| 摘要 |
MEASURING-WHILE DRILLING SYSTEM AND METHOD HAVING ENCODER WITH FEEDBACK COMPENSATION A measuring-while-drilling system having a downhole, motor-driven acoustic signal generator includes a motor speed control circuit featuring feedback compensation which accounts for varying loading conditions on the motor. The acoustic signal generator is motor driven at speeds for imparting to well fluid an acoustic signal having phase states representative of data derived from measured downhole conditions. The motor control circuit includes circuitry which forms a phase locked loop for driving the motor at a reference phase and at a substantially constant carrier frequency producing speed in the absence of data of one logic state and also includes circuitry for temporarily changing the speed of the motor, according to a pre-programmed function, to effect, a predetermined amount of phase change in the carrier signal upon data of the predetermined logic state. Upon occurrence of the data, the motor control circuit takes control away from the phase locked loop and changes the speed of the motor from the carrier frequency producing speed for accumulating a prescribed portion of the predetermined amount of phase change. To accumulate the remainder of the predetermined amount, it then returns the speed of the motor to the carrier frequency producing speed at which time it returns control of the motor to the phase locked loop. During operation feedback from the motor is provided to update the value of the prescribed amount according to loading conditions on the motor. This minimizes the time period required for the phase locked loop circuit to precisely establish the predetermined amount of phase change in the acoustic signal at the carrier frequency. |
