Abstract:
An apparatus for the control of fluid flow through a valve. A single shaft-mounted cam moves translationally along the length of the shaft, stopping sequentially at positions adjacent to and in operative engagement with an actuator disposed on or near a valve body. Once in position with a predetermined valve, the cam, which is also coupled to a rotational member, is rotated, thus causing an eccentric portion of the cam to engage the actuator in such a way so as to force the valve to open or close. A flow detection system is integrated into a main fluid transport conduit, allowing sensed flow variations to be sent to a controller. The controller uses a comparison algorithm to determine what fluid settings in the valve are necessary to effect a desired fluid flow through the valve, and prepares an input signal to be sent to one or more motors controlling the translational and rotational motion of the cam. Capping devices and an enclosure with a safety door can be included to protect personnel and the ambient environment against fluid spillage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to a fluid dispensing system with a device for controlling the flow of fluids through a plurality of valves, and more particularly to a shaft-mounted cam that through a combination of translational and rotational motion can sequentially open or close one or more valves to precisely control the ratio of fluids added to a fluid mixture.  
           [0002]    The use of valves and valve actuators to control the flow of gases and liquids in a fluid combining process or system is well known in the art. One area where precise actuation and control of the valves is critical is in chemical processes, where a large number of valves are employed, often in an extensive array of piping, conduit, ducting or related fluid carrying and containing equipment. The attendant level of monitoring and process control necessary to ensure that these larger, more complex valving systems are performing their intended tasks has rendered manual control of such systems difficult. In response, automated valve control was developed, and with the advent of computer-controlled systems, even more sophisticated ways to control and monitor any given chemical process have become commonplace. While the more precise, predictable control over valve closure and opening associated with automated devices has enabled improved system functionality, it has come with system cost, weight and complexity burdens.  
           [0003]    Many of today&#39;s modem chemical processes, including oil or petroleum refining, food and drug manufacturing and electric generation, rely extensively on the complex interconnection of pumps, piping and valves to effect a particular chemical conversion or mixture. One of the more frequently used forms of chemical processing involves the use of a fluid dispensing system, wherein a single fluid transport conduit permits multiple fluids to be selectively injected into a main stream to create a final mixed product for dispensing. However, there are situations in which fluid dispensing systems, although potentially beneficial, have not found application. One example is the preparation of etchants for metals in the metallurgical laboratory. They are usually prepared in small quantities (typically 100 ml or less), and owing to their reactivity with metals are corrosive and hazardous by nature. Typically, these etchants are recipes comprising a mixture of constituents formulated to react with a given metal. As such, precise control over the ratios to ensure a quality etchant mixture is necessary. While such precise control with prior art systems embodying fluid dispensing features is possible, their reliance on multiple dedicated pumps or redundant valve and actuator engaging configurations results in complex, expensive systems that require that each actuator must be equipped with numerous dedicated devices in order to control multiple valves.  
           [0004]    Another especially acute problem involves the precise control of minute quantities of fluids. When small quantities of injectants are being mixed, such as with medicament samples, acid etchants and related chemical reagents, the lack of a simplistic fluid dispensing system, which can meter precise amounts of the desired fluids reliably, affordably and safely is a hindrance to the creation of application-specific fluids. In response to ever-increasing demands that end product mixtures be of extremely high quality, with minimal contamination, waste and risk of exposure of personnel or the environment to hazardous substances, existing systems have added backup and redundant componentry, exacerbating system cost and complexity. Depending on the size of the fluid transport conduit in a fluid dispensing system, the driver fluid in the conduit&#39;s main stream could be either a conventional liquid (most notably water) carrier or an immiscible gas (most notably air) being drawn into the main stream through a supply valve. With a liquid-based driver system, a “pusher” fluid is used to move the injectant through the main stream of the conduit and into the dispensing unit. By using an “all liquid” approach (i.e.: liquid pusher and liquid injectant), the potential for an extremely accurate final mixture exists, due in part to the incompressibility of the liquids. However, the present inventors have discovered that the size of the conduit effects the mixing process. If the conduit is too large, the discrete volume of fluid in the conduit tended to collapse and mix with the pusher fluid. Likewise, if the size of the conduit is too small (as can be the case when small quantities of injectant are used), friction effects can dominate, resulting in slow dispensing speeds and higher power requirements.  
           [0005]    In metallurgical laboratories, metallurgists and metallurgical technicians routinely prepare etchants, mixtures of acids, solvents, and salts, which are used to etch metallographic samples, thus revealing microstructure and other features. The preparation of etchants, as it is currently practiced in the metallurgical lab, entails: the transfer of acids and solvents from the bottles in which they are supplied to smaller containers to facilitate handling; the measurement of volumetric quantities of these acids and solvents using graduated cylinders; and the mixing of the same in a container along with mass quantities of salts, if required. The handling and measurement activities are time consuming and entail significant risk to both personnel and the environment. Alternatively, some laboratories transfer acids and solvents from the containers in which they were purchased into an individual dispenser for each reagent, or insert a bottle top dispenser into each bottle in which a reagent is purchased. After the etching operation is completed, the etchant must be neutralized prior to disposal. Many laboratories perform the pH neutralization procedure with sodium hydroxide pellets. Because sodium hydroxide is highly reactive in acid, as are related acid neutralizers, it must be added slowly to minimize foaming and spatter. Personnel performing this operation check the neutralization process frequently using litmus paper to determine the pH of the solution. This can be tedious, time-consuming, and potentially dangerous to personnel, adjacent laboratory equipment and the ambient environment. In addition, it is frequently the case that too much neutralizer is added, thus necessitating the addition of more acid in an ad hoc process to ensure that an acceptable pH (typically in the range of 6 to 8) is reached prior to disposal. Not only does the prolonged exposure due to this back-and-forth process present additional risks to personnel, equipment and the environment, but it generates additional quantities of waste product as well.  
