DE2833037C2 - Device for measuring the mass flow of a flowable medium by evaluating twists in a continuous line that is curved at least in sections by means of Coriolis forces - Google Patents

Description

The invention relates to a device for measuring the mass flow of a flowable medium by evaluating twists in a continuous line that is curved at least in sections by Coriolis forces of the flowable medium according to the preamble of claim 1.

The invention also relates to an alternative device for measuring the mass flow of a flowable medium by evaluating forces exerted on a continuous line which is curved at least in sections and in which Coriolis forces of the flowable medium occur, according to the preamble of claim 2.

With such known mass flow meters with a housing in which the ends of the curved pipe are flexibly connected to the housing, attempts have already been made to determine mass flows of flowable media by evaluating the effects of the Coriolis forces (DE-OS 14 98 446).

In detail, in one embodiment of the known mass flow meter, rings are attached to the ends of the curved line, the outer surfaces of which fit into passages in the housing. The rings should be flexible enough that their deformation as a result of a transverse movement of the curved line does not generate any significant restoring forces. Nor should they oppose any large counterforces to rotations of the line ends about their horizontal axis. For this purpose, the rings are made of soft rubber or similar materials. The curved line is suspended on springs which allow a transverse movement in the direction of oscillation perpendicular to the direction of flow. The ends of the curved line are thus mounted in the housing with limited mobility. - There is also a further connection between the middle part of the curved line and the housing, namely via a bushing connected to the middle part, which is pivotally mounted in a block in the housing so that an oscillating movement can be imparted to the bushing. The curved line is therefore not self-supporting in relation to the housing. - In this embodiment, it is provided that, as a result of the oscillating movement of the flowing medium, a force couple acts on the bearings through the curved line, so that the springs bend, the bending of which can be measured with strain gauges. Since the operation of this known mass flow meter does not depend on the dynamic properties of the curved line, in particular whether and how resonance frequencies of the curved line are to be coordinated with one another when excited by the drive and deflected by Coriolis forces, nothing is disclosed about this.

In a variant of this known embodiment, bearings replace the above-mentioned soft rubber rings, and piezoelectric sensors on these bearings replace the above-mentioned strain gauges.

In another embodiment of the known mass flow meter, an air pressure sensor was provided because of the flexible couplings at the ends of the curved tube in order to avoid zero point wanderings and sensitivity fluctuations as far as possible.

In another embodiment of the known mass flow meter, the curved line is made of flexible material, while movable pressure gauges at the ends of the line are omitted. This curved line made of flexible material is also supported at three points within the housing and is therefore not self-supporting.

In yet another embodiment of the known mass flow meter, a central U-shaped curved section of the line is connected to fixed inlet and outlet nozzles via easily deformable bellows. The U-shaped curved section is also supported by an arm on the housing base. This embodiment therefore has a curved line interrupted by the pressure-sensitive bellows. This line is therefore not firmly attached to the housing with its open ends and, for this reason and because of the additional support in its middle, does not form a cantilever boom.

None of these embodiments of the known mass flow meter has been used commercially for mass flow measurement, presumably because they were not sufficiently accurate and were difficult to maintain. It is known, for example, that the Coriolis effects are very small compared to other forces that can occur in such known complicated mass flow meters, e.g. air resistance, inertia forces and forces caused by inhomogeneities in the fluid whose flow is to be measured. In particular, maintenance problems of the flexible couplings, the need for frequent recalibration of any measuring device whose function depends on couplings or flexible sections, represent serious obstacles to long-term commercial use.

The state of the art also includes a measuring device for determining physical properties of a flowable medium, especially its density or specific gravity, in which the medium is passed through a U-shaped, continuous line which is firmly attached with its open ends in a holder (US-PS 34 49 940). A frame is coupled to the U-shaped line, to which spring rods are attached in order to isolate the U-shaped line from the influence of the environment and to ensure that its operation does not depend on changes in its own natural resonance frequency as a result of temperature effects of the fluid, but is instead essentially determined only by the resonance frequency of the spring rods. The U-shaped line and the spring rods are set into vibration about an axis of vibration by a drive device, and only the amplitude of the vibrations of the line about this axis of vibration is recorded and evaluated in order to determine the material properties, in particular the density of the medium. - This measuring device is not intended for determining the mass flow by evaluating rotations of the U-shaped curved pipe around a second deflection axis that is different from the first axis of oscillation by means of Coriolis forces. Therefore, nothing is disclosed about the dimensioning of the resonance frequency around the first axis of oscillation in relation to a resonance frequency of the U-shaped curved pipe around a second deflection axis.

