Feedforward judgment of the drive signal of the piezoelectric actuator

Abstract

The present invention relates to a method for determining a setpoint voltage of a piezoelectric actuator system to achieve a desired optical response, e.g., optical power, of the piezoelectric actuator system. The method is based on a mathematical model OP(V,T,n) that describes the relationship between applied voltage and optical response. Calibration and use of the model requires determining a transition time count value. During use, for example, at given intervals, the model is updated based on actual measurements of temperature and transition time count values. Based on the updated model, a setpoint voltage required to achieve the desired optical response is determined, and the setpoint voltage is applied to the piezoelectric actuator.

Description

[Technical field]

[0001] The present invention relates to piezoelectric actuator systems, in particular those used in optical imaging systems such as compact cameras and smartphones, and to determining the drive signal for such systems. [Background technology]

[0002] Piezoelectric actuators have several advantages that can be utilized in optical systems requiring high precision, such as optical imaging systems used to adjust the focus of an optical system.

[0003] Well-known problems with piezoelectric actuators are hysteresis and piezoelectric creep. Piezoelectric creep describes the slow readjustment of crystalline domains over time at a constant input voltage. When the operating voltage of a piezoelectric actuator is changed, the remnant polarization continues to change even after the voltage change is complete, which manifests as a slow creep. A feedback setup that iteratively adjusts the output based on a comparison of the desired and actual response of the actuator can eliminate the possible deviations by compensating for the effects of hysteresis and creep. However, to achieve a feedback setup, the feedback system must measure the actuator response accurately and with the desired high bandwidth. This can increase the complexity and manufacturing cost, as well as the power consumption and response time of the system.

[0004] Therefore, there is a need for a piezoelectric actuator system that addresses one or more of these problems. Summary of the Invention [Problem to be solved by the invention]

[0005] It is an object of the present invention to improve the control of piezoelectric actuators, particularly optical assemblies used in optical assemblies such as optical imaging systems, thereby improving the accuracy of the response of the piezoelectric actuator, particularly in a feedforward configuration where control is achieved without real-time measurement of the actuator response.

[0006] According to a first aspect of the present invention, there is provided a method for determining a set point voltage for a piezoelectric actuator system to achieve a desired optical response, e.g. optical power, of the piezoelectric actuator system, the piezoelectric actuator system including an optical member, such as an optical lens having an adjustable optical response, and a piezoelectric actuator, the method comprising: - providing a mathematical model OP(V,T,n) describing a relationship between the voltage applied to the piezoelectric actuator, temperature, transition time count values ​​as variables, and the optical response, the mathematical model including a curve-fitted polynomial, the transition time count values ​​relating to at least one transition time value of the piezoelectric actuator for changing from a first state to a second state; calibrating a mathematical model, - determining a calibrated transition time count value based on determining at least one transition time value between a first and a second state of the piezoelectric actuator; - obtaining a calibration temperature of the piezoelectric actuator; - determining one or more calibration parameters OP(V,T,n) of the mathematical model based on one or more calibration voltages and one or more optical responses associated with the one or more calibration voltages applied to the piezoelectric actuator; -After a period of time or use of the piezoelectric actuator, obtaining a temperature of the piezoelectric actuator; determining a transition time count value based on determining at least one actual transition time value between a first and a second state of the piezoelectric actuator; - updating the mathematical model OP(V,T,n) by determining a set point voltage based on the updated mathematical model OP(V,T,n) and applying the set point voltage to the piezoelectric actuator.

[0007] Since piezoelectric actuators have electrical capacitance and the piezoelectric response depends on the capacitance, determining the transition time count value related to the capacitance can improve the accuracy of the model.

[0008] Furthermore, the relationship between the response and capacitance of the piezoelectric actuator is not significantly dependent on the time-dependent polarization of the piezoelectric material, i.e., the creep effect of the piezoelectric actuator. Therefore, including information about the capacitance of the piezoelectric actuator in the model can improve the accuracy of the determination of the optical response for a given input voltage, since the actual creep level is taken into account.

[0009] Additionally, including temperature in the model allows for determination of actuator or optical responses that are compensated for temperature changes.

[0010] Advantageously, by determining a set point voltage to the piezoelectric actuator to achieve a desired optical response, it is possible to eliminate the need for closed loop control of the voltage applied to the piezoelectric actuator, thereby eliminating the need for real-time measurements of the optical response.

[0011] While piezoelectric actuators are used in connection with controlling optical elements to control optical responses, piezoelectric actuators may also be used in connection with other devices controllable by the piezoelectric actuator to control other actuator responses.

[0012] The set point voltage or voltage applied to the piezoelectric actuator is related to the electric field on the electrodes of the piezoelectric element, i.e. the internal voltage of the piezoelectric material. The applied voltage may be a voltage in the electrical circuit used to power the piezoelectric actuator, such as an analog or digital voltage reference of an amplifier, the output voltage of an amplifier, the terminal voltage of the piezoelectric actuator, etc. Thus, the method may also be viewed as a method of determining the electric field on the piezoelectric actuator based on the determination of the set point voltage.

