Multi-way spool distributor

Abstract

Multi-way spool flow distributor (10) having: - a casing (6), - a motor (7) with a rotating shaft, - a transmission shaft (8), - a mechanism (20) for transforming rotary motion into linear motion, - a stationary cage (1), - a spool element (2), which takes the linear motion from the mechanism (20) and is precisely coupled inside the cage (1), wherein the mechanism (20) is provided with a rotating nut element (3), mechanically connected to the transmission shaft (8) and with a translating cylinder (4) integrally fixed to the spool (2), in which - the nut element (3) is provided on a radially external cylindrical surface (11), with a helical groove (12) hollowed out in a radially internal direction, and - the translating cylinder (4) is equipped with a small blade (13) which extends in a radially internal direction and fits into the helical groove (12).

Description

[0001] MULTI-WAY SPOOL DISTRIBUTOR

[0002] DESCRIPTION

[0003] Technical Field of the Invention

[0004] The present invention relates to a flow modulating distributor for position control systems in highly dynamic and high-precision pneumatic actuators.

[0005] Background Art

[0006] As is known, a control valve regulates the flow of a fluid (for example, gas, steam, water, or chemical compounds) through a circuit of any industrial and civil plants. Such fluid is typically called process fluid. The regulation of such process fluid can be made by means of an actuator that, through the control fluid present in the actuator circuit, provides the driving force necessary to modify the control valve's opening-closing percentage as desired.

[0007] In figure 1 a diagram is shown of a flow distribution system S among the most commonly used ones. In the diagram the valve V and the actuator A are identified. In the circled box, instead, P indicates the flow distributor of the valve position control system. The control system is also called positioner and it is fluidly connected to the two outlets Al and A2 of the actuator, through respective flow lines LI and L2. The positioner performs a control action, to which the present innovation relates, and an emergency action. The non-regulating emergency action can be either open, closed, in place, or can be a combination of the sameand does not affect the following description. It must be noted that other devices can be present for an on-off emergency action.

[0008] In the following description, both of the known art and its issues, and the present invention, reference will be made to a double-acting, linear motion actuator with two pressure chambers, both with and without an elastic element. However, the description also applies to angular motion and single-acting actuators with an elastic element, where instead of a pressure difference (AP), the pressure in one chamber is read and, if necessary, the elastic force at different positions of the elastic element is known or detected during the calibration.

[0009] It is also known that the position control system of a valve or of a positioner is a device mounted on the actuator that can manage the flow of the control fluid through a control strategy, developed using algorithms managed by a microprocessor. More recently, control valve positioners have been significantly improved, partially thanks to the use of digital devices capable of monitoring key variables and implementing more complex control and data recording algorithms.

[0010] According to the known art, a positioner comprises a multi-way flow distributor. There are various types of distributors for positioners, for example, diaphragm valves or on-off valves pulsed at different frequencies, but the most effective in terms of flow coefficient amplitude and simultaneous control and dynamics of the same is the spool type discussed here. It comprises a stationary cage and a mobile element, concentric to the cage, which moves with a translational motion. The mobile element, or the spool, translates within the cage, and the relativeposition of the cage and the spool defines appropriate passages for the control fluid, which passages vary depending on the operating conditions.

[0011] In order to meet the requirements for high dynamics and high precision, the actuator must have a high positioning speed (specifically, a high displacement speed combined with high and controllable positive and negative accelerations), it must be stable in the desired position, and be able to manage small displacements and compensate for variations in external force.

[0012] In greater detail:

[0013] - in order to achieve a high actuator movement speed, the flow distributor must be capable of delivering / discharging high flow rates to the two outlets connected to the actuator;

[0014] - in order to achieve high actuator accelerations, the flow distributor must be capable of managing high speeds of flow rate changes;

[0015] - finally, in order to achieve stability in the desired position, capability of managing small movements, and to compensate for variations in external force, the flow distributor must be capable of managing the control of pressures at the two outlets connected to the actuator.

[0016] These requirements could be met by means of spool flow distributors according to the known art, only if the following could be guaranteed:

[0017] - an almost complete absence of translational-radial clearances between the cage and the spool, and a perfect coaxiality among the components themselves;

[0018] - an almost complete absence of stresses due to thermal deformationsand unwanted mechanical deformations.

