The energy supply of an AGV during operation

An automated material flow using AGVs, so-called “automated guided vehicles”, is associated with high acquisition costs. To achieve rapid amortisation of this investment, maximum utilisation of the vehicles must be ensured. On the one hand, this is realised through a storage system solution that is optimally designed for the general conditions. Energy supply of AGV is also crucial in maintaining efficiency. It is also realized through intelligent software control. On the other hand, it must also be ensured that the AGVs have as little downtime as possible. This can be done by operating them efficiently during loading or servicing processes. Next, various options for increasing efficiency are considered and their effects on the design of AGVs are analysed.

Discharging the energy storage system during operation

The discharge of an energy storage system during operation can vary depending on various framework conditions. For example, the payload, the acceleration and speed of various activities all influence energy consumption. Additionally, the route profile contributes to energy consumption. Intelligent control systems can optimise energy management by analysing energy consumption. They navigate AGVs efficiently between their operating locations and distribute activities to different vehicles based on charge status. However, this requires the precise and valid recording of various pieces of information. These need transmission to the control system in real time.

Data acquisition and transmission

Application-specific decisions can or must be made based on recorded temperature, current and voltage values. It is therefore important to interpret the data correctly. For example, thermal inertia plays an important role in temperature measurement. In voltage measurement, the voltage drop via the ohmic resistance of cables is key. Current measurement must consider high-frequency interference during operation of electric motors. In addition to hardware options, such as sense lines when recording voltages, other options exist. The sampling rate and mathematical filters can help obtain valid values.

If the data relevant to sequence control is recorded by a central energy management unit in the form of a DC/DC converter, the bus load can be reduced. This happens during communication with the control unit. Additionally, a standardised time stamp can be guaranteed and an immediate response can be made to critical operating conditions.

The supply of the drives

The supply voltage of electric motors influences their torque and speed. The efficiency of the drive also depends on the voltage applied. Typically, the motor is designed to work optimally at a certain nominal voltage. For this reason, a regulated supply voltage is useful for high efficiency. The clamping voltage of energy storage devices varies depending on the type and chemical composition, state of charge, and current requirement.

To ensure a stable voltage supply even under fluctuating load conditions, an appropriately designed DC/DC converter is required. An example is the DDL4848-48. Depending on the length of the cables to the drive, cable compensation might be useful. Increased voltage depending on the current flow can compensate for cable length.

Increased efficiency through regenerative braking

As described at the beginning, the operating time of the AGV is decisive for the efficiency and amortisation period of the warehouse. It makes sense not to burn energy via resistors during braking. Instead, energy should be fed back into the energy storage system. The prerequisite for this is that the installed DC/DC converter can transfer energy bidirectionally, i.e., in both directions. During recuperation or regeneration, the vehicle’s electric motors act as generators. They behave like a voltage source accordingly. A suitable DC/DC converter recognises this and then changes the direction of energy flow. This flows back into the energy storage system. Depending on the design and operation of the vehicles, efficiency can increase. Consequently, runtime until the next charging stop may extend by up to 40%.

Another positive aspect of energy recovery is that less heat is generated in the vehicle. The braking energy is converted into electrical energy rather than heat.

Conclusion

Overall, recuperation using bidirectional DC/DC converters in AGVs offers a win-win situation. It improves energy efficiency and lowers operating costs. It also reduces environmental impact and optimises vehicle performance.