GB2352150A

GB2352150A - Telemetry system in which data signals are modulated on power signals

Info

Publication number: GB2352150A
Application number: GB0009130A
Authority: GB
Inventors: David Sirda Shanks
Original assignee: AXON INSTR Ltd
Current assignee: AXON INSTR Ltd
Priority date: 1999-04-14
Filing date: 2000-04-14
Publication date: 2001-01-17
Anticipated expiration: 2020-04-14
Also published as: GB9908386D0; GB0009130D0; GB2352150B

Abstract

A telemetry system is capable of transmitting both power and data signals between a master unit 10 and at least one slave unit 50 over a transmission system 12, the master unit being for example at the surface and the slave unit(s) being located in a borehole or oil well. The master unit may use pulse width modulation of a power signal from driver 24 to send data to the slave unit. Power signals received by the slave unit are fed to regulators 56,58 to provide a local power supply for the slave unit. Data signals from the slave unit may be encoded by frequency shift keying at generator 64, synchronised with the pulses of the power signal, for transmission to the master unit. The master unit may include two channels 26-34 and 36-40 for reading the data and synchronising the power and data transmissions to the frequency of the transmission system 12. Data signals sent from the master unit may be detected by a timer circuit 70 and used to control downhole valves, actuators or motors, while the data signals from the slave unit may represent the outputs of downhole sensors, e.g. monitoring a pump. The data signals may be encrypted, e.g. using a Hamming code.

Description

1 2352150 I "Telemetry System" 2 3 The present invention relates to a

telemetry system. 4 5 Telemetry systems are typically used in the oil and gas 6 industry to transmit data from measuring devices, 7 sensors or the like located downhole to receivers 8 located at the surface. Conventional systems use 9 transmission mediums such as drilling fluid or mud in 10 which to transmit the signals between the downhole and 11 surface locations. In addition, mono-conductor 12 instrument cables and single- or three-phase power 13 cables are often used to transmit data communications 14 in addition to their primary function. 1. 5 16 Such conventional systems typically require at least 17 two individual power sources; one at the surface to 18 drive the receiving circuitry and at least one-downhole 19 to drive the remote circuitry. This duplication of 20 power sources increases the cost of the system and may 21 make the system unreliable, as more components are 22 required.

2 2 Furthermore, the power source downhole has limitations 3 associated with it in that the power output from the 4 source is restricted due to the remoteness of the 5 source. For example, the downhole power source may 6 comprise batteries that have a limited power output and 7 also a limited lifetime before they must be either 8 replaced or recharged. 9 10 According to a first aspect of the present invention 11 there is provided a telemetry system, the system 12 comprising a master unit, and at least one slave unit 13 remote from the master unit, the master and slave units 14 communicating via a transmission system, wherein the 15 telemetry system is capable of transmitting power and 16 data transmissions between the units. 17 18 According to a second aspect of the present invention, 19 there is provided a method of transmitting p ower and 20 data from a master unit to at least one slave unit 21 remote from the master unit, the master and slave units 22 communicating via a transmission system, the method 23 comprising the steps of 24 generating a power transmission at the master unit; 26 generating a data transmission and synchronising 27 the data transmission with the power transmission 28 at the master unit; 29 transmitting the power transmission and data transmission via the transmission system to the 31 slave unit; and 32 recovering the power and data transmissions at the 33 slave unit.

3 2 According to a third aspect of the present invention, 3 there is provided a method of transmitting data to a 4 master unit from at least one slave unit remote from the master unit, the master and slave units 6 communicating via a transmission system, the method 7 comprising the steps of 8 generating a power transmission at the master unit 9 and transmitting the power transmission to the 10 slave unit; 11 recovering the power transmission at the slave 12 unit; 13 generating a data transmission at the slave unit 14 and synchronising the data transmission with the is power transmission; 16 transmitting the data transmission via the 17 transmission system to the master unit; and is recovering the data transmission at the master 19 unit. 20 21 Optionally, the method may include the further steps of 22 dividing the data transmission into a series of 23 sub-windows; 24 transmitting a specified data transmission from the slave unit to the master unit; 26 receiving the specified data transmission at the 27 master unit; 28 determining which of the sub-windows reliably 29 transmitted the specified data transmission.

31 The sub-windows that did not reliably transmit data are 32 typically ignored for subsequent transmissions. This 33 technique may be used where the transmission system is 4 1 particularly noisy and increases the chances of 2 reliably retrieving data transmissions.

3 4 According to a fourth aspect of the present invention, there is provided a method of receiving and converting 6 power and data transmissions sent from a master unit to 7 at least one slave unit remote from the master unit, 8 the master and slave units communicating via a 9 transmission system, the method comprising the steps of receiving a power transmission at the slave unit; 11 dividing the power transmission into two channels; 12 rectifying and regulating the power transmission 13 in a first channel; and 14 recovering the data transmission in a second is channel.

16 17 According to a fifth aspect of the present invention, 18 there is provided a method of receiving data 19 transmitted by a master unit from at least one slave unit remote from the master unit, the master and slave 21 units communicating via a transmission system, the 22 method comprising the steps of 23 receiving the data transmission at the master 24 unit; filtering and conditioning the data transmission; 26 and 27 regenerating the transmitted data.

28 29 Optionally, the method may include the further steps of dividing the data transmission into a series of 31 sub-windows; 32 transmitting a specified data transmission from 33 the slave unit to the master unit; 1 receiving the specified data transmission at the 2 master unit; 3 determining which of the sub-windows reliably 4 transmitted the specified data transmission.

6 The sub-windows that did not reliably transmit data are 7 typically ignored for subsequent transmissions. This 8 technique may be used where the transmission system is 9 particularly noisy and increases the chances of reliably retrieving data transmissions.

11 12 Pulse-width modulation is typically used to facilitate 13 data transmission from the master unit to the slave 14 unit. The power transmission is typically modulated with the data transmission using pulse-width 16 modulation.

17 18 Frequency-shift keying (FSK) is typically used to 19 facilitate data transmission from the slave unit to the master unit. The FSK frequencies are typically 21 superimposed on a carrier frequency. The carrier 22 frequency is typically the same frequency as the power 23 transmission frequency. The data transmission is 24 typically synchronised to the "high" cycle of the power transmission. Alternatively, the data transmission may 26 be synchronised to the "low" cycle of the power 27 transmission, or optionally to both the low and high 28 cycles. 29 30 Where more than one slave unit is used, the data 31 transmission from the master unit to the slave unit 32 typically includes an address of the slave unit. This 6 1 allows several slave units to receive commands from a 2 single master unit.

3 4 The data transmissions preferably include data error detection and/or correction. The data error detection 6 and/or correction typically comprise a Hamming code, or 7 other suitable technique.

8 9 The power and data communication is typically provided across a single- or three-phase alternating current 11 (ac) transmission system. The transmission system may 12 be, for example, a power cable, instrument cable, or 13 other types of electrical cable.

14 is The master unit and/or the slave unit are preferably ac 16 coupled to the transmission system using capacitors.

