Torque tube digital differential pressure sensor
DESCRIPTION 
     1. Technical Field 
     This invention relates to differential pressure transducers, and more particularly, to differential pressure transducers having a digital output and a low sensitivity to common mode line pressure errors. 
     2. Background Art 
     A servo torque-balance pressure transducer with low sensitivity to common mode line pressure is described in U.S. Pat. No. 3,664,237, issued to Paros. This analog output sensor achieves low sensitivity to common mode line pressure through delicate adjustment of lever arm distances between the two bellows pressure inputs and a flexible pivot. The lever arm adjustment is designed to equalize the moments produced about the pivot axis when the same line pressure is applied to both bellows. Differential pressure between the bellows produces a resultant toque which may be measured using conventional servo techniques. The adjustment mechanism is complex and difficult to manufacture, and requires inordinate skill to reduce the common mode line pressure errors to acceptable levels. One difficulty in this design is that since the full-scale differential pressure range is determined by the effective areas of the bellows and the lever arm distances to the pivot, adjustments to the lever arm distances are directly proportional to full scale and, therefore, must be made with great precision. Common mode line pressure errors can nevertheless be reduced to an acceptable level with this device, since it produces an analog output which is inherently limited in accuracy. However, an analog output is less desirable than the more accurate direct digital-type signal because a digital signal is more compatible with digital computers and control systems. The greater accuracy of the digital sensor makes common mode line pressure errors more apparent than they would be with a device producing a less accurate analog output. Thus conventional differential pressure mechanisms are inherently incapable of providing optimum performance when used with a digital sensor since their common mode pressure errors are an unacceptably high percentage of their differential pressure sensitivity. 
     In a paper delivered at the 27th Annual ISA Conference and Exhibit in October, 1972, and published in ISA Transactions, Vol. 12, 1973, pp. 173-1979, Paros describes the design of a &#34;Precision Digital Pressure Transducer.&#34; This sensor employs two coaxial opposing bellows, with sealed ends attached to a pivotal suspension system. The bellows are mounted with the open ends adapted to receive pressure inputs such that a different pressure between the two bellows causes a resultant torque which stresses a vibrating quartz crystal whose frequency change is a measurement of the differential pressure. No provision for reducing the common mode line pressure error is described. This error is caused by differences between the bellows effective area or lever arm distance to the pivot and is directly proportional to full scale. 
     U.S. Pat. No. 4,382,385, issued to Paros, describes a digital differential pressure transducer with reduced common mode line pressure effects. Differential pressue is applied across a single pressure-sensing diaphragm or bellows, and the force induced by the differential pressure is coupled to a stress-sensitive resonator either directly or through a force-transmitting structure. The resonator, or at least a portion of the force-transmitting structure, is surrounded by a first bellows to isolate the resonator from the pressure-sensing diaphragm or bellows. A second bellows is provided to counteract the pressure-induced force applied to the resonator by the first bellows. The pressure-induced forces generated by the first and second bellows are substantially smaller than the pressure-induced forces generated by the pressure-sensing diaphragm or bellows. The common mode line pressure error of the transducer resulting from a mismatch between the pressure-induced forces generated by the first and second bellows and expressed as a percentage of the full-scale differential pressure range is relatively low because the error is proportional to the ratio of the force mismatch to the force generated by the pressure-sensing diaphragm or bellows. 
     A number of load-sensitive resonators are known. In an unstressed state, under constant environmental conditions, a vibrating element has a unique resonant frequency determined by its dimensions and material composition. This resonant frequency increases under tensile loading and decreases under compressive loading. The resonant frequency should be a true and accurate measure of the applied load. For optimum performance, stress-sensitive resonators should have a well-defined resonant frequency. Energy losses from the resonator reduce the &#34;Q&#34; or quality factor of the resonator, thereby making the resonant frequency less well defined. One cause of energy loss from a resonator is friction between the resonator and the surrounding environment. The surrounding environment can also, with time, alter the performance of the resonator, thus further degrading its performance. Consequently, it is desirable for resonators to operate in a vacuum or inert atmosphere. U.S. Pat. No. 4,406,966, issued to Paros, describes techniques employing bellows which isolate load-sensitive resonators from the force-producing external environment. 
     Force-sensitive crystal resonators are described in U.S. Pat. No. 3,984,111, issued to Kritz, and U.S. Pat. No. 3,093,760, issued to Tarasevich, in which loads are applied to crystals near the nodal points. 
