Patent Number: 047117575
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The circuit shown in FIG. 1 includes a plurality of sensing coils L, each bearing a respective designation Ln, where n represents the spatial position of the respective coil along the path being monitored, starting from the lowest point of the path. Thus, coil L1 is at the lowest point of the path and coil L22 is at the highest point. Coils L are annular in form and are arranged so that each coil surrounds the path being monitored and permits passage of a member made of a material which can influence the effective impedance of each coil L. The coil array further includes two coils P, bearing respective designations P1 and P2. Coils P are located outside of the path being monitored. Preferably, all coils L, P are electrically and physically identical. The coils are connected in series pairs, as shown, with each pair being connected between a.c. supply voltage terminals 36 and 38. A voltage divider composed of two resistors 40 and 42 is also connected between terminals 36 and 38. Resistors 40 and 42 can have identical resistance values. The circuit further includes eight differential amplifiers 51 to 58, each having a signal input 60, a reference input 62, an output 64 and a feedback resistor 66 connected between signal input 60 and output 64. The reference input 62 of each differential amplifier is connected to the connection point 68 between resistors 40 and 42 to receive a reference voltage. Each pair of coils L, L and L, P has a connection point, or center tap, at which a voltage equal to the voltage at connection point 68 appears when both coils have the same effective impedance. This will occur if the impedance-influencing member is in a position in which it influences the impedances of both coils of a pair equally or in which it does not influence the impedance of either coil of the pair. On the other hand, if the impedance-influencing element is in a position where it influences the impedance of only one coil of a pair, then a voltage imbalance occurs between the coils of the pair and the voltage at the associated connection point will differ from that at connection point 68. The connection point between each pair of coils is connected via a respective resistor 72 to the signal input 60 of a respective one of amplifiers 51-58. It will be noted that in the illustrated circuit, the coils which are connected electrically to form a coil pair are not spatially directly adjacent one another along the path being monitored. For example, the coils at positions 1 and 3 (L1 and L3) form an electrically connected pair, as do those at positions 8 and 11 (L8 and L11). As a general rule, the coils of a given coil pair should be separated from one another by no more than three intervening coils associated with other coil pairs. Therefore, after the impedance-influencing member produces a voltage imbalance across one coil pair, resulting in a voltage representing a logic "1" at the associated connection point, this imbalance will be maintained when, during further member travel, an imbalance has been produced at at least the next succeeding coil pair. This relationship, which has been previously proposed, will be described in greater detail below. The outputs 64 of amplifiers 51-58 provide a parallel eight-bit Gray code representing the current position of the impedance-influencing element along the path being monitored. According to a novel feature of the invention, one or more of differential amplifiers 51-58 has its signal input 60 connected to more than one connection point via respective resistors 72. This means that the value of at least one output bit is influenced by movement of the impedance-influencing member along several portions of the path being monitored. This enables the position responses of the detector circuit to be directly coded into a suitabale eight-bit Gray code. FIG. 1 shows only one possible coding pattern according to the invention. Other patterns can be produced by changing the connection arrangements between center points and amplifier inputs 60. If the coils of each coil pair are electrically balanced when both coils are either fully penetrated or not penetrated at all by the impedance-influencing member, no interaction will occur betwee the multiple inputs to any one differential amplifier. The electrical separation of the inputs to a differential amplifier is further enhanced by connecting each differential amplifier to coil pairs which are physically separated to such an extent that, at any given time, only one of the coil pairs connected to a given amplifier will be unbalanced. The number of positions to be monitored can be varied by providing a suitable number of sensing coils, electrically connected in coil pairs, and by connecting the coil pair center points in an appropriate pattern to the eight amplifiers 51-58. FIG. 2 is a pictorial elevational view showing the lower portion of the sensing coil assembly of FIG. 1 and the upper portion of a control rod drive line 76 penetrating the four lowest coils L1-L4. Each coil has the same designation "Ln" as in FIG. 1 and the coils are spatially arranged in order of their respective "n" designations. The center point connections between electrically connected coil pairs are shown along the left-hand side of FIG. 2. At least the upper portion of line 76 is made of a material selected to vary the effective impedance of each coil which it penetrates and constitutes the abovementioned impedance-influencing member. In the position illustrated, line 76 fully penetrates coils L1 and L3 so that this electrically connected coil pair is electrically balanced and the potential at its center connection point has a value representing logic "0". While coils L2 and L4 are also penetrated by line 76, coils L5 and L7 are not. Therefore, coil pairs L2, L5 and L4, L7 are electrically unbalanced and the potentials at their respective center connection points have a value representing logic "1". Finally, all coils above coil L4 are not penetrated by drive line 76 so that all remaining coil pairs are electrically balanced. Therefore, reverting to FIG. 1, when drive line 76 is in the position shown in FIG. 2, amplifiers 52 and 53 will produce a logic "1" output and all other amplifiers will produce a logic "0" output. If the upper end of drive line 76 moves upwardly to penetrate coil L5, coil pair L2, L5 is no longer unbalanced and amplifier 52 then produces a logic "0" output. When the upper end of drive line 76 is below coil L1 so that no coil is penetrated, all coil pairs are electrically balanced so that all amplifiers 51-58 produce a logic "0" output. On the other hand, when the upper end of drive line 76 is at the highest point of the path being monitored, so that all L coils are penetrated, coils P1 and P2 are not penetrated. Therefore, coil pairs L20, P1 and L21, P2 are electrically unbalanced and cause amplifiers 53 and 56 to produce logic "1" outputs. As a result, the monitoring circuit is capable of distinguishing between the situation in which all L coils are penetrated and that in which no L coil is penetrated. The logic outputs provided by amplifiers 51-58 for all positions of the upper end of drive line 76 are shown in the following Table, where the top of drive line 76 is below coil L1 in POSITION 0, penetrates only coil L1 in POSITION 1, etc. and penetrates all L coils in POSITION 22. ______________________________________ POSITION 58 57 56 55 54 53 52 51 ______________________________________ 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 2 0 0 0 0 0 0 1 1 3 0 0 0 0 0 0 1 0 4 0 0 0 0 0 1 1 0 5 0 0 0 0 0 1 0 0 6 0 0 0 0 1 1 0 0 7 0 0 0 0 1 0 0 0 8 0 0 0 1 1 0 0 0 9 0 0 0 1 0 0 0 0 10 0 0 1 1 0 0 0 0 11 0 0 1 0 0 0 0 0 12 0 0 1 0 0 0 0 1 13 0 0 1 0 1 0 0 1 14 0 0 0 0 1 0 0 1 15 0 1 0 0 1 0 0 1 16 0 1 0 0 1 0 0 0 17 0 1 0 0 0 0 0 0 18 1 1 0 0 0 0 0 0 19 1 0 0 0 0 0 0 0 20 1 0 0 0 0 1 0 0 21 1 0 1 0 0 1 0 0 22 0 0 1 0 0 1 0 0 ______________________________________ The binary values shown in the preceding Table, which appear at the outputs of amplifiers 51-58, thus constitute an 8-bit Gray code identifying the position of the upper end of drive line 76. It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.