METHOD AND APPARATUS FOR TEMPERATURE CONTROL

Abstract: The invention relates to a method to control the temperature for sealing devices used in the packaging industry which sealing devices are provided with electrically heated elements made of temperature dependent resistance material. The electrical current is divided in pulses whereby the length of the pulses is adjusted by means of a controlled switch (6). During the interruptions between the said current pulses, separate measuring current pulses of a constant and known value are generated which pulses will cause an electric voltage drop over the resistance element (2). Said voltage drop will be proportional to the temperature of the resistance element. The voltage drop will in a regulator (7) be compared with a set potential corresponding to a desired temperature whereby the sensed potential difference between the voltage drop measured and the set potential value is adapted to control a driving device (9) which will control the length of the heat generating current pulses.

Description: METHOD AND APPARATUS FOR TEMPERATURE CONTROL TECHNICAL FIELD The present invention relates to a method of monitoring and controlling the temperature of heat-generating devices which are supplied with electric current and are cyclically caused to generate thermal energy in connection with the execution of a work process, for example sealing by surface fusion of thermoplastic coatings preferably in the production of packages, these devices displaying one or more electrically conductive resistor elements of temperature-dependent resistor material, preferably in the form of heating strips, and the heat-generating current supply to the resistor elements during each work cycle is split into current pulses whose pulse length is controlled by a controllable current switch device. BACKGROUND ART Within the packaging industry, it often occurs in connection with the production of packages that packaging material surfaces are sealed or fused to one another with the aid of supplied heat and pressure. Such sealings are often made using so-called heat-sealing jaws, i.e. pairwise movable devices between which the packaging material surfaces intended for sealing are accommodated and compressed together. In at least one of these heat-sealing jaws, there are provided heat emitting elements which supply sufficient sealing heat for the thermoplastic surfaces pressed against one another to fuse together for the formation of a tight and mechanically durable seal. If insufficient thermal energy is supplied, the seal will be untight or weak, while excessive thermal energy may give rise to an uncontrolled sealing zone, while thermal energy which is supplied at excessively high temperatures will cause burn damage to the material in the form of discoloration and, at worst, charring or carbonisation. In packing or filling machines which, in order to raise capacity, operate with several pairs of independent sealing jaws, the problem moreover arises that different temperatures or different quantities of supplied thermal energy in the different jaws will give different sealing results. OUTLINE OF THE INVENTION There is thus a need in the art for a technical solution to the problem of monitoring and controlling heat supply and temperature in the sealing jaws and in governing the different sealing jaws in such a way that they give an even and uniform sealing result. A solution to this problem is provided by the present invention which is characterized in that, during the pulse gap between the above-mentioned current pulses through the previously mentioned metallic elements, measurement current pulses are generated of constant and known current strength, these measurement current pulses giving rise to a voltage across the resistor elements, the voltage, or a potential value related to this voltage, constituting a relative measurement of the temperature of the resistor elements, the above mentioned voltage or potential value being compared with a preset norm potential value, and the potential difference between the norm value and the sensed value being caused to control drive means for the above-mentioned controllable current switch device in such a manner that the above-mentioned current pulse lengths are increased or reduced, respectively, in response to the outcome of the comparison between the preset norm value and the ascertained voltage drop across the resistor elements caused by the measurement current. As was mentioned in the foregoing, the technical problem is per se previously known and has caused major difficulties, and the solution which has previously been employed was to individually adjust the sealing jaws so that, to the greatest possible extent, they are equal or, in usual parlance, that they "match one another". Most generally, the heat-generating and heat- emitting devices of the sealing jaws consist of thin strips of electric resistor material which are disposed on an insulating substrate which, in turn, is disposed on a steel rail. The difficulty in causing these sealing jaws to be absolutely alike in thermal terms resides in the fact that the heating strips are not entirely uniform in thickness, width, length etc., and that the thermal insulation against the substrate may differ between the various sealing jaws, which entails uncontrolled