Magnetic measurement tools attract attention

Tag : Precision Measuring Tools

One of the primary military applications for magnetic-fieldmeasurement is detecting submarines. For example, thesubmarine-hunting Orion P3C military aircraft has a long tail boomto house the magnetometer far away from the engines and othersources of interference. Other military uses for magneticmeasurements include instrumentation of small-caliber shells in thedevelopment of ranging fuses ( Reference 4 ). The instruments inside the shell count the rotations of theshell as it spins through the earth’s magnetic field. Becausethe magnetometer knows the turns ratio of the barrel rifling, afuse circuit can calculate the distance the shell has traveled, soit can then burst over a target. Methods such as a time delay afterfiring are less accurate because the bullet speed varies withpowder charge and gun condition.
Countless other applications exist in the industrial, scientific,and medical fields. Industrial customers may simply need to verifythe north and south poles on magnets used in motors. Paul Elliot,owner of magnetic-field-sensor-vendor Magnetic Sciences, reportsthat oil-pipeline installers need to measure the pipes to ensurethat no latent magnetism resides in the steel. Many industrialusers must measure the field of a magnet to ensure that it has notlost its strength. Another industrial use is to verify whethershipping containers are emitting a magnetic field that is greaterthan the legal limit.
Scientific uses of magnetic measurements includedisk-drive-read-head research. The behavior of material in intensemagnetic fields is an area of active study, and it is oftennecessary to measure the intense field that resistive,room-temperature, and superconducting magnets produce duringresearch.
One of the most common medical uses of magnetic measurement is toverify the field uniformity in MRI (magnetic-resonance-imaging)machines. The field in the tunnel of an MRI machine must be uniformto parts per million. Sensitive instruments can measure the fieldat a point, but a sensor array that distributes 24 to 32 sensors onan arc is more useful. Technicians rotate this sensor inside thetunnel of an MRI machine so that sensors sweep out in a sphere. Ifthe magnetic field is uniform at the periphery of a sphere, theuniformity can only improve anywhere inside that sphere. Inaddition, magnetic sensors around an MRI machine block out theeffects of passing cars or elevators. The sensor feeds back thesignal to a 3-D Helmholtz coil, a device for producing a region ofnearly uniform magnetic field. This coil ensures that outsideinfluences do not affect the field inside the MRI tunnel.
Another area of medical research involves the susceptibility ofhumans to the magnetic fields that surround us. This area of studyis more controversial because power lines and electric cars emitapproximately 1-mG (milligauss) fields, whereas the earth’smagnetic field is 500 mG. Still, MRI machines can be of concern tohealth-care workers who operate them. The ICNIRP (InternationalCommission on Nonionizing Radiation Protection) guidelines foroccupational static-magnetic fields are 200 mT (millitesla) forcontinuous exposure; 2000 mT for short-term, whole-body exposure;and 5000 mT for exposure to arms and legs, according to Ian JWalker, vice president of sales for sensor-distributor and-integrator GMW Associates. “These field levels are high andindicate the lack of evidence for biological effects from dcfields,” he says ( Reference 5 ).
The use of gaussmeters to measure magnetic fields in homes hasuncovered another valuable application. Homes with 60-Hz fieldsoften have wiring errors such that the neutral leg of the dc wiringreturns through the ground wire or plumbing. The current conductorsare far apart and form a loop, so they generate larger magneticfields than those in properly operating wiring. Whether the fieldsthemselves could cause injury is debatable, but it is always moredesirable to have wall power in the wires and not the pipes of yourhome. Sense and “sensorability”
The availability of such a diverse array of magnetic measurementsrequires a wide selection of sensors to properly characterize themagnetic field ( Reference 6 and Figure 3 ). One of the most basic is a simple inductive sensor comprising acoil with a magnetic core. It can measure ac fields and may alsopick up electric fields. The response of the core material alsolimits the upper frequency that the sensor can detect. These typesof sensors find use in inexpensive gaussmeters, often targeting thehealth-care market.