     U.S. Pat. No. 3,470,400, issued to Weisbord, describes a single-beam force transducer with an integral mounting system which effectively decouples the beam vibrations from the mounting points through a spring and mass arrangement. 
     U.S. Pat. No. 3,238,789, issued to Erdley, describes a load-senstitive, closed-end tuning fork consisting of two tines or bars vibrating 180 degress out of phase, such that the reactive forces and moments cancel. 
     3. Disclosure of Invention 
     It is an object of this invention to provide a pressure transducer with an inherently digital-type output. 
     It is another object of this invention to provide a pressure sensor with high sensitivity, accuracy, and stability. 
     It is still another object of this invention to provide a pressure transduer of small size and weight, with low power consumption. 
     It is yet another object of this invention to provide a differential pressure transducer with low sensitivity to common mode line pressure errors. 
     Differential pressure is applied across a single pressure-sensing pressure vessel, such as a diaphragm, bellows, or Bourdon tube. The resulting load is coupled to a stress-sensitive resonator by means of a torque shaft and force-transmitting structure. The resonator is isolated from the presssure input applied to the outer surface of the pressue vessel by a sealed, flexible, cylindrical torque tube which coaxially surrounds the torque shaft and is connected at spaced-apart locations to the torque shaft and an isolation barrier separating the resonator from the pressure vessel. The axial rigidity of the torque tube restricts axial movement of the torque shaft, while the relatively high torsional compliancy of the torque tube allows relatively free rotation of the torque shaft. Thus, the resonator may operate in a vacuum or inert atmosphere, common mode pressures applied to the isolating torque tube do not generate any torques, and line pressure errors are therefore minimized. 
     In one embodiment, the force-generating pressure vessel is a bellows which is connected to a pivotally mounted structure and the resulting torque is transmitted by the torque shaft to a pivotally mounted structure which applies a load to the resonator. Flexible linkages may be connected between the torque shaft and the pivotally mounted structure such that the pivots are substantially relieved of cross axis stress due to common mode line pressure effects on the isolating, sealed, flexible torque tube which surrounds the torque shaft. 
     In another embodiment, the force-generating pressure vessel is a Bourdon tube and the directly resulting torque is transmitted through the torque shaft to load a stress-sensitive resonator. The resonator is isolated from the pressure input side by the flexible, sealed torque tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view illustrating a conventional single-beam force transducer with integral mounting isolation; 
     FIG. 2 is a plan view illustrating a conventional closed-end tuning fork sensor. 
     FIG. 3 is an isometric view of a torque tube digital differential pressure sensor employing a pressure-sensitive bellows that generates a torque which is applied to a stress-sensitive resonator through a torque tube, the external surface of the bellows being isolated from the rsonator torque tube. 
     FIG. 4A is an isometric view of a torque tube digital differential pressure sensor of the type illustrated in FIG. 3 employing a pair of flexure hinges supporting the torque shaft to restrict movement of the torque shaft perpendicular to the axis of the torque shaft. 
     FIG. 4B is an isometric view of a torque tube differential pressure sensor of the type illustrated in FIG. 3 employing flexible linkages transmitting torque from the pressure-sensitive element to the torque shaft to allow axial movement of the torque shaft. 
     FIG. 5 is an isometric view of a torque tube digital differential pressure transducer employing a Bourdon tube as the pressure-sensitive element. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The present invention is equally applicable to load-sensitive resonators of various shapes and configurations; however, simplicity and clarity, only the application to flexurally vibrating, force-sensitive beam and closed-end tuning fork devices will be described in detail, it being understood that the same or similar principles apply in the general case. 
     FIG. 1 illustrates a conventional single-beam force transducer 2 with integral mounting isolation, as disclosed in the aforementioned patent to Weisbord. The transducer 2 consists of a flexurally vibrating center beam 4, two sets of isolator masses 6 and isolator springs 8 extending from each end of the beam 4 to mounting surfaces 10. Axial forces, applied along the longitudinal axis of the transducer 2 stress the vibrating beam 4, thereby changing its resonant frequency in accordance with the magnitude of the applied loads. The isolator masses 6 and isolator springs 8 are designed to decouple the reactive forces and moments generated by the vibrating beam 4 from the mounts 10, thus reducing the energy losses. As a result, the transducer 2 has a high &#34;Q&#34; so that its resonant frequency is an acurate representation of the applied forces. The &#34;Q&#34; can also be maximized by operating the transducer 2 in a vacuum. The beam 4 may be driven at its resonant frequency using electrodes 12 and oscillator circuitry in any conventional manner, such as is described in U.S. Pat. No. 3,479,536, issued to Norris. 
