Method and system for managing hybrid-mode electromechanical actuator actuation

Abstract

According to an embodiment of the present disclosure, a method for driving a playback waveform to an electromagnetic actuator through a transducer drive system may include: operating the transducer drive system in a first mode, wherein the electromagnetic actuator is driven with the playback waveform in a closed loop to form a closed-loop voltage drive system including a negative impedance; operating the transducer drive system in a second mode, wherein the electromechanical actuator is driven with the playback waveform in an open loop; and operating a mode switch for switching the transducer drive system to operate between the first mode and the second mode.

Description

[0001] Related applications

[0002] This disclosure claims priority to U.S. Provisional Patent Application No. 63 / 213,612, filed on June 22, 2021, which is incorporated herein by reference in its entirety.

[0003] This disclosure cross-references U.S. Patent No. 10,828,672 to Stahl et al., entitled “Driver Circuitry” and assigned to Cirrus Logic, Inc. (hereinafter referred to as the “Stahl patent”), U.S. Patent Publication No. 2021 / 0175869 to Taipale, entitled “Methods and Systems for Detecting and Managing Amplifier Instability” (hereinafter referred to as the “Taipale patent application”), and U.S. Patent Publication No. 2021 / 017477 to Marchais et al., entitled “Methods and Systems for Estimating Coil Impedance of an Electromagnetic Transducer” (hereinafter referred to as the “Marchais patent application”). The Stahl patent, the Taipale patent application, and the Marchais patent application are all incorporated herein by reference in their entirety. Technical Field

[0004] The present disclosure generally relates to using an amplifier to drive a haptic vibration load in both open-loop and closed-loop modes of operation, and managing signals sent to the amplifier when switching between these two modes of operation. Background Art

[0005] Vibrotactile transducers, such as electromechanical actuators like linear resonant actuators (LRAs), are widely used in portable devices like mobile phones to generate vibration feedback to the user. Various forms of vibrotactile feedback produce different tactile sensations on the user's skin and are likely to play an increasingly important role in human-computer interaction in modern devices.

[0006] An LRA can be modeled as a mass-spring electromechanical vibration system. When driven with a properly designed or controlled drive signal, the LRA can generate certain desired forms of vibration. For example, a clear and noticeable vibration pattern on a user's finger can be used to produce the sensation of a mechanical button click. This noticeable vibration can then be used as a virtual switch to replace the mechanical button.

[0007] FIG1 illustrates an example of a vibrotactile system in a device 100. The device 100 may include a controller 101 configured to control a signal applied to an amplifier 102. The amplifier 102 may then drive a tactile transducer 103 based on the signal. The controller 101 may be triggered by a trigger to output a signal. The trigger may, for example, include a pressure sensor or force sensor on a screen or virtual button of the device 100.

[0008] Among various forms of vibrotactile feedback, tonal vibrations of duration can play an important role in notifying the user of a device of certain predefined events, such as incoming calls or messages, emergency alerts, and timer reminders. To effectively generate tonal vibration notifications, it may be desirable to operate the tactile actuator at its resonant frequency.

[0009] The resonant frequency f0 of the tactile transducer can be approximately estimated as:

[0010]

[0011] Where C is the compliance of the spring system, and M is the equivalent moving mass, which can be determined based on the actual moving parts in the tactile transducer and the mass of the portable device holding the tactile transducer.

[0012] The vibration resonance of a tactile transducer may vary over time due to sample-to-sample variations of individual tactile transducers, mobile device component variations, temporal component variations due to aging, component variations due to self-heating, and usage conditions such as the varying strength with which a user grips the device.

[0013] FIG2A shows an example of a linear resonant actuator (LRA) modeled as a linear system including a mass-spring system 201. LRAs are nonlinear components that can behave differently depending on, for example, the applied voltage level, operating temperature, and operating frequency. However, under certain conditions, these components can be modeled as linear components.

[0014] FIG2B shows an example of an LRA modeled as a linear system, including an electrical equivalent model of the LRA's mass-spring system 201. In this example, the LRA is modeled as a third-order system with electrical and mechanical elements. Specifically, Re and Le are the DC resistance of the coil-magnet system and the coil inductance, respectively; and Bl is the magnetic factor of the coil. The driver amplifier outputs a voltage waveform V(t) having an output impedance Ro. The terminal voltage V can be sensed across the terminals of the tactile transducer. T (t). The mass-spring system 201 moves at a velocity u(t), which can be related to the back electromotive force V BEMF Proportional.

[0015] An electromagnetic load such as an LRA can be connected to a load through its impedance Z LRA To characterize, such as coil impedance Z coil and mechanical impedance Z mech The sum is shown as:

[0016] Z LRA =Z coil +Z mech (2)

[0017] Coil impedance Z coil Then a direct current (DC) resistor Re may be included in series with the inductor Le:

[0018] Z coil =Re+s*Le(3)

[0019] Mechanical impedance Z mech It can be defined by three parameters, including: the resistance R at resonance RES , which represents the resistance of the mechanical friction of the mass-spring system representing the tactile transducer; the capacitance C MES , which represents the capacitance of the equivalent moving mass M of the mass-spring system representing the tactile transducer; and the inductance L CES , which represents the compliance C of the mass-spring system of the tactile transducer. The electrical equivalent of the total mechanical impedance is R RES 、C MES 、L CES The Laplace transform of this parallel connection is described as:

[0020]

[0021] The resonant frequency f0 of the tactile transducer can be expressed as:

[0022]

[0023] The quality factor Q of LRA can be expressed as:

[0024]

[0025] See formula (6), which describes the resistance Re and R RES The parallel connected sub-expressions of ), and in Figure 2B, these resistors are shown connected in series, which may not seem intuitive. However, this may be the case where the driving voltage Ve is oscillating but then suddenly turns off and goes to zero. The voltage amplifier shown in Figure 2B can be considered to have a low source impedance, ideally zero source impedance. Under these conditions, when the driving voltage Ve goes to zero, the voltage amplifier effectively disappears from the circuit. At this point, the topmost terminal of the resistor Re in Figure 2B and the resistor RRES The bottom terminal is grounded, and therefore the resistors Re and R RES are actually connected in parallel, as reflected in equation (6).

