WO1993002347A1

WO1993002347A1 - Viscometer

Info

Publication number: WO1993002347A1
Application number: PCT/NL1992/000134
Authority: WO
Inventors: Cornelis Blom
Original assignee: Vaf Instruments BV
Current assignee: Vaf Instruments BV
Priority date: 1991-07-23
Filing date: 1992-07-22
Publication date: 1993-02-04
Anticipated expiration: 1994-01-23
Also published as: CN1069337A; NL9101288A

Abstract

A viscometer provided with a transducer for the conversion of viscosity parameter of a fluid into an electrical signal. This transducer comprises a support element (22), an oscillation device (24), which at one end is firmly fastened on the support element (22) and at the other, free, end has an oscillating body (25). In addition the transducer comprises a magnetic drive coil (SZ) and a detection coil (SD) for initiating and maintaining oscillation of the oscillation device (24). The oscillating body (25) is provided with a magnet (26), the drive and detection coils being adjacent to the oscillating body.

Description

Viscometer.

The invention relates to a viscometer provided with a trans¬ ducer for the conversion of a viscosity parameter of a fluid into an electrical signal, comprising a support element, an oscillation device, which at one end is firmly fastened to the support element and at the other, free, end has an oscillating body, and a magnetic drive coil and a detection coil for initiating and maintaining oscillation of the oscillation device. Such a viscometer is disclosed by US Patent 4,488,427. In the case of the known meter the support element is formed by a block or plate in which a hole has been provided. The oscillation device comprises a tube whose internal diameter is larger than that of the hole and one end of which is attached to the block coaxially with the hole. The other end of the oscillation device has an oscillating body or mass which seals the tube at that end. The tube is made to oscillate in a torsional mode. For this purpose a stiff rod is fitted in the internal volume of the tube in a position clear of the tube walls, one end of which rod is fastened to the free end of the tube, whilst the free end of the stiff rod extends through the hole in the support block to beyond said support block. Fastened to the free end of the stiff rod is a crossbar which, at its free ends, has magnets that interact with magnetic drive and detection coils. A feedback loop connected between said coils serves to initiate and maintain oscillation of the oscillation device via the stiff rod. The object of the invention is to provide a viscometer of the type mentioned in the preamble whose construction is as simple as possible.

This object is achieved, according to the invention, in that the oscillating body is provided with a magnet and in that drive and detection coils are adjacent to the oscillating body.

In known systems, the excitation system for causing the oscillation device to oscillate, which system in principle consists of drive and detection coils and the magnets interacting therewith, is always outside the volume containing the fluid whose the viscosity is to be measured. In general this causes sealing problems, or else an indirect drive, for example the stiff rod, has to be applied. Since, according to the invention, the magnet is positioned in the oscillating body, the advantage achieved is that sealing measures can be dispensed with, whilst a direct drive is possible because of which the viscometer according to the invention is more accurate.

It is observed that a fluid property detection system is known from the US Patent US-A- ,005,599 and comprises an oscillating body provided with a magnet and a magnetic drive coil arranged adjacent said body and outside of a container enclosing the oscillating body. The oscillating body is fastened to a support element by means of torsion strips. By the drive coil and the magnet the oscillating body and torsion strips are made to oscillate in a torsion mode. However, a detection coil is not used for deriving a measure of the amplitude of the oscillation as in the viscometer according to this invention.

In contrast to the viscometer according to this invention, in the system known from the US Patent US-A-4,005,599 a frequency measurement principle is used instead of the amplitude measurement method of the invention. The amplitude measurement results in an improved resolution.

Furthermore, in the known system two frequencies are measured by feeding back the signal in phase and shifted in phase. Thereby a new balanced situation should occur in switching over, resulting in a lower response rate of the measurement. For, during a sufficiently known period it should be measured at one of the frequencies in order to let the transient phenomenon to decay.

The US Patent US-A-3,763t69 discloses a density measurement, in which the translation vibration is used instead of the rotational vibration according to the invention. Consequently, the drive and detection coils are arranged such that a translation movement is caused and measured. The rotational vibration could possibly occur, in which the hollow tube follows a rotating ellipse. This rotational vibration, however, should be obviated for keeping the viscosity influence in the measuring result as small as possible.

Further embodiments and developments of the invention are described in the accompanying subordinate claims.

The invention will be explained in more detail below by reference to the diagrams, in which: Figure 1 shows diagrammatically an embodiment of the invention;

Figure 2 shows diagrammatically the position of the coils and the oscillating body of an embodiment according to the invention; Figure 3 shows a preferred embodiment according to the invention; Figure 4 shows the baseplate of the embodiment according to Figure 3; Figure 5 shows the electrical diagram of an embodiment according to the invention.

The invention is based on the damping effect of a fluid on a vibrating oscillating element submerged therein. A feedback system is used to maintain the oscillating body in mechanical vibration by supplying energy to the system in order to compensate for viscous and other inherent mechanical and electrical losses. This is achieved by means of an amplification loop in the feedback system. For example the complex shear viscosity can be determined by measuring the resonance frequency of the oscillation device and its damping.

