Abstract:
Operative monitoring of track brakes involves passing an electric current through a winding of a brake magnet, measuring the electric current and comparing the temporal progression of the measured current with a saved temporal progression of a reference current. Comparison is achieved by calculating the difference between the measured current and the reference current. The difference between measured current and reference current may be temporally integrated and compared with a threshold value. To determine a magnetic coupling between track brake and track, a calculation may be made as to whether the measured current has local minima and/or local maxima during the activation of the track brake, only the temporal progression of the measured current as the current increases being subjected to a comparison with the reference current. The current may be activated in pulses, with the comparison of the measured current and the reference current being restarted with each pulse.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of priority to International Patent Application No. PCT/EP2009/004162 filed 10 Jun. 2009, which further claims the benefit of priority to German Patent Application No. 10 2008 029 31.1 filed 20 Jun. 2008, the contents of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    The disclosed embodiments relate to a method for the operative monitoring of track brakes for rail vehicles. Such a method is known from DE 101 55 143 A1. 
         [0003]    Modern rail vehicles are usually equipped, in addition to a pneumatic brake system, with a magnetic track brake which is embodied as an eddy current brake or magnetic track brake. 
         [0004]    In solid tracks, “suspension” is customary in which a brake magnet is held above the rails at a predetermined height of approximately 100 mm by springs. For the braking process, the spring forces are overcome by pneumatic activation cylinders, and the brake magnets are lowered onto the rail, into the work position, from the elevated position. At the same time, the brake is switched on electrically (cf. Wolfgang Hendrichs, “Das statische, dynamische and thermische Verhalten von Magnetschienenbremsen [The static, dynamic and thermal behavior of magnetic track brakes]”, Elektrische Bahnen [Electric Railways], eb, 86th year, issue July 1988, pp. 224-228). 
         [0005]    In traction vehicles, it is also possible to provide a combination of elevated suspension and low suspension. The magnets are then suspended from pressure cylinders or air bellows which are pressed by means of compressed air into the elevated position against a centering stop which is fixed to the truck. When the brakes are activated, the pressure cylinders or air bellows are then are vented, the magnets being lowered into the standby position. In urban network vehicles such as, for example, trams, low suspension is customary. When a magnetic track brake is in the braking position, the brake magnet is generally in frictional contact with the rail. 
         [0006]    In contrast, it is also the case that, in what is referred to as a linear eddy current brake, the brake magnet is held at a distance from the rail, electric solenoids magnetizing pole cores so that when an eddy current brake is switched on (and there is a relative movement of the eddy current brake with respect to the rail owing to the changes in the magnetic flux over time), eddy currents are induced in the travel rail. These eddy currents generate a secondary magnetic field which is opposed to the magnetic field of the eddy current brake. This results in a horizontal braking force which acts in opposition to the direction of travel. However, this requires there to be magnetic coupling between the rail and the brake magnet which depends essentially on the air gap between the brake magnet and the rail. 
         [0007]    In both types of magnet brakes, the effectiveness of the brake is essentially on the respectively correct distance between the brake magnet and rail. 
         [0008]    DE 101 55 143, therefore, proposes a diagnostic and monitoring device for monitoring the distance between the magnetic brake and the travel rail which uses a plurality of distance sensors which measure the air gap between the magnetic brake and the upper side of the rail. As a result, in both types of magnetic brakes it is possible to continuously check both whether the brake magnet is in the travel position and whether it is at the correct distance from the rail in the braking position. 
         [0009]    However, the sensors require additional expenditure and there is the risk that, in the event of failure or malfunction of the sensors, faults are not detected or inappropriate fault signals are generated. 
         [0010]    DE 100 09 331 C2 also proposes the use of sensors which measure the distance between the magnetic brake and the upper edge of the rail and, as a function of the measurement signal, open-loop/closed-loop control device which adjust the distance between the magnetic brake and the upper edge of the rail using actuators. 
       SUMMARY 
       [0011]    The object of the disclosed embodiments is to improve the method for the operative monitoring of track brakes of the type mentioned at the beginning with the effect that satisfactory functioning of the track brake can be checked without using distance sensors. This object is achieved by means of the features specified in patent claims. 
         [0012]    The disclosed embodiments are based on the basic idea that changes in current which are brought about by changes in the magnetic circuit permit an unambiguous conclusion to be drawn about the state assumed by the track brake/rail system. 
