Sensorless Balancing of a Dual-Piston Linear Compressor of a Stirling Cryogenic Cooler

Abstract

Long-life, highly reliable and low-cost miniature Stirling cryogenic coolers are the core components in a wide variety of cryogenically cooled electro-optic, high temperature superconductive and vacuum applications. The linear compressors of such coolers are known to be the major source of harmful vibration disturbances affecting the operation of these inherently vibration sensitive systems. The dual-piston approach to the design of the linear compressor yields intrinsically low vibration export and, therefore, is widely accepted across the industry. However, the residual vibration export originated from unavoidable technological tolerances, natural wear and contamination cannot be completely eliminated. The ultra-low vibration export from a dual-piston compressor may be attained using active balancing. A dedicated digital controller normally relies on internal sensors (e.g. LVDT) for direct measuring of the pistons' motion or an external sensor (e.g. accelerometer or load cell) for measuring the residual vibration export. This eventually complicates the entire design and makes the cooler more expensive and less reliable. The authors disclose the recent efforts made by Ricor towards the development, implementation and testing of a sensorless adaptive feed-forward controller for the balancing of dual-piston compressors. In this novel approach, the detection and synchronization of the pistons' velocities relies solely on the real-time data acquisition (motors' voltages and currents) and the general model of the linear "moving coil" or "moving magnet" motor. The attainable performance of the controller was evaluated through the full-scale testing performed on the Ricor model K535 linear dual-piston Stirling cryogenic cooler.

Experiments with linear compressors for phase shifting in pulse tube crycoolers

“…This information allowed calculations to obtain PV power at the expander and an understanding of the peak-to-peak stroke of the expander without piston position sensors. 9 The testing procedure involved cooling the apparatus with the 4 K GM cryocooler to the low-end temperature and allowing thermal stability of all the apparatus components. The pressure oscillator of the third stage hybrid was then operated at its maximum current limit to obtain maximum cooling at the cold-end.…”

Evaluation of Pressure Oscillator Losses

“…This was accomplished by use of a hot wire anemometer to measure mass flow delivered to the load [1], a pressure transducer to measure the pressure amplitude at the compressor, and a temperature sensor placed between the compressor and cold head to measure the gas temperature as shown in Figure 2. This was accomplished by use of a hot wire anemometer to measure mass flow delivered to the load [1], a pressure transducer to measure the pressure amplitude at the compressor, and a temperature sensor placed between the compressor and cold head to measure the gas temperature as shown in Figure 2.…”