Motor unit and personal care device having motor unit

Abstract

To provide a method of controlling a motor unit such that a constant motor amplitude can be predominantly maintained.SOLUTION: A motor unit comprises: a motor having an armature configured for relative driving motion to a stator; and a motor control unit having a supply circuit that provides a supply voltage to the motor to provide the motor with a set power level for driving and moving the armature, and a measurement circuit for measuring a value of a physical variable indicative of a current flowing through the motor. The motor control unit is configured to interrupt the supply of the supply voltage by the supply circuit, dynamically brake the motor during a braking time interval and measure the value of the physical variable, and compare the measured value of the physical variable with a target value dependent on a supplied power level and the intended movement amplitude of the armature to determine a new set power level, and then provide a new set power level to the motor.SELECTED DRAWING: Figure 7B

Description

[Technical Field]

[0001] The present application relates to a motor unit comprising a motor driven by supplying a set power level to the motor, wherein a motor control unit is configured to control the subsequent power supply of the motor based on measurements of a physical variable. The present application also relates to a personal care device and a method of controlling the motor unit. [Background technology]

[0002] It is known that the power required by a vibration motor to achieve a constant vibration amplitude or speed varies depending on the load applied to the motor. Without proper control of the power level supplied to the motor, the vibration motor's amplitude or speed will decrease as the load increases. One way to measure the motor's amplitude or speed is to directly determine the so-called back electromagnetic force (back EMF) in the motor coil, i.e., the voltage induced in the motor coil by the relative motion between the motor coil (e.g., mounted on the motor's stator) and a permanent magnet (e.g., mounted on the motor's moving part). As the amplitude decreases, the motor's speed decreases, and therefore the back EMF decreases. To enable direct measurement of the back EMF in the motor coil, the drive current through the motor coil is typically reduced to zero to avoid other voltages that would interfere with the back EMF measurement. This means that when the supply voltage to the motor is stopped, the drive current through the motor coil must first dissipate before the back EMF can be measured. If a decrease in the back EMF is determined, more energy must be applied to the motor each subsequent cycle (i.e., a higher power level must be provided) to maintain a constant motor amplitude. The document EP 1 063 760(B1) generally discusses such control methods. This solution requires long interruptions in the motor drive during which the energy in the motor must be dissipated, thus causing a decrease in motor efficiency.

[0003] Instead of determining the back EMF in the motor coil itself, it has been proposed to use a secondary measuring coil to determine the speed of the moving permanent magnet of the vibration motor, but such a solution requires additional components and therefore increases the cost of the motor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] European Patent No. 1 063 760(B1) Summary of the Invention [Problem to be solved by the invention]

[0005] It is an object of the present disclosure to provide a motor unit configured to control a motor, preferably a brushless motor, in a manner that primarily allows for maintaining a constant motor amplitude or speed and that provides improved, or at least different, motor control than known motor units. It is also an object of the present disclosure to provide a method of controlling a motor unit such that a constant motor amplitude may primarily be maintained, which method represents an improvement, or at least a different method, than known methods. Preferably, the motor unit and associated control method provides higher motor efficiency than known motor units / control methods. [Means for solving the problem]

[0006] According to one aspect, there is provided a motor unit comprising: a motor having a stator and an armature, the armature configured for driving movement relative to the stator; and a motor control unit having a supply circuit providing a supply voltage to the motor so as to provide the motor with a set power level for driving the armature into movement; and a measurement circuit for measuring a value of a physical variable indicative of current through the motor, wherein the motor control unit is configured to interrupt provision of the supply voltage by the supply circuit and dynamically brake the motor during a braking time interval, and further to measure the value of the physical variable during the braking time interval, and the motor control unit is further configured to compare the measured value of the physical variable with a target value, the target value depending on a supplied power level and an intended movement amplitude of the armature, determine a new set power level depending on a result of the comparison, and thereafter provide the new set power level to the motor.

[0007] According to one aspect, there is provided a personal care device comprising a motor unit as described above.

[0008] According to one aspect, there is provided a method for controlling a motor unit, preferably a motor unit of a personal care device, comprising the steps of: - providing a motor having a stator and an armature configured for relative driving movement with respect to the stator; - supplying a set power level to the motor to drive the armature into movement, preferably by supplying a pulse width modulated supply voltage to the motor; - dynamically braking the motor during a braking time interval; - measuring the value of a physical variable indicative of the current flowing through the motor within a braking time interval; - comparing the value of the physical variable with a target value, the target value depending on the set power level and the intended amplitude of the movement of the armature; - determining a new set power level depending on the comparison; thereafter providing the new set power level to the motor. [Brief explanation of the drawings]

[0009] The present disclosure will be further clarified by reference to the detailed description of the exemplary embodiments and drawings. [Figure 1] FIG. 1 illustrates elements of an exemplary motor unit comprising a motor, an H-bridge for applying a rectified DC voltage to the motor, and a motor control unit for controlling the H-bridge. [Figure 2A] FIG. 2 is a diagram of relevant elements of the motor unit shown in FIG. 1 during a first commutation phase. [Figure 2B] 2 is a diagram of the relevant elements of the motor unit shown in FIG. 1 during a dynamic braking time interval. [Figure 2C] FIG. 2 is a diagram of relevant elements of the motor unit shown in FIG. 1 during a second commutation phase. [Figure 3] 1 is a diagram of the voltage signal measured as a physical variable representing the current flowing through the motor depending on the supply power level, with three curves shown for three different constant back EMF values. [Figure 4] 1 is a diagram of voltage and current signals versus time for an example with a medium-sized braking time interval, where the relevant disturbance of the intended sinusoidal current flow through the motor can be seen in the time current behavior. [Figure 5] 1 is a diagram of voltage and current signals versus time for an example with a small magnitude braking time interval, where the relevant disturbance of the intended sinusoidal current flow through the motor is seen in the time current behavior. [Figure 6A] 10A-10C are zoomed-in views of simulated voltage signals before, during, and after a dynamic braking time interval for a first level of back electromagnetic force. [Figure 6B] 10A and 10B are zoomed-in views of simulated voltage signals before, during, and after a dynamic braking time interval for a second level of back electromagnetic force that is higher than the first level. [Figure 6C]10A and 10B are zoomed-in views of simulated voltage signals before, during, and after a dynamic braking time interval for a third level of back electromagnetic force that is lower than the first level. [Figure 7A] 5 is a schematic diagram illustrating the relationship between the set power level supplied to the motor and the measurement signal for several examples. [Figure 7B] 1 is a schematic diagram illustrating the relationship between the set power level supplied to the motor and the measurement signal for several examples over a time sequence, and the resulting repeated application of the set power level; [Figure 8] FIG. 1 is a schematic diagram of a motor including a stator and a rotor that may be used in a motor unit as described herein. [Figure 9] 1 is a diagram of a personal care device in which the disclosed motor unit is used; [Figure 10] FIG. 1 is a flow diagram of a method for controlling a motor unit according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] In the context of this specification, "personal care" shall mean the nourishment (or care) of the skin and its appendages (i.e., hair and nails), as well as the teeth and oral cavity (including tongue, gums, etc.), aimed, on the one hand, at preventing disease and maintaining and promoting health, and, on the other hand, at the cosmetic treatment and improvement of the appearance of the skin and its appendages. This includes the maintenance and enhancement of well-being. It includes skin care, hair care, oral care, and nail care. It further includes grooming operations such as beard care, shaving, and hair removal. Thus, "personal care device" means any device for performing such nourishment or grooming operations, for example (cosmetic) skin treatment devices such as skin massage devices or skin brushes; wet razors; electric shavers or trimmers; electric epilators; and oral care devices such as manual or electric toothbrushes, (electric) flossers, (electric) irrigators, (electric) tongue cleaners, or (electric) gum massagers. This does not exclude that the proposed personal care device may have more significant advantages in one or more of these developmental or device areas than in one or more other of these areas. In this description, an electric toothbrush is chosen to illustrate the details of the proposed personal care apparatus. Unless details are specific to an electric toothbrush, the proposed technology can be used in any other personal care device.

