WO1997037284A1 - Process for controlling the operation of a printer, in particular start and stop - Google Patents

Abstract

A process is disclosed for controlling printing by an electrographic printer in which an endless web of information carrier material is conveyed by a positive engagement drive and by a friction drive. The invention relates in particular to a process for controlling start and stop. Also disclosed is a process for synchronising in an accurate position the parallel displacement of sections of a web and a device for driving a motor.

Description

description

Method for controlling the operation of a printer, in particular the start and stop operation

The invention relates to a method for controlling the pressure in an electrographic printer, in which a web of endless carrier material is transported on the one hand by a form-locking drive and on the other hand by a friction drive. In particular, the invention relates to a method for controlling the start operation and the stop operation. The invention further relates to a method for precisely synchronizing the parallel running of web sections of a web, in particular in a printer. In addition, a device for controlling a motor, in particular for setting the vacuum valve in a vacuum brake, is specified.

The present invention relates to a further development of the methods and devices described in PCT / EP95 / 04265. The arrangement features and method steps described in the mentioned patent application are hereby expressly referred to. The content of PCT / EP95 / 04265 is to be added to the disclosure content of the present patent application.

A printer is known from the aforementioned document and from WO 94/27193, which can operate in various operating modes. In the "two-color simplex mode", the web is offset in parallel by at least one web width during transport in the printer. The offset web sections are then guided alongside one another at the transfer printing point. The front of the web is printed during a first printing process. The printed first web section is then fed to the fixing station, transported back, and the front, which has already been printed, is printed with a second color. In single-color duplex operation, the web is turned during transport through the printer, so that a first web section of the web is printed with its front side and a second web section of the web with its rear side at the transfer printing location.

Furthermore, it should also be pointed out to the simplex operation, in which the web is transported and printed by the printer up to twice the width of the above-mentioned web sections.

In the known printing device, the web is transported in the transfer printing area by a positive-locking drive, in which pins generally engage in drive holes on the edge of the web. The fixing station that fixes the toner image contains a friction drive that transports the web. When the printer is started and thus when the web accelerates to the printing speed, the speeds of the various drives must be coordinated with one another in order not to generate excessive tensile forces in the web. In printing operation it is necessary to ensure synchronous transport for both web sections in order to compensate for the length shrinkage of one web section due to the passage through the fixing station compared to the other web section. In addition, trouble-free operation of the printer and the components provided for it must be ensured. To control motors, for example the motors for vacuum valves, paper width adjustment and a pressure force adjustment mechanism, it must be ensured that the motors are blocked at an early stage.

From EP-A-0 432 298 an automatically operating paper loading device for a printer is known, which contains friction drives. In order to compensate for different transport speeds of the paper web to be transported, a loop puller is provided, whose position is scanned by a detector. The speeds of the friction drives are set depending on the signals from the detector.

In DE magazine: Feinwerktechnik und Meßtechnik, Vol. 100, 1992, Issue 8, pp. 339 - 343, K. Mayer, "Precisely controlled stepper motors enable reverse side printing", a printer concept is described in which between a drive for a paper web and fixing rollers, a web storage device is arranged in the form of a loop puller. Two sensors detect the position of the loop puller. Depending on the signals from the detectors, stepper motors for driving the fixing rollers are accelerated or braked. Acceleration and deceleration take place according to predefined speed-time curves, which are stored in the form of tables in a microprocessor.

It is an object of the invention to provide a method and a device which ensure the trouble-free operation when starting, stopping a printer, or which ensure a positionally accurate parallel running of web sections of a web.

According to a first aspect of the invention, a method for controlling the pressure in an electrographic printer according to the features of patent claim 1 is provided. This method ensures perfect start and stop operation of the printer, the web reaching its printing speed in a short time or being stopped in a short time. The web is accelerated and braked in such a way that no overshoot occurs in a web store connected between the friction drive and the positive-locking drive. An overshoot is also largely avoided in the transition area from acceleration of the web and approximately constant printing speed in printing operation. The method according to the invention can be used both for simplex operation with a single web and for duplex and two-color simplex operation, in which two Web sections are transported between the positive locking drive and the friction drive.

