A method for graded constant current dimming control of an adaptive ambient light non-invasive pulse oximeter

Abstract

This invention discloses an adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method, belonging to the field of non-invasive pulse oximeter graded constant current dimming control technology. It involves real-time acquisition of ambient light illuminance in the usage scenario, filtering out interference from power frequency lighting and electronic equipment flicker through double notch filtering, calculating the smoothed effective illuminance, matching it to a pre-calibrated illuminance-dimming level mapping table to obtain the target dimming level corresponding to red and infrared light-emitting diodes, and outputting the matched rated constant current value. An incremental PID algorithm is used to achieve constant current closed-loop control, combined with a full-temperature-range temperature drift compensation mechanism to correct current sampling errors, and a fixed step size is used to achieve smooth current transition when switching between dimming levels. This invention does not rely on the hysteresis feedback of the photoelectric receiver signal, avoiding dimming misjudgments caused by physiological signal fluctuations, balancing the signal-to-noise ratio of the detection signal in high ambient light scenarios and power consumption control in low ambient light scenarios, and effectively filtering out flicker interference.

Description

Technical Field

[0001] This invention relates to the field of non-invasive pulse oximeter graded constant current dimming control technology, and in particular to an adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method. Background Technology

[0002] Non-invasive pulse oximeters are portable medical devices widely used in clinical monitoring, home health monitoring, and the collection of exercise physiological indicators. They utilize photoplethysmography (PPG) technology, which uses specific wavelengths of red or infrared light-emitting diodes to transmit or reflect light through human tissue, collecting photoelectric signals to calculate blood oxygen saturation and pulse rate parameters. With the increasing prevalence of portable pulse oximeters, these devices need to adapt to various usage scenarios, including indoor lighting, outdoor strong light, and high / low temperature environments. The core development direction of the industry is to balance detection accuracy, light source power consumption, operational stability, and lifespan.

[0003] One of the mainstream light-emitting drive solutions for pulse oximeters is the fixed constant current drive solution. This solution involves calibrating the drive current of the red and infrared LEDs to a fixed rated value before shipment. Typically, a suitable current value for medium light intensity is selected to accommodate the acquisition needs of both high and low ambient light scenarios. The circuit structure only includes a basic constant current drive module, requiring no additional sensing or control logic. This solution is low-cost and simple to operate, and is widely used in entry-level home pulse oximeters. However, this solution cannot adapt to complex lighting environments. In high ambient light interference scenarios, the fixed current output light signal is easily overwhelmed by ambient light, leading to insufficient signal-to-noise ratio, inaccurate detection data, or even failure to output values. In low ambient light scenarios, the drive current is redundant, easily causing excessive heat generation and high power consumption, significantly shortening the battery life of portable devices.

[0004] Another mainstream solution is a continuous analog dimming scheme based on photoelectric signal feedback. The logic involves real-time acquisition of the output signal amplitude from the photoelectric receiver. If the signal amplitude is below a preset threshold, the drive current is gradually increased; if the signal amplitude is above the preset threshold, the drive current is gradually decreased. This approach can adapt to different ambient light scenarios to some extent, and its detection accuracy is superior to the fixed-current scheme. It is commonly used in mid-to-high-end pulse oximeters. However, this scheme relies on the delayed feedback of the photoelectric signal, resulting in a slow dimming response. It cannot distinguish whether signal amplitude fluctuations are caused by ambient light interference or physiological changes in human tissue, leading to potential dimming misjudgments. Furthermore, the lack of graded control logic makes current adjustment prone to jumps, causing photoelectric channel signal saturation. The absence of a constant current closed-loop and temperature compensation mechanism means that changes in ambient temperature and device temperature drift can easily cause the actual output current to deviate from the rated value. Flicker interference from power frequency lighting can also cause frequent current fluctuations, affecting detection stability and the lifespan of the light source.

[0005] The two existing driving solutions cannot simultaneously meet the requirements of anti-interference capability, low power consumption and operational stability in multiple scenarios, which has become the core technical bottleneck restricting the performance improvement of portable pulse oximeters. There is an urgent need for a targeted dimming control solution to solve the above problems. Summary of the Invention

[0006] This invention proposes a graded constant current dimming control method for an adaptive ambient light non-invasive pulse oximeter to solve the problems mentioned in the prior art, such as difficulty in balancing power consumption and anti-interference ability, easy misjudgment of dimming jumps, and current deviation caused by temperature drift.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: an adaptive ambient light non-invasive pulse oximeter with graded constant current dimming control, comprising the following steps: First, the reference output parameters of the dual-channel independent constant current drive module at the pulse oximeter probe end are initialized. A calibrated mapping table between ambient light illuminance ranges and LED constant current levels is pre-written into the controller's non-volatile storage medium. The dual-channel constant current drive module is independently connected to the anode power supply terminals of the red and infrared LEDs, respectively. A current sampling unit is connected in series on the common cathode side of the two LEDs. Then, an ambient light sensor embedded inside the probe's light-shielding silicone ring collects the original illuminance value of transmitted ambient light at the probe-to-finger contact interface at a fixed sampling frequency. After filtering out flicker interference from power frequency lighting sources using a built-in dual notch filter unit, the effective ambient light illuminance value is obtained through a sliding weighted recursive calculation. The calculation formula is: ,in To pre-set the forgetting factor, This is the filtered illuminance sample value for the current period. The effective illuminance value of the previous storage cycle; Next, the effective ambient light illuminance value is matched with the pre-stored mapping relationship table to determine the current suitable constant current output level. The constant current drive signal is then output to the red light and infrared light-emitting diodes according to the amplitude corresponding to the level, and the synchronous acquisition enable signal of the photoelectric acquisition channel is triggered simultaneously. Then, the output value of the current sampling unit is collected in real time, the deviation between the actual drive current and the corresponding rated current is calculated, and the PWM duty cycle compensation of the constant current drive circuit is dynamically adjusted to stabilize the output current within the allowable deviation range of the corresponding level. Finally, when a jump in effective ambient illuminance is detected across intervals, the constant current output amplitude is adjusted cycle by cycle according to a preset gradual change rule to avoid jumps in luminous intensity.

