Abstract:
A velocity transducer or Velomitor® that can output electrical signals relating to vibration, despite the transducer being exposed to low levels of gamma-radiation, is disclosed. A DC feedback circuit, which sets up the input stage bias point, keeps the output bias voltage within a usable voltage range as the transducer is exposed to the gamma-radiation. An additional JFET transistor, configured as a current source, helps the DC feedback circuit compensate for increases in the offset voltage of the JFET amplifier. The value of a resistor controlling the gate current of the JFET amplifier is also reduced, such that when the leakage current increases, the offset voltage is reduced.

Description:
The present invention relates to the monitoring of industrial machinery, such as power generating equipment, and, more particularly, to a velocity transducer capable of measuring vibrations in such machinery despite being exposed to low levels of gamma-radiation. 
   BACKGROUND OF THE INVENTION 
   Serious problems with rotating industrial equipment, such as power generating equipment, can often be avoided by monitoring various parameters of the equipment to determine whether such equipment may have an operating problem. Velomitors® 1  are one brand of velocity transducers that are used to provide protection of industrial machinery by measuring equipment vibration. Velomitor® is a registered trademark of Bently Nevada. 
   Velomitors® are used in a number of environments. One application in which these transducers are used is the monitoring of equipment in nuclear plants where the transducers are continuously exposed over time to low levels of gamma-radiation. Velomitors® used in this type of environment are typically exposed to radiation over a long period of time. The exposed Velomitors® will function in this kind of environment for a period of time, but eventually the output bias voltage of these transducers shifts as they are exposed to the radiation over an extended period of time. Ultimately, the output of a transducer will drift close to one of its supply voltage levels, whereupon the transducer stops functioning. This problem arises when a junction field effect transistor (“JFET”) used in the Velomitor® to form a common source amplifier stage is irradiated by the gamma-radiation. The irradiation causes increasing current leakage into the JFET&#39;s gate over time. This increase in gate current, when coupled with a large feedback resistor in series with the gate, produces a high offset voltage at the transducer&#39;s output, driving the transducer&#39;s output toward the device&#39;s rail (supply voltage). 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention is directed to a velocity transducer that can output electrical signals relating to vibration, despite the transducer being exposed to low levels of gamma-radiation. A feedback circuit in the transducer sets up the input stage bias point. The feedback circuit keeps the output bias voltage within a usable voltage range as the transducer is exposed to the gamma-radiation. An additional JFET transistor, configured as a current source, helps to compensate for increases in the offset voltage of the JFET amplifier. In addition, the value of a resistor controlling the gate current of the JFET amplifier is reduced, such that when the gate leakage current increases, the offset voltage is reduced. 
   In an exemplary embodiment of the invention, a transducer for measuring vibrations which is capable of compensating for changes in the transducer&#39;s output voltage caused by radiation exposure comprises an accelerometer for generating acceleration signals in response to vibrations, an integrator for generating velocity signals by integrating the acceleration signals output by the accelerometer, the integrator comprising an amplifier, an alternating current (“AC”) feedback circuit for integrating the acceleration signals input to the amplifier, and a direct current (“DC”) feedback circuit for biasing the amplifier to produce a predetermined output voltage, the DC feedback circuit including a feedback resistor having a predetermined value selected to reduce by a predetermined amount changes in the output voltage of the amplifier due to increases in gate current into the amplifier resulting from the amplifier being exposed to radiation, the AC feed back circuit including a voltage divider circuit formed by first and second resistors, the voltage divider circuit applying a predetermined percentage of feedback voltage to the feedback resistor that causes the feedback resistor to have an effect in the AC feedback circuit as if the value of the feedback resistor were a predetermined multiple of the feedback resistor&#39;s actual value. 
