Control of a personal care device

Abstract

A controller for driving the oscillating motor of a personal care device, e.g. an electric toothbrush or shaver, receives an input signal indicative of a detected electrical characteristic of the moto

Description

B ACKGROUND Powered personal care devices are known to have a resonant motor. The resonant motor is used to cause oscillations of an applicator for delivering vibrations to a user. One example of such a device is a powered toothbrush, which has a motor for oscillating a brush head to clean a user’s teeth. SUMMARY In personal care devices that have a resonant motor for delivering vibrations to a user, it is desirable to drive the motor in a resonant state. By achieving a resonant state, the vibration amplitude is maximised, which can improve the effectiveness of the device. For example, in the case of a motorised toothbrush, an increase in amplitude corresponds to increased movement of the brush head against the user’s teeth or gums, thereby improving the effectiveness of cleaning. However, a challenge arises in that, by bringing the device into contact with a user (e.g. bringing the brush head into contact with the teeth or gums), the user necessarily increases a load against the motor. Additionally, this load can change over time, as the user moves the device across different surfaces (e.g. teeth or gums) and / or with varying levels of pressure. These variations in load cause the resonant frequency of the system to shift, e.g. with an increase in load resulting in an increase in resonant frequency. Conventional personal care devices typically operate at a fixed frequency and voltage and cannot account for the shifts in resonant frequency that occur during usage. Conventional personal care devices therefore cannot maintain a resonant (or near-resonant) state during use. As a result, they may operate at a suboptimal amplitude, i.e. having weak vibrations. Additionally, another consequence of operating outside resonant conditions is that a phase shift occurs between a back electromagnetic force (emf) of the motor and a current through the motor, which results in reduced power efficiency. One option for improving this is to utilise a sensor that directly measures an angular position of the motor over time and can therefore identify any reductions in amplitude that occur due to changes in load. However, given that the motor of a personal care device typically oscillates at a very high frequency (e.g. 100 to 300 Hz) and / or through a small angle (e.g. 1 to 15 degrees), this would require a sensor with a very high sampling rate and / or resolution, which may be expensive and / or bulky. Alternatively, whilst a simple sensor using a hall sensor and magnet may be made small enough for a personal care device, it could only be used to determine relative amplitude rather than absolute amplitude and would therefore require calibration. A custom-made encoder could help, but would be expensive to make, and would affect the overall resonance of the system. The inventors have identified these challenges and have developed improvements. A first aspect of the invention provides a controller for a personal care device, the personal care device having a driver circuit connected to a motor for causing the motor to oscillate, the controller being configured to: receive an input signal indicative of a detected electrical characteristic of the motor while the driver circuit is driving the motor with a first drive signal; determine (calculate, estimate) an operational characteristic of the motor based on the input signal, wherein the operational characteristic of the motor is indicative of a motor oscillation amplitude and / or a power efficiency of the motor; determine a second drive signal to modify the operational characteristic toward a target operational characteristic; and output the second drive signal to the driver circuit for driving the motor according to the second drive signal. This allows the controller to adjust a drive signal to improve the motor’s oscillation amplitude and / or power efficiency when variations in load occur. For example, the controller can adjust the drive signal to modify the motor oscillation amplitude towards a target motor oscillation amplitude, and / or modify the power efficiency towards a target power efficiency. By performing this adjustment based on an input signal that is indicative of a detected electrical characteristic of the motor (e.g. current or voltage), the adjusted (second) drive signal can be determined in a relatively simple manner, e.g. without requiring a dedicated sensor to track the motor’s position. The controller can therefore improve motor performance without in turn requiring bulky or expensive sensors to be added to the personal care device. The controller may be configured to repeatedly perform the steps of receiving an input signal, determining the operational characteristic of the motor, and determining and outputting an adjusted drive signal, in a feedback loop for continual adjustment of the drive signal. This may help account for variations in load that occur over time, during usage of the personal care device. As used herein, the phrase “personal care device ” may refer to any device that is configured to deliver vibrations to a user. Examples of suitable personal care devices include electric toothbrushes, shavers, massagers, and flossers. The device may include its own power source (e.g. a battery) or may be connectable to a separate power source (e.g. mains power). As used herein, the phrase “drive signal” refers to a signal having a drive frequency and drive voltage for causing the motor to oscillate. The drive signal may be a pulse width modulated signal. The first drive signal (which may also be referred to as an initial drive signal) may be referred to as having a first / initial drive voltage and a first / initial drive frequency. The second drive signal (which may also be referred to as an adjusted drive signal) may be referred to as having a second / adjusted drive voltage (which may be different from the first drive voltage), and a second / adjusted drive frequency (which may be different from the first drive frequency). As used herein, the term “motor” may also be referred to as a “resonant motor". The term “resonant motor” does not imply that the motor is necessarily driven in a resonant or near-resonant state. Rather, a resonant motor could be driven out of resonance. The (resonant) motor