1. Technical Field
The present invention relates to a motor controller that detects an abnormality in a power transmission unit arranged between a main spindle and a motor driving the main spindle of a machine tool for processing a workpiece.
2. Related Art
Machine tools with a tool attached to a main spindle are widely used. In such machine tools, driving power from a motor is transmitted to the main spindle through a power transmission unit and rotation of the main spindle allows the tool to process a workpiece.
The power transmission unit includes, for example, a belt and pulleys. Deterioration of the belt over time causes considerable slipping due to looseness, breakage of the belt, etc., and therefore the driving power may not be transmitted appropriately. In order to detect such conditions as soon as possible or prevent the problems in advance, methods have conventionally been proposed that judge the abnormality of a power transmission unit.
For example, in Japanese Unexamined Patent Publication No. S61-103750, a non-contact-type speed sensor disposed near a belt detects the speed of a main spindle, and when a difference between the speed of the main spindle and the speed of a motor exceeds an allowance, it is judged that the power transmission unit has an abnormality.
In addition, Japanese Unexamined Patent Publication No. 2010-137310 discloses a first judgment unit judging that a belt has abnormality when a load of a motor is in a predetermined range and a second judgment unit operated when the first judgment unit judges the abnormality in the belt. The second judgment unit detects the state of another component different from the motor and thereby judges that there is abnormality in the belt. The first judgment unit, for example, monitors an electric current flowing through the motor or includes a torque meter. The second judgment unit includes, for example, a deflection sensor for monitoring the deflection of a tool attached to a main spindle.
However, the technique of the former Patent Literature (No. 61-103750) requires an additional speed sensor, and the technique of the latter Patent Literature (No. 2010-137310) requires a torque meter, a deflection sensor, etc. The addition of a speed sensor, etc., results in an increase in cost.
The present invention has been accomplished in view of such circumstances, and it is an object of the invention to provide a motor controller capable of judging abnormality of a power transmission unit without adding a speed sensor, a torque meter, or the like.