Apparatus and method for controlling a cryocooler by adjusting cooler gas flow oscillating frequency

• US 7,412,835 B2
• Filed: 06/27/2005
• Issued: 08/19/2008
• Est. Priority Date: 06/27/2005
• Status: Active Grant

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Legall, Edwin L., Jiang, Longzhi, Derakhshan, Mark, Mangano, Roy A.
• Primary Examiner(s): DOERRLER, WILLIAM CHARLES
• Application Number:
  US11/167,850
• Publication Number:
  US 20060288710A1
• Time in Patent Office: 1,149 Days
• Field of Search: 62/6, 622/281
• US Class Current: 62/6
• CPC Class Codes: F25B 2309/1411 Pulse-tube cycles characterised by control details, e.g. tuning, phase shifting or general control
• CPC Class Codes: F25B 2309/1428 Control of a Stirling refrigeration machine
• CPC Class Codes: F25B 2400/17 Re-condensers
• CPC Class Codes: F25B 9/10 with several cooling stages
• CPC Class Codes: F25B 9/14 characterised by the cycle used, e.g. Stirling cycle
• CPC Class Codes: F25B 9/145 pulse-tube cycle
• CPC Class Codes: F25D 19/00 Arrangement or mounting of refrigeration units with respect to devices or objects to be refrigerated, e.g. infrared detectors
• CPC Class Codes: F25D 19/006 Thermal coupling structure or interface
• CPC Class Codes: F25D 2400/02 Refrigerators including a heater
• CPC Class Codes: F25D 29/00 Arrangement or mounting of control or safety devices
• CPC Class Codes: G05D 23/1919 characterised by the type of controller

Abstract

Systems and methods are provided for a variable cooling capacity cryocooler for a superconducting magnetic resonance imaging device having a liquid cryogen pressure vessel to provide cryogenic temperatures to a magnet assembly, a vacuum vessel surrounding the pressure vessel and a radiation shield spaced from the cryogen pressure vessel, and a pressure sensor positioned inside the cryogen pressure vessel pressure boundary for sensing pressure variations. A controller for varying the heat removal rate of the cryocooler based on the pressure variations in the cryogen pressure vessel and where the cooling capacity of the cryocooler is adjusted by modifying the speed of the electric power drive (DC or AC motors) or by changing the mechanical transmission ratio between the constant speed electric power drive and the cryocooler displacer/piston to adjust the cooler oscillating frequency of the gas flow. The invention can be adapted to magnets or non magnets systems using Stirling, Gifford-McMahon (“GM”) and Pulse Tube (“PT”) cooler systems. The system will extend component end of life and provide a controllable constant pressure improving image quality for superconducting MRI magnets.

Claims

1. An apparatus for adjusting stroke frequency of a cryocooler as a function of pressure in a cryogen pressure vessel, the apparatus comprising:

a generator for setting an initial stroke rate for the cryocooler by setting alternating or direct current motor speed;

a difference measuring device for continuously measuring in each cycle of operation a pressure difference between a measured pressure and a set pressure;

a controller for calculating in each cycle of operation a required stroke rate or a frequency adjustment for the cryocooler from the measured pressure difference;

a variable speed module for combining in each cycle of operation the stroke rate adjustment with the initial stroke rate to adjust the stroke rate or frequency by adjusting the alternating or direct current motor speed in order to optimize operations of the cryocooler; and

a thermal gasket operably coupled between the cryocooler and a recondenser to provide a thermal interface, and pressure controller to selectively adjust a mechanical pressure across the thermal interface, a the pressure controller including a display to indicate a temperature drop across the thermal interface;

wherein the variable speed module increases the cryocooler stroke rate by increasing the alternating or direct current motor speed when the measured pressure is greater than the set pressure.

2. The apparatus of claim 1, wherein the controller further comprises a proportional integral derivative controller that calculates the cryocooler stroke rate adjustment as a sum of a proportional signal component, a derivative signal component and an integral signal component of the measured pressure difference.

3. An apparatus for adjusting cryocooler stroke speed by adjusting mechanical transmission ratio between a constant speed electric power drive and a cryocooler displacer/piston as a function of pressure in cryogen pressure vessel, the apparatus comprising:

a generator for setting an initial stroke speed for the cryocooler;

a difference measuring device for continuously measuring in each cycle of operation, a pressure difference between a measured pressure and a set pressure; and

a controller for calculating in each cycle of operation a required stroke rate adjustment and therefore the adjusted mechanical transmission ratio based on the measured pressure difference; and

a thermal gasket operably coupled between the cryocooler and a recondenser to provide a thermal interface, and a pressure controller to selectively adjust a mechanical pressure across the thermal interface, the pressure controller including a display to indicate a temperature drop across the thermal interface;

a mechanical transmission for changing the cryocooler stroke rate at each cycle of operation based on the stroke rate adjustment in order to optimize operations of the cryocooler.

4. The apparatus of claim 3, wherein the controller further comprises a proportional integral derivative controller that calculates the stroke speed adjustment as a sum of a proportional signal component, a derivative signal component and an integral signal component of the measured pressure difference.

5. The apparatus of claim 4, wherein the mechanical transmission applies a retardant force to lower the cryocooler stroke rate when the measured pressure is less than the set pressure.

