Patent Publication Number: US-2012046898-A1

Title: Systems and methods for pressure measurement using optical sensors

Description:
BACKGROUND 
     In the late 1950s and 1960s, with the advent of the aerospace era and advanced weapons development, came the requirement for high-frequency pressure sensors to make shock wave, blast, rocket combustion instability, and ballistic measurements. Products developed for this area consist of piezo-electric gauges made of materials such as quartz, tourmaline or polarized ferroelectric ceramics whose electrical resistance changes when the material is subjected to a force. 
     Current dynamic pressure gauges are prone to inaccuracies due to interference from RF waves and external electric or magnetic fields. Such methods are also costly and rely on electro-dynamic (piezoelectric or peizoresistive) material response, which limits detection to one plane. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the present invention may be better understood, and their numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items. 
         FIG. 1  shows a diagram of an embodiment of an optical pressure sensor system. 
         FIG. 2  shows a diagram of an embodiment of optical sensor referred to as a Photonic Doppler Velocimeter (PDV) that can be used in the sensor system of  FIG. 1 . 
         FIG. 3  shows a diagram of an embodiment of a housing and an object used as components in the sensor system of  FIG. 1 . 
         FIG. 4  shows a flow diagram of a method for determining the pressure applied to an object using an optical sensor. 
         FIG. 5A  shows a side view of an embodiment of a test system that can be used to generate calibration data for optical sensor system of  FIG. 1 . 
         FIG. 5B  shows a front view of an embodiment of a gauge mount that can be used in the test system of  FIG. 5A . 
         FIG. 6  shows an embodiment of a computer system that can be used in the optical pressure sensor system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of pressure gauges disclosed herein use optical technology such as Photonic Doppler Velocimetry (PDV) to offer improvements in temporal and spatial resolution over existing electrodynamic pressure gauges. PDV is a particularly attractive diagnostic for experiments involving significant quantities of radiated electromagnetic energy or high-explosives because the PDV components exposed in the experimental environment are immune to electromagnetic interference. Additionally, PDV requires no direct mechanical contact with the measurement surface, and does not require electrical connections on or near the measured surface. Optical pressure gauges are significantly less expensive than existing electrodynamic pressure gauges and can therefore be considered expendable in certain experiments. Possible applications include, among others, measurements of ground shock and air shock from explosions, evaluating shock waves for deflagration/detonation assessment, and understanding and controlling the combustion process in gas turbine engines to achieve increased efficiency, reduced emissions, and lower operating costs. 
       FIG. 1  shows a diagram of an embodiment of an optical pressure sensor system  100  including processor  102 , data analysis module  104 , calibration data  106 , data recorder  108 , optical sensor  110 , housing  112 , and object  114  movable within housing  112 . Object  114  typically includes a retro-reflective surface  116  that faces optical sensor  110  and reflects the optical signals back toward the optical sensor. 
     Housing  112  includes an opening to expose a surface of object  114  to the ambient environment at one end  118 . An opposite end  120  of housing  112  where optical sensor  110  can be positioned can be open, partially sealed, or completely sealed. 
     Object  114  is configured to move in an inner portion of housing  112  when a pressure force is applied to object  114 . Object  114  can be initially positioned in housing  112  at or near the opening at end  118  and moves toward optical sensor  110  at the opposite end  120  of housing  112  when a pressure force in the ambient environment is applied to the exposed surface of object  114 . Housing  112  and object  114  can be dimensioned to allow object  114  to move without reaching end  120  of the housing before the pressure force acting on object  114  has stopped. 
