Discussion - Robotized Transcranial Magnetic Stimulation - page 152

9.3 Contact Pressure Control
With the FT sensor, the pressure of the coil on the head is measured. For coil
placement, the measurement of the contact pressure allows to optimally position
the coil on the head. Furthermore, the contact pressure control monitors the contact
pressure to avoid exposure on the head for increased patient comfort.
We have shown that the presented force-torque control reaches a control fre-
quency of 40 Hz. Our practical tests have illustrated that this update frequency is
sufficient for smooth coil movements and placements. During our experiments, we
have found that the latency of the contact pressure control is roughly 200 ms. This
latency is acceptable to compensate for coil to head distance changes. It is in the
same range as pure motion compensation (see Sect. 1.3.2.3 ) .
For robotized TMS (see Sect. 1.3 ), as in neuro-navigated TMS (cf. Sect. 1.2 ),
the tracking system tracks a marker at the patient's head. This head marker is
commonly attached to a head band. Due to pressure, patients tend to shift the head
band or the head band loosens during treatment (cf. Sect. 3.3 ). A shift of the head
band, however, will lead to a wrong coil position. In the worst case, the head band
is shifted downwards. The robot moves to compensate for the shift. If unrecog-
nized by the operator, the robot pushes the patient downwards, which is a
potentially dangerous situation. With FT-control such a dangerous situation is
avoided. As the force increases, the FT-control stops the robot motion automati-
cally supporting the patient's safety. Note that the same scenario can also happen
when attaching the head marker to spectacles instead of a head band.
For precision, the described and implemented contact pressure control now
guarantees and maintains the contact to the head during stimulation. Without FT-
control, even with the use of head tracking and motion compensation, this is hard
to achieve.
However, as the FT-control is implemented in software, the latency is too large
to stop the robot immediately in an emergency situation. Therefore, we have
developed an independent safety layer for the robotized TMS system that operates
in real-time.
9.4 FTA Sensor
We have presented the novel Force-Torque-Acceleration (FTA) sensor which
combines acceleration measurements with an FT sensor to perform gravity com-
pensation independent from the robot. The calculations necessary for combining
both sensors are performed with an embedded system in real-time. The Embedded
System (ES) runs a continuous monitoring-cycle that performs the necessary
computations for independent gravity compensation and checks the original
readings and the computed values. In an error case, the ES instantaneously triggers