Patent Application
20210187815
The invention relates to a method for operating a temperature control device for the thermal conditioning of preforms made of a thermoplastic material according to the preamble of claim 1. The invention furthermore relates to a temperature control device for the thermal conditioning of preforms according to the preamble of claim 8. Finally, the invention relates to a machine for producing containers from preforms according to the preamble of claim 13.
The production of containers by blow molding from preforms made of a thermoplastic material, for example, from preforms made of PET (polyethylene terephthalate), is known, wherein the preforms are supplied to different processing stations within a blow molding machine (DE 4340291 A1). A blow molding machine typically comprises a temperature control device for the temperature control and/or thermal conditioning of the preforms and a blowing device having at least one blowing station referred to as a forming station, in the region of which the respective previously temperature-controlled preform is expanded to form a container. The expansion is performed with the aid of a compressed gas (generally compressed air) as a forming fluid or pressure medium, which is introduced at a molding pressure into the preforms to be expanded. The method sequence in the case of such an expansion of the preform is explained, for example, in DE 43 40 291 A1. The fundamental structure of a blowing station is described, for example, in DE 42 12 583 A1. Possibilities for the temperature control of the preforms are explained, for example, in DE 23 52 926 A1. Temperature control or thermal conditioning is understood in this case to mean that the preform is heated to a temperature suitable for the blow molding and possibly a temperature profile adapted to the contour of the container to be produced, for example, is applied to the preform. The blow molding of containers from preforms with additional use of a stretching rod is also known.
According to a typical further processing method, the containers produced by blow molding are supplied to a downstream filling unit and filled here with the provided product or filling material. A separate blowing machine and a separate filling machine are thus used. However, combining the separate blowing machine and the separate filling machine to form a machine block, i.e., to form a blocked blowing-filling unit is also known in this case, wherein the blow molding and the filling still take place on separate machine components and in chronological succession.
Producing containers, in particular also in the form of bottles, from thermally-conditioned or temperature-controlled preforms and in this case filling them simultaneously with a liquid filling material, which is supplied as a hydraulic forming fluid or pressure medium for expanding the preform and/or for forming the container at a forming and filling pressure, so that the respective preform is formed into the container simultaneously with the filling, has also already been proposed. Such methods, in which simultaneous molding and filling of the respective container is performed, can also be referred to as a hydraulic forming method or as hydraulic container molding. Assisting this forming by way of the use of a stretching rod is also known here. The preform is also firstly thermally conditioned here before the forming and filling procedure.
In the case of forming of the container from the preform by the filling material itself, i.e., using the filling material as a hydraulic pressure medium, only one machine is still required for the forming and filling of the container, which has an increased level of complexity for this purpose, however. One example of such a machine is disclosed in U.S. Pat. No. 7,914,726 B2. A further example is disclosed in DE 2010 007 541 A1.
With respect to the thermal conditioning of the preforms, the requirements are essentially identical independently of whether in a following step the forming of the preform provided with a suitable temperature profile is carried out by means of introduction of a pressurized gas or by means of a pressurized liquid. The respective temperature control device to be provided for the thermal conditioning of the preforms and the respective method to be applied for operating such a temperature control device are thus equivalent for both known forming methods. The invention described hereafter relates similarly to both described forming methods and the machines and temperature control devices used in this case.
Temperature control devices known in the prior art consist, for example, of multiple so-called heater boxes. These heater boxes are typically arranged stationary along a heating line and preforms are moved through these heater boxes by means of suitable transport units and heated at the same time. Typical transport units consist, for example, of a circulating transport chain. The chain links are formed in this case, for example, by transport mandrels, each of which holds a preform by clamping engagement in the mouth section of a preform and guides it on its circulating movement along the heating line and through the heater boxes.
In general, the temperature control device is constructed modularly, i.e., multiple such heater boxes are arranged as heating modules along the heating line. These can each be identical heater boxes in this case or heater boxes of different constructions.
