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How to test a speaker voice coil(raw part, not assembled to anything)
3

How to test a speaker voice coil(raw part, not assembled to anything)

How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
I am dealing with voice coil.
How can I detect a voice coil whether it has any short-circuit or open circuit defect ? My main concern here is intermittent cases, eg. intermittent short/open (sometimes short, sometimes open, some only can be reveal after it is assembled to be a speaker and full load is applied BUT i need to detect it before it is being assembled.).
The copper wire for the coil is very thin , so the tester shouldn't predamage the coil during testing period.
I need a fast test method and can indicate "GO/NO-GO". The price if possible lower then better.
Thank you.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

Open-circuit shoudl be easy enough to test.  Put it in a paintcan shaker kind of deal to test for opens during vibration.  Not sure of the best method to test for shorts...with an accurate ohmmeter and the known length/guage of wire, you could determine what the resistance should be compared to what it actually is.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

Measuring the resistance will certainly find open circuits, but for short circuits it is unlikely to show much of a change compared to the overall coil manufacturing tolerance.

The best way to test for short circuits in the winding is to measure its inductance. A shorted turn will seriously reduce this. You will need to use a reliable L-C-R bridge; measuring L, the inductance, will tell you if it is open or shorted in one test. Modern bridges, with digital readout, can usually be programmed for "go" "no-go" range limits, so should be quite speedy to use.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
Hello BrianG:
Can you give some example of these bridges that you mentioned?
I agree that a short-turn will decrease the inductance and resistance. BUT an intermittent short-circuit is the main problem. It sometimes show , sometimes doesn't show and, sometimes only show when it is assembled to be a speaker and the speaker is being played for long time under full load.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

There are literally hundreds of LCR bridges to choose from - just do an internet search on "LCR bridge".

Your intermittent problem seems to be one of material specifications. If the enamel insulation on the wire is failing in manufacture or on load, then you need to use a better specification wire. You could try to improve the insulation by coating the wire after coil winding, using either a high-temperature varnish or an epoxy resin.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

2
Dumb and cheap bridge.  Compare it against a known good speaker.  Use an audio oscilator to supply the top and bottom of a bridge.  Top two resistors of the bridge should be 330 ohm resistors.  Use the good speaker on the right bottom side of the bridge and the left "test speaker" on the left.  Measure the AC millivolts between the two speakers.  Use a frequency appropriate to natural resonance of the speaker.  This will show a variety of mechanical and electrical problems.  Have used this method in testing RC snubber networks and it is quick and easy.  Use test pins in a fixture and you should be able to test as fast as the meter will settle.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

We build voice coils for fast tool servo aplications.  To test for winding shorts, we use a BK Precision 875B LCR Bridge (inexpensive) to test the coil inductance.

Dynamic shorts are difficult to find.  Is there any possibility you could use double insulated magnet wire?  This greatly improves the possibility of coil shorts during winding.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

The only way to test for a dynamic open or short is, as mentioned above, to shake the coil and hope that aggravates the condition.  You might be able to simply place the coil under test on a completed speaker and blast away, although it'll probably be a bit noisy.

A decent 4-terminal Kelvin-connected milliohmmeter can easily measure resistances down into the milliohms.

TTFN

RE: How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
A big THANK YOU to all you guys for the fast and nice replies. :)

Hi OperaHouse:
I have to test the coil before it is assembled to be a speaker,that's why i mentioned "raw part".
If I use the method you said,
How should I decide the good and defect differences using the AC milivolt meter ?
The audio oscillator how should I adjust it? Sinewave ? and the frequency ?

Hi sreid:
The BK Precision 875B good ? How accurate is it ? The voice coil I have to test do not have core(or air-cored).
Can 875B shows even if only one turns is short-circuited ?
Need to do any calibration ? Can be done in-house by ourselves ?
I think the wire cannot be double-coated bcos it will run from the drawing.

Hi BrianG:
The wire is very thin. Any coating too thick then it will run from the drawing and the spec.

Thank you again for all the informations. I will try the methods you all mentioned.Hopefully can settle my problem. Please continue to reply me if got any new ideas. Thank you.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

The BK875B is $189 at Digikey.  Hrere's the link to the spec

http://www.bkprecision.com/download/documentation/100-66.pdf

A shorted turn makes a dramatic difference in the coil inductance so a coil with a shorted turn is easy to find.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

Guess we all assumed you were testing assembled speakers prior to installation in a cabinet.  Just testing the coil would be like testing an inductor.  The same bridge method would apply.  Use a sine wave and test a number of coils to see where the numbers generally fall.  Like I said, this is a very simple test.  A better test would be to have a pulse generator put a very large pulse into the coil through a resistor and monitor the decay on a scope.  The ringing should be very consistant.  The pulse could even be large enough to induce some stress without doing damage for a better test.  We did this on electrmagnetic clutch coils where the internal diode connections needed to be tested.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
Hi OperaHouse:
The better test that u said is a "surge/impulse test" , is it ?
I found a few tester company like Baker and Wagne Kerr selling this kind of tester. But the thing is these testers are expensive and I don't know how effient they are. The test is it fool proof ?
I heard that the impulse or surge test is only good for the first few turns, is it correct ?
Because it have to be implement into the production line, so the tester is for operator to use. It must be easy and tell the operator whether is GOOD/BAD. Showing number is not suitable for the operator. Human judgement tends to make mistake and influence by feeling.

Thank you.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
Hi OperaHouse:
Forgot to mention, these Surge tester company claimed that their testers can test for intermittent cases, is it true ??
And they claim that the high voltage surge won't damage/predamage the coil, is it true ?
What is the life-span of these tester ?
Calibration, service or repair is frequently needed ??

Thank you.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

Would two shorted coils next to each other make that large of a difference in inductance?  I can certainly imagine two shorted coils above/below each other making a huge difference due to their separation distance along the wire length, but two next to each other?

RE: How to test a speaker voice coil(raw part, not assembled to anything)

McGyver,

Yes. It makes quite a difference. The Q will go down the sink if you get a single shorted turn, regardless of where in the coil. The Operahouse method could be applied easily, especially if an elevated frequency is being used. Something like 10 kHz seems to be right.

Use as much power as the coil can take for the few seconds that will be needed for the test. Instead of resistors and a reference coil, I would use three coils and the CUT to form a bridge.

A mV meter sometimes takes too long to stabilise for an automated test like this. A scope with settable limits and GO/NOGO output can do the checking and will help a great deal if you are going to automate the test.

The Bruker equipment mentioned is not for tiny coils, but more for motor windings and tens of kilovolts of pulse amplitudes. Testing with such a device will probably produce 100 percent NOGO (even if you had 100 percent GO to start with). Wayne-Kerr? Yes, can do.

 

RE: How to test a speaker voice coil(raw part, not assembled to anything)

A lot of testing is simply to fulfill ISO testing requirements.  Rejecting products off the line is not the way to control quality, it is improving the process so defects aren't made.  I think a quick test is unlikey ti find anything but major failures.

With simply an air core, I would make the coil part of the oscilator circuit.  Probably lowering the gain of the circuit so that a shorted turn might keep it from oscillating.  An air core should be very consistant in frquency.  This would be more sensitive than a bridge circuit.

A site you might find interesting is hhtp://sound.westhost.com/ A lot of fun projects in audio and general theory. Look up project 59 Self Oscillaing Amplifier.  It would be nice to modify this so that more power is dumped into the coil so it is electrically and mechanicaly stressed.  You should at least put some voltage on the coil over what it would see in normal life.  This could mean switching in a power circuit prior to an oscillator or inductance test in an automated tester.  

RE: How to test a speaker voice coil(raw part, not assembled to anything)

How about submerging your coil in water when doing your tests. Should indicate any 'latent' shorting conditions. Adds the expense of a dry cylce.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

1) Do you know what percentage of the coils are failing open or short?  I ask because if the percentage is high it is usually better to fix the process that is creating the problem than is is to test the problem out.

2) Is it possible that the coils are not shorting but that the coil has not been completely bonded?  What is your bonding method?

3) Could you change to a tougher magnet wire varnish?

4)Another way to find shorts is to put the coil in an AC magnetic field from another coil.  A coil with a shorted turn will vibrate due to the induced current in the shorted turn (this is refered to as "Growling).

RE: How to test a speaker voice coil(raw part, not assembled to anything)

I like to blue sky ideas and a couple have mentioned putting the coil in a magnetic field.  Just think if a small cpacitor was actually added in the manufacturing process across the coil. This would have no effect when the speaker was assenbled with the higher inductance of the core. But, it would allow testing without any electrical connections by just placing it in a magnetic field.  Process would be similar to the old grid dip meter measuring absorbsion.  Tuned slightly off frequency there would be a normal current.  Open would be lower current and shorted turns would be higher.  This is just a reminder that it sometimes pays to add things if you can make it easier to test. Got our company to add circuit board edge connector pattern so we could do plug in testing.  Only had to cut one slot in the board and edge connector used up real estate that couldn't be used because of board cutter.

Is this coil an unusual shape or not have a center core.  Coil winding seems like a standard process which is very reliable.   

RE: How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
I think submerging the coil into water won't detect inner layer's short.

The coil I need to test actually come from supplier. They already did DCR test and open-circuit test. But they couldn't detect intermittent ones.

The failing rate is low, BUT one out of ten thousand is still a problem bcos if one speaker with intermittent short goto the customer then I will be killed.

Now the market is very competetive. All is about cost. *sigh*

The vibrating idea is nice,BUT i don't want the operator to judge by his/her eyes whether the coil is vibrating or not. This will go bac to the issue of human judgement again.

The coil is pure round shape. No core. The core will only be insert during the process of assembling into speaker. (the yoke).

We buy the coil from our current supplier bcos they are cheap. But still sometimes one or two out of thousand or ten thousand will have problem bcos the supplier production line is using man-power, not automated. They are like traditional coil company. No robotic arm or anything. Just a very simple winding machine fully control by human, just the turns of the coil is control by machine.

I hope you understand my situation. *sigh*...

RE: How to test a speaker voice coil(raw part, not assembled to anything)

Looks like you found your problem wihtout us...

"We buy the coil from our current supplier bcos they are cheap."

If you cannot afford even 1 in 10,000 coils to fail, maybe going the cheapest route isn't the best path to take.  I'm being serious.  You'll spend quite a number of pennies setting up a test jig, paying somone to stand by it and use it, etc.  Seems the more logical solution would be pay the extra pennies to have a quality coil sent to you in the first place and be done with it.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

You have a tough but not intractable problem.  First you need a PLAN to discover where the problem is.  Recording coil resistance and inductance at incomming inspection would be the first place to start.  Log the data so you'll have a record.  Retain coils that don't pass so they can subsequently be analysed.  The point of this is that a problem can't be solved unless it is first understood.

Since what you really want to do is to improve the quality of speakers shipped to the customer, perhaps a better final test of the speakers could be implemented.  What comes to mind is an automated high power frequency sweep of the speakers with an automatic FFT analysis of the sound comming out of the speaker.  This might identify any number of pathologies that might occur in the speaker assembly.

You can never attain 100% perfection but you can improve it.  And there is always some point where the cost does not make economic sense.

Pragmatically, here's your position;

1) Management want's the failure rate improved.
2) You can't fix a problem until you know what it is.
3) Here's your plan including time and cost.
4) Management can either accept the plan or reject it.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
Hi MacGyverS2000
I totally agree with you.
But I am just a tiny person in the company. They gave me this problem to settle, I cannot just tell them to find a better supplier and make sure the supplier supply 100% good products.
My job is to design or find something to can detect the intermittent short/open circuit of voice coil. That's my task, and I am facing a big headache.*sigh*
The tester will not only be used in production line, but also for customer rejected items. We can use the tester to reveal what is the problem that occur,eg. why speaker no sound come out after a few times of playing by customer.
Thank you.

Hi sreid
Sorry, but can you explain to me about the high power frequency sweep ? And what is FFT analysis ? Maybe I can recommened to my supervisor for the final testing of speakers.

We do have test using Polarity Checker for the speakers, the speaker is connected to the probe then a sound from speaker go into the mic of the tester for analysis. It is for polarity only.

We have random check using nominal load check for the speaker, eg. 1000-hours of nominal load test.

I think we have DCR checking for the incoming coil, but no inductance check.
I went to the supplier side to see they process. They got DRC test for each of the coil, megaohm test for dual-winding coils(test for short between the two winding), polarity checking(using a not fool-proof method by human judgement).

Still, my main task is on the coil itself.*sigh*...

RE: How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
The main problem now is how to check the voice coil for [/b]intermittent short-circuit[/b] and intermittent open-circuit.
I think both is related.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

Perhaps you can also quantify how much such a procedure is going to cost the company.  If you had a full time tech running this exercise, you'd be talking in the neighborhood of $60,000/yr wages/bennies/G&A.  That would seem to be a lot of speaker coils.

You didn't seem to have mentioned how many you need to test in what amount of time.  That would be a major input into the design equation.

A small shaker, ala:
http://www.vibtest.com/vr520.htm
might be adapatable for single or multiple part testing.

TTFN

RE: How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
Hi IRstuff:
U mean u suggest I use a shaker to shake the coil during test ?
I don't quite understand the "$60,000/yr wages/bennies/G&A". Sorry, can you pls explain ?
I don't have the exact number of voice coils to be test, but I think it is around 500k per month.

Thank you.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

Put enough current through it to raise the temperature above ambient while watching its infrared image. Poor connections and shorts should show up as hot spots. Windings bypassed by a short will be cold. Since they are cheap, you can burn a few to establish some testing limits.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

So, in a single shift operation, that's roughly 3200/hr, or about 54/min, assuming 155 hrs/month.

You're going to need something fully automatic and darn fast to keep up with the product flow.  That means a full-time operator.  Assuming wages around $15/hr, that's $30,000/yr.  Assuming some benefits and overhead of 100% of wage, That's $60K/yr for labor and a rather huge investment in machinery plus maintenance.  

Assuming your 1/10,000 bad, that's 50 per month.  So unless they're worth more than $100 each at the stage where they're otherwise caught, you won't break even financially.

TTFN

RE: How to test a speaker voice coil(raw part, not assembled to anything)

The basic idea would be to put enough power into the speaker (if only briefly) to stress the speaker in an effort to find bad speakers (no matter what the problem might be).

An FFT (Fast Fourier Transform) is a mathematical technique to convert a signal in time into a frquency spectrum.  Bad speakers will probably have a spectrum different from a good one.  Stepping the frequency would show up problems that are frequency sensitive.

We had a problem with voice coils making noise in the system.  We locked the bad coils in a magnet housing and did a power frequency sweep.  The bad coils made distinctive horrible noise at certain frequencies.  The problem turned out to be that the coil was not bonding to the aluminum bobbin we wind the coils on.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

Another calculation.  At 500,000 units testing at 1 minute each comes to 8,000 hours a month or 2,000 hours a week.  If an employee works 40 hours a week, that is 50 extra people to hire.  Drop the time to 30 seconds is still 25 people.  Managers always think in terms of people.  Any manual operation is always goint to take about 30 seconds.

While you have speaker failures, You don't indicate they are a majority of one type.  This means you have to do multiple tests and probably a significant number are not even related to the voice coil.  In this high volume application the problem should be controled at the outside vendor.  

I remember some voice coild being wound on an aluminum sleeve, possibly for dampening.  This would make some testing difficult.  Fastest testing would be if you didn't have to attach anything manually.  I keep thinking of that tuned coil idea.  Just think of the coil mounted to the speaker cone. A metal foil on each side of the speaker cone forming a capacitor.  Advertizing guys could call it the "ring of fire."  These could pass down a conveyor at a pretty good speed and be inductivly tested with the bad ones kicked out. Induced frequency could sweep eliminating need for close tolerances. Think you have quite a challenge ahead of you getting testing down to 10 seconds and eliminating just 50% of the failures.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

It "simply" means that he needs to be able to test about 30 at a time, which is why I caveat "huge investment". since that requires a fairly high-speed switching matrix and a very expensive parts handler.

One thing that could bring down the cost would be to have your supplier deliver the coils on some sort of carrier that would clamp the pigtails on known positions for the test setup to connect to.

TTFN

RE: How to test a speaker voice coil(raw part, not assembled to anything)

(OP)
hi all:
I need some time to "digest" all your ideas...haha...I am still a newbie. I have to read a few times to understand a little bit. Appologize for my stupidity.
What my boss have in mind is a test that can be conducted on the voice coil for about 2 to 3 seconds.
We got around 9 production lines.

Sorry that I still cannot understand the FFT method and the "ring of fire" method, can you please explain clearer(including what tools/equipment needed) ? These methods is for speaker test, is it ?

I am now looking at a tester, Wagne Kerr 6815 Impulse Winding Tester (or the Microtest TF-6815, they are the same). It costs about US$7000. From taiwan. The salesman claim it can detect intermittent cases. The theory is that it inputs a HV but low ampere pulse into a master coil and record how the waveform decays. Then a reject percentage is set. The master coil can be remove and all coils for test will be test at the same method but the waveforms are compared to the master's one.It claims that the test can be conduct for each coil for less than one second.
Anyone ever use this kind of tester ? any comment ?
The claims can be trusted ?
IS this test method suitable for me ? Will it damage the coil ?
Someone said that the impulse can only test the coil for the first few turns, is it correct ?
It claim the voltage is 200V to 5000v and the ampere is very low only(some claim nano amp).
Other company like ECG-Kokusai or Baker sell for more expensive, almost double the price.

The website is http://www.waynekerrtest.com/html/products/lcr/6815.htm
 and http://www.microtest.com.tw/english/products/tf-6815.htm .
Hope you guys are free and have a look there and give me advice.

Thank you.

RE: How to test a speaker voice coil(raw part, not assembled to anything)

OK, last post. The problem exists with the coil winder because you have beat every penny out of him. Before you spend these thousands of dollars for testing, go out and buy a couple spools of high quality wire from a known manufacturer. Then make a test run and see if your problems go away.  

I remember a plastics manufacturer that asked us to inspect plastic pellets for specs of carbon.  We told him that this seemed like a process problem and controlling the ramp up of the extruder heaters would eliminate the black specs.  He said he knew that, but he had budget money for inspection and not new capitol investment.  

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