Computer-implemented method of generating PWM control signals

Abstract

Method of generating PWM control signals and a measurement pulse for applying to an actuator component, comprising: receiving a frame synchronisation signal; generating pulse width modulated (PWM) control signals to drive the actuator component at a PWM frequency substantially equal to an integer multiple of the reciprocal of a sampling period; during a series of sensing intervals occurring at a sensing frequency, suspending generating PWM control signals to drive the actuator component, and generating a measurement pulse to measure an electrical characteristic of the actuator component; and scheduling a first sensing interval of the series of sensing intervals to start a time delay after receiving the frame synchronization signal, and setting the time delay, the duration of sensing intervals and the sensing frequency such that each sensing interval overlaps with both or neither of a duration of reading the reference signal and a substantially equal duration of reading the light-sensing signal by the image sensor.

Description

[0001] COMPUTER-IMPLEMENTED METHOD OF GENERATING PWM CONTROL SIGNALS

[0002] Field

[0003] The present application generally relates to a computer-implemented method of generating PWM control signals, for example for application to an SMA actuator assembly. The present application also relates to a corresponding computer program, a computer-readable storage medium and an apparatus comprising an actuator assembly and a controller.

[0004] Background

[0005] There are a variety of types of actuator assembly in which it is desired to provide positional control of a movable part relative to a support structure. Such actuator assemblies may be used in cameras, in which a lens element and an image sensor are moved relative to each other. For example, WO 2013 / 175197 Al discloses a SMA actuator assembly in which a total of four SMA wires are used to move a lens element relative to an image sensor in a plane orthogonal to the optical axis of the lens element, thereby providing optical image stabilization (OIS). An alternative SMA actuator assembly in which SMA wires are used to provide OIS by moving a movable image sensor relative to a lens element is disclosed in WO 2017 / 072525 Al. WO2007 / 113478 Al and WO 2019 / 243849 Al disclose an apparatus in which SMA wire is used to move a lens element relative to an image sensor in a direction along the optical axis of the lens element, thereby providing auto-focus or zoom functionality.

[0006] In each of these types of actuator assembly, an image sensor is arranged in close proximity to an actuator component in the form of an SMA wire. Driving the actuator component, by switching the electrical power through the actuator component, may give rise to electro-magnetic interference (EMI) in the image sensor. Such EMI may become apparent in the form of noise in the image that is captured by the image sensor, thus degrading the quality of the images captured by the image sensor. The problem can be more pronounced in battery-operated devices such as mobile phones, which tend to operate with a restricted power budget. This results in widespread use of pulse width modulation (PWM) techniques that can improve efficiencies, but which can lead to increased EMI.

[0007] The present invention is concerned with avoiding or at least reducing the effect of EMI on the quality of an image captured by the image sensor. Summary

[0008] According to the present invention, there is provided a computer-implemented method of generating PWM control signals and a measurement pulse for applying to an actuator component that drives movement of a movable part relative to a support structure, an image sensor being fixed relative to the movable part or the support structure, the image sensor carrying out correlated double sampling, CDS, so as to read, offset in time by a sampling period, a reference signal and a light-sensing signal, and to output a frame synchronisation signal, wherein the method comprises: receiving the frame synchronisation signal; generating pulse width modulated, PWM, control signals to drive the actuator component at a PWM frequency substantially equal to an integer multiple of the reciprocal of the sampling period; during a series of sensing intervals occurring at a sensing frequency, suspending generating PWM control signals to drive the actuator component, and generating a measurement pulse to measure an electrical characteristic of the actuator component; and scheduling a first sensing interval of the series of sensing intervals to start a time delay after receiving the frame synchronization signal, and setting the time delay, the duration of sensing intervals and the sensing frequency such that each sensing interval overlaps with both or neither of a duration of reading the reference signal and a substantially equal duration of reading the lightsensing signal by the image sensor.

[0009] According to the present invention, there is also provided a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method.

[0010] According to the present invention, there is also provided a computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the method.

[0011] According to the present invention, there is also provided an apparatus comprising: an actuator assembly comprising a support structure and a movable part supported on the support structure, and an actuator component configured to drive movement of the movable part relative to the support structure; and a controller configured to carry out the method.

[0012] Further aspects of the present invention are set out in the dependent claims. The present invention may be embodied as a computer-implemented method or an apparatus. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects.

[0013] Furthermore, the present techniques may take the form of a computer program product. Such a computer program product may be in the form of code and / or data that may be distributed between a plurality of coupled components in communication with one another. The invention may comprise a controller which includes a microprocessor, working memory and program memory coupled to one or more of the components of a system.

[0014] The computer program product may be embodied in a computer readable medium having computer readable program code thereon. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable medium may be, for example, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. The computer-readable storage medium may be, for example, a solid-state memory, a microprocessor, programmed memory such as non-volatile memory (such as Flash), or read-only memory (firmware), or on a data carrier such as an optical or electrical signal carrier.

[0015] All or part of the computer-implemented method according to embodiments of the present invention may suitably be embodied in a logic apparatus comprising logic elements to perform the steps of the above-described methods, and that such logic elements may comprise components such as logic gates in, for example a programmable logic array or application-specific integrated circuit. Such a logic arrangement may further be embodied in enabling elements for temporarily or permanently establishing logic structures in such an array or circuit using, for example, a virtual hardware descriptor language, which may be stored and transmitted using fixed or transmittable carrier media.

[0016] Further aspects of the present invention are set out in the detailed description.

[0017] Brief description of the drawings

[0018] Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows an apparatus comprising an SMA actuator assembly in accordance with embodiments of the present invention;

[0019] Figure 2 shows a schematic block diagram of the control of the SMA actuator assembly and the image sensor;

[0020] Figure 3 shows signals that may be applied to the SMA wires and to the image sensor;

[0021] Figure 4 shows an embodiment of scheduling a sensing interval in accordance with the present invention;

[0022] Figure 5 shows another embodiment of scheduling a sensing interval in accordance with the present invention.

[0023] Detailed description

[0024] Fig. 1 schematically shows an apparatus 1 in accordance with an embodiment of the present invention. The apparatus 1 is, for example, a camera apparatus 1. The apparatus 1 is to be incorporated in a portable electronic device such as a mobile telephone, or tablet computer. Thus, miniaturisation is an important design criterion.

