Fluid machine and method for controlling such a machine

A method for controlling a fluid machine comprising a pump or a compressor for a cryogenic fluid, the fluid machine being connected to an actuator,

• the fluid machine comprising a liner and a piston which together delimit a fluid compression and expansion chamber, the piston and the liner being configured to be in a back and forth reciprocating movement one with respect to the other between a bottom dead centre as the fluid expands and a top dead centre as the fluid is compressed;
• the actuator comprising the following elements connected in series:
• a rotary-shaft motor, a speed-reducing gearbox connected to the shaft of the motor, a mechanical coupling and a connecting rod, the connecting rod being connected to the fluid machine and configured to alternately drive the piston or the liner in a first movement that compresses the fluid and a second movement that expands the fluid,
• the method comprising the following successive steps:
• determining a relative position of the piston or of the liner with respect to a reference critical position when the shaft of the motor is rotating at a first speed;
• and
• temporarily increasing the rotational speed of the shaft of the motor according to the relative position of the piston or of the liner with respect to the reference critical position, so as to reduce a mechanical shock between the speed-reducing gearbox and the connecting rod at the transition between the fluid-expansion movement and the fluid-compression movement.