Abstract:
Logic elements are provided for a reactor period meter trip circuit. For one element, first and second inputs are applied to first and second chopper comparators, respectively. The output of each comparator is O if the input applied to it is greater than or equal to a trip level associated with each input and each output is a square wave of frequency f if the input applied to it is less than the associated trip level. The outputs of the comparators are algebraically summed and applied to a bandpass filter tuned to f. For another element, the output of each comparator is applied to a bandpass filter which is tuned to f to give a sine wave of frequency f. The outputs of the filters are multiplied by an analog multiplier whose output is 0 if either input is 0 and a sine wave of frequency 2f if both inputs are a frequency f.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The invention described herein was made in the course of, or under, a contract with the UNITED STATES ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION. 
    
    
     BACKGROUND OF THE INVENTION 
     A reactor period meter is a device for monitoring a reactor&#39;s period T. With the reactor period below a critical value, that is, it is too short, the power level is increasing too quickly. A reactor scram signal should be initiated by the reactor period meter, indicating the reactor power should be reduced. Such a meter includes means for developing a signal proportional to reactor period and logical elements for determining whether the reactor period indicated by the signal is at a critical scram level. For use in a reactor period meter, such logic elements should be fail safe, and should indicate scram condition with the reactor period at the scram level and it should indicate scram condition with failure of any of the components of the period meter. With D.C. digital, logical elements, failure of the logic elements themselves might not necessarily cause a scram signal to be generated by the meter. Thus, constant checking of the meter in a nonoperational condition is required. 
     It is therefore an object of this invention to provide fail safe logic elements. 
     Another object of this invention is to provide fail safe logic elements for a reactor period meter. 
     SUMMARY OF THE INVENTION 
     For a reactor period meter trip circuit, there is provided an analog logical gate for indicating the value of a first input signal being greater than a first trip level and simultaneously the value of the second input signal being in excess of the second trip level, much like a logical AND gate. The first and second input signals are applied to first and second chopper comparators respectively. The output of each comparator will be 0 if the input applied to it is greater than or equal to a trip level associated with the respective input. If the input is less than the associated trip level, then the corresponding comparator develops a square wave output of frequency f. The outputs of the comparators are summed and applied to a bandpass filter tuned to the frequency f. If the output of the bandpass filter is 0, that indicates that both of the inputs have exceeded their associated trip levels simultaneously. If the output of the bandpass filter is a sine wave of frequency f, then the inputs have not simultaneously exceeded their respective trip levels. 
     A further logic element indicates a condition of the value of either of two input signals being in excess of respective trip levels regardless of their occurring simultaneously, much like a logical OR gate. Each input signal is applied to a chopper comparator which obeys the relationships previously described for the other logic element. The output of each of these comparators is applied to a bandpass filter tuned to the frequency y of the output signal of the comparators. The outputs of these filters are multiplied together by an analog multiplier. The output of the multiplier will be 0 if either of the filters has a 0 output and will be a sine wave of frequency 2y if both are at a y frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a block diagram showing logic elements of an improved reactor period meter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawing, there is shown a reactor period meter with fail safe logic elements. The reactor period meter shown in the drawing is a log N-type meter having an ionization chamber 10 which develops a signal proportional to the neutron flux density for power level of the reactor (not shown). The neutron flux density is proportional to reactor power level. The output of the chamber 10 is applied to log N amplifier 12. The output of log N amplifier 12 is proportional to the log of the reactor power level. The type of meter shown in the drawing has two circuits for determining reactor period. One circuit is limited to high power levels, while the other functions at all reactor power levels. This is more particularly described in U.S. Pat. application Ser. No. 509,971(70) filed Sept. 27, 1974. The present application relates to particular fail safe logic elements incorporated into such a reactor period meter. 
     The output V of log N amplifier 12 is applied to differentiator 14 whose output is proportional to dV/dt. The output of differentiator 14 is applied to a fail safe logical element 16 which determines whether dV/dt is greater than a particular critical value and simultaneously whether V is greater than a predetermined number. The dV/dt comparison tests the critical reactor period while the V comparison tests the reactor power level. Logic element 16 is designed so that at high power levels dV/dt is used to monitor reactor period. 
     The dV/dt signal is applied to chopper comparator 18. A chopper comparator is a device which chops a resulting D.C. signal determined by subtracting from a reference D.C. an applied signal. Only if the subtraction is positive will the resulting D.C. signal be chopped. However, if the subtraction is negative or 0, the output of chopper comparator 18 will be 0. Chopping means that the comparator generates a square wave in response and proportional to the amplitude of the positive resulting D.C. signal. The frequency of the square wave will be equal to that of a reference A.C. signal applied to the chopper comparator. In the drawing, the chopper comparator 18 compares the value of the applied input signal dV/dt signal with a particular D.C. scram level established by scram setting 20. If dV/dt is greater than the D.C. scram level, comparator 18 will chop the positive resulting D.C. signal. The chopped square wave will have a frequency equal to the frequency of an A.C. reference signal, which in this embodiment is a 1 KHz signal developed by A.C. source 22. The actual reference A.C. signal is chosen for convenience. If the D.C. scram level is not exceeded by the dV/dt signal, the output of comparator 18 is 0. Simultaneously, the output of log N amplifier 12 is applied to chopper comparator 24 which compares the value of V with a D.C. power level established by power setting 26. If V is equal to or exceeds the D.C. power level, comparator 24 develops a 0 output. If the V signal is greater than the D.C. power level, then the output of chopper comparator 24 is a square wave of frequency equal to the reference A.C. signal applied to comparator 24 by A.C. source 22. In logic element 16 this frequency is the same as that applied to comparator 18, which in this embodiment is 1 KHz. 
     The outputs of comparators 18 and 24 are superposed or added by summer 28. The summation of chopper comparators 18 and 24 is then applied to a bandpass filter 30. The frequency to which the filter is tuned is equal to that of the reference A.C. signal applied to chopper comparators 18 and 24, which in this embodiment is 1 KHz. As is well known, a bandpass filter will pass any sine wave or frequency approximately equal to the frequency to which it is tuned. Therefore, since the summation of the output comparators 18 and 24 is either a square wave of frequency 1 KHz to which the bandpass filter is tuned or 0, the output of bandpass filter 30 will be a sine wave of the reference frequency or will be 0. Note that a square wave is merely a superposition of a multitude of sine waves so that applying a square wave to a bandbass filter yields an output sine wave at the relative frequency of a square wave. Note that if the comparators have malfunctioned and are developing signals other than at the reference A.C. signal, the output of bandpass filter 30 will be due to the nonreference A.C. frequency signals and of 0 magnitude. 
     The action of the combination of the comparators 18 and 24 in filter 30 is better understood with reference to the following truth table: 
     
