Abstract:
A capacitive sensor to detect toner volume levels in a toner container within an image forming device includes opposed electrodes disposed within the interior of the toner container. The opposed electrodes form a capacitor characterized by an inherent capacitance that varies in response to an amount of toner that exists between the opposed electrodes. A corresponding sensor circuit is electrically coupled to the opposed electrodes and adapted to sense an instantaneous capacitance of the capacitor to determine the amount of toner that exists between the opposed electrodes. The opposed electrodes may have different shapes and configurations, including for example, plates disposed within the toner container or the interior walls of the container itself. The sensor circuit is configured to apply an alternating current signal to the opposed electrodes and sense an output voltage that is indicative of an instantaneous capacitance of the capacitor corresponding to toner volume within the container.

Description:
BACKGROUND 
       [0001]    The invention relates generally to an image forming device, and more particularly to the sensing of toner levels in a toner container. 
         [0002]    During the image forming process, toner is transferred from a toner supply container to toner carrying members and to print or copy media. Inefficiencies in the transfer process cause residual toner to remain on the toner carrying members or other transport members, such as transport belts, intermediate transfer belts/drums, and photoconductive members. Residual toner may also be created during registration, color calibration, paper jams, and over-print situations. This residual toner should be cleaned before it affects the quality of subsequent images. A blade or other cleaning device commonly removes the residual or waste toner and the removed toner is stored in a waste toner container. 
         [0003]    Over time, toner levels in the toner supply container fall while levels in the waste toner container rise. Clearly, it is desirable to know the toner level in these containers. If the toner supply container nears an empty condition, print quality may suffer. Meanwhile, if a waste toner container overfills, the toner will spill into other regions of the image forming device, thus creating a mess and potentially causing print defects or other malfunctions. Estimates of toner use and accumulation based on print or time counts may not be accurate due to variability in factors such as environment, developer age, patch sensing cycles, transfer parameters, and the duration of operation without paper in the transfer path. 
         [0004]    Accordingly, some type of level-sensing may be appropriate in the toner containers. Some known types of toner level sensors include electrical sensors that measure the motive force required to drive an agitator within the container, optical devices using mirrors and toner dust wipers in a container, and other opto-electro-mechanical devices such as a flag that moves with the toner level to actuate a sensor that triggers only when the volume reaches a predetermined level. Unfortunately, there are drawbacks to these known sensors that make these solutions less than ideal. For instance, toner agitation may create unwanted toner dust and the added complication of moving hardware. Furthermore, the addition of moving parts increases component complexity and opportunities for errors. Therefore, existing solutions may not provide an optimal means for detecting toner levels in a toner container within an image forming device. 
       SUMMARY 
       [0005]    Embodiments disclosed herein are directed to a capacitive sensor to detect toner volume levels in a toner container within an image forming device. The capacitive sensor includes opposed electrodes disposed within the interior of the toner container. The opposed electrodes form a capacitor characterized by an inherent capacitance that varies in response to an amount of toner that exists between the opposed electrodes. Thus, capacitance levels may be obtained at various times to obtain an instantaneous toner volume level within the container. A corresponding sensor circuit is electrically coupled to the opposed electrodes and adapted to sense an instantaneous capacitance of the capacitor to determine the amount of toner that exists between the opposed electrodes. The opposed electrodes may have different shapes and configurations, including for example, plates disposed within the toner container or the interior walls of the container itself. Generally, the sensors may be oriented in a vertical configuration so that as toner levels change, the composite dielectric constant of the capacitor changes. The sensor circuit is configured to apply an alternating current signal to the opposed electrodes and sense an output voltage that is indicative of an instantaneous capacitance of the capacitor corresponding to toner volume within the container. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a functional block diagram of an image forming apparatus according to one embodiment; 
