Patent Publication Number: US-8538040-B2

Title: Drivers and methods for driving a load

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation in part of International Application PCT/US2008/052105, with an international filing date of Jan. 25, 2008, which International Application claims the benefit of U.S. Provisional Application No. 60/886,746, filed Jan. 26, 2007. Both previously referenced applications are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates to drivers and methods for driving a load such as a loudspeaker. 
     BACKGROUND OF THE INVENTION 
     Most audible devices rely upon some form of loudspeaker transducer to transform electrical signals into acoustic waves. These transducers are anything but perfect devices, and introduce numerous forms of distortion into the transformation process. One particularly troublesome characteristic of most loudspeakers is the fact that the impedance is non-linear with respect to both frequency and excitation level. A small variation in the loudspeaker can yield a major variation in perceived performance. 
     Prior systems utilize either voltage or current control to address the variable impedance presented to a driver by a loudspeaker. However, controlled acoustic power remains an elusive goal. Generally, a loudspeaker transducer&#39;s impedance increases as the frequency applied to the transducer decreases. Accordingly, a voltage-controlled amplifier driving a loudspeaker transducer is limited by the increasing impedance in that, below a certain frequency, the current put through the increased impedance is too low to produce acceptable levels of sound. A current-controlled amplifier is able to produce sound at these lower frequency, higher transducer impedance points, but suffers from a risk of ruining the loudspeaker. As the impedance increases and the amplifier continues to put out constant current, the voltage can rise unacceptably high, blowing out the speaker. 
     Accordingly, an improved method for controlling a signal applied to a loudspeaker transducer is needed. 
     SUMMARY 
     Aspects of the present invention relate to methods and devices for controlling a command signal applied to a load. According to one aspect of the present invention, current through and voltage across a load are determined and the values of both are used to generate a hybrid control signal. For example, the hybrid control signal may be generated by taking a weighted summation of the current and voltage control signals. A percentage of the difference between the current and voltage control signals may also be added to one of the current or voltage control signals to generate the hybrid control signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a driver according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of a driver according to an embodiment of the present invention. 
         FIG. 3  is a schematic diagram of a system according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide methods and devices for controlling a command signal applied to a load. While embodiments of the present invention may be advantageously used to control command signals applied to a loudspeaker transducer, it will be appreciated that embodiments of the present invention may be used to control a signal applied to any kind of load, particularly loads presenting a variable impedance to an amplifier. Embodiments of the present invention advantageously combine current and voltage control to generate a hybrid control signal representing aspects of both current and voltage control. For example, in some embodiments the hybrid control signal is generated by taking a weighted summation of the current and voltage control signals. In some embodiments, controlled constant electrical power is applied to the load. Certain details are set forth below to provide a sufficient understanding of embodiments of the invention. However, it will be clear to one skilled in the art that embodiments of the invention may be practiced without various of these particular details. In some instances, well-known circuits, digital blocks, control signals, timing protocols, audio elements, and software operations have not been shown in detail in order to avoid unnecessarily obscuring the described embodiments of the invention. 
     By applying hybrid control, some embodiments of the present invention advantageously allow for a loudspeaker to reproduce lower frequencies than would be obtainable using either voltage control, where the current through the loudspeaker may become too small to allow for proper operation or current control, where the danger of blowing out the loudspeaker may limit the loudspeaker operation. 
       FIG. 1  shows a schematic block diagram of a controlled driver  10  according to an embodiment of the present invention. An input signal is applied to a command resistor  20  and then coupled to an amplifier  25 . The amplifier  25  produces a command signal to be applied to a load  30 . As described above, the load  30  may include a loudspeaker transducer or other variable impedance load. A current sensor  32  measures a current through the load  30  and develops a current control signal indicative of the current through the load. Although the current sensor  32  in  FIG. 1  is shown coupled between the load  30  and ground, it is to be understood that the current sensor  32  may take on a different configuration, or be coupled to a different reference voltage, so long as it produces a current control signal indicative of the current through the load. A voltage sensor  35  measures a voltage across the load  30  and develops a voltage control signal indicative of the voltage across the load. The voltage control signal and the current control signal are both received by a controller  40 . The voltage control signal and current control signals may be, for example, voltages or currents. The controller  40  produces a hybrid control signal based on a combination of the voltage control signal and the current control signal. The hybrid control signal is applied to a feedback resistor  45  and ultimately adjusts the command signal applied by the amplifier  25  to the load  30 . 
