How safe are AGV systems? Everything about safety standards, warning devices, and safety sensors.
More and more companies are turning to AGV systems to optimize their processes in production and logistics halls. These fully automated mobile robots operate in the same environment as employees, making safety a paramount concern.
Automated Guided Vehicles (AGVs) are equipped with extensive safety features to prevent accidents involving people, machinery, or objects.
The focus of safety technology is clearly on preventing personal injuries. In this text, you will learn about available protective measures, the safety standards governing the technology, and how safety devices differ from warning signs.
Establishing safety measures at the early stages of development:
When safely deploying mobile robot systems in intralogistics, the primary focus is on minimizing risks. An inherently safe design, such as using rounded edges, eliminates potential risks from the outset.
However, when mobile robot systems and personnel share the same environment, design alone cannot address all safety risks. This is where technical measures like safety sensors come into play. These sensors monitor all directions of the AGV system and detect obstacles within the defined protection zones.
Additionally, AGVs are equipped with acoustic or optical warning devices, such as horns or blue spotlights, to indirectly prevent collisions and accidents by increasing employee awareness.
Behavioral guidelines in the manual provide additional training for employees on handling driverless transport systems.
What safety standards exist?
Mobile robot systems are subject to the machinery directive of the VDI (The Association of German Engineers). Specific safety requirements govern the use of these systems, particularly outlined in the VDI guideline 2510, Sheet 2, “Safety of AGV Systems.” The VDI status report, “Safety Guide,” describes key definitions and responsibilities for manufacturers and users (Mobile Robot Systems – Safety Guide | VDI).
The VDI machinery directive sets clear safety standards for mobile robot systems, which AGV manufacturers follow. Based on these requirements, they derive measures to minimize vehicle risks.
However, safety standards for mobile robot systems are often broadly formulated and usually refer to large, autonomously operated forklifts. The hazard scenarios for small, agile AGVs are often different.
If a standard cannot be met, manufacturers must conduct their own risk assessment. They must clearly explain how their safety device avoids potential risks to people or objects in specific application scenarios.
Comprehensive and complete documentation is particularly important, as manufacturers issue the CE declaration of a product themselves, confirming the machine’s safety.
Bluespot on the SAFELOG L1 (Image: © SAFELOG).
Technical requirements for high performance of the safety devices
Safety devices on AGV systems must always function reliably in all scenarios. They are classified on a risk level scale from A/1 to E/5, with the highest safety level being E/5. Examples of machinery in this category include press brakes, where failure of a safety system could result in serious injury to the operator.
While the Machinery Directive was written with large AGVs in mind, it applies universally to all size categories. The worst-case scenario in an accident involving such a large vehicle, for example an autonomous forklift, would be the death of the person involved.
Consequently, these devices are categorized as D/4 for safety hazards. Since there is no distinction between large and small transport vehicles, mobile robots also fall into this category. Therefore, the safety technology of a mobile robot system must meet all the requirements of safety level D/4.
Furthermore, the safety controller must continuously monitor all electronically verifiable safety systems and evaluate their data.
For operators planning and constructing a safety device, the following approach is recommended:
- Identify the hazard: Determine what poses a risk.
- How can the mobile robot system detect the hazard source?
- How does the transport system evaluate the hazard situation?
- How can the hazard be eliminated or prevented?
For example, the vehicle must stop or slow down if a person is detected within the safety zones.
To avoid errors and failures, electrical safety devices on mobile robot systems are implemented with dual-channel redundancy or electrical pulses. In the dual-channel variant, two cables carry the same signal. In the electric pulse method, a signal is sent to the safety controller at a specified frequency.
The safety controller evaluates the feedback from the systems and triggers an immediate stop of the vehicle if no feedback is received. The dual execution enhances the reliability of all safety systems in mobile robot systems, contributing to fault tolerance.
Contact systems
Safety sensors are divided into contact and non-contact systems. With contact systems, the safety solutions take effect as soon as the AGV touches a person or an object. These include, for example, bumpers, safety bars and emergency stops.
Bumper
Bumpers were initially used with safety bars in the first AGVs. Located at the front and rear of a mobile robot, they served as mechanical safety devices. In the event of a collision, they absorbed part of the impact force and triggered a stop of the vehicle.
Today, manufacturers primarily use bumpers to protect sensors and other sensitive electronic components. They also serve as a backup system for safety sensors, preventing workers from getting their feet caught under a moving AGV. The construction is mounted 30 mm above the ground, ensuring that a foot cannot be run over when wearing safety shoes.
This provides additional protection for workers when using mobile robots as workpiece carriers or transport vehicles for delivering components to the assembly line.
Close-up of the bumper (Image: © SAFELOG).
Emergency stop
The emergency stop allows warehouse employees to directly intervene in the driving behavior of the mobile robot in an acute danger situation. This safety button differs from an emergency stop switch. While the emergency stop switch immediately cuts off the power supply to a device or machine, the emergency stop causes the device to stop immediately without cutting off the power. Instead, all actuators are passively switched off, ensuring the AGV stops safely and immediately.
The emergency stop is normatively positioned and clearly marked in each direction of the AGV’s travel, making it quickly and easily accessible in a dangerous situation. When the emergency stop is triggered, the mobile robot stops immediately. After the danger is cleared, employees must pull or turn the red button to unlock it. They then acknowledge the process and press the button again to switch the AGV back to driving mode.
Since the button must meet a certain performance level, it is connected to the safety control system, which evaluates the status of the emergency stop.