[0026] Electromagnetic transducers, such as LRAs or microspeakers, may have slow response times. FIG3 is a graph of an example response of an LRA, depicting an example drive signal for the LRA, the current through the LRA, and the back electromotive force (back EMF) of the LRA, where such back EMF may be proportional to the velocity of the moving element (e.g., coil or magnet) of the transducer. As shown in FIG3 , the rise time of the back EMF may be slow as energy is transferred to the LRA, and some “ringing” of the back EMF may occur after the drive signal ends as the mechanical energy stored in the LRA is released. In the context of a tactile LRA, this behavioral characteristic may result in a “mushy” feeling click or pulse, rather than a “crisp” tactile response. Therefore, it may be desirable for the LRA to instead have a tactile response similar to Figure 4 The response shown, where there is minimal ringing after the drive signal has ended, and can provide a more "crisp" tactile response in a tactile context. Therefore, it may be desirable to process the drive signal so that when the processed drive signal is applied to the transducer, the velocity or back EMF of the transducer is closer to that of the transducer. Figure 4 velocity or back EMF. Summary of the Invention

[0027] According to the teachings of the present disclosure, disadvantages and problems associated with gracefully switching the drive mode of an amplifier when coupled to an electromechanical load between an open-loop mode and a negative impedance closed-loop mode may be reduced or eliminated.

[0028] According to an embodiment of the present disclosure, a method of driving a playback waveform to an electromagnetic actuator through a transducer drive system may include operating the transducer drive system in a first mode, wherein the electromagnetic actuator is driven with the playback waveform in a closed loop to form a closed-loop voltage drive system including a negative impedance, operating the transducer drive system in a second mode, wherein the electromechanical actuator is driven with the playback waveform in an open loop, and operating a mode switch for switching the transducer drive system to operate between the first mode and the second mode.

[0029] According to these and other embodiments of the present disclosure, a transducer drive system for driving a playback waveform to an electromagnetic actuator may include an output for generating the playback waveform and a control subsystem configured to operate the transducer drive system in a first mode, wherein the electromagnetic actuator is driven in a closed loop with the playback waveform to form a closed-loop voltage drive system that includes a negative impedance, operate the transducer drive system in a second mode, wherein the electromechanical actuator is driven in an open loop with the playback waveform, and operate a mode switch for switching the transducer drive system to operate between the first mode and the second mode.

[0030] The technical advantages of the present disclosure will be apparent to those skilled in the art from the drawings, descriptions, and claims included herein. The objects and advantages of the embodiments will be realized and achieved at least by the elements, features, and combinations particularly pointed out in the claims.

[0031] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the claims set forth in the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] A more complete understanding of the present embodiment and its advantages may be obtained by referring to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals indicate like features, and wherein:

[0033] FIG1 shows an example of a vibrotactile system in a device known in the art;

[0034] 2A and 2B each show an example of a linear resonant actuator (LRA) known in the art modeled as a linear system;

[0035] FIG3 shows a graph of an example waveform of an electromagnetic load known in the art;

[0036] Figure 4 A graph illustrating an example desired waveform of an electromagnetic load according to an embodiment of the present disclosure;

[0037] Figure 5 A block diagram illustrating selected components of an example mobile device according to an embodiment of the present disclosure;

[0038] Figure 6 A block diagram illustrating selected components of an example integrated haptic system according to an embodiment of the present disclosure;

[0039] Figure 7 An example transducer drive system for improving transducer dynamics when switching operating modes between open loop and negative impedance closed loop according to an embodiment of the present disclosure is shown;

[0040] Figure 8The embodiment of the present disclosure shows the multiplier input value based on the embodiment of the present disclosure. Figure 7 an example classification table of operating modes of an example transducer drive system;

[0041] Figure 9 The embodiment according to the present disclosure is shown Figure 7 a graph of an example voltage drive signal and an example sense terminal voltage for an example transducer drive system illustrating different waveform sub-portions associated with tactile terms of a tactile event;

[0042] Figure 10 A graph showing an example acceleration versus time curve according to an embodiment of the present disclosure is shown, which shows the acceleration versus time curve with changing Figure 7 Examples of problems associated with negative impedance feedback correction levels in transducer drive systems;

[0043] Figure 11 The embodiment according to the present disclosure is shown Figure 7 The original transducer driving signal and sensing terminal voltage V of the example transducer driving system T (t) Example graph of the transfer function between;

[0044] Figure 12 An example of a linear resonant actuator (LRA) modeled as a linear system and including a negative resistance according to an embodiment of the present disclosure is shown;

[0045] Figure 13 The embodiment according to the present disclosure is shown Figure 7 Example waveforms of an example transducer drive system illustrating the benefits of compensation;

[0046] Figure 14 The embodiment of the present disclosure is shown Figure 7 a graph of an example acceleration versus time curve for an example transducer drive system with applied compensation;

[0047] Figure 15 The embodiment according to the present disclosure is shown Figure 7 a graph of example waveforms associated with an example transducer drive system of FIG. 1 illustrating problems associated with driving very large playback waveforms during a closed-loop mode of operation; and

[0048] Figure 16 The embodiment according to the present disclosure is shown Figure 7 Graph of example waveforms for an example transducer drive system depicting management of control signals to strike a balance between preventing clipping and maintaining closed-loop steady-state performance. DETAILED DESCRIPTION

[0049] The following description sets forth example embodiments according to the present disclosure. Further example embodiments and implementations will be apparent to those of ordinary skill in the art. In addition, those of ordinary skill in the art will recognize that various equivalent technologies may be applied in place of or in combination with the embodiments discussed below, and all such equivalents should be considered to be included in this disclosure.

[0050] Various electronic devices or smart devices may have transducers, speakers, and acoustic output transducers, such as any transducer for converting a suitable electrical drive signal into an acoustic output (such as an acoustic pressure wave or mechanical vibration). For example, many electronic devices may include one or more speakers or microphones for sound generation, such as for playback of audio content, voice communication, and / or for providing audible notifications.

[0051] Such a speaker or microphone may include an electromagnetic actuator, such as a voice coil motor, which is mechanically coupled to a flexible diaphragm, such as a conventional microphone cone, or which is mechanically coupled to a surface of the device, such as a glass screen of a mobile device. Some electronic devices may also include an acoustic output transducer capable of generating ultrasonic waves, such as for proximity detection applications and / or machine-to-machine communication.

