Abstract:
A movement distance sensor operates as a differential transformer with a primary coil, at least two secondary coils. A circuit configuration is connected to the secondary coils in order to determine a movement distance. The circuit configuration includes a multiplexer, an A/D converter, and a digital logic module. The multiplexer is connected to the first and the second secondary coil via an input stage. The output of the multiplexer is connected to the input of the A/D converter and the output of the A/D converter is passed to the input of the digital logic module. The latter determines the movement distance from the input signals received alternately from the multiplexer. The measurement configuration exhibits high reliability with low error tolerances.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority, under 35 U.S.C. § 119(e), of provisional patent application No. 60/710,253 filed Aug. 22, 2005; the application further claims the priority, under 35 U.S.C. § 119(a), of German patent application No. 10 2005 035 316.9, filed Jul. 28, 2005; the prior applications are herewith incorporated by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a measurement configuration having a movement distance sensor which operates on the principle of a differential transformer and has a primary coil as well as a first and a second secondary coil, and having a circuit configuration whose circuitry is connected to the first and second secondary coils in order to determine a movement distance. 
   A movement distance sensor operating on the principle of a differential transformer, also referred to as a VDT (variable differential transformer), is suitable for direct measurement of linear movements with high resolution in the range between a few thousandths of a millimeter and more than one meter. Owing to the high accuracy and the robustness, movement distance sensors such as these are used in particular for mechanical position determination in the field of aviation. By way of example, exact position determination is essential for control of the landing flaps or control surfaces. However, movement distance sensors such as these are also worth using for determination of the position of doors, in particular for an automatic door control system. 
   A movement distance sensor operating on the principle of a differential transformer comprises a primary coil as well as two secondary coils which are wound on an appropriately designed cylindrical body with a central hole. In this case, the primary coil is wound centrally on the cylindrical body, and the two secondary coils are wound in front of and behind the primary coil, so that this results in a secondary coil/primary coil/secondary coil sequence along the cylindrical body. A core composed of ferromagnetic material can move freely in the central hole, and can be connected to the object to be measured via a connecting element. 
   The length extent of the ferromagnetic core is defined such that its position within the central hole influences the magnitude of the magnetic flux between the primary and secondary coils. When the core is moved from its rest position in the center of the cylindrical body, that is to say at the same level as the primary coil, in the direction of one of the secondary coils, then this increases the magnetic flux between the primary coil and that secondary coil in whose direction the core has moved. In a corresponding manner, the magnetic flux is reduced for that secondary coil from which the core has moved away. If a sinusoidal excitation voltage is fed to the primary winding, an alternating voltage is induced in the secondary coils, whose magnitude is dependent on the axial movement of the core. The two secondary voltages induced in the secondary coils form the output signal of the described movement distance sensor, and are evaluated in order to determine the position of the core which can move in the central hole. 
   Normally, the output signals from a VDT sensor, that is to say the voltages induced in the secondary coils, are processed by means of analog circuits or by means of analog modules in order to determine the distance of movement. Since the position of the movement distance sensor, that is to say the change in position of the ferromagnetic core with respect to its rest position, can be calculated from the difference between the absolute values of the two secondary voltages, it is known, for example, for the output signals from a VDT sensor to be applied to a rectifier. Analog modules are also normally used to form the difference between the two rectified secondary voltages. A measurement configuration with evaluation circuits such as these for a VDT sensor disadvantageously has inadequate reliability and error tolerances which can be improved, for applications in aviation. 
   SUMMARY OF INVENTION 
   It is accordingly an object of the invention to provide a measurement configuration, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which has a high degree of reliability and small error tolerances in the determination of the movement distance. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a method of measuring a movement distance. The novel method comprises the following steps: 
   providing a measurement configuration with a variable differential transformer assembly having a primary coil, first and second secondary coils, and an actuating element movably disposed for influencing a voltage induced into the secondary coils; 
   connecting the primary coil to an alternating voltage; 
   measuring a first input voltage at the first secondary coil and a second input voltage at the second secondary coil by scanning the first and second input voltages multiple times during a period of the alternating voltage; 
   passing the multiple-scanned first and second input voltages to a digital logic module; 
   determining a rectified average value of the first input voltage and of the second input voltage in the digital logic module; and 
   calculating a relative position of the actuating element from the average values of the first and second input voltages. 