           [0006]    Other applications for a fluid dispensing system capable of handling acids and solvents exist outside of the metallurgical laboratory. One example is compositional analysis of metals in the chemical laboratory using a technique known as inductively coupled plasma. Prior to analysis, the metal to be analyzed must be placed in liquid solution. To accomplish this, chemists dissolve the metallic sample in mixtures of acids and solvents similar to etchants. In the chemical laboratory, the preparation, neutralization and disposal of these solutions of acids and solvents proceeds in much the same way as it does in the metallurgical laboratory. Similarly, in other contexts, examples of commercially available systems exist in which a peristaltic pump is devoted to each liquid to be dispensed. Such systems may be combined with valve manifolds to redirect liquids to a plurality of locations. Valves in such manifolds are generally activated individually using electromechanical devices such as solenoids. Other commercially available systems use multiple screw driven syringes or multiple syringe pumps to dispense a multiplicity of liquids. In any event, exposure to harsh chemicals can present safety and operability risks that typically require additional costs associated with redundant, protective system componentry.  
           [0007]    Accordingly, there exists a need for a fluid dispensing system that can offer greater simplicity, improved safety to using personnel, improved conservation of constituent fluids, and greater speed of fluid mixture preparation.  
         SUMMARY OF THE INVENTION  
         [0008]    This need is met by the present invention by providing a simple, reliable means for controlling the opening and closing of multiple fluid insertion valves arranged in a common valve manifold without having to rely on the use of complicated, redundant actuators. The current invention preferably employs a cam which can be translated by means of a lead screw across a linear array of valves and rotated to actuate a given valve when located in juxtaposition to that valve. By placing a single pump upstream of the valve manifold and a dispensing nozzle downstream, the multiplicity of pumps can be eliminated. The inventors of the present invention have further recognized that their approach increases throughput of the dispensed final product while avoiding the complexity and redundancy of larger, heavily-arrayed fluid transport conduit systems. One of the chief attributes to the system of the present invention is that by using a single cam on a single shaft as a valve actuator engaging member, thus resulting in a single translation member and a single rotation member, the device is inherently simple and compact. Further system simplicity is ensured by the use of one or more conventional motors to move the cam, such as a stepper motor, servomotor or rotary solenoid. Alternatively, the use of multiple pumps of different sizes could be employed to achieve high volumetric accuracy when small amounts of reagent are to be injected into large amounts of solution. In this case pumps with different capacities may be plumbed either in parallel or in series in such a way that the smaller pump provides greater accuracy during aspiration where as the larger pump provides greater capacity during aspiration and greater speed during dispensing.  
           [0009]    In accordance with one embodiment of the present invention, a fluid dispensing system (also known as an injectant dispensing system) is disclosed. It includes at least one pump for metering precise quantities of fluid to be dispensed; one or more fluid injection lines for transporting a fluid to be dispensed, and one or more valves with valve actuators, each of the valves disposed in one of the fluid injection lines, wherein the fluid injection lines can be in fluid communication with fluid dispensing containers at one end, and with a fluid transport conduit at the other. The fluid transport conduit is also in fluid communication with the pump. Each of the valves can control the flow of a quantity of fluid through one of the fluid injection lines. At least one valve actuator engaging member is coupled to each of the valve actuators so that, based on a control signal, each actuator engaging member can force a respective actuator on the valve to open or close the valve in response to the control signal. Furthermore, a pusher fluid is selectively introduced into the fluid transport conduit to force the flow of a fluid to be dispensed through the fluid transport conduit. In the present context, a pusher fluid is one used as a carrier, such that it moves the injectant fluid through the fluid transport conduit and into the pump. The choice of a particular pusher fluid can effect the way many of the system elements are interconnected. Specifically, the size of the fluid transport conduit, pump size and type can be tailored to the dispensing of small quantities of fluids to minimize or prevent fluid intermixing and residual droplet formation. In addition, the present inventors discovered that if the pusher fluid is a liquid, an optional filter device can be disposed in the pusher fluid injection line to not only reduce contaminant presence but also provide damping for flow stability. A flow detection system is disposed adjacent the fluid injection lines, and includes at least one detector and a controller in electrical communication with the detector, valves and pump such that upon detection and comparison of a flow variation, the controller sends signals to at least one of the pump or valves to control the flow of fluid. This system is especially well-suited to the use of acids, solvents and acid neutralizers.  
