Real-time fine adjustment of the driving current of the light-emitting device of an optical rain sensor

Abstract

The present invention relates to real-time fine adjustment of the current of the light-emitting element of an optical rain sensor using an external adjustment current source. The adjustment current source comprises: a variable current source for providing an output current (I out ), the intensity of which can be adjusted based on an external adjustment signal; a controlled switch connected between a terminal of the light emitting element and an output terminal of the variable current source. The controlled switch is adapted to switch between an on state and an off state in synchronization with a driving current pulse supplied to the light emitting element, so as to turn on and off the variable current source, thereby adjusting the intensity of the variable current source based on the adjustment voltage (V reg ) generates a compensation current signal (Ic), which is superimposed on the driving current pulse passing through the light-emitting element to reduce the imbalance between the first and second light pulses.

Description

Technical Field

[0001] The present invention relates to a rain sensor, and more particularly, to an optical rain sensor for detecting rain and / or water drops using a light emitting device, and to fine current regulation of a driving current of the light emitting device. Background Art

[0002] Optical rain sensors are commonly used as rain-sensitive switches for controlling electrical devices in various applications, such as automobiles and irrigation systems. The detection principle of optical rain sensors is based on detecting part of the infrared light emitted by one or more light-emitting sources and reflected from the inner surface of a transparent substrate, such as a windshield. The light reflected from the windshield is detected by a photodetector and the detected signal is analyzed to determine the change in the intensity of the reflected light. If its outer surface is covered with rain or water droplets, the total internal reflection of the incident infrared light will be reduced and, therefore, the photodetector will detect less reflected light. The reduction in the intensity of the detected reflected light indicates rain or water droplets on the outer surface of the windshield and can be used to trigger the operation of an electrical device, such as the motor of a car wiper.

[0003] Figure 1 An optical rain sensor device 100 of a common type is shown. The sensor design includes two light emitting elements (LED 120 and LED 125) for detecting rain and / or water droplets deposited on one side of a transparent substrate 130 (e.g., a windshield). The two LED elements 120, 125 are arranged on opposite sides of the windshield 130 and are oriented so that the respective light beams are incident on different areas of the windshield 130 at respective incident angles θ1 and θ2 (relative to a perpendicular to the substrate 130). The incident angles θ1 and θ2 may be approximately symmetrical (e.g., -45° and +45°, respectively), such as Figure 1 The emitted light beam is reflected from the windshield onto the photodetector 140, which outputs a detection signal 145, which is generated by an integrated circuit (I C ) 150 is processed for detecting the presence of droplets. Since moisture and / or raindrops 135 are generally unevenly distributed on a wet surface, the intensity of light reflected from LED 120 and LED 125 will be different depending on whether the corresponding light beam is incident on an area covered by droplets. In the case of droplets on the external surface, due to the refraction of light passing through the transparent substrate 130 and the droplets, the total internal reflection will be reduced, so that the signal from LED 120 will be weaker than the signal from LED 125. Any sudden change in the difference between the detected LED signal intensities indicates that there is rain or water droplets on the substrate 130.

[0004] Therefore, if the LED signal is not perfectly balanced, changes in the sensitivity of the rain sensor photodetector due to exposure to sunlight may cause false detections. As a result, any changes in the sensitivity of the photodetector due to sunlight exposure may amplify this imbalance, thereby producing a detection signal signature that may be confused with the detection of actual water drops.

[0005] Typically, a dedicated rain sensor integrated circuit (I C ) 150 controls the luminous intensity of each LED in terms of intensity and frequency, and the integrated circuit outputs a drive current on the corresponding LED channel (for example, A and B) that is suitable for generating light pulses on the LEDs connected to the channel. The amplitude and frequency of the drive current output by the rain sensor IC 150 can be controlled by a digital-to-analog converter (DAC) microcontroller 160. The rain sensor LEDs are typically controlled with a drive current of 20mA amplitude and 100kHz, which can be roughly adjusted by existing integrated circuits in steps of approximately + / -2.5mA. However, this current resolution is not sufficient to compensate for small differences between the drive currents of the LED channels (A or B), and therefore, it is not possible to reduce false detections by adjusting the current on the LED channels of the rain sensor IC 150 to correct for imbalances between LED signals.

[0006] Therefore, there is a need for a rain sensor solution that can reduce false detection of rain and / or water drops due to imbalances in LED channels, and therefore, provide a rain sensor having a detection response that is independent of incident sunlight. Summary of the invention

[0007] The present invention is made in view of the shortcomings and deficiencies of the prior art, and its purpose is to provide a rain sensor device and a regulating current source capable of providing fine current regulation for an optical rain sensor, as well as a rain sensor device using the regulating current source, which can reduce or alleviate the erroneous detection of moisture and / or raindrops on a transparent substrate (e.g., a windshield) due to changes in the sensitivity of the rain sensor photodetector.

[0008] According to the present invention, an optical rain sensor device is provided, comprising: first and second light-emitting elements, adapted to emit first and second light pulses toward an inner surface of a transparent substrate; a photodetector, adapted to detect light from the first and second light pulses reflected by the inner surface of the transparent substrate; a rain sensor controller, comprising: a first driver, adapted to output a first drive current pulse on a first channel, for driving the first light-emitting element to emit the first light pulse; and a second driver, adapted to output a second drive current pulse on a second channel, for driving the second light-emitting element to emit the second light pulse; and a regulating current source connectable between the second channel of the rain sensor controller and terminals of the second light-emitting element; wherein the regulating current source is adapted to apply a compensation current signal at the terminals of the second light-emitting element based on a regulating signal, so that the total current passing through the second light-emitting element increases or decreases, so as to reduce the imbalance between the first and second light pulses.

[0009] According to a further development, the optical rain sensor device further comprises a microcontroller, which is suitable for outputting the regulation signal to control the regulation current source to output a compensation current signal having a given amplitude, wherein the regulation current source is suitable for applying the compensation current signal synchronously with the second drive current pulse, and / or wherein the compensation current signal is a square wave current pulse having an amplitude set based on the regulation signal, and the regulation signal is an analog regulation voltage provided by a digital-to-analog converter output of the microcontroller.

[0010] According to a further development, the regulated current source comprises: a variable current source adapted to provide an output current (I out ), the constant intensity being adjustable based on the adjustment signal; and a controlled switch adapted to be connected between the terminal of the second light emitting device and the output terminal of the variable current source; wherein the controlled switch is adapted to switch between an on state and an off state in synchronization with a second drive current pulse for connecting and disconnecting the variable current source, thereby adjusting the constant intensity based on the constant current (I out ) generates the compensation current signal (I C ).