                                           TABLE I__________________________________________________________________________dV/dt  ≦ D.C. scram          ≦ D.C. scram                  &gt; D.C. scram                          &gt; D.C. scram  level   level   level   levelV      ≦ D.C. power          &gt; D.C. power                  ≦ D.C. power                          &gt; D.C. power  level   level   level   level__________________________________________________________________________Output of  1 KHz sine          1 KHz sine                  1 KHz sine                          0filter 30  wave    wave    wave__________________________________________________________________________ 
    
     Thus, when dV/dt is less than the D.C. scram level, the subtraction of dV/dt from the D.C. scram level will be positive and the output of comparator 18 will be a 1 KHz square wave. This results in filter 30 having a 1 KHz sine wave output regardless of the condition of comparator 24. Similarly, when V is less than the D.C. power level, comparator 24 will have a 1 KHz square wave output which results in filter 30 having a 1 KHz sine wave output regardless of the position of comparator 18. However, if dV/dt is greater than the D.C. scram level, and simultaneously V is greater than the D.C. power level, both comparators 18 and 24 will have 0 outputs, resulting in the output of filter 30 being 0. Since 0 output of filter 30 indicates scram, this logical element 16 functions much like an AND gate. 
     Logical element 40 determines whether the reactor period is too fast by comparing the change in V over a predetermined time interval with a predetermined number and develops a scram signal if the predetermined number is exceeded by the change in V. Two separate comparisons, noncoincident in time but lasting for identical time intervals, are taken and, if either indicates scram condition, a scram signal is to be generated. ΔV sampler 42 develops a new output signal each particular time period. The output signal of sampler 42 represents the change in V over the particular time interval. Similarly, ΔV sampler 44 develops a new output signal each particular time interval equal to the change in V over the particular time interval. The time intervals for each sampler 42 and 44 are equal in duration but are not coterminous. The output of sampler 42 is applied to chopper comparator 46 where it is compared with the D.C. ΔV level established by ΔV setting 48. With the output of sampler 42 less than the D.C. ΔV level, a square wave is developed by comparator 46. The reference A.C. signal which determines the frequency of the square wave is developed in this embodiment by A.C. source 22. Here, the 1 KHz signal is divided by two by frequency divider 50 to give a reference A.C. signal having a frequency of 500 HZ. When the signal from sampler 46 is equal to or exceeds the D.C. ΔV level, then the output of comparator 46 is 0. In the same manner, the output of sampler 44 is applied to chopper comparator 52 whose D.C. reference signal is equal to that of comparator 46 and is also established by ΔV setting 48 and whose A.C. reference signal is also the 500 HZ from divider 50. The same output conditions which governed comparator 46 also govern comparator 52. The output of each comparator 46 and 52 is applied to a bandpass filter, 54 or 56, respectively. Filters 54 and 56 are tuned to the A.C. reference frequency of comparators 46 and 52 which, in this case, is 500 HZ. The purpose of filters 54 and 56 is to convert any square wave developed by comparators 46 and 52 to a sine wave at the same frequency as the reference frequency. The outputs of the filters 54 and 56 are applied to an analog multiplier 58. An analog multiplier is a device which, in response to two sine wave inputs of frequency ω 1  and ω 2 , develops an output of the form 
     
