This invention concerns a device to detect the position of a mobile component moving in accordance with a preset course. More particularly the invention relates to a device for detecting the position of a nuclear reactor control rod, the device comprising a magnet attached to the mobile component and a sensor governed by the local action of the magnet and delivering a signal representing the position of the mobile component.
A known device of this kind comprises sealed tongue relays of the Reed type, which close at the approach of the permanent magnet attached to the rod and provide discrete data on the position of the rod. The failure of one of the relays generates an erroneous indication of the position of the rod and in order to avoid unwarranted shutdowns of the reactor it is necessary to multiply the number of detection devices and control and safety circuits. There is a relatively large number of conductors connecting the relays placed inside the reactor vessel to the processing units outside the reactor, thereby complicating the system as a whole.
It is an object of the invention to provide a detection device having a simple and reliable structure.
The sensor is constituted by an electromagnetic delay line lying at a short distance along the path of the magnet and comprising:
a core of saturable magnetic material in conjunction with a winding propagating high frequency pulses; PA1 a generator connected to the input of the line to emit high frequency pulses; PA1 measuring means to detect local saturation of the core facing the magnet entailing a variation in the magnetic permeability of the core and a change in pulse propagation conditions in the winding; PA1 and processing means co-operating with the measuring means to generate the signal representative of the position of the mobile component.
The value of the load impedance Z of the delay line is judiciously chosen to establish standing wave conditions with variable reflection depending on the position of the magnet. Any configuration of the sensor is acceptable such as to constitute a coaxial, two-wire or single wire line.
The data on the position of the control rod is supplied directly by a reflectometer. This data is the result of either the ratio of the time taken for a pulse to be reflected back in the intermediate zone of the core saturated by the magnet to the time taken by a pulse reflected back at the end of the line, or the variation of the coefficient of reflection and/or of the standing wave ratio in relation to the position of the magnet.
The sensitivity of the sensor can be adjusted by concentrating the magnetic flux on the saturable zone of the core through the addition of pole-pieces or a magnetic circuit of predetermined form. It is also possible to modify the permeability of the core by injecting a bias current in the winding on the line.
The calibration of the distances of the sensor is obtained by dividing the core into several aligned elementary sections, delimited either by discontinuities or by zones saturated by means of auxiliary magnets.