(Translation of German Websites)
Collaborative robots (Cobots) are provided for a human machine collaboration. Additional safeguards like fences or light grids may be widely dispensable.
If a contact between robot and human happens: How much force or pressure is acceptable? Shall the forces or pressures be measured before? And how are they to be measured?
E.g. in case of an unintended access of a person to the working zone a contact between cobot and human can happen. The contact situations are distinguished as follows:
- Quasi static contact: Person or body part (e.g. hand) are clamped, they cannot evade
- Transient contact: Person or body part (e.g. upper arm) are only bumped but not clamped, they can evade
In order to avoid injury thresholds have been established. The thresholds are distinguished as follows:
- Pressure thresholds. They are much important in case of edged shapes e.g. at workpiece or robot tool
- Force thresholds. In most cases they are relevant when contact is spread to a larger surface e.g. at robot arm or at padded areas.
You can find the present valid thresholds in the Technical Specification ISO/TS 15066 or in the public available DGUV-Information FBHM 080.
If the cobot integrator wants to show that the cobot machine conforms to the present standards it seems very clear that the contact forces and pressures are to be measured. Guidelines for selection of measurement points you can find here.
Always both have to be measured, force and pressure. If one number exceeds the threshold the test has failed. But how to measure?
The cobot technology is quite new. However, the requirements for measurement of clamping situations have been widely established: The measurement device has to be fixed at the relevant contact point within the robot working zone. The measurement test has to be done by starting the relevant robot motion. The measurement device is placed at that point where in later application the body part would be exposed. After contact between cobot and measurement device the test results are displayed at the measurement device.
The requirements for measurement of transient contacts are still not consistent. Because of a fixed measuring device evading body parts cannot be considered. Here is a short overview of measurement methods for transient contact which already used or in preparation:
- DGUV-Information FBHM 080 This document of the German Occupational Health and Safety Insurance has been published in 2017. It is applied in practice with good success. The transient cases will be measured by fixed measurement device. Because the measurement device cannot evade during measurement non-relevant parts of the measurement may be ignored. So that only the transient parts of the measurement are considered.
- ISO/DPAS 5672 This specification is still a draft. The transient cases will be also measured by a fixed measurement device. Formulas are provided for extraction of the transient parts from measurement. The final version of the document has to be waited for.
- ISO/DIS 10218-2:2020 This document is still a draft. For measurement of transient cases a movable carriage is required. The carriage has to be loaded by weights according to the bumped body parts. Probably this measuring method is very precise because of less necessary simplifications. The mechanical measurement appliance could require more efforts. We have to wait and look if this measurement method will be still incorporated in the final standard.
- The measurement device is hold at the human body. Obviously, this measuring method is applied in practice frequently. Technically it may be sufficient for a roughly measurement. However, it is less reproduceable. Furthermore, accident risks could arise from the non-finally parametrized robot system. Therefore, this measurement method cannot be recommended
