Severe Noise Environments

The preceding two sections have treated the standard methods of noise reduction applicable to the majority of instrumentation problems. This section describes techniques which may become necessary when very high noise levels are anticipated or experienced.

Electrostatic Fields

Grounding
Generally, when shielding against audio-frequency electrostatic noise (below 20 kHz), it is not good practice to ground the shield at more than one point. The reason for this is that the ground points may be at different voltage levels, causing current to flow through the shield. Current flow in such ground loops can induce noise in the signal-carrying conductors through the same phenomenon that occurs in a transformer.

However, for long cables in severe noise environments, the shield impedance from one end to the other can become significant, particularly with high-frequency noise sources. When this occurs, the noise charges captured by the shield no longer find a low-resistance drain to ground, and the result is a noisy shield. Improved shield performance under such circumstances can often be obtained by grounding the shield at both ends, and/or at intermediate points - preferably at points near any localized sources of electrostatic noise. Multiple-point ground connections may also be necessary when radio-frequency interference (RFI) problems are encountered. At these frequencies the shield, or segments of the shield between grounded points, can display antenna behavior. By experimentally grounding the shield at numerous points along its length, the optimum grounding scheme can be determined.

Although the leadwires are ordinarily the dominant medium for noise induction in a strain gage circuit, noise pickup can also occur in the gage itself. When needed, a simple electrostatic shield can be fabricated by forming an aluminum-foil box over the gage and the unshielded leadwire terminations. If the gaged specimen is small and electrically conductive, aluminum tape with conductive adhesive should be used to connect the cable shield, the gage shield, and the specimen together. Conductive epoxy compounds can also be used for this purpose.

On the other hand, when gages are installed on machinery or other large, conductive test objects, care must be exercised to prevent the occurrence of ground current loops in the shield. In such cases, the foil should be electrically insulated from the machine. But the machine should be grounded with a heavy-gauge copper wire (at least 14 gauge or heavier depending upon application) connected to the single-point ground near the instrument. Care must also be taken to make certain that the shield does not form a short circuit to the gage wiring. If the cable has two shields, then, ideally at least, a double-foil shield should be used over the strain gage. The two shields should be connected together only at the instrument end of the cable.

A word about ground connections is in order. It is important to remember that all conductors are characterized by resistance, inductance, and shunt capacitance. As a result, attention should always be given to the quality of the ground connections. To be effective, a connection to ground should be made with heavy-gauge copper wire, and should be as short as practicable. If the nearest earth ground is too remote, a 6-ft (2-m) copper rod can be driven into the earth to establish a local ground.

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