Control of a light source in a pulse oximeter

Abstract

A method for controlling a light source (3, 3a, 3b) of a pulse oximeter (1) comprises: receiving a first input value (17) indicating an electrical quantity with respect to the light source, wherein the electrical quantity comprises an electric current flowing through the light source and / or an electrical voltage applied to the light source; receiving a second input value (19) indicating an amplitude of a sensor signal (11) generated by a light sensor (5) of the pulse oximeter; determining at least one limit value for the electrical quantity using the second input value, wherein the at least one limit value is associated with the smallest setpoint (t) or the largest setpoint (t + ε) of a setpoint range (21) within which the amplitude is to lie; determining at least one deviation value (Δ) indicating a deviation of the first input value from the at least one limit value;Generating a control signal (23) to control the light source using at least one deviation value, such that the amplitude approaches the setpoint range.

Description

Technical field

[0001] The invention relates to a method for controlling a light source of a pulse oximeter. Furthermore, the invention relates to a control unit, a computer program, a computer-readable medium for executing the method, and a pulse oximeter. State of the art

[0002] A pulse oximeter is generally a device for the non-invasive determination of arterial oxygen saturation by measuring light absorption or light remission when shining a light through perfused body tissue. Such a pulse oximeter can also be used for pulse rate monitoring.

[0003] Pulse oximeters are increasingly used in portable, battery-powered applications. For example, a pulse oximeter can be attached to a patient during emergency transport and remain with them during transfers between different hospital departments. Furthermore, pulse oximeters can be used as plug-in modules for multiparameter patient monitors with limited power budgets. Such applications are driving increasing demand for pulse oximeters with lower power consumption. Disclosure of the invention

[0004] One object of the invention is to provide a method for reducing the power consumption of a pulse oximeter, particularly a battery-operated pulse oximeter, without significant loss of performance. A further object of the invention is to provide a control unit, a computer program, and a computer-readable medium for carrying out such a method, as well as a corresponding pulse oximeter.

[0005] These problems are solved by the subject matter of the independent claims. Advantageous embodiments of the invention are set out in the dependent claims, the following description, and the accompanying figures.

[0006] A first aspect of the invention relates to a method for controlling an (electrical) light source of a pulse oximeter. The pulse oximeter further comprises a light sensor configured to convert a portion of light transmitted and / or reflected by a body part when irradiated with light from the light source into an (electrical) sensor signal.The method comprises: receiving a first input value indicating an electrical quantity with respect to the light source, wherein the electrical quantity comprises an electric current flowing through the light source and / or an electrical voltage applied to the light source; receiving a second input value indicating an amplitude of the sensor signal; determining at least one limit value for the electrical quantity using the second input value, wherein the at least one limit value is associated with the smallest or largest setpoint of a setpoint range within which the amplitude is to lie; determining at least one deviation value indicating a deviation of the first input value from the at least one limit value; generating a control signal to control the light source using the at least one deviation value so that the amplitude approaches the setpoint range.

[0007] The method makes it possible to save power during the operation of the pulse oximeter by, for example, reducing the current flowing through the light source, hereinafter referred to as light source current, on the one hand as long as the quality of the photoplethysmogram (PPG) from the sensor signal is still sufficient and the required accuracy is still given, and on the other hand increasing it if stronger noise occurs or the quality of the sensor signal generally deteriorates.

[0008] To ensure sufficient quality in the determination of oxygen saturation, the pulsatile component of the sensor signal in PPG, also called AC component, should not fall below a certain value.

[0009] In general, the amplitude of the sensor signal behaves approximately linearly with respect to the light source current in every patient. This is because physiological factors that influence the light absorption of perfused body tissue, such as finger thickness or skin color, do not change significantly during a measurement.

[0010] To achieve the target value, the light source current can be adjusted accordingly during operation. However, to avoid excessively large and / or frequent adjustments, a target value range for the amplitude should be defined. As long as the AC component remains within the target value range, no adjustment of the light source current is necessary. If the AC component falls below the lowest target value within the range, the light source current is increased accordingly. Conversely, if the AC component exceeds the highest target value within the range, the light source current is decreased accordingly.

