WO1998003864A1

WO1998003864A1 - Method for measuring medium properties

Info

Publication number: WO1998003864A1
Application number: PCT/NL1997/000434
Authority: WO
Inventors: Matheus Jozef Maria Coolen
Original assignee: Matheus Jozef Maria Coolen
Priority date: 1996-07-22
Filing date: 1997-07-21
Publication date: 1998-01-29
Also published as: NL1003654C2; AU3466397A

Abstract

A method and a device for measuring medium properties, wherein an electric oscillator device is used, which is provided with an oscillation element in the form of a membrane, which is brought into contact with a medium to be measured, and with a displacement sensor, by means of which the displacement of said membrane is measured, and wherein furthermore an oscillator signal is applied to said oscillator device, wherein the membrane is driven by means of the oscillator signal in several frequency ranges in which resonance occurs in the membrane, and the influence of the medium properties (viscosity/density/elasticity, etc.) on the resonance (frequency/amplitude/bandwidth) drift upon a change of one of the properties is determined for each of the frequency ranges, after which the membrane is brought into contact during operation with a medium to be measured, and the resonance drift in the various frequency ranges is measured with appropriate means, and the measured values are fed to an electronic data-processing device, which computes the various medium properties from said values.

Description

Title: Method for measuring medium properties.

The invention relates to a method for measuring medium properties, wherein an electric oscillator device is used, which is provided with an oscillation element in the form of a membrane, which is brought into contact with a medium to be measured, and with a displacement sensor, by means of which the displacement of said membrane is measured, and wherein furthermore an oscillator signal is applied to said oscillator device. The invention furthermore relates to a device for carrying out said method.

From Swiss Patent 683 375 a measuring device is known which membrane comprises a housing and a membrane, which is oscillated by suitable means at a frequency which corresponds with the resonance frequency of the membrane. This known measuring device is mainly used for measuring the presence of a liquid or the level thereof. To that end said membrane is provided in the wall of a housing or a pipe, and the drift of the resonance changes as the degree of contact between the membrane and the liquid increases or decreases, which resonance drift is subsequently measured and from which it can be concluded whether any liquid is present or what level said liquid reaches. It is furthermore stated in said patent specification that it is possible to draw conclusions from the influence of the liquid on the frequency and the amplitude of the oscillation with regard to specific properties of the liquid, such as the viscosity, the density, etc.

A drawback of this known device is the fact that only two measuring values, namely the frequency and the amplitude, of a single resonance, namely the resonance at the resonance frequency, are measured, from which consequently only two material properties can be determined. In that case said material properties can only be determined relatively inaccurately, because also other material properties influence the resonance drift.

The object of the invention is to obviate this drawback and to provide a method and a device by which a great deal more properties of a particular substance can be measured in a much more accurate manner.

The invention thereby utilizes the fact that a membrane which is fixedly clamped in a housing will generally exhibit several frequency ranges, whereby standing waves, that is, resonances, occur in the membrane, and it is furthermore based on the new perception that the influence of the various material properties on the resonance drift is different for the various frequency ranges. In order to accomplish the above objective the method according to the invention is characterized in that the membrane is driven by means of the oscillator signal in several frequency ranges in which resonance occurs in the membrane, whereby the influence of the medium properties (viscosity/density/elasticity, etc.) on the resonance

(frequency/amplitude/bandwidth) drift upon a change of one of the properties is determined for each of the frequency ranges, after which the membrane is brought into contact during operation with a medium to be measured, and the resonance drift in the various frequency ranges is measured with appropriate means, and the measured values are fed to an electronic data-processing device, which computes the various medium properties from said values, and/or which determines the extent to which said properties are different from those of a reference medium.

With the method according to the invention the membrane is oscillated in several frequency ranges in which standing waves, that is, resonances occur. It is possible to determine the influence of each of said properties on the frequency, the amplitude and the bandwidth of the resonance by determining the influence of the various medium properties on the resonance drift for each of said frequency ranges. During operation of the measuring device the membrane is brought into contact with the medium to be measured, whereby the frequency, the amplitude and the width of said oscillation is measured in each of the various frequency ranges. In this manner three measured values are obtained for each frequency range, whereby the influence of each of the material properties is known for each of said measured values. In this manner a large number of measured signals is obtained, from which the various material properties can be computed arithmetically. Furthermore the influence of temperature and pressure can be taken into account in these computations. In this manner it is possible to determine a wide range of material properties with great accuracy. Furthermore it is possible not to determine the material properties themselves, but only the extent to which they deviate from those of a reference medium.

