Servomotor control circuit

A servomotor control circuit for a system including a servomotor for driving a control object, a main control apparatus for producing a servomotor drive command including a target stop position of the servomotor, and a rotation position detection section provided at an output shaft of the servomotor for detecting a rotation position of the servomotor, the servomotor control circuit comprising:

• a receiver section for receiving the servomotor drive command from the main control apparatus;
• a drive control section for controlling drive of the servomotor so that the rotation position of the servomotor attains the target stop position;
• a rotation position change detection section for detecting a change in the rotation position of the servomotor;
• and
• an operation limit check section for checking, based on the change in the rotation position of the servomotor, whether the servomotor has attained the operation limit position, wherein
• the drive control section stops drive of the servomotor without reception of a servomotor drive stop command produced from the main control apparatus, when the servomotor is determined as having attained the operation limit position;
• the operation limit check section determines that the servomotor has attained the operation limit position, when the rotation position is different from the target stop position and the rotation position remains the same for a predetermined period;
• the rotation position change detection section includes a sampling section, a position comparison section and a coincidence count storing section;
• the sampling section samples the rotation position detected by counting pulse signals generated in correspondence to the rotation position of the servomotor by the rotation position detection section, the sampling section sampling the rotation position at a predetermined detection timing, which is different from a detection timing of the rotation position;
• the position comparison section compares a previous rotation position sampled previously with a present rotation position sample presently to check whether both rotation positions coincide each other;
• the coincidence count storing section stores a comparison result of the position comparison section as a coincidence count indicative of the number of coincidence of the rotation positions, the coincidence count storing section incrementing and decrementinq by one the coincidence count when the previous rotation position and the present rotation position coincide each other and differ each other, respectively;
• and
• the operation limit check section determines that the servomotor has attained the operation limit position when the coincidence count exceeds a predetermined value.