Patent Application
20170108038
The invention relates generally to a magnetic levitation system that can be, for example but not necessarily, an active magnetic bearing “AMB”. More particularly, the invention relates to a control device and to a method for controlling a magnetic levitation system. Furthermore, the invention relates to a computer program for controlling a magnetic levitation system.
Magnetic levitation systems, such as e.g. active magnetic bearings “AMB”, are commonly known in the art. Magnetic levitation systems are commonly utilized for supporting a rotating or oscillating object. In many cases, the support at each direction is obtained by balancing attractive forces of two opposite acting magnets and other forces acting on an object to be levitated, where at least one of the magnets is a controllable electromagnet. In principle, it is also possible to balance an attractive force of one controllable electromagnet and other forces, e.g. the gravity force, acting against the attractive force of the electromagnet. The magnetic forces acting in all or some degrees of freedom of the levitated object, e.g. a rotor of an electrical machine, have to be controlled actively because of the inherent instability of the magnetic levitation. The instability is due to the fact that the magnetic attractive force acting between a magnet and an object made of e.g. ferromagnetic material increases when the air-gap between the magnet and the object gets smaller. There are several different kinds of magnetic levitation systems. Some systems use permanent magnets to generate bias magnetic fluxes, others use direct biasing currents to generate the bias fluxes. The biasing is used to overcome static loads, increasing the possible rates of change of magnetic forces and to linearize the magnetic force dependence on control variables.
The magnetic force generated by each electromagnet of a magnetic levitation system can be controlled by controlling the current of the electromagnet under consideration. By controlling the currents of all electromagnets of the magnetic levitation system, a resultant magnetic force can be generated into a desired direction. A control device of the magnetic levitation system constitutes typically an outer control loop and an inner control loop for each degree of freedom of the object to be levitated. The outer control loop receives information expressing the measured or estimated position, and possibly also the velocity, of the object to be levitated and a reference, i.e. desired, position of the object. The outer control loop produces reference values for the currents of the electromagnets acting in the degree of freedom under consideration. The inner control loop receives information expressing the reference values of the currents and the measured or estimated values of the currents. The inner control loop controls the voltages directed to the windings of the electromagnets so that the currents follow the reference values of the currents with a sufficient accuracy. The voltages can be controlled on the basis of the differences between the measured or estimated currents and the reference values of the currents for example with the pulse width modulation “PWM”. Instead of the currents, the control quantities controlled by the inner control loop can be estimated or measured magnetic fluxes generated by the electromagnets or estimated or measured forces directed by the electromagnets to the object to be levitated.
The above-described control principle is, however, not free from challenges. One of the challenges is related to delays created by the outer and inner control loops. The outer control loop has to be tuned to alter the reference values of the currents or other control quantities so slowly that the inner control loop is able to make the currents or other control quantities to follow the changes of the reference values with a sufficient accuracy. If the outer control loop is too fast, i.e. the outer control loop changes the reference values too fast, the differences between the prevailing currents or other control quantities and the reference values may get so big that the position control represented by the outer control loop gets instable. Therefore, as usual in cases having outer and inner control loops, the outer control loop has to be sufficiently slower than the inner control loop. The ability of the inner control loop to generate fast changes in the currents and as well in the forces and in the magnetic fluxes is inherently limited by the inductances of the electromagnets and the upper limits of the available voltages. On the other hand, the outer control loop has to be sufficiently fast in order to provide a sufficiently stiff magnetic suspension. Therefore, it can be quite challenging to construct the outer control loop so that the changes in the reference values of the currents or other control quantities are slow enough for the inner control loop to follow but, on the other hand, the changes are fast enough so as to provide a sufficiently stiff magnetic suspension.
The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.
In accordance with the invention, there is provided a new method for controlling a magnetic levitation system that can be, for example but not necessarily, an active magnetic bearing “AMB”. A method according to the invention comprises:
The one or more voltages are controlled by:
In the above-described method, the one or more voltages are controlled without a need to form one or more reference values which have to react fast enough to changes in loading conditions in order to provide a sufficiently stiff magnetic suspension, but whose changes have to be slow enough in order to keep the differences between the reference values and the corresponding control quantities, e.g. currents, fluxes, or forces, sufficiently small so as to maintain stability of the magnetic suspension.
