Title of Invention	A METHOD AND A DEVICE FOR DETERMINING THE EROSION OF CONTACTS OF AN ELECTROMAGENTIC SWITCHING DEVICE
Abstract	The invention relates to a method for determining the erosion of contacts of an electromagnetic switching device, comprising measuring a mechanical variable (v,b,w), which characterizes a time profile of relative movement between the contacts (18), the relative movement between the contacts (18) being caused by movement of an actuator (6,14,16) , the mechanical variable being measured during a switch-on operation in which the contacts (18) are closed; determining a time (tx) at which the contacts (18) close by evaluating the time profile of the relative movement comprising the steps of using the time profile and the time (tk) at which the contacts (18) close to monitor a distance covered during the switch-on operation by either: determining a distance (s,so,sl) covered by the contacts (18) from a beginning of the switch-on operation until the time at which the contacts close, or determining a distance (d,d0,dl) covered by the actuator from the time at which the contacts close until an end position of the actuator (6); and comparing the distance covered with a stored reference value (so,do).

Title of Invention

A METHOD AND A DEVICE FOR DETERMINING THE EROSION OF CONTACTS OF AN ELECTROMAGENTIC SWITCHING DEVICE

Abstract

The invention relates to a method for determining the erosion of contacts of an electromagnetic switching device, comprising measuring a mechanical variable (v,b,w), which characterizes a time profile of relative movement between the contacts (18), the relative movement between the contacts (18) being caused by movement of an actuator (6,14,16) , the mechanical variable being measured during a switch-on operation in which the contacts (18) are closed; determining a time (tx) at which the contacts (18) close by evaluating the time profile of the relative movement comprising the steps of using the time profile and the time (tk) at which the contacts (18) close to monitor a distance covered during the switch-on operation by either: determining a distance (s,so,sl) covered by the contacts (18) from a beginning of the switch-on operation until the time at which the contacts close, or determining a distance (d,d0,dl) covered by the actuator from the time at which the contacts close until an end position of the actuator (6); and comparing the distance covered with a stored reference value (so,do).

