EXTENDED RANGE PROPORTIONAL VALVE

Abstract: An extended range proportional valve which can control rates of mass flow over continuous low, intermediate and high ranges has a pilot member (68) mounted on an armature (84) of a solenoid which can be dithered onto and off of a pilot opening (56) in a main valve member (30) which seals a main valve opening (20) to control mass flow rates over the low range by varying the duty cycle and/or frequency of a pulse width modulated current in the solenoid coil (72). Intermediate and high flow rates are achieved by dithering the pilot valve member (68) with a duty cycle and/or frequency sufficient to raise the main valve member (30) relatively short and relatively long respective distances from the main valve seat (18).

Description: EXTENDED RANGE PROPORTIONAL VALVE Background of the Invention "" This invention relates to a valve of the proportional flow type operated by an electrical solenoid. More particularly, this invention relates to a valve having a high turn down ratio, i.e., one which can control flow rates ranging from very low, through intermediate, to very high magnitudes . Proportional flow valves find utility in performing mixing and measurement functions. For example, proportional flow valves are used to accurately blend gasolines to achieve desired characteristics, such as particular octane ratings, to mix hot and cold water to obtain a desired temperature, and to dispense compressible and noncompressible fluids, including liquids such as gasoline, and gases such as air and natural gas . Depending on the application for which a proportional flow valve is to be used, it may be necessary to maintain constant flow rates of a very low magnitude as well as constant flow rates of a very high magnitude, and constant flow rates of an intermediate magnitude between said high an low magnitudes. In some prior art proportional valves, a main valve member is lifted off of and lowered onto a main valve seat to open and close the valve . The main valve member can be mounted at the center of a diaphragm. Such a valve is shown in U.S. Patent No. 5,676,342. This valve permits a rate of fluid flow through the valve proportional to the amount of electric current flowing through the coil of the solenoid actuator controlling the valve. In this type of arrangement, the actuator behaves in a linear matter, i.e., the force produced by the solenoid armature is linearly proportional to the current applied to the solenoid. As a result, the solenoid armature works in a linear manner against a closing spring which constantly urges the valve member toward the valve seat. In this way, the distance which the valve member is moved away from the valve seat is proportional to the amount of current applied to the solenoid. Atop the main valve member is a pilot valve seat which surrounds a pilot opening through the center of the main valve member. The plunger of a solenoid above the main valve member carries a pilot valve member which is lowered to seal the pilot valve opening in the main valve member and raised to open the pilot valve opening in the main valve member . There is also a bleed opening in the housing or diaphragm, or through another channel, through which fluid can flow between a reservoir chamber above the diaphragm and an inlet chamber below the diaphragm. This bleed opening is smaller than the pilot opening. When the pilot opening is sealed by the plunger, fluid from the inlet port enters the inlet chamber below the diaphragm and passes through the bleed opening in the diaphragm to the reservoir above the diaphragm. The fluid above the diaphragm urges the diaphragm downwardly toward the main valve seat thereby sealing a main valve opening surrounded by the main valve seat, and closing the valve. When the solenoid is actuated to lift the plunger off of the pilot opening, fluid above the diaphragm is drained through the pilot opening faster than it can enter through the smaller bleed opening thereby lessening the pressure above the diaphragm and causing fluid pressure from the inlet below the diaphragm to force the diaphragm upward thereby lifting the main valve member off of the main valve seat for opening the valve. The valve of the above mentioned U.S. Patent No. 5,676,342 has been found to admirably perform its function. However when very low flow rates are to be maintained, the plunger is moved to a position which enables the diaphragm to lift the main valve member just slightly off of the main valve opening. At this time, the pressure differential between the areas above and below the diaphragm is so great that the main valve member tends to jump when lifted off of the main valve seat thereby preventing attainment of very low flow rates. This occurrence denotes the bottom end of the flow vs. current characteristic. That is, in a valve where flow rate is uniformly diminished by decreasing the current applied to the solenoid coil, flow is abruptly shut off when the solenoid coil current is reduced to a level whereat the main valve member is forced onto the main valve seat . Conversely, while the main valve member is in engagement with the main valve seat and the current induced in the coil of a proportional solenoid valve is gradually increased, a level is reached whereat the main valve member jumps off of the main valve seat to a position whereat the lowest possible flow rate for that valve is achieved. Although this minimum flow rate can be optimized through careful selection of design parameters for the valve's components, it can not be improved sufficiently in cases where precise low flow rates are required. It is also known in the art to operate a solenoid valve at a constant high flow rate by applying to the valve solenoid a full wave AC current for displacing the main valve member from the main valve seat, and at a constant low flow rate by rectifying the AC current to obtain a half-wave AC signal which, when applied to the solenoid coil, enables fluid to pass through the pilot opening but does not provide sufficient lifting force to enable the main " valve member to be lifted off of the main valve seat. Such a valve is the subject of U.S. Patent No. 