Abstract:
A sensor circuit, method, and system for conversion of a high-impedance, broad frequency range signal from a transducer into two low-impedance signals, one containing high-frequency components and another containing low-frequency components of the input signal. The transducer can be a piezoelectric (PE) transducer transforming motion or vibration into a high-impedance electrical signal with a broad frequency range. A low-frequency circuit output can contain the frequencies in the linear region of the transducer&#39;s frequency band. The high-frequency output can contain the natural resonance frequency of the transducer. The circuit includes a low-frequency filter amplifier module and a high-frequency filter amplifier module, both amplifier modules having negative feedback, high input impedance, and low output impedance; the outputs may include a DC bias. The circuit may also include a source follower isolating the filter amplifier modules from each other. Optionally, the source follower is electrically disposed between the transducer and the high frequency filter amplifier module and employs an operational amplifier. The voltage supply for the source follower can be provided by the output of the low-frequency filter amplifier. The circuit may include a variety of means aimed at eliminating noise and temperature sensitivity.

Description:
RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/223,884, filed on Aug. 9, 2000; the entire teachings of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   Transducers are devices used for converting energy from one form to another to measure a physical quantity. A typical transducer converts mechanical force or acceleration into electromagnetic energy. A transducer is mechanically coupled to an object to measure its motion. When this motion is vibrational, usually only certain frequency ranges are of interest. In such cases, a sensor employs a transducer and some means of filtering out the unwanted frequencies. To achieve this filtering, the sensor normally includes: (a) a transducer providing an output signal with a broad frequency range and (b) an amplifying and filtering circuit that is electrically connected to the transducer&#39;s output and eliminates the unwanted frequencies outside a given frequency band of interest. 
   Existing sensors are designed for sensing either low-frequency vibrations or high-frequency vibrations. To obtain signals representing both the low-frequency vibrations and high-frequency vibrations, two sensors must be used. 
   SUMMARY OF THE INVENTION 
   The principles of the present invention teach a sensor that can operate with a single transducer and an electronic circuit having two filtering circuits providing separate outputs. By combining at least two filtering functions into a single circuit that can convert a single transducer output into corresponding electrical signals, the sensor is reduced in size, weight, and cost, and provides improved performance for measurement systems where measurement of multiple frequency signals are needed. For example, the low-frequency, linear region of the transducer signal provides force or motion information, and the high, natural resonance frequency of the transducer signal can be used as a diagnostic signal or other status indicator. 
   One embodiment of the present invention accomplishes this task by using an innovative electronic circuit which converts a high-impedance, broad frequency range signal from a transducer into two low-impedance outputs. Typically, one output provides high-frequency signals and the other output provides low-frequency signals. The circuit may also amplify and/or offset the signal. Output signal offset can be used to put the signal within the delivery range needed by a system using the present invention and to establish a proper bias for electronic components to avoid clipping and saturation in their operation within an embodiment of the invention. 
   The low-frequency output signal can contain, for example, the frequency components corresponding to the linear part of the transducer&#39;s frequency response band, providing force or motion information. The high-frequency output signal can contain, for example, the frequency components corresponding to the natural resonance frequency of the transducer, which may be designed at a select frequency for high sensitivity based on the dynamics of the system being monitored for vibration. The high-frequency signal can be used as a diagnostic signal or other status indicator. 
   The transducer can be, for example, a piezoelectric (PE) transducer transforming a sensed force or mechanical vibration into a corresponding high-impedance electrical signal with a broad frequency range, where the transfer function is essentially linear with low hysteresis. 
   In one embodiment, the electronic circuit includes a low-frequency filter amplifier module and a high-frequency filter amplifier module. Both filter amplifier modules have high input impedance, low output impedance, and negative feedback. The negative feedback may be provided by respective single capacitors. The filter amplifier circuit outputs may be DC-biased to provide sufficient signal swing without clipping or saturating circuit components. 
