Understanding collaborative robot safety and risk assessment

In today’s industrial environment, there is an increasing need for flexible machines to work autonomously and adapt to changing production conditions quickly and efficiently.

Human-robot collaboration is one of the answers to these flexible needs. It aims to fit together the best qualities of both humans and robots to reduce manufacturing cost and time.

But an important source of concern for a collaborative workplace is safety, i.e., to protect workers against the dangers posed by speed, movement, and force of robots because an impact with a moving robot can cause serious injury.

Robots can cause severe and fatal injuries to workers during interventions such as maintenance, unjamming, adjustments, and set-up. Traditionally, robots were physically separated behind physical barriers such as safety fences, but a collaborative robotic system seeks to remove these barriers and enables closer interactions between operators and robots.

According to the International safety standard, risk assessment is the first step in understanding and eliminating hazardous work environments. The following are some of the very common hazardous situations in a collaborative environment.

Common safety hazards

  • Movement of the robotic arm, gripper, or other retention devices in the direction of the worker.
  • Failure to limit force and failure to limit the speed
  • An unexpected, sudden movement of a robot or deviation from its intended path.
  • Projected objects from the robot, striking a worker after a failure of fixtures, grippers, or other mechanical parts.
  • A sudden release of stored energy or workers in close proximity to the nozzle of the injection unit.
  • The worker in close proximity to live electric components or contact with live parts or parts accidentally becomes life due to a short circuit or insulation problem.
  • Loss of balance and fall from height due to slippery parts or detective/poor access, causing fractures or death.

The single most important steps in providing safety and risk assessment are identifying the possible hazardous situations, determining the level of risk, and implementing the requirements of the risk reduction measure(s) to reduce the risk to an acceptable level.

The factors to be taken into consideration when estimating the risk are as follows:

  • The severity of harm – Severity of injuries or damage to health (e.g., slight, serious, or death) and the extent of harm (e.g., one or several persons)
  • Probability of occurrence of harm – Exposure of workers to the hazard, the occurrence of a hazardous event, and the technical and human possibilities to avoid or limit the harm.
  • Frequency of exposure to the hazard – Need for access to the hazard zone (e.g., for normal operation, correction of malfunction, maintenance, or repair); nature of access (e.g., manual feeding of materials); time spent in the hazard zone; and number of persons requiring access
  • Duration of exposure to the hazard and frequency of access.
  • Probability of occurrence of a hazardous event – Reliability and other statistical data; accident history; the history of damage to health; and risk comparison
  • Possibility of avoiding or limiting harm – Different persons who can be exposed to the hazard(s), (e.g., skilled, or unskilled); how quickly the hazardous situation could lead to harm (e.g., suddenly, quickly, or slowly); any awareness of risk (e.g., by general information, information for use, by direct observation, or through warning signs and indicating devices on the machinery; the human ability to avoid or limiting harm (e.g., reflex, agility, the possibility of escape); and practical experience and knowledge, (e.g., of the machinery, of similar machinery, or absence of experience)

A risk assessment aims to increase understanding of the hazards and reduce the rate and severity of injuries. It can also increase or maintain operational efficiency through correctly specified and designed risk reduction measures and assure cost-effective, sustainable solutions.

Some of the common modes of operation in terms of safety and strategies around collaborative robots are as follows.

1. Stopped state monitoring or safety-rated monitored stop

The robot stops when the worker enters the collaborative workspace. It continues to monitor until the worker leaves and then resumes working. This mode might resemble a safeguarded robot inside a cage that stops when the worker enters the cage. Still, the difference is that the robot automatically resumes work when the worker leaves.

2. Speed and separation monitoring

The robot slows down when a worker gets closer. The robot motion is allowed only when the separation distance is above a minimum separation distance. Different technologies (laser scanners, safety mats, vision-based systems) are used to detect the operator’s position concerning the robot. The robot maintains a determined speed and separation barrier between itself and the worker. The relative speed of the worker and the robot is considered for minimum distance requirements.

3. Power and force limiting

It is an inherent design and control of the robot. In this mode, the power and force of the robot are limited. When the robot makes contact with a human being or any object with a certain force value, it stops immediately. The robot knows the required amounts of force to pick up a load and to move it. When it recognizes an increase in torque or force required for movement, such as collision, the robot arm safely stops.

4. Hand guiding or gesture assistance robots

The worker is in direct contact with the robot. He guides and trains the robot. The robot assists the worker in tasks in which a force has to be exerted. The robot motion is allowed only through the direct input of the operator. The hand guiding part is close to the end effector and consists of an enabling device and an emergency stop.

Risk reduction measures

For robots working in a collaborative workspace, human safety is one of the most serious and important aspects to be considered. The companies must carry out proactive risk reduction measures to ensure that the robots and humans perform tasks concurrently during production operations. Many standardization bodies deal with the safety in human-robot interactions, and the most influential ones are the International Organization for Standardization (ISO). The inherent safety measures that are typically used on collaborative robots include:

  • The speed and movement paths (trajectory) of the robot are monitored and adjusted based on the operator’s speed and position in the safeguarded space.
  • Limiting the maximum permissible forces or torques, e.g., through drive dimensioning
  • Setting up the robot interfaces reduces the pressure impact or the collision forces transmitted (e.g., rounded robot surfaces, energy-absorbing padding). Installing rubber safety bumpers or safety rings to detect contact and initiate a protective stop.
  • Installing padding on components to increase contact surface, reduce energy transfer rate, and eliminate sharp or pointed edges. Padding can protect corners and edges on components such as fittings, sensors, valves, connectors, lenses, and magnets.
  • Limiting torque, force, or speed through a control system
  • Using pressure-sensitive protective devices (PSPE) or electro-sensitive protective devices (ESPE) to stop or reverse robot movements. Installing non-safety-rated laser scanners to signal warnings of movement and pause the robot at critical times.
  • Post a diagram showing the collaborative space at key points or mark an outline on the floor or install awareness rails
  • Restrict entry – Post caution labels that restrict entry to only authorized personnel.
  • Apply tactile coverings on the robot arm to sense contact and stop the robot via a protective stop.
  • Test the functionality of the following safety functions: force limit, speed limits, restrictive space, axis/pose restrictions, and contact safety sensing devices.
  • Confirm compliance to pain and injury thresholds.

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