Inductive Displacement Sensor Apparatus

U.S. Ser. No. 491,180 (Weckenmann) filed July 23, 1974.

This invention relates to an inductive displacement sensor or indicator utilizing a variable inductance.

One known inductive displacement sensor is the solenoid type of sensor in which a movable ferromagnetic core is moved in and out of a coil, thereby varying the inductance L of the inductor formed by the coil. Two such solenoid sensors may be coupled in a differential arrangement, so that the inductance of one coil is increased while that of another is reduced. Since the correlation of displacement to inductance is not linear, rather expensive electronic equipment is necessary in this type of apparatus. The sensitivity is low and, moreover, the solenoid type of displacement sensor requires a large mechanical length dimension in relation to the usable linear range of the translation characteristic. A further disadvantage is the relatively large bulk of the ferromagnetic core.

It is an object of the present invention to provide an inductive displacement sensor apparatus with a movable part of the smallest possible bulk and weight, so that it may be useful for both static and dynamic measurements of displacement. It is a further object of the invention to provide an inductive displacement sensor apparatus that is as simple as possible in its mechanical and electronic construction and is economical for manufacture in quantity. It is a particular object of the invention to provide an inductive displacement sensor that is suitable for measuring large displacements as well as small displacements.

Briefly, a variable inductor is made with a U-shaped core of ferromagnetic material on which an electromagnet coil is provided, preferably at the bend of the U. A member that acts as a short-circuiting ring on each of the legs of the core is provided which is movable along both legs at once, the movement of the short-circuiting ring being made to follow the displacement to be measured. Means are then provided for generating a signal dependent upon the inductance of the coil and therefore also upon the magnitude of the displacement.

The electronic portion of the means for producing the output signal of the device is particularly simple and effective in accordance with the form of the invention in which a series connection of a comparator having a hysteresis characteristic, a controllable variable inductor and an integrator is provided to form an oscillator. In this case the output of the integrator is connected with an input of the comparator.

A very simple and economical form of the invention is provided when the controllable variable inductor and the integrator are combined in the form of a LR integrator having a resistor connected between the inductor and ground, in which case an operational amplifier is used to serve as the comparator.

The advantages obtained by the invention include, particularly, the fact that the movable part, the short-circuiting ring, as very small bulk and weight, so that the displacement sensor so provided is well suited for dynamic measurements as well as for static measurements. In the case of higher frequencies, beginning at about 100 kHz, one or more thin metal foils perforated to pass over the core legs suffice for the short-circuiting ring. These foils are preferably of copper or silver. Dynamic measurements of displacement made in this fashion are, for example, necessary for measuring the intake air quantity in the intake piping of an internal combustion engine. In that case a diaphragm or a baffle plate can be coupled with the short-circuiting ring, or the short-circuiting ring can itself constitute the diaphragm or baffle plate. The displacement of the diaphragm or baffle plate by the air stream against a restoring force is then a measure for the quantity of air passing through the intake.

A further advantage of the inductive displacement sensor of the present invention is that the output signal can be supplied either as a digital or as an analog signal, as may be desired, to the circuit or device which requires it. A further advantage is to be found in the fact that the legs of the U-shaped core can be extended enough to enable the measurement of long displacements.

Both the mechanical and the electronic construction of the sensor apparatus of this invention is simple and economical. The U-shaped core can be assembled out of two halves fitted together, so that a coil already completed can be mounted on the core. The air gap at the junction of the two parts can be kept small and is in any event negligible in comparison to the air gap between the two legs of the U-shaped core. The electronic circuit is built up of operational amplifiers, so that the entire circuit can be provided by a single integrated circuit unit. There is still a further advantage resulting from the fact that one form of the invention can provide an output frequency that is directly proportional to the amount of displacement. In a further still simpler form of the invention, an output oscillation having a period proportional to the amount of displacement is produced, which means that a frequency can be obtained from the circuit that is inversely proportional to the displacement. Finally, the circuit can be provided with additional features to establish any desired non-linear displacement-frequency translation characteristic or, on the other hand, to improve the linearity of the translation characteristic.

The invention is further described by way of illustrative examples with reference to the annexed drawings, in which:

FIG. 1 is a diagrammatic view, partly in section, of a variable inductor provided with a movable short-circuiting ring;

FIG. 2 is an end view of the device of FIG. 1, as seen from the open end of the core, showing the contour of the short-circuiting ring;

FIG. 3 is a block diagram of the electric portion of the inductive sensor apparatus, utilizing the device of FIGS. 1 and 2;

FIG. 4 is a timing diagram for explaining the operation of the circuit of FIG. 3.

