U.S. patent application number 12/566750 was filed with the patent office on 2010-01-21 for controllable optical lens.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to Ivon F. Helwegen, Bernardus H.W. Hendriks, Albert H.J. Immink, Stein Kuiper, Marco A.J. Van As.
Application Number | 20100014167 12/566750 |
Document ID | / |
Family ID | 32247563 |
Filed Date | 2010-01-21 |
United States Patent
Application |
20100014167 |
Kind Code |
A1 |
Immink; Albert H.J. ; et
al. |
January 21, 2010 |
CONTROLLABLE OPTICAL LENS
Abstract
A controllable optical lens system, comprises a chamber housing
first and second fluids, the interface between the fluids defining
a lens surface. An electrode arrangement controls the shape of the
lens surface and has first and second electrodes. A parameter is
determined by the system dependent on the electrical resistance
through at least one of the lens fluids between the first and
second electrodes. Thus, the series resistance through a lens fluid
is used as a measure of meniscus position.
Inventors: |
Immink; Albert H.J.;
(Eindhoven, NL) ; Hendriks; Bernardus H.W.;
(Eindhoven, NL) ; Kuiper; Stein; (Neerijnen,
NL) ; Helwegen; Ivon F.; (Herten, NL) ; Van
As; Marco A.J.; (Waalre, NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS
N.V.
EINDHOVEN
NL
|
Family ID: |
32247563 |
Appl. No.: |
12/566750 |
Filed: |
September 25, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10599353 |
Sep 26, 2006 |
7612948 |
|
|
12566750 |
|
|
|
|
Current U.S.
Class: |
359/666 |
Current CPC
Class: |
G02B 26/005 20130101;
G02B 3/14 20130101; Y10S 359/90 20130101 |
Class at
Publication: |
359/666 |
International
Class: |
G02B 3/12 20060101
G02B003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2004 |
GB |
0407240.1 |
Claims
1. A controllable optical lens system, comprising: a chamber
housing first (10) and second (12) fluids, the interface between
the fluids defining a lens surface; an electrode arrangement for
electrically controlling the shape of the lens surface, the
electrode arrangement comprising first (14) and second (16)
electrodes; and means (40, 42, 44, 46) for determining a parameter
dependent on the electrical resistance (R.sub.EW) through at least
one of the lens fluids (10) between the first and second
electrodes.
2. A system as claimed in claim 1, wherein the means for
determining is for determining an electrical resistance (R.sub.EW)
and capacitance (C.sub.EW) between the first and second
electrodes.
3. A system as claimed in claim 1, wherein the means for
determining comprises: an ac power source (40); means (42,44) for
analysing the current supplied by the ac power source.
4. A system as claimed in claim 3, wherein the means for
determining further comprises a first resistor (R.sub.m) in series
between the power source and one of the first and second
electrodes, and wherein the means (42,44) for analysing the current
supplied by the ac power source analyses the voltage drop across
the first resistor.
5. A system as claimed in claim 4, wherein the means for analysing
is for determining a time constant for the response of the
system.
6. A system as claimed in claim 4, further comprising a second
series resistor (R.sub.s) which is selectively switched into
circuit with the first resistor (R.sub.m).
7. A system as claimed in claim 6, wherein the means for analysing
is for determining first and second time constants for the response
of the system with and without the second series resistor.
8. A system as claimed in claim 7, wherein the time constants are
obtained by a best fit analysis.
9. A system as claimed in claim 3, wherein the ac power source
signal used for determining is superposed onto the dc power source
signal used for driving the lens.
10. A system as claimed in claim 1, wherein the electrode
arrangement comprises: a drive electrode arrangement comprising a
base electrode (14) and a side wall electrode (16).
11. A system as claimed in claim 10, wherein the side wall
electrode (16) comprises an annular electrode which surrounds the
chamber.
12. A system as claimed in claim 10, wherein the area of overlap of
one of the fluids (10) with respect to the side wall electrode (16)
varies in dependence on the lens surface position, and the side
wall electrode (16) is formed from a material having a higher
resistance than said one of the fluids.
