Abstract:
An outer rotor motor comprises a tubular shaft for maximum motor cooling effect. Coolant may flow through coolant channels of the shaft and the motor in various configurations to carry away the heat. A thermally conductive component may be inserted into the hollow shaft under the stator section to optimize the airflow and cooling. Physical construction of the motor and control algorithms may further enhance motor performance with appropriate sensors. A compact, smooth, and cool operating motor may thus be achieved for applications such as treadmills or other belt drive systems.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention is related to improvements for outer-rotor electric motor systems, particularly where the outer rotor is used to directly drive a belt, such as in treadmills and conveyors.  
         [0003]     2. Discussion of the Related Art  
         [0004]     Outer rotor motors are gaining popularity in many different commercial and residential applications since a direct-drive motor can simplify the overall system structure, increase system reliability and reduce system cost. Traditional treadmills and conveyors have a roller that is driven by conventional AC or DC electric motor through belts and pulleys. In addition, the AC or DC motor normally has a flywheel attached to achieve a smooth speed performance, such as in treadmill applications.  
         [0005]     A typical outer roller motor has a rotating roller supported by end caps and bearings, and a stator and shaft. Permanent magnets are mounted cylindrically inside the roller and form magnetic poles. The stator is firmly mounted on the shaft. The shaft is fixed at both ends to its supporting frames. When the stator windings are energized, they interact with the magnetic field from the magnets and the torque is produced to turn the outer rotor of the motor.  
         [0006]     Outer rotor motors can be used to directly drive a belt where the belt is led directly over the roller surface of an outer-rotor motor. However, challenges remain, especially in thermal cooling for motor surface temperature, motor smoothness at low speed, and motor response to load such as a step-fall on a treadmill belt. The major heat sources are the copper losses and magnetic core losses generated from the stator winding and lamination core. Motor torque ripple such as cogging torque will affect motor smoothness. The cogging torque is due to the interaction between the rotor magnets and the slots of the stator. This represents undesired motor torque output. The motor inertia and the performance of the motor controller contribute to how fast the motor reacts to load disturbances and variations such as step falls in treadmill applications and to the smoothness of the motor.  
         [0007]     Improvement in treadmill applications is desired in each of these three areas. For an outer rotor motor, the stator is inside the rotor housing so heat removal is difficult. One approach is to attach fan type devices to the end caps as disclosed in US patent application publication nos. 2002/0158543 and US 2003/0094867. However, this approach may not be entirely effective to remove heat from inside the motor housing. For example, end caps having ventilation holes may not be effective at removing the heat when the motor is running at lower speed, and by having ventilation holes at both end caps, foreign objects may be sucked in and cause hazards. The motor described in U.S. Pat. No. 6,455,960 relies on the supporting structure for conductive heat dissipation which strategy may become less effective as the motor shaft becomes longer in applications such as treadmills. Also, in certain cases there may not be enough supporting structure available for conductive heat dissipation. In addition, the contact area of the shaft with the supporting frame may be limited, further reducing the effects of conductive heat transfer. As a result, at full load and lower speed, both methods cited above may have difficulties keeping the rotor surface temperature low. Higher rotor surface temperatures can have adverse effects on the life of the belt, and can make an outer rotor motor unsuitable for certain applications.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is directed to the improvement of belt driving systems using a brushless permanent magnet outer-rotor motor, such as in treadmill and conveyors applications, and particularly in treadmill applications where the load can vary and demands for the speed smoothness, speed response, and consistent and controllable belt speed are important. The belt driving system consists of at least one outer rotor motor, one roller, an endless belt and a motor controller. The outer rotor motor drives the belt at commanded speeds. The outer rotor motor is preferably a brushless permanent magnet motor with a sinusoidal back EMF waveform, and the controller can be a sine wave drive based on field orientation control algorithms.  