may be a piezoelectric motor, e.g. an ultrasonic motor. As used herein, the term “operational characteristic” may refer to a characteristic of the motor which would be expected to change as the motor frequency changes (e.g. as the motor is brought into and out of a resonant condition). The operational characteristic is indicative of (e.g. related to or comprises) an oscillation amplitude and / or power efficiency of the motor. For example, the operational characteristic may include a motor oscillation amplitude, a phase shift between a back emf of the motor and a current through the motor, or a combination (e.g. ratio) of these characteristics. The term “target operational characteristic" may refer to a target value for said operational characteristic. The target operational characteristic may comprise (e.g. consist of) a target value for the oscillation amplitude and / or power efficiency. For example, a target power efficiency may comprise (e.g. consist of) a target value for the phase shift between back emf of the motor and a current through the motor, which may be a phase shift of 0. Alternatively, the target power efficiency may comprise (e.g. consist of) a target ratio of oscillation amplitude to said phase shift. In some examples, the target operational characteristic may have a predetermined value. In other examples, the target operational characteristic may be calculated (e.g. by the controller) in a calibration step. For example, the target operational characteristic may be calculated while the motor is being driven at the first drive signal and is not under a load from the user (referred to herein as an “initial unloaded condition”). The target operational characteristic may be representative of a high oscillation amplitude (e.g. an amplitude at the initial unloaded condition) or a high power efficiency (e.g. minimum power at the initial unloaded condition, or a maximum value of a ratio representing amplitude per unit power supplied at the initial unloaded condition). The target operational characteristic may be stored in a memory of the controller or may be stored in another device and transmitted to the controller. The input signal may be indicative of the detected electrical characteristic (e.g. voltage, current) over a first time period. The first time period may comprise a period T of motor oscillations at the first drive frequency, where the period T is the inverse of the first drive frequency. Alternatively, the first time period may consist of a shorter period that comprises a peak in the motor oscillations (for determining motor oscillation amplitude), or may consist of any other time period sufficient to measure a phase shift between a back emf of the motor and a current through the motor (for determining an operational characteristic indicative of power efficiency). The detected electrical characteristic may comprise a current through the motor. Current is simple to detect, with current sensors already being commonplace in personal care devices. Accordingly, this helps the device to maintain a small form factor, without requiring substantial component modifications. Optionally, the operational characteristic of the motor comprises (e.g. consists of) the motor oscillation amplitude. This can help to improve (increase) the oscillation amplitude of the motor, thereby improving device effectiveness. As used herein, the term "motor oscillation amplitude" may also be referred to more simply as a “motor amplitude", "oscillation amplitude”, or “amplitude”. The controller may be configured to determine the motor oscillation amplitude based (at least in part) on a detected electrical characteristic that is indicative of a current through the motor. For example, the motor amplitude may be determined using the equation 0 = — f V — iR — L y- dt, where 6 is the motor’s angular position, Ke is a back emf constant, V Kg dt is the motor voltage, R is the motor resistance, L is the motor inductance, and i is the current through the motor (which may vary over time t). The motor oscillation amplitude corresponds to a peak angular position 0 of the motor over one oscillation period T. The oscillation period T is known since it is the inverse of the drive frequency f of the drive signal, i.e. T = i The above equation can therefore be used to determine the motor oscillation amplitude in dependence of measurable or known variables, as will be discussed in further detail herein. In particular, the motor resistance R, and motor inductance L, and back emf constant Ke can easily be known (e.g. at manufacture) and may be pre-stored in a memory. The motor voltage V may vary over time and can also be deduced based on the drive voltage of the drive signal. For example, the motor voltage V may match the drive voltage, or may be offset therefrom (e.g. by a DC offset). The current i may vary over time t and may be estimated or measured substantially in real-time. For example, the current i may be measured using a current sensor. The measurement of voltage V and current i may involve some error. Any error that results in a DC offset could affect the value of angular position 0 computed using the equation above. Therefore, optionally, the controller is configured to determine the motor oscillation amplitude based on the input signal using a DC error correction technique. For example, the DC error correction may reduce (e.g. remove) a DC error in a detected current. Alternatively or additionally, the DC error correction may account for (e.g. reduce, remove) a DC offset from the known value of the drive voltage (supplied to the motor) to estimate / deduce a value of the motor voltage V. The DC error correction can help improve the accuracy of the determined operational characteristic (e.g. motor oscillation amplitude). Examples of suitable DC error correction techniques include a high pass filter and / or a DC rejection algorithm. Optionally, the target operational characteristic comprises (e.g. consists of) a target motor oscillation amplitude. The target motor oscillation amplitude (which may also be referred to simply as a “target oscillation amplitude” or “target amplitude”) may be preconfigured e.g. as a factory-set value. Alternatively, the target motor oscillation amplitude may be calculated at during usage of the device, e.g. during an initial calibration step when the user is not exerting a load on the motor, as discussed above. By providing a target operational characteristic that comprises (e.g. consists of) a