           [0007]      FIG. 2  is a schematic diagram of an image forming device having a plurality of moveable door assemblies according to one embodiment; 
           [0008]      FIG. 3  is a is a cut-away side view an image forming device illustrating the relative location of toner containers according to one embodiment; 
           [0009]      FIG. 4  is a side section view of a waste toner container including a capacitive waste toner sensor according to one embodiment; 
           [0010]      FIG. 5  is a side section view of a waste toner container including a capacitive waste toner sensor according to one embodiment; 
           [0011]      FIG. 6  is a side section view of a waste toner container including a capacitive waste toner sensor according to one embodiment; 
           [0012]      FIG. 7  is a side section view of a waste toner container including a capacitive waste toner sensor according to one embodiment; 
           [0013]      FIG. 8  is an exploded perspective view of a waste toner container including a capacitive waste toner sensor according to one embodiment; 
           [0014]      FIG. 9  is a graph illustrating a relationship between capacitance values for the capacitive sensor and toner volume according to one embodiment; 
           [0015]      FIG. 10  is an exploded perspective view of a waste toner container including a capacitive waste toner sensor according to one embodiment; 
           [0016]      FIG. 11  is a schematic diagram of a sensor circuit to determine a capacitance of a capacitive sensor according to one embodiment; 
           [0017]      FIG. 12  is a schematic diagram of a synchronous rectifier used in a sensor circuit to determine a capacitance of a capacitive sensor according to one embodiment, and 
           [0018]      FIG. 13  is a schematic diagram of a sensor circuit to determine a capacitance of a capacitive sensor according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The various embodiments disclosed herein are directed to a capacitive type sensor that may be used to sense relative toner levels within a toner container in an image forming device.  FIG. 1  represents an exemplary image forming device in which the capacitive sensor may be implemented. The illustrated image forming device includes a main body  12 , a media tray  98  with a pick mechanism  97  and a multi-purpose feeder  32 , both of which are conduits for introducing media sheets into the device  10 . The media tray  98  is preferably removable for refilling, and located on a lower section of the device  10 . Media sheets are moved from the input and fed into a primary media path. One or more registration rollers  99  disposed along the media path aligns the print media and precisely controls its further movement along the media path. An endless belt  48  forms a section of the media path for moving the media sheets past a plurality of image forming units  100 . Color printers typically include four image forming units  100  for printing with cyan, magenta, yellow, and black toner to produce a four-color image on the media sheet. 
         [0020]    Each image forming unit  100  includes an associated photoconductive unit  50  and a developer unit  40 . An optical scanning device  22  forms a latent image on a photoconductive member  51  in the photoconductive unit  50 . The developer unit  40  supplies toner from a contained volume to the photoconductive unit  50  to develop the latent image. The developed image is subsequently transferred onto a media sheet that is moved past each of the photoconductive units  50  by a transport belt  48 . The media sheet is then moved through a fuser  24  that adheres the toner to the media sheet. Exit rollers  26  rotate in a forward direction to move the media sheet to an output tray  28 , or rollers  26  rotate in a reverse direction to move the media sheet to a duplex path  30 . The duplex path  30  directs the inverted media sheet back through the image formation process for forming an image on a second side of the media sheet. 
