In a multi wheel hydraulic drive vehicle, it is preferable to connect all motors in parallel, with each motor connected by gearing to drive a single wheel. Because all motors are connected in parallel, all motor pressures are equal. If a wheel looses traction to the point where it starts to spin, it acts like a relief valve by limiting system pressure. If pressure requirements exceed the limited pressure available, the vehicle will stop and all available oil flow will be directed to the spinning wheel, thus severely limiting mobility under changing wheel traction conditions.

Various ways to control wheel spin have been provided in the prior art, such as mechanically applying a brake to a slipping wheel or hydraulically throttling flow to the slipping wheel. A disadvantage to the these methods is that braking or hydraulic throttling wastes power, converting it to heat. In addition, this causes overheated brakes or overheated hydraulic oil or larger oil coolers. Also present hydraulic control valves are expensive.

The invention provides a traction control system that reduces torque on a slipping, spinning wheel by detecting when a wheel is slipping and spinning and then reducing the torque applied to the slipping wheel by decreasing the hydraulic motor displacement.

Figure 1 is a view of a vehicle chassis with a schematic view of the electrical system of the inventive traction control system. Figure 2 is a schematic view of the hydraulic system used in the vehicle shown in Figure 1.

Figure 1 is a view of a chassis 10 of a vehicle that uses the inventive traction control system. The chassis 10 is mounted on a first wheel 12, a second wheel, 13, a third wheel 14, and a fourth wheel 15. The first wheel 12 is mechanically connected to a first motor 18, so that the first motor 18 drives the first wheel 12. The second wheel 13 is

SUBSTITUTE SHEET (RULE**

mechanically connected to a second motor 19. The third wheel 14 is mechanically connected to a third motor 20. The fourth wheel 15 is mechanically connected to a fourth motor 21. Each motor drives the corresponding wheel. The first, second, third, and fourth motors 18, 19, 20, 21 are variable displacement motors.

The first wheel 12 and the first motor 18 are mechanically connected to a first end of a front axle 25. The second wheel 13 and the second motor 19 are mechanically connected to a second end of the front axle 25. The third wheel 14 and the third motor 20 are mechanically connected to a first end of a rear axle 26. The fourth wheel 15 and the fourth motor 21 are mechanically connected to a second end of the rear axle 26. The front axle 25 and rear axle 26 are mechanically connected together by a chassis frame 27.

Figure 2 schematically illustrates the hydraulic interconnections of the first, second, third, and fourth motors 18, 19, 20, 21. A first valve 32, a second valve 33, a third valve 34, and a fourth valve 35 are in fluid connection in parallel to a hydraulic pump 30. The first valve 32, second valve 33, third valve 34, and fourth valve 35 are each two position four way valves which are able to switch the direction of flow through the lines connected to an output side of the valves. The first valve 32 is in fluid connection with a first actuator 38, which mechanically controls a swashplate 44 of the first motor 18. The second valve 33 is in fluid connection with a second actuator 39, which mechanically controls a swashplate 45 of the second motor 19. The third valve 34 is in fluid connection with a third actuator 40, which mechanically controls a swashplate 46 of the third motor 20. The fourth valve 35 is in fluid connection with a fourth actuator 41 , which mechanically controls a swashplate 47 of the fourth motor 21. The first motor 18, the second motor 19, the third motor 20, and the fourth motor 21 are in fluid connection in parallel to a hydrostatic pump 50.

