Natural Science Forum / Physics / Acoustics / December 2005

I am familiar having built a system to do this, but with piezos.
The frequencies were much lower, just a touching effect.
You have to deal with resonances at higher frequencies, and
a feedback system might be needed if your changing frequencies.

You could use any mechanical device including a speaker driver.
I think there are other mechanical devices also.

An infra-red detector will indicate movement.
I have used a sensor with sensor and emitter on the same side.
With the mass attached, the resonant frequency will have to be lower than the stimulating
frequency. A tweeter has very little movement in its design. I would use one with
a fs of about 500 Hz. minimum.
greg

1) What is the low frequency limit?
2) What motion of the transducer do you want to follow the applied
voltage....displacement, velocity, acceleration?
3) What is displacement/velocity/acceleration profile vs frequency would
you like to achieve?
4) What are your geometrical constraints in terms of transducer diameter
and proximity of the face of the transducer to the experimental animal
and/or other experimental apparatus?
5) Do you have electrostatic shielding requirements such as those that are
required when making neural or intracellular recordings using high
impedance microelectrodes?

    The speaker will work. Keep in mind that for very low frequencies, a
steady ac current will produce a static deflection (electrodynamic force
working against the retaining spring effective within the speaker
("spider" spring or its equivalent). This relation is no longer valid at
and above the resonant frequency of the speaker.

    Since you want to work from 200 Hz upward, a very small speaker with a
very high resonance frequency is desirable. The tiny speakers mounted in
PC's and Laptops are an example. I have tested PC speakers and found
them to have a resonance around 300-500 Hz. Laptop speakers, being still
smaller, will have a higher resonance frequency.

    The more formal device for such work is the electrodynamic "Shaker"; or
the "Mini-Shaker" that B&K has made (their item # 4810). It has a moving
coil to be driven by a sine wave current via any amplifier. If you get
the instruction booklet with it, it will describe the current needed to
get the velocity or displacement you wish.

    This shaker method may be the more tedious (finding a 4810) way.

    The speaker method you first described can be "calibrated" by using a
micrometer depth gauge (I am inventing here) mounted and set at the
needle (that would connect to the whisker later). The micrometer would
be "zeroed" with no current through he speaker, then backed away, then
ac current at some low frequency should be applied to the voice coil,
then the micrometer turned back in until faint contact is noted, either
by the ticking sound you might hear, or by an electrical continuity
check you might make.

    Angelo Campanella

The reason I ask, I didn't think raw actuators were all that expensive.
The actuators we have used here for Daniel Simons projects starting about 25 years ago,
were constructed by their lab personal out of slabs of about .5 X 3 Cm.
Rods were attached to the ends for contact with the whisker. I designed the
amplifiers which drove up to 300 volts. DC amplifiers
can be had with less drive voltage at reasonable costs.
It seems unreasonable with all your other specs., that using
a quickly set up voice coil system is  going to do the job. Perhaps you can scale down the initial
investigation. Do what your thinking, and just try it out and look at the stylus
under the microscope and other position observing methods.

greg