Heating elements are arranged inside the heater boxes, for example. Near infrared radiators (NIR) are preferably used as heating elements in the prior art, for example, multiple near infrared radiators can be arranged one over another as heating elements in the longitudinal direction of a preform. EP 2 749 397 A1 or also W2011/063784 A2 show one example of such a temperature control device and one example of the typical structure of a heating module referred to above as a heater box.
It is known in the prior art that these temperature control devices are connected to a control unit. This control unit is generally designed in this case such that the preforms are heated inside the temperature control device such that they leave the temperature control device having a desired temperature profile. This is to be understood to mean that both a defined temperature is implemented in the preform and also a defined temperature profile in the longitudinal direction of the preform and possibly also in the circumferential direction of the preform. A temperature profile can also be provided in the radial direction, i.e., inside the wall thickness of the preform.
It is also known in the prior art that, for example, in addition to the above-mentioned heating elements, units can also be provided for cooling the surface of the preforms, for example, units for the targeted application of a cooling airflow to the preform surface. These additional units for cooling can also be incorporated into the mentioned control unit of the temperature control device. The concept of the control unit and the control method comprises units and methods using which a control is executed in the meaning of the English term “open-loop control”, using which a regulation is executed in the meaning of the English term “closed-loop control”, and mixed forms thereof. According to the invention, this relates to the regulation of a temperature control device in the meaning of the English term “closed-loop control”, so that reference is made in the claims to a regulation and a heating regulator.
To be able to regulate and/or control the temperature control device by control technology such that the preforms are provided thermally conditioned in the desired manner at the outlet of the temperature control device, i.e., they have the desired temperature and the desired temperature profile, arranging, for example, a measurement sensor at the outlet of the temperature control devices known in the prior art, for example, a pyrometer, which detects the surface temperature on preforms running past the measurement sensor. The guide value of the regulation in such an embodiment would be the surface temperature of the preforms. This measured value can be compared, for example, to a target value and the regulation of the temperature control device can thus be set to a regulation to a target value for the surface temperature of the preforms.
In known temperature control devices, it is sometimes provided that a differentiation is performed with respect to heating elements arranged one over another in the longitudinal direction of the preforms. A separate heating power is defined, for example, for each of the heating elements arranged one over another, which is regulated and/or controlled by the control unit. For example, preset can be performed in the control unit, for example, by an operator, as to whether and in which way heating elements arranged at different height levels are to heat differently. The control unit can provide for this purpose, for example, a height-specific parameter. Furthermore, it is known that a superordinate power parameter is set, which is applied for all heating elements. The actual heating power specified by the control unit of a radiator level, i.e., all heating elements arranged at an equal height level, then results by multiplication of both parameters. A defined heating profile is settable by the individual setting of the heating power per heating element level. Thus, for example, the heating power in each level can be set so that specific regions of the preform are more strongly heated. At the same time, the heating power can be set overall via the superordinate power parameters shared by all heating elements. The regulation of the temperature control device is performed in this case in the prior art in that the superordinate power parameter is adjusted in dependence on the pyrometer measured value. The pyrometer measured value represents the guide variable of the regulation, i.e., for example, the surface temperature of the preforms. The height-specific parameter is not regulated in most cases in the prior art, but rather results from the desired temperature profile in the axial direction of the preform. It is typically set during the configuration and/or during the breaking in of the machine and is changed as needed by an operator, for example, in the event of a production changeover to other preforms.
Metrologically detecting the wall thickness of finished blown containers and using these measured values as the guide variable of the heating regulation is also known in the prior art. WO 2010/054610 A1 and WO 2007/110018 A1 are examples of this prior art, in which the height-selective regulation of the heating power on the basis of associated sensors is also described.
The temperature prevailing in the temperature control device is metrologically detected in the prior art, for example, by temperature sensors, which can be arranged, for example, in a reflector of the temperature control device. Such a temperature measurement can be used, for example, to establish whether the temperature control device has reached a defined target temperature, from which the introduction of the preforms into the temperature control device is started. It is also possible to provide multiple such temperature sensors.
