Patent Publication Number: US-2018036760-A1

Title: Apparatus and method for the dosed dispensing of a liquid

Description:
The invention relates to an apparatus and a method for the dosed dispensing of a liquid. 
     There is known from the prior art an apparatus by which a dosed dispensing of liquid from an air-tight dispensing vessel takes place. The dispensing vessel has a dispensing opening for the liquid and a compressed-air port so that the dispensing vessel can be pressurized. When a particular pressure is applied for a certain time, the liquid is pushed out of the dispensing vessel for said time. A compressed-air system is provided for the provision of the compressed air. The compressed-air port of the dispensing vessel is connected to the compressed-air system by way of a connecting line. The fill level of the liquid in the dispensing vessel can be measured by means of a sensor device. This ensures that a dispensing vessel in use can be replaced in good time by a new, full dispensing vessel. 
     If, for example, a PUR hot-melt adhesive in liquid form is to be applied by the apparatus to surfaces that are to be bonded, the PUR hot-melt adhesive in the dispensing vessel must be kept at a certain temperature, which limits the choice of possible sensors. The particular nature of the liquid can also rule out those sensors that must have direct contact with the liquid in order to detect the fill level. 
     Capacitive sensors, which operate on the basis of the change in capacitance of an individual capacitor or of an entire capacitor system, have the advantage that they need not come into contact with the liquid when determining the fill level. However, it should be noted that the chemical composition of the liquid for which the fill level is to be determined has an influence on the measurement results of the capacitive sensor. For different liquids, therefore, the measured values of the sensor may differ for the same fill levels, and therefore a liquid-specific calibration of the sensor is required. Use of the capacitive sensor is also limited if the dispensing vessel has a large wall thickness. However, in the case of dispensing vessels which are pressurized in order to dispense the liquid, the wall thickness cannot be reduced as desired due to strength requirements. In addition, when replacing an empty dispensing vessel with a new dispensing vessel, it may be complicated to reattach and realign the sensor on the vessel. 
     The problem addressed by the invention is therefore that of providing an apparatus for the dosed dispensing of a liquid, in particular for dispensing a liquid adhesive such as heated PUR hot-melt adhesive, by which a dosed dispensing of the liquid and the determination of the fill level of the liquid in the dispensing vessel are possible in a simple and reliable manner. 
     The problem addressed by the invention is solved by the combination of features according to claim  1 . Exemplary embodiments of the apparatus according to the invention can be found in the claims dependent on claim  1 . 
     According to claim  1 , the sensor device is connected by way of the connecting line to the dispensing vessel. In addition, the dispensing opening is closable. The sensor device may comprise a pressure sensor which measures the pressure in the connecting line. As an alternative or in addition, the sensor device may comprise an air quantity sensor which measures the quantity of air flowing through or into the connecting line. 
     The apparatus according to the invention has the advantage that, when replacing the vessel, the sensor device is connected directly as a result of connecting the connecting line to the compressed-air port of the dispensing vessel. There is no need for separate attachment and alignment, on the dispensing vessel, of a sensor for determining the fill level. As will be explained in greater detail below, a pressure change can be brought about in the dispensing vessel in various ways via the compressed-air system. The magnitude of the pressure change that results for a particular volume change or for a particular additional quantity of air flowing into the dispensing vessel depends on the air volume within the dispensing vessel. The liquid volume, that is to say the volume taken up by the liquid in the dispensing vessel, can be calculated from the air volume. For this, the air volume is subtracted from the constant total volume. 
     If the pressure is increased in the dispensing vessel for the purpose of determining the fill level, it should be ensured that no liquid is dispensed from the dispensing vessel while doing so. For this reason, the dispensing opening must be closable. The dispensing opening may be assigned a shut-off valve, by which the dispensing opening can be opened and closed. The shut-off valve may be a switching valve, which is actuated via a signal line. 
     In one exemplary embodiment, computer means are provided which calculate the fill level in the dispensing vessel from the measurement result of the pressure sensor and/or of the air quantity sensor. For example, a volume change ΔV can be brought about in the dispensing vessel, which leads to a pressure increase in the dispensing vessel. The air volume in the dispensing vessel, and thus the fill level and/or the liquid volume, can then be calculated from the pressure increase as a function of the volume change ΔV. 