   The electronic circuit may further include a buffer to isolate the filter amplifier modules from each other. In one embodiment, the buffer is electrically disposed between an input to the circuit and an input to the high-frequency filter amplifier module. In an alternate embodiment, the buffer is electrically disposed between the input to the circuit and an input to the low-frequency filter amplifier module. 
   The buffer can be implemented in the form of an operational amplifier arranged in a source follower configuration. The buffer may be unipolar, with one power rail receiving power from the output of the low-frequency filter amplifier module and the other power rail being connected to power return or ground. This arrangement has the advantage of not having to use separate power sources for the low- and high-frequency filter amplifier circuits and the buffer. 
   Because the low-frequency filter amplifier module&#39;s input is electrically connected directly to the transducer, a capacitor or an equivalent circuit can be electrically disposed between the buffer&#39;s output and the high-frequency filter amplifier module&#39;s input to ensure similar input impedances on the inputs of both filter amplifier modules. 
   In an alternative embodiment, the buffer is electrically disposed between the transducer and the low-frequency amplifier module. In this embodiment, a capacitor or an equivalent circuit providing a transducer-like output impedance can be electrically disposed between the buffer&#39;s output and the low-frequency filter amplifier module&#39;s input. 
   The circuit may include a variety of means aimed at eliminating noise and temperature dependence of the circuits&#39; components and characteristics. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a sensor providing low- and high-frequency outputs according to the principles of the present invention; 
       FIG. 2  is an electrical schematic diagram of the sensor of  FIG. 1 ; 
       FIG. 3  is a Bode plot of the calculated frequency responses for the low- and high-frequency outputs of the sensor of  FIG. 2 ; 
       FIG. 4  is a Bode plot of a measured frequency response of the high-frequency output of the sensor of  FIG. 2  at a temperature of 25 degrees Celsius; 
       FIG. 5  is a Bode plot of a measured frequency response of the high-frequency output of the sensor of  FIG. 2  at a temperature of 120 degrees Celsius; 
       FIG. 6  is a Bode plot of a measured frequency response of the low-frequency output of the sensor of  FIG. 2  at a temperature of 25 degrees Celsius; 
       FIG. 7  is a Bode plot of a measured frequency response of the low-frequency output of the sensor of  FIG. 2  at a temperature of 120 degrees Celsius; 
       FIG. 8  is a Bode plot of a measured noise spectrum of the low-frequency output of the sensor of  FIG. 2  at a temperature of 25 degrees Celsius; and 
       FIG. 9  is a Bode plot of a measured noise spectrum of the high-frequency output of the sensor of  FIG. 2  at a temperature of 25 degrees Celsius. 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A description of preferred embodiments of the invention follows. 
     FIG. 1  is a block diagram of an embodiment of a sensor  13  according to the principles of the present invention. The sensor  13  comprises a transducer  1  and an electronic circuit  12 . The electronic circuit  12  includes a low-frequency channel circuit  2  and high-frequency channel circuit  4 . 
   A high-impedance signal from the transducer  1  is processed by the low-frequency channel circuit  2  and high-frequency channel circuit  4 . The low-frequency channel circuit  2  produces a DC-biased, low-impedance, low-frequency signal on a first output terminal  3 ; the high-frequency channel circuit  4  produces a DC-biased, low-impedance, high-frequency signal on a second output terminal  5 . 
   The low-frequency channel circuit  2  includes a low-frequency filter amplifier  6  and a negative feedback path  7 , which is composed of a single capacitor in this embodiment. The low-frequency filter amplifier  6  has a high input impedance and low output impedance. 
   The high-frequency channel circuit  4  includes a high-frequency filter amplifier  8  and a negative feedback path  9 , which is also composed of a single capacitor in this embodiment. The high-frequency filter amplifier  8  has a high input impedance and low output impedance. 
   The electronic circuit  12  also includes a buffer  10 , configured here as a source follower, which provides a high impedance (i.e., isolation) and eliminates cross-coupling between the high-frequency filter amplifier  8  and the low-frequency filter amplifier  6 . In one embodiment, the voltage supply for the buffer  10  is provided by the DC-biased low-frequency output  3  so that the source follower  10  does not need its own power supply. The source follower  10  has a high input impedance and low output impedance. 
