Patent Publication Number: US-2020295740-A1

Title: System and method for narrow band negative feedback control

Description:
This invention was made with Government support under Government Contract Number Ordnance Technology Base Agreement No. 2016-316, Ordnance Agreement No. 1 awarded by DOTC. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Negative feedback is utilized in many systems to reduce unwanted variability in system states. For example, negative feedback can be utilized to stabilize the temperature of an object or reduce fluctuations in the operating frequency of a device (e.g., smooth the optical frequency of a laser). In certain cases, the unwanted variability can occur within a narrow frequency band of a system state, but some degree of variability is needed within different frequency bands or other states. For one example, in the operation of certain optical sensors, the light signals utilized for sensing are phase modulated, but measurement errors can occur if the intensities of the light signals are also amplitude modulated. For this case, wideband negative feedback is commonly employed, but can require high-speed components that can lead to higher electrical power requirements and higher product cost. Narrowband negative feedback control can be utilized in such systems to eliminate the intensity modulation of the light signals while preserving the phase modulation while not requiring high-speed components, thus leading to lower electrical power requirements and lower product cost. Similarly, for example, narrowband feedback control of a system state can be utilized if a digital or analog signal is operating at a fundamental frequency, and a second signal operating at a higher harmonic frequency of that signal is deemed interference and thus harmful to the performance of the system involved. In this case, narrowband negative feedback control can be utilized to remove those signals at the harmonic frequencies close to the fundamental frequency, but without impacting the fundamental frequency itself. 
     Notably, many lower cost systems have extremely limited frequency bandwidths. For example, these systems can include electronic components, actuators, modulators and the like, which are inexpensive and small. Typically, these systems have extremely limited frequency bandwidths. Also, the loop gain of these systems at the noise frequency can be severely limited if the frequency of the desired signal is close enough to the frequency of the noise signal, because the loop gain needs to be small at the frequency of the desired signal to minimize the impact of the noise. Notably, an existing technique for reducing the noise level is to increase the bandwidth of the stabilization loop. However, this technique is relatively expensive and increases the size, weight and power (SWaP) of the circuitry and components utilized. 
     For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for a technique that can be utilized to measure and control the noise in a negative feedback control system at selected frequencies in a manner that does not significantly increase the cost and SWaP of the circuitry and components utilized. 
     SUMMARY 
     Embodiments disclosed herein present techniques for controlling the noise in a high gain, narrowband, negative feedback control system in a manner that minimizes the cost and SWaP of the circuitry and components utilized. 
    
    
     
       DRAWINGS 
       Embodiments of the present disclosure can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which: 
         FIG. 1  is a diagram depicting a Bode plot representing an exemplary wideband negative feedback system that can be utilized to implement one example embodiment of the present invention. 
         FIG. 2  is a schematic block diagram of a system that can be utilized to implement one example embodiment of the present invention. 
         FIG. 3  is schematic block diagram of a system that can be utilized to implement one embodiment of the exemplary system depicted in  FIG. 2 . 
         FIG. 4  is a flow diagram illustrating a method that can be utilized to implement one example embodiment of the present invention. 
         FIG. 5  depicts an example Bode plot of a stable feedback control loop utilizing two feedback loops, which can be implemented in accordance with one example embodiment of the present invention. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to the present disclosure. Reference characters denote like elements throughout the figures and text. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of specific illustrative embodiments in which the embodiments may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense. 
       FIG. 1  is a diagram  100  depicting a Bode plot representing an exemplary wideband negative feedback system that can be utilized to reduce unwanted variability within a narrow frequency band, in accordance with one example embodiment of the present invention. Notably, as indicated in  FIG. 1 , the diagram  100  depicts the system&#39;s loop gain  102  versus frequency  104 . In the system thus represented, a signal generator generates a real signal (e.g., a desired signal plus some noise). A portion of the generated signal is tapped from the system&#39;s output and coupled to an input of a controller. The controller outputs a control signal, which is subtracted from the generated signal. 