In the case of the basic design mentioned above, the viscometer according to the invention is by way of illustration provided with a transducer according to Figure 1, for converting a viscosity parameter of a fluid into an electrical signal. This transducer comprises a support element or baseplate 1, which supports an oscillation device. This oscillation device comprises a torsion rod 2 which is rigidly fastened to the baseplate 1 at right angles thereto, and an oscillating body formed by a cylindrical mass 3 which is in turn rigidly attached to the free end of the torsion rod 2. The combination of torsion rod 2 and cylindrical mass 3 is made to vibrate in a torsional mode, and maintained in vibration. For this purpose an excitation system is used which comprises a magnet 4 and a drive coil 5« As Figure 1 shows, the magnet 4 is fitted in the cylindrical mass 3- Preferably a second drive coil 6 is used. At least the cylindrical mass 3 of the oscillation device, is submerged in fluid or liquid 11 held in the volume 12 bounded by the baseplate 1, a wall 13. for example of cylindrical shape, and a bottom 14. If the viscometer must be used as a flow-type meter, the liquid may, for example, be introduced through an opening in the wall 13 above the oscillating body and taken out by an opening in the bottom 14 or by omitting the bottom, or vice versa. As can be seen from Figure 1 the coils 5 and 6 are fitted adjacent to wall 13 and preferably outside volume 12.

The minimum distance of the lateral face of the cylindrical mass 3 from the wall 13 of the sample holder is determined by the requirement that the shear wave must have almost died down when it reaches the wall 13- In the case of a Newtonian liquid the amplitude of the shear wave is attenuated by a factor 1000 over a distance of 2 mm in the case of a viscosity of 100 MPa-s and a frequency of 400 Hz. Figure 2 diagrammatically shows a preferred embodiment of the excitation system and detection system according to the invention. The cylindrical magnet 4 is shown as an N-S dipole. The cylindrical mass in which the magnet is fitted is not shown in Figure 2. The excitation system comprises the drive coils 5 and 6, and the detection system in a preferred embodiment comprises two detection coils 7 and 8. These detection coils 7 and 8 *_re at right angles to the drive coils 5 and 6. In addition, noise suppression coils and 10 are shown in Figure 2. The cylindrical magnet is preferably magnetised diametrically, as is diagrammatically illustrated by the arrow S N in Figure 2. The magnet is thus positioned between the drive and the detection coils. The vibrations of the oscillation device in torsional mode are brought about by means of an alternating magnetic field, which is generated by an excitation signal supplied to the drive coils. This excitation signal can be generated by a frequency synthesiser. The frequency and the magnitude of the excitation signal can be controlled by means of a microprocessor.

The amplitude of the torsional vibration is measured by means of the detection coils. The detection signal from these detection coils, which in the case of a constant excitation signal is a measure of the viscosity of the liquid in which the cylindrical mass is submerged, can be amplified, filtered by means of a bandpass filter and fed to a voltmeter which is read out by the microprocessor.

As an alternative, however, an electrical feedback loop can be connected between the drive coils and the detection coils, maintaining the oscillation in the torsional mode of the oscillation device, whilst the detection signal can be taken off the detection coils, in which case the ratio of detection signal and excitation signal can be determined by means of a microprocessor as a measure of the viscosity of the fluid to be measured. This will be dealt with further when discussing Figure 5- In order to minimise mechanical coupling between the torsion mass and the baseplate, the chosen moment of inertia of the baseplate is large relative to the moment of inertia of the torsion mass.

The most important advantage of the abovementioned design of the transducer for the viscometer is the very small temperature dependence of the resonance frequency of the oscillation device, with the result that a temperature influence can readily be compensated for by means of a microprocessor. The temperature dependence of the oscillation device can be further reduced by making use of a temperature-independent nickel alloy for the torsion rod. For example, a material is chosen which is known under the designation "Thermelast 4290" and supplied by Vakuumschmelze GmbH. In order to reduce the temperature dependence of the resonance frequency of the oscillation device even further, the torsion rod may additionally be subjected to heat treatment. The cylindrical magnet is for example made from an anisotropic magnetic material commercially available under the designation "VACOMAX 145".

All construction materials near the magnet, such as coil holders and supports are made from a synthetic material, for example a fibre-reinforced phenolformaldehyde. This material is to be preferred because of its rigidity, its expansion coefficient, which approximates very closely that of the copper coils, and its nonconducting properties. Conducting materials would, as a consequence of eddy current effects, reduce the damping of the oscillating mass. In the oscillating mass 3. a cylindrical magnet 4 is fitted, which is magnetised diametrically, as shown in Figure 2 by the arrow S N. The excitation signal is supplied to the drive coils 5 and 6 connected in parallel and having the same winding direction. The direction of the magnetic dipole of the cylindrical magnet 4 relative to the detection coils 7 and 8 is chosen in such a way that the induction voltage of the detection coil is proportional to the angular velocity of the magnet 4.

On this basis, the magnetic dipole is directed towards the edges of the detection coils 7 and 8, as can be seen in Figure 2. This dipole direction has two advantages. Firstly the induced voltage is proportional to the angular velocity of the oscillating mass and secondly the induced voltage is at a maximum for this orientation.

