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6 Aug 2015

Safe human-robot collaboration: from theory to practice

It's all down to the application

Robot applications play an increasingly important role in the automation of production processes. Efficiency increases, the closer human and machine are able to work together.

Robot applications play an increasingly important role in the automation of production processes.  Efficiency increases, the closer human and machine are able to work together.  Static or one-dimensional safeguards are increasingly reaching their limits.  The trend is moving away from full enclosures for robot cells in favour of human-robot collaborations (HRC), which manage without guards where possible, but still guarantee operator safety.  In practice, each application requires a separate safety-related assessment.

Ultimately a safe HRC application is the result of several factors: interplay between normative framework conditions and a complex risk analysis on this basis, selection of a robot with the relevant safety functions, selection of appropriate, additional safety components and finally, validation through a systems integrator.

The classic way to achieve safety in industry is to surround plant and machinery with various mechanical safeguards.  Maximum safety is achieved through the strict separation of work areas.  To raise the economic potential of the HRC, operator and machine must be able to work together as closely as possible.  The result is an increase in demand for intelligent, dynamic safety solutions, in which the safety functions can be adapted more flexibly to the changing protection requirements.

New standards in progress

Robots are classed as partly completed machinery in terms of the Machinery Directive. The two standards ISO 10218 "Safety of Industrial Robots" Part 1: "Robots" and Part 2: "Robot systems and integration" are available for detailed safety requirements.  The German editions of both these parts are published as EN ISO 10218-1:2011 and EN ISO 10218-2:2011 and are listed as harmonised C standards under the Machinery Directive 2006/42/EC.  Part 2: "Robot systems and integration" also contains information on collaborative operation.

When planning an HRC application, robot selection is always an important element for the systems integrator.  EN ISO 10218-1 also includes safe drive functions.  In accordance with EN 61800-5-2 (Adjustable speed electrical power drive systems - Part 5-2: Safety requirements – Functional), these include:  Safe Operating Stop SOS, Safely Limited Speed SLS, Safe Speed Range SSR and Safely Limited Torque SLT.

The requirements of "safety-related parts of the control system" (electrics, hydraulics, pneumatics and software) are clearly defined in Clause 5.2 of EN ISO 10218-2 Robot systems and Integration.  The safety-related parts of the control system must be designed in such a way that they comply with PLd in Category 3 (ISO 13849-1:2006) or SIL2 with single fault tolerance and have an MTTFd of at least 20 years (IEC 62061:2005).

Various methods can be applied to verify and validate safety requirements, including visual inspections, practical tests and measurements.  The systems integrator has to verify or validate over 200 points in all.

The status of the standards is clear, therefore, in theory.  In practice, however, the question remains as to whether an HRC can be implemented safely with this standards framework.  To demonstrate the ways forward, the international standards committee ISO/TC 184/SC2 WG3 was tasked with developing the Technical Specification ISO/TS 15066: "Robots and Robotic Devices - Collaborative industrial robots".  Pilz, the complete safe automation supplier, is a member of this international standards committee and actively works on formulating this specification with robot manufacturers, integrators, notified bodies such as BG and other automation companies.  The current draft ISO/TS 15066 "Robots and Robotic Devices - Collaborative industrial robots" substantiates solutions for safe human-robot collaboration in an industrial environment.

Body area model for pain threshold values

A body area model is defined in the Annex to the Technical Specification (TS).  The model defines points in the corresponding body areas, with details of the respective pain threshold.  Once ISO/TS15066 has been published, these pain threshold values can be applied as validation for safe HRC.  The body area model provides details of the respective pain threshold for each part of the body (e.g. on the head, hand, arm or leg), which marks the start of the pain threshold.  If the application remains within these thresholds during any encounter between human and robot, then it complies with the standard.

The Technical Specification currently has the status of Committee Draft.  As with any new draft of a standard, it's difficult to judge when the final version of ISO/TS 15066 will be available.  In the current roadmap, publication as ISO Stage 60.60 is planned for the end of 2015.