In the electrical schematic illustrated in Figure 1 , a main control module is electrically connected to a steering angle sensor 56, a front solenoid control module 57, and a rear solenoid control module 58. The steering angle sensor 56 is connected to a steering mechanism 59. The front solenoid control module 57 is electrically connected to the first valve 32, a first speed sensor 60, the second valve 33, and a second speed sensor 61. The rear solenoid control module 58 is electrically connected to the third valve 34, a third speed sensor 62, the fourth valve 35, and a fourth speed sensor 63. In operation, the first speed sensor 60 is positioned to measure the speed of the first wheel 12. The second speed sensor 61 is positioned to measure the speed of the second wheel 13. The third speed sensor 62 is positioned to measure the speed of the third wheel 14. The fourth speed sensor 63 is positioned to measure the speed of the fourth wheel 15. The first speed sensor 60 measures the speed of the first wheel 12 and sends an electrical signal to the front solenoid control module 57. The second speed sensor 61 measures the speed of the second wheel 13 and sends an electrical signal to the front solenoid control module 57. The front solenoid control module 57 calculates the speed of the first wheel 12 and the second wheel 13 and sends a signal to the main control module 55. The third speed sensor 62 measures the speed of the third wheel 14 and sends an electrical signal to the rear solenoid control module 58. The fourth speed sensor 63 measures the speed of the fourth wheel 15 and sends an electrical signal to the rear solenoid control module 58. The rear solenoid control module 58 calculates the speed of the third wheel 14 and the fourth wheel 15 and sends a signal to the main control module 55. The steering angle sensor 56 measures the steering angle and sends an electrical signal to the main control module 55. The main control module 55 determines if the chassis is tuming and then whether any individual wheel or wheels are slipping.

If the steering angle sensor 56 sends a signal to the main control module 55, so that the main control module 55 determines the chassis 10 is not turning but is going substantially straight, the main control module 55 determines whether any of the first, second third, or fourth wheels 12, 13, 14, 15 is spinning faster than any other wheel by more than a set threshold amount. If any wheel is found to be going faster than any other wheel by the threshold amount, the motor corresponding to the faster wheel is de-stroked.

In the case shown in Figures 1 and 2, it is found that the third wheel 14 is spinning faster than the first, second, and fourth wheels 12, 13, 15, by more than the threshold difference. As a result, the main control module 55 sends a signals to the front solenoid control module 57 and the rear solenoid control module 58. The front solenoid control module 57 sends signals to the first valve 32 and the second valve 33. The signals from the front solenoid control module 57 to the first and second valves 32, 33, cause the first and second valves 32 and 33 to be in a forward position, which is a stroke position, as shown in Figure 2. The rear solenoid control module 58 sends signals to the third valve 34 and the fourth valve 35. The signals from the rear solenoid control module 58 to the third and fourth valves 34, 35, cause the third valve to be in a return position, which is a de-stroke position, as shown in Figure 2, and cause the fourth valve 35 to be in a forward position, which is a stroke position as shown in Figure 2. The hydraulic pump 30 provides a hydraulic current in the direction indicated by the flow arrows. Since the first valve 32 is in the forward position, hydraulic fluid flows through the first valve 32 as shown by the arrows in Figure 2, moving the piston in the first actuator 38 to a stroke position, which is to the left as viewed in Figure 2. Since the second valve 33 is in the forward position, hydraulic fluid flows through the second valve 33 as shown by the arrows in Figure 2, moving the piston in the second actuator 39 to a stroke position, which

is to the left as viewed in Figure 2. Since the third valve 34 is in the retum position, hydraulic fluid flows through the third valve 34 as shown by the arrows in Figure 2, moving the piston in the third actuator 40 to a de- stroke position, which is to the right as viewed in Figure 2. Since the fourth valve 35 is in the forward position, hydraulic fluid flows through the fourth valve 35 as shown by the arrows in Figure 2, moving the piston in the fourth actuator 41 to a stroke position, which is to the left as viewed in Figure 2. The piston of the first actuator 38 pushes the swashplate 44 of the first motor 18 into a stroke position as shown in Figure 2. The piston of the second actuator 39 pushes the swashplate 45 of the second motor 19 into a stroke position. The piston of the third actuator 40 pushes the swashplate 46 of the third motor 20 into a de-stroke position as shown in Figure 2. The piston of the fourth actuator 41 pushes the swashplate 47 of the fourth motor 21 into a stroke position as shown in Figure 2. The first motor 18, second motor 19, and fourth motor 21 are therefore positioned to provide maximum torque, and the third motor 20 is positioned to provide the minimum torque in this case.

The de-stroking of the third motor 20, causes the third motor 20 to provide less torque to the third wheel 14, which slows the spinning of the third wheel 14 until the third wheel 14 has a speed approximately equal to the speeds of the first, second, and fourth wheels 12, 13, 15. The first, second, third, and fourth speed sensors 60, 61 , 62, 63 will provide input to the front and rear solenoid control modules 57, 58, which will provide input to the main control module 55, which will calculate that the first, second, third, and fourth wheels 12, 13, 14, 15 are spinning at approximately equal speeds within the threshold speed. The main control module 55, sends a signals to the front solenoid control module 57 and the rear solenoid control module 58. The front solenoid control module 57 sends signals to the first valve 32 and the second valve 33. The signals from the front solenoid control module 57 to the first and second

valves 32, 33, cause the first and second valves 32 and 33 to be in the forward position. The rear solenoid control module 58 sends signals to the third valve 34 and the fourth valve 35. The signals from the rear solenoid control module 58 to the third and fourth valves 34, 35 cause the third and fourth valves 34, 35 to be in the forward position. This causes the first, second, and fourth actuators 38, 39, 41 to remain in the stroke position as shown in Figure 2. Since the third valve 34 is has been changed to the forward position, hydraulic fluid flows through the third valve 34 to move the piston in the third actuator 40 from the de-stroke position to the stroke position. The movement of the piston of the third actuator 40 moves the swashplate 46 of the third motor 20 from a de- stroke position to a stroke position to provide maximum torque to the third wheel 14. If the third wheel 14 speeds up again so that it is faster than the other wheels, the cycle is repeated until the slipping third wheel 14 regains traction.

The main control module 55 may be provided with a delay, to prevent a high frequency cycling between stroking and de-stroking.

If the signal from the steering angle sensor 56 indicates to the main control module 55 that the chassis 10 is tuming, the main control module 55 determines the turning radius for each wheel. In addition, a look-up table is used to determine speed tolerances for the turning angle. The wheel speeds are compared according to the turning radius of each wheel. If a wheel with a shorter tuming radius is going faster than a wheel with a longer turning radius and the tolerance specified in the look up table, the wheel with the shorter tuming radius is de-stroked. In addition, if a wheel with the largest turning radius is going faster than the wheel with the second largest turning radius by a tolerance specified in the look up table, the wheel with the largest turning radius is de-stroked. The invention may also have a look-up table for thresholds according to vehicle speeds. This would have a higher threshold for

lower speeds to prevent mechanical backlash of wheels at low speeds and to prevent the providing of false signals.

The invention does not use hydraulic or mechanical resistance to slow a slipping wheel, thus does not waste energy and does not produce heat which can cause hydraulic or mechanical failure. The invention removes power from a slipping wheel and provides this power to the wheels that are not slipping.

The main control module 55, the front solenoid control module 57, and the rear solenoid control 58 form a controller 66 which can be configured in three modules as shown in the preferred embodiment or as one or two modules. The controller 66 may be an electrical controller as shown in the preferred embodiment, such as one or more computers with computer readable memory for directing the computer to perform the desired operations, or may be an analogous hydraulic or mechanical system. In the preferred embodiment, the front solenoid control module 57 was electrically connected to the first speed sensor 60, since both the front solenoid control module 57 and the first speed sensor 60 are electrical. This electrical connection is more generally described as being "in communication with" which in addition to covering electrical connections also covers ways to communicate by radio waves, fiber optics, and mechanical means.

In another embodiment, another algorithm may be used such as averaging the speed of the wheels and determining if one of the wheels is spinning at a speed different than the average by more than a threshold amount.

While the preferred embodiment of the present invention has been shown and described herein, it will be appreciated that various changes and modifications may be made therein without departing from the spirit of the invention as defined by the scope of the appended claims.