[0052] Many electronic devices may additionally or alternatively include more specialized acoustic output transducers, such as tactile transducers, that are customized to generate vibrations for tactile control feedback or notification to the user. Additionally or alternatively, the electronic device may have a connector, such as a socket, for removably mating with a corresponding connector of an accessory device, and may be arranged to provide a drive signal to the connector to drive one or more of the above-mentioned types of transducers of the accessory device when connected. Such an electronic device will therefore include drive circuitry for driving the transducers of the host device or connected accessory with the appropriate drive signal. For acoustic or tactile transducers, the drive signal is typically an analog time-varying voltage signal, such as a time-varying waveform.

[0053] Figure 5 1 shows a block diagram of selected components of an example host device 502 according to an embodiment of the present disclosure. Figure 5 As shown, host device 502 may include a housing 501 , a controller 503 , a memory 504 , a force sensor 505 , a microphone 506 , a linear resonant actuator 507 , a radio transmitter / receiver 508 , a speaker 510 , and an integrated haptic system 512 .

[0054] Housing 501 may include any suitable housing, shell, or other casing for housing the various components of host device 502. Housing 501 may be constructed of plastic, metal, and / or any other suitable material. Furthermore, housing 501 may be adapted (e.g., sized and shaped) to make host device 502 easily transportable on the person of a user of host device 502. Thus, host device 502 may include, but is not limited to, a smartphone, a tablet computing device, a handheld computing device, a personal digital assistant, a laptop computer, a video game controller, or any other device that can be easily transported on the person of a user of host device 502.

[0055] The controller 503 may be housed within the housing 501 and may include any system, device, or apparatus configured to interpret and / or execute program instructions and / or process data, and may include, but is not limited to, a microprocessor, a microcontroller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), or any other digital or analog circuit configured to interpret and / or execute program instructions and / or process data. In some embodiments, the controller 503 interprets and / or executes program instructions and / or processes data stored in a memory 504 and / or other computer-readable medium accessible to the controller 503.

[0056] The memory 504 may be housed within the housing 501, may be communicatively coupled to the controller 503, and may include any system, device, or apparatus (e.g., a computer-readable medium) configured to retain program instructions and / or data for a period of time. The memory 504 may include random access memory (RAM), electrically erasable programmable read-only memory (EEPROM), a Personal Computer Memory Card International Association (PCMCIA) card, flash memory, magnetic storage, optical-magnetic storage, or any suitable selection and / or array of volatile or non-volatile memory that retains data after power to the host device 502 is turned off.

[0057] Microphone 506 can be at least partially housed within housing 501, can be communicatively coupled to controller 503, and can include any system, device, or apparatus configured to convert sound incoming at microphone 506 into an electrical signal that can be processed by controller 503, wherein such sound is converted into an electrical signal using a diaphragm or membrane having a capacitance that varies based on acoustic vibrations received at the diaphragm or membrane. Microphone 506 can include an electrostatic microphone, a condenser microphone, an electret microphone, a microelectromechanical system (MEMS) microphone, or any other suitable condenser microphone.

[0058] The radio transmitter / receiver 508 may be housed within the housing 501, may be communicatively coupled to the controller 503, and may include any system, device, or apparatus configured to generate and transmit radio frequency signals with the aid of an antenna, and to receive radio frequency signals and convert information carried by such received signals into a form usable by the controller 503. The radio transmitter / receiver 508 may be configured to transmit and / or receive various types of radio frequency signals, including, but not limited to, cellular communications (e.g., 2G, 3G, 4G, LTE, etc.), short-range wireless communications (e.g., Bluetooth), commercial radio signals, television signals, satellite radio signals (e.g., GPS), Wi-Fi, etc.

[0059] The speaker 510 can be at least partially housed within the housing 501, or can be external to the housing 501, can be communicatively coupled to the controller 503, and can include any system, device, or apparatus configured to produce sound in response to an electrical audio signal input. In some embodiments, the speaker can include a dynamic amplifier that employs a lightweight diaphragm mechanically coupled to a rigid frame via a flexible suspension that constrains the axial movement of a voice coil through a cylindrical magnetic gap. When an electrical signal is applied to the voice coil, the current in the voice coil generates a magnetic field, causing it to become a variable electromagnet. The magnetic systems of the coil and the driver interact, generating a mechanical force that causes the coil (and thus the attached cone) to move back and forth, thereby reproducing sound under the control of the applied electrical signal from the amplifier.

[0060] The force sensor 505 can be housed within the housing 501 and can include any suitable system, device, or apparatus for sensing force, pressure, or touch (e.g., interaction with a person's finger) and generating an electrical signal or electronic signal in response to such force, pressure, or touch. In some embodiments, such an electrical signal or electronic signal can be a function of the magnitude of the force, pressure, or touch applied to the force sensor. In these and other embodiments, such an electronic signal or electronic signal can include a general-purpose input / output signal (GPIO) associated with an input signal to which tactile feedback is given. The force sensor 505 can include, but is not limited to, a capacitive displacement sensor, an inductive force sensor (e.g., a resistive-inductive-capacitive sensor), a strain gauge, a piezoelectric force sensor, a force sensing resistor, a piezoelectric force sensor, a thin film force sensor, or a force sensor based on a quantum tunneling composite material. For the purposes of clarity and illustration in this disclosure, the term "force" as used herein can refer not only to force, but also to a physical quantity indicating force or similar to force, such as, but not limited to, pressure and touch.

[0061] The linear resonant actuator 507 can be housed within the housing 501 and can include any suitable system, device, or apparatus for generating an oscillating mechanical force across a single axis. For example, in some embodiments, the linear resonant actuator 507 can rely on an AC voltage to drive a voice coil that is pressed against a moving mass connected to a spring. When the voice coil is driven at the resonant frequency of the spring, the linear resonant actuator 507 can vibrate with a perceptible force. Therefore, the linear resonant actuator 507 can be useful in tactile applications within a specific frequency range. Although, for purposes of clarity and illustration, the present disclosure is described with respect to the use of the linear resonant actuator 507, it should be understood that any other type or types of vibration actuators (e.g., an eccentric rotating mass actuator) can be used in place of the linear resonant actuator 507 or in addition to the linear resonant actuator 507. Furthermore, it should be understood that an actuator arranged to generate an oscillatory mechanical force across multiple axes may be used in place of or in addition to the linear resonant actuator 507, or any other type or types of vibration actuators may be used. As described elsewhere in this disclosure, the linear resonant actuator 507 may provide tactile feedback to a user of the host device 502 based on signals received from the integrated haptic system 512 for at least one of mechanical button replacement and capacitive sensor feedback.

