BACKGROUND OF THE INVENTION
The present invention relates generally to web printing presses and more particularly to a method and apparatus for controlling the tension of the web in a printing press including two or more sets of nip rollers with deformable covers.
Web printing presses print a continuous web of material, such as paper. The web travels through nips formed by opposing nip rollers. The web is moved on its way by driven nip rollers which are driven by respective nip roller drivers.
Tension in the web must be maintained within a desired range in order to achieve smooth operation of the printing press. At the same time, the velocity of the web, and hence the rotational speed of nip rollers, must be held relatively constant to achieve good print product quality. Tension in a web span between two nips can be adjusted by controlling the speed of the nip roller driver in the downstream nip.
Multiple types of nip rollers have been used in conventional web printing presses. Some nip rollers were formed with a deformable surface layer such as urethane. Nip rollers with a deformable surface were found to provide less than ideal operation because of uneven ribbon-to-ribbon tension in upstream web spans.
As a result, nip rollers formed with a deformable surface such as microcellular foamed urethane were introduced (see, U.S. Patent Publication No. 2006/0157924 A1 published on Jul. 20, 2006 and incorporated herein by reference). Nip rollers with this type of deformable surface layer were found to overcome the uneven ribbon-to-ribbon tension problem of the earlier nip rollers. However, it was found that any nips with non-rigid covers have an effective gain that is a function of the tension difference across the nip roller, so that if the tension of the web downstream from the nip roller is changed, the tension of the upstream web will also change requiring an additional tension adjustment.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and apparatus which allows for adjustment of the tension of a downstream span of web without affecting the tension of an upstream span of web.
The present invention provides a method for controlling web tension in a web printing press having a plurality of sets of deformable nip rollers, each set of deformable nip rollers driven by a separate driver, the web passing sequentially through each set of deformable nip rollers, each adjacent set of deformable nip rollers forming a web segment therebetween. A first characteristic of the driver of a downstream set of deformable nip rollers is adjusted to set the tension of a first web segment which is between the downstream set of deformable nip rollers and an upstream set of deformable nip rollers. Then, a first characteristic of the driver of the upstream set of deformable nip rollers is adjusted to maintain a constant tension on a second web segment which is immediately preceding the upstream set of deformable nip rollers.
In an alternative embodiment, a second characteristic of the second web segment is measured and the first characteristic of the driver of the upstream set of deformable nip rollers is adjusted based on the measurement of the second characteristic.
The present invention also provides an apparatus for controlling web tension in a web printing press having a plurality of sets of deformable nip rollers, each set of deformable nip rollers driven by a separate driver, the web passing sequentially through each set of deformable nip rollers, each adjacent set of deformable nip rollers forming a web segment therebetween. The apparatus includes a circuit for adjusting a first characteristic of the driver of a downstream set of deformable nip rollers to set the tension of a first web segment which is between the downstream set of deformable nip rollers and an upstream set of deformable nip rollers. The apparatus also includes a controller for adjusting a first characteristic of the driver of the upstream set of deformable nip rollers to maintain a constant tension on a second web segment which is immediately preceding the upstream set of deformable nip rollers.
In an alternative embodiment, the apparatus also includes a sensor for measuring a second characteristic of the second web segment and the controller adjusts the first characteristic of the driver of the upstream set of deformable nip rollers based on the measurement from the sensor.
The first characteristic may be the rotational speed of the driver. The second characteristic may be the tension of the second web segment, the speed of the second web segment, or the amount of torque in the driver of a set of deformable nip rollers immediately preceding the upstream set of deformable nip rollers.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and related objects, features and advantages of the present invention will be more fully understood by reference to the following detailed description of the presently preferred, albeit illustrative, embodiments of the present invention when taken in conjunction with the accompanying drawings wherein:
FIG. 1 is a block diagram of an embodiment of the present invention having closed loop control based on a feedback signal provided from a tension sensor;
FIG. 2 is a block diagram of an embodiment of the present invention having closed loop control based on a feedback signal provided from a velocity sensor;
FIG. 3 is a block diagram of an embodiment of the present invention having closed loop control based on a feedback signal provided from a torque sensor; and
FIG. 4 is a block diagram of an alternative embodiment of the present invention having open-loop control.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
FIG. 1 shows a block diagram of an apparatus for setting the tension of a downstream web without affecting the tension of an upstream web in a web printing press 10 according to one presently preferred embodiment of the invention. Web 12 is moved by driven nips 22, 28 and 34 in the direction indicated by arrow 14. Nips 22, 28 and 34 are formed by nip rollers 20 and 24, 26 and 30, and 32 and 36, respectively. Nip rollers 22 and 24, 26 and 30, and 32 and 36 rotate as indicated by arrows 27 and are formed with a deformable surface as described above and in more detail in U.S. Patent Publication No. 2006/0157924 A1.
Nip rollers 20, 26 and 32 in FIG. 1 are driven by nip roller drivers 38, 40 and 39, respectively. Nip roller drivers 38, 40 and 39 may be, for example, electric motors, or other type of suitable drivers. Drive controllers 48, 50 and 49 provide control signals to nip roller drivers 38, 40 and 39, respectively, to control the rotational speed of respective nip rollers 20, 26 and 32. The control signals are provided from controller 70 to the drive controllers 48, 50 and 49 on respective signal lines 58, 60 and 59, respectively.