[0008] Furthermore, the mapping table is divided into at least six continuous, non-overlapping illuminance intervals based on ambient light illuminance ranging from 0 to the preset maximum permissible interference illuminance value. The boundary values ​​of each illuminance interval are individually calibrated according to the resistance of the LED light intensity output to ambient light, eliminating dimming dead zones. Each illuminance interval corresponds to a set of constant current output ratings for red and infrared LEDs. Low illuminance intervals correspond to low-amplitude constant current output ratings to reduce light source power consumption and heat generation, while high illuminance intervals correspond to high-amplitude constant current output ratings to resist ambient light interference. The difference in constant current output ratings between adjacent intervals is calibrated according to the linear response interval of LED luminous flux-current, ensuring that the junction temperature of the LED at each constant current level remains within the rated operating range, thus extending the lifespan of the light source.

[0009] Furthermore, when filtering out power frequency interference in the sampling channel, a second-order infinite impulse response structure is used to implement dual notch filter units at 50Hz and 60Hz. The -3dB bandwidth of each notch filter unit is set to 2Hz. The original illuminance value obtained by sampling is filtered in real time. During the filtering process, overflow judgment is performed on the sampled value, and abnormal sampled values ​​that exceed the sensor range are eliminated and do not participate in the effective illuminance calculation. Only the slow illuminance fluctuation signal in the frequency range of 0-20Hz is retained, completely eliminating the interference of the flicker component of AC-powered power frequency lighting source on the judgment of ambient light level, and avoiding frequent switching of dimming level due to flicker component.

[0010] Furthermore, when outputting graded constant current drive signals to the red and infrared LEDs, a strict time-division output mode is adopted. Within a single complete light acquisition cycle, the red constant current drive signal, the zero-level dark current acquisition segment, and the infrared constant current drive signal are output sequentially. The duration of each drive signal and the constant current amplitude of the corresponding dimming level satisfy a negative correlation matching relationship, calculated as follows: ,in This is the single-channel reference drive duration at the lowest dimming level. , These are the rated constant current values ​​for the red and infrared LEDs at the lowest dimming level, respectively. , This represents the rated constant current value for the red and infrared LEDs at the current dimming level. , These are the driving durations of the red and infrared light-emitting diodes at the current level, ensuring that the total effective light integration received by the photoelectric receiver remains constant within a single cycle, thus avoiding excessive fluctuations in signal amplitude under different dimming levels.

[0011] Furthermore, when dynamically adjusting the PWM duty cycle compensation of the constant current drive circuit, the difference between the actual current value of the circuit and the rated current value of the corresponding dimming level is used to obtain the current deviation value. The duty cycle adjustment is then calculated cycle by cycle using an incremental PID control algorithm. The calculation formula is as follows: ,in , , These are the proportional, integral, and derivative control coefficients, respectively. This represents the current deviation value for the current control cycle. , These are the current deviation values ​​for the first two control cycles, respectively. The duty cycle increment required for the current cycle is set with an upper limit for the duty cycle adjustment within a single control cycle to prevent high-frequency oscillation of the output current and ensure the accuracy of constant current output.

[0012] Furthermore, when collecting the forward voltage drop of each LED, the detection value of the NTC temperature sensor mounted next to the dual LEDs is read simultaneously. The forward voltage drop temperature coefficient and the resistance temperature coefficient of the current sampling resistor, which are pre-stored in the controller, are retrieved to perform real-time temperature drift compensation on the sampled value of the current sampling circuit. During the temperature drift compensation process, the zero-point drift of the instrumentation amplifier is corrected simultaneously. The compensation coefficient is written to the storage medium through high and low temperature environment calibration before leaving the factory, correcting the current sampling deviation at different operating temperatures. This ensures that within the common operating temperature range of the pulse oximeter, the deviation between the actual value and the rated value of the constant current output is always within the allowable range, avoiding light intensity drift caused by temperature changes.

[0013] Furthermore, when adjusting the constant current output amplitude according to the preset current gradual change step size, if the effective ambient illuminance jumps between two or more intervals, during the process of adjusting the constant current amplitude from the current output value to the target level rated value, the current difference is divided into multiple small adjustment steps. Only one step of the current value is adjusted in each control cycle. The current change in a single adjustment cycle does not exceed the fixed percentage of the lowest level rated current. The dimming level change status flag is not triggered during the gradual adjustment process to avoid false triggering of the level judgment logic during the gradual change process. During the adjustment process, the gain matching coefficient is sent to the photoelectric acquisition channel simultaneously to adjust the amplification factor of the photoelectric conversion link to avoid signal saturation of the acquisition channel caused by sudden changes in light intensity, and at the same time reduce the electrical stress impact of sudden current changes on the light-emitting diode.

