Within industrial manufacturing and assembly most processing steps are accompanied by transporting, handling and positioning of a workpiece. The interfaces between handling system and workpiece are in many cases industrial grippers, which come in various forms, sizes and gripping forces. To this day, most gripping systems are driven by pneumatics \[1\]. On the way to higher energy efficiency, companies are looking for new actuation technologies to replace the energy-intensive and therefore also expensive pneumatic technology. Another motivation for new actuation technologies is the digitalization within industry, which comes with the need for smarter machines, more sensor integration and intelligent condition monitoring.

Commonly used actuators like solenoids and electric engines are in most cases too heavy and large for direct integration into the gripping system. Due to their high energy density, shape memory alloys (SMA) are suited to overcome these drawbacks of conventional actuators. Additionally, they feature self-sensing abilities that allow sensorless monitoring and control of the actuation system \[2\]. Recent research shows various approaches for energy efficient SMA based actuator systems, like the combination of SMA wires with bistable elements \[3\].

Another drawback of conventional gripper design is their moving parts with linear guides and bearings that are prone to wear, especially in abrasive environments. This can be overcome by a compliant gripper design that is based on flexure hinges.

In the proposed work, the design and development process of a compliant gripper for industrial applications with a bistable SMA actuation unit is shown and the functionality is validated by gripping experiments. The gripper is supposed to replace standard pneumatic parallel grippers in pick and place operations for small workpieces.

The design of the bistable mechanism features a 250 µm thick and 6 mm wide buckled leaf spring, which is clamped between two pivot-mounts. Via a 2 mm lever, antagonistic bundles of NiTi wires apply a torque to the pivot mounts and thus let the spring snap from one buckled position to the opposite. Each side is driven by two bundles of 6 SMA wires with a diameter of 100 µm and a length 38 mm. The wires are resistance welded to a stainless-steel sheet metal that ensures electrical and mechanical connection. Resistance welded NiTi-steel joints exhibit high tensile strength and the process shows good automation potential.

The overall stroke in the middle of the bistable spring is 4 mm while the needed SMA stroke is only 1 mm. This leads to transmission ratio of 4 of the SMA stroke. The main benefit of the bistable system is the energy free position holding. Only about 1800 mJ of energy are needed to switch between the two positions at room temperature.

The design of the compliant mechanism features two trapezoidal leaf hinges and two leaf spring joints that basically work as pushing rods and connect to the bistable mechanism. The gripping jaws are replaceable to adapt the gripper to a variety of operational scenarios and different workpiece geometries. Most parts of the gripper are 3D printed. The flexible parts are made of FDM printed PETG. The elements the of actuation mechanism, where higher stiffness and maximum precision is needed, are made from high strength resin via SLA printing.

 

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

\[2\]           S. Langbein and A. Czechowicz, *Konstruktionspraxis Formgedächtnistechnik*, vol. 1. Wiesbaden: Springer Fachmedien, 2013

\[3\]           P. Motzki and S. Seelecke, “BISTABLE ACTUATOR DEVICE HAVING A SHAPE MEMORY ELEMENT,” WO 2017/194591 A1, 2016


Design of a Compliant Industrial Gripper Driven by a Bistable Shape Memory Alloy Actuator

Division / Sponsors: 
Aerospace Division

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

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