Welcome to another video on industrial automation and PLCs.
In this video, we are going to be talking about analog inputs to the PLC.
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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 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 PLC 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.
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