Abstract:
A cantilever feedback mechanism includes an upper cantilever and a lower cantilever. The upper cantilever and lower cantilever have a resistance spring rate to facilitate movement of the bellows over a selected range of resistances. The upper cantilever has downwardly oriented upper cantilever stops. The lower cantilever has upwardly oriented lower cantilever stops that engage the upper cantilever stops. Mean are provided for applying a preload to maintain the lower cantilever stops and the upper cantilever stops engaged until a force urging movement exceeds the preload.

Description:
FIELD 
     There is described an improved form of cantilever feedback mechanism for use in a proportional bellows assembly used in pneumatic controls for pneumatic instruments. 
     BACKGROUND 
     In a process control system that uses pneumatic instrumentation it is very common to use a bellow as a means of providing movement for a feedback control signal. Movement of the bellows must be adjustable to facilitate a range of feedback to allow the user to control the sensitivity of the instrument. A common approach is to use an adjustable valve, proportional valve that will divert or split the feedback signal from the control loop allowing an adjustment of the pressure sent to the bellow by venting the balance to atmosphere. Simply put the control loop feedback pressure to the bellow is regulated by increasing or decreasing exhaust volume through the proportional valve. This design is very effective because it provides a wide range of pressure to the bellow and facilitates very low pressure settings required for the zero movement of the bellows. The challenge of mechanical means of limiting bellow movement is to provide a resistance of the forces introduced by the control loop signal pressure inside the bellow that has a similar range of adjustability to the pneumatic proportional valve. As well, if the design is to be applicable to a variety of applications, it must be universal in design. 
     In a process control system where a proportion valve is used it is most often set where 60-80 percent of the feedback control loop signal is exhausted to atmosphere allowing a higher level of sensitivity for the instrument. Because of this high exhaust rate many users have to account for this gas loss by using larger gas compressors. In remote locations where the Natural Gas is utilized as an instrument supply there is a significant cost as well as an environmental impact associated with this design. 
     United States Patent Application 2008/0078449 (Pesek), entitled “Low Consumption Pneumatic Controller,” discloses a pneumatic instrument that has a proportional bellows assembly which has an upper bellows and a lower bellows. The upper bellows is connected to control pressure. The lower bellows is vented to atmosphere. During operation, changes in control pressure cause an expansion or contraction of the upper bellows. The lower bellows provides a counteracting feedback force to counteract control pressure changes and equalize any resulting force differential in the proportional bellows assembly. In order to provide “tuning” or optimization of the proportional bellows response, a cantilever feedback mechanism is provided that provides proportional band adjustment. This proportional band adjustment is based upon a reduction of any minor motion or hysteresis within the proportional bellows assembly. When operating as intended, the cantilever feedback mechanism provides a proportional adjustment in response to minor movement, without exhausting supply fluid to the surrounding atmosphere. In order to improve functioning of such devices there is a need for an improved cantilever feedback mechanism. 
     SUMMARY 
     There is provided a cantilever feedback mechanism which includes an upper cantilever and a lower cantilever. The upper cantilever and lower cantilever have a resistance spring rate to facilitate movement of the bellows over a selected range of resistances. The upper cantilever has downwardly oriented upper cantilever stops. The lower cantilever has upwardly oriented lower cantilever stops that engage the upper cantilever stops. Means are provided for applying a preload to maintain the lower cantilever stops and the upper cantilever stops engaged until a force urging movement exceeds the preload. 
     After experimenting with a variety of cantilever configurations, it was determined that without a preload hysteresis was unavoidable. The above described cantilever feedback mechanism with opposing stops was developed to enable preload to be applied. 
     The performance of the cantilever feedback mechanism is improved when the upper cantilever and the lower cantilever are identical in shape and spring rate. When there is a difference the movement becomes non-linear, which effects the accuracy of the set point. 
     While there may be different ways of applying a preload, beneficial results may be obtained by positioning spacers between the lower cantilever and the upper cantilever at a central position and at an end remote from a bellows mounting end. The preload is determined by the length of the spacers. The spacers are shorter in length than the combined length of the upper cantilever stops and the lower cantilever stops. 
     The spacers also play a role in determining the resistance spring rate of the upper cantilever and the lower cantilever as they form part of an adjuster assembly. The resistance spring rate is adjusted by a central spacer which is moved and fixed in positioning by loosening and tightening a fastener within a slot in both the upper cantilever and the lower cantilever to provide an adjustable resistance. It will be understood that the resistance is greater as the spacer is moved closer to a bellows mounting end of the upper cantilever and the lower cantilever. 
     For greater accuracy, it is preferred that the upper cantilever stops and the lower cantilever stops are located perpendicular to a centerline of the adjuster assembly on a diametrical centerline of a bellows diameter of a bellows assembly to allow resistance forces to be uniform and not change a natural linear movement of the bellows assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein: 
         FIG. 1  is a schematic view of a pneumatic instrument equipped with a cantilever feedback mechanism. 
