Abstract:
A diaphragm pump having an active feedback system is provided. The diaphragm pump includes a diaphragm pump having a housing including a pumping cavity. The pump cavity having at least one diaphragm dividing the pumping cavity into a first pumping chamber and a first pump actuating chamber. A rod is attached to and reciprocally movable along an axis with the at least one diaphragm with the rod including a ferromagnetic material. An induction coil disposed around the rod wherein relative axial movement between the inductance coil and the ferromagnetic material of the rod varies the inductance of the induction coil. Also provided is a diaphragm pump having an active feedback system in which the rod has an electrically conductive, diametrically tapered portion. A linear displacement sensor is disposed next to the tapered portion which induces a current in the tapered portion and generates an output voltage proportional to a relative position between the linear displacement sensor and the tapered portion.

Description:
This invention relates to active feedback devices for diaphragm pumps, and more particularly for air operated diaphragm pumps. 
     Air operated diaphragm pumps use compressed air to operate diaphragms which alternately draw in and discharge a material to be pumped. Such diaphragm pumps are known in the art and are widely used in pumping a wide variety of materials. Examples are shown in U.S. Pat. Nos. 4,854,832; 4,936,753; and 5,391,060, the disclosures of which are incorporated herein by reference. The diaphragm pumps disclosed in these patents have a first diaphragm coupled to a first end of an axially reciprocating shaft, and a second diaphragm coupled to a second end of the shaft. Each diaphragm constitutes a flexible wall that separates a liquid chamber from an air chamber. The liquid chambers are connected to a common intake manifold and a common discharge manifold, and check valves are positioned at the fluid inlets and outlets of the fluid chambers. 
     In operation, the axially reciprocating shaft is reciprocated by pressurizing the first air chamber while venting the second air chamber, and then venting the first air chamber while pressurizing the second air chamber. As the shaft moves in one direction, the first diaphragm pushes the fluid out of its fluid chamber into the discharge manifold, while the second diaphragm draws fluid into its fluid chamber from the intake manifold. When the diaphragms and connecting rod have traveled a predetermined distance, a mechanical switch is tripped. This mechanical switch in turn shifts a main valve that reverses the pneumatic action to pressurize the second air chamber and reverse the direction of the shaft. As the shaft begins to move in the other direction, the first diaphragm draws fluid into its chamber from the intake manifold, and the second diaphragm pushes the fluid out of its chamber into the discharge manifold. The pump continues this reciprocation until the air supply is stopped. 
     Various factors including the dynamics of the fluid being pumped affect the rate of reciprocation of the connecting rod. In the case of more viscous fluids, for a given air supply pressure, the connecting rod will be caused to reciprocate more slowly thus reducing the output rate of the pump. In attempting to compensate for inequalities between the desired output and the actual output of the pump, passive control systems have been used to measure the pump output and perform some function to increase or decrease the rate of reciprocation of the connecting shaft. One problem with conventional diaphragm pumps having such passive control systems is that they are not readily controllable except by the introduction of external flow measuring devices which add to the complexity and expense of the pump. 
     The foregoing illustrates limitations known to exist in present diaphragm pumps. Thus it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly an alternative diaphragm pump having active feedback monitoring is provided including the features more fully disclosed hereinafter. 
     SUMMARY OF THE INVENTION 
     According to the present invention, diaphragm pumps having active feedback systems are provided. In a first embodiment, the diaphragm pump includes a diaphragm pump having a housing including a pumping cavity. The pump cavity having at least one diaphragm dividing the pumping cavity into a first pumping chamber and a first pump actuating chamber. A rod is attached to and reciprocally movable along an axis with the at least one diaphragm with the rod including a ferromagnetic material. An induction coil disposed around the rod wherein relative axial movement between the inductance coil and the ferromagnetic material of the rod varies the inductance of the induction coil. 
     In a second embodiment, a diaphragm pump is provided having an active feedback system in which the rod has an electrically conductive, diametrically tapered portion. A linear displacement sensor is disposed next to the tapered portion which induces a current in the tapered portion and generates an output voltage proportional to a relative position between the linear displacement sensor and the tapered portion. 
     The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with accompanying drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a partial cross-sectional view of a diaphragm pump having an active feedback system according to one embodiment of the present invention; 
     FIGS. 2A-2C are cross-sectional schematic views of the diaphragm pump shown in FIG. 1 moving through successive stages of a pumping stroke; and 
     FIGS. 3A-3C are cross-sectional schematic views of a diaphragm pump having an active feedback system according to an alternate embodiment of the present invention moving through successive stages of a pumping stroke. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is best understood by reference to the accompanying drawings in which like reference numbers refer to like parts. It is emphasized that, according to common practice, the various dimensions of the component parts shown in the drawings are not to scale and have been enlarged for clarity. 
