Method for functional testing of a magnetic actuator and corresponding system

Abstract

The invention relates to a method for checking the functionality of a magnetically actuated actuator (4) which has a coil (5) to which an electric current can be applied and an armature (6) which can be moved by the magnetic field generated by the coil (5).

Description

The invention relates to a method for checking a magnetic actuator.DE 10 2019 116 221 A1 discloses a monitoring for electromagnetic brakes, in which a coil current is detected in order to detect an armature movement. If deviations of the supply voltage are detected during this, the detected armature movement is discarded.DE 10 2009 018 122 A1 teaches that an abnormal behavior is detected by comparing the current characteristics if a current peak induced by mutual induction also fails due to the absence of movement.In DE 10 2019 116 221 A1, the coil current is checked after the switch-off. In this case, it is examined whether it has a change in the curvature when it falls, in order to verify a correct movement of the armature.It is known from DE 10 2012 213 120 to determine a value tanα for detecting an openly blocked solenoid valve, which is based on the temporal gradient of the coil current. This is compared with a threshold value which is selected such that, if tanα is less than the threshold value, an openly blocked solenoid valve can be reliably assumed.Furthermore, US 2020 / 0355295 A1 discloses examining the exponential rise and fall during the energization and deactivation of the coil. The loading and unloading times are compared to detect problems with the valve.DE 37 15 591 A1 teaches that for control of the switching of solenoid valves the inductance of the coil is determined by current measurement. In this case, the current provided for the armature determination is temporarily switched off and, after a certain time, the current value is measured together with a current reference value and the maximum switch-on current.In US 2011 / 0 198 167 A1, a plurality of measured values of the current are taken and the inductance of the coil is determined therefrom. A time at which the inductance drops is determined. At this time, the current value is used and compared with a threshold value to determine the operating state of the valve.Parking brakes are known which use a magnetically actuated actuator, in particular a lifting magnet, for locking the parking brake. In order to be able to ensure the functionality of the parking brake, it is necessary to monitor the function of the magnetically actuated actuator.It is problematic when monitoring the function of a magnetically actuated actuator that there are actuators whose inductances at the respective end positions of the moved part of the actuator are very similar, so that it is not possible to draw a conclusion from the inductance value itself as to whether or not the movable part of the actuator has assumed a desired position. In addition, the coil resistance of the actuator has a high temperature influence, which makes a functional check more difficult.Proceeding from this, it is an object of the invention to specify a method which enables a simple and reliable checking of the functionality of a magnetically actuated actuator.The object is achieved by a method having the features of independent claim 1. Preferred embodiments are the subject matter of the dependent claims. A system comprising a magnetic actuator is the subject of the subordinate claim 11.According to a first aspect, a method for verifying the functionality of a magnetically actuated actuator is disclosed. The actuator has a coil to which an electric current can be applied and an armature which can be moved by the magnetic field which is generated by the coil. The method comprises the following steps:First, a movement of the armature is effected by applying an electric current to the coil. The magnetic field generated by the coil thereby exerts a force on the armature, which causes a movement, in particular a linear displacement of the armature.Subsequently, a plurality of current measurement values of the electric current through the coil are determined, namely during a period in which the external energization of the coil is switched off and in which, with the actuator being properly functioning, the armature moves relative to the coil. In other words, the course of the current through the coil is determined after deactivation of the external voltage supply, which causes a current flow through the coil, namely when a movement of the armature takes place with correct functioning of the actuator.A plurality of values of a time constant are then calculated on the basis of the determined current measurement values, said values describing the decay of the amplitude of the electric current over time. In particular, the values of the time constants are calculated based on the time difference between two temporally consecutive current measurement values and based on the values of these consecutive current measurement values. Despite the disconnection of the external power supply, a current flows, namely on the one hand due to the dissipation of the magnetic field in the coil, and on the other hand due to the induction of a current due to the movement of the armatureA decay coefficient is then calculated by forming the quotient from two calculated values of the time constant.Finally, it is decided based on the calculated decay coefficient whether or not the armature has been moved. If it is decided that no movement of the armature has occurred, this indicates a defect of the actuator and a driver warning can be carried out.The technical advantage of the proposed method is that reliable testing of the functionality of a magnetically actuated actuator is possible in a technically simple manner, since the function test is not influenced by temperature influences and manufacturing fluctuations by determining the decay coefficient by means of forming the quotient of time constants.According to one exemplary embodiment, the decay coefficient is formed from the quotient of the maximum value and the minimum value of the calculated values of the time constant. This provides a reliable measure for the assessment of the functionality of the actuator.According to an embodiment, the values of the time constant are calculated based on the following formula: wherein Δt time interval between the current measurement values I n and I n+1; I n current measurement value at time n; I n+1 current measurement value at time n+1;As a result, a discrete time constant can be formed on the basis of current measurement values following one another in time. The time profile, in particular the amplitude differences of the time constant during the period in which the movement of the armature is to take place with a functional actuator, has a significance about whether or not the armature has been moved. Namely, the current decay behavior changes significantly depending on whether or not the armature could be moved (for example, due to jamming).According to one exemplary embodiment, the decision as to whether the armature has been moved is made on the basis of a comparison of the calculated decay coefficient with a threshold value. Depending on whether the decay coefficient is above or below the threshold value, it is possible to decide whether or not the actuator has a desired functionality.According to one exemplary embodiment, the armature is evaluated as being moved according to the specification and the functionality of the actuator is thus positively decided if the calculated decay coefficient is greater than or equal to the threshold value. In the case of a movement of the armature, the decay coefficient is greater than in the case of a non-moved armature, so that the threshold value can be considered to be exceeded as an indicator of a movement of the armature.According to one exemplary embodiment, the armature is evaluated as not being moved according to the specification and the functionality of the armature is thus negatively decided if the calculated decay coefficient is less than the threshold value. In the event of a jamming of the armature, the decay coefficient is smaller than in the case of a moving armature, so that the falling below the threshold value can be evaluated as an indicator of a lack of movement of the armature.According to one exemplary embodiment, the measured current values are ascertained during a period in which the amplitude of the current decays after the external power supply is switched off via a freewheeling diode of the actuator. The pure decay behavior of the current can thus be evaluated without external energy influences, wherein the decay behavior results solely from the inductive behavior of the coil and the induced current due to the movement of the armature (if it moves as expected).According to one exemplary embodiment, the measured current values are ascertained immediately after the external power supply is switched off or, if the actuator is in the state of magnetic saturation, after the measured current values drop below the saturation current. In this time period, the current flowing through the coil decays in each case, so that a meaningful evaluation of the decay behavior of the current is possible here.According to one exemplary embodiment, the magnetically actuated actuator is a linear actuator, the armature of which can be moved into two end positions by linear movement, wherein the armature can be moved into the first end position by the magnetic field generated by the coil. Actuators of this type are used, for example, in the field of electric parking brakes of vehicles and must therefore be monitored in order to assess the correct functionality of the parking brake.According to one exemplary embodiment, the armature is spring-loaded and is moved into the second end position by the spring force after the energization of the coil has been switched off. This allows automatic resetting of the actuator.According to a further aspect, a system comprising a magnetically actuatable actuator, a measuring device and a computing unit is disclosed. The actuator has a coil to which an electric current can be applied and an armature which can be moved by the magnetic field generated by the coil. The system is configured to perform the following steps for checking the operation of the actuator:causing movement of the armature by applying an electric current to the coil;determining a plurality of measured current values of the electric current through the coil during a time period in which the external energization of the coil is switched off and in which a movement of the armature relative to the coil takes place with the actuator being properly functioning;calculating a plurality of values of a time constant, which describe the decay of the amplitude of the electric current over time, by means of the arithmetic unit, based on the determined current measurement values;calculating a decay coefficient by forming the quotient from two calculated values of the time constant;the arithmetic unit evaluates the decay coefficient and decides, based on the calculated decay coefficient, whether the armature has been moved.According to one exemplary embodiment of the system, the magnetically actuated actuator is a linear actuator, the armature of which has two end positions, wherein the armature is movable into the first end position by the magnetic field generated by the coil.According to one exemplary embodiment of the system, the armature is spring-loaded and, after the energization of the coil has been switched off, can be moved into the second end position by the spring force.The terms "approximately", "substantially" or "about" mean, for the purposes of the invention, deviations from the exact value in each case by + / - 10%, preferably by + / - 5%, and / or deviations in the form of changes which are insignificant for the function.Developments, advantages and possible applications of the invention also result from the following description of exemplary embodiments and from the figures.The invention is explained in more detail below on the basis of the figures using exemplary embodiments. The following are shown: FIG. 1 shows, by way of example, a schematic illustration of a vehicle having an electric parking brake; FIG. 2 shows, by way of example, a system for detecting an electric parking brake, which system has a magnetically actuated actuator; FIG. 3 shows, by way of example, the time profile of a decaying current through the coil of the magnetically actuated actuator; FIG. 4 shows, by way of example, the time profile of the logarithm of the decaying current according to FIG. 3 ; FIG. 5 shows, by way of example, the time profile of the time constant, calculated on the basis of the time profile of the logarithm of the decaying current according to FIG. 4 ; FIG. 6 is a block diagram illustrating the method steps of a method for checking the functionality of a magnetically actuated actuator by way of example.FIG. 1 shows, by way of example and roughly schematically, a vehicle F which has an electrically actuated parking brake on the rear axle. The parking brake has a plurality of magnetically actuated actuators 4 which bring about locking of the parking brake when said parking brake is activated and thus prevent rolling away of the vehicle F. The actuators 4 are provided, for example, on the friction brakes of the rear axle of the vehicle F.FIG. 2 schematically shows an exemplary embodiment of a system 1 comprising a measuring device 2, a computing unit 3 and an actuator 4 of the electric parking brake of the vehicle F.In the exemplary embodiment shown, the actuator 4 is a magnetically actuated linear actuator which interacts with a gearwheel Z of the parking brake in order to secure the parking brake in the activated state. The actuator 4 has a coil 5, in the coil inner opening of which an armature 6 can be displaced linearly. FIG. 2 shows the armature in a first end position, in which the armature 6 is inserted into the coil interior in a first end position. In this state, the armature 6 or a functional element connected thereto does not interact with the gearwheel Z and thus does not block the rotational movement of the gearwheel Z. The armature 6 is prestressed into this first end state by an elastic element, in the exemplary embodiment shown a helical spring.When the coil 5 is energized, a magnetic field is generated which causes a displacement of the armature 6 and positions the latter into an advanced position, namely a second end position (not shown in FIG. 2 ). In this second end position, the armature 6 or a functional element connected thereto interacts with the gearwheel Z and thus blocks the movement of the gearwheel Z.In order to ensure that the parking brake has been locked securely, it is necessary to monitor the functionality of the actuator 4, i.e. in particular to check whether or not the armature 6 could be moved (for example on account of jamming of the armature 6).The function monitoring makes use of the fact that after the energization of the coil 5 has been switched off, i.e. after the deactivation of the external current supply to the coil 5, the decay time of the current depends on whether or not the armature 6 has been moved. After the energization of the coil 5 has been switched off, the energy stored in the coil can be dissipated via a current flow through a freewheeling diode. This current flow or its decay time depends on whether the armature 6 has been moved or not. In the event of the movement of the armature 6, the latter additionally induces a voltage in the coil, as a result of which the decay duration of the current is extended.The decay curve of the current after the external power supply is turned off follows an exponential function and can be described by the following formula:Here, I 0 is the current immediately after the energization is turned off; τ is the time constant;The time constant can be determined by where L is the inductance of the coil 5 and R is the ohmic resistance of the coil 5.The test method described below uses the technical effect that the current flow through the coil 5 is influenced by the movement of the armature 6 and the induced voltage in the coil 5 caused thereby. This influence leads to a change in the decay duration of the current through the coil 5, so that it is possible to draw conclusions about the decay duration of the current whether or not the armature 6 has been moved (for example due to a jamming).It is problematic here that the electrical resistance of the coil 5 has a strong temperature dependence and therefore large fluctuations in the time constant τ can occur.The system 1 has a measuring device 2 and a computing unit 3. The current through the coil 5 can be determined by the measuring device 2, namely a plurality of sampling values during the decay duration of the electric current through the coil 5.The test method initially brings about a movement of the armature 6, the armature 6 being set in motion here preferably by energizing the coil 5, i.e. the application of electric current from an external voltage supply to the coil. The coil 5 is preferably coupled to a free-wheeling diode. After the external voltage supply is switched off, the current circulates through this freewheeling diode and decays. This current decay is exemplarily shown in FIG. 3. During the decay phase of the electric current, a plurality of current measurement values are determined by the measuring device 2, so that these current measurement values form sampling values of the temporal profile of the electric current.Based on these current measurement values, a plurality of time constants τ can be calculated.The above formula 1 can be transformed as follows by applying logarithm on both sides of the equation:FIG. 4 shows the time profile of the logarithm of the current according to FIG. 3 For the current measurement values I 0, I 1, I 2,... and the times t=[t 0, t 1, t 2...], at which these current measurement values are determined, the following is in a time-discrete representation:The time constant τ can be calculated as follows:In general form, the time constant τ can be calculated from the discrete current measurement values as follows:The plurality of current measurement values, which are obtained by sampling the decay curve of the current flowing through the coil 5, result in a plurality of values of the time constant τ for a decay curve. The curve of the values of the time constant τ for the decay curve of the current according to FIG. 3 is shown in FIG. 5.In order to compensate for the temperature influence on the ohmic resistance and the inductance of the coil 5 and for variations in the inductance resulting from manufacturing tolerances, a decay coefficient R is determined based on a pair of time constants τ.Preferably, the maximum value τ max and the minimum value τ min of the time constant τ are determined from the plurality of values of the time constant τ and these are put in relation to one another by quotient formation, whereby the decay coefficient R is formed. Forming the quotient has the advantage that the dependencies on the temperature and the manufacturing tolerances are compensated for as a result.The decay coefficient R is calculated, for example, as follows:This decay coefficient R is greater in the case that the armature 6 is moving than in the case that the armature 6 cannot be moved (e.g. due to jamming of the armature 6).It can therefore be decided by means of a suitably defined threshold value whether or not a movement of the armature 6 has taken place and therefore the actuator 4 functions according to the specification.The measured current values are preferably determined from the point in time at which the external power supply has been switched off and the current through the coil decays on the basis of the energy stored in the system. The determination of the measured current values can then be ended when these are very small, i.e. are in the range of the noise, for example.If the starting current is very large and the actuator 4 is thus in the region of magnetic saturation, the determination of the measured current values can be started from the time from which the current is less than the saturation current.FIG. 6 shows a flow chart illustrating the method steps for checking the functionality of a magnetically actuated actuator.First, the movement of the armature is effected by applying electric current to the coil (S 10).Subsequently, a plurality of current measurement values of the electric current through the coil are determined, namely during a period in which the external energization of the coil is switched off and in which, with the actuator being properly functioning, the armature moves relative to the coil (S 11).Based on the determined current measurement values, a plurality of values of a time constant are calculated, which describes the decay of the amplitude of the electric current over time (S 12).Then, a decay coefficient is calculated by forming the quotient of two values of the time constant (S 13).Based on the calculated decay coefficient, it is decided last whether the armature has been moved and thus the functionality of the actuator is ensured (S 14).The invention has been described above with reference to exemplary embodiments. It is to be understood that numerous changes and modifications are possible without thereby departing from the scope of protection defined by the claims.List of reference characters1 System 2 Measuring device 3 Computing unit 4 Actuator 5 Coil 6 Armature F Vehicle I n, I n+1 Measured current value R Decay coefficient τ Time constant τ max Time constant τ min Time constant Z Gearwheel

Claims

Method for checking the functionality of a magnetically actuated actuator (4) which has a coil (5) to which an electric current can be applied and an armature (6) which can be moved by the magnetic field which is generated by the coil (5), the method comprising the following steps: - causing the armature (6) to move by applying an electric current (S10) to the coil (5); - ascertaining a plurality of measured current values (I n, I n+1) of the electric current through the coil (5) during a period in which the external energization of the coil (5) is switched off and in which, with the actuator (4) in the correct functionality, the armature (6) moves relative to the coil (5) (S11); - based on the determined current measurement values (I n, I n+1), calculating a plurality of values of a time constant (τ) which describe the decay of the amplitude of the electric current over time (S12); - calculating a decay coefficient (R) by forming the quotient of two calculated values of the time constant (τ max, τ min) ( S13); - deciding whether the armature (6) has been moved based on the calculated decay coefficient (R) (S14).Method according to Claim 1, characterized in that the decay coefficient (R) is formed from the quotient of the maximum value and the minimum value of the calculated values of the time constant (τ max, τ min).Method according to claim 1 or 2, characterized in that the values of the time constant (τ) are calculated based on the following formula: τ = Δ t log ( I n ) - log ( I n + 1 ); wherein Δt time interval between the current measurement values I n and I n+1; I n current measurement value at time n; I n+1 current measurement value at time n+1;Method according to one of the preceding claims, characterized in that the decision as to whether the armature (6) has been moved is made on the basis of a comparison of the calculated decay coefficient (R) with a threshold value.Method according to Claim 4, characterized in that the armature (6) is assessed as being moved according to the specification and the functionality of the actuator (4) is therefore positively decided if the calculated decay coefficient (R) is greater than or equal to the threshold value.Method according to Claim 4 or 5, characterized in that the armature (6) is evaluated as not being moved according to the specification and the functionality of the armature (6) is therefore adversely affected if the calculated decay coefficient (R) is less than the threshold value.Method according to one of the preceding claims, characterized in that the measured current values (I n, I n+1) are determined during a period in which the amplitude of the current decays after the external power supply is switched off via a freewheeling diode of the actuator (4).Method according to Claim 7, characterized in that the measured current values (I n, I n+1) are determined directly after the external power supply is switched off or, if the actuator (4) is in the state of magnetic saturation, after the measured current values drop below the saturation current.Method according to one of the preceding claims, characterized in that the magnetically actuated actuator (4) is a linear actuator, the armature (6) of which can be moved into two end positions by linear movement, wherein the armature (6) can be moved into the first end position by the magnetic field generated by the coil (5).Method according to Claim 9, characterized in that the armature (6) is spring-loaded and can be moved into the second end position by the spring force after the energization of the coil (5) has been switched off.System comprising a magnetically actuatable actuator (4), a measuring device (2) and a computing unit (3), wherein the actuator (4) has a coil (5) to which an electric current can be applied and an armature (6) which can be moved by the magnetic field generated by the coil (5), wherein the system (1) is configured to carry out the following steps for checking the function of the actuator (4): - causing a movement of the armature (6) by applying an electric current to the coil (5); - By means of the measuring device (2), determining a plurality of measured current values (I n, I n+1) of the electric current through the coil (5) during a period in which the external energization of the coil (5) is switched off and in which, with the actuator (4) being correctly functioning, the armature (6) moves relative to the coil (5); - based on the determined measured current values (I n, I n+1), calculating a plurality of values of a time constant (τ) which describe the decay of the amplitude of the electric current over time, by means of the arithmetic unit (3); - by means of the arithmetic unit (3), calculating a decay coefficient (R) by forming the quotient from two calculated values of the time constant (τ); the arithmetic unit (3) is used to evaluate the decay coefficient (R) and to decide, on the basis of the calculated decay coefficient (R), whether the armature (6) has been moved.System according to claim 11, characterised in that the magnetically actuated actuator (4) is a linear actuator, the armature (6) of which has two end positions, the armature (6) being movable into the first end position by the magnetic field generated by the coil (5).System according to claim 12, characterised in that the armature (6) is spring-loaded and can be moved into the second end position by the spring force after the energization of the coil (5) has been switched off.System according to one of Claims 11 to 13, characterized in that the actuator (4) is part of a parking brake of a vehicle (F) and, as a function of the position of the armature (6), the actuator (4) locks the parking brake in the blocking position or releases the parking brake.

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