Full Text	Field of the invention The invention relates to a method for determining the erosion of contacts of an electromagnetic switching device. In addition, the invention relates to an electromagnetic switching device with a device for determining the erosion of its contacts. BACKGROUND OF THE INVENTION During the switch-on and switch-off operations of an electromagnetic switching device, arcs occur between the closing or opening contacts. These arcs cause erosion of the contacts over the course of time. It is therefore important for the operational reliability of such a switching device to identify the degree of this erosion in order to be able to draw conclusions on the residual life of the switching device and avoid operational faults by replacing the contacts in good time. EP 0 694 937 Bl has disclosed a method for determining the erosion and therefore the residual life of contacts in switching devices, in which method the so-called contact resilience is determined as a measure for the contact erosion. This contact resilience is the distance which is covered by the magnet armature as the actuator of the switching movement between the beginning of the switch-off operation, i.e. the time at which the magnet armature, which is resting in the end position on a magnet yoke, releases itself therefrom and the time at which the contacts lift off from one another. The time at which the magnet armature lifts off from the magnet yoke is measured by an auxiliary circuit, in which the magnet armature and the magnet yoke form a switch, which is closed if the magnet' armature and the magnet yoke are in contact with one another. As an alternative to this, it is known, for example, from EP 0 878 015 Bl, to determine the time at which the magnet armature separates from the magnet yoke of the magnet drive by means of measuring the voltage at the magnet coil of the magnet yoke. In both methods, a further auxiliary circuit is required for detecting the time at which the contacts lift off from one another, for example a complex auxiliary circuit which is DC-decoupled from the main circuit with the aid of optocouplers and which detects the occurrence of an arc voltage, which is produced by the arc forming when the contacts lift off from one another. As an alternative to the methods known from EP 0 694 937 Bl and EP 0 878 015 Bl, in which the switch-off operation is used to determine the erosion or the residual life, WO 2004/057634 Al has disclosed a method and an apparatus for determining the residual life of a switching device, in which method the change in the contact resilience is measured during the switch-on operation, i.e. when the switching contacts are closed by the magnet, drive. With this known apparatus, a position encoder is arranged on the magnet armature, which position encoder contains markings, for example in the form of measuring contacts, in at least three positions, with which markings the time profile of the magnet armature movement can be detected. The determination of the position of the magnet armature when the contacts close is determined by means of computation from the movement sequence of the magnet armature which is detected with the aid of these position markers. For this purpose, a simple algorithm is used as a result of the low number of position markers assuming that the armature acceleration is constant between a time prior to the closing of the contacts and a time which is between the closing time of the contacts and the time at which the magnet armature is positioned onto the magnet yoke. In practice, however, it has been established ■ that, with such an approach, the time at which the contacts close can only be determined with a low amount of accuracy. OBJECTS OF THE INVENTION The invention is now based on the object of specifying a method for determining the erosion of contacts of an electromagnetic switching device, with which method precise determination of the time at which the contacts close and therefore precise determination of the contact erosion is possible. In addition, the invention is based on the object of specifying an electromagnetic switching device with a device functioning on the basis of this method. SUMMARY OF THE INVENTION As regards the method, the abovementioned object according to the invention is achieved by a method, with which method, during the switch-on operation, a mechanical variable, which characterizes the time profile of the relative movement, which is caused by an actuator, between the contacts, is measured and the time at which the contacts close is determined by evaluating the time profile of the relative movement, and the distance covered up to this time by the contacts or that covered from this time by the actuator up to its end position is detected at least indirectly and is compared with a stored reference value. In this case, the invention is based on the consideration that the time profile of the relative movement at the time at which the contacts close is changed significantly as a result of the high spring force of the contact spring which sets in at this time and which brakes the movement of the actuator, with the result that, by analyzing the time profile of the movement, the time at which the contacts meet one another can be determined directly and reliably without an approximation model of the movement sequence being required for this purpose, as is the case with the document WO 2004/057634 Al mentioned at the outset. The variable characterizing the movement sequence can be measured directly by measuring the velocity, or the acceleration of one of the contacts or both contacts. As an alternative to this, the velocity of an actuator, which causes this relative movement and is coupled mechanically to at least one of the contacts and is actuated by an electromagnetic drive, can also be measured. If the time profile of the movement is measured by a sensor which is coupled mechanically to the actuator, the measurement can take place using a measurement circuit, which is DC-decoupled from the switched circuit or the circuit of the magnetic drive. A suitable sensor may be a displacement sensor, a velocity sensor or an acceleration sensor. If a velocity sensor or an acceleration sensor is used as the sensor, it is particularly easy to determine the time at which the contacts close from this measurement signal. In order in this case to obtain information on the distance covered, its measurement signals still need to be integrated singularly or twofold. As an alternative to the use of such a sensor, it is also possible to measure the mechanical variable by evaluating an electrical or magnetic variable of the electromagnetic drive which is measured during the switch-on operation. The object in relation to the electromagnetic switching device is achieved in accordance with the invention, whose advantages, correspond to the advantages which are specified in relation to the features of the respectively associated with the method. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS For further explanation of the invention, reference is made to the accompanying drawing, in which: figures 1 to 3 each show an electromagnetic switching device in a.basic illustration at various times of the switch-on operation with uneroded contacts, figures 4 to 5 show the electromagnetic switching device at different times of the switch-on operation after a large number of switching cycles when the contacts have suffered significant erosion, figures 6 to 9 each show graphs, in which the voltage across the magnet coil and the current flowing through it, the magnetic force and the spring force, the spacing between the magnet armature and the yoke and the velocity of the magnet armature or its acceleration are in each case plotted over time, and figure 10 shows a schematic illustration of a switching device with a device for improved determination of the erosion of the contacts. Detail Description of the invention As shown in figure 1, an electromagnetic switching device, in the example illustrated a contactor, contains a magnet yoke 2, on which two magnet coils 4 are arranged for magnetic excitation purposes. A magnet armature 6, which is associated with the magnet yoke 2, is mounted in a sprung manner by means of compression springs 8 in a housing 10 (which is only illustrated symbolically) of the switching device. The magnet yoke 2, magnet coil 4 and magnet armature 6 form an electromagnetic drive of the switching device. The magnet armature 6 is connected in a force-fitting manner to a moveable contact link 14 via a contact spring 12. Two stationary contact carriers 16 are associated with the moveable contact link 14. The magnet armature 6 forms the actuator of the magnetic drive for the relative movement between the contact link 14 and the contact carrier 16. The contact link 14 and the stationary contact carrier 16 are each provided with contact pieces or contacts 18, which when new have a thickness D0. The switching contact formed by the moveable contact link 14 and the stationary contact carrier 16 is located in the open position. In this switched-off state, the contacts 18 are at a spacing s0 and the pole faces 20 and 60 of the magnet yoke or the magnet armature 6 are located at a spacing H. When the magnet coils 4 are switched on, the magnet armature 6 is set in motion, counter to the action of the compression springs 8, in the direction towards the magnet yoke 2, as is illustrated by the arrows in the figure. Figure 2 now shows a situation in which the contacts 18 come into contact with one another for the first time, i.e. the magnet armature 6 has covered a distance s0- At this time, the pole faces 20, 60 are located at a spacing d0 = H-s0. This spacing d0 corresponds to the contact; resilience of the switching device with the contacts 18 uneroded. The further closing movement of the magnet armature 6 now takes place counter to the action of the contact spring 12 and the compression spring 8, which is connected in parallel therewith. Since the spring force exerted by the contact spring 12 is considerably greater than the spring force exerted by the compression spring 8, the spring force acting on the magnet armature 6 increases suddenly and brings about a significant change in the course of the closing movement. As things proceed, the magnetic force acting on the magnet armature 6 is greater than the spring force exerted by the compression spring 8 and the contact spring 12, and the magnet armature 6 can move further in the direction towards the magnet yoke 2 until it finally, as is illustrated in figure 3, rests in an end or rest position with its pole faces 60 on the pole faces 2 0 of the magnet yoke 2. Figure 4 now illustrates a situation in which the contacts 18 have already been considerably eroded after a large number of switching cycles and only have a thickness of D1 Correspondingly, the contact pieces 18 in the switched-off state are located at a spacing s1 which is considerably greater . than the spacing s0 in the new state. If the magnet coils 4 are now excited, i.e. the switch-on operation is introduced, the magnet armature 6 moves with increasing velocity in the direction, towards the magnet yoke 2 until, after a distance as shown in figure 5 which corresponds to this spacing si, the contacts 18 come into contact with one another for the first . time. This is the case given a spacing di of the pole faces 20, 60, for which di = H-si likewise applies. The figure now shows that this spacing di, i.e. the contact resilience as a result of the low thickness Di of the contacts 18, is reduced significantly in comparison with the contact resilience in the new state. In the graph shown in figure 6, the current I (curve a) flowing through the magnet coils and the clocked DC voltage U (curve b) present at the magnet coils are plotted against time t. The example illustrated relates to a switching device which is driven by means of a method known, for example, from WO 2005/017933 Al, in order to set the closing velocity at which the contacts, on the one hand, and the poles, on the other hand, meet one another by means of regulating the acceleration of the magnet armature. This figure now shows that the current I continuously decreases until the time tk at which the contacts close, in order to briefly rise again after this time tk. This rise is necessary in order to compensate for the suddenly increased spring force,' which acts from the time tk, on the magnet armature as a result of a correspondingly higher magnetic force. This can clearly be seen in the graph in figure 7 . In this graph, the magnetic force FM (curve c) and the spring force Fs (curve d) are plotted against time t. At the time tk at which the contacts close, the spring force Fs rises suddenly. For a short period, this rise cannot be compensated for by the magnetic force. Only in the further course of things can the magnetic force again exceed the spring force. In the graph shown in figure 8, the distance s of the magnet armature (curve e) and its velocity v (curve f) are likewise plotted against time t. At the beginning of the switch-on operation, the magnet armature (actuator) is located with its pole faces at the spacing H from the pole faces of the magnet yoke. The velocity v at which the magnet armature moves towards the magnet yoke increases continuously after a certain time delay with an approximately constant acceleration. The reason for this is the abovementioned control of the armature movement, which ensures that the velocity of the magnet armature is not too great. At the closing time tk, i.e. once the armature and therefore also the contacts have covered a distance w = s, the velocity v decreases rapidly to a minimum value in order then to rise, as a result of the again increasing magnetic force FM, to the desired value of approximately 0.5 m/s. This decrease in the velocity v, which takes place as a result of the suddenly increasing spring force Fs, is a significant indication of the closing time tk of the contacts.. This closing time tk is then the time t at which v(t+5t) In figure 9, the acceleration b of the magnet armature is ' plotted in a logarithmic scale against time. The curve g shows that the acceleration b rises rapidly to an approximately constant value and experiences a change of mathematical sign at the time at which the contacts close as a result of the decrease in velocity. This change of mathematical sign can be identified particularly easily in the case of an evaluation of the time profile of the acceleration b and can be used to determine the closing time tk. The graphs illustrated in figures 6 to 9 are used fox-explaining, by way of example, the physical conditions present when an electromagnetic switching device is switched on. The decrease in the velocity or change in the mathematical sign of the acceleration illustrated in figures 8 and 9 also results when the electromagnetic switching device is operated in an unregulated fashion or on the basis of another regulation method. If the velocity v or the acceleration is now detected, . with the aid of a suitable sensor, either directly by means of a velocity sensor or acceleration sensor, the closing time tk can be determined particularly easily from its profile. In principle, the closing time tk can also be derived from a signal measured by a displacement sensor by said signal being differentiated once or twice. As shown in figure 10, in the case of a switching device according to the invention a sensor 