The above-described methods for regulating a temperature control device are considered still to be in need of improvement for various reasons. A pyrometer measurement is performed, for example, on the preform outer surface, the temperature of which can certainly deviate substantially from the temperature of the preform inner surface, however. This results, for example, from the fact that the thermal radiation acts by absorption in the preform, and the absorption is greatest on the preform outer surface, since the thermal radiation is firstly incident there and loses intensity falling exponentially during the penetration of the preform. Thermal equalization processes do take place. The thermoplastic materials typically used for the preforms usually have a poor thermal conductivity, however, so that temperature differences only balance out slowly. Changing environmental conditions can also influence the external temperature, for example, the startup of the machine or interruptions in the production operation. The external temperature of the preform upon leaving the temperature control device therefore does not always sufficiently accurately reflect the energy content and the energy distribution in the preform, which define, however, the material distribution in the finished formed containers and thus the container properties. A regulator having the guide variable preform temperature is not considered to be sufficiently robust against interfering variables. The metrological detection of wall thicknesses is linked to some measurement expenditure and is already in need of improvement for this reason. An earlier detection of the guide value than after total completion of the forming procedure would also be desirable.
The present invention is to effectuate an improvement at this point and is to improve the quality of the thermal conditioning of the preforms and thus also the quality of the containers produced therefrom. The robustness of the regulation against interfering variables is also to be improved.
This object is achieved by a method according to claim 1 and/or according to claim 7, by a temperature control device according to claim 8, and by a machine according to claim 13. Further advantageous features are specified in the dependent claims.
It is provided in the core concept of the invention that the regulation of the thermal conditioning of the preforms is carried out on the basis of the guide variable stretching force. The thermal conditioning takes place in the temperature control device, so that the regulation of this device is carried out according to the invention according to a guide value, which has such a direct relationship to the physical variable stretching force that it may be derived therefrom. On the one hand, it is possible to detect the stretching force directly by arranging corresponding force measuring units. On the other hand, it is possible to measure a value from which the stretching force can be computed, for example, with knowledge of further variables or by application of mathematical methods. This value preferably has a proportionality relationship to the stretching force. For the purposes of the regulation, the stretching force does not necessarily actually have to be derived from the detected values. The detected values can also be used directly to regulate the temperature control device, in particular if a proportionality exists, in particular if a linear proportionality exists.
In the case of mechanically driven stretching units, for example, stretching rods, a direct measurement of the stretching force can be metrologically detected, for example, via strain gauges. In the case of hydraulically or pneumatically driven stretching rods, for example, the metrological detection of the pressure of the drive media results in a variable from which the stretching force is derivable. However, in particular the measurement by means of a stretching rod provided with strain gauges and associated measurement technology is very complex. The stretching unit, in particular the stretching rod, therefore preferably has an electrical stretching system which offers the option of current measurement. The stretching force can thus be inferred from the current measurement without additional measurement equipment.
The regulation according to the invention of the temperature control device according to the stretching force cannot be confused with the regulation as described in EP 2 117 806 B1 in paragraph [0057] therein. The stretching force is used there to regulate a parameter of the blowing process, which is only executed after the thermal conditioning, however. These explicitly relates to the regulation of the blowing pressure therein, which is also directly related to the stretching force, because less stretching force is to be applied upon elevation of the blowing pressure and vice versa. There is no point of contact with the regulation of the thermal conditioning in EP 2 117 806 B1. Both above-mentioned processes together, i.e., the thermal conditioning and its regulation and the forming process and its regulation, merely jointly result in the production method to create a container from a preform. Both processes are to be considered separately with respect to regulation.
