Robust Real-Time Robot/Camera Calibration - Robotized Transcranial Magnetic Stimulation - page 59

Chapter 4
Robust Real-Time Robot/Camera
Calibration
For the robotized Transcranial Magnetic Stimulation (TMS) system, tracking of
head and head movements is required for accurate coil targeting (see Sect. 1.3.2.1 ).
Therefore, robot and tracking system must be calibrated. Conventionally, the cal-
ibration is done by tracking a marker at the robot's end effector, i.e., moving the
robot into different poses and measuring the position and orientation of a marker
attached to the end effector. Hence, robot motions can be related to the respective
motion in camera coordinates. Generally, several methods for precise estimation of
the related transformations exist (see below in Sect. 4.1 ).
For the robotized TMS system, the setup is partially mobile allowing greater
flexibility. Figure 4.1 shows the system in mobile configurations. The robot is
attached to a carrier and a tripod carries the tracking system for fast system
assembly (see Fig. 4.1 a). Furthermore, the robot can be mounted to a particular
cart as done in the commercially available SmartMove TM (Advanced Neuro
Technology B.V., Enschede, The Netherlands) (cf. Fig. 4.1 b). The drawback of a
mobile robotic and navigated system is that calibration is required frequently, i.e.,
an additional calibration step has to be performed before each use. Consequently,
the calibration step requires additional time and interferes with the clinicians'
workflow [ 16 ]. Such a system is thus not easily deployable in daily clinical use.
Even worse is the case when the robot and/or the tracking system are moved
after calibration has been performed or, even more dangerous, during treatment.
This will result in the robot moving to a wrong position.
To solve these issues, we introduce a robust online robot-camera calibration
approach for robotized TMS that does not need an additional calibration step
before system start. It uses a marker that is rigidly attached to the robot's third link
for calibration. To use this setup, it is necessary to determine the constant trans-
form S 3 T M from the robot's third link to the marker (cf. Fig. 4.4 ). It is then
possible to perform an online calculation of the calibration matrix whenever the
Parts of this chapter have been already presented in [ 8 , 19 ].