WO1991013327A1

WO1991013327A1 - Displacement detector, especially displacement to frequency transducer for measuring pressure

Info

Publication number: WO1991013327A1
Application number: PCT/HU1991/000008
Authority: WO
Inventors: István ALMÁSI; István LESI; Tamás VÉGHELY
Original assignee: Almasi Istvan; Lesi Istvan; Veghely Tamas
Priority date: 1990-02-22
Filing date: 1991-02-19
Publication date: 1991-09-05
Also published as: HU212353B; HU900891D0; HUT62411A; AU7320191A

Abstract

In a displacement detector, especially a displacement to frequency transducer for measuring pressure, comprising an oscillator (30) with a resilient vibrating reed (15) and signal processing means, the oscillator (30) including inductance members (20, 20') remaining in magnetic coupling, the resilient vibrating reed (15) comprises at least a first and a second leg (5, 5') connected by a coupling member (12) made of a ferromagnetic material, the first leg (5) being free for a displacement (Δx) according to a pressure or other force causing this displacement (Δx), the second leg (5') being fixed, the oscillator (30) including a first inductance member (20) determining a spacing (3) for receiving the coupling member (12), a second inductance member (20') for following the changes of the magnetic coupling between the coupling member (12) and the first inductance member (20), wherein the first and second inductance members (20, 20') are connected to means, especially an operational amplifier (IC1) for generating signals, particularly square wave signals of frequency corresponding to the displacement (Δx).

Description

DISPLACEMENT DETECTOR, ESPECIALLY DISPLACEMENT TO FREQUENCY TRANSDUCER FOR MEASURING PRESSURE

FIELD OF INVENTION

The present invention refers to a displacement detector, especially a displacement to frequency transducer for measuring pressure, comprising an oscillator with a resilient vibrating reed and signal processing means, wherein the oscillator includes inductance members remaining in magnetic coupling to one another. The oscillator produces electric signals characterized by a frequency which depends on a displacement detected by the vibrating reed.

BACKGROUND OF THE INVENTION

In the industrial electronics and control equipments the displacement detectors are well-known and they find wide application as the active measuring elements of different control means.

The displacement is a physical phenomenon which can be the basis of carrying out measurements of weight, mass, pressure, specific weight, etc. when the measured phenomenon results in the displacement. The measurements of such kinds are generally implemented by detecting a displacement in a resilient body and this displacement is converted later into the value of the measured parameter. Another group of the displacement measurements is based on the detection of changes caused in a body of appropriate shape or inner structure by the pressure or force to be measured. The body is generally elastic. The last group includes the different apparatuses applying the phenomenon of piezoelectricity, different semiconductor devices, etc.

The second group includes also the apparatuses wherein an elastic body having determined frequency of vibration is used. It is known that the inner state of a vibrating body, e.g. a vibrating reed or a quartz crystal results in a predetermined value of the resonance frequency of the vibrating body and any change in this inner state has the consequence that the resonance frequency has changed value. Lots of solutions have become known in this field and the common problem of them is that the Q-factor (quality factor) of the oscillating system should be kept at a relatively high level. This requirement can be met by different methods of designing the systems, wherein it is very important to introduce the force or pressure into the system in an appropriate manner.

The solutions shown in the German patent documents DE-C2-25,53,638, DE-C2-34,23,501 and DE-C2-35,13,269 is characterized by that the force is introduced into the mechanical system either directly or by transmission means to the vibrating reed made in the form of an oscillating frame or a tuning fork. The force results rather in an increase of the inner mechanical tension of the vibrating reed and thereby in altered resonance frequency and not in a measurable displacement. Hence, the quality factor (the Q- —factor) has relatively low value, the accuracy of the measurements falls with increasing force values and because of terminating the oscillation process it is impossible to carry out measurements when the force is stronger than a threshold value. A similar principle of the measurements is implemented by the device described in the patent document US-A-4,680,970 issued in 1987. According to this citation a membrane made in the form of a bent annulus is connected to a vibrating reed and the displacement of the membrane takes place when pressure to be measured is applied. The problem of the arrangement should be seen in the connection between the vibrating reed and the membrane. A further problem lyes in nonlinear movement of the bent annulus of the membrane under the influence of the pressure wherein the changing resonance frequency of the membrane influences the accuracy of detection. The quality factor can be kept in a required value range but only with high difficulties and only when the system is designed with much care. Hence, the device proposed can be manufactured only with technologic problems, it is expensive.

