Patent Application
20090219050
The invention concerns a method of detecting a fault in an electrical component carried by a rotary shaft of an electromagnetic retarder. The invention also concerns such an electromagnetic retarder.
The invention applies to a retarder capable of generating a retarding resisting torque on a main or secondary transmission shaft of a vehicle that it equips, when this retarder is actuated.
Such an electromagnetic retarder comprises a rotary shaft that is coupled to the main or secondary transmission shaft of the vehicle in order to exert on it the retarding resisting torque in particular for assisting the braking of the vehicle.
The retarding is generated with field coils supplied with DC current in order to produce a magnetic field in a metal piece made from ferromagnetic material, in order to make eddy currents appear in this metal piece.
The field coils can be fixed so as to cooperate with at least one metal piece made from movable ferromagnetic material having the general appearance of a disc rigidly secured to the rotary shaft.
In this case, these field coils are generally oriented parallel to the rotation axis and disposed around this axis, facing the disc, while being secured to a fixed plate. Two successive field coils are supplied electrically in order to generate magnetic fields in opposite directions.
When these field coils are supplied electrically, the eddy currents that they generate in the disc through their effects oppose the cause that gave rise to them, which produces a resisting torque on the disc and therefore on the rotary shaft, in order to slow down the vehicle.
In this embodiment, the field coils are supplied electrically by a current coming from the electrical system of the vehicle, that is to say for example from a battery of the vehicle. However, in order to increase the performance of the retarder, recourse is had to a design in which a current generator is integrated in the retarder.
Thus, according to another design known from the patent documents EP0331559 and FR1467310, the electrical supply to the field coils is provided by a current generator comprising primary stator coils supplied by the vehicle system, and secondary rotor coils fixed to the rotating shaft, and defining three electrical phases. The field coils are fixed to the rotating shaft while being radially projecting, in order to generate a magnetic field in a fixed cylindrical jacket that surrounds them.
A rectifier such as a diode bridge rectifier is interposed between the secondary rotor windings and the field coils, while also being carried by the rotary shaft. This rectifier converts the three-phase alternating current delivered by the secondary windings of the generator into a direct current supplying the field coils.
Two radially acting field coils consecutive around the rotation axis generate magnetic fields in opposite directions, one generating a field oriented centrifugally, the other a field oriented centripetally.
In operation, the electrical supply to the primary coils enables the generator to produce the supply current to the field coils, which gives rise to eddy currents in the fixed cylindrical jacket so as to generate a resisting torque on the rotary shaft, which slows the vehicle.
In order to reduce the weight and increase further the performance of such a retarder, it is advantageous to couple it to the transmission shaft of the vehicle by means of a speed multiplier, in accordance with the solution adopted in the patent document EP1527509.
The rotation speed of the retarder shaft is then multiplied compared with the rotation speed of the transmission shaft to which it is coupled. This arrangement significantly increases the electrical power delivered by the generator and therefore the power of the retarder.
In the event of malfunctioning of the current rectifier, the electric power transmitted to the field coils decreases, which results in a reduction in the retarding torque that can be exerted by the retarder.
Such a malfunctioning of the retarder may be partial, that is to say concern only one of the electrical phases of the current delivered by the secondary windings, which is then not converted by the rectifier.
The generator being for example of the three-phase type, in this case the retarding torque available drops by approximately one third of its nominal value, so that the driver of the vehicle is not necessarily aware of this drop, all the more so since such a retarder is generally used to supplement a traditional braking system, which makes the difference even less perceptible.
Such a retarder may also be controlled by means of a central processing unit that, from braking commands exerted by the driver, distributes the power demanded of the traditional brakes and that demanded of the retarder. In this case, the driver may not directly note a drop in the retarding torque supplied by the retarder.
In addition, the detection of a malfunctioning of the bridge rectifier or another electrical component carried by the rotary shaft by means of electrical sensors or the like mounted on the rotary shaft requires transmitting data from the rotary shaft to fixed parts of the retarder, which leads to complex solutions.
