WO2006131822A2

WO2006131822A2 - Level sensor with optoelectronic tilt switch

Info

Publication number: WO2006131822A2
Application number: PCT/IB2006/001516
Authority: WO
Inventors: Gianluigi Riva
Original assignee: Thermo Ramsey Italia S.R.L.
Priority date: 2005-06-10
Filing date: 2006-06-08
Publication date: 2006-12-14
Also published as: EP1889017A2; WO2006131822A3

Abstract

A level indicator is described for bulk material that includes at least one probe sensitive to its tilting with respect to the normal vertical position and at least one electronic control circuit having a first section, associated with the probe, and a second remote section connected to the first by means of a pair of conductors. The control circuit detects a signal of the current circulating between the two connected circuit sections and generates a processed signal representative of the position of the probe and the condition of the open circuit or short circuit between the two circuit sections.

Description

"Tilt level sensor"

FIELD QF THE INVENTION

The present invention relates to a level indicator and, in particular, a level indicator to measure the level of both static and moving bulk materials. TECHNICAL BACKGROUND OF THE INVENTION

Level indicators for bulk materials in various applications are well known, from the food industry to mining. In such fields, a level indicator is employed, for example, to signal high levels in silos, hoppers and stockpiles, blockages occurring in the loading/discharging areas of conveyor belts, and the presence or absence of material on conveyor belts.

The known type of level indicators generally comprise a probe and a control circuit related to the probe. The probe is hung in a vertical position in the measurement zone and, if the probe exceeds a pre-set angle of tilt, the control circuit generates a signal indicating the condition of tilting of the same probe.

In other words, the probe is "switched" from a vertical to a tilted condition ~ or viceversa — by the presence or absence of the material whose level is to be measured. For example, to verify that a conveyor belt transports the correct quantity of product, the probe normally operates in a tilted position and is located at such a distance from the belt that the probe remains tilted when the belt transports the correct amount; in the absence of material, or with an excessively low level of material, the probe will become vertical or assume an insufficient tilting to generate the necessary signal. A type of level indicator employed extremely widely in the known art uses a probe fitted with a mercury switch as the means of tilt-sensitivity. The probe fitted with such a sensing element is connected by a cable (often hundred of meters long) to a remote unit generally comprising a delaying device, an amplifier and an output relay.

This embodiment is particularly simple and economic to produce, is reliable and accurate in measurement, but it has various disadvantages. First of all, mercury is a toxic material and can no longer be used, particularly in the food industry.

Secondly, to guarantee operation of the level indicator even with particularly long cables, it is necessary to maintain a sufficiently high voltage and/or current at the terminals of the probe in order to guarantee immunity from interference. Such a requirement can be obtained easily with a mercury switch, but not with other types of switches unless there is additional circuitry and connecting cables. The latter are in many cases the determining factors in the cost of the installation. Furthermore, the maximum voltage that can be used and compatible with the emergency regulations is in practice 24 V, therefore it is necessary to ensure that the circuit is also immune from interference at voltages not exceeding this value even using long cables and in environments with a lot of interference. Furthermore, a mercury-switch as a means of tilt switching gives rise to a sparking contact, i.e. a switching means that cannot easily be used in particularly dangerous environments in the presence of potentially flammable or explosive materials. That requires particular care in the production of the probe case, the inside of which must be practically totally insulated from the external atmosphere, i.e. an extremely high degree o f protection.

EP-A-0639842 discloses a position switch to open and close the electrical supply circuit to a user device with high voltages and currents. The switch includes a tilt- switching device that can be of any type, such as a magnetically actuated reed switch, for example, a mercury-switch or even an opto-electronic type of switch. The switching device controls a power circuit comprising transistors or similar components that are switched between the interrupted state and the conducting state to open or close respectively the electrical supply circuit of the connected application device. In all the known embodiments, whenever high voltages and currents are employed, these constitute a possible source of danger if the cable connecting to the probe is accidentally severed, as can happen for example in mining environments. Furthermore, it is obvious that any interruption or short circuits on the cable cannot be identified, regardless of the nature of the switching device, since the simple function of opening or closing a circuit does not allow the conditions of normal operation (vertical or tilted probe) to be discriminated from the breakdown conditions that could occur along the probe circuit. In the case of a breakdown, it would be necessary to check the continuity of the cable along all its length and the integrity of the circuits contained in the probe, operations that could take considerable time; this is without considering the fact that, normally, the interruption of the cable or the cessation of its operation in whatever way, causes at best the stoppage of the machinery, and in the worst case leads to anomalous situations such as material spillage, blockages, etc.

