Abstract:
A paint booth airflow control system is described for preventing paint particles and other contaminates from entering adjacent paint booth sections by equalizing air pressure differences between adjacent sections. Pressure sensors in communication with adjacent paint booth sections report pressure differences between the adjacent sections to an airflow controller. Responsive to the reported pressure differences the airflow controller adjusts paint booth airflow to equalize the pressure between adjacent sections. In order to maintain pressure measurement accuracy, automatic calibration modules periodically calibrate all pressure sensors to pneumatic references.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the control of airflow between adjacent sections of a paint booth. 
     2. Description of the Prior Art 
     Durable goods such as vehicles and appliances require protective coatings. Generally, protective coatings are added to such objects from inside the protective environment of multi-section enclosures commonly referred to as coating or paint booths. Typically, coatings of several types and pigments are applied to objects as they are conveyed through a paint booth. Usually, only one type of coating is applied per section of paint booth. For example, a first section may be used to apply a primer coat, another section may be used to apply a pigment and a final section could be used to apply a clear coat. 
     No matter what type of coating is being applied, a clean environment inside each paint booth section free of detrimental substances such as dirt, dust and organic solvents must be maintained. The paint booth enclosure alone limits to some degree the amount of coating contaminants present inside each paint booth section. Never the less, a significant amount of contaminants still exist inside each paint booth section. These contaminants must be prevented from coming into contact with freshly applied coatings. 
     One way to prevent contaminants from coming in contact with fresh uncured coatings is to force clean air to flow vertically from vents in the top of the paint booth to returns in the bottom. Downward airflow helps prevent contaminants from becoming suspended inside the paint booth. 
     Unfortunately, the downward airflow used to solve the problem of suspended contaminants creates another problem by generating unequal static pressures between adjacent sections. The higher static pressure of one paint booth section relative to another forces airborne paint particles to migrate to the lower pressure section. Walls placed between adjacent sections only moderately reduce the number of paint particles transported because the walls must have openings through which objects can pass. 
     Transported paint particles become coating contaminants when they drift into other sections. For example, white-pigment paint particles migrating from one paint booth section into another containing an object with an uncured black-pigment finish coat would be disastrous. 
     Prior art airflow control systems have been designed to minimize paint particle migration between adjacent sections of a paint booth. One such system uses ultrasonic anemometer sensors to measure airflow between adjacent paint booth sections. The airflow measurements are sent to an air handling system that increases or decreases the airflow in each paint booth section. Unfortunately, ultrasonic anemometers cannot accurately measure airflow rates as low as those that transport paint particles between sections. 
     Another system uses pressure sensors to measure pressure differences between adjacent paint booth sections. An air handling system responds to the pressure sensors by adjusting the airflow of each section. While this approach is sound in theory, it is unsatisfactory in a practical sense because factory calibrations of low-pressure sensors are very short-lived. In other words, the output of low pressure sensors significantly drift off calibration unacceptably soon, especially when set to measure differential pressures as low as those responsible for the transport of the undesired paint particles. 
     As described above, the purpose of paint booths are to prevent the contamination of uncured coatings by providing an environment free from particles nd substances that would otherwise mix with the coatings applied to an object, harming the object&#39;s finish. While the enclosure of a paint booth along with the introduction of clean air into its sections go towards providing an environment suitable for applying coatings to durable goods such as vehicles and appliances the problem of paint migration between adjacent sections remains. A solution to this problem will provide a major benefit in that costly rework and refinishing of coated objects now common will be eliminated. 
     SUMMARY OF THE INVENTION 
     It has been found that the migration of undesirable paint particles from one paint booth section to another can be significantly reduced by maintaining substantially equal static pressures between paint booth sections. Pressure sensors are used to measure the static pressure inside paint booth sections. An airflow controller uses static pressure measurements from the paint booth sections to determine the level of airflow adjustment needed to equalize the pressure in adjacent sections. Frequent calibration of all pressure sensors during the continuous operation of a paint booth is necessary because pressure sensors rapidly drift away from calibration when used to make measurements with the accuracy needed to determine airflow adjustments. 
     The present invention is a system for calibrating pressure sensors used to measure the static pressure inside adjacent paint booth sections. The system includes a pressure sensor, an amplifier and an amplifier controller for each paint booth section. A processor connects a low-pressure reference and a high-pressure reference to each of the pressure sensors in a sequential manner. The amplifier controller adjusts the amplifier output to zero based on the low-pressure reference. Likewise, the amplifier controller adjusts the amplifier to a preset value based on the high-pressure reference. When referring to “zero” as a procedure it is to be understood that it is unnecessary for any output to physically go to zero volts or zero amperes, etc. Instead the term “zero” in the case of the present invention includes multiple points of calibration. For example, to “zero” an output the user may select a voltage set point of perhaps—100 mV. 