[0062] The integrated haptic system 512 may be housed within the housing 501, may be communicatively coupled to the force sensor 505 and the linear resonant actuator 507, or may include any system, device, or apparatus configured to receive a signal from the force sensor 505 indicating a force applied to the host device 502 (e.g., a force applied by a human finger to a virtual button of the host device 502) and generate an electronic signal for driving the linear resonant actuator 507 in response to the force applied to the host device 502. Figure 6 Details of an example integrated haptic system according to embodiments of the present disclosure are depicted in .

[0063] Although the specific example components above Figure 5 504, force sensor 505, microphone 506, radio transmitter / receiver 508, speaker 510), but the host device 502 according to the present disclosure may include one or more components not specifically listed above. Figure 5 Certain user interface components are depicted, but the host device 502 is not Figure 5One or more other user interface components beyond those depicted may also be included (including but not limited to a keyboard, a touch screen, and a display) to allow a user to interact with and / or otherwise manipulate the host device 502 and its associated components.

[0064] Figure 6 A block diagram illustrating selected components of an example integrated haptic system 512A according to an embodiment of the present disclosure is shown. In some embodiments, the integrated haptic system 512A may be used to implement Figure 5 The integrated tactile system 512. Figure 6 As shown, the integrated haptic system 512A may include a digital signal processor (DSP) 602 , a memory 604 , and an amplifier 606 .

[0065] DSP 602 may include any system, device, or apparatus configured to interpret and / or execute program instructions and / or process data. In some embodiments, DSP 602 may interpret and / or execute program instructions and / or process data stored in memory 604 and / or other computer-readable media accessible to DSP 602.

[0066] The memory 604 may be communicatively coupled to the DSP 602 and may include any system, device, or apparatus (e.g., a computer-readable medium) configured to retain program instructions and / or data for a period of time. The memory 604 may include random access memory (RAM), electrically erasable programmable read-only memory (EEPROM), a Personal Computer Memory Card International Association (PCMCIA) card, flash memory, magnetic storage, optical-magnetic storage, or any suitable selection and / or array of volatile or non-volatile memory that retains data after power to the host device 502 is turned off.

[0067] Amplifier 606 may be electrically coupled to DSP 602 and may include a circuit configured to increase the input signal V IN (e.g., a time-varying voltage or current) to generate an output signal V OUT Any suitable electronic system, device, or apparatus can be used to generate a signal. For example, amplifier 606 can use electrical power from a power supply (not explicitly shown) to increase the amplitude of the signal. Amplifier 606 can include any suitable amplifier class, including but not limited to a Class D amplifier.

[0068] In operation, memory 604 can store one or more haptic playback waveforms. In some embodiments, each of the one or more haptic playback waveforms can define a haptic response a(t) as a desired acceleration of a linear resonant actuator (e.g., linear resonant actuator 507) as a function of time. DSP 602 can be configured to receive a force signal V indicative of a force applied to force sensor 505. SENSE In response to a force signal V indicating the sensed force SENSE In response to or independently of such reception, DSP 602 may retrieve a haptic playback waveform from memory 604 and process such haptic playback waveform to determine a processed haptic playback signal V IN In an embodiment where amplifier 606 is a Class D amplifier, the processed haptic playback signal V IN In response to receiving a force signal V indicative of the sensed force, a pulse width modulated signal may be included. SENSE , DSP602 can cause the processed haptic playback signal V IN Output to amplifier 606, and amplifier 606 can amplify the processed tactile playback signal V IN To generate a tactile output signal V for driving the linear resonant actuator 507 OUT .

[0069] In some embodiments, the integrated haptic system 512A can be formed on a single integrated circuit, thereby achieving lower latency than existing methods of haptic feedback control. By providing the integrated haptic system 512A as part of a single monolithic integrated circuit, latency between the various interfaces and system components of the integrated haptic system 512 can be reduced or eliminated.

[0070] The problem shown in FIG. 3 may be caused by the linear resonant actuator 507 having a high quality factor q, which has a spike in impedance / resistance at the resonant frequency f 0 of the linear resonant actuator 507 .

[0071] Figure 7 An example transducer drive system 700 for improving the dynamic characteristics of an electromagnetic load 701 according to an embodiment of the present disclosure is shown. In some embodiments, the system 700 can be integrated into a host device (eg, host device 502) that includes the system 700 and the electromagnetic load 701.

[0072] In operation, the haptic waveform generator 722 of the system 700 of the host device may generate a haptic playback waveform V ref (t), in the absence of the pilot tone generated by the pilot tone generator 718, the tactile playback waveform V ref(t) can be equivalent to the original transducer drive signal x′(t) including the tactile waveform signal or the audio signal. In some embodiments, the original transducer drive signal x′(t) can be generated based on a stored tactile waveform and / or a dynamically generated tactile waveform stored in a memory accessible to the tactile waveform generator 722 (e.g., memory 604).

[0073] The raw transducer drive signal x′(t) may be received by a combiner 726, which may combine the raw transducer drive signal x′(t) with a correction term from the multiplier 715 to generate a transducer drive signal x(t) so as to effectively cancel some or all of the coil impedance / resistance of the electromagnetic load 701, as described in greater detail below. Also as described below, by effectively reducing the coil impedance / resistance of the electromagnetic load 701, the system 700 may also reduce the effective quality factor q of the electromagnetic load 701, which in turn may reduce rise time and minimize ringing that occurs after the end of the raw transducer drive signal. Although Figure 7 A virtual negative impedance / resistance is depicted applied by the combiner 726, but in some embodiments, a negative impedance / resistance filter may be applied to the raw transducer drive signal x′(t) to generate the transducer drive signal x(t), thereby achieving the same or similar effect of effectively reducing the coil impedance / resistance of the electromagnetic load 701. An example of such a negative impedance / resistance filter is described in U.S. Patent Publication No. 2020 / 0306796 to Lindemann et al., entitled “Methods and Systems for Improving Transducer Dynamics,” assigned to Cirrus Logic International Semiconductor, Inc. (hereinafter referred to as the “Lindemann patent application”), which is incorporated herein by reference in its entirety.