Multiple types of stretching units are known in the prior art. The use of a stretching rod has prevailed in large part in practice. It is therefore advantageously proposed that the stretching unit is a stretching rod driven by an electric stretching rod drive, in particular by a servo motor or linear motor, wherein the current consumption of the electrical stretching rod drive is metrologically detected. Metrologically simple conditions are then provided and the metrologically easily detectable current of the servo motor or linear motor also permits the conclusion of the applied stretching force.
It is preferable in the sense of a simple regulation implementation that only specific characteristic values of the stretching force profile and/or the guide value profile are used for the regulation, for example, local maxima in the curve profile. It is also preferable, for example, for the stretching force curve or the guide value curve to be integrated over a defined range to make the foundation of the regulation a stretching energy or guide value energy resulting therefrom. Multiple of these preferred values can also be taken. This also applies for the case in which the detected measured values are not converted into a stretching force, but rather are used directly for the regulation. The use of the above-mentioned characteristic values and/or an integral over a defined measured value range is also preferable in this case.
The regulation of the thermal conditioning can be improved still further in that an external temperature of the preforms is metrologically detected and supplied to the heating regulator as a second guide value. The temperature control device comprises, on the one hand, heating units for heating the preforms and cooling units for applying a coolant medium to the preforms. In this case, the cooling units, for example, fans, and the heating units, for example, thermal radiators, are each regulated by the heating regulator on the basis of different guide values, in particular, the cooling units are regulated on the basis of the guide value external temperature and the heating units are regulated on the basis of the guide value stretching force and/or a variable related thereto. A reversal of this regulation association is also implementable in the scope of the present invention, however. A dynamic equilibrium and a stable temperature conditioning may be set well by simultaneous heating and cooling. In particular, in this manner a temperature profile can also be set in the radial direction in the preform, i.e., within the preform wall.
Since the stretching force is directly related to the material distribution in the finished container and thus to the container properties, it is preferable for the heating regulator to prioritize the guide value stretching force or the guide value from which the stretching force is derivable over the guide value external temperature.
The drive of the stretching rod not only has to apply the stretching force, but rather also compensate for friction losses which occurred due to the stretching rod movement, for example, at seals through which the stretching rod is guided. To improve the regulation, it is therefore advantageous if these friction forces are metrologically detected and taken into consideration in the regulation, for example, by the detected current values being filtered of the friction component. The friction losses may be detected, for example, by executing no-load strokes of the stretching rod and metrologically detecting them. During a no-load stroke, the stretching rod movement is executed without preform. Friction losses may also be taken into consideration in that the region of the stretching force curve or the guide value curve is considered in which the acceleration of the stretching system against its inertia is completed, but stretching of the preform has not yet begun. The execution of no-load strokes is considered to be preferable, since those items of information about friction losses are obtainable over the entire stretching travel of the stretching rod, while these friction losses are superimposed in the other described case in the regions of the acceleration of the stretching system and the stretching of the preform with the effectively used forces and separation is not possible between friction losses and acceleration and/or stretching forces.
It can be advantageous to subject the metrologically detected values, for example, to filtering for smoothing, before the regulation is based thereon as a guide value.
The above explanations apply similarly to the devices according to the invention.
The fact that the stretching force or a guide value from which the stretching force is derivable is fundamentally suitable to be used as a guide value for a regulation of the temperature control device results, for example, from the strain hardening property of PET, wherein PET stands here as a representative and without restriction of the generality for the class of thermoplastic materials, which have also heretofore been used for the forming production of containers from preforms. PET is solely an example of a particularly suitable thermoplastic preform material. This is because it can be concluded from the strain hardening property that the forces occurring during the forming process play an important role in the later properties of the formed container. On the one hand, the pressure of the forming fluid introduced into the preform acts as a force on the preform, on the other hand, the force, for example, of a stretching rod acts, which guides the resulting container bubble and exerts a stretching force. It has been established on the basis of experiments that the stretching force exerted by the stretching rod, for example, is very sensitively linked to the properties of the finished container, see
A metrologically simple detection of the guide value stretching force or a guide value derivable from which the stretching force is derivable is possible, for example, if a stretching rod driven by an electrical drive is provided. One example of such a stretching rod having electrical stretching rod drive can be found, for example, in above-mentioned EP 2 117 806 B1. In the servo motor disclosed therein, for example, by means of the motor current, the spindle pitch, and the torque constant of the motor KΦ, the force in the direction of the preform longitudinal axis can be ascertained from F(t)=(i(t)−KΦ·2π)/spindle pitch. The motor current itself is a variable which is already suitable as a guide value, since the stretching force is derivable therefrom, namely by computation. The stretching force is even linearly proportional to the current in a good approximation.