     The present invention further provides an airflow controller that uses the static pressure measurements from inside paint booth sections to determine the level of airflow needed to reach static pressure equilibrium between adjacent paint booth sections. Each amplifier is in communication with an airflow controller that may be as simple as a motor controller in command of a blower. The blower in turn generates airflow into the booth section in which pressure is measured by a corresponding pressure sensor. An increase in airflow entering or a decrease in the airflow exiting a booth section will increase the pressure inside the section. Likewise, a decrease in the airflow entering or an increase in the airflow exiting a booth section will result in a decrease in pressure inside the section. Consequently, the pressure inside the booth can be maintained at a level that is equal to that of neighboring booth sections. Equalized pressured between adjacent sections results in minimized airflow between adjacent sections. 
     Also, a method is disclosed in which a micro-controller controls the sequence of automatic zero and automatic span procedures that recalibrate all pressure sensors on an automatically adjustable time schedule. The pressure sensors are recalibrated frequently, in both zero offset and span; as a result all sensors output equivalent signals for an equivalent pressure. This level of accuracy and equivalence of pressure measurement an air flow controller can be set to maintain the pressure of each paint booth section such that pressure differences between sections is reduced to a level that drastically reduces the transport of paint particles between sections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating an embodiment of the present invention. 
     FIG. 2 is a schematic block diagram depicting an automatic calibration module. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description, like reference characters designate like or corresponding parts throughout the several views. Also in the following description, it is to be understood that such terms as high, low, top, bottom, vertical, horizontal, zero, span and the like, are used solely for the purpose of clarity in illustrating the invention, and should not be taken as words of limitations. 
     The present invention minimizes airflow rates of air flowing between adjacent sections of a paint booth by maintaining a static pressure inside each paint booth section to a level substantially equal to the static pressure inside all other paint booth sections. This is accomplished by a periodic cycle of steps that automatically calibrates the zero set point and span of all pressure sensors after which a measurement of the static pressure of each paint booth section is transmitted to an air flow controller that in turn adjusts the static pressure inside each paint booth section. 
     As shown in FIG. 1 a paint booth  100  is made up of adjacent sections  102 ,  103 ,  104 , and  105 . Section  102  is partially closed by partition  106 . Sections  102  and  103  are separated by partition  107 . Sections  103  and  104  are separated by partition  108 . Sections  104  and  105  are separated by partition  109 . Section  105  is closed by partition  110 . Each partition,  106 ,  107 ,  108 , and  109  has an opening  112  through which cars  114  are passed. Furthermore, pressure sensors  116 ,  117 ,  118 , and  119 , one for each section, are pneumatically in communication with the static pressure of their respective paint booth section. For example, pressure sensor  116  is in intermittent communication with the static pressure of section  102  by way of, plumb work  120 , automatic calibration module  122  and plumb work  124 . Likewise, pressure sensors  117 ,  118 , and  119  are in intermittent communication with their respective paint booth sections by way of, plumb work,  126 ,  132 ,  138 , automatic calibration modules  128 ,  134 ,  140 , and plumb work  130 ,  136  and  142  respectively. 
     An airflow controller  144  is adapted to adjust the static pressure of each individual section by controlling rotation speed of blowers  146 . Airflow controller  144  receives a static air pressure measurement, one for each section, from the plurality of automatic calibration modules  122 ,  128 ,  134  and  140 . 
     A sequencer module  148  automatically controls the sequence of the automatic calibration of each individual pressure sensor by way of a control bus  150 . The sequencer module also selects between a high-pressure reference  152  and a low-pressure reference  154  by way of a pneumatic valve  156  at appropriate times during the calibration sequence. Taken together, pneumatic valve  156 , high-pressure reference  152 , and low-pressure reference  154  all inside a dashed box is referred to as a pneumatic reference selector  158 . 
     Turning attention now to FIG. 2 the automatic calibration modules can been examined in greater detail. The contents inside the dashed box of FIG. 2 represent the components that make automatic calibration module  122 . 
     A pneumatic valve  162  having a first pneumatic input port  166  is in communication with the pneumatic reference selector  158 , a second pneumatic input port  168  is in communication with the static pressure of a paint booth section  102 . A pneumatic output port  170  belonging to pneumatic valve  162  is in communication with the first pneumatic input port  172  of pressure sensor  116 . An electronic amplifier  174  having an electrical input port  176  is in communication with the electrical output port  178  of pressure sensor  116 . Electronic amplifier  174  also has a zero adjustment input  180 , a span adjustment input  184  and an electrical output port  184 . 
     An electronic processor  160  having an electrical input  186  connects to the electrical output port  184  of electronic amplifier  174  by way of feedback path  192 . Also, electronic processor  160  has an output  164  for controlling pneumatic valve  162  as well as another output  188  for controlling the zero adjustment of amplifier  174 . Another output  190  is used for controlling the span adjustment of amplifier  174 . Finally, a communication path  194  connects the electrical output of electronic amplifier  174  to airflow controller  144 . 
     A cyclical calibration process can begin by applying a low-pressure reference across both pneumatic inputs of each pressure sensor. This is done in order to set a zero reference output for the amplifier present in each automatic calibration module. For example refer to FIGS. 1 and 2 with particular interest being paid to the calibration of pressure sensor  116 . 
