Abstract
The disclosure relates to a device and a method for controlling the throughflow of blow-molding fluid during the blow molding of containers. It is the intention to provide a control device and a control method which permit a controlled or defined growth and a defined propagation of the container bubble formed by the expanding preform in the pre-blowing phase of the blow molding process without the specification of a specific setpoint value profile or of a setpoint value curve. The object is achieved by means of a control device and a control method having a proportional valve with a variable throughflow cross section, having an actuator for the operation of the proportional valve, having a means for detecting the position of the actuator, and having sensor means for detecting the valve inlet and valve outlet pressure, wherein a time for the attainment of the yield point for the preform, a container volume and a time period for the attainment of the container volume are predefinable, and, by means of a digital controller, during the pre-blowing phase, from the attainment of the yield point until the run duration, a calculation of control values for the operation of the actuator in order to attain the predefined container volume within the predefined time period is performed in automated cyclic fashion, and the actuator is operated in accordance with the calculated control values, wherein, in each calculation cycle, the calculation of the respectively next control value is performed taking into consideration the container volume attained prior to the respective calculation cycle and calculated on the basis of the previous actuator positions and the previous pressure profile.
Claims
1. A device for controlling the throughflow of blow-molding fluid during the blow molding of containers produced from preforms, comprising: a proportional valve having a modifiable throughflow cross section, an actuator for actuating the proportional valve, a means for detecting the position of the actuator, sensor means for detecting the valve inlet pressure and valve outlet pressure and a digital control device, wherein the control device is set up in a programming manner for the purpose of calculating cyclically, in a manner automated to the run-time, control values for actuating the actuator for the attainment of a predefined container volume within a predefined time period during the pre-blow molding phase from the attainment of a predefinable time point, which corresponds to the attainment of the yield point of the preform, wherein in each calculation cycle the calculation of the respectively next control value is effected with consideration to a current container volume attained up to the respective calculation cycle, the current container volume being calculated by way of the previous actuator positions and the previous pressure course, and wherein the control device calculates the control values without reference to a setpoint value profile or setpoint value curve that specifies a control or regulating parameter over time for actuation of the proportional valve.
2. The device as claimed in claim 1, wherein the attainment of the yield point is predefinable as a pressure value.
3. The device as claimed in claim 1, wherein the attainment of the yield point is predefinable as a time point or as a time interval from the start of the pre-blow molding phase or the introduction of the blow-molding fluid and the control device is set up in a programming manner for the purpose of establishing the attainment of the yield point as a result of the evaluation of the valve outlet pressure as acquiring a pressure peak and of calculating a control sequence for actuating the actuator for attaining the yield point up to the predefined time point or expiry of the predefined time interval.
4. The device as claimed in claim 1, wherein the control device is realized with at least one data communications interface which is compatible with at least one industrial protocol standard.
5. The device as claimed in claim 4, wherein the control device is set up in a programming manner with a server service and a user interface for the input of parameterization data and/or for the display of or for the output of sensor data and/or calculation data acquired via the data communications interface or the standard network interface.
6. The device as claimed in claim 5, wherein the server service is realized with at least one evaluation and/or analysis unit for the generation of evaluation and/or analysis results and for the display of or the output of the evaluation and/or analysis results via the data communications interface or the standard network interface.
7. The device as claimed in claim 1, wherein the control device is realized with at least one standard network interface.
8. The device as claimed in claim 1, wherein all components are realized as a pre-assembled unit.
9. A method for controlling the throughflow of the blow-molding fluid during the blow molding of containers produced from preforms, including a proportional valve having a modifiable throughflow cross section, an actuator for actuating the proportional valve, a means for detecting the position of the actuator and sensor means for detecting the valve inlet pressure and valve outlet pressure, comprising: predefining a time point for the attainment of the yield point for the preform, a container volume and a time period for the attainment of the container volume; calculating by means of a digital control, control values for the actuation of the actuator for the attainment of the predefined container volume within the predefined time period, the calculating being performed cyclically in a manner automated to the run-time during the pre-blow molding phase from the attainment of the yield point; wherein the actuator is actuated corresponding to the calculated control values, wherein in each calculation cycle, the calculation of the respectively next control value is effected with consideration to a current container volume attained up to the respective calculation cycle, the current container volume being calculated by way of the previous actuator positions and the previous pressure course, and wherein the control device calculates the control values without reference to a setpoint value profile or setpoint value curve that specifies a control or regulating parameter over time for actuation of the proportional valve.
