Sensors are an indispensable part of mobile robots (AGV and AMR). Sensors are the “eyes and ears” of mobile robots.
Without good, reliable and affordable sensors, we would not have the super performant mobile robots we have today. Mobile robots count on many types of sensors for their basic functions, such as navigation, guidance, safety, material handling, obstacle avoidance, etc.
In this article we will discover what sensors AGVs use and the different applications of sensors utilized in the automated guided vehicle industry. Here you have a quick recap table.
Function Type of Sensor Application Safety Sensors Safe 2D Lidar Safe personnel detection Bumper Vehicle stoppage if contact Encoder Vehicle speed and steering detection Environment Perception 2D & 3D Lidar, Ultrasonic, Camera, Radar Avoid impacts with objects Navigation and Localization 2D & 3D Lidar, Ultrasonic, Camera, Radar Mapping, Localization and Navigation Line sensors (magentic, inductive, optic sensors) Navigation Load Handling Cameras, 2D or 3D LiDAR, Ultrasonic Pallet pocket detection Optical distance sensors or Wire draw encoders Fork Height Sensors Photocells, ultrasonic, inductive Ensure the right load positioning Identification RFID, Laser or Image based bar code scanners Transported material identification
What are the main types of sensors used by AGV?
AGV Safety Sensors
Since their introduction more than 30 years ago, Automated Guided Vehiclesare mobile, collaborative machines that share traffic routes with manned industrial trucks and people.
Reliable person detection systems are required to prevent danger to people in these surroundings. Due to international standards such as EN ISO 3691-4, ANSI B56.5 or the latest ANSI/RIA R15.08 (for autonomous mobile robots), high safety requirements are placed on person-detection systems.
The most important sensors under this category are the Safety Laser Scanners that must comply with most restrictive safety standards.
AGV Obstacle Sensors - Environment Perception
Collisions between automated guided vehicles and objects can be extremely costly and significantly reduce system throughput.
Typically, mobile robots are not covered 3D-360° by rated personnel safety scanners (the “yellow ones”), because it would be really expensive. For this reason, the safety lasers are not able to detect all the objects such as suspended loads or goods protruding from a shelf.
In this case, there other types of AGV obstacle sensor systems, such as multi-layer LiDAR sensors or vision cameras with TOF technology, which can be utilized to detect all objects. 3D sensors also measure the position of goods in real time, optimizing the load handling process of goods.
AGV Navigation Sensors and Localization
Industrial truck automation concentrates on how the vehicle navigates. If the vehicle is unable to localize its position, it is unable to navigate. The vehicle can determine its position on a physical track or with the aid of a map-based localization system (SLAM).
Depending on the AGV navigation technology, we could have different types of sensors such as LiDAR, magnetic tape sensors, cameras, etc.
Load Handling and Identification
Load handling involves high demands not only on automated vehicles but on sensors as well.
The sensors need to be able to detect different surfaces and load geometries so that the load can be correctly identified and positioned.
Non-contact identification systems offer full traceability for goods transported with automated vehicles.
AGV Safety Sensors
Automated guided vehicles need safe personnel protection systems to prevent damage. AGV Safety Systemsmust be reliable and comply with severe international standards.
There are three main types of safety sensors:
- Contact sensors
- Non-contact sensors
- Positioning sensors
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AGV Safety Bumper: Contact Sensors
Contact sensors are typically known as bumpers. They stop the vehicle in case of contact with an object or a person.
The contact force must be low enough to avoid harming someone. For this reason, the vehicle speed and inertia must be coherent with the bumper dimension.
Bumpers use to be mounted on the side of the vehicle to protect the low-speed turning manoeuvresor slow backwards movements, that could not be protected by a non-contact scanner.
AGV Safety Laser Scanner: Non-Contact Sensors
The principle is simple: AGV Safety Lasers scan the surrounding area and slow down or stop the vehicle if they detect an obstacle.
