Or you can use pocket factory's ability that spawns those robots (I forget what it's called).
can anyone help me with this..
just like in castle fight after building a structure it auto train a unit.
maybe like this?
Events
unit finishes construction
constructed structure equal to (your structure)
unit-order constructed structure to train/upgrade to (your unit)
That won't train units forever. This trigger will create one unit when worker builds it.
This trigger is still another good solution.