target motor oscillation amplitude, the controller can prioritise improvements to oscillation amplitude, compared to e.g. improving power efficiency. Optionally, the operational characteristic of the motor comprises (e.g. consists of) a phase shift between a back electromotive force of the motor and a current through the motor. This phase shift is affected by the drive frequency of the drive signal. For example, in an ideal resonating motor, a phase shift of zero indicates that the motor is operating at resonant frequency and thus maximum efficiency. Outside of resonant conditions, a nonzero phase shift will occur, indicating that the motor is operating at suboptimal efficiency. The controller may be configured to determine (calculate, estimate) the back emf of the di motor, e, according to the equation e = V — iR — L —. Again, the values for voltage V, current i, resistance R, and inductance L may be known or measured in the manner discussed above. By monitoring the current and back emf over a period of time (e.g. over the oscillation period T), the phase shift between their waveforms can be determined. Optionally, the target operational characteristic comprises (e.g. consists of) a target phase shift of approximately zero. This helps to optimise motor efficiency since, as discussed above, a phase shift of zero indicates that the motor is at resonant frequency. Based on the comparison of the measured phase shift with the target phase shift of approximately zero, the controller may determine a second drive signal with an adjusted frequency compared to the first drive signal. The second drive signal may have the same or different voltage from the first drive signal. It should be noted that, in normal operating conditions, a perfect resonant motor is not possible. As a result, the drive frequency that gives rise to a maximum amplitude may differ from the drive frequency that gives rise to a phase shift of zero between the back emf of the motor and the current through the motor. In some examples, the controller may be configured to at least partially account for this, e.g. by optimising a balance between a nearmaximum amplitude and a near-minimum phase shift. Accordingly, optionally, the operational characteristic of the motor comprises a combination of both: the motor oscillation amplitude; and the phase shift between a back electromotive force of the motor and a current through the motor. The target operational characteristic may then account for a balance between these two factors. For example, optionally, the target operational characteristic comprises a target ratio between the motor oscillation amplitude and the phase shift. The value of the ratio may be preconfigured to provide an acceptable balance between device effectiveness and power efficiency (which may depend on device / user requirements). For example, the target ratio may be predetermined based on a comparison, at a fixed frequency, of motor oscillation amplitude and phase shift (e.g. plotting motor oscillation amplitude on a y-axis, and phase shift on an x-axis), and selecting the target ratio as the value at which the ratio between these is a maximum, since this corresponds to a maximum amount of motor oscillation amplitude per unit power provided. In variant examples, the balance between amplitude and phase shift may be struck in other ways. For example, the amplitude and phase shift can each be independently controlled so that they each do not fall outside predetermined thresholds. As discussed above, the first drive signal may have a first drive voltage and a first drive frequency, and the second drive signal may have a second drive voltage and a second drive frequency. Optionally, the controller is configured to determine the second drive signal based on the second drive frequency being equal to the first drive frequency. In other words, the controller may be configured to determine the second drive signal with a same frequency as the first drive signal, such that the second drive signal may differ (e.g. only differ) from the first drive signal by having a (second) drive voltage that is non-equal to the (first) drive voltage. This may also be referred to as the controller performing “voltage control”, since the drive voltage can be adjusted while the drive frequency remains unchanged (fixed, constant). In general, at a fixed frequency, as voltage increases, amplitude increases. Conversely, as voltage decreases, amplitude decreases. A voltage control method therefore allows the amplitude to be adjusted (thereby adjusting the effectiveness of the personal care device) without affecting power efficiency / phase shift (which is affected by frequency). Thus, voltage control methods may be particularly suited to examples where the determined operational characteristic comprises a motor oscillation amplitude. Alternatively, optionally, the controller is configured to determine the second drive signal based on the second drive voltage being equal to the first drive voltage. In other words, the controller may be configured to determine the second drive signal with a same voltage as the first drive signal, such that the second drive signal may differ (e.g. only differ) from the first drive signal by having a (second) drive frequency that is non-equal to the (first) drive frequency. This may also be referred to as the controller performing “frequency control” or “resonance tracking”, since the drive frequency may be adjusted while the drive voltage remains unchanged (fixed, constant). Frequency control methods help improve power efficiency of the motor, but may not always achieve maximum amplitude, which might occur at a different frequency. Thus, frequency control methods may be particularly suited for examples in which the determined operational characteristic is indicative of a power efficiency of the motor, e.g. where the operational characteristic comprises a phase shift between a back electromotive force of the motor and a current through the motor. The frequency control technique and voltage control techniques discussed above each help to provide finer control over the motor adjustments, by independently adjusting voltage or frequency. Otherwise, for example, simultaneous adjustments of both voltage and frequency could both impact amplitude in different manners. Independently controlling these electrical characteristics can therefore help provide more predictable control over the motor’s performance. Optionally, the controller is configured