[0011] As used herein, the term "dynamic braking" refers to a motor control state in which the motor terminals are connected to each other, for example, via two switches in an H-bridge, and the current in the motor recirculates through the motor and dissipates in any resistance present in the recirculation circuit (e.g., the resistance of the motor coils and the resistance of the switches in the H-bridge). During the dynamic braking interval, the motor is used as a generator and continues to drive the armature. In the embodiments described herein, the dynamic braking interval does not include a regenerative aspect, i.e., the energy stored in the motor is not supplied to the energy source, because non-regenerative braking has higher energy efficiency than regenerative braking. This does not exclude that the concepts discussed herein can be combined with regenerative braking.

[0012] The motors in this disclosure include a stator and a rotor or armature as moving motor parts. When the moving motor part performs oscillatory rotation, the term rotor is typically used. When the moving motor part performs reciprocating linear motion, the term armature is typically used. For simplicity, only the term "armature" is used below. Therefore, the motors in this disclosure are typically oscillatory motors. Oscillatory motors have a resonant frequency at which input energy (represented by the supplied power level) is most efficiently converted into motor amplitude. Therefore, such motors are regularly driven at a typically fixed drive frequency at or near that resonant frequency. Adding a load to the motor adds damping and shifts the resonant frequency, thereby reducing the motor amplitude when a load is applied. Here, "motor amplitude" refers to either the peak amplitude provided by a linearly reciprocating armature or the peak angular deflection of an oscillatory rotating rotor (see more detailed description below). Because a user may not perceive the varying amplitude of a motor resulting in a varying amplitude of a driven part (e.g., a movable head part of a personal care device), it may be desirable to maintain, or at least approximately maintain, the motor amplitude despite variations in the applied load for an improved consumer experience. Therefore, it may be desirable to measure the instantaneous motor amplitude or determine the value of a physical variable indicative of the motor amplitude and adjust, for example, the supplied power level so that the motor amplitude remains constant, or at least is kept closer to a constant amplitude level than would be the case without adjustment. This means that variations in the applied load may be faster than the regulation loop, or the regulation process may intentionally respond to load changes with only incremental or partial adaptations of the set power level to avoid jitter or generally unstable behavior of the motor amplitude. Thus, motor amplitude constancy may be achieved only approximately, or with some delay.

[0013] Due to the structure of the vibration motor, which comprises at least one permanent magnet and a coil that move relative to each other at a fixed drive frequency during operation, the voltage induced in the coil by the permanent magnet, also known as the back electromagnetic force (or back EMF or BEMF), provides a signal indicative of the motor amplitude (or motor linear or rotational speed). Therefore, determining a signal indicative of the back electromagnetic force provides a measure of the current motor amplitude and thus provides a basis for controlling the motor amplitude, so as to be able to infer the back electromagnetic force from the signal or to establish an indirect correlation with the BEMF from the signal. Therefore, the motor control unit includes a measurement circuit for determining such a signal, which is the value of a physical variable indicative of the current flowing through the motor within the dynamic braking interval. How measuring the value of a physical variable indicative of the current flowing through the motor makes it possible to derive insight into the back electromagnetic force will be explained in more detail below.

[0014] Motor amplitude is typically given by the peak-to-peak value of the linear reciprocating motion of the motor shaft or by the peak-to-peak deflection angle of the oscillatory rotation of the motor shaft. Instead of peak-to-peak amplitude, reference may be made to half amplitude, i.e., peak amplitude, which is understood to be half the peak-to-peak amplitude. Maintaining peak-to-peak amplitude means maintaining peak amplitude in this disclosure. When a load is applied to the oscillatory motor, as already mentioned, the motor amplitude decreases. Since the frequency of the oscillatory motion of the motor is fixed by the drive frequency utilized by the motor control, a decrease in amplitude means a decrease in armature speed. As mentioned above, the armature speed can be determined directly by measuring the back EMF (i.e., the induced voltage in the stator coil of the motor due to the moving permanent magnet(s) attached to the armature), but the back EMF is also determined by the voltage U at the motor due to the motor resistance. R = R·I(t) is 0, and the self-induced voltage U in the motor due to the change in motor current LBack EMF can only be measured directly if the current driven through the motor is reduced to zero for a period of time, such that = L·(dI(t) / dt) also becomes zero. If the motor control does not inherently provide a long enough time interval during which the motor current is zero, then the motor power supply must be interrupted and the energy stored in the motor coils must be dissipated or supplied back to the battery to create a situation in which the back EMF can be measured directly. This allows the current motor amplitude to be determined and defines whether the set power level provided to the motor should be increased or decreased to maintain a constant motor amplitude. Because this is energy inefficient, a more energy-efficient motor control method and respective motor unit are proposed herein, i.e., a method or motor unit that does not require the motor current to be reduced to zero in order to determine the back EMF / motor amplitude.

[0015] One motor control method for determining motor amplitude is discussed in U.S. Patent Application Publication No. 2005 / 0146296(A1), in which, during a motor control phase in which a voltage is applied to a motor and a current is driven through the motor, at least two current values are measured, allowing the motor speed (and therefore the motor amplitude) to be determined. In U.S. Patent Application Publication No. 2005 / 0146296(A1), a single long voltage pulse is applied per half-cycle of the motor's periodic drive, and the current driven through the motor only very roughly approximates a sinusoidal shape. In contrast, the motor control concept proposed herein includes embodiments in which the current driven through the motor is shaped; for example, the current may be shaped to an approximate sinusoidal shape by applying a set power level in the form of multiple voltage pulses per half-cycle, and the length of each voltage pulse may be controlled by pulse-width modulation (PWM).

[0016] According to some aspects of the present disclosure, a motor unit includes a motor and a motor control unit having a supply circuit and a measurement circuit. The motor is driven by applying an electronically rectified supply voltage (e.g., a DC voltage) to the motor through a supply circuit that is part of the motor control unit. To apply the DC voltage to the motor, the supply circuit may include an H-bridge, as commonly known in the art. The H-bridge may be a full H-bridge, although this does not exclude that, for example, a half-bridge circuit may also be used. The motor control unit is used to control the supply circuit to electronically rectify the DC voltage, which may be supplied by a battery or a rechargeable battery. The motor control may be periodic, i.e., the motor control may be characterized by a drive frequency or the duration of a drive period; for example, the drive frequency may be 100 Hz, in which case the length of the drive period is 0.01 seconds. By applying a DC voltage to the motor that periodically changes its polarity, a current that periodically changes its direction is driven through the motor.

[0017] The motor control unit can use pulse-width modulation (PWM) to control the supply circuit to provide a supply voltage in pulses, with the varying duty cycle of these voltage pulses allowing the current driven through the motor to be shaped; for example, PWM can be used to drive a nearly sinusoidal current through the motor. The PWM signal can have a PWM frequency higher than the drive frequency, preferably significantly higher than the drive frequency, with multiple voltage pulses applied to the motor per half drive period. By way of example only, the drive frequency can be approximately 145 Hz and the PWM frequency can be approximately 37 kHz, i.e., 256 voltage pulses can then be applied per drive period (or 128 pulses can then be applied per half drive period), with the duty cycle of each voltage pulse controlled to shape the current flow. As described in U.S. Patent Application Publication No. 2005 / 0146296 A1, to apply one PWM control voltage pulse per half drive period, the PWM frequency must be twice the drive frequency.

[0018] Generally, the drive frequency may be selected to have a value within the range of 10 Hz to 1,000 Hz, preferably within the range of 30 Hz to 500 Hz, and more preferably within the range of 50 Hz to 300 Hz, but is not limited thereto. The PWM frequency may be the same as the drive frequency or may be twice the drive frequency, but a PWM frequency much higher than the drive frequency may provide certain benefits, such as more precise shaping of the current flow through the motor. While switching losses may need to be considered, a frequency that is too high may not balance greater precision with efficiency. Therefore, the PWM frequency may be higher than the drive frequency by a factor within the range of 4 to 10,000, preferably within the range of 10 to 5,000, and more preferably within the range of 20 to 1,000, but is not limited thereto. A single PWM-controlled voltage pulse may have an on-length between 0% and 100% of the full length of the respective PWM period; this relative on-length is typically referred to as the duty cycle. The duty cycle may be controlled analogously or digitally, with the digital resolution selected to be, but not limited to, in the range of 2 to 64 bits, preferably 4 to 16 bits. With 8-bit resolution, the duty cycle of the voltage pulse can take one of 256 levels, ranging from 0% duty cycle to 100% duty cycle. The on-length and off-length of each PWM control voltage pulse may then be controlled by a base clock signal having a respective high frequency to enable 256 levels of duty cycle, and the individual on-lengths of each PWM control voltage pulse per driving period may be stored in a look-up table in the memory unit of the motor control unit. Controlling the duty cycle of the voltage pulse applied to the motor allows the driven motor current to be shaped into an approximately sinusoidal waveform, although this naturally means that the motor current may be shaped to approximately take any other intended shape, such as a triangular shape, trapezoidal shape, etc. For this reason, multiple look-up tables may be provided in the memory unit to enable different current shapes.

[0019] The motor control unit, and more particularly the measurement circuitry of the motor control unit, is configured to determine a value of a physical variable indicative of the current flowing through the motor. A signal representing this value is then generated and provided to a processing unit of the motor control unit. The physical variable may be a voltage, as described in more detail below, but may also be a current. The value (the signal representing the value) is determined during at least a measurement portion of the braking time interval; i.e., the value may be determined at a moment within the braking time interval that does not coincide with the start of the braking time interval, or the value may be determined during a period within the braking time interval that is shorter than the braking time interval, and that may not begin at the start of the braking time interval and / or may not end at the end of the braking time interval. In some embodiments, the value of the physical variable is determined by sampling the physical variable multiple times during the braking time interval and averaging or otherwise combining the sample values. In this disclosure, the phrase "during the braking time interval" means either the moment the braking time interval begins or ends, or any moment or period in between.

[0020] The fundamental insight described herein lies in the realization that the value of a physical variable indicative of the current flowing through the motor measured during a braking time interval is affected by the back EMF, thus making it possible to derive or estimate the back EMF from such measurements. However, the measurement is also affected by other parameters, specifically the non-zero current resulting from the energy stored in the motor when dynamic braking begins, and therefore (as already mentioned) a direct measurement of back EMF is not possible. According to the present disclosure, the measurement of a physical variable indicative of the current flowing through the motor is compared to a predetermined target value that depends on the currently applied power level, i.e., the set power level, and the intended motor amplitude. This comparison makes it possible to determine at least whether the set power level currently provided to the motor needs to be increased or decreased to maintain the intended motor amplitude, or at least to keep the resulting motor amplitude closer to the intended amplitude than would be the case without such control. The underlying insights and control steps are described in more detail below.

[0021] In particular, the motor current I, which may be approximately sinusoidal (a sinusoidal waveform is assumed here), M is at its peak given by: I M,p =(V BATT -V BEMF ) / √(R 2 +R X 2 ) Here, I M,p is the peak motor current and V BATT is the supply voltage or battery voltage, and V BEMF is the back electromagnetic force, R is the sum of all ohmic resistances, and R X is the reactance of the motor coil. If the current through the motor is sinusoidal and driven by a relative PWM signal ranging from 0 to 1, the motor current can be approximated by the following equation: I M =PWMW·I M,p ·sin(ω·t+φ L ) where PWMW can be a number between 0 and 1 representing the PWM weight, and φ L is the phase shift between the supply voltage and the motor current. (Based on the X and R values of the example motor circuit used in the evaluations of this disclosure, φ L (It was found that the sin function is approximately 19.7 degrees) Under the assumption that the dynamic braking time interval begins at the point where the sin function is 1, the voltage V measured in parallel with the motor circuit at the very beginning of the dynamic braking time interval Measure is given as follows: V Measure =-R S PWM (V BATT -V BEMF ) / √(R 2 +R X 2 ) where R Sis the ohmic resistance of the resistor where the voltage is measured. The PWM weighting factor PWMW reflects the set power level delivered to the motor. While the PWM lookup table described above may provide duty cycle values that represent a sinusoidal function with a normalized peak of 1, the PWMW coefficient is a global weighting factor that determines the power level and is applied to all PWM values. PWMW=1 sets the maximum power level delivered to the motor, and PWMW=0 sets the minimum power level and provides no energy to the motor. The PWMW coefficient can take any value between 0 and 1 to set the power level delivered to the motor. For illustrative purposes, PWMW may be 0.35 for one intended motor amplitude under no-load conditions and 0.6 for another intended motor amplitude under no-load conditions.

[0022] The formula thus derived relates the PWMW value, which represents the set power level provided to the motor, to a given back electromagnetic force, V BEMF Measured voltage V Measure In other words, the relationship between the PWM and the measured voltage is linear for any intended amplitude. The linear formula can be rewritten as: V Measure =-A·PWMW or V Measure =-A·PWMW+B where A is just a constant for a given motor amplitude that depends on the back EMF value, and the second variant with an additional constant value B is, in fact, V Measure may include an absolute shift due to the overall structure of the measurement circuit (e.g., due to pull-up or pull-down resistors in the measurement circuit, which may be required because the measurement circuit cannot actually measure negative voltages). Figure 3, described further below, is a diagram of measured voltage as a function of PWMW for three different constant back EMF values.

[0023] This linear relationship is an approximation because, for example, we have assumed that at a constant motor amplitude, the back electromagnetic force experiences no significant phase shift relative to the motor current. In reality, some phase shift occurs, but for the purposes of this discussion, it can be ignored. We have found that assuming a linear relationship is sufficient in most cases. A linear relationship is easy to calibrate; for example, in the above equation, the voltage at PWMW=0 is a known constant, so only one additional point on the curve needs to be measured; for example, the PWMW setting required for the intended motor amplitude with no additional external load on the motor can be used to determine the linear relationship.

[0024] However, if a linear relationship approximation is not sufficient, it is of course possible to apply more complex calibration schemes, for example if the power levels (PWMW values) can be measured at various load conditions, such as two, three, or four different load conditions (such as 0.5N load, 1N load, 1.5N load, 2N load, and / or 2.5N load) to calibrate the relationship between PWMW and measured voltage.

