Arrangement and method for determining a stopper position

Drug delivery device with a container defining a cavity for a drug and a stopper for proximally delimiting the cavity and displacing the drug from the cavity, the drug delivery device further comprising an arrangement for determining a position (x) of the stopper relative to the container, the arrangement comprising an acoustic source configured to emit an acoustic signal and an acoustic sensor configured to detect an acoustic signal, a processing unit for controlling the acoustic source and processing the detected acoustic signal for determining characteristics of the acoustic signal correlated with the position (x) of the stopper.

A method for determining a position (x) of a stopper relative to a container in a drug delivery device, the method comprising the steps of:

• emitting an acoustic signal from an acoustic source,
• detecting an acoustic signal caused by the emitted acoustic signal by means of an acoustic sensor,
• processing the detected acoustic signal for determining characteristics of the acoustic signal correlated with the position (x) of the stopper by means of a processing unit.