[0074] The transducer drive signal x(t) may then be amplified by the amplifier 706 to generate a drive signal V(t) for driving the electromagnetic load 701. In response to the drive signal V(t), the sensing terminal voltage V T (t) can be converted to a digital representation by a first analog-to-digital converter (ADC) 703. Similarly, the sensed current I(t) can be converted to a digital representation by a second ADC 704. The current I(t) can be measured across a resistor R coupled to the terminals of the electromagnetic load 701. s The terminal voltage V T (t) can be sensed by the terminal voltage sensing block 707 (e.g., a voltmeter). Figure 7As shown, the system 700 may include an impedance estimator 714. The impedance estimator 714 may include any suitable system, device, or apparatus configured to sense the terminal voltage V T (t), sensed current I(t), and / or any other measured parameter of the electromagnetic load 701 to estimate one or more components of the electrical and / or mechanical impedance / resistance of the electromagnetic load 701 and generate one or more control signals. For example, one control signal generated by the impedance estimator 714 may include a negative impedance / resistance Re_neg generated based on an estimate of the direct current (DC) coil impedance / resistance Re of the electromagnetic load 701. As another example, the impedance estimator 714 may also generate a voltage offset V OFFSET and current offset I OFFSET , which can be respectively sensed by combiners 710 and 712 from the sensing terminal voltage V T (t) and the sensed current I(t) to eliminate any measurement offset that may be present and detected by the impedance estimator 714. As a further example, and as described in more detail below, the impedance estimator 714 can generate one or more control signals for communication to the haptic state machine 716. In some embodiments, the haptic state machine 716 can operate in the same or similar manner as described in the Marchais patent application.

[0075] Figure 7 Also shown are two band pass filters (BPFs) 730 and 732, which respectively sense the terminal voltage V T Bandpass filters 730 and 732 can filter the haptic playback content of the drive signal V(t) from entering the impedance estimator 714, thereby potentially deviating from an accurate estimate of the DC coil impedance / resistance Re.

[0076] Likewise Figure 7 As shown, mode switch 740 can receive a status indication of which particular portion of the playback waveform is currently being output by tactile waveform generator 722, as well as an indication of the signal level of the playback waveform, and information regarding the operating mode of the playback waveform—whether an open-loop operating mode (e.g., no negative impedance / resistance applied) or a closed-loop operating mode (e.g., negative impedance / resistance applied)—from haptic state machine 716. As described in more detail below, based on the above inputs, mode switch 740 can control three signal levels: (i) the level of the playback waveform (via a scalar value REF_LEVEL applied by multiplier 750); the level of a pilot signal used to assist in impedance / resistance estimation (via a scalar value PILOT_LEVEL applied by multiplier 760); and the level of a negative impedance / resistance feedback correction signal (via a scalar value ATTEN applied by multiplier 715).

[0077] Figure 8 The value of the scalar value ATTEN according to an embodiment of the present disclosure is shown. Figure 7 7. Example classifications of operating modes of the example transducer drive system 700 are shown in FIG. 1 . If the value of ATTEN is zero, then there may be no feedback correction in the example transducer drive system 700, and thus the negative impedance / resistance generated by the multiplier 725 is not actually present in the example transducer drive system 700 without having any effect on the playback waveform. In this case, the transducer drive system 700 may be classified as "open loop". At the other extreme, if the value of the scalar ATTEN is one, then the example transducer drive system 700 may operate as a negative impedance / resistance system, such as described in the references previously incorporated by reference, including but not limited to the Stahl patent, the Taipale patent application, the Marchais patent application, and the Lindemann patent application. In this case, the transducer drive system 700 may be classified as "closed loop". If the value of the scalar ATTEN is assumed to be any value between (but not including) zero and one, then the example transducer drive system 700 may be classified as "partially closed loop". Typically, a partial closed-loop system is briefly utilized in situations such as, but not limited to, transitions between closed-loop and open-loop, during the beginning of a tactile playback waveform whose level is too great for the closed-loop system to provide sufficient voltage drive, or during the abrupt end of a tactile playback waveform whose level is too great for the closed-loop system to provide sufficient voltage drive.

[0078] Figure 9 The embodiment according to the present disclosure is shown Figure 7 Example raw transducer drive signal x′(t) and example sense terminal voltage V TA graph of x′(t) shows different waveform portions associated with haptic terms for haptic events. Terms can be provided to categorize portions of the haptic playback waveform. The start of the waveform can be referred to as onset. For closed-loop systems, the onset of the waveform can also be referred to as overdrive, as closed-loop systems can provide a much stronger raw transducer drive signal x′(t) to cause the moving mass of the electromechanical load 701 to begin moving more quickly. However, “onset” is a term that can be applied equally to both open-loop and closed-loop systems. The abrupt end of the haptic playback waveform can be referred to as braking or damping. Depending on the damping constant of the system, the electromechanical load 701 can have a ringing that decays over time after braking. Typically, haptic product manufacturers prefer that the ringing of the electromagnetic load 701 decay quickly. The term braking implies the presence of some additional components to force the decay to occur quickly. Such additional components can be those that make up a negative impedance / resistance closed-loop system (e.g., those disclosed in the references incorporated by reference), but such braking may not be available for a purely open-loop system. Nevertheless, the term braking is used herein to refer to the portion of the playback waveform that includes the abrupt end and the brief period thereafter during which the ringing of the moving mass is allowed to fully decay. Between these attacking and braking portions of the haptic playback waveform, the haptic playback waveform can be considered to be in steady state, as shown in FIG. Figure 9 shown.

[0079] To further explain Figure 8 Example categories and modes of operation of the example transducer drive system 700 set forth in Figure 9An example classification of portions of a tactile playback waveform described in

[0015] is provided. For some playback waveforms, such as a "click," which is a very brief tactile event consisting of one or at most a few cycles of the tactile playback waveform, it may be desirable to play the startup and steady-state portions of the tactile playback waveform itself in open-loop mode (ATTEN = 0). The reason for playing back the startup and steady-state portions of the tactile playback waveform in open-loop mode is that the designer of the tactile event may not want to design a waveform with a somewhat brief, unpredictable negative impedance / resistance feedback correction term during the waveform itself. That is, to minimize the ringing decay time after the click event, the designer may want to switch the transducer drive system 700 from open-loop mode to a negative impedance / resistance closed-loop mode (ATTEN = 1). In this case, the transition of the feedback correction signal from fully off (ATTEN = 0) to fully on (ATTEN = 1) can be implemented via a step function, or perhaps via a very short ramp from one level to the next. In this way, the click can be "played" as designed and end faster than the open-loop drive amplifier would allow. This switching from open-loop mode to closed-loop mode can provide a “crisp” tactile effect (e.g., when electromechanical load 701 is a linear resonant actuator) that serves as effective tactile feedback to indicate to a user of a device including electromagnetic load 701 that some of the user's input (such as, for example, tapping a specific letter of the alphabet when composing a text message) has been confirmed.

