String to object

hi everyone,
i guess there have already been dozens of people asking this question, and i am sorry to ask again, but i did not find alone:

i would like to create lots of copies of lots of different clips (the original clips are called _root.model_a, _root.model_b, _root.model_c … …) and then asign them a position depending on variables. i am using inside a loop:

    duplicateMovieClip("_root.model_"+utype, "copy"+cnt, cnt);
    [color=Red]"copy"+cnt[/color]._x [color=DarkOrchid]=[/color] ux;
    [color=Red]"copy"+cnt[/color]._y [color=DarkOrchid]=[/color] uy;

then an error message tells me that the thing on the left side of the assignement [color=DarkOrchid][color=Black]"[/color]=[/color]" is not a varriable / property.
i would like it to be a property, but to do so i guess i need to transform the thing in red (a string) to the reference to the object that has the same name.

in this tutorial the problem is solved by putting the 2 critical line inside the clip that will be duplicated and by puting:

    [color=Red]this[/color]._x = ux;
     [color=Red]this[/color]._y = uy;

but i do not have only 1 clip that will be duplicated and i don’t want to put some code in each (if that is possible).
in fact i would like to make an externall call to the object that is called [color=Red]“copy”+cnt[color=Black] instead of an internall call using [/color]this[color=Black].[/color][/color]

please tell me what i should do