     The compressed-air system may comprise a pneumatic cylinder which is directly or indirectly connected to the connecting line. The cylinder having the cylinder volume, the dispensing vessel having the air volume, the connecting line, as well as further lines or line sections of the compressed-air system which connect the dispensing vessel and pneumatic cylinder, form a test system having a corresponding test volume. This test volume can be reduced by reducing the cylinder volume by moving a piston in the cylinder. The cylinder volume is thus reduced by the stroke volume. The pressure sensor measures the pressure increase in the test volume or the pressure prior to actuation of the piston and the pressure after actuation of the piston. By using the general ideal gas equation: 
         P·V=m·R   S   ·T =const.   (1)
 
     where P pressure, 
     V volume, 
     M quantity of air, 
     R S  specific gas constant, and 
     T temperature, 
     the air volume in the dispensing vessel can be determined. Knowing the total volume of the dispensing vessel, the liquid volume taken up by the liquid, which is a measure of the fill level, can be obtained from the air volume in the dispensing vessel. The pressure increase in the test system can in this case be measured at different points since the same pressure is quickly set throughout. 
     The compressed-air system may comprise a throttle valve which is connected to the connecting line. It is thus possible to introduce into the dispensing vessel an air flow that is limited in terms of its magnitude. An air quantity sensor measures the quantity of air in question. If the pressure increase thereby brought about in the test system is determined, it is thus once again possible to calculate the air volume in the dispensing vessel and thus the fill level of the liquid in the dispensing vessel. Particularly in the case of a dispensing vessel that is almost completely empty, which then has a large air volume, the difference between the quantity of air flowing into the dispensing vessel and the total quantity of air supplied to the test system is large whenever measured with the same pressure increase. 
     The pressure system may comprise a proportional valve which is connected (directly or indirectly) to the connecting line. In this case, there is no need for a separate throttle valve. 
     On the one hand, the proportional valve can be used to bring about the pressure change in the dispensing vessel that is necessary in order to determine the fill level. On the other hand, however, it can also provide the pressure for dispensing liquid from the dispensing vessel. However, the compressed-air system may also comprise a switching valve which serves only to provide the pressure for dispensing liquid. A further switching valve may be provided only for generating a pressure change for determining the fill level. For example, it could generate pressure for moving the piston in the cylinder so that the test volume is reduced by the stroke volume in the cylinder. Or it is used, preferably in conjunction with a throttle valve, to generate an air flow that is delivered into the dispensing vessel or into the test system, the latter being composed of the dispensing vessel, the connecting line and the relevant parts of the compressed-air system. 
     A further problem addressed by the invention, that of providing a simple method for the dosed dispensing of liquid and for determining the fill level, is solved by the combination of features according to claim  8 . Exemplary embodiments can be found in the claims dependent on claim  8 . 
     The method according to the invention uses the above-described apparatus for dispensing liquid, wherein the apparatus is operated in a dispensing mode in which the dispensing opening is open. In order to determine the fill level of liquid in the dispensing vessel, the apparatus is operated in a test mode in which the dispensing opening is closed. Both in the dispensing mode and in the test mode, the dispensing vessel is pressurized or the pressure is changed. In the dispensing mode, the pressure serves to push liquid out of the dispensing vessel. In the test mode, the pressure change leads to new state variables P 2 , V 2  at an instant t 2 , from which the air volume in the dispensing vessel can then be determined according to equation 1 in comparison to old state variables P 1 , V 1  at an earlier instant t 1 . 
     Preferably, the compressed-air system in the test mode brings about a pressure change in the dispensing vessel. The pressure change may be brought about by a particular volume change which, as described above, is achieved for example by moving the piston in the pneumatic cylinder. The pressure change and/or the pressure in the dispensing vessel is measured. 
     In one exemplary embodiment, a reference pressure change is determined for a reference fill level in the dispensing vessel. By way of example, the reference fill level may be the fill level of a completely empty dispensing vessel. Such a state can then be associated with corresponding pressure change which is then the reference pressure change. When using a full or half-full dispensing vessel, the pressure change then measured can be compared with the reference pressure change. If the measured pressure change is greater than the reference pressure change, preferably taking account of a safety margin of 0.1 to 0.3 bar or a safety factor of 2 to 5%, this permits the conclusion that the dispensing vessel is not yet (completely) empty. The apparatus can in this case continue to be operated. 
