Patent Publication Number: US-2023136016-A1

Title: Systems, devices, and methods for a digital analog hybrid haptic effects controller

Description:
RELATED APPLICATIONS 
     This application claims the benefit of prior U.S. Application 62/810,174, filed on Feb. 25, 2019, the entire contents of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to systems, devices and methods for providing haptic effects of limited duration. In particular, the present invention is directed to providing techniques for closed loop feedback control of vibration actuators to produce well defined haptic effects of limited duration using a digital-analog hybrid controller. 
     BACKGROUND OF THE INVENTION 
     Haptic actuators for producing vibration effects, i.e., vibration actuators such as eccentric rotating masses, linear resonant actuators, piezo based actuators, etc., are conventionally used in haptically enabled devices to provide vibration effects of moderate to long durations. Such haptic effects present to a user as buzzing or vibrating sensations. Providing a buzzing sensation can be implemented through excitation of a vibration actuator for many, e.g., dozens, hundreds, or even thousands of oscillations. Such vibration effects are implemented through conventional open loop control techniques of the vibration actuators. Precise actuator control over limited durations in these circumstances is not required and would introduce unnecessary costs in device manufacture. 
     In some circumstances, it may be desirable to produce haptic effects of limited duration, wherein a vibration actuator undergoes only a few, e.g., less than ten, oscillations. Such haptic effects may present to a user as clicks rather than buzzes. These types of clicks may be desirable, for example, to provide the sensation and satisfaction of a mechanical response to a touchscreen input. Conventionally, open loop control techniques and hardware are adapted to provide these short duration clicks by, for example, implementing actuator braking. To retain a high-quality well-defined sensation with a sharp edge through open loop braking may require good actuator characterization. Deviation in the actuator from the characteristics of the open loop control scheme can result in an effect that trails off rather than ends sharply. Thus, for example, variance from a specified resonant frequency of a linear resonant actuator can result in degraded limited duration haptic effects. Conventional solutions to this problem include post manufacture characterization of actuator outputs and adjustment of open loop control parameters. 
     Inventions described herein provided improved methods of generating limited duration haptic effects in haptically enabled devices. 
     BRIEF SUMMARY OF THE INVENTION 
     Systems, devices, and methods are provided herein to accommodate closed loop feedback control of vibration actuators to produce precise haptic vibration effects of limited duration. Heretofore, closed loop feedback control has not been applied to vibration actuators because it is believed that conventional vibration effects do not require precise control. Conventional haptically enabled devices also do not include the necessary components for closed-loop control and the introduction of such components is believed to unnecessarily increase the cost of such devices. Digital-analog hybrid control systems described herein serve to inexpensively provide precise closed loop control to haptically enabled devices. 
     Embodiments hereof may include sensors, control circuits, and vibration actuators specifically configured to provide closed loop control capabilities for the production of limited duration vibration effects. Embodiments further may include devices and systems incorporating these components as well as methods of implementing closed-loop control techniques to provide limited duration haptic effects. 
     Embodiments hereof include a haptically enabled device. The haptically enabled device includes a vibration actuator; a sensor, configured to measure a motion characteristic of the vibration actuator, and to output a motion characteristic feedback signal; a digital-analog hybrid control circuit comprising an analog control circuit and at least one processor configured to control the vibration actuator to produce a limited duration haptic effect. The digital-analog hybrid controller is configured to control the vibration actuator by: generating a reference signal representing the limited duration haptic effect at the processor, providing an error signal to the analog control circuit, providing, by the analog control circuit, a command signal to the vibration actuator based on the error signal, sampling the motion characteristic feedback signal, and providing continuous adjustment of the error signal, by the processor, at the sampling frequency according to the motion characteristic feedback signal and the reference signal to cause the analog control circuit to continuously adjust the command signal to minimize an error between the reference signal and the motion characteristic feedback signal. 
     Further embodiments include a method of controlling a vibration actuator by a digital-analog hybrid control circuit comprising an analog control circuit and a processor to produce a limited duration haptic effect. The method includes generating, by the processor, a reference signal, the reference signal representing the limited duration haptic effect. The method further includes providing, by the processor, an initial error signal to the analog control circuit to cause the analog control circuit to generate a command signal for activating the vibration actuator; measuring, by a sensor over time, a motion characteristic of the vibration actuator; outputting, by the sensor, a motion characteristic feedback signal indicative of the motion characteristic; and controlling the vibration actuator to provide the limited duration haptic effect. Controlling the vibration actuator includes sampling, by the processor, the motion characteristic feedback signal, and providing continuous adjustment of the error signal, by the processor, at the sampling frequency according to the motion characteristic feedback signal and the reference signal while providing a command signal by the analog control circuit, wherein providing continuous adjustment of the error signal causes the analog control circuit to continuously adjust the command signal to minimize an error between the reference and the motion characteristic feedback signal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The foregoing and other features and advantages of the invention will be apparent from the following description of embodiments hereof as illustrated in the accompanying drawings. The accompanying drawings, which are incorporated herein and form a part of the specification, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. The drawings are not to scale. 