   The high-frequency channel circuit  4  receives the output of buffer  10  through a capacitor  11 . The capacitor  11  ensures similar input impedances on the input of low-frequency filter amplifier  6 , which is connected directly to the transducer  1 , and on the input of high-frequency filter amplifier  8 . The capacitor  11  also rejects low frequency variations on the output of the buffer  10  caused in part by using the output of the low-frequency output  3  of the low-frequency channel circuit  2  as a voltage supply for the buffer  10 . 
   One use of the sensor  13  is in a motion sensing application in which the transducer  1  is a piezoelectric (PE) transducer detecting motion in a linear, low-hysteresis, high-sensitivity manner. In such applications, a high-frequency output signal of the transducer  1  includes a signal that is representative of the natural mechanical resonance frequency of the transducer  1 , while a low-frequency output signal includes a frequency range of signals that are representative of signals within the frequency range corresponding to the linear, low-frequency motion of the transducer  1 . Thus, a single transducer and single circuit can be provided by the present invention sensor  13  to provide motion information containing the low- and high-frequency signals simultaneously. Such information can presently be provided only by two separate transducers and circuits of prior art sensor systems. Thus, the size, weight, and cost of the motion sensing system is reduced and the performance is improved. 
     FIG. 2  is an electrical schematic diagram of an embodiment of the sensor  13 . Before describing circuit specifics, correspondence between the schematic diagram of FIG.  2  and block diagram of  FIG. 1  is provided. 
   The sensor  13  includes the transducer  1  and electronic circuit  12 . The electronic circuit  12  includes low-frequency channel circuit  2 , high-frequency channel circuit  4 , and buffer  10 . 
   The low-frequency channel circuit  2  includes a low-frequency channel input terminal T 1  and low-frequency channel output terminal T 2 . Similarly, the high-frequency channel circuit  4  includes a high-frequency channel input terminal T 3  and high-frequency channel output terminal T 4 . 
   The buffer  10  is implemented as a unity gain source follower through the use of an operational amplifier U 1 . The buffer  10  receives power from the low-frequency channel output terminal T 2  and power return (i.e., ground) 0. 
   While the schematic diagram of  FIG. 2  is believed sufficient to make the properties of the sensor  13  apparent to a person skilled in the pertinent field, some details and features of the sensor are specifically pointed out. 
   The input impedance of the low frequency channel circuit  2  is equal to the parallel combination of R 11  and the gate resistance of J 6  (i.e., (R J6 ·R 11 )/(R J6 +R 11 )=200 Mohm). Similarly, the input impedance of the high frequency channel circuit  4  is equal to the parallel combination of R 21  and the input impedance of U 1  (i.e., (R U1 ·R 21 )/(R U1 +R 21 )=200 Mohm). 
   The n-channel JFET transistor J 6  amplifies the input signal on its gate, reduces the output noise, and ensures the high input impedance for the low-frequency filter amplifier  6 . The n-channel JFET transistor J 8  provides the same features for the high-frequency. filter amplifier  8 . 
   The p-n-p transistor Q 8 , together with the resistors R 1 , R 14 , R 15 , and R 16  and the capacitors C 5  and C 6 , provide a proper biasing and reduce the noise on the gate of the JFET transistor J 6  in the low-frequency filter amplifier  6 . 
   The p-n-p transistor Q 1 , together with the resistors R 2 , R 6 , R 7 , and R 8  and the capacitors C 11  and C 14 , provide a proper biasing and reduce the noise on the gate of JFET transistor J 8  in the high-frequency filter amplifier  8 . 
   The p-n-p Darlington transistor Q 7  ensures the low output impedance of the low-frequency filter amplifier  6 ; its emitter and collector are connected to the output wires of the low-frequency filter amplifier  6 . The n-channel JFET transistor J 5  ensures the proper regime for the Darlington transistor Q 7 . The p-n-p Darlington transistor Q 3  and the n-channel JFET transistor J 7  provide the same features for the high-frequency filter amplifier  8 . 
   In the low-frequency filter amplifier  6 , the capacitors C 1 , C 2 , and C 7  together with the resistors R 12  and R 13  function as a two-pole, active, low-pass filter with a rise beginning at about 500 Hz and having a peak at about 10 kHz. The capacitor C 10  functions as the negative feedback 7. The resistor R 11  together with the capacitor C 10  functions as a one-pole high-pass filter with the frequency cutoff (−3 dB) of about 1.5 Hz, for the low-frequency filter amplifier  6 . The resistors R 3  and R 4  together with the capacitor C 9  function as a one-pole low-pass pre-filter with the frequency cutoff (−3 dB) of about 460 Hz, for the low-frequency filter amplifier  6 . Thus, the composite low-pass filter has a frequency range (−3 dB) of about 1.5 Hz to 8 kHz. 
