Printing – Processes – Condition responsive
Reexamination Certificate
2002-05-07
2004-07-13
Hirshfeld, Andrew H. (Department: 2854)
Printing
Processes
Condition responsive
C400S061000, C400S070000, C400S076000
Reexamination Certificate
active
06761115
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a clock generator for an imaging device and a method of generating a clock signal for the imaging operation.
When setting an image on a printing form, which is generally clamped on a cylinder, the desired information is transferred to the printing form with the aid of an imaging or image-setting power source. With the imaging power source, the entire area of the printing form whereon an image is to be set is scanned successively or at least to some extent in parallel, if the imaging power source is formed of a number of individual power sources. In this regard, the information is transferred to the printing form in accordance with an original in the form of individual pixels or dots or groups thereof.
In order to achieve high-quality imaging on the printing form, on the one hand, the imaging power source must have adequate focussing properties or the imaging device must have appropriate elements with the aid of which ultimately adequate focussing can be achieved. On the other hand, it is necessary for the pixels or dots, respectively, to be written into the printing form exactly at the assigned position thereof. This places high requirements on the positioning of the imaging power source relative to the printing form, in particular, in the case of high-resolution imaging. The positioning can be carried out by mutually coordinated movements of the printing form and the imaging light source. In this regard, the printing form is generally subjected to a rotational movement and the imaging power source is subjected to a translational movement parallel to the longitudinal axis of the cylinder whereon the printing form is clamped.
An incremental encoder can be used for registering the rotational movement of the printing form. The resolution of conventional incremental encoders is generally lower, however, than the positioning accuracy required in the circumferential direction of the printing form. Therefore, various devices for increasing the resolution in imaging devices and in other devices, respectively, have already been proposed heretofore.
U.S. Pat. No. 5,174,205 discloses a control system for a discharge device for setting an image on a printing form which is clamped on a cylinder. Arranged on the cylinder is a rotary encoder which emits a signal dependent upon the rotational angle of the cylinder. The signal from the rotary encoder is fed to a phase locked loop-circuit, which generates therefrom a signal with a higher resolution, which is synchronized with the original angle signal. The discharge device is controlled based upon the signal generated in this manner, correction data, in particular, for correcting geometric errors of the printing form, being also taken into consideration.
The published German Patent Document DE 42 26 236 C1 describes a device for controlling electronically triggerable devices which are arranged on a rectilinearly reciprocatable carriage, slide or the like. The control device comprises an incremental encoder which outputs a clock signal as a function of the position of the carriage, slide or the like, and a fine-step timing generator which generates a multiplicity of fine-step clock cycles between successive increments of the incremental encoder. A triggering cycle for the electronically triggerable devices, respectively, is generated after a prescribed or predefined number of fine-step clock cycles. In order also to achieve highly accurate triggering of the electronic devices during nonuniform movement of the carriage, slide or the like, the fine-step timing generator is coordinated with the incremental encoder. For this purpose, a desired or nominal value for the number of fine-step clock cycles within the time period between two increments of the incremental encoder is prescribed or predefined. If the incremental encoder outputs a further clock signal before the nominal value has been reached, the fine-step timing generator then generates the fine-step clock cycles missing from the nominal value as quickly as possible, i.e., at the system clock rate. If, on the other hand, the nominal value has been reached before the incremental encoder outputs a further clock signal, the generation of further fine-step clock cycles is then stopped until the further clock signal has been outputted by the incremental encoder.
U.S. Pat. No. 6,057,715 describes a clock generator for generating a clock signal of any desired frequency from a reference clock signal. The clock generator has a counter which in each case counts up at the cycle rate of the reference clock signal and outputs appropriate numeric values. Through the intermediary of a sine-wave table, the clock generator converts successive numeric values into function values of a prescribed or predefined number of sine waves. From the function values, with the aid of a digital/analog converter, an analog sinusoidal signal is generated, which is filtered and fed to a comparator. The comparator converts the sinusoidal signal, by comparison with a prescribed threshold, into a clock signal of square-wave form. The frequency of the thus generated clock signal, in relation to the frequency of the reference clock signal, is determined by the ratio between the number of sine waves which are stored in the sine-wave table as function values for one pass through the counter, and the size of the counter.
The conventional devices, respectively, have the disadvantage that fluctuations in the rotational speed are not or are only inadequately taken into consideration. These fluctuations in the rotational speed therefore lead to a reduction in the achievable accuracy when the clock signals generated by the conventional devices are used for imaging on printing forms.
Furthermore, the published German Patent Document DE 27 29 697 A1 describes a method for interpolating signals from a sine-cosine rotary encoder. The method is based upon the fact that the sine and cosine signals from the rotary encoder, after being digitized by an A/D (analog/digital) transducer or converter, are subjected to the arctan formation in order to calculate the phase angle. The published European Patent Document EP 0 484 576 B1 discloses a device for subdividing analog periodic signals. The interpolation described in this document is based upon a method wherein a comparison phase vector is generated which is varied until it coincides with the actual phase vector from the rotary encoder with a desired accuracy.
A common drawback of these heretofore known methods is that, initially, angle information is produced, which can be used only via at least one further processing step for generating a clock signal. In principle, the angle information is made available only at discrete times, because of the sampling rate of the A/D converter. In order to use this angle information to generate a clock signal of a required frequency, in the case of the heretofore known methods it is necessary to obtain the angle information at a higher frequency than the clock signal. The formation of non-even-numbered interpolation factors is possible only to a restricted extent or with great outlay or expense.
SUMMARY OF THE INVENTION
It is therefore an object of the invention, to provide a clock generator for an imaging device, which, even under unfavorable operating conditions, ensures that high accuracy is achieved when setting an image on a printing form.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a clock generator for generating a clock signal for an imaging device serving for transferring image-setting information to a rotatable printing form, the clock generator comprising an input to which an actual value signal dependent upon an angular position of the printing form is applicable, and an output serving for outputting the clock signal, and further comprising a first comparator having a first input whereto the actual value signal is applicable as an analog signal, a second input to which an analog nominal value signal, respec
Singer Robert
Wiedemann Andreas
Greenberg Laurence A.
Hamdan Wasseem H.
Heidelberger Drunkmaschinen AG
Hirshfeld Andrew H.
Mayback Gregory L.
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