Abstract:
Electronic sensing circuits monitor multiple sized EL cells, referred to as a lamp, and compensate for variations in contrast between cells, while maintaining a fixed contrast between the luminance of the cells and the ambient lighting. In one embodiment, three separate feedback loops monitor the ambient lighting, cell-luminance, and frequency of the excitation voltage and make appropriate adjustments to an adjustable luminance reference. In another embodiment the circuit which drives the EL cells includes a timer and a microprocessor. The timer measures the elapsed time during which the EL display has been operating. The microprocessor adjusts the drive signal to the EL display to correct for aging, based on the elapsed time measured by the timer and an empirically-determined aging parameter. All these adjustments are accomplished while a microprocessor sequences through a large assortment of electroluminescent cells of various sizes.

Description:
This application claims benefit of 60/123,742, filed Mar. 10, 1999, which also claims benefit of 60/134,168, filed May 13, 1999. 
    
    
     BACKGROUND 
     1. Field of Invention 
     The present invention is directed to controllers for electroluminescent lamps. More specifically, the invention involves electronic control circuits that sense and enhance the viewing contrast of various sized cells within electroluminescent (EL) lamps while these cells are dynamically switched. 
     2. Discussion of Related Art 
     EL lamps provide solutions to many lighting problems that require very thin yet rugged structures, which are light and flexible and have an infinite number of sizes and shapes. EL lamps are also fairly inexpensive and simple to construct. The controllers required for EL lamps must obtain a lamp brightness that allows a viewer the ability to distinguish the shapes and information presented by the lamp&#39;s design. The maintenance and control of this brightness becomes more complicated when considering the large voltages involved and variables such as changing ambient lighting, switching between cells of different sizes, extending the limited lifetimes of EL lamps, and operating within the limitations imposed by the lamp construction. 
     EL lamps are also subject to aging effects that vary the brightness of the lamps&#39; output even when driven with a constant amplitude and frequency drive signal. EL lamps are capacitive in nature, and the capacitance of the EL lamp varies with the operating age of the EL lamp (i.e., the time during which the EL lamp has been operated). In addition, the surface area of the EL lamp is a determinant of the amount of capacitance. Construction techniques of various manufacturers are a further determinant of the initial amount of capacitance exhibited by an EL lamp. 
     Inventors have created controllers that supply the large voltages necessary to drive EL lamps and some have added compensation techniques that improve visibility. However, U.S. Pat. Nos. 5,519,288 and 5,517,089 sense only the output voltage to the EL lamp, which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells, nor does it adequately correct for aging. U.S. Pat. No. 5,440,208 senses only the current flowing through the EL lamp, which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells, nor does it adequately correct for aging. U.S. Pat. No. 5,336,978 senses the light output of a single cell, which does not provide proper compensation concerning the issues of changing ambient light nor switched-multiple-sized cells. U.S. Pat. No. 5,293,098 does not sense any of the output conditions of the lamp, thus changing ambient light, switched-multiple-sized cells and aging are not properly compensated. U.S. Pat. No. 5,144,203 delivers only constant power to EL lamp which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells, nor does it adequately correct for aging. U.S. Pat. No. 5,089,748 senses the light output of a single lamp which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells. U.S. Pat. No. 4,443,741 senses the current of a single lamp, which does not provide proper compensation for the issues of changing ambient light and switched-multiple-sized cells. U.S. Pat. No. 4,319,165 adjusts the power factor of a single lamp, which does not provide proper compensation concerning the issues of changing ambient light and switched-multiple-sized cells, nor does it adequately correct for aging. 
     All controllers known heretofore suffer from a number of disadvantages. Firstly, changing ambient light dramatically decreases the viewer&#39;s ability to distinguish information being presented by the lamp. Unfortunately manual adjustments of luminance to compensate for changing ambient light conditions require constant intervention. In addition, high luminance settings can significantly reduce the lifetime of the battery in battery operated EL lamps, while low settings make the lamps ineffective. High luminance settings can also reduce the lifetime of lamps. Presently known controllers cannot adjust luminance when switching between cells of different sizes meaning that large luminance variations and transients result when dynamically switching EL cells within lamps. Proper compensation for EL lamp aging for various sized cells being sequenced within an EL lamp does not exist in the prior art. Further, presently known lamp controllers cannot vary or set the nominal luminance. Finally, presently known lamp controllers can not shut down inverters when lamps are disconnected, resulting in large AC voltages appearing at the controller outputs. 
