Device and method for controlling a vibration device

Abstract

Control system for use with a vibration device (105) comprising a response signal with a counter-electromotive force component, wherein the control system comprises: a control device (116, 816) designed to provide a control signal to the vibration device (105), wherein the control signal comprises an amplitude and a polarity; a sensor (115, 815) designed to detect an electrical parameter of the response signal; and a control device (120, 820) coupled to the sensor and the control device (116, 816), wherein the control device (120, 820) comprises a computing device (122, 822) designed to determine, based on the electrical parameter, a sign of the curvature of a curved section of the counter-electromotive force component of the response signal;wherein the control device (120, 820) is designed to provide a control signal to adjust the amplitude of the control signal such that the vibration device (105) is slowed down, wherein, upon reversal of the polarity of the control signal, the curved section gradually flattens before a change in the sign of its curvature, and wherein the control device (120, 820) is designed to withdraw the control signal upon identification of the change in the sign of the curvature of the curved section of the counter-electromotive force component.

Description

Technical FieldThe present disclosure relates to an apparatus and method for controlling a vibration device. More particularly, the present invention relates to an apparatus and method for stopping a linear resonance actuator.BackgroundVibration devices, which may include haptic actuators, speakers, and crystal oscillators, are used in a variety of products, haptic actuators are generally used with electronic devices to provide a tactile signal to the user, also referred to as haptic feedback. For example, mobile phones are often equipped with a haptic actuator that vibrates to provide a notification to a user upon the arrival of a text message or similar event. As far as various types of haptic actuators have been developed, among which there are the linear resonance actuator, LRA, and the piezoelectric actuators.LRA are based on an induction coil (electromagnet) coupled to a spring that holds a permanent magnet. In operation, the spring-and-mass system moves along a single axis. When a current flows in one direction through the coil, it generates a magnetic field that repels the magnet. When the current flows in the other direction, the magnetic field attracts the magnet. Therefore, the energy transfer between the spring and the coil generates vibrations. The system has a mechanical resonant frequency which is usually in the range of 50-300 Hz. At the resonant frequency, the push-pull drive voltage produces the maximum linear deflection of the sprung mass.Following Lenz's law, due to the vibration of the mass across the actuator, a back-facing electromotive force, BEMF, is generated which counteracts the voltage of the source it has generated. As the mass moves through the coil, it follows a simple harmonic motion which causes it to induce a sinusoidal voltage in the electromagnet. This BEMF is strongest at the resonant frequency of the system. The BEMF is proportional to a drive signal that drives the LRA in the first order. However, the BEMF amplitude depends on the weight and strength of the magnet and the number of turns of the coil of the electromagnet. This means that the BEMF will vary with the LRA and temperature and other electromechanical parameters.The mass of the LRA may be accelerated or decelerated by changing the parameters of a drive signal that drives the actuator, such as the amplitude and phase of the signal. In this way, a desired user feedback can be achieved. However, the LRA does not respond immediately to a change in the amplitude of the drive signal, but instead provides a low pass filter. The speed of the acceleration is proportional to the drive power. Therefore, it is possible to improve the response of the LRA by over-controlling it for a short period of time.Common practice employs override to accelerate and decelerate the oscillations of the LRA. The level to which a system can be overdriven will depend on the characteristics of the LRA, including its resonant impedance and response time. However, it is often difficult to know when to stop the override, although the override allows the LRA to reach a desired vibration amplitude more quickly. This is a particular issue when attempting to stop the oscillation of the LRA completely. If the level of overdrive is too high or applied too long, the LRA will slow beyond the stall level.US 2015 / 0137713 describes a control device for controlling the movement of a linear resonance actuator (LRA). The controller includes a monitor, a primary loop module, and an alternate cycle module. The monitor monitors a counter electromotive force (BEMF) signal from the LRA and generates an indication indicating whether or not movement of the LRA has occurred. The primary loop module controls acceleration and deceleration of the LRA based on the monitored BEMF signal when the indication from the monitor indicates that movement of the LRA has occurred. The alternate cycle module pushes the LRA at a predetermined frequency when the indication from the monitor indicates that no movement of the LRA has occurred.Current approaches require calibrating the system for a particular haptic actuator based on its BEMF response such that the response of the haptic actuator to a particular level of drive signal is known. This limits the use of the system to a particular actuator and requires a relatively complex system.SummaryIt is an object of the invention to address one or more of the above-mentioned limitations. According to a first aspect of the disclosure, there is provided a control system for use with a vibration device comprising a response signal having a backward electromotive force component, the control system comprising a driver configured to provide a drive signal to the vibration device; wherein the drive signal comprises an amplitude; a sensor configured to sense an electrical parameter of the response signal; and a controller coupled to the sensor and the driver; wherein the controller comprises a computing device configured to determine a sign of the curvature of a portion of the response signal based on the electrical