Page 52 - Handbook of Biomechatronics

46 Reva E. Johnson and Jonathon W. Sensinger

Fig. 7 Shape memory alloy actuators are often coupled with a bias element. In this
example, there is a Nitinol (NiTi) spring on the right and stainless steel spring on the
left. When Nitinol is heated, it returns to its original shape and deforms the stainless steel
spring. (Reproduced with permission from Nespoli, A., Besseghini, S., Pittaccio, S., Villa, E.,
Viscuso, S., 2010. The high potential of shape memory alloys in developing miniature
mechanical devices: a review on shape memory alloy mini-actuators. Sens. Actuators A:
Phys. 158(1), 149–160.)

Fig. 8 Microgripper with shape memory alloy (SMA) actuator. (Reproduced with permis-
sion from Mohamed Ali, M.S., Takahata, K., 2010. Frequency-controlled wireless shape-
memory-alloy microactuators integrated using an electroplating bonding process. Sens.
Actuators A: Phys. 163(1), 363–372.)

The main disadvantage of SMAs is that bandwidth is limited and oper-
ational frequency is low, due to slow cooling processes. The phase transitions
require both heating and cooling processes, and because most SMAs have
high heat capacities, they heat up rapidly but cool down slowly. Much of
the work on SMAs focuses on improving bandwidth, and one successful
development is that of magnetic shape memory alloys (MSMAs), which
have higher operating frequencies. Other strategies to improve bandwidth