Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ , CA 0223~027 1998-04-16
- 1 - EH 305 EP
7 . 04 . 1997
Apparatus for e~tabl;~h;ng and/or monitoring a
predetermined f;ll;ng level in a conta;ne~
The invention relates to an apparatus for establishing
and/or monitoring a predetermined filling level in a
5 container, which apparatus comprises:
- a mechanical oscillatory structure, which is
fitted at the level of the predetermined filling
level,
- an electromechanical transducer,
-- which has at least one transmitter,
--- to which an electrical transmission signal
is applied and
--- which excites the mechanical oscillatory
structure to produce oscillations, and
-- which has a receiver
--- which picks up the mechanical oscillations
of the oscillatory structure and converts
them into an electrical reception signal,
- an evaluation unit,
-- which picks up the reception signal and deter-
mines its frequency, compares said frequency
with a reference frequency and generates an
output signal which indicates that the mechani-
cal oscillatory structure is covered by a
2 5 charge material if the frequency has a value
which is less than the reference frequency, and
that it is not covered if the value i8 greater,
and
- a control loop, which regulates a phase differ-
ence existing between the electrical transmis~ion
signal and the electrical reception signal to a
specific, constant value at which the oscillatory
structure oscillates at a resonant frequency.
Filling level limit ~witches of thi~ type are employed in
35 many branches of industry, in particular in the chemical
industry and in the foodstuffs industry. They serve the
~ , CA 0223~027 l998-04-l6
- 2 - EH 305 EP
7 .04.1997
purpose of limit level detection and are used, for
example, as a protection against overfilling or as a
safeguard against pumps r~lnn;ng dry.
DE-A 44 19 617 describes an apparatus for establishing
5 and/or monitoring a predetermined filling level in a
container. This apparatus comprises:
- a mechanical oscillatory structure, which is
fitted at the level of the predetermined filling
level,
- an electromechanical transducer,
-- which has at least one transmitter,
--- to which an electrical transmission signal
is applied and
--- which excites the mechanical oscillatory
structure to produce oscillations, and
-- which has a receiver
--- which picks up the mechanical oscillations
of the oscillatory structure and converts
them into an electrical reception signal,
- an eyaluation unit,
--, which p~cks up the reception signal and deter-
-¢ ~ mines~ its frequency, compares it with a
re~erence frequency and generates an output
signal which indicates that the mechanical
2 5 oscillatory structure is covered by a charge
material if the frequency has a value which is
less than the reference frequency, and that it
is not covered if the value is greater, and
- a control loop, which regulates a phase differ-
ence existing between the electrical transmission
signal and the electrical reception signal to a
specific, constant value at which the oscillatory
structure oscillates at a resonance frequency.
The control loop is formed, for example, in that the
reception signal i8 amplified and fed back to the
transmission signal via a phase shifter.
- CA 0223S027 1998-04-16
- 3 - EH 305 EP
7.04.1997
To date, it has not been possible to employ apparatuses
of this type additionally for measurements in highly
viscous media or in hydrous or viscous foams, since
reliable excitation of the mechAnical oscillatory
structure to produce oscillations at the resonant
frequency is not ensured in these applications.
First investigations which led to the invention described
below showed that the cause of this lies in the fact that
the apparatus described is a complex oscillatory system
composed of the mechanical oscillatory structure, the
electromechanical transducer and the control loop. The
individual components are not completely isolated
electrically and separated mechanically from one another.
Both electrical and mechanical coupling occurs.
The fixed value of the phase difference corresponds to
the resonance of the system when the oscillatory struc-
ture oscillates in gases or in liquids. However, if the
oscillation quality of the apparatus is reduced for any
reason, then the consequence of this is that the fixed
value of the phase difference no longer exists. There is
no frequency at which the mechanical oscillatory
structure executes oscillations having an amplitude
different from zero and the phase difference has the
fixed value. This phase difference cannot be set by the
control loop. Incorrect functioning consequently occurs.
