Safe Contact: Rethinking Robot Interaction

Author: Denis Avetisyan

As robots move closer to humans, ensuring safe physical contact is paramount, and current safety standards may not be enough.

Robot-human collisions progress through distinct phases-an initial, dynamic impact governed by reflexive motion, transitioning to a sustained contact characterized by either a pushing interaction-where the human recoils with decreasing relative velocity-or a crushing interaction-where the human remains stationary-demonstrating that the nature of sustained contact depends on whether the human yields or remains fixed during the impact event.

This review critically analyzes energy-based safety constraints for physical human-robot interaction, highlighting limitations in existing approaches and the need for improved collision avoidance strategies.

Despite advances in collaborative robotics, ensuring safety during physical human-robot interaction remains a fundamental challenge, often relying on simplifying assumptions with unclear consequences. This paper, ‘Physical Human-Robot Interaction: A Critical Review of Safety Constraints’, undertakes a rigorous analysis of commonly adopted safety standards-specifically ISO/TS 15066-by dissecting their underlying derivations and revealing the impact of key modeling choices on both safety and performance. The work demonstrates how these approximations affect system behavior and quantifies resulting performance degradation, emphasizing the central role of energy management in robust safety assessment. Ultimately, this critical evaluation raises the question of how to move beyond current limitations and develop more nuanced, performance-optimizing safety frameworks for increasingly complex human-robot collaborations?

The Limits of Static Safety: A Rational Approach to Human-Robot Interaction

The increasing prevalence of collaborative robots, or ‘cobots’, working alongside humans demands robust safety measures to safeguard against potential harm. As robots move beyond fixed industrial guarding and into shared workspaces, the risk of accidental collisions necessitates protocols exceeding those designed for traditional, isolated industrial automation. These protocols aren’t merely about preventing impact; they must also address the dynamic forces exerted during interaction, considering factors like human sensitivity and the robot’s ability to react to unexpected movements. Effective safety, therefore, isn’t simply a matter of halting the robot upon contact, but rather of anticipating and mitigating risks through a combination of sensing, force control, and intelligent algorithms that prioritize human wellbeing throughout the entire collaborative process. The need for such stringent protocols is paramount, as even seemingly minor impacts can result in injury, hindering the successful integration of robots into human-centric environments.

Conventional robotic safety systems frequently depend on pre-defined, static force or speed limits to prevent collisions and injuries during human-robot interaction. However, this approach overlooks the inherent dynamism of collaborative tasks, where interaction forces are rarely constant. A static threshold, calibrated to worst-case scenarios, fails to differentiate between a harmless incidental contact and a potentially dangerous impact, leading to overly cautious behavior. This can result in the robot unnecessarily halting or reducing its speed, even when the actual interaction poses no threat – effectively treating all forces above a certain level as equally problematic, regardless of their duration, direction, or the context of the interaction. Consequently, the robot’s ability to adapt to changing circumstances and collaborate naturally with a human partner is severely limited, hindering the potential benefits of shared workspaces and complex task completion.

Conventional robotic safety systems, designed to prevent harm during human-robot interaction, frequently employ pre-defined, static force and speed limits. However, these conservative parameters can significantly impede a robot’s performance and the natural flow of collaborative tasks. Studies indicate that rigidly fixed limits can reduce achievable operational speed by as much as 70%, forcing robots to operate far below their potential and creating a cumbersome, unnatural experience for human partners. This restriction arises because static thresholds fail to differentiate between incidental contact – a gentle brush during a shared activity – and genuinely dangerous impacts, leading to unnecessary and frustrating slowdowns or complete stops. The result is a diminished ability for robots to seamlessly integrate into human workspaces and perform tasks with optimal efficiency and fluidity.

Robot motion safety limits, expressed as maximum admissible speed [latex]v_{max}[/latex] and pre-collision kinetic energy [latex]K_{0,max}[/latex], are analytically derived from human pain thresholds and depend on robot and human effective masses, body-region stiffness, and maximum allowable force/pressure, as modeled by a mass-spring-mass system.

Proactive Safety: Shifting from Reaction to Anticipation

Power and Force Limiting (PFL) represents a shift from reactive to proactive safety systems in robotic and automation applications. Rather than simply halting motion upon exceeding a pre-defined force or torque threshold, PFL actively modulates the robot’s behavior to maintain interaction forces within safe boundaries. This is achieved through continuous monitoring of force and torque data, and real-time adjustment of the robot’s control signals – velocity, acceleration, and motor torque – to prevent excessive forces from developing during contact. By actively controlling the interaction, PFL reduces the risk of collisions resulting in damage to equipment or injury to personnel, and allows for safer operation in dynamic environments with unpredictable contact scenarios.