[0025] The apparatus comprises an actuator assembly 2. The actuator assembly 2 comprises a support structure 10 and a movable part 20. The movable part 20 is supported on the support structure 10. The movable part 20 is movable relative to the support structure 10. For example, the movable part 20 may be supported in a manner allowing movement of the movable part 20 relative to the support structure 10 in a plane orthogonal to an axis O. Movement along the axis O may be constrained or prevented. Alternatively or additionally, the movable part 20 is supported in a manner allowing movement of the movable part 20 relative to the support structure 10 along the axis O. Movement orthogonal to the axis O may be constrained or prevented. The axis O coincides with the optical axis O of optical components (such as a lens 3) of the apparatus 1. In general, the movable part 20 may be movable relative to the support structure 10 in any way, e.g. move in any one or any combination of three translational degrees of freedom and three rotational degrees of freedom. The actuator assembly 2 of Fig. 1 comprises one or more SMA elements 30, in particular SMA wires 30. The SMA wires 30 are connected (in tension or otherwise) between the support structure 10 and the movable part 20. In the depicted actuator assembly 2, the SMA wires 30 are connected at their ends to the support structure 10 and / or to the movable part 20 using connection elements 33, for example crimp connections. The crimp connections may crimp the SMA wires 30 to hold the SMA wires 30 mechanically, as well as providing electrical connections to the SMA wires 30. However, any other suitable connections may alternatively be used, and the SMA wires 30 may for example be connected to the support structure 10 and / or to the movable part 20 via one or more intermediate parts (not shown). The SMA wires 30 are capable, on selective contraction, of driving movement of the movable part 20 with respect to the support structure 10 in one or more degrees of freedom.

[0026] The movable part 20 may be supported (so suspended) on the support structure 10 exclusively by the SMA wires 30. However, preferably, the actuator assembly 2 comprises a bearing arrangement 40 that supports the movable part on the support structure 10. The bearing arrangement 40 may have any suitable form for allowing movement of the movable part 20 with respect to the support structure 10. For this purpose, the bearing arrangement 40 may, for example, comprise a rolling bearing, a flexure bearing or a plain bearing.

[0027] The exemplary camera apparatus 1 of Fig. 1 further comprises a lens assembly 3 and an image sensor 4. The lens assembly 3 comprises one or more lenses configured to focus an image on the image sensor 4. The image sensor 4 captures an image and may be of any suitable type, for example a charge coupled device (CCD) or a CMOS device. The lens assembly 3 comprises a lens carrier, for example in the form of a cylindrical body, supporting the one or more lenses. The one or more lenses may be fixed in the lens carrier, or may be supported in the lens carrier in a manner in which at least one lens is movable along the optical axis O, for example to provide zoom or focus, such as auto-focus (AF). The apparatus 1 may be a miniature camera apparatus in which the or each lens of the lens assembly 3 has a diameter of 20mm or less, preferably of 12mm or less.

[0028] In the embodiment shown in Fig. 1, the movable part 20 comprises the lens assembly 3. The image sensor 4 may be fixed relative to the support structure 10, i.e. mounted on the support structure 10. In other embodiments (not shown), the lens assembly 3 may be fixed relative to the support structure 10 and the movable part 20 may comprise the image sensor 4. In either embodiment, in operation the lens assembly 3 is moved relative to the image sensor 4. This has the effect that the image on the image sensor 4 is moved and / or changed in focus. So, optical image stabilization (OIS) or autofocus (AF) or other focus or zoom functionality may be implemented in the apparatus 1.

[0029] The camera apparatus 1 further comprises a controller 8. The controller 8 may be implemented in an integrated circuit (IC) chip. The controller 8 generates drive signals for the SMA wires 30. SMA material has the property that on heating it undergoes a solid-state phase change that causes the SMA material to contract. Thus, applying drive signals to the SMA wires 30, thereby heating the SMA wires 30 by allowing an electric current to flow, will cause the SMA wires 30 to contract and move the movable part 20. The drive signals are chosen to drive movement of the movable part 20 in a desired manner, for example so as to achieve OIS by stabilizing the image sensed by the image sensor 4 and / or to achieve AF by focussing the image on the image sensor 4. The controller 8 supplies the generated drive signals to the SMA wires 30.

[0030] Optionally, the camera apparatus 1 comprises a vibration sensor 6. The vibration sensor 6 may be a gyroscope sensor, for example, although in general other types of vibration sensor 6 (such as an inertial measurement unit, accelerometers, etc) could be used. The vibration sensor 6 detects vibrations that the camera apparatus 1 is experiencing and generates output signals representative of the vibration of the camera apparatus 1. The controller 8 receives the output signals, and generates the drive signals for the SMA wires 30 in response to the output signals, for example so as to counteract the vibrations represented by the output signals. The controller 8 may thus control the SMA wires 30 to achieve OIS.

[0031] In embodiments of the apparatus 1, OIS is performed. In such embodiments, the apparatus 1 may comprise the SMA actuation apparatus described in WO2013 / 175197 Al, or the SMA actuation apparatus of WO 2011 / 104518 Al, or the camera assembly of WO2017 / 072525, each of which is herein incorporated by reference. In other embodiments of the apparatus 1, AF is performed. In such embodiments, the apparatus 1 may comprise the camera lens actuation apparatus of WO2007 / 113478 Al or the SMA actuation apparatus of WO 2019 / 243849, each of which is herein incorporated by reference.

[0032] Generally, however, the apparatus 1 according to the present invention is any apparatus 1 comprising an arrangement in which an actuator component, such as an SMA wire 30, drives movement of an arrangement associated with an image sensor 4. In such an apparatus 1, the drive signals supplied to the SMA wire 30 can result in electro-magnetic interference, EMI, in the image sensor 4. Such EMI can cause interference or noise in the image signal, which can take the form of one or more lines which appear to move across a display screen that displays the image captured by the image sensor 4. The present invention may prevent or at least reduce EMI in the image sensor 4 due to the signals applied to the SMA wire 30.