Close-up emergency stop on the SAFELOG S2 (Image: © SAFELOG).
Non-contact systems
All AGV system manufacturers use non-contact protective devices to guarantee personal and material protection. Non-contact systems detect potential collision hazards using scanners that continuously scan the surroundings. This category includes the laser scanner.
Laserscanner
The laser scanner continuously scans the path of travel and detects obstacles, including people or objects near the AGV, early enough for the vehicle to react and avoid accidents. The mobile robot’s response is significantly influenced by defined warning and protection zones in each direction of travel, monitored by the laser scanner. For compact AGVs, the scanning plane of the safety laser scanner is approximately 15 centimeters above the ground.
The size of the warning and protection zones is configurable and adjusted according to the application and environmental conditions. Most protection zones range from three to five meters, with SAFELOG configuring the largest zone at four and a half meters. The optimal protection zone configuration is calculated based on several factors:
- Weight of the transported goods: How heavy is the AGV when loaded?
- Ground conditions: What coefficient of friction does the AGV need for braking?
- Space conditions: How much free space is available?
The protection zone must fit specific driving situations. In industrial production halls and logistics warehouses, limited space due to narrow aisles, columns, or other obstacles requires careful adjustment. If the warning and protection zone is set too large, the AGV will slow down or stop frequently, losing time at every turn and bottleneck. Therefore, it is crucial to adjust the protection zone to the environmental conditions. As a rule of thumb, the protection zone should be only as large as necessary.
The length of the protection zone is calculated from the stopping distance considering full load and ground conditions, plus a constant of 265 mm. Under ideal conditions, minimal protection zone lengths of 1.200 mm can be achieved. Depending on the design of the AGV and the sensor type, maximum protection zone lengths of 3 to 7 meters can be reached.
During commissioning at the site of operation, the settings of all protection zones of the AGV are checked. During test operations, the AGV will approach an obstacle at maximum speed, if it reliably stops in front of the test object under the given conditions, one requirement for the safe operation of the AGV system is fulfilled, and commissioning can begin.
Installation example of the safety laser scanner from SICK in the SAFELOG X1 (Image: © SICK).
Image processing, ultrasonic sensors, and passive infrared scanners
Ultrasonic sensors, image processing, and infrared scanners are also used as safety technology for the prevention of personal injury, collision detection to avoid collisions with objects, and as navigation aids.
As active safety devices, they can send signals and receive their reflections to provide information about the AGV’s surroundings. In case of danger, they ensure an appropriate warning and response.
The development of these safety devices is undertaken by specialized firms, which are also responsible for certifying the products according to relevant standards and guidelines. The design of safety technology and the programming of the required software are very complex and time consuming. Ultrasonic sensors and infrared scanners must be safety certified to meet all safety requirements and reliability of safety technology.
Image processing is the latest system among safety devices. In this system, the AGV continuously evaluates images of its surroundings. What may still sound like science fiction could soon become reality: recognizing the difference between people and objects through image data analysis. In the future, operators could then adjust the distance based on the type of obstacle. For example, the AGV would continue driving when detecting a column but stop when detecting a person. The prerequisite is that the software can accurately distinguish between a person and an object or always apply the brakes when in doubt.
Warning-devices
Warning devices, compared to the safety devices of mobile transport robots, require active participation from traffic participants. The VDI (The Association of German Engineers) machinery directive mandates at least one acoustic or optical signal as a warning device in automated guided vehicle systems. Ideally, both systems are installed. However, they only have an indirect effect, as traffic participants must actively respond to the warning message.
Acoustic warning signals
With acoustic signals, transport systems announce events such as the start of a drive or changes in direction. During operation, the AGV honks at regular intervals to alert employees to its presence. However, this solution is not very user-friendly. In a hall with a fleet of 20 AGVs, for example, employees would be subjected to constant noise.
Therefore, SAFELOG primarily relies on optical warning displays in its mobile transport robots. The horn in the automated guided vehicles is only used for specific notifications upon customer request.
Visual warning signals
Optical signals indicate different maneuvers of the AGV. The vehicle is equipped with predefined blinking sequences to signal movement direction or turns.
Visual warning signals are displayed through LED blinkers and strips. For example, two seconds before a transport robot departs, all directional indicators of the mobile robot flash. During operation, the automated guided vehicle blinks at a predefined frequency.
While not normatively required, directional indicators serve as an extra feature, facilitating the assessment of AGV maneuvers. Clear indication of the direction of travel makes it easier for employees to interact with the mobile transport robots, especially when starting after a complete stop.
Older SAFLOG AGV models indicate a turning maneuver by turning off the blinker on one side while the other continues blinking. They demonstrate the direction of travel by turning off the blinkers at the rear and leaving the lamps at the front illuminated.
The new generation of devices offers additional display options. With a running light via an LED strip, the AGV indicates the turning process. A brake light flashes during braking. Additionally, the transport systems indicate the direction of travel with white light at the front and red at the rear. The color scheme, inspired by automotive lighting logic, is intuitively understandable for people.
Close-up of warning light (Image: © SAFELOG).
Conclusion
In industry and production, there is an inherent risk of accidents when humans and automated guided vehicle systems operate in mixed environments. Common sense, training, and sophisticated safety technology prevent accidents and ensure smooth transportation in intralogistics. Currently, laser scanners dominate the market for environmental detection. Acoustic and optical warning signals are virtually standard equipment for all AGVs. Furthermore, many additional protective measures are being developed to make the collaboration between mobile transport robots and humans safer.