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EFFECTIVE 8/30/2010
USA based sales, service and manufacturing will relocate
USA based sales, service and manufacturing
will relocate to a new location
in Auburn Hills, Michigan.
Our new address is:
Laboratorio Elettrofisico Walker LDJ Scientific
LE USA Walker Scientific, Inc.
4280 Giddings Road
Auburn Hills, MI 48326
Phone 248-340-7040 Fax 248-340-7045
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20/07/2010
Sm2Co17
ROTOR AND ENCODER MAGNETS
 MAGNETIZED AT THE SAME TIME
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16/06/2009
AMH-5800
Hard Metal Magnetometer
Magnetic characterization of Cemented Carbides
The latest in Magnetic Measurement Technology for the measurement of magnetic properties of Cemented Carbides (WC in Co or Ni matrix) and Semi-Hard Magnetic Materials.
AMH-5800
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05/06/2008
Improved accuracy in magnetic sensor magnetization
2 poles double trace magnetic
encoder |
72-6-poles double trace magnetic encoder |
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02/01/2008
LE USA Walker LDJ Scientific MOVE to Michigan
LE USA WALKER LDJ SCIENTIFIC has relocated its USA based sales, service / repair, and manufacturing location to Lake Orion, Michigan...
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01/01/2008
The adjustable magnetizing coil
 Sliding poles magnetization head.
Free handling space around the magnet.
Sliding poles fit a wide range....
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19/12/2007
Flywheels rotors magnetization
Flywheels is a common way to increase the efficiency of electromechanical converter.....
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16/11/2007
Energy reduction in the magnetizing process of multipolar rotor
The innovation proposed, of production process of magnetizing heads for permanent magnets, consists of an inedit disposition of the coil conductors as well as a configuration of the magnetic circuit....
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27/09/2007
The lowest THD by a perfect sinusoidal magnetization
The lowest THD by a perfect sinusoidal magnetization...
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18/05/2007
Fully Automatic Magnets Calibration
The calibration of the magnet is performed through the measure of the magnetic moment according to the IEC60404-14 standards....
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17/05/2007
Elevators Rotors fully magnetized
Eliminate the stress of working with magnetized magnets. Magnetize your complete rotor following assembly & achieve superior performance and the lowest cogging....
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07/05/2007
New CHINESE CATALOGUES
New chinese catalogues of: Digiflux, Helmholtz coil and Hystergraf...
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05/06/2008 - Improved accuracy in magnetic sensor magnetization |
Abstract
The first part of this paper presents a brief description of the main magnetic encoders magnetization methods, focusing on the iron less coils developed by LEE, able to reach pitches deviations lower than 0.1°. In the second part are listed some points regarding the specification and the test of the magnetic encoders. |
Introduction
Several electromechanics devices use encoders, to detect with accuracy a position or the speed of an actuator. One example is servomotors, where the encoder may give with precision the rotor position, for determining the drivers commands enables, for Park transformations in the a.c. drivers or may give the actual speed for the speed loop of the regulators. |
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Another example is the position control in many devices, where the encoder changes polarity at few angles, in order to start or stop the actuator, as it is at the car windshield wiper. One of the most important applications is electrical power steering.
Magnetic encoders are an alternative to the expensive optical encoders; They have been strongly imposing on the market, pushing research and development to obtain higher resolutions.
Magnetic encoders uses hall sensors to read, with hysteresis, the flux density due to a permanent magnet. They are mostly ferrite, but ever more are growing the rare earth encoders.
On circular target sometimes there are more then one magnetic traces, disposed at different radius, giving different encoding signals, as position and speed.
In the servomotors the target is fixed with the moving part, usually locked with the shaft at accurate angular position; the hall sensors so are fixed with the stator, being that they must be wired for reading and feeding. |
Magnetization
Magnetization process is one of the most important steps in the encoder production. Magnetization, as it is for all the permanent magnets, takes place imposing magnetic field, with the required spatial waveform, strong enough to irreversibly orient all the material magnetic domains. The magnetic field is induced by a strong pulse current in a magnetizing fixture, generated by a capacitor discharge power supply. Conductors disposition, section, wires number, capacity and voltage must be evaluated time by time depending on dimensions and applications. It is common to magnetize only the encoder, oriented according to a mechanical reference, but, when the assembly sequence permits, it is convenient to magnetize the system already assembled, with the encoder fixed on the shaft.
Iron core coils are useful in ferrite magnetization, although not needed: 800kA/m are enough to saturate all kind of ferrite, and the material thickness is commonly so thin that the iron is not saturated.
Classical magnetization method consist of an array of slots filled with only one solid copper wire, forming magnetic wire enclosing ¾ of pole.
