Panel Speaker Temperature Monitoring and Control

Abstract

A panel audio speaker includes a panel and an actuator attached to a surface of the panel and configured to cause the panel to vibrate. The actuator includes a magnetic coil in thermal communication with the panel. The panel audio speaker further includes: a plurality of electrical sensors electrically coupled to the magnetic coil and configured to output time-varying electrical data of the magnetic coil; and an electronic control module in communication with the magnetic coil and the electrical sensors. The electronic control module is configured to perform operations including: supplying current to the magnetic coil; receiving the time-varying electrical data of the magnetic coil; determining electrical energy supplied to the magnetic coil between a first time and a second time; accessing a thermal model of the panel; and determining a change in panel temperature between the first time and the second time.

Description

Technical Field

[0001] This disclosure application generally relates to audio speakers. Background Art

[0002] This specification relates to an actuator including one or more electromagnetic coils and a panel audio speaker featuring such an actuator.

[0003] Many electronic devices can present multimedia content through speakers including outputs providing tones, voice generation, or recording. A panel audio speaker can produce sound by inducing distributed vibration modes in a panel via an electroacoustic actuator. For example, the panel can include a display panel. Typically, the actuator is an electromagnetic or piezoelectric actuator. Summary of the Invention

[0004] This specification describes techniques, methods, systems, and other mechanisms for monitoring the temperature of a panel in a panel audio device.

[0005] A panel audio speaker can include an actuator that includes a magnetic coil that provides a force to the panel to vibrate the panel to generate audible sound waves. The magnetic coil of the actuator can be in thermal communication with the panel such that heat can flow between the magnetic coil and the panel. For example, the coil can be fixed to the surface of the panel by an adhesive.

[0006] The panel can be, for example, a display panel of a mobile phone, a smart watch, or a head-mounted display. It is desirable to predict, measure, and monitor the temperature of the panel. A high panel temperature can cause harm to the user and may cause damage to the panel and connected components. For example, it may be desirable to keep the panel temperature below 45 degrees Celsius to reduce the risk of harm and damage.

[0007] During actuator operation, a control module of the panel audio speaker can supply an electrical audio signal to the magnetic coil and can measure electrical data of the magnetic coil. Based on the electrical data, the control module can determine the amount of energy applied to the magnetic coil over a period of time. Based on the amount of energy applied to the magnetic coil, a thermal model of the panel, and an initial temperature, the control module can determine the final temperature of the panel.

[0008] The control module can determine that the final temperature of the panel violates a limit or a threshold temperature. In response to determining that the final temperature of the panel violates the threshold temperature, the control module can adjust the audio signal supplied to the magnetic coil. For example, the control module can reduce the current of the audio signal supplied to the magnetic coil. Reducing the current of the audio signal supplied to the magnetic coil may cause the panel temperature to increase at a slower rate, stop increasing, or decrease.

[0009] Generally, an innovative aspect of the subject matter described in this specification can be embodied in a panel audio speaker, which includes: a panel; an actuator attached to the surface of the panel and configured to cause the panel to vibrate, the actuator including a magnetic coil in thermal communication with the panel; a plurality of electrical sensors electrically coupled to the magnetic coil and configured to output time-varying electrical data of the magnetic coil; and an electronic control module in communication with the magnetic coil and the plurality of electrical sensors. The electronic control module is configured to perform operations including: providing current to the magnetic coil; receiving the time-varying electrical data of the magnetic coil from the plurality of electrical sensors; determining the electrical energy provided to the magnetic coil between a first time and a second time based on the time-varying electrical data of the magnetic coil; accessing a thermal model of the panel; and determining a change in panel temperature between the first time and the second time based on the electrical energy provided to the magnetic coil and the thermal model of the panel.

[0010] The foregoing and other embodiments may each optionally include one or more of the following features, alone or in combination. In some embodiments, the time-varying electrical data includes one or more of the following: a time-varying current through the magnetic coil; and a time-varying voltage across the magnetic coil.

[0011] In some embodiments, the thermal model of the panel includes one or more of the following: data representing heat transfer from the magnetic coil to the panel; and data representing heat transfer from the panel to the environment.

[0012] In some embodiments, the thermal model of the panel includes a correlation curve between the electrical energy provided to the magnetic coil and the change in panel temperature.

[0013] In some embodiments, determining the electrical energy provided to the magnetic coil includes: determining a time-varying power provided to the magnetic coil based on the time-varying electrical data of the magnetic coil; and integrating the time-varying power between the first time and the second time.

[0014] In some embodiments, the operations further include: determining a first panel temperature at the first time based on the time-varying electrical data of the magnetic coil and the thermal model of the panel; determining a second panel temperature at the second time based on the first panel temperature and the change in panel temperature between the first time and the second time; and adjusting the current provided to the magnetic coil based on the temperature of the second panel.

[0015] In some embodiments, the operation further includes: determining a rate of change of the panel temperature from the change of the panel temperature between a first time and a second time; and adjusting the current supplied to the magnetic coil based on the rate of change of the panel temperature.

[0016] In some embodiments, the electronic control module includes one or more of an audio signal source, an amplifier, and a digital signal processor.

[0017] In some embodiments, the panel includes a display panel.

[0018] Generally, an innovative aspect of the subject matter described in this specification can be embodied in a mobile device that includes a housing and a panel audio speaker.

[0019] In some embodiments, the mobile device includes a mobile phone or a tablet computer.

[0020] Generally, an innovative aspect of the subject matter described in this specification can be embodied in a wearable device that includes a housing and a panel audio speaker.

[0021] In some embodiments, the wearable device is a smart watch or a head-mounted display.

[0022] Generally, an innovative aspect of the subject matter described in this specification can be embodied in a method that includes: supplying current to a magnetic coil of an actuator to cause vibration of a panel, the magnetic coil being in thermal communication with the panel; receiving time-varying electrical data of the magnetic coil from a plurality of electrical sensors electrically coupled to the magnetic coil; determining electrical energy supplied to the magnetic coil between a first time and a second time based on the time-varying electrical data of the magnetic coil; accessing a thermal model of the panel; and determining a change in panel temperature between the first time and the second time based on the electrical energy supplied to the magnetic coil and the thermal model of the panel.

[0023] The foregoing and other embodiments may each optionally include one or more of the following features, alone or in combination. In some embodiments, the time-varying electrical data includes one or more of the following: a time-varying current through the magnetic coil; and a time-varying voltage across the magnetic coil.