         [0009]     The present invention is directed to three major issues of belt driving systems and in treadmill applications in particular: motor smoothness at low speed, motor response to load disturbance such as step fall, and motor surface temperature. By improving the cooling method of the outer rotor motor, such as by utilizing improved heat transfer by convection, the motor can be designed in smaller size and higher power density, and can run at higher efficiency, thus ensuring that the motor surface temperature stays lower to lengthen belt life. In certain conditions such as at light load, the cooling method according to the invention can remove heat generated by belt friction and friction with the deck or belt supporting structure. Thus, the lifetime of the system can be greatly improved.  
         [0010]     The present invention can provide an outer rotor motor having coolant conducting channels to remove heat from motor by passing a coolant fluid (liquid or gas) through channels in the shaft or motor assembly, or both. The present invention can provide one or more paths to allow coolant to pass through the motor and prevent foreign objects, e.g. dirt, from being sucked into the motor such as by having relatively clean coolant such as cooling air come in from inside the supporting frame in treadmill applications. Apertures may be provided in the shaft or the stator core, or both, to provide coolant paths. The present invention can further provide a sealed motor housing for certain applications.  
         [0011]     The present invention can provide an extended lifetime for its associated belt through the lower roller surface temperature at all speed range and load conditions by using additional cooling methods, such as a coolant pump or fan including blower for moving coolant into coolant channels. Such methods need not rely on the motor rotating speed and supporting structure. Alternatively, some methods can utilize apparatus which is associated with a rotating part of the outer rotor motor to increase coolant flow. For example, an additional passive fan may be provided that rotates synchronously with the roller. In some embodiments, a thermally conductive insertion rod can be inserted into the coolant channel inside a stator shaft to increase cooling effects. The inserted rod can be shaped, such as with fins, to increase heat transfer surface areas, for convection, conduction, or both. Such fins, or grooved channels, might be helically arranged to provide for additional coolant movement. Also, the surface of the shaft can be machined, forged or cast to provide such effects integrally.  
         [0012]     The present invention can also reduce motor cogging torque and improve motor running smoothness especially at low speeds by using a motor with a fractional pitch stator winding with, for example, a 21 stator slot and 16 magnetic pole or 8 magnetic pole configuration. Using fractional pitch winding configurations reduces the net cogging torque by making the contribution of cogging from each magnet pole out of phase with those of the other magnets. The present invention can also improve motor running smoothness by using a magnet shaping method to change the geometric shape of the magnets so that the motor cogging torque is minimized and the motor smoothness is improved. The present invention can also improve motor running smoothness by skewing the stator slots or magnets at a small angle less than or equal to one slot pitch. The present invention can use any of the above cogging torque reducing methods singly or in combination to produce a belt driving system, such as a treadmill, of exceptional smoothness.  
         [0013]     In order to achieve fast speed response to load conditions, such as step fall in a treadmill application, an advanced field orientation control algorithm can be used for the motor controller. Fast speed response and accurate speed control require accurate speed and position information of the rotor.  
         [0014]     The present invention can also improve motor speed performance by using a high resolution speed and position sensor. It is particularly desirable in applications such as treadmills to have accurate motor rotor position and speed feedback information. One approach is to use an encoder. In one embodiment of the invention and unlike a traditional encoder having the encoder&#39;s disk mounted onto the rotating shaft, the encoder disk is mounted onto the rotating roller or end cap directly or through an adapter. The encoder-sensing device is then mounted onto the stationary shaft through a hub or adapter.  
         [0015]     Another suitable high resolution device is a resolver. In one embodiment of the invention and unlike a traditional resolver that has the resolver rotor mounted to the rotating shaft and resolver stator mounted to the motor stationary frame, the rotor of the resolver is mounted onto the rotating roller or the end cap, and the stator of the resolver is secured to the non-rotating shaft.  
         [0016]     An addition feature of the invention is to have an outer rotor motor designed to have sinusoidal back EMF for treadmill applications along with the sinusoidal current field orientation control method thus achieving minimum torque ripple and smoothness especially at low speed.  
