Patent Document

The present invention relates to a paint circulating system and method, of a type suitable for use with automated spray finishing processes. 
     BACKGROUND OF THE INVENTION 
     Traditional paint spray systems, of the type employed in car manufacturing for example, usually consist of several separate paint lines, each providing a different coloured paint to a spray booth for distribution to a number of user points (e.g. spray applicators). In general, only one colour is sprayed or in use at any one time, so only one line is actively employed whilst the rest remain ready for use. 
     When a system is not in use because the paint is not being sprayed, it is usual to maintain the spraying pressure and paint velocity in the paint line, by pumping paint from a paint tank around a circuit and back to the tank. This is done for two reasons: firstly, because the liquid paint must be kept moving, otherwise pigmentation may start to settle out in the paint lines; secondly, because the lines must be primed to the required pressure before spraying commences. However, maintaining the lines at pressure is wasteful of energy. 
     To ensure that the paint is at the required pressure for spraying, a Back Pressure Regulator (BPR) is used in combination with the paint pump to regulate and maintain the required fluid pressure and flow at the spray booth. In conventional systems, the BPR is adjusted manually and uses a coil spring acting on a diaphragm to vary the width of a flow passage. This helps to maintain the paint pressure upstream of the BPR by controlling the fluid flow rate returning to the paint tank. Also, in many systems (such as those employing certain types of turbine or lobe pumps) the pump will be set to operate at a fixed pressure and flow rate and the BPR used to maintain the required system pressure. In this type of system, the BPR controls system pressure by adjusting flow rate to compensate for variations in the amount of fluid used at the spray booth. Thus, each line is usually operated in the condition required for spraying, whether the paint is being used or merely circulated. This is extremely inefficient and results a large waste of energy. For example, a system operating 24 hours a day may only be required to spray each individual colour, for, say, 1 hour a day. Each pump would be operated at the pressure and flow rate required to meet the system requirement for 24 hours a day even though the paint is not required to operate at the full system pressure and flow rate for 23 hours of the day. 
     In addition, a pump that is required to provide a higher flow rate and pressure for a longer period of time will suffer a higher rate of wear, requiring maintenance in a much shorter period of time than one that is used more sparingly. 
     It is an object of the present invention to provide a paint circulation system, which alleviates the aforementioned problems. 
     SUMMARY OF THE INVENTION 
     The paint circulating system according to the invention comprises a pump for pumping paint around the system, and a back-pressure regulator (BPR) to substantially eliminate pressure fluctuations of paint upstream of the BPR. Control means control the pump and the BPR to operate in one of a flow mode, wherein a required flow rate of paint around the system is maintained, and a pressure mode, wherein a pressure of paint between the pump and the BPR is maintained. 
     In embodiments of the invention, in the flow mode, the BPR is configured to be de-activated so as to allow paint to flow without varying the flow rate in response to pressure fluctuations. The BPR is preferably an automated type, whereby activation means such as compressed air or a hydraulic fluid is provided to activate and/or de-activate the BPR. The BPR may comprise a diaphragm that is acted upon by a spring or by fluid pressure on one side, and by the paint pressure on the other side. In the flow mode, the control means may be configured to control the pump so as to pump paint at a fixed flow rate. The fixed flow rate is preferably a low flow rate, at or just above a minimum flow rate required for the paint. 
     It is an advantage that the system is able to place the BPR and pump into the flow mode when pressurised paint is not required at the spray booth. In this flow mode there is no need to maintain a high paint pressure in the lines, and the pump can be operated at a steady, low flow rate to reduce energy consumption and wear. 
