U.S. patent application number 13/459642 was filed with the patent office on 2012-08-23 for method and apparatus for sensing, measurement or characterization of display elements integrated with the display drive scheme, and system and applications using the same.
This patent application is currently assigned to QUALCOMM MEMS TECHNOLOGIES, INC.. Invention is credited to Alok Govil.
Application Number | 20120212468 13/459642 |
Document ID | / |
Family ID | 40666776 |
Filed Date | 2012-08-23 |
United States Patent
Application |
20120212468 |
Kind Code |
A1 |
Govil; Alok |
August 23, 2012 |
METHOD AND APPARATUS FOR SENSING, MEASUREMENT OR CHARACTERIZATION
OF DISPLAY ELEMENTS INTEGRATED WITH THE DISPLAY DRIVE SCHEME, AND
SYSTEM AND APPLICATIONS USING THE SAME
Abstract
Methods and systems for electrical sensing, measurement and
characterization of display elements are described. An embodiment
includes integrating the electrical sensing, measurement and
characterization with the display drive scheme. This embodiment
allows for measurement of DC or operational hysteresis voltages
and/or response times of interferometric modulator MEMS devices,
for example, to be fully integrated with the display driver IC
and/or the display drive scheme. Another embodiment allows these
measurements to be performed and used without resulting in display
artifacts visible to a human user. Another embodiment allows the
measurement circuitry to be integrated with the display driver IC
and/or the display drive scheme re-using several existing circuitry
components and features, thus allowing for integration of the
measurement method and its use relatively easily.
Inventors: |
Govil; Alok; (Santa Clara,
CA) |
Assignee: |
QUALCOMM MEMS TECHNOLOGIES,
INC.
San Diego
CA
|
Family ID: |
40666776 |
Appl. No.: |
13/459642 |
Filed: |
April 30, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12369678 |
Feb 11, 2009 |
8169426 |
|
|
13459642 |
|
|
|
|
61027727 |
Feb 11, 2008 |
|
|
|
Current U.S.
Class: |
345/208 |
Current CPC
Class: |
B81C 99/003 20130101;
B81B 2201/042 20130101; G09G 2320/0693 20130101; G09G 2320/0295
20130101; G09G 2310/066 20130101; G09G 3/006 20130101; G09G 3/3466
20130101 |
Class at
Publication: |
345/208 |
International
Class: |
G06F 3/038 20060101
G06F003/038 |
Claims
1. A method of operating a device, comprising: applying a drive
signal across a first and second electrodes of the device, wherein
the drive signal alters a state of the device from a first state to
a second state and back to the first state, wherein the transition
from the first state to the second state and back to the first
state has a duration which causes the transition to be
substantially undetectable to a viewer; and determining at least
one operational characteristic of the device based on an electrical
response of the device to the applied drive signal.
2. The method of claim 1, wherein the transition from the first
state to the second state and back to the first state is completed
in less than 400 microseconds.
3. The method of claim 1, wherein the transition from the first
state to the second state and back to the first state is completed
in more than about 400 microseconds and less than about 4000
microseconds.
4. The method of claim 1, further comprising repeating the applying
and the determining on a periodic time basis.
5. The method of claim 1, further comprising repeating the applying
and the determining on a pseudorandom time basis.
6. The method of claim 1, further comprising repeating the applying
and the determining at a time based on a temperature change.
7. The method of claim 1, further comprising performing the
applying and the determining based on an age of the device.
8. The method of claim 1, wherein the applying is performed
multiple times at various levels and an operational characteristic
is determined for each of the various levels, the method further
comprising: storing information indicative of the operational
characteristic and the level for each of the various levels, and
determining a drive level based on the stored information and a
predetermined operational characteristic.
9. The method of claim 1, wherein determining the at least one
operational characteristic comprises determining one or more of an
actuation voltage, a release voltage and a response time.
10. The method of claim 1, wherein the drive signal comprises one
or more of a sinusoid, a saw tooth and a rectangular pulse.
11. A characterization apparatus, comprising: drive circuitry
configured to apply a drive signal across first and second
electrodes of a device, wherein the drive signal alters a state of
the device from a first state to a second state and back to the
first state, wherein the transition from the first state to the
second state and back to the first state has a duration which
causes the transition to be substantially undetectable to a viewer;
and a processor configured to receive a response signal indicative
of an electrical response of the device to the applied drive
signal, and to determine at least one operational characteristic of
the device based on the response signal.
12. The apparatus of claim 11, wherein the transition from the
first state to the second state and back to the first state is
completed in less than 400 microseconds.
13. The apparatus of claim 11, wherein the transition from the
first state to the second state and back to the first state is
completed in more than about 400 microseconds and less than about
4000 microseconds.
14. The apparatus of claim 11, wherein the processor is configured
to control the drive circuitry to apply the drive signal on a
periodic time basis.
15. The apparatus of claim 11, wherein the processor is configured
to control the drive circuitry to apply the drive signal on a
pseudorandom time basis.
16. The apparatus of claim 11, wherein the processor is configured
to control the drive circuitry to apply the drive signal at a time
based on a temperature change.
17. The apparatus of claim 11, wherein the processor is configured
to control the drive circuitry to apply the drive signal based on
the age of the device.
18. The apparatus of claim 11, wherein the processor is configured
to control the drive circuitry to apply the drive signal multiple
times at various levels, determine an operational characteristic
associated with each of the multiple levels, store information
indicative of the operational characteristics and the levels, and
determine a drive level based on the stored information and a
predetermined operational characteristic.
19. The apparatus of claim 18, wherein the at least one operational
characteristic comprises one or more of an actuation voltage, a
release voltage and a response time.
20. The apparatus of claim 11, wherein the signal comprises one or
more of a sinusoid, a saw tooth and a rectangular pulse.
21. The apparatus of claim 11, wherein the processor is configured
to control the drive circuitry to apply the drive signal to provide
temporal averaging of a displayed color.
22. A characterization device, comprising: means for applying a
drive signal across first and second electrodes of a device,
wherein the drive signal alters a state of the device from a first
state to a second state and back to the first state, wherein the
transition from the first state to the second state and back to the
first state has a duration which causes the transition to be
substantially undetectable to a viewer; and means for receiving a
response signal indicative of an electrical response of the device
to the applied drive signal, and to determine at least one
operational characteristic of the device based on the response
signal.
23. The device of claim 22, wherein the means for applying
comprises drive circuitry.
24. The device of claim 22, wherein the means for receiving
comprises a processor.
25. The device of claim 22, wherein the transition from the first
state to the second state and back to the first state is completed
in less than 400 microseconds.
26. The device of claim 22, wherein the transition from the first
state to the second state and back to the first state is completed
in more than about 400 microseconds and less than about 4000
microseconds.
27. The device of claim 22, wherein the applying means is
configured to apply the drive signal at a time based on a
temperature change.
28. The device of claim 22, wherein the applying means is
configured to apply the drive signal at a time based on an age of
the device.
29. The device of claim 22, the wherein the applying means is
configured to apply the drive signal multiple times at various
levels and the means for receiving is configured to determine an
operational characteristic for each of the various levels, the
device further comprising: means for storing information indicative
of the operational characteristic and the level for each of the
various levels, and means for determining a drive level based on
the stored information and a predetermined operational
characteristic.
30. The device of claim 22, wherein the means for receiving is
configured to determine one or more of an actuation voltage, a
release voltage and a response time.
31. A characterization device comprising: an array of
interferometric modulators; drive circuitry configured to apply a
drive signal across first and second electrodes of a device,
wherein the drive signal alters a state of the device from a first
state to a second state and back to the first state, wherein the
transition from the first state to the second state and back to the
first state has a duration which causes the transition to be
substantially undetectable to a viewer; a processor configured to
receive a response signal indicative of an electrical response of
the device to the applied drive signal, and to determine at least
one operational characteristic of the device based on the response
signal; and a memory device configured to communicate with the
processor.
32. The device of claim 31, further comprising a controller
configured to send image data to the drive circuitry.
33. The device of claim 31, further comprising an image source
module configured to send image data to the processor.
34. The device of claim 33, wherein the image source module
comprises at least one of a receiver, transceiver, and
transmitter.
35. The device of claim 31, further comprising an input device
configured to receive input data and to communicate the input data
to the processor.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/369,678, filed Feb. 11, 2009, titled
"METHOD AND APPARATUS FOR SENSING, MEASUREMENT OR CHARACTERIZATION
OF DISPLAY ELEMENTS INTEGRATED WITH THE DISPLAY DRIVE SCHEME, AND
SYSTEM AND APPLICATIONS USING THE SAME," which claims the benefit
of U.S. Provisional Application No. 61/027,727, filed Feb. 11,
2008, titled "METHOD AND APPARATUS FOR SENSING, MEASUREMENT OR
CHARACTERIZATION OF DISPLAY ELEMENTS INTEGRATED WITH THE DISPLAY
DRIVE SCHEME, AND SYSTEM AND APPLICATIONS USING THE SAME." Each of
the foregoing applications is hereby expressly incorporated by
reference in its entirety.
BACKGROUND
[0002] 1. Field of the Invention
[0003] This invention relates to microelectromechanical systems.
More particularly, this invention relates to methods and apparatus
for improving the performance of microelectromechanical systems
such as interferometric modulators.
[0004] 2. Description of the Related Art
[0005] Microelectromechanical systems (MEMS) include micro
mechanical elements, actuators, and electronics. Micromechanical
elements may be created using deposition, etching, and/or other
micromachining processes that etch away parts of substrates and/or
deposited material layers or that add layers to form electrical and
electromechanical devices. One type of MEMS device is called an
interferometric modulator. As used herein, the term interferometric
modulator or interferometric light modulator refers to a device
that selectively absorbs and/or reflects light using the principles
of optical interference. In certain embodiments, an interferometric
modulator may comprise a pair of conductive plates, one or both of
which may be transparent and/or reflective in whole or part and
capable of relative motion upon application of an appropriate
electrical signal. In a particular embodiment, one plate may
comprise a stationary layer deposited on a substrate and the other
plate may comprise a metallic membrane separated from the
stationary layer by an air gap. As described herein in more detail,
the position of one plate in relation to another can change the
optical interference of light incident on the interferometric
modulator. Such devices have a wide range of applications, and it
would be beneficial in the art to utilize and/or modify the
characteristics of these types of devices so that their features
can be exploited in improving existing products and creating new
products that have not yet been developed.
