Abstract:
A measuring system is disclosed that may include a sensing module that provides a response to a measured quantity, a first mechanical transducer that provides an output signal proportional to the measured quantity, a conversion device that converts the output signal to a second output signal, and a sensor that detects the second output signal and generates an instrument reading within an instrument range of output signals. A bias assembly may forces a second mechanical transducer to provide the second output signal that is detected by the sensor and which generates an instrument reading outside of the specified instrument range when the mechanical transducer is disconnected from the conversion device. A level detector is also disclosed that may include a displacer assembly, a torque rod coupled to the displacer assembly and connected to a magnet, and a non-contact sensor that detects displacement of the magnet assembly within an instrument range.

Description:
TECHNICAL FIELD 
       [0001]    The subject matter disclosed herein relates to mechanisms and methods for indicating a fault condition in an instrument and more particularly mechanisms and methods for indicating a fault condition in a measurement system that includes a mechanical linkage in a transmitting element. 
       BACKGROUND 
       [0002]    Efficient industrial processes require the measurement and control of various of parameters such as temperatures, pressures, levels and flow rate. Instruments are used to effect those measurements. A typical instrument has three components; a sensing element; a transmitting element and an output or indicating element. 
         [0003]    Sensing elements respond directly to the measured quantity, producing a response such as displacement or motion, pressure, or electrical signal. The response is transmitted by a transmitting element which may include linkages, tubing, wiring that provides an output signal. The transmitting elements may comprise one or more transducers. Among the types of transducers that are commonly used are mechanical transducers that convert one form of energy into other form that can be measured easily. For example, a linear force may be converted to torsion, and torsion may be converted to displacement. The output signal of the transmitting element is received by the output or indicating element that displays a representation of the output signal. For example, displays may include, among other devices, a dial with a needle indicator, or digital displays. 
         [0004]    A level transmitter or controller (e.g. Dresser Masoneilan 12400 Series Digital Level Transmitter/Controller) is an example of such an instrument. Level transmitters are used to measure the level of a liquid in a reservoir or vessel and may be incorporated in control systems that control industrial processes in a variety of industries. Level transmitters or controllers may incorporate two-wire field devices coupled to a control room using a two-wire process control loop. Two-wire devices receive power from a process control loop, and communicate over the process control loop. Some instruments use Highway Addressable Remote Transducer (HART®) protocol for sending and receiving digital information across analog wires between the instrument and control or monitoring systems. An exemplary level transmitter or controller may comprise a displacer (sensing element) that is immersed in the liquid. The displacer is coupled to a torque tube. A change in liquid level varies the net weight of the displacer, increasing or decreasing a torsion load on a torque tube and torque rod by an amount directly proportional to the change in liquid level. The torque rod is attached to a magnet assembly comprising a rotating beam with an attached magnet. The rotation of the torque rod results in the angular displacement of the magnet assembly. The displacement of the magnet modifies the magnetic field surrounding a non-contact sensor, producing a signal proportional to the level in the vessel. The signal may be provided to an output readout component that provides a measure of the level of the liquid. These instruments are rugged, reliable and accurate. [hart is example any communication works] 
         [0005]    In rare situations the mechanical linkage between the torque rod and the rotating beam may be disconnected. When that happens, it is possible for the beam and magnet to be in a position where the non-contact sensor provides an erroneous but plausible output. This possibility creates a problem when the instrument is used as a component of a safety instrumented systems (SIS) that is used to achieve or maintain a safe state of a process when unacceptable or dangerous process conditions are detected. The consequences of an erroneous but plausible process condition may be severe. The quality or dependability of an SIS is conveyed by safety integrity levels ratings (SILs). There are four discrete integrity levels associated with SILs. The higher the SIL level, the lower the probability of failure on demand for the safety system and the better the system performance. In some cases, an instrument having a mechanical connection may not be SIL 2 rated unless the instrument provides a way to detect a broken mechanical connection. For example, in the case of Dresser Masoneilan 12400 Series Digital Level Transmitter/Controller, the instrument may not be SIL 2 rated unless there is a way to detect when the magnet assembly is disconnected from the torque rod. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    According to one aspect of the invention, a measurement system is disclosed. The measuring system includes a sensing module that provides a response to a measured quantity. The measurement system also includes a first mechanical transducer that provides a first output signal proportional to the measured quantity and a second mechanical transducer connected to the first mechanical transducer that provides a second output signal associated with the measured quantity. A sensor that detects the second output signal and generates a reading within a specified instrument range of output signals is also included in the measurement system. The measurement system also includes a bias assembly that forces the second mechanical transducer to provide a second output signal that is detected by the sensor as a reading outside of the specified instrument range when first mechanical transducer is disconnected from the second mechanical transducer. The bias assembly provides a way to detect and indicate when the first mechanical transducer is disconnected from the second mechanical transducer. 
