The invention is concerned with a simple, reliable manner of monitoring a process of expelling a variable dose of medicament from an injection device by way of analyzing mechanical feedback of the injection device with reduced requirements on signal data memory space and processing power. According to the invention, specific measurand and mechanical user feedback sensor configurations allow to produce sensor signal data from which individual dosage units of expelled drug are identifiable at moderate sampling rates. Preferred configurations include a force sensor adapted to measure an axial force component as applied to the injection device by a user pressing a proximal injection button, and a direction sensor adapted to measure an azimuth angle of an orientation of the injection device. The specific measurands, or process quantities observed, relate to feedback means of the injection device including a resilient element that produces a number of successive mechanical feedback events proportionate to a dose or quantity of drug expelled from a reservoir of the injection device.
