Master Thesis - Hochschule Bonn-Rhein-Sieg

6. Evaluating the safety Master Thesis Björn Ostermann page 108 of 126
“Soft limits are software-defined spaces that define the restricted space where the robot is
inhibited from exiting or entering while in automatic mode or any mode using speeds above
reduced speed. […] Motion may be restricted to keep the robot inside the defined space or to
prevent the robot from entering or accessing the defined excluded space.
[…]
A safety-rated soft limit shall be set as a stationary zone that cannot be changed without a
system power up condition and shall not be changed dynamically. […]” [70]
� 5.12.4 Dynamic limiting devices
“Dynamic limiting is the automatically controlled change in a robot’s restricted space during a
portion of the robot system’s cycle. Control devices such as, but not limited to, cam operated
limit switches, light curtains [...] may be utilized to further limit robot movement within the
restricted space while the robot performs its task programme provided the device and
associated controls are capable of stopping the robot motion under rated load and speed
conditions and the associated safety controls comply with category 3 of ISO 13849-1, unless a
risk assessment is performed and determines that another category is required.” [70]
While the standard allows “dynamic limiting devices”, such as the flexible fence, it forbids the
inclusion of this fence in the safety concept of the used robots safety trips (see chapter 3.1), since it
demands for the robot’s control to be rebooted for every change.
The connection of safety trips and developed approach is not yet realized, but in order to achieve the
category 3, as requested by this standard and the EN 13849-1 as well (see chapter 6.2.3), this would be
an advantageous step in the future.
The standard therefore can be applied only in parts. Future developments in the field of collaborative
workplaces, “flexible fences” becoming state of the art, will possibly lead to changes in this standard.
6.3 Achieved safety
To estimate the achieved safety of the workplace, the system was tested on its performance. From this
performance the necessary safety distance of the system is calculated in this chapter. During the course
of this chapter the used equipment is treated as if it were safe, knowing well that this is not the case.
All tests were performed on an Intel Core 2 Duo E8200 (2.66 GHz), equipped with 2 GB of RAM,
running WinXP SP2. The developed program was executed in the debug mode of Visual Studio 2008
Version 9.0.30729.1 SP, in order to acquire the different runtimes.
The additional computational overhead of the debug mode has only minor influences in the complete
runtime, since the largest amount of time is used for communication between camera, PC and robot’s
PLC.
The error in measurement is too low to have a visible influence and was thus omitted in this chapter. If
the presented calculations would not be done to show a measurement of the achieved results of this
thesis and the constructed demonstrator, but to calculate the performance of such a system in industrial
praxis, an additional error calculation would be necessary.
Due to the necessary accuracy of the camera, used in the workplace, only 10 frames per second (FPS)
could be delivered by the camera. The camera is able to deliver more frames per second, but when it