[0006] The systems, methods, and devices described herein each have
several aspects, no single one of which is solely responsible for
its desirable attributes. Without limiting the scope, prominent
features will now be discussed briefly. After considering this
discussion, and particularly after reading the section entitled
"Detailed Description of Certain Embodiments" one will understand
how the features described herein provide advantages over other
display devices.
SUMMARY
[0007] One aspect is a method, including applying a signal waveform
between a first electrode and a second electrode of a display
element, where the signal alters a state of the display element
from a first state to a second state and back to the first state,
where the transition from the first state to the second state and
back to the first state is substantially undetectable to human
vision, measuring an electrical response of the display element in
response to the applied signal, and determining at least one
operational characteristic of the display element based on the
measured electrical response.
[0008] Another aspect is an apparatus, including drive circuitry
configured to apply a signal between a first electrode and a second
electrode of a display element, where the signal alters a state of
the display element from a first state to a second state and back
to the first state, where the transition from the first state to
the second state and back to the first state is substantially
undetectable to human vision, feedback circuitry configured to
measure an electrical response of the display element in response
to the applied voltage waveform, and a processor configured to
control the drive circuitry, receive information indicative of the
measured electrical response, and determine at least one
operational characteristic of the display element based on the
measured electrical response.
[0009] Another aspect is a display device, including means for
applying a signal between a first electrode and a second electrode
of a display element, where the signal alters a state of the
display element from a first state to a second state and back to
the first state, where the transition from the first state to the
second state and back to the first state is substantially
undetectable to human vision, means for measuring an electrical
response of the display element in response to the applied voltage
waveform, and means for receiving information indicative of the
measured electrical response and for determining at least one
operational characteristic of the display element based on the
measured electrical response.
[0010] Another aspect is a display device including an array of
interferometric modulators, drive circuitry configured to apply a
signal between a first electrode and a second electrode of at least
one of the interferometric modulators, where the signal alters a
state of the interferometric modulator from a first state to a
second state and back to the first state, where the transition from
the first state to the second state and back to the first state is
substantially undetectable to human vision, feedback circuitry
configured to measure an electrical response of the interferometric
modulator in response to the applied voltage waveform, a processor
configured control the drive circuitry, receive information
indicative of the measured electrical response, and determine at
least one operational characteristic of the interferometric
modulator based on the measured electrical response, and a memory
device configured to communicate with the processor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is an isometric view depicting a portion of one
embodiment of an interferometric modulator display in which a
movable reflective layer of a first interferometric modulator is in
a relaxed position and a movable reflective layer of a second
interferometric modulator is in an actuated position.
[0012] FIG. 2 is a system block diagram illustrating one embodiment
of an electronic device incorporating a 3.times.3 interferometric
modulator display.
[0013] FIG. 3 is a diagram of movable mirror position versus
applied voltage for one exemplary embodiment of an interferometric
modulator of FIG. 1.
[0014] FIG. 4 is an illustration of a set of row and column
voltages that may be used to drive an interferometric modulator
display.
[0015] FIG. 5A illustrates one exemplary frame of display data in
the 3.times.3 interferometric modulator display of FIG. 2.
[0016] FIG. 5B illustrates one exemplary timing diagram for row and
column signals that may be used to write the frame of FIG. 5A.
[0017] FIGS. 6A and 6B are system block diagrams illustrating an
embodiment of a visual display device comprising a plurality of
interferometric modulators.
[0018] FIG. 7A is a cross section of the device of FIG. 1.
[0019] FIG. 7B is a cross section of an alternative embodiment of
an interferometric modulator.
[0020] FIG. 7C is a cross section of another alternative embodiment
of an interferometric modulator.
[0021] FIG. 7D is a cross section of yet another alternative
embodiment of an interferometric modulator.
[0022] FIG. 7E is a cross section of an additional alternative
embodiment of an interferometric modulator.
[0023] FIG. 8 is a block diagram illustrating an example system
configured to drive a display array and measure an electrical
response of selected display elements, such as the interferometric
modulator display device of FIG. 2.
[0024] FIG. 9 is a block diagram illustrating another example of
circuitry that can be used to measure an electrical response of
selected display elements via the same circuitry used to apply a
stimulus to the selected display elements, such as in the
interferometric modulator display device of FIG. 2.
[0025] FIG. 10A is a flowchart illustrating an example of a method
of driving a display element, such as, for example, the
interferometric modulator as illustrated in FIG. 1, where a ramped
drive voltage is used.
[0026] FIG. 10B is a flowchart illustrating a method of calibrating
drive voltages for driving a display element including determining
a drive voltage based on a desired operational characteristic of
the display element.
[0027] FIG. 10C is a flowchart illustrating another method of
calibrating drive voltages for driving a display element including
adjusting a drive voltage based on identifying an error condition
when driving the display element.
[0028] FIG. 11A is an illustration of an example of a ramped
voltage waveform for driving a display element.
[0029] FIG. 11B is an illustration of a sensed electrical response
of drive circuitry connected to the display element that may be
used in the methods illustrated in FIGS. 10A and 10B.
[0030] FIG. 12 illustrates an example of a drive voltage waveform
for driving a display element and a corresponding electrical
response sensed in drive circuitry connected to the display
element, such as may be used in the methods illustrated in FIGS.
10A and 10B.
[0031] FIG. 13A illustrates an example of a drive voltage waveform
and corresponding electrical response indicative of proper
actuation of a display element, such as may be used in the method
illustrated in FIG. 10C.
[0032] FIG. 13B illustrates an example of a drive voltage waveform
and corresponding electrical response indicative of an example of
erroneous actuation of a display element, such as may be used in
the method illustrated in FIG. 10C.
[0033] FIG. 14 is a flowchart illustrating a method for driving a
display element and measuring an electrical response of the display
element to determine a drive voltage to achieve a desired
operational characteristic, where the drive voltage results in a
display state transition that is substantially undetectable to
human vision.
[0034] FIG. 15 illustrates an example of a drive voltage waveform
and corresponding sensed electrical response that may be used in
the method illustrated in FIG. 15.
[0035] FIG. 16A is a block diagram illustrating an example of
circuitry for driving an isolated portion of a display array and
for sensing an electrical response of the isolated area.
[0036] FIG. 16B illustrates an equivalent circuit illustrating the
electrical relationship of capacitance of a display area being
sensed, and capacitances of other display areas not being
sensed.
DETAILED DESCRIPTION
[0037] The following detailed description is directed to certain
specific embodiments. However, other embodiments may be used and
some elements can be embodied in a multitude of different ways. In
this description, reference is made to the drawings wherein like
parts are designated with like numerals throughout. As will be
apparent from the following description, the embodiments may be
implemented in any device that is configured to display an image,
whether in motion (e.g., video) or stationary (e.g., still image),
and whether textual or pictorial. More particularly, it is
contemplated that the embodiments may be implemented in or
associated with a variety of electronic devices such as, but not
limited to, mobile telephones, wireless devices, personal data
assistants (PDAs), hand-held or portable computers, GPS
receivers/navigators, cameras, MP3 players, camcorders, game
consoles, wrist watches, clocks, calculators, television monitors,
flat panel displays, computer monitors, auto displays (e.g.,
odometer display, etc.), cockpit controls and/or displays, display
of camera views (e.g., display of a rear view camera in a vehicle),
electronic photographs, electronic billboards or signs, projectors,
architectural structures, packaging, and aesthetic structures
(e.g., display of images on a piece of jewelry). MEMS devices of
similar structure to those described herein can also be used in
non-display applications such as in electronic switching
devices.
[0038] Methods and systems for electrical sensing, measurement and
characterization of display elements are described. An embodiment
includes integrating the electrical sensing, measurement and
characterization with the display drive scheme. This embodiment
allows for measurement of DC or operational hysteresis voltages
and/or response times of interferometric modulator MEMS devices,
for example, to be fully integrated with the display driver IC
and/or the display drive scheme. Another embodiment allows these
measurements to be performed and used without resulting in display
artifacts visible to a human user. Another embodiment allows the
measurement circuitry to be integrated with the display driver IC
and/or the display drive scheme re-using several existing circuitry
components and features, thus allowing for integration of the
measurement method and its use relatively easily.
[0039] One interferometric modulator display embodiment comprising
an interferometric MEMS display element is illustrated in FIG. 1.
In these devices, the pixels are in either a bright or dark state.
In the bright ("on" or "open") state, the display element reflects
a large portion of incident visible light to a user. When in the
dark ("off" or "closed") state, the display element reflects little
incident visible light to the user. Depending on the embodiment,
the light reflectance properties of the "on" and "off" states may
be reversed. MEMS pixels can be configured to reflect predominantly
at selected colors, allowing for a color display in addition to
black and white.
[0040] FIG. 1 is an isometric view depicting two adjacent pixels in
a series of pixels of a visual display, wherein each pixel
comprises a MEMS interferometric modulator. In some embodiments, an
interferometric modulator display comprises a row/column array of
these interferometric modulators. Each interferometric modulator
includes a pair of reflective layers positioned at a variable and
controllable distance from each other to form a resonant optical
gap with at least one variable dimension. In one embodiment, one of
the reflective layers may be moved between two positions. In the
first position, referred to herein as the relaxed position, the
movable reflective layer is positioned at a relatively large
distance from a fixed partially reflective layer. In the second
position, referred to herein as the actuated position, the movable
reflective layer is positioned more closely adjacent to the
partially reflective layer. Incident light that reflects from the
two layers interferes constructively or destructively depending on
the position of the movable reflective layer, producing either an
overall reflective or non-reflective state for each pixel.
[0041] The depicted portion of the pixel array in FIG. 1 includes
two adjacent interferometric modulators 12a and 12b. In the
interferometric modulator 12a on the left, a movable reflective
layer 14a is illustrated in a relaxed position at a predetermined
distance from an optical stack 16a, which includes a partially
reflective layer. In the interferometric modulator 12b on the
right, the movable reflective layer 14b is illustrated in an
actuated position adjacent to the optical stack 16b.
[0042] The optical stacks 16a and 16b (collectively referred to as
optical stack 16), as referenced herein, typically comprise several
fused layers, which can include an electrode layer, such as indium
tin oxide (ITO), a partially reflective layer, such as chromium,
and a transparent dielectric. The optical stack 16 is thus
electrically conductive, partially transparent, and partially
reflective, and may be fabricated, for example, by depositing one
or more of the above layers onto a transparent substrate 20. The
partially reflective layer can be formed from a variety of
materials that are partially reflective such as various metals,
semiconductors, and dielectrics. The partially reflective layer can
be formed of one or more layers of materials, and each of the
layers can be formed of a single material or a combination of
materials.
[0043] In some embodiments, the layers of the optical stack 16 are
patterned into parallel strips, and may form row electrodes in a
display device as described further below. The movable reflective
layers 14a, 14b may be formed as a series of parallel strips of a
deposited metal layer or layers (orthogonal to the row electrodes
of 16a, 16b) deposited on top of posts 18 and an intervening
sacrificial material deposited between the posts 18. When the
sacrificial material is etched away, the movable reflective layers
14a, 14b are separated from the optical stacks 16a, 16b by a
defined gap 19. A highly conductive and reflective material such as
aluminum may be used for the reflective layers 14, and these strips
may form column electrodes in a display device.
[0044] With no applied voltage, the gap 19 remains between the
movable reflective layer 14a and optical stack 16a, with the
movable reflective layer 14a in a mechanically relaxed state, as
illustrated by the pixel 12a in FIG. 1. However, when a potential
difference is applied to a selected row and column, the capacitor
formed at the intersection of the row and column electrodes at the
corresponding pixel becomes charged, and electrostatic forces pull
the electrodes together. If the voltage is high enough, the movable
reflective layer 14 is deformed and is forced against the optical
stack 16. A dielectric layer (not illustrated in this Figure)
within the optical stack 16 may prevent shorting and control the
separation distance between layers 14 and 16, as illustrated by
pixel 12b on the right in FIG. 1. The behavior is the same
regardless of the polarity of the applied potential difference. In
this way, row/column actuation that can control the reflective vs.
non-reflective pixel states is analogous in many ways to that used
in conventional LCD and other display technologies.
[0045] FIGS. 2 through 5B illustrate one exemplary process and
system for using an array of interferometric modulators in a
display application.
[0046] FIG. 2 is a system block diagram illustrating one embodiment
of an electronic device that may incorporate aspects of the
invention. In the exemplary embodiment, the electronic device
includes a processor 21 which may be any general purpose single- or
multi-chip microprocessor such as an ARM, Pentium.RTM., Pentium
II.RTM., Pentium III.RTM., Pentium IV.RTM., Pentium.RTM. Pro, an
8051, a MIPS.RTM., a Power PC.RTM., an ALPHA.RTM., or any special
purpose microprocessor such as a digital signal processor,
microcontroller, or a programmable gate array. As is conventional
in the art, the processor 21 may be configured to execute one or
more software modules. In addition to executing an operating
system, the processor may be configured to execute one or more
software applications, including a web browser, a telephone
application, an email program, or any other software
application.
[0047] In one embodiment, the processor 21 is also configured to
communicate with an array driver 22. In one embodiment, the array
driver 22 includes a row driver circuit 24 and a column driver
circuit 26 that provide signals to a display array or panel 30. The
cross section of the array illustrated in FIG. 1 is shown by the
lines 1-1 in FIG. 2. For MEMS interferometric modulators, the
row/column actuation protocol may take advantage of a hysteresis
property of these devices illustrated in FIG. 3. It may require,
for example, a 10 volt potential difference to cause a movable
layer to deform from the relaxed state to the actuated state.
However, when the voltage is reduced from that value, the movable
layer maintains its state as the voltage drops back below 10 volts.
In the exemplary embodiment of FIG. 3, the movable layer does not
relax completely until the voltage drops below 2 volts. Thus, there
exists a window of applied voltage, about 3 to 7 V in the example
illustrated in FIG. 3, within which the device is stable in either
the relaxed or actuated state. This is referred to herein as the
"hysteresis window" or "stability window." For a display array
having the hysteresis characteristics of FIG. 3, the row/column
actuation protocol can be designed such that during row strobing,
pixels in the strobed row that are to be actuated are exposed to a
voltage difference of about 10 volts, and pixels that are to be
relaxed are exposed to a voltage difference of close to zero volts.
After the strobe, the pixels are exposed to a steady state voltage
difference of about 5 volts such that they remain in whatever state
the row strobe put them in. After being written, each pixel sees a
potential difference within the "stability window" of 3-7 volts in
this example. This feature makes the pixel design illustrated in
FIG. 1 stable under the same applied voltage conditions in either
an actuated or relaxed pre-existing state. Since each pixel of the
interferometric modulator, whether in the actuated or relaxed
state, is essentially a capacitor formed by the fixed and moving
reflective layers, this stable state can be held at a voltage
within the hysteresis window with almost no power dissipation.
Essentially no current flows into the pixel if the applied
potential is fixed.
[0048] In typical applications, a display frame may be created by
asserting the set of column electrodes in accordance with the
desired set of actuated pixels in the first row. A row pulse is
then applied to the row 1 electrode, actuating the pixels
corresponding to the asserted column lines. The asserted set of
column electrodes is then changed to correspond to the desired set
of actuated pixels in the second row. A pulse is then applied to
the row 2 electrode, actuating the appropriate pixels in row 2 in
accordance with the asserted column electrodes. The row 1 pixels
are unaffected by the row 2 pulse, and remain in the state they
were set to during the row 1 pulse. This may be repeated for the
entire series of rows in a sequential fashion to produce the frame.
Generally, the frames are refreshed and/or updated with new display
data by continually repeating this process at some desired number
of frames per second. A wide variety of protocols for driving row
and column electrodes of pixel arrays to produce display frames are
also well known and may be used in conjunction with the present
invention.
[0049] FIGS. 4, 5A, and 5B illustrate one possible actuation
protocol for creating a display frame on the 3.times.3 array of
FIG. 2. FIG. 4 illustrates a possible set of column and row voltage
levels that may be used for pixels exhibiting the hysteresis curves
of FIG. 3. In the FIG. 4 embodiment, actuating a pixel involves
setting the appropriate column to -V.sub.bias, and the appropriate
row to +.DELTA.V, which may correspond to -5 volts and +5 volts,
respectively. Relaxing the pixel is accomplished by setting the
appropriate column to +V.sub.bias, and the appropriate row to the
same +.DELTA.V, producing a zero volt potential difference across
the pixel. In those rows where the row voltage is held at zero
volts, the pixels are stable in whatever state they were originally
in, regardless of whether the column is at +V.sub.bias, or
-V.sub.bias. As is also illustrated in FIG. 4, it will be
appreciated that voltages of opposite polarity than those described
above can be used, e.g., actuating a pixel can involve setting the
appropriate column to +V.sub.bias, and the appropriate row to
-.DELTA.V. In this embodiment, releasing the pixel is accomplished
by setting the appropriate column to -V.sub.bias, and the
appropriate row to the same -.DELTA.V, producing a zero volt
potential difference across the pixel.
[0050] FIG. 5B is a timing diagram showing a series of row and
column signals applied to the 3.times.3 array of FIG. 2 which will
result in the display arrangement illustrated in FIG. 5A, where
actuated pixels are non-reflective. Prior to writing the frame
illustrated in FIG. 5A, the pixels can be in any state, and in this
example, all the rows are at 0 volts, and all the columns are at +5
volts. With these applied voltages, all pixels are stable in their
existing actuated or relaxed states.
[0051] In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and
(3,3) are actuated. To accomplish this, during a "line time" for
row 1, columns 1 and 2 are set to -5 volts, and column 3 is set to
+5 volts. This does not change the state of any pixels, because all
the pixels remain in the 3-7 volt stability window. Row 1 is then
strobed with a pulse that goes from 0, up to 5 volts, and back to
zero. This actuates the (1,1) and (1,2) pixels and relaxes the
(1,3) pixel. No other pixels in the array are affected. To set row
2 as desired, column 2 is set to -5 volts, and columns 1 and 3 are
set to +5 volts. The same strobe applied to row 2 will then actuate
pixel (2,2) and relax pixels (2,1) and (2,3). Again, no other
pixels of the array are affected. Row 3 is similarly set by setting
columns 2 and 3 to -5 volts, and column 1 to +5 volts. The row 3
strobe sets the row 3 pixels as shown in FIG. 5A. After writing the
frame, the row potentials are zero, and the column potentials can
remain at either +5 or -5 volts, and the display is then stable in
the arrangement of FIG. 5A. It will be appreciated that the same
procedure can be employed for arrays of dozens or hundreds of rows
and columns. It will also be appreciated that the timing, sequence,
and levels of voltages used to perform row and column actuation can
be varied widely within the general principles outlined above, and
the above example is exemplary only, and any actuation voltage
method can be used with the systems and methods described
herein.
[0052] FIGS. 6A and 6B are system block diagrams illustrating an
embodiment of a display device 40. The display device 40 can be,
for example, a cellular or mobile telephone. However, the same
components of display device 40 or slight variations thereof are
also illustrative of various types of display devices such as
televisions and portable media players.
[0053] The display device 40 includes a housing 41, a display 30,
an antenna 43, a speaker 45, an input device 48, and a microphone
46. The housing 41 is generally formed from any of a variety of
manufacturing processes as are well known to those of skill in the
art, including injection molding and vacuum forming. In addition,
the housing 41 may be made from any of a variety of materials,
including, but not limited to, plastic, metal, glass, rubber, and
ceramic, or a combination thereof. In one embodiment, the housing
41 includes removable portions (not shown) that may be interchanged
with other removable portions of different color, or containing
different logos, pictures, or symbols.
[0054] The display 30 of exemplary display device 40 may be any of
a variety of displays, including a bi-stable display, as described
herein. In other embodiments, the display 30 includes a flat-panel
display, such as plasma, EL, OLED, STN LCD, or TFT LCD as described
above, or a non-flat-panel display, such as a CRT or other tube
device, as is well known to those of skill in the art. However, for
purposes of describing the present embodiment, the display 30
includes an interferometric modulator display, as described
herein.
[0055] The components of one embodiment of exemplary display device
40 are schematically illustrated in FIG. 6B. The illustrated
exemplary display device 40 includes a housing 41 and can include
additional components at least partially enclosed therein. For
example, in one embodiment, the exemplary display device 40
includes a network interface 27 that includes an antenna 43, which
is coupled to a transceiver 47. The transceiver 47 is connected to
a processor 21, which is connected to conditioning hardware 52. The
conditioning hardware 52 may be configured to condition a signal
(e.g., filter a signal). The conditioning hardware 52 is connected
to a speaker 45 and a microphone 46. The processor 21 is also
connected to an input device 48 and a driver controller 29. The
driver controller 29 is coupled to a frame buffer 28 and to an
array driver 22, which in turn is coupled to a display array 30. A
power supply 50 provides power to all components as required by the
particular exemplary display device 40 design.