In an older, unpublished application (DE-OS 28 22 087) a device for mass flow measurement by evaluating rotations of a first pipe loop running essentially in one plane was proposed, which is connected on one side with its is clamped into the holder at its open ends and the flowable medium flows through it. A second pipe loop runs in a plane parallel to this, the open ends of which are clamped next to the open ends of the first pipe loop. The two pipe loops are connected in series so that the mass flow flows through them in the same direction. A vibration generator acts on the pipe sections of both pipe loops opposite the clamping points and causes them to vibrate against each other. Both pipe loops act together as vibrating elements in the manner of a tuning fork. With regard to the dynamic properties of each pipe loop, the natural frequency of the torsional vibrations of the pipe loop is preferably approximately equal to the natural frequency of the vibrations perpendicular to the plane of the pipe loop. - These measures are aimed at increasing the sensitivity of the measuring device, i.e. increasing the deflection about the second deflection axis. However, the pipe loops oscillate in resonance in such a way that the torsional deflections are not only largely due to the Coriolis forces generated by the flowing medium in the pipe loops, but are also significantly influenced by disturbing forces that are greater than the Coriolis effect and contribute to amplifying the deflection. Therefore, the mass flow cannot be determined precisely.

The present invention is therefore based on the object of creating a commercially usable device for precise mass flow measurement according to the preamble of claim 1 or claim 2, in which the naturally small effects of the Coriolis forces during the measurement are not distorted by significant disturbing forces superimposed on the Coriolis forces.

This object is achieved by the design of the device for mass flow measurement with the features specified in the characterizing part of claim 1 or claim 2, according to which the curved, continuous line itself essentially acts as a spring and the Coriolis force is essentially counteracted by the spring force of the line.

The device according to the invention as defined in claim 1 makes it possible to detect essentially only the Coriolis forces without significant influence of disturbing forces, since the curved line has no connection points that can be deformed by pressure and is elastically deformable by the Coriolis forces and is firmly attached to the holder with its open ends in such a way that it can swing freely with its center or, in other words, acts like a cantilever. The elastic deformation of the cantilever as a result of the Coriolis forces affects its deflection around the pivot axis. This eliminates disturbing influences from bearing points with increased elasticity, in particular pressure-sensitive independent coupling elements or pivoting devices. It is particularly important that the effect of the Coriolis forces on the curved line is dynamically decoupled from the effects which are otherwise due to the excitation of vibrations by the drive device: For this purpose, the curved line is shaped in such a way and is elastically deformable about the second deflection axis under the action of essentially the Coriolis forces, that its resonance frequency about the first vibration axis, about which the excitation by the drive device occurs, and its resonance frequency about the second deflection axis are different.

Preferably, the resonance frequency about the first oscillation axis, about which the curved line is deflected by the drive device, is lower than the resonance frequency about the second deflection axis, about which the curved line oscillates due to the Coriolis forces.

The Coriolis forces generated by the fluid flowing through the oscillating curved pipe cause the curved pipe to be twisted and deflected about a second deflection axis that lies symmetrically between the two legs of the curved pipe and bisects the first oscillation axis.

The device according to the invention is characterized by an uncomplicated design.

In order to prevent shearing forces on the bracket from affecting the device by attempting to shear off the curved line after its attachment points on the bracket, measures according to claim 4 or 5 are provided.

The compensation of the shear forces is preferably carried out by the measure specified in claim 4, that a further oscillating element is added to the curved, continuous line in such a way that it forms a tuning fork with this line. The further oscillating element has a natural frequency which is essentially the same as that of the curved line and preferably consists of a spring arm on which part of the drive device is arranged. The curved line and the further oscillating element oscillate in opposite phases similar to the tines of a tuning fork, and as with a tuning fork, no vibrations occur in the holder.

The measure specified in claim 5 to eliminate the harmful effects of the shear forces at the fixed attachment points of the curved line has proven to be sufficient in some cases.

In a preferred embodiment of the invention, the elastic deformation of the curved line is measured by measuring sensors and a time measuring element according to claim 10.

The timing element, which is fed with the output signals from the measuring sensors, measures the time shift or lag between the passage of two different points of the curved line through a given oscillation position as a result of the deformation of the line by the Coriolis forces. In this arrangement, the frequency and/or the amplitude of the oscillation of the line does not need to be kept constant.

The described measures according to the invention can also be advantageously provided in a device for measuring the mass flow of a flowable medium according to claim 2, in which instead of the rotations or deflections of a curved line in which Coriolis forces of the flowable medium occur, the forces which attempt to rotate the line about a deflection axis are compensated by counterforces.

Further features of such a device for measuring the Coriolis forces based on the force compensation principle are specified in claim 12.

The invention is explained below with reference to the drawing with 14 figures. It shows

Fig. 1 perspective view of an embodiment of the device for mass flow measurement;

Fig. 2 End view according to Fig. 1 showing the oscillation without flow through the line;

Fig. 3 End view according to Fig. 1 showing the oscillation at the center during flow through the pipe in an upward direction;

Fig. 4 Representation according to Fig. 3 with oscillation in downward direction;

Fig. 5 Block diagram of the drive circuit of the device according to Fig. 1;

Fig. 6 Logic diagram of the evaluation circuit of the device according to Fig. 1;

Fig. 7 Timing diagram of output signals of the device when the line is not flowing;

Fig. 8 Representation according to Fig. 7 with line flowing from left to right;

Fig. 9 Representation according to Fig. 1 of a modified embodiment;

Fig. 10 Circuit arrangement of the drive and evaluation part of the device according to Fig. 9 without showing the part sensing the deflection or deformation;

Fig. 11 Circuit arrangement of the part sensing the deflection for generating the signal B in Fig. 10;

Fig. 12 shows another circuit arrangement instead of that of Fig. 11;

Fig. 13 shows another circuit arrangement instead of that of Fig. 11; and

Fig. 14 shows a circuit arrangement of the synchronous demodulator in Fig. 10, 12 and 13.

