Thermal or magnatic (shape memory alloy ) actuators used in robots

Thermal or magnetic(Shape memory alloys)  actuators

Actuators which are actuated by thermal or magnetic energy.these types of actuators mainly use shape memory alloys.
A shape-memory alloy (SMA, smart metal, memory metal, memory alloy, muscle wire, smart alloy). these are economical, light in weight, possesses max power density.
                  SMA is the combination of different -2 type of alloys like zinc, copper, gold and iron.

 SMA is an alloy which remembers its original shape.when the alloy (shape-memory alloy) is heated then it changes from its deform position to pre deform position. 

there are mainly two types of SMA 

 copper aluminum nickel alloy –  

 
these types of  SMA alloy commercially available and much cheaper than the nickel titanium alloy.

 

nickel titanium alloy

these are the most important type of alloys  and these are costly than other SMA alloys.but they are much applicable then copper aluminum nickel alloy due to their stability, practicability and superior thermo-mechanic performance.
 
 

SMA can exist in  two different phase

we can find out the phase position with the help of temperature and stress

1.martensite

 in this phase, the temperature of the alloy is lower and it can be converted in any shape in this phase.when the temperature of the alloy is increased then it converted into martensite to austenite.

 

2.austenite

 when the temperature of the alloy is higher, then they are in the austenite phase.

Sma_wire Thermal or magnatic (shape memory alloy ) actuators used in robots

  Here  E(T)  represents martensite fraction
T represents the temperature
M(f) is the temperature at which the transition to Martensite is finished during cooling
A(s) is the temperature at which the transformation from martensite to austenite starts.
A(f) is the temperature at which the transformation from martensite to austenite finishes.
 
if we use the alloy again and again, in this case, their is a shift of temperature and properties of the material.
due to repeatability (repeat use of alloy material ),micro-structure and the properties of the alloys material changes .this change effect is known as functional fatigue .

Transition temperature  it is the temperature at which the material changes its own phase from martensite  to  austenite 

 

 

One-way and Two-way shape memory alloy Effects

 ONE WAY SHAPE MEMORY ALLOY  EFFECT         
                                                                      when the material at low temperature [lower than the A(s)] then it can be shaped in any shape until heated above the transition temperature.Upon heating, the shape changes to its original. When the metal cools again it will remain in the hot shape until deformed again.in this case cooling from high temperature does not cause the macroscopic shape change.                             
                                
 TWO WAY SHAPE MEMORY ALLOY  EFFECT 
                                                                      when the material does not remember shapes at different-2 temperature means at low temperature it gains a shape but as the temperature increases higher than the transition temperature then it imparts different shape.it means it forget it’s shaping when the temperature changes from low to high .during  transformation, material changes its micro-structure due to this reason material forgets its earlier shape.  in this shape effect we can not predicts the exact    shape  of the material .above a certain temperature the material loses the two-way memory effect this is called “amnesia

Application of SMA 

  • aircraft
  • robotics
  • medicine
 
 
 
 
 
 
 

 

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