Abstract:
To insure in a simple and economical manner a uniform and continuous dispensing of a fluid from a pressure tank, a method is provided according to which a pressurized gas is introduced into the pressure tank via a proportional valve that is disposed in an inlet line of the pressure tank, the pressure of the fluid located in an outlet line is measured with a first pressure sensor, and an outlet valve in the outlet line is opened and closed. The method also includes the determination of a set pressure value as a function of the measurement result of the first pressure sensor, the transfer thereof to the proportional valve, the measurement of the gas pressure in the inlet line with a second pressure sensor disposed between the proportional valve of the pressure tank, and the transfer of the measurement result to the proportional valve. The invention also provides an apparatus for carrying out the above method.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for dispensing a fluid from a pressure tank, whereby the introduction of a pressurized gas into the pressure tank is provided via a proportional valve that is disposed in an inlet line of the pressure tank, the measuring of the pressure of the fluid located in the outlet line is provided by a first pressure sensor, and the opening and closing of an outlet valve in the outlet line is provided for. The invention further relates to an apparatus for dispensing a fluid from a pressure tank, and includes a proportional valve that is disposed in an inlet line of the pressure tank for introducing a pressurized gas, a first pressure sensor in an outlet line of the pressure tank for measuring the pressure of the fluid located in the outlet line, and an outlet valve in the outlet line. 
     Such methods and apparatus are known, for example, in coating systems for the manufacture of CDs. With these systems, it is important for a uniform and continuous coating of the CDs that the pressure of the lacquer provided at the outlet or dispense valve preferably has a constant predetermined value. With the known systems, the pressure value measured at the first pressure sensor is utilized as an actual value for an adjustment of the proportional valve. However, this results in the problem that the pressure measured at the first sensor drops due to dynamic line and filter pressure losses as soon as the outlet valve is opened. Due to this drop in pressure, the pressure at the proportional valve is readjusted until the actual value again coincides with the prescribed desired value. In this connection, readjustment also takes place during a dispensing process, which leads to imprecision with regard to the applied quantity of lacquer. After the closing of the outlet valve, the dynamic line and filter pressure losses no longer have any effect, and the measured pressure at the first pressure sensor again rises to an increased value. The proportional valve must again be readjusted since the previously introduced gas, which is generally nitrogen, is vented; the readjustment takes place until the actual value again corresponds to the desired value. 
     These control or adjustment processes lead to fluctuations of the dosing volume as well as to a high consumption of nitrogen. Furthermore, concentration changes and possibly a crystallization of the lacquer dissolved in the solvent can result due to solvent evaporation and a discharge of the solvent vapor together with the nitrogen that escapes during the readjustment. In addition, continuous oscillations of the regulator occur during a tank filling state with critical residence gas volumes, which leads to a greater consumption of nitrogen. 
     It is therefore an object of the present invention to provide a method and apparatus for dispensing a fluid from a pressure tank, according to which a uniform and continuous dispensing of the fluid is ensured in a simple and economical manner. 
     SUMMARY OF THE INVENTION 
     The stated object is inventively realized with a method of the aforementioned type in that a desired or set pressure value is determined as a function of the measurement result of the first pressure sensor and is transferred to the proportional valve, and the gas pressure in the inlet line is measured with a second pressure sensor disposed between the proportional valve and the pressure tank and is transferred to the proportional valve. By the measurement of the gas pressure in the inlet line, and the transfer of the measurement results to the proportional valve, the frequent readjustment of the proportional valve described above during opening and closing of the outlet valve is avoided, since no dynamic line and filter pressure losses occur between the proportional valve and the second pressure sensor. This leads to a low consumption of nitrogen since no discharge of nitrogen occurs during closing of the outlet valve, as a result of which also a lower change of the dye concentration in the solvent is achieved. Furthermore, continuous oscillations during the regulation process are suppressed, since the control loop formed by the proportional valve in the second pressure sensor is not oscillatory. As a consequence of the determination of a set pressure value as a function of the measurement result of the first pressure sensor, and transfer of this value to the proportional valve, there is effected an automatic adaptation of the system to changeable disturbance variables, such as tank filling state and filter pressure losses, as a result of which a stable regulating condition is achieved without oscillations and a high dosing precision is also achieved. 
