CA1159927A

CA1159927A - Apparatus for heating a sheet- or web-like material

Info

Publication number: CA1159927A
Application number: CA000374201A
Authority: CA
Inventors: Jacob De Vries
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-04-02
Filing date: 1981-03-30
Publication date: 1984-01-03
Also published as: ES501094A0; US4354095A; MX149423A; PT72673B; BE887919A; PT72673A; BR8101974A; IT8167452A0; GB2073390A; GR74839B; NL8101112A; NL8001944A; FR2479959A1; AU6872781A; ES8201894A1; AU541311B2; ZA811690B; IE810519L; CH650198A5; GB2073390B

Abstract

APPARATUS FOR HEATING A SHEET- OR WEB-LIKE MATERIAL.

Abstract of the disclosure.
An apparatus for heating a sheet- or web-like material during its transport through a processing machine, comprises at least one infrared heating panel facing the path of transport of the material. The apparatus is provided with a controlling circuit for controlling the heat emission of the heating panel in response to a control signal generated by a control means.
The heating panel is switched off if the transport speed of the material becomes smaller than a minimum speed by means of a monitoring circuit which can switch off the controlling circuit, said monitoring circuit being coupled to a detector means reacting to the transport speed. The monitoring circuit may further comprise at least one zone detector means reacting to the presence of the material within a given zone extending transversely to the transport direction of the material on either side of the desired path of transport, wherein the monitoring circuit switchess off the controlling circuit if the material leaves said zone.

Description

Backgr~und of the invention.
The invention relates to an apparatus for heating a sheet- or web-like material during its transport through a processing machine, comprising at least one infrared heating panel facing the path of transport of the material and connected to an ac-source through semiconductor switching means, a controlling ~.

circuit for delivering ignition impulses to the semiconductor switching means, wherein the ignition time within each half per-iod of the supply voltage is determined by a control signal pro-vided by a control ~eans to the controllingcircuit,while the heating panel is switched off if the transport speed of the ma-terial becomes smaller than a minimum speed.
Summary of the Invention ~ t is an object oftheinvention to provide an appar-atus of this type, wherein the switching off of the heating panel for preventing fire or unnecessary power consumption is realized in a very simple manner.
According to the present invention there is provided apparatus for heating a sheet of web-like material during its transport through a proeessing maehine, comprising at least one infrared heating panel faeing the path of transport of the ma-terial, semiconductor switching means for connecting said panel to ana.c. supply, a controlling eircuit for delivering ignition pulses to the semiconductor switching means, control means for providing a control signal to the controlling switch to determine the ignition time within each half period of the supply voltage, a detector responsive to the transport speed of the web through the machine,and amonitoringcircuitcoupled tosaid detectorand arranged tocausethecontrolling aircuitto switchoffthe heating panel if the transport speed of the material through the machine becomes less than a predetermined minimum speed.
Preferably, said monitoring circuit comprising at least one zone detector means reacting to the presence of the material within a given zone extending transversely to the transport di-rection of the material on either side of the desired path of transport, wherein the monitoring circuit switches off the control-ling circuit if the material lea~es said zone. In this manner a timely switching off of the heating panel can be realized at ~.~