     As indicated by the exemplary Bode plot depicted in  FIG. 1 , the system&#39;s feedback loop and its key frequency values are shown along with the frequency band of the unwanted noise (e.g., noise band to suppress)  106  centered at the frequency, f N , and the frequency of the desired signal  108  indicated as the frequency, f s . The feedback loop gain design of this system is represented by the dashed lines  110   a,    110   b.  An integrator is utilized to provide gain that rolls off with frequency at a rate of 20 dB/decade, as indicated by the dashed line  110   b.  A second order, high-pass filter with a corner frequency of approximately the noise frequency, f N , is combined with the integrator to produce a 20 dB/decade roll off ( 110   a ) for the frequencies lower than the noise frequency, f N . Notably, the rule-of-thumb for attaining a suitable degree of stability in a negative feedback system is that the system will be stable as long as the loop gain is adapted to roll off at 20 dB/decade when the loop gain passes through the unity gain crossover frequency. For example, as depicted in  FIG. 1 , two unity gain crossover frequencies, f unity_L    112   a  and f unity_U    112   b,  are shown. 
     The amount of noise attenuation provided by the negative feedback in the system  100  depicted in  FIG. 1  is approximately proportional to the feedback loop gain at the noise frequency, f N . As such, if substantially high levels of noise attenuation are required, the feedback loop gain at the noise frequency, f N , must be relatively high. Nevertheless, the maximum amount of loop gain at the noise frequency is limited by certain elements (e.g., circuits, modulators, actuators, signal propagation delays, etc.) in the control loop that produce imperfections that introduce additional, undesirable frequency responses in the feedback loop. For example, the frequencies indicated as f a    114   a  and f b    114   b  represent the corner frequencies introduced by the above-described limitations or imperfections in the control loop shown. In this case, the loop gain above the corner frequency, f b    114   b  and below the corner frequency, f a    114   a  rolls off at 40 dB/decade. If the overall loop gain at all frequencies is increased to a high enough level, the unity gain crossover frequencies f unity_L    112   a  and f unity_U    112   b  will occur when the gain roll-off is much greater than 20 dB/decade and, consequently, the control loop is likely to be unstable. Therefore, the unity gain crossover frequency f unity_U    112   b  has to be less than f b    114   b,  and the unity gain crossover frequency f unity_L    112   a  has to be greater than f a    114   a.  As such, this diagram indicates that the maximum loop gain possible at the noise frequency, f N  is limited by the corner frequencies f a    114   a  and f 114   b.    
       FIG. 2  is a schematic block diagram of a system  200 , which can be utilized to implement one example embodiment of the present invention. For example, the system  200  can be utilized to implement a stable, negative feedback control loop having a high loop gain within a narrow frequency band. Referring to  FIG. 2 , the system  200  includes a signal generator  202  coupled to a positive terminal of a subtractor  204 . For this embodiment, the signal generator  202  outputs a signal including a nominal amount of noise. More precisely, for example, the signal generator  202  outputs a wanted (e.g., desired) signal and/or wanted noise  203   a  plus an unwanted (e.g., undesired) sinusoidal signal and unwanted noise  203   b.  The output terminal of the subtractor  204  is coupled to an input terminal of a tap  206 . One (e.g., “tapped”) output of the tap  206  is coupled to an input terminal  210  of a feedback controller  208 , and a second output of the tap  206  is coupled to the output terminal  209  of the system  200 . An output terminal  212  of the feedback controller  208  is coupled to the negative input terminal of the subtractor  204 . 
     In operation of the exemplary system  200 , the signal generator  202  generates and outputs a wanted signal, and due to imperfections in the signal generation process, the signal generator  202  also generates and outputs an unwanted signal. The wanted signal can be a sinusoidal signal or noise, or both a sinusoidal signal and noise, as depicted at  203   a.  The unwanted signal can be a sinusoidal signal and/or noise in a narrow frequency band, as depicted at  203   b.  The feedback controller  208  generates a feedback signal, as depicted at  211 , which is an amplitude- and phase-controlled sinusoidal signal having an amplitude and phase that is nearly equal to that of the unwanted signal and/or noise in the narrow frequency band. The subtractor  204  subtracts the feedback signal  211  from the generated signal  203   a  and  203   b.  Notably, for this embodiment, the feedback signal  211  has nearly the same amplitude and phase as the unwanted signal. Consequently, the signal at the output terminal of the subtractor  204  includes the wanted signal and/or wanted noise  205   a  with little or no change, and a substantially amplitude-reduced unwanted sinusoidal signal and/or noise  205   b.  The tap  206  couples one portion of the output signal from the subtractor  204  to the feedback controller  208 , and also couples a second portion of the output signal from the subtractor  204  to the output terminal  209  of the system  200 . The feedback controller  208  adjusts the amplitude and phase of the feedback signal  211  to minimize the amplitude of the unwanted signal at the input terminal  210  of the feedback controller  208 , as described in detail below. 