10. The method as claimed in claim 9, wherein the calculation of the control values is effected in each calculation cycle with the functional aim of growth in the container that is as uniform as possible up to the attainment of the predefined container volume within the predefined time period.
11. The method as claimed in claim 9, wherein in addition, at least one container interim volume and in each case one interim time period for the attainment of the container interim volume are predefinable, wherein the calculation of the control values in each calculation cycle is effected with consideration to all predefined container interim volumes and interim time periods.
12. The method as claimed in claim 9, wherein the attainment of the yield point is predefinable as a pressure value.
13. The method as claimed in claim 9, wherein the attainment of the yield point is predefinable as a time point or as a time interval from the start of the pre-blow molding phase or the introduction of the blow-molding fluid and by means of the digital control determining of the attainment of the yield point is effected as an evaluation of the valve output pressure as a result of the determining of a pressure peak and a control sequence for the actuation of the actuator for the attainment of the yield point is calculated up to the predefined time point or expiry of the predefined time interval.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Further advantages of the disclosure are shown in more detail below together with the description of preferred exemplary embodiments of the disclosure by way of the figures, in which:
(2) FIG. 1 shows a perspective representation of a device for controlling the throughflow of blow-molding air during the blow molding of containers produced from preforms,
(3) FIG. 2 shows a schematic representation of components of the control device according to FIG. 1,
(4) FIG. 3 shows a diagram for illustrating the growth in volume brought about in the preform during the chronological sequence of a stretch blow molding process,
(5) FIG. 4 shows a schematic block diagram of a digital control with input and output variables.
DETAILED DESCRIPTION
(6) FIG. 1 shows the control device 1 with the proportional valve 2, which is a 2/2-way valve and comprises a compressed air inlet 3 and a compressed air outlet 4. The actuation of the proportional valve 2 is effected by means of the electrically actuatable proportional magnet 5 which serves as actuator for the actuation of the proportional valve 2. The proportional magnet 5 is controlled by means of the digital control device 6 which consists of a programmable single-board computer (SBC) which is mounted in a housing and where all the electronic components (CPU, memory, input and output interfaces, D/A converter, DMA controller, etc.) necessary for operation are combined on one single printed circuit board 7. The printed circuit board 7 with various electronic component parts and the CPU 7a can be seen in part in FIG. 1 through the transparently shown front cover of the housing of the control device 6. The printed circuit board 7 is realized for connection to a fieldbus or industrial Ethernet system by way of a data communications interface 8 which extends out of the housing of the control device 6 as an M12 plug-in connector socket. The data communications interface 8 is designed, if necessary, for example, as a fieldbus interface (for example compatible with Profibus, DeviceNet/ControlNet or CANopen) or as an industrial Ethernet interface (for example compatible with Profinet, EtherNet/IP, Ethernet Powerlink or EtherCat). It can be designed to be compatible with several protocol standards at the same time. Via the data communications interface 8, the control device 6 is integratable into fieldbus or industrial Ethernet systems, installation devices and/or a programmable logic plant control system (PLC) which exist at the location. The control device 6 is additionally also connectable via the data communications interface 8 to an external access point for wireless data communication, for instance an industrial wireless access point. In addition, the printed circuit board 7 is realized with a network interface 9 which extends out of the housing of the control device 6 also as an M12 plug-in connector socket and is a standard Ethernet interface. The control device 6 is connectable, for example, to an office network or the Internet via the network interface 9. The control device 6 is additionally also connectable to an external access point for wireless data communication, for instance a wireless access point, via the network interface 9. The housing of the control device 6 is screw-connected to the housing of the proportional valve 2 with the screws 10 and 10′. The printed circuit board 7 of the control device 6 is connected internally via signal lines 24, 24′ (cannot be seen in FIG. 1 due to the