SMALL APPLIANCE HAVING IMPROVED MOTOR SPEED CONTROL - Patent application

Abstract

The present invention relates to a small appliance (1,1'), which is a hair removal device (1) such as an electric shaver or epilator, or an electric toothbrush (1'). The small appliance (1,1') is provided as a hair removal device (1) or a toothbrush (1'), comprising an electric DC motor (5), a detector (10) for obtaining a detector signal indicative of a rotation speed of the electric DC motor (5), and a controller (20) for controlling the electric DC motor (5). The controller is adapted to determine a first parameter indicative of the rotation speed of the DC motor (5) based on the detector signal, determine a second parameter indicative of the rotation speed of the DC motor (5) based on the detector signal, the second parameter being different from the first parameter and the second parameter being determined based on a frequency domain analysis of the detector signal, and adjust a supply voltage of the electric DC motor (5) based on the first parameter and the second parameter. The controller (20) is adapted to perform coarse control for adjusting the supply voltage based on the first parameter and to perform fine control for adjusting the supply voltage based on the second parameter. The present disclosure further relates to a corresponding method.

Description

[Technical field]

[0001] The present invention relates to the field of small appliances such as battery-powered appliances for personal hygiene, in particular hair removal devices such as electric shavers or epilators, and electric toothbrushes. The present invention further relates to a controller and a corresponding method for controlling a DC motor of a small appliance. [Background technology]

[0002] Battery-powered appliances for personal hygiene, such as, for example, electric shavers, epilators, and electric toothbrushes, are well known in the art.

[0003] An electric shaver, also known as a dry razor, electric razor, or simply shaver, is a device with a rotating or vibrating blade or cutting unit. Electric shavers do not usually require the use of shaving cream, soap, or water. Razors may be powered by a small DC motor, which is powered either by a battery or mains power. Many modern ones are powered using a rechargeable battery.

[0004] Epilators are electrical devices used to remove hair by mechanically grasping and plucking multiple hairs at the same time. The manner in which epilators pluck hairs is similar to waxing, but unlike waxing, they do not typically remove cells from the epidermis. Apart from the spring in early spring-type epilators, epilators have no parts that require periodic replacement. Epilators are corded, rechargeable and battery-operated in design.

[0005] The rotation speed of a DC motor decreases when the mechanical load of the motor increases, for example in thick beard sections. At least, this is the case if no measures are taken. In the case of a shaver with a rotary motor, the reduction in motor speed can lead to a reduction in shaving performance. It is therefore desirable to provide a drive control device for a DC motor of a domestic appliance that provides a constant motor speed when the load changes and when the load is constant (not changing and especially not present).

[0006] The applicant's earlier application EP 3327923 A1 discloses a drive control device for driving a DC motor of a domestic appliance, in particular a hair-cutting device such as an electric razor, shaver or epilator, at a constant rotation speed. The full disclosure of this document is incorporated herein by reference. The drive control device comprises detection means for determining a parameter indicative of the actual rotation speed of the DC motor and a closed-loop control device for adjusting the supply voltage to the DC motor based on a control deviation between a target value and an actual value of the parameter indicative of the rotation speed of the DC motor. In order to provide a constant motor speed when the load varies and when the load is constant (not varying and in particular not present), the loop control device has a high-speed control and a low-speed control for adjusting the supply voltage, the high-speed control providing a faster correction of the control deviation than the low-speed control. The loop control device further comprises processor means for determining a switching criterion for switching between the high-speed control and the low-speed control, the closed-loop control being adapted to switch between the high-speed control and the low-speed control based on the switching criterion. The present disclosure further relates to an electric hair and / or skin treatment device.

[0007] Ertl.et al., "A Method for Real-Time Sensorless Speed ​​Control of Brushed DC Motors in Cost Constrained Systems", 2nd Global Power, Energy and Communication Conference (GPECOM), IEEE, pages 152-157, 2020, describes a method for real-time sensorless speed control of brushed DC motors in cost-constrained systems. First, it is proposed to calibrate the motor constants using a current ripple detection method. Once the motor constants are calibrated, it moves to a back electromotive force (BEMF) measurement method for operation. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] European Patent No. 3327923(A1) [Non-patent literature]

[0009] [Non-Patent Document 1] Ertl.et al., “A Method for Real-Time Sensorless Speed ​​Control of Brushed DC Motors in Cost Constrained Systems”, 2nd Global Power, Energy and Communication Conference (GPECOM), IEEE, pages 152-157, 2020 Summary of the Invention [Means for solving the problem]

[0010] It is an object of the present invention to provide a further improved hair removal device, such as an electric shaver or epilator, or an electric toothbrush. It would be advantageous to control the speed of the electric DC motor of the hair removal device or electric toothbrush with high precision under different load conditions, while at the same time providing a reasonable manufacturing cost. In particular, it is desirable that no additional sensors are required to control the speed of the electric DC motor with high precision.

