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What does the future hold for collaborative robots?

07 June 2015

Collaborative robots - sometimes referred to as 'cobots' - are designed to work alongside humans in a so-called 'collaborative workspace'. Stewart Robinson examines the safety issues surrounding their use.

Unlike more traditional machines, which are ‘caged’ by a guarding mechanism, collaborative robots 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.

Cobots are expanding the possibilities of automation as they are often easier to deploy and use. Consequently, more flexible production automation will become increasingly accessible to a wider number of businesses, including SMEs. However, they do represent new safety concerns.

Standards development
The increased need for adequate protection of people from the safety risks associated with industrial robot systems led to the development of an international and European standard which has two parts:

•EN ISO 10218-1:2011 Robots and robotic devices — Safety requirements for industrial robots — Part 1: Robots. Its scope covers scope industrial use, controllers and manipulators.
•EN ISO 10218-2:2011 Robots and robotic devices — Safety requirements for industrial robots — Part 2: Robot systems and integration. Its scope covers robot (see Part 1), tooling, work pieces, periphery and safeguarding.

While both of these already contain some guidance on the use of collaborative robots, with the rapid pace of technological development, it is widely acknowledged that this guidance needs to be enhanced.

Consequently, a Technical Specification (ISO/TS 15066 Robots and robotic devices — Safety requirements for industrial robots - Collaborative operation) was started in 2010, and is currently under preparation, to deal specifically with this situation. The document is currently a Technical Specification, as more application knowledge is needed before publishing a finalised collaborative robot standard.

There are also working groups of the standards organisations reviewing various aspects of human-machine interactions, which will also inform the development of future standards. But 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.

Current requirements
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 requirements needs to be fulfilled, in addition to having visual indication that the robot is in collaborative operation. The four measures are:

Safety-rated monitored stop - This measure requires that when it is detected that a human has entered the collaborative workspace, the robot should be stopped. The stop condition should then be maintained until the human leaves the workspace. 

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.

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 take account of safety distances, which should include the consideration of approach speeds of parts of the human body as described in EN ISO 13855.

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 these measures, 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) 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.

The cobot heralds the dawn of robotic systems that can safely interact with human workers while effectively performing simple industrial tasks. However, it 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 industrial workplace, with the basis for this risk assessment being EN ISO 10218 Parts 1 and 2, alongside the Machinery Directive.

Stewart Robinson is principal engineer and functional safety expert at TÜV SÜD Product Service

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