Pump cassettes with piston and infusion pump systems

An infusion pump system comprising:

• a processing unit;
• a cassette recess comprising:
• a circularly moveable actuator mechanism disposed proximate to a back surface of the cassette recess and operably coupled to the processing unit, and
• a plurality of cassette engagement slots,
• wherein the cassette recess is adapted to receive a pump cassette comprising:
• a rigid body comprising a frame portion, a base portion, a compliant membrane disposed substantially therebetween, and two opposing longitudinal edge sections, wherein the rigid body comprises a controllable fluid pathway defined in part by the compliant membrane and extending from an inlet port to an outlet port, and
• a piston disposed at least partially within the rigid body such that movement of the piston varies a volume of a pump chamber defined within the controllable fluid pathway, wherein the piston comprises a first seal that provides a sealed movable barrier of the pump chamber and a second seal that provides a sealed movable exterior-facing barrier, wherein the first seal is disposed at a distance from the second seal that is longer than a stroke of the piston such that a first path of the first seal does not overlap a second path of the second seal.