17 Most preferably, the system employs separate and 18 discrete capacitors for this purpose. This is known as 19 capacitive coupling and allows any dc bias within the transmissions to be blocked, whilst passing any ac 21 signals.

22 23 The master unit typically comprises a processor to 24 control the operation of the master unit; a power waveform generator; and signal recovery and 26 conditioning circuitry.

27 28 The processor typically applies pulse-width modulation 29 to the power transmission when data transmission is required from the master unit to the slave unit. When 31 not transmitting data, the processor typically defaults 32 the power waveform to a 50% duty cycle.

33 7 1 The power waveform generator typically comprises an 2 analogue driver and a power drive electrically coupled 3 to the analogue driver. The processor typically 4 applies the power transmission to the analogue driver.

The analogue driver typically drives the power driver.

6 The processor typically controls the voltage amplitude 7 of the power transmission.

8 9 The analogue driver typically includes an isolating circuit that isolates the power driver from the 11 processor. Typically, the analogue driver further 12 includes low voltage logic drivers to high voltage 13 driver stages, which in turn drive the power driver.

14 This prevents any damage being caused to the processor.

The power driver typically comprises a field-effect

16 transistor (FET) based push-pull driver.

17 Alternatively, the power driver comprises a bi-polar 18 transistor based push-pull driver, or the like. The 19 power driver typically operates from a variable dc power supply. The master unit typically includes the 21 variable dc power supply.

22 23 The signal recovery and conditioning circuitry 24 typically allows data transmitted by the at least one slave unit to be extracted and recovered from the 26 transmission system. The signal recovery circuit 27 typically includes first and second data channels. The 28 first data channel typically includes a high-speed 29 switch; a filtering system; an automatic gain control (AGC) stage; a comparator stage; and a first counter.

31 32 The high-speed switch typically enables the data 33 transmission to be directed to the first and second 8 1 data channels when the power transmission is high.

2 Alternatively, the high-speed switch directs the data 3 transmission to the first and/or second data channels 4 when the power transmission is low, or when the power transmission is both high and low.

6 7 The filtering system typically removes any noise and 8 background signals from the recovered data. Typically,

9 the filtering system comprises a pair of selective filters. The selective filters typically comprise 11 broad bandpass filters. Alternatively, the selective 12 filters may comprise tuned filters. This allows the 13 filters to differentiate between the FSK frequencies.

14 The AGC stage typically maintains the signal within a 16 set voltage amplitude range.

17 18 The comparator stage typically compares the voltage 19 amplitudes of the FSK frequencies.

21 The slave unit typically comprises a processor to 22 control the operation of the slave unit; rectifying and 23 regulating circuitry in a first channel; recovery and 24 conditioning circuitry in a second channel; and frequency generating and mixing means.

26 27 The rectifying and regulating circuitry typically 28 comprises a half-bridge rectifier to rectify the 29 received power transmission into a dc voltage; and at least one voltage regulator to regulate the dc voltage.

31 32 The recovery and conditioning circuitry typically 33 comprises an amplifier and filtering system; and a 9 1 timer circuit. The amplifier and filtering system 2 typically amplifies or attenuates the signal, and 3 filters the signal. This boosts the amplitude of the 4 signal and removes any background noise or other interference.

6 7 The frequency mixing and generating means typically 8 comprises a frequency-shift keying (FSK) generator; an 9 FSK mixer; and a line driver.

11 The slave unit typically includes an analogue signal 12 conditioning circuit, and at least one analogue-to- 13 digital convertor. The analogue conditioning circuit 14 allows the slave unit to receive and process signals from a plurality of sensors, such as pressure sensors, 16 temperature sensors or the like.

17 18 The slave unit is typically capable of controlling 19 loads.

21 Embodiments of the present invention shall now be 22 described, by way of example only, with reference to 23 the accompanying drawings, in which:

24 Fig. 1 is a schematic block diagram of a telemetry system for use with the present invention; 26 Fig. 2a shows an exemplary power waveform for 27 transmitting power from a master unit at the 28 surface to a slave unit downhole; 29 Fig. 2b shows an exemplary signal transmit waveform for transmitting data from a slave unit 31 to a master unit using frequency-shift keying 32 (FSK); 1 Fig. 3a shows the power waveform of Fig. 2a 2 modulated using pulse-width modulation for 3 transmitting both power and data from a master 4 unit at the surface to a slave unit downhole; Fig. 3b shows how data is encoded in the modulated 6 waveform of Fig. 3a; 7 Fig. 4a shows an exemplary power waveform 8 transmitted on a transmission system to which the 9 telemetry system of Fig. 1 is attached; Fig. 4b shows an exemplary power waveform. for 11 transmitting power from a master unit at the 12 surface to a slave unit downhole; 13 Fig. 4c shows an exemplary signal transmit 14 waveform for transmitting data from a slave unit is to a master unit using frequency-shift keying 16 (FSK); 17 Fig. 4d shows an enlarged portion of the waveform 18 of Fig. 4c; 19 Fig. 5 shows the telemetry system of Fig. 1 coupled to an exemplary 3-phase ac system; 21 Fig. 6 shows a typical VSD generated power 22 waveform for driving a downhole motor pump; 23 Fig. 7a illustrates voltage spikes produced at a 24 star point created at the junction of capacitors in a three-phase system; 26 Fig. 7b shows an exemplary power waveform for 27 transmitting power from a master unit at the 28 surface to a slave unit downhole; 29 Fig. 7c shows an exemplary signal transmit waveform for transmitting data from a slave unit 31 to a master unit using frequency-shift keying 32 (FSK); 1 Fig. 7d shows an enlarged portion of the waveform 2 of Fig. 7c; and 3 Fig. 8 illustrates the telemetry system of Fig. 1 4 coupled to an exemplary 3-phase ac system. 5 6 Referring to the drawings, Fig 1 shows an exemplary 7 embodiment of a telemetry system of the present 8 invention, the system including a master unit 10 and a 9 slave unit 50. The master and slave units 10, 50 communicate with each other via a transmission system 11 12. The transmission system 12 may be a single- or 12 three-phase ac power system, such as a three-phase 13 power cable, or a mono-conductor instrument cable. The 14 transmission system 12 may comprise any electrical is conductor, such as electric cables used to power 1,6 downhole apparatus, such as downhole motor pumps or the 17 like.

18 19 The master unit 10 is typically located at the surface, and the slave unit 50 is typically located remote from 21 the master unit 10, for example in a borehole, oil well 22 or the like. It should be noted that a number of slave 23 units SO may be coupled to the transmission system 12, 24 and the operation of each slave unit So controlled by the master unit 10 at the surface. It should also be 26 noted that the system may use more than one master unit 27 10 if control of the slave unit(s) 50 is required from 28 more than one point in the system.

29 The master and slave units 10, 50 are advantageously 31 coupled to the transmission system 12 using capacitive 32 coupling. Discrete capacitors 14, 52 are coupling or 33 blocking capacitors that couple a signal from a power 12 1 source (discussed later) to the transmission system 12.