     FIG. 2 is a plan view of a conventional closed-end tuning fork 20, as described in the aforementioned patent to Erdley. This device 20 achieves low energy loss, high &#34;Q&#34; operation by driving a pair of closely matched tines 22 180 degrees out of phase, thus cancelling the reactive moments and forces which might be transmitted to a mount 24 from which the tines 22 project. As with the transducer 2 of FIG. 1, the &#34;Q&#34; of the tuning fork 20 can be maximized by operating it in a vacuum. 
     Under constant environmental conditions, the resonant frequency in Hertz, fo, of an unstressed, fixed-ended, flexurally vibrating beam of length L, thickness t, width b, modulus of elasticity E and density d is given by the formula: 
     
         f.sub.o =(constant) (t/L.sup.2)ν(E/d)                   Equation (1) 
    
     Although the resonant frequency is generally a nonlinear function of the applied load F, the first order load sensitivity S F  may be calculated as: 
     
         S.sub.F =(constant) (L.sup.2 F/Ebt.sup.3)                  Equation (2) 
    
     The quantitative relationships between resonant frequency, applied load, and resonator dimensions and composition can be determined from the above formulae. In particular, pressure-induced loads may be generated by having the applied pressure P act on a bellows or diaphragm of effective area A, and equation 2 may be restated in terms of the first order pressure sensitivity S p  as: 
     
         S.sub.p =(constant) (L.sup.2 AP/Ebt.sup.3)                 Equation (3) 
    
     Although a number of load-sensitive resonators may be mechanically stressed to form pressure sensors, the following discussions will illustrate the inventive devices using flexing bar resonators such as the single-beam force transducers with integral mounting isolation and the closed-end tuning fork force sensor. 
     Most resonators will perform more accurately, with less engery loss and with increased stability, when operating in a vacuum or inert atmosphere. Therefore, a differential pressure transducer construction in which the line pressure is in contact with the resonator is undesirable. Yet, isolating structures such as bellows can themselves apply pressure-induced forces to the resonator, and it is not easy to completely compensate for these isolating structures. For example, the pressure transducer described in U.S. Pat. No. 3,642,237 may be designed to isolate the sensing element from the line pressure through a pair of opposing bellows which act as both isolating elements and differential pressure inputs. In order to make these devices insensitive to common mode line pressure errors, the constructions must include a mechanism which must be adjusted to account for differences in bellows effective areas and pivot lever arm distances. Since the full-scale differential pressure range is determined by the bellows effective areas and lever arm distances, the aforementioned adjustments for line pressure errors must be carefully and skillfully made. It is generally impractical to massproduce differential pressure sensors with sufficient precision to maintain the common mode line pressure error at a level which does not degrade the accuracy of highly precise digital stress sensors. Thus, differential pressure-sensing structures which are acceptable for use with an analog stress sensor can easily be unacceptable for use with a digital stress sensor. Indeed, even though, as described in U.S. Pat. No. 4,382,385, common mode line pressure errors are reduced by the ratio of mismatch between the effective area of the isolating/compensating bellows to the effective area of the pressure-sensitive element, significant common mode errors remain. 
     FIG. 3 illustrates an embodiment of a digital differential pressure transducer which isolates its stress-sensitive resonator from the pressure inputs and also has a low sensitivity to common mode line pressure errors. This configuration does not require any adjustment mechanism to reduce line pressure erros to acceptable levels. 
     An airtight enclosure 30 is divided into first and second sections by an isolation barrier 31. A bellows 40 sealed by end cap 42 is mounted in the first section of the enclosure 30 and is attached to a torque-transmitting arm 44 and is sealed to the airtight enclosure 30 at end 46. Pressure ports 48 and 50 in the enclosures are provided such that the interior portion and the exterior portion of the bellows 40 may be independently pressurized. The torque generated about pivot axis 52 is transmitted through torque shaft 54 and arm 56 to stress-sensitive resonator 58 mounted in the second section of the enclosure 30. The torque shaft 54 extends through an aperture 59 in the barrier 31. Torsionally flexible tube 60 is attached and sealed to torque shaft 54 at end 62. Tube 60 is sealed to the barrier 31 over the aperture 59 at end 64. Thus resonator 58 is isolated from the pressure input side containing ports 48 and 50 by the torsionally flexible, sealed torque tube 60, allowing resonator 58 to operate in a vacuum or inert atmosphere 32. 
