Abstract:
Compensation loop device between a web feed system and the input of the web in a paper using machine such as a high speed printer, for example, a laser printer, including a frame provided with sliding feeding rollers, the latter roller being driven by an electrical motor that accelerates or decelerates the web coming into the machine according to the length of a loop.

Description:
This application is a continuation of U.S. Ser. No. 07/359,349, filed May 31, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for generating a compensation loop on a web that is sent to graphic machines, such as printers. 
     2. Description of Related Art 
     In the graphic industry, there is often a problem of problem of providing a continuous web of printing paper coming from a supply, such as a reel or a box containing the web in a folded configuration, to a printer, wherein the printer may work in a intermittent way, while the web, even though it does not have to arrive in a continuous manner and at a constant rate to the printer, however it cannot be suddenly started or stopped in its running because it can be torn or jammed in an excessive quantity at the printer input, where sliding problems might arise. 
     Therefore, it is an object of the present invention to provide a device generating a loop in a web feeding a graphic machine, for compensating the intermittent operation of the graphic machine, because sometimes the web is not required and sometimes there is a strong necessity therefor. 
     Another object is to provide a signal capable of detecting the formation of a loop in a web at the input of the graphic machine. Another object is to stop the web letting in to the graphic machine when a loop formed in the device exceeds a first prefixed length and to start it again when the loop is shorter than a second prefixed length (that is its length is too short). A further object is to prevent the paper from being spoiled by causing stopping or starting of the movement of the web to take place in a gradual way. 
     A known method to signal an excessive shortening or lengthening of the loop is to employ very sensitive microswitches receptively connected both to a tray whereon the loop bottom can lay and to a short rod that can be lifted from the loop when the loop is shortening too much. The microswitches operate a motor controlling the entrance of the web to the graphic machine. 
     This method has the disadvantage of requiring very sensitive microswitches and consequently, the machine is readily disturbed by vibrations typical of these kinds of machines. Moreover, very weak microswitches can fail very often losing their operating and therefore the control of the web loop with consequences that can be easily understood. 
     Another method uses proximity detectors such as capacity detectors by which the capacity variation of an electrode as caused by the presence of an insulating material such as a web is sensed. These kind of detectors have a good strength and are relatively insensitive to vibrations, but they have a drawback of requiring frequent adjustments when the web composition changes or the environmental conditions change the moisture content of said web. 
     SUMMARY OF THE INVENTION 
     Shortly stated, the device of the present invention consists of a frame put between a storage or supply for paper to be printed and a graphic machine, such as a printer. The frame is provided with at least one roller driving a web that comes into the graphic machine after having formed a compensation loop. The roller is driven by a motor, the turning on of which depends on the detection of a prefixed length of the loop by a web presence detector. Particularly, the frame includes two rollers, on of which is idle and is used to take the web around another driving roller in turn driven by the motor (32) and helped in driving the web by counter-rollers which press the web against the roller. 
     According to a preferred embodiment of the present invention, the loop of the web is supported upstream by one driving roller and downstream by another driving roller that is part of the graphic machine receiving the web. 
     According to another preferred embodiment, the web presence detector is composed of a lamp and a photedector assembly by which the presence of the loop of the web in its proximity through the light reflected by the web is detected. 
     According to the last mentioned preferred embodiment, the photodector which detects the presence of the loop of the web, has a digital output, indicating the absence or the presence of the loop without considering small variations of the reflected light. Moreover, the photodector is equipped with an electronic circuit having a narrow hysteresis loop the output of which changes in a clear cut way as much as possible and without uncertainty from a high state to a low state respectively corresponding to the absence or presence of the web. 
     Particularly, the electronic circuit includes a voltage comparator comparing the output of the photedector with a voltage prefixed by a voltage regulator setting the electronic circuit to change its state at a prefixed distance between the photodector and the loop of the web giving place to a predetermined reflected light intensity. 
     In addition, in order to obtain a gradual acceleration up to a maximum speed of the motor of the driving roller and a gradual deceleration to a complete stop of the motor, the voltage comparator is followed by a ramp generator circuit by which the speed of said motor is adjusted varying it from a zero to a maximum value and back from the maximum value to zero in preset times permitting accelerations and decelerations of the motor in order not to spoil the paper web. The features and the advantages of the present invention will be better appreciated from the following description of an embodiment referred to the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a partial perspective view of the compensation device according to the present invention, 
     FIG. 2 is a side cutaway projection view of the device shown in FIG. 1; 
     FIG. 3 shows a simplified electronic circuit connecting a loop presence photodector with an electric motor for advancing a web. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawing and particularly to FIGS. 1 and 2, it can be seen that a compensation loop device 10 according to the present invention includes a frame 12 comprising two first vertical struts 14 and 16, two longitudinal horizontal members 18 and 20 attached to the base of the struts 14 and 16, two shorter vertical struts 22 and 24 and a cross member 26 connecting the shorter struts at their top. The top first vertical struts 14 and 16 support, in the lower part, a first roller 28 which is an idle roller and in the upper part, a second roller 30 which is a driver roller driven by an electric motor 32 by suitable means such as a belt 34. Supporting counter-rollers 36a, 36b, 36c, keep pressed, by means of a paper device 37, a web 38 against a second roller 30 after it has passed around the idle or first roller 28, the web 28 coming from a paper storage, such as a reel or a box (not shown). 
