Ensure magnetic components meet specs

-August 12, 2013

Typical Magnetic Field Levels
Figure 3 gives several examples of magnetic field levels (flux density) in the world around us. Other values that may be important to the manufacturing engineer or technician are given below in gauss (G) cgs values, with tesla (T) SI values in parentheses:

Figure 3. Some example values for familiar magnetic fields.

Magnetic Measurement Tool Box
Many types of equipment are available to make magnetic measurements. For QC professionals, the proper choice of instrument or system for these measurements will depend, in part, on the relative degree of accuracy that is needed, on the components being measured, and on the particular phase of the assembly or manufacturing process in which the measurement is made.  

Some of the instrumentation that is conventionally used in various phases of inspection may test magnets and magnetic assemblies indirectly, such as by measuring motor torque, speaker output power or relay holding current, all of which depend, at least in part, on characteristics of the applied magnetic field. While this type of testing can give an idea of general performance, it may provide questionable results or disclose critical flaws too late in the manufacturing cycle, such as in final preparation stages.

Indirect testing might, for instance, fail to detect a faulty or under-performing magnet in a multiple-magnet assembly, or it might fail to identify a magnet that is only marginally defective before it is integrated into a product. Failure to detect and correct these problems at an early stage can lead to later problems, including final inspection rejections and field failures that require costly revisions or reworking. In the worst case, failure to quickly identify problems related to magnet performance can lead to product recalls and even raise safety concerns at later stages or after the product has shipped.  

The added step of direct testing magnetic components pays for itself in its ability to prevent future problems. For quality control at incoming inspection of permanent magnets, solenoids, or magnetic subassemblies, many technicians prefer to use magnetic instrumentation such as a gaussmeter, fluxmeter or, in some cases, a more complex system such as a hysteresisgraph (B-H looper) or vibrating sample magnetometer (VSM): see Figure 4. These instruments provide a direct method to measure important magnetic characteristics of the item under test.

Figure 4.   A vibrating sample magnetometer, such as this MicroMagTM VSM from Lake Shore Cryotronics, is a high-end option for characterizing magnetic properties of materials. This type of system is ideal for testing nanoparticles or other material samples in powder, solid, liquid or thin film form.

Magnetic testing during the assembly or final performance stages of production is less straightforward than during incoming inspection. Selecting the proper equipment for this phase depends on the specific characteristics that need to be measured.

If the goal is to determine magnetic field strength at a point or in a gap, field uniformity, or field shape, then the Hall effect gaussmeter (see Figure 5, top) is the instrument of choice. Continued improvements in sensitivity, usability, and cost make the Hall effect gaussmeter a welcome addition to the QC technician’s toolbox.  Certainly, other types of gaussmeters are available for a range of applications and offer varying degrees of accuracy. The coil-type AC teslameter, for example, is about as common as the Hall effect gaussmeter among industrial consumers and can be comparable in price. However, the Hall effect product is more accurate and more versatile. Today’s Hall effect gaussmeters offer more probe options and measure both AC and DC fields rather than just AC. The gaussmeter is also useful for field mapping and magnitude determination.  

Figure 5.  The Hall effect gaussmeter (top) and the fluxmeter (bottom) provide much of the data that industrial QC technicians require when testing magnetic components and products, at a considerably lower cost than more complex instrumentation.

The fluxmeter (see Figure 5, bottom) is another option worthy of consideration for magnetic field measurement. This instrument is advantageous for making accurate measurements when flux or flux density is to be measured inside a magnetic conductor such as a steel return path. The fluxmeter is an economical instrument for permanent magnet sorting.

More sophisticated instrumentation may be needed in some instances.  For example, the technician may be faced with making magnetic measurements that are at high frequency (greater than 100 kHz), such as when measuring RF coil fields, induction heating fields or radiated fields from electronic equipment.  Other specialized measurements may be needed when magnetic field strength is at a very low value (less than 50 mG), such as in space or military applications, or when very high accuracy (i.e., better than 0.01 percent) is needed, such as when measurements are needed for calibrating MRI machines or during setup and calibration of other gaussmeters or magnet systems.  Such cases may call for sophisticated tools such as high-frequency coil magnetometers, flux gates, or NMR gaussmeters, respectively.

Standards labs and high-end research facilities may also use expensive, full-scale systems for magnetic materials characterization. Such complex systems include vibrating sample magnetometers (VSMs), B-H loopers, and permeameters. Each of these systems has its strengths and shortcomings. The VSM can be the best option when material samples vary widely in form (powder, liquids, solids, thin films) and when a direct measurement of magnetic moment is required, whereas the B-H looper is set up to handle larger samples.

For the bulk of magnetic field measurements that QC professionals are most likely to encounter in practice, gaussmeters and fluxmeters provide much of the required data at a reasonable cost to the industrial user. Because these two instruments are particularly useful and are becoming more and more familiar to those in QC environments, the next article in this series will examine the use of these instruments in greater detail.

About the Author
Jeff Dierker is a Senior Consulting Engineer at Lake Shore Cryotronics. See his profile.

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