Hydraulic drive system with neutral drift compensation

1. A method associated with a hydraulic drive system having a high pressure storage device and a low pressure storage device, a pump-motor for converting between hydraulic energy and mechanical energy, the pump-motor disposed between the high pressure storage device and the low pressure storage device and including a swash-plate, and comprising the steps of:

displacing said swash-plate from a presumed neutral position when entering one of a motoring mode and a pumping mode;

exiting from said one of a motoring mode and a pumping mode, and said swash-plate moving back toward said neutral position;

adjusting said swash-plate to be within said neutral position before again commencing said displacing step;

sensing of said swash-plate position drifting; and

ending said adjusting step when said sensing is within an operational band about an absolute neutral.

2. A method as recited in claim 1 , further comprising a delay permitting a settling of said swash-plate upon completing said exiting.

3. A method as recited in claim 1 , wherein when said swash-plate is within said operational band no adjusting takes place.

4. A method as recited in claim 1 , wherein said adjusting step only takes place upon said exiting from said motoring mode.

5. A method as recited in claim 1 , wherein said adjusting step comprises the steps of:

taking a first pressure reading at a first time subsequent to said exiting step;

taking a second pressure reading at a second time subsequent to said exiting step and after said first time;

comparing a difference between said first pressure reading and said second pressure reading;

and moving said swash-plate in accordance with said difference.

6. A method as recited in claim 5 , wherein if said difference is positive, said moving is in a negative direction and wherein if said difference is a negative, said moving is in a positive direction.

7. A method as recited in claim 5 , comprising the step of performing said moving step incrementally.

8. A method as recited in claim 5 , comprising the step of ending said adjusting step when said swash-plate is in said neutral position.

9. A method associated with a hydraulic drive system having a high pressure storage device and a low pressure storage device, a pump-motor for converting between hydraulic energy and mechanical energy, the pump-motor disposed between the high pressure storage device and the low pressure storage device and including a swash-plate, and comprising the steps of:

displacing said swash-plate from a presumed neutral position when entering one of a motoring mode and a pumping mode, said neutral position being an operational band of values about an absolute neutral;

exiting from said one of a motoring mode and a pumping mode, and said swash-plate moving back toward said neutral position;

a delay permitting a settling of said swash-plate upon completing said exiting;

adjusting said swash-plate to be within said neutral position before again commencing said displacing step;

sensing of said swash-plate position drifting; and

ending said adjusting step when said sensing is within an operational band about an absolute neutral.

10. A method as recited in claim 9 , wherein said adjusting step only takes place upon said exiting from said motoring mode.

11. A method as recited in claim 10 , further comprising: said adjusting step further including

taking a first pressure reading at a first time subsequent to said exiting step,

taking a second pressure reading at a second time subsequent to said exiting step and after said first time,

comparing a difference between said first pressure reading and said second pressure reading,

and moving said swash-plate incrementally in accordance with said difference, completing said taking and said completing steps prior to each moving step; and

ending said adjusting step when said swash-plate is in said neutral position.

12. A method associated with a hydraulic drive system having a high pressure storage device and a low pressure storage device, a pump-motor for converting between hydraulic energy and mechanical energy, the pump-motor disposed between the high pressure storage device and the low pressure storage device and including a swash-plate, and comprising the steps of:

displacing said swash-plate from a presumed neutral position when entering one of a motoring mode and a pumping mode;

exiting from said one of a motoring mode and a pumping mode, and said swash-plate moving back toward said neutral position;

adjusting said swash-plate to be within said neutral position before again commencing said displacing step, said adjusting step including the sub-steps of taking a first pressure reading at a first time subsequent to said exiting step, taking a second pressure reading at a second time subsequent to said exiting step and after said first time, comparing a difference between said first pressure reading and said second pressure reading, and moving said swash-plate incrementally in accordance with said difference, completing said taking and said completing steps prior to each moving step; and

ending said adjusting step when said swash-plate is in said neutral position; and

sensing of said swash-plate position drifting, said step of ending said adjusting step completing when said sensing is within an operational band about an absolute neutral.

13. A method as recited in claim 12 , further comprising a delay permitting a settling of said swash-plate upon completing said exiting.

14. A method as recited in claim 12 , wherein when said swash-plate is within said operational band no adjusting takes place.

15. A method as recited in claim 12 , wherein said adjusting step only takes place upon said exiting from said motoring mode.