How to get the value of a CKEditor field within a JS function called by onsubmit ?

I have a FORM with JS function for checking the value of some fields and, in particular, the value of the CKEditor field:

<form ... onSubmit='return(check_xxx());'>

Now, with FCKEditor, in order to get the value, I needed to do this:

var oEditor = FCKeditorAPI.GetInstance('field_name');                         
field_value = oEditor.GetHTML();      

With  CKEditor, I try to do this:

field_value = CKEDITOR.instances.editor1.getData('field_name');

but it's not working.    

Somebody can help me? Thanks a lot in advance.