Chapter 3.9
Robot safety
– from “cages” to “cobots”
Robots are versatile and allow for very economic automation solutions. However, they are also quite dangerous.
Large, high-speed robots may cause very serious injury by crushing and impact. Therefore, robot safety has been standardized internationally. Some vital safety rules are presented below.
Robot safety is built on three elements:
- Prevent access by people
- Limit robot motion
- Make physical contact safe
The latter is a rather new concept, often called collaborative robot operation. It means that a specially designed robot system and an operator work within the same workspace or overlapping workspaces. Initially the collaboration workspace and the desired human and robot interaction must be planned. Then the risks are determined, and appropriate measures taken.
What is a “collaborative robot”?
The most advanced safety measure is the use of a collaborative robot, sometimes referred to as a “cobot”.
A collaborative robot operates at limited force and speed. Humans encountering a moving collaborative robot will not be injured, although they may feel some pain. Years of extensive testing have resulted in a list of force, speed, and energy limits that humans can endure when subjected to physical contact with machine elements (see ISO TS 15066).
Collaborative robots moving within these limits may be considered safe for human-robot interaction. However, that does not mean such applications are automatically safe. The entire context of the robot and the tool handled by the robot (the end effector) must be considered carefully.
Example:
- Imagine sitting next to a robot handling a syringe with poisonous liquid. The speed and force of its movements suddenly turn out to be of secondary importance.
Safety Measures:
In the graphics to the right, a human operator and a robot share the light-grey collaborative workspace.
- A hazardous robot must not move in the collaborative area if a person is present.
- Depending on the distance and motion speed of the person, the robot may have to slow down and stop as the person approaches.
Industrial robot safety
Robot safety largely depends on closed "cells" with fencing and access doors. But in many applications monitoring systems are additionally needed or may even replace the robot "cage".
Advanced detection systems:
- In many robot cells, a combination of safety fences with doors, light barriers, and range scanners is used.
Beware of persons remaining inside the hazard zone behind the detection devices. If that is possible, additional safety measures may be needed.
- The most advanced form of detection system is a 3D-camera. It needs to be installed at a sufficient height above the workspace to ensure it “sees” all the areas in which persons may move.
Fencing may thus be used to create a “protected space,” but it must not normally be used to define the “limited space."
A robot colliding with the fence at high speed and force will cause at least some deformation even to a strong fence. This could be a hazard. Also, people can stick their fingers through fences. So, a minimum distance of 120 to 200 mm must remain between the limited space and the outside of the fence (compare the section on "Determining the proper height and distance of guard fencing".
Consequently, do not go for so called “robot-safe” fencing, but use proper limiting devices instead:
- Safety-rated software control of the motion (meeting at least PL = d to (EN) ISO 13849-1 or SIL 2 to (EN) IEC 62061)
- Space limiting or hard stops (stopper blocks and pins)
- External limiting devices (mechanical or proximity switches)
Fence is meant to keep people out, not the robot in.
“Robot-safe” fences – concept or misconception?
Many people ask for “robot-safe” fencing or point to test results of fence manufacturers showing impact resistance of 2000 joules or more.
But, quite frankly: the question itself reveals a misconception about robot safety. Robot safety standards require that robot motion be limited by means other than fencing or other safeguards.
A robot‘s motion range depends on its size. Often this “maximum space” is much bigger than needed for the operation at hand. Since system designers want to use as little floor space as possible, the robot is programmed to keep to a much smaller “limited space”. Then fences, light courtains and range scanners are used to define a “protected space” around the limited space that persons must not enter.
However, a safety distance is practically always needed between the limited space and the protected space. Why? Because the robot needs time to slow down and come to a standstill when a person entering the protected space is detected (by a light-barrier, scanner, camera, or door switch).