         [0017]     By providing an effective motor cooling method, a motor system according to the present invention can be run at higher efficiency and at lower surface temperature, increasing the life of the belt and making the system more reliable. By providing the above-discussed additional advantages to motor operation, a greater smoothness may be achieved in the belt drive systems of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The preferred embodiments of the present invention will be better understood by reference to the drawings where parts are designated by like numerals throughout.  
         [0019]      FIG. 1  illustrates a belt-driving system with an outer-rotor motor for a treadmill application.  
         [0020]      FIG. 2  is a block diagram of the treadmill system of  FIG. 1 .  
         [0021]      FIG. 3  is a cross sectional view of the belt-driving system along line  3 - 3  of  FIG. 1 .  
         [0022]      FIG. 4  illustrates a clamping structure for the stator shaft.  
         [0023]      FIG. 5  is a cross sectional view of the stator shaft.  
         [0024]      FIG. 6  is a cross sectional view of the outer-rotor motor.  
         [0025]      FIGS. 7 and 8  are end and side cross section views respectively of a rotor.  
         [0026]      FIG. 9  is a sectional view along the shaft longitudinal axis showing variations in motor cooling methods.  
         [0027]      FIG. 10  is a cross section view of the motor with variations of the motor cooling methods.  
         [0028]      FIG. 11  illustrates skewing of the motor stator.  
         [0029]      FIG. 12  illustrates cross section view of shaped magnet.  
         [0030]      FIG. 13  illustrates an encoder assembly structure.  
         [0031]      FIG. 14  illustrates the resolver assembly structure.  
         [0032]      FIG. 15  illustrates the back EMF and current waveforms.  
         [0033]      FIG. 16  is a control block diagram of the field orientation control for a motor controller. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     It will be readily understood that the components and methods of cooling, smoothness and fast response of the present invention, as generally described and illustrated in the figures herein, can be designed in a wide variety of different configurations and combinations depending on the specific application for a motor. Thus, the following more detailed description of the embodiments of the system and methods of the present invention, as represented in  FIGS. 1-16 , is not intended to limit the scope of the invention, as claimed, but is merely representative of the presently preferred embodiments of the invention.  
         [0035]     In  FIG. 1 a  belt driving system configured as a treadmill is shown and includes a control panel  4  secured to a frame structure  7 ,  15  and operably connected to the control system  25  ( FIG. 2 ) and wherein the control panel  4  includes at least a set of user controls  8  effective to permit a user to control the speed of a belt  3 . The belt driving system comprises a direct-drive outer-rotor motor  9  forming a first roller i.e., pulley, and a second passive roller  6 , i.e., pulley. The rollers  6 ,  9  rotatably cooperate to provide for longitudinal movement of the belt  3 . Supporting structures are provided as further discussed below. The arrangement of the belt driving system of the present invention allows the motor  9 , desirably to be mounted on the rear of a treadmill platform for increased belt drive efficiency since it acts to pull the belt  3 . The two rollers  6 ,  9  are supported by frame structures  7 ,  15 . Underneath the belt  3  is a supporting bed or deck  5 . The frame structures  7 ,  15  may be supported by a pair of supports  11  and a pair of wheels  17 . A motor controller  50  ( FIG. 2 ) is mounted inside one of the side frame structures  7 ,  15  and can be accessed through a panel  13  in the frame structure  15 .  
         [0036]     In  FIG. 2 , a control block diagram of a treadmill system primarily includes a control panel  4  with user control keyboard  8  and display  23 , a control system  25 , a motor controller  50  and a permanent magnet motor  55  which has a high resolution rotor position and speed sensor  40  and a cooling mechanism  53  configured inside. A user can send commands such as desired belt speed through treadmill control system  25  which can be a part of the control panel assembly  4  to motor controller  50 . The motor controller  50  will control the motor  55  to follow the user&#39;s speed command. The motor controller  50  can also send motor control status back to the treadmill control system  25  that then makes the necessary information available to the user on the panel display  23 . The motor controller  50  accepts standard AC power input  57  such as 110 VAC and 230 VAC.  
