Patent Publication Number: US-10313787-B2

Title: Electromechanical system with predictive back-EMF protection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority is claimed under 37 CFR 1.78 and 35 USC 119(e) to U.S. Provisional Application 62/037,426, filed 14 Aug. 2014, which is incorporated by reference. 
    
    
     BACKGROUND 
     Technical Field 
     This Patent Disclosure relates generally to electromechanical systems that generate back-emf, and more particularly to providing protection from back-emf for such systems. 
     Related Art 
     A Speaker is an electromechanical system that is capable of storing energy in reactive electrical components, as well as in mechanical components like moving masses and compressed springs. 
     The amplifier drives current to the speaker coil (and passive electrical components in the speaker). Mechanical energy stored in the speaker coil and other mechanical components is transformed back into a current that travels back to the amplifier. 
     The magnitude of the back-EMF current can be large compared to driven current. As a result, the total current at the amplifier output can trigger overcurrent protection in situations where only the driven current would not. 
     One approach for protecting against back-emf current is to overdesign the amplifier to handle worst case current. This solution disadvantageous particularly because the worst case current is sporadic and seldom (based on combinations of audio and speaker). 
     While this Background information references audio speaker systems, the Disclosure in this Patent Document is not limited to such applications, but is more generally directed to predictive back-emf protection for electromechanical systems. 
     BRIEF SUMMARY 
     This Brief Summary is provided as a general introduction to the Disclosure provided by the Detailed Description and Drawings, summarizing aspects and features of the Disclosure. It is not a complete overview of the Disclosure, and should not be interpreted as identifying key elements or features of, or otherwise characterizing or delimiting the scope of, the disclosed invention. 
     The Disclosure describes apparatus and methods for predictive back-emf protection adaptable to electromechanical systems, such as providing predictive back-emf protection for an audio speaker system to limit peaking current. 
     According to aspects of the Disclosure, an electromechanical system that generates back-emf (electro-motive force) can include a signal source to provide a source signal, a signal driver receiving an input signal based on a modified source signal, and generating a drive signal, and an electromechanical transducer coupled to receive the drive signal, and to generate a transducer output response, and a back-emf signal response. The signal driver can include a signal processing module to receive the source signal, and to generate the modified source signal, and an amplifier to receive the modified source signal, and to generate the drive signal, the amplifier having a pre-defined output peak current limit. The signal processing module can include a predictive back-emf generator, and a peak current limit control loop including the predictive back-emf generator to generate the modified source signal based on; the source signal, and a feedback predictive back-emf signal, and a peak current reference corresponding to the pre-defined output peak current limit. The predictive back-emf generator can be configured to generate the predictive back-emf signal based on a pre-defined back-emf transfer function as a representation of the back-emf response of the electromechanical transducer to the modified source signal. 
     According to other aspects of the Disclosure, a signal driver circuit is configured for use in a system with a signal source to generate a source signal. and an electromechanical transducer to generate, in response to a drive signal based on the source signal, a transducer output response, and a back-emf signal response. The signal driver circuit can include an signal processing module to receive the source signal, and to generate the modified source signal, and an amplifier to receive the modified source signal, and to generate the drive signal, the amplifier having a pre-defined output peak current limit. The signal processing module can include a predictive back-emf generator, and a peak current limit control loop including the predictive back-emf generator to generate the modified source signal based on: the source signal, and a feedback predictive back-emf signal, and a peak current reference corresponding to the pre-defined output peak current limit. The predictive back-emf generator can be configured to generate the predictive back-emf signal based on a pre-defined back-emf transfer function as a representation of the back-emf response of the electromechanical transducer to the modified source signal. 
     According to Other aspects pf the Disclosure, a signal processor is operable for use in a system with a signal source to generate a source signal, a signal driver including an amplifier to generate a drive signal based on the source signal, and an electromechanical transducer to generate, in response to the drive signal, a transducer output response, and a back-emf signal response. The signal processor can include a predictive back-emf generator, and a peak current limit control loop including the predictive back-emf generator to generate the modified source signal based on: the source signal, and a feedback predictive back-emf signal, and a peak current reference corresponding to the pre-defined output peak current limit. The predictive back-emf generator can be configured to generate the predictive back-emf signal based on a pre-defined back-emf transfer function as a representation of the back-emf response of the electromechanical transducer to the modified source signal. 
     Other aspects and features of the invention claimed in this Patent Document will be apparent to those skilled in the art from the following Disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example audio system  10  with digital audio input  20 ,  21 , including a signal driver  30  with an audio processor  50 , implemented with a DSP  60  and DAC  70 , and with a voltage amplifier  80  driving a speaker  90 , such as can adapted to use predictive back-emf protection according to this Disclosure. 
         FIG. 2  illustrates an example configuration for predictive back-emf processing according to this Disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     This Description and the Drawings constitute a Disclosure for predictive back-emf protection in an electromechanical system, including example embodiments that illustrate various technical features and advantages. 
     This Disclosure is in the context of an example application of adapting predictive back-emf protection to an audio speaker system. 
     In brief overview, a predictive back-emf protection methodology is adaptable to electromechanical systems. A signal processor processes a source signal to provide a modified source signal. A driver converts the modified source signal to a drive signal, converted by an electromechanical transducer into a transducer response, including generating a resulting back-emf signal coupled back to the driver output. A predictive back-emf generator (such as a routine in the signal processor) is characterized by a back-emf transfer function (such as a linear parameterized model of the electromechanical transducer) for transforming an input signal into a transform back-emf representation of a back-emf signal predicted by the back-emf transfer function as a response of the electromechanical transducer to the modified source signal. The signal processor processes the source signal based on the transform back-emf representation to generate the modified source signal input to the driver. 