Picture By SICK
They deliver non-contact detection of people and objects located in the path of an AGV, avoiding the mechanical damage that can be caused when using switching bars or bumpers.
The safety laser scanners used in the AGV industry typically have at least two or three safety fields.
The first field is called the“warning field.” When an obstacle is detected in this field, the AGV slows down.
Then we have the“safety field” that stops the AGV if it detects an obstacle.
Every point of the AGV path must have a specific protective field consistent with actual AGV speed.
An AGV could be equipped with one or more safety sensors depending on the vehicle risk assessment.
AGV Forklift with two Safety Scanners
There are several types of safety laser scanners with different characteristics, such as:
- Scanning range of the protective field
- Scanning angle
- Number of fields
- Number of monitoring cases
- Response time
- Performance Level (PL)
- Overall dimensions (safety laser scanner must be compatible with your mobile platform chassis)
- Possibility of use as a navigation device in case you wish to have a contour-based navigation platform
AGV Safe Positioning: Speed and Steering Angle
There are other more sophisticated methods able todynamically adapt the protective fielddepending on how fast the mobile robot is moving and where it is.
In this case, we need safe positioning absolute encoder sensors that reliably indicate the actual speed of the AGV and its steering angle, to be sure the current speed matches the appropriate field set.
The Safety Encoder detects the speed and steering angle to safely define the Safety Laser
For AGV applications, components that are rated up to Safety Integrity Level 3 (or SIL3) are ideal, as this allows for the highest safety level in the system.
Furthermore, speed data, steering angle data and other parameters, such as load-carrying position and load weight, are important for monitoring vehicle stability.
AGV Environment Perception
The “safety” specification indicated in the previous point is related to “human or personnel” safety and involves situations that could represent a risk for operators in the working area.
Nevertheless, there are other situations that could not strictly be understood as human safety but that could raise other kinds of risk for the infrastructures, materials or the AGV itself.
AGVs can be equipped with collision-avoidance sensors that are not rated as “safety” devices.
For example, the vehicles could mount 2D or 3D LiDAR sensors covering the front side of the vehicle to detect hanging objects in time.
AGV Forklift with 3D LiDAR able to detect suspended loads
Another application could be small compact 2D LiDARs installed on the sides of automated guided vehicles, offering reliable detection of objects protruding from shelves, thus preventing collisions.
Automated Forklift with 2D lateral LiDAR detecting protruding box in a shelf.
A less expensive alternative are ultrasonic sensors that reliably detect objects in the vehicle path. The work principle is the same as the one we have for ultrasonic sensors in car bumpers.
In contrast to optical scanners, such as LiDARs, ultrasonic sensors detect objects regardless of their surface quality and do not suffer light conditions.
Another solution are 3D vision sensors that allow definable spaces to be constantly monitored and can create a point cloud showing all the objects visible in a monitored space.
AGV Sensors for Navigation and Localization
AGVs mount different navigation sensors depending on the navigation method.
Sensors for Laser Navigation (LGV)
In general, AGVs use 2D LiDAR sensors, specially designed for navigation and featuring integrated position calculation of Laser Guided Vehicles.
The 360° horizontal 2D LiDAR detect reflector marks within the working environment, measures the distance away with high precision and provides the on-board computer with precise information about the vehicle's absolute position.
2D LiDAR targeting reflectors for positioning triangulation
The best 2D LiDARs can target reflectors up to 70m away. Multiple measurements made on each reflector are converted into precise reflector and AGV coordinates.
Sensors for AGV Natural Navigation
In contour or natural navigation, a LiDAR sensor scans existing contours around themobile robotenvironment and saves the measurement data in a “map”.
On subsequent trips, the vehicle localizes its position by comparing the map with up-to-the-moment measurement data.
Contour navigation does not require the installation of additional equipment, such as reflectors or magenetic tape, meaning the vehicle’s routes can be altered at any time.