FIG. 5 is a diagram of a controllable inductive device suitable for the circuit of FIG. 3;

FIG. 6 is a diagram of a combination of another controllable inductive device with an integrator;

FIG. 7 is a diagram of a comparator provided with a linearizing circuit;

FIG. 8 is a block diagram of the electronic portion of another apparatus according to the invention which includes a function generator for providing an arbitrary displacement-frequency translation characteristic, and

FIG. 9 is a circuit diagram of a particular simplification of the electronic circuit of FIG. 3.

In order to obtain variable inductance in the device shown in FIG. 1 and FIG. 2, a magnet coil 10 is fitted to the shape of the cross-section of the core legs. The two legs of the magnet core 11 protrude through the respective openings 13 and 14 in the short-circuiting plate 12. This plate operates as a short-circuiting ring for both legs and is freely movable in a direction parallel to both core legs. Its movements must correspond to the displacement to be measured. For example, a fluid level measuring apparatus may be constituted by using a device of the kind shown in FIGS. 1 and 2, in which case the magnetic core legs would dip vertically into a liquid and the short-circuiting ring 12 would be mounted on a float.

The device shown in FIGS. 1 and 2 operates by the provision by the magnet coil 10 of a homogeneous alternating magnetic field between the two legs of the core 11. The lines of force 15 of the field so produced are shown in FIG. 1. Since the short-circuiting member 12 provides a short-circuited winding for each leg of the core, no alternating magnetic field can proceed beyond the short-circuiting ring in the core legs. To a good approximation, therefore, the total magnetic flux is thus limited to proportionality to the displacement of the short-circuiting ring. In accordance with the induction law, the inductance of the magnet coil 10 is then, likewise, varied in proportion to the displacement. In order to obtain the highest degree of linearity, the spacing between the core legs can be varied as a function of displacement. For angular measurements an analogous device can be provided in which the legs of the core, instead of being straight and parallel, are bent into arcs of circles.

In order to reduce to the lowest possible value the temperature dependence of the device that results from the temperature dependence of the conductivity of the short-circuiting ring 12, the operating frequency of the alternating field can be made high (for example 100kHz), in which case the increase of the ohmic resistance of the short-circuiting ring as the result of skin effect can be mitigated in the following manner: manufacture of the short-circuiting ring from a stack of thin metal sheets or foils laminated together, or manufacture of the short-circuiting ring out of a number of turns of thin wire.

In the block diagram of a circuit shown in FIG. 3, there is shown an oscillator in which the frequency depends upon the momentary magnitude of the inductance of the variable inductor. It includes a switching comparator 30 that has a hysteresis characteristic. The comparator has an upper switching threshold U.sub.0 and a lower switching threshold -U.sub.0. Its output is connected both to the output terminal 31 of the oscillator and to the input of a controllable inductive device 32. This inductive device 32 contains a variable inductor of the form shown in FIGS. 1 and 2 to provide a variable inductance. It is, of course, possible to utilize a different kind of variable inductance in this kind of circuit. The output of the variable inductance unit 32 is connected both with the input of a rectifier stage 33 and with the input of an integrator stage 34. The output of the integrator stage 34 is connected to the input of the comparator 30. The output of the rectifier stage 33 is connected to an output terminal 35 which constitutes an analog output of the oscillator.

The operation of the circuit of FIG. 3 is best understood with reference to the timing diagram given in FIG. 4. It should first be assumed that a positive voltage is present at the output of the comparator 30. This voltage is raised or lowered by the inductive unit 32 in accordance with the position of the control of the variable inductance. This raising or lowering results from operation of the variable inductor in dependence on the displacement to be measured, i.e. in accordance with the position of the short-circuiting ring 12. The positive voltage at the output of the inductive unit 32 is integrated by the integrator 34. The higher the voltage at the output of the inductive unit 32, the faster the integration proceeds, i.e. the faster is the voltage rise of the output of the integrator 34. As soon as the upper switching threshold U.sub.0 of the comparator 30 is reached, the voltage at the output of the comparator 30 jumps back to a negative value. The voltage at the output of the inductive unit 32 then changes correspondingly. At the output of the integrator 34, a decreasing voltage is produced that continues to decrease until a lower threshold value -U.sub.0 of the comparator is reached. The output of the comparator 30 then jumps back to a positive voltage value. The magnitude of the voltage at the output of the inductive unit 32 is, accordingly, a measure for the rise time or fall time of the integrator voltage and, therefore, a measure for the frequency of the oscillator.

The rectifying stage 33 rectifies the voltage at the output of the inductive units 32, so that a d.c. voltage signal is provided at the analog output 35 which is proportional to the variable inductance and, accordingly, proportional to the displacement to be measured. The displacement to be measured can thus be indicated at both digital and analog output signals.

