Patent Publication Number: US-6713902-B2

Title: Closed-path linear motor

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 09/780,848 filed Feb. 9, 2001 now U.S. Pat. No. 6,455,957 and entitled “ENCODER”, which is a continuation of U.S. patent application Ser. No 09,415,166 entitled “CLOSED-PATH LINEAR MOTOR”, which is a continuation of U.S. patent application Ser. No. 09/069,324 entitled “CLOSED-PATH LINEAR MOTOR” filed Apr. 29, 1998, now U.S. Pat. No. 5,994,798, which is a continuation-in-part of U.S. patent application Ser. No. 09/031,009 entitled “LINEAR MOTOR HAVING AUTOMATIC ARMATURE WINDING SWITCHING AT MINIMUM CURRENT POINTS” filed Feb. 26, 1998, now U.S. Pat. No. 5,942,817; U.S. patent application Ser. No. 09/031,287 entitled “ENCODER” filed Feb. 26, 1998, now U.S. Pat. No. 5,907,200; U.S. patent application Ser. No. 09/040,132 entitled “MODULAR WIRELESS LINEAR MOTOR” filed Mar. 17, 1998, now U.S. Pat. No. 5,925,943; and U.S. patent application Ser. No. 09/055,573 entitled “WIRELESS PERMANENT MAGNET LINEAR MOTOR WITH MAGNETICALLY CONTROLLED ARMATURE SWITCHING AND MAGNETIC ENCODER” filed Apr. 6, 1998, now U.S. Pat. No. 5,936,319. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a linear motor and, more particularly, to a system and method for driving a plurality of stages along a path. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a linear motor and, more particularly, to a linear motor which is capable of following any path, including a closed path, where continuous or discontinuous motion in one direction is enabled. 
     Linear motors having stationary armatures containing coils and movable stages containing magnets are well known in the art. Also known are linear motors having stationary magnets and moving coils. 
     One type of such linear motors is disclosed in U.S. Pat. No. 4,749,921. The linear motor of the referenced disclosure has a series of armature windings mounted to a base plate, and a stage having a series of magnets that is free to move on the base plate. The stage is urged in the desired direction by applying AC or DC excitation to the coils. When such a linear motor is used in a positioning system, the relationship between the location of the stage and locations of the coils must be accounted for. 
     In one linear motor, commutator contacts are pendant from the stage. The contacts contact one or more power rails, and one or more coil contacts. As the stage moves along the armature, the location of the stage, relative to the armature is automatically accounted for by applying power to the stationary armature windings through the commutator contacts. 
     In other linear motors, it is conventional to employ a service loop of wires between the moving stage and the stationary elements. The location of the stage is updated using a magnetic or optical position encoder on the stage which senses markings on an encoder tape stationary alongside the path of the stage. The location is connected on the service loop to a stationary motor controller. 
     Generally, the important location information is the phase of the stage relative to the phase of the armature. For example, in a three-phase armature, the windings are disposed in repeating sets of three for phases A, B and C. If the center of the A phase winding is arbitrarily defined as 0 degrees, then the centers of the B and C windings are defined as 120 and 240. There may be two, three or more sets of windings as required for the travel distance of the stage. Normally, all A phase windings are connected in parallel. The same is true of all B and C phase windings. Thus, when the location of the stage requires a certain voltage configuration on the particular windings within the influence of the magnets on the stage, besides powering these windings, all of the other windings in the armature are also powered. The maximum force obtainable from a linear motor is limited by the allowable temperature rise in the armature windings. When all windings are powered, whether they contribute to motor force or not, more armature heating occurs than is strictly necessary for performing the motor functions. 
     Some linear motors in the prior art have responded to this heating problem using switches that are closed only to the armature windings actually within the influence of the magnets. 
     The need for a cable loop connecting moving and stationary elements is inconvenient, and limits the flexibility with which a system can be designed. The wiring harness requires additional clearance from the linear motor to prevent entanglement between the motor and any equipment or items that may be adjacent to the linear motor path. In addition, the wiring harness adds additional weight to the moving element of the linear motor. Furthermore, manufacturing of a linear motor employing a wiring harness incurs additional cost of material and assembly labor. Therefore, it would be desirable to eliminate the use of a wiring harness in a linear motor to decrease the cost of assembly, decrease the overall weight of the moving element, and to eliminate the clearance restrictions on the linear motors utility. 
     Most linear motors are manufactured to follow a straight path and to be of a predetermined fixed length. This establishes the length of the armature, and consequently the number of armature windings. In such linear motors, all armature windings lie parallel to each other, with axes thereof generally 90 degrees to the travel direction of the linear motor. In order to make a new linear motor of any specific length, a new assembly must be tooled. Each assembly has a set number of armature windings, a set number of moveable magnets, and, a fixed length wiring harness associated with the moveable element of the linear motor. The cost of producing a linear motor is increased because each assembly must be custom designed to a users needs, with new tooling required for each such design. Therefore, it is particularly desirable to produce a linear motor of a modular design. 
     A modular designed motor would allow easy customization for any desired length armature winding assembly. The cost of manufacturing a particular linear motor would be decreased since the cost of tooling would be minimal. A data base of assembly and outline drawings will be common to all assemblies within a family of linear motors, easing assembly and manufacturing. A stocking of common parts would allow quick assembly of any special length motor assembly, from now readily available parts. The stocking of common parts also decreases overall cost of manufacturing since materials will be bought in bulk from common suppliers. The assembly of any desired length armature winding assembly will enjoy a decreased lead time. As such, a modular designed linear motor provides for a decrease in manufacturing cost, decrease in lead time to assemble, and increases overall utility. 