[0013] According to one embodiment, the relationship describes the optical response to falling and / or rising values ​​of voltage. Advantageously, since the piezoelectric response exhibits hysteresis, the model may advantageously include the response for both falling and rising values ​​of voltage, although the model may be based on either a falling or rising curve.

[0014] According to one embodiment, this relationship solely describes the optical response to a falling or rising value of voltage. Advantageously, the model can be made simpler by including only one of the falling or rising curves. By including only one curve, hysteresis effects can be addressed by first applying the maximum or minimum voltage before applying the set point voltage.

[0015] According to one embodiment, determining the transition time count value and the calibrated transition time count value includes determining a first transition time value between the first and second states of the piezoelectric actuator and determining a second transition time value between the third and fourth states, the transition time count value and the calibrated transition time count value being determined based on a ratio of the first and second transition time values. Advantageously, determining the transition time count value based on a ratio of the first and second transition time values ​​can reduce the temperature dependence of the transition time count value. As used herein, a transition time value may be a time value measured in time, a number of counts, or other quantity related to time.

[0016] According to one embodiment, the first and second states include first and second steady state voltages obtained based on applying first and second voltages to the piezoelectric actuator.

[0017] According to one embodiment, the first steady state voltage is higher than the second steady state voltage, and the transition time value is obtained based on a transition from the first steady state voltage to the second steady state voltage. Advantageously, going from a higher steady state voltage to a lower steady state voltage may result in a transition time value that is less dependent on the actual temperature, thereby resulting in a more accurate transition time count value. However, the reverse operation has also been observed, and going from a lower steady state voltage to a higher steady state voltage may also be advantageous.

[0018] According to one embodiment, the curve fitting polynomial is a predetermined non-linear function of at least the voltage.

[0019] According to one embodiment, the mathematical model consists of the sum of a first term a(T)xL(V) and a second term b(T,C), where a(T) is a polynomial depending at least on the temperature parameter, the first calibration parameter and the actual temperature, L(V) is a polynomial of at least third order depending on at least the voltage, and b(T,C) is a polynomial depending on the temperature parameter, the voltage, the second calibration parameter, the actual temperature and the actual capacitance parameter.

[0020] Determining the mathematical model may include determining first and second calibration parameters based on the mathematical model, OP(V,T,n), at least first and second calibration voltages, and at least first and second optical responses determined based on the at least first and second calibration voltages applied to the piezoelectric actuator. That is, the first and second optical responses may be responses to applying the first and second calibration voltages, or conversely, the first and second calibration voltages may be voltages that result in desired or predetermined first and second optical responses. Alternatively, a single set of a single calibration voltage and a single optical response may be used for calibration, or more than one calibration voltage and corresponding more than one optical response may be used.

[0021] According to one embodiment, the mathematical model further includes a term, OPtherm(T), describing the optical response variation as a function of temperature due to a temperature dependent component, such as a lens, in an optical lens system, which is optically coupled to the piezoelectric actuator system.

[0022] Advantageously, by including the thermal term OPtherm(T), the optical response of the optical element can be compensated for variations in the optical response of other components, i.e., by determining the voltage set points, the optical response variations of other components can be compensated for in the optical actuator system.

[0023] The optical response variation described by the term OPtherm(T) may be the variation of the optical response of an optical element of the piezoelectric actuator system. In a simple example, the term OPtherm(T) may be a linear function of temperature, e.g. the temperature used in the mathematical model OP(V,T,n). Thus, for a given temperature, the thermal function OPtherm(T) gives the optical response variation to be compensated by the piezoelectric actuator system through the determination of the voltage set point.

[0024] According to one embodiment, applying a voltage to the piezoelectric actuator includes first applying a maximum or minimum voltage to the piezoelectric actuator and then applying a set point voltage to the piezoelectric actuator to drive the piezoelectric actuator at a maximum or minimum deformation. Advantageously, by applying an initial maximum or minimum voltage, a simpler mathematical model OP(V,T,n) can be used since only one of the descending or ascending hysteresis curves is required.

[0025] According to one embodiment, the set point voltage is determined in an open loop configuration that does not rely on measuring the actual optical response.

[0026] A second aspect of the present invention relates to a piezoelectric actuator system comprising a piezoelectric actuator, a piezoelectric driver for supplying power to the piezoelectric actuator, a processing unit for determining a set point voltage applied to the piezoelectric actuator by the piezoelectric driver in accordance with the method of the first aspect, and an optical element having a tunable optical response.

[0027] The piezoelectric driver and the processor may be separate components or may be combined into a single component. The piezoelectric driver typically includes an amplifier circuit designed to power the piezoelectric actuator. The processor may implement the mathematical model, for example via a computer program executed by a digital processor, and may include other circuits, such as a memory for storing, for example, parameters of the mathematical model, and a timer circuit for measuring transition time values.

[0028] A third aspect of the present invention relates to the piezoelectric actuator system according to the second aspect, wherein the optical member is a first transparent cover member; a second cover member; - a transparent, deformable, non-fluid material sandwiched between first and second transparent cover members, the one or more piezoelectric elements of the piezoelectric actuator being arranged to enable controllable changes in curvature, displacement and / or orientation of the first or second cover members.