         [0037]     In  FIG. 3 , a cross section of the belt driving system viewed along line  3 - 3  of  FIG. 1  is shown. The outer rotor motor  9  is supported at both ends of a stator shaft  70  by the side frames  7 ,  15 . The tubular portion of the shaft  70  forms a central coolant channel  73 . The shaft  70  is non rotatable and is fixed in position such as by the clamping system as discussed with respect to  FIG. 4  below. The side frames  7 ,  15  can have multiple slots  14 ,  42  for air ventilation. An electric fan  44  is preferably mounted by screws  46  into a first end  74  of the stator shaft  70  to move a coolant fluid, in this embodiment, a gas such as air, through the coolant channels  73 . It will be appreciated that coolant transfer mechanisms  53  such as fans or pumps or like devices that can operate independently of the rate of rotor rotation in the context of the present invention. The fan  44  is electrically connected by a wire  48  to the motor controller  50  and can be turned on and off by the motor controller  50 . Use of the electrically powered fan  44  may lower motor winding temperatures by about 30 degrees centigrade (C) in some applications such as a treadmill.  
         [0038]     The outer-rotor motor  9  comprises the stator shaft  70 , the stator  28 , the rotor  22  which includes roller housing  130  and magnets  24 , a pair of rotor end caps  18 ,  34  and a high resolution rotor sensing mechanism  40 . The stator  28  is firmly mounted to the shaft  70  such as by a key  30  to prevent the stator  28  from rotating. In this embodiment, the rotor  22  is rotatably supported by its end-caps  18 ,  34  and bearings  20 ,  36 . In certain cases, the rotor  22  can be supported directly by the bearings  20 ,  36  and the end-caps  18 ,  34  can be eliminated. Two C-shaped snap rings  76 ,  78  are used to secure the bearings  20 ,  36  and prevent axial movements along the stator shaft  70 . The lead wires  52  from stator  28  go through apertures, i.e., holes  62 ,  64  in the stator shaft  70 , to connect to the motor controller  50 . The sensor mechanism  40  generally has two sections. One section is attached to the rotor  22 , and the other section is clamped to the stator shaft  70 , as further discussed below. Sensor mechanism lead-wires  60  go through hole  177  in the stator shaft  70  to connect to the controller  50 . When the fan  44  is on, air flows in at the first end of the shaft  74  through the central coolant channel  73  and out at a second end of the shaft  72 . Portions of the cooling air can also flow through hole  54 , through motor air gap  26 , then though holes  58  to remove heat from inside the motor  9 . The exemplary holes in the stator shaft  70  are for illustrative purposes. There can be multiple holes for each stated purpose and the holes need not be at a 90 degree angle to the shaft wall surface.  
         [0039]     In applications that require a sealed motor, the ventilation holes  54  and  58  on the shaft preferably no longer exist. Holes for lead wires  64 ,  62  and  177  of the motor and the sensors to come outside will be sealed. The heat generated by the stator  28  can be removed by the air that flows through the coolant channel  73 .  
         [0040]     In  FIG. 4  the clamp mechanism of the ends  72 ,  74  of the stator shaft  70  are shown. The ends  72 ,  74  of stator shaft  70  are clamped down at both ends by top and bottom L shape clamps,  114  and  124  respectively, with clamping screws  116 . A vibration absorptive resilient material  118 ,  122  such as an elastomeric type material can be interposed between clamps  114 ,  124  and the ends  72 ,  74  of stator shaft  70  to insolate the frame  7 , 15  from motor vibration and noise. Also, because in this embodiment heat is primarily transferred out by coolant flow rather than conduction through the mounting system, the resilient material can have a relatively low level of thermal conductivity. Other thermal insolating material such as ceramic can be used to prevent heat transfer from the stator shaft  70  to the frame  7 ,  15 . For example, in the treadmill applications, it is desirable to insulate the frame from sources of heat to protect finishes.  