[0033] Actuator control

[0034] Fig. 2 shows a block diagram that schematically illustrates control of the SMA wires 30 in the apparatus 1, in accordance with an embodiment of the invention. In the illustrated embodiment, the controller 8 comprises a PWM driver 81 and a frequency setter 82, herein embodied by a frequency multiplier 82 (although in general the PWM frequency may otherwise be set by the frequency setter 82). The PWM driver 81 generates pulse width modulated (PWM) control signals P(l-4) to drive the SMA wires 30. In the illustrated embodiment, the PWM driver 81 generates four PWM control signals P(l-4). Each PWM control signal P(l-4) controls a different one of the four SMA wires 30. In general, the PWM driver 81 may generate any number of PWM control signals, to control any number of SMA wires 30. The frequency multiplier 82 sets the frequency f(PWM) of the PWM control signals P(l-4). The frequency multiplier 82 is, for example, a phase locked loop (PLL).

[0035] A master oscillator 83 generates a master clock signal MCLK. The master clock signal MCLK is received by the frequency multiplier 82. The frequency multiplier 82 may multiply and / or divide the received master clock signal MCLK so as to generate a clock signal with the PWM frequency f(PWM). The master clock signal MCLK is also applied to the image sensor 4. The master oscillator 83 may be part of the controller 8, or may generally be implemented in any component of a device in which the apparatus 1 is integrated. The master oscillator 83 may, for example, correspond to the oscillator that provides a clock signal to the main processor of such a device. In some embodiments, a dedicated oscillator or other clock generator may be provided to generate the PWM frequency f(PWM) instead of the master oscillator 83.

[0036] The image sensor 4 comprises an array of pixels 41. The pixels 41 are light-sensitive pixels 41. The array of pixels 41 may be a square array, although in general an array of any shape may be provided. The array of pixels 41 comprises rows and columns of pixels 41. The image sensor 4 further comprises an analogue to digital converter (ADC) 42. The ADC 42 reads the signals generated by the pixels 41, and outputs image data IMG. The image sensor 4 thus captures an image. The image data IMG may be received by a processor (not shown) and displayed on a display of any device incorporating the apparatus 1. The image sensor 4 also generates and outputs a sync signal SI, also referred to as a first sync signal SI herein. The image sensor 4 generates a second sync signal S2 and / or further sync signals. The sync signals SI, S2 may be received by the PWM driver 81.

[0037] Figs. 3 to 5 schematically depict signals that are used by the controller 8 and by the image sensor 4. The signals P1-P4 correspond to the PWM control signals applied to the four SMA wires 30. The CDS signal (described in more detail immediately below) is used by the ADC 42 of the image sensor 4 to read signals generated by the pixels 41. The signals SI and S2 correspond to sync signals that are output by the image sensor 4 and may be received by the controller 8.

[0038] The CDS signal is used by or inherent to the ADC 42. The ADC 42 carries out correlated double sampling (CDS) to read the signals generated by the pixels 41. In the context of this invention, CDS generally refers to obtaining a reference signal and a light-sensing signal of a pixel 41. The reference signal is taken in a known reference state of the pixel 41. The known reference state is for example a dark, non-exposed state of the pixel 41, i.e. a state in which light does not fall onto the pixel 41. The reference signal may also be referred to as a reset signal or dark signal. The light-sensing signal is taken in an unknown measurement state of the pixel 41. The unknown measurement state may be an illuminated state of the pixel 41, i.e. a state in which the pixel is exposed to light. The image sensor 4 generates pixel data based on the reference signal and the light-sensing signal, for example by subtracting the reference signal from the light-sensing signal. CDS reduces noise in data generated by the pixel of the image sensor 4, compared to a situation in which only the light-sensing signal is read.

[0039] As part of carrying out CDS and reading the reference signal and the light-sensing signal, the ADC 42 uses the CDS signal. The CDS signal comprises a reference ramp 55 and a sensing ramp 56. The reference ramp 55 is used by the ADC 42 to read the reference signal of the pixels 41. The duration t(ref) of the reference ramp 55 corresponds to the duration of reading the reference signal. The substantially equal duration of reading the light-sensing signal, i.e. the duration at the beginning of reading the light-sensing signal that is equal to the duration of reading the reference signal by the image sensor 4, is denoted t(ref). The sensing ramp 56 is used by the ADC 42 to read the lightsensing signal of the pixels 41. The duration t(sns) of the sensing ramp 56 corresponds to the duration of reading the light-sensing signal. The duration t(sns) is generally longer than the duration t(ref), because it takes longer to read the comparably larger light-sensing signal than the reference signal from the pixels 41. The time between the start of the reference ramp 55 and the start of the sensing ramp 56 is herein referred to as the sampling period t(CDS). The reference signal and the light-sensing signal are read offset in time by the sampling period t(CDS).

[0040] The PWM control signals P1-P4 each comprise a pulse train. The frequency of the pulse train is the PWM frequency f(PWM). The period between starts of adjacent pulses in the PWM control signals P1-P4 is the PWM period t(PWM). The pulses of the PWM control signals P1-P4 are preferably square pulses as shown in Figs. 3-5, although in general pulses with other shapes may also be used. Switching the PWM control signals P1-P4 thus gives rise to rising or falling edges in the PWM control signals. As shown in Figs. 3-5, the amplitude of the pulses of the PWM control signal P1-P4 is preferably constant, such that the power applied to the SMA wires 30 is controlled solely or at least primarily by adjusting the width of the pulses of the PWM control signals P1-P4. In some embodiments, the amplitude of the PWM control signals P1-P4 may also be adjusted so as to provide additional control of the power provided to the SMA wires 30.

[0041] Preferably, none of the PWM control signals P1-P4 overlap, so the pulses of the PWM control signals P1-P4 are scheduled to be generated sequentially and not concurrently. The PWM control signals P1-P4 may thus be interleaved, as shown in Figs. 3-5. However, in general, some or all of the PWM control signals P1-P4 may overlap, at least in certain situations (such as low temperature operation of the actuator assembly 2).

[0042] The inventors have found that, generally, switching the PWM control signals P1-P4 during the duration t(ref) of the reference ramp 55 and during a substantially equal duration t(ref) of the sensing ramp 56 may result in noise in the image data IMG due to EMI in the image sensor 4. However, avoiding any switching of the PWM control signal P1-P4 during durations t(ref) reduces the duty of the PWM control signals, and so limits the maximum power that can be provided to the SMA wires 30.

