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Controlling Noise in
Instrumentation Circuits
The type of signal transmitted by a sensor can be effected by noise. A rule
of thumb is, the lower the voltage level and the higher the impedance, the
greater the sensitivity to all types of noise. The following information
addresses they types of noise and the commonly accepted solution for them.
Noise
The four major
types of noise effecting instrumentation are:
1. Static
2. Magnetic
3. Cross Talk
4. Common Mode
1. Static noise is
created by the electric field radiated by a voltage source when coupled
capacitively into the instrumentation circuit. The most efficient means to
combat static noise is to place the circuit inside a total coverage shield,
thus isolating the wires form outside influence. A grounded shield
interrupts the static noise and transports it to ground. It is important
that the shield be grounded in order to reduce static noise as an ungrounded
(floating) shield will not.
2. Magnetic noise is
produced by current flowing through other conductors and electrical
equipment such as generators, motors, etc. When current flows through such
equipment, a magnetic field is emitted around the conductor. As this field
passes through the space between the instrument circuit and the conductor, a
current is created in the instrument circuit opposing the magnetic
(transformer action). Noise is superimposed onto the signal in the
instrument circuit due to this current. Twisting the wires in the instrument
circuit causes the noise in the adjacent sections of the wire to be
cancelled. This recommendation is the most effective, and yet least
expensive, means of reducing magnetic noise problems.
3. Cross talk noise is
encountered when the instrument signals of more than one AC circuit is
carried within the same cable. This is particularly true with pulse type of
signals. The inclination is for a signal to be coupled from one pair to
another within the cable creating noise. It is recommended that individual
shielding and isolation of each pair will protect against noise picked up
from adjacent pairs, as well as reducing noise radiated by the pair itself.
4. Common mode is
encountered when grounds are at different potentials in a process plant
causing current to flow between them and a resultant noise. A high common
mode rejection ratio receiver will control common mode interference.
However, a second type of common mode noise will occur even when using a
high quality receiver and becomes a definite concern in thermocouple
extension circuits. Most thermocouples used are grounded type where it is
connected physically and electrically to the process well in which it is
located. Therefore, when a thermocouple circuit shield ( or any nearby
metallic object, such as conduit, tray, building frames, etc.) is at a
different potential than the thermocouple, a charging current will flow in
the extension wires causing a superimposed common mode noise on the signal.
To prevent common mode interference. It is recommended that the shield
circuit be used and grounded only at thermocouple. If multi-pair cables are
used with sensors, then each circuit must be individually shielded and
isolated. The shield circuit then can be maintained at the individual sensor
ground potential right back to the control room.
Modern
Instrumentation
Constant changes are
taking place in controlling, recording and indication equipment used in
industry. Most instruments fall into one of three categories. They are:
1. Potentiometric (and
Bridge)
2. Current Element
3. A to D Converters, Computers
1. Potentiometric (and
Bridge) types have a conditioned signal sent through a mechanical or
electronic chopper to compare to null (or a standard voltage). The output
voltage is then amplified so it can drive a chart recorder pen or indicator.
2. A current flowing in
the instrument circuit is converted to torque to move a chart recorder pen
or indicator in a current element type of instrument. This style of
instrument never uses an electronic circuit. This classification includes
pyrometers.
3. A to D Converters
take an analog signal which is conditioned, amplified and converted to a
digital signal for input into a computer or telemetry transmission.
Additionally, this signal can be amplified and fed into a recorder or used
for time sampling which can experience significant errors due to noise.
Industrial
Installation Hints
Electrical interference
should always be taken into consideration when selecting wire and cable for
a plant. But mechanical factors should also be weighed as well. For example,
a cable being installed in an open tray should be provided with armor
against damage from crushing or falling objects. Armor can safe guard cable
when buried underground form rodents or shifting ground.
Armor can also provide
strain relief for tensile forces hen cables are suspended between two
points. Served wire type armor can also be used for tensile, crush and
impact resistance.
Precautions should be
taken when possible during installation and engineering to reduce the
effects of noise by locating the cables away form noise sources. Equipment
such as power cables, generators, motors, and arc producing equipment
greatly increase the chance of noise on cables. Cross talk can be reduced by
transmitting only signals of the same relative magnitude and excluding any
higher level signals on the same cable.
Data transmission cable
should be shielded to reduce noise pickup form nearby instrument circuits.
Power and control cables should be twisted to reduce magnetic interference
in nearby instrument circuits. In general, locate the cables away from all
noise sources as magnetic and static fields reduce rapidly as distance from
the source increases.
Typical Rules for
Shielded Cables
1. A shielded circuit
should be grounded at the point which the instrument circuit is grounded and
isolated from all other grounds to prevent common mode noise. For example a
grounded and isolated form all other grounds to prevent common mode noise.
For example a grounded thermocouple should have its extension wire shield
grounded at the junction box, control room or any other shield in the cable.
Rather, connect the pair shield in the cable directly to the single pair
shield attached to the sensor.
2. Be sure all shields
are grounded because an ungrounded shield provides no screening of noise.
3. A shield should be
grounded only at one point.
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