WO2000022412A1

WO2000022412A1 - Piezo-electric viscometer

Info

Publication number: WO2000022412A1
Application number: PCT/NL1999/000631
Authority: WO
Inventors: Dirk Hendrik Petrus Snel
Original assignee: V.A.F. Instruments B.V.
Priority date: 1998-10-13
Filing date: 1999-10-13
Publication date: 2000-04-20
Also published as: NL1010308C2

Abstract

Viscometer provided with a transducer for converting a viscosity parameter of a fluid into an electrical signal. The viscometer comprises a support element, a torsion bar, which at one end is joined to the support element and at the other end is provided with an oscillator, a piezo-electric drive device, for bringing the oscillator into rotational oscillation about an axis of rotation essentially coincident with the axis of the torsion bar, and a piezo-electric detection device for detecting the rotational oscillation of the oscillator. The support element is provided with means for fixing said element to a wall bordering a chamber in which the fluid to be measured is located. The piezo-electric drive device and the piezo-electric detection device are housed in a cavity in the oscillator.

Description

Piezo-electric viscometer

The invention relates to a viscometer provided with a transducer for converting a viscosity parameter of a fluid into an electrical signal, comprising a support element, a torsion bar, which at one end is joined to the support element and at the other end is provided with an oscillator, a piezo-electric drive device, for bringing the oscillator into rotational oscillation about an axis of rotation essentially coincident with the axis of the torsion bar, and a piezoelectric detection device for detecting the rotational oscillation of the oscillator. A viscometer of this type is disclosed in US Patent 4 905 499. The known device for detecting the viscosity of a fluid comprises a vibrator produced from piezo-electric elements and which is able to vibrate about its axis of rotation. The vibration is transmitted, by means of a transmission element that is subjected to torsion, to an oscillator made up of a detection head to be immersed in the fluid. The vibration of the detection head is measured by means of a vibration sensor. The vibrator and the detection head are also joined to one another via the vibration element. The viscosity parameter is determined from, inter alia, the detection result of the vibration sensor.

The aim of the invention is to provide a viscometer of the type mentioned in the preamble with which the construction is simpler and the meter can easily be incorporated in a line or other chamber for a fluid, all of this during operation of a process installation to which the line or the chamber belongs.

Said aim is achieved according to the invention in that the support element is provided with means for fixing said element to a wall bordering a chamber in which the fluid to be measured is located and in that the piezo-electric drive device and the piezo-electric detection device are housed in a cavity in the oscillator. By placing the piezo-electric drive device and detection device in a cavity in the oscillator it is possible to place the entire sensor in the fluid and to fix the support element of the transducer to a wall bordering the fluid, so that the sensor can be introduced even into operating process installations, whilst only little space is needed for the transducer.

In one embodiment the piezo-electric drive device comprises a piezo-electric strip which extends transversely to the axis of rotation of the oscillator and some distance away from the latter and which at one end is firmly fixed to the oscillator and at the other end is freely movable in the direction of oscillation of the oscillator, an electrical signal applied to the piezo-electric strip bending the strip in a plane perpendicular to the axis of rotation. Because the piezo-electric strip has to be clamped in the oscillator at one end only, it can be accommodated in its entirety in the oscillator. As a result of the movement of the free end of the piezo-electric strip, and making use of the mass of the strip, a reaction force is generated which drives the detection head. Thus, no further fixed reference is needed for the embodiment constructed in this way.

Depending on the inertia of the system comprising torsion bar and oscillator and the type of fluid, the piezo-electric drive device is provided with a second corresponding piezoelectric strip, which extends parallel to the first piezo-electric strip on the other side of the axis of rotation of the oscillator, and the end of the second strip attached to the oscillator is located diametrically opposite the fixed end of the first piezo-electric strip. Optionally, the free end of at least one piezo-electric strip can be weighted.

A further aim of the invention is to simplify the viscometer even further. Said simplification is achieved in that, in one embodiment of the invention, an acceleration detector is used as detection device, which acceleration detector is connected via a phase shift device to the piezo-electric drive device. By this means the oscillation is automatically maintained with a very simple circuit. An amplitude measurement is used with the viscometer according to the invention, as a result of which not only is a simple circuit possible but a better resolution during measurement can also be obtained.

In an embodiment which is preferably to be used the acceleration detector consists of two piezo-electric strips, which run parallel to one another and in a plane extending parallel to the direction of oscillation of the oscillator and one end of each of said strips being firmly fixed to the oscillator in locations diametrically opposite one another, whilst the other ends of said strips are freely movable in the direction of oscillation of the oscillator and point in opposite directions, and the outputs of the piezo-electric strips are connected to the respective inputs of an instrumentation amplifier, the output of which is connected to the phase shift device.