       FIG. 3  is schematic block diagram of a system  300 , which can be utilized to implement one embodiment of the exemplary system  200  depicted in  FIG. 2 . Specifically, the system  300  depicted in  FIG. 3  includes a high-gain, ultra-narrow feedback controller  308 , which can be utilized to implement one embodiment of the feedback controller  208  depicted in and described above with respect to  FIG. 2 . Referring to  FIG. 3 , for this example embodiment, the feedback controller  308  includes a quadrature (Q) demodulator  314  and an in-phase (I) demodulator  316 . One output (Cos) of a sine/cosine signal generator  318  is coupled to an input of the quadrature (Q) demodulator  314  and a first input terminal of a first multiplier  320 . A second output (Sin) of the sine/cosine signal generator  318  is coupled to an input of the in-phase (I) demodulator  316  and a first input terminal of a second multiplier  322 . The output of the quadrature demodulator  314  is coupled to an input of a first controller  328 , and the output of the first controller  328  is coupled to the second input of the first multiplier  320 . The output of the first multiplier  320  is coupled to a first input terminal of a summer  324 . The output of the in-phase demodulator  316  is coupled to an input of a second controller  330 , and the output of the second controller  330  is coupled to a second input of the second multiplier  322 . The output of the second multiplier  322  is coupled to a second input terminal of the summer  324 . The output terminal of the summer  324  is coupled to an input of a gain stage  326 , and the output terminal of the gain stage  326  is coupled to the output terminal  312  of the feedback controller  308 . 
     In operation of the exemplary system  300 , the signal transmitted from the output terminal of the subtractor  304  is tapped ( 306 ) and coupled to both the Q demodulator  314  and the I-phase demodulator  316 . The demodulators  314 ,  316  are configured to demodulate the respective, incoming signals down to a frequency represented as f demod  which frequencies are selected to be at the noise frequency, f N  or substantially at the middle of the noise frequency band to be attenuated. The demodulated signals are then coupled to the respective controllers  328  and  330 , which are configured to control the amplitudes of the sine wave and cosine wave generated at the noise frequency, f N . Next, the amplitude-controlled sine wave signals and cosine wave signals are algebraically added by the summer  324 . The algebraically summed signal at the output of the summer  324  is then coupled to the gain stage  326 , which amplifies the incoming signal. The amplified signal is then coupled from the output terminal  312  of the feedback controller  308  to the negative input terminal of the subtractor  304 . 
       FIG. 4  is a flow diagram illustrating a method  400 , which can be utilized to implement one example embodiment of the present invention. Referring to  FIG. 4  and the example embodiments depicted in  FIGS. 2 and 3 , the exemplary method  400  begins with a signal generator generating a wanted signal and an unwanted signal ( 402 ). For example, in  FIG. 2 , the signal generator  202  generates a wanted signal that can be a sinusoidal signal or noise or both, as depicted at  203   a.  The signal generator  202  also generates an unwanted signal that can be sinusoidal and/or noise in a narrow frequency band, as depicted at  203   b.  Next, the method generates a feedback signal ( 404 ). For example, in  FIG. 2 , the feedback controller  208  generates a feedback signal that is an amplitude- and phase-controlled sinusoidal signal, as depicted at  211 . The generated feedback signal has an amplitude and phase that are nearly (e.g., substantially) equal to the amplitude and phase of the unwanted signal and/or unwanted narrowband noise. The method then subtracts the feedback signal from the wanted and unwanted signals generated by the signal generator ( 406 ). For example, in  FIG. 2 , the subtractor  204  subtracts the feedback signal  211  from the generated signal  203   a,    203   b.  Since the feedback signal  211  has substantially the same amplitude and phase as that of the unwanted signal  203   b,  the signal at the output of the subtractor  204  includes the wanted signal  205   a  with little or no amplitude change, and an unwanted signal  205   b  with a substantially reduced amplitude  205   b.  The method then couples the signal at the output of the subtractor to an input of the feedback controller ( 408 ). For example, the tap  206  “taps” a portion of the signal at the output terminal of the subtractor  204  and couples that portion to the input terminal of the feedback controller  208 . In turn, the feedback controller  208  then utilizes that portion of the output signal to generate the feedback signal ( 410 ). The tap  206  also couples a second portion of that signal (e.g., controlled signal out) to the output terminal  209  of the system  200 . The method is then terminated. 