The magnetic levitation system can be for example an axial magnetic bearing for supporting an object, e.g. a rotor of an electrical machine, in mutually opposite directions parallel with the axis of rotation of the object. In this case, there are two possible control directions which are mutually opposite to each other. For another example, the magnetic levitation system can be a radial magnetic bearing for supporting an object, e.g. a rotor of an electrical machine, in directions perpendicular to the axis of rotation of the object. In this case, the possible control directions are in a geometric plane perpendicular to the axis of rotation. For a third example, the magnetic levitation system may comprise one or more radial magnetic bearings and one or more axial magnetic bearings. In this case, the magnetic bearings are advantageously controlled separately. For a fourth example, the magnetic levitation system may comprise conical magnetic bearings capable of supporting a rotating object both in the radial directions and in the axial directions.
It is worth noting that also in a case where the magnetic levitation system comprises only one controllable electromagnet for supporting an object against downwards directed loading including the gravity force, there are two possible control directions. One of the control directions is upwards and the other is downwards. If the object is at a position higher than the reference position, the magnetic force directed to the object is changed in the downward control direction, i.e. the upward directed magnetic force is weakened and thus the change of the magnetic force is downwards. This change of the magnetic force improves the ability of the total force acting on the object and including the magnetic force and the gravity force to move the object towards the reference position.
In accordance with the invention, there is provided also a new control device for controlling a magnetic levitation system that can be, for example but not necessarily, an active magnetic bearing “AMB”. A control device according to the invention comprises:
The controller is configured to:
In accordance with the invention, there is provided also a new magnetic levitation system that comprises:
In accordance with the invention, there is provided also a new computer program for controlling one or more voltages directed to one or more windings of a magnetic levitation system so as to control a resultant magnetic force directed to an object to be levitated.
A computer program according to the invention comprises computer executable instructions for controlling a programmable processing system of the magnetic levitation system to:
In accordance with the invention, there is provided also a new computer program product. The computer program product comprises a non-volatile computer readable medium, e.g. an optical disc, encoded with a computer program according to the invention.
A number of exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.
Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in connection with the accompanying drawings.
The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality.
Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which:
In the exemplifying case illustrated in
The magnetic levitation system comprises controllable voltage sources 107x+, 107x−, 107y+ and 107y− for directing controllable voltages to the windings 105x+, 105x−, 105y+ and 105y−. In the exemplifying magnetic levitation system illustrated in
The magnetic levitation system comprises a control device 101 for controlling the magnetic actuator 104. The control device comprises a signal input 102 for receiving the components Px and Py of the position signal, and a controller 103 for controlling the voltages directed to the windings 105x+, 105x−, 105y+ and 105y− in a time-discrete way at temporally successive control periods. The voltages are controlled on the basis of the deviation between the position of the object 108 and the reference position of the object. In this case, the components Px and Py of the position signal represents the deviation of the position because the origin of the coordinate system shown in
dF/dt˜(2φx+×Vx+−2φx−×Vx−)ex+(2φy+×Vy+−2φy−×Vy−)ey, (1)
where the effect of the resistances of the windings is neglected, and Vx+, Vx− and Vy− are the voltages directed to the windings 105x+, 105x− and 105y−, respectively. As indicated by Equation (1), the direction of change of the resultant magnetic force F can be controlled with the aid of the voltages directed to the windings 105x+, 105x−, 105y+ and 105y−.
