Biomedical Engineering Reference
In-Depth Information
For coil positioning by grasping and moving the coil, however, two different
scenarios are considered:
1. Coarse pre-positioning: To access a stimulation target on the head with the
robotized TMS control software, the coil is coarsely pre-positioned by hand-
assisted positioning. Subsequently, the robotized TMS control software is
operated to precisely place the coil on the selected target.
2. Direct placement on the head: The hand-assisted positioning method is applied
to directly position the coil on the patient's head for an immediate stimulation.
Consequently, we multiply the user applied forces and torques with a proportional
factor p and directly transfer them into robot increments. Thus, robot motion will
depend on the force amplitudes. In this way, fast and easy pre-positioning is
supported. While this proportional control is sufficient for the first case, in the
second case the sensor will detect a counterforce once the coil touches the head.
When using a robot control directly depending on F user and M user , the coil will
retract from the head due to this counterforce and coil positioning with contact to
the head will not be possible.
To overcome this situation, the proportional control is modified using a
piecewise linear transfer function instead of the factor p. The factor q is introduced
and chosen depending on the measured forces while retaining p as global pro-
portional component. It is defined as follows:
8
<
2 10 N ;
if F user
1 ;
2 6 ;
2 \20 N ;
F user
10\ F user
q ¼
1
2
ð 5 : 13 Þ
if
2 20 N :
:
if F user
6 ;
The definition of q is additionally visualized in Fig. 5.3 . The translational robot
motion trans for one processing cycle can then be described as:
trans ¼ q p F user :
ð 5 : 14 Þ
Accordingly, the incremental rotational robot motion is calculated based on the
user applied torques M user . Note that q can be easily adapted for specific needs or
individual convenience. E.g., one user might prefer more strength pulling/pushing
the coil than another operator. The above values present the default values used in
the experiments. However, for more convenience, individual user parameters can
be stored and loaded.
As a prerequisite for fast hot-spot search, the force-torque control is integrated
into the TMS software. While manually positioning the coil using the force-torque
control, the relative coil position is continuously calculated out of head tracking
results and the robot's forward calculation. The TMS software is modified to
automatically update the current coil position in the graphical user interface (GUI)
assisting the user targeting the coil on the head. Once the stimulator triggers a
pulse, the TMS software gets informed using the stimulator-to-computer interface
via the stimulator's trigger output, and instantaneously takes the current coil
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