When an AGV is used in areas where also personal works, an appropriate concept for personal safety is required.
- Dangers caused by the vehicle
- Dangers caused by falling loads, especially during load handling and emergency stops
- Dangers occurring during the charging process and caused by the battery technology itself
Ensuring safety with laser scanners with implemented speed-dependent warning and safety fields, the vehicle scans the area far in front of it and far wider than its actual width. The faster it drives, the larger the area its scanner covers. The exact size of the fields can be parametrized and is calculated during commissioning while keeping in mind the braking distance (weight, speed, kind of floor and safety regulations) and validated with brake tests.
Before the identification of obstacles using optical technologies was developed, AGVs were equipped with protruding safety edges called bumpers. Once a force was applied to the bumpers, the vehicle moved into an emergency stop state. According to the Norm „DIN EN 1525:1997 Safety of industrial trucks – driverless trucks and their systems“ the maximum allowed force applied to a bumper before it triggers an emergency stop is 400N. Without much calculation, it is easy to see that big vehicles with high loads and long braking distances equipped with bumper technology are only allowed to travel at very low speeds.
In contrast, the safety concept with laser scanners allows a higher speed. Once the scanner detects an obstacle or a person in its warning field, the vehicle reduces its speed. If a person steps into the protective field, the vehicle immediately triggers an emergency stop. After a programmed time delay of a few seconds, the vehicle goes back to the autonomous mode, switches back to the smallest protective field and tries to slowly navigate around the obstacle.
The AGVs also monitor the environment beyond the warning fields and change their driving direction proactive to avoid unnecessary emergency stops. This behavior is unfamiliar to the operators in the early phases of an an AGV implementation because they tend to try stepping out of a robot’s way.
In our experience though, after a briefing the employees quickly get along with their new “colleagues”along their routes.
Securing the load
The load should not fall off the vehicle under any circumstance. This includes both the situation when the load is being transported and when it is being handled. The vehicle should be designed in a way, that the load is safely positioned and unable to slip. Shocks caused by stops or small bumps in the floor should not be able to displace the load, since this would lead to changes to the vehicle’s center of gravity. When the vehicle’s center of gravity is altered, the braking distance is increased.
During load handling
The mechanics and control devices must be designed in order to avoid a falling of the load while dropping the load from the machine to the vehicle and vice versa. To ensure a safe handling process, the positioning of the vehicle in front of the station needs to be accurate. Also the loadport of the machine and the load handling attachment of the vehicle must be ready for transfer. As the potential danger that arises from falling heavy loads is very high, a safety system relying only on software is not sufficient. Rather, a safety-related communication between vehicle and machine or transfer station is necessary. See also hardware handshake under Communication.
Charging and battery technology safety
The proANT AGVs are powered by lithium iron phosphate (LiFeYPo4) batteries. These batteries are charged with a voltage of 30V. While charging and in operation, the battery’s cells voltage, temperature and charging state are monitored by balancer boards.
The proANT vehicles automatically drive to the charging station when their charging state is low. In order to avoid injuries due to electrical shock, even when the vehicle docks at the contacts of the charging station, the charging current flows only after a safety request.