A reduction in the oscillation quality occurs, for
example, when the movement of the mechanical oscillatory
structure is attenuated, for example by the latter being
immersed in a viscous medium or in a liquid-contAining or
viscous foam. Furthermore, the oscillation quality is
reduced by energy losses within the apparatus, for
example due to instances of material fatigue or instances
of asymmetry, for example on account of asymmetrical
deposit formation, which lead to asymmetrical restoring
forces. In principle, any type of energy 1088, be it
~ CA 0223~027 l998-04-l6
- 4 - EH 305 EP
7 . 04 . 1997
oscillation energy released to a charge _aterial or
energy released to the container via a fastening of the
apparatu~, leads to a reduction in the oscillation
quality.
One object of the invention is to specify an apparatus of
the type mentioned in the introduction, in which a fixed
phase difference, which is independent of the oscillation
quality of the apparatus, exists between the transmission
signal and the reception signal at the resonant frequency
of the mechanical oscillatory structure.
This is achieved according to the invention in that a
reception signal line, which transmits the reception
signal, is connected via an electrical impedance to a
tran~mission signal line which transmits the transmission
signal.
In accordance with one mhodiment of the invention, the
reception signal has three components, namely
- a measurement signal, which is governed by the
oscillation of the mechanical oscillatory struc-
ture,
- a first additional signal, which is governed by
electrical coupling between the transmitter and
the receiver, and
- a second additional signal, which is governed by
mechanical coupling between the transmitter and
the receiver, and
- in which the impedance is determined in such a
way that the first and the second additional
signals have virtually identical amplitudes, and
have a profile which is virtually in antiphase.
In accordance with a further refinement, the transmitters
23 and the receiver 24 are piezoelectric elements.
In accordance with a further refinement, the phase
CA 0223~027 1998-04-16
- 5 - EH 305 EP
7.04.1997
difference between the tran~mission signal and the
reception signal as~umes values between 20~ and 90~ or
between -90~ and -20~, referring to a reference value.
In accordance with a further refinement, the impedance is
a capacitance.
In accordance with one refinement of the invention, the
impedance is a resistance, an inductance or a combination
of at least one resistance and/or at least one inductance
and/or at least one capacitance.
The invention and further advantages will now be
explained in more detail with reference to the figures of
the drawing, in which figures an exemplary ~mhodiment is
illustrated; identical elements are provided with identi-
cal reference symbols in the figures.
Figure 1 shows a longit--~;nAl section through a
mechanical oscillatory structure and an
electromechanical transducer;
Figure 2 shows a diagrammatic illustration of the
transducer of figure 1 and a circuit
connected thereto;
Figure 3a shows the amplitude of the measurement signal
as a function of the frequency for a
high and for a low oscillation quality;
Figure 3b shows the phase of the measurement signal as
a function of the frequency for a high
and for a low oscillation quality;
Figure 4a shows the amplitude of a first additional
signal as a function of the freguency;
Figure 4b shows the phase of the first additional
CA 0223~027 1998-04-16
- 6 - EH 305 EP
7.04.1997
signal as a function of the frequency;
Figure 5a shows the amplitude of a second additional
signal as a function of the frequency
with and without additional electrical
coupling;
Figure 5b shows the phase of the second additional
signal as a function of the frequency;
Figure 6a shows the amplitude of the reception signal
as a function of the frequency for a
high and for a low oscillation quality;
Figure 6b shows the phase of the reception signal as a
function of the frequency for a high
and for a low oscillation quality;
Figure 7a shows the amplitude of the reception signal
as a function of the frequency for a
high and for a low oscillation quality
with additional electrical coupling;
Figure 7b shows the phase of the reception signal as a
function of the freguency for a high
and for a low oscillation quality with
additional electrical coupling;
Figure 1 shows a longitl~A;n~l section through an exem-
plary embodiment of a mechanical oscillatory structure 1.
It has an essentially cylindrical housing ll, which i8
closed off flush at the front by a circular diaphragm 12.
A thread 13 is integrally formed on the housing 11, by
means of which thread the apparatus can be screwed into
an opening (not illustrated), which iB arranged at the
level of the predetermined filling level, in a container.
Other fastening methods which are known to a person
skilled in the art, for example by means of flanges
- CA 0223~027 l998-04-l6
- - 7 - EH 305 EP
7 . 04 . 1997
integrally formed on the housing 11, can likewise be
employed.