to operate in multiple modes. For example, optionally, the controller is configured to operate in two or more of: (i) a voltage control mode in which the controller is configured to determine the second drive signal based on the second drive frequency being equal to the first drive frequency; (ii) a frequency control mode in which the controller is configured to determine the second drive signal based on the second drive voltage being equal to the first drive voltage; and (iii) a mixed mode in which the controller is configured to determine the second drive signal based on the second drive frequency being different from the first drive frequency, and the second drive voltage being different from the first drive voltage. In other words, in the voltage control mode, the controller may adjust a drive voltage of a drive signal without adjusting its drive frequency. In the frequency control mode, the controller may adjust a drive frequency of a drive signal without adjusting its drive voltage. In the mixed mode, the controller may adjust both the drive voltage and the drive frequency of the drive signal. Optionally, each mode may be performed on the basis of a different input signal, as part of a feedback loop to continually adjust the drive signal over time. The modes may be user-selectable (e.g. via a user interface) based on user preferences. Alternatively, the controller may be pre-programmed to operate in multiple modes, e.g. successively (by cycling through multiple modes) or in parallel (at the same time). For example, the controller may be configured to operate in the frequency control to determine an adjusted drive frequency (for improving oscillation amplitude and / or power efficiency), and may (subsequently or in parallel) operate in the voltage control mode to determine an adjusted drive voltage(for improving oscillation amplitude, without affecting power efficiency). However, since the drive frequency that gives rise to a maximum amplitude may differ from the drive frequency that gives rise to maximum efficiency, in examples that adjust the drive frequency and drive voltage in parallel, the resulting adjustments may risk competing with each other. For example, an adjustment to drive frequency can affect oscillation amplitude in a manner which may not be accounted for while concurrently adjusting drive voltage. Thus, it is possible to overshoot or undershoot a target voltage if adjusting voltage and frequency in parallel. Accordingly, in some examples, it may be beneficial to initially operate in the frequency control mode, and subsequently operate in the voltage control mode. This allows the effects on oscillation amplitude caused by the frequency adjustment to be taken into account during the subsequent voltage adjustment, to allow finer tuning of the oscillation amplitude. In other words, optionally, the controller is configured to (initially) operate in the frequency control mode to determine the second drive signal to modify the operational characteristic toward the target operational characteristic based on the second drive voltage being equal to the first drive voltage. The operational characteristic may be indicative of the motor oscillation amplitude and / or power efficiency. The controller may then output the second drive signal to the driver circuit for driving the motor according to the second drive signal. The controller may further be configured to subsequently operate in the voltage control mode to: receive a second input signal indicative of a detected electrical characteristic of the motor while the driver circuit is driving the motor with the second drive signal; determine a motor oscillation amplitude based on the second input signal; and determine a third drive signal to modify the motor oscillation amplitude towards a target operational characteristic (e.g. a target motor oscillation amplitude or a target ratio of amplitude to phase shift) based on the third drive signal having a third drive frequency that is equal to the second drive frequency. The controller may then output the third drive signal to the driver circuit for driving the motor according to the third drive signal. This allows the controller to effectively prioritise an adjustment to frequency (which affects both phase shift and amplitude), and then subsequently further improve the amplitude by adjusting voltage. The target operational characteristic used to determine the third drive signal may be the same as or different from the target operational characteristic used to determine the second drive signal. Of course, the load conditions on the motor may change over time as the user moves the device. However, during normal device usage, load conditions during a first time period (when the motor is driven according to the first drive signal) and a second time period (when the motor is driven according to the second drive signal) can be approximated as being similar, since significant variations in load are likely to occur over a longer time frame than the time period required to determine and output an adjusted drive signal. This is because, in personal care devices, the oscillation period is typically very short (e.g. with oscillations at 100 to 300 Hz), and so the method can be performed extremely rapidly, with the delay between the determination of the second and third drive signals being negligible. For example, the controller may be configured to perform a feedback loop (receiving an input signal, determining an operational characteristic, and determining / outputting an adjusted drive signal) multiple times per second, e.g. at least 5 times per second, e.g. at least 10 times per second, e.g. at least 15 times per second, e.g. at least 20 times per second. Another aspect of the invention provides a personal care device for delivering vibrations to a user, the personal care device comprising: the controller of any preceding claim; the motor; and the driver circuit. Optionally, the personal care device comprises a current sensor for detecting the current through the motor, which may thus be used as the detected electrical characteristic discussed above. The current sensor may be coupled to the controller for providing the input signal to the controller. Optionally, the motor is detachably connectable to an applicator for delivering vibrations to the user. For example, the personal care device may comprise a motorised toothbrush, and the applicator may comprise a detachable toothbrush head. Another aspect of the invention provides a kit having the personal care device and the applicator. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 and 2 show external perspective views of an example personal care device in the form of an electric toothbrush; Figure 3 shows an internal view of an example personal care device, which may be the device of Figures 1 and 2; Figure 4a shows a schematic diagram of the oscillating movement of a brush head from the device of Figure 3 when in use; Figure 4b shows a graphical representation of the movement of Figure 4a; Figure 5 shows experimental data demonstrating the relationship between frequency, oscillation amplitude, and power, useful for understanding examples of the invention; Figure 6 shows a flow diagram of a method performed by an example controller; Figure 7 shows a phase difference between back emf and current in a motor system operating outside a resonant state, useful for understanding examples of the invention; and Figure 8 shows a schematic view of an example controller connected to a driver circuit and motor. DETAILED DESCRIPTION Figures 1 and 2 illustrate external views of an example personal care device 10. In this example, the personal care device 10 is in the form of a handheld appliance, which is in the form of an electric toothbrush. The personal care device 10 comprises a handle 12 which is connectable to an applicator 14. The handle 12 comprises an external body 16 which may be formed from plastics material. The body 16 is generally cylindrical in shape. The handle 12 comprises a user interface. The user interface comprises a user operable button 18 which is located within an aperture formed in the body 16 so as to be depressible by the thumb of a hand which is gripping the body 16 of the handle 12. Optionally, the handle 12 may comprise a display which is positioned to be visible to a user during use of the appliance. The personal care device 10 may be connectable to a remote display, such as a display of a mobile telephone, to enable the user to select operating modes or parameters for the personal care device 10 using the button 18 and / or the remote display. The applicator 14 comprises a stem 20 and a head 22. The stem 20 is elongate in shape, which serves to space the head 22 from the handle 12 to facilitate user operability of the personal care device 10. In this example, the head 22 of the cleaning tool 14 comprises a brush unit 24, which comprises a bristle carrier 26 and a plurality of sets of bristles 28 mounted on the bristle carrier 26. In this example, the brush unit 24 is rigidly connected to the stem 20. Figure 3 illustrates an internal view of an example personal care device 10, which may be the electric toothbrush of Figures 1 and 2. The same reference numerals are used again where relevant. As shown in Figure 3, the personal care device 10 has a power source 30 (e.g. battery), circuitry 32, and a motor 34. The circuitry 32 includes a controller and a driver circuit, which will be discussed in further detail herein with reference to Figure 8. The circuitry 32 is connected to the motor 34 for causing the motor 34 to oscillate when the device is powered on. The motor 34 is connectable to the applicator 14 so that oscillations of the motor 34 cause corresponding oscillations in the applicator 14. Figure 4a shows a schematic diagram of the oscillating movement of the head 22 of the applicator 14 in use. The head 22 oscillates between a first peak position (shown on the left of Figure 4a) and a second peak position (shown on the right of Figure 4a). Figure 4b shows a graphical depiction of this movement, depicting the angular position 0 of the head 22 in dependence of time t, where each peak deviates from the average position of the head 22 by an amplitude value A. For example, the amplitude value A may be at least 1 degree, optionally at least 3 degrees, optionally at least 5 degrees. The amplitude value A may be up to 15 degrees, optionally up to 12 degrees, optionally up to 10 degrees. For example, optionally the amplitude may be 3°, meaning the head 22 will oscillate between radial positions of -3° and +3°. However, the amplitude may also vary over time, e.g., due to changes in load at the head 22 affecting the resonance conditions of the system. The brush head 22 can be expected to oscillate in phase with the motor 34 and with a proportionate (e.g., equal) oscillation amplitude to the motor 34, such that the information described in relation to the brush head 22 in Figures 4a-b equally applies in relation to the movement of the motor. The movement of the brush head 22, applicator 14, and motor 34 can therefore be discussed in interchangeable manners herein. Figure 5 shows a graph of experimental data useful for understanding the relationship between driving frequency, oscillation angle, and power efficiency in a real-world resonant motor system. The graph includes four curves which provide data of oscillation angle (e.g., motor oscillation angle) and power in both an unloaded condition and a loaded condition, across various frequencies. This data was acquired based on the unloaded condition corresponding to 0 grams being applied to the applicator, and the loaded condition corresponding to 200 grams being applied to the applicator. Of course, these values are purely exemplary. In typical use of a personal care device, the load applied can vary over time. In more detail, the curves are labelled as follows: • The dashed curve Po shows the average power consumption of the motor at various frequencies in the unloaded condition. • The solid curve Pl shows the average power consumption of the motor at various frequencies in the loaded condition. • The dashed curve 2Ao shows the peak-to-peak oscillation angle (equating to twice the motor oscillation amplitude) at various frequencies in the unloaded condition. • The solid curve 2Al shows the peak-to-peak oscillation angle (equating to twice the motor oscillation amplitude) at various frequencies in the loaded condition. The results from Figure 5 help illustrate the shifts in motor oscillation amplitude and power that occur in the loaded conditions (dashed curves) compared to the unloaded conditions (solid curves), as explained further below. Considering amplitude, in the unloaded condition, the solid curve 2Ao indicates that a maximum amplitude is achieved at a frequency f of approximately 240 Hz. At this frequency, the peak-to-peak angle 2Ao is approximately 23°, corresponding to an amplitude value Ao ~ 11.5°. However, when the 200g load is applied, and the frequency is maintained at 240 Hz, the amplitude is significantly reduced (with 2Al ~ 11°, i.e. Al ~ 5.5°). To maximise amplitude in the loaded condition, the frequency should be adjusted (increased), in this example to approximately 252 Hz, where the amplitude Al reaches approximately 8°. Now turning to consider efficiency, in the unloaded condition, the solid curve Po indicates that a minimum amount of power is consumed at a frequency / of approximately 230 Hz in this example. At this frequency, an average of 0.5 W of power is consumed. However, when the 200g load is applied, and the frequency is maintained at 230 Hz, the power consumption increases to approximately 1.2 W, which is not at a minimum of the curve Pl. To minimise power consumption in the loaded condition, the frequency should be adjusted (increased), in this example to approximately 249 Hz, to achieve a minimum power Pl in the loaded condition of approximately 0.8 W Thus, as can be seen from Figure 5, changes in load cause the ideal (resonant or near-resonant) frequency of the system to shift. Further, in real-world conditions, the drive frequency that gives rise to a maximum amplitude may differ from the drive frequency that gives rise to a minimum power consumption. For example, in Figure 5, in the unloaded condition, a frequency of approximately 240 Hz results in a maximum amplitude, whereas a frequency of approximately 230 Hz minimises power. The frequencies at which maximum amplitude and minimum power consumption are achieved may therefore be considered to define end points of a range of “near-resonant” frequencies. Any of the frequencies in this near-resonant range may be advantageous for improving amplitude and / or power efficiency. Figure 6 illustrates a flow diagram of a method 100 implemented by an example controller, as discussed further below. In a first step 102, the controller receives an input signal indicative of a detected electrical characteristic of a motor while the motor is being driven according to a first drive signal. For example, the detected electrical characteristic may comprise a current and / or voltage detected through the motor while the motor is being driven according to the first drive signal. In a second step 104, the controller determines an operational characteristic of the motor based on the received input signal. The operational characteristic may be indicative of an oscillation amplitude and / or power efficiency of the motor, as calculated using the detected electrical characteristic. This can be understood further in view of the following theory. In general, a motor’s voltage Vis related to its resistance R, inductance L, current i (which varies over time / ), and back emf e according to the equation V — iR + L^ + e. This equation can be rearranged as follows: di e = V — iR—L—~ (1) dt The back emf e is also related to angular speed co and a back emf constant Ke according to Equation 2: e m = — (2) ‘‘e While the motor is running, angular position 0 will vary over time. Angular speed co is related to angular position 0 according to Equation 3: 6 = i co dt (3) Substituting equations (1) and (2) into equation (3) gives rise to Equation 4: If . di 6 _ I y _ _ £ dt (4) Ke J dt v 7 The motor oscillation amplitude A corresponds to the maximum angular position B over one oscillation period T. Thus, in examples where the determined operational characteristic comprises a motor oscillation amplitude, the controller may be configured to determine the motor oscillation amplitude by evaluating equation (4) (e.g. over the oscillation period 7), to find the maximum angular position B. The oscillation period T can be easily deduced since it is equal to an inverse of the drive frequency f of the known drive signal that is being used to drive the motor. The remaining variables of Equation 4 are also measurable or known, as follows: • The motor resistance R, and motor inductance L, and back emf constant Ke can be easily determined (e.g. at manufacture) and may be pre-stored in a memory accessible by the controller. • The motor voltage V can be deduced based on the drive voltage of the (first) drive signal, which is known. For example, the motor voltage V may match the drive voltage, or may be offset therefrom by a DC offset, which may be accounted for using a DC error technique as discussed herein. • The current i may be measured or calculated based on the detected electrical characteristic in the received input signal of the first step 102. For example, the detected electrical characteristic may comprise a direct measurement of current, obtained using a current sensor. Alternatively or additionally, some examples may determine an operational characteristic that is indicative of a power efficiency of the motor in the second step 104. Such examples may be further understood in view of Figure 7, which shows a graph of motor back emf e and motor current i over time t, at a non-resonant frequency. In an ideal resonant motor operating at resonant frequency, the back emf e and motor current i would be in phase. However, at non-resonant frequencies, a non-zero phase shift A(p is observable between the current i and back emf e, as shown in Figure 7. Thus, in examples where the determined operational characteristic is indicative of a power efficiency of the motor, the second step 104 may comprise evaluating a phase shift between the motor current i and back emf e. Again, the motor current i may be provided via the received input signal in the first step 102. The back emf e may be determined using Equation 1 above, noting that the variables in this equation are measurable or known in the same manner as discussed above for Equation 4. Thus, by acquiring values for current i and back emf e over a period of time (which may be extremely short due to the high frequencies involved in personal care devices), the phase shift A(p can be determined. In some examples, the second step 104 may include a determination of both motor oscillation amplitude A and the phase shift A(p. Optionally, these may be combined into a single value, e.g. by taking a ratio of the motor oscillation amplitude and phase shift. Turning to a third step 106, the controller then determines an adjusted (second) drive signal based on a comparison of the determined operational characteristic from the second step 104 to a target operational characteristic. This step may be implemented, for example, in a PID controller. The purpose of this step is to determine an adjusted (second) drive signal that would modify the value of the operational characteristic toward a target value for said characteristic. The adjusted (second) drive signal may have a different drive frequency and / or drive voltage compared to the initial (first) drive signal. An adjustment to the drive frequency may be determined, for example, based on the relationship shown in Figure 7, to reduce the phase shift. Alternatively, an adjusted drive frequency may be determined based on the relationship shown in Figure 5. For example, data for phase shift and / or oscillation amplitude in an unloaded condition may be pre-stored in a memory accessible by the controller. Thus, when the controller determines a value for phase shift and / or oscillation amplitude that deviates from a target value at the first drive frequency, the controller can identify that a load has been applied, and can correspondingly shift the frequency to increase amplitude and / or reduce phase shift. Alternatively, an adjusted drive voltage may be determined, for example, using Equations 1 and / or 4 above, to identify a voltage value that would bring the measured operational characteristic closer to the target operational characteristic (e.g., target amplitude, phase shift, and / or ratio thereof). Turning to a fourth step 108 of Figure 6, the controller then outputs the adjusted drive signal to a driver circuit for driving the motor according to the second drive signal. Optionally, the controller may then restart the method at the first step 102, thereby providing a feedback loop for continual adjustment of the drive signal. The controller may determine different operational characteristics in each iteration of the method and / or may adjust different variables (voltage or frequency) of the drive signal in each iteration. For example, in a first iteration of method 100, the controller may adjust a power efficiency (e g. phase shift, or ratio of motor oscillation amplitude to phase shift) of the motor, and in a second iteration of method 100, the controller may adjust a motor oscillation amplitude. Figure 8 illustrates a schematic diagram of an example system. The system includes a resonant motor 34, a controller 36, and a driver circuit 38, which are each connected to each other to form a loop. The controller 36 is configured to output a drive signal to the driver circuit 38, which the driver circuit 38 then applies to the motor 34 to for causing the motor 34 to oscillate in accordance with the drive signal. A DC power source 30 is connected to the driver circuit 38 to provide power to the system. The controller 36 and driver circuit 38 may each be implemented, for example, in the circuitry 32 of the personal care device 10 shown in Figure 3. In this example, the driver circuit 38 includes switches 42a-d, which are each connected to a respective MOSFET 44a-d. The controller 36 has a MOSFET driver 46 which is configured to control the switches 42a-42d to provide the drive signal to the driver circuit 38. The driver circuit 38 supplies the drive signal to the motor 34. The drive signal is shown in Figure 8 as a graph of voltage V over time t. The drive signal has a pulse width modulated (PWM) voltage. The duty cycle of the PWM voltage depends on the drive frequency / which is the frequency of the sine-wave modulation signal shown on the graph of Figure 8. The motor 34 is a resonant motor having a resistor R, inductor L, and back emf e. The motor is connected to a load 40, which includes an applicator 14. For example, the applicator 14 may be the brush head 22 of Figure 3. In use, the load exerted onto the motor 34 will vary, e.g., with variations of load on the applicator 14. A sensor (e.g. current sensor) 48 is connected to the motor 34 to detect the electrical characteristic (e.g. current) through the motor 34 while the motor is being driven according to the drive signal. The sensor 48 is connected to the controller 36 and is configured to provide the detected electrical characteristic in an input signal to the controller 36. In this example, the controller 36 is configured to use data from the sensor 48 to determine both an oscillation amplitude of the motor, and a phase shift between back emf and current through the motor. Accordingly, the controller 36 has both an amplitude measurement unit 50 and a phase shift measurement unit 52, each of which are connected to the sensor 48. The amplitude measurement unit 50 is configured to determine the motor oscillation amplitude A using Equation 4 in the manner discussed above, and is configured to output the determined oscillation amplitude A to an amplitude error detection unit 54. The amplitude error detection unit 54 is configured to compare the determined oscillation amplitude A to a target operational characteristic which, in this example, is a target oscillation amplitude At. The error detection unit 54 then outputs an error measurement indicative of a difference between the detected and target oscillation amplitudes to a PID controller 56. The PID controller 56 is configured to adjust the drive voltage and / or drive frequency to reduce the offset between the determined oscillation amplitude and the target oscillation amplitude. Similarly, the phase shift measurement unit 52 is configured to determine the phase shift ^(p using Equation 1 as discussed above, and is configured to output the resulting value to a phase shift error detection unit 58. In this example, the phase shift error detection unit 58 is configured to compare the determined phase shift ^(p to a target phase shift A^t and outputs an error measurement to a PID controller 60. The PID controller 60 is configured to adjust the drive frequency to reduce the error between the determined phase shift and the target 5 phase shift. The PID controllers 56 and 60 can operate in parallel, e.g. with PID controller 56 adjusting a drive voltage V while PID controller 60 adjusts a drive voltage / Alternatively, the PID controllers can operate in succession, e.g., based on successive input signals from the sensor 10 48, to adjust one variable at a time. The PID controllers 56 and 60 are configured to output the adjusted drive signal to the MOSFET driver 46. The MOSFET driver 46 can then control the driver circuit 38 to drive the motor 34 in accordance with the adjusted drive signal, thereby improving the power 15 efficiency and / or oscillation amplitude of the motor system.

