DE10120822C2 - Inductive displacement sensor with linear characteristic behavior - Google Patents

Description

The invention relates to an inductive Position transducer.

An inductive displacement sensor is from FR 2 682 760 A1 known. With this displacement sensor is on an electrically insulating rectangular Carrier is a slider that can be moved along the carrier arranged, which surrounds the carrier in a U-shape and the serves as a measuring head for distance measurement. The measuring head is there made of a ferromagnetic material. On the carrier are a rectangular electrical  Excitation loop and a V-shaped Measuring loop applied. One to the pathogen loop applied AC voltage induced after the Law of induction on the one developing in the measuring head magnetic circuit and an AC voltage over air in the measuring loop. Because of the constantly changing geometric shape of the measuring loop along the Displacement path of the measuring head and that over the Displacement does not change shape of the Excitation loop changes the output signal of the Measuring loop also continuously with a shift of the measuring head along the carrier.

DE 39 13 861 A1 is a similar one Displacement sensor described, in which the measuring head however, an electrically active (inductive) element has, which is part of an externally excitable Resonant circuit is. In addition to the measurement loop a reference loop is provided for observation a control voltage of the resonant circuit is used and by means of changes in flux density, for example due to a change in voltage AC source or changes in Ambient temperature, can be detected. hereby can be the flux density through the inductive element generated magnetic field can be kept constant. The inductive element induces in the Measuring loop a measuring voltage and in the Reference loop a reference voltage.  

Another displacement sensor with an active measuring head is known from DT 26 17 624 A1.

An inductive displacement sensor with an electrically active one Measuring head as well as a reference loop is also in the DE 100 16 540 C1 is disclosed. Both the measuring loop and the reference loop is also there as Evaluation loops are used, the measurement signal being off the quotient of measurement voltage and reference voltage is formed. By forming the quotient in particular temperature drifts and changes in position of the Computationally eliminated measuring head.

A disadvantage of those from DE 39 13 861 A1, DT 26 17 624 A. and DE 100 16 540 C1 emerging It is displacement sensors that the measuring head is active and needed a constant supply of energy that only through moving, carried lines is feasible. Further the constant supply of energy does not lead to one insignificant power consumption. Beyond that arises thereby a not inconsiderable Energy dissipation.

The disadvantage of that described in FR 2 682 760 A1 Arrangement is that temperature influences and Changes in position of the measuring head to unstable measured values lead, that the suggestion is continuous and high operational performance for usable signals is required.  

Furthermore, it is not ensured that the influence the air coupling between the excitation and measurement loops remains constant and therefore as not changing Constant can be subtracted from the measured value.

It is therefore the object of the present invention an inductive displacement transducer of the type described, to further develop that this one measurement signal as stable as possible over a wide range Temperature range and / or operating voltage range without significant interference from mechanical Delivers tolerances while maintaining a low Operating power required, with energy dissipation should be kept as small as possible. Furthermore should also have a simple, technical structure achieved the best possible cost / benefit ratio become.

This object is achieved by the features of claim 1. Advantageous embodiments of the invention are the subject of the dependent claims. In the case of an electrically passive measuring head, the invention provides a reference loop shown in FIG. 1, the amplitude of the magnetic flux passing through it being kept constant by a control process during the measuring process. The measurement voltage thus remains unaffected by the disturbance variables mentioned. The proposed passive measuring head does not require any moving parts or electrical leads and is therefore inexpensive and extremely reliable. The proposed pulsed operation advantageously results in reduced energy dissipation in the various induction processes and thus enables an overall lower power consumption.

Because of the existing regulation by the reference coil the air coupling into the measuring coil remains constant and can be compensated as a measurement path independent size become. The resulting quotient formation The measurement voltage and reference voltage therefore provide immediately the desired measurand.

In a preferred embodiment, the in Reference loop induced reference voltage or the resulting reference current by means of a regulation, preferably a PI control, kept constant. This Measure ensures particularly stable operation of the displacement sensor. By means of the regulation works the displacement sensor is completely autonomous and delivers immediately, without that in the prior art required arithmetic evaluation step, the desired measurement signal.

