Blowing Valve Device of a Blow-Moulding Device

Abstract

In a method for monitoring a blow-moulding device for producing a hollow body, the blow-moulding device includes at least one process valve unit for feeding a process fluid into a preform of the hollow body under process pressure, wherein the process valve unit includes at least one electrically operated valve. During a blow-moulding process, at least one value of an electric current of the electrically operated valve is detected. The method and the device allow early recognition of an ageing process of the process valves, in particular their pilot valves, and/or the recognition of the status of valve properties.

Claims

1. A method for monitoring a blow-moulding device for producing a hollow body, wherein the blow-moulding device has at least one process valve unit for feeding a process fluid into a preform of the hollow body under process pressure, and wherein the process valve unit has at least one electrically operated valve, wherein at least one value of an electric current of the electrically operated valve is detected during a blowing process.

2. The method as claimed in claim 1, wherein a characteristic of the electric current is detected during the blowing process.

3. The method as claimed in claim 1, wherein the current of the valve is measured in successive time intervals.

4. The method as claimed in claim 1, wherein a change in the current is measured.

5. The method as claimed in claim 1, wherein the time of measurement of the current is recorded together with a current value measured at this time.

6. The method as claimed in claim 1, wherein the current is measured at predetermined times.

7. The method as claimed in claim 1, wherein distinctive points of a characteristic of the electric current are detected.

8. The method as claimed in claim 1, wherein the process valve unit has a main valve and at least one pilot valve, wherein the at least one pilot valve controls the main valve, and wherein the characteristic of the electric current of at least one of the at least one pilot valve is detected.

9. The method as claimed in claim 8, wherein the main valve is a pneumatic valve which is operated by the process fluid and is controlled by a control pressure controlled by the at least one pilot valve.

10. The method as claimed in claim 1, wherein a pressure in the region of the interior of the preform is measured during the blowing process, and the time of the pressure measurement is recorded.

11. The method as claimed in claim 1, wherein a pressure in the region of a tank of the process fluid is measured during the blowing process, and the time of the pressure measurement is recorded.

12. The method as claimed in claim 10, wherein the measured pressure values are evaluated taking into account the measured values of the current.

13. A blowing valve assembly of a blow-moulding device for producing a hollow body, wherein the blowing valve assembly has at least one process valve unit for feeding a process fluid into a preform of the hollow body under process pressure, and wherein the process valve unit has at least one electrically operated valve, characterized in that a means for detecting an electric current value of the electrically operated valve during a blowing process is present.

14. The blowing valve assembly as claimed in claim 13, wherein the blowing valve assembly has at least one pressure sensor for measuring the pressure in the region of the interior of the preform.

15. A computer program product for controlling a blowing valve assembly as claimed in claim 13.

16. A computer program product for carrying out the method as claimed in claim 1.

17. A method for monitoring a blow-moulding unit of a plurality of blow-moulding stations for producing a hollow body, wherein each blow-moulding station has at least one process valve unit for feeding a process fluid into a preform of the hollow body under process pressure, and wherein the process valve unit has at least one valve, and wherein each blow-moulding station is associated with at least one sensor, wherein the method has at least the following steps: detecting at least one first sensor value in each blow-moulding station, transmitting a first data value, which is associated with the at least one detected first sensor value of each blow-moulding station, to a data processing unit, and determining a first statistical average of the transmitted first data values in the data processing unit.

18. The method as claimed in claim 17, wherein the method further comprises the step of: defining the first statistical average as a setpoint.

19. The method as claimed in claim 17, wherein the transmitted first data value is the detected first sensor value.

20. The method as claimed in claim 17, wherein the blow-moulding unit has a central controller, and wherein the first statistical average is stored in the central controller.

21. The method as claimed in claim 20, wherein the data processing unit is part of the central controller.

22. The method as claimed in claim 17, wherein the data processing unit is formed by a plurality of decentralized data processing units and these are part of decentralized control units.

23. The method as claimed in claim 22, wherein the first statistical average is stored in the decentralized control units.

24. The method as claimed in claim 17, wherein the method has at least the following further steps: determining whether individual transmitted first sensor values have a predetermined deviation from the first statistical average, and, if an individual first sensor value has such a deviation, identification of the blow-moulding station associated with the sensor value.

25. The method as claimed in claim 17, wherein the method has at least the following further steps: detecting at least one second sensor value in each blow-moulding station in a second blowing process, transmitting the at least one detected second sensor value of each blow-moulding station to the data processing unit, determining a second statistical average of the transmitted second sensor values in the data processing unit.

