FEEDBACK CIRCUIT FOR RADIATION RESISTANT TRANSDUCER
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® are one brand of velocity transducers that are used to provide protection of industrial machinery by measuring equipment vibration.
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 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'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's output, driving the transducer's output to the device's rail (supply voltage).
BRIEF DESCRIPTION OF THE INVENTION
An object of the present invention is to provide a velocity transducer that can output electrical signals relating to vibration, despite the transducer being exposed to low levels of gamma-radiation. In the present invention, 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 sink, 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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a conceptual schematic diagram for a basic velocity transducer.
FIGURE 2 is a simplified schematic diagram for the basic velocity transducer of FIGURE 1.
FIGURE 3 is a more detailed circuit schematic of the simplified schematic shown in FIGURE 2.
FIGURE 4 is a simplified schematic diagram of a radiation resistant velocity transducer according to the present invention.
FIGURE 5 is a more detailed circuit schematic of the simplified schematic shown in FIGURE 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.
FIGURE 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 4Hz to 5kHz; (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) when exposed to a radiation of 3 Mrads, a life expectancy of about four years.
FIGURE 2 is a simplified schematic of the velocity transducer 10 shown in FIGURE I. The accelerometer 12 is depicted in the schematic of FIGURE 2 as being comprised of a variable voltage source V3, providing a direct current ("DC") output voltage of 0V and an alternating current ("AC") or variable signal output voltage of IV, peak to peak, and a capacitor C7 having a value of approximately 1,850 pf. The integrator 14 is depicted in the schematic of FIGURE 2 as being comprised of an operational amplifier Ul 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.
Integrator 14's DC feedback circuit is comprised of a 5V zener diode D2, a second voltage divider circuit consisting of resistors R5 and RIO, and a third, very large resistor R4 having a value of 500 megohms and being connected between the voltage divider and the negative input of operational amplifier Ul. This DC feedback circuit is designed to apply a DC bias voltage of about 1.3V to the negative input of op operational amplifier Ul. Similarly, a voltage source V4 applies a DC bias voltage of about 1.3V DC to the positive input of operational amplifier Ul.
The variable signal output voltage from accelerometer 12 is applied to a voltage divider comprised of resistors Rl and R2. The portion of the variable signal output
voltage across the second resistor, R2, is input to the negative input of operational amplifier Ul of integrator 14 through a capacitor C9 during conditions of equipment vibration.
Integrator 14 includes an AC feedback circuit that functions during conditions of equipment vibration when the accelerator 12 is outputting a variable signal. Integrator 14's AC feedback circuit includes a voltage divider consisting of resistors R6 and R7, and a feedback capacitor C8 connected between the voltage divider and the negative input of operational amplifier Ul. Capacitor C8 has a value of 470 pf.
The bandwidth of velocity transducer 10 is achieved through the use of large values for feedback resistor R4 and feedback capacitor C8. These large values allow transducer 10 to measure vibrations having frequencies below and within the very low frequency range, i.e., the 4 Hz to 5 kHz frequency range noted above.
FIGURE 3 shows a more detailed circuit schematic for the simplified circuit schematic shown in FIGURE 2. In FIGURE 3, accelerometer 12 is again depicted as being comprised of variable voltage source V3 and capacitor C7. In addition, the output signal from accelerometer 12 is again shown as being split between the voltage divider formed by resistors Rl and R2.
The schematic of FIGURE 3 also includes a first JFET transistor, Jl, whose gate functions as the negative input of the operational amplifier Ul shown in FIGURE 2. JFET Jl is part of a common source amplifier stage. The source of JFET Jl is biased at approximately 1.3V by two JFET transistors, J2 and J3, that are each wired as a diode with a voltage drop of approximately 0.6V. The output of accelerometer 12 is again fed to the gate of transistor Jl through capacitor C9. The DC feedback is again provided by zener diode D2 and resistors R5 and R4, also shown in the schematic of FIGURE 3. The drain of JFET Jl is connected to a PNP Darlington amplifier, Ql.
The circuit arrangement shown in FIGURE 3 produces a quiescent output of about 12 volts at the output terminal VDB connected to resistor RIO. The problem with the circuit of FIGURE 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 Jl is substantially zero. This produces a quiescent output voltage of 12 volts at the terminal VDB connected to resistor RIO. In time, however, as JFET Jl is exposed to the gamma-radiation, the gate current, Ig, into the gate of JFET Jl increases linearly over time with the radiation dose to which Jl 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 R4, which has a large resistance value of 500 megohms. After approximately 50 days, the increased gate current flowing into Jl 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.
FIGURE 4 shows a simplified schematic of one embodiment of a radiation resistant velocity transducer 20 according to the present invention, while FIGURE 5 shows a more detailed circuit schematic for the transducer 20 of FIGURE 4. Circuit components shown in FIGURES 4 and 5, which are identical to those circuit components shown in FIGURES 2 and 3, bear the same identifying notations. Thus, for example, the acceleisometer 12 is apin depicted in FIGURES 4 and 5 as being comprised of the variable voltage source V3 and capacitor C7, with the variable signal output of the acceleroitteter 12 being divided between resistors Rl and R2. The divided voltage signal from accelerometer 12 is again input to the operational amplifier Ul or JFET Jl through capacitor C9. The AC feedback circuit in the schematic of FIGURE 4 is like that shown in the schematic of FIGURE 2.
The DC feedback circuit used in the ckcuit of FIGURES 4 and 5 is different from the DC feedback circuit used in the circuit of FIGURES 2 and 3. The DC feedback circuit of FIGURES 4 and 5 includes a feedback resistor R14, which has a value of 5
megohms, a value that is 100 times less than the 500 megohm value of resistor R4, which R14 replaces. Because resistor R14 is 100 times less in value than the value of resistor R4, the circuit of FIGURES 4 and 5 is less susceptible to the effects of the gamma-radiation over time on JFET Jl shown in FIGURE 5. Like in the circuit of FIGURE 2, as JFET ,11 is exposed to the gamma-radiation, its gate current, Ig, increases linearly over time. However, because the 5 megohm value R14 is substantially less than the 500 megohm value of original resistor R4, 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 R14; but, because R14 has a resistance value of 5 megohms, which is 100 times smaller than the 500 megohm value of R4, 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 RIO is "transparent" to the DC feedback circuit shown in FIGURES 4 and 5 because it is isolated by capacitor Cll. However, when the accelerometer 12 begins to produce variable output signals because of its sensing of vibrations, capacitor Cll effectively becomes a short circuit with respect to such variable signals. As such, the variable signal voltage at the output of operational amplifier Ul sees a voltage divider between R15 and R13, with only a small fraction of preferably about .2% being applied across resistor R13. Feedback resistor R14 is connected between this voltage divider and the negative input to operational amplifier U1. The effect of the reduction in voltage applied to R14 by the voltage divider produces causes resistor R14 to have an effect in the DC feedback circuit that is the same as if the value of resistor R14 were the 500 megohm value of original resistor R4. It is as though the 5 megohm value of R14 is multiplied by 100. As such, transducer 20 provides a bandwidth for sensing vibrations between 4Hz and 5 kHz, as in the original circuit for transducer 10 shown in FIGURES 2 and 3.
The DC feedback circuit of FIGURES 4 and 5 also includes a resistor R15, preferably having a value of 10 megohms, and a current source II. Preferably current source II draws a current of about 1.07 uA through resistor R15 to produce a voltage drop of about 10.7 volts across resistor R15, to thereby provide a voltage of about 1.3V that is applied to the negative input of operational amplifier Ul through resistor R14.
As noted above, FIGURE 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 V3 and capacitor C7, while the variable output voltage of accelerometer 12 is divided between voltage divider resistors Rl and R2.
The circuit schematic of FIGURE 5 also includes JFET Jl, whose gate again functions as the negative input of operational amplifier Ul shown in FIGURE 4, and JFETs J2 and J3, which function as diodes to provide the biasing of JFET Jl between its gate and source. The variable voltage output of accelerometer 12 is again fed to the gate of Jl through capacitor C9, and the drain of Jl is again connected to Darlington amplifier, Ql.
The function of current source II in the DC feedback circuit shown in FIGURE 4 is performed by a fourth JFET transistor J4, which, as shown in FIGURE 5, is connected so as to function as a current source. The other components of the DC feedback circuit, i.e., resistors R15 and R14, are also shown in FIGURE 5 as being connected in the same manner as that shown in FIGURE 4 with respect to the current source II, now depicted as JFET J4.
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.

CLAIMS
1. A transducer (10) for measuring vibrations which is capable of compensating
for changes in the transducer's output voltage caused by radiation exposure, the
transducer comprising:
an accelerometer (12) for generating acceleration signals in response to vibrations,
an integrator (14) for generating velocity signals by integrating the acceleration signals output by the accelerometer, the integrator comprising:
an amplifier (Ul),
an alternating current ("AC") feedback circuit (R6, R7, C8) for integrating the acceleration signals input to die amplifier, and
a direct current ("DC") feedback circuit (D2, R5, R10, R4) for biasing the amplifier to produce a predetermined output voltage,
the DC feedback circuit (D2, R5, RIO, R4) including a feedback resistor (R4) having a predetermined value selected to reduce by a predetermined amount changes in the output voltage of the amplifier (Ul.) due to increases in gate current into the amplifier (Ul) resulting from the amplifier (Ul) being exposed to radiation,
the AC feed back circuit (R6, R7, C8) including a voltage divider circuit formed by first and second resistors (R6, R7), the voltage divider circuit (R6, R7) applying a predetermined percentage of feedback voltage to the feedback resistor (R4) that causes the feedback resistor (R4) to have an effect in the AC feedback circuit (R6, R7, C8) as if the value of the feedback resistor (R4) were a predetermined multiple of the feedback resistor's (R4) actual value.
2, The transducer (10) of claim 1, wherein the amplifier (Ul) includes a junction
field effect transistor ("JFET") (Jl) whose gate current is increased when exposed to
radiation over time.
3. The transducer (10) of claim 1, wherein the feedback resistor (R4) has a value
of 5 megohms.
4. The transducer (10) of claim 1, wherein the predetermined multiple of the
feedback resistor's (R4) actual value is 100.
5. The transducer (10) of claim 1, wherein the DC feed back circuit (D2, R5,
RIO, R4) includes a current source (II) for drawing a predetermined current through
the first resistor (R6) to thereby apply a predetermined biasing voltage to an input of
amplifier (Ul) through the feedback resistor (R4).
6. The transducer (10) of claim 5, wherein the first resistor (R6) has a value of
about 10 megohms.
7. The transducer of claim 6, wherein the current source (II) draws a current of
about 1.07 uA through the first resistor (R6) to apply the predetermined biasing
voltage to the amplifier (Ul).
8. The transducer (10) of claim 1, wherein the transducer (10) has a bandwidth
for sensing vibrations of between 4Hz and 5 kHz.
9. The transducer (10) of claim 1, wherein the AC feed back circuit (R6, R7, C8)
includes a second voltage divider (RS, RIO) that causes a predetermined fraction of a
variable signal voltage at the output of the operational amplifier (Ul) to be applied to
the feedback resistor (R4).
10. The transducer (10) of claim 1, wherein the predetermined fraction is about
0.2%.