         [0007]    According to another aspect of the invention, a level detector is disclosed. The level detector includes a displacer assembly, a torque rod coupled to the displacer assembly; a magnet assembly connected to the torque rod; and a non-contact sensor that detects displacement of the magnet assembly within an instrument range. The level detector also includes a bias assembly that displaces the magnet assembly beyond the instrument range when the magnet assembly is disconnected from the torque rod. The bias assembly provides a way to detect and indicate when the connection between the magnet assembly and the torque rod are disconnected. 
         [0008]    In yet another aspect of the invention, the bias assembly in the level detector may include a weight, a compression spring or a torsion spring attached to the magnet assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0010]      FIG. 1  is a block diagram of an embodiment of an instrument with a fault indication mechanism according to one embodiment of the present invention. 
           [0011]      FIG. 2  is a perspective view of an instrument with a fault indication mechanism as implemented in an illustrative level transmitter or controller according to one embodiment of the present invention. 
           [0012]      FIG. 3  is a perspective view of an illustrative embodiment of a fault indication mechanism according to one embodiment of the present invention. 
           [0013]      FIG. 4  is a front view of an illustrative embodiment of a fault indication mechanism according to one embodiment of the present invention. 
           [0014]      FIG. 5  is a perspective view of an illustrative embodiment of a fault indication mechanism according to one embodiment of the present invention. 
           [0015]      FIG. 6  is a front view of an illustrative embodiment of a fault indication mechanism according to one embodiment of the present invention. 
           [0016]      FIG. 7  represents a perspective view of an illustrative embodiment of a fault indication mechanism according to one embodiment of the present invention. 
           [0017]      FIG. 8  is a front view of an illustrative embodiment of a fault indication mechanism according to one embodiment of the present invention. 
           [0018]      FIG. 9  represents a perspective view of an illustrative embodiment of a fault indication mechanism according to one embodiment of the present invention. 
           [0019]      FIG. 10  is a to view of an illustrative embodiment of a fault indication mechanism according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Reference now will be made in detail to embodiments of the invention, one of or more example of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it would be apparent to those skilled in the art that various modifications and variations can be made present invention without departing from the scope or spirit of the invention. For instance, features illustrated and described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention, covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
         [0021]      FIG. 1  is a block diagram of a measuring system  11  according to one embodiment of the invention. In the measuring system  11  a sensing element  13  is coupled to a transmitting element  15 . The transmitting element  15  is in turn coupled to an output indicating elements  17 . Additionally, the transmitting element  15  may provide a signal to a control system  18 , which controls a process. The transmitting element  15  includes a first transducer  19  which may be a mechanical transducer and a second transducer or conversion device  21  which may be a mechanical transducer. The first transducer  19  and the second transducer  21  are connected through a mechanical linkage  23 . Although in the preceding description only two transponders or conversion devices are described, in other embodiments there may be a plurality of transponders that are sequentially coupled. The transmitting element  15  also includes a biasing element or assembly  25  that forces the second transducer  21  to provide a predetermined output signal to the output indicating element  17  when the mechanical linkage  23  is severed or disconnected. The output or indicating element  17  may include a display  26 . The display  26  may be a digital display or an analog display with an indicator  27  that travels between a lower value  28  and an upper value  29 . The range between the lower value  28  and upper value  29  of a quantity that an instrument is designed to measure is the instrument range. The display  26  may also include indicia  30  indicating a value outside of the instrument range. In operation, the biasing element or assembly  25  forces the output or indicating element  17  to indicate a value outside of the instrument range (such as for example within the indicia  30  showing a value outside of the instrument range in a display  26 ) when the mechanical linkage  23  is severed or disconnected. For example, a level transmitter or controller may have a lower value of 0% and an upper value of 120% (the value may exceed a 100% under certain conditions) an indication of a fill level of a vessel. In this example the instrument range would be between 0 and 120%. A value outside the instrument range may be a value significantly over the maximum value, for example, 150%. 
