Ultrasonic transducer

A small, compact, easily mountable ultrasonic transducer for measuring the filling level in a container is provided, which transducer transmits virtually no oscillation energy to the container, having a can-shaped transducer housing (1), an electronics housing (2) connected to the transducer housing (1), a transducer element (4), arranged in the transducer housing (1), for the transmission and reception of ultrasonic pulses, a union ring (3) for the sound-decoupling securing of the ultrasonic transducer on the container, in which ring the transducer housing (1) is supported coaxially with respect to the union ring (3) and is fixed therein against displacement in the axial direction, and two resilient elements (7, 8) which are arranged between the transducer housing (1) and the union ring (3) and by means of which the transducer housing (1) and the union ring (3) are held spaced apart from each other in the radial direction in such a way that there is no direct mechanical coupling between them in the radial direction.

BACKGROUND OF THE INVENTION 
The invention relates to an ultrasonic transducer for measuring the filling 
level in a container. 
An ultrasonic pulse emitted from the ultrasonic transducer is reflected at 
the surface of the filling material. The propagation time of the 
ultrasonic pulse from the transducer to the surface and back is determined 
and the filling level is determined therefrom. 
Ultrasonic transducers of this type are used in many branches of industry, 
for example in the water supply and water disposal sector and in 
chemistry. 
They preferably have a small diameter, in order that the openings in the 
container or in a sound guide tube, in which they are to be installed, can 
likewise be small. 
To produce the ultrasonic pulse, use is normally made of a disk-shaped 
piezoelectric element arranged in a can-like housing, the piezoelectric 
element being excited into oscillation. The bottom of the housing has the 
function of a diaphragm, onto which the oscillations are transmitted and 
through which the ultrasonic pulses are emitted. 
Between the piezoelectric element and the diaphragm there is arranged a 
plastic layer for matching the acoustic impedance of the piezoelectric 
element to the acoustic impedance of the medium into which the ultrasonic 
pulses are to be transmitted. 
A cavity which is left in the housing and is delimited by a circular 
surface facing away from the diaphragm and a cylindrical outer surface of 
the piezoelectric element is filled with a damping layer made, for 
example, from a plastic. This damping layer has the task of minimizing the 
emission of sound energy in the radial direction and in the direction 
facing away from the diaphragm. 
The smaller are the dimensions of the piezoelectric element and hence of 
the ultrasonic transducer, the greater is the transmission frequency. Such 
an ultrasonic transducer having a transmission frequency of, for example, 
50 kHz and correspondingly small dimensions is described in U.S. Pat. No. 
4,130,018. 
However, there is a direct mechanical coupling between the transducer 
element, the damping layer and the housing, with the result that the 
radiation of sound energy in the radial direction and in the direction 
facing away from the diaphragm cannot be completely suppressed. This 
energy is no longer available for the generation of the measurement 
signal. 
Part of this energy is transmitted to the container. This leads, in 
particular in the case of metallic containers which have a very high 
vibration quality, to a restriction of the measurement range of the 
ultrasonic transducer. The duration of the ultrasonic pulse emitted is 
lengthened by the feeding back of the oscillation energy of the container 
to the transducer element. As a result, a relatively large minimum 
distance, referred to below as the "block distance", is required between 
the ultrasonic transducer and the surface of the filling material. If the 
surface is located within the block distance, no measurement of the 
filling level is possible. 
DESCRIPTION OF THE PRIOR ART 
In DE-A 43 11 963, an ultrasonic transducer is described for measuring a 
filling level in a container, having 
a can-shaped transducer housing, 
a transducer element, arranged in the transducer housing, for the 
transmission and reception of ultrasonic pulses, 
a housing completely enclosing the transducer housing as far as a bottom 
surface of the transducer housing, 
the transducer housing being fixed in the housing against displacement in 
the axial and radial direction by a spring claw, which is molded on the 
transducer housing at the end and engages in a groove in the housing, 
a cylindrical cavity, which is arranged between the housing and the 
transducer housing and encloses the transducer element coaxially, and 
two resilient elements which are arranged between the transducer housing 
and the housing and seal off the cavity at the ends. 
Molded onto the housing, on that side facing away from the transducer 
housing bottom, is a connecting part by means of which the ultrasonic 
transducer is intended to be screwed into a flange. This is in turn to be 
fitted on an opening of a container. The ultrasonic transducer, which has 
a diameter which is increased by the cylindrical cavity and the housing 
enclosing the transducer housing, is located in the interior of the 
container, and the connecting part passes through the flange. The assembly 
of such a device is consequently complicated. 