         [0013]    The magnetic coupling between the brake magnets and the rail forms a magnetic circuit and in the case of a magnetic track brake it is dependent on the distance between the brake magnet and the rail. The magnetic coupling, therefore, has an immediate reactive effect on the exciter current which flows through the winding of the brake magnet. If the exciter current is switched on during the lowering of the magnetic track brake, a significant change in the exciter current is obtained as soon as the magnetic coupling takes place. 
         [0014]    The disclosed embodiments, therefore, propose measuring the electric current through the winding of the brake magnet and comparing the time profile of the measured current with a stored time profile of a reference current. This makes available a signal which indicates whether a magnetic coupling has taken place between the brake magnet and the rail, which may be equivalent to the effectiveness or availability of the track brake. 
         [0015]    In an analogous fashion this also may apply to magnetic eddy current brakes in which the time profile of the exciter current depends on the magnetic coupling between the brake magnet and the rail. 
         [0016]    The changing of the magnetic coupling between the brake magnet and the rail may be detected by comparing the expected current profile (reference current) with a measured current profile. 
         [0017]    There is, therefore, no need for additional sensors or for the previously applied visual inspection. The measurement of the exciter current does not require any additional sensors but rather merely a measuring resistor in a control device. The evaluation of the time profile of the exciter current and the comparison with an expected current profile can be performed by a microprocessor in a brake control device. This makes it possible to determine the availability of the track brake or the braking effectiveness thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0018]    The disclosed embodiments will be explained in more detail below with reference to the drawing, in which: 
           [0019]      FIG. 1  shows a basic circuit diagram of a control device for actuating a magnetic track brake. 
           [0020]      FIG. 2  shows a measurement diagram of the time profile of the exciter current, for the cases of effective magnetic coupling and ineffective magnetic coupling between the brake magnet and the rail. 
           [0021]      FIG. 3  shows a diagram of the time profile of the exciter current when the exciter current is switched on in a clocked fashion. 
           [0022]      FIG. 4  shows a measurement diagram of the time profile of the exciter current with and without change in the magnetic coupling between the brake magnet and the track. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows a control device  1  for the actuation of brake magnets which are illustrated in the diagram in  FIG. 1  as inductors L 1  and L 2 . This control device  1  is supplied with electrical energy from an on-board power system such as, for example, a battery  2 , and is protected with respect to both poles of the battery by a fuse  3  and  4 , respectively. 
         [0024]    The two inductors L 1  and L 2  are located in a circuit with a power switch  5  which is actuated by a microprocessor  6  and which connects inductors L 1  and L 2  to the battery voltage of the battery  2 . The two inductors L 1  and L 2  are each in a series connection to shunt resistors R 1  and R 2 , respectively, the voltage drop of which is proportional to the current flowing through the inductors L 1  and L 2 , respectively. This voltage is tapped at the common connecting point between the inductors L 1  and L 2  and the shunt resistors R 1  and R 2  and is transmitted via measuring amplifiers  7  and  8 , respectively, to the microprocessor  6 . The microprocessor  6  is separated from the power component of the circuit by a galvanic isolating means  9 . 
         [0025]    The power switch  5  has a temperature sensor  13  which measures the temperature of the power switch  5  and signals said temperature to the microprocessor  6  via measuring amplifiers  14  and  15  across the galvanic isolating means  9 . 
         [0026]    A free-wheeling diode D 1 , which is assigned a shunt resistor R 3 , and a measuring amplifier  10 , which transmits the diode-monitoring signal to the microprocessor  6  across the galvanic isolating means  9 , lying parallel with the series circuit composed of the power switch  5 , inductors L 1 , L 2  and shunt resistors R 1  and R 2 . 
         [0027]    A pole of the power switch  5 , which is connected to the battery voltage (normally the positive pole) is connected to the microprocessor  6  via a further measuring amplifier  17  across the galvanic separating means  9 . The battery voltage can be measured and monitored in this way. 
         [0028]    The power switch  5  is switched by two signal, specifically an enable signal and a switching signal (ON/OFF) for switching the power switch  5  on and off. The two signals can be fed via external connections  11  and  12  to the control device  1 , specifically also in an expedient way via the galvanic isolating means  9 , wherein it is, of course, also possible to feed these signals to the microprocessor  6  which passes them onto its outputs via the galvanic isolating means. These two signals are logically linked to one another in an “AND” gate  18  and they control the power switch  5  and therefore the switching on and off of the inductors L 1  and L 2  for the brake magnets. 
         [0029]    On the side which is galvanically isolated from the power component (in particular power switch  5 ), there is, in addition to the microprocessor  6 , also a power supply component  19  which makes available a regulated voltage supply for the measuring amplifiers and for the actuation of the power switch  5 . The voltage supply of the specified components is also separated galvanically from the power supply component  19 . 
         [0030]    The current through the inductors L 1  and L 2  is continuously measured by the shunt resistors R 1  and R 2  and signaled to the microprocessor  6 . 
         [0031]    The free-wheeling diode D 1  is monitored by the measuring amplifier  10 , which is embodied as a comparator, and which measures a current through the free-wheeling diode D 1  as a voltage drop at the shunt resistor R 3 . The measuring amplifier  10  therefore transmits a digital signal to the microprocessor  6 . 
         [0032]    The temperature of the power switch is monitored by the temperature sensor  13  and is signaled as an analog signal from the measuring amplifiers  14  and  15  to the microprocessor  6 . The battery voltage is also monitored by the measuring amplifier  17  and is signaled as an analog signal to the microprocessor  6 . All the signals from and to the power component and all the supply voltages of the power component are galvanically isolated from the control component (microprocessor  6  and power supply component  19 ). 
         [0033]    The temperature of the brake magnets is monitored indirectly by means of the current through the inductors L 1  and L 2  in the steady state. The ohmic resistance of the of the inductors L 1  and L 2  changes significantly with the temperature so that given otherwise known parameters in the steady state and a constant current greater than 0 and a time derivative di/dt=0, the current permits conclusions to be drawn about the temperature of the brake magnets, and this is taken into account in the evaluation. 
         [0034]      FIG. 2  shows a diagram of the current (i) which flows through the inductors (brake magnet) L 1  and L 2 , as a function of the time (t). The current profile i 1  shows the case in which the brake magnet is in contact with the rail, while the current profile i 1  shows the case in which the brake magnet is not in contact with the rail. The two current profiles i 1  and i 2  differ significantly with respect to the rate of increase of the current. According to the basic concept of the invention, the rates of increase of the current are evaluated. It is therefore possible to determine whether the brake magnet is in contact with the rails solely by measuring the current across the shunt resistors R 1  and R 2  ( FIG. 1 ). 
         [0035]    According to one variant of the invention, the time profile of the increase in current with contact between the brake magnet and the rail according to curve i 2  in  FIG. 2  is stored in the microprocessor  6  as a “reference current”. This curve I 2  of the reference current can, according to a first variant of the invention, be determined individually for the individual track brake. According to another variant of the invention, it is also possible to define the curve I 2  for the reference current in a purely computational or theoretical fashion and to adapt it to the respective type of track brake or to the individual track brake through suitable standardization factors or by selecting threshold values which are explained below. 
         [0036]    The microprocessor  6  continuously forms the difference between the measured current profile, which is signaled via the shunt resistors R 1  and R 2  and the measuring amplifiers  7  and  8 , and the current profile of the reference current value, and it integrates the difference over time. If this integral of the difference of the current profiles exceeds a predetermined threshold value, the microprocessor  6  signals that there is no contact between the brake magnet and the track. If, conversely, this integral is below the threshold value, the microprocessor signals that there is contact and therefore the track brake is available and effective. However, conversely it is also possible to relate the curve I 2  for the reference current to a track brake which has not been applied and then to operate with inverse logic compared to the previously described case. 
         [0037]    The specified integral of the difference is preferably evaluated only in a predefined time interval which is shorter than the time period within which the current through the inductors L 1  and L 2  has reached its full saturation value or rated value. 
         [0038]    According to one implementation, which is illustrated purely schematically in  FIG. 3 , the switch-on process is divided along a plurality of switch-on pulses P 1  to P 4 , i.e. the power switch  5  in  FIG. 1  is clocked. This results in successive increases and drops in current until the full rated current is reached in phase P 4 . The previously described evaluation of the rates of increase in the current is carried out anew in each of the phases P 1  to P 4 , wherein this evaluation can also be carried out in a time interval which is shorter than phases P 1 , P 2  or P 3 . The evaluation is preferably performed at the start of the respective phase. 
         [0039]      FIG. 4  shows current profiles of the current through the inductors L 1  and L 2  as a function of the time. The curve i 3  shows a current profile of the switch-on current without a change in state of the magnetic coupling between the brake magnet and the rail. Depending on whether the magnetic coupling is present or not, the steepness according to curves i 1  and i 2  in  FIG. 2  is different. If a change in state of the magnetic coupling between the brake magnet and the rail takes place during the switch-on process, the current profile according to curve i 4  shows a significant change to the formation of local extreme values e min and e max. The moment (t 1 ) at which the magnetic coupling occurs, the value of the inductors L 1  and L 2  changes in the equivalent circuit diagram in  FIG. 1  so that the current reaches a local maximum value (e max) and then becomes smaller for a brief time and is renewed after reaching a local minimum e min at the time t 2 , and then increases somewhat more slowly. By evaluating the significant changes, in particular the local extreme values e max and e min in the current i 4 , it is therefore possible to detect a change in state of the magnetic coupling. If, for example as illustrated in  FIG. 4 , the exciter current i 4  for the inductors L 1  and L 2  is already switched on during the phase of the mechanical lowering of the brake magnet in the direction of the rail, the magnetic coupling is formed starting from a certain distance when the air gap between the brake magnet and the rail is reduced, and the current profile according to curve i 4  in  FIG. 4  is measured. 
         [0040]    According to one implementation, the evaluation according to the above method steps is respectively carried out anew after a local minimum e min has occurred. The rate of increase of the current is then therefore evaluated again starting from this time by integrating over time the difference between the stored setpoint curve (reference current) and the measured actual curve, wherein the evaluation can also be carried out in a predefined time window here. Only the positive rates of increase in the current are therefore compared. 
         [0041]    The abovementioned threshold value of the integral can be set as a function of the type of the track brake. 
         [0042]    Since in practice the measured current profiles and interference signals are superimposed, according to one development of the invention the signal of the measured current profile is filtered, specifically preferably with a low-pass filter whose limiting frequency is dependent on the type of track brake. The limiting frequency is of the order of magnitude of less than 50 Hz. 
         [0043]    An increase in the reliability of the detection of local extreme values is obtained by virtue of the fact that the difference between the integrals of the current values of the local extreme values corresponds to at least 30% of the difference between the integrals of the stored reference current, wherein the two integrals are each determined in the same time period. 
         [0044]    In order to detect the magnetic coupling between the brake magnet and the rail it is also possible to sense the time interval (t 2 −t 1 ) between the local extreme values and compare it to those of a standard signal. The time difference t 1 −t 2  within which the current profile varies between the local maximum e max and the local minimum e min is therefore evaluated. In this context, preferably only current profiles in which the difference Delta e between the local maximum e max and the local minimum e min exceeds a predefined limiting value are evaluated. 
         [0045]    In this context it is also possible in turn to determine the integral of the difference between the measured current profile and a stored reference current profile. 
         [0046]    The integral of the difference between the actual current profile and such a reference current profile, which is formed starting from the time of the first extreme value (e max in  FIG. 4 ) by means of an extrapolated part of a curve up to the point when the rated current I rate is reached, can also be formed. A threshold value which is dependent on the type of track brake can then also be formed again here. A magnetic coupling between the brake magnet and rail is only assumed to be present if this threshold value is exceeded. This evaluation of the extrapolated part of the curve can also be limited to a time window, in which case a threshold value which is dependent on the type of track brake then also has to be exceeded again. 
         [0047]    A further criterion may be that the current in the local minimum e min within a time window t 2 −x to t 2 +x exceeds a mean value of the current in this time window. The time period x depends on the type of track brake and is of the order of magnitude of less 1 second. 
         [0048]    A further criterion can be determined as follows. The difference Delta i between the measured current i 4  and the reference current i 3  is formed continuously, and this is illustrated in the curve Delta i in  FIG. 4 . The time t 3  of the maximum of the Delta i is determined from the profile of Delta i, and it is checked whether this is within a time period t 2 −x to t 2 +x. If this is the case, this is also a criterion for the fact that the magnetic coupling between the brake magnet and the track has taken place. The measurement diagram in  FIG. 4  shows that the maximum value of the difference max (Delta i), can also lie outside the time range t 1  to t 2 , for which reason it is appropriate to position said time window on both sides of t 2 . Instead of the difference it is also possible to determine and evaluate the integral of the difference and the maximum value thereof. 
         [0049]    Finally, faults can be eliminated on the basis of fluctuations in the battery voltage, a change in the temperature of the solenoids of the track brake and/or the temperature of the power switch  5  by virtue of the fact that the store setpoint curve of the reference current is standardized with respect to the battery voltage, the temperature of the solenoids of the track brake and/or the temperature of the power switch  5 . The battery voltage and the temperature of the power switch  5  are sensed, according to  FIG. 1 , by the measuring amplifier  17  for the battery voltage, and the measuring amplifiers  14  and  15  for the temperature of the power switch. The temperature of the solenoids of the track brakes is, as explained above, determined indirectly by measuring the steady-state current through the inductors L 1  and L 2 . A steady-state current is defined by virtue of the fact that the current is greater than 0 and the derivative of the current di/dt over time is equal to 0.