Make Collaborative Robots Safe

Man and robot move closer together. This applies not only to service, but also to industry. Cobots can be used flexibly and require significantly less work space than the previously fenced and firmly anchored industrial robots.

With the dissolution of the spatial separation and the direct contact between humans and robots, the risk of being injured increases. To avoid this, a risk assessment is necessary during the design phase. In addition to the robot, the tool attached to the robot arm, possibly moving objects and the planned area of ​​application must also be taken into account.

When robots meet humans

The ISO 10218 standard for industrial robots is fundamental. Among other things, it defines four types of human-robot collaboration (HRC) and the underlying security concepts:

1. Manual guidance at reduced speed

The movement of humans is absorbed and strengthened. The movement control is immediate and remains controllable due to the reduced maximum speed.

2. Stop when entering the collaboration room

The robot carries out its task automatically and stops as soon as it enters the defined collaboration room, which is monitored by sensors. After leaving this room, the robot starts up again automatically.

3. Distance monitoring

The environment of the cobot is monitored dynamically in its movement. Depending on the degree of proximity of a person, the speed is reduced. Falling below the minimum distance triggers the safety stop.

4. Limitation of energy and strength

The energy and force limitation is used in particular in the scenarios of the HRC, in which humans and cobots have to work closely together and the risk of contact is at its highest. In this case, the performance of the robot must be reduced to such an extent that no injuries can occur.

Supplementary information on where raw forces make sense - an example

When applied to an automotive supplier from Saarland, there is a risk of being hit by a robot on the head, upper body and upper extremities. The highest risk is a hit on the head. For possible hits on the head, the measuring system must be equipped with a spring with a spring constant of 75 N / mm². The maximum permitted impact force for the head area is 90 N. Another limit value for a hit in the head area is a surface pressure of 20 N / cm². Both values ​​represent the medical-biomechanical requirements. They come from the BG / BGIA recommendations for the "Risk assessment according to the Machinery Directive - designing workplaces with collaborative robots". The measured value for the impact force in this example was 231 N, the force was distributed over an area of ​​7 cm 2,which corresponds to a surface pressure of 33N / cm². Because this robot did not meet the requirements of the BG / BGIA recommendation, additional measures were necessary such as:

• the force limitation of the robot
• a partial enclosure (only forearm / hand can be hit)
• reducing speed
• an increase in the impact area

This means that the robot can now be used safely with the skilled workers. With the force measurements and the associated safety measures, operators of cobots are fulfilling their responsibility for occupational health and safety, as required, among other things, by the operational safety regulation amended in June 2015.

Limit values ​​for force and pressure

However, ISO 10218 does not specify exact limit values. In practice, the recommendations of the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) have so far been followed in practice. The recently published technical specification ISO / TS 15066 extends and substantiates the ISO standard and defines biomechanical limit values ​​for 29 body regions, which are based on examinations of pain sensation.

The forces and pressures in the event of a collision must be determined for the risk assessment. The IFA recommends the use of a biofidel measuring device. Tüv Süd Industrie Service also uses such measuring devices. Various springs are installed in a force transducer, the constants of which simulate the different body regions. In this way it is possible to determine which forces act on people in the event of an impact. The design and integration of the robot are important to comply with the limit values. Depending on the area of ​​application, it may make sense to install partial enclosures or to limit movement areas and travel distances. In this way, particularly sensitive regions such as the head and neck can be excluded as hit zones. Rounded edges and larger and padded target areas also have a positive effect.Even older systems can be made compatible by limiting the speed, provided that the control system meets the requirements of ISO 10218 Chapter 5.4 ("Safety-related performance of the control system").

If a manufacturer places a cobot on the market, not only the usual technical documents according to the EC Machinery Directive have to be specified. The biomechanical limit values ​​(force and pressure) for the contact situations are also required. The technical specification is not a standard and is still in the testing phase. However, it does provide guidance on how best to protect employees based on the latest research. The experience should also be included in the update of ISO 10218. (kj)