Abstract:
The present invention relates to an electrostatic capacity type encoder which includes a variable capacitance mechanism made on two dimensional planes, random time varying excitation generation system, interlaced fine &amp; coarse cycles and digital signal processing unit. A variable capacitance mechanism which includes stationary and rotating disks facing each other and forming one or more variable capacitors. The transmit electrodes are split to two transmit phases and the receive electrodes to two receive phases, both can be implemented on two dimensional printed circuit boards without third dimension via interconnects. The combination of the 2 by 2 grid provides the four combinations required to detect both position and direction. Another configuration is made of electrostatic capacity type encoder which includes a first and second stationary disks and in between a third rotating disk, made either of a dielectric material with varying geometry or thickness or from a conductive patterns printed on a PCB. A time varying excitation system with no fundamental frequency is described to improve the robustness to external noise and omit the necessacity of excessive shielding. Interlacing mechanism is explained to put several different encoders channels on the same perimeter, minimizing encoder mechanical dimensions. To complete the sensor a digital signal processing unit is presented, where the capacitance value is converted to conventional encoder formats.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an electrostatic capacity type encoder, and more particularly to an improvement of the electrostatic capacity type encoder which electrically detects relative rotary displacement between rotary disks, or linear displacement between two components.  
       PRIOR ART  
       [0002]     In the prior art exists several measuring apparatus using an encoder which converts the displacement of a rotor moving along three distinctive transmitter plates referred as R, S, T or four distinctive transmitter plates referred as F1, F2, F3, F4. The implementation of three or more distinctive plates required three dimensional PCB design with one via interconnection per plate as in U.S. Pat. No. 2,534,505 and U.S. Pat. No. 4,788,546. As capacitive signal is highly prune to external interference, external shielding was suggested in U.S. Pat. No. 5,099,386. In U.S. Pat. No. 6,492,911 a labyrinth was added to further block interference from entering the sensor through the shaft. A cost effective noise rejecting mechanism using differential signals and differential amplifier was introduced in U.S. Pat. No. 3,93,8113, automatic gain control using a second reference channel was introduced in U.S. Pat. No. 4,864,295 to avoid the signal level degradation due to distance changes. In dialectic based transmittive rotors, where the rotor changes the dialectic constant when rotating, a two dimensional rotor structure was used, in U.S. Pat. No. 6,788,220 two such structures were placed one inside the other, making the plastic molding complicated and the rotor teeth sensitive to shock and vibration. In most cases, more then one channel is needed to accurately find an absolute position, many patents have dealt with this issue, in U.S. Pat. No. 3,222,668 a binary tree of channels were placed one inside the other, in U.S. Pat. No. 4,851,835 a sine shaped fine and coarse channels are placed one inside the other, requiring large radial space and in U.S. Pat. No. 3,238,523 the channels are placed side by side requiring large width, U.S. Pat. No. 6,788,220 used a similar fine &amp; coarse principle. Four phase system require an excitation made of two sine or square wave signals with  90  degrees phase shift, in U.S. Pat. No. 2,461,832 an RC filter made this delay, in U.S. Pat. No. 4,238,781 four phase logic was used to generate the signal, in all the cases, this signal frequency is constant, and hence sensitive to noise on its fundamental frequency.  
       SUMMARY OF THE INVENTION  
       [0003]     Capacitive Sensors, as described in previous patents, are complicated to manufacture and sensitive to electrical noise. The proposed invention simplifies the complexity of the system to a two dimensional layout, compacts the design by the use of interlaced channels and improve the noise sensitivity by digitally encrypting the signals. The invention present 5 new subjects: 
    (1) The use of two dimensional (2D) transmitter and receiver plates, each with two sets of capacitor plates, where the 2 by 2 combination provide four distinctive states.     (2) The random generation excitation, were the signal spectra is flat, and not sensitive to fundamental frequencies like motors PWM noise.     (3) The interleave of several channels in the same radial area.     (4) The use of three dimensional (3D) rotor, where the structure is stiffer and the thickness can be twice modulated.     (5) The use of cross correlation geometry for an index channel, with reference to another channel.    
