1. Compact industrial robots are now available that can be cost-effectively integrated into even small production lines.
2. Part of their appeal is that such robots can collaborate with a human operator to offload repetitive tasks that would otherwise tire the operator and lead to mistakes.
3. The problem is that working in close proximity to a moving machine poses safety risks for humans.
4. The key to keeping collaborative robots (cobots) safe is to carefully consider the risks involved and configure the robot and its control system to mitigate potential hazards.
COLLABORATIVE ROBOTS
1. Industrial robots in major manufacturing facilities have long proven their worth in terms of increasing production throughput while reducing costs. Now compact, generalized industrial robots are bringing such benefits to mid and small scale production.
2. Unlike their larger scale counterparts, however, compact robots are designed to operate in cooperation with their human operators rather than in isolation (Figure 1). The two share a workspace, helping to minimize the robot's use of valuable production floor space and improve its cost-effectiveness.
3. Like all powered machinery, these cobots have the potential to cause injury if not utilized properly. Integration of a cobot into a production line, then, requires that careful consideration be given to the issue of operator safety.
4. Factors to keep in mind include the robot's range and speed of motion, the materials it is handling, and the operator's method and frequency of interaction. Once those are understood, appropriate safety-enhancing features can be incorporated into the system design.
5. Regulatory requirements from organizations such as OSHA (Occupational Safety and Health Administration) in the US, CCOHS in Canada, and the European Commission mandate some elements of cobot operational safety.
6. OSHA 29 Code of Federal Regulations (CFR) 1910, for instance, calls for systems to lock out hazardous energy sources during servicing operations (Section 147) and to prevent electrical shocks from occurring during operation (Section 333).
7. Such regulations, however, were developed to apply to all forms of industrial machinery and have not necessarily kept pace with technology. There is relatively little regulation specific to industrial robots in general or cobots in particular.
8. Industry has filled the gap, however, by developing several technical standards specific to industrial robots. These include the IEC 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.
9. 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
1. The road to cobot safety begins with a careful risk assessment of the intended robotic operation and usage model—not just of the robot 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.
2. 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.
3. 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:
Severity of potential injury
Frequency and/or duration of exposure to the hazard
Probability of avoiding the hazard
4. 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 own way to reflect their specific circumstances.
5. 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.
6. 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.
7. 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.
8. 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.
RISK MITIGATION
1. 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.
2. Traditional industrial robot applications have limited human-robot interaction by using cages to keep humans out of the robot's workspace with interlocks to shut down the robot when a human enters the workspace.
Source: https://www.digikey.com/en/articles/how-to-safely-incorporate-cobots-in-industrial-workplaces
