Drive Control Method And Drive System Operating According To Said Method

Abstract

In a drive control method for a vertical mill having a grinding plate rotatable about the vertical axis and being drivable by a drive train that includes an electric motor and a gearbox, a rotational speed of the grinding plate is varied cyclically, especially intermittently. A corresponding drive system operating according to the method is also disclosed.

Description

[0001] The present invention relates to a drive control method, namely a method for controlling a heavy duty drive, in particular a heavy duty drive for a vertical roller mill for comminuting brittle materials such as cement raw material, and to a corresponding drive system operating according to said method.

[0002] Vertical roller mills of the above mentioned type, having a grinding table rotating about the vertical and grinding rollers above the grinding table, are subject to severe mechanical vibrations. The resulting forces and torques can be so powerful that the grinding process has to be stopped in order to prevent damage to the drive train, namely the electric motor and gearbox in particular, or to the plant as a whole.

[0003] In order to minimize such vibrations, the mill operator has hitherto had to design the process parameters, i.e. in particular the contact pressure of the grinding rollers, composition of the material to be ground and amounts of grinding additives such that the vibrations excited remain below a critical level. However, this means undesirable process design limitations which negatively impact many areas. These include the range of products that can be made from the respective ground material obtained, the effectiveness of the mill, the energy input required and the cost-efficiency. Nevertheless, such measures are unreliable, as much experience is required for correct process control and the properties of the natural materials before and after grinding are necessarily always different. This makes it necessary to continuously optimize the process and adjust it to the raw material.

[0004] However, extreme vibration states--known in the industry as "rumbling" of the mill, repeatedly occur, with the result that the mill has to be stopped and then restarted. This is to the detriment of the availability and productivity of the plant. There is also the risk of gearbox and plant damage. To obviate this problem, process control has hitherto been organized particularly defensively in order to prevent these mill vibrations as far as possible. However, the production rate, product quality and the range of manufacturable products suffer as a result.

[0005] Against this background and because of the increasingly exacting requirements in respect of availability, efficiency and Total Cost of Ownership (TCO), the design and arrangement of the electrical and mechanical components of a drive system and of the respective drive train of a heavy duty drive, in particular of a vertical roller mill, are becoming increasingly important.

[0006] For vertical roller mills, drive systems comprising a gearbox and an electric motor in the form of an asynchronous motor, preferably a wound rotor, and a frequency converter feeding the electric motor constitute a preferred solution. Here the mill gearboxes are in practice implemented as variants of bevel or spur gear planetary mechanisms. The purpose of the gearing arrangement is not only speed and torque conversion but also to absorb the axial grinding forces and transfer them into the base.

[0007] Controlling such a drive system for a vertical roller mill essentially presents the following problems in practice:

[0008] In order to be able to ensure optimum process control, the first, apparently trivial task of the drive is to deliver the predefined rotation speed of the grinding table. As the process torque demand at the grinding table fluctuates, speed control is required.

[0009] The load fluctuations and vibration excitations acting on the drive mechanism are influenced by impulsive loads such as those produced when the grinding rollers encounter coarse material, stochastic loads of the grinding process, periodic excitations from the gearbox and mill kinematics, and a varying contact pressure of the grinding rollers. The interaction of these load influences results in a complex load cycle which can even set off resonance vibrations.

[0010] In addition to the drive train vibrations, an unstable, i.e. fluidizing or undulating grinding bed, for example, can also cause extreme vibrational states of the mill, in particular mill rumbling.

[0011] Lastly the grinding of natural products makes it largely unpredictable how the grinding process must be adjusted in order to guarantee quiet running of the mill. It is therefore always a challenge for the operator at the control desk to find the correct process parameters. In the end, although the drive alone can quieten a poorly adjusted process, it cannot correct it.

[0012] The approach proposed here deals with the effect of undesirable rumbling of a mill as the result of a particular surface structure of the grinding bed, and an object of the present invention is accordingly to specify a means of efficiently preventing or at least reducing such rumbling.