         FIG. 1A  is a perspective view of a pneumatic instrument equipped with the cantilever feedback mechanism of  FIG. 1 . 
         FIG. 2  is a side elevation view perspective view of an unacceptable early embodiment of cantilever feedback mechanism. 
         FIG. 3  is a perspective view of a proportional bellow assembly without a cantilever feedback mechanism. 
         FIG. 4A  is an exploded perspective view of the cantilever feedback mechanism of  FIG. 1  and  FIG. 1A . 
         FIG. 4B  is a perspective view of the cantilever feedback mechanism of  FIG. 1 ,  FIGS. 1A and 4A , engaging a bellows. 
     
    
    
     DETAILED DESCRIPTION 
     In the example shown in  FIG. 1 , a pneumatic pressure controller uses a mechanical means of adjusting the feedback element within a pneumatic feedback loop. Referring to  FIG. 1A , mechanical means are attached to base  107 . Use of the mechanical means replaces the pneumatic means and significantly reduces the use of supply gas. The example illustrated in  FIG. 1  shows the primary feedback loop pressure sensing element, bourdon tube  30 , connected to the process line  31 . The bourdon tube  30  expands with an increase in process pressure  38 . The expansion of the bourdon tube  30  connectively moves a flapper  28  closer to a nozzle  33 . The nozzle  33  has a constant gas flow  12  supplied by the relay assembly, generally referenced as  35 , via the relay tube  23 . A fixed orifice  18  helps to ensure a constant flow. The gas pressure at the nozzle orifice  33  remains constant until the flapper  28  moves in relation to the nozzle  33 . When the flapper  28  proximity to the nozzle  33  relationship changes the subsequent pressure change at the nozzle orifice  33  is transmitted back through the relay tube  23  to the relay assembly  35  where it acts against a diaphragm  14  and  16  effectively causing an output pressure change signal to the control element, not shown, through air output  32 . Exhaust  34  created in relay assembly  35  is allowed to exit the assembly  35 . Absolute adjustment, a pressure setting control  20 , of the flapper  28  and nozzle  33  relationship is provided to establish a predetermined set point. Referring to  FIG. 1 , a secondary feedback loop pressure sensing element, bellows  22  and  24  with cross springs  26 , provide means, output proportional tubing  42  shown in  FIG. 1A , of adjusting the sensitivity of the primary feedback loop by opposing the movement of the primary sensing element, bourdon tube  30 . A return spring  44  biases flapper  28  towards a spaced relation with nozzle  33 . Referring to  FIG. 1 , one knowledgeable about the products used within the industry would appreciate that the use of bellows  22  and  24  as a secondary feedback loop sensing element is widely used in the design of pneumatic controllers. Referring to  FIG. 1A , bellows caps  46  and  108  surround bellows  22  and  24  and an adjustment cantilever assembly  40  is used to control movement of bellows  22  and  24 . 
     In an alternative design,  FIG. 2  illustrates a dual cantilever installed on a bellows assembly. The challenge of mechanical means of limiting bellows  22  and  24  movement is to provide a resistance of the forces introduced by the control loop signal pressure inside the bellows  22  and  24  that has a similar range of adjustability to the pneumatic proportional valve. The cantilevers  82 A and  82 B are manufactured to a specific spring rate to correspond with the effective area of the bellows  68 . The spacer  76  is moved and fixed by loosening and tightening the thumb screw  70  within the slot of the cantilevers  82 A and  82 B to provide an adjustable resistance. The resistance is greater as it is moved closer to the bellows  68 . A spacing screw  80  may be tightened or loosened using jam nuts  78  and acts as an end spacer for cantilevers  82 A and  82 B. Two cantilever pins  66 A and  66 B are formed with a 60 degree point that is nested within corresponding holes in the bellows flange  64 A and  64 B. One skilled in the art would appreciate that a slight compression of the bellows is required to contain the two cantilever pins  66 A and  66 B in their nested positions. Accordingly the spacer  76  length is adjusted to achieve the correct spacing. Although this design achieves a range of adjustability it does not provide enough spring force to completely overcome the force of the bellows under normal operating conditions. Even when the spring rates of the cantilevers are increase by 100% and the thumb screw  70  adjusted to highest resistance the resulting movement is not low enough to provide the required functionality. 
