amp questions for driving 'voice coil actuator'?

steve · 4th October 2005, 04:50 PM

Hi...

I am looking for an amp to drive a 'voice coil actuator'... During a conversation with an engineer for the manufacture I was told that they use a simple audio amp for 'testing' purposes to drive these devices, but I'm concerned that my requirements will place an intolerable load on an audio amp. In particular, I will be driving these devices at very low frequency... (expected frequency range will be 0.1 hz to 10hz) and with a DC offset to boot. Another significant problem with these devices is that they utilize a very substantial coil (one device that I'm considering has a 2 lb voice coil) and consequently produce significant back EMF. This same device has an impedence of 2.8 ohms and produces 100 lbs of force at 41 volts. I think that I will only require a small percentage of that force, but this motor has other charateristics that are important (for one... it's surplus). Anyway, my question is this, is there an audio amp design which would be appropriate for such a load, or should I look elsewhere?

thanks,
steve

bogicp · 4th October 2005, 04:58 PM

Hi steve,

do you have any other specifications about your amplifier requirements... for example, output voltage, output current, maximum distortion, etc... ?

Best regards,

boggy

steve · 4th October 2005, 05:34 PM

to be honest..... no.

There is a very long (and mostly irrelevant) story behind this, but the short explanation is that I'm trying to build a device to introduce pressure waves (at low frequency) into tissue. I currently have a device called a 'foredom percussor' which is used for this purpose, but it is limited to a sine wave of fixed amplitude, with only limited control of frequency. I am interested in more complex patterns, and need to have more control of amplitude and frequency, in particular I am looking for feedback control.

I have found a 'voice coil actuator' which should be capable of reproducing the motions and power of the foredom device (at least the low frequency aspects), with the additional capacities that interest me, and it is this device that I mentioned in my previous post.

I would be happy to 'start out' with an amp that is able to produce a quarter of the force that the motor is rated for, I'd guess about 10 volts at 2.8 ohms, but at the same time, I think that this amp might need to deal with wildly unsymeteric swings as well as DC offset, and the back EMF will be horrendous at any sort of speed..

And the truth is that I'm just experimenting here, and I don't know what requirements I'll end up with... It would be nice to just get to a point where I could produce a simple sine wave with the contraption... but I am hoping to eventually utilize a linear transformer as a feedback device. I actually want to introduce a 'feedback' effect in the tissue

hope this helps,
thanks,

steve

bogicp · 4th October 2005, 05:49 PM

hm, i don't know really, how some standard amplifier chip can tolerate currents returned from your device, but loudspeakers has same "practice"...

You can posibly try some "gainclone" project, it's widely available both at this forum, and at world wide web.
It's simple, inexpensive, and with other components (large power supply capacitors... for example) you can do anything with it...
They is like "power opamp", you can do DC with it, etc.
-Current limit is 7A typical
-and you can use +/-20V power supply
-if your currents are too high, you can do "parallel mode" configuration with two LM3886, and then drive low impedance with him...

Ok, I probably speculate only, but there isn't much more informations...

best regards,

-boggy

steve · 4th October 2005, 06:14 PM

Thanks again... I know that I didn't have much info to provide, but you actually addressed my major concern which had to do with the DC offset that I anticipate.... I actually have a couple of 3875's... not to good for the loads that I anticipate.. but I could try to parallel a couple.. Or I could look at the 4870(?)...

I just wanted assurance that I wasn't going to toast my amp by trying to drive such a crazy load.

thanks,
steve

bogicp · 4th October 2005, 06:22 PM

try 3875, but instead of LM4780 (you probably have a typo) look at old LM3886, this chip has double power dissipation capability... and use two of it in parallel mode... then you will probably have much more time for your work, instead to build power amps all the time...

best regards,

-boggy

steve · 4th October 2005, 06:42 PM

I hear you on that one.... thanks again for your help

Steve

jaycee · 4th October 2005, 11:35 PM

I would say this sort of driving capability is beyond a chipamp. I would build a discrete amp with a heavy output stage, probably switching mosfets. The emphasis here would be on the output stage having to provide lots of current and deal with quite a severe back EMF.

It might also be an idea to look at discrete, high current regulated power supply circuits. These are normally very similar to amps in design, but with emphasis on handling current.

CBS240 · 4th October 2005, 11:46 PM

I wonder if this would be a job for a class H type project? Or some type of modulating power supply for slow, high inductive loading. Or maybe a servo motor drive circuit? motor drives are quite common in industrial machinery.

djk · 5th October 2005, 09:49 AM

Just use four LM3886 in parallel with a 30-0-30 500VA transformer, a 25A bridge and a pair of 15,000µF caps.

The LM3886 is protected against back EMF, and has all kinds of protection built in. Cheap too.