[0024] In some embodiments, the thermal model of the panel includes one or more of the following: data representing heat transfer from the magnetic coil to the panel; and data representing heat transfer from the panel to the environment.

[0025] In some embodiments, determining the electrical energy supplied to the magnetic coil includes: determining a time-varying power supplied to the magnetic coil based on the time-varying electrical data of the magnetic coil; and integrating the time-varying power between the first time and the second time.

[0026] In some embodiments, the method further includes: determining a first panel temperature at the first time based on the time-varying electrical data of the magnetic coil and the thermal model of the panel; determining a second panel temperature at the second time based on the first panel temperature and the change in the panel temperature between the first time and the second time; and adjusting the current supplied to the magnetic coil based on the second panel temperature.

[0027] In some embodiments, the method further includes: determining a rate of change of the panel temperature from the change in the panel temperature between the first time and the second time; and adjusting the current supplied to the magnetic coil based on the rate of change of the panel temperature.

[0028] In some embodiments, the method further includes: adjusting the current supplied to the magnetic coil includes reducing the current supplied to the magnetic coil.

[0029] Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a perspective view of an embodiment of a mobile device.

[0031] Figure 2 is Figure 1 a schematic cross-sectional view of the mobile device.

[0032] Figure 3 is a block diagram of an example system configured to monitor the panel temperature in a panel audio device.

[0033] Figure 4 is Figure 3 a block diagram of an example processor of the example system.

[0034] Figure 5A 、 5B and 5C illustrate example graphs and curves for panel audio temperature monitoring.

[0035] Figure 6 is a flowchart of an example process for monitoring the temperature of a panel in a panel audio device.

[0036] Figure 7Schematic diagram of an embodiment of an electronic control module for a mobile device.

[0037] Like reference numerals in the figures denote like elements. Detailed Description

[0038] Generally, an actuator module can be used in a variety of applications. For example, in some embodiments, the actuator module can be used to drive a panel of a panel audio speaker such as a Distributed Mode Loudspeaker (DML). Such a speaker can be integrated into a mobile device such as a mobile phone, a smartwatch, or a head-mounted display. For example, referring to Figure 1 , the mobile device 100 includes a device chassis 102 and a panel 104, and the panel 104 includes a flat panel display (e.g., an OLED or LCD display panel) integrated with a panel audio speaker. The mobile device 100 interacts with a user in a variety of ways, including by displaying images and receiving touch inputs via the panel 104. Generally, the mobile device has a depth (in the z direction) of about 10 mm or less, a width (in the x direction) of 60 mm to 80 mm (e.g., 68 mm to 72 mm), and a height (in the y direction) of 100 mm to 160 mm (e.g., 138 mm to 144 mm). A Cartesian coordinate system is shown for reference in Figure 1 .

[0039] The mobile device 100 also produces an audio output. The panel audio speaker is used to generate the audio output, and the panel audio speaker creates sound by vibrating the flat panel display. The display panel is coupled to an actuator such as a Distributed Mode Actuator or DMA. The actuator is a movable component arranged to provide a force to a panel (such as panel 104) to cause the panel to vibrate. The vibrating panel generates audible sound waves, e.g., in the range of 20 Hz to 20 kHz.

[0040] Generally, the efficiency of the actuator in generating audible sound waves varies according to the frequency depending on the nature of the actuator, the panel, and the coupling between the actuator and the panel. Generally, the actuator / panel system will exhibit one or more resonance frequencies, which represent the frequencies at which the sound pressure level has local maxima according to the frequency. However, it is generally desirable for the panel audio speaker to maintain a relatively high sound pressure level across the entire audio spectrum.

[0041] In addition to generating a sound output, the mobile device 100 can also use the actuator to generate a haptic output. For example, the haptic output can correspond to vibrations in the range of 180 Hz to 300 Hz.

[0042] Figure 1 Also shown are dashed lines corresponding to the Figure 2 indicated cross-sectional direction. Referring to Figure 2, A cross-sectional view of the mobile device 100 shows the device housing 102 and the panel 104. For ease of reference, Figure 2 also includes a Cartesian coordinate system with x, y, and z axes. The device housing 102 has a depth measured along the z direction and a width measured along the x direction. The device housing 102 also has a back panel formed by a portion of the device housing 102 that mainly extends in the x-y plane. The mobile device 100 includes an actuator module 200 that is housed behind the panel 104 in the housing 102 and attached to the back of the panel 104. A pressure-sensitive adhesive (PSA) 240 can attach the actuator module 200 to the panel 104. Generally, the actuator module 200 is sized to fit within a volume constrained by other components in the housing, which include an electronic control module 220 and a battery 230.

[0043] The actuator module 200 can be configured to convert electrical energy into acoustic energy. The actuator module 200 can be controlled by the electronic control module 220. The electronic control module 220 can consist of one or more electronic components that receive input from one or more sensors and / or signal receivers of the mobile device 100, process the input, and generate and deliver a signal waveform that causes the actuator module 200 to provide an appropriate haptic response. The electronic control module 220 can communicate with a magnetic coil 210.

[0044] Reference Figure 2 , the actuator module 200 includes a magnetic coil 210 and a PSA 240. The PSA 240 allows the actuator module 200 to be fixed to the panel 104. The actuator module 200 can be relatively compact. For example, the height of the actuator module (i.e., its dimension in the z direction) can be about 10 millimeters or less (e.g., 8 millimeters or less, 6 millimeters or less, 5 millimeters or less).

[0045] During operation, the electronic control module 220 energizes the magnetic coil 210 by applying a current to the magnetic coil 210. The resulting magnetic flux interacts with the suspended magnet, and the resulting vibration is transmitted to the panel 104.

[0046] The magnetic coil 210 can be constructed using a fine wire suspended within a magnetic field generated by a magnet. When an analog signal, which can be an input voltage signal, passes through the magnetic coil 210, an electromagnetic field is generated. The electromagnetic field signal strength is determined by the current flowing through the coil.