 
         [0009]     Accordingly, it is an object of the present invention to provide an electrostatic capacity type encoder which enables the device to be designed to be smaller, simpler and cheaper to produce, less sensitive to vibration and shock, and less sensitive to electrical noise. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which:  
         [0011]      FIG. 1  illustrates the main components of a single channel rotary encoder made according to the present invention.  
         [0012]      FIG. 2  illustrates the capacitive elements of a linear encoder in further details.  
         [0013]      FIGS. 3   a - 3   b  illustrates the side view and the signal waveforms of the encoder of  FIG. 2 , fitted with a square wave modulated rotor in one primary position.  
         [0014]      FIGS. 4   a - 4   c  illustrates the side view and the signal waveforms of the encoder of  FIG. 2  where the square wave modulated rotor is in another primary positions. Also illustrated is a side view of a sensor with a sine wave modulated rotor.  
         [0015]      FIGS. 5   a - 5   b  illustrates top and side view of a fine/coarse rotary encoder with two receive channels.  
         [0016]      FIGS. 6   a - 6   b  illustrates a fine/coarse linear encoder, with options of one or two receive channels.  
         [0017]      FIG. 7  illustrates the receiver section of a three channels enhancement to the encoder presented in  FIG. 6 .  
         [0018]      FIG. 8  illustrates an interlaced, space saving, alternative to the two channel encoder presented in  FIG. 6  with two exemplary rotor options.  
         [0019]      FIG. 9  illustrates a rotary implementation of interlaced encoder presented in  FIG. 8 .  
         [0020]      FIGS. 10   a - 10   d  illustrates the special embodiment where the second channel is used to detect a single index position, and describe the principle of the “key” autocorrelation matching mechanism.  
         [0021]      FIG. 11  illustrates an interlaced, space saving, alternative to  FIG. 10 .  
         [0022]      FIGS. 12   a - 12   b  illustrates the principle of a linear reflective sensor that requires only two components.  
         [0023]      FIG. 13  illustrates a reflective rotary sensor.  
         [0024]      FIGS. 14   a - 14   b  illustrates one embodiment of a random frequency excitation generator and the related waveforms.  
         [0025]      FIG. 15  illustrates one embodiment of the signal processing units used in the encoder of  FIG. 1 .  
         [0026]      FIG. 16  illustrates one embodiment of the Digital Demodulator Unit core used in the signal processing unit of  FIG. 15 .  
         [0027]      FIGS. 17   a - 17   b  illustrates one embodiment and logic description of a Post Angle Processor unit used in the signal processing unit of  FIG. 15 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     In the following description, for purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without the specific details. In other instances, well known features are omitted or simplified in order not to obscure the present invention.  
         [0029]      FIG. 1  illustrates the main components of the present invention single channel rotary encoder; where an excitation generator  200  generate two perpendicular square signals  201 ,  202  to two sets of metal transmit plates  5  printed on a single plane of a Printed Circuit Board [PCB]. Those static transmitter  1  plates, together with a rotating rotor  2  made of dielectric material, and two sets of static receiving plates  9  imprinted on the receiver  3 , form the four different variable capacitors, needed to electrically detect the relative position of the rotor and the stators. The differential signal received by the receiver plates  9 , is amplified by a differential amplifier  251 , and further amplified by externally gain controlled amplifier  260  that provides constant signal levels. Signal Processor unit  250  demodulates the encoder signal  270 . The use of a square shaped rotor  2  generate a trapezoid signal  287  at the outputs of the Signal Processor  250 , an alternative use of a sine shaped rotor  4  will result with sine and cosine  274 ,  275  signals as known in the art and described in previous patents. For clarification, every electrical cycle made of a combination of four transmitting plates  13 ( a ), two receiving plates  13 ( b ) and one cycle of the rotor pattern  13 ( c ) is enclosed in dashed section.  
