Device for measuring, controlling and/or detecting the filling level in a container

A fill level indicating device of the rod-in-tube type has a pair of propping struts gusseting the vibratory tube to the membrane and to the membrane support. One strut is arranged above the vibratory tube and the other below, with both struts aligned in a common plane which is vertical when the device is installed in a container. The vibration excitation and detection systems are arranged on the membrane at 90.degree. to the plane of the struts, so that with the device installed in a container the direction of vibration is horizontal. The membrane is made with thickened portions extending through its middle in the direction of vibration. Material may be removed from the thickened portions by grinding or filing to tune the device to the desired resonant frequency. In addition or as an alternative to the thickened portions of the membrane, ridges oriented opposite to the direction of vibration may be provided on an end wall of the vibratory tube from which material may be removed to tune the device to the desired resonant frequency.

BACKGROUND OF THE INVENTION 
This invention relates to a device for controlling, measuring and/or 
detecting the filling level in a container. In particular, this invention 
relates to such a device of the type having concentric vibratory elements 
which extend into the container. 
DISCUSSION OF THE PRIOR ART 
Devices for controlling, measuring and/or detecting the filling level in a 
container having a first vibratory element whose vibration gets damped 
when it contacts the filling material are known. In some types of such 
known devices, the vibratory element extends into the container and is 
tube-shaped and in the hollow space of this first tube-shaped vibratory 
element a second rod-shaped vibratory element is arranged which has the 
same resonant frequency as the first vibratory element and is vibrating in 
opposite phase. The first vibratory element is fixed onto an elastic 
membrane which has a vibration exciting and a vibration pick-up system and 
has its outer edge connected by means of a short tube to a plate which 
holds the second rod-shaped vibratory element so that the two vibratory 
elements are connected to each other, making a vibratory system. Devices 
of this type are known, for example, from German patent nos. DE-29 33 618, 
DE-30 11 603 C2 or from DE-31 40 938 C2. By this kind of construction one 
gains the advantage that almost no vibration energy gets transferred to 
the container wall through which the device extends into the container, so 
that these kinds of devices have a high sensitivity even against extremely 
light-weight filling material. 
In practice, however, it has been shown that the membrane, being 
necessarily relatively flexible because of the required vibrations, may be 
deformed by overload, i.e., it may get bent so that the device cannot be 
used any more. 
Such an overload may especially happen when the device is installed 
horizontally in the lower part of a container in order to control minimum 
level. In such a case the filling material flows over the first 
tube-shaped vibratory element which extends into the container. When the 
container is emptied, depending on the density of the filling material, 
the filling material can excite high forces onto this vibratory element by 
which the membrane can be bent. If the membrane is deformed beyond its 
elastic limit, for example by such an emptying process or by a sudden 
impact of falling clumped filling material onto the vibratory element, the 
device cannot work correctly any more or may be completely destroyed. 
Therefore, it is known to protect from above such devices in the interior 
of the containers by means of a protection plate. This, however, means 
increased installation effort and higher costs. In addition, a protection 
plate for such a measuring device is normally installed approximately 
horizontally extending from the container wall into the interior with a 
length according to the length of the vibratory device or a bit longer. As 
the granular or powdered filling material does not flow like a liquid 
under such a protection plate, there will always be a more or less large 
free space (free of filling material) under this protection plate even 
when the container is filled. If the device is installed too near to the 
protection plate, the vibrating rod may extend into this free space, thus 
causing an incorrect signal of the level control instrument. Therefore, 
the protection plate must be installed with a sufficiently large distance 
over the device. Furthermore, there is not always enough space to install 
such a protection plate. To simply make the membrane stronger is not 
possible because this would disturb the vibration characteristics of the 
device. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the invention to create an adequate device 
where the deformations and bendings of the membrane caused by filling 
material can be avoided, so that a protecting plate is not needed, and so 
that parts belonging to the vibratory system, especially the first 
vibratory element itself or the membrane, do not have to be constructed in 
a complicated manner and so that the vibration characteristics of the 
membrane are not detrimentally influenced. 
The solution of this apparently contradictory task is that at the 
transition from the first tube-shaped vibratory element to the membrane 
holding it there is provided at least one propping strut connecting the 
membrane and the first vibratory element, and that the connecting line 
between the vibration exciting system and the vibration pick-up system on 
the membrane is arranged crosswise to the propping strut. 
In one direction the system gets more stiff and strong which protects the 
system against overload while the vibration of the vibratory element and 
of the membrane which acts crosswise is not adversely affected. 