     The action of applying a low-pressure reference across both pneumatic inputs  171  and  172  of pressure sensor  116  begins when sequencer  148  selects low-pressure reference  154  to be applied to the pneumatic reference input  166  of pneumatic valve  162 . The sequencer selects the low-pressure reference of pneumatic reference selector  158  by sending a logic signal to pneumatic valve  156 , commanding it to connect low-pressure reference  154  to pneumatic reference plumb work  143 . Simultaneously, or shortly thereafter the sequencer  148  commands the processor  160  of automatic command module  122  to select the low-pressure pneumatic reference to be applied to the pneumatic input  172  of pressure sensor  116 . Since pressure sensor  116 &#39;s pneumatic input  171  is permanently plumbed to the low-pressure reference  154  of pneumatic reference selector  158  by way of plumb work  145  the differential pressure applied to pressure sensor  116  is zero. 
     At this point, the output voltage of amplifier  174  should be zero. The processor  160  samples the voltage present at output  184  by way of feedback path  192  to processor input  186 . If the voltage is not zero, processor  160  adjusts the output of amplifier  174  by adjusting the control voltage present on the zero input  180  of amplifier  174 . 
     Once the output voltage of amplifier  174  is adjusted to zero, span calibration for pressure sensor  116  may commence. Sequencer  148  selects high-pressure reference  152  to be applied to the pneumatic reference input  166  of pneumatic valve  162 . The sequencer selects the high-pressure reference of pneumatic reference selector  158  by sending an a logic signal to pneumatic valve  156 , commanding it to connect high-pressure reference  152  to pneumatic reference plumb work  143 . Since the pneumatic reference is still connected to pneumatic input  172  of pressure sensor  116  through pneumatic valve  162  from the proceeding zero calibration a high pressure reference will be immediately applied to pneumatic input  172  of pressure sensor  116 . Since pressure sensor  116 &#39;s pneumatic input  171  is permanently plumbed to the low-pressure reference  154  of pneumatic reference selector  158  by way of plumb work  145  the differential pressure applied to pressure sensor  116  becomes non-zero. 
     At this point, the output voltage of amplifier  174  should be equal to a preset value established as a benchmark for all automatic calibration modules. The processor  160  samples the voltage present at output  184  by way of feedback path  192  to processor input  186 . If the voltage does not equal the preset benchmark value, processor  160  adjusts the output of amplifier  174  by adjusting the control voltage present on the span input  182  of amplifier  174 . 
     Once the output voltage of amplifier  174  is adjusted to match the preset benchmark value, measurement of the static pressure of paint booth section  102  may commence. Sequencer  148  selects the static pressure of paint booth section  102  to be applied to the pneumatic input  172  of pressure sensor  116 . The sequencer selects the static pressure of paint booth section  102  by sending a logic signal to pneumatic valve  162 , commanding it to connect pneumatic input  172  of pressure sensor  116  to pneumatic plumb work  124  going to paint booth section  102 . 
     A highly accurate and resolute static pressure measurement is instantaneously transmitted from output  184  of amplifier  174  to airflow controller input  196  by way of communication path  194 . Airflow controller  144  compares the pressure measurement from paint booth section  102  to a static pressure benchmark value established to be the same for all paint booth sections. If the static pressure of section  102  does not equal the benchmark pressure, airflow controller  144  will adjust the rotational speed of blower  146 A by sending a control signal to blower  146 A over blower control bus  198 . If the measured static pressure of paint booth section  102  is less than the benchmark value, airflow controller  144  will send a signal to increase the rotational speed of blower  146 A and the static pressure of paint booth section  102  will increase. Likewise, if the measured static pressure of paint booth section  102  is greater than the benchmark value, airflow controller  144  will send a signal to decrease the rotational speed of blower  146 A and the static pressure of paint booth section  102  will decrease. Very quickly, the static pressure inside paint booth section  102  will settle onto the static pressure benchmark value. 
     The static pressure of the remaining paint booth sections are controlled in the same way as that described for section  102 . In the case of section  103 , the sequencer  148  will address and control automatic calibration module  128  and airflow controller  144  will adjust the rotational speed of blower  146 B. Likewise, sequencer  148  will address and control automatic calibration modules  134  and  140  and airflow controller  144  will adjust the rotational speed of blowers  146 C and  146 D for maintaining the benchmark static pressure for sections  104  and  105 , respectively. 
     The sequencer  148  can be set by the user to calibrate each pressure sensor on a fixed schedule, typically every fifteen minutes or so, depending on environmental conditions such as temperature and humidity. Also, the sequencer  148  could use feedback of drift rate data from the automatic calibration modules to determine an automatic calibration schedule. Regardless of whether the calibration schedule is fixed or automatically adjusted by the sequencer  148 , the process is cyclical in that ever so often the pressure sensors are automatically recalibrated before the static pressure measurements drift far enough away from calibration to produce noticeable errors. 
     Certain modifications and improvements will occur to those skilled in the art upon reading the foregoing description. For example, the pressure sensor in one of the sections could be replaced by a flow meter mounted to measure vertical airflow. Since this airflow measurement would be related to the section&#39;s static pressure, it could be used as a pneumatic reference for all the other sections. Furthermore, instead of referring to external pneumatic references the static pressure of any paint booth section could be used as a pneumatic reference. It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope of the following claims.