Patent Publication Number: US-6713903-B2

Title: Motor system for positioning a load

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electric motor system that may be used to adjust the position of a load in relation to a plane, and it is applicable for positioning a load associated with a platform or stage of a positioning device such as an XY table. 
     BACKGROUND AND PRIOR ART 
     Various kinds of motors are used in industry to move loads and to control their positions. The linear electric motor, i.e. a motor that produces thrust in a straight line by electromagnetic interaction, is one such kind of motor. 
     For example, U.S. Pat. No. 5,808,381 for a “Linear Motor” discloses several configurations of linear motors using permanent magnets in conjunction with coils in air gaps disposed within an area having a magnetic flux. U.S. Pat. No. 5,087,844, also for a “Linear Motor”, discloses a moving coil linear motor with a pair of permanent magnet assemblies on both sides of a central yoke opposing each other via a gap therebetween. Multi-phase, interlaced coil assemblies are disposed in the air gaps such that the force produced in one permanent magnet assembly acts in the same direction as the other. The magnets are shaped in such a way that the variation of magnetic flux density in the direction of motion of the moving part more closely approximates the ideal sinusoidal form than in conventional linear motors. 
     In the field of semiconductor assembly machines, the past decade has seen a shift from rotary motors and ball screw-based translation stages to linear motor based ones. This is so especially in the area of high speed, high precision XY tables used to move a load in two separate axes on a plane. XY tables comprise separate moving tables or platforms, each capable of moving in a single axis. A combination of movement in both axes allows an object to be positioned in any position on a plane. For instance, XY tables can be used as positioning devices in semiconductor assembling equipment, machine tools and measurement equipment. 
     In the conventional design of a linear motor driven XY table, each of the two axes on a plane is actuated by a linear motor in closed loop servo control, while feedback is provided by digital encoders, one for each axis. A variety of designs of linear motor may be used with such design of XY table. 
     Moving coil motors without iron cores are by far the most commonly-used motors for XY tables in the backend semiconductor industry. Whether single phase or multi-phase, they entail the use of a cantilevered coil bracket. The cantilever contributes substantially to the moment of inertia of a moving mass (including the load) about the axes of motion and also about the axis perpendicular to the axis of motion. This is undesirable from the point of view of vibrations in the yaw and pitch modes. Hence one of the goals of the motor designer is to keep the coil assembly as compact as possible and thereby to minimize the cantilever length. Thus, in a multi-phase motor, distributed coil windings are preferred over concentrated windings. Another advantage of distributed coil windings is better dissipation of the heat generated. FIG. 6 c  is an example of concentrated coil windings while FIGS. 6 a  and  6   b  are examples of distributed coil windings. 
     An example of an XY table comprising one linear motor for each axis is disclosed in U.S. Pat. No. 6,244,192 for an “XY Table”. The improvement of this design over the prior art is mentioned as the use of linear motors instead of ball feed screws connected to driver motors when positioning a load along the X and Y axes. Amongst other things, ball feed screws are less accurate due to errors in screw connection. Further, high-speed acceleration and deceleration is not possible. 
     Nevertheless, one problem relating to control of high bandwidth servo controlled motors such as that used in the aforesaid patent is the effect of the offset current in the driver circuit which may vary with time due to changes in temperature and other aging effects in the circuit. This offset current manifests as an undesired motor force equal to the product of the motor force constant and the offset current. The force constant of a motor can be defined as the force produced per unit current. For a multi-phase motor, the force constant is normally based on the peak value of the sinusoidal current in each of its phases. A linear motor having a high force constant is sensitive even to small amounts of driver offset and spurious noise. 
     It is often difficult to meet the contradictory requirements of high precision and high speed while using just one motor, which has only one force constant. When the force constant of the motor is higher, the larger will be the undesirable force from the effects from interferences such as driver offset and spurious noise. Thus, it becomes difficult to control such a motor, especially in maintaining a steady stationary position at the end of its stroke. However, from the point of view of faster operation with a compact motor without need for a very high current from the driving circuit, the force constant of the motor needs to be adequately high. It follows that the disadvantage of controlling each axis with a single motor is that speed has to be traded off against precision. 
     It would be advantageous to be able to deliver a high force employing a high force constant motor to a table holding a load while moving it over a greater distance and also to deliver a lower force when positioning with more precision over a smaller distance is desired. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to seek to provide an improved motor system for positioning a load with greater speed and precision as compared to the prior art as discussed above. 
     According to a first aspect of the invention there is provided a motor system for positioning a load, comprising support means for supporting the load and a first motor and a second motor, the first motor and second motor cooperating to drive the load, whereby the first motor is adapted to provide higher thrust than the second motor and the second motor is adapted to deliver higher positioning accuracy than the first motor during a positioning sequence to position the load. 
     According to a second aspect of the invention there is provided a method for positioning a load, comprising the use of a first motor cooperating with a second motor to drive the load, wherein the first motor provides higher thrust to the load, and the second motor delivers higher positioning accuracy to the load. 
     Using the invention, it is possible to provide two linear motors working in tandem that may move a load on an XY table, to provide fast and accurate positioning of the load. The overall size of a cantilever of a moving coil motor may also be reduced in overall size, rendering the system compact and with a lower moment of inertia about the centre of gravity perpendicular to the axis of motion. Using the invention it may also be possible to facilitate the segregation and dispersion of heat loss over a wider area, thus leading to a lower temperature of operation. 
    
    
     It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of an XY table comprising an X stage and Y stage, which incorporates a motor system according to a preferred embodiment of the present invention; 
     FIG. 2 is an isometric view of the bottom of the XY table of FIG. 1; 
     FIG. 3 is a schematic side view of the XY table looking from direction D of FIG. 1; 
     FIG. 4 is a schematic side view of the XY table looking from direction E of FIG. 1; 
     FIGS. 5 a  and  5   b  are magnified side views of the second motor of FIG. 3; 
     FIGS. 6 a  and  6   b  illustrate designs of interlaced coil configurations of a moving coil motor and moving magnet motor respectively, that are usable with a motor system according to the invention; and 
     FIG. 6 c  illustrates a design of concentrated or non-interlaced coil windings of the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, FIG. 1 is an isometric view of an XY table  40  incorporating a motor system according to a preferred embodiment of the present invention. Magnet assemblies of the XY table  40  have been removed to illustrate the motor system more clearly. The XY table comprises generally a support means such as an X stage  7  and a Y stage  17 , the X stage  7  being movable along an X axis on X axis crossed roller guides  8 , and the Y stage  17  being movable along a Y axis along Y axis crossed roller guides  16 . The support means  7 ,  17  support a load (not shown) to be positioned on a plane. Positions of the X stage  7  are determinable by an X axis encoder  14  whereas positions of the Y stage  17  are determinable by a Y axis encoder  15 . Upper coils  5  are supported on a cantilevered bracket  6  on the X stage  7  which comprise part of a first motor  1  to provide a first, high force constant, electromotive force to the X stage  7 . The first motor  1  is thus meant to provide a high thrust to drive the X stage  7 . 
     FIG. 2 is an isometric view of the bottom of the XY table  40  of FIG.  1 . This view shows lower coils  11  comprising part of a second motor  2  which provides a second electromotive force to the X stage  7 . The lower coils  11  are mounted on a support or back iron  12 . A heat sink  13  is preferably added to the back iron  12  on the side away from the lower coils  11  primarily to facilitate the dissipation of heat generated by the second motor  2  of the XY table  40 . 
     FIG. 3 is a schematic side view of the XY table  40  looking from direction D of FIG.  1 . The X stage  7  is adapted to move in the X-axis, and the Y stage  17  is mounted onto the X stage  7  and is adapted to move in the Y axis on a plane. The Y stage  17  is connectable to an external apparatus requiring positioning in the XY-axis, such as a bond head for a wire bonder (not shown). 
     In relation to the X stage  7 , it is supported by and moveable along X-axis crossed roller guides  8 . The first motor  1  is preferably a moving coil motor providing a high force constant. The stator of the first motor  1  thus consists of an air gap  20 , formed between two rows of permanent magnets  3  affixed in a specific pattern on yokes  4 , made from material of high magnetic permeability. This results in a nominally sinusoidal variation of magnetic flux in the air gap  20  along the X axis of motion. Further, the upper coils  5  are disposed within the air gap  20  to allow translational movement. In the preferred embodiment, the configuration of the coils  5  is made of distributed or interlaced windings, as illustrated in FIG. 6 a . The X stage  7 ,is supported by and movable along the X-axis crossed roller guides  8  with respect to a fixed table base  18 . 
     A combination of an electric current flowing through the upper coils  5  and the magnetic flux results in movement of the cantilevered bracket  6  and X-stage  7  along the X-axis by electromagnetic interaction. The direction of travel of the X-stage in the X-axis may thus be changed by changing the direction of current flowing through the upper coils  5 . 
     The X stage  7  also relies for its motion on a second motor  2 . The second motor  2  is described in more detail with reference to FIGS. 5 a  and  5   b . The first motor  1  preferably has a larger force constant for higher acceleration of the load resulting in faster operation. The second motor  2  has a relatively lower force constant, which allows it to deliver higher positioning accuracy than the first motor  1  for the same amount of current provided to the motors  1 , 2 , to fine-tune the position of the load. 
     In this preferred embodiment, the motor system of the invention is incorporated into the X stage  7 , but it is also possible with suitable modification for the motor system to be incorporated into the Y stage  17 . The first and second motors  1 , 2  are coupled and cooperate to move the lower, heavier X stage  7  so that on the one hand, a large resultant force is derived from both the motors operating in tandem while on the other hand, precise maintenance of position is achieved solely through control of the second motor  2  at the end of a stroke. In order to achieve this, the two motors  1 , 2  are preferably driven and controlled independently of each other by separate drivers. In such an arrangement, although both motors contribute to the motive force while executing a specified movement, servo control of the X stage  7  at the end of the stroke would be effected only by the second motor  2  of lower force constant with higher precision. 
