Voice Coil Rail

[mjkzz]

Mike, forgot, I am using 12bit DAC and an Op-amp, with 2N2222, current is limited to 0.375A so it will not get blown. I doubt it will be able to drive the BEI much, but if I can make it to move by 1mm, I will be happy :-)

As for speaker selection, the one I showed yesterday is something called "long throw" one, I guess it means the coil or the cone move longer than "normal" ones

[mawyatt]

...The non-linearity is probably a combination of things but mostly the cone I suspect. In any case it's relatively easy to correct with digital controls. I'm sure your BEI motor will work better.
...
I would think a larger speaker such as a 8, 10 or even 12" woofer would have better near zero displacement linearity, better load support and longer displacement than a smaller one. Drawback would be larger driver current, but that's easy to solve.

The nonlinearity is for sure a combination of things. Speaker voice coils are designed to have nonlinear force vs displacement to correct the suspension nonlinearities. This can only be used to a certain displacement...the suspension eventually wins.

I would think a smaller, large displacement woofer would be the best for this application. Smaller woofers are designed to have larger displacements with good linearity. Big woofers don't have to move as much for the same SPL.

I am a bit worried about the crossover distortion of the op amp circuit shown. Input offset may cause a fair bit of nonlinearity near equilibrium point.

Ray,

I recall the large woofers (12" & 15") we using in our guitar amps (when I was younger) had a fairly large displacement (I repaired and modified amps to pay for school), they were designed to move a lot of air for high SPL. Probably linearity wasn't an issue, we all used Fuzz Busters anyway....distortion was Good! Either way, it should be easy to correct any reasonable Voice Coil non-linearity with pre-distortion.

Crossover distortion isn't a problem because three things are at play. First this is current mode operation, not voltage mode! Second the closed loop gain of the op amp corrects any distortion to almost non existent levels. The gain is enormous (~100dB) since the operating frequency is very low near DC, so the GBW Product doesn't come into play either. Third the 1K resistor provides current until the Vbe is reached, which will be just below 1ma, which then the transistor will begin providing current, releasing the op amp and preserving it's open loop gain (OLG falls off as output loading). If you use a CMOS op amp, then this 1K resistor might need to be increased to 5K or so since the op amp output driving impedance is higher than bipolar op amps.

Also, offset shouldn't matter either since it should be well below 5mv (good op amps can be as low as a few 10s of uV), and can easily be nulled out, or even corrected with the setting pot or DAC if used.

Best,

Mike

[mawyatt]

Mike, forgot, I am using 12bit DAC and an Op-amp, with 2N2222, current is limited to 0.375A so it will not get blown. I doubt it will be able to drive the BEI much, but if I can make it to move by 1mm, I will be happy :-)

As for speaker selection, the one I showed yesterday is something called "long throw" one, I guess it means the coil or the cone move longer than "normal" ones

Peter,

Even though you've limited the output to 375ma, the single 2N2222 is probably going to get too hot (Vdrop*Iload). I would either add more 2N2222's in parallel (use slight emitter degeneration, 10 ohms for example), add heat sink, or just use a TO-220 package type transistor (TIP Series).

12bit DAC is an excellent choice, you'll have 1 part in 4096 resolution and plenty of bits for your Voice Coil pre distortion look up table!!

Best,

Mike

[ray_parkhurst]

The non-linearity is probably a combination of things but mostly the cone I suspect. In any case it's relatively easy to correct with digital controls. I'm sure your BEI motor will work better.

OK, it seems we are speaking of different things :-). I am talking about circuit, under DC circuit, a speaker is just like another resistor, so a variable voltage source can be thought as current source.

I think you and ray_parkhurst are talking about final output, which is rightly so, it definitely not linear. Beside my junk BEI motor, why can't we remove the speaker cone and replace it with some sort of spring with linearity, I mean for 2mm displacement, it ought be easy to do :-)

Anything you buy will likely have some sort of suspension to keep the coil centered in the magnetic gap, so no spring should be necessary.

[Ludvig Friberg]

Hi!

Very interested in trying this out. Being quite the novice the electrical field I could really need some help. Looking at making a copy of mawyatt´s drawing. I was thinking of using an arduino and a speaker of sorts, that way I can easily program a simple stacking routine with a optocoupler for the camera shutter.

