Patent Publication Number: US-7896102-B2

Title: Pneumatic drive system

Description:
This application claims priority based on an International Application filed under the Patent Cooperation Treaty, PCT/EP2006/001538, filed Feb. 21, 2006. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a pneumatic drive system comprising at least one pneumatic drive having a drive housing and an output drive unit able to be shifted in relation to it by the action of compressed air, said output drive unit possessing an output piston, which in the drive housing divides two working chambers from each other, of which one or both is connected with control valve means serving for the controlled action of compressed air, said control valve means being able to be switched over between several switching settings which include an air economy setting defining a choke cross section. 
     A pneumatic drive system of this type as disclosed in the patent publication WO 02/14698 A1 is employed for crust breaking applications in aluminum processing. The system comprises a pneumatic drive designed as a crust breaker cylinder, whose output drive unit is able to be driven to perform oscillating working movements, said unit while jabbing through any crust layer, which may have accumulated, being dipped into a bath of molten aluminum for some time. A direction setting valve is responsible for the respective working movements since it controls the supply and venting of compressed air into and from two working chambers separated from each other by the output piston of the output drive unit. 
     Furthermore double control valve means act on the control means for the compressed air action and are placed on the connection between the direction setting valve and a respective working chamber. These control valve means may assume different switching settings, one switching setting being responsible for causing the working movement by freeing an air passage. In order to minimize use of air this switching setting is designed as an air economy setting since the fluid passage has a choke cross section which only permits a limited passage through it. Accordingly the degree of filling of the connected working chamber remains at the lowest possible level. 
     If the output drive unit strikes aluminum crust and is therefore subjected to a greater resistance to motion, via the choke cross section an increasing actuating pressure will be gradually built up in the connected working chamber to greater extent until the necessary penetrating force is reached. On arriving in the end of a stroke the output drive unit finally causes a switching over of the control valve means into a locked position in order to avoid further flow of compressed air into the pneumatic drive. 
     Owing to the time necessary for the build up of pressure in the pneumatic drive, when the output drive unit has to penetrate crust on the melt, there are irregular delays in time in the individual working cycles from case to case. 
     A similar arrangement is described in the European patent publication EP 0771396 B1. In this case there is also the description of an alternative design with the possibility of doing without a choke in the control lines. This however entails a continuous intense action of compressed air pressure in the working chambers, something which is a disadvantage as regards the consumption of compressed air. 
     SUMMARY OF THE INVENTION 
     An important object of the present invention is to suggest measures which allow a reduction in the cycle time without any inordinately increased use of compressed air. 
     In order to achieve this aim there is a provision such that as a further switching setting the control valve means have a high power setting which sets a flow cross section which is larger than the choke cross section and that the control valve means have actuating means, which during the supply of compressed air into one working chamber so control the switching over of the control valve means connected this working chamber in a fashion dependent on the air pressure obtaining in at least one working chamber that a switching over takes place from the normally assumed air economy setting into the high power setting, when and at least as long as the output drive unit is subjected to a increased resistance to motion. 
     Accordingly the output drive unit will move as long in the air economy setting as it is not subject to an increased resistance to movement. Owing to the choke cross section then effective the degree of filling of the connected working chamber is restricted to a minimum and consequently the air requirement is also limited. As soon however as an increased resistance to motion applies for the output drive unit, the control valve means responsible for air supply to the respective working chamber will switch back, owing to the change in pressure occurring in the pneumatic drive, into the high power setting and will render possible a more rapid air inflow with an increased flow cross section and accordingly a quick increase in pressure in the connected working chamber. This leads to an increase in the setting force and overcomes the resistance to motion opposing the output drive unit. Following a reduction in the resistance to motion the control valve means may possibly switch back to the air economy setting. Therefore the requirement for a relatively large amount of air only occurs as from or during the operational phase, in which a higher actuating pressure is in fact necessary. In other respects the requirement for air will remain at the choked normal level. Simultaneously the cycle times are also reduced, because the air filling time is substantially shorter in the high power setting than in the air economy setting always maintained in the prior art. 