     In some embodiments, housing  112  is a hollow cylinder and object  114  is a piston positioned in cylindrical housing.  112 . One end of housing  112  is at least partially open to the pressure force and the piston is configured to move in housing  112  when the pressure force is applied to the piston. In further embodiments, housing  112  is configured to provide an air cushion around object  114  to reduce friction between the housing and the object. Other mechanisms for reducing friction between housing  112  and object  114  can be used. Reducing or eliminating friction between the surfaces of housing  112  and object  114  improves consistency in performance of different pressure sensors  110  during measurement. Additionally, consistent performance of sensors  110  allows the same calibration data  106  to be used for pressure sensors  110  that are configured with similar housings  112 , objects  114 , and optical sensors  110 . 
     In some embodiments, housing  112  is cylindrical, and object  114  is a cylindrical piston positioned in the cylindrical housing. Other suitable shapes for housing  112  and object  114  can used. For example, object  114  can be an elastic membrane positioned over an opening of housing  112  that deflects inwardly in the housing when the membrane is subject to a pressure force. Optical sensor  110  can be configured to measure the velocity of deflection instead of translational movement of object  114 . The deflection of the membrane and return to initial position can be taken into account in determining the pressure force. 
     Although gravity may cause object  114  to rest on the inner surface of housing  112 , the resulting friction will be insignificant compared to the pressure accelerating forces in transient, high-pressure applications. In low-pressure applications, low friction, low mass piston materials can be used to minimize the fiction between housing  112  and object  114 . 
     In some embodiments, sliding friction will be eliminated completely by using flexible membrane material rather than a sliding piston/cylinder. In such embodiments, the position and velocity of the flexing material will be carefully calibrated to pressure. 
     Optical sensor  110  is positioned to emit an optical signal on object  114 . In some configurations, optical sensor  110  is configured at one end of housing  112  to emit the optical signals toward object  114 . Components of optical sensor  110  can be configured in other suitable location(s) in system  100  to emit and detect optical signals to and from object  114  when object  114  is stationary and in motion. 
       FIG. 2  shows a diagram of an embodiment of optical sensor  110  referred to as a Photonic Doppler Velocimeter (PDV) that can be used in system  100 . (Note  FIG. 2  is based on a diagram of a PDV in  FIG. 1  in O.T. Strand et al. Compact system for high-speed velocimetry using heterodyne techniques, Review of Scientific Instruments 77, 083108 (2006)). Optical sensor  110  includes laser  202  that emits optical signals f 0 ) through a fiber optic material to collimator/probe  204 . Probe  204  emits laser light signal f 0  on a reflective surface  116  of object  114 . When object  114  is moving, the reflective light is Doppler-shifted, as indicated by symbol f d . Probe  204  collects Doppler-shifted light signals f d  reflected from object  114  and sends the light signals f d  to detector  206 . Detector  206  also received optical signals f 0  from laser  202  via a fiber optic material. 
     PDV is a Doppler-heterodyne procedure that measures the beat frequency between an unshifted, near-infrared reference light wave that propagates at wavelength λ 0  (or frequency f 0 =c/λ 0 , where c is the speed of light) and the Doppler-shifted light reflected off a moving surface. Mixing the unshifted reference laser signal at frequency f 0  with the Doppler-shifted reflected signal at instantaneous frequency f 1  produces a beat frequency 
         f ( t )=| f   0   −f   1 |=2 v ( t )/λ 0 ,
 