13. A system as claimed in claim 12, wherein the side wall
electrode (16) is formed from a non-metal.
14. A system as claimed in claim 1, wherein the first liquid (10)
comprises a polar or conductive liquid and the second liquid (12)
comprises a nonconductive liquid.
15. A method of sensing the lens position of a controllable optical
lens, the lens comprising a chamber housing first and second
liquids (10,12), the interface between the liquids defining a lens
surface and an electrode arrangement (14,16) for electrically
controlling the shape of the lens surface, the electrode
arrangement comprising first and second electrodes (14,16), wherein
the method comprises: determining a parameter dependent on the
electrical resistance (R.sub.EW) through at least one of the lens
liquids between the first and second electrodes; and using the
parameter to determine the lens surface position.
16. A method as claimed in claim 15, further comprising determining
an electrical capacitance (C.sub.EW) between the first and second
electrodes.
17. A method as claimed in claim 15, wherein determining a
parameter comprises determining a charging time constant for the
lens
18. A method as claimed in claim 17, wherein determining a
parameter comprises determining two charging time constants for the
lens, one with and without an additional known resistance
(R.sub.s), and further determining the lens capacitance (C.sub.EW)
and resistance (R.sub.EW) from the two time constant
measurements.
19. A method as claimed in claim 17, wherein determining a charging
time constant comprises driving the lens with an AC voltage.
20. A method as claimed in claim 17, wherein determining a charging
time constant comprises driving the lens with the superposition of
a DC voltage and a lower voltage square wave AC voltage.
21. A method as claimed in claim 17, wherein determining a charging
time constant comprises driving the lens with the superposition of
a DC voltage and a lower voltage sinusoidal wave AC voltage and
measuring the phase relation between the voltage and the induced
current through the lens.
22. A method as claimed in claim 17, wherein the time constant is
obtained by a best fit analysis.
23. A method as claimed in claim 17, wherein the time constant is
obtained using a look up table.
Description
[0001] This is a continuation of Ser. No. 10/599,353, filed Sep.
26, 2006 and is incorporated by reference herein.
[0002] This invention relates to a controllable optical lens, in
particular using the so-called electrowetting principle (also known
as electrocapillarity).
[0003] An electrowetting lens comprises a chamber housing two
non-miscible fluids, such as an electrically insulating oil and a
water based conducting salt solution, and the meniscus between
these fluids defines a refractive index boundary and therefore
performs a lens function. The shape of the meniscus is electrically
controllable to vary the power of the lens. The fluid may comprise
a liquid, vapour, gas, plasma or a mixture thereof.
[0004] The electrical control of the lens shape is achieved using
an outer annular control electrode, and the electrowetting effect
is used to control the contact angle of the meniscus at the outside
edge of the chamber, thereby changing the meniscus shape.
[0005] The basic design and operation of an electrowetting lens
will be well known to those skilled in the art. By way of example,
reference is made to WO 03/069380.
[0006] Electrowetting lenses are compact and can provide a variable
focusing function without any mechanical moving parts. They have
been proposed in various applications, particularly where there are
space limitations and where power consumption is to be kept to a
minimum, for example use as an autofocus camera lens in a mobile
phone.
[0007] It has been recognised that sensing the lens condition is
desirable, to provide a feedback control function. Due to slow
charging of the insulator (between the electrodes and the fluids)
the relation between the voltage and the exact position of the
oil-water meniscus is subject to drift, and a feedback system can
compensate for this. If a zoom lens is implemented with multiple
variable lenses, it may not be possible to uniquely derive the lens
characteristics from optical measurements through the multi-element
lens system. It is also therefore desirable to be able to measure
the shape of each individual meniscus in such a system.