As normally the loading of the filling material acts from top downwards it 
is functionally that the connecting line from the vibration exciting 
system to the vibration pick-up system and thus the vibration direction is 
horizontal. As the vibration is excited in a horizontal plane the propping 
strut is oriented vertically (i.e., from top downwards or vice-versa) and 
thereby can act against the forces onto the vibrating device which are 
caused by the downward flowing or falling filling material without 
impeding the vibration of the vibratory element. 
It is especially useful for the protection against overload if on the top 
side as well as on the bottom side of the first vibratory element a 
propping strut is fixed in the vertical direction. Thereby the two 
propping struts can be arranged in alignment in a common vertical plane 
which is at the same time the vertical diameter plane of the tube-shaped 
first vibratory element. Advantageously the vibratory element may be of a 
simple tube shape. This is reinforced with the propping strut(s) to 
counteract relatively high forces acting from the top downwards, without 
bending the membrane. 
It is also useful that at least one of the propping struts extends to the 
edge of the membrane and to the tube-shaped support of the membrane. By 
this the forces absorbed by the propping struts can be conducted to the 
tube-shaped support of the membrane to protect the membrane itself from 
these forces. In the horizontal direction, however, the membrane stays 
flexible so that the vibrations in this direction are practically not 
impeded. A vertical additional load caused by the filling material is, 
however, efficiently anticipated. 
The propping strut or struts may end even with the outer edge of the 
tube-shaped support of the membrane or rise above a little. Thereby in any 
case the forces are conducted via the propping struts into the support of 
the membrane. 
The propping struts may be flat plates whose axial dimensions along the 
outside of the first vibratory element correspond to the radial dimension 
on the membrane or are larger. The propping strut or struts may be 
bevelled or rounded on the edge opposite to the side which is fixed to the 
first vibratory element and the bevelling may be about 45.degree.. Thus 
with minimum material an effective reinforcement of the membrane and of 
the vibratory system is obtained in the direction of the forces caused by 
the filling material without disturbing the vibration characteristic in 
the cross direction and thereby the function of the instrument. 
With hitherto known devices of this kind it is usual to make the necessary 
frequency adjustment of the vibratory elements by machining thinner the 
membrane on a lathe, e.g. until the resonant frequency is reached. Because 
of the propping strut or struts according to this invention, however, this 
is not possible. 
One execution of the invention provides on the side of the membrane from 
which the tube-shaped vibratory element and the propping struts start, 
beside the propping struts and/or on the closed end wall of the first 
vibratory element, material thickenings which may be reduced, e.g. by 
filing, in order to adjust the resonant frequency of the system. In spite 
of the propping struts by this provision a frequency adjustment is 
possible. 
Thereby the thickness of the membrane may be chosen thinner from the very 
beginning than it would be necessary for the correct resonant frequency 
and by means of the material thickenings the membrane may be reinforced 
more than necessary to reach the required frequency. By continuously 
reducing the material thickenings the resonant frequency can be adjusted. 
The material thickenings may have the shape of one or more ridges which may 
preferably lie in the vibrating plane. Such a ridge which functionally 
extends over a space of the diameter can easily and quickly be reduced by 
means of a file in order to make the frequency adjustment. 
In addition or instead of this the material thickening at the end of the 
vibratory element may be constructed as a ridge in the vertical direction. 
By this also at this position the frequency adjustment can be made by 
removing material without disruption by the propping strut(s). 
The material thickenings or ridges on the membrane may be bevelled towards 
the membrane and/or the material thickenings or ridges on the end wall of 
the first vibratory element may be bevelled towards the end wall. Thereby 
one avoids the accumulation of filling material in crevices or corners 
which may otherwise be formed by these ridges. 
Seemingly contradictory a membrane thickness is chosen thinner than 
necessary and in addition across the membrane at least one reinforcing 
ridge is provided which functionally lies in the plane of the vibration, 
i.e. in the preferred embodiment in a horizontal plane, and which has a 
thickness which is sufficient to increase the spring force of the membrane 
so that the desired resonant frequency is reached. In order to compensate 
for tolerances which occur in practice, this reinforcing ridge on the 
membrane is chosen thicker than necessary so that the resonant frequency 
of the outer first vibratory element is too high. A frequency adjustment 
can then easily be made by reducing the height of the reinforcing ridge by 
filing or grinding. 
Over all the invention provides a device which can withstand vertical 
forces caused by filling material so that the membrane does not get bent, 
without any protection shield or other extraneous device, and where the 
vibrations in the horizontal direction are not disturbed so that an exact 
level control is possible. Thereby, the first vibratory element can be 
constructed advantageously as a simple tube-shaped vibratory element with 
circular cross section, which is inexpensive and reliable.

