Abstract:
Individual measuring sensors each have a different specific signal type and must therefore be appropriately connected to the analog measuring inputs. An adjustable voltage with adjustable current limitation, or an adjustable current with adjustable voltage clamping is used. The voltage or current is connected to the measuring sensors and the corresponding signal is detected and classified according to the sensor type. This permits the automated integration of the measuring sensor. The corresponding assembly can form part of the complete module.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on and hereby claims priority to PCT Application No. PCT/EP2004/05271 filed on Oct. 29, 2004 and German Application No. 10351356.6 filed Nov. 4, 2003, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a method for identifying analog measuring sensors in measurement and automation technology circuits where each of the individual measuring sensors has a specific signal type. Besides this, the invention also relates to an associated device. 
   Devices for measurement and automation technology frequently have analog measurement inputs. The common signal types here are +/−10 V, +/−20 mA, 4 to 20 mA, 50 mV (thermocouples) or resistance measurements, for example for PT 100 or PT 1000Currently, the signal type is mostly set manually on the automation equipment, whether by hardware using switches/coding plugs or by means of software using electronic switches. Any incorrect setting generally leads to a malfunction, in the worst case indeed to the destruction of the measurement input. 
   With the related art, therefore, the signal type is prescribed. This means that in order to avoid errors and damage, the correct setting must be checked. 
   U.S. Pat. No. 6,115,654 A discloses a sensor/interface system, and an associated operating method for this system, with which it is possible to recognize individual sensors when different sensors are connected to the system and all the sensors are activated simultaneously. For this purpose, the sensor signals are input into a data processing device. This system is used, in particular, with permanently installed sensors in an aircraft, to enable faults which are present in the system to be recognized and/or incipient faults to be predicted in good time, by the monitoring effected during the operation of the system. 
   SUMMARY OF THE INVENTION 
   Starting from this last related art, it is one possible object of the invention to specify a method, using which measurement sensors can be automatically recognized in respect of their signal type. Besides this, it is to devise a device with which the method can be carried out in a simple manner. 
   The inventor proposes a method and device to automatically recognize signal type so that a correct setting can be made, also automatically, or, in the event of an incorrect setting, a warning message generated. 
   The method is based on the making of measurements on the sensor which is connected, from which the different current/voltage characteristic curves of he sensor types are recognized. For these measurements, an adjustable voltage source with a current limiter which can also be adjusted, or an adjustable current source with adjustable voltage limitation, are both suitable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
       FIGS. 1 and 2  show circuit diagrams of the measuring equipment for measuring sensor characteristics, 
       FIG. 3  shows a summary of different characteristic curves, and 
       FIGS. 4 and 5  show decay curves for cooling processes for thermocouples with unipolar and antipolar responses. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     FIGS. 1 and 2  represent circuit diagrams of the test electronics. In these,  1  represents an adjustable voltage source and  2  and adjustable current limiter,  3  an adjustable current source and  4  an adjustable voltage limitation, with connections being provided for the sensor which is to be tested. The two alternatives are equivalent. For the measurements, an adjustable voltage source with a current limiter which can also be adjusted, or an adjustable current source with adjustable voltage limitation, are both suitable. To measure the characteristic curve, the voltage or the current should be varied, and both measurement values recorded at the terminals to the sensor. 
     FIG. 3  summarizes the characteristic curves,  11  to  16 , for the relevant signal types from measuring sensors other than thermocouples, which are discussed further below: the plot shows the sensor current I in milliAmperes (mA) along the abscissa against the sensor voltage U in Volts (V) on the ordinate. It can be clearly seen that all the characteristic curves are different, and thus can be unambiguously detected. The case which is hardest to recognize for the voltage-current sensors, a zero signal, is shown. In the case of a non-zero signal, the corresponding flanks are offset. For unipolar sensors, the inflection in the characteristic curve starts at zero. A characteristic which can also be unambiguously detected. 
   In order to recognize thermocouples, the decay curves must be analyzed. The recognition of thermocouples is effected through the response of the element to excitation by a current.  FIGS. 4 and 5  show the oscillograms of such an excitation with different polarities, with the abscissa indicating the time in seconds and the ordinate the. voltage in volts. The large time constant of the decay process and the reversal of the polarity can be clearly seen in  FIG. 5 . 