   In the high-frequency filter amplifier  8 , the capacitors C 16  and C 12  together with the resistors R 22  and R 5  function as a two-pole high-pass filter with the frequency range (−3 dB) of about 19 kHz to 52 kHz; the capacitor C 16  also functions as the negative feedback 9. The resistor R 1  together with the capacitor C 4  function as a one-pole low-pass pre-filter with the frequency cutoff (−3 dB) of about 80 kHz, for the high-frequency filter amplifier  8 . 
   In the embodiment of  FIG. 2 , the values and configuration of the components involved in filtering signals within sensor  13  are designed according to the principles of filter design commonly known in electrical engineering and similar arts and described, for example, in Adel S. Sedra &amp; Kenneth C. Smith, Microelectronic Circuits 787-89, 792-93 (2d ed. 1987). Alternatively, active filter design principles may be used for both the low-frequency filter amplifier  6  and high-frequency filter amplifier  8  as well as for pre-filtering of an input signal. An active or passive biquad filter design can be used. 
   The capacitor C 4  on  FIG. 2  corresponds to the capacitor  11  on FIG.  1 . Its function is to establish on the input of the high-frequency filter amplifier  8  the input characteristics similar to those provided by the transducer  1  on the input of the low-frequency filter amplifier  6 . To achieve this, the characteristics of capacitor C 4  should be similar to the characteristics of the transducer&#39;s capacitance, represented in  FIG. 2  as C 3 . 
   The capacitor C 15  provides additional DC decoupling between the input and output of the high-frequency filter amplifier  8 . 
   The operational amplifier U 1  is configured as a source follower and functions as the buffer  10 . The resistors R 17 , R 10 , and R 21  establish the proper input offset, and the capacitor C 8  provides DC decoupling for the input of the buffer  10 . The power for the operational amplifier U 1  is provided by the output of the low-frequency filter amplifier  6 . 
   The low-frequency filter amplifier  6  has a two-wires output and, therefore, includes both the voltage source V 2  (24 V DC) and the current source I 1  (4 mA). The voltage source V 1  and current source I 2  provide the same functions for the high-frequency filter amplifier  8 . 
   In the sensor  13  shown in  FIG. 2 , each output T 2 , T 4  provides the same sensor gain of 2 mV/pC and the same maximum output swing of 5V. For a 50 pC/g transducer 1, each output provides the same sensor  13  sensitivity of 100 mV/g. 
     FIG. 3  is a Bode plot of magnitude responses of the low-frequency channel circuit  2  and the high-frequency channel circuit  4  for the embodiment of the sensor  13  shown in FIG.  2 . The Bode plot is representative of the low- and high-frequency outputs of the sensor  13  when the transducer input to the circuit  12  is a swept sine wave having an amplitude of 1 volt. The Bode plot was obtained by simulating the circuit of  FIG. 2  using ORCAD®, a standard electronics design and simulation software program. The units of the horizontal logarithmic axis are frequency. The vertical linear axis is showing magnitude of the output in volts. 
   A circuit embodying the schematic circuit of  FIG. 2  was implemented on a breadboard and its frequency response was tested at room temperature (25 degrees Celsius) and elevated temperature (120 degrees Celsius). This test yielded practically the same results as were obtained by computer simulations.  FIGS. 4 ,  5 ,  6 , and  7  are sinesweep plots of that breadboarded circuit and have the same axes as the Bode plot of  FIG. 3  for comparison purposes. 
     FIG. 4  is a Bode plot of a measured frequency response of the high-frequency output of the implementation of the sensor  13  of  FIG. 2  at the temperature of 25 degrees Celsius. For the reference frequency of 27.38 kHz, the −3 dB points are measured to be 18.7 kHz and 52.8 kHz as expected for the two-pole high-pass filter configuration discussed above and shown on FIG.  2 . 
     FIG. 5  is a Bode plot of a measured frequency response of the high-frequency output of the implementation of the sensor  13  of  FIG. 2  at the temperature of 120 degrees Celsius. 
     FIG. 6  is a Bode plot of a measured frequency response of the low-frequency output of the implementation of the sensor  13  of  FIG. 2  at the temperature of 25 degrees Celsius. For the reference frequency of 100 Hz, the −3 dB points are measured to be 1.3 Hz and 9.07 kHz as expected for the two-pole low-pass filter configuration discussed above and shown on FIG.  2 . 
     FIG. 7  is a Bode plot of a measured frequency response of the low-frequency output of the implementation of the sensor  13  of  FIG. 2  at the temperature of 120 degrees Celsius. 
   Results from the simulation and measured response of the breadboard circuit implementing the sensor  13  shown on  FIG. 2  indicate that the maximum deviation of the gain is about 6% and the bias deviation is about 2 volts DC in the temperature range of 25-120 degrees Celsius. 
   The following table provides approximate performance specifications for the implementation of the sensor  13  of FIG.  2 . These are exemplary specifications for a particular embodiment provided for illustrative purposes and not intended to limit the principles of the present invention. 
   
