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The Basics of Designing for Safety with Collaborative Robots

A primer on collaborative robots, how to conduct safety assessments, and how to design safe implementations for robot/human coworking spaces.

The robots are coming…to work alongside us. If you work in a plant or a warehouse, you may be increasingly presented with opportunities to work alongside mechanical beings. As innovation continues to advance in robotics and manufacturing, and companies deploy the new technology, the potential danger robots pose grows as well. If manufacturers don’t consider the safety risks and address them immediately in the design phase, the robots they produce will likely fail standard testing procedures, resulting in deployment delays and financial losses.

In the Beginning: From Dinosaurs to Dexterous Mammals

Collaborative robots (cobots) emerged in the mid-1990s and have advanced continuously since then. Cobots are built to work alongside humans in a shared workspace, often performing hazardous, sensitive, or mundane tasks while their human coworkers focus on higher value-add activities. Companies that have paired robot functionality with human potential have realized tremendous financial and productive gains.

The ancestors of cobots were large, powerful, and often dangerous industrial robots. They were utilized for specific tasks and kept in restricted areas (or even cages) to protect human employees. These robots were created for specific, programmed tasks and not capable of interacting with humans, which meant they could easily cause injury or worse if a human wandered into the path of the industrial robot’s blind movements.

Cobots today have been designed so they can integrate into shared workspaces in close proximity with humans. Cobots now work alongside or even directly with humans, assisting with various tasks. Example tasks might be assisting with lifting items, assembly of parts, inspecting items, or handling hazardous materials. Cobots may be lighter than their predecessors, the industrial machines, and some are even designed to move between multiple tasks.

Schematic diagram of the KDMG-KOLROBOT instrument. (Source: IFA - Technical information COBOTS: Collaborative robots - Test apparatus)

Resources for Understanding Cobot Risks

Just because these robots are advertised as “collaborative” and intended to operate as cageless robots alongside humans does not necessarily mean cobots are safe. A “collaborative” robot holding a knife is in all likelihood no longer safe to operate without additional safety measures. When designing a collaborative robotic cell, engineers must take into account the intended application of the cobot and the environment it might be placed in, then develop a safety plan for each use case.

ISO/TS 15066 and RIA TR 15.606 are the newest and most important safety standards applicable to collaborative robotic cells in the industry today. These highlight the safety requirements for each of the four types of collaborative robotic cells:

  1. Safety-rated monitored stop
  2. Hand guiding
  3. Speed and separation monitoring
  4. Power and force limiting

Additional standards provide additional information on other hazards that may exist in collaborative robotic cells.

  • UL 1740 (Robotics and Robotic Equipment)
  • ANSI RIA R15.06 (Industrial Robots and Robot Systems)
  • CAN/CSA Z434 (Industrial Robots and Robot Systems)
  • EN/ISO 10218-1
  • EN/ISO 10218-2

Prototype of the force/pressure instrument in the application. (Source: IFA - Technical information COBOTS: Collaborative robots - Test apparatus)

Evaluating Risk

While ISO/TS 15066 (RIA TR 15.606) outlines four different types of collaborative operation, the main form of collaboration used in the industry today is the power- and force-limited application. This is where the potential impact between a robot-human collision is limited by the robot itself—either inherently in the design or through the use of a safety-rated robot controller, which can limit parameters such as speed and force. 

The first step in the evaluation as required by ISO/TS 15066 & (RIA TR 15.606) is to perform a risk assessment in accordance with EN/ISO 12100. This risk assessment should consider the entire collaborative workspace, access, clearances, and how an operator will interact with the robot. Each reasonably foreseeable contact situation between the operator and robot must be considered. It is important to consider if any stationary structures are involved and other aspects like the end-effector and workpiece.

The risk assessment must determine the nature of each type of contact from the following two situations:

Transient contact. A contact in free space where the robot and human can recoil.

Quasi-static contact. Often called “clamping” there is where there is extended contact that results from being constrained against an object.

Based on the type of contact, ISO/TS 15066 & (RIA TR 15.606) assign specific impact force and pressure limits for each part of the human body. These values are pain thresholds which were derived from testing conducted by the University of Mainz on pain onset levels when force is applied to various parts of the human body.

Once each potential contact situation has been classified and the limits established, each must be tested or evaluated. The method used depends on the specific type of impact.

For quasi-static contact situations, a proper test device which mimics key characteristics of each part of the human body must be used. This includes having a proper spring rate and damping material which simulates the shore hardness of that particular area. This also includes a pressure mapping component such as film. Some of these devices are currently available in the market today. The test device is properly anchored, and each potential collision is carried out. The data from each collision is recorded and compared to the limit values published in ISO/TS 15066 & (RIA TR 15.606)

Transient contact situations can be evaluated in two separate ways. In the first method, both the robot system mass and the body region mass must be accounted for. This means that the test device must be attached to an apparatus that allows it to move freely and must also be the same mass as the effective mass (MH) of the body part being tested. The effective mass is defined in table A.3 of ISO/TS 15066 (RIA TR 15.606). The same test procedure as the quasi-static procedure is then followed.

The second method of evaluation utilizes mathematics as opposed to physical testing. For each body region the maximum permissible energy transfer can be calculated as a function of the maximum force or maximum pressure values given in table A.2 of ISO/TS 15066 (RIA TR 15.606). The calculation will show the maximum speed at which the robot can travel for each given situation.

Force-time signal of a human-robot collision (Source: IFA - Technical information COBOTS: Collaborative robots - Test apparatus)

Interpreting the Results

Once testing is complete and guidelines for the thresholds of force and power for a particular cobot has been established, designers must switch to bringing all potential collisions within allowable ranges. There are many options for a collision that goes beyond allowable ranges. In many cases simply slowing down the robot can make a world of difference. In others, utilizing an end-effector which offers softer material and more suitable geometry may get a designer over the hump. While everyone seeks to operate in a “cageless” manner, the truth is that not every application needs to, or can, operate in this manner. In some cases, guarding or external protective measures may be required.

Consider Safety Early to Avoid Harm Later

Testing and safety validation of a collaborative robot cell is a necessary and rigorous activity to ensure worker safety. Manufacturers of these force- and power-limited robots offer users and integrators fantastic features to allow them to build and use a safe collaborative robotic cell. However, these are not in fact safe out-of-the-box. Safety should be a paramount consideration at the start of a design project. When safety is designed into a project and validated accordingly, we can ensure that humans and cobots can work in harmony for years to come.

Ryan Braman is a test engineering manager with TUV Rhineland.

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