Vacuum Gripper System Based on Bistable SMA Actuation

This paper presents the design and the realization of an innovative SMA actuated bistable vacuum suction cup. The sealed, compact and fully integrated design enables the positioning and transport of inherent stable components in mobile and stationary applications. The bistable actuator mechanism based on SMA wires combined with a bistable spring represent an energy-efficient, noiseless gripping system without the need for compressed air. Additionally, the self-sensing effect of the SMA enables a sensorless condition-monitoring and energy-efficient control. The mechanics consists of antagonistic SMA wires, which are laterally arranged and connected to the bistable spring via levers. The membrane is directly connected to the bistable spring. The actuation of the wires leads to a rotational movement of the levers thus changes the state of the bistable spring, which directly deforms the membrane. When the membrane is sealed connected to the workpiece, the deformation of the membrane generates a vacuum. The integrated microcontroller electronics manages the joule heating of the wires by measuring the transmitted electrical energy. By applying an electrical energy to the pre-strained SMA wire, the wire heats up and contracts due to the phase transformation from martensite to austenite. The contraction of the wire is accompanied by a significant change in electrical resistance, which enables a resistance based strain feedback. The integrated electronics is able to correlate this resistance change to the actual state of the bistable spring, which leads to a position feedback of the membrane. This allows an adequate electrical energy deposition in the SMA wire by turning-off the heating directly after the position toggle of the membrane. Thereby, a successful position toggle is ensured independent from the ambient temperature and the real supply voltage. The new position of the membrane is then held by the bistable spring without the use of additional energy. This concept leads to a reliable gripping system with fast actuation times.