Patent Publication Number: US-8541728-B1

Title: Signal monitoring and control system for an optical navigation sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/286,584, filed Sep. 30, 2008. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to a user interface device, and more particularly to a user interface device including an optical navigation sensor and a guard-sensor to enable the optical navigation sensor. 
     BACKGROUND 
     Optical navigation sensors are commonly used in devices, such as an optical computer mouse, trackball or touch pad, for interfacing with personal computers and workstations. One technology used for optical navigation sensors relies on sensing light reflected from a surface using an array of photosensitive elements or detectors, such as photodiodes. Generally, outputs of the individual elements in the array are combined using signal processing circuitry to detect and track motion of a pattern or image in the reflected light and from that tracking to derive the motion of the surface relative to the array. 
     The optical navigation sensor described above will receive very weak signals when tracking on dark surfaces, and is subject to signal saturation when tracking on light surfaces. When this happens, the estimation of displacements become erratic and unreliable, hence affecting the overall performance of the optical navigation sensor. 
     SUMMARY 
     A gain control circuit is used to control strength of signals from an array of photo-detectors (PDs) in an optical navigation sensor. Generally, the circuit includes a number of transimpedance-amplifiers (TIAs) each comprising an input coupled to at least one of the PDs in the array to receive a current signal therefrom and generate an automatic gain control (AGC) signal in response thereto, and a controller coupled to outputs of the number of TIAs to receive the AGC signal therefrom. The controller includes logic to execute a signal gain adjustment algorithm and to adjust a gain of a signal processor coupled to the array of PDs in response to the AGC signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and various other features of the control system and method will be apparent upon reading of the following detailed description in conjunction with the accompanying drawings and the appended claims provided below, where: 
         FIG. 1  is a functional block diagram of an optical navigation sensor including a gain control circuit; 
         FIG. 2  is a schematic block diagram illustrating a gain control loop in an optical navigation sensor; 
         FIGS. 3A and 3B  illustrate embodiments of a signal gain adjustment algorithm; 
         FIG. 4  is a schematic block diagram illustrating coupling of a transimpedance-amplifier (TIA) of the gain control circuit to an array in an optical navigation sensor; 
         FIG. 5  is a schematic block diagram of an array, signal processor and gain control circuit receiving an automatic gain control signal (AGC) from a signal strength algorithm executed in the signal processor; and 
         FIG. 6  is a schematic block diagram illustrating a plurality of TIAs of the gain control circuit, each coupled to a number of photo-detectors (PDs) located in different contiguous areas of the array. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed generally to optical navigation sensors and more particularly to a control circuit and method for use with an optical navigation sensor included in an input device to sense displacement of the device relative to a surface. 
     Optical navigation sensors, for example, an optical computer mouse, trackballs, and the like, may input data into and interface with personal computers and workstations. For purposes of clarity, many of the details of optical navigation sensors in general and optical sensors for optical devices, such as an optical computer mouse, trackball or touch pad, in particular that are widely known and are not relevant to the present control system and method have been omitted from the following description. Optical navigation sensors are described, for example, in commonly assigned U.S. Pat. No. 7,138,620, entitled, “Two-Dimensional Motion Sensor,” by Jahja Trisnadi et al., issued on Nov. 21, 2006. 
     A gain control circuit and method of the present disclosure monitors and controls strength of signals from an array in an optical navigation sensor used to sense movement of the optical navigation sensor, or device in which it is included, relative to a surface. The array, which comprises multiple photosensitive elements, such as photodiodes (PDs), determines a direction and magnitude of movement by detecting changes in a pattern of light reflected from the surface. Generally, the circuit includes a number of transimpedance-amplifiers (TIAs) each comprising an input coupled to at least one PD in the array to receive a current signal therefrom and generate an automatic gain control (AGC) signal in response thereto. A controller coupled to outputs of the TIAs adjusts or modulates gain in a signal processor of optical navigation sensor coupled to the array and/or modulates an intensity of illumination of the surface to control strength of signals from the array. 
