling channels. An overall linear armature of a required substantial length with a minimal transverse thickness results. The structure allows coolant flow parallel to the length of the coil assembly along the height and length of the exterior surfaces of the coil assembly, the flow being between the coil assembly and the conforming coil enclosure, thus preventing heat from the coils escaping into the surrounding air.
While in one embodiment the coil assembly is partially encapsulated in e.g. a cast rigid epoxy which has a relatively low thermal conductance and a minimal thickness to provide a short heat path to the flowing coolant, preferably no such epoxy is present between most of the actual coil surfaces and the coolant. In order to prevent short-circuiting of the flowing coolant in the cast recesses, in one embodiment an integrally cast longitudinal spacer is provided extending over part of the length of the recesses in the cast encapsulant forming the coolant channels. The spacers terminate short of the ends of the recesses so that a cast divided annular channel is formed on each of opposite surfaces of the encapsulated coil assembly. Sealing of the channel is provided by a sheet metal or plastic closure member which with the ends of the assembly form an enclosure around the assembly of coils. The member may be adhered to or otherwise connected to the encapsulation block surrounding the respective recesses. The insulation block also contains a coolant inlet and plenum with coolant bores directing coolant to one end of the sealed recesses and a coolant outlet and plenum with coolant bores directing pumped coolant from the other end of the recesses to the coolant outlet.
In another embodiment called a centerpole cooling arrangement, physical insulating spacers are placed between each coil and extending between the outside enclosure (can or housing), and an inside enclosure, with a gap therebetween. Cooling fluid flows along the outside of the coils and through a gap between the coils and the inner enclosure and between the coils and inner surface of the
Hazelton Andrew J.
Novak W. Thomas
Wasson Ken G.
Enad Elvin
Jones Judson H.
Klivans Norman R.
Nikon Corporation
Skjerven Morrill & MacPherson LLP
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