WO2011023853A2

WO2011023853A2 - Method and equipment for measuring the surface height of a material bed conducted on a conveyor belt

Info

Publication number: WO2011023853A2
Application number: PCT/FI2010/050668
Authority: WO
Inventors: Martti Larinkari; Jukka LEPPÄNEN; Jouni VEHVILÄINEN; Nikolai Vatanski
Original assignee: Outotec Oyj
Priority date: 2009-08-27
Filing date: 2010-08-25
Publication date: 2011-03-03
Also published as: FI122454B; WO2011023853A3; FI20095882A0; FI20095882A

Abstract

Method and equipment for measuring the height of the surface of a material bed (2) conveyed on a conveyor belt (1). In the method, there is used a laser telemeter (3; 3'), which is a laser telemeter (3; 3') scanning over the material bed, operated according to a measurement principle based on the time of propagation of light.

Description

METHOD AND EQUIPMENT FOR MEASURING THE SURFACE HEIGHT OF A MATERIAL BED CONDUCTED ON A CONVEYOR BELT

FIELD OF INVENTION

The invention relates to a method defined in the preamble of claim 1. The invention also relates to equipment defined in the preamble of claim 5.

BACKGROUND OF INVENTION

In continuously operated sintering, there is nowadays generally used a conveyor-type sintering device, where a material bed is first formed on the conveyor belt. The material bed may consist for example of pellets, which by means of a high-temperature treatment, i.e. sintering, are hardened so that the pellets can be fed to further treatment, for instance to a smelting furnace .

In the sintering of a material bed, through the material bed located on the conveyor belt, and simultaneously through the conveyor belt itself, there is first conducted hot gas, so that the temperature of the material bed rises, for instance in the case of ferroalloy pellets, up to the range of 1,300-1,600° C. At a high temperature-, the fragile pellets react with the hot gas and are hardened in the process.

Thereafter the hardened pellets forming the material bed are cooled by conducting cooling gas through the material bed and the conveyor belt. Thus a conveyor belt employed in continuously operated sintering is used under remarkable fluctuations of temperature.

In order to obtain an advantageous sintering result, the material bed must be essentially even throughout the whole width of the conveyor belt. For measuring the evenness of the material bed, there are used for instance wires that are spaced apart over the conveyor belt, said wires being at one end connected to a common bar installed in parallel to the conveyor belt, so that the wires move along the top surface of the material bed. Moreover, in the measuring device there are installed separate bars for the top surface of the material bed, said bars defining the allowed lower and upper limits of the top surface of the material bed. If the position of the bar connected to the measuring wires falls outside the allowed position of the bars making up the lower and upper limits, an emergency notice is sent in order to be able to adjust the material bed at the desired level before the sintering process begins. For measuring the surface height of a material bed, there also are used optical measurement devices installed at the edge of the conveyor belt. In a similar way as the wires described above, said optical measurement devices measure only the top surface of the material bed with respect to the proceeding direction of the conveyor belt. By means of this equipment, it is not, however, possible to detect for instance potholes or apertures made in the material bed, i.e. spots where the amount of material to be sintered is slight or nonexistent. Such potholes or apertures in the material bed may cause even large damages to the conveyor belt, because normally the conveyor belt as such is resistant to a temperature of about 500° C only, whereas in the material bed sintering zone, the temperature in the top part of the material bed may rise up to 1,300° C and over.

The closest prior art is described in the publication WO 02/16866 Al, where the measuring technique of the surface height of a material bed is further attempted to be improved by utilizing a laser telemeter functioning according to an optical triangulation principle, in which telemeter, laser light focused from a laser light source to the material bed draws a line on the surface of the material bed, and the reflected light streak is detected by a CCD camera that is advantageously installed at an angle of 30 - 40° with respect to the laser light source. The real height of the material bed is calculated from an image obtained after calibration by using computer vision functions. The obtained image is compared with an image representing a predetermined surface height. If the obtained image for some parts deviates from the image of a desired material bed, alarm notice is given of the deviation, and on the basis of the obtained information, the material bed is changed to a desired level for example by adjusting the speed of the conveyor belt before the material bed is subjected to the various steps of the thermal treatment. The adjusting of the conveyor belt speed serves for adjusting the height of the material bed, so that for example when the material bed height should be increased, the conveyor belt speed is slowed down, in which case more material is allowed to be supplied on the conveyor belt from the material feed device with an even supply.

