Patent Application
20090185280
This invention relates to a controllable optical component, in particular using the so-called electrowetting principle (also known as electrocapillarity). The invention relates for example to optical lenses or diaphragms.
An electrowetting lens comprises a chamber housing two non-miscible liquids, 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.
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 inside edge of the chamber, thereby changing the meniscus shape.
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.
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 for use in mobile phone cameras.
Electrowetting lenses have also been recognised as good candidates for use as autofocus camera lenses. An advantage of the electrowetting lens over conventional autofocus lenses is a high focusing speed. It is typically possible to change focus from an object at infinity towards an object at 5 cm within 10 ms.
Electrowetting lenses are actuated electrostatically and therefore use very little power when driven by a dc voltage. However, they require a voltage much higher than voltages supplied by batteries, and the driver circuitry that generates the required voltages has significant power consumption. In particular, a dc-to-dc converter is required to supply these voltages.
According to the invention, there is provided a controllable optical system, comprising:
an optical component comprising a chamber housing first and second fluids, the interface between the fluids defining a boundary surface and an electrode arrangement for electrically controlling the shape of the boundary surface;
a drive circuit for controlling the signals applied to the electrode arrangement, and operable in. an operative mode and an inoperative, power saving mode;
a drive signal storage capacitor for maintaining a signal on the electrode arrangement when the drive circuit is in the inoperative mode;
a sensing arrangement for sensing a boundary surface condition; and
a feedback control loop for switching the drive circuit between the operative mode and the inoperative, power saving mode, in dependence on the sensing arrangement output.
The system of the invention provides power saving by intermittently operating the drive circuit. The switching between operative states of the drive circuit is controlled in dependence on a boundary surface condition, so that the correct functioning of the system is ensured for different operative conditions. This boundary surface may define a lens surface or a diaphragm opening.
The drive signal storage capacitor is designed to ensure that the ripple as a result of the intermittent operation on the electrode arrangement signal is within desired boundaries. This will depend on various parameters, such as the drive condition of the lens (for example if the lens is being driven to a steady state condition or if focus changes are taking place) and the extent to which the lens has aged. The feedback system enables these factors to be taken into account.
The feedback control loop preferably comprises a timing generator for providing an enable signal to the drive circuit, and this signal may have a controllable duty cycle and/or frequency, and may be synchronised with the frame rate of the image sensor.
The feedback control loop may additionally be for controlling the electrode arrangement drive signals to implement an autofocus function in a lens system. Furthermore, the same lens setting sensing arrangement can be used for providing an autofocus feedback signal.
The sensing arrangement may comprise a capacitance sensing arrangement for measuring a capacitance between first and second electrodes of the electrode arrangement.
Alternatively, the sensing arrangement can comprise an image sensor, voltage measurement circuitry for measuring an electrode arrangement voltage or current measurement circuitry for measuring a current flowing to the electrode arrangement.
The invention also provides a method of driving a controllable optical component, the component comprising a chamber housing first and second fluids, the interface between the fluids defining a boundary surface and an electrode arrangement for electrically controlling the shape of the boundary surface, wherein the method comprises:
sensing a boundary surface condition; and
controlling the electrode arrangement to cycle between operative and inoperative states, wherein in the operative state an electrode arrangement drive signal is provided to the electrode arrangement, and in the inoperative state the electrode arrangement drive signal is provided by a storage capacitor, the cyclic control of the electrode arrangement being controlled in dependence on the sensed boundary condition.
Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:
The left part of
The optical power of an electrowetting lens is determined by the radius of the meniscus formed at the interface of the two liquids. The radius can be derived from the contact angle θ (shown in
where θ is the angle the meniscus makes with the wall, V the voltage applied, Y
High voltages are required to drive the lens (of the order of 100V) and for this reason, the driver circuit is the main contributor to the power consumption of the lens system.
The invention provides power saving by intermittently operating the drive circuit, and storing the drive signal on a storage capacitor. The switching between operative/inoperative states of the drive circuit is controlled in dependence on a lens setting condition, so that the correct functioning of the system is ensured for different operative conditions.
Before describing a lens system of the invention (as one preferred application of the invention), a conventional autofocus lens system will be described with reference to
The camera comprises a lens module 30 which receives incoming light and focuses the image onto an image sensor 34 in the form of a CCD or image sensor array.
The electrowetting lens module 30 contains a number of conventional lenses in addition to the electrowetting lens, and a driver to generate the high voltage signals to focus an image at a specific distance. The driver may in fact be external to the lens/camera module 30.
The image sensor 34 senses the incoming light and converts it to an electrical signal that can be stored in a memory, for example in RGB or YUV format.
A signal processor 36 includes an autofocus algorithm 38, a sharpness detection device 40 and a video signal processor 42. The sharpness signal is derived from the high frequency content of the image. The autofocus algorithm 38 derives a focus error signal from differences in the sharpness signals at specific moments, and a control loop of the autofocus algorithm 38 generates a lens driver signal 39 to drive the lens.
The invention provides a control system in which the drive circuit is operated intermittently. In order to derive a drive scheme which minimises the power consumption, it is first necessary to analyse the lens charging characteristics. These determine the ripple on the lens drive voltage if this voltage is supplied by a storage capacitor.
If the lens is driven from a storage capacitor, there will be a change in lens focus as the voltage drops due to capacitor discharge. There is also a need to charge the capacitor initially to the operating voltages.
A practical value for the lens capacitance Clens is 100 pF and for the lens resistance Rparellel is 100 MΩ. The additional storage capacitor Cstorage may have a capacitance of 10 nF, by way of example.
The driver IC is required to generate a driving voltage of about 100 V from the (typically) 2.5 V supply voltage, and consumes approximately 24 mW. The majority of this power consumption is due to switching losses for generating the high voltage inside the driver and is not caused by driving the lens.
For the calculations below, it is assumed that a discharge of Clens of 1% is allowed, and that this voltage ripple will not adversely influence the imaging characteristics. Thus, when the lens is focussed with 100V, the driver can be switched off until the voltage over the lens is 99 V.
The time it takes to re-charge Clens and Cstorage from 99 to 100 V depends on the internal resistance of the power supply. It can be assumed that this takes of the order of 100 μs.
The discharge time can be calculated with the following formula:
The oscillation period can be set using this equation. With the example figures given, the discharge time (to 99%) is 10 ms. This can be used to set the oscillation frequency with which the drive circuit is controlled.
The power consumption of the lens and driver becomes:
(100 μs/10 ms)×24 mW=0.24 mW.
Thus, the use of a sufficiently large storage capacitor, together with the appropriate switching electronics, enables the power consumption to be decreased by a factor 100.
One possible implementation of an intermittent drive scheme is to base the drive scheme on a worst-case scenario, that is safe for every lens in every situation. This approach is simple to implement, and requires no feedback control. This does not, however, optimise the power savings. This will also provide a slower start-up.
The preferred approach of the invention is to provide feedback so that the drive scheme can be tailored to the prevailing conditions, and different start up drive conditions can also be provided.
The drive circuit 50 is shown separately, and comprises a high voltage source 52 and a switch 54 for controlling the application of the high voltage to the lens 30 and additional storage capacitor 56. in accordance with the invention, a sensing arrangement 58 is provided, and this forms part of a feedback control loop which controls the drive circuit 50.
The sensing arrangement 58 detects a lens setting, and this may be direct detection of the physical shape of the lens, or else it may be based on analysis of the image captured by the lens. In each case, the sensing arrangement 58 provides a feedback signal for the control system 60 which provides an enable signal 62 to the drive circuit 50. The control system 60 is typically implemented as a PI-controller of a micro-controller or digital signal processor. The control system 60 determines the switching frequency for the enable signal 62 and also the duty cycle of this signal.
In the example shown in
In practice, the units 70 and 72 can be implemented as a single timing controller, having three inputs (frame rate for synchronisation 74, frequency 68 and duty cycle 66). The output from the timing controller is again the enable signal 62.
The control system 60 is also controlled in dependence on the frame rate, provided as input 74.
During initial (startup) operation of the lens system, the enable signal 62 is kept high to enable a fast response as the lens charges to an operating voltage. When the desired lens voltage, or a stable lens condition, is detected by the sensing arrangement 58, the control system operates to a closed loop control mode, and the enable signal 62 is modulated to provide power savings.
The drive circuit 50 also has a control input 76, and this goes high when the lens system is turned on.
As shown in
The initial pulse provides a start-up condition to enable a fast initial response time.
As shown above, the oscillation period is set depending on the RC time constant of the lens and additional capacitor arrangement. As a result of differences between lenses and environmental conditions (such as temperature and humidity) this oscillation frequency may need to be calibrated periodically.