     Pursuant to one preferred specific embodiment of the invention, for the determination of the set pressure value only those measurement results of the first pressure sensor are used that were measured with the outlet valve opened in order to prevent pressure changes, which occur when the outlet valve is opened or closed, from influencing the determination of the desired or set value. For a constant dosing volume flow, only the pressure at the external sensor with the valve opened is relevant and of interest. In this connection, only those measurement results of the first pressure sensor are used that were measured after a specific period of time after the opening of the outlet valve in order that oscillations that occur during the opening will have no influence upon the determination. In order to achieve a uniform dispensing of the fluid, for the determination of the set pressure value a measurement result of the first pressure sensor determined over a measurement interval is used. 
     The set pressure value, in addition to being determined as a function of the measurement result of the first pressure sensor, is preferably determined as a function of the measurement result of the second pressure sensor in order to achieve a better uniformity and suppression of disturbance variables. A pressure difference between the inlet and outlet line is preferably measured, whereby in one specific embodiment of the invention, the set pressure value is determined as a function of the measured pressure differential. 
     Pursuant to a particularly preferred specific embodiment of the invention, the determination and/or transfer of the set pressure value is carried out only when the outlet valve is closed to order to ensure that during a dispensing process no change of the prescribed set pressure value, and a readjustment possibly connected therewith, occur. 
     Pursuant to a further specific embodiment of the invention, the filling state height of the pressure tank is determined as a function of the measurement results of the first and second pressure sensors in order to provide an automatic indication thereof and to be able to correct the thereby resulting disturbance variables during the determination of the set pressure value. 
     For an automatic indication of a filter state, the state of a filter located in the outlet line is preferably determined as a function of the measurement results of the first pressure sensor. From the automatic indication it can be determined when a filter change is necessary. In this connection, the filter state is preferably determined as a function of a difference of the measurement results of the first pressure sensor with the outlet valve closed and opened. During the determination of the filter state, preferably only those measurement results are used that were measured after conclusion of a predetermined period of time after the closing or after the opening of the outlet valve in order that oscillations that result during the closing or opening do not have an influence upon the determination. 
     The object of the present invention is realized with an apparatus of the aforementioned type in that a control unit is provided for the determination, as a function of the measurement result of the first pressure sensor, of a set pressure value that is to be provided to the proportional valve, and a second pressure sensor is provided between the proportional valve and the pressure tank for measuring the gas pressure in the inlet line and for transferring the measurement result to the proportional valve. With such an apparatus, the advantages described above in reference to the method are achieved. For a particularly simple and economical embodiment of the invention, the second pressure sensor is preferably integrated in the proportional valve. 
     Pursuant to a further advantageous specific embodiment of the invention, a differential pressure sensor is provided and is disposed between the inlet and the outlet line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be subsequently explained with the aid of one preferred specific embodiment with reference to the drawings; in the drawings: 
     FIG. 1 shows an inventive apparatus for dispensing a fluid; 
     FIG. 2 is a flow diagram that illustrates the automatic determination of a set pressure value; 
     FIG. 3 is a flow diagram that illustrates the determination of a filter state and a volume flow; and 
     FIG. 4 is a graph showing dead and measurement times of pressure sensors. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a coating system  1  for CDs  2 . 
     The coating system  1  has a pressure tank  4  with an inlet line  5  and an outlet line  6 . The inlet line  5  is connected to an upper side of the pressure tank and communicates with an upper region of the pressure tank  4 . The outlet line  6  is similarly connected to an upper side of the pressure tank  4 . However, the outlet line  6  extends through the interior of the pressure tank  4  and communicates with the interior of the pressure tank in a lower region thereof. The pressure tank  4  is partially filled with a lacquer  8  for coating the CDs  2 . Due to the weight of the lacquer  8 , it;completely fills a lower portion of the pressure tank  4 . Disposed in the region located above the lacquer is a pressurized gas  10 , such as nitrogen. 
