FT-Control - Robotized Transcranial Magnetic Stimulation - page 91

motion compensation is still active. Note that for this purpose the force is trans-
formed into coil coordinates applying the transform E T C from end effector to coil.
5.2.3.2 Response to Head Motion
The contact pressure control maintains the contact of the coil with the head and the
optimal contact pressure even when the head moves. It is combined with the
existing motion compensation (cf. Sect. 1.3.2.3 ). Therefore, three separate control
sequences are operated in parallel. Coil movement control allows the user to
change the coil position relative to the head H T C during stimulation, e.g., to adjust
coil rotation for an optimal stimulation. Head movement control records the
position of the head band T T H continuously using the tracking system. Due to
registration ( Sect. 1.2.1 ), head motion is detected in this way and a new robot
position is calculated to compensate for that motion (cf. Sect. 1.3.2.3 ) . Pressure
and motion compensation control is the main cycle. First, we measure and cal-
culate the applied forces. In case of a collision, we stop the robot instantaneously.
If the pressure of coil to head is larger than the acceptable threshold, we lift the
coil for 0.1 mm. In case of insufficient pressure we move the coil down for
0.1 mm. Therefore, we adapt the coil to head transform H T C . Based on the current
values of coil position and head position, we calculate a new end effector position
R T E and move the robot. For calculating R T E also the constant transforms from
tracking system to robot R T T (obtained by calibration, see Chap. 4 ) and from end
effector to coil C T E (obtained by coil registration ( Sect. 1.3.2.2 ) ) are applied. The
control cycle for the constant pressure control is illustrated in the lower part of the
diagram shown in Fig. 5.4 .
5.2.4 Data Acquisition for Evaluation of FT-Control
5.2.4.1 Coil Calibration and Gravity Compensation
For the presented FT-control mechanisms, an accurate compensation of the tool's
weight is essential. However, as pointed out in Sect. 5.1.3 , the coil's heavy supple
cable also affects the FT-measurements. As the coil calibration is a rigid cali-
bration, the influence of the supply cable is averaged during computation. Thus,
depending on the spatial orientation, the supply cable results in errors of the
gravity compensation.
By determination of these errors, we can estimate if the implemented coil
calibration and gravity compensation method is suitable for the specific case of
TMS coils. Therefore, we test eight different TMS coils that are already utilized in
the robotized TMS system (cf. Sect. 1.1.3 for an overview). The specific coil
parameters and types are listed in Table 5.1 . This list highlights that beside