|4. Transmission System Dynamics
A major difference between electronic and pneumatic transmission systems is the time required
for signal transmission. In an electronic system there are no moving parts, only the state
of the signal changes. This change occurs with virtually no time lost.
Signal Transmission for Electronic and Pneumatic Signals
As we stated previously, mechanical movement takes place whenever any pneumatic process
signal changes. When devices move mechanically, time is lost. In addition, pneumatic
systems, because they contain moving parts, are higher maintenance and subject to
vibration, as well as rotational or gravitational mounting problems. However, pneumatic
systems are still in place in many plants because they are safer than electrical systems
in certain environments containing potentially explosive atmospheres.
|4.1 Transmission lag
Pneumatic Transmission Signal Lag
The figure above shows the time lost with a pneumatic system. This figure represents a
system using 3/16 ID tubing for the transmission line. As shown at the bottom of the
graphic, in short distances, the effect of time is small. Under 200 feet, a signal can
change 15 psi to 3 psi (the span of a pneumatic device) in roughly 0.4 seconds or less.
This lost time represents the time needed to make up the air volume difference in the line
(either replacing or releasing the air volume). A lag of 0.4 seconds is not critical, but
as the distance of the signal line increases, so does the lag. At 400 feet, the lag time
rises to about 1.3 seconds. At 1000 feet, the time is nearly 7 seconds -- in some
processes, a critical period of time. Note: This time measurement represents the time
required for the signal to travel from the sensing device to the controlling device. If
there is a change and the controller responds to it immediately, the amount must be
doubled for the signal transmission to reach a final control device. Pneumatic devices are
best used for safety applications, simplicity, and for valve actuators,
always in applications where the line length is kept under 100 feet; otherwise electronic
signals should be used.
|4.2 Transmitter gain
A transmitter's gain, that is the ratio of the output of the transmitter to the input signal, is constant
regardless of its output. In other words, an electronic transmitter's gain will remain
constant whether it's output is 0% of span (4 mA) or 100% of span (20 mA) or any other
point between those extremes.
Transmitter Gain for an Electronic Transmitter
|4.3 Smart Transmitters
So far, the discussion has centered around electronic and pneumatic transmitters. The
input and output of both of these types of transmitters is an analog signal -- either a mA current or air
pressure, both of which are continuously variable. There is another kind of transmitter --
the "smart" transmitter.
Smart Transmitter Components and Function
The figure above illustrates functions of a smart
transmitter. They can convert analog signals to digital signals (A/D), making
communication swift and easy and can even send both analog and digital signals at the same time as denoted by D/A.
A smart transmitter has a number of other capabilities as well. For instance, inputs
can be varied, as denoted by A/D. If a temperature transmitter is a smart transmitter, it
will accept millivolt signals from thermocouples and
resistance signals from resistance temperature devices (RTDs),
Components of the smart transmitter are illustrated in the lower figure. The
transmitter is built into a housing about the size of a softball as seen on the lower
left. The controller takes the output signal from the transmitter and sends it back to the
final control element. The communicator is shown on the right.
The communicator is a hand-held interface device that allows digital
"instructions" to be delivered to the smart transmitters. Testing, configuring ,
and supply or acquiring data are all accomplished through the communicator. The
communicator has a display that lets the technician see the input or output information.
The communicator can be connected directly to the smart transmitter, or in parallel any
where on the loop.
|4.4 Smart transmitter microprocessor-based features
Smart transmitters also have the following features:
- Signal conditioning
Smart transmitters can be configured to meet the demands of the process in which they
are used. For example, the same transmitter can be set up to read almost any range or type
of thermocouple, RTD, or thermistor. Because of this, they reduce the need for a large
number of specific replacement devices.
The range that the smart transmitter functions under can be easily changed from a
remote location, for example by the technician in a control room. The technician or the
operator has access to any smart device in the loop and does not even have to be at the
transmitter to perform the change. The operator does need to use a communicator, however.
A communicator allows the operator to interface with the smart transmitter. The
communicator could be a PC, a programmable logic controller (PLC), or a hand-held device.
The type of communicator depends on the manufacturer.
Re-ranging is simple with the smart transmitter. For instance, using a communicator,
the operator can change from a 100 ohm RTD to a type-J thermocouple just by reprogramming
the transmitter. The transmitter responds immediately and changes from measuring
resistance to measuring millivoltage.
There is a wide range of inputs that a smart transmitter will accept. For instance,
with pressure units, the operator can determine ahead of time whether to use inches of
water, inches of mercury, psi, bars, millibars, pascals, or kilopascals.
Another characteristic of a smart transmitter is its ability to act as a stand-alone
transmitter. In such a capacity, it sends the output signal to a distributed control system (DCS) or a PLC.
4.4.4 Signal conditioning
Smart transmitters can also perform signal conditioning, scanning the average signal
and eliminating any "noise" spikes.
Signals can also be delayed (dampened) so that the response does not fluctuate. This is
especially useful with a rapidly changing process.
Finally, a smart transmitter can diagnose itself and report on any problems in the
process. For example, it can report on a circuit board which is not working properly.
4.4.6 Summary of smart transmitter benefits
There are distinct advantages in using a smart transmitter. The most important include
ease of installation and communication, self-diagnosis, improved and digital reliability.
Smart transmitters are also less subject to effects of temperature and humidity than
analog devices. And although vibration can still affect them, the effects are far less
than with analog devices. Smart transmitters also provide increased accuracy. And because
can replace several different types of devices, using them allows for inventory reduction.