GB2236399A - Telemetry apparatus for magnetic flux sensors - Google Patents

Abstract

Telemetry apparatus for magnetic flux sensors comprises: at least one transmitter comprising at least one sensor (1, 2, 3); at least one receiver (data collection unit); at least one conductor (e.g. co-axial line) for transmission of electrical signals from said at least one transmitter to said at least one receiver; and at least one multiplexer switcher for controlling said transmission of electrical signals. There may be a plurality of said sensors, e.g. an array of single axial-vector sensitive magnetometers, e.g. flux gate magnetometers. The at least one multiplexer switcher may enable or disable supply of electrical power to at least one said transmitter, at least one electrical signal thereby being able to be transmitted via at least one said conductor, in the absence of said electrical power, and such that during that absence the corresponding at least one transmitter will be self sustaining for at least one predetermined period of time. <IMAGE>

Description

TELEMETRY Telemetry relates to transmitting from at least one transmitter means to at least one receiver means, electrical signal(s) comprising and/or constituting information. For the transmitting, the electrical signal(s) may be transmitted from at least one transmitter means to at least one receiver means via at least one conductor means (e.g. at least one transmission line), the number of said conductor means being e,g, in mathematical relationship with the number of transmitter means and/or the number of receiver means. Any transmitter means may comprise at least one sensor means for sensing information so as to provide at least one said electrical signal, which may be a "raw signal to be processed by at least one processor means.Any said processor means may be comprised by at least one transmitter means, or by at least one receiver means, or by at least one conductor means, or by any combination selected therefrom. Any said sensor means may be constituted in any suitable manner, e.g. comprise at least one magnetometer means, for instance magnetometer means available from BARTINGTON INSTRUMENTS LTD. of Spendlove Centre, Enstone Road, Charlbury, Oxon. OX7 3PQ, England. Some examples of such magnetometer means comprise high quality fluxgate magnetometers designated MAG-O1 and !RG-OlH (see later below).

A fluxgate magnetomer means may comprise: at least one magnetisable core; at least one magnetising conductor disposed relative to a corresponding said core such that AC current passed through said core will magnetise said core or demagnetise said core, e.g. so as to enable it to become suitably magnetically saturated or suitably magnetically unsaturated, the magnetic permeability of said core being variable in response to said AC current; and at least one inductor disposed relative to said core and said at least one conductor such that at least one electrical signal current will be induced in said at least one inductor in response to change in detectable magnetic flux density relative to time, in said core, the magnetic flux density being external and/or internal of said core.

Transmission of electrical signal from at least one transmitter means to at least one receiver means via at least one conductor means may be controlled by at least one multiplexer switching means. Any multiplexer switching means may be comprised by at least one said transmitter means, or by at least one said receiver means, or by at least one said conductor means, or by any combination selected therefrom.

One aspect of the present invention provides telemetry apparatus, comprising: at least one transmitter means, comprising at least one sensor means (optionally at least one magnetometer means, e.g. at least one flux gate magnetometer means )for sensing magnetic flux to derive transmittable information therefrom; at least one receiver means;at least one conductor means for transmission of electrical signals from said at least one transmitter means to said at least one receiver means (said conductor means optionally comprising at least one transmission line), the number of said conductor means optionally being in mathematical relationship with the number of said transmitter means and/or the number of said receiver means; and at least one multiplexer switching means for controlling said transmission of electrical signals, the number of said multiplexer switching means optionally being in mathematical relationship with the number of said transmitter means and/or the number of said receiver means and/or the number of said conductor means.

A second aspect of the present invention provides multiplexer switching means for the first aspect of the invention, said multiplexer switching means comprising: at least one electrical storage means for being charged by at least one supply of current (e.g. AC or DC) such that said at least one electrical storage means will sustain or tend to sustain at least one operating function of said multiplexer switching means, and optionally thereby at least one operating function of at least one said transmitter means (e.g.

at least one operating function of at least one said magnetometer means), Said at least one supply of current may be comprised by, or not comprised by, said telemetry apparatus of the present invention.

