Temperature compensation for liquid lens

Abstract

Temperature compensation for a liquid lens. An optical assembly comprising a variable focal length lens assembly, the variable focal length lens assembly comprising a variable focal length lens and an actuation unit, an energy absorption rate of the variable focal length lens assembly absorbing energy depending on an applied control signal. The optical assembly comprises a control unit configured to control a focal length setting of the variable focal length lens by providing a corresponding control signal, and to apply a default control signal to provide a default focal length and a default energy absorption rate. The control unit provides a thermal stabilization function defining by applying a varying control signal related to a varying focal length and applying a compensating control signal related to a compensating focal length, wherein the varying control signal or the compensating control signal is provided such that the corresponding related energy absorption rate is greater than the default energy absorption rate, and the other one of the varying control signal or the compensating control signal is provided such that the corresponding related energy absorption rate is less than the default energy absorption rate.

Description

Technical Field

[0001] This invention generally relates to the specific design of optical devices that allow for rapid adjustment of probe properties in relation to desired measurement conditions and provide stable measurement conditions, particularly for surveying or mapping equipment such as coordinate measuring machines (CMMs) or total stations. Background Technology

[0002] Inspecting post-production workpieces on coordinate measuring machines (CMMs) is a common practice to verify the accuracy of predefined object parameters, such as the object's dimensions and shape. Furthermore, in many industrial applications, there is a focus on inspecting the surfaces of unknown objects. This measurement can typically be provided using a CMM or any other suitable type of scanning equipment.

[0003] In a conventional 3D coordinate measuring machine (CMM), the probe head is supported to move along three mutually perpendicular axes (in the X, Y, and Z directions). This allows the probe head to be guided to any point within the CMM's measurement volume and to measure objects using a measurement sensor (probe or probe unit) carried by the probe head. Such a probe unit can be designed as a tactile probe or optical sensor, for example, based on the principles of triangulation or interferometry, to provide surface measurements.

[0004] In a simple form of the machine, suitable transducers or linear encoders mounted parallel to the respective axes determine the position of the probe head relative to the machine base, and thus determine the coordinates of the measurement point on the object illuminated and / or imaged by the sensor. To provide the mobility of the probe head, a typical coordinate measuring machine may include: a frame structure on which the probe head is disposed; and a drive mechanism for moving the frame components of the frame structure relative to each other.

[0005] The advantage of using (2D) optical sensors is that they can avoid contact with the part being measured, thus reducing the risk of potential deformation due to contact compared to tactile probes.

[0006] However, the type of optical probe used to measure a particular workpiece must be selected in such a way that the measurement properties of the probe are suitable for the shape or topography of the workpiece. For example, to measure a borehole, the probe may preferably include a prism or mirror to emit a measurement beam at a predetermined angle (e.g., 90°) relative to the extension axis of the probe stylus. According to another example, the optics may have to vary depending on the desired measurement resolution or magnification.

[0007] To provide various suitable properties, it is often necessary to frequently change the probes attached to the CMM probe head. Furthermore, multiple specified probes for corresponding measurement requirements can be used to ensure accurate measurement of different workpieces.

[0008] Therefore, coordinate measuring machines and corresponding probes typically include interfaces that allow for mechanical and optical connections (and especially electrical connections) between the two components. Through such interfaces, the probe can be attached to the probe head relatively quickly and effortlessly, and is carried and positioned by the probe head for measurement purposes. For example, such an optical-mechanical interface is known according to EP 2356401B1.

[0009] According to this method, the entire probe must be replaced when deviation measurement is required. Due to typical measurement conditions (e.g., in industrial processes), the frequency of negative mechanical effects at the probe and CMM is quite high. Therefore, the overall service life of such probes and interfaces is very limited. Summary of the Invention

[0010] Therefore, the object of the present invention is to provide an improved detection system that allows measurement conditions to be quickly and easily adjusted to the required measurement requirements.

[0011] Another objective of this invention is to provide flexible CMM and probe designs for different workpieces to be measured.

[0012] Another object of the present invention is to provide an improved adjustable measurement sensor that provides accurate and reliable measurement conditions.

[0013] The idea behind this invention is based on using a variable focal length lens (tunable lens) whose focal length varies according to an applied control signal. The force applied to the lens can be controlled by the control signal. A specific force provides a corresponding deformation of the lens, which leads to a change in its optical properties and thus a change in its focal length.

[0014] This force can be generated by means of an actuator (e.g., a voice coil or piezoelectric) that applies a defined pressure to the lens, or by means of one or more electrodes that provide a variable current and / or voltage. The lens may include an external membrane that encloses one or more fluids. The fluid can provide variable optical and chemical properties (e.g., density) depending on the applied current. For example, a force can be applied to the lens by pressing the membrane or by applying an electric current to the fluid.

[0015] Figure 2a and Figure 2b An example of a variable focal length lens capable of modifying its focal length is shown. The implementation of this variable focal length lens is described in more detail below.

[0016] A variable focal length lens can be implemented as a deformable lens. Such a lens can consist of a container filled with an optical fluid and sealed with an elastic polymer membrane. The deflection of the lens is proportional to the pressure in the fluid. The lens may include an electromagnetic actuator (e.g., a voice coil) for applying pressure to the container. Therefore, the focal length of the lens is controlled by the current flowing through the coil of the actuator.

[0017] The optical power of a variable focal length lens can therefore be increased with increasing current. The zero-current starting point is set during production and can vary depending on the lens. The slope of the optical power can be affected by the mechanical properties of the film, which can also be varied to provide different properties (e.g., range). The focal length also depends on temperature.