Further advantages, features, and details of the invention result from the exemplary embodiments described hereafter with reference to schematic drawings. In the figures:
The fundamental structure known from the prior art of a forming machine 10 is shown in
After sufficient thermal conditioning, the preforms 14 are transferred by a transfer wheel 122 to a drivable working wheel 110, which is arranged so it is capable of rotation, i.e., revolving around a vertical machine axis MA, or to forming stations 16 which are arranged distributed around the circumference on the working wheel 110. The working wheel 110 is equipped with a plurality of such forming stations 16, in the region of which both forming of the preforms 14 into the schematically illustrated containers 12 and also filling of the containers 12 with the provided filling material take place. The forming of each container 12 takes place simultaneously with the filling in this case, wherein the filling material is used as a pressure medium during the forming. In blowing machines, in contrast, no filling takes place on this working wheel 110, but rather at a later point in time on a filling wheel having filling stations.
After the forming and filling, the finished formed and filled containers 12 are removed by a removal wheel 124 from the working wheel 110, transported further, and supplied to an output line 126. The working wheel 110 revolves continuously in the production operation at a desired revolution speed. During one revolution, the insertion of a preform 14 into a forming station 16, the expansion of the preform 14 to form a container 12 including filling with a filling material and possibly including stretching, if a stretching rod is provided, and the removal of the container 12 from the forming station 16 take place. A stretching unit, for example, a stretching rod, is provided for executing the present invention.
According to the embodiment in
Different thermoplastic materials can be used as the material for the preforms 14. Polyethylene terephthalate (PET), polyethylene (PE), polyethylene naphthalate (PEN), or polypropylene (PP) are mentioned by way of example. The dimensioning and the weight of the preforms 14 are adapted to the size, the weight, and/or to the design of the containers 12 to be produced.
A variety of electrical and electronic components are typically arranged in the region of the temperature control device 116. In addition, the heating units 118 are provided with moisture-sensitive reflectors. Since filling and forming of the containers 12 using the liquid filling material takes place in the region of the working wheel 110, it is preferably to be ensured to avoid electrical problems that an inadvertent introduction of moisture into the region of the temperature control device 116 is avoided. This can be performed, for example, by a partition unit 132, which offers at least a spray protection. In addition, it is also possible to temperature control transport elements used in the region of the transfer wheel 122 for the preforms 14 suitably or to apply pressurized gas bursts to them in such a way that adhering moisture cannot reach the region of the temperature control device 116.
Handling of the preforms 14 and/or the containers 12 is preferably carried out using tongs and/or clamping spikes or mandrels to be applied at least in regions from the inside or from the outside to the mouth section 22 with a retaining force. Such handling means are also well-known from the prior art.
The forming machine 10 is equipped with measurement sensors for the purpose of its control and/or for the purpose of its regulation. It is thus typical, for example, for a temperature sensor 160 to be arranged in the temperature control device 116 in order to be able to measure a temperature of the temperature control device 116. Furthermore, it is known in the prior art that on the outlet side of the transport unit 120, which revolves clockwise, a temperature sensor 162 is arranged, which is designed, for example, as a pyrometer and detects a surface temperature, for example, on thermally-conditioned preforms 14 running past. Finally, performing measurements on finished containers 12 using measurement sensors is also known in the prior art. Thus, for example, a wall thickness measurement sensor 164 can be arranged on the output line 126 to detect the wall thickness of a container guided past it. The above-mentioned sensors can also be formed in this case by multiple sensors arranged vertically offset, for example, to carry out a temperature measurement along the preform longitudinal axis or, for example, to execute a wall thickness measurement along the container longitudinal axis. Multiple temperature sensors 160 can also be arranged in the temperature control device 116.
