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Hysteresisgraph with discontinuous sweep mode
Journal of Magnetism and Magnetic Materials 19 (1980) 260- 262 © North-Holland Publishing Company
HYSTERESISGRAPH WITH DISCONTINUOUS SWEEP MODE J. PLASSARD Equipe de Recherche Mat~riaux Magnbtiques, CNRS, 92190 Meudon-Bellevue, France
The present paper describes a system which permits the tracing of hysteresis loops of magnetic materials submitted to a "sequential" magnetizing field. Based on sampling techniques the set-up provides, in addition to the well-known capability of tracing ac loops on an X- Y recorder, the possibility to reduce drastically the heat dissipation. Moreover, the sequential mode of operation permits the direct tracing of some interesting curves, such as the initial magnetization curve, to be easily carried out.
memorizing techniques with logic phase shifters [3]. Using this last technique to solve problem (1), the system we propose also offers an efficient solution to problem (2) and presents, in addition, some interesting features.
1. Introduction Investigating magnetic materials, for fundamental research and for applications, requires a good knowledge of the hysteresis loops with various conditions. The general principle of the hysteresisgraph is well known and currently two main types of set-up are of interest: in the first the d.c. magnetizing field is slowly swept, while in the second the magnetizing field is produced at a rather low audio frequency (typical values: 50 Hz to 1 kHz). Although it cannot face all the situations, the a.c. hysteresisgraph - more convenient from a technical point of view - is mostly used since it is well known that for many materials the a.c. loops and the d.c. loops are equivalent. In addition, some materials need to be investigated at audio frequencies. However, two main problems arise when using a.c. hysteresisgraphs: (1) owing to the limit of the tracing speed of X - Y recorders, it is not possible to obtain a direct graphic recording of the loop which has to be displayed on a scope. (2) In some cases and for some materials, the power dissipation per time unit in the material itself and in the magnetizing winding makes difficult, or impossible, accurate measurements since the thermal equilibrium may not be reached, mostly in the low temperature range, as pointed out by different authors (see ref. [ 1] for example). Problem (1) received many solutions in the past by using first mechanical or diode switches, then pulse sampling systems with the phase shifted by an electrical motor [1,2] and more recently sampling and
2. General description Fig. 1 shows the block diagram of tile Whole system and indicates the main functions to be programmed to perform a loop analysis. The principle of sampling and memorizing is well known and has been already described, for example in ref. [3]. In our actual set-up the magnetizing current and the scanning are controlled by means of frequency dividers, counters and phase shifters, all of them being driven by a programmable master clock. The scanning is performed step by step, the step duration being equal to the clock period. Tile logic integrated circuits are of CMOS type, which permits the magnetizing sweep frequency to reach about 10 kHz; using faster circuits such as TTL or ECL types would increase significantly the frequency limit. Several functions are programmable, as described below. Function 1 controls the scanning definition, i.e. the number N of sampling points during a cycle, while function 2 controls the clock frequency F and the a.c. magnetizing frequency Fo, related by the formula F = NF o .
Functions 3, 4 and 5 are specific to the "sequen260
3". Plassard /Hysteresisgraph with discontinuous sweep mode
261
3. Sequential mode
O,OER I
:Oi
FUNCTION
--
14---
, , I . ANALOG ~
%,
SWITCH ,Z~ MAGNETIZING~
CURRENT . GENERATOR
PNOGRAMMEN
I NUMBENOFSAMPLINGPOINTS Z MAGNETIZINGFIELDFREQUENCY CLOCK FBEOUENCY , SEQUENCEOUNATION J INTERVAL DURATION ~ SEQOEN°E CDHPOSITIOII 5 SECTION TORESCANNED-SO}~SYNCHBO 6 SCANNING-TF~ACING ...-I 7 AUTO-STOPONsrNGL.EPOINT 11
, WAVEFORM | - .........
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~ .
.
.
.
.
.
.
.
.
.
.
.
SAMPLE
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Fig. 1. Schematic block diagram of the whole set-up showing the programmable functions.
tial" mode of operation used to solve problem 2 mentioned above; they will be examined in more detail below. Adjusting functions 6 and 7 enables us to choose the final result plotted on the recorder, such as, among others: (a) to trace a single loop only and stop; (b) to reach any point of the loop and stay; (c) to stop at any point previously chosen on the curve, and (d) to follow the evolution of such a point as a function of any external parameter applied to the sample, such as temperature and pressure. For example, if we stop at the tip of the loop and we increase the amplitude of the magnetizing field from zero to its maximum value, we will obtain the magnetization curve.