     Linear motors using a series of stationary armature windings and moving magnets require a means to dissipate heat from the coils. Linear motors having cold plates mounted on one edge of an armature winding are known in the art. Alternatively, armature windings having cooling coils or channels are also well known in the art. Examples of such armatures are disclosed in U.S. Pat. No. 4,839,545. These armatures use stacked laminated magnetic material. 
     Linear motors having non-magnetic armatures are also known, an example of which is disclosed in U.S. Pat. No. 4,749,921. The linear motor of the referenced disclosure has a non-magnetic armature which includes a coil support structure composed of an aluminum frame or a serpentine cooling coil. In the embodiment having an aluminum frame, heat is carried away from the coils of the armature via the aluminum frame and a side plate which functions as a heat sink. Alternatively, a serpentine coil may be employed to effect more uniform cooling within the armature. The serpentine coils support the overlapping coils while the coils and the armature are cast in a block of settable resin. However, the incorporation of such a coil has the disadvantage of increasing costs because of the complexity of assembly and material expenses. Furthermore, while the use of the settable resin prevents the occurrence of eddy currents, the thermal conductivity of the settable resin is significantly less than that of metals which it replaces and thus reduces the power dissipation capacity of the linear motor. 
     Linear motors are increasingly being employed in manufacturing equipment. In such equipment, nominal increases in the speed of operation translate into significant savings in the cost of production. Therefore, it is particularly desirable to produce as much force and acceleration as possible in a given linear motor. An increase in force generated requires either an increase in magnetic field intensity or an increase in current applied to coils of the armature. In a permanent magnet linear motor, the available magnetic field intensity is limited by the field strength of available motor magnets. Power dissipated in the coils increases at a rate equal the square of the current. Attendant heat generation limits the force that may be achieved without exceeding the maximum armature temperature. Therefore, improvements in the power dissipation capacity of linear motors provide for increases in their utility. 
     SUMMARY 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     The present invention provides a system and method for a linear motor. The linear motor system includes a plurality of armature windings arranged to form a path, such as may be straight, curved, opened or closed. A plurality of stages are moveable along the path. The stages include motor magnets that provide a magnet field. An encoder sensor is connected for movement with the stages so as to sense relative movement between the stages and the path. The encoder sensor further is operative to provide an encoder signal having an electrical characteristic indicative of relative movement between the stages and the path. A subset of the plurality of stages has independent application of armature power from a motor controller and independent armature switching and independent position communication from the stages back to the motor controller. 
     To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a simplified schematic diagram linear motor system according to an embodiment of the invention. 
     FIG. 1B is a transverse cross section taken along II—II in FIG.  1 . 
     FIG. 2 is a cross section taken along A—A in FIG. 1B, showing the switching magnet and switching sensors which control application of drive power to armature windings. 
     FIG. 3 is a cross section taken along C—C in FIG. 1B, showing, the relationship between the switching magnet and motor magnets. 
     FIG. 3A is a cross section taken along C—C in FIG. 1B, showing, the positional relationship between the switching magnets and the motor magnets. 
     FIG. 3B is a cross section taken along C—C as in FIG. 3A, where the movable stage has moved to the right from its position in FIG.  3 A. 
     FIG. 4 is cross section taken along B—B in FIG. 1B showing the relationship between magnetic zones in the encoder magnet and the encoder sensors. 
     FIG. 4A shows a shape of a beveled magnetic zone about one of the encoder sensors from FIG.  4 . 
     FIG. 4B shows the relationship between the output of the encoder sensors located at the left and right ends of the encoder magnets in FIG. 4, and the beveled magnet zone in FIG.  4 A. 
     FIG. 4C shows another shape of a beveled magnetic zone about one of the encoder sensors from FIG.  4 . 
     FIG. 5 is a schematic diagram showing an embodiment of a wireless linear motor employing active communications elements on the movable stage. 
     FIG. 6 is a schematic diagram showing an embodiment a wireless linear motor employing an active command-response position feedback system. 
     FIG. 7 is a cross section similar to FIG. 1B, except that provision is made in the path for controlling a second movable stage along the same path. 
     FIG. 8 is a cross section similar to FIG. 1B, except that provision is made in the path for controlling any desired number of stages along the same path. 
     FIG. 9 is a cross section similar to FIG. 1B, except that provision is made in the path for controlling two or more stages along the same path. 
     FIG. 10 is a cross section similar to FIG. 1B, except that provision is made in the path for controlling three or more stages along the same path. 
     FIG. 11 is a schematic diagram of a wireless linear motor employing an active command-response system with memory on-board the movable stage. 
     FIG. 12 is a diagram showing a path adapted for open-loop control of a movable stage over one section and closed-loop control over another section. 
     FIG. 13 is a diagram showing several path modules connected together to form a path. 
     FIG. 14 is a diagram showing a preferred embodiment of a path module having three encoder sensor groups spaced along the path of the module. 
     FIG. 15 is a diagram showing an embodiment of two path modules coupled together, one module having a sensor, and another module without a sensor. 
     FIG. 16 is a diagram showing an alternative embodiment of a path module having a single sensor. 