4th October 2005, 04:50 PM	#1
steve diyAudio Member Join Date: Feb 2003 Location: wi	amp questions for driving 'voice coil actuator'? Hi... I am looking for an amp to drive a 'voice coil actuator'... During a conversation with an engineer for the manufacture I was told that they use a simple audio amp for 'testing' purposes to drive these devices, but I'm concerned that my requirements will place an intolerable load on an audio amp. In particular, I will be driving these devices at very low frequency... (expected frequency range will be 0.1 hz to 10hz) and with a DC offset to boot. Another significant problem with these devices is that they utilize a very substantial coil (one device that I'm considering has a 2 lb voice coil) and consequently produce significant back EMF. This same device has an impedence of 2.8 ohms and produces 100 lbs of force at 41 volts. I think that I will only require a small percentage of that force, but this motor has other charateristics that are important (for one... it's surplus). Anyway, my question is this, is there an audio amp design which would be appropriate for such a load, or should I look elsewhere? thanks, steve

4th October 2005, 11:46 PM	#9
CBS240 diyAudio Member Join Date: Aug 2005 Location: K-town	I wonder if this would be a job for a class H type project? Or some type of modulating power supply for slow, high inductive loading. Or maybe a servo motor drive circuit? motor drives are quite common in industrial machinery. __________________ All the trouble I've ever been in started out as fun......

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4th October 2005, 04:58 PM	#2
bogicp diyAudio Member Join Date: Oct 2005	Hi steve, do you have any other specifications about your amplifier requirements... for example, output voltage, output current, maximum distortion, etc... ? Best regards, boggy

4th October 2005, 05:34 PM	#3
steve diyAudio Member Join Date: Feb 2003 Location: wi	to be honest..... no. There is a very long (and mostly irrelevant) story behind this, but the short explanation is that I'm trying to build a device to introduce pressure waves (at low frequency) into tissue. I currently have a device called a 'foredom percussor' which is used for this purpose, but it is limited to a sine wave of fixed amplitude, with only limited control of frequency. I am interested in more complex patterns, and need to have more control of amplitude and frequency, in particular I am looking for feedback control. I have found a 'voice coil actuator' which should be capable of reproducing the motions and power of the foredom device (at least the low frequency aspects), with the additional capacities that interest me, and it is this device that I mentioned in my previous post. I would be happy to 'start out' with an amp that is able to produce a quarter of the force that the motor is rated for, I'd guess about 10 volts at 2.8 ohms, but at the same time, I think that this amp might need to deal with wildly unsymeteric swings as well as DC offset, and the back EMF will be horrendous at any sort of speed.. And the truth is that I'm just experimenting here, and I don't know what requirements I'll end up with... It would be nice to just get to a point where I could produce a simple sine wave with the contraption... but I am hoping to eventually utilize a linear transformer as a feedback device. I actually want to introduce a 'feedback' effect in the tissue hope this helps, thanks, steve

4th October 2005, 05:49 PM	#4
bogicp diyAudio Member Join Date: Oct 2005	hm, i don't know really, how some standard amplifier chip can tolerate currents returned from your device, but loudspeakers has same "practice"... You can posibly try some "gainclone" project, it's widely available both at this forum, and at world wide web. It's simple, inexpensive, and with other components (large power supply capacitors... for example) you can do anything with it... They is like "power opamp", you can do DC with it, etc. -Current limit is 7A typical -and you can use +/-20V power supply -if your currents are too high, you can do "parallel mode" configuration with two LM3886, and then drive low impedance with him... Ok, I probably speculate only, but there isn't much more informations... best regards, -boggy

4th October 2005, 06:14 PM	#5
steve diyAudio Member Join Date: Feb 2003 Location: wi	Thanks again... I know that I didn't have much info to provide, but you actually addressed my major concern which had to do with the DC offset that I anticipate.... I actually have a couple of 3875's... not to good for the loads that I anticipate.. but I could try to parallel a couple.. Or I could look at the 4870(?)... I just wanted assurance that I wasn't going to toast my amp by trying to drive such a crazy load. thanks, steve

4th October 2005, 06:22 PM	#6
bogicp diyAudio Member Join Date: Oct 2005	try 3875, but instead of LM4780 (you probably have a typo) look at old LM3886, this chip has double power dissipation capability... and use two of it in parallel mode... then you will probably have much more time for your work, instead to build power amps all the time... best regards, -boggy

4th October 2005, 06:42 PM	#7
steve diyAudio Member Join Date: Feb 2003 Location: wi	I hear you on that one.... thanks again for your help Steve

4th October 2005, 11:35 PM	#8
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	I would say this sort of driving capability is beyond a chipamp. I would build a discrete amp with a heavy output stage, probably switching mosfets. The emphasis here would be on the output stage having to provide lots of current and deal with quite a severe back EMF. It might also be an idea to look at discrete, high current regulated power supply circuits. These are normally very similar to amps in design, but with emphasis on handling current.

5th October 2005, 09:49 AM	#10
djk diyAudio Member Join Date: Feb 2001 Location: USA	Just use four LM3886 in parallel with a 30-0-30 500VA transformer, a 25A bridge and a pair of 15,000µF caps. The LM3886 is protected against back EMF, and has all kinds of protection built in. Cheap too.

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