     Referring now to the drawings, shown in FIG. 1 is a diaphragm pump  2  having two pumping cavities formed between an air cap  4  and a fluid cap  6  and an active feedback system according to the present invention. Each cavity includes a pumping chamber  8  and a pump actuating chamber  12  that are separated by a pumping diaphragm  10  spanning the width of the cavity. Typically, pumping chambers  8  receives a fluid for pumping, however, particulate matter such as powders may also be pumped. Typically, pump actuating chambers  12  are provided with air as the motive gas, however, other gases such as nitrogen may be utilized for this purpose. 
     Diaphragms  10  are generally circular membranes typically made of a relatively flexible material, e.g., rubber or a thermoplastic elastomer (TPE), and having an outer peripheral portion  20  that is clamped or otherwise held in a stationary position against the pump housing. Diaphragms  10  also include a centrally located portion  21  and a working portion  22  that joins the central and peripheral portions. The central portion  21  is typically clamped between a pair of rigid backup washers  24 ,  25  and secured by a threaded bolt  9  which passes through centrally located holes in the backup washers and the diaphragm into the ends of connecting rod  11  as shown. 
     Rigid backup washers  24 ,  25  are typically metal castings that provide rigid support for diaphragms  10  during operation of diaphragm pump  2 . The working and central portions of the diaphragm are displaced in a reciprocating manner as described above to drive liquid out of the pump. O-rings  13  disposed on connecting rod  11  seal each pump actuating chamber  12  from the other. Each pumping chamber  8  is connected to an intake valve  14  and exhaust valve  16 , with (i) intake valve  14  connected through an intake manifold  15  to a source of fluid or other material to be pumped through the pump and (ii) the exhaust valve  16  connected to an exhaust manifold  17 . By introducing a motive fluid such as pressurized air into one pump actuating chamber  12 , the pressure acts on the diaphragm spanning the air cap causing the diaphragm to move toward its fluid cap  6 . This displaces the fluid being pumped from the pumping chamber  8  and forces it to travel out the exhaust valve  16  and exhaust manifold  17 . As the diaphragm moves, it pulls connecting rod  11  which, in turn, pulls the other diaphragm away from its corresponding fluid cap and toward its corresponding air cap, thereby drawing fluid into the other pumping chamber through its intake valve  14  and intake manifold  15 . At the end of the stroke, the pressurized air cap is exhausted and the exhausted air cap is pressurized, reversing the motion. Thus, the diaphragm pump accomplishes a nearly constant flow of pumping through the pump by continuously driving the connecting rod back and forth in the pump. 
     According to the present invention, active feedback apparatus are provided which anticipate an output condition of a pump by reading and interpreting internal device conditions and performing some function to compensate for inequalities before they occur at the output. This is accomplished by directly and continuously monitoring the position of the diaphragms at any time during-the pump&#39;s operation. In double diaphragm pumps, because the connecting rod forms a solid link between the diaphragms, a change in its position directly represents that of the diaphragms. Thus, the output of the diaphragm pump is directly proportional to the movement of the connecting rod. 
     Generally, the active feedback apparatus according to the present invention operate by measuring the movement of the connecting rod. The movement of the connecting rod is measured in terms of its position (i.e., displacement). The rate of reciprocation (i.e., velocity) or change in the rate of reciprocation (i.e., acceleration) of the connecting rod can also be derived by measuring the displacement of the connecting rod with respect to time. 
     Shown in FIG. 1 is a first embodiment of the present invention in which a diaphragm pump  2  having a housing  3  is provided with an active feedback apparatus having an inductance coil  30  which includes an insulated wire wound about a reciprocally movable connecting rod  11 . Inductance coil  30  is disposed around and does not contact connecting rod  11 , and thus does not affect, the motion of the rod. By this design, the non-contact operation of the inductance coil provides an added inherent benefit of virtually infinite life. 
     Inductance coil  30  may be manufactured from any electrically conductive wire which is externally insulated. Preferably, the conductive wire is a copper wire or “music wire.” Music wire is a high carbon, low alloy steel with a smooth finish and typically having a gauge of 25 to 32. As will become apparent to those skilled in the art, the dimensions of the inductance coil are dependent upon the diameter and stroke of the connecting rod. 
     Inductance coil  30  is connected via leads  31  to a standard LC-type oscillator (not shown) that produces a sinusoidal waveform (i.e., one having an amplitude change as a sine function such as alternating current). In response to the inductance of the inductance coil  30 , the alternating current produces a position signal that is representative of the linear position of the connecting rod relative to the inductance coil as described in greater detail below. A suitable oscillator may include a Colpitts oscillator, which is well known in the art. 
     Connecting rod  11  includes a ferromagnetic material such that relative axial movement between inductance coil  30  and the ferromagnetic material of rod  11  varies the inductance of the coil. Connecting rod  11  reciprocates within a connecting tube  32  that is located between and connects pump actuating chambers  12 . Preferably, connecting tube  32  is made of an electrically insulating material to electrically isolate the inductance coil from connecting rod. Alternately, connecting rod  11  may be coated with an epoxy to electrically isolate the inductance coil  30  from the rod  11 . For example, a suitable coating may include an epoxy resin manufactured by Dow Chemicals of Midland, Mich., as product no. DER331 mixed with a polysebasic polyanhydride (PSPA) manufactured by Cambridge Industries of America of Newark, N.J. However, any other suitable non-conductive coating may be used. 
     Preferably, connecting rod  11  is formed of two connected halves of different materials, a ferromagnetic half  26  and a non-ferromagnetic half  27 . Ferromagnetic half  26  is made from a material which can be attracted magnetically and, preferably, is made of iron or nickel. Non-ferromagnetic half  27  is made of a material which cannot be attracted magnetically and, preferably, is made of stainless steel or plastic. Ferromagnetic half  26  and non-ferromagnetic half  27  are connected, preferably, by a threaded screw  28 . By this construction, upon moving connecting rod within connecting tube  32  as shown in FIGS. 2A-2C and described in greater detail below, the movement of the non-ferrous metal alone within inductance coil does not affect the resultant impedance of the coil. 
     Referring now to the drawings, shown in FIGS. 2A-2C is a cross-sectional schematic that illustrates the motion of two diaphragms  10  as they move through successive stage of a pumping stroke within the pumping chambers  8  and pump actuating chambers  12  of diaphragm pump shown in FIG.  1 . Operation of the pump is as described above with respect to the diaphragm pump  2  shown in FIG.  1  and is accomplished by first introducing pressurized air to the left diaphragm  10  shown in FIG. 2A causing it to exert force on the pumping chamber and expel the fluid within. This motion also causes diaphragm  10  to draw fluid into its respective fluid chamber. When the diaphragms and connecting rod  11  have traveled through the position shown in FIG. 2B to the predetermined distance shown in FIG. 2C, pressurized air is then introduced to the right diaphragm  10  to reverse the motion back through the positions shown in FIG.  2 B and then back to FIG.  2 A. By alternating the introduction of pressurized air to the left and right diaphragms in this manner, the pumping motion of the diaphragm pump is continuously repeated. 
     In operation, when the connecting rod is centered as shown in FIG. 2B, a median impedance is produced in inductance coil  30 . As shown in FIG. 2A, as the center of the rod, which divides the ferromagnetic half  26  and non-ferromagnetic half  27  of the connecting rod, travels to the right the amount of ferromagnetic material inside the inductance coil increases. This, in turn, increases the impedance of the inductance coil thereby causing the current drawn to be reduced. Bridge processing circuitry (not shown) such as that described in U.S. Pat. No. 4,667,158, the disclosure of which is herein incorporated by reference, is used to detect the amount of current drawn and, from this, determine the incremental linear position of the rod relative to the housing  3 . Conversely, in moving connecting rod to the left through the position shown in FIG. 2B to the position shown in FIG. 2C, the amount of ferromagnetic material in inductance coil  30  decreases thereby decreasing the impedance of the coil and causing the current drawn, which is detected by the bridge processing circuitry described above, to be increased. 
     Thus, to summarize, by moving the ferromagnetic half of connecting rod  11  into the coil, the mass of ferromagnetic material in inductance coil  30  changes as the connecting rod moves. This, in turn, changes the inductance coil impedance with the impedance increasing proportionally to the amount of the ferromagnetic half contained within the coil. In this manner, the inductance coil  30  may be used as a variable inductor in a resonant circuit to determine the position of connecting rod  11  from the inductance of the coil. 
     According to another embodiment of the present invention, shown in FIGS. 3A-3C is a cross-sectional schematic that illustrates the motion of two diaphragms in a diaphragm pump  42  similar to that shown in FIGS.  1  and  2 A- 2 C which incorporates a linear displacement sensor  46  with the following additional modifications. Provided between backup washers  25  is a connecting rod  44  having a diametrically tapered portion  45  located in the middle of the rod. Tapered portion  45  is made of an electrically conductive material. Linear displacement sensor  46  is located in the center body of the pump housing  3  as shown and mounted perpendicular to the connecting rod  44  so that in the center position shown in FIG. 3B, a face  49  of the sensor falls in the middle of the tapered portion  45 . 
     In operation, when connecting rod  44  is centered as shown in FIG. 3B, linear displacement sensor  46  produces a median voltage. As connecting rod  44  travels to the right as shown in FIG. 3A, the taper decreases the distance between face  49  of linear displacement sensor  46  and connecting rod  44 . As described in greater detail below, this causes the sensor to produce a lower voltage output. Conversely, as the rod shifts to the left as shown in FIG. 3C, the taper increases the gap with face  49  thereby increasing the voltage output of linear displacement sensor  46 . 
     Preferably, linear displacement sensor  46  is a non-contact sensor which uses a magnetic field (also known as an eddy-current field) across face  49  to induce a current in a metal piece placed in the magnetic field. By measuring the power loss caused by the current induced in the metal piece, the proximity of the metal piece with respect to face  49  can be determined. Preferably, a non-contact linear displacement sensor having an analog output such as a LD701 Series sensor available from Omega Engineering Inc., Stamford, Conn. is used to determine the position of connecting rod  44  based upon the output voltage detected. By this design, the non-contact operation of the linear displacement sensor provides an added inherent benefit of virtually infinite life. 
     Preferably, when using an OMEGA LD701 Series linear displacement sensor, electrically conductive tapered portion  45  is manufactured using a mild steel, a stainless steel, brass aluminum, or copper. When using an OMEGA LD701 Series linear displacement sensor in this fashion, by providing a 14-30 Vdc, 20 mA excitation voltage to leads  48 , a magnetic field is provided across face  49 . 
     Upon moving connecting rod  44  sequentially from the position shown in  3 A to  3 C, typical output voltages ranging from 1-9 volts, respectively, are obtained which correlate with the position of tapered connecting rod  44 . These output voltages are inputted via leads  48  to a controller or computer device (not shown) which then determines the position of connecting rod  44  from the voltage signal and can perform additional signal processing and control functions. 
     The resultant position signals produced by both the inductance coil and the displacement sensors described above are analog and therefore have infinite resolution such that they can be easily converted into a control signal for the pump device using electronic signal processing devices and techniques known in the art. In this fashion, all elements of an analog position signal can thus be used to determine instantaneous position, velocity, and acceleration of the connecting rod thus control the pump accordingly. The inductance coil and displacement sensors described above also provide the advantage that they do not contact the connecting rod and therefore do not wear the rod or otherwise impede its motion. 
     Although described above with respect to using a particular displacement sensors, it will become readily apparent that other displacement sensors which convert the distance between its sensing face and a moving object to an electronic signal may be utilized. 
     An important advantage provided by the active feedback apparatus according to the present invention is that by sensing the exact position of a connecting rod as a function of time, a more accurate means for accurately measuring the actual displacement of the rod in real time is provided. For example, the sensing a sudden change in velocity in mid-travel of the connecting rod could be used to detect a ruptured or failed diaphragm or cavitation problem. 
     Moreover, based upon the information received using the active control devices according to the present invention, corrective action may also be implemented. For example, it is normal for connecting rods in diaphragm pumps to over-travel after the mechanical switching device has been switched. The amount of overtravel will vary, however, with the speed of operation due to the momentum of the backup washers and connecting shaft and the time it takes for the mechanical shifting device to effect the reversal of the motion of the connecting rod. By using active control feedback provided by the present invention, the amount of overtravel can be detected and compensated for in real time by using a computer controller. 
     Thus, based upon the information provided using the active sensing devices according to the present invention by themselves or when used in conjunction with additional sensors (e.g., pressure transducers or thermocouples) various abnormal conditions may be diagnosed and corrected. 
     Thus, according to the present invention active feedback apparatus are provided which, by the introduction of sensors and minor modifications to existing diaphragm pump components, produce an output signal proportional to the position of a diaphragm pump connecting rod. Additional benefits are realized by virtue of the minor nature of the component modifications which facilitate the retrofitting of existing pumps to allow field conversion. Moreover, the analog output signal produced by the active feedback apparatus is very versatile and easily converted to permit diagnostic and control functions to be performed on a pump. 
     While embodiments and applications of this invention have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein described. For example, although the present invention is shown and described above with respect to monitoring the volumetric displacement of a diaphragm pump, various other output parameters may be anticipated by reading and interpreting internal device conditions by monitoring the connecting rod position. For example, actual dispensing/metering control, stall prevention, noise suppression, etc. may be actively compensated for by reading the position of the connecting rod and performing some function to compensate before they occur at the output. 
     It is understood, therefore, that the invention is capable of modification and therefore is not to be limited to the precise details set forth. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims without departing from the spirit of the invention.