[0056] The network interface 27 includes the antenna 43 and the
transceiver 47 so that the exemplary display device 40 can
communicate with one or more devices over a network. In one
embodiment, the network interface 27 may also have some processing
capabilities to relieve requirements of the processor 21. The
antenna 43 is any antenna known to those of skill in the art for
transmitting and receiving signals. In one embodiment, the antenna
transmits and receives RF signals according to the IEEE 802.11
standard, including IEEE 802.11(a), (b), or (g). In another
embodiment, the antenna transmits and receives RF signals according
to the BLUETOOTH standard. In the case of a cellular telephone, the
antenna is designed to receive CDMA, GSM, AMPS, or other known
signals that are used to communicate within a wireless cell phone
network. The transceiver 47 pre-processes the signals received from
the antenna 43 so that they may be received by and further
manipulated by the processor 21. The transceiver 47 also processes
signals received from the processor 21 so that they may be
transmitted from the exemplary display device 40 via the antenna
43.
[0057] In an alternative embodiment, the transceiver 47 can be
replaced by a receiver. In yet another alternative embodiment,
network interface 27 can be replaced by an image source, which can
store or generate image data to be sent to the processor 21. For
example, the image source can be a digital video disc (DVD) or a
hard-disc drive that contains image data, or a software module that
generates image data.
[0058] Processor 21 generally controls the overall operation of the
exemplary display device 40. The processor 21 receives data, such
as compressed image data from the network interface 27 or an image
source, and processes the data into raw image data or into a format
that is readily processed into raw image data. The processor 21
then sends the processed data to the driver controller 29 or to
frame buffer 28 for storage. Raw data typically refers to the
information that identifies the image characteristics at each
location within an image. For example, such image characteristics
can include color, saturation, and gray-scale level.
[0059] In one embodiment, the processor 21 includes a
microcontroller, CPU, or logic unit to control operation of the
exemplary display device 40. Conditioning hardware 52 generally
includes amplifiers and filters for transmitting signals to the
speaker 45, and for receiving signals from the microphone 46.
Conditioning hardware 52 may be discrete components within the
exemplary display device 40, or may be incorporated within the
processor 21 or other components.
[0060] The driver controller 29 takes the raw image data generated
by the processor 21 either directly from the processor 21 or from
the frame buffer 28 and reformats the raw image data appropriately
for high speed transmission to the array driver 22. Specifically,
the driver controller 29 reformats the raw image data into a data
flow having a raster-like format, such that it has a time order
suitable for scanning across the display array 30. Then the driver
controller 29 sends the formatted information to the array driver
22. Although a driver controller 29, such as a LCD controller, is
often associated with the system processor 21 as a stand-alone
Integrated Circuit (IC), such controllers may be implemented in
many ways. They may be embedded in the processor 21 as hardware,
embedded in the processor 21 as software, or fully integrated in
hardware with the array driver 22.
[0061] Typically, the array driver 22 receives the formatted
information from the driver controller 29 and reformats the video
data into a parallel set of waveforms that are applied many times
per second to the hundreds and sometimes thousands of leads coming
from the display's x-y matrix of pixels.
[0062] In one embodiment, the driver controller 29, array driver
22, and display array 30 are appropriate for any of the types of
displays described herein. For example, in one embodiment, driver
controller 29 is a conventional display controller or a bi-stable
display controller (e.g., an interferometric modulator controller).
In another embodiment, array driver 22 is a conventional driver or
a bi-stable display driver (e.g., an interferometric modulator
display). In one embodiment, a driver controller 29 is integrated
with the array driver 22. Such an embodiment is common in highly
integrated systems such as cellular phones, watches, and other
small area displays. In yet another embodiment, display array 30 is
a typical display array or a bi-stable display array (e.g., a
display including an array of interferometric modulators).
[0063] The input device 48 allows a user to control the operation
of the exemplary display device 40. In one embodiment, input device
48 includes a keypad, such as a QWERTY keyboard or a telephone
keypad, a button, a switch, a touch-sensitive screen, or a
pressure- or heat-sensitive membrane. In one embodiment, the
microphone 46 is an input device for the exemplary display device
40. When the microphone 46 is used to input data to the device,
voice commands may be provided by a user for controlling operations
of the exemplary display device 40.
[0064] Power supply 50 can include a variety of energy storage
devices as are well known in the art. For example, in one
embodiment, power supply 50 is a rechargeable battery, such as a
nickel-cadmium battery or a lithium ion battery. In another
embodiment, power supply 50 is a renewable energy source, a
capacitor, or a solar cell including a plastic solar cell, and
solar-cell paint. In another embodiment, power supply 50 is
configured to receive power from a wall outlet.
[0065] In some embodiments, control programmability resides, as
described above, in a driver controller which can be located in
several places in the electronic display system. In some
embodiments, control programmability resides in the array driver
22. Those of skill in the art will recognize that the
above-described optimizations may be implemented in any number of
hardware and/or software components and in various
configurations.
[0066] The details of the structure of interferometric modulators
that operate in accordance with the principles set forth above may
vary widely. For example, FIGS. 7A-7E illustrate five different
embodiments of the movable reflective layer 14 and its supporting
structures. FIG. 7A is a cross section of the embodiment of FIG. 1,
where a strip of metal material 14 is deposited on orthogonally
extending supports 18. In FIG. 7B, the moveable reflective layer 14
is attached to supports at the corners only, on tethers 32. In FIG.
7C, the moveable reflective layer 14 is suspended from a deformable
layer 34, which may comprise a flexible metal. The deformable layer
34 connects, directly or indirectly, to the substrate 20 around the
perimeter of the deformable layer 34. These connections are herein
referred to as support posts. The embodiment illustrated in FIG. 7D
has support post plugs 42 upon which the deformable layer 34 rests.
The movable reflective layer 14 remains suspended over the gap, as
in FIGS. 7A-7C, but the deformable layer 34 does not form the
support posts by filling holes between the deformable layer 34 and
the optical stack 16. Rather, the support posts are formed of a
planarization material, which is used to form support post plugs
42. The embodiment illustrated in FIG. 7E is based on the
embodiment shown in FIG. 7D, but may also be adapted to work with
any of the embodiments illustrated in FIGS. 7A-7C, as well as
additional embodiments not shown. In the embodiment shown in FIG.
7E, an extra layer of metal or other conductive material has been
used to form a bus structure 44. This allows signal routing along
the back of the interferometric modulators, eliminating a number of
electrodes that may otherwise have had to be formed on the
substrate 20.
[0067] In embodiments such as those shown in FIG. 7, the
interferometric modulators function as direct-view devices, in
which images are viewed from the front side of the transparent
substrate 20, the side opposite to that upon which the modulator is
arranged. In these embodiments, the reflective layer 14 optically
shields the portions of the interferometric modulator on the side
of the reflective layer opposite the substrate 20, including the
deformable layer 34. This allows the shielded areas to be
configured and operated upon without negatively affecting the image
quality. Such shielding allows the bus structure 44 in FIG. 7E,
which provides the ability to separate the optical properties of
the modulator from the electromechanical properties of the
modulator, such as addressing and the movements that result from
that addressing. This separable modulator architecture allows the
structural design and materials used for the electromechanical
aspects and the optical aspects of the modulator to be selected and
to function independently of each other. Moreover, the embodiments
shown in FIGS. 7C-7E have additional benefits deriving from the
decoupling of the optical properties of the reflective layer 14
from its mechanical properties, which are carried out by the
deformable layer 34. This allows the structural design and
materials used for the reflective layer 14 to be optimized with
respect to the optical properties, and the structural design and
materials used for the deformable layer 34 to be optimized with
respect to desired mechanical properties.
[0068] The following description is directed to methods and devices
used to provide, monitor and adapt drive voltages of a wide variety
of MEMS elements, such as MEMS switches, and other elements having
deflected or deformed electrodes and/or mirrors. Although the
specific examples discussed use interferometric modulators as the
elements, the principles discussed may apply to other MEMS elements
as well.
[0069] Display devices like those based on interferometric
modulator technology may be measured and characterized
electronically and/or mechanically. Depending on the display
technology, these measurements can form a part of calibration of
the display module (the display "module" referred to herein
includes the display panel, the display driver, and associated
components such as cabling, etc.), and the measurement parameters
may be stored into a non-volatile memory (e.g., NVRAM) in the
display module for future use. As discussed above with reference to
FIG. 3, the interferometric modulators operate based on a potential
difference applied to them. FIG. 3 shows that the interferometric
modulators are in either the relaxed (or released) state or in the
actuated state, depending on the magnitude of the potential
difference applied between their electrodes. As shown, the changing
of one state to another happens according to a hysteretic
characteristic with a stability (or hold) window, where the device
holds its current state when the applied potential difference falls
within the hold window. As used herein, a "bias voltage" refers to
a potential difference that falls within the hold window.
Accordingly, as shown in FIG. 3, there are five input voltage
difference ranges in some embodiments. Each of the five voltage
difference ranges has a title reflecting its effect on the state of
the interferometric modulator. Starting from the left of FIG. 3,
the five voltage difference ranges are: 1) negative actuate
("Actuated"); 2) negative hold ("Stability Window"); 3) release
("Relaxed"); 4) positive hold ("Stability Window"); and 5) positive
actuate ("Actuated").
[0070] Based on theoretical understanding of the devices and past
experimental results, approximate values of the thresholds between
these input voltage difference ranges may be known, but in order to
more optimally operate the interferometric modulator array, the
threshold voltages can be measured with more precision. For
example, as described further herein, the thresholds may vary from
device to device, lot to lot, over temperature, and/or as the
device ages. Threshold values may accordingly be measured for each
manufactured device or group of devices. One method of measuring
the threshold voltages is to apply inputs of various voltage
differences while monitoring the state of the interferometric
modulators through observation of the optical characteristics of
the interferometric modulators. This may be accomplished, for
example, through human observation or by use of an optical
measurement device. Additionally or alternatively, the state of the
interferometric modulators may be monitored through electronic
response measurement. In some embodiments, the array driver 22 of
the display array 30, discussed above, may be configured to measure
electrical responses of display elements in order to determine the
state and/or operational characteristics of the display elements
according to the methods discussed below.
