User guide Owner's manual

User manual for the ZEN 3-channel X-LMS controller Soft dB inc.
3.3.2 Adaptative step size and identification technique in noisy conditions
The normalization of the adaptative step size of the identification module is done at high
level on the PC based on the number of coefficients used and the amplitude of the white
noise. In theory, if the primary noise to be controlled is not activated, the use of Mu=1
obtains the fastest convergence rate and great precision of the identified model. However,
if the primary source generates noise, the use of a constant Mu=1 during the whole
identification process is no longer possible.
When the primary source is activated or if external noise is present during the
identification process, the use of the following technique obtains better convergence and
greater model precision. This technique is called adaptative step size commutation:
1) To start, set Mu=1 and launch the identification process.
2) When convergence is stable, reduce Mu by a factor of two.
3) Theoretically, after a period of time that is twice as long as the previous step,
convergence should be 3 dB better. Validation of this result is not possible
since the technique used to compute the convergence is distorted by external
noise at the error sensor. This means that the convergence indicator of the
identification module will not show a 3 dB increase.
4) Reduce Mu by a factor of two again and wait for a period of time that is twice
as long as the time in step #3.
5) Repeat step #4 until the desired convergence value is achieved. To estimate
this convergence value, add 3 dB by commutation of the adaptative step size
to the convergence obtained in step #2.
The stabilization time discussed in step #2 is difficult to measure since convergence is
very fast when Mu=1. As a guideline, the use of a first stabilization time of 10 seconds
for Mu=1 is conservative. The table below shows an example of the use of the
commutation technique.
Number of Mu
commutations
Mu used Time length of the
optimization
Estimated
Convergence
1 1 10 s -10 dB (measured)
2 0.5 20 s -13 dB
3 0.25 40 s -16 dB
4 0.125 1 min 20 s -19 dB
5 0.0625 2 min 40 s -22 dB
6 0.03125 5 min 20 s -25 dB
Total --- 10 min 30 s -25 dB
ZEN User Manual p. 19