Abstract:
An electronic cook top control system has a cooktop including a heating element. An electronic controller is operatively connected to the cooktop. A rotary position input is operatively connected to the electronic controller. The electronic controller controls a heating level of the cooktop in a first manner in response to rotation of the rotary position input in a first direction. The electronic controller controls the heating level of the cooktop in a second manner in response to rotation of the rotary position input in a second direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/567,920 filed on Dec. 7, 2006, which is a continuation of U.S. patent application Ser. No. 11/138,564 filed May 26, 2005, which is a continuation of U.S. patent application Ser. No. 10/118,294 filed Apr. 8, 2002, which is a continuation-in-part of U.S. patent application Ser. No. 09/973,096 filed Oct. 9, 2001, now abandoned, each of which are hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to the field of electronic controls and more specifically to an electronic power control system for cooktop heating elements. 
         [0003]    Conventional controls for electric cooktops utilize so-called “infinite switches.” The infinite switch comprises a bimetal switch to control an electric heating element. Current flowing in the bimetal switch causes it to physically move through a process of heating and cooling. This movement causes the switch contacts to open and close, thereby, controlling the power applied to the heating element. 
         [0004]    The infinite switch uses pulse width modulation to control the power output, and thus the temperature of the heating element. Rotation of the infinite switch changes the relationship of the closed and open times or duty cycle. As the switch is rotated to a higher setting the contacts remain closed for a longer period of time, raising the heating element temperature. Conversely, rotating the switch to a lower setting causes the contacts to remain closed for a shorter period of time, lowering the heating element temperature. 
         [0005]    Recently, electronic controls have been increasing in popularity. Electronic controls are capable of providing a more precise level of heating. Further, associated digital controls are easier to read than an analog dial, allowing the quick setting of desired heat levels. Electronic controls are also capable of providing advanced features, such as a safety lockout. 
         [0006]    Analog controls remain desirable because their associated rotational control knobs are often easier to manipulate and more convenient for the user than the button-type controls conventionally associated with electronic controls. Likewise, using a duty cycle to control the level of heating remains desirable, because it allows the heating elements to provide very low levels of heat, including levels suitable for warming operations. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a power control system for an electric heating element. The control system comprises a communication bus, a controller connected to the communication bus, a variably resistive device connected to the controller, a digital display connected to the controller, and a power unit connected to the communication bus, the power unit having a power output. 
         [0008]    According to another aspect, the present invention provides a method of controlling a power output comprising the steps of: inputting power setting information to an electronic controller by a variably resistive device, and adjusting a duty cycle of a power output by the electronic controller according to the angular position of the variably resistive device. 
         [0009]    According to yet another aspect, the present invention provides a power control system for controlling a plurality of heating elements. The control system comprises a first rotational control input having a first range of angular rotation and a second range of angular rotation, a first heating element, and a second heating element. A position of the control input in the first range controls the first heating element and a position of the control input in the second range controls the second heating element. 
         [0010]    According to a further aspect, the present invention provides a power control system for controlling a plurality of heating elements. The control system comprises a first rotational control input, a second rotational control input having a first range of angular rotation and a second range of angular rotation, a first heating element, a second heating element, and a third heating element. The second heating element is a bridge element positioned between the first element and the third element. The first control input controls the first heating element. A position of the second control input in the first range controls the third heating element, and a position of the second control input in the second range causes the first control input to concurrently control the first heating element, the second heating element, and the third heating element. 
