Abstract:
Systems and methods for improved ring laser gyroscope devices through mix ratio optimization are provided. In one embodiment, a ring laser gyroscope device comprises: a laser block assembly having a cavity therein that defines a ring shaped laser beam path around the laser block assembly, the cavity containing a fill gas mixture comprising Helium and Neon, wherein the laser block assembly is characterized as having a Neon depletion life limiter; and a readout assembly optically coupled to the laser block assembly. The readout assembly outputs a laser intensity monitor (LIM) voltage that represents optical energy within the cavity. The fill gas mixture has a Helium to Neon ratio richer in Neon than a ratio that would produce a peak LIM voltage from the readout assembly.

Description:
BACKGROUND 
       [0001]    Ring Laser Gyro (“RLG”) devices are a measurement tool used to calculate the angular rotation around a specified axis. An RLG measures the angular rotation around a specified axis by splitting a polarized laser beam in opposite directions within an enclosed cavity and measuring frequency difference of the two beams. RLG design utilizes a mixture of helium and neon gas within the enclosed cavity. Excitement of the gas mixture generates the light for forming the polarized laser beam. However, loss of neon pressure within the cavity over time, either through leaks or consumption of the neon by the RLG&#39;s cathode, causes a degradation of laser intensity as measured by the RLG&#39;s readout assembly. RLGs that ultimately fail due to the depletion of neon are said to have neon depletion as their life limiter. 
         [0002]    For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for improved systems and methods for extending the usable life of RLGs having neon depletion as their life limiter. 
       SUMMARY 
       [0003]    The Embodiments of the present invention provide methods and systems for improved ring laser gyroscope devices through mix ratio optimization and will be understood by reading and studying the following specification. 
         [0004]    In one embodiment, a ring laser gyroscope device comprises: a laser block assembly having a cavity therein that defines a ring shaped laser beam path around the laser block assembly, the cavity containing a fill gas mixture comprising Helium and Neon, wherein the laser block assembly is characterized as having a Neon depletion life limiter; and a readout assembly optically coupled to the laser block assembly. The readout assembly outputs a laser intensity monitor (LIM) voltage that represents optical energy within the cavity. The fill gas mixture has a Helium to Neon ratio richer in Neon than a ratio that would produce a peak LIM voltage from the readout assembly. 
     
    
     
       DRAWINGS 
         [0005]    Embodiments of the present invention can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which: 
           [0006]      FIG. 1  is a simplified block diagram of a laser block assembly for a ring laser gyroscope of one embodiment of the present invention; 
           [0007]      FIG. 2  is a graph illustrating LIM voltage verse fill gas mixture ratio for a LIM voltage test of one embodiment of the present invention; and 
           [0008]      FIG. 3  is a flow chart illustrating a method of one embodiment of the present invention. 
       
    
    
       [0009]    In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to the present invention. Reference characters denote like elements throughout figures and text. 
       DETAILED DESCRIPTION 
       [0010]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. 
         [0011]      FIG. 1  is a simplified block diagram of a laser block assembly  105  for a ring laser gyroscope (RLG)  100  of one embodiment of the present invention. Laser block assembly  105  is triangular in shape having three sides ( 106 - 1 ,  106 - 2  and  106 - 3 ) and three blunted corners  107 - 1 ,  107 - 2  and  107 - 3 . Laser block assembly  105  includes two cathodes  110  and  112  and an anode  114  each positioned on the respective sides  106 - 1 ,  106 - 2  and  106 - 3  of the laser block assembly  105 . In other embodiments, the laser block assembly may alternately include two anodes and a single cathode. Mirrors  120 ,  122  and  124  are located as shown at the blunted corners  107 - 1 ,  107 - 2  and  107 - 3  of laser block assembly  105 , located between each of the sides  106 - 1 ,  106 - 2  and  106 - 3 . Within laser block assembly is a cavity  130 , which in conjunction with the mirrors  120 ,  122  and  124  forms a ring shaped laser beam path around the laser block assembly. The performance of RLG  100  is observed by coupling optical energy information from the cavity  130  to a readout assembly  140 . The readout assembly, in one body that, includes a beam combining mechanism plus two photo diodes  142 . The readout assembly provides to voltage signals from which the difference frequency and hence the rotation information may be obtained. In addition to the rotation information, the readout assembly, in this embodiment, also provides a voltage signal correlated with a laser intensity called the Laser Intensity Monitor (LIM). This LIM signal, referred to herein as the “LIM voltage” provides information regarding the optical energy within the cavity  130  of RLG  100 . As would be appreciated by one of ordinary skill in the art upon reading this specification, there are a number of mechanisms for obtaining the LIM voltage. The particular mechanism used for obtaining the LIM voltage is not pertinent with respect to embodiments of the present invention, and any such mechanism may be used in conjunction with realizing readout assembly  140 . Cavity  130  is filled with a fill gas mixture  132  of Helium and Neon and laser block assembly  105  is characterized as having neon depletion as its life limiter. 
         [0012]    One of ordinary skill in the art after reading this specification would appreciate that  FIG. 1  illustrates a simplified block diagram that provides sufficient detail to facilitate an enabled written description of embodiments of the present invention. Additional details not shown regarding the physical structure and electronic circuitry associated with a laser block assembly for a ring laser gyroscope are considered within the knowledge and skill of one of ordinary skill in the art and are not discussed herein. In one embodiment  FIG. 1  represents a laser block assembly having the same physical structure of a Honeywell GG1308 sensor. In addition, one of ordinary skill in the art upon reading this specification can appreciate whether any particular laser block assembly they are using can be characterized as having neon depletion as its life limiter. 
         [0013]    Embodiments of the present invention differ from the prior art in that the fill gas mixture  132  of Helium and Neon within cavity  130  is not optimized to initially provide a peak LIM voltage at the onset of the RLG  100 &#39;s service life. Rather, cavity  130  is filled with a fill gas mixture  132  of Helium and Neon that will produce a sub-optimal LIM voltage on the onset of the RLG  100 &#39;s designed service life by altering the gas mixture ratio as described below with respect to  FIGS. 2 and 3 . The present inventors have found that the gas mixtures provided by embodiments of the present invention have the benefits of providing extended life and reduced thermal sensitivity when compared to designs using gas mixtures that maximize the initial LIM voltage from the onset. 
         [0014]    Embodiments of the present invention optimize fill gas mixture  132  to provide for a longer service life by giving up a certain level of LIM voltage margin at the early stages of RLG  100 &#39;s life. Currently existing life constraints are overcome by understanding the physics of having neon depletion as a life limiter and compensating for it. This process is described below with reference to the graph shown generally at  200  in  FIG. 2  and the flow chart illustrated in  FIG. 3 . 
         [0015]    The process begins at  310  in  FIG. 3  with determining a peak LIM voltage ratio for a given laser block assembly under test. As discussed above, the laser block assembly under test is known to have a neon depletion life limiter. In one embodiment, determining a peak LIM voltage ratio is accomplished through a LIM voltage test such as illustrated in  FIG. 2 . The purpose of the LIM voltage test is to test cavity fill gas mixtures of different Helium to Neon ratios, all under equivalent temperature conditions, to identify the Helium to Neon ratio that produces a peak LIM voltage. In  FIG. 2 , the total pressure, that is, the partial pressure of Helium plus the partial pressure of Neon, is held constant while the mix ratio of Helium partial pressure to Neon partial pressure is varied to produce the data for each of the different mixture ratios.  FIG. 2  provides a plot at  210  of the Neon pressure within the cavity for each tested mixture ratio.  FIG. 2  also provides a plot at  220  of the resulting LIM voltage for each tested mixture ratio. Values for these curves between each data point may be interpolated or extrapolated using best curve fit algorithms known in the art. The gas mixture ratio associated with the peak LIM voltage (shown generally at  230 ) is referred to herein as the “peak LIM voltage ratio” (shown generally as  231 ). For the example test illustrated in  FIG. 2 , the peak LIM voltage ratio occurs at a Helium to Neon ratio of 20:1. As would be appreciated by one of ordinary skill in the art upon reading this specification, the peak LIM voltage ratio for embodiments using other laser block assemblies may occur at ratios other than 20:1. 
         [0016]    As revealed by  FIG. 2 , selecting a fill gas mixture that has a Helium to Neon ratio richer in Neon than the peak LIM voltage ratio (such as shown at  235 ), will result in a relatively lower LIM voltage output from the readout assembly. As would be appreciated by one of ordinary skill in the art upon reading this specification, in collecting the data for  FIG. 2  testing is performed at a standard testing temperature. This curve of  FIG. 2  indicates that the sensitivity over the operating temperature range of the RLG at the gas mixture at  235  will be less than for the gas mixture at  230 . Testing performed by the inventors has shown that at the mix ratio at  235  (versus the mix ratio at  230 ), the LIM voltage over the operating temperature range of the RLG remains more stable. Further, deviations over time as the device ages are less over the operating temperature range. 
         [0017]    Prior to this disclosure, one of ordinary skill in the art would consider intentionally selecting a mix ratio that did not produce a peak LIM voltage at the onset of the devices life as sub-optimal and undesirable. However, as revealed by  FIG. 2 , by selecting such an initial fill gas mixture ratio, as neon depletion occurs and the pressure of neon in the cavity drops, the LIM voltage will actually improve over time until it reaches the peak LIM voltage at  230 . After reaching the peak LIM voltage at  230 , the LIM voltage will then begin to slowly decrease over time as a function of the continued neon depletion, as would have occurred if the peak LIM voltage ratio  231  were initially selected for the fill gas mixture. In other words, if the selected fill gas mixture has a Helium to Neon ratio of 15:1 (shown generally at  235 ), it will have a Helium to Neon ratio richer in Neon than the peak LIM voltage ratio of 20:1 (shown at  231 ). The resulting usable life of the laser block assembly will be increased at least by the amount of time it will take for neon depletion to raise the LIM voltage from its starting level shown at  235  to the peak LIM voltage level shown at  230 . For example for the Honeywell GG1308 sensor, the inventors have found that a fill gas mixture of 15:1 will result in approximately a 2-times life increase over a fill gas mixture of 20:1 for that device. Accordingly, the method proceeds in  FIGS. 3 to 320  with selecting a fill gas mixture that has a Helium to Neon ratio richer in Neon than the peak LIM voltage ratio. 
         [0018]    As would be appreciated by one of ordinary skill in the art after reading this specification, when a LIM voltage output from a readout assembly drops below a certain minimum LIM voltage threshold level, the LIM voltage output will be insufficient to drive the ring laser gyroscope circuitry receiving the output, resulting in RLG performance degradation and associated bit faults. Accordingly the mix ratio for the fill gas mixture selected at  320  should not (when the laser block assembly is expected to be in operation) result in a LIM voltage that will be below the minimum LIM voltage threshold level. In at least one alternative embodiment however, if it is known that some period of time will pass before the laser block assembly will need to be in an operational state, it may by initially manufactured with a Helium to Neon ratio sufficiently rich in Neon to result in an LIM voltage below the minimum LIM voltage threshold level. In such a case, it would be understood that sufficient neon depletion would need to occur prior to placing the laser block assembly into service to bring the LIM voltage within tolerance. 
         [0019]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.