System For Wireless Communication Along A Drill String

Abstract

A wireless communication and drill string telemetry system. The communication system is used for communicating information along a drill string between a boring tool and a boring machine. An insulator assembly provides an electrically insulated gap between the drill string communication path and a soil engaging electrode for the electrical return path. A transmitter assembly includes a data transmitter for encoding and transmitting a data signal. A signal coupler couples the data signal to the drill string and provides a controlled electrical connection between the drill string communication path and the soil engaging electrode. The signal coupler comprises a transformer and a current regulating circuit to adjust a voltage across the transformer's primary winding. A receiver assembly is disposed proximate the drilling machine and includes a toroidal pickup coil and a signal processing assembly. The pickup coil has an annulus and is positioned such that the drill string communication path passes through the annulus. The pickup coil produces a signal voltage in response to a signal current on the drill string that the signal processing assembly processes to extract the data signal.

Description

FIELD OF THE INVENTION

The present invention relates to the field of horizontal directional drilling and in particular to wireless communication of information along a drill string.

SUMMARY

The present invention is directed to a signal coupler for use with a drill string communication system. The signal coupler is adapted to provide a controlled electrical connection between the drill string communication path and a soil engaging electrode. The signal coupler comprises a transformer having a winding and a control circuit adapted to adjust a control signal across the winding to maintain a desired current amplitude in the controlled electrical connection.

The invention is also directed to a drill string communication system. The drill string communication system comprises a drill string, a soil engaging electrode, and a signal coupler. The signal coupler provides an electrical connection between the drill string and the soil engaging electrode and comprises a winding.

The invention is further directed to a system for data communication in a drilling operation. The system comprises a drill string, a soil engaging electrode, a transmitter assembly and a receiver assembly. The transmitter assembly is supported at the drill string and adapted to transmit a signal comprising a data signal along the drill string between the drill string and the soil engaging electrode. The receiver assembly detects the signal and extracts the data signal.

Further still, the present invention is directed to a method for communicating information along a drill sting. The method comprises insulating a first end of a drill string from a second end of the drill string, generating a signal on the first end of the drill string using a transformer having a winding, maintaining a substantially constant current in the winding, and extracting the signal at the second end of the drill string.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a horizontal directional drilling operation having a drill string communication system in accordance with the present invention.

FIG. 2 is a side view of an insulator assembly in accordance with the present invention.

FIG. 3 is a block diagram of the communication system of the present invention.

FIG. 4 is a schematic of the transmitter assembly of the present invention.

FIG. 5 is a schematic of the receiver assembly of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Horizontal directional drilling (HDD) permits installation of utility services or other products underground in an essentially “trenchless” manner, minimizing surface disruption along the length of the project and reducing the likelihood of damaging previously buried products. A directional drilling operation involves use of an HDD machine to advance a boring tool attached to a drill string along a preplanned borepath through the earth. As the boring tool is advanced through the Earth, information about the operation and location of the boring tool must be known in order to maintain the borepath. Generally, a tracking receiver is used on the surface of the ground to wirelessly track the progress of the boring tool and receive information concerning the boring tool's operation. However, efficient communication of accurate information from the boring tool to an operator at the HDD machine remains a need in the industry, particularly where surface access is not possible, is inconvenient, or is dangerous. The invention of the present application is directed to wireless communication of information along the drill string between the boring tool and the HDD machine.

With reference now to the drawings and to FIG. 1 in particular, shown therein is an HDD system 10. The HDD system 10 comprises a drilling machine 12, a drill string 14, and a downhole tool assembly 16. The drilling machine 12 is operatively connected to the drill string 14 at an uphole end 18 of the drill string. The downhole tool assembly 16 generally comprises a drill bit 19 and is operatively connected to a downhole end 20 of the drill string 14. The downhole tool assembly 16 may include any of a variety of tools and electronics suitable for use in tracking the position and orientation of the downhole tool assembly 16 during an HDD operation. Typically, the drill string 14 is comprised of a series of interconnected pipe sections, or may be a continuous length of tubing. For purposes of the present invention, the drill string 14 may be, any length of pipe or tubing providing an electrically conductive path.

The present invention comprises a communication system 24 for wireless communication of downhole tool information along the drill string 14. The drill string 14 may comprise a single pipe drill string or dual pipe drill string. The communication system 24 uses the drill string 14 as a data conductor and the soil of the earth to provide an electrical return path. This operating arrangement has the advantage of requiring no wireline to carry data and power. The present invention provides for the transfer of data from the downhole tool assembly 16 to the drilling machine 12 without requiring the intervention of a conventional tracking receiver and RF data link.

