ROLLER MILL AND METHOD FOR CONTROLLING A ROLLER MILL

Abstract

The subject matter of the invention is a roller mill comprising two rollers which are arranged in parallel, are pressed one against the other and rotate in opposite directions, wherein one of the rollers can be displaced orthogonally with respect to the axial direction of this roller, and two drives, which drives are each assigned to one of the two rollers, and each have an electric motor, a master of the electric motors predefines for the electric motors a setpoint value for the rotational speed of the torque as a reference, and a reference of a follower electric motor of the electric motors comprises the actual value of the torque or of the rotational speed of the master electric motor multiplied by a load distribution factor.

Claims

1. A roller mill comprising two rollers which are arranged in parallel, are pressed one against the other and rotate in opposite directions during operation, wherein one of the rollers can be displaced orthogonally with respect to the axial direction of this roller, and two electric motors, which electric motors are each assigned to one of the two rollers, wherein a setpoint value for the rotational speed or the torque is predefined as a reference to a control of a master electric motor of the electric motors, wherein a reference for a control of a follower electric motor of the electric motors is based on an actual value of the torque or of the rotational speed of the master electric motor multiplied by a load distribution factor.

2. The roller mill as claimed in claim 1, wherein the load distribution factor is determined taking into account a contact pressure of the rollers, wear of the individual rollers or the contact pressure and the wear.

3. The roller mill as claimed in claim 2, wherein the wear of the individual rollers is quantified by the quotient of a reduction in diameter of a roller and a quantity of material which has been previously milled by this roller.

4. The roller mill as claimed in claim 1, wherein the load distribution factor is determined taking into account the diameters of the rollers.

5. The roller mill as claimed in claim 1, wherein the load distribution factor is defined by means of an operator of the roller mill.

6. The roller mill as claimed in claim 1, wherein the actual value of the master electric motor multiplied by the load distribution factor is compared with a corresponding actual value of the follower electric motor by means of a subtraction, and wherein the reference for the control of the follower electric motor is based on the setpoint value and the value resulting from the subtraction.

7. The roller mill as claimed in claim 6, wherein the compared value is regulated by means of a regulator.

8. A method for controlling a roller mill, comprising: providing a roller mill comprising two rollers which are arranged in parallel, are pressed one against the other and rotate in opposite directions during operation, wherein one of the rollers can be displaced orthogonally with respect to the axial direction of this roller, and two electric motors, which electric motors are each assigned to one of the two rollers. predefining a setpoint value for the rotational speed or the torque as a reference for a control of a master electric motor of the electric motors; determining an actual value of the torque or of the rotational speed of the master electric motor; multiplying of the actual value of the master electric motor by a load distribution factor; and including the result from the step of multiplying in the reference for a control of a follower electric motor of the electric motors.

9. The roller mill as claimed in claim 2, wherein the load distribution factor is determined taking into account the diameters of the rollers.

10. The roller mill as claimed in claim 3, wherein the load distribution factor is determined taking into account the diameters of the rollers.

11. The roller mill as claimed claim 2, wherein the actual value of the master electric motor multiplied by the load distribution factor is compared with a corresponding actual value of the follower electric motor by means of a subtraction, and wherein the reference for the control of the follower electric motor is based on the setpoint value and the value resulting from the subtraction.

12. The roller mill as claimed claim 3, wherein the actual value of the master electric motor multiplied by the load distribution factor is compared with a corresponding actual value of the follower electric motor by means of a subtraction, and wherein the reference for the control of the follower electric motor is based on the setpoint value and the value resulting from the subtraction.

13. The roller mill as claimed claim 4, wherein the actual value of the master electric motor multiplied by the load distribution factor is compared with a corresponding actual value of the follower electric motor by means of a subtraction, and wherein the reference for the control of the follower electric motor is based on the setpoint value and the value resulting from the subtraction.

14. The roller mill as claimed claim 9, wherein the actual value of the master electric motor multiplied by the load distribution factor is compared with a corresponding actual value of the follower electric motor by means of a subtraction, and wherein the reference for the control of the follower electric motor is based on the setpoint value and the value resulting from the subtraction.

