Exemplary embodiments of the invention relate to a method for monitoring a screw centrifuge.
The screw centrifuge to be monitored may have been configured as a solid-bowl screw centrifuge, or as a screen-bowl screw centrifuge, for instance.
EP 0 798 046 A1 discloses a centrifuge drive with two motors—a primary motor and a variable-speed motor—and with a three-stage transmission. On three shafts a torque is either introduced into the transmission or picked up from it.
DE 10 2006 028 804 A1 discloses a screw centrifuge with a centrifuge drive with two motors—a primary motor and a variable-speed motor—and with a three-stage transmission. In this case, torques are capable of being introduced into the first transmission stage and the second transmission stage or capable of being picked up from these two transmission stages on a total of at least four shafts, in which connection, furthermore, the first and second transmission stages are particularly preferably capable of being driven (and, as a rule, are also driven) on at least three shafts, the first motor feeding a torque both into the housing and into the first transmission stage on two shafts.
In the course of the operation of a screw centrifuge, the difference in rotational speed between the drum and the screw is set by the mechanism of the machine or by the actuation of a variable-speed motor. However, in some applications of the screw centrifuge—that is to say, in the course of a separation of a product from solids conveyed out of the drum by the screw—under, for the most part, indeterminate operating conditions the so-called “stick-slip effect” may arise. This stick-slip effect between drum and screw is associated with strong surges of torque (sometimes even with fluctuating directions of torque), which load the drivetrain and, under certain circumstances, may even lead to damage and to stoppage of the plant if they are not noticed in time.
In contrast, exemplary embodiments of the invention are directed to a method for monitoring a screw centrifuge, with which an onset of a stick-slip effect can be ascertained in good time.
According to an embodiment, a method for monitoring a screw centrifuge, in particular a solid-bowl centrifuge or a screen-bowl centrifuge, including the following: a rotatable drum, a rotatable screw arranged in the drum, a main or primary motor at least for driving the drum, a drive motor for driving the screw (within the scope of this document, this also means, in particular, influencing the difference in rotational speed of the screw relative to the drum), which may be the main or primary motor or a secondary motor, and also a transmission arranged between the motor or motors and the drum and screw, transmission input shafts for the main motor and the drive motor for the screw, wherein one or more pulse-generators are arranged at least on the transmission input shaft for the screw, to each of which a proximity-sensor is assigned, with the following steps:
a) providing the screw centrifuge, and processing of a product with the screw centrifuge, in the course of which the product is separated from solids which are conveyed out of the drum by the screw,
b) (repeated, continual) determining a current angular velocity and determination of a mean angular velocity of the transmission input shaft for the screw over time,
c) (repeated) evaluating the measurements from step b), and
d) outputting a warning signal and/or variation of one or more operating parameters of the screw centrifuge, to the extent that dynamic changes of angular velocity are ascertained in the course of the evaluation.
In step b), the measurements of the current angular velocity of a last period—for example, the last 10 seconds—are, for example, averaged, and the mean value is continually updated to this extent. In addition, the current angular velocity is determined, and changes of this value in comparison with the mean value are then registered. Dynamic changes are, in particular, periodic changes.
In this regard, a reference time for one revolution is preferentially ascertained in the no-load state, and deviations from this time are converted into corresponding angles in the course of the subsequent measurements.
According to another embodiment, there is a method for monitoring a screw centrifuge, in particular a solid-bowl screw centrifuge or a screen-bowl screw centrifuge, which includes the following: a rotatable drum, a rotatable screw arranged in the drum, a primary motor for driving the drum and a secondary rotor for driving the screw, and also a transmission, arranged between the motors and the drum and screw, with transmission input shafts for the primary motor and the secondary motor, an elastic element between an output shaft of the secondary motor and the transmission input shaft for the secondary motor, wherein pulse-generators are arranged on the output shaft of the secondary motor and on the transmission input shaft on both sides of the elastic element, to each of which there are assigned proximity-sensors, with the following steps:
a) providing the screw centrifuge, and processing of a product with the screw centrifuge, in the course of which the product is separated from solids which are conveyed out of the drum by the screw,
b) (repeated) measuring a relative angular offset over time between the output shaft and the transmission input shaft on both sides of the elastic element connecting these shafts,
c) (repeated) evaluating the measurements from step b), and
d) outputting a warning signal and/or variation of one or more operating parameters of the screw centrifuge, to the extent that dynamic changes—that is to say, changes that are not constant over a predeterminable time-interval—of angular offset are ascertained in the course of the evaluation. In this regard, dynamic changes are, in particular, periodic changes.