thermal dissipation. A further factor is that the sealing jaw strips become worn with time, with the result that their thermal properties, i.e. the heat they generate become different. The solution to the problem proposed in the present invention entails that the temperature of the heating strips is controlled by means of a regulator while the engagement time of the sealing jaws against the sealing substrate is controlled using known and preferably mechanical means. Thus, the method according to the present invention takes as its point of departure monitoring the heating strips of the sealing jaws so that they always have the same (adjustable) temperature and, by such means, controlling the quantity of heat supplied and the sealing result. Should, hence, the sealing result not be that desired, the temperature of the heating strips is adjusted until the desired sealing result is attained. If several sealing jaws are employed, the temperature of the sealing strips in the different sealing jaws may automatically be adjusted to the same level irrespective of whether the heating strips are worn or whether the sealing jaws displays different electric and thermal properties. However, the present invention need not be applied solely for regulating the temperature in sealing jaws, but there is also a technical need in the art to be able to measure and register the temperature in, for example, filaments and thermal elements which are supplied with electric current. The reason for the need to be able to measure and register the temperature in, for example, filaments, thermal elements and heating strips is that the temperature gives a diagnostic indication of the condition of the heat- generating device. The service life of heating strips or heating elements is greatly influenced by an excessively high temperature, which is not the case in, for example, the filaments in light bulbs, for which reason there is a need in the art to be able continuously to monitor the temperature in heating strips and filaments in order to be able to replace defective strips and filaments before they "burn out" and cause operational stoppage. As was mentioned above, it is also of vital interest to be able to regulate the temperature in the heating strips in sealing devices, since a controlled sealing temperature is of decisive importance to be able to attain optimum sealing results in the shortest possible time. An excessively high sealing temperature results in burn damage to the sealing object, and an excessively low temperature results in poor sealing or a long sealing time. There is, thus, a great need to be able to regulate the temperature in, for example, heating strips and filaments and to register the measurement in order to be able to monitor the condition of the installation, and in order to optimise the sealing results in a sealing unit by regulating the temperature. It is a known fact that the vast majority of metals have a temperature- dependent resistance. Many so-called resistor materials consist of metal alloys which have a highly positive temperature-dependent resistance, i.e. the resistance increases with increased temperature. The function of temperature resistance is, for most metals and alloys, linear within relatively wide temperature ranges. Since the temperature in, for example, heating strips or heating coils in a heating element which is powered by a current source is, in addition to being affected by the thermal generation caused by the current flow through the heating strips, also influenced by factors such as thermal dissipation to the substrate and to the ambient atmosphere, it is difficult to continuously be able to control and monitor the temperature other than by sensing the temperature of the heating strip using a sensor. However, such measurement is complicated, slow and only loosely accurate, while, on the other hand, the method according to the present invention as proposed herein is exact, rapid, continuous and simple to carry into effect. In addition, a sensor of known type often becomes worn out or consumed and often alters its characteristics with the result that measurement accuracy is reduced. Such drawbacks do not burden the method and the apparatus according to the present invention. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS One preferred embodiment of the present invention with its application in sealing devices for packaging materials will now be described in greater detail hereinbelow, with particular reference to the accompanying schematic Drawings, in which: Fig. 1 shows an apparatus for temperature measurement and temperature regulation in sealing devices in a packing or filling machine; and Fig. 2 a shows a diagram illustrating the length of the electrical pulses through the heating band initially when the bands are relatively cold. In Fig. 2 b is illustrated how the electrical pulses are shorter when the heating bands have reacted the working temperature Fig. 3 is a diagram showing the temperature as a function of and resistance in a metallic resistor material of the type employed in heat- generating heating strips or heating coils. DESCRIPTION OF PREFERRED EMBODIMENT In the embodiment of the present invention which is described herein and which has its practical application in sealing devices in the packaging industry, the