To perform this calibration, the lens capacitance and additional capacitance can be charged, and the drive circuit 50 then turned off. Time can then be measured until the lens is discharged to a certain level, from which the RC time constant can be calculated. In particular,
t
discharge≈5×RP(Clens+Cstorage)
It is not necessary to completely discharge the capacitances in order to determine the RC time constant.
The determined RC time constant can then be used in the manner explained above to obtain the desired oscillation frequency.
The oscillator 70 and the enable signal 62 need to be synchronized with the frame rate of the lens system, and this is the purpose of the frame rate input 74 in
As outlined above, the invention uses a feedback of the lens condition in order to determine the appropriate control of the driver circuit.
It has already been recognised that sensing the lens condition is desirable, to provide a feedback control function for other purposes. For example, due to slow charging of the insulators (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. 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 optical power of the lens. Sensing the lens conditions in this way has been proposed in the past as a way of maintaining a constant focal depth over time.
This capacitive sensing approach can also be used to provide the required feedback signal. Because the additional capacitor is large compared to the lens capacitance, a small capacitance difference does need to be detected. A bridge circuit can be used for this purpose.
An alternative approach is to use the existing image sensor to measure a change in focus of a specific object. Again, this information can be used as a feedback signal for the control system.
In this case, if the camera or the object move relatively to each other, the focus of the object will also change, and this will require refocusing by the autofocus control loop, rather than repetition of the same drive voltage. The system therefore requires intelligence to distinguish between a required refocusing operation, or a required recharging operation. This additional intelligence can easily be provided. For example, objects can be recognised, or the content of the image can be detected. If the sharpness of an object changes even when the object maintains an identical size and shape, then the lens is discharging and the driver circuit needs to be enabled. If the sharpness changes and the object appearance also changes, then the autofocus algorithm needs to be operated.
A further alternative is to use voltage measurement of the electrode arrangement drive voltage. Measuring voltage using a high impedance measurement circuit will not significantly influence the discharge time, and a FET source-follower circuit can be used for this purpose. The voltage measurement operation will itself consume power, and to reduce the overall power consumption, this measurement can be sampled.
A further possibility is to measure the current flowing to (or from) the lens. The voltage drop can be obtained by measuring the current flow through the additional capacitance, by applying a resistance in series with the additional capacitor and measuring the voltage across the resistor. Again, this current measurement will also consume power, and can therefore be sampled, to minimise this additional power consumption.
Alternative to the feedback measurements described above, the charge flow needed to charge the additional storage capacitor when the drive circuit is switched on can be measured, and this provides an indication of the leakage currents, and can thereby determine the standby time of the driver circuit.
The lens may comprise a more complicated electrode arrangement than is the simple base and annular electrodes as in the example above.
The invention can be applied to the use of electrowetting lenses in cameras, for example for mobile telephones. Other uses of electrowetting lenses will however be apparent to those skilled in the art, such as in production lines or other object processing operations, where image analysis or image projection is required.
Multiple optical lenses can be used in optical arrangements. For example, a electrowetting zoom lens can be formed from two lens elements.
The invention may be used within an optical diaphragm, for providing a variable size optically transmissive opening.
As shown in
Two possible boundary fluid boundaries are shown in
The additional capacitor introduced for storing the drive voltage will typically be at least 10 times larger than the lens capacitance, and preferably 100 times larger or more. Although it will preferably be implemented as an additional circuit element, it is also possible to alter the lens or diaphragm (or other switchable optical component) design to provide much higher capacitance, for example through the use of different dielectric coatings. This is intended to be within the scope of the invention, so that the drive signal storage capacitor may be an integral part of the materials used in the design.
A dc drive scheme as described above has the advantage of low charge flow, as repeated charging operations as in an ac drive scheme are not required. However, due to the physical properties of the materials, insulator charging can occur, leading to a deterioration in the lens properties. This can be overcome by applying a reverse polarity short pulse periodically. This can remove excessive charge build up but a sufficiently short duration can be used to avoid change in the boundary shape. This pulse can, for example, be generated by connecting the lens to a full bridge circuit, and this optional element is shown as 80 in
Only one detailed example has been given above, and in respect of a lens system. Other applications and detailed implementations will be apparent to those skilled in the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0425611.1 | Nov 2004 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2005/053643 | 11/7/2005 | WO | 00 | 3/17/2009 |