4,503,887 to Johnson et al . It is further known in the art to vary the degree of displacement of a pilot valve member from a pilot valve seat in a proportional valve by applying power to the valve's solenoid coil in the form of a periodically pulsed DC current, the amount of current varying with the length of "on" and "off" times of the pulses, sometimes referred to as pulse width modulation. Pulse width modulation for this purpose is disclosed in U.S. Patent No. 5,294,089 to LaMarca and U.S. Patent No. 5,676,342 to Otto et al . None of the foregoing approaches has provided a solution to the problem of making a proportional solenoid valve with a high turn-down ration, i.e., one which enables continuous variation of flow rate from very high and intermediate levels during which the main valve member is displaced from the main valve seat, to low levels during which the main valve member remains seated for sealing the main valve opening, and fluid flow is limited to passage through the pilot opening. Summary of the Invention According to the invention, low flow rates are achieved over a continuous range, without lifting the main valve member off of the main valve seat, through pulse width and/or frequency modulation of the current applied to the coil of a proportional solenoid valve. For low flow rates, e.g., gas flowing at a rate of 0.5 standard cubic feet per minute (scfm) to 5.0 scfm, the solenoid armature or plunger is oscillated or dithered onto and off of the pilot valve seat on the main valve member with a duty cycle during which the pilot opening is exposed to inlet fluid under " pressure for a portion of the cycle, and the pilot opening is closed for the balance of the cycle thereby maintaining the main valve member on the main valve seat and limiting fluid flow to a path through the pilot opening. For increasingly greater flow rates, the duty cycle of the solenoid armature is adjusted to increase the proportion of the cycle during which the pilot opening is exposed to the fluid, and thereby increase the rate of fluid flow through the pilot opening. As the rate of fluid flow approaches a level that can allow control of the displacement of the main valve member from the main valve seat without the problem of jumping which is encountered at lower flow rates, the duty cycle of the solenoid current is further adjusted to enable the pilot valve to remain open long enough to raise the main valve member from the main valve seat a distance corresponding to a desired intermediate rate of flow whereat the rate of flow through the pilot opening is supplemented by limited flow through the main valve opening. Flow at intermediate mass flow rates is permitted as the main valve member is lifted to a position a short distance from the main valve seat. Higher flow rates, to which the contribution of flow through the pilot opening becomes insignificant, are achieved as the main valve member is lifted further away from the main valve seat. It is therefore an object of the invention to provide a single proportional flow valve which can provide continuous variation of flow rates over a range heretofore unrealizable . Another object of the invention is to provide a proportional flow valve with a solenoid actuator which can be energized by a current having a variable duty cycle for dithering a pilot valve member onto and off of a pilot seat on a main valve member for enabling a continuous range of low flow rates through a pilot opening in the valve without raising the main valve member from the main valve seat. Still another object of this invention is to provide apparatus for modulating flow through the pilot opening in the seated main valve member without reaching the critical flow rate at which open the main valve member is lifted of off the main valve seat. A further object of the invention is to provide a valve of the type described above wherein the duty cycle and/or frequency of the pulse width modulated solenoid current can be adjusted to enable the pilot valve to remain open long enough to raise the main valve member from the main valve seat in degrees corresponding to a desired rate of intermediate or high volume fluid flow. Still another object of the invention is to maintain continuity between low flow, intermediate flow, and high flow rates in a proportional solenoid valve as a transition takes place from a range of low flow rates only through the pilot opening (main valve closed) through intermediate flow rates having significant components passing through both the pilot and main valve openings, to high flow rates which occur principally through the main valve opening. Other and further objects of the invention will be apparent from the following drawings and description of a preferred embodiment of the invention in which like reference numerals are used to indicate like parts in the various views . Description of the Drawings Fig. 1 is a cross sectional view of a proportional flow "" valve in accordance with the preferred embodiment of the invention, the solenoid actuator being deenergized and the valve closed. Fig. 2 is a view similar to Fig. 1, but showing the valve while permitting a low range of mass flow rates . Fig. 3 is a view similar to Fig. 1, but showing the valve while permitting an intermediate range of mass flow rates. Fig. 4 is a view similar to Fig. 1 but showing the valve while permitting a high range of mass flow rates . Fig. 5 is a schematic block diagram depicting the power supply for the solenoid of Figs. 1 - 4. Description of the Preferred Embodiment Referring to Figs. 1 - 4 of the drawings, a proportional flow valve 10 chosen to illustrate the present invention includes a valve body 12 having a fluid inlet port 14, a fluid outlet port 16, and main valve seat 18 surrounding a main orifice 20. The outlet port 16 resides within a hollow elbow having a right angular bend 24 which joins a horizontal section 22 and an a vertical section 28, the latter terminating at the main valve seat 18. A main valve unit 30 includes a main valve member 32 slidably mounted within vertical section 28 of outlet port 16 for reciprocal axial movement. The main valve member 32 has a generally circular