Abstract:
A radar level gauging system for determining a level of a surface of a product in a tank, comprising a propagation device arranged to allow microwave signals to propagate towards the surface, and to return a reflection of the microwave signals from the surface and at least two radar level gauge (RLG) units. Each RLG unit includes transceiver circuitry, processing circuitry connected to the transceiver circuitry and adapted to determine the level based on a relation between transmitted and received signals, and a communication interface. The system further comprises a communication link interconnecting the RLG units, for communicating control data between the RLG units, the communication link comprising an isolating interface, ensuring galvanic isolation between the RLG units. 
     One unit may hereby communicate configuration data to the rest of the RLG units which results in a simplified configuration of the sensor system and thus more cost effective compared to the prior art solutions.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to a radar level gauging system for measuring process variables in a tank, and in particular the exchange of control data in such a system. 
       TECHNICAL BACKGROUND  
       [0002]    Radar level gauging systems can be arranged in a tank, in order to provide information about process variables of a product in the tank. Examples of process variables that may be determined by such sensors are filling level, product composition, inflow, etc. 
         [0003]    In such gauging systems, function independence is sometimes required. For example, safety regulations require that a high level alarm sensor and/or an overfill alarm sensor function independently of a level measurement sensor as well as of each other. Functional independence here means that a fault in one system does not render the other system(s) inoperative. 
         [0004]    Such independence can be achieved by having several RLG units galvanically isolated from each other. This results in a system where each RLG unit is implemented with fully redundant hardware and software in order to meet both operational and service requirements. In case of level measurement and alarms, each RLG unit within the sensor system needs components for measuring the filling level, and each RLG unit needs an interface for receiving configuration data. Such RLG systems are used especially (but not exclusively) in marine applications, e.g. mounted on board tank ships. 
         [0005]    In the case of advanced gauging systems, a large number of configuration parameters are required, and service commands are communicated to the sensor in order to determine its status. In systems with several independent sensors of this or similar type, the independency requirements make the system complex and expensive. In addition, operation of the system becomes time consuming and difficult, as large amounts of data must be communicated to each sensor. 
       GENERAL DISCLOSURE OF THE INVENTION  
       [0006]    It is therefore an object of the present invention to provide an improved sensor system which overcomes, or at least mitigates, the above discussed problems of the prior art. 
         [0007]    These and other objects are achieved by a radar level gauging system for determining a level of a surface of a product in a tank, comprising a propagation device arranged to allow microwave signals to propagate towards the surface, and to return a reflection of the microwave signals from the surface and at least two radar level gauge (RLG) units, each RLG unit including transceiver circuitry connected to the propagation device, the transceiver circuitry being adapted to transmit a microwave signal to the propagation device and to receive a reflected signal related to the reflection, processing circuitry connected to the transceiver circuitry and adapted to determine the level based on a relation between transmitted and received signals, and an interface for communicating measurement data acquired by the RLG unit externally of the RLG unit. The system further comprises a communication link interconnecting the RLG units, for communicating control data between the RLG units, the communication link comprising an isolating interface, ensuring galvanic isolation between the RLG units. 
         [0008]    The isolating interface is adapted to ensure that charge-carrying particles cannot be transferred between sensors via the communication link, i.e. there is no conductive connection between the RLG units, and there is no electrical current flowing directly from one RLG unit to the next via the communication link. Energy and/or information can still be exchanged over the communication link by other means, however, such as by capacitance, induction, electromagnetic waves, optical, acoustic, or mechanical means. 
         [0009]    One unit may hereby communicate configuration data to the rest of the RLG units which results in a simplified configuration of the sensor system and thus more cost effective compared to the prior art solutions. 
         [0010]    According to one embodiment, the RLG units are functionally independent, such that, if a fault occurs in one of said RLG units, any other one of said RLG units remain operable. The isolating interface of the communication link ensures that the communication link can communicate of various control data between the RLG units without compromising the independent functionality of the different RLG units. 
         [0011]    The control data may be configuration data, e.g. data related to the physical properties of the sensor installation or to user set parameters. Such configuration data that typically is the same for several sensors in the system, e.g. data related to the properties or geometry of the tank, may be communicated via the communication link from one sensor to the others. 
         [0012]    If parts of the configuration data is manually entered, such data only needs to be input to one unit, resulting in increased usability, and a more efficient system. It also reduces the risk that configuration data differs between sensors. 