[0014] Furthermore, when the calculated effective ambient light illuminance value is lower than the preset minimum illuminance threshold, which corresponds to a scenario where there is no external light interference and the light is completely blocked, the constant current output is locked at the lowest dimming level to minimize the power consumption and heat generation of the light source. When the effective ambient light illuminance value is higher than the preset maximum illuminance threshold, which corresponds to a scenario where the probe is completely detached and external light directly enters the sensor, an audible and visual alert for probe adhesion abnormality is triggered, and the constant current output is locked at the highest dimming level to avoid excessively high junction temperature of the light-emitting diode and accelerated aging caused by prolonged over-limit output.

[0015] Furthermore, during the actual current value acquisition process of the drive circuit, a high-precision milliohm-level metal film sampling resistor with a temperature coefficient not exceeding 50ppm / ℃ is connected in series in the common cathode circuit of the light-emitting diode. The differential voltage signal across the sampling resistor is amplified by a high common-mode rejection ratio instrumentation amplifier and then input to the controller's 12-bit or higher resolution analog-to-digital conversion channel. The gain value of the instrumentation amplifier is calibrated according to the resistance value of the sampling resistor and the sampling voltage value under the maximum rated current to ensure that the amplified output voltage under the maximum current does not exceed the reference voltage of the analog-to-digital conversion channel. The analog-to-digital sampling time is set at the conduction stabilization stage of each PWM cycle to avoid the spike noise at the moment of switching, ensuring that the current detection accuracy meets the deviation control requirements of graded dimming, and providing accurate sampling basis for constant current closed-loop control.

[0016] Furthermore, when multiple consecutive sampling cycles determine that there is no effective human tissue adhesion through the dark current amplitude of the photoelectric receiver, the constant current output path of the dual-channel light-emitting diodes is completely cut off, the enable port of the constant current driver chip is turned off, and the driver chip is placed in a sleep state to further reduce static power consumption. Only the low-power periodic sampling mode of the ambient light sensor is retained, and the sampling frequency is reduced to less than one-tenth of that in the working state. When an effective human tissue adhesion signal is detected and the effective ambient light illuminance value is continuously and stably reached for a preset duration, the constant current output of the current adaptive dimming level is automatically restored, which greatly reduces the power consumption of the light source in the non-working state and extends the single-charge battery life of the pulse oximeter.

[0017] Compared with existing technologies, the beneficial effects of this invention are: This invention achieves adaptive dimming for different ambient light scenarios by combining direct ambient light acquisition, pre-calibrated illuminance with a constant current level mapping table, and graded dimming technology. It effectively solves the defects of existing fixed constant current solutions, such as insufficient signal-to-noise ratio under high ambient light and excessive power consumption and heat generation under low ambient light. At the same time, it avoids the problems of existing continuous feedback dimming solutions, such as reliance on photoelectric signal lag feedback and susceptibility to dimming misjudgment caused by physiological signal fluctuations. This invention significantly improves the detection stability and equipment endurance under different lighting environments.

[0018] This invention achieves high-precision constant current output and shock-free dimming transition through incremental PID constant current closed-loop control, dual notch filtering for power frequency interference, full-temperature range temperature drift compensation, and smooth current transition across different levels. It effectively solves the defects of existing dimming solutions, such as frequent current jitter caused by power frequency flicker interference, current deviation caused by ambient temperature changes, and signal saturation caused by current jumps during dimming. It not only ensures the accuracy of constant current output under multi-interference scenarios, but also reduces the electrical stress impact of current sudden changes on light-emitting diodes, effectively extending the life of the light source.

[0019] This invention's technical solution is compatible with various pulse oximetry probes, including transmissive and reflective types. It only requires adding a low-power ambient light sensing unit to the existing pulse oximeter hardware, resulting in low hardware modification costs and a simple batch calibration process. It is suitable for various application scenarios such as home health monitoring, clinical bedside monitoring, and outdoor sports monitoring, and is compatible with pulse oximeter products of different power consumption levels, possessing high industry promotion value. The low-power control logic in non-working state can further reduce the standby power consumption of portable devices, adapting to the usage needs of wearable pulse oximetry products with small-capacity batteries, and automatically adapting to the usage needs of all scenarios without requiring manual parameter adjustments by the user. Attached Figure Description

[0020] Figure 1 This is the overall flowchart of the adaptive ambient light non-invasive pulse oximeter graded constant current dimming control proposed in this invention; Figure 2 This is a flowchart of the ambient light acquisition, filtering, and dimming level matching process of the present invention; Figure 3 This is a flowchart of the dual-path light-emitting driver time-division output control of the present invention; Figure 4 This is a flowchart of the constant current drive closed-loop regulation and temperature drift compensation of the present invention; Figure 5 This is a flowchart of the dimming level cross-zone jump and abnormal state locking of the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Reference Figures 1 to 5 This invention discloses a graded constant current dimming control method for an adaptive ambient light non-invasive pulse oximeter, including initial parameter configuration, real-time ambient light sampling and filtering, dimming level matching, constant current closed-loop control, and cross-level gradual adjustment full-process control.

[0023] The method operates with a low-power microcontroller as the core control carrier. The microcontroller is externally connected to a dual-channel independent adjustable constant current drive module, an ambient light acquisition unit, a current sampling unit, a temperature detection unit, and a photoelectric signal acquisition link. After powering on, the device first completes the initialization configuration of each peripheral interface. It then reads the pre-calibrated mapping table between ambient light intensity ranges and LED constant current levels from the built-in non-volatile storage medium, initializes the initial output parameters of the dual-channel constant current drive module, and sets the initial drive current of the red and infrared LEDs to the intermediate rated values. The dual-channel constant current drive module is independently connected to the anode power supply terminals of the 660nm wavelength red LED and the 940nm wavelength infrared LED, respectively. A low-temperature drift current sampling resistor is connected in series on the common cathode side of the two LEDs, and all drive current flowing through the LEDs returns to ground through the sampling resistor.

