A mobile medical pump has a tubular cartridge unit with a first end having an interface to an infusion line and a second, open end having a first engagement interface, with a piston inserted in the cartridge and engaging an inner diameter of the cartridge, such that translation of the piston within the cartridge will draw in or expel liquid to or from the cartridge, a case housing having an opening for inserting the cartridge and an internal second engagement interface at a depth that, with a cartridge fully inserted and the first and second engagement interfaces joined, an infusion line may be connected to the first end external to the case, and a piston rod within the case housing, the piston rod having a permanent magnet at a first end with a planar surface orthogonal to a longitudinal axis of the piston rod, and a threaded opening at an opposite end engaging a rotatable translation screw driven by a translation drive system including an electric motor. With the piston rod set at a preprogrammed position, inserting a charged cartridge into the case to engage the first and second engagement interfaces, causes the magnet end of the piston rod to securely contact a steel plate embedded in the piston, such that translation of the piston rod by the translation drive system will expel liquid from the cartridge, and holding the piston rod motionless will hold the piston motionless, preventing any discharge of liquid from the cartridge.