Control system for a magnetic bearing and associated control method

Abstract

Control system for a magnetic bearing and associated control method. The control system (1) of a magnetic bearing (2) comprises a first control branch (BR1) having at least one control means (3) configured to drive at least one first servo axis of the magnetic bearing. The control system (1) further comprises a second control branch (BR2) identical to the first control branch (BR1), and selection means (5). Figure 1

Description

Title of the invention: Control system for a magnetic bearing and associated control method Technical field of the invention

[0001] The present invention relates to the control of magnetic bearings.

[0002] The present invention relates more particularly to a control system for a magnetic bearing and a method for controlling the control system. Prior art

[0003] Classically, a magnetic bearing control system is based on a centralized or distributed type architecture.

[0004] A centralized type magnetic bearing control system includes at least one position control node implementing a servo algorithm for the position of a rotor in the magnetic bearing, and amplification nodes driving servo axes of the magnetic bearing from information generated by the servo algorithm of the control node.

[0005] In a distributed magnetic bearing control system, some or all of the tasks performed by the position control node are performed by axis control nodes. This task distribution makes it possible to spread the computing power across all the elements constituting the system and to be more flexible with regard to the number of axes to be managed by the magnetic bearing control system.

[0006] However, a failure of a node in either of the architectures compromises the proper functioning of the magnetic bearing control system so that the servo axis of the magnetic bearing driven by said faulty node is no longer driven, compromising the proper functioning of the magnetic bearing control system and causing the magnetic bearing to fail.

[0007] It is therefore proposed to overcome all or part of these drawbacks by improving the availability of the magnetic bearing control system, the control system being of the centralized or distributed type. Summary of the invention

[0008] In view of the foregoing, the invention proposes a control system for a magnetic bearing comprising a first control branch having at least one control means configured to drive at least one first servo axis of the magnetic bearing.

[0009] By "bearing servo axis", we mean the axis which is defined by two diametrically opposed coils of the stator of the magnetic bearing.

[0010] The control system further comprises a second control branch identical to the first control branch, and selection means configured to activate the control means of the second control branch upon failure of the control means of the first control branch so that the activated control means of the second branch drives the first servo axis or configured to activate the control means of the second control branch upon receiving an activation signal of the second control branch so that the control means of the first and second branches drive the first servo axis.

[0011] Substituting the faulty control means of the first branch with the control means of the second branch connected to the same servo axis when the control means of the first branch is faulty (passive or "cold" substitution) allows the bearing control to continue.

[0012] Activation of the control means of the second control branch by the selection means upon receipt of an activation signal of the second control branch makes it possible to simultaneously drive the first servo axis so as to inject more power for the driving of said axis during transient phases and to obtain active or "hot" redundancy so that the control of said axis is not interrupted unlike passive redundancy which requires an instant switching of the control means.

[0013] Advantageously, the control means of the first branch includes a first axis control node configured to drive the first servo axis and the control means of the second branch includes a first axis control node configured to drive the first servo axis, the first axis control node of the first branch and the first axis control node of the second branch being connected to the selection means.

[0014] Preferably, the control means of the first branch includes at least one second axis control node configured to drive a second servo axis, and the control means of the second branch includes at least one second axis control node configured to drive the second servo axis, the axis control nodes of the first branch being connected to each other in series such that the second node of the first branch is connected to the first node of the first branch and the axis control nodes of the second branch being connected to each other in series such that the second node of the second branch is connected to the first node of the second branch.

[0015] Advantageously, the node among the axis control nodes of the first branch that is connected only to a single axis control node of the first branch is connected to the corresponding axis control node of the second branch.

[0016] Preferably, the control means of the first branch includes a second axis control node configured to drive a second servo axis and the control means of the second branch includes a second axis control node configured to drive the second servo axis, the second control node of the first branch and the second control node of the second branch being connected to the selection means.

[0017] Advantageously, the control means of the first branch includes a position control node and a first amplification node configured to drive the first servo axis, and the control means of the second branch includes a position control node and a first amplification node configured to drive the first servo axis, the first amplification node of the first branch being connected to the position control node of the first branch and the first amplification node of the second branch being connected to the position control node of the second branch, the position control node of the first branch and the position control node of the second branch being connected to the selection means.