failures of the processing machine causing the web tension of the web-like material to drop out without the transport speed immediately decreasing.
Brief Description of the Drawings The invention will hereinafter be explained in fur-ther detail by reference to the drawings, in which some - 2a -~ , 1 1 599~7 embodiments of the apparatus according to the invention are shown.
Fig. 1 schematically shows the arrangement of a heating panel of the apparatus according to the invention with respect to a web-like material.
Fig. 2 is a front view of the heating panel of Fig. 1.
Fig. 3 is a block diagram of an embodiment of the apparatus according to the invention, wherein the heat emission is a function of the temperature of the material.
Fig. 4 is a block diagram of an embodiment of the apparatus according to the invention, wherein the heat emission of the heating panel is a function of the transport speed of the~ material.
Fig. 5 is a block diagram of an embodiment of the apparatus according to the invention, wherein the heat emission of the heating panel is manually adjustable.
Fig. 6 is a block diagram of the controlling unit used with the apparatus of Fig. 1 through 5.
Fig. 7 is a block diagram of a part of the apparatus of Fig. 3.
Fig. 8 is a block diagram of a part of the apparatus of Figs. 3 and 4.
Fig. 9 is a block diagram of the monitoring circuit used with the apparat~s of Figs. 1 through 4.
Fig. 10 is a simplified diagram of the monitoring circuit used with the apparatus of Fig. 5.
Fig. 11 shows some voltages which can occur in the monitoring circuit of Fig. 10.
Fig. 12 schematically shows the arrangement of two zone detector means on both sides of two heatin~ panels arranged opposite each other.
Fig. 13 is a block diagram of a part of the monitoring circuit, to which the zone detector means of Fig. 12 are connected.
Detailed description of the preferred embodiments.
-Fig. 1 schematically shows the arrangement ofa heating panel 1 of an apparàtus for heating a material web 2 which is passed through a processing machine, such 25, for instance, a printing press. Only two guide rollers 3,4 of the processing machine are shown in Fig. 1. The heating panel 1 ise~uipped with a plurality of infrared elements 5 (see Fig. 2), which are provided in the form of infrared quart~ tubes.
Becausé of the elevated temperature (2100C) of the tungsten filament of these quartz tubes, the infrared elements 5 provide short- to medium-wave infrared radiation (1000 to 3000 nm), which-offers major advantages.
First of all, the infrared elements 5 have a low thermal inertia, so that, if required, the maximum heat emission is available about 0.5 s after switching on the heating panel 1, while there is no longer any heat emission as early as about 0.2 s after switching offf the heating panel 1. Further, virtually no heat is released to the layer of air between the heating panel 1 and the web 2, so that the efficiency is high.
Moreover, the short-wave infrared radiation penetrates deeply into the web 2, so that there is optimum heating of the material.

1 1 59~27 In the case of a rotary offset machine, wherein a suitable ink is used, drying of the ink is thus introduced, causing the quality and the processability of the web 2 following the prin-ting operation to be substantially improved.
Finally, the heating panel 1 is provided with two blowers 6 for cooling the terminal connections o~ the infrared elements 5.
~ The heat emiss;on of the heating panel 1 is deter-mined by a controlling unit 7 in response to a control signal provided by a control means, as will be explained hereinafter.
To this end, the controlling unit 7 comprises a plurality of thyristors, which are indicated schematically in Figs. 3,4 and 5 by a block 8 and are included in the power supply lines of the infrared elements 5. Further, the controlling unit 7 comprises a controlling circuit 9 for delivering ignition impulses to the gate electrodes 10 of the thyristors 8. The time of ignition of the thyristors 8 with respect to the zero passages of the supply voltage is determined by the magnitude of the control signal.
As shown in Fig. 6, the controlling circuit 9 is provided with a detector 11, which at each zero passage delivers an impulse to a timing circuit 12, an input 13 of which receives the control signal. The control signal, the magnitude of which can vary from O to 5 V, determines within each half period of the supply voltage the time with respect to the zero passages at which an output impulse with a predetermined duration appears at an output 14 ot the timing circuit 12. Since a varying of the heat emission of the heating panel l from 30 to 100 percent of the maximum heat emission is sufficient, the output 14 of the timing circuit 12 delivers, at a control signal of 0 V, an output impulse at such a time that the heating panel 1 delivers about 30 percent of the maximum heat emission.
According to the embodiment described, the infrared elements 5 are connected groupwise to a three-phase ac-source, so that three successive ignition impulses are necessary. The first ignition impulse is formed by the output impulse of the timing circuit 12. The next two ingition impulses are obtained by means of two delay means 15 and 16, which are series-connected to the output 14, and the outputs 17 and 18 of which provide the second and the third ignition impulse, respectively. In order to ensure a reliable ignition for the respective thyristors 8, the ignition impulses are each con-verted with the aid of an oscillator 19 and three mixing circuits 20 into a series of ignition impulses, which impulse series appear respectively at outputs 21, 22 and 23, as indicated in Fig. 6. These outputs 21-23 are coupled in a suitable manner to the gate electrodes 10 of the thyristors 8.
Fig. 3 shows an embodiment of the apparatus according to the invention wherein the control signal is a function of the témperature of the web 2. In this case the control means 24, which applies the control signal to the input 13 of the controlling circuit 9, comprises a temperature detector 25 which, in the transportdirection of the web 2, is mounted beyond the heating panel 1, as shown in Fig. 1. The temperature detector 25, which may be, for exampl~, an optical pyrometer, delivers an output signal which is proportional to `7 the temperature of the passing web 2.
The temperature detector 25 is connected to an input of a control circuit 27, an output 28 of which delivers the control signal which is inversely proportional to the temperature of the web 2. The control circuit 27 has a second input 29, to which a manually operable adjusting device 30 is connected ~or adjusting the desired temperature of the web 2.
According to Fig. 7, which shows the control circuit 27 in more detail, the adjusting device 30, provided in the form of a potentiometer is connected to the non-inverting input of an operational amplifier 31,-which is connected as an integrator and the inverting input of ~hich is coupled to the temperature detector 25. The output of the amplifier 31 delivers the control signal and forms the output 28 of the control circuit 27. As the output signal of the temperature detector 25 increases, i.e., at rising temperature, the magnitude of the control signal at the output 28 will decrease, and therefore the heat emission of the heating panel 1 as well, and conversely. In this manner, an equilibrium is reached at a temperature determined by the adjustment of the potentiometer 30.
Further, the ou~put signal of the temperature detector 25 is applied to an amplifier 32, to which an indicator 33 is connected which indicates the prevailing temperature of the web 2. The control circuit 27 further comprises a comparator 34 for comparing the output signal of the temperature detector 25 with a fixed reference value, which corresponds to a given minimum temperature. When the temperature output signal drops below this reference value, the comparator 34 turns on a transistor 1 1 ~9927 35 causing the output 28 to be short-circuited and the control signal to be fixed at the value zero. As a result, a failure -- procluced, for example, by a wire rupture or the like -- does not have the effect of the heating panel 1 becoming completely energized, since there would otherwise be the possibility of fire breaking out.
In the embodiment shown in Fig. 3, a monitoring circuit 36 is provided for switching off or disconnecting the controlling circuit 9 when the transport speed of the web 2 drops below a given value. The controlling circuit 9 then can no longer supply any ignition impulses to the thyristors 8, so that the heating panel 1 no longer emits any heat. Accordingly~ energy savings can be obtained while the web 2 is being passed at a low running speed through the processing machine, and an imper-missible increase in temperature of the material is prevented when the web 2 is brought to a rapid standstill.
An input 37 of the monitoring circuit 36 receives a control voltage from a converter 38, an input 3~ of which is connected to a detector 40. The detector 40, provided in the form of an inductive transducer, co-operates with a round disc 41 which is coupled with the guide roller 3 and has a number of schematically indicated metallic projections 42 uniformly distributed on the periphery thereof. The sensor 40 thus supplies an impulse signal, the fre~uency of which corresponds to the transport speed of the web 2. The convertor 38 converts this impulse signal into the aforementioned control voltage.
The converter 38 and the monitoring circuit 36 will be further explained hereinafter.