     However, if the measured pressure change corresponds to the reference pressure change, or if the measured pressure change is close to the reference pressure change, it must be assumed that the dispensing vessel is completely empty and must be replaced. By way of example, the apparatus may have display means which indicate an excessively low fill level. As an alternative or in addition, a stop signal may be generated in this case. Instead of the reference pressure change, an absolute reference pressure can also be used as the basis. 
     A predetermined value can be predefined for the pressure change or for a pressure in the dispensing vessel, the quantity of air required for the pressure change or for building up the pressure being measured. The larger the quantity of air, the greater the air volume in the dispensing vessel. Detecting the required quantity of air has the advantage that, for an almost empty dispensing vessel, relatively large values are measured for the required quantity of air. The measurement accuracy thus increases as the liquid volume or fill level decreases. This enables relatively precise information regarding the fill level for a completely or almost completely empty dispensing vessel. It is also possible to predefine a value for the quantity of air to be supplied and then to measure the resulting pressure change. However, this can lead to measurement inaccuracies since small or smaller pressure changes are to be expected for a completely empty dispensing vessel. 
     For a reference fill level, a reference quantity of air can be determined in the dispensing vessel in the context of a reference measurement, a measured quantity of air being compared with the reference quantity of air. Here, too, the reference fill level may be the fill level of an (almost) completely empty dispensing vessel (for example 1 to 3% of the total volume of the dispensing vessel. For such a fill level, the quantity of air for generating a particular pressure change in the dispensing vessel or a particular pressure therein is determined. When the quantity of air required to obtain the predetermined value for the pressure change or the pressure is then determined for a partially filled dispensing vessel, this average quantity of air can be compared with the reference quantity of air. As long as the measured quantity of air is less than the reference quantity of air, the fill level is greater than the fill level at the time of performing the reference measurement. 
     In one exemplary embodiment, the apparatus is operated alternately in the dispensing mode and then in the test mode. A dispensing interval or a block of two, three or more dispensing intervals is thus always followed by a test interval. If in each case a particular quantity of liquid is to be dispensed in a dispensing interval (setpoint value), the test interval following the dispensing interval is used to determine how large a quantity of liquid has actually been dispensed in the dispensing interval (actual value). To this end, the fill level in the dispensing vessel at the end of the dispensing interval is compared with the fill level in the dispensing vessel at the start of the dispensing interval. As the fill level at the start of a dispensing interval n, use can be made of the fill level at the end of a preceding dispensing interval n−1. By comparing the actual value with the setpoint value, quality control can be carried out for each individual dispensing interval. If, for example in the context of series production, a particular quantity of adhesive is applied to a component by the apparatus according to the invention in one dispensing interval, it is possible in the subsequent test mode to make a decision as to whether said component should be rejected on account of an excessively large difference between the setpoint value and the actual value. For a dispensing vessel that is being emptied, the comparison of setpoint value to actual value can also be used to track the pressure by which the adhesive is being pressed out of the dispensing vessel. For example, the pressure can be raised if the actual value is moving increasingly further away from the setpoint value as the dispensing vessel empties. 
     Regardless of the above-described comparison of setpoint value and actual value of a dispensing interval, different pressures can be applied to the dispensing vessel in the dispensing mode depending on the fill level. For example, as the fill level decreases, the pressure can be increased via a function that has been determined beforehand and then stored. To this end, the fill level can be determined at regular intervals in the test mode. By virtue of a higher pressure, it is possible to compensate for a certain temporal delay in the dispensing of liquid in response to the pressurization of the dispensing vessel. The greater the air volume in the dispensing vessel, the softer and less precise is the dispensing behavior of the apparatus. This effect can be compensated by increasing the pressure with which the liquid is pushed out of the dispensing vessel. 
    
    
     
       The invention will be explained in greater detail with reference to the exemplary embodiments shown in the drawing, in which: 
         FIG. 1  shows a block diagram for a first exemplary embodiment of the apparatus according to the invention; 
         FIG. 2  shows a block diagram for a second exemplary embodiment, and 
         FIG. 3  shows a block diagram for a third exemplary embodiment. 
