Collaborative robots must meet certain safety requirements before they are allowed to work together with humans. © Universal Robots

In this application, three robot arms of the UR3e model work together with the aim of handing the visitor a flashlight engraved on site. To do this, the first robot takes a small flashlight from a tray and hands it to the second robot so that it can then unscrew the battery cover. A third robot then takes a battery from another tray and inserts it into the flashlight. The first UR3e closes it again. Now robot number 2 places the lamp in a laser engraving unit. In the final step, the first robot removes the engraved lamp and hands it over to the trade fair visitor.

Division into two areas

In order to design a collaborative application flexibly and economically, it is advisable to first divide the robot's workspace into two areas: the "normal area" and the "collaboration area".

In the normal area, humans and robots work separately from each other. The usual safety parameters apply here: the robot operates at high speeds and its shutdown does not need to be set sensitively.

In the collaboration space, however, the worker and robot arm come into contact. Here, the values of the safety parameters such as speed, force and power are reduced. The robot works at a reduced speed and is switched off in a highly sensitive manner as soon as it detects a collision. The basic principle of "as small as possible, as large as necessary" should always apply to the collaboration space, as it must be proven for this area that no hazards to humans can occur during collaboration. The larger the collaboration space, the greater the effort required to verify safety.

The workspaces can be separated by external sensors, such as a light curtain, or by setting up safety levels in the robot's user interface. If the robot moves through these levels, the switchover from one area to the other is triggered.

These two areas are also defined for the flashlight application. The three robot arms are initially enclosed at the sides and from behind - this is the normal area in which they work at high speed and generate cycle times. No force and pressure measurement is required for the risk assessment to ensure safety. Human intervention in this area causes the robots to stop immediately.

The normal area ends at the front of the application and the collaboration area begins. The separation of the two areas is defined by an integrated safety light grid - if someone reaches through the light grid, this puts the robots into a safety stop. Two functions, which are unique in the robotics market to date, support validation as part of the risk assessment: the safety-monitored stop time and the safety-monitored stop distance. They guarantee that no collision occurs when a person enters the normal area - which otherwise cannot be ruled out despite the safety stop being activated, for example due to a certain overtravel distance of the robot.

On the other hand, there is a safety level parallel to the light grids for robot number 1 - if the robot passes through this level, it switches to a reduced mode. The UR3e then moves at a slower speed and is more sensitive to the effects of force. Its possible working space is limited to a narrow tunnel at about chest height of the human. This is where the robot arm finally hands the flashlight to the visitor. This limitation has the simple advantage that there can be no jamming between the table and the robot in the defined collaboration space, only a collision in free space, which simplifies risk assessment.

The next step is to focus on the entire safety system of a cobot in order to make its collaboration space truly safe. UR's e-Series robot models, for example, are equipped with a total of 17 configurable safety functions. They limit the angular position and speed of the joints, the position and speed of the tool center point (TCP for short) and the elbow joint in Cartesian space, as well as the robot's impulse, force and power. All safety functions have been certified by TÜV Nord in accordance with EN ISO 13849-1:2008 with performance level d category 3.

Most of the 17 functions are used in the flashlight application: Not only are a safety stop and an emergency stop integrated, but also far more sophisticated safety systems. In detail: To ensure reliable HRC, the robot's force and power limiters are used in the application. This means that if, contrary to expectations, a jam does occur, this safety function reduces the maximum forces and pressures that occur in the jam. Secondly, the pulse limiter ensures that the robot stops immediately if a collision in free space is too hard.

Furthermore, the described tunnel in the collaboration space is realized by the TCP position limitation function. It can be used to position safety levels in such a way that they result in the tunnel shown, to which the space in which humans and robots can meet is limited.

Finally, three other safety functions ensure that robot number 1 can successfully switch between the two workspaces: TCP speed monitoring ensures that the robot does not move faster after passing through the safety level than the maximum transfer energies that would allow it to do so in the event of a collision. With the "Output: Reduced mode" function, the robot bypasses the light grid so that it does not stop itself when passing through. The "Output: non-reduced mode" switches robots number 2 and 3 to a safety stop as soon as the first UR3 bridges the safety light barrier and hands the flashlight to the trade fair visitor.

Asking the right questions

A detailed examination reveals two crucial facts: To make an HRC application safe, far more safety functions are required than just the familiar force and power limitation. Furthermore, other or additional safety functions are required than the emergency stop and safety stop functions specified in EN ISO 10218-1:2011. And: The term "cobot" is not a protected term. This also means that it is not defined which additional functions a robot must have in order to ultimately be a suitable cobot for a safe HRC application.

When purchasing a collaborative robot, users should therefore rely less on the statements of the ISO standard for industrial robots, but rather ask what other functions the cobot's safety controller provides them with. It is also essential to pay attention to the performance level and that this has been certified by an independent testing body. If the collaborative robot meets all of these requirements, nothing stands in the way of safely integrating HRC.

Andreas Schunkert, Head of Technical Support Western Europe at Universal Robots / ag