The invention will be explained in more detail below with reference to the drawings. In the drawings:

Figure 1 shows, diagrammatically, one embodiment of the transducer of the viscometer according to the invention;

Figure 2 shows a cross-section through a practical embodiment of the transducer of the viscometer according to the invention;

Figure 3 shows a cross-section along the line III-III in Figure 2; Figure 4 shows a cross-section along the line IV-IV in Figure 2; Figure 5 shows a circuit diagram of the electrical part of the viscometer according to the invention; and

Figure 6 shows a front view, partially in cross-section, of one embodiment of the viscometer system according to the invention incorporated in a pipeline.

The invention is based on the insight that a fluid has an effect, in particular a damping effect, on a vibrating oscillation element that is immersed therein. A feedback system can be used to keep the oscillation element in mechanical vibration by supplying energy to the system to compensate for viscous and other inherent mechanical and electrical losses. This is achieved by means of amplifiers in the feedback system. For example, the complex shear viscosity can be determined by measuring the resonance frequency of the oscillation element and the damping thereof.

Using the basic concept mentioned above, the viscometer according to the invention is, by way of example, provided with a transducer for converting a viscosity parameter of a fluid into an electrical signal. Said transducer comprises a meter housing 1 and a support element or baseplate 2 firmly joined thereto. An oscillation device is supported by said baseplate 2, which oscillation device consists of a torsion bar 3. which is fixed rigidly to the baseplate 2 and perpendicular thereto, and an oscillator 4, which is made up of a cylindrical mass 4 which, in turn, at one end is firmly joined to the free end of the torsion bar 3. The combination of torsion bar 3 and cylindrical mass 4 is brought into, and maintained in, vibration in torsion mode. A piezo-electric drive device 5 is used for this purpose. As can be seen from Fig. 1. the drive device 5 is accommodated in the cylindrical mass 4.

At least the cylindrical mass 4 of the oscillation device is immersed in a fluid or liquid 6 which is present in the space which is delimited by the baseplate 2 and a, for example cylindrical, wall of the housing 1. Said viscometer is suitable as a continuous flow meter and the fluid to be measured can, for example, be introduced through openings 7 in the baseplate 2, can flow over the torsion bar 3 and the cylindrical mass 4 and discharge through the top opening in the housing 1 , or vice versa. The housing 1 , with the oscillator 4 accommodated therein, can be integrated in a pipe of a pipe system in order to measure, continuously or at any desired point in time, a viscosity parameter of the fluid or liquid flowing through the pipe system. For a static measurement the top opening of the housing can be closed.

An excitation signal in the form of an alternating voltage is fed to the drive device 5 via the feed conductor 8 of the drive device 5 which is fed through the torsion bar 3. Said excitation signal can. for example, be generated by a frequency synthesiser. The frequency and magnitude of the excitation signal can be controlled by means of a microprocessor. The cylindrical mass 4 is brought into rotational oscillation, i.e. torsional vibration, about an axis of rotation which is coincident with the axis of the torsion bar 3 by feeding the excitation signal to the drive device 5.

The amplitude of the torsional vibration is measured by means of the piezo-electric detection device 9. The detection output signal from the detection device 9 is fed to the outside via the output conductor 10 fed through the torsion bar 3. At a constant excitation signal, the detection signal is a measure for the viscosity of the fluid in which the cylindrical mass has been immersed and can be amplified, in a manner which is not shown, filtered by means of a bandpass filter and fed to a voltmeter that is read by the microprocessor.

The piezo-electπc detection device is likewise accommodated in the cavity of the oscillator 4.

The temperature sensor 1 1 is accommodated in the wall of the oscillator. The output signal from said sensor 1 1 is fed to the outside via the conductor 12.

Figures 2. 3 and 4 show an oscillator according to one embodiment of the invention. The feed and discharge conductors of the drive and the detection device are not shown in the cross-section in Fig. 2. The oscillator 4 is made of a non-magnetic or non-magnetisable material. A very suitable material is stainless steel of austenitic structure, such as stainless steel 316. A plastic could suffice in some applications. The oscillator 4 is closed at the top and bottom. One end of the torsion bar 3 is firmly joined to the bottom closure 13 of the oscillator 4, the other end of the torsion bar 3 being firmly joined to the baseplate 2.