Two oscillating bars 14 which point into the container
are integrally formed on the diaphragm 12 on the outside
of the housing 11. These bars are caused to oscillate
perpendicularly to their longitll~;n~l axis by means of an
electromechanical transducer 2 arranged in the interior
of the housing 11.
However, the invention is not restricted to mechanical
oscillatory systems having two oscillating bars; it can
also be employed in limit switches which have only one or
no o~cillating bars. In the cases mentioned last, for
example only the oscillating diaphragm comes into contact
with a charge material situated in the contA;ner.
The transducer 2 has three annular piezoelectric elements
arranged in a stack. A respective metal ring 21, 22 is
arranged at both ends of the stack. The metal ring 21
facing the diaphragm bears on pre~sure pins 121, which
are integrally formed on an outer annular surface of the
diaphragm 12. A tensioning bolt 3 pointing into the
interior of the housing ll is provided in the center of
the diaphragm 12. This bolt is provided with insulation
31 and passes through the transducer 2. A nut 32 is
screwed onto that end of the tensioning bolt 3 which is
remote from the diaphragm. This nut bears on the metal
ring 22 remote from the diaphragm. The nut 32 is
tightened. The diaphragm 12 is thus pretensioned.
The two piezoelectric elements facing the diaphragm
operate as transmitters 23 and the piezoelectric element
remote from the diaphragm serves as a receiver 24. Figure
2 shows a diagrammatic illustration of the transducer 2
and an electrical circuit connected thereto. Each of the
transmitters 23 and the receiver 24 has two electrodes
which are in each case arranged on their annular surfaces
CA 0223~027 1998-04-16
- 8 - EH 305 EP
7.04.1997
and of which a respective first electrode is connected
via a line 4 in each case to a reference potential, for
example ground. The respective second electrode of the
transmitters 23 is in each case connected to a transmis-
sion signal line 5. The second electrode of the receiver24 is connected to a reception signal line 6.
The piezoelectric elements, that is to say the transmit-
ters 23 and the receiver 24, are in each case polarized
parallel to the longitudinal axis of the stack. If an AC
voltage is present on the transmission signal line 5,
- then the transmitters 23 execute thickness oscillations.
The height of the stack oscillates correspo~;ngly. Since
the stack is clamped in by the tensioning bolt 3, the nut
32 and the pressure pins 121 and is coupled to the
diaphragm 12, the diaphragm 12 is excited by these
thickness oscillations to produce flexural vibrations.
The oscillating bars 14 are fixedly connected to the
diaphragm 12 at their ends. Flexural vibrations of the
diaphragm 12 consequently cause the oscillating bars 14
to oscillate perpendicularly to their longit~A;n~l axis.
An oscillation of the oscillating bars 14 correspo~A;ngly
leads to a flexural vibration of the diaphragm 12, which
in turn effects a thickness oscillation of the stack.
This thicknes~ oscillation leads to a change in the
voltage drop across the receiver 24. A correspo~A;ng
reception signal E is available via the reception signal
line 6.
The amplitude A of this electrical reception ~ignal E is
larger, the larger the mechanical oscillation amplitude
of the oscillating bars 14 is. To exploit this fact, the
apparatus is preferably operated at its resonant
freguency fr. The mechanical oscillation amplitude is a
maximum at the resonant frequency f r .
If a harmonic oscillator is considered as an example of
CA 0223~027 1998-04-16
.
- 9 - EH 305 EP
7.04.1997
an ideal oscillatory system, then its oscillation
amplitude has a single maximum as a function of the
oscillation frequency. The phase difference between the
oscillation excitation and the oscillation of the
oscillator experiences a sudden phase change of 180~ in
the region of this maximum. At the resonant frequency,
the oscillation amplitude is a maximum and the phase
difference is 90~.
Based on the same fundamental physical principle, a fixed
phase relationship between the transmission signal and
the reception signal E exists in the case of resonance in
the present apparatus, too. The fixed value of this phase
difference is dependent on the polarization o$ the
transmitters 23 and of the receiver 24 and on the mechan-
ical and electrical oscillation properties of theapparatus. Measurements have shown that the values lie,
as a rule, between 20~ and 90~ or between -90~ and -20~,
referring to a reference point, for example 0~ or 180~.
