Patent Publication Number: US-9905170-B2

Title: Control of LED array in a liquid crystal display assembly

Description:
TECHNICAL FIELD 
     The disclosure relates generally to control of an LED array in display assembly having a liquid crystal layer. 
     BACKGROUND 
     Many devices include various forms of displays, such as liquid crystal displays. Liquid crystal displays do not produce light by themselves and require some type of light source or backlighting to produce a visible image. 
     SUMMARY 
     A display assembly includes a display unit having a liquid crystal layer and an LED array configured to illuminate the liquid crystal layer. The display unit is configured to display an image. A driver circuit is operatively connected to the LED array and configured to control a luminance of the LED array. The LED array may include one or more LED light sources. A control module is operatively connected to the display unit and includes a processor and tangible, non-transitory memory on which is recorded instructions for executing a method for controlling the LED array in the display unit. The control module is programmed to obtain a junction temperature (T J ) of the LED array, via the driver circuit. The control module may be programmed to enter one of a plurality of stages based at least partially on the junction temperature (T J ). 
     The junction temperature (T J ) is based at least partially on a first voltage (V 1 ), a second voltage (V 2 ) and a predetermined coefficient (T coefficient ). The junction temperature (T J ) may be defined as: T J =[(V 2 −V 1 )*T coefficient ]. The driver circuit may be programmed to apply a predefined first current to the LED array for a first time interval. The driver circuit may be programmed to apply a predefined second current to the LED array for a second time interval. In one embodiment, the predefined first current is about 10% of the maximum operating current (of the LED array) and the predefined second current is about 95% of the maximum operating current. The control module may be programmed to obtain the first voltage during the first time interval and the second voltage (V 2 ) during the second time interval, via a voltage-measuring device operatively connected to the LED array. 
     A video image adjustment module may be operatively connected to the control module and the display unit. The video image adjustment module is configured to control an appearance of the image displayed by the display unit. The control module is programmed to determine if image content is available for displaying the image. 
     The control module may be programmed to enter a first stage when the image content is not available and the junction temperature (T J ) is at or below a first threshold temperature (T 1 ). In the first stage, the control module may be programmed to set the image to black, via an image blanking signal to the video image adjustment module. In the first stage, the control module may be programmed to set the LED array to a maximum luminance, via a commanded luminance signal to the driver circuit, thereby accelerating preheating of the liquid crystal layer. The first control module may be programmed to exit the when the image content is available. 
     The control module may be programmed to enter a second stage when the image content is available and the junction temperature (T J ) is at or below the first threshold temperature (T 1 ). In the second stage, the control module may be programmed to send an image brightness signal to the video image adjustment module to lower brightness of the image. In the second stage, the control module may be programmed to set the LED array to a maximum luminance, via a commanded luminance signal to the driver circuit. The control module may be programmed to exit the second stage when the junction temperature (T J ) is above the first threshold temperature (T 1 ). 
     The control module may be programmed to enter a third stage when the image content is available and the junction temperature (T J ) is at or above a second threshold temperature (T 2 ). The second threshold temperature is above the first threshold temperature. In the third stage, the control module may be programmed to set the LED array to a maximum luminance for a predefined time (t 0 ), via a commanded luminance signal to the driver circuit such that the junction temperature (T J ) extends beyond a predefined maximum temperature of a de-rating curve. In the third stage after the predefined time (t 0 ), the control module may be programmed to reduce the commanded luminance signal such that the luminance of the LED array extends towards a descending portion of the de-rating curve. The control module may be programmed to exit the third stage when the luminance has reached the de-rating curve. 
     The control module may be programmed to enter a fourth stage when the image content is available and the junction temperature (T J ) is between the first threshold temperature (T 1 ) and the second threshold temperature (T 2 ). In the fourth stage, the control module may be programmed to reduce the power supplied to the LED array by the driver circuit to a predetermined rating, such that the predetermined rating is less than a maximum rating of the LED array. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic fragmentary view of a display assembly having an LED array, a liquid crystal layer and a control module; 
         FIG. 2  is a flowchart of a method stored on and executable by the control module of  FIG. 1 ; and 
         FIG. 3  is an example graph displaying current delivered to the LED array on the vertical axis and time on the horizontal axis; and 
         FIG. 4  is an example de-rating curve for the LED array of  FIG. 1 , displaying luminance of the LED array on the vertical axis and junction temperature on the horizontal axis. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numbers refer to like components,  FIG. 1  schematically illustrates a display assembly  10 . Referring to  FIG. 1 , the display assembly may be part of a device  12 . The device  12  may be a mobile platform, such as, but not limited to, standard passenger car, sport utility vehicle, light truck, heavy duty vehicle, ATV, minivan, bus, transit vehicle, bicycle, robot, farm implement, sports-related equipment, boat, plane, train or any other transportation device. The device  12  may be a non-mobile platform, including, but not limited to, a desktop computer, telephone, tablet and may take many different forms and include multiple and/or alternate components. 