The piezo-electric drive device accommodated in the oscillator 4 comprises a piezoelectric strip 14 which extends transversely to the axis of rotation of the oscillator 4 and some distance away from the latter. The piezo-electric strip 14 is firmly fixed at one end to the oscillator, whilst the other end is freely movable in the direction of oscillation of the oscillator 4. When an electrical signal or voltage is fed to the piezo-electric strip, said strip is bent in a plane perpendicular to the axis of rotation of the oscillator 4. As a result of this bending, and by making use of the mass of the piezo-electric strip 14, a reaction force is generated which drives the oscillator 4. It can also be seen from Figure 2 that the temperature sensor 11 is accommodated in the oscillator 4, in this case in the bottom closure 13 of said oscillator. Temperature measurement is essential in order to be able to make necessary temperature corrections. Thus, for measurement, onlv the oscillator 4 has to be introduced into a line or chamber and the support element 2 fixed to the line or the wall bordering the chamber by means of fixing means, which are not shown. The possible construction of the fixing means can be left to a person skilled in the art.

The piezo-electric drive device is clearly visible in the cross-section in Figure 3. In the embodiment in Figures 2 and 3 the piezo-electric drive device is provided with a second piezo-electric strip 15 which extends on the other side of the axis of rotation of the oscillator 4. Thus, two piezo-electric strips 14 and 15 which run parallel to one another are used. That end of the second piezo-electric strip 15 which is fixed to the oscillator is located diametrically opposite the fixed end of the first piezo-electric strip 14. The piezo-electric strips 14 and 15 are firmly clamped in the associated wall elements 16 and 17 of the oscillator 4.

In order to increase the mass inertia of the free ends of the piezo-electric strips 14 and 15, said ends can be weighted. This embodiment is not shown, but any person skilled in the art can conceive a weighting construction. In this embodiment the detection device is an acceleration detector. An embodiment of the acceleration detector which is preferably to be used can be seen more clearly in Fig. 4. Said acceleration detector comprises two supports 18, 19 which are firmly fixed to the oscillator 4. Said supports 18 and 19 support, on one side, the respective piezo-electric strips 20 and 21. The other ends of the strips 20 and 21 are able to move freely. When the oscillator 4 rotates, for example in the direction of the arrow R, during the back and forth rotational vibration of the oscillator 4 as a consequence of the supply of an alternating voltage to the piezo-electric strips 14 and 15, the forces Kl and K2, respectively, are exerted on the piezo-electric strips 20 and 21 as a consequence of the inertia of said strips and in particular of weights mounted on the free ends thereof (not shown). As a result of the forces Kl and K2 the piezo-electric strips 20 and 21 are bent, as a result of which a detection output signal of the piezo-electric strips is generated and can be sampled. The viscosity parameter can be determined from the amplitude of the detection output signal and that of the signal supplied to the oscillator drive coil.

The mass inertia of the strips 20 and 21 can be increased, in a manner which is not shown, by weighting the free ends thereof.

As can be seen from Figures 2, 3 and 4 the driving piezo-electric strips 14 and 15 run perpendicular to the detecting strips 20, 21.

As an advantageous alternative, however, an electrical feedback circuit can be connected between the piezo-electric strips 14 and 15 and the detection device consisting of the piezo-electric strips 20 and 21, by which means the oscillation in the torsion mode of the oscillation device is automatically maintained, it being possible to determine, by means of, for example, a microprocessor, the viscosity of the fluid or liquid to be measured from the ratio of the amplitude of the detection signal and that of the excitation signal supplied to the strips 14 and 15. Said feedback circuit in the measurement circuit of the viscometer according to the invention is shown in Fig. 5.

The piezo-electric strips 20 and 21 are shown diagrammatically as piezo elements in Fig. 5 and designated as PI and P2. The piezo elements PI and P2 are connected via parallel resistors Rl and R2 to an instrumentation amplifier LA. The polarities are indicated. In the circuit diagram shown in Fig. 5, which is preferably to be used, the feedback circuit consists of the said instrumentation amplifier LA, the filter FI, the phase shift device PD, the automatic gain control AGC and the amplifier VI . The driving piezo-electric strips 14 and 15, indicated as piezo elements P3 and P4, are connected to the output of the amplifier VI. The oscillator 4 shown in Figures 1, 2, 3 and 4 is brought into, and maintained in, vibration in the torsion mode by means of the series circuit comprising the piezo elements PI and P2, the feedback circuit and the piezo elements P3 and P4.

The signal produced at the output of the phase shift device FD and the signal at the connection point of the amplifier VI and the automatic gain control AGC are fed to the inputs 12 and 13 of the microprocessor μP which derives a viscosity output signal on the basis of the ratio of the amplitudes of the said signals, which viscosity output signal is produced at the output O2. A relationship exists between the viscosity signal at the output O2 of the microprocessor μP and the signals fed thereto, which relationship can either be determined experimentally or can be calculated. The signal originating from a temperature sensor mounted in the wall of the oscillator 4, which temperature sensor is indicated in Figure 5 by Rt, is fed to the amplifier V2, the output signal of which is fed to the input II of the microprocessor μP. A temperature signal is produced at the output Ol of the microprocessor. The signal originating from the temperature sensor is preferably used by the microprocessor μP to perform temperature compensation. In order to keep the mechanical coupling between the torsion mass (torsion bar and oscillator) and the baseplate 2 as small as possible, the moment of inertia of the baseplate 2 is chosen to be high compared with the moment of inertia of the torsion mass.

