Measuring Magnetic Quality
Measuring magnetic quality is a critical step in the characterization of magnets, whether for production, research, or quality assurance. There are several methods and approaches to evaluate the quality of magnets, depending on the specific requirements and properties to be measured.
Basic procedures and measured values
The basic properties include magnetic flux density (B), which measures the strength of the magnetic field. It is expressed in Tesla (T) or Gauss (G), where 1 T = 10,000 G. Magnetic field meters (gaussmeter or teslameter) are used for measurement.
Another important parameter is the magnetic field strength (H), which describes the intensity of the magnetic field flowing through the magnets. This is measured in Ampere per meter (A/m), Amperes per centimeter (A/cm), or Oersted (Oe), where 1 A/m is about 0.01257 Oe. gaussmeters or teslameters are also used here.
The coercivity (I_Hc) is a measure of the strength of the magnetic field required to reduce the magnetization of the magnets to zero. Its unit is Oersted (Oe), Ampere per meter (A/m) or Kiloampere per meter (kA/m), and it is measured with a permagraph.
Remanence (Br) describes the remaining magnetization of a magnet when the external magnetic field is removed. It is also expressed in Tesla (T) or Gauss (G) and can be measured with a permagraph or a fluxmeter equipped with a calibrated Helmholtz coil.
Another key criterion is the energy product (BHmax), which is the maximum product of flux density (B) and field strength (H). It represents the maximum energy a magnet can store. The unit is Mega-Gauss-Oersted (MGOe) or Kilojoule per cubic meter (KJ/m³). This measurement is also usually made with a permagraph.
These parameters and the associated measurement methods provide a comprehensive basis for evaluating magnetic quality and play a critical role in material development and optimization of magnetic applications.
The flux density field strength measurement is one of the most common methods. Magnetic field meters are used to determine the magnetic flux density at different points of the magnet. In addition, magnetic flux measurement is carried out with a flux meter, which determines the Br value of a magnet, i.e. the maximum flux density in the saturated state.
Another important method is hysteresis measurement, which is performed using a permagraph - or, as you will learn below, the MagInspect Magnet Analyzer. This method plots the hysteresis curve of the magnet and provides key parameters such as coercivity, remanence and energy product. These values are crucial for assessing the performance of a magnet.
The temperature stability of a magnet is tested to determine how its properties change at different temperatures. This is done using climatic chambers and magnetic meters. Finally, aging tests are used to determine the long-term stability of a magnet. It is tested how the magnetic properties develop under different environmental conditions over a longer period of time.
The practical procedure for these measurements begins with magnet preparation, which ensures that the magnet is clean and free of contamination. The measuring equipment used is then carefully calibrated in accordance with a valid manufacturer's certificate. The measurements themselves are performed according to the specific instructions provided by the instrument manufacturers. Finally, the data is analyzed to evaluate the quality of the magnet and to identify any potential for improvement.
These measurement methods and procedures ensure a comprehensive and accurate assessment of magnet quality, which is essential for both research and industrial applications.
Classic magnetic field measurement (gaussmeter/teslameter)
Measuring magnetic flux density (B) or magnetic field strength (H) with a gaussmeter or teslameter, especially with a Hall probe, is a precise but often challenging task. The difficulties and requirements described can be summarized as follows:
Challenges in magnetic field measurement
- Inhomogeneous magnetic fields: Magnetic fields, especially for permanent magnets, are rarely homogeneous. They diverge, resulting in different readings depending on the position of the Hall probe.
- Measurement reproducibility: Without a positioning device, it is difficult to place the Hall probe in exactly the same position, which affects the repeatability of the measurements.
- A magnetic field measurement with a Hall probe is always a point measurement of the field strength at a magnet; a fluxmetric measurement with a Helmholtz coil is much more accurate. Here you really get absolute values that take into account the entire magnetic volume of the magnet and are position-independent
Practical solutions for magnetic field measurement
- Fixing the Hall Probe: A device for accurately positioning the Hall probe relative to the magnets is essential. This can be done using a mechanical fixture or a computerized positioning device.
- Establish reference points: Defined and documented measurement positions facilitate comparability of measurements.
Conversion between flux density B and magnetic field strength H
- In air, where the relative permeability is μr = 1, the relationship between magnetic flow density (B) and magnetic field strength (H): B = μ0 * H where B is the magnetic flow density in Tesla (T), H the magnetic field strength in Ampere per meter (A/m), μ0 the magnetic field constant (μ0 = 4 * Pi * 10-7 T * m / A)
- It follows: H = B / μ0
- This simple conversion makes it possible to switch between B and H when one of the two was measured.