     FIG. 17 is a diagram of a linear motor with a path in a racetrack shape. 
     FIG. 18 is an enlarged view of a portion of a curved section of the path of FIG.  17 . 
     FIG. 19 is a diagram of a linear motor having path with multiple levels and wherein one portion of the path crosses over or under another portion of the path. 
     FIG. 20 is a diagram of a linear motor path consisting of two connected spirals, including multiple crossovers. 
     FIG. 21 is a diagram of a linear motor path in the shape of a Moebius band. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1A, there is shown, generally at  10 , a linear motor according to the invention. A movable stage  12  is supported and guided in any convenient manner along a path  14 . Path  14  includes therein repeating sets of three armature windings  16 A,  16 B and  16 C for receiving, respectively, phases A, B and C of three-phase drive power produced by a motor controller  18 . Phase A of the drive power from motor controller  18  is connected on a phase-A conductor  20 A to terminals of normally-open phase-A switches  22 A. Each phase-A switch is connected to its associated phase-A armature winding  16 A. Similarly, phase-B and phase-C drive power are connected on phase-B and phase-C conductors  20 B and  20 C to terminals of phase-B and phase-C switches  22 B and  22 C, all respectively. Armature windings  16 A,  16 B and  16 C of each set are non-interleaved. That is, they lie side by side, not overlapping as is the case in some prior art linear motors. 
     All switches  22 A,  22 B and  22 C remain open, except the switches associated with the particular armature windings  16 A,  16 B and  16 C that are within the influence of motor magnets on movable stage  12 . The closed switches  22 A,  22 B and  22 C that are closed in this manner are indicated as  22 A′,  22 B′ and  22 C′, thereby apply power to corresponding armature windings  16 A′,  16 B′ and  16 C′. As moveable stage  12  moves along path  14 , those of switches  22 A,  22 B and  22 C which newly come under the influence of the magnets on movable stage  12  close, and those moving out of the influence of the magnets are opened. Thus, at any time, only the armature windings  16 A′,  16 B′ and  16 C′ which can contribute to generating a force on movable stage  12  are powered. The remainder of armature windings  16 A,  16 B and  16 C, not being useful for contributing to the generation of force, remain in a quiescent, unpowered, condition. This contributes to a reduction in power consumption, and a corresponding reduction in heating compared to prior-art devices in which all armature windings are powered, regardless of whether they are position to contribute to force. 
     In an application where “open-loop” drive of movable stage  12  is satisfactory, motor controller  18  produces the required sequence of phases to drive stage  12  in the desired direction. However, one desirable application is a “closed-loop” drive system in which motor controller  18  receives feedback information from movable stage  12  indicating either its position along path  14 , or increments of motion along path  14 . A closed-loop system permits accurate control of position, velocity and acceleration of movable stage  12 . 
     The prior art satisfies the requirement for position feedback using wiring between movable stage  12  and motor controller  18 . This is inconvenient in some applications, and impractical in others. Impractical applications including travel of movable stage  12  along a path  14  which is closed upon itself. An example of such a path is an oval or “race-track” pattern of value in a robotic assembly operation, to be described in greater detail later in this specification. That is, movable stage  12  continues in a forward direction repeatedly traveling in the same direction on path  14 . Wiring between the movable and stationary elements for such an application is either difficult or impossible to accomplish. 
     The embodiment of the invention in FIG. 1A includes a communications device  24  which wirelessly informs motor controller  18  about the position and/or incremental motion of movable stage  12 . Communications device  24  is preferably a linear encoder which does not require connecting cables between stationary and movable elements, as will be explained. 
     In the preferred embodiment, at least some of the position or motion information is developed at stationary locations off movable stage  12 , without requiring the transmission of position information. 
     It can be seen from the simplified drawing of Fig. IA, and the description above, that linear motor  10  requires the following actions: 
     1) control of switches  22 A,  22 B,  22 C 
     2) feedback of position or motion data 
     3) drive power generation related to position (or motion-derived position). 
     Referring to FIG. 1B, a cross section through path  14 , looking at the end of movable stage  12  reveals a plurality of motor magnets  160 ,  162  below a plate  26 . Lower surfaces of motor magnets  160 ,  162  are maintained closely parallel to an upper surface of armature windings  16 A,  16 B and  16 C. Although it does not form a part of the present invention, armature windings  16 A, B, C, may be wound on stacked laminations of magnetic metal. In this case, the lower surface of motor magnets  160 ,  162  are maintained closely parallel to an upper surface of the stacked laminations. Some applications may benefit from the reduction in static load on movable stage  12  provided when armature windings  16 A,  16 B and  16 C contain no magnetic material. For purposes of later description, motor magnets  160 ,  162  are referred to as motor magnets. Armature windings  16 A, B and C are energized as necessary to interact with motor magnets  160 ,  162  whereby a translational force is generated on movable stage  12 . 
     A pendant arm  28  extends downward from plate  26 . Pendant arm  28  has attached thereto a switching magnet  30  and an encoder magnet  32 , both movable with movable stage  12 . A rail  34 , affixed to path  14 , rises generally parallel to pendant arm  28 . Rail  34  has affixed thereto a plurality of longitudinally spaced-apart switching sensors  36  facing switching magnet  30 , and a plurality of longitudinally spaced-apart encoder sensors  38  facing encoder magnet  32 . 
