Resonant stator balancing of free piston machine coupled to linear motor or alternator
A beta-type free-piston Stirling cycle engine or cooler is drivingly coupled to a linear alternator or linear motor and has an improved balancing system to minimize vibration without the need for a separate vibration balancing unit. The stator of the linear motor or alternator is mounted to the interior of the casing through an interposed spring to provide an oscillating system permitting the stator to reciprocate and flex the spring during operation of the Stirling machine and coupled transducer. The natural frequency of oscillation, ω s , of the stator is maintained essentially equal to ω p 1 - α p k p and the natural frequency of oscillation of the piston, ω p , is maintained essentially equal to the operating frequency, ω o of the coupled Stirling machine and alternator or motor. For applications in which variations of the average temperature and/or the average pressure of the working gas cause more than insubstantial variations of the piston resonant frequency ω p , various alternative means for compensating for those changes in order to maintain vibration balancing are also disclosed.
1. An improved, beta-type Stirling machine, including a reciprocating displacer and a reciprocating piston, drivingly coupled to a linear electro-magnetic-mechanical transducer, including a stator having an armature winding, the displacer, piston and stator all mounted within a casing, the improvement comprising:
the stator being mounted to the interior of the casing through an interposed spring permitting the stator to reciprocate and flex the spring during operation of the Stirling machine and coupled transducer.
2. An improved Stirling machine and coupled transducer in accordance with claim 1 wherein the reciprocation of the piston, displacer and stator is along a common axis of reciprocation.
3. An improved Stirling machine and coupled transducer in accordance with claim 2 and further comprising means for varying the net spring constant of the spring interposed between the casing and the stator.
4. An improved Stirling machine and coupled transducer in accordance with claim 3 , the means for varying the net spring constant comprises a second spring also linking the stator to the casing, the second spring having an adjustable spring constant.
5. An improved Stirling machine and coupled transducer in accordance with claim 4 wherein the second spring comprises a gas spring having differential leakage valves including at least two oppositely directed, parallel connected check valves connected between a back space of the Stirling machine and a cylinder of the gas spring and at least one flow rate controlling valve in series with one of the check valves.
6. An improved Stirling machine and coupled transducer in accordance with claim 2 , the piston having a spring coupling between the piston and the casing, the piston to casing spring coupling having a net spring constant k p , wherein the Stirling machine and coupled transducer further comprises a means for varying the spring constant k p .
7. An improved Stirling machine and coupled transducer in accordance with claim 6 wherein the means for varying the spring constant k p comprises a means for translating the mean piston position.
8. An improved Stirling machine and coupled transducer in accordance with claim 7 wherein the means for varying the spring constant k p comprises a DC voltage source in series connection to the armature winding.
9. An improved Stirling machine and coupled transducer in accordance with claim 2 wherein the natural frequency of oscillation, ω s of the stator is essentially equal to
ω
p
1
-
α
p
k
p
and the natural frequency of oscillation of the piston, ω p , is essentially equal to the operating frequency, ω o of the coupled Stirling machine and transducer.
10. An improved Stirling machine and coupled transducer in accordance with claim 2 wherein the natural frequency of oscillation, ω s , of the stator is within 20% of
ω
p
1
-
α
p
k
p
and wherein the natural frequency of oscillation of the piston, ω p , is within 20% of the operating frequency, ω o of the coupled Stirling machine and transducer, wherein α p is the spring constant of spring coupling between the displacer and the piston and k p is the spring constant of spring coupling between the piston and the casing.
11. An improved Stirling machine and coupled transducer in accordance with claim 10 wherein the relationships of claim 9 are both within 10%.
12. An improved Stirling machine and coupled transducer in accordance with claim 11 wherein the relationships of claim 9 are both within 5%.
13. An improved Stirling machine and coupled transducer in accordance with claim 2 and further comprises a force transducer connected between the casing and the stator.
14. An improved Stirling machine and coupled transducer in accordance with claim 13 wherein the force transducer comprises a secondary linear motor.