Page 483 - Handbook of Biomechatronics

Bioinspired and Biomimetic Micro-Robotics for Therapeutic Applications 477

The rotating magnetic field, B (T), which the micro-robot should align
with, can be applied by permanent magnets or electromagnets, the details of
which will be partially given in the coming subsections. The magnetic
torque is given by

(16)
T ¼ M B

where M (Nm/T) denotes the effective magnetization vector of the mag-
netic head. Because at least one of the vectors M (Nm/T) and B (T) is of
alternating direction, the magnetic torque, T (Nm), is expected to be time
dependent rather than a constant. One of the earliest examples of such
actuation is given by Honda et al. (1996). Authors experimented with a
cm-scale, rigid, and helical robot with a magnetic head, and the robot
was submerged in silicone oil to keep Re number smaller than unity. Rota-
tion of the rigid helix results in helical-wave propagation, which in return
generates the forward thrust to propel the robot.
Now that we know the basics for helical micro-swimmers, we can move
on to more profound examples. Mahoney et al. (2011) experimented on the
visually assisted velocity control scheme on a magnetically driven helical
robot with compensation for gravitational attraction. The helical robot used
in this study consists of a magnetic body with a rigid helical tail glued on one
side, and its total length is approximately 6mm. Authors observed that the
gravitational pull disturbs the alignment between magnetic head of the heli-
cal swimmer reducing the torque and pulling the swimmer downwards. To
be able to overcome this problem, they modified the external magnetic field
with real-time visual feedback, also known as “visual-servoing” that can
depend on various imaging approaches and used for medical robotics
(Zhang et al., 2017), and “gravity compensation.” Gravity compensation
means generating the necessary force component in the direction opposite
to the gravity, and this force is generated by tilting the axis of the robot with a
pitch angle as it rotates. Hence, a portion of the propulsive thrust cancels the
gravitational pull. This is a very good example of position and velocity
control, one being along the direction of gravity and the other being per-
pendicular to that, respectively. An illustration of this method, with the
visual servoing and controlled motion of the tilted micro-swimmer, is given
in Fig. 5.
Visualization, for example, and detecting the location and orientation
with respect to surroundings, is one of the key elements in the control stud-
ies as demonstrated in Mahoney et al. (2011). But in that particular example,