     The advantages as described turn out to be quite considerable, if the pneumatic drive system is employed as a crust breaker system in aluminum production or, respectively, processing. Owing to the short operating cycle time the saving of air has proved to be immense. Simultaneously there is if required an increased setting force with only a short delay in order for example to break through an aluminum crust or to strip off solidified aluminum material fouling the output drive unit. Owing to the pressure-controlled actuation there is furthermore the advantage that the build up of pressure in the working chamber responsible for the current working movement occurs in a manner varied to suit the level of the resistance to movement, which is to be overcome. It is therefore possible to ensure that in the high power or high force phase there is always sufficient compressed air in the pneumatic drive as is required for overcoming the resistance to movement which is just current. 
     Further advantageous developments of the invention are defined in the dependent claims. 
     Although the principle of the invention may also be applied in rotary and pivotal drives, its employment in linear drives is more especially advantageous. 
     In the case of the at least one linear drive it is preferably a question of a pneumatic cylinder with a piston rod, which is able to be utilized as a crust breaking cylinder. Use is however not restricted to crust breaker applications. 
     The actuating means for the control valve means are in particular so designed that they control the switch over operation in a fashion dependent on the air pressure obtaining in the working chamber connected with the control valve means. 
     On encountering a resistance to motion such air pressure rises and will cause switching over from the air economy setting to the high power setting. 
     The switching setting of the control valve means is preferably determined by the currently assumed setting of a control valve member of such control valve means. This member is preferably urged toward the air economy setting by the input pressure present (at the input) at the control valve means. The output pressure obtaining at the output side of the control valve means, i.e. on the side of the connected working chamber, acts on the control valve member in the direction opposite to the high power setting. There are also spring means effective in this direction. When the force of the spring means and the setting force resulting for the output pressure are in all greater than the setting force resulting from the input pressure, switching over to the high power setting takes place. If the setting force of the spring means is able to be adjusted or varied, there is the possibility of individual setting of the switch over threshold. 
     The spring means preferably serve to ensure that in the pressure-less state the control valve means assume the high power setting. If—more particularly via an upstream direction setting valve—the operating pressure is increased, it is possible using a choke means placed on the actuating duct tapping the output pressure, to cause a delayed build up of the setting force resulting from the output pressure so that the control valve means immediately assume the air economy setting. 
     A further advantage may be produced, if the control valve means have a third switching position, in which the compressed air goes through a flow cross section smaller than the choke&#39;s cross section. This switching setting will be termed the hold setting, because it takes effect to hold the output drive unit in its end of stroke position. The hold setting of the control valve means takes effect in a manner dependent on the position of the output drive unit, when the latter gets near or reaches the end of stroke position. Switching over may be caused mechanically, for example owing to a plunger-like setting member cooperating with the output drive unit, or however also electrically using suitable position sensor means. The reduced flow cross section in the hold setting avoids an excessive filling of the connected working chamber compensates simultaneously for any leakage so that the output drive unit is held fast and does not perform any oscillating movements. A design is considered to be optimum in which the flow cross section left free in the hold setting has a size, which taking into consideration the operating pressure present, sets a flow rate which is substantially equal to the leakage in the pneumatic drive. Accordingly the degree of filling with air in the connected working chamber does not increase or only slightly increases, although the air connection is not closed down, as is mandatory in the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following the invention will be explained with reference to the accompanying drawing. The single FIGURE ( FIG. 1 ) shows the pneumatic drive system as a simplified circuit diagram in a preferred embodiment, which is more particularly but not exclusively suitable for crust breaker applications. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The pneumatic drive system generally referenced  1  comprises at least one pneumatic drive  2  which may conveniently be a linear drive. It is provided with a control means generally referenced  3  serving for control during operation. 
     The pneumatic drive  2  is in principle of any desired construction. For instance it could be in the form of a linear drive without a piston rod. For instance it can be a pneumatic cylinder having a piston rod  4 . 
     The pneumatic drive  2  includes a housing which is termed the drive housing  5  and has a certain longitudinal extent, in whose internal space a linearly sliding drive piston  6  is located, which is combined with the above mentioned piston rod  4  to constitute a moving unit termed a output drive unit  7 . This output drive unit  7  is able to be shifted longitudinally to perform either an outward or an inward working movement  8   a  and  8   b  in relation to the drive housing  2  linearly. 