Fig. 1 shows a device for mass flow measurement 10 , which has a stationary holder 12 with a U-shaped line 14 attached to it, which is designed as a self-supporting protruding boom. The U-shaped line 14 consists of a resilient tubular material, for example of beryllium, copper, tempered aluminum, steel, plastic. The U-shaped line 14 can have legs that converge, diverge or also run at an angle to one another. The line 14 has no interruptions. The line 14 has an inlet opening 15 and an outlet opening 16 , which are connected to one another by the inlet leg 18 , the middle leg 19 and the outlet leg 20. The legs 18 and 20 are parallel to one another, and the middle leg 19 is directed perpendicular to both. Other shapes of the line which is curved at least in sections are also usable, for example those in which the line has the shape of a continuous or a partial curve.

The shape of the U-shaped conduit 14 is not critical from a physical point of view, but the frequency characteristics are important. In the embodiment according to Fig. 1, which allows deformation, it is important that the resonance frequency about the axis WW is different from that about the axis OO , and preferably the resonance frequency about the axis WW is the smaller.

A spring arm 22 is attached to the legs 18, 20 and carries an excitation coil 24 and a drive sensor coil 23 at its end facing the center leg 19. A magnet 25 which engages the excitation coil 24 and the drive sensor coil 23 is attached to the center leg 19. The drive circuit 27 , which will be explained in detail later, serves to generate an amplified force in response to the signal from the drive sensor coil 23 in order to set the U-shaped line 14 into oscillating vibrations about the axis WW at its natural frequency. Although the U-shaped line 14 is supported on the bracket 12 as a cantilever, a sufficient oscillation amplitude about the axis WW can be achieved due to the fact that it oscillates at the resonant frequency. The line 14 oscillates essentially about the axis WW at the inlet and outlet openings 15, 16 .

At the intersections of the central leg 19 and the legs 18, 20, a first measuring sensor 43 and a second measuring sensor 44 are arranged. The measuring sensors 43, 44 are optical sensors; they give significant output signals when adjacent points of the line 14 pass through a reference plane at the center of the oscillation. An evaluation circuit arrangement 33 , which will be described later, is provided to determine and display the mass flow as a function of the time shifts of the output signals generated by the measuring sensors 44 and 43 .

The operation of the device 10 is explained in more detail with reference to Fig. 2, 3 and 4, which show the basic principle of the invention in a simplified manner. When the line 14 is set into oscillation without the flowable medium flowing through it, the inlet leg 18 and the outlet leg 16 bend about the axis WW without torsion. As shown in Fig. 2, the middle leg 19 maintains a constant angular position to the axis OO during the oscillation. However, when the line 14 is flowing through, the flowable medium through the inlet leg 18 generates a first Coriolis force perpendicular to the flow direction and perpendicular to the axis WW , while the flow in the outlet leg 20 generates a second Coriolis force which is again perpendicular to the radial direction of the flow but opposite to the first Coriolis force since the flow is in the opposite direction. As shown in Fig. 3, when the central leg passes through the center of the oscillating vibration, the Coriolis forces generated in the inlet leg 18 and the outlet leg 19 form a force couple on the U-shaped conduit 14 , whereby the central leg 19 is rotated at an angle about the axis OO . This deformation is both a bending deformation and a torsional deformation in the inlet leg 18 and the outlet leg 20. Due to the choice of frequency and the shape of the U-shaped conduit 14, practically the entire counterforce to the Coriolis force couple is an elastic spring deformation, which makes complicated measurement of the forces caused by the velocity and the inertia forces unnecessary.

At a constant frequency and amplitude, the measurement of the angular deflection of the center leg 19 about the axis OO at the nominal center of the oscillation provides an accurate indication of the mass flow.

An essential aspect of the invention is that the measurement of the deflection of the central leg 19 relative to the non-deflected central plane about the axis OO by means of the time difference between the times when the front leg, ie the inlet leg 18 , swings through the central plane and the rear leg, ie the outlet leg 20 , swings through the central plane, makes it unnecessary to maintain a constant frequency and amplitude when a subtraction of the time differences thus formed for the upward movement of the line according to Fig. 3 and the downward movement according to Fig. 4 is carried out. By driving the U-shaped line 14 at its resonant frequency, measurements can be made in a manner described in detail later without the need for simultaneous control of the amplitude. If measurements are made in only one direction, ie in the upward direction according to Fig. 3, it is necessary to maintain a precise angular alignment of the center leg 19 to the center plane. This requirement can also be avoided by subtracting the time measurements in the upward direction according to Fig. 3 and in the downward direction according to Fig. 4.

In the downward direction of the oscillation of the line according to Fig. 4, the direction of the Coriolis force couple is reversed compared to Fig. 3 and also, as shown in Fig. 4, the direction of the deflection as a result of this force couple is reversed.

