Abstract:
The present specification provides a method, apparatus and system for sensing a signal with automatic adjustments for changing signal levels. A novel fractional peak discriminator circuit is provided which can be incorporated into a system for measuring periodic signals from moving elements. The circuit can be used regardless of whether the periodic signals are detected using optics, magnetic detector or other methods.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present specification claims priority from PCT Patent Application PCT/US2010/057022 filed on Nov. 17, 2010 and U.S. Provisional Patent Application 61/262,365 filed Nov. 18, 2009, the contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present specification relates generally to signal processing and more specifically relates to a method, apparatus and system for sensing a signal with automatic adjustments for changing signal levels. 
       BACKGROUND 
       [0003]    Various methods have been used to track the rotational speed of anything from car wheels to turbine engines. There are various approaches for capturing these rotational speeds. One approach is to attach a small generator, to a shaft or other rotating device which puts out a voltage proportional to the speed the generator is turning. This approach is often not desirable as it involves an additional mechanical connection. 
         [0004]    Another approach is a magnetic tachometer having a Hall-Effect device, which changes voltage when a magnetic field passing through the tachometer changes. The voltage output is used to trigger an electronic circuit. These devices depend on the pulse being of a certain size to trigger the circuit and any attendant noise on the signal wire to be small enough not to trigger the circuit. 
         [0005]    Another approach is optical. In this case a photo transistor or photo diode senses reflected light and when the light increases or decreases, a change in current occurs in attached circuit. This current is translated into a voltage, which is captured as noted above. 
         [0006]    In these approaches there are three common factors: a. A rotating or oscillating machine; b. Coupling method (mechanical, magnetic, optical); c. A Circuit for detecting a voltage, voltage change, or current. 
         [0007]    The electronics related to these approaches are tasked with ignoring electronic noise and detecting a true signal. Various methods have been used to achieve these tasks, but common methods comprise filtering noise electronically, while ensuring the signal level would be great enough or of sufficiently different frequency not to be filtered or ignored. 
         [0008]    Of these approaches, optical systems are often selected. Optical systems include a light source and a photo sensor and a detector. The photo sensor can be, for example, a phototransistor or a photodiode or a charge coupled device (CCD). Light is emitted from the light source onto the moving part. (“Moving” captures all types of movement, including rotations and oscillations). The photo sensor captures reflections. The detector detects a difference contrast (lightness or darkness) on the moving. 
         [0009]    In general, optical approaches face the problem of having enough light illuminating something of sufficient contrast to provide a signal big enough above the ‘noise floor’ to be considered valid. More specifically, problems involved in the optical approach include sufficient light; a paint spot, marked tape, color patch, (or some other optically differentiating part of the surface that add a different reflectance to the illuminating light source, and as detected by the photo sensor) which didn&#39;t fall off, fade, become tarnished, dirty, or discolored; and an electronic circuit that was tolerant of possible changes over time, optical path changes/variations, and rotating speeds. General electronics filtering and technology can be used for these purposes. 
         [0010]    One method of handling these signals, since they do not change markedly, is to run them through a Phase-Lock-Loop. This is an electronic circuit that ‘seeks’ to oscillate in phase and at the same frequency as an incoming signal; but if there are some skips or small variations in the incoming signals, it will keep the output frequency steady. Thus, it is noise tolerant. This can be used since the signal does not change frequency suddenly. A car, for instance, will not come to a halt without braking, nor instantly go to sixty miles per hour without accelerating to that speed. (The only times things stop suddenly is because they hit something and the output of the tachometer is not likely to be instance under such circumstances.) 
         [0011]    Another signal processing method is to take the AC signal (i.e., the changing part of the signal as opposed to the average or DC signal) and compare its crests to another voltage and when the crest exceeded the comparing voltage, an output pulse would be generated by the electronics to the system monitoring the speed. However this works when the signal does not change amplitude appreciably. As the moving mechanical part speeds or slows, there is a tendency for the signal to get small, since the reflected light or magnetic field is passing quicker, but this is again a slow variation with speed. However, this requires that an operator turn a knob (variable resistor) to adjust the comparison voltage to the right level to get valid output pulses. 
       SUMMARY 
