Cobots: Putting safety first

Making sure that cobots are inherently safe is a challenge that has been steadily addressed with the implementation of various safety standards. The development of applicable safety standards started with ISO 10218-1:2006 (updated in 2011) and ISO 10218-2:2018. Part 1 covered just robots (manipulator and controller) while Part 2 covered the robot system/cell and the application. Since 2016 the ISO/TS 15066:2016 specifies the safety requirements for collaborative industrial robot systems and the work environment, and supplements the requirements and guidance on collaborative industrial robot operation given in ISO 10218 1 and ISO 10218 2.

It is these standards that enable businesses today to create an environment where cobots can operate alongside humans full-time, performing cooperative tasks and this has resulted in the creation of a variety of dedicated solutions.

Cobots were developed to be deployed to work safely side-by-side with humans. “By taking over the most laborious and potentially dangerous tasks cobots can enhance the working environment for human employees by freeing up their time to focus on more creative and less physically demanding tasks,” said Mark Gray, UK sales manager at Universal Robots. 

“Cobots from Universal Robots incorporate a number of safety functions designed to significantly decrease the risk of harm,” said Gray.  “Functions include highly sensitive sensors which will instantly halt all of its operations should it come into contact with an unexpected object.”

However, according to Oliver Giertz, product manager for servo/motion and robotics for the EMEA region at Mitsubishi Electric, the classic image of humans working alongside robot partners is quite a limited definition of modern collaborative robotics. “With appropriate safety considerations it is possible to use robots that would more traditionally be thought of as industrial robots within a ‘collaborative environment’. Mitsubishi Electric has coined the term ‘cooperative robotics’ to encompass this wider definition of collaboration,” he said.

It requires additional thinking on safety to bring a faster, more powerful robot into a classic collaborative robot environment. “It extends collaboration beyond the typical low speed, low torque desktop collaborative robots. While a conventional cobot is brought to a stop by the small increase in torque when an object (human operator) is touched for example, a more powerful robot requires the use of additional safety technologies,” said Giertz. 

Mitsubishi Electric has addressed this with its MELFA robot with additional safety skin. Based on an industrial robot, this alternative breed of cobot is covered in a pressure sensitive skin that reacts to contact to bring the robot to an immediate stop – a good solution where people may occasionally experience proximity to the robot, rather than continuously. 

“In such scenarios higher cycle speeds can be achieved to maximise productivity,” continued Giertz. “On this basis we can further extend the definition of cobots to look at environments where humans are not working alongside the robot but require regular access to the robot cell. In such applications adjustable safety zones (for example using safety scanners) combined with functions such as safe limited speed (SLS), safe limited position (SLP) and safe torque range (STR) progressively limit the position, speed and torque of the robot as the human approaches. Although there is still a risk assessment process required for each application and the environment it is situated in, such a solution can enable powerful robots to be used safely in a cooperative environment.”

Typical uses for classic cobots include pick-and-place applications, handling operations between different production steps or for follow-the-line applications where the robot has to follow precisely a specified trajectory.

In human-robot collaborations (HRCs) like this, the workspaces of humans and robots overlap both spatially and chronologically to combine the strengths and advantages of the machine – such as reliability, endurance and repeat accuracy – with human strengths of dexterity, flexibility and the capacity to make decisions. 

Jochen Vetter, consulting services manager for Pilz GmbH & Co, warns that even cobots cannot provide safety on their own. It is important to understand that there are no safe robots, 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,” he said. 

The Technical Specification ISO/TS 15066 ‘Robots and Robotic Devices - Collaborative industrial robots’, plays a key role here, making it possible to implement safe human-robot collaborations following appropriate validation. The specification describes four types of collaboration as protection principles.
 

• Safety-rated monitored stop

• Hand guiding

• 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 Annex to ISO/TS 15066 describes a body model. It provides information for each part of the body –  on the head, the hand, the arm or the leg – about the respective collision limit values. If the application remains between these limits when a human encounters a robot, then it is compliant. These pain threshold values are used in practice to validate a safe HRC and form the basis for implementing the application with ‘power and force limiting’. 

Pilz has developed a collision measuring device – the PRMS – 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, evaluating them in software and comparing them with ISO/TS 15066 specifications. 

Vetter highlights the big challenge in the basic risk assessment for robot applications: “The boundaries between separate working areas for man and machine have ceased to exist.” As well as 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. The next steps – the ‘safety concept’ and ‘safety design’ – cover the ‘human element’ as well as selection of the correct components. These are usually a combination of interlinked intelligent sensors and control systems that make the necessary dynamic working processes possible. The selected safety measures are then documented and implemented in the ‘system integration’ step. This is followed by ‘validation’, when the previous steps are scrutinised again. 

Many cobots are designed to be moved around a facility, so as production teams learn how to deploy and redeploy their cobots the associated safety systems must become almost ‘plug-and-play’. “There is little point in investing in any robotics application, only to implement a safety system that restricts the very workflow efficiency you are trying to enhance,” said Dr Martin Kidman, a machinery safety product specialist at SICK (UK). “A well-designed safety system will safeguard machinery uptime, minimise stop-start operations and carefully consider the space needed to enable a safe protective area around the robot. A risk assessment must still be carried out, even if the robot manufacturer has incorporated features into the design that reduce risk.” 

There are two parts to any safety equation – Firstly, is the hardware inherently safe?  Secondly, is the way it is used safe? System manufacturers and integrators are required to conduct checks of the structural safety measures taken by the robot manufacturer. They must also consider any hazards or risks that may remain and design the robot systems accordingly.

“For example, it is just as important to consider the design of the robot tool or end-effector chosen for the task, the workpiece itself, or other machines that may be within the workspace,” said Kidman.

It is EN ISO 10218 that details the hazards and requires the safety-related parts of control systems to be designed to comply with PLd (ISO 13849) or SIL2 (IEC 62061), while ISO TS 15066 focuses specifically on the safety of collaborative robots. 

Depending on whether the application is for a cooperative, collaborative or interactive use of robots it is often necessary to incorporate additional measures to reduce risk. These additional measures often come in the form of sensing technologies and systems such as safety light curtains or laser scanners that detect human presence or determine the speed at which humans are moving towards the hazardous area and their distance from it. 

As the use of cobots in the manufacturing sector increases, it is vital to remember that contact between cobots and humans can lead to the possibility of collision. Even when employing a robot designed from the outset to work collaboratively with humans, risk assessments should always be completed before the cobot is used – and for all of its possible applications. Remember that there are no safe robots, only safe robot applications.