2 Capacitors 14, 52 block any direct current (dc) bias 3 that may be applied to the signal, but do not affect 4 any alternating current (ac) signal that is simultaneously transmitted. When considering dc, the 6 capacitors 14, 52 act as open circuits as, at zero 7 frequency (dc), the reactance of a capacitor is 8 infinite.

9 A power input stage 16 provides power for the telemetry 11 system, and may be either an ac or dc power source..

12 The power input stage 16 is electrically coupled to one 13 or more dc voltage regulators 18. Voltage regulators 14 18 provide local power supplies (dc) for the circuitry in the master unit 10. Generally, different components 16 within the master and slave units 10, 50 operate using 17 a plurality of different voltages, depending upon the 18 various specifications of these components.

19 The master unit 10 includes a processor 20 which, among 21 other functions, controls the operation of the 22 telemetry system. One output of the processor 20 is 23 electrically coupled to an analogue driver stage 22, 24 the driver stage 22 being electrically coupled to a high voltage ac power driver 24. The output.of the 26 power driver 24 is electrically coupled (via the 27 coupling capacitor 14) to the transmission system 12. 28 29 The power driver 24 may be a field-effect transistor 30 (FET) or a bi-polar transistor based push-pull drive 31 stage, that typically operates using a variable but 32 relatively large dc voltage power supply. The dc power

33 supply is typically rated from 20 to 500 volts, 13 1 although voltages outwith this range may also be used.

2 The particular voltage used is dependent upon the 3 loading conditions and losses in the transmission 4 system 12, and can be varied accordingly.

6 The power driver 24 is preferably electrically isolated 7 from the processor 20 to prevent damage to the 8 processor 20. Thus, the analogue driver stage 22 9 includes isolating circuits and low voltage logic drivers to a high voltage drive stage, which in turn 11 drives the gates of the FET or bi-polar transistor 12 power driver 24.

13 14 The master unit 10 further includes a signal recovery circuit 26 that retrieves data transmitted (via the 16 transmission system 12 as will be described) by the 17 slave unit 50. The processor 20 controls operation of 18 the signal recovery circuit 26. The recovered data 19 from the signal recovery circuit 26 is processed by a filtering system 28 that further extracts the received 21 information from any noise or other background

22 interference mixed with the recovered data from the 23 slave unit 50.

24 The output from the filtering system 28 is fed into a 26 signal-conditioning unit that includes an automatic 27 gain control (AGC) stage 30, and a comparator stage 32.

28 The output of the comparator stage 32 is fed into a 29 first counter 34. The processor 20 can read the value in the first counter 34.

31 32 In certain embodiments of the present invention, the 33 raw signal from the slave unit 50 is additionally fed 14 1 into a second data channel that includes a second 2 signal recovery circuit 36 to extract data from the 3 power waveform on the transmission system 12. The 4 output from the second signal recovery circuit 36 is fed into a timer circuit 38 that performs pulse-width 6 measurements on the data extracted from the power 7 waveform. The output of the timer circuit 38 is fed 8 into a second counter 40, the value in the second 9 counter 40 being read by the processor 20.

11 Operation of the master unit 10 is typically. controlled 12 by a remote station (not shown) that is electrically 13 (or otherwise) coupled to the master unit 10 via a 14 serial data link 46, such as an RS232/485 serial data port. The remote station may be, for example, a 16 personal computer located remotely from the master unit 17 10.

18 19 The slave unit 50 includes a half-wave rectifier and heat dissipation unit 54. This unit 54 extracts power 21 transmitted via the transmission system 12 to the slave 22 unit 50 as will be described. As with the master unit 23 10, the slave unit 50 has a matched pair of voltage 24 regulators 56 and one or more low voltage dc regulators 58 to provide local power supplies for the circuitry in 26 the slave unit 50. The various components within the 27 slave unit 50 typically operate using a number of.

28 different dc voltages and the number and type of 29 regulators 58 is chosen accordingly.

31 The slave unit 50 is provided with a processor 60 to 32 control the operation thereof. The processor 60 is 33 electrically coupled to a line driver 62 that transmits is 1 data (via the coupling capacitor 52) onto the 2 transmission system 12.

3 4 In certain embodiments, the slave unit 50 transmits data to the master unit 50 (via the transmission system 6 12) using frequency-shift keying (FSK), as will be 7 described. A frequency generator 64 is used to 8 generate the two required modulating frequencies F1, F2- 9 The frequencies F1, F2 are then mixed by a frequency mixer 66 to combine data from the processor 60 with 11 carrier frequency Fc: and the modulating frequencies F3-, 12 F2 13 14 The slave unit 50 further includes a signal recovery circuit 68 to extract data from the transmission system 16 12 generated by the master unit 10. A timer circuit 70 17 is used to perform pulse-width measurements on the data 18 extracted by the signal recovery circuit 68.

19 The slave unit 50 is provided with an analogue signal 21 conditioning circuit 74 and one or more analogue-to 22 digital (A/D) convertors 76. The analogue conditioning 23 circuit 74 and the A/D convertors 76 allow a plurality 24 of different types of instrumentation and/or sensors (not shown) to be coupled to the system. Thus, the 26 slave unit 50 monitors these instruments and sensors 27 and transmits data procured by them to the master unit 28 10 for collection and analysis.

29 The slave unit 50 can accept a wide range of sensors 31 and any electronic sensor that can be conditioned and 32 measured using a processor can be used with the system.

33 Typical sensor inputs to the analogue signal 16 1 conditioning circuit 74 comprise either analogue 2 sensors with voltage outputs, or those with frequency 3 or pulse outputs. Typical examples of analogue sensors 4 that may be used to collect information include pressure sensors, temperature sensors, accelerometers 6 and fluid depth sensors (resistive or capacitive).

7 Typical examples of frequency or pulse output sensors, 8 include shaft speed indicators, high accuracy pressure 9 and temperature sensors and flow meters. These are exemplary only, and the range of applications will be 11 apparent to those skilled in the art.

12 13 The analogue sensors coupled to the system can be 14 powered from the low-level regulators 58 in the slave unit 50. The voltage or current outputs from the 16 sensors is typically amplified and/or filtered in the 17 analogue signal conditioning circuit 74 if required, 18 and the (amplified and/or conditioned) outputs fed into 19 the multiplexed A/D convertor 76. The outputs are then fed to the processor 60 for transmission in 21 digital format.

22 23 The data system architecture within the system 24 typically operates using 16 or 24 bit data words for transmission, and read values can be transmitted to the 26 master unit 10 as A/D counts in either 16 or 24 bit 27 words, depending upon the required accuracy and 28 resolution of the measurements. 29 30 Where pulse or frequency signals are output from the 31 sensors, a reciprocal counter could be used to measure 32 the frequency locally in the analogue signal 33 conditioning unit 74. The processor 60 typically forms 17 1 part of the reciprocal counter to minimise or reduce 2 the electronics required in the slave unit 50.