     In embodiments of the invention, when in the pressure mode, the BPR is configured to be activated so as to respond to variations in the paint pressure to maintain a substantially constant pressure upstream of the BPR. In the pressure mode, the pump is preferably configured to deliver paint at a predetermined pressure. The pump may be a variable speed, or variable capacity pump responsive to a control signal to maintain the predetermined pressure. A pressure sensor may be provided at the pump outlet, or other suitable position in the system, to provide a pressure signal as a basis for the control signal. The control means may be arranged to receive the pressure signal and to provide the control signal to the pump for maintaining the predetermined pressure. 
     It is an advantage that, when paint is required at the spray booth, the system can be placed into the pressure mode by activating (i.e. turning on) the BPR and operating the pump to deliver paint at a high pressure, thereby ensuring that the paint is delivered to the spray booth at the required flow rate and pressure. 
     In embodiments of the invention, the controller may be operable to switch the system between the flow mode and the pressure mode in response to a demand signal. The demand signal may be provided from a plant scheduling or ‘job queue’ data processing apparatus. 
     In one embodiment of the invention, the controller comprises a control card for mounting in a programmable controller or computer device. The control card is preferably provided with a plurality of input and output terminals for receiving signals from sensors in the system and for providing control signals to the BPR and the pump. The control card may be provided with a data link to a graphics system for set-up and monitoring purposes. 
     The control card may include a plurality of channels for controlling a plurality of paint circulating systems, each providing paint to a spray booth. Each of the plurality of paint circulating systems may provide a different colour of paint to the spray booth. It is an advantage that the system can operate in a manner that will allow the “job queue” data to control circulating system operating parameters. “Job queue” data is defined as the data collected by software that monitors the position of parts throughout an automotive OEM, Tier 1 or industrial plant once the parts have been loaded on a conveyor system. The job queue data may be used to provide demand signals to the colour valves to turn on and turn off the supply of paint to the applicators in the spray booth. In the same manner, with the system of the present invention, the job queue data can now be used to provide demand signals that automatically pressurize or de-pressurize the circulating system, depending on the needs at the applicator. This ability provides great savings with regard to paint wear (shear) energy usage and general pump component wear. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Particular embodiments of the invention are illustrated in the accompanying drawings wherein:— 
         FIG. 1  is a schematic representation of a known paint circulation system; 
         FIG. 2  is schematic representation of a paint circulation system according to the present invention; and 
         FIG. 3  is schematic representation of a controller for use in the paint circulation system of  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a paint circulation system  10  includes a paint tank  11  containing a reservoir of liquid paint. A pump  12  is operable to supply paint from the paint tank  11 , optionally through a paint filter  13 , to a spray booth  14 . The spray booth  14  typically includes one or more applicators  16 . For example these may be spray nozzles manipulated by robot arms. Any unused paint flows past the spray booth and is returned to the paint tank  11  via a BPR  15 . 
     In this set-up, the BPR  15  is employed to control the upstream pressure in the system at the desired level, typically 5 to 10 bar when the paint is in use. The BPR  15  typically includes a diaphragm, one side of which is acted upon by a coiled spring. 
     The pressure of paint entering the BPR  15  urges the diaphragm against the spring force to open up a passage for paint. Any reduction in paint pressure results in the diaphragm moving under the spring force, tending to close the passage. This acts as a restriction to the flow of paint, which means that a greater pressure drop occurs across the BPR  15  so that the upstream pressure is maintained. The spring force acting on the diaphragm is pre-set so that the BPR  15  acts to maintain a set upstream pressure. 
     The known circulating system of  FIG. 1  is based on pump flow rates being set to provide the maximum flow demand from the paint take-offs (i.e. applicators  16 ), assuming all are in use at the same time. As paint line pressure drops due to paint usage, the BPR  15  closes to reduce the fluid flow returning to the paint tank  11  thus maintaining the desired line pressure. 