[0071] Often times, the behavior of a display device changes with
the age of the display device, with variations in temperature of
the display, with the content of the images being displayed, etc.
Display devices may have one or more electrical parameters that
change in relation to the optical response or optical state. As
discussed above, the interferometric modulator is set to an
actuated state when the electrostatic attraction between the
reflective layer and the optical stack is great enough to overcome
the mechanical restorative forces working to hold the reflective
layer in the relaxed state. Because the reflective layer, the
optical stack, and the gap between them form two conductive plates
separated by a dielectric, the structure has a capacitance. Also,
because the capacitance of the structure varies according to the
distance between the two plates, the capacitance of the structure
varies according to the state of the interferometric modulator.
Therefore, an indication of the capacitance can be used to
determine the state of the interferometric modulator.
[0072] In one aspect, an indication of the capacitance can be
obtained, for example, by sensing the current or charge used to
change the voltage applied between the reflective layer and the
optical stack. A relatively high amount of current or charge
indicates that the capacitance is relatively large. Similarly, a
relatively low amount of current or charge indicates that the
capacitance is relatively small. The sensing of current or charge
may be accomplished, for example through analog or digital
integration of a signal representing the charge or current.
[0073] Similar characteristics can apply to LCD display technology
where the capacitance of the device is related to the resulting
optical brightness of the cell at a certain temperature. In
addition to the operational characteristics of display element
possibly changing with age, the operational characteristics can be
affected by the temperature of the display elements. The
temperature of a display element can depend on the past optical
response states that were displayed, and, thus, the operational
characteristics could vary independently for each display element
in the display array of the display device.
[0074] In one embodiment, the relevant characteristics of the
display device, like hysteresis voltages and response times for
interferometric modulator MEMS devices and brightness-voltage
relationship for LCD devices, are measured after manufacturing at
the factory during a calibration procedure. This information can
then be stored in a memory the display module used for driving the
display device. Since the characteristics of the display device may
also change with temperature and aging, for example, the effects of
temperature and aging on these characteristics (e.g., temperature
coefficient) may be studied, measured and also hardwired or stored
in the memory of the display module. In spite of this
post-manufacturing characterization, however, the calibration
margins built into the display device may not allow for
unpredictable changes in the characteristics of the display device.
In some cases, the lifetime and quality of a display device may be
improved by performing recalibration of the device after a certain
period of use (e.g., one year), on a random length periodic basis,
based on changes in temperature, etc. In other cases, the drive
scheme may be robust enough to compensate for changes in
characteristics of the display device without such recalibration.
Examples of such recalibration and robust drive schemes are
discussed below.
[0075] FIG. 8 is a block diagram illustrating an example system 100
configured to drive a display array 102 and measure an electrical
response of selected display elements, such as the interferometric
modulators 12a and 12b of FIG. 1. The display array 102 comprises m
columns by n rows of N-component pixels (e.g., N may be 3 display
elements including red, green and blue, for example). The system
100 further includes a column driver comprising 2 or more digital
to analog converters (DACs) 104 for supplying two or more drive
voltage levels as well as a column switch subsystem 106 for
selecting the columns to which data signals are supplied. The
system 100 further includes a row driver circuit comprising two or
more DACs 108 for supplying two or more drive voltage levels as
well as a row switch circuit 110 for selecting which row to strobe.
Note that the row and column drivers that are directly connected to
the display array in this schematic are shown as composed of
switches, but several methods discussed below are applicable to
alternative driver designs including a full analog display driver.
Note that while drive voltages are discussed herein, other drive
signals, such as drive currents or drive charges may be used.
[0076] The row and column driver circuitry including the DACs 104
and 108 and the switches 106 and 110 are controlled by digital
logic of an array driver 112. As discussed above in reference to
FIGS. 2 and 3, the row/column actuation protocol contained in the
digital logic of the array driver 112 may take advantage of a
hysteresis property of interferometric modulator MEMS devices. For
example, in a display array comprising interferometric modulators
12 having the hysteresis characteristics of FIG. 3, the row/column
actuation protocol can be designed such that during row strobing,
display elements in the strobed row that are to be actuated are
exposed to an actuation voltage difference (e.g., about 10 volts),
and display elements that are to be relaxed are exposed to a
voltage difference of close to zero volts, as shown in FIGS. 4 to
5. After the strobe, the display elements are exposed to a steady
state voltage difference known as the bias voltage (e.g., about 5
volts) such that they remain in whatever state the row strobe last
put them. After being written, each display element sees a
potential difference within the "stability window" of 3-7 volts in
this example. However, as discussed above, the characteristics of
the display elements may change with time and/or temperature or may
respond more quickly or slowly to different drive voltage levels.
As such, the array driver 112 and the DACs 104 and 108 may be
configured to supply variable voltage levels, depending on the
embodiment.
[0077] In addition to the drive circuitry discussed above
(including the DACs 104 and 108 and the switches 106 and 110, and
the array driver 112), the remaining blocks of the system 100 are
able to apply further electrical stimulus to selected display
elements, as well as to be able to measure the electrical response
of selected display elements in the display array 102. In this
example, digital-to-analog converters (DACs) 114 and 116 supply
additional voltages to the display array 102 via the column and row
switches 106 and 110, respectively. In general, these may represent
internal or external voltage supply inputs to the row and column
drive circuitry.
[0078] In this example, a direct-digital-synthesis (DDS1) block 118
is used to generate the electrical voltage stimulus that is added
on the top of the voltage level produced by the DAC 114 connected
to the column switch 106. The stimulus signal produced by the DDS1
block 118 may be produced by several alternative means such as an
electrical oscillator, a saw-tooth waveform generator, etc. which
are familiar to those skilled in the art. In various embodiments,
the stimulus may be current or charge, or even a controlled output
impedance.
[0079] In the example shown in FIG. 8, the electrical response of
the display array 102 is measured in the form of electrical current
flowing through the display array 102 resulting from application of
the electrical voltage stimulus to the row and/or column electrodes
via the row and/or column switches 106 and 110, respectively. Other
forms of measured electrical response can include voltage
variations, etc. A trans-impedance amplifier 120 (shown in FIG. 8
as a resistor 120A followed by an amplifier 120B) may be used to
measure the electrical response. The display element(s) for which
the measured electrical response corresponds depends on the states
of the column and row switches 106 and 110. In alternative
embodiments, analog, digital, or mixed-signal processing may be
used for the purpose of measurement of the electrical response of
the display array 102.
[0080] In one embodiment, the electrical response of a display
element is measured directly by measuring the current through the
input terminals of the trans-impedance amplifier 120. In this
embodiment, the profile and/or peak values, or other
characteristics known to skilled technologists, can be used to
identify certain operational characteristics of the display
element.
[0081] In another embodiment, operational characteristics of the
display element being measured can be characterized by additional
post processing of the electrical response output from the
trans-impedance amplifier 120. An example of using post processing
techniques to characterize the capacitance and the resistive
component of the impedance of an interferometric modulator using
the circuitry of FIG. 8 is now discussed.
[0082] Since an interferometric modulator can be considered a
capacitor, a periodic stimulus, such as that which could be applied
using the DDS1 118, will result in a periodic output electrical
response with a 90.degree. phase lag. For example, the DDS1 118
could apply a sinusoidal voltage waveform, sin(wt), to the column
electrode of the display element. For an ideal capacitor, the
electrical response of the display element would be a time
derivative of the applied stimulus, or cos(wt). Thus, the output of
the trans-impedance amplifier 120 would also be a cosine function.
A second DDS, DDS2 122, applies a cosine voltage waveform that is
multiplied by the output of the trans-impedance amplifier 120 at
multiplier 124. The result is a waveform with a constant component
and a periodic component. The constant component of the output of
the multiplier 124 is proportional to the capacitance of the
display element. A filter 126 is used to filter out the periodic
component and result in an electrical signal that is used to
characterize the capacitance, and therefore the actuated or
unactuated state, of the display element.
[0083] For a display element that is an ideal capacitor, the output
of the trans-impedance amplifier 120 is a pure cosine function for
the example where the applied stimulus is a sine function. However,
if the display element exhibits any non-capacitive impedance, due
to leakage for example, the output of the trans-impedance amplifier
120 will also contain a sine component. This sine component does
not affect the measurement of the capacitance, since it will be
filtered out by the filter 126. The sine component can be detected
and used to characterize the resistive portion of the impedance of
the display element.
[0084] A periodic voltage waveform similar to the stimulus applied
by the DDS1, sin(wt) for example, is multiplied by the output of
the trans-impedance amplifier 120 at a multiplier 128. The result
is an electrical response that includes a constant component and a
periodic component. The constant component is proportional to the
resistive portion of the impedance of the display element being
measured. A filter 130 is used to remove the periodic component
resulting in a signal that can be used to characterize the
resistive portion of the impedance of the display element.
[0085] The outputs of the filters are converted to the digital
domain by use of a dual analog to digital converter (ADC) 132. The
output of the dual ADC 132 is received by the array driver 112 for
use in carrying out the methods discussed below.
[0086] In the example circuitry shown in FIG. 8, the
characterization stimulus is applied to a column electrode and the
electrical response is measured via a row electrode. In other
embodiments, the electrical response can be measured from the same
electrode, row or column, for example, to which the stimulus is
applied. FIG. 9 is a block diagram illustrating an example of
circuitry 150 that can be used to measure an electrical response of
selected display elements via the same circuitry used to apply a
stimulus to the selected display elements, such as in the
interferometric modulator display device of FIG. 2. The circuit 150
comprises transistors N1 and P1 which mirror the current from the
current source transistors N2 and P2 used to drive the V.sub.out
signal applied to the display element. Accordingly, the current
I.sub.out is substantially equal to the current used for driving
the V.sub.out signal. Measuring the electrical response of the
I.sub.out signal may, therefore, be used to determine operational
characteristics of the interferometric modulators such as whether
the interferometric modulators are in a high or low capacitance
state. Other circuits may also be used. The circuit 150 shown in
FIG. 9 is applicable to alternative driver IC designs or drive
schemes for supplying a voltage waveform V.sub.out. The circuit 150
depicted in the schematic of FIG. 9 can be used in current conveyor
circuits and in current feedback amplifiers, and can apply an
electrical voltage stimulus to the display array area and
simultaneously replicate the current (response) to a different pin
(I.sub.out) for purposes of electrical sensing.