The noise suppression coils 9 and 10 are connected in series with the detection coils 7 and 8, respectively, but the winding direction of the noise suppression coils 9 and 10 is opposite to that of the detection coils 7 and 8, respectively. This configuration minimizes the noise, including that caused by external fields.

Because of the symmetric arrangement of the coils, the cross¬ talk between the drive coils 5 and 6 and the detection coils 7 and 8 is very small. Figure 3 shows a preferred embodiment of a transducer of the flow-type for a viscometer. The transducer comprises a cylinder 1 with a bore 16 through which the fluid flows according to the arrows PI and P2. Cylinder 15 is provided with a recess 17 for fitting the drive, detection and noise suppression coils, which are not shown. These coils are mounted on the coil formers 18-21 (see Figure 4) which are fixed by screws to the baseplate 22. The rod 23 is fastened to the baseplate 22 or forms an integral part thereof, the torsion rod 24 being rigidly attached to this assembly. Fastened to the free end of the torsion rod 24 is the oscillating mass 25, within which the magnet 26 is located. The oscillating mass 25 is made to vibrate according to arrow P3 by means of the alternating magnetic field produced by the drive coils and of the magnet 26. The baseplate 22, after the coil formers 18-21 with the coils mounted thereon have been screwed to said baseplate, is screwed onto the cylinder 15 by means of screws of which only screw 27 is visible in Figure 3- The cylinder 1 is additionally provided with a temperature sensor 29 whose output signal is used for temperature compensation.

The baseplate is shown separately in Figure 4. This shows the position of the coil formers 18-21. The baseplate is additionally provided with holes 30 for the passage of the fluid whose viscosity para¬ meter is to be measured. These holes provide for a laminar flow along the oscillating body 25.

Figure 5 illustrates the electrical diagram corresponding to the transducer according to Figures 3 and 4. For the sake of simplicity the drive coils are shown as one coil SZ and the combination of the detection coils and noise suppression coils is shown as one coil SD. Connected between the coils SZ and SD is a feedback loop T, which incorporates successively the amplifier V2, the filter Fl, the automatic gain control AGC and the amplifier V3. The signal appearing at the output of the filter Fl and the signal at the connection point of the amplifier V3 and the automatic gain control AGC are fed to the inputs 12 and 13 of the microprocessor μP, which, on the basis of the ratio of said signals derives a viscosity output signal which is emitted at output 02. Between the viscosity signal at the output 02 of the microprocessor μP and the signals applied to said microprocessor there is a relationship which can either be determined experimentally or can be calculated. The signal originating from the temperature sensor 29 is supplied to an amplifier VI, whose output signal is fed to input II of the microprocessor μP. A temperature signal appears on output 01 of the microprocessor.

The signal originating from the temperature sensor 29 is supplied, via the amplifier VI, to the microprocessor μP, which carries out a temperature compensation. In the context of experiments conducted with a viscometer according to the invention, it was found that the temperature dependence of the resonance frequency was less than 3 ^• 10~⁵ %C^'1. The advantage of this low temperature dependence of the resonance frequency is that compensation by means of the temperature measured can be carried out by means of the microprocessor.

Furthermore the detection system of the viscometer according to the invention is very simple and comprises only a small number of components.

Claims

CLAIMS 1. Viscometer provided with a transducer for the conversion of a viscosity parameter of a fluid into an electrical signal, comprising a support element, an oscillation device which at one end is firmly fastened to the support element and at the other, free, end has an oscillating body, and a magnetic drive coil and detection coil for initiating and maintaining oscillation of the oscillation device charac¬ terised in that the oscillating body is provided with a magnet and in that the drive and detection coils are adjacent to the oscillating body. 2. Viscometer according to Claim 1, characterised in that the oscillating body is placed in a volume intended to contain a fluid and bounded by a wall, and the drive and detection coils are placed adjacent to said wall outside the volume.

3. Viscometer according to Claims 1 or 2, characterised in that the plane of the drive coil is at right angles to that of the detection coil.

4. Viscometer according to Claim 3. characterised in that the drive and/or detection coils are each formed by coils placed opposite one another. > Viscometer according to one of Claims 1-4, characterised in that the magnet is cylindrical and is magnetised diametrically.

6. Viscometer according to one of Claims 1-5, characterised in that the direction of the magnetic dipole of the magnet relative to the detection coil is chosen in such a way that the induced voltage of the detection coil is proportional to the angular velocity of the magnet.

7. Viscometer according to Claim 6 characterised in that the magnetic dipole is directed towards the edges of the detection coil.

8. Viscometer according to one of the previous claims in which the oscillation device comprises a torsion rod connected to the oscillating body, characterised in that the torsion rod consists of a temperature- independent nickel alloy.

9- Viscometer according to one of the previous claims, characterised in that a noise suppression coil having opposite winding direction is connected in series with a detection coil. 10. Viscometer according to one of Claims 2-9, characterised in that the wall of the volume in which the oscillating body is positioned extends as far as the support element and thus bounds the volume for a fluid, and that the support element is provided with holes for the passage of the fluid whose viscosity parameter is to be measured.