According to a further aspect of the invention, a method for the precise synchronization of the parallel running of web sections of a web made of continuous material is specified according to the features of claim 11. In this method, two control loops are used, fine adjustment of the paper transport for the two web sections being carried out by changing the braking forces and rough adjustment when leaving the fine control area by changing the pressing forces on the web sections.

Furthermore, in a further aspect of the invention, a device for controlling a motor is specified according to the features of claim 21. This device ensures that the blocking of a motor can be displayed in a simple manner and can be easily evaluated by microprocessors.

Embodiments of the invention are described below with reference to the drawings. In it show:

Figure 1 is a schematic representation of the transport of a paper web in a printer with a

Positive locking drive and a friction drive ^" ,

FIG. 2 shows the transport path of a web in a printer, the returned web passing through a turning device,

FIG. 3 shows a schematic view of the functional units used in a printer for regulating the web transport of two web sections, FIG. 4 angular positions of the loop pullers serving as web stores,

FIG. 5 characteristic curves for start and print operation,

FIG. 6 shows a flow chart for the process steps for the positionally precise synchronization of the parallel run of the web sections, and

Figure 7 shows a circuit arrangement for driving a

Motors in the printer.

As mentioned, the present invention relates to a further development of the device described in PCT / EP95 / 04265. In the following, reference is also made to an electrographic printing device described in WO 94/27193, in which two web sections arranged in parallel can be moved and printed simultaneously by the printer. The basic features of the hardware structure required for the function of the present invention are explained below. Details of this hardware structure can be found in the mentioned document PCT / EP95 / 04265.

According to FIG. 1, parts of a printing device are described in which a web 1 made of continuous carrier material passes through the transport path of the printer twice, namely with a first web section A (A web) and with a second web section B ( B-Bahn). The web sections A and B run largely synchronously through the upper part of the printing device from a caterpillar drive 3 at least up to a fixing roller pair 8/1, 8/2 lying parallel next to one another. After the paper feed, the web section A is moved forward by the form-fitting caterpillar drive 3 and arrives at the pair of fixing rollers 8/1, 8/2, which contains a friction drive. Thereafter, the web 1 is deflected, returned and runs as a web section B in a juxtaposition to the web Section A the same distance from the caterpillar drive 3 to the pair of fixing rollers 8/1, 8/2. In the case of paper return, the web 1 is either turned over in order to be able to also print on its back or not turned over in order to apply a further printed image to the front already printed on. The pair of fixing rollers 8/1, 8/2 fixes the toner image on the web sections A, B by means of thermal pressing. When passing through the pair of fixing rollers 8/1, 8/2, the web section A is heated, as a result of which the web 1 shrinkage results. The distance between transport holes on the edge of the web 1, into which transport pins of the caterpillar drive engage, is shortened, for example by 0.06%. Due to the paper return, this has the consequence that the web section B in the caterpillar drive 3 is transported more slowly than the web section A. The two web sections A, B of different speeds are gripped by the pair of fixing rollers 8/1, 8/2 which is on its entire width has a constant transport speed.

FIG. 2 shows a hardware structure which ensures that the effect caused by the shrinkage does not lead to malfunctions in the printing operation. The web 1 of the recording medium is printed with a toner image in the transfer printing area 2 on its front side. The form-fitting caterpillar drive 3 has transport pins which engage in corresponding edge holes (shown schematically as white omissions in the web '1). The web section A is fed in the transfer printing area 2 with the front, the web section B with the back of the web 1. The web section A passes a belt store in the form of a first loop puller 6/1 and is driven by a friction drive within a fixing station 8 after passing a first vacuum brake 7/1. The web 1 is then fed to a turning device 10, turned there and fed again to the form-fitting caterpillar drive 3 as the web section B. In the transfer area 2, the back of the web 1 is simultaneously printed with its front. Thereafter, the track section B is parallel ~ _Λ „„, _o ^ PCT / DE97 / 00545

WO 97/37284

7 lel led to the web section A to a second loop puller 6/2 and a second vacuum chamber 7/2 and then, driven by the friction drive, arrives at a paper output, not shown. In the return loop of the web 1, a belt store 11 in the form of a further loop puller is arranged behind the turning device 10. This tape store 11 serves only to compensate for tolerances and for form synchronization when inserting forms of different lengths.