cross section and axially extending circumferentially spaced parallel vanes 34, two of which can be seen in the drawings . The outer circumference of the main valve member 32 is profiled to accept an upper diaphragm support washer 36 having a planar lower annular surface and a diaphragm retaining ring 38 having a planar upper annular surface. Sandwiched between the lower annular surface of upper diaphragm support washer 36 and upper annular surface of diaphragm retaining ring 38 for movement with the main valve member 32 is the central area of an annular flexible diaphragm 17 which serves as a pressure member for the valve 10. A bonnet plate 40 is secured to the top of the valve body 12 by suitable fasteners 42. Disposed between the bonnet plate 40 and a raised circumferential ridge 44 on the top of the valve body 12 is the outer circumference of diaphragm 17 which is fixedly held on its top side by the bonnet plate 40, and on its bottom side by the raised circumferential ridge 44 of the valve body 12 and a seal 46 inside and concentric with the ridge 44. Seal 46 cushions the underside of the diaphragm 17 and prevents leakage of fluid at the interfaces between the bonnet plate 40, valve body 12, and diaphragm 17. An annular retaining clip 48 captured in a groove circumscribing the main valve member 32 urges the upper diaphragm support washer 36 toward the central region of diaphragm 17 to secure diaphragm 17 against diaphragm retaining ring 38. The vanes 34 are notched to received an annular main valve seal 50 below retaining ring 38. Main valve seal 50 is preferably fabricated from an elastomeric material. The main valve unit 30 includes main valve member 32, upper diaphragm support washer 36, diaphragm retaining ring 38, diaphragm 17, retaining clip 48, and main valve seal 50, all of which move toward and away from the main valve seat 18 as a unit. During such movement, an intermediate annular portion 54 of diaphragm 17 is free to flex and stretch while the periphery of diaphragm 17 is held fixedly in place. Axial movement of the main valve unit 30 takes place with the vanes 34 of main valve member 32 guided within a vertical cylindrical wall of the outlet port 16 leading from the main valve seat 18. Within the main valve member 32, running along its central axis, is a pilot passageway in the form of a circular bore 56 surrounded at its upper end by a pilot valve seat 58 and opening at its lower end into the outlet port 16. The pilot passageway 56 is selectively opened and closed by a pilot valve sealing member 68. A main valve spring 60 is compressed between a shoulder 62 formed with the bonnet plate 40 and the top surface of the upper diaphragm support washer 36 thereby urging the main valve unit 30 downwardly into engagement with the main valve seat 18. The fluid inlet port 14 is bounded by the underside of the main valve unit 30 (including diaphragm 17) and the exterior surface of vertical section 28 of outlet port 16. A reservoir 64 occupies the open volume above the main valve unit 30. The diaphragm 17 is impermeable to the fluid to be controlled by the proportional flow valve 10. A bleed passageway 66 in the bonnet 40 and valve body 12 enables fluid communication between the reservoir 64 and inlet port 14 so that fluid from the inlet port 14 can enter the reservoir 64 above the main valve unit 30. The bleed passageway 66 has a smaller cross section than the smallest cross section of pilot passageway 56 so that fluid can flow through the pilot passageway 56 faster than through the bleed passageway 66 when the pilot passageway 56 is open. When the pilot valve is closed, as shown in Fig. 1, i.e., when pilot valve sealing member 68 engages pilot valve seat 58, and when the main valve is closed, i.e., when main valve seal 50 engages main valve seat 18, fluid cannot " flow from the fluid inlet port 14 to the fluid outlet port 16. When the pilot valve is open, i.e., when pilot valve sealing member 68 is not in engagement with pilot valve seat 58, and the main valve is closed, as shown in Fig. 2, a fluid can flow from the fluid inlet port 14 to the fluid outlet port 16 only through the bleed hole passageway 66 into the reservoir 64, and then from reservoir 64 through pilot passageway 56. Such fluid flow is therefore limited to a low range of mass fluid flow rates, the actual rate of flow being dependent on the relative time during which the pilot valve is open versus the time during which the pilot valve is closed. When main valve seal 50 is out of engagement with main valve seat 18, fluid flow can occur through the space between the vanes 34 of main valve member 32. The exposed area of the openings between the vanes 34 increases as the main valve unit 30 rises thereby correspondingly increasing the rate of flow from the fluid inlet port 14 to the fluid outlet port 16. Initially, for example when the main valve member is removed from the main valve seat by a distance equal to or less than 25% of the diameter of the main valve opening, flow through the main valve opening is restricted and the rate of flow through the pilot opening constitutes makes a significant contribution to the total rate of flow through the valve, i.e., the sum of the mass flow rates through both the main valve opening and pilot valve opening. Under the above-described condition where the main valve member is removed from the main valve seat by a distance equal to or less than 25% of the diameter of the main valve opening, -li¬ mass flow through the valve can occur over an intermediate range of rates, greater than the low range to which the valve "" is restricted when flow is limited to the pilot opening . Once the main valve member is removed from the main valve seat by a distance greater than 25% of the diameter of the main valve opening, a high range of mass flow rates is achievable. Flow at high rates occurs principally through the main valve opening, and the amount of flow through the pilot opening becomes negligible. In order to achieve low flow rates solely through the pilot opening of the valve, i.e., while the valve is in the state shown in Fig. 2, the pilot valve member is dithered onto and off of the pilot