     The signal processing method of the present disclosure is applicable to both speckle and non-speckle based optical navigation sensors comprising either one or more one-dimensional (1D) arrays or one or more two-dimensional (2D) arrays of PDs. The 2D array may be either a periodic, 2D comb-array, which includes a number of regularly spaced photosensitive elements comprising 1D or 2D periodicity, a quasi-periodic 2D array (such as one comprising Penrose tiling), or a non-periodic 2D array, which has a regular pattern but doesn&#39;t include periodicities. 
     In an embodiment, the optical navigation sensor is a speckle-based system, which senses movement based on displacement of a complex intensity distribution pattern of light, known as speckle. Speckle is the complex interference pattern generated by scattering of coherent light off a rough surface and detected by a photosensitive element, such as a photodiode, with a finite angular field-of-view (or numerical aperture). However, it will be appreciated by those skilled in the art that the method and circuit of the present disclosure is not limited to speckle-based systems, and can be used with other types of illumination, including coherent and non-coherent light sources, and images in which the signal captured by the optical navigation sensor comprises a strong spatial frequency matching a period or spacing of PDs in the array. 
     A functional block diagram of an optical navigation sensor including a gain control circuit is shown in  FIG. 1 . Referring to  FIG. 1 , an optical navigation sensor  100  generally includes a light source or illuminator  104 , such as a Vertical-cavity surface-emitting laser (VCSEL), and illumination optics  106  to illuminate a portion of a surface  108 , imaging optics  110  to map or image a pattern in light reflected by the surface, and an array  112  to sense or detect change in the pattern. Although shown in the figure as ellipses resembling a lens, the illumination optics  106  and imaging optics  110  can include any number of lenses, prisms, and reflectors to illuminate the surface  108  or the array  112 . The array  112  includes one or more two-dimensional (2D) arrays each comprising a number of photosensitive elements, such as photodiodes (PD)  114 , on which light reflected from the surface  108  is received. Current signals from PDs  114  in the array  112  are combined by a signal processor  116  to provide measurements or data (Åx, Åy data  118 ) on the magnitude and direction of displacement of the optical navigation sensor  100  or an input device, such as an optical computer mouse, in which it is included relative to the surface  108 . The optical navigation sensor  100  further includes a gain control circuit  120  for controlling the signal processor  116  and/or the illuminator  104  to automatically control strength of signals originating from the array  112 . 
     The gain control circuit will now be described in detail with reference to  FIGS. 2 and 3 , where  FIG. 2  illustrates a gain control loop of the gain control circuit in an optical navigation sensor, and  FIGS. 3A and 3B  illustrate methods of generating an automatic gain control (AGC) signal according to embodiments. 
     Referring to  FIG. 2 , in one embodiment the gain control circuit  202  includes one or more current-to-voltage converters, such as a single-ended, transimpedance-amplifier (TIA  204 ) to receive a current signal from a number of PDs  206  in an array  208  and generate an AGC signal that is coupled to a controller  210  in the gain control circuit. Each of the TIAs  204  is directly coupled to PDs  206  extending across the array  208  to substantially eliminate errors that can arise when monitoring signals originate from PD located outside a contiguous area of the array. 
     The AGC signal output by the TIA  204  is an output voltage signal given by the expression AGC=g*I IN , where g is a predetermined gain having units in volts/ampere, and I IN  is the current signal received from the PDs  206 . 