     
    
    
       FIG. 1  shows a simplified block diagram for a first exemplary embodiment of the invention. A dispensing vessel  10 , which is air-tight and rigid, is partially filled with a liquid. A fill level line  11  indicates the fill level of the liquid within the dispensing vessel  10 . Air is located above the fill level line  11 , and the liquid is located below said line. An air-filled volume V L  (air volume) and a liquid-filled volume V F  (liquid volume) are thus obtained in the dispensing vessel  10  depending on the fill level (see fill level line  11 ). While the volumes V L  and V F  depend on the fill level in the dispensing vessel  10  and are therefore variable, the sum of the two volumes V L , V F  is constant and corresponds to a total volume of the dispensing vessel V G . 
     When the dispensing vessel  10  is in the use position shown in  FIG. 1 , a dispensing opening  12  for the liquid is provided at a lower end of the dispensing vessel. A shut-off valve  13  is assigned to the dispensing opening  12 . The dispensing opening  12  can be opened and closed by the shut-off valve  13 . 
     A compressed-air port  14  is provided at an end opposite the dispensing opening  12 . Connected to said compressed-air port  14  is a connecting line  15  which connects the dispensing vessel  10  to a compressed-air system  16 . In the exemplary embodiment shown here, the compressed-air port  14  and the dispensing opening  12  are arranged diametrically to one another, which is not absolutely necessary. It is sufficient if the dispensing opening  12  is positioned in such a way that the liquid is in front of this dispensing opening  12  and dispensing without air is possible. In the present case, gravity ensures this. 
     When the air-tight dispensing vessel  10  is pressurized by the compressed-air system  16  via the connecting line  15  and the compressed-air port  14 , liquid is pressed out of the dispensing vessel  10  through the dispensing opening  12  and the open shut-off valve  13 . By way of example, the vessel  10  may be a glue cartridge containing PUR hot-melt adhesive. Hot-melt adhesive can thus be applied by the apparatus to components or surfaces to be bonded. The dispensing vessel  10  must be kept at a temperature such that the hot-melt adhesive remains liquid. It may thus have heating means or connections for a heating medium for heating the liquid in the dispensing vessel. 
     The compressed-air system  16  has a first switching valve  17  which is configured as a 3/2-way valve. The switching valve  17  can be switched into a first switching position and into a second switching position.  FIG. 1  shows the first switching position, which corresponds to a spring-loaded rest position of the first switching valve  17 . This rest position occurs when no signal current is present at the first switching valve (“normally closed”). In the rest position, a first inlet  18  is connected to an outlet  19 . In the second switching position, the first inlet  18  and the outlet  19  are isolated from one another. In this case, in the nomenclature of the block diagram, the outlet  19  is connected to a second inlet  20  of the first switching valve, the second inlet  20  being configured as a blind inlet. In fact, in the second switching position, the first switching valve is thus closed so that no air can escape through the outlet  19  via a node point  21 . 
     A manually adjustable pressure regulator  22  is disposed upstream of the first inlet  18  of the first switching valve  17 . A pressure P M , which is provided by a pressure supply  24 , is applied to an inlet  23  of the pressure regulator  22 . From the main pressure P M , the pressure regulator  22  generates an adjustable pressure P E . Via a (pressure) line  25 , which connects the outlet  24  of the pressure regulator  22  to the first inlet  18  of the first switching valve  17 , this pressure P E  can be switched to the dispensing vessel  10  by way of the first switching valve  17 . When the shut-off valve  13  is open, liquid is thus pushed out of the dispensing vessel  10  through the dispensing opening  12 . If the dispensing of liquid is to be interrupted, the shut-off valve  13  is closed. 
     The compressed-air system  16  has a second switching valve  26 . This switching valve  26  is also configured as a 3/2-way valve. A first inlet  27  of the second switching valve  26  is connected to the pressure supply  24 . An outlet  28  of the second switching valve  26  can be depressurized via a second inlet  29  when the second switching valve  26  is in the switching position shown in  FIG. 1  (“normally open”). This is a first switching position or a spring-loaded rest position. When a signal current is present, the second switching valve  26  switches into a second switching position, in which the first inlet  27  is connected to the outlet  28 . The main pressure P M  is thus applied to the outlet  28  of the second switching valve  26 . 
