Abstract:
A motor system having a motor and an encoder is built using a metal bellows coupling for anti-rotation of encoder housing with high torsional stiffness and capacity to survive large radial and axial misalignment (both static and dynamic) in a very compact space.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a motor system having a motor, an encoder and a flexible coupling. More particularly, the present invention relates to the flexible coupling having a provision by which the stators of the motor and the encoder are connected.  
       BACKGROUND OF THE INVENTION  
       [0002]     The basic components of a motor generally include a rotor that spins inside a housing (i.e., a stator) that does not move. The rotor spins in the electromagnetic field contained in the stator. A shaft is generally connected to the spinning rotor thereby transferring the rotational movement to a load connected to the shaft. A motor system usually includes an encoder (or resolver) to control the operation of the motor system. The encoder is connected to the motor system to provide the position and speed information of the rotor of the motor system. This information may be used by a user to control the operation of the motor system using, for example, an external motor controller with associated electronics.  
         [0003]     Housed rotary optical encoders are the most common type of encoders used in a motor system to provide the rotary position of the motor. A housed rotary optical encoder typically includes a housing (i.e., a stator) to support precision bearings and a shaft with an optical disk attached thereto. The shaft of the rotary optical encoder is usually rigidly coupled to the shaft of the motor to detect the rotational position of the motor.  
         [0004]     A flexible stator coupling is used in the housed rotary optical encoder to prevent rotation of the encoder housing with respect to the motor housing while allowing radial and axial misalignment, both static and dynamic. Despite its flexibility in the radial and axial directions, the coupling must have high torsional stiffness in order to prevent undesirable dynamic positioning errors from the encoder.  
         [0005]     Couplings for the purpose of joining housed encoders and motors are commercially available which are stiff torsionally. While these couplings have been quite successful in a majority of applications, they experience fatigue failures in certain applications that require a large amount of radial and axial misalignment.  
         [0006]     Bellows couplings have been used for connection of encoder and motor stators which are formed from elastomeric materials, however these are not suitable for certain applications which require high positional accuracy, both static and dynamic. Metal bellows couplings have also been used to couple encoder shaft to motor shaft, whereby the motor and encoder stators are rigidly coupled. In this arrangement, the coupling diameter is small since it is mounted to the shaft, therefore the torsional stiffness is lower and the positional errors in operation are higher. The smaller diameter of the shaft-coupling also allows a lesser amount of radial and axial misalignment.  
       SUMMARY OF THE INVENTION  
       [0007]     The above-identified problems are solved and a technical advance is achieved in the art by providing a method and system that connects the motor and the encoder with a flexible coupling thereby achieving a high torsional stiffness in the motor system, along with obtaining capacity for handling larger amounts of radial and axial misalignment without experiencing fatigue damage.  
         [0008]     In accordance with an aspect of the invention, there is provided a bellows coupling that connects the housing of a motor (i.e., motor stator) and the housing of an encoder (i.e., encoder stator).  
         [0009]     In accordance with another aspect of the invention, there is provided a motor system comprising a motor, having a shaft and a housing, capable of driving a load connected to the shaft of the motor; an encoder, having a shaft and a housing, capable of detecting the rotational position of the shaft of the motor; a flexible coupling capable of connecting the housing of the motor to the housing of the encoder, wherein the shaft of the motor and the shaft of the encoder are connected with a rigid connection.  
         [0010]     Other and further aspects of the present invention will become apparent during the course of the following detailed description and by reference to the attached drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  illustrates simplified diagram of the motor system including a motor, an encoder and a stacked type flexible coupling of the present invention;  
         [0012]      FIGS. 2A, 2B ,  2 C,  2 D illustrate an embodiment of the flexible coupling of the present invention;  
         [0013]      FIG. 3  is a graph showing the test result of the torsional resonance of the motor system of the present invention;  
         [0014]      FIG. 4  illustrates four accelerometers located on the surface of the encoder of the motor system; and  
         [0015]      FIG. 5  illustrates simplified diagram of the motor system including a motor, an encoder and a concentric type flexible coupling of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     One aspect of the present invention is directed to the connection between a motor and encoder. In particular, a flexible bellows coupling is used to connect the encoder housing (i.e., encoder stator) with the motor housing (i.e., motor stator) in a motor system. It is assumed that the shaft of the motor and the encoder are rigidly connected.  
         [0017]     In the present invention, the convolutions of the bellows coupling can be oriented in two ways, i.e., stacked and concentric. In the stacked embodiment, the convolutions are stacked one on top of another. In the concentric embodiment, the convolutions are in concentric layers outward from a central axis.  
