{HEAD ~ A scanning head} 7, wherein each of the three electric motors is connected with its own negative resistor, each resistor having a resistance value that is adjustable independent of the other two negative resistors, by {TAIL ~ means} of hardware or software 
{ HEAD ~ A scanning head } 7 , wherein each of the three electric motors is connected with its own negative resistor , each resistor having a resistance value that is adjustable independent of the other two negative resistors , by { TAIL ~ means } of hardware or software
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