Abstract:
The invention concerns a coating plant for the coating of construction units with a coating medium, in particular a paint system for the lacquer finish of motor vehicle body parts, with a dosing pump, the coating medium supplied with a certain delivery (F m ) proportioned, and a pressure control valve arranged upstream before the dosing pump to adjust a coating medium pressure (p v) at the entrance of the dosing pump, as well as a control unit to adjust the pressure control valve a controlled variable of the pressure difference (Δp) through the dosing pump independently of the delivery of the dosing pump and the changing viscosity of the lacquers to an essentially constant desired value (Δp TARGET ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of the provisional patent application 60/778,342 for COATING PLANT AND ASSOCIATED COATING PROCESS, filed on Mar. 2, 2006 which is incorporated by reference herein its entirety. This claim is made under 35 U.S.C. § 119(e); 37 C.F.R. § 1.78; and 65 FR 50093. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention concerns a coating plant for the coating of construction units with a coating medium, in particular a paint system for the lacquer finish of motor vehicle body parts as well as an associated operating procedure in accordance with the operating requirements.  
       BACKGROUND  
       [0003]     From EP 1,287,900 A2 and from “Technical Manual Color Volume Control”, page 32 (1994) of the company DRY a coating plant is well-known including a color print automatic controller and a dosing pump supplying a rotation atomizer with coating medium. Pressure sensors measure the coating medium pressure before and behind the dosing pump and supply an electronic control interacting with the color print automatic controller through a pressure controller valve designed as a proportional valve. With this well-known coating plant, a controlled variable regulates either the initial pressure of the color print automatic controller or the color flow rate.  
         [0004]     Unfavorably the well-known coating plant is however subject to the construction unit wear of the dosing pump and the color print automatic controller, which leads to a short service life of these construction units. This applies in particular to the color print automatic controller, whose sealing ring experiences very strong washing after a certain actual working time.  
         [0005]     Beyond that, dosing inaccuracies can occur with the conventional coating plants, which lead in extreme cases to a lacquer overspray and/or underspray, which expresses itself on the construction units being coated as a coating error.  
       SUMMARY  
       [0006]     One embodiment of the invention is to improve the initially described well-known coating plant.  
         [0007]     This embodiment can include a coating plant and an associated operating procedure.  
         [0008]     The invention is based on the technical realization that the construction unit load of the dosing pump and the color print automatic controller causes strong variations in pressure differentials or drops through the dosing pump during the coating process. This leads to a large positive pressure difference where the pressure before the dosing pump is larger than the pressure behind the dosing pump resulting in a pump slip and a higher discharge rate than desired with small changes of flow rate (so-called “Brushes”). With a negative pressure difference where the pressure before the dosing pump is smaller than the pressure behind the dosing pump, the fluctuations of the pressure difference lead to an interruption of the necessary lacquer volume, which causes the disturbing lacquer downstream errors (i.e. underspray and/or overspray) in the worst case. Beyond that the fluctuations of the pressure difference through the dosing pump contribute also to the unwanted mechanical load of the dosing pump and the color print automatic controller.  
         [0009]     The pressure difference through the dosing pump, and thus disturbing dosing inaccuracies and mechanical loads, are not only affected by the varying quantity of the delivered coating medium. Rather, the pressure difference changes with a change to a coating medium with another viscosity, or with an installation of another color print automatic controller with another pressure speed ratio.  
         [0010]     The invention covers therefore the general technical teaching of keeping as constant as possible the pressure differential through the dosing pump during the coating process independently of the delivered coating quantity, the viscosity of the coating medium, and/or the pressure speed ratio of the assigned color print automatic controller in order to avoid negative effects on the dosing accuracy and the service life of the assigned construction units.  
         [0011]     The coating plant according to an embodiment of the invention includes a control unit, which interacts with the pressure control valve to adjust as a controlled variable the pressure difference through the dosing pump, independently of the delivery of the dosing pump, to an essentially constant desired value. With this embodiment of the invention, the pressure difference through the dosing pump is the controlled variable, whereas with the initially described state of the art the initial pressure of the pressure control valve and/or the color flow rate was regulated.  
         [0012]     A control unit automatically maintains a constant differential pressure through the dosing pump, i.e. without a measurement and a feedback of the actual value of the pressure difference. Controlling the pressure difference, as compared to regulation of the pressure difference, provides favorable results with respect to minimizing oscillation inclination, simple technical realization, fast reaction to pressure jumps and color quantity alterations, and making compensation for dead time of the proportional valve possible.  
         [0013]     The possibility exists in the context of the invention of regulating the pressure difference through the dosing pump by a control unit. This means that the actual value of the pressure difference through the dosing pump is measured and an automatic controller is supplied, which then the color print automatic controller, in order to regulate the pressure difference through the dosing pump to the desired value. An automatic controller can for example be a conventional PID automatic controller, however different automatic controller types can also be applicable in the context of the invention.  
         [0014]     Furthermore the possibility exists of combining a regulation with a control, in the context of the invention, as the regulation overlays, for example with a pilot control, which interconnects the advantages of the regulation on the one hand and the control on the other hand.  
         [0015]     In one embodiment of the invention, the automatic control of constant pressure difference through the dosing pump is a so-called parameter control, i,e. no regulation, so that the actual value of the pressure difference through the dosing pump does not have to be measured,  
         [0016]     For this embodiment a first pressure sensor is used in the example of the invention, which measures the coating medium pressure downstream of the dosing pump, i.e. at the exit of the dosing pump. The measured coating medium pressure at the exit of the dosing pump is supplied to the control unit which interacts with the pressure control valve in such a way to operate as a function of the coating medium pressure measured downstream of the dosing pump according to a given control behavior so that the pressure difference through the dosing pump keeps the desired value constant.  
         [0017]     The control of the pressure control valve by the control unit is made in the example of the invention indirectly by an inserted, connected in series, proportional valve. The proportional valve interacts with the control unit electrically and interacts with the color print automatic controller pneumatically.  
         [0018]     Preferably the control behavior of the control unit is given by an essentially linear control characteristic, whereby the control characteristic defines the connection between the pressure and the interaction with size for the pressure control valve and/or that the resulting pressure measured at the exit of the dosing pump, defines the connection between the pressure and the inserted, connected in series, proportional valve. The linear control characteristic includes an axis intercept value and an upward gradient, whereby the axis intercept value is fixed as a function of the desired value of the pressure difference through the dosing pump and the actual pressure speed ratio of the system from the proportional valve and the color print automatic controller, while the upward gradient of the control characteristic is a function of the speed ratio of the system from the proportional valve and the pressure control valve. To the control characteristic the following formula applies: 
 
