PLC Analog Inputs and Signals

Welcome to another to another video
on industrial automation and PLCs. In this video, we are going to be
talking about analog inputs to the PLC. Before we get into today’s
video, if you love our videos, be sure to click the
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of new RealPars videos. This way you never
miss another one! What do we mean by analog input? Analog values are continuous values, such as temperatures
and pressures. While we tend to think of
them in digital terms, at some point we
have to convert them from the continuous world
into our digital world, and this is where
analog inputs come in. Voltage signals in the range of
plus or minus 20 volts or so and current signals in milliamps are commonly used as both
analog inputs to PLCs. Every PLC manufacturer
has I/O modules to deal with these kinds of signals, along with other, more
specialized modules. The simplest of these types is the voltage input, so
we will tackle it first. Here we have a simple circuit
that converts the position of a knob into a 0 to 5 volts signal and connected to a generic PLC voltage analog input. When the knob is on zero, the input
to the PLC will be zero volts, and at the other end of the knob’s
rotation, the PLC will see 5 volts. What the PLC program sees as the result
of the conversion of the voltage to a number depends on the
PLC and its configuration. For instance, some of the older
Allen-Bradley SLC models would return an integer ranging from 0
to 8191 for an input of 0 to 5 Volts. Usually, the newer models of PLC will allow
more flexibility in how the value is returned, so that you can scale the numeric value
to whatever your application needs. The PLC program can use this input to control (for example) the
brightness of a light bulb, the output of a heat source, the speed of
a motor, or how fast a counter counts. While the voltage input is capable of almost
everything we need in general applications, because of electrical noise
interference from other devices, current mode signal transmission
is a better choice for signals that need to travel
over any real distance. For this reason, many
applications use current inputs. From the PLC end, a current
mode interface is generally a voltage mode interface with an
added high-precision resistor. While the resistor is often
inside the PLC module and switched in by a jumper or connection
position, it is effectively the same. This is an example of a device
powered current source. The field device is getting its
power from another source. The current signal is sourced from the
positive terminal on the measuring device, traveling to the positive
terminal of the PLC. Since there is a limit to how much
voltage is available in the loop; we usually limit the overall resistance on
the measuring end of the loop to 500 Ohms. Often this is called out as part of the
field device installation instructions. Another common type of analog device
uses the 4 to 20 milliamps signal as its power source, reducing the
required complexity on the field end. Here is an example of a
“loop-powered” arrangement. The main addition to the circuit is the
separate power supply on the PLC end. Note that the positive and negative
measuring device connections are different from the previous example. The current comes from the power supply in the positive terminal
of the loop-powered device and out the negative terminal and
into the positive PLC terminal. What we have covered so far
have been generalized inputs, used where the field signal is set up
as a conditioned input for the PLC. The next types of inputs we will cover are specific to two different types of
common temperature measurement devices that are common enough to be worth
building specific modules for. There are thermocouples and resistance
temperature detectors (RTDs.) The older type of temperature
device is a Thermocouple. This type of device
generates a small signal, in the millivolt range, depending on
the temperature at the “junction,” the point where two different
metals touch to create the voltage. A thermocouple measurement module is designed to process
these tiny signals into temperature readings, and they have several
somewhat picky features. The wires used to connect the
thermocouple to the module must be made specifically for
that type of thermocouple the wrong kind of wire will
make the signal useless. Because the signals are
at such a low voltage, the wires need to be well shielded and carefully routed away
from higher voltage wiring. Finally, these sensors need
“cold junction compensation,” a feature which is built into
most thermocouple modules, but some require external
devices for this purpose. The temperature range
of a thermocouple is specified by a single letter, I.e., a “Type J” thermocouple is a common
variety with well-known characteristics. RTD or Resistance
Temperature Detector Inputs The newer RTD or resistance
temperature detector is generally more robust and
accurate than the thermocouple, but the older sensors
are still quite common because they are both designed
into older equipment, and capable of operating at temperature
ranges that RTDs are not made to handle, including up to white heat
(1600°C ) in unique applications. As the name implies, the resistance of
the device changes with its temperature, so measuring the output is
relatively straightforward. They generally come in three-
and four-wire configurations, and several temperature ranges. This video has been a basic introduction to the most common sorts of analog
inputs used in PLC-based applications. Each of the types covered
here has its quirks, and the ways the PLC inputs are built differ between manufacturers
and PLC models, so we have just covered
general principles without enough detail to implement
them in any given PLC system. We have also not attempted to
cover every type of device. Each manufacturer has unique
ways of handling measurements, and there are many other sorts of
sensors that we have not included here. We at RealPars hope you have enjoyed this
introduction to PLC analog interfaces, and hope that you will come back and
watch our other automation videos. We are working diligently
to cover more topics and improve the information available, so feel free to let us know if you want us to include a specific topic. Thanks for watching. Make sure that you head
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