[0019] Therefore, there is a need to define a flow distributor that overcomes the drawbacks described.

[0020] Summary of the Invention

[0021] A purpose of the present invention is achieved through a multiway spool flow distributor equipped with features that solve the problem of translational-radial clearances between the cage and the spool, and of the lack of coaxiality among the components themselves, and the problem of the thermal stresses induced by the working fluid, which, being a gas, typically compressed air, changes its temperature through expansion or through compression.

[0022] These and other purposes and advantages are achieved, according to the invention, by means of a multi-way spool flow distributor having the characteristics set forth in here appended independent process claim.

[0023] Further preferred and / or particularly advantageous embodiments of the invention are described according to the characteristics set forth in the appended dependent claims.

[0024] Brief Description of the Drawings

[0025] The invention will now be described with reference to the accompanying drawings, which illustrate a non-limiting embodiment, in which:

[0026] - figure 1 is a diagram of a flow distribution system according to the known art,

[0027] - figure 2 is a perspective view of a flow distributor of a distribution system, according to one embodiment of the present invention,- figure 3 is a cross-section that illustrates the detail of a spool and of a cage of the flow distributor of figure 2,

[0028] - figure 4 is a functional diagram of the detail of figure 3,

[0029] - figure 5 shows the functional diagram of figure 4 in one operating condition,

[0030] - figure 6 is a longitudinal cross-section of the flow distributor of figure 2,

[0031] - figure 7 shows the detail of the motion transmission organs of the distributor of figure 2,

[0032] - figure 8 is a perspective view of a further detail of the spool, of the cage, and of the outer casing of the flow distributor of figure 2, and - figure 9 is a larger-scale detail of the coupling between the cage and the casing of figure 8.

[0033] Detailed description

[0034] With reference to the above figures, it is specified that for a more extensive description of the operation, reference will be made to doubleacting actuators, but the following, with the appropriate simplifications, is also valid for single-acting actuators.

[0035] For single-acting actuation, simply one of the two outlets of the distribution system (for example, the one in figure 1) must be plugged and the other outlet be connected to the actuator's single chamber. The distribution system will be capable of controlling the pressure and the flow rate at the outlet connected to it.

[0036] With reference to figure 2, the flow distributor 10 according to the present invention comprises:- a radial external casing 6,

[0037] - a motor 7 with a rotating shaft, such as an electric motor,

[0038] - a shaft 8 for transmitting the rotary motion induced by the electric motor,

[0039] - a mechanism 20 for converting the rotary motion coming from the motor 7, with the transmission shaft 8 into linear motion,

[0040] - a stationary cage 1,

[0041] - a translating element or spool 2, which takes the linear motion from the mechanism 20 and is precisely coupled inside the cage 1.

[0042] With reference to figure 3, which shows a typical cross-section of cage and spool, and to figure 4, which is a functional diagram of what is illustrated in figure 3, the cage 1 and the spool 2 are affected by the flow of a control fluid supplied by the supply line connected to S. Depending on the relative position between cage and spool, the pressure / flow rate can be sent or discharged to or from the first connecting line LI by combining a first charging line Bl and an outlet line El. Similarly, the pressure / flow rate can be sent or discharged to or from the second connecting line L2 by combining a second charging line B2 and an outlet line E2. In figures 3 and 4, reference 1 indicates the lines of the left half, and reference 2 indicates the lines of the right half.

[0043] Furthermore, the relative position of the spool 2 with respect to the cage 1 creates narrow fluid passage sections (restrictions) between the supply line S and the connecting lines LI and L2 and between the connecting lines and the outlet lines El and E2. In particular:

[0044] - Al is the restriction between the first connecting line LI and the firstoutlet line El,

[0045] - A2 is the restriction between the second connecting line L2 and the second outlet line E2,

[0046] - Bl is the restriction between the supply line S and the first connecting line LI, and

[0047] - B2 is the restriction between the supply line S and the second connecting line L2.

[0048] What has just been described is schematized, by using the same symbols, in figure 4, where Pl and P2 refer to the pressure values existing in the respective connecting lines LI and L2.