In accordance with the invention, according to variants of the methods described above, it is possible in each instance to notice the incipient onset of the stick-slip effect in good time. This makes it possible to output a warning, to find a better operating point by procedural measures or by a variation of settings—such as a variation of the difference in rotational speed, of the rotational speed of the drum, or of the product feed quantity—or, in case this is necessary for the purpose of protecting the machine, to switch off the decanter.
The elastic element is preferentially an elastic coupling. But the elastic element may also be constituted by a drive belt if a belt drive has been provided between the output shaft and the transmission input shaft for the secondary motor.
A torque-dependent twist-angle of the coupling (or of the belt drive) between the secondary motor and the transmission input shaft on both sides of the elastic element is preferentially measured with high temporal resolution, and harmonic changes of this angle are detected. By virtue of this measurement or these measurements, the stick-slip effect can be detected particularly well in good time. In the case of a belt drive, where appropriate a gear ratio is incorporated correspondingly into the ascertainment.
In this case it is advantageous, and advantageous for the purpose of ensuring a good measurement, if the pulse-generators on the two shafts have been arranged in a fixed angular relationship, for instance with a phase shift—that is to say, with a corresponding angular offset—preferentially with a phase shift between 0° and 360°, and if the pulse-generators have been configured in such a manner that in the course of rotations of the output shaft one pulse or two or more pulses of the pulse-generators are sensed per revolution. Especially in the latter case, a measurement result can be achieved that can be evaluated particularly well.
The output signals of the proximity-sensors are preferentially read out or registered at a high sampling-rate or sampling-frequency by the control device, which with a suitable software measuring program, constitutes a measuring system, the sampling-rate being greater, preferentially several times greater, than the frequency of revolution of the transmission input shaft. Accordingly, it is expedient if the sampling-rate for a screw speed between 1000/min and 10,000/min amounts to between 2.5 kHz and 250 kHz. The stiffer the elastic element—in particular, the elastic coupling—has been designed to be, the higher the resolution of the measurement of the angle of torsion is intended to be. For this purpose, a correspondingly high sampling-rate of the sensors is required. Theoretically, eigenfrequencies may in fact perturb the measuring process. It is therefore preferred that no eigenfrequencies of the elastic element lie within the range of measurement of the system, since otherwise eigenfrequencies possibly cannot be distinguished, or can only be distinguished with difficulty, from oscillations by reason of stick-slip effects. But should an eigenfrequency of the elastic element nevertheless lie within the range of measurement and be excited/measured, in principle this is not problematic. For in this case it is possible to change one or more control parameters for actuating the centrifuge appropriately, in order to get out of the eigenfrequency range.
In the course of the further evaluation of the measurements, it is advantageous to ascertain a dynamic change of the angular offset between the output shaft and the transmission input shaft based on the result of a mathematical transformation method, in particular by means of a fast Fourier transformation method, since such a method permits the developing stick-slip effect to be detected particularly well in good time.
The invention will be discussed in more detail in the following with reference to the drawing on the basis of embodiments. Shown are:
The screw centrifuge includes a centrifuge drive 4 for rotating the drum 1 and the screw 2. For this purpose, the centrifuge drive 4 includes a primary motor 5 and a secondary motor 6—also called a variable-speed motor—and also a transmission 7, arranged between the motors 5, 6 and the drum 1 and the screw 2, into which, in operation, both motors 5, 6 feed a torque. If a secondary motor 6 is not present, the one motor that is then present is called the main motor and not the primary motor.
In this case, by way of example the main or primary motor 5 is coupled via a belt-type drive 8 with a first input shaft 9 of the transmission 7, and the variable-speed motor 6 is coupled via an output shaft 10 and an elastic coupling 12 with a preferentially second transmission input shaft 11 of the transmission 7.
A control device 13 actuates the motors 5, 6, to which it is connected in wireless manner or via lines 14, 15.
