PLC Analog Inputs and Signals

Welcome to another to another blog post on industrial automation and PLCs. In this lesson, we are going to be talking about PLC Analog Signals and Inputs.

What do we mean by PLC analog inputs?

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 PLC 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.

Voltage-Mode Analog Inputs

In the image below we have a simple circuit that converts the position of a 0 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.

Current-Mode PLC Analog Inputs

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.

Below you can find 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.

Below 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.)

Thermocouple Inputs

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 lesson 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 analog 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 inputs, and hope that you will come back and watch our other automation lesson.

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.

We hope that you found it interesting, and that you will come back for more of our educational blogs. share your thoughts and ideas directly in the comments.

See you next week,

The RealPars Team

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