valve seat by a current having a frequency and duty cycle which rapidly permits and interrupts the flow of fluid through the pilot opening so as to maintain sufficient pressure in the reservoir 64 to prevent the inlet pressure beneath the diaphragm from lifting the main valve member off of the main valve seat. The rate of flow through the pilot opening need not be limited to a single magnitude. By varying the frequency and/or duty cycle of the pulse width modulated solenoid current, the relative time during which the pilot valve opening is exposed to fluid within the reservoir 64, versus the time the pilot opening is sealed by the pilot valve member, can be varied to continuously increase or decrease the rate of fluid flow through the pilot opening while preventing the pressure in the reservoir 64 from decreasing enough to permit the diaphragm be raised from the main valve seat. Depending on the frequency and pulse width of the solenoid current, the valve will alternate between the off state shown in Fig. 1 and the on state shown in Fig. 2 to permit low rates of fluid flow without opening the main valve, that is, without lifting the main valve member from the main valve seat . Surmounting the bonnet plate 40 is a solenoid actuator 70. The solenoid actuator 70 includes a coil 72 of electrically conductive wire wound around a spool 74 made of non-electrically and non-magnetically conductive material. Suitable terminals are provided for connection to a source of electric current for energizing the solenoid coil 72. A housing 76 of magnetic material, surrounds the solenoid coil 72. A stationary armature or plugnut 78 is located within the upper portion of the spool 74. A core tube 80 extends downwardly from the plugnut 78 and through the remainder of the spool 74. Surrounding the lower portion of the core tube 80 is a collar 82 which is, in turn, fastened to the upper portion of the bonnet plate 40. Fastening between the core tube 80 and collar 82, and between the collar 82 and bonnet plate 40 can be by press fit, welding, crimping, threading or in any other conventional manner of forming a sturdy and fluid tight connection as will be known to those skilled in the art. Slidably axially disposed within the core tube 80 is a movable armature 84 of magnetic material. Mounted on the movable armature 84 near its lower end is a circumferential flange 86. A pilot valve spring 88 surrounding the movable armature 84 is compressed between circumferential flange 86 and the bottom surface of collar 82 and urges the movable armature 84 downwardly away from plugnut 78. The upper face of the movable armature 84 and lower face of the plugnut 78 are correspondingly profiled so that the two faces mesh as the movable armature 84 moves toward the plugnut 78. At its lower end, the movable armature 84 carries the pilot valve sealing member 68 formed of resilient material. When solenoid coil 72 is deenergized (Fig. 1) and the fluid inlet port 14 of proportional flow valve 10 is connected to a source of pressurized fluid, e.g. a gasoline pump, the fluid is forced through the bleed channel 66 into the reservoir 64 above the main valve unit 30. The area of the top of the main valve unit 30 exposed to the fluid is greater than the area of the bottom of the main valve unit 30 exposed to the fluid. Hence, the force of the fluid on the top of main valve unit 30, combined with the force of the spring 60, holds main valve seal 50 against main valve seat 18 to close the proportional flow valve 10. When solenoid coil 72 is first energized by an electric current (Fig. 2), movable armature 84 is attracted to plugnut 78, and hence begins to move upwardly against the force of spring 88. As movable armature 84 rises, it moves pilot valve sealing member 68 away from pilot valve seat 58, thereby permitting inlet fluid to flow through passageway 56 into outlet port 16 which is at the lower outlet pressure. Because the effective flow rate through the pilot passageway 56 is greater than the effective flow rate through the bleed channel 66, the pressure above the main valve unit 30 and diaphragm 17 begins to decrease. Although the pilot opening in the illustrated preferred embodiment of the invention is of larger diameter than the bleed opening, it is possible to have a greater effective flow rate through the pilot opening than through the bleed opening even if the pilot opening has the smaller diameter when the flow channels are such that turbulence retards the rate of flow through the bleed channel relative to the rate of flow through the pilot opening. "~ If the frequency and pulse width of the solenoid current are sufficient to raise the pilot valve sealing member 68 from the pilot valve seat 58 for a large enough proportion of time, the upward force of the fluid inlet pressure on the main valve unit 30 begins to exceed the downward force of the fluid pressure on the main valve unit 30, the main valve unit 30 begins to rise (Fig. 3), and main valve unit 30 moves away from main valve seat 18. Main valve seal 50 disengages main valve seat 18 and communication between fluid inlet port 14 and fluid outlet port 16 through the spaces between vanes 34 of main valve member 32 is enabled, thereby initially permitting intermediate range fluid flow from inlet port 14 to outlet port 16. The main valve unit 30 continues to rise until pilot valve seat 58 engages pilot valve sealing member 68, i.e., the pilot valve is closed. As a result, high pressure fluid cannot escape from the reservoir 64. As fluid entering the reservoir 64 builds up, the downward force on the main valve unit 30 increases until it, in combination with the downward force of the spring 60, again exceeds the upward force of the inlet fluid against the bottom of main valve unit 30. The result is downward movement of the main valve unit 30. However, as soon as the main valve unit 30 begins to move downwardly, pilot valve 68 opens, once again permitting high pressure fluid above the main valve unit 30 to escape through passageway 56 to the fluid outlet port 16. An equilibrium position (Fig. 4) is quickly established in which main valve unit 30 constantly oscillates a very short distance