Telemetry apparatus of the present invention may energise or power said at least one transmitter means (e.g. power said at least one magnetometer means) via at least one corresponding said multiplexer switching means of the present invention at at least one appropriate time, e.g. according to a predetermined program for periodically switching on or off of said at least one supply of current to at least one said multiplexer switching means. Thus, the telemetry apparatus may time the coming onto or off of respective multiplexer switching means, whereby at least one operating function of at least one transmitter means may be synchronised, or otherwise timed, relative to at least one operating function of at least one said receiver means.

The present invention also includes methods of telemetry utilising the above aspects of the invention.

Further aspects of the invention are described later below with reference to the accompanying diagrams.

One advantage provided by the present invention is that the at least one multiplex switching means may enable or disable supply of electrical power to at least one said transmitter means at any required time, at least one electrical signal thereby being able to be transmitted via said at least one conductor means, in the absence of said electrical power. During that absence the corresponding at least one said transmitter means may be self sustaining for at least one predetermined period of time.

The accompanying diagrams (Figs. 1 to 22) are by way of example of one embodiment of the invention; these drawings do not restrict the present invention, which includes all novel disclosures comprised by the present specification and the diagrams.

In general, a magnetometer array comprises any suitable number of magnetometer means in any suitable distribution(s). A magnetometer array may sense vector components of magnetic fields. Any magnetometer sensor may comprise at least one suitable magnetometer means SECTION 1 TRIPLE MAGNETOMETER ARRAY This text describes the construction and performance of a 3 channel magnetometer system for laboratory use.

The LMA 03 comprises two components: i. A magnetometer array consisting of three single axial vector-sensitive magnetometer sensors each connected to a coaxial cable. Each magnetometer may be internally addressed to transmit data on any transmission channel. (See diagrams 1 and 2) ii. A Data Collection Unit (DCU) which supplies power to the array and outputs the magnetic measurement values via three output ports.

Each port provides analogue voltage output and visual display output. Each port may be internally addressed to output data from any data transmission channel. (See diagram 3) THE SENSORS These are of the fluxgate magnetometer type. The sensing element employs a dual balanced core and therefore radiates very little electromagnetic noise. A feedback solenoid and a high gain feedback amplifier maintain the core material close to the null condition and thereby ensure very high measurement linearity. The magnetometer has a measuring range of + 200 guT and will tolerate exposure to high field strengths without damage.

The sensors are individually trimmed for calibration and offset and are therefore fully interchangeable between channels.

The sensors are housed in a rugged Acetal cylindrical enclosure suitable for laboratory use (see diagram 4). The sensor number and the channel number are marked at one end of the enclosure by a replaceable label. The position of the label within a recess also identifies the magnetic polarity of the sensor.

MULTIPLEXER SYSTEM Data from each magnetometer is multiplexed sequentially onto the data transmission line sixteen times per second.

THE DATA COLLECTION UNIT (see diagram 5) This A.C. mains powered unit provides the power and drive signals required by the array. It is housed in a low profile plastic enclosure. On the front panel there are three 4 % digit liquid crystal displays associated with ports 1-3.

These display directly in UT units and the least significant digit gives a maximum resolution of 10 nT. During over range the polarity and most significant digit remain displayed but the other digits appear blank. Situated on the rear panel are the A.C. supply inlet and switch, the connector for the array and three analogue output connectors associated with ports 1-3.

MINIATURISATION Consideration has been given to the future requirement for miniaturisation of the sensors. All components used in this design are available in "Small Outline" packaging and would be surface mounted. Several of the "standard packaged" components used in the sensors have a ferrous metal content which prevents the magnetometer elements being mounted close to these components. This will not be the case for their surface-mount counterparts. A sample batch of all the component surface--mount counterparts have been measured for magnetic remanence and susceptibility and found to be suitable.

TELEMETRY The analogue data transmission system described overcomes the problems which can occur in a digital system These are:i. Suitable 14-16 bit A-D converters are not readily available in small outline surface mount technology, for incorporation in a miniaturised version.

ii. The incorporation of A-D converters would add to the final trimming and calibration complexity.

iii. Power would be increased.

iv. Component count would increase and therefore reliability would potentially be decreased.

