Wheel slip correction system

The present invention relates to wheel slip correction systems and, more particularly, to wheel slip correction systems for DC electric motor driven vehicles having multiple motors driving multiple wheels of the vehicle.

Wheel slip correction systems commonly used in locomotive applications in DC electric traction motors are coupled in driving relationship to wheel/axle assemblies of the locomotive. Because the locomotive wheels are steel running on steel rails there is a high likelihood of one or more wheels slipping either from effects of load or rail conditions. Various methods have been used to overcome wheel slip and improve traction including mechanical methods, such as applying sand to a rail, and electrical methods such as by removing or decreasing electrical power to a motor driving the slipping wheel.

One method of electrically correcting for wheel slip is disclosed in U.S. Pat. No. 6,634,303. In this patent, a plurality of DC electric motors connected for driving respective wheels of a locomotive have their field windings shunted by a resistance so that at least some armature current bypasses the field winding so that the motors are normally operated in a field weakened mode. Upon detection of a wheel slip, a switch that is connected in series circuit with the resistance is opened so that full armature current is forced through the field winding of the motor associated with the slipping wheel. The increase in field current increases motor flux which in turn increase motor back EMF to thereby reduce the effective voltage at the motor armature causing a drop in motor current and a concomitant reduction in motor torque. Reduced torque allows the wheel to slow thereby correcting for the wheel slip. An obvious disadvantage of this system is the power loss in all the resistive loads connected in parallel with the field windings of each of the motors during normal operation, particularly since wheel slipping is a random, short-term event.

FIG. 1 is a simplified diagram of one form of individual motor control on a multi-motor propelled vehicle;

FIG. 2 is a simplified schematic representation of a motor control system for reducing torque on an individual motor basis when all motors are coupled to a common power source;

FIG. 3 is a modification of FIG. 2 using partial field shunting;

FIG. 4 is a modification of FIG. 2 using non-slipping wheel field shunting to correct wheel slip;

FIG. 5 is another embodiment using direct reduction of motor armature voltage without field shunting by inserting a voltage reduction apparatus in series with the armature;

FIGS. 6A–6C illustrate embodiments of the voltage reduction apparatus of FIG. 5 ;

FIG. 7 illustrates use of a chopper circuit for motor armature voltage reduction for wheel slip correction;

FIG. 8 illustrates use of the armature voltage apparatus of FIG. 5 as applied to a system having separate field control; and

FIG. 9 is an illustration of the system of FIG. 8 in which all motor fields are in series and powered from a single control.

Locomotives used for heavy haul applications and for passenger applications that are powered by DC electric traction motors are typically controlled on a whole vehicle basis. For example, a conventional diesel-electric locomotive uses a diesel engine to drive an alternator producing electric power. The alternator output is controlled to establish a desired voltage on a DC bus. All of the traction motors on the locomotive are connected to the DC bus and locomotive speed or power is controlled by varying the excitation on the DC bus so that all motors are concurrently effected. Similarly, any change in operation of one of the motors effects the operation of all other motors due to the alternator armature reaction. It is to be noted that the various embodiments disclosed herein while directed specifically to a locomotive are applicable to any vehicle having multiple driven wheels where some wheels can slip or slide with respect to others of the driven wheels. Further, since there are multiple wheels and multiple drives and controls. Terms used in the singular shall also be read to encompass the term as if it was stated in its plurality. This reasoning shall also apply in the inverse. For example, the use of the term “invention” shall be construed as also meaning the term “inventions” whereas the use of the term “inventions” shall also be construed to mean the term “invention.”

Although any one motor may not produce the exact same speed or torque for a given excitation voltage, in a locomotive application where each motor is connected to a gear box coupled to a wheel/axle set, the speed of each motor is constrained by the gear ratio and associated wheel diameter. Each wheel may experience different rail interface friction or adhesion characteristics. Accordingly, in order to achieve maximum tractive effort, it is desirable to control each motor separately or at least the motors in one truck should be controlled differently from other motors or groups of motors.