         [0030]      FIG. 2  illustrates the capacitive elements of a linear encoder in further details: the transmitter  1  is made of a printed circuit board where on the isolated substrate  16  two sets  5 , 6  of interlaced capacitor plates are fabricated, the metal patterns are made using lithographic techniques known in the art. The transmitter plates are divided into two groups, the “main” set of identical plates  5 , 7  connected with interconnect lines  18  and driven by one phase  202  of the excitation generator  200  and the “shifted” set  6 , 8  driven by the orthogonal excitation signal  201  through another interconnect line  18 . The receiver  3  is made of a similar printed circuit board where on the isolated substrate  17  two sets  9 ,  10  of interlaced capacitor plates are fabricated, those sets are connected with interconnect lines  19  and drive a differential receiver amplifier  251 . The combination of two transmit and two receive plates creates a matrix of four dependent variable capacitors, used to electrically detect the relative position of the rotor and the stator, this matrix is farther explained later. The pattern printed on the receiver and transmitter is made of two non intersecting areas that can be fabricated using planar two dimensional techniques, without the need of a third dimension “via” interconnect between different layers of the PCB. The square shaped profile rotor  2  is made of any dielectric material known in the art like polycarbonate, the thickness of the capacitor is amplitude modulated by the pattern. Moving the rotor along the transmitter and receiver results in a trapezoid shaped signal. An optional rotor  4  where the thickness is sine modulated, will generate sine and cosine outputs. For clarification, enclosed in dashed section  13 , are a set of four adjacent transmitter plates  5 , 6 , 7 , 8  on the transmitter  1  that interacts with one set of two adjacent receiver plates  9 , 10  on the receiver  2  and one set of “thick” part  11  and “thin” part  12  on the rotor  2 .  
         [0031]      FIG. 3   a  illustrates one electrical cycle  13  cross section view of the encoder from  FIG. 2  made of transmitting plates  5 , 6 , 7 , 8  driven by excitation generator  200 , mounted on isolated transmitter substrate  16  facing a square wave modulated rotor  2  “thick”  11  and “thin”  12  sections. Facing the rotor are the receiver plates  9 ,  10  mounted on receiver isolation substrate  17 , and driving the differential amplifier  251 . The capacitance matrix is made of four distinct combinations of receiver and transmitter plates that resemble the four combinations of 0, 90, 180 and 270 electrical degrees: 
        (1) Combination of “main” transmitter plate  5  to “positive” receiver plate  9  form the first pair which is equivalent to 0 electrical degree,     (2) The inverse of this combination which is equivalent to 180 degrees, made by the pair “main” transmitter plate  7  and “negative” receiver plate  10 .     (3) The “shifted” transmitter plate  6  to “positive” receiver plate  9  pair, equivalent to 90 degrees.     (4) The inverse of this combination which is equivalent to 270 degrees, made by the pair “shifted” transmitter plate  8  and “negative” receiver plate  10 .          
         [0036]     In this drawing the thick rotor part  11  is symmetrically facing the pair “shifted” transmitter plate  6  to “positive” receiver plate  9 , this capacitor pair has larger capacitance compared to the opposite pair “shifted” transmitter plate  8  to “positive” receiver plate  10 , the capacitances of the two other pairs are equal and the sum of them after the differential amplifier is zero. The waveform illustration of  FIG. 3   b , contains the two incoming excitation signals  201   202 , the positive input signal  20  to the differential amplifier  251  that have a high positive peak  22 , and the negative input signal  21  to the differential amplifier that have a lower positive peak  23 , the output signal  24  of the differential amplifier has a positive peak  25 .  
         [0037]      FIG. 4   a  illustrates the same encoder, in the mechanical position where the thick rotor part  11  is symmetrically facing the pair “shifted” transmitter plate  8  to “negative” receiver plate  10 , this capacitor pair has larger capacitance compared to the opposite pair “shifted” transmitter plate  6  to “negative” receiver plate  9 . As in  FIG. 3   a , the capacitances of the two other pairs are equal and the sum of them after the differential amplifier is zero. Shown in the waveform illustration of  FIG. 4   b , are the two incoming excitation signals  201   202 , same as in  FIG. 3   b , the differential amplifier positive input signal  20  which has a low positive peak  23 , and the differential amplifier negative input signal  21  which has a high positive peak  22 . The output signal of the differential amplifier  24  has a negative peak  26 .  
         [0038]      FIG. 4   c  illustrates a side view of the encoder from  FIG. 2 , using sine shaped rotor  4 , with the same encoder transmitter  1  and receiver  3  patterns as in  FIG. 4   a.    
         [0000]     Rotary &amp; Fine/Coarse Implementation  
         [0039]      FIG. 5   a  illustrates a side view of a fine/coarse rotary encoder where box  26  holds the transmitter  1  and receiver  3  firmly in place. The rotor  2  is connected to shaft  28 , on the radial axis of the encoder, the fine channel  34  thick parts are placed on the inner ring and the coarse channel  35  thick  11  and thin  12  parts are placed on the outer ring.  
         [0040]      FIG. 5   b  illustrates the layout of the transmitter  1 , receiver  3  and rotor  2  of a fine/coarse rotary encoder with two receive channels. The transmitter  1 , has one electrical cycle outer coarse ring  31 , made of four excitation plates  5 , 6 , 7 , 8  connected as two pairs and an inner fine ring  32  with eight electrical cycles made of thirty two excitation plates connected to two interconnect rings  18 . Both fine and coarse rings are driven simultaneously from the same excitation generator  201 . For clarification one set out of the eight fine electrical poles is enclosed in dashed section  13 ( a ), The rotor  2  has two sets of thick dielectric teeth, a) A single half circle long tooth  35  for the coarse channel. b) Eight small 1/16 of circle long teeth  34  for the fine channel. All thick parts are placed on the rotor thin homogenous substrate  33  that provide the strength of the rotor, which is firmly connected to shaft  28 . For clarification, a set of a single fine electrical pole is enclosed in the dashed section  13 ( c ). The receiver  3  has two sets of receiving plates: A pair of coarse plates  37 , connected to differential amplifier  252  and eight pairs of fine receive plates  38  connected to differential amplifier  251 . For clarification, a receive set of a single fine electrical pole is enclosed in the dashed section  13 ( b ). Small clearances  39  in the transmitter  31  and receiver  37  plates permit the wiring to be kept in a two dimensional structure.  
         [0041]      FIG. 6   a  illustrates a linear fine and coarse sensor made of transmitter  40 , rotor  41  and receiver  42 . The transmitter  40  comprises of two sets of thin transmit plates of the fine channel  43 , and two symmetrical pairs of larger transmit plate sets that make the coarse channels  44 , both channels are driven by the same two clock phases of the excitation generator  200 . Interconnect wires  50  connect together each of the excitation sets, while maintaining the two dimensional structure. The rotor  41  has two sets of thick  11  and thin  12  parts, one fine set for the fine channel  45  and two symmetrical thick parts  46  for the coarse channel. The receiver  42 , is made of two pairs of receiving elements for the fine channel  48  mating the fine channels of the rotor  45  and transmitter  43  and driving a differential amplifier  251 . Two pairs of symmetrical coarse channel receiving plates  49  mating the rotor channel  46  and transmitter plates  44  are driving a second differential amplifier  252 . Receiver interconnect wires  55  are used to connect together the two sets of coarse receiving plates  49  using a two dimensional layout.  
         [0042]      FIG. 6   b  illustrates another embodiment of the fine/coarse encoder, where the transmitter  40  is using the same fine  43  and coarse  44  elements as  FIG. 6   a , however they have a different interconnect scheme  54 , where the fine transmit plates are driven from one oscillators  200 , and the coarse transmit plates  44  are driven from a second oscillators  203  orthogonal to the first one  200 . One way of making orthogonal clocks is having one clock frequency double then the other, as known in the art. The rotor  41  is the same as in  FIG. 6   a . The receiver is made of the same fine  48  and coarse  49  elements as  FIG. 6   a ; however they have a different interconnect scheme  51 , where both the fine  48  and coarse  49  plate pairs drive the same differential amplifier  251 . This modification requires driving more signals to the transmitter plate and the dynamic range is smaller, however it requires only one input channel and might be cost effective in some cases.  