In summary, it can be generally stated that the U-shaped conduit 14 , which has prescribed frequency characteristics although it has only general characteristics of shape, is oscillated only about the axis WW . The flow through the U-shaped conduit 14 causes its resilient deformation which results in a measurable angular movement of the central leg 19 about the axis OO in a first angular direction during one direction of oscillation and then in the opposite direction during the other direction of oscillation. Although with amplitude control the flow can be determined by direct measurement of the deformation, a preferred method without this requirement is to determine the time difference between the instants at which the front and rear edges of the central leg 19 move through the center plane and to subtract these time differences for upward and downward movement.

This also eliminates irregularities in the measurement result that result from incorrect alignment of the U-shaped line 14 relative to the center plane.

The electronic part of the device is explained in more detail using Fig. 5 to 8:

The drive circuit 27 detects the signals generated by movement of the magnet 25 in the drive sensor coil 23. A detector 39 compares the voltage generated by the drive sensor coil 23 with a reference voltage 37. The output level of the amplifier 41 feeding the excitation coil 25 is therefore a function of the speed of the magnet within the drive sensor coil 23. In this way, the amplitude of the oscillation of the U-shaped line 14 can be easily controlled.

The circuit according to Fig. 5 provides additional information. The output signal of the amplifier 41 is a sinusoidal signal corresponding to the resonance frequency of the U-shaped line 14. Since the resonance frequency is determined by the spring constant and the mass of the oscillating system, and since the spring constant is constant and the mass only changes depending on the density of the medium flowing through the line, any change in frequency is a function of the change in density of the medium flowing through the line. By evaluating the frequency, the density of the medium flowing through the line can thus be determined.

The construction and operation of the circuit 33 is explained with reference to Fig. 6 and the timing diagrams in Figs. 7 and 8. The evaluation circuit arrangement 33 is connected to measuring sensors 43 and 44 which produce output signals when the flags 45 and 46 which are attached to the central leg 19 pass the corresponding measuring sensor located adjacent to the central plane AA of the oscillation of the line 14. The measuring sensor 43 is connected to inverting amplifiers 47 and 48 , while the measuring sensor 44 is similarly connected to inverting amplifiers 49 and 50. The output line 52 of the inverting amplifier 50 forms a positive signal on a set side of a flip-flop 54 as a result of the double inversion. An output line 56 of an inverting amplifier 48 also forms a positive signal on a reset side of a flip-flop 54 . The flip-flop 54 is set when a positive output signal is output from the measuring sensor 44 and is reset when a positive output signal is subsequently output from the measuring sensor 43 .

Similarly, a line 58 carries the inverted output signal of the measuring sensor 43 via the inverting amplifier 47 to a set side of a flip-flop 60 , while a line 62 passes the output signal of the inverting amplifier 49 to a reset side of the flip-flop 60. The flip-flop 60 is thus set when the output signal of the measuring sensor 43 is negative and reset when the output signal of the measuring sensor 44 is subsequently negative. The output of the flip-flop 54 is connected to an AND gate 64 via a line 63. The AND gate 64 and an AND gate 66 are both connected to the output of an oscillator 67. When the flip-flop 54 has an output signal, the signal of the oscillator 67 is passed through the AND gate 64 to a line 68 and thus to a countdown input of an up/down counter 70 . When the flip-flop 60 outputs, the output of the oscillator 67 is fed through the AND gate 66 to the line 69 and to the count-up input of the counter 70 .

In operation, circuit 33 generates a count down signal at the frequency of oscillator 67 to increment counter 70 for a period of time during which sensing sensor 44 is activated prior to activation of sensing sensor 43 during downward movement of U-shaped conduit 14. A count up signal is applied to counter 70 for a period of time during which sensing sensor 43 is activated prior to activation of sensing sensor 44 during upward movement of U-shaped conduit 14 .

In Fig. 7, waveforms of the circuit 33 are shown for the condition that the U-shaped pipe oscillates without flow of the medium, but in which the vanes 45 and 46 are not exactly statically aligned with the plane AA . As can be seen from the timing diagram according to Fig. 7, the measuring sensor 44 initially switches prematurely at the ideal time shown by the vertical lines of the upstroke on the upstroke and switches late on the downstroke as a result of the incorrect alignment of the vane 46. On the other hand, the sensor 43 switches late on the upstroke and switches negatively early on the downstroke. However, if the output signals of the flip-flops 54 and 60 are analyzed and if it is taken into account that these flip-flops give either count-down or count-up signals to the counter 70 , it is shown that the flip-flop 54 , which responds to the leading edge of the Output signals of the measuring sensors 43 and 44 , produces an output signal on the upstroke, while in view of the unchanged orientation of the flags 45 and 46, the flip-flop 60 produces a similar output signal on the downstroke. Accordingly, over a complete cycle, the counter 70 is first counted backwards with a limited number of pulses by the output of the flip-flop 54 via the gate 64 , and then counted forwards in the same way by the output of the flip-flop 60 via the gate 66. The resulting count of the counter 70 thus shows zero, which is indicative of the non-flowing line.

