Ultra Wide Band System - Trilateration

One of the most important building blocks for the drone referee is a method for positioning. At all times, the drone state, namely the set {X,Y,Z,Yaw}, should be known in order to perform the refereeing duties. Of the drone state, Z and the Yaw are measured by either the drone sensor suite or other programs as they are required for the low-level control of the drone. However, in order to localize w.r.t. the field and to find X and Y, a solution has to be found. To this end, several concepts were composed. Of those concepts, trilateration using Ultra Wide Band Anchors (UWB) was realized. First, the rejected concepts are shortly listed, followed by a detailed explanation of the UWB system.

Concepts

The positioning system must provide an accurate enough signal at a high enough sample rate for position control. As ball out of pitch detection is to be detected using image processing, an accuracy of 0.5 meters was chosen as minimum accuracy for the positioning system. This means that a circle with a radius of half a meter designate the position of the drone. For the sample rate a minimum of 10 measurements per second was chosen. However, as most of the technologies listed here are very new or still in development, this information could not be obtained for all of them. The following concepts will be discussed:

• Trilateration using a Laser Range Extender
• Trilateration using ultra-wideband beacons
• Positioning using Bluetooth beacons
• Positioning using image processing

Trilateration using a Laser Range Extender

A laser range extender can be used to determine the distance between the laser itself and an object reflecting the beam. A company was found that produces, small lightweight laser range extenders that can rotate. By placing three poles at known positions around the field, and measuring the distance to those poles, the drone can be positioned using trilateration. Trilateration is a type of positioning algorithm, an easy way of understanding is that if a circle with the distance between each pole and the drone were to be drawn around every pole, only one intersection point would remain. If the height is also known, the drone state is thus known. Unfortunately, the sensor was out of stock for the duration of the project, and no alternative was found at it is an emerging technology. As such this concept was abandoned.

Figure 1: Overview Laser Range Extender Concept

Trilateration using Ultra-Wideband Beacons

Similar to the previous concept are Ultra-Wideband Beacons. These are small sensors communicating with one another over the ultra-wide band radio spectrum. Given the time between sending and receiving, the distance can be estimated with a promised accuracy of around 10 centimeters. By placing at least three beacons at known positions and placing one beacon on the drone, trilateration can again be applied to find the drone position w.r.t. to the stationary beacons. The beacons around the field are referred to as anchors and the beacon on the drone is referred to as the tag. Multiple tags can be in the system as well as more than three anchors providing more accuracy through redundancy, making it a very extendable concept. This method is very new, and only a few manufacturers are available. Ready-to-use systems are still rare. There is a company, DecaWave, offering evaluation kits that are instantly usable without additional development, but these are expensive and maybe too fragile for use on a drone.

Positioning using Bluetooth Beacons

This concept is based on creating a grid of uniquely identifiable Bluetooth beacons around the edges of the field. By evaluating the signal strength of the different Bluetooth beacons and their ID numbers, a normally distributed position estimate can be obtained. There are some start-ups that are developing this approach, but not much is known about range and accuracy. However, individual beacons are quite cheap at around 10 euros. Worst case scenario it would be a good method to support or supplement another concept using sensor fusion.

Positioning using Image Processing

A more traditional method also applied by the soccer robots of TechUnited is to use visual methods and position the drone based on recognized field points and line segments. The algorithm used by TechUnited draws a number of line from the middle of the frame to the edges of the field of vision. Along these lines, field line points are detected based on the transition from dark to light to dark. This generates a point cloud that can be matched on the known layout of the field. If this algorithm can be used, it will have no additional material costs and it can be based on the existing code from TechUnited. In order to apply such a method on the drone, it should be adjusted to have a variable height. However, perturbations in the roll and pitch of the drone as well as the narrower field of vision might make this algorithm difficult to apply. Finally, it is questionable how scalable this solution is with growing field sizes, as lines become sparser.

Concept Choice

The different concepts were compared based on a number of categories as shown below. Each specification was given a certain weight and concepts received points based on their score. By summing up all category scores, a final score is obtained. Based on this score, as well as a prediction of the usability and ease of implementation and development for the different solutions, the Ultra-Wideband System was chosen. The table with categories is shown below.

Table 1: Concept Comparison

Implementation

Introduction

From DecaWave, the TREK1000 evaluation kit was acquired. This kit and its content are described in detail on the following page. http://www.decawave.com/products/trek1000 This kit contains four beacons, of which three are intended to be used as anchors and the fourth as a tag. If required, additional beacons are available to extend the system to four anchors and up to eight tags. Most importantly, the beacons have already been programmed. The most essential part is the DWM1000 chip, which facilitates the UWB communication. In this evaluation kit, this chip has been incorporated into a PCB, which means that the time intensive process of creating our own beacons and programming them to communicate and give the distances between anchors and between the anchors and the tag can be skipped. This also means that the system is bound to limitations of the programming, which means there is a maximum of four anchors. However, the ARM processors on the PCBs are reprogrammable with the right tool and the source code is available from DecaWave on request. In the future, a system can be made especially for the drone. In this project, the existing programming was used. The implemented system has three anchors, A0, A1 and A2 and one tag, T1. If all are powered, the distances between the anchors of the tag are measured and known to each anchor. By connecting an anchor to a computer and using either the DecaWave application or reading out the serial connection, the measurement can be obtained. Furthermore, A0 also gives a measurement report containing the distance between the anchors. Using this measurement report, a coordinate system for the anchors can be set up. If the anchors are placed in a predetermined coordinate system, the positions can be confirmed. Figure 2 shows the different names and definitions as well as the working principle of the system. All distances are measured and compiled in two messages, one containing distances to the tag and the other containing distances between anchors. By connecting anchor A0 to a computer and establishing a serial connection in Matlab, the messages can be received.

Table 1: Concept Comparison