Patent Number: 046719199
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention is illustrated in FIG. 2 and includes neutron sensors 10 and current-to-voltage amplifiers 15 similar to those used in the prior art circuit illustrated in Fig. 1. According to the present invention, both of the neutron sensors 10 may be connected to a single one of the current-to-voltage amplifiers 15, as indicated by the dashed line and as in the prior art circuit of FIG. 1, or each neutron sensor 10 may supply a current signal as the sole input to one of the current-to-voltage amplifiers 15. The analog voltage output by the current-to-voltage amplifiers 15 is supplied to an analog/digital converter 40 for input to a microprocessor 45. The analog/digital converter 40 and microprocessor 45 may be implemented on a single board computer such as an Intel 88/40. Depending on the particular analog/digital converter 40 and microprocessor 45 and monitoring system requirements, separate current-to-voltage amplifiers 15 may each be connected to separate analog-digital converters 40 or, as indicated by the dashed line, be connected to the same analog-digital converter on separate channels. Similarly, there may be one or more microprocessors 45 for each analog/digital converter 40. Mere replacement of the analog circuits illustrated in FIG. 1 by the digital circuitry illustrated in FIG. 2, may simplify the alignment process and the speed with which the power level monitoring circuit responds to transients; however, there is no automatic reduction in noise. It is possible to reduce the effects of nuclear noise by proper selection of the algorithm performed by the microprocessor 45. The selection of an algorithm is governed by several factors. First, the algorithm should be capable of reducing noise without total loss of the ability to detect transients. Secondly, power level monitoring circuits which operate in the power range of a nuclear reactor are required to supply a signal indicating rate of change of the power level, so that a "trip" can be generated when the rate of change has a magnitude above a specified value, i.e., indicating a sudden change in power, which may be a transient condition. One algorithm which meets these requirements is defined by alpha-beta tracker equations which are commonly used in radar applications. Application of the alpha-beta tracker equations to processing in radar systems is described in J. A. Cadzow, Discrete-Time Systems, Prentiss-Hall, 1973, sections 2.6 (pages 63-66) and 8.11 (pages 272-278). Applying the equations described in Cadzow to the power level monitoring circuit illustrated in FIG. 2, the digital voltage output by the analog/digital converter 40 can be represented by f(k). The power level p(k), rate of change of power level p(k) and predicted power level p.sub.p (k) are defined by equations (1)-(3) below. EQU p.sub.p =p(k-1)+T p(k-1) (1) EQU p(k)=p.sub.p (k)-.alpha.[f(k)-p.sub.p (k)] (2) ##EQU1## The power level p(k) is an estimate or smoothed output for the current sampling period in which the effects of noise have been reduced. The predicted power level p.sub.p (k) is a prediction of the estimated power p(k) for the immediately following sampling period. The length of the sampling period is represented by T, .alpha. and .beta. are constants which determine the dynamic response of the power level monitor. The interrelationship of equations (1)-(3) is visually represented by the block diagram illustrated in FIG. 3. The input sample signal f(k) is converted by adder 110 and multiplied by constants .alpha. and .beta./T in multipliers 120 and 130. The resulting signals are input to adders 140 and 150, respectively. The outputs of adders 140 and 150 are supplied to registers 160, 170 and 180 which provide a delay of T. The output of register 160 is summed with the output of multiplier 130 to provide the rate of change of the power level p(k). The output of register 180 is multiplied by the length of the sampling period T in multiplier 190 prior to being summed with the output of register 170 in adder 200 to provide the predicted power level p.sub.p (k). The predicted power level p.sub.p (k) is multiplied by negative one so that it is subtracted from the sampled signal f(k) by adder 110 and is also summed with the output of multiplier 120 to produce the smoothed power level p(k). Selection of appropriate values for the constants .alpha. and .beta. is explained in Cadzow in section 8.11 (pages 272-278) using the Z-transform which is 8.11 (pages 272-278) using the Z-transform which is throughly discussed on pages 144-175 of Cadzow. The Z-transform of equations (1)-(3) are illustrated as a block diagram in FIG. 3B and appear below as equations (4)-(6). EQU P.sub.p (z)=z.sup.