15. The roller mill as claimed claim 10, wherein the actual value of the master electric motor multiplied by the load distribution factor is compared with a corresponding actual value of the follower electric motor by means of a subtraction, and wherein the reference for the control of the follower electric motor is based on the setpoint value and the value resulting from the subtraction.

16. The roller mill as claimed in claim 11, wherein the compared value is regulated by means of a regulator.

17. The roller mill as claimed in claim 12, wherein the compared value is regulated by means of a regulator.

18. The roller mill as claimed in claim 13, wherein the compared value is regulated by means of a regulator.

19. The roller mill as claimed in claim 14, wherein the compared value is regulated by means of a regulator.

20. The roller mill as claimed in claim 15, wherein the compared value is regulated by means of a regulator.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0017] The invention will be explained in more detail below using exemplary embodiments and with reference to the figures.

[0018] In the drawings:

[0019] FIG. 1 shows a schematic illustration of a radial section of a roller mill from the prior art;

[0020] FIG. 2 shows a roller mill with two drives from the prior art;

[0021] FIG. 3 shows a schematic illustration of the signal flow in a roller mill with a master-follower control from the prior art in an initial phase;

[0022] FIG. 4 shows a schematic illustration of the signal flow in a roller mill with a master-follower control from the prior art in a production phase;

[0023] FIG. 5 shows a schematic illustration of the signal flow in a roller mill according to the invention in a first exemplary embodiment; and

[0024] FIG. 6 shows a schematic illustration of the signal flow in a roller mill according to the invention in a second exemplary embodiment; and

[0025] FIG. 7 shows an exemplary relationship between the wear of two rollers and the selection of a load distribution factor.

[0026] Reference symbols used in the drawings are summarized in the list of reference symbols. Basically, identical parts are provided with the same reference symbols.

WAYS OF IMPLEMENTING THE INVENTION

[0027] FIG. 5 shows a schematic illustration of the signal flow in a roller mill according to the invention in a first exemplary embodiment. A superordinate control, for example by means of direct inputting of the operator or by means of a distributed control system (DCS), predefines a setpoint value 61 as a reference for the rotational speed to a frequency converter 5 of a master electric motor 2. An actual value 62, resulting from the regulation of a rotational speed regulator (not illustrated) of the frequency converter 5 of the master electric motor 2, of the torque of the master electric motor 2 is multiplied by a load distribution factor 64 in a multiplier 65. The load distribution factor 64 can be defined, for example, by manual inputting by the operator or regulation of the load distribution factor 64, intended therefor, which input or regulation can optionally also include additional measurement values such as, for example, the roller diameter. A value which results therefrom is transferred as a setpoint value to a torque regulator (not illustrated) of a frequency converter 5 of a follower electric motor 2. The wear of the individual rollers in relation to one another can be influenced by the load distribution factor 642.

[0028] Analogously to FIG. 3, it is also possible that in an initial phase until a defined load threshold is reached or by manual switching over to predefine as a reference the identical setpoint value for the rotational speed to the two frequency converters. Both frequency converters are therefore regulated with respect to the rotational speed in the initial phase. It is optionally also possible for the system to be configured as a speed follower. In this context, instead of the actual value of the torque of the master electric motor in the case of the torque follower, the actual value of a rotational speed of the master electric motor is used as a reference for the follower electric motor in the production phase. Therefore, the value which is obtained after the multiplication by the load distribution factor is also a rotational speed value which is then predefined as a reference to the frequency converter of the follower electric motor. It is possible to predefine, as two variations of the speed follower concept, a setpoint value for the rotational speed and alternatively a setpoint value for the torque as reference for the control of the master electric motor.