as pilot valve 68 is repeatedly opened and closed. The location of the main valve unit 30 as it oscillates is determined by the position of movable armature 84 and, hence, pilot valve sealing member 68. This position also determines the spacing between main valve member 32 and main valve seat 18, and hence determines the rate of flow through the main valve opening. Whether intermediate or high mass flow rates are obtained is determined by the extent to which the main valve member is raised from the main valve seat, which is in turn set according to the position of movable armature 84 is a function of the duty cycle and/or frequency of the pulse width modulated current applied to solenoid coil 72, the preferred method of current control on solenoid activated proportional flow control valves being by pulse width modulation (PWM) . With pulse width modulation, as employed in prior art proportional solenoid valves, a fixed frequency variable duty cycle square wave is applied to the coil of the solenoid in order to vary the current in the coil in a linear fashion, thereby varying the force exerted by the solenoid on the valve actuating mechanism, and thus changing the flow through the valve. The use of a square wave signal has two distinct advantages over the use of a linear amplifier to control of the solenoid current. First, the switching type of controller has much greater efficiency than a linear amplifier. Second, the proper choice of the fixed switching frequency of the square wave can provide a small variation in solenoid current that translates into a mechanical dither of the raised solenoid armature which, in turn, reduces the effects of static friction and mechanical hysteresis in the valve. By carefully controlling the mechanical dither via pulse width modulation and/or frequency modulation, selection of a desired rate of mass flow through the pilot opening is possible over a range of flow rates without opening the main valve. This range is herein referred to as a low range of mass flow rates . Intermediate and high flow rates are achieved by increasing the duty cycle of the pulse width modulated solenoid current so that the magnitude of flow through the pilot opening is great enough to relieve the pressure in the reservoir above the main valve member thereby permitting the main valve member to rise off of the main valve seat . If the pulse width modulation voltage has a 50% duty cycle, the current flowing through the solenoid coil 72 will be 50% of maximum. As a result, the movable armature 84 will rise though one half its maximum stroke between its position when the main valve is closed (Fig. 1) and its position when the valve is fully open (Fig. 4), i.e., when its upper face engages the lower face of the plugnut 78. Consequently, the main valve unit 30 will be permitted to rise through just 50% of its maximum rise, and hence main valve unit 30 will be spaced from main valve seat 18 about % of the maximum spacing. Thus, approximately % of the rate of maximum flow through the valve will be permitted between fluid inlet port 14 and fluid outlet port 16. If the voltage is on 75% of the time and off 25%, i.e., there is a 75% duty cycle, movable armature 84 will rise through 3/4 of its maximum stroke, and as a result approximately 3/4 of the rate of maximum flow through the valve will be permitted between fluid inlet port 14 and fluid outlet port 16. It will be appreciated, therefore, that the rate of high volume flow through the main valve is proportional to the amount of current supplied to the solenoid coil 72. Intermediate and high mass flow rates can be achieved depending on the maximum stroke of the solenoid armature and the diameter of the main valve opening. For example if the pulse width modulation voltage has a 25% duty cycle, the current flowing through the solenoid coil 72 will be 25% of maximum. As a result, the movable armature 84 will rise though one quarter its maximum stroke. Consequently, the main valve unit 30 will be permitted to rise through just 25% of its maximum rise and main valve unit 30 will be spaced from main valve seat 18 about 1/4 of the maximum spacing. If the diameter of main valve opening is greater than 25% of the maximum stroke of the movable armature 84, flow will be in the intermediate range. When operated at high flow rates, i.e., whereat fluid flow is primarily across the main valve seat, the valve of the instant invention behaves like the valve of U.S. Patent No. 5,294,089. That valve is a fluid assisted design, which by the control of a small pilot orifice, allows the solenoid to effectively position the diaphragm which, in turn controls the flow through a much larger orifice. This type of valve typically has a turn down ratio of about 10 to 1 in flow over its control range. As in the case of the aforementioned prior art valve, control of armature position is most precise when a pulsed DC source is applied to the solenoid coil 72, as compared to simply varying the amplitude of a continuous DC current. Prior art valves are operable only in the intermediate and high ranges. Pulsing the current in such valves imparts a dither to the movable armature 84 with an amplitude that is very small in comparison with the displacement of the main valve member from the main valve seat . Hence the dithering has negligible effect on flow rate which is determined by the exposed area of the openings between the vanes 34, and which increases as the main valve unit 30 rises . In the valve of the present invention, low rates of flow occur solely through the pilot opening. To achieve low flow rates over a continuous range, the pulse width and frequency of the dithered pilot valve sealing member are varied to determine the rate of fluid flow through the valve. It has been found that pulsing the pilot solenoid over a carefully controlled range of pulse durations will allow precise control of flow through the pilot flow opening in the valve without causing the diaphragm to open the main valve by raising the main valve member from the main valve seat. By simultaneous variation of the pulse width and frequency of the wave form applied to the solenoid coil, a close approximation