     Referring now to FIG. 2, switching sensors  36  are evenly spaced along rail  34 . Each switching sensor  36  is preferably positioned on rail  34  aligned with its respective armature winding  16 . In the embodiment shown, switching sensors  36  are Hall-effect devices. Switching magnet  30  has a length in the direction of travel roughly equal to the length of travel influenced by the magnetic fields of motor magnets  160 ,  162 . This length is variable in dependence on the number of motor magnets used. In the illustrated embodiment, the length of switching magnet  30  is sufficient to influence nine switching sensors  36 . That is, nine armature windings  16  (three sets of phases A, B and C) are connected at any time to their respective power conductors  20  for magnetic interaction with motor magnets  160 , 162 . 
     Switching sensors  36  control the open and closed condition of respective switches, as previously explained. Any convenient type of switch may be used. In the preferred embodiment, the switches are conventional semiconductor switches such as thyristors. Since semiconductor switches, and the technique for controlling their open/closed condition are well known to those skilled in the art, a detailed description thereof is omitted. 
     Referring now to FIG. 3, the underside of plate  26  includes nine motor magnets  160  equally spaced therealong. In addition, an additional motor magnet  162  is disposed at each end of the array of nine motor magnets  160 . Motor magnets  160 ,  162  are tilted as shown in a conventional fashion to reduce cogging. It will be noted that the length of switching magnet  30  is approximately equal to the center-to-center spacing of the end ones of the set of nine full motor magnets  160 . This length of switching magnet  30  defines the span S of the active portion of linear motor  10 . That is, only those of armature windings  16  that lie within the span S receive power. As armature windings  16  enter the span S, they receive power, as they exit the span S, power is cut off. 
     Additional motor magnets  162 , being outside the span, do not contribute to the generation of force because armature windings  16  below them are unpowered. However, additional motor magnets  162  perform an important function. It is important to the function of linear motor  10  that the magnetic field strength along plate  26  be generally sinusoidal. In the absence of additional motor magnets  162 , the magnetic fields produced by the two motor magnets  160  at the ends of span S depart substantially from sinusoidal due to fringing effects. This produces ripple in the force output. The presence of additional motor magnets  162 , by maintaining substantially sinusoidal magnetic field variations along motor magnets  160 , avoids this source of ripple. 
     Additional motor magnets  162  are shown with widths that are less than that of motor magnets  160 . It has been found that a narrower width in additional motor magnets  162  produces satisfactory results. However, it has also been found that a wider additional motor magnet  162  does not interfere with the function. From the standpoint of manufacturing economy, it may be desirable to employ only a single size magnet for both motor magnets  160  and additional motor magnets  162 , thereby reducing stocking costs, and assembly costs. 
     Referring now to FIG. 3A, the positional relationships of switching magnet  30  and motor magnets  160 ,  162  are shown, using a reduced set of  5  motor magnets interacting with  4  armature windings, for purposes of explanation. As movable stage  12  moves, switching magnet and motor magnets  160 ,  162  move together with it, maintaining the same relative positions. As movable stage  12  moves along, those switching sensors  36  adjacent switching magnet  30  turn on their respective switches. Switching sensors  36  that are not adjacent switching magnet  30  maintain their respective switches turned off. In the condition shown, switching sensors  36  centered on armature windings  16 - 2 ,  16 - 3 , and  16 - 4  are adjacent switching magnet  30 , and these armature windings are connected to drive power. The switching sensors  36  centered on armature windings  16 - 1 .  16 - 5  and  16 - 6  are not adjacent switching magnet  30 , and therefore, these switching sensors  36  maintain armature windings  16 - 1 ,  16 - 5  and  16 - 6  cut off from drive power. The centers of all motor magnets  160  shown are offset from the centers of the armature windings  16  most closely adjacent. Therefore all turned-on armature windings  16  produce force by the interaction of their magnetic fields with the magnetic fields of the three nearest motor magnets  160 . 
     Referring now to FIG. 3B, movable stage  12  has moved to the right from its position in FIG. 3A until the center of the right-hand motor magnet  160  is centered over the center of armature winding  16 - 5 . In this relationship, the end of switching magnet  30  just reaches a position adjacent switching sensor  36 . This is a minimum-current position. Thus, at this instant, switching sensor  36  closes its switch to connect armature winding  16 - 5  to its power source. In this center-overlapped condition, armature winding  16 - 5  is incapable of generating a force. Thus, the current in armature winding  16 - 5  is at a minimum, and the switching takes place at minimum current to armature winding  16 - 5 . Similarly, at about this same instant, the left-hand end of switching magnet  30  passes off the switching sensor  36  aligned with armature winding  16 - 2 , thereby cutting off power to armature winding  16 - 2 . The center of left-hand motor magnet  160  is aligned with the center of armature winding  16 - 2  at this time. Thus, the current to armature winding  16 - 2  is minimum at this time. The above switching at minimum current reduces electrical switching noise which would be generated if switching were to take place at times when an energized armature winding  16  is generating force, or a deenergized armature winding  16  would generate a force immediately upon energization. 
     For a three-phase drive system, a minimum of five motor magnets is required to interact at any time with a minimum of four armature windings, or vice versa. If additional force is desired, magnets can be added in increments of four. That is, the number of magnets=5+4L where L is an integer, including zero. The number of armature windings in span S=(number of motor magnets in span S)−1. The embodiment in FIGS. 2 and 3 employ 5+(4×1)=9 magnets. The positioning of the magnets is such that the center-to-center spacing of the extreme ends of the 9 magnets is equal to the center-to-center spacing of  8  armature windings. 