         [0011]    According to a further aspect, the present invention provides a method of controlling a plurality of power outputs comprising steps of: inputting power setting information to an electronic controller by a variably resistive device, the electronic controller adjusting a duty cycle of a first power output according to a position in a first predetermined range of positions of the variably resistive device, and the electronic controller adjusting a duty cycle of a second power output according to position in a second predetermined range of positions of the variably resistive device. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0012]      FIG. 1  is a schematic representation of a power control system connected to an electric cooktop according to an embodiment of the present invention; 
           [0013]      FIG. 1A  is a schematic representation of a control scheme of a power control system according to an embodiment of the present invention; 
           [0014]      FIG. 2  is plot of power output according to an embodiment of the present invention; 
           [0015]      FIG. 3  is schematic representation of a control scheme of a power control system according to another embodiment of the present invention; 
           [0016]      FIG. 4  is schematic representation of a control scheme of a power control system according to a further embodiment of the present invention; 
           [0017]      FIG. 5  is schematic representation of a control scheme of a power control system according to a further embodiment of the present invention; 
           [0018]      FIG. 6  is schematic representation of a control scheme of a power control system according to a further embodiment of the present invention; 
           [0019]      FIG. 7  is schematic representation of a control scheme of a power control system according to a further embodiment of the present invention; 
           [0020]      FIG. 8  is schematic representation of a control scheme of a power control system according to a further embodiment of the present invention; and 
           [0021]      FIG. 9  is a schematic representation of power and communication connections of a power unit and user interface units according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    The present invention provides a rotational control knob to operate a power controller which provides a duty cycle-controlled power output.  FIG. 1  is a schematic representation of an embodiment of the present invention in which a power control system  10  is provided for an electric cooktop  12 . The power control system  10  includes a power unit  14  and a plurality of user interface units  16 ,  16   s.  The user interface units  16 ,  16   s  are connected to the power unit  14  by a communication bus  18  and the power unit  14  is connected to individual heating elements  20  of the cooktop. The heating elements  20  are electrically resistive and are heated by current flowing through them. 
         [0023]    The power unit  14  includes an electronic controller for controlling power output to the heating elements  20 . Further, the power unit  14  is connected to an electronic oven control unit  22 . The oven control unit  22  controls various operations of an oven (not shown), including the initialization of an oven cleaning cycle. The oven control unit  22  communicates bi-directionally with the power unit  14  via a two-line oven control communication bus  23  for synchronizing certain operations between the operation of the oven by the oven control unit  22  and the operation of the cooktop heating elements  20  by the power unit  14 . Specifically, by way of the oven control communication bus  23 , the power unit  14  is capable of instructing the oven control unit  22  to lockout or prevent the initiation of a cleaning cycle or other operation when one or more of the heating elements  20  are in use. Likewise, the oven control unit  22  is capable of instructing the power unit  14  to lockout the powering of any cooktop heating element  20 , such as when a cleaning cycle has been initiated or after a lockout button has been pressed. As used herein, the term “lockout” refers generally to the disabling of control or operation of some aspect of the power control system  10 . 
         [0024]    Each user interface unit  16 ,  16   s  includes a potentiometer  24 ,  24   s  and a power level display  26 ,  26   s.  Each master user interface unit  16  further includes an electronic controller  28 . A knob is attached to manually control the rotation of the potentiometer  24 ,  24   s.  The potentiometer  24 ,  24   s  acts as a rotational control input device. An angular position of the potentiometer  24 ,  24   s , and thus the knob, is determined by the electronic controller  28  based upon known values representing the relationship between angular position and potentiometer resistance. The angular position is communicated to the power unit  14  via the communication bus  18 . Display information is communicated by the power unit  14  back to the electronic controller  28  via the communication bus  18 . It is contemplated that other variably resistive devices, such as rheostats, or other analog input means can be substituted for the potentiometers  24 ,  24   s  according to the present invention. 