         [0021]    The exemplary image forming device  10  comprises a main body  12  and two door assemblies  11 ,  13 . As used herein, the term “door assembly” is intended to refer to a door panel that is movably or detachably coupled to the main body  12 . Exemplary door assemblies  11 ,  13  may simply comprise a door panel and any mounting hardware that permits relative movement between the main body  12 , including but not limited to hinges and link arms or pivot arms. As indicated below, other components may be coupled to the door assemblies  11 ,  13 . The first door assembly  11  is located towards a top side of the image forming device  10  while the second door assembly  13  is located towards a lateral side of the image forming device  10 . 
         [0022]    Each door assembly  11 ,  13  is movable between a closed position as represented in  FIG. 1  and an open position as shown in  FIGS. 2 and 3 . In one embodiment the second door assembly  13  is pivotally attached to the main body  12  through a pivot  14 . The pivot  14  may attach the main body  12  and second door assembly  13  at a variety of locations, such as towards a lower edge  15 . In the open orientation, the door assembly upper edge  16  is spaced from the main body  12 . One or more modules may be coupled to the first and second door assemblies  11 ,  13 . For instance,  FIG. 2  shows a belt module  20  coupled to the second door assembly  13 . The belt module  20  may include an image transfer belt, a document transport belt, or other belt commonly used in image forming devices  10 . The schematic illustrations provided in  FIGS. 1 and 3  show one embodiment of an image forming device  10  where belt module  20  includes an endless belt  48  implemented as a transport belt. The belt module  20  further includes a pivoting structure (not explicitly identified) that allows the belt  48  to come into alignment with the image forming units  100 . An example of an image forming device  10  incorporating this type of belt module  20  and door assembly  13  is provided in commonly assigned U.S. patent application Ser. No. 10/804,488, filed 19 Mar. 2004, the contents of which being incorporated by reference herein in its entirety. 
         [0023]    Other modules may be coupled to the second door assembly as well. For example, some portion or the entire image forming unit  100  may be coupled to the second door assembly  13 .  FIG. 3  shows exemplary image forming units  100  that are constructed of a separate developer unit  40  and a photoconductor unit  50 . The developer unit  40 , including a developer member  45 , may be positioned within an opening  18  in the main body  12  whereas the photoconductor unit  50  may be mounted to the second door assembly  13  along with the aforementioned belt module  20 . In a closed orientation as illustrated in  FIG. 1 , the second door assembly  13  is positioned adjacent to the main body  12  with the photoconductive member  51  of the photoconductor unit  50  positioned adjacent the developer member  45  of the developer unit  40 . In an open orientation as illustrated in  FIG. 3 , the second door assembly  13  is moved away from the main body  12  separating the photoconductor unit  50  and belt module  20  from the developer unit  40 . This configuration provides direct and easy user access to the developer unit  40 , photoconductor unit  50 , and the belt module  20 . 
         [0024]    As indicated above, the developer member  45  supplies fresh toner to develop latent images that are formed on the photoconductive member  51 . The fresh toner is stored within developer container  62 . Over time, this fresh toner is consumed either as printed images or as waste toner. As images are developed and as the printer is used, some of the waste toner will move into one or more waste toner containers within the image forming device  10 . In the embodiment shown, a waste toner container  60  is disposed adjacent the belt module  20 . In one embodiment, the waste toner container  60  is forms a part of the belt module  20 . The waste toner container  60  is configured to store accumulated waste toner that is removed from the endless belt  48 . In one embodiment, the waste toner container  60  and endless belt  48  are replaceable as a single belt module  20  unit. In one embodiment, the waste toner container  60  is separable and replaceable independent of the endless belt  48 . Other waste toner containers  60  may store accumulated waste toner that is removed from the photoconductive members  51 . 
