IE45473B1 - A digital motor control method and apparatus for measuring-while-drilling - Google Patents

Abstract

Measuring method during the drilling for momentarily to vary the speed of an acoustic signal generator driven by a motor, with respect to its normal speed constant, in order to introduce an appropriate shift in said acoustic signal. The method consists in producing two digital signals in response to said variation in speed of the generator, the difference between said digital signals being representative of the phase shift of the acoustic signal which is caused by the variation in speed of the motor , and stopping said variation in speed of the generator, with respect to its normal speed, when the difference between said digital signals reaches a predetermined value representative of a predetermined phase shift. [FR2366586A1]

Description

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 ac‘ lo cording 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 Ho. 3,309,656, issued March 14, 1967 to Godbey entitled Logging-While-Drilling System. Xn 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 derive.d in response to the sensed downhole condition is initially encoded into binary format, and the acoustic signal generator is driven at speeds so -2I 3 ' that the phase of a constant frequency carrier wave generated in the drilling fluid is indicative of tha data. In particular, a non-return to zero type PSK mode is used wherein th? the so 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 fcr each occurrence of a logic 1 da.a 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 frecuency introduce errors and/or inefficiencies into the system.

As a practical matter, however, the phase of the acoustic signal cannot be changed instantaneously in re-, sponse to data of the predetermined value. Inherent delays are introduced by the physics ox the system. The motor control circuitry which operates the motor-driven acoustic generator is adjusted accordingly to effect optimum response of the generator. Past proposals, such as those o£ the above-refer eneed Godbey and Patton patents, and that of U.S. Patent Ho. 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 -3by varying the speed of* the motor irt 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.

F The above proposals failed to overcome the problems associated with changes in the environmental conditions of the logging-while-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 _5 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 pre20 cisely 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 under30 shoot 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 minimise the period of ΰ 1 7 .) the return, the proposals either allowed inaccuracies to he introduced into the system or provided an unnecessarily slow encoding/data transmission system.

In addition to the motor speed problem just discussed, the qu.'.iity 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 vl;ccsi :y or density of the drilling fluid v.v.ry the length . tin' ?3d?d to recu'-pi the motor speed back to the carrier ω 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 id 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 above-mentioned patents involve an analogy implementation of the motor control cir20 cuitry. 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 25° C to greater than 175° C, causing inherent changes in the device characteristics of the analog circuitry. Furthermore, the 30 analog approach suffers due to the rugged environment encountered during drilling conditions. The extreme vibrations and shock received by the analog circuitry not only reduces its aS473 longevity but also tend to render the circuitry out cf adjustment. Still further, analog circuits are relatively expensive to manufacture and test.

The various aspects of the intention are directed to alleviating it least sane of the abate noted disadvantages.

According to one aspect cf the invention, there is provided a ireasuring-while-drilling method for effecting downhole measurements in a well during drilling thereof and far transmitting to the surface an acoustic signal representative 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 frcm its nontally constant rate, in dependence upon said measurements, to effect a desired phase change in said acoustic signal, the method including the step cf changing the speed of the generator away frcm its normal rate, arid further including the steps of generating a pair of digital signals in response to said generator speed changing steps the difference between said digital signals being indicative cf the change in phase of the acoustic signal caused by the change in speed of the generator; and stopping said generator speed change away from its normal rate when the difference between said digital signals reaches a predetermined value indicative cf a predetermined phase shift.

According to another aspect of the invention, there is provided a measuring-while-drilling systsn far effecting downhole measurements in a well during drilling thereof and for transmitting to the surface an acoustic signal representative cf said measurements through fluid within the well with a motor-driven acoustic signal generator arranged to operate at a normally constant speed which can be momentarily changed in dependence upon said measurements to effect a desired change in the phase of said acoustic signal, said system comprising: means far changing the speed cf the generator away from said normal speed: means, including at least csie digital accumulator coupled to receive a signal indicative cf idle instantaneous speed of the generator, fer generating a pair of digital signals in response to said speed change, toe difference between said digital signals being indicative of the charge in phase of the acoustic signal caused by the change in speed of the generator; aid means far stepping said generator speed change away frcm its normal speed when the difference between said digital signals reaches a selected value indicative of a predetermined phase shift.

I The invention will new be described, by way of example only, 4q with reference to the accaipanying drawings, of which: :-3473 Figure 1 is a schematic drawing showing a general well drilling and data measuring system according to the invention; Figure.2 is a black 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 10 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-while15 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. • <13473 As the drilling system 10 drills a well-defining borehole 14, the measuring-while drilling 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.