In order that the mechanical oscillatory structure is
made to oscillate at its resonant frequency fr~ a control
loop is provided, which regulates the phase difference
existing between the electrical transmission signal and
the electrical reception signal E to a specific, constant
value ~R. An exemplary embodiment of a control loop of
this type is illustrated in Figure 2. In that case, the
reception signal E is fed back to the transmission signal
via an amplifier 7 and a phase ~hifter 8, which shifts
its phase by the specific, constant value ~R. The
amplifier 7 should be dimensioned such that the self-
excitation condition is satisfied. The mechanicaloscillatory structure is consequently excited by means of
the transducer 2 to produce oscillations at its resonant
frequency. If the oscillatory structure is covered by the
charge material, then the re~o~nt frequency fr has a
lower value than if the oscillatory structure oscillates
freely. The fixed value of the phase difference, which is
CA 0223~027 1998-04-16
- 10 - EH 305 EP
7.04.1997
assumed at the resonant frequency fr~ is, on the other
hand, independent of whether or not the oscillatory
structure iB covered by the charge material.
The reception signal E is applied via the reception
signal line 6 to the input of an evaluation unit 9. Its
frequency is determined by means of a frequency-measuring
circuit 91 and the result is fed to a comparator 92. The
latter compares the measured frequency with a reference
frequency fR stored in a memory. If the measured
frequency is less than the reference frequency fR, the
evaluation unit 9 emits an output signal which indicates
that the mechanical oscillatory structure is covered by
a charge material. If the freguency has a value which is
greater than the reference frequency fR, then the evalua-
tion unit 9 emits an output signal which indicates thatthe mec~n;cal oscillatory structure is not covered by
the charge material. The output signal is, for example,
a voltage which assumes a correspon~;ng value or a
current which has a correspo~;ng value or on which a
signal current in the form of pulses having a correspond-
ing frequency or a correspo~;ng duration is superposed.
In a harmonic osctllator, attenuation or reduction of the
oscillation quality effects a reduction in -the maximum
amplitude in the case of resonance. In such a case, the
phase increase as a function of the frequency proceeds
continuously rather than abruptly, to be precise it
proceeds more 81Owly, the greater the attenuation or the
reduction of the oscillation quality is. Overall,
however, even with very great attenuation, a phase change
of a total of 180~ ensues and a phase difference of 90~
exists at the resonant frequency. The fixed value,
corresponding to reso~nce, of the phase difference of
90~ always exists and is assumed at the resonant
frequency.
In contrast to an ideal oscillator, in the abovementioned
CA 0223~027 1998-04-16
- 11 - EH 305 EP
7.04.1997
apparatus, couplings of an electrical and mechanical
nature exist between the transmitters 23, the receiver 24
and the mechanical oscillatory structure. The mechanical
coupling is essentially governed by the mechanical
clamping-in of the transducer 2. Thus, for example, a
transmission signal exciting the transmitter 23 leads to
a reception signal E even when the o~cillating bars 14
are restrained and are consequently not moving.
The electrical coupling exists between the transmitters
23 and the receiver 24. These are not electrically
independent of one another, rather there exists a, as a
rule capacitive, connection between them. This connection
is illustrated in Figure 2, in the form of an equivalent
circuit diagram, by the capacitance C* which is inserted
between the transmission signal line 5 and the reception
signal line.
The reception signal E is consequently composed of three
components, namely a measurement signal EM~ a first
additional signal EmeCh governed by the mechanical
coupling, and a second additional signal Eel governed by
the electrical coupling.
E = EM + EmeCh+ Ee1
The measurement signal EM is based on the oscillation of
the mechanical oscillatory structure and ha~ a frequency-
dependent amplitude AM(f) and a frequency-dependent phase
~M(f). Phase in each case denotes the phase offset of
the respective component of the electrical reception
signal E with reference to the electrical transmission
signal.
