Velocity regulated, open current loop, variable voltage, variable frequency, linear induction motor drive for an elevator car door

A method of driving a linear induction motor, the secondary of which is connected to an elevator car door, comprising:

• cyclically, in each of a series of fixed unit time periods
• a) providing a given number of electrical pulses per unit distance of elevator door travel;
• b) determining the number of pulses provided in said step a);
• c) accumulating the count of pulses determined in said step b) to provide a position signal;
• d) determining average velocity over said unit time period from the number of pulses determined to have been accumulated in said step b);
• e) providing a velocity command as a function of position utilizing the position provided in step c);
• f) providing a velocity error as the difference between the velocity command generated in step e) and the average velocity determined in said step d);
• g) generating a force command as a proportional and integral function of said velocity error;
• h) determining a predetermined desired phase as a function of said force command;
• i) determining the phase differential experienced by the secondary of said linear induction motor as a consequence of the velocity determined in said step d);
• j) providing a total phase signal in response to the summation of phases provided in said steps h) and i);
• k) providing a pulse width signal as a function of the vector sum of a predetermined magnetizing current and said force command, times the sine of said total phase signal;
• and
• l) repetitively in a second series of time periods, connecting an invariant DC voltage to a winding of said linear induction motor primary having the same polarity as the sign of the sine of said total phase signal for a length of time determined by said pulse width signal.