         [0022]      FIG. 2  illustrates another embodiment implemented in a level transmitter or controller  31 . The level transmitter or controller  31  includes a displacer assembly  33 , a transmitter mechanism assembly  35  and an instrument assembly  37 . The displacer assembly  33  includes a displacer  39  with an extension rod  43  connected to a displacer hanger  45  that is in turn connected to a torque arm  47 . The displacer assembly  33  is an example of a sensing element  13  as illustrated in  FIG. 1 . The transmitter mechanism assembly  35  may include torque tube  49 , surrounded by a torque tube housing  51  and incorporating a torque rod  53 . The torque rod  53  is connected to a sensor assembly  55 . The sensor assembly  55  includes a biasing mechanism  57 , a noncontact sensor  59  (such as for example, a Hall effect sensor) and an output display  61 . The sensor assembly  55  may additionally provide an output signal to a control system. In operation, the displacer assembly  33  is disposed in contact with liquid in a reservoir or vessel. When the liquid level changes the relative weight (vertical force) external by the displacer  39  on the torque arm  47  will increase or decrease depending on the change. The torsion on the torque tube  49  will increase or decrease by an amount proportional to the change in the liquid to the change in liquid level. The torsion on the torque tube  49  causes a rotation of the torque rod  53 , which in turn causes the magnet assembly  63  to rotate. The displacement of the magnet assembly  63  modifies the magnetic field surrounding the noncontact sensor  59  producing an analog signal proportional to the level in the reservoir or vessel. If the torque rod  53  is disconnected from the magnet assembly  63  the biasing mechanism will force the magnet assembly to be displaced a predetermined amount, and will produce a signal that is outside of the instrument range. In this embodiment, a number of devices can be characterized as transponders or conversion devices. For example, the displacer  39  converts a change in buoyancy to a force that is applied to the torque arm  47 . The torque arm  47  converts the force applied by the displacer  39  to a moment or torque on the torque rod  53 . Magnet assembly  63  converts the torque applied by the torque rod  53  into a change in the magnetic field detected by the non-contact sensor  59 . 
         [0023]    The level transmitter or controller  31  may be coupled to a control system such as control system  18  in  FIG. 1 . The non-contact sensor provides an analog output signal proportional to the displacement of the magnet assembly or the level in the reservoir or vessel. The analog signal is converted into an error-free digital signal that can be processed by an on-board micro-controller. After the signal has been processed, the digital result is converted to a 4-20 mA analog output signal. A HART digital signal is superimposed to the 4-20 mA analog output signal. The instrument is powered through the 2-wire series loop over a process control loop to the process control system, to enable the monitoring or control of a process. The process control system may include a fault indicator that provides a fault signal when that magnet assembly  63  is displaced beyond the instrument range. Although in this example, communication Other technologies are available to communicate the analog output signal to the control system  18 , such as for example, radio frequency, fiber optic, and electric line telemetry, among others may be used to transmit the output signal to the control system  18 . 
         [0024]    Illustrated in  FIGS. 3 and 4  is an embodiment of a sensor assembly  55 . The sensor assembly  55  includes a magnet assembly  63  that is connected to the torque rod  53  by means of a coupling flange  65  and a coupling lamella or flexure  67 . The lamella or flexure  67  is connected to the magnet assembly  63  by a pin  69 . The magnet assembly  63  may include a beam  71  that is free to rotate about the axis of the torque rod  53 . The movement of the beam  71  is constrained by a U-Flexure assembly  73  that sits on pivot  75 . A magnet  77  is disposed on the beam  71 , and changes in the magnetic field due to rotation of the beam  71  are detected by the non-contact sensor  59 . A weight  79  is provided and attached to the beam  71 . If the connection between the torque rod  53  and the beam  71  is severed, such as for example if the torque rod is detached or disconnected from the coupling flange  65 , the weight  79  forces the magnet assembly  63  to rotate so that the reading of the non-contact sensor  59  falls outside of the instrument range, indicating a fault condition. 
         [0025]    Illustrated in  FIGS. 5 and 6  is a second embodiment of a sensor assembly  55 . The sensor assembly  55  is disposed in a housing with a wall  81 . A compression spring  83  is secured on the wall  81  and on the beam  71 . The compression spring is under compression when the sensor assembly is disposed in the housing so that if the connection between the torque rod  53  and the beam  71  is severed the compression spring  83  will exert a force on the beam  71 . The force will cause the magnet assembly  63  to rotate so that the reading of the non-contact sensor  59  falls outside of the instrument range, indicating a fault condition. 
         [0026]    Illustrated in  FIGS. 7 and 8  is a third embodiment of a sensor assembly  55 . A torsion spring  85  with a first end  87  and a second end  89  is disposed on the torque rod  53 . An anchor plug  91  is secured to the torque rod  53  to provide a connection point for the second end  89  of the torsion spring  85 . An anchor rod  93  is attached to the beam  71  to secure the first end of the torsion spring  85 . When assembled, the torsion spring  85  is under a torsion load. If the connection between the torque rod  53  and the beam  71  is severed the torsion spring  85  will exert a force on the beam  71 . The force will cause the magnet assembly  63  to rotate so that the reading of the non-contact sensor  59  falls outside of the instrument range, indicating a fault condition. 
         [0027]    Illustrated in  FIGS. 9 and 10  is a fourth embodiment of a sensor assembly  55 . An anchor block  95  is secured to the torque rod  53 . A spring support  97  is secured to the anchor block  95 , and a spring support  99  is secured to the beam  71 . A torsion spring  101  with a spring arm  103  is disposed on the spring support  97 . The spring arm  103  is placed below the spring support  99 . If the connection between the torque rod  53  and the beam  71  is severed the torsion spring  101  will exert a force on the beam  71 . The force will cause the magnet assembly  63  to rotate so that the reading of the non-contact sensor  59  falls outside of the instrument range, indicating a fault condition. 
         [0028]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.