[0024] The ambient light acquisition unit uses a digital illuminance sensor, embedded on the outer edge of the probe's light-shielding silicone ring. The sensor surface is covered with a diffuser, and the field of view is set to 60 degrees. It only receives ambient light incident from the skin through the gap in the light-shielding ring, avoiding direct reception of light emitted from its own LED. The sensor collects the original illuminance value of transmitted ambient light at the interface between the probe and the fingertip at a fixed sampling frequency of 200Hz. The sampled signal is sent to a built-in dual notch filter unit to filter out flicker interference from power frequency lighting sources. The effective ambient light illuminance value is then calculated using a sliding weighted recursive method, as shown in the formula: The forgetting factor in the formula is a preset fixed value used to balance the illuminance response speed and signal smoothness. The illuminance sample value after double notch filtering in the current period is weighted and calculated with the effective illuminance value stored in the previous period to output a stable illuminance value that can be directly used for level judgment. Applied to the real-time ambient light sampling process, it can effectively filter out occasional illuminance spikes and avoid frequent changes in dimming level. All parameters in the calculation process are in lux, and the result after weighted calculation is still in lux, which meets the physical dimension requirements of illuminance. The microcontroller performs interval matching between the calculated effective ambient light illuminance value and the pre-stored mapping table to determine the current suitable constant current output level. According to the amplitude corresponding to the level, it outputs constant current drive signals to the red and infrared light-emitting diodes in a time-division manner. Simultaneously, it sends a synchronous acquisition enable signal to the photoelectric acquisition channel through a dedicated IO port. The drive photoelectric link only performs effective signal integration during the emission period.

[0025] During the control process, the microcontroller collects the output voltage of the current sampling unit in real time, converts it into the actual drive current value, calculates the deviation between the actual drive current and the corresponding rated current, and dynamically adjusts the PWM duty cycle compensation of the constant current drive circuit to stabilize the output current within the allowable deviation range of the corresponding level.

[0026] When a jump in effective ambient illuminance is detected across intervals, the microcontroller adjusts the constant current output amplitude periodically according to a preset gradual change rule, so that the current smoothly transitions to the target level value, avoiding the impact of the light intensity jump on the acquisition link.

[0027] This invention also discloses a method for constructing a mapping table. The mapping table is divided into six continuous, non-overlapping illuminance intervals based on ambient light illuminance ranging from 0 to a preset maximum permissible intrusion illuminance value. The boundary values ​​of each illuminance interval are calibrated in a standard darkroom environment. During calibration, the probe is fixed on a standard human fingerprint, and an adjustable standard light source is used to simulate ambient intrusion light of different intensities at the probe gap. Starting from 0 lux, the illuminance is gradually increased, and the driving current of the light-emitting diode is gradually decreased after each adjustment of the illuminance until the signal-to-noise ratio of the photoelectric receiver meets the measurement requirements. The current value at this point is recorded as the rated driving current of that illuminance point. The range between adjacent illuminance recording points is thus formed as a dimming interval, and there is no dimming dead zone.

[0028] Each illuminance range corresponds to a set of constant current output ratings for red and infrared LEDs. The low illuminance range corresponds to a low amplitude constant current output rating to reduce power consumption and heat generation of the light source, while the high illuminance range corresponds to a high amplitude constant current output rating to resist ambient light interference. The difference in constant current output ratings between adjacent ranges is calibrated according to the linear response range of LED luminous flux-current, ensuring that the junction temperature of the LEDs at each constant current level is always within the rated operating range, thus extending the lifespan of the light source.

[0029] This invention also discloses the specific implementation logic for power frequency interference filtering. When filtering power frequency interference within the sampling channel, a second-order infinite impulse response structure is used to implement dual notch filter units at 100Hz and 120Hz, corresponding to the twice-frequency flicker components of a 50Hz and 60Hz AC-powered lighting source, respectively. The -3dB bandwidth of each notch filter unit is set to 2Hz. The filter coefficients are pre-calculated using the bilinear transform method and stored in the microcontroller's memory, achieving deep attenuation of the flicker components without the need for additional hardware filtering circuitry.

[0030] The microcontroller performs real-time filtering on the sampled raw illuminance values. During the filtering process, overflow judgment of the sampled values ​​is performed simultaneously, and abnormal sampled values ​​that exceed the sensor's range are eliminated. Abnormal values ​​are not included in the effective illuminance calculation and are directly replaced by the effective illuminance value of the previous cycle. The filtering link only retains slow illuminance fluctuation signals in the 0-20Hz frequency range, which can accurately identify slow-changing scenarios such as probe light leakage and displacement, and completely eliminate the interference of flicker components from AC-powered power frequency lighting sources on the judgment of ambient light levels, avoiding frequent switching of dimming levels due to flicker components.

[0031] This invention also discloses a control logic for time-division driving and timing matching. When outputting graded constant current drive signals to the red and infrared LEDs, a strict time-division output mode is adopted. The total duration of a single complete light acquisition cycle is fixed at 10 milliseconds. Within the cycle, the red constant current drive signal, the zero-level dark current acquisition segment, the infrared constant current drive signal, and the signal processing idle segment are output sequentially. The zero-level dark current acquisition segment has a fixed duration of 0.5 milliseconds. During this period, both LEDs are turned off, and the photoelectric link acquires the ambient light DC component and the circuit dark current value as a baseline.

