Automated guided vehicle systems (AGVs) have become an indispensable part of modern intralogistics. To ensure that these autonomous vehicles operate safely at all times, they must be able to come to a reliable stop. However, the stopping distance of an AGV system is not a simple constant. It is the result of a complex interaction between sensor systems, control systems, and mechanical components. Short, verified switching times of the brakes and intelligent monitoring are therefore critical to safety.
To ensure that automated guided vehicle systems always stop purposefully on their way through the factory halls and do not cause damage to materials or persons, reliable and precisely operating safety brakes are indispensable. The requirements of the applicable standards are clear: The braking system must be able to bring the vehicle to a safe stop within the sensor detection range even under adverse conditions such as maximum rated load or on a downhill slope. The basis for this are safety brakes that operate on the fail-safe principle. In the de-energized state, they are safely closed by spring force. They reliably apply the required braking torque even in the event of an emergency stop, power failure, or wire breakage.
Stopping Distance: Theory vs. Practice
The total stopping distance of a vehicle, however, is longer than the pure braking distance. "In theory, without a driver, the human reaction distance is eliminated and the brake would engage immediately upon demand and decelerate the vehicle," explains Andreas Merz, Product Manager at mayr® power transmission in Mauerstetten." “In reality, however, each brake has a certain time delay or switching time, the so-called t1 time, during which the braking torque builds up. Add the time delays of the upstream sensor system, the control system and the brake contactor.”
These system time delays result in braking not commencing instantaneously upon demand, but with a delay. “And during these time delays, the AGV continues to move of course, or even accelerates, e.g. due to a downhill slope,” adds Andreas Merz. “In both cases, the braking distance gets longer.”
The situation is further aggravated by unnecessarily long time delays and switching times or incorrectly dimensioned safety brakes. It is therefore important for users to select brakes with the shortest possible, verified switching times that are maintained throughout the entire service life.
When the Brake Gets Smart
This is precisely where intelligent monitoring solutions such as the retrofittable ROBA® brake-checker® module come into play. It operates without additional sensors and provides comprehensive functional monitoring: By analyzing characteristic current and voltage waveforms in the supply line to the brake, the ROBA® brake-checker® draws unambiguous conclusions about its operating state. The solution is more than a mere replacement of release monitoring and wear monitoring with microswitches and proximity sensors: By recording additional measured variables such as switching times, voltages, or the coil resistance of the brake, the ROBA® brake-checker® enables advanced analyses and conclusions regarding complex interrelationships. In combination with the ROBA® gateway module, the solution becomes network-compatible: The two modules are connected via an IrDA interface, enabling the brake data to be retrieved continuously via an Ethernet connection. A robust mechanical component thus becomes a smart, data-supplying unit that also addresses the spirit of the upcoming EU Machinery Regulation, which places a stronger focus on the reliability of control and software functions.
"Currently, we offer the modules as a compact unit in a housing that is mounted on the DIN rail," explains Andreas Merz. "For increasingly compact designs, highly integrated solutions are sometimes required." We have therefore integrated the solution onto a single chip that provides the complete functionality of the ROBA® brake-checker® and ROBA® gateway. The controller is integrated directly into the user's control system, saving significant additional space. "This variant is currently undergoing trials with initial customers." For an AGV manufacturer, the integration of such a system can significantly simplify the conformity process. Instead of relying on blanket maintenance intervals, the manufacturer can draw on specific, verified condition data and thus demonstrate safety without any gaps.
Stable Operation and Reduced Energy Consumption
A key practical advantage of the module is also its ability to supply the brake with a stable voltage even when the battery voltage fluctuates – a typical scenario in mobile applications. It provides a regulated output voltage and thereby ensures that the brake releases safely at all times and maintains defined switching times. This ensures the longest possible reliable operation, even when the battery voltage is dropping.
In addition, after release, the module intelligently reduces the holding voltage. This reduces the energy consumption of the brake by up to 90%, thereby further extending the range of the AGV by reducing the load on the vehicle battery.
Reliable Protection for Lifting Devices as Well
The combination of proven safety brakes and intelligent monitoring electronics represents the optimal solution for the high safety requirements placed on autonomous vehicles. It not only increases operational safety and availability, but also prepares the vehicles for the regulatory requirements of the future. This also applies to lifting devices on AGVs or autonomous industrial trucks, as well as robotic arms mounted on AGVs. Safety is just as important here as it is for the vehicles themselves. As Andreas Merz points out, the consequences of an overloaded brake are particularly severe in this context: ""In the case of vertically moving axes or robotic arms, the brakes can be overloaded, for example, due to the increase in speed before the brake engages." This could further extend the braking distance or, in the worst case, result in a load crash. Reliable components, correct brake dimensioning and intelligent monitoring that is aware of the brake's condition reliably prevent such scenarios.