         [0025]    A capacitive sensor  70  may be incorporated into either the fresh toner container  62  or waste toner container  60  to provide an indication of the relative toner levels contained therein. This capacitive sensor  70  may be implemented as a parallel plate sensor, though other types may be implemented. Accordingly,  FIG. 3  shows a simplified, dashed-line representation of parallel plates to symbolize a capacitive sensor  70  located within each of the fresh toner containers  62 . Further description of the details of exemplary capacitive sensors  70  are described herein in the context of the waste toner container  60 . It should be understood that the teachings and concepts provided herein are applicable to a capacitive sensor  70  installed in other toner containers  60 ,  62 . 
         [0026]      FIGS. 4 and 5  illustrate a side cross section view of an exemplary waste toner container  60  including a capacitive toner sensor  70 . The waste toner container  60  includes a storage volume  64  formed within the inner walls  66  container  60 . A cleaner blade  68  is disposed at the exterior of the storage volume  64  and abuts the endless belt  48  to remove waste toner from the surface of the belt  48  (see  FIGS. 1 ,  3 ). Waste toner passes through a waste toner inlet  72  and collects within the storage volume  64 . 
         [0027]    In the embodiment shown, the waste toner container  60  includes sensor circuitry  76  in an adjoined sensor housing  74 . The sensor circuitry  76  is described in greater detail below. The sensor circuitry  76  may include additional functionality, including for example patch sensing circuitry. However, in at least one embodiment, the sensor circuitry  76  includes circuitry to detect an instantaneous capacitance between electrodes  80  in the capacitive sensor  70 . 
         [0028]    In the embodiments shown in  FIGS. 4 and 5 , the capacitive sensor  70  is implemented as a parallel plate sensor including a pair of opposed, plate-type electrodes  80 . In  FIG. 4 , the plate-type electrodes  80  are oriented parallel to each other, with the face of each electrode  80  facing substantially perpendicular to the process direction (which is perpendicular to the page). In  FIG. 5 , the plate-type electrodes  80  are oriented parallel to each other, with the face of each electrode  80  facing substantially parallel to the process direction. In each case, the electrodes  80  are oriented generally vertically so that as toner accumulates in the interior volume  64 , the waste toner will fill the space between the electrodes  80 . The plate-type electrodes  80  may be secured to side walls  66  via standoffs  82  or other mounting features. In one embodiment, the plate-type electrodes  80  are electrically insulated from the walls  66  of the waste toner container  60 . However, the plate-type electrodes  80  are electrically coupled to the sensor circuitry  76  as indicated by the dashed-line connection  84  shown. Those skilled in the art will understand that there are a variety of techniques that can be used to electrically couple the electrodes  80  to the sensor circuitry  76 . For example, in one embodiment, an electrical connection may be established from the electrodes  80  using conductive hardware (e.g., screw, bolt, rivet) to which a wire ring terminal (not specifically shown) is secured. In this manner, an insulated wire (also not shown) may be run between the conductive hardware and a connection terminal at the sensor circuitry  76 . Other means of coupling the electrodes  80  to the sensor circuitry  76  may be used. 
         [0029]    Further, other types of electrodes  80  may be used. For example,  FIGS. 6 and 7  illustrate embodiments in which the electrodes  80 A,  80 B,  80 C have different forms. Specifically,  FIG. 6  shows a pair of opposed rod-like electrodes  80 A secured to a bottom surface  86  of the waste toner container  60 . In  FIG. 7 , a rod- or plate-type electrode  80 B is contained within the storage volume  64  and a metallic interior wall  66 A forms an opposed electrode  80 C. Other electrode shapes, including curved, cylindrical, coaxial, and other shapes as would occur to those skilled in the art may be implemented for the electrodes  80 . 
         [0030]    Regardless of the form of the electrodes  80 , a capacitor is formed between the electrodes  80 . As the level of toner within the storage volume  64  rises, the toner displaces the air or gas between the electrodes  80 . Toner generally includes a different dielectric constant than air. Thus, a change in the value of the capacitor occurs due to a change in the composite dielectric constant of the substance between the electrodes  80 . Generally, the capacitance relationship for an ideal capacitor is provided by: 
         [0000]    
       
