Auto injector with detection of used cartridge and associated method

An auto-injector for administering a medicament, comprising:

• a housing;
• a cartridge receiver configured to receive a cartridge comprising a first stopper;
• 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 resistance sensor configured to provide a resistance signal indicative of resistance against movement of the plunger rod;
• and
• a processing unit coupled to the drive module and to the resistance sensor;
• wherein the processing unit is configured to:
• control the drive module to move the plunger rod towards the extended plunger rod position;
• determine present plunger rod position;
• receive the resistance signal;
• determine a cartridge parameter based on the resistance signal and the present plunger rod position;
• and
• control the drive module to adjust the movement of the plunger rod based on the cartridge parameter, and
• wherein the processing unit is configured to control the drive module to adjust movement of the plunger rod if the cartridge parameter is indicative of the present plunger rod position having reached a plunger rod threshold and the resistance signal being indicative of resistance against movement of the plunger rod below a low resistance threshold.