     The internal space unit drive housing  5  is divided up by the output drive piston  6  into a first rear working chamber  12  and a second front working chamber  13  having the piston rod  4  extending in it. 
     The first working chamber  12  is joined to a first fluid control line  14 , and the second working chamber  13  is connected with a second fluid control line  15 . These two control lines  14  and  15  are also a component of the control means  3  like a direction setting valve  16 , with which the two control lines  14  and  15  are connected at their ends remote from the pneumatic drive  2 . 
     By way of the direction setting valve  16  the action of compressed air in the two working chambers  12  and  13  may be controlled in order to cause the currently desired working movement  8   a  and  8   b  of the output drive unit  7 . The direction setting valve  16  may connect, dependent on the switching setting assumed by it, either the one ( 14 ) or the other ( 15 ) control line with a compressed air source  17 , while it simultaneously vents the respectively other control line  14  and  15  to the atmosphere  18 . The source  17  of compressed air supplies compressed air at a certain operating pressure. 
     The direction setting valve is in the example a 5/2 directional valve. It is biased by a spring means  22  into a home position as shown in  FIG. 1 , in which the second control line  15  is connected with the compressed air source  17  and the first control line  14  is vented. By means of an electrical or electromagnetic actuating device  23  the direction setting valve  16  may be switched over into the opposite switching setting. 
     The direction setting valve  11  may as such be a directly operated or pilot valve. For producing the desired functionality, it may be also made up of several functionally linked individual valves, as for example two 3/2 directional valves. 
     In a preferred embodiment of application the pneumatic drive  2  is designed in the form of a crust breaker cylinder. In this respect a hammer element  24  is arranged on the end portion, located outside the drive housing  5 , which is suitable for breaking through a crust on the surface of an aluminum melt or some other molten metal bath. In this case the pneumatic drive  2  is typically arranged with its longitudinal direction vertical and with the piston rod  4  extending downward. With the output drive unit  7  retracted—this condition is indicated in FIG.  1 —the hammer element  24  assumes a position some distance clear of the material crust. For penetrating the crust the output drive unit  7  is driven to perform its extending working movement  8   a , it dipping into the aluminum melt with the hammer element  24  to the fore so that it breaks through any crust present. 
     First and second control valve means  25  and  26 , which operate separately, are connected with the two working chambers  12  and  13 . The first control valve means  25  are placed on the first control line  14  and the second control valve means  26  are placed on the second control line  16 . Supplementing the direction setting valve  16  they render possible a particular form of controlled compressed air actuation of the respectively connected working chamber  12  and  13 . 
     The control valve means  25  and  26  respectively possess a valve input  27  connected with the direction setting valve  16  and a valve output  28  connected with the working chamber  12  and  13  to be controlled. 
     The two control valve means  25  and  26  are able to be switched over between different switching settings. In this respect the two control valve means  25  and  26  may alternatively assume a high power setting  29   29 , an air economy setting  30  and a hold setting  31 . The FIGURE shows an operating state in which the first control valve means  25  is in the high power setting and the second control valve means  26  is in the air economy setting. 
     Preferably the two control valve means  25  and  26  are respectively constituted by a control valve, which has a control valve member  32  able to be selectively set in one of three positions, such valve being purely diagrammatically indicated in the drawing. In the case of the control valve member  32  it may be a question of a piston slide or spool for example. 
     All three switching settings share the feature that they open up a compressed air connection between the direction setting valve  16  and the working chamber  12  and  13  connected therewith. The only difference is the size of the flow cross section made available. The passage of air is not completely shut off in any switching setting. 
     The flow cross section cleared in the air economy setting  30  will be termed the choke cross section. It is smaller than the rated cross section of the respectively connected control line  14  and  15  and causes a choking of the compressed air flowing through it. If the output drive unit  7  can therefore move without hindrance, there is therefore an output pressure at the valve output  28  which is lower than the supplied operating pressure, such output pressure being present as the current operating pressure in the connected working chamber  12  and  13  as well. 