Figure 3a shows the amplitude AM(f) and Figure 3b the
phase ~(f) of the measurement signal E~ as a function
of the frequency f. The curves can be determined computa-
tionally by simulation calculations, for example by
CA 02235027 l998-04-l6
- 12 - EH 305 EP
7.04.1997
finite element calculations.
They can be measured experimentally by connecting the
transmission signal line 5 to a frequency generator and
by determining the phase and amplitude of the oscillation
of the oscillating bars 14 as a function of the frequency
of the frequency generator, using a la~er vibrometer, for
example.
In each of the two figures, the continuous line corres-
ponds to an apparatus having a high oscillation quality
and the dashed line corresponds to an apparatus having a
low oscillation quality. In both cases, both the ampli-
tude AM (f) and the phase ~M(f) of the measurement signal
have the profile which is typical of a harmonic oscilla-
tor and has already been described above.
The two additional signals EmeCh~ Eel each have an
essentially constant amplitude AmeCh~ Ael and an essen
tially constant phase ~mech~ ~el. In this case, too,
phase denotes the phase offset of the respective
component of the electrical reception signal E with
20 reference to the electrical transmission signal.
Figure 4a shows the amplitude Amech and Figure 4b the
phase ~mech of the first additional signal EmeCh as a
function of the fre~uency f. The curves can be determined
computationally by simulation calculations, for example
by finite element calculations. They can be measured
experimentally when the other two signal components,
namely the measurement signal EM and the second addi-
tional signal Eel, are suppressed. If their amplitudes
have a value of almost zero, then the reception signal E
30 is equal to the first additional signal EmeCh and can be
measured by means of an oscilloscope, for example.
The measurement signal EM can be eliminated by
restraining the oscillating bars 14 such that they are
CA 0223~027 1998-04-16
- 13 - EH 305 EP
7.04.1997
mechanically fixed. The second additional signal Eel can
be avoided by electrical insulation of the receiver 24,
for example in the form of a grounded metallic shield. It
is also recommendable, if possible, to use short lines,
in order to keep down the coupling in of electrical
signals of any type.
The continuous line in Figure 5a shows the amplitude Ael
and the continuous line in Figure 5b the phase ~el of
the second additional signal Eel as a function of the
frequency f. These curves, too, can be determined by
simulation calculations. They can be recorded experimen-
tally by, for example, employing non-polarized piezoelec-
tric elements as transmitters 23 and as receiver 24. In
these, no mechanical motion at all i8 generated by a
transmission signal, and the reception signal E
consequently corresponds to the second additional signal
Eel based on electrical coupling. The latter signal can
likewise be measured by means of an oscilloscope.
amplitude8 Amech~ Ael and the phases A~ -Ch~ of
the additional signals EmeCh~ Eel are virtually frequency-
independent and are unambiguously related to the
mechanical structure of the respective apparatus and to
the electrical and mechanical properties of the
transmitters 23 and of the receivers 24. The amplitude
AmeCh is typically substantially greater than the ampli-
tude Ael. The phases ~mech~ ~el~ are, typically, virtu-
ally identical or offset virtually by 180~ with respect
to one another. The latter case is illustrated in Figure
4b and Figure 5b. If these two additional signals have
the same phase over the frequency range, then it is
necessary either to interchange the terminals of the
electrodes of the receiver 24a or to rotate the polariza-
tion of the latter through 180~. The latter scenario can
be achieved, for example, by rotating the piezoelectric
element. As a result, the two additional signals have the
desired opposite, that is to say offset hy 180~ with
CA 0223~027 l998-04-l6
~ - 14 - EH 3 0 5 EP
7 . 04 . 1997
respect to one another, phase. The same result is, of
course, achieved by manipulating the transmitters 23 in
a correspo~;ng manner. In practice, the situation may
also arise where the pha8e8 ~mech~ ~el differ by an
5 amount which is distinctly different from 0~ or 180~. The
following then applies in an analogous manner. In this
exemplary embodiment, a difference of 180~ is selected
since the fundamental relationships can be illustrated
more simply and more clearly by doing 80.