perspective) to sensor means 23, 23′ (cannot be seen in FIG. 1 either also due to the representation), which are integrated into the proportional valve 2, for acquiring the valve inlet and valve outlet pressure and which extend out of the housing of the proportional valve 2 via the connection base 11. The control device 6 receives its power supply via the power connection 12 which extends out of the housing of the control device 6 also as an M12 plug-in connector socket. In addition, the control device 6 is connected to the proportional magnet 5 and the Hall sensor 15 via the combined and correspondingly multicore control/signal line 13 and the control/signal connection 14. The proportional magnet 5 is controllable electrically per current via the control/signal connection 14. The control/signal connection 14 at the same time includes a signal connection, via which the control device 6 receives signals from the Hall sensor 15 which is fitted onto the proportional magnet 5. All components of the control device 1 are realized as a common structural unit which is compact in design. To increase the compactness further, the control device 6 can be integrated into the housing of the proportional valve 2 in a modified design or all components of the control device 1 shown in FIG. 1 can be realized in a common housing. The compact design enables simple integration into blow molding stations of blow molding installations. As, in this connection, each blow molding station is equipped with its own control device according to the disclosure, individual blow molding stations inside the same blow molding installation can be equipped differently and container forms that differ from one another can be produced and/or different material mixtures can be processed in said blow molding stations. As a result of the compact and integrated realization, it is possible in a simple manner to retrofit existing blow molding installations by exchanging the existing valve unit for control devices that are designed as the control device 1, as a result of which older installations are able to be retrofitted with a correspondingly performant control platform and current output and input interfaces. In place of arranging the pressure sensors 23, 23′ directly inside the proportional valve 2, it is also possible to acquire the valve inlet and valve outlet pressure in a modified design as a result of the connection to external sensors which are already present inside the further blow molding installation and are arranged, for instance, inside a compressed air supply line to the valve inlet 3 and a compressed air connection line to the blow mold after the valve outlet 4.
(7) FIG. 2 shows a simple schematic representation of components of the control device 1. A valve tappet 16, which ends in a cone-shaped manner, is arranged in the proportional valve 2 so as to be linearly movable. The valve tappet 16 is movable downward in opposition to the force of the spring 18 by the pin-shaped armature 17 which serves as actuator, the pressure medium inlet 3 being connected to the pressure medium outlet 4. The throughflow cross section, which is produced in relation to the boundary surface 2a of the valve body of the proportional valve 2 at the cone-shaped end of the valve tappet 16 in the open position, is continually modifiable and dependent on the linear position of the armature 17. Said armature is also mounted so as to be linearly movable inside a coil winding 19, which surrounds it, in the housing of the proportional magnet 5. The linear position (status) of the armature 17 inside the coil winding 19 is dependent on the current, which is controlled by the control device 6 via the combined control/signal line 13 and is applied in each case to the coil winding 19. The armature 17 is continually movable downward as a result of increasing the current, pressing the valve tappet 16 downward in opposition to the force of the spring 18. In the open position, the pressure in the valve tappet 16 is equalized as a result of the central compensation channel 20. The Hall sensor 15, by means of which the respective position of the armature 17 is acquirable, is fitted onto the proportional magnet 5. For this purpose, the armature 17 is realized at its upper end with the permanent magnet means 21 and the Hall sensor 15 with the sensor means 22 which detects the relative distance between the permanent magnet means 21 and itself in a contactless manner. A signal representing the respective position of the armature 17 is transmitted by the Hall sensor 15 via the combined control/signal line 13 to the control device 6. Pressure sensors 23 and 23′ are additionally arranged in the proportional valve 2 as sensor means for acquiring the valve inlet pressure (by the pressure sensor 23) and the valve outlet pressure (pressure sensor 23′) and are connected to the control device 6 via the signal lines 24 and 24′.