The largest drawback of inductive sensors is their inability tomeasure dc fields. Hall-effect sensors overcome this problem. AHall-effect sensor yields an output voltage proportional to themagnetic-field strength. Hall-effect sensors work in only one axis,but vendors can mount three devices together for three-axismeasurement that provides enough information to detect theearth’s magnetic field or, just as useful, to subtract itfrom the real dc field of interest. The downside of these sensorsis that they drift over time and temperature, making accuratemeasurements difficult.
Flux-gate sensors, which indicate the direction of theearth’s magnetic field, can also measure dc fields and can bemore sensitive than Hall-effect devices. A flux-gate sensor worksby using an ac electric current to sweep a permeable core throughits magnetic-saturation curve. The property of the core determineshow many ampere-turns—the magnetomotive force developed by acoil through which a current flows—are necessary to achievesaturation. The presence of a dc field in the core reduces theamount of current necessary to achieve saturation in one magneticdirection and increases the current necessary when you try to drivethe core in the opposite magnetic direction. It is easy to measuresmall currents, and, as such, it is possible to measure smallfields ( Reference 7 ). If you excite the flux gate fast enough, it can easily measure60-Hz fields and other ac fields into the audio range.
Texas Instruments’ Burr-Brown division offers the DRV401chip, which both excites and measures the response of the core in amagnetoinductive magnetometer, similar to a flux gate. By drivingthe core to a certain current and then reversing, the partestablishes a natural oscillation. With no applied magnetic field,the duty cycle of the oscillations is precisely 50%. With anapplied external field, the duty cycle changes, showing both themagnitude and the direction of the applied magnetic field. Thefrequency range of this technique extends to 100 kHz. This chipprovides the magnetic sensing in current sensors formagnetic-product manufacturer Vacuumschmelze.
A broad range of magnetic sensors uses the principle ofmagnetoresistance. A magnetoresistive material changes itsresistance in the presence of a magnetic field. Irish physicist andengineer William Thomson, more commonly known as Lord Kelvin, in1856 discovered the theoretical basis for the phenomenon, and laterdevelopment of technologies allowing the deposition of thinmetallic films popularized these sensors. Because the fielddirectly changes the resistance, this class of sensor can measureboth dc and ac fields, and, because the sensors are resistive, youcan use them at high frequencies, which accounts for their use indisk drives. These sensors can employ AMR(anisotropic-magnetoresistance), GMR (giant-magnetoresistance), orTMR (tunneling-magnetoresistance) techniques. Japanese physicistTerunobu Miyazaki discovered in 1995 that you can use the TMRtechnique at room temperature. Since the emergence of thatbreakthrough, manufacturers of disk-drive-read heads have adoptedTMR sensors to reach the fast response and high bit rates thatmodern drives require. AMR sensors, available from Honeywell andothers, find use in anything from compasses to gear-toothdetection.
For example, Maxim offers the 16-bit, RISC-microcontroller-basedMAXQ-7665 smart data-acquisition system that interfaces tomagnetoresistive sensors; it also integrates an analog front end, aprogrammable-gain amplifier, and bridge excitation. The devicemeasures the steering angle for yaw and traction control inautomotive applications. The microprocessor core has amultiply/accumulate instruction, allowing the device to performcalculations and DSP-type filtering, according to Mike Mellor,staff engineer at Maxim. The device also integrates a CAN(controller-area-network) bus and UART.
Sensors employing NMR (nuclear-magnetic-resonance) technology basetheir measurement on atomic properties, so they are highlyaccurate, and you can use them as primary standards. Theirresolution approaches one part per billion, and their resonance isbased on the spin states of a proton in a hydrogen nucleus. Theproton-procession magnetometer subjects water or otherhydrogen-rich samples to an intense magnetic field and then allowsthe field to collapse; a second inductor then measures the weakresonance of the protons. The magnetic field of the earth wouldresult in a resonant frequency of 1.5 kHz. The Overhauser type ofNMR sensor excites the hydrogen atoms in water with RF energy ofnearly 45 MHz; the sensor absorbs energy at resonance, and thisfrequency is proportional to the magnetic field. The measurement isprecise, has no drift, and measures the three-axis field becausethe effect is nondirectional. These sensors are more expensive thanthe other types, however, and have a unique drawback: The waterinside freezes in cold climates and ruins the internal vessel,according to Brian Richter, president of GMW Associates. Thisscenario could happen, for example, if a user leaves the sensor onan airport runway in a frozen climate. NMR sensors also need auniform field across the measuring vessel, and they work only on dcand slow-ac fields.