         
           
             
               
                 
                   C 
                   = 
                   
                     0.225 
                     * 
                     K 
                     * 
                     
                       ( 
                       
                         A 
                         D 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where C=capacitance in picoFarads, K=dielectric constant of the material filling the space between the electrodes  80 , A=area of overlap between the electrodes  80 , and D=distance between the electrodes  80 . The dielectric constant K is a numerical value that relates to the ability of the material between the electrodes  80  to store an electrostatic charge. According to equation (1), if a higher dielectric material replaces a lower one, the total capacitance increases. Furthermore, an increase in electrode area A and/or a decrease in separation distance D will each produce an increase in capacitance. 
         [0031]    Notably, the sensor  80  arrangement for the capacitive sensor  70  does not approach an ideal parallel plate capacitor because there are large fringe fields around the plate edges caused by a relatively large sensor  80  separation. Therefore, equation (1) does not precisely represent the characteristics of the capacitive sensor  70 . However, the present discussion is provided to describe the underlying relationship between dielectric constants and capacitance that allow the capacitive sensor  70  to work in the various embodiments disclosed herein. 
         [0032]    The instantaneous capacitance for an ideal capacitive toner sensor  70  may be determined by: 
         [0000]    
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       0.225 
                       * 
                       
                         K 
                         air 
                       
                       * 
                       
                         ( 
                         
                           
                             A 
                             air 
                           
                           
                             D 
                             air 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       0.225 
                       * 
                       
                         K 
                         toner 
                       
                       * 
                       
                         ( 
                         
                           
                             A 
                             toner 
                           
                           
                             D 
                             toner 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where D air  and D toner  are fixed and equal in the case of a parallel plate toner sensor  70 . Note however, that the sensors  80  may also be tilted relative to one another so that the distance D 1  between the sensors  80  is smaller towards the top of the sensors  80  as compared to the distance D 2  at the bottom of the sensors (as shown in  FIG. 5B ). This decreasing distance D may cause the capacitance to increase at a higher rate for a given amount of collected waste toner at the top of the sensors  80  as compared to that at the bottom of the sensors. The variables A air  and A toner  relate to the relative amount of toner that fills the space between the electrodes  80 . Initially, A air  will be at a maximum and A toner  will be zero. As toner fills the storage volume  64 , A toner  will increase and A air  will decrease. The variable K air  refers to the dielectric constant for air (about 1) and K toner  refer to the dielectric constant for toner (about 1.5 in one embodiment). Different toner formulations may have dielectric constants other than 1.5 as used in the present example. Further, the dielectric constants K air  and K toner  may change slightly over time and over different environmental conditions. However, for ease of calculation, they may be considered constant, particularly when the change in the dielectric constants is small relative to the amount of change in the variables A air  and A toner . Thus, equation (2) may be reduced to: 
         [0000]        C≈A   air +1.5 *A   toner   (3) 
         [0000]    which shows that as the amount of toner in storage volume  64  increases, the higher the resultant measured capacitance. Therefore, by measuring the instantaneous capacitance of the capacitive sensor  70 , one may determine the relative amounts of air and toner that fill the space between the electrodes  80 . The approximations provided by equations (2) and (3) indicate the trend that capacitance decreases with increased sensor  80  spacing and increases with increased sensor  80  area. These equations further indicate the approximate linear relationship between dielectric constant and capacitance in this situation. 
