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Collaborative robot safety issues

28 September 2015

Stewart Robinson highlights the safety issues that need to be considered as collaborative robot solutions come to market.

Collaborative robots, or cobots, are designed to work alongside humans to perform tasks simultaneously. While cobots will see more flexible production automation becoming increasingly accessible to a wider number of businesses, they do present 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 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. 

• EN ISO 10218-2:2011 Robots and robotic devices — Safety requirements for industrial robots — Part 2: Robot systems and integration. 

The current standards describe four separate measures that can be used to provide risk reduction in order to ensure that humans are not exposed to unacceptable risks when working collaboratively and at least one of these measures must be fulfilled, in addition to having visual indication that the robot is in collaborative operation.

The four measures include:
1. 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. 

2. Hand guiding: In this mode the human can guide the robot at the end effector by hand, the tool adapted on the robot arm with which the robot performs tasks. 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 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.

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 the 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.

Necessary enhancements
While EN ISO 10218 already contains some guidance on the use of cobots, it is widely acknowledged that, with the rapid pace of technological development, 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 a Technical Specification, as more application knowledge is needed before publishing a finalised cobot standard.

ISO/TS 15066 covers:
• The design of the collaborative work space.
• The design of the collaborative operation.
• Methods of collaborative working.
• Changing between methods.
• Operator controls for different applications.

The methods of collaborative working ‘speed and separation monitoring’ and ‘power and force limiting’ are particularly elaborated on within ISO/TS 15066, receiving a lot of coverage within the Technical Specification.

Health & Safety Executive
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 in the UK. 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 that the HSE considers need more consideration within ISO/TS 15066. 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.

The cobot sees the dawn of robotic systems that can safely interact with human workers while effectively performing simple industrial tasks. However, as contact between the cobot and the operator can lead to the possibility of collision, a complete risk assessment should be completed before the cobot is used.

Stewart Robinson is principal engineer and Functional Safety Expert at TÜV SÜD Product Service.

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