Low Consumption Pneumatic Controller

Abstract

A pneumatic controller for controlling a process advantageously reduces fluid consumption by providing a proportional adjustment to a feedback signal. The pneumatic controller comprises a pneumatic control stage, a process pressure detector, and a feedback proportioning device. The feedback proportioning device uses a feedback cantilever component to provide the proportional adjustment of the feedback signal, thereby reducing the fluid consumption of the pneumatic controller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings wherein like reference numerals identify like elements in the several figures, in which:

FIG. 1 is a graphical representation of an example pneumatic controller comprising a cantilever feedback adjustment;

FIG. 2 is an expanded view of a cantilever feedback adjustment;

FIG. 3 is a graphical representation of an eccentric cam adjuster for a pneumatic controller.

DETAILED DESCRIPTION

The example pneumatic controller uses mechanical feedback element to adjust or proportion the feedback signal within a servo control loop to substantially reduce the fluid consumption during operation. With reference to FIG. 1, an example pneumatic controller 10 will be described. The pneumatic controller 10 comprises a pneumatic control stage comprising a relay 13, a feedback assembly 12 having a Bourdon tube assembly 32 and a nozzle-flapper assembly 22, including a nozzle valve 17 and a summing beam-flapper 21, and a proportional feedback device 37. To operate the controller 10, a supply fluid 11, such as natural gas, is connected to an inlet 14 of the relay 13. The relay 13 provides a pneumatic control stage to drive a control valve actuator 16 with a control pressure 20 to position a flow control element 31 within the control valve 33 which controls a process flow 50 through the control valve 33. The control pressure 20 used to actuate the control valve actuator 16 is derived from a pressure associated with supply fluid 11 connected to the relay 13 and is determined, in part, by a pneumatic control signal from the nozzle-flapper assembly 22.

One skilled in the art should appreciate that at initial startup of the pneumatic controller 10, an internal relay valve 23 in the relay 13 opens and the supply fluid 11 flows through a relay chamber 24 and a control chamber 29 within the relay 13 to build the control pressure 20 in the actuator 16. As shown in FIG. 2, a pneumatic restriction 43 at the control chamber inlet 18 creates a lag or delay during pressurization of the relay chamber 24 and the control chamber 29 to provide fluid flow to the actuator 16 until a predetermined or operational force balance across the relay 13 is achieved, as described in detail below. During operation, the control pressure 20 results from the modulation of a nozzle pressure 30 by the nozzle-flapper assembly 22 connect to a control inlet 19 of the relay 13 via pressure shunting action. That is, the relay valve 23 operates across a force balance primarily established by supply fluid pressure 11 acting upon the area ratio of an upper diaphragm 26 and a loading diaphragm 27 in the relay 13 with an additional bias spring force generated by an inlet spring 51 and a relay chamber spring 52. It is generally understood that by controlling the nozzle pressure 30 acting upon the loading diaphragm 27, a supplemental force directly related to the nozzle pressure 30 controls the relay valve 23 position, and therefore, the control pressure 20 to the actuator 16.

The shunting action of the nozzle-flapper assembly 22 previously described results from the relative position of the summing beam-flapper 21 with respect to the nozzle valve 17. The changes in relative position in the nozzle-flapper assembly 22 create a variable fluid restriction which causes corresponding changes in nozzle pressure 30. More specifically, the relative position of the nozzle valve 17 with respect to the summing-beam flapper 21 is determined, in part, by a process pressure 40 related to the downstream process fluid flow 50. To sense or detect the process pressure 40, the Bourdon tube assembly 32 is directly connected to the downstream process fluid flow 50. As the Bourdon tube assembly 32 is pressurized, it will expand or contract in correspondence to the changes in process pressure 40. Accordingly, it should be appreciated that an increase in process pressure 40 causes an expansion of the Bourdon tube assembly 32 subsequently moving the summing beam-flapper 21, from the left end designated A, resulting in movement towards the nozzle valve 17, effectively increasing a restriction at the nozzle valve 17, to increase the pressure on the loading diaphragm 27 in the relay 13 which subsequently opens the relay valve 23 creating an increase the control pressure 20 to the actuator 16. Likewise, a decrease in process pressure 40 allows the Bourdon tube assembly 32 to contract reducing the restriction presented by the nozzle-flapper assembly 22 thereby lowering the fluid pressure on the loading diaphragm 27 causing the control pressure 20 to the actuator 16 to decrease. In the example pneumatic controller 10, the Bourdon tube assembly 32 is used as a process feedback detector or element, but one of ordinary skill in the art appreciates that other feedback elements such as a bellows assembly may also be used.