It is necessity to produce a level indicator for bulk material, or however for the aforementioned applications, that resolves the disadvantages of the prior art. On that premise, one object of the present invention is to propose an improved level indicator that supplies a large number of indications on the state of the probe, not only regarding its position, but also regarding the operational condition of the probe and the same level indicator.

Another object of the present invention is to propose a level indicator that can work with particularly low voltages and currents in order to avoid safety problems, but with high immunity from interference in difficult environments.

A further object of the present invention is to propose a level indicator for bulk material that can be employed with high reliability and effectiveness both in applications where the probe is normally vertical and in applications where the probe is normally tilted. SUMMARY OF THE INVENTION

These objects are achieved by means of the present invention which relates to a level indicator comprising at least one probe sensitive to its tilting with respect to a vertical reference axis, the probe having a case with a means of vertical suspension above the site of measurement; at least one electronic circuit having at least one first control circuit section housed in the case of the probe and at least one second circuit section in a remote position with respect to the probe, wherein the first circuit section and the second circuit section are connected electrically by means of a twin- conductor cable. The electronic control circuit includes means of detecting the signal of the current circulating between the first circuit section and the second circuit section and of giving a processed signal indicating the position of the probe and the condition of the connection between the first circuit section and the second circuit section.

In particular, the electronic control circuit sets up at least one lower threshold value and at least one upper threshold value in order to define a range of variation of the current signal, together with at least one intermediate threshold value comprised within this range.

The processing of the current signal and the setup of different thresholds with which to compare such signal can therefore supply a greater number of information than those provided for to define the condition of tilting of the probe, i.e. information on the possibility that there are conditions of short-circuit or open-circuit on the cable that connects the two circuit sections, for example.

The probe preferably includes at least one opto-electronic switch tilting device in order to identify the tilting of the probe with respect to the vertical reference axis. The employment of an opto-electronic switch tilting device in the probe in the first place eliminates the presence of mercury that could come into contact with the materials measured and/or be released into the atmosphere. That furthermore allows applications of the present invention in environments where a sparking contact, such as a reed switch or similar could not be used, for example in the presence of dust in suspension in the air, of flammable or explosive materials or the like. Furthermore, the use of an opto-electronic switch tilting device ensures the reliability of the level indicator even in extreme environmental conditions (low temperatures, high humidity etc.) and allows a high degree of protection for the probe to be obtained in a simple and inexpensive way.

According to a preferred aspect of the present invention, the conductors of the cable connecting the two circuit sections are two non-polarized conductors. Several advantages can be obtained from this configuration. The employment of a twin-conductor cable, already present in the installations of the prior art, allows level indicators already installed to be replaced by those of the present invention without it being necessary to replace the cable, which may be the element of greatest cost of the entire device, above all if the cable is particularly long and/or with particular protective covering.

The use of non-polarized conductors noticeably simplifies the installation also for unskilled staff.

Furthermore, the configuration of the level indicator according to the present invention, with separate circuit sections connected by a twin-conductor cable, allows a particularly low supply voltage to be maintained on the cable connecting the two sections, therefore eliminating the potential risks of electrocution in the case in which the cable is accidentally severed.

The level indicator according to the present invention comprises furthermore means of commutation in order to allow the operation of the level indicator with normally vertical probe or normally tilted probe. As will be explained in more detail below, the same level indicator can therefore be used for two different applications, adapting the processing and the interpretation of the current signals in the most effective way for the various types of employment with static or moving material. The invention furthermore relates to a process to determine the state of the probe in a level indicator of the type described above, wherein there are provided the steps of: (a) detecting the signal of the current circulating between the first circuit section and the second circuit section of the level indicator, in which the first circuit section is associated with the probe;

(b) processing the current signal in order to obtain a processed signal representative of the position of the probe and the condition of the connection between the first circuit section and the second circuit section.

In particular, step (b) comprises the comparison of the processed signal with a plurality of pre-set threshold values that include at least one lower threshold value and at least one upper threshold value in order to define a range of variation of the current signal, together with at least one intermediate threshold value comprised within such range. hi order to detect the correct operation of the level indicator, a condition of open circuit between the two circuit sections is identified when the current signal remains lower than the lower threshold value for a pre-set time period, while a condition of short circuit between the two circuit sections is identified when the current signal remains above the upper threshold value for a pre-set time period.