When the line is flowing, the measuring sensor 43 is activated earlier than in Fig. 7 as a result of the deflection of the middle leg 19 by the Coriolis force pair, as shown in Fig . 8. The measuring sensor 44 is activated later for the same reason. During the upward stroke, the flip-flop 54 is therefore activated for a longer period than under the condition shown in Fig. 7, since the incorrect alignment of the flags 45, 46 is added to the deflection of the middle leg 19 by the Coriolis force pair during the upward movement. On the other hand, during the downward movement, ie when the trailing edge of the output signals of the measuring sensors 43, 44 is generated, the Coriolis force pair is reversed, so that the measuring sensor 43 is deactivated earlier and the measuring sensor 44 later. The flip-flop 60 is thus activated for a shorter period of time. As can be seen from the ratio of the activation times of the two flip-flops, the count down period of counter 70 is substantially longer than the count up period due to activation of flip-flop 60. The resulting larger count on the count down side of counter 70 provides an accurate indication of flow over one oscillation period. The count in counter 70 after a given number of oscillations is directly proportional to the mass flow in U-shaped conduit 14 during that period of time. The number of oscillations can be determined, for example, by counting the number of activations of, for example, flip-flop 54 on count down counter 71 which is connected to the output of flip-flop 54 by line 72. Upon the occurrence of N output signals from flip-flop 54 , count down counter 71 is activated which in turn activates logic sequencer 74 . The logic sequencer 74 is connected to the oscillator 67 and at the frequency of the oscillator 67 it first latches a decoder driver 77 via line 78 and then resets the up/down counter 70 via line 75. Until the logic sequencer 74 is again activated after N output signals by the flip-flop 54 , the display 80 shows the accumulated count of the counter 70 at the time of the query and accordingly shows the mass flow for the period N oscillations.

The total mass flow for a selected reset period is similarly determined by applying the output of counter 70 to digital integrator 82 which is connected to a crystal oscillator 84. Thus, the counts of counter 70 are integrated over time by the fixed frequency of oscillator 84 and the integral is provided to latch decoder driver 85 which in turn is connected to display 87 to produce a display of total mass flow for the period from the last activation of resetter 88 , ie a switch connected to digital integrator 82 .

The density factor can be determined independently of the mass flow measurements by activating flip-flop 90 at the clock frequency of the output of flip-flop 54 over line 92. The output of flip-flop 90 is coupled to an AND gate 94 which, when flip-flop 90 is activated, causes the count of oscillator 84 to be passed to the counter latch driver. With the timing information in the form of counts from oscillator 84 , and with the oscillation period from flip-flop 90 , the count in the counter latch driver is a function of the density of the medium in U-shaped conduit 14. Accordingly, display 98 displays the density factor. Since the density factor is not a linear function of the oscillation period of U-shaped conduit 14 , the reading of display 98 must be further evaluated, either by means of a table or by a microprocessor to display density or specific gravity.

In summary, the flow meter 10 can give an instantaneous mass flow, a cumulative mass flow over a given period, density information for the medium and, if necessary, a volumetric flow by dividing the mass flow by the density. This is done, as tests have shown, with accuracies of 0.1 or 0.2%. For example, an accurate measurement of a gas flow at very low velocities is possible in an accurate manner. There is no need to regulate the amplitude or frequency of the device 10 .

A modified embodiment of a mass flow measuring device 100 is shown in Fig. 9 which is similar in some respects to the device 10. The device 100 has a support 102 and a U-shaped conduit 104 attached to the support 102 and extending freely from the support 102. The U-shaped conduit 104 has an inlet 105 and an outlet 106 which are connected to one another by an inlet leg 108 and an outlet leg 109. The legs 108 and 109 can be pivoted about points 112 and 114 along the axis W'-W' to oscillate the conduit 104 about the axis W'-W' .

This can be facilitated, for example, by reducing the wall thickness of the line 104 at the pivot points 112 and 114 ; however, a line 104 with an unchanged wall thickness can also be used. The middle leg 116 connects the inlet leg 108 and the outlet leg 109 , so that a U-shaped line 104 is obtained.

In contrast to the device 10, the line 104 can preferably have a lower resistance to bending about the axis of the Coriolis force than about the oscillation axis W'-W' , since the Coriolis force deflection is zero. Magnets 118 attached to the center leg 116 by supports 119 cooperate with an excitation coil 120 to cause the U-shaped line 104 to oscillate. Preferably, the excitation coil 120 is carried on a cantilevered leaf spring 122 which is attached adjacent to the axis W'-W' and has a natural frequency which is substantially equal to that of the U-shaped line 104 filled with the fluid medium. The arrangement of the magnet 118 and the excitation coil 120 can also be reversed on the line 104 and the leaf spring 122. The leaf spring 122 can also be completely dispensed with if the holder 102 forms a very large mass compared to the mass of the U-shaped line 104 and the medium flowing through it.

In most cases, however, it is preferable to allow the U-shaped conduit 104 and the leaf spring 122 to oscillate at the same frequency but 180° out of phase in order to balance the forces of the device 100 and avoid vibration of the mount 102 .