3 4 In addition to sensor measuring capabilities, the system could be utilised to control loads. As the 6 system can support two-way communication, any 7 electronic control that can be implemented with the 8 local processor 60 can be implemented using the 9 telemetry system. For example, the slave unit 50 may be used to control solenoids to operate and control 11 actuators, hydraulic valve mechanisms, motors or other 12 apparatus that open valves, or other similar functions.

13 14 Operation of the telemetry system shall now be described. The processor 20 in the master unit 10 16 applies a power waveform to the driver 22 under command 17 from the remote station. The driver 22 drives the 18 power driver 24 that applies a square-wave power 19 waveform (Fig. 2a), the power waveform being transmitted to the transmission system 12 through the 21 coupling capacitor 14.

22 23 Fig. 2a shows an exemplary power signal waveform that 24 is transmitted from the master unit 10 to the slave unit 50. The frequency of the waveform may be any 26 suitable frequency; a typical frequency range may be 27 from 10 millihertz (mHz) to 6 kilohertz (kHz) although 28 frequencies outwith this range may be used. Where 29 there is even a moderate bandwidth on the transmission system 12, the frequencies used to transmit power from 31 the master unit 10 to the slave unit 50 will be from 32 100 Hz to 100 kHz.

33 18 1 The amplitude of the waveform is variable and is 2 dependent upon the loading conditions and losses of the 3 transmission system 12. The voltage amplitude of the 4 square-wave power waveform (Fig. 2a) is controlled by the processor 20 using a regulator (e.g. one of the 6 plurality of regulators 18). By controlling the 7 amplitude of the power waveform, using a processor 20, 8 the amplitude may be monitored and adjusted either 9 manually or automatically to set and keep the amplitude constant in varying operating conditions.

11 12 The slave unit 50 receives an attenuated power input 13 from the transmission system 12 through the coupling 14 capacitor 52. Any background noise or other interference will be added to the power signal during 16 transmission from the master unit 10 to the slave unit 17 50, thus resulting in a degraded signal being detected 18 at the slave unit 50. The power is rectified through 19 the half-bridge rectifier 54 and is then regulated in the regulating units 56, 58 to provide the local power 21 supply for the various circuitry within the slave unit 22 50.

23 24 Fig. 2b illustrates how data may be transmitted from the slave unit 50 to the master unit 10. Data is 26 transmitted using frequency-shift keying (FSK) in a 27 continuous stream during transmission. Two FSK 28 frequencies F1, F2 are superimposed on a carrier 29 frequency Fc. In the example shown in Fig. 2b, the carrier frequency F, is the same frequency as the power 31 waveform of Fig. 2a, and the data transmission is 32 synchronised to the "high" cycle of the power waveform 33 shown in Fig. 2a.It should be noted that the data may 19 1 be synchronised to the "low" cycle, or in both the high 2 and low cycles. This synchronisation allows the master 3 unit 10 to correctly detect the data transmission from 4 the slave unit 50.

6 The frequencies F1, F2 used to transmit data from the 7 slave unit 50 are typically in the order of several 8 hundred kilohertz (kHz). For example, the transmit 9 frequencies F1, F2 from the slave unit 50 to the master unit 10 may be 300 kHz for a logic one and 100 kHz for 11 a logic zero. Thus, if a logic one is to be 12 transmitted, then the higher of the two FSK frequencies 13 (i.e. Fj) will be transmitted for the duration of the 14 high cycle of the power waveforml, and if a logic zero is to be transmitted, the lower of the two FSK 16 frequencies (i.e. F2) is transmitted for the duration of 17 the high cycle of the power waveform.

18 19 The two FSK frequencies F1, F2 are preferably not multiples of one another.to minimise the occurrence of 21 false detections. The two frequencies are typically 22 also at least a factor of two different. Although this 23 increases the amount of bandwidth required on the 24 transmission system 12, it allows for the recovery of highly attenuated signals. Where there is significant 26 inductance on the transmission system 12, much lower 27 frequencies may be used. This.reduces the speed of the 28 system, but does not affect the ability of the system 29 to transmit and receive data. Low carrier frequencies may be used (in the order of a few hertz) with very 31 high frequency data carriers to increase data recovery 32 in noisy environments, such as that downhole. Where 33 low frequencies are required, the system may also be 1 used with fractions of a hertz for the carrier, and a 2 logic zero frequency of, for example, 100 Hz and a 3 logic one frequency of, for example, 350 Hz.

4 Power across the slave unit 50 can be adjusted to 6 provide the power supplies necessary for the type of 7 electronics being operated. For example, for any 8 instrument systems being operated downhole, +15 volts 9 is normally required. Thus, the ac power across the (downhole) slave unit 50 will be in the order of +30 11 volts to maintain the power supplies at a stable level 12 (due to losses etc).

13 14 The data recovery circuit 26 in the master unit 10 operates asfollows. The low-level signal transmitted 16 by the slave unit 50 is sensed using a sense resistor 17 42. The signal from the slave unit So develops a 18 voltage across the sense resistor 42 as the output of 19 the push-pull power driver 24 is effectively an ac ground.

21 22 The value of sense resistor 42 is typically twenty 23 times the resistive value of the transmission system 24 12. For example, if the resistive value of the transmission system 12 is 10 ohms (Q) from a master 26 injection point 44 to the slave unit 50, then the sense 27 resistor 42 would have a value of 2000. The value of 28 this sense resistor 42 can be chosen to match the 29 particular transmission system 12.

31 The raw signal from the sense resistor 42 is then 32 processed by the first data channel that includes the 21 1 first signal recovery circuit 26, and is fed through an 2 analogue high-speed switch (not shown, but forms part 3 of the signal recovery circuit 26). The processor 20 4 or a local zero-crossing detection circuit or the like, enables data to be directed to the first data channel 6 only when the power waveform is high, thus facilitating 7 the synchronisation. The data channel then only 8 receives and processes valid segments of the recovered 9 data. It should be noted that the triggering mechanism for directing data into the data channel may be 11 configured to allow transmission when the power 12 waveform is low, or both when it is high and low.

13 14 The sampled data is then fed through the filtering system 28 which, in simple applications, typically 16 comprises a single broad bandpass filter. In noisy 17 applications, it is preferable to use a pair of 18 selective filters designed for each transmit frequency 19 F1, F2 - It may also be necessary in exceptionally noisy environments to use tuned filters.

21 22 The signal recovered from the filtering system 28 is 23 then fed through an automatic gain control (AGC) stage 24 30. The AGC stage 30 maintains the amplitude of the recovered signal within a set amplitude range. The 26 frequency response of the AGC stage 30 is typically 27 sufficient to allow the AGC amplifier to correct for 28 changes in amplitude over one cycle of either the 29 transmission medium power frequency or the telemetry system power frequency, whichever is the higher 31 frequency. The AGC stage 30 performs two functions.

