We have presented a simple but effective method to design flexible actuators. This process relies on understanding the behavior of a simple unit cell element built out of SMA sheet. The unit cell effectively uses the properties of flat SMA sheets: it operates in the bent region where more force is generated; it minimizes the nonbent SMA; and it heats up only the bent regions. However, the force generated by this unit cell does not scale up well, and an array of them is needed to increase the force generated. Building an actuator out of an array of unit cells increases its complexity but provides advantages, including the control of expansion length, trajectory, and generated force. Given the current technologies, including 3-D printing and laser cutting, a variety of support structures can be built to create an actuator with a given behavior. We have shown three types of configurations: linear, rotational, and surface. The linear actuator has been tested for endurance and can easily perform over 10,000 repetitions under load without breaking. These actuators have been tested in actual systems, such as the battery-operated HexRoller robot that uses six actuators connected in a chain. The robot demonstrates that this SMA actuator is power efficient compared with other SMA designs that cannot operate with batteries. We have also showed that a rotational version of this type of actuator is comparable with an electromagnetic motor.
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