Abstract:
There is disclosed a process instrument comprising a housing and an antenna secured to the housing. A process adaptor is associated with the antenna and housing for securing the instrument to a process vessel to define a process seal. A control in the housing generates or receives a high frequency signal. The control comprises an electromagnetic radiating element. A body supports the radiating element in the housing proximate the antenna for rotation at any angular orientation without affecting the process seal.

Description:
CROSS REFERENCE 
       [0001]    There are no related applications. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a process control instrument and more particularly, to a through air radar process control instrument, such as a level sensor or rangefinder. 
       BACKGROUND OF THE INVENTION 
       [0003]    Industrial processes often require measuring the level of liquid or other material in a tank. Many technologies are used for level measurement. With contact level measurement some part of the system, such as a probe, must contact the material being measured. With non-contact level measurement the level is measured without contacting the material to be measured. One example is non-contact ultrasound, which uses high-frequency sonic waves to detect level. Another example is use of high-frequency or microwave RF energy. Microwave measurement for level generally uses either pulsed or frequency modulated continuous wave (FMCW) signals to make product level measurements. This method is often referred to as through air radar. Through air radar has the advantage that it is non-contact and relatively insensitive to measurement errors from varying process pressure and temperature. Known radar process control instruments operate at frequency bands of approximately 6 Ghz or 24 Ghz. 
         [0004]    A through air radar measurement instrument must convert a high frequency electrical signal to an electromagnetic wave. An oscillator is used to generate the high frequency signal. An antenna, such as a waveguide or horn, is operatively associated with the oscillator. A microwave frequency (26 GHZ, for example) radiation beam is propagated downward from the antenna, and reflected off the surface of the material being measured to the antenna where the signal is received. The product level is calculated from the total time of propagation of the signal. 
         [0005]    A difficulty can be encountered when a metal object is located in or around the radiated electromagnetic field. A reflection from a metal object can cause a false target situation, in which the system evaluates the product to be at a level indicated by the reflected signal from the object and not from the actual product. Typical false target objects in tanks are mixers, nozzles, ladders and tank walls. It is well known that the electromagnetic field pattern radiated by a waveguide and/or antenna structure has a characteristic commonly referred to as “polarization”. This term refers to the alignment or orientation of the electric and magnetic field components of the radiated wave. A common polarization characteristic is called “linear polarization”, in which the radiated electric and magnetic fields are oriented at ninety degrees with respect to each other. Linear polarization is common in many RF systems as most simple antenna structures are known to radiate in this manner. Other forms of polarization exist but they are typically the result of more complex and expensive antenna/circuit structures. 
         [0006]    A characteristic of the linearly-polarized radar signals is that orientation of the beam, i.e., orientation of the electric and magnetic fields, will produce a different radar reflection if the object in the beam is not “fully illuminated” by the beam (such as from a large, flat surface). Typical false target objects in a tank are small and asymmetric in the beam (unlike the large, flat liquid surface) and, therefore, orientation of the beam can have a large effect on the susceptibility of the radar to “see” and, therefore be disturbed, by, these smaller unwanted objects. 
         [0007]    It is advantageous, therefore, for the radar transmitter to have a convenient form of varying, or rotating, the orientation (“polarization”) of the radar beam to minimize the effect of unwanted objects in the radar beam in the actual installation. 
         [0008]    An alternative is disclosed in Janitch et al. U.S. Pat. No. 7,106,248, assigned to the assignee of the present application. The instrument described therein utilized a specialized coupling between the housing and antenna to allow independent rotation to achieve optimum orientation of the radar signal. This design requires numerous machined metal parts, O-rings and specially formed electrical components to facilitate independent rotation of the transmission signal relative to the electronics housing. 
         [0009]    The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner. 
       SUMMARY OF THE INVENTION 
       [0010]    In accordance with the invention there is provided an improved through air radar process control instrument. 
         [0011]    Broadly, in accordance with one aspect of the invention, there is disclosed a process instrument comprising a housing and an antenna secured to the housing. A process adaptor is associated with the antenna and housing for securing the instrument to a process vessel to define a process seal. A control in the housing generates or receives a high frequency signal. The control comprises an electromagnetic radiating element. A body supports the radiating element in the housing proximate the antenna for rotation at any angular orientation without effecting the process seal. 
         [0012]    It is a feature of the invention that the body comprises a substrate rotatably mounted in the housing so that the radiating element can be independently oriented relative to the process vessel. The substrate may comprise a circuit board. 