 kd=k 1 +k 2 ·p   H  
 
 with 
 
 kd: Control size for the control of the proportional valve 
 
 pH: measured pressure behind the dosing pump 
 
 k 1 : Axis intercept value of the control characteristic 
 
 k 2 : Upward gradient of the control characteristic. 
 
         [0019]     The control parameters k 1  and k 2  of the control characteristic are adjusted as follows, in order to provide the desired pressure difference through the dosing pump:  
         k   ⁢           ⁢   1     =       Δ   ⁢           ⁢     p   Soll       η         
         k   ⁢           ⁢   2     =     1   η         
 
 with 
 
 Δp Soll  Desired value of the difference of pressure over the dosing pump, 
 
 π Speed ratio of the system from the pressure control valve and the upstream proportional valve. 
 
         [0020]     During an adjustment of the control parameters k 1 , k 2  on those values managing independently the desired pressure difference through the dosing pump during the delivery, results from the following formula:  
               Δ   ⁢           ⁢   p     =       p   V     -     p   H                   =       η   ·   kd     -     p   H                   =       η   ·     (       k   ⁢           ⁢   1     +     k   ⁢           ⁢     2   ·     p   H           )       -     p   H                   =         η   ·   k     ⁢           ⁢   1     +       (         η   ·   k     ⁢           ⁢   2     -   1     )     ·     p   H                     =     Δ   ⁢           ⁢     p   Soll                 
 