[0011] In a first aspect of the present invention, a small appliance is provided, the small appliance being a hair removal device or a toothbrush. - an electric DC motor; a detector for obtaining a detector signal indicative of the rotation speed of an electric DC motor; a controller for controlling an electric DC motor, the controller comprising: determining a first parameter indicative of a rotational speed of the DC motor based on the detector signal; determining a second parameter indicative of a rotational speed of the DC motor based on the detector signal, the second parameter being different from the first parameter, the second parameter being determined based on a frequency domain analysis (### spectrum analysis) of the detector signal; and adjusting a supply voltage of the electric DC motor based on the first parameter and the second parameter. The controller can be adapted to perform coarse control for adjusting the supply voltage based on the first parameter and fine control for adjusting the supply voltage based on the second parameter.

[0012] In a further aspect of the invention, a corresponding controller for controlling an electric DC motor of a small appliance, in particular a hair removal device or a toothbrush, is presented. The controller is adapted to obtain a detector signal indicative of a rotation speed of the electric DC motor, determine a first parameter indicative of the rotation speed of the DC motor based on the detector signal, determine a second parameter indicative of the rotation speed of the DC motor based on the detector signal, the second parameter being different from the first parameter and the second parameter being determined based on a frequency domain analysis of the detector signal, and adjust a supply voltage of the electric DC motor based on the first parameter and the second parameter. The controller can be adapted to perform a coarse control for adjusting the supply voltage based on the first parameter and a fine control for adjusting the supply voltage based on the second parameter.

[0013] In a further aspect of the invention, a method for controlling an electric DC motor of a small appliance is presented, the small appliance being a hair removal device or a toothbrush. The method includes the steps of obtaining a detector signal indicative of a rotation speed of the electric DC motor, determining a first parameter indicative of the rotation speed of the DC motor based on the detector signal, determining a second parameter indicative of the rotation speed of the DC motor based on the detector signal, the second parameter being different from the first parameter, the second parameter being determined based on a frequency domain analysis of the detector signal, and adjusting a supply voltage of the electric DC motor based on the first parameter and the second parameter. Based on the first parameter, a coarse control can be performed to adjust the supply voltage, and based on the second parameter, a fine control can be performed to adjust the supply voltage.

[0014] In yet another aspect of the present invention, there is provided a non-transitory computer readable storage medium having stored therein a corresponding computer program comprising program code means for causing a computer to perform the steps of the controller or control method disclosed herein when the computer program is executed on a computer, as well as a computer program product which, when executed by a processor, causes a computer to perform the control method disclosed herein.

[0015] Preferred embodiments of the invention are defined in the dependent claims. It is to be understood that the claimed controller, method, computer program and medium may have similar and / or identical preferred embodiments as the claimed apparatus, in particular as defined in the dependent claims and disclosed herein.

[0016] In conventional controllers for small appliances, such as those described in the aforementioned EP 3327923 A1, the supply current or voltage may be averaged using a low pass filter, and the averaged current or voltage may be used to control the DC motor. The inventors have recognized that an advantage of this approach is that no dedicated rotation sensor is required. However, the inventors have further recognized that motor parameters such as winding resistance or manufacturing tolerances may affect the control loop. This may lead to device-to-device variations in the provided rotation speed. One aspect of the invention is based on the idea of ​​performing a frequency domain analysis of the detector signal. During operation, electric DC motors may cause ripples in the supply current and voltage due to, for example, commutator switching, time-varying load conditions, and back magnetization. The frequency domain analysis makes it possible to obtain real RPM (revolutions per minute) information that is independent of, or at least not significantly affected by, motor parameters such as coil resistance, value of the measurement shunt resistor, and their tolerances.

[0017] A further advantage of the proposed approach is that manufacturing costs may be reduced since hardware requirements in terms of device tolerances may be relaxed, which may further help to reduce dependency on certain sources of circuit elements since they can be more easily replaced.

