Digital method of wavelength control in a feedback velocimeter

A digital method of wavelength control is described for a feedback velocimeter designed to measure the speed of steel strip. The measurement system, which has been described in an earlier paper, utilises the property of hysteresis of the steel strip and employs a wavelength-selective magnetic detector and a writing head upstream of the detector. Feedback causes a constant-wavelength magnetic pattern to be written onto the strip, making the frequency of the detector output a direct measure of the strip speed. In practice, slight variations in the wavelength occur which can be attributed to variable phase effects in the writing process. The advantages of further controlling the wavelength by digital means are described and the theory of the method is developed. The steady-state and transient performance of two different configurations is examined, one of which is clearly to be preferred. A measurement accuracy of better than ¿¿ 0.05% of actual speed is obtainable using this system in the presence of simulated disturbance such as might be expected in practice.