~, 1 1 59g27 Fig. 4 shows an embodiment of the apparatus according to the invention which is likewise equipped with the contIolling unit 7, but wherein the control signal supplied at the input 13 is a function of the transport speed of the web 2. In this case, control means 43 is constituted by the detector 40 and by the converter 38 acting as a control circuit, the output voltage delivered by the converter 38 being used as the control signal. Just as in the embodiment of Fig. 3, use is made of the monitoring circuit 36, the input 37 of which likewise receives the output voltage of the converter 38.
The converter 38, more deta~ls of which are shown in Fiy. 8, receives at the input 39 thé impulse signal of the detector 40, which signal is converted by means of a Schmitt trigger 44 and a monostable multivibrator 45 into impulses having a predetermined duration T. These impulses appear at an output 46 of the multivibrator 45 and control an analogue multiplexer 47, the anaiogue input of which is connected to the output of a buffer amplifier 48. This buffer amplifier 48 provides an output voltage which can be adjusted by means of a potentiometer 49. Impulses thus appear at the output of the multiplexer 47, which correspond in duration to the duration of the output impulses of the mùlti-vi~rator 45, while the amplitude is determined by the adjustment of the potentiometer 49. The output of the multiplexer 47 is connected to a low-pass filter 50, which supplies an output dc-voltage, the magnitude of which is a function of the frequency and the amplitude of the impulses received. Finally, an amplifier 51 is provided with which the dc-voltage is brought to the desired level for the control signal.

