Cryocooler Frequency Control for MRI Pressure Stability
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Solution Overview
Problem
Conventional cryocooler systems in zero boil-off superconducting magnet assemblies have excess cooling capacity, leading to increased wear on sealing components and reduced component life, as they operate at a constant oscillating frequency, which is not adaptable to variable heat load conditions, causing pressure oscillations that affect the homogeneity of the magnetic field in MRI systems.
Innovation Solution
A controller adjusts the cryocooler displacer/piston stroke frequency using a proportional integral derivative (PID) controller and variable mechanical transmission ratios to match the cryocooler's cooling capacity with the variable heat load requirements, ensuring a stable pressure environment by adjusting the oscillating frequency of the gas flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the cryocooler operates at a constant optimal oscillating frequency to achieve maximum cooling capacity, then the cooling performance is improved, but the wear on sealing components increases and component life is reduced
Solution Approach 1:
The patent applies dynamics by transitioning the cryocooler from constant frequency operation to variable frequency operation. The oscillating frequency is dynamically adjusted based on real-time heat load conditions, allowing the system to operate at lower frequencies during steady-state conditions to reduce wear on sealing components while maintaining adequate cooling performance.
Solution Approach 2:
The patent implements parameter changes by modifying the oscillating frequency parameter of the cryocooler based on operational conditions. During transient cool-down phases, the frequency is increased to maximize cooling capacity, but during steady-state operation, the frequency is reduced to minimize component wear and extend reliability.
2Productivity
If the cryocooler operates at constant maximum cooling capacity, then the heat load requirements are satisfied, but pressure oscillations occur in the cryogen reservoir affecting magnetic field homogeneity
Solution Approach 1:
The patent applies dynamics by making the cryocooler's operating frequency variable rather than constant. The system dynamically adjusts the oscillating frequency based on the difference between actual and target pressure values in the cryogen reservoir, thereby stabilizing pressure while maintaining adequate heat load removal capability.
Solution Approach 2:
The patent implements feedback control by continuously monitoring the pressure in the cryogen reservoir and using this information to adjust the cryocooler's oscillating frequency. The controller compares actual pressure with target pressure and modifies the frequency accordingly, creating a closed-loop system that maintains pressure stability and magnetic field homogeneity.
3Productivity
If the cryocooler frequency is increased to meet variable heat load demands during transient conditions, then the cooling performance is improved, but the wear on sealing components increases
Solution Approach 1:
The patent applies periodic action by using high-frequency operation only during transient cool-down phases when maximum cooling capacity is needed, then transitioning to lower frequency periodic operation during steady-state conditions. This time-based strategy concentrates wear during necessary transient periods while maintaining reliability during extended steady-state operation.
Solution Approach 2:
The patent implements dynamics by adjusting the operating frequency according to the thermal state of the system. During transient conditions requiring high cooling capacity, the frequency is increased temporarily, but during steady-state operation, the frequency is reduced to minimize wear on sealing components and extend their operational life.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution extends the life of cryocooler components, maintains a stable magnetic field, and reduces oscillating stresses in superconducting magnet coils by dynamically adjusting the cooling capacity to meet changing heat load demands, thereby improving the operational stability and efficiency of MRI systems.
Implementation Method 1
The main component of the recondencing system is the cryocooler, which is the heat sink or the heat removal source of this system thru a heat exchanger in thermal contact with the cryogen
Implementation Method 2
vacuum insulated liquid helium reservoir integrated into a cryostat with a radiation shield
Implementation Method 3
A proportional integral derivative (PID) controller determines an adjustment to the initial electric motor rate to adjust stroke rate
Data Source
AI summary
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.