     Referring to  FIG. 1 , the assembly  10  includes a display unit  14  including a liquid crystal layer  16  and an LED array  17  having at least one LED source  18 . It is to be appreciated that any number of LED sources  18  may be employed in the LED array  17 . The LED array  17  is configured to illuminate the liquid crystal layer  16 , from any relative position, including but not limited to the side or back of the liquid crystal layer  16 . An optical guide  20  may be positioned adjacent to the LED array  17  such that the light (“L” in  FIG. 1 ) emitted by each LED source  18  passes through the optical guide  20 . Referring to  FIG. 1 , the display unit  14  includes a first polarizing filter  22  to polarize the light L. The light L may subsequently pass through a TFT array  24 , the liquid crystal layer  16 , and a color filter  26 . The light L must further pass through a second polarizing filter  28 . Referring to  FIG. 1 , the display unit  14  is configured to display an image I. The display unit  14  may employ any type of display technology known to those skilled in the art. 
     Referring to  FIG. 1 , a control module  30  is operatively connected to the display unit  14 . The control module  30  includes at least one processor  32  and at least one memory  34  (or any non-transitory, tangible computer readable storage medium) on which are recorded instructions for executing method  200 , shown in  FIG. 2 , for controlling the LED array  17  in the display unit  14 . The memory  34  can store control module-executable instruction sets, and the processor  32  can execute the control module-executable instruction sets stored in the memory  34 . The control module  30  of  FIG. 1  is specifically programmed to execute the steps of the method  200  (as discussed in detail below with respect to  FIG. 2 ). 
     Referring to  FIG. 1 , a driver circuit  36  may be operatively connected to and configured to receive commands from the control module  30 . The driver circuit  36  is configured to deliver a PWM current signal  38  to the LED array  17 . A voltage-measuring device  39  may be employed to measure the LED differential junction voltage  40 . The assembly  10  may include a thermistor  41 . 
     Referring to  FIG. 1 , the control module  30  delivers a commanded luminance signal  42  to the driver circuit  36 . The driver circuit  36  delivers an LED junction voltage signal  44  to the control module  30 . As is understood by those skilled in the art, luminance is the luminous intensity projected on a given area and direction. Luminance is an objectively measurable attribute, with a unit of candela per square meter. Brightness is a subjective attribute of light, which may be scaled. 
     Referring to  FIG. 1 , a video image adjustment module  48  may be operatively connected to the control module  30  and the display unit  14 . The video image adjustment module  48  is configured to control an appearance of the image I displayed by the display unit  14 . The video image adjustment module  48  is configured to receive an input video content signal  52  from the control module  30  and send an output video content signal  50  to the display unit  14 . The control module  30  is configured to send an image brightness signal  54  and an image blanking signal  56  to the video image adjustment module  48 . 
     In the embodiment shown, the control module  30 , the driver circuit  36  and the video image adjustment module  48  are part of a control unit  60 . The control unit  60  of  FIG. 1  may be an integral portion of, or a separate module operatively connected to, other modules of the device  12 . A user interface  62  may be operatively connected to the control unit  60 . The user interface  62  is configured to convey requests from a user (not shown) regarding various settings, including but not limited to, the level of image brightness, dimming and contrast. 
     Referring to  FIG. 1 , the driver circuit  36  may provide power to the LED array  17  using pulse width modulation (PWM). The average value of voltage or current delivered is controlled by turning a switch (not shown) on and off at a fast rate. This allows control of the power delivered to the LED array  17 . The liquid crystal layer  16  may include a layer of molecules between two transparent electrodes. When a voltage is applied across the liquid crystal layer, the liquid crystal molecules re-orient themselves. By controlling the voltage applied across the liquid crystal layer  16  in each pixel, through the output video content signal  50 , light can be allowed to pass through in varying amounts thus constituting different levels of gray. 
     Referring to  FIG. 1 , the driver circuit  36  includes a power supply  70  that supplies power to the LED array  17  and is controlled by a PWM control unit  72 . The PWM control unit  72  provides a high frequency periodic drive signal of varying pulse width to direct the power supply, based at least partially upon a measured current from a current sensor  74  and a reference signal. In one embodiment, the drive signal is a square wave oscillating between 0 and 12 volts with a frequency of 30 kHz. The driver circuit  36  may include an oscillator  76  that provides a low (or high) frequency oscillating signal to the power supply  70  and the PWM control unit  72 . Any type of driver circuit  36  known to those skilled in the art may be employed. 
