Patent Number: 
Section: claims

1. A collimator control method for controlling a collimator thatincludes motor-driven blades, comprising the steps of:using an encoder to encode rotations made by a motor;using a decoder to decode a signal sent from the encoder;using a sensor to sense whether the blades exist at zero positions;positioning, when the collimator enters a homing phase in a first control state, the blades on the basis of a blade sense signal sent from the sensor, and if the sensor has not sensed the blades but the rotational velocity of the motor has decreased to fall below a predetermined velocity, switching the first control state to the second control state;switching the first control state to the second control state when the collimator enters a working phase in the first control state and when an overshoot made by the blades returning to the zero positions exceeds a predetermined limit;forcibly reversing, when the second control state is designated, the rotating direction of the motor so as to withdraw the blades, and if an operator performs a manipulation to reset the collimator to the first control state, resetting the collimator to the first control state based on tentative zero positions or the first control state based on the sense signal sent from the sensor according to whether the sensor has failed. 2. The collimator control method according to claim 1, wherein the rotational velocity is obtained as a measured value of a time required for the motor to make a unit rotation. 3. The collimator control method according to claim 2, wherein the measured value is a value measured by counting the number of clock pulses. 4. The collimator control method according to claim 3, wherein the pulse-repetition rate of clock pulses is expressed as follows:fbase=Rcoff·V0·Ncyc/60where Rcoff denotes a resolution coefficient, V0 denotes the number of rotations made by the motor with no load imposed on the motor, and Ncyc denotes the number of encoder pulses per rotation of the motor. 5. The collimator control method according to claim 1, wherein an indication signifying that the first control state has been switched to the second control state is displayed. 6. A collimator control apparatus for controlling a collimator that includes motor-driven blades, comprising:an encoder that encodes rotations made by a motor;a decoder that decodes a signal sent from the encoder;a sensor that senses whether the blades exist at zero positions; anda control device that when the collimator enters a homing phase in a first control state, positions the blades on the basis of a blade sense signal sent from the sensor, that if the sensor has not sensed the blades but the rotational velocity of the motor has decreased to fall below a predetermined velocity, switches the first control state to the second control state, that when the collimator enters a working phase in the first control state, if an overshoot made by the blades returning to the zero positions exceeds a predetermined limit, switches the first control state to the second control state, that when the second control state is designated, forcibly reverses the rotating direction of the motor so as to withdraw the blades, and that if an operator performs a manipulation to reset the collimator to the first control state, resets the collimator to the first control state based on tentative zero positions or the first control state based on the sense signal sent from the sensor according to whether the sensor has failed. 7. The collimator control apparatus according to claim 6, wherein the control device obtains the rotational velocity as a measured value of a time required for the motor to make a unit rotation. 8. The collimator control apparatus according to claim 7, wherein the measured value is a value measured by counting the number of clock pulses. 9. The collimator control apparatus according to claim 8, wherein the pulse-repetition rate of clock pulses is expressed as follows:fbase =Rcoff·V0·Ncyc/60where Rcoff denotes a resolution coefficient, V0 denotes the number of rotations made by the motor with no load imposed on the motor, and Ncyc denotes the number of encoder pulses per rotation of the motor. 10. The collimator control apparatus according to claim 6, further comprising a display device on which an indication signifying that the first control state has been switched to the second control state is displayed. 11. A radiography system including an X-ray tube, a collimator that includes motor-driven blades and reshapes an X-ray beam irradiated from the X-ray tube to a subject of radiography, and a control apparatus that controls the collimator, wherein the control apparatus comprises:an encoder that encodes rotations made by a motor;a decoder that decodes a signal sent from the encoder;a sensor that senses whether the blades exists at zero positions; anda control device that when the collimator enters a homing phase in a first control state, positions the blades on the basis of a blade sense signal sent from the sensor, that if the sensor has not sensed the blades but the rotational velocity of the motor has decreased to fall below a predetermined velocity, switches the first control state to the second control state, that when the collimator enters a working phase in the first control state, if an overshoot made by the blades returning to the zero positions exceeds a predetermined limit, switches the first control state to the second control state, that when the second control state is designated, forcibly reverses the rotating direction of the motor so as to withdraw the blades, and that if an operator performs a manipulation to reset the collimator to the first control state, resets the collimator to the first control state based on tentative zero positions or the first control sate based on the sense signal sent from the sensor according to whether the sensor has failed. 12. The radiography system according to claim 11, wherein the control device obtains the rotational velocity as a measured value of a time required for the motor to make a unit rotation. 13. The radiography system according to claim 12, wherein the measured value is a value measured by counting the number of clock pulses. 14. The radiography system according to claim 13, wherein the pulse-repetition rate of clock pulses is expressed as follows:fbase =Rcoff·V0·Ncyc/60where Rcoff denotes a resolution coefficient, V0denotes the number of rotations made by the motor with no load imposed on the motor, and Ncyc denotes the number of encoder pulses per rotation of the motor. 15. The radiography system according to claim 11, further comprising a display device on which an indication signifying that the first control state has been switched to the second control state is displayed.