DE29606249U1 - Coil spring made of a shape memory alloy - Google Patents

Description

spiral spring made of a shape memory alloy

The innovation relates to a spiral spring made of a shape memory alloy according to claim 1.

As a result of increasing miniaturization in microelectronics, there is an increasing need for microcomponents with mechanical, optical or fluidic functions. Actuators with millimeter and submillimeter dimensions are required, for example, for manipulating small objects (microrobotics) or liquid quantities (microfluidics) or as mechanical actuators (microelectronics, microoptics).

A major problem in the development of microactuators is the simultaneous generation of high forces and large travel distances. The desired functions cannot usually be achieved by simply miniaturizing conventional macroscopic actuators, since macroscopic system properties such as heat transfer or friction scale non-linearly. In addition, miniaturization is severely limited by restrictions in manufacturability. This results in the need to develop new concepts that are compatible with microsystem technology.

According to the invention, the spiral spring made of a shape memory alloy as described in claim 1 is proposed to solve this problem. Preferred embodiments of the spiral spring are described in the further claims.

The spiral spring according to the invention made of a shape memory material preferably has dimensions in the millimeter or sub-millimeter range. It enables both the generation and the detection of movements and forces in both directions perpendicular to the spring plane. Due to its simple geometry the spiral spring is easy to manufacture using modern microtechnical processes and has a high degree of micro-

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niaturizability. The generation of movement and force is based on the shape memory effect caused by the crystalline phase transformation of the material. To operate, the spiral spring is thermally cycled between the phase transformation temperatures of the material. The heating of the material can be carried out, for example, electrically by internal current heating (Joule heat), whereby the required voltages and powers are compatible with those of microelectronics.

Shape memory materials show a thermoelastic phase transformation when they transition from a martensitic phase below the phase transition temperature of the material to an austenitic parent phase by heating above the phase transition temperature. Below the phase transformation temperature, the shape memory material can be plastically deformed by several percent. The material remains in the deformed state until it is heated above the phase transformation temperature, where it returns to its original shape (memory shape) with considerable force generation. This effect is also called a one-way effect because it only occurs when the temperature changes in one direction (heating). The memory shape can be adjusted using certain thermomechanical processes. The shape memory effect can also be adjusted during reverse transformation (cooling). In this case, it is also referred to as a two-way effect. However, the maximum force generation during reverse transformation is significantly lower. A technically interesting shape memory material is NiTi, for example, which allows reversible strain changes of up to 8%.

In the case of the spiral spring, the shape memory effect can be exploited by choosing, for example, the planar shape as the memory shape. In this case, when the spring is heated above the phase transformation temperature, it works against all external forces that cause a deflection in the vertical direction.

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If the spiral spring is deflected downwards, for example by attaching a weight, this deflection can be largely reversed when heated above the phase transformation temperature, whereby lifting work is carried out. If a certain spring deflection is chosen as a memory shape, the spring works accordingly when heated above the phase transformation temperature against all external forces that cause a deflection other than the specified one.

An important application of the planar design of the spiral spring is its use as an actuating element for positioning and switching functions in both directions perpendicular to the plane in an environment with narrowly limited geometric dimensions. Compared to shape memory membrane actuators or actuators based on other transducer principles, very high forces and travel distances are achieved. Compared to previously common shape memory bending elements, a higher proportion of shape memory material is used to generate force and movement. Positioning and switching functions are required in many areas of microsystem technology, e.g. micro-optics, micro-fluidics or micro-robotics.

Further applications are in clamping or holding devices, where it is not the movement that is used, but rather the generation of force with suppressed shape recovery in ambient temperatures above the phase transformation temperature. In addition to applications as force or movement generating components, applications as force or movement sensors are also possible in principle.

Pig 1 shows the structure of the spiral spring. In the design shown, the spring thickness and width are constant. The number of spring arms 4, 5 is 2, the number of turns of each spring arm is 3/2. The spring arms are each attached with one of their ends to a mounting surface 1, 2. In general, the

Spring thickness D, spring width B, number of spring arms M and number of turns N are to be determined depending on the desired application in order to fulfill the required force-travel relationship. Instead of constant thicknesses and widths, further increases in the proportion of active shape memory material can be achieved by variable design of the thickness or width profiles. The electrical control A can be carried out both analogously with constant current and digitally by defined supply of current pulses.

Thin films, foils or rolled sheets made of shape memory material with typical thicknesses D in the range of 20 to 200 μm serve as the starting material for producing the spiral springs. Thin films made of shape memory material (D < 20 μηη) can be produced using sputtering processes and subsequent thermomechanical treatment. Foils made of shape memory material in the thickness range of 50 μηη can be produced using the "melt spinning" process, for example. Sheet thicknesses above 100 μηη can be produced by rolling shape memory materials produced using melt metallurgy. Within certain limits, the thickness can also be adjusted to the desired value using chemical etching processes.

The microstructuring of the starting material into the desired shape of the spiral spring can be carried out by laser cutting, erosion or by a lithographic process. The electrolytic photoetching process is particularly preferred as a lithographic process. A suitable photoresist is applied to the material, which is then optically exposed by shadowing or direct laser writing processes and subsequently developed. The microstructured photoresist produced in this way serves as a protective mask in the subsequent electrolytic etching process, in which the unprotected parts are selectively removed in an electrolytic bath in the presence of an electric field. A significant advantage of the lithographic process is the possibility of

Possibility of cost-effective mass production of spiral springs through parallel production in a batch process.

The advantages of this spiral spring can be summarized as follows:

- A comparatively high proportion of the shape memory material can be used to generate force and movement. This means that correspondingly high forces and large travel distances can be achieved at the same time; the movement can be generated in both directions along the spring axis.

The movement occurs frictionlessly without bearings or joints.

The simple geometry of the spiral spring allows for easy production and high miniaturization.

Manufacturing using microstructuring processes in microelectronics allows mass production using batch processes.

By combining the spiral spring elements, additional functions (e.g. push-pull adjustment function, gripping function) can be created, which have a high application potential for microsystem technology.

These advantages add to the favorable properties that can generally be achieved for spiral springs:

- The movements can be controlled electrically with voltages and powers compatible with those of microelectronics ;

Due to the phase transformation, deflections of the spiral spring are linked to changes in the internal resistance value, which can be exploited by feedback for positioning functions or force sensor functions.

Claims (3)

&bull; · &bull; · Research Center Karlsruhe, 2 April 1996 Karlsruhe GmbH PLA 9637 Rü/he ANR 5661498 Protection claims: 1. Spiral spring made of a shape memory alloy with: a) at least one first (1) and one second fastening surface (2), b) a connecting piece (3), c) at least two spring arms (4, 5) bent in the form of turns of a spiral, wherein the coils of the spring arms are spaced apart from each other and each spring arm is arranged between the coils of the other spring arm, the spring arms have two ends, the first end being connected to the fastening surface (1, 2) and the second end being connected to the connecting piece (3). 2. Spiral spring according to claim 1, in which the turns of the spring arms lie in a common plane. 3. Spiral spring according to claim 1 or 2 with a connecting piece (3) in the form of a rod.