As far as identifying the position of the probe is concerned, it is necessary to distinguish between possible applications of the level indicator according to the present invention, considering above all real operating conditions that involve the occurrence of impacts and/or high vibrations that are equivalent to highly unstable current signals. In particular, the level indicator can be set up for operation with normally vertical probe, for example in the case of finding the level of a heap of material in a silo. In this case, the condition of vertical probe is identified when the current signal is unstable or is comprised between the intermediate threshold value and the upper threshold value for a pre-set time period, while a condition of probe in tilted position is identified when the current signal remains comprised between the lower threshold value and the intermediate threshold value for a pre-set time period. Alternatively, the level indicator according to the present invention can be set up for operation with normally tilted probe, for example in the case of checking whether there is material on a conveyor belt in motion. In this case, a condition of probe tilted is identified when the current signal is unstable or remains comprised between the lower threshold value and intermediate threshold value for a pre-set time period, while a condition of probe in vertical position is identified when the current signal remains comprised between the intermediate threshold value and upper threshold value for a pre-set time period. According to a particular aspect of the present invention, the pre-set periods of time within which the actual commutation of the signal is determined can be set up with different durations depending on whether the applications are with normally vertical probe or with normally tilted probe. For example, the measurement of the level of material in a silo must generally be made more quickly than checking the absence of material on a conveyor belt.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and characteristics of the present invention will be clear from the following description, which is non-limiting and made with reference to the enclosed drawings by way of example, in which: - Figures IA and IB are schematic illustrations of possible applications of a level indicator; - Figure 2 represents a possible embodiment of the probe of a level indicator according to the present invention;

- Figure 3 is a scheme of the first circuit section associated with the probe shown in Figure 2; - Figure 4 is a scheme of the second circuit section of the unit of control of a level indicator according to the present invention; and

- Figures 5 A and 5B are diagrams that illustrate schematically some aspects of the operation of a level indicator according to the present invention as a function of the possible applications illustrated in Figures IA and IB. MODES OF CARRYING OUT THE INVENTION

Some applications of a level indicator are shown schematically In Figures IA and IB. The illustrations are extremely schematic in this case so that they can refer to level indicator types already well-known, as well as to level indicators according to the present invention. For example, Figure IA shows a bucket elevator 1 for bulk material that must be transferred to a heap 2. Near the top of heap 2 a probe 3 of a level indicator is hung, so suspended as to remain normally vertical until the moment in which the accumulation of material will cause probe 3 to rest on the top of the heap and, consequently, to tilt. Probe 3, for example, is encapsulated in a cylindrical case fitted at one extremity with suitable means of suspension, such as an eyelet 8, for example. Probe 3 is connected to a control center 5 through a cable 4 having at least two conductors. The control center 5 can comprise optical and/or acoustic means of signaling the tilting condition of probe 3, as well as electronic and/or electromechanical means of commutation in order to act on the operation of elevator 1, for example.

A conveyor belt is illustrated schematically in Figure IB that transports bulk material 7. Probe 3 is normally kept tilted by the presence of material 7 on belt 1, while it returns to the vertical position when no material is present on the belt 6, or is present in insufficient amounts. Also in this case, probe 3 is connected to a control center 5 through a cable 4 having two conductors.

Figure 2 shows a probe 3 of a level indicator according to the present invention. A first circuit section 10 that constitutes part of the control unit of the level indicator is contained in the cylindrical case 11 of the probe.

According to a particular aspect of the present invention, probe 3 comprises in particular an opto-electronic switch tilting device 13. This consists of a device in which there is a light-emitting diode and a phototransistor sensitive to the light emitted by the diode. Between these two elements there is a sphere, housed in a mobile way in a substantially conic-section housing, which can enable or interrupt the passage of light between the two elements according to the tilting of the same device. An example of a device which could be used in the present invention is that identified as RBS 31040 for the general standard model, distributed by Active Switch & Sensor Ltd. (UK). A similar component with a conical tilting of 15° of the conic housing and identified as RBS 310403 has been used in a prototype that has given favorable results. Making reference now also to the schemes of Figures 3 and 4, first circuit section 10 housed in case 11 of probe 3 comprises a plurality of components which allow a variable signal produced by the phototransistor of opto-electronic device 13 to be identified as a function of the position assumed by the probe 3. The two-conductor cable 4 (Figures IA and IB) that connects the first circuit section 10 to the second circuit section 20 illustrated in Figure 4 is coupled to a pair of clamps 14. There is a limited value voltage on clamps 14, for example an AC voltage maintained at a constant value of 24 Volts, coming from corresponding terminals 24 on the second circuit section 20. Since alternate current is used in operation, the two conductors of the cable 4 do not need to be polarized. The second circuit section 20 is located in the control centre 5 (Figures IA and IB) that it is placed in remote position with respect to the first circuit section 10 associated with probe 3, and the two sections 10 and 20 are connected by a single cable 4 thus forming an electrical circuit in which a variable current flows. Due to the configuration of the level sensor according to the present invention, it is possible to keep the two sections at particularly high distances (up to approximately 1000 m.), and therefore to use a cable of corresponding length while maintaining a particularly low voltage on the connecting cable 4. Referring now explicitly to the diagram of Figure 4, the second circuit section 20 comprises essentially an initial power supply portion 21 that supplies the necessary voltages for the operation of the components present in the same circuit section and the 24 Volt alternating current that is supplied to terminals 24 to be fed to the first circuit section 10 of the probe 3.

The second circuit section 20 comprises furthermore the components necessary in order to process the signal of the current that flows along cable 4 between the two circuit sections 10 and 20. These components comprise in particular a programmable microcontroller 25, such as for example an 8-bit microcontroller identified as PIC 16F676, distributed by Microchip Technology Inc. (USA).

The microcontroller 25 can be programmed in order to process the signal of the current circulating between the two circuit sections in such a way as to give a signal representative of the position of the probe and the condition of connection between two circuit sections 10 and 20. The current signal is an analogue signal that varies in a particularly irregular way. Probe 3 is in fact subject to impacts and vibrations during operation, due to the irregularity of arrival of the material whose level is to be found, and also to the presence of other equipment that can induce vibrations in the probe. To obtain meaningful information, the analogue current signal is subjected to sampling and filtration operations in order to give a processed signal representative of the position of the probe and the condition of the connection between the two circuit sections. The signal thus obtained is compared with a plurality of pre-set threshold values that include at least one lower threshold value and at least one upper threshold value in order to define a range of variation of the current signal, together with at least one intermediate threshold value comprised in such range.

As a function of the state of operation and/or position of the probe, microcontroller 25 commands in output a portion of circuit 26 comprising a LED in order to signal the condition of tilting of the probe and a relay to switch an electrical circuit controlled by the level indicator. In the same way, microcontroller 25 also commands a portion of circuit 27 comprising a LED in order to signal the conditions of operation of the probe and/or the cable, as well as a relay to switch a circuit signaling the breakdown and/or interruption of the electrical supply to the machine with which the level indicator is associated.

One or more microswitches 23 allow the operation of the level indicator to be adapted to the various applications, i.e. with normally vertical probe or normally tilted probe. In particular, microswitches 23 allow the setting up of the different time periods within which the switching of the signal should be discriminated.

Figures 5A and 5B show some diagrams that allow the operation of the level indicator according to the invention to be illustrated more clearly.

For example, Figure 5A schematically illustrates the possible traces of the signal processed by the level indicator in the case of operation with normally vertical probe, as illustrated for example in Figure IA. Furthermore, the threshold values Ll, L2 and

L3 with which the signal is compared are illustrated in the diagram.

Initially, before the initial moment t_s, the level indicator alone is assumed to be in operation, while all the equipment connected, or however near the measurement site, is switched off. The signal is therefore found stablilized at a level PV comprised between an intermediate threshold value L2 and an upper threshold value L3, i.e. comprised in a value range within which it is assumed that the probe is in perfectly vertical position.

The machine that transfers the material to the site of measurement is switched on at t_s. The vibrations to which the probe is subject produce an unstable signal that could give rise to an indefinite condition. This state is assumed by microcontroller 25 to be maintenance of the vertical position of the probe.

When the material reaches the desired level at the moment t_cl, the probe tilts and rests on the top of the heap. The level of the current signal rises to a level PT comprised between the lower threshold value Ll and the intermediate threshold value L2. If the signal remains in this condition for a pre-set period time D₁, at t_c2 microcontroller 25 identifies this state as the effective change of the probe from the vertical to the horizontal or tilted position, or at least to a position no longer vertical, and interrupts the machine that accumulates the material. In the operation with normally tilted probe, such as the situation illustrated schematically in Figure IB for example, the reference diagram is that illustrated in Figure 5B.

Similarly to the previous case, before the initial moment t_s, the level indicator alone is assumed to be in function, while the conveyor belt and all the equipment near the site of measurement is switched off. The signal is found stablilized at a level PT comprised between the lower threshold value Ll and the intermediate threshold value L2, i.e. comprised in a range of values within which the probe is assumed to be tilted.

