GoalRef Technology Basics: Low Frequency Magnetic Fields & Coils

[Pinkyishy]

Hi,

I am interested in the basics of goalref technology however there is not many material out there.

What i want to know is why does the magnetic fields have to be low frequency? and how does it detect if the ball has fully crossed the line? And lastly what is the purpose of the coils within the ball?

Any help to clarify these is appreciated.

Thanks

 

[anorlunda]

What i want to know is why does the magnetic fields have to be low frequency? and how does it detect if the ball has fully crossed the line? And lastly what is the purpose of the coils within the ball?

It is against our rules to post something like that without giving a link so that we can read it too. But in this case I did a search on "goalref technology" and found this

GoalRef is an association football goal-line technology system developed by German research institute Fraunhofer in association with Select Sport. It works by detecting the passage of the ball using magnetic induction.

Was that your source?

 

[anorlunda]

This video shows testing of the goalref versus the hawk eye video camera system. It can be clearly seen that the tests included cases where they had a backboard that bounced the ball back before it completely crossed the line. That doesn't tell you how it works, but only that it was tested.

 

[Pinkyishy]

It is against our rules to post something like that without giving a link so that we can read it too. But in this case I did a search on "goalref technology" and found this

The GoalRef system relies on the principle of electromagnetic induction.[3] A low frequency magnetic field is generated around the goal, which is monitored by coils installed in the goal posts and crossbar.

I am wondering why they need a low frequency magnetic field and how electromagnetic induction plays a part in detecting if the ball has fully crossed the line.

 

[anorlunda]

When a conductor passes through a magnetic field, a current is induced in the conductor. That current can be detected by the device making the magnetic field. I is the same principle as a transformer with one winding of the transformer energized by a voltage and not moving. The secondary side of the transformer is short circuited and it moves, that causes a change in currents in both the primary and secondary sides.

Just because they use a low frequency, does not mean they must. I presume they tried it and selected whatever frequency makes it work best.

If you are interested in the physics, this video may help.


It sounds like in the goalref, the goal cage makes the primary, and the secondary is inside the ball. Detecting a change in the primary current means that the ball passed through. I see from the press reports, that 4 different systems for goal detection have been approved. I'm sure that each has strong and weak points, and vulnerabilities.

 

[Pinkyishy]

Another question:

Companies like GoalRef place wires in both the ball and the goal. The wires in the goal create a magnetic field that extends across the goal line. The wires in the ball disturb that magnetic field when they pass through it

How does the wires placed around the goal produces magnetic field and if it did, how would the field look like?

 

[Nugatory]

How does the wires placed around the goal produces magnetic field and if it did, how would the field look like?

Pass a current through a wire, you get a magnetic field. What it looks like depends on the details of how the wire is positioned and whether and how the current varies.

 

[Pinkyishy]

It sounds like in the goalref, the goal cage makes the primary, and the secondary is inside the ball. Detecting a change in the primary current means that the ball passed through

But how do we know if the ball has fully passed through and not just half of the ball?
Does the disruption through the goal cage's magnetic field vary depending on how much of the ball goes through the magnetic field?

 

[anorlunda]

But how do we know if the ball has fully passed through and not just half of the ball?
Does the disruption through the goal cage's magnetic field vary depending on how much of the ball goes through the magnetic field?

That I really can't answer. The signal would depend on the speed of the ball and how far it travels. I would guess that they tested it with varying percent of the ball over the line, from 0% to 100%. Then they look at the results.

If the ball stopped halfway over the line, then bounced back, the current change versus time might look different than if it passed by smoothly.

These are all guesses. Perhaps goalref has published some papers or reports that give more detail. I did not find any when I searched goalref.

Goalref and FIFA must have tested all those questions. But do you trust either organization?

 

[Pinkyishy]

Thanks for all your help.

The reason I started this thread is because we have to make presentations about physics in the industry and I chose my topic to be goal-line technology however seems like understanding and explaining technology isn't one of goalref and FIFA's top priorities so now I am struggling on what to add to my presentation. We need to produce a word document with 2500 words and I am currently at 1700/2500 words.

I uploaded word document if you want to have a look.Help will be much appreciated.

Thanks

EDIT: This work is due in 17th of July so I am worried if I will have time to finish it.

 

[anorlunda]

I would love to read it, but because of security reasons I will never open a DOCX file that comes from a stranger. Neither should you. If you post a PDF version, I'll look at it.

 

[Pinkyishy]

Here you go

 

[anorlunda]

I read your paper. It has only words, not numbers or plots. So to help you out, here is how I would design it. I have no idea if goalref is really designed like this.

I would put 2 sets of coils in the goal posts, separated in distance by the width of the goal line. Then I get 2 signals as the ball passes through.

Here is how the signals versus time might look as the ball passes the goal line and stays in.

Here is the case where the ball goes all the way over, then bounces out some time later.

Here is how they might look if the ball went halfway over the line, came to a full stop, then bounced back out.

You could generate a whole family of such plots, one for each percentage of the ball over the line.

I think you can see how a computer looking at those curves can come to conclusions about what happened including what percent of the ball crossed the line.

I did not calculate anything for those curves. They are just eyeball estimates. But they illustrate the principles much better than words.

 

[sophiecentaur]

You would need several (3?) coils in the ball for different orientations and it could be spinning fast.
It’s a much harder than Hawkeye in tennis, I think.
It always amazes me that people believe that simulated movie is ‘real’ with an almost religious gullibility. At least it kills pointless arguments with the umpire.