         [0041]     In  FIG. 5  an exemplary tubular stator shaft  70  is shown. Flat areas  166 ,  180  are configuring in the ends  72 ,  74  of the stator shaft  70  for clamping. The plurality of holes  54 ,  58 ,  62 ,  64 ,  177  in the wall of the stator shaft  70  allow for cooling air or wires to pass in and out of the stator shaft  70 . Grooves  152 ,  160  on the wall of the stator shaft  70  used in conjunction with C-shaped snap rings  76 ,  78  ( FIG. 3 ) serve to prevent the bearings from moving along the axial direction. A notch  156  on the stator shaft  70  fits the key  30  ( FIG. 3 ) to prevent stator  28  from rotating. In the case of a skewed stator, the notch  156  is not necessary.  
         [0042]      FIG. 6  shows a cross sectional drawing of an exemplary outer-rotor motor  9  with belt  3 . The exemplary motor  9  is twenty-one slots  139 , sixteen pole  132 , 134  outer-rotor motor  9 . The rotor  22  with its associated roller housing  130 , i.e., that part of the rotor sleeve outside of the magnet mounting area, drives the belt  3 . It will be understood that a roller sleeve or the roller housing may encompass other structures whereby the drive surface need not have the magnets of the motor directly mounted therein. The belt  3  may be pre-tensioned to prevent belt slip. Magnets  132 ,  134  are mounted cylindrically inside the roller housing  130 . The roller housing  130  may be made of ferric material and functions as the magnetic flux return path. The thickness of the housing  130  should meet both mechanical strength and no flux saturation requirements. Secured to the stator shaft  70  is a stator core lamination  138 . The air gap  26  exists between the rotor  22  and the stator core lamination  138 . An exemplary slot  139  retains copper windings  140  of the stator  28 . The key  30  is used to prevent the stator  22  from rotating on the stator shaft  70 . An aluminum or otherwise thermally conductive member  148  may be placed in the coolant channel  73  underneath the stator core lamination stack. The shape of the member  148  is not limited to the finned shape shown. As discussed, alternative forms of shapes for the interior or exterior surface area of the shaft  70  may be had for increasing the contact area with the cooling air. The member  148  increases the heat transfer area with the cooling air moving through the coolant channel  73 . By changing the shape of the member  148 , the air pressure between through-holes  54 ,  58  in the shaft  70  may be adjusted so that the cooling air flowing through the air gap  26  can be optimized along with the cooling air flowing through the coolant channel  73 . A finned member  148  as shown and discussed herein may further lower motor winding temperatures by 10 degrees Celsius.  
         [0043]      FIGS. 7 and 8  show an exemplary rotor assembly. A set of permanent magnets  241 ,  242 ,  250 ,  252 ,  254 ,  256  is secured over, e.g., a ten inch length in the axial, i.e., longitudinal, direction onto the inside of the cylindrical roller housing  130  to form the rotor  22 . Each magnet has either north or south polarity and arranged in an alternative sequence of north-pole  241  then south pole  242 , etc. There may be gaps, e.g.,  240 ,  246  between the magnets. The rotor rotates and is supported by the end caps  18 ,  34 , which are pressed in and stopped at raised edges  248 ,  258  inside the roller housing  130 . The locations of raised edges  248 ,  258  here are only for illustrative purposes and may vary along the axial roller direction. The rotor preferably has either 16 or 8 poles with a 21-slot stator configuration.  
         [0044]      FIGS. 9 and 10  depict motor cooling configuration. Cooling air flows in from a first end  326  of a semi-hollow tubular shaft  370  having a solid insertion rod  344  placed therein, then flows out a second end  290 . A portion of the cooling air may flow through first end holes  314 ,  340  in the shaft wall then through a air gap  310 , then through second end holes  302 ,  356 , then finally flow out through the second end of the shaft  290 . A set of additional holes  296  through end cap  298  may assist cooling-air to exit and carry heat away.  