The conveyor belt is switched on at the moment t_s and, similarly to the previous case, the impacts and the vibrations to which the probe is subject produce an unstable signal that could give rise to an indefinite condition. In this case, this state is assumed by microcontroller 25 to be maintenance of the tilted position of the probe. As soon as there is insufficient material on the conveyor belt at the moment _tci, the probe changes to a vertical position. The level of the signal of current goes to a level PV comprised between the intermediate threshold value L2 and upper threshold value L3. If the signal remains in this condition for a pre-set time period Dj_f, at the moment t_c2 the microcontroller 25 identifies this state as the effective change of the probe from the tilted position to the vertical position and stops the conveyor belt. The pre-set periods of time or delays Di and D_f can be different and are set up according to the applications and the operating requirements. For example, the Di delay can be shorter than the D_f delay because in the case of detecting the level of a heap problems could arise if the maximum level is exceeded, while the absence of material on a conveyor belt may not cause serious problems if it is detected with a greater delay. The availability to set up various time delays offered by the level indicator according to the present invention renders its application more versatile in various situations.

As far as verifying the correct operation of the level indicator is concerned, in both the applications illustrated by the diagrams of Figures 5 A and 5B a condition of open circuit between the two circuit sections is identified when the current signal remains lower than the lower threshold value Ll for a pre-set period time, while a condition of short circuit between the two circuit sections is identified when the current signal exceeds the upper threshold value L3 for a pre-set time period. Also in this case, it can be advantageous to be able to set up a temporary time period for each specific type of verification, in order to be able to return to the desired setup delay in the case of normal operation.

Claims

1. A level indicator comprising at least one probe sensitive to the tilting with respect to a vertical reference axis, said probe having a case with means of vertical suspension above the site of measurement; at least one electronic control circuit having at least one first circuit section housed in the case of said probe and at least one second circuit section in a remote position with respect to said probe, wherein said first circuit section and said second circuit section are connected electrically by means of a twin-conductor cable, characterized by said electronic control circuit including means of detecting the signal of the current circulating between said first circuit section and said second circuit section and generating a processed signal representative of the position of said probe and the condition of the connection between said first circuit section and said second circuit section.

2. The level indicator according to Claim 1, wherein said electronic control circuit is set up with at least one lower threshold value and at least one upper threshold value in order to define a range of variation of said current signal, together with at least one intermediate threshold value comprised within said range.

3. The level indicator according to Claim 1, wherein said probe includes at least one opto-electronic switch tilting device in order to detect the tilting of the probe with respect to the vertical reference axis.

4. The level indicator according to Claim 1, wherein said conductors of the cable connecting said first circuit section and said second circuit section are two nonpolarized conductors.

5. The level indicator according to Claim 1, wherein said electronic control circuit comprises means of commutation for the operation of the level indicator with normally vertical probe or with normally tilted probe.

6. A process to determine the state of a probe in a level indicator according to any of the previous Claims, characterized by comprising the steps of:

(a) detecting the signal of the current circulating between a first circuit section and a second circuit section of the level indicator, at least said first circuit section being associated with said probe;

(b) processing said current signal in order to generate a processed signal representative of the position of said probe and of the condition of the connection between said first circuit section and said second circuit section.

7. The process according to Claim 6, wherein said step (b) comprises the comparison between said processed signal and a plurality of pre-set threshold values that include at least one lower threshold value and at least one upper threshold value in order to define a range of variation of said current signal, together with at least one intermediate threshold value comprised within said range.

8. The process according to Claim 6 or 7, wherein a condition of open circuit between said first circuit section and said second circuit section is identified when the current signal remains below said lower threshold value for a pre-set time period, and a condition of short circuit between said first circuit section and said second circuit section is identified when said current signal remains in excess of said upper threshold value for a pre-set time period.

9. The process according to Claim 6 or 7, wherein said level indicator is set up for operation with normally vertical probe, and wherein a condition of probe in said vertical position is identified when the current signal is unstable or remains comprised between said intermediate threshold value and said upper threshold value for a pre-set time period, and a condition of probe in tilted position is identified when said current signal remains comprised between said lower threshold value and said intermediate threshold value for a pre-set time period.

10. The process according to Claim 6 or 7, wherein said level indicator is set up for operation with normally tilted probe, and wherein a condition of probe in said tilted position is identified when the current signal is unstable or remains comprised between said lower threshold value and said inteπnediate threshold value for a pre- set time period, and a condition of probe in vertical position is identified when said current signal remains comprised between said intermediate threshold value and said upper threshold value for a pre-set time period.

11. The process according to any of Claims 8 to 10, wherein said pre-set time periods are set up with durations differing between the operation with normally vertical probe and the operation with normally tilted probe.

12. The process according to Claim 6, wherein the signal of the current circulating between said first circuit section and said second circuit section of the level indicator is an analogue signal that is subjected to sampling and filtration operations in order to generate said processed signal representative of the position of said probe and of the condition of the connection between said first circuit section and said second circuit section.