Figure 6a shows the amplitude A(f) and Figure 6b the
phase ~(f) of the reception signal E. The two curves
result from the phase- and amplitude-accurate
superposition of the three above-described components of
the reception signal E.
Ee~ = AMei~M + Amechei ~ + Aelei~SSel
Both curves each have four regions I, II, III, IV, which
are described in a greatly simplified manner below. In a
first region I, the first additional signal is
predominant, since it has the largest amplitude AmeCh~ The
20 pha8e ~mech of this signal differs from the phases ~M(f)
amd ~el by about 180~. The resulting amplitude A~f)
consequently corresponds approximately to the amplitude
AmeCh(f) reduced by the sum of the amplitudes AM(f) and
Ael .
25 A(f) - AmeCh(f) - (AM(f) + Ael)
The resulting phase ~(f) is 180~ in this region I.
In a second region II, the measurement signal EM assumes
the commanding role on account of its increasing ampli-
tude AM(f), which exceeds the amplitude AmeCh of the first
additional signal. Its phase ~(f) is 0~ in this region
II. The amplitude A of the resulting signal consequently
corresponds approximately to the ~um of the amplitudes of
CA 0223~027 1998-04-16
- 15 - EH 305 EP
7 . 04 . 1997
the measurement signal AM(f) and of the second additional
signal Ael(f), which sum is reduced by the amplitude AmeCh
of the first additional signal.
A(f) - AM(f) + Ae1 - Amech
Before the region boundary between region I and region
II, the amplitude A(f) of the reception signal E
decreases considerably. In this frequency range, the
phase ~(f) of the reception signal E decreases from 180~
to 0~. In the region II, the amplitude A rises and the
phase ~(f) r: -;n~ unchanged at 0~. The resonant fre-
quency fr lies between the region II and a region III.
The measurement signal EM correspo~ingly has a sudden
phase change of 180~. This signal is also predominant in
the region III on account of its amplitude AM(f), which
is now decreasing but still exceeds the amplitude AmeCh of
the first additional signal EmeCh~ Consequently, in the
region III, the amplitude A of the reception signal
essentially corresponds to the sum of the amplitudes
AM(f) of the measurement signal and of the first
additional signal AmeCh(f)~ which sum iB reduced by the
amplitude Ael(f) of the second additional signal.
A(f) - AM(f) + AmeCh ~ Ae1
It decreases with the frequency in accordance with the
decrease in the amplitude of the measurement signal
AM(f). The phase is 180~ in this region III.
In a region IV, the amplitude of the measurement signal
AM(f) falls below the amplitude of the first additional
signal AmeCh~ In this region IV, the amplitude A of the
reception Bignal E decreases asymptotically to a final
30 value, which corresponds to the difference between the
amPlitUdes Amech and Ae1 of the two additional signals.
A(f) - AmeCh - Ae1
CA 0223~027 l998-04-l6
- - 16 - EH 305 EP
7 . 04 . 1997
The phase Q~(f) remains at a value of 180~.
The phase difference between the electrical transmission
signal and the electrical reception signal E has, as a
function of the frequency, two sudden phase changes each
of 180~ in opposite directions to one another. There are
consequently two frequencie6 at which the phase A~(f) has
the fixed ~alue ~R which corresponds to resonance and
was described at the beg;nn;ng~ in this case 90~, namely
at the region boundary between the region I and the
region II and at the region boundary between the region
II and the region III. The first frequency, which is
referred to as the antiresonant frequency far below, is
insignificant, since the amplitude of the electrical
reception signal E is negligibly small in this case. The
frequency-determining element of the control loop is
consequently inactive. As a result, the feedback is
interrupted and the self-excitation condition cannot be
satisfied. The second frequency is the resonant frequency
fr of the system. It is the critical frequency during
operation of the apparatus and is set automatically by
the control loop.
It goes without saying that the values of the phases A~,
~mech~ ~el of the individual signals EM~ EmeCh~ Eel are
different for different apparatuses, but their fundamen-
tal profile applies to all of the apparatuses mentionedand can be compr~h~n~e~ with reference to the description
of the exemplary ~hodiment.