         [0021]     The accurate adjustment of the optimal control parameters k 1 , k 2  sets the knowledge of the speed ratio π ahead of the system from the proportional valve and the pressure control valve. With an exchange of the pressure control valve or with a change to a coating medium with another viscosity, the speed ratio π is not known, so that the control parameters k 1 , k 2  must be determined. Preferably the determination of the control parameters k 1  and k 2  takes place in the context of an adaptation, whereby also the coating medium pressure is measured and evaluated upstream of the dosing pump. The adaptation of the control behavior takes place then via an adaptation unit, which is connected with the two pressure sensors at the input side and which adapts control behavior of the control unit as a function of the measured coating medium pressure upstream of the dosing pump and the measured coating medium pressure downstream of the dosing pump.  
         [0022]     Preferably the adaptation of the control behavior of the control unit takes place iteratively and/or recursively. An iterative adaptation of the control behavior means that the control behavior, successively in multiple steps, appropriately becomes the optimal control behavior, which is necessary, in order to keep the pressure difference through the dosing pump constant. A recursive adaptation in the sense according to invention means that from the current control behavior of the control unit an improved control behavior is computed in each case.  
         [0023]     The adaptation of the control behavior of the control unit can take place during the normal coating process or in separate adaptation phases. Beyond that the adaptation can take place during the normal coating process constantly or at certain time intervals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:  
         [0025]      FIG. 1  is a schematic representation of a coating plant according to one embodiment of the invention;  
         [0026]      FIG. 2  is a simplified schematic flow diagram of the adaptation procedure according to one embodiment of the invention for the adjustment of the control behavior.  
         [0027]      FIG. 3  is a graph of time versus pressure; and  
         [0028]      FIG. 4  is a graph of pressure verses electrical control signal.  
     
    
     DETAILED DESCRIPTION  
       [0029]     The coating plant according to one embodiment of the invention is represented schematically in  FIG. 1  agrees partly with the initially described state of the art in accordance with EP 1,287,900 A2 and illustrates a conventional atomizer  1 , which is supplied via a volumetric working dosing pump  2  with lacquer whereby the dosing pump  2  is attached through a color print automatic controller  3  to a color line  4 , which interacts with a color print pressure sensor (p≈ lacquer )  8 .  
         [0030]     The color print automatic controller  3  can be configured in conventional way, which is described for example in EP 1,376,289 A1, which is incorporated by reference herein in its entirety.  
         [0031]     In operation the color print automatic controller  3 , at the entrance of the dosing pump  2 , regulates a color print pressure p v  as a function of an actuating pressure p steuer , which the color print automatic controller  3  is supplied by a proportional valve  5 , whereby the proportional valve  5  is attached to a control air line  6 , which interacts with a control air pressure sensor (p≈ AIR )  10 .  
         [0032]     The proportional valve  5  is controlled by a control unit  7  with an electrical control signal kd, whereby the system from the proportional valve  5  and the color print automatic controller  3  exhibits a speed ratio π=p v /kd, i.e, with a control of the proportional valve  5 , the electrical control signal kd provides at the exit of the color print automatic controller  3  a coating medium pressure p v =kd·π.  
         [0033]     During the control of the proportional valve  5 , the control unit considers the coating medium pressure p H  at the exit of the dosing pump  2 , whereby the pressure p H  is measured by a pressure sensor  8 . The control of the proportional valve  5  by the control unit  7  takes place then according to the following linear control characteristic: 
 
 kd=k 1 +k 2 ·p   H  
 
 The control parameters k 1  and k 2  are as follows thereby:  
         k   ⁢           ⁢   1     =       Δ   ⁢           ⁢     p   Soll       η         
         k   ⁢           ⁢   2     =     1   η         
 
         [0034]     During such an optimal attitude of the control parameters k 1 , k 2  then the desired pressure difference Δp Soll  produced the dosing pump  2 , becomes from the following derivation:  
               Δ   ⁢           ⁢   p     =       p   V     -     p   H                   =       η   ·   kd     -     p   H                   =       η   ·     (       k   ⁢           ⁢   1     +     k   ⁢           ⁢     2   ·     p   H           )       -     p   H                   =         η   ·   k     ⁢           ⁢   1     +       (         η   ·   k     ⁢           ⁢   2     -   1     )     ·     p   H                     =     Δ   ⁢           ⁢     p   Soll                 
 