A special oscillation system of improved quality factor is shown in the patent document US-A-4,773,493 issued in 1988. The improvement is ensured by fixing a body of predetermined mass in the node point of a vibration holder comprising a double reed, in the node point determined by the vibration of basic frequency. The body shows not only relatively high frequency but also low frequency vibrations during the measurements. A basic problem is that at higher frequency values in a predetermined region more node points can be experienced, their place is changing, so the accuracy of the measurements can be sometimes very low. Because of the problem of accuracy are the solutions shown in the patent documents US-A-4,614,245 issued in 1986, further in US-A-4,574,639 issued in 1986 and in US-A-4,739,664 issued in 1988 problematic.

In the solutions determining the state of the art the basic problem can be seen in the fact that the force (pressure) is introduced rather by a manner resulting in a change of the mechanical tension of the vibrating member and not in a real displacement.

SUMMARY OF THE INVENTION

The object of the present invention is to realize a displacement detector capable of measuring pressure with high reliability and high value of the quality factor. The invention is based on the recognition that the pressure or force to be measured should be transmitted to the measuring system without causing any remarkable change in the internal pressure state of the system and especially of the vibrating element. The recognition depicted above can be recapitulated thereby that a vibrating element should be applied which shows alteration of the natural frequency when it is under influence of a pressure or force causing a displacement of a part of this vibrating element. This means that a resilient vibrating reed is to be applied which is fixed at one end, has a free end for receiving a force or pressure causing displacement of this end and another free end being in vibration. In this way a relatively rigid vibrating system can be realized with controlled basic frequency.

Hence, the invention is a displacement detector, especially a displacement to frequency transducer for measuring pressure, comprising an oscillator equipped with a resilient vibrating reed and signal processing means, the oscillator including inductance members remaining to one another in magnetic coupling. According to the invention the resilient vibrating reed comprises at least a first and a second leg connected by a coupling member made of a ferromagnetic material, wherein the first leg is free for a displacement according to a pressure or other force causing this displacement, the second leg is fixed, the oscillator includes a first inductance member determining a spacing for receiving the coupling member, a second inductance member for following the changes of the magnetic coupling between the coupling member and the first inductance member, wherein the first and second inductance members are connected to means, especially an operational amplifier for generating signals, particularly square wave signals of frequency corresponding to the displacement.

The displacement detector is preferably realized according to the invention with means for generating signals which are constituted by an^' operational amplifier having an inverting input connected to the first inductance member and a non inverting input connected to the second inductance member, wherein the inverting input is advantageously through a first condenser in feed-back connection with the of the operational amplifier.

In the displacement detector realized according to the invention the accuracy of the measurements can be improved when the resilient vibrating reed is a prestretched element having first and second legs lying in a distance from one another in the direction of the displacement. The prestretched resilient vibrating reed is preferably made with legs determining a V- or W-shape body with peak angle at most 20°, preferably about 1° to 2°.

A very fine alteration of the n ural resonance frequency can be ensured when the resilient vibrating reed includes a third leg lying in parallel with the first and second legs. The transmission of the distortion forces between the ends of the legs and the coupling member of the vibrating reed is restricted in an advantageous embodiment of the proposed displacement detector realized -according to the invention if at least one of the legs is made with an inner and an outer resilient contacting members, the inner contacting members formed at the coupling member and the outer resilient contacting members formed at the other end of the legs, wherein the resilient contacting members have width smaller than the width of the legs. In a rather simple embodiment of the displacement detector realized according to the invention the first and second inductance members comprise respective iron cores and coils of preferably different number of turns. The first and second inductance members comprise advantageously coils of different number of turns prepared on a common iron core consisted of L-shaped iron parts determining at one end the spacing and connected at other end to respective poles of a permanent magnet especially made of rare earth metals, e.g. samarium. A second possibility of detecting the displacement can be ensured by the advantageous embodiment of the proposed detector realized according to the invention, wherein a light source, especially a light emitting diode is applied for illuminating the vibrating reed, especially the coupling member thereof and a light detector serves for detecting at least one position of the illuminated part of the vibrating reed, especially that of the coupling member when the latter vibrates in the spacing.