         [0043]     When the coarse channel resolution is not enough to distinguish between two fine cycles, a second coarse channel is added. The receiver section of a three speed unit is illustrated in  FIG. 7 : made of a fine channel  60 , a symmetric pair of coarse 1   61  channels and a symmetric pair of coarse 2   62  channels driving three differential amplifiers  251 ,  252 ,  253  respectively. In this example there are eight fine segments  60  per one coarse 1  cycle  61 , and the fine position in one coarse cycle is found by simple divide mathematics as known in the art. To distinguish between the different coarse 1  cycles  61  a second coarse 2   62  with a slightly different pitch is used. For example: in  FIG. 7  there is a ratio of seven coarse 1   61  cycles to eight coarse 2   62  cycles. Using the principle of a Vernier caliper, as known in the art, the exact coarse cycle can be found. The receiver plates are interconnected together with interconnection wires  63  on a single two dimensional plane, the rotor and transmitter are based on the same principle as the two speed sensor of  FIG. 6   a  and need not be further explained.  
         [0000]     Several Channels of the Same Perimeter  
         [0044]     Putting several speed tracks side by side as in  FIG. 6  and  FIG. 7  requires a large space,  FIG. 8  illustrates an interlaced, space saving, alternative to the two channel encoder presented in  FIG. 6 . This encoder is made of a transmitter  70 ; receiver  76  and one of two rotor options  73  or  87 , the two different channels coarse and fine are interlaced one within the other within the same width. Half of the transmitter  70  area is made of fine transmit plates  71  and the other half is made of the coarse transmit plates  72 , both are driven by the same excitation generator  200 . The receiver structure  76  is similar to the transmitter, with coarse channel  78  plates connected to differential amplifier  252 . The fine channel plates  77  are connected together with interconnect wires  79  that are interlaced between the coarse plates in order to achieve a two dimensional plane layout. The fine channel plates are driving differential amplifier  251 . To match the interconnect wires  88  in the receiver, two more pairs of fine transmit plates  80  are placed on the transmitter  70 . The rotor  73  in this example is split to fine segments  85  and coarse segments  86 . The principle of operation is: The pitch of each of the rotor sections fine  85  and coarse  86 , is twice that of the transmitter and receiver section fine  71 , 77  and coarse  72 , 78 . Creating four possible combinations of interactions: 
        (1) Case marked by shaded area  81 : coarse rotor segment  86  facing coarse transmit  72  and receive  78  sections, resulting in a signal proportional to the coarse channel relative position.     (2) Case marked by shaded area  82 : fine rotor segment  85  facing fine transmit  71  and receive  77  sections, resulting in a signal proportional to the fine channel relative position.     (3) Case marked by shaded area  83 : fine rotor segment  85  facing coarse transmit  72  and receive  78  sections, resulting zero integral of orthogonal channels.     (4) Case marked by shaded area  84 : coarse rotor segment  86  facing fine transmit  71  and receive  77  sections, resulting zero integral of orthogonal channels.        
 
         [0049]     Also presented in  FIG. 8  is another, more energy efficient, option of a triple thickness level interlaced rotor  87 , using the same transmitter  70  and receiver  76 . The rotor is made of fine segment lines thickness modulated by the coarse waveform, where half of the fine lines are thinner in the coarse “thin” area  88  and thicker in the coarse “thick” area  89 . The principle of operation is: As the pitch of each of the coarse modulation of the rotor is equal to the transmitter and receiver sections fine  71 , 77  and coarse  72 , 78 , there are only two combinations of interactions: 
        (1) Case marked by shaded area  82 : four fine rotor segment  89  and four half height fine rotor segment  88  facing fine transmit  71  and receive  77  sections, resulting with a signal proportional to the fine channel location.     (2) Case marked by shaded area  83 : a course cycle made of four fine thick rotor segment  89  and four thin (half height) fine rotor segment  88  facing coarse Transmit  72  and receive  78  sections, resulting with a signal proportional to the coarse channel location.        