[0047] The magnetic coil 210 is attached to the surface of the panel 104, and the surface of the panel 104 also moves in concert. The magnetic coil 210 can be fixed to the surface of the panel 104 by an adhesive (such as a pressure-sensitive adhesive, a liquid adhesive, etc.). The movement of the panel causes a disturbance in the air around it, thereby generating sound. In the case where the input signal is a sine wave, the panel 104 will pulsate (e.g., in and out), which pushes air when it moves and generates an audible sound representing the signal frequency. The intensity of the movement of the panel 104 and the pushing of the surrounding air, and thus the speed, can be determined at least in part based on the input signal applied to the magnetic coil 210.

[0048] The magnetic coil 210 can be in thermal communication with the panel 104. When in thermal communication with the panel, heat can flow or transfer from the magnetic coil 210 to the panel 104, and from the panel 104 to the magnetic coil 210. For example, when the electronic control module 220 drives the magnetic coil 210, current flows through the magnetic coil 210, heating the magnetic coil 210. Then the heat from the magnetic coil 210 can be transferred to the panel 104.

[0049] During the operation of the actuator module 200, the magnetic coil temperature may increase, resulting in an increase in the panel temperature as well. As the panel 104 receives heat from the magnetic coil 210, the panel 104 may also dissipate heat to the environment. Therefore, during operation, the panel temperature may rise at a slower rate of change than the magnetic coil temperature, and the panel temperature may remain lower than the magnetic coil temperature.

[0050] When the actuator module 200 is not operating, the magnetic coil temperature may decrease, causing the panel temperature to decrease as well. During an extended period when the actuator module 200 is not operating, the magnetic coil 210 and the panel 104 can reach thermal equilibrium. Therefore, when no current passes through the magnetic coil 210 for an extended period, the magnetic coil 210 and the panel 104 can reach the same temperature.

[0051] Figure 3 FIG. is an example system 300 configured to monitor the temperature of the panel in a panel audio device. The system 300 includes an electronic control module 220, a magnetic coil 210, and a panel 104. The electronic control module 220 includes a signal generator 340, a processor 310, a digital-to-analog converter (DAC) 330, an amplifier 360, a current sensor 312, a current analog-to-digital converter (ADC) 316, a voltage sensor 314, a voltage ADC 318, a clock 320, and a memory 350 that can store a panel thermal model 352 and an initial panel temperature 354.

[0052] Although in Figure 3A specific configuration of system 300 is shown, but other configurations are also possible. For example, in some embodiments, certain components may not be included in the illustrated electronic control module 220. For example, the signal generator 340, the clock 320, and / or the amplifier 360 may not be included in the electronic control module 220. In some embodiments, certain components may be combined into a single component. For example, the amplifier 360 may include the processor 310, the DAC 330, or both. In some examples, the processor 310 may include the memory 350, the DAC 330, the current ADC 316, the voltage ADC 318, or all of these.

[0053] Generally, the operation of system 300 is as follows. The magnetic coil 210 may communicate with the electronic control module 220, for example, via a wired or wireless connection. The magnetic coil 210 may receive an electrical signal that has been output from the amplifier 360 as an input. When the electrical signal is applied to the magnetic coil 210, the magnetic coil temperature may increase, and in turn, the panel temperature may also increase.

[0054] The electrical sensors may measure the time-varying electrical data of the magnetic coil 210. For example, the current sensor 312 may measure the time-varying current passing through the magnetic coil 210, and the voltage sensor 314 may measure the time-varying voltage across the magnetic coil 210. The processor 310 may determine the amount of energy supplied to the magnetic coil 210 over a period of time based on the measured coil current and coil voltage. Based on the supplied energy, the panel thermal model 352, and the initial panel temperature 354, the processor 310 may determine the final temperature of the panel 104. Based on the final temperature of the panel 104, the processor 310 may determine to adjust the electrical signal provided to the magnetic coil 210.

[0055] The signal generator 340 may be an audio signal source that generates an audio signal. For example, the signal generator may generate a digital audio signal that represents the audible sound to be produced by the panel 104.

[0056] The processor 310 may be, for example, a digital signal processor (DSP). The processor 310 may receive the audio signal from the signal generator 340. The processor 310 may process the audio signal, for example, by decoding, filtering, decompressing, transforming, and modulating the audio signal. In some examples, the processor 310 may adjust the audio signal by increasing or decreasing the power level of the audio signal. The processor 310 may output the adjusted digital audio signal to the DAC 330.

[0057] The DAC 330 may convert the digital audio signal into an analog electrical signal. For example, the analog electrical signal may be an alternating current (AC) electrical signal. The DAC 330 may output the analog electrical signal to the amplifier 360.

[0058] Amplifier 360 can amplify an analog electrical signal. For example, amplifier 360 can amplify an analog electrical signal by increasing the voltage, current, or power of the analog signal. Amplifier 360 can output the amplified electrical signal to magnetic coil 210.

[0059] Magnetic coil 210 is excited by the amplified electrical signal output by amplifier 360. As current from the amplified electrical signal flows through magnetic coil 210, the temperature of magnetic coil 210 may increase. Panel 104 in thermal communication with magnetic coil 210 can receive the heat transferred from magnetic coil 210, resulting in an increase in the panel temperature.

[0060] Current sensor 312 can measure the current flowing through magnetic coil 210. Current sensor 312 can be any suitable type of current sensor. For example, current sensor 312 can be a fluxgate, Hall effect, or inductive current sensor. Current sensor 312 can output an analog signal representing the measured current to current ADC 316. Current ADC 316 can convert the analog signal representing the measured coil current into a digital current signal. Current ADC 316 can output the coil current to processor 310.

[0061] In some examples, current sensor 312 can output a digital signal representing the measured coil current. In these examples, system 300 may not include current ADC 316, and current sensor 312 can provide the coil current directly to processor 310.

[0062] Voltage sensor 314 can measure the voltage across magnetic coil 210. Voltage sensor 314 can be any suitable type of voltage sensor. For example, voltage sensor 314 can be a resistive or capacitive voltage sensor. Voltage sensor 314 can output an analog signal representing the measured voltage to voltage ADC 318. Voltage ADC 318 can convert the analog signal representing the measured voltage into a digital voltage signal. Voltage ADC 318 can output the coil voltage to processor 310.

[0063] In some examples, voltage sensor 314 can output a digital signal representing the measured coil voltage. In these examples, system 300 may not include voltage ADC 318, and voltage sensor 314 can provide the coil voltage directly to processor 310.

[0064] Processor 310 can receive the coil current and coil voltage from current ADC 316 and voltage ADC 318. Processor 310 can determine the panel temperature based on the coil current and coil voltage. Reference Figure 4 describes determining the panel temperature based on the coil current and coil voltage.