Typically, LiDAR sensors are electro-mechanical scanning systems, they can be 2D or 3D sensors.They count on a range of mechanical moving parts, adding complexity of assembly and affecting the sensor’s robustness. They are expensive but provide good resolution over a scanning range of 360 degrees.
3D LiDAR Sensor mapping the environment
Good and reliable LiDAR technology remains expensive, and for this reason sensor manufacturers are developing new solutions such as solid-state LiDAR.
Solid-state LiDARs emit pulses of light that are spread across a wide field of view by means of a diffuser.
They are less expensive, smaller and more robust. On the other hand, they provide less resolution measurements and scan maximum 120 degrees.
AGV Safety Laser Scanners Suitable for Navigation Purposes
At the end of the day, safety lasers are LiDAR sensors with criticalspecifications such as low failure rate that enable the safety classification.
Some safety scanners can be used for both functions contemporaneously, “safety” and “navigation”.They combine safety technology and a measurement value output in a single device.
Mobile Robot using the safety scanner for navigation purposes.
The scanners send measurement data about the scanned environment to the navigation controller, which then uses this data for localization and navigation. Of course, in this case, the safety scanner must have a goodangular resolution, typically 0.1° and count on a measurement range of up to 50m.
Meanwhile and independently of the navigation feature, the scanner monitors the protective fields. The safety laser scanner’s intelligent dual-use function allows the option of designing compact, more cost-effective vehicles.
This application is typically used in mobile platforms, autonomous carts, piggyback vehicles, etc.
Sensors for Line Guidance (Magnetic and Optic Navigation)
We all love Autonomous Mobile Robots that perform trackless Natural Navigation. On the other hand, sometimes it is practical (or less expensive) to adopt a Line Guidance Navigation Technology.
Line Guidance is a straightforward and cost-effective type of navigation for automated guided vehicles (AGVs) or mobile platforms.
It functions through a sensor on the vehicle that detects and tracks a line on the floor. The sensor outputs the deviation relative to the vehicle's center, which enables precise positioning of the vehicle in the line.
Magnetic Tape Sensor for AGV
In this case, AGVs are guided by a magnetic tape induction signal adhered to the floor, which are detected by a sensor underneath the vehicle.
The magnetic sensor will measure how far the AGV is from the center of the tape and provide the information to the motor controller, which will then adjust the steering so the vehicle remains at the center of the track.
Typically, the sensor is located at 30mm above the floor and ensures the height fluctuates within +/-10mm max as the AGV moves along the track.
Optic Sensors for AGV
The same concept is applied. In this case, the guideline for the vehicle is painted or taped onto the floor.
In general, the AGV is equipped with a camera that detects the line and calculates the deviation from the line, which is provided to the vehicle’s controller as an analogue voltage or on a digital interface.
Optical line sensors detect conventional luminescent adhesive tape regardless of the background, contamination or surface defect.
Identifying Points of Interest (POI) and Fine Positioning
The navigation solutions such as line guidance or magnetic grids are not able to permanently establish an absolute position of the robot.
For this reason, the vehicles need to receive additional information about their current location at the POI - Point of Interest (workstations, crossings, loading stations, etc.).
The POI is equipped with Optical markers, QR Codes, Data Matrix or RFID tags.
Somerobots are equipped with smart vision sensors that read the 2D code and report the exact positional deviation and angular position (pose) of the marker to the vehicle.
Load Handling Sensors for AGVs
Forklift AGV - Pallet Pocket Detector
Robot Forklifts have some special requirements regarding sensors. For example, the Pallet Pocket Detection sensors (also known as pallet finder) that are required to detect the pockets in the pallet to accurately pick up the pallet.
The task assigned to the robot is therefore to detect pallets in a certain loading area or shelf, compute their positions and angles and engage them with a very low failure rate.
Mobile robots must be accurate enough to navigate the forks into the pallet pocket.