           [0010]    Optionally, to meet the need of ensuring that the highly accurate approach of using a liquid pusher with a small bore conduit could be replicated without the aforementioned speed and power drawbacks, the present invention further may include an immiscible gas as the pusher in small conduit lines (such lines being commonly associated with the use of acid reagents for etchant solutions). Thus, by using a gaseous pusher fluid for small mixture quantities, where an appropriate amount of conduit is placed between the pump and the fluid injection valves in the absence of a liquid pusher, the aspiration of the fluid could be accurately metered, resulting in precise mixtures to be dispensed. Advantages of this approach include the use of a smaller, more simplistic fluid transport conduit, as well as reducing the need to dilute or mix the fluid with a water-based main stream carrier. As another option, the fluid dispensing system can include a capping mechanism adapted to be disposed in the container apertures, thus acting as a stopper to prevent unintended release of fluids from the container. Furthermore, the capping mechanism permits the flow of fluid to and from the container under normal operating conditions by being operatively responsive to pressure differentials arising out of putting fluid into and taking fluid out of the container. The capping mechanism, which is operatively responsive to a pressure differential across the aperture in each of the containers, can include the following features: a generally cylindrical body; at least one threaded groove disposed on the body&#39;s outer surface such that a complementary threaded top can be fit thereon; at least one recess disposed in an outer surface of the body and axially distant from the threaded groove. The recess is adapted to receive an O-ring to facilitate better sealing as well as easier, safer removal. At least one aperture is disposed therein to receive a fluid injection line. The capping mechanism itself may include at least one elastic vent member with at least one slit and at least one channel disposed therein; and at least one membrane plate with at least one recess and at least one channel disposed therein, where the recess is in substantially axial alignment with the slit. Slits placed in the compliant members can respond to pressure differentials between the inside and outside of the container, which then permits the insertion and withdrawal of fluid. In the alternative, the capping mechanism may include: a plurality of passages; at least one venturi; a plurality of generally spherical stoppers disposed within a chamber in the body such that they are seatably responsive to a pressure differential in the fluid transfer line that extends between the container and the fluid transport conduit, such that, upon exposure to a pressure differential, the generally spherical stoppers change their seating arrangement against the aperture. Another desirable attribute of the present invention is its incorporation of fluid containment devices that permit relatively “handsfree” fluid dispensing system operation of handling acids, solvents and dispensing liquid neutralizer. For example, the inventors discovered that when the mixing process involves hazardous substances, such as acids and related etchants, exposure of the vapors and liquids to personnel and sensitive equipment could be minimized through the use of an appropriate capping mechanism. The features of the capping mechanism permit the uninhibited access of fluid to and from the fluid container while simultaneously minimizing the chance of liquid spillage or inadvertent venting of corrosive or noxious vapors. With the inclusion of features such as this, the present invention greatly increases the efficiency of dispensing and neutralization processes by integrating improved safety features into the dispensing system&#39;s inherently simple design. As another option, a filter is disposed in the fluid injection lines to provide fluid damping. Additional options to the flow detection system include specific detector features. For example, the detector can be either an ultrasonic or optical detector, where more specifically, in embodiments using the optical detector, it can be an IR detector. Another option is the inclusion of a neutralizer with integral dye indicator into the fluid dispensing system. The neutralizer comprises a base in liquid solution mixed with a dye indicator. Upon addition to an acidic solution, the pH changes. When the pH range of 6 to 8 is achieved, the solution undergoes an abrupt color change. One example of such a neutralizer is triethanolamine, although other solutions, including sodium hydroxide, can be used. Another option includes an enclosure to house one or more of the various components of the fluid dispensing system such that they are disposed within the enclosure. Preferably, the enclosure contains the pump and valve assembly, and is sized to conveniently fit in a fume hood, or on top of a table designed to house fluid reagents, thus providing a compact, autonomous container for the fluid dispensing system.  
           [0011]    In accordance with another embodiment of the present invention, a cam assembly is disclosed. The apparatus comprises a shaft, a rotational member, a cam, and a cam driver. The shaft and rotational member each include an axis of rotation along their respective length. The cam moves in at least two degrees of freedom, where the first is preferably a translational movement operatively responsive to its threaded relationship with the turning shaft, and the second is preferably a rotational movement operatively responsive to the interaction between complementary mating cam and rotational member surfaces. Furthermore, each cam degree of freedom movement is independently responsive to shaft and rotational member motion, caused in turn by the cam driver coupled to the shaft and rotational members. In the present context, “independently responsive” means that even though the cam is coupled to both the shaft and the rotational member, it does not require the simultaneous movement of both to perform its intended function. To take up as little space as possible, that shaft can be disposed concentrically inside a hollow portion of the rotational member, and the cam can be disposed on an outer surface of the rotational member so that the axes of rotation of the shaft, cam and rotational member are coaxial. This space-saving feature is highly desirable in volume-limited applications, such as when working with hazardous substances, where the entire assembly might need to be located in a fume hood or similar device. Moreover, the cam driver need not be a single motor, but instead can comprise a first motor for imparting translational movement to the shaft, and a second motor for imparting rotational movement to the rotational member.  
           [0012]    In accordance with another embodiment of the present invention, a flow control apparatus for porting fluids is disclosed. The apparatus comprises a housing and a plurality of valves, in addition to the cam assembly described in the previous embodiment. The housing supports the plurality of valves, as well as the shaft, cam and rotational member. Each one of the valves include a valve actuator, that, on one end, is connected to the valve such that movement of the actuator opens or closes the valve. The other end of the actuator engages the eccentric portion of the cam such that rotational changes in the cam produce changes in the actuator&#39;s position. In addition, a flow detection system with at least one ultrasonic or optical sensor may be included, and, in the case of an optical detector, operable in either the IR or visible band. This flow detection system can be integrated into a microprocessor-based controller to ensure accurate and repeatable quantities of mixing fluids are being drawn into the mixing region of the pump from their containers. As with the aforementioned fluid dispensing system, this apparatus is especially well-suited to the use of acids, solvents and acid neutralizers.  
