Safety issues in modern applications of robots

S.P. Gaskill, S.R.G. Went

Reliability Engineering and System Safety, 53, 1996

Summary

Industrial Robot design and selection is today closely related with the robot's final application and its integration with the other machines, several hazard aspects are involved due to movements and high energy.

One extreme difficulty in taking decisions related to safety is the fact that all robots use programmable electronic systems (defined with the acronym PES), which are complex to evaluate. Programmable Electronic Systems are reliable and offer a wide functionality, wider than hard wired control systems, however they are complex and it may be impossible to give any prediction regarding safety, since systematic failures, especially software faults, could cause unexpected actions from the device.

For this reason in EN60204-1:1993 sub clause 12.3.5, 'Safety of Machinery - Electrical Equipment of Machines: Part 1 "General Requirement"', there is a preference for hard-wired electro-mechanical components for emergency stop functions; where this is not possible other measures should be used (for example self diagnostic checking features). On June 14th 1989 the European Community signed the Machinery Directive (in UK it is known as "The Supply of Machinery Regulations 1992"), which today is the basic directive in the robotic field, requiring that certain technical documentation, including the technical construction file, must be available any time for inspection.

At present every robot maker has to provide the proper documentation, including conformity about the integration in the cell where the robot is installed, also all documents must be available within at least 10 year from the first machine's operation.

The European Law in the mentioned regulation is providing an appendix, the EHSR (Essential Health and Safety Requirements), where general terms are reported for operators’ safety. Currently the EN775:1992 (ISO 10219) ‘Manipulating Industrial Robots – Safety’, bases technical measures for preventions of accidents on two principles: 1)Absence of people in the safeguard space during automatic operation; 2) the elimination of hazards or at least their reduction during interventions (e.g., teaching program verification) in the safeguards space.

Acceptable levels of safety are regulated by IEC1508 'Functional safety: safety-related systems', which tends to minimize risk, but still  today tests and trials are not enough to prove the safety of a robots and therefore a risk based, quality approach throughout the lifecycle of the machine is performed.

In the international standard IEC1508, systematic errors are taken under consideration under the concept of "target safety integrity levels", which are chosen according to the amount of risk reduction attributed to the related system in order to reduce the overall risk to a tolerable level.

In the European Union law there are primarily two ways to conform the technical measure required:

Designer are assisted by the standard EN292, ‘Safety of Machinery - Basic Concepts for design’, interpreting EHSRs (Essential Health and Safety Requirements) guiding the producer through: determining the boundary of the system (space, time), identifying and describing the nature and consequences of the constraints of the system (specially hazards related with human robot interaction during the life cycle of the robot), assigning a risk level for each possible hazard and finally ensuring that safety is adequate.

Safeguarding is regulated by EN775 'Manipulating Industrial Robots - Safety" where prevention of accidents is based on two fundamental principles: absence of people in the safeguard space during automatic operations; elimination of hazards or at least their reduction during interventions (such as teaching program verification) in the safeguard space. The standard, based on different steps, uses safety lifecycle as a key framework defining: objectives to be achieved, requirements to meet the objective, the scope of each phase, the required inputs for each phase and the deliverable to comply with the requirements.

Key Concepts

Standards and Regulations