parameter; wherein the controller is configured to provide a control signal to adjust the amplitude of the drive signal such that the vibrator slows down and to decrease the drive signal due to identifying a change in sign of the curvature. The oscillation device can be, for example, a haptic actuator, such as an LRA.Optionally, the drive signal comprises a polarity and the controller is configured to slow the vibrator by reversing the polarity of the drive signal.Optionally, identifying a change in sign of the curvature includes identifying a change from a negative curvature to a positive curvature.Optionally, identifying a change in sign of the curvature includes identifying that the portion of the response signal has a curvature of zero. The portion of the response signal may be substantially flat, for example.Optionally, the sensor may be configured to sense the electrical parameter at a plurality of sampling times associated with a sampling pattern to obtain a first measurement, a second measurement, and a third measurement; and wherein the computing device is configured to compare the second and third measurements with the first measurement and determine the sign of the curvature based on the comparison.Optionally, wherein the vibration device comprises a resonant frequency, wherein the control system comprises a frequency control device configured to adjust the drive signal such that the vibration device is driven at the resonant frequency.Optionally, the driver is a current driver to provide a current signal, and wherein the sensor is a voltage sensor to sense a voltage across the vibrator.According to a second aspect of the disclosure, there is provided a method of controlling a vibrator configured to provide a response signal comprising a back electromotive force component, the method comprising providing a drive signal to drive the vibrator, the drive signal comprising an amplitude; detecting an electrical parameter of the response signal; adjusting the amplitude of the drive signal such that the vibrator is slowed; determining a sign of the curvature of a portion of the response signal based on the electrical parameter; and de-energizing the drive signal based on identifying a change in the sign of the curvature.Optionally, wherein the drive signal comprises a polarity, and wherein adjusting the amplitude of the drive signal such that the vibrator slows down comprises reversing the polarity of the drive signal.Optionally, wherein identifying a change in sign of the curvature comprises identifying a change from a negative curvature to a positive curvature.Optionally, wherein identifying a change in sign of the curvature comprises identifying that the portion of the response signal has a curvature of zero.Optionally, wherein determining the sign of the curvature comprises sensing the electrical parameter at a plurality of sampling times to obtain a first measurement, a second measurement, and a third measurement; comparing the second and third measurements to the first measurement; and determining the sign of the curvature based on the comparison.Wherein the oscillation device optionally comprises a resonant frequency, wherein the method comprises setting the drive signal such that the oscillation device is driven at the resonant frequency.wherein the drive signal is optionally a current signal and wherein the electrical parameter is a voltage.According to a third aspect of the disclosure, there is provided a vibrator system comprising a vibrator comprising a response signal having a backward electromotive force component and a control system, the control system comprising a driver configured to provide a drive signal to the vibrator; the drive signal comprising an amplitude; a sensor configured to sense an electrical parameter of the response signal; and a controller coupled to the sensor and to the driver; wherein the controller comprises a computing device configured to determine a sign of the curvature of a portion of the response signal based on the electrical parameter; wherein the controller is configured to provide a control signal to adjust the amplitude of the drive signal such that, first, the vibrator slows down and, second, the drive signal is decreased due to identifying a change in sign of the curvature.According to a fourth aspect of the disclosure, there is provided a mobile device comprising a vibrator system; the vibrator system comprising a vibrator comprising a response signal having a backward electromotive force component and a control system, the control system comprising a driver configured to provide a drive signal to the vibrator; the drive signal comprising an amplitude; a sensor configured to sense an electrical parameter of the response signal; and a controller coupled to the sensor and to the driver; wherein the controller comprises a computing device configured to determine a sign of the curvature of a portion of the response signal based on the electrical parameter; wherein the controller is configured to provide a control signal to adjust the amplitude of the drive signal such that, first, the vibrator slows down and, second, the drive signal is decreased due to identifying a change in sign of the curvature.According to a fifth aspect of the disclosure, there is provided a controller for use with a driver for providing a drive signal to a vibrator and a sensor for sensing an electrical parameter of a response signal of the vibrator, the controller comprising a computing device configured to determine a sign of the curvature of a portion of the response signal based on the electrical parameter; wherein the controller is configured to provide a control signal to adjust an amplitude of the drive signal such that the vibrator slows down and to decrease the drive signal based on identifying a change in the sign of the curvature.Optionally, the response signal comprises a back-facing electromotive force component.Optionally, the drive signal comprises a polarity, and wherein the controller is configured to slow the vibrator by reversing the polarity of the drive signal.Optionally, identifying a change in sign of the curvature includes identifying a change from a negative curvature to a positive curvature.Optionally, identifying a change in sign of the curvature includes identifying that the portion of the response signal has a curvature of zero.Optionally, the computing device may be