[0029] The second cover member may be transparent, reflective, or partially reflective. Thus, light may be transmitted through the optical member, reflected from the optical member, or partially transmitted and partially reflected. A controllable change in curvature may be used to cause a change in optical power. A change in displacement may be used to cause a displacement of a light beam transmitted through or reflected from the optical member. A change in orientation, such as a change in the angle of the first transparent cover member relative to the second cover member, may be used to cause a change in the propagation direction of a light beam transmitted through or reflected from the optical member.

[0030] A fourth aspect of the present invention relates to a camera module including a piezoelectric actuator system according to the second aspect, wherein the piezoelectric actuator system includes an optical lens, and the camera module further includes an image sensor arranged to receive light transmitted through the optical lens.

[0031] A fifth aspect of the present invention relates to an electronic device including the piezoelectric actuator system according to the second aspect.

[0032] In general, the various aspects and embodiments of the invention may be combined and combined in any manner possible within the scope of the invention. These and other aspects, features and / or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. [Brief description of the drawings]

[0033] Embodiments of the present invention will now be described, by way of example only, with reference to the drawings in which: [Figure 1A] 1 shows a piezoelectric actuator system. [Figure 1B] FIG. 2 shows a top view of a piezoelectric actuator system. [Figure 1C] FIG. 2 shows a top view of a piezoelectric actuator system. [Figure 1D] 1 shows a camera module including a piezoelectric actuator system. [Figure 2A] A curve OP(V) showing the relationship between the optical power OP and the voltage V applied to the piezoelectric actuator is shown. [Figure 2B] Demonstrates the piezoelectric creep effect. [Figure 3A] 1 shows the relationship between optical power and capacitance of a piezoelectric actuator system. [Figure 3B] 4 illustrates an example of determining the first and second transition time values. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] FIG. 1A shows a piezoelectric actuator system 100 including a piezoelectric actuator 101 and an optical lens 120. The optical power of the optical lens can be adjusted by a force or torque generated by the piezoelectric actuator on the optical lens. The piezoelectric actuator system 100 further includes a piezoelectric driver 110 that provides power to the piezoelectric actuator 100, and a processing unit 111 that determines a set-point voltage Vx to be applied by the piezoelectric driver 110 to the piezoelectric actuator to achieve a desired actuator response, such as a desired optical power of the optical lens 120. Equivalently, the set-point voltage Vx may be the output of the driver 110, the input to the driver, or another voltage or analog or digital reference to the voltage applied to the piezoelectric actuator 101. Here, it is understood that the optical power and the focal length are equivalent measures.

[0035] The piezoelectric actuator 101 may include one or more piezoelectric elements, such as a ring-shaped piezoelectric element as shown in FIG. 1B or multiple piezoelectric elements as shown in FIG. 1C, mounted on a surface of the first and / or second transparent cover members 121, 122. Other configurations of the piezoelectric actuator are possible. For example, the piezoelectric actuator may be disposed between the first and second transparent cover members 121, 122 and connected to the inwardly facing surfaces of the cover members 121, 122.

[0036] The optical lens is composed of a first transparent cover member 121, a second transparent cover member 122, and a transparent, deformable, non-fluid body 123 sandwiched between the first and second transparent cover members.

[0037] The piezoelectric actuator 101 is arranged to produce a controllable change in curvature of the first or second cover members. The force or torque that the piezoelectric actuator produces on the first and / or second transparent cover members can cause the transparent cover members to bend into a concave or convex shape, thereby imparting optical power to the incident light 125.

[0038] In general, the piezoelectric actuator system 100 may be configured with an optical member 130, such as a lens 120, and other optical components that provide an optical response, such as optical power. For example, the optical member 130 may be configured as in FIG. 1A with first and second transparent cover members 121, 122, but the piezoelectric actuator 101 may be positioned to displace and / or change the orientation of one of the transparent cover members 121 relative to the other 122. For example, as described above, the piezoelectric actuator 101 may be positioned between the cover members 121, 122, such that changing the angle between the two cover members can, for example, change the direction of a beam transmitted through the optical member 130. In another embodiment, one of the transparent cover members 121, 122 is replaced by a mirror, allowing the deflection of the reflected beam to be adjusted. Thus, examples of optical responses include optical power, change in optical axis direction, and the like, including combinations thereof.

[0039] Although some examples herein refer to the piezoelectric actuator system 100 as comprising a lens 120, the piezoelectric actuator system 100 generally includes an optical member 130.

[0040] FIG. 1B is a top view of a piezoelectric actuator system 100 including a ring-shaped piezoelectric actuator 101 arranged to induce bending of a transparent cover member 121.

[0041] FIG. 1C is a top view of a piezoelectric actuator system 100 that includes two or more individually controllable piezoelectric actuators 101.