When applying the method according to the publication WO 02/16866 Al for measuring the surface height of a material bed to be conducted to thermal treatment, the measurement as such can be carried out without mechanically touching the material bed itself. This is particularly advantageous, because the material bed can be formed of fragile, moist pellets that are easily broken by mechanical contact. Moreover, the measuring device is free of any mechanically wearing parts. Further, in the measuring of the surface height of a material bed, there are not needed any such measuring devices that could present an obstacle for a possible mechanical smoothing out of the material bed prior to the thermal treatment thereof .

However, the measuring based on optical triangulation of the surface height of a material bed has a few drawbacks. The components required in the known system are expensive. For ensuring the security of the equipment, CE certification must be obtained. The equipment is difficult to tune, because it features a nonlinearity phenomenon caused by the angle left between the laser light source and the CCD camera. In the long run, this has a directly weakening effect on the accuracy of the measuring result . The equipment comprises at least two physical devices, and therefore it is complicated to install and takes up a lot of space. The existence of several different devices is problematic also with respect to maintenance, because for instance the replacing of the equipment by a new one requires that many different devices are disconnected and reinstalled.

OBJECT OF INVENTION

The object of the invention is to eliminate the above mentioned drawbacks .

A particular object of the invention is to introduce a method and equipment utilizing optical laser distance measurement with all advantages of the known material bed optical laser distance measurement method and equipment, without their drawbacks, i.e. so that it is cheaper, more secure, easier to tune and simpler to install and maintain.

SUMMARY OF INVENTION

The method according to the invention is characterized by what is set forth in claim 1. Further, the equipment according to the invention is characterized by what is set forth in claim 5.

According to the invention, in the method the surface height of a material bed is measured by a laser telemeter scanning over the material bed and operated according to a measurement principle based on the time of propagation of light. Respectively, in the equipment according to the invention, the laser telemeter is a laser telemeter scanning over the material bed and operated according to a measurement principle based on the time of propagation of light.

An advantage of the invention is that it is cheap, safe, easy to tune and simple to install and maintain. Savings in expenses are obtained mainly in components, because the components are cheaper than in a system according to the publication WO 02/16866 Al. The improved safety is connected to the fact that scanning laser telemeters based on the measurement of the time of propagation of light are already available as CE certified, and thus it is remarkably easier to obtain CE certification for the whole system. The laser classification of the laser telemeter is lower (class 2) than in a system operated according to the triangulation principle, where effective 3B class lasers must be used. The safety requirements of class 3 lasers are remarkably higher than with class 2 laser equipment. Tuning is simpler than before, because the nonlinearity phenomenon typical of a system operated according to the triangulation principle does not occur in equipment based on the measurement of the time of propagation of light. In the long run, this has a direct effect on the accuracy of the measurement results. The equipment according to the invention can be realized as one single physical unit, in which case installation and maintenance is easy, and the equipment does not take up a lot of space . The improvement with maintenance is connected to the fact that only one device must be replaced by a new one, which means that also the availability of spare parts is better than before. Moreover, the structure of the laser telemeter already is classified as suitable in industrial conditions (IP65 protection feature) , which means that the protection of the system can be realized more cheaply than before.

In one embodiment of the method, a laser light source is used for generating laser light pulses with a predetermined frequency and wavelength; the laser light pulse signal generated by the laser light source is divided, by means of a dichroic mirror, into a reference signal, which is immediately reflected to a reference receiver, and into a measurement signal, which is directed towards the material bed; the reflection of the measurement signal from the material bed is detected in the measurement receiver; the phase difference between the reference signal and the measurement signal is calculated; on the basis of the phase difference, the distance of the light source from the surface of the material bed is calculated; and the preceding steps are repeated in succession at several different measurement points in a transversal direction over the surface of the whole material bed in order to define its height profile.

In one embodiment of the method, the measurement at each measurement point is carried out with two or several different pulse frequencies.

In one embodiment of the method, the speed of the conveyor belt is adjusted on the basis of the measured height profile in order to adjust the surface height of the material bed.

In one embodiment of the equipment, the laser telemeter includes a laser light source, which is arranged to generate laser pulses with a predetermined wavelength and frequency; a dichroic mirror, which is arranged to divide the laser light pulse signal into a reference signal and a measurement signal; a reference receiver, which is arranged to receive the reference signal reflected from the dichroic mirror; a measurement receiver, which is arranged to receive from the material bed the reflection of the measurement signal that has penetrated the dichroic mirror; and a calculator device which is arranged to calculate the phase difference between the reference signal and the measurement signal, and on the basis of the obtained phase difference, to calculate the distance of the laser light source from the surface of the material bed.

In one embodiment of the equipment, the laser telemeter includes a movable mirror for focusing the measurement signal on different points of measurement in the material bed.

In one embodiment of the equipment, the laser telemeter includes a frequency synthesizer, which is arranged to control the laser light source in order to generate pulse queues with different frequencies.