The friction drive comprises a fixing roller 8/1 and a pressure roller 8/2, the driven fixing roller 8/1 being heated. In a fixing gap 9 between the two rolls 8/1 and 8/2, the web 1 is pressed, heated and transported forward.

According to FIG. 3, the functional units for producing a synchronous parallel run of the two web sections A and B comprise several assemblies. First of all, a loop puller unit 6 is to be mentioned, which contains the two loop pullers 6/1 and 6/2, each loop puller 6/1, 6/2 having an angle sensor 12/1, 12/2, which sends its signals to a controller ¬ supply assembly 20. The loop pullers 6/1, 6/2 are pretensioned by a spring mechanism 13. The respective torque is set by an adjusting device 14. In the event that only one track passes through the transport path, both loop pullers 6/1, 6/2 can be rigidly coupled.

Another important assembly is the vacuum device 15, which comprises a suction pump 15a, which is connected to controllable valves 16/1, 16/2, which provide negative pressure on sliding surfaces for the web sections A, B. This vacuum device 15 thus forms vacuum brakes 7/1, 7/2, with which each track section A, B can be acted upon separately with a braking force which can be set via the valves 16/1, 16/2. The setting of valves 16/1, 16/2 is carried out by a vacuum control module 21. In addition to the fixing roller 8/1 and pressure roller 8/2 already mentioned, the fixing unit 8 has a pivoting mechanism 18 which is set via a drive 19. As can be seen in FIG. 3, the force acting in the fixing gap 9 on the web sections A and B can be set by the pivot mechanism 18. The driven fixing roller 8/1 is driven by a stepper motor 17, which receives its current impulses from a power electronics module 21, and also rather provides the control signals for the motors of the valves 16/1, 16/2. Another power electronics module 22 provides current pulses for a stepper motor 23 of the positive-locking drive 3, which, as mentioned, is designed as a caterpillar unit. The assemblies 20, 21, 22 receive information from a higher-level control (not shown) via the line 24.

The angular positions of the loop pullers 6/1, 6/2, which are important for regulating the transport of the web sections A, B, are explained below with reference to FIG. 4. Each slack extractor 6/1, 6/2 can pivot about an axis of rotation 28 between an upper mechanical stop 26 and a lower mechanical stop 27. Before these stops 26, 27 are reached, the angle sensors emit signals which define an upper error range 0 and a lower error range U. The working range of the loop pullers 6/1, 6/2 is defined by R. The middle of the handle puller work area is labeled MR. MA defines the mean of the positions of the two loop pullers 6/1, 6/2. RD is the control deviation of the two loop pullers 6/1, 6/2 according to the loop differential control. RL denotes the rule deviation of MA with respect to the center MR of the work area R.

In printing operation, by changing the speed of the fixing roller 8/1 via the control electronics 20, the content of the web storage is realized by the loop pullers 6/1, 6/2, regulated to a medium value. The mean value MA of the positions of the two loop pullers 6/1, 6/2 serves as the controlled variable, ie an average value of the signals of the two associated angle sensors 12/1 and 12/2. For example, the setpoint is given the value MR, ie the mean value of the working range of the loop pullers 6/1, 6/2. The control works in such a way that the value RL, ie the control deviation of MA with respect to MR, is close to zero. This type of control is referred to as loop length control.

As a further control process, a loop difference control is carried out, in which the difference between the angular positions of the loop puller 6/1 for the web section A and the loop puller 6/2 for the web section B is regulated to zero, i.e. the control deviation RD should be minimized. If a purely proportional control algorithm is used, a permanent, low control deviation can remain. In order to carry out the loop difference control, the vacuum brakes 7/1, 7/2 are used as actuators, the valves 16/1, 16/2 being acted upon by the control. As a result of the braking force generated on the sliding surfaces 7/1, 7/2 of the vacuum brakes, the slippage of the web section A and that of the web section B in the fixing gap 9 between the rollers 8/1 and 8/2 is changed relative to one another.

Another possibility of changing the slip in the fixing roller nip 9 is to change the pressing force on the web section A or the web section B by adjusting the pivoting mechanism 18. A low force increases the slip, an increase in the force reduces it. In the mentioned patent application PCT / EP95 / 04265 further details of the structure of the loop pullers 6/1, 6/2, the pivot mechanism 18 and their effect on the loop length control and the loop difference control are described. In particular, reference should be made to the pressure force adjustment mechanism described in the mentioned patent application, which, in contrast to the The pivot mechanism 18 shown in FIG. 3 can set different pressing forces for the web section A and the web section B, an increase in the pressing force for the web section A preferably resulting in a corresponding reduction in the pressing force for the web section B. As a result, a noticeable change in slip in the web sections A, B can be achieved even with slight changes in force.

When operating a printer, a distinction must be made between start operation, in which the web must be accelerated from standstill to the printing speed, printing operation, in which the web is essentially transported forward at the printing speed, and stop operation, in which the web is braked back to zero speed. In the start mode and in the stop mode, both a positive locking drive for the caterpillar unit and a friction drive for the fixing unit must be coordinated with one another in terms of their transport speeds. Therefore, according to the present invention, a separate control process for the start and stop operation is provided.

FIG. 5 shows a frequency-time diagram, on the basis of which the control process for starting operation is explained. The diagram is divided into a start phase and a subsequent phase with continuous printing. The characteristic curve 40 in continuous printing operation approximately defines the printing speed, which is essentially constant. Due to paper tolerances, the transport speed for the web 1 can change slightly, which is compensated for by fine regulation of the speed of the fixing roller 8/1. The stepping motor 17 of the fixing roller 8/1 (cf. FIG. 4) and the stepping motor 23 of the caterpillar unit 3 are controlled by the control by means of current pulses with a predetermined frequency; this frequency thus defines the peripheral speed of the fixing roller 8/1 or that of the caterpillar wheel of the caterpillar unit 3. The stepping motor 23 is according to driven a linear characteristic 42, ie it has a constant acceleration.

A further characteristic curve 44 is stored in an EPROM, which the controller accesses, which specifies a frequency-time behavior of the current pulses for the stepping motor 17 of the fixing roller 8/1. This characteristic curve 44 has a larger gradient than the characteristic curve 42. This means that after starting at 45, the fixing roller 8/1 transports the web 1 faster than the caterpillar unit 3 can. Accordingly, the length of the web 1 stored in the loop pullers 6/1, 6/2 is reduced, so that the loop pullers 6/1, 6/2 proceed from a central position around MR (see FIG. 4) in the direction of the lower error range U be moved. The mean value of the signals emitted by the angle sensors 12/1, 12/2 also moves in the direction of the signal value which corresponds to the error range U. When a lower storage value Su is reached, which is, for example, just below MR, point 46 on characteristic curve 44 is reached. The control then no longer increases the frequency of the current pulses delivered for the stepper motor 17, but keeps it constant. Section 49 results in the frequency-time characteristic. Since the caterpillar unit 3 continues to transport the web 1 at the increasing speed according to the characteristic 42, the mean value MA or that of the signals of the angle sensors 12 / 1, 12/2 and reaches an upper storage value So, which, for example is just above the MR value.

Position 48 is reached in the frequency-time characteristic. Now the controller outputs current pulses for the stepping motor 17 according to the stored characteristic curve 44, ie the stepping motor 17 is accelerated more than the stepping motor 23, with the result that the loop pullers 6/1, 6/2 have a decreasing length of the path 1 save. Section 50 is run through in the frequency-time diagram. At point 52, the value MA or the mean signal of the two angle sensors 12/1, 12/2 is again below the value MR. pleased and signals that the lower storage value Su has been reached. Then the stepping motor 17 is again supplied with a constant frequency of current pulses, so that the peripheral speed of the fixing roller 8/1 remains constant. The loop pullers 6/1, 6/2 now store an increasing amount of web 1 again.

Keeping the frequency of the current pulses constant and increasing the frequency of the current pulses for the stepping motor 17 continues until approximately the point 54 on the frequency-time characteristic curve 56 is reached. The then achieved transport speed of the web 1 corresponds approximately to the printing speed, whereupon a transition is made to the continuous printing operation, in which a control process with PI control behavior begins in order to keep the transport speed of the web 1 essentially constant. As can be seen, the frequency-time characteristic curve 56 for the current pulses of the stepping motor 17 is composed of different sections with increasing frequency and with constant frequency. In this way, a smooth and fast acceleration of the web 1 is achieved without the loop pullers 6/1 and 6/2 vibrating strongly and possibly reaching the error areas O and U (see FIG. 4). In addition, the overshoot in the transition area from start operation to continuous printing operation remains low. It should also be noted that the invention is not limited to linear characteristics. Rather, sections that are non-linear in sections, e.g. sinusoidal characteristics are used.