As already said before, the influence of the medium properties on the drift of the resonance in the various frequency ranges can be determined by computation, but this is a rather labourious method. In an advantageous embodiment of the method according to the invention, in order to overcome this problem, the influence of the medium properties on the drift of the resonance in the various frequency ranges is determined by bringing the membrane into contact with a reference medium whose properties are known, and by varying respectively one or more of said properties to a known degree and recording their influence of the resonances.

The invention furthermore relates to a device for carrying out the method according to the invention, comprising an electric oscillator device in the form of a housing, which is provided with an oscillation element in the form of a membrane which is fixedly attached thereto, which is otherwise displaceable, for contact with a medium to be measured. Furthermore said device comprises a displacement sensor which measures displacement of the membrane, and an electric connecting device connected to said oscillation element and to said displacement sensor, which applies an oscillator signal to said oscillation element and which delivers one or more measuring signals. This device is characterized in that a frequency generator for generating said oscillator signal is provided, which is arranged in such a manner that frequencies are generated at least in those ranges in which the standing waves required in the membrane are generated and wherein the oscillation source is mechanically coupled to said membrane.

The measurement is carried out by means of oscillations of a flat membrane, so that contamination or clogging up of the measuring device is practically excluded. The absence of penetrating parts in the liquid makes the measuring device insensitive to contamination, for example bacterial contamination, and the medium to be measured is not contaminated either. The measuring device is also highly suitable for use in the foodstuffs industry, depending on the selection of materials. Since the oscillations of the membrane are preferably generated and measured perpendicularly to the membrane, the membrane is substantially insensitive to film formation on the membrane, which often has an adverse effect on a measuring process. The invention will be explained in more detail with reference to the drawing.

Figure 1 is a schematic sectional view, not to scale, of an embodiment of a device according to the invention; Figures 2a and 2b show by way of example the oscillation pattern of a membrane of the device according to the invention when passing through a particular frequency range for a membrane which moves in air and for a membrane which is in contact with a liquid on one side respectively; and

Figure 3 is a block diagram of an embodiment of an electronic processing circuit of a device according to the invention.

Figure 1 schematically shows an embodiment of the device according to the invention. This device is used for measuring medium properties, in particular typical medium properties, such as the density, the viscosity, the elasticity, etc. The device according to Figure 1 comprises a housing 1 and a membrane 2 attached thereto, which is movable per se and which functions to make contact with a medium M to be measured, which is present outside housing 1. Disposed within housing 1 is an oscillation element 3 in the form of a loudspeaker, whose coil is mechanically coupled to a membrane 2 via a connection 4. Present within housing 1 is furthermore a displacement sensor 5 in the form of a microphone, which indirectly senses the displacement of the membrane 2 by measuring the pressure changes in housing 1. The Figure finally shows a processing circuit 6 present within housing 1, which processing circuit could in principle also be provided outside housing 1 instead of inside said housing. Provided within housing 1, but not shown in Figure l for easy reference, is furthermore an electric connecting device, which applies an external oscillator signal to oscillation element 3 and which passes one or more measuring signals outside housing 1. The electronic processing circuit 6 and the electric connecting device, which may consist of a number of leads and an electric terminal which is accessible outside housing 1, will be discussed in more detail later herein with reference to Figure 3. In the embodiment shown in Figure l measuring device 1 is attached, by means of a flanged joint, for example, to the wall of a pipe, through which a medium M may be passed, which medium will be in contact with membrane 2 in that case. Instead of bringing the membrane into contact with a medium that flows through a pipe, it is also possible, of course, to attach device 1 to the wall of a chamber (not shown) , in which a stationary medium is present, which is in contact with membrane 2. Oscillation element 3, which may be of the loudspeaker type, generates standing oscillations on membrane 2. More in particular a commercially available loudspeaker, whose speaker coil is rigidly connected^' to the centre of the membrane via connection 4, has been used as an oscillation element for a prototype of the device according to the invention. Essential for oscillation source 3 is, however, that it sets membrane 2 oscillating.

The operation of the device according to Figure 1 is as follows. Oscillation element 3 exerts a force, preferably a sinusoidal force, on the membrane at a selected frequency. Oscillation element 3 is thereby driven by an external frequency generator. The frequency generator thereby passes through a frequency range whose limits are determined by the physical properties of the measuring device, which are in turn determined by the desired measuring range. Membrane 2 and housing 1 thereby behave partially as a damped mass spring system. The displacements of membrane 2 upon being oscillated are measured by displacement sensor 5, which in this case takes place indirectly by measuring the pressure changes generated by the displacements within housing 1. Of course it is also possible to measure the displacements of the membrane with a direct displacement sensor. When oscillation element 3 sets membrane 2 oscillating, standing waves are generated in the membrane at various frequencies, which cause membrane 2 to start oscillating to a smaller or larger extent, that is, so-called resonance peaks are formed at certain frequencies. Figure 2a shows the oscillation pattern of an oscillated membrane for the situation wherein the membrane is in contact with air.

