FTA-Sensor: Combination of Force/Torque and Acceleration - Robotized Transcranial Magnetic Stimulation - page 113

roughly 5 % of the tools zero force. This is reasonable consindering the calibration
accuracy and noise in the recordings.
6.2.3 Latency
As the FTA sensor's main task is monitoring of the forces and torques in real-time,
and to stop the robot in an emergency situation, the FTA sensor's latency is of high
interest. Therefore, we have estimated the time for the computation cycle on the
FTA sensor. This computation time is an indicator for the maximum latency, as in
the worst-case a full computation cycle must be performed before an error or
collision is detected with the FTA sensor.
The mean maximum latency lat is 1.0 ms with a standard deviation of 0.03 ms.
The maximum latency is 1.25 ms. Therefore, the FTA sensor runs the monitoring
cycle in real-time with a frequency of up to 1,000 Hz.
These results show that the emergency circuit of the robot is interrupted in less
than 1.25 ms after the collision has occurred. On average, the emergency stop is
set in less than 1.0 ms. Thus, the FTA sensor is almost 200 times faster than using
a software based FT-control to stop the robot, as shown in Sect. 5.3.3 .
6.2.4 Realistic Worst-Case Estimate
Finally, we have conducted crash tests to simulate worst-case scenarios that could
occur during application. During these tests the robot hits an iron plate to simulate
a collision. For evaluation, we have measured the time until the robot stops and
and the distance the robot moves after the collision. We have conducted this test
with and without enabled emergency stop of the FTA sensor.
Figure 6.13 shows the distances the robot still has moved after the crash into the
plate. Robot speeds in the range from 1-100 % of the maximal robot speed are
shown. Without using an external emergency stop, the robot moves almost
100 mm after the impact into the object until the robot's hardware envelope stops
the robot. The moved distance is independent from the robot speed. When using
the FTA sensor to control the external emergency stop, the robot stops almost
immediately for slow robot speeds (slower than 10 % of maximum robot speed).
This is the typical robot speed range in which the robotized TMS system operates
[ 3 , 4 ]. For fast robot speeds, however, the robot moves up to 53 mm (at maximum
robot speed) into the object. This is due to the latencies until the brakes of the
robot react to stop the robot.
As the average maximum latency of the FTA sensor to set the emergency stop
signal is only 1 ms (cf. Sect. 6.2.3 ) which would result only in a very short
distance (even at high speed), the robot has an additional latency: Fig. 6.14 shows