Cryocooler and method for operating cryocooler

A method of operating a cryocooler, the cryocooler including an expander provided with (1) an expander motor having a motor rotary shaft, (2) a displacer connected to the motor rotary shaft to reciprocate linearly by rotation of the motor rotary shaft so as to change a volume of an expansion space of a working gas by the reciprocation, and (3) a rotary valve connected to the motor rotary shaft to be rotated by the rotation of the motor rotary shaft so as to control intake and exhaust of the working gas into the expansion space, the method comprising:

• measuring a pressure of the working gas;
• detecting a feature point appearing periodically in the measured pressure during an operation of the cryocooler;
• acquiring a motor drive waveform indicating a command rotation speed of the motor rotary shaft determined to vary within one rotation of the motor rotary shaft;
• and
• outputting the motor drive waveform in synchronization with the feature point appearing periodically.

A cryocooler comprising:

• an expander motor having a motor rotary shaft;
• a displacer connected to the motor rotary shaft to reciprocate linearly by rotation of the motor rotary shaft so as to change a volume of an expansion space of a working gas by the reciprocation such that the displacer passes through a top dead center where the volume of the expansion space is a maximum within a first angle range in one rotation of the motor rotary shaft and passes through a midpoint between the top dead center and a bottom dead center where the volume of the expansion space is a minimum within a second angle range following the first angle range in the one rotation of the motor rotary shaft;
• and
• a controller configured to operate the expander motor to reduce a rotation speed of the motor rotary shaft in the second angle range as compared to the first angle range.