[0011] The process can be computer-implemented.

[0012] The term "sensor signal" can refer to an analog or digital signal. "Amplitude" can specifically refer to the amplitude of the AC component of the sensor signal.

[0013] The term "electrical quantity" can also refer to an electrical quantity based on current and / or voltage, and / or an electrical quantity that influences current and / or voltage. For example, the electrical quantity could also be at least one of the following: an adjustable series resistor connected to the light source; a frequency at which the light source is switched on and off; a time ratio between the on-phases, in which the light source is switched on, and the off-phases, in which the light source is switched off. For example, the frequency could be a clock frequency and / or the time ratio a duty cycle in the context of pulse-width modulation of the light source.

[0014] The term "setpoint," as in "lowest setpoint" or "highest setpoint," can refer to a predetermined constant. However, it is also possible that the lowest or highest setpoint is varied during operation, or that both setpoints are varied, for example, depending on a (measured or estimated) change in certain physiological factors of the respective patient.

[0015] A second aspect of the invention relates to a control unit. The control unit comprises means configured to carry out the method described above and below.

[0016] The means can include hardware and / or software modules. In particular, the means can include a processor configured to execute the (computer-implemented) procedure. Additionally, the means can include memory and / or a data communication interface for wireless and / or wired data communication with peripheral devices, such as a smartphone, smartwatch, tablet, laptop, PC, or ventilator. Alternatively, the control unit can be implemented solely as hardware, for example, in the form of an ASIC (application-specific integrated circuit) or FPGA (field-programmable gate array) device.

[0017] It should be noted that features of the procedure described above and below may also be features of the control unit (and vice versa).

[0018] A third aspect of the invention relates to a pulse oximeter. The pulse oximeter comprises a light source, a light sensor, and a control unit, as described above and below. The light sensor is configured to convert a portion of light transmitted and / or reflected by a body part when irradiated with light from the light source into an (electrical) sensor signal. The pulse oximeter, for example its control unit, can be configured to determine an oxygen saturation (sO2) and / or a pulse from the sensor signal by absorption spectroscopy.

[0019] The term "light source" can refer, for example, to a light-emitting diode, a laser diode, an incandescent lamp, or a combination of at least two of these. In particular, the light source can be configured to emit light in at least two different predetermined wavelength ranges, such as red light, (near-)infrared light, or green light. Examples of suitable wavelength ranges are 660 nm, 750 nm to 850 nm, and 905 nm to 940 nm.

[0020] The term "light sensor" can refer to, for example, a photodiode, a photocell, a CMOS sensor (CMOS = complementary metal-oxide-semiconductor), a CCD sensor (CCD = charge-coupled device), or a combination of at least two of these examples.

[0021] The term "pulse oximeter" can also refer to a CO oximeter.

[0022] The pulse oximeter can, for example, be designed as a clip for attachment to a part of the body, such as a finger, an earlobe or a wrist.

[0023] The pulse oximeter can be designed so that the light source and the light sensor can be arranged on the same side of the body part or on opposite sides of the body part.

[0024] Further aspects of the invention relate to a computer program and a computer-readable medium on which the computer program is stored.

[0025] The computer program includes instructions that cause a processor – for example, a processor of the control unit described above and below – to execute the procedure described above and below when the computer program is run by the processor.

[0026] The computer-readable medium can be volatile or non-volatile data storage. For example, the computer-readable medium can be a hard drive, a USB storage device (USB = Universal Serial Bus), RAM (random-access memory), ROM (read-only memory), EPROM (erasable programmable read-only memory), EEPROM (electrically erasable programmable read-only memory), flash memory, or a combination of at least two of these. The computer-readable medium can also be a data communication network that allows the downloading of program code (e.g., via the internet) or a cloud.

[0027] It should be noted that features of the procedure described above and below may also be features of the computer program and / or the computer-readable medium (and vice versa).

[0028] The following describes various embodiments of the invention. These embodiments are not to be understood as limiting the scope of the invention.

