Specification of a AGV system in practice
The technical challenge in the development of an AGV system is adapting it to the production environment. Without specification, no efficient material flow system with AGVs can be designed.
Not only the vehicles are created to fulfill the customer requirements. Also the communication between the vehicles and the production machines needs to be perfectly integrated in order to generate transport requests according to the material demand, to guarantee a smooth workflow and to meet safety requirements.
An AGV project begins with the definition of specifications. A close cooperation with the customer is important, in order to collect the external conditions and the system requirements. They are necessary for a successful implementation of a material flow system with AGVs.
The vehicle fleet should be available both for the average production capacity and during production peaks. The following conditions affect the size of the vehicle fleet:
- Output level, rate of production and range of products
- Production environment: length of the transport paths, amount of transports per hour
- Maximum possible speed, depending on the width of the gangways an on the amount of human movement
- Charging stations: distance, charge time / transport time ratio
- Order management: amount of empty runs
This calculation should be done for the average production capacity and for production peaks of 100% production capacity.The result shows the required number of vehicles and charging stations for both production levels. The size of the fleet bases on this estimation.Monitoring capacities is important for the design of the transportation system. A deep knowledge of the production processes is fundamental. This requires a close cooperation with the customer.
After defining the number of charging stations, their position must be set in the layout. As described in battery technology, the vehicles can and should be charged during running operations, in order to increase battery life. The charging pauses are of a short duration of about 40-60 seconds, while the vehicle is handling a load on docking on a transfer station. We call this opportunity charging.
In addition to the opportunity charging, separate charging stations can be implemented. The fleet management software can send the vehicles, when they have no transport orders to fulfill, or when the battery management requires it. The vehicles are supplied with charging contact surfaces. Only when the vehicle has docked onto the charging station and the contact has been confirmed with a hardware-handshake, the charging current starts to flow. As soon as the battery management detects that the batteries are fully loaded or overheating, the charging process is terminated.
The battery capacity is also an important factor when planning the positioning of loading stations in the area layout to make sure that seamless 100%-productivity can be provided.
Interface to machines and loadports
The vehicles must communicate with machines ot the load ports placed in front of them.
For a safe load transfer, the following points must be checked:
- The vehicle is the right position (see navigation)
- Both sides (machine and vehicle) are ready for load transfer
- A general emergency stop is possible (for instance, when a worker sees that the load handling is not working correctly, he/she has to be able to cancel the process on both vehicle and machine with just one E-STOP button)
We have created various methods of implementing this hardware-handshake between vehicle and transfer station.
WLAN – coverage
The WLAN coverage of the production site needs to be measured in order to determine the amount and the positioning of access points. If the coverage of WLAN is too weak, the vehicles may not be able to communicate among each other or with the control station.
Walls and contours
The laser scanner works in a specific height, usually just above the floor. Only contours on this height are detected by the vehicle. Glass walls or highly reflective surfaces could affect the navigation of the vehicle. In this case, special solutions should be considered.
Although the floor should be as flat as possible, small ramps and edges can be handled.
The surface of the floor influences the braking distance and parametrization of the protective and warning fields of the security laser. Different floor types can lead to different concepts of chassis and wheels.
Required space – width of gangways, oncoming traffic, roundabouts and overtaking
The required aisle widths are regulated by the legislator. Specifically in Germany, the Arbeitsstättenverordnung (Technische Regeln für Arbeitsstätten ASR A1.8 – Verkehrswege) needs to be considered. The aisle width depends on the size of the vehicle, which is designed based on the size, shape and weight of the transported load.
The production layout needs to be analyzed with a focus on aisle widths. Bottlenecks, escape routes and transport paths and transfer points must be defined. Vehicles standing at the transfer points and narrowing the paths must be taken into account. This will be a part of all planning processes from step one, as all of these factors have an influence on the maximum speed of the AGVs and therefore on the capacity planning.
To operate a vehicle fleet, a fleet control system is required. InSystems calls it AIC (AGV Interface Controller).
The AIC has an interface with the customer order management software (MES / ERP) and translates the material needs into transport assignments to the vehicles. It works similar to a taxi control center, assigning the transport order to the most suitable vehicle. The criteria include the distance to the transport station, avoiding empty runs and the battery state of the vehicle.
The interfaces to the existing production control systems and machines, as well as time-out behavior and emergency situations like fire alarms must be taken into account.
The vehicle and the load handling equipment need to be individually designed according to the expected load. The handling heights influence the way all the components (battery, drive units, vehicle contoller) integrate in the chassis. The center of gravity should be low enough, so that the vehicle doesn’t tilt during emergency brakings. During load handling, the load should be on the back of the vehicle, so that it doesn’t slip during braking procedures.
When the machines have different transfer heights, following solutions can be implemented:
- a lift is integrated into the vehicle (disadvantage; the vehicle is heavier and drains more battery power)
- loadports with an integrated lift are built at the outfeed of the machine
- the machines are adjusted to the same transfer height