Method for non-contact measurement of the velocity and/or the length of an extrudate moved in the longitudinal direction, of a cable in particular

A method for non-contact measurement of the velocity and/or the length of an extrudate moved forward in the longitudinal direction, in particular of a cable during the production, with the following steps:

• a first optical beam is directed transversely to the extrudate at one side thereof and received by a first receiving surface of a first image sensor on a preset first location on the opposing side of the extrudate,
• at preset points in time, the first diffraction patterns represented at the shadow boundaries on the first receiving surface are analysed with respect to a specific trait characterising the surface contour, and the trait characterising the surface contour is memorised,
• a second optical beam directed transversely to the extrudate is directed to a second receiving surface of the first or of a second image sensor, said second receiving surface being arranged on a second location at the same side as the first receiving surface which is spaced apart from the first location of the first receiving surface in the advancing direction of the extrudate,
• second diffraction patterns represented at the shadow boundaries on the second receiving surface are analysed with respect to at least one trait characterising the surface contour,
• in an evaluation device, it is determined after which span of time a characteristic trait of a first diffraction pattern acquired on the first receiving surface correlates approximately with a characteristic trait of a second diffraction pattern acquired on the second receiving surface, and
• the advancing speed of the extrudate, and from this the advanced length of the extrudate is determined from the distance of the receiving surfaces or the representation locations, respectively, of the characteristic trait of the first and second diffraction patterns on the receiving surfaces and the time span.