Patent Publication Number: US-2003221498-A1

Title: Control knob to position encoder interface with self-aligning rotary guide

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001] Not Applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002] Not Applicable.  
       BACKGROUND OF THE INVENTION  
       [0003] The present invention relates in general to position encoders for control panels, and, more specifically, to interfacing a manual control knob with a rotary position encoder when an aperture in the control panel for receiving the knob is misaligned with a shaft of the position encoder.  
       [0004] Use of a rotary position encoder (e.g., a quadrature pulse encoder or a potentiometer) is popular in many types of control panels, such as a volume control knob for an automotive audio system. The encoder is typically mounted to a printed circuit board that also carries other components such as a visual display (e.g. vacuum fluorescent or LCD), membrane switches, and a microcontroller. A bezel for covering the control panel includes apertures for the control knob and other switches. In the interior space of the control panel, the control knob is coaxially attached to a rotating shaft of the position encoder. The distal end of the knob extends through a respective aperture in the bezel so that it can be manually grasped by a person for adjusting the shaft position (e.g., increasing or decreasing audio volume).  
       [0005] The aperture opening of the bezel must be properly aligned with the location of the shaft of the rotary position encoder in order to prevent interference between the knob and the edge surfaces of the aperture. The printed circuit board has a pre-specified size and shape and may include position registration features that align with corresponding features of the bezel or other intervening parts. However, manufacturing processes have inherent tolerances such that there are minor variations between actual parts and the position of the encoder shaft can only be controlled within these tolerances. Likewise, the size and location of the aperture is subject to its own tolerance. The tolerances may “build up” in incompatible directions, causing a misalignment severe enough that a crash condition between the knob and the aperture is created.  
       [0006] One typical prior art solution to this problem has been to provide in the nominal design of the control panel a sufficient gap between the knob and the aperture to accommodate the misalignment corresponding to the worst case tolerance build up. This has typically required a gap of about 1 millimeter, which is undesirable for several reasons. When there is a misalignment, the knob is off-center in the gap (i.e., the gap is uneven around the circumference). The uneven gap has a negative impact on the ornamental appearance of the control panel and may give the appearance of poor craftsmanship. Even when the knob is not perceptibly off center, the large 1 millimeter gap still gives an undesirable, uncraftsmanlike appearance.  
       [0007] Another prior art solution has been to employ a spring-loaded shaft to compensate for misalignment between the encoder and the aperture for the knob. This has allowed for use of a tighter fitting aperture around the knob, but the spring-loaded shaft adds expense to the control panel. Since the axis of shaft rotation is not coaxial with the axis of rotation of the knob in this design, a more complex and failure prone joint must be employed between the shaft and the knob. Furthermore, the tactile feel of the knob rotation suffers.  
       SUMMARY OF THE INVENTION  
       [0008] The present invention provides a cost effective and easily manufactured knob to encoder interface with the advantage of avoiding rubbing or interference between the knob and aperture without introducing a large gap at the periphery of the knob.  
       [0009] In one aspect of the invention, a control panel is provided comprising a circuit board, a rotary position encoder mounted to the circuit board and having a rotating shaft with a first rotation axis, and a cover having an aperture. A knob is rotationally received in the aperture with a second rotation axis. The knob has an open interior space and a radial guiding rib projecting into the interior space. A guide part rotates with the rotating shaft and has a guide groove slidably engaging the radial guiding rib whereby the radial guiding rib drives rotation of the rotating shaft in response to manual rotation of the knob. The radial guiding rib shifts in position within the guide groove in response to misalignment between the first rotation axis and the second rotation axis. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010]FIG. 1 is a perspective, exploded view of portions of a control panel.  
     [0011]FIG. 2 is a rear perspective, exploded view of a knob to encoder interface of the present invention.  
     [0012]FIG. 3 is a front perspective, exploded view of the interface of FIG. 2.  
     [0013]FIG. 4 is a perspective view showing more of the interior of the knob of FIGS. 2 and 3.  
     [0014]FIG. 5 is a side cross section through an assembled bezel, knob, guide, encoder, and circuit board.  
     [0015]FIG. 6 is a perspective exploded view of the control panel employing the present invention.  
     [0016]FIG. 7 is a partial cross-sectional view showing the sliding relationship of the guiding rib within a guide groove during knob rotation.  
     [0017]FIG. 8 is a rear perspective view showing how a knob is assembled between a bezel front plate and a bezel backing plate.  
     [0018]FIG. 9 is a front perspective view of an assembled guide part and encoder prior to bringing the circuit board together with a bezel assembly. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0019] Referring to FIG. 1, a control panel comprises a bezel assembly  10  and a printed circuit board  11 . Bezel assembly  10  houses a plurality of knobs and push buttons including a knob  12  being received in an aperture  13 . Knob  12  is mechanically coupled to a rotary position encoder  14  via its rotating shaft  15 . Encoder  14  generates a control signal in response to rotation of shaft  15 , such as an audio volume control signal when the control panel is attached to a head unit of an automotive audio system. Other uses include encoders for adjusting other audio parameters (e.g., tone, balance, fade), climate control parameters (e.g., temperature) on a climate control module, light dimming for a display, and many other applications. The control panel comprising the bezel and circuit board joined together is typically mounted to the front of a sheet metal enclosure for an electronic module (not shown).  