[0087] There are several ways in which measured electrical
responses, such as those sensed by the systems shown in FIGS. 8 and
9, can be used as a feedback signal to affect the operation of the
display driver circuitry. For example, the measured information may
be analyzed in the digital domain, e.g., using the digital logic of
array driver 112 and/or a processor configured to control the array
driver 112 (e.g., the processor 21 and array driver 22 shown in
FIG. 2) and then used to adaptively drive the display array 102.
The measured electrical responses may also be used to complete a
feedback loop in the analog domain (e.g., using the outputs of the
DACs 104, 114, 108 and/or 116, or using the output of the DDS1 118
shown in FIG. 8). Examples of methods of driving interferometric
modulator display elements using measured electrical responses as
feedback are illustrated in FIGS. 10A-10C.
[0088] FIG. 10A is a flowchart illustrating an example of a method
200A of driving a display element, such as, for example, the
interferometric modulator as illustrated in FIG. 1, where a ramped
drive voltage is used. In one embodiment, the method 200A can be
performed by the array driver 112 for controlling the drive
circuitry (e.g., the DACs 104, 108 and 114, the switches 106 and
110, and the DDS1 118) shown in FIG. 8 to display images on the
display array 102. In other embodiments, a processor such as the
processor 21 in FIG. 2, can perform the method 200A. The method
200A provides a method of adapting drive voltage levels by applying
a gradually increasing or decreasing voltage waveform to a display
element and discontinuing the application of the voltage waveform
when a change in state of the display element is sensed. In this
way, the applied voltages, including drive voltages to actuate or
release the display element, can be changed only as much as
necessary, thereby conserving power.
[0089] The method 200A starts at block 202 where the array driver
112 applies a drive voltage between a first electrode and a second
electrode of a display element. The first electrode may be one of
the movable reflective layers (column electrodes) 14 and the second
electrode may be one of the row electrodes 16 of the
interferometric modulators 12 illustrated in FIG. 1. The drive
voltage applied at block 202 may be a voltage at the bias voltage
within the hysteresis window (e.g., 3-7 volts as discussed above),
or, alternatively may be a static voltage level outside of the
hysteresis window. As used herein, a static voltage is a voltage
that is non-varying over time, such as over an actuation period.
The static drive voltage difference applied to the two electrodes
at block 202 may be supplied by one or more of the DACs 104 or 108
(FIG. 8) to the column and/or row electrodes, respectively.
[0090] After the initial drive voltage is applied at the block 202,
the method 200A continues at block 204, where the array driver 112
ramps the level of the drive voltage from a first level (e.g., the
static voltage level applied at block 202) to a second level. FIG.
11A is an illustration of an example of a ramped voltage waveform
for driving a display element that may be used in the method 200A.
In FIG. 11A, the initial drive voltage applied at the block 202 is
a 5 volt bias voltage 302 (the static voltage applied in block
202). At approximately 2 ms, a ramped voltage waveform 304 is
applied at block 204 in the method 200A. The ramped voltage
waveform 304 continues to be increased until a measured electrical
response, as sensed by electrical sensing feedback circuitry such
as the trans-impedance amplifier 120 in FIG. 8, monitors an
electrical response of the display element at block 206. For
example, the trans-impedance amplifier 120 may sense a change in
the current to or from the display element, indicating a change in
state of the display element.
[0091] In this example, the monitored electrical response is
indicative of a change of state of the interferometric modulators
12 of FIG. 1. FIG. 11B is an illustration of a sensed electrical
response that may be sensed with the electrical sensing feedback
circuitry connected to the drive circuitry of the display element
using the method 200A illustrated in FIG. 10A. At about 4 ms, the
sensed electrical current shows a sharp rise 306 to a level of
about +5 milliamps. The sensitivity of the amplifier to the sensed
electrical current can depend on the resistance of the circuitry
being used for sensing. For example, in an embodiment such as that
shown in FIG. 8, the resistance of the resistor 120A may be chosen
to result in a output amplitude that is easily measurable,
depending on the feedback circuitry. Upon detecting the rise 306 in
the sensed current in block 206, the method 200A continues to block
208, where the ramped voltage waveform is discontinued as shown at
308 in FIG. 11A and reduced to the static (bias) voltage level of 5
volts at 310 to allow the interferometric modulator to remain in
the actuated state. In the example shown in FIG. 11A, the ramped
voltage results in actuation of the display element at about 6
volts. This is merely an example actuation level and other levels
of voltage may result in actuation, depending on the design of the
display element.
[0092] Although described above with respect to an actuation
signal, a release signal can also be applied by the array driver
112 at the block 202 of the method 200A. For example, as shown in
FIG. 11A at about 6 ms, a release procedure is initiated and a
ramped voltage waveform 312 is applied. The ramped voltage 312,
applied at the block 204 of method 200A reduces the drive voltage
from the initial 5 volts (that was applied at the block 202) to
about 4 volts. When the ramped voltage waveform reaches about 4
volts, interferometric modulator 12 releases and the electrical
sensing circuitry measures a sharp decline 314 in the sensed
current (sensed at the block 206) to a level of about -3 milliamps,
indicating that the display element has released. Upon sensing the
decline in current at 314 due to the change in IMOD state, the
method 200A continues to block 208, where the ramped drive voltage
waveform is discontinued and the drive voltage is reduced (see 316)
to the 5 volt bias voltage level at 318 such that the display
element remains in the released state. Once again, the voltage and
current levels shown in FIG. 11 are exemplary only, and other
levels may be indicative of actuation and or release of a display
element. The ramped voltage waveform applied at the block 204 may
be applied using the DDS1 118 illustrated in FIG. 8.
[0093] In some embodiments, the rate of increase or decrease of the
ramped voltage waveform is at a predetermined rate that is slow
relative to the response time of the display element when an
actuation and/or release event occurs. In this way, the change in
voltage levels from the bias level to the actuation and/or release
voltage levels can be minimized. In another embodiment, the rate of
increase and/or decrease in the ramped voltage waveform is
calibrated and chosen in order to achieve a desired operational
characteristic of the display element, such as, for example
response time.
[0094] FIG. 10B is a flowchart illustrating a method 200B of
calibrating drive voltages for driving a display element. In one
embodiment, the method 200B can be used to determine an operational
threshold drive voltage based on a desired operational
characteristic of the display element, e.g., response time. The
method 200B includes a calibration portion, blocks 220 to 234,
which, in one embodiment, can be performed at the time of
manufacture of the display element for initial calibration. In this
embodiment, the process 200B can be performed by an external
processor connected to the display array, such as a test stand, for
example.
[0095] In another embodiment, the calibration blocks 220 to 234 can
also be included in logic coupled to the display array so that the
calibration can be performed at other times in order to recalibrate
the display element. For example, the recalibration may be done on
a periodic basis based on the age of the display element, on a
pseudo-random basis, based on temperature, etc. In this embodiment,
the method 200B can be performed using the array driver 112 for
controlling the drive circuitry (e.g., the DACs 104, 108 and 114,
the switches 106 and 110, and the DDS1 118) shown in FIG. 8 to
display images on the display array 102. In other embodiments, a
processor such as the processor 21 in FIG. 2 can perform the method
200A. After calibration, the array driver 112 may determine a drive
voltage (e.g., an initial drive voltage level and/or a ramped
voltage rate) in order to achieve a desired operational
characteristic.
[0096] At block 220, the array driver 112 applies a drive voltage
between a first electrode and a second electrode of a display
element. The first electrode may be one of the movable reflective
layers (column electrodes) 14 and the second electrode may be one
of the row electrodes 16 of the interferometric modulator
illustrated in FIG. 1. The drive voltage applied at block 220 may
be a static voltage at a bias voltage level within the hysteresis
window (e.g., 3-7 volts as discussed above), or, alternatively may
be a static voltage outside of the hysteresis window. By selecting
different static voltage levels outside of the hysteresis window,
an operational characteristic of the display element in response to
a static, i.e., non-ramped, drive voltage may be determined.
Operational characteristics that may be affected by the various
static drive voltage levels applied at the block 220 include
response time, maximum sensed current level, amount of stiction,
release voltage level, actuation voltage level, etc. The static
drive voltage difference applied to the two electrodes at block 220
may be supplied by one or more of the DACs 104 or 108 to the column
and/or row electrodes, respectively.
[0097] At block 222, the array driver 112 ramps the level of the
drive voltage from a first level, e.g., the static voltage level
applied at block 202, to a second level. The rate of increasing or
decreasing ramped voltage levels (slope of ramp) may be varied for
multiple calibration tests. In this way, the operational
characteristic(s) of the display element may be determined for the
various ramped voltage rates. Operational characteristics that may
be affected by the various ramped voltage rates applied at the
block 222 include response time, maximum current level, amount of
stiction, release voltage level, actuation voltage level, etc. The
ramped voltage waveform applied at the block 222 may be applied
using the DDS1 118 illustrated in FIG. 8.
[0098] In some embodiments, where the DDS1 118 is faster than the
DAC 114, the DDS1 118 is used to supply the variable portion of the
signal and the DAC 114 is used to supply the static portion of the
signal. In addition in some embodiments, the DDS1 118 may be
configured to generate the waveforms autonomously. In some
embodiments, a DDS is configured to generate a static voltage, and
one or more DACs may be used to generate a variable portion of the
signal. In some embodiments, one or more DACs or DDS's may be used
to generate either or both of the variable and static portions of
the signal.
[0099] The method 200B continues at block 224, where the array
driver 112 monitors the electrical sensing feedback circuitry
(e.g., the trans-impedance amplifier 120) for the electrical
response of the display element. The monitoring functions performed
at the step 224 are similar to those discussed above in reference
to the block 206 of the method 200A. For example, the
trans-impedance amplifier 120 may sense a change in the current to
or from the display element, indicating a change in state of the
display element. At the block 226, the array driver 112 that is
receiving the monitored electrical response detects a change of
state of the display element. The change of state may be an
actuation or a release of the display element. Upon detecting the
change of state of the display element at the block 226, the array
driver 112 discontinues the ramping of the drive voltage (if a
ramped voltage was applied at the block 222) at block 228 and the
method 200B continues to the block 230, where information
indicative of the drive voltage is stored, e.g., the static voltage
level applied at the block 220 and/or the ramped voltage rate
applied at the block 222. In addition, at the block 230, the array
driver 112 stores information indicative of the change of state of
the display element and optionally an operational characteristic of
the display element.
[0100] The remaining blocks of FIG. 10B are discussed in reference
to FIG. 12. In one embodiment, a response time of the display
element is monitored. FIG. 12 illustrates an example of a drive
voltage waveform for driving a display element and the
corresponding electrical response sensed in drive circuitry (e.g.,
the row and/or column electrodes in the row or column switches 110
and 106) connected to the display element, such as may be used in
the methods illustrated in FIGS. 10A and 10B. The example of FIG.
12 shows the drive voltage transitioning from a bias voltage level
where the display element is stable, e.g. in a released state. At
time 320, a static drive voltage is applied (e.g., at the block 220
in the method 200A) that results in actuation of the display
element. The sensed electrical response, current in this example,
exhibits a first current spike 322 indicating that the voltage
across the electrodes has changed abruptly, followed by a current
"bump" 324 which is indicative of the actuation event. The time
between the current spike 322 and the current bump 324 is
indicative of the response time (an operational characteristic) of
the display element in response to the applied drive voltage. After
the current bump 324 is sensed by the electrical sensing circuitry,
the drive voltage is discontinued at the block 228 (FIG. 10B) and
returned to the bias voltage level at 326. When the drive voltage
is reduced to the bias voltage level at 326, the sensed electrical
response exhibits another spike 328 indicating that the voltage
difference between the electrodes of the display element has been
abruptly reduced.
[0101] The determination of the response time of the display
element is an example of one type of operational characteristic
that may be determined at the block 226 (FIG. 10B) and stored in
reference to the applied voltage level (the static voltage level
and/or the ramped voltage rate) at the block 230. In some
embodiments of the display array 202, the response time is reduced
at higher or more quickly ramped voltage levels (e.g. where a
strong electrostatic attraction causes the movable element to
rapidly switch states, where at higher temperatures the spring
constant is reduced for the restoring mechanical element, and the
like). Other operational characteristics that may be determined and
stored in reference to the applied voltage waveforms include
maximum sensed current level, amount of stiction, release voltage
level, actuation voltage level, etc. At decision block 234, the
array driver 112 controlling the calibration method 200B determines
if more calibration cases remain to be tested. If more tests
remain, the blocks 220 to 234 are repeated for multiple drive
periods until no more tests remain and the method 200B proceeds to
block 236.
[0102] At the block 236, the array driver 112 determines a drive
voltage (the static voltage level applied at the block 220 and/or
the ramped voltage rate applied at the block 222) based on the
information stored at the block 230 to achieve a desired
operational characteristic. For example, it may be desired to
achieve a response time below a certain time threshold in order to
more quickly display an image on a display array comprising the
display elements for which the drive voltages and characteristics
were calibrated. In another example, it may be desired to keep the
peak current level below a certain value in order to keep
temperatures below a certain level.
[0103] In some embodiments, the methods 200A and 200B may be
performed in unison. For example, the functions performed at the
block 236 may be performed in conjunction with the method 200A to
perform the actuation and release functions of the display element
until another calibration process (e.g., the functions at the
blocks 220 to 234) is performed at a later time. It should be noted
that certain blocks of the methods 200A and 200B may be omitted,
combined, rearranged, or combinations thereof.
[0104] The methods illustrated in FIGS. 10A and 10B are examples of
methods that provide feedback by sensing the electrical response of
drive circuitry, for example, where the feedback detects that a
display element has been properly actuated or relaxed in response
to a given drive voltage. Another embodiment provides feedback that
may be used to sense when a display element has not actuated or
released properly. Such feedback may be used to adjust the drive
voltages to correct the erroneous actuation and/or release
states.
[0105] FIG. 10C is a flowchart illustrating another method 200C of
calibrating drive voltages for driving a display element including
adjusting a drive voltage based on identifying an error condition
when driving the display element. In one embodiment, the method
200C can be used for calibrating the drive voltages of certain
display elements for initial testing during or after manufacture of
a display array. This could be done in parallel with the method
200B discussed above. In this embodiment, the process 200C can be
performed by an external processor connected to the display array,
such as a test stand, for example. In another embodiment, the
method 200C can be used for adjusting the drive voltage of display
elements during operation upon detecting a failure to actuate a
display element while the array driver 112 is driving the display
array 102 to display an image. This later embodiment will be
discussed in the example shown in FIG. 10C.
[0106] The method 200C starts at block 250, where the array driver
112 applies a drive voltage between a first electrode and a second
electrode of a display element, wherein the drive voltage is at a
level predetermined to result in the display element being in a
first of a plurality of display states. The first electrode may be
one of the movable reflective layers (column electrodes) 14 and the
second electrode may be one of the row electrodes 16 of the
interferometric modulators 12 illustrated in FIG. 1, or vice versa.
The drive voltage applied at block 250 may be at a level that has
been predetermined to result in actuation of a released display
element (e.g., a voltage magnitude above the bias voltage range), a
level that has been predetermined to result in release of an
actuated display element (e.g., a voltage level lower in magnitude
that the bias voltage range), or a voltage level that has been
predetermined to keep the display element in the current display
state (e.g., a voltage magnitude within the bias voltage hysteresis
window as discussed above).
[0107] As discussed above in reference to FIG. 12, release and/or
actuation of a display element can be identified by observing
certain electrical response characteristics that can be measured by
feedback circuitry. At block 252, the feedback circuitry is used to
measure an electrical response of the display element in response
to the drive voltage applied by the drive circuitry at the block
250. The feedback circuitry may comprise elements such as the
trans-impedance amplifier 120 in FIG. 8. At block 254, a processor
receives information indicative of the electrical response measured
at the block 252. The array driver 112 analyzes the characteristics
of the measured electrical response in order to identify an error
in operation of the display element.
[0108] An example of a correct actuation and an example of an
erroneous actuation of display elements will now be discussed. FIG.
13A illustrates an example of a drive voltage waveform and
corresponding electrical responses indicative of proper actuation
of an interferometric modulator, such as may be used in the method
200C illustrated in FIG. 10C. In this example, a released
interferometric modulator 12 is driven to move from a released
state to an actuated state. The initial voltage difference between
the two electrodes is at a level 331 that is below the actuation
voltage threshold level (e.g., within the bias voltage level)
V.sub.act in FIG. 13A. At a time point 330, the drive voltage is
increased to a level 333 above V.sub.act. Beginning at the time
point 330, the feedback circuitry measurement, current in this
example, shows an initial spike 332 followed by a second bump 334.
The second bump is indicative that the interferometric modulator 12
has actuated properly. At a second time point 336, the drive
voltage is reduced to the level 331 below V.sub.act (within the
bias voltage region). At the time point 336, a feedback current
exhibits a single spike 338. There is no second bump similar to the
bump 334 in the feedback current. This lack of a second bump is
indicative that the display element properly remained in the
actuated state after the time point 336.
[0109] FIG. 13B illustrates an example of a drive voltage waveform
and corresponding electrical responses indicative of an example of
erroneous actuation of an interferometric modulator 12, such as may
be used in the method illustrated in FIG. 10C. This example is a
case where the bias voltage level is incorrectly calibrated at a
level that is outside of the bias voltage window. The
interferometric modulator 12 may be incorrectly calibrated due to
changes in the characteristics of the display element due to age
and/or the temperature of the display element, for example.
[0110] In this example, the initial voltage between the electrodes
is at a level 340 that is below the "bias voltage level", i.e., the
level to sustain the interferometric modulator 12 in the current
state. At a time point 342, the voltage between the electrodes is
increase to a level 344 above the actuation voltage level V.sub.act
in order to actuate the interferometric modulator 12. The feedback
current exhibits a first spike 346 followed by a second bump 348
that is indicative of a proper actuation of the interferometric
modulator 12.
[0111] At a second time point 350, the voltage between the
electrodes is returned to the initial voltage level 340. The
feedback current exhibits a first spike 352 followed by a second
bump 354. This is indicative that the interferometric modulator 12
has erroneously released due to the voltage being lowered to the
level 340 that is outside of the bias voltage window (between the
voltage levels V.sub.rel and V.sub.act). By detecting the current
bump, the array driver 112 can identify that an error has occurred
at block 254 of the method 200C. Subsequent to identifying that an
error in operation of the interferometric modulator 12 has
occurred, the array driver 112 can adjust the drive voltage at
block 256 to be at a level greater than V.sub.rel and less than
V.sub.act thereby resulting in a properly tuned interferometric
modulator 12 that remains actuated. The array driver 112 can
determine the adjusted drive voltage level using a method such as
discussed above in reference to FIG. 10B.
[0112] Skilled technologists will readily be able to use similar
methods to identify proper actuation voltage thresholds of an
interferometric modulator 12. For example, if the interferometric
modulator 12 is in the actuated state and the drive voltage applied
between the electrodes is supposed to result in releasing the
interferometric modulator 12, but the interferometric modulator 12
does not release, then the array driver 112 can adjust the voltage
at the block 256 to a lower level until the interferometric
modulator 12 properly releases. In another example, if the
interferometric modulator 12 is in the released state and the
voltage applied at the block 250 is supposed to actuate the
interferometric modulator 12, but the interferometric modulator 12
does not actuate, the array driver 112 can adjust the drive voltage
to a higher value at the block 256 until the interferometric
modulator 12 actuates properly.
[0113] In one embodiment, the method 200C includes an optional
block 258 where the array driver 112 stores information indicative
of the adjusted drive voltage for later use. The adjusted voltage
can be stored with information cross-referencing it to a specific
interferometric modulator 12. The array driver 112 can then use the
adjusted value at a later time when the specific interferometric
modulator 12 is being actuated and/or released again. The voltage
levels stored at the optional block 258 may include bias voltage
levels, release voltage levels and/or actuation voltage levels,
depending on the embodiment.
[0114] FIG. 14 is a flowchart illustrating an example of a method
500 for driving an interferometric modulator 12 and measuring an
electrical response of the interferometric modulator 12 to
determine a drive voltage to achieve a desired operational
characteristic, where the drive voltage results in a display state
transition that is substantially undetectable to human vision. The
method 500, in one embodiment, enables drive voltage levels and/or
ramped drive voltage rates (as discussed above in reference to the
methods 200A and 200B of FIGS. 10A and 10B) to be characterized
during operation of the display array 102 in order to adapt to
changes in drive voltages quickly. Drive voltage levels may change
due to changing conditions such as age and/or temperature of the
interferometric modulator 12. The method 500 can be performed by
the array driver 112 for controlling the drive circuitry (e.g., the
DACs 104, 108 and 114, the switches 106 and 110, and the DDS1 118)
shown in FIG. 8 to display images on the display array 102. In
other embodiments, a processor such as the processor 21 in FIG. 2
can perform the method 500.