     The nitrogen  10  is fed into the pressure tank  4  from a nitrogen supply  12  that is connected to the inlet line  5 . Disposed in the inlet line  5  between the nitrogen supply  12  and the pressure tank  4  is a valve unit  14  having a proportional valve  15  and an internal pressure sensor  16  that is disposed between the proportional valve  15  and the pressure tank  4 . 
     Provided adjacent to the upper side of the pressure tank  4  is a differential pressure sensor  18  having a first sensor element  19  that is disposed in the inlet line  5 , and a second sensor element  20  that is disposed in the outlet line  6 . Above the sensor element  20 , in the outlet line  6 , are furthermore disposed a filter  22 , a pressure sensor  24 , as well as an outlet valve  26 . Downstream of the valve  26  (viewed from the pressure tank  4 ) the outlet line  6  opens to a coating station  28  in which CDs  2  are supplied with the lacquer  8  when the valve  26  is opened. 
     The valve unit  14 , the differential pressure sensor  18 , the pressure sensor  24 , as well as the valve  26  are respectively connected to a control unit  30 . The control unit  30  is provided with an input portion  31  that receives the measurement results of the internal pressure sensor  16 , of the differential pressure sensor  18 , and of the pressure sensor  24 . An output portion  32  of the control unit  30  is connected to the valve  26  in order to control the opening and closing of the valve  26 . The output portion  32  is furthermore connected to the proportional valve  15  of the valve unit  14  in order to prescribe for the proportional valve  15  a set pressure value calculated by the control unit  30 . An output of the internal pressure sensor  16  is also connected to the proportional valve  15  in order to provide an actual pressure value to the proportional valve  15  in the inlet line  5 . The proportional valve  15  of the valve unit  14  is in the position, due to the prescribed set pressure value from the control unit  30 , as well as the actual pressure value from the internal pressure sensor  16 , to adjust the pressure in the inlet line  5  to the set pressure valve. Thus, a control loop is formed within the valve unit  14  into which a set pressure value is externally introduced by the control unit  30 . 
     During operation of the coating system  1 , the pressure tank  4  is brought to a prescribed pressure level by introducing the pressurized nitrogen. Due to the pressure that exists in the pressure tank, the lacquer  8  that is in the pressure tank is pressed upwardly in the outlet line  6  in the direction of the valve  26 . When the valve  26  is closed, the system is essentially static, and no flow of lacquer takes place. If a CD  2  is disposed in a coating position in the coating station  28 , the valve  26 , as controlled by the control unit  30 , is opened for a specific period of time that is necessary for applying a coat of lacquer on the substrate  2 . With the valve  26  opened, due to the pressure that exists in the pressure tank  4 , lacquer  8  flows through the outlet line  6  to the coating station  28 . After the valve  26  is closed, a static systems again results in which no lacquer flows. 
     So that in successive coating steps a uniform quantity of lacquer is applied to the substrate  2 , the pressure in the system, especially in the vicinity of the outlet  26 , must be kept at a constant value when the outlet valve  26  is opened. This is achieved by keeping the pressure in the outlet line  6 , especially in a region downstream of the filter  22  (seen from the pressure tank  4 ) at a constant level. For this purpose, a set pressure value is prescribed for the valve unit  14  for regulating the pressure in the system, which set pressure value is related to the pressure measured by the pressure sensor  24 . 
     The calculation of the set pressure value will be explained subsequently with the aid of the flow diagram of FIG.  2 . 
     In a first block  40 , the pressure P ext  is measured at the first pressure sensor  24 . This measured value is conveyed further to a decision block  42  that establishes whether the measured values are derived from a period of time in which the outlet valve  26  is opened or closed. 
     If the measurement results are derived from a period of time in which the outlet valve  26  is closed, then the measurement results are conveyed further to a block  44 , which from the measurement results calculates an average pressure value P ext, OFF, average . 
     If the measurement results are derived from a period of time in which the valve  26  is opened, the measurement results are conveyed further to a block  46  that calculates an average pressure value P ext, ON, average . 
     After the calculation of the average pressure value P ext, ON average , this value is conveyed further to a block  48 . In the block  50 , as a function of the desired volume stream of the lacquer to the coating station, a desired pressure value P desired  is prescribed. This pressure value P desired  is conveyed further to the block  48 . 
     In a block  52 , a pressure P int  is measured at the pressure sensor  16  of the valve unit  14 . The measurement results are conveyed further to a block  54 , in which an average pressure value P int, average  is calculated therefrom. This average pressure value is conveyed further to the block  48 . 
     In the block  48 , a roughly approximated or corrected set pressure value P desired, corrected, rough  is calculated as a function of the pressure values introduced in the block  48 . The roughly approximated set pressure value results from the following equation: 
     