The place of the application of the displacement detector proposed according to the invention can be detached by a distance up to 30 - 50 meters from the place of evaluating the measurements' results when the output of the means for generating signals are connected by optoelectronic coupling unit to means for signal processing which are preferably connected over the optoelectronic coupling unit to the output of the operational amplifier, wherein the signal processing means include a voltage stabilizer connected to a load resistor, whereby the output of the voltage stabilizer is coupled with a series member including a differentiating and rectifying circuit, a monostabile multivibrator (square wave signal generator) and an integrator for generating output signals. According to another possibility the signal processing means include a voltage stabilizer and a series member consisted of a freqeuncy to voltage transducer and a filtering unit, wherein the series member is arranged on the output of the optoelectronic coupling unit. The series member generates direct current (or voltage) level following the changes of the frequency characterizing the signals produced by the transducer of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described in more detail with reference to the enclosed drawings showing some preferred embodiments of the displacement detector of the invention by way of examples only. In the drawings

Fig. 1 illustrates the perspectivic view of a two leg vibrating reed applicable in the detector of the invention,

Fig. 2 shows the top view of the vibrating reed illustrated in Fig. 1, Fig. 3 illustrates the top view of a three leg vibrating reed applicable in the detector of the invention.

Fig. 4 is the circuit diagram of a preferrred embodiir. nt of the detector of the invention realized with detached inductance members. Fig. 5 is a circuit diagram of another preferred embodiment of the detector of the invention with inductance members applied on a common iron core, the circuit diagram presented with connection of contacts of an IC3130 type integrated circuit. Fig. 6 illustrates in cross-section a preferred embodiment of the common iron core with coils applied for realizing the circuit diagram of Fig. 5, Fig. 7 shows a block diagram of a preferred embodiment of a displacement measuring unit applying the detector of the invention, Fig. 8 illustrates a block diagram of another preferred embodiment of a displacement measuring unit applying the detector of the invention and Fig. 9 is the cross-section of another preferred embodiment of the detector of the invention equipped with a light source and a light detector.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODI¬ MENTS

The displacement detector of the invention is based on a novel vibrating reed 15 which is realized generally according to Figs. 1 and 2 or to Fig. 3. This vibrating reed 15 of length H is a two or three leg body comprising legs 5 and 5' (Fig. 1 and 2) or 5, 5¹ and 5" (Fig. 3) which lye parallel to one another. The legs 5 and 5¹, further 5^» and 5" are detached by a long aperture of width S and are connected to a coupling member 12 made of a non permanent magnetic material, e.g. steel. In the legs 5, 5' and 5" of width K resilient contacting members 7, 7' and 7" are prepared at the coupling member 12 and further resilient contacting members 9, 9' and 9" near to end parts 1 and 2 of length A of the legs 5, 5' and 5". The resilient contacting members 7, 7', 7", 9, 9' and 9" are thin parts of the legs 5, 5' and 5", their width is about 1/10 of the width K of the legs 5, 5' and 5". By their use it is intended to limit the process of transferring mechanical distortions within the material of the vibrating reed 15 from the end parts 1 and 2 to the coupling member 12 when the end parts 1 and 2 are connected to a fixation and an element transferring the displacement to be measured. The vibrating reed 15 is rigidly clamped at end part 2 of the leg 5^■ and its end part 1 belonging to the leg 5 (and 5") is free, it can be exposed to the action of a force causing a displacement _\x whereby a distance x₀ changes. The vibrating reed 15 is generally a V-shaped body, the legs thereof determine an opening of the distance x₀ which gives a prestretch of the legs 5, 5¹, 5" and an angle between the legs 5 and 5' in the range up to 20°, preferably 1° to 2° (Fig. 1).