       FIG. 3  illustrates another alternative design where pneumatic means is used in a two bellow system whereby an opposing bellows  58 A is charged with gas pressure regulated by an integrally mounted regulator  50 . Regulator  50  is mounted to gauge block  52  and attached by fittings  48 . Bellows  58 A is maintained in position by upper bellows retaining flange  62 A and lower bellows retaining flange  62 B. The regulator pressure travels through output pressure tubing  56  and causes the bellows  58 A to expand against an externally mounted bellows clamp  60 . By adjustably regulating the charging pressure within bellows  58 A a pneumatic spring is created allowing adjustable resistance of any forces introduced by the opposing bellows  58 B. A pressure gauge  54  is mounted downstream of the regulator  50  on gauge block  52  to indicate the charging pressure to the resistance bellows  58 A. Because the available supply pressure to the bellows  58 B and the available charging pressure to the resistance bellows  58 A are of equal values one would assume that when the resistance bellow  58 A is charged with the maximum value available to the control loop very little movement of the two bellow assembly would occur when maximum loop pressure is introduce to bellow  58 B. As well further movement could be adjusted by reducibly regulating the charging pressure in bellows  58 A below that of bellows  58 B. However in practical testing significant movement occurs within the assembly when full loop pressure is introduced to bellows  58 B when the resistance bellow  58 A is charged with equal pressure. It was concluded that in order to achieve the minimum movement requirements the charging pressure in bellows  58 A would have to exceed the loop pressure in bellows  58 B by a significant margin. In order to facilitate this, additional components would be required effecting the practical application of this design versus other alternatives. 
     Referring to  FIG. 4A  and  FIG. 4B , a dual cantilever spring assembly manufactured to a specific spring rate and preload to match the force created by the maximum feedback loop charging pressure within a bellows is described. Bellows  68  are movable within deep bellows cap  108  and shallow bellows cap  46 . A top bellows cap  100  and a bottom bellows cap  102  are also present at the top and bottom of the bellows  68 , respectively. Top bellows cap  100  contains o-ring seals which come into contact with deep bellows cap  108 . It is preferred that pivot pins  84 A and  84 B be precisely located perpendicular to the centerline  96  of the proportional set-point adjuster assembly which includes a cap screw  90 , center sleeve  86 , top lock nut  72  and bottom lock nut  74  on the diametrical centerline  98  of the bellows diameter to allow resistance forces to be uniform and not change the natural linear movement of the bellows assembly. The Pivot Pins  84 A and  84 B are designed with a specified predetermined length so that when they touch together they act as stops against the cantilever forces in order to allow preload forces to be applied. The rear sleeve  88  and center sleeve  86  are also designed to a corresponding predetermined equal length  106 A and  106 B which is shorter than the combined length of the Pivot Pins  84 A and  84 B. When cap screw  90  is tightened, the cantilevers  92 A and  92 B are compressed against the center sleeve  86  and rear sleeve  88 . The desired preload is achieved by calculating the preload required to achieve zero bellows movement and designing the center sleeve  86  and rear sleeve  88  to the correct length. Distance  106 A and  106 B will then be of equal height. One skilled in the art would understand that infinite preload and spring rate can be achieved by altering a combination of spring rate of the cantilevers  92 A and  92 B and preload of the assembly. The application of this design as proportional adjustment of a pneumatic feedback loop using a bellows assembly requires that the spring rate and preload of the dual cantilever assembly be designed to match the Bellows force created by the feedback loop charging pressure. 
     Without preload there is an inherent hysteresis due to the various contact points within the cantilever assembly. The preload solves this problem. It is required that a setting of 1 on the scale would limit the bellows to zero movement. Different settings on the scale are achieved by moving center sleeve  86  along upper cantilever slot  94 A and lower cantilever slot  94 B. With the cantilever assembly preloaded to a force greater than that created from the maximum feedback loop pressure this can be achieved. Through our testing we were not able achieve zero movement without pre-loading even with larger and heavier cantilevers. The relationship between the pivot pin  84 A and  84 B, rear sleeve  88  and center sleeve  86  dictate the amount of preload the design has. When using a larger bellows  68  or higher pressure instrument supply pressure, preload pressure can be increased by simply shortening the center sleeve  86  and rear sleeve  88 . The design of the assembly operates best when both Cantilevers  92 A and  92 B are identical in shape and spring rate so the movement is uniform in a linear direction. During testing when non identical cantilevers were used the movement was not straight up and down due to the weaker cantilever moving more that the stronger one. This movement could be referred to as an arc rather that a straight line. The arc type movement changes the relationship of the flapper  28  and the nozzle  33  of the assembly on the horizontal plane which affects the accuracy of the set point. 
     In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. 
     The following claims are to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, and what can be obviously substituted. Those skilled in the art will appreciate that various adaptations and modifications of the described embodiments can be configured without departing from the scope of the claims. The illustrated embodiments have been set forth only as examples and should not be taken as limiting the invention. It is to be understood that, within the scope of the following claims, the invention may be practiced other than as specifically illustrated and described.