Important notes on magnetic field measurement
- Calibration of the measuring device: Regular calibration of the magnetic field measuring device ensures precise measurements.
- Shielding of interference fields: In order to minimize external magnetic interference fields, the measuring range should be shielded or at least well characterized.
With these measures and basics, the measurement of the magnetic properties can be made more effective and reproducible.
The uniformity of magnetization in multipole rotors is a critical quality feature that provides information about the performance of magnetizing coils or the condition of individual magnets on a magnetized rotor. To evaluate this property, the Magnetic Field Meter MP-4000 in combination with the Magnetic Field Tester Rotor-Check 300 is used.
In this procedure, the magnetic red to be tested is positioned on a special mount that is driven by a synchronous or stepper motor. The rotor performs a complete rotation of 360°. During this process, a flexible Hall probe is precisely positioned on the rotor surface to record the magnetic fields.
The MP-4000 Magnetic Field Meter graphically records the entire magnetization curve along the circumference of the rotor. The data obtained is not only stored, but can also be exported for detailed analysis in Microsoft Excel. This functionality provides a comprehensive assessment of magnetization quality and supports efficient documentation and optimization of production processes.
This method is particularly useful in production and quality assurance, as it enables both precise measurements and a clear visualization of the results. As a result, deviations in magnetization can be quickly recognized and corrected in a targeted manner, which contributes to ensuring consistently high product quality.
The MP-4000 magnetic field measuring device offers an extended functionality, which enables even very short magnetization impulses to be presented precisely and graphically recorded. This property makes it a valuable instrument for monitoring magnetizing systems, in which the top field strength of the magnetizing impulse is measured for quality assurance. The device can save and analyze impulses with a duration of ≥ 0.1 ms, which enables an extremely detailed representation of magnetization processes.
Practical information on measurement with magnetic field measuring devices
When using a magnetic field measuring device for measuring magnetic fields, the correct positioning of the Hall probe plays a central role. There are a variety of different measuring probes on the market, which differ in particular by the distances of the Hall chip to the measuring surface. Since there are no fixed standards for this, different results can occur when measuring. Such deviations are often the cause of discussions or disputes, especially when accepting measured values in areas with divergent magnetic fields (such as the residual field measurement).
Comparison of measurement methods
While the measurement with gaussmeter/teslameter is an inexpensive solution, it is not always the best choice, especially when measuring individual magnets. For a higher precision and reproducible results, the fluxmetrical measurement with a Helmholtz coil is preferable. This method offers much better accuracy because it captures the entire magnetic flow of a magnet and determines the Br value on the basis of it. As a result, deviations that arise from local differences in the field distribution can largely be avoided.
For particularly precise applications, such as the characterization of individual magnets, fluxmetrical measurement is still the first choice.
Fluxmetrical measurement with Helmholtz coil
Measurement of the magnetic flow density B (mT) or magnetic flux φ (mVs / mWb)
The magnetic flux density and magnetic moment of a magnet can be measured very accurately using a fluxmeter, such as the FLUX-CHECK 250, in combination with a helmet-holt coil. This method offers a significant increase in accuracy compared to field strength measurement with a teslameter because the entire magnet in a helmet-holt coil is measured, regardless of its exact position in the coil. In contrast, a Hall probe detects only selective readings for the teslameter, which can lead to inaccuracies.
Advantages of Fluxmetric Measurement
- Captures the entire magnetic volume: The method measures the total effect of the magnet and is therefore insensitive to slight positional variations.
- Accurate calculation of magnetic flux density: The measurement of the magnetic volume can be reliably calculated.
- Accuracy and reliability: The Helmholtz coil provides a uniform magnetic field measurement that is independent of local field strength variations.
Features of the FLUX-CHECK 250 Fluxmeter
- Induced pulse curve: The instrument records the entire induced curve during the measurement and calculates the integral of the curve, which is displayed as the flux value φ in mVs or mWb.
- Drift-free measurements: Unlike conventional analog fluxmeters, the FLUX-CHECK 250 has no drift. Prior to measurement, a trigger value is set to start recording. The drift value before this point is automatically set to zero and considered as an offset.
