Shape Memory Alloy Actuator: A Thermal Heat Engine

Heat Source

Heater power 60%

Cooling / fan rate 40%

Load mass 0.30 kg

Actuator State

Austenite 0%Martensite 100%

Temperature

22.0 °C

Stroke

0.0 mm

Lift height

0.0 mm

Work / cycle

0.00 mJ

Cycles completed

Phase

Martensite

How it works

A NiTi (Nitinol) actuator spring is trained so its austenite shape is short. Below its transformation temperatures it is soft, twinned martensite and can be stretched by a load; heating it electrically drives a diffusionless lattice shear back to austenite, and the spring recovers its short trained length — lifting the weight. This simulator tracks the austenite fraction ξ with the classic cosine (Liang–Rogers) transformation model, which reproduces the real thermal hysteresis loop:

Heating (A_s ≤ T ≤ A_f):
  ξ_heat(T) = ½[1 − cos(π (T − A_s)/(A_f − A_s))]

Cooling (M_f ≤ T ≤ M_s):
  ξ_cool(T) = ½[1 + cos(π (M_s − T)/(M_s − M_f))]

ξ only ever moves toward the curve for the
current direction of dT/dt → hysteresis loop.

Spring length:  L(ξ) = L_M − ξ (L_M − L_A)
Work per cycle: W = m g (L_M − L_A)

Heater power — sets the electrical (Joule) heating rate while the heater is on.
Cooling / fan rate — sets how fast the spring loses heat to ambient air (Newton's law of cooling) once the heater turns off.
Load mass — the weight the actuator lifts each cycle; the stroke itself is set by the material's trained transformation strain, not the load, as long as the load stays below the plateau stress — the defining feature of an SMA actuator.
Auto-Cycle — a thermostat that switches the heater off once the spring fully reaches austenite (A_f) and back on once it fully cools to martensite (M_f), turning the actuator into a self-running thermal heat engine.

Real-world relevance: this exact heat-engine principle — resistively heated NiTi wire/spring lifting a load, then air-cooling to reset — drives SMA robotic grippers, camera-shutter and valve actuators, and self-deploying spacecraft hinges.

Ambient

Austenite (trained/short) Martensite (soft/long)

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