In one embodiment of the equipment, the equipment includes a control device, which is arranged to adjust the conveying speed of the conveyor on the basis of the signal given by the laser telemeter. The method and equipment are particularly useful for measuring the height of a pellet bed conveyed on the conveyor belt of a strand sintering furnace prior to the heating steps. In addition, the equipment can be used for calculating the volume of the material flow on the belt conveyor, in which case for example by adjusting the supply of crushed ore or the like, or the speed of the belt conveyor, a desired volume flow in a defined target can be achieved. Respectively, the system can be used for defining the transversal surface area of the material bed by comparing the measurement profile obtained from the system with the reference profile formed by an empty belt surface. Further, the method and equipment can be used for measuring the height and/or height profile of a bottom pellet layer fed from a bottom pellet silo to the conveyor belt of a strand sintering furnace.

LIST OF DRAWINGS

In the specification below, the invention is explained in more detail with reference to exemplary embodiments and to the appended drawing, where

Figure 1 is a schematical illustration of one embodiment of the equipment according to the invention, and

Figure 2 is a schematical illustration of a laser telemeter included in the equipment illustrated in Figure 1.

DETAILED DESCRIPTION OF INVENTION

Figure 1 shows how pellets are fed to form a material bed 2 to the conveyor belt 1 of a strand sintering furnace. The fragile and moist pellets P obtained from pelletizing are conducted by means of a tripper conveyor 12 serving as a feed device to form a material bed 2 on the belt 1 of the belt conveyor, which conveys pellets through the sintering steps 13. Above the material bed 2, and essentially near to the point where the material bed 2 is formed, there is installed a laser telemeter 3 for measuring the height of the surface of the material bed 2. The laser telemeter 3 is operated according to a measurement principle based on the time of propagation of light. The laser telemeter 3 is used for scanning over the surface of the material bed, essentially in a transversal direction with respect to the conveying direction of the bed, in order to form a height profile for the bed. At the first end of the conveyor belt 1, there is provided a bottom pellet silo 16, in which there is collected part of the sintered pellets that have already passed through the sintering furnace. From the bottom pellet silo 16, the sintered pellets are fed to form a bottom pellet layer 17 on top of the bare conveyor belt, which is generally made of perforated steel band, to protect it against excessive heat. Above the bottom pellet layer 17, and essentially near to the point where it is formed, there is installed a second laser telemeter 3' for measuring the height of the surface of the bottom pellet layer 17. The laser telemeter 3' is operated according to a measurement principle based on the time of propagation of light. The laser telemeter 3 ' is used for scanning over the surface of the bottom pellet layer, essentially in a transversal direction with respect to the bed conveying direction in order to form a height profile for the bed. The feed aperture of the bottom pellet silo 16 is provided with a gate element 18, by means of which the thickness of the bottom pellet layer 17 can be adjusted. If there are disturbances in the feed of the bottom pellets (for example blockages) , they can easily be detected by the above described arrangement.

Figure 1 also shows a control device 11, which is on the basis of the signal received from the laser telemeter 3 advantageously arranged to adjust the conveying speed of the conveyor 1. If it is detected, from the measurement results of the laser telemeter 3, that the pellet bed is too low, the control device 11 can automatically decrease the running speed of the driving motor of the conveyor belt 1, so that a higher pellet bed is formed on the conveyor belt. Respectively, if it is detected from the measurement results of the laser telemeter 3 that the pellet bed is too high, the control device 11 can automatically increase the running speed of the driving motor of the conveyor belt 1, so that a lower pellet bed is formed on the conveyor belt. Advantageously the control device 11 can be arranged, on the basis of a signal received from a second laser telemeter 3', to adjust the position of the gate element 18 in order to adjust the thickness of the bottom pellet layer 17.

Further, in the conveying direction, before the sintering furnace, there can be arranged mechanical smoothing devices, controlled by the laser telemeter 3, for smoothing out the pellet bed. As an alternative, the laser telemeter 3 can be arranged to give an alarm for the operating staff, when the height profile of the pellet bed deviates from the desired, in which case the smoothing out of the pellet bed can be carried out manually.