To change the control point of the control valve 33, the pneumatic controller 10 provides an adjustment means 25 connected to the nozzle-flapper assembly 22 to establish a fixed or minimum pressure shunt in the nozzle-flapper assembly 22. That is, a set point of the pneumatic controller 10 is established by adjusting the absolute position of the nozzle valve 17 relative to the summing beam-flapper 21. In the example pneumatic controller 10, a cammed lever device 36 moves the nozzle valve 17 relative to summing beam-flapper 21 to provide the previously described predetermined shunt or “bleed” through the nozzle valve 17. By establishing this predetermined shunt, the nozzle pressure 30 provides a predetermined force on the loading relay 27 to generally fix the control pressure 20 to the actuator 16. It is also generally known that disturbances within the process (i.e. buffeting forces within the valve or changes in flow demand downstream of the valve) may cause deviations in control element position 31 that will affect process control (i.e., open loop control using only the aforementioned set point control is insufficient to control the process). To minimize such disturbances from affecting the process, process controllers provide a means for an adjustable negative feedback in a closed loop control strategy, as described in detail below.

Conventional pneumatic controllers often use a proportional band valve connected between the control pressure and atmosphere to ratiometrically proportion or adjust the pressure feedback through a feedback or proportional bellows (i.e., an adjustable negative feedback means). Conventional pneumatic controllers use the proportional band valve as a pressure divider to develop feedback pressure in the proportional band bellows based on a percentage of the controller's output pressure. It is generally understood that changing the setting of the proportional band valve provides for a different percentage of feedback pressure relative to the applied output pressure and ultimately results in a different proportional gain for the controller. The proportional band setting on the controller is used to tune the response of a process loop in response to set point changes and load upsets that occur in the process, but the proportional band valve continuously exhausts the supply fluid to the atmosphere which generally wastes large amounts of supply fluid.

The example pneumatic controller 10 reduces its consumption by replacing the proportional band valve with a cantilever feedback mechanism 60 that provides a proportional band adjustment without the bleed associated with the proportional band valve. As shown in FIGS. 1 and 2, a proportional bellows assembly 41 is pneumatically connected to the control pressure 20 and mechanically attached to the summing beam-flapper 21 as a process control signal detector. The proportional bellows assembly 41 comprises an upper bellows 55 and a lower bellows 56. The upper bellows 55 is connected to the control pressure 20. The lower bellows 56 is vented to atmosphere. As such, the proportional bellows assembly 41 may detect and respond to changes in control pressure 20 to provide a feedback force through the summing beam-flapper 21 to counteract pressure changes at the nozzle valve 17 and equalize a force differential that exists across the relay 13. During operation, changes in control pressure 20 are fed to the proportional bellows assembly 41, which causes a corresponding expansion or contraction of the upper bellows 55 which imparts a feedback force, relative to the right end of the summing beam-flapper designated as L to the summing beam-flapper 21 to counteract nozzle valve forces resulting from increases or decreases in the nozzle pressure 30.

To provide “tuning” or optimization of the pneumatic controller response, the cantilever feedback mechanism 60 provides a proportional band adjustment. The proportional band adjustment is based on a reduction, or division, of the motion imparted to the summing beam-flapper 21 through the proportional bellows assembly 41 as a result of a given change in the process pressure 40. It should be appreciated that for a given change in process pressure 40, the upper bellows 55 of the proportional bellows assembly 41 displaces the end of a cantilever feedback mechanism 60 by an amount that is directly proportional to the effective area of the proportional bellows assembly 41 and indirectly proportional to a spring rate or stiffness resulting from the cantilever feedback mechanism 60 in combination with a stiffness in the proportional bellows assembly 41.

