Abstract:
A level transmitter for use in a process application measures height of a product in a tank. The level transmitter includes a microwave antenna directed into the tank. A low power microwave source sends a microwave signal through the microwave antenna. A low power microwave receiver receives a reflected microwave signal. Measurement circuitry coupled to the source and receiver initiates transmitting of the microwave signal and determines product height based upon the received, reflected signal. Output circuitry coupled to a two-wire process control loop transmits information related to product height over the loop. Power supply circuitry in the level transmitter coupled to the two-wire process control loop receives power from the loop which powers the level transmitter including the microwave source and the microwave receiver.

Description:
This is a Divisional patent application of U.S. Ser. No. 08/937,730, filed Sep. 25, 1997, now abandoned, which is a Continuation of U.S. Ser. No. 08/486,649, filled Jun. 7, 1995, now U.S. Pat. No. 5,672,975. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to level measurement in industrial processes. More specifically, the present invention relates to measurement of product level height in a storage tank of the type used in industrial applications using a microwave level gauge. 
     Instrumentation for the measurement of product level (either liquids or solids) in storage vessels is evolving from contact measurement techniques, such as tape and float, to non-contact techniques. One technology that holds considerable promise is based on the use of microwaves. The basic premise involves transmitting microwaves towards the product surface and receiving reflected microwave energy from the surface. The reflected microwaves are analyzed to determine the distance that they have traveled. Knowledge of the distance traveled and storage vessel height allows determination of product level. Since it is known that microwaves travel at the speed of light, the distance that a microwave travels can be determined if the time of travel is known. The time of travel can be determined by measuring the phase of the return wave and knowing the frequency of the microwave that was transmitted. Further, the time of travel can be measured using well-known digital sampling techniques. 
     One standard in the process control industry is the use of 4-20 mA process control loops. Under this standard, a 4 mA signal represents a zero reading and a 20 mA signal represents a full scale reading. Further, if a transmitter in the field has sufficiently low power requirements, it is possible to power the transmitter using current from the two-wire loop. However, microwave level transmitters in the process control industry have always required a separate power source. The level transmitters were large and their operation required more power than could be delivered using the 4-20 mA industry standard. Thus, typical prior art microwave level transmitters required additional wiring into the field to provide power to the unit. This additional wiring was not only expensive but also was a source of potential failure. 
     SUMMARY OF THE INVENTION 
     A level transmitter measures height of product in a tank such as those used in industrial process applications. The level transmitter is coupled to a two-wire process control loop which is used for both transmitting level information provided by the level transmitter and for providing power to the level transmitter. The level transmitter includes a microwave antenna directed into the tank. A low power microwave source sends a microwave signal through the antenna into the tank. A low power microwave receiver receives a reflected microwave signal. Measurement circuitry coupled to the low power microwave source and to the low power microwave receiver initiates transmitting of the microwave signal and determines product height based upon the reflected signal received by the receiver. Output circuitry coupled to the two-wire process control loop transmits information related to product height over the loop. Power supply circuitry coupled to the two-wire process control loop receives power from the loop to power the level transmitter. 
     In one embodiment, the measurement circuitry includes a first clock coupled to the source for periodically initiating the microwave signal at a first clock rate. A second clock coupled to the receiver periodically gates the received signal at a second clock rate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a microwave level transmitter in accordance with the invention. 
     FIG. 2 is a block diagram showing electrical circuitry of the level transmitter of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a diagram which shows microwave level transmitter  10  operably coupled to storage tank  12 . Storage tank  12  is the type typically used in process application and contains fluid (product)  14 . As used herein, product can be a liquid, a solid or a combination of both. Level transmitter  10  includes housing  16  and feedhorn  18 . Transmitter  10  is coupled to two-wire loop  20 . Two-wire loop  20  is a 4-20 mA process control loop. In accordance with the invention, transmitter  10  transmits information related to product  14  height over loop  20 . Further, transmitter  10  is completely powered by power received over loop  20 . In some installations, transmitter  10  meets intrinsic safety requirements and is capable of operating in a potentially explosive environment without danger of causing an ignition. For example, housing  16  is tightly sealed to contain any ignition, and circuitry in housing  16  is designed to reduce stored energy, thereby reducing potential ignition. 
