Abstract:
A probe defining a transmission line is used with a measurement instrument including a pulse circuit connected to the probe for generating pulses on the transmission line and receiving reflected pulses on the transmission line. The probe comprises an adapter for mounting to a process vessel. A conductive outer sleeve is secured to the adapter. A center conductor is coaxial with the outer sleeve for conducting the pulses. The conductive outer sleeve has a connector end proximate the adapter and a distal end to extend into a processed liquid. The outer sleeve has a geometric discontinuity defining a referenced target producing an impedance change at a position between the adapter and the distal end at a location in a process vapor region above a processed liquid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       MICROFICHE/COPYRIGHT REFERENCE 
       [0003]    Not Applicable. 
       FIELD OF THE INVENTION 
       [0004]    This invention relates to process control instruments, and more particularly, to a guided wave radar probe reference target. 
       BACKGROUND 
       [0005]    Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level. 
         [0006]    Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and low cost time domain reflectometry (TDR) instruments. 
         [0007]    In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a probe, that serves as a transmission line, in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments. 
         [0008]    In one form, a guided wave radar (GWR) transmitter uses a coaxial probe that functions as an electrical transmission line into the process vessel. The GWR measurement process begins with an electrical pulse that is launched along the probe from one end. A typical coaxial probe  10  is illustrated in  FIG. 3 . A TDR circuit identifies impedance discontinuities along the length of the probe, as shown in the impedance chart of  FIG. 3 . One source of an impedance discontinuity occurs at the vapor to liquid interface due to the difference in the relative dielectrics of the media. The TDR circuit detects, and locates in time, the reflected signal from the interface. Another source of an impedance discontinuity can be a change in geometry in the transmission line. This is a convenient method for producing a known reference location, called a fiducial (FID) in the probe. The difference in the TDR time measurements of the fiducial to the vapor to liquid interface is used to calculate the liquid level. Another impedance discontinuity exists at the end of the probe (EOP). With this type of probe and TDR circuit an increased impedance creates a positive reflected signal and a decrease in impedance creates a negative reflected signal, as shown in the echo curve of  FIG. 3 . As is apparent, the probe, impedance chart and echo curve in  FIG. 3  are aligned to illustrate physically along the probe where the impedance changes occur and the resultant echo curve caused by these impedance changes. 
         [0009]    The velocity of the signal propagation is a function of the relative dielectric of the medium. A problem occurs when the relative dielectric of the vapor varies due to changes in temperature, pressure or vapor composition. A known solution to this problem is to create an impedance discontinuity at a known location in the process vapor region, called a reference target. The reference target is used to measure the actual propagation velocity in the vapor. The measured propagation velocity is used for a more accurate level measurement. This technique is illustrated in  FIG. 4  which shows a probe  12  including a target sleeve  14  on the probe center conductor  16 . There are impedance changes which occur at each end of the target sleeve  14 , as illustrated. This produces a negative reflection at the leading edge and a positive reflection at the trailing edge of the reference target. This results in transmission losses, which results in a smaller level signal. Also, the negative reflection can be confused as a level signal. 
         [0010]    The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner. 
       SUMMARY 
       [0011]    As described herein, a guided wave radar probe employs an outer reference target to compensate for changes in signal transmission propagation speed due to varying media dielectric. 
         [0012]    There is disclosed in accordance with one aspect of the invention a probe defining a transmission line for use with a measurement instrument including a pulse circuit connected to the probe for generating pulses on the transmission line and receiving reflected pulses on the transmission line. The probe comprises an adapter for mounting to a process vessel. A conductive outer sleeve is secured to the adapter. A center conductor is coaxial with the outer sleeve for conducting the pulses. The conductive outer sleeve has a connector end proximate the adapter and a distal end to extend into a processed liquid. The outer sleeve has a geometric discontinuity defining a referenced target producing an impedance change at a position between the adapter and the distal end at a location in a process vapor region above a processed liquid. 
         [0013]    There is disclosed in accordance with another aspect a probe comprising a connector for electrical connection to a pulse circuit. A conductive outer sleeve is secured at a connector end to the connector and has a distal end to extend into a process liquid. A center conductor is coaxial with the outer sleeve for conducting the pulses. The outer sleeve has a geometric discontinuity defining a reference target producing an impedance change at a position between the connector and the distal end at a location in a process vapor region above a process liquid. 
         [0014]    There is disclosed in accordance with a further aspect a probe comprising a connector for electrical connection to the pulse circuit. A conductive outer sleeve is secured at a connector end to the connector and has a distal end to extend into the process liquid. A center conductor is coaxial with the outer sleeve for conducting the pulses. The probe has a reference target producing an impedance change at a position between the connector end and the distal end at a location in a process vapor region above a process liquid, wherein any reflected pulse at the reference target location has an opposite polarity to a process liquid reflected pulse at the process liquid level. 