The communication system 24 comprises an insulator assembly 26, a transmitter assembly 28, and receiver assembly 30. The insulator assembly 26 provides an insulated gap (not shown) for an electrical circuit created for the wireless communication. The transmitter assembly 28 is adapted to couple an electrical signal current 32 onto the drill string 14. Because the drill string 14 is an electrical conductor, a signal current 32 propagates along a drill string communication path 33 between the downhole tool assembly 16 and the drilling machine 12 in the direction of arrows 35 up the drill string, while arrows 36 are used to illustrate the return signal path. Although there are signal losses to the ground 34, a portion of the signal current 32 reaches the drilling machine 12 where it passes through the receiver assembly 30 and thence to ground 34. To prevent signal loss, one skilled in the art will appreciate that the drill string 14 may be coated with an insulating material to insulate the drill string from the soil. The soil of the ground 34 forms a return connection for the signal path as shown by the arrows 36, with the drill bit 19 on the downhole end 20 of the drill string 14 acting as a conductive soil engaging electrode. The present invention also anticipates an alternative embodiment in which the drill bit 19 may be comprised of a ceramic or other nonconductive material. In such a case, the downhole tool assembly 16 may further comprise a separate conductive structure to function as the soil engaging electrode.

To establish the intended communication circuit, the drill string communication path 33 (the drill string 14) may be electrically insulated from the soil engaging electrode and the drill bit 19. The insulator assembly 26 is provided for this purpose and is connected between the downhole end 20 of the drill string 14 and the downhole tool assembly 16. Preferably, the insulator assembly 26 comprises a section of nonconductive material. More preferably, the insulator assembly 26 is comprised of a fiberglass or epoxy composite material. Alternatively, the insulator assembly 26 may be of ceramic or other nonconductive material.

With reference now to FIG. 2, the insulator assembly 26 of FIG. 1 is shown therein. The insulator assembly 26 may comprise one or more drill pipe insulating sections 38 of insulating construction. A first end 43 of the section 38 is preferably connectable to a housing 45 of the downhole tool assembly 16. Preferably, the transmitter assembly 28 is supported within the housing 45.

The insulating section 38 preferably comprises a center section 40 made of nonconductive material with metal ends 42 for connecting the insulator assembly 26 to the downhole end 20 of the drill string 14 and the downhole tool assembly 16. Fiberglass material may be used to form the center section 40. The metal ends 42 may be connected to such a fiberglass center section 40 with an adhesive. The adhesive may be an aerospace grade high strength adhesive. The insulator assembly 26 is preferably constructed to be of substantially the same outside diameter as the drill string 14. The insulating section 38 may alternatively serve to house the transmitter assembly 28 or other electronics.

Referring now to FIG. 3, there is shown therein a block diagram of the communication system 24. The insulator assembly 26 is located between the drill string communication path 33 of the drill string 14 and the soil engaging electrode of the drill bit 19. As discussed above, the insulator assembly 26 may provide a housing for the transmitter assembly 28. The transmitter assembly 28 preferably comprises a data transmitter assembly 44 and a signal coupler 46.

The data transmitter 44 is adapted to encode and transmit a data signal. The data signal preferably includes information related to the downhole tool assembly 16, the drilling bit 19 and the boring operation. The transmitter 44 may obtain information from a processor 48 which in turn receives data from various sensors 50. The processor 48 is preferably adapted to format the data signal for a variety of communication protocols. For example, bidirectional data communication is possible using RS-232 format; data output via RS-485 format is also available for full compatibility with wireline operations. Data is preferably encoded with a phase-modulated waveform, and more preferably using two cycles of a 180 Hz carrier signal per bit period.

The signal coupler 46 is adapted to provide a controlled electrical connection for the electrical communication. The signal coupler 46 preferably comprises a transformer 52 having a primary winding 54 and a secondary winding 56. A first side 58 of the secondary winding 56 is connected to the drill string 14 and a second side 60 of the secondary winding is connected to the drill bit 19 on the downhole tool assembly 16. Preferably, the transformer 52 is a step-up transformer, but may alternatively be a step-down transformer or a unity gain isolation transformer.

