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A piston position drift control for a free-piston device. The control includes a passage connecting internal volumes of the device, the passage being substantially shorter than an acoustic wavelength of the device; and a check valve in the passage for controlling fluid communication between the inte

A piston position drift control for a free-piston device. The control includes a passage connecting internal volumes of the device, the passage being substantially shorter than an acoustic wavelength of the device; and a check valve in the passage for controlling fluid communication between the internal volumes, the check valve having an opening pressure not less than approximately 20% of a maximum pressure differential of the device at a maximum stroke. The control is passive, requires no active control once in service, provides low susceptibility to damage or fouling, and is amenable to adjustment or repair if needed. In addition, the control is small and inexpensive, and functions across the entire operational range of a free-piston device it supports. A related free-piston device including the piston position drift control is also provided.

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1. A piston position drift control for a free-piston device having a reciprocating piston with an imperfect seal between internal volumes adjacent to the piston, the control comprising:a passage connecting the internal volumes, the passage being substantially shorter than an acoustic wavelength of t

1. A piston position drift control for a free-piston device having a reciprocating piston with an imperfect seal between internal volumes adjacent to the piston, the control comprising:a passage connecting the internal volumes, the passage being substantially shorter than an acoustic wavelength of the device; and a check valve in the passage for controlling fluid communication between the internal volumes, the check valve having an opening pressure not less than approximately 20% of a maximum pressure differential of the device at a maximum stroke. 2. The piston position drift control of claim 1, wherein the opening pressure is not greater than approximately 50% of the maximum pressure differential of the device at the maximum stroke.3. The piston position drift control of claim 1, wherein the opening pressure is approximately 20 psi.4. The piston position drift control of claim 1, wherein the piston position is regulated in a range of +/?1 mm from a mean average position.5. The piston position drift control of claim 1, wherein the maximum pressure differential is approximately 75 psi.6. The piston position drift control of claim 1, wherein a flow rating of the check valve is such that the valve is not opening at an approximate low-range of piston stroke, increasingly opening in an approximate mid-range of piston stroke, and opening fully in an approximate high range of piston stroke.7. The piston position drift control of claim 6, wherein the valve allows increasing flow in the approximate mid-range of piston stroke due to increasing opening of the valve and in the approximate high range of piston stroke due to increasing pressure differential.8. The piston position drift control of claim 1, wherein the check valve allows flow in a direction that corrects for leakage across the imperfect seal.9. A free-piston device comprising:a reciprocating piston with an imperfect seal between internal volumes adjacent to the piston; a piston position drift control including a passage, between the internal volumes, that is substantially shorter than an acoustic wavelength of the device; and a check valve in the passage for controlling fluid communication between the internal volumes, the check valve having an opening pressure not less than approximately 20% of a maximum pressure differential of the device at a maximum stroke. 10. The free-piston device of claim 9, wherein the opening pressure is not greater than approximately 50% of the maximum operating pressure of the device at the maximum stroke.11. The free-piston device of claim 9, wherein the opening pressure is approximately 20 psi.12. The free-piston device of claim 9, wherein the piston position is regulated in a range of +/?1 mm from a mean average position.13. The free-piston device of claim 9, wherein the maximum pressure differential is approximately 75 psi.14. The free-piston device of claim 9, wherein a flow rating of the check valve is such that the valve is not opening at an approximate low-range of piston stroke, increasingly opening in an approximate mid-range of piston stroke, and opening fully in an approximate high range of piston stroke.15. The free-piston device of claim 14, wherein the valve allows increasing flow in the approximate mid-range of piston stroke due to increasing opening of the valve and in the approximate high range of piston stroke due to increasing pressure differential.16. The free-piston device of claim 9, wherein the check valve allows flow in a direction that corrects for leakage across the imperfect seal.17. A piston position drift control for a free-piston device having a reciprocating piston with an imperfect seal between internal volumes adjacent to the piston, the control comprising:means for connecting the internal volumes; and means for passively allowing fluid communication between the internal volumes when a pressure differential between the internal volumes is not less than approximately 20% of a maximum pressure differential of the device at a maximum stroke.