         FIG.  1    is a schematic diagram illustrating aspects of a haptically enabled device in accordance with embodiments hereof. 
         FIGS.  2 A and  2 B  are schematic diagrams illustrating a digital-analog hybrid control circuit implemented via an integrated circuit according to embodiments hereof. 
         FIG.  3    is a flow chart of an actuator control process consistent with embodiments hereof. 
         FIGS.  4 A and  4 B  are charts showing results of LRA testing consistent with embodiments hereof. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Specific embodiments of the present invention are now described with reference to thefigures. The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Embodiments described herein relate to haptically enabled devices. Haptically enabled devices consistent with embodiments herein may be configured as smartphones, tablet computing devices, smart watches, fitness bands, haptic enabled wearable devices, glasses, virtual reality (VR), augmented reality (AR), and/or mixed reality (MR) headsets, handheld gaming devices, personal computers (e.g., a desktop computer, a laptop computer, etc.), televisions, interactive signs, and/or other devices that can be programmed to provide a haptic output to a user. Haptically enabled devices consistent with embodiments hereof include devices having one or more vibration actuators for delivering vibration effects to the haptically enabled device. In embodiments hereof, haptically enabled devices may also include user input elements, e.g., control elements such as triggers, buttons, joysticks, joypads, touchscreens, touchpads, etc., to permit a user to interact with a computer system. Haptically enabled devices may further include peripheral devices configured to augment the capabilities of other devices, haptically enabled or not. 
     Haptically enabled devices consistent with embodiments hereof may include processing systems. Processing systems consistent with embodiments described herein include one or more processors (also interchangeably referred to herein as processors, processor(s), or processor for convenience), one or more memory units, audio outputs, user input elements, a communication unit or units, and/or other components. Processors may be programmed by one or more computer program instructions to carry out methods described herein. Communication units consistent with the present invention may include any connection device, wired or wireless, that may transmit or communicate with peripheral devices. 
     In embodiments hereof, haptically enabled devices may be provided separately from processing systems configured to provide haptic control signals to the haptically enabled device. Such haptically enabled devices include vibration actuators and the required control circuity and power sources to activate the vibration actuators. Haptically enabled devices provided separately from processing systems may be, for example, wearable devices intended for communication with a central processing system. Haptically enabled devices according to these embodiments may include wrist-bands, rings, leg-bands, finger attachments, gloves, eye-glasses, and other types of devices configured to provide haptic outputs. 
     Embodiments hereof relate to closed-loop feedback control of vibration actuators via digital-analog hybrid controllers to produce haptic effects of limited duration. Feedback control systems consistent with embodiments hereof are configured to reduce and/or minimize errors between intended haptic effects, represented by a reference signal, and an output haptic effect, represented by a motion characteristic signal. Reference signals represent haptic effects intended to be produced by vibration actuators. In response to the reference signals, feedback control systems as described herein control haptic outputs, which are measured by sensors outputting motion characteristic signals. The motion characteristic signals are used by the feedback systems to minimize errors in the haptic output. 
     As used herein “vibration actuator” refers to an actuator configured to produce a haptic effect by oscillation or vibration in response to a command signal. Vibration actuators consistent with embodiments hereof are capable of producing haptic effects by oscillating or vibrating at 1 Hz or more. Haptic effects of limited duration refer to haptic effects having a duration of less than 100 ms. The length of a limited duration haptic effect may change according to the frequency of actuator. For example, one oscillation of an actuator at 10 Hz requires 100 ms, and a limited duration haptic effect may be 100 ms or less. In contrast, at 1,000 Hz, one oscillation requires just 1 ms, and a limited duration haptic effect may encompass 15 oscillations, taking approximately 15 ms. In embodiments, limited duration haptic effects may have durations less than 100 ms, less than 50 ms, 30 ms, less than 25 ms, less than 20 ms, and/or less than 15 ms. In embodiments, limited duration haptic effects may employ vibration actuators operating between 1 Hz and 1000 Hz for durations between 15 ms and 50 ms. Selection of limited duration haptic effect durations may be performed based on the type of actuator being used, the amount of force or displacement provided by the vibration actuator, and/or by the type of effect that is sought by the designer. In embodiments, the duration of the limited duration haptic effect may be determined according to a representative transient time of the vibration actuator producing the haptic effect. Limited duration haptic effects may be produced by a vibration actuator performing anywhere between 1 and approximately 15 oscillations, where the number of oscillations delivered may be selected according to the frequency of the vibration actuator. Embodiments hereof further relate to closed-loop feedback control of vibration actuators to produce sharp haptic effects of limited duration. As used herein, “sharp haptic effects” refers to haptic effects having an abrupt cut-off at the completion of the effect. 
