Method and device for detecting faults during transport of a web

#Field of the Invention

#The invention relates to a method for detecting faults during transport of a web, in particular, for detecting a web break during the transport of a paper web in a web-fed rotary printing machine, wherein the speed of the web is determined by contact-free or contactless measurement, in accordance with the Doppler principle, and is evaluated thereafter.

#The invention also relates to a device for detecting a fault during a web transport, in particular, for detecting a web break during the transport of a paper web in a web-fed rotary printing machine, the device including a detector for determining the speed of the web free of any contact, in accordance with the Doppler principle, and supplying the determined speed value to an evaluation unit disposed downline therefrom, as viewed in the transport direction of the web.

#FIG. 1 is a diagrammatic and schematic side elevational view of a web-fed rotary printing machine with several measurement positions;

#FIG. 2 is an enlarged fragmentary view of FIG. 1 showing a different embodiment of the invention wherein a microwave detector having one microwave antenna is located at least at one of the measurement positions;

#FIG. 3 is a view like that of FIG. 2 wherein a microwave detector having two microwave antennas is shown in another embodiment;

#Referring now to the drawings and, first, particularly to FIG. 1 thereof, there is shown therein a diagrammatic and schematic side elevational view of a web-fed rotary printing machine of in-line construction, having a reel changer 1, four printing units 2a to 2d, a hot-air dryer 3 and a cooling roller unit 4. A paper web 5 is unwound from a paper reel 6 in the reel changer 1 and passes successively through the printing units 2a to 2d, the hot-air dryer 3, and the cooling roller unit 4 and is then supplied to a further processing unit, for example, a folder. The local speed of the web 5 is determined at a number of measurement positions by contact-free measurement using microwave detectors 10a to 10g, and the results of these measurements are fed to an evaluation unit 20 via signal lines 12a to 12g as signals proportional to the speed of the web. In order to determine the local speeds of the web 5 at the various measurement positions, each microwave detector has a microwave transmitter and receiver, it being possible for the transmitter and receiver to be two separate microwave antennas or the same microwave antenna.

#The measurement of the speed of the web 5 is based in this case on the Doppler principle, which describes the physical phenomenon that transmitters and receivers, which are moved relative to one another, do not assign the same wavelength and frequency, respectively, to a wave. In this case, one also speaks of a frequency shift. When transmitted and reflected waves are received, the frequency shift effect is multiplied. Furthermore, the Doppler effect is angle-dependent, i.e., depends upon the angle between the propagation direction of the wave and the relative direction of movement of transmitter and receiver, no Doppler effect occurring in the case of mutually perpendicular directions. For the frequency shift resulting from the Doppler effect for the case wherein a stationary transmitter transmits waves which are reflected by a moving object and are received by a likewise stationary receiver, the following relationship applies: Δ ⁢ ⁢ f = f ( 1 1 + v c ⁢ ( ± cos ⁢ ⁢ α ∓ cos ⁢ ⁢ β ) - 1 )

#wherein Δf is the frequency shift