Patent Publication Number: US-11042026-B2

Title: Transducer-induced heating and cleaning

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/463,224, filed Feb. 24, 2017, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Electronic optical sensors are widely used for generating electronic images. Often, such sensors (e.g., “cameras”) are located in places remote to a viewer. The remote locations include places (e.g., external to vehicles) where contaminants (e.g. moisture and/or dirt) from the environment can cloud or otherwise obscure the camera lens, such that degraded images are generated by a camera having an obscured lens. The degradation of the image quality can decrease safety or security in many applications. Various techniques for automatically cleaning the camera lenses include water sprayers, mechanical wipers, or air jet solutions. Such approaches are not practical or too costly in a variety of applications. 
     SUMMARY 
     In described examples, a transducer vibrates a lens element at a first frequency and a different second frequency. Controller circuitry selects one of a cleaning mode and a heating mode in response to an estimated temperature of the lens element. The controller circuitry activates the transducer to vibrate the lens element at the first frequency in response to the controller circuitry selecting the cleaning mode. The controller circuitry activates the transducer to vibrate the lens element at the second frequency in response to the controller circuitry selecting the heating mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example computing device  100  for controlling a transducer coupled to a lens element. 
         FIG. 2  is a cross-section view of an example camera lens cover system. 
         FIG. 3  is a waveform diagram of an impedance response of an example camera lens cover system over a broad frequency range. 
         FIG. 4  is a waveform diagram of an impedance response of an example camera lens cover system over a reduced frequency range. 
         FIG. 5  is a plot diagram showing a linear relationship between the impedance response of an example camera lens cover system and operating temperatures thereof while operating at a selected operating frequency of 20 kHz. 
         FIG. 6  is a flow diagram of an example process for estimating a temperature of an example camera lens cover system in response to an impedance measurement of the example camera lens cover system. 
         FIG. 7  is an isometric view of an example camera lens cover system. 
         FIG. 8  is an external view of example foreign contaminant volumes for an example camera lens cover system. 
         FIG. 9  is a block diagram of an example signal generator of an example camera lens cover system. 
         FIG. 10  is a flow diagram illustrating an example method of foreign contaminant removal from an exposed surface of the example camera lens cover system. 
         FIG. 11  is a top view of an example vehicle including example camera lens cover systems. 
     
    
    
     DETAILED DESCRIPTION 
     In this description: (a) the term “portion” means an entire portion or a portion that is less than the entire portion; (b) the term “housing” means a package or a sealed subassembly/assembly, which can include control circuitry, a transducer, lenses and an imaging sensor in a local environment that is sealed from an outside environment. 
     Ultrasonic vibration of lens surfaces (including lens covers) of camera systems (e.g. automotive systems including rear view and/or surround view systems) can be more cost effective than various water sprayer, mechanical wiper, or air jet solutions. As described herein, piezoelectric transducers (e.g., within a camera housing) can be monitored in a feedback loop structure without including a thermocouple in the feedback loop. The piezoelectric transducer is controlled by estimating a temperature of a piezoelectric transducer, such that, for example, the buildup of heat (which can be caused by activating the piezoelectric transducer) is limited by a comparison to a temperature threshold. The limiting of the buildup of heat helps prevent the permanent depolarization of the piezoelectric transducer (e.g., which could adversely affect the ability of the piezoelectric transducer to vibrate). 
     The apparatus and methods described herein for controlling and operating a piezoelectric transducer can help ensure that the temperature of the piezoelectric transducer does not reach more than one-half a Curie temperature (in degrees Celsius) of the piezo material of the transducer being controlled. In example embodiments, the transducer lifetime can be extended by the avoidance of operating the piezoelectric transducer at potentially damaging temperatures. 
       FIG. 1  is a block diagram of a computing device  100  for controlling a transducer (in a lens cover system  132 ) coupled to a lens element (in the lens cover system  132 ). For example, the computing device  100  is, or is incorporated into, or is coupled (e.g., connected) to an electronic system  129 , such as a computer, electronics control “box” or display, controllers (including wireless transmitters or receivers), or any type of electronic system operable to process information. 
     In example systems, a computing device  100  includes a megacell or a system-on-chip (SoC) that includes control logic such as a central processing unit (CPU)  112 , a storage  114  (e.g., random access memory (RAM)) and a power supply  110 . For example, the CPU  112  can be a complex instruction set computer (CISC)-type CPU, reduced instruction set computer (RISC)-type CPU, microcontroller unit (MCU), or digital signal processor (DSP). The storage  114  (which can be memory such as on-processor cache, off-processor cache, RAM, flash memory, or disk storage) stores one or more software applications  130  (e.g., embedded applications) that, when executed by the CPU  112 , perform any suitable function associated with the computing device  100 . The processor is arranged to execute code (e.g., firmware instructions and/or software instructions) for transforming the processor into a special-purpose machine having the structures—and the capability of performing the operations—described herein. 
