DEFECT DETECTION DEVICE

摘要

A defect detection device operates on the eddy-current principle uses an excitation coil (16) placed around the column (10) to provide a varying magnetic field producing the eddy currents and a probe (15) placed inside the column to detect magnetic fields arising from the eddy currents. A signal generating and processing means (22, 23, 24, 25, 26, 27) is connected to the excitation means and the probe to provide a drive signal for the excitation means and to process a signal output by the sensor. The signal generating and processing means includes a filtering means for filtering out harmonics and noise present in the sensor output signal without at the same time introducing significant phase delay into the sensor output signal. The device preferably employs a chopper amplifier (426) to achieve the filtering, the chopper being strobed in dependence on the excitation signal, and a locking circuit ensures that the strobe signal is 90 ° behind the excitation signal, which is conveniently a square wave. Use then of a further strobe signal 90 ° behind the existing strobe signal enables ready detection of the phase difference between the excitation signal and probe output signal, this phase difference being used to drive, e.g., a meter providing an indication of degree of defect in the column.