           [0013]    In accordance with yet another embodiment of the present invention, a fluid dispensing system is disclosed. The fluid dispensing system comprises, a pump, a fluid transport conduit, a valve assembly and a flow detection system, and a dispensing unit in fluid communication with the valve assembly to accept fluid from the fluid transport conduit. The fluid transport conduit provides a containment path through which the injectant fluids can be circulated. The flow detection system (similar to the previously described fluid dispensing system) is in fluid communication with the fluid transport conduit, as is the valve assembly. The valve assembly contains a plurality of valves, each of which includes a valve actuator. While the valves are designed to be either open or closed, they could optionally be coupled to a feedback-based controller to provide flow rate control. The valve assembly itself comprises a housing, shaft, rotational member, cam and cam driver similar to that of the previous embodiment. Optionally, the fluid dispensing system further comprises at least one container for supplying the injectant, where the container is in fluid communication with the valve assembly and fluid transport conduit. The aperture of the container may have a capping mechanism similar to that described in conjunction with the previous fluid dispensing system embodiment. As with the prior fluid dispensing system embodiment, an enclosure can be included to house one or more of the individual components within the fluid dispensing system. Additionally, the fluid dispensing system includes safety and convenience features, such as a dispensing unit including a dispensing nozzle to facilitate the introduction of fluid in the fluid transport conduit into a receiving container (such as a beaker), a mixing device (such as a magnetic stirrer) to improve mixing of dispensed fluid in the receiving container, a door disposed on the enclosure to prevent fluid spillage from escaping, an interlock that prevents the fluid dispensing system from operating until the door is closed, a drain disposed within the dispensing unit to collect any fluid spillage inside the dispensing unit, a pressure relief valve to protect the fluid transport conduit from becoming overpressurized, and a waste receptacle attached to the drain and pressure relief valve. Optionally, the fluid dispensing system can accommodate various pusher fluids in a fashion similar to that of the previous embodiment fluid dispensing system. Also as with the previous embodiment fluid dispensing system, a neutralizer with integral dye indicator can be added to facilitate efficient neutralization of the dispensed fluids, which can include, among others, acids, solvents and acid neutralizers.  
           [0014]    In accordance with still another embodiment of the present invention, a method for controlling the amount of fluid flowing through at least one of a plurality of valves is disclosed. The method comprises the steps of placing at least one fluid container in operative communication with at least one valve, arranging a valve actuator to be in mechanical communication with the valve, mounting a cam to both a shaft and a rotational member such that the cam is operatively responsive to movements in the shaft and rotational members, placing a cam driver to provide translational and rotational movement to the cam through the shaft and rotational member, and controlling the opening or closing of the valve in response to a predetermined process condition. This last step is accomplished by receiving an input from a control mechanism, sending a control signal from the control mechanism to the cam driver, translating the cam until it is aligned with the valve actuator, then rotating the cam to force engagement between it and the valve actuator to open the valve until a desired amount of fluid is injected into the fluid transport conduit. Optionally, the method is accomplished with a device that has the shaft axis of rotation coaxial with the rotational member axis of rotation, and where the control mechanism comprises a microprocessor-based controller. The method may also include installing a flow detection system, whereby air pockets or bubbles injected into either the fluid transport conduit or the fluid injection lines can be sensed, then correlating the sensed value against a predetermined fluid volume to be dispensed, then calculating a flow adjustment signal to send to the cam driver to adjust the valve to remain open for an additional period to ensure adequate quantities of fluid are dispensed. Other features that may be incorporated include an aperture in the fluid container with a capping mechanism such that when the fluid is flowing neither to nor from the container, the capping mechanism prevents the fluid from escaping from the container, as well as to facilitate the flow to or from the container during such periods that fluid transport is necessary. Such capping mechanisms having already been described herein.  
           [0015]    In accordance with still another embodiment of the present invention, a method for preparing metallurgical etchants is disclosed. The steps of this method include: placing at least one fluid container with a fluid to be dispensed disposed therein in operative communication with at least one valve; arranging a valve actuator to be in mechanical communication with the valve; placing a fluid injection line in fluid communication with the valve such that the fluid injection line is also in operative communication with the fluid container; placing a fluid transport conduit in fluid communication with the fluid injection line; placing at least one pump for metering precise quantities of the fluid to be dispensed in fluid communication with the fluid transport conduit, thereby establishing fluid communication between the pump and the fluid container; selectively introducing a pusher fluid into the fluid transport conduit to force the flow of the fluid to be dispensed through the fluid transport conduit; monitoring the flow of the fluid to be dispensed through the fluid injection line with a flow detection system; controlling the opening or closing of the valve in response to a predetermined process condition by receiving an input from the controller, and sending a control signal from the controller to the valve actuator, thereby forcing engagement between the valve actuator and the valve to an extent dictated by the control signal such that the valve adjusts a flow of the fluid to be dispensed; and operating the pump to move a predetermined amount of the fluid to be dispensed from the fluid container, through the valve, fluid injection line, fluid transport conduit, and into a dispensing unit in fluid communication with the fluid transport conduit so as to accept fluid therefrom. The flow detection system itself comprises at least one detector placed in sensor communication with the fluid injection line and a controller in electrical communication with the detector, valve and pump such that upon detection and comparison of a flow variation, the controller sends signals to at least the pump or valve to control the flow of the fluid to be dispensed. Optionally, the fluid to be dispensed by the method is an acid, solvent or acid neutralizer. The method may further include a step to neutralize the etchant after use by dispensing an acid neutralizer with an integral dye indicator contained therein to indicate when a desired pH level is attained. This step could obviate the need to iteratively adjust the pH of the spent etchant.  