[0032] The duration of each drive signal and the constant current amplitude of the corresponding dimming level are negatively correlated, and the calculation formula is as follows: In the formula, the reference drive duration is the single-channel emission duration at the lowest dimming level, and the reference rated current for the red and infrared channels is the corresponding constant current output value at the lowest dimming level. The rated constant current values ​​for the red and infrared channels at the current level are obtained from the mapping table. The drive duration of the red and infrared LEDs at the current level is calculated. During the calculation, the reference drive duration is in milliseconds, and the units for both the reference current and the current at the current level are milliamperes. The current ratio is dimensionless, and the calculated current drive duration is in milliseconds, which meets the time dimension requirements. Through the matching design of duration and current, the total effective light integration received by the photoelectric receiver can be kept constant within a single cycle, avoiding excessive fluctuations in signal amplitude at different dimming levels and reducing the dynamic range requirements for photoelectric link gain adjustment.

[0033] This invention also discloses the incremental PID implementation logic for constant current closed-loop control. When dynamically adjusting the PWM duty cycle compensation of the constant current drive loop, the microcontroller uses a fixed control cycle of 1 millisecond. It calculates the current deviation value by subtracting the actual current value of the acquired loop from the rated current value of the corresponding dimming level. The duty cycle adjustment is then calculated cycle by cycle using an incremental PID control algorithm. The calculation formula is as follows: In the formula, the proportional, integral, and derivative control parameters are fixed values ​​set at the factory. The current deviation is the difference between the actual current and the target current in each control cycle. The system stores historical deviation values ​​from the previous two control cycles for calculation of the derivative and integral terms, and finally outputs the PWM duty cycle increment that needs to be adjusted in the current cycle. The unit of current deviation is milliamperes (mA), the dimension of the proportional coefficient is per milliampere of duty cycle, and the dimensions of the integral and derivative coefficients are consistent with the proportional coefficient after conversion through a fixed control cycle. The calculated duty cycle increment is a dimensionless value, corresponding to the duty cycle adjustment range of the PWM output, with completely unified dimensions.

[0034] The duty cycle adjustment within a single control cycle has an upper limit, and the adjustment range in a single instance cannot exceed 2% of the full duty cycle, thus avoiding high-frequency oscillations in the output current and ensuring the accuracy of the constant current output. The incremental algorithm does not have the problem of integral saturation, which can avoid overshoot during rapid current adjustment and prevent excessive instantaneous current from causing electrical stress on the LED.

[0035] This invention also discloses a specific implementation method for temperature drift compensation. When acquiring the forward voltage drop of each LED, the microcontroller synchronously reads the detection value of the NTC temperature sensor mounted next to the pads of the dual LEDs. The distance between the NTC device and the LED pins is no more than 2 mm, which can accurately reflect the temperature value near the operating junction temperature of the LED.

[0036] The microcontroller retrieves the forward voltage drop temperature coefficient of the LED, the resistance temperature coefficient of the current sampling resistor, and the zero-point drift coefficient of the instrumentation amplifier, all calibrated before leaving the factory under high and low temperature environments. It then performs real-time temperature drift compensation on the sampled values ​​of the current sampling circuit. The compensation coefficient is calculated using linear interpolation based on the current detection temperature, correcting current sampling deviations at different operating temperatures. This ensures that within the common operating temperature range of the pulse oximeter, the actual value of the constant current output remains within the allowable range compared to the rated value, avoiding light intensity drift caused by temperature changes. The temperature drift compensation parameters are calibrated at 0°C, 25°C, and 40°C before leaving the factory and written to a non-volatile storage medium. These parameters are automatically retrieved during device operation, eliminating the need for manual calibration by the user.

[0037] This invention also discloses a current gradient control logic for cross-level dimming. When the effective ambient illuminance jumps between two or more ranges, during the adjustment of the constant current amplitude from the current output value to the target level rated value, the microcontroller divides the total current difference into multiple small adjustment steps. In each control cycle, only one step of the current value is adjusted, and the current change in a single adjustment cycle does not exceed one-quarter of the lowest level rated current, ensuring a smooth current adjustment process without abrupt changes.

[0038] During the gradual adjustment process, the system pauses the dimming level judgment logic and does not respond to new level matching results. It must wait until the current adjusts to the target value and stabilizes for 100 milliseconds before restarting the illuminance matching and level judgment process to avoid false triggering of the level judgment logic during the gradual adjustment. During the current gradual adjustment, the microcontroller synchronously sends a gain matching coefficient to the photoelectric acquisition channel, adjusting the amplification factor of the photoelectric conversion link according to the current adjustment ratio. This ensures that the output signal of the photoelectric link is always within the optimal input range for analog-to-digital conversion, preventing signal saturation of the acquisition channel due to sudden changes in light intensity, and reducing the electrical stress impact of current surges on the LEDs.

[0039] This invention also discloses a protection control logic for abnormal illuminance thresholds. The system is set with a minimum illuminance threshold and a maximum illuminance threshold. When the calculated effective ambient light illuminance value is lower than the preset minimum illuminance threshold, corresponding to a scenario where the probe is fully attached and there is no external light interference, the microcontroller locks the constant current output at the lowest dimming level to minimize light source power consumption and heat generation, reducing the temperature rise of the finger contact surface. When the effective ambient light illuminance value is higher than the preset maximum illuminance threshold, corresponding to a scenario where the probe is detached or the gap is too large, allowing direct external light to enter, the microcontroller triggers an audible and visual alert for abnormal probe attachment, and simultaneously locks the constant current output at the highest dimming level to prevent the level judgment logic from malfunctioning within the threshold range. The rated current value of the highest dimming level is set as the upper limit of the rated current for long-term operation of the LED, ensuring that even with continuous output, the junction temperature of the LED will not become too high, effectively avoiding the problem of accelerated aging of the light source caused by prolonged over-limit output.

[0040] This invention also discloses a hardware implementation scheme for the current sampling circuit. During the actual current value acquisition process of the drive circuit, a high-precision metal film sampling resistor with a temperature coefficient not exceeding 50 ppm per degree Celsius is connected in series in the common cathode circuit of the light-emitting diode. The sampling resistor has an accuracy of 1%, and its rated power meets the power consumption requirements under the maximum current, generating almost no additional heat.

[0041] The differential voltage signal across the sampling resistor is amplified by a high common-mode rejection ratio zero-drift instrumentation amplifier and then input to the microcontroller's 12-bit or higher resolution analog-to-digital converter channel. The gain of the instrumentation amplifier is calibrated according to the resistance value of the sampling resistor and the sampling voltage value under the maximum rated current to ensure that the amplified output voltage under the maximum current does not exceed the reference voltage of the analog-to-digital converter channel, thus making full use of the dynamic range of the analog-to-digital converter.

[0042] The analog-to-digital sampling time is set during the conduction stabilization phase of each PWM cycle to avoid the current spike noise at the moment of switching. Each sampling continuously collects 8 points and performs an arithmetic average to further reduce the impact of random noise, ensure that the current detection accuracy meets the deviation control requirements of graded dimming, and provide accurate sampling basis for constant current closed-loop control.

[0043] The current setting pin of the constant current drive chip is connected to the PWM output channel of the microcontroller through a first-order RC filter network. The PWM signal is smoothed into a DC control voltage by the RC network. The constant current output current can be linearly adjusted by adjusting the duty cycle. No additional digital-to-analog converter chip is required, which can effectively reduce hardware costs.

[0044] This invention also discloses low-power control logic in the non-operating state. When the dark current amplitude and reflected light intensity of the photoelectric receiver determine that there is no effective human tissue adhesion after three consecutive acquisition cycles, the microcontroller completely cuts off the constant current output path of the dual-channel light-emitting diodes, closes the enable port of the constant current driver chip, and puts the driver chip into a sleep state to reduce static power consumption. At the same time, the microcontroller is configured to a low-power stop mode, retaining only the low-power periodic sampling mode of the ambient light sensor, reducing the sampling frequency to less than one-tenth of that in the operating state. At this time, the standby current of the entire probe circuit can be reduced to the microamp level.

[0045] When the ambient light sensor detects a jump in illuminance that matches the characteristics of human body contact, it triggers a microcontroller wake-up interrupt, and the system starts the initialization process. After a valid human tissue contact signal is detected and the valid ambient light illuminance value remains stable for 2 seconds, the constant current output of the current dimming level is automatically restored, which greatly reduces the power consumption of the light source in non-working state and extends the battery life of the pulse oximeter's built-in battery on a single charge.

[0046] Example 1

[0047] Users use a clip-on pulse oximeter for daily pulse oximetry monitoring in their living rooms. The indoor lighting is a 50Hz power-driven LED ceiling light. During the day, there is natural light entering from the window. When the user first clamps the probe, it is not fully pressed down, resulting in light leakage at the edges. Subsequently, the user gradually presses the probe down until it is fully in place.

[0048] The device initially operates in low-power standby mode. The ambient light sensor collects ambient illuminance at a frequency of 1Hz. When a user inserts their finger into the probe clip, the sensor detects a rapid drop in illuminance from the normal indoor illuminance of 3000 lux to 520 lux, triggering system wake-up. After startup, the microcontroller first processes the original illuminance sample value through a dual notch filter link, attenuating the 100Hz lamp flicker component to less than 1% of its original value. Subsequently, through sliding weighted calculation, the effective illuminance value is obtained as 490 lux, matching the corresponding dimming level to level 5, with a target output current of 10 mA.

[0049] Since the initial current value is 0 when the system wakes up from the hibernation state, the microcontroller starts the current gradual change process, which divides the total current difference of 10 mA into 20 adjustment steps, adjusting by 0.5 mA every 1 millisecond, and smoothly reaches the target current value after 20 milliseconds. During the adjustment process, a gain adjustment signal is sent to the optoelectronic link at the same time to ensure the stability of the signal amplitude.

[0050] After the current stabilizes, the incremental PID control loop starts, acquiring the actual current value every 1 millisecond, calculating the deviation, and adjusting the PWM duty cycle. At this time, the ambient temperature is 25 degrees Celsius, the temperature drift compensation coefficient is 1, and the current stabilization accuracy is controlled within 2% of the target value. As the user gradually presses the probe, the effective illuminance slowly decreases from 490 lux to 4 lux. A smooth transition process is initiated every time a dimming interval is crossed, gradually reducing the output current to the lowest level of 2 mA, with no current jumps throughout the process.

[0051] The code snippet for the core control logic in this scenario is as follows: / / Core logic for effective illuminance calculation and level matching #define ALPHA 0.3 float E_filtered = double_notch_filter(raw_adc); / / Double notch filter float E_eff = ALPHA * E_filtered + (1-ALPHA) * E_last; uint8_t target_level = find_map_level(E_eff); / / Match the map level if(target_level != current_level) { start_gradual_ramp(target_level); / / Start gradient adjustment } E_last = E_eff; In this scenario, dual notch filtering can completely filter out the power frequency flicker interference of household LED lights, avoiding frequent dimming level jumps caused by flicker; the graded constant current mechanism automatically increases the current during the light leakage stage to ensure the signal-to-noise ratio, and automatically reduces the current after full bonding to reduce heat generation and power consumption; the current gradient mechanism avoids signal saturation and measurement jumps caused by current jumps during the adjustment process, solving the problems of high power consumption in low light, poor anti-interference ability in strong light, and dimming jumps in traditional fixed current solutions.

[0052] Example 2

[0053] When a user uses a pulse oximeter to monitor blood oxygen levels after exercise in a winter outdoor environment with an ambient temperature of 0 degrees Celsius and high outdoor natural light, and the user's arm swings naturally while walking, there are intermittent minor light leaks between the probe and the finger.

[0054] After the device is woken up, the NTC temperature sensor detects that the current operating temperature is 0 degrees Celsius. The microcontroller retrieves the pre-stored temperature drift compensation parameters, calculates the compensation coefficient between the sampling resistor value and the amplifier zero point, and corrects the current sampling value in real time. In low-temperature scenarios, the full-link temperature drift compensation mechanism solves the constant current output deviation problem caused by device parameter drift in outdoor low-temperature environments, and avoids the decrease in ambient light resistance due to insufficient output current at low temperatures, or the aging of the light-emitting diode junction temperature due to excessive current. The graded constant current dimming mechanism automatically increases the drive current in high-illuminance outdoor environments to ensure that the light output intensity meets the acquisition requirements under strong ambient light. In low-illuminance environments where clothing blocks the light, it automatically reduces the drive current to reduce ineffective power consumption and heat generation at the contact surface. The dual notch filter can effectively filter out high-frequency PWM flicker interference from the screen of the user's portable electronic device, avoiding misjudgment and frequent switching of dimming level caused by flicker components. The current gradient mechanism addresses intermittent light leakage during walking, preventing sudden increases and decreases in current due to brief changes in illuminance. This reduces the electrical stress impact of sudden current changes on the LED and avoids signal saturation in the photoelectric receiving channel caused by rapid changes in light intensity. The low-power standby mechanism automatically enters sleep mode when the user removes the probe and puts it in their pocket, retaining only low-frequency ambient light sampling, which significantly reduces battery power consumption in low-temperature environments and improves the device's battery life.

[0055] In the actual calibration process, all mapping table parameters, compensation coefficients, and control parameters can be calibrated in batches using automated testing fixtures, eliminating the need for manual debugging of individual devices and effectively reducing calibration costs during mass production.

[0056] During calibration, the fixture outputs ambient light of a fixed intensity through a standard illuminance source, collects the actual current value of the circuit through a high-precision ammeter, automatically records the parameter values ​​under different illuminance and temperature and writes them into the device's storage medium. Once the parameters are written, the device can operate normally without subsequent manual maintenance.

[0057] During operation, the system will not experience a decrease in dimming accuracy due to the long-term aging of the LEDs. The constant current closed-loop control can offset the current deviation caused by the aging of the LEDs' forward voltage drop in real time. As long as the LEDs can conduct normally, the output current can be stabilized at the target rated value, effectively extending the overall service life of the equipment.

[0058] Reference Figure 1This flowchart illustrates the overall execution process of the adaptive ambient light non-invasive pulse oximeter's dimming control. First, the system initializes the baseline parameters of the dual-path light-emitting drive module and constructs a mapping table between ambient light illuminance and constant current level. Then, the sensor collects the intrusive illuminance at the probe interface in real time and filters out power frequency interference before outputting the effective value. Next, the system matches the appropriate dimming level based on the effective illuminance value and outputs a graded constant current drive signal with corresponding amplitude. During operation, the system collects the actual current and on-state voltage drop in real time, dynamically compensates and adjusts the duty cycle to stabilize the output current within the allowable range. If a jump in illuminance across zones is detected, the system smoothly adjusts the output amplitude according to a preset step size.

[0059] Reference Figure 2 This flowchart details the ambient light signal processing and level determination mechanism. After acquiring the raw illuminance data, the acquisition end filters out specific frequencies of power frequency lighting interference through a dual notch filter window, effectively preserving the true illuminance fluctuations in the low-frequency band. After obtaining the effective ambient light illuminance, the system uses a pre-stored continuous non-overlapping interval mapping table for comprehensive determination. When the effective illuminance is below the preset lower limit, the constant current output is directly locked to the lowest dimming level; if it exceeds the preset upper limit, the probe is determined to be in poor contact and an immediate prompt is triggered, while the system is locked to the highest dimming level, thereby avoiding the impact of extreme light interference on the accuracy of blood oxygen detection.

[0060] Reference Figure 3 This flowchart illustrates the timing allocation and drive signal generation logic of the dual light source in a pulse oximeter. Upon entering a single optical signal acquisition cycle, the system employs a time-division output mode to avoid crosstalk between red and infrared light. The controller sequentially activates the red constant current drive signal, the zero-level interval segment, and the infrared constant current drive signal. In the time-division drive process, the specific duration of each signal is dynamically set according to the currently adapted dimming level, and the duration exhibits a strict negative correlation with the constant current amplitude. Based on this, the system synchronously triggers an optical signal acquisition enable signal that perfectly matches the constant current output timing, ensuring spatiotemporal consistency between the acquisition and emission ends.

[0061] Reference Figure 4 This flowchart illustrates the refined closed-loop control steps of the underlying hardware feedback loop. In the sampling stage, the system uses a milliohm-level sampling resistor to acquire the electrical signal of the LED cathode circuit. After amplification by the instrument and high-resolution analog-to-digital conversion, the actual current value is obtained. To eliminate the influence of temperature on component characteristics, the system simultaneously acquires the detection value from the probe's built-in temperature sensor and performs temperature drift compensation on the forward conduction voltage drop based on pre-stored relationships. After obtaining the accurate current deviation, the system executes an incremental proportional-integral-derivative (PID) control algorithm to calculate the duty cycle adjustment amount cycle by cycle and controls the adjustment amount within the limiting threshold, ultimately ensuring constant current accuracy and system stability.