Various ways of controlling individual traction motors on a vehicle using multiple motors have been developed. As an example, the motor excitation voltage may be controlled on a per motor or per truck basis by using separate phase controlled rectifiers for each motor or group of motors to produce excitation that matches the motor torque to rail adhesion.

FIG. 1 is an example of an individual motor control proposed by applicant. The alternator used for electric power generation can be separated into multiple alternators 12 A to 12 N each supplying power to either a single traction motor 14 A– 14 N via corresponding rectifier circuits 16 A– 16 N or to groups of motors such as all motors on one truck. Each alternator 12 A– 12 N is regulated separately by a corresponding regulator 18 A– 18 N. In actual practice, a typical power alternator has multiple armature windings and multiple field controllers so that the alternators 12 A– 12 N may be multiple armature windings on a common shaft with common housing.

FIG. 2 illustrates another method for implementing individual motor control using a conventional excitation system in which the power alternator/rectifier 20 supplies DC electric power to a common bus 22 . Each motor 14 A– 14 F has an armature winding 24 A– 24 F (sometimes referred to herein as simply an “armature”) connected in series circuit with a corresponding field winding 26 A– 26 F (sometimes referred to herein as simply a “field”). In this embodiment, each field winding 26 is shunted by a controllable switching device 28 A– 28 F, such as a thyristor, that is serially connected with a field boost power source 30 A– 30 F. The power source may be a phase-controlled rectifier powered with AC voltage from an auxiliary winding of the alternator of source 20 or a DC to DC converter powered from a separate DC source such as the locomotive's battery. It is also possible to incorporate an additional field winding (not shown) into each motor and use the boost power source to energize this winding during a wheel slip. In the system of FIG. 2 , the field boost arrangement increases current through the field winding of a motor 14 driving a slipping wheel. The increased current increases motor flux causing an increase in armature back EMF that reduces the effective voltage on the armature (effective voltage is applied voltage minus the armature generated voltage or counter EMF) thereby reducing motor current and torque so that the slipping wheel can slow and regain traction. The advantage of the system of FIG. 2 is that the boost circuit is disconnected except during a wheel slip condition so that power usage is minimal.

FIG. 3 illustrates another form of field boost but eliminates the need for a separate power source. In this embodiment, a small portion of the motor field windings 26 A– 26 F are shunted by the controllable switching devices 32 A– 32 F. Each device 32 is normally conducting. When a wheel slip is detected, the device 32 associated with the motor driving the slipping wheel is open circuited forcing the motor current to pass through the normally bypass winding section. This increases motor flux and again reduces motor torque to allow the slipping wheel to slow and regain traction. This system has the advantage of reduced power drain during normal operation since the switching devices 32 absorb little power.

FIG. 4 shows still another embodiment of a wheel slip correction system that is operated in a manner to avoid power loss during normal locomotive operation. In this embodiment, each motor field winding 26 A– 26 F includes a field shunt circuit comprising a controllable switching device 34 A– 34 F in series with a current limiting resistor 36 A– 36 F. In prior art systems such as U.S. Pat. No. 6,634,303, a switching device and series resistance are connected in parallel with a motor field and the switching device is normally conductive so that power is dissipated in the resistance during normal motor operation, i.e, when an associated wheel is not slipping. The system of FIG. 4 avoids this problem by maintaining the devices 34 in a non-conductive mode during normal, non-wheel slip conditions. Upon detection of a wheel slip, the devices 34 associated with each wheel except the slipping wheel are gated into conduction so that the torque or speed of the non-slipping wheels are increased while at the same time a reduction in excitation voltage on the DC bus is implemented. The effect is to maintain vehicle speed while allowing the slipping wheel to recover. Since wheel slip is a transient effect with the correction system, power dissipation in the resistors 36 is minimized.

FIG. 5 illustrates another embodiment of wheel slip correction using voltage adjustment in the armature circuit. A device 40 A– 40 F is inserted in series with each motor 14 (shown between armature winding 24 and field winding 26 although it will be recognized that the device may be outside the motor). The devices 40 may take various forms, examples of which are shown in FIGS. 6A , 6 B and 6 C. Each device 40 is designed to reduce voltage applied to the motor armature so that motor current and torque are reduced so as to allow the wheel associated with the motor to regain adhesion and correct for wheel slip.

FIG. 6A illustrates a simple solution to reducing armature voltage by connecting a resistance 42 in series with the armature and power source. A switching device 44 , such as a thyristor, is connected in parallel with resistance 42 and controlled to normally be conductive to allow current to bypass the resistance. Upon detection of a wheel slip, device 44 is switched out of conduction forcing current through the resistance. The resulting IR drop reduces the voltage on the armature to achieve the effect described above.

FIG. 6B is a modification of the device of FIG. 6A that incorporates a capacitor 46 in parallel with the resistance 42 . A diode 48 is connected in series with the parallel combination of the resistance and capacitor. The switch 44 bypasses the diode and resistance-capacitor combination. When a wheel slip is detected, the switch 44 opens and the current diverts through the capacitor-resistance combination. The capacitor is selected so as to produce a high charge quickly to force a rapid reduction of armature voltage. The resistance provides a discharge path for the capacitor and could be controlled to dissipate the charge over a long time period.

Still another modification is shown in FIG. 6C in which the capacitor 46 is coupled to a separate charging source 50 . Source 50 may be the vehicle battery or a separate winding on the vehicle alternator. This circuit allows the amount of armature voltage reduction to be controlled by the magnitude of charge on the capacitor and also eliminates the capacitor charging time to effect a faster response. FIG. 6C also includes a second switch 52 bypassing the capacitor and a second diode 54 in series with the switch 44 . These two components allow the capacitor 46 to be charged with an opposite polarity to allow power to be inserted in series with the armature. It will be noted that while these systems are shown in relation to individual motors, in propulsion systems in which more than one motor is connected in parallel such as when all motors on one truck are controlled together, the device 40 could be used on a per truck rather than a per motor basis.

FIG. 7 illustrates a system in which the device 40 is replaced by a chopper circuit 56 to control the voltage applied to each motor. The use of the chopper circuit requires the addition of the flyback diodes 58 to allow inductive current to continue through the motor when the chopper circuit is switched off.

FIGS. 8 and 9 illustrate two embodiments of a series wound but separately excited DC motor in which the device 40 can be used to control armature current without changing field excitation. FIG. 8 shows a per motor control system while FIG. 9 shows a motor group control system. The advantage of FIG. 9 is that individual armature voltage can be controlled in a system in which multiple fields are powered from a single control unit. Any of the devices 40 of FIGS. 6A , 6 B and 6 C could be used in these embodiments. In both FIGS. 8 and 9 , the armatures of multiple motors are connected in parallel to a single power source thus simplifying the motor control scheme.

While the various embodiments of the invention have been described with reference to a method for correcting for a wheel slip or wheel slide condition (wheel slide occurring when one or more of the wheels slide on a rail and turn slower as opposed to a wheel slip in which a wheel turns faster, the slide condition normally occurring when the system is in a braking mode), it will be recognized that the ability to control individual wheels in the various embodiments also allows for adjustment of the speed of each wheel so as to correct for wheel diameter differences. In other words, a wheel diameter difference may cause a smaller diameter wheel to always be in a slipping mode if the same torque is applied to that wheel as compared to a slightly larger diameter wheel. Since wheels may wear differently and some wheels may be changed without changing other wheels on the vehicle, there is often some slight difference in wheel diameter among the several wheels of a locomotive or similar type multi-axle driven vehicle. The system disclosed herein allows continual compensation for such wheel diameter differences just as though such differences were caused by wheel slippage due to load or loss of traction.

While the invention have been described in what is presently considered to be the preferred embodiments, various modifications will become apparent to those skilled in the art. Accordingly, it is intended that the invention not be limited to the specific disclosed embodiments but be interpreted within the full scope and spirit of the intended claims.