     Referring to  FIG. 2 , there is shown a system  20  according to the present invention, wherein equivalent components to those shown in  FIG. 1  have the same reference numeral. In this case an electric variable speed pump  22 , referred to hereafter as a smart pump, pumps the paint from the paint tank  11  to the spray booth  14 . Although the smart pump described herein is an electric pump, it will be appreciated by those of skill in the art that alternative pumps may be used, for example air driven or hydraulically driven pumps. The smart pump  22  includes a pressure sensor  24 . Paint not used in the spray booth  14  is circulated back to the paint tank  11  via an automatically controlled BPR  25 , referred to hereafter as a smart BPR. The smart BPR is of a type that can be activated and de-activated by way of a suitable control mechanism, for example compressed air or hydraulic fluid. An example of such a regulator is described in the applicants&#39; concurrently filed United Kingdom patent application entitled “Back Pressure Regulator”, the contents of which are hereby incorporated by reference. The smart pump  22  and the smart BPR  25  are controlled from a controller  26 . A signal from the pressure sensor  24  is provided as an input to the controller  26 . The controller  26  may be a PLC or other suitable programmable device. In an exemplary embodiment, the controller comprises a smart card, as will be described in more detail below. 
     The controller  26  is configured to control the smart pump  22  and the smart BPR  25  so that these will operate in either a flow mode or a pressure mode. The mode may be determined from job queue data. 
     When paint is needed at the applicators  16  (as per job queue data), the system  20  will be operated in the pressure mode. The controller  26  will issue a command signal that will cause the smart BPR to be activated so that it will operate to maintain the upstream pressure according to a predetermined set pressure. The user will also have pre-set the desired system pressure into a memory of the controller  26 , for example via a laptop or PC input during initial start up. The controller  26  is programmed to control the pump speed so that the pressure will be maintained, by means of a suitable control loop. The pressure sensor  24  transmits the actual pressure in the paint line to the controller  26 , which reacts by using the control loop to output a signal that controls the speed of the smart pump  22 . For example if the paint pressure drops in the line below the set pressure due to usage at the applicators  16 , the pump  22  will speed up in order to maintain pressure. Note: The smart BPR  25  will initially dynamically reduce the amount of fluid returning to the paint tank  11  in order to maintain the set pressure. The smart pump  22  only speeds up once the BPR  25  can no longer maintain the system pressure. 
     When material is not in demand (as per job queue data) the system  20  will be operated in flow mode. The user will have input the minimum flow rate required to meet the desired minimum paint velocity as recommended by material supplier and the controller will control the smart pump  22  to operate at the speed required to provide this minimum flow rate. In addition the controller  26  will issue a command to de-activate the smart BPR  25 . The smart BPR  25  will no longer operate to maintain the upstream pressure, so that the only system back pressure will be due to the pipework frictional resistance. Energy usage will now be at a minimum. 
     Referring to  FIG. 3 , more detail is shown of an exemplary controller  26  for controlling the smart pump  22  and smart BPR  25  of the system  20  of  FIG. 2 . This controller  26  includes a smart card  30 . The smart card  30  typically comprises one or more printed circuit boards (PCBs) housed in a plastic carrier and mountable to a DIN rail in a purpose built or an existing control panel. The smart card  30  contains circuitry that includes a programmable memory and a processor. Alternatively, the smart card may include an interface for communicating with an external processor, for example a PLC or a computer. The smart card  30  may include a plurality (e.g. 8) channels, each channel on the card being used to control one of a number of paint lines, each of which may provide a different colour, feeding the spray booth. Each channel on the smart card  30  includes a number of input/output terminals. These include:
         A digital input  41  for receiving a system mode signal   An input  42  for receiving a signal (e.g. 4-20 mA) from the pressure sensor  24     An output  43  for providing a signal (e.g. 4-20 mA) corresponding to a frequency to an AC frequency inverter  32  for controlling the speed of the smart pump  22     An output  44  (e.g. capable of driving 24 v at 50 mA) for controlling the switching of a valve  34  for connecting/disconnecting a supply of compressed air  36  to the smart BPR  25 .       