[0062] Reference Figure 5This flowchart illustrates the system's handling mechanism for complex ambient light fluctuations and smooth transitions between different light levels. When ambient light intensity experiences a significant jump across different ranges, the controller prevents transient steps in the drive current. The system then switches to a preset current gradient step size, gradually adjusting the constant current output amplitude towards the target level, ensuring that the current change in a single cycle does not exceed a fixed percentage of the rated current. During this gradual current amplitude adjustment process, the drive module synchronously sends a gain matching signal to the optical signal acquisition channel, coordinating with the receiver to adjust the amplification gain synchronously, effectively preventing signal saturation distortion in the acquisition channel caused by sudden changes in luminous intensity.

[0063] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for graded constant current dimming control of an adaptive ambient light non-invasive pulse oximeter, characterized in that, Includes the following steps: S1. Initialize the reference constant current output parameters of the dual-channel light emission drive module of the pulse oximeter, and pre-build a mapping relationship table between the ambient light illuminance range and the light emission drive constant current level. The dual-channel light emission drive module is used to drive the red light emission diode and the infrared light emission diode respectively. S2. By using an ambient light sensor deployed on the contact surface of the pulse oximeter probe, the original value of ambient light interference illuminance at the contact interface between the probe and the human body detection part is collected in real time, and the effective ambient light illuminance value is output after filtering out the power frequency interference in the sampling channel. S3. Match the effective ambient light illuminance value with the pre-stored mapping table to determine the current adaptive constant current dimming level. According to the determined dimming level, output the corresponding amplitude graded constant current drive signal to the red light and infrared light-emitting diodes, and synchronously trigger the light signal acquisition enable signal that matches the constant current output timing. S4. Real-time acquisition of the forward voltage drop of each LED and the actual current value of the drive circuit. Based on the deviation between the actual current value and the corresponding rated current value, dynamically adjust the PWM duty cycle compensation of the constant current drive circuit to stabilize the output current of the circuit within the allowable deviation range of the corresponding dimming level. S5. When a jump in the effective ambient light intensity value is detected across intervals, the constant current output amplitude is adjusted according to the preset current gradual change step size.

2. The adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method according to claim 1, characterized in that, The mapping table is divided into multiple continuous and non-overlapping illuminance intervals according to the ambient light illuminance from low to high. Each illuminance interval corresponds to a set of constant current output ratings for red and infrared light-emitting diodes. The low illuminance interval corresponds to the low amplitude constant current output rating, and the high illuminance interval corresponds to the high amplitude constant current output rating. The difference in constant current output rating between adjacent intervals is calibrated according to the linear response interval of the luminous flux of the light-emitting diode.

3. The adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method according to claim 2, characterized in that, The process of filtering out power frequency interference in the sampling channel is as follows: the original illuminance value is filtered using a dual notch filter window of 50Hz and 60Hz, retaining the effective illuminance fluctuation signal in the frequency range of 0-20Hz, and eliminating the influence of light interference from power frequency lighting sources on the judgment of ambient light level.

4. The adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method according to claim 1, characterized in that, The red and infrared light-emitting diodes are output with corresponding amplitude-level constant current drive signals. Specifically, a time-division output mode is adopted, and the red light constant current drive signal, the zero-level interval segment, and the infrared light constant current drive signal are output sequentially within a single optical signal acquisition cycle. The duration of each drive signal is negatively correlated with the constant current amplitude of the corresponding dimming level.

5. The adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method according to claim 1, characterized in that, The dynamic adjustment of the PWM duty cycle compensation of the constant current drive circuit is specifically as follows: the difference between the actual current value of the circuit and the rated current value of the corresponding dimming level is used to obtain the current deviation value, and the duty cycle adjustment amount is calculated cycle by cycle through the incremental PID control algorithm. The duty cycle adjustment amount in a single control cycle does not exceed the preset limit value.

6. The adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method according to claim 5, characterized in that, When the forward voltage drop of each LED is collected in real time, the temperature detection value of the built-in temperature sensor of the probe is collected simultaneously. Based on the pre-stored LED temperature-forward voltage drop correspondence, the sampling value of the current detection circuit is compensated for temperature drift.

7. The adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method according to claim 1, characterized in that, The adjustment of the constant current output amplitude according to the preset current gradual change step size is specifically as follows: during the process of adjusting the constant current amplitude from the current output value to the target level rated value, the current change in a single adjustment cycle does not exceed a fixed percentage of the rated current, and a gain matching signal is sent to the optical signal acquisition channel simultaneously during the adjustment process.

8. The adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method according to claim 1, characterized in that, When the calculated effective ambient light illuminance value is lower than the preset minimum illuminance threshold, the constant current output is locked at the lowest dimming level; when the effective ambient light illuminance value is higher than the preset maximum illuminance threshold, a probe adhesion abnormality warning is triggered, and the constant current output is locked at the highest dimming level.

9. The adaptive ambient light non-invasive pulse oximeter graded constant current dimming control method according to claim 1, characterized in that, During the numerical acquisition of the actual current value of the drive circuit, a milliohm-level sampling resistor is connected in series in the cathode circuit of the light-emitting diode. The sampled voltage signal is amplified by the instrumentation amplifier and then input to the analog-to-digital conversion channel of the controller. The sampling resolution is not less than 12 bits.

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