[0013] One reason for mill rumbling caused by the surface structure of the grinding bed is that the mill, as a resonant system, reacts to stochastic excitations from the grinding process e.g. with regular relative movements between the grinding rollers and the grinding bed. These regular relative movements result from the particular natural frequency of the mechanics and kinematics of the grinding rollers which are disposed in a movable, in particular swiveling manner above the grinding table and the grinding bed produced thereon during mill operation. The grinding rollers act on the grinding bed on the one hand because of their own weight and because of their movable, in particular swivel mounting. In addition, the action of the grinding rollers on the grinding bed can be intensified still further by an additionally applied contact pressure.

[0014] The above mentioned object is achieved by a method for drive control of a vertical roller mill having the features as claimed in claim 1. The object is also achieved by a drive system having the features of the parallel device claim. The vertical roller mill, also referred to here and in the following sometimes merely as the mill for short, comprises a grinding table rotating about the vertical, wherein the grinding table can be driven by means of a drive train comprising at least one electric motor and usually a gearbox and is driven during operation of the mill. The method is characterized in that the rotation speed of the grinding table is cyclically varied.

[0015] In the case of a drive system designed to carry out such a method and possibly one or more of the embodiments described below, namely a drive system for a vertical roller mill comprising a grinding table rotating about the vertical, the drive system comprises at least one electric motor, optionally a frequency converter feeding the electric motor, a gearbox between the at least one electric motor and the grinding table and optionally a sensor system for obtaining vibration-relevant measured values, in particular vibration-relevant measured values in the form of torque or speed measurements with respect to a rotation speed of a rotating component of the vertical roller mill or to at least one driving and/or supporting torque acting in or on the gearbox. The drive system is characterized by a speed variation device with which the rotation speed of the grinding table can be varied, wherein the speed variation device is designed and set up to operate according to the method outlined above and described in greater detail below, and carries out such a method during operation of the drive system.

[0016] In short, the invention is therefore a method and a device for drive control of a heavy duty arrangement in the form of a drive system in which the cyclical variation of the rotation speed of the grinding table has the aim of creating no regular structure in the grinding bed surface, i.e. no undulation of the grinding bed. Accordingly, the purpose of the method and the device operating according to said method is to prevent a possible cause of mill rumbling at source.

[0017] The advantage of the invention is that by cyclically varying the rotation speed of the grinding table, rumbling of the mill can be prevented or eliminated or at least reduced without having to stop the grinding process, and that this result is achieved by comparatively simple intervention in the overall system, namely by appropriately controlling the electric motor. The prevention, elimination or reduction of the vibrations will hereinafter be referred to as prevention for short.

[0018] Cyclically varying the rotation speed of the grinding table is to be understood as meaning varying the rotation speed of the grinding table using an, in particular, variable speed profile in which the time-averaged speed profile corresponds to the setpoint rotation speed of the grinding table. A special form of such cyclical variation of the rotation speed of the grinding table is periodic variation of the rotation speed of the grinding table, e.g. sinusoidally varying the rotation speed of the grinding table about a setpoint rotation speed of the grinding table.

[0019] Advantageous embodiments of the invention are set forth in the sub-claims. References used indicate the further development of the subject matter of the main claim by the features of the respective sub-claim. They are not to be understood as a waiver of the achievement of independent, concrete protection for the feature combination of the sub-claims referred to. In addition, in respect of interpretation of the claims in the case of a closer concretization of a feature in a subordinate claim it is assumed that such a limitation is not present in the respective preceding claims. Lastly it is pointed out that the method specified here can also be further developed according to the dependent device claims and vice versa.

[0020] In an embodiment of the method, cyclical or periodic variation of the rotation speed of the grinding table is activated in a time-controlled manner at predefined or predefinable, in particular equidistant points in time for a predetermined or predeterminable time period, e.g. every five minutes for ten seconds at a time. The cyclical or periodic variation of the rotation speed of the grinding table is then not continuously active. Outside of the activation of the cyclical or periodic speed variation, "normal" rotation speed of the grinding table is produced.

[0021] In an alternative embodiment of the method, the cyclical or periodic variation of the rotation speed of the grinding table is activated as a reaction to a vibration-relevant measured value fluctuation exceeding a predefined or predefinable limit value.