         [0035]     The determination of the optimal values of the control parameters k 1  and k 2  requires knowledge of the speed ratio π of the system from the proportional valve  5  and the color print automatic controller  3 . After an exchange of the color print automatic controller  3  by another color print automatic controller with another pressure speed ratio π the control parameters k 1 , k 2  must be adapted to the changed pressure speed ratio of the color print automatic controller  3 . Also with a change of the assigned coating medium and a change of the viscosity of the coating medium due to changes in the speed ratio π, likewise makes an adjustment of the control parameters k 1 , k 1  necessary.  
         [0036]     The coating plant according to one embodiment of the invention includes an adaptation unit  9 , which is connected with the pressure sensor  8  and measures a further pressure sensor  10  of the coating medium pressure p, before the dosing pump  2 . The adaptation unit  9  adjusts the control parameters k 1 , k 2  in the context of an adaptation procedure, that in  FIG. 2  is represented in the form of a flow chart.  
         [0037]     With the first adaptation of the control parameters first the values for the speed ratio, π the desired value becomes Δp Soll  for the difference through the dosing pump  2  and the initial values k 1   alto  and k 2   alto  initialize for the control parameters k 1  and k 2 , whereby the defaults on assumptions of the speed ratio π are based.  
         [0038]     Subsequently, a so-called Brush or coating medium application process has been waiting, which lasts longer than one second. It concerns a change of flow rate in delivered coating averaging one, to which an opening is based to the main needle of the atomizer  1 . The consideration of only relatively long persisting Brushes (or coating medium application process) with a length of time of 1 second is meaningful, since the length of time of shorter Brushes is not sufficient in order to let transients on engagement fade away.  
         [0039]     Subsequently, the adaptation unit  9  measures over the two pressure sensors  8 ,  10 , the pressure pV 1  before the dosing pump  2  and the pressure pH 1  behind the dosing pump  2 . Beyond that the adaptation unit  9 , in this first operating point, seizes also the color quantity of Fm 1 . The values pV 1 , pV 2 , and Fm 1  are available here in the control anyway and do not have to be additionally measured.  
         [0040]     Subsequently, the next Brush or coating medium operation process has been waiting, which lasts longer than one second and is not concerned therefore with transients on engagement.  
         [0041]     In this second operating point, then again the values from the control that are selected and stored become pV 2 , pH 2  and Fm 2  for the pressure p before the dosing pump  2 , the pressure p H behind the dosing pump  2  and the color quantity of F m . Also here it can be used that the values pV 2 , pH 2  and Fm 2  available and therefore do not need to be additionally measured.  
         [0042]     Then becomes examined whether the two operating points are sufficiently far from each other, in order to make a meaningful measurement possible. For this comparison, the absolute value of the difference between Fm 1  and Fm 2  of the measured color quantities are consulted for the adaptation of operating points in an educated manner and with a minimum value. If the formed distance between the two operating points is too small, the second operating point is rejected.  
         [0043]     Otherwise then k 2   alto  becomes a computed from the past values k 1   alto , the control parameter optimized values k 1  NEU, k 2  NEU using the following formulas:  
         k   ⁢           ⁢     2   Neu       =     k   ⁢           ⁢       2   Alt     ·         p     H   ⁢           ⁢   1       -     p     H   ⁢           ⁢   2             p     V   ⁢           ⁢   1       -     p     V   ⁢           ⁢   2                   
         k   ⁢           ⁢     1   Neu       =     k   ⁢           ⁢     1   Alt     ⁢       Δ   ⁢           ⁢     p   SOLL           p     V   ⁢           ⁢   1       -           p     V   ⁢           ⁢   1       -     p     V   ⁢           ⁢   2             p     H   ⁢           ⁢   1       -     p     H   ⁢           ⁢   2           ·     p     H   ⁢           ⁢   1                   
 
         [0044]     Subsequently, the control unit  7  then with the optimized values k 1   NEU  and k 2   NEU  works the control parameter, whereby in Figure the adaptation of the process, in order to optimize and reach the control behavior of the control unit  7  that the pressure difference Δp through the dosing pump  2  is kept as close as possible to the given desired value Δp Soll .  
         [0045]     Those formulas for the adaptation of the control parameters k 1 , k 2  result from the following mathematical-physical derivation.  
         [0046]     Described first the behavior of the coating plant by the following equations: 
 
 kd=k 1 +k 2 ·p   H   (1) 
 
pv=kdπ  (2) 
 
Δ p=p   v   −p   H   (3) 
 
 with 
 
 kd: Control size for the control of the proportional valve  5 , 
 
 pv measured pressure before the dosing pump  2 , 
 
 P H : measured pressure behind the dosing pump  2 , 
 
 k 1 : Axis intercept value of the control characteristic, 
 
 k 2 : Upward gradient of the control characteristic, 
 
 Δp Soll  Desired value of the pressure difference through the dosing pump  2 , 
 
 Δp actual value of the pressure difference of pressure through the dosing pump  2 , 
 
 πSpeed ratio of the system from the pressure control valve  3  and the upstream proportional valve  5 . 
 