     The controller  210  includes computer circuitry or logic to execute a signal gain adjustment algorithm and to adjust a gain in a signal processor  212  coupled to the array  208  in response to the AGC signal. Generally, as in the embodiment shown, the signal processor  212  includes a number of differential transimpedance amplifiers (DIFF-TIAs  214 ) each comprising inputs coupled to number of PDs  216  in the array  208  to receive current signals therefrom and output a voltage signal (V OUT ) generated in response to a difference between the received current signals. V OUT  is given by the expression V OUT =g*(I IN+ , −I IN− ), where g is a predetermined gain having units in volts/ampere, and I IN+  is the current applied to a non-inverting input and I IN−  is the current applied to an inverting input. The signal processor  212  also includes one or more amplification stages  218  following the DIFF-TIAs  214  to amplify the voltage signals generated by the DIFF-TIAs and output quasi-sinusoidal signals or waveforms (CC, CS, SC, SS), which are further processed in the signal processor to provide data on the magnitude and direction of displacement of the optical navigation sensor relative to the surface. Where the amplification stages  218  include single ended amplifiers, as shown, the output signal (V SIG     —     OUT ) can be expressed as follows: V SIG     —     OUT =G*V IN , where G is a predetermined unitless gain of the amplifier, and V IN  is the input voltage received from the DIFF-TIAs  214 . In another embodiment (not shown), the amplification stages  218  can include differential amplifiers having a second input coupled to a predetermined reference or offset voltage. In this embodiment, the output signal is expressed as V SIG     —     OUT =G*(V IN1 −V IN2 ) where V IN1  is the voltage applied to one of the inputs and V IN2  to another. 
     In one version of this embodiment, the controller  210  is configured to output an integration time control signal to adjust or modulate an integration time over which the DIFF-TIAs  214  integrate the received current signals to generate the voltage signals, thereby adjusting gain in the signal processor  212 . If the AGC signal is too weak, below a specified or predetermined minimum, the controller  210  executing the signal gain adjustment algorithm operates to increase the time over which the DIFF-TIAs  214  integrate the received current signals, thereby increasing gain in the signal processor  212  and reducing if not eliminating errors in the displacement data. Conversely, if the AGC signal is too strong or exceeds a specified or predetermined maximum, the controller  210  decreases the time over which the DIFF-TIAs  214  integrate the received current signals, thereby avoiding errors in the displacement data that can result from saturating amplifiers in the signal processor  212 . 
     Optionally or additionally, where the signal processor  212  further includes one or more amplification stages  218  following the DIFF-TIAs  214 , the controller  210  is configured to output an amplification gain control signal to adjust or modulate gain of the amplification stages. If the AGC signal is below the predetermined minimum the controller  210 , executing the signal gain adjustment algorithm operates to increase gain in the amplification stages  218 . If the AGC signal exceeds the predetermined maximum, the controller  210  decreases gain in the amplification stages  218 . 
     In certain embodiments, the controller  210  can be configured to output an illuminator driver setpoint control signal to adjust or modulate illumination from an illuminator (VCSEL  220 ). In particular, the illuminator driver setpoint control signal is coupled to a driver (VCSEL driver  222 ) used to power the illuminator (VCSEL  220 ). The controller  210  executes the signal gain adjustment algorithm and operates the VCSEL driver  222  to increase electrical power applied to the illuminator (VCSEL  220 ), or to increase a duty-cycle of the VCSEL driver  222  if the AGC signal is below the predetermined minimum and to decrease the applied power or duty-cycle if the AGC signal exceeds the predetermined maximum. 
     Although the controller  210  and the signal gain adjustment algorithm executed therein is described above as controlling a single parameter, i.e., an integration time, amplification stage gain or illumination intensity, it will be appreciated that the controller  210  and algorithm can be operated to simultaneously or sequentially modulate one or more of these parameters to control the strength of signals from the array  208 . For example, in certain embodiments, such as those used in a wireless computer mouse, the controller  210  and algorithm can be configured to decrease power to the illuminator (VCSEL  220 ) or duty cycle of the VCSEL driver  222  if the AGC signal exceeds the predetermined maximum, thereby reducing power consumption. In the same embodiment, the controller  210  and algorithm can be configured to increase amplifier gain if the AGC signal is below a predetermined minimum, thereby increasing the dynamic range of a signal out of the array  208  while minimizing an increase in power consumption. 
