Abstract:
The invention is related to an electric field proximity detection system suitable for use as a touch sensitive keyboard or to be used in close proximity without direct contact. In an embodiment of a circuit useful in the system, an AC signal is coupled to a single electrode functioning as an antenna radiating an electric field through a high impedance circuit. A conductive object in close proximity disturbs the field causing a voltage change across nodes of the high impedance circuit that is compared by a detector circuit that generates a DC output indicating an object is close to the electrode. In another embodiment, the circuit couples to an analog multiplexer to control a plurality of electrodes. In another embodiment, a row and column address scheme coupling a plurality of electrodes and increases resolution without substantially increasing complexity. The circuits may be integrated in a semiconductor to reduce size and cost. The electric field proximity detection system extends to applications related to object detection such as remote sensing, motion detection and remote controls.

Description:
BACKGROUND  
       [0001]     This invention relates to electric field proximity detection systems and particularly to electric field proximity circuits for touch sensitive or close proximity keyboards.  
         [0002]     Many keyboards today are activated by depressing a keypad. A touch sensitive keyboard provides a similar I/O device, but is activated by merely touching rather than depressing each keypad. Touch sensitive keyboards are used in a variety of applications such as handheld computing devices (e.g., PDA), musical instruments, elevator switches, appliances, bank ATMs, and computers. Other types of touch sensitive keyboards can be activated by an object such as a finger or stylus touching the keypads displayed on a computer or terminal. A close proximity keyboard does not require contact, but is activated when an object (e.g., stylus or finger) is detected in close proximity to the keypad. Close proximity means the object is at a distance that should be detected for activation. The close proximity keyboard is especially useful where physical contact would result in undue wear or contamination in a sterile or hazardous environment.  
         [0003]     In the past, touch sensitive and close proximity keyboards have used complicated detection systems. Some of these detection systems are described in U.S. Pat. Nos. 6,452,514 B1, 5,572,205, and 5,594,222, and U.S. Published Patent Application No. 2002/0130848 A1. These patents and application describe use of two spaced apart electrodes or conductive pads such as a transmit electrode and a receiver electrode with a keypad. An object is detected as close to the keypad when it interferes with a signal coupled or transmitted between the two electrodes or pads. U.S. Pat. No. 3,971,013 takes a different approach using a gas panel to detect gas discharge due to contact to the gas panel. Due to the complexity of these detection designs, the costs impact the adoption rates of these types of keyboards.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention relates to several embodiments of electric field proximity detection systems. The systems are suitable for touch sensitive keyboards and/or close proximity keyboards. In one embodiment, each keypad uses an electrode to radiate an electric field. In others, each keypad has a plurality of electrodes. The electric field is disturbed by touching an associated area (e.g., a keypad) or by a conductive object in close proximity to that area. A circuit that can sense when the electric field is disturbed and sends an output to activate a response and/or to a controller for further analysis and in some cases activation of a response.  
         [0005]     In other features, the electric field proximity detection systems use row and column address schemes to multiplex the electrodes. The row and columns can be nested to permit higher resolution. The address schemes generally reduce complexity, and costs by reducing the addresses. The electric field proximity circuits, arrangements of electrodes, and address schemes are not limited to the keyboards and extend to applications related to object detection such as remote sensing, motion detection and remote control. By increasing the resolution using the row and column schemes, the waveform characteristics change accordingly. The controller can store the different loaded state waveforms characteristics for various objects for analysis and identification of the object types, and distance and duration of the object.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  illustrates an object in close proximity to an electrode associated with a keypad of an electric field proximity keyboard.  
         [0007]      FIG. 2  illustrates a circuit for an electrode suitable for use in an electric field proximity keyboard.  
         [0008]      FIG. 3  illustrates an electric field proximity keyboard with an address to each electrode.  
         [0009]      FIG. 4  illustrates a row and column address scheme for an electric field proximity keyboard.  
         [0010]      FIG. 5  illustrates a nesting row and column address scheme for an electric field proximity keyboard.  