     In summary, as depicted in  FIGS. 2 through 4 , and in accordance with an example embodiment of the present invention, a feedback controller includes a dual-phase demodulator to reduce noise by generating quadrature (Q) and in-phase (I) error signals. The error signals are integrated to generate control signals for controlling the amplitudes of quadrature and in-phase continuous (cosine/sine) wave signals. These continuous wave signals are summed together, and the summed signal is amplified and then subtracted from the controlled signal. Thus, in accordance with the above-described teachings of the present description, a stable feedback control loop is provided with a high level of loop gain within a narrow frequency band. 
     Notably, the exemplary approach described above utilizes two feedback loops, with one loop controlling the quadrature signal and the other loop controlling the in-phase signal.  FIG. 5  depicts an example Bode plot  500  that illustrates this approach. Specifically,  FIG. 5  depicts the in-phase or quadrature loop bandwidth measured at the output of the Q-phase or I-phase demodulator (e.g.,  314  or  316  in  FIG. 3 ) involved. As indicated at  502 , the signal at the demodulated frequency f demod is frequency translated down to essentially zero frequency (e.g., f demod minus the lower frequency of the noise band, or f N_L . Therefore, the frequency band of the noise signal to be controlled is from approximately zero frequency to 1/2Δf, where Δf is the entire width of the frequency band of the noise to be attenuated. Notably, although the desired signal  504  is presumably filtered out within the controller utilized, the effective frequency of the desired signal at the output of the demodulator is f demod −f s . To attenuate the noise signal at the desired level, the loop gain required at 1/2Δf can be represented as G reg . The loop gain in dB, as a function of frequency, can be approximated as 
     
       
         
           
             
               
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     For example, utilizing the narrowband negative feedback system and method described above, the center of the noise frequency band f N  can be set to 80 kHz, the signal frequency f s  set to 40 kHz, and the noise frequency band Δf set to 10 Hz. Therefore, utilizing the above-described narrowband, negative feedback control system and method, the loop gain at the frequency band of the noise at the demodulator output is approximately 58 dB, and the loop gain is significantly less than unity at the frequency of the desired signal. Therefore, it is important to note that embodiments of the present invention can achieve high attenuation of the noise signal without significantly impacting the level of the desired signal. In contrast, if the narrowband negative feedback system and method described above is not utilized, and the wideband feedback system loop gain at the frequency of the noise signal is 58 dB, then the loop gain at the frequency of the desired signal will be at about 46 dB (the frequency of the desired signal is merely one octave away from the frequency of the noise signal), which will significantly impact the desired signal. 
     Note that the above-described example embodiments assume that first order feedback loops are being utilized. However, if higher attenuation is required at the frequency of the noise signal, then in a different embodiment, a second order feedback loop could be utilized. For example, the second order loop could utilize two integrators in series, but to maintain the stability of the loop, one of the integrators would have a zero in its frequency transfer function. The zero in the frequency transfer function would function to flatten the frequency response of the second integrator before the loop gain crosses unity, and thus ensure that the loop gain rolls off at −20 dB/decade through the unity gain crossover frequency. Also note that, in other embodiments, other types of controllers can be utilized in the above-described narrowband feedback control loops, such as, for example, a proportional, integral and derivative (PID) controller. 
     It should be understood that elements of the above described embodiments and illustrative figures may be used in various combinations with each other to produce still further embodiments which are explicitly intended as within the scope of the present disclosure. 
     EXAMPLE EMBODIMENTS 
     Example 1 includes a feedback controller, comprising: a phase-sensitive quadrature controller configured to generate a first control signal associated with a controlled signal; a phase-sensitive in-phase controller configured to generate a second control signal associated with the controlled signal; a summer configured to add the first control signal and the second control signal; and a subtractor configured to subtract the summed first and second control signals from an uncontrolled signal. 