       
           P   desired, corrected, rough   =P   desired +(P int, average   −P   ext, average ) 
       
     
     This value is conveyed further to a block  56 , in which an average value P desired, corrected, rough, average  is calculated, which in turn is conveyed further to a block  58 . 
     In a block  60 , the pressure differential ΔP tank  between the inlet line and the outlet line is measured and is conveyed further to a block  62 , where the measurement results are averaged. The measured pressure differential ΔP tank  is related to the fill height of the lacquer  8  in the pressure tank  4 , which is calculated therefrom. The averaged pressure differential ΔP tank , average is transferred the block  58 . In a block  64 , a tolerance range ΔP tolerance  for successive measurements is determined and transferred to the block  58 . 
     In the block  58  it is determined whether the last roughly approximated, averaged set pressure value P desired, corrected, rough, average  obtained in the block  56  relative to a previously obtained value lies within the tolerance range. This is determined as follows: 
     
       
         (( P   desired, corrected, rough, average, j+1   −P   desired, corrected, rough, average, j ) 2 ) 0.5   &gt;ΔP   tolerance , 
       
     
     where j+1 indicates the last calculated average, and j indicates the previously calculated average value. 
     When the above relationship is fulfilled, then the following applies 
     
       
         
           P 
           desired, corrected, j+1 
           +P 
           desired, corrected, rough, average, j+1 
         
       
     
     i.e. the newly approximated set pressure value corresponds to the average value determined in Block  56 . 
     If the above relationship is not fulfilled, then the newly approximated set pressure value is determined with the aid of the following equation: 
     
       
           P   desired, corrected, j+1   =P   desired, corrected, j +(Δ P   tank, average, j+1   −ΔP   tank, average, j ). 
       
     
     The regulation or adjustment with the aid of the measurement results of the differential pressure sensor  18  is effected on the basis of a greater measurement precision of the differential pressure sensor  18  relative to the pressure sensor  24 , as a result of which the frequency of “rough” readjustment procedures and in particular the amplitude of the readjustment changes are reduced. Consequently, less nitrogen is used. 
     The new, approximated set pressure value P desired, corrected  that results from the determination in the Block  58  is subsequently conveyed further to the Block  66 . In the Block  66 , it is established whether the coating system is within a start phase, i.e. for example within the first five coating cycles. If the system is not within a start phase, then the set pressure value determined in the Block  58  is conveyed via the Block  68  to a Block  70  in order there to form the new, approximated set pressure value P desired, corrected, new . 
     However, if the system is within a start phase, in a Block  72  an adapted or an approximated start set pressure value P desired, corrected, start  is determined, among others, with the aid of the characteristics of the lacquer, the geometrical relationships of the system, the pressure difference at the differential pressure sensor, and the volume flow of the lacquer. 
     V start  can be determined on the basis of the geometrical relationships, the pressure relationships in the outlet line, as well as the characteristics of the lacquer, whereby due to the lack of measurement results one proceeds on the basis that the pressure at the pressure sensor is P ext =P desired . If the filter is new, the filter constant K filter, start  corresponds to the manufacturing specifications. If the filter is used, it corresponds to the last determined and stored value. Alternatively, it is also possible to determine the filter constant in that prior to the actual coating of a CD, a test output cycle is carried out in which the filter constant is then determined. 
     This start value determination is necessary since when the system is initiated P ext, ON  is not known. The set value correction provides a roughly approximated start value. 
     The approximated start set pressure value is subsequently conveyed further to the Block  70 , where it forms the new, approximated set pressure value P desired, corrected, new . 
     This value is conveyed further to the Block  74 , in which it is determined whether the system is presently in a static or dynamic state. If the system is in a static state, i.e. if no medium flows, then the new, approximated set pressure value P desired, corrected, new  is transferred via a Block  76  to a Block  78 . If it is determined in the Block  74  that the system is in a dynamic state, i.e. a medium flows, then the new, approximated set pressure value P desired, corrected, new  is not conveyed further to the Block  78 , but rather the previous value found in the Block  78  is retained. 
     In the Block  78 , the approximated set pressure value P desired, corrected, new  is converted into units utilizable for the proportional valve  15  and, for presetting a set pressure value, is transmitted to the valve as P valve  in the Block  80 . 
     The determination of the filter state will be explained subsequently with the aid of the flow diagram of FIG.  3 . 
     To determine the filter state, the pressure P ext  must also be evaluated with the outlet or dispense valve  26  closed. The difference between the pressure P ext, open  measured at the pressure sensor  24  with the outlet valve opened and P ext, closed  with the outlet valve  26  closed resulted from the filter pressure loss and the line pressure loss in the conduit in conformity with the following equation: 
     
       
         
           P 
           ext, closed 
           −P 
           ext, opened 
           =ΔP 
           filter 
           +ΔP 
           conduit, 
         
