Vibrator-type level sensor

A vibrator-type level sensor having a detecting pipe (20) in which an inner vibration member (22) with piezo-electric devices (8a) (8b) (8c) is mounted, the detecting pipe (20) and the inner vibration member (22) constituting a folded cantilever.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
1. FIELD OF THE INVENTION 
The present invention relates to a level sensor for detecting a level of 
powdery, granular or liquid material in a container, and more particularly 
to a level sensor for detecting the level by change of vibration of a 
member thereof at the detection of such material. 
2. DESCRIPTION OF THE RELATED ART 
A typical conventional vibrator-type level sensor has a structure and 
operation as described hereafter with reference to FIG. 1. The 
vibration-type level sensor has a vibration member 2 supported by a 
diaphragm 4. An outer side 2a of the vibration member 2 is projected from 
the pipe 6 and inner side 2b of the vibration member 2 is kept in the pipe 
6. On the inner side 2b of the vibration member 2, receiving 
piezo-electric device 8a and a vibrating piezo-electric device 8b are 
provided. 
The vibrating piezo-electric device 8b vibrates the vibration member 2. 
This vibration is detected and converted into electric signal by the 
receiving piezo-electric device 8a. The converted electric signal is 
amplified and impressed to the vibrating piezo-electric device 8b. 
Therefore, the vibration member 2 oscillates at a frequency determined in 
accordance with an eigenfrequency of the vibration member 2. 
When powdery, granular or liquid material as material to be detected 
touches the vibration member 2, the vibration stops or decreases. The 
detecting circuit provided in a case 16 detects this change, thus detects 
the touching by the material to be detected. Therefore, the level sensor 
provided in a container can detect the level of the material to be 
detected. 
In the prior art vibrator-type level sensor, the use of the diaphragm 4 
reduces the strength and therefore shortens the level sensor's life. 
Further, when the powdery material is stuck on the diaphragm 4, the stuck 
powdery material causes an erroneous detection. 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a vibrator-type level 
sensor having long life and accurate detection. 
The vibrator-type level sensor in accordance with the present invention 
comprises: 
a hollow member one end of which is fixed and the other end of which is 
blockaded, 
an inner vibration member which is provided in the other end of the hollow 
member and one end of which is fixed to the other end of the hollow 
member, 
vibrating means for driving the inner vibration member, and 
detecting means for detecting a decrease of vibration of the inner 
vibration member. 
While the novel features of the invention are set forth particularly in the 
appended claims, the invention, both as to organization and content, will 
be better understood and appreciated, along with other objects and 
features thereof, from the following detailed description taken in 
conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The structure of a preferred embodiment of a vibrator-type level sensor 
embodying the present invention is described in detail with reference to 
FIG. 2. As shown in FIG. 2, as case 16 of metal substance, is configurated 
substantially in a cylinder-shape having a cap 16a. The case 16 has a 
weight 95 and an electric circuit (not shown) therein. In front of the 
case 16, a base pipe 58 having a screw 12 is projected from the case 16. 
One end of a detecting pipe 20 with thinner thickness than the base pipe 
58 is fixed to the base pipe 58. Another end of the detecting pipe 20 is 
closed by a tapped tip 24 which is welded to the detecting pipe 20. An 
inner vibration member 22 is provided in the inner space of the detecting 
pipe 20, and one end of a vibration member 22 is screwed to the tapped tip 
24. Therefore, a folded cantilever is formed by the detecting pipe 20 and 
the inner vibration member 22. On the inner vibration member 22, a 
vibrating piezo-electric device (not shown) and a receiving piezo-electric 
device 8a are provided. 
The vibrating piezo-electric device vibrates the inner vibration member 22. 
This vibration is detected and converted into electric signal by the 
receiving piezo-electric device 8a. The converted electric signal is sent 
to the electric circuit through a lead wire 26a. The electric circuit 
amplifies this converted electric signal and impresses it to the vibrating 
piezo-electric device (not shown) through a lead wire 26b. Therefore, the 
inner vibration member 22 oscillates at a frequency determined in 
accordance with an eigenfrequency of the folded cantilever formed by the 
detecting pipe 20 and the inner vibration member 22. The weight 95, which 
weighs enough larger than the detecting pipe 20 and the inner vibration 
member 22, stabilizes the vibration of the inner vibration member 22. 
Instead of the weight 95, a thick pipe for the base pipe 58 may be used as 
weight. 
When powdery, granular or liquid material as material to be detected 
touches to the detecting pipe 20, the vibration of the inner vibration 
member 22 stops or decreases. The electric circuit detects this change, 
thus detects the touching of the material to be detected. Therefore, the 
level sensor provided in a container can detect the level of the material 
to be detected. 