[0008] A conventional electrowetting lens has a bottom electrode
and a circumferential wall electrode. It has been proposed that the
capacitance across the electrodes can be measured to provide
feedback about the shape of the lens. In particular, the shape and
the position of the meniscus changes when a voltage is applied, so
that the effective size of the annular electrode changes (the
effective size depends on the area of water in contact with the
electrode, which changes as the meniscus position changes). A
resulting change in capacitance can be measured, and this
capacitance has been considered to be a reasonably accurate
parameter for measuring the strength of the lens.
[0009] The use of measured capacitance to determine the lens
position requires the thickness and dielectric constant of the
insulating coating to be known. This thickness may be subject to
variations form batch to batch.
[0010] According to the invention, there is provided a controllable
optical lens system, comprising: [0011] a chamber housing first and
second fluids, the interface between the fluids defining a lens
surface; [0012] an electrode arrangement for electrically
controlling the shape of the lens surface, the electrode
arrangement comprising first and second electrodes; and [0013]
means for determining a parameter dependent on the electrical
resistance through at least one of the lens fluids between the
first and second electrodes.
[0014] The invention is based on the recognition that the
resistance in series with the lens capacitance will also change in
response to lens power due to the fact that the conducting liquid
changes shape. Thus, the series resistance can also be used as a
measure of meniscus position.
[0015] The means for determining may be for determining an
electrical capacitance and resistance between the first and second
electrodes. Thus, resistance information may be used to supplement
capacitance information.
[0016] The means for determining may comprise an ac power source
and means for analysing the current supplied by the ac power
source. This enables the derivation of the temporal response of the
current through the lens in reaction to an ac stimulus. The
temporal response is dependent on both the capacitance and
resistance across the lens electrodes. A relatively small ac power
source signal may be used, superposed onto the dc drive voltage of
the lens.
[0017] The means for determining may further comprise a first
resistor in series between the power source and one of the first
and second electrodes, wherein the means for analysing the current
supplied by the ac power source analyses the voltage drop across
the first resistor.
[0018] This analysis may yield a time constant for the response of
the system.
[0019] In a further embodiment, the means for determining further
comprises a second series resistor which is selectively switched
into circuit with the first resistor. This enables first and second
time constants to be determined for the response of the system with
and without the second series resistor. In turn, the resistance and
capacitance values can be derived from two time constants.
[0020] The invention also provides a method of sensing the lens
position of a controllable optical lens, the lens comprising a
chamber housing first and second fluids, the interface between the
fluids defining a lens surface, and an electrode arrangement for
electrically controlling the shape of the lens surface, the
electrode arrangement comprising first and second electrodes,
wherein the method comprises:
[0021] determining a parameter dependent on the electrical
resistance through at least one of the lens liquids between the
first and second electrodes; and
[0022] using the parameter to determine the lens surface
position.
[0023] This method uses the resistance as the, or an additional,
parameter in providing feedback concerning the lens meniscus
position.
[0024] Determining the parameter may comprise determining a
charging time constant for the lens, or determining two charging
time constants for the lens, one with and without an additional
known resistance. In this case, the lens capacitance and resistance
can be derived from the two time constant measurements.
[0025] The feedback can be obtained using an AC probe voltage, or
the superposition of a DC voltage and a lower voltage square wave
AC voltage.
[0026] The time constant can be obtained by a best fit analysis
and/or using a look up table.
[0027] Examples of the invention will now be described in detail
with reference to the accompanying drawings, in which:
[0028] FIG. 1 shows a known design of electrowetting lens;
[0029] FIG. 2 shows how resistance through the lens liquid varies
with meniscus position;
[0030] FIG. 3 shows a first example of measurement of the
invention;
[0031] FIG. 4 shows a circuit in a lens system of the
invention;
[0032] FIG. 5 shows an experimental implementation of the circuit
of FIG. 4 for producing the results of FIGS. 6 to 9;
[0033] FIG. 6 shows the temporal response for the lens with two
different voltage drive conditions;
[0034] FIG. 7 shows an analysis of the plots of FIG. 6;
[0035] FIG. 8 shows the temporal response for the lens with two
different voltage drive conditions and with an additional resistor
in circuit; and
[0036] FIG. 9 shows an analysis of the plots of FIG. 8.