[0018] The inventors further recognized that it is possible to provide a fast adjustment based on a first parameter in combination with a slower but finer adjustment based on a second parameter based on a frequency domain analysis. Thus, even small appliances with limited processing power may benefit from the frequency domain analysis, even if the frequency domain analysis may not be performed in real time, but with a certain delay and / or only at intervals. It should be understood that the controller may be adapted to continuously perform a coarse / fast control for adjusting the supply voltage based on the first parameter and to continuously perform a fine / slow control for adjusting the supply voltage based on the second parameter. In other words, both may be performed continuously and / or in parallel during the operation of the small appliance, rather than being performed only once for calibration. Thus, even if a small appliance with limited processing power is not able to quickly perform a fine control in case of a fast load change, the parallel coarse control may provide a fast adaptation of the supply voltage based on the first parameter.

[0019] A further advantage of the proposed solution is that the control can be quite flexible with regard to the implementation of the detector. For example, a detector for obtaining a detector signal indicative of the rotation speed of an electric DC motor can be implemented at low cost using an analog-to-digital converter (ADC) measuring the current or voltage, for example at a series or shunt resistor or a voltage divider coupled to the motor or to the power supply. Such an ADC can for example be part of a microcontroller that is provided anyway in the electrical appliance for the operation of the human-machine-interface (HMI).

[0020] The following provides a brief explanation and definition of some terms used throughout this application.

[0021] As used herein, a small appliance or appliance may refer to an appliance for personal hygiene or personal use, such as, for example, a hair removal device or a toothbrush. The term small appliance may refer to a handheld electronic device.

[0022] As used herein, a computer may refer to a device adapted to execute machine-readable instructions, including, but not limited to, a microcontroller (μC), an ASIC, an FPGA, a microprocessor, or a general-purpose computer.

[0023] As used herein, a controller may be implemented by discrete electronic circuit elements, but may also be implemented using integrated circuits, such as, for example, a microcontroller, an ASIC, an FPGA, etc. A control circuit may be implemented using a combination of hardware and software adapted to operate the hardware to function as the control circuit.

[0024] As used herein, the term "continuously" may indicate that both the coarse control based on a first parameter and the fine control based on a second parameter are active and operable during use of the epilation device or toothbrush, e.g., while the motor is running, and that both the coarse / high speed control and the fine / low speed control are performed during the entire shaving time while the motor is running, or during the entire tooth brushing session while the motor is running.

[0025] As used herein, the terms high precision and low precision may be understood as relative terms and may indicate that the control based on the second parameter may provide a higher precision than the control based on the first parameter. For example, high precision may be at least twice as accurate as the low precision, in particular at least three times as accurate, in particular at least four times as accurate, in particular at least ten times as accurate. As used herein, precision may refer to the maximum deviation or standard deviation of an expected value of RPM, revolutions per minute, etc. A fast / coarse control may provide, for example, a precision of ±8% in terms of RPM, and a slow / fine control may provide, for example, a precision of ±1% in terms of RPM. Similarly, the terms fine and coarse, or fast and slow, and the like, may be understood as relative terms and may indicate, for example, that the control based on the second parameter may be finer and / or slower than the control based on the first parameter, in particular at least two times, in particular at least three times, in particular at least four times, in particular at least ten times finer and / or slower. Similarly, the terms fast refresh rate and slow refresh rate can be understood as relative terms, which may indicate that a first parameter is determined (e.g., with lower accuracy) at a higher refresh rate than a second parameter that may be determined (e.g., with higher accuracy) at a lower refresh rate. For example, at a higher refresh rate, the determination of the first parameter may be at least twice as fast, in particular at least four times as fast, in particular at least ten times as fast, as determined at a lower refresh rate. For example, the first parameter may be repeatedly determined at a refresh rate of at least 0.5 kHz, in particular at least 1 kHz, in particular at least 2 kHz, in particular at least 10 kHz, in particular at least 100 kHz. For example, the second parameter may be repeatedly determined at a refresh rate of 100 kHz or less, in particular at least 10 kHz or less, in particular at least 2 kHz or less, in particular at least 1 kHz or less, in particular at least 0.5 kHz or less.

[0026] The small appliance is a hair removal device or an electric toothbrush.The small appliance may be a battery-powered appliance for personal hygiene, which may be a hair removal device, in particular a shaver or epilator, or an electric toothbrush.