[0011] In a further development, the controlled switch is adapted to switch to an on-state when the second drive current pulse is above a predetermined current threshold condition.

[0012] In a further development, the controlled switch comprises a switching transistor and a first resistive element connected between a base terminal and an emitter terminal of the switching transistor, the emitter terminal of the switching transistor being applied with the compensation current signal (I C) to the terminal of the second light-emitting device, the first resistor element is connected between the second drive channel and the terminal of the second light-emitting element to sense the second drive pulse and generate a voltage difference across the first resistor element, and when the second drive current pulse is greater than the predetermined current threshold, the voltage difference causes the switching transistor to be disconnected.

[0013] In a further development, the variable current source comprises a driving transistor adapted to adjust the current based on said regulating signal (V Reg ) to provide the output current (I out ).

[0014] In a further development, the variable current source comprises a second resistive element connected between the emitter terminal of the drive transistor and the reference potential, the second resistive element being selected to be regulated based on the regulation signal (V Reg ) provides the compensation current signal amplitude (I C )’s expectations are fine-tuned.

[0015] The present invention also provides a regulating current source for finely regulating the driving current pulse provided to the light-emitting element of the optical rain sensor, comprising: a variable current source, adapted to provide an output current (I out ), the constant intensity being adjustable based on an external adjustment signal; a controlled switch adapted to be connected between a terminal of the light-emitting device and the output terminal of the variable current source; wherein the controlled switch is adapted to switch between an on state and an off state in synchronization with a drive current pulse supplied to the light-emitting device, for connecting and disconnecting the variable current source, thereby adjusting the constant intensity based on the adjustment signal (V Reg ) to generate a compensation current signal (I C ), used to reduce the imbalance between the first and second light pulses.

[0016] In a further development, the controlled switch is adapted to switch to an on-state when the drive current pulse is above a predetermined current threshold condition.

[0017] In a further development, the controlled switch comprises a switching transistor and a first resistive element connected between a base terminal and an emitter terminal of the switching transistor, the emitter terminal of the switching transistor being applied with the compensation current signal (I C ) to the terminal of the light-emitting device, and the first resistor element is suitable for being connected between the driving channel providing the driving current pulse and the terminal of the light-emitting element to sense the driving pulse current and generate a voltage difference, which causes the switching transistor to be disconnected when the driving current pulse is greater than a predetermined current threshold.

[0018] In a further development, the variable current source comprises a variable current source adapted to adjust the current based on the regulation signal (VReg ) provides the output current (I out ) of the drive transistor, regulating the signal (V Reg ) is the regulation voltage applied to the base terminal of the drive transistor.

[0019] In a further development, the variable current source comprises a second resistive element connected between the emitter terminal of the drive transistor and the reference potential, the second resistive element being selected to be regulated based on the regulation signal (V Reg ) provides the amplitude of the compensation current signal (I C )’s expectations are fine-tuned.

[0020] The present invention also provides a method for finely adjusting the drive current of a light emitting element of an optical rain sensor, the optical sensor comprising first and second light emitting elements adapted to emit first and second light pulses toward an inner surface of a transparent substrate, a photodetector adapted to detect the first and second light pulses reflected from the inner surface of the transparent substrate, and a rain sensor controller, the rain sensor controller comprising: a first driver adapted to output a first drive current pulse on a first channel for driving the first light emitting element to emit the first light pulse; and a second driver adapted to output a second drive current pulse on a second channel for driving the second light emitting element to emit the second light pulse; the method comprising: receiving amplitudes of first and second photodetection signals corresponding to the reflected first and second light pulses detected by the photodetector, respectively; determining whether a difference between the received amplitudes of the first and second photodetection signals satisfies a given compensation condition associated with an imbalance between the first and second light pulses; applying a compensation current signal on the second channel using an externally regulated current source when the compensation condition is satisfied; and wherein the compensation current signal is adapted to increase or decrease a total current passing through the second light emitting element (based on the relationship between the amplitudes of the first and second photodetection signals, and, for example, to reduce the imbalance between the first and second photodetection signals).

[0021] In a further development, when the amplitude of the second photodetection signal is lower than the amplitude of the first photodetection signal, the method also includes: monitoring the amplitudes of the received first and second photodetection signals; gradually increasing the amplitude of the compensation current signal within a given current range, while the monitored amplitudes of the received first and second photodetection signals satisfy the compensation conditions, and the amplitude of the compensation current signal is lower than the maximum value of the current range; and if the compensation conditions are still met when the amplitude of the compensation current signal reaches the maximum value of the current range, increasing the second drive current through a compensation step, and setting the amplitude of the compensation current signal to the minimum value of the current range.

[0022] In a further development, when the amplitude of the second photodetection signal is higher than the amplitude of the first photodetection signal, the method further comprises: reducing the second drive current by a compensation step, and applying the compensation current signal on the second channel (B) with an amplitude equal to the maximum value of the given current range; monitoring the amplitudes of the received first and second photodetection signals; gradually reducing the amplitude of the compensation current signal within the given current range, while the monitored amplitudes of the received first and second photodetection signals satisfy the compensation condition, and the amplitude of the compensation current signal is higher than the minimum value of the current range; if the compensation condition is still satisfied when the amplitude of the compensation current signal reaches the minimum value of the current range, reducing the second drive current by a compensation step, and setting the amplitude of the compensation current signal to the maximum value of the current range.

[0023] The accompanying drawings are incorporated into and form a part of the specification for the purpose of explaining the principles of the invention. The accompanying drawings should not be construed as limiting the invention to the illustration and description of how to make and use the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Further features and advantages will become apparent from the following and more detailed description of the invention as illustrated in the accompanying drawings, in which:

[0025] Figure 1 A conventional optical rain sensor arrangement is schematically shown.

[0026] Figure 2 Schematically shows an optical rain sensor device having at least two light emitting elements according to an embodiment, wherein the at least two light emitting elements are driven by driving current pulses outputted by a rain sensor integrated circuit on corresponding channels (channels A and B) to emit alternating light pulses, and one of the driving channels (channel B) is coupled to a regulating current source to compensate for an imbalance between the emitted alternating light pulses;

[0027] Figure 3 The schematic diagram in more detail shows Figure 2 The connection of the regulated current source in channel B of the optical rain sensor device is shown to the light-emitting element.