The circuit shown in FIG. 5 likewise contains an inductive variable component 32. The rectangular voltage wave coming from the comparator 30 is integrated in the integrator 50, so that a triangular current wave appears at the output of the integrator 50. This triangular wave is differentiated by the transformer 51, so that at the output of the variable inductive component 32 there is again produced a rectangular voltage wave. This occurs because the triangular current wave at the output of the integrator 50, in accordance with the induction law, produces a rectangular induced voltage across a primary winding which can be taken off from the unloaded secondary winding that is subject to the same encircling magnetic flux. This secondary winding can actually be provided on top of the magnet winding 10 on the U-shaped magnetic core 11. By thus providing the induced voltage as an output by transformer action from the variable inductor, a galvanic separation is obtained. The direct current stability of the oscillator lost by this arrangement can be restored by provision of a very lightly loaded direct current parallel branch circuit. This parallel branch circuit is provided by the resistor 52 bridging the variable inductor 32. Without the provision of the resistor 52, there would not be sufficient stability of the oscillator because of the integration drift of the integrator 34. The advantages of using the variable inductive component 32 in the oscillator circuit are to be found particularly in the fact that the output frequency is directly proportional to the displacement to be measured.

FIG. 6 shows a circuit for a combination of the variable inductive component 32 with the integrator 34. This circuit of an inductive integrator consists of an operational amplifier 60 provided with a feedback resistor 61. A variable inductor 62 is connected in series with the inverting input of the operational amplifier 60 and can preferably be a variable inductor of the type shown in FIGS. 1 and 2. This circuit practically represents a reversal of the usual differentiator, in which a capacitor is provided in series with the inverting input of the operational amplifier. When the arrangement shown in FIG. 6 is used in a circuit of the type of FIG. 3, there results considerable simplification of the arrangement of FIG. 3, particularly the type of arrangement according to FIG. 3 in which instead of the variable inductive component 32 of FIG. 3, a circuit like that of FIG. 5 is used. Nevertheless, when the circuit of FIG. 6 is used, the output frequency is then inversely proportional to the displacement to be measured, hence proportional to the period of one cycle of the output frequency. Furthermore, in order to obtain an analog output signal, the arrangement of the rectifying stage 33 shown in FIG. 3 can no longer be used. The frequency of the triangular output voltage wave of the integrator 34 must now be differentiated by a differentiator, not shown in the drawing, in order to again obtain a rectangular signal wave. This rectangular signal is then rectified by a rectifier stage (not shown) at the output of which the desired analog output signal appears.

The circuit shown in FIG. 7 represents a comparator having a hysteresis characteristic by means of which the linearity of the inductive displacement sensor can be improved by variation of the threshold voltages U.sub.0 and -U.sub.0 respectively. Experiments have shown that when a U-shaped core is used that has a constant and not too small spacing between the legs, deviations from linearity appear at higher frequencies. As the displacement to be measured increases, the frequency increases at a rate less than that proportional to the displacement. A compensation for this effect can be provided by making one or both of the threshold voltages U.sub.0 and -U.sub.0 dependent on frequency, i.e. reducing their absolute value for higher frequencies. The input of the comparator in FIG. 7 is, accordingly, connected over a resistor 70 with the inverting input of an operational amplifier 71, the output of which is connected over two resistors 72 and 73 with the inverting input of a second operational amplifier 74. This second operational amplifier 74 is bridged from output to input by a feedback path formed by the parallel combination of the resistor 75 and the capacitor 76 and its output is connected back, through a resistor 77, to the inverting input of the first operational amplifier 71. The connection point of the two resistors 72 and 73 is connected on one hand with the output of the comparator 30 and, on the other hand, over a bridge circuit composed of four diodes 78, 79, 80 and 81, to ground. The oppositely poled diodes 78 and 79 form one adjacent pair of bridge arms between the comparator and ground and the oppositely poled diodes 80 and 81 form the other pair of bridge arms. A Zener diode 82 is connected across the transverse diagonal bridge of this bridge (a diagonal which is conjugate to the comparator-ground diagonal).

