GB1592994A - Motor control method and apparatus for measuring-while-drilling - Google Patents

Description

PATENT SPECIFICATION

Application No 6469/80 ( 22) Filed 27 Sept 1977 Divided out of No 1592993 Convention Application No 727685 Filed 29 Sept 1976 Convention Application No 727686 Filed 29 Sept 1976 Convention Application No 727687 Filed 29 Sept 1976 in (l' ( 33) United States of America (US) ( 44) Complete Specification published 15 July 1981 ( 51) INT CL 3 GO 8 C 23/00 ( 52) Index at acceptance G 4 F 10 XX ( 72) Inventors JAMES I BEARD and GERALD A STROM ( 54) A MOTOR CONTROL METHOD AND APPARATUS FOR MEASURING-WHILE-DRILLING ( 71) We, SCHLUMBERGER TECHNOLOGY CORPORATION, a corporation organized and existing under the laws of the State of Texas 5000 Gulf Freeway, P O Box 1472, Houston, Texas 77001, U S A, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-

This invention relates to data measuring of downhole conditions within wells during drilling and more particularly relates to apparatus and methods for telemetering data in such operations using an acoustic signal transmitted through the drilling fluid during drilling.

Various logging-while-drilling techniques for telemetering data representing downhole conditions during drilling of a well have been suggested One approach uses a technique which imparts an acoustic signal, modulated according to the sensed conditions, to the drilling fluid, i e, the drilling mud, for transmission to the entrance of the well where it is received and decoded by uphole electronics circuitry.

This basic technique is described in detail in U S Patent No 3,309,656, issued March 14, 1967 to Godbey entitled "LoggingWhile-Drilling System " In this system the modulated signal is applied to the drilling fluid using an acoustic signal generator which includes a movable member for selectively interrupting the drilling fluid At least part of the flow of the drilling fluid is through the acoustic generator, and the movable member selectively impedes this flow, transmitting a continuous acoustic wave uphole within the drilling fluid.

The acoustic signal is preferably phase shift keyed modulated, as disclosed in U S.

Patent No 3,789,355, issued January 29, 1974, to Patton entitled "Method and Apparatus For Logging While Drilling " According to phase shift keyed (PSK) modulation, the data derived in response to the sensed downhole condition is initially encoded, into binary format, and the acoustic -signal generator is driven at speeds so that the phase of a constant frequency carrier wave generated in the drilling fluid is indicative of the data In particular, a non-return to zero type PSK mode is used wherein the phase of the carrier signal is changed only upon each receipt of data of a predetermined value For example, for data encoded in binary, the phase of the carrier wave may be changed for each occurrence of a logic 1 data bit.

Ideally the phase change of the carrier signal would be instantaneous upon occurrence of the data of the particular value This is because the downhole telemetering unit is continuously transmitting data to the, uphole receiving instruments where the data in turn is continuously decoded Any delays in effecting the phase change and in returning the acoustic signal to its carrier frequency introduce errors and/or inefficiencies into the system.

As a practical matter, however, the phase of the acoustic signal cannot be changed instantaneously in response to data of the predetermined value Inherent delays are introduced by the physics of the system.

The motor control circuitry which-operates the motor-driven acoustic generator is adjusted accordingly to effect optimum ( 21) ( 62) ( 31) ( 32) ( 31) ( 32) ( 31) ( 32) ( 11) 1 592 994 1,592,994 response of the generator Past proposals, such as those of the above-referenced Godbey and Patton patents, and that of U.S Patent No 3,820,063, issued June 25, 1974 to Sexton et al and entitled "Logging While Drilling Encoder," have proposed several circuits for implementing the motor control circuitry In the Patton and Sexton et al patents, the speed of the motor was to be temporarily varied such that, upon returning of the motor speed back to the carrier frequency producing speed, the desired amount of phase change would be accumulated In the Sexton et al patent, this was accomplished by varying the speed of the motor in a first direction until a predetermined amount of phase shift had been accumulated The motor speed was then returned in the other direction to the carrier frequency producing speed for a predetermined duration of time, thereby attempting to accumulate the remainder of the desired amount of the phase change.

The above proposals failed to overcome the problems associated with changes in the environmental conditions of the loggingwhile-drilling system These variable conditions can deleteriously affect the precision with which the speed of the acoustic generator drive motor is returned to the constant carrier frequency producing speed (the carrier speed) during the phase changing (during modulation) The proposals appeared to suggest tuning of the respective systems such that the return approximated the accumulating of the desired amount of change and approximated terminating the return when the speed of the motor had reached the carrier speed The proposals, however, failed to detect the actual speed of the motor which would allow termination of the return precisely upon reaching the carrier speed In failing to detect the actual motor speed, the proposals failed in providing a system which would allow the return to be in the shortest possible period of time; i e, failed in providing a system which would allow the driving of the drive motor at maximum excitation yet which would obviate undershoot or overshoot of the carrier speed The proposals relied on a separate phase and frequency adjusting and maintaining circuitry to adjust the phase and frequency to the proper values after approximate return to carrier speed to account for the undershoot and overshoot.

Such adjusting and maintaining circuitry, however, required a relatively long time to change the motor speed any substantial amount, thereby failing to minimize the period of the return By failing to minimize the period of the return, the proposals either allowed inaccuracies to be introduced into the system or provided an unnecessarily slow encoding/data transmission system.

In addition to the motor speed problem just discussed, the quality of the modulation also suffers For example, changes in the loading on the acoustic generator drive motor caused by changes in the pressure or the flow rate or the viscosity or density of the drilling fluid vary the length of time needed to return the motor speed back to the carrier frequency producing speed This time variance varies the amount of phase accumulated during the return to the carrier frequency producing speed, causing a longer period of time to be needed in generating the proper amount of phase change at the carrier frequency This longer period of time allows the introduction of inaccuracies into the system and/or decreases the rate of data transmission which otherwise would be obtainable.