The above-mentioned vibrator type level sensor can be installed to a 
container 90, inwhich a granular material 80 to be detected is put, in 
various way as shown in FIG. 3. A vibrator-type level sensor 1a is mounted 
to a side wall of the container 90 by a screw 12 and a nut 13. A 
vibrator-type level sensor 1b is mounted to the side wall of the container 
90 by mounting a flange 60 to the container 90 by screws. When it is 
difficult to mount the level sensor to the side wall of the container 90, 
a sensor 1c having a longer base pipe 58 of metal material may be used or 
a sensor 1d having a flexible pipe 64 may be used. The sensor 1d has a 
second base pipe 59 which weighs larger value than the detecting pipe 20. 
As a result of using the flexible pipe 64, a transport volume can be made 
small by bending the flexible pipe 64. 
FIG. 4 shows a block diagram of the electric circuit provided in the case 
16. In this embodiment, the vibrating piezo-electric device 8b, the 
receiving piezo-electric device 8a, an input circuit 30, an amplifier 32, 
and an output circuit 36 constitute an oscillation circuit. The vibrating 
piezo-electric device 8b receives output from the output circuit 36 and 
vibrates the inner vibration member 22. The receiving piezo-electric 
device 8a provided on the vibration member 22 converts this vibration into 
electric signal. The converted electric signal is inputted to the 
amplifier 32 through the input circuit 30. Output of the amplifier is 
impressed to the vibrating piezo-electric device 8b through the output 
circuit 36. Therefore, the inner vibration member 22 vibrates at a 
frequency determined in accordance with an eigenfrequency of the folded 
cantilever formed by the detecting pipe 20 and the inner vibration member 
22. In other words, the oscillation circuit oscillates at the frequency 
determined in accordance with the eigenfrequency of the folded cantilever. 
Output of the oscillation circuit, i.e., output of the amplifier 32 is 
detected by a detection circuit 38. The enveloped output from the 
detection circuit 38 is inputted to a comparator 40. In the comparator 40, 
an amplitude of the output signal from the detection circuit 38 is 
compared with an amplitude of a reference signal. When touching of the 
granular material to the detecting pipe 20 reduces the output signal 
smaller than the reference amplitude, the comparator issues an output 
which drives a relay 42 for switching a contact 46. 
A control circuit 34 is provided for reducing a beat of the inner vibration 
member 22 by controlling a gain of the amplifier 32. For preventing the 
erroneous operation, an initialization circuit 44 prohibits the operation 
of the relay 42 during about 3 minutes from closing a main switch of the 
level sensor. 
The eigenfrequency of a cantilever is given by the following equation: 
##EQU1## 
where: L: length of cantilever 
E: young's modulus of cantilever 
P: density of cantilever 
A: square measure of cantilever 
I: moment of inertia of cantilever 
a: modulus of frequency 
FIG. 5 shows relation between a resonance frequency and a value of a ratio 
Lp/Lb when Lp is changed and Lb is held constant, where Lp is a length of 
the detecting pipe 20 and Lb is a length of the inner vibration member 22. 
In FIG. 5, a curve f.sub.1 shows the eigenfrequency of the level sensor. 
The curve f.sub.1 is plotted by experimentation data. A curve f.sub.2 
shows a first mode oscillation of the detecting pipe 20, and a curve 
f.sub.3 shows a second mode oscillation of the detecting pipe 20. Both of 
the curves f.sub.2 and f.sub.3 are plotted basing on calculation. As shown 
in FIG. 5, the eigenfrequency of the level sensor is almost constant 
irrespective of the value of the ratio Lp/Lb. The curve f.sub.1 is 
approximate to a calculation curve of an eigenfrequency of the inner 
vibration member 22. Therefore, it is clear that the inner vibration 
member 22 compulsory vibrates the detecting pipe 20. 
FIG. 6 shows relation between the output voltage from the oscillation 
circuit and the value of the ratio Lp/Lb. In FIG. 6, the output voltage 
rapidly falls around the ratio Lp/Lb of 1. In FIG. 6, in a range shown by 
B, a phase of the impressed signal to the vibrating piezo-electric device 
2b is same as that of the output signal from the receiving piezo-electric 
device 2a. Therefore, the electric circuit of FIG. 4 is suitable for using 
in the range B. 
In a range shown by C, a phase of the impressed signal to the vibrating 
piezo-electric device 2b is different from that of the output signal from 
the receiving piezo-electric device 2a. For using in the range C, 
therefore, the phase adjustment amplifier is suitable instead of the 
amplifier 32. In this case, the touching of the granular material can be 
detected by detecting the phase difference signal from the phase 
adjustment amplifier. 
FIG. 7 shows a block diagram of the electric circuit of other embodiment of 
the present invention. The vibrating piezo-electric device 8b receives 
output from an oscillator 50 through an output circuit 36, and vibrates 
the inner vibration member 22. The detecting piezo-electric device 8c 
provided on the vibration member 22 converts this vibration into electric 
signal. The converted electric signal is inputted to the amplifier 32 
through the input circuit 30. Output of the amplifier 32 is detected by a 
detection circuit 38. The enveloped output from the detection circuit 38 
is inputted to a comparator 40. In the comparator 40, an amplitude of the 
output signal from the detection circuit 38 is compared with an amplitude 
of a reference signal. When the touching of the granular material to the 
detecting pipe 20 reduces the output signal smaller than the reference 
amplitude, the comparator issues an output which drives a relay 42 for 
switching a contact 46. 
The electric circuit of FIG. 7 is operable irrespective of a phase 
difference between an impressed and output signal. Therefore, the electric 
circuit can be used both in ranges B and C, i.e., in a whole range of A in 
FIG. 6. 
FIG. 8 shows a block diagram of the electric circuit provided in the case 
16. In this embodiment, the vibrating piezo-electric device 8b, the 
receiving piezo-electric device 8a, an input circuit 30, an amplifier 32, 
and an output circuit 36 constitute an oscillation circuit. In the same 
manner as the circuit of FIG. 4, the inner vibration member 22 vibrates at 
a frequency determined in accordance with the eigenfrequency of the folded 
cantilever formed by the detecting pipe 20 and the inner vibration member 
22. Output of the oscillation circuit, i.e, output of the amplifier 32 is 
inputted to a frequency comparator 54. In the frequency comparator 54, a 
frequency of the output signal from the oscillation circuit is compared 
with a reference frequency from a reference frequency oscillator 56. 
When the touching of the granular material to the detecting pipe 20 changes 
the frequency of the oscillation circuit, a frequency comparator 54 issues 
an output which drives a relay 42 for switching a contact 46. 
FIG. 9 shows a frequencies fa and fb of the free and contacted state of the 
detecting pipe 20. In this embodiment the reference frequency is 
established with a limit frequency fl shown in FIG. 9. 
Instead of the frequency comparator 54, a filter having characteristic show 
by a curve A may be used. In this case, the frequency shift can be 
detected by detecting a change of an output of the filter. 
There are some cases where the oscillation stoppes at touching of the 
granular material. For these cases, a vibration detection circuit 52 
detects the stop of the oscillation is provided. 
FIG. 10 shows a block diagram of other embodiment of the present invention. 
In this embodiment, the detecting piezo-electric device 8c is provided on 
the inner vibration member 22. The detecting piezo-electric device 8c 
detects the vibration of the inner vibration member 22, and output the 
detected signal to the frequency comparator 54. 
FIG. 11 shows a block diagram of still other embodiment of the present 
invention. The vibrating piezo-electric device 8b receives output from a 
sweep oscillator 70 through an output circuit 36, and vibrates the inner 
vibration member 22. The detecting piezo-electric device 8c provided on 
the vibration member 22 converts this vibration into electric signal. The 
converted electric signal inputted to the amplifier 32 through the input 
circuit 30. The output from the amplifier 32 is inputted to the frequency 
comparator 54. In the frequency comparator 54, a frequency of the output 
signal from the amplifier 32 is compared with a reference frequency. 
FIG. 12 shows a vibrator-type level sensor suitable for detecting a level 
of liquid material or material having little apparent specific gravity. 
Because the liquid material or material having little apprent specific 
gravity such as powder of styrofoam, little influences the vibration of 
the inner vibration member 22, there are some cases where the level of its 
material cannot be detected. In this embodiment, a paddle 95 which 
increases a sensitiveness is fixed to the detecting pipe 20. The paddle 98 
is provided parallel to a surface of the material to be detected. 
As has been described in detail for various embodiments, the vibrator-type 
level sensor in accordance with the present invention has accurate 
detection even after long time service, as a result of adoption of the 
structure that the inner vibration member 22 is put into the detecting 
pipe 20, and the structure may dispense with the use of diaphragm. By 
eliminating the diaphragm, which is mechanically weak and is cause of 
unstable oscillation by adhering the granular material or the like, the 
operational life time of the vibrator-type level sensor can be prolonged 
and more accurate detection can be obtained. 
Although the invention has been described in its preferred form with a 
certain degree of particularity, it is understood that the present 
disclosure of the preferred form has been changed in the details of 
construction and the combination and arrangement of parts may be resorted 
to without departing from the spirit and scope of the invention as 
hereinafter claimed.