The also proposed low-resistance operation of the Measuring loop or reference loop has the advantage that the displacement sensor and the circuit arrangement become less sensitive to external Interferences, such as B. electromagnetic or capacitive interference.

From the following description of drawings illustrated embodiments result further features and advantages of the invention, wherein identical or functionally the same features identical reference numerals.

The following show in detail:

Fig. 1 is a perspective view of a Wegmessaufnehmers invention;

Fig. 2 is a schematic plan view of an array of excitation, measuring and reference loop according to the invention;

Fig. 3 is a block circuit diagram of a circuit arrangement according to the invention a Wegmessaufnehmers operating in the Figures 1 and 2 shown.

Fig. 4 characteristic curves measured on a displacement transducer according to the invention voltage over measuring head displacement with PI controller stage.

The displacement sensor shown in FIG. 1 has a measuring head 10 (position transmitter) which is designed to be displaceable in an x direction shown along an arrangement of conductor loops, specifically an excitation loop 20 , a measuring loop 30 and a reference loop 40 . However, the linearly displaceable measuring head 10 represents only one exemplary embodiment and can describe any displacement curve, for example a circular or elliptical displacement curve.

The measuring head 10 consists of a magnetically permeable material, in the present example a ferrite core, but can also be formed from a material with ferromagnetic properties. When an electrical alternating voltage 50 is applied to the excitation loop 20 , a magnetic field, indicated by magnetic field lines 60 , is formed, the flux lines of which penetrate the inside of the excitation loop 20 and the inside of the measuring loop 30 and the reference loop 40 and in the area of the ferrite core 10 due to the magnetisability of the ferrite material be bundled or concentrated in a known manner. The magnetic circles which form in the area of the ferrite core 10 therefore penetrate the measuring loop 30 and the reference loop 40 in this area to a greater extent. Since the measuring head 10 only appears passively, this is a so-called "passive measuring head".

In contrast to the reference loop 40 , the measuring loop 30 has a triangular course along the distance measuring section x. Therefore, the proportion of the magnetic field lines penetrating the measuring loop 30 at the level of the ferrite core 10 varies approximately linearly with the distance x, whereas in the case of the reference loop 40 the proportion is constant over the distance x.

The position x can thus be calculated from the linear deviation of the measurement current induced in the measurement loop 30 after prior calibration. By forming the quotient of the measured current 12 determined in the measurement loop and the determined reference current 13 in the reference loop, interferences occurring in the measurement loop and the reference loop can be largely eliminated at the same time.

FIG. 2 schematically shows the electrical measuring arrangement of the displacement sensor shown in FIG. 1. The measuring loop and the reference loop are operated with low impedance, the impedances R2 and R3 being only a few ohms with R2.3 <= 10 ohms, e.g. B. 2 ohms, correspond to the working frequency. Because of the negligible voltage drop across the measuring and reference loop, this results in a pure current measurement operation of the arrangement shown, U1 the voltage applied to the excitation loop, i1, i2, i3 the currents flowing in the respective conductor loops and R1, R2, R3 the respective ones Impedances of the individual loops mean. Due to this low-resistance operation of the displacement sensor, it is almost insensitive to external interference such as EMC interference.

When an electrical alternating voltage is applied to the excitation loop 20 , specifically in the present example a square-wave signal which oscillates for only about 1 μsec and oscillates with about 10 kHz, the excitation loop 20 (briefly) builds up a magnetic field, which is concentrated via the ferrite core and couples into both the measurement and the reference loop. The coupling via the ferrite core takes place in the measuring loop 30 in a surface segment S2 and in the reference loop 40 in a surface segment S3. In addition, the current loops couple one another via air, that is to say in particular the excitation coil 20 via air into the measuring and reference coil and, by means of a mutual induction, the measuring loop 30 and the reference loop 40 mutually, the contribution of the mutual induction of the reference loop 40 into the measuring loop 30 the direct influence on the actual measurement signal dominates.

