The present invention pertains to a method of controlling a pellet mill, said pellet mill comprising a number of rollers mounted inside a cylindrical die for pressing material to be pelletized through perforations provided in said cylindrical die The pellet mill further comprises one or more adjustment mechanisms for adjusting the distance between the rollers and the die, and a drive mechanism for rotationally driving the die and rollers. Typically, the rollers are driven by the friction between the die, the material to be pelletized and the rollers and the drive mechanism thus merely drives the die.
In order to maximize the output of such a pellet mill, it is normally driven close to maximum load and thus just minor fluctuations may lead to overload of the pellet mill Typically, such overload is not detected until the safety mechanism, e g. in the form of a shear pin, breaks, which leads to disengagement of the roller support, whereby the rollers and roller support may rotate with the die, when blocking of the pellet mill has been encountered. Subsequently, it will be necessary to disassemble the pellet mill in order to empty the space between the rollers and the die and insert a new shear pin for the safety mechanism before restarting the pellet mill
From EP 472 016 A2 it is known to provide a measurement of the rotational speed of the die and the rollers and to compare the peripheral speed of these, in order to detect a possible slip, which is taken as an indication of malfunction of the pellet mill. Suitable countermeasures can then be taken, e g. in the form of reduced supply of material or change of the composition of the material supplied in order to resume proper function of the pellet mill.
It is the object of the present invention to provide an improved method of controlling a pellet mill of the above kind, whereby there can be provided an early warning of an overload of the pellet mill and initiation of suitable corresponding countermeasures for avoiding total stop of the pellet mill in such situations.
According to the present invention, the above-mentioned and further objects are obtained by the controlling method comprising the steps of controlling the position of the rollers by means of an adjustment mechanism to a predetermined distance between the roller and the die, and limiting the available forces for the adjustment mechanism, whereby the rollers will give way if larger forces are necessary for maintaining the predetermined distance. In case of an increase of the distance between the rollers and the die, this is taken as an indication of an overload of the pellet mill and load-reducing countermeasures are taken, e g. in the form of a reduced supply of material.
The position of the rollers may be controlled by means of an adjustment mechanism, e.g in the form of hydraulic piston cylinder units and the position is measured, typically by the physical position of said hydraulic piston cylinder unit In accordance with the present invention, the position of the rollers is controlled to a predetermined distance between roller and die and the forces available for the adjustment mechanism is limited, in the case of a hydraulic system, by limiting the hydraulic pressure available, whereby the rollers will give way if larger forces are necessary for maintaining the position of the rollers and accordingly, such yield of a roller leading to non-maintenance of the predetermined distance between roller and die, can be taken as an indication of an overload of the pellet mill.
Furthermore, this yield of a roller will allow part of the material to be pelletized to be transported to the next following roller, where it may be pressed through the die. The limitation of the available forces for the adjustment mechanism also limits the possible unbalance between the rollers and thus the strain on the structural parts of the pellet mill.
In this connection it may be advantageous to allow a predetermined increase in the distance before initiating the load-reducing countermeasures. This will allow an automatic load-distribution between the rollers which may lead to overcoming the tendency to overload of a single roller and subsequent resumption of normal processing of the material
In a preferred embodiment, the forces available for the adjustment are limited to a level equal to the level necessary for keeping the position under normal conditions plus a predefined value, preferably e.g 10% thereof
The suitable countermeasures for reducing the load on the pellet mill may comprise one or more of the following.
i) reducing the supply of material to the pellet mill and/or to a specific roller, for which an indication of overload has been registered.
ii) increasing the distance between the die and the rollers or, if overload is related to a specific roller, between the die and a specific roller, for which an overload has been indicated
iii) allowing a short overload by short term increasing the hydraulic pressure for one or more rollers
iv) a combination of the above-mentioned
In a preferred embodiment, the measurements of the positions of the individual rollers are used in a closed loop control for the positions of the rollers It is furthermore preferred that an individual position control is performed and in connection with the slip measurement for the individual roller, individual initiation of countermeasures is performed for each of the rollers
In an advantageous supplementary embodiment of the method, a further measurement of the rotational speed of each of the rollers can be performed and in case of detection of a non-rotating roller or a roller rolling at a reduced peripheral speed compared to the peripheral speed of the die, this can be taken as an indication of slip between the roller and the die, and suitable load-reducing countermeasures can be taken in such a situation
In the following detailed part of the present description, the invention will be explained in more detail with reference to the exemplary embodiments of methods of controlling a pellet mill according to the invention in connection with a pellet mill as shown in the drawings, in which
The pellet mill shown in
Various possibilities for performing the position measurements could be simple linear position detectors, rotational position detectors or, in case of hydraulic drive for controlling the position of the rollers, hydraulic fluid volume measurement, measuring the amount of hydraulic fluid delivered to the hydraulic cylinders for controlling the position of the rollers.
The measurement of the rotational speed of rollers and die, respectively, may be performed by means of optical sensors, inductive sensors, etc.
The controller shown in
In
By the provision of this maximum available force, a position measurement of the rollers can indicate an overload of the individual roller by the adjustment system not being able to keep the desired position
The motor control for driving the die may include a frequency converter for adjustable rotational speed of the die. This provides the possibility of adjusting the set point for the rotational speed of the die, in order to offer the possibility of maximizing the output of the pellet mill.
In connection with the above drawings, some preferred features of the present invention have been illustrated, however, it should be emphasized that not all of the above features need to be employed in order to implement the present invention, which is solely to be restricted by the appended claims Thus, it is possible to implement the present invention with only one or preferably at least two rollers and also more than three rollers as illustrated may be used. Furthermore, the different measurements may be reduced to fewer measurements than indicated in the appended figures. Also the individual supply of material may be omitted and a common supply for the pellet mill may be used Preferably this supply can be controlled in a suitable manner in accordance with the present invention.
Number | Date | Country | Kind |
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11164246.8 | Apr 2011 | EP | regional |
11164249.2 | Apr 2011 | EP | regional |
11164251.8 | Apr 2011 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2012/057726 | 4/27/2012 | WO | 00 | 1/31/2014 |