         [0011]      FIG. 6  illustrates a circuit for a plurality of electrodes suitable for use in an electric field proximity keyboard.  
         [0012]      FIG. 7A  compares the waveforms of the AC reference signal with the unloaded electric field signal of an electrode when no object is in close proximity to a keypad.  
         [0013]      FIG. 7B  compares the waveforms of the AC reference signal with the loaded electric field signal of an electrode when an object is in close proximity to a keypad.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     The following description includes the best mode of carrying out the invention. The detailed description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is determined by reference to the claims.  
         [0015]     We assign each part, even if structurally identical to another part, a unique reference number wherever that part is shown in the drawing figures. A dashed circle indicates a portion of a figure that is enlarged in another figure. The figure showing the enlarged portion is indicated by a reference number tied to the dashed circle.  
         [0016]      FIG. 1  illustrates an electric field proximity keyboard  20  that includes a substrate  16  such as a printed circuit board (PCB) and one or more keypads each with an electrode  14 . Each electrode can be made, for example, by conductive patterns on the PCB and is connected to an AC signal source described further in connection with  FIG. 2 . The electrode  14  functions as an antenna radiating an electric field  22  and is located over a non-metallized portion  26  of the substrate  16  to reduce loading of the electric field  22 . A conductive object such as a finger  10  in close proximity to the electrode  14 , for example, at a distance  24 , will disturb, i.e., reduce the intensity of electric field  22 . This is referred to as electric field attenuation or loading. The electrode  14  may be protected from the environment and physical contact by a low loss dielectric layer  18  such as a solder mask or another other dielectric media (not shown) over the layer  18 . Suitable materials include a polycarbonate cover, glass, plastic, wood and other non-conductive materials that do not overly attenuate the electric field  22 .  
         [0017]      FIG. 2  illustrates a circuit  66  for a single electrode of an electric field proximity keyboard. Horowitz and Hill,  The Art of Electronics  (Second Edition, 1989) describe electronic circuit design and is incorporated by reference herein. This circuit  66  can be used for an electrode in a keyboard or another electric field proximity detection system although other arrangements are possible as will be described below. It should be also understood that any circuit lines that cross each other are not electrically connected unless the intersection is covered by a solid dot. The circuit  66  includes an electrode circuit  72  (i.e., modeling the electrode  14 ), an oscillator circuit  70  that includes multi-stage feedback oscillator amplifiers  81 ,  83 , and  87 , and RC low pass filters such as the following resistor and capacitors: R 2 -C 1 , R 3 -C 2 , and R 4 -C 3 . The oscillator circuit  70  produces an AC signal, for example, a 64 KHz signal, at a first node  71 . A high impedance circuit such as a resistor R 8  (e.g., 2 k ohms to 100 M ohms) isolates the AC signal from the electrode circuit  72  illustrated as a variable impedance parallel RC circuit with a resistor R 9  and a capacitor C 4 .  
         [0018]     The detector circuit will detect the loading of the electrode when an object such as a human finger  10  is in close proximity or in contact with the electrode by comparing the voltage difference of the AC signal source with the electric field voltage at the electrode  14 . More specifically, when there is no conductive object near the electrode  14 , the electrode circuit  72  is in an unloaded state. In this state, resistor R 9  has a high resistance of 1 M ohm or more and capacitor C 4  has a low capacitance such as 1-10 picofarads and the voltage at a second node  73  is substantially identical to the voltage at the first node  71 . Thus, resistor R 9  and capacitor C 4  represent the impedance of the body (specifically the finger  10 ) as it approaches the electrode  14 .  
         [0019]     When a conductive object is close to electrode  14 , the electrode circuit  72  is loaded and grounded and the resistor R 9  goes as low as 1 k ohm, while the capacitance of capacitor C 4  increases to 100 picofarads or more and the voltage at second node  73  is attenuated in an amount dependent on the distance  24  of the conductive object such as a finger  10  to the electrode  14  in  FIG. 1 . Voltage waveforms for unloaded and loaded states is discussed in connection with  FIGS. 7A and 7B .  