Pallet detection is made by three main types of sensors:
- Camera (monocular or stereo)
- 2D or 3D time-of-flight Laser Range Finder (LRF). Lasers that can operate in complete darkness are not affected by lighting conditions.
- Ultrasonic sensors
Automated Forklift - Fork Height Sensors
Automated Forklifts require precise positioning of loads and effective control of rapid movements for transporting materials. It is mandatory to count on resilient sensors to guarantee precise and reliable operations.
There are two types of forks height sensors:
- Optical distance sensors are based on time-of-flight technology and enable continuous determination of the fork height without causing wear. These sensors are used up to a height of 12m.
- Wire draw encoders are a combination of wire draw mechanism and encoder. They record the drum rotation, which is proportional to the length, and provide the corresponding data. Typically, draw encoders have a maximum measuring length of up to 10m, perfect for positioning the lift height of forklift trucks.
Load On Board Sensors
Picking up, securing and safely transporting cargo is one of the most important tasks of any automated guided vehicle (AGV) or autonomous mobile robot (AMR). In order to successfully complete this task, the proper sensors are required.
There are several methods and sensors to ensure the right load positioning. All of them works with the same principle: If the load is not in the desired position, sensors will send a signal to disable the vehicle movement.
For example, I have mentioned pallet pocket detection before. Once the forklift AGV has correctly navigated into the pallet, the box, the bin, etc., it is mandatory ensure the load located in the correct position to avoid problems during its transportation.
AGVs can mount photocells on the forks aiming to the ceiling. If the load is located too close to the edge, the photocell beam would be engaged, stopping the AGV.
- Another solution is to use inductive safety sensors for detecting metal unit loads. Inductive sensors are able to sense ferrous and nonferrous metals like aluminum. The range for an inductive sensor is generally smaller than other sensing methods (under 100mm). The small sensing range and detection of only metal help ensure the mobile robot is in the right position and has the correct cargo.
- Finally, ultrasonic sensors are also a great option for detecting boxes, bins, wire mesh crates and totes. They emit a sound cone that offers a wide angle of detection and is independent of color and material. This means the ultrasonic sensor will be able to accurately sense and detect the cargo regardless of whether the object is black, reflective, solid or even a slotted bin.
Identification Sensors for AGVs
In general, AGVs do not need to identify the material they are transporting. Typically, this task is assigned to the Management Software.
The Management Systems tell the AGV “A” to go and pick material “W” in position “345A.” So when the AGV “A” arrives to position “345A,” it picks material and assumes that it is material “W”.
In certain applications, this info is not complete and the AGV need to identify what is in position 345A or simply must confirm that the material in 345A is really W (as it was supposed to be).
Mobile Robots use different types of sensors to identify material. RFID, laser-based bar code scanners and image-based code readers are common and efficient solutions for identification tasks used by AGVs.
- RFIDs offer a reliable identification of concealed or contaminated objects, as no visual contact with the RFID tag is necessary. RFIDs identify large objects with undefined tag positions due to large reading distances (even 1m). This means pallets or trolleys can be identified during the AGV’s approach. Once read, the tag data is verified via the warehouse management system, supporting consistent traceability of goods flows.
Moreover, they offer a high level of counterfeit protection and data protection due to encrypted data transmission.
- Laser-based bar code scanners can read the widely used code bars or 2D codes. The most efficient sensors are able to identify materials at various distances with different object sizes due to a large depth of field and large aperture angle.
- Image-based code readers are the evolution of typical laser-based scanners. Thanks to sophisticated decoding algorithms Image-based can handle degradations in code quality caused by damage, reflections, quiet zone violations and differences in material types and surfaces. Image-based ID readers can also read 2D codes such as data matrix that hold a much larger volume of data, providing redundancy that can enable the code to be read even when it is damaged. An image-based barcode reader is able to interpret multiple barcodes in any orientation within a single view. With no moving parts, their normal lifespan is considerably longer than laser scanners.
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