     The flow cross section made available in the high power setting  29  is larger than the choke cross section. It renders more especially possible an unchoked access of air and preferably corresponds to the rated cross section of the control lines  14  and  15 . 
     The minimum flow cross section is provided in the hold setting  31 . This cross section is even substantially smaller than the choke cross section effective in the air economy setting  30  to be described infra. The two control valve means  25  and  26  are provided with functioning first and, respectively, second actuating means  36  and  37 . They are responsible for seeing that the associated control valve means  25  and  26  assume the high power setting  29  or the air economy setting  30 . Switching over into the hold setting  31  cannot on the other hand be caused by them. 
     For the switching over into the hold setting  31  first and second further actuating means  38  and  39  are responsible, which in contradistinction to the completely pressure dependent first and second actuating means  36  and  37  are preferably activated or deactivate completely dependently on the linear position of the output drive unit  7 , and they have priority over the first and the second actuating means  36  and  37 . When the output drive unit  7  reaches a position relevant for the switching over into the hold setting  31 , the switching over operation will occur irrespectively of whether the control valve means  25  and  26  have so far assumed the high power setting  29  or the air economy setting  30 . 
     The first and the second actuating means  36  and  37  are in a position of controlling the switching over of the associated control valve means  25  and  26  in a manner dependent air pressure obtaining in at least one working chamber. The control is on the basis in particular of the pressure which currently obtains in the working chamber  12  and  13  and which in the present case is the same as the output pressure obtaining at the valve output  28 . The design is best such that the normally, when the output drive unit  7  is able to move freely, the air economy setting  39  is assumed and starting at this point switching over takes place into the high power setting  29 , when the output drive unit  7  in the course of its working motion  8   a  and  8   b  is subject to a higher resistance to motion and accordingly the working pressure then obtaining in the working chamber  12  and  13  just being supplied with compressed air rises to a predetermined switch over threshold. 
     In order to render this switching over operation possible in a particularly simple manner, each respective control valve member  32  is in the working example provided with two oppositely aligned first and second air action faces  42  and  43 . 
     Action of compressed air on the first actuating face  42  produces a setting force toward the air economy setting and action of air on the second air action face  43  produces a setting force toward the high power setting  29 . 
     The first compressed air actuating face  42  is subjected to the input pressure present at the valve input  27  via a first actuating duct. Via a second actuating duct  45  the second compressed air actuating face  43  is subjected to the output pressure obtaining at the valve output  28 . In addition spring means  46  are present, which exert a setting force also effective toward the high power setting on the control valve member  32 . 
     The setting force of the spring means  46  is preferably adjustable, something which is indicated symbolically by an oblique arrow. 
     On the second actuating duct  45  there is preferably a choke means  47 , which causes a time delayed build up of pressure in the second air actuating face  43 . 
     Ignoring the hold setting  31  for the present, more particularly the operation in steps of the pneumatic drive system  1  is possible as explained in the following. 
     The explanation will begin in a home position with the output drive unit  7  drawn as far as possible into the drive housing  5  and with the system in s pressureless state. In this case the two control valve means  25  and  26 —if the further actuating means  38  and  39  were not present—are held by the force of the spring means  46  in the high power setting  29  allowing the maximum flow rate. 
     Starting at this point the direction setting valve  16  is switched over into the second switching over position (not illustrated) with the compressed air source  17  turned on so that the first control line  14  receives compressed air at the operational pressure level and simultaneously the second control line is vented. The compressed air entering through the first control line  14  will flow through the first control valve means  25  located in the high power setting and will act on the output drive unit  7  in the extension direction so that the unit is driven to perform the outward working movement  8   a.    
     The compressed air then expelled by the working piston  6  from the second working chamber  13  then passes by way of the second control valve means  26  held by the spring means  46  in the high power setting also allowing unrestricted passage of air and through the following direction setting valve  16  to the atmosphere  18 . Since atmospheric pressure obtains in the control line  15 , the switching setting of the second control valve means  26  is not affected during the venting phase. 