         [0020]     The detector circuit  74  indicates when an object is close to the electrode. When a conductive object is close to electrode  14  ( FIG. 1 ) modeled by electrode circuit  72 , the detector circuit  74  senses a voltage drop at second node  73  with respect to the reference voltage at first node  71 . A differential operational amplifier  86  uses the voltage at first node  71  as its inverting input and the voltage at second node  73  as its noninverting input. In an alternative, the second node  73  can be the inverting input and the first node  71  as the noninverting input. The output  80  is coupled to a diode D 1  for conversion to a DC output  82  indicating an object is in close proximity. A sample hold capacitor C 5  connected to diode D 1  reduces noise in DC output  82 . The closer the object to the electrode  14 , the larger electric field attenuation as indicated by a drop in the DC output  82 . The DC output  82  is coupled to a controller  84  with addresses  92 . One suitable programmable integrated circuit for the controller  84  is the Microchip PIC16F77 made by Microchip Technology, Inc. in San Jose, Calif. performs logic to analyze DC output  82  as described below.  
         [0021]      FIG. 3  illustrates an electric field proximity keyboard  40  having twelve keypads associated with twelve electrodes  28 ,  29 ,  30 ,  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37 ,  38  and  39  that connect to corresponding I/O addresses  01 ,  02 ,  03 ,  04 ,  05 ,  06 ,  07 ,  08 ,  09 ,  10 ,  11 , and  12  that are inputs to an analog multiplexer  90  discussed in connection with  FIG. 6 .  
         [0022]      FIG. 4  illustrates a row and column address scheme for an electric field proximity keyboard  65 . The AC signal described earlier can be multiplexed to a pair of electrodes associated with each keypad using the address scheme. Each electrode of the pair radiates its own electric field. Thus, if one electrode fails, the other electrode can still independently radiate and sense disturbance of its own electric field. For example, keypad  15  has an electrode  41  connected to an address represented by row  1  and an electrode  45  connected to an address represented by column  1 . The product of the rows and columns (m×n) will equal the total number of keypads, while the sum of the rows and columns (m+n) will equal the number of addresses.  FIG. 4  illustrates the scheme with 12 keypads made up of three rows and four columns and seven addresses. As shown, the AC signal is coupled to the electrodes  41 ,  42 ,  43 , and  44  on row  1 , to the electrodes  49 ,  50 ,  51 , and  52  on row  2 , and to the electrodes  57 ,  58 ,  59 , and  60  on row  3 . The AC signal is coupled to electrodes  45 ,  53  and  61  on column  1 , to electrodes  46 ,  54  and  62  on column  2 , to electrodes  47 ,  55  and  63  on column  3 , and to electrodes  48 ,  56 , and  64  on column  4 . An individual keypad  15  can be viewed as a 1×1 electrode pair of row electrode  41  and column electrode  45 . In contrast to the keyboard  40  shown in  FIG. 3 , where the number of I/O addresses equal the number of keypads, this addressing scheme scales increasingly well as the number of keypads increases. For example, I/O addresses made up nine rows and 16 columns will form an array of (9×16) 144 electrode pairs with only (9+16) 25 I/O addresses. This reduces the complexity of the addressing scheme, manufacturing costs and has advantages as discussed in connection with  FIG. 6 .  
         [0023]     In another embodiment shown in  FIG. 5 , the address scheme can be used on a keypad such as keypad  75  on an electric field proximity keyboard  67  by nesting the row and column addresses to form a larger array of electrodes. For example, the electric field keyboard  67  is defined by an array of keypads with three rows (m 1 =3) and four columns (n 1 =4) such as the keypads  75 ,  76 ,  77 , and  78 , where each of the keypads such as keypad  75  is further defined by an array with three rows (m 2=3 ) and four columns (n 2=4 ) of electrode pairs. The electric field proximity keyboard  67  includes a total of 12 (m 1 ×n 1 ) keypads and a total of (m 2 ×n 2 ) 12 electrode pairs associated with each keypad such as keypad  75 . Thus, the address scheme has a total of 144 (m 1 ×n 1 )×(m 2 ×n 2 ) electrode pairs but only 25 (m 1 ×m 2 )+(n 1 ×n 2 ) I/O addresses.  
