Abstract:
A load measuring device, especially load cell, with one ore more strain gages preferably connected in form of a bridge circuit, and feed lines for excitation of the strain gage and of the strain gages respectively and for signal derivation, with an integrated analogue/digital converter to convert the analogue measuring signals of the at least one strain gage into a digital signal, and with a bus interface to output the digital signal to a digital bus, the load measuring device comprising an equipment for potential separation between the bus interface and the at least one strain gage.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a load measuring device, particularly a load cell, with one or more strain gages preferably connected in form of a bridge circuit, and feed lines for excitation of the strain gage and of the strain gages respectively and for signal derivation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Load measuring devices are for example used in form of load cells to examine the weight of an equipment, e.g. of tanks or similar during the usage. A load cell according DE-U-29613833.9 is for this reason equipped within a cavity with strain gages, connected to form a bridge circuit. The strain gages are equipped at the inner surface of an external cover of the cavity. Action of outer force to the cover results as usual known in a measuring signal of the bridge circuit. In the described solution of the state of the art are two strain gages used for the pressure range und two further strain gages for the tension range. At the known solution the analogue measuring signal is fed by bushing filters through an electrical conductive closing plate out of the cavity to an evaluator equipment and there evaluated as an analogue signal. 
         [0003]    DE 29 06 669 A1 describes a load measuring device with strain gages, where the strain gages und the evaluator equipment can be used potential-free. 
         [0004]    A load measuring device with the features of the preamble of claim  1  is known from US 2005/0137821 A1. 
       SUMMARY OF THE INVENTION 
       [0005]    An object of this invention is to provide a load measuring device which allows the connection to a digital measuring bus, leads to a low interference liability and is easy in use and connection. 
         [0006]    The solution of this object is described by the features of claim  1 . 
         [0007]    Especially in accordance with the invention the load measuring device is provided with an integrated analogue/digital converter for converting the analogue measuring signal of the at least one strain gage to a digital signal and has a bus interface to output the digital signal to a digital bus. 
         [0008]    It is an object of the invention that the load measuring device can be directly connected to digital bus systems. Therefore the derivation of an analogue measuring signal is not more necessary. The integration of the analogue/digital converter and of the bus interface in the load measuring device reduces the cabling expense and the influence of interference signals. Especially also in comparison with solutions, where an external bus interface is connected with an analogue signal output of the load measuring device, the cabling expense is reduced significantly. The assembly and the installation are very simple, because only the output of the load measuring device has to be connected with the used bus system. The interference liability is strongly reduced, especially by using digital signals. 
         [0009]    An especially advantageous enhancement of the load measuring device within the scope of the invention has a bus interface which is also suitable for feeding the energy for the supply of the at least one strain gage. An external supply of the measuring system is then not necessary. 
         [0010]    Especially with the realization of a force measuring cell, especially of a load cell, the load measuring device within the scope of the invention has a housing, in which the strain gages are mounted in a known manner to an inner surface. 
         [0011]    The bus interface of the load measuring device within the scope of the invention can for example be equipped for a connection with a PROFIBUS, an INTERBUS or a MODBUS. Especially simple and versatile is the use of an actuator/sensor bus (AS-i-Bus), whereas the bus interface of the load measuring device within the scope of the invention includes an AS-Interface (actuator/sensor-Interface, ASI). It is within the purview of the invention that one can change between different bus protocols. 
         [0012]    Especially an AS-Interface bus usually has no connection to the electrical ground and is therefore potential-free. Therefore it is important that the sensor connection is symmetrically with reference to the electrical ground and also a low capacity to the electrical ground is demanded. But for example concerning load cells usually the strain gages are at the inner surface of the grounded housing and are there bonded for example. These constructions can result in high capacities to the electrical ground, which leads to a restriction for the use with a digital measuring bus. Unsymmetrical irradiation leads to a reduced electromagnetic compatibility. 
         [0013]    The load measuring device is within the scope of the invention is equipped with a electrical isolation or potential separation between the bus interface and the at least one strain gage to reduce for example the interference liability resulting from signal cross talk. To prevent that high capacities, which can for example occur between the strain gages and the housing of the load measuring device, will have a negative impact to the bus system, an embodiment of the invention comprises the equipment for the potential separation in a way, that the coupling capacity between the areas which are separated by the potential separation, is less than 15 pF, preferably less than 10 pF. In this way one obtains an effective capacitive decoupling. It is proofed that a negative influence on the bus system by an unsymmetrical irradiation into the bus lines can thus be prevented or reduced. 
         [0014]    Especially in an embodiment of the invention when the bus interface also feeds the supply of the at least one strain gage, a special embodiment for potential separation comprises a toroidal core transformer in the supply path for the supply of the at least one strain gage. The small coupling capacity will be achieved besides other ways by the suitable choice of the core material, e.g. a ferrite core. Other provisions are the suitable choice of the distance of the coils of the transformer from the core, a low capacity design of the printed-circuit board, on which there are placed the different components, and the selection of a sealing compound with little coupling capacity for the toroidal core transformer and/or a separate shielding of the primary coil and the secondary coil, whereas also the primary shielding can be connected with a wire of the primary coil and the secondary shielding with a wire of the secondary coil. 
         [0015]    A special embodiment achieves a small coupling capacity among other things by spatially separating the at least one primary coil and the at least secondary coil of the toroidal core transformer in a manner, that the coupling capacity reaches the wished small amount. For this reason one can wind especially the windings of the separate coins in a concentrative manner on the ring core of the transformer, so that the wires of the separate windings are basically positioned one upon the other and not side by side. These arrangements can be done both on the one hand for the primary coils and on the other hand for the secondary coils. 
         [0016]    In a further development within the scope of this invention the primary coil of the toroidal core transformer is electrically shielded in a special way. There are two shields, whereas one comprises a close capacitive coupling with the primary coil and the other a low capacitive coupling with the primary coil. This can for example be achieved by choosing the thickness of the insulating layers between the primary coil and the corresponding shield. A further step for an effective capacitive decoupling is the provision that the second shield, which has a smaller capacitive coupling, is arranged in two parts, whereas the two parts are electroconductively connected. Especially in an advantageous embodiment is one part of the second shielding arranged within the primary winding of the primary coil and the second part outside of the primary winding, especially advantageous also outside of the first shielding. 
         [0017]    To further reduce the coupling capacity, e.g. to values less than 1 pF, one embodiment of the invention keeps the first and the second shielding advantageously on the same potential, so that no electric charge can flow between both shieldings. Thereby are the toroidal core and the secondary coil from the side of the primary coil electrically no more visible, so that the measuring system with at least one strain gage at the secondary side of the toroidal core transformer can be grounded without influencing a digital bus, which is coupled to the primary side if applicable. 
         [0018]    For the control of the potential of both shieldings a special embodiment of the invention provides an active shield control. 
         [0019]    As a matter of course all described provisions can also be combined. 
         [0020]    One preferential embodiment of the invention comprises a digital isolator with a low coupling capacity, preferentially less than 4 pF, very preferentially less than 2 pF, which transfers the digital measuring signals for example from the analogue/digital converter to the bus interface or to an interconnected microprocessor. Such a digital isolator can vice versa also be used to transfer control signals to the measuring system. For digital isolators can be used for example optocouplers. One embodiment of the invention uses in a very effective manner air-core transformers. This technology uses transformers on chip basis which comprises for example ten times increased velocities with a ten times reduced need of energy at the same time combined with linear transfer functions without reducing the insulation rigidity. An insulation rigidity of for example 5 kV is possible. 
         [0021]    A further development within the scope of this invention provides the complete integration of the load measuring device except for the bus interface in one housing. The housing can be hermetically sealed to limit the influence of environmental impact. Furthermore can be provided that the components of the bus interface itself except for the bus connection are also integrated into the housing. 
         [0022]    A special embodiment of the load measuring device within the scope of this invention comprises a real time clock together with an equipment for storage of errors. In this way it is ensured that in the case of an error during the operation this error can be chronologically allocated. Also in the case of a claim under guarantee it is possible to evaluate whether the equipment is used beyond admissible parameter ranges or not. 
         [0023]    A further special embodiment of the load measuring device within the scope of this invention comprises an equipment for temperature measurement of the sensor environment. With this provision it is possible by means of corresponding software to detect temperature failings of the measuring chain and if necessary to compensate them. 
         [0024]    The load measuring device within the scope of this invention may have a microprocessor for controlling purpose. Advantageously has the microprocessor a boot sector by means of which one can execute a programming, e.g. a software update. Especially realizing a load measuring device which is hermetically sealed, a possibility for programming from outside is specially advantageous. 
         [0025]    The microprocessor can be provided for example between the bus interface and the equipment for potential separation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention will be explained in detail on the basis of the enclosed figures, which show schematically special embodiments. Thereby shows 
           [0027]      FIG. 1  the schematic assembly of one embodiment of a load measuring device within the scope of this invention, 
           [0028]      FIG. 2  a detail of an equipment for potential separation of an embodiment within the scope of this invention, and 
           [0029]      FIG. 3  a circuit diagram of one embodiment of a load measuring device within the scope of this invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0030]    In  FIG. 1  the numeral  1  indicates a bridge circuit with four strain gages  3 , as realized for example in load cells. Normally two of these strain gages are provided in the pressure environment and two more of the strain gages in the tension environment of the cover of a hermetically sealed housing, which is not shown in the schematic drawing of  FIG. 1 . Pressure on this cover leads to a change of the resistance, which can be evaluated to indicate the load. Feed line  5  connects the bridge circuit  1  with a processing unit  7 , which comprises an analogue/digital converter. Additionally to the analogue/digital conversion the processing unit  7  can also amplify and/or filter the signal. Not shown is an equipment for temperature measuring, which can also be provided within the housing and which signals can be also be transferred in digital form. The digital signals of the measuring bridge  1 , the temperature measuring equipment and possibly further sensor equipments are transferred via the signal path  11  to a microprocessor  13 . 
         [0031]    Otherwise can the microprocessor  13  via the signal path  11  transfer control signals to the processing unit  7 . So can for example a signal of the microprocessor be used to control a tare equipment which is not shown in the schematic drawing of  FIG. 1 . 
         [0032]    The signal path  11  comprises at least one digital isolator  10  which provides a potential separation. This may be for example an optocoupler which provides an intrinsic potential separation. Especially simple is the use of an air-core transformer, e.g. an iCoupler of Analog Devices Company, especially an iCoupler AduM240x1. 
         [0033]    The microprocessor  13  is connected with a real time clock  15  with an error memory, which provides an monitoring of the individual actions, to show for example in case of warranty, whether the equipment was used within the range of the admissible parameters, and to provide the possibility for a chronological assignment of the occurred errors. To guarantee the independency of the real time clock there can be provided a battery back up  31 . 
         [0034]    The elements provided left of the imaginary line  33  in  FIG. 1  will be declared below as bus interface of this embodiment. 
         [0035]    The microprocessor  13  transfers via path  19  measuring signals to a bus component  17 , which can among others also comprise a noise filter and/or a protection circuit against overvoltage. Double arrow  21  marks the bus connection of the bus interface. Especially the use of a AS-Interface device network is suitable, so that the bus interface is provided as AS-Interface. 
         [0036]    With other embodiments the bus component can be switched for connection of different bus systems and different bus protocols respectively. 
         [0037]    The load measuring signals can be transmitted for example via a 16 bit value. 
         [0038]    From the bus connection  21  or from the bus component  17  supply energy is derived out of the bus signal an supplied via the energy path  23  as electrical supply. A branch  25  provides the supply of the microprocessor  13  with energy. A branch  27  serves for providing possibly necessary energy for the process unit  7 , this means especially for the analogue/digital converter, where applicable for the amplification, the tare process, the filtering or for the temperature measuring. Branch  29  symbolize the supply of the measuring bridge  1  and is understood only schematically to show the energy path. The supply of the measuring bridge  1  can for example be provides by the processing unit  7 . To ensure the potential separation between bus line and measuring bridge, a DC/DC-converter  9  is provided, which is provided in one embodiment as toroidal core transformer. 
         [0039]    The whole arrangement of  FIG. 1  except the bus connection  21  can be comprised within one housing, so that a compact and rugged arrangement is guaranteed. 
         [0040]    The applied coils of the toroidal core transformer may comprise for example 10 to 20 windings, whereas on the secondary side are to coils and on the primary side three coils. On the secondary side are these coils connected for two-way rectification and on the primary side for producing an oscillator effect. Also possible are other embodiments of toroidal core transformers with for example only one primary coil and only one secondary coil. The primary coil and the secondary coil are spatially separated in a way that the coupling capacity results in less than 10 pF. The primary coil on the one hand and the secondary coil on the other hand may have a special arrangement, to guarantee effectively a spatial separation. For this reason the windings of the separate coils can be concentrated provided on the toroidal core in a way that the wires of the separate coils are arranged mainly not side by side but one upon the other, so that the distance between the primary and the secondary coil on the toroidal core is as far as possible. 
         [0041]    A specially preferred embodiment comprises a toroidal core transformer with a special arrangement of the shielding as shown in  FIG. 2 . This figure shows the toroidal core transformer  41  in detail. The toroidal core  77  comprises for example a core material with high specific resistance of 100 kOhmmeter, e.g. the core material M13 of Epcos Company. Arranged on that is a secondary winding coil  45 , which comprises three windings in the shown embodiment example. The primary coil  43  comprises also three windings, whereas the number of windings can be adopted to the necessary facts. The primary coil  43  is shielded by electrical shieldings  47  and  49 , whereas the second shielding  49  is provided two-part. The first shielding  47  is over a thin insulating layer  75  arranged on the primary coil  43  in a way that there results in a narrow capacitive coupling C 1 . This arrangement is as inside as outside surrounded by thick insulating layers  71  and  73 , which are again surrounded by the two parts of the second shielding  49 . Therefore has the second shielding  49  a smaller capacitive coupling C 2  to the primary coil. 
         [0042]    This thin insulation  75  is for example 0,1 mm thick, but comprises an insulation rigidity of for example 5 kV. The shieldings  47 ,  49  can for example comprise copper foils with insulations that there will not arise a short circuit winding when the foil overlaps. 
         [0043]      FIG. 3  shows a circuit arrangement of one embodiment of a load measuring device according to the present invention which makes visible how the DC/DC converter  9  is equipped with such a toroidal core transformer  41 . One recognizes in particular the primary coil  43 , the secondary coil  45  and both shieldings  47  and  49 . Numeral  51  denotes the resultant capacity C 1  between the primary coil  43  and the first shielding  47  and numeral  53  denotes the resultant capacity C 2  between the second shielding  49  and the primary coil  43 . 
         [0044]    The primary coil  43  is connected with the bus line  55 , by this way corresponding particularly to part of the primary side of the energy path  23  ( FIG. 1 ). The secondary coil  45  is via a voltage regulator  67  with rectifier connected to the measuring bridge  1 , which comprises in described manner strain gages, which are arranged at the inner surface of the cover of a load cell. Especially recognizable is that one connection of the measuring bridge can be grounded, which is implied by the electric ground  65 . This grounding possibility is advantageous especially for sensor elements, which have a close coupling with ground due to physical reasons. 