Figure 2b shows the oscillation pattern for the same membrane for the situation in which membrane 2 is in contact with a liquid. It is apparent from these two Figures that the resonance peaks, which occur in the situation in which the membrane is in contact with a liquid, are shifted in comparison with the resonance peaks for the situation in which the membrane is in contact with air. The term shifted in this context means that for the situation in which the membrane is in contact with the liquid, the resonance peaks occur at a different frequency, exhibit a different amplitude, and that furthermore the bandwidth of the resonance peaks is different than for the situation wherein the membrane is in contact with air. It has become apparent that the influence of the various material properties to be measured on the drift of the resonance peaks of the membrane is different with different frequencies. For each device 1 the influence of the various materials properties on the drift, or the changed drift, of the resonance peaks at the various frequencies is first determined by bringing membrane 2 into contact with a reference medium having known material properties. This established data is recorded and stored in an electronic data-processing device. Then the reference medium is substituted for a medium whose material properties are to be measured, and the drift of the resonance peaks, that is, the frequency at which they occur, the amplitude and the band width of each of said resonance peaks is measured, and the various material properties are computed by means of the measured values that are already known. In this manner a great many material properties of the medium to be measured can be determined with great accuracy, and if said material properties deviate from the desired material properties to an unacceptable extent, a control action may be undertaken.

Oscillation element 3 is thereby driven by a frequency generator. A drawback of a frequency generator is the fact that under certain circumstances the output signal has a frequency value which if different from the input frequency value. This might lead to an inaccuracy in the measurement. In order to prevent this the frequency generator is calibrated prior to every measurement on the basis of the resonance frequency of the housing 1. Housing 1 has a highly accurate resonance frequency, which is determined by its configuration. By comparing the frequency of the output signal of the frequency generator with the resonance frequency of the housing and adjusting the frequency generator in case of a deviation, a highly accurate measurement can be obtained.

Resonance peaks which are connected with the construction of the device are those resonance peaks whose frequency does not change when the device is brought into contact with another medium. In the drawing these are the peaks which are indicated at 8. Of these peaks only the amplitude will change. With the other peaks the frequency, the amplitude as well as the bandwidth will change. The shift thereof is indicated by arrows. Some of said arrows cross either other arrows or the oscillation curve thereby. It will be apparent the measurements will interfere with one another in the range of said crossings. Consequently, in order to obtain usable measuring signals, measurements can only be carried out on those resonance peaks which do not influence each other upon shifting. Membrane 3 must be constructed in such a manner, therefore, that there are a number of resonance peaks within the desired measuring range, which shift in such a manner upon being measured that they are not influenced by other resonance peaks.

As is shown in Figure 3 , displacement sensor 5 is connected by means of a lead 20 to an amplifier 30 which forms part of a series circuit with successively an n-th order band-pass filter 31, a top detector 32 and an n-th order low-pass filter, which latter filter produces an amplitude signal 34 as a measuring signal. Amplifier 30 amplifies the output signal of the displacement sensor 5 in such a manner that it rises above the noise level. Since several mutually different signal strengths are required for different types and thicknesses of the membrane, amplifier 30 is configured such that it can be adjusted once for each membrane so as to be able to deliver and process a maximum signal. Since medium M is in many cases circulated by a pump, and since oscillations may also be transferred to medium M and thus to membrane 4 by other mechanical means, the measuring signal contains all kinds of interfering influences. Band-pass filter 31, which is connected after amplifier 30, only passes the desired signal and cuts off all interference signals that have a higher or a lower freguency than the pass band.

The top detector 32, which is connected after band-pass filter 31, determines the amplitude of the signal which passes through band-pass filter 31. Top detector 32 may include a capacitor thereby, which is charged by means of a resistor via a diode and which can be discharged again by means of a leakage resistor.

Low-pass filter 33, which is connected after top detector 32, limits the rate at which the output signal of top detector 32 rises, in order to damp strong interference signals which might cause the peaks to rise strongly until a predetermined moment.

As is shown in Figure 3, a medium temperature sensor 37 is provided on membrane 2 within housing 1, which is connected to an A/D converter 36 via lead 50 for delivering a medium temperature signal as a measuring signal. Since the viscosity and also other properties of a great many liquids are temperature-dependent, it is important to know the temperature of the medium.