[0028] Figure 4A Example of connecting to Figure 2 The driving current I of the light emitting elements of channels A and B is shown in LED_A and I LED_B The amplitude of the drive current pulse I output to drive channels A and B A and I B The photoelectric detection signal I measured by the photodetector of the optical rain sensor device when the light-emitting elements on each channel are unbalanced and have the same amplitude and frequencyPD The amplitude of

[0029] Figure 4B shows the driving current I LED_A and I LED_B The amplitude of the photoelectric detection signal IPD is measured under the following conditions: the driving current pulse I A In a higher state (i.e. Figure 4B 20 mA) and compensate the current signal I C By adjusting the current source and the driving current pulse I B Applied synchronously to channel B to compensate Figure 4A The imbalance shown is measured;

[0030] Figure 5 Schematically shows an implementation of a regulated current source with a controlled switch and a variable current source according to an embodiment;

[0031] Figure 6 yes Figure 5 A circuit diagram of the configuration of the controlled switch and the variable current source shown;

[0032] Figure 7 It is used for Figure 5 A flow chart of compensation operation for compensating for imbalance between light pulses emitted by light emitting elements of an optical rain sensing device;

[0033] Figure 8 shows the compensation operation period Figure 5 The compensation operation is performed on the driving current I A and I B The compensation operation is performed when the light pulse emitted by the light emitting element on channel A (LED A) is stronger than the light pulse emitted by the light emitting element on channel B (LED B) with the same amplitude, wherein: (a) shows the amplitude I of the photodetector signal measured when LED B is turned on and when LED A is turned on during the compensation operation, respectively. PD_LED_B_ON and I PD_LED_A_ON (b) shows the drive pulse I output to channel A during the compensation operation A (c) shows the drive pulse I caused by applying a rough adjustment during the compensation operation B (d) shows the compensation current signal I output by adjusting the current source during fine current regulation C (e) shows the change in amplitude during the compensation operation until reaching time t comp The total current I previously passed through LED B LED_B The amplitude increases at the time t compAmplitude I PD_LED_A and I PD_LED_B Become balanced;

[0034] Fig. 9 shows the compensation operation period Figure 5 The change of various signal amplitudes of the regulated current source and the optical rain sensing device over time is shown. The compensation operation is performed on the driving pulse I A and I B The compensation operation is performed when the light signal emitted by LED A is weaker than the light signal emitted by LED B with the same amplitude, wherein: (a) shows the amplitude I of the photodetector signal measured when LED B is turned on and when LED A is turned on during the compensation operation, respectively. PD_LED_B_ON and I PD_LED_A_ON ; (b) shows the driving pulse I on channel A A (c) shows the drive pulse I caused by applying a rough adjustment during the compensation operation B (d) shows the compensation current signal I output by adjusting the current source C (e) shows the change in amplitude during the compensation operation until reaching time t comp The total current I previously passed through LED B LED_B The amplitude decreases at the time t comp Amplitude I PD_LED _ A and I PD_LED_B Become balanced. DETAILED DESCRIPTION

[0035] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the present invention are shown. However, the present invention may be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided to make the disclosure comprehensive and complete and to fully convey the scope of the present invention to those skilled in the art. The same reference numerals represent the same elements.

[0036] Figure 2 The optical rain sensor device 200 according to an embodiment of the present invention is schematically shown. The rain sensor device 200 includes a Figure 1 The optical sensor 210 of the optical sensor 110 described above includes at least two light emitting elements 225, 220 (which may be referred to as a first light emitting element 225, a second light emitting element 220), such as two infrared LEDs, which are arranged on the same side of a transparent substrate 230 and are oriented so that the light beams emitted by the LEDs 225, 220 are incident on different positions of the inner surface of the substrate 230. The respective incident angles θ1 and θ2 of the LEDs 225, 220 may be symmetrical, as shown in FIG. Figure 1However, the incident angles θ1 and θ2 may also be different from each other, depending on the specific sensor configuration, and may have Figure 2 , and the values ​​shown in are different from those in (not 0°). The photodetector 240 is disposed in the middle region between the LED elements 225, 220, for detecting the reflected light pulses 236, 238 (which may be referred to as) of each light emitting element 225, 220, and outputting a photodetection signal 245 indicating the detected intensity of the reflected light pulses 236, 238. The optical sensor 210 is controlled by a dedicated rain sensor integrated circuit 250. The rain sensor IC 250 includes two embedded slew rate controlled LED drivers 252, 254, which output drive currents on respective channels A and B, respectively, for driving the LED elements 225, 220. The first driver 252 outputs a first drive current pulse (I ) on a first channel (channel A). A ), which is used to drive the light emitting element 225 to emit a light pulse with a given intensity and frequency, and the second driver 254 outputs a second driving current pulse (I B ), for driving the light emitting element 220 to emit a second optical signal with a corresponding intensity and frequency. The rain sensor IC 250 also includes a sensitive receiver 256, which receives and processes the detection signal 245 output by the photodetector 240, for detecting the presence of raindrops and water droplets 235 on the windshield 230. The IC 250 may also include additional input channels (for example, for inputting ambient light measurement values), and an interface for communication (not shown).

[0037] As described above, the rain sensor IC 250 can provide a driving current pulse I with a desired amplitude and frequency on the corresponding LED driving channels A and B. A and I B The light emitted by each LED element 225, 220 is controlled separately. A and I B The amplitude, frequency, and phase of the LEDs may be initially selected based on the intended application of the rain sensor and / or ambient light conditions, and then dynamically adjusted in response to control signals from the microcontroller 260. Typically, the light emitting elements of a conventional rain sensor are driven by current pulses having an amplitude of 20 mA and a frequency of 100 kHz. However, this does not allow for adjustment of the drive current on LED channels A and B by an amount smaller than the minimum current resolution of the rain sensor IC (typically about 2.50 mA). In order to provide fine adjustment of the LED drive current, the present invention provides an externally regulated current source 270 that may be connected to one of the LED channels (e.g., LED channels 270) of the LED rain sensor IC 250. Figure 2 Channel B) in order to apply the compensation current I Cto the rain sensor LED elements 220 connected to the corresponding LED channels, such as Figure 2 shown.