[0080] As another example, a designer of haptic events in a device including electromagnetic load 701 may wish to operate a long "buzz" event entirely within a negative impedance / resistance closed-loop operating mode (ATTEN=1). The reason for this desire for a long "buzz" is that in closed-loop mode, the start and stop times of such an event can be minimized, contributing to a more pleasing haptic effect for the user, and furthermore, the steady-state envelope level of the buzz (typically played at the resonance of electromagnetic load 701) can be more consistently maintained at a target level across unit-to-unit and temperature variations. If, during a long buzz event, a sudden input from a user who may be pressing a virtual button of the device including electromagnetic load 701 is acknowledged by a click haptic effect, without much delay between the user input and the click haptic effect, it may be desirable to interrupt the long buzz by quickly decreasing the value of ATTEN, then playing an open-loop click effect with ATTEN=0, and then increasing ATTEN to 1 to quickly stop the click after playing, and then returning to complete the duration of the long buzz. At each of these transitions, the level of ATTEN must be managed.

[0081] It may also be necessary to manage the levels of the playback waveform itself, as well as the pilot signal (required for impedance / resistance estimation to close the control loop), in order to provide the most seamless haptic experience to the user.

[0082] Figure 10 A graph showing an example acceleration versus time curve of the moving mass of an electromagnetic load 701, according to an embodiment of the present disclosure, illustrates issues associated with varying the level of negative impedance feedback correction of the example transducer drive system 700 (e.g., via the value of ATTEN). In particular, Figure 10 This illustrates a particular problem that can be associated with switching between open-loop and closed-loop modes—not only are the amounts of time spent during actuation and braking / inhibiting different, but the drive waveform levels during steady state are also different. If, for whatever reason, there is a motivation to switch between open-loop and closed-loop output drive, and the product designer wishes to maintain (at least) a constant haptic effect in steady state, then the product designer must compensate in some way. Figure 10 A wide range of level differences is shown.

[0083] Figure 11 : shows the raw transducer drive signal x′(t) and the sense terminal voltage V at various levels of the control signal ATTEN of an example transducer drive system 700 according to an embodiment of the present disclosure. T An example graph of the transfer function between (t) (which may represent the back EMF of the electromagnetic load 701). Figure 11 By revealing the transfer function V T (t) / x′(t) varies as a function of the level of negative impedance / resistance feedback correction applied, illustrating the above reference to Figure 10 The reason why the problem discussed may exist. In particular, when the amount of negative impedance / resistance feedback correction is modulated by the ATTEN scalar of multiplier 715, then the sense terminal voltage V T The ratio of (t) to the raw transducer drive signal x'(t), which may be referred to as sensitivity, increases monotonically as a function of increasing ATTEN.

[0084] Brief turn Figure 12 , Figure 12 An example of an electromagnetic load 701 modeled as a linear system is shown, according to an embodiment of the present disclosure, the linear system including electrical models of electrical components 1202 and mechanical components 1204, and including a negative resistance resistor 1206 having a negative impedance Re_neg inserted in series with the electromagnetic load 701. The addition of the negative impedance Re_neg can reduce the quality factor q because it effectively subtracts from the DC resistance Re, thereby reducing the overall DC electrical impedance.

[0085] In fact, there is no negative resistor. Instead, the example transducer drive system 700 can be configured to operate substantially similar to Figure 12 the circuit shown, including a mathematical model of a negative impedance Re_neg in series with a mathematical model of an electromagnetic load 701. In operation, if a physical resistor with a negative impedance Re_neg could actually be placed in series with the electromagnetic load 701, the example transducer drive system 700 (e.g., at the output of the combiner 726) could actually calculate the voltage V m that would appear at the junction of the negative impedance Re_neg and the DC resistance Re, as Figure 12 shown. The calculated voltage V m can then be used to drive the electromagnetic load 701.

[0086] In essence, the example transducer drive system 700 can implement a sensorless speed control feedback loop for the electromagnetic load 701. The feedback loop can use a dynamic estimate of the parameters of the electromagnetic load 701 and generate feedback (e.g., the negative impedance Re_neg) to cancel out most of the electrical and mechanical impedance of the electromagnetic load 701. In the case of the DC coil resistance Re, its estimate must be very accurate (e.g., < 1% error) for the feedback loop of the example transducer drive system 700 to achieve stability and achieve the desired negative impedance effect. The electrical and mechanical impedance of the electromagnetic load 701 can change in response to stimuli applied to it (e.g., the amplitude and frequency of the drive signal V(t)), ambient temperature conditions, and / or other factors.

[0087] See Figure 12 , the sensitivity H of the electromagnetic load 701 can be given as:

[0088]

[0089] At mechanical resonance, Zmech = res and |Zle| << Re_neg, so the sensitivity at resonance H0 can be given by:

[0090]

[0091] Therefore, to compensate for the problem of changing the level of the negative impedance / resistance feedback correction via the control signal ATTEN, the scale factor REF_LEVEL that will be applied to the haptic playback waveform can be given by:

[0092]

[0093] Thus, the scaling factor REF_LEVEL can be a compensation term that can be used to compensate or normalize the output drive signal as a function of the control signal ATTEN (particularly at resonance, but this is typically the frequency that drives the haptic playback waveform during steady state).

[0094] Figure 13 Example waveforms of an example transducer drive system 700 illustrating the benefits of compensation are shown according to an embodiment of the present disclosure. In particular, Figure 13 The left side of the graph depicts the sensitivity V not normalized by the scale factor REF_LEVEL T (t) / x′(t) and the sense current I(t), and Figure 13 The right side of the graph depicts the sensitivity V normalized by the scaling factor REF_LEVEL. T (t) / x′(t) and the sensed current I(t).