[0022] Vibration-relevant measured values are all the measured values obtained or obtainable in respect of the mill, the evaluation of which indicates mechanical vibrations of the mill, in particular such mechanical vibrations as are termed rumbling. Possible means of obtaining such vibration-relevant measured values include sensors in the form of vibration monitors, vibration sensors or the like which are e.g. mounted on the mill framework or other parts of the mill structure. Alternatively or additionally possible are also sensors which are assigned to the drive train where they acquire vibration-relevant measured values. For example, it can be provided that the power draw of the electric motor is measured by means of such a sensor and vibration-relevant data is derived from an electric motor power draw correlated with varying load situations so that in this respect measured values relating to motor current drawn during operation of the mill in each case are also an example of vibration-relevant measured values.

[0023] If such a measured value exceeds a predefined or predefinable limit value or possibly repeatedly exceeds such a limit value, in particular repeatedly exceeds it within a predefined or predefinable period of time, this indicates existing or impending mill rumbling. On the basis of such a detection or prediction of mill rumbling, cyclical or periodic variation of the rotation speed of the grinding table is activated in order to eliminate or at least reduce mill rumbling or prevent it from occurring in the first place.

[0024] In this variant of the method, the cyclical or periodic varying of the rotation speed of the grinding table does not therefore take place continuously but only as required, namely if monitoring of the respective value obtained automatically indicates the necessity of counteracting an existing, undesirable vibration or risk of vibration in the form of mill rumbling in each case.

[0025] In an embodiment of the method, cyclically varying the rotation speed of the grinding table takes the form of periodically varying the speed, wherein a frequency and/or an amplitude of the signal waveform underlying the periodic variation of the rotation speed of the grinding table and, alternatively or additionally, possibly also the signal waveform itself is increased or reduced, i.e. changed, as a function of a vibration-relevant measured value fluctuation exceeding a predefined or predefinable threshold value. In this variant of the method, the periodic variation of the rotation speed of the grinding table is therefore itself varied on the basis of automatic evaluation of the above mentioned measured value. The threshold value in question can be below or above the limit value mentioned earlier. In the case of continuous variation of the rotation speed of the grinding table, a threshold value below a limit value indicating existing or impending rumbling is a possibility. The attainment or exceeding of such a threshold value then indicates that the already occurring periodic variation of the rotation speed of the grinding table is insufficient to prevent or eliminate mill rumbling. In the case of periodic variation of the rotation speed of the grinding table solely as a function of a monitored limit value, the threshold value additionally considered in this embodiment of the method will be above such a limit value. Exceeding of the threshold value is then an automatically evaluable indication that periodically varying the rotation speed of the grinding table is insufficient for preventing or eliminating mill rumbling.

[0026] As a countermeasure, both in the case of grinding table speed variation that is active continuously or only when required or intermittently, the periodic variation is itself varied, e.g. by increasing the period of the underlying signal waveform. This is aimed at changing the undulation of the grinding bed and the aim of the thereby achieved change in the undulation of the grinding bed is in turn that no natural frequencies in the overall mill system are excited by the resulting movements of the grinding rollers because of grinding bed undulation and therefore mill rumbling is prevented or eliminated, in any case at least reduced, namely e.g. in its frequency and/or vibration intensity and/or echo speed.

[0027] A sinusoidal profile, a triangular profile, a rectangular profile or a ramp profile, for example, can be used for periodically varying the rotation speed of the grinding table. Such profiles can easily be generated using a signal generator or the like and, in the case of software implementation of the method, by appropriate mathematical expressions or families of characteristics.