[0038] Figure 3 The schematic diagram in more detail shows the connection of the external regulated current source 270 to one of the LED channels (channel B) and the drive current I LED_B Compensation. Figure 3 As shown, the regulating current source 270 will compensate the current I C The compensation current I is applied to a current path between one terminal of the light emitting element 220 (the cathode in the example shown) and the output terminal of the corresponding LED channel of the rain sensor IC 250. C is added to the driving current I supplied to the light emitting element 220 by the LED driver 254 of the rain sensor IC 250 itself. B , so that the total current I LED_B =I B +I C The light emitting element 220 is driven, while the light emitting element 225 of another LED channel connected to the rain sensor IC 250 is driven by the current pulse I provided by the corresponding LED driver 252. A The corresponding driving current I LED_A The regulating current source 270 can be driven by the regulating signal V Reg To control, for example, it is the analog voltage V output from the DAC analog output of the microcontroller 260 DAC , to output a compensation current with a magnitude I C , which can be based on the measured photodetection signal I PD As will be described later. Thus, fine current regulation can be easily achieved with the aid of the microcontroller DAC 260 by suitable software. For example, a standard 10-bit DAC typically provides a current regulation resolution of 3 μA, which can be used for this purpose. As will be described later, this allows a compensation current I having a precise value of 0 to 3 mA to be superimposed on the LED channel connected to the regulation current source 270 C This current regulation is sufficient to achieve fine current regulation with step sizes smaller than the typical minimum resolution of about 2.50mA provided by the LED drivers of conventional rain sensor ICs.

[0039] Note that although Figure 3A single light emitting element 220 or 225 is shown on each LED channel A and B of the rain sensor IC 250, however, the optical sensor 210 may include one or more LEDs connected in series to the same LED channel, for example, to cover a larger rain detection area. In addition, the optical sensor device 200 may include more than two LED channels and be controlled by a rain sensor IC having multiple channel drivers. The photodetector 240 is also not limited to Figure 3 The photodiode shown may also be replaced by other suitable photodetection devices that achieve the same purpose.

[0040] If the LED signals 236, 238 from the light emitting elements 225, 220 (i.e., the light pulses emitted by LEDs A and B reflected by the transparent substrate 230) are well balanced, then in the absence of rain on the substrate 230, the drive pulses I A and I B During the high state, the photodetector 240 detects similar light intensities and outputs detection signals 245 of approximately the same amplitude. The rain sensor IC 250 receives and processes the photodetection signals 245. The microcontroller 260 analyzes the measured amplitude I of the photodetection signals 245. PD_LED_A_ON and I PD_LED_B_ON , and the difference I PD_LED_A_ON -I PD_LED_B_ON Any sudden change in will be interpreted by the microcontroller 260 as corresponding to the detection of raindrops. However, since the sensitivity of the photodetector changes when exposed to sunlight, the drive current pulses I supplied to the light emitting elements 225, 220 A and I B The LEDs 260 and 261 may be configured to detect raindrops. The LEDs 260 and 262 may be configured to detect raindrops. The LEDs 260 and 263 may be configured to detect raindrops. The LEDs 260 and 264 ...

[0041] Figure 4A An exemplary situation is shown in which no water droplets are deposited on the transparent substrate 230, but although the LEDs on channels A and B are pulsed with current pulses I having the same amplitude and frequency A and I B However, when LED B is turned on, the photodetection signal I measured by the photodetector 240 is PD The amplitude, I PD_LED_B_ON , than the amplitude I measured when LED A is turned on PD_LED_A_ONLow, which is due to the imbalance between the light pulses emitted by LEDs A and B (ie, the imbalance between the LED signals 236, 238).

[0042] The drive signals I provided by the LED drivers 252 and 254 on channels A and B respectively A and I B是 Preferably, the driver signal I is a square wave pulse with an amplitude of 20 mA and a frequency of 100 kHz. However, depending on the specific application and operating conditions, the driver signal I A and I B It may have an amplitude, frequency and / or waveform different from that shown in the figure. A and I B The LEDs connected to channels A and B are out of phase, so that the LEDs connected to channels A and B emit light in alternating half cycles. Therefore, the photodetector 240 outputs a photodetection signal I with an amplitude of 1.1 μA when detecting the reflected LED signal 236 from the light emitting element 225. PD , while a lower amplitude of 0.1 μA is output when the reflected LED signal 238 from the light emitting element 220 is detected. This is because the light emitting element 220 emits a lower light intensity under the same 20 mA driving current.

[0043] Figure 4B It is shown that in order to adjust the imbalance between the detected LED signals 236 and 238, the compensation current signal I output by the external adjustment current source 270 is adjusted. C The compensation current signal I C is a current pulse having the same frequency and synchronized with the drive current pulse on the channel to be compensated (i.e., channel B in this embodiment). Figure 4B In the illustrated case, the photodetector 240 outputs a detection signal I having an amplitude of 1.1 μA upon detecting the reflected LED signal 238 from the light emitting element 220 (which is now driven by a total current of 22 mA). PD This is because the detection signal I of 20mA and similar strength (1.1 PD ) when driving the light emitting element 225, the signal is now driven by a total current of 22 mA, which is the result of a drive current I of 20 mA. B The output current is generated by adding the 2 mA compensation current applied by the external regulated current source 270.

[0044] Compensation current signal I C The drive current pulse I BSynchronization can be achieved using the reference clock of the rain sensor IC itself. However, not every rain sensor IC that is commonly used to control an optical rain sensor provides external access to the internal reference clock.

[0045] If you will refer to Figure 5 As explained above, the external regulating current source 270 can use the driving current signal itself to adjust the compensation current signal I C Synchronized with the drive current signal on the channel to be compensated.

[0046] Figure 5 The schematic diagram shows a regulating current source 270, which provides a compensation current signal I C , the compensation current signal I C The driving current signal I B The driving current signal I of the rain sensor IC 250 on the channel (channel B) to be compensated B Each of the light emitting elements 225 and 220 is connected to its own power supply voltage V LED_A , V LED_B and the outputs of driving channels A and B of the rain sensor IC 250. The regulating current source 270 applies a compensation current signal I on the path connecting the light emitting device 220 to the driver 254 of channel B. C , with the drive current I B When in the high state, it increases (or decreases) the total current I across the light emitting element 220 LED_B , thus, reducing the imbalance between the light pulses emitted by the light emitting elements 225, 220. The regulating current source 270 includes a variable current source 280 that provides a DC current I OUT , whose strength can be based on the input regulation signal (V Reg ) is changed, for example, by the analog output (V DAC ) provides an analog voltage signal. For example, for a 2V V DAC , an I of 2mA can be achieved OUT .

