Why are Risk Assessments Necessary for a Collaborative Application?

With any industrial robot application, adhering to the applicable industry and OSHA safety standards is critical to ensuring the safety of anyone that may interact with it intentionally or unintentionally. This involves several steps that begin at the design stage by performing a task-based risk assessment, then ensuring that the prescribed risk reduction measures are correctly chosen through verification, and then tested to ensure that they work as designed by validation.

If you are not experienced with performing these tasks, partnering with a robot supplier or integrator is advised, but certainly not required. However, it should be noted that the party performing the robotic system integration is responsible for being knowledgeable of, and adhering to all applicable standards necessary for safe integration and use of robotic applications.

Guidelines for performing this due diligence can be found in the newly revised OSHA Technical Manual (OTM) Section IV: Chapter 4 under the topic of, “Industrial Robot Systems and Industrial Robot System Safety”, or in ANSI RIA TR 15.306-2016 Task-based Risk Assessment Methodology.

The OSHA Technical Manual (OTM) Section IV: Chapter 4 is a free educational resource for experienced and inexperienced users alike. This document is a collective effort between OSHA, the National Institute for Occupational Safety and Health (NIOSH) and the Association for Advancing Automation (A3), and sets expectations for the integrator, employer and user by referring to applicable OSHA regulations and industry standards.

ANSI RIA TR 15.306, is the go to source document for the methodology behind how complete a Task-based Risk assessment as specified by ANSI RIA 15.06-2012.  It covers topics such as: Responsibilities of the user and Integrator, Life cycle requirements and responsibilities, The Risk Assessment process, Process documentation, Verification and validation.

Collaborative Safety 101

At Yaskawa, our experts are often asked, “Do I really need to do a risk assessment if I am using a Power and Force Limiting (PFL) collaborative robot?” The answer is, “Yes!” ANSI/RIA R15.06-2012 (Part 2) specifies “the integrator [Entity doing the integration] shall perform a risk assessment to determine the risk reduction measures to adequately reduce the risks presented by the integrated application.” Likewise, the ANSI RIA TR 15.606-2016 [ISO TS15066 2016] collaborative robot safety specifications support this requirement.

While PFL equipped robots are inherently built to work with or alongside humans, using one in name or design is not enough to make an application safe. For an application to qualify as collaborative and be deemed safe, the entire robotic system and application must be assessed. This includes the robot, workpiece, work envelope, end-of-arm tooling (EOAT), fixtures et.al – as the amount of force and pressure that can be exerted during contact could result in injury. ANSI RIA TR 15.606-2016 Annex A. shows that different parts of the human body are able to withstand different contact forces and pressures (bio mechanical limits), and it is within the scope of the risk assessment to define the tasks where contact may occur, what body area(s) may be affected, and the contact type involved clamping (Quasi-static), or Dynamic impact (Transient) where the person is not clamped or pinched during the impact. If any crucial area of the robotic system fails to meet specified safety standards during the risk assessment, then the application cannot be approved as collaborative until the identified risk is mitigated appropriately.

Risk Assessment Contents

Each collaborative robot application – no matter how similar to another robot setup – should have its own risk assessment. Though equipment may be identical, robot systems will most likely be performing unique processes and manipulating different parts. That said, when compiling a risk assessment, it will generally cover:

• Document Identification – this section certifies the validity of the risk assessment and includes information such as the date, name of the project, project number, revision status and the names of those involved in the process.

• Workcell Description – this is for general information about the robotic system, including the how, why and where it is intended to be installed and used. The motivation behind the automation project, methods of risk reduction being used and a list of robot/peripheral details (i.e., model, manufacturer, serial number, date of manufacture, etc.). Any supporting documentation (i.e., operating manuals, specifications) that were consulted should be listed, along with the date read.