           [0016]    In accordance with still another embodiment of the present invention, a method for preparing metallurgical etchants is disclosed, comprising the steps of: placing at least one fluid container with a fluid disposed therein in operative communication with at least one valve of a plurality of valves; arranging a valve actuator to be in mechanical communication with the valve; mounting a cam to both a shaft with an axis of rotation along its length and a rotational member with an axis of rotation along its length such that the cam is independently responsive to rotation of the shaft and rotational member; placing a cam driver for translating and rotating the cam relative to the valve in operative communication with both the shaft and rotational member; and controlling the opening or closing of the valve in response to a predetermined process condition. The step of controlling includes the following: receiving an input from a control mechanism; sending a control signal from the control mechanism to the cam driver; translating the cam until the cam is aligned with the valve actuator; and rotating the cam to force engagement between it and the valve actuator to an extent dictated by the control signal such that the valve actuator forces the valve to adjust a flow of the fluid therethrough. Optionally, the fluid to be dispensed is an acid, solvent or acid neutralizer, where the neutralizer can include an integral dye indicator.  
           [0017]    Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:  
         [0019]    [0019]FIG. 1 is a schematic of a general fluid flow path according to an embodiment of the present invention;  
         [0020]    [0020]FIG. 2 is a schematic illustration of an enclosed fluid dispensing system of an embodiment of the present invention;  
         [0021]    [0021]FIG. 3 is a schematic illustration of a valve arrangement according to an embodiment of the present invention;  
         [0022]    [0022]FIG. 4 is a top view of a flow control apparatus with housing, cam assembly and a plurality of valves;  
         [0023]    [0023]FIG. 5 is an isometric view highlighting the cam assembly of FIG. 4;  
         [0024]    [0024]FIG. 6 is an illustration of a reagent fluid containment bottle with a stopper capping mechanism in accordance with an embodiment of the present invention; and  
         [0025]    [0025]FIG. 7 is an illustration of an alternate embodiment stopper capping mechanism. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Referring first to FIG. 1, a general flowpath for a continuous fluid flow system  1  is shown. Pump  12  moves a reagent (comprising a mixture of individual fluids  13 , each of which are stored in container  14 ), through a conduit  15  to a fluid dispensing unit  16 , which holds a fluid receptacle  28 . Preferably, pump  12  is a metering syringe pump powered by a stepper motor (not shown), wherein during the suction phase, it draws fluid  13  out of container  14  into fluid injection line  22 , past a valve  18 . This process is repeated with as many fluids as is necessary to achieve the desired mixture. Once this is accomplished, pump  12  then pumps the mixture into the main stream of conduit  15 . Once the fluid is dispensed into fluid receptacle  28 , it is mixed, preferably through a magnetic stirrer  17 . A series of valves  18  are employed to control the introduction of fluids  13  into conduit  15 .  
         [0027]    One or more flow sensors  21  (alternately referred to as detectors) are used in various locations in the flowpath to detect fluid flow. Sensor  21  works by sensing the presence of air pockets, either inherently present in the conduit  15  or fluid injection line  22 , or in the form of injected bubbles  20 . Preferably, the sensor includes a detector and a signal transmitter, both of which are mounted to a common board, such as a printed circuit board (PCB)  21 A,  21 B and  21 C, where PCBs  21 A,  21 B and  21 C are adapted to fit around the fluid injection lines  22 , conduit  15  or pressure relief tube  27 B, depending on the application. Although the sensors  21  are shown notionally mounted to three separate PCBs  21 A,  21 B and  21 C, they could also be mounted to a single elongate PCB (not shown). In the present invention, sensors  21  may be either ultrasonic, optical, or any other type of device capable of sensing flow changes and converting the sensed signal into a machine or human readable flow number. One suitable optical sensor includes light- or infrared-emitting diodes (LEDs and IREDs, respectively) arranged to transmit a signal to a phototransistor. Although the sensors could also be used to monitor flow rate, it is for measuring discrete quantities of fluid to be injected that they find their primary use in the present invention. For example, each cycle of metering pump  12  (which may be controlled by the aforementioned stepper motor) is designed to suction a precise quantity of fluid  13  present in fluid injection line  22 . However, the presence of air pockets (not shown) in the fluid injection line  22 , which is indiscriminately drawn up into the pump  12  for mixing, can result in less than the desired quantity of fluid  13  to be drawn up into pump  12  for mixing. The presence of sensors  21  on PCB  21 A is designed to prevent these inaccuracies by permitting the density, frequency or spacing of these air pockets within the fluid injection line  22  to be detected, then correlated with the amount of fluid  13  to be aspirated through the use of an automated feedback control arrangement, which is usually a microprocessor-based device such as controller  23 . The automated feedback control arrangement will typically utilize algorithms to detect the presence of air pockets in the fluid injection line  22  picked up by sensors  21  on PCB  21 A. This precise interactive control of the fluid metering components ensures reliable, highly repeatable resulting mixtures.  
