A linear compressor and a method for controlling a compressor are provided. The compressor may include a piston that reciprocates within a cylinder, a linear motor that supplies a driving force to the piston, a discharge device through which a refrigerant compressed in the cylinder by the reciprocating motion of the piston is discharged, a pressure changing device that changes a variation rate of pressure applied to the piston before the piston reaches a virtual discharge surface (VDS) during the reciprocating motion, to prevent collision between the piston and the discharge device. The virtual discharge surface may be formed on at least a portion of the discharge device facing a compression space within the cylinder.

A piston pump group for a brake system and a control method thereof are provided, and the piston pump group includes: a piston, a pump body provided with an operating chamber, and a transmission mechanism used for driving the piston to move in the operating chamber, where the transmission mechanism includes a lead screw transmission assembly, a follower, and a planetary gear assembly used for transmitting power to the lead screw transmission assembly, the follower is fixedly connected to the piston, the lead screw transmission assembly is used for driving the follower to move relative to the operating chamber, and a limiting member used for limiting the movement of the follower is disposed between the lead screw transmission assembly and the follower.

A pump system for generating a volume flow of a dialysis solution in a dialysis machine, the pump system includes at least one piston pump having a piston that cooperates with a working fluid, which exerts a force on a conveying means, and in particular on a membrane. The pump system includes setting means, with which a conveying volume of the at least one piston pump per piston stroke can be reduced, with the setting means having at least one of a position-variable mechanical piston stop to reduce the piston stroke, means to reduce a quantity of the working fluid, and means to reduce a volume of a conveying chamber that cooperates with the conveying means and that contains the dialysis solution to be conveyed.