This design employs Time Division Multiplexing to transmit analogue data from the magnetometers to the DCU. The 3 magnetometer system has been designed and tested with the object of proving the design when a number of channels are connected to the line. The line is alternately occupied between supplying power from the DCU to the sensors and transmitting analogue information in sequence from the sensors to the DCU. This method has the following advantages over comparable methods:i. Low component count for good reliability.

ii. Low power requirements. Special cables are therefore not required and because the power is switched off during measurements high currents do not circulate and therefore a possible source of electrical and magnetic errors is avoided.

All trials have been carried out over a 100 metre length of solid polyethylene core 50 ohm coaxial cable. This type of cable is recommended, however operation over 100 metres of low grade twisted pair cable is possible with some degradation of performance. Moderate lengths of unshielded cable can be included in the coaxial line with no degradation of performance.

THE SENSORS Mechanical Materials: Acetal enclosure.

Weight: 200g.

Dimensions: length 150mm.

diameter 40mm.

Electrical Voltage: 9-15 V Current: 2.2 mA &commat; O pT, 3.2 mA e e @ 200 pT.

Environmental Sealing: Splash proof, for laboratory use only. For use up to 75% relative humidity.

Operating Temperature Range: -10 to +706C.

Fluxpate Element Dimensions: length 30mm, diameter 5mm.

Position: axial to enclosure, with centre of sensitive volume at centre of label on enclosure.

Markings Label: carries probe and channel identification numbers.

Magnetic Polarity Conventions: produces positive values when label end of enclosure is pointed to magnetic North.

Channel Selection Binary coded appropriate programming of five solder pads: binary coded solder pads will select for data transmission over one of the available channels. Two or more sensors may not be allocated the same channel. Selection to same channel will not cause permanent damage. but data from that channel will be erroneous.

ARRAY CABLE Recommended/ 82 metres of coaxial cable supplied type RG58 CU.

Alternative: May include up to 10 metres of low grade twisted pair cable.

Loop Resistance: 10 ohms maximum permitted.

Capacitance: 20 nF maximum permitted.

Connector: TNC.

THE DATA COLLECTION UNIT Mechanical Materials: ABS enclosure.

Weight: 1,750g.

Dimensions: width 260mm height 63mm depth 255mm Electrical Supply Voltage: 240 V Power: 8 W.

Fuse Type: 250 mA. anti-surge 20 mm (spare included in IEC receptacle).

Environmental Sealing: Not sealed. Eor laboratory use only for use up to 75% relative humidity.

Operating Temperature Range: -lOC to + 70 C.

Array Connector Type: TNC on rear panel.

Supply voltage: +13V pk at 16Hz Supply current: 200mA short circuit protected (indefinite) Data Ports via : i Visual Display: 3 x 4K digit Liquid Crystal Display on front panel.

Displays data in the range 200 pT with + 10 nT resolution.

Overload indication by blanking off 4 least significant digits with preservation of sign.

ii Analog Output: 3 x BNC connectors on rear panel provide t 8 V Channels ver Port Each Port may be internally programmed to operate on any one of the available channels. Ports may be set to operate on a common channel. Channel selection is via internal 3 x 5 way binary coded switches.

SYSTEM PERFORMANCE Off set/Temyerature Coefficient Offset: less than 10 nT at room temperature.

Temperature Coefficient of Offset: + 1.5 nT/'C.

Calibration/TemDerature Coefficient Calibration: * 1%.

Intercalibration : # 0.1 %.

Temperature Coefficient of Calibration: + 130 ppm/'C.

Linearitv O to + 200 auT: 0.01 % worst case.

Bandwidth (see diagram 13) -3 dB: 4 Hz Roll-off: - 24 dB per octave.

Cross Talk Adjacent channel: better than v 80 dB.

Remote channel: better than - 80 dB.

( < lLSD) Measurina Ranee Measuring Range: + 200 pT.

Noise Noise: not greater than 10 nT.

Overload Recovery Time: not greater than 100 ms.

Proximity Effects An offset error of not more than 10 nT occurs with the sensing element positioned 10 mm from a ferrous surface.

Reproqrammina the Array CAUTION: Circuitry contains CMOS devices. Take normal ESD precautions when reprogramming the sensors.

i. Remove sensor identification label.

ii. Remove radial screws retaining the sensor outer cylinder.

iii. Reprogramme in accordance with diagrams 2 and 9. NOTE: No two sensors may occupy the same channel number.

iv. Replace in reverse order and fit new identification label.