     In view of the above disadvantages of the prior art, it is an object of the present invention to provide an ambient sensor that automatically corrects for environmental changes in light directed onto the lamp, making lamp information readable, allowing lamps to age more slowly, and allowing battery operated systems to last longer. It is also an object of the present invention to provide an EL lamp system which operates independently of the aging effects of its EL elements. It is another object of the present invention to provide an EL lamp system which compensates for the age of its EL elements. It is a further object of the present invention to provide adjustable gains in the ambient feedback loop so that a controlled maximum increase in lamp luminance can be selected. It is another object of the present invention to provide appropriate feedback time constants that allow response time of the corrections to track desired changes. It is a still further object of the present invention to provide an automatic ambient control that integrates with selectable settings for lamp luminance, aging in the lamps, and dynamic switching of various sized cells within the lamp. It is yet another object of the present invention to provide frequency and voltage compensation that automatically adjusts the luminance quickly for applications requiring sequencing cells of different sizes. It is still another object of the present invention to provide constant cell-to-cell and long-term luminance to extend lamp lifetime while a lamp is being sequenced through cells of various sizes. It is a further object of the present invention to provide manual luminance control that is independent of AC line input variations and allow for accurate and repeatable levels of luminance for lamps of various constructions. 
     Further objects and advantages include providing a controller which removes many constraints on lamp designs. This approach can also eliminate many quality control problems with lamp construction. The controller can shut down the inverter whenever the lamp is disconnected, and can shut down the inverter when excessive loads or overheating occur. This approach widens the operational range for lamps and cell sizes. Still further objects and advantages will become evident with the following descriptions and drawings. 
     SUMMARY OF THE INVENTION 
     The above objects are each achieved according to an aspect of the present invention by providing electronic sensing circuits which monitor the multiple-sized electroluminescent cells of the lamp and make adjustments to compensate for variations in the contrast between the cells, while maintaining a fixed contrast between the lamp luminance and the ambient lighting. Separate feedback loops monitor the ambient lighting, the cell luminance and the frequency of the excitation voltage and make appropriate adjustments to an adjustable luminance reference. The adjustments occur while a microprocessor sequences through an assortment of electroluminescent cells of various sizes. In another embodiment, instead of having a feedback loop to monitor the lamp luminance as it decreases due to aging, the circuit which drives the EL lamp includes a timer and a microprocessor. The timer measures the elapsed time during which the EL display has been operating. The microprocessor adjusts the drive signal to the EL display to compensate its brightness to be independent of its age, based on the elapsed time measured by the timer and an empirically-determined aging parameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood by reference to the appended figures, in which: 
     FIG. 1 is a block diagram showing the two-feedback control loops and two additional control paths according to a preferred embodiment of the invention; 
     FIG. 2 is a schematic diagram showing the connection with a controller; and 
     FIG. 3 is a block diagram showing an alternate aging compensation means, including a timer and microprocessor according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described with reference to preferred embodiments. One typical embodiment of the invention is shown in FIG. 1 where blocks show four separate and distinct paths by which the luminance of the lamp is controlled. These paths are as follows: (1) “Reference luminance path” where a user will adjust the desired luminance of the lamp; (2) “Ambient path” where environmental lighting incident on the lamp will control the luminance of the lamp; (3) “Aging feedback loop” where aging, lamp construction, and efficiency of the lamp are eliminated and do not affect the luminance of the lamp; and (4) “Cell-to-cell feedback loop” where cell-size or combinations of cells do not affect the luminance of the cell or cells being illuminated. 
     All four paths operate in harmony with one another and allow a reference luminance adjust  32  to set a nominal lamp luminance through ambient amplifier  30 , aging comparator  26 , cell comparator  22 , inverter regulator  10 , DC-to-AC inverter  12 , and EL lamp with multiple cells  14  around which each of the other three paths will operate. An “ambient path” is composed of ambient photo detector  16 , ambient amplifier  30 , aging comparator  26 , cell comparator  22 , inverter regulator  10 , DC-to-AC inverter  12 , EL lamp with multiple cells  14 , where an “aging feedback loop” is composed of aging photo detector  20 , aging amplifier  28 , aging comparator  26 , cell comparator  22 , inverter regulator  10 , DC-to-AC inverter  12 , EL lamp with multiple cells  14 , and where a “cell-to-cell feedback loop” is composed of frequency to voltage translator  18 , rectification &amp; scaling  24  and cell comparator  22 , inverter regulator  10 , DC-to-AC inverter  12 , EL lamp with multiple cells  14 . It will be seen that each of these loops makes use of the section composed of inverter regulator  10  and DC-to-AC inverter  12 . For ease of reference, this group of components will be referred to herein as the “EL drive section”. 
     The “reference luminance path” uses the reference luminance adjust  32  to generate a level for the ambient amplifier  30 , which is compared with a signal from the “ambient path” to form a signal to the aging comparator  26 , which is compared to a signal from the “aging feedback loop” to form a signal to the cell comparator  22 , which is compared to a signal from the “cell-to-cell feedback loop” to form a signal to inverter regulator  10 , which varies the DC voltage into the DC-to-AC inverter  12 , which in turn sets the luminance of the EL lamp with multiple cells  14 . 
