Drug delivery device including motion detection system

A method of operating a drug delivery device comprising:

• a motor including a rotatable output shaft and a pinion attached to the output shaft;
• a cartridge retainer configured to receive the cartridge of medicament, the cartridge retainer moveable between an open position and a closed position;
• a controller;
• and a motion detection system including an optical encoder and at least one element operatively coupled to the pinion of the motor and configured to rotate with the pinion such that the at least one element passes through the optical encoder;
• and one or more switches configured to detect or confirm whether the cartridge retainer has been properly closed, the method comprising:
• the one or more switches detecting or confirming that the cartridge retainer has been properly closed;
• the optical encoder sending electrical pulses to the controller as the at least one element passes through the optical encoder;
• and
• the controller monitoring rotation of the output shaft via the electrical pulses and detecting a position error of the motor or a jamming of a drive train of the drug delivery device.