Process Signals

SIGNALS
There are three kinds of signals that exist for the process industry to
transmit the process variable measurement from the instrument to a
centralized control system.
1. Pneumatic signal
2. Analog signal
3. Digital signal





Pneumatic Signals
Pneumatic signals are signals produced by changing the air pressure
in a signal pipe in proportion to the measured change in a process
variable. The common industry standard pneumatic signal range is
3–15 psig. The 3 corresponds to the lower range value (LRV) and the
15 corresponds to the upper range value (URV). Pneumatic signalling
is still common. However, since the advent of electronic instruments
in the 1960s, the lower costs involved in running electrical signal wire
through a plant as opposed to running pressurized air tubes has made
pneumatic signal technology less attractive.

Analog Signals
The most common standard electrical signal is the 4–20 mA current
signal. With this signal, a transmitter sends a small current through a
set of wires. The current signal is a kind of gauge in which
4 mA represents the lowest possible measurement, or zero, and 20
mA represents the highest possible measurement.
For example, imagine a process that must be maintained at 100 °C.
An RTD temperature sensor and transmitter are installed in the
process vessel, and the transmitter is set to produce a 4 mA signal
when the process temperature is at 95 °C and a 20 mA signal
when the process temperature is at 105 °C. The transmitter will
transmit a 12 mA signal when the temperature is at the 100 °C
setpoint. As the sensor’s resistance property changes in
response to changes in temperature, the transmitter outputs a
4–20 mA signal that is proportionate to the temperature changes. This
signal can be converted to a temperature reading or an
input to a control device, such as a burner fuel valve.
Other common standard electrical signals include the 1–5 V (volts)
signal and the pulse output.

Digital Signals
Digital signals are the most recent addition to process control signal
technology. Digital signals are discrete levels or values that are
combined in specific ways to represent process variables and also carry
other information, such as diagnostic information. The methodology
used to combine the digital signals is referred to as protocol.
Manufacturers may use either an open or a proprietary digital
protocol. Open protocols are those that anyone who is developing a
control device can use. Proprietary protocols are owned by specific
companies and may be used only with their permission. Open digital
protocols include the HART® (highway addressable remote
transducer) protocol, FOUNDATION™ Fieldbus, Profibus, DeviceNet,
and the Modbus® protocol.
(See Module 8: Communication Technologies for more information
on digital communication protocols.)