         [0025]    Each electronic controller  28  controls its respective display  26 ,  26   s  based upon the display information received from the power unit  14 . Each power level display  26 ,  26   s  is a two-digit seven-segment light-emitting diode (LED) display for indicating a power level or setting based on a level chosen by the user using the respective potentiometer  24 ,  24   s.  The power level is displayed on the display  26 ,  26   s  as “LO” indicating the lowest setting, “HI” indicating the highest setting, or as a number from 1.0 to 9.0 in predetermined increments, indicating an intermediate setting. A larger number indicates a higher level of power. The power level display  26 ,  26   s  is also used for displaying other messages, as further explained herein, including warning messages and error codes. It is contemplated that other types of digital displays can be substituted for the two-digit LED display  26 ,  26   s,  such as a liquid crystal displays (LCDs), plasma displays, mechanical displays, cathode ray tubes (CRTs), vacuum fluorescent displays (VFDs), discrete LEDs, discrete LEDs arranged in a clock-like fashion, LED bar graphs, and the like. 
         [0026]    The display  26 ,  26   s  is also used in the present embodiment to display a visual indication that the respective heating element  20  has been locked out of operation by displaying “--”. The oven control unit  22  includes a buzzer or other audible warning device to emit an audible warning. Further, using the oven control communication bus  23 , the power unit  14  can instruct the oven control unit  22  to emit an audible warning tone when a user attempts to operate the heating elements  20  that have been locked out. Thus, the power unit  14  can cause an audible tone to be generated without requiring a separate audible warning device to be provided to the power unit  14 . 
         [0027]    In  FIG. 1A , a simple control scheme is illustrated by way of example. The power output to a heating element  20 ′ is controlled by turning a respective potentiometer  24 ′ through its entire or full range of angular rotation. A small segment or range of the angular rotation is used to turn the heating element  20 ′ completely off. The potentiometer  24 ′ is provided with a physical detent, or other tactile indication or the like, to indicate when the “off range” is correctly engaged The term “single potentiometer” is used herein with reference to a potentiometer operating to control a single heating element over the potentiometer&#39;s entire range, such as the potentiometer  24 ′ shown in  FIG. 1A . 
         [0028]    In the embodiment of  FIG. 1 , the user interface units  16 ,  16   s  are provided in pairs consisting of a master unit  16  and a slave unit  16   s.  The potentiometer  24   s  and the display  26   s  of the slave unit  16   s  are connected to the controller  28  of the master unit  16 . The master unit  16  communicates with the power unit  14  for both user interface units  16 ,  16   s  via the communication bus  18 . 
         [0029]    The power unit  14  also delivers pulse width modulated output current to each heating element  20 . The power unit  14  controls current and/or voltage to each heating element  20  to produce the desired output power to power the heating elements  20 . 
         [0030]    The duty cycle of the output current delivered to each heating element  20  is determined by the angular position of a respective one of the potentiometers  24 ,  24   s.  Duty cycle is expressed as a ratio of current on-time to the period (sum of current on-time and off-time). As explained above, the power level provided to each heating element  20  is displayed on the respective power level display  26 ,  26   s.    
         [0031]    In the embodiment of  FIG. 1 , the output power provided to the heating elements  20  is fixed as 240 VAC, which would typically be provided from two-phase utility power. It should be appreciated that maximum output power is equal to the maximum output voltage multiplied by the unmodulated output current. Thus, it is contemplated that the voltage of the output power could also be modulated, in addition to the duty cycle of the current, by the power unit  14  to control the output power. For example switching from 240 VAC to 120 VAC, by utilizing a single phase of the two-phase utility power, could be used to provide additional control, especially for achieving lower power outputs. 
         [0032]    For a single potentiometer, such as in the example of  FIG. 1A , the relationships between angular position, display information and output power are determined according to Table 1, below. The output power is expressed as a percentage of maximum output power, or the duty cycle times 100 percent. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                   
                 Power 
                 Output 
               
