FTA-Sensor: Combination of Force/Torque and Acceleration - Robotized Transcranial Magnetic Stimulation - page 100

Fig. 6.1 Communication setup for the FTA sensor. The embedded system reads data from the
IMU and the FT sensor. It is connected to the emergency circuit via a relay. The embedded
system provides a serial connection (RS-232) to the host system or robot. An optional USB
connection is also provided. Furthermore, two additional I/O ports can be used for interaction
6.1.2 Embedded System for Real-Time Monitoring
We use an Embedded System (ES) for implementation of the above calculations in
real-time. Figure 6.1 shows the communication setup for the embedded system.
We use a relay for Galvanic isolation of the controlling circuit from the controlled
e-stop circuit. Beside a standard communication interface via a serial port (RS-
232), the ES also provides an optional USB communication interface, e.g. for
programming or debugging. Buttons or switches can be directly linked to the ES
via the two user Input/Output (I/O).
The embedded system's main task, however, is monitoring the sensor readings.
To stop the robot instantaneously in case of an error or collision, the ES is directly
linked to the robot's external emergency stop. For monitoring and data processing
the embedded system runs the computation cycle continuously as illustrated in
Fig. 6.2 . First, the ES reads the pure voltages V from the force-torque sensor and
checks feasibility and security thresholds for the single readings. In case of an error,
the system interrupts the e-stop channel. The voltages are transferred to forces and
torques using the individual sensor's calibration FT T V . These values are checked
again. Next, the ES reads the accelerations from the IMU and directly verifies the
readings. Now, it compensates for gravity taking the tool's weight and centroid into
account. The resulting user forces and torques are tested again for collisions or
errors. If requested and enabled by the user, a contact point transformation is
performed in the next step. For instance, this transformation allows to shift the
contact point into the coil's handle for optimized user interaction. After a last check
for this cycle, the user-applied forces and torques are available for the host system.
As an important additional safeguard, the ES runs a processor watchdog to stop the
robot in case of a system or processor fault. Also, the system continuously monitors
the power state. An execution counter is increased during each cycle.