     Referring to  FIG. 1 , the video image adjustment module  48  may include a first signal processing unit  80  for adjusting image brightness (based on the image brightness signal  54  from the control module  30 ), contrast, hue, saturation and sharpness. The video image adjustment module  48  may include a second signal processing unit  82  for processing the image blanking signal to blank the display of video information during predefined intervals, such as horizontal and vertical retrace intervals which occur between line and field image trace intervals, respectively. The video image adjustment module  48  may include a third signal processing unit  84  for scaling display resolution, de-interlacing, controlling aspect ratio, controlling frame rate conversion, color point conversion, color space conversion, noise reduction and various image enhancements. Any type of video image adjustment or processing units known to those skilled in the art may be employed. 
     Referring now to  FIG. 2 , a flowchart of the method  200  stored on and executable by the control module  30  of  FIG. 1  is shown. The method  200  may include first, second, third and fourth stages, A, B, C and D. A typical control mechanism uses thermal protection during high temperature ambient, and reduces luminance of image when the internal display temperature increases. This reduction in luminance may result in a display that is difficult or impossible to see. Method  200  obviates these difficulties and increases the useful operating temperature range of a liquid crystal display. Method  200  employs an LED junction voltage measurement to estimate LED junction temperature, in place of stand-alone thermistor  41 . Method  200  need not be applied in the specific order recited herein. Furthermore, it is to be understood that some steps may be eliminated. The start and end of the method  200  are indicated by “S” and “E,” respectively. “Yes” and “no” are indicated by “Y” and “N,” respectively. 
     Referring to  FIG. 2 , method  200  may begin when the control module  30  is “awake” or otherwise powered. In block  202  of  FIG. 2 , the control module  30  is programmed to obtain a junction temperature (T J ). The junction temperature (T J ) may be defined as the highest operating temperature of the semiconductor(s) in the LED array  17 . In one example, the junction temperature (T J ) is taken to be the junction temperature of one pre-selected LED source  18  in the array  17 . In another example, the junction temperature (T J ) is taken to be the mathematical average of the junction temperatures of some or all of the LED sources  18  in the array  17 . In another example, the junction temperature (T J ) is taken to be a weighted average of the junction temperatures of some or all of the LED sources  18  in the array  17 . The junction temperature is generally higher than the temperature of the exterior surface or case of the LED array  17 . A two-step process of obtaining the junction temperature (T J ) is used, in place of a stand-alone thermistor  41 . 
       FIG. 3  illustrates a trace  102  with current (I) delivered to the LED array  17  on the vertical axis and time (t) on the horizontal axis. Referring to  FIG. 3 , the driver circuit  36  (shown in  FIG. 1 ) is configured to apply a two-step sequence between a first duty-cycle  104  and a second duty-cycle  106 . Referring to  FIG. 3 , during the two-step sequence, the driver circuit  36  is programmed to apply a predefined first current  110  to the LED array  17  for a first time interval and a predefined second current  108  to the LED array  17  for a second time interval. In one embodiment, the predefined first current  110  is about 10% of the maximum operating current and the predefined second current  108  is about 95% of the maximum operating current. The control module  30  may be programmed to obtain the first voltage (V 1 ) during the first time interval and the second voltage (V 2 ) during the second time interval, via the voltage-measuring device  40 . 
     The junction temperature (T J ) is obtained by using a thermal coefficient (T coefficient ) such that:
 
 T   J =[( V   2   −V   1 )* T   coefficient ].
 
The thermal coefficient (T coefficient ) may be obtained by calibration, i.e., plotting a series of voltage versus temperature readings as the current of the LED array  17  is incremented in a test cell and obtaining the slope of the relationship (T coefficient =ΔV/ΔT). In one example, T coefficient  is 2 mV/° C. By synchronizing the two-step measurement with other system information, such as oscillators, measurement error due to noise can be minimized.
 
     In block  204  of  FIG. 2 , the control module  30  is programmed to determine if image content is available, i.e., if any digital values for the pixels forming the image I is available, in the display unit  14 . If the image content is available in block  204 , the method  200  proceeds to block  214  as described below. If not, the method  200  proceeds to block  206 . In block  206 , the control module  30  is programmed to determine if the junction temperature (T J ) is at or below a first threshold temperature (T 1 ). If so, the method  200  proceeds to block  208 . If not, the method  200  proceeds to block  214 . 