     The CPU  112  includes memory and logic circuits that store information that is frequently accessed from the storage  114 . The computing device  100  can be controlled by a user operating a UI (user interface)  116 , which provides output to and receives input from the user during the execution the software application  130 . The UI output can include indicators such as the display  118 , indicator lights, a speaker, and vibrations. The UI input can include sensors for receiving audio and/or light (using, for example, voice or image recognition), and can include electrical and/or mechanical devices such as keypads, switches, proximity detectors, gyros, and accelerometers. For example, the UI can be responsive to a vehicle operator command to clear an exterior surface of a backup camera of the vehicle. 
     The CPU  112  and the power supply  110  are coupled to I/O (Input-Output) port  128 , which provides an interface that is configured to receive input from (and/or provide output to) networked devices  131 . The networked devices  131  can include any device (including test equipment) capable of point-to-point and/or networked communications with the computing device  100 . The computing device  100  can be coupled to peripherals and/or computing devices, including tangible, non-transitory media (such as flash memory) and/or cabled or wireless media. These and other such input and output devices can be selectively coupled to the computing device  100  by external devices using wireless or cabled connections. The storage  114  is accessible, for example, by the networked devices  131 . The CPU  112 , storage  114 , and power supply  110  are also optionally coupled to an external power source (not shown), which is configured to receive power from a power source (such as a battery, solar cell, “live” power cord, inductive field, fuel cell, capacitor, and energy storage devices). 
     The transducer controller  138  includes control and signaling circuitry components for resonating a transducer of the lens cover system  132 , such that the lens element can be cleaned by expelling foreign material (e.g., shaken clean of moisture). As described hereinbelow, the transducer controller  138  includes a temperature calculator  140  for determining a temperature of the lens cover system  132 , such that, for example, the transducer of the lens cover system  132  is operated within a safe range of operating parameters. 
       FIG. 2  is a cross-section view of an example camera lens cover system. The camera lens cover system  200  generally includes a lens element  220 , a seal  230 , a housing  240 , a transducer  250 , and a camera  260 . The camera  260  includes a camera lens  262 , a camera base  264 , a photodetector  272 , and controller circuitry  274 . The transducer  250  is operable to vibrate at a selected frequency (such as a factory-selected frequency or an operator-selected frequency) for motivating the dispersal of the moisture  210  (or other foreign materials) from the exterior (e.g., upper) surface of the lens element  220 . 
     The lens element  220  is a transparent element elastically captivated in a distal (e.g., upper) portion of the housing  240 . The lens element  220  is arranged to receive light from surrounding areas and to optically couple the received light to the photodetector  272  (e.g., via the camera lens  262 ). The lens element  220  is arranged to protect the camera lens  262  against moisture  210  intrusion, for example. The moisture  210  can be in the form of frost, water drops, and/or a film of condensation. Foreign materials (such as the moisture  210  and dirt particles) can block and/or diffuse light, such that at least some of the received light is prevented from reaching the camera lens (e.g., compound lens)  262 . In an embodiment, the lens element  220  can be a focusing lens (e.g., for refractively focusing light). 
     A seal  230  (such as a rubber seal) is arranged to elastically captivate the lens element  220  to the housing  240  and to seal a cavity (e.g., in which the camera lens  262  is arranged) against intrusion of moisture  210  into the cavity. The intrusion of moisture  210  and other foreign substances into the cavity can facilitate condensation inside the lens cover system that can obstruct the camera&#39;s view. Moisture inside the lens cover system can also damage the controller circuitry  274  electronics and/or the pixels (e.g., pixel cells) of the photodetector  272 . The cavity extends inwards from the lens element  220  to a proximal (e.g., lower) portion of the housing  240 . 
     The cavity is also formed by the camera base  264 , which is coupled to (or formed as part of) the housing  240 . The camera base  264  can include a photodetector  272  and controller circuitry  274 . The photodetector  272  can be a video detector for generating electronic images (e.g., video streams) in response to the focused light coupled through the lens element  220  and the camera lens (which can include lenses). The controller circuitry  274  can include: (a) a printed circuit board; (b) circuitry of the transducer controller  138 ; and (c) and the circuitry of the temperature calculator  140  for controlling the lens cover system  132  (e.g., where such circuitry and the lens cover system are arranged in a feedback loop structure). The controller circuitry  274  is coupled to external power, control, and information systems using wiring and/or optical conduits (such as fibers). 
     The transducer  250  is mechanically coupled to the lens element  220 . The transducer  250  can be affixed to the lens element  220  by an intervening adhesive layer (e.g., a high-temperature resistant epoxy). In operation, the transducer  250  is arranged to vibrate (e.g., at a selected frequency) the lens element  220  in response to transducer driver signals. The transducer driver signals are controllably modulated, such that the transducer  250  is controllably excited in response to the transducer driver signals. The transducer driver signals can be amplitude modulated, such that vibrating lens element  220  can controllably expel moisture  210  and other such foreign material from the external surface of the lens element  220  (e.g., external to the cavity). 