The approaches proposed in the abovementioned patents involve an analog implementation of the motor control circuitry Because the motor control circuitry operates at a relatively low frequency, the analog approach has resulted in a system which may operate at a less than optimum data encoding/decoding rate Furthermore, such analog circuitry suffers from the inherent disadvantages of instability over wide ranges of temperature, resulting in a less than optimally dependable system More specifically, normal temperatures encountered within a borehole during drilling vary from 250 C to greater than 1750 C, causing inherent changes in the device characteristics of the analog circuitry Furthermore, the analog approach suffers due to the rugged environment encountered during drilling conditions The extreme vibrations and shock received by the analog circuitry not only reduce its longevity but also tend to render the circuitry out of adjustment.

The various aspects of the invention are directed to alleviating at least some of the above noted disadvantages.

According to one aspect of the invention, there is provided a measuring-while-drilling method for effecting downhole measurements in a well during drilling thereof and for transmitting to the surface an acoustic signal respresentative of said measurements through fluid within the well, wherein said acoustic signal is produced by a motor-driven acoustic signal generator whose speed is momentarily changed from a normally constant rate providing a carrier frequency signal, in dependence upon said measurements, to effect a selected phase change for modulating said carrier signal, the method comprising the steps of:

(a) changing the generator speed away 1,592,994 from the normal rate to accumulate a portion of the selected phase change; (b) returning the generator speed to the normal rate to thereby accumulate the remainder of said selected phase change; (c) generating a control signal for terminating step (a) when the phase change accumulated in step (a) reaches a prescribed value; and (d) adjusting said prescribed value in response to the phase change accumulated during at least one preceding step (b) so as to tend to maintain the total amount of the actual phase change accumulated during each pair of successive steps (a) and (b) substantially equal to the selected phase change.

According to another aspect of the invention, there is provided a measuringwhile-drilling system for effecting downhole measurements in a well during drilling thereof and for transmitting to the surface an acoustic signal representative of said measurements through fluid within the well with a motor-driven acoustic signal generator arranged to operate at a normally constant speed for providing a carrier frequency which is momentarily changed in dependence upon said measurement to effect selected phase change for modulating said acoustic signal, said system comprising:

first means for changing the generator speed away from the normal rate to accumulate a portion of said selected phase change; second means for returning the generator speed to the normal rate to thereby accumulate the remainder of said selected phase change; further comprising:

means for generating a control signal for terminating the operation of said first means when the amount of phase change accumulated by the operation of said first means reaches a prescribed value; and means for adjusting said prescribed value in response to the phase change accumulated by at least one preceding operation of the second means so as to tend to maintain the total amount of the actual phase change accumulated during each pair of successive operations of the first and second means substantially equal to the selected phase change.

The invention will now be described by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic drawing showing a general well drilling and data measuring system according to the invention; Figure 2 is a block diagram of downhole telemetering apparatus utilized in the system of Figure 1; Figure 3 is a circuit schematic of logic circuitry utilized within the downhole telemetering apparatus of Figure 2; Figure 4 is a set of exemplary waveforms illustrating operation of the downhole telemetering apparatus; and Figure 5 is a functional block diagram depicting targeting compensation circuitry utilized in the apparatus of Figure 3.

Referring now to the drawings, Fig 1 shows a well drilling system 10 in association with a measuring-while-drilling system 12 embodying the invention For convenience, Figure 1 depicts a land based drilling system, but it is understood that a sea based system is also contemplated.

As the drilling system 10 drills a welldefining borehole 14, the measuring-whiledrilling system 12 senses downhole conditions within the well and generates an acoustic signal which is modulated according to data generated to represent the downhole conditions The acoustic signal is imparted to drilling fluid, commonly referred to as drilling mud, in which the signal is communicated to the surface of the borehole 14 At or near the surface of the borehole 14 the acoustic signal is detected and processed to provide recordable data representative of the downhole conditions This basic system is now well-known and is described in detail in the above referred U S Patent No.

3,309,656 to Godbey.

The drilling system 10 is conventional and includes a drill string 20 and a supporting derrick (not shown) represented by a hook 22 which supports the drill string within the borehole 14.

The drill string 20 includes a bit 24, one or more drill collars 26, and a length of drill pipe 28 extending into the hole the pipe 28 is coupled to a kelly 30 which extends through a rotary drive mechanism 32.

Acutation of the rotary drive mechanism 32 (by equipment not shown) rotates the kelly which in turn rotates the drill pipe 28 and the bit 24 The kelly 30 is supported by the hook via a swivel 34.

Positioned near the entrance to the borehole 14 is a conventional drilling fluid circulating system 40 which circulates drilling fluid, commonly referred to as mud, downwardly into the borehole 14 The mud is circulated downwardly through the drill pipe 28 during drilling, exits through jets in the bit 24 into the annulus and returns uphole where it is received by the system 40.

The circulating system 40 inlcudes a mud pump 42 coupled to receive the mud from a mud pit 44 via a length of tubing 46 A desurger 48 is coupled to the exit end of the mud pump 42 for removing and surges in the flow of the mud from the pump 42, thereby supplying a continuous flow of mud at its output orifice 50 A mud line 52 1,592,994 couples the output orifice 50 of the desurger to the kelly 30 via a gooseneck 54 coupled to the swivel 34.

Mud returning from downhole exits near the mouth of the borehole 14 from an aperture in a casing 56 which provides a flow passage 58 between the walls of the borehole 14 and the drill pipe 28 A mud return line 60 transfers the returning mud from the aperture in the casing 56 into the mud pit 44 for recirculation.