     The control module  30  may be programmed to enter a first stage A when the image content is not available and the junction temperature (T J ) is at or below a first threshold temperature (T 1 ). An example of this stage is a remote start for the device  12  during the winter season. The first stage A includes blocks  206 ,  208 ,  210  and  212 . 
     In block  208 , the control module  30  is programmed to set the image display to black, via the image blanking signal  56  to the video image adjustment module  48 . In block  210 , the control module  30  is programmed to set the LED array  17  to the maximum luminance, causing the display unit  14  to warm more quickly, via the commanded luminance signal  42  to the driver circuit  36 . This results in a faster response time of the liquid crystal layer  16  (and less blur) when the image does appear later. In other words, the internal heat generated by the LED array  17  is employed to accelerate the pre-heating of the liquid crystal layer  16  when the image is inactive. Control module  30  may be programmed to exit the first stage A when the image content is available. 
     In block  214 , the control module  30  is programmed to determine or verify that the junction temperature (T J ) is at or below the first threshold temperature (T 1 ). If so, the method  200  proceeds to second stage B, which may also be referred to as the low-ambient temperature stage. If not, the method  200  proceeds to block  224 . The second stage B includes blocks  216 ,  218 ,  220  and  222 . An example of second stage B is a night time starting of the device  12  during the winter season. The temperature of the display unit  14  is low and the image display is less than full intensity. When the display unit  14  is first turned on during a relatively low temperature condition, image motion blur may be present. 
     In block  216 , the control module  30  is configured to determine if a dimming request (“R” in  FIG. 2 ) (by the user interface  62  of  FIG. 1 ) is less than 100%. In other words, the image display is less than full intensity. If so, the method  200  proceeds to blocks  218  and  220 . If not, it proceeds to block  224 . The dimming request (“R”) may conveyed by a user to the control module  30  via the user interface  62 . The PWM control unit  72  of the driver circuit  36  may perform dimming (upon command of the control module  30 ) by turning the power to the LED on and off at a sufficiently fast rate such that the flicker is not detectable by the human eye. 
     In block  218 , to help the liquid crystal layer  16  warm more quickly, the control module  30  is programmed to set the LED array  17  to a maximum luminance, via a commanded luminance signal  42  to the driver circuit  36 . This leads to a faster response time of the liquid crystal layer  16 , and less image blur. In block  220 , the control module  30  is programmed to lower the image brightness, via the image brightness command  54  from the control module  30  to the video image adjustment module  48 . The control module  30  may be programmed to exit the second stage B when the junction temperature (T J ) is above the first threshold temperature (T 1 ). In block  220 , the controller is programmed to determine if the junction temperature (T J ) is less than the first threshold temperature (T 1 ). If so, the method  200  loops back to block  216 . If not, the method  200  proceeds to block  224 . 
     The control module  30  may be programmed to enter a third stage C when the image content is available and the junction temperature (T J ) is at or above a second threshold temperature (T 2 ), which is above the first threshold temperature (T 1 ). Referring to  FIG. 3 , in block  224 , the control module  30  is programmed to determine if the junction temperature (T J ) is at or above the second threshold temperature (T 2 ). If so, the method  200  begins the third stage C, which includes blocks  226 ,  228 ,  230 ,  232  and  234 . If not, the method  200  proceeds to block  236 . An example application of the third stage C is a device  12  that is started after soaking in a desert sun. The temperature of the display unit  14  is high enough that normally the backlight intensity is reduced, making the image difficult to see. In one example, the first threshold temperature (T 1 ) is −40° C. and the second threshold temperature (T 2 ) is 75° C. 
     In block  226 , the control module  30  is programmed to begin a timer  86  (see  FIG. 1 ) for a predefined time (t 0 ). The timer  86  may be operatively connected to the control module  30  or may be an integral portion of the control module  30 . In block  228 , the control module  30  is programmed to set the LED array  17  to a maximum requested luminance for the predefined time (t 0 ), via the commanded luminance signal to the driver circuit  36  such that the junction temperature (T J ) extends beyond a predefined maximum temperature  306  of a de-rating curve  301 , an example of which is shown in  FIG. 4 . 
     Referring to  FIG. 4 , an example de-rating curve  301  for the LED array  17  is shown, displaying luminance (or intensity) of the LED array  17  on the vertical axis  302  and junction temperature (T J ) on the horizontal axis  303 . The de-rating curve  301  is not shown to scale and it is to be understood that the numbers shown are intended to be non-limiting examples. The de-rating curve  301  includes a flat portion  304  that ends at the predefined maximum temperature  306 . In block  228 , the control module  30  is programmed to allow the LED array  17  to exceed the predefined maximum temperature  306  along a first curve portion  308  to reach a shifted maximum temperature  310  (see  FIG. 4 ). 