22 is coupled directly to the magnet armature 6, which sensor 22 can be in the form of a velocity sensor, acceleration sensor or displacement sensor. This sensor 22 is used to detect the relative movement of the contacts 18 indirectly and to evaluate it in an evaluation • device 25. In the evaluation, the time tk is determined from the change in the acceleration b or the decrease in the velocity v. The remaining distance d (contact resilience) or the distance s covered up until this time tk can be taken directly from the distance/time profile w(t) of the actuator (magnet armature 6). In this case, the evaluation device 25 can also take on the function of the differentiation or integration of the movement signal produced by the sensor 22. As an alternative to this, a sensor 24 can be arranged on the moveable contact link 14. In the case of a displacement sensor, the distances so and si can be measured directly. In the event of a velocity sensor, the velocity v can be determined directly as a function of time. In this case, the closing time tk is the time at which the movement ends and the velocity v of the moveable contact 18 is equal to zero. In the exemplary embodiment, the sensors 22, 24 are coupled mechanically to the moving parts, the magnet armature 6 or the moveable contact 18. In principle, however, sensors which function in contactless fashion can also be used, which sensors measure the spacing between the relevant moving part and a stationary housing part. As an alternative to this, it is also possible to measure the current I flowing through the magnet coils 4 and the magnetic flux $ with an induction coil 26, in order to determine from this the acceleration acting on the magnet armature 6, by means of a method known for example from DE 195 44 207 C2. If the time tk at which the contacts close is known, this can be used to determine, depending on the sensor used, either directly or indirectly the distance s covered up to this time by the magnet armature 6 and therefore by the contacts 18. If the. distance si is known for the example in figure 4, it is possible to draw a conclusion directly on the erosion D0-Di and therefore also on the residual life of the contacts by means of a comparison with a stored reference value so- The following relationship results for the erosion Do-Di Do-Di = (Si-s0)/2 with the precondition that the erosion D0-Di is distributed uniformly over the contacts which are positioned opposite one another. As a mathematical equivalent to this, the spacing di of the pole faces from the magnet yoke and the magnet armature can also be calculated from the distance si. This then results from the difference from the stored value H for the spacing of the pole faces in the open state and the distance covered, where di = H - si. In this case, the following equation applies for the erosion Do-D1 Do-D1 = (d0-di)/2. If the spacing di is measured directly as the distance, which is covered by the actuator (magnet armature) from the time tk up to. its end position, the erosion Do-Di can be calculated directly with the above equation if the spacing d0 (contact resilience) in the case of unused contacts is stored as the reference value. We Claim: 1. A method for determining the erosion of contacts of an electromagnetic switching device, comprising: measuring a mechanical variable (v,b,w), which characterizes a time profile cf relative movement between the contacts (18), the relative movement between the contacts (18) being caused by movement of an actuator (6,14,16), the mechanical variable being measured during a switch-on operation in which the contacts (18) are closed; determining a time (tx) at which the contacts (18) close by evaluating the time profile of the relative movement; characterized by comprising the steps of: using the time profile and the time (tk) at which the contacts (18) close to monitor a distance covered during the switch-on operation by either: determining a distance (s,s0,Si) covered by the contacts (18) from a beginning of the switch-on operation until the time at which the contacts close, or determining a distance (d,d0,di) covered by the actuator from the time at which the contacts close until an end position of the actuator (6); and comparing the distance covered with a stored reference value (s0,d0). 2. The method as claimed in claim 1, wherein the mechanical variable is measured by a sensor, which is coupled mechanically to the actuator. 3. The method as claimed in claim 1 or 2, wherein the sensor is selected from the group consisting of a velocity sensor, an acceleration sensor, and a displacement sensor. 4. The method as claimed in claim 1 wherein the actuator (14,16) is. moved by an electromagnetic drive (2,4,6), and the mechanical variable is measured by evaluating an electrical or magnetic property, which is measured during the switch-on operation, of the electromagnetic drive. 5. A device for determining the erosion of contacts of an electromagnetic switching device, the electromagnetic switching device, comprising: a pair of contacts (18) which are moveable relative one another and which are brought into contact with each other after a switch-on operation; an actuator (6,14,16) which moves to cause relative movement between the contacts (18); the device comprising: - a sensor (22) mechanically coupled to the actuator to measure a mechanical variable and determine a time profile of relative movement between the contacts based on the mechanical variable; and An evaluation device (25) to : Determine a time at which the contacts close by evaluating the time profile of the relative movement; Use the time profile and the time at which the contacts close to monitor a distance covered during the switch-on operation by either: determining a distance covered by the contacts from a beginning of the switch-on operation until the time at which the contacts close, or determining a distance covered by the actuator from the time at which the contacts close until an end position of the actuator; and compare the distance covered with a stored reference value (s0/d0). 6. The device as claimed in claim 5, wherein the sensor is selected from the group consisting of a velocity sensor, an acceleration sensor and a displacement sensor. ' . 7. The device as claimed in claim 5, wherein The device comprises an electromagnetic drive to move the actuator, and The sensor measures an electrical or magnetic property (I,(j>) of the electromagnetic drive (2,4,6) and determines the mechanical variable by evaluating the property. ABSTRACT TITLE: "A method and a device for determining the erosion of contacts of an electromagnetic switching device". The invention relates to a method for determining the erosion of contacts of an electromagnetic switching device, comprising measuring a mechanical variable (v,b,w), which characterizes a time profile of relative movement between the contacts (18), the relative movement between the contacts (18) being caused by movement of an actuator (6,14,16) , the mechanical variable being measured during a switch-on operation in which the contacts (18) are closed; determining a time (tx) at which the contacts (18) close by evaluating the time profile of the relative movement comprising the steps of using the time profile and the time (tk) at which the contacts (18) close to monitor a distance covered during the switch-on operation by either: determining a distance (s,so,sl) covered by the contacts (18) from a beginning of the switch-on operation until the time at which the contacts close, or determining a distance (d,d0,dl) covered by the actuator from the time at which the contacts close until an end position of the actuator (6); and comparing the distance covered with a stored reference value (so,do).