Reroqramminq the DCU (see diagram 5) i. Remove DCU lid.

ii. Reprogramme the port(s) in accordance with the diagram.

The ArraY Connector This connects via a TNC plug to the TNC socket on the rear of the DCU.

NOTE: The DCU supplies 13V at a maximum current of 200mA.

The instrument will tolerate array cable short circuit conditions indefinitely.

Analoa OUtPuts The + 8V analogue outputs available on the rear panel should be loaded by not less than lic ohms. The signal levels at the highest resolution are in the sub-millivolts and multiple earthing and therefore ground loops should be avoided when connecting external test equipment.

Warm-uP Time A few (not more than two) minutes warm-up time are required to achieve ultimate stability. The system may.be left on indefinitely.

GENERAL DESCRIPTION OF CIRCUITRY (see diagram 6) The crystal controlled clock within the DCU periodically switches power onto the 50 ohm coaxial line which supplies the sensors. This is referred to as the "power phase During this time capacitors within the power supply of each sensor store sufficient energy to supply the sensors during the time for which the supply is switched off. The time interval between the power phases is referred to as the "read phase". At the commencement of the read phase programmable channel select timers within the DCU commence a timing sequence the length of which is determined by the programming switches. The commencement of the timing sequence is signalled to identical timers within the sensors via a trigger circuit. The delay length is equal in time periods to the channel number.At the end of the predetermined time period an analogue transmission gate within the sensor connects the magnetometer output to the line, and a sample and hold circuit within the DCU stores this value until refreshed. This occurs 16 times per second. The outputs from the sample and hold circuits are filtered in a -12dB per octave low pass filter. The analogue information is available as a voltage on the analogue output sockets and as a re-scaled voltage for presentation to three liquid crystal digital panel meters on the front of the DCU.

The DCU (see diagram 7) The power supply module supplies + l5V to the sample and hold devices IC 6, 7, 8 and filter circuit op-amps IC 9, 10, 11.

+15V is supplied via power switch TR1 to the array and is regulated to +5V for the digital circuitry.

Output from the 32.768Khz oscillator comprising X1 and part of IC1 is divided by IC2 to a frequency of 16Hz for control of the "power phase" (TP5). This power switching to the array is via current limited transistor TRi.. Low leakage diodes D1 and D2 which connect the power circuitry to the line are configured to prevent the flow of leakage currents from the power transistor. At the end of the power phase the line is momentarily (lObwus) held to approximately OV by TR4, and this event provides triggering information for the timers within the array. When TP5 goes low the programmable timers IC3, 4, 5, are preset to the value on the channel select switches SW3, 2, 1 respectively.TP5 going high (read phase) removes the preset and timing commences under the control of clock pulses (TP6) generated by Q5 of IC2. The clock information (1024Hz) is delayed by 70;its (IC12b) so that the start of the channel measurement will not coincide with varying analog data on the line. The read signal for ports 1, 2, 3 is supplied by the positive going pulse on TP3, 2, 1 respectively. This occurs when the contents of the programmable. timers ICs 5, 4, 3 respectively are equal to zero. The duration of the pulses which control the sample and hold circuits IC8, 7, 6 respectively is determined by monostable circuits IC13b, IC13a and IC12a.

The Data Transmission Line is connected to the terminal pins ss and 5, designated input High and Low. Connection of the sample and hold circuits to the line is effected when J1 is connected between SK6 and 7. With J1 connected between SK7 and 8 the analogue outputs appearing at Port 1, 2 and 3 may be trimmed to zero using potentiometers RV1, 2, 3 respectively. The potentiometers RV4, 5, 6 scale the f 8V full-scale analogue signal to + 2V full scale to drive the digital panel metres DPM 1, 2, 3 respectively. To scale DPM outputs to + 199.99 using potentiometers RV4, 5, 6 inject a signal of + 8.0001V into SK7 relative to SK8 with J1 removed.

The filter circuits constructed around ICs 9, 10, 11 are -12dBs per octave maximally flat (Butterworth response) and provide a -3dB point of 7Hz. Ultimate frequency response for the system is determined by the combined sensor and DCU response.

The liquid crystal panel metres may be replaced without removal of PC23 and are supplied pre-calibrated by Bartington Instruments.

The Sensors (see diagram 8) The timing circuitry within the sensor is essentially the same as within the DCU. Detection of supply voltage removal from the line occurs in the Schmitt trigger circuit comprising D6, RlO, TR5, Rill, R12, R13 and part of IC1. A narrow trigger pulse resets the 1024Hz clock divider IC6 and the timing sequence is commenced under the control of the crystal oscillator X1 and part of IC1. At the end of the timing sequence the transmission gate IC3, pins 9 to 10, closes and connects the output of the magnetometer filter ICS, pin 1. The commencement of the line connection is delayed by approximately 5O'is to prevent overlap of consecutively timed channels.

Power is supplied to the sensor circuitry via low leakage diode D5. This voltage appears at current limiting resistor R9 and is stored at capacitor C5. Transistor combination TR3, TR4 provide an 8kHz 4V p-p signal which is boosted via C10, C12, D1, D2, D3 to 15V (Vb) to provide a low current supply to IC3, which during operation will always be substantially above the peak line voltage.

IC3 Analogue Devices AD7510 DI is a dielectrically isolated analog transmission gate which can withstand momentary application of voltages up to 25V in excess of the supply voltage. At all times this device is operated very well within its safe operating range.

The 13V p-p l6kHz signal appearing at IC3, pins 12 and 13, is converted to -12V (Vc) to provide the necessary negative supply for IC3 and IC5.

The magnetometer section comprises the following:- A probe. (See diagrams8g9XIOXll and 12) This carries 5 windings. Windings 1 and 2 are the excitation windings and receive pulses of current at a repetition frequency of 256Hz via TR1, TR2, C2, C2, C3. Windings 1 and 2 surround wire cores made of high magnetic permeability metal alloy consisting of approximately 80% Nickel and 20% Iron. The magnetic flux generated in the windings 1 and , 2 repeatedly drive the cores in and out of magnetic saturation. Any static incident magnetic field B will therefore be modified by the changing permeability of'the cores to generate an alternating magnetic field. This alternating field is detected by induction in winding 3.Winding 3 is a combined signal pick-up and feedback winding. The signal produced by the probe in the presence of a magnetic field appears across C6, which whilst IC3, source and drain pins 16 and 15 are momentarily closed stores the peak value. This voltage is filtered and fed to the integrating amplifier ICS, C17, and fed back to Null the detected magnetic flux producing the signal. In this way the magnitude of the voltage appearing at IC5, pin 7, is in proportion to the feedback constant of the winding W3 and the value of R3 and is a linear analog representation of the measured flux density. Calibration of the magnetometer is accomplished by select-on-test modification of the value of R3.This value is further filtered in passive filter R6, C8, the corner frequency of which is 1SHz and provides protection against aliasing of the measured alternating value and the line sampling frequency whilst preserving maximally flat frequency response within the bandwidth specified for the system.

Probe windings W4 and W5 provide trimming of any small residual offset errors within the magnetometer and derive their current via select-on-test resistors R1 or R2 from precision +5V (Vd) voltage source D9, which also supplies the digital circuitry.

BANDWIDTH Diagram 13 shows the analog output response at the DCU to alternating magnetic field for any channel.

SECTION 2 MULTIPLE MAGNETOMETER ARRAY INTRODUCTION This text describes an array of fluxgate magnetometers which interface to a computer via an RS 232 serial interface. The magnetometery and telemetry methods are as previously described but the possible number of sensors has been expanded and the sensor circuitry miniaturised by the use of surface mount technology.

GENERAL DESCRIPTION (See diagrams 14,15 and 16) The system comprises two parts: 1) A Linear Array of axially sensing vector magnetometers housed in cylinders which are spliced into a cable.

2) A Data Collection unit which houses two units, the Array Control Module and the Analogue to Digital Converter. The Array Control Module supplies power to the array, and synchronises and receives time division multiplexed analogue data from the array.

The A-D converter, under computer control, selects any of the available analogue channels for conversion into serial digital format.

PATA COLLECTION UNIT (See diagrams 15 and 16) The Data Collection unit comprises a rack enclosure which houses the Arra control module and the A-D converterJserial interface.

This unit interfaces the magnetometer array to a computer via an RS232 link.