configured to receive a first measurement, a second measurement, and a third measurement of the electrical parameter; wherein the measurements are obtained at multiple sampling times; and wherein the computing device is configured to compare the second and third measurements with the first measurement and determine the sign of the curvature based on the comparison.Optionally, the control device comprises a selection device for selecting a level of the drive signal, which is coupled to the computing device.Brief Description of the DrawingsThe invention is described in more detail below by way of example and with reference to the accompanying drawings, in which: FIG. 1 is a diagram of a vibrator system; FIG. 2 is a flow chart of a method for braking a vibration device; FIG. 3 is an electrical system for modeling a linear resonance actuator; FIG. 4 is a time chart showing waveforms of a drive signal and a response signal of a haptic actuator; FIG. 5 is a time chart illustrating the waveforms of an envelope signal, a drive signal, and a back electromotive force, BEMF signal; FIG. 6 ais a time graph showing the change of a BEMF signal when the haptic actuator is controlled with a negative override having an ideal duration; FIG. 6 bis a time graph showing the change of a BEMF signal when the haptic actuator is controlled with a negative override that is too short; FIG. 6 cis a time graph showing the change of a BEMF signal when the haptic actuator is controlled with a negative override that is too long; FIG. 7 is a time chart illustrating operation of the system of FIG. 1 as it changes from maintaining vibration to stopping vibration; FIG. 8 is a diagram of a haptic system provided with a frequency tracking device; FIG. 9 is a flow chart of a method for tracking an operating frequency of a vibration device.DESCRIPTION OF THE INVENTIONFIG. 1 illustrates a vibration system 100. The system includes a vibrator 105, a regulator 110, a sensor 115, and a controller 120.The vibration device 105 may be any device configured to provide inductive or capacitive oscillations. The vibrator may also be a mechanical assembly including a sprung magnet. For example, the vibration device may be a speaker, a crystal oscillator, or a haptic actuator such as a linear resonance actuator, LRA.The regulator 110 may be a current regulator or a voltage regulator. The regulator 110 has an input for receiving a control signal from the controller 120 and an output for providing a drive signal, such as a drive current signal or a drive voltage signal, to the haptic actuator 105. The regulator 110 is configured to adjust the drive signal using the control signal from the controller 120.The sensor 115 is coupled between the vibrator 105 and the controller 120. The sensor 115 may be a voltage sensor or a current sensor and is configured to sense an electrical parameter. The electrical parameter is associated with a system waveform, which may be a voltage waveform or a current waveform. The sensed electrical parameter may be, for example, a DC voltage or a representation of the voltage, such as a filtered value of the voltage. In another example, the sensed electrical parameter may be a direct current or a representation of the current, such as a filtered value of the current. The sensor 115 may be implemented in various ways. For example, the sensor 115 may be implemented with a preamplifier coupled to an analog / digital converter, ADC.The controller 120 is coupled to the regulator 110 and the sensor 115. The controller includes a memory 128, also referred to as a register, a computing device 122, a comparator 124, and a selection device 126, also referred to as a drive level selection device. The controller 120 may be implemented as a digital controller or an analog controller.The memory 128 has an input for receiving a value of the electrical parameter from the sensor 115 and an output coupled to the computing device 122. The memory may be implemented as a digital register or as multiple capacitors for sample and hold. The computing device 122 has an input for receiving the value of the electrical parameter or measurement from the memory 128 or directly from the sensor 115 and an output for providing a difference value or slope value between multiple measurements. The computing device 122 is configured to determine a curvature of a portion of the response signal based on the electrical parameter. The calculator 122 may be a subtracter or a slope calculator to calculate a slope or a gradient between two points.The comparator 124 has a first input for receiving the difference value or slope value from the computing device 122, a second input for receiving a target value, and an output for providing a control signal to select a drive level. For example, the comparator 124 receives a difference between two values and compares the difference to the target. The target may be a reference zero value to determine whether the difference is positive or negative. The comparator 124 may be implemented as a subtractor.The computing device 122 and the comparator 124 may be implemented as a digital or an analog circuit. For example, the computing device 122 and the comparator 124 may be implemented as two comparator amplifiers. In an alternative embodiment, the computing device 122 and the comparator 124 may be a single digital computing device.The drive level selector 126 has an input for receiving the control signal from the comparator 124 and an output for providing a control signal to the regulator 110. The selector 126 may be coupled to a plurality of drive levels (not shown). After the control signal is input, the selection device 126 is designed to select one of a plurality of drive levels. For example, the selection means may be configured to select one of a positive level, a target level, an initial level and a negative override level, also referred to as a stop level. The stopping level may depend on the limit of the electrical operating means for the system or on the tolerance of the vibration device. For example, the stopping level may be a voltage of the negative rail or a current generated from the voltage of the negative rail. The drive level selector 126 may be implemented as a multiplexer.Optionally, the selector 126 may also include an input for receiving an operating signal. The operation signal may be a signal for starting or stopping the vibrator 105 depending on a certain circumstance. Considering the example of a mobile phone, the operation signal may be triggered, e.g., by an incoming call or an incoming message. Alternatively, the operating signal can be triggered by the user touching the screen or the keyboard of the mobile telephone. The control signal includes an amplitude parameter for changing an amplitude of the envelope signal generated by the envelope generator 112. The control signal may be used to reverse the polarity of the drive signal to decelerate the vibrator. For example, when the drive signal is a drive current, active overdrive may be achieved by adjusting the amplitude (DC level) of the current signal. The deceleration may be achieved by changing a positive amplitude to a negative amplitude that inverts the phase of the drive current signal. The control signal may also be used to reduce the amplitude of the drive signal to zero, as a result of which the drive signal is effectively decreased.The regulator 110 includes an envelope generator 112, a carrier generator 114, and a driver 116 such as a current driver or a voltage driver. The envelope generator 112 has an input for receiving the control signal from the control loop 126. The carrier generator 114 has an input for receiving an amplitude parameter from the envelope generator 112 and an output for providing a signal to the driver. The control device 116 has an input for receiving a signal from the carrier generator 114 and an output for providing a control signal to the haptic actuator 105. In operation, carrier generator 114 receives the output from envelope generator 112 to control the amplitude of its output signal. The driver 116 then receives the output from the carrier generator and provides an appropriate current or voltage to the vibrator. The regulator 110 may be implemented in various ways. For example, a current regulator may include a current driver, such as a differential H-bridge, coupled to a current control loop. The current control loop may use, for example, a PWM or a linear control technique.FIG. 2 shows the steps of a method for braking a vibration device that provides a response signal using the system from FIG. 1.In step 210, a drive signal is provided by the regulator 110 in order to drive the oscillation device 105. The drive signal has an amplitude, a polarity and a drive frequency. The regulator 110 may provide a current drive signal or a voltage drive signal, for example.In step 220, an electrical parameter of the response signal is detected by the sensor 115. For example, the sensor 115 detects a voltage across the vibrator.In step 230, the amplitude of the drive signal is adjusted such that the vibration device is decelerated. The polarity of the drive signal may be reversed to decelerate the vibrator. This can be achieved by selecting a negative override signal. For example, the controller 120 of FIG. 1 generates a control signal to reverse the amplitude of the drive signal in a certain period of time.In step 240, the controller 120 determines a sign of the curvature of a portion of the response signal based on the electrical parameter. The sign of the curvature may be positive, negative, or zero. For example, the controller 120 determines a shape of the waveform in a range arranged around a maximum or a minimum of the waveform. This can be achieved by detecting three measurements on the waveform, e.g. a first point S1 located around the maximum (or minimum) of the waveform and two other points S2 before S1 and S3 after S1. The sensor 115 may transmit multiple sampling points to the memory 112. The computing device 122 may then compute a difference or slope between S 1, S 2 and S 1, S 3 to identify the sign of the curvature in a region of the response signal. For example, the computing device may be configured to verify in a positive half cycle of the response signal whether the following inequality i) S1>S2and ii) S1>S3are satisfied. If i) and ii) are true, then the waveform is convex, and if both i) and ii) are false, then the signal is concave. If only either i) or ii) is true, then the drive signal and the response signal are not in phase. In a preferred embodiment, the control system should be locked in frequency such that the drive signal and the response signal are in phase. If the measurements are performed in the negative half cycle, the measurements may be rectified.In step 250, the drive signal is deasserted due to identifying a change in sign of the curvature of the portion of the response signal. For example, if the controller 120 has a curvature that is initially negative (concave), the controller may provide a control signal to stop the drive signal when the curvature just begins to become positive (convex). This can be performed at a time when the curvature is zero (flat). The de-assertion of the drive signal could be accomplished by reducing the amplitude of the drive signal to zero.This approach provides a simple, flexible and low-power method for actively stopping vibration of a vibrator. This method is compatible with various vibration devices and does not require calibrating the device's response to a particular drive signal. The response time to stop the vibrator is also reduced without significantly increasing the complexity of the system. When the vibrator is driven with a current signal, the above method can be applied while the device is driven. Therefore, no interruption of the drive signal is required until the vibrator has actually stopped.FIG. 3 shows an electrical system 300 for modeling an LRA. The LRA is modeled as a first resistor Rs in series with a first inductance Ls connected to a second resistor Rp in parallel with a second inductance Lp and a capacitor Cp. The damped oscillatory motion of the system, which is formed by the spring and the magnetic mass of the LRA, is due to the components Rp. Lp and Cp. The components Rs and Ls are used to represent the electrical behavior of the internal coils (electromagnets) used to attract and repel the mass of the LRA.The circuit will resonate at a frequency which depends primarily on the values of Cp and Lp. Assuming that the filtering action of Rsand Lsis well above the resonance of the circuit, then the current and voltage at resonance will depend purely on the sum of the resistors Rsand Rp. When driven with a drive signal that approximates the resonant frequency, an LRA will resonate and