[0018] This method preferably involves the use of a variable focal length lens as mentioned in coordinate measuring equipment (such as a coordinate measuring machine (CMM)).

[0019] This method also involves a multi-sensor CMM that incorporates an imaging system (e.g., a Hexagon Optiv machine). The imaging system typically includes an objective lens and a camera. The CMM allows for accurate positioning of the imaging system in the x, y, and z directions. The objective lens is preferably a telecentric lens, such that the magnification is independent of the distance from the object to the objective lens. The CMM (typically its z-axis) is used to position the telecentric lens at a certain distance relative to the object being focused on in the image. This distance is also known as the working distance. It is a fixed distance and depends on the design of the lens optics (typically around 100 mm).

[0020] Tunable lenses (variable focal length lenses) allow the working distance to become variable, for example, between 80mm and 100mm. Depending on the setting of the tunable lens, an object (or a portion thereof) can be focused at different distances. Because the lens can change its focal length very quickly (<10ms), this measurement process can be faster and more efficient compared to the conventional method of using mechanical CMM translation (typically along the z-axis) to set a new focal plane.

[0021] To accurately measure the x, y, z coordinates of an object feature (e.g., the object's surface, its edge, or a borehole), the feature can first be focused. Then, the coordinates of the imaging system (moved by the CMM) and its working distance can be combined. When using a tunable lens, focus settings should be considered.

[0022] Therefore, the working distance can be calibrated based on the lens tuning signal (e.g., voltage or current). The correlation can be a linear function or a higher-order polynomial, or a lookup table interpolating values ​​between values ​​in a table. Calibration artifacts can be utilized or the CMM itself can be used (because it provides a clearly defined displacement of the sensor (lens) relative to the object) to determine the calibration.

[0023] The correlation between the working distance and the lens tuning (control) signal is typically (very) temperature sensitive. The temperature of a tunable lens is affected by ambient temperature and also by the lens's own energy absorption or power consumption. Actual power consumption depends primarily on the focal point setting of the tunable lens. Therefore, lens calibration preferably takes temperature conditions into account (each lens should be calibrated individually for different temperatures). Thus, the lens temperature must be known, preferably measured inside or very close to the lens to reduce hysteresis. Since the lens tuning signal, in turn, affects the temperature, compensation is preferably provided dynamically, forming a closed-loop control system.

[0024] When the control signal (lens tuning signal) changes, the lens typically changes its focus very rapidly (an instantaneous response to force, usually within milliseconds). Using the tuning signal (especially if it is an electric current), the actual power consumption and therefore the lens temperature change. This results in subsequent focus drift (focus adjustment time, typically several seconds to several minutes). Because the temperature sensor is usually physically located away from the optical lens medium, temperature measurements may only be accurate after a certain period of time (temperature sensing lag, typically several seconds to several minutes).

[0025] These two time constants are usually different, which reduces the overall accuracy of the system even when temperature is taken into account to compensate for the temperature effect of the lens. Figure 1 The diagram illustrates the drift in working distance after the tuning signal is changed to achieve different focal lengths. Although the measured temperature (temperature-dependent calibration of the focus) is taken into account, the actual working distance drifts until the system reaches a new thermal equilibrium (setup time ~2 minutes). During this period, accurate measurements are not possible.

[0026] The present invention relates to a method for overcoming the settling effect as described above.

[0027] To overcome this adjustment effect after a change in focus tuning, the lens can operate continuously with a constant average energy or power consumption. When the lens focus changes (e.g., for measurement), this can be done quickly and the power consumption can be compensated for subsequently (especially immediately afterward).

[0028] This invention relates to an optical assembly including a variable focal length lens assembly. The variable focal length lens assembly includes a variable focal length lens capable of modifying its focal length and an actuation unit configured and arranged relative to the variable focal length lens, such that the actuation unit provides setting and changing of the focal length according to a control signal. The energy absorption rate of the variable focal length lens assembly depends on the applied control signal.

[0029] In the context of this invention, energy absorption is specifically understood as any form and type of energy transferred to or absorbed by the variable focal length lens or actuation unit, such as thermal energy generated by applying current to a circuit. Such energy transfer to the variable focal length lens assembly typically results in a temperature change or provides a specific temperature level.

[0030] Energy absorption within a limited time period or duration can also be understood as the corresponding power consumption of a specific component of a variable focal length lens assembly.

[0031] Furthermore, the amount of energy absorbed per unit time, or the power consumption, depends on the corresponding energy emission rate or energy absorption rate. This energy absorption rate can be influenced, for example, by the temperature difference between components or by a corresponding control signal applied to one of the components (i.e., by the applied current, voltage, or frequency).

[0032] The optical components also include a control unit configured to control the focal length setting of the variable focal length lens by providing appropriate control signals, and configured to apply a default control signal to provide a default focal length and thereby provide a default energy absorption rate (steady-state condition).

[0033] Specifically, a default control signal is set or selected such that the focal length of the tunable lens provided is between the maximum and minimum focal lengths applicable to the tunable lens. In particular, the default focal length is essentially in the middle of the focal length range provided by the tunable lens.

[0034] According to the present invention, the control unit includes a thermal stabilization function for compensating for energy absorption by varying the energy absorption rate, which differs from the default energy absorption rate.

