Magnetic method using feedback to measure the speed of steel strip

A noncontact method is described for measuring the speed of steel strip to an accuracy of about 0.1% over the range of speeds typically found in steel mills. The method is magnetic in character and relies on the presence of a small amount of hysteresis in the material of the strip. A wavelength-sensitive reading head of multiple construction detects magnetisation along the strip and is connected in a feedback configuration to a writing head placed upstream of the reading head. With sufficient amplification, an unstable system results, oscillating at a frequency approximately proportional to the strip speed. Phase shifts within the feedback loop cause a departure from linearity in the speed/frequency relationship, an effect minimised by a further feedback loop which performs phase correction.