26. The method as claimed in claim 25, wherein the method has at least the following further steps: determining whether individual transmitted second sensor values have a predetermined deviation from the first statistical average, or determining whether individual transmitted second sensor values have a predetermined deviation from the second statistical average, and, if an individual second sensor value has such a deviation, identification of the blow-moulding station associated with the sensor value.

27. The method as claimed in claim 26, wherein, after the second statistical average has been determined, individual second sensor values which deviate from the second statistical average or alternatively from the first statistical average are first of all determined, and then a new second statistical average is formed with the remaining second sensor values, and, in the event of a deviation of this new second statistical average from the first statistical average, this new second statistical average is defined as a setpoint for the blow-moulding stations.

28. The method as claimed in claim 25, wherein the transmitted second data value is the detected second sensor value.

29. The method as claimed in claim 28, wherein the transmitted first data value is the difference between the detected first sensor value and the stored setpoint.

30. The method as claimed in claim 23, wherein the transmitted second data value is the detected second sensor value and wherein the transmitted second data value is the difference between the detected second sensor value and the stored setpoint.

31. The method as claimed claim 17, wherein the at least one sensor measures at least one of the following parameters: pressure in the hollow body to be produced, pressure in a fluid tank, current of a pilot valve, position or movement of the piston of a process valve.

32. A control system of a blow-moulding unit having a plurality of blow-moulding stations, wherein the blow-moulding unit has a central control unit and wherein each blow-moulding station is associated with at least one sensor, wherein the blow-moulding unit has at least one data processing unit, wherein the at least one data processing unit has evaluation means for forming a statistical average from data which are based on the sensor values obtained from the sensors, and wherein the blow-moulding unit has storage means for storing the determined statistical average as a setpoint.

33. The control system as claimed in claim 32, wherein there is a machine controller which, together with the at least one data processing unit, forms a central control unit.

34. A computer program product for carrying out the method as claimed in claim 17.

35. The method as claimed in claim 11, wherein the measured pressure values are evaluated taking into account the measured values of the current.

36. The method as claimed in claim 3, wherein the current of the valve is measured in successive time intervals of from 0.1 ms to 1 ms.

37. The method of claim 25, wherein, in case of deviation of the second statistical average from the first statistical average, the method comprises the step of defining the second statistical average as a setpoint.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] A preferred embodiment of the invention is described below by means of the drawings, which serve merely for explanation and should not be interpreted as restrictive. In the drawings:

[0075] FIG. 1 shows an exemplary example of a process valve having a pneumatic main valve and a pilot valve in the closed state of the main valve in a schematic illustration;

[0076] FIG. 2 shows the process valve according to FIG. 1 in the open state of the main valve;

[0077] FIG. 3 shows a unit, as used in a blow-moulding machine, with valve block and actuator box;

[0078] FIG. 4 shows a longitudinal section through a pilot valve;

[0079] FIGS. 5a to 5c show a portion of a graphical representation of measured values during a blowing process;

[0080] FIGS. 6a and 6b show a graphical representation of measured values during part of a blowing process;

[0081] FIGS. 7a to 7c show a graphical representation of measured values during part of three different blowing processes;

[0082] FIGS. 8a to 8c show a graphical representation of measured values during a complete blowing process;

[0083] FIG. 9 shows a schematic illustration of part of a blow-moulding unit according to the invention in a first embodiment;

[0084] FIG. 10 shows a schematic illustration of part of a blow-moulding unit according to the invention in a second embodiment;

[0085] FIG. 11 shows a schematic illustration of part of a blow-moulding unit according to the invention in a third embodiment;

[0086] FIG. 12a shows a longitudinal section through a process valve according to the invention in a first embodiment in the open state;

[0087] FIG. 12b shows a longitudinal section through a process valve according to the invention in a first embodiment in the closed state;

[0088] FIG. 13a shows a longitudinal section through a process valve according to the invention in a second embodiment in the open state;

[0089] FIG. 13b shows a longitudinal section through a process valve according to the invention in a second embodiment in the closed state;

[0090] FIG. 14a shows a longitudinal section through a process valve according to the invention in a third embodiment in the open state;

[0091] FIG. 14b shows a longitudinal section through a process valve according to the invention in a third embodiment in the closed state;

[0092] FIG. 15 shows a schematic illustration of a blow-moulding unit according to the invention;

[0093] FIG. 16 shows a schematic illustration of part of a blow-moulding unit according to FIG. 15.

DESCRIPTION OF THE INVENTION

[0094] FIGS. 1 and 2 show a typical process valve. However, the invention is not restricted to this type of valve, but can also be used in other valves which use an electronically actuated auxiliary or main valve. In particular, the invention can be used for pilot-controlled process valves which have a pneumatic main valve and at least one pilot valve, in particular at least one solenoid valve. In other embodiments, the main valve is directly controlled.