- Simultaneous display: In addition to the flux value, the instrument can visually display the induced curve for easier analysis.
This fluxmetric method combines high precision with ease of use and is ideal for magnet characterization. This technology is a far superior alternative, especially in applications where a selective Hall probe measurement is not sufficient.
If the measurement constant of the connected Helmholtz coil is known, the fluxmeter, such as the FLUX-CHECK 250, provides an even more detailed and directly interpretable display of the magnetic parameters.
Additional functions of the FLUX-CHECK 250
- Direct display of the magnetic moment: Using the stored measurement constant of the Helmholtz coil, the Fluxmeter can directly calculate and display the magnetic moment of a magnet. This saves time in subsequent data analysis and reduces the likelihood of calculation errors.
- Magnetic flux density calculation: If the magnetic volume is known, the magnetic flux density can be displayed directly in A/cm or MT units. This is particularly useful for applications where an immediate assessment of magnetic quality is required.
- Parameter entry in the instrument: The user-friendliness of the instrument is increased by the possibility of entering the required parameters, such as the coil constant or the magnetic volume, directly into the instrument. This automates the calculations and provides accurate results immediately.
Advantages of the FLUX-CHECK 250 Fluxmeter functionality
- Fast evaluation: no additional manual calculations required.
- Versatility: The ability to derive multiple parameters (e.g. flux, momentum and momentum density) from a single measurement makes the instrument versatile.
- Ease of use: Integrated parameter entry allows less experienced users to obtain accurate results without in-depth knowledge of the calculations.
These advanced features make the Fluxmeter an indispensable tool for quality assurance and magnet characterization, especially in production and laboratory environments.
Permagraph measurement
Measurement of Hysteresis Curve, Coercive Field Strength (I_Hc), Remanence (BR) and Energy Product (BHMax) with a Permagraph
The measurement of the hysteresis curve provides important magnetic parameters such as the coercive field strength (I_Hc), the remanence (Br) and the maximum energy product (BHMax).
Detailed description of the procedure, measurement principles and disadvantages
Magnetization and demagnetization process
- A material sample is brought into the magnetic field of a magnetizer yoke or an electromagnetic coil
- This field is continuously built up to maximize the magnetization of the material, and then gradually reduced to measure the demagnetization
Recording of the hysteresis curve
- Induction coils measure the change in the magnetic flux density B
- Hall sensors or other field sensors record the magnetic field strength H
- The combination of the data results in a complete hysteresis curve. This shows the magnetic properties of the material
Results from the Hysteresis curve
- Coercive field strength (I_Hc): The external field that is necessary to reduce the magnetization to zero
- Remanence (Br): The remaining magnetization after removing the magnetic field.
- Energy product (BHMax): The maximum product of BB and HH, which represents the maximum energy in the magnets
Approach
- Calibration of the Permagraph: The device is calibrated before measurement to ensure precise and reproducible results
- Place the magnet: The sample is placed in the measuring field of the instrument. Accurate positioning is important for consistent results
- Measurement cycle: The instrument automatically performs magnetization and demagnetization. Hysteresis curve data is recorded during the process
- Data analysis: The software analyzes the measured signals and calculates the magnetic parameters
Disadvantages of the permagraph
- High acquisition costs: Permagraphs are technically complex and therefore expensive. This investment is difficult to justify for many industrial companies, especially for occasional quality assurance measurements.
- Difficult to use: Calibration and measurement require trained personnel, which adds to the cost of ownership.
- Limited to laboratory conditions: The permagraph is less suitable for use in production or in the field due to its sensitivity and the fact that it is often stationary.
Alternative methods
Industrial companies often use fluxmetric methods because they are less expensive and sufficiently accurate to measure Br. However, the coercivity and energy product cannot be determined with these methods.
Summary
A permagraph is a powerful instrument that accurately measures the most important magnetic parameters of a magnet. Due to its high cost and laborious operation, its use is usually limited to laboratory environments. For less extensive measurement requirements, such as the determination of the Br value, the fluxmetric method can be an economical alternative.
MagInspect Magnet Analyzer
Typical applications for MagInspect (or a Permagraph)
Material testing
- Determination of magnetic properties of materials for quality assurance
- Analysis of coercive field strength (I_Hc), remanence (Br), and energy product (BHMax) to evaluate material quality.