Full Text

Field of the invention
The invention relates to a method for determining the erosion of contacts of an electromagnetic switching device. In addition, the invention relates to an electromagnetic switching device with a device for determining the erosion of its contacts.
BACKGROUND OF THE INVENTION
During the switch-on and switch-off operations of an electromagnetic switching device, arcs occur between the closing or opening contacts. These arcs cause erosion of the contacts over the course of time. It is therefore important for the operational reliability of such a switching device to identify the degree of this erosion in order to be able to draw conclusions on the residual life of the switching device and avoid operational faults by replacing the contacts in good time.
EP 0 694 937 Bl has disclosed a method for determining the erosion and therefore the residual life of contacts in switching devices, in which method the so-called contact resilience is determined as a measure for the contact erosion. This contact resilience is the distance which is covered by the magnet armature as the actuator of the switching movement between the beginning of the switch-off operation, i.e. the time at which the magnet armature, which is resting in the end position on a magnet yoke, releases itself therefrom and the time at which the contacts lift off from one another. The time at which the magnet armature lifts off from the magnet yoke is measured by an auxiliary circuit, in which the magnet armature and the magnet yoke form a switch, which is closed if the

magnet' armature and the magnet yoke are in contact with one another.

As an alternative to this, it is known, for example, from EP 0 878 015 Bl, to determine the time at which the magnet armature separates from the magnet yoke of the magnet drive by means of measuring the voltage at the magnet coil of the magnet yoke.
In both methods, a further auxiliary circuit is required for detecting the time at which the contacts lift off from one another, for example a complex auxiliary circuit which is DC-decoupled from the main circuit with the aid of optocouplers and which detects the occurrence of an arc voltage, which is produced by the arc forming when the contacts lift off from one another.
As an alternative to the methods known from EP 0 694 937 Bl and EP 0 878 015 Bl, in which the switch-off operation is used to determine the erosion or the residual life, WO 2004/057634 Al has disclosed a method and an apparatus for determining the residual life of a switching device, in which method the change in the contact resilience is measured during the switch-on operation, i.e. when the switching contacts are closed by the magnet, drive. With this known apparatus, a position encoder is arranged on the magnet armature, which position encoder contains markings, for example in the form of measuring contacts, in at least three positions, with which markings the time profile of the magnet armature movement can be detected. The determination of the position of the magnet armature when the contacts close is determined by means of computation from the movement sequence of the magnet armature which is detected with the aid of these position markers. For this purpose, a simple algorithm is used as a result of the low number of position markers assuming that the armature acceleration is constant between a time prior to the closing of the contacts and a time which is between the closing time of the contacts and the time at which the magnet armature is positioned onto the magnet yoke. In practice, however, it has been established

■ that, with such an approach, the time at which the contacts close can only be determined with a low amount of accuracy.

OBJECTS OF THE INVENTION
The invention is now based on the object of specifying a method for determining the erosion of contacts of an electromagnetic switching device, with which method precise determination of the time at which the contacts close and therefore precise determination of the contact erosion is possible. In addition, the invention is based on the object of specifying an electromagnetic switching device with a device functioning on the basis of this method. SUMMARY OF THE INVENTION
As regards the method, the abovementioned object according to the invention is achieved by a method, with which method, during the switch-on operation, a mechanical variable, which characterizes the time profile of the relative movement, which is caused by an actuator, between the contacts, is measured and the time at which the contacts close is determined by evaluating the time profile of the relative movement, and the distance covered up to this time by the contacts or that covered from this time by the actuator up to its end position is detected at least indirectly and is compared with a stored reference value.
In this case, the invention is based on the consideration that the time profile of the relative movement at the time at which the contacts close is changed significantly as a result of the high spring force of the contact spring which sets in at this time and which brakes the movement of the actuator, with the result that, by analyzing the time profile of the movement, the time at which the contacts meet one another can be determined directly and reliably without an approximation model of the movement sequence being required for this purpose, as is the case with the document WO 2004/057634 Al mentioned at the outset.
The variable characterizing the movement sequence can be measured directly by measuring the velocity, or the acceleration of one of the contacts or both contacts. As an alternative to

this, the velocity of an actuator, which causes this relative movement and is coupled mechanically to at least one of the contacts and is

actuated by an electromagnetic drive, can also be measured.
If the time profile of the movement is measured by a sensor which is coupled
mechanically to the actuator, the measurement can take place using a
measurement circuit, which is DC-decoupled from the switched circuit or the
circuit of the magnetic drive.
A suitable sensor may be a displacement sensor, a velocity sensor or an
acceleration sensor.
If a velocity sensor or an acceleration sensor is used as the sensor, it is
particularly easy to determine the time at which the contacts close from this
measurement signal. In order in this case to obtain information on the distance
covered, its measurement signals still need to be integrated singularly or twofold.
As an alternative to the use of such a sensor, it is also possible to measure the
mechanical variable by evaluating an electrical or magnetic variable of the
electromagnetic drive which is measured during the switch-on operation.
The object in relation to the electromagnetic switching device is achieved in
accordance with the invention, whose advantages, correspond to the advantages
which are specified in relation to the features of the respectively associated with
the method.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
For further explanation of the invention, reference is made to the accompanying
drawing, in which:
figures 1 to 3 each show an electromagnetic switching device in a.basic
illustration at various

times of the switch-on operation with uneroded contacts,
figures 4 to 5 show the electromagnetic switching device at different times of the
switch-on operation after a large number of switching cycles when the contacts
have suffered significant erosion,
figures 6 to 9 each show graphs, in which the voltage across the magnet coil
and the current flowing through it, the magnetic force and the spring force, the
spacing between the magnet armature and the yoke and the velocity of the
magnet armature or its acceleration are in each case plotted over time, and
figure 10 shows a schematic illustration of a switching device with a device for
improved determination of the erosion of the contacts.
Detail Description of the invention
As shown in figure 1, an electromagnetic switching device, in the example illustrated a contactor, contains a magnet yoke 2, on which two magnet coils 4 are arranged for magnetic excitation purposes. A magnet armature 6, which is associated with the magnet yoke 2, is mounted in a sprung manner by means of compression springs 8 in a housing 10 (which is only illustrated symbolically) of the switching device. The magnet yoke 2, magnet coil 4 and magnet armature 6 form an electromagnetic drive of the switching device. The magnet armature 6 is connected in a force-fitting manner to a moveable contact link 14 via a contact spring 12. Two stationary contact carriers 16 are associated with the moveable contact link 14. The magnet armature 6 forms the actuator of the magnetic drive for the relative movement between the contact link 14 and the contact carrier 16.

The contact link 14 and the stationary contact carrier 16 are each provided with contact pieces or contacts 18, which when new have a thickness D0. The switching contact formed by the moveable contact link 14 and the stationary contact carrier 16 is located in the open

position. In this switched-off state, the contacts 18 are at a spacing s0 and the pole faces 20 and 60 of the magnet yoke or the magnet armature 6 are located at a spacing H.
When the magnet coils 4 are switched on, the magnet armature 6 is set in motion, counter to the action of the compression springs 8, in the direction towards the magnet yoke 2, as is illustrated by the arrows in the figure.
Figure 2 now shows a situation in which the contacts 18 come into contact with one another for the first time, i.e. the magnet armature 6 has covered a distance s0- At this time, the pole faces 20, 60 are located at a spacing d0 = H-s0. This spacing d0 corresponds to the contact; resilience of the switching device with the contacts 18 uneroded. The further closing movement of the magnet armature 6 now takes place counter to the action of the contact spring 12 and the compression spring 8, which is connected in parallel therewith. Since the spring force exerted by the contact spring 12 is considerably greater than the spring force exerted by the compression spring 8, the spring force acting on the magnet armature 6 increases suddenly and brings about a significant change in the course of the closing movement.
As things proceed, the magnetic force acting on the magnet armature 6 is greater than the spring force exerted by the compression spring 8 and the contact spring 12, and the magnet armature 6 can move further in the direction towards the magnet yoke 2 until it finally, as is illustrated in figure 3, rests in an end or rest position with its pole faces 60 on the pole faces 2 0 of the magnet yoke 2.
Figure 4 now illustrates a situation in which the contacts 18 have already been considerably eroded after a large number of switching cycles and only have a thickness of D1
Correspondingly, the contact pieces 18 in the switched-off state are located at a spacing s1 which is considerably greater . than the spacing s0 in the new state. If the magnet coils 4 are now excited, i.e. the switch-on operation is introduced,