The heating unit 118 illustrated by way of example in
The heater box 118 shown in a schematic sectional view in
A counter reflector 207, which reflects thermal radiation incident thereon back in the direction toward the preform 14 and thus back into the heating tunnel 211, is arranged opposite to the near-infrared radiators 209. The heating tunnel 211 is terminated on the bottom by a bottom reflector 212. The preform 14 is protected against thermal radiation on the mouth side by a support ring shield 205, since the mouth region having thread formed thereon is supposed to be protected from unnecessary heating. The support ring shield 205 is arranged in this case on the handling unit 203, which can be part of a circulating chain as explained with respect to
The temperature sensor 160 schematically shown in
In addition, the preforms 14 can also be provided with a temperature profile in the circumferential direction. This is known, for example, for preforms which are subsequently to be shaped into non-round containers, for example, into oval containers.
To improve the transient behavior of the temperature control device, a feedforward control k2 is integrated into the regulator according to
The heating power Pheat0 and the fan power Pfan0 represent set powers for these actuators and describe an operating point or a base point. These powers are changed in dependence on the guide variables.
According to the invention, a regulation on the basis of a metrologically detected variable is provided, which is derivable from a stretching force or is the stretching force itself. A regulation according to variables determined therefrom can also be provided, for example, on the basis of the stretching force. Therefore, it is to be explained hereafter on the basis of an example how this variable can be respectively provided for the regulation.
For example, the first and/or the second peak (peak1 and/or peak2) or also an integral over a range of the stretching force curve, for example, arranged essentially between these two peaks, is suitable for the use as a guide value. Such an integral represents stretching work. The forming process during the forming of a preform into a container is, in simplified form, the introduction of forming energy into a preform to produce a container. This forming energy is divided into thermal energy (temperature control of the preform) and into mechanical energy (radial and axial expansion of the preform). If one introduces more thermal energy, less mechanical work is necessary for the forming.
The mechanical forming work is composed of the application of a forming fluid to the preform at a defined forming pressure and/or having a defined volume flow and of the force applied by the stretching rod. The force applied by the stretching rod can be determined as stated above by metrological detection of the motor current.
The suitability of the second peak as a guide value is illustrated and explained hereafter. The measurement results compiled in
The time between leaving the heating and measuring the stretching force is described from the aspect of the energy content as the dead time Tf. An energy loss due to convection does occur during this time, however, the energy content only changes insignificantly and can therefore also be neglected in regulation.
The energy Qpreform contained in the preform is decisively determined by the energy Qheat introduced by the temperature control device. The energy Qcool is removed by the surface cooling. If the temperature control device is in a steady operating state, which is achieved after a heating time, surrounding components are heated and emit longwave secondary radiation, which introduces the interference energy Qinterference into the preform. This interference energy also acts on the surface temperature by way of the factor KQϑ. Since the process is set to the steady state of the temperature control device, an absence of this energy results in deviations in the process, since the preform does not have the total energy required for the blowing process. The amount of secondary radiation which is emitted may be estimated by the temperature of the reflector plate ϑPT100. However, since the relationship between energy Qinterference and ϑPT100 is not accurately known, it is described by the nonlinear relationship NLϑQ. The temperature ϑPT100 results with a PT-1 behavior in dependence on the heating power Pheat. Due to the good absorption of the longwave secondary radiation, it has influence on the surface temperature of the preform. This is depicted by means of the factor KQϑ. It describes how the surface temperature also changes due to the interference energy.
The entry temperature of the preform ϑ0 is a further source of interference. This can change depending on the storage of the preform. If it rises, the surface temperature and the energy content thus also increase.