The center leg 16 carries magnets 125, 126 depending from it. The magnet 125 engages a sensor coil 128 on the bracket 102 , while the magnet 126 similarly engages a coil 129 on the bracket 102. The magnet 125 extends into the excitation coil 131 which is concentric with the sensor coil 128 , while the magnet 126 engages the excitation coil 132 which is concentric with the sensor coil 129. Deflection sensors 133 and 134 are shown in simple representation in Fig. 9, but in detail in Figs. 11 to 13; they are arranged adjacent to the junction of the inlet legs 108 and 109 with the center leg 116 .

Fig. 10 shows details of the circuit not shown in Fig. 9. The sensor coils 128 and 129 are connected in series such that the movement of the magnets 125 and 126 in the sensor coils 128 and 129 produces a sinusoidal signal A with an amplitude proportional to the speed of the U-shaped line 104 .

This signal, the magnitude of which is proportional to the speed of movement of the magnets 125 and 126 and is a function of the amplitude of oscillation of the line 104 , is applied through an AC amplifier 135 to a diode 136 which passes only the positive portion of the sinusoidal signal to the capacitor 137. The input from the diode 136 and the capacitor 137 to a differential amplifier 138 is determined by the magnitude of the sinusoidal signal. The differential amplifier 138 compares this input with a reference voltage VR ₁ . When the voltage of the capacitor 137 exceeds the reference voltage, the differential amplifier 138 outputs an amplified signal. The output signal from the AC amplifier 135 , which is a sinusoidal signal in phase with the oscillation of the line 104 and whose magnitude is determined by a signal from the differential amplifier 138 , excites an excitation coil 120 to maintain the desired oscillating motion of the line 104. The signal A is also fed to a bridge of resistors 140, 141, 142 and a photoresistor 143. A resistor 144 is in a feedback loop between the resistors 140 and 142 , and the output from the junction of the resistors 140, 142 and 144 is connected, for example, to the negative input of the differential amplifier 145. A variable light source 147 is connected through the resistor 148 to the output of a servo amplifier 150 , on which a servo compensator 152 forms a feedback loop. The signal B , which is an AC signal proportional to the small non-zero deformation of the U-shaped line 104 produced as will be described later with reference to Figs. 11, 12 and 13, is fed through resistor 153 to an input of the servo amplifier 150. The output of the servo amplifier 150 is compared with the voltage VR ₂ through the servo compensator and fed through a resistor to the light source 147. As a function of the magnitude of the signal B with respect to VR ₂ at the servo amplifier 150, the intensity of the light source 147 is controlled. For example, as the intensity of the light source 147 increases, the resistance of the photoresistor 143 is reduced, causing the voltage between the negative and positive inputs of the differential amplifier 145 to change accordingly. The output of the differential amplifier 145 is thereby 180° out of phase with the signal A . As signal B increases, light source 147 is attenuated and the resistance of photoresistor 143 is increased, thereby amplifying the output of differential amplifier 145 in phase with signal A. The output of differential amplifier 145 is connected to excitation coils 131 and 132 , which are connected in series in opposite directions. As shown in Fig. 9, current flowing through excitation coils 131 and 132 produces a torque to attract magnet 125 and repel magnet 126 , both of which are connected to center leg 116. This torque across center leg 116 cancels the deflection of center leg 116 due to the Coriolis forces generated by flow through line 104 .

Resistors 155, 156 or 157 can be connected to the excitation coils 131 and 132 via a switch 159 in order to achieve an optional load and to set a larger or smaller torque on the center leg 116. The output of the series-connected excitation coils 131 and 132 is connected to an input of a synchronous demodulator 162 , which will be described in more detail with reference to Fig. 14.

The output of the synchronous demodulator 162 provides a DC signal proportional to the mass flow, which can be displayed or further processed.

According to Fig. 11, the deflection sensors 133 and 134 in Fig. 9 consist of a left and right flag 164, 165 projecting from the line 104. A left flag 166 and a right flag 167 are attached to the holder 102. When the center leg 116 oscillates, the flags 164 and 165 interrupt the light beam coming from the light sources 169, 170 so that it does not reach the photo sensors 181, 182. The point at which the flags 164, 166 and 165, 167 interrupt the light beam is preferably at the level of the center of the oscillation of the center leg 116 , but the flags on one side can be slightly offset from those on the other side. If the center leg 116 is deflected at an angle relative to the holder 102 as a result of the Coriolis forces generated by the flow of the medium in the U-shaped line 104 , there is a change in the time course between the coverage of the light beam by the flags 164, 165 and 166, 167. The time difference and the direction depend on the generated Coriolis forces and the direction of oscillation at a constant oscillation speed of the center leg 116. The photosensor 181 is connected to the reset sides of the flip-flops 185, 186 , with an inverting amplifier 188 arranged in front of the flip-flop 186. Differentiating capacitors 191, 192 are each connected to a reset input. Similarly, the photosensor 182 is connected to the set side of the flip-flop 185 and through the inverting amplifier 189 to the set side of the flip-flop 186 through the differentiating capacitors 193, 194. When the flags 164 and 166 close, a positive signal is output by the photosensor 181 which activates the reset side of flip-flop 185 and when flags 165 and 167 close, a similar positive signal is generated by photosensor 182 to activate the set side of flip-flop 185. Flip-flop 185 is activated for the period between the closing of the respective flag assemblies. On the other hand, when flags 164, 166 and 165, 167 open, a falling edge or negative signal is generated by photosensors 181, 182 which activates flip-flop 186 through inverting amplifiers 189. Flip-flop 186 is activated for the period between the opening of the respective flag assemblies. The outputs of the flip-flops 185, 186 are applied to the inputs of the differential integrator 198 via the resistors 195 and 196. An integrating capacitor 200 is associated with the resistor 195 , while the integrating capacitor 201 is associated with the resistor 196 .

