Abstract:
A controller for controlling the flow rate of compressed air to a spray paint gun as well as to a comparable tool which is optimally operative dependant upon receiving a specified flow rate of air includes a flow control valve which regulates the air flow rate therethrough based on a pressure differential signal received from an air flow meter. The pressure differential signal transmitted to the air flow controller is balanced against an opposing spring biased needle in the flow controller.

Description:
RELATED APPLICATIONS 
   This application is a continuation-in-part of application Ser. No. 09/772,834 filed May 7, 2001, now U.S. Pat. No. 6,516,707, which is a division of application Ser. No. 09/322,425, filed May 28, 1999, and patented as U.S. Pat. No. 6,223,645. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus and method for controlling the flow of compressed air, and in particular for controlling the flow rate of compressed air to a paint spray system and to other which are optimally operatice dependent upon receiving a specified flow rate of air. 
   2. Background of the Invention 
   Many factories use compressed air as a source of power for operating various types of production equipment. “Compressed air, ” which is sometimes referred to as “pressurized air” or referred in spray paint operations as “atomization air, ” is defined as free air that has been pressed into a volume that is smaller than it normally occupies. Controlled expansion of the compressed air can be used as a source of power to operate a wide range of pneumatically powered tools. 
   Compressed air is typically supplied from onsite or nearby compressors and piped through a distribution system to a downstream point of use. Paint spraying operations for painting various types of manufactured products, including especially automobiles and airplanes, is one typical use of compressed air. 
   In a spray paint operation, a paint fluid, which can be in the form of either a liquid or a fine powder, is mixed together with compressed air in a spray gun nozzle in order to atomize the paint into extremely fine particles and to transfer the paint particles onto the surface of the item being painted. One commonly used spray paint gun, referred to in the industry as a high volume low pressure (HVLP) spray gun, generates high volumes of low pressure air which transfers the paint particles to the surface of the article being painted with a relatively low velocity. Other uses of compressed air include pneumatically powered machine tools, drills and wrenches, and other pneumatically powered items which are optimally operative dependent upon receiving a specified flow rate of the pressurized air into the tool. 
   Major changes in downstream air demand create varying loads on the compressors. Air compressors are typically controlled according to system supply pressure, coming online as the system air pressure at the compressor drops below a threshold pressure and going offline at a higher cutout pressure. This is a rather crude method of controlling air supply, especially as air compressors are normally quite slow to respond to change. 
   Industrial compressed air systems are commonly controlled by pressure regulation, meaning, by regulating the nominal air pressure at a certain point in the system. A pressure regulator might be placed, for example, at or near the compressor, at one or more points on the distribution line, or on a hose which used to supply air to the tool. The major disadvantage with this method is that measuring air pressure at only one particular point in the system is not necessarily a good indicator of the air pressure at another point in the system. Air pressure drops as it flows through the system, and the amount that the air pressure drops from one point to the next varies greatly depending on the specific installation and also on varying conditions of usage during the course of the day. In many cases a compressed air system supplies not only spray guns but also other devices used in a paint shop such as sanders, polishers, screw drivers, drills and so forth. The intermittent operation of such other tools will affect the air pressure throughout the system. 
   For paint spray operations in particular, one commonly used method for determining whether a sufficient amount of compressed air is being delivered to the spray gun is to place a pressure gage on the cap of the spray gun immediately after the painter has set the spray gun for proper atomization of the paint but before he actually begins painting. Many operators, however, find this extra step to be a great inconvenience which interrupts their painting operations, and so they often do not do it. Another method to checking whether a sufficient amount of compressed air is being delivered to the spray gun is to attach an air pressure regulator and gage to the handle of the gun. However, attaching a pressure regulator to the gun naturally increases its weight. Over a period of time, muscle fatigue sets in, thereby causing the operator to use unnatural arm and wrist actions, which cause overspray or underspray conditions and other flaws in the paint job. 
   More importantly, regulating the nominal pressure at any one point in the system does not necessarily mean that the proper amount of air, or even any air is flowing at another point downstream. There may be a blockage in the spray nozzle of a paint gun, or a break in the line or some other problem in the system. 