     where v(t) is the time-varying speed of object  114 . A detector converts the optical signal to an electrical signal (voltage) that is proportional to the instantaneous beat frequency. The detected signal power is proportional to the time-averaged output intensity 
         I ( t )≅ I   0   +I   1 +2·√{square root over ( I   0   ·I   1 )} cos( 2 π· f ( t ))·t+φ,
 
     where I 0  and I 1  are the transmitted and received laser signal intensities, respectively, and φ is a phase constant. Short-time Fourier transforms can be used to calculate the spectral content of the instantaneous frequency and instantaneous velocity, since both are effectively constant over the small time interval needed to measure velocities. 
     In some embodiments, a four-channel PDV system designed by David Holtkamp et al. of Los Alamos National Laboratory (LANL) of Los Alamos, N. Mex. and constructed by National Security Technologies of Las Vegas, Nev. can be used as optical sensor  110 . The PDV system can be excited with an IPG Photonics ELR-Series, narrow-band (&lt;30 kHz), single-mode laser (λ=1549.44 nm) at a power level of 1.6 W (0.4 W/PDV-channel). The high-resolution PDV signals are digitally recorded at constant digital sample rate, for example, of f s =1/Δt=6.25 GHz. Other suitable sample rates can be used. 
     PDVs were developed as an alternative velocimetry diagnostic technique to the velocity interferometer system for any reflector (VISAR) and Fabry-Pérot [4] interferometers for short-range, high-velocity shock experiments. The PDV uses a heterodyne method that has many of the advantages of the VISAR and other optical systems while avoiding many of their disadvantages. The PDV is compact, relatively inexpensive, and can be assembled fairly easily from commercially available parts. (See, for example, O.T. Strand, et. al, Compact System For High-Speed Velocimetry Using Heterodyne Techniques, Review Of Scientific Instruments 77, 083208 (2006)). The derived velocity time history is directly related to the frequency of the beat wave form, so there is no need for extra components in the system to resolve velocity ambiguities. The data are recorded on digital data recorder  108 , which can provide recording lengths sufficient to capture the amount of data required to determine the pressure profile. Data analyses with Fourier transform techniques allow the heterodyne method to observe multiple discrete velocities and even velocity dispersion. The PDV is robust against high-intensity fluctuations of light reflected from object  114  moving at high-speed. The PDV system does not suffer from data ambiguity due to short-time signal loss, since the velocity information is encoded in the frequency of the recorded signal. This is in contrast with a VISAR, where continuous measurement of the phase is required for a true velocity record. 
     Referring again to  FIG. 1 , data recorder  108  typically receives data signals from optical sensor  110  and provides digitized data signals to computer processor  102 . An example of a data recorder  108  that can be used in system  100  is a digitizer/oscilloscope, model number (TDS6804B) commercially available from Tektronix Corporation of Beaverton, Oreg. (USA). Other suitable data recording devices can be used, however. 
     Computer processor  102  can include components and execute logic instructions including data analysis module  104  to receive data from recorder  108  based on optical signals from optical sensor  110 . The optical signals include signals that are reflected off object  114  and the data represents velocity of object  114  after object  114  is exposed to a pressure force. Analysis module  104  can further determine the velocity of object  114  from the data  114  and determine the pressure force applied to object  114  based on the velocity of object  114 . 
     As an example of functions performed by analysis module  104 , the frequency and velocity spectral content of the signal can be obtained by short-time Fourier transforms of the digitized beat signal. The signal frequency is directly proportional to the projectile velocity 
     
       
         
           
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     The signal frequency can be treated as a constant in the small time-subinterval nΔt, over which each of a series of fast Fourier transforms (FFTs) are calculated. A user-selectable integer n determines the frequency (or velocity) interval size: Δf=1/(nΔt). The sample rate f s  is typically several times the highest frequency component (at least twice to avoid aliasing), which is proportional to the highest projectile velocity during a launch. 
     Temporal and velocity resolution can be adjusted during data processing after the measurements are taken. As n increases, so does the frequency resolution—with smaller and more (=n/2) frequent (velocity) components obtained. The direct tradeoff is decreased time resolution, where an increased time subinterval nΔt corresponds to fewer and sparser time measurements. A series of 50% overlapping, Hamming windowed, short-time Fourier transforms x(v)=ℑ(I(v(t)) of the digitized PDV signal records can be used to calculate the spectral content of the instantaneous velocity v. The spectral content can be calculated and displayed in decibels as a two-dimensional spectrogram 
         S ( v   i   ,T   k )=10 log 10 [|x ( v   i   ,T   k | 2 ], 
     where x(v i , T k ) is the fast Fourier transform of the k th  subrecord of the beat signal intensity I(Δf i , T k ) centered about time T k  and velocity v i . 
     
       
         
           
             
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     Once a velocity profile of object  114  is calculated over the time period when the pressure force is applied, analysis module  104  can access and use calibration data  102  that maps different velocities to corresponding pressure forces to determine the pressure force applied to the object  114 . Calibration data  106  can be derived from pressure measurements taken with electrodynamic pressure sensors or other suitable pressure sensors in the vicinity of housing  112 . With regard to housing  112 , when end  120  of housing  112  is closed, backpressure can build up between object  114  and end  120  as object  114  moves toward end  120 . The backpressure force typically prevents object  114  from moving as far or as quickly through housing  112  as object  114  would move if end  120  were open or at least partially open to relieve the backpressure. When end  120  is closed, an isentropic correction can be applied to account for the backpressure in determining the pressure force acting on object  114  at open end  118 . 
     Referring to  FIG. 3 , a diagram of an embodiment of housing  112  and object  114  used as components in the sensor system  100  of  FIG. 1  are shown. Housing  112  can have a sealed, an unsealed, or partially sealed end  120 , and the pressure profile may be determined using the derivative of the velocity over time 
         P ( t )=ρ L ( dv/dt )  Equation (1)
 
     where ρ is the mass density of object  114 , L is the axial length of object  114 , and dv/dt is the piston acceleration over time based on the velocity profile determined from the optical data from optical sensor  110 . 
     The pressure profile when end  120  of housing  112  is sealed can be determined using the Equation (1) above with an isentropic correction to account for the backpressure: 
         Pi=ρL ( dv/dt )/(1−( x   0   /x ( t )) γ )  Equation (2)
 