The controller 103 comprises a functional section 110 for selecting, for each of the temporally successive control periods, a control direction CD so that changing the resultant magnetic force F in the selected control direction improves the ability of the resultant magnetic force to decrease the deviation of the position. The controller 103 comprises a functional section 111 for setting, for each of the temporally successive control periods, the voltages Vx+, Vx−, Vy+ and Vy− in accordance with the selected control direction CD so as to decrease the deviation of the position by changing the resultant magnetic force with the aid of the voltages. In this exemplifying case, a value of each of the voltages Vx+, Vx−, Vy+ and Vy− is selected from a finite set of selectable voltage values, i.e. ≈+UDC, ≈0, ≈−UDC, and the selection is accomplished by setting appropriate values to the voltage selectors Sx+, Sx−, Sy+ and Sy−. The functional section 111 can be implemented for example with the aid of a selection look-up table for outputting the voltage selectors Sx+, Sx−, Sy+ and Sy− on the basis of a look-up key comprising an indicator of the selected control direction CD. As the value of each of the voltages V″, Vx−, Vy, and Vy− is selected from the finite set of selectable voltage values, there are only a finite number of selectable voltage combinations which correspond to different control directions. Therefore, the control direction CD cannot be selected freely but the control direction is selected from a finite set of selectable control directions. In cases where voltages are continuously controllable, the control direction can be selected more freely.
As mentioned earlier, the functional section 111 of the control device 101 can be implemented with the aid of a selection look-up table for outputting the voltage selectors Sx+, Sx−, Sy+ and Sy− on the basis of a look-up key comprising an indicator of the selected control direction. The selection look-up table can be for example according to Table 1 shown below. In Table 1, ‘+’ means that ≈+UDC is directed to the winding under consideration, ‘0’ means that ≈zero voltage is directed to the winding, and ‘−’ means that ≈−UDC is directed to the winding. The row of table 1 is determined on the basis of the control direction and the column is determined according to the winding under consideration. The control directions are denoted in the same way as in
In a control device according to an exemplifying and non-limiting embodiment of the invention, the functional section 109 shown in
The reference vector R may represent for example a desired resultant magnetic force F or desired values of some other control quantities defining the operation of the magnetic levitation system, such as e.g. a vector of balances of squares of the currents (ix+2−ix−2)ex+(iy+2−iy−2)ey, where ix+, ix−, iy+ and iy− are the currents of the windings of the magnetic levitation system. The use of the predetermined rule for producing the reference vector is depicted with blocks 115a in
The above-described control principles are based on the assumption that direction of change of the resultant magnetic force F is sufficiently close to the direction of the following voltage vector:
V=(Vx+−Vx−)ex+(Vy+−Vy−)ey, (2)
As presented earlier in Equation (1), the rate of change vector dF/dt of the resultant magnetic force F is proportional to the following vector:
dF/dt˜(2φx+×Vx+−2φx−×Vx−)ex+(2φy+×Vy+−2φy−×Vy−)ey,
As can be seen, the direction of the voltage vector V is the direction of the change of the resultant magnetic force F, i.e. the angle between V and dF/dt is zero, if the magnetic fluxes φx+, φx−, φy+ and φy− are mutually equal, i.e. φx+=φx−=φy+=φy−.
Although this assumption is applicable in many cases, there can be situations where the quality of the control can be improved by using a more accurate model for selecting the voltages directed to the windings of the magnetic levitation system.
In a control device according to an exemplifying and non-limiting embodiment of the invention, the functional section 110 is configured to maintain a correction model for correcting the selection of the control direction. The correction model is depicted with a block 115 in
A control device according to an exemplifying and non-limiting embodiment of the invention is configured to determine a temporal length of each of the temporally successive control periods on the basis of (i) the control values Cx and Cy indicating the required changes of the control quantities being controlled, e.g. the x- and y-components of the resultant magnetic force F, and (ii) a fact that the voltages set for the control period under consideration at least partly determine a rate of change of each of the control quantities under consideration. In many cases this approach reduces the switching frequency of the voltage sources because switching is made only when needed. For example the functional entity 111 which selects the voltages can be provided with computing capacity for determining the temporal lengths of the control periods. A simpler and more straightforward approach is to use control periods having a constant temporal length.