[0029] FIG. 6 shows a schematic illustration of the signal flow in a roller mill according to the invention in a second exemplary embodiment. In addition to FIG. 5, feedback of the actual value of the torque of the follower electric motor 2 is present. The setpoint value of the torque of the follower electric motor 2 from the multiplication by the load distribution factor is compared with the actual value of the torque of the follower electric motor 2 by means of a subtraction. The difference which is formed in this way between the setpoint value and the actual value of the torque of the follower electric motor 2 is transferred to a regulator 66, which regulator 66 can be, for example, a PID regulator. The regulator 66 regulates the difference of the torque of the follower electric motor 2 and converts the regulated signal into a rotational speed value using the area moment of inertia of the roller 1 which is connected to the follower electric motor 2. This direct coupling between the torque and the rotational speed is ensured by the mechanical coupling of the rollers by means of the material in the milling gap. As a result of the mechanical coupling of the two rollers, increasing the circumferential speed of one roller gives rise to an additional force which acts tangentially on the second roller and reduces the required force or torque in order to maintain or increase the circumferential speed of the second roller to the same degree. In this context, the ratio between the two roller radii corresponds to the transmission ratio in a gear mechanism with a transmission ratio in the vicinity of 1. The output of the regulator 66 is added to the original setpoint value 61 for the rotational speed and then transferred as a setpoint value to the frequency converter of the follower electric motor 2.

[0030] Analogously to FIG. 5, an optional initial phase or a refinement as a speed follower are also possible in both variants in FIG. 6. In the variant of the speed follower in which a setpoint value is predefined for the rotational speed as a reference for the control of the master electric motor, the conversion of the regulator using the area moment of inertia is eliminated, the the signals relate to rotational speed values with the exception of the load distribution factor.

[0031] FIG. 7 shows an exemplary relationship between the wear of two rollers and the selection of a load distribution factor 115. In the diagram, the wear 112 of a roller, in the form of the reduction in the roller diameter, is plotted against the rotational work 111 already performed by this roller. The rotational work 111 is to be understood here as being the cumulated torque, necessary for the milling of the previously milled material, plotted against the time required for the milling. The two curves 113, 114 represent the wear 112 of two rollers of a pair of rollers as a function of the rotational work 111. The curve 114 shows a greater degree of wear of the corresponding roller than the wear of the roller illustrated in the curve 113. In the illustrated case, the load factor 115 is then selected such that the roller with the accumulated greater previous wear bears a smaller part of the load necessary for the milling.

[0032] In general, the load distribution factor can be a positive real number including zero. In the case of identical accumulated wear of the two rollers, the load distribution factor should assume the value of one. The greater the difference between the accumulated wear values of the two rollers, the further the corresponding load distribution factor is away from the value of one. Depending on which of the two rollers has a greater degree of wear, the value of the load distribution factor tends toward zero here or toward infinity. In practice, the load distribution factor tends to vary between 0.8 and 1.2.

[0033] In the preceding case, the objective is to achieve, during the selection of the load factor, as far as possible the same wear of the rollers of a pair of rollers, in order, for example, to exchange both rollers in a maintenance operation and to maximize the time between two maintenance operations. However, other objectives when selecting the load distribution factor are also possible, such as, for example, the greater degree of wear of the roller which has already worn to a greater degree, and the protection of the roller which has worn to a lesser degree. Furthermore, it is ensured that the energy required is minimized, since, in particular in comparison with the solution in which both motors are provided with the same rotational speed references, it is ensured that only the energy required for milling is supplied.

LIST OF REFERENCE NUMBERS

[0034] 1 Displaceable roller

[0035] 1 Fixed roller

[0036] 2 Master electric motor

[0037] 2 Follower electric motor

[0038] 3 Cardan shaft

[0039] 4 Planetary gear mechanism

[0040] 5 Frequency converter of the master electric motor

[0041] 5 Frequency converter of the follower electric motor

[0042] 61 Setpoint value of the rotational speed

[0043] 62 Actual value of the master electric motor

[0044] 63 Reference for follower electric motor

[0045] 64 Load distribution factor

[0046] 65 Multiplier

[0047] 66 Regulator

[0048] 111 Rotational work of a roller

[0049] 112 Wear of a roller

[0050] 113 Curve of the displaceable roller

[0051] 114 Curve of the fixed roller

[0052] 115 Curve of the load distribution factor