Apparatus for driving spindles of machine tools with induction motors

Apparatus for driving a spindle of a machine tool or the like with an induction motor comprising:

• pulse generating means coupled to said spindle for generating a series of pulses at a short interval;
• a reversible counter for counting the number of said pulses;
• gate means which samples a count of said reversible counter with an interval shorter than a response speed of a spindle driving system including said induction motor;
• a first register connected to said reversible counter through said gate means for storing the count of said counter;
• a second register to which a content of said first register is transferred at the time of sampling;
• first calculating means for calculating difference between contents of said first and second registers;
• input means delivering a speed instruction signal and an angular position instruction signal of said spindle;
• a first adder responsive to said angular position instruction signal and an output signal from said first register for forming a position error signal;
• a transfer switch for selecting either one of said speed instruction signal and said position error signal;
• a second adder for adding together a signal selected by said transfer switch and an output signal of said first calculating means for forming a torque instruction signal representing a torque to be produced by said induction motor;
• and
• second calculating means for sequentially calculating values of currents supplied to said induction motor based on an output signal of said first register and said torque instruction signal.