• Application Assessment – this part of the process provides all required information about the robotic application, the potential hazards involved, and how to mitigate them. From basic robot and application descriptions to technical/specification data of the multiple devices being used, a lot of information is listed. A risk estimation with evaluation criteria is also given, along with a findings summary that states all the potential hazards for the robotic workcell. Hazards should be ranked in order of severity, so those with significant risk can be addressed first.

• Assessment Conclusion – this section offers next steps for what actions will be taken to reduce or eliminate the identified risks, so that human workers can safely work with or alongside the collaborative robot.

• Other Documentation – any supporting documents or further explanations should be attached. Note: for each risk reduction process or similar process, a proof should be cited, if applicable.

Cobot Safety Testing: Verification vs. Validation

Two key concepts that arise around the idea of safety and risk assessments are verification and validation – as the robot system manufacturer or integrator shall provide for the verification and validation of the design and construction or robot systems, including appropriate safeguarding devices.

• Verification – asks the question, “Did we build the safety system correctly?” During Verification, actions like safety analysis, task-based risk assessment, calculations (i.e., stopping distance calculations) and simulations are performed.

• Validation – asks the question, “Did we build the right thing?” During validation, user tests under real life conditions take place. This would include force limit testing, speed limit testing, testing the robot’s restricted space, determining any pose restrictions using functional safety, or testing safety sensing devices.

Verification comes before validation and is performed during the design stage – as it is an engineer’s analytical or mathematical effort to confirm that the safety circuit will achieve the risk assessment’s required performance level. Relevant guidance for verification can be found in RIA TR R15.306-2016, and ISO 13849. Likewise, OSHA points out that employers should also be following the guidance provided in RIA TR R15.706-2019 under, “User Responsibilities.” On the other hand, validation is ensuring that the required safety function that you identified is reliably achieved in the machine’s safety system. In essence, this is providing objective evidence that the mitigation measures required have been successfully completed and tested, ensuring the utmost worker safety.

Key to verification (where PFL is concerned) is the idea of transient vs. quasi-static contact – as the peak force and pressure of the robot must remain in acceptable limits from the start of the contact and abate after .5 seconds. It is important to note that ANSI RIA TR 15.606-2016 states, “The robot system shall be designed to adequately reduce risks to an operator by not exceeding the applicable threshold limit value for quasi-static and transient contacts, as defined by the risk assessment.”

• Transient Contact (Dynamic)– contact between an operator and part of a robot system, where the operator body part is not clamped and can recoil or retract from the moving part of the robot system, with the duration being <=.5 seconds.

• Quasi-static Contact (Clamping) – contact between an operator and part of a robot system, where the operator body part can be clamped between a moving part of a robot system and another fixed or moving part of the robot cell, with the duration being <=.5 seconds.

Furthermore, the Biomechanical limit values for the relevant contact events on the exposed body regions shall be [analyzed] for the most stringent limits. These “worse case” threshold limit values for the transient and quasi-static events shall be used in determining the proper level of risk reduction. Design or measures shall be implemented so that the effects of the identified contacts remain below these threshold limit values.

Cobot Application Considerations

That being said, what are some items to consider when it comes to PFL applications?

• Evaluate contact events, including reasonably foreseeable misuse.

• Evaluate the contact events with respect to the Pressure/Force limits.

• A risk assessment shall consider all collaborative tasks and associated workspaces, including at a minimum:

  • Robot characteristics (i.e, load, speed, power, etc.)

  • End effector hazards

  • Layout (i.e., separation distance between the robot and the operator)

  • Operator location with respect to the robot arm

  • Fixture design and related hazards

  • Design and location of any manually controlled guiding device

  • Application specific hazards

  • Limitations caused by operator PPE

  • Environmental conditions (i.e., chemicals, radiation, etc.)

  • Performance criteria of the associated safety functions

To dive deeper into the risk assessment topic, including quasi-static and transient contact, watch our very informative webinar on The Why and How of Cobot Safety Testing. Plus, gain helpful insights on how to measure collision forces and pressures with Stefan Clemens, an industry expert from GTE CoboSafe^®.