Tha< drilling system 10 is conventional and includes a 15 drill string 20'and a supporting derrick (not shown) represented'by a hook 22 which supports the drill string 20 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. Actuation of the rotary drive mechanism 32 (by equipment not shown) rotates' the kelly 30 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 3o downwardly through the drill pipe 28 during drilling, exits through jets in the bit 24 into the annulus and returns ' ' 8 uphole where it is received by the system 40. The circulating system 40 includes 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 any 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 couples the output orifice 50 of the desurger to the felly 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 • £3473 . decoded into meaningful information representative of the downhole conditions; and the decoded information is recorded amd displayed by the unit 74. .

Oxic- sueh uphole data receiving and decoding system 70 5 is described in U.S.· Patent No. 3,886,(495 to Sexton et al., issued May 27, 1975, entitled Uphole Receiver Por Logging tr · · • While-Drilling System.

The downhole acoustic signal generating unit 68 is supported within one of -the downhole drill collars 26 by a lo suspension mechanism-79 and generally includes a modulator having at least part of· the flow of the mud passing through it. The modulator 80 is controllably driven' for •selectively interrupting tha 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 energising the cartridge 82. A plurality of centralizers 85 are provided to position the modulator'80, the cartridge 82, and 2Ό 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 now well-known in the art. St 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 io ' : ί *5 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 raduc5 txan drive linkage 96 which drives the rotor. The cartridge 62 is c-erabiy cor ner feed tc the linkage 36 for rotating the r&':o~ 9z. 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 1·; 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 SO is'shown and described in · detail in U.S. patent No. 3,764,970 to Manning which is 2o 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-Nhile-Drilling Tool by Patton et al., U.S. Patent No. 3,792,429, issued February 12, 1374, aid in Logging-Wnile-Drilling Tool'1 by section et al., -,- U.S. Patent No. 3,770,006, issued Ncwentoer 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 ,1-5473 deviation), .weight on hit,' .torque,, .etc. . The .sensors 100 may be provided for monitoring safety parameters, such .as for detecting over pressure^ rones (resistivity measurements) and fluid entry characteristics by measuring the temperature of .the drilling mud within the annulus 53. Additionally, radiation sensor's may be provided, such as gamma ray sensitive sensors for discriminating between shale and sand and fer depth correlation.

The data encoding circuitry 101 is conventional and includes a multiplex arrangement far encoding the signals ' from the sensors into binary and then serially transmitting them over' a data line·. A suitable multiplex encoder arrangement is disclosed in detail in the above referenced Sexton-et al. patent', U.S. Patent No. 3,820,063.

IS The· cartridge 82 also-includes a 'motor 102 coupled to the linkage 96, and motor control circuitry l'O4 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. 2o The motor 102 is a conventional two-phase AC induction motor which, in the preferred embodiment, is driven at 60 Ha by the motoe control circuitry 102. Use of an 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 100 and encoding circuitry 101 aad 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 30 . signal transmitted in the drilling mud at the proper phase • and (2) for changing the frequency of the acoustic signal . 12 and returning it to the carrier frequency to thereby change the phase thereof by a predetermined value e*.s rapidly as possible in response to the encoded data. . In the preferred . emfer? 'ii ;ants w;·. -rein he data from the censors IOC is encoded in binary code, the* phase change is one of 180 dagr-ses.

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 φΑ, ¢3 and their complements ΦΑ, ΦΒ. The phase signals are 90 degrees out of phase from one another. The voltage control oscillator circuit 114 is conventional, and the phase signal generator 112 includes conventional circuitry for generating approximately 50 percent duty cycle waveforms 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 φΑ, φΒ at the desired 50 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 α 3 ·17 3 generator 112, and the .voltage controlled oscillator circuit 114, advantageously implements a phase locked loop circuit.

The phase' and frequency maintaining circuitry includes ι - . . . a tachometer 120 coupled to the motor 102 for producing a series ef pulses, whose repetition rate is indicative of the frequency at which the motor 102 is driven. Xn the preferred embodiment the tachometer 120 is selected to generate six cycles per revolution of the motor. This ratio in lo combination with the design of the modulator 80, the design qf 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 ωφ having a 360 Hz Ϊ5 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, ω^, and a relatively high frequency motor frequency signal ωΜ· For ex20 ’ ample, the loop frequency signal L is produced at a 24 Hz frequency and the motor frequency signal is produced at a 720 Hz frequency when the motor is operating at 60 Hz. The conditioning circuit 122 it 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 ω^,-and to·a 24 Hertz loop reference frequency signal to selectively generate 30 a VCO control signal on a line 126 which is operatively H 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 ω,, and a low pass filter (not shown) coupled co the output 5 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 and signals tc ba indicative « of the motor 102 deviating from the carrier frequency or phase. In response to the control signal on the line 126, lo 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. u.s. Patent wo. 3,870,063, shows and describes another phase locked loop circuit oper15 ating 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 from the sensors 100 is implemented digitally. 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.

