Fabrication

Fabrication of a SMA prototype consists of three major steps. Refer to Bill of Materials for the supplies you'll need to get started.

Pre Stretching of the nitinol spring
Crimps that connect a regular spring and a nitinol spring
A 14-16 AWG wire that connects both crimps together

Step 1: How do we "treat" SMAs?

Heating is the only way that most memory metals retain their original shape. Since heat is the property that determines the shape of the metal, heat is the first property used for manipulation for formation.

The spring is hot/cold worked (stretched) by 3% when it is in the martensite phase.
The spring is then heated to austenite finish (AF) to recover its shape.
The spring is then cooled to a martensite phase.

Pre-stretching of a nitinol spring at martensite phase

Step 2: Connecting Crimps

We used two crimps to connect nitinol and a regular spring. These crimps hold the springs together or in other words provide a strong grip, lowering the retraction force of the nitinol spring after each contraction. It is very important to make sure that the ends of these springs are tightly bound to these crimps since the retraction force during the contraction gets very high and if the strength of the attachment point of these springs aren’t strong enough, they might break from the middle and fly off.

Crimps connecting the nitinol and a regular spring together

Step 3: Crimping

Crimping is a style of joining wires to metal fittings that are then used to connect the wire to other wiring components or to other wires. For our application, we used a ring connector that had a ring width of 0.38 inch and a no.10 screw size which are usually suitable for 14- to16- gauge insulated connectors.

To achieve the necessary compressive force, it’s best to use a crimping tool when attaching crimp connectors to a wire. Pliers or a hammer will work in a pinch, but using such tools can result in a loose connection that ultimately results in an open circuit. Here, since we only needed about 1.5 inch of wire to connect both crimps we used any random insulated wire and inserted it into a hole of the crimp making a big tight knot such that it wouldn’t slip or pass back through that hole when maximum horizontal force was applied.

using a random wire to make a tight knot to connect two crimps such that they provide enough strength to overcome the retraction force after each contraction of nitinol spring

crimps connected together with a random sized wire. The ring connector is then used to connect the two springs together.

Powering SMAs

SMAs uses a maximum 12V battery power and a MOSFET circuit to operate. You would need a N-type power MOSFET and a 10k ohms resistor connected to any pin with a squiggly line that denotes a PWM pin on an arduino. PWM stands for pulse width modulation and it is a technique used in controlling the brightness of LED, speed control of DC motor, controlling a servo motor or where you have to get analog output with a digital means.

The Arduino IDE has a built in function "analogWrite( )" which can be used to generate a PWM signal. The frequency of this generated signal on pin 5 and 6 will be about 980 Hz and on other pins will be about 490 Hz. We can give the value from 0-255 using this function. For example:

analogWrite(0) means a signal of 0% duty cycle.

analogWrite (127) means a signal of 50% duty cycle.

analogWrite (255) means a signal of 100% duty cycle.

Further information about PWM signal and duty cycle can be found in this website.

Schematic of SMA, powered using a 12V battery and an Arduino Uno.

Arduino Code for SMA

const int pin_9 = 9;

void setup() {

// put your setup code here, to run once:

pinMode(pin_9, OUTPUT);

}

void loop() {

// put your main code here, to run repeatedly:

analogWrite(pin_9,150);

delay(3000);

analogWrite(pin_9,0);

delay(3000);

}

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