         [0018]      FIG. 1  illustrates a simplified diagram of the motor system  10  of the present invention showing a stacked type flexible stator coupling  200  that connects a motor  100  and an encoder  300  of the present invention. The motor includes a motor stator  101 , a motor bearing  103  and a motor shaft  105 . The encoder includes encoder stator  301 , encoder bearings  303  and an encoder shaft  305 . In this embodiment, a bolt/nut type connection is used between the flexible coupling and the motor, and a sleeve type connection is used between the flexible coupling and the encoder.  
         [0019]      FIGS. 2A, 2B ,  2 C,  2 D illustrate an embodiment of the flexible coupling of the present invention that can be used to connect the motor and encoder as shown in  FIG. 1 . Referring to  FIG. 2A , the flexible coupling of the present invention includes three main parts, i.e., a first part  201  having a sleeve, a second part  203  having a bellows and a third part  205  having a flange. The third part also includes holes  207  through which bolts may be used. Referring back to  FIG. 1 , the sleeve of the first part of the flexible coupling is configured to receive the outer surface of the encoder stator  301  and the flange of the third part of the flexible coupling is configured to attach to the side surface of the motor stator  101 . In this embodiment, the flexible coupling of the present invention is a bellows coupling and made of stainless steel (e.g., 321SS). Alternatively, other materials may be used which meet the criteria for high torsional stiffness and capacity for high misalignment for the bellows coupling.  
         [0020]      FIGS. 2B, 2C ,  2 D illustrate the dimensions of the flexible coupling of the present invention in this embodiment. In particular,  FIG. 2B  illustrates the dimensions of the flexible coupling when viewed from the flange of the third part  205 .  FIG. 2C  is a cross-sectional view of the flexible coupling when cut along the line A-A as indicated in  FIG. 2B .  FIG. 2D  illustrates an exploded view of a portion of the flexible coupling as indicated A in  FIG. 2C .  
         [0021]     Referring to  FIG. 2B , exemplary dimensions of the flexible coupling include the outer diameter (3.625″) and the inner diameter (2.324″). Additionally, the material thickness of the flexible coupling is 0.008 inches in this embodiment.  
         [0022]     Table I shows working conditions during the movement of the flexible coupling.  
                             TABLE 1                       Working Conditions During Operation                                    Temperature Max.   150° C., 302 F.           Torsional Stiffness (±30%)   36,076 Nm/rad           Material Thickness    0.20 mm, 0.008 in           Plies   1           Convolutions   4           Fatigue Life   Infinite           Dynamic Radial Offset   ±0.14 mm, ±0.0055 in           Operating Torque    0.42 Nm, 60.0 ozin           Static Radial Offset   ±0.24 mm, ±0.0095 in           Static Axial Offset   ±0.76 mm, ±0.030 in                      
 
         [0023]     An experiment has been performed to test the flexible coupling built according to the embodiment as described above. The motor system embodying the present invention shows nearly the same frequency of torsional resonance as the flexible coupling previously used.  FIG. 3  illustrates torsional resonance test results in a motor system built according to the present invention showing the amplitude values varying depending on the frequency. The amplitude values are measured by several accelerometers  307 ,  309 ,  311 ,  313  located on the surface of the encoder of the motor system as shown in  FIG. 4 . The result shows that a possible mode shape occurs at the frequency range between 912-920 Hz, nearly identical to the other non-bellows style of coupling.  
         [0024]     A second experiment was performed to verify the life of the bellows coupling when operated under combined radial and axial misalignment. The bellows coupling survived  113  million cycles under a 0.009 inches radial misalignment in combination with 0.012 inches radial run-out with no degradation or damage. The original non-bellows coupling was tested in a similar manner under lower levels of offset (0.004 inches radial runout) and failed due to fatigue crack propagation after as little as 4 million cycles.  
         [0025]      FIG. 5  illustrates a simplified diagram of the motor system  20  of the present invention showing a concentric type flexible stator coupling  500  that connects a motor  400  and an encoder  600  of the present invention as an alternative embodiment. The motor includes a motor stator  401 , a motor bearing  403  and a motor shaft  405 . The encoder includes encoder stator  601 , encoder bearings  603  and an encoder shaft  605 . The flexible coupling in this embodiment is now concentric instead of stacked. While  FIG. 5  shows a concentric flexible coupling having one convolution, two or more convolutions may be used as well within the scope of the present invention.  
         [0026]     Although illustrative embodiments of the present invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments, and that various changes and further modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention, which is defined in the claims, below. For an example, while the flexible coupling of the present invention connects the motor stator and the encoder stator having a similar diameter, this invention may also be applied easily to the motor stator and encoder stator having different diameter without significant modification. Additionally, while the flexible coupling of the present invention uses a flange type connection and a bolt/nut type connection, other types of connections may be used as well within the scope of the invention.