[0028] In another or alternative embodiment of the method, a change in a respective signal waveform with which the rotation speed of the grinding table is periodically varied is initiated as a function of a vibration-relevant measured value fluctuation exceeding a predefined or predefinable threshold value. This variant of the method is characterized by parallels with the changing (frequency, amplitude or signal waveform) of the periodic variation of the rotation speed of the grinding table already described above. Here, however, it is not the period of the periodic variation of the rotation speed of the grinding table that is changed as a function of the exceeding of the threshold value, but the signal waveform underlying said periodic variation. The statements made above may be referred to regarding the position of the threshold value and for the use of this variant of the method either for continuous or merely as-required or intermittent variation of the rotation speed of the grinding table. As a result, this variant is designed to change the grinding table speed variation according to the automatically detectable exceeding of the threshold value which indicates that mill rumbling cannot be prevented or eliminated using the existing periodic variation of the rotation speed of the grinding table. This change is accomplished by changing the signal waveform. This also produces a change in the undulation of the grinding bed. As in the case of the method variant described above, this is designed to ensure that no natural frequencies are excited in the overall mill system due to the resulting movements of the grinding rollers because of the grinding bed undulation, so that in this way the mill rumbling is prevented or eliminated, in any case at least reduced.

[0029] The two variants for changing the periodic variation of the rotation speed of the grinding table, namely changing the period and changing the signal waveform, in particular quantitative and/or qualitative changing of the signal waveform, are self-evidently also combinable.

[0030] Because it is particularly a matter of preventing a grinding bed surface regularity that is conducive to resonances, another possibility is to apply the individual methods in a quasi-random manner. A software implementation of such a method provides all the possible variants of the method as functional units or the changing of the period and the changing of the signal waveform as parameterizable functional units. Based on a random number generator or the like, individual functional units and/or parameters for the parameterization of the functional units or combinations of possibly individually parameterized functional units are then selected in order to counteract existing or impending mill rumbling detected on the basis of the measured value monitoring outlined above.

[0031] In a specific embodiment of individual method variants described above, torque or rotation speed measurements are used as vibration-relevant measured values and are acquired. The further description of the approach proposed here will be based, without loss of generality, on such torque or speed measurements. These will now be subsumed under the term measured values. To obtain such measured values, a sensor system, i.e. at least one sensor incorporated in the sensor system or belonging to the sensor system, is used to measure a rotation speed of a rotating component of the drive train and/or at least one driving and/or supporting torque acting on the gearbox.

[0032] A corresponding embodiment of the drive system outlined above is characterized by such a sensor system for obtaining a vibration-relevant measured value in the form of a torque or speed measurement, wherein by means of the sensor system a rotation speed of a rotating component of the vertical roller mill and/or at least one driving and/or supporting torque acting in or on the gearbox can be measured and is measured during operation.

[0033] In this connection it should be noted that the use of sensors, e.g. sensors for acquiring torque or rotation speed measurements in the drive train (drive sensor system) instead of vibration sensors on the mill structure has apparently not been taken into consideration hitherto. However, according to the inventor's insight, mechanical vibration of the mill and therefore also rumbling of the mill can also be detected using such measurements. It is actually even the case that such measured values replicate respective process events even more directly, as the events in the grinding mechanism become much more clearly apparent, according to the inventor's insight, in the degree of rotational freedom of the drive than in the vibration level of the mill structure as a whole. This is because the rumbling is a periodic collapse of the supporting effect of the grinding bed on the grinding rollers. Associated with the loss of this supporting effect, e.g. because of undulation of the grinding bed or yielding aside of a fluidized material to be ground, there also arises a breakdown of the loading torque exerted by the grinding bed on the grinding table therefore directly on the drive train. This torque characteristic is directly detectable in said measured values, namely even before the grinding rollers move so strongly that the vibration also perceptibly spreads to the rest of the mill structure. Undesirable states such as mill rumbling or incipient mill rumbling can be detected earlier and more precisely in this way. Comparative studies have shown that detection of mill rumbling via vibration sensors usually takes place after a few seconds at the earliest. In this time the drive is subject to just over a hundred load cycles (at a rumble frequency of e.g. 15 Hz). Evaluation of the drive sensor system allows rumbling to be identified after just three to ten load cycles, i.e. in less than one second. The advantage of using a sensor system of this kind and of the thereby obtained measurements is therefore that, because mill rumbling can be detected earlier, countermeasures, i.e. even emergency shutdown of the drive ("emergency stop"), for example, but self-evidently also the cyclical or periodic varying of the rotation speed of the grinding table highlighted here, can be initiated earlier and therefore the stressing of the drive, but also of the mill as a whole, by mill rumbling can be reduced.