[0029] According to one embodiment, the at least one limit value can be determined by multiplying the first input value by a quotient of the smallest or largest setpoint and the second input value. In other words, the at least one limit value can change during operation depending on a change in the first input value, i.e., the electrical quantity with respect to the light source, or in the second input value, i.e., the amplitude of the sensor signal.

[0030] According to one embodiment, the at least one deviation value can be determined by subtracting the first input value from the at least one limit value.

[0031] According to one embodiment, the at least one limit value can include a lower limit value associated with the smallest setpoint. In this case, a first deviation value can be determined by subtracting the first input value from the lower limit value. The control signal can then be generated using the first deviation value.

[0032] According to one embodiment, the limit L can be below 2m to be determined according to L2m=tA1×L1, where t represents the smallest setpoint, L1 the first input value, and A1 the second input value. The lower limit could, for example, be a value of the electrical quantity where the light source, such as its brightness and / or color, is set so that the amplitude of the sensor signal (approximately) matches the smallest setpoint.

[0033] According to one embodiment, the at least one limit value can include an upper limit value associated with the largest setpoint. In this case, a second deviation value can be determined by subtracting the first input value from the upper limit value. The control signal can then be generated using the second deviation value. The upper limit value can, for example, be a value of the electrical quantity at which the light source, for example its brightness and / or color, is set such that the amplitude of the sensor signal (approximately) matches the largest setpoint.

[0034] According to one embodiment, the upper limit L 2M to be determined according to L2m=t+εA1×L1, where t + ε represents the largest setpoint (here a sum of the smallest setpoint t and a predetermined distance ε between the smallest and largest setpoint), L1 represents the first input value and A1 represents the second input value.

[0035] According to one embodiment, the first and second deviation values, and in particular their signs, can be compared. In this case, the control signal can only be generated, or the average value determined, if the sign of the first deviation value matches the sign of the second deviation value. Thus, with minimal computational effort, it can be determined whether the first input value, for example, the current light source current, lies within or outside a limit range defined by the lower and upper limits. Due to the (approximately) linear relationship between the current light source current and the current amplitude of the sensor signal, this in turn allows a conclusion to be drawn as to whether the current amplitude lies within or outside the setpoint range. If it lies within the setpoint range, no adjustment of the light source current is necessary.

[0036] According to one embodiment, an average value can be determined from the first and second deviation values. The control signal can then be generated using this average value as the basis for the at least one deviation value. The average value can, for example, be an arithmetic, geometric, or quadratic mean. This can simplify any subsequent calculations.

[0037] According to one embodiment, an adjustment value can be determined using at least one deviation value and an assignment rule that assigns an adjustment value to each of the possible deviation values. In this case, an output value can be determined using the first input value and the adjustment value, in particular by adding the first input value and the (positive or negative) adjustment value. The control signal can then be generated using the output value. "Assignment rule" can be, for example, a mathematical function or a lookup table. The assignment rule can, for example, be stored in a memory of the control unit described above and below.

[0038] According to one embodiment, the assignment rule can be a sigmoid function or be based on a sigmoid function. A "sigmoid function" can generally be understood to be a function with an S-shaped graph. The sigmoid function can include a linear or approximately linear section. Such a section can be relevant, among other things, for the stability of the method.

[0039] According to one embodiment, the allocation rule can be defined as follows: f(Δ)=2×S1+2−Δ−S, where f (Δ) represents the adjustment value, Δ represents the at least one deviation value, and S represents a maximum permissible (variable or constant during operation of the pulse oximeter) amount of the adjustment value or a step size.

[0040] According to one embodiment, an approximation P for the term 2 can be -Δare determined based on a series expansion, preferably a Taylor series, and particularly preferably a Maclaurin series. In this case, the assignment rule can be defined as follows: f(Δ)=2×S1+P−S.

[0041] This allows for improved computational efficiency compared to implementations where term 2 -Δ Instead of P, the power consumption of the pulse oximeter can be calculated. This further reduces the power consumption of the pulse oximeter. Furthermore, this simplifies the implementation of the method as hardware and / or software.

[0042] According to one embodiment, the approximation P can be defined as follows: if−Δ≥0, then P=1+∑n=1N(k×(−Δ))nn! and / or if−Δ<0, then P=11+∑n=1N(−k×(−Δ))nn!.