   In accordance with an added feature of the invention, the method comprises sequentially scanning each of the first and second input voltages and passing the first and second input voltages through a multiplexer and an analog-to-digital converter to the digital logic module. Preferably, the movement distance of the actuating element is determined in the digital logic module from the input signals received alternately from the multiplexer. 
   In a preferred mode of the method, the calculating step comprises calculating: 
           Q   =         V   1     -     V   2           V     1   ,   rms       +     V     2   ,   rms                 
where Q is a normalized movement distance, V 1  and V 2  are the absolute magnitudes of the first and second input voltages induced in the first and second secondary coils, respectively, and V 1,rms  and V 2,rms  represent root mean square values of the first and second input voltages.
 
   In accordance with an additional feature of the invention, the digital logic module provides offset correction for the input signals received alternately from the multiplexer. 
   With the above and other objects in view there is also provided, in accordance with the invention, a measurement configuration, comprising: 
   a variable differential transformer assembly including a primary coil, a first secondary coil and a second secondary coil, an actuating element movably disposed for influencing a voltage induced in the secondary coils, and a circuit configuration connected to the secondary coils for measuring a displacement of the actuating element;
 
the circuit configuration including a multiplexer connected to receive and to sequentially output a signal representing the voltages induced in the first and second secondary coils, an A/D converter for outputting a digital signal, and a digital logic module connected to receive and to process the digital signal;
 
the digital logic module being configured to calculate from the digital signals an amount of displacement of the actuating element.
 
   In other words, the measurement configuration has a movement distance sensor which operates on the principle of a differential transformer and has a primary coil as well as a first and a second secondary coil, and has a circuit configuration whose circuitry is connected to the first and second secondary coils in order to determine a movement distance. The objects of the invention are achieved in that the circuit configuration comprises a multiplexer, an A/D converter and a digital logic module, the multiplexer in each case being connected to the first and the second secondary coil via an input stage, with the circuitry of the output of the multiplexer being connected to the input of the A/D converter, and with the output of the A/D converter being passed to the input of the digital logic module which is designed to determine the movement distance from the input signals received alternately from the multiplexer. 
   A first step of the invention is in this case based on the idea that the reliability of a circuit configuration and hence of a measurement configuration as such can be improved by reducing the number of modules required. In a circuit configuration for a VDT sensor, this can be achieved by passing the output voltages from the two secondary coils to a common measurement channel as early as possible. This is done by in each case connecting the first and second secondary coils to a multiplexer via an input stage. Since the multiplexer switches backward and forward alternately between the induced voltages from the first and second secondary coils, modules in a simple form can be used for measurement of both voltages in the circuit configuration following the multiplexer. The use of separate input stages is necessary for level matching. Reducing the number of modules improves the overall reliability. 
   In a further step, the invention is based on the idea that the use of a digital logic module for determination of the movement distance makes it possible to avoid the use of software. This reliably avoids any software-dependent errors. The logic operations to be carried out in a digital logic module are predetermined such that they are fixed. By way of example, a CPLD (Complex Programmable Logic Device), an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit) is suitable for a logic module such as this. Apart from this, when using a digital logic module, there is no need for a digital computer, which in turn conceals a certain risk of failure. 
   Finally, in a third step, the invention identifies the fact that the measurement accuracy of the circuit configuration can be improved by using identical modules to carry out as many of the logic operations that are required for determination of the movement distance as possible. In the present case, this is achieved by passing the two induced voltages from the secondary coils to a common measurement channel as early as possible. Since, in terms of circuitry, the output of the multiplexer is connected to the input of the A/D converter, this also results in an increase in the measurement accuracy, since all the logic operations are carried out digitally. This is because this avoids any offset, which is naturally caused by analog modules, in the further voltage signals derived from the two induced voltages. Since the movement distance is determined by subtraction, any offset such as this reduces the measurement accuracy. 