[0034] In one embodiment of the method, the electric motor is fed by a frequency converter and the rotation speed of the grinding table is cyclically or periodically varied by means of appropriate control of the frequency converter. By means of a frequency converter, the described cyclical or periodic varying of the rotation speed of the grinding table by appropriate control of the electric motor, but also the changing of the period or the changing of the underlying signal waveform and combinations thereof, can be comparatively easily achieved. An alternative to a frequency converter is a superposition gear with which the above described cyclical or periodic varying of the rotation speed of the grinding table can be achieved in basically an equivalent manner.

[0035] The method and the drive system operating according to said method are based on the cyclical or periodic variation of the rotation speed of the grinding table and on a speed variation device designed for that purpose. Individual aspects of the functionality of the speed variation device have already been described above. The functionality of the speed variation device, the optional acquisition and processing of the measured values in question functionally upstream of the cyclical or periodic variation of the speed, and the resolution of the variation of the rotation speed of the grinding table functionally downstream of the speed variation device can be realized in hardware and/or software. In the case of software realization, the invention is also a computer program having program coding means for executing all the steps of the method described here and in the following when the computer program is run on a controller or the like for a drive system for a vertical roller mill. Further, the invention is therefore also a digital storage medium having electronically readable control signals which can interact with a programmable controller for a drive system for a vertical roller mill such that such a method can be executed. Finally, the invention is also a drive system of the above mentioned type which comprises a processing unit and a memory, wherein such a computer program is loaded into the memory and is executed during operation of the drive system by the processing unit thereof.

[0036] An exemplary embodiment of the invention will now be explained in greater detail with reference to the accompanying drawings. Equivalent items or elements are provided with the same reference characters in the figures.

[0037] It should also be pointed out that the approach described here and individual and possibly combined embodiments can also be combined with the approach proposed and specific embodiments described in the same applicant's parallel application attributable to the same inventor and having the applicant's internal reference number 201312099 (official application number not yet known). In this respect, the complete disclosure content of this parallel application, especially having regard to the therein described pattern recognition and the action on the basis of a detected pattern, is implied in the description presented here.

[0038] The exemplary embodiment should not be interpreted as a limitation of the invention. On the contrary, within the scope of the present disclosure, changes and modifications are possible, especially such modifications and combinations that, for example, as a result of combinations or modifications of individual features or elements or method steps contained in the general description, in the descriptions of various embodiments, and in the claims, and illustrated in the drawings, can be comprehended by persons skilled in the art as far as the achievement of the object is concerned and, as a result of combinable features, lead to a novel subject matter or to novel method steps and/or sequences of method steps.

[0039] FIG. 1 shows a greatly simplified schematic representation of a vertical roller mill comprising a grinding table driven by means of a heavy duty drive,

[0040] FIG. 2 shows a plan view onto the grinding table and grinding bed,

[0041] FIG. 3 shows a periodically varied rotation speed of the grinding table of the vertical roller mill, and

[0042] FIG. 4 shows a drive system of the vertical roller mill incorporating a controller which causes the rotation speed of the grinding table of the mill as shown in FIG. 3 to be cyclically and periodically varied.

[0043] FIG. 1 shows a greatly simplified schematic representation of a vertical roller mill 10 for comminuting brittle material, e.g. cement raw material. The vertical roller mill 10 comprises a grinding table 12 rotatable about the vertical. The grinding table 12 is driven by means of a heavy duty drive in the form of a motor, in particular an electric motor 14, and, in the example shown here, by means of a gearbox 16 located between electric motor 14 and grinding table 12. The gearbox 16 is shown here, without loss of further generality, as bevel-gear teeth with following planetary gearing not shown in greater detail. The gearbox 16 can also comprise spur-gear teeth or the like and/or a preceding or following planetary gearing or the like.

[0044] The vertical roller mill 10 comprises at least one driven shaft. In the illustration in FIG. 1, the vertical roller mill 10 comprises a motor shaft 18 and a grinding table shaft 20. All the means for transmitting the driving force of the electric motor 14 to the grinding table 12 are termed the drive train. Here the drive train comprises at least the electric motor 14, the motor shaft 18, the gearbox and the grinding table shaft 20.