         [0045]    The circuit of  FIG. 3  comprises an active shielding control. For this is provided an operational amplifier  57 , e.g. an operational amplifier AD628 of Analog Devices Company, which is supplied by the secondary side  45  of the toroidal core transformer  41 . Should the regulator range of the operational amplifier  57  not be sufficient, can the normal provided supply voltage be increased for example by an additional secondary coil. The voltage divider with the resistances  61  and  59 , which for example have the value each 100 kOhm leads the output of the operational amplifier  57  to the half of the supply voltage, so that the output can be driven in positive or negative direction. The first shielding  47  will be kept via a resistance  63  with for example 100 MOhm at the potential of halve of the supply voltage. The operational amplifier is provided with a high input resistance and a internal protection circuit against over voltage. Especially by the use of digital isolators for the transfer of the digital measuring signals and digital control signals respectively and by the use of a DC/DC converter  9  in the supply path it is guaranteed, that there is an effective potential separation with small coupling capacity along the imaginary line  35 . Possible high capacities of the measuring device comprising strain gages and out of this possibly resulting negative influences by unsymmetrical irradiations will be “faded out” along the imaginary line  35 , so that the negative influence on a bus system will be reduced and avoided respectively, which will be connected to the bus connection  21  ( FIG. 1 ) and the bus line  55  ( FIG. 3 ) respectively. This allows in particular the use of an AS-Interface device network. 
         [0046]    The microprocessor  13  can be implemented in a way that a special analysis of the measuring signal of the measuring bridge  1  provides a error detection of the strain gages with which one can detect for example a coming off from the underground or a wire breakage. There can be provided a software which determines important parameters of the strain gages  3 , e.g. bridge unbalance, characteristic line errors and temperature errors and compensates these errors by software. Further can the measuring signals by software be smoothed and additional disturbing signals be eliminated. Further on it is possible to report errors of the measuring device and of the environment conditions to the bus master. 
         [0047]    Because of the use of internal voltage converters the provided circuit of the load measuring device according to the present invention is suitable for network-independent battery supply for any place of use. For this purpose it is possible to switch by software the microprocessor into a power saving mode. 
         [0048]    The software can also be organized in a way which allows an interval operation so that for example only one measuring value per hour is evaluated, and that these measuring values will be later time corresponding read out. This is especially advantageous with battery supply because of reduced energy expense. 
         [0049]    The microprocessor comprises a boot sector which allows at any time a software update from outside. This is especially advantageous since for example load cells are sealed hermitically. 
         [0050]    The invention provides the integration of a digital interface especially an AS-Interface within a load cell. The digital interface allows the direct connection to separate strain gages or to strain gages bridge circuits. Special embodiments guarantee a small coupling capacities between bus system and measuring device so that a high electromagnetic compatibility (EMC) is guaranteed. By “complete fading out” of the ground it is possible to run a measuring device at a digital bus, even when the measuring device has a strong galvanic or capacitive connection with the housing or the ground and therefore not being suited for the bus operation or only with high expense. This is especially important with load measuring devices and load cells respectively, which are provided with strain gages which are attached to the inner surface of the cover or housing of load cell which is grounded. 
         [0051]    The term “load measuring device” which is used in this context comprises especially pressure measuring cells, force measuring cells and load cells. These serve for example to monitor the weight of tanks or of reinforcements in the scope of civil engineering or of components of mechanical engineering. Adequate provided load measuring devices according to the present invention are also suitable as sensors for safety engineering, e.g. as emergency stop switches and/or for safety doors, if applicable for example up to the category 4 of the norm DIN EN954-1. 
       REFERENCE SIGN LIST 
       [0000]    
       
           1  strain gage measuring bridge 
           3  strain gages 
           5  electrical feed line 
           7  processing unit, comprising an AID-converter 
           9  DC/DC-converter 
           10  air-coil transformer 
           13  microprocessor 
           15  real time clock 
           17  bus component 
           19  signal path 
           21  bus connection 
           23  energy path 
           25 , 27 , 29  branch from energy path 
           31  battery back up 
           33  bus interface boundary 
           35  potential separation line 
           41  toroidal core transformer 
           43  primary coil 
           45  secondary coil 
           47  first shielding 
           49  second shielding 
           51  capacity C 1   
           53  capacity C 2   
           55  bus line 
           57  operational amplifier 
           59  resistance 
           61  resistance 
           63  resistance 
           65  electrical ground 
           67  voltage regulator with rectification 
           71 , 73 , 75  insulation layer 
           77  toroidal core 
       
     
         [0084]    While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them. Many variations, modifications, additions and improvements of the embodiment described are possible.