Abstract: An endless loop of fabric of the type used in papermaking is provided with a row of optically detectable marks aligned in the direction of motion of the fabric. A beam of light pulses from a strobe light is directed to a position on the fabric through which the row of marks passes and the position of the marks as illuminated by the strobe light is observed. When the observed marks are stationary, the speed of the fabric can be determined based on the frequency of the light pulses from the strobe light and the spacing between the marks on the fabric. The speed of the fabric can be controlled to a desired speed by setting the strobe pulse frequency and adjusting a variable speed drive for the motor driven rollers driving the fabric until the marks as illuminated by the strobe light appear stationary. Variations in the speed of the fabric result in variations of the position of the marks as illuminated by the strobe light, allowing equipment problems causing the speed variations to be diagnosed.