of a linear correspondence between current and flow rate in the low flow range can be obtained, as it has heretofore been done in the intermediate and high flow ranges. Moreover, the transition from low flow range to the intermediate flow range can be made transparent with no abrupt discontinuity in the current vs. flow characteristic, as can be done in the transition from the intermediate flow range to the high flow range. For low flow rates, the on time of the pulse must be within a range that allows the solenoid to lift the pilot valve member from the pilot seat but does not allow the pilot valve member to expose the pilot opening sufficiently to cause the diaphragm to lift the main valve member from the main valve seat. Also, the frequency of the current applied to the solenoid coil must be limited to a range over which the armature of the pilot solenoid will continue to operate in a pulsing mode. Balancing of three mechanical parameters enables achievement of a continuous range of low flow rates, each of which can be selected by controlling the frequency and pulse wave duty cycle of the solenoid coil current. These mechanical parameters are pilot orifice area, effective bleed channel area and diaphragm hold down spring constant and spring force. The area of the pilot orifice is a major controlling factor in achieving a wide range of low flow rates. As the cross sectional area of the pilot opening increases, so too does the range of available low flow rates or turn down ration of the low flow region of the current vs . flow rate characteristic . The bleed channel of a proportional solenoid valve balances the pressures and forces above and below the diaphragm. The cross sectional area of the bleed channel is typically smaller than the cross sectional area of the pilot opening through the main valve member. Exposure of the pilot opening by lifting of the pilot valve member from the pilot valve seat causes a pressure imbalance across the diaphragm which urges the valve main member away from the main valve seat. Conversely, sealing of the pilot opening balances the pressures on both sides of the diaphragm thereby allowing it to be closed in response to a mechanical force, e.g., from a spring. The size of the bleed channel is somewhat critical. If the bleed area is too small, pressure in the reservoir will decrease so rapidly during the opening phase of the pulse cycle as to cause the diaphragm to lift the main valve member prematurely, thus limiting the high end of the low flow range. A bleed area which is too large, while potentially extending the flow range obtained by dithering the pilot valve member onto and off of the pilot seat, would interfere with the needed unbalancing of the pressures on either side of the diaphragm need for displacing the main valve member from the main valve seat for transition to the high flow range, i.e., across the main valve seat. It has been found that by placing on top of the diaphragm, a spring having an appropriate spring constant and spring force, it is possible to keep the main valve member in a closed position, i.e., sealing the main valve opening, thereby allowing operation at higher duty cycles and frequencies, thus maximizing the low flow range. By balancing solenoid duty cycle and frequency, pilot opening area, bleed channel area, and diaphragm spring constant and spring force, high turn-down ratios, i.e., wide ranging flow rates, can be achieved by a single proportional solenoid valve. Example 1 In a proportional solenoid valve having a circular pilot opening .078 inches in diameter, a bleed channel .073 inches in diameter, and a diaphragm hold-down spring with a spring force of 1.5 lbs. a low flow range of 0.5 - 5.0 scfm was obtainable by varying the pulse width duty cycle and frequency of the solenoid coil current from 8% and 20 Hz to 50% and 25 Hz, respectively. Depending on the size and design of the valve, frequencies as high as 40 Hz or more, when combined with appropriate duty cycles, can be effective in obtaining low flow rates over a substantial range . "~ Referring now to Fig. 5 of the drawings, a square-wave generator 101 applies current in the form of pulsed DC signals to the coil 72 of the proportional valve solenoid 70. The duty cycle, i.e., the percentage of on-time vs. off-time for a single cycle of the square wave signal is controlled by a pulse width modulator 103 the construction of which will be known to those skilled in the art. A frequency setting circuit 105 is also provided for setting the number of cycles per second of the pulsed DC signal produced by the generator 101. The construction of the frequency setting circuit will also be known to those skilled in the art. A manual control device, e.g., the control lever on the handle of a gasoline pump, can be mechanically linked to a transducer for sending signals to a digital microcontroller 107 which is connected to the pulse width modulator circuit 103 and frequency adjusting circuit 105 for simultaneously adjusting the frequency and duty cycle of the DC pulses applied to the solenoid coil by the generator 101. The microcontroller 107, pulse width modulator circuit 103, and frequency setting circuit 105, may be designed and/or programmed so that narrow pulses are applied, i.e., the pulsed waveform has a low duty cycle, for enabling low flow rates at which time the solenoid armature is dithered for allowing flow only through the pilot opening of the proportional valve while preventing lift off of the main valve member from the main valve seat. Moreover, the duty cycle and frequency of the solenoid coil current may be adjusted to increase the rate of flow through the pilot opening while still preventing main valve member lift-off. Flow rate is still further increased by enlarging the duty cycle of the solenoid coil current beyond a percentage whereat lift-off of the main valve member from the main valve seat occurs . It has been found that by employing an extended range proportional valve in accordance with the invention, a substantially linear relationship between flow rate and pump handle position may be achieved over a range from very low flow rates to very high flow