[0049] In an ideal system, to solve the technical problem related to the need for high speeds, high accelerations, and stable actuator positioning, the flow distributor 10 must be capable of modulating:

[0050] - the charging flow coefficient between the supply line S and the first connecting line LI and between the supply line S and the second connecting line L2, and

[0051] - the discharging flow coefficient between the first connecting line LI and the first outlet line El and between the second connecting line L2 and the second discharging line E2.

[0052] Furthermore, the flow coefficients as a function of the relative position between spool 2 and cage 1 must be unique, repeatable, and have an increasing or decreasing but monotonous trend depending on the direction of displacement of the spool with respect to the cage.

[0053] In particular, considering the displacement of the spool in one direction to be arbitrarily positive and the displacement in the oppositedirection to be negative, the following relationships must be valid for positive displacements:

[0054] - the flow coefficients Bl between the supply line S and the first connecting line LI and A2 between the second connecting line L2 and the second discharging line E2 are increasing, and

[0055] - the flow coefficients B2 between the supply line S and the second connecting line L2 and Al between the first connecting line LI and the first discharging line El are decreasing.

[0056] Increase and decrease will obviously be reversed for negative displacements.

[0057] From the above, it can be seen that (in an ideal system) there is always one and only one relative position between the spool and the cage, in which the two pressure values Pl, P2 in the corresponding first LI and second delivery lines

[0058] L2 coincide. This position is defined by the term "zero pos." Returning to figure 3 and to the related functional diagram (figure 4), as can be seen from the physical structure, for each leftward displacement of the spool 2 with respect to the cage 1, restrictions Al and B2 simultaneously increase and restrictions Bl and A2 simultaneously decrease. The contrary occurs for rightward displacements.

[0059] Assuming that the distributor 10 is connected to two finite and closed volumes VI and V2, any leftward displacement will cause an increase in P2 and a consequent decrease in Pl, and the contrary occurs rightwards.

[0060] Now, also with reference to figure 5, the following is defined:- Q_L1E1 is the flow rate of the control fluid from the first supply line LI to the first discharge line El,

[0061] - Q_SL1 is the flow rate of the control fluid from the supply line S to the first outlet line LI,

[0062] - Q_SL2 is the flow rate of the control fluid from the supply line S to the second outlet line L2, and

[0063] - Q_L2E2 is the flow rate of the control fluid from the second outlet line L2 to the second discharge line E2.

[0064] Following the above considerations about the flow coefficients of the restrictions, a position can be found in which Pl = P2 and the net flow rate to and from the first connecting line LI and the second connecting line L2 is equal to zero.

[0065] In this situation, it will result between the flow rates Q_SL1 = Q_L1E1 and simultaneously Q_SL2 = Q_L2E2. The sum of Q_L1E1 + Q_L2E2 defines the system's consumption under steady-state conditions.

[0066] The percentage of the pressure value Pl = P2 at the desired value with respect to the pressure in the supply line S depends on the ratio between the flow coefficients of the restrictions Bl, B2 and the respective flow coefficients of the restrictions Al, A2, defined during the design and construction phase.

[0067] As described above, it will always be possible to identify a relative position between spool 2 and cage 1 that generates a pressure Pl equal to the desired value between 0% and 100% of the pressure in the supply line S, and it will always be possible to identify a relative position between spool 2 and cage 1 that generates a pressure P2 equal to the desired valuebetween 100% and 0% of the pressure in the supply line S.

[0068] To have a real system as close as possible to the ideal one, the following conditions must be met:

[0069] 1. The geometry of spool 2 and cage 1 must be as perfect as possible.

[0070] 2. The clearance between spool and cage must be as close to zero as possible.

[0071] 3. Sharp edges of the spool and of the cage, with joints between cylindrical surfaces and flat surfaces lying on a single plane perpendicular to the axis of movement.

[0072] 4. Edges of the spool and of the cage with minimal covering (overlap) so that even the slightest relative displacement of the spool with respect to the cage causes significant changes in the flow coefficient.

[0073] 5. Spool 2 to be moved with high dynamics, therefore not with a very high mass.

[0074] 6. Large passage cross-section, i.e., high flow coefficients, as a function of the dimensions.

[0075] Compliance with conditions 1-6 described above would allow a precise control at high dynamics, provided that the relative positioning between spool and cage is precise and virtually free of clearances.

