How safe is your cobot application?

According to the International Federation of Robotics by 2020, there will be 3 million industrial robots hard at work in factories all over the world. As the types and technologies multiply, more compact, lightweight and adaptable robots offer the tantalising prospect of becoming an almost ‘plug and play’ commodity.

The future of robots lies in their ability to work safely alongside humans. Affordable and easy to use, so-called collaborative robots, or ‘cobots’, are already being deployed for tasks such as machine tending, pick and place assembly and materials loading and unloading.

It’s always important to remember that there are two parts of the safety equation: Firstly, is the hardware inherently safe?  Secondly, is the way it is used safe?

Traditional industrial robots have achieved a good record in keeping workers safe, caged behind fences and segregated by guards and interlocks. But, as more robots leave their enclosures and help to create flexible workflows, the challenges for safety standards and systems are clear.

Safe human-robot interaction

The international harmonised standard EN ISO 10218-2 specifies safety requirements for the integration of a robot and, unless the results of the risk assessment determine otherwise, requires the safety-related parts of control systems to be designed to comply with at least PLd (ISO 13849) or SIL2 (IEC 62061). As a supplement to this standard, the technical specification ISO TS 15066 contains further requirements and guidance on the safety of industrial collaborative robots.
Even if the robot manufacturer has incorporated features into the design that reduce risk, a risk assessment must still be carried out. System manufacturers and integrators of robot systems are required to conduct thorough checks of the structural safety measures taken by the robot manufacturer. They must also consider any hazards or risks that may remain and design the robot systems accordingly. For example, it’s just as important to consider the design of the robot tool or end-effector chosen for the task, the workpiece itself, or other machines that may be within the workspace.

What defines a collaborative robot?

How to define a collaborative robot operation is the subject of controversy and debate. Simply put, a collaborative robot application is just one that allows robots to be present in the same workspace as people. The standards consider the safe interaction of humans with active robots in terms of space and time.

This could be:
- At the same time
- At alternating times
- Under power but not moving
- At reduced speed
- At reduced force
- Working on a part together or apart.

Let’s consider the different ways human and robots can work together:

Coexistence

Even where there is no human intervention during the production process itself, it will still be necessary for a person to enter the robot's workspace, e.g. for maintenance. The workspace must be suitably guarded, and access doors must be interlocked. The interlock must ensure that any hazards are brought to a safe state before someone can reach them though the guard. To achieve this, the guards should be positioned according to EN ISO 13855.

Cooperation

In cooperative applications, operator and robot complete the necessary stages of a process in the same workspace at different times, for example, an operator loading and unloading robot cells. Here too, technical protective measures are required. Depending on the risk assessment and how the loading and unloading system is set up, it may be appropriate to use opto-electronic protective devices such as safety light curtains or safety laser scanners.

Interactive

When an operation is fully interactive and humans and robots interact in the same process at the same time, the force, speed, and movement paths of the robot must be restricted.  If they are available, inherent safety measures or additional safety measures, such as limiting torque through the drives or safety-related parts of the system controller, can be used to minimize risk. Force, speed, and movement paths must also be monitored and controlled based on the actual degree of risk. This degree of risk is also dependent upon the distance between human and robot.

Quite often, additional measures to reduce risk must be taken by the system integrator in these applications and usually, the most hazardous part of the system is the end-effector. Sensing technologies and systems such as safety light curtains or laser scanners could be deployed to detect human presence or determine the speed at which humans are moving towards the hazardous area and their distance from it.

Methods

The Technical Specification ISO/TS 15066 provides additional and detailed guidance for collaborative robot operations when a robot and people share the same space. It describes a number of methods (safety features) that can be used to implement safe collaborative operations. There could be more than one method involved in an operation:

- Safety-rated monitored stop: for a robot to safely stop under power and then automatically restart if a person walks away.

- Hand guiding: e.g. to teach the robot specific motion paths or to use the robot for power assistance.

- Speed and separation monitoring: for a robot to speed up, slow down and come to a stop, depending on how far away a person is from it.

- Power and force limiting: Of all the methods outlined in the standard, power and force limiting is the most controversial, because it considers the physical contact, whether it be intentional or unintentional, between a robot and a person.

Risks are reduced, either by making the system inherently safe or by keeping hazards below threshold limits. The standard states that contact between a human and a robot can be modelled and the energy resulting in a fully inelastic contact can be calculated by taking into account the payload of the robot and the part of the body undergoing contact.

The standard provides detailed guidance for quasi-static contact, where a body is trapped between the robot and a surface (such as crushing and clamping) and for transient contact, a dynamic impact when the robot strikes a person.

Productivity

Of course, there is no point in investing in a collaborative robot application, if you then need to implement a safety system that restricts the very workflow efficiency you are trying to enhance. So, a well-designed safety system will safeguard machinery uptime, minimise stop-start operations and carefully consider the space needed to enable a safe protective area around the robot.

As production teams learn how to deploy and redeploy cobots around the factory, their associated safety systems will need to be integrated and become almost as ‘plug and play’ as the systems themselves.  

SICK’s Safe Robots Area Protection is a convenient, easily integrated, single-source safety system for collaborative human-robot applications that satisfies Performance Level PLd/SIL2 in accordance with EN ISO 13849-1/EN 62061.

The solution deploys a SICK safety laser scanner and a SICK Flexi Soft controller to create a warning field outside two protective fields. Depending on how close the person is to the robot, the robot is either slowed to a safe speed or brought to a stop.

Dr Martin Kidman is SICK (UK) Product Specialist - Machinery Safety  |   FS Engineer (TÜV Rheinland)