Position control system

A position control system comprising:

• a stepping motor;
• an encoder for producing polyphase signals having different phases from one another according to a rotation of said stepping motor;
• current position signal generating means for generating from said polyphase signals a current position signal indicative of a current position of said stepping motor;
• first reference position generating means for generating an open loop reference position command for subjecting said stepping motor to an open loop position control;
• second reference position generating means for generating a closed loop reference position command for subjecting said stepping motor to a closed loop position control;
• error calculating means for calculating a difference between said closed loop reference position command and said current position signal and for outputting an error signal indicative of the thus calculated difference;
• operating electric angle calculating means for subjecting said error signal to a compensation calculation for said closed loop position control to obtain an operating electric angle signal indicative of a torque of said stepping motor;
• angle adjusting means for subjecting said stepping motor to reciprocal movements during said open loop position control using said open loop reference position command to detect a torque angle error caused by a positional error between said stepping motor and said encoder, and for generating an angle adjusting signal proportional to said torque angle error;
• control signal generating means for generating a control signal according to said open loop reference position command during said open loop position control and for generating said control signal according to said operating electric angle signal, said current position signal and said angle adjusting signal during said closed loop position control;
• current command generating means for generating current commands according to said control signal, said current commands each expressed by a predetermined mathematical function, and said current commands being indicative of polyphase currents to be supplied to said stepping motor;
• and
• driving means responsive to said current commands for generating said polyphase currents and for supplying said polyphase currents to said stepping motor to drive said stepping motor.