The measuring-while-drilling system 12 includes a downhole acoustic signal generating unit 68 and an uphole data receiving and decoding system 70 The acoustic signal generating unit 68 senses the downhole conditions and imparts encoded acoustic signals to the drilling fluid The acoustic signal is transmitted by the drilling fluid to the uphole receiving and decoding system 70 for processing and display.

To this end, the receiving and decoding system 70 includes a signal processor 72 and a record and display unit 74 The processor 72 is coupled by a line 76 and a pressure transducer 78 to the mud line 52 The encoded acoustic signal transmitted uphole by the drilling fluid is monitored by the transducer 78, which in turn generates electrical signals to the processor 72 These electrical signals are decoded into meaningful information representative of the downhole conditions; and the decoded information is recorded and displayed by the unit 74.

One such uphole data receiving and decoding system 70 is described in U S.

Patent No 3,886,495 to Sexton et al, issued May 27, 1975, entitled "Uphole Receiver For Logging-While-Drilling System.

The downhole acoustic signal generating unit 68 is supported within one of the downhole drill collars 26 by a suspension mechanism 79 and generally includes a modulator 80 having at least part of the flow of the mud passing through it The modulator 80 is controllably driven for selectively interrupting the flow of the drilling fluid to thereby impart the acoustic signal to the mud A cartridge 82 is provided for sensing the various downhole conditions and for driving the modulator 80 accordingly The generating unit 68 also includes a power supply 84 for energizing the cartridge 82 A plurality of centralizers are provided to position the modulator 80, the cartridge 82, and the supply 84 centrally within the collar 26.

The power supply 84 is now well-known in the art and includes a turbine 86 positioned within the flow of the drilling fluid to drive the rotor of an alternator 88.

A voltage regulator 90 regulates the output voltage of the alternator 88 to a proper value for use by the cartridge 82.

The modulator 80 is also well-known in the art It includes a movable member in the form of a rotor 92 which is rotatably mounted on a stator 94 At least part of the flow of the mud passes through apertures in the rotor 92 and in the stator 94, and rotation of the rotor selectively interrupts flow of the drilling fluid when the apertures are in misalignment, thereby imparting the acoustic signal to the drilling fluid The rotor 92 is coupled to gear reduction drive linkage 96 which drives the rotor The cartridge 82 is operably connected to the linkage 96 for rotating the rotor 92 at speeds producing an acoustic signal in the drilling fluid having ( 1) a substantially constant carrier frequency which defines a reference phase value, and ( 2) a selectively produced phase shift relative to the reference phase value at the carrier frequency The phase shift is indicative of encoded data values representing the measured downhole conditions.

In the preferred embodiment the drive linkage 96 and the designs of the rotor 92 and stator 94 are chosen to generate 1/5 of a carrier cycle in the acoustic signal for each revolution of the motor 102.

A suitable modulator 80 is shown and described in detail in U S Patent No.

3,764,970 to Manning which is assigned to the assignee of this invention Other suitable modulators 80 are described in the above-referenced Patton and Godbey patents, as well as in "Logging-WhileDrilling Tool" by Patton et al, U S Patent No 3,792,429, issued February 12, 1974, and in "Logging-While-Drilling Tool" by Sexton et al, U S Patent No 3,770,006, issued November 6, 1973.

Referring now to the cartridge 82, it includes one or more sensors 100 and associated data encoding circuitry 101 for measuring the downhole conditions and generating encoded data signals representative thereof For example, the sensors 100 may be provided for monitoring drilling parameters such as the direction of the hole (azimuth of hole deviation), weight on bit, torque, etc The sensors 100 may be provided for monitoring safety-parameters, such as for detecting over pressure zones (resistivity measurements) and fluid entry characteristics by measuring the temperature of the drilling mud within the annulus 58 Additionally, radiation sensors may be provided, such as gamma ray sensitive sensors for discriminating between shale and sand and for depth correlation.

The data encoding circuitry 101 is conventional and includes a multiplex arrangement for encoding the signals from the sensors into binary and then serially transmitting them over a data line A suitable multiplex encoder arrangement is 1,592,994 5 disclosed in detail in the above referenced Sexton et al patent,' U S Patent No.

3,820,063 The cartridge 82 also includes a motor 102 coupled to the linkage 96, and motor control circuitry 104 for controlling the speed of the motor 102 for rotating the rotor 92 of the modulator 80 at the proper speeds to effect the desired acoustic signal modulation The motor 102 is a conventional two-phase AC induction motor for the motor 102 is not critical, as other types of motors, such as a d c servomotor, are suitable.

The motor control circuitry 104 is shown in relation to the motor 102, to the sensors and encoding circuitry 101 and to the modulator 80 in Fig 2 The motor control circuitry 104 includes circuitry ( 1) for maintaining the substantially constant carrier frequency of the acoustic signal transmitted in the drilling mud at the proper phase and ( 2) for changing the frequency of the acoustic signal and returning it to the carrier frequency to thereby change the phase thereof by a predetermined value as rapidly as possible in response to the encoded data In the preferred embodiments wherein the data from the sensors 100 is encoded in binary code, the phase change is one of 180 degrees.

The motor control circuitry 104 includes a motor switching circuit 110, such as a conventional dc-ac inverter, for supplying two-phase power to the two-phase motor 102.

A phase signal generator 112 and a voltage controlled oscillator (VCO) circuit 114 are provided to generate to the motor switching circuit 110 a pair of phase signals OA, SIB and their complements i, <B.

The phase signals are 90 degrees out of phase from one another The voltage control oscillator circuit 1 14 is conventional, and the phase signal generator 112 includes conventional circuitry for generating approximately 50 percent duty cycle wave forms and their complements In the preferred embodiment the VCO circuit 114 operates at slightly higher than 240 Hertz during carrier frequency operation This frequency accounts for inherent "slip" of the induction motor 102 and provides a frequency multiplication factor of four necessary for the phase signal generator 112 to provide the phase signals OA, OB at the desired 60 Hertz frequency For convenience of description, the slip of the motor will hereafter be assumed negligible.