         [0024]     Still referring to  FIG. 5 , the first row of keypads such as keypads  75 ,  76 ,  77 , and  78  are activated by their associated electrodes coupled to address rows  1 ,  2 , and  3 . The second and third rows of keypads are activated by their associated electrodes coupled to address rows  4 ,  5 , and  6 , and  7 ,  8 , and  9 , respectively. The first column of keypads  75 ,  69 , and  79  are activated by their associated electrodes coupled to column addresses  1 ,  2 ,  3 , and  4 . Likewise, the second, third and fourth column of keypads are activated by their associated electrodes coupled to column addresses  5 ,  6 ,  7 , and  8 , column addresses  9 ,  10 ,  11 , and  12 , and column addresses  13 ,  14 ,  15 , and  16 , respectively. The increase in electrodes per keypad increases resolution of the detected object and keeps the design simple.  FIG. 4  shows an enlarged view of an electrode array for a single keypad  75 .  
         [0025]     By increasing the number of electrodes associated with each keypad, a larger number of smaller and overlapping electric fields can be generated and sensed so that together the electrodes for a given area (e.g., keypad) act like a phased array antenna or a multi-aperture antenna to closely resolve the approaching object. The resulting signals sent to the microcontroller can be analyzed as to the shape and conductivity of the object in close proximity to the keypad to validate or invalidate the object as activating the keypad. The shape and pattern of electrodes to be used include common planar antenna structures that can be printed onto the PCB such as rectangular, circular, spiral, looped, serpentine and inter-digital structures.  
         [0026]      FIG. 6  illustrates a circuit  68  for an electric field proximity keyboard with an AC signal source coupled through a high impedance circuit such as resistor R 8  at the second node  73  to the output of the analog multiplexer  90  such as the 64-channel 491AMUX1-64 multiplexer manufactured by Quad Tron, Inc. in Feasterville, Pa. Horowitz and Hill,  The Art of Electronics  (Second Edition, 1989) describe analog multiplexers and is incorporated by reference herein. The plurality of analog multiplexer inputs are the I/O addresses to the electrodes. A plurality of multiplexers can be also connected in parallel to increase I/O addresses, that is, the channel capacity. The AC signal is selected and connected to each of the plurality of I/O addresses  91  that are the inputs of the analog multiplexer for a predetermined time and a control command  92  of the controller  84  selects the I/O address to connect. The controller  84  switches the AC signal to each of the I/O addresses  91  that interface with the electrodes associated with the keypads. The controller  84  is programmed as is known in this field to compensate for background noise, to determine the distance and time duration the object must be from the electrode to be in close proximity, and to compare the object to known signatures (e.g., shapes and conductivity). The functions on the circuits can be miniaturized in a semiconductor substrate in a known manner in an integrated circuit to reduce the size and the cost of manufacturing the circuits.  
         [0027]      FIG. 7A  compares the waveform of the AC signal from the oscillator circuit with the electric field signal of an electrode with no object (e.g., finger) in close proximity. In this unloaded state, the electric field voltage is not attenuated and the voltage at the first node  71  and the second node  73  ( FIG. 2 ) are substantially identical and can be represented by waveform  97  (dashed line). The DC output  82  from the detector D 1  is represented by the waveform  96  (solid line). The small peak-to-peak ripple is due to the sample and hold charge capacitor C 5  in the circuit that reduces noise and ripple.  
         [0028]      FIG. 7B  compares the waveform of the AC signal at node  71  versus the electric field signal of an electrode when the object is in close proximity to the electrode  14  ( FIG. 1 ). In this loaded state, the electric field voltage at the second node  73  is attenuated as depicted by the waveform  99  (solid line), and the AC signal at the first node  71  is also affected as represented by the waveform  98  (dash line). The DC output  82  follows waveform  99  due to the attenuation and the change in DC output  82  is analyzed in the controller  84  shown in  FIG. 6  and identifies the change as an object touching or in close proximity to the electrode  14 .