Design Considerations for Robotic Welding Cell Safety
Sergio Aguilar of Omron STI explores the inherent differences in robotic welding cells that make it important to develop a safety strategy for each cell and select the optimal technologies to match that strategy.
Inherent differences in robotic welding cells make it important to develop a safety strategy for each cell and select the optimal technologies to match that strategy.
Robotic welding cells deliver remarkably high levels of quality and productivity at a substantially lower total cost of ownership than just a few years ago. Yet today’s robots cannot function without human interaction so robots, people and other machines need to be protected from one another. A safety strategy should be developed for every robotic welding cell – this involves risk assessment and risk reduction.
The usual approach is to provide fences around the cell and an entry way that enables operators to enter to load and unload parts but keeps people out of harm’s way when the cell is operating. Primary alternatives for safeguarding the cell include physical barriers, light curtains, safety mats and laser scanners. This article will consider robotic welding cell safety in detail and consider the advantages and disadvantages of various risk reduction measures.
The increasing popularity of robotic welding cells can be traced to a number of factors. The amount of time and level of skill required to program robots has been substantially reduced, and the flexibility of the latest generation of robots makes it possible for them to go from part to part very quickly as long as the work holding tooling is designed to be quickly changed. The level of skill and experience required to operate a robotic welding cell is usually less than that which is required to produce high quality manual welds.
The dollar cost of robotic welding cells has dropped substantially over the past decade, due largely to cost reductions in the electronic equipment that makes up a high proportion of their value. Over the same period, the speed, accuracy and other capabilities of robots designed for welding applications has steadily increased.
ROBOTIC WORK CELL SAFETY CONCERNS
However, robotic welding cells present significant safety concerns. Robots lack the intelligence of a human operator, and in the event of a programming error or hardware malfunction they have the potential to unexpectedly move large distances at a high rate of speed.
The welding operation itself generates intense light flashes, smoke and fumes, and high electric currents. Protecting employees against these hazards is required to comply with regulations and, to protect a company’s most valuable assets, its employees. The same equipment that prevents injuries also provides an opportunity to make a positive impact on the bottom line. This is because the cost of a work-related injury goes far beyond hospital and medical costs.
Additional costs that commonly result from an accident include rehabilitating and retaining the injured worker, time spent by supervision and management on the incident, machine downtime, and possible litigation* (see sidebar).
According to a poll conducted by the Liberty Mutual Group, 61 percent of executives claim for every dollar spent on investments in workplace safety three dollars are saved. OSHA’s Office of Regulatory Affairs provides similar, although even more encouraging results, suggesting four to six dollars saved for every dollar invested. Furthermore, 95 percent of executives in Liberty Mutual’s poll believe workplace safety has a positive impact on a company’s financial performance.
The poll by Liberty Mutual also revealed that 40 percent of the executives reported that one dollar spent on direct accident costs generates from three to five dollars of indirect costs. So consider that an accident with direct medical and compensation payments of $15,000 will likely cost between $45,000 and $75,000 more in indirect cost. Now consider this: Indirect costs account for the majority of the accident expenses and are typically not covered by insurance.
The primary regulations on robotic safety are the Robotic Industries Association ANSI/RIA 15.06−1999 (R2009) American National Standard for Industrial Robots and Robot Systems – Safety Standards, and the RIA Technical Report RIA TR R15.106‑2006. The purpose of these documents is to provide guidelines for industrial robot manufacture, remanufacture and rebuild; robot system installation; and methods of safeguarding to enhance the safety of personnel associated with the use of industrial robots.
Minimum performance standards are specified for safety circuit integrity including electronic, pneumatic, hydraulic, electrical and mechanical components. For example, the new standard requires two separate stopping circuit functions for the robot, one for safety stops initiated by safeguarding devices and the other for emergency stops.
It is important to note, however, that this standard has been recently revised. ANSI/RIA R15.06–2012 was approved and adopted in early April 2013. The 2012 revision is a national adoption of ISO 10218–1 & –2, which only address the manufacture and integration of a robot. The RIA committee is working on new Technical Reports, (R15.306 & R15.406) to address risk assessment and safeguarding which are not included in the new standards.
DEVELOPING A SAFETY STRATEGY
The standard directs that companies that use robots develop a safety strategy beginning with identifying the machine limits and functions that pose a potential hazard. The degree of risk due to the hazard is then estimated in order to provide a basis for judgment at later stages. The risk assessment should consider the severity of potential injury, frequency of exposure and probability of injury. A risk evaluation is then performed to determine whether additional safety measures are needed to reduce the risk.