[0027] The first parameter may indicate the amplitude of the detector signal. This may also include an amplitude difference, for example, a difference relative to a reference amplitude. The second parameter may indicate a frequency component of the detector signal. This may also include a frequency difference relative to a reference frequency. For example, the first parameter may represent the amplitude (or magnitude) of the motor current or voltage of a DC motor, or the amplitude (or magnitude) of the supply voltage. A higher motor voltage generally corresponds to a higher rotation speed. A lower motor voltage generally corresponds to a lower rotation speed. The relationship between the applied motor currents or voltages may be provided, for example, by a look-up table. This is because the amplitudes can generally be measured and evaluated quickly, even with limited processing power. This allows for a first parameter with a high refresh frequency. The determination of the second parameter, determined based on a frequency domain analysis of the detector signal, may be available with a lower refresh frequency, but with a higher accuracy.

[0028] The detector signal may be indicative of at least one of (a) the motor current of the electric DC motor, (b) the motor voltage of the electric DC motor, and / or (c) the supply voltage. The advantage is that these parameters can be determined with little effort. On the one hand, the current ripple of the motor current can be evaluated in the frequency domain to obtain the second parameter. On the other hand, to obtain the first parameter, it is possible to evaluate the back electromagnetic force, also called back electromotive force, as an indicator of the rotation speed. For this, the motor current can be measured and the formula U mot =i * R mot + The rotation speed can be calculated by the back electromotive force, where i is the measured current and R motis the resistance of the electric motor and the back electromotive force is the voltage contribution due to the back electromagnetic force. The motor current can be measured by the voltage drop across a series resistor connected in series with the electric DC motor. Similarly, it is possible to evaluate the motor voltage of the electric DC motor. On the one hand, the average value of the motor voltage can be used as the first parameter. On the other hand, the fluctuations of the motor voltage may be evaluated in the frequency domain to obtain the second parameter. A third option is to evaluate the supply voltage of the small appliance. The lower the supply voltage of the appliance, the lower the expected rotation speed of the electric DC motor. In this case too, the amplitude or magnitude of the supply voltage may be evaluated to obtain the first parameter and to obtain the second parameter. Advantageously, the first parameter indicative of the rotation speed of the DC motor and the second parameter indicative of the rotation speed of the DC motor are determined based on the same detector signal. The advantage of evaluating the same detector signal is that no separate dedicated sensors are needed to obtain the first and second parameters indicative of the rotation speed of the DC motor.

[0029] The controller may be adapted to perform a coarse control for adjusting the supply voltage based on the first parameter. The controller may be adapted to perform a fine control for adjusting the supply voltage based on the second parameter. Similarly, the controller may be adapted to perform a fast control for adjusting the supply voltage based on the first parameter and the controller may be adapted to perform a slow control for adjusting the supply voltage based on the second parameter. As indicated above, the controller may be adapted to continuously perform a coarse / fast control for adjusting the supply voltage based on the first parameter and continuously perform a fine / slow control for adjusting the supply voltage based on the second parameter. In other words, both may be performed continuously and / or in parallel during operation of the small appliance. The first parameter may be determined, for example, based on amplitude measurements with low effort at a high refresh rate. It is therefore possible to react quickly to changes in the load of the electric motor. The second parameter may be determined, for example, based on frequency domain analysis at a lower refresh rate with higher accuracy. If the load does not change much, it is possible to provide high accuracy control even with limited processing power.

[0030] The controller may comprise a first controller and a second controller. The first controller may be adapted to adjust the supply voltage of the electric DC motor based on a first parameter indicative of the amplitude of the detector signal. The second controller may be adapted to adjust the supply voltage of the electric DC motor based on a second parameter indicative of the frequency content of the detector. The first controller and the second controller may be cascaded. For example, the second controller may be used as an external controller and the first controller may be used as an internal controller. Thereby, it is possible to respond quickly to fast changes by the inner controller and respond slower but with higher accuracy based on the outer controller.

[0031] In an improvement, the second controller may be adapted to provide a set point for the first controller. Additionally or alternatively, the second controller may be adapted to provide a correction to the first controller. Additionally or alternatively, the second controller may be adapted to adjust the feedback path of the feedback loop of the first controller. In other words, the second, more accurate but possibly slower controller provides an input to the first controller; however, the first controller is perhaps less accurate. The advantage of this approach is that it combines the advantages of both approaches by overcoming the disadvantages of both approaches.

[0032] The detector signal may be sampled at at least twice the maximum rotational frequency of the electric DC motor, in particular the detector signal is sampled at at least twice the maximum rotational frequency multiplied by the number of commutations per revolution of the electric motor, an advantage being that the rotational frequency can be determined based on a frequency domain analysis of the detector signal, and the detector signal is sampled at a rate that is high enough to reduce the effects of artifacts.