   In another exemplary embodiment of the invention, a transducer for measuring vibrations which is capable of compensating for changes in the transducer&#39;s quiescent output voltage caused by radiation exposure comprises an accelerometer for generating acceleration signals in response to vibrations, an integrator for generating velocity signals by integrating the acceleration signals output by the accelerometer, the integrator comprising an operational amplifier, an alternating current (“AC”) feedback circuit for integrating the acceleration signals input to the amplifier, and a direct current (“DC”) feedback circuit for biasing the amplifier to produce a predetermined quiescent output voltage, the DC feedback circuit including a feedback resistor connected to operational amplifier&#39;s input, the feedback resistor having a predetermined value selected to reduce by a predetermined amount changes in the quiescent output voltage of the amplifier due to increases in gate current into the amplifier resulting from the amplifier being exposed to radiation, the AC feed back circuit including a voltage divider circuit formed by first and second resistors, the feedback resistor being connected between the voltage divider circuit and the amplifier&#39;s input, wherein the voltage divider circuit applying a predetermined percentage of feedback voltage to the feedback resistor that causes the feedback resistor to have an effect in the AC feedback circuit as if the value of the feedback resistor were a predetermined multiple of the feedback resistor&#39;s actual value. 
   In yet another exemplary embodiment of the invention, a transducer for measuring vibrations which is capable of compensating for changes in the transducer&#39;s quiescent output voltage caused by radiation exposure comprises an accelerometer for generating acceleration signals in response to vibrations, an integrator for generating velocity signals by integrating the acceleration signals output by the accelerometer, the integrator comprising a junction field effect transistor (“JFET”) amplifier, an alternating current (“AC”) feedback circuit for integrating the acceleration signals, and a direct current (“DC”) feedback circuit for biasing the amplifier to produce a predetermined quiescent output voltage, the DC feed back circuit including a feedback resistor connected to JFET amplifier&#39;s input, the feedback resistor having a predetermined value selected to reduce by a predetermined amount changes in the quiescent output voltage due to increases in gate current into the JFET amplifier resulting from the JFET amplifier being exposed to radiation, the AC feed back circuit including a voltage divider circuit formed by first and second resistors, the feedback resistor being connected between the voltage divider circuit and the JFET amplifier&#39;s input, wherein the voltage divider circuit applies a predetermined percentage of feedback voltage to the feedback resistor that causes the feedback resistor to function as if the value of the feedback resistor were a predetermined multiple of the feedback resistor&#39;s actual value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a conceptual schematic diagram for a basic velocity transducer. 
       FIG. 2  is a simplified schematic diagram for the basic velocity transducer of  FIG. 1 . 
       FIG. 3  is a more detailed circuit schematic of the simplified schematic shown in  FIG. 2 . 
       FIG. 4  is a simplified schematic diagram of a radiation resistant velocity transducer according to the present invention. 
       FIG. 5  is a more detailed circuit schematic of the simplified schematic shown in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is directed to a velocity transducer that provides electrical measurements relating to vibration despite being exposed to low levels of gamma-radiation. 
     FIG. 1  shows a conceptual schematic of a velocity transducer or Velomitor®  10  that is used to measure vibration of industrial equipment in an environment in which the equipment is subjected to low levels of gamma-radiation. The velocity transducer  10  includes an accelerometer  12  preferably in the form of a piezoelectric crystal, which produces electrical acceleration signals caused by a squeezing of the crystal due to vibrations. The velocity transducer  10  also includes an integrator  14  that integrates the acceleration signals output by accelerometer  12  so as to produce a velocity signal at the output of transducer  10 . 
   The operating characteristics of transducer  10  are typically as follows: (1) a bandwidth for measuring vibration frequencies ranging from 4 Hz to 5 kHz; (2) a response range for measuring vibrations of 0 to 50 inches per second; (3) a temperature operating range of −55° C. to 121° C.; and (4) acceptable bias drift after a total dose of 3 Mrads, maximum, giving a life expectancy of about four years. 
     FIG. 2  is a simplified schematic of the velocity transducer  10  shown in  FIG. 1 . The accelerometer  12  is depicted in the schematic of  FIG. 2  as being comprised of a variable voltage source V 3 , providing a direct current (“DC”) output voltage of 0V and an alternating current (“AC”) or variable signal output voltage of 1V, peak to peak, and a capacitor C 7  having a value of approximately 1,850 pf. The integrator  14  is depicted in the schematic of  FIG. 2  as being comprised of an operational amplifier U 1  including both AC and DC feedback circuits. Before being affected by gamma radiation, integrator  14  produces a quiescent DC output bias voltage of about 12V. 
   The DC feedback circuit of integrator  14  is comprised of a 5V zener diode D 2 , a second voltage divider circuit consisting of resistors R 5  and R 10 , and a third, very large resistor R 4  having a value of 500 megohms and being connected between the voltage divider and the negative input of operational amplifier U 1 . This DC feedback circuit is designed to apply a DC bias voltage of about 1.3V to the negative input of op operational amplifier U 1 . Similarly, a voltage source V 4  applies a DC bias voltage of about 1.3V DC to the positive input of operational amplifier U 1 . 
