Abstract
The invention relates to a detection system in a screening device for screening material, e.g. aggregate, ore or similar, comprising at least one screening decks, the at least one screening deck having a screening surface comprising one or more screening modules. The system comprises a sensor arranged at or near at least one screening deck of the screening device. The sensor is arranged such that it can detect objects present leaving the at least one screening deck. The invention also relates to a method for detection of objects in a screening device, and use of the detection system.
Claims
1. A method for detection of objects in a screening device comprising one or more screening decks, said method comprising: transmitting signals from an ultrasound sensor in relation to a mass flow from the one or more screening decks, said ultrasound sensor being arranged such that it can detect objects leaving said at least one screening deck; and differentiating, by a control unit operably connected to the ultrasound sensor, between signals from the sensor resulting from the material to be screened and signals from the sensor resulting from a foreign object, such as a dislodged screening module or liner element, by applying one or more predefined threshold values, wherein a threshold value is based on a time an object is present in the range of the sensor.
2. The method according to claim 1 further comprising the step of: controlling an operation parameter of the screening device based on an outcome of said comparison of signals received by said control unit from said ultrasound sensor with said threshold values.
3. The method according to claim 1, wherein the sensor is arranged to detect objects present outside a predefined area adjacent to a discharge of the at least one screening deck.
4. The method according to claim 3, wherein the predefined area is at least in part defined by a ballistic trajectory.
5. The method according to claim 4, wherein the ballistic trajectory has a starting point at or near a discharge end of the at least one screening deck.
6. The method according to claim 1, wherein the ultrasound sensor is a rangefinder.
7. The method according to claim 1, wherein said ultrasound sensor is arranged on a structure which is independent of the screening device.
8. The method according to claim 1, further comprising the step of determining whether or not a foreign object, such as a dislodged screening module or liner element, is present in the mass flow based on a comparison of signals received by said control unit from said ultrasound sensor with said predefined threshold values.
9. The method according to claim 8, further comprising the step of controlling an operation parameter of the screening device based on an outcome of said comparison of signals received by said control unit from said ultrasound sensor with said predetermined threshold values.
10. A method for detection of objects in a screening device comprising at least one screening deck, said method comprising: transmitting signals from an ultrasound sensor in relation to a mass flow from the at least one screening deck, the ultrasound sensor being arranged such that it can detect objects leaving the at least one screening deck outside of a predefined area that is adjacent to a discharge of the at least one screening deck, wherein the mass flow of the screened material will be found within the predefined area after leaving the at least one screening deck; and determining, by a control unit arranged to receive signals from the ultrasound sensor, whether or not a foreign object, such as a dislodged screening module or liner element, is present in the mass flow based on a comparison of signals received by the control unit from the ultrasound sensor.
11. The method according to claim 10, further comprising the steps of: defining threshold values for the signal from the ultrasound sensor; and determining whether or not a foreign object is present in said mass flow by applying the threshold values.
12. The method according to claim 10, wherein the ultrasound sensor is a rangefinder.
13. The method according to claim 10, wherein said ultrasound sensor is arranged on a structure which is independent of the screening device.
14. The method according to claim 11, further comprising the step of: determining whether or not a foreign object, such as a dislodged screening module or liner element, is present in the mass flow based on a comparison of signals received by the control unit from the ultrasound sensor with the defined threshold values.
15. The method according to claim 14, further comprising the step of controlling an operation parameter of the screening device based on an outcome of the comparison of signals received by the control unit from the ultrasound sensor with the defined threshold values.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of embodiments of the present invention, with reference to the appended drawings, where the same reference numerals may be used for similar elements, and wherein:
(2) FIGS. 1a and 1b are perspective views of a screening device equipped with a detection system according to one exemplary embodiment of the invention.
(3) FIG. 2 is a top view of the screening device in FIGS. 1a and 1b.
(4) FIG. 3 is a top view of the screening device equipped with the detection system according to another exemplary embodiment of the invention.