   In what follows, details are given of the different application possibilities. 
   The distinguishing characteristic of voltage sensors which correspond to the characteristic curve  15  is that they apply a defined voltage to the measurement input up to the current limit, which is due to the technical realization using electronic circuits. The common sensors generally have an output voltage range of 0 to 10 V or (bipolar) of +/−10 V. The maximum current which these sensors can supply lies mostly in the range of 5 mA up to around 50 mA. The method described recognizes such a voltage sensor by injecting a variable current into the sensor (e.g. from −100 mA up to +100 mA). While this is done, the voltage at the terminals is monitored. If a voltage sensor is connected, it will hold the terminal voltage almost constant in the region of its current supply capability, because its internal resistance is small, and then at the current limit there is a step increase in the voltages. Alternatively, it is also possible to use a variable voltage source with current limitation for the measurements. In this case, the current consumption of the sensor will switch over abruptly when the sensor voltage is exceeded. 
   The distinguishing characteristic of current sensors which correspond to the characteristic curve  16  is that they inject a defined current into the measurement input up to their voltage limit, which is due to the technical realization using electronic circuits. The common sensors generally have an output current range of 0 to 20 mA, 4 to 20 mA or (bipolar) of +/−20 mA. The maximum voltage which these sensors can supply is mostly less than +/−15 V. The method described recognizes such a current sensor by applying a variable voltage to the sensor, e.g. from −100 mA up to +100 mA. While this is done, the current at the terminals is monitored. If a current sensor is connected, then in the region of its output current it will cause a step change in the terminal voltage, between the maximum output voltage values. The measurement of the characteristic curve  16  can also be effected by connecting up a variable voltage source and monitoring the output current. 
   Resistive sensors can also be detected using a variable voltage or current source. This gives characteristic curves,  13  or  14  as applicable, which are nearly linear over the entire range. From their slope it is then also possible to distinguish different types, e.g. PT100 or PT1000. Four-wire measurements can also be made, by connecting the test signal to the supply wires and using the other wires for the test measurements. 
   Line short-circuits corresponding to the characteristic curve  12  have the same characteristics as a voltage source of 0 V with a low internal resistance, with the difference that no voltage limitation occurs across the measurement range. 
   A line break, corresponding to the characteristic curve  11 , has the same response as a current sensor with 0 mA output current, i.e. high internal resistance, with the difference that no current limitation occurs across the measurement range. 
   Thermocouples have a response which is initially similar to a relatively high-resistance voltage source, a far distant line short-circuit, or even a low-resistance resistive sensor (PT100). They are distinguished by selective excitation of the thermoelectric effect, using an injected current. This current causes warming of one connection point, e.g. the measurement location, and a cooling of the other point, e.g. the compensation location. The site of the warming/cooling is swapped by reversing the polarity of the excitation current, so as to exploit the Peltier effect. 
   After the excitation current is switched off, the response of the thermocouple can be detected, this taking the form of a decaying voltage source with a time constant of around one to  10  seconds or more, producing either the characteristic curve  41  shown in  FIG. 4  or the characteristic curve  51  shown in  FIG. 5 . Depending on the thermal time constant of the two thermoelectric junctions, the polarity is the same as the excitation voltage in the case of  FIG. 4  and the opposite for  FIG. 5 . 
   Automatic recognition of the signal type for measuring sensors has the following advantages:
         avoidance of any damage to the measurement inputs   avoidance of malfunctions in the plant, and hence avoidance of damage to the plant   shorter times to put plant into service, due to the automatic diagnosis   recognition of faults in connected measuring transducers, line breaks and short-circuits   the normal commercially-available sensors can be detected with no specific enhancements.       

   The recognition or identification of the measuring sensors can in practice be carried out directly where they are used. The measurement device for this purpose can be realized as a separate device or can equally well be integrated into the module which is to be used. This results in considerable simplifications for practical use, because staff do not need to carry out separate checks on the individual measuring sensors, but rather can make the connections without testing them. The testing then takes place in the plant containing the modules. 
   The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).