Research and development
- Investigation of new magnetic materials and alloys (e.g. AlNiCo, Ferrite, SmCo or Neodymium)
- Optimization of magnetic properties for specific applications
Production control
- Monitoring the magnetization of components during production
- Quality assurance of magnetized components such as B. rotors or permanent magnetic sensors
Measurement methods and advantages of the MagInspect procedure
The MagInspect Magnet Analyzer combines accurate measurements of the hysteresis curve with a cost-effective and flexible approach.
Measurement procedure
- Pulse measurement: The integrated Hall probe measures the magnetic field strength H and the magnetic flux B. The controller generates magnetization and demagnetization pulses, gradually saturating and demagnetizing the magnet with 10-14 pulses. The hysteresis curve is built up in steps and automatically stored.
- Measured values are stored in the controller and can be imported using Microsoft Excel (Data Streamer). A spline function with an Excel macro accurately calculates the hysteresis curve
Advantages of the MagInspect procedure
- High flexibility: The small pulse probe allows measurements regardless of magnetic geometry. It can also be used on magnets that are difficult to access (e.g., rotors)
- Cost efficiency: MagInspect is significantly less expensive than a permagraph and does not require external magnetizer yokes or complicated pole inserts.
- Simple measurement structure: Compensation coils are unnecessary because the pulse method works with abandoning fields
- Automation and integration: The entire measurement process is automatic and the results can be easily integrated into digital systems
- Quick procedure: For series measurements, the instrument can quickly perform a quality check of the Br and I_Hc values for series measurements with a single magnetizing and demagnetizing pulse
Disadvantages of the MagInspect method
- Limited temperature measurements: Temperature dependent hysteresis measurements with heated pole inserts are not possible with MagInspect. Such measurements still require a permagraph with special pole inserts
- Accuracy in high precision applications: Although MagInspect achieves the accuracy of a permagraph, the permagraph remains superior in high-precision scientific applications because of its closed magnetic circuit.
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Comparison: MagInspect - Permagraph |
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|---|---|---|
| Feature | MagInspect | Permagraph |
| Flexibility | High flexibility in different geometries | restricted by special magnetization yokes |
| Costs | much cheaper | high investment costs |
| Measuring process | Pulse measurement with integrated Hall probe | Magnetizing yoke with Hall probes and compensation coils |
| Temperature-dependent measurements | not possible | with special pole inserts |
| Automation | fully automatic fast process | fully automatic, but more complex structure |
| Area of application | industry, quality control, research | research, laboratories, highly precise material analyzes |
MagInspect is an innovative and cost-effective solution optimized for industrial applications and rapid quality inspection. It is particularly suitable when cost, flexibility and ease of use are important. MagInspect is an affordable and easy-to-use test instrument that can record the magnetization and demagnetization curve using an impulse method. It consists of a control unit and an external pulse probe with innovative measurement electronics.
A particular advantage of this method is that the geometry of the object to be measured is largely free. In contrast, measurements with a permagraph require special magnetic geometries.
However, the Permagraph remains the first choice for scientific applications or special measurements under controlled conditions.
The MagInspect Magnetic Analyzer's measurement process
The impulse measurement probe has a built-in Hall probe with newly developed measurement electronics that can record the Hall probe's measurements very accurately with a 16-bit A/D converter at up to 10 kHz. The control unit generates the individual magnetizing or demagnetizing pulses, the measuring electronics of the probe picks up the magnetic peak field strength (H-value) and transmits it to the control unit for storage. The magnetic flux (B-value) of the magnet is measured with the probe between the individual pulses, which increase in strength during the magnetizing and demagnetizing process, and is also stored in the control unit. The whole measuring process is automatic, 10-14 pulses are generated for magnetization and demagnetization. The magnetic material can be selected in the control unit in order to use the correct magnetizing and demagnetizing parameters.
Based on the individual H and B values, the controller generates the hysteresis curve in steps that are displayed on the color screen. Microsoft Excel offers the DataStreamer import function, which can receive data directly from the controller. After transfer, the exact hysteresis curve is calculated and displayed using a mathematical spline function.
This measurement method also provides a quick way to compare the I_Hc values of a material. If the I_Hc value of magnetized magnets is to be determined, a fixed demagnetization pulse can be triggered manually and the H and B values displayed directly on the control unit. A quality check can be performed in series.
Process description MagInspect
This lecture was held on the symposium electromagnetism on February 27, 2025 at the Künzelsau campus of the Heilbronn University by Heinz-Dieter List.