[0076] The operation is based on the movement of the spool with respect to the cage, a movement that must be precisely controlled and be such that it does not produce radial forces, which would cause friction and elastic micro-deformations between spool 2 and cage 1.

[0077] Due to the dimensions of the spool 2, it is preferable to use themotor 7 with a rotating drive shaft 8 and then to convert the rotary motion into linear motion through the mechanism 20.

[0078] The cage 1 is positioned in the seat of the casing 6 through O-ring seals that do not guarantee a perfectly uniform circumferential deformation. Consequently, the position of the longitudinal axis of the cage 1 cannot be determined with absolute precision, which should coincide with the longitudinal axis of movement of the spool 2, in order to get a relative motion of the latter which is free from jamming and / or friction due to radial loads.

[0079] On the other hand, the mechanism for converting rotary to linear motion should be absolutely free of clearances with respect to the linear-rotary motion. Therefore, it must accept (without producing significant radial loads) a lack of coaxiality (not constant) between the rotating shaft and the linear motion. The two conditions in state-of-the-art mechanisms (for example, preloaded ball screws, screw-lead with elastic coupling, etc.) are mutually contradictory.

[0080] The rotating part rotates around its own axis (which may not coincide perfectly with the axis of the component in linear motion), taking into account that eccentric movements will inevitably also occur.

[0081] With reference to figures 6 and 7, in order to solve the problem of translational-radial clearances and the lack of coaxiality between cage and spool, according to the present invention, a new mechanism 20 has been designed to transform rotary motion into linear motion. The mechanism 20 comprises:

[0082] - a rotating nut element 3, mechanically connected to the transmissionshaft 8, and

[0083] - a translating cylinder 4.

[0084] The nut element 3 is a rotating cylinder and is provided on a radial external cylindrical surface 11 with a helical groove 12 (or rotating nut) recessed in a radial internal direction.

[0085] As can be seen in figure 2, the cylinder 4 is equipped with an external anti-rotation device 5 that slides in a linear guide. The coupling between the anti-rotation device and the guide is achieved with minimal clearance, but without compromising the free sliding of the device, so avoiding interference that could generate unwanted friction. To allow the free movement and minimize any rotation due to the clearance, the contact point of the guide is radially distant from the axis of rotation.

[0086] The translating cylinder 4 is integrally fixed to the spool 2 and equipped with a small blade 13 that extends in a radially internal direction and fits into the helical groove 12 made in the nut element 3. This blade 13 has an inclination (or pitch) determined by an elastic preload such that the blade 13 touches both lateral shoulders 12', 12" of the helical groove 12 of the nut element. The preload of the spring must be such as to guarantee the desired axial force without additional deflections. In this way any micro-rotation of the nut element 3 (even with a rotation reversal) will produce a linear movement as the blade 13 is always in contact with both shoulders 12', 12" of the helical groove 12. By doing this, therefore, the problem of possible linear / rotational clearances is solved, even in the event of wear of the blade 13 and / or the nut element 3.Furthermore, any micro-eccentric rotations of the nut element 3 are absorbed by the lifting or deepening of the blade 13 in the helical groove 12 without causing significant radial loads. The same applies to not perfectly coaxial movements of the translating cylinder 4 with respect to the rotating nut element 3. The spool 2 is therefore free to follow its own travel path defined by the axis of the cage 1. In this way also the problem of a possible lack of coaxiality between cage 1 and spool 2 is solved.

[0087] To ensure a safe and precise control of the relative position between spool and cage, as described above, it must be sure that the cage 1 cannot undergo any axial micro-movement. This is ensured by the mechanically constrained positioning of the cage 1 in the casing 6, as better explained below.