6. A variable cooling capacity cryocooler in a zero boiloff cryogen cooled recondensing superconducting magnet assembly including superconducting magnet coils suitable for magnetic resonance imaging comprising:

a cryogen pressure vessel to contain a liquid cryogen reservoir to provide cryogenic temperatures to the magnet coils for superconducting operation;

a radiation shield surrounding the cryogen pressure vessel and spaced from the cryogen pressure vessel;

a cryocooler for removing heat at a first rate from the cryogen pressure vessel resulting from the provided cryogenic temperature to the magnet coils;

a pressure sensor positioned outside the cryogen pressure vessel for sensing pressure variations;

a thermal gasket operably coupled between the cryocooler and a recondenser to provide a thermal interface, and a pressure controller to selectively adjust a mechanical pressure across the thermal interface, the pressure controller including a display to indicate a temperature drop across the thermal interface; and

a controller for controlling the cryocooler cooling capacity based on the pressure variations;

wherein the heat removal rate of the cryocooler decreases with the internal gas flow oscillation frequency.

7. The variable cooling capacity cryocooler of claim 6, wherein the cryocooler heat removal rate is proportional to the cryocooler pulse frequency or stroke rate.

8. The variable cooling capacity cryocooler of claim 7, wherein the variable cooling capacity cryocooler further comprises:

a variable pulse frequency module electrically coupled to the alternating or a direct current motor for regulating the pulse frequency of the alternating or direct current motor.

9. The variable cooling capacity cryocooler of claim 8, wherein the controller further comprises a proportional integral derivative controller that calculates a pulse frequency adjustment as a sum of a proportional signal component, a derivative signal component and an integral signal component of the sensed pressure variations.

10. The variable cooling capacity cryocooler of claim 9, wherein the variable heat load capacity cryocooler further comprises:

a combiner for combining the pulse frequency adjustment with an initial pulse frequency that is proportional to the first rate of heat removal;

wherein the combiner is electrically coupled to the variable pulse frequency to control the cryocooler heat removal from the cryogen pressure vessel.

11. The variable heat load capacity cryocooler of claim 10, the variable cooling capacity cryocooler further comprising:

a recondenser; and

a thermal interface between the cryocooler and the recondenser;

wherein a temperature sensor is positioned in the region proximate to the thermal interface.

12. The variable heat load capacity cryocooler of claim 11, the variable heat load capacity cryocooler further comprising:

an electrical heater in thermal contact with the recondenser and stably responsive to variations in the temperature of the recondenser as sensed by the temperature sensor.

13. The variable heat load capacity cryocooler of claim 12 wherein the thermal interface includes a heat sink and the temperature sensor is positioned on the recondenser proximate to the heat sink.

14. The variable heat load capacity cryocooler of claim 13 wherein the temperature sensor further comprises a ruthenium oxide cryogenic sensor.

15. The variable heat load capacity cryocooler of claim 14 wherein the thermal interface further comprises an indium gasket.

16. A method for controlling a cryocooler of a zero boiloff cryogen cooled recondensing superconducting magnet assembly including superconducting magnet coils suitable for magnetic resonance imaging comprising:

setting an initial stroke speed for an alternating current motor in the cryocooler;

continuously measuring in each cycle of operation a pressure difference between a measured pressure at a cryogen pressure vessel and a set pressure;

calculating in each cycle of operation a required stroke speed adjustment for the alternating current motor based on the measured pressure difference;

providing a thermal gasket operably coupled between the cryocooler and a recondenser to provide a thermal interface, and a pressure controller to selectively adjust a mechanical pressure across the thermal interface, the pressure controller including a display to indicate a temperature drop across the thermal interface;

combining in each cycle of operation each the stroke speed adjustment with the initial stroke speed of the alternating current motor; and

adjusting the stroke speed of the alternating current motor based on the combined initial stroke speed and the adjusted stroke speed for the alternating current motor in order to optimize operations of the cryocooler.

17. The method of claim 16, wherein the calculated stroke speed adjustment further comprises a sum of a proportional signal component, a derivative signal component and an integral signal component of the measured pressure difference.

18. The method of claim 17, wherein the adjusting is performed by a variable speed module to changes the stroke rate by changing the speed of an alternating current motor.

19. A method for controlling a cryostat for a zero boiloff cryogen cooled recondensing superconducting magnet assembly including superconducting magnet coils suitable for magnetic resonance imaging comprising:

receiving a pressure signal indicative of the pressure within the cryostat,

receiving a stroke frequency signal indicative of the pulse rate of a cryocooler coupled to the cryostat;

receiving a set pressure signal indicative of a selected operating pressure for the cryostat;

calculating a pressure difference signal indicative of a difference between the received pressure signal and the received set pressure signal;

controlling the cryostat when the calculated pressure difference pressure exceeds a predetermined value;

providing a thermal gasket operably coupled between the cryocooler and a recondenser to provide a thermal interface, and a pressure controller to selectively adjust a mechanical pressure across the thermal interface, the pressure controller including a display to indicate a temperature drop across the thermal interface; and

wherein controlling the cryostat is one or more adding heat to the cryostat, adjusting the stroke frequency of the cryocooler, and adding heat and adjusting the stroke frequency of the cryocooler.

20. The method of claim 19, wherein the calculated adjustment further comprises a sum of a proportional signal component, a derivative signal component and an integral signal component of the determined pressure difference.

21. The method of claim 20, wherein adding heat is performed by a two stage cryostat.

22. The method of claim 21, wherein the cryocooler uses a variable speed module to adjust stroke frequency.

23. The method of claim 21, wherein heat is added by a heater element thermally coupled to the cryostat.