     Directly after the supply of air into the first control, line  14  the first control valve means  25  switch over into the air economy setting  30 . This is because the operating pressure so far obtaining in the entire first control line  14  is able to act on the first air actuating face  42  without limitation, at the second air actuating face  43  however owing to the intermediately placed means  47  initially only a low actuating pressure obtains. The design is such that the thrust force applied by way of the first air actuating face  42  toward the air economy setting is greater than the sum of the thrust force acting at the second air actuating face  43  and the setting force of the spring means  46 . 
     Following the switching over into the air economy setting  30  owing to the choke cross section now effective there is an output pressure lower than the output pressure, such output pressure also taking effect in the connected first working chamber  12  where it is responsible for the advance motion of the output drive unit  7 . 
     Even if after a certain time a constant actuating pressure should obtain in the entire second actuating duct, the first control valve means  25  will dwell in the air economy setting, because the design of the first and the second actuating means  36  and  37  is such that the above mentioned actuating pressure corresponding to the valve output pressure to a maximum extent together with the spring means  46  can exert a setting force to a maximum extent, which is under the opposite actuating force on the basis of the valve input pressure. 
     As long as the output drive unit  7  does not strike any obstacle, it is extended with the reduced output pressure of the first control valve means  25 , the degree of filling of the first working chamber  12  being relatively low in accordance with the low output pressure. 
     When the output drive unit  7  reaches its stroke position at maximum extension, by switching over the direction setting valve  16  it is possible to cause a reversed progression of motion, the second control valve means  26  behaving like the first control valve means  25  did previously and the first control valve means  25  previously behaving like the second control valve means  26  did. 
     However the operational behavior does change when the output drive unit  7  is subjected to a greater resistance to movement during the one or the other working movement  8   a  and  8   b . During extension this may be because the hammer element  24  of the output drive unit  7  strikes material crust to be penetrated. During retraction such a resistance may be entailed by solidified materials from the melt, which cling to the extended end section of the piston rod  4 . 
     In the case of such an operating stage the working pressure in the working chamber  12  or  13  presently subject to compressed air will increase. The speed of the increase in pressure is dependent on the size of the choke cross section for the air to flow through. 
     Since the working pressure effective in the operated working chamber  12  or  13  acts via the second actuating duct on the control valve member  32  as well, the actuating force, effective in the high power setting direction, will at some time exceed the opposite actuating force effective at the first air actuating face  42 . The switch over threshold force responsible for the time of switch over may be influenced and set by mutual matching of the area sizes of the two air actuating faces and the setting force of the spring means  46 . 
     In the case of a typical application an operating pressure may be 6 bar, this meaning a working chamber pressure of 2 bar in the air economy setting, the switch over threshold for switching over into the high power setting lying at a working chamber pressure of approximately 2.5 bar. 
     Owing to the switching over into the high power setting the supplied compressed air has larger flow cross section available for it. Accordingly the working pressure obtaining in the connected working chamber  12  or  13  rises in a short time to a maximum equal to the operating pressure supplied to the control valve means so that the output drive unit  7  is acted on by a considerable fluid setting force, on the basis of which it is able to overcome the resistance to movement, i.e. in the present case for example the material crust to be penetrated. 
     As soon as the output drive unit  7  may be moved with a lower resistance again, as a rule the pressure in the working chamber will drop again owing to the dynamics of the system so that at the control valve means  32  a new resulting actuating force will become established tending to switch over into the air economy setting and there will be a corresponding switching back into the air economy setting  30 . 
     Even if the control valve means  25  and  26  after switch over to the high power setting cannot be switched into the air economy setting owing to the dynamics of the system while the continued working movement, there is even so a considerable air economy effect, because the switching into the high power setting for the individual working movements always only takes place, when an increased resistance to movement occurs. In many cases this will not be the case so that operation taking full advantage of the air economy function is possible. 
     Further advantages are possible if the control valve means  25  and  26  render possible the above mentioned additional switching over into a hold setting  31 . 
     For this purpose the further actuating means  38  and  39  are so designed that they shift the control valve means  25  or  26 , which currently serve for the supply of compressed air into a working chamber  12  or  13 , into a hold setting, which renders possible a reduced flow rate, when the output drive unit  7  reaches an end of stroke position or a position just short of the end of a stroke. Owing to this position-dependent switching over it is possible to ensure that in the end of stroke positions, when the output drive unit  7  is unable to move farther, the compressed air can flow at a further-reduced flow rate into the connected working chamber  12  or  13  as long as the direction setting valve  16  is not switched over. 