     The torque tube 60, which is preferably of cylindrical configuration, has a length that is sufficiently long to make the torque tube 60 relatively compliant to torsional stress. As a result, the torque shaft 54 is relatively free to rotate without significant restraint from the torque tube 60. However, the axial rigidity of the torque tube 60 restrains the torque tube 60 from axial movement. A cover plate 61, connecting the torque tube 60 to the torque shaft 54, is preferably relatively rigid to restrain axial movement of the torque shaft 54 without significantly restraining the torsional stress transmitted to arm 54 and stress-sensitive resonator 58. 
     Torque-transmitting arm 44 can move up or down only between gaps 38 until it contacts limit stops 34 attached to airtight enclosure 30 at position 36, thus protecting resonator 58 from excessive stress due to overpressure. In order to eliminate unwanted forces and torques, pivot axis 52 may be mechanically defined and constrained by adding single axis pivots at positions 66 and 68. 
     Although the embodiment illustrated in FIG. 3 utilizes a bellows 48 as the pressure-sensing element, it will be understood that other pressure-sensitive pressure vessels, such as diaphragms, Bourdon tubes and the like, may also be used. 
     FIG. 4A illustrates an embodiment of a torque tube digital differential pressure sensor including flexural supports which relieve axial stresses in structural parts. A bellows 140 sealed by end cap 142 is attached to a torque-transmitting arm 170 that is constrained to rotate about the pivot axis 152 by flexural pivot 72 that is supported by arm 74 attached at position 76. The torque from torque-transmitting arm 170 is transmitted by bracket 78 to torque shaft 154 at attachment 80, allowing torque shaft 154 to align coaxially with pivot axis 152 and flexural pivot 72. Flexure 82 in arm 74 allows displacement of torque shaft 154 along pivot axis 152 to relieve cross axis stress in flexure pivot 72 caused by thermal expansion of torque shaft 154 or axial stress in tube 160 induced by pressure. Torque shaft 154 transmits torque through bracket 84 that is connected to arm 156. Arm 156 is constrained to rotate about pivot axis 152 by flexural pivot 86 that is supported by arm 88 attached at position 90. Torque generated about pivot axis 152 is transmitted by arm 156 to stress-sensitive resonator 158. Flexure 92 in arm 88 allows displacment of torque shaft 154 along pivot axis 152 to relieve cross axis stress in flexure pivot 86. Counterbalance weights 162 and 164 are attached to torque-transmitting arm 170 and bracket 78, respectively, so that no torque is generated about pivot axis 152 by gravity or acceleration, since the center of gravity can be balanced to coincide with the pivot axis 152. 
     FIG. 4B illustrates an embodiment of a portion of a torque tube digital differential pressure sensor including flexible linkages which relieve axial stresses in flexural pivots. Torque-transmitting arm 94 is constrained to rotate about pivot axis 190 by flexural pivot 180 supported by support 198 constrained at position 200. The torque from torque-transmitting arm 94 is transmitted to torque shaft 210 through axially flexible linkages 104 at attachment point 106. Axially flexible linkages 104 relieve cross axis stress in flexural pivot 180 caused by thermal expansion of torque shaft 210 or axial stress in tube 240 induced by pressure. 
     FIG. 5 illustrates another embodiment of a torque tube digital differential pressure transducer, including an airtight enclosure 260 having pressure inlet ports 202 and 212. Port 202 is sealed to and communicates with the interior of pressure-sensitive Bourdon element 214. Port 212 communicates with the exterior of Bourdon element 214. Differential pressure applied between ports 202 and 212 causes Bourdon element 214 to generate a torque about axis 116. This torque is transmitted through shaft 120 and arm 122 to stress-sensitive resonator 220. Flexible torque tube 124 is sealed at shaft end 128 and at a barrier 221 dividing enclosure 260 into two sections at end 126, thus isolating resonator 220 from fluids entering port 212. Resonator 220 can operate in a vacuum or inert atmosphere and still measure the loads applied by Bourdon element 214 which are induced by pressure differentials in ports 202 and 212.