generate a back electromotive force, BEMF, whose amplitude is proportional to the drive signal. The amplitude will build by following a low pass filter response that describes an exponential decay time that tends to a final value.FIG. 4 shows the waveforms of a drive signal 410 and a response signal 420, also referred to as output voltage V out. The response signal 420 includes an inductive component 422, a BEMF component 424, and a drive signal component 426.In this example, the drive signal 410 is a current drive signal having a rectangular wave profile. For example, the output current may change polarity between -50 mA and +50 mA. The dotted line represents the intended drive waveform and the solid line represents the current as it appears across the actuator due to RC effects. FIG. 4 illustrates the operation of the LRA at or near resonance. In this operating state, the current signal 410 changes polarity at a frequency equal to or close to the resonant frequency of the haptic actuator. Thus, the reverse EMF voltage 424 and the current drive signal 410 are in phase. If the haptic actuator were operated at a different frequency, then the shape of the system waveform 420 would change, as the BEMF signal 424 is phase shifted from the drive signal 410.The output voltage V out420 across the LRA can be expressed by equation (1) as follows: where I is the current drive, R is the resistance of the vibrator, L is the inductance of the LRA, and V BEMF is the back EMF voltage.A drive period extends between times t 0 and t 4. At t0, current drive 410 begins to rise. Between times t 0 and t 1, current 410 increases from a negative current value to a positive current value. During this time, the di / dt component of V out is positive. The output voltage 420 shows a steep, positive peak.At t 1, the current driver 410 has reached its highest positive value. Between times t 1 and t 2, current drive 410 remains substantially constant. During this time, the di / dt component of V out is zero or close to zero. The IR component of V out is a positive constant value. Therefore, the output voltage 420 corresponds to a positive DC component (IR) added to a positive V BEMF- component.At t 2, the current drive 410 begins to decrease. Between times t 2 and t 3, the current decreases from a positive current value to a negative current value. During this time, the di / dt component of V out is negative.At t 3, the current driver 410 has reached its highest negative value. Between times t 3 and t 4, the current drive 410 remains substantially constant. During this time, the di / dt component of V out is zero or close to zero. The IR component of V out is a negative constant value. Therefore, the output voltage 420 corresponds to a negative DC component (IR) added to a negative V BEMF- component.The output voltage is usually not measured during the time windows t1-t0 and t3-t2, since any measurement performed during these time windows would be disturbed by the transient response of the current regulator and the voltage sensor. Instead, the measurement of the output voltage is carried out outside these time windows. During time slots t 2- t 1 and t 4- t 3, a voltage sensor may be used to sense output voltage V out= IR+V BEMF.FIG. 5 illustrates the waveforms of an envelope signal 510, a drive signal 520, and a BEMF signal 530. The envelope signal 510 generated by the envelope generator 112 modulates the amplitude of the drive signal 520. In this example, the drive signal is turned on at time t 1 and turned off at time t 3.The BEMF signal 530 is modulated by a response envelope. After time t 1, the BEMF signal 530 begins to gradually rise to achieve a maximum target amplitude at time t 2. Between times t2 and t3, the maximum amplitude of the BEMF signal remains constant. At time t3, the amplitude of the BEMF signal begins to decrease to reach its minimum target value at time t4. The envelope signal may be used to generate an overdrive to accelerate or decelerate the vibrating mass of the LRA to provide the desired user feedback.FIG. 6 illustrates how the response of the LRA changes for different envelope signals corresponding to different override sequences.FIG. 6 ashows the waveform of a first envelope signal 610 and the corresponding BEMF signal 620. In this example, the envelope signal 610 provides ideal override control. Between times t1 and t2, the amplitude of the envelope signal is above a target value. The target value may be a driving level set by the user. For example, the user sets a target value of 1V and the system drives the device at 1.5V for a short period of time until the amplitude of the response signal is 1V. This time period corresponds to a positive overdrive, accelerating the mass of the LRA. At time t2, the amplitude of the envelope is decreased and maintained at the target value until time t3. No acceleration takes place between times t2 and t3, and the velocity of the mass remains constant. At time t3, the amplitude of the envelope signal becomes negative, as a result of which the polarity of the drive signal is reversed. This time period corresponds to a negative overdrive, slowing the mass of the LRA. This is illustrated by the decreasing amplitude of the BEMF signal 620. At time t4a, the amplitude of the envelope signal is zero and the mass of the LRA rests without speed. In this example, the time interval between t3 and t4a corresponding to the negative override takes just long enough to stop the movement of the mass at the point when the override signal is not applied. In other words, the negative overdrive has been applied for a precise period of time to achieve the desired effect.FIG. 6 b shows the waveform of a second envelope signal 630 and the corresponding BEMF signal 640. In this example, the envelope signal 630 provides a negative overdrive between t 3 and t 4 b, which is too short to achieve the desired effect. As a result, the movement of the mass continues for a certain period after the application of the negative overdrive has ended. At time t 4 a, the mass is still in motion and only stops moving at time t 5.FIG. 6 cshows the waveform of a third envelope signal 650 and the corresponding BEMF signal 660. In this example, the envelope signal 650 provides excessive override time. In this example, the time duration between t 3 and t 4 ccorresponding to the negative override is too long. As a result, the movement of the