A non-contact and online ink thickness sensor for printing machines using the photoacoustic effect

We have developed an ink thickness sensor by employing a photoacoustic technique in order to enable the online measurement of ink thickness for printing machines such as a sheet-fed press. This sensor enables the online measurement of black ink thickness, which was impossible using the conventional methods. In order to detect the photoacoustic signals from the ink on a rotating ink roller using a non-contact technique, the halogen light chopping frequency was made to coincide with the pipe acoustical resonance frequency of the photoacoustic cell. With this method, even though the photoacoustic cell is an open cell, the sensitivity of the photoacoustic cell could be improved as compared to the conventional non-resonant closed cell. Further, it was clarified both experimentally and theoretically that the photoacoustic signal strength of black ink corresponds to an ink film thickness when the optimum halogen light chopping frequency is selected. In addition, a study is made on the characteristics of the photoacoustic signal strength variation for the clearance between the resonance tube and the ink roller.