     [0020] In conventional control panels, knob  12  typically included an integral socket to fixedly receive shaft  15 . Thus, the precise location of knob  12  relative to aperture  13  was controlled by the relative position of encoder  14  to aperture  13 . Due to the tolerance build up between encoder  14  and aperture  13 , the size of aperture  13  was enlarged to allow knob  12  to turn freely within aperture  13  even when a misalignment occurred.  
     [0021] One embodiment of a knob-to-encoder interface of the present invention for allowing free knob rotation even in the presence of a misalignment and without enlarging the aperture is shown in FIG. 2. A guide part  20  is retained on shaft  15  via a D-clip  21  and is received by a knob  22  in an open interior space  24 . D-clip  21  has a D-shape matching the outside shape of shaft  15  and the inside shape of an internal shaft-retaining socket within a bore  28  of guide part  20 , so that guide part  20  is locked on and rotates with shaft  15 .  
     [0022] Guide part  20  has a plurality of radial guide grooves  23  which slidably receive at least one radial guiding rib  25  that projects into interior space  24 . Radial guiding rib  25  drives rotation of guide part  20 , and consequently shaft  15 , by bearing against the walls of corresponding groove  23  in response to manual rotation of knob  22 . Radial guiding rib  25  shifts radially in position within guide groove  23  in response to misalignment (i.e., offset) between the rotation axis of shaft  15  and the rotation axis of knob  22  as the two rotate. While only one guiding rib  25  is needed to transfer the rotation from knob  22  to guide part  20 , a radial alignment rib  26  is also provided diametrically opposite from rib  25  in order to help with the insertion of rib  25  into a groove  23  during assembly. As shown in FIG. 3, guide part  20  may also include guide ramps  27  which are sloped in order to rotate knob  22  by bearing against the leading edge of alignment rib  26  during insertion.  
     [0023] As shown in FIG. 4, guiding rib  25  may include one or more transverse secondary ribs  30  and  31 . In order to be slidable within a groove  23 , the transverse width of guiding rib  25  is slightly less than the width of groove  23  (e.g., about 0.4 mm less). The groove must be wide enough to provide sufficient play so that the guiding rib can pivot slightly (since the radial line of the rib may not pass through the central axis of the shaft). Thus, a groove width of about 3 mm may be employed. A countervailing concern for the width of rib  25  is that it should be less than about 60% of the thickness of the knob wall in order to avoid molding sink-deformation of the outside knob surface proximate to the rib. Therefore, the width of rib  25  at the interface with the knob outer wall is reduced (while still maintaining the necessary transverse thickness for driving the guide part) by using transverse secondary ribs  30  and  31 .  
     [0024]FIG. 5 is a cross section of an assembled control panel. The bezel includes a front plate  32  and a back plate  33  that retain knob  22  between them. Aperture  13  is bounded by a collar  34 . Knob  22  has an annular flange  35  at its lower end. Back plate  33  has an annular collar  36  for cooperating with collar  34  to sandwich flange  35  therebetween and to allow smooth rotation of knob  22  without jitter or wobble. In one preferred embodiment, alignment rib  26  extends a farther distance toward the lower end of knob  22  than guiding rib  25 . In addition, a sloped edge  37  may be provided to facilitate the self-alignment of knob  22  and guide part  20  during insertion.  
     [0025] Due to the knob to encoder interface of the present invention, a knob clearance  38  is maintained that is determined by tolerances involving the plastic molding of the knob and the bezel plates, and not by any tolerances in locating the encoder. Thus, clearance  38  can be kept in the range of about 0.1 to about 0.15 millimeters.  
     [0026]FIG. 7 shows how the interface operates to contend with an offset between the rotation axes of the knob and the encoder shaft. Knob  22  has a center axis of rotation  40 . The encoder shaft and guide part  20  have a center axis of rotation  41 . In a first position shown by solid lines, guiding rib  25  extends a first distance into groove  23  toward axis  40 . By turning knob  22 , rib  25  bears against a side wall of groove  23  and drives them both toward a second position shown by dashed lines. During rotation, rib  25  slides radially in groove  23  because of the offset in their axes of rotation. In the second position, rib  25  has slid outward in groove  23 .  
     [0027] In a preferred method of assembling the knob to encoder interface, knob  22  is assembled to bezel plates  32  and  33  as shown in FIG. 8. Knob  22  and other knobs or push buttons (not shown) of the control panel are inserted into their respective apertures in front plate  32 . Then back plate  33  is attached to front plate  32  (e.g., by snapping, use of attachment screws, or heat staking), thereby capturing the knobs and push buttons in place. As shown in FIG. 9, guide part  20  is mounted on the shaft of rotary position encoder  14  which has already been mounted to circuit board  11 .  
     [0028] Guide part  20  and the encoder shaft turn freely together. The circuit board assembly having the encoder and guide part is then brought together with the bezel assembly (i.e., cover) having the knob captured in the aperture. As the two are brought together, the internal rib(s) of the knob are inserted into a respective groove(s) in the guide part. If there are more than one rib, then their spacing matches the spacing of grooves of the guide part. If a rib is not already aligned with a groove during insertion, it first contacts a guide ramp so that the two self-align. By providing an alignment rib which is relatively thinner than the radial guiding rib and which extends farther toward the guide part, the self-alignment during said step of bringing together is improved. By being thinner and substantially diametrically opposite from the guiding rib, the alignment rib does not interfere with mutual rotation of the knob and the guide part.  
     [0029] Although the guide part has been shown as a separate piece, it could be formed integrally with the encoder shaft and could have a different number of grooves. If only one rib is provided on the knob, then the guide part does not need grooves that are diametrically aligned.