Between the housing and the bottom surface there is a gap, in which 
residues of the filling material can accumulate. This is in particular 
disadvantageous if the filling material is an aggressive material or if 
the ultrasonic transducer is consecutively exposed to different filling 
materials which, in particular, react chemically with one another. 
In the case of the ultrasonic transducer according to DE-A 43 11 963, an 
emission of sound energy in the radial direction is very severely reduced 
by the cylindrical cavity. A transmission of sound energy to the 
container, however, also occurs via the connecting part. 
SUMMARY OF THE INVENTION 
It is an object of the invention to specify a small, compact, easily 
mountable ultrasonic transducer which can be rotated about its 
longitudinal axis, for measuring the filling level in a container, which 
transducer transmits virtually no oscillation energy to the container. 
To this end, the invention comprises an ultrasonic transducer for measuring 
a filling level in a container, having 
a can-shaped transducer housing, 
an electronics housing connected to the transducer housing, 
a transducer element, arranged in the transducer housing, for the 
transmission and reception of ultrasonic pulses, 
a union ring for the sound-decoupling securing of the ultrasonic transducer 
on the container, 
in which ring the transducer housing is supported coaxially with respect to 
the union ring and is fixed therein against a displacement in the axial 
direction, and 
two resilient elements, which are arranged between the transducer housing 
and the union ring and by means of which the transducer housing and the 
union ring are held spaced apart from each other in the radial direction 
in such a way that there is no direct mechanical coupling between them in 
the radial direction. 
According to one refinement of the invention, one open end of the 
transducer housing has at its open end a shoulder extending radially 
outward, which rests on a ledge on the interior of the union ring. 
According to a further refinement of the invention, the transducer housing 
is fixed in the union ring by an obtuse angled circular ring segment, 
which is fixed in a groove arranged in the union ring, extends radially 
into the opening in the union ring and rests with a circular ring segment 
surface that faces toward the ledge on an annular surface of the shoulder 
of the transducer housing that faces away from the ledge, and by the 
shoulder of the transducer housing resting on the ledge of the union ring. 
According to a refinement of the invention, the resilient elements are O 
rings fixed in grooves arranged in the union ring. 
According to another refinement of the invention, an open end of the 
transducer housing has threaded holes for connecting to the electronics 
housing. 
According to another refinement of the invention, an open end of the 
transducer housing has an internal thread for connecting to the 
electronics housing. 
According to another refinement of the invention, the transducer housing is 
supported in the union ring such that it can rotate about its longitudinal 
axis. 
According to a refinement of the invention, a resilient element for sound 
decoupling in the axial direction is arranged in a groove that is arranged 
in a shoulder ring which is molded on the union ring on a side facing 
toward the container.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The exemplary embodiment, represented in FIG. 1, of an ultrasonic 
transducer comprises three basic elements, a can-shaped transducer housing 
1, an electronics housing 2 and a union ring 3. The transducer housing 1 
is intended to be inserted into the union ring 3 and the electronics 
housing 2 is to be mounted subsequently. The ultrasonic transducer is thus 
of modular and compact construction. 
FIG. 2 shows a longitudinal section through the transducer housing 1. It 
comprises a hollow cylinder 11, which is sealed at the end by a diaphragm 
12 and consists of a plastic, preferably of a thermoplastic of high 
chemical resistance, for example polyvinyl chloride (PVC) or polyvinyl 
difluoride (PVDF). 
Molded on one open end of the hollow cylinder 11 is a shoulder 13, which 
extends radially outward. In the exemplary embodiment shown, the 
electronics housing 2 is screwed to the transducer housing 1. For this 
purpose, the shoulder 13 has threaded holes 14. 
It is equally possible for the hollow cylinder 11 to have an internal 
thread at its open end, into which thread the electronics housing 2 is 
intended to be screwed. 
Molded onto the hollow cylinder 11, on an annular surface 15 of the 
shoulder 13 facing toward the diaphragm, is a shoulder ring 16, whose 
function will be explained in more detail further below. 