(5) FIG. 4 is an enlarged side view of a part of a screening device equipped with a detection system according to one exemplary embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
(6) FIGS. 1a, 1b and 4 illustrate a screening device 1 comprising three screening decks 2, each of which consists of a plurality of screening modules 3. Each screening module 3 has one apertured section. The apertured section has a first, upper surface intended to receive and carry material to be screened, a second, lower surface opposite the first surface, and a circumferential surface. The apertures extend from the first surface to the second surface. The screening device 1 is equipped with a detection system 4 which here comprises three sensors 5. The sensors 5, which, according to the example embodiment, are rangefinders of ultrasound type, are arranged such that they can detect objects present near a discharge 6 of the respective screening deck 2. In this embodiment (see also FIG. 2), each sensor 5 is attached to a side wall arranged downstream of the screening decks 2. The sensors 5 are connected to a control unit 7 and arranged to transmit signals in a direction generally perpendicular to a mass flow from the screening decks 2. Each of the sensors 5 are arranged such that their respective range of measurement lies outside of a ballistic trajectory A of the sorted material when it leaves the discharge 6 of the screening deck but still within an area in which at least a part of a detached screening module 3 (or similar) would be present after leaving the discharge of the screening deck 2. This trajectory can be determined by empirical experiments or calculated in advance taking into consideration e.g. the working parameters of the screening device (amplitude; frequency, etc.), the properties of the material to be sorted and the properties of possible foreign objects (e.g. screening modules and liner elements used in the screening equipment). However, in order to avoid that any foreign objects in the mass flow are missed, it is also possible, and possibly preferable, to arrange the sensors 5 such that their range of measurement has a certain overlap with the ballistic trajectory A of the screened material. Therefore, the control unit 7 is preferably arranged to differentiate between signals from the sensors 5 resulting from the screened material and signals from the sensors 5 resulting from a foreign object, such as a dislodged screening module or liner element, preferably by applying predefined threshold values. The threshold values can for example be based on material type or size of the material to be screened. This way, since the screening modules 3 are made of a different material and/or is larger in size than the material that is screened in the screening device 1, they can be detected by the detection system 4 when passing the area covered by the sensors 5. As soon as a screening module 3 or similar is detected, the screening device 1 can be stopped for maintenance. The threshold may also be based on the time which an object is present in the range of the sensor 5. Since a liner element or a screening module 3 normally will be of greater size and made from different materials than the sorted material, it will be present in the range of the sensor/s for a longer time, or at least under a different length of time than particles of the sorted material. Therefore, it is possible to base threshold values on the time an object is present in the range of the sensor/s. The control 7 unit may also, for example, be programmed to have different threshold values which thereafter may be used to determine whether a foreign object exists in the mass flow or not. The threshold levels may also or alternatively be based on the size or on the material of the objects present. In other words, the control unit 7 may be arranged to differentiate between signals from the sensor 5 resulting from the material to be screened and signals from the sensor resulting from a foreign object, such as a dislodged screening module 3 or liner element, by applying predefined threshold values. Different materials will reflect acoustic waves differently and objects of different size will also reflect acoustic waves differently. Hence, it is possible to differentiate between e.g. a screening module made from e.g. rubber covered metal and gravel particles to be screened. Some or all of the sensors 5 according to the invention are capable of determining the area of the object which is detected and since e.g. a screening module in most cases will have a larger surface than the particles of the material to be screened, it will be possible to determine presence of a foreign object based on the area of the object that is detected by the senor/s. These threshold values can be used alone or in combination with one or more of the others. It is also possible to arrange the control unit such that it takes readings of a plurality of sensors 5 into consideration. For example, detection of foreign objects made by two or more sensors 5 is most likely more reliable than a reading made by a single sensor 5.
(7) FIG. 3 illustrates the screening device 1 equipped with the detection system 4 according to another exemplary embodiment of the invention. In this embodiment, each sensor 5 is attached to the side wall opposed to the discharge 6 of the screening deck. Here, the sensors 5 transmit signals in a direction generally parallel with the mass flow from the screening decks 2, instead of generally perpendicular to the mass from the screening decks 2.
(8) FIG. 4 illustrates a detail of the system according to the invention. Here it is disclosed in more detail how a flow of material leaves the inclined screening deck 2 and as can be seen a screening module 3 has detached from the screening deck 2 and follows the flow of material leaving the screening deck 2. Due to its size, the screening module 3 will at least in part and at least during a certain amount of time protrude from the flow of material flowing along trajectory A. This protrusion of the screening module 3 can be detected by sensor 5 as has been disclosed above. In FIG. 4, the sensor is arranged as shown in FIGS. 1a, 1b and 2, i.e. transmitting in a direction generally perpendicular to the direction of the mass flow.
(9) The skilled person realizes that a number of modifications of the embodiments described herein are possible without departing from the scope of the invention, which is defined in the appended claims. For instance, the sensor can be arranged on a support structure independent from the screening device. In another embodiment, the sensor is attached to a side wall of the screening device. An ultrasound sensor can be a piezoelectric or capacitive transducer. In addition to ultrasound sensors, suitable sensors include laser, radar, sonar, lidar. It would also be possible to use photogrammetry for this purpose. Photogrammetry is suitable for applications where it is necessary to detect and differentiate between elements having different properties (size, color, speed, etc.), which makes it useful in the current invention. A combination of different types of sensors is also conceivable. The skilled person also realizes that even though only two conveyer belts are indicated in the figures, it is of course possible, and often preferable, to have further conveyer belts, containers, chutes or similar. In FIGS. 1a and 1b, for example, two further conveyer belts (or chutes or containers) would be preferable in order to keep separated the flow from the respective screening decks from each other. It would also be possible to arrange one or more sensors 5 below the lowermost screening deck to be able to detect foreign objects that falls through the deck. Normally, screening modules and other foreign objects will be transported together with the screened material along the screening deck but the possibility that may fall downwardly cannot be excluded and thus one or more sensors at that position may be advantageous. Even though the system is indicated as comprising three sensors, it is not a requirement to be able to perform the invention. One, two or more than three sensors are of course also conceivable.