[0043] Here, N represents a predetermined order of the series expansion and k a predetermined factor. For example, N can be a natural number between 1 and 10, preferably 5, and / or k can be a percentage between 0 and 1, preferably between 0.5 and 1.0, most preferably 0.75.

[0044] This enables a particularly computationally efficient and / or particularly easy-to-implement approximation without noticeably affecting the accuracy of the method.

[0045] According to one embodiment, the difference between the smallest and the largest setpoint can be up to 10 nanoamperes, preferably up to 5 nanoamperes, and particularly preferably up to 1 nanoampere.

[0046] According to one embodiment, the smallest setpoint can be between 1 and 5 nanoamperes, preferably between 1 and 3 nanoamperes, and particularly preferably between 2 nanoamperes.

[0047] With such current values, a good compromise between accuracy and energy efficiency could be achieved in practice.

[0048] Theoretically, the light source current can range between 3 mA and 50 mA. In practice, the light source current can also take on values ​​outside this range, depending on the individual patient.

[0049] According to one embodiment, the pulse oximeter can further include a battery for powering at least one electrical or electronic component of the pulse oximeter, in particular the light source, or of the entire pulse oximeter. This makes it possible to carry the pulse oximeter on the go.

[0050] According to one embodiment, the pulse oximeter may further include a display unit for displaying at least one value determined using the sensor signal, for example, an oxygen saturation or a pulse. Brief description of the drawings

[0051] The following describes embodiments of the invention with reference to the accompanying drawings. Neither the description nor the drawings are to be understood as limiting the scope of the invention. Fig. Figure 1 shows a pulse oximeter according to an embodiment of the invention. Fig. Figure 2 shows a diagram illustrating a target value range for use in a method according to an embodiment of the invention. Fig. Figure 3 shows a diagram illustrating an allocation rule for use in a method according to an embodiment of the invention.

[0052] The figures are purely schematic and not to scale. If the same reference symbols are used in different drawings, these reference symbols denote identical or equivalent features. Embodiments of the invention

[0053] Fig. Figure 1 shows a pulse oximeter 1 comprising an (electric) light source 3, a light sensor 5, and a control unit 7. The light sensor 5 is designed to convert a portion of the light transmitted and / or reflected by a body part 9, for example, a finger, an earlobe, or a similarly thin body part, when illuminated by light from the light source 3, into an (electrical) sensor signal 11.

[0054] In this example, the control unit 7 is designed to determine an oxygen saturation (sO2) and / or a pulse from the sensor signal 11.

[0055] Additionally, the pulse oximeter 1 can include a display unit 13 for displaying at least one value and / or at least one graph relating to the sensor signal 11, in particular relating to oxygen saturation and / or pulse rate. The display unit 13 can, for example, be arranged in and / or on a housing of the pulse oximeter 1.

[0056] It is also possible that the pulse oximeter 1 includes a battery 15 for powering the pulse oximeter 1. This would allow the pulse oximeter 1 to be worn on the go.

[0057] In this example, the light source 3 comprises a first light-emitting diode 3a for emitting light in a first wavelength range, for example, 660 nm, and a second light-emitting diode 3b for emitting light in a second wavelength range that differs from the first, for example, 905 nm to 940 nm. The control unit 7 can be configured to alternately switch the light-emitting diodes 3a and 3b on and off during operation of the pulse oximeter 1. The light source 3 can also comprise only a single light-emitting diode with a suitably variable wavelength range. Other types of light sources, such as laser diodes or incandescent lamps, are also possible.

[0058] The light sensor 5 can, for example, include a photodiode, a photocell, a CMOS sensor, a CCD sensor, or a combination of at least two of these examples.

[0059] The pulse oximeter 1 can, for example, be designed as a clip for attachment to body part 9 and / or as a CO-oximeter.

[0060] In this example, the pulse oximeter 1 is designed such that the light source 3 and the light sensor 5 are arranged on opposite sides of the body part 9 when the pulse oximeter 1 is in operation. The light sensor 5 thus predominantly receives the portion of light transmitted by the body part 9 when the light source 3 illuminates the body part 9.

[0061] Alternatively, the pulse oximeter 1 can be designed such that the light source 3 and the light sensor 5 are arranged on the same side of the body part 9 during operation of the pulse oximeter 1.

[0062] The control unit 7 comprises means configured to execute a specific procedure for controlling the power supply to the light source 3, as described in more detail below. The means may include hardware and / or software modules. In particular, the means may include memory and a processor. A computer program may be stored in the memory, and the processor may be configured to execute the procedure by running the computer program. Additionally, the means may include a data communication interface for wireless and / or wired data communication with peripheral devices, such as a smartphone, smartwatch, tablet, laptop, PC, or ventilator.

[0063] The control unit 7 can also be implemented exclusively as hardware, for example in the form of an ASIC or FPGA chip.

[0064] The procedure includes the following steps.

[0065] In a first step, the control unit 7, for example in a corresponding hardware and / or software module of the control unit 7, receives a first input value 17, which indicates an electrical quantity with respect to the light source 3, and a second input value 19, which indicates the current amplitude of the sensor signal 11. The two input values ​​17 and 19 can be measured and / or estimated (i.e., calculated) values. The electrical quantity includes an electric current currently flowing through the light source 3 and / or an electric voltage currently applied to the light source 3. The electrical quantity can also be an electrical quantity based on the current and / or voltage and / or an electrical quantity influencing the current and / or voltage.For example, the electrical quantity can also be at least one of the following: an adjustable series resistor connected upstream of the light source 3; a frequency at which the light source 3 is switched on and off; a time ratio between the on phases, in which the light source 3 is switched on, and the off phases, in which the light source 3 is switched off. For example, the frequency can be a clock frequency and / or the time ratio a duty cycle in the context of pulse-width modulation of the light source 3. The amplitude can, in particular, be an amplitude of an AC component of the sensor signal 11.

[0066] In a second step, at least one limit value for the electrical quantity is determined using the second input value 19, i.e., depending on the current amplitude of the sensor signal 11. The at least one limit value is assigned to the smallest setpoint t or the largest setpoint t + ε of a setpoint range 21 in which the amplitude is to lie (see Fig. 2).

[0067] In a third step, at least one deviation value is determined, which indicates a deviation of the first input value 17 from at least one limit value.

[0068] In a fourth step, a control signal 23 is generated to control the light source 3 using at least one deviation value, so that the amplitude approaches the target value range 21.

[0069] The method makes it possible to save power during the operation of the pulse oximeter 1 by, for example, reducing the current flowing through the light source 3 on the one hand as long as the quality of the photoplethysmogram (PPG) from the sensor signal 11 is still sufficient and the required accuracy is still given, and on the other hand increasing it if stronger noise occurs or the quality of the sensor signal 11 generally deteriorates.

[0070] The difference between the smallest setpoint t and the largest setpoint t + ε can be, for example, up to 10 nanoamperes, preferably up to 5 nanoamperes, and particularly preferably up to 1 nanoampere. Additionally or alternatively, the smallest setpoint t can be between 1 and 5 nanoamperes, preferably between 1 and 3 nanoamperes, and particularly preferably 2 nanoamperes. Such current values ​​have proven particularly suitable in practice with regard to the achieved accuracy and energy efficiency.

[0071] It is possible that a value below the limit L 2m and an upper limit L 2M for the electrical quantity. In this case, a first deviation value Δ1 can be determined by subtracting the first input value 17 from the lower limit L. 2m or a second deviation value Δ2 by subtracting the first input value 17 from the upper limit L 2M The control signal 23 can then be generated using the two deviation values ​​Δ1, Δ2.

[0072] For example, the limit L can be below 2m to be determined according to L2m=tA1×L1.

[0073] Additionally or alternatively, the upper limit L 2M to be determined according to L2M=t+εA1×L1.

[0074] L1 represents the first input value 17, here an electric current flowing through the respective light-emitting diode 3a or 3b, and A1 represents the second input value 19.

[0075] Accordingly, the two deviation values ​​Δ1, Δ2 are defined as Δ1=(tA1−1)×L1 or Δ2=(t+εA1−1)×L1.

[0076] Additionally, an average value Δ can be determined from the two deviation values ​​Δ1, Δ2, for example as the arithmetic mean according to Δ=Δ1+Δ22.

[0077] The average value Δ can also be a geometric or quadratic mean.

[0078] The control signal 23 can then be generated using the average value Δ.

[0079] For example, the signs of the two deviation values ​​Δ1 and Δ2 can be compared. In this case, the average value Δ is only determined if the signs match. Thus, with minimal computational effort, it can be determined whether the first input value 17, for example, the current light source current, is within or outside a range defined by the two limit values ​​L. 2m , L 2M the defined limit range. Due to the (approximately) linear relationship between the current light source current and the current amplitude of the sensor signal 11, this in turn allows a conclusion to be drawn as to whether the current amplitude lies within or outside the setpoint range 21. If it lies within the setpoint range 21, no adjustment of the light source current is necessary.

[0080] It is possible to obtain an adjustment value using the average value Δ and an allocation rule 25 (see Fig. 3) assigns an adjustment value to each of the possible deviation values. An output value can then be determined from the adjustment value and the first input value 17, for example by adding the first input value 17 and the (positive or negative) adjustment value. The output value can then be used to generate the control signal 23.

[0081] The assignment rule 25 can, for example, be stored in the memory of the control unit 7 in the form of a mathematical function or a lookup table.

[0082] In particular, assignment rule 25 may be a special sigmoid function. The sigmoid function may be defined as follows: f(Δ)=2×S1+2−Δ−S, where f(Δ) represents the adjustment value and S represents a maximum permissible (variable or constant during operation of pulse oximeter 1) value of the adjustment value or a step size. This prevents unwanted fluctuations and / or jumps in the amplitude adjustment.

[0083] To reduce the computational effort, especially when using a microcontroller, an approximation P can be used instead of the term 2 as an option. -Δ calculated in the sigmoid function, so that it reads: f(Δ)=2×S1+P−S.

[0084] The approximation is performed, for example, according to the following equation: 2−Δ≈apprx(−Δ)={1+∑n=1N(k+(−Δ))nn!,−Δ≥011+∑n=1N(−k×(−Δ))nn!,−Δ<0

[0085] Here, N represents a predetermined order of the series expansion and k a predetermined factor. For example, N can be a natural number between 1 and 10, preferably 5, and / or k can be a percentage between 0 and 1, preferably between 0.5 and 1.0, most preferably 0.75. The number N influences the accuracy of the approximation and the computational effort. With N = 5, a good compromise between accuracy and computational effort can be achieved.

[0086] Such a series expansion can also be called a Maclaurin series. This allows for a particularly computationally efficient and / or particularly easy-to-implement approximation without noticeably affecting the accuracy of the method.

[0087] As in Fig. As can be seen from 3, the assignment rule 25 includes a longer (approximately) linear section around the zero point, which is important for the stability of the calculation.

[0088] Finally, it should be noted that terms such as "have", "comprise", "include", "with", etc. do not exclude any other elements or steps, and indefinite articles such as "a" or "an" do not exclude any variety.

[0089] It is further noted that features or steps described with reference to one of the foregoing embodiments may also be used in combination with features or steps described with reference to other of the foregoing embodiments.

[0090] Reference numerals in the claims are not to be understood as limiting the scope of the subject matter defined by the claims. List of reference symbols 1 pulse oximeter 3 light source 3a first light-emitting diode 3b second LED 5 light sensor 7 Control unit 9 Body part 11 Sensor signal 13 Display unit 15 batteries 17 first input value 19 second input value 21 Setpoint range 23 Control signal 25 Assignment rule S largest permissible amount of the adjustment value or step size t smallest setpoint t + ε largest setpoint Δ Deviation value, average value

Claims

[1] Method for controlling a light source (3, 3a, 3b) of a pulse oximeter (1), wherein the pulse oximeter (1) further comprises a light sensor (5) which is designed to convert a light component transmitted and / or reflected by a body part (9) when irradiated with light from the light source (3, 3a, 3b) into a sensor signal (11), the method comprising: Receiving a first input value (17) indicating an electrical quantity related to the light source (3, 3a, 3b), the electrical quantity comprising an electrical current flowing through the light source (3, 3a, 3b) and / or an electrical voltage applied to the light source (3, 3a, 3b); Receiving a second input value (19) indicating an amplitude of the sensor signal (11); Determining at least one limit value for the electrical variable using the second input value (19), wherein the at least one limit value is assigned to the smallest setpoint value (t) or the largest setpoint value (t + ε) of a setpoint value range (21) in which the amplitude is to lie; Determining at least one deviation value (Δ) indicating a deviation of the first input value (17) from the at least one limit value; Generating a control signal (23) for controlling the light source (3, 3a, 3b) using the at least one deviation value (Δ) so that the amplitude approaches the target value range (21). [2] Method according to claim 1, wherein the at least one limit value is determined by multiplying the first input value (17) by a quotient of the smallest target value (t) or the largest target value (t + ε) and the second input value (19); and / or wherein the at least one deviation value is determined by subtracting the first input value (17) from the at least one limit value. [3] Method according to one of the preceding claims, wherein the at least one limit value comprises a lower limit value associated with the smallest target value (t), wherein a first deviation value is determined by subtracting the first input value (17) from the lower limit value, wherein the control signal (23) is generated using the first deviation value; and / or wherein the at least one limit value comprises an upper limit value associated with the largest target value (t + ε), wherein a second deviation value is determined by subtracting the first input value (17) from the upper limit value, wherein the control signal (23) is generated using the second deviation value. [4] Method according to claim 3, wherein the first deviation value and the second deviation value are compared with each other and the control signal (23) is only generated if the sign of the first deviation value matches the sign of the second deviation value. [5] Method according to claim 3 or 4, wherein an average value (Δ) is determined from the first deviation value and the second deviation value and the control signal (23) is generated using the average value (Δ) as the at least one deviation value (Δ). [6] Method according to one of the preceding claims, wherein an adaptation value is determined using the at least one deviation value (Δ) and an assignment rule (25) which assigns an adaptation value to each possible deviation value; wherein an output value is determined using the first input value (17) and the adjustment value, in particular by adding the first input value (17) and the adjustment value; wherein the control signal (23) is generated using the output value. [7] Method according to claim 6, wherein the assignment rule (25) is a sigmoid function or is based on a sigmoid function. [8] Method according to claim 6 or 7, where the assignment rule (25) is defined as follows: f(Δ)=2×S1+2−Δ−S, where f(Δ) stands for the adjustment value, Δ for the at least one deviation value (Δ) and S for a largest permissible amount (S) of the adjustment value. [9] Method according to claim 8, where an approximation P for the term 2 -Δbased on a series expansion, preferably a Taylor series, particularly preferably a Maclaurin series, and the assignment rule (25) is defined as follows: f(Δ)=2×S1+P−S. [10] Method according to claim 9, where the approximation P is determined as follows: if −Δ≥0, then P=1+∑n=1N(k×(−Δ))nn! and / or if −Δ<0, then P=11+∑n=1N(−k×(−Δ))nn!; where N stands for a predetermined order of the series expansion and k for a predetermined factor. [11] Method according to one of the preceding claims, wherein a distance between the smallest setpoint (t) and the largest setpoint (t + ε) is up to 10 nanoamperes, preferably up to 5 nanoamperes, particularly preferably up to 1 nanoampere; and / or wherein the smallest setpoint value (t) is between 1 and 5 nanoamperes, preferably between 1 and 3 nanoamperes, particularly preferably 2 nanoamperes. [12] Control unit (7) comprising means configured to carry out the method according to any one of the preceding claims. [13] Pulse oximeter (1), comprising: a light source (3, 3a, 3b); a light sensor (5) which is designed to convert a light component transmitted and / or reflected by a body part (9) when irradiated with light from the light source (3, 3a, 3b) into a sensor signal (11); a control unit (7) according to claim 12. [14] A computer program comprising instructions which, when the computer program is executed by the processor, cause a processor to carry out the method according to any one of claims 1 to 11. [15] A computer-readable medium on which the computer program according to claim 14 is stored.

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