Need help with position reaction~!


I run into a problem with the position reaction...
When I manually do the position reaction I can see the controler of the master object is "world space postion" in the reaction manager. But when I run the maxscript as blow, the controller is just position...
Anybody knows how to assign the world space postion as the master object's controller?

my script:
y = $BoxA.pos

x = $BoxB.transform
preTranslate x [0,0,15]
a = x.pos

--Assign a reactor
cont = $BoxB.pos.controller = position_Reactor()

reactTo cont $BoxA.pos.controller

animate on at time 50 $BoxA.pos = $BoxA.pos + [0,0,50]

-- Creat new states
slidertime = 50

createReaction cont

-- Set the reaction parameters

setReactionState cont 2 a

selectReaction cont 1

setReactionInfluence cont 1 100

setReactionStrength cont 1 1

setReactionValue cont 1 y