   In order to determine the movement distance, the multiplexer passes the voltages induced in the first and second secondary coils alternately to the input of the A/D converter. In a corresponding manner, the digitized voltage signal from the first secondary coil and the digitized voltage signal from the second secondary coil are passed alternately to the input of the digital logic module. The movement distance can be deduced by forming the difference between the absolute values of the two voltages. Since the sum of the root mean square values of the voltages from the two secondary coils is constant irrespective of the position of the ferromagnetic core, it is normal to emit, as the output value for determination of the movement distance, the difference, normalized with respect to this sum, between the absolute values of the two induced voltages. 
   The digital logic module is advantageously designed to sample the input signals which are received alternately from the multiplexer. In this case, the measurement accuracy can be increased by increasing the sampling rate. The use of sampling makes it possible to determine the measured voltage profiles of the associated root mean square value, the respective peak value or the mean value of the rectified signal, within a predetermined time period. Both the root mean square value, the peak value or the mean value can be used to form the difference between the induced voltages, and thus to determine the movement distance. 
   It is also advantageous for the digital logic module to be designed at least for magnitude formation, for addition, for subtraction and for division of input signals, or of signals which are derived from the input signals. A logic module such as this makes it possible to form the difference between the absolute magnitudes of the induced voltages, and allows normalization of the difference with respect to the sum of the root mean square values, using the following formula: 
           Q   =         V   1     -     V   2           V     1   ,   rms       +     V     2   ,   rms                 
where Q is the normalized movement distance, V 1  and V 2  respectively denote the absolute magnitude of the induced voltages, and V 1,rms  and V 2,rms  respectively denote the root mean square values of the two induced voltages.
 
   In this case, all of the logic operations for determination of the movement distance are carried out in the logic module. It is, of course, also possible to carry out only some of the required logic operations in the logic module, and to carry out the other required operations in a different manner, for example in a downstream component. 
   In a further advantageous refinement of the invention, the digital logic module is designed to provide offset correction for the input signals which are received alternately from the multiplexer. In this case, the logic module determines any offset by integration of a known voltage profile over a predetermined time period and subtracts this from the digitized profiles of the induced voltages in order to determine the movement distance. As already mentioned, this improves the measurement accuracy. Since the offset is taken directly from the measured signal, the offset correction is carried out during operation of the measurement configuration. In the case of sampling, the integration for determination of the offset corresponds to formation of the sum of the sample values. The digital logic module is advantageously an FPGA. 
   In a further advantageous refinement of the invention, the circuit configuration comprises a monitoring unit, which is connected to the digital logic module, to the multiplexer and to a voltage generator, with the primary coil being connected to the voltage generator, and with the monitoring unit being designed to produce an excitation voltage (which is correlated in time with the clock cycle of the logic module) for the voltage generator and to switch the multiplexer, correlated with the clock cycle of the logic module. 
   One period must be sampled as exactly as possible for each determination of the root mean square value and in particular for determination of the mean value of the absolute magnitude of the induced voltages. For this reason, it is advantageous to derive the excitation voltage for the primary coil from the clock cycle of the logic module. If the multiplexer is switched such that it is correlated in time with the clock cycle of the multiplexer, then it is easily possible to detect one or more periods of the respectively induced voltages for the logic operations of the logic module. This further improves the measurement accuracy. 
   Once more in sum, the two VDT (variable differential transformer) input voltages that are induced in the secondary coils from the alternating voltage in the primary coil are sequentially scanned multiple times over a period of the a.c. voltage. The digital logic module (CPU) then forms from the two induced voltages a mean value, i.e., a rectified average voltage. The synchronization is effected by a generator (for driving the VDT) that is clocked by the CPU. The position is then calculated by forming the quotient of the difference of the voltages and the sum of the voltages. 
