It is intended to be applied in Architectural Technology to move flexible and ... create shadows over buildings, streets and squares in order to decrease electrical ... The spring used for the string of the bow is ... and no vibrations are generated.
Nitinol Based Actuator for Architectural Technology in Hot Climate Countries M. A. Callejas1, Dr. J.I.P. Calero2 1 2
Lirio9 Architecture, Sevilla, Spain Building Structures Department, University of Sevilla, Spain
Abstract: Introducing a new Nitinol based actuator combining the two interesting attributes of this material: shape memory and superelasticity, in a simple and reliable device. It is intended to be applied in Architectural Technology to move flexible and lightweight surfaces (canopies) to create shadows over buildings, streets and squares in order to decrease electrical power consumption on air cooling systems in hot climate countries. Shadow generator elements which activate with the hot morning sunlight and return to their original position at night, with no need for user intervention, no sounds or vibrations, no motors, low maintenance and high reliability. Keywords: Actuator, Nitinol, Shape Memory, Superelasticity, Architecture Technology, Moving Textiles Introduction The use of shape memory alloys (SMA) like Nitinol, is quite extended in Medicine, Engineering and other fields, but not in Architecture, except for specific isolated cases [1]. Nitinol has shape memory ability which means that if an element is deformed in martensitic condition (cold state), when heated over its activation temperature, it transforms into austenitic state (hot state) and returns to its original shape. It also shows greater strength and elasticity. This superelastic condition can be obtained if the alloy transformation temperature is lower than the room temperature. The actuator described in this paper takes advantage of these properties, to be applied in Architectural Technology. Actuator Design The device is simple. It is only formed by two parts: a bow and a string (see Fig. 1). The first one is a superelastic Nitinol rod and the second one is a spring, also Nitinol made but with shape memory ability. The superelastic rod tested has 2.7mm diameter, 500mm length and a austenite final transformation temperature of Af=0ºC. The material surface is formed by a polished and oxidized layer of TiO2 which makes this alloy corrosion resistant and therefore suitable for outdoor applications.
Fig. 1: Actuator Design
The spring used for the string of the bow is composed of Nitinol wire with a transformation temperature of Af=55ºC and 750µm diameter. The number of turns is 20, the outer diameter 6.5mm and the inner 4.8mm. (see Fig.2) The ends of the spring have the shape of a hook in order to allow its connection with levers, rings, wires, etc. Two tiny holes are made at the ends of the superelastic rod (800µm diam.) Through these holes both ends of the spring are inserted in such a way that the hooks remain on the other side of the aforementioned holes in order to allow the connection of the activator to other elements. The gap between the rod and the spring is filled with electrical insulation. It is necessary to separate them because if not, the electricity flows all over the actuator and not only over the spring. This becomes an unnecessary waste of energy. Also, this filled gap and the hook shape itself provide mechanical fastening to avoid its gliding. Initially, the rod is straight form set. When curved it stores energy that can be controlled liberated. This control is made by the spring which, in its cold condition (martensite), prevents the rod from recovering the straight shape. This way, forces are balanced, but when the SMA gets over its transformation temperature, it returns to its original shape and the bow ends get closer as the superelastic rod becomes weaker than the austenite spring. When the SMA cools down, the stronger bow extends the spring to its maximum length in a daily actuation cycle. The simplicity of this actuator makes it highly reliable and long-lasting because there are no gears, mechanisms, wheels or parts that might fail. This system has very low maintenance and because of this simplicity and the excellent behavior of the superelastic rod on load and unload cycles, it is expected to have a very long service life. The arc shape actuator has already been tested before [2],[3]. In these cases, usually a simple Nitinol wire is used for the string and therefore, the actuation distance is about 10% the length of the wire which is variation of the Nitinol length when transforms from martensite to austenite state. In the actuator presented in this paper, the string is not a wire but a spring so it has greater linear development. It can be electrically activated. 3A at 5V of direct electric current is applied so the spring activates or transforms in four seconds, moving point B to point B’ with a 7N force and fixed point A. It also has a different configuration as the central point of the arc can be fixed so the ends can move to actuate. (see Fig. 1)
ACTUATOR FEATURES Heating time 4s Cooling time 60s Dimensions (off) 260mm x 180mm Force delivered 7N Af (spring) 55ºC Actuation distance 160mm SUPERELASTIC WIRE Length 500mm Diameter 2.7mm Af 0ºC SMA SPRING ᴓ1 spring wire 0.75mm ᴓ2 outer diam. 6.3mm ᴓ3 inner diam. 4.8mm Turns 20 Fig. 2: Actuator Features Table Architectural applications Case 1: Window Canopy The actuator has been tested on a light window canopy structure. Several concentric arc shaped ribs provide support for the textile element to generate shadow over a building facade window (see Fig. 3)
Fig. 3: Window Canopy These ribs are articulated on two little beams fitted to the facade wall, which handles the overall lightweight ensemble. All of them turn around these two points but the first rib remains still and parallel to the facade. The actuator is fitted to this first rib on one end and to the last moving rib. This way, when the temperature raises and activates the SMA, it turns to deploy the textile providing shade to the window. The facade wall works as a block to limit this turning movement (see Fig. 4) In the evening, when the temperature falls, the SMA cools down, the actuator returns the canopy to its previous position and a free view of the night sky is displayed. 2/4
The whole movement of the canopy is quite silent and no vibrations are generated. The system can be implemented on existing canopies. The textile fabric can also be cleaned periodically as well as the ribs structure which can be easily disassembled to be repaired or substituted.
the actuator are inside the hollow articulated bars and also along the guide-wire If more strength is needed, for example due to the use of a heavier fabric, the actuators can be doubled at each point and/or raise the number of acting lines. Also, as in the window canopy case, it is easy to install and remove from existing canopies.
Fig. 4: Window Canopy (deactivated and activated actuator detail) Case 2: Patio Canopy In this second case, the textile is set horizontally, like the ones used to cover and shade patios, streets and squares. They usually are static or moved by a manual pulleys system or electric motors. This fabric hangs from several galvanized steel guide-wires with incorporated rings. One actuator end is fitted to this cable and the other one to two articulated rods connected to the steel cable rings. This way, when the SMA activates, these bars rotate and separate the rings. The sum of the horizontal resultant of all the actuaries forces makes the canopy to unfold, covering thus the space in shade (see Fig. 6). This happens in the morning when the temperature is high. In the evening-night, the SMA cools down again and the actuator returns the canopy to the open position. As previously said, the system is autonomous but it also can be user controlled. Electricity is provided by photovoltaic cells, integrated in the patio south patio wall, which heat the SMA through Joule effect. It can be directly used during the day or stored into batteries in a simple electrical circuit that can also include a switch and a PWM (Pulse Width Modulation) to optimize the electric current supply therefore avoiding overheating the Nitinol. The electric wires needed to provide this electricity to
Fig. 5: Patio Canopy 3/4
and not another, has been chosen specifically due to this characteristic. No corrosion signs are visible after three months of outdoor testing, neither in the rod or the spring. The actuator has been tested with hot air stream and also with direct electric current supply provided by photovoltaic cells which worked as expected. The results were quite predictable since the entire system is quite simple in its motion and electric components. As we present this paper, we are also working with small solar amplification devices to add to the SMA spring in order to get the system working directly with the sun heat. We also believe that this idea of a superelasticmemory shape arc configuration can be reproduced at different sizes, including micro scale. Perhaps to be used in different Industry or Medicine. Conclusions These are only two of many potential applications of this material. We need to keep working to develop more systems that help us reduce electrical energy consumption, take care of our planet and its resources and, overall, improve our lives, which is a main goal for Architecture. References
Fig. 6: Patio Canopy (deactivated and activated actuator detail) Results Tests results satisfy the objective searched which is the creation of mobile shadow generator elements with the help of Nitinol and its shape memory and superelasticity features. The actuator works as expected in a 1:1 model for the window canopy and in a 1:2 model for the patio canopy. Movement is even better as it is not dangerous or too abrupt. Four seconds for the activation and sixty seconds for deactivation. Although the spring shows some plastic deformation after several load cycles, this is not a problem as it only decreases in a small percentage the actuation distance. The superelastic rod does not show fatigue signs either, which is not surprising, because this material,
[1] Decker&Yeadon LLC, Smart Screen, Homeostatic Facade System, www.deckeryeadon.com [2] G.S.Mammano, Attuatore lineare a memoria di forma con configurazione ad arco, (2012) XXXXI Convegno Nazionale AIAS #22 [3] R. Dunlop, A. Garcia, A Nitinol Wire Actuated Stewart Platform (2002), Proceedings 2002 Australasian Conference on Robotics Automation, Auckland, 122-127 [4] M. Addington, D. Schodek, Smart Materials and Technologies for the architecture and design professions (2005), 105-108 [5] W. Huang, Shape Memory Alloys and their Application to Actuators for Deployable Structures (1998) [6] D.K.Sung, Metal that breathes, www.dosuarch.com [7] Hoberman Associates, Adaptive Building Initiative, www.adaptivebuildings.com
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