     where γ is the specific heat ratio of air and is equal to 1.4 at standard day conditions, x 0  is the initial position of object  114  in housing  112 , and x is the position of object  114  in housing  112  after the pressure force has acted on object  114 . 
     Optical pressure gauge system  100  can use a variety of different optical sensors  110  in addition to or instead of a PDV as long as the optical sensor  110  is capable of providing Doppler-shifted frequency data at intervals that are sufficient for determining the velocity and acceleration of object  114  over the time period that pressure is applied to object  114 . 
     In another embodiment, sensor system  100  includes housing  112 , a first opening in one end  118  of housing  112  and object  114  positioned in an inner portion of housing  112 . The opening in end  118  allows object  114  to be exposed to a pressure force. Object  114  is configured to move in the inner portion of housing  112  when the pressure force is applied to object  114 . Optical sensor  110  is positioned to emit optical signals on object  114  and to detect reflected optical signals from object  114 . The velocity of object  114  and the pressure force exerted on object  114  are determined from frequency changes between the optical signals and the reflected optical signals. The frequency changes in the optical signals are proportional to changes in corresponding surface velocities of object  114 . The optical frequency measurements are insensitive to radiofrequency waves, as well as external electric or magnetic fields. 
     Referring to  FIG. 4 , a flow diagram of a method  400  for determining the pressure applied to an object using an optical sensor is shown. Method  400  may be implemented a logic instructions executable by a computer processor. Process  402  receives data based on the optical signals and the reflected optical signals. The data may be provided by a digitizer that converts analog optical signals to digital electrical signals. The data represents the velocity of the object after being exposed to a first pressure force. 
     Process  404  can include accessing calibration data to determine the velocity of the object from the data received in process  402 . The calibration data can be implemented in data tables, as an equation, or other suitable format for deriving velocity from the data from the optical sensor. The calibration data will typically be the same for systems using the same dimensions and types of physical components. Note that systems with different dimensions and types of physical components will require a set of calibration data that was generated for the particular configuration. 
     Process  406  includes determining the first pressure force applied to the object based on the velocity of the object, as further described in the discussion of  FIG. 3  herein. 
     Once a velocity profile of object  114  is calculated over the time period when the pressure force is applied, analysis module  104  can access and use calibration data  106  that maps different velocities to corresponding pressure forces to determine the pressure force applied to the object  114 . Calibration data  106  can be derived from pressure measurements taken with electrodynamic pressure sensors or other suitable pressure sensors in the vicinity of housing  112 . For example,  FIG. 5A  shows a side view of an embodiment of a test system  500  that can be used to generate calibration data for optical sensor system  100 . In the embodiment shown, test system  500  includes mounting structure  502  for gauge mount  504 , gun barrel  506 , and gas gun  508 . Gas gun  508  generates pressure pulses by firing a burst of air (without a projectile) down gun barrel  506  toward gauge mount  504 . 
       FIG. 5B  shows a front view of an embodiment of gauge mount  504  including three piezoelectric sensors  512   a ,  512   b ,  512   c  (collectively, “ 512 ”) and two PDV sensors  514   a ,  514   b  (collectively, “ 514 ”). Piezoelectric sensors  512  can be positioned at varying distances from the center of gauge mount  504 . In the embodiment shown, pressure forces from gas gun  508  are measured by calibrated piezoelectric sensors  512  at offset radii of 2.125 inches, 0.0 inches, and 1.5 inches from center of gauge mount  504 . The pressure force is also measured by uncalibrated piezoelectric sensors  512  at offset radii of 0.65 inches from center of gauge mount  504 . The pressure signals from the calibrated piezoelectric sensors  512  are compared to pressure signals derived from PDV sensors  514  to generate calibration data  106  for PDV sensors  514 . 
     Note that although gauge mount  504  is shown with three piezoelectric sensors  512  and two PDV sensors  514  in a specific configuration, additional or fewer numbers of sensors can be used in different configurations. Further, calibration data  106  generated for specific types and arrangements of optical sensor systems  100  can be used for all systems  100  having the same characteristics. 
     Referring to  FIGS. 1 ,  4 , and  6 ,  FIG. 6  illustrates a block diagram of a computer system  600 , according to some embodiments that can be used to implement processor  102 , data analysis module  104 , and method  400 . The computer system  600  includes a processor  610  coupled to a memory  620 . The memory  620  can be operable to store program instructions  630  such as analysis module  104  that are executable by the processor  610  to perform one or more functions. It should be understood that the term “computer system” can be intended to encompass any device having a processor that can be capable of executing program instructions from a memory medium. In a particular embodiment, the various functions, processes, methods, and operations described herein may be implemented using the computer system  600 . For example, controller  102  or any components thereof, may be implemented using the computer system  600 . 
     The various functions, processes, methods, and operations performed or executed by the system  600  can be implemented as the program instructions  630  (also referred to as software or computer programs) that are executable by the processor  610  and various types of computer processors, controllers, central processing units, microprocessors, digital signal processors, state machines, programmable logic arrays, and the like. In an exemplary, non-depicted embodiment, the computer system  600  may be networked (using wired or wireless networks) with other computer systems. 
     In various embodiments the program instructions  630  may be implemented in various ways, including procedure-based techniques, component-based techniques, object-oriented techniques, rule-based techniques, among others. The program instructions  630  can be stored on the memory  620  or any computer-readable medium for use by or in connection with any computer-related system or method. A computer-readable medium can be an electronic, magnetic, optical, or other physical device or means that can contain or store a computer program for use by or in connection with a computer-related system, method, process, or procedure. Programs can be embodied in a computer-readable medium for use by or in connection with an instruction execution system, device, component, element, or apparatus, such as a system based on a computer or processor, or other system that can fetch instructions from an instruction memory or storage of any appropriate type. A computer-readable medium can be any structure, device, component, product, or other means that can store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. 
     The illustrative block diagrams and flow charts depict process steps or blocks that may represent modules, segments, or portions of code that include one or more executable instructions for implementing specific logical functions or steps in the process. Although the particular examples illustrate specific process steps or acts, many alternative implementations are possible and commonly made by simple design choice. Acts and steps may be executed in different order from the specific description herein, based on considerations of function, purpose, conformance to standard, legacy structure, and the like. 
     While the present disclosure describes various embodiments, these embodiments are to be understood as illustrative and do not limit the claim scope. Many variations, modifications, additions and improvements of the described embodiments are possible. For example, those having ordinary skill in the art will readily implement the processes necessary to provide the structures and methods disclosed herein. Variations and modifications of the embodiments disclosed herein may also be made while remaining within the scope of the following claims. The functionality and combinations of functionality of the individual modules can be any appropriate functionality. Additionally, limitations set forth in publications incorporated by reference herein are not intended to limit the scope of the claims. In the claims, unless otherwise indicated the article “a” is to refer to “one or more than one”.