The present invention relates to control systems for electric motor powered traction vehicles such as locomotives, transit vehicles or off-highway vehicles and, more particularly, the invention relates to a method for controlling such a vehicle in a manner to correct for wheel slip or slide during propulsion and electrical retarding, respectively.
Locomotives and transit vehicles as well as other large traction vehicles are commonly powered by electric traction motors coupled in driving relationship to one or more axles of the vehicle. Locomotives and transit vehicles generally have at least four axle-wheel sets per vehicle with each axle-wheel set being connected via suitable gearing to the shaft of a separate electric motor commonly referred to as a traction motor. In the motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power (e.g., an engine-driven traction alternator) and apply torque to the vehicle wheels which exert tangential force or tractive effort on the surface on which the vehicle is traveling (e.g., the parallel steel rails of a railroad track), thereby propelling the vehicle in a desired direction along the right of way. Alternatively, in an electrical braking mode of operation, the motors serve as axle-driven electrical generators. Torque is applied to their shafts by their respectively associated axle-wheel sets which then exert braking effort on the surface, thereby retarding or slowing the vehicle""s progress. In either case, good adhesion between each wheel and the surface is required for efficient operation of the vehicle.
Maximum tractive or braking effort is obtained if each powered wheel of the vehicle is rotating at such an angular velocity that its actual peripheral speed is slightly higher (motoring) or slightly lower (braking) than the true vehicle speed (i.e., the linear speed at which the vehicle is traveling, usually referred to as xe2x80x9cground speedxe2x80x9d or xe2x80x9ctrack speedxe2x80x9d). The difference between wheel speed and track (or xe2x80x9cgroundxe2x80x9d) speed is referred to as xe2x80x9cslip speed.xe2x80x9d There is a relatively low limit value of slip speed at which peak tractive or braking effort is realized. This value, commonly known as maximum xe2x80x9ccreep speed,xe2x80x9d is a variable that depends on track speed and rail conditions. So long as the maximum creep speed is not exceeded, slip speed is normal and the vehicle will operate in a stable microslip or creep mode. If wheel-to-rail adhesion tends to be reduced or lost, some or all of the vehicle wheels may slip excessively, i.e., the actual slip speed may be greater than the maximum creep speed. Such a wheel slip condition, which is characterized in the motoring mode by one or more spinning axle-wheel sets and in the braking mode by one or more sliding or skidding axle-wheel sets, can cause accelerated wheel wear, rail damage, high mechanical stresses in the drive components of the propulsion system, and an undesirable decrease of tractive (or braking) effort.
Many different systems are disclosed in the prior art for automatically detecting and recovering from undesirable wheel slip conditions. Typically, differential speeds between axle-wheel sets or rate of change of wheel speed or a combination of these two measurements are used to detect wheel slip. Speed is monitored and if found to exceed predetermined differentials or rates of change, power to the motors is reduced in an attempt to bring speed to a value at which traction is regained. The disadvantage of such systems of wheel slip control is that the controlled variable, i.e., creep speed, is also the signal used to enable correction.
Among the several objects of the present invention may be noted the provision of a wheel slip and wheel slide correction system in which the monitored or trigger variable is different from the control variable, and a wheel slip and wheel slide system in which the variable used for detecting a wheel slip or wheel slide is a function of vehicle velocity while the variable used to correct for wheel slip or wheel slide is a function of wheel acceleration. In illustrative form, the invention is illustrated as a method for correcting for wheel slip or wheel slide in an electric traction motor propelled vehicle such as a locomotive. The locomotive includes a motor control system for establishing a torque request for controlling the torque produced by at least one electric traction motor coupled in driving relationship to at least one wheel of the locomotive. The inventive system obtains values corresponding to the actual vehicle velocity from auxiliary measuring devices and computes from that velocity corresponding wheel rotational velocity and wheel acceleration values for the vehicle. Tachometers are used to measure actual wheel velocity and the values of actual wheel acceleration are obtained as a first derivative of the wheel velocity values. The system compares actual wheel velocity to the equivalent computed wheel velocity to determine if the wheel creep speed is greater than desired. If creep speed is greater than some minimum value, a wheel slip or wheel slide signal is generated which actuates the torque correction circuit. A torque correction circuit computes the difference between the equivalent wheel acceleration value derived from actual vehicle velocity and compares that to the computed wheel acceleration derived by measuring wheel velocity. The difference between the measured wheel acceleration and the equivalent wheel acceleration is then applied as a correction factor to the torque request signal so as to adjust the actual motor torque in a manner to correct with wheel slip or slide conditions. In addition to the basic function as described above, the system also implements a substantial torque reduction function if the actual wheel velocity varies from the computed wheel velocity by more than some larger amounts such as, for example, 20% in propulsion or 30% in braking modes. The system also monitors the actual locomotive speed to inhibit operation of the torque correction circuit unless the locomotive speed actually exceeds some minimum velocity so as to prevent shutdown during initial start up of a locomotive from rest.
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