         [0033]    Using these principals, a capacitive toner sensor  70  may be implemented within the exemplary waste toner container  60  using a variety of electrodes  80 . The embodiments shown in  FIGS. 8 and 10  depict two different embodiments. Other embodiments are certainly possible. In the embodiment shown in  FIG. 8 , the capacitive toner sensor  70 A includes first and second plate electrodes  80 D,  80 E that are offset from each other. In one embodiment, the plate electrodes  80 D,  80 E include a surface area in the range between about 80 to 120 cm 2  and are spaced apart between about 2-4 mm, thereby providing a nominal capacitance of between about 30-35 pF for an empty waste container  60 . As suggested above, the spacing between the electrodes  80 D,  80 E may vary from a larger value (e.g., about 4 mm) at the bottom to a smaller value (e.g., about 2 mm) at the top of the electrodes  80 D,  80 E. With exemplary electrodes  80 D,  80 E of this size and with a toner dielectric constant K toner  of about 1.5, the nominal capacitance for a full waste toner container  60  may increase to a value between about 40-50 pF. Of course, these numbers are merely representative of one embodiment. The relative values and ranges may change depending on a particular configuration.  FIG. 9  shows the relationship between the capacitance and waste toner volume for the exemplary capacitive sensor  70 . 
         [0034]      FIG. 9  shows two sets of data One set (identified by triangles) represents capacitance measurements taken before the front door assembly  13  is opened while the other set (identified by squares) represents capacitance measurements taken after the front door assembly  13  is closed. As indicated above, the waste toner container  60  is positioned adjacent an endless belt  48  that is mounted to a front door assembly  13 . This door assembly  13  is opened and closed periodically by users who need to access the interior volume  18  of the image forming device  10 . For instance, the door assembly  13  may be opened to replace developer units  40  or clear paper jams. The door  13  motion tends to disturb or jostle the waste toner container  60  and distribute the level of waste toner contained therein. This agitation tends to improve the reliability of the data set obtained after the front door assembly is closed. However, as the graph in  FIG. 9  shows, the capacitance measurements may increase or decrease following a single open-close cycle of the front door assembly  13 . 
         [0035]    To further improve the distribution of waste toner within the waste toner container  60 , one or both of the plate electrodes  80 D,  80 E may be perforated. In the embodiment shown in  FIG. 8 , the plate electrode  80 E nearest the waste toner inlet  72  is perforated. The perforated plate electrode  80 E still serves to create the desired capacitor while allowing waste toner to pass through and fill the interior volume  64 . Otherwise, the space between the plate electrodes  80  may not fill evenly with waste toner, which may decrease the effectiveness of the sensor  70 A. 
         [0036]    In an embodiment of a capacitive sensor  70 B illustrated in  FIG. 10 , the inner walls  66  of the waste toner container  60  are lined with electrically conductive material  88 . Accordingly, the opposing vertical walls  66  on either side of the interior volume  64  form electrodes  80 F,  80 G of the exemplary capacitive sensor  70 B. The conductive material  88  may include, for example, pliable metallic tape or sheet metal. Materials  88  having high electrical conductivity may be desirable. In one embodiment, the conductive material  88  is adhered to the inner walls  66 . In one embodiment, the conductive material is secured to the inner walls  66  with securing hardware. In one embodiment, the conductive material  88  is molded into the inner walls  66 . 
         [0037]    In creating electrodes  80 F,  80 G at the walls  66  of the waste toner container  60 , the interior volume  64  is maximized. This configuration eliminates concerns about toner packing and toner flow. Thus, the resulting capacitance is purely a function of the volume of waste toner collected between the two electrodes  80 F,  80 G. However, the electrodes  80 F,  80 G may be spaced farther apart than in the embodiment shown in  FIG. 8 . Because of the increased spacing between the electrodes  80 F,  80 G, the resulting capacitance and capacitance variation may decrease. For instance, with the embodiment shown, the capacitance of an empty box may be between about 6-8 pF. The capacitance when full of waste toner may be approximately 10-11 pF. The decreased range may make it more difficult to sense small changes in capacitance. However, if the sensor circuitry  76  includes an appropriate sensitivity and filtering capability, this type of capacitive sensor  70 B may be appropriate. 
         [0038]    To that end, the sensor circuitry  76  may be implemented using a number of techniques. One approach uses the principles of a feedback amplifier U 1  as shown in  FIG. 11  to determine the capacitance of the capacitive sensor  70 . Once the capacitance is determined, the volume of waste toner in the waste toner container  60  may be determined using correlation data similar to that shown in  FIG. 9 . As is well known to those skilled in the art, the input/output relationship of the feedback circuit in  FIG. 11  is described by the equation. 
         [0000]    
       
