GB2054853A - Device for sensing the presence of contents at a certain level in a container - Google Patents

Abstract

A device for sensing the presence or absence of contents in a container comprises a probe in the form of a rod 1 which projects through a seal 4 from a "dry" side of the device to the "wet" side of the device. An exciter 15 on the dry side is provided for exciting a first rod 2 which is mounted on the dry side and which is mechanically coupled to the probe rod 1 to cause the probe rod to vibrate. A detector 17 on the dry side detects the vibration in a third rod 3 which is mounted on the dry side and which is mechanically coupled to the probe rod 1. The first 2, probe 1 and third 3 rods have slightly different natural frequencies which are successively greater or less than one another such that when vibration of the probe rod 1 is undamped, in use, the three rods vibrate together but when the vibration of the probe rod 1 is damped, in use, by contents in the container, the third rod 3 is not forced to vibrate sufficiently by its coupling to the first rod 1 with as great an amplitude. Means are provided for discriminating between the response of the detector 17 to the amplitude of vibration of the third rod 3 when the vibration of the probe 1 is or is not damped by the contents and for providing a corresponding output signal. <IMAGE>

Description

SPECIFICATION Device for sensing the presence of contents at a certain level in a container The invention relates to a device for sensing the presence or absence of contents, such as a liquid or powder, at a certain level, at which the device is fitted, in a container.

One such device comprises a probe in the form of a rod which projects through a seal from a socalled "dry" side of the device to the so-called "wet" side of the device where in use the probe rod extends into the container and is exposed to any contents in the container. When the probe rod is clear of the contents in the container, it is caused to vibrate at its natural frequency by an exciter on the dry side and a detector on the dry side detects the vibration of the probe rod. When the level of contents in the container rises to immerse the projecting end of the probe rod, the vibrations of the probe rod are damped and the detector responds accordingly.

A device of the kind described above has the advantage that it may be used as a failsafe high level sensor and in this respect is superior to many known ultrasonic sensors which require the presence of the container contents between a transmitter and a receiver to transmit an ultrasonic signal. However, a disadvantage of known devices of the kind described is that they operate within a very narrow band width and the natural frequency, and hence the amplitude of vibration, varies significantly as a result for example of changes in temperature or other operating parameters, and this can lead to unreliable output, in the absence of constant supervision.

In accordance with the present invention"a device for sensing the presence or absence of contents, at a certain level, at which the device is fitted in a container comprises a probe in the form of a rod which projects through a seal from a "dry" side of the device to the "wet" side of the device, where, in use, the probe rod extends into the container and is exposed to any contents in the container; an exciter on the dry side for exciting a first rod which is mounted on the dry side and which is mechanically coupled to the probe rod to cause the probe rod to vibrate; and a detector on the dry side which detects the vibrations in a third rod which is mounted on the dry side and which is mechanically coupled to the probe rod, the arrangement being such that the first, probe, and third rods have slightly different natural frequencies which are successively greater or less than one another such that when vibration of the probe rod is undamped, in use, the three rods vibrate together but when the vibration of the probe rod is damped, in use, by contents in the container, the third rod is not forced to vibrate sufficiently by its coupling to the first rod with as great an amplitude, and means are provided for discriminating between the response of the detector to the amplitude of vibration of the third rod when the vibration of the probe rod is or is not damped by the contents and for providing a corresponding output signal.

Experiments show that with this construction, maintenance of high amplitude vibration of the third rod in the undamped state of the probe rod is substantially unaffected by temperature changes and the device can be left free running for long periods without supervision.

The device can be used with a wide variety of container contents, for example water, 81' plastic beads, and baby powder. The device can also be positioned at any orientation in a container, and can thus be positioned at the point most convenient for the user.

In use when the probe rod is undamped, it seems that the rods of each frequency adjacent pair of rods, that is to say the first and probe rods and the probe and third rods, vibrate at the same frequency because the rod with the higher natural frequency approaches the end of its half period first but this period is extended by energy from the other rod which is still moving. All three rods are therefore forced to vibrate at the same frequency.

However, when the vibration of the probe rod is damped, the energy reaching the third rod from the first rod is too small to sustain any, or at least as great a, mutual oscillation.

Preferably, the exciter and the detector are electro-mechanical transducers, such as magnetic coils or piezoelectric crystals and the output signal will be an electrical signal for indication or control purposes. Where the exciter is an electromagnet, the first rod should be at least partly ferromagnetic or have a ferromagnetic part mounted on it in order to receive driving energy from the electromagnet. Of course, if a piezoelectric exciter is used, a ferromagnetic exciter part will not be required.

In one arrangement the detector provides an electrical output which is fed to the exciter via a positive gain feedback loop. The amplifier gain may then be set such that when the probe rod is undamped, the feedback is sufficient to maintain the system oscillating, but insufficient to sustain a positive gain in the feedback loop when the probe rod is damped. Alternatively, the amplifier gain could be increased so that the system continues to oscillate even when the probe rod is damped. The signal amplitude in the feedback loop would then depend upon whether the probe rod was damped or not by the container contents and the output signal would be derived from the amplitude in the feedback loop.

As an alternative the loop could be open and the exciter input supplied by a frequency pretuned to the system natural frequency. The output signal would then be derived from the detector in dependence upon the amplitude of the signal received from the third rod.

The device may further comprise a series of rods mechanically coupled on the dry side, with every second rod acting as a probe rod extending into the container, the exciter being arranged to excite the first rod of the series, and the detector being arranged to respond to vibrations of the last rod of the series, and in any group of three adjacent rods of the series, incorporating a central probe rod, the natural frequencies of the three rods successively increasing or decreasing. The probe rods may be mounted at the same level so that they are all damped or undamped at the same time, thereby resulting in a system of increased sensitivity. Alternatively, the probe rods may be arranged at different levels and means may be provided to discriminate the vibrations in the end rod of the series in dependence upon which of the rods are damped by the contents in the container.

With this arrangement more than one level could be monitored by a common system.

In the present context, the term rod is not intended to be limited to a beam of circular section. In fact, it is preferable if all the rods are made with a substantially rectangular section. In circular beams the stiffness is the same in every lateral loading direction and on occasions such beams have been observed to adopt rotary motion as well as lateral vibrations. However, with beams of rectangular cross-section, the direction of significant vibration will be known, since they will vibrate in their least stiff plane, and the damping of the vibration of the rod will be enhanced by contact with the container contents, A similar effect may be achieved by appropriate attachments to the probe rod or by shaping the projecting end of the probe rod in the form, for example, of a paddie.

Sensitivity can be increased by making the system frequency as low as possible so that the probe rod sweeps out a large volume. However, if the system frequency is too iow it will be adversely effected by ambient noise. Preferably, each rod has a natural frequency lying in the range 800 Hz to 1200 Hz. Sensitivity can be increased still further by increasing the probe rod width transverse to its vibration direction.

Preferably, the natural frequency of the probe rod is 90% that of the first rod. Furthermore, it is also particularly convenient if the natural frequency of the third rod is 85% that of the probe rod.

An example of the device in accordance with the present invention is illustrated in the accompanying drawings, in which: Figure 1 is a section through the device mounted on a container wall; Figure 2 is a diagram of a positive feed-back circuit for connection between the exciter and detector; Figure 3 is a perspective view of the first, probe, and third rods; Figure 4 shows the device mounted to a substantially vertical side wall of a container; and Figure 5 shows the device and a modification of the device fitted to a substantially horizontal top wall of a container.

The device illustrated in Figure 1 has a probe rod formed by a stainless steel beam 1 of rectangular cross-section, a first input rod formed by a mild steel beam 2 of rectangular crosssection, and a third, output rod formed by a stainless steel beam 3. also having a rectangular cross-section. The beams 2, 3 are riveted on, opposite sides to one end of the beam 1, and the assembly of three beams is eutectic soldered into a sealing plate 4. The sealing plate 4 is clamped to a flange part 5 having a central aperture 6 by a clamp ring 7 having a central aperture 8 coaxial with the aperture 6 of the flange part 5. The plate 4 is sandwiched between the clamp ring 7 and the flange part 5 and the clamp ring is secured to the flange part by means of bolts 9, the plate 4 sealing the aperture 6.

As shown, the device is mounted to a mounting pad 10, having a central aperture 11, by means of bolts 12, the mounting plate 11 being fixed around an aperture 13 in a container wall 14. The beam 1 extends through the apertures 6, 11 and 13 into the container, while the beams 2, 3 extend away from the beam 1 through a seal in the plate 4 and through the aperture 8 in the clamp ring 7.

The beam 1 should extend as far as possible into the container and it is desirable to make the depths of the aperture 6, 11, and 13 as small as possible.

The beams 1, 2, 3 are positioned with respect to the sealing plate 4 so that dynamic balance is achieved between all the wet side parts and all the dry side parts about the centre line of the sealing plate 4. The sealing plate 4 thus forms a node.

This positioning of the beams 1, 2, 3, means that the actual resonant frequencies of the beams bear a fairly close relationship to those predicted by calculation. In this example, the first beam 2 has a natural frequency of 1200 Hz, the probe beam 1 has a natural frequency of 1073 Hz, and the output beam 3 has a natural frequency of 914 Hz.

Thus, the probe beam 1 has a natural frequency of substantially 90% of that of the input beam (infant 89.4%), and the output beam 3 has a natural frequency substantially 85% that of the probe beam 1 (in fact 85.2%). When this system is run free as a closed loop system the frequency of oscillation of all the parts is 931 Hz.

An electromagnet 15 is mounted adjacent the first beam 2 to cause the beam, in use, to vibrate.

In use, a magnetic circuit 1 6 passes through the electromagnet and through part of the beam 2 and the beam will be attracted towards the electromagnet. During a first half cycle of an A C.

current input to the electromagnet, the beam 2 will move towards the electromagnet, while during the second half cycle the beam will move back towards its rest position against the electromagnetic force and then the cycle will be repeated, thus causing the beam to vibrate. It has been found that the retarding effect of the electromagnet 15 on the beam 2 when the beams move away from the electromagnet can be greatly reduced by connecting a diode across the electromagnet. Alternatively, a permanent magnet may be positioned in the magnetic circuit 1 6 which enables more efficient energy use.

Vibrations from the beam 2 are transmitted to the beam 1 which is caused to vibrate and vibration of the beam 1 then causes vibration of beam 3.

When the free end of the beam 1 on the "wet" side of the container wall 14 is not immersed in any container contents, all three beams 1, 2, 3 will vibrate with the same frequency. However, when the free end of the beam 1 is immersed in container contents, its vibrations will be damped and the frequency of the beam 3 will be correspondingly affected.

A piezoelectric crystal 1 7 is soldered to the beam 3 to enable vibrations of the beam 3 to be detected. A cover 1 8 is push fitted over the clamp ring 7 to protect the various dry side components.

The assembly of the probe beam 1 the input beam 2, and the output beam 3 is illustrated more clearly in Figure 3 and it should be appreciated that this assembly could alternatively be made as a unitary bifurcated construction.

In this example, a positive feedback circuit is provided connecting the piezoelectric crystal 1 7 to the electromagnet 1 5. The circuit is illustrated in Figure 2 and comprises a gain preset 18 connected to an amplifier 19. The amplifier 19 is connected to the electromagnet 1 5 via a pulse shaping circuit 20. An amplitude monitor 21 is connected into the positive feedback circuit between the amplifier 19 and the pulse shaping circuit 20, the output of the monitor 21 enabiing an operator to monitor the amplitude of the voltage being fed back through the circuit.In use, movement of the crystal 1 7 caused by vibration of the beam 3, causes a variable voltage to be fed through the feed-back circuit to the electromagnet 1 5. This causes a corresponding variable current to pass through the electromagnet 15 which vibrates the input beam 2. It has been determined experimentally that with a suitabie choice of input and output polarities the assembly of three beams 1, 2, 3 can be made to self oscillate at least when the probe beam 1 is undamped. Any change in the fed back voltage which will correspond to a change in the vibration frequency of the beam 3, caused in turn by change in vibration of the beam 1 due to that beam being damped, can be detected by the user via the amplitude monitor 21.

If the amplifier gain is set appropriately, the system can be arranged such that osciliation will cease when the probe rod 1 is damped. This gives a very high wet to dry signal ratio.

The device may be rnounted in a vertical side wall of a container, for example as shown in Figure 4, and in this position it is preferable if the probe beam 1 is arranged so that its wider faces are substantially vertical in order to minimise the buiid up of material when the container contents is dry matter such as a powder.

The device may also be mounted in a substantially horizontal wall of a container, as shown in Figure 5. Where it is desired to detect the presence of contents at a level remote frnm, the container top wall, an extension tube 22 is provided, the sealing plate 4, the first and third beams 2, 3, the electromagnet 15, and the crystal 1 7 all being positioned at the lower end of the extension tube and wires running back through the extension tube to allow the amplitude of the fed back voltage to be monitored.

Claims (14)

1. A device for sensing the presence or absence of contents, at a certain level, at which the device is fitted, in a container, the device comprising a probe in the form of a rod which projects through a seal from a "dry" side of the device to the "wet" side of the device where, in use, the probe rod extends into the container and is exposed to any contents in the container; an exciter on the dry side for exciting a first rod which is mounted on the dry side and which is mechanically coupled to the probe rod to cause the probe rod to vibrate; and a detector on the dry side which detects the vibrations in a third rod which is mounted on the dry side and which is mechanically coupled to the probe rod, the arrangement being such that the first, probe, and third rods have slightly different natural frequencies which are successively greater or less than one another such that when vibration of the probe rod is undamped, in use, the three rods vibrate together but when the vibration of the probe rod is damped, in use, by contents in the container, the third rod is not forced to vibrate sufficiently by its coupling to the first rod with as great an amplitude, and means are provided for discriminating between the response of the detector to the amplitude of vibration of the third rod when the vibration of the probe rod is or is not damped by the contents and for providing a corresponding output signal.

2. A device according to claim 1, wherein the exciter and the detector are electro-mechanical transducers.

3. A device according to claim 2, wherein at least one of the exciter and the detector is a magnetic coil.

4. A device according to claim 2 or claim 3, wherein at least one of the exciter and the detector is a piezoelectric crystal.

5. A device according to any of claims 2 to 4, wherein the detector provides, in use, an electrical output, the device further comprising a positive gain feed-back loop connected between the detector and the exciter, the feed-back being at least sufficient to maintain the system oscillating when the probe rod is undamped.

6. A device according to any of the preceding claims, further comprising a series of rods mechanically coupled on the dry side, with every second rod acting as a probe rod extending into the container, the exciter being arranged to excite a first rod of the series, and the detector being arranged to respond to vibrations of the last rod of the series, and in any group of three adjacent rods of the series, incorporating a central probe rod, the natural frequencies of the three rods successively increasing or decreasing.

7. A device according to any of the preceding claims, wherein all the rods are beams each with a substantially rectangular cross-section.

8. A device according to any of the preceding claims, wherein the natural frequency of each rod lies between 800 Hz and 1200 Hz.

9. A device according to any of the preceding claims, wherein the natural frequency of the probe rod is substantially 90% that of the first rod.

1 0. A device according to any of the preceding claims, wherein the natural frequency of the third rod is substantially 85% that of the probe rod.

1 A device according to claim 1, substantially as described with reference to the accompanying drawings.

12. A container fitted with a device according to any of the preceding claims.

1 3. A container according to claim 12, when dependent on claim 6, wherein the probe rods are arranged at different levels.

14. A container according to claim 12, when dependent on claim 6, wherein the probe rods are mounted at substantially the same level.