[0043] In accordance with the present invention, the PWM frequency f(PWM) is equal to an integer multiple of the reciprocal of the sampling period t(CDS). So, the period t(PWM) between subsequent PWM pulses is equal to or a fraction of the sampling period t(CDS). This ensures that any rising or falling edge of a PWM pulse that takes place during duration t(ref) of the reference ramp 55 also takes place during the substantially equal duration t(ref) of the sensing ramp 56, as illustrated by arrows 59 in Figs. 3-5. Any noise due to EMI resulting from such rising and falling edges thus appears in both the reference signal and the light-sensing signal. Because CDS removes the reference signal from the light-sensing signal so as to create image data IMG, such noise is not present or at least reduced in the final image data IMG. Setting the PWM frequency f(PWM) equal to an integer multiple of the reciprocal of the sampling period t(CDS) thus reduces noise in the image sensor 4 due to EMI from the SMA wires 30. The PWM frequency f(PWM) may typically be set in the range from 90kHz to 600kHz, for example about 200kHz.

[0044] The sampling period t(CDS) is a specification of the image sensor 4. The sampling period t(CDS) may thus be known at the time of implementing the controller 8. In instances in which the sampling period t(CDS) is not accurately known to the designer of the controller 8, one approach to determine a suitable PWM frequency f(PWM) is to sweep all possible PWM frequencies and determine empirically which contribute least noise to the resultant image. It is then possible to set the PWM frequency f(PWM), taking this knowledge into account, such that the image is degraded as little as possible. Some degree of error in setting the PWM frequency f(PWM) equal to an integer multiple of (including equal to) the reciprocal of the sampling period t(CDS) may be acceptable. So, the PWM frequency f(PWM) need not be exactly equal to an integer multiple of the reciprocal of the sampling period t(CDS), but may be substantially equal. For example, the PWM frequency f(PWM) may be in a range from 0.9 to 1.1, preferably from 0.95 to 1.05, further preferably from 0.98 to 1.02 times, particularly preferably from 0.995 to 1.0045 times, the integer multiple of the reciprocal of the sampling period t(CDS).

[0045] In addition to generating the PWM control signals P1-P4, the controller 8 may measure an electrical characteristic, such as the resistance, of the SMA wires 30. The length of the SMA wire 30 is a function of the resistance of the SMA wire 30. The measured electrical characteristic may thus provide a measure of the length of a respective SMA wire 30, and so ultimately allows determination of the position of the movable part 20 relative to the support structure 10. The determined position of the movable part 20 relative to the support structure 10 may be compared to a desired position of the movable part 20 relative to the support structure 10, and the PWM control signal P1-P4 may be adjusted to bring the movable part 20 closer to the desired position. So, the controller 8 may comprise closed loop control to generate the PWM control signals P1-P4. The measured electrical characteristic, or a measure (such as the length of the SMA wires 30) derived from the measured electrical characteristic may be fed back to the closed loop control.

[0046] Scheduling of sensing intervals The controller 8 may determine the electrical characteristic of an SMA wire 30 during a respective sensing interval 50. The sensing interval 50 may be part of a series of sensing intervals 50. So, the electrical characteristic of one or more SMA wires 30 may be determined at repeated intervals. The frequency of the sensing intervals 50 is referred to herein as the sensing frequency. The sensing frequency is the reciprocal of the sensing period t(Rmeas) shown in Figures 4 and 5. Typically, the sensing frequency is less than the PWM frequency. The sensing frequency may be in the range from 2kHz to 5kHz, for example.

[0047] During the sensing interval 50, the PWM control signals P1-P4 that are used to drive the SMA wires 30 may be suspended or supressed. This is schematically shown in Figs. 3 and 5, in which during the sensing interval 50 the PWM control signals P1-P4 are shown in dotted lines, indicating that the PWM control signals P1-P4 are not output, so not applied to the actuator component, during the sensing interval 50. The generation of the PWM control signals P1-P4 may effectively continue during the sensing interval 50, but the PWM control signals P1-P4 may be suppressed and not output to drive the actuator component during the sensing interval 50. So, at a point in time after the sensing interval 50, the same PWM control signal is applied to the actuator component as would be applied to the actuator component at the point in time if the sensing interval 50 had not been scheduled. The PWM control signals P1-P4 scheduled during the sensing interval 50 are omitted. In general, specifically the PWM control signal P2 of the SMA wire 30 to which the measurement pulse 50 is to be applied may be suspended, or all PWM control signals P1-P4 may be suspended.

[0048] Fig. 4 shows an alternative embodiment, in which the generation of PWM control signals P1-P4 is paused at the start of the sensing interval 50 and resumes at the end of the sensing interval 50. The PWM control signals P1-P4 resume at the end of the sensing interval 50 at the state in which the PWM control signals P1-P4 are at the start of the sensing interval 50. In essence, a gap of duration equal to the sensing interval 50 is provided in the PWM control signals P1-P4. The PWM control signals P1-P4 scheduled during the sensing interval 50 are delayed by a duration equal to the duration of the sensing interval 50. Pausing and resuming the PWM control signals P1-P4 may be preferable over suppressing the PWM control signals P1-P4 as per Figs. 3 and 5, because it may be ensured that the correct relative power is delivered to the actuator components regardless of the asynchronous timing of the sensing intervals 50. The exact power delivered to the actuator components may thus be more precisely controlled. Fig. 3 schematically depicts a situation in which the sensing interval 50 is scheduled without taking the CDS signal into account. As shown, in such a situation, the sensing interval 50 may overlap (i.e. occur concurrently) with the reference ramp 55 but not the sensing ramp 56. So, while the PWM control signals P1-P4 are suspended during the reference ramp 55, they are not suspended during the duration t(ref) of the sensing ramp 56. Fig. 3 shows that as a result, a falling edge of a pulse of the PWM control signal P2 occurs during the duration t(ref) of the sensing ramp 56. Such a falling edge does not occur during the duration of the corresponding reference ramp 55. As a result, EMI may affect reading the light-sensing signal by the ADC. Such EMI is not cancelled by CDS, because the reference signal is not affected by EMI. So, the image data IMG that is output by the ADC 42 may undesirably include a contribution due to EMI.

[0049] The present invention is concerned with addressing this problem. In accordance with the present invention, the controller 8 schedules the sensing intervals 50 so as to avoid or reduce an imbalance in noise in the reference signal and the light-sensing signal. Figs. 4 and 5 depict examples of how the sensing intervals 50 may be scheduled.

