IEEE Robotics & Automation Magazine - September 2016 - 67

power consumption of only 0.5 W, it has a high range of
motion and a relatively high torque output of 34.1 mN∙m,
with a torque-to-weight ratio of 486 mN∙m/g, filling the gap
of the tradeoff among size, efficiency, range of motion, and
output torque.
This article focuses on the modeling, characterization, and
control of the novel actuator and its implementation in a multitile robotic origami. Our X-SMA actuator is used for
motion in one direction, which can also be extended to
bidirectional motion with an additional antagonistic actuator
[18] or elastic bias element. The main contributions of this
research are
1) a novel low-profile SMA torsion actuator with high torque
output and low power consumption
2) analytical modeling of the X-SMA torsional actuator
3) characterization of the actuator in terms of temperature,
position, velocity, and torque in three different loading conditions, both by experiments and validation of the model
4) closed-loop control of the actuator by integrating thin-film
strain sensors for position feedback
5) experimental and modeling results via closed-loop control
of the Robogami prototype.
The Actuator Design and Fabrication
The proposed actuator is made of a thin sheet of Ni50Ti50
SMA (M alloy, Memry GmbH) with a 0.1 mm thickness. Its
low-profile, slim, X -like shape has dimensions of
12 # 8 # 2 . 5 mm 3, which closes upon heating, as depicted
in Figure 1(a). It is possible to achieve approximately 180c
of deflection if one end of the actuator is fixed and the
other is kept in plane at an angle of 0c. The cylindrical

hinge forms a circular curvature with a higher circumference compared to the U geometry obtained by bending,
resulting in a higher surface area for activation, a higher
range of deflection and torque output, and a prolonged
actuator lifetime [18]. Likewise, deformation of a U-shaped
metal sheet to its initial flat form is not possible, because of
the plastic deformation of the material that forms a curvature at the hinge. Therefore, introducing curvature adds an
advantage as well as extra elasticity and the ability to reset
the actuator to its initial open configuration.
The fabrication and training process of the actuator consists of three steps. First, an unprocessed SMA sheet is cut in
a rectangular form with holes using a laser micromachining
station (a LAB 3550, Inno6 Inc.). The machined sample
is then bent along a 2-mm-diameter steel rod, forming a
U shape. Then both ends are squeezed together so the rod
remains tightly within the hinge, and the ends are clamped
between two jig plates with grooves using bolts and nuts
[Figure 1(c)]. Finally, the assembly is placed into a hightemperature furnace (Nabertherm GmbH) and annealed at
400 cC for 30 min to induce the shape.
We use customized microheaters to concentrate the
thermal activation zone, as depicted in Figure 1(d). The
heaters are made of an Inconel-polyimide (Kapton-backed
Ni-Cr alloy) thin-film layer, fabricated to provide heat to
the actuator [Figure 1(d)]. They have multiple resistive
paths, patterned in parallel to generate heat upon current
flow. This mesh design adds high flexibility to the film
when folded or stretched, with negligible loading impact
on the actuator. Placing the heater inside the actuator
hinge, as in Figure 1(b), with the Kapton side facing the

Omega SMA Actuator

SMA

Microheater
M1 Screws

Open

5 mm
Closed

Heating

5 mm
(a)

(b)

5 mm

Rod

5 mm

Annealed Actuator

5 mm

5 mm
(d)

Figure 1. (a) The X-SMA actuator is set to open configuration at room temperature, and it closes when thermally activated. (b) The
actuator and flexible microheater layer assembly. (c) Clamping jig plates for annealing. (d) The laser-machined heater and actuator
after the annealing process.

September 2016

IEEE ROBOTICS & AUTOMATION MAGAZINE

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2016