     The controller  40  may develop the hybrid control signal based on the current and voltage control signals in a variety of ways. If the controller  40  passes the current control signal only, the driver  10  operates as a current controlled driver. If the controller  40  passes the voltage control signal only, the driver  10  operates as a voltage controlled driver. In embodiments of the present invention, the hybrid control signal developed by the controller represents a combination of both the voltage and current control signals. In some embodiments, the controller  40  may be set to take a weighted summation of the current control signal and the voltage control signal to produce the hybrid control signal. In some embodiments, a weighted average may be taken of the current control signal and the voltage control signal. In some embodiments, the controller  40  selects the hybrid control signal to be at some point in between the values of the current control signal and the voltage control signal. That is, the controller  40  selects a point from, for example, 0 to 100 percent between the voltage control signal and the current control signal where, for example, 0 percent represents the current control signal, and 100 percent represents the voltage control signal. Generally, the controller computes a difference between the two signals and adds a certain percentage of that difference on to either the current or voltage controlled signals. Adding 70.7 percent of the difference between the current and voltage controlled signals to the voltage controlled signal will generally yield a controlled constant electric power. In other embodiments, the percentage may be different to achieve a constant power based on irregularities of the amplifier or load. In still other embodiments, a different hybrid combination of current and voltage control is used that may not yield constant electric power. In other embodiments, the percentage is between 0 and 100. In some embodiments, the percentage is 50 percent. In still other embodiments, the percentage is between 20 and 80 percent. Generally, any percentage may be used. The percentage chosen will depend on the desired amplifier performance and the characteristics of the load. 
     In some embodiments, the method used to combine the current control signal and the voltage control signal is set for the driver  10  and the driver  10  continues to utilize the same combination ratio throughout its operation. In other embodiments, the method for combining the control signals, such as how much each signal is weighted in determining the hybrid control signal, varies according to each application of the amplifier, or indeed in some embodiments is constantly adjusted during operation of the driver  10  according to the desired performance of the amplifier, characteristics of the load  30 , and/or characteristics of the audio input signal. In some embodiments, the music genre detection is used to determine how the control signals are combined—classical music may be treated differently than, for example, rap music. Additionally, the current and voltage feedback signals may be independently weighted by frequency in some embodiments. In this manner, one of the voltage or current control signals could be more heavily weighted at certain frequencies to address limitations of the loudspeakers or protect their operation. 
     The above discussion described a driver according to an embodiment of the present invention that may employ both current and voltage control using a current control signal generated by the current sensor  32  and a voltage control signal generated by the voltage sensor  35 . In some embodiments, it may be desirable to manipulate the current or voltage control signal, or both. For example, some applications may have high electromagnetic field (EMF) emissions, such as magnetic actuators. It may be desirable to reduce or eliminate the EMF emissions. Some applications may be resonant systems having high peak-to-average ratios, such as digitally-modulated radio transmitters. 
     Accordingly, as shown in  FIG. 1 , manipulators  37  or  38 , or both, may be provided to manipulate the current or voltage control signals, or both. The manipulator  37  receives the voltage control signal from the voltage sensor  35  and outputs a manipulated version of the voltage control signal. The manipulator  38  receives the current control signal from the current sensor  32  and outputs a manipulated version of the current control signal. The controller  40  may then generate the hybrid control signal based on a combination of the manipulated voltage control signal and the manipulated current control signal. In this manner, the controller  40  can be set to combine the received current and voltage sense signals in a particular manner, such as to achieve constant power control; however, the current and voltage signals it receives may be previously manipulated by the manipulators  37  and  38  to effect the resultant combination. The manipulators  37  and  38  may manipulate the respective voltage and current control signals according to any variable, including frequency, time, finite state, and the like. Accordingly, one or both of the manipulators  37  and  38  may include any type of filter, as well as one or more attenuators to reduce or block the amplitude of a signal, either entirely or in a frequency-dependent manner. 
     In one example, the driver  10  may be used to control a system or component having high back electromotive force that runs the risk of damaging the component, such as a magnetic actuator that may be found, for example on an automotive shock absorber. As the controller  40  implements a particular combination of the current and voltage control signals, high force may result if the controller  40  is compensating for a condition that will occur over a fairly long period of time (as opposed to a temporary perturbation of the system). Accordingly, the manipulators  37  and  38  may receive information from other sensors in the system, or they may simply analyze the voltage or current control signals or both to determine a chronic condition exists, and attenuate the magnitude of the current control signal coupled to the controller  40 . 
     In another example, the driver  10  may be used to control a loudspeaker responsive to an input audio signal. Some audio signals will have predictable control issues. For example, a singer having a high-pitched voice may damage a speaker if allowed to continue singing for a prolonged period of time. Accordingly, when the high-pitched singer begins, the manipulators  37  and  38  may initially allow the voltage and current control signals to couple through to the controller  40  as normal. However, after a period of time, the manipulator  38  may attenuate the current control signal applied at the frequencies of concern. 
     In still another example, the driver  10  may used in resonant systems having high peak-to-average ratios, where peak events occur that consume significantly more power than the average state, such as in CDMA modulation for cell phones. In this example, a peak event may be passed by the manipulators  37  and  38  as normal; however, after a prolonged time, the manipulator  38  may attenuate the current control signal. 
     As described generally above, information may be shared between the manipulators  37  and  38 . The manipulators  37  and  38  may also, or in addition, receive information from other components of the system that can assist in a determination of how or when to manipulate the current and voltage control signals. In some embodiments, one manipulator may be used to manipulate both the current and voltage control signals. Although an analog implementation is shown in  FIG. 1 , a digital implementation may also be used, including digital filters that may employ algorithms or digital functions for which there is no suitable analog counterpart. 
       FIG. 2  shows a schematic block diagram of a driver  150  according to an embodiment of the present invention. An input signal  100  is presented to command resister  101  which, in conjunction with feedback resistor  102 , controls the output voltage of operational amplifier  103 . The output of op amp  103  drives non-inverting power amplifier  104 , the output of which is capable of driving an output transducer  107  at the desired power. Although the invention is described in terms of “op amps,” other forms of differential amplifiers may alternatively be used, where appropriate. Additionally, various resistive elements used to implement the op amps in  FIG. 1  are not shown in the diagram of  FIG. 1  to avoid obscuring the disclosed embodiment of the invention. 
     Power amplifier  104  drives transducer  107  through resistor  105 . The resistor  105  is a current sensing resistor and may form part of an embodiment of the current sensor  32  shown in  FIG. 10   p  amp  106  may also form part of an embodiment of the current sensor  32  shown in  FIG. 1  and converts the voltage drop across  105  (proportional to the current through transducer  107 ) into a voltage indicative of current through transducer  107 . Accordingly, op amp  106  outputs the current control signal. Op amp  108 , directly measures the voltage across transducer  107  and is an embodiment of the voltage sensor  35  shown in  FIG. 1 . Op amp  108  therefore outputs the voltage control signal. The gain of op amp  106  is assumed to be whatever is required to yield the same voltage as is output from op amp  108  when transducer  107  exhibits the expected nominal impedance. In other words, no difference voltage will exist between op amps  106  and  108  when transducer  107  impedance is nominal in the embodiment shown in  FIG. 2 . 