The design of the transmission 7 and of the control device 13 is preferentially such that a difference in rotational speed between the rotational speed of the drum 1 and the rotational speed of the screw 2 is adjustable in operation.
In operation, a dependence of the difference in rotational speed between the drum 1 and the screw 2 on the slippage and on the loading-state of the screw centrifuge cannot be avoided. Under, for the most part, indeterminate operating conditions in the course of the conveying of the centrifuged solids by the screw 2, the stick-slip effect discussed above arises, associated with strong surges of torque.
For the purpose of early detection of the onset of the effect, the screw centrifuge is provided with a monitoring device or a measuring system. This monitoring device makes it possible to measure a—torque-dependent—twist-angle of an elastic element—here, the coupling 12—between the output shaft 10 of the secondary motor 6 and the transmission input shaft 11 with high temporal resolution, and to detect changes (in particular, harmonic changes) of this angle.
For this purpose, the monitoring device includes two or more proximity-sensors 18, 19, linked to the control device 13, and pulse-generators 16, 17 respectively assigned to the proximity-sensors.
Pulse-generator 16 has been arranged on the output shaft 10 of the secondary motor 6 and configured in such a way that one signal or two or more signals is/are capable of being sensed per revolution. For instance, pins have been arranged or formed on shaft 10 at two points on shaft 10 that are offset by 180° in relation to one another. Assigned to pulse-generator 16 is proximity-sensor 18 which has been arranged in such a manner and which has been designed in such a manner that in the course of rotations of the output shaft 10—here, per revolution—it senses one pulse of pulse-generator 16 or, per revolution, two or more pulses of the pulse-generators 16, 16′.
Pulse-generator 17, on the other hand, has been arranged on the transmission input shaft 11 and configured, in turn (like pulse-generator 16), in such a way that one signal or two or more signals are capable of being sensed per revolution. Assigned to pulse-generator 17 for this purpose is proximity-sensor 19, which has been arranged in such a manner and which has been designed in such a manner that in the course of rotations of the transmission input shaft 11—here, per revolution—it senses one pulse of pulse-generator 17 or, per revolution, two or more pulses of the pulse-generators 17, 17′.
The pulse-generators 17, 16 have been arranged on the two shafts 10 and 11 in a fixed angular relationship, for instance with a phase shift—that is to say, with a corresponding angular offset. By way of example, this angular offset amounts to 90° (see
Since the pulse-generators or initiators 16, 17 have been arranged on both sides of the elastic element—here, the coupling 12—in this way it is possible to record the angular offset between the pulse-generators 17, 16 over time. The proximity-sensors 18, 19 (which have been designed, for instance, as inductive proximity-sensors, Hall sensors, or reed-type contact sensors) are for this purpose monitored by the control device 13, which with a suitable software measuring program constitutes a measuring system, at a sufficiently high sampling-rate or sampling-frequency. This sampling-rate amounts to 100 kHz, for instance.
Based on the measurement signals of the proximity-sensors 18, 19 linked to the control device 13, the current angular offset between the pulse-generators 16, 17 in operation is determined during the rotation of the drum 1 and the screw 2. Without torque loading, the measured angular offset coincides with that in the case of a reference measurement that was recorded, for instance, at the time of an initial installation of the machine (for example, 90° in
On the other hand, a temporally constant torque leads to a static deflection of the coupling 12 and therefore to a different phase shift or angular offset. This static angular offset is of no significance for the onset of the stick-slip effect.
Rather, in the event of an onset of the stick-slip effect a dynamic torque arises which brings about a dynamic change of the angular offset between the output shaft 10 and the transmission input shaft 11. This dynamic change of the angular offset is relevant here.
Depending on the number of pulses that each of the pulse-generators 16, 17 provides per revolution, it is possible for the angular offset between the pulse-generators 16, 17 to be determined, even several times per revolution of the shafts 10, 11.
For instance, in the case of two pulse-generators 16, 16′ and 17, 17′ in each instance and a phase offset of 90° between the four pulse-generators, four angular offsets can be ascertained per revolution of the shafts 10, 11.