So far I have looked at a MCP4725 as DAC, but I need some more components. What OP-AMP should I get? If I need more volts I guess I need some gain on it? I have some tip120 somewhere can I use those? Something resembling a BOM for an Arduino build would be very nice, but perhaps a bit much to ask for.

Anyways, very exiting stuff. Keep it up! Looking forward to see where you guys take this.

[mawyatt]

Ludvig,


The TIP120 should work fine as the output Darlington. A LM358 Dual OP Amp is a good general purpose device that is cheap and readily available.

I'll let others discuss the Arduino, as I have no experience with this device.

Best,

Mike

Hi!

Very interested in trying this out. Being quite the novice the electrical field I could really need some help. Looking at making a copy of mawyatt´s drawing. I was thinking of using an arduino and a speaker of sorts, that way I can easily program a simple stacking routine with a optocoupler for the camera shutter.

So far I have looked at a MCP4725 as DAC, but I need some more components. What OP-AMP should I get? If I need more volts I guess I need some gain on it? I have some tip120 somewhere can I use those? Something resembling a BOM for an Arduino build would be very nice, but perhaps a bit much to ask for.

Anyways, very exiting stuff. Keep it up! Looking forward to see where you guys take this.

[mjkzz]

@Ludvig Friberg, I think you can build Mike's circuit on a piece of breadboard (such as breadboard shield for Arduino). And if your DAC is surface mount version, you can get some breakout PCB to make it easier to build on breadboard. After that, you are good to go :-)

Hmmm, why didn't I think of uisng Arduino and where is my breadboard shield :-)

[Ludvig Friberg]

Thanks!

Ordering up the parts needed tonight. When/if anything good comes of this I will post details here.

[mawyatt]

Thanks!

Ordering up the parts needed tonight. When/if anything good comes of this I will post details here.

Ludvig,

If you are building the unidirectional current source (my 1st schematic) you can place the voice coil in either position as shown in both schematics, the unidirectional and bi-directional versions.

The first schematic (unidirectional) doesn't require as much Vcc supply for the op-amp since the voice coil drop is on the collector side of the driver transistor. If the load (coil) has a low impedance and thus low voltage drop then this isn't a big deal. The 2nd schematic (bidirectional) places the coil in series with the output transistor emitter(s) and thus the coil voltage drops adds to the sense resistor voltage. This can be a big deal if the voltage drop across the coil is large, requiring a higher Vcc supply for the op amp. Note the op amp must supply the required output voltage plus the forward biased Vbe for the output transistors, in the Darlington case that's 2 Vbes...so you'll need a +Vcc supply that is about ~3 volts greater than required output voltage to allow for the op amp voltage output limits.

You shouldn't have to worry about this unless you plan on producing high coil currents, then the power dissipation of the components comes into play.

Anyway, I'm sure your design will work fine!!

Best,

Mike

[Ludvig Friberg]

Perhaps I am blind but I can only see one sketch? And it says Bi-Directional. It is a sketch on on paper with grid.

[mawyatt]

Perhaps I am blind but I can only see one sketch? And it says Bi-Directional. It is a sketch on on paper with grid.

See page two of this thread.

[mjkzz]

After some debugging on my PCB, the result is pretty good, I can step current at 0.125ma increment, that is pretty good resolution. Now, the load is just a resistor, but will put the speaker as load. Circuit can handle up to 375ma which is limited by setting a parameter via PC.

Forgot to mention, I blew the fuse in my multi-meter on mA mode, else I can show the number in mA

This video shows the circuit is being controlled by a PC application and each key press is 0.5ma, ie, 4 minimum steps. The app needs a bit refreshing but it is awesome to see how precise it can be.

[Ludvig Friberg]

So I was blind!

Strangeness, I looked through the thread 5 times. I blame my phone, now browsing on computer instead.


Looking good mjkzz!!

Are you selling the pcb?

[ray_parkhurst]

The 0.125mA current step would yield 0.25um steps on my 1.5" speaker, 800 steps from 0 to 100mA, with total displacement 200um.

This should be small enough for any practical stacking application.

[mjkzz]

Ludvig, my PCB is probably useless to you as it has microprocessor to handle communication with PC, camera triggering, and rotary dial etc, all integrated into one. It is not what you would want unless you want to use it as a module and program your Arduino to talk to it via serial. I also need to do some more changes to PCB. However if you want to be a beta tester, you can get it at cost (the USB to serial OR bluetooth module is kinda expensive), but PM me please.