     A lens cover system  132  can include the transducer  250 , the housing  240 , the seal  230 , and the lens element  220 . As described hereinbelow, the temperature of the lens cover system can be estimated in response to (e.g., as a function of) electrical properties of the lens cover system. The transducer  250  can be controllably excited in response to the estimated temperature to efficiently remove obscuring foreign material (including potentially obscuring foreign material) from the lens element  220  without exceed a threshold temperature limit. (Indeed, the transducer  250  can be destroyed or degraded when operated for prolonged periods at excessively elevated temperatures.) 
     The impedance response of the lens cover system  132  varies according to the temperature of the lens cover system  132 . As described herein, the relationship between the estimated temperature of the lens cover system  132  and the measured electrical impedance of the lens cover system is substantially linear within a frequency range.  FIG. 3 , as described hereinbelow, shows an impedance response of an example lens cover system over a frequency range of selected temperatures. 
       FIG. 3  is a waveform diagram of an impedance response of an example lens cover system over a broad frequency range. The waveform diagram  300  includes a lens cover system impedance response  310 . The lens cover system impedance response  310  shows the impedance in Ohms over a frequency range between 10 kHz to around 1 MHz. The example lens cover system can be a lens cover system such as the system  200  described hereinabove. 
     The “zeros” of the impedance response correspond to the series resonance properties, which correspond to the electromechanical vibration properties (such as resonance) of a lens cover system that includes the example transducer. The electromechanical resonances of the system occur at frequencies in which relatively larger vibration amplitudes occur for a variable electrical input amplitude stimulus. For example, electromechanical resonances occur at frequency ranges  321 ,  322 , and  323 . The zeros are indicated by valleys (such as valley  301 ) in the curve  310 . A reduced frequency range (e.g., for a “zoomed in” view) of the impedance response  310  is described hereinbelow with respect to  FIG. 4 . 
       FIG. 4  is a waveform diagram of an impedance response of an example lens cover system over a reduced frequency range. The waveform diagram  400  includes an example lens cover system impedance response  410 . The lens cover system impedance response  410  shows the impedance response of the example lens cover system over a reduced frequency range (e.g., with respect to the frequency range shown in  FIG. 3 ). 
     The lens cover system impedance response  410  includes discrete temperature curves for indicating the lens cover system impedance at a discrete temperature selected from a range of temperatures. The range of selected discrete temperatures extends from a temperature of − (minus) 40° C. to a temperature of 60° C., where the temperature represented by each temperature curve differs from the represented temperature of an adjacent temperature curve by 20° C. The range of temperatures encompasses operating temperatures potentially encountered in operation of the example lens cover system in various example applications. 
     For example, the temperature curve  412  shows the example lens cover system impedance response in Ohms over a frequency range of around 20 kHz to 40 kHz at a temperature of −40° C., whereas the temperature curve  414  shows the lens cover system impedance response in Ohms over a frequency range of around 20 kHz to 40 kHz at a temperature of 60° C. The lens cover system impedance response  410  includes a valley  401 , which indicates a resonance of the example lens cover system at around 29 kHz for all illustrated temperature responses. 
     At frequencies below 150 kHz (e.g., as shown by the lens cover system impedance response  310 ), the gain of an impedance response generally decreases as the temperature increases, such that the gain of the lens cover system impedance response  410  is inversely related to temperature within a selected operating frequency range (e.g., with exceptions occurring around the locations of resonant frequencies of the example transducer). The change in the impedance over temperature is linear (e.g., having a constant slope and having a change in impedance that is proportional to a step change in temperature). 
     For example, the vertical spacing (e.g., for a given frequency) between each temperature curve between temperature curves  412  and  414  of a selected operating frequency is equal (e.g., substantially equal). The selected operating frequency is selected from a frequency included in a linear response region, such as the linear response region extending between, at least, 20 kHz and 25 kHz). The equal spacing of the temperature curves between temperature curves  412  and  414  is indicative of a linear relationship between an operating temperature and a measured impedance at a selected operating frequency. 
     The temperature of the example transducer can be determined in response to a measurement of the impedance of the example transducer. For example, the example transducer is excited to vibration (e.g., in response to amplitude-modulated driver signals) at a frequency of a frequency range in which the change in the transducer impedance over frequency is linear. 
     The dependent variable temperature T as a function of the impedance variable Z for the example transducer is expressed as the linear equation:
 
 T=− 0.29 *Z+ 392.6  (1)
 
which has a coefficient of determination R 2  value of R 2 =0.9932 (e.g., which is substantially linear, and wherein the constant “−0.29” is a slope of the linear equation, and the constant “392.6” is a y-intercept of the linear equation). The dependent variable temperature T as a function of the impedance variable Z for the example transducer can also be expressed as the parabolic equation:
 
 T=A*Z   2   +B*Z+C   (2)
 
where A, B and C are constants. When A=0, Equation (2) is reduced to the linear form (such as the form of Equation (1)). Accordingly, the selected operating frequency is selected from within a frequency region within which the relationship between the estimated temperature and the measured impedance is determinable as a quadratic function (e.g., according to the Equation (2)).
 
     The determined relationship between the estimated temperature of the lens cover system and the actual (e.g., empirically measured) temperature of the example lens cover system is substantially linear when the coefficient of determination R 2  value is at least 0.95, for example. As the value of the coefficient of determination R 2  approaches unity, the statistical variance between an estimated value using the linear equation and the actual value is minimized. As the value of coefficient of determination recedes from unity, errors in the estimation increase, which can result in any of: (a) decreased temperature operating range; (b) increased safety margins; and (c) decreased life of the lens cover system. 
       FIG. 5  is a plot diagram showing a linear relationship between the impedance response of an example lens cover system and operating temperatures thereof while operating at a selected operating frequency of 20 kHz. As described herein, the operating temperature of a lens cover system can be estimated by measuring the impedance of the lens cover system at a selected operating frequency, and by converting the impedance measurement to an estimated temperature (e.g., according to the relationship described by Equation (1) hereinabove). The conversion of the impedance measurement to an estimated temperature can be executed in response to calculating the Equation (1) result, and/or by indexing a lookup table to retrieve a result according to Equation (1). 
     Plot  500  shows the close statistical correlation between estimated curve  520  (EST) and a corresponding empirically measured curve  510  (MEAS). The actual (e.g., simulation value of) temperature is shown by the empirically measured curve  510 . The estimated temperature (e.g., calculated using Equation (1)) is shown by the estimated curve  520 . The estimated curve  520  can be estimated in simulations by controlling the temperature (e.g., from −60° C. to 40° C.) to derive impedance measurements (e.g., ranging from around 1150 Ohms to 1475 ohms) of the example lens cover system. The empirically measured curve  510  and the estimated curve  520  are statistically correlated to a high degree. 
     As shown by the plot  500 , the relationship between the estimated temperature of the lens cover system and the actual (e.g., empirically measured) temperature of the example lens cover system is linear (e.g., substantially linear). The maximum error (e.g., determined in simulations between corresponding points of the estimated curve  520  and the empirically measured curve  510 ) shown by plot  500  is 3.7° C. Accordingly, the lens cover system temperature can be accurately estimated using a simple linear equation. 
     For example, a temperature error 3.7° C. of a temperature estimate is sufficiently accurate, such that the example lens cover system can be safely operated when the transducer controller maintains the estimated operating temperature of the example lens cover system below a temperature threshold for estimated temperatures. As described hereinbelow, the operating temperature threshold for estimated temperatures can be selected in response to the error margin of the estimated temperature measurement and the Curie temperature of the example transducer. The Curie temperature threshold can be a temperature threshold beyond which the permanent polarization of piezoelectric materials of the example transducer is degraded (for example, the Curie temperature threshold can be half of the Curie temperature). Accordingly, the temperature threshold is for delineating (e.g., an upper limit of) an operating temperature range below which the example transducer can be activated without (e.g., accelerated) depolarization. 
     In an embodiment, the impedance data over a range of temperatures for a selected operating frequency can be measured at discrete temperatures and stored as a lookup table in memory (e.g., which reduces processing requirements for calculating the equation otherwise calculated to determine an instant operating temperature). Simple (e.g., one-dimensional) linear interpolation can be used to more precisely determine the operating temperature (e.g., depending on a particular application of the described techniques, such as measuring a temperature outside a vehicle for determining a control decision described hereinbelow). 
     In an embodiment, impedance data measured over a range of temperatures and over a range of operating frequencies can be stored. The impedance data can be measured at discrete temperatures and discrete operating frequencies and stored as a lookup table in memory (e.g., such that firmware would not have to be programmed for controlling specific transducers, each of which can be operated mutually different frequencies according to a selected transducer and a selected application). Simple (e.g., two-dimensional) linear interpolation can be used to more precisely determine the operating temperature for a selected operating frequency. 
       FIG. 6  is a flow diagram of an example process for estimating a temperature of an example lens cover system in response to an impedance measurement of the example lens cover system. The flow  600  can be performed by hardware circuits exclusive of programming commands. For example, the example process can be executed by apparatus including analog and/or digital control circuits (such as registers, adders, multipliers, voltage generators, and comparators) that are arranged (e.g., pipelined) according to the process  600 , described hereinbelow. 
     The flow  600  begins at operation  610 , in which an example transducer is activated (e.g., electrically excited at a selected frequency by assertion of amplitude-modulated transducer driver signals). For example, the amplitude-modulated transducer driver signals are asserted to effect excitation of the example lens cover system at the selected frequency of 20 kHz (which is a frequency at which a linear relationship exists between the temperature of the example lens cover system and the impedance of the lens cover system). The flow continues to operation  620 . 
     At operation  620 , the impedance (e.g., effective impedance) of the activated example lens cover system is measured. The impedance can be measured in response to a voltage drop resulting from coupling the example transducer to the asserted amplitude-modulated transducer driver signals, for example. Because the example lens cover system is excited at 20 kHz, the measured impedance is derived in response to example transducer excitation at the selected frequency of 20 kHz. The flow continues to operation  630 . 
     At operation  630 , the measured impedance is converted to an estimated temperature. The estimated temperature is determined according to the linear relationship between the impedance of the example lens cover system and the operating temperature of the example lens cover system. For example, the measured impedance can be converted to the estimated temperature by circuits operating according to the function of Equation (1), and/or the measured impedance can be converted to the estimated temperature in response to indexing a lookup table with values for creating the output of Equation (1). The lookup table includes addressable values that can be addressed using the independent variable (e.g., the measured impedance) as the index, and that are output as results for providing or determining the value of the dependent variable. For example, the addressable values are determined (e.g., pre-calculated before or after deployment of the system  100 ) according to Equation (1). The flow continues to operation  640 . 
     At operation  640 , the temperature is compared against a temperature threshold. The temperature threshold can be determined in response to the Curie temperature threshold and a safety margin. The safety margin can be selected in response to the Curie temperature threshold, the maximum expected error of the estimated lens cover system temperature, and a margin for “derating” the lens cover system for increasing product lifetime (e.g., increasing the mean-time-between-failure reliability factor) of the example lens cover system. The flow continues to operation  650 . 
     At operation  650 , the activation state of the transducer is toggled (e.g., activated when the example transducer is in a deactivated state, or is deactivated when the example transducer is in an activated state) in response to the comparison at operation  640 . For example, the example transducer is deactivated if the temperature indicates that the example lens cover system has an operating temperature that approaches a self-damaging temperature. The example transducer can be deactivated when the comparison at operation  640  indicates that the estimated temperature exceeds half of the Curie temperature (in degrees Celsius) of the example transducer. 
     The process  600  can be invoked each time the example transducer is activated. The length of a periodic interval (e.g., fixed period) of time the example transducer is activated can be limited (for example) for the purpose of periodically re-invoking the process  600 , which in turn limits the accumulation of heat from operating the example transducer. The example transducer can be deactivated in response to the expiration of fixed period of time during which the example transducer is activated. The length of time selected for limiting the activated time of the example transducer can be selected in view of the rate of accumulation of heat during operation at the selected operating frequency and the relative sizes of safety margins. Accordingly, the rise of temperature of the example lens cover system is controllably limited below levels that are likely to permanently (e.g., without repair) damage the example transducer (e.g., without incurring the space, cost, and reliability considerations otherwise encountered by coupling a thermocouple to the example transducer). 
       FIG. 7  is an isometric view of an example camera lens cover system. The camera lens cover system  700  generally includes a transducer  710 , power wires  720 , a lens element  730  and bonding agent  740 . The transducer  710  of the camera lens cover system  700  can be a cylindrical transducer such as transducer  250 , described hereinabove, that is arranged to apply ultrasonic vibrations for cleaning and/or heating a camera lens cover. 
     The transducer  710  is arranged to vibrate a mechanically coupled lens cover (e.g., the lens element  730 ) in response to being driven by an electronic amplifier at frequencies ranging from around 20 kHz to 2.0 MHz. The transducer  710  can be driven at a given excitation frequency and a resulting impedance sensed by coupling signals to and from the transducer via the power wires  720 . The resulting impedance can be affected by temperature, mechanical characteristics, electrical characteristics and the frequency at which the transducer is driven, for example. A lens element  730  is secured to a distal surface of the transducer  710  by a bonding agent  740  (e.g. epoxy) disposed (e.g., as a circular shape) between the distal surface of the transducer  710  and an adjacent portion of a surface of the lens element  730 . The seal between the transducer element and the lens element  730  helps prevent the intrusion of moisture into a sealed cavity (e.g., which can be formed by a camera base, the transducer  710 , the lens element  730 , and the bonding agent  740 ). 
     Environmental moisture (e.g., water drops, water droplets and/or a film of condensation) can adhere to an exterior surface of the lens element  730 . The moisture can occlude light from being clearly received by a camera lens in the sealed cavity. The transducer  710  is operable to vibrate at a selected frequency for motivating the dispersal of the moisture (or other foreign materials) from the exterior (e.g., outer) surface of the lens element  730 . When droplets of moisture and/or a film of condensation remain on the exterior surface of the lens element  730 , the remaining moisture can cause saturation of the image sensor optically coupled to the camera lens when, for example, incident light encounters the exterior surface of the lens element  730  at an oblique angle. 
     The exterior surface of the lens element  730  can be formed without a “lip” (or can be formed with a lip arranged with channels extending therethrough), which provides a path for moisture migration during vibration. Vibration of the sealed lens element urges moisture along the path for moisture migration, for example, because the vibration helps overcome surface tension of the moisture (which otherwise helps the moisture to adhere to itself as well as to adhere to the outer surface of the lens element  730 ). As described hereinbelow with respect to  FIG. 8 , the transducer  710  is arranged to (e.g., both) vibrate at the selected frequency and to generate thermal energy for heating the lens element  730 . 
       FIG. 8  is an external view of foreign contaminant volumes for an example camera lens cover system. Water drops “contaminate” a lens surface (e.g., of lens element  730 ), such that a view through the lens surface is blocked or otherwise obscured. In an example, a camera lens cover system is vertically oriented, such that the lens element  730  is level, and such that moisture is not removed by gravity (e.g., for the purpose of illustration) during the moisture removal stages  810 ,  820 , and  830 . In the example, the multi-stage cleaning diagram  800  includes a large-volume cleaning stage  810  (e.g., for generally removing drops greater than around 15 μL in volume, such as drop  812 ), a medium-volume cleaning stage  820  (e.g., for generally removing drops less than around 15 μL in volume, such as drop  822 ), and a small-volume cleaning stage  830  (e.g., for removing residual moisture, such as droplets  832 ). 
     In the large-volume cleaning stage  810 , the transducer is arranged to vibrate in a first mode at a first selected frequency such that water drops of around 4-10 mm (or greater) diameter are dispersed (e.g., atomized or otherwise reduced in size) in response to vibration generated at the first selected frequency. In the first mode (in stage  810 ), a large-volume cleaning excitation signal is applied to the transducer to generate vibration at the first selected frequency. The first selected frequency can be a frequency in a frequency range at which electromechanical resonances occur. The first selected frequency can be characterized by a relatively high frequency vibration that consumes a relatively high amount of power. The large-volume cleaning stage  810  can be followed by the medium-volume cleaning stage  820 . 
     In the medium-volume cleaning stage  820 , the transducer is arranged to vibrate in a second mode at a second selected frequency, such that water drops (or droplets) of around 1-4 mm diameter are dispersed (e.g., atomized or otherwise reduced in size) in response to the vibration generated at the second selected frequency. In the second mode (in stage  820 ), a medium-volume cleaning excitation signal is applied to the transducer to generate vibration at the second selected frequency. The second selected frequency can be a frequency in a frequency range at which electromechanical resonances occur. The second selected frequency can be a frequency that is lower than the first selected frequency. The first selected frequency can be characterized by a relatively low frequency vibration that consumes a relatively low amount of power. The medium-volume cleaning stage  810  can be followed by a small-volume cleaning stage  830 . 
     In the small-volume cleaning stage  830 , the transducer is arranged to vibrate in a third mode at a third selected frequency, such that water droplets of around 0-1 mm diameter are evaporated (e.g., atomized or otherwise dispersed) in response to the heat and vibration generated at the third selected frequency. In the third mode (in stage  830 ), a heating excitation signal is applied to the transducer to generate vibration at the third selected frequency. The third selected frequency can be a frequency in a frequency range at which electromechanical resonances occur. The water droplets of around 0-1 mm diameter are difficult to remove by vibrations because, for example, the surface tension of the water as well as the relatively high van der Waals forces exerted between the surface of the lens cover and the water. 
     The third selected frequency can be a frequency that is higher than the first selected frequency. The third selected frequency can be characterized by a relatively high frequency vibration that consumes a relatively high amount of power. The heat generated by the transducer is thermally coupled to the lens element via the bonding agent  740  interposed between the transducer  710  and the lens element  730 . The heat transferred to the lens element  730  helps remove any residual droplets, condensates on the lens element. 
     A control system described hereinbelow with respect to  FIG. 9  is arranged to control the amount of heat generated so as to not overheat the piezoelectric material (which can damage the transducer actuator) and to avoid exceeding safe touch temperatures on the surface of the transparent element  730 . The transducer temperature can be estimated by measuring the impedance of the lens cover system as described hereinabove. 
       FIG. 9  is a block diagram of an example signal generator of an example camera lens cover system. For example, the signal generator  900  is arranged to control signals for driving a transducer of the lens cover system, to monitor the transducer performance and to change aspects of the drive signals in response to the monitored transducer performance. 
     The signal generator  900  includes a voltage (V) boost circuit  902  that is arranged to receive power (such as 12 volts direct current input from a vehicle power system) and to generate a 50-volt potential (e.g., surge-protected potential) from the received 12-volt input power. The 50-volt potential is modulated as described hereinbelow for driving a transducer of the camera lens cover system. 
     The signal generator  900  also includes an embedded core (such as a microcontroller unit MCU)  910  for executing instructions to transform the embedded core into a special-purpose machine for executing the functions of the camera lens cover system controller  920 . For example, the camera lens cover system controller  920  includes control algorithms  922 , pulse width modulation (PWM) signal generation circuit  924 , temperature estimation and regulation circuit  926  and system monitoring and diagnostics circuit  928 . Such functions are described hereinbelow with respect to  FIG. 10 . 
     The camera lens cover system controller  920  is arranged to select operating parameters (such as cleaning modes, heating modes, cleaning stages, frequencies and operating temperatures) for the camera lens cover system in response to monitoring the camera lens cover system transducer and to control the PWM switching controller  930  in response to the selected operating parameters. For example, the camera lens cover system controller  920  is arranged to control PWM switching times of the PWM switching controller  930 . The PWM switching controller  930  is arranged to signal the PWM PreDriver  940  in response to the switching times received from the PWM switching controller  930 . The PWM PreDriver  940  generates control signals for toggling (e.g., actuating) the switches of the Class D driver  950 . The Class D Driver  950  is a full-bridge rectifier that is arranged to generate +/−50 volts (e.g., 100 volts peak-to-peak) for driving the transducer  960 . The sense circuitry  970  generate current and voltage signals for sensing impedance of the camera lens cover system and transducer  960 , which are monitored (e.g., buffered) by the transducer monitor  980 . The monitor signals are coupled via a multiplexer (MUX)  990  to the analog-to-digital converter (ADC)  990  for sampling. The embedded core  910  is arranged to receive the sampled current and voltage signals, to compute (via the camera lens cover system controller  920 ) new PWM signaling, to perform temperature estimation and regulation and to perform system monitoring and diagnostics as described hereinbelow with respect to  FIG. 10 . 
       FIG. 10  is a flow diagram  1000  illustrating an example method of foreign contaminant removal from an exposed surface of the example camera lens cover system described herein. At  1002 , the process begins in the camera lens cover system controller described hereinabove. At  1010 , the camera lens cover system controller waits a period of time (e.g., waits for the system start signal) before identifying and/or determining the existence (presence) of contaminants at  1014 . If the wait duration is not expired, the wait duration is updated at  1012 , and the process loops back to  1010 . 
     At  1014 , after the wait period has expired, a frequency measurement device monitors the resonant frequency the example camera lens cover system to identify (for example) the amount of contaminant disposed on the exposed surface. For example, the amount of a contaminant can be determined in response to a measured frequency response of the camera lens cover system and comparing the measured frequency response to a database that includes known frequency responses for given types and amounts for specific contaminants. At  1020 , the camera lens cover system controller determines whether a contaminating material exists (is present) on the camera lens cover, such that at least one operation for cleaning the camera lens cover is initiated. If “YES,” then at  1028 , the temperature of the example camera lens cover system is determined, and types of cleaning and heating are selected in response to the determined temperature as described hereinbelow. If “NO,” then system checks are performed at  1030 . 
     If at  1020  the presence of a material is not indicated (“NO”), then at  1030 , the process initiates system monitoring and diagnostics tests (e.g., during which the camera lens cover system is self-tested). At  1032 , a decision is made as to whether to disable the system. For example, the determination whether to disable the system can be determined in response to the nature of faults diagnosed at  1030 , a response to a user input and/or a response to whether the power has been turned off to the system. If the system is to be disabled (“YES”), then at  1034 , the system is shut down. If the system is not to be disabled (“NO”), the process loops back to  1010  and waits for a specified duration before the process starts again. 
     As described hereinabove, at  1028 , the temperature of the example camera lens cover system is determined. For example, the temperature of the example camera lens cover system can be determined in response to (e.g., as a function of) an operating frequency of an activated transducer as described hereinabove, or by a temperature sensing device (such as an externally coupled thermocouple). The process continues at  1050 , for example. 
     If at  1050  the determined temperature is below the freezing point of water (e.g., within a margin of error), then at  1052 , the camera lens cover system controller generates a heating signal for a specified duration (e.g., time period). For example, in response to a comparison of the determined temperature to the freezing point of water, the camera lens cover system controller can generate a heating excitation signal in a heating mode for warming the camera lens cover system as described hereinabove by exciting the transducer at a frequency at which the transducer generates relatively large amounts of heat, which occurs at frequency locations where the impedance response is small for a given voltage input. For example, the heating excitation signal in a heating mode for warming the camera lens cover system can be generated at a valley (such as the valley of the frequency range  321 , which includes a relatively low impedance value of a selected frequency range) of a lens cover system impedance response curve (such as described hereinabove with respect to  FIG. 3  and  FIG. 4 ). 
     At  1054 , the temperature of the example camera lens cover system is determined. After the temperature is determined (e.g., after initiation of a heating or cleaning mode), the process continues at  1040 , after which the process initiates further operations (described hereinbelow) to ensure, for example, the transducer is operated within a safe operating region of temperatures. 