In the preferred embodiment the circuitry for maintaining the carrier frequency and phase of the acoustic signal in the absence of selected data signals, in combination with the motor switching circuit 110, the phase signal generator 112, and the voltage controlled oscillator circuit 114, advantageously implements a phase locked loop circuit.

The phase and frequency maintaining circuitry includes a tachometer 120 coupled to the motor 102 for producing a series of pulses whose repetition rate is indicative of the frequency at which the motor 102 is driven In the preferred embodiment the tachometer 120 is selected to generate six cycles per revolution of the motor This ratio in combination with the design of the modulator 80, the design of the drive linkage 96, and the 60 Hz speed of the motor 102, results in the generation of an acoustic signal within the drilling mud having a 12 Hz carrier frequency and in the generation of a tachometer output signal C O T having a 360 Hz frequency.

A tachometer signal conditioning circuit 122 is coupled to the output of the tachometer 120 for providing a relatively low frequency loop frequency signal, c L, and a relatively, high frequency motor frequency signal co M For example, the loop frequency signal w OL is produced at a 24 Hz frequency and the motor frequency signal Co L is produced at a 720 Hz frequency when the motor is operating at 60 Hz The conditioning circuit 122 is conventionally implemented using zero crossing circuitry and frequency multiplying/dividing circuitry:

Completing the phase locked loop circuitry is a phase detector circuit 124 The phase detector circuit 124 is responsive to the loop frequency signal w Ls and to a 24 Hertz loop reference frequency signal GOLF to selectively generate a VCO control signal on a line 126 which is operatively coupled to the VCO circuit 114 via a loop switch 128 The phase detector 124 is conventional and may include a set/reset flip-flop (not shown) responsive to the signals (OLD 6 LF and a low pass filter (not shown) coupled to the output of the flip-flop The output of the detector 124 generates the VCO control signal as a function of the difference per loop cycle between the WL and,LF signals to be indicative of the motor 102 deviating from the carrier frequency or phase In response to the control signal on the line 126, the VCO circuit 114 changes the excitation frequency supplied to the motor 102 via the inverter 110 to return the motor to and maintain it in phase and frequency lock.

The above referred Sexton et al patents, U.S Patent No 3,870,063, shows and describes another phase locked loop circuit operating on similar principles.

The circuitry for changing the speed of the motor 102 to thereby change the phase of the acoustic signal in response to data 1,592,994 6 1,592994 6 from the sensors 100 is implemented digitally in the illustrated and preferred embodiment The digital implementation effects a frequency and phase change in the acoustic signal rapidly yet in an extremely accurate manner The size of the package for the motor control circuitry has been reduced over that of previously proposed analog systems due to the digital implementation, and reliability over wide environmental ranges is achieved.

However, the invention is also suitably implemented in analog systems if so desired.

As will be described, the circuitry for changing the speed of the motor operates initially to decelerate the speed of the motor 102 and then to accelerate it for accumulating the total phase change of 180 degrees Although an acceleration/ deceleration sequence is operable, the deceleration/acceleration sequence results in the motor 102 operating in a higher torque range and thus in the modulating of the acoustic signal more predictably and in a shorter period of time.

The speed changing circuitry operates the switch 128 and a set of acceleration and deceleration switches 130, 132, which respectively control the voltage input to the VCO circuit 114 In the illustrated embodiment, the acceleration switch 130 has one terminal and commonly connected to the input of the VCO circuit 114 and to one terminal of the loop switch 128 It has its other terminal commonly coupled to a ramp voltage producing network and to the deceleration switch 132 via a resistor Rl.

The ramp voltage need not be limited to a linearly changing voltage For example it may change substantially exponentially with time As illustrated an RC timing circuit comprising the series connection of a resistor R 2 and capacitor C between a voltage V, and circuit ground produces an exponentially increasing range voltage.

Accordingly, when the loop switch 128 is open, the acceleration switch 130 is in the closed position and the deceleration switch 132 is opened, the input to the VCO circuit 114 is a ramp voltage, effecting an output from the VCO circuit 114 which increases with time and thus effecting acceleration of the motor which is an increasing function with time This assures that the phase change in the acoustic signal is accomplished as rapidly as possible.

The deceleration switch 132 has one terminal commonly connected to the resistor Rl and thus to the switch 130 It has its other terminal connected to circuit ground When the acceleration switch 130 is closed and the deceleration switch 132 is in the closed position, the capacitor C, which had been discharged through the resistor RI to circuit ground by closing of the switch 132, remains discharged In the preferred embodiment upon closing of the switch 130, the discharged capacitor C produces a voltage level at the input of the VCO circuit 114 which causes the output of the VCO circuit 114 to step down to approximately 180 Hz from its otherwise constant carrier frequency producing output of approximately 240 Hz.

The speed changing circuitry includes a targeting phase accumulator 140, a motor frequency detector 142 and a control logic circuit 144 In response to input signals from the targeting phase accumulator 140 and from the motor frequency detector 142, the control logic circuit 144 generates a set of control signals X, X, and Z on a set of lines 145, 146, 147 to the switches 128, 130, 132 respectively These signals are generated in a sequence, approximately initiated by data from the sensors 100, which: ( 1) initially opens the loop switch 128 to take control away from the phase lock loop; ( 2) closes the acceleration switch (the deceleration switch 132 already having been closed) to cause a low voltage level to be supplied to the VCO circuit 114 to thereby cause rapid deceleration of the motor 102, and thus change the frequency of the acoustic signal to approximately 180 Hz; ( 3) to open the deceleration switch 132while leaving closed the acceleration switch to begin acceleration of the speed of the motor 102 back toward the carrier frequency producing speed; and, ( 4) thereafter to open the acceleration switch and to close the loop switch 128 to return control of the motor 102 back to the phase lock loop when the carrier frequency producing speed has been achieved by the motor 102.