(8) FIG. 3 shows the container expansion of a preform during an exemplary chronological sequence of a stretch blow molding process (production of a container) as a volume/time diagram, the abscissa axis representing the course of time and the ordinate axis representing the growth in volume. The stretching operation, with the horizontal bar extended linearly in the blow molding mold, starts initially at time t.sub.1 (at which the preform has the initial volume Vol. The preform is stretched in the longitudinal direction. At time t.sub.2 (at which the preform simply has a small growth in volume compared to the initial volume V.sub.0, brought about solely by longitudinal stretching) the introduction of the blow-molding air begins via the proportional valve 2. This can be triggered, for example, by a corresponding control signal from a higher-level plant control system (PLC) via the data communications interface 8 of the control device 1. At time t the yield point of the preform is attained. At said moment, the volume of the container is ΔV0 which represents the initial volume for the control process during the pre-blow molding phase. The growth in the container, brought about up to now as a result of the introduction of the blow-molding air since the time t.sub.2, is calculatable by way of the previous pressure course acquired by way of the pressure sensors 23 and 23′, the actuator positions of the proportional magnet 5 acquired with the Hall sensor 15 (and consequently the course of the variable of the opening cross section of the proportional valve 2). At time t4, the end of the stretching operation is attained and the horizontal bar is fully extended. At this point in time, the container has the already strongly increased volume ΔV1 as a result of the expansion of the container once the yield point has been exceeded as a result of further introducing the blow-molding air. The growth in the container brought about up to now as a result of the introduction of the blow-molding air is calculatable by way of the previous data acquired by the sensors. At time t.sub.5, the end of the pre-blow molding phase of the stretch blow molding process is attained, at which the expansion of the container balloon has attained the final volume ΔV2 which is defined for the pre-blow molding phase. Once time t.sub.5 has been attained, the pre-blow molding phase ends and the final blow molding phase begins (also designated as the main blow molding phase), in which the container, under a sharply increased pressure level, is brought into its desired final form and the definitive final volume V.sub.max is brought about. The time period A consequently characterizes the extent of the pre-blow molding phase during the stretch blow molding process which, in practice, is approximately 200 ms. The graph B symbolizes the change in the volume of the preform and consequently at the same time the growth of the container balloon up to the time t.sub.5. The curve C symbolizes the growth in the volume brought about purely by the longitudinal stretching of the preform with the extending of the horizontal bar. The diagram in FIG. 3 consequently shows that the essential growth in the volume of the container balloon is brought about as a result of the introduction of the blow-molding air in the pre-blow molding phase. The slight changes in the volume brought about both by only the extending of the horizontal bar according to curve C and those in the final blow molding phase (corresponding to the difference in volume V.sub.max−ΔV2) can consequently be ignored in practice for the control procedure. As an alternative to this, said changes in the volume can be taken into consideration in the control model by corresponding absolute or percentage blanket variables or, insofar as they are easily assessable and determinable, can be predefinable as additional parameters.
(9) FIG. 4 shows a simplified schematic block diagram of embodiments of the digital control 25 according to the disclosure with input and output variables. The specification of the desired container final volume V.sub.max at time t.sub.max, which corresponds to the end of the control operation, serves as a global default variable 26 for the control 25. In a simple design of the control, this corresponds to the default of the volume V.sub.max for the time t.sub.5 (the end of the control time period for the pre-blow molding phase) corresponding to the representation according to FIG. 3. In alternative designs, the volume difference V.sub.max−ΔV2 brought about in the final blow molding phase and/or the change in volume brought about only by the extending of the horizontal bar according to curve C in FIG. 3 can be taken into consideration additionally in the control model for the control up to time t.sub.5 by deducting corresponding absolute or percentage blanket variables. Insofar as can be estimated or determined through preliminary tests, the volume ΔV2—where applicable additionally minus the change in volume brought about only by the extending of the horizontal bar—can also be predefined directly as default variable 26 at time t.sub.5 corresponding to the representation according to FIG. 3. Insofar as the proportional valve is also utilized for the introduction of the blow-molding air in the final blow molding phase, the change in volume from ΔV2 to V.sub.max brought about in the final blow molding phase can additionally be determined in a device pass by way of the sensor values and, automated in the control model, can be taken into consideration for the volume ΔV2 to be attained up to the defined time t.sub.5. By way of the default variable 26, in an automated cyclical manner the control 25 calculates the manipulated variable 27 as control value x.sub.CMD.sub.k which is, as the result of each individual calculation cycle, in each case a discrete control value (i.e. a certain current) for actuating the proportional magnet 5 at the next control time t.sub.k, wherein k=0..n is valid. When calculating the control value x.sub.CMD.sub.k for the control time t.sub.k, according to the technical control model of the control 25, the container volume V.sub.k-1 attained already in each case up to the current calculation cycle is taken into consideration, which container volume is calculated by way of the previous actuator positions {tilde over (x)}.sub.k-1 acquired with the Hall sensor 15 up to time k−1 and the sequences of the valve inlet pressure {tilde over (P)}1.sub.k-1, and of the valve outlet pressure {tilde over (P)}2.sub.k-1 acquired with the pressure sensors 23 and 23′. The values acquired by the Hall sensor 15 and the pressure sensors 23 and 23′ are written, for this purpose, for example, continuously in DMA registers inside the memory of the control device 6 and are buffered continuously by the control device 6 at least during the run-time during the time period A.
(10) To calculate the manipulated variable 27 as control value x.sub.CMD.sub.k, the digital control 25 is set up in a programming manner with instructions with which is imaged a correspondingly suitable, technical control model, which is derived from the general laws of fluid mechanics with the known relationships between the variables volume flow and mass flow {dot over (m)}.
