How to scan the position of the quadrupole to measure the emittance

[QinJohn]

Hi All,
I'm a new to the elegant and I want to simulate the quadrupole scan method to measure the emittance. I have found an example in the ElegantExamples, in that example, the scan parameters are strength of the quadrupoles. Now I want to scan the quadrupoles' position, which can also lead to the change of the transfer matrix. When I change the parameter to the position, the results showed as follows:

Vertical emittance couldn't be determined
Horizontal emittance couldn't be determined
Horizontal emittance couldn't be determined
Vertical emittance couldn't be determined
Vertical emittance couldn't be determined
Horizontal emittance couldn't be determined
Vertical emittance couldn't be determined
Horizontal emittance couldn't be determined
Vertical emittance couldn't be determined
Vertical emittance couldn't be determined

The attachment is my input file .Any one can help me to solve this question? Thank you !

[michael_borland]

John,

The problem seems to be that this method is very senstive to errors and to the fact that the beam size only goes through a minimum in one plane. I was able to improve matters slightly by using two stronger quadrupoles, see attached. Perhaps this will give a clue.

--Michael