The vibrating reed 15 is made at least partly of a soft ferromagnetic material, i.e. at least the coupling member 12 can be magnetized, however it does not consist of a permanent magnet. The legs 5, 5', 5" may be consisted of any ceramic, plastic and composite materials showing required high elasticity.

The vibrating reed 15 cooperates ^•with a first and a second conductance members 20, 20' equipped with respective coils L;-. and L₂. The first conductance member 20 and its coil L-L is arranged so as to determine a spacing 3 receiving the coupling member 12 of the vibrating reed 15 (Fig. 4) . The first and second inductance members 20 and 20' have respective iron cores 19 (Fig. 4) or the first coil L-_j. form ah inner coil arranged on a common iron core 19 (Fig. 5) and the second coil L₂ is prepared on the surface of the first coil 20 (Fig. 5 and 6) . In the last embodiment the first coil L_x is generally made with at least four times more turns than the second coil L₂. The iron core 19 as shown in Fig. 6 is consisted of two L shaped part 21 and a permanent magnet 23 made preferably of rare earth metals, e.g. on the basis of samarium, wherein the longer legs of the L-shaped parts 21 determine the spacing 3 and the shorter ends are connected to one another through the permanent magnet 23. The permanent magnet 23 gives a magnetic biasing effect to the iron core. The vibrating reed 15 cooperates with the first and second inductance members 20, 20' ensuring the operation of the detector of the invention on the basis of the magnetic coupling. This is realized thereby that the first and second coils L-L and L₂ are connected to an integrated circuit IC-L forming an operational amplifier. The inverting input of the integrated circuit IC-_j. is connected to a common point of a first resistor R-L and a first condenser C-L. The condenser C₂ is inserted between the corresponding input and the output of the integrated circuit IC-^ The output of the integrated circuit is connected also to the second coil L₂ and is bridged over by a diode D_x for limiting harmful current flow. According to commonly known principles of realizing an operational amplifier the outputs of the integrated circuit IC-L are bridged over by a second condenser C₂ and resistors R₃ and R for adjusting the parameters of the operation. The second coil L₂ is connected to a second resistor R₂. The values of the second condenser C , second resistor R , resistors R₃ and R are determined according to known methods after preparing the circuit in order to ensure the required working point parameters.

The vibrating reed 15 and the first and second inductance members 20 and 20' constitute an oscillator 30, wherein the electric members are connected with the integrated circuit IC-L which is the basis of means 50 for generating signals. The oscillator 30 and the means 50 form together a detecting unit 70 (Figs. 4 and 5). The outputs of the detecting unit 70 are connected to a signal processing means 100 which can be realized in different ways (Figs. 7 and 8) . The signal processing means 100 should generate a signal which corresponds to the changes of the output signals of the detecting unit. The signal generated can be e.g. a current or voltage level or a series of digital signals, etc.

According to Fig. 8 the signal processing means 100 are realized so that the output signals, generally square wave signals of the detecting unit 70 are received by a voltage stabilizer 110 connected to one pole of a load resistor R_t. This resistor is connected to a source of a supply voltage +U_T. The two poles of the load resistor R_t are connected to a differentiating and rectifying circuit 120 which drives a monostabile multivibrator 130 connected in turn to an integrator 140 forwarding an output voltage level according to the frequency of the input signal received by the voltage stabilizer 110.

In Fig. 7 another possibility of realizing the signal processing means 100 is shown. The detecting unit 70 is connected through an optoelectronic coupling unit 71 to a frequency/voltage transducer 72. The output of the latter is connected to a filtering unit 73 which forwards direct voltage signals at an output 74. The frequency/voltage transducer 72 and the filtering unit 73 are connected to a voltage stabilizer 110 receiving the input supply voltage of U_τ level.