[0065] Reference Figure 4, the processor 310 includes a power calculator 410, an energy calculator 420, a temperature change calculator 430, a panel temperature calculator 440, a panel temperature limiter 450, and a signal adjuster 460. The processor 310 may also optionally include a temperature change rate calculator 470.

[0066] The power calculator 410 of the processor 310 can receive the time-varying coil current 404 and the time-varying coil voltage 402 from the current ADC 316 and the voltage ADC 318 respectively. For example, the coil current 404 can be indicated in amperes (A). For example, the coil voltage can be indicated in volts (V). Based on the coil current 404 and the coil voltage 402, the power calculator 410 can calculate the power 412 of the magnetic coil 210. Specifically, the power calculator 410 can multiply the coil current 404 and the coil voltage 402 at a specific time to calculate the power 412 at that specific time. The power calculator 410 can continuously calculate the time-varying power 412. The power 412 can be indicated in watts (W), for example. Figure 5A An example graph of the time-varying power 412 is shown.

[0067] Reference Figure 5A , the power 412 can be represented on the graph according to time. Generally, when the actuator module 200 is in operation, the power 412 can increase, decrease, or remain stable over time. For example, due to changes in the audio volume (e.g., music or voice volume), the audio signal may increase and decrease in power over time.

[0068] At Figure 5A , a graph of the power 412 is plotted over a time period including a first time 510 and a second time 520. The first time 510 can be, for example, a time period shortly after initially exciting the magnetic coil 210. The second time 520 can be a time later than the first time 510.

[0069] The energy calculator 420 of the processor 310 can receive the time-varying power 412 from the power calculator 410. The energy calculator 420 can also receive the clock time 424 from the clock 320. Based on the time-varying power 412, the energy calculator 420 can calculate the energy 422 supplied to the magnetic coil 210. Specifically, as Figure 5A shown, the energy calculator 420 can integrate the time-varying power 412 between the first time 510 and the second time 520 to determine the total energy 422 supplied between the first time 510 and the second time 520.

[0070] At Figure 5AIn this case, the energy 422 is represented by the area under the curve representing the time-varying power 412. The energy 422 can be indicated, for example, in joules (J). Generally, a higher power level maintained over a longer period results in a larger area under the curve and thus a larger amount of energy supplied to the magnetic coil. The energy calculator 420 can output the energy 422 to the temperature change calculator 430.

[0071] The temperature change calculator 430 can receive the energy 422 from the energy calculator 420 and receive the panel thermal model 352 from the memory 350. In some examples, the panel thermal model 352 can be an experimental model. For example, experiments can be performed on the panel 104 or a similar panel to determine the panel temperature performance in response to the excitation of the magnetic coil 210. The experiments can include exciting the magnetic coil 210 at known power levels for different durations and measuring the resulting temperature of the panel 104. The resulting temperature of the panel 104 can be measured, for example, directly using a temperature sensor or indirectly based on the resistance of the magnetic coil. In some examples, the electronic control module can determine that the magnetic coil 210 and the panel are likely at approximately the same temperature.

[0072] Figure 5B An example temperature characteristic curve of the panel 104 is shown. This characteristic curve can be generated by exciting the magnetic coil 210 with an audio signal at a stable known power for a period of time and then turning off the audio signal. As Figure 5B shown, the panel temperature 540 and the actuator temperature 550 can be represented on the graph according to time. For example, the temperature change 432 can be indicated in degrees Celsius (°C). At time 542, the audio signal is turned on and the magnetic coil 210 is excited at a constant power. At time 544, the audio signal is turned off.

[0073] Between time 542 and time 544, the actuator temperature 550 and the panel temperature 540 increase. For example, the actuator temperature 550 increases due to being excited by the audio signal, and the panel temperature 540 increases due to heat transfer from the magnetic coil 210. The actuator temperature 550 can change temperature more rapidly than the panel temperature 540 because the magnetic coil 210 has a lower thermal mass than the panel 104. After time 544, when the audio signal is turned off, the actuator temperature 550 and the panel temperature 540 decrease.

[0074] Temperature characteristic curves can be generated for various power levels and durations. The processor 310 can obtain the temperature change rate at a given temperature for a specific power level from the temperature characteristic curve. For example, the temperature characteristic curve can indicate that the panel temperature 540 changes 1 degree Celsius (°C) per watt per minute at an initial temperature of 35 °C. The temperature characteristic curve of the panel 104 can be generated by experiment and stored in the memory 350.

[0075] In some examples, the panel thermal model 352 can be a mathematical model. For example, the panel thermal model 352 can include data representing heat transfer from the magnetic coil 210 to the panel 104, data representing heat transfer from the panel 104 to the environment, or both. The panel thermal model 352 can also include a model of the panel temperature behavior in response to excitation of the magnetic coil 210. The data can take into account factors such as the specific heat capacity of the panel 104, the contact surface area between the magnetic coil 210 and the panel 104, and the total surface area of the panel 104. The data can also take into account factors such as ambient temperature variations, panel vibration frequency variations, and excitation continuity.

[0076] In some examples, the panel thermal model 352 can be a mathematical model that can be updated and verified experimentally. For example, the panel thermal model 352 can be generated mathematically for predicted panel temperatures. Experiments can then be performed on the panel 104 or a similar panel to verify and / or update the panel thermal model 352. The experiments can include exciting the magnetic coil 210 at a stable power level for different durations and generating a temperature characteristic curve as Figure 5B shown. The resulting temperature of the panel 104 can be provided as feedback to the panel thermal model 352 to update the mathematical model.

[0077] In some examples, the panel thermal model 352 can be calibrated during a calibration phase of operation. For example, a preliminary thermal model can be programmed into the memory 350. During the calibration phase, the electronic control module 220 can excite the magnetic coil 210 at a known power level and can measure the panel temperature. The panel thermal model 352 can then be updated based on the panel temperature measured during the calibration phase.

[0078] In some examples, instead of or in addition to the calibration phase, the panel thermal model 352 can continue to be updated during operation. For example, an audio signal can be applied to the magnetic coil 210 between a first time and a second time. When the audio signal is applied to the magnetic coil 210, the actuator temperature 550 and the panel temperature 540 increase.