The aim of the invention is to propose a solution for detecting at lower cost a malfunctioning of an electrical component carried by the rotary shaft.
To this end, the object of the invention is a method of detecting a fault in an electrical component carried by a rotary shaft of an electromagnetic retarder, this retarder comprising primary stator coils, a control box for injecting into these coils a current having an intensity corresponding to a theoretical intensity dependent on a set intensity value, a sensor delivering a signal representing an actual intensity value of the current flowing in these primary coils, a rotary shaft carrying secondary windings defining several phases and field coils as well as a current rectifier interposed between the secondary windings and the field coils, this method consisting of comparing, in the control box, the theoretical intensity and the actual intensity so as to identify a fault in the event of a difference between the theoretical intensity and the actual intensity greater than a threshold value.
The invention thus makes it possible to identify the presence of an electrical problem at the electrical component carried by the rotary shaft simply by analysis of the electrical behaviour of the primary coils when they are excited. It is thus not necessary to provide a device for the transmission of data between the rotary shaft and a fixed part of the retarder, which makes it possible to use a fault detector having a very simple design.
The invention also concerns a method as defined above, consisting of determining a difference between the theoretical intensity and a minimum or maximum value taken by the actual intensity of the current actually passing through the primary coils over a predetermined interval of time.
The invention also concerns a method as defined above in which the theoretical intensity is determined in the control box from the set intensity value and data representing a transfer function of the retarder.
The invention also concerns a method as defined above, consisting of taking into account the set intensity value as the value representing the theoretical intensity.
The invention also concerns a method as defined above, consisting of slaving, from the control box, the current injected into the primary coils to the signal delivered by the current sensor, and providing primary coils having a time constant three times greater than the time constant of the secondary coils.
The invention also concerns a method as defined above, consisting of slaving, from the control box, the current injected into the primary coils to the signal delivered by the sensor, with a slaving having a reaction time sufficiently long to be insensitive to a fault in an electrical component carried by the rotary shaft.
The invention also concerns a method as defined above, consisting of providing a slaving having a cutoff frequency Fc satisfying the relationship Fc<1/3.2.pi.T2, in which Fc is expressed in hertz and in which T2 is the time constant of the secondary winding expressed in seconds.
The invention also concerns a method as defined above, consisting of using inductive measuring turns as an actual current sensor.
The invention also concerns an electromagnetic retarder comprising primary stator coils, a control box for injecting into these primary coils a current having an intensity corresponding to a theoretical intensity dependent on a set intensity value, a sensor delivering a signal representing an actual intensity value of the current flowing in these primary coils, a rotary shaft carrying secondary windings defining several phases and field coils as well as a current rectifier interposed between the secondary windings and the field coils, and means of comparing the theoretical intensity with the actual intensity in order to identify an operating fault in an electrical component carried by the rotary shaft in the event of a difference between the theoretical intensity and the actual intensity greater than a threshold value.
The invention also concerns an electromagnetic retarder as defined above, comprising means of slaving the current injected into the primary coils to the signal delivered by the sensor, and primary coils having a time constant greater that three times the time constant of the secondary windings.
The invention also concerns an electromagnetic retarder as defined above, comprising means of slaving the current injected into the primary coils to the signal delivered by the sensor, in which this slaving has a cutoff frequency Fc satisfying the relationship Fc<1/3.2.pi.T2, in which Fc is expressed in hertz and in which T2 is the time constant of the secondary windings expressed in seconds.
The invention also concerns an electromagnetic retarder as defined above in which the sensor comprises one or more measuring field turns wound with the primary coils.
The invention will now be described in more detail and with reference to the accompanying drawings, which illustrate an embodiment thereof by way of non-limitative example.
In
This casing 2, which is fixed, encloses a rotary shaft 7 that is coupled to a transmission shaft, not visible in the figure, such as a main transmission shaft to the vehicle wheels, or secondary such as a secondary gearbox output shaft via the speed multiplier 6. In a region corresponding to the inside of the cover 3 a current generator is situated, which comprises fixed or stator primary coils 8 that surround rotor secondary windings, secured to the rotary shaft 7.
These secondary windings are shown symbolically in
An internal jacket 9, cylindrical in shape overall, is mounted in the main casing 2, being slightly spaced apart radially from the external wall of this main casing 2 in order to define a substantially cylindrical intermediate space 10 in which a cooling liquid of this jacket 9 circulates.
This main casing, which also has a cylindrical shape overall, is provided with a channel 11 for admitting cooling liquid into the space 10 and a channel 12 for discharging cooling liquid out of this space 10.
This jacket 9 surrounds several field coils 13, which are carried by a rotor 14 rigidly fixed to the rotary shaft 7. Each field coil 13 is oriented so as to generate a radial magnetic field while having an oblong shape overall extending parallel to the shaft 7. The various field coils 13 are interconnected with each other so as to form a dipole.
In a known fashion, the jacket 9 and the body of the rotor 14 are made from ferromagnetic material. Here the casing is a castable piece based on aluminium and sealing joints intervene between the casing and jacket 9; the cover 3 and the piece 4 are perforated.
The field coils 13 are supplied electrically by the rotor secondary windings 5 of the generator via a bridge rectifier carried by the rotary shaft 7. This bridge rectifier can be the one that is marked 15 in
In the example in
Thus the first phase SA is connected to the two diodes 15A and 15D, which are connected respectively to the first and second terminal of the load 13. The second phase 5B is connected to the diodes 15B and 15E, which are themselves connected respectively to the first and second terminal of the load 13. The third phase is connected to the diodes 15C and 15F, which are themselves connected respectively to the first and second terminal of the load 13.
In operation, each arm of the rectifier delivers in the load 13 a current having the appearance of the sinusoidal positive parts of the voltage signal of the phase corresponding to this arm, this current being zero when the voltage in question is negative.
The three phases being offset with respect to one another by a third of a period, they deliver in the load a substantially constant current, having an appearance corresponding to the sum of the positive parts of the sinusoids of the three phases.
As can be seen in
Bringing the retarder into service consists of injecting into the primary coils 8 an excitation current coming from the electrical system of the vehicle and in particular the battery, so that the current generator delivers an induced current on its secondary windings 5. This current then supplies the field coils 13 in order to produce a resisting torque retarding the vehicle.
The excitation current is injected into the primary coils 8 by means of a control box 19, shown in
This control box 19 comprises an input able to receive a control signal representing a level of retarding torque demanded of the retarder.
This input can be connected to a lever or the like that is actuated directly by a driver of the vehicle. This lever may be able to move gradually between two extreme positions, namely a maximum position corresponding to a demand for maximum resisting torque and a minimum position in which the retarder is not acted on.
When the driver places this lever in an intermediate position, the retarder is controlled by the box 19 in order to exert on the rotary shaft 7 a resisting torque proportional to the position of the lever, with respect to the maximum retarding torque available. In other words, the input of the control box 19 receives a control signal that corresponds to a value lying between zero and one hundred percent.
This input can also be connected to a braking control box that autonomously determines a control signal for the retarder. This braking control box is then connected to one or more braking actuators that the vehicle has. In this case, the driver does not act directly on the retarder but it is the braking control box that, from the various parameters, controls the retarder and the traditional brakes of the vehicle.
The control box 19, visible in
On reception of a control signal corresponding to a non-zero value, the control box 19 determines a set intensity value Ci of the excitation current to be injected into the primary coils 8, and applies, via its module PU, to the primary coils 8, a voltage U for injecting a current corresponding to this set intensity Ci.
The current injected into the primary coils 8 has a theoretical intensity It that increases until it reaches the set value Ci. The level of the theoretical current It is determined in the control box from a transfer function Ft that depends in particular on the inductance and electrical resistance of the primary coils 8 so as to represent the electrical behaviour of the primary coils in transient mode.
As visible in
A difference between the theoretical current It and the actual intensity le greater than a predetermined value signifies a malfunctioning of an electrical component of the rectifier 15, such as in particular the destruction of a diode.
This is because, when a diode is defective, it becomes permanently either electrically conductive or non-conductive. This causes an electrical imbalance in the three phases 5A, 5B and 5C of the secondary windings 5, which generates a so-called mutual current in the primary coils 8.
This phenomenon is visible in the graph in
As can be seen in this figure, the mutual currents resulting from this defective diode interfere with the current passing through the primary coils. Thus, instead of having a substantially constant appearance, the current Ie actually flowing in the primary coils 8 has a sinusoidal appearance of high amplitude. This sinusoid has a frequency linked to the speed of the rotary shaft 7.
In normal operation of the retarder, the actual current curve le is substantially merged with the theoretical current curve It.
Thus the detection from the control box 19 of a difference between the actual current Ie and the theoretical current It greater than a predetermined value makes it possible to detect a fault in the rectifier 15 mounted on the rotary shaft 7. This detection is made without contact, that is to say without having to transmit data issuing from sensors mounted on the rotary shaft 7 to a fixed part of the retarder.
The predetermined difference value is advantageously twenty percent of the value of the theoretical current It since, as can be seen in
Basing the fault detection on a comparison of the actual current le with the theoretical current It makes it possible in particular to effect a pertinent detection including when the retarder is in transient mode.
It is also possible to provide a detection based on a comparison of the actual current le with the set current value, provided that the retarder is in continuous operation.
In the case in
Because of the sinusoidal oscillations caused by the mutual currents resulting from a defective diode, the comparison of the theoretical current It with the actual intensity le can consist of determining the maximum or minimum value taken by the actual intensity le for a predetermined period corresponding to several rotation periods of the shaft 7 and comparing this maximum or minimum with the set value Ci.
As shown schematically in
The control box comprises, in the aforementioned manner, power electronics PU controlled by a corrector CR so as to inject the excitation current Ii into the primary coils 8, which gives rise to the current induced in the secondary windings 5. The actual intensity le is subtracted at 50 from the set intensity value Ci in order to constitute an input signal for the corrector CR controlling the power electronics PU.
When the corrector receives a negative signal as an input, it controls the power electronics PU in order to reduce the current injected and, when it receives a positive signal as an input, it controls the power electronics in order to increase the current injected.
As shown schematically in
The theoretical current It is determined in the control box 15 from the set value Ci, on the basis of the transfer function Ft that in particular represents the intensity response of the primary coils 8 to the application of a voltage U.
In order to ensure reliable detection of a fault in a diode, the slaving of the injected current does not compensate for the disturbances due to the mutual currents in the case of a defective diode.
This can be obtained by sizing the primary coils so that they have a time constant T1 greater than N times the time constant T2 of the secondary windings 5, N designating a natural integer. Advantageously N is chosen greater than or equal to 3 so that this time constant T1 is greater than three times the time constant T2 so as to ensure optimal independence of the detection.
This can also be obtained by the choice of a sufficiently slow slaving vis-à-vis the frequency of the oscillations due to the mutual currents. Such a slaving is thus insensitive to the disturbances introduced by a malfunctioning of an electrical component carried by the rotary shaft. In this case, the slaving of the injected current is chosen so as to have a cutoff frequency Fc satisfying the relationship Fc<1/(2.N.pi.T2), in which Fc is expressed in hertz and T2 in seconds, pi representing the number having a value close to 3.14. In a similar manner, N is a natural integer that is advantageously chosen as equal to three.
The invention thus makes it possible to detect, without contact, a fault in an electrical component of the rotor, this component being able to be a diode or a transistor of the rectifier 15, but this component also being able to be a secondary winding 15A, 15B or 15C.
The example described above concerns a retarder in which the generator comprises three-phase secondary windings, but the invention also applies to a retarder comprising secondary windings having a different number of phases, equal at a minimum to two.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0553939 | Dec 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2006/002750 | 12/15/2006 | WO | 00 | 7/7/2008 |