         [0045]     The shape or arrangement, or both, of the holes  296  on the end cap  298  can be designed to assist airflow away from the motor inside housing, especially when motor runs at higher speed. In addition, or alternatively, a fan type device  300  such as a concentric series of fan or propeller blades, or the like, can be secured to the rotating roller housing  130 , or secured to the end cap  298 , which is also rotating, to assist in the movement of air.  
         [0046]     A set of longitudinal holes or stator coolant channels  306  through the stator core lamination stack can allow air flow through, thus further removing heat. In this case, the cooling-air flows in from the first end  326  of the semi-hollow shaft  370 , then through the first end holes  314 ,  340  of the shaft, then through both air gap  310  and lamination holes  306 , then through the second end holes  302 ,  356  of the shaft or holes on the end-cap  296 , or both, to carry heat away.  
         [0047]     Effective cooling methods can improve motor performance greatly in terms of efficiency, motor power density, motor size and the life of the motor. The temperature of the motor may also have direct impact on other components such as the belt on treadmill and conveyor. Therefore, the illustrated cooling methods and its variations herein are important objects of the present invention.  
         [0048]     Motor cogging torque can have a great effect on the smoothness of the motor operation, especially in treadmill application. Referring to  FIG. 6 , cogging torque is due to the interaction between the rotor magnets  132 ,  134  and slots  139  of the stator  28 . The cogging torque can be identified as Tcog=−½ φg2 dR/dθ, where φg is the air gap flux and R is the net reluctance seen by the flux φg. The primary component of R is the air gap reluctance Rg. Therefore if the air gap reluctance varies with position, cogging torque will be generated. Setting φg to zero is not possible since φg must be maximized to produce the desired motor mutual torque. Thus, the cogging torque can only be eliminated by making the air gap reluctance constant with respect to position.  
         [0049]     Therefore, in order to provide smoother motor and belt performance, some embodiments of the motor of the present invention may use fractional pitch winding configuration, which reduces the net cogging torque by making the contribution of the dR/dθ from each magnet pole out of phase with those of the other magnets. The basic idea is to arrange the number combination of the stator slots and rotor magnets, such that, the overall magnetic flux distribution will remain unchanged or the change is minimized, while the rotor is rotating. In the ideal case, the cogging torque sums to zero at all positions. In reality, however, some residual cogging torque remains.  FIG. 6  shows a 21 slot and 16 pole fractional pitch winding configuration. Variations of the fractional pitch-winding configuration include 15-slot stator for smaller diameter motors and 27 slot stator for larger diameter motors. The number of magnet poles may vary as necessary with each configuration.  
         [0050]     Referring to  FIG. 11 , the technique of skewing may also be used within the present invention to reduce cogging torque. This technique can be accomplished by slanting or skewing the slot edges  498 ,  500  of a slot  496  with respect to the magnet edges  492 ,  494  of a magnet  490 . Also, this technique can be accomplished by skewing the magnets. The idea of skewing is to let the relative position of the slot and magnet to be different for different sections of the motor. Thus the cogging torque generated in different sections of the motor will tend to cancel each other.  
         [0051]     Referring to  FIG. 12 , the technique of magnet shaping may also be used within the present invention to reducing cogging torque by controlling the shape of the cross section of the magnets  491 . Usually finite element analysis is required for magnet shaping design. By shaping the magnets, the air gap  493  between the stator tooth lamination  495  and the magnets  491  will no longer be even. Then the magnet flux distribution will tend to create a smaller cogging torque. Thus, fractional pitch winding techniques, skewing techniques, magnet shaping or combinations of them, can reduce cogging torque significantly.  
         [0052]     In treadmill applications, load varies significantly when people walking or running on it. Traditionally a flywheel is used to reduce or smooth the speed variation as load changes. By using the outer rotor motor belt driving system, the flywheel is no longer needed. However this poses higher requirements on the motor controller system to provide fast torque response and achieve accurate speed control.  