       [0012]    The present specification provides a signal conditioner circuit comprising: a filter for receiving an electrical signal generated by a sensor; said sensor configured to detect periodic movement and generate said electrical signal based on said periodic movement; said filter for generating a filtered signal having a peak where said electrical signal is greatest; a peak detector configured to receive said filtered signal and to detect said peak and generate a detected-peak signal that holds said peak; a peak-divider configured to receive said detected-peak signal and to divide said detected-peak signal by a predetermined amount and thereby generate a divided-peak signal; a comparator configured to receive said divided-peak signal and said filtered signal; said comparator configured to generate and output a pulse when a comparison between said divided-peak signal and said filtered signal results in a determination that said filtered signal exceeds said divided-peak signal. 
         [0013]    The circuit can further comprise a pulse generator configured to generate a further pulse, based on said pulse; said further pulse conditioned for monitoring equipment. 
         [0014]    The sensor can be an optical sensor. 
         [0015]    The sensor can be a magnetic sensor. 
         [0016]    The periodic movement can be rotational. Each peak can represent a single rotation, or a plurality of said peaks can represent a single rotation. The periodic movement may correspond to rotation of a turbine in a jet engine. 
         [0017]    The periodic movement can be oscillatory. The periodic movement can correspond to reciprocating movement of a piston. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  shows a signal sensing system in accordance with a first embodiment. 
           [0019]      FIG. 2  shows a signal sensing system in accordance with another embodiment. 
           [0020]      FIG. 3  shows a block diagram of an exemplary implementation of the signal conditioner circuit of  FIG. 1  and  FIG. 2 . 
           [0021]      FIG. 4  shows a specific circuit diagram giving an example of how the block diagram of  FIG. 3  can be implemented. 
           [0022]      FIG. 5  shows another specific circuit diagram giving an example of how the block diagram of  FIG. 3  can be implemented. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0023]    Referring to  FIG. 1 , a signal sensing system is indicated generally at  50 . System  50  comprises a light source  54 , a light sensor  58  and a signal conditioner circuit  62  connected to the light sensor via a link  66 . A moving element  70  is provided having an optical marker  74  disposed thereon. In a present example, moving element  70  is rotating in the direction “A”. 
         [0024]    Light source  54  emits light  78  which is reflected off the surface of element  70 . Different amounts of light are reflected each time marker  74  passes in front of light  78 . Light source can be from a fibre optic cable, or ambient light, or other source. Light sensor  58  captures reflected light  79  as it is reflected from element  70 . Link  66  may be a signal wire or even a radio channel. In general link  66  is configured to introduce as little error as possible to any signal  80  captured by light sensor  58 . 
         [0025]    System  50  is a simplified illustrative example. It is to be understood however that the term “move” and its variants (e.g. “moving”) can refer to any type of movement, including rotation and oscillation. A piston is an example of an oscillating element. In a practical application, moving element  70  can be, for example, an engine turbine used on an aircraft. On review of this specification, other practical applications will occur to those skilled in the art. 
         [0026]    Notable characteristics of a periodic signal from a rotating or oscillating object, such as element  70 , are that a periodic signal does change markedly in amplitude or frequency and that most noise is at most about sixty percent of the real signal amplitude. (If such characteristics are not met, the desired functionality from the rotating element will be nearly unworkable in any case.) Based on this characteristic, circuit  62  can be configured to capture the present peak signal, and then a fixed fraction of that signal can be used to validate a rotational signal. Such a circuit  62  can be configured to adapt to the time, speed, and optical/magnetic variations that can occur in tachometer systems. 
         [0027]    Optical marker  74  is any type of contrasting mark such as a reflective tape, or paint, which changes the level of reflected light  79 . 
         [0028]    Light sensor  58  can be implemented as a phototransistor, photodiode, or charge couple device (CCD) or the like. Light sensor  58  captures reflected light  79  and generates an electrical signal  80  (e.g voltage or current) that is substantially proportional to the amount of reflected light  79  captured by light sensor  58 . 
         [0029]    Signal conditioner circuit  62  receives the electrical signal from sensor  58 . Signal conditioner circuit  62 , which may be referred to as a fractional peak discriminator circuit, processes the electrical signal from sensor  58  and outputs an output signal to monitoring equipment (not shown). 
         [0030]    Signal conditioner circuit  62  will be discussed in greater detail below. However, before proceeding further it is to be understood that other types of sensing modalities may be used to obtain the electrical signal that is processed by signal conditioner circuit  62 . Referring now to  FIG. 2 , another signal sensing system is indicated generally at  50   a.  System  50   a  is a variant on system  50 , and so like elements bear like references except followed by the suffix “a”. 
         [0031]    Of note is that in system  50   a , light source  54  and marker  74  are eliminated. In their place, a magnetic element  75   a  is provided on the surface of moving element  70   a.  Magnetic element  75   a  emits a magnetic field  81   a , which is periodically detected by a magnetic sensor  58   a , used in place of optical sensor  58 . Magnetic sensor  58   a  is thus configured to generate an electrical signal  80   a . Magnetic sensor  58   a  can be based on a hall-effect detector, in which case a voltage signal is generated that is proportional to the detected magnetic field  81   a.  Magnetic sensor  58   a  thus generates an electrical signal  80   a  that is received by signal conditioner circuit  62   a.    