In more detail and referring to the waveforms depicted in Figure 4, the targeting phase accumulator 140 generates a TPA control signal on the line 148 a period of time, referred to as the integrating period IP, corresponding to the accumulation of a predetermined amount of phase change, after a transition start (hereafter TS) timing signal has been generated on a line 149 At the beginning of one integrating period, IP, the logic control circuit 144 is actuated to generate the X, X, and Z control signals to open the loop switch 128 and to close the acceleration switch 130 and to maintain closure of the deceleration switch 132, thereby causing deceleration of the motor 102.

In effect, the targeting phase accumulator 140 is a differential integrating circuit That is, during the integrating period, the targeting phase accumulator effectively is integrating the difference between a 720 Hertz motor reference 1,592,994 1,592,994 frequency signal, CO-MR, on a line 150 and the motor frequency signal, o)M, on a line 152.

In the illustrated embodiment, the signals CM and,o MR are integrated The difference between these integrated signals produces an indication of the amount of phase which is being accumulated due to speed changes of the motor 102 When the difference between the integrated values of the signals on the lines 150, 152 reaches a predetermined value due to the deceleration of the motor speed, the targeting phase accumulator 140 generates the TPA signal on the line 146, causing the control logic circuit 144 to open the switch 132 This permits the beginning of the rapid acceleration of the speed of the motor back toward the carrier frequency producing speed.

As above indicated for the illustrated embodiment, the motor reference frequency signal (OMR on the line 150 is a 720 Hz signal This results in sixty cycles of the motor reference frequency signal being produced for each cycle of the 12 Hz carrier frequency Accordingly, thirty cycles of the ct WMR signal correspond to 180 degrees of phase of the 12 Hz carrier.

Since a finite time is required to return the motor speed to the 60 Hz, carrier frequency producing speed, phase shift additional to that effected by the deceleration is accumulated during the return With a typical load on the motor, it has been ascertained that approximately 65 degrees of carrier phase change is accrued in the process of returning the speed of the motor 102 back from the 45 Hz frequency to the carrier frequency producing speed of 60 Hz Accordingly, it is necessary to accumulate 115 degrees of phase change in the targeting phase accumulator 140 prior to the generation of the TPA signal and thus of the beginning of the acceleration of the speed of the motor back towards 60 Hz.

Since 30 cycles of the W 9 MR signal correspond to 180 degrees of carrier phase shift, the targeting phase accumulator 140 needs to accumulate 115/180 x 30 = 19 cycles or counts Eqn I as the difference between the integrated wo M and integrated cl)MR signals The calculation in Eqn 1 is conditioned upon the characteristic linear relationship between phase loss and phase gain of the acoustic signal as a function of the changing of the motor frequency signal 6 M The amount of additional phase accumulated due to return of the motor speed varies with motor loading However, because the phase and frequency maintaining circuitry operates with inputs at twice the carrier frequency of 12 Hz, it acts to pull the motor speed into lock at 180 degrees of phase change even when the phase changing circuitry results in a range of 91-269 degrees of phase change Also, as will be described subsequently, the targeted value of 115 degrees of phase change is updated and modified according to loading conditions on the motor 102.

This updating allows the frequency changing circuitry to effect nearly the precise amount of phase change desired when it returns the speed of the motor back to substantially the carrier frequency producing speed, at which time it gives control back to the phase and frequency maintaining circuitry 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.

In the illustrated embodiment, to provide the differential integration the targeting phase accumulator 140 includes a pair of digital accumulator circuits in the form of a motor frequency counter 154 and a tach reference frequency counter 156 The motor frequency counter 154 is pfesettable to a value indicative of a desired amount of phase loss (i e, the target value of 115 degrees) due to the deceleration of the motor during the integrating period The counter 154 is preset or updated after every encoding by a targeting compensation circuit 157 for adjusting the target value according to loading conditions on the motor 102 For purposes of simplifying the description of the targeting phase accumulator, it wil be assumed that the targeting compensation circuit 157 is maintaining the target value of 115; i e, no changes in the loading of the motor 102 are occurring.

The targeting phase accumulator 140 also includes a digital comparator 158 The digital comparator 158 is coupled to the outputs of the counters 154, 156 and determines when the tach reference frequency counter 156 has been incremented by a value of 19 more than the motor frequency counter 154 Upon this condition, the comparator 158 generates the TPA signal to the motor control logic circuit 144, indicating that the target value of 115 degrees of phase change has been accumulated.

The motor frequency detector 142 and the control logic circuit 144, as shown in detail in Fig 3, effect acceleration of the speed of the motor 102 back to the 60 Hz carrier frequency producing speed The detector 142 comprises a digital integrator which includes a pair of presettable counters 160, 162 which are coupled to the output of an R/S flip-flop 164 The flip-flop 164 has its clock input coupled to the line 8 1,592,994 8 152 for receiving the motor frequency signal w OM and generating an ENABLE signal through a pair of gates 166, 168 to the counters 160, 162 via a line 170 The ENABLE signal on the line 170 is generated upon the absence of the Z control signal on the line 147 to the reset terminal of the flipflop 164 The Z control signal on the line 147 is removed by the control logic circuit 144 upon generation of the TPA signal (at the end of the integration period IP) on the line 148 from the targeting phase accumulator 140.