 
         [0052]      FIG. 9  illustrates a rotary implementation of the interlaced encoder presented in  FIG. 8 , made of a transmitter  90 , receiver  93  and rotor  96 . Half of the transmitter board  90  perimeter is made of fine  91  segments and the other made of coarse segments  92 , both driven by the same excitation generator  200 . Half of the receiver  93  perimeter area is made of the coarse  98  segments driving differential amplifiers  252 , and the other half by the fine segments  97  driving differential amplifier  251 . Interconnect wires  99  keep the design two dimensional. The rotor  96 , is made of fine thick segments  95  where half of them  94  are missing in the area defined as “thin” by the coarse cycle, in this example there are only two capacitance levels. Note that two fine segments  100  are only half fine period length, complementing each other, making the coarse cycle exactly one half of a circle. The principle of operation is similar to the one described on linear rotor  87  of  FIG. 8 .  
         [0000]     Single Index Autocorrelation Generating Mechanism  
         [0053]     Incremental encoders require an index channel that generates an index mark only once per full rotation. As a fine channel with N poles can provide N such index marks in one full rotation, a second index channel is required to single out one position from others.  FIG. 10   a  illustrates an enhancement of the rotary encoder of  FIG. 5 , made of transmitter  101 , receiver  107  and rotor  104 , where the coarse channel of  FIG. 5  is replaced with a set of plates (five in this example) placed unevenly around the perimeter of the transmitter, receiver and rotor. The transmitter  101  have additional index channel transmit plates  102  placed externally to the fine transmitter plates  103 , driven by one of the excitation generator  200  phases. On the rotor  104 , some of the fine teeth  105 , are made longer  106  to form the index marking. In the receiver  107 , the fine channel plates  108  drive the differential amplifier  251  as in  FIG. 5 . An additional non differential index channel  109  is added on the perimeter driving the positive input of differential amplifier  252 . This amplifier negative input is driven through a voltage divider  110  by one of the input signals from the fine channel. The divider ratio is calculated to be less then the ratio of index teeth total area to the fine teeth total area, in this example five to sixteen. To further explain the “key” principle,  FIG. 10   b  illustrate the case where all of the index teeth  106  in the rotor, the transmitter  102  and receiver  109  are aligned, forming five capacitors and high signal strength.  FIG. 10   c  illustrates one of the cases where they are not aligned, forming only two capacitors and lower signal strength. To achieve aligning only in one position over the rotation, the teeth are arranged with an increasing prime distance between them to both directions. In this example: two and three spaces,  FIG. 10   d  presents an alternative negative logic rotor  111 , where the signal is minimal when the rotor and stators are aligned.  
         [0054]      FIG. 11  illustrates an interlaced space optimization of the design presented in  FIG. 10 , similar to the interlacing principle presented in  FIG. 9  where the two channels are interlaced on the same radial space. The symmetry of the rotor  119  is broken by removing  121  some of the fine segments  120 , in the receiver  115  a third channel  118  is added and interlaced in the same perimeter with the two existing fine sets of plates  116  &amp;  117 , the enlargement view  114  illustrate the index receive plate  118  replacing a fine pair made of one positive fine receiver plate  123 , and two halves of two negative plates  122 . Interconnect wire  125  maintain the connectivity of the negative receiver channel  117 ; The fine channel plates  116 ,  117  drive differential amplifier  251 . Differential amplifier  252  is driven by the non differential index channel and by one of the fine channels through voltage divider  110 , in a similar configuration to  FIG. 10 . The transmitter  126  has the trivial set of one transmit channel pairs.  
         [0000]     Reflective Capacitor Structure  
         [0055]      FIG. 12   a  illustrates the principle of a linear reflective sensor that requires only two components: transceiver  150  and rotor  158 . The transceiver  150  is made of two transmitting pairs of plates  152  and  153  driven by oscillator  200  and two symmetrical sets of receiver plate pairs: ( 154 ,  155 ) and ( 156 ,  157 ) connected with interconnect wires  160  on a two dimensional plane to differential amplifier  251 . Half of the rotor area is covered with conductive segments  159  that according to the position, couple the transmit plates to the positive  155 ,  156  set of receive plates, to the negative  154 ,  157  set of receive plates or to a combination of both. This square shaped rotor generates a trapezoid wave vs. position. An alternative sine shaped rotor  161  can be used to achieve sine and cosine outputs.  