         [0015]    It is a feature that the outer sleeve has a uniform inner diameter from the adapter to the reference target. The outer sleeve inner diameter increases at the reference target. The outer sleeve has a uniform inner diameter from the reference target to the distal end. 
         [0016]    It is another feature that the outer sleeve has a stepped up inner diameter at the reference target. 
         [0017]    It is a further feature that the probe produces a single reflected pulse at the reference target location. 
         [0018]    It is yet another feature that the probe produces a reference reflected pulse at the reference target location of an opposite polarity to a process liquid reflected pulse at the process liquid level. 
         [0019]    It is another feature that any reflected pulse at the reference target location has a like polarity to an end of the probe reflected pulse produced at a distal end of the center conductor. 
         [0020]    Other features and advantages will be apparent from a review of the entire specification, including the appended claims and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is an elevation view of a guided wave radar instrument including a probe with a reference target; 
           [0022]      FIG. 2  is a block diagram of the transmitter of  FIG. 1 ; 
           [0023]      FIG. 3  is schematic representation of the operation of a prior art GWR probe without a reference target; 
           [0024]      FIG. 4  is a schematic representation of the operation of a prior art GWR probe with a reference target; 
           [0025]      FIG. 5  is a sectional view of a guided wave radar probe with reference target in accordance with the invention; 
           [0026]      FIG. 6  is a detailed view taken from  FIG. 5 ; and 
           [0027]      FIG. 7  is a schematic representation of the operation of the probe of  FIG. 5   
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Referring to  FIG. 1 , a process instrument  20  is illustrated. The process instrument  20  uses pulsed radar in conjunction with equivalent time sampling (ETS) and ultra-wide band (UWB) transceivers for measuring level using time domain reflectometry (TDR). Particularly, the instrument  20  uses guided wave radar for sensing level. While the embodiment described herein relates to a guided wave radar level sensing apparatus, various aspects of the invention may be used with other types of process instruments for measuring various process parameters. 
         [0029]    The process instrument  20  includes a control housing  22 , a probe  24 , and a connector  26  for connecting the probe  24  to the housing  22 . The probe  24  is mounted to a process vessel (not shown) using a flange  28 . The housing  22  is then secured to the probe  24  as by threading the connector  26  to the probe  24  and also to the housing  22 . The probe  24  comprises a high frequency transmission line which, when placed in a fluid, can be used to measure level of the fluid. Particularly, the probe  24  is controlled by a controller  30 , described below, in the housing  22  for determining level in the vessel. 
         [0030]    As described more particularly below, the controller  30  generates and transmits pulses on the probe  24 . A reflected signal is developed off any impedance changes, such as the liquid surface of the material being measured. A small amount of energy may continue down the probe  24 . 
         [0031]    Guided wave radar combines TDR, ETS and low power circuitry. TDR uses pulses of electromagnetic (EM) energy to measure distanced or levels. When a pulse reaches a dielectric discontinuity then a part of the energy is reflected. The greater the dielectric difference, the greater the amplitude of the reflection. In the measurement instrument  20 , the probe  24  comprises a wave guide with a characteristic impedance in air. When part of the probe  24  is immersed in a material other than air, there is lower impedance due to the increase in the dielectric. When the EM pulse is sent down the probe it meets the dielectric discontinuity, a reflection is generated. 
         [0032]    ETS is used to measure the high speed, low power EM energy. The high speed EM energy (1000 foot/microsecond) is difficult to measure over short distances and at the resolution required in the process industry. ETS captures the EM signals in real time (nanoseconds) and reconstructs them in equivalent time (milliseconds), which is much easier to measure. ETS is accomplished by scanning the wave guide to collect thousands of samples. Approximately eight scans are taken per second. 
         [0033]    Referring to  FIG. 2 , the electronic circuitry mounted in the housing  22  of  FIG. 1  is illustrated in block diagram form as an exemplary controller  30  connected to the probe  24 . As will be apparent, the probe  24  could be used with other controller designs. The controller  30  includes a digital circuit  32  and an analog circuit  34 . The digital circuit  32  includes a microprocessor  36  connected to a suitable memory  38  (the combination forming a computer) and a display/push button interface  40 . The display/push button interface  40  is used for entering parameters with a keypad and displaying user and status information. The memory  38  comprises both non-volatile memory for storing programs and calibration parameters, as well as volatile memory used during level measurement. The microprocessor  36  is also connected to a digital to analog input/output circuit  42  which is in turn connected to a two-wire circuit  44  for connecting to a remote power source. Particularly, the two-wire circuit  44  utilizes loop control and power circuitry which is well known and commonly used in process instrumentation. The two-wire circuit  44  controls the current on the two-wire line in the range of 4-20 mA which represents level or other characteristics measured by the probe  24 . 