On the other hand, adding a computing unit would obviously permit any type of calculation to be performed.
This specific mode, controlled by functions 3, 4 and 5, allows us to apply the magnetizing field only during "sequences"; separated by "intervals" of zero field (see fig. 2). The time dependence of the magnetizing field applied during a sequence may be adjusted by mean of the waveform converter (function 4) according to the type of tracing needed: to trace a full hysteresis loop we must use a symmetric field. We consider the symmetric sawtooth as the best time dependence law, since it results in a narrower dB/dt spectrum which improves the capabilities of the sampler and of the integrator. However, the waveform converter can also provide a sine wave which may be of interest for some materials. On the other hand, the waveform converter can deliver a driving signal composed of any combination of linear segments, allowing any type of scanning to be performed: section of a loop only, minor loops, etc. Function 5 controls the choice of scanning and the synchronized scope is used for visualizing the sequence. Fig. 2 shows three actual tracings obtained on a given ferrimagnetic sample: line 1 corresponds to the magnetizing field, line 2 to the induction. The left-hand part is the continuous mode, the right-hand part a sequential mode. Cycle a is traced with the continuous mode, while cycles b and c are traced with sequential modes, the ratio between the interval and the sequence being respectively 20 and 600; frequency F o equals 1 kHz. The small gap which appears at the lower tip of the cycle c is due to the influence of drifts, from the sample (thermal drift) and from the electronics during the tracing; the very long tracing time for loop c (75 mn) has been so chosen in order to check the capabilities of the set-up for long intervals at a given frequency. Such drifts could, of course, be reduced by taking appropriate care. In the sequential mode of operation it can be seen that the induction is set down to zero at the end of each sequence by adjusting the amplitude of the last maximum of the magnetizing field. The first advantage of the sequential mode of operation results from the reduction of the heat dissipation due to the Joule losses in the magnetizing
262
J. Plassard /Hysteresisgraph with discontinuous sweep mode
continuous
®
sequential
®
©
Fig. 2. Line 1 : magnetizing current versus time; line 2: induction versus time. a: loop with continuous sweep mode; b and c: loops with sequential mode. Refer to the main text for details.
winding and the hysteresis losses in the material. For most materials it is sufficient to apply the magnetizing field during very short sequences (a few cycles only) in order to obtain the correct hysteresis loop. With the actual electronics presently used we have verified that the interval duration can be about 1000 times longer that the sequence duration without affecting the final tracing of the loop. Using such a very large ratio obviously results in an increase of tire tracing time, but the heat dissipation per time unit is drastically reduced, which leads to interesting experimental features. (1) The thermal equilibrium of the sample is reached more easily: measurements as a function of temperature, including low temperatures, become possible or easier. (2) Since the magnetizing current is applied only during short periods separated by long intervals, tire maximum field can be increased for a given geometry of the magnetizing coil; such a feature could be of interest when investigating hard magnetic materials. The second advantage of the sequential mode of operation is based on the use of the interval duration in order to submit the sample to a given physical treatment. One among the most interesting possibilities is, for example, the introduction during the interval of a demagnetizing sequence with a decreasing a.c. magnetic field. For many materials such a demag-
netizing process could be done very quickly by using a high frequency field. It is then possible to trace directly the initial magnetization curve, the permeability curves, etc. In addition, it must be pointed out that another interesting feature of the sequential mode is the possibility of using an integrator of the dc type, since it can be set down to zero during each interval. DC integrators are indeed more suitable to process the induced voltages whose spectrum may include very low frequencies. On the other hand, the possibility of using a d.c. analogic integrator working in good conditions allows us to avoid the use of a numeric integrator that would need much faster samplers.
4. Conclusion The sequential mode of operation which is proposed for a.c. hysteresisgraphs presents many advantages. In addition to improvements of the electronics capability, such a mode of operation results in (1) a reduction which could be very important - of the heat dissipation in the sample and the magnetizing coil, and (2) in the possibility of tracing directly the variations of the magnetic properties of the material.
Acknowledgements The author wishes to thank M. Guyot for many valuable discussions about the physical requirements for the magnetic applications and V. Cagan for advice and improvements of the manuscript.
References [1] W.A. Manly, Jr., IEEE Trans. Magn. MAG-7 (1971) 442. [2] Wm. A Geyger, Magnetic Amplifier Circuits (McGraw-Hill, New York, Maidenhead, 1957), several references cited in bibliography. [3] M. Pasta and G.P. Soardo, Proc. 2nd lnteln. Conf. on Soft-Magnetic Mater. (1975)p. 275.