-1 p(z)+z.sup.-1 T P(z) (4) EQU P(z)=P.sub.p (z)+.alpha.[F(z)-P.sub.p (z)] (5) ##EQU2## Since the outputs illustrated in FIG. 3B are all derived from a single input, the following transfer functions H.sub.1 (z)-H.sub.3 (z) can be defined. EQU P(z)=H.sub.1 (z)F(z) (7) EQU P(z)=H.sub.2 (z)F(z) (8) EQU P.sub.p (z)=H.sub.3 (z)F(z) (9) Dividing both sides of equations (4)-(6) by the Z-transform F(z) of the input signal f(k), incorporating the transfer function relationships of equations (7)-(9) and rearranging the terms, results in the following equations (10)-(12). EQU z.sup.-1 H.sub.1 (z)+z.sup.-1 T H.sub.2 (z)-H.sub.3 (z)=0 (10) EQU H.sub.1 (z)-(1-.alpha.)H.sub.3 (z)=.alpha. (11) ##EQU3## Solving equations (10)-(12) for H.sub.1 (z)-H.sub.3 (z) results in the following equations (13)-(15). ##EQU4## The denominator polynomial which is common to all three of the fractions above is known as the characteristic equation which defines the system poles. Solving for the poles of the characteristic equation yields equation (16) below. ##EQU5## Assuming a critically damped system is desired, the term (.beta..sup.2 +.alpha..sup.2 +2.alpha..beta.-4.beta.) is set to zero with the result that .alpha.=2.sqroot..beta.-.beta.. Substituting for in equations (13)-(15) produces the following equations (17)-(19). ##EQU6## Thus, a critically damped power monitor using alpha-beta tracker equations has a double pole of z=1-.sqroot..beta.. Implementation of an alpha-beta tracker power level monitor requires selection of a sampling period length T and a value for .beta., from which the value of .alpha. can be found. The sampling period length T will be determined by the speed of the neutron sensor 10, analog/digital converter 40, and the requirements of the equipment which receives the signals output by the microprocessor. A discussion of how to select the value of .beta. can be found on page 278 of Cadzow and in Benedict, T. R. and Bordner, G. W., "Synthesis of an Optimal Set of Radar Track--While Scan Smoothing Equations," IRE Transactions on Automatic Control, Vol. AC-7, No. 4 (July, 1962) pages 27-32. The value of .beta. affects the degree of noise reduction and system response speed. For applications such as data logging of the neutron flux in a nuclear reactor, a value of .beta. equal to or very close to zero is preferable, because the effects of noise will be minimized. However, the response time will be very slow. Therefore, in neutron flux monitors which must generate trip signals, the value of .beta. is increased (up to a maximum of 1) until statisfactory system response time is achieved. The amount of noise suppression provided by an alpha-beta tracker is reduced as .beta. is increased; therefore, .beta. should be selected to be as small as possible while meeting the system response time requirements. Once the value of has been selected, the value of .alpha. can be found as 2.sqroot..beta.-.beta. and a program for microprocessor 45 can be easily written from the block diagram in FIG. 3A. When implemented, a power level monitor using alpha-beta tracker equations will provide noise suppression and "fast follow" capibility for responding to transients in the neutron flux. In addition, both the rate of power level change p(k) and a predicted next power level p.sub.p (k) are automatically produced by the alpha-beta tracker equations. Also, alignment of such a power level monitor is considerably simplified due to the noise suppression capabilities of the alpha-beta tracker equations and the use of digital processing which eliminates the need for adjusting a variable resistor in a rate/lag circuit 20 as in the prior art. The many features and advantages of the present invention are apparent from the detailed specification, and thus it is intended by the appended claims to cover all such features and advantages of the power level monitor which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.