Measuring-while-drilling method and system having a digital motor control

摘要

A measuring-while-drilling system has a digitally implemented motor speed control circuit for controlling a downhole, motor-driven acoustic signal generator. The acoustic generator is motor driven at speeds for imparting to well fluid an acoustic signal having phase states representative of encoded data derived from measured downhole conditions. The digital motor control circuit drives the motor at a substantially constant, carrier frequency producing speed in the absence of data of one logic state and temporarily changes the speed of the motor to effect a predetermined phase change in the carrier signal upon data of the predetermined logic state. For returning the motor speed to the carrier frequency producing speed during phase changes, the digital motor control circuit includes a first digital integrating circuit for providing a first digital signal having a value indicative of the constant value of the carrier frequency integrated over a time period beginning substantially upon the occurrence of the particular data; it has a second digital integrating circuit for providing a second digital signal having a value indicative of the instantaneous speed of the acoustic generator integrated over the time period; and it has a digital comparator which is responsive to the first and second digital signals for generating a control signal effective to change the speed of the motor when the difference between the first and second digital signals reaches a predetermined value.