[0042] FIG. 1D illustrates a camera module 150 including the piezoelectric actuator system 100 and an image sensor 152 arranged to receive light passing through the optical lens of the actuator system. Additional optical components, such as a fixed optical lens of an optical lens system 151, may be arranged optically together with the variable lens 120. The purpose of the optical lens system 151 may be to provide additional optical power, correct optical errors, or other optical adaptations. In this manner, the optical lens system 151 is optically connected to the piezoelectric actuator system 100. The camera module 150 may be used in an electronic device 159, such as a smartphone.

[0043] Transparent and deformable non-fluid 123 is in contact with the inward surfaces of first and second transparent cover members 121, 122. As shown in Figs. 1A to 1C, lens 120 constitutes portion 121 constituting a lens area surrounded by piezoelectric actuator 101, and piezoelectric actuator 101 is arranged so that light can pass through the lens area without being blocked. Non-fluid 123 is arranged so as to cover at least the lens area, but may extend beyond the extension line of non-fluid 123 toward the outer periphery of first and second transparent cover members 121, 122.

[0044] The transparent deformable non-fluid lens body 123 is preferably made from an elastic material. Because the lens body is non-fluid, no liquid-tight enclosure is required to hold the lens body, and there is no risk of leakage. In a preferred embodiment, the lens body 123 is made from a soft polymer, which can include many different materials, such as silicone, polymer gels, polymer networks of cross-linked or partially cross-linked polymers, and miscible oils or combinations of oils. The elastic modulus of the non-fluid lens body can be greater than 300 Pa to avoid deformation due to gravity during normal operation. The refractive index of the non-fluid lens body can be greater than 1.3. The non-fluid body 123 can have a refractive index equal to, substantially equal to, or close to the refractive index of the transparent cover members 121, 122 to reduce reflections at the boundaries of the non-fluid body 123.

[0045] The transparent cover members 121, 122 are generally slab-like and may be curved or flat, or a combination thereof. The transparent cover members 121, 122 may be made from a number of different materials, such as acrylic, polyolefin, polyester, silicone, polyurethane, glass, etc. At least one of the first and second cover members 121, 122 arranged to be deformed by the actuator has a suitable stiffness and thickness to be bendable by actuation of the piezoelectric actuator 101. In general, the material of the first and / or second cover members 121, 122 may be formed of a material with a Young's modulus in the range of 5 MPa to 100 GPa to obtain the required stiffness. For example, the Young's modulus of borosilicate glass is 63 GPa, and that of fused silica glass is 72 GPa.

[0046] FIG. 2A shows a curve OP(V) showing the relationship between the optical power OP and the voltage V applied to the piezoelectric actuator 101. FIG. 2 shows that the hysteresis effect of the piezoelectric actuator, where the optical power OP depends on the start and stop voltages V applied to the piezoelectric actuator 101, increases with increasing voltage range, resulting in a larger amount of hysteresis. It has been confirmed that the optical power OP value for voltages falling from a certain maximum voltage Vmax follows the same falling OP(V) curve for the entire voltage range Vmin to Vmax. This observation can be utilized for hysteresis compensation according to an embodiment of the present invention. The rising curve is similar, and the same rising OP(V) curve will follow the entire voltage range Vmin to Vmax from a certain minimum voltage Vmin.

[0047] Thus, initially, a maximum or minimum voltage Vmax, Vmin can be applied to the piezoelectric actuator to drive the piezoelectric actuator at a maximum or minimum deformation or optical power OP. After the maximum or minimum deformation or optical power is reached, a desired set point voltage Vx is applied to the piezoelectric actuator 101 of the piezoelectric actuator system 100. As described below, the set point voltage Vx is the voltage that is determined so that the piezoelectric actuator achieves a desired optical power OPx.

[0048] Thus, by initially applying a maximum or minimum voltage that is located above or below the set point voltage Vx, the transition from the maximum or minimum voltage to the set point voltage Vx will follow a descending or ascending curve, i.e., the optical power OP for a given voltage V will follow or substantially follow the hysteresis curve of OP(V).

[0049] The hysteresis curve OP(V), or at least one of the descending or ascending curve portions of OP(V), may be calculated at any time for any given piezoelectric actuator system 101, as described below.

[0050] The optical power vs. voltage relationship OP(V) also depends on the temperature T of the piezoelectric actuator 101. The temperature dependence is indicated by arrow 201 and curve 202 which is valid at temperatures higher than those for which the solid OP(V) curve is valid. Thus, the voltage required to obtain a given optical power OP decreases with increasing temperature T. For the same actuation voltage V, the optical power OP increases with increasing temperature T.

[0051] Thus, the thermal drift of the piezoelectric actuator system 100 may mean that for the same voltage V, the optical power increases with increasing temperature. This optical power vs. temperature relationship is typically opposite to that of a plastic lens that may be used in the camera module 150, such as the optical lens system 151. Thus, the thermal drift of the piezoelectric actuator 101 may be partially compensated by the thermal drift of the optical lens system 151, and vice versa.

[0052] To reduce the thermal drift of piezoelectric actuator system 100 with respect to optical power, or to achieve or approach a desired thermal drift of piezoelectric actuator system 100, optical lens module 151 may be designed such that the thermal drift of module 151 compensates for or partially compensates for the thermal drift of piezoelectric actuator system 100 with respect to optical power versus temperature.