Figure 3 also shows a housing temperature sensor 38 provided within housing 1, which senses the temperature of housing 1, which sensor is connected to the A/D converter 36 by means of a lead 54 for delivering a housing temperature signal as a measuring signal. The fact is that in a particular environment the measuring device as a whole will become too hot to ensure a reliable operation. Housing temperature sensor 38 delivers housing temperature signal 54 directly to the A/D converter 36, in order to be able to determine externally whether the housing temperature is still within a safe range.

Processing circuit 6 also comprises a power supply circuit 42, which receives voltage from central processing unit (CPU) 60 via leads 43, generates power supply voltages therefrom and produces power supply monitoring signals 45 as measuring signals. More in particular power supply circuit 42 comprises a converter, which generates all internally required supply voltages from said external voltage. The supply voltages of +5V and -5V are applied to the A/D converter 36 as supply voltage monitoring signals for monitoring the voltage.

Finally processing circuit 6 as shown in Figure 3 comprises a further detector 41, which receives the external oscillator signal via lead 39 and which produces as a measuring signal a compensation signal representing a voltage drop over lead 39 and further leads of the connecting device.

As already said before, processing circuit 6 is also supplied from the external oscillator signal. Since the external oscillator signal may be supplied via a long cable and it is not known how long this cable will be and what diameter it will have, the extent of the voltage drop over said cables is not known, either. The external oscillator signal is furthermore an amplified output signal from an external oscillator, whereby the amplifier will not deliver a constant external oscillator signal at all times. In order to be able to eliminate all these influences, which are partially caused by the length of the cables and long-term drifts in the external amplifier and the oscillator, the top value of the external oscillator signal is measured continuously by the A/D converter.

The processing device is connected to a central processing unit (CPU) , which comprises a microprocessor 60, which composes a characteristic pattern of the medium to be measured, from which the viscosity, the elasticity and/or the density are determined through comparison with predetermined measuring results. The microprocessor controls the voltage- controlled oscillator incorporated in the processing device for generating the external oscillator signal for driving oscillation element 3 in the measuring device.

Central processing uni 60 is capable of retrieving signals present in the A/D converter via lead 55 and of storing, analysing and further processing said measured data.

The oscillator signal from oscillation element 3 is software-controlled.

The range of the oscillator is adjusted in such a manner by said software that the specific parts of the characteristic pattern of the medium to be measured are gone through. The software computes the extent of the deviation of the measured properties and the subsequent control action of a control device -^'(not shown) for bringing said values to the

*. desired level again.

Claims

C A I M S

1. Method for measuring medium properties, wherein an electric oscillator device is used, which is provided with an oscillation element in the form of a membrane, which is brought into contact with a medium to be measured, and with a displacement sensor, by means of which the displacement of said membrane is measured, and wherein furthermore an oscillator signal is applied to said oscillator device, characterized in that the membrane is driven by means of the oscillator signal in several frequency ranges in which resonance occurs in the membrane, the influence of the medium properties (viscosity/density/elasticity, etc.) on the resonance

2. Method according to claim 1, characterized in that the influence of -each of the medium properties on the drift of the resonance in the various frequency ranges is determined by bringing the membrane into contact with a reference medium having known properties, and changing respectively one of said properties to a predetermined degree and determining the influence of said change on the drift of the resonances.

3. Method according to claim 1 or 2, characterized in that said oscillator signal is generated by means of a frequency generator, and that the operation of said frequency generator is calibrated prior to every measuring cycle by adjusting the output signal of the frequency generator at the resonance frequency of the housing to which the membrane is attached.

4. Device for carrying out the method according to claim 1, 2 or 3, comprising an electric oscillator device in the form of a housing, which is provided with an oscillation element in the form of a membrane which is fixedly attached thereto, which is otherwise displaceable, for contact with a medium to be measured, with an electric displacement sensor which measures displacement of the membrane, and with an electric connecting device connected to said oscillation element and to said displacement sensor, which applies an oscillator signal to said oscillation element and which delivers one or more measuring signals, characterized in that a frequency generator for generating said oscillator signal is provided, which is arranged in such a manner that frequencies are generated at least in those ranges in which the standing waves (resonances) are generated and wherein the oscillation source is mechanically coupled to said membrane.

5. Device according to claim 4, characterized in that the membrane exhibits resonance peaks at such frequencies that when the membrane is brought into contact with a medium to be measured and when changes in the properties of said medium occur, the various resonance peaks do not interfere with each other.