If the situation now arises where the mechanical oscilla-
tory ~tructure is damped or has a reduced oscillation
quality, then the amplitude AM(f) and phase ~M(f) of the
measurement signal exhibit the profile illustrated by
dashed lines in Figures 3a and 3b. The amplitude AM(f)
rises and falls considerably more 810wly with the
~requency and has a distinctly lower maximum value. The
phase ~M(f) does not exhibit a sudden phase change, but
CA 0223~027 1998-04-16
- 17 - EH 305 EP
7.04.1997
rather rises continuously with the frequency. The greater
the reduction in the oscillation quality of the system
i8, the lower the maximum value of the amplitude is and
the lower the gradient of the phase i8. However, the
phase ~M(f) always reaches the values 0~ and 180~
asymptotically and it is still 90~ at the resonant
frequencY. The additional signals EmeCh and Eel rem
unchanged.
The amplitude A(f) and phase ~(f) of the reception
signal E resulting from the amplitude- and phase-accurate
superposition of the three components clearly differ from
the example mentioned first, in which there was no
reduction in the oscillation quality. The maxima of the
amplitude A(f) are a great deal less pronounced and the
phase A~(f) has two continuous phase changes in opposite
directions to one another instead of the two sudden phase
changes each of 180~ in opposite directions to one
another. The maximum phase difference is clearly less
than 180~. Dep~n~;ng on the oscillation quality of the
system, it iB even less than 90~.
If, therefore, damping of the mechanical oscillatory
structure, for example in foam or in a viscous medium, or
a different type of reduction in the oscillation quality
of the system, for example caused by loosening of the
mechanical connection between the piezoelectric elements
operating as transmitters 23 or as receiver 24 and the
mechanical oscillatory structure, occurs, then the phase
difference between the electrical transmission signal and
the electrical reception signal E admittedly still has,
as a function of the frequency, two continuous phase
changes in opposite directions to one another, but the
maximum phase difference may be very small. The maximum
phase difference is smaller, the smaller the interval
between the resonant frequency fr and the antiresonant
frequency far is.
0223~027 1998-04-16
- 18 - EH 305 EP
7.04.1997
A fixed phase relationship, correspon~;ng to resonance,
between the transmission signal and the reception signal
E is established by the control loop, which excites the
mechanical oscillatory system to produce oscillations at
the resonant frequency fr. In order that the apparatus
described is functional in the uncovered state in con-
junction with a high oscillation quality, the fixed phase
difference ~R is 90~ in the exemplary ~hodiment shown
here. It is produced by the phase shifter 8 in the
exemplary embodiment ~hown.
If the previously described situation where the phase
~(f) of the reception signal no longer assumes this
fixed value ~R over the-entire frequency range now
occurs on account of the properties of the charge mater-
ial or on account of a reduction in the oscillationquality, then reliable excitation of the mechanical
oscillatory system is no longer possible. The apparatus
is consequently not functional.
According to the invention, this problem is solved by
connecting the transmission signal line 5 to the recep-
tion signal line 6 via an electrical impedance Z. This
constitutes an additional electrical coupling. This
connection is arranged electrically in parallel with that
section of the control loop which comprises the amplifier
7 and the phase shifter 8. The impedance Z is, for
example, a resistance, a capacitance, an inductance or a
combination of the components mentioned.
The impedance Z has an effect both on the amplitude Ael
and on the phase ~el of the second additional signal. It
must be dimensioned such that the amplitude Ael of said
second additional signal is as far as possible equal to
the amplitude AmeCh of the first additional signal, and
that the phase ~el of said second additional signal is
shifted through 180~ with respect to the phase ~mech.
Investigations have shown that in most cases it is
CA 0223~027 1998-04-16
- 19 - EH 305 EP
7.04.1997
sufficient to use a capacitor having a correspo~ing
capacitance, for example a capacitance of a few pico-
farads. The value of the capacitance can either be
determined in advance by means of model calculations or
can be determined by a series of mea6urements to be
carried out using a tunable capacitor.
If the two additional signals Emech and Eel have virtually
the same phase, then it iB necessary either to inter-
change the terminals of the electrodes of the receiver 24
or to rotate the polarization of the latter through 180~.