     The web 38 after coming down from the roller 30 forms a loop 40 which during the operation can take positions 40a and 40b (see FIG. 2) and then comes up to another roller 42 fixed by brackets, such as a bracket 44 to the body (48) of a graphic machine. 
     The proper shape of the loop 40 is detected by a proximity detector consisting of a lamp or light emitter means 50 and of a photodector 52, sensing a light ray emitted by the lamp 50 and reflected or diffused by the loop 40 of the web 38 that is running in the direction of the arrows 54 and 56 (see FIG. 2). 
     The upper ends of the struts 14 and 16 are joined by a bridge 57, supporting the motor 32 and the device 37 by which the counterrollers 36a, 36b, 36c are placed against the roller 30 and one of the struts has a housing 58 for controlling circuits for the web 38 provided with knobs and displays 59a, 59b and 59c. 
     An electronic circuit for detecting the loop 40, as shown in FIG. 3, includes a phototransistor 62 connected by a load resistor 64 to a continuous input voltage +V R  and by a coupling resistor 66 to a not inverting input of a voltage comparator circuit 68 that through a feedback resistor 70, whose value can be determined by a person skilled in the art, has a narrow hysteresis loop to promote a clear cut and definite change of its input state. The inverting input of the voltage comparator 68 is connected through a coupling resistor 72 to a potentiometer 74 whose adjustment influences the sensitiveness of the circuit 60 and therefore the distance between the loop 40 and the proximity detector 48 causing the state change at the output of the comparator 68. 
     The output of the comparator 68 is connected to an input of a ramp generator 76 that upon receiving at its input either an upwardly or a downwardly stepping signal for transforming the signal at the output in a ramp signal extending the time duration of its step variation. The ramp signal is taken to a second input by a feedback connection 78 to give linearity thereto and it is applied to an input of a driving amplifier 80 the purpose of which is to drive an electric motor, such as the motor 32, having a speed proportional to an applied voltage. 
     DESCRIPTION OF OPERATION 
     The operation of the present invention is as follows: a web 38 to be printed, is drawn from a storage and is passed around the rollers 28 and 30 of the frame 12 and then, after having formed the loop 40, comes to a roller 42 of the graphic machine. 
     The motor 32 rotates at its maximum speed driving the roller 30 and thus the web 38, the loop of which takes the positions 40, 40a, 40b. While the loop goes from the position 40 to the position 40b, the light reflected from the emitter 50 to the photodector 52 changes from an intensity almost zero to an increasing intensity, making the phototransistor of FIG. 3 more and more conductive until its collector voltage gets down to a value at which it commutates the output of the voltage comparator 68 from its high value to the low one. At that point the ramp detector 76 initiates a descending ramp at its output causing the motor 32 to gradually decelerate or slow down until it stops. Once the motor 32, is stopped the roller 42 keeps on drawing the web 48 in such a way that its loop goes from the position 40b to the position 40 where the lacking of light impinging onto the photodector 52 restores a high voltage at the output of the voltage generator 68 and a gradual return of high voltage according to a rising ramp at the output of the ramp generator 76. This in turn causes the motor to gradually accelerate or speed up again which, in turn, causes the web 38 to form again the loop passing from the position 40 to the position 40b and the operating cycle of the motor 32 is repeated. 
     If for any reason the graphic machine stops, the web 38 should never be drawn by the roller 42 and the loop would quickly extend below the position 40b causing the motor 32 to stop according to the characteristics of the ramp of the generator 76, thus ensuring the same web to stop gently. In the same way, the raising of the loop above the position 40 would cause the motor 32 to operate according to the characteristics of the ramp ensuring the restarting of the web 38 without being torn. 
     The hereinabove described embodiment is a preferred and not a limitating form of the present invention, and it is obvious that changes and substitutions with functionally equivalent elements for one or more components of the described device will lead to embodiments to be considered all within the scope of the invention. 
     For example, the ramp circuit 36 could be substituted for by a stepping function generator or by a microprocessor providing a series of numerical data that is above to provide gradual acceleration and deceleration control for the motor 32.