Separate standard for personal care robots in the non-industrial sector

ISO 13482 (Robots and robotic devices – safety requirements for personal care robots) is the first safety standard to deal with direct contact between human and robot in the non-industrial sector.  ISO 13482 was developed as a C standard in the international standards working group ISO/TC 184/SC2 WG 7, in which Pilz is actively involved.  The standard was published as an ISO version on 1 February 2014 and is currently being translated into German.

Steps toward a safe HRC application

When the normative specifications are implemented, the fact that robot cells are classified as a machine under the Machinery Directive means that each step of the conformity assessment procedure must be completed.  It should be noted that the robot itself is only regarded as partly completed machinery; it is not until the gripper or the necessary tool for the respective application is in place that the robot achieves a specific purpose and must then be regarded as final machinery.  The integrator or user becomes the manufacturer of the machinery and is responsible for the safety-related inspection, including CE marking.

One of the most important points en route to achieving a safe robot application is to produce a risk analysis in accordance with EN ISO 12100.  The risk analysis should include identification of the applicable harmonised standards and regulations, determination of the machine's limits, identification of all the hazards in each of the machine's lifecycle phases, actual estimation and assessment of the risk, plus the recommended approach for reducing the risk.  On robot applications, the challenge for the risk assessment is the dissolution of the boundaries between the two work areas for human and machine.  The operator's movement must be considered in addition to the hazards emanating from the robot. However, these cannot always be calculated in terms of speed, reflexes or the sudden approach of additional people.

Based on the risk analysis, the next steps are the "safety concept" and "safety design", including component selection.  The results from the "risk analysis" and "safety concept" are used to document the selected safety measures in the risk assessment and to implement these in the "system implementation".  This is followed by the "validation", in which the previous steps are re-examined.  Have the protective measures been implemented correctly?  Has the safety concept been designed correctly in association with the machine control system and has it been implemented in accordance with the safety regulations?  Validation is essential for proving that machines are safe.  The check lists in EN ISO 10218-2 provide additional guidance for robot applications.  By attaching the CE mark, the integrator ultimately confirms that the robot cell and its assured properties meet all the legal requirements of the Machinery Directive 2006/42/EC when used in accordance with its intended purpose.

Selection of robot and safety components

A wide range of robot systems are available on the market and are suitable for various application areas.  Although they form the basis of a safe robot application, a safety-related assessment of the application and any additional components and systems is always required in order to implement a safe HRC.

Reacting to a safety-related event with a total shutdown can only be regarded as a last resort. Reactions should no longer be triggered only by simple logic connections, but by complex states or results of intricate calculations, to which the safety function must react appropriately.

In future we will need safety systems that are considerably more intelligent so that work areas in which human and machine collaborate can be designed safely, even without guards.  These systems may be part of the actual robot control system, for calculating the robot's movements safely for example.  This enables the path of the robot arm to be calculated in advance.  In many cases however, safe motion functions such as these will not be enough to achieve the safety objective.  Combinations will often be needed, which will include near-field protection (e.g. tactile sensors or infrared sensors on the robot arm), personal protection equipment (safety goggles and clothing) and safe sensor technology for monitoring the detection zone.

For dynamic safety concepts, these sensors must be able to assess events in a clearly graduated manner.  For example, they should be able to distinguish whether a person is within the potential action radius of a hazardous movement (warning zone) or has already accessed a zone with an increased safety requirement (detection zone).  It must be possible to adjust these zones dynamically and to track the movements of a machine or robot, for example.  SafetyEYE, the safe 3D camera system from Pilz, is able to monitor warning and detection zones safely in 3D.  Integration of sensor, control and actuator technology opens up new freedoms when it comes to planning dynamic process cycles and work areas in which human and robot interact safely.

Summary

Interaction between human and robot increasingly requires new technologies and solution approaches to guarantee that they cooperate safely.  Practically every robot application is different.  A safe robot application can only be achieved if the safety concept is implemented using the right choice of robot and its safety functions, combined with intelligent safety components.  Safety plays a key role in robot applications.  As the aspiration is to achieve the closest possible collaboration between human and machine, new standards have already been published.  The industry's desire for "powerful" robots for HRC will be the challenge of the future when it comes to complying with the normative specifications for functional safety.

Robotics person with helmet

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