     In embodiments, vibration actuators consistent with embodiments hereof may include macrofiber composite actuators, capable of producing vibration effects at frequencies between 1 Hz and 10,000 Hz. In further embodiments, vibration actuators consistent with embodiments hereof may include piezoelectric material based vibration actuators, such as piezoceramic actuators, capable of producing vibration effects at frequencies between approximately 1 Hz and 10,000 Hz. In further embodiments, vibration actuators consistent with embodiments hereof may include LRAs, capable of producing vibration effects at frequencies between approximately 50 Hz and 500 Hz. Other types of vibration actuators, such as ERM actuators, configured to deliver haptic effects through vibrating components in the frequency range of 1 Hz and 10,000 Hz may be employed with embodiments hereof. 
     Some vibration actuators consistent with embodiments hereof, such as LRAs, are designed to provide a resonant response to a frequency input, and frequently have a high Q-factor or narrow bandwidth. Such actuators are constructed to minimize damping to provide greater efficiency. Thus, when provided with a command signal at the resonant frequency of the vibration actuator the vibration haptic response is maximized. To prevent wasted energy, such actuators are constructed to minimize friction and other sources of damping. When a command signal to the vibration actuator is ceased, the vibration actuator will still oscillate several times at its resonant frequency. Creating a strong haptic effect requires a commensurately powerful signal which, without damping, will cause the vibration actuator to oscillate several times before slowing to a stop. For conventional uses of vibration actuators, this is an acceptable result, as tens of milliseconds of free oscillations after cessation of a command signal does not degrade a vibration or buzzing haptic effect of several hundred milliseconds. On the contrary, tens of milliseconds of free oscillations will significantly distort an intended 15 millisecond haptic effect. 
     Closed loop control of vibration actuators that oscillate at high frequencies requires high frequency measurements of the motion of such actuators as well as high frequency control schemes. An actuator oscillating at 1000 Hz cannot be reliably controlled by a control scheme providing commands at 500 Hz. Conventional mobile devices are typically not equipped with digital signal processing circuitry sufficient to implement digital control schemes at a high frequency. While processing units specifically tailored for high frequency digital control exist, the addition of such to a mobile device may represent an unacceptable increase in expense for the mobile device. 
     Embodiments herein describe the use of hybrid digital-analog control systems configured to provide high-frequency control through the use of dedicated analog integrated circuits in combination with digital processing units. Digital processing units consistent with embodiments hereof may include the central processing units of mobile devices. Accordingly, high frequency closed loop control schemes may be added to haptically enabled devices inexpensively. 
       FIG.  1    is a schematic diagram illustrating aspects of a haptically enabled device  100  in accordance with embodiments hereof. The haptically enabled device  100  includes one or more vibration actuators  105 , an analog control circuit  102 , one or more motion characteristic sensors  107 , and a housing  101 . Optionally, the haptically enabled device  100  further includes a display  106 , at least one processor  108 , at least one memory unit  120 , one or more user input elements  110 , one or more audio outputs  109 , and one or more communication units  112 . 
     The vibration actuators  105  include actuators configured for oscillation or vibrate in response to a command signal. Vibration actuators  105  are configured to produce haptic effects when oscillating at frequencies in excess of 50 Hz. Vibration actuators  105  may include actuators configured with a spring-mass oscillatory system, such as linear resonant actuators (LRAs) and voice coil actuators. Vibration actuators  105  consistent with embodiments hereof are configured to produce oscillatory effects ranging between approximately 50 Hz and 1000 Hz. 
     Motion characteristic sensors  107  include sensors and transducers configured to measure motion. Motion characteristic sensors  107  are configured to measure a motion characteristic of the vibration actuator  105  of the haptically enabled device  100 . Motion characteristic sensors  107  include sensors configured to determine motion characteristics of actuator components. Such motion characteristics may include, for example, vector values such as displacement, force, velocity, momentum, angular velocity, angular momentum, and acceleration as well as scalar values such as speed, distance, and acceleration magnitude. Other motion characteristics may include oscillatory characteristics such as frequency, amplitude, and phase. In embodiments, direct measurement of one or more of the above motion characteristics may be used to determine values for other motion characteristics. For example, direct measurement of acceleration may be used to indirectly determine velocity and/or displacement. In some examples, system parameters may be stored in a memory for use in such determinations. For example, a system’s mass may be stored as a parameter and combined with a measurement of acceleration to permit a determination of force. In an example of a motion characteristic sensor  107 , the motion characteristic sensor  107  is configured to determine motion characteristics of a moving mass of a vibration actuator  105 . In another example, a motion characteristic sensor  107  is configured to measure strain of a spring associated with a spring-mass actuator system. 
     The motion characteristic sensor  107  may be, for example, an accelerometer. A motion characteristic sensor  107  may be implemented as an accelerometer and/or may be a transducer specifically selected for detecting motion characteristics of the vibration actuator  105  and/or may be a transducer included within the haptically enabled device  100  for other purposes. For example, haptically enabled devices  100  frequently include accelerometers for tilt-control or step-counting purposes. Such an accelerometer may provide motion characteristics information as a motion characteristic sensor  107 . In optional embodiments, a motion characteristic sensor  107  implemented as an accelerometer is oriented to detect motion in the same axis of movement as the vibration actuator  105  is oriented to produce movement. 
     An analog control circuit  102  for use in an embodiment hereof may be a collection of components configured for controlling the vibration actuators  105 . In embodiments, a control circuit  102  may include an integrated circuit containing components dedicated to providing the haptic control functionality. For example, the control circuit  102  may include an application specific integrated circuit (“ASIC”), a programmable gate array (“PGA”), a field programmable gate array (“FPGA”), system on a chip (“SoC”), or other type of integrated circuit. In further embodiments, the control circuit  102  may be implemented entirely in hardware components and may include various electronics components, e.g., capacitors, resistors, op-amps, etc., configured to perform the functionality discussed herein. 
     Optional components of the haptically enabled device  100  further include a display  106 , at least one processor  108 , at least one memory unit  120 , user input elements  110 , audio outputs  109 , and one or more communication units  112 . 
     The haptically enabled device  100  may include one or more processors  108  and one or more memory units  120 . The processors  108  may be programmed by one or more computer program instruction stored in the memory unit(s)  120 . The functionality of the processor  108 , as described herein, may be implemented by software stored in the memory unit(s)  120  or another computer-readable or tangible medium, and executed by the processor  108 . As used herein, for convenience, the various instructions may be described as performing an operation, when, in fact, the various instructions program the processors  108  to perform the operation. 
     The various instructions described herein may be stored in the memory unit(s)  120 , which may comprise random access memory (RAM), read only memory (ROM), flash memory, and/or any other memory suitable for storing software instructions. The memory unit(s)  120  may store the computer program instructions (e.g., the aforementioned instructions) to be executed by the processor  108  as well as data that may be manipulated by the processor  108 . 
     The processor  108  is configured to operate in conjunction with the analog circuit  102  to provide closed loop control of the vibration actuators  105  as a digital-analog hybrid controller, as discussed in greater detail below. 
     User input elements  110  for use with embodiments hereof may include any elements suitable for accepting user input. These may include buttons, switches, dials, levers, touchscreens, touchpads, and the like. The user input elements  110  may further include peripherally connected devices, such as mice, joysticks, game controllers, keyboards, and the like. User input elements  110  may further include cameras, radar devices, lidar devices, ultrasound devices, and other devices configured to remotely capture user gestures. 
     A communication unit  112  in accordance with embodiment hereof may include one or more devices or components configured for external communication. The communication unit may include wired communication ports, such as USB ports, HDMI® ports, A/V ports, optical cable ports, and any other component or device configured to receive or send information in a wired fashion. The communication unit may further include wireless communication devices, such as BLUETOOTH® antennas, WI-FI® antennas, cellular antennas, infrared sensors, optical sensors, and any other device configured to receive and/or transmit information wirelessly. In further embodiments, the communication unit  112  may include ultrasound speakers and microphones configured to transmit information via ultrasonic soundwaves. 
     A display  106  for use with embodiments hereof maybe a screen for providing a visual output to a user. The display  106  may include touchscreen capabilities (and therefore serve as a user input element  110  as well). The display  106  may be of any size, shape, or configuration to fit the needs of the haptically enabled device  100 . In some embodiments of haptically enabled device  100 , such as a wearable device configured for delivering haptic effects, no display  106  is required. In embodiments, the display  106  may include a head-mounted display, such as a VR, AR, or MR headset, goggles, and/or other VR/AR/MR display device. In embodiments, the display  106  may be projected, either onto a surface or for display in the air. 