[0035] Because a different energy absorption rate (different from the default rate) is applied, the thermal energy that the variable focal length lens assembly can absorb changes accordingly, and the thermal state (temperature) of at least one component of the variable focal length lens assembly is changing. As summarized above, this temperature change can lead to a further change in focal length due to thermal drift.

[0036] The thermal stabilization function is defined by applying a variation control signal related to the change focal length and a compensation control signal related to the compensation focal length, wherein the variation control signal or the compensation control signal is provided such that the corresponding energy absorption rate is greater than the default energy absorption rate, and the other of the variation control signal or the compensation control signal is provided such that the corresponding energy absorption rate is less than the default energy absorption rate.

[0037] Specifically, the variation control signal is related to the focal length, through which measurements should be performed using the optical components. This focal length can be set by applying the variation control signal, and measurements can be performed simultaneously with the application of the variation control signal. Due to different energy absorption rates (different from the default energy absorption rate), the thermal state of the optical components will change. To compensate for this change in thermal state, a compensation control signal is applied to counteract the thermal change.

[0038] Specifically, the thermal stabilization function is defined by: obtaining or determining the amount of energy absorbed by applying a control signal that provides a varying energy absorption rate, and deriving the difference between the absorbed energy and the energy to be absorbed when a default energy absorption rate is applied during the corresponding (variable) time period. Based on this difference, a compensation energy absorption is derived, which is defined by a compensation control signal and a compensation duration for applying the compensation control signal. The compensation control signal and the compensation duration are provided and applied to the lens assembly such that the resulting compensation energy absorption offsets the aforementioned difference.

[0039] In other words, by applying a variable energy absorption rate, a variable energy absorption deviation is provided relative to the variable default energy absorption (which occurs when the default control signal is applied for the same duration as the variable control signal). By providing compensating energy absorption, the total energy absorption deviation becomes smaller than the variable energy absorption deviation. The total energy absorption deviation corresponds to the difference between the energy absorbed when the variable energy absorption rate is applied during the variable time period and the compensating energy absorption rate is applied during the compensating duration, and the energy absorbed when the default energy absorption rate is applied for the sum of the variable time period and the compensating duration.

[0040] In other words, the thermal stabilization function is defined by obtaining at least one variable energy parameter that provides a determination of the variable energy absorption. The variable energy parameter may include at least one of a change control signal, a change energy absorption rate, and a change duration (e.g., for applying the change control signal).

[0041] Based on the variable energy absorption, the variable energy absorption deviation relative to the default energy absorption is obtained or derived (e.g., calculated). The variable energy absorption deviation can be defined by the energy absorbed due to the variable energy absorption and the energy to be absorbed when the default energy absorption rate is applied within the corresponding time period (corresponding to the variable energy absorption).

[0042] The compensation control signal and compensation duration are determined, and these provide the compensation energy absorption. The compensation energy absorption provides a deviation from the default energy absorption. Similarly, the compensation energy absorption deviation can be defined by the energy absorbed due to the compensation energy absorption and the energy absorbed when the default energy absorption rate is applied within the corresponding time period (corresponding to the compensation energy absorption).

[0043] Determine the compensation control signal and compensation duration so that the total energy absorption deviation provided by the variable energy absorption and the compensation energy absorption is less than the variable energy absorption deviation.

[0044] Specifically, the thermal stabilization function can be configured to apply a control signal to the variable focal length lens assembly during the compensation duration.

[0045] This means that by applying a compensation signal (after providing the variable energy absorption), it is possible to compensate for different energy consumptions (different from the corresponding default energy consumption) that have exceeded or fallen below the corresponding default energy consumption.

[0046] Applying a compensation control signal during the compensation duration provides the compensation energy absorption rate, allowing the average power consumption to be substantially maintained. The average power consumption can be limited by continuously applying a default control signal.

[0047] Specifically, the variable energy absorption deviation corresponds to the difference between the energy absorbed when the default energy absorption rate is provided and the energy absorbed when the variable energy absorption rate is provided over the same duration.

[0048] Specifically, the compensation energy absorption deviation corresponds to the difference between the energy absorbed when the default energy absorption rate is provided and the energy absorbed when the compensation energy absorption rate (which is different from the variable energy absorption rate) is provided over the same duration.

[0049] Specifically, the total energy absorption deviation corresponds to the difference between the energy absorbed when the default energy absorption rate is provided and the energy absorbed when the compensated energy absorption rate and the variable energy absorption rate are provided, where both energy absorptions are provided over the same duration.

[0050] The total energy absorption of the lens can be defined by the sum of the first (variable) energy absorption and the second (compensated) energy absorption, and the total energy absorption deviation can be the difference between the total energy absorption and the default total energy absorption, which can be defined by the default energy absorption rate and the sum of the first duration and the second duration.

[0051] In one implementation, a compensation control signal is applied continuously after the variation control signal is applied. Specifically, the compensation control signal is applied immediately after the variation control signal is applied. In another implementation, a superposition of the compensation control signal and the variation control signal is provided, i.e., these signals are provided at least partially during the same time period, for example, in the case of providing a high-frequency current (see below).

[0052] According to the implementation method, the total energy absorption deviation may correspond to the sum of the variable energy absorption deviation and the compensated energy absorption deviation. Alternatively or additionally, the compensated energy absorption deviation may correspond to the difference between the energy absorbed when the compensated control signal is applied during the compensation duration and the energy absorbed when the default control signal is applied during the compensation duration.

[0053] In one implementation, variable energy absorption can limit variable power consumption, and compensation control signals and compensation durations can provide compensation power consumption, and can provide compensation control signals and compensation durations such that the deviation between variable power consumption and variable default power consumption is greater than the deviation between total power consumption and corresponding total default power consumption.

[0054] Specifically, the energy absorbed by the variable focal length lens assembly can provide for the power consumption of the variable focal length lens and / or the actuation unit and / or the defined temperature or temperature change of the variable focal length lens and / or the actuation unit. More specifically, the various (electronic) signals applied to the variable focal length lens assembly can result in signal-related power consumption (depending on the corresponding time period of the applied signal), i.e., changes in the energy (and temperature / thermal) state of the assembly.

[0055] In one implementation, the compensation control signal may include a first compensation control signal and a second compensation control signal, wherein the first compensation control signal is different from the second compensation control signal. Therefore, corresponding thermal compensation can be provided in the steps, for example, based on previous measurement steps with different focal lengths.

[0056] Specifically, the default control signal and / or compensation control signal include a high-frequency current. The default control signal may include a first default control signal and a second default control signal, at least one of which can provide the high-frequency current. When applying a current with a frequency f... HF (frequency f) HF At currents higher than the frequency that a variable focal length lens can mechanically follow, this current will cause power consumption (and heat generation) without requiring the lens to mechanically follow the signal. If the power consumption P is selected accordingly... HF This can then be used to keep the total power consumption constant: P1 + P HF1 =P2+P HF2 = constant.

[0057] P1+P HF1 This power consumption could be due to the application of a default control signal to apply a default focal length. The default control signal includes a first default signal (I1) and a second default signal (I...). HF P2 can be the power consumption caused by the application of a changing control signal, and P HF2 The power consumption could be due to the application of a compensation control signal provided as a second high-frequency current.

[0058] Alternatively, P1 can be the power consumption caused by applying a different focal length (different from the default focal length) by applying a first change control signal, and P HF1 The power consumption could be due to the application of a second change control signal as the first high-frequency current. P2 could be due to the application of a first compensation control signal, and P... HF2 The power consumption could be due to the application of a second compensation control signal, which is a second high-frequency current.

[0059] To further minimize the impact of residual high-frequency modulation, the exposure time T is selected. exp It is beneficial to ensure that the average high-frequency current is zero during the exposure duration. Therefore, the exposure time or HF frequency should be selected such that the exposure time corresponds to an integer number of HF cycles, i.e., T. exp =N / f HF N is an integer.

[0060] In an implementation, the variable energy absorption may exceed the variable default energy absorption and the compensation energy absorption may be lower than the compensation default energy absorption, or the variable energy absorption may be lower than the variable default energy absorption and the compensation energy absorption may exceed the compensation default energy absorption.

[0061] According to one implementation, the absolute value of the compensation for energy absorption deviation can be less than or equal to the absolute value of the variation in energy absorption deviation. This means that the deviation from the average power consumption can be reduced or even completely compensated.

[0062] Specifically, the absolute value of the compensated energy absorption deviation can be greater than the absolute value of the varied energy absorption deviation, where the difference between varied energy absorption and varied default energy absorption includes a different algebraic sign compared to the difference between total energy absorption and total default energy absorption. In this case, the deviation in average power consumption can be "over-compensated" or "excessively compensated."

[0063] According to one embodiment, the actuation unit may include a voice coil or an electrode connected to a variable focal length lens.

[0064] A voice coil actuator (also known as a non-commutated DC linear actuator) is a type of direct-drive linear motor. This type of actuator consists of a permanent magnetic field assembly (e.g., a combination of permanent magnets and iron steel) and a coil assembly. The current flowing through the coil assembly interacts with the permanent magnetic field and generates a force vector perpendicular to the direction of the current. The force vector can be reversed by changing the polarity of the current flowing through the coil. Non-commutated DC linear actuators can move in both directions, have a relatively constant force throughout the stroke, and can be used for open-loop or closed-loop position or force applications.

[0065] Two types of voice coils are typically available. Moving coil actuators generally consist of a coil wound around a spool, which can be made of a variety of non-magnetic materials, and a permanent magnetic field assembly that moves in and out of the spool. This permanent magnetic field assembly consists of a steel housing and a concentric permanent magnet assembly at its center. Moving magnet actuators provide a stationary coil and a moving magnet assembly. This construction prevents the coil leads from needing to move during operation. Encapsulated and operating similarly to the moving coil design (except for the exposed coil replacing the moving magnet assembly), moving magnet devices have a piston that moves within a cylindrical coil tube to accommodate the permanent magnetic field assembly.

[0066] In cases where the lens comprises a liquid whose chemical or optical properties change according to an applied current, electrodes for an actuation unit can be provided. The focal length can then be changed by applying a corresponding current (directly) to the liquid in the lens.

[0067] In one embodiment, at least one variable energy parameter (e.g., absorption rate and / or variation duration) includes or provides at least one variation control signal that provides the variable energy absorption rate and / or applies the variation control signal and / or the variation duration of the variable energy absorption rate.

[0068] Each of the parameters mentioned above can provide a determination of the varying energy absorption. For example, if the time period for which the varying signal is applied is known or remains constant (e.g., to keep the thermal effect moderate), information about the current applied during that time period will be sufficient to calculate the varying energy absorption. The reverse is also true.

[0069] In one implementation, the thermal stability function can be defined by the following steps:

[0070] • Obtain a control signal that provides a variable energy absorption rate.

[0071] • Determine the duration of the applied change control signal, wherein the change energy absorption is defined by the change energy absorption rate and the change duration, and wherein the change default energy absorption is defined by the default energy absorption rate and the change duration, and

[0072] Specifically, after applying the change control signal, a compensation control signal is applied during the compensation duration, thereby providing a compensation energy absorption rate, wherein the compensation energy absorption is defined by the compensation energy absorption rate and the compensation duration, and wherein the compensation default energy absorption is defined by the default energy absorption rate and the compensation duration.

[0073] In one implementation, applying a variation control signal during the variation duration and a compensation control signal during the compensation duration can correspond to the total energy absorption of the variable focal length lens assembly. Additionally or alternatively, the variation energy absorption deviation can correspond to the difference between the energy absorbed when the variation control signal is applied during the variation duration and the energy absorbed when the default control signal is applied during the variation duration.

[0074] In addition to the compensation control signal, the change control signal may include a first change control signal and a second change control signal, wherein the first change control signal is different from the second change control signal.

[0075] According to one embodiment, a compensation control signal can be defined such that the sum of the squared current values ​​obtained by applying a default control signal (which may include a first default signal and a second default signal) corresponds to the sum of the squared current values ​​obtained by applying a change control signal and a compensation control signal.

[0076] Referring to the method described above, a second coil that works in conjunction with the voice coil can be implemented in a tunable lens, for example, by winding an additional wire together with the wire of the first coil. The two coils can be driven with different currents I1 (e.g., according to a first default signal or according to a variation control signal) and I2 (e.g., according to a second default signal or according to a compensation control signal). Since the magnetic field originates from the sum and direction of the currents in the two coils, the focus can be set by the sum of the currents, including their polarities. For example, if I1 = -I2, then the total current I... position The current is zero, and no magnetic field is generated for focusing. On the other hand, power consumption is independent of polarity, so the cases I1 = -I2 and I1 = I2 are the same. In the second case, the total current I... position It is positive and generates a magnetic field.

[0077] If the sum of the square currents I thermal 2 =I1 2 +I2 2 If it is a constant, then the average power consumption will remain constant. By following and I position =I1+I2 generates the desired magnetic field by selecting I1 (variation control signal) and I2 (compensation control signal), making focus setting possible.

[0078] To further improve thermal stability, changes in internal or coil resistance caused by factors such as temperature can be compensated for. For this purpose, the voltage across the coil can be measured, and the current can be adjusted so that not only the current but also the power loss remains constant, P1+P2=|U1I1|+|U2I2|=constant.

[0079] In one implementation, the thermal stabilization function can provide the application of a compensation control signal such that the sum of the thermal energy absorbed by the variable focal length lens assembly by applying the variation control signal and the compensation control signal substantially corresponds to the energy absorbed by applying the default control signal within the sum of the variation duration and the compensation duration.

[0080] One aspect of the invention relates to a measuring device for measuring distance to an object, the measuring device comprising at least one sensor and at least one optical element configured and arranged to provide desired light propagation. The measuring device includes an optical assembly according to any of the embodiments described above, and the at least one optical element includes the variable focal length lens assembly.

[0081] The measuring device can be implemented as a coordinate measuring machine or a geodetic surveying instrument (e.g., a total station). Optical elements can be configured and arranged to influence and shape the laser beam emitted by the laser source of the device, or can be configured and arranged to influence and / or guide the light received by the light receiving unit of the device.

[0082] The present invention also relates to a method for thermally stabilizing a variable focal length lens assembly, particularly a computer-implemented method. The variable focal length lens assembly includes a variable focal length lens capable of modifying its focal length and an actuation unit configured and arranged relative to the variable focal length lens, such that the actuation unit provides setting and changing of the focal length according to a control signal, wherein the energy absorption rate of the variable focal length lens assembly depends on the applied control signal.

[0083] The method includes: applying a default control signal to the actuation unit to provide a default focal length and a default energy absorption rate; and applying a variable control signal to provide a variable focal length, the variable control signal providing a variable energy absorption rate different from the default energy absorption rate and thereby providing a variable energy absorption deviation.

[0084] The method further includes: applying a compensation control signal to provide a compensation energy absorption rate, wherein the compensation control signal is provided such that the total energy absorption deviation obtained by applying the variation control signal and the compensation control signal is less than the variation energy absorption deviation.

[0085] The present invention also relates to a computer program product comprising program code stored on a machine-readable medium, or implemented by electromagnetic waves including program code segments, and having computer-executable instructions for performing the above methods, particularly when executed on a control unit of an optical component according to any of the above embodiments. Attached Figure Description

[0086] The method and apparatus according to the invention will be described or explained in more detail below by way of example only, with reference to the schematic examples shown in the accompanying drawings. Specifically,

[0087] Figure 1 The diagram shows the change in working distance based on the application of corresponding control signals;

[0088] Figures 2a to 2b An embodiment of the variable focal length lens assembly according to the present invention is shown;

[0089] Figures 3a to 3d Different operating modes of the variable focal length lens assembly according to the present invention are shown; and

[0090] Figure 4 The workflow of the method according to the present invention is shown. Detailed Implementation

[0091] Figure 2a and Figure 2b A cross-sectional view of an embodiment of the variable focal length lens assembly 10 (tunable lens) according to the present invention is shown.

[0092] The actuation unit 11 is mounted on the variable focal length lens 15. The actuation unit 11 includes an actuation member 12, which is capable of applying force or pressure to the lens in the direction indicated by the arrow. Figure 2a The variable focal length lens assembly 10 is shown in a specific first focusing position.

[0093] The actuating member 12 is provided by a voice coil actuator and can move according to a defined current applied to the voice coil. That is, the force applied to the lens can be decreased or increased.

[0094] Figure 2b A variable focal length lens assembly 10 in an increased actuation state is shown. The provided optical power is increased accordingly. It can be seen that the actuating element 12 is configured to apply an increased force to the outer part 16 of the lens. This increased pressure causes the fluid 17, which is enclosed inside the lens by means of a flexible membrane 18, to be forced out of the outer part 16 to the central region of the lens. In this way, the volume of the central portion of the lens increases, and the curvature of the boundary surface of the lens increases accordingly.

[0095] The downward movement of the actuating member is provided by changing the corresponding control signal to the actuating unit 11. The control signal is provided by the control unit 20. By changing the control signal (e.g., the applied current or voltage), the amount of (thermal) energy consumed or absorbed by the variable focal length lens assembly 10 within a defined time period varies. Consequently, the thermal state or temperature of the variable focal length lens assembly 10 is affected accordingly. Compensation for this effect is described in the present invention and in conjunction with the following. Figures 3a to 3d And it is described in more detail above.

[0096] In other words, membrane 18 comprises an elastic or deformable polymer. The fluid 17 forming the lens contains an optical fluid, which is sealed using the elastic deformable polymer membrane. Electromagnetic actuator 11 provides pressure applied to container 16 and thus changes the curvature of the lens. The optical power of lens 15 can be controlled by changing the current flowing through voice coil 11.

[0097] Because an electric current is applied, the lens focal length can be tuned within milliseconds.

[0098] The variable focal length lens assembly 10 can be designed to provide a "push-pull" characteristic, which means that the lens curvature can be deflected from concave to convex.

[0099] The voice coil can provide reduced hysteresis for temperature measurement at the lens. Measuring the temperature as close as possible to the lens itself may be beneficial. One opportunity is to use the voice coil itself as a thermistor. Temperature sensing can be achieved by time-multiplexing the drive current and using the same coil, or alternatively by integrating a second coil that can be used for sensing in parallel with the first coil. To avoid magnetically induced voltages from the first drive coil, it is helpful in this case to reverse the second coil midway, giving it a net zero winding.

[0100] In another implementation, the second coil does not need to be reversed midway. The second coil can be implemented as a conventional unidirectional coil acting in the opposite direction to the primary coil, and the "net zero" effect can be achieved by changing (increasing) the current in the primary coil. Thus, power loss can be increased rather than decreased. Therefore, the default energy absorption rate can be related to the maximum current of the primary coil. The second coil can then be used to increase power loss with a smaller actuation force.

[0101] If the temperature sensing within the coil reacts faster than the optical changes in focus, the measured temperature signal can be delayed and low-pass filtered to match the lens's thermodynamics.

[0102] As described above, a variable focal length lens assembly 10 is proposed for more precise measurement purposes and for integration into a (multi-sensor) CMM.

[0103] Therefore, the variable focal length lens assembly 10 can be advantageous when performing coordinate measurements using an image stack. The image stack is formed when acquiring multiple images at different focal settings (and therefore different working distances). The lens setting corresponding to the sharpest image of the object feature enables the calculation of the working distance and ultimately the x, y, z coordinates of the feature. For this purpose, knowledge of ambient temperature and temperature-dependent calibration functions can also be considered. As a measure of sharpness, image contrast or the steepness of the transition from dark image features to bright image features can be addressed.

[0104] As a further improvement to the above coordinate measurement method, not only can the image with the highest contrast be selected, but the maximum contrast can also be determined by fitting the contrast value to the lens signal as a function and determining its maximum value.

[0105] Another advantage of using the optical components according to the invention relates to relative measurements. In some applications, the focus is not on the absolute x, y, z coordinates of an object feature, but on its relative coordinates or distance relative to another object feature, such as the height of a step. Preferably, both features are within the range of the z-scan provided by the tunable lens and within a field of view (FOV) (i.e., without the need to reposition the CMM probe), allowing the relative distance to be determined very quickly and with maximum accuracy. Inaccuracies associated with the absolute position of the telecentric lens and the entire measurement system (repositioning of the probe or its lens) fall outside the calculations, i.e., do not affect the measurement.

[0106] Another advantage of using the optical assembly according to the invention relates to obtaining a "constantly sharp image". When performing a z-scan, features at different z-distances are focused one after another. Features that are out of focus are blurred. The consistently sharp image can be calculated from the image stack by selecting the focused portion of the image and combining it into a new image. The variable focal length lens assembly 10 of the invention provides continuous acquisition of consistently sharp images without causing temperature drift.

[0107] Furthermore, some CMMs provide positioning of the imaging system not only in the x, y, and z directions but also in one or more rotational axes. This element of a (multi-sensor) CMM is called a wrist. This allows the imaging system to move and position in 5DoF, which enables very flexible use of the device (the sixth degree of freedom is rotation about the optical axis, which offers little advantage). Due to the relatively compact design of the variable focal length lens assembly 10 according to the invention, combination with the CMM wrist can be achieved in a correspondingly advantageous (simple) manner.

[0108] Figures 3a to 3d Embodiments of power consumption compensation according to different operating modes of the present invention are shown. These operating modes can be applied to [the following] according to [the present invention]. Figure 2a and Figure 2bVariable focal length lens assembly.

[0109] According to the present invention, in order to overcome the lens adjustment effect after a focus tuning change (see above), the lens can be operated continuously with a constant average power consumption. When the lens focus is changed for measurement (changing focal length), this can be done quickly, and subsequent or prior compensation can be made based on energy or power consumption (compensating focal length) (especially immediately after or immediately before).

[0110] Figure 3a This shows the energy or power consumption based on the default operating mode when the default control signal is applied to the lens assembly. This mode could be the standby mode of the corresponding measuring machine (such as a CMM), during which no measurement is performed. The lens is set to a defined (default) focus state, for example, in the middle of the lens's operating range. Power consumption P m It is constant.

[0111] To perform the measurement, the lens can be set to another focusing state to provide the desired focal length. This is done by applying a variation control signal for a much shorter time than the lens's adjustment time (see above). An image can be acquired while providing the desired focal length, and then the lens can be set to another (compensated) focusing state by applying a compensation control signal for, for example, the same duration. Figure 3b Choose the second (compensated) focusing state, such that the power consumption P2 of the compensated state compensates for the power consumption P1 of the changing measurement state. If the duration t is different in state 1 and state 2, then it can be calculated by P1t1 + P2t2 = P m (t1+t2) is used to consider different durations. In other words, energy consumption is compensated to become the same as in the default mode for the same duration. The compensation control signal (and the duration for which it is applied) is determined and applied separately. The durations t1 and t2 are much shorter than the adjustment time, typically t1+t2<1 second.

[0112] According to another operation of the variable focal length lens assembly, the lens can be controlled by applying a series of varying control signals to execute a series of focusing states (e.g., to determine the distance to the object). Following these different focusing states can be additional settings for compensating focusing states to compensate for energy consumption by applying corresponding compensation control signals. Figure 3c The applied compensation control signal (level and duration) is limited so that the deviation power level (from P) is controlled. m The region enclosed by the deviation and the compensation duration corresponds to the region enclosed by the deviation power level and the variation duration of the control signal.

[0113] The compensated focusing state can also be decomposed into multiple compensated focusing states, which allows the above series to continue while maintaining thermal equilibrium. Figure 3d Similarly, these enclosed areas correspond to each other. The entire sequence is preferably much shorter than the lens adjustment time.

[0114] Figure 4 An implementation of the workflow of the method according to the present invention is shown.

[0115] The method provides, in step 100, the application of a default control signal that provides a constant default energy or power consumption (110). For example, in order to perform a desired measurement by means of an optical component, a defined variation control signal is applied (step 200). This defined variation control signal provides different focal lengths of the desired tunable lens and thus has a deviation from the default power consumption (210). The deviation in power consumption results in a change in the thermal state (temperature) of the variable focal length lens assembly, which may lead to focal length drift (lens adjustment).

[0116] To avoid adjusting and stabilizing the optical properties (focal length) of the lens, a compensation control signal is applied (step 300). The compensation control signal provides the application of different compensation power consumptions, which (as can be seen together with the deviation power consumption applied with the varying control signal) provide a return to a constant default power consumption.

[0117] As a result, the average energy (power) consumption of the lens assembly can remain constant, which provides a constant temperature for the assembly. Specific adjustments to the lens can be avoided.

[0118] Regarding a specific application of the above measurement method, the lens can initially be positioned in a steady state with an average focal length of 90mm (default control signal). The user wants to measure an object within a target distance range of 91mm to 93mm. Therefore, the lens is tuned to a focal length of 93mm (variation control signal). In the next step, the focal length is gradually changed from 93mm to 91mm in increments of, for example, 100μm. At each step, an image is captured using a camera. During each step, for thermal compensation, lens tuning continues without capturing camera images until 87mm. Afterward, the lens is tuned back to the steady-state position of 90mm.

[0119] Although the invention has been shown above with reference to some specific embodiments, it must be understood that many modifications and combinations can be made to the different features of the embodiments, and that different features can be combined with each other or with coordinate measuring machines known in the art.

Claims

1. An optical assembly, the optical assembly comprising: a variable focus lens assembly (10), the variable focus lens assembly (10) comprising: a variable focus lens (15), the variable focus lens (15) being capable of modifying a focal length of the variable focus lens (15), and an actuation unit (11), the actuation unit (11) being configured and arranged relative to the variable focus lens (15) such that the actuation unit (11) provides for a setting and changing of the focal length in dependence of a control signal, wherein an energy absorption rate of the variable focus lens assembly (10) absorbing energy depends on the applied control signal, and a control unit (20), the control unit (20) being configured to: control the focal length setting of the variable focus lens (15) by providing a respective control signal, and apply a default control signal to provide for a default focal length and a default energy absorption rate, wherein the control unit (20) provides a thermal stabilization function for compensating for an absorption of energy at a varying energy absorption rate different from the default energy absorption rate, the thermal stabilization function being defined by: applying a varying control signal related to a varying focal length, and applying a compensating control signal related to a compensating focal length, wherein the varying control signal or the compensating control signal is provided such that the respective related energy absorption rate is greater than the default energy absorption rate, and the other one of the varying control signal or the compensating control signal is provided such that the respective related energy absorption rate is less than the default energy absorption rate, characterized in that the default control signal and the compensating control signal both comprise a high frequency current, and an overlap of the compensating control signal and the varying control signal is provided, wherein the high frequency current is a current with a frequency higher than a frequency the variable focus lens is mechanically able to follow.