These angular offsets are ascertained with the aid of the proximity-sensors 18, 19 and the control device 13, and are recorded over a period of time, and then an amplitude spectrum or the amplitude spectrum of the sequence is ascertained via a transformation, for instance an FFT (fast Fourier transform). In the case of an evaluation of four angular offsets per revolution, it is possible for oscillations up to a frequency of twice the rotational speed of the motor to be detected.
Under load, on the other hand, the flexible coupling 12 becomes twisted, so that the relative angular position of the shafts 10 and 11 in relation to one another varies. This variation is analyzable.
The angular offset in the example lies alternately above and below 60°. This is due to the fact that in the case of one of the pulse-generators 16, 17 the two edges are not situated 180° opposite of each other, but this is of no significance for the evaluation, since this frequency is just no longer detectable.
In the lower region of
It is conceivable to evaluate only the rising edges of
The described method can be used in principle for the most diverse decanters with driven or even braked transmission input shaft 11. In the case of drives with an elastic belt drive between the secondary motor 6 and the transmission input shaft, it is likewise conceivable to establish a dynamic angular deviation of the two belt pulleys from the normal gear ratio, and to ascertain the incipient stick-slip effect by an appropriate evaluation.
According to this set-up, the main motor 5 is designed to drive the drum 1 and the screw 2. Therefore, two belt-type drives 8a, 8b are provided, which couple the main motor 5 both with the first input shaft 9 of the transmission 7 and directly with a second transmission input shaft 11 of the transmission 7.
The control device 13 serves for actuating the motor 5.
The design of the transmission 7 and of the control device 13 is preferentially such that a difference in rotational speed between the rotational speed of the drum 1 and the rotational speed of the screw 2 is adjustable in operation.
Here too, under—for the most part—indeterminate operating conditions the stick-slip effect discussed above may arise in the course of the conveying of the centrifuged solids by the screw 2, associated with strong surges of torque.
For early detection of the onset of the effect, the screw centrifuge is provided with a variant of the monitoring device or, to be more exact, with a measuring system. This monitoring device makes it possible to measure torque-dependent fluctuations of the rotations of the transmission input shaft 11 with high temporal resolution, and to detect changes (in particular, harmonic changes) of this angle.
For this purpose, the monitoring device includes one or more proximity-sensors 18, linked to the control device 13, and pulse-generators 16 respectively assigned to said proximity-sensors.
Pulse-generator 16 has been arranged on the transmission input shaft 11 and configured in such a way that one signal or two or more signals are capable of being sensed per revolution.
In the course of the processing of a product with the screw centrifuge, in the course of which the product is separated from solids which are conveyed out of the drum 1 by the screw 2, a determination now takes place—in advance in the load-free state and/or repeatedly at intervals or incessantly, again and again in operation—of a mean angular velocity of the transmission input shaft for the screw over time. Then an evaluation of the measurements and an output of a warning signal and/or variation of one or more operating parameters of the screw centrifuge take place, to the extent that dynamic changes of angular velocity are ascertained in the course of the evaluation that satisfy a predetermined condition (for instance, an exceeding of a limiting value of the deviation). Also, in this way, an onset of the stick-slip effect can be detected in good time, and a progression of this effect can therefore, as a rule, be prevented at an early stage.
Also, in this variant of the monitoring method, the transmission input shaft 11 for the screw 2 could alternatively be driven by a secondary motor (with or without elastic element 12) instead of by a belt drive 8b.
Although the invention has been illustrated and described in detail by way of preferred embodiments, the invention is not limited by the examples disclosed, and other variations can be derived from these by the person skilled in the art without leaving the scope of the invention. It is therefore clear that there is a plurality of possible variations. It is also clear that embodiments stated by way of example are only really examples that are not to be seen as limiting the scope, application possibilities or configuration of the invention in any way. In fact, the preceding description and the description of the figures enable the person skilled in the art to implement the exemplary embodiments in concrete manner, wherein, with the knowledge of the disclosed inventive concept, the person skilled in the art is able to undertake various changes, for example, with regard to the functioning or arrangement of individual elements stated in an exemplary embodiment without leaving the scope of the invention, which is defined by the claims and their legal equivalents, such as further explanations in the description.
Number | Date | Country | Kind |
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10 2016 116 391.0 | Sep 2016 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2017/064255 | 6/12/2017 | WO | 00 |