     Referring now to FIG. 4, encoder magnet  32  includes alternating magnetic zones alternating with north and south polarities facing encoder sensors  38 . Accordingly, each encoder sensor  38  is exposed to alternating positive and negative magnetic fields as encoder magnet  32  passes it. The zones at the extreme ends of encoder magnet  32  are beveled magnetic zones  42 . Beveled magnetic zones  42  produce an increasing or decreasing magnetic field as it moves onto or off an encoder sensor  38 . Beveled magnetic zones  42  are illustrated as linear ramps. Motors using such linear ramps have been built and tested successfully. However, a shape other than a linear ramp may give improved results. It is known that the magnetic field of a motor magnet decreases as the square of the distance from the magnet. Thus, to have an increase in magnetic field at one beveled zone that is substantially equal to the decrease in the magnetic field at the opposite magnetic zone, the bevel shape may be described by a squared law. 
     Referring momentarily to FIG. 4A, a shape of beveled magnetic zone which satisfies the rule that, for equal increments of motion of beveled magnetic zone  42 ′, there are equal changes in magnetic field at encoder sensor  38  is represented by the equation: 
     
       
         
           y=a+bx 
           2  
         
       
     
     where: 
     y is the distance from the surface of the magnet to encoder sensor  38 , 
     x is the position along beveled magnetic zone  42 ′, and 
     a and b are constants. 
     Experience dictates that other factors besides the square law above affects the relationship between magnetic field and distance. The shape of beveled magnetic zones  42 ′ may require modification from the square law to account for such other factors. 
     Referring now to FIG. 4B, when the ideal shape of beveled magnetic zones  42 ′ is attained, the outputs of the encoder sensors at the left and right ends of encoder magnet  32  should approximate the figure. That is, the sum of the signal from the left beveled magnetic zone  42 ′, and the signal from the right beveled magnetic zone  42 ′ should remain about constant. 
     Returning now to FIG. 4, each encoder sensor  38  is preferably a Hall-effect device. A Hall-effect device produces a current when exposed to one magnetic polarity (north or south) but is insensitive to the opposite magnetic polarity. Encoder sensors  38  are disposed into encoder sensor groups  40  consisting of four encoder sensors  38  spaced in the direction of travel. Each encoder sensor group  40  is spaced from its neighboring encoder sensor group by a distance D. Distance D is seen to be equal to the center-to-center distance between the beveled magnetic zones  42  at the ends of encoder magnet  32 . The four encoder sensors  38  in each encoder sensor group  40  are spaced in the direction of travel of movable stage  12  in relation to the center-to-center distance between magnetic zones in encoder magnet  32 . For purposes of description, the center-to-center distance between magnetic zones of like polarity is considered to be 360°. Thus, the center-to-center distance between adjacent magnetic zones is considered to be 180°, and the distance between the center of a zone and its edge is considered to be 90°. 
     It is conventional for encoders to produce a sine and a cosine signal, relatively 90° out of phase, for use in detecting the direction of incremental motion of a stage. With magnetically actuated Hall-effect devices, this conventional technique presents a problem in that a Hall effect device responds only to one magnetic polarity (north or south) and is insensitive to the opposite polarity. To solve this problem, each encoder sensor group  40  includes one encoder sensor  38   s + for producing a sine+ output, and a second encoder sensor  38   s − for producing a sine− output. Encoder sensor  38   s − in encoder sensor group  40  is spaced 180° in the direction of travel from its companion encoder sensor  38   s +. When the sine+ and sine− signals are added in motor controller  18 , the desired sinusoidal sine signal is available. A cosine+ encoder sensor  38   c + is spaced  90 ° in the direction of travel from sine+ encoder sensor  38   s +. A cosine− encoder sensor  38   c − is spaced 180° in the direction of travel from its companion cosine+encoder sensor  38   c +. When the cosine+−and cosine− signals are added in motor controller  18 , the desired cosine signal is generated. 
     The spacing D between encoder sensor groups  40  is such that, as a particular encoder sensor  38  in one encoder sensor group  40  is aligned with beveled magnetic zone  42  at one end of encoder magnet  32 , its counterpart is aligned with beveled magnetic zone  42  at the opposite end of encoder magnet  32 . As illustrated, for example, when sine+ encoder sensor  38   s + in the left-hand encoder sensor group  40  is aligned with the center of the left-hand beveled magnetic zone  42 , its counterpart sine+ encoder sensor  38   s + is aligned with the right-hand beveled magnetic zone  42  at right end of encoder magnet  32 . 
     All corresponding encoder sensors  38  are connected in parallel to a line connected to motor controller  18 . Four separate lines are illustrated to carry the +/− sine/cosine signals. As movable stage  12  moves along, the encoder sensor  38  coming into alignment with beveled magnetic zone  42  at one end of encoder magnet  32  produces an increasing signal while the encoder sensor  38  moving out of alignment with beveled magnetic zone  42  at that end produces a decreasing signal. Since all corresponding encoder sensor signals are added, the signal transition, as one encoder sensor group  40  becomes active, and its neighbor encoder sensor group  40  becomes inactive is smooth, without a discontinuity that would interfere with detecting motion. One skilled in the art will understand that the above spacing can be increased by 360° between any +/− pair of encoder sensors  38  without affecting the resulting output signal. Also, in some applications, since the outputs of sine encoder sensors are, in theory, 180° out of phase with each other, both sine encoder outputs could be applied to a single conductor for connection to motor controller  18 . In other applications, four separate conductors, as illustrated, may be desired. 