     FIG. 2 is a block diagram of level transmitter  10  coupled to a process control room  30  over two-wire process control loop  20 . Control room  30  is modeled as resistor  32  and voltage source  34 . Transmitter  10  controls the current I flowing through loop  20  in response to height of product  14  in tank  12 . 
     Electric circuitry carried in housing  16  of transmitter  10  includes voltage regulator  40 , microprocessor  42 , memory  44 , digital-to-analog converter  46  coupled to analog output circuitry  48 , system clock  50  and reset circuitry  52 . Microprocessor  42  is connected to UART  54  which controls digital I/O circuit  56  and is coupled to current loop  20  through DC blocking capacitors  58 . UART  54  can also be a part of microprocessor  42 . Microprocessor  42  is also coupled to display module  60  for providing a display output and to transceiver circuitry  70 . 
     Transmitter housing  16  includes microwave transceiver circuitry  70  which includes clock- 1   72  and clock- 2   74 . The output of clock- 1   72  is coupled to step generator  76  which provides an input signal to microwave circulator  78 . Microwave circulator  78  is coupled to antenna  18  and provides an input to impulse receiver  80 . Impulse receiver  80  also receives an input from clock- 2   74  and provides an output to analog-to-digital converter  82 . 
     In operation, transmitter  10  is in communication with control room  30  over loop  20  and receives power over loop  20 . Voltage regulator  40  provides regulated voltage outputs to electronic circuitry in transmitter  10 . Transmitter  10  operates in accordance with instructions stored in memory  44  under the control of microprocessor  42  at a clock rate determined by system clock  50 . A reset and watchdog circuit  52  monitors the supply voltage to the microprocessor and memory. During power on, circuit  52  provides a reset signal to microprocessor  42  once the supply voltage has reached a sufficient level to allow operation of microprocessor  42 . Additionally, microprocessor  42  periodically ides a “kick” signal to watchdog circuit  52 . If these kick pulses are not received by circuit  52 , circuit  52  provides a reset input to microprocessor  42  to thereby restart microprocessor  42 . 
     Microprocessor  42  receives data from circuitry  70  through analog-to-digital converter  82  to determine product level height. Clock- 1   72  operates at a first clock frequency f 1  and clock- 2   74  operates at a second frequency f 2 . Clock- 1   72  acts as a “start transmit” clock and clock- 2   74  operates as a “gate receiver” clock, and the clocks are slightly offset in frequency. That is, f 2 =f 1 +Δf. This provides a digital sampling technique described in the ISA paper entitled “Smart Transmitter Using Microwave Pulses to Measure The Level Of Liquids And Solids In Process Applications,” by Hugo Lang and Wolfgang Lubcke of Endress and Hauser GmbH and Company, Maulburg, Germany. Product height is calculated by determining which cycle of clock- 2   74  coincides with a received microwave pulse. In one embodiment, clock- 1   72  is set for a frequency of between 1 MHz and 4 MHz, depending upon such condition at the installation as the maximum distance to be measured and current consumption requirements of the circuitry. Clock- 2   74  is synchronized to clock- 1   72 , but varies in frequency by between about 10 Hz and 40 Hz. The difference in frequency (Δf which provides a difference in clock rates) between clocks  72  and  74  determines the update rate of transmitter  10 . It is possible to obtain a higher received signal level by integrating received pulses over several cycles at the expense of reduced update rates. 
     The signal of clock- 2   74  provides a gating window which sweeps through the incoming signal at a rate determined by Δf. Impulse receiver  80  gates the incoming microwave signal using the f 2  signal from clock- 2   74 . The output of impulse receiver  80  is a series of pulses. These pulses will vary in amplitude dependent upon the noise or spurious reflections contained in the received signal. When the receipt of the microwave echo from the product surface is coincident with the gate pulse from clock- 2   74 , a larger output pulse results, and is converted to a larger value by analog-to-digital converter  82 . Microprocessor  42  calculates distance by determining which cycle of clock- 2   74  provided the largest output pulse from receiver  80 . Microprocessor  42  determines distance by knowing which gate pulse caused the largest output pulses from impulse receiver  80  as determined by analog-to-digital converter  82 . Product height is determined by the equation: 
     
       
         Level−Tank Height−Distance of Pulse Travel  Eq. 1  
       
       
         
           
             
               
                 