The displacement detector of the invention can be implemented also with an auxiliary supervisor system comprising a light source 22, e.g. a light emitting diode projecting a thin light beam to the coupling member 12 having a polished surface. In the way of propagation of the light beam after the coupling member 12 a light detector 24 is arranged. Hence, an optic signal may complete the electric signal received from the first and second inductance members 20 and 20' (Fig. 9). .

In a realized embodiment of the invention the vibrat¬ ing reed 15 is a two or three leg member having length H = 18 mm, legs of width K = 3.5 mm, a spacing along the legs S = 0.5 mm, wherein the length A of the end parts 1 and 2 is 3.5 mm, further the length E of the coupling member 12 is 3 mm. The coupling member 12 may have also a circular edge, e.g. with radius about 10 mm. When prepared from steel of high quality, e.g. steel applied for razor blades the vibrating reed 15 has natural frequency f₀ of more than about 200 Hz and less than about 500 Hz. In a prestretched vibrating reed 15 the natural frequency can be altered with high accuracy by about 100 Hz when the dispalcement acting on the end part 1 of the leg 5 causes^' a respective change in the distance x₀ between the end parts l and 2 of the legs 5 and 5' .

As for the electric elements the coil L_x of the first inductance member 20 consists generally of about 2500 turns and the coil L₂ of the second inductance members 20' of about 600 turns. The integrated circuit IC_λ can be realized by an operational amplifier IC3130, the first condenser C₂ is a 22 nF unit. The displacement detector of the invention operates as follows:

By contacting the first and second coils L*L and L₂ of the first and second conductance members 20 and 20' with a voltage source the vibrating reed 15 is pressed to begin to move. The magnetic flux generated by the current flowing through the first inductance member 20 makes the vibrating reed 15 to leave the basic, generally middle position in the direction determined by the current flow. This position is fixed by the permanent magnet 23 which is arranged in that manner that the first inductance member 20 does not negativate its magnetized state, it helps to maintain the pole arrangement. Hence, after starting the vibrating reed

15 it reaches one of the dead points and this causes the magnetic flux to be reduced within the first coil L^ Under this influence the current becomes very weak and the vibrating reed 15 can return to its other dead point because of the elastic forces. In this time the current in the first inductance member 20 increases and reaches its maximal value when the vibrating reed 15 is maximally detached therefrom. After that the vibrating reed 15 returns to the first dead point and this oscillation process is repeatedly continued with the natural frequency f₀ of the vibrating reed 15. This frequency characterizes the movement of the coupling member 12.

When applying a force on the end part 1 of the leg 5 of the vibrating reed 15 the natural frequency f₀ of the latter changes. This means, the coupling member 12 moves with a changed speed between the ends of the iron core 19 within the spacing 3. Hence, the magnetic flux in the second inductance member 20' changes and this change is detected by the integrated circuit IC-L generating square wave output signals now of frequency differing from that which is resulted from the previous state. Hence, a displacement

is transformed into a change of a frequency value. The frequency values are implemented by squaxe wave signals generated by the integrated circuit IC-^ In the signal processing means 100 the series of the signals of given frequency are converted into a voltage level and this level received at the output 74 represents the value of the displacement, i.e. the output 74 forwards a voltage level characterized by alteration when a displacement of the end part 1 occurs.

The displacement detector of the invention requires, of course, a previous adjustment before beginning the measurements. This can be done according to the known principles and this step does not need further comments. In the displacement detector of the invention the vibrating reed 15 is a freely oscillating member and its natural frequency is influenced only in a very narrow range by the different distortion effects introduced by the end parts 1 and 2 of the legs 5, 5' and 5". Hence, the natural frequency is practically dependent only on the force causing the displacement of the free end of the vibration reed 15.