The cantilever feedback mechanism 60 provides a proportional band adjustment by changing the effective length, and therefore the spring rate, of a cantilever 65. That is, the effective length of the cantilever 65 is adjusted by moving the proportional band adjuster 68 to a different position. As shown in FIGS. 1 and 2, the proportional band adjuster 68 is a clamping device arranged to slides along the cantilever 65 and may be secured by any means generally known in the art such as a rotating fastener (i.e. a thumb screw arrangement). One skilled in the art should appreciate that various arrangements of the cantilever 65 and the proportional band adjuster 68 may be used to align the two components. For example, a slot traversing the length of the cantilever 65 may accommodate a fastener in the proportional band adjuster 68 or the proportional band adjuster 68 may incorporate a recess (not shown) that “straddles” the cantilever to maintain alignment without departing from the spirit and scope of the example feedback adjustment means.

In tuning the feedback of the pneumatic controller 10, the relocation of the proportional band adjuster 68 causes the stiffness of the cantilever 65 to change as the length of the flexible portion of the cantilever 65 changes. Thus, the combination of the process pressure acting in the proportional bellows assembly 41 and the stiffness supplied by the cantilever 65 results in an adjustable displacement imparted to the sunning beam-flapper 21 to control to control pressure 20 to the actuator 16. For example, moving the proportional band adjuster 68 to the right in reference to FIG. 2 reduces the stiffness of the cantilever 65 and results in more summing beam displacement resulting from a change in pressure in the proportional bellows assembly 41. In addition to the modification of displacement due to the above described change in the position of the proportional band adjuster 68, additional amplification may occur to alter the effect of the cantilever's stiffness (i.e., both upper and lower bellows 55 and 56 have an associated spring rate that operates in combination with the stiffness of the cantilever 65).

For example, as the proportional band adjuster 68 is positioned to the right, the effective length of the cantilever 65 is increased. As the effective length of the cantilever 65 is increased, more of the displacement of the proportional bellows assembly 41 directly transfers to the summing beam-flapper 21 yielding a multiplicative effect on the stiffness of the cantilever 65. This increasing feedback may not be directly proportional to the length of the cantilever 65. In fact, it should be appreciated by one of ordinary skill in the art that this multiplicative effect may be approximately logarithmic with respect to the change in position of the proportional band adjuster 68 and the inherent spring rate of the proportional bellows assembly 41 which may exert an additional force related to the displacement length of the upper bellows 55. A logarithmic relationship may be desirable in the application of the controller as it enhances tuning sensitivity of the proportional gain adjustment when the proportional band becomes large (i.e., feedback supply sensitivity in increased). One of ordinary skill in the art may also appreciate various cantilever arrangements may provide other travel/spring rate relationships such as a “leaf spring” arrangement or a variable thickness or width of the cantilever.

To change the feedback signal in operation, the adjuster 68 is moved along length of the cantilever 65. As previously described, if the proportional band adjuster 68 is moved all the way to the right of the cantilever 65 in FIG. 2, all of the control pressure 20 change feeds back to the proportional bellows assembly 41. Thus, as the control pressure 20 increases, the proportional bellows assembly 41 will expand and move the summing beam-flapper 21 away from the nozzle 17 to command a decrease in the control pressure 20 from the relay. Similarly, when the proportional band adjuster 68 is moved all the way to the left of the cantilever 65, the combined stiffness of the cantilever feedback mechanism 60 and the proportional bellows assembly 41 may resist the process pressure 40 thus decreasing the displacement of the summing beam-flapper 21 from the nozzle. This movement increases nozzle resistance thereby increasing pressure on the loading diaphragm 27 subsequently increasing the control pressure 20. As a result, the example pneumatic controller provides a proportional band adjustment without exhausting supply fluid to the surrounding atmosphere.

