Sh Sjirk Koekebakker

Iterative learning control with basis functions for media positioning in scanning inkjet printers

In printing systems, the positioning accuracy of the medium with respect to the print heads directly impacts print quality. In a regular document inkjet printer, the main task of the media positioning drive is to shift the medium after the printhead has finished a pass. Most media have the tendency to deform while it is being printed due to variations in temperature and moisture content. In order to improve print quality, we propose to move the medium during printing to counteract the deformation. These small scale trajectories are performed in an operating regime of which the dynamics considerably differ from the regular transportation step. Using iterative learning control with basis functions for both positioning tasks, the positioning accuracy of the drive is improved substantially; while keeping numerical cost low.

A Waveform Design Method for a Piezo Inkjet Printhead Based on Robust Feedforward Control

The printing quality delivered by a Drop-on-Demand (DoD) piezo inkjet printhead is limited mainly due to the residual oscillations in the ink channel. The maximal jetting frequency of a DoD inkjet printhead can be increased by quickly damping the residual oscillations and thus bringing an ink channel to rest after jetting an ink drop. In this paper, we propose a robust optimization-based method to design the input actuation waveform for the piezo actuator in order to improve the damping of the residual oscillations in the presence of parametric uncertainties in the ink-channel model. The proposed method obtains a robust actuation pulse by minimizing the tracking error under the parametric uncertainty. Experimental results with a small-droplet DoD inkjet printhead are presented to show the efficacy of the proposed method and the significant improvement in the ink drop consistency.

Enhancing Flatbed Printer Accuracy and Throughput: Optimal Rational Feedforward Controller Tuning Via Iterative Learning Control

Advanced control methods potentially enable performance improvements in printing systems for minor additional costs. The aim of this paper is to develop a control framework that is capable of delivering throughput and accuracy enhancements for an industrial flatbed inkjet printer. The proposed method involves iterative learning control with a rational feedforward parameterization to enable varying position references which are required for printing. Experimental results highlight the efficacy of the proposed method in a comparison with related pre-existing learning control approaches.

Piezo printhead control : jetting any drop at any time

Full flexible use of inkjet printhead units in printing systems requires consistent generation of drops with any given volume and velocity at any moment and place desired. True drop-on-demand is currently hampered by physical phenomena in the printhead. These are residual vibrations and crosstalk resulting from conventional jets. This chapter presents control strategies to overcome these problems. First, with experiment-based control the drop characteristics are measured and the jet pulse that activates the jetting of a drop is optimised. Choosing a proper jet pulse structure, one can deal with single-channel residual vibration, multi-channel crosstalk, and even generalise optimisation over each bitmap to be printed. Secondly, with a model-based control approach, optimised jet pulses can be derived without additional measurement equipment. Considering the inkjet mechanism as an uncertain system and designing a robust pulse allows to deal with differences between model and real system. Both the experiment- and model-based method result in strongly improved drop characteristics, which is experimentally verified and thereby provide very valuable steps towards adaptive printing systems.

Minimization of cross-talk in a piezo inkjet printhead based on system identification and feedforward control

The printing quality delivered by a drop-on-demand inkjet printhead is severely affected by the residual oscillations in an ink channel and the cross-talk between neighboring ink channels. For a single ink channel, our earlier contribution shows that the actuation pulse can be designed, using a physical model, to effectively damp the residual oscillations. It is not always possible to obtain a good physical model for a single ink channel. A physical model for a multi-input multi-output (MIMO) inkjet printhead is made even more sophisticated by the presence of the cross-talk effect. This paper proposes a system identification-based approach to build a MIMO model for an inkjet printhead. Additionally, the identified MIMO model is used to design new actuation pulses to effectively minimize the residual oscillations and the cross-talk. Using simulation and experimental results, we demonstrate the efficacy of the proposed method.

Design and modeling aspects in multivariable iterative learning control

Iterative Learning Control (ILC) can significantly improve the performance of systems that perform repeating tasks. Typically, several decentralized ILC controllers are designed and implemented. Such ILC designs tacitly ignore interaction. The aim of this paper is to further analyze the consequences of interaction in ILC, and develop a solution framework, covering a spectrum of systematic decentralized designs to centralized designs. The proposed set of solutions differs in design, i.e., performance and robustness, and modeling requirements, which are investigated in detail. The benefits and differences are demonstrated through a simulation study.

Parameterized Iterative Learning Control: Application to a wide format inkjet printer

In this paper, a new Parameterized Iterative Learning Controller (PILC) is discussed. A fixed structure feedforward controller is put into a setting for iterative learning control and adapted to work in a repetitive environment. The fixed structure keeps the number of inline calculations low and enables proper adjustment to changing tasks. Within the ILC setting, suitable compensations can be found in a variable environment and specific penalties can be given to errors in trial domain. Further, by taking into account possible interaction between the trials, the PILC works properly with changing initial conditions over the trials. Successful implementation of the PILC on the carriage of a professional wide format inkjet printer shows its practical applicability.

Combining ILC and repetitive control to handle repeating, event-triggered disturbances in precision inkjet printing

Learning and repetitive control are powerful instruments in handling recurring disturbances. Repetitive control properly handles constantly repeating variations, while iterative learning control is well-equipped when it comes to handling event triggered deviations. Neither controller is well equipped to adequately deal with repetitive disturbances, which are only present during limited, but varying, periods of time. These are often seen in precision handling systems such as production inkjet printers. This paper combines ILC and RC using a structure which originated in multi-period repetitive control. It is shown that this enables full suppression of the repeating event-triggered disturbances. The approach is successfully demonstrated in an illustrative simulation, as well as by using experimental data from a precision inkjet printing setup.

Identification of cyclic disturbances in positioning systems via compressive sensing

In industrial precision positioning systems the measurement position is hardly ever the same as the location of the actuator. The properties and imperfections of the actuator and the underlying components between the sensor and the actuator mainly lead to deterministic reproducible position errors. The advantage of these systematic cyclic disturbances is that they can be compensated for, once identified. In this paper we use nonuniform sampling combined with Compressive Sensing (CS) to identify high spatial frequency disturbances in positioning systems when the spatial sample period is limited. The proposed strategy is implemented on the paper positioning unit of a wide format printer, to identify the cyclic disturbances in the positioning of paper with respect to the printheads. Based on CS, we present a strategy to identify the cyclic disturbances in the paper positioning from randomly obtained relative position error measurements. Experiments with a limited spatial sample period show that the high disturbance frequencies are also successfully identified.

A new Robust Delay-Variable Repetitive Controller with application to media transport in a printer

Repetitive Control (RC) is an effective tool to reject the repetitive disturbances and their harmonics. However, the effectiveness of RC decays when the time period of the RC does not exactly match with the period of the repetitive disturbance. RC also leads to reduced performance in between harmonics in terms of degraded sensitivity (more noise sensitive). High Order Repetitive Control (HORC) is an RC structure, which allows to shape the sensitivity function to make it either more robust for time period of the repetitive disturbances, or more noise robust. In addition, standard RC will also fail in systems with spatial repetitive disturbances, since the time period of this kind of repetitive disturbances may vary. Delay-Varying Repetitive Control (DVRC) can cope with spatial disturbances, however, it lacks the robustness attained by HORC. By combining these two RC structures, HORC and DVRC, a new Robust Delay-Variable Repetitive Controller (RDVRC) is obtained. The RDVRC is successfully applied to measurement data from a media transport system of an industrial printer. Robust DVRC will be useful for many motion control systems with repetitive tasks, as disturbances often repeat along the motion path.