     An aspect of the signal gain adjustment algorithm is the order in which adjustments to the illuminator power (or duty cycle), integration time and amplifier gain are made. An embodiment of the signal gain adjustment algorithm is illustrated in  FIGS. 3A and 3B , where  FIG. 3A  is a flowchart illustrating increase of signal strength and where  FIG. 3B  is a flowchart illustrating decrease of signal strength. Referring to  FIG. 3A , the order in the algorithm of adjustments used to increase signal is to adjust illuminator power first (block  302 ), then integration time (block  304 ), then amplifier gain (block  306 ). Illuminator power can be increased by increasing the duty-cycle or fraction of time in a given period in which power is applied to the illuminator. The amplifier gain can be increased by increasing a gain of one or more individual amplifiers or by switching one or more amplifiers into a chain of amplifiers in the amplification stage. The reverse of this order is used to decrease signal strength, thereby achieving an optimized signal-to-noise-ratio (SNR). Referring to  FIG. 3B , the order of adjustments made in the algorithm to decrease signal is to adjust amplifier gain first (block  308 ), then integration time (block  310 ), then illuminator power (block  312 ). 
     In another embodiment of signal gain adjustment algorithm (not shown), the order of adjustments can be selected to reduce power consumption. In particular, to increase signal strength the order of adjustment can be to first adjust illuminator power, then amplifier gain and then integration time (i.e., VCSEL duty-cycle). Similarly, to the embodiment described above, a reverse order can be used to decrease signal. The order of adjustments used to decrease signal strength, while reducing power consumption and maintaining reasonable SNR, is to adjust integration time (i.e. VCSEL duty-cycle), then amplifier gain and finally illuminator power. 
     In other embodiments, the TIAs of the gain control circuit can be coupled in parallel with the DIFF-TIAs of the signal processor to shared PDs in the array. By shared PDs, it is meant PDs that are coupled directly to TIAs in the gain control circuit and are coupled to DIFF-TIAs in the signal processor. In one embodiment of this version, shown in the  FIG. 4 , each of the single-ended TIAs  402  in the gain control circuit is coupled to an input of one of the DIFF-TIAs  404  in the signal processor  406  to split off and route from the PDs  408  in the array  410  to the TIA. The TIA converts the current signal to a voltage to generate the AGC signal that is then coupled to a controller (not shown in this figure), which executes a signal gain adjustment algorithm and operates to adjust or modulate the integration time of the DIFF-TIAs, gain of an amplification stage  412 , and/or intensity of illumination from an illuminator (not shown in this figure). 
     In another embodiment, shown in  FIG. 5 , the AGC signal is derived from DIFF-TIAs in the signal processor. Referring to  FIG. 5 , current signals from PDs  502  in the array  504  are converted to voltage signals by DIFF-TIAs  506  in the signal processor  508 . The voltage signals are amplified in amplification stage  510  and the result processed using a signal strength calculation  512  or algorithm to determine signal strength of quasi-sinusoidal signals (CC, CS, SC, SS). As in the embodiments described above the AGC signal derived from signal strength calculation  512  is coupled to a controller  514 , which executes a signal gain adjustment algorithm and operates to adjust or modulate the integration time of the DIFF-TIAs  506 , gain of an amplification stage  510 , and/or intensity of illumination from an illuminator (not shown in this figure). When used in speckle based optical navigation sensors this embodiment has the added advantage of enabling the gain loop to respond to reduction in signals (CC, CS, SC, SS) from the array  504  due to fading of detected speckle spatial frequencies. In addition, the AGC signal derived from signal strength calculation  512  is not affected by the stray-light, i.e., the light not reflected or scattered from a tracking surface, which may vary from part to part due to component placement tolerances. 
     The signal strength can be determined using a number of different calculations or algorithms including: (i) calculation of peak-to-peak amplitude; (ii) calculation of standard deviation; and (iii) calculation of an average of magnitudes squared of phasor vectors derived from the signals in logarithm scale (SIGLOG function) of the array. 
     Example embodiments of each of these different calculations or algorithms for determining signal strength from a comb-array in a speckle-based optical navigation sensor are described in detail below. 
     Consider a block of N sample frame pairs with T 1  and T 2  frame intervals from two sensor areas within an array (sensor 1  and sensor 2 ), each sample frame from each sensor area contains following signals output from the differential trans-impedance amplifiers:
 