     Also provided is a pneumatic cylinder  40  which is disposed downstream of the second switching valve  26 . The cylinder  40  has an inlet  30  and an outlet  31 . If the main pressure P M  is switched to the inlet  30  of the cylinder  40  by way of the second switching valve  26 , a piston  32  of the cylinder  40  pushes the air located in the cylinder  40  into the line  32  via the outlet  31 . If it is assumed that the cylinder volume V Z  corresponds to the volume that can be pushed out of the cylinder  40  by the piston, the remaining cylinder volume is zero in an upper dead center of the piston  32 . 
     Connected to the pressure line  33  is a pressure sensor  34 , by which the pressure in the pressure line  33  and thus also in the dispensing vessel  10  can be measured. 
     The apparatus can be operated in a dispensing mode and in a test mode. In the dispensing mode, the shut-off valve  13  is open. The switching valves  17 ,  26  are in the switching positions shown in  FIG. 1 . Liquid is pushed out of the dispensing vessel  10  via the dispensing opening  12  by the pressure P E  generated by the pressure regulator  22 . By switching the first switching valve  17 , the dispensing can be timed. For example, if the first switching valve  17  is in the open first switching position for  10  seconds, then liquid will be dispensed from the dispensing vessel  10  for these  10  seconds. 
     In the test mode, the first switching valve  17  is in the second switching position, in which the outlet  19  is closed. The shut-off valve is closed. At an instant t 1 , at which the piston  32  is in the position shown in  FIG. 1 , a pressure P 1  is determined by the pressure sensor  34 . At this instant, a test volume V 1  of a test system is composed of the air volume V L  in the dispensing vessel  10  and the cylinder volume V Z  in the cylinder  40 . The volumes V 15 , V 33  of the lines  15 ,  33  or of all line sections located between the cylinder  40  and the dispensing vessel  10  must also be taken into account. The second switching valve  26  is then brought into the second switching position so that the piston  32  pushes the volume V Z  out of the cylinder  40 . At the end of the movement of the piston  32  at an instant t 2 , a new pressure P 2  thus exists in the dispensing vessel, this new pressure being greater than the pressure P 1  since the test volume of the test system is now smaller. The volume V 2  at instant t 2  corresponds to the volume V 1  minus V Z . According to the general gas equation (see equation (1)): 
       ( V   L   +V   Z   +V   33   +V   15 )· P   1 =( V   L   +V   33   +V   15 )· P   2    (2)
 
     where V L  air volume in the dispensing vessel; 
     V Z  cylinder volume; 
     V 33  volume of the pressure line  33 ; 
     V 15  volume of the connecting line  15 ; 
     P 1  pressure at the instant t 1 ; 
     P 2  pressure at the instant t 2 . 
     From equation 2, the air volume V L  can be calculated by transformation. Knowing the total volume V G  of the dispensing vessel  10 , the quantity V F  or the fill level to be determined can be obtained directly from the air volume V L . 
       FIG. 2  shows a block diagram for a further exemplary embodiment. Components or features which are similar or identical to components or features of  FIG. 1  are provided with the same reference signs. This also applies to the exemplary embodiment shown in  FIG. 3 . 
     The basic structure of the apparatus shown in  FIG. 2  corresponds to the structure of the apparatus  1 . Reference is therefore made to what has been stated in respect of  FIG. 1 . Instead of the cylinder  40  shown in  FIG. 1 , a throttle valve  35  having an inlet  36  and an outlet  37  is disposed downstream of the second switching valve  26 . In the first switching position of the second switching valve  26 , as is also the case in the exemplary embodiment of  FIG. 1 , the outlet  28  is connected to the second inlet  29 . However, the outlet  28  is not depressurized as a result but rather is closed in an airtight manner. 
     Provided in addition to the pressure sensor  34  is an air quantity sensor  38  which measures the quantity of air flowing through the compressed-air line  33 . The line  33  connects the outlet  37  of the throttle valve  35  to the connecting line  15 . 