[0050] As shown in Figs. 4 and 5, the image sensor 4 outputs the first sync signal SI and a second sync signal S2. The first sync signal SI may be a line synchronization signal SI, also referred to as an HSYNC signal herein. The line synchronization signal SI is a sync signal that is output by the image sensor 4 before reading a new line of the pixel array of the image sensor, for example. The second sync signal S2 may be a frame synchronization signal S2, also referred to as a VSYCN signal herein. The frame synchronization signal S2 is a sync signal that is output by the image sensor 4 before reading a new frame of the image sensor, for example. The frequency of the frame synchronization signal S2 may typically be in the range from 20Hz to 240Hz. The frequency of the frame synchronization signal S2 may thus be significantly smaller than the PWM frequency or reciprocal of the CDS period. The frequency of the frame synchronization signal S2 is typically also less than the sensing frequency. Although Figs. 4 and 5 show a single pulse of the frame synchronization signal S2 only, it will be appreciated that the frame synchronization signal S2 typically comprises a series of pulses at the frequency of the frame synchronization signal S2.

[0051] There is typically a deterministic relationship between output of the frame synchronization signal S2 and performing CDS, in particular performing the first instance of CDS following the frame synchronization signal S2. In particular, CDS may start a fixed time period after the frame synchronization signal S2 is output. The fixed time period may be 0. Subsequent instances of CDS may be carried out at a fixed frequency, herein referred to as the ADC frequency. The ADC frequency is the reciprocal of the ADC period t(ADC) shown in Figures 4 and 5. The ADC frequency may be in the range from 40kHz to 400kHz, for example. The ADC frequency is smaller than the reciprocal of the CDS period, and so smaller than the PWM frequency.

[0052] In accordance with the present invention, the controller 8 schedules a first sensing interval 50 of a series of sensing intervals 50 to start a time delay T after receiving the frame synchronization signal S2. The time delay T and the duration of the sensing interval 50 are set such that the first sensing interval 50 (after receiving the frame synchronization signal S2) straddles or avoids the reference ramp 55 and duration t(ref) of the sensing ramp 56. As such, it may be ensured that the first sensing interval 50 does not result in EMI in the image data IMG. The sensing frequency is set such that subsequent sensing intervals 50 also straddle or avoid the reference ramp 55 and duration t(ref) of the sensing ramp 56. As such, it may be ensured that the subsequent sensing intervals 50 does not result in EMI in the image data IMG.

[0053] Figures 4 and 5 show three alternatives for the arrangement of the sensing interval 50. Typically, only one of these options will be implemented in an embodiment of the invention. Although the possible arrangement of sensing intervals 50 is explained with reference to a single sensing interval 50, it will be appreciated that other sensing intervals 50 within the series of sensing intervals 50 may be arranged in a similar manner by appropriate setting of the sensing frequency.

[0054] According to a first alternative, the sensing interval 50 may be arranged between subsequent pairs of CDS ramps, i.e. between subsequent pairs of reference ramp 55 and sensing ramp 56. In embodiments in which the sensing ramp 56 follows the corresponding reference ramp 55, as shown in Figures 4 and 5, the sensing interval 50 may be arranged after the sensing ramp 56 (for example after the duration r(ref) thereof) and before the subsequent reference ramp 55. In embodiments in which the reference ramp 55 follows the corresponding sensing ramp 56, not shown, the sensing interval 50 may be arranged after the reference ramp 55 and before the subsequent sensing ramp 56. The sensing interval 50 may be located generally during the time labelled "SAFE AREA" in Figures 4 and 5. The sensing interval 50 may thus have a maximum duration t(SI)lmax depicted in Figures 4 and 5. The duration t(SI)lmax may generally be long enough for applying a measurement pulse 51, and so arranging the sensing interval 50 at this time may be preferable. According to a second alternative, the sensing interval 50 be arranged between the CDS ramps of a pair of CDS ramps, in particular after the first ramp and before the second ramp. In embodiments in which the sensing ramp 56 follows the corresponding reference ramp 55, as shown in Figures 4 and 5, the sensing interval 50 may be arranged after the reference ramp 55 and before the corresponding sensing ramp 56. In embodiments in which the reference ramp 55 follows the corresponding sensing ramp 56, not shown, the sensing interval 50 may be arranged after the sensing ramp 56 (for example after the duration r(ref) thereof) and before the corresponding reference ramp 55. The sensing interval 50 may have a maximum duration t(SI)2max depicted in Figures 4 and 5. The duration t(SI)2max is generally shorter than duration t(SI)lmax, and so may be suitable for scheduling the sensing interval 50 only in some situations.

[0055] According to a third alternative, the sensing interval 50 may straddle the CDS ramps of a pair of CDS ramps. So, the sensing interval 50 may start before the first ramp and end after the second ramp of a pair of CDS ramps. In embodiments in which the sensing ramp 56 follows the corresponding reference ramp 55, as shown in Figures 4 and 5, the sensing interval 50 may be arranged to start before the reference ramp 55 and end after the corresponding sensing ramp 56 (for example after the duration r(ref) thereof). In embodiments in which the reference ramp 55 follows the corresponding sensing ramp 56, not shown, the sensing interval 50 may be arranged to start before the sensing ramp 56 and end after the corresponding reference ramp 55. The sensing interval 50 may have a minimum duration t(SI)3min depicted in Figures 4 and 5.

[0056] So, the sensing interval 50 may either i) overlap with both the duration t(ref) of reading the reference signal and the substantially equal duration t(ref) of reading the light-sensing signal by the image sensor 4, or else ii) overlap with neither the duration t(ref) of reading the reference signal and the substantially equal duration t(ref) of reading the light-sensing signal by the image sensor 4. In the former instance i), PWM control signals P1-P4 are suppressed during both the reference ramp 55 and duration t(ref) of the sensing ramp 56, and so EMI due to PWM control signals does not affect the reference and light-sensing signals. In the latter instance ii), PWM control signals P1-P4 may be applied during both the reference ramp 55 and duration t(ref) of the sensing ramp 56, and so EMI due to the PWM control signals P1-P4 is ultimately cancelled in the image data IMG due to CDS.

