What does resonace peak mean when it is represented with dB

It sounds like you are working with an optical drive actuator? The specs are usually rather vague, but you can assume that zeta is about

0.0325 to give 24dB peak. The mm/V spec is often specified at a few frequencies, but 0.85mm/V is probably the low frequency gain spec before the resonant peak.

I hope that helps.

fred

Hi Fred,

Yes it is an optical drive actuator. Are you also doing optical disk servo? My msn is: snipped-for-privacy@hotmail.com.

I think 0.0325 is square of zeta and zeta is 0.18. I have much confusion on reading spec of the actuator. From an article, I read the resonance frequency as of the spring suports the OPU and the 1st mode frequency as the resonace frequency of the VCM motor. Am I right? Another confusion is about the quality factor Q(e.g. equals 16): what information does it convey? I think the resonance frequecy and resonance frequency peak can tell what the 2-order system is. If quality factor is used to determine resonance peak, what is the relation between Q value and resonance peak?

Best Regards, Thomas

Q is the ratio of the energy stored in the resonant system(1) to the energy dissipated in one cycle of oscillation. Damping lowers the Q. Q is roughly the ratio of resonant amplitude to amplitude at a frequency remote from resonance. Q ans zeta are different ways to specify or measure the same thing.

Jerry ___________________________________

1) for a mass and spring, the energy sloshes back and forth between kinetic in the mass and potential in the spring.

Hi Thomas,

I worked on optical drive servo until last year. I think zeta should be

0.035 - you can check by drawing a Bode plot. You should get around 24dB peak. Zeta=0.18 is too high. The device behaves like a spring and a mass with negligible viscous damping which is why zeta is so low. Zeta and Q are related in reciprocal fashion with a factor of 2 included.

fred

Hey Fred:

Is this one of those systems where you bring the resonance into the loop and control it, or is it one where you put a big ol' notch in your controller and hope the resonance doesn't move too much in production?

Hi Tim,

I have seen both approaches used and I have heard that gain scheduled methods are also used. Optical pickup actuators are quite precision devices, so their characteristics are reasonably well controlled. The main challenge is the nonlinear S-curve transfer characteristics of the laser array and the requirement that modern optical drives must be able to play all types of media; CD, DVD, RMS etc. with different LASER wavelengths. "Blue Ray" should be very interesting, but I'm out of the loop with this technology now.

fred

Hi Tim:

Notch filter is used to suppress the higher frequency resonance of the actuater. What doya mean with "bring the resonance into the loop and control it"?

Thomas

The S-curve can be searched open loop. We are meeting with how to stabilize the focus in the beginning. We want to tune it with PID, but the critical P is hard to find.

How doya think about "blue ray" and "hd-dvd", both is difficult to control and they are very different. But the boss want to make compatibility of these two and also DVD and CD.

Thomas

If the system is well behaved at the resonant frequency of the plant you can damp out the resonance with differential control. For example, I have a client/former employer who's systems generally have a poorly defined, low-Q resonance around 0.1 to 1Hz, and another, much higher frequency one, that is rather high-Q and well-defined for any given product line. The loop is closed well above the 1Hz point but below the higher resonance. To keep the higher resonance from interfering with stability we notch it out.

Here is an extremely ugly and rough root-locus plot of what you'll see if you wrap that resonance:

| | | | | | --- | / \ | / x| | | | | | | | | ===

(from email) > Tim:

The root locus is for a controller where the resonance is brought within the active bandwidth of the controller. By placing a zero close to f=0 (with a differentiator) you add damping to the resonance, which acts to increase the effective damping coefficient of the system.