Upon an increase of the heating power, the preform located in the last heating module receives less additional energy because of the short remaining dwell time in the temperature control device. A preform which is just at the beginning of the temperature control device at the point in time of the power increase, in contrast, will already have the full additional energy content. Since every preform can be viewed as an energy accumulator between these two cases, it is obvious that a higher-order transmission behavior without harmonics results.
Since this observation is also carried out for the surface cooling, the main dynamics of the temperature control device are represented by 4 PT-n elements having degree k.
A model of the control loop suitable for the regulator of the temperature control device has been described with respect to
Since the temperatures and forces are dependent on the energy state of the preform, it may additionally be established that the amplifications Kuy in
The base regulator is supposed to regulate the surface temperature and the stretching force as guide values. The heating units and the surface cooling still remain actuators. The use of a decentralized regulator enables a simple implementation.
Since the stretching force has a better correlation to the container quality than the surface temperature, it is preferably used as the manipulated variable for the temperature control device.
It is indirectly possible using the guide variables stretching force and surface temperature to specify the radial temperature profile in the preform. If the energy content (stretching force) is kept constant, the temperature in the interior of the preform has to rise upon reduction of the surface temperature. The internal temperature may thus be specified or maintained indirectly using this mode of action of the regulator.
The control loop described with respect to
For example, the first or second peak and the stretching work can be selected as guide variables. Since all three variables describe the energy content in the preform after the thermal conditioning, in the above explanations, all three possible guide variables are described under the term “stretching force” and/or Fstretch. The stretching work is considered to be preferable as the guide variable, since it takes into consideration the entire process sequence during the forming.
To approximately compensate for the transient state of the temperature control device, i.e., the state before reaching a thermal equilibrium, the deviation from the already known stable temperature is connected as power to the temperature control device by the factor k2. An improvement of the startup behavior can thus also be achieved using the base regulator explained with respect to
Pheat0 and nfan0 represent the powers set in the formula for the actuators and therefore describe the operating point. The signs in the subtraction are exchanged in comparison to the standard controller. This results from the relationships described hereafter. Thus, a higher heating power reduces the stretching force, and a greater airflow cools the surface more strongly.
Δheating power˜−Δstretching force
Δairflow˜−ΔTsurface
Since the essential interfering factors such as ambient temperature or soiling of the temperature control device only change very slowly, low dynamic requirements for the regulator result therefrom. The steady accuracy of the process, and thus a constant container quality in the course of the container production, has priority.
To achieve static accuracy, because of the non-integrating property of the main control loops, I components are provided in both regulators. To additionally achieve better dynamics of the closed-loop, PI regulators are used. The use of a PID regulator is precluded, since the D component can be selected to be very small because of the process noise, so as not to cause the control loop to oscillate.
Several main functions of the regulator are described hereafter, for example, how the guide variables stretching force can be generated.
Upon the start of the stretching, the stretching force, the stretching travel, and the bottle interior pressure are recorded. After ending the process, these measurements series are transmitted by means of an OPC interface to a computer for visualization. This computer processes the data, for example, for the export as a CSV file. The curve can thus be manually analyzed thereafter. However, the real-time analysis of the curve in the form of a guide value is necessary for the described regulator.
To ascertain the stretching force, the effective value of the current of the last millisecond is output in each case by the stretching rod drive. This is computed on a drive-internal FPGA, which also ensures the position regulation by means of a cascade regulation having guide variable generator. At a sampling rate of 1 ms, a large information loss thus does not result, since the entire time range is detected by effective value formation of the last millisecond. Due to the motor-internal effective value formation, noise caused by the converter is already filtered out. However, the generated stretching force curve is still subject to a substantial noise component. Therefore, filtering with a discrete PT1 filter is provided as the first, although optional step.