[0037] FIG. 10 shows a different lens design which can be used in a
system of the invention; and
[0038] FIG. 11 shows drive and sense circuitry for use with the
lens of FIG. 10.
[0039] FIG. 1 schematically shows a known electrowetting lens
design. The left part of FIG. 1 shows the interior of the lens. The
lens comprises a chamber which houses a polar and/or conductive
liquid such as a salted water based component 10 (referred to below
simply as the water) and a nonconductive liquid such as an oil
based component 12 (referred to below simply as the oil). A bottom
electrode 14 and a circumferential side electrode 16 control the
power of the lens. The side electrode is separated from the liquid
by an insulator which forms the side wall of the chamber, and this
insulator acts as a capacitor dielectric layer during electrical
operation of the lens. This operation will be well known to those
skilled in the art, and reference is made to WO 03/069380.
[0040] The invention provides a controllable optical lens system,
in which the electrical resistance through the lens between the
first and second electrodes is used to provide a lens power
feedback function.
[0041] FIG. 2 shows schematically how the lens shape changes the
resistance. In FIG. 2, both R and C increase when the lens position
is changed from that in the left part of the Figure to that in the
right part of the Figure. Typical resistance values of the liquid
for a lens with a diameter of approximately 1 cm are 500.OMEGA. (a
typical liquid is for example 0.1M KCl in water). The resistance of
the liquid may be much lower if a higher salt concentration is
provided, for example to prevent the liquid from freezing.
[0042] The current practical situation is somewhat more complicated
due the fact that the electrodes of the electro-wetting lens are
ITO layers with a considerable resistance, also in the order of
500.OMEGA.. To reduce the electrode resistance, metal electrodes
may be employed, at least for the side wall electrode which is not
in the light path.
[0043] One possible implementation of the invention is therefore
simply to measure the lens resistance, and this could be performed
with a simple dc analysis.
[0044] However, instead of simply measuring resistance, a combined
resistance and capacitance measurement can be performed, and this
is equivalent to performing a complete impulse response
measurement. The detailed examples of the invention set out below
use measurements which depend both on the resistance and
capacitance of the lens.
[0045] One possibility is to directly drive the electro-wetting
lens with a square wave voltage and measure the current waveform
(for example by converting the current waveform to a voltage
waveform) and sample this by an analogue to digital converter and
process this signal further digitally.
[0046] FIG. 3 shows how sampling of the current waveform in
response to an ac probe signal can be used to derive information
concerning the time constant of the lens, which depends on the
product of the resistance and capacitance.
[0047] In case of a square wave voltage, the RC time constant leads
to an exponential signal shape for the current with a time constant
T=RC as indicated in FIG. 3. A sinusoidal voltage can instead be
applied as the ac voltage. In this case, the phase relation between
the voltage and the induced current can be used to determine the RC
time constant. It will be clear to a person skilled in the art that
there are more ways to determine RC values.
[0048] In case of a square wave voltage, the circles in FIG. 3
indicate a possible sampling of the curve to be able to measure the
impulse response of the lens. From this impulse response
measurement, more accurate information can be derived about the
actual lens position.
[0049] The RC time constant can provide a unique relationship to
the lens position, particularly when they increase or decrease
together, as schematically shown in FIG. 2. Thus, a simple time
constant measurement as above will suffice. This can, as one of
various possible embodiments, be achieved by sampling the voltage
drop across a known resistance R.sub.m placed between the ac power
source and the lens.
[0050] However, it may also be desirable to distinguish between the
values of R and C, and this can be achieved by repeating the
measurement after adding a small and accurately known resistor
R.sub.s, in series with the electro-wetting lens.
[0051] The time constant then will be T'=(R+R.sub.s)C. The
capacitance C can then be calculated as C=(T'-T)/R.sub.s and the
value R of the electro-wetting lens can also be calculated.