[0095] Figure 14 Graph showing an example acceleration versus time curve of the moving mass of the electromagnetic load 701 with compensation for the scale factor REF_LEVEL of the example transducer drive system 700 applied, according to an embodiment of the present disclosure. Figure 10 compared to, Figure 14 It is shown that during the steady-state portion of the haptic playback waveform, the variability of acceleration as a function of the control signal ATTEN is minimized due to the compensation applied by the scaling factor REF_LEVEL. However, Figure 14 shows that even with such compensation, ringing may occur during starting, and ringing may occur during braking, depending on the value of ATTEN.

[0096] Figure 15 Graphs showing example waveforms associated with an example transducer drive system 700 illustrating the problems associated with driving very large playback waveforms during a closed-loop mode of operation, in accordance with an embodiment of the present disclosure. In particular, Figure 15The problems that may arise when driving very large raw transducer drive signals x′(t) when the negative feedback system is fully in closed-loop operation mode (e.g., ATTEN=1) are demonstrated. In existing approaches, the negative impedance / resistance closed-loop system provides significant amplification of the raw transducer drive signal x′(t) during startup and braking / inhibition in order to cause the mass of the electromagnetic load 701 to move quickly during startup and stop quickly during braking. In fact, this feature is part of the motivation for using closed-loop systems in tactile vibration products. However, when a particularly strong playback waveform is sent to such a closed-loop system, the startup and braking portions of the resulting signal sent to the amplifier may exceed the voltage levels that the amplifier can drive. In this case, signal clipping may occur, as shown by the sense terminal voltage V at voltage CLIP_LEVEL. T (t) Clipping is shown. Clipping is generally undesirable in haptic vibration systems because it can damage the electromagnetic load in the long term and produce an undesirable haptic experience in the short term. During the steady-state portion of the waveform, clipping may not be an issue (because the designer of the haptic playback waveform may have understood the amplification gain associated with the closed-loop system when selecting the level of the playback waveform to maximize the steady-state level without clipping).

[0097] One way to strike a balance between the closed-loop level in steady state and "some" closed-loop benefit during starting and braking is to dynamically adjust the ATTEN level, especially during the starting and braking portions of the playback waveform.

[0098] Figure 16 A graph of example waveforms of an example transducer drive system 700 is shown, depicting the management of the control signal ATTEN to strike a balance between preventing clipping and maintaining closed-loop steady-state performance, in accordance with an embodiment of the present disclosure. In particular, during startup, when the closed-loop system is providing significant gain, the ATTEN value can be briefly reduced to prevent clipping. The amount of reduction to the level designated as LEVEL_1 can be a function of the amplitude of the original transducer drive signal x′(t) and the voltage level at which the amplifier 706 clips. The rate at which ATTEN is reduced at startup can be a function of LEVEL_1 and the frequency of the playback waveform (e.g., most likely the resonant frequency, as it is the typical drive frequency and indicates how quickly the amplifier output will reach the point of clipping). The amount of time that ATTEN can remain at LEVEL_1, as Figure 16 , which is shown as duration_onset, can be a function of the frequency of the original transducer drive signal x'(t). Finally, just as a brief reduction in ATTEN can be provided during startup, it may also be desirable to provide another brief reduction during braking. In this case, ATTEN can be maintained at LEVEL_1 for an amount of time, such as Figure 16denoted as duration_brake, may be a function of the frequency of the original transducer drive signal x′(t).

[0099] Although the foregoing discusses applications to linear electromagnetic loads, it will be understood that systems and methods similar or identical to the disclosed systems and methods may be applied to other linear or nonlinear systems.

[0100] Furthermore, although the above contemplated the use of a negative impedance / resistance filter to implement the model of the LRA, in some embodiments, a mathematical equivalent of the LRA may be used in place of the model.

[0101] As used herein, when two or more elements are referred to as being “coupled” to each other, the term means that the two or more elements are in electronic or mechanical communication, as the case may be, whether indirectly or directly, with or without intervening elements.

[0102] This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that would be understood by a person of ordinary skill in the art. Similarly, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that would be understood by a person of ordinary skill in the art, where appropriate. In addition, in the appended claims, references to a device or system or component of a device or system adapted to, arranged to, capable of, configured to, enabled to, operable to, or operable to perform a particular function include that device, system, or component, regardless of whether it or that particular function is activated, turned on, or unlocked, as long as the device, system, or component is so adapted, arranged, capable of, configured to, enabled to, operable, or operable. Therefore, the systems, devices, and methods described herein may be modified, added to, or omitted without departing from the scope of this disclosure. For example, the components of the systems and devices may be integrated or separated. In addition, the operations of the systems and devices disclosed herein may be performed by more, fewer, or other components, and the methods described may include more, fewer, or other steps. In addition, the steps may be performed in any appropriate order. As used in this document, "each" refers to each member of a set or each member of a subset of a set.

[0103] Although exemplary embodiments are shown in the drawings and described below, the principles of the present disclosure may be implemented using any number of techniques, whether currently known or not. The present disclosure should not be limited in any way to the exemplary embodiments and techniques shown in the drawings and described above.

[0104] Unless specifically noted otherwise, items depicted in the drawings are not necessarily drawn to scale.

[0105] All examples and conditional language described herein are intended for teaching purposes to help readers understand the present disclosure and the concepts contributed by the inventors to the advancement of the art, and are to be interpreted as not being limited to these specific examples and conditions. Although the embodiments of the present disclosure have been described in detail, it should be understood that various changes, substitutions, and modifications may be made thereto without departing from the spirit and scope of the present disclosure.

[0106] Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages. Additionally, other technical advantages may become apparent to one of ordinary skill in the art after reviewing the preceding drawings and descriptions.

[0107] To assist the Patent Office and any reader of any patent issuing pursuant to this application in interpreting the appended claims, applicants wish to note that unless the phrase "means for" or "step for" is expressly used in a particular claim, they do not intend that any of the appended claims or claim elements be invoked under 35 U.S.C. §112(f).

Claims

1. A method for driving a playback waveform to an electromagnetic actuator via a transducer drive system, the method comprising: operating the transducer drive system in a first mode wherein the electromagnetic actuator is driven in a closed loop with the playback waveform to form a closed-loop voltage drive system including a negative impedance; operating the transducer drive system in a second mode in which an electromechanical actuator is driven in an open loop with the playback waveform; and An operating mode switch is provided for switching the transducer drive system to operate between the first mode and the second mode.

2. The method according to claim 1, wherein The closed-loop voltage drive system utilizes a feedback correction signal added to the playback waveform.

