The data handling of many sensors that scan a single target, called multi-sensor networking, is an important capability to support large object applications. Many sensors are required when the target object is larger than what a single sensor is able to scan, or when multiple views are required to capture key features of an object.
There are four main challenges that arise in multi-sensor networks: (1) sensor wiring, (2) sensor discovery, assignment, and mapping, (3) sensor alignment to a common coordinate system, and (4) sparse or dense network processing.
In Part 1 we will discuss the first two of these challenges.
Creating a successful, robust network of sensors requires careful design around a star topology. Unlike bus topologies such as USB or Firewire, a star-type network such as Ethernet continues to function even if a node stops functioning. In a bus topology, a malfunction anywhere along the main wire connecting nodes will cause the entire network to fail.
In a star topology, the wires connecting the sensor to the central computer must support power, data, synchronization, and laser safety. This can be achieved using a LMI Master hub and simple CAT5e cabling.
A Master hub distributes power, microsecond synchronization and laser safety in a multi-sensor network. Synchronization data is broadcast to all sensors and includes a timestamp, encoder stamp, and the status of direct factory inputs (like photocells) wired to the hub. Sensor data is streamed through a Gigabit Ethernet switch to a central PC for processing.
Sensor Discovery, Assignment, and Mapping
Once a network of sensors is wired and powered up, software is needed to enumerate each sensor on the network, physically identify it (usually by serial number), and enable it for use in a layout (Fig. 1).
Common layouts are “wide”, “ring”, or “opposite”. Each sensor is assigned a location in a layout and its physical to logical mapping is recorded. This mapping is essential in order to later align and stitch data or to simply replace a sensor that is no longer functioning.
The following are examples of the most common sensor layouts:
Wide Layout for Scanning Large Objects (Fig. 2) - Multiple sensors are used to measure large objects that are wider than a single sensor’s field of view, such as automotive parts and assemblies.
Opposite Layout for Determining Object Thickness (Fig. 3) - Two sensors perform top and bottom differential measurements to calculate true thickness when the object cannot be referenced to a known surface like a conveyor.
Angled or Ring Layout for Measuring Entire Object Circumference (Fig. 4) - Multiple sensors are set up at an angle or in a ring to eliminate occlusions and scan the entire circumference of an object, as in log scanning applications.
Stay tuned for Solving the Challenges of Multi-Sensor Networking Part 2. coming end of June (includes sensor alignment to a common coordinate system, and sparse vs. dense network processing) ...