How to safely incorporate cobots in industrial workplaces
own way to reflect their specific circumstances. One of the insights that ISO/TS 15066 has brought to the industry, however, is a quantitative definition of physical contact between robot and human that is non-injurious. This definition is especially important in cobot applications, where physical contact is highly likely or even intended. The standard defines two types of contact: transient and quasi-static ( Figure 3 ). Situations in which the human can readily move away from contact with the robot, such as a robot part bumping against the operator’s arm, are considered transient. When the human is trapped between the robot and a fixed object, such as a robotic gripper pressing the operator's hand against the tabletop, the contact is considered quasi-static. The limits for force of contact in a cobot application are based on the human threshold of pain. Collaborative robots must be configured so that any contact, intended or otherwise, will be below the pain threshold. Force limit values vary depending on what body part is involved. Head contact has a much lower pain threshold than arm contact, for instance. Further, quasi-static contacts have lower thresholds than transient contacts.
Situation
Contact type
Intended
Transient
Robot-Human Contact
Accidental
Quasi- stationary
Result of failure
Figure 3: Robot-human contact—accidental or anticipated—falls into two categories: transient and quasi-stationary. (Image source: Richard A. Quinnell)
Once risks have been identified and evaluated, the critical question to ask for each is, "Is this an acceptable level of risk?" In most cases, a negligible or very low risk is tolerable and everything else will require one or more forms of mitigation.Choosing an appropriate form of risk mitigation followed by re-evaluation of the risk are thus the next steps along the road to robot safety, to be repeated until all risks have been reduced to acceptable levels.
cages to keep humans out of the robot's workspace with interlocks to shut down the robot when a human enters the workspace. For cobot applications, where robots need to share a collaborative workspace with humans, other methods are needed. The industry has identified four key approaches for collaborative robot- human interaction: n Safety-rated monitored stop n Hand-guiding n Speed and separation monitoring n Power and force limiting Developers will need to determine which approach or combination of approaches best fits their application. The safety-rated monitored stop works well in applications where the operator interacts with the robot only under specific conditions, such as loading or
Figure 2: Risk level assessment requires examining the severity and likelihood of possible injuries. (Image source: Richard A. Quinnell)
Once the risks are identified, each must be evaluated. This evaluation categorizes each such interaction as a negligible, low, medium, high, or very high risk using three key criteria: n Severity of potential injury n Frequency and/or duration of exposure to the hazard n Probability of avoiding the hazard A representative risk evaluation tree is shown in Figure 2 . The severity of injury ranges from minor, such as cuts or bruises that completely heal in a few days, to serious, resulting in permanent damage or death. Exposure ranges from low (occasional) to high (frequent or continual), and avoidance probability ranges from likely to not possible. Evaluators can quantify these criteria in their
itself, but the entire application and operating environment. A robotic system handling sharp-edged sheets of metal, for instance, creates different risks than those of a system handling cardboard boxes. Similarly, risk assessment for a robot equipped with a gripper will differ from that of a robot with a drill or soldering iron. Thus, developers must fully understand the system's scope of operations, the robot's movement characteristics, the workspace and workflow, and other similar factors in order to identify the potential risk sources in robot operation. These sources include any possible robot-human interaction—whether intended, inadvertent, or resulting from equipment failure—that might result in an injury of some kind.
61508 standard on functional safety, the ISO 12100 standard on design for machine safety, and the ISO 10218-1 and -2 standards on safety for industrial robots. Most recently, industry has released the ISO/TS 15066 technical standard on collaborative robot safety. Only some sections of these standards are defined as requirements for robotic system design. The rest are recommendations that provide developers and operators with detailed guidelines for ensuring safe interaction of robots and humans. Cobot risk assessment The road to cobot safety begins with a careful risk assessment of the intended robotic operation and usage model—not just of the robot
Risk mitigation avenues
Some of the most preferred methods for risk mitigation include redesigning the process or layout of the robotic workspace to eliminate the hazard or to minimize exposure by limiting human interaction with the robot. Traditional industrial robot applications have limited human-robot interaction by using
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