[0053] Thus, the thermal behavior of the camera module 150 system that does not include the piezoelectric actuator system 100, i.e., the thermal behavior of the temperature-dependent components of the camera module, such as the fixed optical lens module 151, can be designed, for example by material selection, dimensions and mechanical design, to obtain optimal performance of the entire camera module 150 or to compensate for the thermal dependency of the piezoelectric actuator system 100 over a given operating temperature range.

[0054] As described in more detail below, this athermal design may be addressed by a mathematical model that describes the relationship between optical power OP and voltage V by a mathematical term OPtherm(T), which describes the temperature dependence of optical power OP of a temperature-dependent component, such as the optical lens system 151, as a function of temperature T.

[0055] FIG. 2B illustrates the piezoelectric creep effect due to the time-dependent polarization of the piezoelectric material in a constant electric field. As shown by the initial curve OP(V) (similar to OP(V) in FIG. 2A) and the creep effect curve 211, the piezoelectric creep causes a bump in the OP(V) curve of the piezoelectric actuator system 100, i.e., the voltage required to reach a certain optical power decreases. The lower and upper parts of the curve, e.g., near the minimum and maximum voltages Vmin and Vmax, are less affected by the creep effect. The rate of creep decreases logarithmically with time, so that most of the creep occurs within, e.g., the first hour of biasing, which is equivalent to applying a voltage V to the piezoelectric actuator 101. The piezoelectric actuator 101 retains some voltage history. The remnant polarization gradually decreases over time without the application of an electric field. In practice, however, creep levels are maintained with regular use of the piezoelectric actuator system 100. A piezoelectric actuator system 100 that has experienced some creep may be less susceptible to further creep than an unused piezoelectric actuator system 100 .

[0056] After applying voltage V, or a different voltage V, to piezoelectric actuator 101 for a short time, the piezoelectric actuator begins to operate according to curve 211 with reduced hysteresis and improved linearity.

[0057] Thus, the use of the piezoelectric actuator 101 to achieve accurate and predictable setting of the optical power OP is complicated by temperature, creep and hysteresis dependencies. However, it has been found that the relationship between the optical power OP and the capacitance of the piezoelectric actuator 101 is constant, or substantially constant, for a given piezoelectric actuator 101 and is independent of the voltage bias history, i.e., creep effects.

[0058] It should be noted that these dependencies are relative to the piezoelectric elements of the piezoelectric actuator 101. Because the piezoelectric actuator 101 drives the lens 120, the optical power OP of the piezoelectric actuator system 100 will be subject to these dependencies as well.

[0059] FIG. 3A is a diagram showing the relationship between the optical power OP and the capacitance C of the piezoelectric actuator system 100, that is, the capacitance of the piezoelectric element(s) of the piezoelectric actuator 101.

[0060] Therefore, by measuring the capacitance C of the piezoelectric actuator 101 for a given voltage V, this relationship will give correct information about the optical power at that voltage OP(V), regardless of the level of creep at the time of the measurement.

[0061] According to one embodiment, the following mathematical model OP(V,T,n) describes the relationship between optical power OP and voltage V for the descending or ascending curve illustrated in FIG. 2A. OP(V,T,n)=a(T)xL(V)+b(T,n)

[0062] Thus, this equation determines the optical power OP for a falling or rising voltage V as the sum of the product term a(T) × L(V) and the term b(T,n), which depends on the variables temperature T and transition time count value n. The coefficient L(V) is a curve-fit polynomial.

[0063] This mathematical model can be expanded to include the term OPtherm(T) as follows: OP(V,T,n)=a(T)xL(V)+b(T,n)+OPtherm(T) OPtherm(T) describes the dependence of the optical power OP on the temperature T of a temperature-dependent component, such as a lens in the optical lens system 151, i.e., the thermal drift of the temperature-dependent component mentioned above. OPtherm(T) can be obtained by measuring the thermal drift and determining a mathematical expression of the thermal drift as a function of temperature T.

[0064] The transition time count value n relates to the transition time x of the piezoelectric actuator 101 when the piezoelectric actuator changes from a first state X1 to a second state X2.

[0065] The first and second states X1, X2 may be steady-state voltages V1, V2 resulting from first and second power supply values ​​P1, P2, such as first and second voltages V1, V2, or first and second charges applied to the piezoelectric actuator. Note that, although the embodiment of the present invention is based on driving the piezoelectric actuator 101 based on a voltage controller power signal, it does not exclude driving the piezoelectric actuator 101 based on a current or charge control power signal. In practice, like the steady-state voltages, the voltage signals such as the first and second voltages V1, V2 can be represented by digital or analog values.

[0066] Thus, the voltages V1, V2 can be measured via electrical connections to the terminals of the piezoelectric actuator 101. The first and second states X1, X2 can also be steady-state optical powers OP1, OP2 (or other optical responses) resulting from the applied first and second voltages V1, V2 and measured via an optical detector such as the image sensor 152.

[0067] The transition time count value n can be measured in various ways, for example, by using a timer that starts in response to the initiation of application of a first voltage value V1 when the actuator 101 is in a known state, such as a first state X1, and stops when the actuator 101 reaches a second state X2 in response to an applied second voltage value V2. Either of the first and second voltage values ​​V1, V2 may be a zero value, such as zero voltage.

[0068] The transition time count value n may be a time, a count of an oscillator clock signal, or some other representation of transition time.

[0069] According to one embodiment, the transition time count value n is determined based on determining a first transition time value x1 between the first and second states X1, X2 resulting from the first and second voltage values ​​V1, V2 and determining a second transition time value x2 between the third and fourth states X3, X4 of the piezoelectric actuator resulting from the third and fourth voltage values ​​V3, V4. Based on the first and second transition time values ​​x1, x2, the transition time count value is determined based on a ratio n=x1 / x2 of the first and second transition time values.

[0070] In this manner, the transition time count value n may be determined based on one or more transition time values ​​x1, x2. Advantageously, it has been found that a count value n determined from a ratio of transition time values ​​x1, x2 is at least to some extent temperature independent. A count value n determined from a single transition time value x1 is also feasible, but may require some adaptation of the model OP(V,T,n).

[0071] FIG. 3B shows an example for determining the first and second transition time values ​​x1, x2. Thus, the determination of the first transition time value x1 is made by the time from the moment when the second voltage value V2 is applied, i.e., after the first state X1 is reached in response to the first voltage value V1, to the moment when the second state X2 is reached. The same is true for the determination of the second transition time x2. In FIG. 3B, the second and third voltage values ​​V2, V3 are the same, and as a result, the second and third states X2, X3 are also the same. The direction of change between states X1-X4 may be changed, for example, so that the first state X1 has a higher value than the second state X2. Again, all voltage power supply values ​​V1-V4 may be zero values, such as zero voltage. Obviously, a certain tolerance may be utilized to start and stop the time measurement, such as the tolerance for entering the second state X2.

[0072] It has been found that when an oscillator is used to determine the transition time value x, the frequency of the oscillator may have a temperature dependency. In order to accurately determine the transition time value, e.g. in milliseconds, the frequency needs to be stable. However, it has been found that in the case of a transition from a first high voltage V1 to a second lower voltage V2 of the piezoelectric actuator 101, the determined count value n correlates well with the capacitance of the piezoelectric actuator 101 and thus with the optical power OP of the piezoelectric actuator system 100. Thus, the transition time value x (or the first and / or second transition time values ​​x1, x2) may be obtained based on the transition from the first reached first and / or third steady state voltages V1, V3 to the second and / or fourth lower steady state voltages V2, V4.

[0073] The relationship between the voltage V and the optical power OP varies depending on the type of the piezoelectric actuator system 100. Therefore, a calibration of the model OP(V, T, n) is necessary. The calibration procedure includes determining a calibrated transition time count value nCAL. The determination of the calibrated transition time count value nCAL follows the procedure for determining the transition time count value n described above.

[0074] The calibration procedure further includes obtaining a calibration temperature TCAL of the piezoelectric actuator. The calibration temperature, as well as other temperatures of the piezoelectric actuator, can be obtained from a temperature sensor configured in the piezoelectric actuator system 100 or a temperature sensor configured in the camera module 150 or electronics 159.

[0075] The calibration further includes determining one or more optical powers, such as first and second optical powers OP1, OP2, based on one or more corresponding calibration voltages, such as first and second calibration voltages V1, V2, applied to the piezoelectric actuator 101.

[0076] The optical powers OP1, OP2 can be determined in various ways. For example, a first optical power OP1 is determined from a user-determined or predetermined distance between the camera module 150 and a screen placed in front of the camera module, and the first voltage V1 is a voltage that provides a focused image of the screen. The second voltage V2 and the optical power OP can be similarly determined based on another distance to the screen. Alternatively, the second optical power OP2 can be an optical power that focuses at infinity, and the second voltage can be a voltage that focuses at a relatively large distance, such as infinity, for example 10 m.

[0077] Calibration may be performed at the time of manufacture of camera module 150 or electronics 159. Alternatively, calibration may be performed or repeated by a user of the camera module or electronics, for example based on an installed calibration algorithm.

[0078] Calibration can also be based on statistical techniques using estimated distances to various objects.

[0079] Function values ​​of a curve-fit polynomial L, which is a function, such as a nonlinear function, of voltage V, are determined for one or more calibration voltages. For example, first and second function values ​​L(V1) and L(V2) may be determined from first and second calibration voltages V1, V2.

[0080] In one example, the curve fitting polynomial L(V) is L(V)=v6xV^6+v5xV^5+v4xV^4+v3xV^3+v2xV^2+v1xV+v0, It is expressed as a 6th degree curve fitting polynomial of the form, where v0 through v6 are coefficients empirically determined for a given type of piezoelectric actuator system 100 or piezoelectric actuator 101. L(V) gives the characteristic shape of the OP(V) curve of FIG. 2A that is independent of temperature, creep, and type of lens used in piezoelectric actuator system 100. The curve-fit polynomial L(V) may be expressed as a polynomial of other orders, and is typically expressed as a polynomial of at least the third order or higher.