         [0028]    A bubble injection mechanism controls, via bubble injection valve  19 A, the introduction of bubbles  20  of an immiscible gas, such as air, into conduit  15  to provide thorough and precise quantities of the mixed reagent being discharged from pump  12 . It is noted that in the event a liquid “pusher” is desired over an immiscible gas, the bubble injection valve  19 A (or an equivalent) could be utilized, preferably in the same general location. Similarly, if a liquid pusher is used, a filter device can be added to ensure that particulate contamination is not introduced into the mixed reagent, as well as providing damping benefits to ensure proper fluid injection, mixing and transport. Sensor  21  mounted to PCB  21 B can be used to detect flow of fluid  13  from container  14 . In another adaptation, sensor  21  can be mounted to PCB  21 C to detect the presence of any flow through pressure relief tube  27 B and valve  24  into waste receptacle  25 . Bubbles  20  can provide, in addition to a “pusher” fluid to move fluid  13 , contamination reduction features, which due to the scrubbing action of bubbles  20  as they traverse conduit  15  remove fluid droplets from the line that could contaminate a subsequent mixture, as well as optional flow rate measuring capability, as previously mentioned. In performing their flow measuring function, the bubbles  20  are first injected by bubble injection valve  19 A into the conduit  15  upstream of the location where the fluid reagents  13  are to be inserted. One or more of the sensors  21  are placed downstream of the reagent insertion location, such as on PCB  21 B, and usually through either optical or sonic means, detects the flow rate based on the bubble flux. The sensor  21  sends a signal, typically in the form of a voltage, to a controller  23  for comparison to a predetermined flow constant. Based on a comparison of the measured flow rate with the flow constant, the controller  23  can provide active feedback to determine how much and how fast fluid reagent should flow through the main stream of the conduit  15 , and then into either fluid receptacle  28  or waste receptacle  25  through pressure relief valve  24 , which is included as a system safety measure. Drain  26  and waste tube  27 A are situated on a lower surface of fluid dispensing unit  16  to ensure that any spilled fluid is also routed to waste receptacle  25 . Dispensant control valve  19 B is coupled to the controller  23  (coupling line not shown to minimize drawing complexity) to ensure that reagent is isolated from fluid receptacle  28  during periods where pressure relief valve  24  is activated. Similarly, dispensant control valve  19 B is closed when pump  12  is aspirating liquids during its suction phase.  
         [0029]    As shown in FIG. 2, a continuous fluid dispensing system  1  includes an enclosure  10 , a pump  12  with a motor  29  (which is typically a stepper motor or servomotor), a fluid dispensing unit  16  a part of which includes a fluid dispensing nozzle  16 A, and a fluid transport conduit  15 . The exterior dimensions of fluid dispensing system  1  are such that the system can fit in a conventional laboratory fume hood  2  with sliding glass front door  2 A, and on top of a stand  3 , under which a plurality of fluid containers  14  can be stored. Passage of fluid injection lines  22  from enclosure  10  to stand  3  can be accomplished by mating apertures (not shown) on respective surfaces of the two. While in the preferred embodiment the pump  12  can be the aforementioned syringe pump, the inventors recognize that other types of pumps capable of precise metering of the desired fluid are equally valid substitutes. The space defined by fluid dispensing unit  16  is user-accessible via an opening in an upstanding wall (not shown) of enclosure  10 , with such opening covered by a safety door  31  slidably mounted on the upstanding wall and positioned to block user access to fluid dispensing unit  16  and fluid dispensing nozzle  16 A during operation. An optional safety interlock system (not shown) is added as a failsafe way to ensure fluid dispensing system  1  does not operate until safety door  31  is closed, thus preventing the inadvertent discharge of fluid  13  to the environment, the user, or both. Housing  32  is used to support the plurality of valves  18 , which are used to fluid connect conduit  15  and pump  12  to dispensing unit  16  and dispensing nozzle  16 A. Housing  32  is preferably placed at an incline to further ensure that any fluids in the main stream of conduit  15  drain, by the force of gravity, to the lowermost position, thus reducing the incidence of conduit wall buildup of droplets. Enclosure  10  further includes ventilation holes  33 A and  33 B, as well as exhaust tube  33 C and exhaust stack  33 D to facilitate venting of enclosure  10  and dispensing unit  16 . Exhaust stack  33 D can be further integrated into the ventilation duct (not shown) of fume hood  22 , which can in turn be pumped away by facility ventilation systems (not shown). The ventilation enabled by ventilation holes  33 A and  33 B, exhaust tube  33 C and exhaust stack  33 D is useful in situations where fluid  13  emits noxious or corrosive fumes, which absent purging airflow through the enclosure  10 , could present a hazard to users as well as hasten the degradation of exposed components.  
         [0030]    Referring now to FIG. 3, one representative valve  18  of the plurality of valves  18  housed in housing  32  is shown. The valve  18  includes a valve stem  40  that is biased by a spring  41  in a closed position. To open valve  18 , a pushrod actuator  42  is forced by cam  51  (described in more detail later) against a rear stop member  43  of valve stem  40 , thus causing valve stem  40  to overcome the spring bias, and creating an open path for fluid  13  to be suctioned up by pump  12  to flow from container  14 , through fluid injection line  22 , and into conduit  15 . O-rings  44  are placed in grooves  45  of valve stem  40  to provide leak-resistant sealing around valve inlet  46 . Each of the valves  18  are mounted inside housing  32 , as well as to cover plate  47 . Container  14 , which holds a supply of fluid  13 , is situated vertically below valve  18  so that excess fluid could be gravity-fed back into the container  14 . Flow sensor  21  is mounted on PCB  21 A, which is designed to fit around the fluid injection line  22 . The connection between container  14  and valve  18  is secured and sealed by gland nut  48  and ferrule  49 . In a preferred embodiment, the containers  14  are bottles, and are constructed of a material that can withstand chemical attack from the fluid therein. Where the fluid reagents are corrosive (such as an acid), the fluid-exposed components, including tubes, lines, conduits, containers, seals and O-rings are made from glass, fluoroelastomers such as Viton®, perfluoroelastomers such as Kalrez®, or related material such as Teflon® or polytetrafluoroethylene (PTFE).  