[0079] During the duration between the second time and a third time, the audio signal can be turned off or can be reduced to a lower power such that the heat from the actuator no longer causes the panel temperature 540 to increase. The duration can be equal to or longer than a threshold duration at which the actuator temperature 550 becomes approximately equal to the panel temperature 540. The processor 310 can then measure the resistance of the magnetic coil 210 to determine the actuator temperature 550 and, thus, the measured panel temperature.

[0080] The processor 310 may also determine a calculated panel temperature at a third time based on the panel thermal model 352. The processor 310 may then compare the measured panel temperature based on the actuator temperature 550 with the calculated panel temperature based on the thermal model. The processor 310 may calculate an error between the measured panel temperature and the calculated panel temperature. The processor 310 may provide the error as feedback to adjust one or more variables of the panel thermal model 532.

[0081] In an example, the initial panel temperature 354 is 33 °C. An audio signal is applied to the magnetic coil 210 between a first time T1 and a second time T2. At time T2, the audio signal is turned off and remains off until time T3. The duration between T2 and T3 is a duration longer than a threshold duration during which the actuator temperature 550 becomes approximately equal to the panel temperature 540.

[0082] The processor 310 measures the resistance of the magnetic coil 210 at time T3. Based on the resistance, the processor determines that the actuator temperature is 40 °C and thus determines that the measured panel temperature is 40 °C. The processor 310 determines a calculated panel temperature of 42 °C based on the panel thermal model 352. The processor 310 calculates an error of 2 °C. The processor 310 provides the error as feedback to adjust the panel thermal model 532.

[0083] At a third time, the audio signal may be applied to the magnetic coil 210 again. The processor 310 may use the measured temperature at the third time as the initial panel temperature 354 for the next calculation of the final panel temperature 442. In the above example, the measured panel temperature of 40 °C may be used as the initial panel temperature 354 for the next calculation of the final panel temperature 44 - e.g., the panel temperature at a fourth time T4.

[0084] In some examples, the panel thermal model 352 may include a panel thermal model curve 530 that represents the association between energy and panel temperature change, as Figure 5C shown. The panel thermal model curve 530 may be generated based on a temperature characteristic curve as Figure 5B shown. For example, the temperature characteristic curve may provide the rate of temperature change at a specific power level at a given temperature. From multiple temperature characteristic curves, the rate of temperature change for a certain amount of energy may be determined. In some examples, multiple panel thermal model curves 530 may be generated for multiple initial temperatures.

[0085] Referring to Figure 5C the panel thermal model curve 530 may be represented on a graph according to energy. Generally, the panel temperature change increases with an increase in energy. For example, for a larger amount of energy supplied to the magnetic coil 210, the panel temperature may change by a greater amount. Although Figure 5CA curve with an approximately logarithmic shape is shown. The shape of the panel thermal model curve 530 can vary according to the characteristics of the panel. The shape of the panel thermal model curve 530 can be, for example, linear, exponential, or parabolic.

[0086] The panel thermal model 352 can be programmed into the memory 350. The processor 310 can then access the panel thermal model 352 from the memory 350 to determine the panel temperature change 430.

[0087] Using the panel thermal model curve 530, the temperature change calculator 430 can calculate the panel temperature change associated with the energy 422. In Figure 5C the example, the temperature change calculator 430 calculates the panel temperature change 432. The panel temperature change 432 represents the change in the panel temperature between the first time 510 and the second time 520. The temperature change calculator 430 can output the panel temperature change 432 to the panel temperature calculator 440.

[0088] The panel temperature calculator 440 can receive the panel temperature change 432 from the temperature change calculator 430 and the initial or first panel temperature 354 from the memory 350. The initial panel temperature 354 can be the panel temperature at the first time 510. In some examples, at or before the first time 510, the processor 310 can determine the initial panel temperature 354 and store the initial panel temperature 354 in the memory 350.

[0089] In some examples, the processor 310 can determine the initial panel temperature 354 based on the magnetic coil temperature at the first time 510. In some examples, the panel thermal model 352 can include an association between the panel temperature and the magnetic coil temperature.

[0090] In some examples, the processor 310 can determine that the panel temperature may be the same as the magnetic coil temperature. For example, based on the panel thermal model 352, the processor 310 can determine that when the magnetic coil 210 is not energized for a specific duration, the panel 104 reaches the same or approximately the same temperature as the magnetic coil temperature. Therefore, the processor 310 can determine that the initial panel temperature 354 is the same as the magnetic coil temperature based on the magnetic coil 210 not being energized for a specific duration.

[0091] In some examples, the first time 510 can be a time shortly after initially energizing the magnetic coil 210 after a specific duration in which the magnetic coil 210 was not energized. In these examples, the processor 310 can determine that the initial panel temperature 354 at the first time 510 is the same as the magnetic coil temperature at the first time 510.

[0092] In some examples, the processor 310 may determine the magnetic coil temperature based on the resistance of the magnetic coil 210 during operation of the actuator module 200. For example, the processor 310 may determine the resistance of the magnetic coil 210 based on the coil current 404 and the coil voltage 402 when the actuator module 200 is operating. The magnetic coil 210 may have a known resistance temperature coefficient. Thus, based on the resistance of the magnetic coil 210, the processor 310 may determine the magnetic coil temperature. Based on the correlation between the magnetic coil temperature and the panel temperature, the processor 310 may determine the initial panel temperature 354.

[0093] In some examples, the processor 310 may determine the magnetic coil temperature based on the resistance of the magnetic coil 210 when the actuator module 200 is not operating. For example, the processor 310 may provide a pilot tone to the magnetic coil 210. For example, the pilot tone may be a low amplitude and / or low frequency tone that does not cause the panel 104 to produce an audible sound. The processor 310 may measure the coil current 404 and the coil voltage 402 when exciting the magnetic coil 210 with the pilot tone. Based on the resistance of the magnetic coil 210 and the known resistance temperature coefficient, the processor 310 may determine the magnetic coil temperature. Based on the correlation between the magnetic coil temperature and the panel temperature, the processor 310 may determine the initial panel temperature 354.

[0094] In some examples, the initial panel temperature 354 may be the previously calculated final panel temperature 442. For example, the processor 310 may determine the final panel temperature 442 at the second time 520 based on the energy 422 and the panel thermal model 352. The processor 310 may store the final panel temperature 442 at the second time 520 in the memory 350 for later reference as the initial panel temperature 354 in a new calculation of the final panel temperature 442.