Arranged in the transducer housing i is a transducer element 4, which 
serves alternately for the emission and for the reception of ultrasonic 
pulses. This is, for example, a piezoelectric element, which is excited 
into pulsed oscillations by an electronic circuit which is not shown in 
the figures. Following each emission of an ultrasonic pulse, the 
transducer element 4 serves as a receiving element. As a result of the 
ultrasonic pulse reflected at the surface of the filling material, the 
transducer element 4 is excited into oscillations. The resulting 
piezoelectric voltage is fed via connecting lines to a further electronic 
circuit, likewise not shown, which determines the propagation time of the 
ultrasonic pulse, determines therefrom the filling level and makes a 
signal corresponding to the filling level available for further processing 
and/or indication. 
To match the acoustic impedance of the transducer element 4 to the acoustic 
impedance of the medium into which the ultrasonic pulses are to be 
transmitted, a matching layer 41 made of a plastic is arranged between the 
transducer element 4 and the diaphragm 12. 
A cavity which remains in the transducer housing 1 and is delimited by a 
circular surface facing away from the diaphragm and a cylindrical outer 
surface of the transducer element 4 is filled with a damping layer 42 made 
of a plastic. This damping layer has the task of minimizing the emission 
of sound energy in the radial direction and in the direction facing away 
from the diaphragm. 
The transducer housing I is intended to be arranged in a union ring 3, 
which is shown in longitudinal section in FIG. 3. The said ring comprises 
three portions 31, 32, 33, of which the first portion 31 has an internal 
diameter which is slightly greater than the external diameter of the 
shoulder 13 of the transducer housing 1. The second and third portions 32, 
33 have in each case an internal diameter which is slightly greater than 
the external diameter of the hollow cylinder 11 of the transducer housing 
1. 
There is thus produced between the portions 31, 32 a ledge 311, on which 
the annular surface 15 of the transducer housing 1 rests in the assembled 
condition. The second portion 32 has, at that end facing the first portion 
31, a recess 321 for accommodating the shoulder ring 16. The shoulder ring 
16 serves to center the transducer housing 1 in the union ring 3 during 
assembly. 
The second portion 32 has on the outside the shape of a hexagonal screw, 
and there is formed on the third portion 33 an external thread 331, which 
is intended to be screwed from the outside into an opening in the 
container, which opening is not shown. The ultrasonic transducer can 
equally well be welded into the container opening or can be fastened 
thereto by means of a flange. The external shape of the portion 33 is then 
to be constructed appropriately. 
On that side facing the third portion 33, the second portion 32 has on the 
outside a shoulder ring 322. In the latter, on that side facing the third 
portion 33, there is molded a groove 323 for the accommodation of a 
resilient element 5, for example a seal made of an elastomer. This 
resilient element 5 effects decoupling of sound in the axial direction 
between the union ring 3 and the container. 
The transducer housing 1 is fixed in the axial direction in the union ring 
3. For this purpose, the first portion 31 has at its end a groove 312 
extending in its interior. In the assembled state, the groove 312 aligns 
with the side of the shoulder 13 of the transducer housing 1 facing away 
from the hollow cylinder. An obtuse angled circular ring segment 9, for 
example made of spring steel and shown in FIG. 4, is intended to be 
clamped into the groove 312, said circular ring segment extending radially 
into the interior 30 of the union ring 3 and resting with a circular ring 
segment surface facing toward the interior on an external annular surface 
17 of the shoulder 13 of the transducer housing 1. 
This fixing acts only in the axial direction. The transducer housing 1 and 
the electronics housing 2 which is connected thereto are thus supported in 
the union ring 3 such that they can rotate about a longitudinal axis 6 of 
the transducer housing 1. 
The second and third portions 32, 33 of the union ring 3 have in each case 
at the end a groove 324, 332, which is arranged in their interior, for the 
accommodation in each case of a resilient element 7, 8. The latter are, 
for example, Viton O rings, Kalrez seals or 
polytetrafluoroethylene-encased Viton seals. Since the resilient element 8 
comes into contact with the filling material of the container, it is 
necessary here to use a material which is matched to the filling material 
and which, if appropriate, has a high chemical resistance. 
By means of the two resilient elements 7, 8, the transducer housing 1 and 
the union ring 3 are held spaced apart from each other in the radial 
direction in such a way that there is no direct mechanical coupling 
between them in the radial direction. 
Along the hollow cylinder 11, in the portion delimited by the two resilient 
elements 7, 8, there is a tubular cavity between the transducer housing 1 
and the union ring 3. In this portion, virtually no sound energy is 
transmitted from the transducer housing 1 to the union ring 3, since there 
is a considerable sound impedance step at the two interfaces between the 
transducer housing 1 and the cavity and between the cavity and the union 
ring 3.