[0095] The process valve shown schematically here has a pneumatic main valve 1 and a pilot valve 2 in the form of an electromagnetic valve, i.e. a solenoid valve.

[0096] The main valve 1 has a valve housing 10 with a control chamber 13, a process pressure inlet channel 15 and a process pressure outlet channel 16. A switching piston 12 is held slidably in the valve housing 10, being sealed with respect to the valve housing 10 by means of high-pressure seals 14. It is usually of rotationally symmetrical design. In the closed state of the main valve 1, as shown in FIG. 1, the switching piston 12 rests on a valve seat 17 and thereby closes a connection between the process pressure inlet channel 15 and the process pressure outlet channel 16. In the open state according to FIG. 2, this connection is open. In the open state of the main valve 1, the process gas can thus pass from the process gas source (not shown here) into the preform or blank (likewise not shown here) and inflate the latter to the desired final shape of the hollow body, in particular the PP or PET bottle.

[0097] The control chamber 13 of the main valve 1 is connected to a control air source 3 via a control air line 30. The pressurized control air acts in the control chamber 13 on a control surface 121 of a piston head 120 of the switching piston 12. The resulting control pressure generates a force on the switching piston 12 which is greater than the force of the process gas pressure acting via the process pressure inlet channel 15 on the opposite side of the switching piston 12. As a result, the switching piston 12 is held in its closed position.

[0098] If the control pressure is lowered, the process gas presses the switching piston 12 upward, and the main valve 1 is opened.

[0099] The switching of the control pressure is accomplished by means of said pilot valve 2. As shown here, this is connected to the control chamber 13 via the control air line 30. It is furthermore connected via a control air inlet channel 300 to the control air source 3 and via a control air outlet channel 301 to the surroundings or to a control air recovery unit. For the rapid venting of the control chamber, a control air switching chamber 31 can be provided, as shown here, which ensures minimum switching times by means of an additional venting channel 32. A process valve of this kind is described in detail in WO 2015/010216 A1, for example. It serves merely as an example for understanding possible functions of the main valve and of the at least one pilot valve.

[0100] In other embodiments, the control pressure actuated by means of the pilot valve 2 serves to open the main valve 1. In further embodiments, a first pilot valve having a first control pressure is used for opening and a second pilot valve having a second control pressure is used for closing the main valve 1. In further embodiments, control is accomplished by means of a 5/2-way pilot valve, in which either air admission takes place at the top and venting takes place at the bottom or vice versa.

[0101] FIG. 4 shows an example of a pilot valve 2 of the kind typically used in such process valves. This is just an example. Other types of pilot valve can also be used. The pilot valve 2 shown here has a housing in which an electromagnet coil 22 and a plunger 23 passing through the coil 22 are arranged. A spring 24 is arranged at one end of the plunger 23. The plunger 23 can be moved against the force of this spring 24 by means of the electromagnet. As a result of the movement of the plunger 24, a connection to the pressure supply is released. The connection to the pressure supply is provided with the reference numeral 25. In the no-load state, a connection to the control chamber of the main valve is created. The corresponding outlet is provided with the reference numeral 26. In addition, an outlet 27 to a muffler is preferably provided.

[0102] More than one process valve is usually necessary for producing a hollow body from a blank. A preblowing valve and at least one main blowing valve are usually present. Furthermore, at least one recovery valve and/or one vent valve are preferably present. As shown in FIG. 3, these valves are preferably arranged in a common valve block 4, which has at least one, preferably a plurality of, process pressure inlet and outlet channels for connection to the process gas source. By means of the process gas, the preform is inflated into the desired shape of the final hollow body. For this purpose, the valve block 4 has a blowing nozzle 40, on which the blank or preform 7 to be inflated is secured.

[0103] The valve block 4 furthermore has at least one pressure sensor 41, which measures the pressure of the process gas in the region of the nozzle 40. This corresponds, with minimal and thus negligible delay, to the pressure which prevails in the interior of the blank 7.

[0104] The individual pilot valves 2 of the individual process valves are connected via respective electronic control connections 20 to an actuator box 5, which in turn is connected directly or via further control subunits to the machine controller 6 or which itself is already part of the machine controller 6. The actuator box 5 and the machine controller 6 are referred to below as a control unit.

[0105] The control unit controls the pilot valves 2 and thus the process valves. It also receives the measurement data of the at least one pressure sensor 41.

[0106] According to the invention, the control unit, preferably the actuator box 5, measures the current of at least one, preferably all, pilot valves 2. The current can be measured at a predetermined time or the time at which a specific current value is reached can be determined. The time of the counterpeak of the current at the end of counterinduction is preferably determined when the pilot valve 2 is switched on and off. In the most preferred embodiment, the current measurement takes place at predetermined time intervals, for example every 0.1 ms.