HYSTERESISGRAPH WITH DISCONTINUOUS SWEEP MODE J. PLASSARD Equipe de Recherche Mat~riaux Magnbtiques, CNRS, 92190 Meudon-Bellevue, France
The present paper describes a system which permits the tracing of hysteresis loops of magnetic materials submitted to a "sequential" magnetizing field. Based on sampling techniques the set-up provides, in addition to the well-known capability of tracing ac loops on an X- Y recorder, the possibility to reduce drastically the heat dissipation. Moreover, the sequential mode of operation permits the direct tracing of some interesting curves, such as the initial magnetization curve, to be easily carried out.
memorizing techniques with logic phase shifters [3]. Using this last technique to solve problem (1), the system we propose also offers an efficient solution to problem (2) and presents, in addition, some interesting features.
1. Introduction Investigating magnetic materials, for fundamental research and for applications, requires a good knowledge of the hysteresis loops with various conditions. The general principle of the hysteresisgraph is well known and currently two main types of set-up are of interest: in the first the d.c. magnetizing field is slowly swept, while in the second the magnetizing field is produced at a rather low audio frequency (typical values: 50 Hz to 1 kHz). Although it cannot face all the situations, the a.c. hysteresisgraph - more convenient from a technical point of view - is mostly used since it is well known that for many materials the a.c. loops and the d.c. loops are equivalent. In addition, some materials need to be investigated at audio frequencies. However, two main problems arise when using a.c. hysteresisgraphs: (1) owing to the limit of the tracing speed of X - Y recorders, it is not possible to obtain a direct graphic recording of the loop which has to be displayed on a scope. (2) In some cases and for some materials, the power dissipation per time unit in the material itself and in the magnetizing winding makes difficult, or impossible, accurate measurements since the thermal equilibrium may not be reached, mostly in the low temperature range, as pointed out by different authors (see ref. [ 1] for example). Problem (1) received many solutions in the past by using first mechanical or diode switches, then pulse sampling systems with the phase shifted by an electrical motor [1,2] and more recently sampling and
2. General description Fig. 1 shows the block diagram of tile Whole system and indicates the main functions to be programmed to perform a loop analysis. The principle of sampling and memorizing is well known and has been already described, for example in ref. [3]. In our actual set-up the magnetizing current and the scanning are controlled by means of frequency dividers, counters and phase shifters, all of them being driven by a programmable master clock. The scanning is performed step by step, the step duration being equal to the clock period. Tile logic integrated circuits are of CMOS type, which permits the magnetizing sweep frequency to reach about 10 kHz; using faster circuits such as TTL or ECL types would increase significantly the frequency limit. Several functions are programmable, as described below. Function 1 controls the scanning definition, i.e. the number N of sampling points during a cycle, while function 2 controls the clock frequency F and the a.c. magnetizing frequency Fo, related by the formula F = NF o .
Functions 3, 4 and 5 are specific to the "sequen260
3". Plassard /Hysteresisgraph with discontinuous sweep mode
261
3. Sequential mode
O,OER I
:Oi
FUNCTION
--
14---
, , I . ANALOG ~
%,
SWITCH ,Z~ MAGNETIZING~
CURRENT . GENERATOR
PNOGRAMMEN
I NUMBENOFSAMPLINGPOINTS Z MAGNETIZINGFIELDFREQUENCY CLOCK FBEOUENCY , SEQUENCEOUNATION J INTERVAL DURATION ~ SEQOEN°E CDHPOSITIOII 5 SECTION TORESCANNED-SO}~SYNCHBO 6 SCANNING-TF~ACING ...-I 7 AUTO-STOPONsrNGL.EPOINT 11
, WAVEFORM | - .........
t SA'P
~ .
.
.
.
.
.
.
.
.
.
.
.
SAMPLE
X-Y RECOBDER
Fig. 1. Schematic block diagram of the whole set-up showing the programmable functions.
tial" mode of operation used to solve problem 2 mentioned above; they will be examined in more detail below. Adjusting functions 6 and 7 enables us to choose the final result plotted on the recorder, such as, among others: (a) to trace a single loop only and stop; (b) to reach any point of the loop and stay; (c) to stop at any point previously chosen on the curve, and (d) to follow the evolution of such a point as a function of any external parameter applied to the sample, such as temperature and pressure. For example, if we stop at the tip of the loop and we increase the amplitude of the magnetizing field from zero to its maximum value, we will obtain the magnetization curve.