   The variable signal output voltage from accelerometer  12  is applied to a voltage divider comprised of resistors R 1  and R 2 . The portion of the variable signal output voltage across the second resistor, R 2 , is input to the negative input of operational amplifier U 1  of integrator  14  through a capacitor C 9  during conditions of equipment vibration. 
   Integrator  14  includes an AC feedback circuit that functions during conditions of equipment vibration when the accelerometer  12  is outputting a variable signal. The AC feedback circuit of integrator  14  includes a voltage divider consisting of resistors R 6  and R 7 , and a feedback capacitor C 8  connected between the voltage divider and the negative input of operational amplifier U 1 . Capacitor C 8  has a value of 470 pf. 
   The low frequency bandwidth of velocity transducer  10  is achieved through the use of large values for feedback resistor R 4  and feedback capacitor C 8 . These large values allow transducer  10  to measure vibrations having frequencies within a very low frequency range, i.e., the 4 Hz to 5 kHz frequency range noted above. 
     FIG. 3  shows a more detailed circuit schematic for the simplified circuit schematic shown in  FIG. 2 . In  FIG. 3 , accelerometer  12  is again depicted as being comprised of variable voltage source V 3  and capacitor C 7 . In addition, the output signal from accelerometer  12  is again shown as being split between the voltage divider formed by resistors R 1  and R 2 . 
   The schematic of  FIG. 3  also includes a first JFET transistor, J 1 , whose gate corresponds to the negative input of the operational amplifier U 1  shown in  FIG. 2 . JFET J 1  is part of a common source amplifier stage. The source of JFET J 1  is biased at approximately 1.3V by two JFET transistors, J 2  and J 3 , that are each wired as a diode with a voltage drop of approximately 0.65V each. The output of accelerometer  12  is again fed to the gate of transistor J 1  through capacitor C 9 . The DC feedback is again provided by zener diode D 2  and resistors R 5  and R 4 , also shown in the schematic of  FIG. 3 . The drain of JFET J 1  is connected to a PNP Darlington amplifier, Q 1 . 
   The circuit arrangement shown in  FIG. 3  produces a quiescent output of about 12 volts at the output terminal VDB connected to resistor R 10 . The problem with the circuit of  FIG. 3  results from the circuit being exposed to the gamma-radiation that is present in the environment in which transducer  10  must operate. When transducer  10  is new, the gate current into JFET J 1  is substantially zero. This produces a quiescent output voltage of 12 volts at the terminal VDB connected to resistor R 10 . In time, however, as JFET J 1  is exposed to the gamma-radiation, the gate current, Ig, into the gate of JFET J 1  increases linearly over time with the radiation dose to which J 1  is exposed. As the gate current increases, the quiescent output voltage of transducer  10  decreases to compensate for the increased gate current, Ig, by “bleeding off” the increased gate current through feedback resistor R 4 , which has a large resistance value of 500 megohms. After approximately 50 days, the increased gate current flowing into J 1  decreases the quiescent output bias voltage of velocity transducer  10  by approximately 3 volts. For each 50 days thereafter, the output voltage of velocity transducer  10  decreases by approximately 3 volts until, ultimately, it approaches zero, so that the device no longer functions to provide any vibration measurements. The difficulty presented by this change in output voltage is compounded by the fact that the affected transducer  10  can not be readily replaced for a period of at least a year to two years, since the plants in which transducer  10  is typically located are sealed and operated for such period without access to them due to the radiation to which they are exposed. 
     FIG. 4  shows a simplified schematic of one embodiment of a radiation resistant velocity transducer  20  according to the present invention, while  FIG. 5  shows a more detailed circuit schematic for the transducer  20  of  FIG. 4 . Circuit components shown in  FIGS. 4 and 5 , which are identical to those circuit components shown in  FIGS. 2 and 3 , bear the same identifying notations. Thus, for example, the accelerometer  12  is again depicted in  FIGS. 4 and 5  as being comprised of the variable voltage source V 3  and capacitor C 7 , with the variable signal output of the accelerometer  12  being divided between resistors R 1  and R 2 . The divided voltage signal from accelerometer  12  is again input to the operational amplifier U 1  or JFET J 1  through capacitor C 9 . The AC feedback circuit in the schematic of  FIG. 4  is like that shown in the schematic of  FIG. 2 . 