   The difficulty in delivering a proper amount of compressed air to a spray paint nozzle is further exasperated by the fact that paint viscosity varies due to temperature fluctuations. If the temperature of the paint varies, the amount of paint fluid delivered to the nozzle of the spray gun also varies. Therefore, to compensate for the change in viscosity of the paint fluid, the amount of compressed air delivered to the spray gun nozzle must be adjusted. This type of adjustment is not easily done with only pressure regulation, and at a minimum requires a great deal of testing and trial and error to achieve the proper settings. Additionally, in many typical spray paint operations a single compressor is used to deliver compressed air to a number of output points. Each point is located a different distance from the air compressor, and so the pressure drops from the compressor to one point or the other will differ. Further, hoses which deliver air from the output point on the wall to the spray gun in the paint booth often differ in length, diameter or both, which greatly affects the pressure drop from one end of the hose to the other. Additionally, different types of spray paint guns, which require different amount of pressurized air, may be employed. The number of variables which are encountered during the course of operations thus increase to the point where it is nearly impossible to control the amount of compressed air delivered to the spray paint gun based merely on regulating the nominal air pressure at any particular point in the system. 
   Accordingly, an improved means for controlling the flow rate of compressed air delivered to a spray paint gun, as well as to other types of pneumatic tools which are optimally operative dependent upon receiving a specified flow rate of compressed air is desired. 
   SUMMARY OF THE INVENTION 
   A method and apparatus for monitoring and compensating the flow rate of compressed air delivered to a paint spray gun and other comparable pneumatic tools comprising both air flow rate and pressure based control of the compressed air system is disclosed. The invention includes an apparatus and method for measuring the air flow rate between two points in the system, comparing the measured flow rate to a desired flow rate, and then adjusting the flow rate in response to a difference between the measured flow rate and the desired flow rate, if any, and for also regulating the ultimate pressure in the system. 
   Generally, the present invention of an apparatus and method for controlling the air flow rate to a spray paint gun comprises: providing a source of compressed air; providing a source of paint fluid; mixing the paint fluid with the compressed air to thereby atomize the paint fluid and thereby transfer the atomized paint fluid to a substrate; measuring the flow rate of the compressed air by measuring a pressure differential across a fixed orifice located at a point downstream from the source of compressed air but upstream from where the compressed air is mixed with paint fluid; comparing the measured pressure differential to a desired pressure differential; and, in response to a difference between the measured pressure differential and the desired pressure differential, if any, adjusting the flow rate so that the measured pressure differential will equal the desired pressure differential. The above-described apparatus and method for controlling air flow rate is also preferably used in combination with a pressure regulating circuit that controls the ultimate pressure in the compressed air system particularly when the air flow rate drops to zero. 
   In the present invention, a pressure differential signal is generated by directing the compressed air through a fixed orifice obstruction, and by measuring the relative pressure difference between two points on either side of the orifice. The pressure differential signal is transmitted to a control device which compares the measured pressure signal to a desired pressure differential signal, and if there is a difference makes an automatic correction to adjust the flow rate to a desired amount. When the pressure differential signal is zero, which means that the air flow rate is also zero because the tool has been deactivated, the system automatically reverts to pressure regulation circuit. Thus, by controlling the rate of air delivered to the spray gun or other tool, as opposed to merely controlling the nominal air pressure at a single point in the system, the operator is assured that the proper amount of compressed air will be delivered to the tool. Additionally, when the tool is deactivated, the pressure regulation circuit assures that an overpressurization of the system will not occur. 
   The present invention of an air flow rate control method and apparatus provides a number of significant advantages over mere pressure regulation methods in the prior art. Such advantages include providing consistent air flow to the pneumatic tool regardless of supply hose diameter or length. The monitoring of flow rate can be used to detect various problems in the system such as pinched hoses, malfunctions of the tools, and other unexpected abnormalities in the demand or supply of the pressurized air. 
   Installations such as body shops normally include several air outlets so that the painter can disconnect the spray gun and move to various locations within the spray booth. Such installations now require several pressure regulators to be installed inside the spray booth. By using the air flow control of the present invention, a single supply pipe can be installed circling the spray booth area and the proper amount of compressed air will be delivered to the tool regardless of the location of the operator. 