The most sensitive magnetometers are SQUIDs(superconducting-quantum-interference devices). “They caneasily detect the magnetic field from the nerve impulses in yourbrain or your heart,” says AlphaLab’s Lee. “Youhave to shield them very well since a truck driving by a half-mileaway can add more magnetic fields.” Weighing your options
Selling for less than $300, inductive-measurement devices are themost economical ( Figure 4 ). You use them to measure the magnetic fields from ac sources,such as power lines and motors. These instruments can also helpfind wiring breaks because the magnetic field collapses when itencounters the broken wire. Inductive sensors come in bothsingle-axis and triaxis versions. For example, the FW Bell divisionof Sypris Test and Measurement offers the series 4100 meters, whichcan measure ac fields at frequencies higher than 25 Hz in threeaxes. One unit in the series, the Bell-4180, sells for $324.
If high sensitivity is a necessity for your application, you maywant to consider a flux-gate sensor. For example, the Mag-03MCflux-gate-magnetometer probe from Bartington Instruments has 70- to1000-µT sensitivity; this three-axis, dc to 3-kHz instrumenthas ±0.5% accuracy. The probe, available from GMWAssociates, costs $3190 and interfaces with the Mag-03DAMdigitizer, which sells for $5750. GMW Associates provides NationalInstruments LabView drivers for this device and othermagnetometers.
Hall-effect sensors are more versatile and their prices—$500to $800 for a single-axis device and more than $1000 for athree-axis unit—reflect this feature. The FW Bell 5100 seriesHall-effect sensor measures dc fields, has 2% accuracy, and canmeasure frequencies as high as 20 kHz at 1G to 20 kG. The model5180 has 1.1% accuracy, measures frequencies as high as 30 kHz, andranges to 30 kG. It also has peak hold, relative mode, and analogand USB outputs. The 5180 costs $1325, and another unit in theseries, the 5170, sells for $985. FW Bell also makes two benchtopinstruments. The model 6010 Hall-effect gaussmeter sells for $2492,and the 7010 single-channel gaussmeter/teslameter costs $4365 ( Figure 5 ). The 7010, with an accuracy of ±0.5% dc ±2%, cansimultaneously measure and display flux-density, frequency,temperature, minimum, maximum, peak, and valley parameters. Thethree-channel model 7030 gaussmeter/teslameter sells for $6864.
You can use AlphaLab’s $380, 10,000G DC magnetometer, withoverall accuracy of ±2% at 30 to 110°F, for both dc andac measurements ( Figure 6 ). The unit has a pseudo-root-mean-square response, and it operatesat 45 to 2000 Hz. A high-stability version is also available. Themeter comes with a NIST (National Institute of Standards andTechnology)-traceable-calibration certificate. Another line ofmeters, Hirst Magnetic Instruments’ gaussmeters, includes theHall-effect VGM01, which connects to your PC through an RS-232interface. Metrolab’s $3980 THM1176 sensor integrates threeorthogonal Hall-effect elements onto one IC ( Figure 7 ). The USB instrument provides a 0 to 20T field range, a dc to1-kHz passband, a three-axis Hall-effect sensor in a 193.54-mm 3 footprint, and a USB interface that can also interface to anoptional $1730 PDA (personal digital assistant). Both the sensorand the PDA include software and are available from GMW Associates.
