Abstract:
An inductive displacement sensor having a displaceable or rotatable, magnetically permeable measuring head, at least one measuring loop whose geometric shape varies in response to the longitudinal/rotary displacement of the measuring head, and at least one excitation loop by which a magnetic flux can be generated in the measuring head. The flux penetrates the at least one measuring loop at any point of the longitudinal/rotary displacement, essentially in the region of the measuring head, and inducing an electric measuring signal (i 2 ). A resonant circuit, which is electrically decoupled from the outside, is arranged on the measuring head and is excited in correct phase relation by a short voltage pulse after a number of, preferably, 10 to 20 free oscillations.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an inductive displacement sensor. 
   An inductive displacement sensor of the before-mentioned kind has been disclosed, for example, in DE 100 16 540 C1 and FR 2 682 760 A1. The disclosed displacement sensor comprises an electrically insulated rectangular substrate with an inductor arranged thereon, which can be displaced along the substrate, which embraces the substrate in the form of a U and which serves as a displacement measuring head. 
   The measuring head consists of a ferromagnetic material. An excitation loop of rectangular shape and a measuring loop of V-shape are provided on the substrate. When an alternating voltage is applied to the excitation loop, it induces, according to Faraday&#39;s Law, an alternating voltage in the measuring loop across the magnetic circuit forming in the measuring head and across air. Due to the geometric shape of the measuring loop, which varies continuously along the displacement path of the measuring head, and due to the shape of the excitation loop, that does not change over the displacement path, the output signal of the measuring loop likewise varies continuously as the measuring head is displaced along the substrate. 
   DE 39 13 861 A1 describes a similar displacement sensor, but with a measuring head that comprises an electrically active (inductive) element which is part of a resonant circuit that can be excited from the outside. Preferably, the inductive element consists of a core coil with a magnetically soft core of substantially E-shaped configuration. The center limb of that core coil is arranged to extend above the measuring and reference loop. 
   In addition to the measuring loop, the displacement sensor described in DE 39 13 861 A1 comprises a reference loop of, preferably, rectangular shape which serves to observe a control voltage of a resonant circuit that can be acquired from the variations in flux density occurring, for example, as a result of voltage variations of the alternating voltage source or variations in ambient temperature. This permits the flux density of the magnetic field produced by the inductive element to be kept at a constant level. The inductive element induces a measuring voltage in the measuring loop and a reference voltage in the reference loop. For keeping the reference voltage at a constant level, the alternating voltage exciting the resonant circuit can be adjusted to a predetermined threshold voltage by means of a P or a PI controller, depending on the reference voltage. 
   A displacement sensor comprising a reference loop of the before-mentioned kind is disclosed also in unpublished German Patent Applications Nos. 101 20 822.7 and 100 16 540.0. In the case of those arrangements, both the measuring loop and the reference loop are used as evaluating loops, the measuring signal being derived as the quotient of measuring voltage and reference voltage. By forming the quotient it is possible, especially, to arithmetically eliminate any temperature drifts and positional variations of the measuring head. 
   In most of the cases, the before-mentioned measuring head is connected to an external evaluation circuit via a conductor, or without a conductor, for example by optical means or a radio link. The evaluation circuit serves for computing the respective displacement distance of the inductor on the substrate as a function of the measuring voltage determined and, if desired, in addition to the before-mentioned correction by means of the reference loop. 
   It should be noted in this connection that the reference loop and the measuring loop can be arranged on substrates of any desired shape, or even without a substrate, as long as no-contact sampling by the measuring head remains possible. Preferably, a printed circuit board is used as a substrate. 
   Now, it is a disadvantage of the displacement sensor described in DE 39 13 861 A1 and DE 100 16 540.0 that the measuring head is of the active type so that, consequently, it requires a permanent energy supply which can be realized only by lines that move together with the head. 
   The measuring arrangement described in FR 2 682 760 A1 is connected with the disadvantage that temperature influences and positional variations of the displacement sensor may lead to unstable measured values. In addition, a relatively high operating power is rendered necessary by the permanent excitation required if useful signals are to be obtained. 