{ CC,CS,SC,SS}   k,t,s  
 
where sub-index “k” denotes the location of a frame pair within the block (k=1, 2 . . . N); sub-index “t” denotes the T 1  or T 2  frame interval within a frame pair (t=T 1  or T 2 ); and sub-index “s” indicates which sensor area the signals come from (s=sensor 1 , or sensor 2 ). The corresponding in-phase (I) and quadrature (Q) signals for processing motion along two diagonal directions (“+” and “−” directions) can be derived as follows:
 
 I   +,k,t,s   =CC   k,t,s   −SS   k,t,s  
 
 Q   +,k,t,s   =CS   k,t,s   +SC   k,t,s  
 
 I   −k,t,s   =CC   k,t,s   +SS   k,t,s  
 
 Q   −k,t,s   =CS   k,t,s   −SC   k,t,s  
 
     The block-averaged motion across T 1  frame interval along the two diagonal directions can be estimated from the phase angles of the following “b-vectors”: 
               b     +     ,   x   ,     T   1           =       1     2   ⁢   N       ⁢       ∑     k   =   1     N     ⁢           ⁢       ∑     s   =     sensor   ⁢           ⁢   1         sensor   ⁢           ⁢   2       ⁢           ⁢       [         I     +     ,   k   ,     T   1           ×     I     +     ,     (     k   -   1     )     ,     T   2             +       Q     +     ,   k   ,     T   1           ×     Q     +     ,     (     k   -   1     )     ,     T   2               ]     s                         b     +     ,   y   ,     T   1           =       1     2   ⁢   N       ⁢       ∑     k   =   1     N     ⁢           ⁢       ∑     s   =     sensor   ⁢           ⁢   1         sensor   ⁢           ⁢   2       ⁢           ⁢       [         Q     +     ,   k   ,     T   1           ×     I     +     ,     (     k   -   1     )     ,     T   2             -       I     +     ,   k   ,     T   1           ×     Q     +     ,     (     k   -   1     )     ,     T   2               ]     s                         b     -     ,   x   ,     T   1           =       1     2   ⁢   N       ⁢       ∑     k   =   1     N     ⁢           ⁢       ∑     s   =     sensor   ⁢           ⁢   1         sensor   ⁢           ⁢   2       ⁢           ⁢       [         I     -     ,   k   ,     T   1           ×     I     -     ,     (     k   -   1     )     ,     T   2             +       Q     -     ,   k   ,     T   1           ×     Q     -     ,     (     k   -   1     )     ,     T   2               ]     s                         b     -     ,   y   ,     T   1           =       1     2   ⁢   N       ⁢       ∑     k   =   1     N     ⁢           ⁢       ∑     s   =     sensor   ⁢           ⁢   1         sensor   ⁢           ⁢   2       ⁢           ⁢       [         Q     -     ,   k   ,     T   1           ×     I     -     ,     (     k   -   1     )     ,     T   2             -       I     -     ,   k   ,     T   1           ×     Q     -     ,     (     k   -   1     )     ,     T   2               ]     s                 
and the block-averaged motion across T 2  frame interval along the two diagonal directions can be estimated from the phase angles of the following “b-vectors”:
 
               b     +     ,   x   ,     T   2           =       1     2   ⁢   N       ⁢       ∑     k   =   1     N     ⁢           ⁢       ∑     s   =     sensor   ⁢           ⁢   1         sensor   ⁢           ⁢   2       ⁢           ⁢       [         I     +     ,   k   ,     T   2           ×     I     +     ,   k   ,     T   1             +       Q     +     ,   k   ,     T   2           ×     Q     +     ,   k   ,     T   1               ]     s                         b     +     ,   y   ,     T   2           =       1     2   ⁢   N       ⁢       ∑     k   =   1     N     ⁢           ⁢       ∑     s   =     sensor   ⁢           ⁢   1         sensor   ⁢           ⁢   2       ⁢           ⁢       [         Q     +     ,   k   ,     T   2           ×     I     +     ,   k   ,     T   1             -       I     +     ,   k   ,     T   2           ×     Q     +     ,   k   ,     T   1               ]     s                         b     -     ,   x   ,     T   2           =       1     2   ⁢   N       ⁢       ∑     k   =   1     N     ⁢           ⁢       ∑     s   =     sensor   ⁢           ⁢   1         sensor   ⁢           ⁢   2       ⁢           ⁢       [         I     -     ,   k   ,     T   2           ×     I     -     ,   k   ,     T   1             +       Q     -     ,   k   ,     T   2           ×     Q     -     ,   k   ,     T   1               ]     s                         b     -     ,   y   ,     T   2           =       1     2   ⁢   N       ⁢       ∑     k   =   1     N     ⁢           ⁢       ∑     s   =     sensor   ⁢           ⁢   1         sensor   ⁢           ⁢   2       ⁢           ⁢       [         Q     -     ,   k   ,     T   2           ×     I     -     ,   k   ,     T   1             -       I     -     ,   k   ,     T   2           ×     Q     -     ,   k   ,     T   1               ]     s                 
where the sub-index “x” and sub-index “y” denote the X and Y coordinates of the phasor diagram in which these “b-vectors” can be displayed; and “b x ” and “b y ” are the two components of a “b-vector” in the phasor diagram.
 
