Analysis of Goal Line Technology from the perspective of an electromagnetic field based approach Solutions11 months ago - IT, IoT - United States (US) - 234 views
Analysis of Goal Line Technology from the perspective of an electromagnetic field based approach
1. IntroductionThe ‘Wembley Goal’ in the World Cup final 1966 between England and Germany is probably the most famousgoal in the history of football. The question whether it was a goal or not has been an open problem for decades. In2012, after a long period of evaluation of different technologies, the International Football Association Board(IFAB) approved the use of Goal Line Technology (GLT) in official football matches. The objective of a GLTsystem is to provide a clear indication to the referee –typically on a special watch- whether the ball has fullycrossed the line. Fraunhofer IIS conducted research work on the electromagnetic field based solution GoalRef,which is one of the currently licensed GLT systems
2. RequirementsThere are several requirements a GLT system has to fulfill before it is allowed to be used in official matches.These are stated in the FIFA testing manual (FIFA, 2012) and they range from material tests in the lab to systemtests under real game conditions and aim to guarantee a fully functional GLT system. There are three stages on theway to the pitch that have to be passed. First of all, in order to get licensed by FIFA, a system check guarantees thesystem to work even under adverse conditions. This has to be checked in a slightly modified manner for everystadium during the final installation test. This is usually done by an independent test institute and guarantees theinstalled system to work like the system tested during the system check. Finally, before kick-off the refereeperforms some short tests to ensure that the system is working properly.
The present paper focusses on the accuracy of a GLT system. These are based on dynamic tests with a targetwall that has to be passed with an accuracy of ±3cm and a slider test that allows to determine the offset where agoal is triggered (±2cm). Fig. 1 shows the areas relevant to the slider test. Balls in the no goal/goal areas alwayshave be detected as no goal/goal. The area in between (±2cm from the back side of the goal line) is a tolerancearea, in which a goal and no goal decision will be accepted.
3. Solution Approaches & Influencing Factor
3.1. Solution approaches
Today there are two approaches for realising a GLT system. One is a video-based approach and the other is anelectromagnetic field based approach.The video-based approach generally consists of several (e.g. 2×7) high-speed cameras with fast lenses mountednearby each of the goals (Hawkeye, 2013, Goalcontrol, 2013) and an infrastructure to connect all cameras to a highperformance computer cluster. To guarantee best view the cameras are mounted on different positions above thefield at the catwalk or other construction elements of the stadium roof. The data of all 14 cameras is streamed to Rafael Psiuk et al. / Procedia Engineering 72 ( 2014 ) 279 – 284 281the computer cluster, where sophisticated image processing algorithms detect the relevant ball (as well as todistinguish it from similar objects) and decide whether the ball has fully crossed the line.In an electromagnetic field based system the ball may be equipped with electronic circuitry for transmittingand/or receiving and/or reflecting electromagnetic signals and therefore require electronic components inside theball,
3.2. Influencing factors
3.2.1. Ball speed
The most influencing factor is the movement of the ball itself. Typical ball speeds during a game are 50-80km/h, but can sometimes reach speeds up to 140 km/h. Fig. 2 shows the relation between speed and displacementof the ball for different frame rates. The figures can also be used for the measuring rate of electromagnetic fieldbased approaches. At high speeds of 140 km/h the ball travels app. 4 cm within a period of1 ms, which is the time between two frames at a frame rate of 1000 Hz. A resolution of 1 cm requires a frame rateof app. 4000 Hz. In practice additional positions between two frames can be calculated using algorithms todetermine the trajectory of the ball, so that lower frame rates are possible.
3.2.2. Ball Compression
Depending on its speed, the ball will be compressed when hitting against an obstacle like a player’s leg or thegoalpost. The rule of the game define that a goal is given, when the ball has completely crossed the goal line.Hence when a ball is compressed, its diameter decreases, which has to be taken into account. This is a challengefor both approaches.282 Rafael Psiuk et al. / Procedia Engineering 72 ( 2014 ) 279 – 284
3.2.3. Light conditions
Obviously light conditions as sun, shadow, rain or snow do not have any influence for any kind ofelectromagnetic field based system but have to been taken carefully into account for all kind of video basedsystems. The illumination of the scene strongly influences the choice of cameras and lenses.Relatively slow changes of light conditions (e.g., cloudy sky) will easily be balanced by the signal processing,whereas rapid changes as snow fall or heavy rain will be a real challenge for the processing unit – as well as sharpedges of bright light and shadow nearby the goal or even between both goalposts
3.2.4. Occlusion of the ball
In football games the ball is often hidden by players. It can seem that the ball is completely occluded, but inmost cases, the ball can partly be seen from other perspectives. Hence video based systems have to use severalcameras to generate different perspectives of the goal to ensure to detect parts of the ball even under heavymasking effects.Electromagnetic field approaches do not have this problem since the low frequencies used are immune againsthuman bodies and environmental conditions like rain or snow. As a result, the detection accuracy is not affected byobstacles and the system works even under full occlusion
.4. GoalRef System
The electromagnetic field based solution GoalRef is based on a RFID approach. It consists of a readergenerating an exciter signal at a low frequency of 119 kHz, the exciter loop, which is a current-carrying conductorembracing the goal frame, loop antennas in the ball, one or more receiving antennas around the goal frame and areceiving unit inside the reader.
Fig. 3 (a) shows the working principle of the GoalRef system. It shows a top view on a ball within the systemsrange. The reader with the exciter loop antenna creates an electromagnetic field (HExciter) perpendicular to the goalframe. This field vector stimulates the purely passive loop antenna in the ball, which reflects back a part of theexciter field (HBall) which is received by antennas attached to the goal frame. One receiving antenna consists oftwo orthogonally arranged loop antennas, the so called main- and the frame-antenna (HMain, HFrame). Thesereceiving antennas are symmetrically arranged around the exciter loop wire, with the loop conductor placed exactlyin the middle of the receiving antennas. With this antenna construction it is possible to determine the orientation ofthe incident magnetic field vector (HBall) of the backscatter signal of the ball.
.5. Test setup
In order to determine the accuracy of the electromagnetic field based approach a static test at one goal in the labhas been performed which is based on the slider test as stated in the FIFA testing manual. The exact point at whichthe GLT system triggers and indicates a goal is measured. The goal frame is equipped with an exciter wire andseveral receiving antennas mounted around the goal frame. The signals of the antennas are recorded by the readerand analysed to derive the goal decision. The goal decision is generated in the PC and the information is sent to thereferee watch. During the test, the referee watch has been observed.Fig.
The test has been performed with a test ball with a single effective coil perpendicular to the exciter field. Theintention of the test is to determine the system performance limit without any influencing factors
.Therefore a slider is placed on the goal line to slowly move the ball (approx. 1cm/s) across the line. A precisemeasurement of the location of the coil when the system is triggered is possible in this test.
Negative values indicate that the system detects a goal before the goal line whereas positive values indicate thatthe detection was behind the goal line. The mean values are +3.0 mm at 0.8 m distance and -0.6 mm at 3.6 mdistance from the goal post. Since the measurement has been done manually, there could be some reaction timebetween observing the goal decision on the watch and the stop of the movement of the ball. Taking this intoaccount the system detection threshold is pretty close to the real zero point. The standard deviation is below 2 mmfor both positions.
This paper has tried to give an overview to the actual status of GLT for football, to the different solutionapproaches and the influencing factors in real operation. The presented electromagnetic field based solutionGoalRef offers some advantages over video based systems. It can detect the ball even under full occlusion, sinceno line of sight is necessary and it offers simple detection of the ball with an update rate of 2500 Hz. Finally the labmeasurement results show the potential precision of this system approach, which could be down to a fewmillimeters.