     The “ambient path” uses the ambient photodetector  16  to monitor the incident light on the EL lamp  14  to generate a signal proportional to the incident light, which is compared in the ambient amplifier  30  with the desired level from the reference luminance  32 . This signal will be applied to the aging comparator  26  and compared to a signal from the “aging feedback loop” to form a signal to the cell comparator  22 . The cell comparator  22  will compare this signal to the signal from the “cell-to-cell feedback loop” to form a signal to the inverter regulator  10 , which in turn will change the input signal to the DC-to-AC inverter  12 . The DC-to-AC inverter  12  will then change the luminance on the EL lamp with multiple cells  14 . 
     The “aging feedback loop” uses the aging photo detector  20  to monitor the luminance of the lamp  14  and generate a signal proportional to the average lamp luminance; this signal will then be applied to the aging amplifier  28 . The output of the aging amplifier  28  will be compared in the aging comparator  26  to a composite signal made up of information from the ambient photodetector  16  and the desired reference luminance  32 . Whenever the signal from the aging amplifier  28  is different than the signal from the ambient amplifier  30  the output of the aging comparator  26  will change the magnitude of the signal into the cell comparator  22 . This in turn will activate the inverter regulator  10  to supply a DC output signal to the DC-to-AC inverter  12 , which increases or decreases the luminance to the EL lamp with multiple cells  14 . The feedback control loop is satisfied when the aging comparator  26  inputs are equal. 
     The “cell-to-cell feedback loop” uses the frequency-to-voltage converter  18  to generate a signal proportional to frequency and voltage so that the rectification and scaling  24  can generate a signal. The cell comparator  22  compares this rectified and scaled signal to a composite signal made up of information from the aging comparator  26 . Whenever the rectified and scaled signal from  24  is different than the signal from the aging comparator  26  the output of the cell comparator  22  will activate the inverter regulator  10 . The inverter regulator  10  then supplies a DC output signal to the DC-to-AC inverter  12 , which increases or decreases the luminance to the EL lamp with the multiple cells  14 . The feedback control loop is satisfied when the cell comparator  22  inputs are equal. 
     From the description above, a number of advantages of the present invention become evident. Firstly, ambient sensing acts as an automatic reference level adjustment which eliminates user adjustments and allows for optimum performance. Secondly, the inverter regulator provides a stable and constant voltage to the DC-to-AC inverter  12  that is a function of four different signals. Further, with the “cell-to-cell feedback loop” as the inner loop, cell switching can occur at high speeds. Another advantage is that the “ambient path” can have a selectable response time so that changes due to moving shadows or unwanted changes in environmental lighting can be eliminated. Further, aging and ambient photo detectors  16  and  20  can be physically located on the EL lamp with multiple cells  14 . 
     The operation of the preferred embodiment can best be understood by referring to FIG. 2, which shows how the preferred embodiment integrates into a controller. FIG. 2 consists of several detailed groups of components. The power source consists of line input and fuse  38  along with the AC to DC converter  40  out of which an unregulated DC voltage is available. The control elements include inverter regulator  42 ; reference luminance adjust pot  46 ; ambient amplifier  48 ; ambient gain adjust  50 ; aging amplifier  52 ; aging comparator  54 ; and cell comparator  56 . A further component is the DC-to-AC Inverter  44 . Lamp switching elements and I/O connector consist of the regulator  58 , micro-controller  60 , EL cell drivers  62  and I/O connector  66 . Lamp elements consist of EL lamp with multiple cells  64 , aging photo detector  68  and ambient photo detector  70 . 
     AC power to operate the preferred embodiment and the associated groups or elements in a controller is input through the line input and fuse  38 , and then into the AC to DC converter  40  out of which an unregulated DC voltage is generated on line  100  relative to the converters return path  102 . This unregulated DC voltage is distributed and used to operate the inverter regulator  42 , regulator  58 , ambient amplifier  48 , aging amplifier  52 , aging comparator  54 , and cell comparator  56 . To simplify and reduce the cost of the system, the operational amplifiers are being operated from a single voltage power supply, but could just as easily be operated from a bipolar power source. 
     The unregulated DC line  100  provides the regulator  58  with power to generate the direct current for the micro-controller and the resistor divider. The resistor divider is made up of resistor  168  and the reference luminance adjust potentiometer  46 . The resistor  168  limits the maximum voltage at line  114  and thus limits the maximum voltage available for the reference luminance adjust potentiometer  46 . 
     When a user adjusts the reference luminance adjust potentiometer  46  the voltage level at line  112  changes and sets the operating point for the non-inverting input of the ambient amplifier  48 . 
     The voltage at the inverting input of the ambient amplifier  48  is determined by the ambient gain adjust  50  and the ambient photo detector  70 . If the ambient light increases, the photo detector  70  would decrease resistance and force the output of the ambient amplifier  48  to increase The output voltage V 118  at line  118  of the ambient amplifier is equal to: 
     