[0047] The magnitude I is achieved by connecting a controlled switch 290 between the output of the variable current source 280 and the terminals of the light emitting device 220 to be compensated. OUT And the driving current I applied to the LED channel to be compensated (channel B) B Synchronous square pulse signal I C The controlled switch 290 is connected in series with the variable current source 280 and is operable to be coupled to the drive current pulse I on channel B. B The switching between the high and low states of the current signal I is synchronously switched between the on and off states. More specifically, for example, when the current signal IB When the current is higher than a given threshold, the controlled switch 290 can be driven based on the driving current signal I B For a typical application, where a drive current with an amplitude of about 20 mA is used, a current threshold of 1 mA is suitable for detecting a drive current pulse I B When the variable current source 280 switches from the low state to the high state, and thus closes the controlled switch 290, the current I OUT Output to the cathode terminal of the light emitting device 220. When the driving current signal I B When the current falls below the predetermined current threshold, the controlled switch 290 automatically switches to the off state, thereby disconnecting the variable current source 280 from the channel B. Therefore, when the controlled switch 290 is turned off, the output current I OUT , the total current I applied to the light emitting element 220 on the light channel B LED_B The corresponding driving current signal I provided by the driver 254 on channel B is B and the compensation current I provided by the variable current source 280 C =I OUT The sum of .

[0048] exist Figure 6 An exemplary embodiment of a threshold current controlled switch 290 is shown in FIG. For simplicity, a current pulse I is applied to channels A and B. A and I B The rain sensor LED drivers 252 and 254 are shown as ideal current sources. The controlled switch 290 includes a switching transistor Q1, such as a pnp bipolar transistor 292. When the voltage difference between the base and emitter terminals of the transistor is equal to or greater than a threshold voltage V be，Q1 When the pnp bipolar transistor Q1 is turned on, the threshold voltage is the characteristic parameter of the transistor Q1. B The current threshold condition for automatically turning on or off the switching transistor Q1 synchronously can be achieved by connecting a resistor R1 between the emitter and base terminals of the transistor Q1, which is inserted between the cathode of the light emitting element 220 and the output of the LED driver 254. Therefore, when the current I R1 The pnp transistor Q1 is turned on when the following conditions are met:

[0049]

[0050] Since the current at the base terminal of the switching transistor Q1 is very small, the current I R1 Approximately equal to the driving current I on LED channel B B. By using the driving current I for driving the LED element 220 B The range and characteristics of the switching transistor Q1 threshold voltage V be,Q1 By selecting the value of resistor R1, the desired current threshold condition for opening / closing controlled switch 290 can be easily achieved. For example, for the threshold voltage V be,Q1 =0.6V and resistor R1 =330 ohms, when the current I R1 When the current becomes higher than 1.8 mA, the switching transistor Q1 is automatically turned on.

[0051] Now refer to Figure 6 The implementation of the variable current source 280 is described below. Figure 6 As shown, the variable current source 280 is a voltage-controlled current source that includes a transistor Q2, such as a bipolar npn transistor 294, and a resistor R2 connected between the emitter terminal of Q2 and a reference potential (such as ground). Considering the base current I b The collector current I c , emitter current I e It is very small compared to the collector I c have approximately the same value. The emitter current I e corresponds to the current through resistor R2 and depends on the voltage V applied to the base of transistor Q2 Reg (For example, the voltage V DAC ) and the characteristic threshold voltage V be,Q2 .as follows:

[0052]

[0053] Compensation current signal I C The range can then be determined by the resistor R2 and the threshold voltage V be,Q2 Select a suitable value to set. For example, use resistor R2 = 900 ohms, V be,Q2 The npn bipolar transistor Q2 of about 0.6V can be based on the regulated voltage signal V output from the microcontroller 260 varying between 0.6 and 3.3V. Reg The collector current I of Q2 can be easily changed between 0 and 3 mA. c，Q2 . Collector current I c，Q2 is applied to the collector of the switching transistor Q1 of the controlled switch 290, and thus corresponds approximately to the current I output by the voltage-controlled current source 280. OUT Therefore, the compensation pulse I C The amplitude can be easily adjusted by adjusting the voltage signal V DACFurthermore, synchronization of the controlled switch 290 with the drive current pulses on channel B described above allows for real-time adjustment of the imbalance between the two LED signals.

[0054] The amount of compensation necessary to correct for imbalances caused by differences in the inherent characteristics of the LEDs and / or due to exposure to sunlight can be determined based on the photodetector signal 245 received by the rain sensor IC 250. The photodetector current I output to the microcontroller 260 PD_LED_A_ON and IPD_LED_B_ON Can be used to determine whether a compensation operation should be applied on one of the drive channels (e.g., channel B). Figures 7 to 9 Describes the photodetector current I output by the rain sensor IC 250 PD_LED_A_ON and I PD_LED_A_ON Exemplary compensation operations performed by microcontroller 260 .

[0055] Figure 7 2 shows a flow chart of the compensation operation performed by the microcontroller 260. When the measured value I of the photodetection signal 245 is received PD_LED_A_ON and I PD_LED_B_ON When the microcontroller 260 is based on the amplitude I PD_LED_A_ON and I PD_LED_B_ON The relationship between the amplitude I and the predetermined compensation condition that should be satisfied when applying current compensation on one driving channel (e.g., channel B) determines whether a compensation operation is required to correct the imbalance between LEDs A and B. For example, the predetermined compensation condition may include the amplitude I PD_LED_A_ON and I PD_LED_B_ON The difference between is greater than a given amount.

[0056] When the compensation condition is met, the microcontroller 260 calculates the compensation condition according to the measured value I PD_LED_A_ON and I PD_LED_B_ON The type of compensation to be applied is determined by the relationship between the photodetection current measured when LED A is on (and LED B is off) and the photodetection current measured when LED B is on (and LED A is off). If compensation is required, the microcontroller 260 will perform a compensation operation to increase (or decrease) the total current through LED B, as will be described below. The microcontroller 260 can then continue to receive and detect these values ​​I continuously (or at predetermined time intervals). PD_LED_A_ON and I PD_LED_B_ON , for example, to compare received values ​​and decide in real time whether further compensation is required.

[0057] Now refer to Figure 8 The magnitude of the photodetector signal 245 is stronger when LED A is on (I PD_LED_A_ON >I PD_LED_B_ON) is used to increase the total current I across LED B LED_B compensation operation.

[0058] Figure 8 The two LEDs on channels A and B are initially driven by current pulses I of the same magnitude (e.g., 20 mA). B and I A The result of the compensation operation performed by the microcontroller 260 when the intensity of the light signal emitted by the light emitting element 220 on channel B (LED B) is less than the intensity of the light signal emitted by the light emitting element 225 on channel A (LED A) is shown in FIG. 2 . Due to this imbalance, the photodetector 240 detects less reflected light when LED B is turned on, and therefore, the photodetection signal I PD The amplitude I PD_LED_B_ON Initially, the amplitude I is measured when LED A is on (and LED B is off). PD_LED_A_ON Lower, such as Figure 8 (a) shown.

[0059] In this case, if the amplitude I PD_LED_A_ON and I PD_LED_B_ON If the difference between meets the predetermined compensation condition, the microcontroller 260 determines that compensation is required and outputs a regulating signal V for controlling the regulating current source 270. Reg The output compensation current I C To achieve smooth compensation, the compensation can be done within a given range (e.g., from 0 to the maximum value V Reg，MAX ) Gradually increase the regulation voltage V Reg , for example, so that the regulating current source 270 can adjust the compensation current signal I C The amplitude gradually increases from 0 to the maximum value I C，MAX Current regulation is performed. Figure 6 In the example of C，MAX is the maximum regulation voltage V Reg，MAX The factor (I C，MAX =V Reg，MAX / R2). Compensation current I C The rate of increase is preferably controlled by limiting V Reg The speed of change of V is limited to a fixed rate, for example, less than or equal to 0.1 mA / s, thereby avoiding Reg Any sudden change of the photodetector signal 245 caused by, for example, a real water droplet is used to avoid being compensated by the current I C By filtering the photodetector signal 245 in the microcontroller 260 with a digital high pass filter, sudden changes in the photodetector signal 245 due to water droplets can be distinguished from other types of changes.

[0060] In the regulation signal V Reg During fine current regulation under the control of , the microcontroller 260 can continuously monitor and compare the measured value I of the photodetector signal 245. PD_LED_A_ON and I PD_LED_B_ON , and by gradually increasing the regulation voltage V reg While maintaining the compensation current I C The amplitude gradually increases, while the difference (I PD_LED_A_ON -I PD LED_B_ON) The compensation condition is continuously met or until the maximum compensation current I is reached C，MAX (i.e. maximum regulation voltage Vreg，MAX ). If the measured value I PD_LED_B_ON and I PD LED_A_ON At the maximum compensation current I C，MAX When the imbalance, that is, the difference (I PD_LED_A_ON -I PD_LED_B_ON ) still satisfies the compensation condition, the microcontroller 260 may decide to apply the compensation step size of the driving current I by instructing the rain sensor IC 250 to increase the driving current output by the driver 254 on channel B by a predetermined value. B For example, the compensation step can be selected to be the minimum current resolution provided by the rain sensor IC 250 and / or to correspond to the maximum compensation current I C，MAX itself (in Figure 8 In the example, 2.5 mA is used to ensure a smooth transition. Then the regulated voltage V Reg And the compensation current I C At the same time, the photodetector signal 245 continues to be monitored and the measured value I needs to be reset by the microcontroller 260. PD_LED_A_ON， I PD_LED_B_ON Compare to determine whether further fine current regulation is needed. When the compensation conditions are still met, gradually increase the regulation voltage V from 0 Reg Until the compensation current I C The amplitude (the measured value I PD_LED_B_ON , I PD LED_A_ON The compensation condition is no longer met) or until the maximum compensation current I C，MAX The microcontroller 260 can perform further fine current regulation. PD_LED_A_ON and I PD_LED_B_ON , it is necessary to apply several sequences of fine current adjustments followed by coarse adjustments until the desired balance is achieved between the LED signals 236, 238 and the measured photodetector current IPD_LED_A_ON and I PD_LED_B_ON The stage at which the conditions for compensation are no longer met. Figure 8 In the case shown, when the measured value I is reached PD_LED_A_ON and I PD_LED_B_ON The stage (t = t comp , Figure 8 ), two fine current regulation sequences are applied first, followed by coarse regulation. After this stage, the light pulses from LED A and LED B are considered to be compensated, and the microcontroller 260 performs the control of the rain sensor IC 250 and the regulated current source 270 to maintain the drive current pulse I B and compensation pulse I C The amplitude (which is at time t comp reached), such as Figure 8 (c) and Figure 8 (d) When t>t comp shown.

[0061] Fig. 9 The figure shows that when both LEDs are powered by a current I having the same value (e.g. 20 mA) B and I A When driven, when the light signal emitted by LED B is stronger than the light signal emitted by LED A (I PD_LED_A_ON PD_LED_B_ON ), the compensation operation is performed by the microcontroller 260. In this case, when LED B is turned on (and LED A is turned off), the photodetector 240 detects more reflected light, and therefore the measured value I of the photodetector signal 245 is PD_LED_B_ON is higher than the value I measured when LED A is on (and LED B is off) PD_LED_A_ON If the difference |I PD_LED_A_ON -I PD_LED_B_ON | If the compensation condition is met, the microcontroller 260 applies a compensation operation to reduce the total current on LED B. When the microcontroller 260 determines that the total current on LED B should be reduced to correct the imbalance between LEDs A and B, the microcontroller 260 controls the regulating current source 270 to gradually reduce the corresponding regulating voltage V Reg To reduce the current I that may be output to channel B C The adjustable voltage V Reg During this fine current regulation under the control of , the microcontroller 260 can monitor and compare the amplitude I of the photodetector signal 245. PD_LED_A_ON and I PD_LED_B_ON , and continue to gradually reduce the compensation current I C The amplitude of the difference |I PD_LED_A_ON -I​PD_LED_B_ON | The compensation condition is met or until the minimum compensation current I that can be output by the current regulating source 270 is obtained C，MIN (For example, I C，MIN =0). If the minimum compensation current I C，MIN The measured value I PD_LED_B_ON and I PD_LED_A_ON Still unbalanced, that is, the difference |I PD_LED_A_ON -I PD_LED_B_ON If the conditions for applying compensation are still met, the microcontroller 260 can instruct the rain sensor IC 250 to output the driving current I on channel B by the driver 254. B Reduce the amount corresponding to the predetermined compensation step to determine the applied drive current I B This compensation step can have a rough adjustment with reference Figure 8 To ensure that the compensation of the total current in the LEDs on channel B is performed in a smooth manner, once the drive current I B By reducing the compensation step, the microcontroller 260 can simultaneously output an amplitude of V Reg，MAX The regulated voltage V Reg , which is sufficient to control the regulating current source 270 to apply the compensation current I on channel B C , the compensation current I C With the drive current I realized by the compensation step B The value of the reduction equivalent to I C，MAX If the microcontroller 260 first detects the measured value I PD_LED_B_ON and I PD_LED_A_ON The compensation current I C Already at minimum value I C，MIN , the microcontroller 260 can drive the current I B The compensation current is set to the maximum value I C，MAX ,like Fig. 9 (c) and Fig. 9 Then, the microcontroller 260 generates a voltage from the maximum amplitude V Reg，MAX Gradually reduce the regulation voltage V Reg To perform fine current regulation, C，MAX Smoothly reduce the compensation current I output by the regulating current source 270 C The amplitude of the photodetector signal 245 is PD_LED_A_ON and I PD_LED_B_ON The comparison is performed (continuously or at predetermined time intervals). The compensation current I C The gradual decrease in the amplitude continues until I PD_LED_A_ON and I PD_LED_B_ONThe difference between them no longer meets the compensation condition or until the compensation current I C Until it reaches zero.

[0062] If I C Reach the minimum value I C，MIN (e.g. zero) when the measured value I PD_LED_A_ON and I PD_LED_B_ON If the microcontroller 260 again instructs the rain sensor IC 250 to adjust the driving current output by the driver 254 on channel B by reducing the driving current by another compensation step, and adjusts the voltage V Reg Set it to the maximum value V again Reg，MAX A further sequence of fine current regulation followed by coarse current regulation can be applied until the photodetector amplitude I PD_LED_A_ON and I PD_LED_B_ON The desired balance between the two is reached and the compensation condition is no longer met. PD_LED_A_ON and I PD_LED_B_ON In the next fine current regulation sequence, the current becomes balanced and the compensation condition is no longer met (e.g. Fig. 9 (e) Time t comp As shown in FIG. 1 ), the microcontroller 260 interrupts the fine current regulation operation and maintains the compensation current I C (ie, regulating voltage V Reg ) and drive current I B The amplitude is at a balanced value, such as Fig. 9 (c) and 9(d) at t>t comp Similar to reference Figure 8 The compensation operation described is performed in increments, and the compensation current I C The reduction rate of is preferably limited to, for example, less than 0.1 mA / s.

[0063] The combination of fine current regulation via regulated current source 270 and coarse current regulation provided by rain sensor IC 250 itself allows the photodetector signal I to be adjusted in a smooth manner. PD_LED_A_ON and IPD_LED_B_ON A balance is achieved between the compensation operation and the detection accuracy while ensuring that any sudden changes in the photodetector signal caused by real water droplets cannot be masked by the compensation operation or cannot be detected.

[0064] In summary, the present invention solves the main limitations of existing optical rain sensors by providing an externally regulated current source that allows fine adjustment of the imbalance between the LED signals to be achieved in real time. In addition, the above technology is able to synchronize itself with the driving signal of the LED driver of the rain sensor IC, which solves the disadvantage of existing rain sensor ICs that do not have a signal that can be used to ensure synchronization of the external current source. Moreover, the solution of the present invention uses few components, is low cost and robust. It also takes up little space on the PCB, which is an additional advantage because the PCB area of ​​the optical rain sensor device is limited.

[0065] Finally, although the present invention has been described above with reference to an optical rain sensor for a windshield, the principles of the present invention may also be advantageously applied to other applications, including industrial cameras and window systems, sensors for ambient light measurement, which are used for the control of headlights / lights, head-up displays and air conditioning.

[0066] Reference numerals

[0067] 100 Optical sensor device

[0068] 110 Optical Sensor

[0069] 120, 125 light emitting element

[0070] 130 transparent substrate

[0071] 135 Raindrops

[0072] 140 Photodetector

[0073] 145 Detection Signal

[0074] 150 Rain Sensor IC

[0075] 160 DAC Microcontroller

[0076] A, B LED channels

[0077] 200 Optical sensor device

[0078] 210 Optical Sensor

[0079] 225, 220 Light-emitting element

[0080] 230 transparent substrate

[0081] 235 Raindrops

[0082] 236, 238 Reflected light pulses

[0083] 240 Photodetector

[0084] 245 Detection Signal

[0085] 250 Rain Sensor IC

[0086] 252, 254 Rain Sensor IC LED Driver

[0087] 256 Rain Sensor IC Controller

[0088] 260 DAC Microcontroller

[0089] 270 Externally regulated current source

[0090] 280 Variable Current Source

[0091] 290 Controlled switch

[0092] 292 Switching transistor

[0093] 294 Driver transistor

[0094] A, B LED channels.

Claims

1. An optical rain sensor device, comprising: first and second light emitting elements (225, 220) adapted to emit first and second light pulses toward an inner surface of a transparent substrate (230); a photodetector (240) adapted to detect light from the first and second light pulses that is reflected by the inner surface of the transparent substrate (230); A rain sensor controller, comprising: A first driver (252) adapted to output a first driving current pulse on a first channel to drive the first light emitting element (225) to emit a first light pulse; and A second driver (254) adapted to output a second driving current pulse on a second channel to drive the second light emitting element (220) to emit a second light pulse; and a regulating current source (270) connectable between the second channel of the rain sensor controller and the terminal of the second light emitting element; wherein the regulating current source (270) is adapted to apply a compensation current signal at the terminal of the second light-emitting element (220) based on the regulating signal, thereby increasing or decreasing the total current on the second light-emitting element (220) to reduce the imbalance between the first and second light pulses; The regulating current source (270) comprises a variable current source (280) adapted to provide an output current (I out ), the intensity of which is adjustable based on the adjustment signal, the variable current source (280) comprising a driving transistor (294) adapted to provide the output current (I out ).

2. The optical rain sensor device according to claim 1, further comprising: A microcontroller (260) adapted to output the regulating signal to control the regulating current source (270) to output a compensation current signal with a given amplitude; The regulating current source (270) is adapted to apply the compensating current signal (I C ), and / or The compensation current signal is a square current pulse having an amplitude set based on the adjustment signal, and the adjustment signal is an analog adjustment voltage provided by the digital-to-analog converter output of the microcontroller (260).