[0045] The vertical roller mill 10 as a whole is a resonant system. During operation of the vertical roller mill 10, the electric motor 14 causes the grinding table 12 to rotate. On the grinding table 12 there is, as a result of the grinding process and as a result of supplied material to be ground, a grinding bed 22, i.e. a mixture of ground material and material to be ground. The grinding effect is achieved by a grinding roller 24 or a plurality of grinding rollers 24 pressing onto the grinding bed 22 and the rotating grinding table 12 because of their weight on the one hand, but on the other hand in some cases also because of additionally applied forces which are applied e.g. by means of a hydraulic cylinder or the like engaging with a swivel-mounted grinding roller 24.

[0046] FIG. 2 shows a simplified schematic plan view of the grinding table 12 with the grinding bed 22 and the (here) two grinding rollers 24. The radial dotted lines within the grinding bed 22 are to indicate an undulation of the grinding bed 22 that frequently arises during the grinding process. Such undulation of the grinding bed 22 is a possible cause of the mill rumbling that is to be prevented using the approach presented here. If the grinding bed 22 is undulating, it is easy to see that the swivel-mounted grinding rollers 24 follow the surface of the grinding bed 22 and the thereby caused upward and downward movement of the grinding rollers 24 is transmitted to the mill 10 in the form of vibrations. If the natural frequency of the mill 10 is excited in this way, resonance can even be set up.

[0047] Such vibrations have hitherto been detected by means of a sensor system disposed on the mill framework (vibration sensor; not shown). As soon as a vibration measurement acquired by the sensor system exceeds a limit value, the electric motor 14 is stopped and the mill 10 is subsequently restarted.

[0048] Here it is proposed that a rotation speed of the grinding table 12 is cyclically, in particular periodically, varied. To illustrate this, FIG. 3 shows a normally constant rotation speed 26 of the grinding table 12 apart from operational fluctuations, and a rotation speed 28 that is periodically varied according to the approach proposed here, namely a rotation speed 28 of the grinding table 12 that is varied symmetrically about the original constant rotation speed 26. The frequency underlying the periodic variation of the rotation speed 28 of the grinding table 12 is in the 0.1 Hz range, for example. The amplitude of the periodic variation is e.g. in the range of 1% of the effective setpoint speed without the variation. In the example shown, the periodic variation of the rotation speed of the grinding table 12 has an underlying triangular signal waveform. Other signal waveforms such as a sinusoidal signal, a square-wave signal, a ramp-shaped signal, etc. are also possible. The illustrated periodic variation of the speed 26 is a special form of a cyclical variation of the speed. The feature of such a periodic variation of the speed 26, but also of each more general cyclical variation of the speed 26 of the grinding table 12, is that the average value over time of the varied speed 26 results in the setpoint speed and that the varied speed values continually assume or pass through the setpoint speed value.

[0049] According to the inventor's insight, varying the rotation speed of the grinding table 12 prevents mill rumbling from occurring at all, because the cyclical or periodic variation of the rotation speed of the grinding table 12 prevents the formation of a regular undulation of the grinding bed 22 as shown in FIG. 2. Varying the rotation speed of the grinding table, especially periodically varying the rotation speed of the grinding table, produces a local displacement of the wave contour in the surface of the grinding bed 22 compared to a state arising in the case of an unvaried rotation speed. This disrupts the regular excitation of the grinding rollers 24, i.e. throws them out of their rhythm somewhat, and no resonance is produced. According to the inventor's insights, a slight variation (ranging from 1 to 5%) is sufficient to achieve the desired effect. As a result, the mill 10 rumbles less frequently, which advantageously impacts availability and the operator's freedoms in terms of process design. In addition, the mill and drive mechanics are subject to much less stress.

[0050] This is a method for drive control of the mill 10 in that, in this way, a surface structure of the grinding bed 22 resulting from the driving of the grinding table 12 is reacted to by indirectly or directly varying the rotation speed of the grinding table 12 in a cyclical or periodic manner.