       
           V   118   ≈V   ref (1+ R   50   /R   apd ) 
       
     
     where V ref  is the voltage from the reference luminance adjust pot  46 , R 50  is the resistance of the ambient gain adjust pot  50  in the feedback path of the ambient amplifier  48 , and R apd  is the resistance of the ambient photo detector  70 . The resistance R apd  is inversely proportional to the ambient light falling on its surface: 
     
       
           R   apd ≈1 /L   amb   
       
     
     Where L amb  is the ambient light falling onto the surface of the photo detector. When R apd  is substituted into the equation for the line  118  voltage V 118 , the following relationship is valid: 
       V   118   ≈V   ref (1 +R   50   L   amb ) 
     The ambient gain adjust pot  50  allows for various gains and operational ranges, but allows the desired relationship of voltage at line  118  to be proportional to the ambient luminance. 
     Line  118  attaches to the Butterworth filter made up of the resistor  120  and the capacitor  122 . This filter provides a slow response to ambient lighting that changes quickly, such as when there are shadows from passing objects. Line  124  attaches to the non-inverting input of the aging comparator  54 . Note that the non-inverting input does not load down the filter, and so the voltage at line  124  has the same DC level as the voltage at line  118 . 
     The resistor divider made up of resistors  172  and  170 , provides a voltage on line  126  to the non-inverting input of the aging amplifier  52 ; the physical properties of the EL lamp with multiple cells  64  determine these resistors and fix the operating point. 
     The fixed resistor  132  and the aging photodetector  68  determine the line  128  voltage of the inverting input on the aging amplifier  52 . If the light from the EL lamp with multiple cells  64  increases, the photo detector  68  would decrease in resistance and force the output of the aging amplifier  52  to increase. The output voltage V 134  at line  134  of the aging amplifier is equal to: 
     
       
           V   134   ≈V   126 (1 +R   132   /R   age ) 
       
     
     Where V 126  is the voltage at line  126 . R 132  is the resistance of the resistor  132  in the feedback path of the aging amplifier  52 ; Rage is the resistance of the aging photo detector  68 . The resistance R age  is inversely proportional to the light coming from EL lamp with multiple cells  64 . 
     