The stop operation is described below, in which the web 1 is brought to a standstill, starting from a transport speed, approximately equal to the printing speed. The control process then takes place such that 17 current pulses are specified for the motor in accordance with a falling frequency-time characteristic. This frequency-time characteristic has a greater inclination than the frequency-time characteristic of the stepping motor 23 for the caterpillar 3. The web storage realized by the loop pullers 6/1, 6/2 is emptied in sections faster by the transport of the fixing roller 8/1 than web material by the transport of the caterpillar unit 3 web material is supplied later. When the lower storage value Su for the web storage is reached, the speed of the fixing roller 8/1 is then kept constant in accordance with a constant frequency of the current pulses for the stepping motor 17 until the upper storage value So is again reached in the web storage. The keeping constant and the lowering of the speed continue until the speed of the web 1 is reduced to zero. The web 1 is preferably stopped in a position in which the loop pullers 6/1, 6/2 store a large length of web material, ie the position of the loop pullers 6/1, 6/2 is in the vicinity of the upper error area 0 .

The control process for setting a uniform web transport of the web section A and the web section B is described below. In this control process, by setting the vacuum in the vacuum brake 7/1 for the web section A and the vacuum brake 7/2 for the web section B, and by adjusting the pressure force adjusting mechanism and thus the pressure force on the web section A and the pressure force on the Web section B the slip is adjusted with respect to the rotation of the fixing roller 8/1 so that the difference in transport speeds between the two web sections is minimal. The controlled variable for this control process is the length difference of the lengths of the web section A or the web section B stored in the loop pullers 6/1, 6/2.

The following variables are used to describe the control process:

FA contact force acting on web section A, FB contact pressure acting on web section B, PA the negative pressure acting on web section A, PB the negative pressure acting on web section B, TA angular position of the loop puller for web section A, TB angular position of the loop puller for web section B, n running variable 1, 2, 3, 4, ..., n _max , dT angle difference between TB - TA, dF difference of pressure forces (FA - FB) / 2, dP negative pressure difference (PB - PA) / 2, Fstart initial state for FA, FB, Pstart initial state for PA, PB,

Tn switching threshold for the difference TB - TA for a value of the running variable n, Tsynch switching threshold for a synchronization stop, dFup value of the increase in the pressing force for the web section

A, dFdown Reduction of the pressing force for the web section A, Pmin minimum negative pressure, Fmax maximum pressing force, ⁿ max maximum number of switching thresholds T.

The process steps shown in Figure 6 relate essentially to the printing operation. At the beginning of the control process (step 60), a print start (step 62) takes place at time t = 0, which has already been described in detail further above. An initialization is then carried out in step 64, ie a check is made as to whether the loop pullers 6/1, 6/2 are in a central position of the working area R, preferably close to the value MR (cf. FIG. 4). . The preset control parameters are then output by a service person's diagnostic system (step 66). The service technician can also re-enter various control parameters in this diagnostic system in order to adjust the printer to different types of paper, printing speeds, etc. In the subsequent process step 68, the sequence control checks whether the printing operation has stopped. If this is the case, the process branches to the beginning, ie to step 60. In process step 70, depending on the value dT, ie depending on the difference in the angular position of the loop pullers 6/1, 6/2, a pressure difference dP is set on the vacuum brakes 7/1, 7/2 according to a proportional control algorithm. According to the selected control algorithm (step 72), the power electronics module 21 receives the value Pstart - dP as the setpoint PAsetpoint for the vacuum on the vacuum brake 7/1 and the value Pstart + as the corresponding setpoint PBsetpoint for the vacuum on the vacuum brake 7/2 dP specified. The pressure force adjustment technology is given as the forces FA for the web section A the value Fstart + dF and for the force FB for the web section B the value Fstart - dF.

In the following step 74, it is checked whether one of the target values PA target or PB target is lower than a predetermined minimum pressure Pmin. If this is the case, the values PAsoll and PBsoll are set to this value Pmin.

It is then checked whether the difference dT from the angular position of the loop puller 6/1 for the web section A and the angular position of the loop puller 6/2 for the web section B is less than zero. If this is the case, which has the practical meaning that the web length stored in the loop puller 6/2 is smaller than the web length stored in the loop puller 6/1, the method branches to step 80. Here it is checked whether the force difference dF is greater than zero. If this is not the case, a branch is made to step 66. Otherwise, the value dF is set to zero in step 82.

If the value dT is greater than zero in step 78, it is checked in the subsequent step 84 whether the angle difference dT is greater than a critical value Tsynch. Is this the If so, a so-called synchronization stop is triggered in subsequent step 86, that is to say the printing operation is stopped in accordance with the stop operation described further above, and at a later point in time at which a defined initial state is restored, printing operation can be continued with step 66 .

In the event that the value dT does not exceed the critical value Tsynch, it is checked in step 88 whether the angle difference dT is less than a preset threshold value T for the angle difference. There are different threshold values Tn, which are identified with the running variable n (eg n = 1, 2, 3, ...). As n increases, the threshold value Tn also increases, ie T- ^ <T2 <T3 ... <T _n . If it is the case that dT <Tn, then in step 98 it is checked whether the angle difference dT is smaller than the previous switching threshold Tn-1. Furthermore, an AND condition is used to check whether the value dFdown is greater than zero. In the event that this is the case, the step variable n is reduced by 1 in step 100, ie the lower switching threshold Tn-1 is used in the further control. In addition, the differential force dF is reduced by dFdown in step 102. The reductions according to steps 100 and 102 have the practical meaning that the malfunction has been eliminated in the control process and that a lower force difference dF and a lower switching threshold Tn-1 can now be used to fulfill the control task. If the AND condition is not fulfilled in step 98, the process branches immediately to step 66. It should be mentioned that the control circuit according to steps 66 to 88 and 98 to 66 represents the normal state, wherein sufficient slip is generated only by the suction effect of the vacuum brakes 7/1, 7/2 to keep the angle difference dT essentially constant to hold or downsize.

If it is determined in step 88 that the angle difference dT is greater than the current switching threshold Tn for the win- keldiffer, then it branches to step 90, where it is checked whether the set force difference dF for the two web sections A, B is greater than a critical maximum value of the pressing force Fmax. If this is the case, the process branches to step 98, which has already been explained above. Otherwise, in the subsequent step 92, the differential force dF is increased by the value dFup, ie there is an increase in the pressure force on the web section A and a corresponding relief of the web section B. This allows the relative transport speed between the web section A and the web section B to be changed become, ie the web section B can assume a larger slip value. In the following step 94, it is checked whether the run variable n has reached its maximum value. If this is the case, the process branches to step 98. Otherwise, the run variable is increased by 1 in the subsequent step 96, ie in the subsequent control process a higher value Tn + 1 is used as the switching threshold for the angle difference T.

As can be seen from the example according to FIG. 6, the pressure force adjustment by increasing the pressure force FA in the web section A according to step 92 or by reducing the pressure force FA in the web section A according to step 102 has the effect of a coarse control which only takes place after a predetermined angle difference has been exceeded Tn is activated. In the normal application, by adjusting the vacuum in the vacuum brakes 7/1, 7/2 according to step 70, the difference in the angular positions of the loop pullers 6/1 and 6/2 is corrected.

In the event that the vacuum brakes 7/1, 7/2 work only insufficiently, for example if the web has 1 edge holes, control is nevertheless carried out in the sense of minimizing the angle difference dT by adjusting the pressure force in accordance with the steps 92 and following.

As mentioned earlier, the power electronics module 21 controls the vacuum valves 16/1 and 16/2 to the vacuum pressure brakes 7/1 and 7/2. Motors are provided for this purpose, which are controlled by a direct current output stage. Such a direct current output stage is shown in more detail in FIG. This output stage is of considerable importance for the functioning of the vacuum brakes 7/1, 7/2. This is because it must enable the motors for the valves 16/1, 16/2 to rotate in both directions of rotation and should immediately detect a mechanical blockage of the motor shafts of the motors for the valves 16/1, 16/2. The output stage shown in Figure 7 meets the requirements mentioned. The motor M is supplied by the transistors Tl and T4 via a PTC thermistor R via the operating voltage Ub when running counterclockwise. When running clockwise, a current flows through the transistors T3, T2 and the PTC thermistor R. If the motor shaft of the motor M is blocked, there is a current increase in both directions of rotation and the voltage U _{e n n} , which is tapped at the resistor R, increases . Despite the different directions of motor rotation, a voltage of the same polarity always drops across the resistor R. A simple voltage measurement and a comparison with a reference voltage can generate an error message when the motor shaft is blocked, and the control can accordingly switch off the motor M or the motors of the valves 16/1, 16/2. So far, PTC thermistors have only been connected in series with motor leads. Although it was possible to protect an end stage from destruction, depending on the selected direction of rotation of the motor, voltages of different polarities occurred on the PTC thermistor, depending on the direction in which the current flowed through the PTC thermistor. The detection of a blockage of a motor shaft was therefore not easily possible.

Claims

claims 1. Method for controlling the pressure in an electrographic printer, in which a web (1) made of endless carrier material is moved forward at a first speed by a form-fit drive (3) which engages positively in the web (1) and is fed to a fixing station (8) which the web ( 1) transported by a friction drive (8/1) at a second speed, the first speed and the second speed being substantially equal to the printing speed in stationary printing operation, in which a length of the web (1) is stored by a web store (6/1, 6/2) arranged between the positive locking drive (3) and the friction drive (8/1), a memory sensor (12/1, 12/2) which detects the degree of filling of the web store (6/1, 6/2), a first threshold value signal (So) when an upper store value is reached and a second threshold value signal (Su) when a generated lower storage value, in the starting mode, the friction drive (8/1) is controlled according to a second speed-time characteristic curve (44), the gradient of which is greater than a first speed-time characteristic curve (42) of the positive-locking drive (3), the speed of the friction drive (8/1) is kept substantially constant when the first threshold value signal (So) occurs, and when the second threshold value signal (Su) occurs according to the second speed-time characteristic ( 44) is increased and in which the keeping constant and the increasing of the speed is continued until the printing speed is reached. 2. The method according to claim 1, characterized in that in the stop mode, in which the web is stopped starting from a transport speed equal to the printing speed, the friction drive (8/1) is controlled according to a fourth speed-time characteristic, whose inclination is greater than a third speed-time characteristic of the positive locking drive (3), that the speed of the friction drive (8/1) is kept essentially constant when the second threshold value signal (Su) occurs and is reduced according to the fourth speed-time characteristic curve when the second threshold value signal (So) occurs, and that the keeping constant and the lowering of the speed is continued until the speed of the web (1) is zero. 3. The method according to claim 2, characterized in that the web (1) is stopped in a position in which the web storage (6/1, 6/2) has a high, predetermined filling value. 4. The method according to any one of the preceding claims, characterized in that the web store (6/1, 6/2) contains at least one loop puller (6/1, 6/2), des¬ sen pivot position indicates the level of the web store, that the storage sensor is an angle sensor (12/1, 12/2), the signal of which corresponds to the swivel position of the loop puller (6/1, 6/2). Method according to one of the preceding claims, characterized in that the positive-locking drive (3) has a first stepper motor (23) and the friction drive (8/1) contains a second stepper motor (17) that a controller supplies the first and second stepper motors (23, 17) with current pulses, and that the first, second, third and fourth speed-time characteristic are realized by first, second, third and fourth frequency-time characteristics for the current pulses, which are stored in a memory of the control. 6. The method according to any one of the preceding claims, characterized in that in duplex printing, the web (1) contains a first web section (A) and a second web section (B), both web sections (A, B) by the positive-locking drive (3) and the friction drive (8/1) are driven. 7. The method according to claim 6, characterized in that the web store contains a first loop puller (6/1) for the web section (A) and a second loop puller (6/2) for the web section (B), both loop pullers (6 / 1, 6/2) a first and a .. further angle sensor (12/1, 12/2) is assigned. 8. The method according to claim 7, characterized in that the first threshold signal (So) and the second Schwell¬ value signal (Su) from the mean of the signals of the first and the second angle sensor (12/1, 12 / 2) can be determined. 9. The method according to any one of the preceding claims, da¬ characterized in that the speed of the friction drive (8/1) is set by a controller in printing operation so that the mean value of the signal of the first angle sensor (12/1) and the signal of the second angle sensor (12/2) remains within a defined working range (R). 10. The method according to any one of the preceding claims, characterized in that when a signal value of the first angle sensor and the second angle sensor occurs outside of a defined working range (u, o), the stop operation is initiated. 11. The method according to any one of the preceding claims, characterized in that the first web section (A) is moved forward at a first speed and fed to a station (8) by a form-locking drive (3) which engages in a form-fitting manner in the web (1), which the web (1) is transported by the friction drive (8/1) at essentially the same speed, the web (1) is again fed to the form-locking drive (3) and is fed to the same friction drive (8/1) as the second web section (B), a length of the track section (A) and a length of the track section (B) are stored by a track store (6/1) arranged between the positive-locking drive (3) and the friction drive (8/1), an adjustable braking force (PA, PB) is applied to the track section (A) and the track section (B), the web (1) is adjusted to the web section (A) and the web section (B) between two rolls (8/1, 8/2) acting forces (FA, FB) by an adjusting device, in a first control circuit, the braking forces (PA, PB) are set so that the difference (dT) of the web lengths (TA, TB) in the web stores (6/1, 6/2) remain essentially constant or are regulated to zero, and in the case of an increasing difference (dT) of the web lengths (TA, TB) in the web stores (6/1, 6/2), the forces acting on the web sections (A, B) by the actuating device (FA, FB) can be changed to reduce the difference. 12. The method according to claim 11, characterized in that the difference (dA) is predetermined as a threshold value (Tn) and that when a predetermined threshold value (Tn) is reached, the forces generated by the actuating device (FA, FB) are changed. 13. The method according to claim 11 or 12, characterized gekennzeich¬ net that the forces generated by the actuating device (FA, FB) are gradually increased or decreased in predetermined steps (dFup, dFdown). 14. The method according to any one of the preceding claims, da¬ characterized in that the braking forces (PA, PB) are gradually increased or decreased in predetermined steps (dP). 15. The method according to any one of the preceding claims, da¬ characterized in that the increasing and decreasing the braking forces (PA, PB) or the forces (FA, FB) depending on the level of the threshold (Tn) of the difference or depending on a run variable (n) assigned to the threshold value (T). 16. The method according to any one of the preceding claims, characterized in that the threshold value (Tn) is monitored for exceeding a maximum value (Tsynch), upon reaching which the rail transport is stopped. 17. The method according to any one of the preceding claims, characterized in that it is used to control the pressure in an electrophotographic printer, wherein the station is a fixing station (8) and the positive locking drive (3) is a caterpillar unit (3). 18. The method according to any one of the preceding claims, characterized in that the web storage is designed as a loop puller (6/1, 6/2), the respective swivel position of the stored length (TA, TB) of the web sections (A , B) and that for detecting the swivel position of the respective loop puller (6/1, 6/2) angle sensors (12/1, 12/2) are provided, the signals of the swivel position of the respective loop puller (6/1, 6/2) corresponds. 19. The method according to any one of the preceding claims, characterized in that it is one for duplex operation Printer is used, the width of the photoconductor drum used corresponding to the total width of the two web sections (A, B). 20. The method according to any one of the preceding claims, characterized in that a bridge circuit is used to control the motor for the adjustment of the vacuum valve of the vacuum brake whose bridge branches contain at least one transistor each, the motor being arranged in the diagonal branch of the bridge, the when running in one direction of rotation current is supplied via transistors arranged diagonally in the bridge and when running in the other direction of rotation current is supplied via the other transistors that m the power supply for the bridge circuit is switched a resistor, the voltage drop is evaluated to determine the blockage of the motor shaft. 21. The method according to claim 20, characterized in that a PTC resistor is used as the resistor. 22. The method according to any one of the preceding claims, characterized in that diagnostic means are provided which display the control parameters and control profiles over time and are entered into the control parameters as required.