     In addition, the smart card  30  is provided with a serial communications link  45 . This is used as a data link to a computer  38  (e.g. a PC or a lap-top) that includes a graphics system for use in setting up the smart card, and for monitoring, data-logging and display of system parameters. The computer  38  may also receive data via one or more inputs  47  relating to other operating parameters of the system, for example differential pressures across the paint filter  13 , or level indicators on the paint tank  11 . The smart card  30  may also be provided with a further data link  46  to another, similar smart card, so that a plurality of smart cards may be cascaded in a single control system. 
     In use, set point values are inputted to the smart card  30  at initial start up via the communications link  45  from the laptop or PC  38 . Job queue data from the software that monitors the position of parts being conveyed through the plant reports which paint system (i.e. which colour) needs to be in readiness for production, this data will be received by the smart card  30  to control the smart pump  22  and smart BPR  25  accordingly. The job queue data is transmitted to the smart card  30  by CCR LAN to the monitoring PC  38  or by digital input  41 . 
     The memory on the smart card  30  includes a programmed control algorithm that defines the control loop for the operation of the smart pump  22  in response to the sensed pressure from the pressure sensor  24 , when the system is operating in pressure mode. 
     Sequence of Operation: 
     
         
         
           
             Material is not in use (Job Queue load data shows no immediate need for paint)
           Smart pump  22  operates in flow mode. A: preset frequency setting is equal to the low flow rate required to maintain the specified minimum paint velocity.   The smart BPR  25  is fully unloaded (de-activated).   The system operates at the lowest recommended flow rate with the only pressure being that required to overcome the paint line pressure loss. Therefore paint shear, energy usage and pump wear are at a minimum.   
         
             Material will be needed shortly (before color will be required at the applicators). Information is automatically provided by the job queue load data.
           The smart BPR  25  is activated to provide the pre-set system pressure.   The smart pump  22  is switched to pressure mode. The pressure setting is preset and the controller  26  will operate the smart pump  22  in accordance with the control loop according to the pressure senses at the pressure sensor  24 .   If the system pressure drops due to demand at the applicators  16  the BPR  25  will dynamically close in order to maintain pressure. If the BPR  25  can no longer maintain system pressure the smart pump  22  will automatically speed up, thus maintaining the pressure at the set point.   The system will continue to operate in this mode until the job queue data shows that the paint material is no longer needed.   
         
           
         
       
    
     Material is no longer needed (after color is no longer needed at the spraybooth)
         The smart pump  22  is switched to flow mode. The preset frequency setting is equal to flow rate required to maintain minimum paint velocity in the line.   The smart BPR is fully unloaded (de-activated).       

     It will be recognised that the in the pressure mode, the control of the paint pressure at the spray booth results from a combination of the operation of the smart pump  22  and the smart BPR  25 . Table 1 shows an example of how the paint flow rates provided by the smart pump  22  and through the smart BPR  25  might change as different amounts of paint are taken out through the applicators  16 . In this example there are five applicators, designated A 1 , A 2 , A 3 , A 4  and A 5 . Four different rates of paint usage are shown. 
     In condition 1, the system has been switched into the pressure mode, but there is not yet any paint being taken out through the applicators. The smart pump provides a flow rate of 9 L/min to ensure the required paint pressure at the applicators, and all of this flow circulates around the system through the smart BPR. 
     In condition 2, two applicators are spraying at a rate of 2 L/min, while one is spraying at 1 L/min and the other two are not spraying. The total amount being taken out is 5 L/min. In this condition, instead of the flow through the BPR dropping to 4 L/min and the smart pump continuing to provide a flow of 9 L/min, the amount of paint circulating through the smart BPR has only dropped to 6 L/min, while the smart pump has increased its speed to provide a flow of 11 L/min. 
     Similarly in condition 3, all the applicators are taking out 2 L/min each (a total of 10 L/min), while the smart pump has increased its speed to deliver 13 L/min, and the amount circulating back through the BPR has dropped to 3 L/min. This means that the smart BPR is still controlling the upstream pressure, even though the amount of paint being taken out is more than was originally being provided. The pressure of paint at the spray booth will therefore continue to be maintained by the smart BPR when there in a subsequent increase in the amount being sprayed. 
     In condition 4, the applicators are spraying at their maximum capacity of 3 L/min each (a total of 15 L/min). In this case there is no need to provide any flow through the smart BPR as there can be no further increase in the amount of paint being taken out of the system. The smart BPR therefore closes the line back to the paint tank and all the flow is provided from the smart pump (15 L/min). 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 A1 
                 A2 
                 A3 
                 A4 
                 A5 
                 Pump 
                 BPR 
               
               
                 Condition 
                 L/min 
                 L/min 
                 L/min 
                 L/min 
                 L/min 
                 flow L/min 
                 flow L/min 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 9 
                 9 
               
               
                 2 
                 0 
                 2 
                 2 
                 1 
                 0 
                 11 
                 6 
               
               
                 3 
                 2 
                 2 
                 2 
                 2 
                 2 
                 13 
                 3 
               
               
                 4 
                 3 
                 3 
                 3 
                 3 
                 3 
                 15 
                 0

Technology Category: 7