Without the components indicated by the reference numerals 73 to 76, the circuit would be an ordinary known form of comparator. The two levels of the rectangular output voltage wave are stabilized by the diode network 78-82. The voltage appearing at the output is fed back over the resistor 77 to the input and stabilizes the new condition. This feedback is delayed by the components 73 to 76, which constitute a delay stage. Accordingly, when the input voltage reaches the value U.sub.0, or -U.sub.0, as the case may be, the output voltage then jumps exactly to its opposite value. At higher frequency, as the result of the time delay produced particularly by the capacitor 76, the voltage at the output of the second operational amplifier 74 can no longer reach its final value. The switching operation therefore already takes place at input voltages that are smaller than U.sub.0. The two thresholds thus gradually approach each other. By suitable dimensioning of the components 73 to 76, compensation can be provided for the deviation from linearity of the displacement translation characteristic of the variable inductor.

FIG. 8 shows a generalized circuit, in block form, for obtaining any arbitrarily chosen non-linear displacement-frequency translation characteristic. The present constitution of the circuit of FIG. 8 and its manner of operation correspond basically to the circuit of FIG. 3. Between the variable inductor 32 and the integrator 34, a function generator 84 is interposed. The threshold voltages U.sub.0 and -U.sub.0 are shiftable to any values in a reasonable range by the function generator 84, so that any arbitrarily chosen non-linear displacement-frequency translation characteristic can be obtained. The output signal of the inductive displacement sensor apparatus as a whole can thus represent any arbitrarily chosen function of the displacement. It is even possible to provide to the function generator through the variable inductive component 32 voltages U.sub.1 and -U.sub.1 corresponding to arbitrary parameters, so that the output voltage of the inductive component will be proportional not only to the displacement but also to these parameters. A signal generator 95 is shown in FIG. 8 connected by dashed lines to the inputs of the control switch 83 to signify modifying the output of the displacement sensor by arbitrary parameters in this fashion. In this case, a voltage balanced to ground is varied by a potentiometer 96 moved by a cam 97 driven by a motor 98.

A feature of the comparator is the possibility of selecting threshold and output voltages for the comparator that are proportional to each other or identical. Since the output frequency and, consequently, the cycle period for this frequency are proportional to the comparator output voltage and inversely proportional to the comparator threshold voltage, the output frequency is not subject to influence from variations of these voltages. Unregulated voltages can then be used for this circuit.

FIG. 9 shows a very simple form of circuit for a combination of the variable inductive component 32 with the integrator 34, in which the variable inductor 62 has one terminal of its winding connected both to ground over a resistor 90 and also to the inverting input of an operational amplifier serving as the comparator 30. The non-inverting input of the operational amplifier 30 is connected to the tap of a voltage divider composed of two resistors 91 and 92, the resistor 92 being connected between the tap and ground and the resistor 91 between the tap connection and the output terminal 31. The output of the operational amplifier 30 and the other terminal of the variable inductor 62 are connected to the output terminal 31. The voltage supply connection for the operational amplifier 30 is not shown in the diagram, as these are well known for operational amplifiers and would merely complicate the diagram.

The operation of the circuit shown in FIG. 9 corresponds essentially to the mode of operation previously described. If a certain voltage is present at the output of the operational amplifier 30, the current in the inductor increases and with it the voltage across the resistor 90 increases in very nearly linear fashion. This voltage is supplied to the inverting input of the operational amplifier 30 where it continues to rise until it reaches the voltage that is present at the tap of the voltage divider 91, 92. At that moment, the voltage at the output of the operational amplifier suddenly changes from a positive to a negative voltage. In consequence, a negative voltage then appears at the tap of the voltage divider 91, 92. The voltage across the resistor 90 and, hence also at the inverting input of the operational amplifier, now falls at a very nearly constant rate, until it reaches the voltage now present at the tap of the voltage divider 91, 92. At that moment, a positive voltage again appears at the output of the operational amplifier and the entire cycle is repeated anew.

A triangular voltage wave accordingly is produced at the inverting input of the operational amplifier 30, while at the output of the operational amplifier 30 and, therefore, at the output terminal 31, a rectangular voltage wave appears. The output frequency thus produced is inversely proportional to the displacement to be measured, which is to say that the repetition period of the wave is proportional to the displacement to be measured.

This very simple circuit that requires, in addition to the variable inductor, merely a single operational amplifier and a single resistor, has a high linearity, a very small dependence upon the supply voltage and an adequately small dependence upon temperature. Since this combination has only very small space requirements, the circuit is preferably incorporated in the structure or mounting of the variable inductor, so that the output frequency can be generated directly at the place of measurement and then supplied to an indicator or to a work circuit at another location over electrical connections.

Although for purposes of explanation, reference has been made to a displacement to be measured, it is to be understood that this expression also includes the notion of a displacement to be recorded, the concept of a displacement to be represented in real time by an electrical signal for any type of processing or utilization circuit, to be sampled, and the like.

Although the invention has been described with respect to particular illustrative embodiments, it is to be understood that variations are possible within the inventive concept.