[0052] FIG. 4 shows a system for implementing this approach. The
system has an ac power source 40, and the output is provided to the
lens through a measurement resistor R.sub.m and an additional
series resistor R.sub.s. The voltages on the terminals of the
measurement resistor R.sub.m are monitored by differential
amplifier 42 which enables an instantaneous current measurement to
be obtained. The current profile is analysed after A/D conversion
by converter 44, which then enables the time constant to be
obtained.
[0053] The additional series resistor R.sub.s is selectively
switched into or out of circuit by a shorting switch 46. FIG. 4
shows the lens 48 having series resistance R.sub.EW and capacitance
C.sub.EW.
[0054] FIG. 5 shows an experimental implementation of the circuit
of FIG. 4. The lens in this experimental setup was measured using a
network analyzer. It was found that the lens can be best modeled as
a series connection of a resistor of approximately 735.OMEGA. and a
capacitor of 69 pF. The values of the other components are shown in
FIG. 5. A square waveform with a rise-time of 170 ns was used to
provide the experimental results given below. The large series
resistor of 12 k.OMEGA. is used in the DC supply circuit, to avoid
the output impedance of the DC source being measured by the AC
voltage instead of the electro-wetting lens. In an actual
implementation, the AC drive would be included in the DC
supply.
[0055] The lens is initially connected in series with the 10.OMEGA.
measurement resistor only. The exponential decay of current as
function of time is measured by measuring the voltage across the
measurement resistor R.sub.m.
[0056] The result is shown in FIG. 6 for a DC voltage of 0V (plot
60) and a DC voltage of 100V (plot 62). These DC voltages are used
to create different meniscus positions. Plot 64 shows the step
voltage change provided by the ac source. A clear difference in
time-constant can be observed. The time-constant is measured by
calculating the logarithm of this curve and fitting the result with
a straight line as described by the following equation:
ln ( - t .tau. ) = - t .tau. ##EQU00001##
[0057] A similar procedure can be followed by using a look-up table
with the logarithm followed by a simple least square linear fit.
The result of the logarithm is shown in FIG. 7, with corresponding
plots 70 and 72. The x-axis shows the sample number.
[0058] The extra series resistor R.sub.s of 390.OMEGA. is then
added to the circuit. This makes the total external series
resistance equal to 390+10=400.OMEGA.. The measurement is repeated
showing the results as indicated in FIG. 8 and FIG. 9. Again plots
80 and 90 correspond to a 0V dc drive and plots 82 and 92
correspond to a 100V dc drive.
[0059] The results are summarized in Table 1.
TABLE-US-00001 TABLE 1 Overview of measurement results Measurement
Slope .tau. [ns] R [.OMEGA.] C [pF] 0 V; 10 .OMEGA. -0.00905736
55.204 0 V; 400 .OMEGA. -0.0062758 70.67 880 62.7 100 V; 10 .OMEGA.
-0.0048818 102.42 100 V 400 .OMEGA. -0.0028989 172.5 570 179.6
[0060] It is clear that at the high voltage (100V) the meniscus is
relatively flat and the capacitance is high (179.6 pF) as expected
and corresponding to the right part of FIG. 2. In this example, the
voltage is sufficiently high to give a concave meniscus.
[0061] For the low voltage, the results reflect the left part of
FIG. 1 and a low capacitance of 62.7 pF results.
[0062] However, for the resistance, the measurements are different
to what could be expected. A decrease in resistance is observed for
higher voltages where an increase in resistance with higher
voltages is expected, as shown in FIG. 2. This discrepancy is
attributable to the ITO electrodes 14,16. They have a resistance
approximately equal to the liquid of the electro-wetting lens. For
a high voltage and a flat or concave meniscus the total area of the
liquid that is in contact with the cylinder wall increases leading
to a lower resistance. This effect is apparently larger than the
effect of the reduced volume of conducting solution near the edges
of the cylinder as shown in FIG. 2.
[0063] With correct modeling or testing, the effect on the
resistance and capacitance of the meniscus position can be known in
advance, so that the resistance and capacitance effects can be used
to measure the position of the meniscus. The resistance can be used
alone or in combination with the capacitance to provide a feedback
function.
