WP3-15 1

UltraWideBand (UWB) based indoor positioning

ID	WP3-15_1
Contributor	ACORDE
Levels	Platform, Function
Require	Energy, Raw sensed data from UWB transceiver and IMU
Provide	Navigation Sensor
Input	Raw sensed data from UWB transceiver and low-cost INS
Output	Position
C4D building block	UWB-based indoor positioning system
TRL	4
Contact	fernando.herrera at acorde.com

Building block diagram for WP3-15_1

Description

In COMP4DRONES, ACORDE has tackled the design and developmentof of an Indoor Positioning System (IPS), as a solution for the challenges posed by the construction use case, demo tunnel (UC2-demo 2). ACORDE IPS can be considered a specific instantiation of the building block of C4D architecture shown above. ACORDE IPS is specifically oriented to serve reliable and precise geo-referenced position to a drone flying on a tunnel under construction. Such a drone is in charge of the capture of raw data for tunnel digitization, to be employed for Building Information Modelling or "BIM". The objective of the ACORDE IPS is two-fold, i.e., to ensure a safe navigation along a planned path, and to optimize the accuracy of the navigation data synchronized with the data sensed for digitisalization(e.g., thermography or LIDAR), which can be exploited, for instance, for a faster offline digitalization. The specific environment and the usage scenario posed by the construction stakeholder (ACCIONA) and the drone integrator&operator (FADA-CATEC in C4D), configures a set of requirements in terms of cost of the solution, power consumption, size, weight, precision, and interface capabilities, which, added to the specific geometry of the indoor infrastructure, has lead ACORDE to a novel IPS solution, outlined the following figure.

Indoor Positioning System developed by ACORDE

The solution relies on a “tag node”, to be mounted on the drone, and to provide to it to the geo-reference position, based on a set of measured distances (called ranges) to a set of strategically deployed “anchor nodes”. Range estimation is based on a specific protocol of message exchanges on the Ultrawideband (UWB) spectrum. The tag integrates an INS, so that it can fuse INS information with UWB ranges, for providing a processed tag position. In addition, the tag can also provide at its output interface range data. A standard Mavlink interface supports the provision of these data to the drone navigation system. The posed solution contributes improvements and innovations accounting the scenario and the current state of the art. ACORDE provides a vertical solution, covering the design&implementation of the solution (anchor&tags platforms and their firmware). In addition, a specific design tool, IPS Modellign and Analysis Framework (IPS-MAF), has been developed, which allows to perform a more holistic and efficient analysis and design of this specific type of solutions. All this activity, has been comprised by technical packages WP3-WP6 in C4D, which can be summarized as follows:

• Design and implementation of anchor and tag platforms (including the required board support package (BSP) (WP3-15_1).
• Design and implementation of anchor and tag firmware, including the configuration, positioning algorithms and interfacing (WP4-17).
• Ensuring a robust and enriched communication among anchors, and among the tag (within the drone) and the anchors, for a more robust and improved positioning (WP5-19_ACO).
• Developing an Indoor Positioning System Modelling and Analysis Framework (IPS-MAF) for indoor structures, specifically tunnels (IPS-MAF).
• Providing at the tag a Mavlink [29] interface to support providing both processed positioning information, and “pre-processed” information (ranges) (WP3, WP4, and WP5).

Challenges and Contributions

The stakeholder (ACCIONA) described the digitization scenario at a specific time slot where the construction 7days/24hours activity stopped for 1-2hours. At that time, it is possible to set up some “mobile” anchors, together with the “fixed” anchors, and later let the drone perform a pre-programmed digitization flight, where the tunnel under-construction can present some eventual obstacles (e.g., machines, signals). The posed scenario presents several challenges in relation to other existing real-time location systems:

• The specific long geometry of the tunnel (which challenges UWB range estimation, the cost of the solution and the validity of conventional trilateration algorithms).
• Demand of features for a flexible and agile anchor deployment, which does not oblige surveyors to geo-positioning all anchors, especially mobile ones.
• Need to provide real-time, 3D geo-referenced positioning on the tag (while most solutions focus on 2D relative positioning, commonly computed on a ground platform which centralizes data from anchors).
• Cost and energy/power optimization (the latter important for fixed anchors)
• Need for system-level design tooling. The IPS is a complex system with many parameters and aspects (specific deployment, transmission powers, sensitivities, latencies of the ranging phases, algorithmic alternatives, etc) with potential significant impact on the overall performance. Means for facilitating a holistic design, accounting for those many different aspects at early design stage are required even for experienced designers in the field.

The same scenario also presented the opportunity to exploit the fact that during digitization flight only one drone, i.e., one tag, needs to be serviced. The question is how to exploit this single-tag assumption for a safer flight and better digitalization.

The ACORDE solution tackles these challenges and opportunities. So far, a number of contributions in comparison to other existing solutions are highlighted:

• Customized, cost-effective anchor and tag platforms, specifically designed to cope with the computational needs (specially for tag) and energy/power efficiency needs (specially anchors).
• Novel MAC protocol called Asynchronous Tag trigger, Slotted Anchor response with Deterministic and Random Allocation (ATSA-DRA), specifically adapted to the single-tag assumption, that ensures deterministic latency, while optimizing the number of anchors in view.
• At application level, the solution enables geo-referenced, real-time 3D positioning. Moreover, the application overcomes the challenges of the tunnel geometry. A modified trilateration algorithm has been already developed which enables positioning in regions of limited anchors visibility (coverage) and poor dilution of precision (DOP), where a conventional least-squares based approach is not working. This is specifically required for enabling anchors auto-positioning at the initial phase.
• The development of IPS-MAF, is a qualitative step on ACORDE capabilities for tackling custom design of IPS systems for long indoor infrastructures (tunnels, mines, large pipes, …). It enables a newer system-level design flow (as explained in WP6-21), capable to reveal outcomes and aspects almost imposible to notice or tackle manually.
• The support of a Mavlink interface at the tag side, to provide both completely processed (e.g., position data) and partially processed (e.g., ranges). The latter enable the ACORDE IPS to behave as a complementary positioning sub-system, whose outputs can be fused by integrators with other alternative sensor data. Moreover, ranges transfer is enabled via a “smooth” (in the form of dialect) Mavlink extension contributed by ACORDE in COMP4DRONES.

Anchor at Tag platforms

In C4D ACORDE has performed a customized development for the anchor and tag platforms.

On top of a base mother board design, the tag and the anchor platforms have been implemented. Thanks to its I/O interfaces, the anchor platform supports the different types of anchor nodes defined for the solution, i.e.:

• an external ranging node
• an internal ranging node
• the Configuration, Control and Monitoring (CCM) node (this category can be applied to any of the previous

Indoor Positioning System developed by ACORDE

For the same form factor, the base platform design brings several advantages in relation to devices found in the market:

• more estable clock
• LiPO connector
• better connectivity for configuration purposes

Moreover, the tag platform adds a more powerful microcontroller, the possibility to add an INS for a powerful tag (capable to provide in real-time fused geo-location solution), and an SD card slot (e.g. to enableo local logging).

Moreover, this development has provided ACORDE a strong technological base to further develop with greater flexibility the firmare of the positioning solution (WP4-17), where much of the added value of the solution is concentrated.