     The structure differing from the first exemplary embodiment has essentially no effect on the operation of the apparatus of  FIG. 2  in the dispensing mode. In other words, the second exemplary embodiment does not differ from the first exemplary embodiment with regard to use in the dispensing mode. In the test mode, on the other hand, the test system is reduced by a predetermined volume V Z , but a particular quantity of air, which is detected by the air quantity sensor  38 , is supplied to the test system. The additional quantity of air leads to a particular pressure increase. Again, the state variables are determined before (instant t 1 ) and after (instant t 2 ). The less the dispensing vessel  10  is filled with the liquid, the greater the quantity of air that must be supplied to the test system in order to achieve a particular pressure increase. The quantity of air required for this is thus a measure for the air volume V L  in the dispensing vessel and the fill level in the dispensing vessel. Using the following equation, which is once again based on the general gas equation, the fill level can be determined as a function of the measured quantity of air: 
         m   D   ·R   S   ·T =( V   L   +V   15   +V   33 )·( P   2   −P   1 )   (3)
 
     where m D  quantity of air supplied in time interval between t 1  and t 2 ; 
     T temperature of the supplied quantity of air; 
     R S  specific gas constant; 
     V 33  volume of the pressure line  33 ; 
     V 15  volume of the connecting line  15 ; 
     P 1  pressure at the instant t 1 ; 
     P 2  pressure at the instant t 2 . 
     Compared to the exemplary embodiment of  FIG. 1 , the exemplary embodiment of  FIG. 2  has the advantage that, in order to achieve a predetermined pressure increase or a pressure P 2 , the quantity of air required for this is comparatively large in the case of an almost empty or completely empty dispensing vessel  10 . The measurement accuracy thus increases as the fill level decreases. This is advantageous when the accurate and reliable determination of the fill level of (almost) completely empty dispensing vessels is of particular importance. 
     In the exemplary embodiment of  FIG. 3 , the functions which are fulfilled by the switching valves  17 ,  26  and the throttle valve  35  in the exemplary embodiment of  FIG. 2  are taken over by a proportional valve  39 . When the apparatus is in the dispensing mode, that is to say the shut-off valve  13  is open, the proportional valve  39  provides in the dispensing vessel  10  the pressure that is necessary for dispensing the liquid. However, the proportional valve  39  can also be used in the test mode, in which the shut-off valve  13  is closed. In this case, it supplies an additional quantity of air, which is measured by the air quantity sensor  38 , to the pneumatic test system (here: line  33 , connecting line  15  and dispensing vessel  10  having the test volume V 33 +V 15 +V L . Since with the proportional valve  39  there is the possibility of precisely defining a target pressure value, there is no need for a separate pressure sensor  34 . As also in the exemplary embodiment of  FIG. 2 , the quantity of air required for a pressure increase is measured in order to determine the fill level. 
     The test volume of the test system (air-filled portion of the dispensing vessel  10 , connecting line  15  and line  33 ) can be 1 to 2000 ml, preferably 60 to 350 ml. The cylinder volume V Z  can assume values of 1 to 2000 ml. A preferred range for V Z  extends from 12 to 70 ml. The pressures P 1  and P 2  can be 0.1 to 12, preferably 0.2 to 5 bar. The pressure change P 2 −P 1  brought about by reducing the test volume by the cylinder volume V Z  or by the supplied quantity of air can assume values of 0.02 to 5 bar. The supplied quantity of air can be between 80 and 0.25 mg, preferably between 40 and 0.28 mg. The temperature in the dispensing vessel can be 10 to 200, preferably 20 to 180 or 100 to 170° C. 
     LIST OF REFERENCE SIGNS 
     
         
           10  dispensing vessel 
           11  fill level line 
           12  dispensing opening 
           13  shut-off valve 
           14  compressed-air port 
           15  connecting line 
           16  compressed-air system 
           17  first switching valve 
           18  first inlet 
           19  outlet 
           20  second inlet 
           21  node point 
           22  pressure regulator 
           23  inlet 
           24  outlet 
           25  (pressure) line 
           26  second switching valve 
           27  first inlet 
           28  outlet 
           29  second inlet 
           30  inlet 
           31  outlet 
           32  piston 
           33  (pressure) line 
           34  pressure sensor 
           35  throttle valve 
           36  inlet 
           37  outlet 
           38  air quantity sensor 
           39  proportional valve 
           40  cylinder