{TAIL ~ 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 {HEAD ~ the other two negative resistors}, by means of hardware or software 
{ TAIL ~ 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 { HEAD ~ the other two negative resistors } , by means of hardware or software
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