[0025] When the external load changes, the back EMF changes, which in turn changes the measured voltage V Measure This means that the measured voltage deviates from the above linear curve for the set power level / PWMW, i.e., the measured voltage will be higher if the load decreases (back EMF increases as the motor amplitude increases) or lower if the load increases (back EMF decreases as the motor amplitude decreases). Therefore, if the measured voltage is above the linear curve for the intended amplitude, the set power level, i.e., the PWMW value in the above equation, needs to be decreased so that the measured voltage returns to the linear curve for the intended motor amplitude. In contrast, if the measured voltage is below the linear curve for the intended amplitude, the set power level, i.e., the PWM in the above equation, needs to be increased so that the measured voltage returns to the linear curve for the intended motor amplitude.

[0026] This further means that for a set power level (given PWMW value) during motor operation, a measured voltage that deviates from the linear curve for the intended motor amplitude is an indication of a changed load condition, and the set power level should be changed to a new set power level, i.e., a different PWMW value, that results in a motor amplitude that is essentially the same as, or at least close to, the intended motor amplitude. It has been found that incremental adaptation of the power level is, on the one hand, sufficient to achieve the intended motor amplitude within a short period of time, while, on the other hand, avoids jumps or noticeable jitter in the motor amplitude. Furthermore, applying a threshold between the measured voltage and the calibrated linear curve has been found to be advisable to prevent small deviations below the threshold from resulting in a new set power level. By "threshold," we mean the absolute difference between the measured voltage and the calibrated linear curve. In control systems, this is commonly known as hysteresis. This helps stabilize the motor amplitude and avoid noticeable jitter. This concept is further explained below with reference to Figures 7A and 7B. Essentially, a calibration can be performed for multiple intended amplitude values and a single PWMW value (i.e., at no load conditions), for a linear relationship, or for multiple PWMW values per amplitude in the case of a non-linear relationship, so that for any subsequent measurement, it can be derived (e.g., by interpolation or extrapolation) to which amplitude (and therefore load condition) the measurement relates, and then the PWMW change required to shift the amplitude back to the intended amplitude (i.e., on the target curve) can be calculated.

[0027] To compensate for any errors in the calculations regarding the phase shift of the motor current, the phase shift can also be measured or tracked by a measurement circuit. The measurement circuit can, for example, track the positions of the voltage peaks (two for the sine wave) and their relative positions to the tabulated PWM values. The dynamic braking interval can then be shifted by the amount that the tracked voltage peaks have moved, so that the measurement circuit always measures the recirculation current at approximately the same position relative to the BEMF sine wave.

[0028] As described above, the duty cycle of the PWM control voltage pulses applied during one drive cycle may be selected to generate a sinusoidal drive current through the coil. The duty cycle values during the drive cycle may be stored in a memory unit of the motor control unit; for example, if the PWM frequency is 256 times the drive frequency, 256 duty cycle values may be stored. The duty cycle values may have values between 0 and 1 or between 0% and 100%. For clarity, the duty cycle values may be stored digitally, as described in the following paragraphs.

[0029] In operation, the duty cycle applied to the voltage pulse is the stored duty cycle multiplied by the PMWM coefficient. As mentioned above, the duty cycle may be digitized, for example, with 8-bit resolution so that duty cycle values from 0 to 255 may be used, or with 7-bit resolution so that duty cycle values from 0 to 127 may be used.

[0030] Further, it was stated above that the PWMW value may be between 0 and 1. In what follows, for ease of explanation, it will be assumed that the PWMW is also mapped to a 7-bit digital scheme, where 0 is 0 and 1 is 127. It should be understood that this is a non-limiting assumption.

[0031] As an example, the PWMW value may be set to 63 to achieve the intended peak-to-peak amplitude of the motor shaft, e.g., 0.8 mm. As will be explained in more detail, the measured voltage, i.e., the expected value of the physical variable under no-load conditions for a peak-to-peak amplitude of 0.8 mm, is known from the linear formula described above through pre-calibration. During operation, the set power level may change under varying load conditions; i.e., the weighting factor PWMW is adapted, and the new power level is set to maintain the motor amplitude. Assume that the motor unit starts with a weighting factor PWMW of 63, which results in a peak-to-peak amplitude of 0.8 mm under no-load conditions. Now, for example, if the applied load changes when the personal care device is used and the movable head is pressed against the body part to be treated, the load will cause a decrease in motor amplitude, which in turn will cause a change in the value of the determined physical variable, here the measured voltage. In turn, the decrease in motor amplitude will lead to a decrease in back EMF, causing the measured voltage to decrease below the target value under no-load conditions. During operation, the applied load may at some point be lower than the previously applied load, and then the motor amplitude may increase, resulting in an increase in the back EMF and therefore a higher measured voltage than the expected value of the measured voltage. An exemplary description of the control scheme is provided further below with particular reference to Figures 7A and 7B.

[0032] The above calibration would ideally be a global calibration for the personal care device or even all personal care devices of a given type from a manufacturer, but personal care devices may be equipped with different interchangeable movable heads, each of which may have its own calibrated target function, or tolerances of various parts of the personal care device may require each personal care device to have its own target curve.

[0033] To maintain the intended motor amplitude, the PWMW coefficient may be changed; for example, following the example above with a PWMW of 63 under no-load conditions, the PWMW may need to be higher when a load is applied. For example, the PWMW may need to be increased to 85 to compensate for the additional load; note here that the given values are for illustrative purposes only and are non-limiting. While the required change can be made in a single step, it is also contemplated to change the PMWM only incrementally; for example, the PWMW may be increased from 63 to 64 to set a new power level, and then increased from 64 to 65, etc., after the next measurement, until the value of a physical variable indicative of the current through the motor, e.g., the measured voltage, matches the expected value, i.e., the value of the calibrated linear relationship described above. If the applied load decreases, the PWMW may also be decreased, preferably in incremental steps.

[0034] Such incremental adjustments avoid large adjustments every drive cycle and adjustments that jump rapidly in both positive and negative directions. For example, a signal comparison may indicate that a power level increase should be made from a weighting factor of 63 to 79 due to increased load, but in the incremental control scheme just described, this is not the new set power level that is subsequently applied (i.e., in the next drive cycle). In contrast, only an incremental increase to 64 is applied. For example, at a drive frequency of 145 Hz, by determining the value of a physical variable indicative of the current flowing through the motor each cycle, an adjustment from the highest duty cycle (127) to the lowest duty cycle (0) still takes less than 1 second (i.e., approximately 0.9 seconds), and from 63 to 79 takes 0.11 seconds. This, of course, does not preclude adjustments being made non-incrementally, e.g., a new set power level to be applied each determined period to immediately compensate for the difference in applied load.

[0035] Furthermore, if the comparison between the determined signal and the target signal results in a difference less than the absolute threshold difference value, then no subsequent power level adjustment may be applied, which may effectively avoid adjustment jitter and / or result in a more stable behavior as already mentioned, which in turn means that the new set power level remains the set power level, since nothing has been changed.

[0036] It has already been mentioned that step changes in power level may be used instead of incremental changes. If the measured voltage is not on the target curve for the intended amplitude, the PWMW may be altered as described below. It is clear that the measured voltage lies on a linear curve with the same origin as the target curve, and therefore the slope relating the currently measured voltage to the unknown amplitude can be easily determined based on the known currently set power level / PWMW and the corresponding measured value, as well as the known value of the target curve at PWMW=0 (value B in the above equation). The slope of this curve may be denoted as A'. By vertically projecting the currently measured voltage value at the measured voltage on the PWMW graph onto the target curve along a line rotated 90 degrees, i.e., a line with a slope of A''=-1 / A', the intersection between the rotated line and the target curve defines the new set power level.

[0037] Although this disclosure focuses on linear vibrating or oscillating motors (or resonant motors), the motor control proposed herein is independent of the specific motor type and can operate with all types of motors that can be driven by applying an electronically commutated DC voltage to the motor, for example, all types of brushless DC motors or permanent magnet synchronous motors.

[0038] FIG. 1 is a schematic diagram illustrating some elements of an exemplary motor unit 100 according to the present disclosure. Some elements shown in FIG. 1 are optional, as will be explained. The motor unit 100 includes a motor 101, a motor control unit 110 with a supply circuit 120 including an H-bridge, and a measurement circuit 130. The motor 101 is shown here as being represented by a resistance, an inductance, and a voltage source. As commonly known in the art of electronic commutation, the motor 101 is here arranged in a bridge section of an H-bridge having four controllable switches 121, 122, 123, and 124. The H-bridge can be controlled to apply a DC voltage Vdd at the motor 101, the polarity of the applied DC voltage depending on which of the controllable switches are closed. When switches 121 and 124 are closed to provide a current path and switches 122 and 123 are open, a DC voltage is applied at motor 101 with a positive polarity; when switches 122 and 123 are closed and switches 121 and 124 are open, a DC voltage is applied with a negative polarity. Switches 121, 122, 123, and 124 may be implemented with MOSFETs as is common in the art. For this reason, a diode may be placed in parallel with each MOSFET, as shown in FIG. 1 . Switches 121, 122, 123, and 124 are controlled by controller 140 as indicated by four control signals S1, S2, S3, and S4 that may be output by controller 140. The four control signals S1, S2, S3, and S4 are used to control switches 121, 122, 123, and 124, respectively, i.e., when the switches are closed to allow current to flow through their respective resistances and when the switches are opened to block current through their respective resistances.

[0039] 1 includes an RC low-pass filter 131 having a capacitor 1311 and a resistor 1312, a pull-up resistor 132, and an analog-to-digital converter (ADC) 133, which together are configured to determine the value of a physical parameter indicative of the back electromagnetic force of the motor 101, which here is a voltage. The RC low-pass filter 131 is connected to the positive terminal of the motor 101. The pull-up resistor 132 has one end connected to a reference voltage, here a DC voltage Vdd that may be provided by a battery or accumulator, and functions to stabilize the voltage provided by the RC low-pass filter 131 and raise it to a value optimized for supplying to the ADC 133. The ADC 133 is used here to convert the analog voltage signal to a digital signal and may be configured to sample voltage values within at least a measurement portion of a braking time interval during which the motor 101 is dynamically braked, as described in more detail below. For completeness, it is mentioned that ADC 133 may be part of a hardware component implementing processor 140, e.g., ADC 133 and processor 140 may be implemented together by a microprocessor. RC low-pass filter 131 and pull-up resistor 132 are optional components, and the voltage across switch 123 may be supplied directly to measurement circuit 130, e.g., to ADC 133 or any other component capable of converting and / or comparing voltage signals.

[0040] Dynamic braking is achieved, for example, by closing switches 123 and 124 and leaving switches 121 and 122 open, effectively shorting out motor 101 through the resistance of switches 123 and 124. During such a dynamic braking phase, the kinetic energy of motor 101 is converted to electrical energy. The current flowing through motor 101 at the start of the braking time interval is recirculated through motor 101 via the current path provided by switches 123 and 124, and the current is dissipated in the resistance of switches 123 and 124 as well as the motor resistance. In contrast to regenerative braking, where motor current is supplied back to the battery, the current drop is slower in dynamic braking, making dynamic braking more energetically efficient than regenerative braking.

[0041] 2A to 2C show the effective circuit configuration of the motor unit 100 shown in FIG. 1, divided into a positive half cycle of motor drive when a voltage pulse is applied as shown in FIG. 2A, a braking time interval as shown in FIG. 2B, and a negative half cycle of motor drive when a voltage pulse is applied as shown in FIG. 2C. For simplicity, the controller 140 is not shown in FIGS. 2A and 2C, but it should be understood that the controller 140 is, of course, present to control the H-bridge so that the effective circuit structure as shown is produced.

[0042] In FIG. 2A, switches 121 and 124 are closed and essentially function as resistors, while switches 122 and 123 are open and do not contribute to the effective circuit configuration and are therefore not shown. In this phase, a DC voltage Vdd is provided to the positive terminal of motor 101. As previously explained, the DC voltage Vdd may be applied to the motor at a PWM frequency higher than the drive frequency; FIG. 2A, of course, applies only to the on-period of the voltage pulse application. The provision of the DC voltage Vdd having a positive polarity may be maintained for a first period, which may be interrupted by a dynamic braking time interval. FIG. 2C shows the circuit configuration when a DC voltage of the opposite polarity (i.e., negative polarity) is provided to the motor, switches 122 and 123 are closed, and switches 121 and 124 are open. The provision of the DC voltage Vdd having a negative polarity may be maintained for a second period (again, the DC voltage Vdd may be applied in pulses), which is preferably the same length as the first period. The provision of positive and negative DC voltages may be repeated periodically (at a drive frequency). The period of this periodic function is given by the sum of the first period and the second period. For example, the period may be 6.8966 ms, in other words, the motor is thus driven at a drive frequency of 145 Hz. This is merely an example, and it should be understood that in general, any other drive frequency may be used, such as, for example, 0.5 Hz, 2 Hz, 7 Hz, 13.4 Hz, 29 Hz, 52 Hz, 84 Hz, 112 Hz, 140 Hz, 141 Hz, 142 Hz, 143 Hz, 144 Hz, 146 Hz, 147 Hz, 148 Hz, 149 Hz, 150 Hz, 184 Hz, 250 Hz, 400 Hz, etc.

[0043] 2B shows the circuit configuration during a dynamic braking time interval, with switches 123 and 124 closed and switches 121 and 122 open. The same circuit configuration is typically effective during the "off" phase of PWM voltage pulse application, but for only a short time; the dynamic braking interval is typically longer than the duration of a single voltage pulse and may, for example, span the time interval for applying at least two or more DC voltage pulses. As will be explained in more detail below, it is preferable to apply the shortest possible braking time interval, as this will have minimal effect on the shape of the current flowing through motor 101. Motor 101 is short-circuited via switches 123 and 124, and motor current I M is recirculated through the motor 101 and the resistance of the switches 123 and 124, and the kinetic energy of the motor 101 is converted into electrical energy. The measurement circuit 130 is configured to measure the value of the physical variable indicative of the current flowing through the motor 101 only during the dynamic braking time interval, and preferably only within a measurement portion of the braking time interval that is shorter than the braking time interval itself. That is, the measurement portion may not start at the instant the braking time interval begins, but may start later, and / or may not end when the braking time interval ends, but may end earlier. The measurement may be performed at a single instant, or several measurements may be made within the measurement portion. The signal measured according to the setup as shown in Figures 1 and 2B is the voltage drop V across the resistance of the switch 123. M which is the same voltage that can be measured across the resistance of motor 101 and switch 124, and which voltage is therefore indicative of the back electromagnetic force induced in the motor coils due to the motion of the permanent magnet. This example does not exclude that the signal indicative of the back electromagnetic force is a voltage that can be measured between the positive and negative terminals of motor 101, or that the current flowing through motor 101 actually be measured as the physical variable.

[0044] As already indicated, the DC voltage Vdd may be provided to the motor 101 by pulse width modulation (PWM), i.e., the H-bridge may be controlled to provide the DC voltage Vdd intermittently during first and second periods at a PWM frequency higher than the drive frequency. For example, if the drive frequency is approximately 145 Hz, the frequency at which the DC voltage Vdd is provided by the PWM control pulses may be approximately 37.12 kHz, which means that 256 DC voltage pulses are applied per period and 128 voltage pulses are applied per half period, i.e., during a positive or negative half period (the terms period and cycle are used interchangeably herein). According to the present disclosure, since a braking time interval is proposed, the number of pulses applied per half period may naturally be less than 128 (in the given example). The voltage signal applied by PWM may have a varying length from pulse to pulse; for example, the voltage signal may have a low on-time at the beginning and end of a half-cycle and a high on-time in the center of the half-cycle, so that the shape of the current driven through the motor can be controlled (the sum of the on- and off-times of each applied pulse is typically constant and, of course, determined by the PWM frequency, e.g., 37.12 kHz in this embodiment). In some examples, PWM can apply voltage pulses such that an approximately sinusoidal current is achieved. During the off-time following each on-time, the H-bridge can be controlled to switch to a circuit configuration such as that shown in FIG. 2B for dynamic braking of the motor, as already indicated. For clarity, any other current shape besides a sinusoidal shape, e.g., a trapezoidal shape, a triangular shape, a rectangular shape, etc., may also be contemplated, where it is understood that the current is a periodic function according to the drive frequency.

[0045] Figure 3 shows the measured voltage V as a value of the physical variable that indicates the current through the motor for different constant back EMF values for the PWM weight PWMW. Measure As derived in the previous paragraph, the measured voltage is V Measure =-R S PWM (V BATT -V BEMF ) / √(R 2 +RX 2 ) or more generally, under the discussed assumptions, V Measure 3, the offset value was assumed to be B=0, but it should be understood that the offset B may have a non-zero value due to pull-up resistors that may be used or due to other circuit specifications. Curve 200 shows a curve that is suitable for applications with moderate back EMF values (e.g., V BEMF =0.5 volts), while curve 201 shows the measured voltage for lower back EMF values (e.g., V BEMF <0.5 volts), while curve 202 shows the measured voltage for higher back EMF values (e.g., V BEMF2 shows the measured voltage for a given back EMF value (>0.5 volts). As explained above, a given back EMF value is associated with a given amplitude. Thus, curve 200 can be associated with a given peak amplitude value of a linear reciprocating motor, e.g., 0.8 mm, and can represent a target curve. If the measured voltage is not on target curve 200 (the PWMW is known by the system and represents a set power level), the motor control unit takes action to modify the PWM so that, ideally, in the next measurement, the measured voltage is at least closer to the target curve. It is clear that the space between curves 201 and 202 is filled with curves, each associated with one back EMF value and therefore one motor amplitude. It can therefore be seen that all points between these curves can be assigned to a back EMF value and therefore a motor amplitude. To generate the intended amplitude under no-load conditions, a specific power level needs to be provided at the motor; this power level is designated as P1, as voltage V1 is measured and a respective point 203 on target line 200 is indicated. Now, if the load increases, the measured voltage will decrease; for example, at a given load value, the measured voltage will decrease to V2, and the PWMW will still be at P1, so the respective point in the diagram is 204. The concept is now to increase the PWMW to another, apparently higher value, so that the measured voltage is again on target line 200. A more detailed description of exemplary adaptation process steps is provided further below in connection with FIGS. 7A and 7B. However, for now, it can be stated that the P and V values for various amplitude and load values are determined to enable determination, for example by two-dimensional interpolation, of which amplitude and load value the currently determined P / V combination is associated with, and what P value is required to reach the intended target line under the current load situation.

[0046] 4 shows a current signal 210 flowing through the motor and a voltage signal 211 that can be measured over a time interval of approximately 1.5 periods by the measurement circuit 130 shown in FIG. 1. These signals represent actual measurements, and the measurement circuit includes an RC filter circuit as shown in FIG. 1, although it should be noted that, as already mentioned, the RC filter is an optional circuit element. The RC filter prevents the voltage pulse from dropping to 0 and rising to Vdd. The braking time interval T during which the motor is dynamically braked is B1 2. Negative voltages cannot be measured by the measurement circuit 130, and therefore no negative voltage is visible during the negative polarity phase. It can be seen that signals 210 and 211 reflect the application of a DC voltage by PWM, as voltage signal 211 is pulsed in nature and current signal 210 exhibits ripples associated with the application of voltage pulses. Current signal 210 exhibits a braking time interval T B1 It can be seen that the reactivating phase drops significantly during the braking period T B1 followed by a braking time interval T during which energy is provided to the system until the current signal 210 returns to an approximately sinusoidal curve. B1 The longer the braking time interval T, the stronger the deviation of the current signal 210 from a sinusoidal curve. B1 It decreases after the start of The value of the physical parameter indicating the current flowing through the motor, i.e. the measured voltage in the example described here, is determined over the braking time interval T B1 Measurement part T M1 The measurement may be performed during the measurement part T M1 is the braking time interval T B1 The measured voltage is B1 The measured voltage may be determined at any given moment within the measurement portion T M1 The voltage value may be determined by averaging or otherwise combining several voltage values sampled during the period.

[0047] 5 shows the current signal 220 and the voltage signal 221 for a time interval having a length of approximately three periods, similar to FIG. 4. Each positive half period has a length TB2 The braking time interval T B2 is the braking time interval T B1 . Due to the shorter braking time interval, the distortion of the current signal 211 is less pronounced than the distortion of the current signal 201 shown in FIG. 4 . A shorter braking time interval results in more energy-efficient motor control, since less energy is dissipated during the braking time interval. Furthermore, the intended current shape is less distorted. A sinusoidal current can lead to a quieter overall motor design, since harmonics are less pronounced. The greater the distortion, the stronger the harmonics and the greater the motor noise. Overall, shortening the braking time interval is advisable, but this must be balanced against the accuracy of the measurement of the physical parameter indicative of the current flowing through the motor. The length of the braking time interval shown in FIG. 5 is approximately 5% of the length of the drive period. Generally, the length of the braking time interval should not be limited, but the braking time interval may have a length of approximately 20% or less of the period, preferably approximately 15% or less, more preferably approximately 10% or less, and even more preferably approximately 5% or less, for example, less than 3%. It is not excluded that the length of the braking time interval can be varied from period to period. The braking time interval may also be used to support other aspects of motor control. For example, the motor control unit may be configured to increase the braking time interval when a signal indicates a high load on the motor above a threshold. Such a high load would ultimately result in a large increase in the on-time of the PWM voltage pulse to apply more energy to the motor to overcome the high load and maintain a constant amplitude. However, a sudden release of the load may then lead to a sudden over-energization of the unloaded motor, which may cause problems for the motor. Therefore, the motor control unit may be configured to increase the braking time interval at higher loads above a threshold load in order to actually brake the motor. Another solution may be to not provide a power level above a certain power level threshold.

[0048] FIGS. 6A-6C show zooms of simulated voltage signals within and around the braking time interval for three different back EMF values. The simulations were performed without the RC filter shown in FIG. 1. Without the RC filter, the voltage signal drops immediately at the beginning of the braking time interval. FIG. 6A shows a voltage curve 231 for a given back EMF value, e.g., 1.5 volts. FIG. 6B shows a voltage curve 241 for a higher back EMF value, e.g., 2.5 volts. FIG. 6C shows a voltage curve 251 for a lower back EMF value, e.g., 0.5 volts. In all three FIGS. 6A-6C, three measurement points 232, 233, 234 and 232, 243, 244 and 252, 253, 254 are shown, which in each case occur at three identical instants t0, t1, and t2 after the start of the braking time interval. It is assumed that the measurement time point is constant, i.e., if t0 is selected as the measurement time point, the voltage signal is always measured at t0. Again, assume that FIG. 6A shows the voltage curve when the target amplitude is achieved. Then, FIG. 6B shows a lower load condition, and FIG. 6C shows a higher load condition. FIGS. 6B and 6C show the respective voltage values of the target voltage curve shown in FIG. 6A. FIGS. 6A-6C show that it is practically irrelevant where in the braking time interval the voltage is measured, since the voltage values at each fixed time point behave similarly but with different offsets.

