DIGITAL MOTOR CONTROL METHOD AND APPARATUS FOR MEASURING-WHILE-DRILLING

摘要

A measuring-while-drilling system includes motor speed detection during encoding for returning control to a phase locked loop circuit when the rate of an acoustic signal generator has precisely returned to a carrier frequency producing rate. The measuring-while-drilling system includes the acoustic generator which has a moveable member disposed within the flow of drilling fluid and which is driven at speeds for imparting to the drilling fluid an acoustic signal having phase states representative of encoded data signals derived from measured downhole conditions. The phase locked loop circuit drives the moveable member at a substantially constant rate to thereby effect a substantially constant carrier frequency in the acoustic signal. A frequency changing control circuit is provided for temporarily changing the rate of the member to effect a predetermined phase change in the carrier signal according to the data. The rate of movement of the member is changed in a first direction until a prescribed amount of the predetermined phase change is achieved, and the rate of movement is changed in the opposite direction for accumulating the remainder of the predetermined phase change. A rate detection circuit monitors a signal representing the rate of movement of the member for generating an end-of-return signal which stops the rate of movement in the opposite direction precisely when the rate of the member has been returned to the constant frequency producing rate.