The Chen Yang Technologies CYHT-201 measures dc or ac magneticfields. The instrument has ±2% dc accuracy and ±5% acaccuracy, measures dc to 200-kHz fields, and has a 4½-digitLCD. The company also offers the $350 CYHT-T08A gaussmeter with aHall-effect probe. Yet another handheld teslameter comes fromTel-Atomic. The TeslaMeter 2000 costs $719 and comes with atransverse probe. The Hall-effect device measures sensitivity to2T. Accuracy is ±0.5% for dc measurements and ±2% forac measurements. The company also offers a $150 axial probe and isdeveloping a triaxial probe. Lake Shore Cryotronics offers the $590Model 410 handheld gaussmeter for field measurements of 0.1G to 20kG (0.01 mT to 2T). The device displays measurements in gauss ortesla and ac- or dc-magnetic-field values with resolution to 100mG. Operating functions include maximum hold, filter, relativereading, zero probe, and an audible alarm. Accuracy is ±0.1%full-scale dc and 5% ac, and the frequency response extends to 10kHz. For users needing a small, inexpensive dc gaussmeter, CarlsenMelton offers the $329 GM-200A, which measures to 10,000G with 2%accuracy and resolution of less than 1G. A calibration certificatecosts $50.
At the other end of the pricing scale for Hall-effect meters,Group3 Technology’s $4390 DTM-151 teslameter has 20-bitresolution and ±0.01% accuracy of full-scale for dc orslow-dc fields. For less demanding applications, the $2380 DTM-133with digital-linearity correction has 0.03% accuracy, resolution to10 ppm, and a temperature stability of 100 ppm/°C. Both devicesare available from GMW Associates. Because these Hall-effectdevices drift more than an NMR unit does, customers sometimes buyone NMR instrument to verify the calibration and drift of severalof these less expensive Hall-effect meters.
Micro Magnetics’ TMR sensors can investigate extremely smallareas. The die measures 1.9×1.9 mm, but the actual area thatsenses the magnetic field measures only a few microns across,according to Ben Schrag, project manager of metrology at thecompany. He also notes that you can make an array of sensors on onedie and average the result to get better sensitivity. The $325,bipolar, linear-output STJ-020 magnetic microsensor has a fieldsensitivity of 5 nT. Because the circuit comprises only resistors,the sensor has a frequency response as high as 5 MHz. Sensors withlower resistance have frequency response exceeding 100 MHz.
Metrolab’s NMR-type PT 2025 sensor finds extensive use inmedical-MRI applications ( Figure 8 ). The teslameter achieves 5-ppm absolute accuracy and 0.1-µT(1-mG) resolution for measuring or mapping uniform magnetic fieldsof 0.043 to 13.7T (430G to 137 kG). Optional probe multiplexersenable readout of as many as 64 probes. The instrument operates inMRI- and spectrometer-magnet mapping, precision field control, andmagnetic-sensor calibration. The PT 2025 instrument, for dc orslow, low frequencies, costs $20,650 and is available from GMWAssociates. The $31,580 MFC 3045D-32 NMR-probe array, alsoavailable from GMW Associates, sweeps a sphere and characterizesthe field-strength uniformity for MRI markets.
Magnetoresistive sensors measure weak-dc fields in AlphaLab’sdc milligauss magnetometer. The instrument has a resolution of 0.01mG (1 nT) and a range of ±2000 mG (200 mT). At fixedtemperature, reproducibility is ±0.01 mG (1 nT), and thetemperature coefficients of the offset and of the gain are lessthan 0.01 mG/°C and less than 0.0015%/°C, respectively. The$490 device has a gain accuracy of ±0.5%, and the meteroffset is ±0.5 mG. The $4.95 CY-MVF555 magnetic-field viewerfrom Chen Yang requires no electricity and allows you to directlyview a magnetic field ( Figure 9 ). This ability can expose multiple poles in a magnetic assembly orindicate the field uniformity or fringing around a magnet.
Magnetic measurements are important if you are finding the NorthPole or looking for submarines underwater. Instruments to measuremagnetism can detect a dozen orders of magnitude of field strength.Understanding whether you need to measure ac or dc fields, alongwith the limitations of single- versus three-axis measurements,will help you do the job. If you realize the limitations of thevarious sensors and instrument types, you can ensure that you getan accurate measurement in the most cost-effective way. No matterwhether your instrument costs $50 or $50,000, it represents abeautiful collaboration of the regimes of physics, electronics, andeven optics.