US 3652898 A
A two channel bistable trip circuit, each channel including a variable reference voltage source and voltage comparator circuit. An input signal commensurate with a variable being monitored is delivered to the comparator in each channel and, when the variable reaches preselected levels commensurate with the reference voltages selected for each channel, control signals will be provided. Means are also provided for isolating the control signal of one channel from apparatus normally responsive thereto under certain conditions.
Claims available in
Description (OCR text may contain errors)
United States Patent Steeves [4 1 Mar. 28, 1972 54] DUAL CHANNEL MONITORING 3,311,835 3/1967 Richman ..328/146 APPARATUS 3,474,258 10/1969 Nagy ..307/235  Inventor: Linda A. Steeves, Endicott, NY.
 Assignee: Combustion Engineering, Inc., Windsor,
 Filed: Dec. 27, 1968  Appl. No.1 787,318
 U.S.Cl ..3l7/27, 176/19, 307/235,
[51 1 Int. Cl. ..H02h 3/28  Field ofSearch ..3l7/27,3l,33;340/213; 307/235, 202, 217; 176/19  References Cited UNITED STATES PATENTS 2,345,131 3/1944 Leonard ..3l7/14 3,213,293 10/1965 Finlan et al ..307/235 Primary Examiner-J. D. Miller Assistant Examiner-Harry E. Moose, .l r.
Attorney-Carlton F, Bryant, Eldon H. Luther, Robert L. 01- son, John F. Carney, Richard H. Berneike, Edward L. Kochey, Jr. and Lawrence P. Kessler [5 7] ABSTRACT A two channel bistable trip circuit, each channel including a variable reference voltage source and voltage comparator circuit. An input signal commensurate with a variable being monitored is delivered to the comparator in each channel and, when the variable reaches preselected levels commensurate with the reference voltages selected for each channel, control signals will be provided. Means are also provided for isolating the control signal of one channel from apparatus normally responsive thereto under certain conditions.
9 Claims, 2 Drawing Figures DUAL CHANNEL MONITORING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the monitoring of apparati or processes for safety purposes. More particularly, the present invention is directed to apparatus for sensing plant parameters and generating alarm and safety control signals in response thereto as the monitored parameters approach and exceed predetermined safe levels. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art While not limited thereto in its utility, the present invention is particularly well suited for use in nuclear reactor safety systems. It is imperative, in order to prevent accidents of calamitous proportions, to provide nuclear power plants with safety control systems that are capable of immediately responding to deviations from safe operation of the plant and to terminate reactor operation whenever such deviations occur. Such systems must be characterized by a high degree of reliability and must be so designed as to be operative whenever the power plant is in operation. Such safety systems include means for sensing dangerous levels of neutron flux, temperature, pressure, etc. and for generating signals commensurate with these operational parameters. The safety systems also include circuitry, operating in response to the signals commensurate with the sensed operational parameters, for generating alarm and safety equipment trip signals when a given output level from an operational parameter monitoring transducer is exceeded.
Bistable trip circuits for employment in monitoring systems are well known in the art. However, systems employing the prior art trip circuits have been characterized by certain inherent dificiencies. Thus, for example, reliable circuitry has not previously been available for first providing a pretrip warning or alarm energization signal as the parameter being monitored approached the dangerous level coupled with the ability to thereafter also provide an actual trip signal if the dangerous parameter level was reached. Similarly, prior art circuitry has not been characterized by flexibility which enables selecting various trip and pretrip signal levels. Also, and this is particularly true in the case of nuclear applications, prior art trip units could not readily be bypassed as is necessary in order to disable the safety devices during reactor start up.
SUMMARY OF THE INVENTION The present invention overcomes the foregoing and other disadvantages of the prior art and in so doing provides a novel two channel bistable trip circuit particularly well suited for nuclear reactor safety system applications. The bistable trip circuit of the present invention senses a signal from instrumentation representing the status of a plant parameter, compares the thus sensed parameter status signal to a preset alarm reference level and a preset trip reference level, provides a pre-trip alarm signal as the sensed parameter approaches the alarm level, and provides a trip signal for energizing a safety device when the parameter reaches the danger or trip level.
Circuitry for accomplishing the foregoing comprises a pair of operational amplifiers arranged as voltage comparators with variable hysteresis. Each of the operational amplifiers has a preselected and adjustable reference voltage applied as an input thereto. The input signal commensurate with the monitored parameter is also applied as an input to both operational amplifiers. As this input signal passes the pre-trip alarm set point (reference voltage), the first or alarm amplifier changes state thereby causing, through suitable circuitry, a relay to drop out and the pre-trip alarm to be actuated. If the input signal continues to rise or fall so that it passes through the trip set point, the other operational amplifier changes state causing, again through suitable circuitry, a plurality of trip relays to drop out and a trip indicator to be energized. When the input signal returns to its normal level, the operational amplifiers will return to their initial state and the trip relays will be reenergized. However, both the pre-trip and trip alarms will remain in an energized state until manually reset.
A particularly novel feature of the present invention resides in the fact that the trip" amplifier may be bypassed without interrupting or in any way interfering with the input signal commensurate with the monitored parameter. Bypassing of the trip" circuit is accomplished through application of an external logic voltage which prevents the trip relays from dropping out. In the case of a nuclear reactor, it is highly desirable if not essential to provide for the disabling of the safety device trip circuits during reactor start up and the ability to achieve such bypassing merely through the application of an external voltage as opposed to reconfiguring the circuit constitutes a substantial step forward in the art. It is also to be noted that the trip and pre-trip alarm circuits are independent of each other in accordanc e with the present invention and thus bypassing of the trip" circuit does not result in the pre-trip alarm being bypassed.
BRIEF DESCRIPTION OF THE DRAWING The present invention may be better understood and its numerous advantages will become obvious to those skilled in the art by reference to the accompanying drawing wherein like reference numerals refer to like elements in the figures and in which:
FIG. 1 is a block diagram of the present invention.
FIG. 2 is a schematic drawing of the preferred embodiment of the invention shown in block form in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT With reference now to FIG. 1, the present invention comprises a pair of comparator circuits which are indicated generally at 10 and 12. Comparator 10 functions as the trip" unit comparator while comparator 12 functions as the pretrip alarm unit comparator. Comparators 10 and 12 may, for example, be commercially available operational amplifiers such as Motorola type MC 1433T. The input signal commensurate with the level of the process parameter being monitored is applied as an input to each of parallel connected comparators l0 and 12.
Each of comparators l0 and 12 has a second or reference input signal applied thereto. In the case of trip comparator 10, the reference signal may be manually selected by means of adjusting a reference voltage generator 14. Reference voltage generator 14 may comprise merely series connected potentiometers and a suitable voltage source. Alternatively, a variable reference voltage may be applied to trip comparator 10 by means of a variable reference voltage generator 16 which operates in response to an input signal commensurate with a selected process parameter. Selection of a variable or fixed reference input for trip comparator 10 is accomplished by means of switch S1.
As in the case of the trip" comparator, the second or reference input to the pre-trip alarm" comparator 12 may be manually preset by means of adjusting a reference voltage source 18 or a variable reference voltage may be generated by circuit 20 which is responsive to a monitored process parameter. Selection of a preset fixed or variable reference input to pre-trip" alarm comparator 12 is accomplished by means of switch S2.
The output of trip" comparator 10 is delivered to a buffer amplifier 22 which provides, from the output signal of the trip comparator, sufficient power to energize driver amplifiers 24, 26, 28, 30 and 32. The output of driver amplifier 24 is applied to a trip indicator circuit 34. The outputs of driver amplifiers 26, 28, 30 and 32 are respectively applied to trip relay circuits 36, 38, 40 and 42. The trip relays may employ double wound coils, one coil being operative in the control system test circuit whereby actuation of the control system can be simulated by a test circuit that is completely isolated from the circuitry of the control system. Such test circuitry is shown and described in a copending application of Frank Bevilacqua and Christoffel H. Meijer entitled Test Circuit for Reactor Safety Control System, Ser. No. 677,109, filed Sept. 23, 1967 and assigned to the same assignee as the present invention.
The output of pre-trip alarm comparator 12 is applied to a pair of driver amplifiers 44 and 46. Driver amplifier 44 provides energization for a pre-trip indicator circuit 48. Driver amplifier 46, in the usual instance, provides energization for a pre-trip alarm relay 50 which is connected in a pre trip alarm annunciator circuit.
it is to be noted that means in the form of a switch S3 are provided to enable the application of a second input signal to buffer amplifier 22. This additional signal will be generated by an external logic voltage generator 52 and will be of sufficient magnitude and of proper polarity to prevent an output signal from trip comparator l0, regardless of magnitude, from affecting the state offidriver amplifiers 24-32. Switch S3 will be closed and the external voltage applied to the input of buffer amplifier 22 from generator 52 upon reactor startup. As should be obvious, as a result of the mode of application of the bypass voltage from generator 52 to the input to buffer amplifier 52, the bypass signal does not in any way interfere with or interrupt the input signal from the process parameter monitor and the pre-trip comparator 12 will function in the usual manner when switch S3 is closed.
Turning now to a consideration of the schematic drawing comprising FIG. 2, variable input reference voltage generators l6 and 20 and switches S1 and S2 have been omitted in the interest of clarity. Similarly, the trip relay circuits have been shown merely as two coil solenoids with provision for application of test voltages. The signal commensurate with the monitored process parameter will typically be a ramp input generated by a pressure transducer 60 and developed across resistor R1. This input voltage is shown as being applied directly to the negative input terminal of the operational amplifier which comprises trip" comparator 10 with simultaneous application of this signal to the negative input terminal of the operational amplifier which comprises pre-trip alarm comparator 12. The selected reference voltage from generator 14 is shown as being applied to the other or positive input terminal of comparator 10 while the selected input voltage from reference voltage generator 18 is shown as being applied to the positive input terminal of comparator 12.
The amplifiers comprising comparators l and 12 are high gain operational amplifiers utilizing positive feedback. As previously noted, the operational amplifiers are connected as voltage comparators with variable hysteresis, hysteresis adjustment for the amplifier of comparator being accomplished via potentiometer R2 and hysteresis adjustment for the amplifier comprising comparator 12 being accomplished by potentiometer R3. The response time of the operational amplifier circuits may be adjusted by varying the value of capacitors C1 in the frequency compensating network for each of the operational amplifiers. In this manner, any desired delay up to 200 milliseconds may be introduced between the time the input signal to the amplifier reaches the trip level and the time the trip relays 36, 38, 40 and 42 associated with comparator 10 drop out. Obviously, the pre-trip alarm and trip units may have different delay times so that it is possible to have, for example, a fast pre-trip alarm but a delayed trip.
As previously noted, the output of trip comparator circuit 10 drives buffer amplifier 22. Amplifier 22 comprises a transistor Q1 connected as an emitter follower, the emitter follower connection being dictated by the need for sufficient power to drive amplifiers 26, 28, 30 and 32 and, through these amplifiers, their associated trip relays. it is to be noted that a diode CR1 is connected between trip comparator 10 and the input to buffer amplifier 22. Diode CR1 protects the operational amplifier comprising trip comparator 10 when a test or bypass signal generator 52 (FIG. 1) is applied to the input of buffer amplifier 22 via conductor 54. Restated, diode CR1 ensures that the base-emitter breakdown voltage of transistor Q1 of buffer amplifier 22 will not be exceeded when the output of the operational amplifier comprising the trip comparator 10 goes negative.
As previously noted, a first output derived directly from the emitter of buffer amplifier 22 is applied to the base of each of driver amplifiers 26, 28, 30 and 32. In the embodiment being described, with the circuit configured for an upscale trip point with fixed reference or set points, the outputs of the operational amplifiers in comparators 10 and 12 are positive when the trip unit is in the untripped state. Under these conditions, the driver amplifiers 26, 28, 30 and 32, which may be considered as solid state switches, are in the conductive state and their associated relays are energized. A second output signal taken from the emitter circuit of buffer amplifier 22 is applied to the base of trip-alarm driver amplifier 24. Transistor Q2, which comprises driver amplifier 24, is of the opposite conductivity type from those which comprise driver amplifiers 26, 28, 30 and 32. Accordingly, when the relay driver amplifiers are in their normally conductive state, the trip alarm driver amplifier is biased off. Upon receipt of an input signal of sufficient magnitude and polarity to cause buffer amplifier 22 to change state, either from bypass signal generator 52 or from the process monitoring transducer via the operational amplifier in comparator 10, driver amplifiers 26, 28, 30 and 32 will be turned off and their associated relays will drop out. Simultaneously, transistor Q2 of driver amplifier 24 will be turned on. Diode CR8 provides isolation between transistor amplifier 24 and amplifiers 26-32 while resistor R4 provides base drive for amplifier 24 (Q3) and establishes a bias voltage for turning the driver amplifiers back on.
Trip alarm driver amplifier 24 functions as a triggering transistor for silicon controlled rectifier SCRl in the trip indicator circuit 34. Signals derived from the collector circuit of transistor Q2 are applied to the gate and cathode of SCRl thus turning on the rectifier device and energizing a warning lamp 62. As is well known in the art, the silicon controlled rectifier, which acts as a switch for the trip warning lamp 62, cannot be turned off until the current through it falls below the holding current of the device. Accordingly, even though normal circuit conditions return and the trip relays are pulled back in, and trip alarm driver amplifier 24 deenergized, the trip alarm warning will remain energized until manually reset. Manual resetting of the trip indicator circuit is accomplished via switch S6 which can be used to momentarily interrupt the current to SCR-l. The trip indicator circuit and lamp may be tested by the closing of switch S7.
The output of pre-trip alarm comparator circuit 12 is applied to the base of a normally conductive annunciator relay driver amplifier 46. The output of pre-trip comparator 12 is also applied to the base of pre-trip alarm driver amplifier 44. Driver amplifier 44 is normally nonconductive and functions in the same manner as the driver amplifier 24 to render conductive a switching device, SCR-2, in pre-trip indicator circuit 48 thereby energizing a warning lamp 64. Also as in the case of trip indicator alarm circuit 34, once energized, the pre-trip indicator light 64 will remain on until current to SCR-2 is interrupted by manually breaking the circuit by opening switch S8. The pre-trip indicator circuit and light may be tested by means of switch S9.
Operation of the circuit is as follows: if the input signal from the process monitoring transducer 60 increases until it is greater than the pre-trip alarm set point voltage as provided at the movable arm of switch S4 in voltage generator 18, but less than the trip set point voltage provided at the output of reference voltage source 14, the operational amplifier in comparator 12 will change state and its output will switch from positive to negative. At this time, the output of the operational amplifier in trip comparator 10 will be positive and will remain so. The change in polarity of the output of comparator 12 will cause driver amplifier 46 to stop conducting and pre-trip annunciator relay 50 to be deenergized thereby sounding an audible alarm. Simultaneously, pre-trip indicator driver amplifier 44 will begin to conduct thus triggering SCR-2, in pre-trip indicator circuit 48, into conduction. As SCR-Z begins to conduct, the pre-trip alarm light 64 will be turned on. If the input signal continues to increase until it is greater than the trip set point voltage appearing at the movable arm of switch S5 in reference voltage generator 14, the operational amplifier in trip comparator will change state and its output will go negative causing, in the manner above described, relays 36, 38, 40 and 42 to be deenergized. At the same time, also as above described, trip alarm driver amplifier 24 will be turned on and will in turn trigger SCR-1, in trip indicator circuit 34, into conduction.
If the input signal from the transducer 60 now decreases until it is smaller than the trip set point voltage by a preselected percentage of the set point voltage, this percentage being a function of the hysteresis produced by the positive feedback through potentiometer R2 of comparator 10, the operational amplifier in comparator 10 will revert to its original state and the positive output signal thus provided will, through the action of the buffer amplifier and driver amplifiers, cause the trip relays 36, 38, 40 and 42 to become reenergized. However, as noted above, even though driver amplifier 24 will stop conducting, the trip alarm indicator lamp 62 in circuit 34 will remain energized since SCR-l will continue to conduct. Lamp 62 may, however, be turned off by momentarily depressing switch S6 to reduce the current through SCR-l below its minimum holding current. If the input signal continues to decrease until it is smaller than the pre-trip alarm set point voltage by the amount of hysteresis selected for the operational amplifier of comparator 12 the output of comparator 12 will revert to the positive state thus turning driver amplifier 46 back on and shutting driver amplifier 44 off. Thus, the annunciator relay 50 will pick up shutting off the audible alarm. However, as in the case of the trip-alarm relay, warning lamp 64 in pre-trip indicator circuit 68 will remain energized until switch S8 is manually opened.
For operation with upscale trip points and variable as opposed to fixed preset reference voltages, the circuit operates in the same manner as above described with the exception that the set voltages are not taken from the trip unit itself but are applied thereto from external sources such as voltage generators 16 and 20.
The present invention may also operate with downscale trip with either fixed or variable set points. For downscale trip, the operational amplifiers in comparators l0 and 12 change state from positive (untripped state) to negative (tripped state) as the input signal falls below the set point. If it is desired to switch from upscale to downscale tripping, it is merely necessary to reverse the connections A and B and C and D at terminal board 66.
The present invention is also characterized by low drift in that a temperature compensated Zener diode ZDl is employed to obtain the preset set point or reference voltages. A second Zener diode, ZD2, serves as overvoltage protection at the input to the unit. It is also to be noted that the overall size of the power supply required for the present invention had been reduced by using the negative side of the power supply for the trip and the pre-trip alarm lights, diode CR2 providing protection against inadvertent reversal of the polarity of the supply voltage. Additional diodes CR3-CR7 provide protection for driver amplifiers 26, 28, 30, 32 and 46 from high voltage spikes which are generated when the associated relay coils are deenergized.
While a preferred embodiment has been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the present invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.
What is claimed is:
1. Control signal generating apparatus, said apparatus being responsive to a signal commensurate with a variable being monitored, the signal generating apparatus comprising:
a. means for generating a first reference signal commensurate with a first level of the monitored variable; b. means for generating a second reference signal commensurate with a second level of the monitored variable;
c. first comparator means responsive to the signal commensurate with the monitored variable and the first reference signal for generating a first control signal when the monitored variable reaches a first level;
d. second comparator means responsive to the signal commensurate with the monitored variable and the second reference signal for generating a second control signal when the monitored variable reaches a second level;
e. means responsive to said first control signal for providing a first indication of the level of the monitored variable;
f. means responsive to said second control signal for providing a second indication of the level of the monitored variable; and
g. means connected to said second comparator means for isolating said second indication providing means from said second control signal.
2. The apparatus of claim 1 wherein said first and second reference signal generating means each comprise:
a. an adjustable voltage source.
3. The apparatus of claim 2 wherein said first and second comparator means each comprise:
a. bistable circuit means.
4. The apparatus of claim 3 wherein said bistable circuit means comprise:
a. operational amplifiers with positive feedback.
5. The apparatus of claim 3 wherein said indication providing means each comprise:
a. self-latching alarm means; and
b. means for manually deencrgizing said alarm means.
6. The apparatus of claim 5 wherein said second indication providing means further comprises:
a. means responsive to said second control signal for providing an energization signal for means for regulating the variable being monitored.
7. The apparatus of claim 1 wherein said second indication providing means further comprises:
a. means responsive to said second control signal for providing an energization signal for means for regulating the variable being monitored.
8. The apparatus of claim 1 wherein said isolating means comprises:
a. means for generating and applying at the output of said second comparator means a signal of opposite polarity to said second control signal.
9. The apparatus of claim 3 wherein said isolating means comprises:
a. means for generating a signal of opposite polarity to said second control signal; and
b. means for applying said opposite polarity signal to the bistable circuit means comprising said second comparator means in opposition to said second control signal.