ARRAY CONTROL MODULE This unit provides TDM control and power to the magnetometer array and interfaces stored analogue data via parallel digital multiplexed control to the CIL Microsystems A-D converter.

THE A-D CONVERTER This is a 16 bit A-D converter with serial, interface. It multiplexes analogue data from the array control module end converts this into serial ASCII data for computer processing.

The unit is manufactured by CIL Instruments Ltd, Lancing, West Sussex, England.

CIRCUIT DESCRIPTIONS ARRAY CONTROL MODULE (See diagrams 17 and 18) This comprises a power supply section and a circuit to perform power control and synchronisation of the time division multiplexing of the array telemetry.

MULTIPLEX CONTROL CIRCUIT (See diagram 19) This functions in the following way. Crystal oscillator X1, U1 provides a 31.25ms on and 31.25ms off wave form (16 Hz) via U4. This switches current limited power transistors T1 on and off to provide 13V supply voltage to the array. When the supply is off Counter U2 provides decoded TDM address codes which control analogue multiplexers U8 and U9. These scan the analogue data which appears sequentially on the array line.

Analogue values are stored in capacitors C2 to C32. The 730ps pulse from US ensures that no overlap can occur between channel sampling.

Multiplexers U6 and U7 are parallel addressed on lines AO-A4 by lines form the A-D converter unit to multiplex out the analogue +10V adjustment is provided here but its action is nulled by the software autozero facility.

THE MAGNETOMETERS (See diagrams 20,21 and 22) Detection of supply voltage removal from the line occurs in the Schmitt trigger circuit comprising D6, R10, T5, U1. A narrow trigger pulse resets the 1024Hz clock divider U6 and the timing sequence is commenced under the control of the crystal oscillator X1 and part of U1. At the end of the timing sequence the transmission gate U3, pins 9 to 10, closes and connects the output of the magnetometer filter U5, pin 7 to the transmission line for a period of 82Os. The commencement of the line connection is delayed by approximately 50ps to prevent overlap of consecutively timed channels.

Power is supplied to the sensor circuitry via low leakage diode D5. This voltage appears at current limiting resistor R9 and is stored at capacitor C5. Transistor combination T3, T4 provide an 8kHz 4V p-p signal which is boosted via C10, C12, D1, D2, D3 to 15V (Vb) to provide a low current supply to U3, which duringoperation will always be substantially above the peak line voltage.

U3 Analogue Devices AD7510 DI is a dielectrically isolated analog transmission gate which can withstand momentary application of voltages up to 25V in excess of the supply voltage. At all times this device is operated very well within its safe operating range.

The 13V p-p 16kHz signal appearing at U3, pins 15 and 17, is converted to -12V (Vc) to provide the necessary negative supply for U3 and U5.

The magnetometer section comprises the following: A probe. This carries 3 windings. Windings 1 and 2 are the excitation windings and receive pulses of current at a repetition frequency of 256Hz via T1, T2, C1, C2, C3. Winding 3 is a combined signal pick-up and feedback winding. The signal produced by the probe in the presence of a magnetic field appears across C6, which whilst U3, source and drain pins 16 and 15 are momentarily closed stores the peak value. This voltage is filtered and fed to the integrating amplifier U5, C17, and fed back to null the detected magnetic flux.

In this way the magnitude of the voltage appearing at U5, pin 1, is in proportion to the feedback constant of the winding W3 and the value of R3 and is a linear analogue representation of the measured flux density. This value is further filtered in passive filter R6, C8, the corner frequency of which is lSHz end provides protection against aliasing of the measured alternating value and the line sampling frequency whilst preserving maximally flat frequency response within the bandwidth specified for the system. The precision +5V (Vd) voltage source D9 supplies the digital circuitry.

IVIAG@@@ IVAG@@@@@ 2 High Quality Fluxgate Magnetometers # Choice of probes and instruments for measuremen from 0.1 nT 10 2mT Probes feature small sensitive volume for detailed field plotting Portable Instruments with 4tH2 digit autoranging LCD and analog output facility I Excellent packaging permits operation in in a wide range of environments Probes individually calibrated, interchangeable between MAG-O1 and MAG-Ol H with no adjustments MAG-01 AND MAG-OIH Fluxgate Magnetometers the MAO-Cl is a compacl, portable fluxgate magnetometer.It provides continuous directional measurement by responding to the yield component parallel to the probe detection axis, over the range 1 nT - 2mT, auloranging. It has digital and analog outpuls, and runs on non-magnetic rechargeable batteries.