   Other objects and advantages of the invention will become apparent from the following description which, taken in connection with the accompanying drawings, set forth by illustration and example certain embodiments of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings, which constitute part of the specification and include exemplary embodiments of the present invention, include the following: 
       FIG. 1  is a pneumatic illustration of a first embodiment of a device for controlling the flow rate of compressed air delivered from an air source to a point of use in accordance with the principles of the present invention. 
       FIG. 2  is a pneumatic and electrical illustration of a second embodiment of a device for controlling the flow rate of compressed air in accordance with the principles of the present invention. 
       FIG. 3  is a pneumatic illustration of a typical pressure regulator. 
       FIG. 4  is a pneumatic illustration of a pressure regulator modified in accordance with the principles of the present invention to make itto a flow control valve. 
       FIG. 5  is a pneumatic and electrical illustration of a third embodiment of a device for controlling the flow rate of compressed air in accordance with the principles of the present invention. 
       FIG. 6  is a pneumatic and electrical illustration of a fourth embodiment of a device for controlling the flow rate of compressed air. 
       FIG. 7  is a pneumatic and electrical illustration of a fifth embodiment of a device for controlling the flow rate of compressed air. 
       FIG. 8  is a pneumatic and electrical illustration of a sixth embodiment of a device for controlling the flow rate of compressed air. 
       FIG. 9  is an illustration of an air pressure selector. 
       FIG. 10  is an illustration of an air hold control. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 1 and 2 , the invention disclosed herein relates to a method and apparatus for controlling the air flow rate of compressed air, sometimes referred to as either pressurized air or as atomization air, to a tool. The method and apparatus are particularly useful for use in connection with a spray paint operation, in which the compressed air is mixed with a volume of liquid or powdered paint in order to atomize the paint fluid into minute particles and transfer the paint particles onto the surface of an item being painted. The invention disclosed herein is also adaptable for use with other types of pneumatically powered tools in which the optimal operation of the tool is dependent upon receiving a predetermined or desired flow rate of compressed air, such as, for example, air powered tools that require constant rpm or torque.  FIG. 1  illustrates a first embodiment of the present invention, which essentially comprises a mechanical flow rate control apparatus.  FIG. 2  illustrates a second embodiment of the invention, which essentially comprises an electromechanical flow rate control apparatus. Both embodiments arc based on the principle that the flow rate of a fluid through a passageway of known dimension can be calculated by determining pressure differential between two points in the passage, and by comparing the measured pressure differential to a desired pressure differential, the actual flow rate can then be adjusted to reach and maintain a desired flow rate. Both embodiments also provide a dual means for controlling the compressed air systems, that is, by monitoring and adjusting the air flow rate when the tool is activated and air is flowing through the system and also by monitoring and regulating the overall pressure in the system especially when the tool has been deactivated and there is no air flow through the system. 
   Referring to  FIG. 1 , a mechanical compressed air flow rate control apparatus  10  in accordance with the present invention includes an air inlet  11  for receiving compressed air from an air pressurizing source  12 , such as a compressor, and an air outlet  13  for transmitting the compressed air to a tool  14 , such as a paint spray gun. Between the air inlet  11  and air outlet  13  is a first air flow path  15 , a second air flow path  16 , and a pilot air flow path  17 . Two air flow diverter valves, namely, a first air diverter valve  18  and a second air diverter valve  19 , direct the flow of air from the inlet to the outlet through either the first air flow path  15  or second air flow path  16 . The diverter valves are each essentially a three-way valve which select the direction of air flow. Specifically, the first air diverter valve  18  is connected adjacent to the air inlet  11 , and the first air diverter valve  18  directs the flow of air from the air inlet to either the first air flow path  15  or the second air flow path  16 . Similarly, the second air diverter valve  19  is connected at or near the air outlet  13 , and the second diverter valve receives pressurized air from either the first air flow path  15  or second air flow path  16 , and directs it to the air outlet  13  and thus to the tool  14 . An air flow switch  20  located upstream from the first diverter valve  18  is used to actuate the pilot air flow path  17 , which in turn actuates the first and second air diverter valves (discussed further below). 
   The first air flow path  18  is essentially a pipe or tube structure which provides a pneumatic passageway for the air to flow from the first diverter valve  18  to the second diverter valve  19 . The first air flow path includes an air flow controller  21  located between the first and second diverter valves. The air flow controller  21  includes a fixed orifice obstruction  22  for producing a pressure differential between a first point  23  and a second point  24  in the first air flow path  15 . The internal geometry of the obstruction in the orifice creates a resistance to the air flowing through the first air flow path. The resistance produces a pressure drop between the first point  23  and second point  24 , in the flow path. Of course, the volume of air flowing through a passage of fixed dimension is dependent upon the pressure differential between two longitudinally spaced apart points in the air flow passageway. If the difference in pressure between the two points is zero, then obviously the flow rate is also zero. As the difference in pressure between the two points increases, the flow rate of the air likewise increases. Thus, the pressure difference of air flowing through the orifice can be used to both monitor and control the flow rate. 
   The air flow controller  21  further includes a spring  25  which biases a needle  26 , and a diaphragm  27  for controlling the position of the needle. The needle  26  is positioned laterally to the path of air flowing through the fixed orifice obstruction  22 . Further, the needle  26  protrudes through an opening  28  in the center of the fixed orifice obstruction  22 , and the needle is slidable into and out of the fixed orifice. As a result, the position of the needle controls the amount of air flowing through the fix orifice obstruction. The needle  26  is normally biased by the spring  25  in the open position, meaning that the needle is fully retracted out of the opening  28  so that the air flow passageway in the orifice is completely clear. As the needle  26  gradually protrudes through the opening and into the orifice the flow of air through the orifice becomes partially or fully blocked, which consequently reduces the amount of air flowing through the device. 
   The needle  26 , spring  25  and diaphragm  27  are contained within an air chamber  29  inside the air flow controller  21 , with the diaphragm  27  essentially dividing the air chamber  29  into a first subchamber  30  and a second subchamber  31 , the first subchamber  30  of course being on one side of the diaphragm  27  and the second subchamber  31  being on the other side of the diaphragm  27 . As mentioned, the fixed orifice obstruction  22  produces a pressure differential between a first point  23  and a second point  24  in the first air flow path. A first air portal tube  32  pneumatically connects the first point  23  to the first subchamber  30 . A second air portal tube  33  pneumatically connects the second point  24  in the fixed orifice obstruction to the second subchamber  31 . 
   When the pressure differential between the first point and second point is zero, the spring  25  biases the needle  26  so that the fixed orifice is in the fully open position. As the air flow rate through the fixed orifice increases, a pressure differential will be created between the first point  23  and the second point  24 . 
   The nominal pressure valve at the first point  23  in the flow path is transmitted through the first air portal tube  32  into the first subchamber  30  of the air flow controller  21 , and the nominal pressure valve at the second point  24  in the flow path is transmitted through the second air portal tube  33  to the second subchamber  31  in the air flow controller, so that substantially the same pressure differential that exists between the first and second points in the air flow path is reproduced between the first and second subchambers in the air flow controller. As a result, as the pressure differential between the first and second points in the air flow path becomes increased, the pressure in the first subchamber  30  of the flow controller  21  likewise becomes increased relative to the air pressure in the second subchamber  31 , thereby causing the diaphragm  27  to deflect and depress the needle  26  into the opening  28  in the fixed orifice and thereby balance the air flow. The desired flow rate of the compressed air is achieved when the pressure differential between the first subchamber  30  and second subchamber  31  in the flow controller  2  is balanced against the spring force of the needle  26 . 
   The air flow rate can be manually adjusted by rotating a manual adjusting knob  34  on the flow controller  21 , which contains a counteracting spring  37 . The force of the counteracting spring  37  acts in the opposite direction of the force produced by spring  25 . Thus, by tightening the knob  34 , the counteracting spring  37  urges the needle  26  toward the opening  28  in the orifice which in effect reduces the air flow rate. Conversely, by untightening the knob  34  the spring force of the counteracting spring  37  is reduced which in turn allows the needle to retract from the opening  28  in the orifice and thereby effectively increase the flow rate. 
   As mentioned, the second air flow path  16  is essentially comprised of a pneumatic conduit which extends from the first diverter valve  18  to the second diverter valve  19 . The second air flow path includes a pressure gauge  35  with a pressure relief valve located between the two diverter valves. 
   The air flow controller illustrated in  FIG. 1  operates in substantially the following manner. 
   When the spray gun or other tool  14  is not triggered, the air flow switch  20  located in the supply airline would not be activated and the pilot air path  17  from the air flow switch would not be pressurized. Upon triggering the tool, the air flow switch  20  pressurizes the pilot air path  17  thereby causing the first and second three-way diverter valves  18  and  19  to switch so that the flow of air is directed through the first air flow path  15 . With the diverter valves activated the flow control valve  21  modulates the flow rate based on pressure differential between the first and second points  23  and  24  in the fixed orifice. 
   The spring pressure on the needle  26  adjusts the flow rate through the flow control valve. With no flow, the flow control valve  21  would be wide open. As flow increases, the valve gradually closes based on the pressure drop between the first and second air portal tubes  32  and  33  in the flow controller  21 . The resultant flow rate is based on balancing the pressure differential force of the diaphragm  27  which is produced by the pressure drop across the valve and the force of the spring  25  urging the needle  26  valve open. 
   When the spray gun or tool  14  is triggered off, the air flow of course stops. When the flow through the air flow switch  20  drops to zero, the pilot signal in the pilot air path  17  is exhausted out through the air flow switch  20 . As a result, the first and second air diverter valves  18  and  19 , which are spring biased, automatically switch back to divert the compressed air through the second air path  16  which provides for a pressure control. Since the regulator  35  on the pressure loop  16  is self-relieving, any over pressurization of the system will be relieved. 
   An electromechanical flow control apparatus  40  which produces substantially the same results is shown in FIG.  2 . Referring to  FIG. 2 , the electromechanical air flow controller  40  likewise includes an air supply inlet  41  for receiving compressed air from an air pressurizing source  42 , and an air outlet  43  for transmitting the compressed air to the tool  44 . Between the air inlet and air outlet are a first air flow path  45  and a second air flow path  46 . Also, at or near the air inlet is an air flow switch  47 . The air flow switch is electrically connected to an electronic microprocessor  48  which controls the opening and closing of a downstream solenoid valve  66  (discussed further below). 
   On the electromechanical air flow controller shown in  FIG. 2 , the first air flow path  45  includes a pressure differential air flow control meter  49  and a flow control valve  50 . The pressure differential air flow control meter  49  similarly includes a fixed orifice obstruction  51  for producing a pressure differential between a first point  52  and a second point  53  in the first air flow path. At the first point  52  is a first pressure transducer  55  for measuring the nominal air pressure and for generating an electronic signal in response to the measured nominal air pressure at that first point  52 . At the second point  53  on the flow meter  49  is a second pressure transducer  55  for measuring the nominal air pressure and for generating a second electronic pressure signal responsive to the measured nominal air pressure at that second point  53 . As air flows through the flow meter  49 , the fixed orifice obstruction produces a difference in pressure between the first and second points  52  and  53 , which consequently produces an electronic pressure differential signal generated by the first and second pressure transducers  54  and  55 . The first and second pressure transducers  54  and  55  are each electrically connected to the microprocessor  48 . 
   The flow control valve  50  is located upstream from the differential pressure flow meter  49 . The flow control valve  50  for the electromechanical flow controller likewise includes a diaphragm  56 , spring  57  and needle  58  combination for controlling the air flow through the first air flow path  45 . Specifically, the flow control valve  50  comprises a needle  58  which extends laterally to a passageway  59  in the first air flow path  45 , the needle  58  being slidable through a lateral opening  60  in the passageway  59 . The needle  58  is normally in the fully retracted position, meaning that the passageway  59  is normally open. The air flow rate through the passageway  59  is controlled by sliding the needle  58  through the lateral opening  60  to partially or in some cases completely block the air flow through the passageway  59 . The flow control valve  50  further comprises an air chamber  61  subdivided by the diaphragm  56  into a first subchamber  62  and a second subchamber  63 . The position of the needle  58  is controlled by deflection of the diaphragm  56 . One of the subchambers  63  in the flow control valve  50  is pneumatically connected to a voltage to pneumatic converter  64 . The voltage to pneumatic converter  64  is also electrically connected to the microprocessor  48 . The flow control valve  50  is thus arranged to control the amount of compressed air flowing through the first air path  45  in response to the electronic signals produced by the first and second pressure transducers  54  and  55 . 
   The second air flow path  46  further includes a pressure regulator  65  and a solenoid valve  66  for opening and closing the second air flow path. The electronic microprocessor  48  therefore receives electronic signals from the air flow switch  47  and receives further signals from the first and second pressure transducers  54  and  55  in the differential pressure flow meter  49 , and depending on the signals received, controls the opening and closing of the solenoid valve  66  in the second air flow path  46 , and controls the voltage to pneumatic converter  64  which in turn produces a pneumatic signal to control the position of the needle  58  in the flow control valve  50  in the first air flow path  45 . 
   The electromechanical air flow controller  40  shown in  FIG. 2  operates substantially as follows. When the tool  44  has not yet been triggered, the system is pressurized, but in a static ready condition. The solenoid valve  66  is open, and system pressure is regulated by pressure regulator  65 . Upon triggering the tool  44 , the air flow switch  47  at the air inlet  41  is activated, meaning that the flow of air through the air flow switch  47  produces an electronic signal to that effect which is transmitted to the microprocessor  48 , which in turn transmits a further electronic signal to close the solenoid valve  66 . Thus, air now flows from the air inlet  41  to the air outlet  43  only through the first air path  45 . As air flows through the pressure differential flow meter  49 , a pressure differential between the first and second points  52  and  53  causes the first and second pressure transducers  54  and  55  to generate an electronic pressure differential signal which is sent to the microprocessor. The electronic pressure differential signal is compared to a desired signal, and depending upon the difference in value between the measured signal and the desired signal the microprocessor directs the voltage to pneumatic converter to produce and transmit a pneumatic signal which is sent to the air flow control  50 . The pneumatic signal sent to the air flow controller  50  causes a deflection in the diaphragm  56  to either open the flow control valve to permit a greater flow of pressurized air through the system, or close the air control valve in order to restrict the amount of air flowing through the system. 
   When the tool  44  is triggered off, air flow through the pressure differential flow meter  49  stops. When there is no air flow through the pressure differential flow meter  49 , the electronic signal produced by the first and second pressure transducers  54  and  55  is equal, which causes the flow control processor  48  to open the solenoid valve  66  in the second air flow path  46 , and thereby revert the system back to pressure regulation status. 
     FIGS. 3-10  illustrate additional embodiment of a device for controlling the flow rate of compressed air in accordance with the principles of the present invention. For comparison purposes  FIG. 3  details a typical pressure regulator.  FIG. 4  shows the modifications made to make it a flow control valve. Essentially two signals are required for the device to function as a flow rate controller: flow on/off and flow/pressure rate. Electronically this could be accomplished using one signal where 0 voltage would equal no flow and positive voltage pressure and negative voltage flow. 
     FIG. 5  illustrates a third embodiment of a device for controlling the flow rate of compressed air in accordance with the principles of the present invention. The device in  FIG. 5  adapts pneumatic circuitry to address slow deactivation signal in a high pressure system. 
   In the static state, air enters at 12 into a primary pressure regulator  35 . Due to the logic state of the air pressure selector  2  the bias for the primary regulator is from the static air regulator x 1 . The flow valve FV is fully open due to the pressure on the bias cylinder x 3  via the bias cylinder regulator x 4 . The output  17  is equal to the pressure set on the static air regulator x 1 . 
   For dynamic adjusting of the flow rate, upon airflow, the pressure across the restriction x 5  develops a pressure drop that triggers the airflow switch  20 . The signal for the airflow switch  20  changes the state of the air pressure selector x 2  from static mode to dynamic mode, which in turn changes the control pressure from regulated to main line air. The main line air opens the regulator to a full open position. The airflow switch  20  also activates an airflow timer, x 6  which provides a adjustable delayed control signal. 
   As the air starts to flow through the system to output  17  is flows through x 5 , which develops additional drop providing a feedback to the flow valve for controlling the airflow. The amount of airflow is a ratio between the diaphragm  27  area and the bias cylinder force. If the restriction x 5  is 5 psi at a max flow rate 20 CFM and the diaphragm is 3″ in diameter, the bias cylinder would require 35 PSI for equilibrium. During this adjustment stage of the circuit, the flow valve FV will make the proper corrections in opening to obtain the correct flow rate. The output pressure required to provide the correct flow rate is piped into a holding reservoir for the hold mode. 
   A third mode, referred to as hold mode, is proposed in this method. The hold mode traps the high-pressure feedback air into a reservoir. Upon time out of the flow  20  control timer x 6 , the hold mode on/off valve x 8  is closed and the airflow control valve x 2  is selected to hold. The hold mode changes the bias to the pressure regulator to the current required pressure and holds the output of the flow rate at that pressure until the tool is turned off and back on again. 
     FIG. 6  is a pneumatic and electrical illustration of a fourth embodiment of a device for controlling the flow rate of compressed air. This device and method uses a pressure regulator for controlling both pressure and flow. This method provides feedback to Pressure Transducer  2  when controlling in the pressure mode, and to Pressure Transducers  2  and  3  in the flow mode. The pressure Transducers provide a compression ratio for the air to allow for SCFM control. 
   During the static mode, a signal is received from a controller to signal that the pressure mode is desired. In this mode, the control matches the signal with the pressure on Pressure Transducer  2 . The match is obtained by pulsing the pressure and exhaust solenoid valves to hold the correct pressure in the pressure regulator cavity  61 . By monitoring the cavity and modulating the valves the pressure can be maintain. 
   Upon receiving a flow on signal, the pressure and exhaust solenoid valves are modulated to obtain the correct flow rate drop across Pressure Transducer  2  and Pressure Transducer  3 . 
     FIG. 7  illustrates a fifth embodiment of a device for controlling the flow rate of compressed air. This devise and method uses a single regulator for control of both pressure and flow. In comparison to the device illustrated in  FIG. 5 , which uses a single pressure regulator for both the pressure and flow rate control, the device and method shown in  FIG. 7  adds the bias cylinder to the pressure valve and changes the source of feedback based on the state of the flow rate. 
   In the static mode, no pressure drop is across the restriction x 5  and the air flow switch  20  is in an off state. The off state of switch  20  selects the air flow selector x 2  to static mode and the bias regulator control valve x 9  to off. The output of the air control valve is directed to the pressure/flow regulator  35  control port which adjusts the output to the pressure on the port. The bias regulator valve x 9  is selected to off which vents the bias pressure cylinder to atmosphere. 
   In the dynamic mode, upon air flow through the system, pressure drop occurs at the restriction x 5  which causes the air flow switch  20  to activate. The output signal of the air flow switch  20  changes the air control valve x 2  from static mode to dynamic mode. The output signal from the air control valve directs the low pressure feedback signal from the output side of the restriction x 5  back into the pilot port of the flow pressure regulator. Concurrently, the bias air valve is activated which applies air to the blind end of the bias air cylinder x 3  applying a force on the diaphragm  27  forcing the air control valve  28  open. The amount that the valve will open is in proportion to the force applied on both sides of the diaphragm low pressure  29  and high pressure  31 , and the bias cylinder pressure. 
   When the pressure drop decreases to a preset level the air flow switch  20  will reset the system to static. 
     FIG. 8  illustrates a sixth embodiment of a device for controlling the flow rate of compressed air. In static mode the air flow on and air flow off valves are modulated to control the pressure to the output. The only sensing element used is the Pressure Transducer  2 . 
   In dynamic mode the controller monitors the interface for a flow/pressure signal and a start signal. Upon receiving a valid interface the air flow on valve is modulated to provide the correct flow rate to the device via  17 . The flow rate to the devise is calculated from signals from Pressure Transducer  1  and Pressure Transducer  3  where Pressure Transducer  1  provides the compression ratio of the compressed air and Pressure Transducer  3  provides the pressure differential across a known orifice x 5 . 
   It is to be understood that the embodiments disclosed above are merely exemplary of the invention which may be embodied in various forms. Changes maybe made in the details of construction, arrangement and operation of various elements of the invention without departing from the spirit of the invention. For example, the function pressure differential flow meter  49  which includes two pressure transducers  54  and  55  as described above could be performed by a turbine flow meter or alternatively by a heat flow meter. As a further example, the flow rate control feature of particularly the electromechanical embodiment of the invention shown in  FIG. 2  might be activated by an electronic signal received from the tool rather than just a pneumatic signal. Therefore, specific structural and functional details disclosed above are not to be interpreted as limiting the scope of the invention, but are presented merely as the basis for the claims and for teaching one skilled in the art to various employ the present invention in any appropriately detailed manner especially as defined in the following claims.