(11)
(12)
for fluids calculatable by
{dot over (m)}=![custom character]()
.Math.{dot over (V)}=![custom character]()
.Math.c.Math.A.
with ![custom character]()
Density of the medium
c Mean flow speead A Cross sectional area {dot over (V)} Volume flow.
(13) The pressure force Fp generated by the valve outlet pressure P.sub.2 inside the preform with the inner wall surface A.sub.o acts with
F.sub.p=p.sub.2.Math.A.sub.o
on the container inside surface. The change in volume {dot over (V)} resulting from this, for example when using a blow molding gas on the basis of the general gas law of ideal gases, is calculated by
(14)
wherein R is the general gas constant and ϑ is the gas temperature deemed to be constant in time. On the basis of the technical control model derived from the known principles stated above, the calculation of the respectively next control value 27 for the actuation of the actuator to attain the predefined container volume within the time period predefined for this purpose is effected in each case as a control value sequence which is calculated successively in an automated manner, in each calculation cycle the container volume already attained previously being taken into consideration. To this end, the respectively next control value x.sub.CMD.sub.k is recalculated to the next control time point t.sub.k in each calculation cycle proceeding from the predefined container final volume V.sub.max at time t.sub.max, the container volume V.sub.k-1 already attained up to the respective calculation cycle and calculated by way of the acquired sensor data being taken into consideration. The control value x.sub.CMD.sub.k calculated for the respective control time point t.sub.k consequently corresponds to the throughflow cross section of the proportional valve 2 necessary in each case to attain the residual volume V.sub.max-V.sub.k-1 remaining at said time in the remaining time period t.sub.max−t.sub.k under the given pressure conditions. In this connection, necessary boundary parameters, such as the specific density of the blow-molding fluid used in each case, the isentropic exponent of a blow molding gas used, the valve-specific, pressure-dependent flow speed characteristic value of the proportional valve used and the specific cross sectional area of the proportional valve used which is modifiable in dependence on the position of the actuator, are taken into consideration. The slight changes in volume brought about only by the extending of the horizontal bar and in the final blow molding phase, as shown in conjunction with FIG. 3, are able to be ignored for the control in practice. In addition, variables and parameters which are only modifiable in practice to a small extent, such as, for example, the temperature of the blow-molding fluid, can be taken into consideration simplified as constants, since possible considerable qualitative changes in such boundary parameters are taken into consideration indirectly as a result of the self-correction which is continuous and inherent to the method according to the disclosure (as a result of the consideration of the container volume attained in each calculation cycle).
(15) In a simple realization of the control 25, only the default variable 26, corresponding to the desired container final volume V.sub.max at time t.sub.max is predefined. Recalculation of the manipulated variable 27 as control value x.sub.CMD.sub.k at control time t.sub.k is effected, in this connection, in each case, by way of a corresponding qualitative quality default in the technical model, with which the calculation of the control value x.sub.CMD.sub.k in each calculation cycle is calculated with the aim of a growth in the container that is as uniform as possible overall up to the attainment of the predefined container final volume V.sub.max at time t.sub.max.
(16) The default variables 28 and 28′, which correspond to the attainment of the container interim volume ΔV0 at time t.sub.3 (default variable 28) and the attainment of the container interim volume ΔV1 at time t.sub.4 (default variable 28′) according to the diagram in FIG. 3, are additionally predefined in an alternative realization of the control 25. The recalculation of the manipulated variable 27 as control value x.sub.CDM.sub.k at time t.sub.k is effected, in this connection, in each case, by an interpolation where the interim volumes 28 and 28′, predefined in time, as support points form the basis for the calculation of the container final volume V.sub.max at time t.sub.max.
LIST OF REFERENCES
(17) 1 Control device 2 Proportional valve 2a Boundary surface 3 Compressed air inlet 4 Compressed air outlet 5 Proportional magnet 7 Control device 7 Printed circuit board 7a CPU 8 Data communications interface 9 Network interface 10,10′ Screw 11 Connection base 12 Power connection 13 Control/signal line 14 Control/signal connection 16 Hall sensor 16 Valve tappet 17 Armature 18 Spring 19 Coil winding 20 Compensation channel 21 Permanent magnet means 22 Sensor means 23,23′ Pressure sensor 24,24′ Signal line 25 Control 26,28,28′ Default variable