Sometimes it is convenient to use rare earth magnets as field source, but they must be helped by an inductor coil, with an added axial field applied two times, in both the polarities.
Similar to this method is the axial magnetization, first of one polarity, then of the other, using iron shaped pole, using a core with teeth and an axial inductor, applying the energy two times.
Transverse flux coils, coming from the parent synchronous machine, do not give big advantages, due to the difficulty in shaping the teeth and to the fact that very fast pulses reduce the skin depth; but the idea should be mentioned as not so obvious and suitable for further studies. |
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| Iron core coils have some disadvantage; first they need long pulse time because of the thin skin depth; second the iron attract the encoder so it must be taken off carefully and perpendicular and higher force is required; then it's not convenient also because of the difficulties to insulate the wires from ground. |
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In order to avoid iron disadvantage and magnetize also rare earth permanent magnet sensors LEE uses an iron less innovative technique, thanks also to new production and materials technologies, coupling with a special magnetizer, properly studied for encoders, of very low internal impedance.
At equal magnetizing current, a brief pulse time avoids the conductor overheating during the shot and reduces the average temperature; with the incoming advantage of longer fixture life and higher productivity in terms of shots per hour. The insulator support material also reduces the risk of insulation losses between wires and wires to ground. |
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| Usually capacitor dicharge units coulpled with magnetizing fixtures has a damping waveform rectified by a free weeling diode. The peak current happens when the electrical energy changes completely in magnetic energy, if the resistance can be neglected. |
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| And the peak time comes from the LC circuit oscillation frequency and is |
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So the advantages come from reducing C and L and increasing V. Cycle times of 3s with spikes of thousands of Amperes in less than 20us have been obtained.
Many targets have a metallic back iron plate as a magnetic circuit permeance improvement, giving it an higher flux density pattern, so the magnetizing winding can stay only from one side; in those cases when the back iron is absent higher magnetic efficiency is obtained with a coil on both sides.
In Iron core fixtures FEM analysis is necessary and usefull in calculating iron core coils. |
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| For iron less fixtures gross calculation and optimization may be analytically worked out integrating the Laplace elementary action law along the conductor's paths. The law general expression is also time varying: the elementary field generated in the point P by a current i(t) with tangent t in the point Q, is: |
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| The only difficulty being define the tangent unit vector of the conductors' segments, but with the big advantage of giving fastest results and a very enhanced parametric optimization. Any kind of numerical calculation language is suitable for summing that integration, from the commercial MathCad and Matlab, to the open source Octave. |
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Calibration and Stabilization
Small differences in material grade and volume can be minimized calibrating the magnets: first the magnet is saturated then demagnetized step by step till the total flux falls within a required acceptance range.
The calibrating process has also a stabilizing effect, making the magnet less sensitive to the thermal external oscillations and to the phenomena known as magnetic viscosity.
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Specifications and test
Magnetic encoders are mechanically specified as a common gearing. Follows important definitions:
p: poles pairs; pitch: angular distance between poles pairs;
rr: raise to rise pitch; ff: fall to fall pitch;
The poles accuracy is defined with:
rrs: raise to raise single pitch deviation: difference between maximum positive and negative raise to raise pitch deviation from average or nominal value; |
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fs: fall to fall single pitch deviation: difference between maximum positive and negative fall to fall pitch deviation from average or nominal value;
rffrs: raise to fall fall to rise single pitch deviation: difference between maximum positive and negative rise to fall fall to rise pitch deviation from average or nominal value;
rrt: raise to raise total pitch deviation: difference between maximum positive and negative of the algebraic summation of all the rrs;
fft: fall to fall total pitch deviation: difference between maximum positive and negative of the algebraic summation of all the ffs;
rffrt: raise to fall fall to raise total pitch deviation: difference between maximum positive and negative of the algebraic summation of all the rffrs. |
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| These deviations can be also expressed in percentage. We reached a very high accuracy, in single traces encoder we stand with single pitch deviation lower than 0.1° and total pitch deviation lower than 0.3°Magnetic specification gives maximum value, mean and rms of the flux density at a given distance from the magnet surface, and the typical characteristic values of the harmonic analysis in the space domain, as the total harmonic distortion (THD). It is also a good target raising as high as possible the slope of the polarity changes. |
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| The test is run with an instrument called magnetic scanner, where the magnet rotates fixed with an optical encoder and the flux density is read by a linear hall probe, connected to a gauss meter, so the flux density can be plotted versus the angular position. A proper software calculate all the deviations and the harmonic analysis. |
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| Improved accuracy in magnetic sensor magnetization |
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