When an aAGV is used in areas where humans are present, a thorough safety system is required.
Potential dangers of an autonomous transport vehicle:
- of the driving vehicle
- of a falling load, especially during load handling an in case of a vehicle’s emergency stop
- while charging the vehicle’s batteries and due to the battery technology itself
Ensuring safety of the vehicle
Using laser scanners with implemented speed-dependent warning and safety fields, the vehicle observes 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 and validated through tests while keeping weight, speed, kind of floor and safety regulations in mind.
Before the identification of obstacles using optical technologies was developed, AGVs where 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 using laser scanners consists of warning and protective fields which monitor the immediate area adjacent to the vehicle and far beyond it in driving direction. Once the scanner detects an obstacle or a human in its warning field, the vehicle reduces its speed. If a person steps into the scanner’s protective field, the vehicle immediately triggers an emergency stop. After a programmed time delay of a few seconds, the vehicle reenters its autonomous mode, scanning its smallest protective field and attempting to slowly navigate around the obstacle. The size of the warning and protective fields are velocity-dependent and change its size according to the vehicle’s speed. The size of the fields are calculated depending on the vehicle’s braking distance, parametrized in the laser scanner and validated with a brake test.
The vehicles also monitor the environment beyond the warning fields and change their driving direction in foresight to avoid unnecessary emergency stops. This behavior is unfamiliar to the plant’s workers in the early phases of an aAGV implementation because humans tend to try stepping out of a robot’s way. But in our experience, workers quickly get accustomed to the robot’s behavior after they are introduced to its navigation algorithms.
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 the load is being handled. The the vehicle should be designed in a manner that the load is safely positioned and unable to slip. Commotions caused by stops or irregularities 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
On every point in which the load is passed between the vehicle and a station, a load handling process must be designed. The mechanics and software must be engineered in a manner that the load cannot fall down under any circumstances during the process. To ensure a safe handling process, both the positioning of the vehicle in front of the station needs to be accurate and the conveyors of vehicle and station need to signal their availability.
As the potential danger that arises from heavy loads falling is very high, a safety system relying only on software is not sufficient. Rather, a safety-related communication between vehicle and station is strictly necessary. See hardware handshake under Communication.
Safety while charging the battery and relating to the battery technology
The proANT aAGVs are powered by Lithium-Ironphosphate (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. To avoid injury due to electrical shock, a loading current is only routed to the charging station if the vehicle is covering the charging station’s contacts. To ensure this, the vehicle communicates with the charging station in a similar way as when passing a load.