Voltage signal
systems
The use of variable voltage for
instrumentation signals seems a rather obvious option to
explore. Let's see how a voltage signal instrument might be
used to measure and relay information about water tank
level:
The "transmitter" in this diagram contains
its own precision regulated source of voltage, and the
potentiometer setting is varied by the motion of a float
inside the water tank following the water level. The
"indicator" is nothing more than a voltmeter with a scale
calibrated to read in some unit height of water (inches,
feet, meters) instead of volts.
As the water tank level changes, the float
will move. As the float moves, the potentiometer wiper will
correspondingly be moved, dividing a different proportion of
the battery voltage to go across the two-conductor cable and
on to the level indicator. As a result, the voltage received
by the indicator will be representative of the level of
water in the storage tank.
This elementary transmitter/indicator system
is reliable and easy to understand, but it has its
limitations. Perhaps greatest is the fact that the system
accuracy can be influenced by excessive cable resistance.
Remember that real voltmeters draw small amounts of current,
even though it is ideal for a voltmeter not to draw any
current at all. This being the case, especially for the kind
of heavy, rugged analog meter movement likely used for an
industrial-quality system, there will be a small amount of
current through the 2-conductor cable wires. The cable,
having a small amount of resistance along its length, will
consequently drop a small amount of voltage, leaving less
voltage across the indicator's leads than what is across the
leads of the transmitter. This loss of voltage, however
small, constitutes an error in measurement:
Resistor symbols have been added to the
wires of the cable to show what is happening in a real
system. Bear in mind that these resistances can be minimized
with heavy-gauge wire (at additional expense) and/or their
effects mitigated through the use of a high-resistance
(null-balance?) voltmeter for an indicator (at additional
complexity).
Despite this inherent disadvantage, voltage
signals are still used in many applications because of their
extreme design simplicity. One common signal standard is
0-10 volts, meaning that a signal of 0 volts represents 0
percent of measurement, 10 volts represents 100 percent of
measurement, 5 volts represents 50 percent of measurement,
and so on. Instruments designed to output and/or accept this
standard signal range are available for purchase from major
manufacturers. A more common voltage range is 1-5 volts,
which makes use of the "live zero" concept for circuit fault
indication.
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REVIEW:
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DC voltage can be used as an analog signal
to relay information from one location to another.
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A major disadvantage of voltage signaling
is the possibility that the voltage at the indicator
(voltmeter) will be less than the voltage at the signal
source, due to line resistance and indicator current draw.
This drop in voltage along the conductor length
constitutes a measurement error from transmitter to
indicator.
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