         [0056]      FIG. 12   b  illustrates a side view of the sensor; where: 
        (1) Case marked by shaded area  162 : Two sections of the rotor conductive segments  159  are facing the “main”  152  transmit plate and the positive receive plate  155 .     (2) Case marked by shaded area  163 : One section of the rotor conductive segments  159  face the combination of the “shifted”  153  transmit plate and positive receive plate  155 .     (3) Case marked by shaded area  164 : One section of the rotor conductive segments  159  face the combination of the “shifted”  153  transmit plate and negative receive plate  154 . 
 
 The sum of the last two signals  163   164  is zero. To prevent cross talk between the two transmit channels  152 ,  153 . The rotor conductive segments  159  are made as thin stripes, so every stripe faces only one of the transmit plates. 
         
         [0060]      FIG. 13  illustrates a reflective rotary sensor that requires only two parts: transceiver  170  and rotor  175 . The transceiver  170 , is made of two sets of transmitting plate pairs  172  driven by excitation generator  200  and two sets of positive and negative receiver plates  173  driving a differential amplifier  251 , On the rotor  175  there are reflective segments  176 , that creates a capacitance between the transmit and receive plates. For clarification, enclosed in dashed section  13 ( a ) are a set of two pairs of transmit plates  172  and one pair of receive plates  173  on the transceiver  170  that interacts with one segment  176  of the rotor  175  enclosed in dashed section  13 ( b ).  
         [0000]     Random Waveform Excitation Mechanism  
         [0061]     Previous designs had a constant excitation frequency that was sensitive to external electric noise harmonics at this frequency;  FIG. 14   a  illustrates one embodiment of the random frequency excitation generator  201 , creating two orthogonal clock signals “main”  201  and “shifted”  202 , used to drive the transmitter excitation plates. The random frequency excitation generator is made of a random number generator  205  digitally configuring a clock generator  207 , a two bit gray counter  210 , and a switch  215  that can swap between the two outputs. After every set of four clocks, counted by frequency divider  208 , the random clock generator  205  “shuffle” and generate a new random clock length word  206  to the configurable clock generator  207 . The two bit gray counter  210  generates two orthogonal clock signals out of every four clock  209  cycles using two D-FF as known in the art. Another random bit  216  generated by the random clock generator  205  randomly commands the switch  215  to alternate between the two channels so half the times the “main”  201  leads the “shifted”  202  (case  213  of  FIG. 14   b ) and on the other half, the “shifted”  202  leads the “main”  201  (case  214  of  FIG. 14   b ).  
         [0062]      FIG. 14   b  illustrates three examples of the waveforms generated by the random frequency excitation generator of  FIG. 14   a :           (1) On the first cycle  212  the clock length word  206  is 0×15 and polarity bit  216  is reset, four relatively slow phases are generated and the “main”  201  leads the “shifted”  202  clock.     (2) After a shuffle  211  tick, on the second cycle  213 , the clock length word  106  is 0×03 and polarity bit  216  is reset. Four relatively fast phases are generated and the “main”  201  leads the “shifted”  202  clock.     (3) After another shuffle  211  tick, a third cycle  214  with clock length word  106  set again to 0×03 but with polarity bit  216  set, four similar relatively fast phases are generated with the “shifted”  202  leads the “main”  201  clock. 
