DRIVE SYSTEM FOR VEHICLE PULLING A TOWED VEHICLE

A drive system is provided for a towing vehicle pulling a towed vehicle, such as a tractor pulling a scraper. The drive system includes a diesel engine which drives a generator for generating electrical power. Tractor drive motors are drivingly connected to driven wheels of the tractor. An assist drive motor is drivingly connected to driven wheels of the scraper. A power distribution unit controls distribution of electric power from the generator to the tractor and assist drive motors. A control unit controls the power distribution unit as a function of vehicle speed.

FIELD OF THE INVENTION

The present disclosure relates to a drive system for a vehicle pulling a towed vehicle.

BACKGROUND OF THE INVENTION

A tractor pulling a scraper is an example of a towing vehicle pulling a towed vehicle. Such earth moving towed scrapers are heavy and often operate in soft soils. This results in a rolling resistance which represents a large fraction of the tractor drawbar pull needed to load the scraper and climb steep grades. The weight carried by the scraper tires, combined with the soil conditions on which the machine is normally used, results in a potential for delivering tractive power to move the machine. If this tractive potential can be utilized, the drawbar pull needed from the tractor while loading the scraper and climbing steep grades can be reduced. A reduction in the maximum drawbar pull required allows ballast to be removed from the tractor and lowers the parasitic losses due to the rolling resistance of the entire tractor-scraper system. The operational advantage gained is twofold. Reduced parasitic losses allow for higher speeds and more productive operation during those portions of the operating cycle where full engine power can be utilized. There is also a fuel economy advantage during the entire cycle.

SUMMARY

According to an aspect of the present disclosure, a drive system is provided for a towing vehicle pulling a towed vehicle, such as a tractor pulling a towed scraper. The drive system includes a power generating unit on the tractor, such as a diesel engine which drives a generator for generating electrical power. Tractor drive motors are drivingly connected to driven wheels of the tractor. An assist drive motor is drivingly connected to driven wheels of the scraper. A power distribution unit controls distribution of electric power from the generator to the tractor drive motors and to the assist drive motor. A control unit controls the power distribution unit in response to a vehicle speed signal. The control unit, below a first speed threshold, causes a first greater portion of the power to be applied to the tractor drive motors and causing a second lesser portion of the power to be applied to the assist drive motor. The control unit, above a second speed threshold, causes all of the power to be applied to the tractor drive motors and causes none of the power to be applied to the assist drive motor.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring toFIG. 1, a drive system is provided for a towing vehicle or tractor10which pulls a towed vehicle12. The towing vehicle10may be an agricultural tractor. The towed vehicle12may be a pull-type scraper. The tractor10includes an engine14, such as a Diesel engine, which drives a gearbox16which drives an electric generator18. The generator18provides electrical power to a power electronics unit20. The power electronics unit20distributes electrical power to electric motor and final drive units22which drive corresponding vehicle wheels24. Alternatively, the tractor10may have an engine which drives a generator and which drives a mechanical transmission which provides mechanical power to driven wheels of the tractor. Alternatively, instead of a diesel engine driving a generator, the tractor10may have a known fuel cell and power electronics for generating electrical power. Thus, this invention requires a tractor10with some sort of electric drive train. This would normally be a Diesel electric system somewhat similar to a locomotive. The prime mover could be any sort of combustion engine or fuel cell of adequate capacity, and the electric power would normally be used to power the drive axles of the tractor10.

The towed vehicle12includes an electric assist motor30which receives electrical power from the power electronics unit20. The assist motor30drives an axle32which drives wheels34of the towed vehicle12.

As best seen inFIG. 2, the generator18is operatively connected to the power electronics unit20which is preferably a generator control/inverter. The generator control/inverter20is operatively connected to a left rear inverter40, a left front inverter42, a right front inverter44and a right rear inverter46, each of which drives a corresponding wheel drive motor22. The generator control/inverter20is also connected to an assist drive inverter48which provides electrical power and control to the dive motor30on the towed vehicle12. Units20and40-48are connected together by a DC bus50and by a CAN bus52. Units40-48are connected to drive motors22by four separate identical AC power buses54and by four separate identical M/G data buses56. Unit48is connected to drive motor30by AC power bus58and by an M/G data bus60.

An operator station control unit62receives a desired speed signal from a speed input device64, such as an operator-controlled speed control handle. The operator station control unit62receives a sensed ground speed signal from a speed sensor65. The sensed ground speed may be a ground speed calculated from axle rpm, or true ground speed as sensed by radar or GPS (not shown). The operator station control unit62may also receive information from a steering wheel66and a brake pedal66. Control unit62, a chassis control unit70, a drive train control unit72and an engine control unit74are all connected to the CAN bus52.

One or more of the control units executes the algorithm100illustrated by the flow chart shown inFIG. 3. The conversion of this flow chart into a standard language for implementing the algorithm described by the flow chart in a digital computer or microprocessor, will be evident to one with ordinary skill in the art.

In step102the ECU receives various inputs, including a desired speed signal from speed input device64.

In step104the ECU receives a sensed ground speed value from sensor65.

Then, step106determines the total power or pull required to move the tractor10and the towed vehicle12at the desired speed. Preferably, this determination is done by a speed control loop (not shown) which attempts to maintain a set speed as a function of the sensed ground speed from step104and the desired speed from the speed input device64. In some cases the operator would be given more direct control, perhaps with the position of a foot pedal being interpreted as 0 to 100% pull. The speed control loop, when used may be any of the commonly known types up to and including a PID (Proportional Integral Derivative) algorithm. This is all expressed in terms of pull since the speed ratio between engine and axles is allowed to vary along a constant engine power curve. Answering a request for more pull when the engine is already at maximum power entails slowing down but the speed control loop will recover to the requested speed before allowing the engine to drop below rated power. Also, in some applications, the ratio of pull to axle torque is close enough to constant so that axle torque can be used to determine pull without additional instrumentation.

Step108then determines how the total power from step106should be split between the tractor wheel drive motors22and the towed vehicle drive motor30. This function is illustrated graphically byFIG. 4and may be executed by control unit70. Referring now toFIG. 4, for ground speeds between zero and a first speed threshold (such as 8 miles per hour (mph)) the ECU maintains the percentage or portion of the electrical power or pull which is applied to the tractor drive motors22at a first tractor power level TP1, such as slightly less than 80%, and the ECU maintains the percentage or portion of the electrical power or pull which is applied to the assist drive motor30at a first assist power level AP1, such as slightly more than 20%. For ground speeds between at or above a second speed threshold (such as 10 mph) the ECU maintains the percentage or portion of the electrical power or pull which is applied to the tractor drive motors22at a second higher tractor power level TP2, such as 100%, and the ECU maintains the percentage or portion of the electrical power or pull which is applied to the assist drive motor30at a second lower assist power level AP2, such as zero %. For ground speeds between the first and second speed thresholds, the portion of the electrical power or pull which is applied to the tractor drive motors22preferably varies in a linear manner, as does the portion of the electrical power or pull which is applied to the assist drive motor30.

Step110then determines the required amount of tractor power or pull and the required amount of assist drive power or pull using the values determined by steps106and108.

Step112then generates commands fuel for engine14, current for generator18and currents for motors22and30in order to cause motors22and30to applied the amounts of power or pull determined in step110to the tractor10and to the towed vehicle, respectively.

The result is a drive system wherein an electric motor supplies propulsive power to an axle supporting a towed scraper to reduce the drawbar pull needed from the tractor that is pulling and powering the scraper. The electric power is diverted from the tractor electric drive train components to provide a power assist at low operating speeds. This reduces the need for ballast weights on the tractor, and reduces the empty weight of the machine.

If desired, the power or pull commands could be limited as a function of sensed parameters such as engine output, electric machine currents and the like.