With an updale rate ol 2 readings per second, it is able to monitor slowly changing as well as OC fields.

In the higher specification MAG-01H, sensitivity Is increased by a laclor of 10, so that a limit of O.tnT Is achlevable with appropriate probes, An offset switch backs off high readings to increase the range of the 41/2 digit display, and the x10 resolution Is obtained with a sensitivity switch.

A version of the MAG-01H is produced for geomagnetic yield observallons In combination with a non-magnetic theodolite. In addition to Intensity and declinalion, this instrument permits the measurement ol Inclination to a resolution within 2 seconds ol arc, worldwide.

Full technical specifications for the MAG-01H are glven in the brochure for the MAG-01H Fluxgale declinomeler/ inc@@nomeler - ava@able on request.

The Instruments may be powered either from the mains via the power supply unit or from Internal non magnetic rechargeable batteries. These provide more than 20 hours continuous use. The batterles can be recharged at 12V either fron a mains charger or a vehicle dashboard connector (clgar lighler) Recharging is completed in livr hours.

but overcharging wilit do no damage.

Batiery voltage is continuously monliored an audible bleeper gives advance warning of discharged batteries, and battery voltage is shown on the LCD for 5 seconds after switch-on.

Probes consist of Mu metal cores wound with three coils. A drive coll is supplied with a high purity AC current generating an alternating magnetic field which continuously drives the Mu metal in and out ol saturation, gating in and out the axial component ol any external field inlercepted by the sensor. This effect Is detected by 8 precision pick up coil, to which the magnetometer circuit retums a feedback current that maintains the sensor In nul field. This current is converted to a precise linear voltage measurement of field strength which Is drift-free and stable over a wide measurement range. This analog voltage is senl to the auto-ranging 4 /2 digit display and analog output socket.

four types ol probe are available for axial and transverse measurements at low (200lt T maximum) and high (2mT maximum) field strengths.

The axis of detection of axial field probes types B and D is in line with the cylindrical enclosure. The axis of detection of the mushroom shaped transverse fieid probes types C and E runs diametrically across the end of the short cylindrical encisoure. The magnetic detection axis is clearly marked on each probe. A while spot Indicates the centre of the sensitive volume on the axial probes.

Probe dimenslons and sensitive volume outline are shown diagramaticaily.

AN probes are individually calibrated and interchangeable between the MAG-O1 and MAG-O1H with no adlusiments. A table Illustrates the span of measuring range and resolution.

Probe components are mounted In compact unplasticised PVC enclosures with integral leads. standard length 2 meres variable on request. The whole assembly is rugged, shock-reslstant and hermelically sealed. Extension holders can be provided for gaining access to difficult places.

The probes are notable for their very small sensitive volumes. which make possible the detailed exploration of complex field patterns.

Their outstanding performance derives from superior qualRy control In the selection and preparation of materials.

and a precision feedback system which gives high linearity and measurement accuracy.

MAG-01 ELECTRONICS UNIT : Operating termperah@@@ tange : -10 C to + 50 C Atessuring range : Int to 2mT (dependent on probe) Band width : - 3dB 10 Hz. - 12d8 par octave Calibration accurncy : 0.1 % Cutput impedance : 1 k ohm Temperature coefficient : < ppm/ C Liquid @@stal display fow fleld probes : displays #00.001 = 199.99 T high field probes : displays as above mutlipty by 10 for true value Anslog output : 10 micro volls per nT vè@ 50 ohm BNC sockel low fleld probes : 5 voits full scale high field probes : 2 voits full scale Front panel conlrots : On/off swftch Power supply : 1 - 8 voit 1.1 ampere/tit zero mainlenance Lead/acld rechargeable beliery Battery changer in@el : 2.1 mm sockel d.c.O. 5A max.

polar@y protected 8-18V cont@@uous or interm@@@@ use D@@@@@@lons : 160 x 150 x 62 mm.

(6 @@ x 6 x 21/2 in.) Matertals : high impact ABS Weight : 1,5 kg.