         [0031]    Referring now to FIGS. 4 and 5, flow control apparatus  30  includes housing  32 , which contains a cam assembly  50  and a plurality of fluid injection valves  18 , as well as bubble injection valve  19 A, dispensant control valve  19 B, and pressure relief valve  24 . Each of the valves  18  are connected to an individual container  14 , as well as to conduit  15 . The valves  18  are preferably aligned in such a way so as to be readily accessible to being in mechanical communication with the single cam  51 , either in a single line as shown, or in dual parallel lines with the cam  51  spaced parallel to and equidistant between them (not shown). Once aligned, the cam  51  is then rotated so that its inherent eccentricity will engage the valve&#39;s actuator  42 , thereby forcing a change in the amount of fluid allowed to flow through the valve  18 . By this arrangement, the single cam  51  can control the movement of every valve, one at a time by responding to motor-driven signals from microprocessor-based controller  23 .  
         [0032]    Cam assembly  50  comprises cam  51 , shaft  52 , rotational member  53 , bushing  54 , first motor  55  and second motor  56 . By translating up and down the length of the shaft  52 , cam  51  can be positioned in relation to any one of the valves  18 . Then, by rotating, cam  51  can actuate any one of the valves  18  according to predetermined needs for a particular fluid. Preferably, shaft  52  is a smaller diameter generally cylindrical cross section lead screw shaft, which imparts translational movement to cam  51 . In addition to being mounted to shaft  52 , cam  51  is mounted to the rotational member  53 , which is of larger diameter than shaft  52 . In the present context, when one object is “mounted” to another, it means that the objects are in direct, uninterrupted, contiguous mechanical communication with one another, with no other components in between. Thus, one can either be pivotally or rotatably attached to the other (such as through a hinge, bearing or pivot), or simply supported on the other (such as in an unattached, resting relationship), or the objects can be conventionally attached to each other (such as by bolting, gluing, screwing, welding, soldering, and the like). Rotational member  53 , which includes a larger diameter cam engaging section  53 A, smaller diameter cam driver engaging section  53 B, hollow center section  53 C and generally planar surface  53 D, imparts rotational movement to cam  51 . The axes of rotation of the cam  51 , shaft  52  and rotational member  53  are coaxial, with shaft  52  disposed inside the hollow center section  53 C of rotational member  53 , terminating in a receiving cup (not shown) at a distal end of hollow center section  53 C which, along with bushing  54  disposed between shaft  52  and rotational member  53  at a proximal end of hollow center section  53 C, maintains proper alignment between the shaft  52  and rotational member  53 . Specifically referring to FIG. 3, wherein shaft  52  and rotational member  53  are viewed looking down their mutual axis of rotation, and with bushing  54  removed for clarity, rotational member  53  defines a truncated cylindrical cross section, revealing a generally planar surface  53 D that engages cam  51 , while simultaneously permitting uninhibited connection between shaft  52  and cam aperture  51 B, where the size of aperture  51 B is shown exaggerated and without helical-shaped threads  52 A for clarity. The combined translational and rotational movement of cam  51  is referred to as motion in two degrees of freedom. As used herein, the term “degrees of freedom” coincide with the convention used in solid or continuum mechanics, where a continuous medium in Euclidean space can experience a total of six degrees of freedom of motion: three translational (along each of the x, y and z axes in a Cartesian system), and three rotational along each of the same three axes.  
         [0033]    Shaft  52  is aligned with rotating member  53  by bushing  54 . Shaft  52 , rotational member  53 , first motor  55  and second motor  56  are conventionally mounted to housing  32 , while bushing  54  is mounted to both shaft  52  and rotational member  53 . Translation movement of cam  51  is achieved by using the first motor  55 , disposed at one end of housing  32 , to turn shaft  52 . Helical-shaped threads  52 A extend substantially between opposing ends of the outer surface of shaft  52 , and engage inner surface  51 A of an aperture  51 B in cam  51 , which is complementary threaded. Once cam  51  is put into aligned relationship with pushrod  42  of a selected valve  18 , rotational movement of cam  51  can be achieved by using the second motor  56  disposed at the opposing end of housing  32  to turn rotational member  53 . Upon rotation, eccentric portion  51 C of cam  51  comes into contact with pushrod  42 , forcing it to open or close valve  18  to its desired position, which, in turn, alters the amount of flow through fluid injection line  22 , which is mounted in gland nut  48  and ferrule  49 . While the configuration of FIGS. 4 and 5 depict the use of two motors, one for each of rotational and translational movement, it is noted that a single motor could be used to provide both forces through, for example, a clutch or gearing arrangement between the motor, shaft and rotational member. Regardless of the number of motors used to provide cam  51  movement, it is noted that conventional stepper or servomotors provide reliable, inexpensive power. In addition, while the embodiment depicted in FIG. 5 notionally includes four valves, it is readily appreciated that the present invention can accommodate any number of valves, limited only by the needs of the end use application.  