         [0047]     From the equations (1) and (2) follow: 
 
 p   v =( k 1 +k 2 ·p   H )·π  (4) 
 
         [0048]     Considering now two operating points with different color quantities of Fm 1 , Fm 2  and different coating medium pressures pV 1 , pV 2 , pH 1  and pH 2  before and/or behind the—I dosing pump, then applies in accordance with equation (4) to these two operating points in each case: 
 
 p   v 1=( k 1 +k 2 ·p   H 1)  (5) 
 
 p   v 2=( k 1 +k 2 ·p   H 1)  (6) 
 
         [0049]     From the equations (5) and (6) follow then for the control parameter  
               k   ⁢           ⁢   2     =         p     V   ⁢           ⁢   1       -     p     V   ⁢           ⁢   2           η   ·     (       p     H   ⁢           ⁢   1       -     p     H   ⁢           ⁢   2         )                 (   7   )             
 
         [0050]     For the old, non-optimized value k 2   alto , the control parameter k 2  applies directly:  
               k   ⁢           ⁢     2   Alt       =         p     V   ⁢           ⁢   1       -     p     V   ⁢           ⁢   2           η   ·     (       p     H   ⁢           ⁢   1       -     p     H   ⁢           ⁢   2         )                 (   8   )             
 
         [0051]     For that new, optimized value k 2   NEU , the control parameter k 2  applies then with consideration of the equation (3), fulfilled with optimal control behavior.  
               k   ⁢           ⁢     2   Neu       =         (       p     H   ⁢           ⁢   1       +     Δ   ⁢           ⁢   p       )     -     (       p     H   ⁢           ⁢   2       +     Δ   ⁢           ⁢   p       )         η   ·     (       p     H   ⁢           ⁢   1       -     p     H   ⁢           ⁢   2         )                 (   9   )             
 
         [0052]     From the equations (8) and (9) follow then the adaptation formula for the adaptation of the control parameter k 2 :  
               k   ⁢           ⁢     2   Neu       =     k   ⁢           ⁢       2   Alt     ·           p     H   ⁢           ⁢   1       -     p     H   ⁢           ⁢   2             p     V   ⁢           ⁢   1       -     p     V   ⁢           ⁢   2           .                 (   10   )             
 
         [0053]     Described the derivation of the adaptation formula for the control parameter k 1  in the following one becomes now, from the equations (1) (2) and (3) follows:  
               k   ⁢           ⁢   1     =         p   V     η     -     k   ⁢           ⁢     2   ·     p   H                   (   11   )             
 
         [0054]     If one uses equation (6) in equation (11), then one obtains for the old, non-optimized value k 1   alto , of the control parameter k 1  in the operating point  1  with pV 1 , pH 1  and Fm 1 :  
               k   ⁢           ⁢     1   Alt       =         p     V   ⁢           ⁢   1       η     -           p     V   ⁢           ⁢   1       -     p     V   ⁢           ⁢   2           η   ·     (       p     H   ⁢           ⁢   1       -     p     H   ⁢           ⁢   2         )         ·     p     H   ⁢           ⁢   1                   (   12   )             
 
         [0055]     For that new adapted optimum value k 1  NEU must apply against it:  
               k   ⁢           ⁢     1   Neu       =       Δ   ⁢           ⁢     p   Soll       η             (   13   )             
 
         [0056]     From the equations (12) and (13) follow then finally the adaptation formula for the adaptation of the control parameter k 1 :  
               k   ⁢           ⁢     1   Neu       =     k   ⁢           ⁢     1   Alt     ⁢       Δ   ⁢           ⁢     p   SOLL           p     V   ⁢           ⁢   1       -           p     V   ⁢           ⁢   1       -     p     V   ⁢           ⁢   2             p     H   ⁢           ⁢   1       -     p     H   ⁢           ⁢   2           ·     p     H   ⁢           ⁢   1                       (   13   )             
 
         [0057]     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.