preconditions are that mutually facing or mutually superposed materials which display thermoplastic material layers at least along their common interface or contact surfaces are to be permanently sealed to one another by surface fusion of the plastic material. This sealing is realised by heating of the thermoplastic materials to sealing or fusion temperature while the materials are at the same time urged against one another so that a surface fusion takes place. A sealing device of the type referred to here comprises strip-shaped elements of resistor material in which heat is generated in that an electric current is passed through the strips. These heating strips are mounted on supports and secured in such a manner that an electrically and thermally insulating layer is disposed between the heating strips and the support. Together, the support and heating strip form a so-called sealing jaw which comprises a movable device which may be urged against a substrate or a counterjaw while the material intended for sealing is accommodated between the counterjaw and the sealing jaw. Most generally, the sealing devices function in such a manner that, on each sealing occasion, pulse-like quantities of heat are generated in the strip- shaped elements, whereafter this heat is transferred to the material intended for sealing on each sealing occasion in that the strip-shaped elements are urged against their substrate. Thus, the heat pulses are synchronised with the sealing operation which also includes a period of pressure application and a subsequent cooling period in order that the surface-fused regions be given the opportunity to stabilise by cooling. Each heat pulse is adapted in view of energy, temperature and pulse length so that optimum sealing strength is achieved in the shortest possible time (capacity conditions) and with suitably adapted temperature (not such a high temperature that the material is exposed to burn damage). As was mentioned previously, optimum regulation may be achieved in that the temperature in the heating strips is kept at a constant, adjustable level during the sealing operation. The apparatus schematically illustrated in Fig. 1 displays a sealing jaw 1 with heating strips 2 disposed on the sealing jaw 1. The heating strips 2 of the sealing jaw 1 are connected to a current source 3, 4 which, for example, may be a direct current power source with a voltage of between 24 and 48 volts. In the current circuit, there is further connected a current switch device 6 which, in this case, consists of an MOS transistor. Parallel across the transistor 6, there is disposed a direct current source which emits a constant, adjustable current, for example a current which may be regulated to 1 amp. Parallel across the heating strips 2, there is disposed a regulator apparatus 7 including an amplifier section 10, a switch 11 illustrated schematically here, a memory circuit comprising a capacitor 13 and a PI regulator circuit 14 (PI = proportional integration), including a total aggregation point 12. The aggregation point 12 is connected to a voltage source 8 by means of which an adjustable norm value can be obtained. The circuit further includes drive means 9 which are partly operative to emit a guide voltage to the transistor 6 via its output 16, and partly to control the switch 11 by signals from its output 15. When the sealing jaw 1 is to begin a work cycle, the sealing jaw is urged against its sealing substrate by means of known mechanical devices and before or after sealing pressure is applied, a sealing sequence is commenced in that an electric voltage is connected to the contacts 3 and 4, Since the current switch device or transistor 6 is not conductive when the sequence starts, the direct current regulator 5 will instead allow the passage of a current of, for example, 1 amp., in which event this current passes through the heating strips 2. The current of 1 amp. causes a voltage across the heating strips 2, and this voltage or potential is impressed to the input of the regulator apparatus 7. Since the transistor 6 is not conductive, the switch 11 is in the closed state, for which reason the potential difference across the heating strips 2 amplified by the amplifier 10 will be supplied to the capacitor 13 which will be charged with a voltage which is proportional to the sensed voltage across the heating strips 2. The regulator 14 emits an output potential which is of the opposite polarity to the "norm potential value" which was emitted by the voltage source or transducer 8 and which controls the input to the drive means 9. When the potential at the output 14 of the regulator apparatus 7 has balanced the norm value from the transducer 8, the drive means 9 emits an output signal through the output 16 which "ignites" the transistor 6 which becomes live, in which event the heating strips 2 will be directly connected between the contacts 3 and 4 of the d.c. source and a large current controlled by the resistance of the heating strips will then pass through the heating strips which are, in such instance, heated. The drive means 9 is also a timer which, by means of the switch 11, disconnects the connection between the amplifier 10 and the memory circuit 13 when the transistor 6 is conductive, and a powerful current passes through the heating strips 2. The drive means 9 also functions as a timer for the connection-in frequency of the transistor 6 and also regulates the pulse length of the current through the transistor 6. If the heating strips 2 are cold (as they are at the beginning of the sealing sequence because of resistance difference), the voltage across the strips will, since they are powered with 1 amp. during the measurement pulse, be lower than the voltage will be when the heating strips are hot. As is apparent from Fig. 2a, the pulses 17 are represented by the time which the transistor 6 is conductive, while the pulse gaps 18 represent that time when the transistor 6 is not conductive and, thus, no heat-generating current is led through the heating strips 2. As will be apparent from Fig. 2b, a (pulse-like) measurement current 19 is led through the heating strips 2 during the pulse gaps 18 between the heat-generating current pulses 17. As was mentioned previously, these measurement pulses 19, which are at constant current strength (e.g. 1 amp.) are led, during the gap in the current pulses, through the heating strips 2 and occasion the previously mentioned voltage which increases at the same rate as the temperature of the heating strips 2 increases, and constitutes a relative measurement of the temperature. Each sealing sequence thus entails that the sealing strips, before or after having come into contact with their sealing substrate, are supplied with current pulses 17 which, as is apparent from Fig. 2, initially are relatively long (Fig. 2a) in order that the temperature in the heating strips rapidly rises to the adjusted level, while the temperature of the heating strips, when the adjusted level has been reached, are supplied with steadily shorter current pulses 19 (Fig. 2b) while the voltage across the heating strips 2 is constantly monitored and controlled with the aid of the measurement pulses. As is apparent from Fig. 3, the relationship between the resistance of the heating strips and their temperature is relatively linear, which is an advantage for the regulation function. The time involved for the sealing sequence is selected in such a way that the connecting-in time of each current pulse constitutes at most 80 per cent of the time between two current pulses, which entails that there is always room for measurement pulses in the pulse gaps between the current pulses. The regulator apparatus 7 and the drive means 9, like the MOS transistor 6 and the d.c. regulator 5, constitute components known in the trade which may have different characteristics. The central aspect of the present invention is that the current supply through the heating strips is put into effect using a switching device which, in this particular case, consists of a transistor, and that the current supply is maintained in such a manner that the heat-generating current is split into pulses which generate thermal energy in the heating strips 2. In the pulse gaps between the current supply pulses, measurement current pulses are generated by means of a constant current emitter which are led through the heating strips and then give rise to a voltage which is proportional to the temperature of the heating strips. The voltage may be processed through a regulator unit in such a manner that it results in an output potential which is proportional to the temperature of the heating strips and, with the aid of a supplied potential which constitutes a relative "norm value" for the temperature, the output potential from the regulator is balanced in such a manner that major differences between the norm value potential and the potential emitted from the regulator apparatus 7 entail that the drive means 9 emit long connecting-in pulses to the transistor 6 in order that the temperature in the heating strips will rapidly be altered such that the norm value potential and the potential emitted from the regulator apparatus 7 coincide. When the norm value potential and the emitted potential are equal, the drive means 9 emits only brief connecting-in pulses to the transistor 6 in order that only heat emission and cooling of the heating strips be compensated for, such that the temperature in the strips is kept constant. As was mentioned above, one version relating to a sealing device for packaging materials has been discussed in the described embodiment, but the apparatus may, as has also been intimated, additionally be employed for monitoring the temperature in heating elements or in filaments in order to achieve a constant working temperature in a heating coil or filament. It has also been presupposed in the body of this specification that the above¬ mentioned heating strips are to be metallic, but it has proved that the monitoring and control system according to the invention functions just as well with ceramic heating strips which, in certain cases, have proved to be far superior to conventional metallic heating strips. The present invention has proved to function well and be operationally reliable as well as entailing major savings, since elevated temperatures in filaments and heating strips result in their rapidly being spent and burnt out. The present invention should not be considered as restricted to that described above and shown on the Drawings, many modifications being conceivable without departing from the spirit and scope of the appended Claims.