[0095] Based on the initial panel temperature 354 and the panel temperature change 432, the panel temperature calculator 440 may calculate the final or second panel temperature 442. The final panel temperature 442 may be the temperature of the panel 104 at the second time 520. The panel temperature calculator 440 may calculate the final panel temperature 442, for example, by adding the panel temperature change 432 to the initial panel temperature 354. The panel temperature calculator 440 may output the final panel temperature 442 to the panel temperature limiter 450.

[0096] The panel temperature limiter 450 may compare the final panel temperature 442 with a threshold panel temperature. For example, the threshold may be the maximum allowable panel temperature. In some examples, the threshold panel temperature may be a panel temperature within a buffer range of the maximum allowable panel temperature. For example, the maximum allowable panel temperature may be 45 °C. To provide a 5 °C buffer range, the threshold panel temperature may be set to 40 °C.

[0097] In some embodiments, in addition to the panel temperature calculator 440, the temperature change calculator 430 may also output the panel temperature change 432 to the temperature change rate calculator 470. Based on the panel temperature change 432 and the duration between the first time 510 and the second time 520, the temperature change rate calculator 470 may determine the panel temperature change rate 434. The panel temperature change rate 434 may be indicated, for example, in degrees Celsius per minute (°C / min). The temperature change rate calculator 470 may output the temperature change rate 434 to the panel temperature limiter 450.

[0098] The panel temperature limiter 450 may compare the temperature change rate 434 with a threshold temperature change rate. For example, the threshold may be the maximum allowable temperature change rate. The threshold change rate may vary based on the final panel temperature 442. For example, at a final panel temperature of 35 °C, the threshold change rate may be set to +2 °C / min. At a final panel temperature of 38 °C, the threshold change rate may be set to +1 °C / min. Thus, as the final panel temperature 442 increases, approaching the threshold panel temperature, the threshold change rate may decrease.

[0099] Based on determining that the final panel temperature 442 exceeds the threshold panel temperature, the temperature change rate 434 exceeds the threshold temperature change rate, or both, the panel temperature limiter 450 may determine to output a signal adjustment 452 to the signal adjuster 460. For example, the signal adjustment may be a mathematical function to be applied to the audio signal 458 in order to generate an adjusted audio signal 462.

[0100] The panel temperature limiter 450 may be programmed with rules for determining the signal adjustment 452 for various final panel temperatures 442 and temperature change rates 434. For example, the rule may specify that when the final panel temperature 442 exceeds the threshold panel temperature, the signal adjustment 452 includes a function of reducing the audio signal power by a divisor of 2. In another example, the rule may specify that when the temperature change rate 434 exceeds the threshold temperature change rate, the signal adjustment 452 includes a function of reducing the audio signal power by a factor of one-third. In another example, the rule may specify that when the final panel temperature 442 exceeds the threshold panel temperature, the signal adjustment 452 includes turning off the audio signal.

[0101] In some examples, the panel temperature limiter 450 may be programmed to output the signal adjustment 452 for a specified period of time. For example, in response to determining that the final panel temperature 442 exceeds the threshold panel temperature, the panel temperature limiter 450 may determine to output a signal adjustment 452 that reduces the audio signal power by one-half for a period of one minute. In some examples, after a period of one minute, the panel temperature limiter 450 may automatically remove the signal adjustment 452.

[0102] In some examples, the panel temperature limiter 450 can output signal adjustments that are applied only to certain frequencies of the audio signal 458. In some examples, the panel temperature limiter 450 can output multiple signal adjustments that are applied to multiple frequency ranges of the audio signal 458. For example, the panel temperature limiter 450 can output a first signal adjustment that is applied to a first frequency range of the audio signal 458, and a second signal adjustment that is applied to a second frequency range of the audio signal 458.

[0103] In some examples, the panel temperature limiter 450 can determine to remove the signal adjustment 452. For example, the panel temperature limiter 450 may have previously determined to apply the signal adjustment 452 to the audio signal 458. The panel temperature limiter 450 can continue to monitor the final panel temperature 442 and / or the rate of temperature change 434. When the final panel temperature 442, the rate of temperature change 434, or both return below the programmed threshold, the panel temperature limiter 450 can determine to remove the previously applied signal adjustment 452.

[0104] The signal adjuster 460 receives the audio signal 458 from the signal generator 340, and the signal adjustment 452 from the panel temperature limiter 450. The signal adjuster 460 can apply the signal adjustment 452 to the audio signal 458. For example, for a signal adjustment of reducing by one-half, the signal adjuster 460 can reduce the power of the audio signal 458 by one-half. The signal adjuster 460 outputs the adjusted audio signal 462 to the magnetic coil 210.

[0105] In some examples, instead of or in addition to the processor 310 applying the signal adjustment 452 to the audio signal 458, the processor 310 can send a command to the amplifier 360 to adjust the amplification. For example, the processor 310 can send a command to the amplifier 360 to reduce the amplification of an analog electrical signal by, for example, one-half. The amplifier 360 can then reduce the amplification of the analog electrical signal for a specified period of time, or until a subsequent command is received from the processor 310 to stop reducing the amplification.

[0106] When the power of the audio signal is reduced, the current through the magnetic coil 210 is reduced. Due to the reduced current, the magnetic coil 210 can then increase temperature at a slower rate, stop increasing temperature, or decrease temperature. Due to the thermal communication between the magnetic coil 210 and the panel 104, the panel 104 can similarly increase temperature at a slower rate, stop increasing temperature, or decrease temperature. The processor 310 can continue to monitor the coil current 404 and the coil voltage 402 in order to recalculate the change in panel temperature.

[0107] Figure 6 is a flowchart of an example process 600 for monitoring the temperature of a panel in a panel audio device. The process 600 can be performed, for example, by the electronic control module 220.

[0108] In short, process 600 includes providing current to a magnetic coil of an actuator to cause vibration of a panel, the magnetic coil being in thermal communication with the panel (602), receiving time-varying electrical data of the magnetic coil from a plurality of electrical sensors electrically coupled to the magnetic coil (604), determining electrical energy supplied to the magnetic coil between a first time and a second time based on the time-varying electrical data of the magnetic coil (606), accessing a thermal model of the panel (608), determining a change in the panel temperature between the first time and the second time based on the electrical energy supplied to the magnetic coil and the thermal model of the panel (610), determining a first panel temperature at the first time (612), determining a second panel temperature at the second time based on the first panel temperature and the change in the panel temperature between the first time and the second time (614), and adjusting the current supplied to the magnetic coil based on the second panel temperature (616).

