Electromagnetic Fields: Noise Control

Electromagnetic Fields

Cabling
As with electrostatic noise pickup, the leadwires commonly represent the principal source of magnetic noise induction in strain gage circuits. In intense electromagnetic fields with steep gradients (near motors, generators, and similar equipment), ordinary wire-twisting techniques may prove inadequate. An end view of a conventionally twisted pair can reveal the reason for pickup. As indicated in Fig. 501.4 , even if the induced noise were precisely equal in both wires the amplifier noise output would be zero only if the amplifier had infinite common-mode rejection characteristics - an impossibility. In order to minimize common-mode noise voltages, a special, woven, four-wire cable has been designed which, as seen from the wire end, eliminates the spiral inductive loops (Fig. 501.8). For maximum cancellation of electrostatic fields, pairs of wires (composed of one wire from each plane) are connected in parallel. Referring to the figure, wires 1 and 2 are paralleled to form one conductor; and wires 3 and 4 to form the other. So connected, this type of cable is largely insensitive to magnetic field gradients, both parallel and perpendicular to the cable length. The cable is known as Inter-8 Weave, and is available from: Magnetic Shield Division, Perfection Mica, 740 Thomas Drive, Bensensville, Illinois 60106.

Fig. 501.8 - Woven cable to reduce severe electromagnetic radiation and pickup.

Even though the strain gage is much less frequently the significant medium for magnetic noise induction than the leadwires, different gage patterns have differing sensitivities to noise pickup. For instance, if the gage has both solder tabs at one end, the net noise pickup is less than for a gage with one tab at each end. As shown in Fig. 501.5 , the difference in noise sensitivity results from the relative size of the inductive loop area in each case. It is also worth noting that smaller gages, with more closely spaced grid lines, are intrinsically quieter than large gages.

H-Series NoninductiveGages
In severe magnetic fields, especially those with steep gradients in field intensity, additional measures may be required. For this purpose, Micro-Measurements has developed a special gage configuration, the H-Series , consisting of two identical grids, with one stacked directly above, and insulated from, the other. By connecting the upper and lower gage elements in series so that the current flows in opposite directions through the two grids, the noise induced in the assembly tends to be self-cancelling. This arrangement is particularly effective against magnetic field gradients and their components parallel to the test surface. The dual-element gage is intended to function as one arm of a Wheatstone bridge circuit; and the bridge is usually completed with another gage of the same type, or with a fixed precision resistor. Standard practices are followed when installing the gages; but the Micro-Measurements M-Bond 600/610 adhesive system is recommended for bonding, since this will result in the thinnest glue line, and placement of the grids as close as possible to the specimen surface. Available from Micro-Measurements are two types of dual-element, noninductive stacked gages - linear H06A-AC1-125-700 and a three-gage rosette H06A-AD3-125-700 .

In addition to the strain gage size and pattern, the selection of the gage grid alloy should be given careful consideration. If the grid alloy is magnetic, it will be subject to extraneous physical forces in a magnetic field; and, if magnetoresistive, will undergo spurious resistance changes. Similarly, if the alloy is magnetostrictive, the grid will try to change length in the magnetic field. Isoelastic alloy, for example, should not be used in magnetic fields, since it is both strongly magnetoresistive and magnetostrictive. Stemming from their comparative freedom from magnetic effects, constantan and Karma-type alloys are usually selected for such applications. Constantan, however, at cryogenic temperatures and in high magnetic fields (7-70 Tesla) becomes severely magnetoresistive. The Karma-type alloy is ordinarily preferred for cryogenic service because of its generally superior performance in magnetic fields at very low temperatures.

Magnetic Shielding
When necessary, strain gages can also be shielded from electromagnetic fields to some degree with a magnetic shielding material such as mu-metal. Two or more layers of the shielding material may be required to effect a noticeable improvement. Of course, even this will be ineffective if the source of the magnetic field is beneath the strain gage. When high-frequency fields are encountered, be sure that the material is suitable (high permeability) at the anticipated frequency.

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