[0107] Measurement is carried out using customary and known means. Measurement is carried out, for example, by installing a measuring resistor in the circuit to the pilot valve. Across this resistor there is a voltage drop, the magnitude of which is subsequently amplified. The magnitude of this voltage drop is proportional to the current flowing.

[0108] The measured current values and/or the measured times of the occurrence of distinctive current values, for example the reversal point in the counterinduction, are compared with previous measured values and/or with predefined values. On the basis of a deviation of these values compared with one another, conclusions are drawn about the ageing process and/or the state of the pilot valve 2. In this way, it is also possible to determine and observe valve properties, such as delays, accuracy and wear. If the deviation is such that, based on empirical values, imminent failure of the pilot valve 2 is probable, then an error message is generated by the control unit and there is a request for the corresponding pilot valve 2 to be changed. In one embodiment, compensation of the deviation is initiated by means of the controller before the request to change the pilot valve. In this case, the signal for valve switching is requested earlier, preferably in accordance with the magnitude of a detected delay in the current characteristic of the pilot valve. In this way, the service life of the pilot valve can be extended. In other embodiments, the state of the pilot valve 2 is indicated visually by the control unit, allowing the user to observe the continuous ageing process of the pilot valve 2 at an early stage.

[0109] FIGS. 5a to 5c show a portion of a typical process sequence at the time of the start of the process with the preblowing. In curve a), FIG. 5a shows the pressure characteristic P.sub.T in the process pressure source, i.e. the tank pressure as a function of time t. Curve b) shows the pressure P.sub.R which was measured by the pressure sensor 41 in the valve block 4 in the region of the interior of the blank as a function of time. FIG. 5b shows the voltage signal U of the pilot valve 2 measured or applied in this process as a function of the same time t as in FIG. 5a. Curve c) shows the voltage signal U of the pilot valve 2 for the preblowing valve and curve d) shows the voltage signal U of the pilot valve 2 for the main blowing valve. In FIG. 5c, the measured current value I which matches it is reproduced as a function of the same time t of the pilot valves. Curve e) shows the current value I of the pilot valve of the preblowing valve and curve f) shows the current value I of the pilot valve of the main blowing valve.

[0110] As can be clearly seen in curves e) and f), the current characteristic I has a downward-pointing peak when the pilot valve is switched on and an upward-pointing peak when it is switched off. This designates the time of the end of counterinduction H.sub.1, H.sub.2. According to the invention, these times t.sub.H1 and t.sub.H2 and/or the magnitude of the peak are detected and compared with a predetermined value by the control unit. These data are preferably stored in the control unit or in a cloud for later reuse.

[0111] Viewing FIGS. 5a to 5c together furthermore reveals the pressure behavior P.sub.R in the region of the interior of the blank in the course of the process. At time A, the voltage U is applied to the pilot valve of the preblowing valve. However, owing to the first counterinduction H.sub.1, the pilot valve responds with a time delay. The internal pressure P.sub.R in the blank rises with a time delay. At time t.sub.B in curve b) of FIG. 5a, the pressure rise is for the first time sufficiently distinct from the measuring noise. The pressure at point C at time t.sub.C in curve b) according to FIG. 5a is approximately 50% of the first pressure rise (i.e. the pressure rise up to the first reduction of the pressure). The difference between times t.sub.C and t.sub.B indicates that the main valve has opened completely at the correct time.

[0112] The pressure P.sub.T in the tank has already dropped earlier, as shown by region F in curve a). At point D at time t.sub.D of curve b), the blank also begins to expand radially. According to experience, this has the result that the pressure P.sub.R in the interior of the blank drops for a short time, as can be clearly seen in curve b). This is followed by a period of time with an approximately constant pressure P.sub.R in the interior of the blank, with a subsequent rise. This can also be seen clearly in curve b).

[0113] The pilot valve of the preblowing valve is likewise switched off with a time delay with the second counterinduction H.sub.2. The switching on of the main blowing valve (curve d) in FIG. 5b) and curve f) in FIG. 5c), which took place shortly thereafter, again takes place with a time delay. Point E in curve b) shows the pressure rise after switching on the main blowing valve. As can be seen in curve b), the massive rise in the internal pressure P.sub.R in the partially inflated blank follows with a time delay.

[0114] In a variant of the method according to the invention, not only are the times and/or the level of at least one of the counterinductions H.sub.1 and H.sub.2 determined, but the pressures are also monitored. The pressure P.sub.T in the process tank and/or the pressure P.sub.R in the outlet channel of the valve block and thus in the interior of the blank can be measured. The values are preferably measured in very short time intervals as a function of time. Alternatively, these pressures P.sub.T and/or P.sub.R can also be measured at predetermined times. These values are compared with values measured at earlier times and/or with predetermined values. Deviations are detected. If predetermined limit values and/or predetermined maximum deviations are exceeded and/or undershot, this is indicated.

[0115] Depending on the embodiment, these values are used to monitor ageing processes in the machine, in particular in the valve block and/or in the process gas tank. As an alternative or in addition, they can also be used for controlling the blow-moulding machine, for example in that the process pressure is increased or lowered, the times of actuation of the process valves are changed and/or the switching cycles of the process valves are altered.

[0116] The data of the pressure measurement P.sub.R in the interior of the blank are preferably combined with the data of the current measurement I of the pilot valves and evaluated together. This results, for example, in further knowledge on the state of the process valve, in particular on the ageing process of the remaining parts of the pilot valve and/or of the main valve.

[0117] In another embodiment, the pressure measurement P.sub.R in the interior of the blank and/or the measurement of the tank pressure P.sub.T is carried out as a function of time without the measurement of the flow I of the pilot valves, and information is obtained from this on the status of the valve properties and/or on the ageing process of the process valves. This embodiment is likewise claimed here as a separate invention.

[0118] The method according to the invention and the device according to the invention allow early detection of an ageing process of the process valves, in particular their pilot valves, and/or the detection of the status of valve properties.

[0119] The further invention is described below with reference to preferred embodiments and with reference to the figures:

[0120] FIG. 15 schematically shows a blow-moulding unit BE according to the invention having a plurality of blow-moulding stations BS1 to BS6. There may be two, three or more blow-moulding stations. In this example, there are six stations. Each blow-moulding station BS1 to BS6 is connected to a decentralized data processing unit 8, although it is also possible for one decentralized data processing unit 8 to be assigned to a plurality of blow-moulding stations. The decentralized data processing units 8 are connected to a central control unit 6. The control unit 6 contains the machine controller of the blow-moulding unit. Depending on the embodiment, it has a separate module, which is designed to be functionally and, depending on the embodiment, also locally separate from the machine controller, but communicates with it. This module is in communicating connection with the decentralized data processing units.

[0121] FIG. 16 shows parts of the blow-moulding unit BE according to FIG. 15. Only three blow-moulding stations are shown. Each blow-moulding station is illustrated in simplified form with a single main valve 1. In real blow-moulding stations, however, a plurality of valves, i.e. the abovementioned preblowing valve, one or more blowing valves and one or more recovery valves and vent valves, are present at each station. The main valve 1 is controlled by means of a pilot valve 2. A typical exemplary embodiment of such a valve 1 with a pilot valve 2 is described below with reference to FIGS. 1 and 2.

[0122] The pilot valve 2 is preferably controlled by the central control unit 6 or by a decentralized control unit, hereinafter referred to as actuator box 5. The valve 1 and also other regions of the corresponding blow-moulding station are connected to a sensor unit 9, which contains sensors or receives measured values from sensors.

[0123] The sensors can measure a pressure in the preform during the blowing process. The pressure can be measured over the entire process time, i.e. during preblowing, blowing, recovery and venting. It is also possible for the pressure to be measured only during a period of time of the process or at predefined times. The sensors can also measure other parameters, such as the current of the pilot valve or movements of individual parts of the blow-moulding station, in particular of the valve 1.

[0124] Depending on the embodiment, the sensor unit 9 and the decentralized control unit 5 are designed as a common module or as separate components. In place of a decentralized control unit, i.e. an actuator box 5 provided with control functions, the module can also be designed as a data processing unit without control functions. Control is then performed by the central control unit 6. Control can also be partially transferred from the central control unit 6 to the actuator box 5 or to some other decentralized control unit. It is preferable for simple control commands to be executed by means of the actuator box 5 in order to relieve the load on the central control unit 6.

[0125] FIGS. 1 and 2 show a typical process valve. It has a pneumatic main valve 1 and at least one pilot valve, in particular an electromagnetic valve, preferably a solenoid valve. However, the invention is not restricted to this type of valve. The main valve can also be controlled directly, for example.

[0126] The main valve 1 has a valve housing 10 with a control chamber 13, a process pressure inlet channel 15 and a process pressure outlet channel 16. A switching piston 12 is held slidably in the valve housing 10, being sealed with respect to the valve housing 10 by means of high-pressure seals 14. It is usually of rotationally symmetrical design. In the closed state of the main valve 1, as shown in FIG. 15, the switching piston 12 rests on a valve seat 17 and thereby closes a connection between the process pressure inlet channel 15 and the process pressure outlet channel 16. In the open state according to FIG. 16, this connection is open. In the open state of the main valve 1, the process gas can thus pass from the process gas source (not shown here) into the preform or blank (likewise not shown here) and inflate the latter to the desired final shape of the hollow body, in particular the PP or PET bottle.

[0127] The control chamber 13 of the main valve 1 is connected to a control air source 3 via a control air line 30. The pressurized control air acts in the control chamber 13 on a control surface 121 of a piston head 120 of the switching piston 12. The resulting control pressure generates a force on the switching piston 12 which is greater than the force of the process gas pressure acting via the process pressure inlet channel 15 on the opposite side of the switching piston 12. As a result, the switching piston 12 is held in its closed position.

[0128] If the control pressure is lowered, the process gas presses the switching piston 12 upward, and the main valve 1 is opened.

[0129] The switching of the control pressure is accomplished by means of said pilot valve 2. As shown here, this is connected to the control chamber 13 via the control air line 30. It is furthermore connected via a control air inlet channel 300 to the control air source 3 and via a control air outlet channel 301 to the surroundings or to a control air recovery unit. For the rapid venting of the control chamber, a control air switching chamber 31 can be provided, as shown here, which ensures minimum switching times by means of an additional venting channel 32. A process valve of this kind is described in detail in WO 2015/010216 A1, for example. It serves merely as an example for understanding possible functions of the main valve and of the at least one pilot valve.

[0130] In other embodiments, the control pressure actuated by means of the pilot valve 2 serves to open the main valve 1. In further embodiments, a first pilot valve having a first control pressure is used for opening and a second pilot valve having a second control pressure is used for closing the main valve 1. In further embodiments, control is accomplished by means of a 5/2-way pilot valve, in which either air admission takes place at the top and venting takes place at the bottom or vice versa.

[0131] FIG. 4 shows an example of a pilot valve 2 of the kind typically used in such process valves. This is just an example. Other types of pilot valve can also be used. The pilot valve 2 shown here has a housing in which an electromagnet coil 22 and a plunger 23 passing through the coil 22 are arranged. A spring 24 is arranged at one end of the plunger 23. The plunger 23 can be moved against the force of this spring 24 by means of the electromagnet. As a result of the movement of the plunger 24, a connection to the pressure supply is released. The connection to the pressure supply is provided with the reference numeral 25. In the no-load state, a connection to the control chamber of the main valve is created. The corresponding outlet is provided with the reference numeral 26. In addition, an outlet 27 to a muffler is preferably provided.

[0132] More than one process valve is usually necessary for producing a hollow body from a blank. A preblowing valve and at least one main blowing valve are usually present. Furthermore, at least one recovery valve and/or one vent valve are preferably present. As shown in FIG. 3, these valves are preferably arranged in a common valve block 4, which has at least one, preferably a plurality of, process pressure inlet and outlet channels for connection to the process gas source. By means of the process gas, the preform is inflated into the desired shape of the final hollow body. For this purpose, the valve block 4 has a blowing nozzle 40, on which the blank or preform 7 to be inflated is secured.

[0133] The valve block 4 furthermore has at least one pressure sensor 41, which measures the pressure of the process gas in the region of the nozzle 40. This corresponds, with minimal and thus negligible delay, to the pressure which prevails in the interior of the blank 7. The pressure sensor 41 forms part of the abovementioned sensor unit 9. Depending on the embodiment, further sensors are also present.

[0134] The individual pilot valves 2 of the individual process valves are connected via respective electronic control connections 20 to an actuator box 5, which in turn is connected directly or via further control subunits to the machine controller 6 or which itself is already part of the machine controller 6. Depending on the embodiment, the actuator box 5 has a processor and other means for control and/or data processing. The actuator box 5 and the machine controller 6 are referred to below as a control system.

[0135] The control system controls the pilot valves 2 and thus the process valves. Moreover, it receives the measurement data of the at least one pressure sensor 41 and any other sensors which supply data about the blowing process.

[0136] FIGS. 6a to 6b show a portion of a typical process sequence at the time of the start of the process with the preblowing. In curve b), FIG. 6a shows the pressure P.sub.R which was measured by the pressure sensor 41 in the valve block 4 in the region of the interior of the blank as a function of time t. FIG. 6b shows the voltage signal U of the pilot valve 2 measured or applied in this process as a function of the same time t as in FIG. 6a. Curve c) shows the voltage signal U of the pilot valve 2 for the preblowing valve and curve d) shows the voltage signal U of the pilot valve 2 for the main blowing valve.

[0137] Viewing FIGS. 6a and 6b together furthermore reveals the pressure behavior P.sub.R in the region of the interior of the blank in the course of the process. At time A, the voltage U is applied to the pilot valve of the preblowing valve. The pilot valve responds with a time delay and thus the internal pressure P.sub.R in the blank rises with a time delay. At time t.sub.B in curve b) of FIG. 6a, the pressure rise is for the first time sufficiently distinct from the measuring noise. The pressure at point C at time t.sub.C in curve b) according to FIG. 6a is approximately 50% of the first pressure rise (i.e. the pressure rise up to the first reduction of the pressure). The difference between times t.sub.C and t.sub.B indicates that the main valve has opened completely at the correct time.

[0138] At point D at time t.sub.D of curve b), the blank also begins to expand radially. According to experience, this has the result that the pressure P.sub.R in the interior of the blank drops for a short time, as can be clearly seen in curve b). This is followed by a period of time with an approximately constant pressure P.sub.R in the interior of the blank, with a subsequent rise. This can also be seen clearly in curve b).

[0139] The pilot valve of the preblowing valve is likewise switched off with a time delay. The switching on of the main blowing valve (curve d) in FIG. 6b), which took place shortly thereafter, again takes place with a time delay. Point E in curve b) shows the pressure rise after switching on the main blowing valve. As can be seen in curve b), the massive rise in the internal pressure P.sub.R in the partially inflated blank follows with a time delay.

[0140] FIGS. 7a to 7c show a similar portion of a blowing process, three different blowing processes being superimposed in time in the graphic. FIG. 7a shows the pressure characteristic P.sub.R, measured in the region of the blank, i.e. of the preform, during the preblowing process and at the beginning of the blowing process with increased pressure. FIG. 7b shows the voltage U applied to the pilot valve 2 as a function of time. FIG. 7c simultaneously shows the current I measured at the pilot valve 2 as a function of time.

[0141] The time indications t.sub.A, t.sub.B, t.sub.C, t.sub.D and t.sub.E relate to the solid curve I of a first blowing process. In a second blowing process, illustrated by the dashed curve II, the beginning of the blowing process is delayed, for example because the switching piston 12 of the main valve 1 responds too late and opens too late. In a third blowing process, illustrated by the dotted curve III, the beginning of the blowing process is correct but the pressure increase is delayed, for example because the switching piston 12 of the main valve 1 moves too slowly.

[0142] In a variant of the method according to the invention, the three curves illustrated have been produced by pressure measurement in the same blow-moulding station during three blow-moulding operations separated in time, i.e. the same behavior of the same valves 1 has been recorded at different times. In another variant, the pressure curves of different blow-moulding stations of the same blow-moulding unit are compared with one another. In this case, these curves were produced as close to one another in time as possible. The blow-moulding stations are usually operated with a very small time difference between them. FIGS. 8a to 8c show a complete blowing process of a single blow-moulding station. FIG. 8a again shows the pressure P.sub.R in the blank as a function of time t, FIG. 8b shows the voltage U applied to the individual pilot valves as a function of the same time t, and FIG. 8c shows the current I of the pilot valves as a function of the same time t.

[0143] In region V, preblowing takes place with a preblowing valve, and, in region H, blowing takes place at elevated pressure and using a blowing valve. In region R1, recovery is carried out by means of a first recovery valve, and, in region R2 recovery is continued by means of a second recovery valve or a vent valve or venting takes place directly into the surroundings. Finally, the inflated hollow body is ejected from the blow mould. This is a well-known pressure curve of a typical blowing process. These pressure curves can also have further regions, e.g. if the blowing process is operated with two or more blowing valves connected at successive times in each case at a higher pressure than the previous stage.

[0144] These curves can be recorded in the central controller 6 or in the decentralized data processing unit 8 on the basis of the measured sensor values. The values are preferably measured in very small time intervals, for example in 0.1 ms to 1 ms. The sensor values are thus preferably detected at high frequency, for example 1,000 to 10,000 times per second. As a result, relatively accurate curves can be mapped.

[0145] However, this curve does not have to be recorded. In addition or preferably as an alternative to such a recording, tables of the measured values are produced. The tabular form facilitates statistical processing of the data.

[0146] The values in the tables or in the curves of the same blow-moulding station for various completed blowing cycles (i.e. a blowing process from preblowing to ejection of the hollow body produced) can be compared with one another, and/or the values in the tables or in the curves of the various blow-moulding stations of the blow-moulding unit can be compared with one another for the same blowing cycle or for blowing cycles lying close to one another. In this case, the values in the tables or in the curves of an entire blowing process, or only a portion thereof, or else only distinctive points in the curves and tables, such as maxima, minima and linear regions, can be taken into account in the further evaluation.

[0147] FIGS. 9 to 11 show part of the blow-moulding unit, the interaction of the central control unit 6 and the decentralized data processing unit 8 being evident. The central control unit 6 forms the machine controller. In these figures, it is shown with an actuator box 5 and a valve block 4 of a blow-moulding station. It is connected via corresponding connections, in this case a BUS, to a plurality of such blow-moulding stations, for example to forty stations.

[0148] In the embodiment according to FIG. 9, the actuator box 5 forms the data processing unit 8. It collects the sensor values, maps them in curves and/or tables, statistically evaluates them and delivers the result, e.g. a report on the valve state, a recommendation for correction or a new setpoint for further control, to the central control unit 6 via the BUS system. In this case, the actuator box 5 preferably also has control functions. In this embodiment, the data of the same blow-moulding station, but not the data of different blow-moulding stations, can be compared with one another at different times.

[0149] In the embodiment according to FIG. 10, an additional module in the form of a central data processing unit is present in addition to the actuator box 5, which actuates the valve block 4 in accordance with its own control commands and/or control commands received from the machine controller, depending on the variant of the embodiment. This, together with the machine controller, forms the central control unit. The central data processing unit 8 collects the data of all the blow-moulding stations of this blow-moulding unit, compares them with one another, statistically evaluates them and sends the result to the machine controller, e.g. a report on the valve state, a recommendation for correction or a new setpoint for further control. In this embodiment, the data of a single blow-moulding station and also the data of different blow-moulding stations can now be compared with one another.

[0150] In the embodiment according to FIG. 11, the same construction and the same functioning are present as in the embodiment according to FIG. 10. In addition, however, the central data processing unit has control functions. The machine controller supplies the formulation, i.e. the parameters for producing the desired hollow body, to this central data processing unit and the latter transmits corresponding control commands to the various actuator boxes 5 of the individual blow-moulding stations.

[0151] The method according to the invention permits early detection of malfunctions in blowing processes.

[0152] FIGS. 12 to 14 show alternatives or additions to the pressure measurement in the blank and in the hollow body. The measuring methods and the devices presented here serve to control the valves in a manner which is as accurate and error-free as possible without having to consider the blowing process as a whole. This has the advantage that errors can be detected more easily since the measurements are not exposed to a multiplicity of mutually influencing process factors.

[0153] FIGS. 12 to 14 again show a pneumatically pilot-controlled main valve 1 which is used as a process valve in a blowing process. It can be, for example, a preblowing valve, a main blowing valve, a recovery valve or a vent valve. It is preferably once again pilot-controlled by means of at least one pilot valve 2, in particular a solenoid valve. The same reference numerals were used as for the valve in FIGS. 1 and 2. Identical parts are provided with the same reference signs. The upper part of the valve housing 10, which forms the upper end of the control chamber 13, is provided here with the reference numeral 100.

[0154] According to the invention, sensors are then mounted in or on the valve housing 10 which directly monitor the position and/or the movement of the switching piston 12 and/or draw conclusions about its position and/or its movement from reactions which are caused by the switching piston 12.

[0155] In the embodiment according to FIGS. 12a and 12b, two position sensors 90, 91 are present, which detect the position of the switching piston in its open and in its closed position. One position sensor would be sufficient, but two sensors enable more reliable measurement. In FIG. 12a, the switching piston is shown in the open position and the first position sensor 90 detects the switching piston 12. In FIG. 12b, the valve is shown in the closed position and the second position sensor 91 detects the switching piston 12. Thus it is possible to detect the time at which the piston movement begins and the time at which the open end position of the piston is reached. Suitable position sensors 90, 91 are, for example, proximity sensors, for example optical sensors or Hall-effect sensors. A marking on the piston, shown in black in the drawings, facilitates detection of the position.

[0156] In the embodiment according to FIGS. 13a and 13b, a displacement measuring sensor 92 is present, which measures the displacement of the switching piston 12. The displacement measuring sensor 92 is preferably arranged in the control chamber 13. FIG. 13a again shows the situation when the valve is open and FIG. 13b shows the situation when the valve is closed. As sensors, it is possible, for example, to use displacement measuring sensors with an analog output signal, ultrasonic sensors and load triangulation sensors.

[0157] In the embodiment according to FIGS. 14a and 14b, a vibration sensor 93 is arranged in the valve housing 10. When the switching piston 12 is moved, the housing 10 vibrates and the sensor 93 responds. If the valve does not switch, or switches too slowly or with a delay, this is recorded by the sensor 93. If a plurality of valves is arranged in a common valve block, a single vibration sensor 93 is usually sufficient for all these valves since they are usually actuated successively in time and thus the sensor signal can be unambiguously assigned in each case to a specific valve. It is also possible to use an acceleration sensor instead of a vibration sensor.

[0158] These valves can also be used in other blow-moulding units without the data evaluation and control according to the invention. The corresponding measuring methods as well as the corresponding valves and valve blocks are thus likewise claimed as a separate invention.