[0051] The described cyclical or periodic varying of the rotation speed of the grinding table 12 can be active continuously or in a time-controlled manner. In the case of time-controlled activation of the cyclical or periodic variation of the rotation speed of the grinding table 12 it is possible for the speed variation to be activated at equidistant points in time for a predefined or predefinable time period and then deactivated again.

[0052] The described cyclical or periodic varying of the rotation speed of the grinding table 12 can also be activated as a function of a state detected in relation to the mill 10. This can be done using a sensor system 30 (FIG. 1) assigned in particular to the drive train, i.e. in particular the electric motor 14, motor shaft 18, gearbox 16 or grinding table shaft 20, or to the grinding table 12, to obtain vibration-relevant measured values 32, e.g. torque or rotation speed measurements 32. Specifically vibration-relevant measured values 32 in the form of torque measurements 32 are a measure of the torque or gearbox torque transmitted by means of the gearbox 16, i.e. a measure of a torque which is termed the mechanically effective torque in the drive train, in particular in the gearbox 16, to differentiate it from an electrical torque acting on the electric motor 14. On the basis of such a measured value 32 (or the pattern recognition described in the parallel application having the internal reference number 201312099) the cyclical or periodic speed variation can be activated as required, e.g. whenever an acquired measured value 32 exceeds a predefined or predefinable limit.

[0053] To cyclically or periodically vary the rotation speed of the grinding table 12, a speed variation device 34 (FIG. 1) is provided. This comprises, for example, a signal generator 36 realized in software or hardware and--if the method and its embodiments are implemented in software--a computer program 38 and a processing unit 40 in the form or in the manner of a microprocessor provided for executing the computer program 38. The computer program 38 is loaded into a memory 42 of the speed variation device 34.

[0054] By generating an appropriate cyclical or periodic signal, the signal generator 36 causes the rotation speed of the grinding table 12 to be cyclically or periodically varied by e.g. combining a resulting, cyclical or periodic output signal 44 of the speed variation device 34 with a speed setpoint value 46 and feeding the combination of the two signals 44, 46 (addition or subtraction) to a frequency converter 48 connected upstream of the electric motor 14 in per se known manner which, on the basis of the input signal 50 thus obtained, produces a respective supply voltage, in particular an AC voltage, for driving the electric motor 14. The speed of the electric motor 14 therefore fluctuates with the cyclicality or periodicity predefined by the speed variation device 34 and the resulting rotation speed of the grinding table 12 also fluctuates accordingly.

[0055] Lastly FIG. 4 shows in schematically simplified form that the speed variation device 34 is e.g. a sub-functionality of a controller 52 or the like, i.e. an open-loop control device for controlling the frequency converter 48. In addition to the speed variation device 34 or a software implementation of the speed variation device 34, the controller 52 can also comprise other functional units such as a closed-loop control device 54 for controlling the speed of the electric motor 14 or similar. The controller 52, the frequency converter 48 and the electric motor 14 together constitute a drive system 56 for driving the mill 10. Instead of the frequency converter 48 or in addition to a frequency converter 48, it is also possible for a superposition gear to be used.

[0056] In particular embodiments of the approach described here, the signal waveform underlying the periodic speed variation and/or a period 58 (FIG. 4) of the output signal 44 of the speed variation device 34 underlying the periodic speed variation, for example, can be selected by means of the speed variation device 34.

[0057] Although the invention has been illustrated and described in detail by an exemplary embodiment, the invention is not limited by the example(s) disclosed and other variations may be inferred therefrom by the average person skilled in the art without departing from the scope of protection sought to the invention.

[0058] Individual prominent aspects of the description submitted here may be summarized as follows: specified are a method for drive control of a vertical roller mill 10 having a grinding table 12 rotatable about the vertical, wherein the grinding table 12 can be driven by a drive train comprising an electric motor 14 and a gearbox 16, wherein a rotation speed of the grinding table 12 is varied cyclically, in particular periodically, in order to prevent rumbling of the mill 10, and a drive system 56 operating according to the method with which a rotation speed of the grinding table 12 can be periodically varied.