   In addition, the displacement sensors known from the prior art are, on principle, connected with the problem known as the “air coupling” phenomenon. The air coupling phenomenon is due to electric coupling, according to Faraday&#39;s Law, between the excitation loop and the measuring loop(s) outside the active region of the magnetizable core of the measuring head. 
   SUMMARY OF THE INVENTION 
   Now, it is the object of the present invention to improve a displacement sensor of the before-mentioned kind so that the disadvantages discussed above, especially the before-mentioned air coupling effects, can be avoided as efficiently as possible and with the best possible cost-benefit ratio. 
   The displacement sensor distinguishes itself by the fact that a passive oscillating circuit (resonant circuit) is arranged on the measuring head, which circuit is not connected with the outside by an electric conductor and in which, therefore, electric oscillations can be excited only from the outside, i.e. in the present case causally by inductive coupling in the area of the measuring head. According to the invention, the resonant circuit is excited in correct phase relation by a short voltage pulse after a number of free oscillations. The number of free oscillations is, preferably, 10 to 20. 
   It is the advantage of that arrangement that the displacement sensor can be operated in an energy-saving fashion. In addition, immediate evaluation of the measuring signal is rendered possible after the first half-wave. 
   Further, the oscillating frequency can be lower by approximately two powers of ten, compared with the 5 to 10 MHz customary in the prior art, which means that it is equal to approximately 100 kHz. This considerably facilitates evaluation or processing of the measuring signal. 
   Excitation of the resonant circuit is, preferably, effected by a rectangular excitation loop, i.e., as mentioned before, by a voltage pulse shorter than one half-wave of the respective resonant frequency of the resonant circuit. The voltage pulse excites the resonant circuit of the measuring head via the, preferably, U-shaped ferrite. At the same time it induces, due to the air coupling effect, a position-independent voltage in the measuring loops. In order to avoid the influence of the air coupling effect one does not evaluate the measuring signals during the excitation pulse so that one can be sure that the interfering effects resulting from air coupled induction have definitely died down. Only after the excitation pulse has died down will the voltage induced in the measuring loops be evaluated. This induced voltage is, therefore, based on the induction produced by the resonant circuit free from any air coupling effects. 
   Evaluation of the measuring signal is effected by in-phase rectification of the first few, preferably the second to eighth, amplitudes of oscillation. This is achieved by integrating the amplitudes of oscillation and storing them temporarily in a sample&amp;hold circuit. Thereafter, one measuring cycle is terminated, and the resonant circuit is re-excited in correct phase relation, for example at the moment of the tenth pulse. The measured value is, thus, updated at a frequency of 5 to 10 kHz. 
   In the case of the measuring head that embraces the substrate in U shape, the passive resonant circuit is, preferably, formed by at least one conductor loop arranged on each side of the substrate, which arrangement guarantees that with loop arranged on both sides of the substrate the coupling effect between the resonant circuit and those loops will be maximized. According to one embodiment of the invention, the resonant circuit comprises at least two conductor loops on each side of the substrate. 
   The proposed measuring arrangement provides the further advantages that arranging a reference loop on the substrate for the purpose described above is no longer necessary. Contrary to the oscillating frequency of 5 to 10 MHz customary in the prior art, it is now possible to select a lower oscillating frequency of, for example, 100 kHz for the excitation voltage. This considerably simplifies evaluation of the measuring signal, from a technical standpoint. 
   It should be noted that, on principle, the displacement sensor according to the invention can be employed with the same advantages in measuring arrangements with or without a reference loop and also in measuring arrangements where coupling is effected capacitively instead of inductively. 
   The displacement sensor according to the invention is operated by an external control and evaluation circuit comprising switching elements by means of which, preferably, the first 5 to 10 half-waves of identical polarity are supplied to an integrator via a phase-controlled switch, and the signals, having been integrated in that way, are stored in a sample&amp;hold circuit. The advantage of this circuit arrangement lies in the fact that the rectification process gives rise to little or even no temperature sensitivity and that relatively high signal amplitudes are obtained at the output of the integrator. Further, the updating frequency of the output value of the entire circuit arrangement is in the range of approximately 5 kHz. 