         COS(ω.sub.1 + ω.sub.2) - COS(ω.sub.1 - ω.sub.2) 
    
     if the outputs of both filters 54 and 56 are both at 500 HZ, then the output of multiplier 58 will be sine sin wave of frequency 1 KHz. However, if the output of either filter 54 or filter 56 is 0, then the output of multiplier 58 will be 0. The action of the combination of comparators 46 and 52, filters 54 and 56, and multiplier 58 is better understood with reference to the following truth table: 
     
                                           TABLE II__________________________________________________________________________V        ≦ D.C. Δ V           &gt; D.C. Δ V                  ≦ D.C. Δ V                         &gt; D.C. Δ VSampler 42    level  level  level  leveloutputV        ≦ D.C. Δ V           ≦ D.C. Δ V                  &gt; D.C. Δ V                         &gt; D.C. Δ VSampler 44    level  level  level  leveloutput__________________________________________________________________________Multiplier 58    1 KHz sine           0      0      0output__________________________________________________________________________ 
    
     Thus, when the output of sampler 42 is less than the D.C. ΔV level, comparator 46 develops a 500 HZ signal, and when the output of sampler 42 is greater than or equal to the D.C. V level, comparator 46 develops a 0 output. The same rule governs the operation of comparator 52 with respect to the output of sampler 44. The output of multiplier 58 will be 0 if the output of either chopper is 0 and will by a 1 KHz signal if both are at 500 HZ. Since 0 indicates scram condition, logic element 40 can be said to function like an OR gate. 
     The outputs of filter 30 and multiplier 58 are applied to analog multiplier 60. The reference frequencies have been established so that the outputs of filter 30 and multiplier 58 are both either 0 or a 1 KHz sine wave depending upon whether a scram condition or a nonscram condition exists according to logic elements 16 and 40. The output of multiplier 60 will be governed by the same multiplication rule as for multiplier 58 and will be 0 if either input or both inputs are 0 and will be a 2 KHz sine wave if both inputs are 1 KH signals. The output of multiplier 60 is applied to bandpass filter 61 which is tuned to a frequency of 2 KHz in this embodiment. Thus the output of filter 61 will either be 0 or 2 KHz depending upon the output of multiplier 60. If a component of the meter has failed, the output of multiplier 60 may be a sine wave of frequency of 2 KHz. Filter 61 responds to such a signal with a 0 output. A scram should be generated if the output of filter 61 is 0. This is done by scram reactor circuit 70. 
     Because the logic elements are A.C. they are fail safe. If at any time one of the components of the element fails, the filter 61 will have an output of 0 since the frequencies of its two inputs will not match up. Further redundancy can be provided by examining the output of each chopper individually with a missing pulse detector which for each chopper is 62, 63, 64 and 66. A missing pulse detector might be a retriggerable monostable multivibrator. When it receives a pulse from the comparator, a capacitor is charged and when the pulse ends the capacitor is discharged. If a pulse is missing, the capacitor continues to charge and the excess charge can be detected, showing a missing pulse. 
     To those skilled in the art, the particular selection of A.C. reference frequencies and of filters would be apparent. Also various combinations of the basic logic elements disclosed to perform a variety of logical operations would also be apparent.