   Other features which are considered as characteristic for the invention are set forth in the appended claims. 
   Although the invention is illustrated and described herein as embodied in a measurement configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of the design of a movement distance sensor operating on the principle of a differential transformer; 
       FIG. 2  is a block diagram illustrating a measurement configuration with a movement distance sensor as shown in  FIG. 1 ; and 
       FIG. 3  is a graph of the waveforms for movement distance determination by way of the measurement configuration shown in  FIG. 2 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, the movement distance sensor  1  which operates on the principle of a differential transformer. The sensor may also be referred to as a displacement transducer or a variable displacement transducer. In this case, the movement distance sensor  1  comprises a primary coil  3  and two secondary coils  4  and  5 . The primary coil  3  as well as the secondary coils  4  and  5  are wound with appropriate offsets on a cylindrical body which has a central hole. An actuating element  7  with a core composed of a ferromagnetic material can move freely in the interior of the central hole. Any linear movement of an object to be measured is transmitted by means of a connecting element  8  to the actuating element  7 , in a corresponding manner to the arrows  10  that are shown. 
   Any movement of the actuating element  7  corresponding to the arrows  10  changes the magnitude of the magnetic flux between the primary coil  3  and the secondary coils  4  and  5 . If, for example, the actuating element  7  moves in the direction of the secondary coil  5 , starting from the illustrated rest position, then the magnetic flux which is responsible for the magnetic coupling between the primary coil  3  and the secondary coil  5  is increased. A sinusoidal alternating voltage is used as the excitation voltage for the primary coil  3  for this purpose. 
   In a movement distance sensor  1 , the sum of the root mean square values of the voltages which are induced in the secondary coils  4  and  5  is constant irrespective of the position of the actuating element  7 . The position of the actuating element  7 , starting from its rest position, can be deduced from the difference between the absolute values of the induced voltages. The difference is in this case generally normalized with respect to the sum of the root mean square values, so that the position of the actuating element  7  is calculated as follows: 
           Q   =         V   1     -     V   2           V     1   ,   rms       +     V     2   ,   rms                 
where Q is the normalized movement distance, V 1  and V 2  respectively denote the absolute magnitude of the induced voltages, and V 1,rms  and V 2,rms  respectively denote the root mean square values of the two induced voltages.
 
   The secondary coils  4  and  5  have respective outputs  11  and  12  in order to read the voltages induced in them, with the respective other end of the secondary coils  4  and  5  being at ground potential. For excitation purposes, the primary coil  3  has an input  13 , with the primary coil  3  also being connected with respect to ground potential. 
     FIG. 2  shows a schematic illustration of a measurement configuration  14  which comprises a movement distance sensor  1 , which operates on the principle of a differential transformer, as well as a circuit configuration  16  associated with it. For this purpose, the circuit configuration  16  is connected to the outputs  11  and  12  (which are shown in  FIG. 1 ) of the movement distance sensor  1 , at which outputs the two voltages induced in the secondary coils are produced. For excitation of the primary coil, the movement distance sensor  1  is also connected to the circuit configuration  16  via the input  13 , as can likewise be seen in  FIG. 1 . 
   The circuit configuration  16  comprises a multiplexer  18 , an A/D converter  20  and a digital logic module  22 , which is in the form of an FPGA. The voltages induced in the two secondary coils of the movement distance sensor  1  are connected by means of the outputs  11  and  12  to the multiplexer  18 , via a first input stage  25  and a second input stage  26 . Downstream from the multiplexer  18  in the circuit configuration  16 , both induced voltages are passed to a single measurement channel. For this purpose, the output of the multiplexer  18  is connected, in terms of circuitry, via a signal amplifier  29  to the input of the A/D converter  20 . The A/D converter alternately digitizes the voltages, which are passed to it from the multiplexer  18 , from the first and the second secondary coil of the movement distance sensor  1 . The digitized voltage profiles are passed to the digital logic module  22  via a 16-bit data line  34  from the output of the A/D converter  20  and via a buffer  32 . The buffer  32  is used for temporary storage of the digitized voltage profiles. The signal which is generated by the logic module  22  can be tapped off via the signal output  35  in order to determine the movement distance. 