The latter scenario can be achieved, for example, by
rotating the piezoelectric element. AB a result, the two
additional signals have the desired opposite, that is to
say offset by 180~ with respect to one another, phase.
The same result is, of course, achieved by manipulating
the transmitters 23 in a correspon~; ng manner.
As a ~Ashe~ line in each case, Figure 5a illustrates the
amplitude Ael and Figure Sb the phase A~el of the second
additional signal Eel as a function of the frequency f
for the case where the transmission signal line 5 and the
reception signal line 6 are connected to one another via
a correspo~Aingly dimensioned impedance Z.
In the previously described prior art, the amplitudes
AmeCh and Ael of the two additional signals differ to a
great extent. The signal having the larger amplitude i8
consequently predominant in wide frequency ranges. In the
apparatus according to the invention, on the other hand,
the amplitudeB Amech and Ael are preferably virtually
identical and their phaBeB ~mech and ~el are preferably
virtually opposite. The amplitude- and phase- accurate
sum Ameche ; ~ + Aeli~el has a critical influence on the
reception signal. If the two additional signals have an
OppoBite phage, a difference A~ = Amech ~ Ael between the
amplitudes Amech~ Ael ~f the two signals consequently has
an influence. This difference has a very low value in
CA 0223S027 1998-04-16
- 20 - EH 305 EP
7.04.1997
comparison with the individual amplitudes Amech and Ael of
the additional signals EmeCh~ Eel. The value of A* is
illustrated in Figure 3a.
Assuming that in this case, too, the amplitude Amech ~f
the first additional signal is slightly larger than the
amplitude of the second additional signal, the phase
~mech of the first additional signal is critical for the
frequencies at which the difference A*, now critical, is
greater, in terms of its magnitude, than the amplitude
10 AM ( f) of the measurement signal.
The amplitude A(f) and the phase ~(f) of the resulting
reception signal E are illustrated in Figures 7a and 7b.
According to these figures, the amplitude A(f) assumes
the value A* asymptotically. It has a minimum at an
antiresonant frequency far and a maximum at the resonant
frequency fr. In comparison with the profile illustrated
in Figure 6a, the minimum is a good deal less pronounced
and the difference between the resonant frequency and the
antiresonant frequency i~ distinctly greater. As shown by
the ~he~ line, this is still the case even when the
maximum of the amplitude AM(f) of the measurement signal
is distinctly reduced by attenuation or a reduction in
the oscillation quality.
For the ideal case, where the two additional signals
compensate one another exactly, A* has the value zero and
the amplitude A(f) of the reception signal E is identical
to the amplitude AM(f) of the measurement signal.
In principle, the phase ~(f) of the reception signal has
the same profile as a function of the frequency as in the
example of Figure 6b. The antiresonant frequency far~ at
which the first sudden phase change occurs, is consider-
ably lower in the case illustrated in Figure 7b than in
the case illustrated in Figure 6b. The frequency range in
which the phase ~(f) has the value 0~ is correspo~;ngly
CA 0223~027 1998-04-16
- 21 - EH 305 EP
7.04.1997
wider. The value of the frequency at which the second
sudden phase change occurs is identical in both figures
and corresponds to the resonant frequency f r .
The ~he~ line indicates the profile of the phase ~(f)
for the case where attenuation or a reduction in the
oscillation quality is present. According to this,
although the phase difference between the electrical
transmission signal and the electrical reception signal
E as a function of the frequency has no sudden phase
changes, it does have two continuous phase changes in
opposite directions to one another. Although the maximum
phase difference may be less than 180~, a maximum phase
difference of at least 90~- i8 still ensured even in the
event of very great attenuation or reduction in the
oscillation quality of the system.
For the ideal case where the two additional signals
compensate for one another exactly, in other words A*
assumes the value zero, it i8 true in this case, too,
that the phase ~(f) of the reception signal E is identi-
cal to the phase ~(f) of the measurement ~ignal.
Irrespective of its oscillation quality, the receptionsignal E of an apparatus according to the invention
always has the same fixed phase difference ~R at the
resonant frequency of the mechanical oscillatory
~tructure.