[0018] Preferably, the first branch includes at least one second amplification node configured to drive a second servo axis, and the second branch includes at least one second amplification node configured to drive the second servo axis, the amplification nodes of the first branch being connected to each other in series and the amplification nodes of the second branch being connected to each other in series.

[0019] Advantageously, the amplification node of the first branch connected only to a single amplification node of the first branch is further connected to the position control node of the first branch, and the amplification node of the second branch connected only to a single amplification node of the second branch is further connected to the position control node of the second branch.

[0020] Preferably, the amplification node of the first branch connected only to a single amplification node of the first branch is further connected to the corresponding amplification node of the second branch.

[0021] A method for controlling a control system for a magnetic bearing is also proposed, the system comprising a first control branch having at least one control means driving at least one first servo axis of the magnetic bearing, and a second control branch identical to the first control branch.

[0022] The process comprises:

[0023] - the activation of the control means of the second control branch during the de failure of the control mechanism of the first control branch so that the activated control means of the second pilot branch the first servo axis or

[0024] - the activation of the control means of the second control branch during the reception of an activation signal from the second control branch so that the first and second branches drive the first servo axis. Brief description of the figures

[0025] Other objects, features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0026] [Fig.1]

[0027] illustrates an example of a control system for a magnetic bearing according to the invention;

[0028] [Fig.2]

[0029] illustrates a first example of an embodiment of the control system according to the invention

[0030] [Fig.3]

[0031] illustrates a second example of an embodiment of the control system according to the invention;

[0032] [Fig.4]

[0033] illustrates a third example of an embodiment of the control system according to the invention;

[0034] [Fig.5]

[0035] illustrates a fourth example of an embodiment of the control system according to the invention;

[0036] [Fig.6]

[0037] illustrates a fifth embodiment of the control system according to the invention; and

[0038] [Fig.7]

[0039] illustrates a sixth example of an embodiment of the control system according to the invention. Detailed description of the invention

[0040] Reference is made to [Fig.1] which illustrates an example of a control system 1 of a magnetic bearing driving a magnetic bearing 2.

[0041] In a manner known per se, the magnetic bearing 2 comprises a stator 2a and a rotor 2b placed in the stator 2a.

[0042] The stator 2a comprises stator coils distributed uniformly in the circumferential direction on the inner side of the stator 2a, two diametrically opposed coils powered by electrical power converters.

[0043] Two diametrically opposed stator coils define a servo axis of the magnetic bearing and allow this axis to be controlled.

[0044] The control system 1 comprises a first control branch BRI and a second control branch BR2 identical to the first control branch BRI.

[0045] Each control branch BRI, BR2 includes a control means 3, 4.

[0046] The control system 1 further includes selection means 5 comprising a first control output 51 connected to a first control input 30 of the control means 3 of the first branch BRI and a second control output 52 connected to a first control input 40 of the control means 4 of the second branch BR2.

[0047] The control means 3, 4 of each control branch BRI, BR2 further includes a second input 31, 41 connected to a first sensor 6, and a first control output 32, 42 supplying stator coils to drive a first servo axis of the magnetic bearing 2.

[0048] The control means 3, 4 of each control branch BRI, BR2 further includes a third input 33, 43 connected to a second sensor 8, and a second control output 34, 44 supplying stator coils to drive a second servo axis of the magnetic bearing 2.

[0049] Each first control output 32, 42 supplies the first servo axis of the magnetic bearing comprising two opposed stator coils (not shown) of the bearing 2 and each second control output 34, 44 supplies the second servo axis of the magnetic bearing comprising two opposed stator coils (not shown) of the bearing 2, the stator coils generating a magnetic flux to keep the rotor 2b of the bearing levitated in the stator 2a.

[0050] The sensors 6, 8 measure for example radial or axial displacements of the rotor 2b, the angular position of the rotor 2b, and transmit the measured data by means of control 3, 4 of each branch BRI, BR2.

[0051] The control system 1 drives two servo axes of the bearing 2.

[0052] The control means 3, 4 of each control branch BRI, BR2 is intended to drive the two servo axes of the bearing 2.

[0053] The selection means 5 are capable of activating the control means 4 of the second control branch BR2 when the control means 3 of the first control branch BRI fails so that the activated control means 4 of the second control branch BR2 drives at least one of the first and second servo axes.

[0054] The substitution of the faulty control means of the first branch by the control means of the second branch connected to the same servo axis when the control means of the first branch is faulty (passive substitution or "to cold") allows the bearing control to continue.

[0055] The selection means 5 are further capable of activating the control means 4 of the second control branch BR2 upon receiving an activation signal from the second control branch so that the control means 3, 4 of the first and second branches BRI, BR2 drive at least one of the first and second servo axes.

[0056] Activation of the control means of the second control branch by the selection means upon receipt of an activation signal of the second control branch makes it possible to simultaneously drive the first servo axis so as to inject more power for the driving of said axis during transient phases and to obtain active or "hot" redundancy so that the control of said axis is not interrupted unlike passive redundancy which requires an instant switching of the control means.

[0057] Of course, the control system 1 can control more than two servo axes of the bearing 2, the control means 3, 4 of each control branch BRI, BR2 being able to control said axes.

[0058] Alternatively, the control system 1 can control a single servo axis of the bearing 2, the control means 3, 4 of each control branch BRI, BR2 being able to control said axis.

[0059] Several examples of implementation of the first and second control branches BRI, BR2 are now described.

[0060] In the embodiment examples of the first and second control branches BRI, BR2 described below, the means 3, 4 include axis control nodes, the control system architecture 1 being of the decentralized type.

[0061] Fig. 2 illustrates a first example of the realization of the first and second control branches BRI, BR2.

[0062] The control means 3, 4 of the first and second branches BRI, BR2 comprises a first axis control node 35, 45 and a second axis control node 36, 46.

[0063] Each control node 35, 36, 45, 46 implements a control algorithm for a servo axis of the magnetic bearing 2 determined in particular from the measurements of the sensors 6, 8.

[0064] Each branch BRI, BR2 comprises as many control node(s) as there are servo axis(es) of the bearing 2, for each branch, each control node driving a servo axis of the bearing 2.

[0065] A first control node 35 of the control means 3 of the first BRI branch is connected to the first control input 30 of the control means 3 of the first BRI branch, to the second input 31 and to the first output 32 of said means control 3.

[0066] The second node 36 of the control means 3 of the first branch BRI is connected to the third input 33 and to the second control output 34 of said control means 3.

[0067] The first and second control nodes 35, 36 of the control means 3 of the first BRI branch are further connected to each other in series, for example by a data bus.

[0068] A first control node 45 of the control means 4 of the second branch BR2 is connected to the first control input 40 of the control means 4 of the second branch BR2, to the second input 41, and to the first output 42 of said control means 4.

[0069] The second node 46 of the control means 4 of the second branch BR2 is connected to the third input 43 and to the second control output 44 of said control means 4.

[0070] The first and second control nodes 45, 46 of the control means 4 of the first branch BR3 are further connected to each other in series, for example by a data bus.

[0071] The first nodes 35, 45 of the first and second branch BR2 are able to control the first servo axis, and the second nodes 36, 46 of the first and second branch BR2 are able to control the second servo axis.

[0072] When the bearing 2 includes more than two servo axes, the additional axis control nodes of each branch BRI, BR2 are connected together in series at the second node 36, 46 so that the second node 36, 46 of each branch BRI, BR2 is connected to the first node 35, 45 of said branch.

[0073] When bearing 2 comprises a single servo axis, each BRI, BR2 branch comprises a single axis control node.

[0074] The selection means 5 include for example a first master control node 10 and a second master control node 11 connected in series.

[0075] The first master control node 10 is further connected to the first control output 51 and the second master control node 11 is further connected to the second control output 52.

[0076] Alternatively, the selection means 5 comprise a single master control node connected to the first control output 51 and to the second control output 52.

[0077] Figure 3 illustrates a second example of embodiment of the first and second BRI, BR2 control branches.

[0078] The control nodes 35, 36, 45, 46 and the master control nodes 10, 11 are arranged as described previously.

[0079] This example of an embodiment of the first and second control branches BRI, BR2 differs from the first embodiment illustrated in [Fig.2] in that the node among the axis control nodes 35, 36 of the first BRI branch which is connected only to a single axis control node of the first BRI branch is connected to the corresponding axis control node of the second branch.

[0080] In this case, as the first control node 35, 45 of each means 3, 4 is connected to the second node 36, 46 of said means 3, 4 and to the selection means 5, the second node 36, 46 of each means 3, 4 is connected to the only axis control node of said means 3, 4.

[0081] The second nodes 36, 46 of the means 3, 4 are connected to each other, for example by a bus.

[0082] This additional connection allows communication between the first and second branches BRI, BR2 when the selection means 5 fail or when one of the first nodes 35, 45 fails.

[0083] Figure 4 illustrates a third example of embodiment of the first and second BRI, BR2 control branches.

[0084] We find the control nodes 35, 36, 45, 46 and the selection means 5.

[0085] The first and second control nodes 35, 36 of the middle 3 of the first branch BRI are each connected to the first control output 51 of the selection means 5, and the first and second control nodes 45, 46 of the means 4 of the second branch BR2 are each connected to the second control output 52 of the selection means 5.

[0086] The selection means 5 include, for example, a single master control node 12 connected to the first and second outputs 51, 52 of the control of the selection means.

[0087] In this architecture, each node 35, 36, 45, 46 is directly connected to the selection means 5 so that if one of the nodes 35, 36, 45, 46 of a BRI, BR2 branch fails, the functional nodes continue to communicate with the selection means.

[0088] Now, implementation methods for the embodiments of branches BRI and BR2 of system 1 described previously are described.

[0089] It is assumed that level 2 is driven by the nodes of the first branch BRI, the nodes of the second branch BR2 being deactivated.

[0090] When the selection means 5 detect the failure of one of the axis control nodes 36, 35 of the first branch BRI, said means 5 activate the corresponding node of the second branch BR2 so that the bearing 2 continues to operate when the associated node is activated.

[0091] Alternatively, when the selection means 5 detect the failure of one of the axis control nodes 36, 35 of the first BRI branch, said means 5 deactivate all nodes of the first branch BRI and activate all nodes of the second branch BR2 so that level 2 continues to function when the nodes of the second branch BR2 are activated.

[0092] According to another embodiment, when the selection means 5 receive an activation signal from the second control branch BR2, the selection means 5 activate the nodes of the second branch BR2 so that the nodes of the first and second branches BRI, BR2 drive the bearing 2.

[0093] The activation signal is for example emitted by a master control controller of level 2 (not shown) connected to the selection means 5 by a bus (not shown).

[0094] In the embodiment examples of the first and second control branches BRI, BR2 described above, means 3, 4 include axis control nodes.

[0095] In the embodiment examples of the first and second control branches BRI, BR2 described below, the means 3, 4 include position control nodes and amplification nodes, the control system architecture 1 being of the centralized type.

[0096] Fig. 5 illustrates a fourth example of the embodiment of the first and second control branches BRI, BR2.

[0097] The control means 3, 4 of the first and second branches BRI, BR2 includes a position control node 60, 70, a first amplification node 61, 71, and a second amplification node 62, 72.

[0098] Each branch BRI, BR2 comprises as many amplification nodes as there are servo axes of the level 2, for each branch, each amplification node driving one servo axis of the level 2.

[0099] The position control node 60 of the control means 3 of the first BRI branch is connected to the first control input 30 of the control means 3 of the first BRI branch, to the second input 31 of the first BRI branch and to the third input 33 of the first BRI branch.

[0100] The position control node 60, the first amplification node 61, and the second amplification node 62 of the means 3 of the first BRI branch are connected together in series.

[0101] The first amplification node 61 is connected to the position control node 60 and to the second amplification node 62.

[0102] The first amplification node 61 is further connected to the first output 32 of said control means 3.

[0103] The second amplification node 62 is further connected to the second output 34 of said control means 3.

[0104] The position control node 70 of the control means 4 of the second branch BR2 is connected to the first control input 40 of the control means 4 of the second branch BR2, to the second input 41 of the second branch BR2 and to the third input 43 of the second branch BR2.

[0105] The position control node 70, the first amplification node 71, and the second amplification node 72 of the middle 4 of the second branch BR2 are connected together in series.

[0106] The first amplification node 71 is connected to the position control node 70 and to the second amplification node 72.

[0107] The first amplification node 71 is further connected to the first output 42 of said control means 4.

[0108] The second amplification node 72 is further connected to the second output 44 of said control means 4.

[0109] Each position control node 60, 70 is capable of implementing a control algorithm for a servo axis of the magnetic bearing 2 to determine and transmit control instructions to the amplification nodes 61, 62, 71, 72 connected to said position control node 60, 70 based in particular on measurements from sensors 6, 8.

[0110] When the stage 2 includes more than two servo axes, the additional amplification nodes of each branch BRI, BR2 are connected together in series to the second amplification node 62, 72 so that the first amplification node 61, 71 of each branch BRI, BR2 is connected to the position control node 60, 70 of said branch.

[0111] When the bearing 2 comprises a single servo axis, each branch BRI, BR2 comprises the position control node and a single amplification node connected to said control node.

[0112] Fig. 6 illustrates a fifth example of embodiment of the first and second control branches BRI, BR2.

[0113] The position control nodes 60, 70 and the amplification nodes 61, 62, 71, 72 are arranged as described previously in the fourth embodiment example in [Fig.5].

[0114] This example of an embodiment of the first and second control branches BRI, BR2 differs from the fourth example of an embodiment illustrated in [Fig.5] in that the amplification node of the first branch BRI connected only to a single amplification node of the first branch BRI is further connected to the position control node 60 of the first branch BRI, and the amplification node of the second branch BR2 connected only to a single amplification node of the second branch BR2 is further connected to the position control node 70 of the second branch BR2.

[0115] In this case, as the first amplification node 61, 71 of each means 3, 4 is connected to the position control node 60, 70 of said means 3, 4 and to the second amplification node 62, 72 of said means 3, 4, the second amplification node 62, 72 of each means 3, 4 is connected only to a single amplification node of said means 3, 4.

[0116] The second amplification node 62 of the first branch BRI is connected to the position control node 60 of the first branch BRI and the second amplification node 72 of the second branch BR2 is connected to the position control node 70 of the second branch BR2.

[0117] This additional connection allows the position control node 60, 70 of a BRI, BR2 branch to transmit instructions to all the amplification nodes 61, 62, 71, 72 of said branch redundantly via the first and last amplification nodes of said branch so that if a node located between the first and last node fails or a connection, for example a bus, linking one of the nodes located between the first and last amplification nodes fails, the functional nodes of said BRI, BR2 branch continue to receive instructions from the position control node 60, 70 of said branch.

[0118] Figure 7 illustrates a sixth example of embodiment of the first and second BRI, BR2 control branches.

[0119] The position control nodes 60, 70 and the amplification nodes 61, 62, 71, 72 are arranged as described previously in the fourth embodiment example in [Fig.5].

[0120] This embodiment of the first and second control branches BRI, BR2 differs from the fourth embodiment illustrated in [Fig.5] in that the amplification node among the amplification nodes 61, 62 of the first branch BRI which is connected only to a single amplification node of the first branch BRI is connected to the corresponding axis control node of the second branch BR2.

[0121] In this case, as the first amplification node 61, 71 of each means 3, 4 is connected to the second amplification node 62, 72 of said means 3, 4 and to the position control node 60, 70 of said branch BRI, BR2, the second amplification node 62, 72 of each means 3, 4 is connected only to a single amplification node of said means 3, 4.

[0122] The second amplification nodes 62, 72 of the means 3, 4 are connected to each other, for example by a bus.

[0123] This additional connection allows communication between the first and second branches BRI, BR2 when the selection means 5 are faulty or when one of the first amplification nodes 61,71 is faulty.

[0124] Now, implementation methods for the embodiments of branches BRI and BR2 of system 1 according to the embodiment examples described in figures 5, 6, and 7.

[0125] It is assumed that level 2 is driven by the nodes of the first branch BRI, the nodes of the second branch BR2 being deactivated.

[0126] When the selection means 5 detect the failure of one of the nodes among the position control node and the amplification nodes of the first branch BRI, said means 5 activate the corresponding node of the second branch BR2 so that the bearing 2 continues to operate when the associated node is activated.

[0127] Alternatively, when the selection means 5 detect the failure of a node among the position control node and the amplification nodes of the first branch BRI, said means 5 deactivate all the nodes of the first branch BRI and activate all the nodes of the second branch BR2 so that the bearing 2 continues to operate when the nodes of the second branch BR2 are activated.

[0128] According to another embodiment, when the selection means 5 receive an activation signal from the second control branch BR2, the selection means 5 activate the nodes of the second branch BR2 so that the nodes of the first and second branches BRI, BR2 drive the bearing 2.

Claims

Claims

1. Control system (1) of a magnetic bearing (2) comprising a first control branch (BRI) comprising at least one control means (3) configured to control at least a first servo axis of the magnetic bearing (2), characterized in that the control system (1) further comprises a second control branch (BR2) identical to the first control branch (BRI), and selection means (5) configured to activate the control means (4) of the second control branch (BR2) upon failure of the control means (3) of the first control branch (BRI) so that the activated control means (4) of the second branch (BR2) controls the first servo axis or configured to activate the control means (4) of the second control branch (BR2) upon receipt of an activation signal from the second control branch so that the control means (3, 4) of the first and second branches (BRI,BR2) control the first servo axis.,

2. System according to claim 1, wherein the control means (3) of the first branch (BRI) comprises a first axis control node (35) configured to drive the first servo axis and the control means (4) of the second branch (BR2) comprises a first axis control node (45) configured to drive the first servo axis, the first axis control node (35) of the first branch (BRI) and the first axis control node (45) of the second branch (BR2) being connected to the selection means (5).

3. System according to claim 2, wherein the control means (3) of the first branch (BRI) comprises at least one second axis control node (36) configured to control a second servo axis, and the control means (4) of the second branch (BR2) comprises at least one second axis control node (46) configured to control the second servo axis, the axis control nodes (35, 36) of the first branch (BRI) being connected together in series so that the second node (36) of the first branch (BRI) is connected to the first node (35) of the first branch (BRI) and the axis control nodes (45, 46) of the second branch (BR2) being connected together in series so that the second node (46) of the second branch (BR2) is connected to the first node (45) of the second branch (BR2).

4. The system of claim 3, wherein the node (36) among the axis control nodes (35, 36) of the first branch (BRI) which is connected only to a single axis control node (35) of the first branch (BRI) is connected to the corresponding axis control node (46) of the second branch (BR2).

5. System according to claim 2, wherein the control means (3) of the first branch (BRI) comprises a second axis control node (36) configured to control a second servo axis and the control means (4) of the second branch (BR2) comprises a second axis control node (46) configured to control the second servo axis, the second control node (36) of the first branch (BRI) and the second control node (46) of the second branch (BR2) being connected to the selection means (5).

6. System according to claim 1, wherein the control means (3) of the first branch (BRI) comprises a position control node (60) and a first amplification node (61) configured to drive the first servo axis, and the control means (4) of the second branch (BR2) comprises a position control node (70) and a first amplification node (71) configured to drive the first servo axis, the first amplification node (61) of the first branch (BRI) being connected to the position control node (60) of the first branch (BRI) and the first amplification node (71) of the second branch (BR2) being connected to the position control node (70) of the second branch (BR2), the position control node (60) of the first branch (BRI) and the position control node (70) of the second branch (BR2) being connected to the selection means (5).

7. System according to claim 6, in which the first branch (BRI) comprises at least one second amplification node (62) configured to drive a second servo axis, and the second branch (BR2) comprises at least one second amplification node (72) configured to drive the second servo axis, the amplification nodes (61, 62) of the first branch (BRI) being connected together in series and the amplification nodes (71, 72) of the second branch (BR2) being connected together in series.

8. System according to claim 7, in which the amplification node (62) of the first branch (BRI) connected only to a single node amplification node (61) of the first branch (BRI) is further connected to the position control node (60) of the first branch (BRI), and the amplification node (72) of the second branch (BR2) connected only to a single amplification node (71) of the second branch (BR2) is further connected to the position control node (70) of the second branch (BR2).

9. System according to claim 7, wherein the amplification node (62) of the first branch (BRI) connected only to a single amplification node (61) of the first branch (BRI) is further connected to the corresponding amplification node (72) of the second branch (BR2).

10. Method for controlling a control system (1) for a magnetic bearing (2), the system comprising a first control branch (BRI) comprising at least one control means (3) controlling at least a first servo axis of the magnetic bearing (2), and a second control branch (BR2) identical to the first control branch (BRI), characterized in that the method comprises: - activating the control means (4) of the second control branch (BR2) upon failure of the control means (3) of the first control branch (BRI) so that the activated control means (4) of the second branch (BR2) controls the first servo axis or - activating the control means (4) of the second control branch (BR2) upon receipt of an activation signal from the second control branch so that the first and second branches (BR1, BR2) control the first servo axis.

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