[0033] Determining the first parameter may include averaging the detector signal. Determining the second parameter may include a Fourier analysis of the detector signal, in particular an FFT analysis of the detector signal. The advantage is that the first parameter can be determined with very little processing effort. On the other hand, the second parameter can be determined with very high accuracy, albeit with a higher computational effort. The first parameter can be determined with a high refresh rate (with lower accuracy), whereas the second parameter is determined with high accuracy (with a lower refresh rate).

[0034] The second parameter may indicate a frequency peak of the spectrum, in particular the frequency peak may be an integer multiple of the rotation frequency. The advantage of this embodiment is that the second parameter may directly reflect the rotation speed in revolutions per minute (RPM). The integer multiple factor may result from the natural number of commutator switches during one revolution. Since this parameter is determined by the electric motor used, the frequency of the frequency peak may simply be divided by the number of commutator switches to obtain the actual rotation frequency.

[0035] The first and second parameters are determined in parallel. In an embodiment, the second parameter may be used to adjust the supply voltage of the DC motor if the difference between the first and second frequency peaks of the frequency domain analysis is below a predefined threshold. If the frequency domain analysis shows a significant frequency peak, this means that the DC motor is operating in a steady state. In this case, the second parameter may be used for high-precision control. On the other hand, if the frequency domain analysis does not provide a significant frequency peak, this means that the rotation speed is changing. In this case, the first parameter, which may be indicative of the amplitude of the detector signal, may be used. [Brief description of the drawings]

[0036] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter, the following description being directed to the drawings. [Figure 1] FIG. 1 shows a schematic diagram of a first embodiment of a small appliance in the form of an electric shaver. [Diagram 2] FIG. 2 shows a schematic diagram of a second embodiment of a small appliance in the form of a toothbrush. [Diagram 3] 1 shows a flow diagram of a method according to one aspect of the present disclosure. [Figure 4] 1 shows an example block diagram of a circuit for a small appliance. [Diagram 5] 1 illustrates a first exemplary control loop configuration. [Figure 6]4 illustrates a second exemplary control loop configuration. [Figure 7] 13 illustrates a third exemplary control loop configuration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] FIG. 1 shows a schematic representation of a first embodiment of a battery-powered appliance for personal hygiene in the form of an electric shaver. The appliance is indicated in its entirety in the drawings by the reference number 1. The shaver comprises a shaver body or main body 2 and a shaver head 3, as generally known in the art. The shaver head 3 comprises a cutter 4 having one or more razor blades. The cutter 4 is driven by an electric DC motor 5, which may be located in the main body 2 or in the shaver head 3. The electric motor 5 is connected to the cutter 4 via a power transmission path 6. The cutter 4 can move along a pivot 7. The shaver head 3 can be movable relative to the shaver body 2 to a certain extent, so that the shaver head can adapt to the contours of the user's face. A battery 8, in particular a lithium-ion or NiMH battery, which may be located in the main body 2, can serve as an energy source for powering the small appliance 1.

[0038] The small appliance 1 further comprises a detector 10 for obtaining a detector signal indicative of the rotation speed of the electric DC motor 5, and a controller 20 for controlling the electric DC motor 5. In the diagrams shown in Figures 1 and 2, the detector 10 and the controller 20 may for example be implemented together by a microcontroller having a processor and one or more integrated analog-to-digital converters adapted to carry out the steps as described with reference to Figure 3. An exemplary circuit diagram is further described below with reference to Figure 4.

[0039] Figure 2 shows a further embodiment of a battery-powered appliance 1 for personal hygiene in the form of an electric toothbrush 1'. The further embodiment of the mini appliance may refer to other embodiments of battery-powered appliances for personal hygiene such as epilators or other forms of hair removal devices such as hair removal devices based on light pulses. The toothbrush 1' comprises a main body 2' and a toothbrush head 3'. The toothbrush is driven by an electric DC motor 5.

[0040] Figure 3 shows a flow diagram of a method 100 for controlling an electric DC motor of a small appliance (1, 1'), in particular an epilation device (1) or a toothbrush (1'). In a first step S101, a detector signal indicative of the rotational speed of the electric DC motor is obtained. The detector signal may be indicative of at least one of: (a) the motor current of the electric DC motor, (b) the motor voltage of the electric DC motor, and / or (c) the supply voltage. For example, the motor current through the motor may be measured during operation of the appliance. Additionally or alternatively, the voltage applied to the electric motor 5 and / or the supply voltage of the battery 8 may be measured during operation of the appliance 1, 1'.

[0041] In step S102, a first parameter indicative of the rotational speed of the DC motor may be determined based on the detector signal acquired in step S101. For example, the first parameter may be indicative of the (average) amplitude of the detector signal. In step S103, a second parameter indicative of the rotational speed of the DC motor may be determined based on the detector signal acquired in step S101. In contrast to the first parameter in the given example, the second parameter may be determined based on a frequency domain analysis of the detector signal. Hence, a spectral analysis may be performed to obtain the second parameter. The second parameter may be indicative of a frequency component of the detector signal, for example a frequency peak in the spectral analysis. If the electric motor performs eight commutator switches per revolution, this may result in a frequency peak that is eight times the rotational frequency of the electric motor. The speed of the electric motor may therefore be determined by dividing the peak frequency of the spectrum by the number of commutator switches.

[0042] In step S104, the supply voltage of the electric DC motor is adjusted based on the first and second parameters. For example, the supply voltage may be adjusted by pulse width or pulse frequency modulation. For example, the pulse width and / or pulse frequency may be adjusted directly based on the first and / or second parameters. Additionally or alternatively, the first and / or second parameters may be compared to a desired set point and the supply voltage may be adjusted based on the deviation between the first and / or second parameters and the desired set point. As indicated by the dashed arrows in steps S103 to S102, the second parameter determined in step S103 may additionally or alternatively be provided to step S102 of determining the first parameter, such that the first parameter may be determined or may be influenced by the determined second parameter. For example, the second parameter may be used in an outer control loop and already taken into account in an inner control loop to determine the first parameter. Further details and examples are provided with reference to Figs. 5 to 7.

[0043] FIG. 4 shows an exemplary block diagram of a circuit of a small appliance 1, comprising an electric DC motor 5, a power supply 8 and a circuit 30. The power supply 8 provides electric energy to the electric DC motor 5 and to the circuit 30. In the given embodiment, a DC voltage U is provided. The power supply 8 can be a rechargeable battery. The electric DC motor 5 can be switched on and off and operated by a transistor 40 connected in series with the electric DC motor 5. The circuit 30 comprises a controller 20 for controlling the electric DC motor 5. The controller 20 can regulate the supply voltage provided to the electric DC motor 5 by operating the transistor 40. More precisely, the controller can comprise a processor 21 controlling an output 22 providing a control signal 23 to the transistor 40. For example, the output can be a PWM (Pulse Width Modulation) module providing a PWM signal with a variable duty cycle to the transistor 40. However, other ways of regulating the supply voltage are also possible, such as pulse frequency modulation or the use of a variable resistor.

[0044] In an exemplary embodiment shown in Figure 4, a detector 10 (Figures 1 and 2) for obtaining a detector signal indicative of the rotational speed of an electric DC motor 5 may comprise a resistor 11 connected in series with the electric DC motor 5 and a transistor 40. The higher the current I through the electric motor and the series resistor, the higher the voltage drop across the series resistor 11. The detector optionally calculates the voltage drop U across the series resistor 11. S The detector may further comprise an operational amplifier 12 for sensing and amplifying the voltage at the upper node of the series resistor 11. The operational amplifier 12 may in turn be connected to an analog-to-digital converter 24 of the controller 20. In an embodiment, the detector may be at least partially part of the controller. For example, an ADC input of a microcontroller may be used to directly obtain the voltage at, for example, the upper node of the series resistor 11.

[0045] 4, in addition to or instead of the series resistor 11, the supply voltage U can be obtained. For example, a voltage divider 13, 14 can be provided and the supply voltage can again be determined via an analog-to-digital converter 24 of the controller 20.

[0046] Instead of applying a low-pass filter, as for example in EP 3327923 A1 as cited above in the background section, it is proposed to preserve the frequency components of the detector signal. Thus, the signal reaching the controller 20 still contains frequency components such as ripples generated during the operation of the electric DC motor. These frequency components are a further source of information for determining the rotation speed of the electric DC motor. Since the frequency components are almost independent of tolerances, it is possible to determine the rotation speed of the DC motor with high accuracy, even taking into account the tolerances of the circuit elements. The detector signal may be sampled at at least twice the maximum rotation frequency of the electric DC motor, in particular at least twice the maximum rotation frequency multiplied by the number of commutations of the electric motor per revolution. This ensures that the detector signal is sufficiently oversampled to avoid artifacts.

[0047] As shown in the flow diagram shown in Fig. 3, the controller 20 of Fig. 4 is adapted to determine a first parameter indicative of the rotational speed of the DC motor based on the detector signal, determine a second parameter indicative of the rotational speed of the DC motor based on the detector signal, the second parameter being different from the first parameter, the second parameter being determined based on a frequency domain analysis of the detector signal, and adjust the supply voltage of the electric DC motor 5 based on the first and second parameters. The detector signal may be represented as a motor current I or a voltage drop U across the series resistor 11. S Additionally or alternatively, the detector signals may represent the supply voltage measured via the voltage dividers 13, 14. The first parameter may be determined by averaging the respective detector signals, which can be determined quickly with low processing power. On the other hand, the second parameter may be obtained by performing a Fourier analysis of the detector signals, in particular a computationally efficient FFT analysis. As explained above, the second parameter may be indicative of frequency peaks in the spectrum, in particular frequency peaks that are integer multiples of the rotation frequency.

[0048] 5-7 illustrate exemplary embodiments of motor control based on a first parameter and a second parameter.

[0049] 5, a desired set point, such as a desired rotation speed of an electric DC motor, referred to herein simply as an electric motor or motor, may be provided as an input 51. The input 51 is provided to a summing node that calculates a difference or error between the input 51 and a feedback signal 53. In an exemplary embodiment, the feedback signal 53 may be a second parameter indicative of the rotation speed of the DC motor 5. The feedback may be determined in step 54 based on a detector signal 55 from the electric motor based on a frequency domain analysis. The output 55 of the summing node may be used for precision control in step 56. This may be seen as a second controller adapted to adjust the voltage of the electric DC motor based on a second parameter indicative of the frequency content of the detector signal.

[0050] Advantageously, rather than directly using the output of step 56 to adjust the supply voltage of the electric DC motor 5, the output of step 56 may be provided as a set point 57 to a further summing node 58 which calculates a difference or error between an input 57 and a further feedback signal 59. In an exemplary embodiment, the feedback signal 59 may be a first parameter indicative of the rotational speed of the DC motor 5. The feedback may be determined in step 60 based on a detector signal 61 from the electric motor by evaluating the amplitude of the detector signal. This may be seen as a first controller adapted to adjust the voltage of the electric DC motor based on a first parameter indicative of the frequency content of the detector signal.

[0051] 5, it should be understood that the same detector signal could be used but processed differently in steps 54 and 60. Because the amplitude of the detector signal can be quickly evaluated even in low-cost small appliances with limited processing power, the output 62 of the summing node can be used in step 63 for faster, but less accurate, control.

[0052] 6 and 7 illustrate a further variant, in which the controller is adapted to adjust the supply voltage of the electric DC motor 5 based on the first parameter determined in step 60 and the second parameter determined in step 54. Therefore, in the following, the differences with the embodiment described with reference to FIG. 5 are highlighted. With reference to FIG. 6, instead of providing a set point for the first controller, more precisely for the summing node 58, the output 57′ of step 56, i.e. the output of the second controller, may be used to provide a correction factor for the first controller in step 63′. With reference now to FIG. 7, in addition to or as an alternative to correcting the control parameter in step 63, the output 57″ of step 56, i.e. the output of the second controller, may be used to provide a correction factor for the feedback path in step 60′, so that the accuracy of the fast control loop can be further improved by further using information from the frequency domain analysis.

[0053] In conclusion, a further improved miniature appliance is provided. In particular, the proposed solution may serve to combine the advantages of rapidly controlling the rotational speed of an electric DC motor based on a first parameter indicative of the amplitude of the detector signal, and with high precision based on a second parameter, the second parameter being determined based on a frequency domain analysis of the detector signal.

[0054] While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description is to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0055] It should be understood that the functions performed by the controller may be performed more generally by a small appliance. The controller may be implemented by one or more entities. In an advantageous embodiment, the controller and / or detector may be implemented by circuitry that comprises a microcontroller.

[0056] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0057] The computer program may be stored / distributed on a suitable non-transitory medium, such as, for example, an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as, for example, via the Internet or other wired or wireless communication systems.

[0058] Any reference signs in the claims should not be construed as limiting the scope of the claims.

[0059] Dimensions and values ​​disclosed herein should not be understood as being strictly limited to the exact numerical values ​​recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as "40 mm" is intended to mean "about 40 mm."

Claims

1. A small appliance (1, 1'), said small appliance (1) being a hair removal device (1) or an electric toothbrush (1'), an electric DC motor (5), a detector (10) for obtaining a detector signal indicative of the rotation speed of said electric DC motor (5); a controller (20) for controlling said electric DC motor (5), The controller: determining a first parameter indicative of the rotational speed of the DC motor (5) based on the detector signal; determining a second parameter indicative of the rotational speed of the DC motor (5) based on the detector signal, the second parameter being different from the first parameter, the second parameter being determined based on a frequency domain analysis of the detector signal; adjusting a supply voltage of the electric DC motor (5) based on the first parameter and the second parameter; The small electrical appliance (1, 1'), characterized in that the controller (20) is adapted to perform a coarse control for regulating the supply voltage based on the first parameter, and to perform a fine control for regulating the supply voltage based on the second parameter.

2. 2. The small electrical appliance (1,1') according to claim 1, wherein the first parameter is indicative of an amplitude of the detector signal and the second parameter is indicative of a frequency content of the detector signal.

3. 3. The small electrical appliance (1,1') according to claim 1 or 2, wherein the detector signal is indicative of at least one of: (a) a motor current of the electric DC motor (5); (b) a motor voltage of the electric DC motor (5); and / or (c) a supply voltage.

4. 4. The small electrical appliance (1,1') according to any one of claims 1 to 3, wherein the controller (20) is adapted to continuously perform both a coarse control for regulating the supply voltage based on the first parameter and a fine control for regulating the supply voltage based on the second parameter.

5. 5. The small electrical appliance (1,1') according to any one of claims 1 to 4, wherein the controller (20) is adapted to perform a fast control for regulating the supply voltage based on the first parameter and to perform a slow control for regulating the supply voltage based on the second parameter.

6. 6. The small appliance (1,1') according to any one of claims 1 to 5, wherein the controller (20) comprises a first controller and a second controller, the first controller being adapted to adjust the supply voltage of the electric DC motor (5) based on the first parameter indicative of the amplitude of the detector signal, and the second controller being adapted to adjust the supply voltage of the electric DC motor (5) based on the second parameter indicative of a frequency component of the detector signal.

7. 7. A small appliance (1,1') according to claim 6, wherein the first controller and the second controller are cascaded.

8. 8. A small appliance (1,1') according to claim 6 or 7, wherein the second controller provides a set point for the first controller.

9. A small appliance (1,1') according to any one of claims 6 to 8, wherein the second controller adjusts a feedback path of a feedback loop of the first controller.

10. 10. A small electrical appliance (1,1') according to any one of claims 1 to 9, wherein the detector signal is sampled at at least twice the maximum rotational frequency of the electric DC motor (5), in particular the detector signal is sampled at at least twice the maximum rotational frequency multiplied by the number of commutations per revolution of the electric motor.

11. A small electrical appliance (1,1') according to any one of claims 1 to 10, wherein determining the first parameter comprises averaging the detector signal and / or determining the second parameter comprises a Fourier analysis of the detector signal, in particular an FFT analysis of the detector signal.

12. 12. A small electrical appliance (1,1') according to any one of the preceding claims, wherein the first parameter is determined with less precision at a high refresh rate and the second parameter is determined with greater precision at a lower refresh rate.

13. A small electrical appliance (1,1') according to any one of the preceding claims, wherein the second parameter is indicative of a frequency peak of a spectrum, in particular said frequency peak being an integer multiple of a rotation frequency.

14. 14. The small electrical appliance (1,1') according to any one of claims 1 to 13, wherein the first and second parameters are determined in parallel and the second parameter is used to adjust the supply voltage of the DC motor (5) when a difference between a first and a second frequency peak of a frequency domain analysis falls below a predefined threshold.

15. A method for controlling an electric DC (5) motor of a small appliance (1, 1'), said small appliance (1) being an epilation device (1) or an electric toothbrush (1'), said method comprising: obtaining a detector signal indicative of the rotational speed of the electric DC motor (5); determining a first parameter indicative of the rotational speed of the DC motor (5) based on the detector signal; determining a second parameter indicative of the rotational speed of the DC motor (5) based on the detector signal, the second parameter being different from the first parameter, the second parameter being determined based on a frequency domain analysis of the detector signal; and adjusting a supply voltage of the electric DC motor (5) based on the first parameter and the second parameter; A method, comprising: performing coarse control for adjusting the supply voltage based on the first parameter; and performing fine control for adjusting the supply voltage based on the second parameter.

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