The two complete devices marked with 1 (FIGS. 1-2 and FIGS. 3-4), differ 
only concerning their inner vibratory elements 4. Both devices 1 are used 
to measure, control and/or detect the filling level in a container, and 
have been identified with the same numbers concerning the corresponding 
parts in the following description. 
In both embodiments the device 1 has one first vibratory element 2, whose 
vibration gets damped when it contacts filling material in container 3. 
The first vibratory element 2 extends horizontally into the container 
through its wall 3 and is tube-shaped. In the hollow space of this first 
tube-shaped vibratory element 2 there is arranged a second rod-shaped 
vibratory element 4 which has the same resonant frequency as the first 
vibratory element 2 and which vibrates in opposite phase when the device 
is operated. 
The difference between the two embodiments is that in the embodiment 
according to FIGS. 1 and 2, the second vibratory element 4 is fixed to a 
stiff plate 5, thus acting as a bending vibrator, while in the embodiment 
according to FIGS. 3 and 4 between the stiff plate 5 and the rod-shaped 
vibratory element 4 there is a center of rotation 6 in the form of a 
material recess provided around the circumference of the element 4 so that 
in this case the second vibratory element 4 acts as a rotating vibrator. 
The first vibratory element 2 is fixed to a flexible membrane 7 which bears 
one vibration exciting system 8 (FIGS. 2 and 4) and 180.degree. opposite 
to the system 8 a vibration pick-up system 14. The vibration pick-up 
system 14 is shown in the drawings in hidden lines since the top half of 
FIGS. 2 and 4 are not shown in section. As shown in FIGS. 2 and 4 the 
vibration pick-up system 14 is the mirror image of the system 8 shown in 
the lower half of FIGS. 2 and 4, the device being symmetrical about its 
centerline. The flexible membrane 7 is connected at its outer edge by 
means of a tube-shaped mounting device 9 to the stiff plate 5. The two 
vibration elements 2 and 4 are thus connected together thereby making a 
vibration system in which the forces and deviations caused by the 
vibrations of the two vibratory elements compensate each other, so that 
practically no vibration energy is transferred to the container wall 3. 
Such vibration exciting and pick-up systems and rod-in-tube vibration 
systems are well known in the art. 
At the transition from the first tube-shaped vibratory element 2 to the 
membrane 7 bearing it, there are arranged the two propping struts 10 
connecting the membrane 7 and the first vibratory element 2 in such an 
orientation that the connecting line between the vibration exciting system 
8 and the vibration pick-up system on the membrane 7 lies crosswise (at 
90.degree.) to the plane of the propping struts. 
According to FIGS. 2 and 4 the connecting line from the vibration exciting 
system 8 to the vibration pick-up system 14 runs horizontally and thus 
also the vibration acts in a horizontal plane. According to FIGS. 1 and 3 
on each of the upper side and on the lower side of the first vibratory 
element 2 there is provided a propping strut 10 oriented vertically. Also 
from FIGS. 2 and 4 it can be seen that the propping struts 10 are aligned 
and are arranged in a common vertical plane which at the same time is also 
the vertical diameter plane of the outer, tube-shaped vibratory element 2. 
As the propping struts 10 have a certain thickness their center exactly 
lies in the mentioned vertical diameter plane. 
This shows that in vertical direction there is a reinforcement of the 
vibration system which acts against additional loads caused by the filling 
material while at the same time in horizontal direction the membrane 7 
stays flexible so that the necessary vibration for the level control is 
possible. That means that two apparently contradictory requirements are 
enabled, namely to keep the membrane flexible for vibrations and at the 
same time to reinforce the membrane against excessive loads. 
Each propping strut 10 according to FIGS. 1 and 3 extends to the edge of 
membrane 7 and to its tube-shaped support 9 and ends even with the outer 
edge or the outer periphery of the tube-shaped support 9. Thus from these 
propping struts 10 load forces from the element 2 can be absorbed and 
transferred via the tube-shaped support 9 to the stiff plate 5. Thereby, 
the membrane 7 is be protected from these forces caused by the filling 
material which in general act from the top downward. 
The propping struts 10 according to FIGS. 2 and 4 are flat plates whose 
axial dimension along the outside of the first vibratory element 2 
corresponds approximately to the radial dimension of the membrane 7 or is 
a little larger. Thereby the propping struts are bevelled at the radially 
outer side 10a, which is opposite to the side which is fixed to the 
element 2, the bevelling being at about 45.degree.. If the axial dimension 
of the propping struts 10 on the first vibratory element 2 is larger than 
the radial dimension of the membrane, there is at the top end (or the 
bottom end for the lower strut 10) of the membrane a short straight 
transit 10b to the support 9. 
Due to tolerances in production it may happen that the resonant frequencies 
of the system are not correct. Therefore, the thickness of the membrane 7 
is made thinner than required to get the correct resonant frequency of the 
vibratory element 2. In addition, on the side of the membrane 7 where the 
tube-shaped vibrator element 2 and the propping struts 10 start (i.e., the 
exposed side), beside the propping struts 10, there are material 
thickenings provided which may be reduced, e.g. by filing or grinding, in 
order to adjust the resonant frequency of the system. 
Instead or in addition it also would be possible to provide on the closed 
end wall 11 of the first vibratory element 2 one or more of such material 
thickenings 13. These material thickenings 13 are constructed as one or 
more ridges running in a direction which is 90.degree. to the direction of 
vibration and which have bevelled edges. The resonant frequency of the 
first vibratory element 2 is too low in this case and can be adjusted to 
the right value by taking off material. 
In the disclosed embodiments the material thickening has the shape of a 
reinforcing ridge 12 which extends on both sides of the propping struts 10 
in the vibration direction or plane on membrane 7. 
The reinforcing ridge 12 runs according to FIGS. 1 and 3 over the 
horizontal middle of the membrane 7. In FIGS. 2 and 4 it is shown that 
this reinforcing ridge 12 runs on both sides of the propping struts from 
the outside of the vibratory element 2 to the edge of the membrane 7. As 
it is arranged in vibration direction it can affect the vibration 
characteristics accordingly. 
From FIG. 1 and 3 it appears that this ridge 12 on membrane 7 is bevelled 
towards membrane 7 so that on this horizontally running reinforcing ridge 
12 on both sides of the propping struts 10 no filling material can 
deposit. Thereby the reinforcing ridges 12 enable an easy frequency 
adjustment. The reinforcing ridges 12 are so dimensioned to increase the 
spring force of the membrane 7 so that by removing material from the 
ridges the desired resonant frequency can be obtained. 
By a reasonable stiffening and reinforcing thus in total a device 1 is 
created with a low-cost, simple vibrating system consisting of a first 
tube-shaped vibratory system 2 with a circular cross section and a 
corresponding second vibratory element 4, which can withstand overloads 
due to filling material and still is able to vibrate with the desired 
frequency. 
Thus a protection plate on top of device 1 with the associated expensive 
and complicated construction can be avoided. In spite of the propping 
struts 10, which do not allow machining by lathe the outside of the 
membrane 7, an easy frequency adjustment is nonetheless made possible by 
means of reinforcing ridges 12 which are arranged on both sides of the 
propping struts 10 on the horizontal diameter of membrane 7. 
The device 1 for measuring, controlling and/or detecting whether filling 
material is in a container, has a first vibratory element 2, whose 
vibrations are damped by contact with filling material, whereby this 
vibratory element 2 has the shape of a tube with circular cross section 
and where in its hollow space a second rod-shaped vibratory element 4 is 
arranged which has the same resonant frequency as the first vibratory 
element 2 and vibrates in opposite phase. 
The first vibratory element 2 thereby is supported by a flexible membrane 7 
which has a vibration exciting system 8 and a vibration pick-up system 14 
whereby the vibration exciting system 8 and the vibration pick-up system 
are opposite each other to the center of membrane 7 on a horizontal 
diameter. The outer edge of membrane 7 is connected via a tube-shaped 
support 9 with a stiff plate 5 which bears the second rod-shaped vibratory 
element 4, so that the vibratory elements 2 and 4 are connected together 
and form a vibratory system, where practically no vibrating energy gets 
transmitted to the wall 3 of the container. At the transition from the 
first tube-shaped vibratory element 2 to the membrane 7 bearing the 
vibratory element 2 there is a propping strut 10 mounted to the connecting 
membrane 7 and the vibratory element 2 and acting against excessive loads 
and possible permanent deformation of membrane 7. The connecting line 
between the vibration exciting system 8 and the vibration pick-up system 
14 on the membrane 7 runs crosswise to the propping struts 10 (i.e., at 
90.degree. to the effective direction of the struts 10), so that vibration 
excitation and detection are unimpeded horizontally when the propping 
struts 10 are arranged vertically acting against the loads caused by the 
filling material.