[0057] The delay between the frame synchronization signal S2 and performing CDS, as well as the CDS period t(CDS), may be specifications of the image sensor 4. This delay may thus be known at the time of implementing the controller 8, and the time delay T and the duration of the first sensing interval 50 may thus be set accordingly. In instances in which the delay is not accurately known to the designer of the controller 8, the time delay T and the duration of the sensing interval 50 may be determined empirically by sweeping possible time delays T and durations and determining which contribute the least noise to an image taken by the image sensor 4. Similarly, the ADC period t(ADC) may be a specification of the image sensor 4. The ADC period t(ADC) may thus be known at the time of implementing the controller 8, and the sensing frequency may thus be set accordingly. The sensing period (reciprocal of the sensing frequency) may be set as an integer multiple of (including set equal to) the ADC period t(ADC).

[0058] The ADC period t(ADC) may, alternatively or additionally, be synchronous with a first sync signal SI (e.g. a line synchronization signal SI or HSYNC signal) output by the image sensor 4. The ADC period may be an integer multiple of (including equal to) the period of the first sync signal SI, for example. So, the sensing frequency may be set based on the first sync signal SI. The sensing period (reciprocal of the sensing frequency) may be set as an integer multiple of (including set equal to) the period of the first sync signal SI.

[0059] Figs. 4 and 5 show the duration t(SI)2max. A sensing interval 50 scheduled during the duration t(SI)2max (not shown) overlaps with neither the duration t(ref) of reading the reference signal and the substantially equal duration t(ref) of reading the light-sensing signal by the image sensor 4. In particular, the sensing interval 50 is between the reference ramp 55 and the sensing ramp 56. The time delay is set such that the sensing interval 50 starts after the earlier of the duration t(ref) of reading the reference signal and the substantially equal duration t(ref) of reading the light-sensing signal. The duration of the sensing interval 50 is set shorter than the sampling period t(CDS). The sensing interval 50 ends before the later of reading the reference signal and reading the lightsensing signal.

[0060] The maximum time t(SI)2max is indicated as the entire duration between the reference ramp 55 and the sensing ramp 56. In general, the sensing interval 50 may be shorter than this entire duration between the reference ramp 55 and the sensing ramp 56. The sensing interval 50 may occur at any time and for any duration between the reference ramp 55 and the sensing ramp 56. The sensing interval 50 may occur for a pre-defined minimum duration, to allow for enough time to measure the electrical characteristic of a respective SMA wire 30. In the embodiment of Figs. 4 and 5, the sensing interval 50 is scheduled to take place after the later of the duration t(ref) of reading the reference signal and the substantially equal duration t(ref) of reading the light-sensing signal by the image sensor 4. So, the sensing interval is scheduled during the time t(SI)lmax shown in Figs. 4 and 5. The duration of the sensing interval 50 is set such that the sensing interval 50 ends before the next occurrence of CDS.

[0061] The maximum time t(SI)lmax is indicated as the entire duration between the subsequent pairs of reference and sensing ramps 55, 56. In general, the sensing interval 50 may be shorter than this maximum time t(SI)lmax. The sensing interval 50 may occur at any time and for any duration between the subsequent pairs of reference and sensing ramps 55, 56. The sensing interval 50 may occur for a pre-defined minimum duration, to allow for enough time to measure the electrical characteristic of a respective SMA wire 30.

[0062] Figs. 4 and 5 also show the duration t(SI)3 min. A sensing interval 50 scheduled during the duration t(SI)3min (not shown) overlaps with both the duration t(ref) of reading the reference signal and the substantially equal duration t(ref) of reading the light-sensing signal by the image sensor 4. The time delay T is set such that the sensing interval 50 starts before the earlier one of reading the reference signal and reading the light-sensing signal. The duration of the sensing interval is set longer than the sampling period t(CDS). The sensing interval 50 ends after the later of the duration t(ref) of reading the reference signal and the substantially equal duration t(ref) of reading the light-sensing signal. This allows the sensing interval 50 to be longer than the sensing interval 50 within t(SI)2max, which may be beneficial especially if the sampling period t(CDS) is relatively short and / or if a longer measurement pulse is to be applied to the SMA wire 30.

[0063] The time t(SI)3min is illustrated as taking place for an almost minimum duration that allows straddling both the reference ramp 55 and the sensing ramp 56. In general, the sensing interval 50 may be longer than this minimum duration. For example, the sensing interval 50 may start at any time after the preceding occurrence of CDS, and end at any time before the subsequent occurrence of CDS. In some embodiments, the sensing interval 50 may straddle multiple pairs of reference ramp 55 and sensing ramp 56. This allows even longer sensing intervals 50 to be implemented compared to sensing intervals 50 that straddle a single pair of reference ramp 55 and sensing ramp 56. As described above, the image sensor 4 and the controller 8 may be driven based on the same master clock signal MCLK. So, both the image sensor 4 and the controller 8 may be locked to the same master clock signal MCLK. In some embodiments, the temporal resolution (or time resolution) of the image sensor 4 may be greater, in some instances significantly greater (e.g. by a factor of more than 10, or more than 100, or more than 1000), than the temporal resolution of the controller 8. The clock cycle of the controller 8 may be longer than the clock cycle of the image sensor 4. Due to such an imbalance in the temporal resolution between image sensor 4 and controller 8, the position of the sensing intervals 50 may slowly drift away from the safe areas described above (i.e. t(SI)lmax, t(SI)2max or t(SI)3min) and encroach on the CDS ramps. There is thus a risk of EMI in the image data IMG. To mitigate any such drift, the sensing interval 50 may need to effectively be set to a fractional value of the clock cycle period of the controller 8. In practice, this may be implemented by dithering the sensing period t(Rmeas). For example, it may be required to schedule the sensing interval every 1000.125 clock cycles of the controller 8 to stay in phase with the ADC period t(ADC). In practice, this may be implemented by setting 7 out of 8 sensing periods to 1000 clock cycles and setting 1 out of 8 sensing periods to 1001 clock cycles. So, the period between one or more sensing intervals 50 in the series of sensing intervals may be set different to the period between one or more other sensing intervals 50 in the series of sensing intervals so as to remain in phase with the CDS ramps 55, 56.

[0064] The image sensor 4 may output the sync signal SI at the same rate as carrying out CDS. For every instance of the sync signal SI, CDS may be carried out. Alternatively, as shown in Figs. 4 and 5, the sync signal SI may be output at a rate that is an integer multiple of (including equal to) the rate at which the image sensor 4 carries out CDS. Figs. 4 and 5 show a sync signal SI that is output at twice the rate at which the image sensor 4 carries out CDS.

[0065] In Figs. 4 and 5, the measurement pulse 51 is a square voltage pulse. However, the measurement pulse 51 may in general be any other pulse, for example a current pulse, that allows measuring of the electrical characteristic of the SMA wire 30. The measurement pulse 51 is not necessarily a square pulse, but may be a pulse with a slower or gradual onset and a slower or gradual descent. This may advantageously reduce any EMI in the image sensor 4 due to the measurement pulse 51. In general, the measurement pulse 51 may have any shape.

[0066] In Figs. 4 and 5, the measurement pulse 51 is generated for the entire duration of the sensing interval 50. In other embodiments, the measurement pulse 51 may be generated for part of the duration of the sensing interval 50. The measurement pulse 51 could start and end at any time within the sensing interval 50. In some embodiments, the start time and duration of the measurement pulse 51 may be scheduled so as to avoid switching of the measurement pulse 51 for the duration t(ref) of the reference ramp 55 and for a substantially equal duration t(ref) of the sensing ramp 56. With reference to Figs. 4 and 5, for example, the measurement pulse 51 may be scheduled to start at the beginning of the depicted sensing interval 50 within time t(SI)3min, and end at a time between the reference ramp 55 and the sensing ramp 56. This may further reduce EMI due to the measurement pulse 51.

[0067] In Figs. 3 to 5, the duty cycle (i.e. pulse widths) of the PWM control signals P1-P4 is shown to be constant. In practice, the duty cycle of the PWM control signals P1-P4 may be variable, to thereby control the power that is delivered to the respective SMA wires 30. The duty cycle may be adjustable at fixed intervals, for example within a control loop servo frame. The frequency of duty cycle adjustment (so the frequency of control loop servo frames) may be about 2 kHz, for example 1 to 5 kHz. The inventors have found that adjusting the duty cycle of the PWM control signals P1-P4 at a time between the reference ramp 55 and the sensing ramp 56 may lead to EMI in the image sensor 4. So as to avoid or reduce such EMI, in some embodiments, the controller 8 may schedule modifying the duty cycle of the PWM control signals P1-P4 at a time before the earlier one of reading the reference signal and reading the light-sensing signal and after the later of the duration of reading the reference signal and the substantially equal duration of reading the light-sensing signal. So, the controller 8 may schedule modifying the duty cycle of the PWM control signals P1-P4 outside a time between the reference ramp 55 and the sensing ramp 56.

[0068] Alternatively or additionally, the controller 8 may schedule modifying the duty cycle of the PWM control signals P1-P4 at a time outside the sensing interval 50. The sensing interval 50 may overlap with the entire or a large portion of the time between the reference ramp 55 and the sensing ramp 56, and so avoiding modifying the duty cycle of the PWM control signals P1-P4 during the sensing interval 50 may reduce EMI due to modifying the duty cycle. Scheduling the duty cycle modification with reference to the sampling interval 50 may be computationally more efficient than scheduling the duty cycle modification based on the sync signal SI and / or the sync signal S2. Such scheduling of the duty cycle modification may for example be beneficial if the sensing interval 50 is scheduled during durations t(SI)3min or t(SI)2max shown in Figures 4 and 5. The image sensor 4 may operate in a plurality of different operating modes. The controller 8 may receive data indicative of the operating mode of the image sensor 4. For example, the image sensor 4 may operate in a first mode for capturing high-resolution still images, and in a second mode for capturing a video stream. The relation between the frame synchronization signal S2 and performing CDS, as well as the CDS ramps, may differ in the different operating modes. As such, the controller 8 may take the operating mode of the image sensor 4 into account when scheduling the sensing interval 50. In particular, the controller 8 may set one or more of the time delay, the duration of the sensing interval 50, the sensing frequency and optionally the timing and duration of the measurement pulse 51 to different values for the different operating modes of the image sensor 4. The different values for the different operating modes may be known to the designer of the controller 8 from the specifications of the image sensor 4, or may be determined empirically as described above in relation to a single operating mode.

[0069] As described above, the first sync signal SI may be an HSYNC signal that is output by the image sensor 4. Alternatively, the first sync signal SI may be any other sync signal SI that has a deterministic relationship with performing CDS. The first sync signal SI may or may not be a dedicated signal that is output by the image sensor 4 specifically for the purpose of scheduling the sensing interval 50.

[0070] The present invention also relates to scheduling the modification of the pulse widths of the PWM control signals P1-P4. As described above, the duty cycle of the PWM control signals P1-P4 may be variable, so the pulse widths of the PWM control signals P1-P4 may be variable. This allows control of the power that is delivered to the respective SMA wires 30. In accordance with the present invention, scheduling modification of the pulse width of the PWM control signals P1-P4 may take place with a time delay after receiving the frame synchronization signal S2. The modification may be scheduled so as to avoid modifying the pulse width of the PWM control signals for a duration between reading the reference signal and reading the light-sensing signal by the image sensor, i.e. for the duration between the start of the reference ramp 55 and between the duration t(ref) of the sensing ramp 56, or for the duration between the start of the sensing ramp 56 and the end of the reference ramp 55.

[0071] The present invention has been described in connection with SMA wires 30. The term 'SMA wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires.

[0072] Although the present invention has been described with reference to four SMA wires 30, and four PWM control signals P1-P4 that are applied to the four SMA wires 30, in general the present invention extends to the use of any number of SMA wires 30, and any number of corresponding PWM control signals. For example, a single PWM control signal may be applied to a single SMA wire 30 so as to drive movement of the movable part 20 relative to the support structure 10. The sensing interval 50 for the single SMA wire may be scheduled as described in this application.

[0073] The present invention has been described in connection with SMA wires 30 that, on contraction, move the movable part 20 relative to the support structure 10. SMA wire is advantageous as an actuator in such an actuator assembly, in particular due to its high energy density which means that the SMA wire 30 required to apply a given force is of relatively small size. However, in general, other actuator components may be used instead of the SMA wires 30. Such actuator components, on actuation, move the movable part 20 relative to the support structure 10. Examples of such actuator components include, but are not limited to, voice-coil motors (VCM), MEMS devices or any other components capable of moving the movable part 20 relative to the support structure 10. Such actuator components may introduce EMI in an image sensor 4 that is fixed relative to the movable part 20 or support structure 10. Driving such actuator components in accordance with the present invention may reduce such noise. Measuring an electrical characteristic of these actuator components, for example an inductance of VCMs, may be used to determine an actuation amount of the actuator components. The determined actuation amount may be used to determine the actual position of a movable part 10 relative to the support structure 20.