A control device according to an exemplifying and non-limiting embodiment of the invention is configured to keep, in order to reduce the switching frequency, the voltages unchanged in response to a situation in which a vector norm of the control vector C=Cxex+Cyey indicating the required changes of the control quantities is below a pre-determined limit. The vector norm can be for example the Euclidean norm or some other suitable vector norm.
In many magnetic levitation systems, the windings constitute one or more mutually non-overlapping winding groups so that the windings belonging to a same winding group are capable of generating mutually cancelling components of the resultant magnetic force. The meaning of the term “mutually non-overlapping” is that each winding belongs to only one winding group, i.e. none of the winding belongs to two or more winding groups. For example, in the exemplifying magnetic levitation system illustrated in
Next we consider a winding group of the kind mentioned above, e.g. the winding group constituted by the windings 105x+ and 105x−. The resultant of the magnetic forces generated by the windings of the winding group can be adjusted by using only non-negative changes in the magnetic forces or only non-positive changes in the magnetic forces. This can be understood in the following way. We first assume that a desired change in the resultant of the magnetic forces is accomplished so that some of the magnetic forces are increased, some the magnetic forces are decreased, and possibly some the magnetic forces are kept unchanged. If we want to use only non-positive changes, we decrease all the magnetic forces by an amount that is at least the greatest one of the increases. This does not change the resultant of the magnetic forces but, as a corollary, none of the magnetic forces is increased but each of them is either decreased or kept unchanged. Correspondingly, if we want to use only non-negative changes, we increase all the magnetic forces by an amount that is at least the greatest one of the absolute values of the decreases. This does not change the resultant of the magnetic forces, but as a corollary, none of the magnetic forces is decreased but each of them is either increased or kept unchanged. The operating points of the currents of the windings can be moved upwards by using only non-negative changes in the magnetic forces for adjusting the resultant of the magnetic forces. Correspondingly, the operating points of the currents of the windings can be moved downwards by using only non-positive changes in the magnetic forces for adjusting the resultant of the magnetic forces. This principle is applicable also in e.g. such cases where a magnetic actuator of a radial magnetic bearing comprises six legs and three windings where the windings are positioned 120 degrees apart from each other in the circumferential direction in a corresponding way as the windings 105x+, 105y+, 105x−, and 105y− shown in
In a control device according to an exemplifying and non-limiting embodiment of the invention, the functional section 111 is configured to use, for each of one or more mutually non-overlapping winding groups each constituted by at least two of the windings and capable of generating mutually cancelling components of the resultant magnetic force, only zero voltages and current-decreasing voltages but not current-increasing voltages in response to a need to decrease operating points of control quantities, e.g. currents, of the winding group under consideration. It is to be noted that the above-mentioned “zero voltage” means typically slightly negative voltage because of the resistances of conductors and the conductive state threshold voltages of power electronic components. In the exemplifying case illustrated in
In a control device according to an exemplifying and non-limiting embodiment of the invention, the functional section 111 is configured to use, for each of the one or more winding groups, only zero voltages and current-increasing voltages but not current-decreasing voltages in response to a need to increase the operating points of the control quantities, e.g. the currents, of the winding group under consideration.
It is worth noting that in cases, such as e.g. the case illustrated in
As mentioned earlier, the functional section 111 for selecting the voltages and shown in
In conjunction with a control device according to an exemplifying and non-limiting embodiment of the invention, the selection look-up table comprises a first sub-table, a second sub-table, and a third sub-table. The first sub-table is a current-decreasing sub-table which allows only current-decreasing and zero voltages. The second sub-table is a full-voltage sub-table which allows current-decreasing, zero, and current increasing voltages. The third sub-table is a current-increasing sub-table which allows only current-increasing and zero voltages. The above-mentioned sub-tables for the exemplifying magnetic levitation system shown in
The functional section 111 or some other part of the control device 101 is configured to select the current-decreasing sub-table, Table 2a, for the winding group-X, i.e. the windings 105x+ and 105x−, in response to a need to decrease the operating points of the control quantities, e.g. currents, of the winding group-X. Correspondingly, the functional section 111 or the other part of the control device 101 is configured to select the current-increasing sub-table, Table 2c, for the winding group-X in response to a need to increase the operating points of the control quantities of the winding group-X. Correspondingly, the functional section 111 or some other part of the control device 101 is configured to select the current-decreasing sub-table, Table 2a, for the winding group-Y, i.e. the windings 105y+ and 105y−, in response to a need to decrease the operating points of the control quantities, e.g. currents, of the winding group-y, and to select the current-increasing sub-table, Table 2c, for the winding group-y in response to a need to increase the operating points of the control quantities of the winding group-Y. The need to increase or decrease the operating points is indicated by a control signal OP that is received by the control device 101.
It is worth noting that using the current-decreasing sub-table or the current-increasing sub-table for one winding group and the full-voltage sub-table for another winding group changes the set of the selectable control directions presented in
In a control device according to an exemplifying and non-limiting embodiment of the invention, the functional section 111 is configured to select, for one of the winding groups, either the current-decreasing or the current-sub-table in response to a situation in which either the current-decreasing or the current-sub-table needs to be selected for another one of the winding groups. In this case, the set of possible control directions remains unchanged with respect to a situation in which the full-voltage sub-tables are used for all winding groups. In a case where the operating points of the control quantities of one of the winding groups are at a desirable area and thus these operating points are not wanted to be changed, the current-decreasing and the current-increasing sub-tables can be used for this winding group alternatively on successive control periods.
In a control device according to another exemplifying and non-limiting embodiment of the invention, the functional section is allowed to select the current-decreasing sub-table or the current-increasing sub-table for one of the winding groups and the full-voltage sub-table for another one of the winding groups. The functional section 110 is configured to change the set of the selectable control directions and the boundaries of the sectors s1-s8 shown in
In the exemplifying case illustrated in
The magnetic levitation system comprises controllable voltage sources 207z+ and 207z− for directing controllable voltages to the windings 205z+ and 205z−. In the exemplifying magnetic levitation system illustrated in
The magnetic levitation system comprises a control device 201 for controlling the magnetic actuators 204z+ and 204z−. The control device comprises a signal input 202 for receiving the position signal Pz, and a controller 203 for controlling the voltages directed to the windings 205z+ and 205z− in a time-discrete way at temporally successive control periods. The voltages are controlled on the basis of the deviation between the position of the object 208 and the reference position of the object. The controller 203 comprises a functional section 209 for producing, for each of the temporally successive control periods, a control value Cz at least partly on the basis of the position signal Pz. The control value Cz indicates a direction in which a resultant magnetic force F directed to the object 208 should be changed in order to decrease the deviation of the position.
The controller 203 comprises a functional section 210 for selecting, for each of the temporally successive control periods, a control direction CD so that changing the resultant magnetic force F in the selected control direction improves the ability of the resultant magnetic force to decrease the deviation of the position. The controller 203 comprises a functional section 211 for setting, for each of the temporally successive control periods, the voltages of the windings 205z+ and 205z− in accordance with the selected control direction CD so as to decrease the deviation of the position by changing the resultant magnetic force with the aid of the voltages.
In the exemplifying case illustrated in figure, the windings 205z+ and 205z− constitute a winding group capable of generating mutually cancelling magnetic forces. In a control device according to an exemplifying and non-limiting embodiment of the invention, the functional section 211 is configured to use, for the windings 205z+ and 205z−, only zero voltages and current-decreasing voltages but not current-increasing voltages in response to a need to decrease operating points of control quantities, e.g. currents, of the windings.
In a control device according to an exemplifying and non-limiting embodiment of the invention, the functional section 111 is configured to use, for the windings 205z+ and 205z−, only zero voltages and current-increasing voltages but not current-decreasing voltages in response to a need to increase the operating points of the control quantities, e.g. the currents, of the windings.
The control devices 101 and 201 shown in
The method comprises:
The control of the one or more voltages in the phase 302 comprises the following actions:
In a method according to an exemplifying and non-limiting embodiment of the invention, the action 304 for setting the one or more voltages comprises the following sub-actions illustrated in
A method according to an exemplifying and non-limiting embodiment of the invention comprises using a predetermined rule for producing, on the basis of the deviation of the position, one or more reference values for one or more control quantities defining operation of the magnetic levitation system. The method comprises subtracting, from the reference values, previous reference values corresponding to a previous one of the temporally successive control periods so as to produce one or more control values. The method comprises selecting the control direction on the basis of the one or more control values.
A method according to an exemplifying and non-limiting embodiment of the invention comprises using a predetermined rule for producing, on the basis of the deviation of the position, one or more reference values for one or more control quantities defining operation of the magnetic levitation system. The method comprises subtracting, from the reference values, prevailing values indicative of the one or more control quantities so as to produce one or more control values. The method comprises selecting the control direction on the basis of the one or more control values.
A method according to an exemplifying and non-limiting embodiment of the invention comprises maintaining a correction model for correcting the selection of the control direction. The correction model contains information about characteristics of magnetic circuits of the magnetic levitation system and is configured to receive input information indicative of the selected control direction, the prevailing currents of the windings, and the position of the object.
A method according to an exemplifying and non-limiting embodiment of the invention comprises determining a temporal length of each of the temporally successive control periods on the basis of (i) the one or more control values indicating required changes of the one or more control quantities and (ii) a fact that the one or more voltages set for the control period under consideration at least partly determine a rate of change of each of the one or more control quantities.
A method according to an exemplifying and non-limiting embodiment of the invention comprises keeping, in order to reduce switching frequency, the one or more voltages unchanged with respect to corresponding one or more voltages used during a previous one of the temporally successive control periods in response to a situation in which a vector norm of the one or more control values is below a pre-determined limit.
A method according to an exemplifying and non-limiting embodiment of the invention comprises selecting the control direction from among a set of selectable control directions, e.g. x+, x−, y+, y−, x+/y+, x+/y−, x−/y+, x−/y− shown in
A method according to an exemplifying and non-limiting embodiment of the invention comprises maintaining a selection look-up table for outputting one or more voltage selectors on the basis of a look-up key comprising an indicator of the selected control direction, the one or more voltage selectors being suitable for controlling one or more controllable voltage sources to produce the one or more voltages in accordance with the selected control direction.
In a method according to an exemplifying and non-limiting embodiment of the invention, the selection look-up table comprises two or more sub-tables each outputting the one or more voltage selectors on the basis of the selected control direction. The method comprises selecting, for each of one or more mutually non-overlapping winding groups each constituted by at least two of the windings and capable of generating mutually cancelling components of the resultant magnetic force, one of the sub-tables at least partly on the basis of operating quantities of the winding group under consideration.
In a method according to an exemplifying and non-limiting embodiment of the invention, a first one of the sub-tables allows only current-decreasing and zero voltages, a second one of the sub-tables allows current-decreasing, zero, and current increasing voltages, and a third one of the sub-tables allows only current-increasing and zero voltages. The method comprises selecting the first one of the sub-tables in response to a need to decrease operating points of the operating quantities of the winding group under consideration, and selecting the third one of the sub-tables in response to a need to increase the operating points of the operating quantities of the winding group under consideration.
A method according to an exemplifying and non-limiting embodiment of the invention comprises selecting, for a first one of the winding groups, either the first or third one of the sub-tables in response to a situation in which either the first or third one of the sub-tables needs to be selected for a second one of the winding groups.
In a method according to an exemplifying and non-limiting embodiment of the invention, the position signal constitutes a position vector expressing the position of the object in a planar two-dimensional coordinate system whose origin is at the reference position and where the selectable control directions are defined by first geometric lines intersecting each other at the origin of the planar two-dimensional coordinate system. The method comprises selecting one of the selectable control directions so that changing the resultant magnetic force in the selected control direction improves the ability of the total force to decrease the magnitude of the position vector.
In a method according to an exemplifying and non-limiting embodiment of the invention, the planar two-dimensional coordinate system is divided into sectors by second geometric lines intersecting each other at the origin so that each of the selectable control directions belongs to one of the sectors and a symmetry line of each sector is one of the selectable control directions.
A method according to an exemplifying and non-limiting embodiment of the invention comprises determining a particular one of the sectors to which an opposite vector of the position vector belongs and selecting the control direction which belongs to the determined sector.
A method according to an exemplifying and non-limiting embodiment of the invention comprises using a predetermined rule for producing, on the basis of the position vector, a reference vector of control quantities defining operation of the magnetic levitation system. The method comprises subtracting, from the reference vector, previous reference vector corresponding to a previous one of the temporally successive control periods so as to produce a control vector. Furthermore, the method comprises determining a particular one of the sectors to which the control vector belongs and selecting the control direction which belongs to the determined sector.
A method according to an exemplifying and non-limiting embodiment of the invention comprises using a predetermined rule for producing, on the basis of the position vector, a reference vector of control quantities defining operation of the magnetic levitation system. The method comprises subtracting, from the reference vector, a vector indicative of prevailing values of the control quantities so as to produce a control vector. Furthermore, the method comprises determining a particular one of the sectors to which the control vector belongs and selecting the control direction which belongs to the determined sector.
In a method according to an exemplifying and non-limiting embodiment of the invention, the central angles of the sectors are proportional to magnitudes of sector-specific voltage vectors so that a greater magnitude of the sector-specific voltage vector corresponds to a greater central angle of the corresponding sector. Each sector-specific voltage vector is a vector of the voltages corresponding to the control direction related to the sector under consideration.
In a method according to an exemplifying and non-limiting embodiment of the invention, the ratio sin(αi/2)/Vi is a same for all of the sectors, where αi is the central angle of the i:th sector, Vi is the magnitude of the sector-specific voltage vector related to i:th the sector, and i=1, 2, . . . , N, the N being a number of the sectors.
A method according to an exemplifying and non-limiting embodiment of the invention comprises maintaining a correction look-up table for correcting the selection of the control direction. The correction look-up table contains information about characteristics of magnetic circuits of the magnetic levitation system and is configured to receive input information indicative of the selected control direction, the prevailing currents of the windings, and the position of the object.
A computer program according to an exemplifying and non-limiting embodiment of the invention comprises computer executable instructions for controlling a programmable processing system to carry out actions related to a method according to any of the above-described exemplifying embodiments of the invention.
A computer program according to an exemplifying and non-limiting embodiment of the invention comprises software means for controlling a programmable processing system to control one or more voltages directed to one or more windings of a magnetic levitation system on the basis of a deviation of a position of an object from a reference position of the object so as to control a resultant magnetic force directed to the object. The software means comprise computer executable instructions for controlling the programmable processing system to:
In a computer program according to an exemplifying and non-limiting embodiment of the invention, the software means further comprise computer executable instructions for controlling the programmable processing system to use, for each of one or more mutually non-overlapping winding groups each constituted by at least two of the windings and capable of generating mutually cancelling components of the resultant magnetic force, only zero voltages and current-decreasing voltages in response to a need to decrease operating points of operating quantities of the winding group under consideration.
In a computer program according to an exemplifying and non-limiting embodiment of the invention, the software means further comprise computer executable instructions for controlling the programmable processing system to use, for each of one or more mutually non-overlapping winding groups each constituted by at least two of the windings and capable of generating mutually cancelling components of the resultant magnetic force, only zero voltages and current-increasing voltages in response to a need to increase operating points of operating quantities of the winding group under consideration.
The software means can be e.g. subroutines or functions implemented with a suitable programming language and with a compiler suitable for the programming language and for the above-mentioned programmable processing system.
A computer program product according to an exemplifying and non-limiting embodiment of the invention comprises a computer readable medium, e.g. an optical disc, encoded with a computer program according to an exemplifying embodiment of invention.
A signal according to an exemplifying and non-limiting embodiment of the invention is encoded to carry information defining a computer program according to an exemplifying embodiment of invention.
The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20145520 | Jun 2014 | FI | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FI2015/050378 | 5/29/2015 | WO | 00 |