         [0034]    The microprocessor  36  is also connected to a signal processing circuit  46  of the analog circuit  34 . The signal processing circuit  46  is in turn connected via a probe interface circuit  48  to the probe  24 . The probe interface circuit  48  includes an ETS circuit which converts real time signals to equivalent time signals, as discussed above. The signal processing circuit  46  processes the ETS signals and provides a timed output to the microprocessor  36 , as described more particularly below. 
         [0035]    The general concept implemented by the ETS circuit is known. The probe interface circuit  48  generates hundreds of thousands of very fast pulses of 500 picoseconds or less rise time every second. The timing between pulses is tightly controlled. The reflected pulses are sampled at controlled intervals. The samples build a time multiplied “picture” of the reflected pulses. Since these pulses travel on the probe  24  at the speed of light, this picture represents approximately ten nanoseconds in real time for a five-foot probe. The probe interface circuit  48  converts the time to about seventy-one milliseconds. As is apparent, the exact time would depend on various factors, such as, for example, probe length. The largest signals have an amplitude on the order of twenty millivolts before amplification to the desired amplitude by common audio amplifiers. For a low power device, a threshold scheme is employed to give interrupts to the microprocessor  36  for select signals, namely, fiducial, target, level, and end of probe, as described below. The microprocessor  36  converts these timed interrupts into distance. With the probe length entered through the display/push button interface  40 , or some other interface, the microprocessor  36  can calculate the level by subtracting from the probe length the difference between the fiducial and level distances. Changes in measured location of the reference target can be used for velocity compensation, as necessary or desired. 
         [0036]    Referring to  FIG. 5 , the probe  24  comprises an electrical connector  50  for connection to the probe interface circuit  48 , a conductive outer sleeve  52  secured at a connector end  54  to the electrical connector  50  and having a distal end  56  and a center conductor  58  coaxial with the outer sleeve  52 . 
         [0037]    The electrical connector  50  includes an outer tube  60  housing a dielectric insert  62 . A conductive rod  64  passes through the insert  62 . An electrical terminal  66  is connected to a near end of the rod  64  and in turn to a 50 ohm coaxial cable  68  that connects to the probe interface circuit  48 , see  FIG. 2 . A secondary seal  70  surrounds the terminal  66  and is secured in place using an adapter  72 . The electrical terminal  66  is configured to produce a fiducial. 
         [0038]    The outer sleeve connector end  54  is configured to threadably receive the electrical conductor  50 , via the tube  60 . An adapter body  74  surrounds the distal end of the dielectric insert  62  and receives a primary process seal  76 . A pin  78  passes through the primary process seal  76  and connects the rod  64  to the center conductor  58 . 
         [0039]    The outer sleeve  52  proximate the connector end  54  is threadably received in an adapter in the form of the flange  28 . Other types of adapters could be used in place of the flange  28 . The flange  28  is secured to a process vessel (not shown) so that the probe  24  extends into the process vessel for sensing location of a liquid level interface L, as shown. 
         [0040]    In accordance with the invention, the outer sleeve  52  has a first uniform inner diameter at  80  between the electrical connector  50  and a reference target  82 . The inner diameter is stepped up at the reference target  82 . The outer sleeve  52  has a second uniform inner diameter at  84  between the reference target  82  and the distal end  56 . As such, the inner diameter increases at the reference target  82 . In the illustrated embodiment, the probe has a characteristic impedance of 65 ohms corresponding to the first uniform inner diameter at  80  and a characteristic impedance of 75 ohms at the second uniform inner diameter at  84 . Thus, the reference target  82  causes an increase in impedance which creates a positive reflected signal. 
         [0041]    With the reference target  82  being on the conductive outer sleeve  52 , as described, there is only one target reflection, which reduces transmission losses. Also, the reflection is only positive and cannot be confused as level. This is illustrated in  FIG. 7  with the impedance curve showing an increase in impedance at the fiducial, a further increase in impedance at the reference target  82 , a decrease in impedance at the liquid level and an increase impedance at the end of probe (EOP) corresponding to the outer sleeve distal end  56 . This produces an echo curve showing a positive polarity pulse  90  at the fiducial, a positive polarity target echo  92  at the reference target, a negative polarity pulse  94  at the liquid level and a positive polarity pulse  96  at the EOP. 
         [0042]    Thus, as described, an improved guided wave radar probe, which is used for industrial process level measurement, employs an outer reference target to compensate for changes in signal transmission propagation speed due to varying media dielectric. 
         [0043]    It will be appreciated by those skilled in the art that there are many possible modifications to be made to the specific forms of the features and components of the disclosed embodiments while keeping within the spirit of the concepts disclosed herein. Accordingly, no limitations to the specific forms of the embodiments disclosed herein should be read into the claims unless expressly recited in the claims. Although a few embodiments have been described in detail above, other modifications are possible. Other embodiments may be within the scope of the following claims.