     In block  230 , the control module  30  is programmed to determine if the timer  86  has expired. If the timer  86  has not expired, the method  200  loops back to block  228 . In block  232 , if the timer  86  has expired, the control module  30  is programmed to reduce the commanded luminance signal such that the LED array travels the second curve portion  312 , from the shifted maximum temperature  310  to the point  314  of the de-rating curve  301 . The first and second curve portions  308  and  312  deviate from a typical de-rating curve, bypassing the portion  318  of a typical de-rating curve. 
     The control module  30  may be programmed to exit the third stage C when the luminance has reached the point  314  of the descending portion  320  of the de-rating curve  301 . In block  234 , control module  30  is programmed to determine if the luminance has reached the descending portion  320  (shown as “DP?” in block  234 ) of the de-rating curve  301  (at point  314 ). If so, the method  200  proceeds to block  236 . If not, method  200  loops back to block  232 . 
     The control module  30  may be programmed to enter a fourth stage D when the image content is available; and the junction temperature (T J ) is between the first threshold temperature (T 1 ) and the second threshold temperature (T 2 ). The fourth stage D includes block  236 . In block  236 , the control module  30  is programmed to reduce the power supplied to the LED array  17  by the driver circuit  36  to a predetermined rating, such that the predetermined rating is less than a maximum allowable rating of the LED array  17 . 
     In summary, the method  200  may include four stages, A through D. The first stage A uses the LED array  17  as a thermal source to accelerate the preheating of liquid crystal when the image is inactive. The second stage B uses the LED array  17  and image adjustment/control to increase warming of the liquid crystal layer  16  when the image is active. The third stage C allows full luminance of the LED array  17  for a pre-defined duration while above a predefined second threshold temperature. In the fourth stage D, the power rating of the LED is reduced to a predetermined reduced rating less than the maximum rated capabilities. 
     The control module  30  (and execution of the method  200 ) improves the functioning of the device  12  by improving the readability of the image I observed in the display unit  14 , thereby improving accuracy of user interaction with the device  12 . For example, a user may rely on the readability of the image I to make control decisions for the device  12 , e.g. changing the trajectory of the device  12 . 
     Referring to  FIG. 1 , the control module  30 , the driver circuit  36 , the video image adjustment module  48 , the control unit  60  may each include a respective computer-readable medium (also referred to as a processor-readable medium), including any non-transitory (e.g., tangible) medium that participates in providing data (e.g., instructions) that may be read by a computer (e.g., by a processor of a computer). Such a medium may take many forms, including, but not limited to, non-volatile media and volatile media. Non-volatile media may include, for example, optical or magnetic disks and other persistent memory. Volatile media may include, for example, dynamic random access memory (DRAM), which may constitute a main memory. Such instructions may be transmitted by one or more transmission media, including coaxial cables, copper wire and fiber optics, including the wires that comprise a system bus coupled to a processor of a computer. Some forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, an EPROM, a FLASH-EEPROM, any other memory chip or cartridge, or any other medium from which a computer can read. 
     Look-up tables, databases, data repositories or other data stores described herein may include various kinds of mechanisms for storing, accessing, and retrieving various kinds of data, including a hierarchical database, a set of files in a file system, an application database in a proprietary format, a relational database management system (RDBMS), etc. Each such data store may be included within a computing device employing a computer operating system such as one of those mentioned above, and may be accessed via a network in any one or more of a variety of manners. A file system may be accessible from a computer operating system, and may include files stored in various formats. An RDBMS may employ the Structured Query Language (SQL) in addition to a language for creating, storing, editing, and executing stored procedures, such as the PL/SQL language mentioned above. 
     The detailed description and the drawings or figures are supportive and descriptive of the invention, but the scope of the invention is defined solely by the claims. While some of the best modes and other embodiments for carrying out the claimed invention have been described in detail, various alternative designs and embodiments exist for practicing the invention defined in the appended claims. Furthermore, the embodiments shown in the drawings or the characteristics of various embodiments mentioned in the present description are not necessarily to be understood as embodiments independent of each other. Rather, it is possible that each of the characteristics described in one of the examples of an embodiment can be combined with one or a plurality of other desired characteristics from other embodiments, resulting in other embodiments not described in words or by reference to the drawings. Accordingly, such other embodiments fall within the framework of the scope of the appended claims.