     
       
             
             
           
         
             
                 
             
           
           
             
               Source capacitance 
               1000 pC ± 10% 
             
             
               Output impedance 
               10 ohms 
             
             
               DC output bias (over the temperature range) 
               8-13 V DC 
             
             
               Maximum output voltage 
               5 V peak 
             
             
               Frequency response for the LF output (100 Hz 
               10%: 3 Hz-5 kHz 
             
             
               reference) 
             
             
               Frequency response for the LF output (100 Hz 
               −3 dB: 1.5 Hz-8 kHz 
             
             
               reference) 
             
             
               Frequency response for the HF output (27 kHz 
               −3 dB: 19 kHz-52 kHz 
             
             
               reference) 
             
             
               Gain (each channel) 
               2 mV/pC 
             
             
               Residual noise for the LF output (2 Hz to 20 
               25 μV rms typical 
             
             
               kHz) 
             
             
               Residual noise for the HF output (2 Hz to 100 
               22 μV rms typical 
             
             
               kHz) 
             
             
               Warm-up time 
               5 sec 
             
             
               Power requirements 
               Powered from positive 
             
             
                 
               constant current source 
             
             
               Supply voltage 
               22 to 30 V DC 
             
             
               Supply current (for each channel) 
               4 mA nominal, 2 to 10 
             
             
                 
               mA operating range 
             
             
               Operating temperature 
               −50° C. to +120° C. 
             
             
                 
               (−58° F. to +248° F.) 
             
             
               Non-operating temperature 
               −73° C. to +150° C. 
             
             
                 
               (−100° F. to +302° F.) 
             
             
                 
             
           
        
       
     
   
   In the Bode plots of  FIGS. 8 and 9 , the units of the horizontal logarithmic axis is frequency and units of the vertical linear axis is the voltage spectral density. The measurements were taken at a temperature of 25 degrees Celsius with the input of the circuit connected to ground 0, i.e. with the grounded input, where the transducer behaves essentially as a 1050 pF capacitor. 
     FIG. 8  is a Bode plot of a grounded-input noise response spectrum measured at the output of the low-frequency channel circuit  2  of the implementation of the sensor  13  of  FIG. 2  at a temperature of 25 degrees Celsius. 
     FIG. 9  is a Bode plot of a grounded-input noise response spectrum measured at the output of the high-frequency channel circuit  4  of the implementation of the sensor  13  of  FIG. 2  at a temperature of 25 degrees Celsius. 
   The noise measurements represented by the plots of  FIGS. 8 and 9  show the values of a noise spectrum that are practically identical to such values specified for traditional sensors. Thus, the noise measurements provide confidence that there is little cross-coupling between the low-frequency channel  2  and high-frequency channel  4 . 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 
   The sensor  13  can be implemented, for example, using surface mount technology or chip and wire technology. The analog circuits described herein can be implemented in digital circuitry or signal processing technology. It should be understood that typical techniques of conversion of filters from analog to digital form can be used to implement in digital form the filtering features of the analog circuits described herein. For a digital processing implementation, analog-to-digital and digital-to-analog converters to sample and output the processed signal, respectively, can be used. Further, supporting analog circuitry, such as the buffer  10  and Nyquist filters, may be employed in the digital embodiment.