[0049] FIG. 7A depicts a target line 401 of the measured voltage signal S depending on the currently delivered power level P. The target line 401 represents a line for a given back-EMF value or motor amplitude depending on the PWM, i.e., a line for different load conditions because increasing the load requires increasing the set power level to maintain the amplitude. For example, dot D01 represents the voltage signal S01 measured in a situation where there is no external load and a power level P01 is supplied to the motor to achieve the motor amplitude. If the load increases, a higher power level must be provided to the motor to still achieve the same motor amplitude. For example, in a first load situation associated with a first non-zero external load, a power level P02 is supplied to the motor and signal S02 is measured, while in a second, increased load level, a power level P03 must be supplied to achieve the intended motor amplitude and signal S03 is measured. As explained, the target curves can be assumed to be linear, and each curve can be made available simply by storing the absolute value and slope.

[0050] As previously explained with reference to FIG. 3, the measured voltage signal changes when the load changes. If the current load is zero, a load change can only mean an increase in load. Since the PWMW remains the same, the measured voltage signal drops as the increased load reduces the amplitude and therefore the BEMF. The PWMW then needs to be increased to compensate for the amplitude drop so that the amplitude is maintained and the measured voltage value is again on the target line. During operation, as a load is applied, the applied load may decrease or increase, and therefore the measured voltage signal may also increase or decrease. As a result, if the load decreases, the PWMW needs to be decreased, or if the load increases, the PWMW needs to be increased. The motor control unit may be configured to set a new power level that fully compensates for the changed load, so that the next measurement ideally lies on the target curve.

[0051] Instead of directly adjusting the set power level so that the intended motor amplitude is theoretically achieved in the next period (which may not always be achievable due to the inertia of a practical motor system), an incremental approach may be chosen as already mentioned, which is explained with reference to FIG. 7B. In FIG. 7B, the currently applied power level P is again plotted on the horizontal axis, and the measured voltage signal S is plotted on the vertical axis. Target curve 411 represents a target line for the intended motor amplitude. Dots Dn1 schematically represent the known P value (currently set power level Pn1) and the determined S value (measured voltage signal Sn1), respectively. As explained, the currently applied power level Pn1 may be related to a normalized voltage pulse duty cycle weighting factor of 69 (this, of course, is merely an example used for illustrative purposes only). Any dots above the target line 411 are known to be associated with too high a motor amplitude, i.e., the currently provided power level Pn1 is too high. Instead of significantly reducing the currently applied power level to hit the target line 411 in the next period, the new set power level is increased incrementally; for example, the new set power level may thus be associated with a weighting factor of 68 in the subsequent period, which in turn results in a dot Dn2 associated with power level Pn2 and signal Sn2 in the next period. Of course, the load conditions may change from period to period, but are described here as being constant. As expected, dot Dn2 is closer to the target curve, but indicates that the new set power level Pn2 is still too high. In the next period, a subsequent power level Pn3 associated with a weighting factor of 67 is applied in this way, resulting in a dot Dn3 associated with power level Pn3 and signal Sn3, which is still too high, so the new set power level is again lowered to Pn4 associated with a weighting factor of 66, which in turn results in dot D04. The final incremental adjustment in this example then results in dot D05 being very close to target line 411. In FIG. 7B, two tolerance or hysteresis lines 412 and 413 are shown as dashed lines. To avoid adjustment jitter, the adjustment procedure may stop applying any further adjustments when the P and S value dots are within the bands indicated by these tolerance lines 412 and 413. In this example, the complete adjustment procedure from Dn1 to Dn5 required four adjustment steps. Of course, this is just a schematic example, but four adjustment steps at a drive frequency of 150 Hz require less than 3 ms.

[0052] As mentioned above, the user can change the load situation quite quickly, and as a result of adjustments made after determining dot Dn1, dot Dn6 can be determined instead of the expected Dn2. However, for the adjustment procedure, this is not a problem. Dot Dn6 clearly relates to the set power level Pn2 (weighting factor 68) that, under the given load situation, leads to a motor amplitude that is too low, as implied by signal value Sn6. That is, the currently achieved motor amplitude is below the intended motor amplitude. The adjustment procedure then increases the applied power level (again to weighting factor 69), and dot Dn7 (as determined by signal Sn7) may be determined in the next period, where dot Dn7 is thus, in the shown example, within the band defined by tolerance or threshold or hysteresis lines 412 and 413, and no further adjustments are made until the next determination of the S value (at the current power level Pn1 associated with weighting factor 69) is again outside the tolerance band given by lines 412 and 413.

[0053] The objective of the system and method proposed herein is to make it possible to maintain, or at least stay close to, the intended motor amplitude based on measuring the value of a physical variable indicative of the current flowing through the motor. As explained above, the signal may be sampled at the very beginning of the braking time interval or within the measurement portion of the braking time interval. It has been found that whenever sampling is done in the braking time interval, it contains the relevant motor amplitude information, i.e., the relevant back-electromagnetic force information that needs to be extracted.

[0054] It should be noted here that the individual duty cycle values for the intended current shape (i.e., sinusoidal shape) may be stored in the memory unit of the motor control unit, and therefore, an increase or decrease as described will affect all duty cycle values in the same way by adjusting the weighting factor PWMW. This means that an absolute increase / decrease of 0.5% in a 50% duty cycle will translate into a relative increase / decrease of 1% in all tabulated duty cycle values. In other words, the tabulated duty cycle values are scaled according to the currently defined increase or decrease value.

[0055] 8 is a diagram of an exemplary motor 500 that may be used in a motor unit as proposed in the present application. The motor 500 includes a stator 510 having a coil 511 and an armature 520 with at least one permanent magnet 521; in the illustrated example, two permanent magnets 521 are used, with the coil 511 wound around an E-shaped stator core. A drive shaft 530 is fixed to the armature 520. The stator 510 is fixedly attached to a motor carrier 540, and the armature 520 is attached to the motor carrier 540 by an armature attachment spring 528.

[0056] During operation, an electronically rectified DC current is applied to the coil 511 such that current is driven through the coil 511. In this disclosure, the current through the coil 511 in the illustrated example is referred to as the motor current. The current flow generates an electromagnetic field that interacts with the permanent magnet 521. When current is driven through the coil 511 in a first direction, the electromagnetic force acting on the permanent magnet 521 deflects the armature 520 in one direction; when the direction of current flow is reversed, the armature 520 deflects in the opposite direction, as indicated by the double arrow M. When a DC voltage is applied to the coil 511 such that the polarity of the DC voltage changes periodically, the armature 520 is driven into a periodic linear reciprocating motion. Because the spring-loaded armatures 520, 528 form a spring-mass system, excitation of the spring-loaded armatures 520, 528 can be characterized by a resonant frequency that results in a maximum deflection amplitude. A motor 500 such as that shown in FIG. 7 may typically be driven at a drive frequency of a periodic DC voltage at or near the resonant frequency.

[0057] In the illustrated embodiment, a so-called dynamic vibration absorber 550 is attached to the motor carrier 540 to compensate for vibrations of the motor carrier 540 caused by the cyclic driving of the armature 520. According to Newton's third law (action equals reaction or conservation of impulse in a closed system), the impulse of the driven armature 520 must be compensated by a counter impulse of the motor carrier / stator unit. The counter impulse of the motor carrier / stator unit will result in vibrations of the handle of the device in which the motor is located, unless the motor is completely mechanically decoupled from the handle. Handle vibrations are detrimental to a positive user experience during device operation, and therefore measures, which may take the form of a dynamic vibration absorber, are taken to at least reduce such vibrations. Dynamic vibration absorber 550 includes a mass 551 and a mounting spring 558. To optimally compensate for vibrations of motor carrier 540, the resonant frequency of the dynamic vibration absorber is tuned to the drive frequency (or the drive frequency is set as close as possible to the resonant frequency of the dynamic vibration absorber). It is understood that the dynamic vibration absorber 550 is an optional feature.

[0058] It should be noted that the resonant vibration motor 500 shown is just one example of a motor that may be used in the motor units described herein, and that any brushless DC motor, for example, may be used as well.

[0059] As mentioned above, in the motor 500 described with reference to FIG. 8 , the drive frequency is dominated by the resonant frequency of the dynamic vibration absorber, not the resonant frequency of the spring-loaded armatures 520, 528. The resonant frequencies of the spring-loaded armatures 520, 528 can mean that there is a varying spread of drive and resonant frequencies for the spring-loaded armatures in a series of motors 500. Such differences are known to affect the phase difference between the motion of the armature, and therefore the motion of the back electromagnetic force, and the motion of the excitation force, i.e., the drive function. It has been found that under some conditions, the linear calibration of the target curve between the currently applied power level and the determined signal described herein may no longer adequately describe the system. In such cases, a nonlinear target curve (e.g., a quadratic target curve) may be applied, and / or the position of the damping time interval within each half-cycle may be adjusted to approach the maximum value of the back electromagnetic force. This position can be determined by system simulation or experimentation. Similarly, it has been found that if a motor is used to drive different interchangeable head portions of a personal care device and the inertia of the drivable head portions varies, the calibration of the target curve may be valid for all such interchangeable head portions, and individual target curves may need to be used for the different drivable head portions.

[0060] FIG. 9 is a diagram of an exemplary personal care device 600, realized here as an electric toothbrush. The personal care device 600 includes a handpiece 610 and a head 620 for applying a care procedure, such as brushing teeth. A motor unit housed within the handpiece 610 is configured to drive and move a movable head 621. During operation, the movable head 621 is pressed against a body part, thus applying a load to the motor of the motor unit. As previously explained, such a load can lead to a decrease in the deflection amplitude of the movable head 621; as explained above, the motor unit can measure the applied load and apply an adapted amount of energy to the motor to essentially maintain a constant deflection amplitude. It is not excluded that a complete head 620 can also realize the movable head 621.

[0061] FIG. 10 is a diagram of a method of controlling a motor unit, such as a motor unit of a personal care device, having several steps.

[0062] In step 700, a motor is provided having a stator and an armature configured for relative driving motion with respect to the stator. In step 701, a set power level is supplied to the motor to drive the armature into motion. Pulse width modulation may be used to provide a supply voltage to the motor. In step 702, the motor is dynamically braked during a braking time interval. In step 703, a value of a physical variable indicative of current flowing through the motor is measured within the braking time interval. In step 704, the value of the physical variable is compared to a target value, which depends on the set power level and the intended amplitude of armature motion. In step 705, a new set power level is determined in response to the comparison. Then, in step 706, the new set power level is supplied to the motor after the dynamic braking period has ended. The method then repeats, beginning with step 702, until the loop is interrupted, e.g., the device in which the method is used is turned off.

[0063] The method may include the step of calibrating the target value by assuming a linear relationship between a set power level at a constant amplitude of the armature and a measurement of the physical variable.

[0064] Dimensions and values disclosed herein should not be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise indicated, 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 motor unit, a motor having a stator and an armature, the armature configured for driving movement relative to the stator; A motor control unit, a supply circuit for providing a supply voltage to the motor so as to provide the motor with a set power level for driving the armature into motion; a measurement circuit for measuring a value of a physical variable indicative of a current flowing through said motor; a motor control unit having the motor control unit is configured to interrupt the provision of the supply voltage by the supply circuit, dynamically brake the motor during a braking time interval, and measure a value of the physical variable during the braking time interval; 1. The motor unit of claim 1, wherein the motor control unit is further configured to compare the measured value of the physical variable with a target value, the target value depending on the supplied power level and an intended movement amplitude of the armature, determine a new set power level depending on a result of the comparison, and thereafter provide the new set power level to the motor.

2. 2. The motor unit of claim 1, wherein the motor control unit is configured to set the new set power level higher than the set power level if the comparison result indicates that the load on the motor has increased, and to set the new set power level lower than the set power level if the comparison result indicates that the load on the motor has decreased.

3. 3. A motor unit according to claim 1 or 2, wherein the motor control unit is arranged to increase or decrease the new set power level relative to the set power level by an incremental value, preferably a fixed incremental value.

4. 4. The motor unit of claim 1, wherein the motor control unit is configured to set the new set power level to the set power level when the comparison result indicates that the change in the load on the motor is less than a threshold value.

5. 5. A motor unit according to any one of claims 1 to 4, wherein the motor control unit is configured to measure values of the physical variables at fixed time instances within the braking time interval.

6. 6. A motor unit as claimed in any one of claims 1 to 5, wherein the motor control unit is configured to repeatedly measure the value of the physical variable and determine a new set power level, the previous new set power level becoming the set power level.

7. 7. A motor unit as claimed in any one of claims 1 to 6, wherein the motor control unit is configured to set the new set power level such that changes in load applied to the motor are at least partially compensated for and such that the amplitude of the armature resulting from the new set power level is closer to the intended amplitude of the armature than the set power level.

8. 8. A motor unit according to any one of claims 1 to 7, wherein the motor control unit is configured to periodically control the supply of the supply voltage, preferably the supply voltage being supplied with a periodically changing polarity.

9. 9. A motor unit according to any one of claims 1 to 8, wherein the measurement circuit comprises at least one pull-up or pull-down resistor.

10. 10. A motor unit according to any one of the preceding claims, wherein the motor control unit is configured to modify the target value in dependence on the value of the supply voltage.

11. 11. A motor unit according to any one of claims 1 to 10, wherein the motor control unit is configured to control the measurement circuit to sample the motor current or voltage signal at multiple times within the braking time interval and to determine the value of the physical variable by averaging or otherwise combining the multiple sampled signals.

12. 12. A motor unit as claimed in any one of claims 1 to 11, wherein the motor control circuit is configured to control the motor supply circuit such that the supply voltage is provided to the motor by means of a pulse width modulated signal, preferably the frequency of the pulse width modulated signal being higher than the frequency of the periodic provision of the supply voltage.

13. 13. A personal care device comprising a motor unit according to any one of claims 1 to 12, preferably said personal care device comprising a personal care head configured for driving movement.

14. 1. A method for controlling a motor unit, preferably a motor unit of a personal care device, comprising: providing a motor having a stator and an armature configured for driving movement relative to the stator; supplying a set power level to said motor to drive said armature into movement, preferably by providing a pulse width modulated supply voltage to said motor; dynamically braking the motor during a braking time interval; measuring a value of a physical variable indicative of current flowing through the motor within the braking time interval; comparing the value of the physical variable to a target value, the target value being dependent on the set power level and the intended amplitude of the movement of the armature; determining a new set power level in response to the comparison; thereafter providing the new set power level to the motor.

15. The method of claim 14 further comprising the step of calibrating the target value by assuming a linear relationship between the set power level at a constant amplitude of the armature and the measured value of the physical variable.

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