1 1~99~7 From the above it will be understood that the converter 38 provides an output voltage, the magnitude of which is a function of the frequency of the impulse signal delivered by the detector 40, as well as of the adjustment of the potentiometer 49. The supplied output voltage which constitutes the control signal varies between O and 5 V. The potentiometer 49 allows adjustment of the rate of increase of the control signal and, therefore, of t~e heat emission of the heating pane] 1 at increasing transport speed, by which the transport speed at which the heating panel 1 emits the maximum amount of heat is also adjusted. If desired, the potentiometer 49 can be adjusted in such manner that, at the maximum transport speed within the control range of the converter 38, the heat emission by the heating panel does not constitute the maximum value which can be reached.
The frequency of the impulse signal of the detector 40 must not exceed a predetermined value. For, no new impulse from the detector 40 must be received within the impulse duration T of the impulses generated by the multi-vibrator 45. This maximum frequency determines the control range of the converter 38. Of course, the control range of the con-verter 38 can be adapted in a simple manner to the working speed of the processing machine at which the apparatus according to the invention is used. This can be achieved, for example, by choosing a suitable number of metallic projections 42 of the disc 41.
As already noted, the output of the converter 38 is also connected to the input 37 of the monitoring circuit 36, which is shown in Fig. 9. The monitoring circuit 36 is provided with a comparator 52, the inverting input of which receives the output voltage of the converter 38, while a reference voltage Vref, adjustable by means of a potentiometer 53, is connected to the non-inverting input. The comparator 52 is connected by a time-delay means 54 -- which is active only when the output of the comparator 52 changes from the high to the low level ---to a switching element 55, with which the con-trolling circuit 9 can be switched on and off, for example by interrupting the supply voltage for this controlling circuit 9.
When the output voltage of the converter 37 is greater than Vref, the output of the comparator 52 is at the low level, and the switching element 55 keeps the controlling circuit 9 switched on, so that the heat emission of the heating panel 1 is controlled in the desired manner. When the transport speed of the web 2 drops below the reference value Vref as adjusted with:the potentiometer 53, the output of the comparator 52 changes to the high level, and the switching element 55 at once switches off the controlling circuit 9, so that the heat emission is discontinued. As soon as the transport speed again exceeds the adjusted reference value Vref, the output of the comparator 52 changes from the high to the low level, which change of level is transmitted by the time-delay means 54 with some delay to the switching element 55, so that the controllling circuit 9 and therefore the heating panel 1 are switched on with some delay~ The time-delay element 54 prevents that the controlling circuit 9 is switched on under the action of inter-ferencence impulses.