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 3 473 an aceeleration/deceleration sequence is operable, the . deceleration/acceleration sequence results in the motor 102 operating in a higher torque range and thus in the modu.· ' lating of the acoustic signal more predictably and in a shorter period of time. . .

I · · 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. Xn the illustrated embodiment, the accelera10 tion switch 130.has one terminal commonly connected to the input of the VCO circuit 114 and to one terminal of the loop switch 128. Xt 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 linearally changing voltage. For.example it may change substantially exponentially with time. As illustrated an RC timing circuit comprising the series connection of a resistor R2 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 wil± 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 tenninal commonly connected to the resistor Rl and thus to the switch. 130. Xt has its other terminal connected to circuit ground. When the acceleration switch 130 is closed and the deceleration ΰ 1 ‘, ,) switch 132 is in the closed position, the capacitor C, which had been discharged through the resistor Rl to circuit ground by closing, of the switch 132, remains discharged. In the 3X< ferred embodiment upon closing of the switch 130, the discharged capacitor C produces a voltage level et 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 ? on a set of lines 145, 146, 147 to the switches 128, 130, 132 respectively. These signals are generated in a sequence, appropriately 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 130 (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 132 while leaving closed the acceleration switch 130 to begin acceleration of the '7 17 3 speed',of the motor 102 back toward the carrier frequency, producing speed; and, .(4) thereaftefc to open the 'acceleration switch 130 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 3.02.

Xn more detail and referring to the waveforms depicted ' in Figtira 4, the targeting phase accumulator 140 generates a TPA control signal on the line 148 a period of time, re10 ferred 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 ac15 tuated 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.

Xn effect, the targeting phase accumulator 140 is a differential Integrating circuit. That is,.during the integrating period, the targeting phase accumulator 140 . effectively is integrating the difference between a 720 Hertz motor reference frequency signal, ω^, on a line 150 and the motor frequency signal, uijj, on a line 152. In the illustrated embodi25 ment, the signals ωΜ and 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 pre30 determined 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 Γ32. This permits the-beginning of the 4-U -ί'ΐ 3 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 ωΜρ on the line 150 is a 5 720 Hz signal. This results ir. sixty cycles cf the motor reference frequency signal being produced for each cycle of the 12 Hz carrier frequency. Accordingly, thirty cycles of the a),^ signal correspond to 180 degrees of phase of the Ik Hz carrier. io Since a finite time is required to return the motor speed to the 50 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 df 60 Hz. Accordingly, it is necessary to accumulate 115 degrees of phase change in the targeting phase accumulator 140 prior to 2o the generation of the TPA signal and thus of tha beginning of the acceleration of the speed of the motor back towards 60 Hz. Since 30 cycles of the signal correspond to 180 degrees cf carrier phase shift, the targeting phase accumulator 140 needs to accumulate 115/180 x 30 = 19 cycles or counts EQN. 1 as the difference between the integrated ca^and integrated 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 ω^.

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 5· of 12'‘.Hz, it acts to pull the motor speed, into lock at ISO degrees ef phase change even when the phase changing. • eireuifery results in a range of 91-269 degrees of phase change. Also, as will be described subsequently, the targeted x’aiue of 115 degrees of phase change may be 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 tha carrierfrequency producing speed, at which time it gives control· back to the phase and frequency maintaining I 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. 2o · ' In the illustrated embodiment^ to .provide the differ.ential integration the targeting phase'accumulator 140 includes a pair of digital accumulator circuits in the formof a motor frequency counter 154 and a tach reference frequency counter^ 156. The motor frequency counter 154 is presettable 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. Xn the preferred embodiment the counter 154 is preset or . updated after every encoding by a targeting compensation circuit 157 fo* adjusting the target value according to . loading conditions on the motor 102. For purposes of simplifying the description cf the targeting phase accumulator, it will be assumed that the targeting compensation circuit is mairt&ir.ng the target value of 115j i.a., 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 153 is coupled tc the outputs of the counters 154, 15S and deterio mines 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 153 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 2o digital integrator which includes a pair of presettable counters. 150, 162 which are coupled to the output of an 3/3 flip-flop 164. The flip-flop 164 has its clock input coupled to the line 152 for receiving the motor frequency signal ωΜ 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 flip-flop 164. The Z control signal on the line 147 is removed by the control logic circuit 144 upon generation of 30 the TPA signal (at the end of the integration period IP) on the line 148 from the targeting phase accumulator 140. 2ί· • Because the motor 102 has been decelerated-.to a speed less than 60 Hz at the time of the occurrence of the TEA signal, .the period of the motor frequency signal ωΜ is ' longer than normal. . The purpose of the presettable counters 5 160, .162 is to determine when the period of the motor frequency signal· is indicative that the speed-of the motor has been accelerated back to '60 Ez .after generation of the SPA.signal. To this end, the counters 160, 162 have preset .. lines (not shown) which determine the number of counts the 10- ‘ counters 160, 162 will achieve when the period of the ωΜ signal is proper for 60 Hz operation. The counters 160, 162 'are also responsive to a 24 KHz 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 tothe value which causes a MFD signal to be generated on a-line 174 whenever the 24 KHz . 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 2.0 decreasing with time due to the acceleration of the motor. Eventually the MED 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 gate 186. The flip-flops 180, .184 respectively;generate a y'signal on a line 187 and .the X and 3t 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 X signals.

