Abstract:
A mounting system for an encoder is disclosed. The mounting system prevents rotation about the encoder shaft&#39;s axis of rotation. The mounting system allows translation in a plane perpendicular to the encoder shaft&#39;s axis of rotation.

Description:
BACKGROUND 
     External encoders are used to determine the position and movement of shafts inside a machine or device. Encoders typically produce a stream of encoder pulses as the encoder shaft rotates with respect to the encoder body. There are two general types of external encoders: through shaft designs and ridged mount designs. In general the encoders using the ridged mount designs are typically of higher quality and have better accuracy or higher resolution. Ridged mount designs are attached directly to the device with a ridged mount and use a flexible or compliant coupling between the encoder shaft and the device shaft. Through shaft designs typically use a ridged coupling that attaches the encoder directly to the shaft of the device. A compliant mount couples the encoder body to the side of the device. 
     Ideally, for both types of encoders, the center of rotation of the encoder will be aligned with the center of rotation of the shaft in the device. But in reality there is always some misalignment between the two different centers of rotation. The compliant coupling between the shafts in the ridged mount encoder and the compliant coupling between the encoder body and the device for the through shaft encoder both compensate for the inherent offset between the center of rotation of the encoder and the center of rotation of the shaft in the device. Compliant designs (compliant shaft coupling for ridged mount, and single compliant tether for through shaft mount) have the disadvantage of inducing small inconsistencies in the encoder pulse stream timing. The inconsistencies manifest themselves as cyclic increases and decreases in encoder pulse timing with each revolution of the encoder due to the geometric limitations of these existing designs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric partial view of a ridged mount external encoder attached to a device  100  in an example embodiment of the invention. 
         FIG. 2  is an isometric view of bracket  104  in an example embodiment of the invention. 
         FIG. 3  is a front view of bracket  104  in an example embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-3 , and the following description depict specific examples of the invention. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these examples that fall within the scope of the invention. The features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to the specific examples described below, but only by the claims and their equivalents. 
       FIG. 1  is an isometric partial view of a external encoder attached to a device  100  in an example embodiment of the invention.  FIG. 1  includes the external encoder  102 , bracket  104  and a shaft  106  of the device  100  to be measured. The shaft  106  may be in a device that requires accurate positioning and control of the shaft, for example in a printer. Shaft  106  may be part of a paper feeding system in the printer that requires accurate positioning information so that the position of the paper or media can be controller with respect to the print heads of the device. The external encoder  102  is shown as a ridged mount encoder. In other embodiments, the encoder may be a through shaft design. 
     The shaft of encoder  102  is rigidly coupled to shaft  106  of the device using a coupler. The coupler may be any type of coupler, for example a collar that fits over both shafts and is tightened in place. Bracket  104  is a compliant mounting system that couples the body of the encoder to the device. One part of bracket  104  is rigidly attached to the body of the encoder and another part of bracket  104  is rigidly attached to the device  100 . Bracket  104  is compliant and allows translation of the body of the encoder with respect to the device. Bracket only allows translation of the body of the encoder  102  but does not allow rotation of the body of the encoder with respect to the axis of rotation of the shaft of the device. The unique geometry in bracket  104  is compliant in a way that preserves the encoder pulse stream without inducing a cyclic acceleration/deceleration. 
       FIG. 2  is an isometric view of bracket  104  in an example embodiment of the invention. Bracket  104  comprises: flange  212 , a first side beam  210 , a second side beam  216 , a first pair of legs  208 , a second pair of legs  214  and four mounting tabs  218 . Bracket  104  is formed from a thin flat plate. Each part of bracket  104  has a front face (FF) and a back face (BF). Bracket  104  has flange  212  as the main section. Flange  212  has mounting holes for mounting the encoder  102 . The back face (BF) of flange  212  is visible in  FIG. 2 . A side beam ( 210  and  216 ) is formed at the left and right side of flange  212 . The side beams are formed such that the front faces of the two side beams ( 210  and  216 ) face each other and are perpendicular to the front face of flange  212 . A pair of legs ( 208  and  214 ) are attached to the ends of the two side beams ( 210  and  216 ). The front faces for each pair of legs face each other and are perpendicular to both the front face of flange  212  and the front faces of the two side beam ( 210  and  216 ). A mounting tab  218  is attached to the end of each leg ( 208  and  214 ). In other embodiments the mounting tabs  218  may be replaced by mounting holes formed in the end of each of the legs. 
     A coordinate system can be referenced to bracket  104  with the Z axis perpendicular to flange  212  and the X and Y axis in the plane of flange  212 . When the four mounting tabs  218  are attached to a device, bracket  104  prevents rotation of flange  212  around the Z axis. Bracket  104  allows translation in the plane of flange  212  along both the X and Y axis. Each pair of legs ( 208  and  214 ) allows translation of side beams ( 210  and  216 ) along the Y axis but prevents Z axis rotation. The two side beams allow translation of flange  212  along the X axis but prevent Z axis rotation. Together the bracket allows translation in the plane of flange  212  but prevents rotation around the Z axis. 
     The encoder  102  has a shaft. When the encoder  102  is mounted to the back face (BF) of flange  212 , the shaft sticks through the large mounting hole and aligns with the z axis. Bracket  104  allows translation of the body of the encoder  102  but prevents the body from rotation around the axis of the shaft of the encoder  102 . In one example embodiment of the invention, bracket  104  is fabricated from a thin flat plate, for example sheet metal. The thickness of the flat plate may be between 0.01 inches and 0.1 inches thick, for example 0.02 inches thick. The material may be stainless steel, spring steel, or the like, for example T-301 stainless spring steel sheet, ½ hardened. The front faces of each of the parts of bracket  104  are all formed from the same side of the flat plate. The back faces are all formed from the other side of the flat plate. 
       FIG. 3  is a front view of bracket  104  in an example embodiment of the invention.  FIG. 3  shows that in one example embodiment of the invention, all four legs are the same length d 1 . The side beams are also the same length d 2 . The main mounting hole is centered in flange  212  with equal lengths between the hole center and the two side beams d 3  and equal distance between the hole center and the legs ½ d 2 . In addition the height of the two side beams H 1  are also equal (see  FIG. 2 ). 
     In the examples above, two side beams are shown, one at each end of flange  212 . In other example embodiments of the invention, there may be only one side beam and one pair of legs. In addition, the position of the encoder and the device can be switched with the encoder body attached to the ends of the legs and the device attached to the flange. Other geometries are also possible.