Patent Application
20150053435
The present disclosure relates to a drive system for a vehicle pulling a towed vehicle.
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.
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.
Referring to
The towed vehicle 12 includes an electric assist motor 30 which receives electrical power from the power electronics unit 20. The assist motor 30 drives an axle 32 which drives wheels 34 of the towed vehicle 12.
As best seen in
An operator station control unit 62 receives a desired speed signal from a speed input device 64, such as an operator-controlled speed control handle. The operator station control unit 62 receives a sensed ground speed signal from a speed sensor 65. 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 unit 62 may also receive information from a steering wheel 66 and a brake pedal 66. Control unit 62, a chassis control unit 70, a drive train control unit 72 and an engine control unit 74 are all connected to the CAN bus 52.
One or more of the control units executes the algorithm 100 illustrated by the flow chart shown in
In step 102 the ECU receives various inputs, including a desired speed signal from speed input device 64.
In step 104 the ECU receives a sensed ground speed value from sensor 65.
Then, step 106 determines the total power or pull required to move the tractor 10 and the towed vehicle 12 at 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 step 104 and the desired speed from the speed input device 64. 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.
Step 108 then determines how the total power from step 106 should be split between the tractor wheel drive motors 22 and the towed vehicle drive motor 30. This function is illustrated graphically by
Step 110 then determines the required amount of tractor power or pull and the required amount of assist drive power or pull using the values determined by steps 106 and 108.
Step 112 then generates commands fuel for engine 14, current for generator 18 and currents for motors 22 and 30 in order to cause motors 22 and 30 to applied the amounts of power or pull determined in step 110 to the tractor 10 and 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.
While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it being understood that illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected. It will be noted that alternative embodiments of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations that incorporate one or more of the features of the present disclosure and fall within the spirit and scope of the present invention as defined by the appended claims.
