Abstract:
To provide a simpler and less expensive system for multipoint level measurement, the level measurement system comprises at least two sensor devices operating in accordance with the pulse-echo principle and being connected via a communication network to a signal processing device. The computation-intensive evaluation of echo profiles for calculating distances or ranges is thus shifted from the sensor devices at the measurement locations or sites to the central signal processing device. 
     A clean copy of the Abstract that incorporates the above amendments is provided herewith on a separate page.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office application No. 07018501.2 EP filed Sep. 20, 2007, which is incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The present invention relates to a level measurement system. 
       BACKGROUND OF INVENTION 
       [0003]    In level measurement applications, pulse-echo ranging devices, also known as time-of-flight ranging devices, are commonly used for determining the distance to a target object (e.g. reflective surface of a material in a container) by measuring how long after transmission of a burst of energy pulses the reflected pulses or echoes are received. Such devices typically use ultrasonic pulses or pulsed radar or microwave signals. 
         [0004]    Pulse-echo acoustic ranging devices generally include a transducer and a signal processor. The transducer serves the dual role of transmitting the energy pulses and receiving the reflected energy pulses or echoes. An echo profile is generated from the received energy pulses by amplifying, filtering and envelope shaping, wherein in some point an analog-to-digital conversion is performed. Echo pulses are identified in the digital echo profile by a signal processor, and the distance or range of the target is calculated based on the transmit times of the transmitted energy pulses and the identified echo pulses. The calculated distance or range is then transmitted via a communication network to a process control system using a communications protocol such as analog 4-20 mA, HART, PROFIBUS or FOUNDATION Fieldbus. 
         [0005]    When measurements at different points of the same surface of, e.g., a bulk material or at different locations or sites are required, one complete device is needed for each measurement, which can be very expensive. 
       SUMMARY OF INVENTION 
       [0006]    It is therefore an object of the invention to provide a simpler and less expensive system for multipoint level measurement. 
         [0007]    According to the invention this object is achieved by the level measurement system defined in an independent claim. 
         [0008]    Preferred embodiments of the method according to the invention are specified in the remaining claims. 
         [0009]    A computation-intensive evaluation of the echo profiles for calculating distances or ranges is shifted from the sensor devices at the measurement locations or sites to the central signal processing device. The sensor devices consist only of the minimum components required for emitting and receiving the energy pulses and for providing and transmitting a digitized echo profile to the central signal processing device. Thus, the individual sensor devices are less complex and expensive and also require less energy than complete field devices. The communication network for transmitting the echo profiles to the central signal processing device may be, but does not need to be, of a standard industrial type such as PROFIBUS, FOUNDATION Fieldbus and Ethernet. It can be of any convenient network topology, such as daisy chain or multidrop, and is not limited to be wired but also may be wireless. 
         [0010]    The echo profiles may be digitized before they are transmitted to the central signal processing device. 
         [0011]    It is also possible to transmit analog samples of the respective echo profiles in a manner so as not to interfere with the analog echo profiles from other sensor devices connected to the same communication network. The encoding scheme may then be a current loop, frequency modulation or any other modulation scheme where signal amplitude is represented by an analog coding scheme. The sensor devices located at measurement points cooperatively transmit their echo profiles according to the network protocols and thereby eliminate interference with each other. Typically, such cooperation requires that the devices are operated in a master-slave protocol and the master addresses individual sensor devices by their address. Other networking schemes include the client/server model, in which a sensor device responds to requests from the process controller, and the peer-to-peer model, in which either of two sensors or a sensor and the process controller can initiate a communication session. 
         [0012]    Other analog encoding schemes that require less processing capability at the sensor devices are, e.g., frequency modulation where each sensor device is set at a frequency that will not interfere with that of another sensor device or pulse width modulation the pulses are transmitted in pre-allocated time slots. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention will be now described by way of preferred examples and with reference to the accompanying drawing, in which: 
           [0014]      FIG. 1  shows a block diagram of the level measurement system according to the invention comprising a plurality of sensor devices linked by a communication network to a signal processing device, 
           [0015]      FIG. 2  shows a block diagram of a first embodiment of one of the sensor devices, 
           [0016]      FIG. 3  shows a block diagram of the signal processing device and 
           [0017]      FIG. 4  shows a block diagram of another embodiment of one of the sensor devices. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0018]    Reference is first made to  FIG. 1  which exemplarily shows a simplified schematic diagram of an industrial process automation system  1  including a level measurement system  2  according to the invention. The process automation system  1  comprises a process controller  3  and a plurality of field devices  4 ,  5 ,  6 , such as flow, level and pressure transmitters and control valves, distributed throughout the physical process and communicatively connected together and to the process controller  3  by a superior communication network  7 . The level measurement system  2  itself forms from the process automation system&#39;s  1  perspective the field device indicated by reference number  6 . The level measurement system  2  comprises several sensor devices  8 ,  9 ,  10 ,  11  which operate in accordance with the pulse-echo principle. The sensor devices  8 ,  9  and  11  are radar based whereas the sensor device  10  uses ultrasonic pulses. The sensor devices  8 ,  9 ,  10 ,  11  are located at different measurement locations and connected via a communication network  12  to a common signal processing device  13  which is interfaced to the superior communication network  7 . The sensor devices  8  and  9  measure the level of a bulk material at different points of the material surface. The sensor devices  10  and  11  measure the filling levels of different liquids  15 ,  16 . 
         [0019]      FIG. 2  shows in greater detail and as a representative example for the other sensor devices  8 ,  9 ,  11  an embodiment of the sensor device  10 . While the device  10  and its operation are described in the context of an ultrasonic based pulse-echo acoustic ranging device, it should be understood that the system can also be radar based. The sensor device  10  comprises an ultrasonic transducer  17  which is installed in a tank  18  containing the liquid  15 , or other type of material, with a level determined by the top surface  19  of the liquid  15 . The top surface  19  of the liquid  15  provides a reflective surface which reflects ultrasonic pulses  20  generated from the transducer  17 . The transducer  17  is coupled to a transmitter  21  which is controlled by an internal timer (not shown) or by a control signal  22  to excite the transducer  17  to emit the ultrasonic pulses  20  at predetermined points in time and with a predetermined frequency and amplitude. The echoes or reflected pulses  23  from the surface  19  of the liquid  15 , the wall of the tank  18  and obstacles in the measurement environment (not shown) are received by the transducer  17  and converted into an electrical signal which is then fed to a receiver  24 . The transmitter  21  and the receiver  24  can be implemented individually or be combined to form a transceiver. The receiver  24  comprises an amplifier  25  and an envelope detector  26  for amplifying and thereafter shaping the electrical signal output from the transducer  17 . The output from the envelope detector  26  is then sampled and digitized by an analog-to-digital converter  27 , thus providing a digital echo profile  28 . The output of the receiver  24  is connected to the communication network  12  via a communication interface  29 . This communication interface  29 , which may comprise a buffer (not shown) for buffering the echo profile  28 , digitally transmits the echo profile  28 , if necessary together with an address information, via the communication network  12  to the signal processing device  13 . Transmission may occur at predetermined time intervals or on request by the signal processing device  13 . The transmitter  21  may receive the control signal  22  from the signal processing device  13  or from an adjacent sensor device, e.g. by using a token method, via the communication network  12  and the communication interface  29 . As indicated by reference number  12 ′, the communication network  12  may be of wireless type. 
         [0020]      FIG. 3  shows in greater detail an embodiment of the signal processing device  13 . The signal processing device  13  comprises a microcontroller  30  which operates under a control program stored in read-only memory (ROM)  31 , utilizing parameters stored in non-volatile random access memory (NVRAM)  32 , and provided with a working memory in the form of random access memory (RAM)  33 . The control program includes a signal processing module  34  providing different algorithms for the different types of sensor devices  8 ,  9 ,  10 ,  11 , which different algorithms have access to different parameter sets stored in the NVRAM  32 . The microcontroller  30  is, via a first communication interface  35 , connected to the communication network  12  of the level measurement system  2  for receiving the digitized echo profiles  28  from the sensor devices  8 ,  9 ,  10 ,  11 . The microcontroller  30  is further connected to the superior communication network  7  of the process automation system  1  via a second communication interface  36 . Each received digital echo profile  28  is stored in the RAM  33  for further processing by the microcontroller  30 . The microcontroller  30  selects and executes a matching algorithm which identifies and verifies in the echo profile  28  the true echo from the surface  19  of the liquid  15  and calculates the range of the reflective surface  15 , i.e. the time it takes for the reflected pulses  23  to travel from the reflective surface  19  to the transducer  17 . From this calculation, the distance to the surface  19  of the liquid  15  and thereby the level of the liquid  15  is determined. The second communication interface  36  provides for the export of these data from the level measurement system  2  to the process controller  3  and the import of operating parameters from the process controller  3  or any other programming device (not shown) connectable to the superior communication network  7 . Data may be further visualized on a display  37  of the signal processing device  13 . 
         [0021]      FIG. 4  shows an example of the sensor device  11  as another embodiment of one of the sensor devices  8 ,  9 ,  10 ,  11 , where similar parts to those of  FIG. 2  are indicated by the same reference numerals. Instead of an ultrasonic transducer  17 , the sensor device  11  comprises a microwave transducer  17 ′ which emits microwave (radar) pulses  20 ′ and receives reflected microwave (radar) pulses  23 ′. In the receiver  24 , the electrical signal output from the transducer  17 ′ is digitized at an early stage (analog-to-digital converter  27 ) and then digitally processed, particularly by digital filtering, to the echo profile  28 . This signal processing is done by a microprocessor  38  of relatively low computing power compared to those of conventional radar level transmitters or that of the signal processing device  13 . The microprocessor  38  operates under a control program stored in a read-only memory (ROM)  39  and accesses a working memory in the form of random access memory (RAM)  40  for e.g. buffering the echo profile  28  before digitally transmitting the echo profile  28  via the communication network  12  to the signal processing device  13 .