       FIG. 1  illustrates an example audio system  10  with digital audio input, such as can adapted to use predictive back-emf protection according to this Disclosure. 
     A digital audio source  20  supplies digital audio to a signal driver  30  that includes an audio processor  50 , implemented in this example as a DSP (digital signal processor)  60  and DAC (digital to analog converter)  70  and an audio amplifier  80 . The audio amplifier  80  drives a speaker unit  90 . 
     The audio system  10  can be adapted to provide protection for back-emf using predictive back-emf processing according to the Disclosure. Predictive back-emf protection is based on a linear parameterized description of the speaker (and the gain in the DAC and amplifier). For the example implementation of predictive back-emf according to this Disclosure, a predictive back-emf algorithm is executed by the DSP  60 . DSP predictive back-emf processing (predictive of back-emf) is used to modify the audio stream (digital audio source) processed in the DSP to limit amplifier current due to back-emf peaking. 
       FIG. 2  illustrates an example audio processor  500 , configured for predictive back-emf processing according to this Disclosure. Audio processor  500  implements a peak current limit control loop predictive back-emf processing  600  includes a peak current control loop predictive back-emf processing  600  protects against back-emf current triggering overcurrent protection in the audio amplifier ( FIG 1 .  80 ) when amplifier driven current would not. 
     Audio processor  500  includes a peak current limit control loop  600 , including a modified source signal generator  610  and a predictive back-emf generator  620  that provides a feedback predictive back-emf signal  621 . 
     The modified source signal generator  610  incudes additive and subtractive threshold units  611  and  612 . The threshold units  611  and  612  generate peak current limit thresholds based on an amplifier peak current threshold/reference (for amplifier  80  in  FIG. 1 ) UMAX  613 , and the predictive back-emf signal  621  generated by the predictive back-emf generator  620  (as described below): additive threshold unit  611  generates an additive peak current limit threshold, and subtractive threshold unit  612  generates a subtractive peak current limit threshold. 
     The modified source signal generator  610  includes maximum and minimum function blocks  614  and  615  that generate the modified source signal based on the additive and subtractive peak current limit thresholds  611  and  612  and the input digital audio source signal  21 . Maximum function block  614  generate a maximum function output based on the subtractive peak current limit threshold  612  and the input digital audio source signal  21 . Minimum function block  615  generates a digital modified source signal  630  based on the additive peak current limit threshold  611  and the maximum function output (which is based on the input digital audio source signal  21  and the subtractive peak current limit threshold  612 ). 
     The digital modified source signal  630  generated by the modified source signal generator  610 , based on the predictive back-emf signal  621  generated by the predictive back-emf generator  620 , is input to a DAC  700  for conversion to an analog modified source signal  510 . Modified source signal  510  is input to an audio amplifer, such as the audio amplifier  80  in  FIG 1 . This modified source signal  510  protects against back-emf current triggering overcurrent protection in the audio amplifier ( FIG. 1, 80 ) when amplifier driven current would not. 
     The predictive back-emf generator  620  implements a back-emf model corresponding to a linear model of the back-emf that predicts the back-emf based on past speaker voltage input. The example audio system in  FIG. 1  uses a voltage amplifier  80 , so that the example predictive back-emf processing to provide current limit protection is described in the voltage domain. That is, the predictive back-emf signal  621  is a current, but is expressed in terms of a corresponding voltage over the speaker resistive component, and in particular, the peak current limit threshold  613  for the amplifier is expressed in terms of a voltage UMAX. 
     The example back-emf model implemented in the predictive back-emf model uses an example speaker transfer function model that applies for the lower part of the audio spectrum, including accounting for the current flowing into a speaker. An example speaker transfer function model can be found in J. W. Marshall Leach, Introduction to Electroacoustics &amp; Audio Amplifier Design. Kendall/Hunt Publishing company 2003. 
     With this speaker transfer function model, the current into the speaker can be expressed in terms of voice coil resistance, and back-EMF: 
             i   =     u   (       1     R   E       -           (   Bl   )     2       R   E   2       ⁢     1       j   ⁢           ⁢   ϖ   ⁢           ⁢     M   MS       +     R   MT     +       1     j   ⁢           ⁢   ϖ       ⁢     C   MS               )           
where the variables corresponds to the following physical parameters: R E : voice coil resistance at DC; BI: force factor; M MS : mechanical mass of driver diaphragm assembly; C MS : mechanical compliance of driver suspension; R MT : Total mechanical damping. R MT  is given by
 
     
       
         
           
             
               R 
               MT 
             
             = 
             
               
                 R 
                 MS 
               
               + 
               
                 
                   
                     ( 
                     Bl 
                     ) 
                   
                   2 
                 
                 
                   R 
                   E 
                 
               
             
           
         
       
     
     Other speaker models can be used. 
     Adapting predictive back-emf protection according to this Disclosure allows amplifier design for expected-average operation. Predictive back-emf processing is then used to predict back-emf current peaking based on audio input, and modify the audio stream to compensate for such predicted back-emf current peaking. 
     The Disclosed predictive back-emf protection methodology is adaptable to other electromechanical system, providing protection from back-emf current peaking, such protecting batteries from current peaking. 
     The Disclosure provided by this Description and the Figures sets forth example embodiments and applications illustrating aspects and features of the invention, and does not limit the scope of the invention, which is defined by the claims. Known circuits, functions and operations are not described in detail to avoid obscuring the principles and features of the invention. These example embodiments and applications can be used by ordinarily skilled artisans as a basis for modifications, substitutions and alternatives to construct other embodiments, including adaptations for other applications.