For decades, the fact that robots spent their lives behind safety fences was in line with the current health and safety regulations for automated production. The principles of human-robot cooperation break down these structures: While the unintentional access of third parties continues to be prevented by safety fences, the worker acts within the protected area during his activity and actively collaborates with his 'automated colleague'.

The advantages of collaboration between humans and robots are obvious: while the robot has tireless strength and repeat accuracy, the human contributes his intelligence, experience, perceptiveness and problem-solving skills. From an industrial perspective, this symbiosis forms an ideal unit for assembling components and loading and unloading machines, for example.

Smaller collaborative robots (cobots) are already in use in many companies, without any physical separation. However, the workspaces of robots with a higher handling capacity are still limited by guards. When planning such systems, the question arises for everyone involved: Which normative requirements need to be met?

The normative requirements for the entire cell to be planned must be taken into account, including personnel access (safety doors) and material infeed and outfeed. In addition, the cooperation between humans and robots within the safety cell must be evaluated.

The standards situation

The risk assessment and the definition of appropriate protective measures are carried out using the applicable Machinery Directive and the harmonized standards. The familiar 'standards pyramid' of type A, B and C standards applies here.

When evaluating a robot system - including in an HRC application - two specialist standards (type C standards) from the EN ISO 0218 series 'Industrial robots - Safety requirements' are primarily applied: Part 1 ('Robots') and Part 2 ('Robot systems and integration'). Part 2 is of particular interest to the integrator, as it places specific safety requirements on the integration of the robot system.

Both technical standards were supplemented in 2016 by the technical specification ISO/TS 15066 'Robots and robotic devices - Collaborative robots'. However, it is not listed in the Official Journal of the European Commission and is therefore not a harmonized standard according to the Machinery Directive.

In addition to type A, B and C standards, there are other publications such as DGUV information, TÜV Austria white papers and a VDMA position paper on the topic of safe collaboration between humans and robots. These publications provide valuable guidance and support companies in implementing HRC in accordance with the applicable safety standards.

A revised version of EN ISO 10218 Part 2 has already been available as a draft for three years, in which the specific safety requirements of ISO/TS 15066 have also been incorporated. As soon as the draft has been adopted and published in the Official Journal of the European Commission, the series of standards will fully cover the HRC application and the safety requirements for HRC will be harmonized.

The collaborative work system

Working areas of a collaborative robot system according to EN ISO 10218.

© Schmersal

The iterative risk assessment process forms the basis for the evaluation of work systems and HRC applications. As part of this process, risks are identified, protective measures are defined and the remaining residual risk is evaluated. The iteration is continued until the residual risk is acceptable. Three requirements must be met:

• Conformity of the robot system with EN ISO 10218-1
• Integration of the robot system in accordance with the requirements of EN ISO 10218-2 and other relevant standards, such as EN ISO 11161
• Evaluation of the collaboration according to ISO/TS 15066

The conformity of the robot system with EN ISO 10218-1 is certified by the manufacturer.

Work areas and protection zones

According to EN ISO 10218-2, the following areas of the robot cell must be defined:

• Protected area: Actual protective cell, usually within separating protective devices, e.g. protective fence
• Operating space: Space required by the robot to perform its work task
• Restricted area: Additional area limitation, e.g. software-supported safety-related limitations to reduce the maximum possible design movement space
• Collaboration space: Space within the protected area in which humans and robots can perform tasks simultaneously, i.e. in 'collaborative operation'

Defining the physical boundaries

The definition of these work areas and protection zones is the first step in risk minimization and defines the physical system boundaries of the HRC application. The requirements for personnel access (safety doors) and human-machine interfaces (HMIs) must also be defined. Due to the dynamics of the robot system, additional hazards may arise at the MMS that need to be evaluated. This includes, for example, overtravel characteristics during a safe stop with EMERGENCY STOP devices.

The option of software-supported, safety-related limitation of the workspace is usually already integrated at the factory. It must be parameterized by the integrator in accordance with the specified spaces.

Dynamics of the robot system

Programmable safety controllers can be used to meet HRC-specific safety requirements at the control level.

© Schmersal

The dynamics of the robot system harbor particular potential hazards. In view of payloads of over 2 t and reaches of up to 4 m, a conscientious evaluation as part of the risk assessment is essential. The integrator is supported in this by the 'lists of significant hazards' in Annex A of the EN ISO 10218 series of standards, which explicitly list the risks posed by robotic systems, such as sharp tools on the end effector. The risks are compared with the standard chapters that describe the corresponding countermeasures and protection requirements.

However, the risks posed by humans must also be taken into account, for example if several people are in the collaboration space at the same time as intended. In this case, each person must be protected by an individual control element. Possible examples of this are three-stage enabling devices.

Designing a collaborative operation

ISO/TS 15066 defines three design methods for collaborative operation, which must be evaluated as part of the risk assessment with regard to their suitability for the specific application:

Manual guidance

• Operation: A hand-operated device, usually at the end effector, transmits movement commands to the robot system.
• Type of movement: Manual guidance of the end effector by the human operator.

Speed and distance monitoring

• Mode of operation: Humans and robots work in parallel and independently in the collaboration space.
• Safety distance: A defined (safety) distance is permanently monitored and maintained.
• Speed and/or distance adjustment: If one of the parameters changes, the other is automatically adjusted. The required safety distance is always maintained.
• Safety function: If the distance is not maintained, the robot is brought to a safe standstill.

Power and force limitation

• Mode of operation: Humans and robots work in parallel and independently in the collaboration space.
• Physical contact: Intentional or unintentional contact is possible.
• Limits: Permissible force and power limits are defined in the risk assessment.
• Compliance with the limits: Inherently by the robot or by a safety-related control system.

In order to implement these design methods or requirements, a suitable safety controller is required, the functions of which must be validated in accordance with the applicable standards. Such a control system is available in the Schmersal range, for example, with the 'Protect PSC'. Its logic can be adapted precisely to the respective application.

Collaborative operation with power and force limitation

The risk assessment of an HRC application essentially focuses on mechanical hazards such as crushing and impact. An acceptable residual risk can be achieved by limiting power and force, whereby the biomechanical limit values of ISO/TS 15066 Annex A.3 must be complied with. The end effector plays an important role in complying with these limit values, as its usually sharp edges and corners can quickly lead to the limit values being exceeded. Design solutions such as enlarging the contact surfaces or foam padding should be considered right from the start of risk minimization. It is also possible to limit the force and torque using control technology.

Verification and validation

With regard to the verification and validation requirements, ISO/TS 15066 refers to ISO 10218-2. In addition to the validation of the safety functions in accordance with EN ISO 13849, further evidence may be required, for example through measurements. In practice, force and pressure measurements are carried out in HRC applications to ensure conformity with the normative requirements.

tec.nicum, a member of the Schmersal Group, supports the user in implementing the conformity assessment procedure described, from risk assessment to final verification and validation. In addition, the tec.nicum Academy offers a supplementary seminar on HRC in its training program.

The authors:

Benjamin Bottler is a Safety Consultant at K.A. Schmersal in Wuppertal.

Tristan Willigsecker is a Safety Consultant at K.A. Schmersal in Wuppertal.