the magnet armature 6 moves with increasing velocity in the direction, towards the magnet yoke 2 until, after a distance as shown in figure 5 which corresponds to this spacing si, the contacts 18 come into contact with one another for the first . time. This is the case given a spacing di of the pole faces 20, 60, for which di = H-si likewise applies. The figure now shows that this spacing di, i.e. the contact resilience as a result of the low thickness Di of the contacts 18, is reduced significantly in comparison with the contact resilience in the new state.
In the graph shown in figure 6, the current I (curve a) flowing through the magnet coils and the clocked DC voltage U (curve b) present at the magnet coils are plotted against time t. The example illustrated relates to a switching device which is driven by means of a method known, for example, from WO 2005/017933 Al, in order to set the closing velocity at which the contacts, on the one hand, and the poles, on the other hand, meet one another by means of regulating the acceleration of the magnet armature. This figure now shows that the current I continuously decreases until the time tk at which the contacts close, in order to briefly rise again after this time tk. This rise is necessary in order to compensate for the suddenly increased spring force,' which acts from the time tk, on the magnet armature as a result of a correspondingly higher magnetic force.
This can clearly be seen in the graph in figure 7 . In this graph, the magnetic force FM (curve c) and the spring force Fs (curve d) are plotted against time t. At the time tk at which the contacts close, the spring force Fs rises suddenly. For a short period, this rise cannot be compensated for by the magnetic force. Only in the further course of things can the magnetic force again exceed the spring force.

In the graph shown in figure 8, the distance s of the magnet armature (curve e) and its velocity v (curve f) are likewise plotted against time t. At the beginning of the switch-on operation, the magnet armature (actuator) is located with its pole

faces at the spacing H from the pole faces of the magnet yoke. The velocity v at which the magnet armature moves towards the magnet yoke increases continuously after a certain time delay with an approximately constant acceleration. The reason for this is the abovementioned control of the armature movement, which ensures that the velocity of the magnet armature is not too great. At the closing time tk, i.e. once the armature and therefore also the contacts have covered a distance w = s, the velocity v decreases rapidly to a minimum value in order then to rise, as a result of the again increasing magnetic force FM, to the desired value of approximately 0.5 m/s. This decrease in the velocity v, which takes place as a result of the suddenly increasing spring force Fs, is a significant indication of the closing time tk of the contacts.. This closing time tk is then the time t at which v(t+5t) In figure 9, the acceleration b of the magnet armature is ' plotted in a logarithmic scale against time. The curve g shows that the acceleration b rises rapidly to an approximately constant value and experiences a change of mathematical sign at the time at which the contacts close as a result of the decrease in velocity. This change of mathematical sign can be identified particularly easily in the case of an evaluation of the time profile of the acceleration b and can be used to determine the closing time tk.
The graphs illustrated in figures 6 to 9 are used fox-explaining, by way of example, the physical conditions present when an electromagnetic switching device is switched on. The decrease in the velocity or change in the mathematical sign of the acceleration illustrated in figures 8 and 9 also results

when the electromagnetic switching device is operated in an unregulated fashion or on the basis of another regulation

method. If the velocity v or the acceleration is now detected, . with the aid of a suitable sensor, either directly by means of a velocity sensor or acceleration sensor, the closing time tk can be determined particularly easily from its profile. In principle, the closing time tk can also be derived from a signal measured by a displacement sensor by said signal being differentiated once or twice.
As shown in figure 10, in the case of a switching device according to the invention a sensor 22 is coupled directly to the magnet armature 6, which sensor 22 can be in the form of a velocity sensor, acceleration sensor or displacement sensor. This sensor 22 is used to detect the relative movement of the contacts 18 indirectly and to evaluate it in an evaluation • device 25. In the evaluation, the time tk is determined from the change in the acceleration b or the decrease in the velocity v. The remaining distance d (contact resilience) or the distance s covered up until this time tk can be taken directly from the distance/time profile w(t) of the actuator (magnet armature 6). In this case, the evaluation device 25 can also take on the function of the differentiation or integration of the movement signal produced by the sensor 22.
As an alternative to this, a sensor 24 can be arranged on the moveable contact link 14. In the case of a displacement sensor, the distances so and si can be measured directly. In the event of a velocity sensor, the velocity v can be determined directly as a function of time. In this case, the closing time tk is the time at which the movement ends and the velocity v of the moveable contact 18 is equal to zero.
In the exemplary embodiment, the sensors 22, 24 are coupled mechanically to the moving parts, the magnet armature 6 or the moveable contact 18. In principle, however, sensors which function in contactless fashion can also be used, which sensors measure the spacing between

the relevant moving part and a stationary housing part.
As an alternative to this, it is also possible to measure the current I flowing through the magnet coils 4 and the magnetic flux $ with an induction coil 26, in order to determine from this the acceleration acting on the magnet armature 6, by means of a method known for example from DE 195 44 207 C2.
If the time tk at which the contacts close is known, this can be used to determine, depending on the sensor used, either directly or indirectly the distance s covered up to this time by the magnet armature 6 and therefore by the contacts 18.
If the. distance si is known for the example in figure 4, it is possible to draw a conclusion directly on the erosion D0-Di and therefore also on the residual life of the contacts by means of a comparison with a stored reference value so- The following relationship results for the erosion Do-Di
Do-Di = (Si-s0)/2
with the precondition that the erosion D0-Di is distributed uniformly over the contacts which are positioned opposite one another. As a mathematical equivalent to this, the spacing di of the pole faces from the magnet yoke and the magnet armature can also be calculated from the distance si. This then results from the difference from the stored value H for the spacing of the pole faces in the open state and the distance covered, where
di = H - si.
In this case, the following equation applies for the erosion Do-D1
Do-D1 = (d0-di)/2.

If the spacing di is measured directly as the distance, which is covered by the actuator (magnet armature) from the time tk up to. its end position, the erosion Do-Di can be calculated directly with the above equation if the spacing d0 (contact resilience) in the case of unused contacts is stored as the reference value.

We Claim:
1. A method for determining the erosion of contacts of an electromagnetic switching device, comprising:
measuring a mechanical variable (v,b,w), which characterizes a time profile cf
relative movement between the contacts (18), the relative movement between
the contacts (18) being caused by movement of an actuator (6,14,16), the
mechanical variable being measured during a switch-on operation in which the
contacts (18) are closed;
determining a time (tx) at which the contacts (18) close by evaluating the time
profile of the relative movement;
characterized by comprising the steps of:
using the time profile and the time (tk) at which the contacts (18) close to
monitor a distance covered during the switch-on operation by either:
determining a distance (s,s0,Si) covered by the contacts (18) from a beginning of
the switch-on operation until the time at which the contacts close, or
determining a distance (d,d0,di) covered by the actuator from the time at which
the contacts close until an end position of the actuator (6); and
comparing the distance covered with a stored reference value (s0,d0).

2. The method as claimed in claim 1, wherein the mechanical variable is measured by a sensor, which is coupled mechanically to the actuator.
3. The method as claimed in claim 1 or 2, wherein the sensor is selected from the group consisting of a velocity sensor, an acceleration sensor, and a displacement sensor.
4. The method as claimed in claim 1 wherein
the actuator (14,16) is. moved by an electromagnetic drive (2,4,6), and the mechanical variable is measured by evaluating an electrical or magnetic property, which is measured during the switch-on operation, of the electromagnetic drive.
5. A device for determining the erosion of contacts of an electromagnetic
switching device, the electromagnetic switching device, comprising:
a pair of contacts (18) which are moveable relative one another and which are brought into contact with each other after a switch-on operation; an actuator (6,14,16) which moves to cause relative movement between the contacts (18); the device comprising:

- a sensor (22) mechanically coupled to the actuator to measure a
mechanical variable and determine a time profile of relative movement
between the contacts based on the mechanical variable; and
An evaluation device (25) to :
Determine a time at which the contacts close by evaluating the time
profile of the relative movement;
Use the time profile and the time at which the contacts close to monitor a
distance covered during the switch-on operation by either:
determining a distance covered by the contacts from a beginning of the
switch-on operation until the time at which the contacts close, or
determining a distance covered by the actuator from the time at which the
contacts close until an end position of the actuator; and
compare the distance covered with a stored reference value (s0/d0).
6. The device as claimed in claim 5, wherein the sensor is selected from the
group consisting of a velocity sensor, an acceleration sensor and a displacement
sensor. ' .

7. The device as claimed in claim 5, wherein
The device comprises an electromagnetic drive to move the actuator, and The sensor measures an electrical or magnetic property (I,(j>) of the
electromagnetic drive (2,4,6) and determines the mechanical variable by
evaluating the property.

ABSTRACT

TITLE: "A method and a device for determining the erosion of contacts of an electromagnetic switching device".
The invention relates to a method for determining the erosion of contacts of an electromagnetic switching device, comprising measuring a mechanical variable (v,b,w), which characterizes a time profile of relative movement between the contacts (18), the relative movement between the contacts (18) being caused by movement of an actuator (6,14,16) , the mechanical variable being measured during a switch-on operation in which the contacts (18) are closed; determining a time (tx) at which the contacts (18) close by evaluating the time profile of the relative movement comprising the steps of using the time profile and the time (tk) at which the contacts (18) close to monitor a distance covered during the switch-on operation by either: determining a distance (s,so,sl) covered by the contacts (18) from a beginning of the switch-on operation until the time at which the contacts close, or determining a distance (d,d0,dl) covered by the actuator from the time at which the contacts close until an end position of the actuator (6); and comparing the distance covered with a stored reference value (so,do).

A METHOD AND A DEVICE FOR DETERMINING THE EROSION OF CONTACTS OF AN ELECTROMAGENTIC SWITCHING DEVICE

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