         [0013]    The control data can also, or alternatively, be related to the measurement data acquired by the sensors. This enables each RLG unit to have information about the results from the other sensors, for example to verify that it is tracking the true measurement data. A further advantage is improved signal processing by comparing process data between the different functions. 
         [0014]    Each RLG unit can be adapted to send and receive control data via said communication link, thus forming a completely bidirectional network of sensors. This is particularly advantageous when the control data is data that needs to be communicated from many sensors to many sensors, such as measurement data. 
         [0015]    Alternatively, one of said RLG units may be a “master” unit, adapted to receive said control data from outside said sensor system, and remaining RLG units may be “slave” units, adapted to receive control data via the communication link from the master unit. Such a master unit can thus be used a point of entry for data that should reach all units, such as configuration data or external sensor data. 
         [0016]    If a data input device is used for inputting control data, it is thus sufficient that the master unit comprises such an input device. The master unit can transmit the data to the other RLG units via the communication link. The slave units do thus not require any input devices. 
         [0017]    The communication link may be based on optical signals, and e.g. comprise optical switches in each RLG unit, and optical links connecting these switches. Optical signals are known to provide a galvanically isolated communication link. The communication link may also be implemented e.g. using wireless network technology. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]    This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention. 
           [0019]      FIG. 1  is a perspective, and partly broken away view of a sensor system according to an embodiment of the invention. 
           [0020]      FIG. 2  is a more detailed view of an example of a radar level gauge unit suitable for the system in  FIG. 1 . 
           [0021]      FIG. 3  is a more detailed view of a second example of a radar level gauge unit suitable for the system in  FIG. 1 . 
           [0022]      FIG. 4  is a schematic view of how a communication link according to an embodiment of the invention is implemented between three RLG units. 
           [0023]      FIG. 5  shows an alternative way to connect the units in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION  
       [0024]    The invention will be described with reference to a radar level gauging system  10  in  FIG. 1 , having three radar level gauge (RLG) units  1 ,  2 ,  3  physically integrated inside a protective housing  4 . The housing  4  here comprises an outer cover  4   a,  as well as an inner cover  4   b.  At least the outer cover is adapted to protect the electronics in the gauging system from the outside environment, such as sea water spray if the system is mounted on a tank ship. At least one of the covers  4   a,    4   b  may further be conductive, so as to shield the electronics from electromagnetic disturbance. 
         [0025]    The system  10  has a flange  5  that is mounted on a corresponding flange  6  of an opening in the roof of a tank  7 . The tank may be a stationary process tank as indicated in  FIG. 1 , but may also be a tank on a moving vessel, such as a tank ship. Each RLG unit is arranged to perform measurements of a process variable such as the filling level L or ullage level of a product  8  contained in the tank  7 , by transmitting signals into the tank and receiving signals reflected by an interface  9  between two materials in the tank  7 . Typically, the first material is a liquid product  8  stored in the tank, e.g. gasoline, while the second material is the atmosphere above the product, such as air. 
         [0026]    All three RLG units are here connected to one common propagation device  11 , for allowing signals from the transceivers to into the tank  7 . For this purposes, the system comprises a combiner  12 , arranged to combine signals from each RLG and provide a combined signal to the propagation device, and for separating a reflection received from the tank, so that a portion of this reflection resulting form a signal transmitted from a particular RLG is returned to this particular RLG. The combination/separation process may be based on polarization, frequency modulation, time modulation, etc. In case of polarization, the combiner can be a turnstile junction, as discussed in WO 2003/025523, herewith incorporated by reference. 
         [0027]    The propagation device  11  is arranged to act as an adapter, transmitting electromagnetic waves into the tank  7  to be reflected by the interface  9 . The propagation device can be a free propagating antenna or pipe, arranged in the top of the tank  7 , as illustrated in  FIG. 1 . Alternatively, the propagation device  11  can be a guided wave transmission line. Such a transmission line can be a flexible wire suspended between top and bottom of the tank, or can be a rigid probe extending into the tank. It can be a single wire, twin wire, coaxial, or any other type of suitable transmission line. The propagation device may also be a hollow waveguide for guided propagation of electromagnetic waves. 
         [0028]    The gauging system may be connected to one or several additional sensors  13 . Examples of additional sensors comprise vapor pressure sensors and temperature sensors. 