The signal coupler 46 further comprises a current regulating circuit 62. The regulating circuit 62 is adapted to adjust a voltage across the primary winding 54 to maintain a substantially constant current amplitude in the primary winding. As described below with reference to FIG. 4, the regulating circuit 62 comprises a voltage regulator with an externally available feedback connection. The voltage regulator establishes the voltage amplitude applied to an H-bridge 100 circuit, the legs of which are connected to the primary winding 54 of transformer 52. The low side switches of the H-bridge 100 are not connected directly to a common circuit, but are instead tied together and then connected to circuit common through a shunt resister. The H-bridge 100 current, which circulated through the primary winding 54 of transformer 52, produces a voltage across the shunt resistor which is amplified, filtered, combined with a DC reference voltage, and the resulting composite signal then is applied to feedback connection of the voltage regulator, which completes the essential part of the regulating circuit 62.

One skilled in the art will appreciate alternative arrangements for the transformer 52 and the regulating circuit 62. For example, the transformer 52 may comprise a current-sampling winding and the regulating circuit 62 would be adapted to adjust a voltage across the primary winding to maintain substantially constant voltage amplitude in the current-sampling winding. Alternatively, the transformer 52 may comprise at least one primary winding and at least one secondary winding, with each of the secondary windings having different winding turns ratio. The transformer 52 could also comprise at least one primary winding and a tapped secondary winding. The current regulating circuit 62 would comprise a switch to select the appropriate tap to maintain a best approximation to the desired constant current.

The transmitter assembly 28 as described has the benefit of addressing several performance issues, including operating efficiency at particularly low operating power, operating from conventional battery power sources, the ability to provide adequate telemetry signal current despite large and unpredictable variations in load impedance, and compatibility with existing beacons and other electronic structures used in HDD systems.

In operation, a telemetry transmitter demonstration board operated with a primary DC source power draw of roughly 230 mA from a 3.0 VDC source, or 690 mW. This measurement is typical of low soil impedance conditions, representing a worst-case situation. This power consumption figure includes all operating losses in power converters and regulators. H-bridge 100, and transformer 52. Operation was demonstrated to 800 feet with 690 mW power consumption using a 180 Hz carrier frequency.

As shown in FIG. 3, the transmitter assembly 28 may be operated from a power supply 64. Power supply 64 is supported by a battery power source 65. The power source 65 may comprise batteries between 2.5 and 3.5 VDC. This input operating range includes most common battery arrangements: single cell lithium chemistries, two series-connected alkaline cells, or three NiCd or NiMH cells in series. With minor modifications, the range could be extended beyond 4.0 VDC to permit operation from two series-connected lead-acid gel cells. One skilled in the art will appreciate that battery power sources 65 of many different voltages may be utilized.

A fundamental problem for any drill string telemetry transmitting apparatus is that of efficiently providing useful signal current to a widely varying load impedance. In heavy clay a transmitter drill string signal current of approximately 50 mA may be adequate for most boring applications. Soil impedance is known to be anywhere from a hundred ohms to a thousand ohms. It will be appreciated that a simple fixed voltage drive capable of supplying 50 mA to a thousand ohm load would produce 500 mA in a hundred ohm load.

The present invention allows for reliable communication for compaction boring applications, which require no drilling fluid (known as a dry bore). This allows direct signal coupling to both sides of the transmitter's insulating gap 26. Using an efficient transformer 52 connected across the insulating gap 26 between the drill bit 19 and the drill string 14, provides improved ability to drive current into high impedance loads. It is significant that the reflected load of the transformer 52 appears as a resistance, whereas an induction toroid is an inductive load.

Continuing with FIG. 3, the receiver assembly 30 of the communication system 24 is located proximate the drilling machine 12 (FIG. 1). The receiver 30 comprises a toroidal pickup coil 66 and a signal processing assembly 68. The pickup coil 66 is necessarily electrically isolated from the drilling machine 12 (FIG. 1) to allow for maximum detection of the electrical signal current 32. The pickup coil 66 has an annulus 70 and is positioned such that the drill string 14, and the drill string communication path 33, passes through the annulus 70. The pickup coil 66 may use a ferromagnetic core or a Nylon core. The number of turns may be as many as 4000 turns. The number of required turns may depend on the drill string signal current's 32 amplitude and frequency. For example, use of a 180 Hz carrier may require coil 66 with a large number of turns. The pickup coil 66 is adapted to produce a signal voltage in response to the current 32 on the drill string communication path 33. One skilled in the art will appreciate that other operating frequencies could be used advantageously in accordance with the present invention.