         [0032]      FIG. 3  shows a block diagram representing a possible implementation for signal conditioner circuit  62  (or signal conditioner circuit  62   a ). Signal conditioner circuit  62  comprises a filter  100  which receives signal  80   a  via link  66 . Filter  100  input sends filtered signal  102  to a peak detector  104  and a comparator  108 . The detected-peak signal  106  from peak detector  104  provides input to peak divider  112 . The divided-peak signal  114  provides a second input to comparator  108 , which is configured to make a comparison between filtered signal  102  and divided-peak signal  114 . As will be discussed further below, comparator  108  will generate a pulse when a comparison  109  results in a determination that filtered signal  102  exceeds divided-peak signal  114 . The compared-signal  110  outputted from comparator  108  provides input to pulse generator  116 . Generated-pulse signals  118  are outputted from pulse generator  116  and provide input to monitoring equipment (not shown). 
         [0033]    The operation of signal conditioner circuit  62  will now be discussed in greater detail, which will also provide further understanding as to how signal conditioner circuit  62  may be constructed. As noted above, signal  80  comes from a photo-sensor  58  or magnetic sensor  58   a  or other type of voltage or current-generating device. 
         [0034]    It is contemplated that system  50  may be located within a noisy environment and so noise may be introduced on link  66  or elsewhere, resulting in the acquisition of noise on link  66  (or elsewhere) which will be outside the frequency of interest for the purposes of the tachometer. Accordingly, signal  80  is filtered at filter  100  as a precautionary design practice to reduce or eliminate frequencies outside those of interest. “AC bypassing” techniques and “RC Filtering” techniques can be usual for these purposes. To add dynamic range, it can be desired to amplify the filtered version of signal  100  before doing any peak detection. 
         [0035]    Filter  100  is thus configured to generate filtered signal  102 , which is “well behaved”, in that it signal  102  shows a crest or peak where the optical or magnetic return is greatest. N system  50  or system  50   a , this crest or peak may be a once per revolution. However, multiple peaks may occur where a plurality of markers  74  (or magnetic elements  75   a ) are employed. It will now be apparent that the number of markers  74  (or magnetic elements  75   a ) can be selected according to the different design specifications for system  50  or system  50   a.    
         [0036]    Peak detector  104  comprises a peak-and-hold circuit, which can be implemented through the use of an operational amplifier to impress a voltage on a capacitor as the filtered signal  102  rises to a peak. A diode can also be provided to prevent (or at least reduce the likelihood of) the capacitor from discharging as the voltage declines from the peak, thus storing a voltage charge on the capacitor substantially equal to the peak of the input and filtered signal. This voltage level is transmitted to a voltage buffering circuit (also known as a voltage follower) which isolates the capacitor from discharging. 
         [0037]    Peak divider  112  then buffers and divides the detected peak signal  106  (i.e. the peak voltage) using a voltage divider circuit, which can be implemented using two resistors in series. This can be an adjustable point on the circuit so that any percentage of the peak can be utilized as a comparison voltage to the signal peaks that follow. 
         [0038]    Divided-peak signal  114  is the transmitted to comparator  108  to be compared with filtered signal  102 . Comparator  108  is thus provided with the incoming signal train of pulses representing the optical pulses (or magnetic pulses) of element  70 , and a percentage of the peak of the previous signal. Since the signal peaks are fairly constant from cycle-to-cycle, this is a substantially reliable method of detecting the next peak of the signal. 
         [0039]    Comparator  108  thus determines when signal  102  is greater than the divided-peak signal  114 . Compared-signal  110  will thus be a ‘high’ voltage (such as 2.4 volts to 5 volts) during the period when the incoming signal is of a greater voltage than the chosen percentage of the peak signal. Compared-signal  110  is transmitted to pulse generator  116 . 
         [0040]    Compared-signal  110  may or may not be sufficient to meet the requirements of the equipment monitoring the rotation speed of the mechanical system, therefore there a pulse generator  116  is provided which meets the voltage and amplitude needs of the monitoring equipment. It will now be apparent though that depending on the monitoring equipment, pulse generator  116  may be obviated. 
         [0041]    Generated pulse signals  118  are then sent to the monitoring equipment, which lets the monitoring equipment know when the optical marker  74  (or magnetic element  75   a ) has been detected moving past on the moving element  70 . 
         [0042]    Referring now to  FIG. 4  and Table I, a specific but non-limiting example of how circuit  62  can be implemented is provided, which is indicated generally as circuit  62   c . 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Block Element 
                 Part Reference 
                 Part Description 
               
               
                   
                   
               
             
             
               
                   
                 Filter 100 
                 R1 
                 Resistor, 10 kohms, 
               
               
                   
                   
                   
                 5%, 0.1 Watt 
               
               
                   
                   
                   
                 U1B 
               
               
                   
                 Filter 100 
                 R2 
                 Resistor, 100 ohms, 
               
               
                   
                   
                   
                 5%, 0.1 Watt 
               
               
                   
                 Filter 100 
                 C1 
                 Capacitor 0.1 uF, 25 V, 
               
               
                   
                   
                   
                 10% 
               
               
                   
                 Filter 100 
                 O1 
                 Integrated Circuit 
               
               
                   
                   
                   
                 Operational Amplifier; 
               
               
                   
                   
                   
                 Prec JFET 
               
               
                   
                 Peak-detector 104 
                 D1 
                 Schottky Diode, 
               
               
                   
                   
                   
                 100 mA, VR = 45 V 
               
               
                   
                 Peak-detector 104 
                 C4 
                 Capacitor 1 uF, 16 V, 
               
               
                   
                   
                   
                 10%, C1206 
               
               
                   
                 Peak-detector 104 
                 O2 
                 Integrated Circuit 
               
               
                   
                   
                   
                 Operational Amplifier 
               
               
                   
                   
                   
                 Prec JFET 
               
               
                   
                 Peak Divider 112 
                 R10 
                 Resistor, 5 Kohms 
               
               
                   
                 Peak Divider 112 
                 O3 
                 Integrated Circuit 
               
               
                   
                   
                   
                 Operational Amplifier 
               
               
                   
                   
                   
                 Prec JFET 
               
               
                   
                 Comparator 108 
                 R11 
                 Resistor 15 Kohms, 
               
               
                   
                   
                   
                 5%, 0.1 W 
               
               
                   
                 Comparator 108 
                 C9 
                 Capacitor 0.01 uF 
               
               
                   
                 Comparator 108 
                 O4 
                 Operational Amplifier 
               
               
                   
                   
                   
                 LT3941S8 
               
               
                   
                   
               
             
          
         
       
     
         [0043]    Referring now to  FIG. 5  and Table II, a further specific, but non-limiting example of how circuit  62   d  can be implemented is provided. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Block Element 
                 Part Reference 
                 Part Description 
               
               
                   
                   
               
             
             
               
                   
                 Filter 100 
                 R1 
                 Resistor, 10 kohms, 
               
               
                   
                   
                   
                 5%, 0.1 Watt 
               
               
                   
                   
                   
                 U1B 
               
               
                   
                 Filter 100 
                 R2 
                 Resistor, 100 ohms, 
               
               
                   
                   
                   
                 5%, 0.1 Watt 
               
               
                   
                 Filter 100 
                 C1 
                 Capacitor 0.1 uF, 25 V, 
               
               
                   
                   
                   
                 10% 
               
               
                   
                 Filter 100 
                 O1 
                 Integrated Circuit 
               
               
                   
                   
                   
                 Operational Amplifier; 
               
               
                   
                   
                   
                 Prec JFET 
               
               
                   
                 Peak-detector 104 
                 D1 
                 Schottky Diode, 
               
               
                   
                   
                   
                 100 mA, VR = 45 V 
               
               
                   
                 Peak-detector 104 
                 C4 
                 Capacitor 1 uF, 16 V, 
               
               
                   
                   
                   
                 10%, C1206 
               
               
                   
                 Peak-detector 104 
                 O2 
                 Integrated Circuit 
               
               
                   
                   
                   
                 Operational Amplifier 
               
               
                   
                   
                   
                 Prec JFET 
               
               
                   
                 Peak Divider 112 
                 R10 
                 Resistor, 5 Kohms 
               
               
                   
                 Peak Divider 112 
                 O3 
                 Integrated Circuit 
               
               
                   
                   
                   
                 Operational Amplifier 
               
               
                   
                   
                   
                 Prec JFET 
               
               
                   
                 Peak Divider 112 
                 C51 
                 Capacitor, 1.0 uF, 25 V, 
               
               
                   
                   
                   
                 5% 
               
               
                   
                 Peak Divider 112 
                 R32 
                 Resistor, 25.5 Kohms, 
               
               
                   
                   
                   
                 1%, 0.1 W 
               
               
                   
                 Peak Divider 112 
                 R33 
                 Resistor, 25.5 Kohms, 
               
               
                   
                   
                   
                 1%, 0.1 W 
               
               
                   
                 Peak Divider 112 
                 O5 
                 Integrated Circuit 
               
               
                   
                   
                   
                 Operational Amplifier 
               
               
                   
                   
                   
                 Prec JFET 
               
               
                   
                 Comparator 108 
                 R11 
                 Resistor 15 Kohms, 
               
               
                   
                   
                   
                 5%, 0.1 W 
               
               
                   
                 Comparator 108 
                 C9 
                 Capacitor 0.01 uF 
               
               
                   
                 Comparator 108 
                 O4 
                 Operational Amplifier 
               
               
                   
                   
                   
                 LT3941S8 
               
               
                   
                   
               
             
          
         
       
     
         [0044]    In circuit  62   d , modifications were made to the peak divider  112  to reduce (and, as much as possible, minimize) jitter in the divided-peak signal  114 . 
         [0045]    This was achieved by operational amplifier O 5  and the associated components as shown in  FIG. 5 . The added amplifiers output is summed with (connected to) the output of the voltage buffering amplifier to create a Buffered Peak Signal. 
         [0046]    It will now be apparent that one of the advantages of provided by this specification is a means to sense of an optical or magnetic signal that automatically adjusts for a changing signal level, but not incorrectly trigger on random electronic noise. 
         [0047]    Combinations, subsets and variations of the foregoing are contemplated.