               
                 Potentiometer 
                 Potentiometer Angle 
                 Level 
                 (% of max. 
               
             
          
           
               
                 Position 
                 Minimum 
                 Maximum 
                 Display 
                 power) 
               
               
                   
               
               
                  1 
                 330 
                 318 
                 Lo 
                 1 
               
               
                  2 
                 318 
                 306 
                 1.0 
                 2 
               
               
                  3 
                 306 
                 294 
                 1.2 
                 3 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 23 
                  66 
                  54 
                 8.5 
                 90  
               
               
                 24 
                  54 
                  42 
                 9.0 
                 95  
               
               
                 25 
                  42 
                  30 
                 Hi 
                 100  
               
               
                   
               
             
          
         
       
     
         [0033]    Since the power level is controlled electronically, the relationship between the potentiometer angular position and the power output can be non-linear and even non-uniform such that the relationship cannot be expressed as an equation. For example, the power level is incremented in steps of 0.2 from 1.0 to 3.0 and in larger steps of 0.5 from 3.0 to 9.0. This allows more control in the lower heating ranges, which is useful for cooking and keeping food warm. Turning the potentiometer to above 330 degrees and below 30 degrees, in the off range, turns the power completely off. As referred to herein, zero degrees is at a 12 o&#39;clock position on the potentiometer and succeeding degrees are measured in a clockwise fashion. 
         [0034]    Alternatively, as embodied in the various alternative control schemes of  FIGS. 3-8 , one potentiometer can be used to control two or more power outputs, and thus two or more heating elements. A potentiometer being used in this way is referred to herein as a “dual potentiometer.” According to this alternative embodiment of the present invention, one portion of the total angular rotation of a dual potentiometer controls power to a first element and the other portion of the angular rotation controls power to both the first element and a second element. Table 2, below, illustrates the operation of a dual potentiometer according to this alternative control scheme. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Dual Potentiometer Angle from 0° 
                 Power 
                 Output 
               
             
          
           
               
                 Potentiometer 
                 Left Side 
                 Right Side 
                 Level 
                 (% of max. 
               
             
          
           
               
                 Position 
                 Minimum 
                 Maximum 
                 Minimum 
                 Maximum 
                 Display 
                 power) 
               
               
                   
               
             
          
           
               
                  1 
                 196 
                 190 
                 170 
                 164 
                 Lo 
                 1 
               
               
                  2 
                 201 
                 196 
                 164 
                 159 
                 1.0 
                 2 
               
               
                  3 
                 207 
                 201 
                 159 
                 153 
                 1.2 
                 3 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
                 . 
               
               
                 23 
                 319 
                 313 
                 47 
                 41 
                 8.5 
                 90 
               
               
                 24 
                 324 
                 319 
                 41 
                 36 
                 9.0 
                 95 
               
               
                 25 
                 330 
                 324 
                 36 
                 30 
                 Hi 
                 100 
               
               
                   
               
             
          
         
       
     