. Fig. 5 illustrates a simple embodi.ment of the apparatus according to the invention, which is particularly suitable for use with a machine for processing sheet-like materials, such as, for example, a sheet-fed offset machine.
The control signal, supplied to the input 13 of the controlling unit 7, in this case originates from a manually operable ad-justing device 56, which may be constituted, for example, by a potentiometer or by a multiple-position switch.
In this embodiment, a detector 57 provided just before the heating panel 1, viewed in the transport direc-tion of the material, emits a low-level signal in the presence of a sheet, and a high-level signal in the absence of a sheet.
This binary signal is suppli~ed to a monitoring circuit 58, which can switch on and off the controlling circuit 9 of the control-ling unit 7.
The monitoring circuit 58 (see Flg. 10) com-prises two RC-circuits RlCl and R2C2, by means of which it is established whether the binary signal of the detector 57 has the low or the high level, respeciively, for too long a period of time. In the former case, there is a sheet in front of the detector 57 and, therefore, in front of the heating panel 1 as well, while the processing machine is at a standstill or at least is transporting the material at a speed which is too low. The heating.panel 1 is then switched off so as to prevent the material from overheating, which could cause fire to break out. In the latter case, no successive sheet appears within the period determined by the time constant ~lC1, and the heating panel 1 is switched off in order to avoid unnecessary energy consumption.
Shown in Fig. 11, a-e, are the voltages Vl, V2, V3 and V4 occuring in the monitoring circuit 58 and the switching state of the controlling circuit 9 and, therefore, of the heating ~anel 1. The voltage Vl corresponds to the output signal of the detector 57, while V2 is the voltage on the capacitor Cl, and V3 the voltage on the capacitor C2. V4 is the collector voltage of the transistor 59.
The resistances Rl and R2 are adjustable, so that the respective time constants RlCl and R2C2 can be adapted as required.
The operation of the monitoring circuit 58 is as follows:
If no sheet of material is observed for some time by the detector 57, the voltage V2 on the capacitor Cl increases until a zener diode 60 turns on, which causes the transistor 61 to turn on as well. The voltage level at which this takes place is indicated with a broken line in Fig. llb.
This causes the transistor 50 to be switched off and a relay 62 connected in the collector line to become inoperative, by which the controlling circuit 9 is switched off.
If a new sheet of material follows before the zener diode 60 turns on, the transistor 59 remains in the con-ducting state, and the controlling circuit 9 is not swltched off.
The voltage Vl has a low value when the detec-tor 57 observes a sheet. As a result, the voltage V3 can decrease, so that, upon reaching a value indicated by a broken line in Fig. llc, a zener diode 63 turns on, which causes a transistor 64 to turn on. As a result, the transistor 61 becomes conductive and the transistor 59 is switched off, so that the relay 62 again becomes inoperative and the controlling circuit 9 is switched off.
If the sheet has passed before the zener diode 63 turns on, the transistor 59 remains conductive, and th~
control~ing circuit 9 is not switched off.
From the above it appears that with the use of the apparatus according to Fig. 5 a favourable energy con-sumption can be realized in the processing of sheet-like materials with the heating panel 1 emitting heat only when material occurs in front of the heating panel. Furthermore, overheating of the material during standstill or a very low transport speed is pre-vented, since the heating panel is timely switched off.
Fig. 12 schematically shows the arrangement of two heating panels on both sides of a material web 65, which arrangement may be used in a rotary offset press for example.
The material web 65 only partial shown is guided in tensioned condition between the heating panels 1 and extends along a roller 66 to a folder, for example (not shown in Fig. 12). The control of the heat emission of the heating panels 1, not shown in Fig. 11, can be as a function of the temperature of the material web 65 (Fig. 3) or as a function of the transport speed of the material web 65 (Fig. 4), as desired.
Although with both control methods the heating panels 1 are automatically switched off by the monitoring circuit 36 if the transport s~eed of the web 65 becomes smaller than the adjusted minimum speed, it could occur under circumstances, for I 1 5~9~7 example at a failure of the folder, that, because of a dropout of the web tension, the web 65 contacts a heating panel-l, which is still operating because the transport speed is not yet smaller than the adjusted minimum speed. In this case fire could easily break out.
According to the invention this disadvantage can be obviated by means of a plurality of detectors 67 connected to a part of the monitory circuit 36 shown in Fig. 13. At the arrangement of Fig. 12 a detector 67 is mounted on both sides of the heating panels 1. The detectors 67 known per se provide a binary signal having the first binary value at the presence of the web 65 within a zone 68 shown by a dotted line on either side of the desired path of transport of the web (shown by the web 65) and the other binary value at the absence of the web 65 in the zone 68.
- According to Fig. 13 the monitoring circuit 36 comprises an AND-input circuit 69 with four inputs 70 and a OR-input 71 with two inputs 72, to which inputs 70, 72 the detec-tors 67 can be connected. The outputs of both input circuits 69, 71 are coupled with a time-delay means 73 which supplies a change of state of the output signal of the input circuits 69, 71 after lapse of a time-delay to a switching means 74 if no new change of state occurs within the time-delay. The switching means 74 can switch on and off the controlling circuit g and, therefore, the heating panels 1 in response to the signal supplied by the time-delay means 73.
The time-delay of the time-delay means 73 is adjus-table by means of a manually operated adjusting device 75.

1 1 5992'7 ,16-The time-delay means 73 prevents that short during movements of the web 65 beyond the zone 68 could cause a switching off of the heating panels 1.
If the detectors 67 are connected to the inputs 70 of the AND-input circuit 69 the heating panels 1 are switched off when the web 65 is outside of the zone 68 at one of the detectors 67, while, if the detectors 67 are connec~
ted to the inputs 72 of the OR-input circuit 71, the heating panels 1 are switched off when the web 65 is outside of the zone 68 at all detectors ~7.
It is noted that both input circuits can have a different plurality Gf inputs 70, 72 respectively, than shown in Fig. 13.
The detectors 67 also detect an eventual rupture of the web 65 and the complete absence of the web 65.
The invention is not restricted to the embodiments described above, which can be varied in a number of ways within the scope of the invention.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for heating a sheet of web-like material during its transport through a processing machine, comprising at least one infrared heating panel facing the path of transport of the material, semiconductor switching means for connecting said panel to an a.c. supply, a controlling circuit for delivering ig-nition pulses to the semiconductor switching means, control means for providing a control signal to the controlling circuit to determine the ignition time within each half period of the supply voltage, a detector responsive to the transport speed of the web through the machine, and a monitoring circuit coupled to said detector and arranged to cause the controlling circuit to switch off the heating panel if the transport speed of the ma-terial through the machine becomes less than a predetermined minimum speed.