32 It compensates for the difference in amplitude from the 33 high carrier to the low carrier frequency received 22 1 (i.e. the difference in amplitudes between F, and F2) 2 In addition, there may also be variations in the 3 amplitude of the received signal over the time period 4 of the high cycle of the power waveform. frequency (i.e.

variations in amplitude of the signal during a single 6 bit transmission). The AGC stage 30 must be able to 7 react quickly enough to compensate for these changes 8 without becoming unstable. Thus, the frequency 9 response of the AGC stage 30 is related to the frequency of the power waveform, and the bandwith of 11 the AGC stage 30 is typically 10 times greater than the 12 power waveform frequency (i.e. ten times greater than 13 the baud rate).

14 The recovered signal is then fed into the comparator 16 stage 32, the output of which is fed into the first 17 counter 34. The comparator stage 32 compares the 18 signal level of each of the two FSK frequencies F1, F2 19 to establish which is present. The output of the comparator stage 32 is a signal that contains either 21 one of the two FSK frequencies F1, F2The first 22 counter 34 then counts the number of pulses in the 23 signal from the comparator stage 32, and the processor 24 20 reads the value in the first counter 34 to determine which of the two FSK logic frequencies F1, F2 are 26 present (i.e. either the frequency relating to a logic 27 one or zero).

28 29 The slave unit 50 transmits in a continuous stream of digital data (i.e. ones and zeros), with each high 31 cycle of the power waveform containing one of the two 32 FSK frequencies F3., F2 representing either a logic one 33 or zero. The process is thus continued for each high 23 1 cycle of the power waveform to determine whether a one 2 or a zero was transmitted in each high cycle. Once the 3 processor 20 has determined whether a one or a zero was 4 sent in each high cycle, the transmitted digital data from the slave unit 50 may be reconstructed by the 6 processor 20.

7 8 The slave unit 50 may also transmit bursts of 9 transmitted data in a poll response mode. In the poll response mode, there are three states for transmission 11 from the slave unit 10 to the master unit 50: a logic 12 one, a logic zero and a "none" state. Thus, when not 13 requested to transmit data the slave unit 50 ceases 14 transmission. This poll response mode is typically used where multiple slave units 50 are operating on the 16 same transmission system.

17 18 Figs 3a and 3b illustrate a power and data transmission 19 waveform.1 respectively, for the transmission of data from the master unit 10 at the surface to the slave 21 unit 50. Data is transmitted from the master unit 10 22 to the slave unit 50 using pulse-width modulation. Use 23 of this technique allows the signal recovery circuitry 24 in the slave unit 50 located downhole to be less complex than that in the master unit 10, thus reducing 26 the size, cost, complexity and power consumption of the 27 slave unit 50.

28 29 Fig. 3a illustrates the power waveform transmitted when data is being transmitted from the master unit 10 to 31 the slave unit 50,. In order to transmit digital data, 32 the widths of the pulses in the waveform are modified 33 to represent either a digital zero or one.This 24 1 technique is termed pulse-width modulation. Fig. 3b 2 illustrates the difference in pulse-widths between a 3 logic one and zero as an example. There are typically 4 three different pulse-widths (frequencies) used, each relating to either a logic one, a logic zero or an idle G state. The idle state is typically used to aid 7 specific command recovery in the slave units 50. For 8 example, where there is more than one slave unit 50 9 coupled to the system, each unit 50 remains in the idle state and polls the data transmissions from the master 11 unit 10 until it receives a command intended for that 12 particular unit 50 identified by a command string. 13 14 When data is transmitted from the master unit 10 to the is slave unit 50 using pulse- width modulation, the signal 16 received at the slave unit 50 is fed through a second 17 ac coupling capacitor 72 into a signal recovery circuit 18 70 that includes an amplifier and filtering system. 19 The signal is amplified or attenuated, depending upon 20 the application. 21 22 The value of the coupling capacitor 72 is determined by 23 the relative frequency of the main transmission system 24 power, and the frequency of the telemetry power carrier 25 F,. The value is chosen so that the capacitive 2G decoupling acts as a high-pass filter to remove 27 substantially all of the transmission system power 28 waveform whilst recovering as much of the telemetry 29 system power waveform as possible. 30 31 The requirement to either attenuate or amplify the 32 signal after decoupling depends upon the attenuation of 33 the high-pass filter described above. As the signal is 1 superimposed on the power waveform, it will have a 2 substantial peak-to-peak voltage at the slave unit 50 3 connection. If this large voltage signal is decoupled 4 without any substantial losses, the recovered signal fed to the first stage amplifiers in the signal 6 recovery circuit 68 will exceed the supply rails and 7 will thus require to be attenuated.

8 9 However, if the signal is decoupled with a substantial amount of low frequency rejection (i.e. through a high- 11 pass filter), then the signal fed to the first stage 12 amplifier will be relatively small and will thus 13 require to be amplified. The requirement to amplify or 14 attenuate the signal is dependent upon the relative frequency of the power waveform to the transmission 16 medium frequency.

17 18 The recovered and filtered signal is then fed into a 19 processor-controlled timer circuit 70. The timer circuit 70 may be replaced by a re-triggered 21 monostable. The timer circuit 70 allows pulse-width 22 measurements to be taken to determine whether a one or 23 a zero was sent. The processor 60 can then reconstruct 24 the data transmission from the master unit 10 to the slave unit 50 by analysing and recording each pulse- 26 width in turn to determine the sequence of ones and 27 zeros in the data transmission. 28 29 Data sent by the master unit 10 to the slave unit 50, 30 or vice versa, is typically encrypted by use of a 31 Hamming Code, or any other suitable data error 32 detection and correction encoding scheme. The data 33 from the master unit 10 can also include the address of 26 1 the slave unit 50 in the command so that several slave 2 units 50 may receive different and individual commands 3 from the same master unit 10.

4 Where the transmission system 12 is particularly noisy G or there is a large degree of background interference,

7 it is often not possible to determine from the method 8 described above whether a logic one or zero was 9 transmitted. To overcome this, the recovered data is not measured as one of two frequencies in windows 11 delineated from the power waveform, but each data 12 detection window that is seen by the processor 20 at 13 the surface is sub-divided into several sub-windows.

14 Figs. 4a to 4d illustrate this technique.

i's 1,6 To operate correctly using this sub-dividing technique, 17 it is preferable to use a second data channel within 18 the master unit 10 that includes the second signal 19 recovery circuit 36, the timer circuit 38 and the second counter 40.

21 22 The telemetry system may he coupled to any transmission 23 system 12. For example, it may be coupled to existing 24 power cabling that is used as a power source for a downhole motor pump. Fig. 4a shows a typical power 26 waveform that may be present on the transmission system 27 12 and may be, for example, a power waveform that is 28 driving a downhole motor. The second data channel in 29 the master unit 10 is used to determine the fundamental operating frequency of the power waveform for the 31 downhole motor. The processor 20 within the master 32 unit 10 uses the second counter 40 to establish the 33 frequency at which the power waveform on the 27 1 transmission system 12 is operating at, using a similar 2 technique as described above to determine whether a 3 zero or a one was sent. The processor 20 then 4 synchronises the transmitted power for the slave unit 50 (waveform shown in Fig. 4b) to the same frequency, 6 or a multiple thereof, as the power frequency of the 7 transmission system 12. Thus, the power and data 8 transmissions are synchronised to the frequency of the 9 power transmission system 12 over which they are transmitted (i.e. they are synchronised with the source 11 of the noise which can cause a loss of signal) thus 12 reducing the effect of the noise.

13 14 Fig. 4b shows an exemplary power waveform for transmission of power and/or data from the master unit 16 10 to the slave unit 50. The waveform shown in Fig. 4b 17 is similar to that shown in Fig. 2a and may operate 18 over the same frequency range (i.e. in the order of a 19 few mHz to several kHz).

21 Fig. 4c illustrates the data transmission from the 22 slave unit 10 downhole to the master unit 50 at the 23 surface. The waveform is similar to that shown in Fig.

24 2b wherein the data transmission is superimposed upon and synchronised to the high cycle of the power 26 waveform. Although the example shows data being 27 superimposed on and synchronised to the high cycle, it 28 should be noted that data could also be superimposed on 29 and synchronised to the low cycle or both. 30 31 Frequency-shift keying (FSK) is used to transmit data 32 from the slave unit 50 to the master unit 10. In the 33 example shown in Fig. 4c, F, which represents a logic 28 1 one is 200 kHz and F2 which represents a logic zero is 2 90 kHz. As with the previous example, the two FSK 3 frequencies F1, F2 are preferably not multiples of one 4 another to minimise the occurrence of false detections.

The two frequencies F1, F2 are typically also at least a 6 factor of two different. Although this increases the 7 amount of bandwidth required on the transmission system 8 12, the telemetry system allows for the recovery of 9 highly attenuated signals. Where there is significant inductance on the transmission system 12, much lower 11 frequencies may be used. This reduces the speed of the 12 system, but does not affect the ability of the system 13 to transmit and receive data. Low carrier frequencies 14 may be used (in the order of a few hertz) with very high frequency data carriers to increase data recovery 1)6 in noisy environments, such as those downhole. Where 17 low frequencies are required, the system may also be 18 used with fractions of a hertz for the carrier, and a 19 logic zero frequency of 100 Hz and a logic one frequency of 350 Hz.

21 22 The frequency used to transmit data to and from the 23 slave unit 50 is typically several hundred kilohertz 24 (kHz). For example, the transmit frequencies from the slave unit 50 to the master 10 may be 200 kHz (FI) for a 26 one and 90 kHz (F2) for a zero. Thus, if a logic one is 27 to be transmitted, then the higher of the two FSK 28 frequencies F, (i.e. 200 kHz) will be transmitted for 29 the duration of the high cycle of the power waveform, and if a logic zero is to be transmitted, the lower of 31 the two FSK frequencies F2 (i.e. 90 kHz) is transmitted 32 for the duration of the high cycle of the power 33 waveform.

29 1 2 However, where the transmission system 12 is 3 particularly noisy, for example where the transmission 4 system 12 is also used to drive a downhole motor, it may not be possible to determine whether a logic one or 6 zero was sent from the basic counter discrimination 7 technique. A further technique is used to aid in 8 discriminating between a logic one and zero which sub- 9 divides each of the data windows in the data transmission waveform into a series of sub-windows. An 11 example of a sub-window is shown in Fig. 4d, which is 12 an enlarged view of one of the data windows from the 13 waveform. in Fig. 4c. The data window is sub-divided 14 into a number of sub-windows, such as ten shown in Fig.

is 4d. Each of the ten sub-windows is then studied and 16 measurements taken to determine which of the two FSK 17 frequencies (i.e. F, or F2) is present within that sub- 18 window. 19 20 Every high period (the receive window) of the slave 21 unit power waveform (Fig. 4c) is segmented in the 22 processor code into smaller time slots (typically ten 23 per receive window). When the system is first 24 initiated, or on command from the master unit 10, the 25 slave unit 50 transmits a specified pattern of ones and 26 zeros to calibrate the transmission data windows. The 27 master unit 10 receives and processes this pattern and 28 determines from the pattern received the reliability of 29 the recovered data. The reliability of the recovered 30 data indicates which of the sub-windows in the received 31 window has reliably transmitted a one or a zero. The 32 sub-windows in which a one or a zero cannot be reliably 33 recovered are mapped as being "not usable" in the 1 memory of the processor 20 and are thus not used for 2 data recovery. In this way, the reliability of the 3 system is increased, as the test transmission allows 4 the system to assess which sub-windows are being affected by noise and other interference, these sub 6 windows then being ignored for future transmissions.

7 This technique allows for enhanced reliability and also 8 the ability to allow the system to be calibrated to 9 particular environments.

11 Thus, this technique provides a method of data 12 transmission and recovery that uses sub-divided and 13 synchronised data recovery windows to enhance the noise 14 immunity of the system, and also the use of a is calibrating pattern to allow the master unit 10 to 16 determine the reliable portions of the recovered data 17 transmitted by the slave unit 10.

18 19 A particular use of the telemetry system of the present invention would be on a three-phase ac electrical 21 system, such as system 100 shown in Fig. 5. System 100 22 includes a three-phase surface transformer 102 that 23 transmits power to a motor load or three-phase 24 transformer 104 located, for example, downhole. The transmitted power waveform on any one-phase may be 26 similar to that shown in Fig. 4a.

27 28 The three-phase system has inherent cable resistance 29 and inductance schematically illustrated by resistors 106. The three-phase system 100 also has inherent 31 cable capacitance between each of the cables of the 32 system 100, schematically shown by capacitors 108, and 31 1 cable capacitance between the cables and earth, 2 schematically shown by capacitors 110.

3 4 The master unit 10 is ac coupled to each of the three- phases using capacitors 112 in parallel, and similarly 6 the slave unit 50 is ac coupled to each of the three- 7 phases using capacitors 114 in parallel. This creates 8 a balanced voltage condition at the coupling point of 9 the master unit 10 (i.e. the coupling capacitors 112).

The slave unit 50 is coupled through three coupling 11 capacitors 110 creating a balanced point with respect 12 to the three-phases at the connection point for the 13 slave unit 50 (i.e. at the coupling capacitors 114).

14 The system could then be synchronised as previously described to the fundamental supply frequency of the 16 three-phase ac system. Thus, any noise from motors, 17 noisy power drives or other large machines coupled to 18 the three-phase system could be reduced by calibrating 19 the data recovery system using the technique described above. 21 22 The telemetry system could be used to monitor a 23 downhole pump (not shown) connected in place of the 24 transformer 104. The slave unit 50 may measure the 25 pressure above and below the pump in addition to 26 monitoring the temperature of the motor, the vibration 27 levels from the motor, the shaft rotation speed and any 28 other variables using appropriate sensors coupled to 29 the slave unit 50 as described above. The slave unit 30 50 would typically be mounted on the pump motor. 31 32 It is common practice to use variable speed drives 33 (VSDs) to provide the electrical supply for such 32 1 downhole motor pumps. VSDs generate a substantial 2 amount of electrical noise on the cables of the three- 3 phase system 100 that can cause considerable 4 interference with the data telemetry system. Fig. 6 shows a typical VSD generated power waveform for 6 driving a downhole motor pump.

7 8 The telemetry system in certain embodiments has a 9 particular advantage when used with this type of system as the power and data communications can be 11 synchronised to the fundamental frequency of the three 12 phase power system 100 transmitting power to the 13 downhole motor, and thus the interference can be 14 reduced so that data transmission is less affected.

is 16 A further use of the telemetry system is to couple the 17 slave unit 50 to a star point 220 in a three-phase 18 transformer or motor 204 (Fig. 8) and transmit the 19 telemetry carrier at very low frequencies to allow transmission of data through the inductance of the 21 windings. The three-phase ac system 200 shown in Fig.

22 8 is similar to the system 100 in Fig. 5, except that 23 the windings of the downhole motor or transformer 204 24 are coupled in a star point formation.

26 VSDs typically produce large voltage spikes at the star 27 point 220 at every switching point of the power 28 frequency generator of the VSDs. Fig. 7a illustrates 29 the large voltage spikes that are typically created at the star point 220.

31 32 Once the master unit 10 is synchronised to the 33 fundamental power frequency of the three-phase system 33 1 200, the interference with the telemetry system can be 2 calibrated out using the slave unit 50 calibration 3 patterns described above. This allows the master unit 4 10 to identify the time zones (e.g. the sub-windows) in which data cannot be reliably recovered due to 6 interference. In any recovery slot, the master unit 10 7 can map any of the n segments (five shown in Fig. 7d) 8 of the recovered data as unusable by determining from 9 the calibration pattern whether a logic one or logic zero can or cannot be discriminated.

11 12 Figs 7b, c and d illustrate an exemplary power waveform.

13 for transmitting power from a master unit 10 at the 14 surface to a slave unit So downhole; an exemplary is signal transmit waveform. for transmitting data from a 16 slave unit 50 to a master unit 10 using frequency-shift 17 keying (FSK); and an enlarged portion of the waveform.

18 of Fig. 7c, respectively, similar to those of Figs 4b 19 to 4d. 20 21 This application would typically require a low-pass 22 filter to be included in the filtering system 28 of the 23 master unit 10 and the filtering system included as 24 part of the signal recovery circuit 68 in the slave 25 unit 50, to remove interference from the higher 26 frequency ac power operation. 27 28 Referring to Fig. 8, it should be noted that the slave 29 unit 50 does not require to be ac coupled using 30 capacitors to the star point 220. Isolation from the 31 three-phase system 200 is not required in this 32 particular application. Indeed, as a result of the 33 attenuation of the telemetry power frequency F. and the 34 1 FSK carrier frequencies F1, F2, the system offers 2 improved performance when the coupling capacitors are 3 omitted.

4 The sensors used to monitor data at the slave unit 50 6 are typically the same as those described above. The 7 power and data transmission of the telemetry system can 8 be synchronised to the fundamental power frequency of 9 the three-phase transmission system as before.

11 It should be noted that due to the large inductances 12 that are typical of three-phase motors and 13 transformers, the telemetry power and data 14 transmissions are typically lower in frequency and can range from a few hertz to a few kilohertz. The speed 16 of the power and data transmissions typically vary 17 according to cable length and the individual motor or 18 transformer used in the specific application.

19 Thus, there is provided a telemetry system that, in 21 certain embodiments, allows for both power and 22 communication transmissions across a single- or three- 23 phase ac power system. The power system may be a 24 three-phase power cable, monoconductor instrument cable, or other electrical conduits. The system uses 26 frequency-shift keying (FSK) and pulse-width modulation 27 to allow for the transmission of data across the 28 system. 29 30 The system in certain embodiments is flexible in that 31 it allows for a number of slave units to be located 32 remotely from one or more master units, the master 33 units being used to control the operation of the slave 1 units. The slave and master units in certain 2 embodiments are coupled to a single transmission 3 medium. The system in certain embodiments can also 4 support the use of more than one master unit to control the slave units from more than one point within the 6 system.

7 8 There is also provided a method of transmitting pulse- 9 width modulated power over a transmission medium and recovering this as both power and data. There is also 11 provided a method of transmitting frequency-shifted 12 data that is synchronised to a received power waveform.

13 14 Modifications and improvements may be made to the foregoing without departing from the scope of the 16 present invention.

Claims

36 1 CLAIMS

2 1. A telemetry system comprising a master unit, and 3 at least one slave unit remote from the master unit, 4 the master and slave units communicating via a transmission system, wherein the telemetry system is 6 capable of transmitting power and data transmissions 7 between the units.

8 9
2. A telemetry system according to claim 1, wherein pulse-width modulation is used to facilitate data 11 transmission from the master unit to the slave unit.

12 13
3. A telemetry system according to either preceding 14 claim, wherein the power transmission is modulated with is the data transmission using pulse-width modulation.

16 17
4. A telemetry system according to any preceding 18 claim, wherein frequency-shift keying (FSK) is used to 19 facilitate data transmission from the slave unit to the master unit.

21 22 S. A telemetry system according to claim.4, wherein 23 the FSK frequencies are superimposed on a carrier 24 frequency.

26 6. A telemetry system according to claim 5, wherein 27 the carrier frequency is the same frequency as the 28 power transmission frequency.

29 7. A telemetry system according to any preceding 31 claim, wherein the data transmission is synchronised to 32 the "high" cycle of the power transmission.

33 37 1 8. A telemetry system according to any preceding 2 claim, wherein the data transmission is synchronised to 3 the "low" cycle of the power transmission.

4 9. A telemetry system according to any preceding 6 claim, wherein the data transmission is synchronised to 7 both the low and high cycles of the power transmission.

8 9 10. A telemetry system according to any preceding claim, wherein more than one slave unit is located 11 remote from the master unit.

12 13 11. A telemetry system according to claim 10, wherein 14 the data transmission from the master unit to the or each slave unit includes an address of the slave unit.

176 17 12. A telemetry system according to any preceding 18 claim, wherein the data transmissions include data 19 error detection and/or correction.

21 13. A telemetry system according to claim 12, wherein 22 the data error detection and/or correction comprises a 23 Hamming code, or other suitable technique.

24 14. A telemetry system according to any preceding 26 claim, wherein the transmission system comprises a 27 single- or three-phase alternating current (ac) 28 transmission system. 29 15. A telemetry system according to any preceding 31 claim, wherein the transmission system comprises a 32 power cable, instrument cable, or other type of 33 electrical cable.

38 2 16. A telemetry system according to any preceding 3 claim, wherein the master unit and/or the slave unit 4 are ac coupled to the transmission system.

6 17. A telemetry system according to any preceding 7 claim, wherein the master unit comprises a processor to 8 control the operation of the master unit; a power 9 waveform generator; and signal recovery and conditioning circuitry.

11 12 18. A telemetry system according to claim 17, wherein 13 the processor applies pulse-width modulation to the 14 power transmission when data transmission is required is from the master unit to the slave unit.

16 17 19. A telemetry system according to claim 17 or claim 18 18, wherein the power waveform generator comprises an 19 analogue driver, and a power driver electrically coupled to the analogue driver.

21 22 20. A telemetry system according to claim 19, wherein 23 the processor applies the power transmission to the 24 analogue driver.

26 21. A telemetry system according to any one of claims 27 17 to 20, wherein the processor controls the voltage 28 amplitude of the power t. ransmission.

29 22. A telemetry system according to any one of claims 31 19 to 21, wherein'the analogue driver includes an 32 isolating circuit that isolates the power driver from 33 the processor.

39 1 23. A telemetry system according to any one of claims 2 17 to 22, wherein the signal recovery and conditioning 3 circuitry allows data transmitted by the at least one 4 slave unit to be extracted and recovered from the transmission system.

6 7 24. A telemetry system according to any one of claims 8 17 to 23, wherein the signal recovery circuit includes 9 first and second data channels.

11 25. A telemetry system according to claim 24, wherein 12 the first data channel includes a high-speed switch; a 13 filtering system; an automatic gain control (AGC) 14 stage; a comparator stage; and a first counter. 15 16 26. A telemetry system according to claim 25, wherein 17 the filtering system removes a substantial portion of 18 noise and background signals from the recovered data. 19 27. A telemetry system according to any preceding 21 claim, wherein the or each slave unit comprises a 22 processor to control the operation of the slave unit; 23 rectifying and regulating circuitry in a first channel; 24 recovery and conditioning circuitry in a second -channel; and frequency generating and mixing means. 26 27 28. A telemetry system according to claim 27, wherein 28 the rectifying and regulating circuitry comprises a 29 half-bridge rectifier to rectify the received power transmission into a dc voltage; and at least one 31 voltage regulator to regulate the dc voltage. 32 1 29. A telemetry system according to either claim 27 or 2 claim 28, wherein the recovery and conditioning 3 circuitry typically comprises an amplifier and 4 filtering system; and a timer circuit.

6 30. A telemetry system according to any one of claims 7 27 to 29, wherein the frequency mixing and generating 8 means typically comprises a frequency-shift keying 9 (FSK) generator; an FSK mixer; and a line driver.

11 31. A telemetry system according to any preceding 12 claim, wherein the slave unit includes an analogue 13 signal conditioning circuit, and at least one analogue 14 to-digital convertor.

16 32. A telemetry system according to claim 31, wherein 17 the analogue conditioning circuit allows the slave unit 18 to receive and process signals from one or more sensors 19 coupled to the slave unit.

21 33. A telemetry system according to any preceding 22 claim, wherein the slave unit is capable of controlling 23 loads.

24 34. A method of transmitting power and data from a 26 master unit to at least one slave unit remote from the 27 master unit, the master and slave units communicating 28 via a transmission system, the method comprising the 29 steps of generating a power transmission at the master 31 unit; 41 1 generating a data transmission and synchronising 2 the data transmission with the power transmission 3 at the master unit; 4 transmitting the power transmission and data transmission via the transmission system to the 6 slave unit; and 7 recovering the power and data transmissions at the 8 slave unit.

9 35. A method of transmitting data to a master unit 11 from at least one slave unit remote from the master 12 unit, the master and slave units communicating via a 13 transmission system, the method comprising the steps of 14 generating a power transmission at the master unit is and transmitting the power transmission to the slave unit; 17 recovering the power transmission at the slave 18 unit; 19 generating a data transmission at the slave unit and synchronising the data transmission with the 21 power transmission; 22 transmitting the data transmission via the 23 transmission system to the master unit; and 24 recovering the data transmission at the master unit.

26 27 36. A method according to either claim 34 or claim 35, 28 wherein the method includes the further steps of 29 dividing the data transmission into a series of sub-windows; 31 transmitting a specified data transmission from 32 the slave unit to the master unit; 42 1 receiving the specified data transmission at the 2 master unit; and 3 determining which of the sub-windows reliably 4 transmitted the specified data transmission.

6 37. A method according to claim 36, wherein the sub- 7 windows that did not reliably transmit data are ignored 8 for subsequent transmissions.

9 38. A method of receiving and converting power and 11 data transmissions sent from a master unit to at least 12 one slave unit remote from the master unit, the master 13 and slave units communicating via a transmission 14 system, the method comprising the steps of receiving a power transmission at the slave unit; 16 dividing the power transmission into two channels; 17 rectifying and regulating the power transmission 18 in a first channel; and 19 recovering the data transmission in a second channel.

21 22 39. A method of receiving data transmitted by a master 23 unit from at least one slave unit remote from the 24 master unit, the master and slave units communicating via a transmission system, the method comprising the 26 steps of 27 receiving the data transmission at the master 28 unit; 29 filtering and conditioning the data transmission; and 31 regenerating the transmitted data.

32 43 1 40. A method according to either claim 38 or claim 39, 2 wherein the method includes the further steps of 3 dividing the data transmission into a series of 4 sub-windows; transmitting a specified data transmission from 6 the slave unit to the master unit; 7 receiving the specified data transmission at the 8 master unit; and 9 determining which of the sub-windows reliably transmitted the specified data transmission.

11 12 41. A method according to claim 40, wherein the sub13 windows that did not reliably transmit data are ignored 14 for subsequent transmissions. is 16 42. A method according to any one of claims 34 to 41, 17 wherein pulse-width modulation is used to facilitate 18 data transmission from the master unit to the slave 19 unit or vice versa. 20 21 43. A method according to claim 42, wherein the power 22 transmission is modulated with the data transmission 23 using pulse-width modulation. 24 25 44. A method according to any one of claims 34 to 43, 26 wherein frequency- shift keying (FSK) is used to 27 facilitate data transmission from the slave unit to the 28 master unit or vice versa. 29 30 45. A method according to claim 44, wherein the FSK 31 frequencies are superimposed on a carrier frequency. 32 44 1 46. A method according to claim 45, wherein the 2 carrier frequency is the same frequency as the power 3 transmission frequency.

4 47. A method according to any one of claims 34 to 46, 6 wherein the data transmission is synchronised to the 7 "high" cycle of the power transmission.

8 9 48. A method according to any one of claims 34 to 47, wherein the data transmission is synchronised to the 11 "low" cycle of the power transmission.

12 13 49. A method according to any one of claims 34 to 48, 14 wherein the data transmission is synchronised to both the low and high cycles of the power transmission.

16 17 50. A method according to any one of claims 34 to 49, 18 wherein the data transmissions include data error 19 detection and/or correction.

21 51. A method according to claim 50, wherein the data 22 error detection and/or correction comprises a Hamming 23 code, or other suitable technique.

24