[0074] The foregoing has described some embodiments of the present invention, but the present invention is not limited to these embodiments. The scope of the invention is defined in the appended claims.

Claims

Claims1. A computer-implemented method of generating PWM control signals and a measurement pulse for applying to an actuator component that drives movement of a movable part relative to a support structure, an image sensor being fixed relative to the movable part or the support structure, the image sensor carrying out correlated double sampling, CDS, so as to read, offset in time by a sampling period, a reference signal and a light-sensing signal, and to output a frame synchronisation signal, wherein the method comprises: receiving the frame synchronisation signal; generating pulse width modulated, PWM, control signals to drive the actuator component at a PWM frequency substantially equal to an integer multiple of the reciprocal of the sampling period; during a series of sensing intervals occurring at a sensing frequency, suspending generating PWM control signals to drive the actuator component, and generating a measurement pulse to measure an electrical characteristic of the actuator component; and scheduling a first sensing interval of the series of sensing intervals to start a time delay after receiving the frame synchronization signal, and setting the time delay, the duration of sensing intervals and the sensing frequency such that each sensing interval overlaps with both or neither of a duration of reading the reference signal and a substantially equal duration of reading the lightsensing signal by the image sensor.

2. The computer-implemented method according to claim 1, wherein the time delay and the sensing frequency are set such that each sensing interval starts after the later of i) the duration of reading the reference signal and ii) the substantially equal duration of reading the light-sensing signal, and wherein the duration of the sensing interval is set so that the sensing interval ends before the next occurrence of CDS.

3. The computer-implemented method according to claim 1 or 2, wherein the time delay and the sensing frequency are set such that each sensing interval starts before the earlier of i) reading the reference signal and ii) reading the light-sensing signal, and wherein the duration of the sensing interval is set longer than the sampling period so that the sensing interval ends after the later of i) the duration of reading the reference signal and ii) the substantially equal duration of reading the light-sensing signal.

4. The computer-implemented method according to any preceding claim, wherein the method comprises setting a reciprocal of the sensing frequency to an integer multiple of a period of a line synchronization signal output by the image sensor.

5. The computer-implemented method according to any preceding claim, wherein the PWM control signals are asynchronous to the sensing intervals.

6. The computer-implemented method according to any preceding claim, wherein the generation of PWM control signals is paused at the start of the sensing interval and resumes at the end of the sensing interval.

7. The computer-implemented method according to any one of claims 1 to 5, wherein the generation of PWM control signals continues during the sensing interval, and wherein the PWM control signals are suppressed and not output to drive the actuator component during the sensing interval.

8. The computer-implemented method according to any one of the preceding claims, wherein the image sensor is driven based on a master clock signal, and wherein the method comprising receiving the master clock signal and using the master clock signal to generate the PWM control signals and to schedule the sensing intervals.

9. The computer-implemented method according to any one of the preceding claims, wherein the temporal resolution of scheduling the sensing intervals is less than the temporal resolution of the image sensor, and wherein the period between one or more sensing intervals in the series of sensing intervals is set different to the period between one or more other sensing intervals in the series of sensing intervals so as to remain in phase with the CDS ramps.

10. The computer-implemented method according to any preceding claim, wherein the measurement pulse is generated for the entire duration of the sensing interval.

11. The computer-implemented method according to any preceding claim, wherein the measurement pulse is a current pulse and the electrical characteristic is a measure of the resistance of the actuator component, and wherein during generation of the measurement pulse, the methodcomprises measuring the voltage across the actuator component and determining the measure of the resistance based on the current pulse and the measured voltage.

12. The computer-implemented method according to any preceding claim, comprising deriving a measure of the actuation amount of the actuator component based on the measured electrical characteristic, wherein generating the PWM control signals comprises closed loop control and feeding back the measure of the actuation amount to the closed loop control.

13. The computer-implemented method according to any preceding claim, wherein generating the PWM control signals comprises generating PWM control signals for an SMA element acting as an actuator component, the SMA element, on actuation, driving movement of the moveable part relative to the support structure.

14. The computer-implemented method according to any preceding claim, comprising generating PWM control signals to drive each of a plurality of actuator components at a PWM frequency substantially equal to an integer multiple of the reciprocal of the sampling period, such that the plurality of actuator components drive movement of the moveable part relative to the support structure, wherein the actuator component is one of the plurality of actuator components.

15. The computer-implemented method according to claim 14, comprising generating the PWM control signals to drive each of the plurality of actuator components such that none of the PWM control signals overlap.

16. The computer-implemented method according to claim 14 or 15, comprising, during the sensing interval used to measure an electrical characteristic of the one actuator component, suspending generating PWM control signals for each of the actuator components.

17. The computer-implemented method according to any preceding claim, further comprising modifying a duty cycle of the PWM control signals at a time before the earlier one of reading the reference signal and reading the light-sensing signal and after the later of the duration of reading the reference signal and the substantially equal duration of reading the light-sensing signal.

18. The computer-implemented method according to any preceding claim, comprising modifying a duty cycle of the PWM control signals at a time outside the sensing interval.

19. The computer-implemented method according to any preceding claim, further comprising receiving data indicative of one of a plurality of different operating modes of the image sensor, and setting at least one of the time delay, the duration of the sensing interval, the sensing frequency and the timing and duration of the measurement pulse to different values for the different operating modes of the image sensor.

20. A computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to any preceding claim.

21. A computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the method according to any one of claims 1 to 19.

22. An apparatus comprising: an actuator assembly comprising a support structure and a movable part supported on the support structure, and an actuator component configured to drive movement of the movable part relative to the support structure; and a controller configured to carry out the method according to any one of claims 1 to 19.

23. The apparatus according to claim 22, further comprising an image sensor fixed relative to the support structure or the movable part, wherein the image sensor is configured to carry out correlated double sampling, CDS, so as to read, offset in time by a sampling period, a reference signal and a light-sensing signal, and to output a sync signal prior to CDS.

24. The apparatus according to claim 22 or 23, wherein each actuator component comprises an SMA element, wherein the SMA element is arranged, on actuation, to drive movement of the moveable part relative to the support structure.

25. The apparatus according to any one of claims 22 to 24, comprising a plurality of actuator components arranged, on actuation, to drive movement of the moveable part relative to the support structure.

Images