2. Apparatus according to claim 1, wherein the moni-toring circuit includes a first time-delay means which switches on the controlling circuit when a given length of time has lapsed since the transport speed has exceeded the minimum speed.

3. Apparatus according to claim 1, wherein the moni-toring circuit comprises an adjusting device for adjusting the minimum speed at which the controlling circuit is switched off.

4. Apparatus according to claim 3, wherein the moni-toring circuit comprises at least one zone detector means react-ing to the presence of the material within a given zone extend-ing transversely to the transport direction of the material on either side of the desired path of transport, and the monitoring circuit switches off the controlling circuit if the material leaves said zone.

5. Apparatus according to claim 4, wherein the moni-toring circuit comprises two or more zone detector means and at least one input gate circuit, said at least one input gate cir-cuit controlling a switching means through a second time-delay means for switching the controlling circuit on and off.

6. Apparatus according to claim 5, wherein the time-delay of the second time-delay means is adjustable.

7. Apparatus according to claim 3, wherein the detector means is coupled to a first input of said control means and wherein the control signal provided by said control means increases at increasing transport speed of the material and the control signal is also supplied to the monitoring cir-cuit, said control means having a second input to which an ad-justing means is connected for adjustment of the rate of increase of the control signal at increasing transport speed.

8. Apparatus according to claim 7, wherein the detector means provides a pulse signal, the frequency of which is proportional to the transport speed of the material, wherein the control means is formed as a convertor comprising a pulse shaper which, in response to the pulse signal, delivers output pulses with a predetermined pulse duration to the switch input of an analogue switching means, an analogue input of which receives a dc-voltage, the value of which is determined by said adjusting means, while an analogue output is connected to a low pass filter, an output, of which controls an amplifier which provides the control signal.

9. Apparatus according to claim 1, wherein, for pro-cessing of sheet-like material, the detector means is mounted, seen in the transport direction, just before the heating panel and provides a binary signal having the first binary value at the presence of a material sheet opposite the detector means and having the second binary value at the absence of a material sheet opposite the detector means, wherein the monitoring circuit in response to said binary signal, switches on the controlling circuit at the presence of a material sheet and switches off the controlling circuit if within a first predetermined period after the passage of a material sheet no subsequent sheet is detected by said detector means.

10. Apparatus according to claim 9, wherein the monitoring circuit also switches off the controlling circuit if a material-sheet remains longer than a second prede-termined period opposite of the detector means.

11. Apparatus according to claim 10, wherein both said periods are adjustable.

12. Apparatus according to claim 10 or 11, wherein the monitoring circuit comprises two timing circuits which, upon the expiration of said first and second period, respectively, provide an output signal, wherein said first timing circuit is returned to zero each time the binary signal of the detector means goes from the second to the first binary value, while said second timing circuit is returned to zero each time the binary signal of the detector means goes from the first to the second binary value, wherein the outputs of both timing cir-cuits operate a switching means for switching on and off the controlling circuit.

13. Apparatus according to claim 3 or 6, wherein the control means comprises an adjusting device for adjustment of the desired temperature of the passing material, wherein a temperature detector delivers an output signal propor-tional to the temperature of the passing material to the con-trol means, while the control signal provided by the control means is inversily proportional to said temperature of the passing material, wherein the control means further comprises a comparator for comparing the output signal of the temperature detector with a fixed reference value, said comparator fixing the control signal at a value corresponding to the minimum heat emission of the heating panel if the output signal of the tem-perature detector is smaller than the reference value.

14. Apparatus for heating a sheet- or web-like ma-terial during its transport through a processing machine, com-prising at least one infrared heating panel facing the path of transport of the material and connected to an ac-source through semiconductor switching means, a controlling circuit for deli-vering ignition impulses to the semiconductor switching means, a control means for supplying a control signal to the control-ling circuit to determine the ignition time within each half per-iod of the supply voltage, a detector means responsive to trans-port speed of the web through the processing machine, and a moni-toring circuit coupled to the detector means, said monitoring circuit comprising at least one zone detector means responsive to the presence of the material within a given zone extending transversely to the transport direction of the material on either side of the desired path of transport, and being arranged to switch off the controlling circuit if the material leaves said zone.

15. Apparatus according to claim 14, wherein the moni-toring circuit comprises two or more zone detector means and at least one input gate circuit, said at least one input gate cir-cuit controlling a switching means through a second time-delay means for switching the controlling circuit on and off.

16. Apparatus according to claim 15, wherein the time-delay of the second time-delay means is adjustable.

17. An apparatus for heating a web-like material during its passage through a processing machine, which comprises: at least one heating panel facing the path of travel taken by a web-like material through the processing machine, said panel being equipped with at least one short-to-medium wave infrared radiation producing element; a controlling circuit having means for varying the heat emission of the radiation producing elements as a function of the magnitude of a control signal; and a moni-toring circuit arranged to switch off the radiation producing elements if the rate of passage of the web-like material through the processing machine becomes less than a reference rate and before it comes to a stop.

18. An apparatus for heating a web-like material during its passage through a processing machine which comprises:
at least one heating panel facing the path of travel taken by a web-like material when passing through a processing machine, said panel being equipped with at least one short-to-medium wave infrared radiation producing element connected to an AC
power supply through a semiconductor switching means; a controlling circuit for delivering ignition pulses to the semiconductor switching means, wherein the ignititon time with-in each half period of the power supply voltage is determined by a control signal in such a manner that the emission of the radiation producing element varies as a function of the magnitude of the control signal; and a monitoring circuit arranged to switch off the radiation producing element if the rate of the passage of the web-like material through the processing machine becomes less than a reference rate and before it comes to a stop.

19. The apparatus according to claim 18, wherein the monitoring circuit includes a first time-delay means whereby the radiation producing element is switched on after a given length of time has elapsed from when the rate of passage of the web-like material through the processing machine exceeds the reference rate.

20. The apparatus according to claim 18 or 19, wherein the monitoring circuit further comprises a means for adjusting the reference rate at which the radiation producing elements are switched off.

21. The apparatus according to claim 18, wherein the monitoring circuit further comprises a zone detector means cap-able of measuring deflections of the web-like material from its intended path through the processing machine whereby the monitoring circuit switches off the radiation producing elements if the web-like material is deflected more than a predetermined amount as it passes the zone detector.

22. An apparatus for heating sheet-like material during its passage through a processing machine, which comprises:
at least one heating panel facing the path of travel taken by a material when passing through a processing machine, said panel being equipped with at least one short-to-medium wave radiation producing element connected to a power supply through a semi-conductor switching means; a controlling circuit for delivering ignition pulses to the semiconductor switching means wherein the ignition time within each half period of the supply voltage is determined by a control signal in such manner that the emission of the radiation producing element varies as a function of the magnitude of the control signal; a monitoring circuit arranged to switch off the radiation producing element if the rate of passage of the sheet material becomes less than a reference rate and be-fore it comes to a stop; said monitoring circuit further comprising a detector means for detecting the presence of sheet-like material before it reaches the heating panel;

said detector means providing a binary signal whereby the controlling circuit is energized when a sheet material is opposite the detector and de-energized if no sheet material is opposite the detector means within a first predetermined time interval.

23. The apparatus according to claim 22 wherein the monitoring circuit, further comprises:
means for switching off the controlling circuit when sheet material remains opposite the detector means for a second predetermined time interval.

24. The apparatus according to claim 22 wherein said monitoring circuit further comprises means for independently adjusting the first and second predetermined time intervals.

25. An apparatus for heating a sheet-like material during its passage through a processing machine which comprises at least one heating panel facing the path of travel taken by the sheet-like material when passing through a processing machine, said panel being equipped with at least one short-to-medium wave infrared radiation producing element connected to an AC power supply through a semiconductor switching means; a controlling circuit for delivering ignition impulses to the semiconductor switching means wherein the ignition time within each half period of the power supply voltage is determined by a control signal in such manner that emission of the radiation producing element varies as a function of the magnitude of the control signal; a monitoring circuit when by the radiation producing element will be switched off if the rate of passage of the sheet-like material through the processing machine becomes less than a reference rate and before it comes to a stop; said monitoring circuit further comprising a zone detector means capable of measuring deflections of the sheet-like material from its intended path through the processing machine whereby the monitoring circuit switches off the radiation producing elements if the sheet-like material is deflected more than a predetermined amount as it passes the zone detector.