Disclosed herein is an auto injector for administering a medicament, the auto injector comprising:a housing；a cartridge receiver configured to receive a cartridge comprising a first stopper and a cartridge compartment containing the medicament, the cartridge compartment having a first cartridge subcompartment containing a first medicament component of the medicament and a second cartridge subcompartment containing a second medicament component of the medicament； a drive module coupled to move a plunger rod between a retracted plunger rod position and an extended plunger rod position, the plunger rod being configured to move the first stopper；a processing unit coupled to the drive module, wherein the processing unit is configured to control the drive module to move the plunger rod from a first plunger rod position to a mix plunger rod position with a mix plunger rod speed, wherein the mix plunger rod position is selected to position the first stopper in a position wherein the first medicament component is mixed with the second medicament component, and to provide an onset signal after a number of completed inversions of the auto injector has been performed.
