Strip speed measurement using distributed infra-red detectors

A non-contact method for measuring the speed of electrically conducting strip over the range of speeds found in industry is described. The method is thermal in character and relies on the ability of the strip to store heat for short periods of time. A wavelength-sensitive reading head consisting of an array of equally spaced infra-red detectors senses the heat pattern present on the moving strip. The signals from the array elements are combined and the array acts as a spatial filter. The resultant signal is then fed back to a heater, or writing head, placed upstream of the reading head, resulting in a closed loop time-delay feedback system which oscillates at a frequency proportional to the strip speed. Phase shifts within the feedback loop cause a departure from linearity in the speed/frequency relationship. This effect is minimised by a digital feedback loop which performs phase correction. The method has been implemented using inexpensive pyroelectric detectors, an accuracy of 0.4% being obtained with strip of adequate emissivity. Use of high-performance detectors would enhance performance, particularly for low emissivity material