[0081] The calibration further includes determining one or more calibration parameters aCAL, bCAL by using a mathematical model OP(V,T,n), one or more calibration voltages V1, V2, and one or more optical powers OP1, OP2 associated with the calibration voltages V1, V2. Thus, the calibration parameters are determined based on the model, with the calibration voltages and the measured optical response as known inputs to the mathematical model. The calibration voltages V1, V2 may be determined as voltages that result in a desired or predetermined optical response, e.g., optical power. Alternatively, the optical response when a predetermined voltage is applied may be determined.

[0082] The calibration parameters aCAL and bCAL can be determined from the model OP(V,T,n) where a(T) and b(T,n) are independent of the voltage V, by substituting OP1, OP2 and the first and second function values ​​L(V1) and L(V2), respectively, as two equations OP1(L(V1),T,n) and OP2(L(V2),T,n). The following two equations aCAL = (OP1-OP2) / (L(V1)-L(V2)) and bCAL = OP1(V1) - aCAL x (L(V1)) Based on this, aCAL and bCAL can be determined as follows:

[0083] The calibration parameters aCAL, bCAL correspond to the behavior of the piezoelectric actuator system 100 at the time of calibration, i.e., the actual temperature and creep level. With this information, a calibration curve OP(V) can be constructed for decreasing or increasing voltage.

[0084] The determined calibration parameters aCAL, bCAL, nCAL and TCAL are stored in the memory of the processing unit 111, for example.

[0085] The model OP(V,T,n) can be used to determine the optical power OP for a given voltage V, a given temperature T and a given creep level. To do so, it is necessary to determine the model parts a(T) and b(T,n) for real conditions, i.e., for real temperature T and real transition time count value n. The temperature T can be measured by a temperature sensor as described above, and the transition time count value n can be determined as described above, for example using the method described in connection with FIG. 3B.

[0086] The model parts a(T) and b(T,n) can be expressed by the following polynomials. a(T)=a2x(T^2-TCAL^2)+a1x(T-TCAL)+aCAL b(T,n)=b2x(T^2-TCAL^2)+b1x(T-TCAL)+bCAL+(n-nCAL)xΔb / Δn.

[0087] Obviously, other representations of a(T) and b(T,n), such as higher degree polynomials, are also valid.

[0088] The element Δb / Δn, and the coefficients a1, a2, b1, and b2 are empirically determined values.

[0089] By iteratively adapting the mathematical model and the model to the actual conditions, a set-point voltage Vx can be determined to achieve a desired optical power OPx, as shown in Figure 2A. The set-point voltage Vx may be determined based on an iterative algorithm that uses a model OP(V,T,n) to determine a voltage V that gives, or at least approximates, the desired optical power OPx.

[0090] Since the desired optical power OPx can be achieved without measuring the actual optical power except during the calibration procedure, the desired optical power OPx can be determined in an open loop configuration that is not dependent on measuring the actual optical power OP.

[0091] In the above examples, the model OP(V,T,n) has been described with a(T) depending on temperature T and the polynomial L(V) depending on voltage V, however in other examples the coefficient a(T) could further depend on the transition time count value n, i.e. the coefficient could take the form a(T,n). Similarly, the polynomial could further depend on the transition time count value n and / or the temperature T, i.e. the polynomial could have the form L(V,T,n).

[0092] For example, a polynomial is L(V,T)=v6(T)xV^6+v5(T)xV^5+v4xV^4+v3xV^3+v2xV^2+v1xV+v0+vT*T^2 It can be in the form Or, the form L(V,T,n) further includes a term that depends on n.

[0093] In another example, the polynomial L(V) is defined as two or more polynomials L1(V), L2(V) for different temperature ranges. By defining different polynomials L(V) for different temperature ranges or defining the polynomial as dependent on the temperature T and / or the transition time count value n, the accuracy of the determined optical power OP for a given voltage V can be improved.

[0094] Since the piezoelectric actuator system 100 is not limited to working with the lens 120, but generally with the optical member 130, it is understood that the optical model OP(V,T,n) applies to other optical responses, such as beam deflection. Therefore, an equivalent model OR(V,T,n) may be formulated for other optical responses. For convenience, the optical model OP(V,T,n) used herein applies to any optical response.

[0095] Although the present invention has been described with reference to specific embodiments, it should not be construed as being limited to the examples presented. The scope of the present invention is to be interpreted in the light of the attached set of claims. In the context of the claims, the term "comprising" or "comprises" does not exclude other possible elements or steps. Furthermore, the terms "a" and "an" should not be interpreted as excluding a plurality. Furthermore, the use of reference signs in the claims for elements shown in the figures should not be interpreted as limiting the scope of the present invention. Furthermore, individual features recited in different claims may be advantageously combined, and the recitation of these features in different claims does not exclude that a combination of features is not possible or advantageous.

Claims

1. 1. A method for determining a set point voltage (Vx) for a piezoelectric actuator system (100) to achieve a desired optical response of the piezoelectric actuator system, the piezoelectric actuator system including an optical element (130, 120) having an adjustable optical response (OP) and a piezoelectric actuator (101), the method comprising: providing a mathematical model OP(V,T,n) describing the relationship between the optical response (OP) and voltage (V), temperature (T), and transition time count values ​​(n) applied to the piezoelectric actuator as variables, said mathematical model comprising a curve-fitted polynomial (L), said transition time count values ​​(n) relating to at least one transition time value (x) of the piezoelectric actuator for changing from a first state (X1) to a second state (X2); calibrating a mathematical model, - determining a calibrated transition time count value (nCAL) based on determining the at least one transition time value (x) between the first and second states (X1, X2) of the piezoelectric actuator; - obtaining a calibration temperature (TCAL) of said piezoelectric actuator; calibrating a mathematical model by determining one or more calibration parameters (aCAL, bCAL) of said mathematical model OP(V,T,n) based on one or more calibration voltages (V1,V2) and one or more optical responses (OP1,OP2) associated with said one or more calibration voltages (V1,V2) applied to a piezoelectric actuator; - after a period of time or use of the piezoelectric actuator, - obtaining the temperature (T) of said piezoelectric actuator; - determining the transition time count value (n) based on determining at least one actual transition time value (x) between the first and second states of the piezoelectric actuator; - determining a set point voltage (Vx) based on an updated mathematical model OP(V,T,n) and applying said set point voltage (Vx) to said piezoelectric actuator; updating the mathematical model OP(V, T, n) by A method comprising:

2. the relationship describes the optical response to decreasing and / or increasing values ​​of voltage (V), The method of claim 1.

3. the relationship describes only the optical response to decreasing or increasing values ​​of voltage (V); The method of claim 2.

4. determining the transition time count value (n) and the calibration transition time count value (nCAL), determining a first transition time value (x1) between the first and second states (X1, X2) of the piezoelectric actuator; determining a second transition time value (x2) between the third and fourth states (X3, X4); the transition time count value (n) and the calibration transition time count value (nCAL) are determined based on a ratio of the first and second transition time values ​​(x1, x2); The method according to any one of claims 1 to 3.

5. the first and second states (X1, X2) include first and second steady-state voltages (V1, V2) obtained based on applying first and second voltages (V1, V2) to the piezoelectric actuator; The method according to any one of claims 1 to 4.

6. the first steady-state voltage (V1) is greater than the second steady-state voltage (V2), and the transition time value (x) is obtained based on a transition from the first steady-state voltage (V1) to the second steady-state voltage (V2); The method according to claim 5.

7. the curve-fit polynomial (L) being a predetermined non-linear function of the voltage (V); The method according to any one of claims 1 to 6.

8. the mathematical model is composed of a first term a(T)×L(V) plus a second term b(T,n), where a(T) is a polynomial depending on at least a temperature parameter (T), a first calibration parameter (aCAL) and an actual temperature (Ta), L(V) is a polynomial of at least the third order depending on a voltage (V), and b(T,n) is a polynomial depending on the temperature parameter (T), the voltage (V), the second calibration parameter (bCAL), the actual temperature (Ta) and the actual capacitance parameter (Ca); The method according to any one of claims 1 to 7.

9. The mathematical model further includes a term OPtherm(T) describing the optical response variation due to temperature dependent components in the optical lens system (151) as a function of temperature (T); the optical lens system is optically connected to the piezoelectric actuator system (100); The method according to any one of claims 1 to 8.

10. applying the voltage to the piezoelectric actuator includes first applying a maximum or minimum voltage (Vmax, Vmin) to the piezoelectric actuator to drive the piezoelectric actuator to a maximum or minimum deformation, and applying the set point voltage (Vx) to the piezoelectric actuator after the maximum or minimum deformation is reached. The method according to any one of claims 1 to 9.

11. The set point voltage (Vx) is determined in an open loop configuration that does not rely on measuring the actual optical response (OP). The method according to any one of claims 1 to 10.

12. A piezoelectric actuator (101), a piezoelectric driver (110) that supplies power to the piezoelectric actuator (101), a processing unit (111) that determines a set point voltage (Vx) that the piezoelectric driver applies to the piezoelectric actuator according to the method of claim 1, and an optical element (130, 120) having a tunable optical response. A piezoelectric actuator system (100) comprising:

13. The optical member (130, 120) - a first transparent cover member (121), - a second cover member (122), a transparent, deformable, non-fluid body (123) sandwiched between said first transparent cover member (121) and said second cover member (122); one or more piezoelectric elements of the piezoelectric actuator are arranged to enable controllable changes in curvature, displacement and / or orientation of the first or second cover members; The piezoelectric actuator system (100) of claim 12.

14. The piezoelectric actuator system (100) according to any one of claims 12 to 13, comprising an optical lens (120), the camera module further comprising an image sensor (152) positioned to receive light transmitted through the optical lens (120); A camera module (150) including a piezoelectric actuator system.

15. An electronic device (159) comprising a piezoelectric actuator system (100) according to any one of claims 12 to 13.

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