Collaborative robots, or so-called cobots, are quickly gaining market share. Their ease-of-use enables integrators and end-users to start working with cobots in places where traditional robots do not fit. Having a cobot in your factory or logistic center without a safety cage around it immediately raises an important question: How to guarantee the safety of the operators working with the cobot? It is often assumed that the safety functions of the cobot itself are sufficient to ensure a safe application. However, it takes more than just that to realize a safe system. This blog discusses different aspects that contribute to a safe cobot system.
When is a cobot system safe?
A safe cobot system means that an operator will not be harmed by working with the cobot. Physical contact is allowed, but its impact must be limited. Researchers from the universities of Mainz and Nagoya have conducted several tests to determine the maximum allowed impact on different parts of the human body. They have considered two situations:
- Quasi static contact in which part of the human body is clamped
- Transient contact in which the operator is hit by the robot but recoils from the moving part.
The exact results of this analysis can be found in this Technical Specification of ISO
Important safety aspects
Besides the safety functions of the cobot itself, several other important aspects contribute to the safety of the cobot system:
The cobot plays a central role in safety. With its safety PLC it ensures a limit on the measured force, speed and power of each part of the robot, and a limit on the area the robot can move in. Furthermore, to minimize impact, the robot should not have sharp edges.
The gripper at the end of the robot arm often moves with the highest velocity, and therefore needs careful consideration. As the gripper is often a custom part, the safety of this part is largely overlooked. All edges should be rounded with at least a diameter of 5 mm to minimize impact at cobot speeds up to 1,5 m/s. Furthermore, actuating the gripper should not result in bruises. The mechanism must not allow for the operator’s fingers to get stuck in the gripper.
Just as the cobot and its gripper, any objects in the direct environment of the cobot should have rounded corners. After all, the cobot can get in contact with these objects. Consider a situation where the cobot hits an operator’s arm, and the operator is at that moment leaning on a conveyor. This is an aspect which is hard to satisfy and is often overlooked, especially when the robot is not positioned statically, but e.g. on an AGV. It is then necessary to scale down the maximum velocities of the cobot.
Cobots often work together with peripheral systems. For example, the cobot picks boxes from a conveyor belt where boxes slide into position by a pusher. As this typically happens outside of a fenced area, these peripheral systems need to comply with the same safety regulations as the cobot itself. For example, the minimum distances of a pusher to the belt to avoid clamping is an important feature to consider.
Not only the gripper, but also the product a gripper is holding can be a safety risk for an operator. Some examples are: sheets of metal handled by a cobot have a high risk of cutting or a gripper handling a bottle of hair spray from which spray might be accidentally released during movement. Hence, it is important to consider the safety of the products a cobot will handle.
Safety measures to take into account
In addition to the safety of the cobot system, there are multiple measures that should be considered for each installation:
- The software that determines the motion of the robot should take care of not letting the cobot make unexpected moves. Operators get used to robot behavior quickly, and will anticipate on its moves based on its usual behavior. Unexpected cobot moves might lead to sudden operator moves with accompanying safety hazards. Furthermore, the cobot should ideally be controlled such that it does not unnecessarily move in the operator area.
- Extra sensors can be added to optimize the safety of the area in which both operator and cobot work. Examples are safety scanners, light curtains and pressure mats. Installing such sensors enables the cobot to slow down when an operator is in close range.
- Also, when working with cobots the use of personal protective equipment should be considered. Some examples are:
– If a palletizer cobot drops a box of 8 kg from a height of 2 meters, an operator needs to wear safety shoes.
– The nozzle of a gluing cobot could get partly blocked with glue spraying in an unforeseen direction, for which goggles are needed.
- In any case, the area in which the cobot operates should be marked with floor marking. For any other residual risks, other forms of signing should be considered, to warn the operator for the autonomous actions of the cobot nearby.
- Lastly, the operator should in no case feel withheld be press the emergency button when a hazardous situation occurs.
More detailed information of typical situations that can occur and how to deal with them can be found in the following ISO standards: 15066, 10218-2 and the references therein.