Upstream web span 16 is formed between nips 22 and 28, while downstream web span 18 is formed between nips 28 and 34. As one of ordinary skill in the art will readily recognize, additional nips may be provided in the printing press 10 within the context of the present invention, and the principles of this invention apply in that event as well. A tension sensor 92 directly measures the tension in upstream web span 16 and provides a corresponding output signal on line 93 to controller 70. Controller 70 also includes an input 60 which is used by an operator to set the tension of downstream web span 18 according to principles of the invention.
In the embodiment of FIG. 1, closed loop control is used to maintain the tension of upstream web span 16 at a constant value when an operator desires to change the tension of the downstream web span 18. In particular, when an operator inputs a tension adjustment for upstream web span 18 to controller 70 via input 80, controller 70 first calculates an updated value for the control signal 59 so that drive controller 49 will adjust the rotational speed of nip roller driver 39 thereby changing the tension of downstream web span 18 to the desired value. Since the change in tension of downstream web span 18 would also result a change in the upstream web span 16 when using deformable nip roller, controller 70 also changes the rotational speed of nip roller driver 40 in a manner that will maintain the tension of upstream web span 16 at a constant value. This requires a decrease in the rotational speed of nip roller driver 40 when the tension of downstream web span 18 is increased (by increasing the speed of nip roller driver 39), and an increase in the rotational speed of nip roller driver 40 when the tension of downstream web span 18 is decreased (by decreasing the speed of nip roller driver 39). In particular, controller 70 receives a signal on line 93 from tension sensor 92, and compares the received tension signal with the desired tension level and adjusts, in a closed loop manner, the control signal 60 so that the result of the comparison between the received tension signal and the desired tension level is zero. In this manner, whenever an operator changes the tension of downstream web 18, the tension of upstream web 16 will be automatically kept constant by closed loop control via the feedback signal provided from tension sensor 92. By maintaining the tension on upstream web span 16 at a constant level, the change of tension on downstream web span 18 is invisible to all preceding nips and spans.
Although the embodiment of FIG. 1 shows a single controller which separately sets and controls the speed of nip roller drivers 38, 39 and 40, as one of ordinary skill in the art will readily recognize, each nip roller driver 38, 39 and 40 could be separately controlled, and that the closed-loop control is only applied in this illustrative embodiment to control the nip roller driver which is upstream from the web span having its tension changed. Of course, as one of ordinary skill in the art will readily recognize, the techniques of this invention can be applied to allow adjustment of any span in a printing machine without affecting the tension of any other span. Thus, in the embodiment of FIG. 1, nip roller drives 38 and 39 are shown being set by controller 70, but could alternatively could be set by other means known to one of ordinary skill in the art. Also, although FIG. 1 shows a controller 70 which compares the signal from tension sensor 92 to a desired value to calculate the feedback control signal 60 to apply to drive controller 50, as one of skill in the art would readily recognize, this could alternatively be done in the analog domain using a comparator and other ancillary circuitry, where the comparator compares an analog signal from the tension sensor 92 to a predetermined value representing the desired tension setting. Finally, although FIG. 1 shows drive controllers 48, 49 and 50 as separate from controller 70, as one of ordinary skill in the art will understand, the drive controller functions could alternatively be implemented either within controller 70 or separately within nip roller drivers 38, 39 and 40.
FIG. 2 shows a first alternative embodiment of the present invention which also relies on closed loop control to maintain the tension of upstream web span 16 at a constant value when the tension of downstream web span 18 is intentionally changed by the operator. The only change from the embodiment of FIG. 1 is that a velocity sensor 95 is provided to monitor the velocity of upstream web span 16 and provide a corresponding velocity signal 98 to controller 70 (and tension sensor 92 is omitted). Since the velocity of the web span is proportional to the tension thereof, controller 70 compares the measured velocity signal 98 to the desired velocity value and generates a control signal 60 for the drive controller 50 connected to nip roller driver 40 which, because of the closed loop nature of the system, will cause the difference between the measured velocity signal 98 and the desired velocity value to quickly become zero, even after the tension on downstream web span 18 is changed.
FIG. 3 shows a second alternative embodiment of the present invention which also relies on closed loop control to maintain the tension of upstream web span 16 at a constant value when the tension of downstream web span 18 is intentionally changed by the operator. The only change from the embodiment of FIG. 1 is that a torque sensor 90 is provided in nip roller driver 38 to provide a torque signal 57 to controller 70 (and tension sensor 92 is omitted). Since the torque of the nip roller driver 38 is proportional to the tension on upstream web span 16, controller 70 compares the measured torque signal 57 to a toque value that corresponds to the desired tension on upstream web span 16 and generates a control signal 60 for the drive controller 50 connected to nip roller driver 40 which, because of the closed loop nature of the system, will cause the difference between the measured torque signal 57 and the desired torque value to quickly become zero, even after a change on the tension on downstream web span 18.
FIG. 4 shows a third alternative embodiment of the present invention which, unlike the other embodiments, does not rely on closed loop control to maintain the tension of upstream web span 16 and thus does not require any sensor to monitor the tension of upstream web span 16. Instead, when the operator inputs a tension change for downstream web span 18, controller 70 automatically calculates the amount of change necessary for both control signal 59 and control signal 60 based on characteristics stored in memory. These characteristics can be defined empirically or using predictive models, as understood by one of ordinary skill in the art, and, for example, control signal 60 could be a fixed percentage of control signal 59.
While the present invention has been particularly shown and described with reference to the preferred embodiments and various aspects thereof, it will be appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the appended claims be interpreted as including the embodiments described herein, the alternatives mentioned above, and all equivalents thereto.
Patent Application
20100181360