[0064] The effect of the high electrode resistance can also be
enhanced so that the reduction in resistance as the electrode area
is increased becomes more pronounced. Thus, higher resistance, for
example non-metal, electrodes can be used to enhance this effect
and thereby make the electrode resistance dominant over the
resistance of the conductive lens liquid. For example, the wall
electrode may be formed from a higher resistance material than the
conducting liquid.
[0065] The resulting extra resistance in the electrodes will have
an effect on the response time of the lens because of the small
capacitance of the lens. The response time of the lens will however
be limited by fluid dynamics. When driving the lens with a large
symmetrical AC voltage to prevent charging of the lens, this series
resistance in the electrodes could however cause unwanted
dissipation.
[0066] There are therefore numerous ways to use resistance
measurements to provide lens power feedback, and different schemes
will be appropriate for different electrode and lens materials. The
invention is intended to cover any such use of resistance
information as a sensing parameter, and this resistance does not
necessarily need to be extracted from the parameter, for example
the time constant may be obtained without subsequently deriving the
resistance value. As an example, information about the time
constant can also be derived from the phase relation between a
sinusoidal applied voltage and the induced current through the lens
by this voltage.
[0067] The electrowetting lens described above has a bottom
electrode and a circumferential wall electrode. Due to this
circular symmetric structure of the lens, aberrations in the shape
of the meniscus due to gravity or other effects cannot be
compensated for. It is also not possible to measure asymmetric
changes in the shape of the meniscus using this electrode
configuration.
[0068] This invention can also be applied to a lens design having
angularly spaced electrode segments. These can be used to determine
the local shape characteristics of the lens at different angular
positions around the lens. In this way, asymmetry can be detected.
With appropriate design of the control electrode arrangement, this
detected asymmetry can then be corrected.
[0069] FIG. 10 shows an arrangement in which the wall electrode 16
is split into a number of axial electrodes 100. A resistance (and
optionally also capacitance) measurement can then be carried out
for the path between each individual axial electrode and the bottom
electrode 14.
[0070] In this way, for each of the electrodes 100, a measurement
with respect to the bottom electrode 14 can be measured
independently. This then provides not only information about the
global strength of the lens, but also the asymmetric shape of the
meniscus.
[0071] The segmented side wall electrode also allows each electrode
100 to be driven independently with a driving voltage that is a
function of the measured values and the required shape and strength
of the lens. When different voltages are applied across the
circumference, the angle the liquid will make with the cylinder
wall will vary across the circumference leading to a deformation of
the meniscus. This can be used to compensate for lens aberrations
due to gravity.
[0072] The driving voltages can be calculated continuously such
that the aberration is minimized at each orientation of the lens
with respect to the field of gravity. A block diagram of a system
for implementing this control scheme is shown in FIG. 11.
[0073] Each electrode 100 connects to a measurement circuit 110 as
described above, to perform the sensing operation. When driving the
electrodes 100, the angle of the conducting liquid (water) with the
wall of the lens cylinder will vary as function of the voltage.
Control of the voltages can be achieved using a loop-filter 112
which receives error values from a comparator 114. The comparator
114 compares the measured values with reference values for the
desired optical power. The loop filter can implement a linear
control scheme, for example PI (proportional-integral) control
using an integrating loop filter. The changes may be relatively
slow, so it may also be possible to calculate the voltages with a
DSP (digital signal processor) based on the output of the
comparison between the measured values and the reference values. In
that case it is more easy to apply more sophisticated non-linear
control schemes.
[0074] Voltage drivers 116 controlled by the filter or DSP provide
the drive voltages to the electrodes 100.
[0075] The feedback information provided by the invention, in both
the single electrode scheme or the segmented electrode scheme, can
be used in conventional manner to provide feedback control of the
lens power, in the same way that capacitance measurements alone
have previously been proposed for this purpose.
[0076] Various other modifications will be apparent to those
skilled in the art.
* * * * *