Butterfly valve

1. A butterfly valve configured to be placed on a pipe between a vacuum chamber with a pressure sensor and a vacuum pump, and configured to control pressure in the vacuum chamber, the butterfly valve comprising:

a motor coupled to a rotary shaft and including an encoder;

a butterfly valve element configured to rotate about the rotary shaft in an opening direction and a closing direction; and

a control unit configured to adjust an opening degree of the butterfly valve element based on a pressure value of the vacuum chamber, the pressure being detected by the pressure sensor, wherein

the control unit includes an abnormal vibration detection program configured to determine whether abnormal vibration is occurring in the motor based on a variation amount of the opening degree of the butterfly valve element within a predetermined time period, the variation amount being calculated based on a pulse output from the encoder, while at least the pressure value of the vacuum chamber is stable,

the predetermined time period is shorter than a cycle of a minute variation in a normal state, which is a variation of the opening degree of the butterfly valve element to keep the pressure in the vacuum chamber stable, and

when a difference between (i) an actual variation amount corresponding to a difference between an opening degree of the butterfly valve element at a starting time point of the predetermined time period and an opening degree of the butterfly valve element at an ending time point of the predetermined time period and (ii) an accumulated variation amount obtained by accumulation of the variation amount of the butterfly valve element from the starting time point to the ending time point is equal to or larger than a predetermined threshold, the abnormal vibration detection program is configured to distinguish minute variation of the opening degree of the butterfly valve element caused when the motor is abnormally vibrating from the minute variation in the normal state, and determine that the abnormal vibration is occurring in the motor.

2. The butterfly valve according to claim 1 , wherein

the abnormal vibration detection program is configured to calculate first number of pulse counts for calculation of the actual variation amount and second number of pulse counts for calculation of the accumulated variation amount based on a pulse output by the encoder,

the first number of pulse counts is calculated by counting up when the butterfly valve element rotates in the opening direction or closing direction or by counting down when the butterfly valve element rotates in an opposite direction to the direction for which the first number of pulse counts is counted up,

the actual variation amount is a difference between the first number of pulse counts at the starting time point and the first number of pulse counts at the ending time point,

the second number of pulse counts is calculated by counting up irrespective of the direction of rotation of the butterfly valve element, and

the accumulated variation amount is a difference between the second number of pulse counts at the starting time point and the second number of pulse counts at the ending time point.

3. The butterfly valve according to claim 2 , wherein

when a determination that abnormal vibration is occurring in the motor is consecutively made a predetermined number of times within multiple consecutive predetermined time periods, the abnormal vibration detection program is configured to give a warning that abnormality has occurred in the motor.

4. The butterfly valve according to claim 3 , wherein the motor is a servo motor.

5. The butterfly valve according to claim 2 , wherein the motor is a servo motor.

6. The butterfly valve according to claim 1 , wherein

when a determination that abnormal vibration is occurring in the motor is consecutively made a predetermined number of times within multiple consecutive predetermined time periods, the abnormal vibration detection program is configured to give a warning that abnormality has occurred in the motor.

7. The butterfly valve according to claim 6 , wherein the motor is a servo motor.

8. The butterfly valve according to claim 1 , wherein the motor is a servo motor.