     Thus, the peak-to-peak amplitude of the comb-array signals within a block of N sample frame pairs can be computed based on the following equation:
 
 A   pp =max { I   +,k,t,s   ,Q   +,k,t,s   ,I   −,k,t,s   Q   +,k,t,s } k=1,2, . . . ,N;t=T     1     ,T     2     ;s=sensor1,sensor2 −min { I   +,k,t,s   ,Q   +,k,t,s   ,I   −,k,t,s   ,Q   −,k,t,s } k=1,2, . . . ,N;t=T     1     ,T     2     ;s=sensor1,sensor2  
 
     Since these in-phase and quadrature signals are zero-mean, the standard deviation of the comb array signals within a block of N sample frame pairs can be calculated based on the following equation: 
               A   stddev     =         1     8   ⁢   N       ⁢       ∑     k   =   1     N     ⁢           ⁢       ∑     t   =     T   1         T   2       ⁢           ⁢       ∑     s   =     sensor   ⁢           ⁢   1         sensor   ⁢           ⁢   2       ⁢           ⁢     {         (     I     +     ,   k   ,   t   ,   s         )     2     +       (     Q     +     ,   k   ,   t   ,   s         )     2     +       (     I     -     ,   k   ,   t   ,   s         )     2     +       (     Q     -     ,   k   ,   t   ,   s         )     2       }                     
and the comb array signal SIGLOG function is defined as the average of the magnitudes squared of the “b-vectors” mentioned above in logarithm scale:
 
     
       
         
           
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     An embodiment of the SIGLOG function calculation is to take separate averages of the magnitudes squared of the “b-vectors” for the two orthogonal directions (“+” and “−” directions), and then take the minimum of the two averages. The SIGLOG function is this minimum in logarithm scale: 
     
       
         
           
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     In another embodiment, shown in  FIG. 6 , the gain control circuit includes multiple TIAs  602 A through  602 D, each comprising an input coupled to a number of PDs  604  located in a contiguous area of an array  606  different from PDs coupled to another of the TIAs. The controller (not shown in this figure) further includes logic to execute an algorithm or perform a calculation to determine a spatial distribution of an intensity or level of light across the array  606  using AGC signals (AGC 1 -AGC 4 ) from the TIAs  602 A- 602 D. Information on the spatial distribution of light level across the array  606  can be used, for example, to determine a height and/or angle between the array and a tracking surface relative to which it is displaced. This information or measurement can also be used to determine if the optical navigation sensor, and more particularly the array  606 , is properly assembled within manufacturing tolerances into an input device, such as a computer mouse, in which it is included. The embodiment illustrated in  FIG. 6  show a configuration in which four different single-ended TIAs  602 A- 602 D are each coupled to multiple PDs  604  in the array  606  to determine the light level or beam position across the array. However, it will be appreciated that other configurations including a greater or lesser number of TIAs or PDs coupled to each TIA can also be used without departing from the scope of the present disclosure. 
     For example, in one embodiment the TIAs  602  can include a number of TIAs coupled to one or small number of PDs  604  located near a peripheral edge of the array  606  outside of the area normally illuminated by light originating from the system illuminator and reflected from a tracking surface. These TIAs  602  coupled to PDs  604  near the edge of the array  606  can be used primarily or solely for determining a photocurrent due to stray light, i.e., light not reflected from a tracking surface, which can then be subtracted from a signal out of the TIAs  602  or DIFF_AMPs  608  to improve accuracy of the of the gain control circuit or optical navigation sensor. The TIAs  602  may include a number of TIAs coupled to one or small number of PDs  604  located near a center of the array  606  so the accuracy of the AGC signal derived from the outputs of the TIAs is less susceptible to component placement tolerances in assembly as well as changes in illuminator beam spot size. 
     In the description, for purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the control system and method of the present disclosure. It will be evident; however, to one skilled in the art that the present control system and method may be practiced without these specific details. In other instances, well-known structures, and techniques are not shown in detail or are shown in block diagram form in order to avoid unnecessarily obscuring an understanding of this description. 
     Reference in the description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the control system or method. The appearances of the phrase “one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. The term “to couple” as used herein may include both to directly connect and to indirectly connect through one or more intervening components. 
     The foregoing description of specific embodiments and examples have been presented for the purpose of illustration and description, and although described and illustrated by certain of the preceding examples, the signal monitoring method and control system disclosed herein are not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the control system and method to the precise forms disclosed, and many modifications, improvements and variations within the scope of the disclosure are possible in light of the above teaching.