     Alternatively, in applications where the requirement of position control at the end of the motion stroke is not particularly stringent, the motors may be connected in series or in parallel, to be driven and controlled by a single driver and a single controller respectively. This would simplify driver and controller circuits while retaining the advantages of compactness and better dispersal of the heat generated. 
     FIG. 4 is a side view of the XY table  40  looking from-direction E of FIG.  1 . The Y stage  17  is illustrated with a single Y-axis motor  19 . A Y-axis cantilevered bracket  21  holds a series of Y-axis motor coils  22  and is attached to the Y stage  17 . The Y-axis motor coils  22  are disposed in an air gap formed between top and bottom permanent magnets  23 . The permanent magnets  23  create a magnetic flux in the air gap. The principle of motion of the Y stage  17  is similar to that of the first motor  1  for the X stage and will not be repeated. As previously mentioned, a second motor may optionally be included for the Y stage  17  although it is not preferred in the described embodiment. 
     In the described embodiment, the Y motor  19  for the Y stage  17 , which is lighter and hence easier to control, is of conventional design, preferably of moving coil type (see FIG. 6 a ) mounted on the Y stage  17  and supported on linear crossed roller guides  16 . These linear crossed roller guides  16  of the Y-stage are in turn mounted on the top side of the X-stage  7 , orthogonal to the orientation of the X-axis crossed roller guides  8  of the X stage  7 . The Y motor  19  receives feedback from the Y-axis digital encoder  15 . 
     Also illustrated in FIG. 4 is the X-axis encoder  14  for the X stage  7 . The design of the heat sink  13  is also shown in more detail. Both the first and second motors  1 , 2  receive feedback from the digital X-axis encoder  14 . 
     FIG. 5 a  is a magnified side view of the second motor  2  of FIG. 3, which is positioned below the X stage  7 . The stator of the second motor  2  includes a multiplicity of lower coils  11  preferably in the form of distributed or interlaced windings as illustrated in FIG. 6 b . The lower coils  11  are mounted on a magnetically permeable support or back iron  12 . At the base of the back iron  12 , a heat sink  13  is arranged to dissipate heat from the second motor  2 . In a design of the second motor  2 , permanent magnets  9  are affixed on a yoke  10 , made from material of high magnetic permeability. The yoke  10  is mounted on the underside of the X stage  7 . The preferred embodiment is of a moving magnet design, using slotless distributed coil windings  11 , because a slotless design does not give rise to cogging forces, which may complicate control. 
     FIG. 5 b  is a view of the second motor  2  looking from direction F of FIG. 5 a.    
     A positioning sequence to position the load will now be described. In order to actuate movement of the X stage  7 , current is delivered through the upper coils  5  and lower coils  11 , such that electromagnetic interaction forces are generated to move the X stage  7  in one direction by combination of the first motor  1  and the second motor  2 . The combination of forces results in accelerated movement of the X stage  7  to a desired position on the X-axis. As the X stage  7  reaches its destination, the first motor  1  is deactivated, for example, by cutting off the current-flow in the upper coils  5 . However, the second motor  2  continues to generate a force for fine-tuning the position of the X stage  7 . 
     The force takeoff elements of the first and second motors  1 , 2  are situated at different heights from the centre of gravity (C.G.) of the total moving mass of the X stage. They are so arranged that the product of the force (F 1 ) of the first motor  1  with its offset (a) from the C.G. is nominally equal to the product of the corresponding quantities (F 2 , b) for the second motor  2  (see FIG.  3 ). This can be expressed as: 
     
       
         
           F 
           1 
           ×a=F 
           2 
           ×b 
         
       
     
     This ensures that there is almost no residual moment on the X-axis crossed roller guides  8  during motion, leading to a higher operational life for the guides. 
     Since the force demands of the X stage  7  are split between the first and second motors  1 , 2 , each motor is smaller in size than in the case where only one motor is provided for the X stage  7 . In particular, this translates to a smaller and compact coil assembly  5 , which results in a shorter cantilevered bracket  6 , thus leading to a higher dynamic stiffness of the coil bracket  6  and also a lower moment of inertia of the XY table  40  as a whole. Other variations in design, employing a similar tandem motor arrangement can also be conceived by those ordinarily skilled in the art. 
     The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.