       
     
     whereby with a constant tank pressure and volume flow the pressure loss in the conduit is also constant. However, over a longer period of time the filter pressure loss can increase, even at constant tank pressure, if the filter becomes clogged. Consequently, the approximated set pressure value P desired, corrected, new  must be increased to the same extent. 
     If in so doing a threshold value is reached, an alarm message appears that indicates that the filter is used up and must be exchanged. 
     Furthermore, the filter state should be indicated during the operation. The calculation of the filter state will be described with the aid of the flow diagram of FIG.  3 . 
     In a Block  90 , the pressure P ext  is measured in the outlet line  6  at the pressure sensor  24 . The measured pressure values are conveyed further to a Block  92  in which it is determined whether the outlet or dispense valve  26  is opened or closed. If the outlet valve  26  is closed, the measured pressure values P ext  are transferred to a Block  94  that calculates an average value of the measured pressure values with the valve  26  closed of P ext, OFF, average . 
     If it is determined in Block  92  that the outlet valve  26  is opened, then the measured pressure values P ext  are conveyed to a Block  96  that calculates an average value P ext, ON, average  from the pressure values P ext  measured with the valve open. 
     Subsequently, in a Block  98  the volume flow of the lacquer, and in Block  100  the pressure loss in the outlet line ΔP conduit  are determined. The average value P ext, OFF, average , the average value P ext, ON, average  as well as the pressure loss in the line ΔP conduit  are transferred to a Block  102  in which a pressure loss in the filter ΔP filter  is calculated. The pressure loss in the filter is determined with the aid of the following equation: 
     
       
         Δ P   filter   =ΔP   ext, OFF, average   −ΔP   ext, ON, average   −ΔP   conduit.   
       
     
     Subsequently, in a Block  104 ; an average value ΔP filter, average  of the pressure loss at the filter is calculated. 
     Subsequently, in a Block  106 , a filter constant K filter  is calculated with the aid of the determined filter pressure loss and with the aid of the volume flow. This is done with the aid of the following equation: 
     
       
           K   filter   =ΔP   filter, average   ·A   filter /( V ·η medium ) 
       
     
     This value is transferred to a Block  108  in which is determined if the system is in a start phase. 
     If the system is in a start phase, then it is determined in a further decision Block  110  if previously a filter change took place or not. If a filter change took place prior to the new start, then in a Block  112  for the above calculation of the start set pressure value P desired, corrected, start  the filter constant K filter, start  is specified the same as the filter constants of a new filter K filter, new . This value K filter, new  is determined from the manufacturing specifications in the Block  114  and is transferred to the Block  112 . 
     If it is established in the Block  110  that no filter change took place prior to the new start, then in a Block  116 , for the starting value determination, the filter value constant K filter, start  is specified the same as the last determined and stored, prior to the shutdown of the system, value K filter, observed . 
     If it is determined in the Block  108  that the system is not in a start phase, the filter constant K filter  calculated in the Block  106  is conveyed via a Block  118  to the Block  120  as an instantaneous filter state factor K filter, instantaneous . In the Block  120 , the filter state factor is calculated by the following equation: 
     
       
         Filter state factor= K   filter, instantaneous   /K   filter, new . 
       
     
     The filter state factor indicates by how much the instantaneous filter pressure loss is greater than the original. In this way, it can be better estimated how long the old filter can still be used before a filter change is necessary. 
     FIG. 4 shows the pressures measured at the internal pressure sensor  16  and at the pressure sensor  24  over an opening cycle of the valve  26 . The upper curve shows the pressures measured at the internal pressure sensor  16 , while the lower curve, which varies relatively significantly, shows the pressures measured at the pressure sensor  24 . 
     As can be seen from FIG. 4, the pressures measured at the internal pressure sensor  16  are independent of whether the outlet valve  26  is closed or opened, and are relatively constant. 
     However, the pressures measured at the pressure sensor  24  vary relatively significantly during opening or closing of the outlet valve  26 . In this connection, directly after the closing there results a relatively significant drop in pressure, which subsequently again increases in order to then again drop. Thus, in a time interval T 1  after the opening of the outlet valve  26  relatively significant fluctuations of the pressure result. After the time interval T 1 , there is a phase having a relatively constant pressure. After the closing of the outlet valve  26 , there is a relatively significant increase in pressure, which subsequently again drops and rises. Thus, in a time interval T 2  after the closing of the valve relatively significant fluctuations result. After the time interval T 2 , there is again a phase having a relatively constant pressure. 
     For the above calculation of a corrected set pressure value, as well as for the calculation of the filter state factor, therefore exclusively measurement results are used that originate from a phase in which the pressure fluctuations resulting from the opening and closing of the valve have essentially subsided. These phases are indicated in FIG. 4 as measurement T 1  for measurements with the valve opened, and measurement T 2  for measurements with the valve closed. 
     Although the apparatus has been described with the aid of a preferred specific embodiment assuming a coating system for CDs, the apparatus is not limited thereto. 
     The specification incorporates by reference the disclosure of German priority document 199 14 203.3 filed Mar. 29, 1999, German priority document 199 37 606.9 filed Aug. 9, 1999 and International priority document PCT/EP00/02155 of Mar. 11, 2000. 
     The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims.