I've developed a small sample code that could be a good start point for your program.
Note that sheet metal document is a subtype of PartDocument.
It has a lot of specific methods and properties.
Please refer Inventor API Help for more details.
Sub FindSheetsByThickness()
'target thickness - search criteria
Dim TargetThickness As Double
TargetThickness = 0.05 'cm
'-------------------------------
Dim oAssyDoc As AssemblyDocument
Set oAssyDoc = ThisApplication.ActiveDocument
Dim oAssyDef As AssemblyComponentDefinition
Set oAssyDef = oAssyDoc.ComponentDefinition
'select set object of main assembly
Dim oSSet As SelectSet
Set oSSet = oAssyDoc.SelectSet
Dim oDoc As Inventor.Document
For Each oDoc In oAssyDoc.AllReferencedDocuments
'verify document type (we are interested in metsl sheets only)
If oDoc.ComponentDefinition.Type = kSheetMetalComponentDefinitionObject Then
'extract thickness of this sheet part
Dim oDef As SheetMetalComponentDefinition
Set oDef = oDoc.ComponentDefinition
Dim Thickness As Double
Thickness = oDef.Thickness.Value 'returned in cm (base length units)
If Math.Abs(Thickness - TargetThickness) < 0.00001 Then
'thickness is equal to target thickness
'find all child components for this part document
'at all assembly levels
Dim oColl As ComponentOccurrencesEnumerator
Set oColl = oAssyDef.Occurrences.AllReferencedOccurrences(oDoc)
If oColl.Count > 0 Then
'select all found components
Dim oOcc As ComponentOccurrence
For Each oOcc In oColl
Call oSSet.Select(oOcc)
Next oOcc
End If
End If
End If
Next
Beep
End Sub
Hope this helps.
Cheers,
Vladimir Ananyev
Developer Technical Services
Autodesk Developer Network