Because the motor 102 has been decelerated to a speed less than 60 Hz at the time of the occurrence of the TPA signal, the period of the motor frequency signal co M is longer than normal The purpose of the presettable counters 160, 162, is to determine when the period of the motor frequency signal coai is indicative that the speed of the motor has been accelerated back to 60 Hz after generation of the TPA signal, To this end, the counters 160, 162 have preset lines (not shown) which determine the number of counts the counters 160, 162 will achieve when the period of the (,o M signal is proper for 60 Hz operation The counters 160, 162 are also responsive to a 24 K Hz high frequency reference signal on a line 172 which provides a high frequency clocking signal to the counters for incrementing them The counters 160, 162 are preset to the value which causes an MFD signal to be generated on a line 174 whenever the 24 k Hz reference signal on the line 172 causes the number of counts accumulated by the counters 160, 162 to exceed the preset value The period of the ENABLE signal on the line 170 is decreasing with time due to the acceleration of the motor Eventually the MFD signal on the line 174 is not generated for a given period of the ENABLE signal Upon this condition, the motor 102 is operating once again at the carrier frequency producing speed.

Operation of the motor frequency detector 142 is better understood when considering the control logic circuit 144 as shown in Fig 3 The control logic circuit 144 includes three R/S flip-flops 180, 182, 184 and a NAND gage 186 The flip-flops 180, 184 respectively generate a V signal on a line 187 and the X and X signals on the lines 146, 145 The gate 186 is coupled to the lines 146, 187 for generating the Z signal on the line 147 as a function of the X and Y signals.

The flip-flops 180, 184 are responsive to the TS timing signal on the line 149 and are set upon the occurrence of data or a predetermined logic state as sensed by the sensors 100 Setting of the flip-flop 184 causes a logic I and a logic 0 to be generated as the X and X signals, thereby closing and opening the acceleration and loop switches 130, 128 respectively The flip-flop 180 generates a logic 0 as the Y signal on the line 187 upon its being set by the TS signal The Y signal is then coupled to the gate 186 for generating a logic zero state of the Z signal Upon the occurrence of the TPA signal at the end of the integration period IP, the TPA signals on the line 148 clocks the flip-flop 180, changing the V signal to a logic one During this interval, the Z signal has maintained the deceleration switch 132 closed and has disabled operations of the flip-flop 182 by way of the reset input.

Recapitulating, upon generation of the TS timing signal and thus at the beginning of the integration period IP, the X, X, and Z signals have respectively closed the switch 130, opened the switch 128, and maintained closure of the switch 132, causing deceleration of the motor 102.

At the end of the integration period when the targeting phase accumulator 140 has indicated that the desired 115 degrees of phase has been accumulated, as indicated by the TPA signal on the line 148, the flipflop 180 changes state This results as a logic 0 is applied to its data input and the TPA signal is applied to its clock input This change of state generates a logic 1 as the Y signal on the line 187, causing a logic 0 to be, generated on the line 147 as the Z signal.

This opens the deceleration switch 132, ending the deceleration phase of the motor change and beginning the acceleration change.

Referring now additionally to the motor frequency detector 142, as is also illustrated in detail in Fig 3, when the Z signal on the line 147 changes to a logic 0, the flip-flops 164 and 182 become unlatched A logic 1 applied to the data input of the flip-flop 164 is then clocked thereinto by the motor frequency signal c OM producing a logic zero at one input of the gate 166 Another input of the gate 166 receives the w M signal on the line 152 The gates 166, 168 thereby generate the ENABLE signal on the line to the counters 160, 162 for presetting them at the beginning of every cycle of the WM signal The counters then begin counting at a 24 k Hz rate, as determined by the 24 k Hz signal on a line 172.

At the end of the ENABLE signal, i e, at the end of one cycle of the motor frequency signal Cwm, if a carry has occurred out of the counter 162, i e, if a logic 0 has been generated on the line 174 as the MFD signal, the flip-flop 182 remains in the reset state (having been placed into the reset state by the Z signal on the line 147 upon the occurrence of the X signal going to the logic zero state, indicating the end of the 1,592,994 1,592,994 modulation) Only upon the conditions that a logic 1 is provided on the line 174 to the flip-flop 182 when a logic 1 ENABLE signal occurs will a clock signal be provided via a line 188 to the flip-flop 184 Unless a clock signal is provided via the line 188, the flipflop 184 maintains the X and X signals in the logic 1, logic 0 states as respectively set by the TS timing signals.

When the counters 160, 162 indicate that the period of the ENABLE signal; i e, the period of one cycle of the motor frequency signal %M has been reduced to a value corresponding to a motor frequency of 60 Hz, no carry out of the counter 162 will occur The logic 1 needed to change the state of the flip-flop 182 is thereupon generated This provides a clock signal to and changes the state of the flip-flop 184, which in turn changes the states of the X and X signals, thereby closing the loop switch 128 and opening the acceleration switch 130.

For purposes of simplifying the description of the phase and frequency maintaining circuitry and of the carrier frequency maintaining circuitry, it has heretobefore been assumed that the targeting compensation circuit 157 has been maintaining the target value of the targeting phase accumulator 140 at a constant 115 degrees of phase This corresponds to no changing in the loading on the motor 102 During actual well drilling operations, however, there are loading changes on the motor 102 These loading changes are quasi-static in that they usually change only very slowly with time.

The targeting compensation circuit 157 detects these changes in loading on the motor 102 and adjusts the preset of the targeting phase accumulator 140, i e, the targeting value heretofore identified as 115 degrees, to cause the total phase shift provided by first the deceleration and then the acceleration of the motor during encoding to be the total desired amount.

Because the compensation circuit operates continuously, no prior knowledge of the loading conditions on the motor 102 is necessary.

Referring now to Figure 5, the targeting compensation circuit 157 includes a targeting correction circuit 190 and an end of transition (EOT) phase accumulator 192.