         [0053]     There are at least two types of high resolution rotor position sensing devices that are suitable for use in outer rotor motor applications according to the invention. First such device is encoder. An encoder  372  is shown in  FIG. 13 . The encoder  372  consists of a collar  378 , a disk  382 , a hub  392 , a LED  380  and a sensing circuit PCB  384 . The pass-through LED  380  and sensing circuit PCB  384  may be a commercially available integrated module. The collar  378  is firmly attached to the end cap  376  or roller housing  374  and rotates with either. The disk  382  is attached firmly to the collar  378 . The collar  378  may be a part of the structure of end cap  376  or roller housing  374  so it rotates. The hub  392  is non-rotatably, firmly mounted to the shaft  398  by setscrews  396 , 400 . The LED  380  and sensing circuit PCB  384  are attached to the hub by screws  386 ,  408 . As an alternative mounting method, the LED and sensing circuit can also be mounted over the disk from the inner diameter instead of outer diameter. The disk  382  can be made of either glass or film type materials depending on the temperature requirements. The encoder disk can also have commutation channels build in thus to eliminate the Hall sensor devices. Encoder can provide higher resolution of motor rotor position information, e.g., over 1000 lines per revolution.  
         [0054]     A dust cover  388  is attached to the hub by screws  390 ,  402  to prevent foreign objects from contaminating the encoder. The dust cover  388  can also be attached to the roller. In this case, it will rotate with the roller. The encoder  372  could be mounted either inside the end cap  376  or outside the end cap  376 .  
         [0055]     A second such higher resolution rotor position sensing device can be a resolver  410  as shown in  FIG. 14 . It is suitable for use with the present invention to provide rotor position and speed information. A resolver stator includes stator winding  442 ,  438  and winding transformer  430 . It is mounted on the motor shaft  434 . A resolver rotor includes rotor winding  424 ,  427  transformer winding  428 , and housing  426 . It is mounted to the roller housing  420 . Resolvers may be considered as inductive position sensors, which have their own rotor windings  424 ,  427  and stator windings  442 ,  438  which are shifted by 90 degrees. The windings transfer energy from stator to rotor by means of an auxiliary rotary transformer  428 ,  430 . No slip ring and brush are necessary, therefore reducing the cost and increasing the reliability of the device.  
         [0056]      FIG. 15  shows the three phase sinusoidal back EMF waveforms  450 ,  454 ,  456  and motor phase current waveforms  452 . The outer-rotor motor of the present invention is designed to have three phase sinusoidal back EMF  450 ,  454  and  456  for treadmill applications. The motor controller is designed to have a sinusoidal current waveform  452  that matches with the motor sinusoidal phase back EMF and achieves the minimum torque ripple and fast torque response.  
         [0057]      FIG. 16  is a block diagram of motor controller  50 . Advanced field orientation control algorithms for permanent magnet synchronous motors can be used to achieve fast torque response and accurate speed control. A speed command  460  is taken from the high level control system  25  such as input from the user controls  8  on a treadmill control panel  4 . A high resolution speed feedback signal is derived from either the encoder  372 , or the resolver  410 , at the feedback interface  459 . The difference between the speed command and the speed feedback is the input of the speed regulator  462 . The speed regulator  462  may use PID or PI control algorithms. A feed forward controller may be added to the speed regulator  462  to further improve speed response. The output of the speed regulator is a torque current command in synchronous frame. A current sensing device senses at least two phases of motor currents. Then the phase current is transferred into synchronous frames from stationary frame by blocks  478 ,  482 . The difference between current commands and current feedbacks is the input of the current regulators  464 ,  466 . The current regulators  464 ,  466  can be a simple PI regulator. The outputs of the current regulators  464 ,  466  are the voltage commands in synchronous frame. The voltage commands in synchronous frames are transferred to voltage commands in stationary frame by block  468 . The voltage command in stationary frame is the input of the Space Vector PWM (SVPWM) module  470 . The SVPWM generates six PWM signals that control the six IGBT power devices of the inverter  474  powered by an AC/DC converter  472 . The three phase outputs of the inverter are connected to the outer-rotor motor  476 .  
         [0058]     While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.