Plate capacitor for dosage sensing

An apparatus for use with a drug injection pen that dispenses a fluid, the apparatus comprising:

• a dosage measurement system adapted to receive a motion from a dosage injection mechanism disposed within the drug injection pen, the dosage measurement system including:
• a substrate;
• a sensing capacitor disposed on the substrate, the sensing capacitor including a dielectric layer disposed between a base plate and an adjustable plate;
• and
• a lifting tab attached to the adjustable plate and positioned to engage an undulation pattern disposed on a component attached to the dosage injection mechanism, wherein the lifting tab is adapted to physically change a separation distance between the adjustable plate and the base plate in a reciprocal manner in response to the motion of the dosage injection mechanism and engagement with the undulation pattern, wherein a capacitance of the sensing capacitor changes in response to the engagement of the lifting tab with the undulation pattern and the motion of the dosage injection mechanism.