rates, thereby enabling linear flow control over a turn-down ratio of as much as 100 to 1 or more. In designing an extended range proportional valve in accordance with the invention, it is preferable to model the operation of the valve by examining the response of the valve to a PWM (pulse width modulated) control voltage that is applied to the coil of the solenoid operator. This voltage waveform causes a variation in the position of the armature of the solenoid. The motion of the armature of the solenoid, in turn, causes a variation in rate of mass flow through the valve. The motion of the armature can be described by a standard second order differential derived from a free body diagram of the armature and all relevant forces acting on it, including gravity, return spring force, and the magnetic force of attraction. Md 2 x/dt 2 + Bdx/dt + Kx = F-F 0 where x = Displacement of the armature from its initial position in meters F = The magnetic attraction force on the armature in newtons t = time in seconds M = Mass of armature in kilograms B = Friction force on the armature in newton/meter/sec K = Spring constant of armature spring in newton/meter Fo = The initial force on the armature that must be overcome to start motion, in newtons The dynamics of the electric circuit of the solenoid coil, which is driven by the PWM excitation voltage, are described by the following relationships. During the 'ON' period of the PWM signal E = N dφ/dt + IR During the 'OFF' period of the PWM signal Nc dt + IR = 0 Where Φ = Total flux in webers , which links the turns of the solenoid coil I = Coil current in solenoid R = Resistance of solenoid coil E = Voltage on solenoid coil when during on period of PWM signal N = Number of turns in the solenoid coil The coil current in the solenoid and the magnetic attraction force on the armature in newtons are both functions of the total flux which links the turns of the solenoid coil, and the displacement of the armature from its initial position, i.e., I = f (Φ, x) and F = f (Φ, x) Both of the above relationships are non-linear functions, that are dependent upon the geometry of the solenoid operator and the materials from which the valve components are constructed. Solutions to the foregoing equations may be obtained by modeling the mechanical and electrical elements of the valve on a digital computer by use of circuit solver software, such as the commercially available SPICE program. In such a model, the electrical driver circuitry is directly modeled by electrical elements, and the mechanical components are represented by corresponding electrical analogs. The magnetic coupling of back emf (Ndφ/dt) , core position, current, and solenoid force can be modeled with the use of an element that accepts tabular data about the solenoid's parameters. This tabular data can be extracted from a magnetic finite element analysis of the solenoid over a range of operating conditions with solutions obtained for various values of core position and coil excitation. An example of a commercially available software solver capable of performing this analysis on a digital computer is EMSS by Ansoft of Pittsburgh PA. This solver integrates magnetic finite element analysis programs with a version of the SPICE program. By modeling this problem in such a solver, a solution in the form of a time variant waveform that represents the displacement x, i.e., the displacement of the armature from its initial position, can be obtained. In the range of low mass flow rates, the total mass flow through the valve is equal to pilot flow only. That is, the main valve member remains seated on the main valve seat thereby preventing flow through the main valve opening. Using the displacement, x, as determined by the solver, the mass flow of a gas or liquid through the pilot opening of the main valve member can be calculated from the following relationships. Where the fluid passed through the valve is a gas: Mpuot fgas) = ( K Pi C d π x Di N 12 ) / ( T 1/2 ) , where γ = gas constant M = Mass flow per unit of time Ro = degrees Rankine x = Displacement of the armature from its initial position in inches K = Constant (Ro 1 2 ) / unit temp. = [(γ-l)/2γ / ( (Pι/P 2 ) (γ_1)/ γ - 1)] - (1/γ ) Pι= Inlet pressure in psia P 2 = Pressure downstream of main valve seat C d = Discharge coefficient Di≡ Pilot sealing surface diameter N i2 = Ratio of actual flow to sonic flow per unit area at given values of total temperature and pressure [ ( p 2 /Pχ ) 2 γ - ( p 2 /Pι) <Y+1 ) / 7 ( (γ- D /2 ( 2 /γ+i ) ) '^ / ^ ) ] ^ 2 T = Inlet temperature in Ro Where the fluid passed through the valve is a gas: 1/2 Mpiiot (liquid) = C d x Di ( 2g c p ( Pi - P 2 ) ) where g c = gravitational constant (386 in-lbm/lbf-sec 2 ) p = density (lbm/in 3 ) The total mass flow through the valve equals mass pilot flow until the displacement of the main valve member from the main valve seat, i.e., diaphragm stroke, X d > 0 In order to determine when the main valve member is lifted from the main valve seat, thereby unsealing the main valve opening for increasing the mass flow rate through the valve, the relationship between the changes in pressure, temperature and volume occurring within the valve can be considered as follows. The Ideal Gas Equation is known to be M = PV/RT where P = pressure in diaphragm chamber V = volume in diaphragm chamber R = perfect gas constant M = mass of gas in diaphragm chamber Taking the derivative of the Ideal Gas Equation: m/M = p/P +v/V + t/T = 0 Where m = change in mass M v = change in volume V p = change in pressure P t = change in temperature T Assuming a polytropic process, the relationship of pressure change to volume change is calculated from the following: P = nPAdXa / V, where A d = diaphragm area X d = diaphragm movement n = number between 1 (for constant temperature) and γ (for constant entropy) γ = ratio of specific heats Solving for X d gives the diaphragm displacement: By varying the duty cycle of the pulse width modulated current in the solenoid coil, and/or the frequency of the current, to dither the pilot valve member onto and off of the pilot valve seat, mass flow rates can be achieved over a continuous low range. When