FTA-Sensor: Combination of Force/Torque and Acceleration - Robotized Transcranial Magnetic Stimulation - page 108

6.1.5.4 Realistic Worst-Case Estimate
Concluding, we perform a set of crash tests with the robot to estimate the time
needed to a full stop after an impact in relation to the robot speed. Furthermore, we
measure the distance the robot moves after the impact until the full stop. There-
fore, we connect the FTA sensor to a host computer to continuously query the
force, torque, and acceleration data from the FTA sensor. To this end, we place
rubber foam covered by an iron plate next to the robot. First of all, we measure the
position of the plate in robot coordinates (position when the robot's end effector
touches the plate). Starting from an initial position roughly 500 mm above the
plate, we move the robot downwards to crash into the iron plate. The setup is
illustrated in Fig. 6.8 . During robot motion, we are recording the forces, torques
and accelerations from the FTA sensor. Additionally, we are recording the cor-
responding host computer's system time. Also, we measure the robot end effector
position when the robot stops. We repeat this crashtest with different robot speeds
ranging from 1-100 % of its maximum speed. Furthermore, we are performing the
crashtest with the FTA's emergency stop enabled and without external emergency
stop (e-stop bridged). As a security limit we are using 10 N. As the end effector is
aligned vertically, the main impact on the FTA sensor will be detected as the z-
directed force f z .
By evaluating the recorded data, we can estimate the time needed for a full
robot stop t stop by comparing the time-point at the detected impact t ð f z [ 10 N)
with the time-point at the maximum force t ð max ð f z ÞÞ which is the time-point of the
robot stop:
Fig. 6.8 Setup for realistic
worst-case estimate. Once the
robot hits the iron plate, the
time is measured until the
robot stops