[0109] More specifically, process 600 includes providing current to a magnetic coil of an actuator to cause vibration of a panel, the magnetic coil being in thermal communication with the panel (602). For example, the magnetic coil may be attached to the surface of the panel such that when the temperature of the magnetic coil increases, the panel temperature may also increase.

[0110] Process 600 includes receiving time-varying electrical data of the magnetic coil from a plurality of electrical sensors electrically coupled to the magnetic coil (604). For example, the time-varying electrical data may include a time-varying current through the magnetic coil and a time-varying voltage across the magnetic coil.

[0111] Process 600 includes: determining electrical energy supplied to the magnetic coil between a first time and a second time based on the time-varying electrical data of the magnetic coil (606). For example, the first time may be a time of zero seconds. For example, the second time may be a time of sixty seconds. Based on the current data and voltage data of the magnetic coil, an electronic control module may determine the time-varying power supplied to the magnetic coil. For example, the time-varying power may include a steady power of 10 W supplied to the magnetic coil over a duration of sixty seconds. Based on this power, the electronic control module may determine the electrical energy supplied to the magnetic coil. For example, based on a power of 10 W for 60 seconds, the electronic control module may determine 600 joules of electrical energy.

[0112] Process 600 includes accessing a thermal model of the panel (608). For example, the thermal model of the panel may be accessed from the memory of the electronic control module. The thermal model of the panel may include a correlation curve between the energy supplied to the magnetic coil and the change in the panel temperature. The thermal model of the panel may include data representing heat transfer from the magnetic coil to the panel and from the panel to the environment. The thermal model of the panel may be generated using, for example, mathematical calculations, experimental results, or both. The thermal model of the panel may be generated before operation, during a calibration phase, during operation, or a combination of these.

[0113] Process 600 includes: determining a change in panel temperature between a first time and a second time (610) based on the electrical energy provided to the magnetic coil and a thermal model of the panel. For example, based on 600 joules of electrical energy and a panel thermal model that correlates the energy with a change in temperature, an electronic control module can determine a +3 °C change in panel temperature between zero seconds and sixty seconds.

[0114] Process 600 includes determining a first panel temperature at a first time (612). The first panel temperature can be stored in the memory of the electronic control module. The first panel temperature can be based on measurements taken when applying a pilot tone to the magnetic coil. For example, the first panel temperature can be 38 °C.

[0115] Process 600 includes determining a second panel temperature at a second time (614) based on the first panel temperature and the change in panel temperature between the first time and the second time. For example, based on a first panel temperature of 38 °C and a +3 °C change in panel temperature, the electronic control module can determine a second panel temperature of 41 °C.

[0116] Process 600 includes: adjusting the current provided to the magnetic coil based on the second panel temperature (616). For example, the electronic control module can compare the second panel temperature to a threshold panel temperature. For example, the threshold panel temperature can be 40 °C. The electronic control module can determine that the second panel temperature of 41 °C exceeds the threshold panel temperature of 40 °C. In response to determining that the second panel temperature exceeds the threshold panel temperature, the electronic control module can determine to adjust the current provided to the magnetic coil. For example, the electronic control module can determine to adjust the current by reducing the current (e.g., reducing it by a factor of one-half or one-third).

[0117] Reference Figure 7 , an exemplary electronic control module 220 of a mobile device such as mobile device 100 includes a processor 310, a memory 350, a display driver 730, a signal generator 340, an input / output (I / O) module 750, and a network / communication module 760. These components communicate electrically with each other (e.g., via signal bus 702) and communicate electrically with the actuator module 200.

[0118] The processor 310 can be implemented as any electronic device capable of processing, receiving, or transmitting data or instructions. For example, the processor 310 can be a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or a combination of these devices.

[0119] The memory 350 has various instructions, computer programs, or other data stored thereon. The instructions or computer programs may be configured to perform one or more of the operations or functions described with respect to the mobile device. For example, the instructions may be configured to control and coordinate the following operations: the display of the display via the display driver 730, the signal generator 340, one or more components of the I / O module 750, one or more communication channels accessible via the network / communication module 760, one or more sensors (e.g., biometric sensors, temperature sensors, accelerometers, optical sensors, barometric sensors, humidity sensors, etc.), and / or the actuator module 200.

[0120] The signal generator 340 is configured to generate an AC waveform having a varying amplitude, frequency, and / or pulse profile suitable for the actuator module 200 and generating an acoustic and / or tactile response via the actuator. Although described as a separate component, in some embodiments, the signal generator 340 may be part of the processor 310. In some embodiments, the signal generator 340 may include an amplifier, for example, as an integral or separate component.

[0121] The memory 350 may store electronic data that can be used by the mobile device. For example, the memory 350 may store electronic data or content such as audio and video files, documents and applications, device settings and user preferences, timing and control signals or data for various modules, and data structures or databases, etc. The memory 350 may also store instructions for reconstructing various types of waveforms that the signal generator 340 can use to generate signals for the actuator module 200. The memory 350 may be any type of memory, such as random access memory, read-only memory, flash memory, removable memory, or a combination of other types of storage elements or such devices.

[0122] As briefly discussed above, the electronic control module 220 may include Figure 7 the various input and output components represented as the I / O module 750. Although the components of the I / O module 750 are represented as a single item in Figure 7 , the mobile device may include multiple different input components, including buttons, microphones, switches, and dials for receiving user input. In some embodiments, the components of the I / O module 750 may include one or more touch sensors and / or force sensors. For example, the display of the mobile device may include one or more touch sensors and / or one or more force sensors that enable a user to provide input to the mobile device.

[0123] Each component of the I / O module 750 may include dedicated circuitry for generating signals or data. In some cases, the component may generate or provide feedback for application-specific input corresponding to a prompt or user interface object presented on the display.