The flip-flips 180, 184 are responsive to the TS timing· sign;.il on the line 149 and are set upon the occurrence of data cf a predetermined logic state as sensed by the sensors 100. Setting· of the flip-flop 184 causes a logic 1 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 ISO generates a logic 0 as the ϊ signal on the line 187 upon its being set by the TS signal. The Ϋ signal is then coupled to the gate 186 for generating a logic zero 3tate of the Z signal.

Upon the occurrence of the TPA signal at the end of the integration period IP, the TPA signal or the line 148 clocks the flip-flop 180, changing the Ϊ signal to a logic one.

During this interval, the 2 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 flip-flop 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 be•5 ginning the acceleration change. .iaferring 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 t . 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 ω^, producing a logic zero at one input of the gate 166. Another input of the gate .166 receives the ωΜ signal on the line 132. The gates 166, 168 thereby generate the ENABLE signal on the line 170 to the counters 160, 162 for presetting them at the beginning of every cycle of the ωΜ signal. ' The counters then begin counting af a 24 kHz rate, as determined by the 24 kHz 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 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 occur25 rence of the X signal going to’the logic zero state, indicating the end of the 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 flip-flop 184 maintains the X and Z 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 has been reduced to a value corresponding to a motor frequency of 60 Hz, no carry out of the counter l-ί 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 10 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 heretofore been assumed that the targeting compensation circuit 157 has been 15 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 20 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 25 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 4ΰΐ73 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 bn 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 1Ό · 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 15 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 ω,κ and the motor’frequency signal.exceeds a predetermined value 2o 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 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 20-2 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 occurrence 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 ωΜ and to the TS timing signal for producing an integrated motor frequency signal on a line 204. The integrated motor frequency signal is indicative of io 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 program;· mable 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, 2.04 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 aco.ustic signal due to operation of the frequency changing circuitry. A latch circuit (not shown) Is coupled to the output of the comparator 200. Upon the condition that the digital 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. 4.3473 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 occur. ring encoding.

The preset counter 210 is a conventional up/down counter implemented using a pair of se'rially 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. Xn the illustrated V 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 on a line 222 and to ah error pulse, E? from the error pulse generator 216. When the error pulse EP is of a sufficient du ration according to the frequency of the signal, a pulse is generated from the output of the counters to provide the CP clock pulse to the preset counter 210. The C? pulse is also coupled to the counters in generator 212 for resetting them. Accordingly, by choosing any of various frequencies for the io uTC 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. Xn the preferred embodiment a frequency of approximately 380 Hz is used for the targeting compensation reference frequency signal 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 i 8473 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 ter• ’ ninal and which is reset by the EOT signal. Accordingly, · the SR signal on the line 220 is generated as either a logic 1 or logic' 0 depending on which of the.x or ΓΟΤ 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 transistion start circuit 230 generates a pulse as . the TS timing signal upon the occurrence of data of a pre15 determined logic state as sensed by the sensors 100 and encoded by the encoding circuitry 101. Xn 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 1 signal has been 20 encoded by the encoding circuit 101 and generates the TS timing signal accordingly.

The transistion start circuit 230 is suitably described in‘the above-referenced Sexton et al. patent,. U.'S. 3,820,063, which previously has been incorporated by reference.