Claims

WHAT WE CLAIM IS:

1. A displacement detector, especially displacement to frequency transducer for measuring pressure, comprising an oscillator with a resilient vibrating reed and signal processing means, the oscillator including inductance members remaining in magnetic coupling with one another, characterized in that the resilient vibrating reed (15) comprises at least a first and a second legs (5, 5') connected by a coupling member (12) made of a ferromagnetic material, the first leg (5) being free for a displacement (AX) according to a pressure or other force causing this displacement ( X) the second leg (5') being fixed, the oscillator (30) including a first inductance member (20) determining a spacing (3) for receiving the coupling member (12), a second inductance member (20') for following the changes of the magnetic coupling between the coupling member

(12) and the first inductance member (20), wherein the first and second inductance members (20, 20^») are connected to means, especially an operational amplifier (IC_T for generating signals, particularly square wave signals of frequency corresponding to the displacement ( X) •

2. The displacement detector according to claim 1, characterized in that the means for generating signals (50) is constituted by an operational amplifier (IC_j,) having an inverting input connected to the first inductance member (20) and a non inverting input connected to the second in¬ ductance member (20¹), wherein the inverting input is in feed-back connection with the output of the operational amplifier (IC-^ .

3. The displacement detector according to claim 2 , characterized in that the feed-back connection is realized by inserting a first condenser (C_α) .

4. The displacement detector according to any of claims 1 to 3, characterized in that the resilient vibrating reed (15) is a prestretched element having first and second legs (5, 5') lying in a distance (x₀) from one another in the direction of the displacement ( X) • 5. The displacement detector according to any of claims 1 to 4, characterized in that the resilient vibrating reed (15) includes a third leg (5") lying in parallel with the first and second legs (5,

5').

6. The displacement detector according to any of claims l to 5 , characterized in that at least one of the legs (5, 5', 5") is made with an inner and an outer re¬ silient contacting members (7, 7', 7", 9, 9', 9"), the inner contacting members (7, 7*, 7") formed at the coupling member (12) and the outer resilient contacting members (9, 9', 9") formed at the other end of the legs (5, 5', 5"), the re¬ silient contacting members (7, 7', 7", 9, 9', 9") having width smaller than the width of the legs (5, 5', 5").

7. The displacement detector according to any of claims 4 to 6, characterized in that the distance (x₀) is determined by an angle of the legs (5, 5', 5") being at most 20°, preferably about 1° to 2°.

8. The displacement detector according to any of claims 1 to 7, characterized in that the first and second inductance members (20, 20') comprise respective iron cores (19) and coils (L_lf L₂) of different number of turns.

9. The displacement detector according to any of claims 1 to 8, characterized in that the first and second inductance members (20, 20') comprise coils CL_{l f} L₂) of different number of turns prepared on a common iron core (19) consisted of L-shaped iron parts (21) determining at one end the spacing (3) and connected at other end to a permanent magnet (23) especially made of rare earth metals.

10. The displacement detector according to any of claims 1 to 9 , characterized in comprising a light source, especially a light emitting diode (22) illuminating the vibrating reed (15) , especially the coupling member (12) thereof and a light detector (24) detecting at least one position of the illuminated part of the vibrating reed (15) , especially that of the coupling member (12) when vibrating in the spacing (3) .

11. The displacement detector according to any of claims 1 to 10 , characterized in that the output of the means for generating signals (50) are connected by opto¬ electronic coupling unit (71) to means (100) for signal processing.

12. The displacement detector according to claim 11 , characterized in that the means (100) for signal processing are connected over the optoelectronic coupling unit (71) to the output of the operational amplifier (IC-_j , the means (100) including a voltage stabilizer (110) connected to a load resistor (R_t) , whereby the output of the voltage stabilizer (110) is coupled with series member including a differentiating and rectifying circuit (120) , a monostabile multivibrator (130) and an integrator (140) for generating output signals.

13. The displacement detector according to claim 11 , characterized in that the means (100) for signal processing are connected over the optoelectronic coupling unit (71) to the output of the operational amplifier (IC₂) , the means (100) including a voltage stabilizer (110) and a series member consisted of a freqeuncy to voltage transducer (72) and a filtering unit (73) , the series member being arranged on the output of the optoelectronic coupling unit (71) .