A significant advantage of the design described above for the transducer for the viscometer is the very low temperature dependence of the resonance frequency of the oscillation device, so that the microprocessor is easily able to perform the temperature compensation by means of the measured temperature.

Moreover, the electrical part of the viscometer is very simple and contains only a small number of components.

The most significant advantage of the viscometer according to the invention is that it requires very little maintenance (low cleaning frequency).

Furthermore, the viscometer according to the invention has, especially by use of the detection device and the circuit diagram according to Fig. 5, the advantage that the translation of the oscillator is compensated for.

Ln the event of translational vibration, which in practice virtually always occurs as a side effect, the piezo-electric strips 20 and 21 will move in opposing directions to one another, so that the voltages which are fed to the instrumentation amplifier IA do not give rise to any change in output. In the event of rotational vibration, which is what is desired with this embodiment, the voltages generated are summed and an effect on the output of the amplifier IA is thus detectable.

Figure 6 shows an embodiment of the invention, in which the transducer of the viscometer according to the invention is integrated in a pipe section 22, which is provided with fixing flanges 23 and 24 for incorporating the pipe section 22 in a line system. For the purposes of illustration, the pipe section 22 has been cut open at the location of the viscometer. The oscillator 4 is joined to the base 2 via the torsion bar 3. The torsion bar 3 and the base 2 are hollow for feeding through the feed and output leads for the piezo-electric strips and temperature sensor accommodated in the oscillator 4. The base 2 is firmly connected to the pipe section 22 by means of the closure element 25. The closure element 25 is accommodated in the bore 26 of the pipe section 22 and is screwed tightly into the latter by means of the screw thread 27. The feed and output leads coming from the base 2 and combined in the cable 28 run to the electronic circuit of the viscometer incorporated in the housing 29.

Claims

1. Viscometer provided with a transducer for converting a viscosity parameter of a fluid into an electrical signal, comprising a support element, a torsion bar, which at one end is joined to the support element and at the other end is provided with an oscillator, a piezoelectric drive device, for bringing the oscillator into rotational oscillation about an axis of rotation essentially coincident with the axis of the torsion bar, and a piezo-electric detection device for detecting the rotational oscillation of the oscillator, characterised in that the support element is provided with means for fixing said element to a wall bordering a chamber in which the fluid to be measured is located and in that the piezo-electric drive device and the piezo-electric detection device are housed in a cavity in the oscillator.

2. Viscometer according to Claim 1, characterised in that the piezo-electric drive device comprises a piezo-electric strip which extends transversely to the axis of rotation of the oscillator and some distance away from the latter and which at one end is firmly fixed to the oscillator and at the other end is freely movable in the direction of oscillation of the oscillator, an electrical signal applied to the piezo-electric strip bending the strip in a plane perpendicular to the axis of rotation.

3. Viscometer according to Claim 2, characterised in that the piezo-electric drive device is provided with a second corresponding piezo-electric strip, which extends parallel to the first piezo-electric strip on the other side of the axis of rotation of the oscillator, and in that that end of the second strip attached to the oscillator is located diametrically opposite the fixed end of the first piezo-electric strip.

4. Viscometer according to Claim 2 or 3, characterised in that the free end of at least one piezo-electric strip is weighted.

5. Viscometer according to one of Claims 1 - 4, characterised in that the detection device is an acceleration detector, the viscosity parameter being determined from the amplitude of the signal emitted by said detector and that of the signal fed to the piezo-electric drive device.

6. Viscometer according to Claim 5, characterised in that the acceleration detector is connected to the piezo-electric drive device via a phase shift device.

7. Viscometer according to Claim 6, characterised in that the acceleration detector consists of two piezo-electric strips, which run parallel to one another and in a plane extending parallel to the direction of oscillation of the oscillator and one end of each of said strips being firmly fixed to the oscillator in locations diametrically opposite one another, whilst the other ends of said strips are freely movable in the direction of oscillation of the oscillator and point in opposite directions, and in that the outputs of the piezo-electric strips are connected to the respective inputs of an instrumentation amplifier, the output of wluch is connected to the phase shift device.

8. Viscometer according to Claim 6, characterised in that the free end of at least one piezo-electric strip is weighted.

9. Viscometer according to Claim 7 or 8, characterised in that the piezo-electric strips of the piezo drive device run perpendicularly to those of the detection device.

10. Viscometer according to one of the preceding claims, characterised in that a temperature sensor is incorporated in the wall of the oscillator.