                   Level 
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                       Tank 
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                       Height 
                     
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                             Δ 
                           
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                           f 
                         
                         
                           f 
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                       · 
                       
                         C 
                         
                           2 
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                   2 
                 
               
             
             
               
                 
                   
                     One 
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                     Way 
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                     Distance 
                      
                     
                         
                     
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                     of 
                      
                     
                         
                     
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                     Pulse 
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                     Travel 
                   
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                           R 
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                           Δ 
                         
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                         f 
                       
                       
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                   Eq 
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                   3 
                 
               
             
           
         
                 
         
             
         
      
     
     where: 
     f 1 =clock  1  frequency 
     f 2 =clock  2  frequency 
     Δf=f 2 −f 1    
     R=Receive sample pulse which detected return to echo (R=O to f 1 /Δf) 
     Analog-to-digital converter  82  should have a fairly fast conversion rate, for example 0.5 μs, when the transmit rate (clock  1 ) is 2 MHz since a sample must be taken after every transmit pulse to see if an echo is present, converter  82  should have a sampling rate which must at least equal the frequency of clock- 1   72 . One example of such an analog-to-digital converter is the sigma-delta converter described in co-pending U.S. patent application Ser. No. 08/060,448 entitled SIGMA DELTA CONVERTER FOR VORTEX FLOWMETER. The resolution of analog-to-digital converter  82  is not particularly critical because only the presence or absence of a pulse is significant. 
     To further improve performance of transmitter  10 , the receive and transmit circuits in circuitry  70  are electrically isolated from each other. This is important so that transmit pulses are not incorrectly detected by the receiver as the echo pulse. The use of microwave circulator  78  permits accurate control of the source impedance and the receive impedance. The microwave circulator provides isolation between transmit and receive circuitry. Further, circulator  78  prevents the transmit pulse from causing the received circuit to ring. One example circulator is a three-port device which only allows signals from the transmit circuit (step generator  76 ) to reach antenna  18  and incoming signals from antenna  18  to reach receive circuitry  80 . Electrical isolation between transmit and receive circuits may be obtained by other techniques known to those skilled in the art. For example, circulator  78  may be removed and a separate transmit and receive antenna implemented. Further, circuit isolation techniques may be employed which provide isolation between transmit and receive circuits along with a delay circuit such that a received pulse was not received until after any “ringing” from the transmit pulse had faded. In another embodiment, microwave antenna  18  is replaced by a probe which extends into tank  12  shown in FIG.  1 . This embodiment may also include a circulator. 
     Based upon the detection of an echo pulse by microprocessor  42  through analog-to-digital converter  82 , microprocessor  42  determines the height of product  14  in tank  12 . This information can be transmitted digitally over two-wire loop  20  using digital circuit  56  under the control of UART  54 . Alternatively, microprocessor  42  can control the current level (between, for example, 4 and 20 mA) using digital-to-analog converter  46  to control output circuit  48  and thereby transmit information over two-wire loop  20 . In one embodiment, microprocessor  42  can be set to provide a high output (for example 16 mA) on loop  20  if the product level is either above or below a threshold level stored in memory  44 . 
     In one preferred embodiment, microprocessor  42  comprises a Motorola 68HC11. This is a low power microprocessor which also provides high speed operation. Another suitable microprocessor is the Intel 80C51. Low power memory devices are preferred. In one embodiment, a 24 Kbyte EPROM is used for program memory, 1 Kbyte RAM is used for working memory and a 256 byte EEPROM non-volatile memory is provided. A typical system clock for a microprocessor is between about 2 MHz and 4 MHz. However, a slower clock requires less power but also yields a slower update rate. Typically, power supply  40  provides efficient conversion of power from the control loop into a supply voltage. For example, if the input power supply is 12 volts and the level gauge electronics require 4 mA, the power supply must efficiently convert this 48 mwatts into a usable supply voltage, such as 5 volts. 
     The invention provides a number of significant advancements over the art. For example, transmitter  10  is completely powered by power received over two-wire current loop  20 . This reduces the amount of wiring required to place transmitter  10  at a remote location. Microprocessor  42  is also capable of receiving commands over two-wire current loop  20  sent from control room  30 . This is according to a digital communications protocol, for example the HART® communication protocol or, preferably, a digital communications protocol having a dc voltage averaging zero. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.