In times of digitalization and optimization of manufacturing processes, industry is looking to add functionality and intelligence to their machines and actuators. In production lines, e.g. for the assembly of complex geometric parts like car doors and the car chassis, special gripping systems are applied. Commonly used gripping systems are mostly fitted to exactly one geometric shape and if there are some component modifications in the parts to handle, the reconfiguration of the gripping system must be done manually \[1, 2\]. This paper presents the design and the realization of an innovative handling system for plane work pieces in production and assembly industry. The system consists of an adaptive end-effector with energy-efficient vacuum grippers. Shape memory alloy (SMA) actuator wires are used to achieve active reconfiguration of the gripping arms as well as to activate the vacuum suction cups. The SMA technology enables the system to be versatile, and the inherent self-sensing features allows for advanced maintenance and control strategies.

First the basic design and operating principles of the SMA actuator components in the end-effector and the suction cups are presented and their performance is characterized.

The end-effector prototype can adjust to a variety of geometries, from planar up to spherical shapes. The design of the end-effector is modular and consists of four independent quarters with two degrees of freedom (DOF) each. Using an SMA-wire actuator working against a mechanical spring, each arm can rotate 90° in plane (first DOF). Each arm has an additional second joint, which allows a rotation out of plane in the range of 30° (second DOF), which is also achieved by an SMA-spring actuator configuration. To ensure energy, the previous design of the SMA based end-effector \[3\] has been expanded by a brake mechanism in the first DOF to hold the arm power free in any desired position within its 90° rotation radius. This braking mechanism is also an SMA-spring configuration, in which the spring pushes a brake shoe against the housing of the end-effector. Due to a high friction between housing and brake shoe any position in the 90° range can be held energy free without heating the SMA-wire. To open the brake, its SMA wire is activated and the end-effector arm of the first DOF can be positioned by feedforward or position control.

The actual grippers to handle workpieces are realized by SMA-based vacuum suction cups mounted on top of each end-effector arm. This vacuum cup is based on a bistable SMA actuator principle for energy-free holding of payloads \[4\] and has been improved with regard to assembly, fatigue behaviour and control. The control and the adjustment of the end-effector’s actuator systems in the first, second DOF and in the brakes are performed by a separate microcontroller and a power electronics. The whole system is attached to a UR5 Universal Robot and communicates with the central UR robot control.

 

\[1\]         G. Monkman, S. Hesse, R. Steinmann, and H. Schunk, *Robot Grippers*. Weinheim: WILEY-VCH, 2007.

\[2\]         Destaco, “Lightweight End Effector Tooling,” 2017. \[Online\]. Available: http://www.destaco.com/lightweight-tooling.html. \[Accessed: 10-Jan-2018\].

\[3\]         Paul Motzki *et al.*, “Design and Validation of a Reconfigurable Robotic End-effector Based on Shape Memory Alloys,” *IEEE/ASME Trans. Mechatronics*, vol. 24, no. 1, pp. 293–303, 2019.

\[4\]         F. Welsch, S.-M. Kirsch, P. Motzki, M. Schmidt, and S. Seelecke, “Vacuum Gripper System Based on Bistable SMA Actuation,” in *ASME 2018 Conference on Smart Materials, Adaptive Structures and Intelligent Systems*, 2018, no. 51944, p. V001T04A014.


Adaptive Material Handling System Based on Shape Memory Alloy Actuators

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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