Traditional safety systems often rely on static force or velocity thresholds to trigger a shutdown, which can be overly conservative and limit operational efficiency. Power and Force Limiting (PFL) overcomes these limitations by dynamically adjusting to the specific characteristics of each interaction. Rather than a fixed limit, PFL continuously monitors and regulates force and power in real-time, responding to changes in contact conditions and allowing for higher speeds and more nuanced control. This adaptive approach prevents excessive force development during both sudden impacts and prolonged contact, enabling safer operation across a wider range of tasks and environments without unnecessary interruptions caused by false positives.

Power and Force Limiting (PFL) achieves enhanced safety by simultaneously restricting both force and power during human-robot interaction. Force limitations address peak impact forces, preventing injury from sudden collisions, while power limitations manage the energy delivered during sustained contact. This dual constraint is critical because force alone does not account for the duration of interaction; a low-force, long-duration contact can still pose a risk. By limiting power – defined as the rate of work done, or [latex]P = F \cdot v[/latex] where F is force and v is velocity – PFL effectively caps the total energy transferred. This allows for significantly higher permissible speeds and more dynamic movements compared to systems relying solely on fixed force thresholds, which often necessitate slow, cautious operation to remain within safety margins.

Admissible end-effector velocities vary significantly across body regions depending on the safety formulation-transient contact (green), quasi-static contact per ISO/TS 15066 (orange), or a potentially-clamped scenario (blue)-with boxplots representing velocity limits influenced by configuration-dependent apparent mass [latex]m_{\mathbf{u}}(q)[/latex] and stars indicating scalar limits derived from the constant ISO-based mass [latex]m_{R}^{ISO}[/latex].

Standardizing Collaborative Safety: The Role of ISO/TS 15066

ISO/TS 15066 is a technical specification detailing safety requirements for robotic systems used in collaborative human-robot workspaces. It addresses hazards present during integration, operation, and maintenance of these systems, and provides guidance on conducting risk assessments and implementing appropriate safety measures. Unlike standards focused on safeguarding against industrial robot hazards where the robot operates within a protected space, ISO/TS 15066 specifically addresses scenarios where robots and humans share the same workspace, necessitating different safety considerations. The standard defines parameters for collaborative robot operation, including speed and separation monitoring, safety-rated stop functions, and force/pressure limits, allowing for a structured approach to validating the safety of collaborative applications.

Power and Force Limiting (PFL) mode, as detailed in ISO/TS 15066, establishes a safety-rated monitored stop condition for collaborative robots based on detected forces and pressures. This method defines safe interaction scenarios by limiting the robot’s energy output when contact with a human is detected, bringing it to a controlled stop or reducing force to a pre-defined threshold. Validation of these scenarios involves comprehensive risk assessments and testing to ensure that the robot’s behavior remains within acceptable safety parameters under anticipated contact conditions. The standard requires specification of the robot’s maximum force and pressure limits, as well as the duration and location of potential contact, to determine appropriate safety responses and validate the overall system safety.

ISO/TS 15066 assesses collaborative robot safety by establishing acceptable risk levels through quantified energy and force/pressure limits during human-robot interaction. However, strict adherence to these limits, particularly without incorporating advanced body-part detection, can significantly reduce robot speed. Studies indicate a potential speed reduction of up to 46% when robots operate within defined safety parameters but lack the ability to differentiate between various body parts and adjust force/pressure accordingly. This reduction in speed is a direct consequence of maintaining conservative safety margins to account for the most sensitive human tissue contact scenarios.

A simplified mechanical model of a robot-human collision, comprising effective masses for the robot ([latex]m_R = 3.0[/latex] kg) and human ([latex]m_H = 1.0[/latex] kg) coupled by a spring ([latex]k = 5.0[/latex] N/m), demonstrates that with an initial robot velocity of [latex]v_0 = 1.0[/latex] m/s, both masses reach a similar velocity of approximately [latex]v^* \approx 0.75[/latex] m/s after a spring compression peak at 0.6 s.

