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

5.Thermal or magnetic(Shape memory alloys)  actuators

Actuators which are actuated by thermal or magnetic energy.these types of actuators mainly uses 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 alloy like zink,copper,gold and iron.
 SMA is an alloy which remember its original shape.when the alloy (shape-memory alloy) is heated then it change form its deform position to pre deform position. 

there are mainly two types of SMA 

 copper aluminium nickel alloy –  

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

nickel titanium alloy

these are most important type of  alloys  and these are costly than other SMA alloys.but they are much applicable then copper aluminium 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 increase then it converted into martensite to austenite.

2.austenite

 when the temperature of the alloy is higher , then they are in ausenite 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 repeat-ability (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 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 gain a shape but as the temperature increases higher than the transition temperature then it imparts different shape.it means it forget it’s shape when the temperature is change form 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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