Claims: WHAT IS CLAIMED IS: 1. A method of monitoring and controlling the temperature of heat- generating devices which are supplied with electric current and are cyclically caused to generate thermal energy in connection with the execution of a work process, for example sealing by surface fusion of thermoplastic coatings preferably in the production of packages, said devices displaying one or more resistor elements of temperature-dependent resistor material, preferably in the form of heating strips or heating bodies, the heat-generating current supply to the resistor elements during each work cycle being, during each working cycle, split into current pulses whose pulse length is controlled by a controllable current switch device (6) characterized in that during the pulse gaps (18) between said current pulse (17), there are generated measurement current pulses (19) through said elements (2), said measurement current pulses being of constant and known current strength, and giving rise to a voltage across a resistor element (2), said voltage or a potential value relating to this voltage constituting a relative measurement of the temperature of the resistor elements (2); that said voltage potential value is compared in a regulator apparatus (7) with a preset norm potential value; and that the potential difference between norm value and sensed value is caused to control drive means (9) for said controllable current switch in such a manner that said current pulse lengths are increased or reduced, respectively, in response to the outcome of the comparison between preset norm value and ascertained voltage across the resistor elements (2) occasion by the measurement current. 2. The method as claimed in Claim 1, characterized in that initiation of the current supply sequences to said heat-generating devices (1) will, like the length of the sequences, be effected with the aid of external control means (7, 9) which, in each work cycle, give a constant sequence length. 3. The method as claimed in Claim 2, characterized in that initiation of the current supply sequences in a sealing apparatus consisting of heating strips (2) disposed on a sealing jaw (1) is effected when the heating strips are brought into contact with their sealing substrate; and that the sequence is terminated before the heating strips (2) have been removed from the sealing substrate. 4. The method as claimed in Claim 1, characterized in that said current pulses (17, 19) are initiated by drive means (9) at a fixed frequency; and that the pulse lengths at most constitute eighty per cent of the time between two consecutive current pulses. 5. The method as claimed in Claim 4, characterized in that the length of the current pulses (17, 19) which corresponds to the connecting-in time of said controllable switch device (6) is governed by a regulator apparatus (7) coupled-in during the pulse gap of the current pulses, the apparatus sensing the potential across the heating strips (2) supplied with a constant measurement current, this potential being compared with a preset norm potential value which has been fed into a regulator (12) and which is caused to control said switch device (6) which preferably consists of a power transistor, e.g. an MOS transistor which, on activation, leads heat-generating current through the heating strips (2). 6. An apparatus for controlling the temperature in heat-generating devices supplied with electric current, and preferably disposed to be employed in a sealing apparatus for plastic materials, characterized by: a) Heat-generating devices displaying electrically conductive heating strips (2) with temperature-dependent resistance; b) A current supply source which is connectable to said heating strips (2) with a view to generating heat in said heating strip or strips (2) by means of pulsed current supply; c) A current switching device (6) disposed in the current supply circuit of the heating strips (2); and d) Means for sensing the voltage across said heating strips (2) and, in response to the sensed results, for controlling drive means (9) and thereby the connecting-in time of said current switching device (6). 7. The apparatus as claimed in Claim 6, characterized in that said heat- generating device consists of a sealing apparatus comprising heating strips (2), the apparatus being intended for sealing of thermoplastic material or packaging materials provided with thermoplastic material which, with the aid of the sealing apparatus and the supply of heat and pressure, are joined together by surface fusion of mutually facing plastic layers. 8. The apparatus as claimed in Claim 6, characterized in that the heating strips (2) consist of a metallic or ceramic material of temperature-dependent resistance. 9. The apparatus as claimed in Claim 6, characterized in that the current switching device (6) consists of a semiconductor, preferably a transistor.

Inventors: INGVERT CLAES, INGVERT, CLAES
Applicants: INGVERT CLAES, TETRA LAVAL HOLDINGS & FINANCE, INGVERT, CLAES, TETRA LAVAL HOLDINGS & FINANCE S.A.
Application year: 1996
Main classification: G05D23
IPC codes: B29C65, B65B51, G05D23