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I need an actuator with a few mm of travel and it has to be very responsive to a very low frequency (DC 10 Hz) sine wave generated by a function generator.

I'm thinking of getting a cheap subwoofer and hooking it up to my function generator via a power amp.

My questions:

  1. Can I use a speaker as a pseudo-linear actuator or is the speaker designed to pull back the membrane?
  2. Will the voice coil blow when it's used at very low frequencies with enough power to actually displace the membrane by a few mms?

Edit:

The load is a PCB assembly weighing around 100 grams. I want to move that assembly up and down in a controlled way at different frequencies which I'm generating with a function generator. I also have linear stages but they are slower and noisy and I've thought about a mechanical contraption with a stepper motor and cam shaft. The actuator solution seems more attractive because I can control the motion more easily and I expect the latency to be better.

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    \$\begingroup\$ If you tell us more about the stuff you want to displace we may be able to provide some better suggetions. \$\endgroup\$
    – jpc
    Commented Mar 26, 2011 at 1:41
  • \$\begingroup\$ Speakers depend on the cone moving back and forth to keep air flowing and keep the voice coil cool. DC will, in general, destroy the coil. \$\endgroup\$
    – endolith
    Commented Mar 26, 2011 at 15:34

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The actuator used to position a hard-drive would match those attributes. Funnily enough they are called voice-coil actuators!

I guess with any solenoid type of actuator that needs to operate at very low frequencies you probably need to design your drive circuit to not overheat the windings even with a DC current.

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  • \$\begingroup\$ Good idea. We've got stacks of old hard drives around. I'll take one apart and see how much power the actuator has. My load is going to be around 100grams so I might have to gang up a few of them. \$\endgroup\$
    – Andrew Queisser
    Commented Mar 26, 2011 at 14:02
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A subwoofer will work just fine. Something like this would work, you'd get about 12mm of approximately linear travel.

You'll have to adapt for the response down to DC. I don't mean just the woofer itself which will have some roll off under fs but amplifiers will roll off at a couple hz (cheap amplifier will be higher) and just the interconnects used in the system generally form a HPF at 5-7hz. Basically you'll need some equalization to keep the excursion response flat. Most off the self EQ will not work at these frequencies so you'll probably have to deal with it by just adjusting the magnitude of your input signal, i.e. pre filter the source.

Don't send the woofer DC if you can avoid it but when operating in free air it doesn't take much power at all to move cone a few mm, i doubt you'd have heating issues with the voice coil.

Most subwoofers you see used as such are in enclosures. Its the job of the enclosure to provide spring resistance to the driver. It takes a lot more power to move the cone with its in a sealed enclosure than it does when its just sitting on the table. A 9V battery will move a subwoofer cone a millimeter or two.

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Woofers have been used to move mirrors for simple laser light displays, but you probably won't get much displacement out of a cheap woofer.

A better idea might be a solenoid with a weak spring attached.

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