Claims

1. A controller for a personal care device, the personal care device having a driver circuit connected to a motor for causing the motor to oscillate, the controller being configured to:receive an input signal indicative of a detected electrical characteristic of the motor while the driver circuit is driving the motor with a first drive signal;determine an operational characteristic of the motor based on the input signal, wherein the operational characteristic of the motor is indicative of a motor oscillation amplitude and / or a power efficiency of the motor;determine a second drive signal to modify the operational characteristic toward a target operational characteristic; andoutput the second drive signal to the driver circuit for driving the motor according to the second drive signal.

2. The controller according to claim 1, wherein the operational characteristic of the motor comprises the motor oscillation amplitude.

3. The controller according to claim 2, wherein the controller is configured to determine the motor oscillation amplitude based on the input signal using a DC error correction technique.

4. The controller according to claim 2 or claim 3, wherein the target operational characteristic comprises a target motor oscillation amplitude.

5. The controller according to any preceding claim, wherein the operational characteristic of the motor comprises a phase shift between a back electromotive force of the motor and a current through the motor.

6. The controller according to claim 5, wherein the target operational characteristic comprises a target phase shift of approximately zero.

7. The controller according to any preceding claim, wherein the operational characteristic of the motor comprises a combination of both:motor oscillation amplitude; andphase shift between a back electromotive force of the motor and a current through the motor.

8. The controller according to claim 7, wherein the target operational characteristic comprises a target ratio between the motor oscillation amplitude and the phase shift.

9. The controller according to any preceding claim, wherein the first drive signal has a first drive voltage and a first drive frequency, wherein the second drive signal has a second drive voltage and a second drive frequency, and wherein the controller is configured to:determine the second drive signal based on the second drive frequency being equal to the first drive frequency; ordetermine the second drive signal based on the second drive voltage being equal to the first drive voltage.

10. The controller according to claim 9, wherein the controller is configured to operate in multiple modes including two or more of:(i) a voltage control mode in which the controller is configured to determine the second drive signal based on the second drive frequency being equal to the first drive frequency;(ii) a frequency control mode in which the controller is configured to determine the second drive signal based on the second drive voltage being equal to the first drive voltage; and(iii) a mixed mode in which the controller is configured to determine the second drive signal based on the second drive frequency being different from the first drive frequency, and the second drive voltage being different from the first drive voltage.

11. The controller according to claim 10, wherein the controller is configured to operate in the frequency control mode to:determine the second drive signal to modify the operational characteristic toward the target operational characteristic based on the second drive voltage being equal to the first drive voltage; andoutput the second drive signal to the driver circuit for driving the motor according to the second drive signal; andwherein the controller is configured to subsequently operate in the voltage control mode to:receive a second input signal indicative of a detected electrical characteristic of the motor while the driver circuit is driving the motor with the second drive signal;determine a motor oscillation amplitude based on the second input signal;determine a third drive signal to modify the motor oscillation amplitude of the motor towards a target operational characteristic based on the third drive signal having a third drive frequency that is equal to the second drive frequency; andoutput the third drive signal to the driver circuit for driving the motor according to the third drive signal.

12. A personal care device for delivering vibrations to a user, the personal care device comprising:the controller of any preceding claim;the motor; andthe driver circuit.

13. The personal care device according to claim 12, wherein the motor is detachably connectable to an applicator for delivering vibrations to the user.

14. A kit comprising:the personal care device according to claim 13; andthe applicator.

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