SMAs don’t like it hot

Chad Schneider
Key Technologies Inc. Baltimore, Md.

Edited by Lawrence Kren

Unlike ordinary metals that grow slightly when heated to moderate temperatures, shape-memory alloys contract a significant amount. This property makes them useful for actuators. One SMA alloy called Flexinol from Dynalloy Inc., Costa Mesa, Calif., is a nickel-titanium wire that shrinks between 2 and 5% of its overall length when heated to an activation temperature of between 70 or 90C, depending on diameter. Contraction force scales with cross-sectional area, which, in this case, is about 166 N/mm″.

The idea of using SMAs for actuators isn’t new, though a lack of detailed analytical models describing behavior of the materials has limited their use. Such models define input for open-loop control systems. However, closed-loop feedback systems that compensate for ambient temperature get around some of these limitations.

Electrical design
Electric current can heat SMA wire to its activation temperature in less than a second, though convective cooling takes significantly longer. Figure on a cycle time of about 3 to 10 sec, depending on ambient temperature and wire diameter. Careful control of input power is key because SMAs are highly susceptible to overheating that can anneal and permanently damage them.

Either analog controllers that vary wire voltage, or digital controllers that change wire current using a variable-duty-cycle, pulse-widthmodulated (PWM) signal, can control SMA-based actuators. Optimizing PWM duty cycle and duration is tricky and strongly depends on ambient conditions and a thorough understanding of SMA system thermodynamics. The primary concern is preventing thermal overshoot that can damage the wire. When practical, automatically adjust SMA current input for changes in ambient temperature.

Also, it’s usually a good idea to stop current flow at completion of the stroke to lower the risk of overheating. In one configuration, electrical leads touch a carbonsteel insert and neodymium magnets, creating a simple single-pole, double-throw switch. An embedded processor monitors the switch and stops current flow at the proper time. Alternatively, an analog circuit could employ a normally closed, momentary switch mounted such that it opens the circuit at stroke completion.

As with most mechanical devices, there is a delay between full actuation of the SMA and switch. Delay of the temperature-compensating switch may let excess power reach the SMA wire, as well as hurt power efficiency. Multiplying the PWM duty cycle by duration gives a relative indicator of power efficiency. A 100% duty cycle would seem the most efficient application of power, though thermal overshoot can be a problem. In most cases, a 65% duty cycle ensures proper actuation under all conditions and minimizes overshoot.

Two alternative digital control schemes may also work, though they need further development and testing. The first “front loads,” or initially sets, the duty cycle to 100% until just before reaching the minimum duration time. The duty cycle then lowers for the remainder of SMA actuation and shuts off when the switch closes.

The other approach delivers a constant, say, 10 to 15% duty-cycle “trickle” current. The trickle current slightly heats and preloads the SMA wire, reducing the amount of work needed to heat it to activation temperature. An electrical current capable of heating the wire to its activation temperature in >1 sec is safe for continuous loading, say Dynalloy engineers. A downside to this scheme is that power continuously heats the wire, which may be a problem for battery-powered devices. A thermistor could signal the controller to preheat the wire as needed.

Mechanical design
Besides contracting rather than expanding in response to a temperature rise, as do ordinary metals, heat applied to SMAs also induces unidirectional strain. An external force is needed to return the wire to its original length when temperature normalizes, the amount of which varies by wire size and alloy.

Designing an SMA-based actuator involves the calculation of actuator force, Factuation, the force developed by the wire, Fwire, and the external force, Fextend needed to stretch the SMA back to its original length. The wire must be sized such that:

Factuation > Fwire – Fextend.

A spring typically applies the extension force, though a second SMA wire opposing the first also works. However, it’s tough to build in mechanical compliance with the latter configuration. Mechanical compliance helps lower sensitivity to mounting and travel tolerances, which can cut reliable wire travel distance as well as complicate actuator design.

Actuator travel need not be tightly controlled in some applications. For example, one approach uses an overcenter mechanism that nearly eliminates sensitivity to variations in SMA wire contraction. Here, the actuator completes the stroke after the wire moves sufficiently past the overcenter point. In this way, the stroke beginning and end points, and thus overall actuator travel, can be tightly controlled without regard to wire mounting and travel tolerances. The microliter pump, for instance, holds a stroke tolerance of ±0.001 in.

Take care when mounting SMA wire in a device. Soldering can excessively heat and damage SMA alloy. Instead, crimps are the best way of establishing an electrical connection and can also attach the wire to an actuator. Note that any surfaces touching an SMA wire act as a thermal sink. Thermal sinks reduce wire contraction and promote uneven heating and stress concentrations, which can lead to early failure.

As a rule, SMA-wire-based actuators work best in high-cycle, low-frequency applications. When properly designed, such actuators can last tens of millions of cycles.

Make contact
Dynalloy Inc., www.dynalloy.com
Key Technologies Inc., www.keytechinc.com