[0115] At block 502, the array driver 112 (FIG. 8) applies a
voltage waveform between a first electrode and a second electrode
of an interferometric modulator 12, where the voltage waveform
alters a state of the interferometric modulator 12 from a first
state to a second state and back to the first state. The voltage
waveform applied at the block 502 results in the interferometric
modulator 12 being altered from a released state to an actuated
state and back to the released state, or vice-versa. In other
words, the optical characteristics of the selected interferometric
modulator 12 (or interferometric modulators 12) is momentarily
disturbed for the measurement of the electrical response of the
interferometric modulator 12, but the interferometric modulator 12
is quickly returned to display the original optical response such
that a human observer is not aware of the change of state. As noted
above, in some embodiments the interferometric modulator 12 can
switch states at .about.10 kHz, much faster than human vision can
detect. Note that when a new image is "ripped" on the display array
(e.g., via a line-at-a-time drive scheme), it is usually desirable
that a human user should not be able to perceive the process of one
image being overwritten with another. A suitably fast scan rate or
rip rate is chosen for this purpose. When the image content is
changing anyway, a slight momentary disturbance of the content for
the purpose of measurement can be easily masked from a user.
[0116] FIG. 15 illustrates an example of a drive voltage waveform
and corresponding sensed electrical response that may be used at
the block 502 in the method 500 illustrated in FIG. 15. In this
example, a saw-tooth voltage waveform 520 is applied between the
electrodes of the display element. In one embodiment, the voltage
waveform applied at the block 502 has a duration from start to
finish less than about 400 microseconds. However, some embodiments
may use voltage wave forms having end-to-end time durations from
about 400 microseconds to about 4000 microseconds or larger. The
waveform 520 starts with the display element in the released state
due to the voltage level being at a level 522 below the release
voltage (V.sub.rel) of the display element. The waveform 520 then
ramps up to a level 524 above the actuation voltage level
(V.sub.act) and then ramps down to a level 526 below the Vrel
level. Thus the display element transitions from the released state
to the actuated state and back to the released state faster than
can be detected by the user.
[0117] Other waveform shapes such as square waves, and sinusoidal
waves, for example, can be applied at the block 502 in the method
500. The specific waveforms chosen may depend on the specific
technology and choice of algorithm. The mechanism to apply the
waveform may be similar to those described above in reference to
FIG. 8.
[0118] While the voltage waveform is being applied at the block
502, the feedback circuitry (e.g., the trans-impedance amplifier
120) is monitored at block 504 to measure an electrical response of
the display element in response to the applied waveform. As
discussed above in reference to the methods illustrated in FIGS.
10A, 10B and 10C, an electrical current of the display element can
be monitored to determine if and when an element is released and/or
actuated in response to a given voltage level and/or voltage ramp
rate. In FIG. 15, the sensed current typically exhibits a peak 528
when the voltage level exceeds V.sub.act and another peak 530 when
the voltage declines below V.sub.rel. The current peak 528 is
indicative that the display element has transitioned from the
released state to the actuated state. The current peak 530 is
indicative that the display element has transitioned back to the
released state. The timing of the sensed current peaks exhibit
different characteristics depending on the timing of the actuation
and/or release of the display element in response to the applied
voltage waveform.
[0119] The feedback circuitry discussed above in reference to FIG.
8 may by used to measure the electrical response at the block 204.
The array driver 112 receives information indicative of the
electrical response measured at the block 504, and at block 506
determines at least one operational characteristic of the display
element based on the measured electrical response. The response
time of the display element may be determined at the block 506. The
response time may vary based on the applied peak voltage level
and/or the voltage ramp rate. In addition, the operational
characteristic may include one or more of release voltage levels,
actuation voltage levels and bias voltage levels. These voltage
levels may also vary as a function of temperature of the display
element, age of the display element, etc.
[0120] At optional block 508, the array driver 112 may store
information indicative of the operational characteristic determined
at the block 506 and store information indicative of the voltage
levels applied at the block 502 to which the operational
characteristics correspond. The voltage level information stored at
the block 508 may include peak voltage levels, voltage ramp rate,
voltage waveform shape, voltage waveform time duration, and others.
The operational characteristics information stored at the block 508
can include response time to actuate or release the display
element, actuation voltage levels, release voltage levels, bias
voltage levels, etc. Release and actuation voltage levels may also
be a function of the ramped voltage rate of the waveform, and this
information may also be stored at the block 508.
[0121] After information has been stored at the block 508, the
method 500 optionally continues to block 510, where the array
driver 112 can determine a drive voltage level and/or ramp rate to
apply to a display element based on the information stored at the
block 508 and a desired operational characteristic. In one
embodiment, the operational characteristic may simply be actuation
or release of the display element in order to adapt these voltage
levels to changing environmental conditions or age of the
interferometric modulator 12. In this embodiment, the processor or
array driver may determine the minimum voltage amplitude to actuate
the display element. In another embodiment, the operational
characteristic may be a desired response time. In this embodiment,
the voltage level and/or the voltage ramp rate that best provides
the desired response time is determined at the optional block
510.
[0122] The functions performed at the block 502, 504, 506, and
optionally 508 may be performed on a periodic basis, on a
pseudorandom basis, based on a temperature level or change in
temperature of the display element or display device, based on the
age of the display element, or other basis.
[0123] The determination of drive voltage levels at the optional
block 510 may be performed just prior to the array driver 112
signaling the display elements to display image data during the
normal image writing phase. The determination of drive voltage
levels at the optional block 510 may also be performed on a
periodic basis, on a pseudorandom basis, based on a temperature
level or change in temperature of the display element or display
device, or based on the age of the display element.
[0124] Each of the methods discussed above in reference to FIGS.
10A, 10B, 10C and 14 involve measuring an electrical response of a
display element. There are various methods of sensing different
portions of a display array of display elements. For example, it
may be chosen to sense an entire display array in one test. In
other words, feedback signals from all the row electrodes (or
column electrodes) may always be electrically connected to the
trans-impedance amplifier 120 shown in FIG. 8. In this case, the
timing of the column electrodes being signaled, and the rows being
signaled, may be synchronized by the array driver such that
individual display elements, pixels or sub-pixels (e.g., red, green
and blue sub-pixels) may be monitored at certain times. It may also
be chosen to monitor or measure one or more specific row or column
electrodes at one time and optionally switch to monitor other row
and column electrodes at other times, and repeating with different
rows and/or columns until the entire array is monitored. Finally,
it may also be chosen to measure individual display elements and
optionally switch to monitor or measure the other display elements
until the entire array is measured.
[0125] In one embodiment, one or more selected row or columns
electrodes may be permanently connected to the stimulus and/or
sense circuitry while the remaining row or columns electrodes are
not connected to the stimulus and/or sense circuitry. In some
embodiments, extra electrodes (row or column) are added to the
display area for the purpose of applying the stimulus or sensing.
These additional electrodes may or may not be visible to a viewer
of the display area. Finally, another option is to connect and
disconnect the stimulus/drive and/or sense circuitry to a different
set of one or more row or column electrodes via switches or
alternative electrical components.
[0126] Embodiments of the systems and methods discussed above may
be applied to monochrome, bi-chrome, or multicolor displays. In
some embodiments, groups of pixels for different colors are
measured by suitable choice of row and column electrodes. For
example, if the display uses an RGB layout where Red (R), Green
(G), and Blue (B) sub-pixels are located on different column lines,
areas of individual colors may be measured via application of
stimulus only to the `Red` columns and sensing on the rows.
Alternatively, the stimulus may be applied to the rows, but sensed
only on the `Red` columns.
[0127] In many display technologies, application of a drive pulse
on a given row or column may result in undesirable effects on
neighboring rows or columns. This undesirable effect is commonly
called crosstalk. Crosstalk affects many display technologies
including IMOD, LCD and OLED. In one embodiment, sensing or
feedback circuitry is provided to sense existence of these
undesirable effects and compensate. The signal from the area of
interest can be isolated from the signal or interference from other
regions of a display via various methods.
[0128] FIG. 16A is a block diagram illustrating an example of
circuitry for driving an isolated portion of a display array and
for sensing an electrical response of the isolated area. A voltage
stimulus V.sub.in is applied to a selected set of column electrodes
540 and a current signal is sensed via a trans-impedance amplifier
542 with low input impedance (Z) from a selected set of row
electrodes 544. Thus, a display area 550 is sensed. Display areas
555 and 560 are portions of the column electrodes 540 and the row
electrodes 544, respectively, which are not sensed.
[0129] FIG. 16B illustrates a circuit 580 illustrating the
electrical relationship of capacitance of the display area 550
sensed, and the capacitances of the display areas 555 and 560 not
sensed. Capacitor C2 represents the capacitance of the display area
555, C3 represents that of the display area 560 and C1 represents
that of the display area 550 that is isolated and sensed. The
current consumed by C2 is supplied by V.sub.in and goes directly to
ground. The current through C1, that is the desired current to be
sensed, is also supplied by V.sub.in, but may be affected by the
capacitance C3 before it reaches the trans-impedance amplifier 542.
However, the current through C1 may be forced to go almost entirely
to the trans-impedance amplifier 542 via choice of an appropriately
low input impedance of the trans-impedance amplifier 542 as
compared to the impedance of the capacitance C3. In this case,
there is substantially no signal current via C3. Thus, from the
example circuit 580, only the current through C1, the area 555, is
sensed by the amplifier. Any area of the display can be selected
via corresponding choice of the row and column electrodes. Note
that in the example circuitry of FIG. 16B, the remaining electrodes
not included in the isolated area 550 are depicted as being
connected to ground, however, they could be connected to any
voltage level.
[0130] While the above detailed description has shown, described,
and pointed out novel features as applied to various embodiments,
it will be understood that various omissions, substitutions, and
changes in the form and details of the device or process
illustrated may be made without departing from that which has been
disclosed. As will be recognized, the present invention may be
embodied within a form that does not provide all of the features
and benefits set forth herein, as some features may be used or
practiced separately from others.
* * * * *