         [0035]    The specific numbers or values shown in Tables 1 and 2 are given by way of example and can be modified as appropriate to meet the needs of a particular application. 
         [0036]      FIG. 2  is a plot of potentiometer position versus duty cycle (in percent of maximum power) as embodied by the control schemes of Tables 1 and 2 above. As set forth in Tables 1 and 2, each “potentiometer position” relates to an angular range of potentiometer rotation. Thus, although the potentiometer rotates smoothly throughout its range, the duty cycle is controlled in discrete steps corresponding to the specific ranges of potentiometer rotation set forth in Tables 1 and 2. The minimum duty cycle of the present embodiment is 1%, as shown in  FIG. 2 . 
         [0037]      FIG. 3  shows another embodiment in which a dual potentiometer  124  is arranged to control a dual heating element  120 , having concentrically arranged inner heating element  120   b  and outer heating element  120   a.  The left portion  124 L of the angular rotation of the dual potentiometer  124 , from 190 to 330 degrees, controls power to the inner heating element  120   b  only, and the right portion  124 R of the angular rotation of the dual potentiometer  124 , from 170 to 30 degrees, controls both heating elements  120   a,    120   b  simultaneously. 
         [0038]      FIG. 4  shows another embodiment using a dual potentiometer  224   a  to control a single heating element  220   a  and a separate bridge heating element  220   b.  The bridge heating element  220   b  provides heating between the single heating element  220   a  and a second heating element  220   c  spaced apart from the single element  220   a.  The dual potentiometer  224   a  operates similarly to the dual potentiometer  124   a  of the embodiment of  FIG. 3 . Specifically, the left portion  224   a L of the angular rotation of the dual potentiometer  224   a  controls power to the single heating element  220   a  only, and the right portion  224   a R of the angular rotation of the dual potentiometer  224   a,  controls both the single heating element  220   a  and the bridge element  220   b  simultaneously. Power to the second single heating element  220   c  is controlled by a single potentiometer  224   b.    
         [0039]      FIG. 5  shows an embodiment using two potentiometers  324   a,    324   b  to control three heating elements: two single heating elements  320   a,    320   c  and a bridge heating element  320   b . The first potentiometer  324   a  controls the first single heating element  320   a  around its entire angular rotation  324   a   1 . The second potentiometer  324   b  is a “modified single potentiometer,” wherein  324   b  controls the second single heating element  320   c  over most of its angular rotation  324   b M, except that a small range  324   b B of the angular rotation is used to enable bridge control. A physical detent, or the like, indicates that the second potentiometer  324   b  is set on the bridge control range  324   b B. When bridge control is enabled by the second potentiometer  324   b,  the first potentiometer  324   a  simultaneously controls all three heating elements  320   a - c  over its entire angular rotation  324   a   2 . This allows all three heating elements  320   a - c  to be easily and accurately set to the same power level. 
         [0040]      FIG. 6  shows an embodiment which uses principles from both the embodiment of  FIG. 4  and the embodiment of  FIG. 5 . Like the embodiment of  FIG. 5 , a second potentiometer  424   b , being a modified single potentiometer, controls only a second single heating element  420   c  over most of its angular rotation  424   b M and places the first potentiometer  424   a  in bridge control mode at a bridge control range  424   b B. The first potentiometer  424   a  of  FIG. 6  is a dual potentiometer and operates much like the first potentiometer  224   a  of  FIG. 4 , controlling the first heating element  420   a  over the left portion of rotation  424   a L 1  and controlling both the first heating element  420   a  and the bridge heating element  420   b  over the right portion  424   a R 1  of angular rotation. When the first potentiometer  424   a  of  FIG. 6  is placed in bridge mode by the second potentiometer  424   b,  the first potentiometer  424   a  controls all three heating elements  420   a - c  over either portion  424   a L 2 ,  424   a R 2  of its angular rotation. 
         [0041]      FIG. 7  is a variation on the embodiment of  FIG. 6 . The first potentiometer  524   a  normally acts as a dual potentiometer, independently controlling the first heating element  520   a  over its left portion  524   a L and controlling both the bridge element  520   b  and the first heating element  520   a  over its right portion  524   a R. When bridge control is enabled, the first potentiometer  524   a  acts as a single potentiometer. That is, when the second potentiometer  524   b,  being a modified single potentiometer, is placed in its bridge range  524   b B, the first potentiometer  524   a  controls all three heating elements  520   a - c  over its entire range  524   a E of angular rotation. This provides more precise control of power than the scheme of  FIG. 6 . 
         [0042]      FIG. 8  is an additional embodiment for controlling two single heating elements  620   a,    620   c  and a bridge heating element  620   b.  First and second potentiometers  624   a,    624   b  are both dual potentiometers. The first potentiometer  624   a  controls the first single heating element  620   a  over the left portion  624   a L of its angular rotation and controls both the first single heating element  620   a  and the bridge heating element  620   b  simultaneously over the right portion  624   a R of its angular rotation. The second potentiometer  624   b  controls the second single heating element  620   c  over the right portion  624   b R of its angular rotation and controls all three heating elements  620   a - c  simultaneously over the left portion  624   b L of its angular rotation. When the second potentiometer  624   b  is controlling all three heating elements  620   a - c , the first potentiometer  624   a  is disabled from controlling any of the heating elements  620   a - c.    
         [0043]    Referring again to  FIG. 1 , thermal limiters  30  are provided to prevent the heating elements  20  from overheating and potentially causing damage, such as when the heating elements  20  are covered by a flat glass cooking surface. Each limiter  30  comprises two bimetallic thermostatic switches or limiter elements: a high temperature switch and a low temperature switch. 