     If at  1056  the type and size of detected contaminating material indicates the contaminating material is to be reduced in size, then at  1058 , a cleaning signal is generated for cleaning the example camera lens cover system. For example, the camera lens cover system controller can select a cleaning mode in response to the size of detected contaminating material. The cleaning mode can be selected, such that the cleaning signal can be generated as one of a large-volume cleaning excitation signal, a medium-volume cleaning excitation signal and a small-volume cleaning excitation signal. The large-volume cleaning excitation signal can be generated at a frequency conducive to resonating larger size drops of water (for example), whereas the medium-volume cleaning excitation signal can be generated at a frequency conducive to resonating medium size drops of water (for example) and the small-volume cleaning excitation signal can be generated at a frequency conducive to heating small droplets of water. After the large-volume or medium-volume cleaning signal is generated and applied at  1058 , the process continues at  1054  where the temperature of the example camera lens cover system is determined (e.g., to ensure, the transducer is operated within a safe operating region of temperatures). 
     If at  1060 , the size of detected contaminating material indicates small droplets of water (for example), such that drying is indicated, then at  1062 , a heating signal is generated for cleaning the example camera lens cover system. For example, the camera lens cover system controller can generate a heating signal in a heating mode such that water droplets of around 0-1 mm diameter are evaporated (e.g., atomized or otherwise dispersed) in response to the heat and vibration generated in response to the heating signal (e.g., applied at a frequency different from the respective frequencies of the applied cleaning signals). After the heating signal is generated and applied at  1062 , the process continues at  1054  where the temperature of the example camera lens cover system is determined (e.g., to ensure, the transducer is operated within a safe operating region of temperatures). 
     After the temperature is determined (e.g., again) at  1054 , the process continues at  1040 , where a decision is made to determine whether the temperature of the example camera lens cover system exceeds a temperature threshold. For example, the temperature threshold can be half of the transducer Curie temperature, such that the transducer is controlled to operate within a safe temperature range. If “YES,” the process proceeds to disable the applied signal (e.g. heating or cleaning signal) at  1042 . At  1044 , cooling of the transducer and/or exposed surface is initialized (e.g., by entering a delay period during which the heating or cleaning signal is disabled, such that additional heat is not generated). At  1046 , the process determines the latest temperature, and in response at  1048 , a decision is made to determine whether the transducer temperature has finished cooling. For example, the decision can be made in response to the information determined at  1046 , such that the temperature can be determined to be below a selected temperature threshold. In an example, the temperature threshold can be half of the transducer Curie temperature, such that the transducer is controlled to operate within a safe temperature range. In another example, the temperature threshold can be less than the transducer Curie temperature. If “YES” (e.g., when finished cooling), then at  1048 , the process loops back to  1010 . If “NO,” then at  1048 , the process loops back to  1046 , determines a latest temperature and loops back to  1048  (e.g., for additional cooling). 
     If “NO” at  1040  (e.g., when the transducer temperature does not exceed the temperature threshold), then at  1016 , a decision is made to determine whether the cleaning process is complete. If “YES,” then the process starts again at  1010 . If “NO” at  1016 , then at  1018 , the cleaning signal duration is updated and the process loops back to  1020  for additional testing and potential cleaning operations. 
       FIG. 11  is a top view of an example vehicle including example camera lens cover systems. The vehicle  1110  includes a vehicle body that includes an interior space sheltered from an exterior environment. The vehicle  1110  includes at least one camera coupled to the vehicle body, where each camera includes a lens element, where the lens element is transparent and is exposed to the exterior environment. The vehicle also includes at least one apparatus that includes a transducer arranged to vibrate the lens element at a selected operating frequency when operating in an activated state. 
     The vehicle  1110  further includes controller circuitry  1150  coupled to the vehicle, wherein the controller circuitry  1150  includes a user interface arranged to receive commands generated in response to an operator operating the vehicle  1110  from the interior space of the vehicle, wherein the controller circuitry  1150  is arranged to measure an impedance of the apparatus while the transducer is operating at the selected operating frequency, wherein the controller circuitry  1150  is arranged to determine an estimated temperature of the apparatus in response to the measured impedance, wherein the controller circuitry  1150  is arranged to compare the estimated temperature of the apparatus against a temperature threshold for delineating an operating temperature range of the apparatus, and wherein the controller circuitry  1150  is arranged to toggle an activation state of the transducer in response to comparing the estimated temperature of the apparatus against the temperature threshold. The controller circuitry  1150  can be arranged to measure an impedance of the apparatus in response to commands received from the operator operating the vehicle from the interior space of the vehicle  1110 . The controller circuitry  1150  can also arranged to measure an impedance of the apparatus in response to the operator starting the vehicle  1110 . 
     The controller circuitry  1150  includes a display  1160  (which can also include a touch screen) for displaying a synoptic view  1140  in response to each video signal of a local view  1430  of a local camera (CAM)  1420 . 
     Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.