The circuit arrangement shown in FIG. 3 has a low voltage supply 100 for the electrical supply of the excitation loop 20 . The voltage supply 100 is designed in particular as a rectangular pulse generator which provides voltage pulses with a pulse duration of approximately 0.5-5 μsec and a signal frequency of approximately 5-50 kHz. This voltage applied to the excitation loop 20 is set by means of the output signal 115 of a proportional / integral (PI) controller 110 such that the reference current amplitude î3 (see FIG. 2) is constant during the measuring process.

Of the controller 110 to be supplied actual value is measured at the reference loop 40 reference current amplitude I3 is regulated to a predefinable desired value. The reference current is first evaluated via a sample circuit 140 with regard to the current peaks occurring during the pulse operation. The output signal supplied by the sample, namely a voltage value, is amplified by means of an amplifier stage 120 and finally fed to the controller 110 , where it is compared with a reference voltage and the current in the exciter is thus re-regulated.

The measurement current amplitude î2 measured on the measurement loop 30 is likewise first evaluated using a further sample 150 , the resulting voltage value is then amplified and is then available at the output 160 as a measurement signal.

Finally, FIG. 4 shows an error profile measured on a displacement transducer according to the invention, ie the difference between the setpoint and actual value of the measurement voltage present at output 160 in FIG. 3 over the measuring head displacement x.

Advantageously, both of the above inductive displacement transducer as well as the one described above Circuit arrangement simultaneously on a single Printed circuit board arranged and thus form a unit.

Claims (10)

1. Inductive displacement sensor with a displaceable, magnetically permeable passive measuring head ( 10 ), with at least one measuring loop ( 30 ), the geometric shape of which changes depending on the displacement path of the measuring head ( 10 ), with at least one excitation loop ( 20 ), by means of the in a magnetic flux ( 60 ) can be generated for the measuring head ( 10 ), which passes through the measuring loop ( 30 ) at every point of the displacement path in the area of the measuring head ( 10 ) and induces an electrical measuring signal ( 12 ), and with at least one reference loop ( 40 ) , the geometric shape of which does not change over the displacement path and which is penetrated by a magnetic flux ( 60 ) at each point of the displacement path, in the area of the measuring head ( 10 ), the reference current amplitude passing through the reference loop ( 40 ) being replaced by that at the excitation loop ( 20 ) applied voltage is kept constant, the excitation loop ( 20 ) with a pulse excitation voltage / current is applied. 2. Displacement sensor according to claim 1, characterized in that the excitation loop ( 20 ) is pulsed with a excitation voltage / current which oscillates in the preferred range of 0.5-5 µsec and preferably oscillates in the range of 5-50 kHz. 3. Displacement sensor according to claim 1 or 2, characterized in that the excitation loop ( 20 ) is acted upon by a rectangular excitation voltage / current. 4. Displacement sensor according to one of claims 1 to 3, characterized in that the measurement signal of the displacement sensor ( 10 ) results from the quotient of the measurement voltage amplitude or measurement current amplitude (î2) and reference voltage amplitude or reference current amplitude (î3). 5. Displacement sensor according to one of the previous ones Claims, characterized in that the induced reference current amplitude (î3) using a regulation, in particular a PI regulation, can be regulated to a predeterminable value. 6. Displacement sensor according to one of the preceding claims, characterized in that the measuring loop ( 30 ) and / or the reference loop ( 40 ) are operated or operated with low resistance ( 70 , 80 ). 7. Displacement sensor according to one of claims 1 to 6, with an electrical circuit arrangement comprising a voltage / power supply ( 100 ) for the excitation loop ( 20 ), characterized by a control stage ( 110 ), the output signal of the reference loop ( 40 ) of the control stage ( 110 ) is supplied as the actual value, the actual value is compared with a predefinable target value, and the output of the control stage ( 110 ) is connected to the input of the voltage / power supply ( 100 ) of the excitation loop ( 20 ). 8. Displacement sensor according to claim 7, characterized in that the control stage ( 110 ) is designed as a PI controller. 9. Displacement sensor according to claim 7 or 8, characterized in that the measuring loop ( 30 ) and / or the reference loop ( 40 ) are closed or closed via an impedance ( 70 , 80 ) of preferably 0.1-10 ohms. 10. Displacement sensor according to one of claims 7 to 9, characterized in that the Circuit arrangement on a circuit board is arranged together with the displacement sensor.