3. The method according to claim 2, wherein: The electromagnetic actuator comprises a haptic actuator, and wherein the playback waveform signal plays back discrete haptic events during one or more of the first mode and the second mode.

4. The method according to claim 2, further comprising: When operating in the first mode, driving a first portion of a single haptic event by the transducer drive system as the playback waveform; as well as When operating in the second mode, a second portion of the single haptic event is driven by the transducer drive system as the playback waveform. 5 . The method of claim 4 , further comprising ramping the feedback correction signal from a first level to a second level via the mode switch when transitioning between the first mode and the second mode.

6. The method according to claim 5, wherein: The playback waveform associated with the single haptic event is played completely in the second mode and then switched to the first mode at the end of the single haptic event to minimize braking time, and the method also includes controlling the transition between the first mode and the second mode by the mode switch.

7. The method according to claim 5, wherein: The playback waveform associated with the single tactile event plays each of its three phases, namely, a starting phase, a steady-state phase, and a braking phase, and the method also includes using the mode switch to control the transition between the first mode and the second mode at the phase boundaries of the three phases.

8. The method of claim 5, further comprising ramping the playback waveform from one level to another via the mode switch when transitioning between the first mode and the second mode.

9. The method according to claim 8, wherein The level of the playback waveform is a function of the feedback correction signal.

10. The method of claim 2, further comprising, during the first mode: ramping down the feedback correction signal level during a brief start-up portion of the playback waveform via the mode switch to avoid clipping of the playback waveform driven to the electromechanical actuator; and The feedback correction signal level is ramped up by the mode switch after a brief start-up portion of the playback waveform so that a desired signal level is reached during a steady-state portion of the playback waveform.

11. The method according to claim 10, wherein: The minimum level of the feedback correction signal level to which the mode switch is ramped is a function of the level of the playback waveform and the maximum level of the playback waveform at which no clipping occurs.

12. The method according to claim 10, wherein: The minimum level of the feedback correction signal to which the mode switch is ramped is a function of the level of the playback waveform and the maximum level of the playback waveform at which a user perceives an undesirable electromechanical actuator effect.

13. The method of claim 2, further comprising injecting a pilot signal into the playback waveform for controlling the level of the feedback correction signal.

14. The method of claim 13, further comprising ramping the pilot signal level from one level to another via the mode switch when transitioning between the first mode and the second mode.

15. The method according to claim 14, wherein The pilot signal level is a function of the level of the feedback correction signal such that the pilot signal level driven to the electromechanical actuator remains constant in steady state for any level of the feedback correction signal.

16. The method of claim 14, further comprising disabling the pilot signal during the second mode.

17. The method of claim 13, further comprising modulating the feedback correction signal based on an estimate of the impedance of the electromechanical actuator.

18. A transducer drive system for driving a playback waveform to an electromagnetic actuator, the transducer drive system comprising: An output terminal for generating a playback waveform; as well as A control subsystem configured to: operating the transducer drive system in a first mode wherein the electromagnetic actuator is driven in a closed loop with the playback waveform to form a closed-loop voltage drive system including a negative impedance; operating the transducer drive system in a second mode in which an electromechanical actuator is driven in an open loop with the playback waveform; and An operating mode switch is provided for switching the transducer drive system to operate between the first mode and the second mode.

19. The transducer drive system according to claim 18, wherein: The closed-loop voltage drive system utilizes a feedback correction signal added to the playback waveform.

20. The transducer drive system according to claim 19, wherein: The electromagnetic actuator comprises a haptic actuator, and wherein the playback waveform signal plays back discrete haptic events during one or more of the first mode and the second mode.

21. The transducer drive system according to claim 19, wherein: The transducer drive system is further configured to: When operating in the first mode, driving a first portion of a single haptic event as the playback waveform; as well as When operating in the second mode, a second portion of the single haptic event is driven as the playback waveform.

22. The transducer drive system according to claim 21, wherein: The mode switch is further configured to ramp the feedback correction signal from a first level to a second level when transitioning between the first mode and the second mode.

23. The transducer drive system according to claim 22, wherein: The playback waveform associated with the single haptic event plays in its entirety in the second mode and then switches to the first mode at the end of the single haptic event to minimize braking time, and wherein the mode switch is further configured to control transitions between the first mode and the second mode.

24. The transducer drive system of claim 22, wherein: The playback waveform associated with the single tactile event plays each of its three phases, namely, a starting phase, a steady-state phase, and a braking phase, and wherein the mode switch is further configured to control the transition between the first mode and the second mode at the phase boundaries of the three phases.

25. The transducer drive system of claim 22, wherein: The mode switch is further configured to ramp the playback waveform from one level to another when transitioning between the first mode and the second mode.

26. The transducer drive system according to claim 25, wherein: The level of the playback waveform is a function of the feedback correction signal.

27. The transducer drive system of claim 19, wherein: The mode switch is further configured to, during the first mode: ramping down the feedback correction signal level during a brief attack portion of the playback waveform to avoid clipping of the playback waveform driven to the electromechanical actuator; as well as The feedback correction signal level is ramped up after a brief attack portion of the playback waveform so that a desired signal level is reached during a steady-state portion of the playback waveform.

28. The transducer drive system of claim 27, wherein: The minimum level of the feedback correction signal level to which the mode switch is ramped is a function of the level of the playback waveform and the maximum level of the playback waveform at which no clipping occurs.

29. The transducer drive system of claim 27, wherein: The minimum level of the feedback correction signal to which the mode switch is ramped is a function of the level of the playback waveform and the maximum level of the playback waveform at which a user perceives an undesirable electromechanical actuator effect.

30. The transducer drive system of claim 19, further comprising a pilot tone generator configured to inject a pilot signal into the playback waveform for controlling the level of the feedback correction signal.

31. The transducer drive system of claim 30, wherein: The mode switch is further configured to ramp the pilot signal level from one level to another when transitioning between the first mode and the second mode.

32. The transducer drive system of claim 31 , wherein: The pilot signal level is a function of the level of the feedback correction signal such that the pilot signal level driven to the electromechanical actuator remains constant in steady state for any level of the feedback correction signal.

33. The transducer drive system of claim 31 , further comprising disabling the pilot tone generator from generating the pilot signal during the second mode.

34. The transducer drive system of claim 30, wherein: The transducer drive system is further configured to modulate the feedback correction signal based on an estimate of the impedance of the electromechanical actuator.

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