The EOT phase accumulator 192 computes the total amount of phase accumulated during each encoding, i e, that which is caused by the deceleration and acceleration of the motor 102, and generates an EOT signal on a line 194 to the targeting correction circuit 190 when the desired total phase shift for the encoding has been accumulated In the illustrated and preferred embodiment, this phase shift is 180 degrees for binary encoded data The targeting correction circuit 190 is responsive to the EOT signal and adjusts the preset value of the targeting phase accumulator 140 via a line 195 according to whether more or less than 180 degrees of phase has been accumulated by the accumulator 192.

The EOT phase accumulator 192 is in effect another differential integrator circuit similar to that implemented for the targeting phase accumulator 140 The accumulator 192 generates the EOT signal when the difference between the integrated motor reference frequency signal ( O MR and the motor frequency signal' co O M exceeds a predetermined value corresponding to the total desired amount of phase change In the illustrated and preferred embodiment, the differential integrating circuit includes a reference counter 196, a tachometer counter 198, and a comparator 200.

The reference counter 196 is responsive to the motor reference frequency signal )MR on the line 150 and to the TS timing signal on the line 149 for generating an integrated motor reference frequency signal on a line 202 to the comparator 200 The integrated motor reference frequency signal is indicative of the value of the carrier frequency integrated over the time period beginning upon the occurence of the TS signal, i e, upon the occurrence of selected data from the encoding circuitry 101 The TS timing signal resets the counter 196 at the beginning of each IP integration period.

The tachometer counter 198 is responsive to the motor frequency signal w O M and to the TS timing, signal for producing an integrated motor frequency signal on a line 204 The integrated motor frequency signal %M is indicative of the value of the instantaneous motor speed integrated over the IP integration period beginning upon the occurrence of each TS timing signal Similarly to the reference counter 196, the tachometer counter 198 is reset by the TS signal Although not shown, the tachometer counter 198 is a programmable counter and has programming inputs set to a value corresponding to a 180 degrees phase shift.

According to the described system, this value is a count of thirty Presetting of the tachometer counter 198 allows a difference of 180 degrees of phase to be indicated when the integrated signals on the lines 202, 204 achieve the same digital value.

The comparator 200 is coupled to the lines 202, 204 for detecting when the digital values of the integrated signals from the counters 196, 198 become equal This indicates that 180 degrees of phase has been accumulated in the acoustic signal due to ' 1,592994 10 operation of the frequency changing circuitry A latch circuit (not shown) is coupled to the output of the comparator Upon the condition that the digital S values become equal, the comparator 200 sets the latch circuit for generating the EOT signal on the line 194 The latch circuit is reset by the TS timing signal.

The targeting correction circuit 190 includes a preset counter 210, a correction pulse generator 212, up/down steering logic 214, and an error pulse generator 216 The targeting correction circuit 190 is responsive to the EOT signal on the line 194 and to the X signal on the line 145 for generating a signal on the line 195 which updates the preset Value of the motor frequency counter 154 in the targeting phase accumulator 140 according to whether more or less than 180 degrees of phase shift has been accumulated during the encoding Accordingly, the motor loading compensation for one encoding is based on a previous encoding; or, stated in other terms, the correction for motor loading during a given encoding is compensation for the next occurring encoding.

The preset counter 210 is a conventional up/down counter implemented using a pair of serially connected, four bit, up/down counters The preset counter 210 receives a clock pulse on a line 217 from the correction pulse generator 212 whenever the total accumulated phase shift during an encoding differs by more than a predetermined value from the targeted value of 180 degrees In the illustrated embodiment, because each count of the motor frequency counter 154 corresponds to 6 degrees of phase shift accumulated, each CP pulse generated to the preset counter 201 either increments or decrements the target value of the motor frequency counter 154 by 6 degrees.

Whether the counter 210 increases or decreases in value depends upon a steering pulse SP generated on a line 220 from the up/down steering logic 214.

The correction pulse generator 212 includes a pair of serially connected four bit binary counters which are reset by the TS timing signal The counters are responsive to a targeting compensation reference frequency signal w TC on a line 222 and to an error pulse, EP from the error pulse generator 216 When the error pulse EP is of a sufficient duration according to the frequency of the c)TC signal, a pulse is generated from the output of the counters to provide the CP clock pulse to the preset counter 210 The CP pulse is also coupled to the counters in generator 212 for resetting them Accordingly, by choosing any of various frequencies for the w TC signal, the amount of overshoot or undershoot of the accumulated phase shift which triggers adjustment of the targeting value of the preset counter 210 is adjustable In the preferred embodiment a frequency of approximately 380 Hz is used for the targeting compensation reference frequency signal o O TC The error pulse generator 216 is responsive to the X signal on the line 145 and to the EOT signal on the line 194 In the preferred embodiment the generator 216 is an EXCLUSIVE-OR circuit for producing the EP signal having a pulse width indicative of the time difference between the returning of control to the phase and frequency and maintaining circuitry (as indicated by the change of state of the X signal) and achieving of the 180 degrees total.

phase (as indicated by the EOT signal) The time difference translates into a specific number of degrees of phase shift which either exceeds or is less than the targeted value of 180 degrees.

The up/down steering logic 214 is responsive to the EOT signal on the line 194 and to the X signal on the line 145 for generating the SP signal on the line 220.

The up/down steering logic in the preferred embodiment is an RS flip-flop having its clock terminal coupled to receive the X signal, having a logic 1 impressed on its data input terminal and which is reset by the EOT signal Accordingly, the SP signal on the line 220 is generated as either a logic 1 or logic 0 depending on which of the X or EOT signals first occurred, thereby indicating whether control has been returned to the phase and frequency maintaining circuit, i e, the phase lock loop, before or after 180 degrees of phase has been accumulated.