mass stops at time t4a, but then starts moving again after this time. Between times t 4 aand t 4 c, the mass oscillates in an opposite direction (opposite phase). After time 4c, the mass slows down again and stops moving at time t6.FIG. 7 illustrates the waveforms of an envelope signal 710, a drive signal 720, a BEMF signal 730, a composite drive and BEMF signal 740, and a curvature state indicator 750. In this example, the haptic system is locked in frequency, e.g., at the resonant frequency of the LRA.The haptic system operates in three phases, referred to as first, second, and third phases. In the first phase, also referred to as the drive phase, the drive signal is used to maintain the speed of the mass. In the second phase, also referred to as the deceleration phase, the drive signal is used to stop the movement of the mass by operating the LRA in reverse. In the third phase, which is also referred to as the idling phase, no drive signal is applied. The system may also be operated in a fourth intermediate phase in which the amplitude of the drive signal is reduced.Between times t 0 and t 1, the haptic system is in the actuation phase. During this time, the amplitude of the envelope signal 710 has a first value A. In this example, the drive signal 720 is a square wave signal having the amplitude A and the frequency F corresponding to the resonant frequency of the LRA. The drive signal 720 changes polarity every half cycle, changing between a positive value and a negative value. The BEMF signal 730 and the drive signal are in phase. Composite drive and BEMF signal 740, also referred to as a system waveform, includes multiple cycles associated with a time period T. Each cycle includes a positive half cycle 742 and a negative half cycle 744. The positive half cycle 742 has a convex portion of the outer shape corresponding to a positive curvature.Between times t 1 and t 2, the haptic system is in the intermediate phase in which the amplitude of the drive signal 720 has decreased to a value B that is less than A. The BEMF signal 730 and the drive signal are still in phase. The amplitude of composite drive and BEMF signal 740 decreases. This phase is optional.At time t 2, the polarity of the drive signal is reversed to decelerate the LRA. This can be accomplished by exchanging the positive and negative signals at the LRA. The reversal of the drive signal may be achieved, for example, using an H-bridge driver, with the LRA load between the two branches of the driver. The phase of the carrier signal is reversed and the amplitude of the envelope signal is -C. At this time, the LRA is driven in reverse, and the system waveform has a shape that changes from convex (positive curvature) to concave (negative curvature).Between times t 2 and t 3, the haptic system is in the active deceleration phase. The BEMF signal 730 and the drive signal 720 are out of phase. Composite drive and BEMF signal 740 has an upper portion 746 (746a, 746b, 746c) and a lower portion 748 (748a, 748b) for each time period T. The upper portion 746 has a concave portion of the outer shape corresponding to a negative curvature. However, the degree of concavity of the upper portion decreases with time. In other words, the upper portion has a concave portion of the outer shape that becomes less and less concave. This is illustrated by the three upper portions 746 a, 746 band 746 c. The upper portion 746a has a portion that is more concave than the corresponding upper portion 746b. The upper portion 746c includes a portion that is substantially flat, corresponding to a zero curvature. At time t 3, the amplitude of the envelope signal 710 is reduced to zero. As a result, the drive signal is zero and the haptic system enters the idle phase. Therefore, the system waveform 740 becomes less concave as the LRA slows until it becomes flat and changes back to be convex as it begins to accelerate again. By monitoring the change in sign of the curvature, the end of the deceleration point can be determined. The sign of the curvature can be detected in both the positive half cycle and the negative half cycle of the waveform.The shape of the system waveform 740 may be identified by monitoring and comparing multiple sample points on the waveform. In an exemplary embodiment, three sampling points are monitored and used to determine whether the waveform is concave or convex. For example, a first point S1 is chosen at or near the center of the positive half cycle of the waveform, a second point S2 is chosen before S1, and a third point S3 is chosen after S1. The sampling points S1, S2 and S3 have an amplitude A1 or A2 or A3.The curvature of the system waveform can be determined by comparing the amplitude of the three points S1, S2 and S3. In the positive half cycle 746 of the system waveform A2-A1>0 and A1-A3<0, the system waveform has a concave shape. However, if A2 - A1 < 0 and A1 - A3 > 0, then the system waveform has a convex shape. Whether the shape of the curvature is concave or convex may be determined by the calculator 122 and the comparator 124.It is also possible to select three sampling points N1, N2 and N3 having an amplitude B1, B2 and B3 on the negative half cycle of the system waveform. In the negative half cycle 748 of the system waveform B2-B1<0 and B1-B3>0, the system waveform has a concave shape. However, if B2-B1> 0 and B1-B3<0, then the system waveform has a convex shape.In this example, three points are monitored and compared for each and per half period of the system waveform. However, the sampling rate may be changed as required. For example, the sampling points could be monitored and compared every other period. The number of points to be monitored may also be adjusted. For example, the number of points may be at least three or greater than three. The greater the number of monitored points, the higher the accuracy of the system. In addition, a minimum amplitude threshold may be used to select the sampling points when the sampling points are selected within a small range.Therefore, the proposed method and system uses the sinusoidal nature of the BEMF signal along with the drive signal to monitor when the composite waveform changes from concave to convex. At this point, the LRA has just stopped. This mechanism is independent of any analog shifts or drive levels because it is based on the comparison of relative points on the system waveform.The decelerating intermediate phase is optional. The active braking may be applied directly. For example, the positive and negative signals are reversed over the LRA and the amplitude level may be maintained. This provides a strong deceleration, but one that is well proportional to the current drive level.Vibration devices are generally operated at a frequency typical of the device. Depending on the conditions, this frequency may have a drift as a function of temperature, external load conditions or time, for example. For this reason, it may be necessary to use a control system which can track the operating frequency of the device.FIG. 8 illustrates another haptic system 800. The haptic system 800 is similar to the haptic system 100 of FIG. 1 to which certain components of the circuit are added or modified. The other components have the reference numerals as indicated above and shown in FIG. 1. For the sake of brevity, the description of these common components will not be repeated.In this example, the controller 820 is provided with a frequency controller to track an operating frequency of a haptic actuator, in this case an LRA. The frequency controller includes a memory 828, a computing device 822, a comparator 825, and a control loop 827.The memory 828 has an input for receiving an electrical parameter value from the sensor 815 and an output coupled to the computing device 822. The calculator 822 may be a slope calculator to calculate a slope or a gradient between two points. The computing device 822 has an input for receiving measurements from the memory 828 and two outputs: a first output for providing a first calculated value to the comparator 124, and a second output for providing a second calculated value to the comparator 825. The second calculated value may be a difference value or a slope value.The comparator 825 has a first input for receiving the second calculated value from the computing device 822, a second input for receiving a target value, and an output for providing an error signal.The control loop 827 has an input for receiving the error signal from the comparator 825 and an output for providing a control signal to the carrier generator 114. The control signal may include a period parameter to change the period of the signal generated by the carrier generator 114.The control loop 827 is coupled to the selector 126. For example, the control loop 827 may provide a signal indicating that the drive signal is locked to the response signal in terms of frequency. Frequency value of response signal to the selector 126. Likewise, the selector 126 may provide an amplitude level to normalize the measurements obtained from the sensor. The control loop 827 may be implemented as a phase control loop PLL or as a frequency control loop FLL or as a delay control loop DLL, either in analog or digital form.The carrier generator 814 may be coupled to the comparator 124 (connection not shown) to provide a signal to the comparator to enable the inversion of the comparisons of the computations when the drive phase is the negative half cycle.The regulator 810 includes an envelope generator 112, a carrier generator 814 and a driver 816. Carrier generator 814 has an input for receiving a control signal from control loop 827. The carrier generator 814 may be configured to behave like a digital PLL by varying its period in accordance with the output from the control loop 827. The driver 816 may be implemented as an H-bridge driver to provide a current or voltage across the haptic, which in this example is represented as an LRA.FIG. 9 is a flow diagram of a method for controlling the frequency of a haptic actuator using the system of FIG. 8.In step 910, a sampling pattern is defined that includes at least a first sampling time and a second sampling time. For example, the sampling pattern may be defined such that when the response signal and the drive signal have the same frequency and are in phase, a measurement of the response signal obtained in the first sampling time is equal to another measurement of the response signal obtained in the second sampling time.In step 920, an electrical parameter of the response signal, such as a current value or a voltage value, is detected by the sensor 115 at the first sampling time to obtain a first measurement. In step 930, the electrical parameter at the second sampling time is detected by the sensor 115 to obtain a second measurement. The measurements may be stored in memory 828.In step 940, a difference between the first measurement and the second measurement is calculated by the calculation device 822. The computing device 122 may retrieve the measurement from the memory 828 to perform its calculation.In step 950, the drive signal is adjusted based on the difference. For example, the drive frequency of the drive signal may be changed such that the difference between the first measurement and the second measurement is reduced. For example, the computing device 822 may provide a difference value to the comparator 825. Comparator 825 then compares the difference value to a target value and provides an error signal to control loop 827. Control loop 827 then provides a control signal to carrier generator 114.The operating frequency of the haptic device may be tracked using at least one pair of measurements within a time period T corresponding to one cycle of the response signal. The measurement pair can be monitored at different rates. For example, the measurement pair may be monitored for each half cycle of the response signal. Alternatively, the measurement pair may be monitored every N half cycles of the response signal, where N is an integer greater than one. Using this approach, it is possible to operate the haptic device at its resonant frequency using a relatively simple system.One skilled in the art will appreciate that modifications to the disclosed arrangements are possible without departing from the disclosure. Although the system of the disclosure has been described with reference to haptic actuators such as LRA, the system could also be used with other types of vibration devices. For example, the system could be used with a speaker. Accordingly, the above description of the specific embodiment is provided by way of example only and not for purposes of limitation. It will be apparent to those skilled in the art that minor changes can be made without significant changes to the operation described above.

