A few more wild guesses here :
- you say the resonant frequency is 18kHz, is it measured or computed? In the
later case you may easily be off by 20% or more compared to your device - at
20kHz you don't reach the resonance...
- you detect the contact of the mass with a counter electrodes it seems, the
contact duration will be very short (impact) and the contact force may be
small, and what about the roughness of material in contact, and the materials
used - in short you may have a hard time to detect the contact even if it
happens...

You may want to look in the papers about 'vibro scanning method' from the team
of T. Masuzawa at the U. of Tokyo, they had similar problems - but not for
acceleration measurement.

Good luck,
Franck


"-----Original Message-----
"From: Vic Kley [mailto:[email protected]]
"Sent: Thursday, 07 March, 2002 18:26
"To: [email protected]
"Subject: Re: [mems-talk] inertial sensor testing
"
"
"Just because it resonates doesn't mean that the AMPLITUDE of
"the resonance
"is +/- 10 microns. This will depend on the spring constant over the
"operating range (presumably +/11 microns or more) and the
"excitation energy.
"Another way to put this is turn up the volume! At 18khz an
"audio drive would
"work just fine, and you can buy mucho watts (excitation
"energy) real cheap.
"
"Vic
"
"----- Original Message -----
"From: ll 
"To: 
"Sent: Thursday, March 07, 2002 2:26 PM
"Subject: [mems-talk] inertial sensor testing
"
"
"> Dear group members,
">
"> Now I suffer a problem on inertial switch testing. My switch
"is made of
"> spring-mass structure which can sensitive to acceleration.
"When there is a
"> accelaration, the switch will be in ON state. Now I put it on a
"> shaker(sinusoidal input) to test it using the theory that
"the displacement
"> of my switch's mass will be larger when my switch is vibrating at its
"> resonant frequency. My switch's resonant frequency is about
"18kHz (it is a
"> little high:). ), the mass need to travel 10micron to reach
"its ON state.
">
"> SO here comes my PROBLEM: I sweep my shaker output frequency
"from 100Hz to
"> 20kHz BUT can't get any ON signal!!!
">
"> Does anyone has experience of this situation?
">
"> It can be vibrated at its resonant frequency even this
"frequency is high,
"> is that right?
">
"> Is 10 micron a large displacement for micro device to travel?
">
"> Is there other method to detect if the device does vibrate
"and also its
"> resonant frequency?
">
"> Please pay attension to my long story and give me wonderful reply:)
">
"> Best,
">
"> Vickie
"> _______________________________________________
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