2. The optical assembly according to claim 1, the thermal stabilization function being defined by: obtaining at least one varying energy parameter, the at least one varying energy parameter providing a determination of a varying energy absorption, based on the varying energy absorption, obtaining a varying energy absorption deviation, and determining a compensating control signal and a compensating duration, the compensating control signal and the compensating duration providing a compensating energy absorption, the compensating energy absorption providing a compensating energy absorption deviation, wherein, the compensating control signal and the compensating duration being determined such that a total energy absorption deviation provided by the varying energy absorption and the compensating energy absorption is less than the varying energy absorption deviation.

3. The optical assembly according to claim 2, characterized in that the total energy absorption deviation corresponds to a sum of the varying energy absorption deviation and the compensating energy absorption deviation, and / or the compensating energy absorption deviation corresponds to a difference between an energy absorbed when applying the compensating control signal for the compensating duration and an energy absorbed when applying the default control signal for the compensating duration.

4. The optical assembly according to claim 2 or 3, characterized in that the varying energy absorption defines a varying power consumption (PI), The compensation control signal and the compensation duration provide a compensation power consumption (P2), and The compensation control signal and the compensation duration are provided such that the deviation between the varying power consumption and the varying default power consumption is larger than the deviation between the total power consumption and the corresponding total default power consumption (P m ).

5. The optical assembly according to any one of claims 1 to 3, characterized in that The compensation control signal comprises a first compensation control signal and a second compensation control signal, wherein the first compensation control signal is different from the second compensation control signal, and / or The variation control signal comprises a first variation control signal and a second variation control signal, wherein the first variation control signal is different from the second variation control signal.

6. The optical assembly according to claim 2 or 3, characterized in that The variation energy absorption exceeds a variation default energy absorption, and the compensation energy absorption is lower than a compensation default energy absorption, or The variation energy absorption is lower than the variation default energy absorption, and the compensation energy absorption exceeds a compensation default energy absorption.

7. The optical assembly according to claim 2 or 3, characterized in that The absolute value of the compensation energy absorption deviation is lower than or equal to the absolute value of the variation energy absorption deviation, and / or The absolute value of the compensation energy absorption deviation is greater than the absolute value of the variation energy absorption deviation, wherein the difference between the variation energy absorption and a variation default energy absorption comprises a different algebraic sign compared to the difference between the total energy absorption and a total default energy absorption.

8. The optical assembly according to any one of claims 1 to 3, characterized in that The actuation unit (11) comprises a voice coil or an electrode connected to the variable focal length lens.

9. The optical assembly according to claim 2 or 3, characterized in that The at least one variation energy parameter comprises at least one of: a variation control signal providing the variation energy absorption rate, a variation duration at which the variation control signal is applied, and a variation energy absorption rate.

10. The optical assembly according to claim 6, characterized in that The thermal stabilization function is defined by: obtaining a variation control signal providing the variation energy absorption rate, determining a variation duration at which the variation control signal is applied, wherein the variation energy absorption is defined by the variation energy absorption rate and the variation duration, wherein the variation default energy absorption is defined by the default energy absorption rate and the variation duration, applying the compensation control signal and thereby providing a compensation energy absorption rate for a compensation duration after applying the variation control signal, wherein the compensation energy absorption is defined by the compensation energy absorption rate and the compensation duration, wherein the compensation default energy absorption is defined by the default energy absorption rate and the compensation duration.

11. The optical assembly according to any one of claims 1 to 3, characterized in that The compensation control signal is defined such that a sum of squared currents resulting from applying the default control signal corresponds to a sum of squared currents resulting from applying the variation control signal and the compensation control signal.

12. The optical assembly according to claim 10, characterized in that The thermal stabilization function provides for application of the compensation control signal such that the sum of thermal energy absorbed by the variable focal length lens assembly by application of the variation control signal and the compensation control signal substantially corresponds to the energy absorbed by application of the default control signal over the sum of the variation duration and the compensation duration.

13. A measuring device for measuring a distance to an object, the measuring device comprising at least one sensor and at least one optical element configured and arranged to provide a desired light propagation, characterized in that the measuring device comprises an optical assembly according to any one of claims 1 to 12, and the at least one optical element comprises the variable focal length lens assembly (10).

14. A method for thermal stabilization of a variable focal length lens assembly (10), the variable focal length lens assembly comprising: a variable focal length lens (15) capable of modifying a focal length of the variable focal length lens (15), and an actuation unit (11) configured and arranged with respect to the variable focal length lens such that the actuation unit provides for setting and changing of the focal length in dependence of a control signal, wherein an energy absorption rate at which the variable focal length lens assembly absorbs energy depends on the applied control signal, wherein the method comprises the steps of: applying a default control signal (100) to the actuation unit to provide for a default focal length and a default energy absorption rate, and applying a variation control signal (200) to provide for a variation focal length, the variation control signal providing for a variation energy absorption rate different from the default energy absorption rate and thereby providing for a variation energy absorption deviation, and applying a compensation control signal (300) to provide for a compensation energy absorption rate, wherein the compensation control signal is provided such that the total energy absorption deviation resulting from application of the variation control signal and the compensation control signal is less than the variation energy absorption deviation, characterized in that the default control signal and the compensation control signal each comprise a high frequency current, and the compensation control signal and the variation control signal are provided superimposed, wherein the high frequency current is a current with a frequency higher than the variable focal length lens is mechanically capable of following.

15. The method according to claim 14, the method being a computer implemented method.

16. A computer program product comprising a program code stored on a machine readable medium and having computer executable instructions for performing the method according to claim 14 or 15 when executed on a control unit of an optical assembly according to any one of claims 1 to 12.

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