The induced signal voltage is presented to the signal processing assembly 68. The signal processing assembly 68 comprises a processor 72 and electronics to amplify and filter the induced signal voltage. The processor 72 may execute an efficient cross correlation phase-tracking demodulation scheme. The processor 72 may then communicate data or information to a display 74.

The telemetry receiver 30 may also contain a number of power supply sections to provide operating power from either a +6.25 VDC supply or a +12 VDC supply. The telemetry receiver 30 operates with a primary DC source power draw of roughly 20 mA from a +6.25 VDC source. Performance has been demonstrated up to 800 feet with 125 mW power consumption at a 180 Hz carrier frequency.

The novel current regulation and supply section driving the transformer 52 is beneficial. Circuits directly involved with the signal generation are shown in FIG. 4. H-bridge 100 drives the signal transformer 52 primary winding 54. Because all H-bridge 100 current must return to circuit common through the low-side switches, it is convenient to use a small shunt resistor 102 to sense low-side switch current. The resulting shunt voltage is part of a regulator loop modifying operation of switched-mode power converter 104, which supplies operating power to the H-bridge 100.

If load current is too low, voltage at the feedback terminal (FB) of converter 104 is lower than required, causing converter output voltage to increase. This higher voltage is applied to the high side of the H-bridge 100, increasing the transformer's 52 primary voltage, which increases the transformer's secondary voltage, which increases load current until the loop comes into regulation. Likewise, if load current is too high, voltage on the feedback terminal of converter 104 is higher than required, causing regulator output voltage to decrease. This reduces voltage applied to the transformer 52 primary winding 54, which reduces voltage across the transformer 52 secondary winding 56, reducing the drive current.

H-bridge 100 and transformer 52 drive voltage increases may be limited due to device rating limitations. For this reason, a voltage comparator 106 changes state if the current regulator loop 62 attempts to force the power converter's 104 output voltage beyond the desired limit. If the comparator 106 is triggered, multiplexer 108 switches the power converter's 104 feedback connection from the current regulation loop 62 to the converter's normal feedback arrangement. The normal feedback arrangement of the power converter 104 is set for the converter's highest allowable output voltage, with the current regulation loop 62 reducing voltage from this maximum value if load impedance is low (a condition, which, if there were no current regulating circuit 62, would cause H-bridge 100 current to be much higher than necessary). In the preferred embodiment, the power converter 104 output voltage lower limit is actually determined by the feed-forward path through diode 110 in the power converter. Although this feed forward connection may allow signal current 32 (FIG. 1) to increase beyond the normal set point value for very low soil impedances, actual power dissipation is relatively low because the power converter's output voltage is low.

The transformer 52 provides a 1:2 voltage step-up to the load. Thus, when power converter 104 produces a +12 Volt output, the transformer 52 primary winding 54 drive voltage is ±12 Volts and the transformer 52 secondary winding 56 presents a nominal ±24 Volt drive to the load. Transistors 112 and 114 interface to the processor 48. Only two microprocessor lines are required—the first is an ENABLE which is high when the transmitter is to deliver power to the load (see 112), the second is a serial DATA line (see 114) which is expanded into complementary H-bridge drive signals by gates 116 and 118.

The signal current regulator circuit loop 62 is also detailed in FIG. 4. The H-bridge 100 current from the low-side switches passes through 102, a 0.100 ohm shunt resistor. The resulting voltage signal is applied to the input of a lowpass filter 120 with gain of +20 dBV and cutoff frequency of approximately 100 Hz (the transformer signal frequency of this embodiment is 180 Hz). The lowpass filter output signal is then applied to a lowpass filter 122 with gain of +20 dB but cutoff frequency of 10 Hz. The result is a +40 dBV second-order lowpass filter with the same phase as the bridge current signal.

The filtered and amplified H-bridge 100 drive signal is applied to a non-inverting summing amplifier 124. The other input to amplifier 124 is derived from reference diode 126, a +2.50 VDC reference diode. The current setpoint is determined by the voltage established by voltage divider resistors 128, 130, buffered by amplifier 132, and applied to the other input of the non-inverting summing amplifier 124. The summed signal at the output of 124 has two components—one fixed by the divider ratio of 128-130, the other ultimately dependent on primary winding 54 current delivered by the H-bridge 100. The summing amplifier 124 itself has a gain of unity, or 0 dBV.

