Vibration reduction in a set-up of two split type stirling cryocoolers

At the University of Twente research is in progress on performing magnetocardiography in clinical conditions. For the magnetic measurements High-Tc SQUIDs will be used, which will be cooled by commercial cryocoolers. These coolers consist of separate compressor and displacer modules. The compressors are of the dual opposed pistons type. Since the magnetic measurements are sensitive for movements of magnetic materials as well as for movements of the sensors, it is advantageous to minimise the vibrations caused by the cryocoolers. This is achieved by:
Using coolers with dual opposed pistons in the compressor modules.

Using two coolers and combining the two displacer modules into one rigid unit and operating them in a dual opposed manner.

The remaining vibrations in the axial direction are about 1 ms⁻² for the compressors and 0.2 ms⁻² for the displacer unit. In order to reduce these vibrations, the automated control system described in this paper was developed. It uses an adaptive feed-forward algorithm based on the work of Wu¹ and Boyle et al.² and is capable of controlling the vibrations of the two compressors and the displacer unit for multiple harmonics simultaneously. Thus the vibration levels are reduced down to 0.02 ms⁻² and 0.01 ms⁻² for the compressors and the displacer unit respectively.