   According to another embodiment of the displacement sensor, two V-shaped measuring loops, provided in mirror-symmetrical arrangement, are preferably arranged on both sides of the substrate. This provides the possibility to use the sum voltage of the two V-shaped measuring loops, provided in inverse-parallel arrangement on both sides of the substrate, as a reference and to keep it permanently constant with the aid of a regulating circuit. The differential voltage then represents the position-dependent measured value. 
   As a result of this feature, almost full compensation of the temperature sensitivity of the evaluation electronics, especially the rectifiers and the integrator, is achieved. In addition, any mechanical variation of the transducers, for example the printed circuit and/or the ferrite, will be effectively compensated. 
   Further, the influence of any changes in distance between measuring head and printed circuit, in vertical and horizontal direction, is considerable reduced. And linearity, especially at the terminal points of the substrate, is likewise decisively improved by the before-mentioned compensation of marginal effects. 
   The invention will be described hereafter by reference to the drawing and certain embodiments from which further features and advantages of the invention can be derived. 

   
     BRIEF DESCRIPTION OF THE DRAWING(S) 
     In the drawings: 
       FIG. 1  shows a diagrammatic representation of an inductive displacement sensor according to the prior art; 
       FIG. 2  shows a perspective view of a preferred embodiment of the displacement sensor according to the invention; 
       FIG. 3  shows a block diagram of a preferred embodiment of a circuit arrangement for operation of the displacement sensor illustrated in  FIG. 2 ; 
       FIG. 4  shows a schematic circuit diagram of one embodiment of the synchronous rectifier illustrated in  FIG. 3 ; and 
       FIG. 5  shows a perspective view of a displacement sensor with a rotatable measuring head. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The displacement sensor shown in  FIG. 1  comprises a measuring head  10  (position transducer) which can be displaced, in the indicated x direction, along an arrangement of conductor loops, namely an excitation loop  20 , a measuring loop  30  and a reference loop  40 . It should be noted in this connection that the measuring head  10 , which is adapted to be linearly displaced, is to be regarded only as one possible example and that, on principle, any displacement curve is possible, including for example a circular or elliptical displacement curve. 
   The measuring head  10  consists of a magnetically permeable material, in the present case of ferrite (ferrite core), although it may also be made from a ferromagnetic or ferrimagnetic material. When an electric alternating voltage  50  is applied to the excitation loop  20 , a magnetic field, indicated by magnetic lines of force  60 , forms whose flux lines penetrate the inner region of the excitation loop  20  and the inner regions of the measuring loop  30  and the reference loop  40 , and will be bundled or concentrated in the known way in the region of the ferrite core  10 , due to the magnetizability of the ferrite material. The magnetic circuits forming in the region of the ferrite core  10 , therefore, penetrate the measuring loop  30  and the reference loop  40  more strongly in that region. Given the fact that the measuring head  10  has a passive function only, it can be described as a “passive measuring head”. 
   Contrary to the reference loop  40 , the measuring loop  30  follows a triangular course along the measuring distance x. Consequently, the portion of the magnetic lines of force that penetrate the measuring loop  30  at the level of the ferrite core  10  varies in approximately linear relation to the measuring distance x, whereas in the case of the reference loop  40  the same portion is approximately constant over the measuring distance x. 
   It is, thus, possible, after prior calibration, to calculate the position x based on the linear deviation of the measuring current induced in the measuring loop  30 . By forming the quotient of the measuring current i 2  determined in the measuring loop  30  and the reference current i 3  determined in the reference loop  40 , it is thus possible, in the way known from the cited prior art, to largely eliminate any interfering influences that may occur simultaneous in the measuring loop  30  and the reference loop  40 . 
     FIG. 2  shows a preferred embodiment of the displacement sensor according to the invention. The displacement sensor comprises a carrier substrate  104  on which two oppositely arranged V-shaped measuring loops  100 ,  102  and one rectangular excitation loop  106  are applied. The conductor loops  100 ,  102  and  106  are operated by an electronic circuit, which is arranged in a housing  108  and which will be described hereafter in more detail by reference to FIG.  3 . According to the invention, a measuring head  110 , adapted to be displaced along the carrier substrate  104 , comprises a coil/capacitor arrangement connected to form a resonant circuit  112 . 