Referring again to Figure 2 the TS timing signal is produced in a conventional way by a transition start circuit 230 The transition start circuit 230 generates a pulse as the TS timing signal upon the occurrence of data of a predetermined logic state as sensed by the sensors 100 and encoded by the encoding circuitry 101 In the illustrated and preferred embodiment, the encoding circuitry 101 encodes the data from the sensors 100 into binary and the transition start circuit 230 detects whenever a logic I signal has been encoded by the encoding circuit 101 and generates the TS timing signal accordingly.

The transition start circuit 230 is suitably described in the above-reference Sexton et al patent, U S 3,820,063.

As above described, it thus will be apparent that motor speed detection during encoding, whether taken singularly or in combination with motor loading 1,592,994 1,592,994 combination, is an outstanding aid in reducing systems inaccuracies and/or in increasing the speed of data transmission.

Attention is directed to our co-pending United Kingdom Patent Application No.

40117/77 (Serial No 1592993), from which the subject matter of the present application has been divided, and to our copending United Kingdom Patent Application No 8006491 (which is also a divisional application divided from application No 40117/77) (Serial No.

1592993).

Claims (13)

WHAT WE CLAIM IS:-

1 A measuring-while-drilling method for effecting downhole measurements in a well during drilling thereof and for transmitting to the surface an acoustic signal representative of said measurements through fluid within the well, wherein said acoustic signal is produced by a motordriven acoustic signal generator whose speed is momentarily changed from a normally constant rate providing a carrier frequency signal, in dependence upon said measurements, to effect a selected phase change for modulating said carrier signal, the method comprising the steps of:

(a) changing the generator speed away from the normal rate to accumulate a portion of the selected phase change; (b) returning the generator speed to the normal rate to thereby accumulate the remainder of said selected phase change; (c) generating a control signal for terminating step (a) when the phase change accumulated in step (a) reaches a prescribed value; and (d) adjusting said prescribed value in response to the phase change accumulated during at least one preceding step (b) so as to tend to maintain the total amount of the actual phase change accumulated during each pair of successive steps (a) and (b) substantially equal to the selected phase change.

2 h The method of claim 1, wherein said carrier signal is modulated in response to intermittently occurring data, and the step of adjusting the prescribed value during one modulation is effected in response to actual phase change accumulated during the previous modulation.

3 The method of claim I or 2, wherein the motor is initially decelerated and then accelerated to normal speed, and the prescribed value for phase change accumulation is assigned to the deceleration stage.

4 The method of any of claims 1 to 3, wherein the control signal generating step comprises generating a signal representative of the carrier frequency, generating a signal representative of the substantially instantaneous speed of the acoustic generator, integrating the carrier frequency signal and the instantaneous speed signal over a time period beginning with the initiation of a speed change, and generating said control signal when the difference between said integrated signals reaches a predetermined value.

The method of claim 4, wherein said step of adjusting said prescribed value is effected in response to the difference between said integrated signals upon the condition that the motor speed has been returned to substantially its normal rate after generation of said control signal.

6 A measuring-while-drilling system for effecting downhole measurements in a well during drilling thereof and for transmitting to the surface an acoustic signal representative of said measurements through fluid within the well with a motordriven acoustic signal generator arranged to operate at a normally constant speed for providing a carrier frequency which is momentarily changed in dependence upon said measurements to effect a selected phase change for modulating said acoustic signal, said system comprising:

first means for changing the generator speed away from the normal rate to accumulate a portion of said selected phase change; second means for returning the generator speed to the normal rate to thereby accumulate the remainder of said selected phase change; further comprising:

means for generating a control signal for terminating the operation of said first means when the amount of phase change accumulated by the operation of said first means reaches a prescribed value, and means for adjusting said prescribed value in response to the phase change accumulated by at least one preceding operation of the second means so as to tend to maintain the total amount of the actual phase change accumulated during each pair of successive operations of the first and second means substantially equal to the selected phase change.

7 The system of claim 6, wherein the control signal generating means includes a presettable accumulator circuit for generating said control signal.

8 The system of claim 7, wherein said adjusting means generates a correction signal to said control signal generating means for adjusting the presetting of said accumulator circuit when said total phase change differs from said selected phase by at least a predetermined value.

9 The system of claim 7 or 8, wherein said control signal generating means comprises a differential integrating circuit means for generating the control signal lo 1 1,592,994 when a predetermined value is exceeded by the difference between ( 1) an integrated carrier frequency signal representing the value of the carrier frequency integrated over a time period beginning substantially upon the occurrence of a particular value of a downhole measurement, and ( 2) an integrated instantaneous generator speed signal representative of the integral of the instantaneous generator speed integrated over said time period, said presettable accumulator circuit providing one of said integrated signals.

The system of claims 8 and 9, wherein the adjusting means generate said correction signal in response to the difference between said integrated instantaneous generator speed and said integrated carrier frequency upon the return of generator speed to its normal speed.

11 The system of claim 10, further including means for generating a signal for presetting said accumulator circuit to said prescribed value corresponding to a predetermined portion of said selected phase change, said adjusting means generating said correction signal in response to phase accumulated during a previously occurring momentary speed change.

12 The system of any one of claims 6 to 11, wherein the first and second means for changing the generator speed comprise respectively means for decelerating said motor and means for accelerating said motor, and wherein the adjusting means is responsive to the motor speed to sense hanging loading conditions on the motor.

13 The system of any one of claims 6 to 12, further including one or more sensors in the well, an acoustic generator disposed within the flow of the well fluid for interrupting said flow at a controlled rate, a motor coupled between the sensor and acoustic generator for effecting the fluid flow interruption at a rate controlled in accordance with the sensor output signal, and a phase and frequency maintaining circuit operative to drive the generator at said normally constant speed in the absence of a sensor signal of a predetermined value, wherein the adjusting means corrects the prescribed value after motor speed has returned to its normal speed.

B D STOOLE, Chartered Patent Agent, Agent for the Applicants.

Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa, 1981 Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A IAY, from which copies may be obtained.

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