         [0029]    In the illustrated example, the first RLG unit  1  is used for level measurement, i.e. to provide a continuously updated value of the filling level or ullage in the tank. The second RLG unit  2  is used as an high level alarm, i.e. to provide an alarm signal when the filling level exceeds a predefined level, e.g. equal to 95% of the tank volume. The third RLG unit  3  is used as an overfill alarm, i.e. to provide an alarm signal when the filling level exceeds a predefined overfill level, e.g. 98% of the tank volume. 
         [0030]    Various safety requirement, e.g. marine safety regulations, require that the sensors in  FIG. 1  are functionally independent, i.e. that a fault in one sensor does not render the other systems inoperable. Note that this requirement can be fulfilled also when several RLGs are physically integrated and share the same propagation device, as illustrated in  FIG. 1 . 
         [0031]      FIG. 2  shows a generalized schematic block diagram of an example of radar level gauge suitable for use in the system in  FIG. 1 . 
         [0032]    The radar level gauge  1  includes transceiver circuitry  21 , also referred to as a microwave module, connected to the propagation device  11 . The radar level gauge  1  also includes timing circuitry and processing circuitry, here illustrated as a transceiver control and signal processing block  22 , adapted to control the transceiver circuitry  21  and to determine a process variable of the product in the tank  7 , based on a relation between waves transmitted and received by the transceiver  21 . 
         [0033]    The transceiver control and signal processing block  22  is further connected to a communication interface  23  which is adapted to provide communication externally of the radar level gauge  1  over a two wire interface  24 . The two-wire interface  24  has a combined function of both communicating externally of the RLG unit  1 , and for receiving power for operation of the RLG unit  1 . In the two-wire interface  24  data can be communicated by a protocol superimposed on the current flowing through the interface. Examples of such protocols are HART, Profibus and Foundation Fieldbus. A serial bus  25  is connected to the interface  24 , enabling communication between the RLG unit  1  data and a central control unit  26 . 
         [0034]    In addition to the power provided by the two-wire interface  24 , additional power can be provided on separate wire pairs (not shown), e.g. connected to the microwave unit  21 , processing circuitry  22  and/or communication interface  23 . 
         [0035]    The RLG unit  1  is typically placed in a hazardous zone, where intrinsic safety regulations have to be fulfilled. Preferably, all power supply is therefore made in an intrinsically safe manner, avoiding the need for explosion protective capsulation of the entire gauging system. 
         [0036]    The RLG unit  1  can also comprise a non volatile memory, e.g. an EPROM  28 , for storing various configuration data required for the signal processing. Such data may be communicated to the gauge  1  over bus  25  and interface  24 . For this purpose, the bus  25  may also be connected to a user interface  29  for receiving manually entered configuration data. The user interface  29  may also be used for service applications, and be adapted to display information such as tank spectra, level related information, etc. The user interface  29  may be located in vicinity of the RLG unit  1  or remote, e.g. in a control room. 
         [0037]    Alternatively, as illustrated in  FIG. 3 , the communication between the communication interface  23  and the central control unit  26  is provided by an industrial loop (e.g. a 4-20 mA loop), where the measurement data is communicated by means of an analogue current value. In this case, a current control unit  33  is arranged in the communication interface to regulate the current in the lines  31 ,  32 , in accordance with a measurement result determined by the data processing block  22 . 
         [0038]    A person skilled in the art realizes that other types of communication protocols might be used, such as for example a four-wire connection in which case the power supply to the radar level gauge  1  is supplied through two of the four wires, and the communication is provided through the remaining two wires. 
         [0039]    In use, during each measurement cycle, the processing circuitry  22  controls the transceiver circuitry  21  to generate and transmit a measurement signal to be transmitted into the tank  7  by the propagation device  11 . This signal can e.g. be a pulsed signal (pulsed level gauging) or a continuous signal with a frequency varying over a certain range (Frequency Modulated Continuous Wave, FMCW), or any other suitable signal modulation for tank gauging. In case of pulsed level gauging, the signals generated by the transceiver  21  can be DC pulses with a length of about 2 ns or less, with a pulse repetition frequency on the order of 1 MHz, and modulated on a carrier of a 4-11 GHz frequency (microwaves). In case of FMCW, the signals may lie in a frequency band having a bandwidth of e.g. 0.5-3 GHz, and a center frequency of e.g. around 6 GHz, 10 GHz or 26 GHz. Average output power levels can be in the mW to μW area. As mentioned above, the propagation device  11 , here the antenna, acts as an adapter, enabling the signal generated in the transceiver  21  to propagate into the tank  7  as electromagnetic waves, which can be reflected by the surface  9  of the material  8 . 
         [0040]    A tank signal, i.e. the correlation of the transmitted signal and its echo, or a mix of transmitted and reflected signals, is received by the transceiver  21  via the propagation device  11 , and communicated to the processing circuitry  22 . The processing circuitry  22  determines a measurement result based on a relation between the emitted and received waves. 
         [0041]    For satisfactory operation, each RLG unit is preferably provided with configuration data, for example defining the geometry of the tank, the characteristics of the product in the tank, etc. Such configuration data may be provided via bus  25  and interface  24 , using the user interface  29 . Alternatively, an input interface  30 , separate from the interface  23 , is connected to the circuitry  22 , providing direct access to the RLG unit  1 . The interface  30  may be used e.g. to connect a manual input device such as a keyboard directly to the RLG unit. 
         [0042]    As illustrated in  FIG. 4 , the gauging system  10  further comprises a communication link  41  interconnecting the RLG units  1 ,  2 ,  3  while at the same time including a galvanically isolating interface, ensuring that electrical charge is not transferred between the units via the link. Such a galvanically isolating interface can be realized in different ways, and examples include an optical interface, an induction interface, or a short-range wireless interface, such as Bluetooth. Although referred to as “a” link, the link  41  may include several link segments, each segment connecting two of the RLG units  1 ,  2 ,  3  in the system  10 . 
         [0043]    The link may be connected in series, as indicated in  FIG. 4 , but may alternatively be connected in parallel (see  FIG. 5 ). In  FIG. 2 , the link  41  is indicated to be connected to the communication interface  23 . If required, a separate communication driver may be provided for the communication link  41 . 
         [0044]    In  FIG. 4 , the communication link  41  is an optical link, where each segment connecting two units  1 ,  2 ,  3  comprises optical switches  42  in either end, and an optical guide, e.g. an optical cable  43  extending between the switches  42 . By arranging the optical switches  42  facing each other on opposing surfaces of adjacent RLG units  1 ,  2 ,  3 , the optical guide may be omitted. 
         [0045]    The communication link  41  allows communication between the sensors  1 ,  2 ,  3 , and possibly any external sensors  13 , without compromising the functional independence. This may reduce the need for providing data separately to each sensor in the gauging system  10 . 
         [0046]    For example, as each RLG unit  1 ,  2 ,  3  is mounted in the same opening in the tank, the geometrical conditions will be identical for each unit. Therefore, tank related configuration data required by the processing unit, and stored in the EPROM  28  will be identical. Such configuration data may now be inputted only to one of the sensors, and then provided to the remaining sensors using the communication link  41 . 
         [0047]    As indicated in  FIG. 1 , it may now be sufficient to provide one user interface  29  for the entire sensor system  10 . This significantly reduces cost and complexity, as the user interface typically is an expensive part of the system. 
         [0048]    The RLG unit  1  that is arranged to receive data from outside the system can be regarded as a “master” unit, and the remaining RLG units, arranged to receive this data via the communication link  41 , can be regarded as “slave” units. In the illustrated example of a radar level gauging system  10 , the master sensor is preferably the RLG unit  1  used for continuous level measurement. 
         [0049]    Another type of information that may be treated in a similar way is measurement data from the external sensor  13 . Again, such data may be of use to all sensors, but is advantageously only received from the outside by one of the sensors and then communicated to the others using the communication link  41 . According to one embodiment, the external sensor data is received by the master RLG unit and forwarded to the slave RLG units. 
         [0050]    The communication link  41  may also be used to communicate measurement data from one sensor to another in the sensor system. For example, in the case of an radar level gauging system  10 , it may be advantageous for one RLG unit to know what filling level is detected by the other RLG units. Such information may be used to detect an erroneous measurement value, or some malfunction. 
         [0051]    The person skilled in the art realizes that while the present invention has been described above primarily with respect to a radar level gauging system, the invention is by no means limited to this application. On the contrary, the invention may advantageously be applied to any sensor system, where communication between a plurality of functionally independent sensors is desired.