PSD analysis: warning about modal covariance matrix

[AnsysNewUser]

All,

I am doing a PSD analysis, and when I solve the random vibration solution, a warning message shown as below:

*** WARNING ***
One or more diagonal terms of the modal covariance matrix (i.e., when mode i = mode j) is negtive; The 1 sigma results may be incorrect as a result of the input PSD curve being improperly represented.

What is the possible reason for this? How can I debug the problem?

Any input are highly appreciated!

Thanks.

AnsysNewUser

 

[AnsysNewUser]

Hi,

Anybody could help on this?

AnsysNewUser

 

[Transient1]

Have you run a modal analysis without any constraints for the first six modes? This is a good check for issues with the dynamic behavior of the model.