       
           R   age ≈1 /L   age   
       
     
     Where L age  is the light from the EL lamp with multiple cells  64  hitting the surface of the aging photo detector  68 . When R age  is substituted into the equation for the line  134  voltage V 134 , the following relationship is valid: 
     
       
           V   134   ≈V   126 (1 +R   132   .L   age ) 
       
     
     When the aging photodetector is monitoring only one of the cells on the EL lamp with multiple cells  64  the capacitor  130  averages the pulses of light so that the voltage at line  134  is constant over the duration of the sequence time of the lamp  14 . The sequence time of the lamp  14  is defined as the time it takes to step through and illuminate all the cells. If multiple cells are monitored or multiple photo detectors are used to monitor the cells, the capacitor  130  can be reduced in size. 
     The aging comparator  54  has been implemented with capacitor  140  connected between the output and the inverting input, thus forming an integrator. This integrator has a gain that is inversely proportional to the product of resistor  136  and feedback capacitor  140 . The voltage at line  142  is dependent on the magnitudes of the voltage on line  124  and line  134  When the voltage on line  124  is larger than the voltage on line  134 , the voltage on output line  142  will increase in a positive direction at a rate proportional to the integrator&#39;s time constant: 
     
       
         τ αge   ≈R   136 C 140   
       
     
     where R 136  is the resistance of the resistor  136  and C 140  is the capacitance of the capacitor  140 . The time constant for this loop is very long and must be long enough to average the light pulses from the cells being monitored. 
     The cell comparator  56  has been implemented with capacitor  148  connected between the output and the inverting input, thus forming an integrator. This integrator has a gain that is inversely proportional to the product of resistor  144  and feedback capacitor  148 . The voltage at line  150  is dependent on the magnitudes of the voltages on line  142  and line  146 . When the voltage on line  142  is larger than line  146 , the output line  150  will increase in a positive direction at a rate proportional to the integrator&#39;s time constant: 
      τ cell   ≈R   144 C 148   
     where R 144  is the resistance of the resistor  144  and C 148  is the capacitance of the capacitor  148 . The time constant for this loop is much smaller than for the aging loop and thus it responds much more quickly to switching between cells of various sizes. 
     The level of the voltage at line  150  determines the output voltage of the adjustable converter regulator  42 . The resistor  152  and zener  137  form a limiter on the voltage that can be applied to the DC-to-AC inverter  44 . Capacitor  104  filters the unregulated DC voltage on line  100  and the capacitor  106  filters noise reflected back from the DC-to-AC inverter  44 . 
     When the voltage at line  108  increases, the output line  154  of the DC-to-AC Inverter  44  will increase and cause more light to be emitted from the EL lamp with multiple cells  64 . The increase in voltage at line  108  also increases the voltage applied to the frequency-to-voltage converter made up of capacitor  156 , resistor  160 , diode  161 , resistor  164  and resistor  166 . The voltage that appears at line  146  is dependent on the frequency and voltage of cell being activated at any given time. Frequency and voltage levels are a function of the characteristics of the lamp being activated. 
     The micro-controller  60  controls the sequencing, duration and order in which the cells of the lamp are illuminated; it also enables the DC-to-AC inverter  44  via a trace that passes through the I/O connector  66  and loops on the EL lamp with multiple cells  64 . The EL cell drivers  62  are activated via a bus of signals  178  and perform the high power switching of the voltages being applied to the cells of the lamp. 
     Operation of the aging feedback and cell-to-cell feedback loops can be more clearly demonstrated by assuming the system has stabilized at some reference luminance as dictated by the setting on pot  46  and a constant ambient light value. Assume now that the aging photo detector  68 , placed onto the EL lamp with multiple cells  64 , senses a decrease in light level due to aging of the lamp. This decreased light level causes the resistance of the photodetector  68  to increase, which causes more voltage to appear at line  128 . The voltage at line  134  must decrease to bring the voltage level at line  128  back to equal the voltage on line  126 . Since the response of the aging amplifier  52  has been heavily filtered, line  134  slowly decreases in voltage. The aging comparator  54  begins to integrate this voltage and line  142  moves in a positive direction. The voltage at line  142  determines the steady state luminance of a particular cell and the cell-to-cell feedback loop will quickly force the voltage at line  146  to equal the new voltage on line  142 . This is accomplished by increasing the voltage at line  150  and thus increasing the voltage at the inverter regulator output. The DC-to-AC inverter  44  quickly responds and increases the voltage at the resistor divider formed by resistor  164  and resistor  166 . This in turn makes the voltage at the inverting and non-inverting inputs of the cell comparator  56  equal, thus stabilizing the operational amplifier output. The bandwidth of the cell-to-cell feedback loop is much larger than that of the aging feedback loop and as such can acquire or easily follow changes of the voltage at line  142 . Once the voltages are equal the output voltage of the DC-to-AC inverter  44  will have increased to a level such that the EL lamp with multiple cells  64  is emitting enough light to return the resistance of the aging photo detector  68  to its original value. 
     Switching between cells of different sizes can cause large changes in the luminance of the cell being activated unless the frequency and voltage are monitored and used for controlling the voltage applied to the EL lamp with multiple cells  64 . This task is handled by the cell comparator loop as discussed in the operation of the aging feedback loop. 
     In a second embodiment, a timer which is attached to the EL lamp circuitry replaces the aging feedback loop. FIG. 3 schematically illustrates a circuit  100  in accordance with this second preferred embodiment of the present invention. Here, a display of EL lamps  102  includes multiple EL cells  105 . A step-up transformer  106  receives a power signal  108  from a power-switching transformer  110 . The power-switching transformer  110  provides a stepped-up power signal in the form of power pulses to drive the EL cells  105 . The power-switching transformer is driven by an analog signal provided from a digital-to-analog (D/A) converter  112 . The analog signal provided from the D/A converter  112  corresponds to a digital word provided from a microprocessor  114 . 
     The brightness of the light output from an individual one of the EL cells  105  is dependent upon the amplitude and frequency of a sinusoidal voltage signal provided to drive the EL cells  105 . 
     No matter what the shape of the periodic voltage waveform employed to drive the EL cells  105 , the frequency and amplitude of the voltage signal provided to drive the EL cells  105  is dependent upon the digital signals provided from the microprocessor  114  to the D/A converter  112 . 
     In general, the brightness of the cells  105  can be considered to vary proportionally with the operating age of the EL lamp  102 . Furthermore, cells  105  of different manufacturers generally have different light output brightness characteristics. 
     In accordance with this embodiment, the aging characteristic is determined empirically. For example, if the brightness is considered to vary proportionally with operating age, an EL lamp  102  of a particular manufacturer is operated over time and the light output measured and the proportionality constant determined. For all EL lamps  102  of this manufacturer, the proportionality constant is now “predetermined”. 
     That is, the microprocessor  114  measures the “operating age” of the EL lamp system  102  using an elapsed time clock  108 . The microprocessor then corrects for the aging process by varying either the amplitude of the periodic voltage waveform employed to drive the EL cells  105  or by varying the frequency of the periodic voltage waveform employed to drive the EL cells  105 . In one embodiment, the microprocessor  114  measures the operating age of the EL cells  105  every hour and adjusts the periodic voltage waveform accordingly. An electrically erasable programmable read-only memory (EEPROM)  116  may be provided into which the elapsed operating time may be recorded and held when the circuit  100  is powered down. The microprocessor  114  also receives an input from sense resistor divider  118  as a representation of the drive voltage. 
     Accordingly, this invention provides several advantages over conventional approaches to controlling EL lamps: 
     a. Automatically maintains the contrast between the EL lamp and the ambient lighting; 
     b. Automatically maintains the luminance as the lamp ages, thus increasing the useful lifetime; 
     c. Uses the frequency and voltage for maintaining constant luminance of a cell; 
     d. Eliminates stringent fabrication requirements of EL lamps; 
     e. Provides for manual luminance settings; 
     f. Allows one DC-to-AC Inverter to operate with multiple sized cells; and 
     g. Provides high speed switching between cells of various sizes. 
     Although the description above contains the details of preferred embodiments, these should not be construed as limiting the scope of the invention, but as merely illustrative of the invention. Indeed, variations of the invention will be readily apparent to those skilled in the art and also fall within the scope of the invention. Thus the appended claims and their legal equivalents should determine the scope of the invention.