When the circuit loop 62 is in regulation, the signal from non-inverting summing amplifier 124 passes through multiplexer 108 and thence to the feedback input pin (FB) of power converter 104. The power converter regulation loop is designated to maintain the feedback pin (FB) at +1.230 V. If, for example, the output of non-inverting summing amplifier 124 is lower than +1.230 V, the power converter 104 will increase output voltage in response to the low feedback signal. This will increase the voltage on the high-side switches of H-bridge 100, which in turn increases the voltage appearing across the transformer 52 primary winding 54.

The transformer's 52 load impedance reflects as a resistance. As the transformer's primary 54 voltage increases, the transformer's secondary 56 voltage increases, drive current increases, and the reflected load appears as a higher current on the low-side switches of H-bridge 100. This is the load current sensed by the shunt resistor 102.

The non-inverting summing amplifier 124 has two signal components. The signal component derived from voltage reference diode 126 is fixed, whereas the component related to H-bridge 100 current is controlled by the output voltage of the power converter 104. The power converter's 104 output voltage climbs until low-side (transformer primary) current reaches the desired value, at which operating point the output of non-inverting summing amplifier 124 reaches +1.230 Volts and the power converter regulator loop is in balance.

If ground impedance is unusually high, the loop 62 will attempt to increase power converter 104 output voltage beyond desirable limits. This condition is detected by Schmitt trigger 106, which compares the output of non-inverting summing amplifier 124 with the power converter's 104 normal feedback arrangement at resistors 134 and 136. Resistors 134 and 136 are selected to divide the power converter output signal and produce +1.230 Volts at the power converter's maximum desired voltage. During normal operation, the power converter 104 output will be lower than the maximum value and the signal at resistors 134 and 136 will be less than +1.230 Volts.

If the regulator circuit 62 attempts to drive the power converter 104 voltage beyond the maximum level, Schmitt trigger 106 will change states and connect the power converter's feedback pin to 134-136 rather than the output of non-inverting summing amplifier 124. Thus, the power converter loop is forced to limit the power converter's 104 output voltage at a safe value when soil impedance is very high. For lower soil impedances, the power converter loop dynamically adjusts the converter's 104 output voltage to maintain a constant current in the transformer primary 54 until the power converter output voltage falls below the forward path voltage at the cathode of 110, which establishes the minimum possible voltage applied to primary winding 54 in this embodiment.

The signal processing assembly 68 electronics are shown in FIG. 5. The pickup coil 66 provides an input signal related to the time derivative of the phase-modulated data signal from the coil. The signal is characteristically a rapidly rising or falling edge with exponentially decaying oscillations. Resistors 200 and 202 dampen the oscillations and provide a return path for an instrumentation amplifier's 204 input bias currents. The instrumentation amplifier 204 provides a gain of +46 dBV to the pickup coil 66 signal while providing very good rejection of common-mode noise signals. Amplifier 206 is part of a compensation loop which lowpass filters DC and low frequency signals and applies these signals to the reference pin of amplifier 204, effectively canceling out DC offset and low frequency noise.

The output of initial amplifier 204 is applied to a lowpass filter 208, having a cutoff frequency of roughly 1.6 Hz. This low cutoff frequency causes the stage to behave much like an integrator—it captures and holds the essential edge energy of the amplified pickup coil 66 signal while strongly attenuating the high frequency decaying oscillations of the input waveform. The existing lowpass filter 208 has 0 dBV gain, although it could be used to provide additional signal amplification.

The lowpass filtered (integrated) waveform at the output of filter 208 is applied to the input of instrumentation amplifier 210, having a stage gain of +46 dBV. Amplifier 212 provides additional offset and noise compensation identical to that described for amplifier 206. The output of amplifier 210 is applied to a passive lowpass filter 214 at the input of a non-inverting amplifier 216 providing switch-selectable gains from 0 dBV to +30 dBV in +6 dBV increments. Switch 218 allows the user to determine a gain setting which provides reliable data communication response without introducing unacceptable signal instability from residual noise. The instant apparatus uses a mechanical switch 218, but gain selection could be placed under software control.

Finally, the amplified signal is applied to a non-inverting summing amplifier 220. The other input of the non-inverting amplifier 220 summing arrangement is provided by a +2.50 volt reference diode 222. The resulting level-shifting operation converts the bipolar signal from amplifier 216 into a unipolar signal symmetric about +2.50 Volts to fit the microprocessor's 72 A/D converter input window.

The processor 72, shown in FIG. 5, contains an A/D converter which digitizes the waveform at the output from amplifier 220. The processor 72 also performs the correlation sums, selects the appropriate phase tracking delay, and extracts the phase-modulation data. The resultant output data may be presented in either RS-232 or RS-485 format, or any other format needed to satisfy design requirements.

The present invention comprises a communication system 24 for wireless communication, or drill string telemetry, along the drill string 14. As described in the principal preferred construction presented herein, data transfer is assumed to take place from the subsurface transmitter assembly 28. However, this invention contemplates providing data transfer from a surface transmitter assembly to a subsurface receiver assembly, as for establishing two way communication and the benefit of surface control over subsurface functions.

Various modifications can be made in the design and operation of the present invention without departing from the spirit thereof. Thus, while the principal preferred construction and modes of operation of the invention have been explained in what is now considered to represent its best embodiments, which have been illustrated and described, it should be understood that the invention may be practiced otherwise than as specifically illustrated and described.

Claims

1. A signal coupler adapted for use with a drill string communication system, the signal coupler adapted to provide a controlled electrical connection between the drill string and a soil engaging electrode, the signal coupler comprising: a transformer having a winding; anda control circuit adapted to adjust a control signal across the winding to maintain a desired current amplitude in the controlled electrical connection.

2. The coupler of claim 1 wherein the transformer comprises a secondary winding.

3. The coupler of claim 2 wherein a first side of the secondary winding is connected to the drill string and a second side of the secondary winding is connected to the soil engaging electrode.

4. The coupler of claim 1 wherein the transformer comprises a unity gain isolation transformer.

5. The coupler of claim 1 wherein the transformer further comprises a current-sampling winding and wherein the regulating circuit is adapted to adjust a voltage across the primary winding to maintain substantially constant signal amplitude in the current-sampling winding.

6. The coupler of claim 1 wherein the transformer comprises a step-down transformer.

7. The coupler of claim 1 wherein the control circuit comprises a regulating circuit.

8. The coupler of claim 7 wherein the regulating circuit is chosen from the group consisting of a voltage regulating circuit, a current regulating circuit, and a control impedance circuit.

9. The coupler of claim 1 wherein the control signal comprises a voltage.

10. A drill string communication system comprising: a drill string;a soil engaging electrode; anda signal coupler for providing an electrical connection between the drill string and the soil engaging electrode, the signal coupler comprising a winding.

11. The system of claim 10 further comprising: a data transmitter assembly to encode and transmit a data signal along the drill string; anda receiver assembly to process and extract the data signal.

12. The system of claim 11 further comprising a regulating circuit adapted to adjust a voltage across the winding to maintain an acceptable bit error rate at the receiver assembly.

13. The system of claim 10 further comprising a regulating circuit adapted to adjust an operating parameter across the winding to maintain a substantially constant signal amplitude in the winding.

14. The system of claim 10 wherein the signal coupler further comprises an insulator assembly, the insulator assembly defining an electrically insulated gap between the drill string and the soil engaging electrode.

15. A system for data communication in a drilling operation, the system comprising: a drill string;a soil-engaging electrode;a transmitter assembly adapted to transmit a signal comprising a data signal along the drill string;a receiver assembly to detect the signal and extract the data signal.

16. The system of claim 15 wherein the signal comprises a electrical current.

17. The system of claim 15 further comprising an electrically insulated gap between the drill string and the soil engaging electrode.

18. The system of claim 15 wherein the receiver assembly comprises a pickup coil to produce a signal voltage in response to the signal on the drill string.

19. The system of claim 18 wherein the pickup coil defines a toroid.

20. The system of claim 15 wherein the transmitter assembly is located at a downhole end of the drill string.

21. The system of claim 20 further comprising: a second transmitter assembly located at an uphole end of the drill string; anda second receiver assembly located at a downhole end of the drill string.

22. A method for communicating information along a drill string comprising: insulating a first end of a drill string from second end of the drill string;generating a signal on the first end of the drill string using a transformer having a winding;maintaining a substantially constant signal amplitude in the winding; andextracting the signal at the second end of the drill string.

23. The method of claim 22 further comprising adjusting a control signal across the primary winding to reduce bit errors.

24. The method of claim 22 wherein the signal is encoded in a signal current.

25. The method of claim 23 wherein the control signal comprises a voltage.