Design of a thermal bimorph actuator

Lact       length of the active beam in µm
Lpas      length of the passive beam in µm
Lcon      length of the connecting link in µm
wid        width of the beam in µm
thc         thickness of the beam in µm
ΔT         temperature variation in °C
α            thermal expansion coefficient in 1/K
ymod     Young's modulus in GPa
sel         number denoting the selected result.
              Use 1 for deflection and 3 for force at the free end of the actuator

Design of a thermal bimorph actuator


A thermal bimorph actuator consists of a hot and cold region which are connected together. Unlike the bimetallic actuator which is made of two materials of different thermal expansion coefficient, the bimorph actuator is made of one material. Also while the whole bimetallic actuator is raised to a higher temperature, for a bimorph actuator only a portion of it is heated.

For the actuator shown above, if a current is passed through the entire system between the two fixed pads, the higher Joule heating in the thinner active beam would cause its expansion while the temperature of the cold arm will remain relatively unchanged. The cold arm is connected to the fixed pad via a passive beam of the same cross-sectional area as the active beam. The passive beam allows flexibility of the cold arm and at the same time can be used to control the deflection based on its length. The hot and cold arms are connected together by a connecting link which has an influence on the deflection.

This design interface can be used to determine the deflection and actuation force for a thermal bimorph actuator as shown above. The deflection and force are estimated at the free tip of the actuator. The influence of the different geometric features of the actuator on its deflection can be examined.

The plot shows the variation of deflection of the actuator in relation to the ratio of the length of the passive beam to that of the active beam (Lpas/Lact) given all geometric features including the active length as given in the design form. It shows that there is a particular length of the passive beam which gives the maximum deflection output. This critical length is dependent on the cross-sectional area of the beam. Using the cross hair tool, the deflection for a particular passive beam length can be obtained from the graph.


-The default material is Silicon with a Young's modulus of 180GPa. 
-The coefficient of thermal expansion of Silicon can be assumed to be 2.3e-6 K-1
-The passive beam and active beam has the same cross sectional area.
-The two arms are made of the same material.
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