[0088] However, considering the conditions 1, 2, 3, 4, and 6 cited above, according to which even microns can be relevant, it is appropriate to consider that external stresses due to the locking constraint can cause deformations. In fact, the distributor 10 is designed to manage changes in pressure and flow rate of gaseous fluids. Therefore, sudden temperature changes (due to expansion of the gas or to the contact with the hot gas supply) may occur that are uncontrollable, non-synchronous, and non-uniform both in the casing 6 and in the spool 2-cage 1 assembly. These thermal variations produce dimensional variations between casing 6, cage 1, and spool 2 which, in the event of mechanical blockage, would induce extremely high stresses and deformations in the cage 2, both in radial and axial directions, with potentially disastrous consequences.Regarding radial stresses, the problem is solved by elastically positioning the cage 1 in the radial direction on elastomeric seals that ensure a gap between cage 1 and casing 6 equal to or greater than the maximum foreseeable thermal deformation in the radial direction. Specifically, and with reference to figures 8 and 9, the cage 1 is positioned in the casing 6, the housing seat of which 6' comprises a radially internal cylindrical surface 6a. The cylindrical surface 6a has an internal diameter larger than the external diameter of a cylindrical surface la, that is radially external to the cage 1. The gap 17 separating the cage from the casing is large enough to allow relative thermal deformations between the two components without exchange of mechanical stresses, but at the same time it is small enough to maintain the compression of elastomeric sealing elements 18, such as O-rings, and therefore the perfect seal even in the most unfavourable combination of positioning and thermal deformations. This means that the position of the longitudinal axis of the cage 1, in which the spool 2 will slide, cannot be determined with absolute precision.

[0089] Regarding axial stresses, the problem is solved by constraining the cage 1 axially with respect to the casing 6, with one end blocked by a first stop element 14 and with the other end blocked by an elastic preload stop made of an elastic element 15 and a second stop element 16 that guarantees the elastic preload. In this way, the cage is securely axially blocked, but with a more than acceptable stress without deformation. In the event of thermal expansion of casing 6-cage 1, the preload of the spring varies negligibly, maintaining the stress more than acceptablewithout deformations. In this way, the stress in the direction of the longitudinal axis that the two pieces, cage 1 and casing 6, exchange remains almost constant even in the presence of thermal deformations, simultaneously ensuring a safe and fixed relative positioning.

[0090] In addition to the methods of implementing the invention, as described above, it is to be understood that numerous further variations exist. It should also be understood that these methods of implementing are merely exemplary and do not limit the scope of the invention, nor its applications, nor its possible configurations. On the contrary, although the above description enables a skilled craftsman to implement the present invention at least according to one exemplary configuration, it is to be understood that numerous variations of the described components are conceivable, without departing from the scope of the invention, as defined in the appended claims.

Claims

CLAIMS1. Multi-way spool flow distributor (10) comprising:- a casing (6),- a motor (7) with a rotating shaft,- a shaft (8) for transmitting the rotary motion induced by the motor (7) with a rotating shaft,- a mechanism (20) for transforming the rotary motion coming from the motor (7) with a rotating shaft into linear motion,- a stationary cage (1),- a spool element (2), which takes the linear motion from the mechanism (20) and is precisely coupled inside the cage (1),the flow distributor being characterized in that the mechanism (20) comprises a rotating nut element (3), mechanically connected to the transmission shaft (8) and a translating cylinder (4) integrally fixed to the spool (2), in which- the nut element (3) is provided on a radially external cylindrical surface (11) with a helical groove (12) hollowed out in a radially internal direction, and- the translating cylinder (4) is equipped with a small blade (13) that extends in a radially internal direction and fits into the helical groove (12).

2. Flow distributor (10) according to claim 1, wherein the blade (13) has an inclination determined by an elastic preload such that the blade (13) touches both lateral shoulders (12', 12") of the helical groove (12) of the nut element (3).

3. Flow distributor (10) according to claim 1 or 2, wherein the translating cylinder (4) is equipped with an external anti-rotation device (5) which is in a linear guide.

4. Flow distributor (10) according to anyone of the preceding claims, wherein the cage (1) is elastically housed in a radial direction in a housing seat (6') of the casing (6) by means of elastomeric sealing elements (18).

5. Flow distributor (10) according to claim 4, wherein between a cylindrical surface (6a), radially internal, of the housing seat (6') of the casing (6) and a cylindrical surface (la), radially external, of the cage (1) a gap (17) is defined which distances the cage (1) from the casing (6).

6. Flow distributor (10) according to anyone of the preceding claims, wherein the cage (1) is constrained in the axial direction with respect to the casing (6) with one end blocked by a first stop element (14) and with the other end elastically blocked by means of an elastic element (15) and a second stop element (16) which ensures the elastic preload.

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