     The controller  40  of  FIG. 1  is implemented in  FIG. 2  as a potentiometer  110  and a voltage follower  109 . The outputs of op amps  106  and  108 , the current and voltage control signals, each drive one end of potentiometer  110 . The wiper of potentiometer  110  drives voltage follower  109 , which in turn drives feedback resistor  102 . At one end of potentiometer  110  travel, op amp  109  outputs a voltage representative of the voltage across transducer  107  (controlled voltage operation); and at the other end of potentiometer  110 , op am  109  will output a voltage representative of the current through transducer  107  (controlled current operation). Due to the equivalent gains of op amps  106  and  108 , the position of potentiometer  110  will be inconsequential when transducer  107  impedance is nominal. The potentiometer operates as a voltage divider between the voltage control signal and the current control signal, and positioning the wiper at an appropriate position results in an output hybrid control signal that combines the values of the current and voltage control signals as described above. Accordingly, where 0 represents a position of the wiper yielding constant current control, and 1 represents a position of the wiper yielding constant voltage control, the wiper may be set to any intermediate position to achieve a hybrid control, as described above with reference to percentages. 
     In that op amp  109  drives feedback resistor  102 , overall amplifier loop feedback is therefore continuously variable from voltage to current control by potentiometer  110 . Potentiometer  110  may be adjusted from controlled voltage operation, through controlled power operation, to controlled current operation of the amplifier. When adjusted to reflect relative efficiency at the operating points to be linearized, availability of both voltage and current control components allow the present invention to automatically equalize transducer performance. Although an analog implementation is shown in  FIG. 2 , it should be understood that embodiments of the present invention may be implemented using digital circuits and control blocks as well. 
     The potentiometer  110  may be set at a particular level for operation of the system in, for example, controlled current, controlled power, or controlled voltage operation, or somewhere in between. In some embodiments, as described above, the potentiometer  110  may be adjusted based on characteristics of the signal applied to the load  30 , the load  30  itself, or both. For example, as described above, manipulators may be implemented to effectively change the combination of current and voltage control signals. Operation of the manipulators accordingly may dynamically determine a setting for the potentiometer  110 . 
     The drivers  10  and  150  shown in  FIGS. 1 and 2  generally may form part of an amplifier utilized in a loudspeaker system. The drivers  10  and  150  in some embodiments may form a driver for one or more loudspeakers. The drivers  10  and  150  in some embodiments may be included in a pre-driver for an amplifier system, or may reside in a modulator of an amplifier. 
     A system  300  according to an embodiment of the present invention is shown in  FIG. 3 . An audio input signal is provided to an amplifier  310 , which is configured to drive one or more loudspeakers, such as loudspeakers  320  and  330  shown in  FIG. 3 . One or more drivers according to an embodiment of the present invention is present in the amplifier  310  to receive the audio signal and drive one or both of the speakers  320  and  330  using the hybrid control methods described above. In some embodiments, however, the hybrid control method is used only to control audio signals corresponding to certain frequencies of the audio input signal, in particular embodiments, to certain low frequencies. While in some embodiments, the hybrid control methods described herein are applied to all frequencies of the audio signal, in some embodiments of the present invention the hybrid control mechanisms are applied selectively to certain frequencies, and in some embodiments lower or bass frequencies. This is because at lower frequencies, the impedance of the loudspeaker may generally be more suitable for hybrid control than at higher frequencies where the impedance curve may be less appropriate. 
     Accordingly, in some embodiments, the hybrid control techniques described are applied only to portions of an input signal corresponding to frequencies below a threshold frequency. The threshold frequency may generally be between 100 Hz up to about 6 kHz. In one embodiment, the hybrid control methods described are applied to portions of an input audio signal having frequencies at or below 2 kHz. 
     Loudspeakers may have a crossover frequency specifying the appropriate frequencies within the audio signal for individual transducers to reproduce. For example, in the embodiment of  FIG. 3 , the transducer  330  may be intended to produce bass sounds, and use of the hybrid control methods described may be advantageous below 200 Hz. The transducer  320  may receive the higher frequency portions of the audio signal and use of the hybrid control methods described may be advantageous at other frequencies for the transducer  320 , such as frequencies where the transducer  320  exhibits undesirable impedance variation. In some embodiments, the frequencies at which the hybrid control methods are applied are set based on characteristics of the loudspeaker transducers. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.