En@@@mental sealing to @@@@ PROBES Operating termp. range : .30 C to + 80 C Calibration accuracy : 0.1 % Linearity : 0.02 % Malerlals : unplastichsed PVC enclosure Connecting cable (replaceable) : 4 core screened 2 mehe with 6 pole Flscher connactor A@@@native fengths of cable are avattable on request Environnement seafing : submersible in water to 100 meltes Shock reststance : drop tested from 0.5 metre onto PVC noor 3 times Dimenskons - ses diagram of Probe mechanical delalls (prevlous page) Measuring range/resolution - see table (previous page) Low Field Probes B and C temp. coefficient of calibration : 10 ppm/ C over full range of operating temperatures Offset error : < 1 nT High Field Probes D and E temp. coefficient of calibration : 30 ppm/ C over full range of operating temperatures Offset error : < 10 nT ACCESSORIES Power base/bech @@@ stand : @c mains battery charger, dual voltage, selectable for 110V or 240V Vehicle dashboard connector : 12V dc Terylene carrying beg Operation Manual Service manual (option) The wording of the appended claims and abstraet is to be regarded as imported into the above description.

Claims (15)

C1'AIio"

1. Telemetry apparatus, comprising: at least one transmitter means, comprising at least one sensor means for sensing magnetic flux to derive transmittable information therefrom; at least one receiver mrenas; at least one conductor means for transmission of electrical signals from said at least one transmitter means to said at least one receiver means- ; and at least one multiplexer switching means for controlling said transmission of electrical signals.

2. Telemetry apparatus as claimed in claim 1, wherein there is a plurality of said sensor means.

3. Telemetry apparatus as claimed in claim 2, wherein said plurality of sensor means comprises a plurality of directionally sensitive magnetometer means.

4. Telemetry apparatus as claimed in claim 3, wherein at least one said magnetometer means comprises at least one flux gate magnetometer means.

5. Telemetry apparatus as claimed in any one of claims 1 to 4, wherein the nunber of said conductor means is/are in mathematical relationship with the number of said transmitter means and/or the number of said receiver means.

6. Telemetry apparatus as claimed in any one of claims 1 to 5, wherein the number of said multiplexer switching means is/are in mathematical relationship with the number of said transmitter means and/or the number of said receiver means and/or the number of said conductor means.

7. Telemetry apparatus as claimed in any one of claims 1 to 6, wherein at least one said multiplexer switching means comprises: at least one electrical storage means for being charged by at least one supply of current such that said at least one electrical storage means will sustain or tend to sustain at least one operating function of said multiplexer switching means.

8. Telemetry apparatus as claimed. in claim 7, wherein said at least one electrical storage means will sustain or tend to sustain at least one operating function of said multiplexer switching means, and thereby at least one operating function of at least one said transmitter means.

9. Telemetry apparatus as claimed in claim 8, wherein said operating function of at least one said transmitter means is at least one operating function of at least one said sensor means.

10. Telemetry apparatus as claimed in any one of claims 1 to 9 when adapted to power said at least one transmitter means via at least one corresponding said multiplexer switching means at at least one appropriate time.

11. Telemetry apparatus as claimed in claim 10, wherein said powering is according to a predetermined program for periodically switching on or off at least one supply of current to at least one said multiplexer switching means, such that said telemetry apparatus will time the coming onto or off of respective multiplexer switching means, whereby at least one operating function of at least one transmitter means may be synchronised, or otherwise timed, relative to at least one operating function of at least one said receiver means.

12. Telemetry apparatus as claimed in any one of claims 1 to 11, wherein at least one said multiplexer switching means will enable or disable supply of electrical power to at least one said transmitter means at any required time, at least one electrical signal thereby being able to be transmitted via said at least one conductor means in the absence of said electrical power, such that during that absence the corresponding at least one said transmitter means may be self sustaining for at least one predetermined period of time.

13. (reemetry apparatus as claimed in claim 1, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

14. Multiplexer switching means for telemetry apparatus as claimed in any one of claims 1 to 13, comprising: at least one electrical storage means for being charged by at least one supply of current such that said at least one electrical storage means will sustain or tend to sustain at least one operating function of said multiplexer switching means.

15. Multiplexer switching means as claimed in claim 14, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.