         
           
             
               
                 
                   
                     Vout 
                      
                     
                         
                     
                      
                     1 
                   
                   = 
                   
                     Vbias 
                     - 
                     
                       
                         ( 
                         
                           Ci 
                           Cf 
                         
                         ) 
                       
                       * 
                       Vi 
                        
                       
                           
                       
                        
                       n 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where Cf is a known, fixed reference capacitance value and Ci represents the instantaneous capacitance of the capacitive sensor  70 . The value of Cf may be set at any appropriate value, including at a value near the expected value of Ci. The output Vout 1  of the feedback amplifier varies in relation to the comparative values of the capacitors Ci, Cf. The voltages Vin and Vbias are also predetermined values. Thus, equation (4) may be rewritten as follows 
         [0000]    
       
         
           
             
               
                 
                   Ci 
                   = 
                   
                     
                       
                         Vbias 
                         - 
                         
                           Vout 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       
                         Vi 
                          
                         
                             
                         
                          
                         n 
                       
                     
                     * 
                     Cf 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0000]    to provide the instantaneous capacitance of the capacitive sensor  70  as a function of a measured amplifier U 1  output voltage Vout 1 . 
         [0039]    Capacitors are, by their very nature, energy storage devices that block DC current. Therefore, the input voltage Vin should include an AC component. In one embodiment, the input voltage Vin includes a square wave signal. Consequently, the feedback amplifier U 1  produces an AC output with a DC offset that is generated by the voltage Vbias. In order to use equation (5), the AC portion in the output voltage Vout 1  should be converted to a DC signal that is representative of the AC amplitude and the DC offset removed Accordingly, the output voltage Vout 1  may be rectified and filtered with a conventionally known rectifier  90  and a conventionally known low pass filter (LPF)  92 . A conventional first order RC filter may be used for the LPF  92 , though it should be understood by those skilled in the art that other types of filters including Butterworth and higher order filters, may be used. 
         [0040]    The rectifier  90  may be implemented using conventional diode rectifiers. However, in one embodiment, a synchronous rectifier  90  as shown in  FIG. 12  is used. A synchronous rectifier  90  is generally known to have good noise rejections. In the illustrated embodiments the synchronous rectifier  90  is implemented using a unity gain amplifier U 3  with reversible polarity. A switch U 2  (e.g., a multiplexer or other switching device) is toggled synchronously with the input voltage Vin to provide the polarity reversal every half cycle of the input voltage Vin. With this implementation, equation (4) may be modified as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     Vout 
                      
                     
                         
                     
                      
                     2 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           Ci 
                           Cf 
                         
                         ) 
                       
                       * 
                       
                         AVERAGE 
                          
                         
                           ( 
                           
                              
                             Vin 
                              
                           
                           ) 
                         
                       
                     
                     + 
                     Vbias 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    which again may be rewritten as follows 
         [0000]    
       
         
           
             
               
                 
                   Ci 
                   = 
                   
                     
                       
                         
                           Vout 
                            
                           
                               
                           
                            
                           2 
                         
                         - 
                         Vbias 
                       
                       
                         AVERAGE 
                          
                         
                           ( 
                           
                              
                             Vin 
                              
                           
                           ) 
                         
                       
                     
                     * 
                     Cf 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0000]    to provide the instantaneous capacitance of the capacitive sensor  70  as a function of a measured LPF  92  output voltage Vout 2 . 
         [0041]    In an embodiment shown in  FIG. 13 , additional improvements may be achieved by closing the feedback loop around the entire sensor circuit  76 A rather than around the first stage amplifier U 1  as shown in  FIG. 11 . To achieve this modified feedback loop, an additional switch U 4  is added to the output Vout 2  of the LPF  92 . This switch U 4  modifies the DC output into an AC signal that is 180 degrees out of phase with the input signal Vin. The sensor circuit  76 A further includes a summer to remove the bias voltage Vbias before the low pass filter  92 . Thus, the bias voltage Vbias need not be subtracted from the output voltage Vout in calculating the instantaneous capacitance Ci of the capacitive sensor  70 . Closing the feedback loop in this way tends to reduce sensitivity to distortion in the rectifier stage and allows the use a low cost op-amp U 1 . Furthermore, one may design most of the gain into the low pass filter stage where the signal has only low frequency content to relieve the first stage (which generally handles high frequencies) of requiring high gain or large amplitude signals. Consequently, this circuit advantageously rejects noise at frequencies other than that of the input signal Vin. This noise filtering is an important characteristic since capacitance sensors tend to pick up ambient noise. In this particular application, the capacitor plates may be relatively large and may tend to pick up an extraordinary amount of ambient noise. 
         [0042]    The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. For example, the sensor circuitry described herein may be implemented using discrete components. However, those skilled in the art will recognize that microcontroller-based sensors may be incorporated into programmable devices, including for example microprocessors, DSPs, ASICs, or other stored-program processors. The present embodiments are, therefore to be considered in all respects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.