   It should be noted that the illustrated arrangement of the rectangular excitation loop  106  and the two oppositely directed V-shaped measuring loops  100 ,  102  on one side represents only one of the possible embodiments and that the preferred embodiment comprises a double-sided rectangular excitation loop  102  and V-shaped measuring loops  100 ,  102  provided in inverse-parallel arrangement on both sides. That arrangement provides the fourfold output voltage (double excitation voltage plus double measuring voltage) which already considerably reduces the parasitic position-dependency of the displaceable measuring head  110 . 
     FIG. 3  shows one embodiment of a circuit arrangement for operation of the displacement sensor illustrated in FIG.  2  and for evaluation of the measuring signal. A logic module  201  controls on the one hand, via a control line  203 , an excitation generator  200 , which serves to generate an excitation pulse, and on the other hand, via control lines  205 ,  207 , a synchronous rectifier  206 ,  208 . The rectangular excitation loop  106  illustrated in  FIG. 2  is connected to the output of the excitation generator  200 . The V-shaped measuring loops  100  and  102  illustrated in  FIG. 2  are connected to the inputs of the two identically configured synchronous rectifiers  206 ,  208 . 
   The inner circuit arrangement of the illustrated synchronous rectifier unit  206 ,  208  is shown in FIG.  4 . The unit  206 ,  208  comprises a preamplifier  300  and electronic switches  301 ,  302  and  303 . Further, the unit  206 ,  208  comprises an integrator  304  and a sample&amp;hold circuit  305 . 
   At the beginning of a measuring cycle, the excitation generator  200  sends a short pulse to the excitation loop  106  thereby exciting the resonant circuit  112  (FIG.  2 ). As long as the excitation pulse is applied, the switches  301  and  303  illustrated in  FIG. 4  remain open, while the switch  302  is closed. The voltage signal induced during that period of time in the measuring loops  100 ,  102  is not transmitted by the preamplifier  300 . Once the excitation pulse has died down, sinusoidal voltages are induced in the measuring loops  100  and  102  via the resonant circuit  112 . The switch  301  is now opened in correct phase relation, for one half-wave of the first approximately eight sinusoidal oscillations at a time, and the signal present at the output of the preamplifier  300  is integrated by means of the integrator  304 . 
   At the end of the measuring cycle, the switch  303  is closed shortly, and the before-mentioned integrated signal value is transmitted to the sample&amp;hold circuit  305 . Thereafter, the integration capacitor of the integrator  304  is discharged via the switch  302 , whereby the circuit is initiated for the next measuring cycle. The oscillating circuit  112  is then again excited in correct phase relation by a short excitation pulse, whereby the measuring cycle is re-started. 
   The output signals of the two synchronous rectifier units  206 ,  208  are added or summed up by an operational amplifier  210 , or are subtracted one from the other by an operational amplifier  212 . Given the fact that the arithmetic sum of the voltage induced in the two oppositely directed V-shaped measuring loops  100 ,  102  remains constant, irrespective of the position of the measuring head  110  along the carrier substrate  104 —the geometric sum of the inner surfaces of the measuring loops being equal at any point of the displacement path of the measuring head  110 , due to the opposite direction of the measuring loops  100 ,  102 —the arithmetic sum can be used as reference output (reference voltage). 
   The reference voltage is then compared with a stable voltage source  214  via a PI controller, for example an amplifier  216 . Using the differential voltage, the excitation generator  200  then regulates the before-mentioned arithmetic sum so as to keep it on a constant amplitude. Thus, the value of the measuring signal and, consequently, the position-dependent output voltage  218  of the output amplifier  212 , derived by subtraction, remain constant regardless of the temperature of the displacement sensor and the before-mentioned electronic components and regardless of any mechanical influences and voltage variations. 
   Thus, while there have been shown and described and pointed out fundamental novel features of the present invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the present invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.