Calibration of runout error in a digital printing system

A method for controlling a printer, which includes a one or more print bars configured to eject droplets of ink at respective locations onto a moving blanket in an image area of the printer, thereby forming an image on the moving blanket, the method comprising:

• advancing the continuous blanket at a constant speed through the image area over one or more monitoring rollers, which are positioned in proximity to the respective locations of the one or more print bars and contact the blanket so as to be rotated by advancement of the blanket, each monitoring roller comprising an encoder;
• receiving a signal from the encoder in each monitoring roller indicative of a rotation angle of the monitoring roller;
• during a calibration phase, collecting the signal from the encoder in each of the monitoring rollers over multiple rotations of the monitoring rollers while the blanket is advanced at the constant speed through the image area;
• computing runout correction factors for the monitoring rollers responsively to the collected signal;
• and
• during an operational phase subsequent to the calibration phase, synchronizing ejection of the droplets from the print bars using the computed runout correction factors,
• wherein computing the runout correction factors comprises calculating the runout correction factors as a function of an angle of rotation of each of the monitoring rollers,
• wherein calculating the runout correction factors comprises detecting, based on the signal, variations in a speed of rotation of each of the one or more monitoring rollers as a function of the angle of rotation and computing the runout correction factors so as to compensate for the variations in the speed, and
• wherein the runout correction factors for each monitoring roller are based on a ratio between an average speed of the rotation of the monitoring roller and a specific speed of rotation measured during the calibration phase in each of a multiplicity of angular sectors.

A printing system, comprising:

• a continuous blanket;
• an image-forming station, which comprises:
• a set of motorized rollers, which are coupled to advance the blanket at a constant speed through an image area of the image-forming station;
• one or more print bars, which are configured to eject droplets of ink at respective locations onto the blanket in the image area so as to create an image on the blanket;
• and
• one or more monitoring rollers, which are positioned in proximity to the respective locations of the print bars and contact the blanket so as to be rotated by advancement of the blanket, each monitoring roller comprising an encoder configured to output a signal indicative of a rotation angle of the monitoring roller;
• a transfer station, which is configured to transfer the image from the blanket to a print medium;
• and
• a control unit, which is configured to collect, during a calibration phase, the signal from the encoder in each of the one or more monitoring rollers over multiple rotations of the monitoring rollers while the blanket is advanced at the constant speed through the image area and to compute runout correction factors for the one or more monitoring rollers responsively to the collected signal, and which is further configured to synchronize, during an operational phase subsequent to the calibration phase, ejection of the droplets from the one or more print bars using the computed runout correction factors,
• wherein the control unit is configured to compute and apply the runout correction factors as a function of an angle of rotation of each of the one or more monitoring rollers,
• wherein the control unit is configured to detect, based on the signal, variations in a speed of rotation of each of the one or more monitoring rollers as a function of the angle of rotation and to compute the runout correction factors so as to compensate for the variations in the speed, and
• wherein the runout correction factors for each monitoring roller are based on a ratio between an average speed of the rotation of the monitoring roller and a specific speed of rotation measured during the calibration phase in each of a multiplicity of angular sectors.

A method for controlling a printer, comprising:

• advancing a continuous blanket at a constant speed through an image area of the printer over one or more monitoring rollers, which are positioned in proximity to respective locations of one or more print bars in the image area and contact the blanket so as to be rotated by advancement of the blanket, each monitoring roller comprising an encoder;
• receiving a signal from the encoder in each monitoring roller indicative of a rotation angle of the monitoring roller;
• during a calibration phase, collecting the signal from the encoder in each of the monitoring rollers over multiple rotations of the monitoring rollers while the blanket is advanced at the constant speed through the image area;
• computing runout correction factors for the monitoring rollers responsively to the collected signal;
• during an operational phase subsequent to the calibration phase, forming an image on the blanket while advancing the blanket through the image area by ejecting droplets from the one or more print bars onto the blanket and synchronizing ejection of the droplets using the computed runout correction factors;
• and
• transferring the image from the blanket to a print medium,
• wherein computing the runout correction factors comprises calculating the runout correction factors as a function of an angle of rotation of each of the monitoring rollers,
• wherein calculating the runout correction factors comprises detecting, based on the signal, variations in a speed of rotation of each of the one or more monitoring rollers as a function of the angle of rotation and computing the runout correction factors so as to compensate for the variations in the speed, and
• wherein the runout correction factors for each monitoring roller are based on a ratio between an average speed of the rotation of the monitoring roller and a specific speed of rotation measured during the calibration phase in each of a multiplicity of angular sectors.