| 摘要 |
<p>A LWD tool (50) is disclosed comprising, an encoder for generating a signal in the borehole fluid flowing therethrough, a brushless DC motor (100) coupled to the encoder, a position sensor (110) coupled to the motor for sensing the rotational position thereof, motor drive electronics (191) coupled to motor for driving the motor, a microprocessor (91) coupled to the position sensor (110) and to the drive electronics (191) for controlling the drive signals to the motor based on the actual and desired positions of the motor, and a magnetic positioner (130) which is coupled to one of the drive shafts (102,124) of the system. By controlling the drive signal to the motor, the speed of the motor is controlled, thus effecting changes in frequency and/or phase of the signal. The magnetic positioner (130), includes inner S magnets (130aS) extending in a first arc, inner N magnets (130aN) extending in a second arc, outer S magnets (130bS) extending in a third arc, and outer N magnets (130bN) extending in a fourth arc, where the inner magnets rotate with the drive shaft. The magnetic positioner (130) is provided to force the rotor (160) of the encoder (18) into an open position relative to the stator (150) in the case of loss of power. The magnetic positioner is also used to aid in the decelaration and acceleration of the encoder during phase shifting. <IMAGE></p> |