     Audio outputs  109  include devices configured to provide an audio output to a user. Audio outputs  109  may include speakers as well as audio output ports, such as headphone jacks, configured for delivering an audio signal to a speaker or headphones. Audio outputs  109  may further include any hardware and/or antennas necessary for wireless transmission of audio signals, for example, via Bluetooth protocol. 
       FIGS.  2 A and  2 B  illustrate digital-analog hybrid control systems consistent with embodiments hereof.  FIG.  2 A  illustrates a digital-analog hybrid control system  111  consistent with embodiments hereof. The digital-analog hybrid control system  111  includes an analog control circuit  102  and a digital control circuit  208 . The digital control circuit  208  includes at least a processor  108 , memory unit  120 . The digital-analog hybrid control system  111  further includes an analog to digital converter (ADC)  121  and a digital to analog converter (DAC)  122 . As shown in  FIG.  2 A , the ADC  121  and DAC  122  may be part of the digital control circuit  208 . The ADC  121  and DAC  122  may be separate components and/or may have their functionality included as part of the processor  108 . In further embodiments, the ADC  121  and DAC  122  may be part of the analog control circuit  102  and/or may not be included in either the analog control circuit  102  or digital control circuit  208 . The analog control circuit  102  and the digital control circuit  208  cooperate to provide high frequency control of the vibration actuator  105  so as to produce haptic effects of limited duration. 
     The analog control circuit  102  is implemented as an integrated circuit in  FIG.  2 A . As illustrated in  FIG.  2 A , the analog control circuit  102  is an integrated circuit configured as a PID controller. The illustrated embodiment of the analog control circuit  102  is by way of example only and additional or different analog circuit components and controller schemes may be used without departing from the invention. 
     The processor  108  receives or generates a reference signal. The reference signal represents a desired haptic output. The reference signal is a time-varying signal that represents desired values of a motion characteristic measured over time. The reference signal may be a time-varying signal of any motion characteristic, including each of those discussed herein. For example, the reference signal may be an acceleration over time. The reference signal may represent the desired motion characteristics of the vibration actuator  105 . In embodiments, the reference signal may represent a desired motion characteristic of a different component of the haptically enabled device  100  that is coupled to the vibration actuator  105 . The reference signal may be generated by the processor  108  of the haptically enabled device  100 , received from the at least one memory unit  120 , and/or may be received from a source external to the haptically enabled device  100 . For example, where the haptically enabled device  100  is implemented as a wearable device, such as a bracelet, for providing haptic effects, the reference signal may be delivered to the processor  108  from a processor of a larger system with which the wearable device is associated. In embodiments, the reference signal may track the same parameter as the motion characteristic sensor  107 , e.g., the reference signal may indicate a desired acceleration over time when the motion characteristic sensor  107  is an accelerometer. In further embodiments, the reference signal may track a different parameter form the motion characteristic sensor  107 . For example, the reference signal may indicate a desired velocity over time when the motion characteristic sensor  107  is an accelerometer. 
     The processor is configured to receive a motion characteristic feedback signal  222  from the motion characteristic sensor  107 . The motion characteristic feedback signal  222  is converted from an analog signal to a digital signal by the ADC  121 . The motion characteristic sensor  107  is configured to detect, measure, and/or determine at least one motion characteristic of the vibration actuator  103  and deliver the motion characteristic feedback signal  222  based on the motion characteristic to the processor  108 . As discussed above, the motion characteristic sensor  107  may deliver a motion characteristic feedback signal  222  based on a directly measured motion characteristic, e.g., an acceleration measured by an accelerometer, and/or may deliver a motion characteristic feedback signal  222  derived from a measured motion characteristic, e.g., a velocity signal derived from an acceleration measured by an accelerometer. The motion characteristic feedback signal  222  may also be based on motion measurement of a part of the haptically enabled device  100  that is coupled to the vibration actuator  105 . Once received, the motion characteristic feedback signal  222  is converted from an analog signal to a digital signal for processing by the processor  108 . 
     The processor  108  receives (or generates) the reference signal and receives the motion characteristic feedback signal  222  and outputs the error signal  223  to the analog control circuit  102 . The processor  108  compares the reference signal to the motion characteristic feedback signal  222  to determine an error between them. Based on the error, the processor  108  generates a digital error signal that is converted by the DAC  122  to analog error signal  223  that is then delivered to the analog control circuit  102 . 