Figure 2 illustrates, schematically and by way of example, the structure of one laser telemeter 3 or 3' . The_ laser telemeter 3 includes a laser light source 4, which is arranged to generate laser pulses with a predetermined wavelength and frequency. Further, the laser telemeter 3 includes a dichroic mirror 5, which is arranged to divide the laser light pulse signal to a reference signal S_ref and a measurement signal S_m. A reference receiver 6 is arranged to receive the reference signal S_ref reflected from the dichroic mirror. A measurement receiver 7 is arranged to receive the reflection of the measurement signal that has penetrated the dichroic mirror from the surface of the material bed 2 or 17. The reference receiver 6 and the measurement receiver 7 convert optical signals to electrical signals, and the analog to digital converters 14, 15 in the calculator device 8 convert electrical signals to digital signals. The calculator device 8 calculates the phase difference between the reference signal S_ref and the measurement signal S_m, and on the basis of the phase difference, it calculates the distance of the laser light source from the surface of the material bed 2 or 17. In addition, the laser telemeter 3 includes a movable mirror 9 for focusing the measurement signal S_m to different points of measurement in the material bed. A frequency synthesizer 10 is arranged to control the laser light source 4 for adjusting the repetition frequency of the laser light pulses. The measurement can be carried out at the same measurement point in succession, with a lower and with a higher pulse repetition frequency for improving the measurement accuracy.

The invention is not restricted to the above described embodiments only, but many modifications are possible within the scope of the inventive idea defined in the appended claims.

Claims

1. A method for measuring the height of the surface of a material bed (2; 17) conveyed on a conveyor belt (1) , in which method the height of the material bed surface is measured with a laser telemeter, (3; 3'), characterized in that the surface height of the material bed is measured with a laser telemeter (3; 3') scanning over the material bed, operated according to a measurement principle based on the time of propagation of light.

2. A method according to claim 1, characterized in that

a) a laser light source (4) is used for generating laser light pulses with a predetermined frequency and wavelength,

b) the laser light pulse signal generated by the laser light source (4) is divided, by means of a dichroic mirror (5) , into a reference signal (S_ref) , which is reflected immediately to a reference receiver (6) , and a measurement signal (S_ra) , which is directed towards the material bed (1) ,

c) the reflection of the measurement signal from the material bed is detected in the measurement receiver (7) ,

e) the phase difference between the reference signal (S_ref) and the measurement signal (S_m) is calculated,

f) on the basis of the phase difference, the distance of the laser light source from the surface of the material bed is calculated, and

g) steps a) - f) are repeated in succession at several different measurement points in a transversal direction over the whole surface of the material bed in order to define its height profile.

3. A method according to claim 1 or 2 , characterized in that the measurement at each measurement point is carried out with two or several different pulse frequencies.

4. A method according to any of the claims 1 - 3, characterized in that on the basis of the measured height profile, the speed of the conveyor belt is adjusted for adjusting the height of the surface of the material bed.

5. Equipment for measuring the height of the surface of a material bed (1) conveyed on a conveyor belt, said equipment including a laser telemeter (3; 3')_/ characterized in that the laser telemeter (3; 3') is a laser telemeter scanning over the material bed, operated according to a measurement principle based on the time of propagation of light.

6. Equipment according to claim 5, characterized in that the laser telemeter (3; 3') includes

- a laser light source (4) , which is arranged to generate laser pulses with a predetermined wavelength and frequency,

- a dichroic mirror (5) , which is arranged to divide the laser light pulse signal to a reference signal (S_ref) and a measurement signal (S_n,) ,

- a reference receiver (6) , which is arranged to receive the reference signal (S_ref) reflected from the dichroic mirror,

a measurement receiver (7) , which is arranged to receive the reflection of the measurement signal that has penetrated the dichroic mirror from the material bed, and

- a calculator device (8) , which is arranged to calculate the phase difference between the reference signal (S_ref) and the measurement signal (S_m) , and on the basis of the phase difference, to calculate the distance of the laser light source from the surface of the material bed.

7. Equipment according to claim 6, characterized in that the laser telemeter (3; 3') includes a movable mirror (9) for focusing the measurement signal to different points of measurement in the material bed.

8. Equipment according to claim 6 or 7, characterized in that the laser telemeter (3; 3') includes a frequency synthesizer (10) , which is arranged to control the laser light source (4) for generating pulse queues with different frequencies.

9. Equipment according to any of the claims 5 - 8, characterized in that the equipment includes a control device (11) , which is on the basis of the signal received from the laser telemeter (3) arranged to adjust the conveying speed of the conveyor (1) .

10. The use of a method according to any of the claims 1 - 4 and/or equipment according to any of the claims 5 - 9,

- for measuring the height and/or height profile of a bottom pellet layer (17) fed from a bottom pellet silo (16) to the conveyor belt (1) of a strand sintering furnace,

- for measuring the height of a pellet bed (2) conveyed on the conveyor belt (1) of a strand sintering furnace,

- for adjusting the transversal surface area of an ore bed in the processing of crushed ore,

- for defining the volume flow of material.