The output signal B of the differential integrator 198 depends on the periods of activation of the flip-flops 185, 186. When the center leg 116 merely oscillates without deflection, the time differences between the opening and closing of the vanes are constant and the input signals to the differential integrator 198 are equal to one another, so that no signal B is generated. When Coriolis forces are generated, the center leg 116 is deflected clockwise on one stroke and counterclockwise on the other stroke, so that the closing of the vanes on one side occurs before one stroke and after the other stroke, while the other side of the vanes closes late on the first stroke and early on the next stroke. The activation of the flip-flops 185 and 186 thus does not occur for equal periods of time, and the differential integrator will output a corresponding direct current signal B in the plus or minus sense, depending on the phase of the deflection of the center leg 116 relative to the up and down stroke.

Another embodiment for the same purpose is shown in Fig. 12. Adjacent to the intersection of the inlet leg 108 and the middle leg 116 and the outlet leg 109 and the middle leg 116 , the line 104 is loaded with deflection-dependent resilient variable resistors. The variable resistors 204, 205 are connected to the resistors 207 and 208 as a bridge which is applied to a voltage source and is connected to the AC differential amplifier 210. With a simple oscillating movement of the U-shaped line 104 , the variable resistors 204, 205 change uniformly and give identical input signals to the amplifier 210 . When deflected due to the Coriolis forces, one resistance of the variable resistors 204, 205 increases while the other decreases, so that different input signals are applied to the amplifier 210 and an output signal is produced in the form of an alternating current signal which is proportional in magnitude and direction to the different resistance values of the variable resistors 204, 205 .

The output of amplifier 210 is fed to a synchronous demodulator 211 which, in conjunction with signal A, produces a DC output signal proportional in magnitude and direction to the deflection of U-shaped line 104 due to Coriolis forces. Synchronous demodulator 211 is similar to previously described synchronous demodulator 162 described with reference to FIG. 14.

A similar arrangement for generating the signal B is shown in Fig. 13. Here, however, a pivotable member 215 is centrally attached to the center leg 116 and carries a drag bar 217 which is mounted on the member 215 so as to be freely rotatable in equilibrium. Piezo crystals 219, 220 are arranged between the drag bar 217 and the center leg 116. When the center leg 116 undergoes a simple oscillating movement, the drag bar 217 follows this movement without rotating about the member 215. However, when the line 104 is deformed as a result of Coriolis forces occurring, the center leg 116 rotates relative to the drag bar 217 , whereby forces in the opposite direction act on the crystals 219, 220 , which emit signals due to the piezoelectric effect. The outputs of the crystals 219, 220 are connected to the AC differential amplifier 222 , which in turn is connected to the synchronous demodulator 224 to produce, in conjunction with the signal A , a DC signal B whose magnitude and direction is proportional to the deflection of the U-shaped line 104. Instead of the crystals 219, 220, variable resistors 204, 205 according to Fig. 12 can be used.

The synchronous demodulator 162 in Fig. 10, which is similar to the synchronous demodulators 211, 224 , is described with reference to Fig. 14. The input signal in the form of an alternating current signal is applied via line 225 to the primary winding 227 of a transformer. The output of the two opposite ends of secondary windings 228 is 180° out of phase. Switches in the form of FET transistors 230, 231 are located in the outputs of the secondary windings 228. A comparator 233 connected to the signal A produces positive or negative output signals depending on the ratio of the signal A to a reference voltage VR ₃ . The output signal of the comparator 233 is a positive or negative square wave and is applied to the inverting amplifier 235 which inverts the signal. One part of the square wave signal turns on the FET transistor 230 while turning off the other transistor 231 , and the other part of the signal turns on the transistor 231 and turns off the transistor 230. The part of the input signal on line 225 that is in phase with the signal A is fed into the RC circuit 237 formed by the resistor 238 and the capacitor 239 , which outputs a DC signal that is proportional to the RMS value of the input signal to the RC circuit 237. This DC signal can be displayed or further processed.