         [0013]    It is another feature of the invention to provide a feed bearing attached to the body operatively engaging a feed end of the antenna so that the body is free to rotate in the housing guided by the antenna. 
         [0014]    It is an additional feature of the invention to provide a clamping element biasing the body with the feed bearing engaging the antenna. 
         [0015]    It is yet another feature of the invention to provide a detent device in the housing operatively engaging the body to maintain the body in a select angular orientation. 
         [0016]    It is yet another feature of the invention that the radiating element comprises an oscillator circuit including microstrip resonators. 
         [0017]    It is an additional feature of the invention that the antenna comprises a horn antenna. 
         [0018]    There is disclosed in accordance with a further aspect of the invention a through air radar sensor comprising a housing and an antenna secured to the housing. A process adaptor is associated with the antenna and housing for securing the sensor to a process vessel to define a process seal. A mounting bracket is secured in the housing. A control circuit in the housing generates or receives a high frequency signal. The control circuit comprises a transceiver circuit board rotationally mounted to the antenna. The transceiver circuit board includes an electromagnetic radiating element. The transceiver board supports the radiating element in the housing proximate the antenna for rotation at any angular orientation without affecting the process seal. 
         [0019]    There is disclosed in accordance with a further aspect of the invention a process control instrument comprising a housing and an antenna secured to the housing. Means are associated with the antenna and the housing for securing the instrument to a process vessel to define a process seal. A control in the housing generates or receives a high frequency signal. The control comprises an electromagnetic radiating element. Means are provided for supporting the radiating element in the housing proximate the antenna for rotation at any angular orientation without affecting the process seal. 
         [0020]    Further features and advantages of the invention will be readily apparent from the specification and from the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a perspective view of a process control instrument in accordance with the invention; 
           [0022]      FIG. 2  is an elevation view of the process control instrument of  FIG. 1  mounted in a process vessel 
           [0023]      FIG. 3  is a sectional view of the process control instrument of  FIG. 1 ; 
           [0024]      FIG. 4  is a partial sectional, perspective view of the process control instrument of  FIG. 1 , illustrating a rotatable RF transceiver circuit board; 
           [0025]      FIG. 5  is an exploded, plan view of an antenna and the RF transceiver circuit board of the process control instrument of  FIG. 1 ; and 
           [0026]      FIG. 6  is a block diagram of a circuit on the RF transceiver circuit board. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Referring to  FIG. 1 , a process control instrument  10  according to the invention is illustrated. The process control instrument  10  uses micro power impulse radar (MIR) in conjunction with equivalent time sampling (ETS) and ultra-wideband (UWB) transceivers for measuring a level using time domain reflectometry (TDR). Particularly, the instrument  10  uses through air radar for sensing level. While the embodiments described herein relate to an MIR level sensing apparatus, various aspects of the invention may be used with other types of process control instruments for measuring various process parameters, such as a rangefinder, as will be apparent to those skilled in the art. 
         [0028]    The process control instrument  10  includes a control housing  12 , an antenna  14 , and a process adapter  16  for connecting the antenna  14  to the housing  12 . The process adapter  16  is typically mounted to a process vessel V, see also  FIG. 2 , using a threaded fitting  18 . Alternatively, a flange may be used. 
         [0029]    The instrument  10  uses pulse-burst radar technology with ETS circuitry. Short bursts of microwave energy are emitted and subsequently reflected from a surface. The distance is calculated by the equation 
         [0000]        D =(velocity of EM propagation)*transit time (round trip)/2. 
         [0000]    Level is then calculated by applying a tank height value. ETS is used to measure the high speed, low power electromagnetic (EM) energy. The high speed EM energy (1,000 ft/μs) is difficult to measure over short distances and at the resolutions required in the process control 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 vessel to collect thousands of samples. The round trip event on a 65 ft. tank takes only 133 nanoseconds in real time. After it is reconstructed in equivalent time it measures 200 milliseconds. 
         [0030]    Referring particularly to  FIG. 3 , the housing  12  comprises a base  20  and a cover  22  threaded to the base  20  to define an enclosed space  24 . The cover  22  has a top opening  26  closed by a glass plate  28 . A bottom of the base  20  includes a downwardly depending neck  30 . The neck  30  is receivable in the process adaptor  16 . The process adapter  16  is generally cylindrical and connects to an antenna housing  31  narrowing downwardly from the housing  12  to a conical closed end  32 . In the illustrated embodiment, the process adapter  16  and antenna housing  31  comprise a unitary structure. 
         [0031]    The antenna  14  comprises an antenna horn  33  embedded in potting compound  34  within the antenna housing  31  and process adapter  16 . The potting compound  34  also serves to permanently and rigidly attach the process adaptor  16  to the housing base  20 . The antenna horn  33  includes an upper feed end  36  that projects into the housing space  24 . 
         [0032]    A control  40  in the housing space  24  generates or receives a high frequency signal, as described below. The control  40  comprises a mounting bracket  42  fixedly secured to the base  20  in any known manner. First and second main circuit boards  44  and  46  are fixedly secured to the bracket  42  using fasteners  48  and  50 , respectively. A bezel  52  is secured in the housing. A rotating RF transceiver circuit assembly  54  is rotationally mounted relative to the fixed mounting bracket  42 , and thus antenna horn  33 , and is electrically connected to the first main circuit board  44  by a flexible cable  94 , see  FIG. 4 . 
         [0033]    Referring also to  FIGS. 4 and 5 , the rotating transceiver circuit assembly  54  comprises a plate  56  connected to a transceiver circuit board  58 . An antenna feed bearing  60  extends downwardly from the transceiver circuit board  58  and is sized to be telescopically received in the antenna horn feed end  36 , as shown in  FIG. 3 . The feed bearing  60 , and thus the transceiver circuit board  58 , are free to rotate about a vertical axis, guided by the antenna horn  33 . 
         [0034]    The transceiver circuit board  58  is generally circular and includes a notched outer edge  61 . The transceiver circuit board  58  extends through a slot  62  in a shield plate  64 . Thus, the notched edge  61  projects through the shield plate  62  into an area  65  of the housing space  24  accessible by a user of the device. The notched edge  61  creates a serrated surface that provides traction for a user&#39;s finger or tool to rotate the transceiver circuit board  58 . 
         [0035]    The notched edge  61  also provides a detent capability for holding the transceiver board  58  in a desired rotational orientation. Particularly, two detent spring clips  66  are mounted to the second main circuit board  46 . The spring arms of the clips  66  drop into the notches along the notched edge  61  to detent the transceiver circuit board  58 . A hold down spring clip  68  is also mounted to the second printed circuit board  46  to press downwardly on the transceiver circuit board  58  to bias it against the horn antenna feed end  36 , as shown in  FIG. 3 . 
         [0036]    The main circuit boards  44  and  46  include electrical circuitry for supplying power to the control  40 , and a control circuit to provide measurement functions, display control, configuration, general operation and the like for sensing level and interfacing with other peripherals and control equipment, as is well known to those skilled in the art. The particular circuitry does not form part of the present invention and is not described in detail herein. 
         [0037]    The transceiver circuit board  58  contains the necessary circuitry to produce a microwave signal, transmit the signal to a liquid or other surface, receive and process the radar return signal into a so-called “video” wave form from which the locations of the radar echoes can be determined. In the illustrated embodiment of the invention, the main circuitry generates and sends to the transceiver circuit board  58  control signals such as time variable gain, window, run/stop and end of ramp that control the radar scanning process. 
         [0038]    A block diagram of the circuit on the transceiver circuit board  58  is illustrated in block diagram form in  FIG. 6 . 
         [0039]    A main oscillator circuit  72  generates a square wave. The main oscillator  72  is a crystal controlled oscillator with a typical frequency in the 3-5 MHz range. In an illustrated embodiment of the invention, the frequency is 3.6864 MHz. A divide-by-two circuit  74  produces a pulse repetition frequency of 1.8432 MHz. This is done to reduce the power requirement in delay locked loop (DLL) logic gates. A DLL timing circuit  76  generates a precise, controlled timing delay between two logic transitions, referred to as transmit, or TX, pulse and receive, or RX, pulse. The transitions are on the order of 0 to approximately 100 nanoseconds, according to the value of a ramp signal input from a ramp generator  86 . The delay lock loop in conjunction with a sampling detector  78  performs the function of equivalent time sampling on the transmitted RF signal in accordance with previous, well known designs. 
         [0040]    A transmit pulse generator  80  and a receive pulse generator  82  are controlled by the DLL timing circuit  76  and use a single high-frequency switching transistor to generate a very fast, less than one nanosecond, pulse to excite a harmonic oscillator  84 . The TX pulse creates the transmit pulse out the antenna  14  towards the radar target, such as a liquid level surface. The RX pulse is delayed from the TX pulse by an amount determined by the ramp generator  86  and the DLL timing circuit  76 . The purpose of the RX pulse is to gate the sampling detector  78  and listen for TX pulses which have been reflected by a distant target, and have returned to the antenna  14  after a delay dependent on the target distance. 
         [0041]    The harmonic oscillator  84  represents a Colpitts oscillator comprised of a high frequency HJFET and tuned via microstrip resonators to oscillate at approximately 13 GHz while being rich in second harmonic (26 GHz) content. It is primarily the 26 GHz component that is transmitted. This allows the use of smaller antennas which achieve smaller radiated beam widths. 
         [0042]    The antenna  14  is not electrically connected to the circuit  58 . Instead, the antenna  14  uses an antenna horn  33  that is placed over the oscillator transistor and microstrips of the harmonic oscillator  84 , as generally depicted in  FIG. 3 , whereby the RF energy from the harmonic oscillator  84  is directly coupled into the antenna horn  33  after radiating directly from the circuit elements themselves. As is apparent, a wave guide could also be used with, or instead of, the antenna horn  33 . 
         [0043]    The sampling detector  78  comprises a microwave diode that is placed inside the antenna horn  33  proximate the harmonic oscillator  84 . TX pulses that are reflected by the liquid surface are received by the antenna  14  and conducted to the sampling diode  78  where they are mixed and detected with the delayed RX pulse to perform the equivalent time sampling function. A preamp  88  comprises a fixed gain stage to amplify signal from the sampling detector  78 . A variable gain bandpass amplifier  90  comprises a bandpass amplifier tuned to the frequency of the video or down-converted signal which is a result of the equivalent time sampling process. Also, the amplifier  90  provides a variable gain, as controlled by the signal “time variable gain”. In radar, since the strength of echoes decreases with increasing distance, a time variable gain circuit is used to increase the gain of the receiver with increasing distance from the transmitter to offset the effect of diminishing radar signal strength with distance. 
         [0044]    Thus, the circuitry on the transceiver circuit board  58  operates to generate the transmission signal and the signal is launched directly from the transceiver circuit board  58 . By rotating the transceiver circuit board  58 , the signal pattern from the harmonic oscillator  84  can be oriented to optimize false target rejection, such as the ladder L and the pipe P in  FIG. 2 . This allows the assembly of the housing  12  and process adaptor  16  with the antenna  14  to be installed in any rotational direction in the process vessel V simplifying the installation process. Also, the RF signal is generated on the transceiver circuit board  58  directly into the antenna horn  33  without the need of multiple intermediate components. Only a single antenna feed bearing  60  is required along with a sheet metal grounding ring and PTFE washer  92 , see  FIG. 5 . The bearing  60  allows the transceiver circuit board  58  to rotate relative to the housing  12 . The three spring clips  66  and  68  are used to hold the transceiver circuit board  58  at the desired rotational position and against the top of the antenna horn  33 . Thus, a minimal number of inexpensive parts are required to accomplish the desired rotational orientation. 
         [0045]    The ribbon cable  94  comprises a flexible conductor for connecting the transceiver circuit board  58  to the first main circuit board  42 . 
         [0046]    As described above, the antenna horn  33  is contained within the process adaptor  16  and antenna housing  31  and both are fixed relative to the housing  12  using the epoxy potting compound  34 . Use of the potting compound  34  allows for the use of a single process connection with both aluminum and steel housings as well as plastic housings. The potting compound  34  serves to locate and support the antenna horn  33  so that it could be made from relatively thin material, thus lowering costs. Since the process adaptor  16  completely surrounds the antenna horn  33 , the antenna horn  33  can be made from a material based on optimal signal propagation rather than on chemical compatibility. 
         [0047]    In the illustrated embodiment of the invention, an electromagnetic radiating element comprises the transistor and microstrip resonators of the harmonic oscillator  84 . The invention is not limited to such radiating element, but could alternatively use other elements, such as a loop launcher or the like. Moreover, the characteristics of the propagation signal described herein are by way of example only. The invention is not intended to be limited to any particular frequency or wavelength. Instead, the invention is particularly directed to a through air radar sensor having structure rotationally supporting a radiating element within the housing to achieve optimization of launcher position independent of position of the antenna and housing assembly. 
         [0048]    Thus, in accordance with the invention, there is provided an improved through air radar level process control instrument.