Claims

A control system for use with a vibration device (105) comprising a response signal having a counterelectromotive force component, the control system comprising: a driver device (116, 816) configured to provide a drive signal to the vibration device (105); wherein the drive signal comprises an amplitude and a polarity; a sensor (115, 815) configured to sense an electrical parameter of the response signal; and a controller device (120, 820) coupled to the sensor and the driver device (116, 816), wherein the controller device (120, 820) comprises a computing device (122, 822) configured to determine a sign of the curvature of a curved portion of the counterelectromotive force component of the response signal based on the electrical parameter; wherein the controller (120, 820) is configured to provide a control signal to adjust the amplitude of the drive signal such that the vibrator (105) slows down, wherein upon reversing the polarity of the drive signal, the curved portion gradually flattens prior to changing its sign of the curvature, and wherein the controller (120, 820) is configured to decrease the drive signal based on identifying the change in the sign of the curvature of the curved portion of the counterelectromotive force component.The control system of claim 1, wherein identifying a change in sign of the curvature comprises identifying a change from a negative curvature to a positive curvature.The control system of claim 1, wherein identifying a change in sign of the curvature comprises identifying that the curved portion of the counterelectromotive force component of the response signal has a curvature of zero.The control system of claim 1, wherein the sensor (115, 815) is configured to sense the electrical parameter at a plurality of sampling times associated with a sampling pattern to obtain a first measurement, a second measurement, and a third measurement; and wherein the computing device (122, 822) is configured to compare the second and third measurements with the first measurement and determine the sign of the curvature based on the comparison.The control system of claim 4, wherein the vibrator (105) comprises a resonant frequency, the control system comprises a frequency controller (828, 822, 825, 827) configured to adjust the drive signal such that the vibrator (105) is driven at the resonant frequency.The control system of claim 1, wherein the driver (116, 816) is a current driver (116, 816) to provide a current signal, and wherein the sensor (115, 815) is a voltage sensor to sense a voltage across the vibrator (105).A method of controlling a vibrator (105) configured to provide a response signal comprising a counter electromotive force component, the method comprising: providing a drive signal to drive the vibrator (105), the drive signal comprising an amplitude and a polarity; detecting an electrical parameter of the response signal; adjusting the amplitude of the drive signal such that its polarity is reversed and the vibrator (105) is slowed; determining a sign of the curvature of a curved portion of the counter electromotive force component of the response signal based on the electrical parameter; wherein, upon reversing the polarity of the drive signal, the curved portion gradually flattens before changing its sign of the curvature and decreasing the drive signal due to identifying the change in the sign of the curvature of the curved portion of the counter electromotive force component.The method of claim 7, wherein identifying a change in sign of the curvature comprises identifying a change from a negative curvature to a positive curvature.The method of claim 7, wherein identifying a change in sign of the curvature comprises identifying that the curved portion of the counterelectromotive force component of the response signal has a curvature of zero.The method of claim 7, wherein determining the sign of the curvature comprises: acquiring the electrical parameter at multiple sampling times to obtain a first measurement, a second measurement, and a third measurement; comparing the second and third measurements to the first measurement; and determining the sign of the curvature based on the comparison.The method of claim 7, wherein the vibration device (105) comprises a resonant frequency, the method comprising adjusting the drive signal such that the vibration device (105) is driven at the resonant frequency.The method of claim 7, wherein the drive signal is a current signal and wherein the electrical parameter is a voltage.A vibrator system (100) comprising: a vibrator (105) comprising a response signal having a counter electromotive force component; and a control system according to claim 1.A mobile device comprising a vibration device system (100) according to claim 13.A controller (120, 820) for use with a driver (116, 816) for providing a drive signal to a vibrator and a sensor (115, 815) for sensing an electrical parameter of a response signal of the vibrator, wherein the drive signal comprises an amplitude and a polarity, and wherein the response signal comprises a counterelectromotive force component; wherein the controller (120. 820) comprises: a computing device (122, 822) configured to determine a sign of the curvature of a curved section of the counterelectromotive force component of the response signal on the basis of the electrical parameter; wherein the control device (120, 820) is configured to provide a control signal to adjust the amplitude of the drive signal such that its polarity is reversed and the oscillation device is slowed down, wherein upon reversal of the polarity of the drive signal the curved section gradually flattens before changing its sign of the curvature, wherein the control device (120, 820) is configured to decrease the drive signal on the basis of identifying the change of the sign of the curvature of the curved section of the counterelectromotive force component.The controller (120, 820) of claim 15, wherein identifying a change in sign of the curvature comprises identifying a change from a negative curvature to a positive curvature.The controller (120, 820) of claim 15, wherein identifying a change in sign of the curvature comprises identifying that the curved portion of the counterelectromotive force component of the response signal has a curvature of zero.The controller (120, 820) of claim 15, wherein the computing device (122, 822) is configured to receive a first measurement, a second measurement, and a third measurement of the electrical parameter; wherein the measurements are obtained at multiple sampling times; and wherein the computing device (122, 822) is configured to compare the second measurement and the third measurement with the first measurement and determine the sign of the curvature based on the comparison.The controller (120, 820) of claim 15, comprising a selector (126) for selecting a level of the drive signal coupled to the computing device (122, 822).

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