The previously described device 100 uses deflection sensors 133, 134 to determine the magnitude and direction of small initial deflections of the U-shaped conduit 104 due to Coriolis forces and generates a DC signal proportional in direction and magnitude to this deflection. The DC signal B is a feedback signal which determines the moment generated by the excitation coils 131 and 132 or a counterforce which prevents significant forces beyond the initial deflection. The sensor coils 128 and 129 , which maintain the frequency of vibration of the U-shaped conduit 104 through the drive circuit described above, produce a signal A which is in phase with the Coriolis forces and ensures precise control of the excitation coils 131, 132 , synchronization of the output signal of the AC amplifier 135 for driving the U-shaped conduit 104 , and precise demodulation of the synchronous signal of the excitation coils 131 and 132 to produce a DC output proportional to the flow velocity.

Claims (12)

1. Device for measuring the mass flow of a flowable medium by evaluating twists in a continuous line that is curved at least in sections by Coriolis forces of the flowable medium that flows through the line, with a drive device that causes the line to vibrate about a first oscillation axis relative to a holder to which the line is attached, and with a measuring sensor device that detects the twists caused by the forces that twist the line about a second deflection axis that is different from the first oscillation axis, characterized in that, in order to detect essentially the Coriolis forces, the curved, continuous line ( 14 ) has no connection points that can be deformed by pressure, is elastically deformable by the Coriolis forces and is firmly attached with its open ends in the holder ( 12 ) in such a way that it protrudes from the holder as a cantilever and has a resonance frequency about the first oscillation axis (WW) that differs from its resonance frequency about its second deflection axis (OO) is different. 2. Device for measuring the mass flow of a flowable medium by evaluating forces exerted on a continuous line which is curved at least in sections and in which Coriolis forces occur from the flowable medium which flows through the line, with a drive device which causes the line to oscillate about a first oscillation axis relative to a holder to which the line is attached, and with a measuring sensor device which measures the forces which attempt to rotate the line about a second deflection axis which is different from the first oscillation axis, characterized in that in order to detect essentially the Coriolis forces, the curved, continuous line ( 14 ) has no connection points which can be deformed by pressure, is elastically deformable by the Coriolis forces and is firmly attached with its open ends in the holder ( 12 ) in such a way that it protrudes from the holder as a cantilever and has a resonance frequency about the first oscillation axis (WW) which differs from its resonance frequency by its second deflection axis (OO) is different. 3. Device for mass flow measurement according to claim 1 or 2, characterized in that the resonance frequency about the first oscillation axis (WW) is lower than the resonance frequency about the second deflection axis (OO) . 4. Device for mass flow measurement according to one of claims 1-3, characterized in that a further oscillatable element is provided which is suitable for oscillating in the manner of a tuning fork to the curved, continuous line ( 14 ). 5. Device for mass flow measurement according to one of claims 1-4, characterized in that the mass of the curved, continuous line ( 14 ) with the medium contained therein is very small compared to that of the holder ( 12 ). 6. Device according to claim 4, characterized in that the drive device which sets the curved line ( 14 ) into oscillating vibrations has a magnet ( 25 ) fastened to the curved line ( 14 ) or the further oscillatable element and, adjacent to the magnet, a drive sensor coil ( 23 ) and an excitation coil ( 24 ) which are attached to the further oscillatable element or the curved line ( 14 ), and in that the drive device comprises a current source which feeds the excitation coil and which can be controlled by a signal emitted by the drive sensor coil ( 23 ). 7. Device according to claims 4 or 6, characterized in that the further oscillatable element consists of a spring arm ( 22 ) fastened to the first oscillation axis (WW) of the curved line ( 14 ). 8. Device according to claims 1 or 6, characterized by such a design of the drive device that the curved line ( 14 ) and the further oscillatable element oscillate with constant frequency and amplitude about the first oscillation axis. 9. Device according to claims 6 and 8, characterized in that the drive device comprises a peak value detector fed with the signal generated by the drive sensor coil ( 23 ) with an amplifier which feeds into the excitation coil ( 24 ) such a current that a preselected oscillation amplitude of the curved line is maintained according to the feedback principle. 10. Device according to one of claims 1, 3-7, characterized in that a first measuring sensor ( 43 ) and a second identical measuring sensor ( 44 ) are arranged adjacent to the curved line ( 14 ) and symmetrically to the deflection axis (OO) , that each measuring sensor ( 43, 44 ) is set up to emit an output signal when the curved line ( 14 ) passes a predetermined oscillation position, and that a time measuring element ( 54, 60, 64, 66, 70 ) is provided which is supplied with the output signals of the measuring sensors ( 43, 44 ) and measures a time shift between the output signals which is proportional to the mass flow. 11. Device according to claim 10, characterized in that the time measuring element ( 54, 60, 64, 66, 70 ) subtracts the time shifts measured for one of the two directions of oscillation from the time shifts measured in the other direction of oscillation. 12. Device according to claim 2, characterized in that for measuring the Coriolis forces according to the compensation principle, the measuring sensor device consists of a deflection detector (deflection sensors 133, 134 ) for determining the deflection of the curved line due to the elastic deformation by the Coriolis forces about the deflection axis (OO) located centrally between the side legs of the line, a circuit controlled by the deflection detector which feeds currents into excitation coils ( 131, 132 ) which are arranged in such a way that they counteract a deflection of the line about the deflection axis with a counterforce, and a measuring device which measures the counterforce by means of the currents.