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

Claims (15)

1. CLAIMS:1. A measuring-while-drilling method for effecting downhole measurements in a well during drilling thereof and for transmitting to the surface of an acoustic siqnal representative 5 c; .ia. d measirements through fluid within the well, wherein said acoustic signal is produced by a motor-driven acoustic signal generator whose speed is momentarily changed from its normally cc.ittant rate, in dependence upon said measurements, to effect a desired phase change in said acoustic signal, the method lc including the step of changing the speed of the generator away from its normal rate, and further including the steps of generating a pair of digital signals in response to said generator speed changing step, the difference between said digital signals being indicative of the change in phase of the acoustic signal 35 caused by the change in speed of the generator, and stopping said generator speed change away from its normal rate when the difference between said digital signals reaches a predetermined value indicative of a predetermined phase shift.

2. The method according to Claim 1, wherein the step of changing the speed of the generator away frcm its normal rate 20 includes tne step of initially decelerating the speed of the motor to a first relatively low value: said method further including the step of accelerating the speed of tne motor to thereby return said speed to said normally constant rate value. 25

3. The method according to Claim 1 or 2, further including tne steps of sensing downhole parameters and generating data in binary format representative of the sensed parameters, wherein tile step of changing the speed of the generator is performed only upon the generation of data of a particular logic state. j.j

4. The method of any one of the previous claims, wherein the step of generating a pair of digital signals comprises: a) generating a first signal representative of the normally constant generator speed; bi generating a second signal representative of the 35 instantaneous speed of the acoustic generator; and c) digitally integrating the first signal and the second signal over a time period beginning substantially upon the occurrence of a sensed downhole parameter value.

5. The method of Claim 4, wherein the step of digitally intergrating includes the step of presetting to a predetermined value a programmable counter responsive to one of the first and second signals. 5

6. The method of any one of the previous claims', wherein said momentary generator speed change includes a reduction of motor speed and further includes, after stopping the motor speed reduction, accelerating the speed of the motor toward the normally constant rate at a changing rate of acceleration. 10

7. 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 15 normally constant speed which can be momentarily changed in dependence upon said measurements to effect a desired change in the phase of said acoustic signal, said system comprising; means for changing the speed of the generator away from said normal speed; 20 means, including at least one digital accumulator coupled to receive a signal indicative of the instantaneous speed of the generator, for generating a pair of digital signals in response to said speed change, the difference between, said digital signals being indicative of the change in phase of the 25 acoustic signal caused by the change in speed of the generator; and means for stopping said generator speed change away from its normal spaed when the difference between said digital signals reaches a selected value indicative of a predetermined phase shift. 30

8. The system of Claim 7, wherein said digital accumulator provides at its output one of said pair of digital signals having a value indicative of the instantaneous speed of the generator integrated ever a time period beginning substantially upon the initiation of the speed change away from the normal 35 speed, and said digital signal generating means includes another digital accumulator providing the other of said pair of digital signals at its output having a value indicative of the normally 43 173 constant speed of the generator integrated over said time period.

9. The system of Claim 8, wherein the digital accumulators respectively comprise digital counters. J.C. The ί/stem of Claim 9, wherein sard predetermined ph·?; shift is less than the full phase shift jenersted by the moKe .rary speed change, with the remainder of the full phase shif- occurring as the motor is returned to the normal speed, and at least one of said counters is a programmable counter preset to a count state corresponding to said predetermined chase shift.

10. 11. Tiie system of any one of Claims 7 to lo, wherein said stopping means includes a digital comparator responsive to said first and second digital signals.

11. 12. The system of any one of claims 7 to 11, wherein the speed change away from the normal speed is a deceleration, and further comprising means for generating a ramp signal in response to the selected value of the difference between the digital signals being reached for effecting acceleration of the motor speed as an increasing function with time.

12. 13. The system of any one of Claims 7 to 12, comprising an acoustic signal generator comprising a rotary valve transmitter having a rotor disposed for selectively interrupting the downward passage of the drilling fluid, said system further including a motor for rotating said rotor, one or more sensors for sensing the downhole conditions and generating encoded sensor signals representative thereof, and a control circuit coupled to the sensor and to the motor for controlling operation of the motor in response to the sensor signals, and further including; a tachometer coupled to said motor for providing instantaneous motor speed to the digital signal generating means, said digital signal generating means and the motor speed change stopping means being part of the control circuit which further includes a phase and frequency maintaining circuit operative to maintain the motor speed at the normally constant speed in :· j Xj.3 47 3 accordance with a reference signal, laid means for changing the speed of the motor away from its normal speed including means far disconnecting the phase and frequency maintaining circuit from rhe motor.

13. 14. A measuring-while-drilling method substantially as herein described with reference to the accompanying drawings.

14.

15. A measuring-while-drilling system substantially as herein described with reference to the accompanying drawings.