The Physics of Interaction: Effective Mass and Reduced Impact

The magnitude of impact force experienced during physical interaction is fundamentally linked to the effective mass of the colliding body segments. This isn’t simply the physical mass, but rather a representation of how a segment resists acceleration – influenced by its mass and its rotational inertia. A segment with a higher effective mass will generate a greater impact force for the same velocity change, due to its increased resistance to deceleration. Consequently, understanding and accurately calculating this effective mass is crucial for robotic systems designed to safely interact with humans; it allows for precise prediction of impact forces and informs the design of compliant control strategies that minimize potential harm. [latex]F = ma[/latex] highlights this relationship, but in the context of complex human-robot interaction, considering the rotational components of effective mass becomes paramount for a complete and accurate model.

Traditional impact dynamics calculations often treat colliding bodies as isolated systems, yet robot-human interaction fundamentally involves a transfer of momentum between two interconnected masses. A more nuanced approach utilizes the concept of reduced mass, [latex]\mu = \frac{m_1 m_2}{m_1 + m_2}[/latex], which effectively represents the combined inertia of both the robot and human segment involved in the collision. This isn’t simply an averaging of masses; it encapsulates how the total kinetic energy is distributed during impact, influencing the resulting force and deformation. By factoring in the reduced mass, researchers can develop collision models that more accurately predict the forces experienced by a human partner, crucial for ensuring safe and compliant robotic behavior and enabling precise adherence to safety standards like ISO/TS 15066. Ignoring this interconnectedness can lead to overestimation of impact forces, potentially triggering unnecessary safety shutdowns or, conversely, underestimation, jeopardizing human safety.

Accurate determination of safe human-robot interaction necessitates precise calculation of force and power limits, a requirement directly addressed by the ISO/TS 15066 framework. Utilizing concepts like effective and reduced mass allows for a refined modeling of impact dynamics, moving beyond simplistic approaches. This detailed analysis enables the establishment of conservative safety parameters; current estimations, based on these principles, suggest a maximum admissible pre-collision speed of 0.15 m/s. This value represents a cautious threshold designed to minimize risk, factoring in uncertainties in system modeling and human sensitivity. Consequently, these biomechanical considerations aren’t merely theoretical; they are foundational for designing collaborative robots that prioritize human safety and adhere to stringent international standards.

The pursuit of safe physical human-robot interaction demands a relentless cycle of testing and refinement, mirroring the scientific method itself. The article’s deep dive into energy-based control and passivity constraints isn’t about achieving a perfect, immutable standard, but about iteratively reducing the space for error. As John Dewey observed, “Education is not preparation for life; education is life itself.” This sentiment applies equally to robotics; safety isn’t a destination, but a continuous process of learning through interaction and responding to anomalies. The limitations of current standards, such as ISO 15066, aren’t failures, but opportunities to refine understanding and move closer to genuinely safe collaboration.

What’s Next?

The preceding analysis reveals, perhaps unsurprisingly, that simply meeting a standard isn’t synonymous with achieving genuine safety. ISO 15066 provides a framework, certainly, but a calculation adhering to its limits remains merely that – a calculation. The field now requires a shift from demonstrable compliance to robust verification of energetic behavior during interaction, not just in simulated scenarios. Any seeming confirmation of current models warrants a second look; the tendency to extrapolate safe behavior from controlled conditions is a persistent hazard.

Future work must grapple with the inherent uncertainty in human behavior. Current approaches largely treat the human as a predictable impedance, a simplification that will inevitably fail. A hypothesis isn’t belief – it’s structured doubt – and the current literature demonstrates a notable lack of models explicitly accounting for unpredictable human actions or the complex dynamics of soft tissue contact. Exploration of adaptive control strategies, capable of learning and reacting to unanticipated events, appears essential.

Ultimately, the pursuit of safe physical human-robot interaction isn’t an engineering problem alone. It’s a question of defining acceptable risk. A truly rigorous approach demands not just minimizing energy transfer, but quantifying and communicating the remaining uncertainty to the human operator. It is not enough to build a robot that doesn’t hurt someone; the system must also convey how much it might, and the conditions under which that possibility increases.

Original article: https://arxiv.org/pdf/2601.19462.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

The Limits of Static Safety: A Rational Approach to Human-Robot Interaction

Proactive Safety: Shifting from Reaction to Anticipation

Standardizing Collaborative Safety: The Role of ISO/TS 15066

The Physics of Interaction: Effective Mass and Reduced Impact

What’s Next?

See also: