Abstract:
An externally-powerable sensor is described for detecting ultrasonic energy and for providing an accurate indication of the level of such energy to an external monitoring device over a wide dynamic range. A first electrical signal proportional to the detected ultrasonic energy is chopped by the output of a free-running multivibrator to generate a signal having a frequency differing from the frequency of the first signal by an amount within the audio frequency range. Such difference signal is selectively amplified to concentrate the spectral energy of the difference component in a lower portion of the audio range. A low pass filter extracts, from the so-concentrated difference signal, a modified audio signal whose frequency content is at the lower end, illustratively 0-6 KHz, of the spectrum of the concentrated difference signal. The output of the low pass filter is processed to generate a DC output current that is proportional to the detected ultrasonic energy level. The sensor is connectable to a  4 - 20  mA current loop which serves to power the sensor electronics as well as to carry the DC output current to the monitoring device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention pertains to ultrasonic sensors and more specifically to ultrasonic sensors having signal processing subassemblies that may be remotely powered.  
           [0002]    An ultrasonic sensor, which typically utilizes a transducer that produces an electrical output in response to received ultrasonic energy, is used to locate and measure leaks or defects in pipes and the like as well as to detect excess friction within mechanical devices. The transducer output is coupled to a signal processing subassembly that derives a measurement signal proportional to the transducer output. The measured ultrasonic energy is generally in the range of 20-100 KHz, which is too high in frequency to be heard by a human being. Thus, the signal processing subassembly is sometimes adapted to frequency shift the detected signal into the 0-20 Khz audio range. In some cases, the subassembly includes facilities for deriving monitoring a DC signal proportional to such audio signal.  
           [0003]    Because of the low levels of the ultrasonic signals detected by a sensor of this type, it has been necessary to augment the detected signal using a high-gain preamplifier before it can be further processed. In order to prevent such amplified signal from saturating the heterodyne and measurement circuitry, it has been common to attenuate the preamplified signal. Unfortunately, with such arrangements the maximum device sensitivity—i.e., the dynamic range of ultrasonic signal inputs that can be accurately processed by the sensor—has been limited typically to the range of 30-40 DB.  
         SUMMARY OF THE INVENTION  
         [0004]    Such limitations on dynamic range are minimized with an ultrasonic sensor in accordance with the invention. In an illustrative embodiment, the preamplified output of an ultrasonic transducer is chopped by the output of a free-running oscillator which generates a selectable frequency differing from the frequency of the transducer output by an amount within the audio frequency range. The output of the chopper includes a signal having a frequency equal to the difference of the frequencies of the transducer and the oscillator.  
           [0005]    The frequency components of such difference signal are selectively amplified to concentrate the spectral energy of the difference signal in a lower portion of the audio range. A low pass filter extracts, from the so-concentrated difference signal, a modified audio signal whose frequency content is at the lower end, illustratively 0-6 KHz, of the spectrum of the concentrated difference signal.  
           [0006]    Preferably, the output of the low pass filter is coupled to an AC to DC converter which provides a DC voltage that is proportional to the ultrasonic energy level detected by the transducer. As an additional feature of the invention, the AC to DC converter utilizes a compression network that prevents the converter output from saturating over the enhanced dynamic range of the input signal.  
           [0007]    The improved sensitivity provided by the arrangement of the invention allows the sensor to be employed effectively with an external power supply. Illustratively, the sensor is connected to a 4-20 mA current loop which may serve both as a vehicle for powering of the signal processing subassembly of the sensor as well as for carrying an output current proportional to the DC voltage generated by the sensor. Such current may be generated by a suitable voltage to current converter in the signal processing subassembly of the sensor.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    These and other aspects, features and advantages of the invention are further set forth in the following detailed description of an illustrative embodiment thereof taken in conjunction with the appended drawing, in which:  
         [0009]    [0009]FIG. 1 is a pictorial representation of an externally powerable ultrasonic sensor that is adapted for enhanced measurement sensitivity in accordance with the invention;  
         [0010]    [0010]FIG. 2 is a representation of external facilities for powering, controlling, and monitoring measurement signals from the sensor of FIG. 1;  
         [0011]    [0011]FIG. 3 is a block diagram of the signal processing subassembly in the sensor of FIG. 2; and  
         [0012]    [0012]FIGS. 4A and 4B are schematic diagrams of the components of the signal processing subassembly of FIG. 3. 
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring to the drawing, FIG. 1 shows an illustrative embodiment of an ultrasonic sensor  9  which may be remotely powered, monitored and adjusted from a suitable external system  10  through an I/O cable  11 . The sensor  9  includes a hollow moisture-resistant housing  12  formed from a conductive material. A transducer mount  13  is secured in one end  14  of the housing  12 , and supports a conventional ultrasonic transducer  15  that assumed to be formed from a single piezoelectric crystal.  
         [0014]    The transducer  15  may be coupled in a conventional manner through the mount  13  to a workpiece (not shown) which is to be measured ultrasonically for flaws, defects, excess friction or the like. The transducer  15  is conventionally operable to convert measured ultrasonic oscillations in a frequency range of 20-100 KHz into a corresponding electrical signal. The transducer  16  may be arranged to detect either structure-borne or acoustic ultrasonic signals in a conventional manner. The electrical signal at the output of the transducer  15  is coupled to the input of a signal processing subassembly  16  located in the interior of the housing  12 .  
         [0015]    The I/O cable  11  is directly connected to the subassembly  16  through the housing  12  via a moisture resistant cable fitting  17  for applying power to the subassembly from an external current loop  18  (FIG. 2). Such current loop, which is illustratively a 4-20 mA current loop, forms part of the system  10 . Advantageously, the cable  11  may also be adapted to provide digital control signals from the system  10  for remotely adjusting parameter(s), such as the sensitivity and/or carrier frequency, of the subassembly  16  (FIG. 1)  
         [0016]    As indicated below, the subassembly  16  is adapted to generate a first DC output signal and a second audio output signal each proportional to the transducer output signal. Such DC and audio output signals are respectively coupled through the I/O cable  11  to a monitor  19  (FIG. 2) in the current loop  18  and to a suitable audio detector  20  in the external system  10 .  
         [0017]    As shown in FIG. 3, the subassembly  16  includes a preamplifier  21  to which the 20-100 KHz electrical signal from the transducer  15  is applied. The resultant amplified signal is then subjected to a heterodyne mode where it is effectively frequency-shifted into the audio range. This operation may be illustratively implemented in an optimum manner by chopping the output of the preamplifier  21  with a chopper circuit  22  in the manner indicated below, and applying the chopped signal to the input of a high gain linear amplifier  23 . The chopper circuit  22  operates at a rate governed by an adjustable-frequency oscillator  24 . The chopped signal has a frequency spectrum with components representing the sum of, and the difference between, the frequency at the output of the transducer  15  and the carrier frequency at the output of the oscillator  24 . The carrier frequency is adjusted so that the difference component is in the audio frequency range.  
         [0018]    The oscillator  24  is illustratively tuned to a carrier frequency in the 30-50 KHz range (e.g., 37 KHz), although adjustability over other suitable ranges, such as 20-100 KHz, may also be used. Such expanded range may be particularly appropriate when the ultrasonic energy being measured is at the high end of the 20-100 KHz range to assure that the difference component at the output of the chopper  22  is within the audio range.  
         [0019]    The gain of the linear amplifier  23  is preferably more pronounced at the lower end of the frequency range of the incoming chopped signal, so that the spectrum of the difference component, and in particular a lower portion of such spectrum, will be augmented. Such selective augmentation of the difference component serves to concentrate the spectral energy of the difference component in a band significantly below the 20 KHz band typically appearing at the audio output of previous ultrasonic sensors.  
         [0020]    A low pass filter  26  at the output of the linear amplifier  23  eliminates any residual sum frequency components resulting from the action of the chopper circuit  22  and further narrows the band of the concentrated difference component from the amplifier  23  to a range of about 0-6 KHz. The resultant output of the filter  26  is proportional to the ultrasonic energy detected by the transducer  15 .  
         [0021]    The audio output from the filter  26  is coupled to the I/O cable  11  for application to the audio detector  20  (FIG. 2), which may be associated with a suitable utilization device such as a spectrum analyzer (not shown). The output of the filter  26  (FIG. 4A) is also applied to the input of an AC-DC converter  27 . The DC output of the converter  27  is proportional to the detected ultrasonic signal level of the transducer  15 . Such DC voltage is applied to a voltage to current converter  28  which generates a DC output current proportional to the detected ultrasonic signal level.  
         [0022]    Such DC output current is connected, through the ground conductor (not shown) of the I/O cable  11  and a grounded negative lead  32  (FIG. 2) of the current loop  18 , to a fixed DC supply  36  disposed in the loop  18 . The monitor  19  is connected to a positive lead  35  of the loop  18 . Voltage from the supply  36  is applied, through the positive lead  35  and the positive power conductor (not shown) of the I/O cable  11 , to a conventional voltage regulator  37  (FIG. 3) in the signal processing subassembly  16 . The voltage regulator  37  derives a regulated positive voltage, designated VCC, necessary to power the components of the subassembly  16 . Illustratively, VCC is 10 volts.  
         [0023]    [0023]FIGS. 4A and 4B depict an illustrative circuit arrangement for the components of the subassembly  16 . The preamplifier  21 , whose maximum gain may typically be 100 DB or greater, conventionally includes an operational amplifier  41  having a parallel RC negative feedback loop having a capacitor  42  and a variable resistor  43 . The non-inverting input of the amplifier  41  is biased to one half of the supply voltage VCC. When the resistor  43  is adjusted to present maximum resistance in the feedback loop, the amplifier  41  operates as a charge amplifier wherein the 20-100 KHz output voltage from the transducer  15  is coupled directly to the inverting input of the amplifier  41 . For other settings of the resistor  43 , the amplifier  41  acts as a conventional voltage amplifier.  
         [0024]    As indicated above, the spectrum of the amplified 20-100 KHz signal at the output of the preamplifier  21  is altered by the chopper circuit  22 . Illustratively, the chopper circuit  22  includes a voltage divider utilizing a pair of resistors  46  and  47  and the collector-emitter path of a transistor  48  that are connected in series between VCC and ground. The output signal from the preamplifier  21  is coupled, through a capacitor  51 , to the junction of the resistor  46  and the collector of the transistor  48 . The base of the transistor  48  is coupled to the output of the oscillator  24 , which may be conventionally embodied as a free-running multivibrator. The selection of the carrier frequency of the oscillator  24  is implemented by adjustment of a variable resistor  52 . Advantageously, such adjustment may be remotely accomplished by digital control signals from the system  10  (FIG. 2), in which case the variable resistor  52  may be embodied in programmable digital form. In an appropriate case, the variable resistor  52  may be shunted with an auxiliary resistor (not shown) to raise the carrier frequency by an amount sufficient to help assure that the difference frequency component at the output of the chopper circuit  22  is in the audio range as indicated above.  
         [0025]    The chopper circuit  22  is so configured that the resistance of the resistor  46  is much greater than that of the resistor  47 . Under the circumstances, the 20-100 KHz output of the preamplifier  21  is chopped by being periodically driven essentially to ground when the transistor  48  conducts. Such chopping action occurs at the carrier frequency of the oscillator  24 .  
         [0026]    The components of the chopped signal are applied through a capacitor  56  and a resistor  57  to the high gain linear amplifier  23 , which is illustratively embodied as an inverting operational amplifier  54 . The amplifier  54  has a parallel RC negative feed back loop including a resistor  58  and a capacitor  59 . The value of the resistor  58  (illustratively 470K ohms) is advantageously made about 50 times greater than that of the resistor  57 .  