   Furthermore, the A/D converter  20  has an associated comparison voltage generator  30  in order to provide a comparison voltage. In addition, a monitoring unit  37  is provided, is connected to the digital logic module  22 , controls the multiplexer  18  via appropriate connections corresponding to the clock cycle of the logic module  22 , and supplies an excitation voltage to the primary coil of the movement distance sensor  1  via the connection  13 . The excitation voltage is produced by a voltage generator  38  for this purpose. In the present case, the multiplexer  18  is switched using the clock cycle of the logic module  22 , and the primary coil of the movement distance sensor  1  is also supplied with an alternating voltage based on the clock cycle of the logic module  22 . 
   The use of the measurement configuration shown in  FIG. 2  to determine a movement distance is explained in more detail by  FIG. 3 . For this purpose,  FIG. 3  uses a graph to show the waveforms of various voltages measured or derived in the circuit configuration  16  shown in  FIG. 2 . In this case, the magnitude of the voltages is plotted on the ordinate, and the time on the abscissa. 
   The waveform (in each case measured for one period) of the voltages induced in the two secondary coils is shown in the upper block  39  of the graph. In order to determine the movement distance, the waveform of the first secondary voltage  42  induced in the first secondary coil is first of all measured by sampling in the digital logic module exactly for one period, corresponding to the illustrated first time window  40 . In order to allow exactly one period to be measured, the primary coil of the movement distance sensor is excited using the clock cycle of the logic module. The first sampling time  41  thus corresponds exactly to the length of one period of the first secondary voltage  42 . 
   In the second time window  45 , the multiplexer  18  is switched to the voltage induced in the second secondary coil. In order to take account of any stabilization times in the modules that are used, no further measurement is carried out by the logic module during a stabilization time  46 . Once the stabilization time  46  has elapsed, the logic module once again samples the second secondary voltage  52  induced in the second secondary coil, for exactly one period of the waveform. This is illustrated in the third time window  50 . In this case, once again, the second sampling time  51  corresponds exactly to one period duration of the second secondary voltage  52 . If the waveform of the first secondary voltage in the first time window  40  is compared with the waveform of the second secondary voltage  52  in the third time window  50 , this illustrates the phase shift between the first and the second secondary voltages  42  and  52 , respectively, resulting from the design of the movement distance sensor  1 . However, this is irrelevant to the determination of the movement distance itself. 
   Once the sampling of the second secondary voltage  52  has been completed and the multiplexer  18  has switched again, the process once again waits for the stabilization time  46  of the component to elapse, in the fourth time window  53 . 
   Once the stabilization time  46  has elapsed—as illustrated in the fifth time window  54 —the waveform of the first secondary voltage  42 ′ is measured by sampling at a subsequent time, for exactly one period. 
   The lower block  55  of the graph shows the formation of the magnitude of the voltage profiles illustrated in the upper block  39 . The profile of the magnitude of the first secondary voltage  56  and  56 ′ can thus be seen in the first and fifth time windows  40  and  54 , respectively. The profile of the magnitude of the second secondary voltage  57  is shown in the third time window  50 . 
   The difference between the absolute magnitudes of the waveform of the first and second secondary voltages is required in order to calculate the movement distance. In consequence, in the illustrated case, the movement distance can in each case be determined once the two sampling times  41  and  51  and a stabilization time  46  have elapsed. This is indicated by the sum symbols Σ that are shown. It has been found that, in terms of error tolerances, it is best to use the mean value of the sampled absolute values measured for one period in order to form the difference between the absolute magnitudes. This rectifier mean value is formed by summation of the magnitude of the sampled voltage values, after which the sum is divided by the number of sample values.