As explained with respect to
The above image shows the fundamental sequence of an exemplary algorithm. It is firstly identified in which ranges of the data the values to be ascertained and/or the friction are located. The noise is subsequently reduced by means of a discrete PT1 filter. The mean friction force is now determined. This occurs after the acceleration of the stretching rod during the constant travel up to the incidence on the cup of the preform. This force is subtracted as an offset from the curve, since it results from friction losses. The determination of the maxima and of the stretching force integral are now performed. To specify ranges in the stretching force curve for the respective variables, the travel of the stretching rod is used. This is also recorded with the same resolution as the stretching force. This travel is used as the x axis of the stretching force curve to define in which region the friction force occurs, the peaks are located, and the stretching work is to be determined. The regions can overlap in this case.
The algorithm passes through the array in which the travel is recorded and determines the index of the respective travel range limits. The stretching force curve is subsequently traversed value by value. If the value is located between the limits, the maximum deflection (in positive or negative direction) and the stretching work are determined for the peaks.
The ranges can be set manually, for example, or an automated definition of the positions of the peaks can also be performed and ranges can be defined therefrom. Alternative methods for establishing the ranges are also possible.
All determined values of the function are subsequently output. Therefore, either the first or second process peak and the stretching work can then be linked as the guide variable “stretching force” to the input of the regulator. The value which best reflects the quality of the bottle can thus be selected depending on the process. Optionally, only one of these values can also be output and used as a guide variable, for example, only the stretching force.
It is possible to define a stretching force curve as a reference curve. This reference curve is provided, for example, in stored form and is subtracted, for example, from the presently detected curve. To eliminate influence of phase offset by way of the filter, the reference curve is filtered using the same filter constant, for example, before subtraction from the present curve. Furthermore, the resulting curve thus also remains free of noise. If the process and/or the stretching force curve is identical to the reference curve, the value 0 is output as the value for peak 1 and peak 2 and for the stretching work. It is thus not necessary upon use of the reference curve to explicitly specify a target value for the stretching force.
A change of the blowing pressure results in a change of the stretching force. It is therefore advantageous for a stable regulation if the regulator switches off upon change of the blowing pressure and the settings of the temperature control device are frozen. If one presumes that the temperature control of the preform has not changed in the time of the switching off, after the pressure change, the actual value for the stretching force can be assumed as the new target value and the regulation can be switched on again. This also applies for the change of another blowing parameter.
The blowing procedure is typically carried out in such a manner that after the preform 14 is inserted into the blow mold 4, locking of the blowing station 3 occurs and firstly the stretching rod 11 is moved into the preform 14 with simultaneous blowing pressure assistance in such a way that the preform 14 does not shrink radially onto the stretching rod 11 due to the axial stretching. In this phase, a blowing pressure P1 is supplied. After the stretching procedure is completely carried out, the complete expansion of the container bubble into the final contour of the container 12 is performed by application of a higher blowing pressure P2. The maximum internal pressure P2 is maintained until the container 12 has reached a sufficient dimensional stability due to cooling. After reaching this dimensional stability, the blowing pressure supply is switched off and the stretching rod 11 is retracted again from the blow mold 4 and thus out of the blown container 12.
The threaded rod shown therein is connected via a coupling to a motor shaft of the servo motor 49. In the exemplary embodiment illustrated therein, the motor shaft and the threaded rod extend along a common longitudinal axis, so that the threaded rod is arranged as an extension of the motor shaft. In particular a gear-free connection of the motor shaft to the threaded rod is assisted in this way.
The coupling of the servo motor 49 via a threaded rod, a coupling element 46, and the stretching rod carrier 41 having the stretching rod 11 provides a system which is rigid in relation to external loads and nonetheless highly dynamic.
A present stretching force can be inferred in a simple manner from a metrological detection of the motor current of the servo motor 49. A regulation of the temperature control device can be performed, as explained above, in dependence on the stretching force metrologically detected by the detection of the motor current.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 010 970.2 | Nov 2017 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/081937 | 11/20/2018 | WO | 00 |