         [0027]    The difference component of the chopped signal is concentrated by the amplifier  23  to enhance the spectral energy of such difference component at frequencies well below 20 KHz. Such enhancement results from the high nominal amplification (e.g., 20-30 DB) presented by the amplifier  23  and, preferably, by selectively reducing the gain presented to the higher audio frequencies of the difference component. For this purpose the value of the feedback capacitor  59  is made relatively large, for example 470 pF. Such large capacitance also serves suppress the sum components of the chopped signal.  
         [0028]    The so-concentrated difference signal from the linear amplifier  23  is applied to the input of the low pass filter  26 , illustratively a third order low pass filter that is embodied using an operational amplifier  61 , resistors  62 ,  63  and  64  and capacitors  66  and  67 . The values of such resistors and capacitors are chosen such that the 3 DB break point for the filter  26  occurs at about 6 KHz. The resulting 0-6 KHz audio signal at the output of the filter  26  has an amplitude proportional to the measured ultrasonic energy at the transducer  15 , and is applied both to the AC to DC converter  27  and to the I/O cable  11 .  
         [0029]    The converter  27 , which is illustratively a full-wave rectifier, includes a operational amplifier  71  (FIG. 4B). The audio output from the filter  26  is applied through a resistor  72  to the non-inverting input of the amplifier  71 . A feedback path extends from the output of the amplifier  71  to the inverting input thereof, and operates as a compression network. Specifically, such feedback path has a first branch that includes a resistor  73 , and a second parallel branch including a zener diode  76  in series with a resistor  77  and a diode  78 . With this arrangement, when the audio input voltage to the converter  27  rises above a threshold value that causes the voltage across an output resistor  79  to trigger on the zener diode  76 , the gain of the amplifier  71  will be reduced to prevent saturation at the output of the converter  27 . Because of the action of the compression network just described, the effective dynamic range of the converter  27  may be increased by up to 15 DB.  
         [0030]    The voltage to current converter  28  illustratively includes an operational amplifier  81 . The output of the AC to DC converter  27 , generated across a parallel RC path including a resistor  82  and a capacitor  83 , is applied to the non-inverting input of the amplifier  81  through a resistor  84 . The output of the amplifier  81  is coupled to the base of a transistor  86 , whose collector-emitter path is connected to a load resistor  87  and through a resistor  88  to the non-inverting input of the amplifier  81 . The inverting input of the amplifier  81  is biased from a voltage divider consisting of resistors  91  and  92  connected in series between VCC and ground. With this arrangement, the current through the load resistor  87  is proportional to the DC input voltage to the amplifier  81 . Such current constitutes a DC indication proportional to the ultrasonic energy to be externally monitored in the external current loop  18  (FIG. 2).  
         [0031]    A principal advantage of the arrangement of the invention just described is that the sensitivity of ultrasonic measurement is significantly greater than that of prior art ultrasonic sensor arrangements. This effect is particularly evident in the case where the sensor  9  is powered with the 4-20 mA current loop  18  as indicated above. Using such loop, the sensor gain setting is first initialized by adjustment of the variable resistor  43  (FIG. 4A) so that the current at the collector of the transistor  86  (FIG. 4B) of the voltage to current converter  28  is 4 mA when a minimal ultrasonic level is detected. Such adjustment is advantageously done remotely by means of digital control signals from the external system  10  (FIG. 2), in which case the variable resistor  43  (FIG. 4A) may be embodied in programmable digital form. Once such adjustment is done, the spread between maximum and minimum values of the measured output current from the converter  28  (FIG. 4B) during a working measurement corresponds typically to a 50 DB dynamic range of the detected ultrasonic energy.  
         [0032]    In the foregoing, the invention has been described in connection with a preferred arrangement thereof. Many variations and modifications will now occur to those skilled in the art. For example, while for purposes of illustration the DC output quantity from the subassembly  15  has been described as a current proportional to the detected ultrasonic signal, such quantity may also be a DC voltage obtained by substituting a voltage amplifier for the voltage to current converter  28 . In such case, the 50 DB dynamic range of the sensor  9  (FIG. 1) may be represented by various output voltage ranges, typically 0-5 or 0-10 volts. It is accordingly desired that the scope of the appended claims not be limited to or by the specific disclosure herein contained.