Grippers for the Micro Assembly Containing Shape Memory Actuators and Sensors

Shape memory alloys (SMA) show a high ratio of work capacity per material volume. This makes the application of SMA especially useful in micron-sized systems. The development of robotic grippers is one important prerequisite for the successful automation of the assembly of micro systems. Therefore the SMA may also play a role, for example, as actuators in micron-sized grippers. This paper presents the development of micron-sized grippers. Due to a special relation between the electrical resistance and the shape change of a NiTi-wire the actuator may also be used simultaneously as a sensor. Besides these functional properties a superelastic SMA may be used for structural purposes, i.e. as solid-state flexure hinges. The sensoric features of binary and ternary NiTi-based alloys are investigated using different Ti50Ni50-xCux alloys with 0 < x < 25 at-%. Besides the chemical composition the functional properties are affected by the thermo-mechanical treatment (TMT). Different TMT of Ti50Ni50-xCux alloys and their influence on the functional properties are compared. The effect of the TMT on the amount and the stability of the shape memory effect has to be taken into account if the optimum alloy and condition for the use in grippers for micro assembly are investigated. The function and the properties of the developped gripping devices are demonstrated by prototypes

Design and application of shape memory actuators

The use of shape memory alloys in actuators allows the development of robust, simple and lightweight elements for application in a multitude of different industries. Over the years, the intermetallic compound Nickel-Titanium (NiTi or Nitinol) together with its ternary and quaternary derivates has gained general acceptance as a standard alloy. Even though as many as 99% of all shape memory actuator applications make use of Nitinol there are certain properties of this alloy system which require further research in order to find improvements and new markets: 
• Lack of higher transformation temperatures in the available alloys in order to open the field of automotive applications (Mf  temperature > 80 °C) 
• Non-linearity in the electrical resistivity in order to improve the controllability of the actuator,
• Wide hysteresis in the temperature-vs.-strain behaviour, which has a signi-ficant effect on both, the dynamics of the actuator and its controllability. 
Hence, there is a constant strive in the field towards an improvement of the related properties. However, these improvements are not always just alloy composition related. There is also a tremendous potential in the thermomechanical treatment of the material and in the design of the actuator. Significant improvement steps are already possible if the usage of the existent materials is optimized for the projected application and if the actuator system is designed in the most efficient way. This paper provides an overview about existent designs, applications and alloys for use in actuators, as well as examples of new shape memory actuator application with improved performance. It also gives an overview about general design rules and reflects about the strengths of the material and the related opportunities for its application

Influence of the Thermal Treatment on the Stability of Partially Constrained Recovery of NiTi Actuator Wire

NiTi shape memory wire may be used for actuation purposes in flexible robotic grippers, which have to be able to handle objects of different size, shape or weight. Therefore it is advantageous to develop an electrically driven shape memory actuator, which may perform any combination of shape change and exerted force within the following limiting boundaries: - free recovery: gripping of a very small and lightweight object, - constrained recovery, gripping of an object with maximum size and weight. Several NiTi actuator wires are fabricated and annealed between 400 and 600°C after cold working in the martensitic state. After prestraining each wire is embedded in a silicone matrix material. The polymer works as a bias spring and is able to store elastic deformation energy. This paper investigates the influence of thermal treatment on the stability of the exerted force between the two boundaries of completely free and constrained recovery, the "partially constrained recovery". The stability of recovery strain and stress is measured in a test assembly, in which different modes of partially constrained recovery are simulated. The work is supplemented by dilatometric measurements carried out with each actuator wire before and after the test procedure

First cycle shape memory effect in the ternary NiTiNb system

The usage of shape memory alloys (SMAs) in coupling devices depend considerably on the phase transformation temperatures (PTTS) of the material. It is important that SMAs in such devices feature austenite start temperatures (A$_{\rm s}$-temperatures) above room temperature for storage purposes as well as low martensite start temperatures (M$_{\rm s}$-temperatures) in order to guarantee a high level of mechanical resistance service. It is well known that the addition of Niobium to the NiTi-System results in an increase in the width of the PTT-hysteresis, in particularly for the first phase transformation cycle. The present work studies the influence of different amounts of Nb both as a comparison between binary and temary alloys and as a comparison of diferent levels of Nb (9 and 21 at. %) on. the first cycle shape memory effect. The constrained shape memory effect is simulated using a tensile test machine and the dependence of the recovery stress on the predeformation is determined. The dependence of this functional property on cold working and subsequent annealing is considered; special emphasis is placed on the recovery stress and how it is affected by the thermomechanical treatments. In the present work, solutionannealed material and thermo-mechanically treated material is characterized on the basis of calorimetrie (differential scanning calorimetry - DSC) and mechanical experiments