The example pneumatic controller 10 may also provide an alternate means to secure the proportional band adjuster 68 to the cantilever 65. FIG. 3 shows a clamping arrangement to secure a proportional band adjuster to the cantilever 65 without a rotational fastener directly clamping to the cantilever 65. In the locking lever assembly 168, a spring component 185, such as a Belleville spring, provides a mechanical compliance during a camming action to prevent distorting the cantilever 65 or permanently elongating the shaft 181. Similar to the previously described proportional band adjuster, the example locking lever assembly 168 is positioned along the cantilever at the desired position. A locking lever 180 rotates about a pin 182 within an adjuster clamp 187 offset from the central axis, Z, of the locking lever 180. As the locking lever 180 engages the clamp 187 by rotating in a clockwise direction, an adjuster shaft 181 is drawn towards the cantilever 65, compressing the spring 185, to provide a spring biased/compliant load on the cantilever 65 which secures the locking lever assembly 168 in the desired position. Additionally, a pair of spacers 191 and 192 may be provided to avoid damaging the cantilever 65 and provide alignment during engagement of the adjuster clamp 187. To provide a means to adjust the Belleville spring load, an adjusting nut 184 may be threadably attached to the shaft 181 to control the compression depth of the locking lever assembly 168. One skilled in the art may appreciate that other compliant means could also be used to provide the momentary elongation duration the camming action such as a coil spring or a polymer.

While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims. Although certain apparatus, methods, and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. To the contrary, this patent covers all apparatus, methods, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.

Claims

1. A pneumatic controller for controlling a process comprising: a pneumatic control stage providing a process control signal to a control element;a pneumatic feedback assembly providing a feedback control signal representative of the process to the pneumatic control stage, wherein the feedback control signal modifies the process control signal; anda cantilever feedback assembly connected to the pneumatic feedback assembly wherein the cantilever feedback assembly provides an adjustment to the feedback control signal.

2. The pneumatic controller of claim 1, wherein the cantilever feedback assembly further comprises a bellows assembly.

3. The pneumatic controller of claim 1, wherein the pneumatic control stage comprises a relay.

4. The pneumatic controller of claim 1, wherein the pneumatic feedback assembly further comprises a Bourdon tube and a nozzle-flapper assembly.

5. The pneumatic controller of claim 1, wherein the cantilever feedback assembly includes a cantilever and an adjuster to adjust the stiffness of the cantilever such that the stiffness of the cantilever provides a predetermined feedback control signal.

6. The pneumatic controller of claim 5, wherein the stiffness of the cantilever is proportional to at least one of the length of the cantilever, the thickness of the cantilever or the width of the cantilever.

7. The pneumatic controller of claim 5, wherein the adjuster comprises a clamping assembly.

8. The pneumatic controller of claim 7, wherein the clamping assembly comprises a rotational fastener.

9. The pneumatic controller of claim 7, wherein the clamping assembly comprises an eccentric cam fastener.

10. The pneumatic controller of claim 1, wherein the cantilever feedback assembly reduces a supply fluid consumption of the pneumatic controller.

11. A feedback proportioning device for a pneumatic process controller having a pneumatic control stage and a pneumatic feedback assembly, the feedback proportioning device comprising: a feedback detector providing a feedback signal representative of a control signal produced by the pneumatic control stage; anda cantilever assembly providing a predetermined adjustment of the feedback signal.

12. The feedback proportioning device of claim 11, wherein the feedback detector comprises a bellows assembly.

13. The feedback proportioning device of claim 12, wherein the cantilever assembly includes a cantilever and an adjuster.

14. The feedback proportioning device of claim 13, wherein the predetermined adjustment of the cantilever assembly comprises changing the stiffness of the cantilever.

15. The feedback proportioning device of claim 14, wherein stiffness of the cantilever is directly related to at least one of the length of the cantilever, the thickness of the cantilever or the width of the cantilever.

16. The feedback proportioning device of claim 16, wherein the length of the cantilever is determined by a position of the adjuster.

17. The feedback proportioning device of claim 16, wherein the adjuster comprises a clamping arrangement.

18. The feedback proportioning device of claim 17, wherein the clamping arrangement comprises a rotational fastener.

19. The feedback proportioning device of claim 17, wherein the clamping arrangement comprises an eccentric cam fastener.

20. The feedback proportioning device of claim 14, wherein stiffness of the cantilever provides a logarithmic relationship relative to a displacement of the bellows assembly.