     Owing to the constantly maintained action of compressed air it is possible to achieve the major advantage over a complete turning off that leakages occurring in the system are compensated for and the air pressure existing in the pressurized working chamber normally never falls below a value permitting motion of the output drive unit  7  in relation to the drive housing  5 . 
     This is more particularly relevant in the case of employment as a crust breaker cylinder, when it is a question of securely locking the retracted and therefore elevated output drive unit  7  and preventing even the least downward movement. 
     Preferably the flow cross section of the control valve means  25  and  26 , which is open in the hold setting  31 , is so related to the acting operating pressure that the permissible flow rate is at least essentially equal to the leakage occurring in the pneumatic drive  2 . Preferably the permitted flow rate is at least equal to or slightly above the leaked flow occurring, which for example takes place between the output piston  6  and the drive housing  5 . 
     For the detection of that axial position of the output drive unit  7 , at which the switching over of the control valve means  25  and  26  into the hold setting  31  is to be caused, the further actuating means  38  and  39  are fitted with suitable responsive means  48  and  49 . These responsive means  48  and  49  are located preferably on or in the drive housing  5 , and in the particular working example are designed to produce a purely mechanical switch over of the control valve means  25  and  26 . 
     For the purpose of mechanical activation they preferably include in each case at least one plunger-like setting member  48   a  and  48   b  which so extends into the path of motion of the output drive unit  7  that it is struck and shifted by it on reaching the desired switching over position. 
     Preferably the responsive means  48  and  49  are direct components of the control valve means  25  and  26 . This opens up the particularly advantageous possibility of installing the control valve means  25  and  26  directly on or in the drive housing  5 , as is indicated in chained lines in  FIG. 1 . For reasons clarity of the drawing the control valve means  25  and  26  are illustrated in  FIG. 1  as being separate from the drive housing  5  and the reference numerals  48  and  49  are employed twice to make it clear which responsive means  48  and  49  belong to which control valve means  25  and  26 . 
     A exclusively mechanical switching over offers the advantage that it is possible to do without electrical means. However it would be quite possible to provide the responsive means  48  and  49  in the form of sensors detecting the position of the output drive unit  7  and which on activation produce an electrical sensor signal, on the basis of which an electrical switching over of the control valve means  25  and  26  is caused into the hold setting  31 . 
     At this point it is to be noted that in principle the switching over between the high power setting  29  and the air economy setting  30  can be caused by electrical signals, if the relevant pressure parameters are detected by pressure switches or pressure sensors. 
     In conjunction with the further actuating means  38  and  39  there is, on starting the above mentioned course of operation, a change such that the output drive unit  7  is moved initially briefly at a reduced speed, because the control valve means associated with the working chamber being vented are held in the hold setting  31  by the further actuating means until the output drive unit has cleared the response range of the responsive means  48  and, respectively,  49 . 
     As long as the output drive unit  7  dwells in the response range of the responsive means  48  or  49 , the associated control valve means  25  and  26  assume the hold setting  31  irrespectively of the working pressures obtaining of the working chambers  12  and  13 . The switching setting is in this case set in a fashion dependent on the position of the output drive unit  7 . It is only clear of this response range that the switching of the position of the control valve means  25  and  26  is controlled in a pressure dependent manner between the air economy setting  30  and the high power setting. 
     As already hinted at least the two control valve means  25  and  26  may be designed as a unitary subassembly with the pneumatic drive  2 . Furthermore, the direction setting valve  16  can be a component of this subassembly. 
     The pneumatic drive system  1  may comprise more than the one pneumatic drive  2 , each pneumatic drive then preferably having its own first and second control valve means  25  and  26 . 
     The direction setting valve  15  may on the contrary serve for the simultaneous operation of several parallel-connected pneumatic drives  2 . 
     Departing from the working example the control valve means  25  and  26  associated with the one pneumatic drive  2  may be present in the singular. They are then preferably either on the first control line  14  or on the second control line  15  dependent on which stroke direction is associated with the functionality which is strived at.