         [0034]    Referring now to FIG. 6, capping mechanism  60  acts as a stopper to be placed in the aperture  14 A of container  14  to allow the insertion and removal of fluid from container  14  while simultaneously limiting exposure of the fluid (not shown) disposed therein to the ambient environment, in order to inhibit spillage of the fluid or release of vapors. Capping mechanism  60  is made up of a body  60 A, with threads  60 B disposed on the outer surface thereof to engage a complementary threaded inner surface of top  60 C and body disengaging nut  60 L. Vent membrane  60 D and membrane plate  60 E, each with substantially centrally disposed channels  60 F,  60 G, respectively are axially-aligned disk-like members that fit in chamber  60 K disposed in the top of body  60 A such that they rest on ledge  60 M. Vent membrane  60 D, which is typically made of a compliant elastic material, such as Viton®, includes a plurality of slits  60 H disposed circumferentially about channel  60 F. These slits  60 H can open in response to pressure differentials across the surface of vent membrane  60 D. Recesses  60 J, substantially axially aligned with slits  60 H, permit fluid communication between chamber  60 K (which itself is in fluid communication with the gaseous region inside container  14  above the liquid line  13 A by virtue of passage  60 N being of slightly greater diameter than fluid injection line  22 ) and the ambient environment. Top  60 C, through threaded engagement with threads  60 B, secures vent membrane  60 D and membrane plate  60 E in an axially fixed position relative to chamber  60 K. Body disengaging nut  60 L, with internal threads (not shown) to engage threads  60 B of body  60 A, is used to gently but firmly remove capping mechanism  60  from aperture  14 A. Fluid injection line  22  can frictionally engage channels  60 F and  60 G to secure fluid injection line  22  in place. Passage  60 N is axially disposed in body  60 A and extends from the bottom of the chamber  60 K through to the bottom plug portion  60 P, thereby allowing gas in container  14  to be vented through slits  60 H and recesses  60 J upon return of liquid through fluid injectant line  22  to container  14 . During aspiration of liquid into fluid injectant line  22  as a result of suction applied to fluid injectant line  22 , air enters the container  14  through slits  60 H, recesses  60 J, chamber  60 K, and passage  60 N. Note that a second membrane plate (not shown) identical to membrane plate  60 E could be situated under vent membrane  60 D to create a stacked, sandwich structure. Such a configuration could be included in the event that additional support of vent membrane  60 D is desired. The portion of capping mechanism  60  designed to fit inside the aperture can optionally include one or more O-ring grooves (not shown) with inserted O-rings  70 .  
         [0035]    Alternatively, for containers  14  which have external threads on the neck of the bottle (not shown), the capping arrangement previously described can be simplified; in this case including solely an oversized variant (not shown) of threaded top  60 C with a smaller opening (not shown) sized to accommodate the fluid injection line  22 , membrane plate  60 E and vent membrane  60 D. The internal threads on the oversized top would engage the external threads on the neck of the container  14 , while an arrangement of vent membrane  60 D and one or more membrane plates  60 E can be axially disposed between the threaded top and the top of the neck of container  14 . Fluid is transferred either into or out of the container  14  in the same manner as above. As previously discussed, two membrane plates  60 E may be used to sandwich a single vent membrane  60 D in this arrangement.  
         [0036]    Referring now to FIG. 7, an alternate embodiment of the capping mechanism shown in FIG. 6 is shown, with capping mechanism  160  and spherical-shaped stopper members  161 ,  162 , which together comprise a two-way vent. During the suction phase, where fluid  13  is being dispensed from container  14  through fluid injection line  22 , a partial vacuum is created in fluid injection line  22  which, due to it being in fluid communication with venturi  163 C through fluid  13 , the gaseous region above the liquid line  13 A, and the gap between fluid injection line  22  and access tube  164  in capping mechanism  160 , draws in higher pressure ambient air from outside the container  14 . For ambient air to reach venturi  163 C, it is necessary that it push smaller sphere  161  out of the way. The weight of smaller sphere  161  is such that the incoming air is of sufficient pressure to cause smaller sphere  161  to raise up off of small seating throat  165 , thus admitting air into container  14  via passages  163 A and  163 B and venturi  163 C. The incoming air, which is in fluid communication with the gaseous region inside container above liquid line  13 A through gaps between fluid injection line  22  and access tube  164 , exerts pressure on fluid  13 , pushing it up and into fluid injection line  22 . When the pressure is equalized, smaller sphere  161  reseats on small seating throat  165 . During the fluid input phase, the process is reversed. Increased pressure in the fluid injection line  22  forces smaller sphere  161  even more forcefully against small seating throat  165 . In addition, the higher pressure overcomes the gravitational force on larger sphere  162 , and lifts it off large seating throat  166 , placing a vent port  167  (and the lower pressure ambient air) in fluid communication with the higher pressure gaseous region situated above liquid line  13 A in container  14 . Larger sphere  162  is massive enough so as to positively reseat upon return to pressure equilibrium between container  14  and the ambient environment, and in so doing, reduces the likelihood that the enclosed fluid will evaporate. It is also noted that smaller sphere  161  and larger sphere  162  could both have their seating enhanced by the addition of O-rings (not shown). Ambient conditions are defined as those which exist outside of a fluid&#39;s primary container, and typically include pressures and temperatures found in normal industrial or laboratory settings. Thus, if the fluid resides in a bottle, the environment outside the bottle is considered “ambient”, even if the bottle is itself contained within another, larger enclosure. The reduction in the likelihood of evaporation is important for fluids with high vapor pressures, such as acids and solvents. The portion of stopper  160  above the aperture of the container  14  can optionally have grooved outer surface, to engage a threaded top (not shown for clarity). The top facilitates easier, safer removal; by screwing the top down, it interacts with the grooves in capping mechanism  160  to gently, but smoothly lift capping mechanism  160  out of the container aperture, thereby preventing a recoil or snapping action when the capping mechanism  160  finally disengages from the container  14 . As with the previous embodiment, the portion of capping mechanism  160  beneath the aperture can optionally include O-ring grooves  168 . Their inclusion, in conjunction with inserted O-rings  170 , also helps prevent the sudden, often violent snapping action of the container upon removal of capping mechanism  160 . Fluid injection line  22  is friction fitted into the uppermost portion of capping mechanism  160 , with sealing provided by an additional set of O-rings  171  disposed near the top.  
         [0037]    Having described the invention in detail and by reference to the aspects thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention which is defined in the appended claims: