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Old 27th December 2011, 04:43 PM   #1
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Default Precise cone position control

is it possible to accurately control the position of the speaker cone? i'm planning on using a speaker to position some lightweight (<0.5g) mirrors for a laser project. would it be possible if i varied current instead of voltage? i'm looking at frequencies around 4KHz.
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Old 27th December 2011, 07:28 PM   #2
T101 is offline T101  Bulgaria
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They are doing it all the time with CD player laser assembly lenses - moving iron surrounded by coils.
You'll have to have either spring or sufficient weight to counter the motor force for precise positioning. Maybe high compliance (soft suspension), low Qms (allot of losses-shock absorber like-opposite of spring effect) and low Qes (strong motor)
Have in mind distortion, it is something you can't compensate for unless you use MFB (motion feedback) with proper resolution and I'm not sure for 4 kHz.

Best Regards!
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Old 27th December 2011, 07:47 PM   #3
jcx is offline jcx  United States
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it always good to see the performance specs of pro gear to get an idea of what the limits of a diy hack might be

in this case search for galvanometer mirrors, a quick search give 2 kHz broadband and 12 kHz resonant for purpose designed galvo mirror hardware
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Old 27th December 2011, 07:54 PM   #4
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Quote:
Originally Posted by bittuthegr8 View Post
is it possible to accurately control the position of the speaker cone? i'm planning on using a speaker to position some lightweight (<0.5g) mirrors for a laser project. would it be possible if i varied current instead of voltage? i'm looking at frequencies around 4KHz.
Speakers have an impedance which varies with frequency, but the impedance does not change with voltage when used in itís linear power range .
The amount of current that flows through the speaker is dependent on the voltage applied and the speakerís impedance.
The lower the impedance, the higher the current flow with a given drive voltage.
Increasing the impedance will decrease the current flow with a given drive voltage.

Since the speaker impedance is fixed, you canít vary current without varying voltage.

Assuming you are planning a simple device such as the one below from (Laser Science Projects - Intermediate Projects - www.LaserFX.com) , more voltage (or louder music) will make a larger size pattern, while the pattern is determined by combining frequencies.


Frequencies above 16 Hz or so will appear as a continuous line due to persistence of vision.
At low frequencies, the speaker has much more excursion per given voltage, so a bigger pattern can be made with less power using lower frequencies to drive the speaker.
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Old 28th December 2011, 12:15 AM   #5
Squeak is offline Squeak  Denmark
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The read/write head in a HDD is controlled by a voicecoil, so very, very good precision is possible. But as already said, to have it you need some kind of feedback control.
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Old 28th December 2011, 12:49 AM   #6
gootee is offline gootee  United States
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First, what are the positioning requirements, for your system?
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Old 28th December 2011, 01:29 AM   #7
gootee is offline gootee  United States
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There are many threads here about using cone position or "motional" feedback (aka MFB).

Here is just one: Motional Feedback Subwoofer
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Old 28th December 2011, 08:57 AM   #8
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Originally Posted by weltersys View Post
Since the speaker impedance is fixed, you canít vary current without varying voltage.
Assuming you are planning a simple device such as the one below from (Laser Science Projects - Intermediate Projects - www.LaserFX.com) , more voltage (or louder music) will make a larger size pattern, while the pattern is determined by combining frequencies.
i meant varying the current but not keeping the voltage constant because the force on the coil is proportional to the current flowing through it.

Quote:
Originally Posted by gootee View Post
There are many threads here about using cone position or "motional" feedback (aka MFB).

motional feedback seems like a good idea, the capacitance method seems to be the easiest to implement. Tech

also when the speakers are run there will probably be a hissing noise, would it be alright if i cut a few slits into the cone
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