Safety for robots in collaborative applications
06 October 2020
In industrial production, smart machines have been integral to many plants for a long time. Today, original equipment manufacturers, as well as other tier suppliers, are making use of new technologies for collaborative robot applications. The challenge is to guarantee safety and minimise the chance of injury when people and machines work together.
Robots designed for use in collaborative applications, sometimes referred to as cobots, are expanding the possibilities of automation, as they are often easier to deploy and use. As obstacles like system interoperability are overcome, manufacturers will integrate increasing numbers of robots. Consequently, flexible production automation will become increasingly accessible to a wider number of businesses.
ISO 8373 defines an industrial robot as: “An automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes, which may be either fixed in place or mobile for use in industrial automation applications.”
Cobots are designed to work alongside humans in a ‘collaborative workspace’, an area where the robot and the human can perform tasks simultaneously. This means that, unlike more traditional machines which are ‘caged’ by a guarding mechanism, robots used in collaborative applications often operate in the human-occupied workspace without safety fencing. However, not all ‘collaborative robots’ are guard-free, depending on their function and related safety requirements. Besides the robot itself, the collaborative robot might include an ‘end effector’, that is the tool adapted on the robot arm with which the robot performs tasks. In tandem with this new way of working, there comes new safety concerns.
The increased need for adequate protection of people from the safety risks associated with industrial robot systems led to the development of EN ISO 10218:2011, an international and European standard which covers the safety requirements for industrial robots. The standard replaced the 2006 version, and is in two parts:
• EN ISO 10218-1:2011 Robots and robotic devices – Safety requirements for industrial robots: Part 1: Robots.
• EN ISO 10218-2:2011 Robots and robotic devices – Safety requirements for industrial robots: Part 2: Robot systems and integration.
In order to ensure that humans are not exposed to unacceptable risks when working collaboratively the current standards describe four separate measures that can be used to provide risk reduction. It is required that at least one of these is fulfilled, in addition to having visual indication that the robot is in a collaborative operation.
The four measures are:
1. Safety-rated monitored stop
This measure requires that when a human has been detected entering the collaborative workspace, the robot shall be stopped. The stop condition shall then be maintained until the human leaves the workspace.
2. Hand guiding
In this mode the human can guide the robot at the end effector, by hand. Additional requirements for safety include safe-limited speed monitoring, a local emergency stop and the use of an enabling device, which is a three-position device that has to be held in the centre position.
3. Speed and separation monitoring
In this mode, the robot must maintain a specified separation distance from the human and operate at a predetermined speed. This measure requires careful risk assessment and needs to consider safety distances, which should include the speed of approach of parts of the human body as described in EN ISO 13855.
4. Power and force limiting by inherent design or control
In this mode, the power and force of the robot actuators need to be monitored by safety related control systems to ensure that they are within limits established by a risk assessment.
In all four of the measures described above, the safety related control system that provides this functionality needs to meet either:
• Safety Performance Level d (PLd), with Category 3 architecture (the identified level to which the safety related parts of a control system resist faults and their subsequent behaviour if a fault occurs) as outlined within the standard EN ISO 13849 or;
• Safety Integrity Level 2 (SIL 2) with hardware fault tolerance (HFT) 1, as set out in EN [IEC] 62061.
Safety first
While EN ISO 10218 contains some guidance on the use of collaborative robots, with the rapid pace of technological development it was widely acknowledged that this guidance needed to be enhanced. Consequently, a Technical Specification (ISO/TS 15066 Robots and robotic devices – Safety requirements for industrial robots: Collaborative operation) was published in 2016.
ISO/TS 15066 covers:
• The design of the collaborative workspace
• The design of the collaborative operation
• Methods of collaborative working
• Changing between collaborative/non-collaborative and different methods of collaboration
• Operator controls for different applications
‘Speed and separation monitoring’ and ‘power and force limiting’ are particularly elaborated in ISO/TS 15066. This includes recommendations for ‘biomechanical limits’ of pain thresholds for specific parts of the human body.
There are also working groups from the standards organisations reviewing various aspects of human-machine interactions, which will inform the development of future standards. For now, EN ISO 10218 Parts 1 and 2 and the ISO/TS 15066 specification defines the safety requirements for the sphere of collaborative robots, with the most relevant published guidance being contained in EN ISO 10218.
Guidelines from HSE
Published in 2012, a Health & Safety Executive (HSE) Research Report, (RR906) – Collision and injury criteria when working with collaborative robots, also offers some useful guidance.
The introduction to the HSE report states that “this study explored the safety, reliability and evidence for the force limits defined by the draft TS 15066, and of the methods for testing them. It also addressed whether the proposed approach in the draft TS 15066 is likely to adequately protect people from the risks. Risk assessment of potential collision scenarios, human reliability and behaviour issues, and equipment failure modes and rates are discussed, as is the adequacy of personal protective equipment against collision injuries.”
The report identifies several areas, within ISO/TS 15066, that the HSE believes needs more consideration. For example, it concludes that the psychological, behavioural and organisational aspects affecting the level of human-robot collision risk, along with the effects of human movement velocities, are not strongly represented in ISO/TS 15066. The HSE also points out that the frequency of injury is not included in the criteria for acceptable collision limits.
While the advent of the cobots offer exciting possibilities for industry, some end-effectors may create hazards – especially as contact between the collaborative robot and the operator can lead to the possibility of collision. It is vital that a complete risk assessment is undertaken before a cobot is deployed, as you would with any machinery in the workplace. This must cover the intended use of the cobot, as well as any reasonably foreseeable misuse. The basis for this risk assessment must be EN ISO 12100, in order to provide a presumption of conformity with the Machinery Directive.
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