   The DC feedback circuit used in the circuit of  FIGS. 4 and 5  is different from the DC feedback circuit used in the circuit of  FIGS. 2 and 3 . The DC feedback circuit of  FIGS. 4 and 5  includes a feedback resistor R 14 , which has a value of 5 megohms, a value that is 100 times less than the 500 megohm value of resistor R 4 , which R 14  replaces. Because resistor R 14  is 100 times less in value than the value of resistor R 4 , the circuit of  FIGS. 4 and 5  is less susceptible to the effects of the gamma-radiation over time on JFET J 1  shown in  FIG. 5 . Like in the circuit of  FIG. 2 , as JFET J 1  is exposed to the gamma-radiation, its gate current, Ig, increases linearly over time. However, because the 5 megohm value R 14  is substantially less than the 500 megohm value of original resistor R 4 , the effect of increases in gate current Ig on the output of transducer  20  is substantially less. As the gate current Ig increases, the output offset voltage of transducer  20  will again decrease to compensate for the increased gate current, Ig, by “bleeding off” the increased gate current through feedback resistor R 14 ; but, because R 14  has a resistance value of 5 megohms, which is 100 times smaller than the 500 megohm value of R 4 , the change in the quiescent output voltage will be substantially less. For example, where the output of transducer  10  was 3V after 50 days of radiation exposure, the change in the output of transducer  20  would be 30 mV after 50 days of radiation exposure. 
   Resistor R 13  is “transparent” to the DC feedback circuit shown in  FIGS. 4 and 5  because it is isolated by capacitor C 11 . However, when the accelerometer  12  begins to produce variable output signals because of its sensing of vibrations, capacitor C 11  effectively becomes a short circuit with respect to such variable signals. As such, the variable signal voltage at the output of operational amplifier U 1  sees a voltage divider between R 15  and R 13 , with only a small fraction of preferably about 0.2% being applied across resistor R 13 . Feedback resistor R 14  is connected between this voltage divider and the negative input to operational amplifier U 1 . The effect of the reduction in voltage applied to R 14  by the voltage divider produces causes resistor R 14  to have an effect in the DC feedback circuit that is the same as if the value of resistor R 14  were the 500 megohm value of original resistor R 4 . It is as though the 5 megohm value of R 14  is multiplied by 100. As such, transducer  20  provides a bandwidth for sensing vibrations between 4 Hz and 5 kHz, as in the original circuit for transducer  10  shown in  FIGS. 2 and 3 . 
   The DC feedback circuit of  FIGS. 4 and 5  also includes a resistor R 15 , preferably having a value of 10 megohms, and a current source I 1 . Preferably current source I 1  draws a current of about 1.07 μA through resistor R 15  to produce a voltage drop of about 10.7 volts across resistor R 15 , to thereby provide a voltage of about 1.3V that is applied to the negative input of operational amplifier U 1  through resistor R 14 . 
   As noted above,  FIG. 5  shows a more detailed circuit schematic for the radiation resistant velocity transducer  20 . Here again, the accelerometer  12  is comprised of variable voltage source V 3  and capacitor C 7 , while the variable output voltage of accelerometer  12  is divided between voltage divider resistors R 1  and R 2 . 
   The circuit schematic of  FIG. 5  also includes JFET J 1 , whose gate again corresponds to the negative input of operational amplifier U 1  shown in  FIG. 4 , and JFETs J 2  and J 3 , which function as diodes to provide the biasing of JFET J 1  between its gate and source. The variable voltage output of accelerometer  12  is again fed to the gate of J 1  through capacitor C 9 , and the drain of J 1  is again connected to Darlington amplifier, Q 1 . 
   The function of current source I 1  in the DC feedback circuit shown in  FIG. 4  is performed by a fourth JFET transistor J 4 , which, as shown in  FIG. 5 , is connected so as to function as a current source. The other components of the DC feedback circuit, i.e., resistors R 15  and R 14 , are also shown in  FIG. 5  as being connected in the same manner as that shown in  FIG. 4  with respect to the current source I 1 , now depicted as JFET J 4 . 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.