[0124] As described above, the network / communication module 760 includes one or more communication channels. These communication channels may include one or more wireless interfaces that provide communication between the processor 310 and an external device or other electronic devices. Generally, the communication channels may be configured to transmit and receive data and / or signals that can be interpreted by instructions executed on the processor 310. In some cases, the external device is part of an external communication network configured to exchange data with other devices. Generally, the wireless interface may include, but is not limited to, radio frequency, optical, acoustic, and / or magnetic signals, and may be configured to operate over a wireless interface or protocol. Example wireless interfaces include radio frequency cellular interfaces, fiber optic interfaces, acoustic interfaces, Bluetooth interfaces, near field communication interfaces, infrared interfaces, USB interfaces, Wi-Fi interfaces, TCP / IP interfaces, network communication interfaces, or any conventional communication interface.

[0125] In some embodiments, one or more communication channels of the network / communication module 760 may include a wireless communication channel between the mobile device and another device such as another mobile phone, tablet, or computer. In certain cases, the output, audio output, haptic output, or visual display element may be directly transmitted to another device for output. For example, a sound alert or visual warning may be transmitted from the mobile device 100 to a mobile phone for output on that device, and vice versa. Similarly, the network / communication module 760 may be configured to receive input provided on another device to control the mobile device. For example, a sound alert, visual notification, or haptic alert (or instructions therefor) may be transmitted from an external device to the mobile device for presentation.

[0126] The actuator technology disclosed herein can be used, for example, in a panel audio system designed to provide acoustic and / or haptic feedback. The panel may be a display system, such as an OLED based on LCD technology. The panel may be part of a smartphone, tablet, or wearable device (e.g., a smartwatch or a head-mounted device such as smart glasses).

[0127] Although some embodiments have been described in detail above, other modifications are possible. Additionally, other mechanisms may be used to perform the systems and methods described in this document. Additionally, the logical flow depicted in the figures does not require the particular order or sequence shown to achieve the desired result. Other steps may be provided or steps may be eliminated from the described flow, and other components may be added to or removed from the described system. Accordingly, other embodiments are within the scope of the appended claims.

[0128] Other embodiments are in the appended claims.

Claims

1. A panel audio speaker, comprising: a panel; an actuator attached to a surface of the panel and configured to cause vibration of the panel, the actuator including a magnetic coil in thermal communication with the panel; a plurality of electrical sensors electrically coupled to the magnetic coil and configured to output time-varying electrical data of the magnetic coil; and an electronic control module in communication with the magnetic coil and the plurality of electrical sensors, wherein the electronic control module is configured to perform operations including: providing current to the magnetic coil; receiving the time-varying electrical data of the magnetic coil from the plurality of electrical sensors; determining electrical energy supplied to the magnetic coil between a first time and a second time based on the time-varying electrical data of the magnetic coil; accessing a thermal model of the panel; and determining a change in panel temperature between the first time and the second time based on the electrical energy supplied to the magnetic coil and the thermal model of the panel.

2. The panel audio speaker according to claim 1, wherein the time-varying electrical data includes one or more of the following: a time-varying current passing through the magnetic coil; and a time-varying voltage across the magnetic coil.

3. The panel audio speaker according to claim 1, wherein the thermal model of the panel includes one or more of the following: data representing heat transfer from the magnetic coil to the panel; and data representing heat transfer from the panel to the environment.

4. The panel audio speaker according to claim 1, wherein the thermal model of the panel includes a correlation curve between the electrical energy supplied to the magnetic coil and the change in panel temperature.

5. The panel audio speaker according to claim 1, wherein determining the electrical energy supplied to the magnetic coil includes: determining a time-varying power supplied to the magnetic coil from the time-varying electrical data of the magnetic coil; and integrating the time-varying power between the first time and the second time.

6. The panel audio speaker according to claim 1, wherein the operations further include: determining a first panel temperature at the first time from the time-varying electrical data of the magnetic coil and the thermal model of the panel; determining a second panel temperature at the second time based on the first panel temperature and the change in panel temperature between the first time and the second time; and adjusting the current supplied to the magnetic coil based on the second panel temperature.

7. The panel audio speaker according to claim 1, wherein the operations further include: determining a rate of change of the panel temperature from the change in panel temperature between the first time and the second time; and adjusting the current supplied to the magnetic coil based on the rate of change of the panel temperature.

8. The panel audio speaker according to claim 1, wherein the electronic control module includes one or more of an audio signal source, an amplifier, and a digital signal processor.

9. The panel audio speaker according to any one of claims 1-8, wherein The panel includes a display panel.

10. A mobile device, comprising: a housing; and a panel audio speaker according to any one of claims 1-9.

11. The mobile device according to claim 10, wherein the mobile device includes a mobile phone or a tablet computer.

12. A wearable device, comprising: a housing; and a panel audio speaker according to any one of claims 1-9.

13. The wearable device according to claim 12, wherein the wearable device is a smart watch or a head-mounted display.

14. A method for monitoring and controlling the temperature of a panel, comprising: providing a current to a magnetic coil of an actuator to cause vibration of the panel, the magnetic coil being in thermal communication with the panel; receiving time-varying electrical data of the magnetic coil from a plurality of electrical sensors electrically coupled to the magnetic coil; determining electrical energy supplied to the magnetic coil between a first time and a second time based on the time-varying electrical data of the magnetic coil; accessing a thermal model of the panel; and determining a change in panel temperature between the first time and the second time based on the electrical energy supplied to the magnetic coil and the thermal model of the panel.

15. The method according to claim 14, wherein the time-varying electrical data includes one or more of the following: a time-varying current passing through the magnetic coil; and a time-varying voltage across the magnetic coil.

16. The method according to claim 14, wherein the thermal model of the panel includes one or more of the following: data representing heat transfer from the magnetic coil to the panel; and data representing heat transfer from the panel to the environment.

17. The method according to claim 14, wherein determining the electrical energy supplied to the magnetic coil includes: determining a time-varying power supplied to the magnetic coil from the time-varying electrical data of the magnetic coil; and integrating the time-varying power between the first time and the second time.

18. The method according to claim 14, further comprising: determining a first panel temperature at the first time from the time-varying electrical data of the magnetic coil and the thermal model of the panel; determining a second panel temperature at the second time based on the first panel temperature and the change in panel temperature between the first time and the second time; and adjusting the current supplied to the magnetic coil based on the second panel temperature.

19. The method according to claim 14, further comprising: determining a rate of change of the panel temperature from the change in panel temperature between the first time and the second time; and adjusting the current supplied to the magnetic coil based on the rate of change of the panel temperature.

20. The method according to any one of claims 14-19, wherein adjusting the current supplied to the magnetic coil includes: reducing the current supplied to the magnetic coil.

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