Patent Number: 055685285
Section: claims

1. A method for compensating for the non-linearity of an LVDT-type sensor utilized to determine the position of each control rod of a control rod cluster assembly during selective step movement of each control rod into and out of a nuclear reactor core, comprising the steps of: a) applying an excitation current of a first preselected frequency to a primary of said LVDT-type sensor for inducing at a secondary of said sensor a secondary voltage representative of the position of each control rod of said control rod cluster assembly relative to a fully inserted position within said reactor core so long as the relationship between said secondary voltage and the position of each control rod is a linear relationship; and  b) switching the frequency of said excitation current to a second preselected frequency when the relationship between said secondary voltage and the position of each control rod becomes non-linear to restore said linear relationship.  a) an LVDT-type sensor communicating electromagnetically with a control rod cluster assembly and operable to determine the position of each control rod of said control rod cluster assembly relative to a fully inserted position within said reactor core; and  b) a signal conditioning module for applying an excitation current of a first preselected frequency to a primary of said LVDT-type sensor for inducing at a secondary of said sensor a secondary voltage representative of the position of each control rod of said control rod cluster assembly relative to a fully inserted position within said reactor core so long as the relationship between said secondary voltage and the position of each control rod is a linear relationship and including means for switching the frequency of said excitation current to a second preselected frequency when the relationship between said secondary voltage and the position of each control rod becomes non-linear to restore said linear relationship. 2. The method as in claim 1, wherein said first preselected frequency is approximately 60 cycles. 3. The method as in claim 1, wherein said second preselected frequency is approximately 20 cycles. 4. The method as in claim 1, wherein said excitation current is applied at a first preselected frequency of 60 cycles during step movement of each said control rod of said control rod cluster assembly over a range of between approximately 0 and 220 steps. 5. The method as in claim 1, wherein said excitation current is applied at a second preselected frequency of 20 cycles during step movement of each said control rod of said control rod cluster assembly over a range of between approximately 221 and 231 steps. 6. An apparatus for compensating for the non-linearity of an LVDT-type sensor utilized to determine the position of each control rod of a control rod cluster assembly during selective step movement of the control rods into and out of a nuclear reactor core, comprising: 7. The apparatus as in claim 6, wherein said signal conditioning module includes a first frequency generator means for generating said first preselected frequency having a value of approximately 60 cycles and a second frequency generator means for generating said second preselected frequency having a value of approximately 20 cycles. 8. The apparatus as in claim 7, wherein said first frequency generator means generates said excitation current at a frequency of approximately 60 cycles during step movement of each said control rod of said control rod cluster assembly over a range of between approximately 0 and 220 steps and said second frequency generator means generates said excitation current at a frequency of approximately 20 cycles during step movement of each said control rod of said control rod cluster assembly over a range of between approximately 221 and 231 steps.