On the other hand, adding a computing unit would obviously permit any type of calculation to be performed.
This specific mode, controlled by functions 3, 4 and 5, allows us to apply the magnetizing field only during "sequences"; separated by "intervals" of zero field (see fig. 2). The time dependence of the magnetizing field applied during a sequence may be adjusted by mean of the waveform converter (function 4) according to the type of tracing needed: to trace a full hysteresis loop we must use a symmetric field. We consider the symmetric sawtooth as the best time dependence law, since it results in a narrower dB/dt spectrum which improves the capabilities of the sampler and of the integrator. However, the waveform converter can also provide a sine wave which may be of interest for some materials. On the other hand, the waveform converter can deliver a driving signal composed of any combination of linear segments, allowing any type of scanning to be performed: section of a loop only, minor loops, etc. Function 5 controls the choice of scanning and the synchronized scope is used for visualizing the sequence. Fig. 2 shows three actual tracings obtained on a given ferrimagnetic sample: line 1 corresponds to the magnetizing field, line 2 to the induction. The left-hand part is the continuous mode, the right-hand part a sequential mode. Cycle a is traced with the continuous mode, while cycles b and c are traced with sequential modes, the ratio between the interval and the sequence being respectively 20 and 600; frequency F o equals 1 kHz. The small gap which appears at the lower tip of the cycle c is due to the influence of drifts, from the sample (thermal drift) and from the electronics during the tracing; the very long tracing time for loop c (75 mn) has been so chosen in order to check the capabilities of the set-up for long intervals at a given frequency. Such drifts could, of course, be reduced by taking appropriate care. In the sequential mode of operation it can be seen that the induction is set down to zero at the end of each sequence by adjusting the amplitude of the last maximum of the magnetizing field. The first advantage of the sequential mode of operation results from the reduction of the heat dissipation due to the Joule losses in the magnetizing
262
J. Plassard /Hysteresisgraph with discontinuous sweep mode
continuous
®
sequential
®
©
Fig. 2. Line 1 : magnetizing current versus time; line 2: induction versus time. a: loop with continuous sweep mode; b and c: loops with sequential mode. Refer to the main text for details.
winding and the hysteresis losses in the material. For most materials it is sufficient to apply the magnetizing field during very short sequences (a few cycles only) in order to obtain the correct hysteresis loop. With the actual electronics presently used we have verified that the interval duration can be about 1000 times longer that the sequence duration without affecting the final tracing of the loop. Using such a very large ratio obviously results in an increase of tire tracing time, but the heat dissipation per time unit is drastically reduced, which leads to interesting experimental features. (1) The thermal equilibrium of the sample is reached more easily: measurements as a function of temperature, including low temperatures, become possible or easier. (2) Since the magnetizing current is applied only during short periods separated by long intervals, tire maximum field can be increased for a given geometry of the magnetizing coil; such a feature could be of interest when investigating hard magnetic materials. The second advantage of the sequential mode of operation is based on the use of the interval duration in order to submit the sample to a given physical treatment. One among the most interesting possibilities is, for example, the introduction during the interval of a demagnetizing sequence with a decreasing a.c. magnetic field. For many materials such a demag-
netizing process could be done very quickly by using a high frequency field. It is then possible to trace directly the initial magnetization curve, the permeability curves, etc. In addition, it must be pointed out that another interesting feature of the sequential mode is the possibility of using an integrator of the dc type, since it can be set down to zero during each interval. DC integrators are indeed more suitable to process the induced voltages whose spectrum may include very low frequencies. On the other hand, the possibility of using a d.c. analogic integrator working in good conditions allows us to avoid the use of a numeric integrator that would need much faster samplers.
4. Conclusion The sequential mode of operation which is proposed for a.c. hysteresisgraphs presents many advantages. In addition to improvements of the electronics capability, such a mode of operation results in (1) a reduction which could be very important - of the heat dissipation in the sample and the magnetizing coil, and (2) in the possibility of tracing directly the variations of the magnetic properties of the material.
Acknowledgements The author wishes to thank M. Guyot for many valuable discussions about the physical requirements for the magnetic applications and V. Cagan for advice and improvements of the manuscript.
References [1] W.A. Manly, Jr., IEEE Trans. Magn. MAG-7 (1971) 442. [2] Wm. A Geyger, Magnetic Amplifier Circuits (McGraw-Hill, New York, Maidenhead, 1957), several references cited in bibliography. [3] M. Pasta and G.P. Soardo, Proc. 2nd lnteln. Conf. on Soft-Magnetic Mater. (1975)p. 275.