     In a preferred embodiment of linear motor  10 , a third encoder sensor group  40  (not shown) is disposed midway between the illustrated encoder sensor groups  40 . This has the advantage that, during the transition of beveled magnetic zones  42  at the ends of encoder magnet  32  from one encoder sensor group  40  to the next encoder sensor group  40 , resulting departures of the encoder signal due to tolerances in the lengths of encoder magnet  32 , and the precise spacing of encoder sensor groups  40  is at least partially swamped out by the signal generated by an encoder sensor group  40  located midway between the ends of encoder magnet  32 . 
     Referring again to Fig. IA, it will be recognized that the functions of communications device  24  are satisfied by the above-described wireless magnetic system for communicating the motion of movable stage  12  to motor controller, without requiring any active devices on movable stage  12 . One limitation on such a system is the difficulty in producing closely spaced alternating magnetic zones in encoder magnet  32 . Thus, the positional resolution of such a system is relatively crude. 
     Referring now to FIG. 5, one solution to the resolution problem includes a conventional encoder tape  44  in a fixed location along path  14 , and a conventional optical encoder sensor  46  on movable stage  12 . Encoder tape  44  is ruled with fine parallel lines. Optical encoder sensor  46  focuses one or more spots of light on encoder tape  44 , and detects the changes in light reflected therefrom as lines and non-lines pass in front of it. Generally, optical encoder sensor  46  produces sine and cosine signals for determining motion. Since the parallel lines on encoder tape  44  are closely spaced, very fine resolution is possible. An optical encoder system can be added to the less precise magnetic encoder system in order to obtain enhanced position resolution. 
     The sine and cosine outputs of optical encoder sensor  46  are applied to a pulse generator  48 . The output of pulse generator  48  is applied to a transmitter  52 . Transmitter  52  transmits the pulse data to a data receiver  54 . Although the system is shown with antennas, implying that transmission and reception use radio frequency, in fact, any wireless transmission system may be used. This includes radio, optical (preferably infrared), and any other technique capable of transmitting the information, without requiring connecting wires, from movable stage  12  to stationary motor controller  18 . 
     The embodiment of the invention of FIG. 5 has the disadvantage that transmitter  52  is active at all times. Since the system is wireless, the illustrated apparatus on movable stage  12  is battery operated. Full-time operation of transmitter  52  reduces battery life. 
     Referring now to FIG. 6, an embodiment of the invention adds to the embodiment of FIG. 5, a command transmitter  56  in motor controller  18 , a receiver  58  and a counter  50  in movable stage  12 . In this embodiment, transmitter  52  remains off until commanded through receiver  58  to transmit the count stored in counter  50 . The command to transmit is sent from command transmitter  56  to receiver  58 . Although this embodiment requires that receiver  58  remain active at all times, the power drain of a solid state receiver is generally lower than that of a transmitter. As in prior embodiments, any wireless technology may be used in receiver  58  and command transmitter  56 . 
     In one embodiment of the invention, the magnetic encoder system may be omitted, and the entire encoder operation may be accomplished using optical encoder sensor  46  facing optical encoder tape  44 , and transmitting the position or motion data from the stage using electromagnetic means, such as described above. 
     Referring now to FIG. 7, an embodiment of the invention is shown in which it is possible to drive more than one movable stage  12  along path  14 . Each movable stage  12  requires independent application of armature power from motor controller  18 , independent armature switching and independent position communication from the movable stage back to motor controller  18 . The embodiment in FIG. 7 continues to show movable stage  12 , but adds a second rail  34 ′ on the second side of path  14  for use by a second movable stage (not shown). The second movable stage is similar to movable stage  12 , except that a pendant arm  28 ′ (not shown), supporting switching and encoder magnets (not shown), if in a visible position, would be located on the left side of the drawing. Second rail  34 ′ includes encoder sensors  38 ′ and switching sensors  36 ′, corresponding to the encoder and switching sensors of the embodiment of FIG.  1 B. Conductors  20 ′A, B and C carry motor drive power, separately generated in motor controller  18 , to the switches feeding power to the armature windings  16 A, B and C, along paths separate from conductors  20 A, B and C. In this manner, the second stage is separately controlled, and its motion is separately fed back to motor controller  18 . 
     Referring now to FIG. 8, there is shown an embodiment of the invention adapted to controlling and driving two movable stages  12  (and  12 ′, not shown). In this embodiment, rail  34 ′, besides supporting encoder sensor  38  and switching sensor  36 , also supports, spaced below, a second encoder sensor  38 ′ and a second switching sensor  36 ′. It will be understood power to armature windings  16 A, B and C is independently controlled by separate switches that feed motor power from conductors  20 A, B and C, when influenced by switching magnet  30 , and from conductors  20 ′A, B and C when influenced by switching magnet  30 ′. 
     Referring to FIG. 9, a second movable stage  12 ′ is shown, for use with rail  34 ′ of FIG.  8 . Second movable stage  12 ′ includes a pendant arm  28 ′, on the same side of movable stage  12  of FIG. 8, but extending further downward to accommodate an encoder magnet  32 ′ and switching magnet  30 ′ aligned with second encoder sensors  38 ′ and second switching sensors  36 ′, respectively. It would be clear to one skilled in the art that more than two movable stages could be controlled by adding additional elements to rail  34 ′, and by installing suitably long pendant arms  28 ,  28 ′ . . .  28   n  to each movable stage  12 . 
     The present invention is not limited to two movable stages on a single path. Any number of movable stages may be controlled independently along the same path  14 . Referring to FIG. 10, for example, three rails  34 ,  34 ′ and  34 ″ are spaced parallel to each other outward from path  14 . Each of rails  34 ,  34 ′ and  34 ″ includes thereon encoder sensors  38 ,  38 ′ and  38 ″, and switching sensors  36 ,  36 ′ and  36 ″. Each movable stage  12 ,  12 ′ and  12 ″ (only movable stage  12  is shown) includes a pendant arm  28 ,  28 ′ and  28 ″ (only pendant arm  28  is shown) adjacent to the sensors on its respective rail  34 , etc. Encoder magnets  32 ,  32 ′ and  32 ″ (only encoder magnet  32  is shown), and switching magnets  30 ,  30 ′ and  30 ″ (only switching magnet  30  is shown) are installed on their respective pendant arms. With the interleaving of pendant arms  28 , etc. between rails  34 , etc., as many stages  12 , etc. as desired may be accommodated, driven and controlled on a single path  14 . 
     In some applications, it may be desirable to have closed-loop control in some regions of the path for precise positioning, but where open-loop control may be desirable over other regions of the path. Referring to FIG. 12, a region of closed-loop control  60 , along path  14  receives drive power from motor controller  18  on a first set of conductors  20 A, B, and C through magnetically actuated switches  22 A, B and C, as previously described. Position or motion feedback in region  60 , as previously described, permits motor controller  18  to accurately control the position and velocity of movable stage  12 . A region of open-loop control  62 , along path  14  receives drive power from motor controller  18  on a second set of conductors  20 ′A, B and C. When movable stage  12  is in region  62 , motion feedback is either not generated, or is not responded to by motor controller  18 . Instead, motor controller  18  generates a programmed phase sequence for open-loop control of movable stage  12 . This drives movable stage at a predetermined speed. Once a region of closed-loop control is attained, movable stage  12  resumes operation under control of motor controller  18 . 
     It is also possible to provide path switching, similar to the switching used on railroads, to direct movable stage  12  flexibly along different paths. 
     Referring now to FIG. 11, an embodiment, similar to that of FIG. 6, adds a memory  64  for receiving commanded motion information. Once commanded motion information is stored, it is continuously compared with the content of counter  50  until a commanded condition is attained. During the interval between storage of the information, and the accomplishment of the commanded condition, transmitter  52  may remain quiescent. In some applications, receiver  58  may also remain quiescent during such interval, thereby consuming a minimum amount of battery power. 
     Referring now to FIG. 13, the power consumption of the above-described system is independent of the length of path  14 , since only active armature windings  16  are energized. Consequently, it is convenient to be able to construct a path  14  of any length by simply adding path modules  66  end to end. Each path module  66  includes at least one armature winding  16 , an associated portion of rail  34  and conductors  20 A, B and C. Conductors  20 A, B and C on adjacent path modules are connected together by connectors  68 . Path modules  66  are illustrated to contain three armature windings  16 A, B and C. It will be understood that switching sensors, together with their semiconductor switches, for the contained armature windings are mounted on the portion of rail  34  associated with that path module  66 . In addition, position feedback, if magnetic encoder sensing is used, is also included on suitable path modules  66 . As noted above, encoder sensors are spaced relatively widely apart. In a preferred embodiment, each path module should be long enough to contain at least one encoder sensor group. One system of this sort has been long enough to contain  9  armature windings ( 3  repetitions of phases A, B and C armatures). 
     Referring now to FIG. 14, a preferred embodiment of a path module  70  includes armature windings, as described above, plus three encoder sensor groups  40  spaced D/2 apart (D is the center-to-center spacing of beveled magnetic zones  42  at the ends of encoder magnet  32 ). Path module  70  extends a distance D/4 beyond the outer encoder sensor groups  40 . In this way, when the next path module  70  is connected end to end, the distance between the nearest encoder sensor groups  40  on the mated path modules  70  is D/2 as is desired. Path modules  70  are connected together to form a path  14 ′ of any desired length or shape. 
     Referring now to FIG. 15, another preferred embodiment includes two path modules  72 , 74  having armature windings, as described above. One module has an encoder sensor group  40 , and another module does not contain an encoder sensor. Path modules  72 ,  74  are connected together to form a path  14 ″ such that encoder sensor groups  40  in path modules  72  are spaced D/2 apart (D is the center-to-center spacing of beveled magnetic zones  42  at the ends of encoder magnet  32 ). Any desired path  14 ″ can be achieved using a combination of path modules  72  and  74 . It is understood by one skilled in the art that other arrangements of path modules  72 ,  74  can be used to form any desired shape or length path  14 ″ and any other desired spacing of encoder sensor groups  40 , so long as provision is made for spacing encoder sensor groups  40  a desired repeating distance apart. One embodiment includes a modular path module from which encoder sensor groups are omitted. However, provision is made for clamping, or otherwise affixing, encoder sensor groups  40  anywhere along the assembled modular path. When affixing the encoder sensor groups  40 , the appropriate spacing (D, D/2, D/4, etc.) is observed to ensure that the encoding signal is produced without distortion or dropouts during transitions from one path module to another. 
     Referring now to FIG. 16, an alternative embodiment of a path module  76  includes armature windings, as described above, and an encoder sensor group  40 . Modules  76  are connected together to form a path  14 ″ such that encoder sensor groups  40  in path modules  76  are spaced D/2 apart (D is the center-to-center spacing of beveled magnetic zones  42  at the ends of encoder magnet  32 ). Any desired length or shape path  14 ′″ can be achieved using a combination of path modules  76 . 
     The connection of signals and power along linear motor  10 , especially in the case of modular devices, has been described with wires and connectors joining wires in adjacent modules. Other techniques for carrying signals and power may be employed without departing from the spirit and scope of the invention. For example, instead of using wires, conductive traces on a rigid or flexible substrate may be used. 
     It will be noted that path  14  is shown as containing curves. It is a feature of the present invention that path  14  is not restricted to a straight line, as is frequently the case with the prior art. Instead, due to the nature of the present invention, linear motor  10  can be arranged to follow any desired path, including a straight path, curved path  14  as shown, or a closed path wherein movable stage  12  can repeatedly trace a closed path, moving in a single direction, or moving back and forth to desired locations anywhere along the open or closed path. 
     Referring now to FIG. 17, a linear motor  10 ′ includes a path  14 ′ which is closed on itself in a racetrack pattern. That is, path  14 ′ includes straight and parallel runs  78  joined by curved ends  80 . Movable stage  12  is driven, as described to any point on path  14 ′. In the preferred embodiment, movable stage  12  may continue in one direction indefinitely, or may move in one direction, then in the other, without limitation. This freedom of movement is enabled by the wireless control and feedback described herein. 
     Dashed box  82  in FIG. 17 is expanded in FIG. 18 to enable description. 
     All armature windings  16 A,  16 B and  16 C include an axis  84 , illustrated by a line in each armature winding. All axes  84  in runs  78  lie substantially parallel to each other, as shown in armature windings  16 A and  16 B at the lower left of the figure. Axes  84  in curved ends  80 , however, do not lie parallel to each other. Instead, axes  84  in curved ends  80  are tilted with respect to each other so that they lie across the shortest transverse distance of path  14 ′. In this way, repeating sets of armature windings  16 A,  16 B and  16 C at enabled to generate the desired force for urging movable stage  12  along path  14 ′. 
     One skilled in the art will recognize that accommodation must be made in the actuation times of switches  22 A,  22 B and  22 C for the tilting of axes  84  in curved ends  80 . One possibility includes adjusting an upstream-downstream dimension of armature windings  16 A,  16 B and  16 C so that center-to-center dimensions between end ones of each set of four such windings in curved ends  80  remains the same as the center-to-center dimensions between corresponding windings in runs  78 . In this manner, the span S of four armature windings  16  remains the same in curved ends  80  as the span S of 5+(n×4) motor magnets  160  (n=0, 1, 2, . . . ) in straight runs  78 . Switching sensors  36  are located along curved ends  80  so that their respective switches are actuated at minimum-current times, as previously explained. 
     A racetrack shape, as in FIGS. 17 and 18 do not exhaust the possible shapes of path that can be attained with the present invention. Any shape can be accommodated. 
     Referring now to FIG. 19, a multilevel path  86  is equally within the contemplation of the present invention. A lower portion  88  of path  86  passes under an upper portion  90 , thereof. Movable stage  12  may be positioned anywhere on path  86 . In cases where two or more movable stages  12  are employed on path  86 , the possibility exists that one movable stage  12  may cross on upper portion  90  at the same time that a second movable stage  12  on lower portion  88  passes under upper portion  90 . 
     Referring now to FIG. 20, a further illustration of a multilevel path  86 ′ includes a down spiral  92  aside a down and up spiral  94 . Spirals  92  and  94  are connected into a single path  86 ′ by crossing elements  96  and  98 . Spiral paths are frequently seen in conveyor systems to increase the residence time of objects in a location. For example, in a bakery operation, spirals are frequently used to permit time for newly baked goods to cool, before being discharged to packaging or further processing. 
     To illustrate the complete flexibility of the present invention, a path may be laid out as a Moebius band  100 , as shown in FIG. 21. A Moebius band is characterized as having only a single edge and a single surface, rather than having two edges and two surfaces, as in other examples of paths in the above description. A toy Moebius band is constructed by making a half twist in a strip of paper and then connecting the ends together. One proves that the strip has only a single surface by drawing a line down the center of the strip. Eventually, the end of the line meets the beginning of the line without having turned the strip over. Similarly, one can draw a line along the edge of the strip, and find the end of the line joining the beginning of the line, without crossing over from one edge to the other, since the strip has only a single edge. 
     The views of paths in the foregoing must not be considered to be top views. Indeed, important applications of the invention include those in which movable stage  12  is located below its path. Especially in the case where the path includes magnetic material, motor magnets  160 , and additional magnets  162  in movable stage  12  may be relied on to support movable stage by magnetic attraction to the magnetic material in the path. Other types of support are equally within the contemplation of the invention. In some cases, some portions of the path may be below and supporting movable stage  12 , and other portions of the path may be above movable stage  12 , as movable stage completes a full traverse of the path. 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.