| 摘要 |
A web of moving material is successively scanned by a light source, such as a laser beam, and changes of intensity of the transmitted, reflected or scattered light from the material is detected to provide an indication for discerning flaws in the web. A once per scan line digital signal from the scanner controls a voltage controlled oscillator connected in a phased locked loop which is used for controlling the generation of a precision ramp analog voltage that follows the angular scan position of the light source on the material. The light signal from the web is applied to a comparator for providing a digital pedestal signal in which the first light passing the comparator is the leading edge of the web while the last light passing during a scan line is the trailing edge or the end of the web. If no pedestal signal is present indicative of a break in the web, a synthetic maximum width pedestal signal is substituted to enable the inspection system to continue to function. The digital pedestal signal or synthetic maximum width pedestal signal and the precision ramp are applied to leading and trailing edge, position-to-voltage converters. The precision ramp voltage is sampled and held by the voltage converters which generate leading and trailing edge position analog voltage outputs. These voltage outputs are applied to a leading and trailing edge cross web, down web position tracking rate filter which is controlled by edge position tracking rate and web speed circuits. The filtered leading and trailing edge outputs are applied to margin set circuits and then are converted by voltage-to-edge, position converter circuits into digital active scan pulses that accurately define and track the active scan interval of interest where flaws are to be detected. |