3. The optical rain sensor device according to any one of claims 1 to 2, wherein: The regulated current source (270) comprises a controlled switch (290) adapted to be connected between the terminal of the second light emitting device and an output terminal of the variable current source (280); The controlled switch (290) is adapted to switch between an on state and an off state in synchronization with the second drive current pulse to connect and disconnect the variable current source (280), thereby adjusting the output current (I out ) generates the compensation current signal (I C ).

4. The optical rain sensor device according to claim 3, wherein: The controlled switch (290) is adapted to switch to an on state when the second drive current pulse is above a predetermined current threshold condition.

5. The optical rain sensor device according to claim 3, wherein: The controlled switch (290) includes a switching transistor (292) and a first resistance element (R1) connected between a base terminal and an emitter terminal of the switching transistor (292). The emitter terminal of the switching transistor (292) applies the compensation current signal to the terminal of the second light emitting element (220), The first resistor element (R1) is connected between the second drive channel and the terminal of the second light-emitting element (220) to sense the second drive current pulse and form a voltage difference across the first resistor element (R1), so that the switching transistor (292) is turned on when the second drive current pulse is greater than a predetermined current threshold.

6. The optical rain sensor device according to claim 3, wherein: The variable current source (280) includes a second resistive element (R2) connected between the emitter terminal of the drive transistor (294) and the reference potential, the second resistive element (R2) being selected to provide a compensation current signal (I C )’s amplitude.

7. A regulating current source (270) for finely regulating a driving current pulse provided to a light emitting device of an optical sensor (210), comprising: A variable current source (280) adapted to provide an output current (I out ), the constant intensity can be adjusted based on an external adjustment signal, the variable current source (280) includes a driving transistor (294), which is suitable for providing the output current (I out ), the regulation signal being a regulation voltage applied to the base terminal of the driving transistor (294); a controlled switch (290) adapted to be connected between a terminal of the light emitting device and an output terminal of the variable current source (280); The controlled switch (290) is adapted to switch between an on state and an off state in synchronization with a driving current pulse provided to the light emitting device to connect and disconnect the variable current source (280), thereby generating a compensation current signal (I C ), the compensation current signal (I C ) is superimposed on the driving current pulse at both ends of the light emitting device to reduce the imbalance between the first and second light pulses.

8. The regulated current source (270) according to claim 7, wherein: The controlled switch (290) is adapted to switch to an on state when the drive current pulse is above a predetermined current threshold condition.

9. The regulating current source (270) according to claim 7 or 8, wherein: The controlled switch (290) includes a switching transistor (292) and a first resistance element (R1) connected between a base terminal and an emitter terminal of the switching transistor (292). The emitter terminal of the switching transistor (292) applies a compensation current signal (I C ),and The first resistor element (R1) is suitable for being connected between a driving channel providing a driving current pulse and the terminal of the light emitting element to sense the driving pulse current and form a voltage difference that disconnects the switching transistor (292) when the driving current pulse is greater than a predetermined current threshold.

10. The regulated current source (270) of claim 8, wherein: The variable current source (280) includes a second resistive element (R2) connected between the emitter terminal of the drive transistor (294) and a reference potential, the second resistive element (R2) being selected to provide a desired fine adjustment of the compensation current signal amplitude based on the adjustment signal.

11. A method for finely adjusting a driving current of a light emitting element of an optical rain sensor, the optical rain sensor comprising: first and second light emitting elements (225, 220) adapted to emit first and second light pulses toward an inner surface of a transparent substrate (230); a photodetector (240) adapted to detect first and second light pulses reflected from an inner surface of the transparent substrate (230); and a rain sensor controller, comprising a first driver (252) and a second driver (254), the first driver being adapted to output a first drive current pulse on a first channel for driving a first light emitting element (225) to emit a first light pulse, and the second driver being adapted to output a second drive current pulse on a second channel for driving a second light emitting element (220) to emit a second light pulse, the method comprising: Receive the amplitude (I PD_LED_A_ON , I PD_LED_B_ON ), which correspond to the reflected first and second light pulses detected by the photodetector (240), respectively; Determine the amplitudes (I PD_LED_A_ON , I PD_LED_B_ON ) satisfies a given compensation condition related to the imbalance between the first and second light pulses; and When the compensation condition is met, a compensation current signal is applied to the second channel (B) using an external regulating current source (270), wherein the regulating current source (270) comprises a variable current source (280) adapted to provide an output current (I out ), the intensity of which is adjustable based on an adjustment signal, the variable current source (280) comprising a driving transistor (294) adapted to provide the output current (I out ); The compensation current signal is suitable for being based on the amplitude (I PD_LED_A_ON ,I PD_LED_B_ON ) to increase or decrease the total current passing through the second light emitting element (220) so as to reduce the imbalance between the first and second photodetection signals.

12. The method according to claim 11, wherein: When the amplitude of the second photodetection signal (I PD_LED_B_ON ) is greater than the amplitude of the first photodetection signal (I PD_LED_A_ON ) is lower, the method further comprises: Monitor the amplitudes (I PD_LED_A_ON , I PD_LED_B_ON ); The amplitude of the compensation current signal is gradually increased within a given current range, and the monitored amplitudes (I PD_LED_A_ON , I PD_LED_B_ON ) satisfies the compensation condition, and the amplitude of the compensation current signal is lower than the maximum value of the current range (I C，MAX );as well as If the compensation condition is met, the amplitude of the compensation current signal reaches the maximum value of the current range (I C，MAX ), then the second drive current (I B ) and setting the amplitude of the compensation current signal to the minimum value of the current range (I C，MIN ).

13. The method according to claim 11, wherein: When the amplitude of the second photodetection signal (I PD_LED_B_ON ) is greater than the amplitude of the first photodetection signal (I PD_LED_A_ON ) is higher, the method further comprises: The second drive current (I B ), and the compensation current signal is equal to the maximum value (I C，MAX ) is applied to the second channel (B); Monitor the amplitudes (I PD_LED_A_ON , I PD_LED_B_ON ); The monitored amplitudes (I PD_LED_A_ON , I PD_LED_B_ON ) satisfies the compensation condition and the amplitude of the compensation current signal is higher than the minimum value of the current range (I C，MIN ), while gradually reducing the amplitude of the compensation current signal within the given current range; and If the compensation condition is still met, the amplitude of the compensation current signal reaches the minimum value of the current range (I C，MIN ), the second drive current (I B ), and the amplitude of the compensation current signal is set to the maximum value of the current range (I C，MAX ).

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