     Example 2 includes the feedback controller of Example 1, wherein the feedback controller is a narrowband feedback controller. 
     Example 3 includes the feedback controller of any of Examples 1-2, wherein the first control signal is a quadrature-phase control signal, and the second control signal is an in-phase control signal. 
     Example 4 includes the feedback controller of any of Examples 1-3, wherein the first control signal is out of phase with the second control signal. 
     Example 5 includes the feedback controller of any of Examples 1-4, wherein the first control signal is an amplitude-controlled cosine wave signal, and the second control signal is an amplitude-controlled sine wave signal. 
     Example 6 includes the feedback controller of any of Examples 1-5, wherein the controlled signal is tapped and coupled to an input terminal of the feedback controller. 
     Example 7 includes the feedback controller of any of Examples 1-6, further comprising a sine/cosine generator coupled to a reference input of a quadrature demodulator and a reference input of an in-phase demodulator, and configured to generate a cosine wave signal at the output of the quadrature demodulator and a sine wave signal at the output of the in-phase demodulator. 
     Example 8 includes the feedback controller of any of Examples 1-7, wherein the feedback controller comprises a high gain, narrowband feedback controller. 
     Example 9 includes the feedback controller of any of Examples 1-8, wherein the feedback controller comprises a negative feedback controller. 
     Example 10 includes the feedback controller of any of Examples 1-9, wherein the feedback controller includes a dual-phase demodulator. 
     Example 11 includes a method for feedback control, comprising: generating a wanted signal and an unwanted signal; generating an amplitude-controlled and phase-controlled sinusoidal feedback signal; subtracting the feedback signal from the wanted signal and the unwanted signal; outputting a controlled signal associated with the subtraction of the feedback signal from the wanted signal and the unwanted signal; and utilizing a portion of the controlled signal to generate the amplitude-controlled and phase-controlled sinusoidal feedback signal. 
     Example 12 includes the method of Example 11, wherein the outputting the controlled signal comprises a signal generator generating at least a real signal. 
     Example 13 includes the method of any of Examples 11-12, wherein the generating the amplitude-controlled and phase-controlled sinusoidal feedback signal comprises generating a cosine wave control signal. 
     Example 14 includes the method of any of Examples 11-13, wherein the generating the amplitude-controlled and phase-controlled sinusoidal feedback signal comprises generating a sine wave control signal. 
     Example 15 includes the method of any of Examples 11-14, further comprising tapping the controlled signal, and coupling the tapped controlled signal to an input terminal of a feedback controller. 
     Example 16 includes the method of any of Examples 11-15, wherein the utilizing the portion of the controlled signal to generate the amplitude-controlled and phase-controlled sinusoidal feedback signal comprises coupling the portion of the controlled signal to an input of a high gain, narrowband, negative feedback controller, and the high gain, narrowband, negative feedback controller generating the amplitude-controlled and phase-controlled sinusoidal feedback signal in response to the portion of the controlled signal. 
     Example 17 includes a system, comprising: a signal generator configured to generate a controlled signal including an unwanted signal; a feedback controller, wherein an output of the signal generator is coupled to an input of the feedback controller, wherein the feedback controller is configured to produce an output that is an amplitude- and phase-controlled sinusoidal signal having an amplitude and phase that is nearly equal to that of the unwanted signal in a narrow frequency band; and a subtractor, wherein the output of the signal generator is coupled to a first input of the subtractor, and the output of the feedback controller is coupled to a second input of the subtractor. 
     Example 18 includes the system of Example 17, wherein the subtractor is configured to subtract a signal at the second input from a signal at the first input. 
     Example 19 includes the system of any of the examples 17-18, wherein the feedback controller includes a phase sensitive quadrature demodulator configured to generate a first control signal associated with the controlled signal, and a phase sensitive in-phase demodulator configured to generate a second control signal associated with the controlled signal. 
     Example 20 includes the system of any of the examples 17-19, wherein the feedback controller comprises a high gain, narrowband, negative feedback controller. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the presented embodiments. Therefore, it is manifestly intended that embodiments be limited only by the claims and the equivalents thereof.