the rate of pilot mass flow is increased to a magnitude whereat the differential pressure across the main valve member causes it to be initially raised from the main valve seat, mass flow through the pilot opening in the main valve member is supplemented by limited mass flow through the main valve opening which is partially blocked by the main valve member being in close proximity to the main valve opening. While the main valve member is displaced from the main valve seat a distance equal to or less than 25% of the diameter of the main valve opening, mass flow rates over an intermediate range can be achieved. Once the main valve member is raised from the main valve opening by a distance position greater than 25% of the diameter of the main valve opening, mass flow rates over a high range can be achieved Once the main valve opening is unsealed, the mass flow rate throughout the intermediate range of flow rates can be calculated as follows. Mtotai = mass flow rate through the extended range proportional valve Mtotai ® Xd < .25 D2 = Mdiaphragm + M p n ot Where D 2 = diameter of the main valve opening diaphragm = mass flow rate through the main valve opening MpHot = mass flow rate through the pilot opening As main valve member displacement increases and the main valve member is no longer in close proximity to the main valve opening, the rate of mass flow through the pilot opening in the main valve member becomes insignificant relative to the rate of mass flow through the main valve opening and can be ignored. Hence, the mass flow rate throughout the high range of flow rates can be calculated as follows. Mtotai @ Xd > .2ΞD2 = Mdiaphragm Mdiaphragm (gas) = (K P x A λ N 12 ) / ( T 1 ^ ) Mdiaphra m (liquid) = A λ (2g c p (P x - P 2 ) ) 1/2 , where Ai = X d C d Dιπ = effective area of main valve opening The effective area of the main valve opening when the main valve member is displaced from the main valve seat by less than 25% of the diameter of the main valve opening is equal to the area of the main valve opening across which an equal pressure drop occurs under similar conditions when the main valve member is sufficiently displaced from the main valve seat so as not to affect mass flow rate through the main valve opening. Example 2 In an extended range proportional valve that was constructed in accordance with the preferred embodiment of the invention for controlling the flow of natural gas (methane gas constant used) , the following parameter values applied. K = Gas constant (Ro 1/2 ) /unit temp. = [((ratio of specific heats, γ-l) /2γ) ( (P1/P2) (γ-1) /γ-1) ] - (l/γ) = 23.14 P 2 = Inlet pressure in = 79.7 psia Cd = Discharge coefficient .35 (takes into account loss due to inlet restriction) £>ι = Pilot sealing surface diameter = .056" N 12 = Ratio of actual flow to sonic flow per unit area at given values of total temperature, and pressure = P 2 = .95P 2 = 15 . 72 psia Therefore N 12 . .4507 [ (P 2 /PX 2/y - (P 2 /Pι) <y+1> /y / ( (y-1) /2 (2/ (y + l) ) <y+1) / (y'1> ) ] 1/2 T = Inlet temperature in degrees Rankine (Ro) = 527 d-Di = main orifice = .328" - (.1652 to .326) M = Mass of armature in kilograms = .0277 B = Friction force on the armature in newton/meter/second = 9.0 K = Spring constant in newton/meter = 2185 F 0 = Initial force on the armature that must be overcome to start motion, in newtons = 1.338 R = Resistance of solenoid coil = 6.5 ohms N = Number of turns in the solenoid coil = 850 It is to be appreciated that the foregoing is a description of a preferred embodiment of the invention to which variations and modifications may be made without departing from the spirit and scope of the invention. For example, this invention could also be applied to a pilot operated proportional solenoid valve design wherein pressure on a rigid piston, instead of a flexible diaphragm, is used to lift the main valve member.

Claims: What is claimed is: 1. A proportional flow valve for selectively controlling the rate of flow of fluid over an intermediate range of mass flow rates between a contiguous low range of mass flow rates and a contiguous high range of mass flow rates, comprising: a valve body including an inlet port, an outlet port, and a main valve seat mounted in said body and having an inlet side exposed to said inlet port and an outlet side exposed to said outlet port, a main valve member movably mounted within said valve body into and out of engagement with the main valve seat to close and open the valve, said main valve unit having a pilot opening extending therethrough and a pilot seat surrounding said pilot opening, a pilot valve member movably mounted within said valve body into engagement with the pilot seat for sealing the pilot opening thereby preventing fluid flow from said inlet port to said outlet port through said pilot opening and out of engagement with the pilot seat for exposing the pilot opening thereby permitting fluid flow from said inlet port to said outlet port through said pilot opening, a solenoid actuator having an armature on which said pilot valve member is mounted for movement therewith, and a coil for producing a flux as a function of an electrical current flowing therein, said armature being movable in response to said flux, and alternating current electrical energizer means operatively connected to said coil for selectively inducing therein an alternating current having a characteristic with a magnitude selectable from a range of magnitudes for disengaging said pilot valve member from said pilot valve seat for a time long enough to cause sufficient flow of said fluid through said pilot opening for creating a differential pressure across said main valve member sufficient to disengage said main valve member from said main valve seat by a distance equal to a fraction of a diameter of said main valve opening for causing flow of said fluid through said pilot opening at a mass flow rate in said intermediate range of flow rates .

Inventors: FREISINGER PAUL W, HALLER JOHN J, HOLBOROW PETER A, FREISINGER, PAUL, W., HALLER, JOHN, J., HOLBOROW, PETER, A.
Applicants: AUTOMATIC SWITCH CO, AUTOMATIC SWITCH COMPANY
Application year: 2000
Main classification: F16K31
IPC codes: F16K31