Hydraulic motor and brake control system and method of controlling the same

1. A hydraulic drive system for a wheeled vehicle comprising:

a hydraulic motor operably connected to at least one vehicle wheel, the hydraulic motor having a fluid input and a fluid output and operable at a motor activation pressure;

a hydraulic pump supplying fluid at a fluid pressure;

a reservoir tank fluidly connected to the pump to supply fluid to the pump;

a first hydraulic sub-circuit connecting the pump to the motor to provide a motor input pressure at the fluid input on the motor;

a hydraulic brake operably engaging the motor and operable at a brake deactivation pressure;

a brake sub-circuit connecting the pump to the brake;

the first hydraulic sub-circuit further comprising a high limit valve connected between the pump and the motor and a low limit valve connected fluidly parallel to the high limit valve and between the pump and the motor wherein:

the low limit valve restricts fluid flow to the motor after an initial pressure is reached;

the high limit valve restricts fluid flow to the motor until the brake deactivation pressure is reached; and

the initial pressure is less than the brake deactivation pressure; and

wherein operation of the hydraulic drive system restricts the rotation of the motor until the brake is operably disengaged from the motor.

2. The hydraulic drive system of claim 1 wherein the motor activation pressure is greater than the initial pressure and less than the brake deactivation pressure.

3. The hydraulic drive system of claim 1 wherein the brake sub-circuit is an open circuit from the pump to the brake.

4. The hydraulic drive system of claim 1 wherein the low limit valve and the high limit valve restrict fluid flow to the motor when the fluid pressure is between the brake deactivation pressure and the initial pressure.

5. A method of controlling fluid flow in a hydraulic circuit between a pump, hydraulic brake and a hydraulic motor, the hydraulic circuit driving a wheeled vehicle, the method comprising the steps of:

a) restricting the fluid flow from the pump to the motor after an initial pressure is reached;

b) restricting fluid flow from the pump to the motor until a motor activation pressure is reached; and

c) restricting fluid flow from the pump to the motor until a brake deactivation pressure is reached.

6. The method of claim 5 wherein step b) further includes allowing fluid flow from the pump to the motor after a brake deactivation pressure is reached, wherein the brake deactivation pressure is greater than the motor activation pressure.

7. The method of claim 5 further comprising the step of:

c) allowing unrestricted fluid flow from the pump to the brake at all pressures.

8. The method of claim 7 further comprising the step of:

d) placing the brake and motor in independent hydraulic sub-circuits.

9. A method of controlling fluid flow in a hydraulic circuit between a pump, hydraulic brake and a hydraulic motor, the hydraulic circuit driving a wheeled vehicle, the method comprising the steps of:

a) restricting the fluid flow from the pump to the motor after an initial pressure is reached;

b) allowing fluid flow from the pump to the motor after a motor activation pressure is reached; and

c) dampening variations in a motor input pressure of the motor by:

1) sensing the variations in motor input pressure;

2) providing a valve control signal that is responsive to the sensed variations in motor input pressure;

3) using the valve control signal to variably restrict flow of fluid from a fluid output from the motor when the motor input pressure is below a set pressure level and to enable flow of fluid from the fluid output of the motor when the motor input pressure is above the set pressure level; and

4) setting the set pressure level such that the variations in the motor input pressure are dampened.

10. The method of claim 9 further comprising the step of:

d) selecting the set pressure level such that free rotation of the motor is inhibited when the vehicle is operated on an inclined surface.