Current developments in the field of robot applications are characterised by the desire to allow man and machine to work as closely as possible with each other. Instead of man and machine cooperating by means of defined static transfer points, both partners, in the future, will collaborate in a shared workspace and be deployed together on a flexible basis, such that they can exploit their respective strengths.
A new type of industrial robot is ready for such a task, called a cobot. A cobot is a combination of the words ‘collaboration’ and ‘robot’. These lightweight robots can move loads of about 10 kg and have a sensory, tactile capability. As service robots, they are intended to ‘give humans a hand’ with physically burdensome or monotonous tasks. Typical uses include pick-and-place applications, handling operations between different production steps or follow-the-line applications where the robot has to follow precisely a specified trajectory, for example, for bonding tasks or when tracing a contour.
In human-robot collaborations (HRC) like this, the workspaces of humans and robots overlap, both, spatially and chronologically. By contrast with cooperation, humans and robots share a single working space in the case of HRC. This combines the strengths and advantages of the machine – reliability, endurance and repeat accuracy, with human strengths including flexibility and the capacity to make decisions. The most conspicuous difference between ‘classic’, enclosed robot applications and human-robot collaboration is that collisions between machines and humans are a real possibility. But they must not be allowed to result in any injuries. This makes it important to take up safety measures and deal with their challenges.
Only the application can be safe
Despite new cobots, robots cannot provide safety on their own. There are no safe robots; there are only safe robot applications. Safety results from the interaction of normative boundary conditions, the risk analysis that is based on it, the selection of a robot with the corresponding safety functions and the matching additional safety components, and finally, from validation.
This means that the ISO/TS 15066 ‘Robots and Robotic Devices - Collaborative industrial robots’, plays a pivotal role. This technical specification makes it possible to implement safe HRC, following appropriate validation. Four types of collaborations are described in ISO/TS 15066 as protection principles:
Safety-rated monitored stop
Speed and separation monitoring
Power and force limiting
When implementing a safe HRC, system integrators can choose one of these types of collaboration or a combination of them for their application.
The technical specification is, moreover, the first standard that provides detailed information on pain thresholds for various parts of the body in its Annex A. These values form the basis for implementing the application with ‘power and force limiting’.
In practice, it has been found that HRCs can often be achieved by combining a ‘speed and separation monitoring’ and ‘power and force limiting’ in ISO/TS 15066.
The Annex to technical specification ISO/TS 15066 describes a body model. It provides information for each part of the body, for instance, the head, hand, arm or leg and about the respective collision limit values. If the application remains between these limits when a human encounters a robot, then it is standard-compliant. These pain threshold values are used in practice to validate a safe HRC. Pilz has developed a collision measuring device, PROBms, to measure forces and speeds. Equipped with springs and appropriate sensors, the device can record precisely the forces generated in a collision with a robot, evaluate them in software and compare them with the specifications from ISO/TS 15066.
CE Marking: The final step
Manufacturers of robot applications are, likewise, subject to the principle that, by law, if they are exporting to the European Union, they must carry out a conformity assessment procedure with CE marking. Attaching the CE mark confirms that the robot application meets all the necessary health and safety requirements. The challenge in the basic ‘risk assessment’ is that the boundaries between separate working areas for man and machine have ceased to exist. Including the hazards presented by a robot, the human’s movements need to be taken into consideration. But they are not always calculable in terms of speed, reflexes or the sudden arrival of another person.
They then follow the ‘safety concept’ and ‘safety design’ steps, which involve selection of the components. These are usually an amalgamation of intelligent sensors that are interlinked, and control systems that make the necessary dynamic working processes possible in the first place. The selected safety measures are then documented in the risk assessment and implemented in the ‘system integration’ step. This is followed by ‘validation’, when the previous steps are scrutinised again.
The ultimate safe robot or safe sensor technology to cover all possible safety scenarios, in practice, has not been achieved yet. The demands for safety technology always depend on the respective application. Safe robot cells can only be set up within the overall context of the robot, tool and work piece, including any associated machinery, such as, the conveyor technology. This means that every application calls for a separate safety assessment in practice.
Implementing HRCs in an industrial environment is definitely going to increase. However, its growth will be heavily dependent on innovations in the fields of sensor technology and robotics. Together, automation engineers, robot manufacturers, integrators and notified bodies will be able to turn the vision of a robot workmate into reality, on a step-by-step and application-by-application basis.
Pilz has been actively involved with robot manufacturers, integrators, notified bodies in defining this pioneering standard for human-robot collaboration in the industrial environment. The company supports users with a services portfolio tailored to the individual life cycle phases of a robot system – from process analysis to risk assessment and beyond to CE marking. A specific training package on robot safety completes the range of services.
Courtesy: Pilz India