     The analog control circuit  102  receives the error signal  223 . A control portion  104  of the analog control circuit  102  acts as a PID controller on the error signal  223  to produce an unamplified command signal  224 . The unamplified command signal  224  of the control portion  104  is amplified by an amplifier portion  103  to produce a command signal  221 . 
     The command signal  221  is output to the vibration actuator  105  to cause a haptic output. As the vibration actuator  105  is driven by the command signal  221 , the haptic output of the vibration actuator  105  is measured by the motion characteristic sensor  107 . 
     The processor  108  receives the motion characteristic feedback signal  222  and compares it to the reference signal to continuously adjust the error signal  223 , and thus the command signal  221  that is output to the vibration actuator  105 , to minimize the error between the reference signal and the motion characteristic feedback signal  222 . 
     As used herein, continuous adjustment means that a signal output by the processor  108 , e.g., the error signal  223 , is adjusted on an ongoing basis during the output of that signal to adjust a haptic effect or output. For digital applications, it is understood that continuous adjustment includes repeated discrete adjustments. Continuous adjustment, as used herein, does not include the use of measurements of haptic outputs for use in the adjustment of parameters for future haptic effects, even if performed on a regular basis. In embodiments, continuous adjustment may be performed digitally at frequencies in excess of 500 Hz, in excess of 1 kHz, 5 kHz, 10 kHz, and 20 kHz. In embodiments, the motion characteristic feedback signal  222  is sampled at a frequency equal to or in excess of the continuous adjustment frequency. In embodiments, the motion characteristic feedback signal  222  is sampled at a frequency of at least two times the continuous adjustment frequency. These definitions of “continuous adjustment” apply to all embodiments and uses of this term discussed herein. 
     In the digital-analog hybrid control system  111 , the processor  108  performs the simple calculations to produce the error signal  223  based on the reference signal and the motion characteristic feedback signal  222 . These relatively simple calculations (e.g., as compared to calculations performed by the PID control of the analog control circuit  102 ) may be performed by a central processing unit found in conventional mobile devices, without the requirement of a dedicated and specialized digital signal processor. The analog control circuit  102  performs the more complex calculations of the PID control scheme, or any other suitable control scheme. The hardwired analog nature of the analog control circuit  102  permits the analog control circuit  102  to perform the control calculations more efficiently and in a cheaper package than a digital version. 
     In embodiments, the processor  108  is further configured to receive or generate an adjusted reference signal during or immediately subsequent to the performance of haptic effect. Required haptic outputs may be determined based on a user interaction with the haptically enabled device  100 , and such requirements may change on an ongoing basis. The processor  108 , which may operate as the central processing unit of the haptically enabled device  100 , may update or adjust the reference signal as required. 
     In embodiments, the processor  108  is further configured to adjust characteristics of the analog control circuit  102 . The analog control circuit  102  may be implemented as an integrated circuit having adjustable parameters, such as an FPGA. The processor  108  may be configured to adjust the parameters of the FPGA so as to adjust the parameters of a control scheme implemented by the analog control circuit  102 . In embodiments, the processor  108  may be configured to adjust the parameters of the FPGA to correspond to one from among a plurality of predefined control schemes to be implemented by the analog control circuit  102 . For example, the control scheme parameters of multiple potential FPGA programmings may be tuned to produce different control results, e.g., different gains or different damping. In embodiments, the multiple FPGA programmings may be configured for optimal performance in driving vibration actuators  105  at differing frequencies. The processor  108  may switch between the multiple FPGA programmings to select a preferred analog control circuit  102  according to the reference signal (and desired haptic effect). 
     In embodiments, the digital-analog hybrid control system  111  may include a plurality of analog control circuits  102 . Each analog control circuit  102  may differ in control scheme parameters. For example, the control scheme parameters of multiple analog control circuits  102  may be tuned to produce different control results, e.g., different gains or different damping. In embodiments, the multiple analog control circuits  102  may be configured for optimal performance in driving vibration actuators  105  at differing frequencies. The processor  108  may switch between the multiple analog control circuits  102  to select a preferred analog control circuit  102  according to the reference signal (and desired haptic effect). 
     In an additional embodiment, as illustrated in  FIG.  2 B , a digital-analog hybrid control system  311  may employ a switch  301  to permit switching between open loop and closed loop control. The digital-analog hybrid control system  311  may include each of the same elements as the digital analog control system  111 , including an analog control circuit  102 , a digital control circuit  208 , a vibration actuator  105 , and one or more motion characteristic sensors  107 . The digital-analog hybrid control system  311  further includes a switch  301  and a summation circuit  302 . 
     The digital-analog hybrid control system  311  may operate as follows. During open loop operation, the switch  301  may be in position A. During open loop operation, the switch  301  provides a direct control path between the digital control circuit  208  and the vibration actuator  105 . The digital control circuit  208  outputs a reference signal  224 . During open loop operation, the reference signal  224  is the same as the command signal  221 , which is received by the vibration actuator to control its output. 
     During closed loop operation, the switch  301  may be in position B. During closed loop operation, the switch  301  provides a path between the digital control circuit  208  and the summation circuit  302 . The digital control circuit  208  provides the reference signal  224  for closed loop operation of the analog control circuit  102 . The summation circuit  302  receives the reference signal  224  from the digital control circuit  208  and the motion characteristic signal  222  and outputs the error signal  223  as the difference between the reference signal  224  and the motion characteristic signal  222 . 
     Thus, according to the digital-analog hybrid control system  311 , the vibration actuator  105  may be controlled alternatively by open loop or closed loop control according to the requirements of the haptic enabled device  101 . 
     In embodiments, the analog control circuit  102  may implement any suitable control scheme. For example, the analog control circuit  102  may implement a lead compensation controller. Lead compensation control may be advantageous when implemented with an LRA due to lag in the LRA system at resonant frequencies. When the LRA is excited at a resonant frequency, the initial frequency response demonstrates phase lag with respect to the input signal. Lead compensation control may act to counter this lag and reduce the error between the reference signal and the motion characteristic feedback signal  222 . In other examples, the analog control circuit  102  may implement a proportional controller, a proportional derivative (PD) controller, a proportional integral derivative (PID) controller, a proportional integral (PI) controller, a lead-lag compensation controller, and/or any other appropriate controller. 
     The digital-analog hybrid control system  111  is advantageous when applied to the production of limited duration haptic effects, i.e., effects having a duration of less than 100 ms. In embodiments, limited duration haptic effects may be between 5 and 50 ms and use between 1 and 10 oscillations of the vibration actuator  105 . Because of the limited duration of the haptic effects produced by embodiments hereof, the digital-analog hybrid control system  111  operates to provide continuous adjustment of the command signal  221 . Such continuous adjustment means that the command signal  221  is adjusted based on the motion characteristic feedback signal  222  many times during even a very short haptic effect. In embodiments, the motion characteristic feedback signal  222  may capture motion of the vibration actuator  105  at a sampling frequency in excess of 500 Hz, in excess of 1 kHz, in excess of 5 kHz, in excess of 10 kHz, and/or in excess of 20 kHz. The processor  108  may perform updates to the error signal  223  at the same rate as the sampling frequency of the motion characteristic feedback signal  222 . 
     In further embodiments, different portions of a digital-analog hybrid controller may be implemented in either digital or analog forms. For example, in an embodiment, the entire control loop, including the error signal, may be implemented in analog circuitry where the digital processor supplies only the reference signal to the analog portion of the control loop. In further embodiments, the digital processor may handle a larger portion of the control loop. For example, in a control system employing a PID control scheme, the digital processor may perform the steps necessary for the P (proportional control) aspects of the control scheme, while the analog circuitry is configured to perform the steps required for the I (integral control) and D (derivative control). In such embodiments, the digital processor is configured to transmit the proportional control signal to the analog control circuit as well as either or both of the error signal and the reference signal. Further embodiments may include the digital processor and the analog circuit each performing any aspects of the implemented control scheme. 
       FIG.  3    depicts a flow chart showing a process  400  of providing closed-loop feedback control of a vibration actuator. The process  400  may be performed by a digital-analog hybrid control system  111  as discussed herein. As further discussed herein, any portion of a controller suitable for implementing process  400  may be implemented digitally and any portion may be implemented in analog. The closed loop feedback control implemented by process  400  may be understood as providing close tracking of a desired reference signal that may include sharp or abrupt starts and stops. For example, the process  400  may provide controlled damping to the controlled system so as to provide a sharp cut-off or abrupt stop to a haptic effect. As discussed above, closed loop feedback control may be used for only a portion of a delivered haptic effect, for example, to eliminate excess vibration at the end of a haptic effect. Such embodiments are consistent with the process  400  discussed below. 
     In an operation  402 , the process  400  includes receiving (or generating) a reference signal. The reference signal represents a haptic effect that the haptically enabled device is attempting to produce. The goal of the process  400  is to reduce the error between the measured haptic effect, i.e., as measured by a motion characteristic feedback signal, and a haptic effect that is intended to be produced by the reference signal. Embodiments discussed herein are well suited for producing sharp haptic effects of less than 50 ms, less than 30 ms, less than 20 ms, less than 15 ms, and less than 10 ms. 
     In an operation  404 , the process  400  includes providing an initial error signal to an analog control circuit to generate a command signal to cause the vibration actuator to deliver the limited duration haptic effect. An initial value of the error signal is selected to initiate motion of the vibration actuator and cause the limited duration haptic effect. The initial value of the error signal is determined according to the reference signal and the known characteristics of the feedback system, including at least the vibration actuator, the components that it is coupled to, and the sensor. Although feedback from the sensor will act to minimize errors between the reference signal and the motion characteristic signal (i.e., the measured haptic effect), selecting an initial error signal value close to what is necessary to achieve the desired output serves to minimize errors in the early portions of the haptic effect. 
     In an operation  406 , the process  400  includes measuring, by a sensor, one or more motion characteristics of a haptically activated component of the haptically enabled device. In embodiments, the sensor is a motion characteristic sensor as discussed herein. Motion characteristics may include vector values such as displacement, velocity, momentum, angular velocity, angular momentum, and acceleration as well as scalar values such as speed, distance, and acceleration magnitude. The motion characteristic may be measured directly or may be derived from a directly measured value. The motion characteristic sensor may be vibrationally coupled, directly or indirectly to the vibration actuator. 
     In an operation  408 , the process  400  includes outputting, by the sensor, a motion characteristic feedback signal indicative of the motion characteristic that is used for feedback control of the vibration actuator. 
     In an operation  410 , the process  400  includes providing an updated error signal to the analog control circuit. The updated error signal is based on a difference between the motion characteristic feedback signal and the reference signal. 
     In an operation  412 , the process  400  includes providing continuous adjustment of the error signal by the processor according to the motion characteristic feedback signal and the reference signal while continuously providing the command signal by the analog control circuit. The continuous adjustment of the error signal minimizes an error between the reference signal and the motion characteristic feedback signal. The motion characteristic feedback signal measures the output haptic effect and thus the continuous adjustment serves to control the vibration actuator to control the output haptic effect. The feedback system reduces and/or minimizes errors between the intended haptic effect, represented by the reference signal, and the output haptic effect, represented by the motion characteristic signal. In embodiments, continuous adjustment of the command signal is performed at a rate equaling that of the rate at which the motion characteristic feedback signal is sampled. 
     The above describes an illustrative flow of an example process  400  of providing closed loop control of a vibration actuator to produce limited duration haptic effects, according to embodiments described herein. The process as illustrated in  FIG.  3    is exemplary only, and variations exist without departing from the scope of the embodiments disclosed herein. The steps may be performed in a different order than that described, additional steps may be performed, and/or fewer steps may be performed. 
     Systems and methods consistent with the closed loop control schemes described herein may permit the use of actuators having wider variance of characteristics than is conventional. As discussed above, accurate open loop control of actuators requires that the actuator characteristics fall within a narrow range. Characteristics outside of that range will result in aberrant behavior from the actuators. The use of closed loop controllers consistent with those described herein, however, permit actuators with out of specification characteristics to perform as well as actuators that are within specifications. This wider range of acceptable characteristics permits the use of less expensive actuators. 
     Experiments were performed on twenty LRAs, ten of which were within specification and ten of which failed quality control.  FIG.  4 A  shows resonant frequencies of the twenty LRAs, illustrating ten within specification and ten out of specification.  FIG.  4 B  shows the acceleration response of five LRAs within specification and 5 LRAs out of specification in producing a sharp limited duration haptic effect. These LRAs were controlled via closed loop control schemes consistent with embodiments hereof. As shown in  FIG.  4 B , both the in specification and out of specification LRAs showed excellent limited duration responsiveness. Thus, there are provided systems, devices, and methods of using digital-analog hybrid closed loop control systems to provide precise control of vibration actuators during limited duration haptic effects. The precise control methods enabled by embodiments herein permit the production of limited duration haptic effects having sharp or abrupt finishes. While various embodiments according to the present invention have been described above, it should be understood that they have been presented by way of illustration and example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. It will also be understood that each feature of each embodiment discussed herein, and of each reference cited herein, can be used in combination with the features of any other embodiment. Stated another way, aspects of the above methods of rendering haptic effects may be used in any combination with other methods described herein or the methods can be used separately. The following paragraphs describe additional aspects and embodiments of the invention.