Free Piston Stirling Cooler Control via Collision-Adaptive Voltage Limiting
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Solution Overview
Problem
Current control systems for free-piston Stirling heat pumps and coolers face challenges in maximizing heat pumping rate without causing damaging collisions between internal and stationary components, especially during startup when temperature variations are significant, and over time due to component aging or gas leakage.
Innovation Solution
A digital data processing control system that senses internal mechanical collisions, repeatedly measures temperatures and drive voltage, stores data associated with collision events, and limits the drive voltage to prevent collisions by generating a surface map of over-stroke voltage versus temperature, allowing for self-calibration and recalibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the linear motor drives the pistons at maximum amplitude to maximize heat pumping rate, then productivity is improved, but the risk of damaging collisions between pistons and internal components increases
Solution Approach 1:
The control system continuously monitors the actual piston position and drive voltage, comparing them against the calibrated surface map to detect when collision conditions are approaching. This feedback mechanism allows the system to operate at maximum amplitude while automatically adjusting to prevent collisions, thus maintaining both high productivity and reliability
Solution Approach 2:
The system performs preliminary calibration by intentionally driving the piston to collision points during setup to map the relationship between drive voltage, temperature, and collision conditions. This pre-established surface map enables subsequent operation to avoid collisions while maintaining maximum amplitude, resolving the contradiction between productivity and reliability
2Power
If the drive voltage amplitude is increased to maximize piston reciprocation amplitude, then heat pumping rate is improved, but the likelihood of piston collision with internal components worsens
Solution Approach 1:
The system dynamically adjusts the maximum allowable drive voltage based on real-time temperature conditions by referencing the calibrated surface map. As temperatures change during operation, the system automatically updates the voltage limits, enabling maximum power output at each temperature state while avoiding collision conditions, thus resolving the contradiction between power and harmful factors
3Productivity
If the system operates without collision detection to maintain continuous maximum amplitude operation, then productivity is improved, but reliability deteriorates due to undetected collisions
Solution Approach 1:
The control system uses feedback from temperature sensors and drive voltage monitoring to continuously check operating conditions against the calibrated surface map. This non-intrusive feedback mechanism provides collision detection without requiring additional mechanical sensors that would interrupt operation, thus maintaining productivity while improving reliability through undetected collision prevention
4Reliability
If the drive voltage is limited to prevent collisions, then reliability is improved, but the heat pumping rate decreases
Solution Approach 1:
The system performs preliminary calibration to establish the exact boundary between safe and collision conditions in the surface map. During operation, it operates as close as possible to this boundary without exceeding it, thus maximizing heat pumping rate while maintaining collision-free operation. This resolves the contradiction by eliminating the need for conservative voltage limits
Solution Approach 2:
The system changes the maximum drive voltage parameter dynamically based on temperature conditions using the calibrated surface map. At each temperature state, it uses the highest voltage that prevents collision, thus optimizing the balance between reliability and productivity across the entire operating range rather than using a fixed conservative limit
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
Enables maximum piston amplitude and efficient heat transfer while preventing collisions, ensuring reliable operation across varying temperatures and component changes, thereby maximizing heat pumping rate and minimizing downtime.
Implementation Method 1
a linear electromagnetic motor... drives the power piston in reciprocation
Implementation Method 2
Stirling cycle machines have been known for nearly two centuries... The operation of the Stirling machine transfers ('pumps') heat from the cold end to the warm end
Data Source
AI summary
A method for use in controlling a free piston Stirling machine having a cold end and a warm end and driven by a linear motor having an armature winding to which a drive voltage is applied. The method comprises (a) sensing internal mechanical collisions; (b) repeatedly sensing the temperature of the cold end, the temperature of the warm end and the drive voltage; (c) storing, as associated data, a value representing drive voltage, the temperature of the cold end and the temperature of the warm end at the time of sensed collisions; and (d) limiting the drive voltage to less than the drive voltage that was sensed at a collision and is stored in association with stored warm end and cold end temperatures that are proximate currently sensed warm end and cold end temperatures.