 
 Signal Processing 
         
         [0066]      FIG. 15  illustrates one embodiment of the signal processing units used in the encoder of  FIG. 1 : where the excitation generator  200  is driving the transmitter fine  1  and coarse  31  capacitor plates, forming two variable capacitors a) Fine: using either a square rotor  2  or a sine shaped rotor  4 , and receiver plates  3 . b) Coarse using rotor  33  and receiver plates  36 . The fine receiver capacitor plates  3  are driving the signal processing unit  250  through differential input amplifier  251 , the four levels signal is amplified to a constant amplitude range signal  270  using a digital variable gain controlled amplifier  260 . The Analog to Digital Converter [ADC]  261  convert the input signal to digital format, processed by the Digital Demodulator Unit [DDU]  262 , that in case that a sine patterned rotor  4  is used, creates sine  272  and cosine  273  digital words. The two words are converted by Digital to Analog Converters [DAC]  263  to analog signals  274  &amp;  275 . Optionally, in the case that a square patterned rotor  2  is used, the same DDU  262  trapezoid shaped output words  272  &amp;  273  are referred as “S” &amp; “C”. A suitable Analog to Digital Converter  261  includes AD7450 12 bit ADC manufactured by Analog Devices, of Norwood, Mass. A suitable DAC  263 , include AD5446 14 bit DAC manufactured by Analog Devices, of Norwood, Mass., The core of DDU circuit  262  is made as VHDL code in a programmable FPGA including Cyclone manufactured by Altera Corporation of San Jose, Calif. or as software written in a programmable processor including the C8051F002 CPU made by Silicon Laboratories of Austin, Tex. or as part of custom made ASIC. A suitable variable gain controlled amplifier  260  is made of a DAC8043A multiplying DAC manufactured by Analog Devices, of Norwood, Mass., configured as a feedback to an operational amplifier known in the art. The Coarse receiver capacitor plates  36  are driving a similar signal processing unit  255  through input amplifier  252 . An optional Post Angle Processor [PAP]  264  convert the two fine orthogonal “S”  272  &amp; “C”  273  digital words to a digital fine angle  277 . A similar PAP unit  265 , convert the two coarse  276  digital words to a digital coarse angle  278 . an Absolute Angle Processor [AAP]  266  generate an absolute position  279  out of those two digital words, using fine and coarse absolute position finding algorithms known in the art.  
         [0067]      FIG. 16  illustrates one embodiment of the Digital Demodulator Unit core used in the signal processing unit of  FIG. 15 . Each of the four signal levels of the input signal  270 , representing one of the four combinations of the two excitation  200  phases {(S+C), (S−C), (−S−C), (−S+C)}, is latched by one of the four latches  281  synchronized to the four states of excitation generator  200  using a 2to=b  4  de-multiplexer  282 . Those four levels are added/subtracted by adders  283  to decode out of them the sine and cosine signals. Digital low pass filters  284  filter the signal and limit the bandwidth as known in the art. The Automatic Gain Control digital command  271 , is generated by a PID filter  286  that compares a Maximum Range Function [MRF]  285  of the two signals  272 ,  273  amplitude to a reference value, and the result  271  of the PID mechanism  286  calculation drive the digital variable gain controlled amplifier  260  of  FIG. 15 , The design of a PID controller is commonly known in the art. The implementation of the Maximum Range Function [MRF]  286  is dependent of the rotor structure; a) Sum of squares of the sine  274  and cosine  275  signals generated using a sine shaped rotor  4 . b) MAX function for “S” 264  and “C”  265  signals generated using a square shaped rotor  2 . The implementation of comparator, multiplier and adder are known in the art.  
         [0068]      FIG. 17   a  illustrates one embodiment of a Post Angle Processor [PAP] used in the signal processing unit of  FIG. 15  to generate a fine digital angle  277  out of the “S”  272  &amp; “C”  273  signals, requiring a square shaped rotor  2 . The logic  293  compare the “S” &amp; “C” signals and decide to swap  295  the signals “S” and “C” using switch  290  so the smaller signal is the nominator and the larger one the denominator of the divider  291 , the logic also defines if it is needed to invert the “S” signal in two of the cases. The division result is added to the center angle  296  of the quarter of a circle selected by the logic  293 . This signal can be converted to a potentiometer style analog value using DAC  263  or transmitted to an external computer via a communication link  297 , both known in the art.  
         [0069]      FIG. 17   b  illustrates the waveform of the two signals S  272  &amp; C  273  and a mating look up table  298  implementation of the logic  293 . The table has one colon per circle quadrant and one row per logic function. To detect the quadrant, two inequalities  294  are tested to define the quadrant, a) if (“S”&gt;“C”) and b) if (“S”&gt;−“C”), generating two commands: A command  295  to switch  290  and a center angle value selection  296 , added by adder  292  to form the center angle for the specific quadrant. The implementation of comparator, divider and adder are known in the art.  
         [0070]     Thus, a method and apparatus for two dimensional layout, space saving interlaced channels with high noise immunity electric capacitive sensor has been described. While the methods and apparatus of the present invention have been described in terms of the above illustrated embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described. The present invention can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of restrictive on the present invention.