Claims

1.-13. (canceled)

14. A drive control method for a vertical roller mill having a grinding table rotatable about a vertical axis, the method comprising: driving the grinding table by way of a drive train having an electric motor and a gearbox, and cyclically varying a rotation speed of the grinding table.

15. The method of claim 14, wherein the cyclical variation of the rotation speed of the grinding table is activated under time control for a predefined period of time.

16. The method of claim 15, wherein the rotation speed is activated at predefined points in time.

17. The method of claim 16, wherein the predefined points in time are equidistant.

18. The method of claim 14, wherein the cyclical variation of the rotation speed of the grinding table is activated in response to a fluctuation of vibration-relevant measured values that exceeds a predefined limit.

19. The method of claim 18, wherein the vibration-relevant measured values comprise at least one of a rotation speed of a rotating component of the vertical roller mill and a driving torque and supporting torque operating in or on the gearbox.

20. The method of claim 14, wherein the rotation speed is cyclically varied by periodically varying of the rotation speed, and wherein at least one of an amplitude and a period duration of the periodic variation of the rotation speed of the grinding table is increased or reduced as a function of a fluctuation of vibration-relevant measured values that exceeds a predefined limit.

21. The method of claim 20, wherein the vibration-relevant measured values comprise at least one of a rotation speed of a rotating component of the vertical roller mill and a driving torque and supporting torque operating in or on the gearbox.

22. The method of claim 14, wherein the rotation speed is cyclically varied by periodically varying of the rotation speed with a rotation speed profile selected from a sinusoidal profile, a rectangular profile, a square-wave profile and a ramp-profile.

23. The method of claim 22, wherein a change in a signal waveform with which the rotation speed of the grinding table is periodically varied is initiated when a fluctuation of vibration-relevant measured values exceeds a predefined limit.

24. The method of claim 23, wherein the vibration-relevant measured values comprise at least one of a rotation speed of a rotating component of the vertical roller mill and a driving torque and supporting torque operating in or on the gearbox.

25. The method of claim 14, wherein the electric motor is fed by a frequency converter, and wherein the rotation speed of the grinding table is cyclically varied by appropriate control of the frequency converter.

26. The method of claim 14, wherein vibration-relevant measured values are obtained by measuring with a sensor system a rotation speed of a rotating component of the drive train or at least one driving torque or supporting torque, or both, operating in or on the gearbox.

27. A computer program product embodied on a non-transitory storage medium and comprising a computer program having program instructions which, when loaded into a memory of a control device controlling a drive for a vertical roller mill having a grinding table driven by at least one electric motor and a drive train comprising at least one gearbox for rotation about a vertical axis, and executed by the control device, causes the control device to cyclically vary a rotation speed of the grinding table.

28. A non-transitory digital storage medium comprising a computer program having program instructions which, when loaded into a memory of a programmable controller controlling a drive system for a vertical roller mill having a grinding table driven by at least one electric motor and a drive train comprising at least one gearbox for rotation about a vertical axis, and executed by the programmable controller, cause the programmable controller to cyclically vary a rotation speed of the grinding table.

29. A drive system for a vertical roller mill having a grinding table rotatable about a vertical axis, the drive system comprising: an electric motor driving the grinding table, a frequency converter feeding the electric motor, a gearbox arranged between the electric motor and the grinding table, and a rotation speed varying device configured to cyclically vary a rotation speed of the grinding table.

30. The drive system of claim 29, further comprising a sensor system for measuring vibration-relevant measured values.

31. The drive system of claim 30, wherein the vibration-relevant measured values comprise at least one of a rotation speed of a rotating component of the vertical roller mill and a driving torque and supporting torque operating in or on the gearbox.

32. A controller of a drive system of a vertical roller mill having a grinding table rotatable about a vertical axis, wherein the drive system comprises an electric motor driving the grinding table, a frequency converter feeding the electric motor, and a gearbox arranged between the electric motor and the grinding table, the controller comprising a processing unit and a memory into which a computer program having program instructions which, when loaded into the memory and executed by the processing unit, cause the controller during operation of the controller to cyclically vary a rotation speed of the grinding table.