         [0044]    The high temperature switch in each limiter  30  is connected directly to a corresponding heating element  20 . The high temperature switch opens at temperatures above t hi , such as 500 degrees Celsius, thus disconnecting power from the heating element  20 . Once the heating element  20  cools below t hi , the high temperature switch closes, reconnecting power to the heating element  20 . It is contemplated that the high temperature switch could be connected in a different manner, for example by being connected via the controller of the power unit  14  rather than directly to the heating element  20 . 
         [0045]    The low temperature switch in each limiter  30  is connected to the power unit  14 . The low temperature switch opens when the temperature falls below t lo , such as 50 or 70 degrees Celsius. When the low temperature switch is closed, the power unit  14  causes a heat warning to be displayed on the seven-segment power level display  26 ,  26   s,  such as “HE” for element, “HS” for hot surface, “HC” for hot cooktop, or other appropriate display, indicating that the cooking surface at the respective heating element  20  is too hot to touch. Alternatively, a warning lamp or indicator could be used to display the heat warning. 
         [0046]    As a further alternative, the low temperature switch or limiter element can be replaced by a timing mechanism which causes the heat warning to be displayed for a predetermined period of time, after which the respective heating element  20  should have predictably fallen below t lo . The timing mechanism can be implemented by the electronic controller of the power unit  14 , or by some other known means. Nonvolatile memory, such as an EEPROM, can be provided to the power unit  14  to retain timing information in the event of a power failure. 
         [0047]      FIG. 9  illustrates a communication and power connection arrangement according to an embodiment of the present invention including a power board  714  and two master user interface units  716 L,  716 R. Communication between the master user interface units  716 L,  716 R and the power board  714  is accomplished by a one wire serial communication bus or wire  718  provided in a wiring harness  730 . In addition to the communication wire  718 , the 5-wire harness  730  also includes +12 VDC, ground, +5 VDC, and an identification wire. With the exception of the identification wire, each of the 5 wires is connected from the power unit  714  to each of the master user interface units  716 L,  716 R. 
         [0048]    The identification wire  732  carries a +5V identification signal from the power unit  714  to the right master user interface unit  716 R, telling the unit  716 R that its position is “right.” Since there is no connection between the identification wire  732  and the left master user interface unit  716 L, the unit  716 L will not receive the identification signal, causing the unit  716 L to identify its position as “left.” It should be appreciated that the “right” and “left” positions can be transposed without departing from the present invention. 
         [0049]    Potentiometer angle information from a master interface unit  716 L,  716 R or a slave user interface unit  716 LS,  716 RS is digitally encoded by the microprocessor in the respective master user interface unit  716 R,  716 S and sent to the power unit  714  via the communication bus  718 , similarly to that described above with reference to  FIG. 1 . Likewise, digital display information is sent from the power unit  714  to the user interface units  716 L,  716 R via the communication bus  718 . An identification code is included in each communication to identify the sender or recipient user interface unit as the left master unit  716 L, the left slave unit  716 LS, the right master unit  716 R, the right slave unit  716 RS. The identification code also indicates whether the corresponding potentiometer is being used as a single or dual potentiometer, whereby the power board  714  controls the user interface unit  716  and its corresponding heating element according to the appropriate set of data, as exemplified in Tables 1 and 2. 
         [0050]    A 3-bit identification code is shown in the following table: 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Left/ 
                 Master/ 
                 Single/ 
               
               
                   
                 Right 
                 Slave 
                 Dual 
               
               
                   
                 Pair 
                 Unit 
                 Element 
               
               
                 Description 
                 (b 2 ) 
                 (b 1 ) 
                 (b 0 ) 
               
               
                   
               
             
             
               
                 Left pair, Master unit, Single element 
                 0 
                 0 
                 0 
               
               
                 Left pair, Master unit, Dual element 
                 0 
                 0 
                 1 
               
               
                 Left pair, Slave unit, Single element 
                 0 
                 1 
                 0 
               
               
                 Left pair, Slave unit, Dual element 
                 0 
                 1 
                 1 
               
               
                 Right pair, Master unit, Single element 
                 1 
                 0 
                 0 
               
               
                 Right pair, Master unit, Dual element 
                 1 
                 0 
                 1 
               
               
                 Right pair, Slave unit, Single element 
                 1 
                 1 
                 0 
               
               
                 Right pair, Slave unit, Dual element 
                 1 
                 1 
                 1 
               
               
                   
               
             
          
         
       
     
         [0051]    The remaining wires in the wiring harness  730  are used for providing operating voltages to the user interface units  716 L,  716 LS,  716 R,  716 RS. 
         [0052]    It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.